AN UNUSUAL TYPE OF H...H INTERACTION - IR-H...H-O AND IR-H...H-N HYDROGEN-BONDING AND ITS INVOLVEMENT IN SIGMA-BOND METATHESIS

A series of metal hydride complexes is described (1, 6, and 7) contain ing X-H...H-Ir (X = O or N) hydrogen bonds. The H...H distance is shor t, about 1.8 Angstrom, as in the N-H...H-Ir system independently disco vered recently (1994) by Ramachandran and Morris, but much shorter tha n the prior (1990) O-H...H-Ir system of Milstein et al. A J-coupling o f 2-4 Hz in the NMR spectrum is often found between H-a and H-b in X-H -a...H-b-Ir. Exchange between H-a and H-b occurs on the NMR time scale , probably via proton transfer to give hydrogen complexes X...-(H-2)-M as intermediates. The interaction activates the Ir-H bond for reactio ns such as substitution, and cyclometalation. NMR data indicates an H- bond strength of 4.3 +/- 0.8 kcal/mol in one case (7). The species giv e sigma-bond metathesis reactions and easily lose H-2 to give a cyclom etalated derivative, a reaction which is reversible in some cases. Rea sons for the formation of a strong H-bond in this case are proposed: t he close approach possible for H-a and H-b; the absence of lone pair r epulsion; and the high polarizability of the Ir-H bond.