A range of spin-crossover temperature T1/2>300 K results from out-of-sphere anion exchange in a series of ferrous materials based on the 4-(4-imidazolylmethyl)-2-(2-imidazolylmethyl)imidazole (trim) ligand, [Fe(trim)2]X2 (X=F, Cl, Br, I): comparison of experimental results with those derived from density functional theory calculations.

The synthesis and characterization of [FeII(trim)2]Cl2 (2), [FeII(trim)2]Br2MeOH (3), and [FeII(trim)2]I2MeOH (4), including the X-ray crystal structure determinations of 2 (50 and 293 K) and 4 (293 K), have been performed and their properties have been examined. In agreement with the magnetic susceptibility results, the Mössbauer data show the presence of high-spin (HS) to low-spin (LS) crossover with a range of T1/2 larger than 300 K (from approximately 20 K for [FeII(trim)2]F2 (1) to approximately 380 K for 4). All complexes in this series include the same [Fe(trim)2]2+ complex cation: the ligand field comprises a constant contribution from the trim ligands and a variable one originating from the out-of-sphere anions, which is transmitted to the metal center by the connecting imidazole rings and hydrogen bonds. The impressive variation in the intrinsic characteristics of the spin-crossover (SCO) phenomenon in this series is then interpreted as an inductive effect of the anions transmitted to the nitrogen donors through the hydrogen bonds. Based on this qualitative analysis, an increased inductive effect of the out-of-sphere anion corresponds to a decreased SCO temperature T1/2, in agreement with the experimental results. Electronic structure calculations with periodic boundary conditions have been performed that show the importance of intermolecular effects in tuning the ligand field, and thus in determining the transition temperature. Starting with the geometries obtained from the X-ray studies, the [FeII(trim)2]X2 complex molecules 1-4 have been investigated both for the single molecules and the crystal lattices with the local density approximation of density functional theory. The bulk geometries of the complex cations deduced from the X-ray studies and those calculated are in fair agreement for both approaches. However, the trend observed for the transition temperatures of 1-4 disagrees with the trend for the spin-state splittings ES (difference EHS-ELS between the energy of the HS and LS isomers) calculated for the isolated molecules, whereas it agrees with the trend for ES calculated with periodic boundary conditions. The latter calculations predict the strongest stabilization of the HS state for the fluoride complex, which actually is essentially HS above T=50 K, while the most pronounced stabilization of the LS state is predicted for 4, in line with the experimental results.