Anion doping as a probe of cooperativity in the molecular spin-crossover compound [FeL2][BF4]2 (L=2,6-di{pyrazol-1-yl}pyridine)

The complex dications in the cooperative spin-crossover compound [FeL2][BF4](2) (2,6-di(pyrazol-1-yl)pyridine) pack through pi-pi interactions into a 2-D layered structure (a terpyridine embrace motif). The effects of doping the larger ClO4- ion into this lattice have been investigated. The bulk solids [FeL2][ClO4](x)[BF4](2-x) are isostructural with [FeL2][BF4](2) when x = 0.30 and 0.98, and isostructural with (stucturally distinct) [FeL2][ClO4](2) when x = 1.89. When x = 1.68, powder samples are a mixture of both phases, but crystalline material adopts purely the ClO4- structure. Increasing the perchlorate content in the lattice for 0 = x = 1.68 causes a small decrease in T-1/2 and a narrowing of hysteresis in their spin-crossover, but with no significant reduction in cooperativity. It also leads to more pronounced decreases in Delta H [by up to 3.2(5) kJ mol(-1)] and Delta S [by up to 10(2) J mol(-1) K-1] for the transition by DSC. Single crystals of formula [FeL2][ClO4](y)[BF](2-y) (y = 0.44 and 1.13) are isostructural with the pure BF4- salt. While their molecular structures are indistinguishable, the distances between cations in the lattice increase in the doped materials. Weakening of intermolecular pi-pi interactions between cations is the likely reason for the reduced enthalpy of spin-crossover as x increases. However, the biggest stuctural change is an increase in the spacing between the 2-D layers with increased ClO4-. These results suggest that cooperativity in this material is transmitted within the terpyridine embrace layers.