The complex dications in the cooperative spin-crossover compound [FeL2][BF4]2 (2,6-di(pyrazol-1-yl)pyridine) pack through π–π 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 T1/2 and a narrowing of hysteresis in their spin-crossover, but with no significant reduction in cooperativity. It also leads to more pronounced decreases in ΔH [by up to 3.2(5) kJ mol−1] and ΔS [by up to 10(2) J mol−1 K−1] for the transition by DSC. Single crystals of formula [FeL2][ClO4]y[BF4]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 π–π 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.
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