Theoretical investigations of a series of [hexakis(1-(tetrazol-1-yl)alkane-N4)iron(II)]bis(tetrafluoroborate) spin crossover complexes:: Methyl-to-pentyl substituted species in the approximation of free cations

Within this work the density functional theory approach was used to simulate the well-known series of [hexakis(1-(tetrazol-1-yl)alkane-N4)iron(II)] bis(tetrafluoroborate) spin crossover complexes ranging from methyl- to pentyl substituted species. The calculations were carried out in the approximation of free cations thus neglecting intermolecular interactions and solid-state effects. The resulting structural relaxation was compared with the experimentally obtained structure. Furthermore the vibrational frequencies were calculated and compared with experimentally obtained spectra to check the accuracy of the used simplifications. Finally, the spin transition temperatures were calculated using a simple thermodynamic model according to the law of mass action for a system of non-interacting molecules. An interesting behaviour of the calculated spin-transition temperature depending on the number of carbon atoms within the ligand was found. Complexes based on ligands with even and odd number of carbon atoms form two different series. This behaviour turned out to be related with the vibrational contribution to the entropy change of the spin-transition.