FORMATION OF OLEFINS UPON OXIDATION OF MOLYBDENUM ALKYL CARBYNES - ORGANIC RADICAL REACTIVITY IN AN ORGANOMETALLIC RADICAL-CATION

Decomposition of the complexes (eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CR [R = c-C4H9, (CH2)(3)CH3, and CH(CH2-CH2CH3)(2)] in CHCl3 results in conver sion of the carbyne ligand to a terminal olefin. The reaction is initi ated by oxidation and occurs during photolysis in CHCl3 or upon slow d iffusion of O-2 into the reaction mixtures. Corroborating evidence for initiation of the reaction by electron transfer has been obtained by conversion of the butyl carbyne ligand of (eta(5)-C5H5)(CO){P(OMe)(3)} Mo=C(CH2)(3)CH3 to 1-pentene upon electrochemical oxidation. Mechanist ic studies were consistent with H-abstraction by the carbyne radical c ation to yield a cationic carbene complex which forms the olefin in a H-shift process. INDO calculations on the carbyne radical cation [(eta (5)-C5H5)(CO){P(OMe)(3)}Mo=CCH2CH3](.+) indicate stabilization upon be nding the Mo=C-C angle from 180 degrees to 120 degrees. This change in geometry places spin density on the carbyne carbon in the radical cat ion although the initial oxidation occurs from an orbital that is prim arily nonbonding metal d in character. The oxidized carbyne is thus ab le to function as a carbon-centered radical and abstract a hydrogen at om at the carbyne carbon. Although the olefin-forming reaction is gene ral far alkyl carbynes with a hydrogen on C2, the tert-butyl carbyne ( eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CC(CH3)(3) (2e) did not form an olefin u pon oxidation. Instead, photolysis of 2e in CHCl3 yielded the dichloro molybdenum carbyne (eta(5)-C5H5)Cl-2{P(OMe)(3)}Mo=CC(CH3)(3) (5e) via a Cl-abstraction pathway.