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Chiral Tetranuclear μ3-Alkoxo-Bridged Copper(Il) Complex with 2 + 4 Cubane-Like Cu4O4 Core Framework and Ferromagnetic Ground State.

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Title: Chiral Tetranuclear μ3-Alkoxo-Bridged Copper(Il) Complex with 2 + 4 Cubane-Like Cu4O4 Core Framework and Ferromagnetic Ground State.
Authors: Burkhardt, Anja1, Spielberg, Eike T.1, Görls, Helmar1, Plass, Winfried1 sekr.plass@uni-jena.de
Source: Inorganic Chemistry. 4/7/2008, Vol. 47 Issue 7, p2485-2493. 9p. 6 Diagrams, 2 Charts, 4 Graphs.
Subjects: Ferromagnetism, Chitosan, Copper compounds, Hydrogen bonding, Molecular association
Abstract: The sugar-modified Schiff base ligand benzyl 2-deoxy-2-salicylideneamino-c~-D-glucopyranoside H2L, prepared by condensation of salicylaldehyde and the monomeric chitosan analogue benzyl 2-deoxy-2-amino-a-D-glucopyranoside, reacts with copper(ll) acetate to form a self-assembled, alkoxo-bridged tetranuclear homoleptic copper(Il) complex [{Cu(L)}4] (4) with Cu404 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 u3-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 0-4 hydroxy group of the two constituting chiral monomeric building blocks. Therefore the Cu404 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 (H = - J1(S1S3 + S2S4) - J2(S1 + S3)(S2 + S4)) leads to exchange coupling constants of J1 = 64 and J2 = 4 cm1. The observed ferromagnetic coupling can be attributed to the very small Cu-O-Cu bridging angles within the Cu202 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(ll) complex with Cu404 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. [ABSTRACT FROM AUTHOR]
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Abstract:The sugar-modified Schiff base ligand benzyl 2-deoxy-2-salicylideneamino-c~-D-glucopyranoside H2L, prepared by condensation of salicylaldehyde and the monomeric chitosan analogue benzyl 2-deoxy-2-amino-a-D-glucopyranoside, reacts with copper(ll) acetate to form a self-assembled, alkoxo-bridged tetranuclear homoleptic copper(Il) complex [{Cu(L)}4] (4) with Cu404 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 u3-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 0-4 hydroxy group of the two constituting chiral monomeric building blocks. Therefore the Cu404 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 (H = - J1(S1S3 + S2S4) - J2(S1 + S3)(S2 + S4)) leads to exchange coupling constants of J1 = 64 and J2 = 4 cm1. The observed ferromagnetic coupling can be attributed to the very small Cu-O-Cu bridging angles within the Cu202 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(ll) complex with Cu404 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. [ABSTRACT FROM AUTHOR]
ISSN:00201669
DOI:10.1021/ic701777t