Quantification of intramolecular cooperativity in polynuclear spin crossover Fe(ii) complexes by density functional theory calculations

Abstract

The influence of the spin state of nearest neighbours on the spin transition of a given centre has been modelled with density functional theory methods for the linear oligomeric Fe(II) complexes of 4-amino-1,2,4-triazole ligands. The calculated parameter H_coop which is equal to the difference in the LS → HS spin transition energy for a centre with two LS and two HS spin nearest neighbours, respectively, was found to be always positive, varying from 15 to 35 kJ mol⁻¹ depending on the applied model and exchange–correlation functional. On the other hand H_coop was found to be of negligible value for a model of a linear Fe(II) complex of the more flexible alkylene-bis-tetrazole ligands. This corresponds well to the observed cooperative behaviour of the spin transition for the complexes of triazole derivatives and the gradual transition for the above bis-tetrazole ligands. The analysis of the bond distances in the optimised structures points towards elongation of the Fe–N bonds of the iron centre upon changing of the spin of the neighbours from low-spin to high-spin. This effect is related to the rigid nature of the bridging triazole ligand and is assumed to be the primary reason for the observed cooperativity. The presence of the next-to-nearest neighbour effects has been inferred. The effect of the softening of the Fe–N bonds in the LS centre distorted by the presence of the HS neighbours is also observed and its entropic effects are discussed.