How to Predict Activation Barriers – Conformational Transformations of Compounds CH3C(CH2PPh2)3n[CH2P(oTol)2]nMo(CO)3 (n = 1–3): Force Field Calculations versus NMR Data

Abstract

Tripod metal entities tripodM are sterically congested systems. The conformations adopted by compounds CH3C(CH2PPh2)3–n[CH2P(oTol)2]nMo(CO)3 (n = 1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsive forces acting in these molecules. The steric demand of the o-tolyl groups impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the ΔG‡ values for these conformational enantiomerization processes have been determined as ΔG‡298K = 54.3, 57.9, 65.5 kJ·mol–1 for compounds 1, 2, and 3, respectively. By an exhaustive search on a force field generated hypersurface, activation energies of 53, 57 and 69 kJ·mol–1 have been calculated. Thus, the force field approach correctly reproduces the dependence of the activation energy on the degree of o-tolyl substitution. Moreover, the force field simulation also gives an insight into the individual microsteps of the enantiomerization pathways.