Vanadium(V) Tartrato Complexes:  Speciation in the H₃O⁺(OH^-)/H₂VO₄^-/(2R,3R)-Tartrate System and X-ray Crystal Structures of Na₄[V₄O₈(rac-tart)₂]·12H₂O and (NEt₄)₄[V₄O₈((R,R)-tart)₂]·6H₂O (tart = C₄H₂O₆^4-)

Abstract

A study of the aqueous H3O+(OH-)/H2VO4-/(2R,3R)-tartrate system has been performed at 273 K in a 1.0 mol/L Na+(Cl-) ionic medium using 51V NMR spectroscopy. In this relatively complicated system, more than 12 different species were observed. Ligand concentration, vanadate concentration, and pH variation studies were carried out, particularly for the range of pH 5.8-8.0 and for pH 2.4. Chemical shifts, vanadium-ligand stoichiometry, and also composition and formation constants for some, but not all, species are given. Despite some reduction of vanadium(V) to vanadium(IV) in an acidic medium at pH approximately 2.4, the stoichiometries of the principal species in solution at this pH were determined. Electrospray ionization mass spectra for some solutions were obtained and were in accordance with the conclusions drawn from the speciation studies. A series of crystalline vanadium(V) tartrato complexes M4[V4O8(tart)2].aq were also prepared and characterized. X-ray diffraction studies of Na4[V4O8(rac-tart)2].12H2O (1) and (NEt4)4[V4O8((R,R)-tart)2].6H2O (2) revealed unique tetranuclear [V4O8(tart)2]4- ions for which the {V4O4} rings have boat conformations.

Structural and spectral properties of tartrato complexes of vanadium(V) from quantum chemical calculations

“…Table 3 contains assignments for selected parts of the vibrational spectra of 1-3. Data for the whole pertinent frequency range are given in the supplementary material and complement the experimentally recorded IR and Raman spectra reported in [16,17]. We show the calculated harmonic vibrational frequencies obtained with BP86 and B3LYP functionals in WI basis as fairly well coinciding with the measured spectra.…”

“…Experimentally observed IR and Raman fundamentals were extracted from the previously published spectra, and compounds were prepared based on established synthetic procedure [16,17].…”

“…2. Selected bond lengths for all the three anions calculated with M06, BP86, and B3LYP functionals with WI basis together with experimental XRD crystal structure data [16,17] are given in Table 2. Similar set for all investigated functionals can be found in Table S1 of the supplementary material.…”

“…The following dinuclear and tetranuclear tartrato complexes of vanadium(V) previously isolated from aqueous-ethanolic solution and characterized by X-ray structure analysis and spectral methods (infrared (IR), Raman, UV-Vis, 51 V NMR) [16,17] were included in our computational study:…”

“…While the tetranuclear anion [17] always contains tartrato groups of the same enantiomeric form, formation of the dinuclear complexes is stereospecific [16]. Continuous effort to understand the stereospecific aspects of vanadium(V) tartrato complexes leads us to take a closer look at their spectral properties.…”