Tuning intramolecular electron transfer of cyanide bridged [Co2Fe2] squares through chemical modifications in solid and solution

Abstract

Cyanide bridged heterometallic complexes exhibiting intramolecular electron transfer associated with a spin conversion (so-called electron-transfer-coupled spin transition, ETCST) have attracted considerable attention for future applications in new generation molecular switching devices. Yet, the rational control of intermetallic electron transfer, especially in bistable molecules in solid and solution, remains challenging. In this work, we report three structurally related [Co₂Fe₂] molecular square complexes, {[Co(bpy)₂]₂[Fe(tp′)(CN)₃]₂}(PF₆)₂ (1), {[Co(dmbpy)₂]₂[Fe(tp′)(CN)₃]₂}(PF₆)(OTf)·2MeOH (2), and {[Co(dtbbpy)₂]₂[Fe(tp′)(CN)₃]₂}(PF₆)·2MeOH·3EtOH (3), (tp′ = hydrotris(3-methylpyrazol-1-yl)borate, bpy = 2,2′-bipyridine, dmbpy = 4,4′-dimethyl-2,2′-bipyridine, and dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, OTf = trifluoromethanesulfonate), which showed different ETCST behavior with electronic configurations in the high-spin (HS: [(Co^II_HS)₂(Fe^III_LS)₂]) and low-spin (LS: [(Co^III_LS)₂(Fe^II_LS)₂]) states in solid. Solution experiments revealed that the substituents of the bipyridyl ligand lead to different ETCST behavior, which was caused by the changes in Co^II/III redox potentials. Moreover, hydrogen bonding interactions of the terminal cyanide group on the Fe sites with solvent molecules of MeOH, EtOH, and n-PrOH shifted the equilibrium temperature between the HS and LS states in the square molecules.