Dinuclear Vanadium Research Articles

Synthetic V(V)-peroxido-zwitterionic species catalyze benzene oxidation under mild reaction conditions

A series of dinuclear complexes were synthesized, involving vanadium with N-methylglycine (sarcosine)/N,N’-dimethylglycine in aqueous media and H2O2, capable of exerting catalytic activity. Specifically, the ternary V(V) tetraperoxido dinuclear species (NH4)2[V2O2(O2)4(CH3NH2CH2COO)] (symmetric conformer) (1), (NH4)2[V2O2(O2)4(CH3NH2CH2COO)] (asymmetric conformer) (2), K2(Η2Ο)[V2O2(O2)4(CH3NH2CH2COO)] . H2O (3) (asymmetric conformer), and K2(Η2Ο)2[V2O2(O2)4{(CH3)2NHCH2COO)}] (4) (asymmetric conformer) were synthetically isolated in crystalline form. Compounds 1 and 2 are the same, whilst with profound conformational differences exemplified through pH-specific chemistries. The new materials 1–4 were characterized by elemental analysis, FT-IR, Raman, NMR, and UV–Visible spectroscopy, cyclic voltammetry, thermogravimetric analysis (TGA-DTG), and X-ray crystallography. Further support of the spectroscopic and structural profile was achieved through Bond Valence Sum (BVS) and Hirshfeld molecular mapping calculations. In all of the studied compounds, pH appears to be a crucial factor in the synthesis and isolation. The physicochemical characterization of the hybrid materials justified further use of compound 3 as a potential hydroxylation catalyst of benzene under a variable set of mild reaction conditions. The products of the catalytic reaction systems investigated were identified and quantified by gas chromatography–mass spectrometry (GC-MS) and gas chromatography–flame ionization detection (GC-FID), respectively. The results a) exemplify the involvement of the hybrid dinuclear vanadium compounds in the pursued benzene transformations, and b) distinctly demonstrate the structural and chemical characteristics justifying the emergence of catalytic action of vanadium under mild conditions (40 °C) in reactions toward organic substrates (e.g. benzene) as the basis for the pursuit of selective industrial applications, affording readily available useful compounds.

A combined experimental and theoretical study to demonstrate the importance of V2O4 synthon in the crystal packing of an oxo-bridged dinuclear vanadium(V) complex with V2O4 core

A dinuclear oxo-bridged vanadium(V) Schiff base complex, (µ-O)2[V(O)(L)]2, [where HL = 2-((2-aminocyclohexylimino)methyl)-6-ethoxyphenol] has been synthesized and further characterized with standard spectroscopic methods. Structure of the complex has been determined with the help of single crystal X-ray diffraction study. The complex consists of two cyclohexane rings of which one is disordered over its carbon and hydrogen atoms. Moreover, several supramolecular assemblies observed in the crystal packing of the title complex have been studied using DFT calculations and several computational tools (QTAIM, NCI plot and MEP surface analyses).

Atroposelective Synthesis of C-C Axially Chiral Compounds via Mono- and Dinuclear Vanadium Catalysis.

Axially chiral compounds with rotationally constrained σ-bonds that exhibit atropisomerism are lucrative synthetic targets because of their ubiquity in functional materials and natural products. The metal complex-catalyzed enantioselective fabrication of axially chiral scaffolds has been widely investigated, and thus far, considerable progress has been made. Over the past two decades, we have developed a highly efficient strategy for constructing axially chiral biarenol derivatives using chiral mono- and dinuclear vanadium complexes. These complexes are readily prepared from vanadium(IV) salts and Schiff base ligands (generated from the condensation of (S)-tert-leucine and di- or monoformyl-(R)-1,1'-bi-2-naphthol (BINOL) derivatives) under O2 and act as highly active catalysts for highly stereoselective C-C bond formation. In particular, the vanadium complex-catalyzed enantioselective oxidative coupling of 2-naphthols 1 under oxygen or in air, which is a green oxidant, affords the desired axially chiral molecules in high yields and high stereoselectivity (up to quantitative yield and 97% ee), along with water as the sole coproduct. This coupling reaction tolerated various functional groups (such as halogens, alkoxys, and boryls) and avoided overoxidation of coupling products.The key feature of dinuclear vanadium(V) catalysts such as (Ra,S,S)-5a is an outstanding mode of the homocoupling reaction, in which a single molecule of the catalyst activates two molecules of the starting material (e.g., 2-naphthols) simultaneously. With this "dual activation" mechanism, the oxidative coupling promoted by the dinuclear catalyst proceeds in an intramolecular manner. The homocoupling rate using 5 mol % of the dinuclear vanadium(V) complex (Ra,S,S)-5a was measured to be 111 times faster than that of the mononuclear vanadium(IV) complex (S)-4a bearing a half motif of the dinuclear vanadium complex.In the case of the heterocoupling reaction utilizing two different kinds of arenol derivatives, only a starting arenol having lower oxidation potential seems to be activated by the mononuclear vanadium complex. The reaction rate of the heterocoupling using either mono- or dinuclear vanadium complexes showed no difference to give the coupling product in high yields but with a different enantioselective manner; chiral mononuclear vanadium(V) complexes showed better enantioselectivites than that of the dinuclear vanadium(V) complexes. A competing heterocoupling study and a linear correlation between the ee of the mononucaler vanadium catalyst and ee of the heterocoupling suggested that the heterocoupling involves an intermolecular radical-anion coupling pathway.In this Account, we summarize the recent advances in vanadium-catalyzed coupling reactions that produced important chiral molecules, such as biresorcinols, polycyclic biphenols, oxa[9]helicenes, bihydroxycarbazoles, and C1-symmetrical biarenols, and their coupling reaction mechanisms. By pursuing vanadium catalysis, we believe numerous additional transformations as well as a renewed interest in catalytic and chemo-, regio-, and enantioselective aryl-aryl bond constructions will be manifested.

Structural and magnetic characterization of mixed-valence vanadium (IV/V) complex with {(VO)2(µ‒O)}3+ core: Theoretical and experimental insights

Using deprotonated forms of tetradentate phenol amine ligands 2-((((1-methylbenzimidazol-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenol (HL1), dinuclear vanadium(IV,V) complex of composition [(L1)2(VIVO)(µ-O)(VVO)](ClO4) (1) was synthesized and characterized by X–ray crystallography, UV–Vis, EPR and magnetic susceptibility measurements. Direct current (DC) variable-temperature magnetic susceptibility measurements on polycrystalline sample of 1 were carried out in the temperature range 1.8–300 K. Electron paramagnetic resonance and magnetic studies for 1 demonstrated the complete localization of the single 3d electron on one of vanadium ion and very weak antiferromagnetic interaction between the paramagnetic vanadium(IV) ions in the crystal lattice. The cyclic voltammogram observed with 1, revealed one oxidation process which is tentatively assigned to VIVVV/V2V. [(L1)2(VVO)2(µ-O)](ClO4)2 (2.2.5CH2Cl2) has been synthesized chemically from 1 and were thoroughly characterized by X–ray crystallography, UV–Vis, 51V NMR and IR measurements.

Synthesis, Characterization, X-Ray Crystal Structures and Antibacterial Activity of Zinc(II) and Vanadium(V) Complexes Derived from 5-Bromo-2-((2-(methylamino)ethylimino)methyl)phenol.

Three new zinc(II) and one vanadium(V) complexes, [Zn2Cl2L2] (1), [Zn2I2L2] (2), [ZnCl2(HL)] (3), and [V2O2(μ-O)2L2] (4), where L is 5-bromo-2-((2-(methylamino)ethylimino)methyl)phenolate, have been synthesized and characterized by elemental analyses, IR and UV-Vis spectra, as well as molar conductivity. Structures of the complexes were confirmed by single crystal X-ray diffraction. Complexes 1 and 2 are isostructural dinuclear zinc compounds, with the Zn atoms in square pyramidal coordination. The Zn atoms in the mononuclear complex 3 are in tetrahedral coordination. Complex 4 is a dinuclear vanadium(V) compound, with the V atoms in octahedral coordination. The complexes were assayed for antibacterial activities by MTT method.

A Combined Experimental and Theoretical Study to Demonstrate the Importance of V2o4 Synthon in the Crystal Packing of an Oxo-Bridged Dinuclear Vanadium(V) Complex with V2o4 Core

A Combined Experimental and Theoretical Study to Demonstrate the Importance of V2o4 Synthon in the Crystal Packing of an Oxo-Bridged Dinuclear Vanadium(V) Complex with V2o4 Core

Cytotoxic oxidovanadium(IV) complexes of tridentate halogen‐substituted Schiff bases: First dinuclear V(IV) complexes with O → VIV = O → VIV = O core

The reaction of potentially N,N,O-tridentate Schiff base ligands, Cl-LH, Br-LH, BrCl-LH and H-LH, with [VIVO(acac)2] in 2:1 ratio in methanol gave the corresponding mononuclear and dinuclear oxidovanadium(IV) complexes, VO(Cl-L)2 (1), VO(Br-L)2 (2), [(BrCl-L)2(H2O)V(μ-O)VO(BrCl-L)2] (3) and [(H-L)2(H2O)V(μ -O)VO(H-L)2] (4), in good yields. The ligands and complexes were fully characterized by elemental analysis and FT-IR spectroscopy. The ligands were also characterized by 1H NMR spectroscopy. The oxidation state of V(IV)O with d1 configuration in all synthesized complexes was confirmed by EPR. Moreover, the structures of 2 and 3 were determined by X-ray diffraction (XRD) analysis which revealed them as mono- and dinuclear vanadium(IV) complexes, respectively, with the ligands coordinated as bidentate chelates. The structure of 3 represents the first example of dinuclear V(IV) complex with O → VIV = O → VIV = O core (Cambridge Structural Database (CSD)​, version 5.42, update of May 2021). The cytotoxicity of ligands and complexes was evaluated towards ovarian (A2780), breast (MCF7) and prostate (PC3) cancer cells at 48 h. While ligands showed modest IC50 values (>42 μM), all complexes turned out to be effective in the range 3.9–17.2 μM. In particular, A2780 and MCF7 cell lines were the most sensitive to the newly synthesized V(IV)O complexes.

Chiral Vanadium(V)-catalyzed Oxidative Coupling of 4-Hydroxycarbazoles

Abstract Enantioselective oxidative couplings of 4-hydroxycarbazoles using a chiral dinuclear vanadium(V) complex have been achieved for the first time. Under the mild reaction conditions, the corresponding dimeric 4-hydroxycarbazoles were obtained in up to 90% ee.

Liquid‐phase oxidation of olefins with rare hydronium ion salt of dinuclear dioxido‐vanadium(V) complexes and comparative catalytic studies with analogous copper complexes

Homogeneous liquid‐phase oxidation of a number of aromatic and aliphatic olefins was examined using dinuclear anionic vanadium dioxido complexes [(VO2)2(salLH)]− (1) and [(VO2)2(NsalLH)]− (2) and dinuclear copper complexes [(CuCl)2(salLH)]− (3) and [(CuCl)2(NsalLH)]− (4) (reaction of carbohydrazide with salicylaldehyde and 4‐diethylamino salicylaldehyde afforded Schiff‐base ligands [salLH4] and [NsalLH4], respectively). Anionic vanadium and copper complexes 1, 2, 3, and 4 were isolated in the form of their hydronium ion salt, which is rare. The molecular structure of the hydronium ion salt of anionic dinuclear vanadium dioxido complex [(VO2)2(salLH)]− (1) was established through single‐crystal X‐ray analysis. The chemical and structural properties were studied using Fourier transform infrared (FT‐IR), ultraviolet–visible (UV–Vis), 1H and 13C nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI‐MS), electron paramagnetic resonance (EPR) spectroscopy, and thermogravimetric analysis (TGA). In the presence of hydrogen peroxide, both dinuclear vanadium dioxido complexes were applied for the oxidation of a series of aromatic and aliphatic alkenes. High catalytic activity and efficiency were achieved using catalysts 1 and 2 in the oxidation of olefins. Alkenes with electron‐donating groups make the oxidation processes easy. Thus, in general, aromatic olefins show better substrate conversion in comparison to the aliphatic olefins. Under optimized reaction conditions, both copper catalysts 3 and 4 fail to compete with the activity shown by their vanadium counterparts. Irrespective of olefins, metal (vanadium or copper) complexes of the ligand [salLH4] (I) show better substrate conversion(%) compared with the metal complexes of the ligand [NsalLH4] (II).

Oxidovanadium(IV/V) complexes bound with a ONS donor backbone: The search for therapeutic versatility in one class of compounds

Among the multifarious approaches to treat health challenges such as microbial infections, cancers, and diabetes, metallotherapy is a very significant one. Many transition metal‐based candidates have been previously developed that displayed promising therapeutic interests. Vanadium is one such biologically important transition metal, and in the present work, we have developed potent bioactive vanadium complexes, namely, [{VVO(hpdbal‐smdt)}2μ‐O] (1), [VVO(hpdbal‐smdt)(bha)] (2), [VVO(hpdbal‐smdt)(cat)] (3), [VIVO(hpdbal‐smdt)(phen)] (4), and [VIVO(hpdbal‐smdt)(bipy)] (5), which were synthesized using a ONS donor‐based ligand, H2hpdbal‐smdt (H2L), and [VO(acac)2] alongside secondary ligands, benzohydroxamic acid (bha), catechol (cat), 1,10‐phenanthroline (phen), and 2,2′‐bipyridyl (bipy), respectively. The complexes were mainly characterized using electron paramagnetic resonance (EPR), 1H‐NMR, 13C‐NMR, and infrared (IR) spectroscopy techniques. The ligand was additionally characterized using a single‐crystal X‐ray diffraction (XRD) technique. The compounds were tested for their therapeutic versatility, namely, glucose uptake activity, free radical scavenging activity, and growth inhibitory action towards resilient microbial strains and liver cancer cells. The oxo‐bridged dinuclear vanadium complex (1) showed promising growth inhibitory activity against Pseudomonas aeruginosa bacterial species. Secondary ligand tethered compound (2) displayed notable toxicity against the human liver cancer cells. All complexes induced glucose uptake by the insulin‐resistant cells and exhibited moderate to high antioxidant potential. pH titration and interaction study of these complexes with bovine serum albumin were conducted to investigate solution stability and protein binding properties.

A new vanadium complex having [OVV(µ-O)2VVO] core with anti-coplanar configuration: Synthesis, crystal structure, DFT calculation, antibacterial and a homogeneous catalyst for epoxidation of alkenes

A new dinuclear vanadium(V) Schiff base complex [VO2(L)]2 with [OVV(µ-O)2VVO] core was synthesized and characterized by FT-IR and 1H NMR spectroscopy and single-crystal X-ray analysis. The X-ray structural analysis revealed a centrosymmetric structure with anti-coplanar configuration, consisting of two symmetry-independent complex parts. In each part, two edge sharing symmetry-related octahedral VVO centers connected two ONN Schiff base ligands. Epoxidation of cis‐cyclooctene was carried out in the presence of this complex, and the experimental procedures were optimized. The optimized experimental conditions were implemented and tested successfully for the epoxidation of some other substituted alkenes. Density functional theory (DFT) calculations were done to obtain the electronic and geometric structures of the complex. The in vitro antibacterial activity of the ligand (HL) and its corresponding vanadium(V) complex was evaluated against three gram-positive (S. aureus, E. faecalis, and B. cereus) and three gram-negative (P. aeruginosa, E. coli, and K. pneumoniae) bacterial strains.

A tetrameric uudd type water cluster encapsulated in a dinuclear vanadium(V) Schiff base complex and its role in the formation of supramolecular assemblies: A joint experimental and theoretical study

A new dinuclear vanadium(V) complex, [(µ-O)2V2O2L2]·4H2O, [where HL = 2-((2-(ethylamino)ethylimino)methyl)-5-(diethylamino)phenol] has been synthesized and subsequently characterized by elemental and spectral studies. Structure of the complex has been confirmed by single crystal X-ray diffraction study. The four lattice water molecules in the crystal structure are assembled to form a tetrameric water cluster (W4), stabilized via hydrogen bonding interactions. These clusters further interconnect with the complex moiety to generate a 1D supramolecular chain of the complex in the solid state. The interaction energies and the possible cooperativity effects have been studied using DFT calculations and the MEP surface analysis.

Di- and Tri-nuclear VIII and CrIII Complexes of Dipyridyltriazoles: Ligand Rearrangements, Mixed Valency and Ferromagnetic Coupling.

The first dinuclear and trinuclear chromium(III) and dinuclear vanadium(III) complexes of N4-R-substituted-3,5-di(2-pyridyl)-1,2,4-triazole (Rdpt) ligands have been prepared by solvothermal complexations under inert atmospheres, and characterized. The reactions of CrIII and VIII with adpt (R = amino) resulted in deamination of the ligand and yielded the dinuclear doubly-triazolate bridged complexes [(dpt−)2Cl4] (1) and [(dpt−)2Cl4] (2). In the case of the CrIII complex 2 this bridging results in a rare example of ferromagnetic coupling for a dinuclear CrIII compound. DFT studies confirm that in 2 the ferromagnetic coupling pathways dominate over the antiferromagnetic pathways, whereas in 1 the reverse occurs, consistent with the observed overall antiferromagnetic coupling in that case. It was also found that the use of different additives in the reaction allows the nuclearity of the CrIII product to be manipulated, giving either the dinuclear system, or the first example of a trinuclear circular helicate for a Rdpt complex, [(dpt)3Cl6]·1¾MeCN·¼DCM (3). Reaction of N4-pydpt (R = 4-pyridyl) with VIII led to an unusual shift of the pyridyl substituent from N4 to N1 of the triazole, forming the ligand isomer N1-pydpt, and giving a dinuclear doubly-triazole bridged complex, [(N1-pydpt)2Cl6]·2MeCN (4). Reaction with CrIII results in loss of the 4-pyridyl ring and a mixture of the di- and trinuclear complexes, 2 and 3. Interestingly, partial oxidation of the VIII in dinuclear complex 4 to vanadyl VIV=O was identified by crystallographic analysis of partially oxidized single crystals, [(VIVO)0.84(VIII)1.16(N1-pydpt)2Cl5.16]·0.84H2O·1.16MeCN (5).

Exploration of Br⋯O halogen bonding interactions in dinuclear vanadium(V) complexes with Schiff base ligands

An oxo-bridged dinuclear vanadium(V) complex, (µ-O)2[V(O)(L)]2, [where HL = 2-((2-(methylamino)ethylimino)methyl)-4-bromo-6-methoxyphenol] has been synthesized and characterized by elemental and spectral analysis. Structure of the complex has been determined by single crystal X-ray diffraction study. The complex generates an infinite 1D chain governed by Br⋯O halogen bond involving the oxygen atoms of the VO2+ unit as electron donor and the σ-hole at the halogen as electron acceptor. The molecular electrostatic potential (MEP) surface of the complex has been computed, which indicates that the most electron rich part corresponds to the oxygen atoms of the VO2+ unit and the existence of a σ–hole (+9.4 kcal/mol) around Br atom, and therefore justifies the formation of a directional halogen bonding interaction. The interaction in the complex has also been characterized energetically by using the Bader’s quantum theory of “atoms in molecules” (QTAIM). The contribution of the halogen bond is found to be ∼−6.3 kcal/mol, which is in the range of typical halogen bonds. The results have been compared with the energies of Br⋯O halogen bonding interactions in a structurally similar oxo-bridged dinuclear vanadium(V) complex.

Selective Cα Alcohol Oxidation of Lignin Substrates Featuring a β‐O‐4 Linkage by a Dinuclear Oxovanadium Catalyst via Two‐Electron Redox Processes

Developing highly efficient catalyst systems to transform lignin biomass into value‐added chemical feedstocks is imperative for utilizing lignin as renewable alternatives to fossil fuels. Recently, the pre‐activated strategy involving the selective oxidation of Cα alcohol of lignin substrates containing (β‐O‐4 linkage mode has been demonstrated to significantly increase the depolymerization efficiency of native aspen lignin from 10–20 to 60 wt.‐%. In this study, we reported the synthesis of a dinuclear oxovanadium complex 2 that is capable of selectively oxidizing the Cα alcohol (80 – 100% selectivity) of various dimeric lignin substrates under a mild condition. Further investigation of catalytic mechanism has revealed that two V=O motifs of complex 2 could serve as proton abstraction sites for both Cα and Cγ alcohol of dimeric lignin substrates, respectively. Interestingly, the dinuclear vanadium intermediate 4 demonstrates the ability to uptake two electrons resulting from the oxidation of Cα alcohol and yields two corresponding mononuclear VIV intermediate 5. The mononuclear VIV intermediate 5 exhibits a characteristic 8‐line EPR spectrum and possesses one unpaired electron determined by the Evans method. The established structure‐reactivity relationships will be able to shed light on the future directions for rational design of highly efficient catalysts for selective oxidation of lignin biomass.

Naphthalene Based Amide‐Imine Derivative and its Dinuclear Vanadium Complex: Structures, Atmospheric CO2 Fixation and Theoretical Support

AbstractThe oxo‐vanadium(V) complex (1) derived from an amide‐imine conjugate, (E)‐N′‐(4‐(diethylamino)‐2‐hydroxybenzylidene)‐2‐(naphthalen‐1‐yl)acetohydrazide (H2L1) is exploited as catalyst for arresting atmospheric CO2 to organic cyclic carbonate by reacting with epoxide. The H2L1 is prepared by facile condensation of 2‐(naphthalen‐1‐yl)acetohydrazide (AC) and 4‐(diethylamino)‐2‐hydroxybenzaldehyde in good yield. The 1 is characterized by single crystal X‐ray diffraction, FTIR and absorption spectroscopic studies.

Synthesis, characterization and DFT studies of a new unsymmetrical dinuclear Vanadium(IV) complex with a bipodal N2O-donor ligand

A new dinuclear vanadium(IV) complex [(VO)2(μ-HBPA)2(μ-SO4)].4.75H2O was synthesized by the reaction between a bipodal N2O-donor ligand N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl)amine (HBPA) and vanadyl sulfate. The complex was characterized by elemental analysis, thermogravimetric, electrochemical analysis, ESI-MS, FT-IR and UV–visible. The structure was determined by single crystal X-ray diffraction. The two non-equivalent vanadium(IV) ions are hexacoordinated being bridge-coordinated by the oxygen atoms from of HBPA and sulfate group. The complex was studied by DFT calculations and showed high polarity.

Stabilization of two conformers via intra- or inter-molecular hydrogen bonds in a dinuclear vanadium(v) complex with a pendant Schiff base: theoretical insight.

A dinuclear vanadium(v) complex, (μ-O)2[V(O)(L)]2, [where HL = 2-methoxy-6-((2-(2-hydroxyethylamino)ethylimino)methyl)phenol] has been synthesized and characterized by spectral and elemental analysis. A single crystal X-ray diffraction study confirms it structure. Two different conformations, stabilized via either intra- or inter-dinuclear hydrogen bonding interactions, co-exist in the solid-state structure. The energies of these intra- or inter-dinuclear hydrogen bonding interactions have been estimated by Density functional theory (DFT) calculations. A ‘Non-covalent interaction’ (NCI) plot has also been used to characterize these interactions.

Chiral Dinuclear Vanadium Complex-Mediated Oxidative Coupling of Resorcinols.

A method for the highly regio- and enantioselective oxidative coupling of resorcinols has been established by using dibrominated dinuclear vanadium(V) catalyst 1c under air. When resorcinols bearing an aryl substituent were applied as substrates to the coupling, axially chiral biresorcinols were obtained as single regioisomers in high yield with up to 98% ee.

Asymmetric oxidative coupling of hydroxycarbazoles: Facile synthesis of (+)-bi-2-hydroxy-3-methylcarbazole

Asymmetric oxidative coupling reactions of hydroxycarbazoles have been established using a chiral dinuclear vanadium complex. To demonstrate the utility of vanadium-catalyzed reactions, we have used them to synthesize (+)-bi-2-hydroxy-3-carbazole in three steps from cyclohexanone and commercially available aniline derivatives.