Synthesis of 4-hydroxy-2, 6-di(pyrazol-1-yl)pyridine, and the Spin state behaviour of its Iron(II) complex salts

Abstract

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.