Solid-state spin crossover of Ni(II) in a bioinspired N₃S ₂ ligand field

The complex Tp^Ph,MeNiS₂CNMe₂ [Tp ^Ph,Me = hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate] features a bioinspired N₃S₂ ligand set supporting a five-coordinate, trigonally distorted square-pyramidal geometry in the solid state. Spin crossover of Ni(II) was demonstrated by temperature-dependent X-ray crystallography and magnetic susceptibility measurements. The crystal lattice contains two independent molecules (i.e., Ni1 and Ni2). At 293 K, the observed bond lengths and susceptibility are consistent with high-spin (S = 1) Ni(II), and both molecules exhibit relatively short axial Ni-N bonds and long Ni-N and Ni-S equatorial bonds. At 123 K, the Ni1 complex remains high-spin, but the Ni2 molecule substantially crosses to a structurally distinct diamagnetic (S = 0) state with significant elongation of the axial Ni-N bond and offsetting contraction of the equatorial bonds. The temperature-dependent susceptibility data were fit to a spin equilibrium at Ni2 [ΔH° = 1.13(2) kcal/mol and ΔS° = +7.3(1) cal mol^-1 K^-1] consistent with weak coupling to lattice effects. Cooling below 100 K results in crossover of the Ni1 complex.