Molybdenum complexes with citrate revisited. A mononuclear [MoVOCl4(H2O)]− ion as a new synthetic entry

Abstract

Products of the reaction of a mononuclear molybdenum(V) compound, (PyH)5[MoVOCl4(H2O)]3Cl2, with citric acid (IUPAC name: 2-hydroxypropane-1,2,3-tricarboxylic acid, abbreviated as H4cit) were characterized by spectroscopic techniques and X-ray structure analysis. When the supply of oxygen was limited, molybdenum retained the initial oxidation state and a tetranuclear Mo(V) compound formed, (MeNC5H5)4[MoV4O8(cit)2]·2H2O (1) (MeNC5H5+ = N-methylpyridinium cation). Otherwise, oxidation of metal to +6 state took place resulting in {(C6H5)4P}[MoVIO2Cl(H2cit)] (3), (PyH)2[MoVIO2(H2cit)2]·Py (4), (PyH)4[{MoVIO2(Hcit)}2(µ2-O)]·2Py (5), (PyH)4[MoVI4O11(Hcit)2]·2H2O·1/2CH3OH (6) and (PyH)6[MoVI4O11(Hcit)2]Cl2·H2O·3/2CH3OH (7) (Py = pyridine, C5H5N; PyH+ = pyridinium cation, C5H5NH+). In one instance, (PyH)2[MoVOCl2(Hcit)]0.39[MoVIO2Cl(Hcit)]0.61·CH3CN (2), a compound that is a solid mixture of Mo(V) and Mo(VI) species was obtained. In all complexes, the citrate coordinated to molybdenum in a chelating manner with at least two oxygen atoms that belonged to deprotonated OH and α-carboxyl groups. In {MoVIO2}2+ species, the two trans positions of the Mo=O moieties were occupied with carboxylate oxygens. In order to assess stabilities of different orientations of citrate ligands with respect to Mo=O, the isomeric forms of [MoVIO2Cl(H2cit)]− and [MoVIO2(H2cit)2]2−, the anions of 3 and 4, were subjected to DFT calculations. Results reveal that the relative energies of isomers are close to one another. A particular orientation of citrate does not seem to have a determining impact. As the case of the [MoVIO2(H2cit)2]2− isomers, which gain stability by intramolecular hydrogen bonds, shows, other factors are at work. For the metal–metal bonded {MoV2O4}2+ compounds, comparison with the known complexes suggests that a concept of a preferential orientation of citrate, i.e., with the alkoxide oxygen situated at the site trans to Mo=O, is not valid.