title: Structural and spectroscopic evidence for the formation of trinuclear and tetranuclear vanadium(III)/carboxylate complexes of acetate and related derivatives in aqueous solution.
abstract: 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.
authors.affiliation: Department of Chemistry, Kent State University, Kent, Ohio 44242, School of Biosciences, University of Exeter, Exeter EX4 4QD, UK.
authors.forename: Riya
authors.initials: R
authors.lastname: Mukherjee
authors.forename: Brenda A
authors.initials: BA
authors.lastname: Dougan
authors.forename: Fiona H
authors.initials: FH
authors.lastname: Fry
authors.forename: Scott D
authors.initials: SD
authors.lastname: Bunge
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Ziegler
authors.forename: Nicola E
authors.initials: NE
authors.lastname: Brasch
mesh_terms: D000085:Acetates / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D012680:Sensitivity and Specificity; D012996:Solutions / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D014639:Vanadium / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D000085:Acetates; D002264:Carboxylic Acids; D009942:Organometallic Compounds; D012996:Solutions; D014639:Vanadium; D014867:Water