Theoretical Study of the Oxidation of Alcohol to Aldehyde by d0 Transition-Metal−Oxo Complexes: Combined Approach Based on Density Functional Theory and the Intrinsic Reaction Coordinate Method

Abstract

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 d0 transition-metal−oxo complexes MO2X2 (M = Cr, Mo, X = Cl; M = Ru, X = O). A two-step mechanism was investigated. The two steps involve addition of the methanol OH bond to an MO linkage to form a M−methoxy complex, MO2X2 + CH3OH = M(O)(OH)Cl2(OCH3) (step 1), and the elimination of formaldehyde from the M−methoxy complex to yield the final products, M(O)(OH)Cl2(OCH3) = M(OH)2X2 + 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), 41.9 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−MO2X2 between the initial reactants, whereas reaction 2 results in the strong adduct CH2O−M(OH)2X2 between final products. It is concluded that only the chromium and ruthenium oxo complexes are efficient reagents for the conversion of methanol to formaldehyde.