Fe^II(pap-5NO₂)₂ and Fe^II(qsal-5NO₂)₂ Schiff-Base Spin-Crossover Complexes: A Rare Example with Photomagnetism and Room-Temperature Bistability

Abstract

We focus here on the properties of Fe complexes formed with Schiff bases involved in the chemistry of Fe^III spin-transition archetypes. The neutral Fe­(pap-5NO₂)₂ (<b>1</b>) and Fe­(qsal-5NO₂)₂·Solv (<b>2</b> and <b>2·Solv</b>) compounds (<b>Solv</b> = 2H₂O) derive from the reaction of Fe^II salts with the condensation products of pyridine-2-carbaldehyde with 2-hydroxy-5-nitroaniline (Hpap-5NO₂) or 5-nitrosalicylaldehyde with quinolin-8-amine (Hqsal-5NO₂), respectively. While the Fe­(qsal-5NO₂)₂·Solv solid is essentially low spin (S = 0) and requires temperatures above 300 K to undergo a S = 0 ↔ S = 2 spin-state switching, the Fe­(pap-5NO₂)₂ one presents a strongly cooperative first-order transition (<i>T</i>↓ = 291 K, <i>T</i>↑ = 308 K) centered at room temperature associated with a photomagnetic effect at 10 K (<i>T</i>_LIESST = 58 K). The investigation of these magnetic behaviors was conducted with single-crystal X-ray diffraction (<b>1</b>, 100 and 320 K; <b>2</b>, 100 K), Mössbauer, IR, UV–vis (<b>1</b> and <b>2·Solv</b>), and differential scanning calorimetry (<b>1</b>) measurements. The Mössbauer analysis supports a description of these compounds as Fe^II Schiff-base complexes and the occurrence of a metal-centered spin crossover process. In comparison with Fe^III analogues, it appears that an expanded coordination sphere stabilizes the valence 2+ state of the Fe ion in both complexes. Strong hydrogen-bonding interactions that implicate the phenolato group bound to Fe^II promote the required extra-stabilization of the S = 2 state and thus determines the spin transition of <b>1</b> centered at room temperature. In the lattice, the hydrogen-bonded sites form infinite chains interconnected via a three-dimensional network of intermolecular van der Waals contacts and π–π interactions. Therefore, the spin transition of <b>1</b> involves the synergetic influence of electrostatic and elastic interactions, which cause the enhancement of cooperativity and result in the bistability at room temperature