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

The reaction of the niobocene complex [Nb­(η⁵-C₅H₄SiMe₃)₂(H)­(CO)] (<b>1</b>) 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 (<b>2</b>); R = COOMe (<b>3</b>)), 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