Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Abstract

Bis(formazanate)iron(II) complexes undergo a thermally induced <i>S</i> = 0 to <i>S</i> = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (<b>7</b>). In total, six new compounds (<b>2</b>-<b>7</b>) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (<i>T</i>_1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound <b>6</b> which remains high-spin (<i>S</i> = 2) down to 210 K. In the solid state, <b>6</b> was shown to exist in two distinct forms: a tetrahedral high-spin complex (<b>6a</b>, <i>S</i> = 2) and a rare square-planar structure with an intermediate-spin state (<b>6b</b>, <i>S</i> = 1). SQUID measurements, ⁵⁷Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form <b>6b</b> undergoes an incomplete spin-change-coupled isomerization to tetrahedral <b>6a</b>. The complex that contains additional OMe donors (<b>7</b>) results in a six-coordinate (NNO)₂Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (<i>T</i>_1/2 = 444 K). The available experimental and computational data for <b>7</b> suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal-ligand π-covalency.