Combined Spectroelectrochemical and Theoretical Study of Electron-Rich Dendritic 2,5-Diaminothiophene Derivatives: <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine <b>11</b> is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene <b>4</b>_<b>1</b> as well as to the corresponding high-molar third dendrimer generation <b>8</b> containing the same thiophene-2,5-diamine core. The dendritic compound <b>11</b> can be reversibly oxidized in three separated steps to yield the corresponding stable monocation <b>11</b>^<b>•+</b>, dication <b>11</b>^<b>2+</b>, and tetracation <b>11</b>^<b>4+</b>. A well resolved ESR spectrum of the corresponding cation radical <b>11</b>^<b>•+</b> with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to <b>4</b>_<b>1</b>^<b>•+</b>. The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds <b>4</b>_<b>1</b>^<b>•+</b>, <b>11</b>^<b>•+</b>, and <b>8</b>^<b>•+</b> was confirmed. The computed single occupied molecular orbital (SOMO) for trication <b>11</b>^<b>•3+</b> is completely different compared to the SOMO of the corresponding monocation <b>11</b>^<b>•+</b>, and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation <b>11</b>^<b>4+</b>. The estimated theoretical limit energy of the lowest optical transition S₀ → S₁ is 2.90 eV, and it can be achieved for the 3D dendrimer generation

Redox Reactions of Nickel, Copper, and Cobalt Complexes with “Noninnocent” Dithiolate Ligands: Combined in Situ Spectroelectrochemical and Theoretical Study

The redox properties of copper, nickel, and cobalt complexes (MePh₃P)­[M­(bdt)₂] with the ligand benzene-1,2-dithiolate (bdt) and synthesized complexes (MePh₃P)­[M­(bdtCl₂)₂] with the ligand 3,6-dichlorobenzene-1,2-dithiolate (bdtCl₂) have been studied by cyclic voltammetry and in situ EPR–UV/vis/NIR spectroelectrochemistry. The addition of chlorine substituents to the 3- and 6-positions of the benzene ring not only facilitates the reduction of [M­(bdtCl₂)₂]⁻ complexes but also leads to the remarkable stabilization of [M­(bdtCl₂)₂]^2– dianions in solution. In contrast to the EPR-silent copper complexes, the solutions of nickel samples exhibit a broad singlet EPR signal at room temperature which becomes anisotropic at 100 K with a characteristic rhombic pattern. Cathodic reduction of copper and cobalt complexes leads to paramagnetic species having an EPR signal with splitting from ^63,65Cu for copper and from ⁵⁹Co for cobalt samples, confirming a strong contribution of the central atom with substantial delocalization of the unpaired spin onto the central atom. B3LYP/6-311g*/pcm calculations of the monoanions as well as of their oxidized and reduced forms were performed. The spin density of all open-shell ground states calculated for the investigated complexes in different redox states corresponds well to the experimental spectroelectrochemical data

Osmium(III) Analogues of KP1019: Electrochemical and Chemical Synthesis, Spectroscopic Characterization, X‑ray Crystallography, Hydrolytic Stability, and Antiproliferative Activity

A one-electron reduction of osmium­(IV) complexes <i>trans</i>-[Os^IVCl₄(Hazole)₂], where Hazole = 1<i>H</i>-pyrazole ([<b>1</b>]⁰), 2<i>H</i>-indazole ([<b>2</b>]⁰), 1<i>H</i>-imidazole ([<b>3</b>]⁰), and 1<i>H</i>-benzimidazole ([<b>4</b>]⁰), afforded a series of eight new complexes as osmium analogues of KP1019, a lead anticancer drug in clinical trials, with the general formula (cation)­[<i>trans</i>-Os^IIICl₄(Hazole)₂], where cation = H₂pz⁺ (H₂pz­[<b>1</b>]), H₂ind⁺ (H₂ind­[<b>2</b>]), H₂im⁺ (H₂im­[<b>3</b>]), Ph₄P⁺ (Ph₄P­[<b>3</b>]), <i>n</i>Bu₄N⁺ (<i>n</i>Bu₄N­[<b>3</b>]), H₂bzim⁺ (H₂bzim­[<b>4</b>]), Ph₄P⁺ (Ph₄P­[<b>4</b>]), and <i>n</i>Bu₄N⁺ (<i>n</i>Bu₄N­[<b>4</b>]). All complexes were characterized by elemental analysis, ¹H NMR spectroscopy, electrospray ionization mass spectrometry, UV–vis spectroscopy, cyclic voltammetry, while H₂pz­[<b>1</b>], H₂ind­[<b>2</b>], and <i>n</i>Bu₄[<b>3</b>], in addition, by X-ray diffraction. The reduced species [<b>1</b>]⁻ and [<b>4</b>]⁻ are stable in aqueous media in the absence of air oxygen and do not react with small biomolecules such as amino acids and the nucleotide 5′-dGMP. Cell culture experiments in five different human cancer cell lines (HeLa, A549, FemX, MDA-MB-453, and LS-174) and one noncancerous cell line (MRC-5) were performed, and the results were discussed and compared to those for KP1019 and cisplatin. Benzannulation in complexes with similar structure enhances antitumor activity by several orders of magnitude, implicating different mechanisms of action of the tested compounds. In particular, complexes H₂ind­[<b>2</b>] and H₂bzim­[<b>4</b>] exhibited significant antiproliferative activity <i>in vitro</i> when compared to H₂pz­[<b>1</b>] and H₂im­[<b>3</b>]

Charge and Spin States in Schiff Base Metal Complexes with a Disiloxane Unit Exhibiting a Strong Noninnocent Ligand Character: Synthesis, Structure, Spectroelectrochemistry, and Theoretical Calculations

Mononuclear nickel­(II), copper­(II), and manganese­(III) complexes with a noninnocent tetradentate Schiff base ligand containing a disiloxane unit were prepared in situ by reaction of 3,5-di-<i>tert</i>-butyl-2-hydroxybenzaldehyde with 1,3-bis­(3-aminopropyl)­tetramethyldisiloxane followed by addition of the appropriate metal­(II) salt. The ligand H₂L resulting from these reactions is a 2:1 condensation product of 3,5-di-<i>tert</i>-butyl-2-hydroxybenzaldehyde with 1,3-bis­(3-aminopropyl)­tetramethyldisiloxane. The resulting metal complexes, NiL·0.5CH₂Cl₂, CuL·1.5H₂O, and MnL­(OAc)·0.15H₂O, were characterized by elemental analysis, spectroscopic methods (IR, UV–vis, X-band EPR, HFEPR, ¹H NMR), ESI mass spectrometry, and single crystal X-ray diffraction. Taking into account the well-known strong stabilizing effects of <i>tert</i>-butyl groups in positions 3 and 5 of the aromatic ring on phenoxyl radicals, we studied the one-electron and two-electron oxidation of the compounds using both experimental (chiefly spectroelectrochemistry) and computational (DFT) techniques. The calculated spin-density distribution and localized orbitals analysis revealed the oxidation locus and the effect of the electrochemical electron transfer on the molecular structure of the complexes, while time-dependent DFT calculations helped to explain the absorption spectra of the electrochemically generated species. Hyperfine coupling constants, <i>g</i>-tensors, and zero-field splitting parameters have been calculated at the DFT level of theory. Finally, the CASSCF approach has been employed to theoretically explore the zero-field splitting of the <i>S</i> = 2 MnL­(OAc) complex for comparison purposes with the DFT and experimental HFEPR results. It is found that the <i>D</i> parameter sign strongly depends on the metal coordination geometry

Molecular Structure, UV/Vis Spectra, and Cyclic Voltammograms of Mn(II), Co(II), and Zn(II) 5,10,15,20-Tetraphenyl-21-oxaporphyrins

The 5,10,15,20-tetraphenyl-21-oxaporphyrin complexes of Mn­(II), Co­(II), and Zn­(II) have been crystallized and studied by X-ray diffraction, NMR and UV/vis spectroscopy, and mass spectrometry as well as cyclic voltammetry. The X-ray structure of the earlier described Cu­(II) complex is also reported. All complex structures possess a five-coordinate, approximately square-pyramidal geometry with a slight deviation of the heteroaromatic moieties from planarity. The packing structures are characterized by parallel strands of complex molecules interacting by weak hydrogen bonds. In the case of Zn­(II) an octahedral complex has also been isolated using a side-chain hydroxy functionalized oxaporphyrin ligand; the structure was verified by NMR and EXAFS spectroscopy. Cyclic voltammetry studies reveal that the reduction of the complex bound Mn­(II), Co­(II), and Zn­(II) ions is a ligand-centered process whereas the first oxidation step depends on the metal ion present

Stable Radical Trianions from Reversibly Formed Sigma-Dimers of Selenadiazoloquinolones Studied by In Situ EPR/UV–vis Spectroelectrochemistry and Quantum Chemical Calculations

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>h</i>]­quinolines and 8-substituted 9-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>f</i>]­quinolines with R₇, R₈ = H, COOC₂H₅, COOCH₃, COOH, COCH₃, and CN has been studied by in situ EPR and EPR/UV–vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from <i>N</i>-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV–vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations

Marked Stabilization of Redox States and Enhanced Catalytic Activity in Galactose Oxidase Models Based on Transition Metal <i>S</i>‑Methylisothiosemicarbazonates with −SR Group in Ortho Position to the Phenolic Oxygen

Reactions of 5-<i>tert</i>-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde <i>S</i>-methylisothiosemicarbazone and 5-<i>tert</i>-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde <i>S</i>-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M­(acac)₂ as template source at 107 °C afforded metal complexes of the type M^IIL¹ and M^IIL², where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H₂L¹) and/or thiophenyl (H₂L²) group in the ortho position of the phenolic moiety. Demetalation of NiL¹ in CHCl₃ with HCl­(g) afforded H₂L¹. The latter reacts with Zn­(OAc)₂·2H₂O with formation of ZnL¹. The effect of −SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML²]⁺ (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV–vis–NIR measurements. In solution, the oxidized CuL² and NiL² display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL² complex shows reversible reduction, reduction of NiL², CuL¹, and NiL¹ is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the ³[CuL²]⁺ and ²[NiL²]⁺ oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL²/air/TEMPO and the CuL²/TBHP/MW­(microwave-assisted) oxidation systems

Marked Stabilization of Redox States and Enhanced Catalytic Activity in Galactose Oxidase Models Based on Transition Metal <i>S</i>‑Methylisothiosemicarbazonates with −SR Group in Ortho Position to the Phenolic Oxygen

Reactions of 5-<i>tert</i>-butyl-2-hydroxy-3-methylsulfanylbenzaldehyde <i>S</i>-methylisothiosemicarbazone and 5-<i>tert</i>-butyl-2-hydroxy-3-phenylsulfanylbenzaldehyde <i>S</i>-methylisothiosemicarbazone with pentane-2,4-dione (Hacac) and triethyl orthoformate in the presence of M­(acac)₂ as template source at 107 °C afforded metal complexes of the type M^IIL¹ and M^IIL², where M = Ni and Cu, with a new Schiff base ligand with thiomethyl (H₂L¹) and/or thiophenyl (H₂L²) group in the ortho position of the phenolic moiety. Demetalation of NiL¹ in CHCl₃ with HCl­(g) afforded H₂L¹. The latter reacts with Zn­(OAc)₂·2H₂O with formation of ZnL¹. The effect of −SR groups and metal ion identity on stabilization of phenoxyl radicals generated electrochemically was studied in detail. A marked stabilization of phenoxyl radical was observed in one-electron-oxidized complexes [ML²]⁺ (M = Ni, Cu) at room temperature, as demonstrated by cyclic voltammetry, EPR spectroscopy, and UV–vis–NIR measurements. In solution, the oxidized CuL² and NiL² display intense low-energy NIR transitions consistent with their classification as metal-delocalized phenoxyl radical species. While the CuL² complex shows reversible reduction, reduction of NiL², CuL¹, and NiL¹ is irreversible. EPR measurements in conjunction with density functional theory calculations provided insights into the extent of electron delocalization as well as spin density in different redox states. The experimental room temperature spectroelectrochemical data can be reliably interpreted with the ³[CuL²]⁺ and ²[NiL²]⁺ oxidation ground states. The catalytic activity of synthesized complexes in the selective oxidations of alcohols has been studied as well. The remarkable efficiency is evident from the high yields of carbonyl products when employing both the CuL²/air/TEMPO and the CuL²/TBHP/MW­(microwave-assisted) oxidation systems

Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane

Six dinuclear vanadium­(V) complexes have been synthesized: NH₄[(VO₂)₂(^HLH)] (NH₄[<b>1</b>]), NH₄[(VO₂)₂(^{<i>t</i>‑Bu}LH)] (NH₄[<b>2</b>]), NH₄[(VO₂)₂(^ClLH)] (NH₄[<b>3</b>]), [(VO₂)­(VO)­(^HLH)­(CH₃O)] (<b>4</b>), [(VO₂)­(VO)­(^{<i>t</i>‑Bu}LH)­(C₂H₅O)] (<b>5</b>), and [(VO₂)­(VO)­(^ClLH)­(CH₃O)­(CH₃OH/H₂O)] (<b>6</b>) (where ^HLH₄ = 1,5-bis­(2-hydroxybenzaldehyde)­carbohydrazone, ^{<i>t</i>‑Bu}LH₄ = 1,5-bis­(3,5-di-<i>tert</i>-butyl-2-hydroxybenzaldehyde)­carbohydrazone, and ^ClLH₄ = 1,5-bis­(3,5-dichloro-2-hydroxybenzaldehyde)­carbohydrazone). The structures of NH₄[<b>1</b>] and <b>4</b>–<b>6</b> have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit −N–N–. In two pockets of NH₄[<b>1</b>], two identical VO₂⁺ entities are present, whereas, in those of <b>4</b>–<b>6</b>, two different VO₂⁺ and VO³⁺ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO³⁺ in <b>4</b>–<b>6</b>, charge density measurements on <b>4</b>, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH₄[<b>1</b>], NH₄[<b>2</b>], and <b>4</b>–<b>6</b> were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with <i>tert</i>-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst

Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane

Six dinuclear vanadium­(V) complexes have been synthesized: NH₄[(VO₂)₂(^HLH)] (NH₄[<b>1</b>]), NH₄[(VO₂)₂(^{<i>t</i>‑Bu}LH)] (NH₄[<b>2</b>]), NH₄[(VO₂)₂(^ClLH)] (NH₄[<b>3</b>]), [(VO₂)­(VO)­(^HLH)­(CH₃O)] (<b>4</b>), [(VO₂)­(VO)­(^{<i>t</i>‑Bu}LH)­(C₂H₅O)] (<b>5</b>), and [(VO₂)­(VO)­(^ClLH)­(CH₃O)­(CH₃OH/H₂O)] (<b>6</b>) (where ^HLH₄ = 1,5-bis­(2-hydroxybenzaldehyde)­carbohydrazone, ^{<i>t</i>‑Bu}LH₄ = 1,5-bis­(3,5-di-<i>tert</i>-butyl-2-hydroxybenzaldehyde)­carbohydrazone, and ^ClLH₄ = 1,5-bis­(3,5-dichloro-2-hydroxybenzaldehyde)­carbohydrazone). The structures of NH₄[<b>1</b>] and <b>4</b>–<b>6</b> have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit −N–N–. In two pockets of NH₄[<b>1</b>], two identical VO₂⁺ entities are present, whereas, in those of <b>4</b>–<b>6</b>, two different VO₂⁺ and VO³⁺ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO³⁺ in <b>4</b>–<b>6</b>, charge density measurements on <b>4</b>, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH₄[<b>1</b>], NH₄[<b>2</b>], and <b>4</b>–<b>6</b> were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with <i>tert</i>-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst