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Formations of Mixed-Valence OxovanadiumV,IV Citrates and Homocitrate with N-Heterocycle Chelated Ligand

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journal contribution
posted on 2008-10-06, 00:00 authored by Can-Yu Chen, Zhao-Hui Zhou, Hong-Bin Chen, Pei-Qiang Huang, Khi-Rui Tsai, Yuan L. Chow
Dimeric mixed-valence oxovanadium citrate [V2O3(phen)3(Hcit)]·5H2O (1) (H4cit = citric acid, phen = 1,10-phenanthroline) was isolated from a weak acidic medium. It could be converted quantitatively into a tetrameric oxovanadium citrate adduct of 1,10-phenanthroline [V2O3(phen)3(Hcit)2(phen)3O3V2]·12H2O (2). This was supported by the trace of infrared spectra and X-ray diffraction patterns. The two compounds feature a bidentate citrate group that chelates only to one vanadium center through their negatively charged α-alkoxy and α-carboxy oxygen atoms, while the other β-carboxy and β-carboxylic acid groups are free to participate in strong intramolecular and intermolecular hydrogen bonding [2.45(1) in 1 and 2.487(2) Å in 2], respectively. This is also the case of homocitrato vanadate(V/IV) [V2O3(phen)3(R,S-H2homocit)]Cl·6H2O (3) (H4homocit = homocitric acid), which features a binding mode similar to that found in the R-homocitrato iron molybdenum cofactor of Mo-nitrogenase. Moreover, the homocitrato vanadate(V) [VO2(phen)2]2[V2O4(R,S-H2homocit)2]·4H2O·2C2H5OH (4) is isolated as a molecular precursor for the formation of mixed-valence complex 3. The V−Oα-alkoxy and V−Oα-carboxy bond distances of homocitrate complexes 3 and 4 are 1.858(4) and 1.968(6)av and 2.085(4) and 1.937(5) Å, respectively. They are shorter than those of homocitrate to FeVco (2.15 Å). The γ-carboxy groups of coordinated homocitrato complexes 3 and 4, and the free homocitrate salt Na3(Hhomocit)·H2O (5), form strong hydrogen bonds with the chloride ion and the water molecule [2.982(5) in 3, 2.562(9) in 4, and 2.763(1) Å in 5], respectively.