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

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• 作者：Masayuki Nihei ; Yoshihiro Sekine ; Naoki Suganami ; Kento Nakazawa ; Akiko Nakao ; Hironori Nakao ; Youichi Murakami ; Hiroki Oshio
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：March 16, 2011
• 年：2011
• 卷：133
• 期：10
• 页码：3592-3600
• 全文大小：1070K
• 年卷期：v.133,no.10(March 16, 2011)
• ISSN：1520-5126

文摘

A series of cyanide bridged Fe鈭扖o molecular squares, [Co₂Fe₂(CN)₆(tp*)₂(dtbbpy)₄](PF₆)₂路2MeOH (1), [Co₂Fe₂(CN)₆(tp*)₂(bpy)₄](PF₆)₂路2MeOH (2), and [Co₂Fe₂(CN)₆(tp)₂(dtbbpy)₄](PF₆)₂路4H₂O (3) (tp = hydrotris(pyrazol-1-yl)borate, tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, bpy =2,2鈥?bipyridine, dtbbpy =4,4鈥?di-tert-butyl-2,2鈥?bipyridine), were prepared by the reactions of [Fe(CN)₃(L)]^{鈭?/sup> (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 T_1/2 = 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_LS2CoII_HS2]) at 330 K, while a low-temperature (LT) phase ([FeII_LS2CoIII_LS2]) 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 [FeIII_LS2CoII_HS2] and [FeII_LS2CoIII_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) 鈫?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鈭抳is-NIR absorption spectral measurements and cyclic voltammetry in solution revealed that the different electronic states in 1鈭?b>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鈭抳is-NIR spectra confirmed that 1 exhibited a one-step CTIST in butyronitrile, of which T_1/2 varied from 227 to 280 K upon the addition of trifluoroacetic acid.}