Acidity of Benzoic Acids Bearing the (CO)5Cr═CN(CH3)2 Group

Benzoic acids 2a and 2b, bearing the (CO)5Cr═CN(CH3)2 group in the p- and m-position, and the corresponding benzoic acids 2c and 2d, in which the rotation of the aminocarbene moiety was blocked by the presence of a methyl group, were prepared together with the corresponding N,N-dimethylamido acids 3a−d. The measurement of pKa values in EtOH and DMF revealed that the (CO)5Cr═CN(CH3)2 group is a very weak electron acceptor (σp = 0; σm = 0.14). The restriction of the rotation of the aminocarbene moiety did not significantly influence its electronic properties. The obtained results are in accordance with earlier findings that the relatively strong acidity of carbene complexes bearing hydrogens at the α-position to the carbene atom is due to the resonance stabilization of the anion.