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 Hcoop 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.

Vibrational Coupling of Nearest Neighbors in 1-D Spin Crossover Polymers of Rigid Bridging Ligands. A Nuclear Inelastic Scattering and DFT Study

“…This effect leads to the destabilization of the LS state in a HS matrix and to a corresponding destabilization of the HS state in a LS matrix, compared to their "same spin" environments. The calculated differences of the LS Ñ HS transition energies for the LS and HS nearest-neighbors was found to be always positive, varying from 15-35 kJ/mol depending on the applied model and exchange-correlation functional [28]. On the other hand, the calculated temperature dependencies of the vibrational contribution to the entropy for the LS Ñ HS transition in the presence of neighbors with different spin imply that entropic effects act against the enthalpic stabilization of the LS state by the LS host.…”

“…On the basis of previous work using trimeric and pentameric model molecules of the 4-amino-1,2,4-triazole (atrz) complex of Fe(II) [28] we are now extending our discussion to the properties of heptameric and nonameric model molecules. As an overview, the corresponding models used in the present work are shown in Scheme 1.…”

“…In order to get more insight into the microscopic nature of the cooperativity between the spin centers we started to perform density functional theory (DFT) calculations for a variety of spin isomers of increasing complexity. On the basis of previous work using trimeric and pentameric model molecules of the 4-amino-1,2,4-triazole (atrz) complex of Fe(II) [28] we are now extending our discussion to the properties of heptameric and nonameric model molecules. As an overview, the corresponding models used in the present work are shown in Scheme 1.…”

“…We have shown before [28] that the rigid amino-triazole ligand is misdirected when it bridges two iron ions of different spin. The consequence is that the presence of the HS neighbors leads to an elongation of the Fe-N distances for a given center, compared to the situation when the center has LS neighbors.…”

“…It is worth to note that the calculated NIS spectra and the principal geometrical parameters Scheme 1. Trimeric Fe 3 (atrz) 6 (H 2 O) 6 Cl 6 (a); pentameric Fe 5 (atrz) 12 (H 2 O) 6 Cl 6 (b); heptameric Fe 7 (atrz) 18 (H 2 O) 6 Cl 6 (c); and nonameric Fe 9 (atrz) 24 (H 2 O) 6 Cl 6 (d) model molecules used in calculations in [28] (trimeric and pentameric) and in this study (heptameric and nonameric). In each case, the two terminal iron centers, with three coordinated waters are assumed to be in the high-spin state.…”