Carbaboranerhodium complexes with Rh–Rh, Rh–Cu and Rh–Au bonds; crystal structure of [NEt4][Rh2(µ-σ : η5-C2B9H8Me2)(CO)3(η5-C2B9H9Me2)]

Abstract

The compound [NEt₄][Rh(CO)(PPh₃)(η⁵-C₂B₉H₉Me₂)] has been prepared and used to synthesise the dimetal complexes [RhM(CO)(PPh₃)₂(η⁵-C₂B₉H₉Me₂)](M = Cu or Au) and [RhCu(CO)(PMe₃)(PPh₃)(η⁵-C₂B₉H₉Me₂)]. Protonation of [NEt₄][Rh(CO)(PPh₃)(η⁵-C₂B₉H₉Me₂)] gives an unstable hydride [RhH(CO)(PPh₃)(η⁵-C₂B₉H₉Me₂)]. In contrast, treatment of [NEt₄][Rh(CO)₂(η⁵-C₂B₉H₉Me₂)] with HBF₄·Et₂O affords a dirhodium complex [Rh₂(CO)₃(η⁵-C₂B₉H₉Me₂)₂] in which one carbaborane fragment bridges the Rh–Rh bond via an exopolyhedral B–H⇀Rh bond. This species can be deprotonated to give an anionic complex [Rh₂(µ-σ : η⁵-C₂B₉H₈Me₂)(CO)₃(η⁵-C₂B₉H₉Me₂)]^–, the [NEt₄]⁺ salt of which may be obtained directly from [NEt₄][Rh(CO)₂(η⁵-C₂B₉H₉Me₂)] by treatment of the latter with [NHMe₃]Cl. An X-ray crystallographic study established the structure of the anion. One rhodium centre is ligated by a CO molecule and by a C₂B₉H₉Me₂ cage in the normal η⁵ bonding mode, the other carries two CO groups and an η⁵-C₂B₉ cage. However, a boron atom β to the carbons in the pentagonal open [graphic omitted] face of this cage forms a σ bond to the other rhodium atom [B–Rh 2.05(3)Å], so that this C₂B₉H₈Me₂ group bridges the Rh–Rh bond [2.876(3)Å]. The NMR data for the new compounds are reported and discussed.