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

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

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

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

π‑Extended and Curved Antiaromatic Polycyclic Hydrocarbons

Synthesis of antiaromatic polycyclic hydrocarbons (PHs) is challenging because the high energy of their highest occupied molecular orbital and low energy of their lowest unoccupied molecular orbital cause them to be reactive and unstable. In this work, two large antiaromatic acene analogues, namely, cyclopenta­[<i>pqr</i>]­indeno­[2,1,7-<i>ijk</i>]­tetraphene (CIT, <b>1a</b>) and cyclopenta­[<i>pqr</i>]­indeno­[7,1,2-<i>cde</i>]­picene (CIP, <b>1b</b>), as well as a curved antiaromatic molecule with 48 π-electrons, dibenzo­[<i>a</i>,<i>c</i>]­diindeno­[7,1,2-<i>fgh</i>:7′,1′,2′-<i>mno</i>]­phenanthro­[9,10-<i>k</i>]­tetraphene (DPT, <b>1c</b>), are synthesized on the basis of the corona of indeno­[1,2-<i>b</i>]­fluorene. These three antiaromatic PHs possess a narrow energy gap down to 1.55 eV and exhibit high kinetic stability under ambient conditions. Moreover, these compounds display reversible electron transfer processes in both the cathodic and anodic regimes. Their cation and anion radicals are characterized by in situ vis–NIR absorption and electron paramagnetic resonance spectroelectrochemistry. The X-ray crystallographic analysis confirms that while CIP and CIT manifest planar structures, DPT shows a curved π-conjugated carbon skeleton. The synthetic strategy starting from <i>ortho</i>-substituted benzene units to construct five-membered rings in this work provides a unique entry to novel pentagon-embedding or curved antiaromatic polycyclic hydrocarbons. In addition, besides the detailed chemical and physical investigations, microscale single-crystal fiber field-effect transistors were also fabricated