Spin crossover in monoadducts of Co(Salen) with pyridine and imidazole: a quantum chemical study

Abstract

Complexation of N,N′-ethylene- and N,N′-butylene-bis(salicylideneiminato)cobalt(II) (Co(salen)) with nitrogen-containing heterocyclic ligands (pyridine, imidazole) and investigation into the mechanism responsible for the spin crossover behavior of these complexes have been computationally studied using the DFT B3LYP*/6-311++G(d,p) method. The results of calculations of geometric characteristics and energy parameters are in good agreement with an available experimental data. An addition of a base molecule to Co(salen) has been shown to be accompanied by the narrowing of the energy gap between the high-spin and the low-spin electronic states up to the values typical for spin crossover cobalt complexes. According to the calculations, the energy barrier of spin-forbidden rearrangement of a monoadduct of N,N′-butylene-bis(salicylideneiminato) Co(II) with pyridine does not exceed of 6 kcal/mol. This finding allows one to consider the adducts of Co(salen) with nitrogen-centered ligands as the prospective spin crossover systems. The computational investigation into the spatial and electronic structure of N,N′-butylene-bis(salicylideneiminato) Co(II) dimer predicts the simultaneous existence in a crystal cell of two types of molecules with different metal spin states.