Kinetic and Thermodynamic Acidities of Pentacarbonyl(cyclobutenylidene)chromium Complexes. Effect of Antiaromaticity in the Conjugate Anion. An Experimental and Computational Study^†

The deprotonation of pentacarbonyl[(3-diethylamino-2,4-dimethyl)cyclobut-2-ene-1-ylidene]chromium (1d) and pentacarbonyl[(3-diethylamino-4-methyl-2-phenyl)cyclobut-2-ene-1-ylidene]chromium (1e) leads to antiaromatic conjugate anions by virtue of their being cyclobutadiene derivatives. Rate constants for the deprotonation of 1d and 1e by P2-Et and pK_a values were determined in acetonitrile. Gas-phase B3LYP calculations of 1d, 1e, and their respective conjugate anions, using a generalized basis set, were also performed. Furthermore, for purposes of comparison with carbene complexes of similar structures, but having conjugate anions that are not antiaromatic, corresponding calculations were performed on pentacarbonyl[3-diethylamino-2,5-dimethyl)cyclopent-2-ene-1-ylidene]chromium (5), [dimethylamino(methyl)carbene]pentacarbonylchromium (3a), and [dimethylamino(iso-propyl)carbene]pentacarbonylchromium (3b) and their respective conjugate anions, and solution-phase pK_a and kinetic measurements were carried out for 3a and 3b. Major points of interest include the effect of antiaromaticity on the kinetic and thermodynamic acidities of 1d and 1e, the large effect of the phenyl group on the gas-phase acidity of 1e, the strong attenuation of the acidities and the effect of the phenyl group in acetonitrile, and the position of the CC double bonds in the cyclobutadiene ring of the conjugate anion of 1e.