Polynuclear oxomolybdates(VI): Products of inadvertent oxidation of molybdenum(V) species

Abstract

Unplanned oxidation of molybdenum(V) in the presence of carboxylic acid afforded three fully-oxidized polynuclear oxomolybdates. In very similar reaction mixtures, the initial oxidation state of the metal ion was retained and species with the frequently-encountered dinuclear {MoV(O)(μ2-O)2MoV(O)}2+ core formed. The chosen carboxylic acid contained either two COOH groups or another functional group, i.e., OH moiety. The carboxylic acid was introduced with the idea that with both of its functional groups engaged in coordination discrete clusters composed of the {MoV(O)(μ2-O)2MoV(O)}2+ units and the organic linkers would form. Reaction systems with dimethylmalonic acid (dmmH2) yielded two products, (Bu4N)2[Mo3O8(dmm)2] (1) with a trinuclear anion and [{MoV2O4Py2(dmmMe)}2(OCH3)2] (2) (Py = pyridine, dmmMe− = monomethyl dimethylmalonate ester) as the non-oxidized counterpart. The dmm2− ion engaged in coordination to Mo(VI) centers both carboxylate groups, whereas the dmmMe− ion coordinated to Mo(V) centers via its carboxylate moiety in a bidentate bridging mode. Reactions with mandelic (H2man) acid resulted in {(C6H5)4P}2[Mo4O10(OCH3)4Cl2] (3) with a tetranuclear core and in (PyH)3[MoV2O4Cl4(Hman)] (4). Only the non-oxidized compound 4 contains coordinated mandelate ion, whereas the role of the acid in the synthesis of 3 became that of a templating agent. (PyH)4[Mo8O26Py2] (5) with an octanuclear anion is a fully-oxidized compound obtained from the reaction mixture with succinic acid. The compounds were characterized by X-ray structure analysis, bond valence sum (BVS) calculations, IR vibrational and 1H NMR spectroscopy. The TG and DSC experiments were performed on fully-oxidized compounds. Powder X-ray diffraction has confirmed molybdenum(VI) oxide as the end product of the thermal decomposition of (Bu4N)2[Mo3O8(dmm)2] (1) and (PyH)4[Mo8O26Py2] (5) in the air atmosphere.