Five 2,6-Di(pyrazol-1-yl)pyridine-4-carboxylate Esters, and the Spin States of their Iron(II) Complexes

Two phenyl ester and three benzyl ester derivatives have been synthesized from 2,6- di(pyrazol-1-yl)pyridine-4-carboxylic acid and the appropriate phenyl or benzyl alcohol using N,N’- dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF₄]₂·6H₂O in acetone yielded the corresponding [FeL₂][BF₄]₂ complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe(L³)₂][BF₄]₂·2MeN₂2 (L³ = 3,4-dimethoxyphenyl 2,6-di{pyrazol-1-yl}pyridine-4- carboxylate), one of which is crystallographically characterised as high-spin while the other exhibits the onset of spin-crossover above room temperature. The other complexes are similarly low-spin at low temperature but exhibit gradual spin-crossover on heating, except for an acetone solvate of [Fe(L⁵)₂][BF₄]₂ (L⁵ = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T½ = 273 K with 9 K thermal hysteresis.