MECHANISTIC STUDIES ON METAL TO LIGAND HYDROGEN TRANSFER IN THE THERMAL-REACTIONS OF H(MU-H)OS3(CO)10(CNR) - EVIDENCE FOR PROTON BARRIER TUNNELING IN A METAL TO LIGAND HYDROGEN TRANSFER

The thermal reactions of the series of complexes X(mu-X)Os3(CO)10(CNR) (R = CH3, C6H5, p-C6H4CH3, omicron-C6H4CH3, 2,6-C6H3(CH3)2; X = H, D, 1a-e, 1a-e-d2) have been studied. In all cases one or more of three products are formed: (mu-H) (mu-eta-1-CN(H)R)Os3(CO)10 (2a,c-e) resulting from metal to nitrogen hydrogen transfer, (mu-H) (mu-eta-2CHNR)Os3(CO)10 (3b-d) resulting from metal to carbon hydrogen transfer, and (mu-H)2Os3(CO)9(CNR) (4c-e) resulting from loss of carbon monoxide. The reaction shows significant steric and electronic components. Thus the metal to nitrogen hydrogen-transfer product is favored with more basic isocyanides (R = CH3 > p-C6H4CH3 > C6H5) while the relative amount of metal to carbon hydrogen transfer decreases with increasing bulkiness of the isocyanide (R = p-C6H4CH3 > omicron-C6H4CH3 > 2,6-C6H3(CH3)2). Variable-temperature (3-70-degrees-C) kinetics on the methyl derivative 1a, by H-1 NMR spectroscopy in benzene-d6, showed that metal to nitrogen hydrogen transfer is a first-order process whose rate is very sensitive to even trace amounts of moisture. In the temperature range 32-70-degrees-C, very large isotope effects and a very large difference between calculated DELTA-E(aH)D and DELTA-E(oH)D suggest that metal to nitrogen hydrogen transfer in 1a proceeds with a significant proton barrier tunneling component. A Bell tunneling model calculation was performed to estimate the barrier width. Addition of base to the reaction quenches the large isotope effects. At 65-degrees-C the phenyl derivative 1b gives only the metal to carbon hydrogen-transfer product 3b and shows only a small inverse isotope effect (k(H)/k(D) = 0.6), suggestive of reversible hydrogen transfer. A comparison of the relative yields obtained from thermolysis of the p-tolyl derivative 1c and its deuteriated analogue at 80-degrees-C in benzene under vacuum and carbon monoxide atmosphere gives further evidence that metal to nitrogen and metal to carbon hydrogen-transfers proceed by distinctly different mechanisms.