Reactivity of group 15 complexes

Trialkylstibines and bismuthines R₃E (E = Sb, Bi) were intensely studied in reactions with Lewis acidic group 13 trialkyls, yielding Lewis acid base adducts. Adduct formation was also observed in reactions of distibines and dibismuthines with group 13 metal complexes. Recently, we studied oxidation reactions of R₃E as well as insertion reactions of low valent distibines and dibismuthines R₄E₂ with group 16 elements as well as organometallic group 16 element compounds.

Oxidation reactions of R₃Sb

Even though chalcogenostiboranes R₃SbE (E = O, S, Se; R = alkyl, aryl) have been initially prepared more than 150 years ago by Carl Jakob Löwig and Eduard Schweizer by redox reaction of Et₃Sb with elemental sulfur and selenium,[1] reports on solid state structures of R₃SbE bearing an unsupported terminal Sb=E double bond are almost unknown, Ph₃SbS represents the only structurally characterized triorganylthiostiborane.[2,3] Pebler et al. described the short Sb-S bond in Ph₃SbS (2.244(1) Å) as partial double bond, resulting from a d_π–p_π interaction,[3] while Otera et al. investigated the bonding situation in Me₃SbS and calculated a positive charge of +0.6 at the Sb atom, indicating the Sb-S bond to be best described as polar single bond with some ionic stabilization.[4] Vibrational spectroscopy was used to clarify the bonding situation. The Sb-E stretching vibration frequencies for a Sb-E single bond and Sb=E double bond were calculated using Gordy's rules and compared with the experimental values for Me₃Sb=S, Et₃Sb=S and Et₃Sb=Se. However, since the experimental values fall in between the calculated values, a clear distinction between both bonding situations was not possible. Therefore, it is still unclear whether the Sb-E bond in chalcogenostiboranes should be described as a polar single bond or as a real double bond (Scheme 1).

We synthesized Et₃Sb=S and Et₃Sb=Se, determined their solid state structures by single crystal X-ray diffraction and investigated the bonding situation by quantum chemical calculations.[5] The Sb-S bond length of Et₃Sb=S (2.381(7) Å) is in between the calculated values for the Sb-S single bond (Sb–S 2.43 Å) and the Sb=S double-bond (2.27 Å) and  significantly elongated compared to that in Ph₃Sb=S (2.244(1) Å). Et₃Sb=Se is the first structurally characterized trialkylselenostiborane and shows the shortest Sb–Se bond length (2.4062(8) Å) reported, to date, which perfectly agrees with the calculated Sb=Se double bond (Sb–Se 2.40 Å). However, according to quantum chemical calculations, the shortening of the Sb-E bonds in Et₃Sb=S and Et₃Sb=Se results from a strong electrostatic interaction between the central Sb atom and the chalcogen bonding partner as both bonds are strongly polarized, Sb(δ+)–E(δ−) (vide infra). Natural bond orbital (NBO) analysis suggests the presence of three lone pair orbitals on the chalcogen atom. One of the lone pair orbitals consists of the valence shell s orbital (S: 89%; Se: 92%), the remaining two are pure valence shell p orbitals orthogonal to the Sb-E bond, thus precluding the existence of a Sb-E double bond in both cases due to p-interactions.

References

[1] C. Löwig, E. Schweizer, Liebigs Ann. Chem. 1850, 75, 315.

[2] a) A. Hantzsch, H. Hibbert, Ber. Dtsch. Chem. Ges. 1907, 40, 1508; b) W. J. C. Dyke, W. J. Jones, J. Chem. Soc. 1930, 1921; c) R. A. Zingaro, A. Merijanian, J. Organomet. Chem., 1964, 1, 369; d) G. N. Chremos, R. A. Zingaro, J. Organomet. Chem. 1970, 22, 637.

[3] L. Kaufmann, Chem. Ber. 1908, 41, 2762; W. J. Lile, R. C. Menzies, J. Chem. Soc. 1950, 617.

[4] a) J. Pebler, F. Weller, K. Dehnicke, Z. Anorg. Allg. Chem. 1982, 492, 139; b) F. Weller, K. Dehnicke, Naturwissenschaften 1981, 68, 523; c) G. S. Zhdanov, V. A. Pospelov, M. M. Umanski, V. P.Glushkova, Dokl. Akad. Nauk SSSR (Russ.) (Proc. Nat. Acad. Sci. USSR) 1953, 92, 983.

[5] S. Heimann, D. Bläser, C. Wölper, R. Haack, G. Jansen, S. Schulz, Dalton Trans, 2014, 43, 14772.