Mononuclear copper(II) complexes with polypyridyl ligands: synthesis, characterization, DNA interactions/cleavages and in vitro cytotoxicity towards human cancer cells

DNA being the necessary element in cell regeneration, controlled cellular apoptosis via DNA binding/cleaving is considered an approach to combat cancer cells. The widely prescribed metallodrug cisplatin has shown interactions with the guanine-N7 center, and a plethora of complexes are continually developed to enhance crosslinking properties as well as covalent and non-covalent interactions. Two pentadentate ligands, L1 (1-(6-(1H-benzo[d]imidazol-2-yl)pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) and L2 (1-(6-(1-methyl-1H-benzo[d]imidazol-2-yl)pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine), were synthesized together with their respective copper(II) complexes [1](ClO4)2 and [2](ClO4)2, which crystallized in a trigonal bipyramidal fashion. Different analytical and spectroscopic methods confirmed their formation, and their redox behaviour was also examined. The interactions of salmon sperm DNA (ss-DNA) with these two complexes were explored using absorbance spectroscopy, and they both exhibited a binding affinity (Kb) of ∼104 M-1. Fluorescence quenching experiments with ethidium bromide (EB)-bound DNA (EB-DNA) were also performed, and Stern-Volmer constant (KSV) values of 6.93 × 103 and 2.34 × 104 M-1 for [1](ClO4)2 and [2](ClO4)2, respectively, were obtained. Furthermore, DNA conformational changes due to the interactions of both complexes were validated via circular dichroism. We also assessed the DNA cleavage property of these complexes, which resulted in the linearization of circular plasmid DNA. This finding was supported by studying the growth of MDA-MB-231 breast cancer cells upon treatment with both Cu(II) complexes; IC50 values of 5.34 ± 1.02 μM and 0.83 ± 0.18 μM were obtained for [1](ClO4)2 and [2](ClO4)2, respectively. This validates their affinity towards DNA, and these insights can be further utilized for non-platinum based economical metallodrug development based on first row transition metals.

Systematic Development of Vanadium Catalysts for Sustainable Epoxidation of Small Alkenes and Allylic Alcohols

The catalytic epoxidation of small alkenes and allylic alcohols includes a wide range of valuable chemical applications, with many works describing vanadium complexes as suitable catalysts towards sustainable process chemistry. But, given the complexity of these mechanisms, it is not always easy to sort out efficient examples for streamlining sustainable processes and tuning product optimization. In this review, we provide an update on major works of tunable vanadium-catalyzed epoxidations, with a focus on sustainable optimization routes. After presenting the current mechanistic view on vanadium catalysts for small alkenes and allylic alcohols' epoxidation, we argue the key challenges in green process development by highlighting the value of updated kinetic and mechanistic studies, along with essential computational studies.

Mononuclear/Binuclear [VIVO]/[VVO2] Complexes Derived from 1,3-Diaminoguanidine and Their Catalytic Application for the Oxidation of Benzoin via Oxygen Atom Transfer

Ligands H₄sal-dag (I) and H₄Brsal-dag (II) derived from 1,3-diaminoguanidine and salicylaldehyde or 5-bromosalicylaldehyde react with one or 2 mol equivalent of vanadium precursor to give two different series of vanadium complexes. Thus, complexes [V^IVO(H₂sal-dag) (H₂O)] (1) and [V^IVO(H₂Brsal-dag) (H₂O)] (2) were isolated by the reaction of an equimolar ratio of these ligands with [V^IVO(acac)₂] in MeOH. In the presence of K⁺/Cs⁺ ion and using aerially oxidized [V^IVO(acac)₂], the above reaction gave complexes [K(H₂O){V^VO₂(H₂sal-dag)}]₂ (3), [Cs(H₂O){V^VO₂(H₂sal-dag)}]₂ (4), [K(H₂O){VO₂(H₂Brsal-dag)}]₂ (5), and [Cs(H₂O){V^VO₂(H₂Brsal-dag)}]₂ (6), which could also be isolated by direct aerial oxidation of complexes 1 and 2 in MeOH in the presence of K⁺/Cs⁺ ion. Complexes [(H₂O)V^IVO(Hsal-dag)V^VO₂] (7) and [(H₂O)V^IVO(HBrsal-dag)V^VO₂] (8) were isolated upon increasing the ligand-to-vanadium precursor molar ratio to 1:2 under an air atmosphere. When I and II were reacted with aerially oxidized [V^IVO(acac)₂] in a 1:2 molar ratio in MeOH in the presence of K⁺/Cs⁺ ion, they formed [K(H₂O)₅{(V^VO₂)₂(Hsal-dag)}]₂ (9), [Cs(H₂O)₂{(V^VO₂)₂(Hsal-dag)}]₂ (10), [K₂(H₂O)₄{(V^VO₂)₂(Brsal-dag)}]₂ (11), and [Cs₂(H₂O)₄{(V^VO₂)₂(Brsal-dag)}]₂ (12). The structures of complexes 3, 4, 5, and 9 determined by single-crystal X-ray diffraction study confirm the mono-, bi-, tri-, and tetra-anionic behaviors of the ligands. All complexes were found to be an effective catalyst for the oxidation of benzoin to benzil via oxygen atom transfer (OAT) between DMSO and benzoin. Under aerobic condition, this oxidation also proceeds effectively in the absence of DMSO. Electron paramagnetic resonance and ⁵¹V NMR studies demonstrated the active role of a stable V(IV) intermediate during OAT between DMSO and benzoin.

Immobilization of Zn4 complex on functionalized Layered HUS-7: Synthesis, Structural Investigation and Catalytic Activity

Tetranuclear Zn (Zn4) complex was selected for the immobilization on novel layered Hiroshima University Silicate (HUS)-7. Modification of HUS-7 was carried out with sulfopropylsilyl agent followed by Zn4 complex incorporation....

New VIVO-complexes for oxidative desulfurization of refractory sulfur compounds in fuel: synthesis, structure, reactivity trend and mechanistic studies

A series of 5-coordinate oxidovanadium(iv) complexes based on 2-(2'-hydroxyphenyl)imidazole (HPIMH), with substituent groups of different electronegativities on the phenolic para position (HPIMX; X = -H, -Br, -OMe and -NO2), were synthesized and characterized. Three of these complexes were characterized by single crystal X-ray diffraction, [VIVO(PIMH)2], [VIVO(PIMBr)2] and [VIVO(PIMNO2)2], as well as a dioxidovanadium(v) compound ([VVO2(PIMH)(PIMH2)]). The complexes were tested for their catalytic activities in the oxidation of dibenzothiophene (DBT), the major refractory organosulfur compound found in fuel. The nitro substituted compound [VIVO(PIMNO2)2] had the highest catalytic oxidation activity followed by: [VIVO(PIMH)2] > [VIVO(PIMBr)2] > [VIVO(PIMMeO)2]. The decrease in activity is attributed to the different electronegativities of the substituent groups, which influence the electron density on the metal center, the V[double bond, length as m-dash]O bond distances and infrared stretching bands. Geometry index (τ) values calculated from single crystal X-ray diffraction (SC-XRD) data and DFT studies provided further insights on the trend in activity observed. SC-XRD, EPR, 51V NMR and UV-Vis spectroscopies, and DFT studies were instrumental in studying the mechanism of the catalyzed reaction and proposal of intermediate species. Both radical and non-radical pathways are plausible for the catalytic oxidation and participation of reactive oxygen species in both pathways is also postulated.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Synthesis, structure, solution behavior, reactivity and biological evaluation of oxidovanadium(iv/v) thiosemicarbazone complexes

The synthesis and characterization of an oxidovanadium(iv) [VIVO(L)(acac)] (1) and of two dioxidovanadium(v) [VVO2(L')] (2) and [VVO2(L)] (2a) complexes of the Schiff base formed from the reaction of 4-(p-fluorophenyl) thiosemicarbazone with pyridine-2-aldehyde (HL) are described. The oxidovanadium(iv) species [VIVO(L)(acac)] (1) was synthesized by the reaction of VIVO(acac)2 with the thiosemicarbazone HL in refluxing ethanol. The recrystallization of [VIVO(L)(acac)] (1) in DMF, CH3CN or EtOH gave the same product i.e. the dioxidovanadium(v) complex [VVO2(L)] (2a); however, upon recrystallization of 1 in DMSO a distinct compound [VVO2(L')] (2) was formed, wherein the original ligand L- is transformed to a rearranged one, L'-. In the presence of DMSO the ligand in complex 1 is found to undergo methylation at the carbon centre attached to imine nitrogen (aldimine) and transformed to the corresponding VVO2-species through in situ reaction. The synthesized HL and the metal complexes were characterized by elemental analysis, IR, UV-Vis, NMR and EPR spectroscopy. The molecular structure of [VVO2(L')] (2) was determined by single crystal X-ray crystallography. The methylation of various other ligands and complexes prepared from different vanadium precursors under similar reaction conditions was also attempted and it was confirmed that the imine methylation observed is both ligand and metal precursor specific. Complexes 1 and 2 show in vitro insulin-like activity against insulin responsive L6 myoblast cells, higher than VIVO(acac)2, with complex 1 being more potent. In addition, the in vitro cytotoxicity studies of HL, and of complexes 1 and 2 against the MCF-7 and Vero cell lines were also done. The ligand is not cytotoxic and complex 2 is significantly more cytotoxic than 1. DAPI staining experiments indicate that an increase in the time of incubation and an increase of concentration of the complexes lead to the increase in cell death.

Radical reactions of diamine bis(phenolate) vanadium(iii) complexes. Solid state binding of O2 to form a vanadium(v) peroxo complex

[VCl₃(THF)₃] reacted with the sodium salt of a tripodal diamine bisphenolate ligand precursor Na₂L2 to give a paramagnetic d² complex [V(L2)Cl] (2). The reaction of 2 with oxygen is strongly dependent on the experimental conditions, affording [VO(L2)Cl] (6) or [V(η²-O₂)(L2)Cl] (7). The formation of 7 involves the direct addition of O₂ to V(iii) in the solid state with oxidation to V(v) without significantly disturbing the structure of 2. DFT calculations showed that compound 7 is an intermediate in the formation of 6 from 2. The reaction involves the cleavage of the η¹-O-O bond in a proposed dimeric species. The overall reaction of 2 moles of vanadium(iii), complex 2, and one mole of O₂ to yield two moles of product 6 is a favourable process with ΔG₂₉₈ = -38.3 kcal mol^-1. 7 is the first non-oxido peroxidovanadium(v) complex obtained directly from the reaction of a crystal and the second example of a structurally characterized complex of that type. Reactions of V(L1)Cl (1) and V(L2)Cl (2) with one equivalent of the nitroxyl radical TEMPO in toluene also result in the formation of oxido V(v) complexes, VO(L1)Cl (5) and VO(L2)Cl (6). The reaction of VO(OⁱPr)₃ with Na₂L2 afforded [VO(L2)(OⁱPr)] (8) in high yield. A major isomer having the V[double bond, length as m-dash]O moiety in the equatorial plane was characterised by X-ray diffraction although solution NMR data showed the presence of a minor species with the oxido ligand trans to the tripodal nitrogen, as in 6. Complexes 6 and 8 are very active and selective sulfoxidation catalysts of thioanisole, but no enantiomeric excess was obtained.

Interaction of [VIV O(acac)2 ] with Human Serum Transferrin and Albumin

[VO(acac)₂ ] is a remarkable vanadium compound and has potential as a therapeutic drug. It is important to clarify how it is transported in blood, but the reports addressing its binding to serum proteins have been contradictory. We use several spectroscopic and mass spectrometric techniques (ESI and MALDI-TOF), small-angle X-ray scattering and size exclusion chromatography (SEC) to characterize solutions containing [VO(acac)₂ ] and either human serum apotransferrin (apoHTF) or albumin (HSA). DFT and modeling protein calculations are carried out to disclose the type of binding to apoHTF. The measured circular dichroism spectra, SEC and MALDI-TOF data clearly prove that at least two VO-acac moieties may bind to apoHTF, most probably forming [V^IV O(acac)(apoHTF)] complexes with residues of the HTF binding sites. No indication of binding of [VO(acac)₂ ] to HSA is obtained. We conclude that V^IV O-acac species may be transported in blood by transferrin. At very low complex concentrations speciation calculations suggest that [(VO)(apoHTF)] species form.

Paramagnetic dioxovanadium(IV) molecules inside the channels of zeolite BEA--EPR screening of VO2 reactivity toward small gas-phase molecules

Interaction of small gas-phase molecules (NO, N2O, O2, CO) with VO2 radicals inside the channels of a dealuminated SiBEA zeolite was investigated by means of electron paramagnetic resonance (EPR), infrared (IR), and mass (QMS) spectroscopies to provide direct insights into the chemistry of a unique paramagnetic state of vanadium - VO2 molecules. A facile way of forming VO2 inside the channels of SiBEA via thermal reduction of VO2(+) precursor cations was shown. Dioxovanadium(IV) was identified based on its unusual EPR signal which, as compared with the typical monooxovanadium(IV) (VO(2+) cation), is featured by rhombic symmetry and a positive Aiso value leading to a hyperfine splitting as large as 32 mT. VO2 molecules exhibit reducing properties transforming N2O and O2 into vanadium intrachannel cage adducts comprising of reactive oxygen species (O(-) and O2(-), respectively). Interaction with CO led to its oxidation to CO2, while paramagnetic NO acted as a scavenger for VO2 radicals producing diamagnetic adducts. The observed reactivity was rationalized in terms of spin-pairing, electron transfer, and oxygen transfer processes. As a result new chemical pathways of vanadium reactivity were demonstrated which were not observed so far either in the homogeneous molecular systems or supported vanadium materials.

Vanadium(V) complexes of a tripodal ligand, their characterisation and biological implications

The reaction of the tripodal tetradentate dibasic ligand 6,6'-(2-(pyridin-2-yl)ethylazanediyl)bis(methylene)bis(2,4-di-tert-butylphenol), H2L(1)I, with [V(IV)O(acac)2] in CH3CN gives the V(V)O-complex, [V(V)O(acac)(L(1))] 1. Crystallisation of 1 in CH3CN at ∼0 °C gives dark blue crystals of 1, while at room temperature it affords dark green crystals of [{V(V)O(L(1))}2μ-O] 3. Upon prolonged treatment of 1 in MeOH, [V(V)O(OMe)(MeOH)(L(1))] 2 is obtained. All three complexes were analysed by single-crystal X-ray diffraction, depicting a distorted octahedral geometry around vanadium. In the reaction of H2L(1) with V(IV)OSO4 partial hydrolysis of the tripodal ligand results in the elimination of the pyridyl fragment of L(1) and the formation of H[V(V)O2(L(2))] 4 containing the ONO tridentate ligand 6,6'-azanediylbis(methylene)bis(2,4-di-tert-butylphenol), H2L(2)II. Compound 4, which was not fully characterised, undergoes dimerization in acetone yielding the hydroxido-bridged [{V(V)O(L(2))}2μ-(OH)2] 5 having a distorted octahedral geometry around each vanadium. In contrast, from a solution of 4 in acetonitrile, the dinuclear compound [{V(V)O(L(2))}2μ-O] 6 is obtained, with a trigonal bipyramidal geometry around each vanadium. The methoxido complex 2 is successfully employed as a functional catechol-oxidase mimic in the oxidation of catechol to o-quinone under air. The process was confirmed to follow a Michaelis-Menten type kinetics with respect to catechol, the Vmax and KM values obtained being 7.66 × 10(-6) M min(-1) and 0.0557 M, respectively, and the turnover frequency is 0.0541 min(-1). A similar reaction with the bulkier 3,5-di-tert-butylcatechol proceeded at a much slower rate. Complex 2 was also used as a catalyst precursor for the oxidative bromination of thymol in aqueous medium. The selectivity shows quite interesting trends, namely when not using excess of the primary oxidizing agent, H2O2, the para mono-brominated product corresponds to ∼93% of the products and no dibromo derivative is formed.

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.

Thirty years through vanadium chemistry

The relevance of vanadium in biological systems is known for many years and vanadium-based catalysts have important industrial applications, however, till the beginning of the 80s research on vanadium chemistry and biochemistry did not receive much attention from the scientific community. The understanding of the broad bioinorganic implications resulting from the similarities between phosphate and vanadate(V) and the discovery of vanadium dependent enzymes gave rise to an enormous increase in interest in the chemistry and biological relevance of vanadium. Thereupon the last 30years corresponded to a period of enormous research effort in these fields, as well as in medicinal applications of vanadium and in the development of catalysts for use in fine-chemical synthesis, some of these inspired by enzymatic active sites. Since the 80s my group in collaboration with others made contributions, described throughout this text, namely in the understanding of the speciation of vanadium compounds in aqueous solution and in biological fluids, and to the transport of vanadium compounds in blood plasma and their uptake by cells. Several new types of vanadium compounds were also synthesized and characterized, with applications either as prospective therapeutic drugs or as homogeneous or heterogenized catalysts for the production of fine chemicals. The developments made are described also considering the international context of the evolution of the knowledge in the chemistry and bioinorganic chemistry of vanadium compounds during the last 30years. This article was compiled based on the Vanadis Award presentation at the 9th International Vanadium Symposium.

Oxidovanadium(iv) and dioxidovanadium(v) complexes of hydrazones of 2-benzoylpyridine and their catalytic applications

V^V-polymer-supported compounds, their neat analogues and the corresponding peroxido-complexes are prepared and applied as catalyst precursors for the oxidation of isoeugenol.

Hydroxyquinoline derived vanadium(IV and V) and copper(II) complexes as potential anti-tuberculosis and anti-tumor agents

Several mixed ligand vanadium and copper complexes were synthesized containing 8-hydroxyquinoline (8HQ) and a ligand such as picolinato (pic(-)), dipicolinato (dipic(2-)) or a Schiff base. The complexes were characterized by spectroscopic techniques and by single-crystal X-ray diffraction in the case of [V(V)O(L-pheolnaph-im)(5-Cl-8HQ)] and [V(V)O(OMe)(8HQ)2], which evidenced the distorted octahedral geometry of the complexes. The electronic absorption data showed the presence of strong ligand to metal charge transfer bands, significant solvent effects, and methoxido species in methanol, which was further confirmed by (51)V-NMR spectroscopy. The structures of [Cu(II)(dipic)(8HQ)]Na and [V(IV)O(pic)(8HQ)] were confirmed by EPR spectroscopy, showing only one species in solution. The biological activity of the compounds was assessed through the minimal inhibitory concentration (MIC) of the compounds against Mycobacterium tuberculosis (Mtb) and the cytotoxic activity against the cisplatin sensitive/resistant ovarian cells A2780/A2780cisR and the non-tumorigenic HEK cells (IC50 values). Almost all tested vanadium complexes were very active against Mtb and the MICs were comparable to, or better than, the MICs of drugs, such as streptomycin. The activity of the complexes against the A2780 cell line was dependent on incubation time presenting IC50 values in the 3-14 μM (at 48 h) range. In these conditions, the complexes were significantly (*P<0.05-**P<0.001) more active than cisplatin (22 μM), in the A2780 cells and even surpassing its activity in the cisplatin-resistant cells A2780cisR (2.4-8 μM vs. 75.4; **P<0.001). In the non-tumorigenic HEK cells poor selectivity toward cancer cells for most of the complexes was observed, as well as for cisplatin.