Aerial Oxidation of a V^IV–Iminopyridine Hydroquinonate Complex: A Trap for the V^IV–Semiquinonate Radical Intermediate

The reaction of 2,5-bis­[<i>N</i>,<i>N</i>′-bis­(2-pyridyl-aminomethyl)­aminomethyl]-<i>p</i>-hydroquinone (H₂bpymah) with VO²⁺ salts in acetonitrile or water at a low pH (2.2–3.5) results in the isolation of [{V^IV(O)­(Cl)}₂(μ-bpymah)], the <i>p</i>-semiquinonate complex [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH), the cyclic mixed-valent hexanuclear compound [{V^V(O)­(μ-O)­V^IV(O)}­(μ-bpymah)]₃, and [(V^VO₂)₂(μ-bpymah)]. [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) is an intermediate of the radical-mediated oxidation of [{V^IV(O)­(Cl)}₂(μ-bpymah)] from O₂. At lower pH values (2.2), a reversible intramolecular electron transfer from the metal to the ligand of [{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) is induced with the concurrent substitution of chlorine atoms by the oxygen-bridging atoms, resulting in the formation of [{V^V(O)­(μ-O)­V^IV(O)}­(μ-bpymah)]₃. The metal complexes were fully characterized by X-ray crystallography, infrared (IR) spectroscopy, and magnetic measurements in the solid state, as well as by conductivity measurements, UV–vis spectroscopy, and electrochemical measurements in solution. The oxidation states of the metal ions and ligands were determined by the crystallographic data. The [{V^IV(O)­(Cl)}₂(μ-bpymah)]–[{V^IV(O)­(Cl)}₂(μ-bpymas)]­(OH) redox process is electrochemically reversible. The V^IV ion in the semiquinonate compound exhibits a surprisingly low oxophilicity, resulting in the stabilization of OH^– counterions at acidic pH values. An investigation of the mechanism of this reaction reveals that these complexes induce the reduction of O₂ to H₂O₂, mimicking the activity of enzymes incorporating two redox-active centers (metal–organic) in the active site

Bis(hydroxylamino)triazines: High Selectivity and Hydrolytic Stability of Hydroxylamine-Based Ligands for Uranyl Compared to Vanadium(V) and Iron(III)

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the U^VIO₂²⁺ exhibit high selectivity for the uranyl moiety. The bis­(hydroxylamino)­(triazine) ligand, 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat), a strong binder for hard metal ions (Fe^III, Ti^IV, V^V, and Mo^VI), reacted with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in aqueous solution and resulted in the isolation of the complexes [U^VIO₂(bihyat)­(H₂O)], [U^VIO₂(bihyat)₂]^2–, and {[U^VIO₂(bihyat)­(μ-OH)]}₂^2–. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H₂bihyat and the pH. Reaction of H₂bihyat with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in CH₃CN gave the trinuclear complex [U^VI₃O₆(bihyat)₂(μ-bihyat)₂]^2–, which is the major species in organic solvents. The dynamics between the U^VIO₂²⁺ and the free ligand H₂bihyat in aqueous and dimethyl sulfoxide solutions; the metal binding ability of the H₂bihyat over pyridine-2,6-dicarboxylic acid (H₂dipic) or glutarimidedioxime for U^VIO₂²⁺, and the selectivity of the H₂bihyat to bind U^VIO₂²⁺ in comparison to V^VO₄^3– and Fe^III in either U^VIO₂²⁺/V^VO₄^3– or U^VIO₂²⁺/Fe^III solutions were examined by NMR and UV–vis spectroscopies. The results revealed that H₂bihyat is a superior ligand for U^VIO₂²⁺ with high selectivity compared to Fe^III and V^VO₄^3–, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body

Synthesis, Bonding, and Reactivity of Vanadium(IV) Oxido–Fluorido Compounds with Neutral Chelate Ligands of the General Formula <i>cis</i>-[V^IV(O)(F)(L_N–N)₂]⁺

Reaction of the oxidovanadium­(IV)–L_N–N species (L_N–N is bipy = 2,2′-bipyridine or bipy-like molecules) with either BF₄^– or HF and/or KF results in the formation of compounds of the general formula <i>cis</i>-[V^IV(O)­(F)­(L_N–N)₂]⁺. Structural and spectroscopic (electron paramagnetic resonance) characterization shows that these compounds are in the tetravalent oxidation state containing a terminal fluorido ligand. Density functional theory calculations reveal that the V^IV–F bond is mainly electrostatic, which is reinforced by reactivity studies that demonstrate the nucleophilicity of the fluoride ligand in a halogen exchange reaction and in fluorination of various organic substrates

Synthesis, Bonding, and Reactivity of Vanadium(IV) Oxido–Fluorido Compounds with Neutral Chelate Ligands of the General Formula <i>cis</i>-[V^IV(O)(F)(L_N–N)₂]⁺

Reaction of the oxidovanadium­(IV)–L_N–N species (L_N–N is bipy = 2,2′-bipyridine or bipy-like molecules) with either BF₄^– or HF and/or KF results in the formation of compounds of the general formula <i>cis</i>-[V^IV(O)­(F)­(L_N–N)₂]⁺. Structural and spectroscopic (electron paramagnetic resonance) characterization shows that these compounds are in the tetravalent oxidation state containing a terminal fluorido ligand. Density functional theory calculations reveal that the V^IV–F bond is mainly electrostatic, which is reinforced by reactivity studies that demonstrate the nucleophilicity of the fluoride ligand in a halogen exchange reaction and in fluorination of various organic substrates

Interaction of Chromium(III) with a <i>N</i>,<i>N</i>′‑Disubstituted Hydroxylamine-(diamido) Ligand: A Combined Experimental and Theoretical Study

Reaction of hydroxylamine hydrochloride with prop-2-enamide in dichloromethane in the presence of triethylamine resulted in the isolation of the <i>N</i>,<i>N</i>′-disubstituted hydroxylamine-(diamido) ligand, 3,3′-(hydroxyazanediyl)­dipropanamide (Hhydia). The ligand Hhydia was characterized by multinuclear NMR, high-resolution electrospray ionization mass spectrometry (ESI-MS), and X-ray structure analysis. Interaction of Hhydia with <i>trans</i>-[Cr^IIICl₂(H₂O)₄]­Cl·2H₂O in ethanol yields the ionization isomers [Cr^III(Hhydia)₂]­Cl₃·2H₂O­(<b>1</b>·2H₂O) and <i>cis/trans-</i>[Cr^IIICl₂(Hhydia)₂]­Cl·2H₂O (<b>2</b>·2H₂O). The X-ray structure analysis of <b>1</b> revealed that the chromium atom in [Cr^III(Hhydia)₂]³⁺ is bonded to two neutral tridentate <i>O</i>,<i>N</i>,<i>O</i>-Hhydia ligands. The twist angle, <i>θ,</i> in [Cr^III(Hhydia)₂]³⁺ is 54.5(6)⁰, that is, very close to an ideal octahedron. The intramolecular hydrogen bonds developed between the N–OH group of the first ligand and the amidic oxygen atom of the second ligand and vice versa contribute to the overall stability of the cation [Cr^III(Hhydia)₂]³⁺. The reaction rate constant of the formation of Cr­(III) complexes <b>1</b>·2H₂O and <b>2</b>·2H₂O was found to be 8.7(±0.8) × 10^–5 M^–1 s^–1 at 25 °C in methyl alcohol and follows a first-order law kinetics based on the biologically relevant ligand Hhydia. The reaction rate constant is considerably faster in comparison with the corresponding water exchange rate constant for the hydrated chromium­(III). The modification of the kinetics is of fundamental importance for the chromium­(III) chemistry in biological systems. Ultraviolet-visible and electron paramagnetic resonance studies, both in solution and in the solid state, ESI-MS, and conductivity measurements support the fact that, irrespective of the solvent used in the interaction of Hhydia with <i>trans</i>-[Cr^IIICl₂(H₂O)₄]­Cl·2H₂O, the ionization isomers­[Cr^III(Hhydia)₂]­Cl₃·2H₂O (<b>1</b>·2H₂O) and <i>cis/trans-</i>[Cr^IIICl₂(Hhydia)₂]­Cl·2H₂O (<b>2</b>·2H₂O) are produced.The reaction medium affects only the relevant percentage of the isomers in the solid state. The thermodynamic stability of the ionization isomers <b>1</b>·2H₂O and <i>cis/trans-</i><b>2</b>·2H₂O, their molecular structures as well as the vibrational spectra and the energetics of the Cr^III– Hhydia/hydia^– were studied by means of density functional theory calculations and found to be in excellent agreement with our experimental observations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

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^IVO­(hybeb)] was combined with its oxidized analogue (Ph₄P)­[V^VO­(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^IVO­(hybeb)]^2–/[V^VO­(hybeb)]⁻ redox potential and the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [V^IVO­(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

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H₂bihyat. Water-Assisted Activation of the Mo^VIO Bond and Reversible Dimerization of <i>cis</i>-[Mo^VIO₂(bihyat)] to [Mo^VI₂O₄(bihyat)₂(H₂O)₂]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat) with <i>cis</i>-[Mo^VIO₂(acac)₂] in tetrahydrofuran resulted in isolation of the mononuclear compound <i>cis</i>-[Mo^VIO₂(bihyat)] (<b>1</b>). The treatment of Na₂Mo^VIO₄·2H₂O with the ligand H₂bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>2</b>) and <i>trans</i>-[Mo^VI₂O₄(bihyat)₂(H₂O)₂] (<b>3</b>) at pH values of 3.5 and 5.5, respectively. The structures for the three molybdenum­(VI) compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while in the two dinuclear compounds, each molybdenum atom is in a distorted pentagonal-bipyramidal environment of two bridging and one terminal oxido groups, a tridentate (O,N,O) bihyat^2– ligand that forms two five-membered chelate rings, and a water molecule trans to the terminal oxido group. The dinuclear compounds constitute rare examples containing the {Mo₂^VIO₂(μ₂-O₂)}⁴⁺ moiety. The potentiometry revealed that the Mo^VIbihyat^2– species exhibit high hydrolytic stability in aqueous solution at a narrow range of pH values, 3–5. A subtle change in the coordination environment of the five-coordinate compound <b>1</b> with ligation of a weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic than that in <b>1</b>, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of the oxido group nucleophilic attack on organic substrates, resulting in oxidation of the substrate, in the active site of molybdenum enzymes such as xanthine oxidase. Theoretical calculations in the gas phase were performed to examine the influence of water on the dimerization process (<b>1</b> → <b>2</b>/<b>3</b>). In addition, the molecular structures, cis/trans geometrical isomerism for the dinuclear molybdenum­(VI) species, vibrational spectra, and energetics of the metal–ligand interaction for the three molybdenum­(VI) compounds <b>1</b>–<b>3</b> have been studied by means of density functional theory calculations

Hafnium(IV) Chemistry with Imide–Dioxime and Catecholate–Oxime Ligands: Unique {Hf₅} and Metalloaromatic {Hf₆}–Oxo Clusters Exhibiting Fluorescence

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal–organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(μ3-O)2(μ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV–vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters’ band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of −44.6 ppm is considerably higher than the −29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices