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 [Mo V OCl 4 (H 2 O)] 3 Cl 2 , with citric acid (IUPAC name: 2-hydroxypropane-1,2,3-tricarboxylic acid, abbreviated as H 4 cit) 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, (MeNC 5 H 5 ) 4 [Mo V 4 O 8 (cit) 2 ]·2H 2 O ( 1 ) (MeNC 5 H 5 + = N -methylpyridinium cation). Otherwise, oxidation of metal to +6 state took place resulting in {(C 6 H 5 ) 4 P}[Mo VI O 2 Cl(H 2 cit)] ( 3 ), (PyH) 2 [Mo VI O 2 (H 2 cit) 2 ]·Py ( 4 ), (PyH) 4 [{Mo VI O 2 (Hcit)} 2 ( µ 2 -O)]·2Py ( 5 ), (PyH) 4 [Mo VI 4 O 11 (Hcit) 2 ]·2H 2 O·1/2CH 3 OH ( 6 ) and (PyH) 6 [Mo VI 4 O 11 (Hcit) 2 ]Cl 2 ·H 2 O·3/2CH 3 OH ( 7 ) (Py = pyridine, C 5 H 5 N; PyH + = pyridinium cation, C 5 H 5 NH + ). In one instance, (PyH) 2 [Mo V OCl 2 (Hcit)] 0.39 [Mo VI O 2 Cl(Hcit)] 0.61 ·CH 3 CN ( 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 {Mo VI O 2 } 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 [Mo VI O 2 Cl(H 2 cit)] – and [Mo VI O 2 (H 2 cit) 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 [Mo VI O 2 (H 2 cit) 2 ] 2– isomers, which gain stability by intramolecular hydrogen bonds, shows, other factors are at work. For the metal–metal bonded {Mo V 2 O 4 } 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.