Rhodium and iridium beta-diiminate complexes - Olefin hydrogenation step by step
Budzelaar PHM, Moonen NNP, de Gelder R, Smits JMM, Gal AW
EUROPEAN JOURNAL OF INORGANIC CHEMISTRY (4): 753-769 APR 2000

Abstract:
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 stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14-e complex LMeRh(COE) (COE = cyclootene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N-2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LM,Rh(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. Rhodium(I) complexes of L-Me and L-Cl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky beta-diiminate ligands allow direct observation 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 systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, 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.