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

We focus here on the properties of Fe complexes formed with Schiff bases involved in the chemistry of FeIII spin-transition archetypes. The neutral Fe­(pap-5NO2)2 (1) and Fe­(qsal-5NO2)2·Solv (2 and 2·Solv) compounds (Solv = 2H2O) derive from the reaction of FeII 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 (T↓ = 291 K, T↑ = 308 K) centered at room temperature associated with a photomagnetic effect at 10 K (TLIESST = 58 K). The investigation of these magnetic behaviors was conducted with single-crystal X-ray diffraction (1, 100 and 320 K; 2, 100 K), Mössbauer, IR, UV–vis (1 and 2·Solv), and differential scanning calorimetry (1) measurements. The Mössbauer analysis supports a description of these compounds as FeII Schiff-base complexes and the occurrence of a metal-centered spin crossover process. In comparison with FeIII 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 FeII promote the required extra-stabilization of the S = 2 state and thus determines the spin transition of 1 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 1 involves the synergetic influence of electrostatic and elastic interactions, which cause the enhancement of cooperativity and result in the bistability at room temperature.