Rhodium and Iridium Complexes of Lutidine-Based NHC/Amino Pincer Ligands: from Monodentate to Tetradentate Coordination Enabled by C–H Activation

A series of rhodium and iridium complexes derived from lutidine-based ligands (lutidine, 2,6-dimethylpyridine) with NHC and amino side-donor functions have been prepared and characterized. Deprotonation of the functionalized imidazolium salts, [(t)BuHImCH(2)PyCH(2)NR(2)]Br, by the bridging methoxo ligands of the dinuclear complexes [M(mu-OMe)(cod)](2) affords [MBr(cod)(kappa C-(t)BuImCH(2)PyCH(2)NR(2))] (M = Rh and Ir) complexes from which a series of Rh(I) and Ir(I) complexes including [M(cod)(kappa C-3,N,N '-(t)BuImCH(2)PyCH(2)NR(2))](+), [Rh(CO)(kappa C-3,N,N '-(t)BuImCH(2)PyCH(2)NR(2))](+), and [IrBr(CO)(2)(kappa C-(t)BuImCH(2)PyCH(2)NR(2))] are readily accessible by halide abstraction and/or carbonylation reactions. In contrast, direct metalation of imidazolium salts with the dinuclear compounds [M(mu-Cl)(cod)](2) (M = Rh and Ir) in the presence of potassium acetate and potassium iodide, a well-established synthetic route to M(III) species, provides access to unusual di-iodido M(III) cyclometalated compounds [MI2{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}] in low yield. Experimental studies combined with DFT calculations suggest that cyclometalated M(III) hydrido [MH(CH3CN){kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}](+) compounds and square-planar cyclometalated M(I) [M{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}] species resulting from their deprotonation by acetate could be intermediates involved in the formation of these compounds. Based on the observed formation of elemental rhodium, disproportionation of square-planar cyclometalated M(I) complexes to afford M(0) and the cationic M(II) species [M{kappa C-4,C ',N,N '-(CH(2)CMe(2)ImCH(2)PyCH(2)NR(2))}](+) is proposed. Reaction of the latter with iodide (I-) would regenerate the M(I) intermediate to give an iodo radical (I center dot) that in turn could dimerize to form diiodine I-2. In this regard, DFT calculations have shown that the oxidative addition of diiodine to the cyclometalated M(I) intermediates leading to the di-iodido M(III) cyclometalated compounds is a highly exergonic process.