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

The synthesis of the novel C 3-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 (2 J P–H = 115.4 Hz) and a much smaller cis coupling (2 J P–H = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir(H)(κ5 P,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.