Controlled Intramolecular Electron Transfers in Cyanide-Bridged Molecular Squares by Chemical Modifications and External Stimuli
Controlled Intramolecular Electron Transfers in Cyanide-Bridged Molecular Squares by Chemical Modifications and External Stimuli
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DOI：
10.1021/ja109721w
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发表时间：
2011-03-16
影响因子：
15
通讯作者：
Oshio, Hiroki
中科院分区：
文献类型：
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作者：
Nihei, Masayuki;Sekine, Yoshihiro;Oshio, Hiroki
一系列氰化物桥联的铁 - 钴分子方块，[Co₂Fe₂(CN)₆(tp*)₂(dtbbpy)₄](PF₆)₂·2MeOH（1），[Co₂Fe₂(CN)₆(tp*)₂(bpy)₄](PF₆)₂·2MeOH（2）以及[Co₂Fe₂(CN)₆(tp)₂(dtbbpy)₄](PF₆)₂·4H₂O（3）（tp = 氢三（吡唑 - 1 - 基）硼酸盐，tp* = 氢三（3,5 - 二甲基吡唑 - 1 - 基）硼酸盐，bpy = 2,2′ - 联吡啶，dtbbpy = 4,4′ - 二叔丁基 - 2,2′ - 联吡啶）通过[Fe(CN)₃(L)]⁻（L = tp或tp*）与Co²⁺以及双齿配体（bpy或dtbbpy）在甲醇中的反应制备得到。在这些分子方块中，铁离子和钴离子通过氰根离子交替桥联，形成大环四核核心。变温X射线结构分析和磁化率测量证实，1在固态下在T₁/₂ = 275和310K处呈现两步电荷转移诱导的自旋转变（CTIST）。1中的铁离子和钴离子分别是低自旋（LS）的Fe(III)和高自旋（HS）的Co(II)离子，在330K的高温（HT）相（[(FeLS₂CoHS₂II)-Co - III]）中是这样描述的，而在260K以下，具有低自旋Fe(II)和Co(III)离子的低温（LT）相（[(FeLS₂CoLS₂III)-Co - II]）占主导。X射线结构分析表明，在298K的中间（IM）相，1呈现出[(FeLS₂CoHS₂II)-Co - III]和[(FeLS₂CoLS₂III)-Co - II]物种以2∶2比例的位置有序性。在对1进行的光磁实验中，通过在5K激发Fe(II)→Co(III)的价间电荷转移（IVCT）带观察到从低温相到高温相的光诱导CTIST，并且被困的高温相以两步方式热弛豫到低温相。另一方面，在整个测量的温度范围内，2和3分别处于高温相和低温相，并且没有观察到CTIST。溶液中的紫外 - 可见 - 近红外吸收光谱测量和循环伏安法表明，1 - 3中不同的电子态可归因于分别在配体tp和bpy上引入甲基和叔丁基对铁和钴离子d轨道的去稳定化作用。紫外 - 可见 - 近红外光谱的温度依赖性证实，1在丁腈中呈现一步CTIST，当加入三氟乙酸时，其T₁/₂从227变化到280K。
A series of cyanide bridged Fe-Co molecular squares, [Co2Fe2(CN)(6)(tp*)(2)(dtbbpy)(4)](PF6)(2)center dot 2MeOH (1), [Co2Fe2(CN)(6)(tp*)(2)(bpy)(4)](PF6)(2)center dot 2MeOH (2), and [Co2Fe2(CN)(6)(tp)(2)(dtbbpy)(4)](PF6)(2)center dot 4H(2)O (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 T-1/2 = 275 and 310K 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 ([(FeLS2CoHS2II)-Co-III]) at 330 K, while a low-temperature (LT) phase ([(FeLS2CoLS2III)-Co-II]) 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 [(FeLS2CoHS2II)-Co-III] and [(FeLS2CoLS2III)-Co-II] 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 T-1/2 varied from 227 to 280 K upon the addition of trifluoroacetic acid.