One-step and two-step spin crossover binuclear iron(III) complexes bridged by 4,4′-bipyridine

Two binuclear iron(III) complexes, [L1FeIII(bpy)FeIIIL1](BPh4)2 (1) and [L2FeIII(bpy)FeIIIL2](BPh4)2 (2), were synthesized and characterized, where H2L1 and H2L2 denote bis(salicylicdeneaminopropyl)methylamine and bis(3-methoxysalicylideneaminopropyl)methylamine, respectively, and bpy denotes 4,4′-bipyridine and BPh4− denotes tetraphenylborate. Complexes 1 and 2 consist of one and two crystallographically unique Fe sites, respectively, while they have a similar binuclear complex-cation [LnFeIII(bpy)FeIIILn]2+ (n = 1, 2) bridged by 4,4′-bipyridine and two tetraphenylborate ions as the counter anions. The magnetic susceptibility measurements of 1 and 2 showed one-step and two-step spin crossover (SCO), respectively. The four saturated six-membered chelate rings at the aminopropyl moieties of 1 exhibit disorder throughout one-step SCO. The two chelate rings of one Fe site of 2 exhibit disorder but the other two of another Fe site do not. The different SCO behaviors of 1 and 2 were ascribed to one and two crystallographically unique Fe sites and the order/disorder at the saturated six-membered chelate rings of aminopropyl moieties.