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

DIEGO M. ANDRADA; MARTIN E. ZOLOFF MICHOFF; RITA H. DE ROSSI; ALEJANDRO M. GRANADOS

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

ROYAL SOC CHEMISTRY

Año: 2010 vol. 12 p. 6616 - 6624

1463-9076

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 determinedin 50% MeCN:50% water at 25 1C. In addition, solvation energy and proton affinities valuesfor M-SR (M = Cr and W) in the gas phase and in acetonitrile have been computed at DFTlevel. Although there is not a linear correlation between the calculated proton affinities andthe measured pKas, the calculations reveal that when solvent effects are taken into accountthe substituted compounds studied show differences in their proton affinities. There is a goodcorrelation between the change in cavitation energy (DGcav) for the Fischer carbene complexesand log P of the thioalkyl substituents. In proton transfer reactions with amines, steric effectsare more important for W complexes with respect to their Cr analogues as a consequence ofdifferences in transition state progress. On the other hand, in reactions with OH, hydrophobicityof the R substituent is responsible for the observed changes in intrinsic kinetic acidities, which issupported by the good correlation obtained between log k0 and log P. W complexes are moresensitive to hydrophobic effects due to the tighter solvation sphere with respect to their Crcounterparts. However, in the limit of log P = 0, the energy involved in the solventreorganization process is the same regardless of the metal.