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

摘要

A series of cyanide bridged Fe-Co molecular squares, [Co_2Fe_2(C-N)_6(tp*)_2(dtbbpy)_4](PF_6)_2·2M_eOH (l), [Co_2Fe_2(CN)_6(tp*)_2(bpy)_4](PF_6)_2 •2M_eOH (2), and [Co_2Fe_2(CN)_6(tp)_2(dtbbpy)_4](PF_6)_2-4H_2O (3) (tp = hydrotris(pyrazol-l-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-l-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 Co~(2+) 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 ([FeⅢ_(LS2)CoⅡ_(HS2)]) at 330 K, while a low-temperature (LT) phase ([FeⅡ_(LS2)CoⅢ_(LS2)]) with LS Fe(ll) and Co(IIl) 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 [FeⅢ_(LS2)CoⅡ_(Hs2)] and [FeII_(LS2)CoⅢ_(LS2)] 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 terf-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.