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 [Fe_zZn_1−z(bpp)₂][BF₄]₂ (bpp=2,6-bis{pyrazol-1-yl}pyridine; z=0.88, 0.72 and 0.27) and [Fe_zNi_1−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 (ΔV_SCO) behaves differently in the Zn- and Ni-doped crystals. For the FeZn crystals, the relationship between ΔV_SCO and z is continuous for z≥0.3 but is steeper than expected, so ΔV_SCO≈0 for z=0.27. In contrast ΔV_SCO in the FeNi materials shows only a small variation between 0.83≥z≥0.46, before decreasing more strongly at higher dilution. ΔV_SCO 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 ΔV_SCO for the Zn-doped materials reflects their T_1/2 values, which are lower than for their FeNi counterparts. The contribution of T_1/2 to this behavior is especially evident at high metal dilution.