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 <i>N</i>,<i>N&#8217;</i>-dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF₄]₂&#183;6H₂O in acetone yielded the corresponding [Fe<i>L</i>₂][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(<i>L</i>³)₂][BF₄]₂&#183;2MeNO₂ (<i>L</i>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(<i>L</i>⁵)₂][BF₄]₂ (<i>L</i>⁵ = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at <i>T</i>_&#189; = 273 K with 9 K thermal hysteresis.

Abstract licence: CC BY

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