Tracking the Formation of a Polynuclear Co16 Complex and Its Elimination and Substitution Reactions by Mass Spectroscopy and Crystallography

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• 作者：Yue-Qiao Hu ; Ming-Hua Zeng ; Kun Zhang ; Sheng Hu ; Fu-Fang Zhou ; Mohamedally Kurmoo
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：May 29, 2013
• 年：2013
• 卷：135
• 期：21
• 页码：7901-7908
• 全文大小：401K
• 年卷期：v.135,no.21(May 29, 2013)
• ISSN：1520-5126

文摘

We present the syntheses and structures of the biggest chiral cobalt coordination cluster, [Co₁₆(L)₄(H₃L)₈(N₃)₆](NO₃)₂路16H₂O路2CH₃OH (1, where H₄L = S,S-1,2-bis(1H-benzimidazol-2-yl)-1,2-ethanediol). 1 consists of two Co₄O₄ cubes (Co₄(L)₂(H₃L)₂) alternating with Co₂(EO-N₃)₂Co₂ (Co₄(L)₂(H₃L)₂(N₃)₂), bridged by the benzimidazole and azide nitrogen atoms to form a twisted ring. The ligand adopts both cis and trans forms, and all the rings have the same chiralilty. ESI-MS of 1 from a methanol solution of crystals reveals the fragment [Co₁₆(L)₄(H₃L)₈(N₃)₆+2H]⁴⁺, suggesting the polynuclear core is stable in solution. ESI-MS measurements from the reaction solution found smaller fragments, [Co₄(H₃L)₄鈥揌]³⁺, [Co₄(H₃L)₄鈥?H]²⁺, [Co₄(H₃L)₄(N₃)₂]²⁺, and [Co₂(H₃L)₂]²⁺, and ESI-MS from a methanol solution of the solid deposit found in addition the Co₁₆ core. These results and the dependence on the synthesis time allow us to propose the process for the formation of 1, which opens up a new way for the direct observation of the ligand-controlled assembly of clusters. In addition, the isolation of [Co₄(H₃L)₄](NO₃)₄路4H₂O (2) consisting of separate Co₄O₄ cubes with the ligands being only cis in crystalline form supports the proposal. Interestingly, N₃^{鈥?/sup> is replaced by either CH₃O鈥?/sup> or OH鈥?/sup>, and this is the first time that high-resolution ESI-MS is successfully utilized to examine both the step-by-step elimination and substitution of inner bridging ligands in such a high nuclear complex. Increasing the voltage results in stepwise elimination of azide from the parent cluster. The preliminary magnetic susceptibility of 1 indicates ferromagnetic cubes antiferromagnetically coupled to the squares within the cluster, though in a field of 2.5 kOe, weak and slow relaxation is observed below 4 K.}