Structural and spectroscopic evidence for the formation of trinuclear and tetranuclear vanadium(III)/carboxylate complexes of acetate and related derivatives in aqueous solution

The formation of vanadium(III) complexes with nuclearity greater than two is believed to occur in aqueous solution on the basis of potentiometric, electrochemical, and/or UV-vis spectroscopy titration measurements, although structural evidence for this is limited. Upon the addition of 1-2 equiv of acetate, propionate, chloroacetate, trifluoroacetate, or bromoacetate to an aqueous, acidic solution of vanadium(III), trinuclear and tetranuclear complexes are formed. The structures of V4(mu-OH)4(mu-OOCCF3)4(OH2)8]Cl4.7.5H2O (1), V4(mu-OH)4(mu-OOCCH3)4(OH2)8]Cl4.CH3COOH.12H2O (2), V4(mu-OH)4(mu-OOCCH3)4(OH2)8]Cl4.3H2O (3), V3(mu3-O)(mu-OOCCH2Br)6(OH2)3]CF3SO3.H2O (4), V3(mu3-O)(mu-OOCCH2CH3)6(OH2)3]Cl.2H2O (5), V3(mu3-O)(mu-OOCCH3)6(OH2)3]Cl.3.5H2O (6), and V3(mu3-O)(mu-OOCCH2Cl)6(OH2)3]CF3SO3.H2O (7) have been determined by X-ray diffraction. Importantly, electrospray mass spectrometry and 1H NMR measurements suggest that these complexes are not purely solid-state phenomena but are also present in solution. For the vanadium(III)/acetate and vanadium(III)/propionate systems, two paramagnetic 1H NMR signals corresponding to two distinct complexes (species A and B) are observed in the 40-55 ppm region for 0.20 mol equiv of acetate or propionate, at pD 3.44. No corresponding signals are observed for the vanadium(III)/bromoacetate and vanadium(III)/chloroacetate systems under the same conditions or for the vanadium(III)/ trifluoroacetate system using 19F NMR spectroscopy. UV-vis spectra suggest that species B are structurally analogous for the vanadium(III)/acetate and vanadium(III)/propionate systems, whereas structurally different complexes are the major species for the other systems. Diffusion coefficients of species B for the vanadium(III)/acetate and vanadium(III)/propionate systems determined by pulsed-field-gradient spin-echo NMR spectroscopy measurements are (3.0 +/- 0.1) x 10-6 and (3.23 +/- 0.01) x 10-6 cm2 s-1, respectively, and are most consistent with species B being trimeric, rather than tetranuclear, complexes.