Potentiometric study of the molybdenum(VI)–benzilic acid system. Structural characterisation and electrochemical properties of [NH4]2[MoO2{O2CC(O)Ph2}2]·2H2O

Abstract

The formation of complexes between molybdate and benzilate (2-hydroxy-2,2-diphenylacetate), Hbza^–, ions has been investigated in the range pH 2–7 by potentiometric measurements. Computer treatment of the potentiometric data revealed the formation of two complexes the [MoO₄]^2–, Hbza^–, H⁺ stoichiometries of which are [1,2,2] and [2,2,4]; log β₁₂₂= 17.35, log β₂₂₄= 29.07. Compared to other less bulky hydroxycarboxylate ligands previously studied, this result indicates that the phenyl groups of benzilate only prevent the formation of [2,2,5] dimeric complex which presumably has a double oxo bridge. The mononuclear [1,2,2] complex has been isolated and its structure determined by X-ray analysis of its ammonium salt, [NH₄]₂[MoO₂{O₂CC(O)Ph₂}₂]·2H₂O; monoclinic, space group P2₁/n, a= 8.106(2), b= 25.702(8), c= 13.628(4)Å, β= 90.62(2)° and Z= 4. The redox properties of this complex salt were also investigated and compared with those previously found for an analogous complex containing two thiobenzilate ligands.