Disobedient boron clusters: synthesis, structure and bonding of hypoelectronic open and closo osmaborane clusters

Abstract

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 characterization of open hypoelectronic osmaborane clusters [(Cp*Os)B₁₀H₁₅] (1) and [(Cp*Os)₂B₆H₁₀] (2) obtained from the pyrolysis of an in situ generated intermediate formed via fast metathesis of [(Cp*OsBr₂)₂] (Cp* = η⁵-C₅Me₅) and [LiBH₄·THF], with [BH₃·SMe₂]. Following a similar methodology, when [BH₃·THF] was utilized under modified reaction conditions, hypoelectronic trimetallic closo-metallaboranes [(Cp*Os)₃B₅H₆] (3) and [(Cp*Os)₃B₆H₇] (4) were isolated. Cluster 1 represents the first synthesized 11-vertex arachno-metallaborane 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 the Wade–Mingos rules, thus rendering them hypoelectronic metallaboranes. Clusters 3 and 4 exhibit unusual, less spherical deltahedral shapes and can be generated from the parent canonical closo-[B_nH_n]²⁻ (n = 8, 9) by performing diamond–square–diamond (DSD) rearrangements. Clusters 3 and 4 are two SEPs deficient relative to the required electron count and can be classified as hypoelectronic closo clusters.