Activation of CH bonds by transition metals: IV. Partial dehydrogenation of tricyclohexylphosphine by iridium(I) and rhodium(I) leading to the formation of a metal—olefin complex and a five-coordinated metal dihydride, which is a hydrogenation catalyst

Abstract

Reaction of tricyclohexylphosphine with [(COT) 2MC1] 2 (M = Rh, Ir, COT = cyclooctene) in refluxing toluene, yields the five-coordinated metal(III) complexes H 2[P(C 6H 11) 3] 2MCl and the four-coordinated metal(I) complexes [P(C 6H 11) 3] M[P(C 6 H 9)(C 6H 11) 2]Cl, in which one cyclohexyl group has been dehydrogenated to a cyclohexene group, the double bond of which occupies the fourth coordination site. In refluxing toluene, triisopropylphosphine reacts with [(COT) 2MCl] 2 to give the five-coordinated complex H 2[P(C 3H 7) 3] 2MCl, which suggests a similar dehydrogenation. Reaction of the partly dehydrogenated products with CO yields [P(C 6H 11) 3]M[P(C 6H 9)(C 6H 11) 2]Cl(CO) in which the double bond is not coordinated to the metal. Hydrogen adds oxidatively to the iridium(I) compound, but does not react with the rhodium(I) compound. The mechanism of the dehydrogenation is discussed. The complexes H 2(PR 3) 2MCl are catalysts for hydrogenation of olefins at 100°C and at 1 atmosphere. 1,5-Cyclooctadiene is especially readily hydrogenated at this temperature.