Theoretical study of the oxidation of alcohol to aldehyde by d(0) transition-metal-oxo complexes: Combined approach based on density functional theory and the intrinsic reaction coordinate method

A combined density functional (DF) and intrinsic reaction coordinate (IRC) method has been applied to the mechanistic study of methanol oxidization to formaldehyde by the d(0) transition-metal-oxo complexes MO(2)X(2) (M = Cr, Mo, X = CI; M = Ru, X = O). A two-step mechanism was investigated. The two steps involve addition of the methanol O-H bond to an M=O linkage to form a M-methoxy complex, MO(2)X(2) + CH3OH = M(O)(OH)Cl-2(OCH3) (step 1), and the elimination of formaldehyde from the M-methoxy complex to yield the final products, M(O)(OH)Cl-2(OCH3) = M(OH)(2)X(2) + CH2O (step 2). The calculated vibrational adiabatic intrinsic barriers were 23.7 kcal/mol (Cr), 16.2 kcal/mol (Mo), and 21.4 kcal/mol (Ru) for the addition process (1), as well as 23.1. kcal/mol (Cr), 33.3 kcal/mol (Mo), and 7.4 kcal/mol (Ru) for the elimination step (2). The enthalpies of the overall oxidation process were computed to be 3.1 kcal/mol (Cr), 41.4 kcal/mol (Mo), and -1.9 kcal/mol(Ru). The IRC trajectories revealed that reaction 1 is initiated by the formation of the weaker adduct CH3OH-MO(2)X(2) between the initial reactants, whereas reaction 2 results in the strong adduct CH2O-M(OH)(2)X(2) between final products. It is concluded that only the chromium and ruthenium oxo complexes are efficient reagents for the conversion of methanol to formaldehyde.