Solid-State Spin Crossover of Ni(II) in a Bioinspired N3S2 Ligand Field

The complex TpPh,MeNiS2CNMe2 [TpPh,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.