Disobedient Boron Clusters: Synthesis, Structure and Bonding of Hypoelectronic Open and Closo Osmaborane Clusters

Abstract

The hypoelectronic clusters pose enduring challenges to the well-established Wade's electron counting rule and its modified versions. Here, we report the syntheses and structural characterizations of open hypoelectronic osmaborane clusters [(Cp*Os)B₁₀H₁₅] (1), [(Cp*Os)₂B₆H₁₀] (2) obtained from the pyrolysis of an in situ generated intermediate obtained from fast metathesis of [(Cp*OsBr₂)₂] (Cp* = η⁵ -C₅Me₅) and [LiBH₄·THF], with [BH₃·SMe₂]. Under similar methodology, when [BH₃·THF] was utilized with modified reaction conditions, hypoelectronic trimetallic closo-metallaboranes [(Cp*Os)₃B₅H₆] (3) and [(Cp*Os)₃B₆H₇] (4) have been isolated. Cluster 1 represents the first synthesized 11-vertex arachnometallaborane derived from the 13-vertex closo-henicosahedron by removal of two adjacent five-connected vertices. Cluster 2 adopts a nido-{Os₂B₆} core, generated from a 9-vertex closo-tricapped trigonal prism by removing one five-connected vertex. Notably, Clusters 1 and 2 each fall short by one skeletal electron pair (SEP) according to Wade-Mingos' rule, thus rendering them as hypoelectronic metallaboranes. Clusters 3 and 4 exhibit unusual, less spherical deltahedral shapes and can be generated from parent canonical closo-[B_nH_n] ^2- (n = 8, 9) by performing diamond-square-diamond (DSD) rearrangements. 3 and 4 are two SEP deficient than required and can be classified as hypoelectronic closo.