Theoretical Studies on Alkene Addition to Molybdenum Alkylidenes

Abstract

The addition of ethene to Mo(NH)(CHR)(OR‘)2 (R = H, Me; R‘ = CH3, CF3) has been studied with both ab initio molecular orbital and density functional theory calculations. Geometry optimizations were carried out with the HF/3-21G, HF/HW3, and B3LYP/HW3 methods. The energies were further evaluated with the MP2/HW3 and B3LYP/HWF (HWF basis set is equivalent to the 6-311G** basis set) methods. Ethene significantly favors attacking on the CNO face. The attack on the COO face by ethene is disfavored by 12.3 and 18.8 kcal/mol for R‘ = CH3 and CF3, respectively. The transition structure for the CNO face addition is in a distorted trigonal bipyramidal geometry, with the NH and one of the OR‘ groups axial. The calculated activation energy is low for R‘ = CH3, and it is significantly lower for R‘ = CF3. In agreement with the experiment, the syn alkylidene is calculated to be more stable than the anti rotamer (R = CH3). This is apparently due to the stabilizing agostic interaction involving the anti-H of the syn rotam...