Thermochemistry of oxygen transfer between rhenium and phosphine complexes.: A density functional study

The thermochemistry of the oxygen transfer from Cp*ReO3 to PPh3, recently studied experimentally, has been investigated by density functional (DF) calculations. Both gradient corrected (BP86) and hybrid (B3LYP) DF methods were employed. The goal of the study was twofold. First, we evaluate the accuracy of the computational methodology for describing a transition metal oxygen multiple bond, and we check some assumptions that were made to establish the experimental thermochemistry of this complex transfer reaction. In this way we validate a computational strategy which we apply in the second part to calculate the Re-O bond dissociation energies in the complexes LReO3, L = CH3, C6H5, Cl, F, OH, and NH2. A high level of calculation on appropriate models, including enthalpy corrections and solvent effects, is required to compute enthalpy values of all reaction steps in good agreement with experiment. The B3LYP approach with flexible basis sets leads to Re-O bond energies of analogous complexes LReO3 (L = Cp, Cp*, and CH3) of 109, 113, and 153 kcal/mol, respectively; the value calculated for L = Cp* agrees very well with the experimentally derived value of 116 kcal/mol. The structure of the complex with L = Cp is similar to that with L = Cp*, but the Re-O bond is slightly more covalent. Overall, oxygen abstraction by PPh3 including formation of the dimer (LReO2)(2) is exothermic for L = Cp* and Cp, but endothermic for L = CH3. The experimentally not characterized dimer (LReO)(2)(mu-O)(2) with L = CH3 is significantly more stable with respect to its monomers than the analogous dimer with L = Cp. This may be due to a direct Re-Re interaction since the metal-metal distance of (CH3ReO)(2)(mu-O)(2) is calculated to be 2.59 Angstrom, but is 3.14 Angstrom for (CpReO)(2)(mu-O)(2). The strength of the P-O bond of OPPh3 is calculated to be 124 kcal/mol, which is somewhat smaller than the most favorable experimental value of 133 kcal/mol.