Coordination asymmetry in μ-oxido divanadium complexes: Development of synthetic protocols

Coumarin-Ensembled Vanadium(V) Compounds and Their Affinity Studies Toward Biological Thiols Probed by Fluorescence Spectroscopy

Fluorescence spectroscopic studies of a pair of new oxido-vanadium(V) compounds with biological thiols, such as homocysteine (Hcy), cysteine (Cys), and glutathione (GSH), have been investigated in this article. Despite notable progress in vanadium-thiol chemistry, no attention has been paid to exploring vanadium-based optical probes to study their interaction with biothiols. For this purpose, two oxido-vanadium(V) compounds, 1 and 2, have been prepared involving a tridentate ONO donor-based luminescent coumarin-derived ligand. Single crystal X-ray diffraction analysis, NMR (¹ H, ¹³ C, and ⁵¹ V) spectroscopy, XPS, and DFT calculations have been used to establish their identities. The vanadium center in these compounds has a distorted octahedral environment. In compound 2, a methanol molecule is coordinated to the vanadium(V) center in the trans position of the terminal oxido moiety. The latter exerts a strong trans-labilizing influence on the coordinating methanol. Both 1 and 2 are weakly fluorescent. Photophysical investigations of the vanadium complexes in aqueous media at physiological pH (7.4) in the presence of various biothiols and amino acids showed significant fluorescence enhancement (83-fold) of the vanadium complexes, specifically with Hcy. The specific affinity of the complexes for Hcy remained unchanged even in the presence of other biothiols and amino acids. Kinetic investigation reveals pseudo-first order behavior of the compound with Hcy. Mechanistic studies have manifested that Hcy-induced reduction triggers the decomplexation of the vanadium compound, followed by hydrolysis and subsequent cyclization. Time-correlated single photon counting suggested that the radiative rate constant (k_r ) of 1 and 2 in the presence of Hcy serves as the prime factor for the fluorescence enhancement of the medium. Compound 1 has been tested efficiently for Hcy measurement in blood plasma rendering it suitable for practical applications.

Experimental and Quantum Mechanical Characterization of an Oxygen-Bridged Plutonium(IV) Dimer

We report the synthesis and characterization of K₄ {[PuCl₂ (NO₃ )₃ ]₂ (μ₂ -O)}⋅H₂ O, which contains the first known μ₂ -oxo bridge between two Pu^IV metal centers. Adding to its uniqueness is the Pu-(μ₂ -O) bond length of 2.04 Å, which is the shortest of other analogous compounds. The Pu-(μ₂ -O)-Pu bridge is characterized by the mixing of s-, d-, and p-orbitals from Pu with the p-orbitals of O; the 5f-orbitals do not participate in bonding. Natural bond orbital analysis indicates that Pu and O interact through one 3c-2e σ_Pu-O-Pu and two 3c-2e π_Pu-O-Pu bonding orbitals and that the electron density is highly polarized on the μ₂ -O. Bond topology properties analysis indicates that the Pu-(μ₂ -O) bond shares both ionic and covalent character. Quantum mechanical calculations also show that the dimer has multiconfigurational ground states, where the nonet, septet, quintet, triplet, and singlet are close in energy. This work demonstrates the interplay between experimental and computational efforts that is required to understand the chemical bonding of Pu compounds.

A spherical deca-vanadophosphate covalent assembled all-inorganic open framework

Through covalent connection of phosphate groups, an all-inorganic open framework, [NH₄]₉[P₄V₁₀O₃₄]_1.5·3H₂O (1), has been prepared by a hydrothermal reduction-condensation method. It is built from a vanadophosphate (VPO) unit of [P₄V₁₀O₃₄]^6-, which represents a rare example of a spherical {V₁₀-type} structure among the VPO family. Further studies also demonstrate that compound 1 is an n-type semiconductor.

51V magic angle spinning NMR spectroscopy and quantum chemical calculations in vanadium bio-inorganic systems: current perspective

In recent years, 51V magic angle spinning (MAS) NMR spectroscopy has been widely used to characterize vanadium centers in biology, biomimetic complexes, and inorganic compounds of medicinal and industrial relevance. It has been demonstrated that 51V NMR parameters are sensitive probes of the coordination geometry and chemical environment of the metal center, beyond the first coordination sphere. To establish the relationships between NMR parameters and structure of the vanadium centers, over the past decade a large series of coordination complexes have been analyzed by MAS NMR spectroscopy. It has been demonstrated that the interpretation of the NMR parameters requires the use of theoretical methods, such as density functional (DFT) theory, whereby the experimental NMR observables are linked to the electronic and structural properties of a molecule. DFT calculations have been successfully employed to not only predict NMR parameters but to also yield valuable information regarding the structure and function of various vanadium compounds. In this report, we review the current state of the field, and present a survey of bioinorganic vanadium complexes as well as vanadium-dependent haloperoxidases analyzed using 51V MAS NMR spectroscopy and DFT calculations, to illustrate the rich information content available from such a combined approach.

Oxidovanadium(IV/V) complexes as new redox mediators in dye-sensitized solar cells: a combined experimental and theoretical study

Corrosiveness is one of the main drawbacks of using the iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs). Alternative redox couples including transition metal complexes have been investigated where surprisingly high efficiencies for the conversion of solar to electrical energy have been achieved. In this paper, we examined the development of a DSSC using an electrolyte based on square pyramidal oxidovanadium(IV/V) complexes. The oxidovanadium(IV) complex (Ph4P)2[V(IV)O(hybeb)] was combined with its oxidized analogue (Ph4P)[V(V)O(hybeb)] {where hybeb(4-) is the tetradentate diamidodiphenolate ligand [1-(2-hydroxybenzamido)-2-(2-pyridinecarboxamido)benzenato}and applied as a redox couple in the electrolyte of DSSCs. The complexes exhibit large electron exchange and transfer rates, which are evident from electron paramagnetic resonance spectroscopy and electrochemistry, rendering the oxidovanadium(IV/V) compounds suitable for redox mediators in DSSCs. The very large self-exchange rate constant offered an insight into the mechanism of the exchange reaction most likely mediated through an outer-sphere exchange mechanism. The [V(IV)O(hybeb)](2-)/[V(V)O(hybeb)](-) redox potential and the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [V(IV)O(hybeb)](2-) were calculated by means of density functional theory electronic structure calculation methods. The complexes were applied as a new redox mediator in DSSCs, while the cell performance was studied in terms of the concentration of the reduced and oxidized form of the complexes. These studies were performed with the commercial Ru-based sensitizer N719 absorbed on a TiO2 semiconducting film in the DSSC. Maximum energy conversion efficiencies of 2% at simulated solar light (AM 1.5; 1000 W m(-2)) with an open circuit voltage of 660 mV, a short-circuit current of 5.2 mA cm(-2), and a fill factor of 0.58 were recorded without the presence of any additives in the electrolyte.

Synthesis, crystal structure and interaction of L-valine Schiff base divanadium(V) complex containing a V2O3 core with DNA and BSA

A divanadium(V) complex, [V2O3(o-van-val)2] (o-van-val=Schiff base derived from o-vanillin and L-valine), has been synthesized and structurally characterized. The crystal structure shows that both of the vanadium centers in the complex have a distorted octahedral coordination environment composed of tridentate Schiff base ligand. A V2O3 core in molecular structure adopts intermediate between cis and trans configuration with the O1V1⋯V1AO1A torsion angle 115.22 (28)° and the V1⋯V1A distance 3.455Å. The binding properties of the complex with calf thymus DNA (CT-DNA) have been investigated by UV-vis absorption, fluorescence, CD spectra and viscosity measurement. The results indicate that the complex binds to CT-DNA in non-classical intercalative mode. Meanwhile, the interaction of the complex with bovine serum albumin (BSA) has been studied by UV-vis absorption, fluorescence and CD spectra. Results indicated that the complex can markedly quench the intrinsic fluorescence of BSA via a static quenching process, and cause its conformational change. The calculated apparent binding constant Kb was 1.05×10(6)M(-1) and the binding site number n was 1.18.

Ligand substitution reactions of a phenolic quinolyl hydrazone; oxidovanadium (IV) complexes

BACKGROUND

Quinoline ring has therapeutic and biological activities. Quinolyl hydrazones constitute a class of excellent chelating agents. Recently, the physiological and biological activities of quinolyl hydrazones arise from their tendency to form metal chelates with transition metal ions. In this context, we have aimed to study the competency effect of a phenolic quinolyl hydrazone (H2L; primary ligand) with some auxiliary ligands (Tmen, Phen or Oxine; secondary ligands) towards oxidovanadium (IV) ions.

RESULTS

Mono- and binuclear oxidovanadium (IV) - complexes were obtained from the reaction of a phenolic quinolyl hydrazone with oxidovanadium (IV)- ion in absence and presence of N,N,N',N'- tetramethylethylenediamine (Tmen), 1,10-phenanthroline (Phen) or 8-hydroxyquinoline (Oxine). The phenolic quinolyl hydrazone ligand behaves as monobasic bidentate (NO- donor with O- bridging). All the obtained complexes have the preferable octahedral geometry except the oxinato complex (2) which has a square pyramid geometry with no axial interaction; the only homoleptic complex in this study.

CONCLUSION

The ligand exchange (substitution/replacement) reactions reflect the strong competency power of the auxiliary aromatic ligands (Phen/Oxine) compared to the phenolic quinolyl hydrazone (H2L) towards oxidovanadium (IV) ion; (complexes 2 and 3). By contrast, in case of the more flexible aliphatic competitor (Tmen), an adduct was obtained (4). The obtained complexes reflect the strength of the ligand field towards the oxidovanadium (IV)- ion; Oxine or Phen >> phenolic hydrazone (H2L) > Tmen.