Reactions of monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2:: An experimental and computational study

Addition of CO to [1,2,4-(Me3C)(3)C5H2](2)CeH, Cp'2CeH, in toluene yields the cis-(Cp'Ce-2)2(mu-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100 degrees C over 7 months. When the solvent is pentane, Cp'Ce-2(OCH2) CeCp'(2) forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'Ce-2(OCH2)CeCp'(2), generating Cp'2CeOCH2COCeCp'(2). The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H-2) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H-2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H-2. The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H-2 to give Cp2CeOMe is exoergic by 50 kcal mol(-1). The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta(2)-CHO), is formed by further reaction with H-2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H-2 by CH4 increases the activation energy by 17 kcal mol(-1).