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

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)trz]BF (Htrz = 4-1,2,4-triazole, trz = 1,2,4-triazolato), 1 the dehydrated and hydrated [Fe(NHtrz)]Cl (NHtrz = 4-amino-1,2,4-triazole), 2 and 2a, and Fe(NHtrz), 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 given by the difference in energies between the LS and HS states and the strain energy . This quantity is non-zero for any spin isomer containing both LS and HS centres. Unlike , which has also been determined experimentally by calorimetric measurements, is independent of the applied functional. Calculations of the temperature dependence of the Gibbs free energy change Δ 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.