| Abstract: | The cis-labilizing effect of carboxylate ligands has been examined. The pseudo-first order kinetics of carbonyl displacement in W(CO)5O2CR- for R = -C(CH3)3, -CH2CN and -CF3 have been determined. Activation parameters for the reactions where R = -C(CH3)3 and -CH2CN were found to be ΔH* = 24.2 [plus or minus] 1.5 kcal/mol, ΔS* = 9.7 [plus or minus] 5.1 eu, ΔH* = 25.3 [plus or minus] 0.8, and ΔS* = 7.7 [plus or minus] 2.5 eu, respectively. These data, coupled with those for R = -CH3 and -H, give a linear Taft plot with ρ* = -0.66. Furthermore, O,O-chelated complexes were observed in the infrared where R = -C(CH3)3 but not where R = -CH2CN. Thus, the carboxylate's labilizing ability is due in part to donation from the distal oxygen atom to the carbonyl carbon atom. Moreover, carboxylate ligands possessing R groups with large negative σ* values lose a cis-CO ligand immediately, yielding the O,O-chelate. W(CO)5CH2CN- was prepared and characterized by X-ray diffraction. Although the W-CH2CN bond was similar to that reported for W-CH3, the cyanomethyl derivative was inert toward carbon dioxide. Similarly, W(CO)5O2CCH2CN- did not undergo decarboxylation. However, the cyanoacetate complex homogeneously catalyzed the decarboxylation of cyanoacetic acid. An equilibrium exists between the catalyst/substrate mixture and the complex, W(CO)5NCCH2COOH, which inhibits the catalysis. The equilibrium constant was determined to be 0.413 at 50 °C, with k[f] = 6.11x10^-3 M^-1sec^-1 and k[r[ = 1.48x10^-2 M^-1sec^-1. Activation parameters for the decarboxylation were ΔH*= 21.0 [plus or minus] 0.7 kcal/mol and ΔS* = -3.4 [plus or minus] 1.9 eu. The rate limiting step is proposed to be loss of a cis-CO ligand with concomitant formation of the intermediate, cis-W(CO)4(O2CCH2CN)(NCCH2COOH)- since the free energy of activation is similar to that for cis-CO loss, 23.0 [plus or minus] 0.9 kcal/mol. The cyanoacetate ligand serves as an intramolecular Bronsted base for the rapid proton transfer step. Molecular modeling studies confirm the proximity of the acidic proton and the basic site in the proposed intermediate... |
|---|