Anion-solvent dependence of bistability in a family of meridional N-donor-ligand-containing iron(II) spin crossover complexes

Five mononuclear spin crossover iron(II) bis-meridional ligand complexes of the general formula [Fe(L)2](X)2·solvent, have been synthesized, where X = BF4- or ClO4-; L = 2-(1-pyridin-2-ylmethyl-1H-pyrazol-3-yl)-pyrazine (picpzpz) or 2-(3-(2-pyridyl)pyrazol-1-ylmethyl)pyridine) (picpypz); solvent = MeOH or EtOH. The magnetic and structural consequences of systematic variation of meridional ligand, solvent, and anion, including a desolvated species, have been investigated. The complex [Fe(picpzpz)2](BF4)2·MeOH, 1·MeOH, displays several unique properties including a two-step spin transition with a gradual higher-temperature step (1T1/2 = 197 K) and an abrupt low-temperature step with hysteresis (2T1/2 = 91/98 K) and a metastable intermediate spin state below 70 K with quench-cooling. Removal of the solvent methanol results in the loss of the abrupt step and associated hysteresis (T1/2 = 150 K)..