Oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands: The role of counterions and water of crystallization

• Crystal structures of three oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands were revealed.
• Dianionic malate ligands adopt a unique bidentate coordination mode via alkoxy and α-carboxylate groups in an oxo-bridged dinuclear anionic Mo(VI) complex.
• Unprecedented icosameric water clusters trapped in the crystal lattice of a mononuclear W(VI) oxo complex were uncovered.
• High affinity of oxo Mo(VI) and W(VI) complexes towards water makes the final outcome of their crystal structures unpredictable.

Crystal structures of three oxo complexes of Mo(VI) and W(VI) with α-alkoxycarboxylate ligands were solved, namely [(CH3CH2)4N]2[Mo2O5(Hmal)2(H2O)2] (H3mal = malic acid) (1), Na6[Mo2O5(cit)2)]·10.5H2O (H4cit = citric acid) (2) and Na2[WO2(H2cit)2]·10H2O (3). In 1, dianionic malate ligands adopt a unique bidentate coordination mode via alkoxy and α-carboxylate groups in the oxo-bridged dinuclear anionic complex, in which two terminal oxo ligands and a water molecule complete the distorted octahedral geometry around the Mo(VI) centre. In compound 2, a similar oxo-bridged dinuclear core, [Mo2O5]2+, is present. However, the distorted octahedral geometry of each Mo(VI) is completed by oxygen atoms originating from a fully deprotonated citrate ligand, adopting a tridentate coordination mode. The mononuclear complex 3, with two terminal oxo ligands and four oxygen atoms originating from two dianionic, bidentately coordinated citrate ligands positioned in a distorted octahedral geometry around W(VI), shows the presence of unique icosameric water clusters trapped within the crystal lattice.

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