C−H Bond Activation Reactions with Ligand Adducts of a β-Diiminate Iridium Dihydride

Addition of neutral ligands to the trigonal prismatic iridium tetrahydride (iPrBDI)IrH4
(BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-iPr2C6H3) induces reductive elimination of
dihydrogen to afford (iPrBDI)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 PiPr3, 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(I)−Ir(III) couples are
preferred.