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

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 Hcoop 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.