Controlled Intramolecular Electron Transfers in Cyanide-Bridged Molecular Squares by Chemical Modifications and External Stimuli

A series of cyanide bridged Fe−Co molecular squares, [Co2Fe2(CN)6(tp*)2(dtbbpy)4](PF6)2·2MeOH (1), [Co2Fe2(CN)6(tp*)2(bpy)4](PF6)2·2MeOH (2), and [Co2Fe2(CN)6(tp)2(dtbbpy)4](PF6)2·4H2O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2′-bipyridine, dtbbpy =4,4′-di-tert-butyl-2,2′-bipyridine), were prepared by the reactions of [Fe(CN)3(L)]− (L = tp or tp*) with Co2+ and bidentate ligands (bpy or dtbbpy) in MeOH. In the molecular squares, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Variable temperature X-ray structural analyses and magnetic susceptibility measurements confirmed that 1 exhibits two-step charge-transfer induced spin transitions (CTIST) centered at T1/2 = 275 and 310 K in the solid state. The Fe and Co ions in 1 are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions, described here in the high-temperature (HT) phase ([FeIIILS2CoIIHS2]) at 330 K, while a low-temperature (LT) phase ([FeIILS2CoIIILS2]) with LS Fe(II) and Co(III) ions was dominant below 260 K. X-ray structural analysis revealed that in the intermediate (IM) phase at 298 K 1 exhibits positional ordering of [FeIIILS2CoIIHS2] and [FeIILS2CoIIILS2] species with the 2:2 ratio. In photomagnetic experiments on 1, light-induced CTIST from the LT to the HT phase was observed by excitation of Fe(II) → Co(III) intervalence charge transfer (IVCT) band at 5 K and the trapped HT phase thermally relaxed to the LT phase in a two-step fashion. On the other hand, 2 and 3 are in the HT and LT phases, respectively, throughout the entire temperature range measured, and no CTIST was observed. UV−vis-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1−3 are ascribable to the destabilization of iron and cobalt ion d-orbitals by the introduction of methyl and tert-butyl groups to the ligands tp and bpy, respectively. Temperature dependence of UV−vis-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T1/2 varied from 227 to 280 K upon the addition of trifluoroacetic acid.