Spin-crossover iron(II) complexes featuring boronic acid and boronic ester groups: synthesis and magnetic properties

Abstract

A series of bis(pyrazol-1-yl)pyridine (1-BPP)-based iron(II) complexes bearing either a boronic acid group or boronic monoester groups, with the general formula [FeII(1-BPP)2](ClO4)2, have been synthesized and characterized in the solid state. The boronic acid or boronic monoester substituents are introduced at the 4-position of the pyridine ring. Despite their close structural similarity, the four iron(II) complexes exhibit markedly different magnetic properties. Complex 1a, which contains a boronic acid group, undergoes an incomplete, thermally induced spin-crossover in the solid state. Conversion of the boronic acid group into monomethyl or monoethyl esters stabilizes the high-spin state in 1b and 1c, respectively. Further substitution with electron-donating tert-butyl groups in 2b enforces a pure low-spin state.