II 2Fe III 2] 2+ molecular switches: [Fe II(bik) 2(N–) 2] spin-crossover complexes containing [Fe III(Tp)(CN) 3] – metalloligands as N-donor]]>

摘要

Three novel mixed valence cyanide-bridged {FeIII2FeII2} square complexes were obtained through the self-assembling of [FeIII(Tp)(CN)3]-or [FeIII(Tp*)(CN)3]?cyanido building blocks with thein situformed [FeII(bik)2(S)2] complex (Tp = hydrotris (pyrazol-1-yl)borate, Tp* = hydrotris (3,5-dimethyl-pyrazol-1-yl)borate, bik = bis(1-methylimidazol-2-yl)ketone, S = solvent). The structures of these three complexes (2, 3and4) are reminiscent of that of our previously published square complex {[FeIII(Tp)(CN)3]2[FeII(bik)2]2}·[FeIII(Tp)(CN)3]2·18H2O·4CH3OH (1). They consist of cyanide-bridged square dicationic complexes, ClO4?(2and3) or BF4?(4) counterions and solvate molecules. The FT-IR cyanide stretching vibrations observed at νCN≈ 2145–60?cm?1are typical of {FeIII–CN–FeII} moieties. The investigation of the magnetic properties of2reveals the occurrence of spin-crossover centered atT1/2= 227?K. The χMTvariation,ca. 7?cm3?mol?1?K, reflects the complete spin-state change occurring on both {FeII(bik)(–NC)2} moieties (–NC represents the cyanido building blocks). The Slichter–Drickamer model leads to a weak cooperativity factor, Γ = 1.6?kJ?mol?1(with Γ < 2RT1/2), which reflects the gradual spin-state change. This is in agreement with the molecular structure of2, which does not present significant intermolecular interactions. The calculated enthalpy and entropy variations associated with the spin-state equilibrium are ΔH= 24?kJ?mol?1and ΔS= 105?J?K?1?mol?1. In contrast,3and4show only partial spin-crossover in the accessible temperature range (2–400?K) as theT1/2are shifted toward higher temperatures (ca.T1/2> 400?K). Although no photomagnetic effect is observed for3, compound4shows a moderate increase in the magnetization upon irradiation at low temperature. This phenomenon is ascribed to the light-induced excited spin-state trapping (LIESST) effect. Interestingly, the complex2also shows a remarkable LIESST effect, which is observed with different laser lights covering the visible and near-infrared range. The resulting χMTvalue obtained in the photoinduced state suggests the occurrence of a ferromagnetic interaction inside the {FeIII–CN–FeII} units.