Dynamic NMR and Quantum-Chemical Study of the Stereochemistry and Stability of the Chiral MoO2(acac)2 Complex in...

The stereochemistry and dynamics of MoO2(acac)2 in benzene, chloroform, and toluene were investigated by variable temperature (1)H NMR, density functional theory (SOGGA11-X, B3LYP), and ab initio (MP2) methods. In solution, an equilibrium between two chiral enantiomers with C2 symmetry was identified, Λ-cis-MoO2(acac)2 and Δ-cis-MoO2(acac)2. The two enantiomers are connected via achiral cis transition states that switch the enantiomeric conformations via a Ray-Dutt, Bailar, and a newly described racemization twisting mechanism. All three mechanisms have similar calculated activation energies. Activation parameters Ea, ΔH(‡), and ΔS(‡) were experimentally determined for the exchange process, with a small, negative ΔS(‡), and a positive ΔH(‡) of 68.1 kJ mol(-1) in benzene, 54.9 kJ mol(-1) in chloroform, and 60.6 kJ mol(-1) in toluene, in reasonable general agreement with the calculations. Trans configurations of MoO2(acac)2 are very much higher in energy than cis and are not relevant in the temperature range experimentally studied, 243-340 K. The enantiomers interconvert within seconds near room temperature and much faster at elevated temperatures. Racemization will thus prevent the use of enantiomerically pure MoO2(acac)2 for chiral catalysis under practical conditions.