Synthesis and characterization of bis(sigma)borate and bis-zwitterionic complexes of rhodium and iridium

Abstract

Building upon the chemistry of Rh–N,S-heterocyclic carbene complex, [(Cp*Rh)(L₂)(1-benzothiazol-2-ylidene)], 2 (Cp*=η⁵-C₅Me₅; L=C₇H₄NS₂) with various monoboranes-Lewis adducts, we explored the chemistry of 2 with BH₃⋅thf at elevated temperature. As a result, mild thermolysis of 2 with BH₃⋅thf led to the formation of bis(sigma)borate [(η⁴-C₅Me₅H)Rh(η²-H₃BL)], 3 and a bis-zwitterionic species [Cp*RhS(BH₂L₂)], 4 with the concomitant release of BH₃⋅bt (bt=benzothiazole). The RhS₃C₂N₂B₂ atoms in 4 generates two six membered rings fused by a common Rh−S bond, which may be considered as a bicycle [4.4.0] cage at the rhodium center. In an effort to generate the iridium analogue of 3, reaction of [Cp*IrCl₂]₂ with Na[H₃B(mbt)] (mbt=2-mercaptobenzothiazole) was carried out that produced bis(sigma)borate complex [(η⁴-C₅Me₅H)Ir(η²-H₃BL)], 1. The solid state X-ray structures of 1 and 3 showed that the Cp*H ligand coordinated to the metal center in a η⁴-fashion. In compound 3, the methyl group is oriented towards rhodium center, whereas it is away from Ir center in 1. In addition, the DFT computations were performed to shed light on the bonding and electronic structures of these compounds.