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

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.