C-H bond activation reactions with ligand adducts of β-diiminate iridium dihydride

Addition of neutral ligands to the trigonal prismatic iridium tetrahydride ((BDI)-B-iPr)IrH4 (BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-(Pr2C6H3)-Pr-i) induces reductive elimination of dihydrogen to afford ((BDI)-B-iPr)IrH2(L) (L = phosphine, tetrahydrothiophene, cyclohexene) compounds. A combination of solution NMR data and infrared spectroscopic studies have established that the phosphine dihydride compounds and the cyclohexene adduct are best described as classical iridium(III) dihydride or "stretched dihydrogen" complexes. The corresponding tetrahydrothiophene adduct exhibits spectroscopic features and reactivity patterns consistent with increased iridium(I) dihydrogen character. In complexes containing large cone angle phosphines such as PCy3 and (PPr3)-Pr-i, ligand dissociation is facile and isotopic exchange in arene substrates is observed. The experimental data support an Ir(III)-Ir(V) oxidative addition-reductive elimination sequence for C-H bond activation, in contrast to more traditional coordinatively saturated precursors, where Ir(l)-Ir(III) couples are preferred.