Labile N-Heterocyclic Carbene Complexes of Iridium

The arene complex [IrH2(eta(6)-(CH6)-H-6)(IMes)]PF6 (3) has been prepared in a one-pot synthesis from conventional starting materials. The facile substitution of its benzene ligand can be exploited in the preparation of many other NHC Ir(III) dihydrides, for which the solvent complexes [IrH2(L)(3)(IMes)]PF6 (L = acetone-d(6), 4; NCMe, 5), the NHC-phosphine compound [IrH2(NCMe)(2)(IMes)(PiPr(3))]BF4 (6), and the water-soluble analogue [IrH2(NCMe)(2)(IMes)(TPPTS)]BF4 (7) constitute representative examples. The reactions of dihydrides 5 and 6 with hydrogen acceptors such as ethylene, propylene, and diphenylacetylene have been examined. Depending on the dihydride and the acceptor, they have led to different Ir(III) complexes with a cyclometalated IMes moiety and hydride, alkyl, or alkenyl ligands: [Ir(R)(IMes')(NCMe)(2)(L)]PF6 {R = H, L = NCMe (8), PiPr(3) (9); L = NCMe, R = Et (10); nPr (11); Z-C(Ph)=CHPh (12)}. Because of their six-membered rings, such cyclometalated compounds have been found to adopt two possible conformations in equilibrium. The rates of exchange between conformers are of the same order as the NMR time scale, and the equilibrium position is governed by steric factors. By reaction with phenylacetylene, compounds 6 and 9 have afforded a common hydride alkynyl complex [IrH(C CPh)(NCMe)(2)(IMes)(PiPr(3))]PF6 (14). Its selective formation as a deuteride isotopomer from the reaction between 9 and PhC CD has proven that Ir(I) species, although nonobserved, are accessible.