Spin and valence isomerism in cyanide-bridged {Fe III2MII} (M = Fe and Co) clusters

Abstract

Two cyanide-bridged V-shaped isostructural trinuclear complexes [{(Tp*)Fe^III(CN)₃}₂M^II(bztpen)]·Sol (M = Fe, Sol = CH₃OH·3H₂O, 1; M = Co, Sol = 2CH₃OH·2H₂O, 2; bztpen = N-benzyl-N,N′,N′-tris(2-methylpyridyl)ethylenediamine; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) were synthesized and characterized. The bztpen ligand serves as a tetradentate capping ligand around the inner metal ion, leaving one pyridyl group intact. Complex 1 exhibits a spin crossover (SCO) behavior between the {Fe^III_LSFe^II_HSFe^III_LS} and {Fe^III_LSFe^II_LSFe^III_LS} spin isomers, while 2 shows both thermally- and photo-induced electron-transfer coupled spin transition (ETCST) property between the {Fe^III_LSCo^II_HSFe^III_LS} and {Fe^III_LSCo^III_LSFe^II_LS} valence isomers. The total entropy changes for 1 and 2 between their corresponding two electronic states were found to be very close with the values of 87.46 and 84.49 J mol⁻¹ K⁻¹, respectively, indicating the comparable thermal energy barriers necessary for either an SCO or ETCST event for such a given system. Furthermore, both complexes undergo desolvation-induced irreversible and sharp magnetic change at high temperatures.