Formations of Mixed-Valence Oxovanadium^V,IV Citrates and Homocitrate with N-Heterocycle Chelated Ligand

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−O_α-alkoxy and V−O_α-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.