Synthesis of 4-Hydroxy-2,6-di(pyrazol-1-yl)pyridine, and the Spin State Behaviour of Its Iron(II) Complex Salts

Treatment of 4-hydroxy-2,6-dibromopyridine with four equivalent of deprotonated pyrazole in hot diglyme affords 4-hydroxy-2,6-di(pyrazol-1-yl)pyridine (L) in low yield. The three complex salts [FeL2]X2 (X− = BF4−, 1; X− = ClO4−, 2; X− = PF6−, 3) have been prepared, and crystallographically characterised as their methanol solvates. The solvate structures contain complexes that are fully high-spin (1 and 3), or in a mixed high:low spin state population at 150 K (2). Bulk samples of 1 and 2 obtained from methanol/diethyl ether contain a second, minor crystal phase that exhibits an abrupt spin-transition near 200 K. Recrystallisation of 1 and 2 from nitromethane/diethyl ether affords powder samples that are highly enriched in this spin-transition phase.