Carbon-13, oxygen-17, and molybdenum-95 nuclear magnetic resonance studies of oxomolybdenum(VI) complexes

Article abstract of DOI:10.1021/ic00132a021

Ligands which contain three facially arranged amine, carboxylate, and/or heterocyclic nitrogen functional groups form stable complexes with the MoO₃ core at pH ～6. ¹³C and ¹⁷O NMR spectra reveal an interesting variety of compositions, inter- and intramolecular labilities, and spatial arrangements of ligand framework and donor atoms for these complexes. Ligands which contain a hydroxyl functional group result in the formation of Mo₂O₅²⁺ dimers. The hydroxyl proton is lost upon coordination, and the strong trans influence of the coordinated alkoxy group weakens the opposing Mo=O bond and causes formation of a μ-oxo-bridged Mo₂O₅²⁺ core. A ranking of trans-influencing ability of common ligand groups is derived from crystal structure data and is shown to be a controlling factor in the structure and composition of oxomolybdenum(VI) complexes. Oxygen-17 NMR chemical shifts of MoO₃-ligand complexes are much more sensitive to trans influences than crystal structure distance measurements. The ¹⁷O chemical shifts of oxomolybdenum(VI) complexes and polyanions reported in the literature reveal a smooth, nonlinear relation to Mo-O bond length. ⁹⁵Mo NMR chemical shifts of MoO₃-ligand complexes are identical within experimental error, and line widths show no relation to complex size.