Tetranuclear [Fe2IIFe2III]2+ molecular switches: [FeII(bik)2(N-)2] spin-crossover complexes containing [FeIII(Tp)(CN)3]- metalloligands as N-donor

Three novel mixed valence cyanide-bridged {(Fe2Fe2II)-Fe-III} square complexes were obtained through the self-assembling of [Fe-III(Tp)(CN)(3)](-) or [Fe-III(Tp*)(CN)(3)](-) cyanido building blocks with the in situ formed [Fe-III(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, 3 and 4) are reminiscent of that of our previously published square complex {[Fe-III(Tp)(CN)(3)](2) [Fe-II(bik)(2)](2)}center dot[Fe-III(Tp)(CN)(3)](2)center dot 18H(2)O center dot 4CH(3)OH (1). They consist of cyanide-bridged square dicationic complexes, ClO4- (2 and 3) or B-4(-) (4) counterions and solvate molecules. The FT-IR cyanide stretching vibrations observed at nu(CN) approximate to 2145-60 cm(-1) are typical of {Fe-III-CN-Fe-II} moieties. The investigation of the magnetic properties of 2 reveals the occurrence of spincrossover centered at T-1/2 = 227 K. The chi mT variation, ca. 7 cm(3) mol(-1) K, reflects the complete spin-state change occurring on both {Fe-II(bik)(-NC)(2)} moieties (-NC represents the cyanido building blocks). The Slichter-Drickamer model leads to a weak cooperativity factor, Gamma = 1.6 kJ mol(-1) (with Gamma < 2RT(1/2)), which reflects the gradual spin-state change. This is in agreement with the molecular structure of 2, which does not present significant intermolecular interactions. The calculated enthalpy and entropy variations associated with the spin-state equilibrium are Delta H = 24 kJ mol(-1) and Delta S = 105 J K-1 mol(-1). In contrast, 3 and 4 show only partial spin-crossover in the accessible temperature range (2-400 K) as the T-1/2 are shifted toward higher temperatures (ca. T-1/2 > 400 K). Although no photomagnetic effect is observed for 3, compound 4 shows 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 complex 2 also shows a remarkable LIESST effect, which is observed with different laser lights covering the visible and near-infrared range. The resulting chi T-M value obtained in the photoinduced state suggests the occurrence of a ferromagnetic interaction inside the {Fe-III-CN-Fe-II} units. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.