The chemistry of isomeric icosaboranes, B20H26. Molecular structures and physical characterization of 2,2′- bi(nido-decaboranyl) and 2,6′-bi-(nido-decaboranyl)

Abstract

The structures of the large neutral boranes 2,2′-bi(nido-decaboranyl), 2,2′-(B₁₀H₁₃)₂, m.p. 178 °C, and 2,6′- bi(nido-decaboranyl), 2,6′-(B₁₀H₁₃)₂, m.p. 154 °C, have been established by single-crystal X-ray diffraction. The 2,2′ isomer is tetragonal, space group l4₁cd, with a= 11.901(3), c= 23.135(4)Å, molecular symmetry C₂; the 2,6′ isomer is orthorhombic, space group Pbca, with a= 14.673(3), b= 19.765(4), and c= 11.580(3)Å. The intercluster B–B bond lengths are respectively 1.692(3) and 1.679(3)Å, and there is a lengthening of 0.015 Å of the adjacent cluster B–B distances when compared with the corresponding distances in decaborane(14) itself. The ¹H-{¹¹B} and ¹¹B n.m.r. behaviour of the two isomeric conjuncto-boranes has been investigated and the results for the 2,2′ isomer are discussed in some detail. Infrared and u.v. spectroscopic data are presented. The unambiguous determination of the structures permits a discussion of possible synthetic mechanisms.