Experimental and Theoretical Studies of the Hydrogenation of α,β-Unsaturated Acids by an 18e Hydride Carbonylniobocene Complex

The reaction of the niobocene complex [Nb­(η5-C5H4SiMe3)2(H)­(CO)] (1) with α,β-unsaturated acids, namely, fumaric and maleic acids, and methylfumarate ester yielded the corresponding carboxylato-containing niobocene derivatives [Nb­(η5-C5H4SiMe3)2(κ1-O-OOC-CH2-CH2-R)­(CO)] (R = COOH (2); R = COOMe (3)), resulting from the hydrogenation of the CC bond and the consequent coordination of a carboxylato moiety on the Nb center. The process represents the first reported example of the hydrogenation of α,β-unsaturated acids mediated by a hydride niobocene complex. Spectroscopic and theoretical studies at the DFT level have allowed for proposing a plausible reaction mechanism. Initial coordination of the CC bond to the Nb center is followed by the insertion of this bond into the Nb–H bond, and the reaction is completed by a proton transfer from the carboxylic moiety to the α-carbon of the alkyl moiety, giving rise to the final κ1-carboxylato ligand.