Introduction

Tetraalkyldistibines R₄Sb₂ such as Me₄Sb₂ and Et₄Sb₂ are long known low-valent organoantimony compounds. Me₄Sb₂ for instance was synthesized for the first time almost eighty years ago by reaction of an Sb-mirror with methyl radicals.[1,2] Since then, alternative synthetic pathways were established and their structures were investigated, in detail.[3]

Distibines Sb₂R₄ and dibismuthines R₄Bi₂ tend to form intermolecular interactions in the solid state, yielding chain-like structures with E···E bond lengths below the sum of the van-der-Waals radii (4.12 Å).[4]. The formation of intermolecular interactions is typically observed for compounds containing small organic substituents, whereas sterically demanding substituents hamper the formation of intermolecular contacts.

These interactions are rather weak and are often interrupted upon melting, resulting in a bathochromic shift between fluid and solid phases, the so-called ''thermochromic'' behavior.[5] For instance, Me₄Sb₂ is yellow in the solid state in liquid nitrogen, red close to its melting point of 17 °C, again yellow in the fluid state but colourless in the gas phase.[6] We recently studied the solid state structures of Et₄Sb₂ and Et₄Bi₂.[7]

The crystals were obtained by in situ crystallization on the diffractometer using an IR laser-asssited technique. Et₄Sb₂ forms endless chains via short intermolecular Sb···Sb contacts of 3.688(1) Å between two distibine units, which agrees very well with values of reported for Me₄Sb₂ (3.645, 3.678 Å) and [(HC=CMe)₂]₂Sb₂ (3.63 Å). In contrast, Et₄Bi₂ doesn't form isolated Bi chains but exists as rather isolated molecules and the closest intermolecular Bi···Bi distance of 4.4598(4) Å is longer than the sum of the van-der-Waals radii (4.14 Å).

According to these structural findings, it isn't surprising that Et₄Sb₂ shows thermochromic behavior, while Et₄Bi₂ only shows an intensification of the red color to a deep red color upon cooling to -103° C.

Refences

[1] For a historical review see: D. Seyferth, Organometallics 2001, 20, 1488.

[2] a) F. A. Paneth, Trans. Faraday Soc. 1934, 30, 179; b) F. A. Paneth, H. Loleit, J. Chem. Soc. 1935, 366.

[3] For review articles see: a) H. J. Breunig, R. Rössler, Coord. Chem. Rev. 1997, 163, 33; b) C. Silvestru, H. J. Breunig, H. Althaus, Chem. Rev. 1999, 99, 3277; c) H. J. Breunig, R. Rössler, Chem. Soc. Rev. 2000, 29, 403; d) Y. Matano, T. Ikegami, in Organobismuth Chemistry, ed. H. Suzuki, Y. Matano, Elsevier, Amsterdam, 2001, p. 107ff; e) G. Balázs, H. J. Breunig, in Unusual Structures and Physical properties in organometallic Chemistry, ed. M. Gielen, R. Willem, B. Wrackmeyer, J. Wiley & Sons, West Sussex (GB), 2002, p. 387ff; f) L. Balázs, H. J. Breunig, Coord. Chem. Rev. 2004, 248, 603; g) H. J. Breunig, Z. Anorg. Allg. Chem. 2005, 631, 621.

[4] Mantina, M.; Chamberlin, A. C.; Valero, R.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. A 2009, 113, 5806.

[5] a) Ashe, III, A. J.; Ludwig Jr., E. G.; Oleksyszyn, J.; Huffman J. C. Organometallics 1984, 3, 337; b) Mundt, O.; Riffel, H.; Becker, G.; Simon, A. Z. Naturforsch. 1984, 39b, 317; c) Mundt, 0.; Riffel, H.; Becker, G.; Simon, A. Z. Naturforsch. 1988, 43b, 952; d) Breunig, H. J. Z. Anorg. Allg. Chem. 2005, 631, 621; e) Breunig, H. J.; Lork, E.; Moldovan, O.; Rat, C. I. J. Organomet. Chem. 2008, 693, 2527; f) Monakhov, K. Y.; Zessin, T.; Linti G., Eur. J. Inorg. Chem. 2010, 322.

[6] A. J. Ashe, III, E. G. Ludwig, Jr., J. Oleksyszyn, J. C. Huffman Organometallics 1984, 3, 337.

[7] a) Kuczkowski, A.; Heimann, S.; Weber, A.; Schulz, S.; Bläser, D.; Wölper, C. Organometallics 2011, 30, 4730; b) Schulz, S.; Heimann, S.; Kuczkowski, A.; Wölper, C.; Bläser, D. Organometallics 2013, 32, 3391.