extfieldsofstudy: Chemistry Medicine

s2fieldsofstudy: Chemistry Biology

text:
Highly selective nickel-catalyzed gem-difluoropropargylation of unactivated alkylzinc reagents

In spite of the important applications of difluoroalkylated molecules in medicinal chemistry, to date, the reaction of difluoroalkylating reagents with unactivated, aliphatic substrates through a controllable manner remains challenging and has not been reported. Here we describe an efficient nickel-catalyzed cross-coupling of unactivated alkylzinc reagen\ts with gem-difluoropropargyl bromides. The reaction proceeds under mild reaction conditions with high efficiency and excellent regiochemical selectivity. Transformations of the resulting difluoroalkylated alkanes lead to a variety of biologically active molecules, providing a facile route for applications in drug discovery and development. Preliminary mechanistic studies reveal that an alkyl nickel intermediate [Ni(tpy)alkyl] (tpy, terpyridine) is involved in the catalytic cycle.

T he important role of organofluorinated compounds in medicinal chemistry, agrochemistry, and materials science 1, 2 has triggered substantial efforts in developing new and efficient methods for the controllable introduction of fluoroalkyl groups into organic molecules 3,4 . Particularly, the preparation of difluoroalkylated compounds have received much attention over the past few years [5][6][7] due to the unique properties of difluoromethylene group (CF 2 ) that can improve the metabolic stability of biologically active molecules and lead to enhanced acidity of its neighboring group, increased the dipole moments as well as conformational changes 8,9 . To date, the transition-metalcatalyzed difluoroalkylations have emerged as a useful strategy for the construction of Ar-CF 2 R bond and thus to access difluoroalkylated arenes efficiently 6,7,[10][11][12][13][14] . However, it remains challenging to adapt this strategy to the formation of Csp 3 -CF 2 R bond 15 . To the best of our knowledge, the reaction of difluoroalkylating reagents with unactivated, aliphatic substrates catalyzed by transition-metal has not been reported so far. Although some approaches to difluoroalkylated alkanes exist, they mainly rely on the transformation of functional groups, such as carbonyl, olefin, and alkyne [16][17][18][19][20] . We envisioned that if the transitionmetal-catalyzed direct introduction of a CF 2 group into aliphatic chain at any position is successful, it would be a significant advance and provide an effective and general method for siteselective synthesis of difluoroalkylated alkanes.
Accordingly, herein, we demonstrate the feasibility of transition-metal-catalyzed cross-coupling of readily available alkylzinc reagents with gem-difluoropargyl bromides. The use of gem-difluoropropargyl bromides is because of their versatile synthetic utility of the carbon-carbon triple bond and their ready availability 21,22 . Thus, this type of precursor would provide attractive and useful difluoroalkylated structures for applications in medicinal chemistry and materials science. However, efficient methods to such a valuable structure are very limited 15,23 . As part of our ongoing study in transition-metalcatalyzed fluoroalkylations 11,12,[24][25][26] , herein, we describe a highly selective Ni-catalyzed cross-coupling between unactivated alkylzinc reagents and gem-difluoropropargyl bromides [27][28][29][30][31] . This method has several advantages: broad substrate scope, including a variety of unactivated alkylzinc reagents and gem-difluoropropargyl bromides; high efficiency and excellent regiochemical selectivity for the formation of gem-difluoropropargylated products; a catalyst loading of NiCl 2 or Ni(dppf)Cl 2 (dppf, 1,1′-bis(diphenylphosphino)ferrocene) as low as 0.5-5 mol%; and practical for gram-scale production.

Results
Optimization of the Ni-catalyzed cross-coupling. We began our initial studies on the cross-coupling of 3-phenylpropylzinc bromide 1a with (3-bromo-3,3-difluoroprop-1-yn-1-yl)triisopropylsilane 2a in the presence of nickel catalyst with a variety of diamine ligands (Table 1, entries 1-8). However, only low yields of desired product 3a were obtained. To our delight, switching diamine ligand with a triamine ligand (tpy) dramatically improved the yield of 3a and no allenic side product was observed (entry 9). The beneficial effect of tpy on the current nickelcatalyzed process is in sharp contrast to previous work, in which diamine ligands were the best ligands for nickel-catalyzed difluoroalkylations 24,32,33 . We also found that the reaction was insensitive to the nature of nickel catalysts (entries [10][11][12][13][14]. All the tested nickel salts showed good reactivities and Ni(dppf)Cl 2 (2.5 mol%) was the best choice, providing 3a in 94% yield upon isolation (entry 17). Alternatively, the use of 4,4′,4″-tri-tert-butylterpyridine (tpy′) as a ligand also produced 3a in a comparable yield (entry 18), thus providing a complementary ligand of choice for this process. No product was obtained in the absence of nickel catalyst (entry 19) and only 9% yield of 3a was produced without ligand (entry 20), thus demonstrating the essential role of both nickel catalyst and ligand in promotion of the reaction.
Scope of the Ni-catalyzed cross-coupling. To ascertain the substrate scope of this method, a wide range of alkylzinc reagents were examined (Table 2). Overall, good-to-high yields of gem-difluoropropargylated alkanes 3 were provided under optimized reaction conditions; even NiCl 2 was used as a catalyst, high product yields were still observed (3a-3c, 3f, 3i, 3n). The reaction exhibits high-functional group tolerance. Substrates 1, bearing important functional groups, such as ether, ester, indole, morpholine, ferrocenyl, and vinyl, all underwent the reactions smoothly (3d-3i, 3n-3q). Importantly, the successful formation of 3j-3l in good yields with intact bromide, chloride, and fluoride provides good synthetic handles for downstream transformations. Most remarkably, an alkylboronic acid ester was also compatible with the reaction conditions (3m), thus highlighting the high chemoselectivity of current nickel-catalyzed process. Moreover, thienyl-containing substrate furnished corresponding product 3r with high efficiency (90% yield). The reaction was readily scalable as demonstrated by the gram-scale synthesis of 3a (72% yield) Table 2 Ni-catalyzed cross-coupling of unactivated alkylzincs 1 with 2a a Bn, benzyl a Reaction conditions (unless otherwise specified): 1 (1.2 equiv in 2 mL DMA), 2a (0.6 mmol, 1.0 equiv), dioxane (4 mL) for 8 h b 5 mol% of NiCl 2 was used c Reaction run in 0.3 mmol scale and 1.5 equiv of 1o was used d 0.5 mol% of Ni(dppf)Cl 2 and 0.5 mol% of tpy were used even when 0.5 mol% of Ni(dppf)Cl 2 was used, thus demonstrating the reliability of current process. In addition to alkylzinc reagents, a variety of gem-difluoropropargylated bromides 2 were investigated (Table 3). Compounds 2 bearing silyl substituents, such as methyldiphenylsilyl and triethylsilyl groups, underwent the reaction smoothly (4a and 4b). Aryl substituted 2 was also applicable to the reaction without observation of allenic side products (4c and 4d). Notably, no defluorination side reaction occurred during the reaction process, in sharp contrast to previous reports, where defluorinations were the major pathway [34][35][36][37] . Remarkably, the alkyl substituted gemdifluoropropargylated bromides 2 were also competent coupling partners and provided corresponding products in good yields (4e-4j), in which the application of this method could efficiently lead to compound 4j, a key intermediate for the preparation of bioactive molecule used for pheromone study 38 . To demonstrate the generality of this approach further, some biologically active molecules containing 2 were also examined. The estrone-derived substrate underwent the reactions smoothly (4k and 4l), even 90% yield of 4k was obtained. The glucose-and prolinecontaining gem-difluoropropargyl bromides were also amenable to the reaction (4m and 4n). Because carbohydrates and amino acids play important roles in life science, it would provide good opportunity to discover some interesting biologically active molecules. Importantly, sulbactam, a drug for the treatment of bacterial infection, derived substrate furnished 4o efficiently, thus featuring the applicability of this protocol in drug discovery and development.
Transformations of compounds 3 and 4. The utility of this approach can also be highlighted by the transformations of resulting gem-difluoroalkylated alkanes. As shown in Fig. 1a, deprotection of 3a with TBAF, followed by [3 + 2] cyclization 39 afforded difluoroalkylated pyrrole 7 in high yield. Sonogashira reaction of 5 with heteroaryl iodides proceeded smoothly, offering a complementary method to the approach illustrated in Table 3. The CF 2 -containing alkane 10 can also be readily prepared through hydrogenation of carbon-carbon triple bond without defluorination, providing a facile route for site-selective introduction of a CF 2 group into an aliphatic chain (Fig. 1b). Notably, compounds 3 and 4 can serve as key intermediates for the synthesis of biologically active molecules. For example, compound 11, a probe for the study of hydrophobic interaction in pheromone reception 38 , was readily prepared by selective Table 3 Ni-catalyzed cross-coupling of 1 with 2 a TES triethylsilyl, TBS terbutyldimethylsilyl a Reaction conditions (unless otherwise specified): 1 (1.5 equiv in 1 mL DMA), 2 (0.3 mmol, 1.0 equiv), dioxane (2 mL) for 8 h reduction of compound 4f (Fig. 1c). In light of a series of pheromone analogs containing Z-difluoroalkylated alkenes 38,40 , this transformation is highly relevant to the synthesis of such a kind of structures. Furthermore, compound 12 derived from gem-difluoroalkylated alkane 3i can also functionalize as a key intermediate for the synthesis of analog of omega-3 fatty acids 13 for therapeutic use (Fig. 1d) 41    metabolites. Importantly, the copper-free click reaction 42 of terminal alkyne 12 with the dansyl fluorophore provided [3 + 2] cycloaddition products 15 with high efficiency (86%, as a mixture of regioisomers) in phosphate-buffered saline (PBS, pH 7.4) at 37 o C (Fig. 1d). Yet, no reaction occurred by using its nonfluorinated counterpart (for details, see Supplementary Methods). Thus, these results clearly demonstrate that the presence of CF 2 group is critical for the high reactivity of 12 in the copper-free click reaction. This reactivity is not only because the lowest unoccupied molecular orbital (LUMO) of the difluoroalkylated alkyne can be lowered by the CF 2 group 43 , but also because the stability of the transition state can be increased through hyperconjugative interactions between the carbon-carbon triple bond π system and vicinal σ* C-F acceptors 44 45,46 , was prepared efficiently through the current nickel-catalyzed process, thus demonstrating the utility of this approach further (Fig. 1e).

Discussion
To gain mechanistic insights into the current reaction, radicalinhibition experiment was conducted. We found that the reaction was almost terminated (giving 3i in a yield of 2%) by the electron transfer inhibitor (1,4-dinitrobenzene, 1.0 equiv) 47 that was added to the reaction mixture of 1i and 2a under standard reaction conditions (Fig. 2a). Furthermore, a ring-expanded product 20 (10% yield) was afforded when α-cyclopropylstyrene 19, which is known to participate in radical rearrangements 48 , was treated with 2a under standard reaction conditions (Fig. 2b). This result confirms the possibility of forming gem-difluoropropargyl radical intermediate during the nickel-catalyzed process. As a direct evidence, the electron paramagnetic resonance (EPR) study of reaction of 2a with spin-trapping agent phenyl tert-butyl nitrone 49 showed a spin adduct of the trapped gemdifluoropropargyl radical 21 (a N = 14.10 G, a H β = 2.01 G) in the EPR sextet spectrum (Fig. 2c and Supplementary Fig. 168), suggesting that the difluoroalkyl group is substituted at the β-carbon atom of nitroxide so that the a N and a H β values are decreased by the strong electron-withdrawing effect of CF 2 group through reducing the interaction of the electron spin with β-H atom as well as the electron pair on the nitrogen atom (usually, a N values are 14-16.5 G for the non-fluorinated dialkyl nitroxides) 49. Moreover, EPR spectrum also showed the EPR signal of spin adduct 21 at the beginning of the reaction ( Supplementary  Fig. 169). Thus, a combination of the above results demonstrates that a gem-difluoropropargyl radical generated via a nickelinitiated single-electron transfer pathway is involved in the reaction.
We also prepared Ni(I) complex [(tpy)Ni I -I] 50, 51 (A1) and Ni (II) complex [(tpy)Ni II Cl 2 ] (D1) 52 as the nickel species to add in the reaction of 1a with 2a, and found that both complexes catalyzed the reaction to afford compound 3a with high yields (Fig. 3a, b), thus demonstrating that the ligand dppf in the nickel catalyst Ni(dppf)Cl 2 just play a trivial role to promote the reaction. We propose that dppf in this precatalyst benefits the formation of an active nickel species that can facilitate the catalytic cycle. To further investigate the detailed mechanism, we prepared methyl nickel complexes [(tpy)Ni-Me] (B1) 53 and [I-Ni(tpy′)Me] (C1) 54 as the catalysts in the mechanistic studies. In C1 tpy′ is used instead of tpy because [I-Ni(tpy)Me] (C1′) is prone to decomposition to [(tpy)NiI] 51 , and because tpy′ as the ligand showed similar reactivity as tpy in the current cross-coupling (Table 1, entry 18). In the presence of B1 catalyst, the reaction could provide compound 3a in high yield (Fig. 3c), as well as a methylated product 22 in a yield in the range of 3-17%, increasing as raising amount of B1 from 5 to 20 mol% (Fig. 3c). On the contrary, the use of C1 (10 mol%) as a catalyst led to only 1% yield of compound 22, albeit formation of 3a in a 73% yield (Fig. 3d). These findings suggest that compared with complex C1, complex B1 is more favorable for the formation of 22. In addition, cross-coupling product 23 was not generated between alkylzinc 1a and C1, and even the stoichiometric reaction of C1 with 1a only afforded 23 in 5% yield (Fig. 3g). Both results indicate that a simple transmetalation of C1 by 1a is unlikely, ruling out a Ni(0/II) redox cycle. On the basis of the above analysis, we envisioned that a Ni(I/III) catalytic cycle is involved in the current reaction and alkyl nickel complex [(tpy)Ni-alkyl] (B) may be the key intermediate. To further support this possible pathway, a stoichiometric reaction of B1 with 2a (1.0 equiv) was conducted and provided 22 in 51% yield (Fig. 3e). However, only 18% yield of 22 was obtained when C1 was used (Fig. 3f). Thus, the reaction via a key Ni(I) intermediate [(tpy)Ni-alkyl] (B) generated by a transmetalation-first pathway between alkyzinc and nickel(I) complex [(tpy)Ni-X] (A) is reasonable.
On the basis of above results 55, 56 , a plausible reaction mechanism involving a Ni(I/III) catalytic cycle is proposed (Fig. 3h). The comproportionation of in situ-generated Ni(0) from the reaction of Ni(II) with alkylzinc reagent and the remaining Ni(II) species generates the Ni(I) complex [(tpy)Ni I -X] (A) 53 , which subsequently produces alkyl nickel complex [TpyNi-Alkyl] (B) through transmetalation with alkylzinc. Because such a terpyridine-based nickel(I) complex B is actually a nickel(II) complex [(tpy −1 )Ni II -alkyl] 53 , a one-electron, ligand-based redox pathway would be occurred in the reaction of B with gemdifluoropropargyl bromide 2 to generate C and difluoroalkyl radical 53 . Recombination of C with difluoroalkyl radical produces nickel(III) complex [Alkyl(Tpy)Ni III (R f )X] (E, R f = CF 2 CCR 2 ), which subsequently undergoes reductive elimination to deliver gem-difluoropropargylated alkane and regenerate A simultaneously.
In conclusion, we have demonstrated a Ni-catalyzed difluoroalkylation of unactivated aliphatic substrates. The reaction proceeds under mild reaction conditions and allows a variety of alkylzinc reagents and gem-difluoropropargyl bromides with high efficiency and excellent chemoselectivity. Importantly, most of the resulting gem-difluoropropargylated alkanes are unknown and can serve as versatile building blocks for the organic synthesis and medicinal chemistry. Preliminary mechanistic studies reveal that the transmetalation between Ni(I) and alkylzinc to generate alkyl nickel complex [alkylNi(tpy)] initiates the reaction, and a Ni (I/III) catalytic cycle is involved in the reaction. This mechanism is different from that of recently reported nickel-catalyzed Negishi arylation of proparylic bromides, which has been suggested through a bimetallic pathway 56 . Because of the synthetic utility of carbon-carbon triple bond and the unique properties of CF 2 , which can significantly improve the metabolic stability of biologically active molecules, this reaction will have applications in life and materials science as well as prompt the research area in fluoroalkylation of unactivated, aliphatic compounds from lowcost fluoroalkyl halides. Further studies to other derivative reactions are now in progress in our laboratory.

Methods
General procedure for nickel-catalyzed cross-coupling. To a 25 mL of Schlenk tube was added Ni(dppf)Cl 2 (2.5 mol%) and tpy (2.5 mol%). The tube was evacuated and backfilled with Ar for three times, then gem-difluoropropargyl bromide 2a (0.6 mmol, 1.0 equiv) and 1,4-dioxane (4 mL) were added. The resulting mixture was stirred at room temperature for 5 min, alkylzinc reagent 1 (0.72 mmol, 1.2 equiv in 2 mL of DMA) was then added dropwise over a period of 5 min. The tube was screw capped and put into a preheated oil bath (40 o C). After stirring for 8 h, the reaction mixture was cooled to room temperature and diluted with EtOAc (50 mL). The resulting mixture was filtered with a pad of cellite. The filtrate was washed with water (15 mL), the organic layer was dried over Na 2 SO 4 , filtered, and concentrated. The residue was purified with silica gel chromatography to give the pure product.
Data availability. The authors declare that all the data supporting the findings of this study are available within the paper and its supplementary information files.


text:
Triamidoamine thorium-arsenic complexes with parent arsenide, arsinidiide and arsenido structural motifs

Despite a major expansion of uranium–ligand multiple bond chemistry in recent years, analogous complexes involving other actinides (An) remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, phosphorus and chalcogenides are reported, and none to arsenic are known; indeed only two complexes with thorium–arsenic single bonds have been structurally authenticated, reflecting the challenges of stabilizing polar linkages at the large thorium ion. Here, we report thorium parent–arsenide (ThAsH2), –arsinidiides (ThAs(H)K and ThAs(H)Th) and arsenido (ThAsTh) linkages stabilized by a bulky triamidoamine ligand. The ThAs(H)K and ThAsTh linkages exhibit polarized-covalent thorium–arsenic multiple bonding interactions, hitherto restricted to cryogenic matrix isolation experiments, and the AnAs(H)An and AnAsAn linkages reported here have no precedent in f-block chemistry. 7s, 6d and 5f orbital contributions to the Th–As bonds are suggested by quantum chemical calculations, and their compositions unexpectedly appear to be tensioned differently compared to phosphorus congeners.

Liddle and co-workers report a series of thorium complexes with a variety of Th-As(H) bonding motifs in this article. This work is challenging both in practice and analysis. This chemistry represents the cutting edge of actinide/main group chemistry. That being said, the work in its present form is not publishable in this or any other journal. A review of the CIF shows the use of DFT calculations to restrain (very) poor refinements to match calculations. For starters, this is the opposite of how science is supposed to work. Theory is modified to fit data not vice versa. The resulting refinements are beyond the limits of credibility with restraints in one case totally 1802! The second terms of the weighting schemes are larger than 100 and in one case 159. The wR2 tops 20% in several structures. Even the space groups are worrisome. The first one is in Pna2(1), which is polar. This is important because few molecular systems adopt this space group. There are some, but normally the molecules have highly acentric shapes and intermolecular interactions (typically hydrogen bonds) create the polar c axis by aligning acentric shapes in a single direction. It is not clear to me how this happens in this system. The work from Liddle and co-workers is normally quite brilliant. This does not fit the mould from this group.

Reviewer #3 (Remarks to the Author):
This manuscript describes the synthesis, structure, and bonding of new thorium-arsenic compounds including the first actinide arsenide, which includes some multiple bonding character. Overall the manuscript is well-written and the compounds are well characterized (although compound 5 needs better combustion analysis), however this report is too close to the previously reported thorium and uranium phosphorus compounds to be published in Nature Communications. Absolutely beautiful compounds and unique to arsenic versus phosphorus but the approach to obtaining these compounds is too cookie-cutter.
Reviewer #1 (Remarks to the Author): Liddle and associates detail the synthesis of several thorium arsenic complexes featuring terminal AsH2, bridging AsH and bridging As units. The compounds are well described and characterized by IR, NMR and X-ray diffraction. The electronic structure is analysed using standard DFT methods and NBO as well as AIM analyses. The research results are clearly presented and certainly of interest to the actinide community. In principle I am willing to recommend acceptance, however, I also believe that the authors should consider for future studies to avoid "unnecessary" fragmentation of their excellent experimental work. Previously they reported on an identical set of compounds, in which As is replaced by P (Ref. 37). Furthermore, nearly identical compounds have also been prepared for uranium as the metal centre. Nevertheless, returning to thorium I concede that these arsenic and P containing compounds are experimentally challenging, but why not write a full paper in which they directly compare these group 15 elements and their influence on the Th-E bond as inferred from DFT studies? However, this is just a minor comment and mostly represents this referee's preference for "complete" Articles over "less complete" Communications. RESPONSE: We thank the reviewer for their positive support. Regarding 'fragmentation' we understand what the reviewer is saying but would like them to know that we do try to publish work when we feel it is complete and when, for example, our Th-P paper was submitted/refereed we had no idea whether the work in the present paper would work out. Further, the Th-P paper is 11 journal pages and our prior U-As paper is 9 journal pages so it's hard to us to see how if we had waited then added this work in how we would have anything other than an unwieldy manuscript. We feel it is not unreasonable to say that the papers naturally define themselves by the U/Th/P/As series of combinations.
Nevertheless, there are also some scientific points that need to be clarified before acceptance of the manuscript: 1. p. 4: "the presence of AsH2 in 2 is confirmed by two As-H stretches at 2051 and 2064 cm-1" However, when I consult the Supplementary material Figure 1, I can only detect one broad absorption. So why do the authors refer to two stretches? Next point which is also valid for Supplementary Figures 2 and 3. There is a clear change upon preparation of the D-labelled complexes 2D, 3D and 5D. In the manuscript the authors only mention that the resonances assigned to As-H shifts into the fingerprint regime upon deuteration. This is certainly true but there is also a clear change in this regime, i.e. one absorption gains intensity upon D-labelling. This stretch should be clearly assigned and mentioned accordingly. There is clear new stretch at ca. 1300 cm-1 in Supplementary Figure S1. Ideally the authors will employ equally scaled IR spectra of labelled and unlabelled compounds and then form the difference spectra. I assume that this will be easy considering that they should have the electronic raw data in hand. I strongly believe that this would be beneficial for the paper. RESPONSE: When supplementary figure 1 is inspected in close detail it can just be made out that there are two overlapping absorptions, but we appreciate that this may well have been lost in the pdf conversion process. However, how well-resolved these two absorbances are varies from ATR-IR to ATR-IR. Therefore, we have re-recorded the spectrum of 2 where the two absorptions are better defined now, and hopefully this can be seen in the new SI file. Regarding the difference suggestion, this is in principle an excellent idea so we looked into this. However, there is an inherent issue to these systems that prevents this suggestion from being realised. The issue is that where heavy pblock elements are concerned multiple vibrational modes tend to end up coupling to the As-H vibrations in large molecules. Thus, the core vibrations in the fingerprint region and C-H region are affected and this does change significantly on changing H to D on As. Therefore, unfortunately but with good reason, the IR spectra of nH and nD are not directly subtractable. Those of us based in Manchester have noticed this since embarking on this An-E collaboration and those based in Regensburg have seen this phenomena many times over their extensive experience of heavy p-block chemistry spanning decades. The analytical frequencies calculations do not really pick up these nuances, and unfortunately the sheer size of these molecules prevents fuller vibrational analyses from being tractable computationally. That said, whilst this is an interesting issue we feel it is peripheral to the manuscript. This also accounts for the 1300 cm -1 absorbance for compound 2, i.e. the different H/D coupled vibrations result in a sharper absorbance for the latter (so something has decoupled) -in this case the AsH 2 at ~2064 cm -1 would be predicted to move to ca 1474 cm -1 which already has moderately intense absorptions so cannot be observed. 2. Did the authors entertain to record RAMAN spectra on 5? It may potentially be useful to gather some information on the strength of the Th-As bond, which is apparently strongly polarized.
RESPONSE: This is a good suggestion and we did look into this before submission. However, a frustrating and recurrent theme with our Tren(TIPS) complexes is their organosilicon-rich nature renders them incompatible with Raman. When we have the laser power high enough to expect a sample response they burn. When we turn the power down we get no response except sample warming. When we dilute the sample we get no response. This is frustrating for us as we use Raman on separate projects involving uranyl and get excellent responses, but here the Tren(TIPS) ligand has thwarted our efforts irrespective of what heteroatom sits in the pocket.
3. What is the thermal stability of these complexes?
RESPONSE: Samples can be handled at room temperature for short periods of time in solution but they tend to decompose. Solids in an inert atmosphere glove box are ok for days but they darken on extended storage.

RESPONSE:
We have added a sentence at the end of paragraph 2 of the introduction highlighting that M-As multiple bonds for even transition metals are quite rare, providing an opportunity to introduce the suggested citations. We have added a number of reviews (Scheer, Scherer, Cummins) then also added some of the early results in the area from Schrock, Scheer, and Wolczanski (references 49-55 in revised manuscript). This takes the paper to 77 references which is ok editorially (10% overshoot usually allowed) but we could not then add further references, but by focusing on reviews the relevant literature is covered one way or another. 5. Would it be possible to include a table which features the computational results for isostructural U/Th-As and U/Th-P compounds (similar to their Table 1 in the manuscript)? This may make it a little easier to draw a direct comparison.
RESPONSE: This is a good suggestion so we have added a table (table 2) compiling the relevant NBO data and modified the discussion section to reflect its inclusion.
6. In their conclusions the authors argue that the simplistic picture 5f vs. 6d which is very frequently advocated by chemists in the actinide community may be too simplistic. This is an important point which makes this contribution particular valuable for the f-block community. They also state that in the case of P vs. As a "tension of 7s vs. 6d vs. 5f" makes this picture more complicated. Based on the computations I completely agree with this statement, but: "How robust are these computations when switching methods/functionals?" So far everything is based on computations and experimentally this information will be very hard to extract …. Although the authors honestly admit that an explanation is hard to provide, but would it be possible to advance a model why this tension may exist for Th-As vs. Th-P? RESPONSE: The referee raises a very interesting point. We've always strived to point out that of course the results have to be viewed as qualitative rather than quantitative. One thing we would say though is that the calculations are internally consistent. We've done a lot of calculations in the past 9 years (>150 easily) all at exactly the same level of theory in terms of functional and basis sets. In addition, STL has overseen them all himself to ensure absolute internal consistency. As a researcher whose PhD was strongly crystallographic, STL has long been aware of how variability of treatments in modeling can be problematic and when independent and embarking on DFT calculations STL decided such work needed a consistent treatment which is why he oversees them himself. So the trend will be consistent even if the magnitudes are perhaps open to debate. It is tempting to speculate on the f vs d vs s split, and we would like to be able to, but we stand by our final statement that more work really needs to be done to fully tease this out.
Reviewer #2 (Remarks to the Author): Liddle and co-workers report a series of thorium complexes with a variety of Th-As(H) bonding motifs in this article. This work is challenging both in practice and analysis. This chemistry represents the cutting edge of actinide/main group chemistry.
RESPONSE: We thank the reviewer for acknowledging the importance of the work.
That being said, the work in its present form is not publishable in this or any other journal. A review of the CIF shows the use of DFT calculations to restrain (very) poor refinements to match calculations. For starters, this is the opposite of how science is supposed to work. Theory is modified to fit data not vice versa.

RESPONSE:
We believe there is a misunderstanding here because our initial text was not complete and clear, so we apologise for the lack of clarity. The As-H were initially located in the difference maps. However, mindful that we were refining one or two H atoms next to Th-As units we restrained them. The DFT was used as simply a guide and this was coupled to a survey of the literature to ensure that sensible bond lengths and angles were used, based on the initial locations from the difference map. Since they are crystallographic restraints and not constraints the models are not totally rigid. In many regards this is no different to the riding models used to refine C-H positions, which is standard nowadays. Indeed, we would suggest that the treatment of the As-Hs is better due to libration effects for C-H bonds but everyone fixes the latter in riding models with no question even though they usually constitute the majority of atoms in structures. This method of E-H treatment has also been previously accepted after multiple rounds of review including by specialist crystallographic reviewers called in specifically to comment on this aspect both at Nature Chemistry and this journal. Clarifying comments have been added to the cif file, where we realise the initial step, that is location in the difference map, had been missed out before from the refinement_special_details sections.
The resulting refinements are beyond the limits of credibility with restraints in one case totally 1802! RESPONSE: We are a bit puzzled by this comment because the number of restraints referred to simply reflects the nature of the disorder. In compound 2 two entire SiPr i 3 groups are disordered each over two locations and each in approximate 0.6:0.4 ratios. So there are in total 62 C, H, and Si atoms split over 124 positions in the presence of Th. This requires restraints to model 1,2-and 1,3-distances as well as rigid bond restraints and thermal motion. This, taken together with other restraints, e.g. the As-H bonds, gets you quite quickly to this range of restraints. Although it would always be desirable for the data to be better in practice they aren't going to be since it has been a real struggle to get good crystals of 2 so this is the best data we could ever get on this compound, but the connectivity and identity is clearcut. We end up with a publishable set of data because the most important bond in the structure, Th-As, is the best determined being between the two heaviest atoms in the structure, with a bond length determined to three decimal places and a su on the third dp in low single figures, which is fine. The structure has a data to parameter ratio of 12.5:1 that is more than enough, the model is well-behaved with essentially no shifts in the final refinement and ellipsoids are fine. Although it is always possible to focus on various crystallographic metrics the data are publishable, confirm the compound identity, and yield the bond length of most importance to a good level of precision. We have also re-refined the other structures, see below.
The second terms of the weighting schemes are larger than 100 and in one case 159. RESPONSE: While the weighting schemes found in the refinement of our reported structures are high based on classical standards, they are not due to any un-modelled disorder or twinning and are merely a genuine reflection of the data obtained. The GoF numbers are also rather good and the refinements are settled.
The wR2 tops 20% in several structures. RESPONSE: Only one compound had a wR2>20% and since submission we have obtained a better dataset for compound 5, (new wR2 = 8% and 23 restraints rather than wR2 = 20% with 1620 restraints) and have re-refined the datasets for compounds 3 and 6 to reduce the number of restraints present, however due to the multiple sets of disordered components within both structures restraints were still required to give chemically sensible models; previously: 3, 1502: 6, 1202. New: 3, 873: 6, 689. We believe that the remaining restraints used are justified and make chemical sense. We would also point out that by definition wR2 is usually 2-3 times larger than R1 values, which are perfectly reasonable in the new cif files supplied (R1 = 10.5, 6.3, 4.3, and 7.2% for 2, 3, 5, and 6, respectively). The data:parameter ratios are also very acceptable: 2, 7252:581; 3, 14715:691; 5, 18521:798; 6, 20018:1092. Even the space groups are worrisome. The first one is in Pna2 (1), which is polar. This is important because few molecular systems adopt this space group. There are some, but normally the molecules have highly acentric shapes and intermolecular interactions (typically hydrogen bonds) create the polar c axis by aligning acentric shapes in a single direction. It is not clear to me how this happens in this system. The work from Liddle and co-workers is normally quite brilliant. This does not fit the mould from this group.
RESPONSE: A search of the CCDC (Based on a search of the CSD database 16 th December 2016, Acta Cryst., 2016, B72, 171-179) finds that there are 7342 examples of structures published in this space group (or its equivalents), so it is not particularly unusual. That said, there is some pseudo-symmetry present in the dataset for 2 (See appendix 1 for Space group determination output for Compound 2). While the c glide on the short axis is clear, the n glide on the intermediate axis is less clear, but we believe it to be present. Although screw axes are more difficult to find than glide planes, considering the condition for observed intensity for the 2 1 screw on the b axis (k=2n) does not fully overlap with the conditions required for the c (l=2n) or n (h+k= 2n) glides we believe there to be a 2 1 screw axis present on the long axis. Putting the assigned space group of Pc2 1 n into a standard setting leads to the space group assignment of Pna2 1 . If the assignment of a 2 1 screw axis is incorrect, the alternate space group assignment would be the centrosymmetric space group Pnam (standard setting Pnma). While the structure can be solved in this space group the mirror plane runs through the N amine -Th-As plane, with the whole molecule being disordered over the mirror plane. Refinement of this did not lead to a more sensible model. Due to the pseudo-symmetry and large amount of disorder present, restraints were required to provide a satisfactory refinement of 2, however we believe the structure and connectivity to be unambiguous. The other space groups (Pbcn, P-1, P2(1)/c) seem perfectly reasonable to us also.

Reviewer #3 (Remarks to the Author):
This manuscript describes the synthesis, structure, and bonding of new thorium-arsenic compounds including the first actinide arsenide, which includes some multiple bonding character. Overall the manuscript is well-written and the compounds are well characterized (although compound 5 needs better combustion analysis), however this report is too close to the previously reported thorium and uranium phosphorus compounds to be published in Nature Communications. Absolutely beautiful compounds and unique to arsenic versus phosphorus but the approach to obtaining these compounds is too cookie-cutter. RESPONSE: With respect to this referee we disagree with this report. As they even themselves point out this paper contains a number of novel 'firsts' and they effectively acknowledge that phosphorus is not arsenic, and thorium is certainly not uranium. If the work were 'cookie-cutter' then it would be an exact match to prior work but this is not the case; experimentally we show we've gone right to the edge of stability with these systems. We therefore feel this work represents an important advance, especially when the new orbital tensioning contrast of P vs As issue emerges as it does.


text:
Why oppositely charged ions of equal radii have different heats of hydration?

Looking for the answer to the title question a number of oversimplifications of the Born model of ion hydration are discussed. They involved: ionic radius, dielectric saturation, structure of water molecules around ions and the nature of ion–water interactions. On the basis of recent literature the last factor—pure electrostatic interactions of alkali metal cations with water molecules but hydrogen bonding of halide anions—has been found to decide on the minimum energy of interactions, the charge transferred between interacting species in equilibrium and the distance between them. Thus, different nature of interactions for cations and anions explains difference in their hydration heats as well as the observation that solvent–solvent interactions in hydrogen bond donor solvents give the important contribution to solvation heats only for anions.

Introduction
The enthalpy of hydration of an ion DH i;hydr (and the corresponding Gibbs free energy DG i;hydr ) plays an important role in the elucidation of a behavior of ions in aqueous solutions including thermodynamic as well as kinetic aspects of a number of ionic reactions considered in courses of general and inorganic chemistry. Usually the starting point for most considerations on the solvation of ions is the familiar Born equation [1] which describes the electrostatic work due to growing polarization of a medium [2] when one mol of a spherical ions of radius r i and charge z i e o is transferred from a vacuum of permittivity e o into a solvent treated as a continuum dielectric medium with a relative permittivity of e s : where N A is Avogadro's constant. The enthalpy of solvation is given by the similar Born-Bjerrum equation [3] but with the additional term involving the temperature derivative of the solvent permittivity: Qualitatively Eqs. (1) and (2) predict correctly higher negative values of DG i;hydr and DH i;hydr for smaller ions with higher charges and in solvents with a higher permittivity. However, a quantitative comparison between experimental and computed values are not satisfactory as was discussed repeatedly by many authors for aqueous solutions of monatomic and univalent ions (in particular alkali metal cations and halide anions for which spherical shape is most adequate). The enthalpy of hydration which does not include the entropy term and can be directly compared with results of some theoretical calculations will be considered here. In general, three problems arise then. The first is that the Born estimates of DH i;hydr are too large in magnitude than absolute values obtained from experimental data with some extrathermodynamic assumption [4] and the differences observed are greater for cations than for anions as shown in Table 1 (Table 1), e.g., for K ? and Fions for which Pauling's crystal radii [5] are similar (133 pm) as well as for Na ? (r i = 116 pm) and F -(r i = 119 pm) if crystal radii of Shannon and Prewitt based on electron density measurements [6] are used. Third, Eq. (2) requires that the intercept of the linear plot of DH i;hydr against 1/r i is equal to zero but it is not the case, in particular for cations (Fig. 1). A number of corrections to the original Born equation were repeatedly proposed and often they are mentioned in modern textbooks but some of them have only historical meaning and it is hard to choose which effect is mainly responsible for the title question.
In general, it is clear that the Born model cannot give the correct results because of its oversimplifications: ions are not rigid spheres with the same radius as in crystals, solvent is not continuum dielectric medium but has a molecular structure, solvent electric permittivity decreases dramatically in the strong electric field near an ion, and finally solvent-ion interactions have different nature depending on chemical properties of a given system and cannot be always limited to pure electrostatic interactions. They all will be briefly discussed below.

Ionic radius and disruption of water structure
The use of the effective ion radius r eff = r i ? d s in the Born equation, i.e., increasing Pauling's radius of cations r i in aqueous solutions by d s & 80 pm, results in a correct plot of DH i;hydr against (r i ? d s ) -1 with the intercept equal to zero [7]. The correction term d s is usually found by fitting experimental data although it can be calculated in some simple models of solutions [8] and can be related to results of statistical mechanical [9] and molecular dynamics [10] simulations. The d s term is different for cations and anions and depends on the solvent nature. It is interesting to note that a recent comparison of experimental DG i;solv values for monatomic ions in 17 solvents using r eff = r i ? d s with Pauling radii r i showed [8] that the d s term depends on solvent Lewis basicity and Lewis acidity for cations and anions, respectively. Thus, it reflects specific, chemical ion-solvent interactions.
On the other hand, the idea of adding the d s term to r i in the Born equation is usually explained in terms of increasing the ionic radius to account for the disruption of solvent structure around this ion [11]. Such procedure reduces the negative Gibbs energy and enthalpy of an ion according to Eqs. (1) and (2) by the energy which is necessary for changing dipolar solvent-solvent interactions around an ion and in particular, hydrogen bonds between water molecules. It clearly explains a discrepancy between experimental and calculated values for each ion given in Table 1. Moreover, higher d s values for cations than for anions (confirmed in recent analysis for monatomic ions [11,12]) can explain smaller hydration of K ? ion than F - (Table 1) due to a stronger breaking of water structure around the cation [11]. Thus, the further discussion of the effective ionic radius should take into account the breaking of the original water structure by some ions as first noted by Bernal and Fowler [13] and explicitly described by Frank and Evans [14,15]. The last authors proposed the model of water structure in aqueous solutions consisting of three concentric layers around an ion: the innermost layer with water molecules strongly oriented to an ion, the second region with the broken original water structure, and the third one with the original H-bonded structure of water molecules a little polarized by the relatively weak ion field at larger distance from an ion. The relative extension of the second layer depends on the ion nature (charge and size) and this layer can dominate in observed properties. Different structure of broken regions for cations and anions formed as a monolayer outside the first coordination shell  Fig. 1 Relationships between the heat of hydration, DH i;hydr , [4] and the reciprocal of ionic radius [5] for monatomic cations and anions was also considered by Bockris with coworkers [16]: for cations, it consists of water monomers some of which liberate with the respect to molecules in the first shell while for anions hydrogen bonds occur between water molecules in the first shell and in the structure broken region.
The classic suggestion of Frank and Evans [14,15] is in agreement with experimental parameters proposed later by other authors to determine quantitatively the structuremaking or structure-breaking character of ions in aqueous solutions. Such parameters based on the activation energy of water exchange caused by the ion, the change of ion entropy, the effect of ions on viscosity of water and the difference of solubility of salts in light and heavy water have been recently tabulated by Marcus [17] with references to original papers. They all indicate that small ions (Li ? , F -) are structure-making ions, K ? ion is slightly structure-breaking and larger ions (Rb ? , Cs ? , Br -, I -) are evidently structure-breaking [14,15,17]. Thus, the opposite behavior of K ? and Fions is in accordance with the difference in their DH i;hydr values discussed above. However, it is not the case for other monatomic ions for which the size determines mainly their effect on water structure and not the sign of the charge. Thus, the assumption that smaller negative values of DH i;hydr for cations than those for anions are caused by stronger disruption of original water structure by positive ions is not correct and another explanation should be considered.

Dielectric saturation
The enormous gradient of the electrostatic potential near the surface of an ion causes the strong polarization of a solvent. It results in an extreme decrease of the relative permittivity e s near the ion (as reviewed in [18]) or in a more realistic model [19] the gradual decrease of e s in a series of concentric spherical layers around ion, each with a different relative permittivity. The smaller value of local permittivity used in the Born equation results in smaller DH i;hydr values in agreement with experimental data. Assuming that a discrepancy between calculated and experimental values of thermodynamic functions of ions depends only on dielectric saturation, Noyes obtained [20] effective dielectric constants which were very small and of course higher for anions than for cations. However, for cations having the electronic structure of an inert gas and charge numbers of 1, 2 and 3 he found [20] that effective dielectric constants depend only on size of an ion but are virtually independent of charge. All other models describing variation of medium relative permittivity with the distance from an ion [18,21] assumed as well the independence of the magnitude and sign of a charge. Thus, the effect of dielectric saturation cannot be responsible for differences between properties of cations and anions in aqueous solutions.

Structural aspects of aqueous solutions
Different arrangements of a water molecule interacting with a cation and with an anion were pointed out in the literature and their discussion is related to various kinds of possible interactions [18]. In general, interactions of a cation with a lone pair at water oxygen atom analogous to H-bonding of an anion were considered [22] or simple electrostatic interactions between an ion and a point-dipole [13,23] or quadrupole [24,25] of water molecule. Concerning ion-dipole interactions Bernal and Fowler noted [13] that the dipole moment of water molecule is not centrally distributed between three atoms and thus, the positive end of a dipole can get closer to anions than the negative end can to cations. That explains experimentally observed stronger hydration of anions as cited by other authors [20,23,26]. Modern X-ray diffraction studies of aqueous KF solutions [27,28] indeed supported a shorter ion-oxygen distance for Fanions (262 pm) than for K ? cations (295 pm) and of course much shorter anion-deuterium distance, e.g., 222-226 pm for Cl --D as obtained from neutron diffraction measurements in LiCl and NaCl solutions [28,29]. However, distances between interacting species should be rather understood as consequences of the nature and energy of interactions as will be discussed later. The equilibrium distance depends on the minimum of interaction energy which in turn corresponds to a compromise between attractive and repulsive forces. On the other hand, Buckingham considering water molecules as electrical quadrupoles found [24,25] a positive contribution to hydration heats for cations but negative for anions due to different orientations of water molecules towards both kinds of ions. The above result explains more negative experimental DH i;hydr values for anions. Nevertheless, the above discussions were restricted to pure electrostatic interactions and cannot explain correctly the title question if interactions have some chemical nature. Bernal and Fowler assumed [13] the planar structure of an anion-H 2 O entity, in which an anion is located in the same line as the H-O bond favorable to hydrogen bonding (Fig. 2a). On the other hand, Buckingham suggested [24,25] that an anion is located between both hydrogen atoms (the symmetry C 2v as in Fig. 2b). Thus, the formation of hydrogen bonds is not possible and interactions of an anion with H 2 O molecules are essentially ion-dipolar and ion-quadrupolar. Similar symmetric orientations of a water quadrupole to ions were also discussed recently by Bockris and Reddy [30]. However, neutron diffraction studies of aqueous solutions of alkali metals chlorides [28,29] as well as X-ray diffraction and infrared spectroscopy investigations [28] of solutions of other halides (cf., examples in [31]) evidently support the structure shown in Fig. 2a for all halide ions in aqueous solutions. For example, the deviation of the Cl --D-O angle from 180°is negligibly small [28,29], close to 0°for LiCl solutions [29]. Thus, dipolar and quadrupole models look unlikely and hydrogen bonding to anions should be taken into account as the primary factor causing different behavior of cations and anions.

Nature of ion-water interactions
The electrostatic nature of interactions between water molecules and alkali metal cations, in particular hard [32] or nonpolarizable small cations, looks quite acceptable and can be confirmed by recent quantum-chemical calculations [33]. However, for anions the role of hydrogen bonding was well recognized in modern literature [4,11,31] and supported by neutron and X-ray diffraction studies [27,28,31]. The above different nature of interactions was mentioned in some discussions on difference in DG i;hydr and DH i;hydr values for cations and anions of the same size but only Sharpe [31] explicitly stated that it ''is very probably a major reason for this, but other factors also appear to be involved''.
Before further discussion, it will be worthy to remember that for monatomic cations with charge numbers of 1, 2 and 3, the function [1 -(DH i;hydr ) exp /(DH i;hydr ) Born ] of the ratio of experimental and calculated hydration enthalpy decreases with increasing ionic radius forming a reasonable curve [20]. However, substantial deviations to lower values were observed for halide anions as well as for Hg 2? , Cu ? and Ag ? cations. Extremely high experimental DH i;hydr values for last two cations (e.g., for Cu ? DH i;hydr = -535 kJ mol -1 [4] whereas for Na ? with a similar size DH i;hydr = -375 kJ mol -1 ) were difficult for explanation. However, at present they can be easily related to more covalent interactions of soft cations with water molecules, e.g., for Ag ? ion (DH i;hydr = -440 kJ mol -1 ) as compared with smaller Na ? ion [34]. The same explanation can be also proposed here for anions. The hydrogen bonding of anions to water molecules in aqueous solutions means the covalent nature of their interactions which are stronger and result in more negative values of DH i;hydr than those for alkali metal cations of similar size.
Different nature of interactions between monatomic cations and anions under consideration was evidently shown in our recent theoretical calculations [33]. The formation of complexes between solvent molecule and ion was considered there for three hard cations: Li ? , Na ? and K ? , which interact with lone electron pairs of O or N atoms of hydrogen bond donor (HBD) solvents (in particular water, methanol, formamide, and ammonia for which experimental DH i;solv values are known [4,35]) and for three anions: F -, Cl -, Br -, which form hydrogen bonds with the same solvents. The total energy of interaction, E total , and the amount of charge, CT, which is transferred in the complex formed were calculated. Moreover, to characterize the nature of interactions the ratio of potential to kinetic electron energy density at bond critical point |V BCP |/ G BCP [36] was calculated using the quantum theory of atoms in molecules (QTAIM). The increase in the |V BCP |/ G BCP ratio indicates a more covalent character of the bond. For interactions of solvent molecules with cations small values of charge transferred to cations were found [33] and the ratio |V BCP |/G BCP \1 as shown in Fig. 3 a linear H-bonded [13]; b normal ion-dipole and ion-quadrupole [24,25] Fig. 3 Covalent character of interactions given by the ratio of |V BCP |/ G BCP plotted against charge transferred, CT, from lone electron pairs at O or N atoms in solvent molecules to cations or from anions to HOor HN-groups in solvent molecules. Data from Reference [33] between alkali metal cations and solvent molecules was supported [33]. On the other hand, for anions the positive values of the Laplacian in combination with larger values of the |V BCP |/G BCP (mostly [1 with the exception of Cland Brions in ammonia [33], as shown in Fig. 3) indicate a partially covalent character of interactions in equilibrium H-bonded complexes. It is also evident from Fig. 3 that for anions the values of charge transferred, CT, are higher than those for cations resulting in stronger bonds with solvent molecules in equilibrium complexes. Thus, distances d between ions and interacting atoms of solvent molecules are shorter. For example, a comparison of oppositely charged ions of similar Pauling radius showed for the complex of water molecule with K ? ion: CT = 0.025 a.u., E total = -76.8 kJ mol -1 and the OÁÁÁK ? distance d = 261.8 pm, whereas for the complex with Fion: CT = -0.112 a.u. (negative charge is transferred in the opposite direction, i.e., from the anion to the solvent molecule), E total = -111.0 kJ mol -1 and the HÁÁÁFdistance d = 139.9 pm [33].
Interaction energies calculated are in good agreement with experimental data supporting a correctness of the proposed model. Namely, for three cations in four solvents a linear correlation was found [33]  which holds for R 2 = 0.976 and the addition of the second parameter DH vap is statistically important with the probability of 99.73% [33]. Equation (4) indicates that solventsolvent interactions are important in solvation heats of anions by water and other HBD solvents but not for cations. The last result is in accordance with earlier analysis of Fawcett who modified the Born equation by multiplying Eq. (1) by the f dd term which describes the effect of dipoledipole interactions and H-bonding between solvent molecules on DG i;hydr values and found that for monatomic ions in aqueous solutions the f dd term is important only for anions [12,37]. The following simple explanation of these different behaviors of cations and anions can be proposed: the formation of hydrogen bonds which have covalent character needs a substantial rearrangement of solvent structure around an anion because these bonds are directional, whereas similar significant rearrangements are not necessary for pure electrostatic, not directional interactions of water molecules with alkali metal cations.
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extfieldsofstudy:
Environmental Science
Medicine

text:
Experimental outgassing of toxic chemicals to simulate the characteristics of hazards tainting globally shipped products

Ambient monitoring analyses may identify potential new public health hazards such as residual levels of fumigants and industrial chemicals off gassing from products and goods shipped globally. We analyzed container air with gas chromatography coupled to mass spectrometry (TD-2D-GC-MS/FPD) and assessed whether the concentration of the volatiles benzene and 1,2-dichloroethane exceeded recommended exposure limits (REL). Products were taken from transport containers and analyzed for outgassing of volatiles. Furthermore, experimental outgassing was performed on packaging materials and textiles, to simulate the hazards tainting from globally shipped goods. The mean amounts of benzene in analyzed container air were 698-fold higher, and those of ethylene dichloride were 4.5-fold higher than the corresponding REL. More than 90% of all containers struck with toluene residues higher than its REL. For 1,2-dichloroethane 53% of containers, transporting shoes exceeded the REL. In standardized experimental fumigation of various products, outgassing of 1,2-dichloroethane under controlled laboratory conditions took up to several months. Globally produced transported products tainted with toxic industrial chemicals may contribute to the mixture of volatiles in indoor air as they are likely to emit for a long period. These results need to be taken into account for further evaluation of safety standards applying to workers and consumers.

Introduction
With the globalized production and trade, most small and large companies import production parts, raw materials and goods from overseas and consumers place their individual orders anywhere in the world [1,2]. To ensure preservation and quality of these goods, chemical agents (e.g. methyl bromide) for pest control or to stop the introduction of non-indigenous species are added either to the shipped items or to transport units [3,4]. Treatment of materials with volatile chemical agents referred to as fumigation is regulated by the international standards of the UN Food and Agriculture Organization (FAO) for phytosanitary measures (ISPM 15 endocrine disrupting effects, the authors have shown that low level exposure to BTEX may induce sperm abnormalities, reduced fetal growth, cardiovascular disease, respiratory dysfunction, asthma, sensitization to common antigens, and more. Health effects were observed at exposure concentrations that were in many cases orders of magnitude below the U.S. EPA reference concentrations (i.e., safe daily exposure level) [28]. The aim of our study was to provide experimental data allowing future risk assessment of possible health risks from products tainted with fumigants and industrial chemicals. Such understanding will increase our ability to control and prevent exposures.
In this study, we have screened the air of 2,027 import containers for VICs and VOCs. We tested whether the concentration of the volatiles exceeded recommended exposure limits and if there is a relationship of the transported goods and the VIC/VOCs measured. Furthermore, goods from suspicious containers were analyzed for outgassing from the transported goods. For a better understanding of the desorption behavior of fumigants from consumer products, we conducted fumigation experiments and analyzed the outgassing of classical fumigants (phosphine, methyl bromide) and ethylene dichloride (1,2-dichloroethane) from packaging materials, textiles and food for detailed time course analyses.

Screening of container air samples
The requirement for an effective monitoring of residual fumigant contamination in the air of imported freight containers led us to develop and validate a mass spectrometry method based on mass spectrometry combined with thermal desorption gas chromatography (TD-GC/MSD), allowing the simultaneous determination of major fumigants such as methyl bromide, sulfuryl fluoride (sulfuryl difluoride), methyl iodide (iodomethane), propylene dichloride (1,2-dichloropropane), ethylene dichloride (1,2-dichloroethane), chloropicrin (trichloronitromethane), and the toxic industrial solvents benzene, toluene and carbon disulfide [13]. The method was also developed to simultaneously detect phosphine along with VOCs in container air samples using a thermal desorption system coupled to a two dimensional gas chromatograph with mass spectrometric and flame photometric detection (TD-2D-GC-MS/FPD). By incorporating simultaneous collection of selected ion monitoring (SIM) and SCAN data, single analysis was previously found sufficient for qualitative screening and quantification of all target compounds [29].
The container sampling was permitted and supported by the Federal Customs Office in Hamburg.Air samples were taken using a tubular steel lance pushed through the container door seal and a silicon tube connected to a Tedlar 1 sample bag in the Vacu-Case™ vacuum pump (both Analyt MTC, Mühlheim, Germany). 1 L of air was taken from each of the 2,027 containers arriving at the Customs Office in the port of Hamburg, Germany. A certified test mixture of 39 compounds in the gas phase was purchased from Scott (Scott Specialty Gasses, PA, USA). Additionally, certified standard gases of methyl bromide (bromomethane), phosphine and sulfuryl fluoride were obtained from Linde (Linde AG, Gases Division Germany, Pullach, Germany). Analytical grade liquid compounds benzene, carbon disulfide, 1,2-dichloroethane, 1,2-dichloropropane, dichloromethane, ethyl benzene, iodomethane, toluene, tetrachloromethane and trichloronitromethane were purchased from Fluka Analytical (Fluka Analytical/Sigma-Aldrich Switzerland, Buchs, Switzerland). The gas chromatograph was run in constant pressure mode using the Deans column switch. Helium 5.0 was used as carrier gas and was further purified using a helium gas filter (Supelcarb HC, Supelco/Sigma-Aldrich, Sigma-Aldrich Switzerland, Buchs, Switzerland) to trap oxygen, water and hydrocarbons as described earlier. Columns were chosen to separate phosphine and sulfuryl fluoride from the VOCs on the first column and to separate phosphine from sulfuryl fluoride on the second one.
Phosphine and sulfuryl fluoride were the first compounds of interest to elute from column #1. The corresponding peak was switched to the second column where the two compounds were separated and eluted to the FPD in phosphorus mode. All other compounds eluting from the first column were analyzed by MS in scan mode for compound identification and in SIM mode for quantification. All VOCs were well separated in the first dimension on the HP-1MS column, while phosphine and sulfuryl fluoride were separated sufficiently in the second dimension on the PLOT column. More details on the method were published elsewhere [29]. Limits of detection and quantification were derived from low concentration standard curves by appropriate equations: Note the conversion factors from μL/m 3 (μL/m 3 = ppb) to μg/m 3 for the target compounds at 23˚C (laboratory temperature): phosphine: 1.4; dichloromethane: 3.50; methyl bromide/ bromomethane: 3.91; 1,2-dichloroethane: 4.07; toluene: 3.79; and benzene: 3.21.

Outgassing of container-origin products
Children toys (n = 23), shoes and socks (n = 15) were taken out from shipping containers and transferred to a desorption chamber. After 24 h an air sample was taken and analyzed by TD-2D-GC-MS/FPD (see above). The samples have been taken from containers, which have exceeded the recommended exposure limit for one of the analyzed toxic industrial chemicals (ethylene dichloride, methylene chloride or toluene). The products (n = 38, not randomized) were placed in an evaporating chamber at room temperature (21˚C, 30% relative humidity) for 24 h. (The 1.24 m 3 outgas volume with continuous ventilation of 1.14 m 3 /h/m 2 provides a good model for a small room of about 11 m 2 , as based on the methods for VOC emissions from construction products, European Commission EUR 17334-Rep no.18), Then the TD-GC/MSD analysis of the residual outgassing chemicals was performed (see above).

Experimental outgassing
We decided to perform standardized experimental fumigation of various products to look into the outgassing kinetics under controlled laboratory conditions. Further we aimed to analyze whether fumigation with these chemicals may affect the properties of the products. In an outset, we have chosen to fumigate socks and packing material (wrapping paper) to elaborate the possible differences in its outgassing patterns. To investigate the desorption behavior of fumigants from consumer products in a detailed time course, we fumigated wrapping paper (80 g/ m 100% cellulose) and nylon socks (85% polyamide and 15% elastane) with 100 ppm phosphine, methyl bromide or 1,2-dichloroethane, for 72 h in a fumigation chamber of 4 L volume (3 independent replicates). After fumigation samples were transferred to a desorption chamber (53 L). At consecutive days, air samples were collected repeatedly from the side of the chamber (see digital abstract) with the help of a gas jumbo syringe (with 1 L volume). After each sampling, the chamber had been ventilated completely with fresh air to simulate natural conditions at a storage room or a consumer home. This procedure has been repeated on the following sampling days till the concentration of fumigants in the air samples reached the detection limit. Air samples (transferred from the gas jumbo syringe into tedlar bags) were analyzed by TD-2D-GC-MS/FPD (see above).
Surface analyses using time-of-flight secondary ion mass spectrometry (Tof-SIMS) were performed to investigate, whether the product surfaces may be especially susceptible for the absorption/desorption of toxic gases (data not shown). Fumigated samples of wrapping paper were analyzed by Tof-SIMS to look into interactions of the fumigant with molecules of the consumer product to reflect the adhesion of the fumigants to the product surface. For each sample (n = 3 independent fumigations, see above), 1 cm have been cut out on dry ice and a depth profile and a surface scan in positive and negative mode have been taken.

Data analysis
For the interpretation of the results, independent, international scientifically based Reference Exposure Levels (RELs) were used. As limit values the chronic RELs released by the US Office of Environmental Health Hazard Assessment, OEHHA were applied [30]. The REL values were as follows: 400 μg/m 3 (102 μL/m 3 , ppb) for methylene chloride (dichloromethane), 400 μg/m 3 (98 μL/m 3 , ppb) for ethylene dichloride (1,2-dichloroethane), 300 μg/m 3 (79 μL/m 3 , ppb) for toluene and 3 μg/m 3 (0.98 μL/m 3 , ppb) for benzene. These RELs are derived from the most sensitive non-cancer health effect reported in medical and toxicological literature for a particular target tissue (either in the nervous, respiratory, cardiovascular or alimentary system or for developmental processes). The values are designed to protect those individuals who live or work in the vicinity of emission sources and who are continuously exposed to these substances.
Data evaluation was performed using descriptive statistics with univariate analysis. Subgroups were formed according to the major categories of the type of goods or contents as declared to the customs authorities and the type of contaminating chemical detected during the course of investigation. These were further subdivided into subsets for data analysis. The statistical analysis was performed with Graph PAD 6.05.

Analysis: Presumably carcinogenic chemicals in container air vs. transported goods
To evaluate the levels of presumably carcinogenic chemicals in containers in correlation with the transported products, we have first analyzed the container air in 2,027 randomly chosen containers arriving at the harbor of Hamburg in the years 2010-2014 by using TD-GC/MS [29]. We then evaluated gas concentrations in the container air and calculated the numbers of transport units with the gas atmosphere higher than the community relevant health based exposure levels (RELs). Previous data have shown that contamination with industrial chemicals appear to provide a greater problem as the fumigants themselves [10,12]. We have now focused mainly on the carcinogens benzene and ethylene dichloride. The analyzed container air was contaminated with both benzene (mean level: 685 μL/m 3 ± 139 SEM) and ethylene dichloride (mean level: 447 μL/m 3 ± 80 SEM) residues (Fig 1). The median values for benzene were much higher than the REL values of 0.98 μL/m 3 (median = 7.6 μL/m 3 ; 95% CI: 6.7, 8.4). The highest maximal concentration found, i.e., 177,158 μL/m 3 , was disturbing, since it exceeded the REL by 180,000-fold. Conversely, for the concentration of ethylene dichloride (EDC) the median values found were not higher than the health-based limit value of 98 μL/m 3 (median = 3.9 μL/m 3 ; 95% CI: 3.4, 4.7). However, the maximal ethylene dichloride concentration was as high as 95,650 μL/m 3 , thus exceeding the REL nearly 1000-fold. The bars in Fig 1 show the measured concentration levels of the different substances (mean ± SD values) within the different product groups investigated. For a better general comparative view these figures only contain the positive values within the same range of log scales and neglect results below the limits of detection.
Further classifications show the amount of ethylene dichloride and benzene concentration in container air grouped by transported goods categories (Fig 1). In all product groups there are containers that show concentrations of ethylene dichloride up to a range of 10-100 times the limit value or higher. Three groups, namely containers transporting cars and vehicle parts, furniture and household goods and shoes reach up to levels more than 1000 times the limit value. While for all other groups the majority of containers (50-75%) remain below the limit value, the majority of containers transporting shoes were found with concentrations above the limit value. 75% of these containers exceeded the limit value for ethylene dichloride, 50% are higher than 10 times the limit value and 25% show concentrations of more than 100 times the limit value. Ethylene dichloride burden of containers transporting bags and accessories is higher than average with more than 50% beyond the limit value. Also for benzene the limit value was exceeded in every commodity group. Again it was the group of containers transporting shoes that revealed most units with high toxicant burden. In this group also the highest benzene concentrations were monitored.
For a better overview, we converted the measured concentrations in container vs products to percentage of container showing values higher as the corresponding RELs, as health-based community exposure values (Fig 2). For this overview, we have included not only the carcinogens benzene and ethylene dichloride, but also toluene and dichloromethane. We found that 98% of all containers, which transported shoes, had benzene air concentration higher than the corresponding REL (0.98 μL/m 3 ). 53% of shoe containers, 33% of the containers transporting wood/paper, 25% of furniture and 9% of foodstuffs containers were contaminated with ethylene dichloride higher than its REL (>98 μL/m 3 ). Containers transporting metal products, car & mechanic parts or construction products had no ethylene dichloride residues that exceeded the REL. Methylene chloride (dichloromethane) amounts above its REL (115 μL/m 3 ) were found mainly in containers with mixed products (50%) or plastic products (29%), followed by wood/paper (22%), metal products (18%) and natural products (14%). Conversely, methylene chloride has not been found in containers transporting foodstuff items. More than 90% of all containers revealed toluene residues higher than the health-based community exposure level of 80 μL/m 3 . Only containers transporting electrical appliances or construction products had less toluene (88%, 86%).
For comparison, we analyzed the amounts of classical fumigant residues (methyl bromide, phosphine) in container air (data not shown). Looking for fumigant residues within the individual container groups, 42% of the container units transporting natural products, 36% of containers with construction products, 31% of foodstuff containers, 27% of containers transporting furniture and 17% of the textile/cloth containers had methyl bromide residues higher than its corresponding REL (1 μL/m 3 ). None of the containers transporting chemical products had any methyl bromide residues (all were <LOD) and in containers transporting other products (wood, paper, metal or plastic products), including shoe containers, the methyl bromide residues were higher than the REL in only 10% of all cases [9-14%]. Since phosphine was found only in 1% of the randomly selected containers, we did not analyze the distribution of this fumigant within the product groups.

Contaminated products outgas chemical residues for several days
We then took several highly contaminated products out of the containers, thereby emphasizing on toys, shoes and socks, assuming that these products, if contaminated, may have the greatest health impact on vulnerable groups such as children. The products (n = 38, note that the sampling was not randomized since we deliberately took products from containers with suspected high volatile concentrations in the air) were placed in an evaporating chamber at room temperature, followed by analysis of the residual outgassing chemicals. The data (Fig 3) show that after one day out of the vested groups, the products were still outgassing benzene and toluene in concentrations higher than the corresponding RELs. More than 50% were outgassing ethylene dichloride and dichloromethane. We decided to let the products outgas for a significant longer time. We took two products, a pair of children's shoes and a dolls playhouse, contaminated simultaneously with both toluene and ethylene dichloride and monitored the outgassing behaviour for several days. We transferred the items into an emission test chamber.

Experimental outgassing of fumigated products
We first fumigated the chosen products with the classical fumigants methyl bromide and phosphine (Fig 4, left and middle panels). The amounts of both phosphine (Fig 4, green, left panel) and methyl bromide (Fig 4, brown, middle panel) emitting from fumigated socks decreased below limit values in the course of 48 h. By contrast, the packaging material fumigated with phosphine was still outgassing after 1 day, whereas paper fumigated with methyl bromide was outgassing for 1 day only (Fig 4). Unlike the other fumigants, ethylene dichloride was outgassing from the products for a longer time period (Fig 4, right panel, blue). After 37 days (887 h) and 43 days (1028 h) the concentrations of ethylene dichloride in the collected air samples from outgassing socks and wrapping paper, respectively, were reaching the detection limit.
When assessing the different experimentally fumigated products by using ToF-SIMS, it became clear that the fumigation itself did not alter the structure characteristics of the analyzed textiles or packing materials (data not shown). The fumigants adsorbed on the surface of the product, without undergoing any further chemical interaction with the respective material.

Discussion
Our data confirm previous studies showing chemical residues in product-transporting containers from us [12,29] and others [10,14]. Since only little experimental data is available yet, actually too little to enable any risk assessment for those individuals dealing with contaminated products (like transport or shop workers, by-standers or private consumers), we have performed experimental fumigation and monitored the outgassing time for several specific chemicals and various product groups. Our study shows that unlike products contaminated with classic fumigants (phosphine and methyl bromide) which outgassed more rapidly, the products tainted with industrial chemicals like ethylene dichloride were still outgassing this compound even after 1.5 months. We assume that within the indicated time period the products have reached their destination in storage, production area (i.e. for construction parts or goods to sell), and in private homes from end-consumers. Chemical agents with health hazard or carcinogenic potential and to which storage workers or consumers are likely to be exposed are toluene, benzene, methylene chloride and ethylene dichloride. All of these compounds belong to a group of organic solvents causing potential occupational and home exposure. The source of these chemicals in import containers and related transported goods is mostly unknown. One possibility is their presence in fumigant formulations; they may be residues from container cleaning processes as well as from product outgassing after the manufacturing process. Ethylene dichloride and methylene chloride were used as pesticide fumigants in South America in the past [5]; or as solvents for resins and fats and as gasoline additives to remove lead; they were also used as chemical intermediates in organic synthesis (e.g for vinyl chloride), as extraction solvents and as precursors for cleaning agents for containers. Toluene and benzene can be used in solvent mixtures, cleaning agents and as intermediates in organic synthesis, or being a part of glue or smear. We observed that the amounts of benzene, toluene, and ethylene dichloride varied in individual containers depending on the transported items. Notably nearly 100% of all shoe containers had benzene and toluene levels exceeding the respective RELs, 53% had higher ethylene dichloride levels and 23% higher methylene chloride levels. No methylene chloride higher than RELs was found in foodstuff containers and <10% of these containers had ethylene dichloride levels above the corresponding REL. 73% of these containers were found with measurable benzene concentrations in the air. Although the reference levels these data refer to are quite low, it is important to note that individual transport units had very high concentrations of benzene, toluene, ethylene dichloride or methylene dichloride. Here we show for the first time that products taken out from contaminated containers were still outgassing these volatile industrial chemicals for several weeks, thus being well in the time window in which the products or production parts reach the end-users or public homes. Although we were not able to detect any impact of the chemical contamination on the tested product surface or its properties yet, further studies are required to address this issue more precisely.
It is known that exposure to volatile chemicals can contribute to a wide range of acute and chronic health effects (like asthma, respiratory diseases, liver and kidney dysfunction, neurological impairment and cancer), however exposure related diseases are difficult to detect in non-occupational settings [23,27,28,31]. References on potential health effects mostly result from occupational exposures or experimental data with laboratory animals. Several studies have been published in which the disease or tumor response of animals exposed to solvents/ industrial chemicals have been measured. Some recent animal experimental data were concentrating on the adverse effects of sub-acute doses (100-1000 mg/m 3 ) of ethylene dichloride showing changes in mice behavior with reduced loco motor and exploratory activities and increased anxiety [32], or on low doses of benzene inducing genotoxic effects [33]. On the other hand, epidemiological data from cohort studies (Weisel 2002) provide credible, but limited evidence that exposures to low dose solvents/industrial chemicals (such as benzene, toluene, ethylene dichloride, methyl chloride) would significantly contribute to the development of chronic diseases and cancer on the basis of cohort studies [27]. The health risk assessment of ambient air concentrations of benzene and toluene has been carried out in service station environments, showing as expected the highest health risk after chronic exposure to the carcinogen benzene [34]. The literature references rely mostly on short term high exposure levels, and focus mostly only on one chemical, thus leaving cumulative or additive effects unreported. Some of the recent epidemiological data, for instance, obtained from occupationally exposed mothers (Infante-Rivard et al. 2005), is more an exception than the rule. In this study of Infante-Rivard and coworkers, an expert exposure assessment method adjusted to low dose occupational solvent mixtures finally allowed to correlate low dose parental exposures with the occurrence of childhood leukemia [35]. Similarly, another recent study [36] has shown an association between resident exposure to solvents and childhood leukemia. Three large population-based case control studies confirmed an increased incidence (OR 1.5-2.2) for non-Hodgkin lymphoma and breast cancer risk following exposure to methylene chloride [19,37]. Although limited to a small number of studies a comprehensive meta-regression analysis of 9 heterogenic studies [38] revealed a significantly increased risk of leukemia (RR = 1.14, 95% CI 1.04-1.26) at exposure levels as low as 10 ppm-benzene-years. As for many other exposure related diseases, individual risk levels for various mixed exposures and the risk of developing leukemia remain largely unclear. Assuming every day exposures (365 days) one could expect an increased risk for the occurrence of leukemia at levels of benzene as low as 27 ppb (μL/m 3 ) benzene per day. No WHO Air Quality Guideline values are available for benzene. However, for this carcinogen the European Air Quality Guideline recommends outdoor exposure levels as low as 5 μg/m 3 for the annual mean [39].

Conclusions
The European WHO office recommended a better source control to reduce the indoor concentrations of VOCs [39]. Our data provide evidence that globally produced transported products tainted with toxic industrial chemicals may contribute to the mixture of VOCs in indoor air as they are likely to emit for longer periods than generally anticipated. Children playing on the floor (or crib), ill and elderly persons in poorly ventilated areas are more vulnerable to such emissions [40]. Based on the findings reported we suggest to evaluate the outgassing potency of globally transported products and production parts further. In addition, it seems advisable to address environmental low dose exposure scenarios more carefully in future epidemiological research by applying adequate exposure assessments.