Role of the hydrophobicity on the thermodynamic and kinetic acidity of Fischer thiocarbene complexes

Abstract

Rate constants for the reversible deprotonation of (CO)₅W=C(SR)CH₃ (W-SR) by OH⁻, water and a number of primary aliphatic and secondary alicyclic amines, have been determined in 50% MeCN:50% water at 25 °C. In addition, solvation energy and proton affinities values for M-SR (M = Cr and W) in the gas phase and in acetonitrile have been computed at DFT level. Although there is not a linear correlation between the calculated proton affinities and the measured pK_as, the calculations reveal that when solvent effects are taken into account the substituted compounds studied show differences in their proton affinities. There is a good correlation between the change in cavitation energy (ΔG_cav) for the Fischer carbene complexes and log P of the thioalkyl substituents. In proton transfer reactions with amines, steric effects are more important for W complexes with respect to their Cr analogues as a consequence of differences in transition state progress. On the other hand, in reactions with OH⁻, hydrophobicity of the R substituent is responsible for the observed changes in intrinsic kinetic acidities, which is supported by the good correlation obtained between log k₀ and log P. W complexes are more sensitive to hydrophobic effects due to the tighter solvation sphere with respect to their Cr counterparts. However, in the limit of log P = 0, the energy involved in the solvent reorganization process is the same regardless of the metal.