Peroxomolybdate(vi)–citrate and –malate complex interconversions by pH-dependence. Synthetic, structural and spectroscopic studies

Abstract

The reaction of potassium molybdate(VI) with biologically relevant ligands, citric and malic acids, in the presence of H₂O₂ was investigated for the effect of pH variations on the product pattern. That with citric acid led to the formation of the monomeric complex K₄[MoO(O₂)₂(cit)]·4H₂O (1) in the pH range 7–9, and dimer K₅[MoO(O₂)₂(Hcit)H(Hcit)(O₂)₂OMo]·6H₂O (2) (H₄cit = citric acid) at pH 3–6 through carboxylate–carboxylic acid hydrogen bonding. The relation with the previously identified K₄[MoO₃(cit)]·2H₂O (4) and K₄[Mo₂O₅(Hcit)₂]·4H₂O (5) were shown. These and other intermediates were shown to react in the pH range 3–6 to give a more stable species 2; the reaction sequence was demonstrated either by the protonation from 1 or the deprotonation of [MoO(O₂)₂(H₂cit)]²⁻ (8). Evidence that 2 exists as a dimer in solution is presented. The reaction with (S)-malic acid afforded Δ-K_2n[MoO(O₂)₂((S)-Hmal)]_n·nH₂O (3) (H₃mal = malic acid) that was oxidized further to oxalato molybdate (11) by H₂O₂. The three complexes 1–3 were characterized by elemental analysis, UV, IR and NMR spectroscopies, in addition to the X-ray structural studies that show citrate and malate being coordinated as bidentate ligands via α-alkoxyl and α-carboxylate groups. The formation of these complexes is dictated by pH and their thermal stabilities varied with the coordinated hydroxycarboxylate ligands.