Rhodium and iridium beta-diiminate complexes - Olefin hydrogenation step by step

The bulky beta-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(-Me)N(2,6-C6H3X2)]( -) (X = Me, L-Me; X = Cl, L-Cl) have been found to be effective in stabiliz ing low coordination numbers (CN) in Rh and Ir complexes. The 14-e complex LMeRh(COE) (COE = cyclootene) has a three-coordinate T-shaped Rh environmen t and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N-2, MeCN, olefins), by the intramolecular coordina tion of a chlorine atom in LClRh(COE), or by an agostic interaction in LM,R h(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2 ,3-dimethylbutene complex actually prefers the allyl hydride structure. Rho dium(I) complexes of L-Me and L-Cl catalyze olefin hydrogenation; hydrogena tion of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky beta-diiminate ligands allow direct o bservation of a number of reactive intermediates or their iridium analogues , including an olefin-dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model s ystems, provide us with snapshots of a plausible hydrogenation cycle. Remar kably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.