gardner_et_al-angewandte_chemie_international_editionpdf

gardner_et_al-angewandte_chemie_international_editionpdf Angewandte Chemie International Edition: DOI: 10.1002/anie.201508600 German Edition: DOI: 10.1002/ange.201508600 Isolation of Elusive HAsAsH in a Crystalline Diuranium(IV) Complex Benedict M. Gardner, GQbor BalQzs, Manfred Scheer,* Ashley J. Wooles, Floriana Tuna, Eric J. L. McInnes, Jonathan McMaster, William Lewis, Alexander J. Blake, and Stephen T. Liddle* Abstract: The HAsAsH molecule has hitherto only been stabilizing groups, to more clearly probe their potential p- acceptor properties when bonded to metal centers. However, proposed tentatively as a short-lived species generated in the combination of a double bond and lone pairs renders electrochemical or microwave-plasma experiments. After dipnictenes increasingly reactive; HNNH is only found in the two centuries of inconclusive or disproven claims of [2, 3]HAsAsH formation in the condensed phase, we report the solid state when coordinated to metals, and only three [4]isolation and structural authentication of HAsAsH in the metal–HPPH complexes are known. Conspicuously, there TIPS22diuranium(IV) complex [{U(Tren)}(m-h :h-AsH)] (3, are no examples of structurally authenticated HEEH (E = As, 222TIPSiiSb, and Bi) in any charge state and therefore little is known Tren = N(CHCHNSiPr) ; Pr = CH(CH)). Complex 223332about these parent molecules. 3 was pre-pared by deprotonation and oxidative homocoupling of an arsenide precursor. Characterization Where HAsAsH is concerned, generation in electro-and computational data are consistent with back-bonding-chemical–IR and microwave plasma–IR experiments has been [5]type interactions from ura-nium to the HAsAsH p*-orbital. proposed, but the assignments, whilst consistent with As H This experimentally confirms the theoretically predicted bonds, were not conclusive regarding the precise nature of excellent p-acceptor character of HAsAsH, and is these transient, surface-absorbed hydrides. In the routine tantamount to full reduction to the diarsane-1,2-diide form. condensed phase, HAsAsH was first proposed as a reaction [6]product in 1810 by Davy and a year later by Gay-Lussac and [7]Dipnictenes REER (E = N, P, As, Sb, Bi; R = H, alkyl, aryl) Th nard. In 1924, Weeks and Druce claimed that the action of are a fundamental class of molecules that have played a stannous chloride on arsenic trichloride in the presence of [1]central role in the development of main-group chemistry. hydrochloric acid produced brown, amor-phous solids [8]Diazenes have been known for decades and are most formulated as HAsAsH. However, in 1957, Jolly, Anderson, prevalent, and though diphosphenes, diarsenes, distibenes, and Beltrami showed that these products are ostensibly and dibismuthenes have all been reported in the past thirty [9]arsenic with adsorbed sub-stoichiometric arsenic hydrides. [1]years, their numbers rapidly decrease down the group. This Thus, HAsAsH has eluded capture, and has most likely never reflects the difficulties of stabilizing multiple bonds between actually been made, which probably reflects the absence of increasingly large nuclei, the importance of dispersion [1a]synthetic methods to construct HAsAsH and prevent forces, and the reduced tendency of heavier p-block subsequent decomposition in the absence of bulky arsenic-elements to catenate, and thus sterically demanding substitu-[1]bound stabilizing groups. Here, more than two centuries after it ents are required to stabilize these linkages. However, it is was first proposed, we report the synthesis and structural fundamentally appealing to study parent REER molecules (R = H), free of structural distortions caused by bulky authentication of HAsAsH in a crystalline diuranium(IV) complex. TIPSTIPS We previously reported that the [U(Tren)] (Tren [*] Dr. B. M. Gardner, Dr. A. J. Wooles, Dr. F. Tuna, Prof. E. J. L. i= N(CHCHNSiPr)) fragment stabilizes reactive 2233McInnes, Prof. Dr. S. T. Liddle [10][11] fragments such as cyclo-P, mono-oxo, terminal School of Chemistry, University of Manchester 5[12]Oxford Road, Manchester, M13 9PL (UK) E-nitrides (U N), and parent U = EH groups (E = N, P, [13–15]mail: steve.liddle@manchester.ac.uk As). The latter of these was prepared by reaction of TIPS[14][16]Dr. G. BalQzs, Prof. Dr. M. Scheer [U(Tren)(THF)][BPh] (1) with KAsH, to give 42TIPS[15]Institut of Inorganic Chemistry, University of Regensburg [U(Tren)(AsH)] (2) followed by deprotonation of 2 2Universit tsstrasse 31, 93053 Regensburg (Germany) E-and abstraction of the potassium cation by a crown ether. mail: manfred.scheer@ur.de [17] Inspired by the works of Herrmann and Huttner, we Dr. J. McMaster, Dr. W. Lewis, Prof. Dr. A. J. Blake wondered whether 2 could undergo oxidative homocoupling School of Chemistry, University of Nottingham TIPSto give HAsAsH stabilized by a bulky [U(Tren)] unit that University Park, Nottingham, NG7 2RD (UK) might preclude decom-position. Supporting information for this article is available on the To prepare 2, the KAsH reagent has to be finely ground, 2WWW under [15]otherwise intractable product mixtures are obtained. How- 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. ever, on one occasion a small crop of dark brown crystals of KGaA. This is an open access article under the terms of the TIPS22[{U(Tren)}(m-h :h-AsH)] (3) was obtained in about Creative Commons Attribution License, which permits use, 2221 % yield. Deducing that 3 is likely formed due to sluggish distribution and reproduction in any medium, provided the original work is properly cited. KAsH reactivity when not ground, therefore resulting in 2 localized excesses of KAsH deprotonating 2 when formed, we 2 Angew. Chem. Int. Ed. 2015, 54, 1 – 6 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 These are not the final page numbers! . Angewandte Communications The ATR-IR spectrum of 3 exhibits one weak As H absorption 11[15]at 2029 cm (2052 and 2031 cm for 2), and this compares 1 to As H absorbances at 2040 and 2000 cm assigned as [5a,b] HAsAsH generated in situ deposited on GaAs surfaces, but is significantly different to As H stretches of 2306 and 2298 [19]calculated for gas-phase HAsAsH. An analytical frequency calculation predicts symmetric and asymmetric As H stretches 1in the IR spectrum of 3 at 2049 and 2028 cm , respectively, Scheme 1. Synthesis of 3 from 1 and KAsH. 2which for the latter compares well to the experimentally 1observed value. The As H stretch at 2029 cm can thus be assigned as the asymmetric stretch-ing mode, because due to repeated the reaction with different ratios of 1:KAsH 2selection rules the symmetric stretch cannot be IR active as 3 varying from 1:1.1 to 1:2 where KAsH was ground. We 2exhibits an inversion center (see below). The symmetric stretch determined 1:1.4 to be the optimal ratio, which reproducibly should be observable in the Raman spectrum of 3, but samples 1affords 3 in circa 50 % crude yield, as determined by H of 3 decompose in the beam, or the inherently weak As H tNMR spectros-copy using 2,4,6-BuCH as an internal stretch cannot be observed at low-/mid-power settings or in 363[18]dilute samples, so this data remains unobtainable. The ATR-IR standard (Scheme 1). Recrystallization reproducibly data for 3 rule out the presence of the Z isomer because, affords 2 in 8 % pure crystalline yield, reflecting the lacking an inversion center, it would exhibit both symmetric and instability of HAsAsH. asymmetric As H stretches, which is not observed Whilst it seems certain that the KAsH deprotonates 2 to 2TIPSexperimentally. To examine this aspect further we prepared give [U(Tren)(AsHK)], and presumably AsH, in situ, it is 3TIPS22[{U(Tren)}(m-h :h-AsD)] (3 D), using previously 222unclear how it promotes oxidative homocoupling to give 3, as [18]unknown KAsD. As anticipated the ATR-IR spectrum of 3 all attempts to identify by-products have been inconclusive. 21However, we note that AsH has precedent for forming MAsH D does not exhibit the absorbance at 2029 cm , but the As D 32stretch could not be observed because from reduced-mass and H (M = Na or K) from M-containing substrates, and that 2considerations this absorbance falls in the fingerprint region KAsH is known to decompose to “KAs” and H, which might 221[16]where a strong and broad absorbance (1410–1490 cm ) provide the redox path to 3. We also note that although 3 is resides. obtained most conveniently by treatment of 1 with excess [18]The molecular structure of 3 is shown in Figure 1; the KAsH, rather than isolating 2 and reacting with further KAsH, 22salient feature is the presence of HAsAsH bridging two TIPSthe latter method is effective, suggesting that the HAsAsH unit [U(Tren)] units. In the solid state, 3 crystallizes over an may be formed by coupling and subse-quently remains isolated inversion center between the two arsenic ions. Although this between two uranium centers. We investigated alternative TIPSmethods of producing 3, by preparing [U(Tren)(AsHK)] and treating it with oxidants to effect homocoupling; however, adding stoichiometric iodine, lead-(II) iodide, TEMPO, pyridine- N-oxide, 4-morpholine-N-oxide, trimethylamine-N-oxide, silver tetraphenylborate, and copper(I) iodide all gave intractable products. We have also separately refluxed and photolyzed 2 to see if dihydrogen elimination to give 3 occurs, but only decomposition occurs under these conditions. These observations underscore the fragile nature of HAsAsH and hence why it was elusive. Although 3 is obtained in poor crystalline yield or moderate yield in crude form, the synthesis is reproducible. On one occasion, after isolating 3, a small crop of light brown crystals deposited from the mother liquor in less than TIPS22 1 % yield. These were identified as [{U(Tren)}(m-h :h-2[18] As)] (4). Although the low yield of 4 has prevented its 2 characterization, its structure serves to support the formula-tion of 3 by virtue of their metrical differences, and under-scores the complex dehydrogenative chemistry that operates for these [16] redox active molecules with polar bonds. Once 3 is crystalline, it has very low solubility in non-Figure 1. Molecular structure of 3 at 120 K with ellipsoids set at polar solvents and it decomposes in polar solvents, so [31]150 % probability. Non-arsenic-bound hydrogen atoms are reliable UV/ Vis/NIR spectra could not be obtained. The H omitted for clarity. An arbitrary pair of HAsAsH hydride positions NMR spectrum exhibits two very broad resonances at circa corresponding to an E isomer have been selected, with the other i5.3 (Pr) and circa 6.2 ppm (CH); we attribute this to the 2pair omitted for clarity. Selected distances [&]: U1–As1 3.1203(7), dinuclear nature of 3 and the absence of As H resonances U1–As1A 3.1273(7), U1–N1 2.256(4), U1–N2 2.273(4), U1–N3 2.261(4), U1–N4 2.709(4), As1– As1A 2.4102(13). to their close proximity to the paramagnetic uranium ions. 2 www.angewandte.org 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2015, 54, 1 – 6 These are not the final page numbers! Angewandte Chemie is consistent with the presence of the E isomer, in the crystal ion in 3 at 298 K (2.7 m) is lower than the theoretical value Bthat was examined the hydride is disordered over two sites, so of 3.58 m for uranium(IV), but this is common for uranium-B[26] the presence of the Z isomer, as opposed to two averaged E (IV).isomers of opposite “hands”, could not initially be dis-counted. To probe the nature of the U As interactions in 3, we However, the ATR-IR data rule out the presence of the Z calculated the electronic structure of the full model using [18]isomer, which is consistent with the greater prevalence of E density functional theory (DFT). With the Z isomer [1, 20]dipnictenes compared to the corresponding Z isom-ers. experimentally ruled out, our discussion focuses on the E The uranium–amide and uranium–amine bond lengths in 3 are [27]isomer. The geometry-optimized structure agrees well with [21]typical of such distances. The U As distances of 3.1203(7) experiment, with bond lengths and angles predicted to within and 3.1273(7) & in 3 are longer than the sum of the single bond 0.05 & and 28, respectively; the DFT model can thus be [22]covalent radii for uranium and arsenic (2.91 &), but are only considered to present a qualitative description of the elec-tronic slightly longer than the formal U As covalent s-bond in 2 structure of 3. The calculated MDC charges and MDC spin qm[15](3.004(4) &). The As As bond length in 3 of 2.4102(13) & is densities at each uranium average + 3.20 and 2.31, [23]consistent with a single rather than double bond, the latter of respectively, which suggests modest net donation of electron [1][28]which tends to be about 2.2 &, and rules out the presence of density to uranium(IV) from the ligands. The arsenic MDC q an (As) unit that when trapped between two transition metals charges average 1.12, which is consistent with the HAsAsH 2[17, fragment carrying a formal 2 charge overall, which is a exhibits As As bond lengths of circa 2.2–2.3 & (see 4 below).TIPS+23a, 24]requirement of being bonded to two uranium(IV) [U(Tren)] When diarsenes with sterically demanding substituents cations for charge neutrality. The calculated As As Mayer bond are bonded to transition metals the As As bond tends to order is 0.97, consistent with the As As single bond suggested lengthen as a result of back-bonding, for example to 2.365 & in 2[25]by the X-ray diffraction data, whereas the U As Mayer bond [(CO)Fe(h-AsPh)], which suggests a significant uranium 422orders average 0.34 and suggest polarized interactions; for to diarsene back-bonding-type interaction in 3 (see below), comparison, calculated As H, U which would also be consistent with a diuranium(IV) N, and U N Mayer bond orders average 0.92, formulation. Further support for the formulation of 3 comes amideamine[18]0.82, and 0.22, respectively. from the crystal structure of 4, which crystallizes in a The top four most energetic electrons in 3 are of essentially different crystal habit to 3. In 4 the As=As distance of pure, non-bonding 5f character and constitute the top four 2.2568(14) & is shorter than the analogous distance in 3 and is [23a]quasi-degenerate (0.05 eV spread) a-spin highest occupied characteristic of As. Also, the U As distances are shorter 2molecular orbitals (HOMOs), which are each singularly than in 3 at 3.0357(7) and 3.0497(8) & that is consistent with occupied. HOMO 4 in the a- and b-spin manifolds comprise the the high charge load of principal U As interactions, and represent formal back-bonding As. 2 from uranium to the p*-orbital of HAsAsH (Figure 3). As The assignment of uranium(IV) ions in 3, suggested by nitrogen-based orbital coefficients intrude into HOMO 4 of 3, the solid-state metrical data, is also supported by magnetic natural bond orbital (NBO) analyses were performed to obtain measurements (Figure 2). A powdered sample of 3 exhibits a localized, clear descrip-tion of the U As interactions. NBO analyses reveal highly polarized U As interactions that comprise an average of Figure 2. Temperature-dependent SQUID magnetization data for 3 plotted as m versus temperature (K) (&) and c vs eff temperature (K) (*) over the range 1.8–298 K. a magnetic moment of 4.0 m at 298 K, which decreases B monotonously to a moment of 1.2 m at 2 K and tends to zero B as would be expected for uranium(IV), which at low temper- ature is a magnetic singlet with residual temperature-inde-Figure 3. The a-spin Kohn–Sham HOMO 4 representation of the 3pendent paramagnetism. The magnetic moment per uranium principal uranium–arsenic interaction in 3 at the 0.05 e & level. Angew. Chem. Int. Ed. 2015, 54, 1 – 6 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 3 These are not the final page numbers! . Angewandte Communications [2] A search of the Cambridge Structural Database (CSD version 91.3 % As and 8.7 % U character, and these interactions 1.17, date: 12/05/15) revealed 30 examples of metal-are also found at the second-order level of perturbation. coordinated HNNH. The arsenic components are essentially pure 4p character, [3] Examples with f-block ions: a) W. J. Evans, G. Kociok-Kçhn, whereas the uranium contributions are 53.2 % 5f and J. W. Ziller, Angew. Chem. Int. Ed. Engl. 1992, 31, 1081; Angew. 45.3 % 6d character with no meaningful 7s or 7p Chem. 1992, 104, 1114; b) W. J. Evans, G. Kociok-Kçhn, V. S. contributions. Leong, J. W. Ziller, Inorg. Chem. 1992, 31, 3592; c) K. G. Wang, When considering the bonding of the HAsAsH fragment E. D. Stevens, S. P. Nolan, Organometallics 1992, 11, 1011. TIPS[4] a) S. Yao, M. Brym, K. Merz, M. Driess, Organometallics 2008, to two [U(Tren)] fragments, an a priori treatment yields 27, 3601; b) N. Etkin, M. T. Benson, S. Courtenay, M. J. two bonding extremes. On one hand, HAsAsH could donate McGlinchey, A. D. Bain, D. W. Stephan, Organometallics 1997, electron density purely from its filled p-orbital to vacant 16, 3504; c) J. C. Green, M. L. H. Green, G. E. Morris, J. orbitals on each uranium center, which would be assigned Chem. Soc. Chem. Commun. 1974, 212. as formally trivalent, with no back-bonding and thus retain [5] a) B. H. Ern , F. Ozanam, M. Stchakovsky, D. Vanmaekelbergh, J.-the As=As double bond. Alternatively, each uranium could N. Chazalviel, J. Phys. Chem. B 2000, 104, 5961; b) B. H. Ern , formally engage in a back-bond-type interaction into the M. Stchakovsky, F. Ozanam, J.-N. Chazalviel, J. Electrochem. vacant p*-orbital of HAsAsH, leading to reduction to give Soc. 1998, 145, 447; c) T. R. Omstead, A. V. Annapragada, K. F. Jensen, Appl. Phys. Lett. 1990, 57, 2543. two uranium(IV) centers and a HAsAsH dianion with an As [6] H. Davy, Philos. Trans. R. Soc. London 1810, 100, 16. As single bond. Interestingly, all attempts to computationally [7] J. L. Gay-Lussac, L. J. Th nard, Recherches Physico-model 3 as diuranium(III) with a formally neutral HAsAsH Chimiques, vol 1, 232, Paris, 1811. met with failure or converged instead to a diuranium(IV) [8] E. J. Weeks, J. G. F. Druce, Chem. News 1924, 129, 31. HAsAsH-dianion spin-state formulation. Previous calcula-[9] W. L. Jolly, L. B. Anderson, R. T. Beltrami, J. Am. Chem. Soc. tions on HAsAsH have predicted it to be an excellent p-1957, 79, 2443. [19, 29][10] B. M. Gardner, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, acceptor ligand, and the combined characterization A. J. Blake, S. T. Liddle, Angew. Chem. Int. Ed. 2015, 54, 7068; data for 3 clearly support the latter bonding picture, that is, Angew. Chem. 2015, 127, 7174. in 3 HAsAsH can be considered as a diarsane-1,2-diide [11] D. M. King, F. Tuna, J. McMaster, W. Lewis, A. J. Blake, E. J. L. resulting from extensive electron transfer from uranium. McInnes, S. T. Liddle, Angew. Chem. Int. Ed. 2013, 52, 4921; In summary, by careful control of reaction conditions we Angew. Chem. 2013, 125, 5021. have been able to isolate the highly reactive HAsAsH unit [12] a) P. A. Cleaves, D. M. King, C. E. Kefalidis, L. Maron, F. Tuna, between two sterically demanding uranium fragments, thus E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, confirming the synthesis of a molecule first proposed over two Angew. Chem. Int. Ed. 2014, 53, 10412; Angew. Chem. 2014, 126, 10580; b) D. M. King, F. Tuna, E. J. L. McInnes, J. McMaster, W. centuries ago. The characterization data for 3 uniformly point to Lewis, A. J. Blake, S. T. Liddle, Nat. Chem. 2013, 5, 482; c) D. M. the HAsAsH unit being formally reduced to its dianionc form by King, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. the two uranium centers, in-line with the predicted excellent Blake, S. T. Liddle, Science 2012, 337, 717. acceptor properties of HAsAsH. This study high-lights the [13] D. M. King, J. McMaster, F. Tuna, E. J. L. McInnes, W. Lewis, capacity of an f-block element, uranium, to bond in a manner A. J. Blake, S. T. Liddle, J. Am. Chem. Soc. 2014, 136, 5619. that is reminiscent of d-block metals, though at one bonding [14] B. M. Gardner, G. BalQzs, M. Scheer, F. Tuna, E. J. L. McInnes, J. extreme with highly polarized U As bonding interactions. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, Angew. Chem. Int. Ed. 2014, 53, 4484; Angew. Chem. 2014, 126, 4573. Complex 3 is an isoelectronic model for a p-alkene complex of [15] B. M. Gardner, G. BalQzs, M. Scheer, F. Tuna, E. J. L. uranium, which is a class of complex yet to be realized under [30] McInnes, J. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, Nat. any experimental conditions.Chem. 2015, 7, 582. [16] W. C. Johnson, A. Pechukas, J. Am. Chem. Soc. 1937, 59, 2068. [17] a) W. A. Herrmann, B. Koumbouris, T. Zahn, M. L. Ziegler, Acknowledgements Angew. Chem. Int. Ed. Engl. 1984, 23, 812; Angew. Chem. 1984, 96, 802; b) G. Huttner, H. G. Schmid, H. Lorenz, Chem. Ber. 1976, 109, 3741. We thank the Royal Society, European Research Council, [18] Full details can be found in the Supporting Information. Engineering and Physical Sciences Research Council, Uni- [19] S. Nagase, S. Suzuki, T. Kurakake, J. Chem. Soc. Chem. versities of Nottingham, Manchester, and Regensburg, the Commun. 1990, 1724. Deutsche Forschungsgemeinschaft, UK National Nuclear [20] C. H. Lai, M. D. Su, J. Comput. Chem. 2008, 29, 2487. Laboratory, and COST Action CM1006 for generously [21] a) B. M. Gardner, S. T. Liddle, Chem. Commun. 2015, 51, 10589; supporting this work. b) J. C. Berthet, M. Ephritikhine, Coord. Chem. Rev. 1998, 178 – 180, 83. Keywords: arsenic ? back-bonding ? density functional [22] P. Pyykkç, M. Atsumi, Chem. Eur. J. 2009, 15, 186. [23] a) H. A. Spinney, N. A. Piro, C. C. Cummins, J. Am. Chem. Soc. theory ? diarsene ? uranium 2009, 131, 16233; b) B. Twamley, C. D. 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Cornish, J. McMaster, W. Lewis, A. J. Blake, S. T. [26] a) S. T. Liddle, Angew. Chem. Int. Ed. 2015, 54, 8604; Angew. Liddle, Chem. Eur. J. 2011, 17, 11266; e) B. M. Gardner, J. Chem. 2015, 127, 8726; b) D. R. Kindra, W. J. Evans, Chem. McMaster, F. Moro, W. Lewis, A. J. Blake, S. T. Liddle, Chem. Eur. Rev. 2014, 114, 8865; c) I. Castro-Rodr%guez, K. Meyer, Chem. Commun. 2006, 1353. J. 2011, 17, 6909; f) S. T. Liddle, J. McMaster, D. P. Mills, 1A. J. Blake, C. Jones, W. D. Woodul, Angew. Chem. Int. Ed. 2009, 48, [27] The Z isomer is calculated to be 24 kcal mol less stable than the 1077; Angew. Chem. 2009, 121, 1097; g) B. M. Gardner, J. McMaster, E isomer. Although this seems to be a large DE value, it should be W. Lewis, S. T. Liddle, Chem. Commun. 2009, 2851. noted that: 1) the ligand reorganization energy of [U- DMBSDMBSt[29] a) T. Sasamori, E. Mieda, N. Nagahora, K. Sato, D. Shiomi, T. (Tren)] (Tren = N(CHCHNSiMeBu)) alone is about 22231Takui, Y. Hosoi, Y. Furukawa, N. Takagi, S. Nagase, N. Tokitoh, 11 kcal mol (Ref. [28b]); 2) in the Z isomer the cis As H clash TIPSJ. Am. Chem. Soc. 2006, 128, 12582; b) T. Sasamori, E. Mieda, N. with Tren C H groups whereas in the E isomer they sit in Nagahora, N. Takeda, N. Takagi, S. Nagase, N. Tokitoh, Chem. pockets formed between staggered triisopropylsilyl groups; 3) the Lett. 2005, 34, 166; c) V. Galasso, Chem. Phys. 1984, 83, 407. energy difference should be regarded as an upper limit as [30] S. T. Liddle, Coord. Chem. Rev. 2015, 293 – 294, 211. dispersion-corrected calculations, which might be anticipated to [31] CCDC 1403870 (3) and 1419459 (4) contain the supplementary provide a more representative picture (Ref. [1a]), proved to be intractable. The calculated symmetric and asymmetric As H crystallographic data for this paper. These data are provided free 1of charge by The Cambridge Crystallographic Data Centre. stretches for the Z isomer are 2069 and 2064 cm . [28] a) D. Patel, J. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, Nat. Commun. 2013, 4, 2323; b) B. M. Gardner, J. C. Stewart, A. L. Davis, J. McMaster, W. Lewis, A. J. Blake, S. T. Liddle, Proc. Natl. Acad. Sci. USA 2012, 109, 9265; c) B. M. Gardner, D. Patel, W. Received: September 14, 2015 Lewis, A. J. Blake, S. T. Liddle, Angew. Chem. Int. Ed. 2011, Published online: && &&, &&&& Angew. Chem. Int. Ed. 2015, 54, 1 – 6 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 5 These are not the final page numbers!