Structural Consequences of Different Metal Compositions in the Doped Spin-Crossover Crystals [FezM1−z(bpp)2][BF4]2 (M=Ni, Zn; bpp=2,6-Bis{Pyrazol-1-yl}Pyridine)

Variable temperature crystallographic characterization of [FezZn1−z(bpp)2][BF4]2 (bpp=2,6-bis{pyrazol-1-yl}pyridine; z=0.88, 0.72 and 0.27) and [FezNi1−z(bpp)2][BF4]2 (z=0.83, 0.72 and 0.32) is presented. Comparison with previously published data confirms the isothermal unit cell volume change during spin-crossover (ΔVSCO) behaves differently in the Zn- and Ni-doped crystals. For the FeZn crystals, the relationship between ΔVSCO and z is continuous for z≥0.3 but is steeper than expected, so ΔVSCO≈0 for z=0.27. In contrast ΔVSCO in the FeNi materials shows only a small variation between 0.83≥z≥0.46, before decreasing more strongly at higher dilution. ΔVSCO in each FeZn crystal is smaller than for its FeNi analogue with a similar composition. As well as the dopant ion ionic radius, the smaller ΔVSCO for the Zn-doped materials reflects their T1/2 values, which are lower than for their FeNi counterparts. The contribution of T1/2 to this behavior is especially evident at high metal dilution.