Reductive Elimination at an Ortho-Metalated Iridium(III) Hydride Bearing a Tripodal Tetraphosphorus Ligand

Abstract

The synthesis of the novel C3-symmetric tripodal, tetradentate ligand 1, bearing only phosphorus atoms as donor groups, is described, starting from commercially available o-tolyldiphenylphosphine, and its molecular structure has been determined by X-ray crystallographic analysis. Coordination to the cationic IrI precursor [Ir(COE)2(acetone)2]PF6 led to a highly unsymmetrical species (90% yield) with four inequivalent phosphorus atoms, as evidenced by 31P NMR spectroscopy. The corresponding 1H NMR spectrum exhibited a pseudo doublet of quartets at δ −5.9 ppm with one large trans P–H coupling (2JP–H = 115.4 Hz) and a much smaller cis coupling (2JP–H = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir(H)(κ5P,P,P,P,C-1)]PF6, which is a rare example of a structurally characterized mononuclear Ir hydride species bearing an ortho-metalated phosphine ligand. This species does not react with hydride sources, but addition of 1 equiv of CF3COOH resulted in facile overall formal protonation of the Ir–C bond. DFT calculations support a pathway involving initial reductive elimination, forming the highly distorted four-coordinate IrI species 2′, followed by protonation at iridium to give the dicationic monohydride species 3, with an activation barrier ΔG† of 28.2 kcal mol–1. Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of 2 with carbon monoxide, yielding the monocationic carbonyl complex [IrI(CO)(1)]PF6 as the sole product.