Structure, Stability, and Electronic and NMR Properties of Various Oxoand Nitrido-Derivatives of [L(Salen)Mn(lll)]+, Where L = None and Imidazole. A Density Functional Study

Structure, stability, and electronic and NMR properties of [(Salen)Mn(lll)]+-derived intermediates/reactants in the epoxidation/amination of unfunctionalized olefins, namely [(Salen)Mn(V)0]+- (1-oxo), [(Salen)Mn(IV)0] (2-oxo), and [(Salen)Mn(V)N] (3), have been studied with the B3LYP density functional method. It has been shown that the 1A, 3A, and 5A states of cationic 1-oxo species are virtually degenerate, while for the neutral 2-oxo species the ground 4A state lies 6.4 kcal/mol lower than 2A. In the nitrido species 3, the 1A state has been shown to be the ground state in agreement with experiment. We have investigated isomerization of 1-oxo and 2-oxo species into unusual [(OSalen)IVIn(lll)]+ (1-N-oxo and 1-peroxo) and [(OSalen)Mn(ll)] (2-N-oxo and 2-peroxo) species, respectively. For cationic species 1, the 1-N-oxo isomers are more stable (by 10-12 kcal/mol) than the 1-oxo isomer and are separated from the latter by 21-22 kcal/mol barriers. On the other hand, 1-peroxo isomers are calculated to be 14-16 kcal/mol higher than the 1-oxo isomer. For neutral species 2, however, both 2-N-oxo and 2-peroxo isomers lie significantly higher in energy than the 2-oxo isomer. It has been shown that coordination of axial imidazole ligand alters relative energies of spin states for 1- and 2-oxo species, destabilizing low-spin states. For singlet states of H2Salen, 1-oxo, and 3, we have calculated 1H,13C, 15N, and 17O NMR chemical shifts using the gaugeindependent-atomic orbital (GIAO) approach.