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

摘要

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[BF 4 ] 2 ·6H 2 O in acetone yielded the corresponding [Fe L 2 ][BF 4 ] 2 complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe( L 3 ) 2 ][BF 4 ] 2 ·2MeNO 2 ( L 3 = 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( L 5 ) 2 ][BF 4 ] 2 ( L 5 = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T ? = 273 K with 9 K thermal hysteresis.