Thermally-induced hysteretic valence tautomeric conversions in the solid state via two-step labile electron transfers in manganese-nitronyl nitroxide 2D-frameworks

Abstract

Near room temperature hysteretic thermo-induced valence tautomerism was discovered in a layered 2D-coordination polymer of manganese(II) with nitronyl nitroxide radicals separated by ClO₄⁻ anions (1). This opens a novel approach towards switchable materials with hysteresis and under ambient conditions with prospects for applications and for investigating solid-state intramolecular electron transfers. Herein, two new compounds with similar layered structures where the anions (X) are BF₄⁻ (2) or PF₆⁻ (3) are presented. Their magnetic behaviors also reveal hysteretic thermo-induced valence tautomeric conversions but in two steps and evidencing a strong effect of the anion. This occurs near room temperature (278–220 K) for 2 and higher for 3 (380–330 K). Their single crystal structures at different temperatures show that this involves two successive thermally-triggered electron transfers with switching between three redox tautomers formulated as {[Mn^II_2−yMn^III_y(NITIm)_3−y(NITRed)_y]X}_n, where y is temperature dependent. Upon cooling from the high-temperature redox-tautomer (y = 0) to the intermediate one (y = 1), half of the manganese(II) centers are oxidized to manganese(III) and 1/3 of the nitronyl nitroxide radicals (NITIm⁻) are reduced to the aminoxyl form (NITRed²⁻). On further cooling, the second half of the manganese(II) centers are oxidized and another 1/3 of the radicals are reduced to reach the low-temperature redox-tautomer (y = 2). Upon reheating, reverse electron transfers occur. This is complementarily supported by X-ray powder measurements, differential scanning calorimetry, and electron paramagnetic resonance and Raman spectroscopies. These multi-stable compounds in which manganese ions exchange reversibly their electron with the nitronyl nitroxide radical are outstanding rare examples of two-step valence tautomerism in the solid state promoted by the polymeric structure.