Chiral Tetranuclear ;c;3-Alkoxo-Bridged Copper(II) Complex with 2 + 4 Cubane-Like Cu4O4 Core Framework and Ferromagnetic Ground State

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• 作者：Anja Burkhardt ; Eike T. Spielberg ; Helmar G&#xf6 ; rls ; Winfried Plass
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：April 7, 2008
• 年：2008
• 卷：47
• 期：7
• 页码：2485 - 2493
• 全文大小：1729K
• 年卷期：v.47,no.7(April 7, 2008)
• ISSN：1520-510X

文摘

The sugar-modified Schiff base ligand benzyl 2-deoxy-2-salicylideneamino-α-D-glucopyranoside H₂L, prepared by condensation of salicylaldehyde and the monomeric chitosan analogue benzyl 2-deoxy-2-amino-α-D-glucopyranoside, reacts with copper(II) acetate to form a self-assembled, alkoxo-bridged tetranuclear homoleptic copper(II) complex [{Cu(L)}₄] (4) with Cu₄O₄ heterocubane core. The chiral complex 4 crystallizes in the space group P2₁2₁2₁. The tetranuclear complex 4 is composed of two dinuclear {Cu(L)}₂ entities linked by the four ;c;₃-bridging C-3 alkoxide oxygen atoms of the sugar backbone. The preorganization of the dimeric {Cu(L)}₂ entities is enforced by strong hydrogen bonds between the phenolate oxygen atom and the C-4 hydroxy group of the two constituting chiral monomeric building blocks. Therefore the Cu₄O₄ core can be classified as a type I or 2 + 4 cubane. The chirality of the structure is confirmed by circular dichroism (CD) spectra, which reveal a significant dichroism associated with the copper centered transitions at around 600 nm. Temperature dependent magnetic susceptibility measurements indicate ferromagnetic exchange interactions in complex 4. Fitting of the experimental data with a two J model based on the 2 + 4 topology c="/isubscribe/journals/inocaj/47/i07/eqn/ic-2007-01777t_m001.gif"> leads to exchange coupling constants of J₁ = 64 and J₂ = 4 cm⁻¹. The observed ferromagnetic coupling can be attributed to the very small Cu−O−Cu bridging angles within the Cu₂O₂ core of the constituting dimeric entities, which are a result of the conformational requirements introduced by the sugar backbone. 4 is not only the first example of an alkoxo-bridged tetranuclear copper(II) complex with Cu₄O₄ core representing the 2 + 4 cubane class with ferromagnetic ground state but also a rare example for the class of molecules combining a ferromagnetic ground state with optical activity. The ferromagnetic S = 2 ground state of 4 is confirmed by magnetization measurements and ESR spectroscopy.