Synthesis of Rhodium and Iridium Complexes Supported by Bis(indolylphosphino)silyl Pincer Ligation: Competitive N-H and C-H Bond Activation by an Ir(I) Species

Abstract

The synthesis of five-coordinate amido hydride PSiP pincer complexes of both RhIII and IrIII was pursued. The preparation of such complexes by a salt metathesis route was initially targeted to assess the synthetic viability of such species. Isolable anilido hydride complexes of both Rh and Ir proved accessible, and rare examples of thermally robust Ir alkylamido hydride complexes were found to be viable synthetic targets. The preparation of amido hydride species by N-H oxidative addition was also pursued. Toward this end, the generation of coordinatively unsaturated (iPr-PSiPInd)MI (M = Rh, Ir) species was investigated. Dehydrohalogenation of (iPr-PSiPInd)RhH(Cl) (1) under N2 afforded the isolable complex [(iPr-PSiPInd)Rh]2(μ-N2) (8a). While the N2 ligand could be displaced to afford (iPr-PSiPInd)RhL (L = PMe3, DMAP, H2NPh), such RhI species proved unreactive toward N-H oxidative addition of aniline. Dehydrohalogenation of (iPr-PSiPInd)IrH(Cl) (2) in benzene solution resulted in activation of the solvent to afford (iPr-PSiPInd)IrH(Ph) (11), which undergoes facile arene exchange with benzene-d6. In cyclohexane solution, treatment with aniline afforded a mixture of N-H and aniline sp2-C-H bond oxidative addition products, with the latter species being favored. Although the observation of arene exchange involving (iPr-PSiPInd)IrH(Ph) suggests that C-H bond oxidative addition is reversible, attempts to drive the reactivity with aniline in the direction of N-H bond oxidative addition were unsuccessful. This reactivity differs significantly from that of previously reported complexes supported by related PSiP ligation. Most notably, it appears that for (iPr-PSiPInd)Ir, products resulting from N-H- and C-H-oxidative addition of aniline substrates do not readily interconvert, although both are postulated to share a common intermediate of the type (iPr-PSiPInd)Ir(NH2Ph).