Combination of density functional theory and calorimetry reveals the microscopic nature of spin state switching in 1D Fe(ii) spin crossover complexes

Abstract

The Gibbs free energy of spin transitions in the heptanuclear models of 1D Fe(II) spin crossover 1,2,4-triazole complexes has been estimated using DFT methods. The complexes modelled were [Fe(Htrz)₂trz]BF₄ (Htrz = 4H-1,2,4-triazole, trz = 1,2,4-triazolato), 1 the dehydrated and hydrated [Fe(NH₂trz)₃]Cl₂ (NH₂trz = 4-amino-1,2,4-triazole), 2 and 2a, and [Fe(NH₂trz)₃](NO₃)₂, 3. For each complex, the electronic energy and the vibrational energies were calculated for a heptanuclear model containing five inner Fe(II) centres in the high-spin (HS) and the low-spin (LS) states. All other possible 18 spin isomers with one to four HS centres were also modelled. Results obtained using different exchange–correlation functionals based on the B3LYP one show that each spin isomer with a particular permutation of HS and LS centres within the pentanuclear linear unit has distinctive electronic and vibrational energies. The electronic energy of each spin isomer was found to be equal to the sum of the adiabatic electronic energy of the spin transition E_ad given by the difference in energies between the LS and HS states and the strain energy H_strain. This quantity is non-zero for any spin isomer containing both LS and HS centres. Unlike E_ad, which has also been determined experimentally by calorimetric measurements, H_strain is independent of the applied functional. Calculations of the temperature dependence of the Gibbs free energy change ΔG of 19 possible spin transitions for heptanuclear model systems reveals that strain effects lead to an additional destabilisation of the spin isomers containing both LS and HS centres. The actual strain pattern depends on the chemical structure of the model molecule.