Theoretical study of the low lying electronic states of oxoX(salen) (X = Mn, Mn-, Fe, and Cr-) complexes

The lowest lying electronic states of oxoX(salen) (X = Mn, Mn-, Fe and Cr) complexes have been studied using complete active space self-consistent field (CASSCF) calculations. These wave functions have been analyzed, via the use of localized and natural orbitals, to identify the electronic structure contributions to the chemistry of these systems. It is found that the electronic structures of all complexes can be rationalized through the use of a common orbital energy diagram. Single point multireference Moller-Plesset (MRMP2) perturbation theory calculations have been performed for the oxoMn(salen) system and these results verify that the CASSCF method gives accurate energy separations for the electronic states. We find that the CASSCF method predicts bound species for all compounds except for the triplet and quintet states of the oxoMn(salen) complex, which spontaneously dissociate in the gas phase. Calculations on the separated species [(5)A Mn(salen) + (3)p O] indicate that the bound singlet species lies some 30 kcal/mol above the dissociated products at the CASSCF level. Attempts to obtain MRMP2 energies for the supersystem of the separated species failed; hence a more accurate value of the dissociation energy could not be determined. We also find, from CASSCF calculations, that the oxoMn(salen) complex is predicted to be a very potent electrophile.