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

International audience; Three novel mixed valence cyanide-bridged {Fe III 2 Fe II 2 } 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 II (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 }·[Fe III (Tp)(CN) 3 ] 2 ·18H 2 O·4CH 3 OH (1). They consist of cyanide-bridged square dicationic complexes, ClO 4 − (2 and 3)or BF 4 − (4)counterions and solvate molecules. The FT-IR cyanide stretching vibrations observed at ν CN ≈ 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 spin-crossover centered at T 1/2 = 227 K. The χ M T 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, Γ = 1.6 kJ mol −1 (with Γ andlt; 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 ΔH = 24 kJ mol −1 and ΔS = 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 andgt; 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 χ M T value obtained in the photoinduced state suggests the occurrence of a ferromagnetic interaction inside the {Fe III –CN–Fe II } units. © 2019 Académie des sciences