Nickel Center Research Articles

Modulating the electrocatalytic activity of mononuclear nickel complexes toward water oxidation by tertiary amine group.

Water oxidation is the bottleneck of water splitting, which is a promising strategy for hydrogen production. Therefore, it is significant to develop efficient water oxidation catalysts. Herein, electrochemical water oxidation catalyzed by three nickel complexes, namely [Ni(bptn)(H2O)](ClO4)2 (1), [Ni(mbptn)(CH3CN)](ClO4)2 (2), and [Ni(tmbptn)(H2O)](ClO4)2 (3) (bptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane, mbptn = 5-methyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane, and tmbptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), is studied under near-neutral condition (pH 9.0). Meanwhile, the homogeneous catalytic behaviors of the three mononuclear nickel complexes were investigated and confirmed by scanning electron microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy and electrochemical method. Complex 1 stabilized by a pentadentate ligand with three N-H fragments homogeneously catalyzes water oxidation to oxygen with the lowest onset overpotential. Complex 2 stabilized by a similar ligand with two N-H groups and one N-CH3 group exhibits relatively higher onset overpotential but higher catalytic current and turnover frequency. However, complex 3 with three N-CH3 coordination environment shows the highest onset overpotential and the highest catalytic current at higher potential. Comparison of catalytic behaviors and ligand structure of the three complexes reveals that the methyl group on the polypyridine amine ligand affects the water oxidation activity of the complexes obviously. The electronic effect of N-CH3 coordination environment leads to higher redox potential of the metal center and potential demand for water oxidation, while it leads to higher reaction activity of high-valent intermediates, which account for higher catalytic current and efficiency of water oxidation. This work reveals that electrocatalytic water oxidation performance of nickel complexes can be finely modulated by constructing suitable N-CH3 coordination.

Spectral, thermal and DFT studies of novel nickel(II) complexes of 2-benzoylpyridine-N4-methyl-3- thiosemicarbazone: Crystal structure of a square planar azido-nickel(II) complex

Novel six Ni(II) complexes of an NNS donor 2-benzoylpyridine-N4-methyl-3- thiosemicarbazone(HL) are synthesized and characterized. Various physicochemical techniques are applied for the study of the coordination behavior of the thiosemicarbazone to the nickel center. In all the complexes, thiosemicarbazone is coordinated in the thiolate form. A four coordinated Ni(II) complex [NiLN3] is crystallized and its molecular and crystal structures are determined by single-crystal X-ray crystallography. Single-crystal XRD reveals that the complex got crystallized in the monoclinic space group P21/n and nickel(II) has a square planar environment. Intramolecular hydrogen bonding interactions make the complex more rigid and in the crystal lattice, the intermolecular hydrogen bonding interactions generate a supramolecular 1 D chain. The chemical reactivity behavior of the HL and six Ni(II) complexes was evaluated with the CAM-B3LYP quantum chemical calculations. The validity of electronic structure principles like Maximum Hardness, Minimum Polarizability, and Minimum Electrophilicity Principle in the study is discussed.

Binding of carbon monoxide at a single nickel center and its oxidative reactivity toward CO2 and O2

AbstractA nickel(0) monocarbonyl complex (1‐CO) supported by a neutral tridentate PP2 ligand reveals significant stability toward CO2, which is clearly different from its analogous nickel monocarbonyl species supported by an anionic PNP ligand. Although both nickel(0)‐CO species possess a similar distorted tetrahedral geometry, the nickel site in 1‐CO is well protected by three phosphorus donors, while (NP2)Ni(CO) possesses a nickel center having a site trans to the amide donor for effective CO2 binding. The current study shows that the structural preference of a tridentate ligand dictates nickel's reactivity toward CO2. Its oxygenation by O2(g) results in the formation of a nickel(II) carbonate species, which is followed by generating phosphine oxide.

One Pot Ultrasonic Assisted {[Ni(tptz)Cl(H2O)2][Ni(tptz)(H2O)3]}-3Cl.5H2O Complex Formation Using Triazine Ligand, XRD/HSA-Interactions, and Spectral and Thermal Investigation

Under room temperature ultrasonic conditions, mixing the neutral 2,4,6-tri(pyridin-2-yl)-1,3,5-triazine (tptz) ligand with Ni(II) salt resulted in the preparation of a unique {[Ni(tptz)Cl(H2O)2]-[Ni(tptz)(H2O)3]}3Cl.5H2O complex with two different nickel(II) centers in a high yield. The crystal structure of {[Ni(tptz)Cl(H2O)2][Ni(tptz)(H2O)3]}3Cl.5H2O revealed the existence of distorted octahedral around both Ni(II) centers. The complex structure was further supported by FT-IR, UV-Vis., CHN-EA, TGA and EDX. The computed HSA was also performed to support the XRD interaction types. The existence of the H2O molecules in the crystal lattice was examined by FT-IR and TG/DTG measurements that proved the presence of coordinated and uncoordinated water molecules. Moreover, the thermal stability of the desired complex was evaluated at open atmosphere via TG/DTG and showed stability up to ~400 °C and multistep thermal decomposition.

Dinickel Catalyzed Vinylidene-Alkene Cyclization Reactions.

A dinickel catalyst promotes reductive cyclization reactions of 1,1-dichloroalkenes containing pendant olefins. The reactions can be conducted with a Zn reductant or electrocatalytically using a carbon working electrode. Mechanistic studies are consistent with the intermediacy of a Ni2(vinylidene) species, which adds to the alkene and generates a metallacyclic intermediate. β-Hydride elimination followed by C-H reductive elimination forms the cyclization product. The proposed dinickel metallacycle is structurally characterized and its stoichiometric conversion to product is demonstrated. Spin polarized, unrestricted DFT calculations are used to further examine the cyclization mechanism. These computational models reveal that both nickel centers function cooperatively to mediate the key oxidative addition, migratory insertion, β-hydride elimination, and reductive elimination steps.

A functional model for quercetin 2,4-dioxygenase: Geometric and electronic structures and reactivity of a nickel(II) flavonolate complex

Quercetin 2,4-dioyxgenase (QueD) has been known to catalyze the oxygenative degradation of flavonoids and quercetin. Recent crystallographic study revealed a nickel ion occupies the active site as a co-factor to support O2 activation and catalysis. Herein, we report a nickel(II) flavonolate complex bearing a tridentate macrocyclic ligand, [NiII(Me3-TACN)(Fl)(NO3)](H2O) (1, Me3-TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, Fl = 3-hydroxyflavone) as a functional model for QueD. The flavonolatonickel(II) complex was characterized by using spectrometric analysis including UV–vis spectroscopy, electrospray ionization mass spectrometer (ESI-MS), infrared spectroscopy (FT-IR) and 1H nuclear magnetic resonance spectroscopy (NMR). The single crystal X-ray structure of 1 shows two isomers with respect to the direction of a flavonolate ligand. Two isomers commonly are in the octahedral geometry with a bidentate of flavonolate and a monodentate of nitrate as well as a tridentate binding of Me3-TACN ligand. The spin state of 1 is determined to be a triplet state based on the Evans' method. Interestingly, electronic configuration of 1 from density functional theory (DFT) calculations revealed that the two singly occupied molecular orbitals (SOMOs) lie energetically lower than the highest (doubly) occupied molecular orbital (HOMO), that is so-called the SOMO-HOMO level inversion (SHI). The HOMO shows an electron density localized in the flavonolate ligand, indicating that flavonolate ligand is oxidized first rather than the nickel center. Thermal degradation of 1 resulted in the formation of benzoic acid and salicylic acid, which is attributed to the oxygenation of flavonolate of 1.

Experimental Electrochemical Potentials of Nickel Complexes

AbstractNickel-catalyzed cross-coupling and photoredox catalytic reactions has found widespread utilities in organic synthesis. Redox processes are key intermediate steps in many catalytic cycles. As a result, it is pertinent to measure and document the redox potentials of various nickel species as precatalysts, catalysts, and intermediates. The redox potentials of a transition-metal complex are governed by its oxidation state, ligand, and the solvent environment. This article tabulates experimentally measured redox potentials of nickel complexes supported on common ligands under various conditions. This review article serves as a versatile tool to help synthetic organic and organometallic chemists evaluate the feasibility and kinetics of redox events occurring at the nickel center, when designing catalytic reactions and preparing nickel complexes.1 Introduction1.1 Scope1.2 Measurement of Formal Redox Potentials1.3 Redox Potentials in Nonaqueous Solution2 Redox Potentials of Nickel Complexes2.1 Redox Potentials of (Phosphine)Ni Complexes2.2 Redox Potentials of (Nitrogen)Ni Complexes2.3 Redox Potentials of (NHC)Ni Complexes

Two Ni-Substituted Trilacunary Keggin-Type Polyoxometalates: Syntheses, Crystal Structures, NLO Studies, and Magnetic Properties.

Two Ni-substituted polyoxometalates (NiSPs), [Ni6(Py)6(H2O)5(μ3-OH)3(PW9O34)]2·10H2O (1), [Ni7(Py)6(Im)(H2O)5O(WO4)(μ3-OH)3(H2PW9O34)]·3H2O (2) (Py = pyridine, Im = imidazole), were successfully hydrothermally synthesized. Compounds 1 and 2 have significantly different configurations by introducing different amounts of imidazole ligands. For compound 1, two malposed {Ni6(Py)6PW9} units that are face to face are bridged by two Ni-O-W bonds to constitute an isolated dimeric structure. Differently, the {Ni7(Py)6(Im)PW9}2 dimer in compound 2 connects with four adjacent dimers by four {WO4} groups in an interesting two-dimensional (2-D) arrangement. The magnetism of compounds 1 and 2 was studied, and magnetic test results demonstrated that both compounds have ferromagnetic interactions between the nickel centers. Meanwhile, the third-order nonlinear optical (NLO) measurements indicated that compound 1 can serve as potential nonlinear optical materials.

γ-NiOOH electrocatalyst derived from a nickel dithiooxamide chelate polymer for oxygen evolution reaction in alkaline solutions

The oxygen evolution reaction (OER) is a key process in number of electrochemical energy conversion technologies that demands the design of efficient electrocatalysts. In this work, catalytically active gamma-nickel oxyhydroxide (γ-NiOOH) was obtained from a two-dimensional (2D) nickel(II) dithiooxamide Ni(dto) chelate polymer via in situ electrochemical transformation in an acetylene black matrix. The transformation was carried out on the surface of a glassy carbon electrode in 1 M KOH solution, in which the nickel(II) centers of the chelate polymer were transformed into nano-sized γ-NiOOH crystallites dispersed in the carbon matrix. The resultant nanostructured catalyst exhibits OER overpotential of 390 mV (at 12 mA/cm2) and a Tafel slope of 328 mV/dec in 1 M KOH solution. The OER overpotential of the catalyst is lower than that of IrO2/C electrode, with the same material loading. The oxygen evolution is initiated by adsorption of hydroxide ions at the Ni metal sites of the layered γ-NiOOH, with a subsequent release of oxygen molecules. This study unfolds the potential of in situ electrochemical transformation of 3d transition metal 2D chelate polymers to fabricate active electrocatalysts.

Antimicrobial Activities of Co (III), Mono and Tri-nuclear Ni Complexes Containing Schiff base Functionalized Imidazolium based Ligands

We reported the antimicrobial activities of cobalt and nickel complexes containing imino-NHC ligands. Complex 2 was synthesized by direct reaction of the insitu generated free carbene from 2-[2-(3-benzylimidazol-1-yl)ethyliminomethyl]phenol ligand with NiCl2 diglyme while complexes 3-5 were previously reported as catalysts in the transfer hydrogenation reaction of ketones. The compounds 1-5 were screened for antimicrobial sensitivity test against four gram-negative bacteria Escherichia Coli (E-coli), Shigella, Klebsiella Pneumoniae (K. Pneumoniae) and Salmonella Typhi (S.Typhi) and a gram positive bacteria Staphylocossus aureus (S.aureus). At a varying concentrations of 100, 200, 300, 400 and 500 µg/mL, significant activities were recorded using disc diffusion methods. The cobalt complex 3 was found to have higher activities compared with the corresponding nickel complexes and among the three nickel complexes, nickel complex with pyridine as wingtip was found to be more active than the one with a benzyl group. Similarly, the nickel centre with mononuclear was found to be more active than the tri-nuclear nickel complex. Except for the cobalt complex 3 no activity was recorded against S. typhi for all the nickel compounds.

Asymmetric interfaces sandwiched between infinite-layer oxides and perovskite oxides

Asymmetric heterointerfaces that bridge two nonisostructural oxides provide valuable opportunities for novel emergent phenomena that may be unavailable for symmetric interfaces. Here we present a theoretical investigation on three different asymmetric interfaces consisting of the infinite-layer nickelate $\mathrm{LaNi}{\mathrm{O}}_{2}$ and the perovskite manganite $\mathrm{LaMn}{\mathrm{O}}_{3}$ (type A, B and C). An alternative crystal geometry, pyramid, is introduced when the planar-type $\mathrm{LaNi}{\mathrm{O}}_{2}$ and the $\mathrm{LaMn}{\mathrm{O}}_{3}$ are jointed at the interface, resulting in strong charge and orbital reconstruction. For type A interface, the magnetic moment per Mn ion has increased by 10% due to the replacement of $\mathrm{Mn}{\mathrm{O}}_{6}$ by $\mathrm{Mn}{\mathrm{O}}_{5}$. For type B interface, in contrast, the magnetic moment grew by 26% for the interfacial Ni ions due to the strong charge transfer between center nickel and apical oxygen. For type C interface, only slightly enhanced $\mathrm{Mn}{\mathrm{O}}_{6}$ distortions are observed and thus the change of charge and orbital occupancy are negligible. Our results demonstrated that an interface-selective orbital occupancy, where the Mn ${e}_{\mathrm{g}}$ orbital preferential occupation alternated from the out-of-plane ${\mathrm{d}}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ state at type A interface to nearly degenerate at type C interface and then to in-plane ${\mathrm{d}}_{{x}^{2}\ensuremath{-}{y}^{2}}$ state at type B interface. The values of relative change of Mn ${e}_{\mathrm{g}}$ orbital occupancy are 15%, 2%, and \ensuremath{-}21%, respectively. The values of relative change at type A and B interface are larger than that achieved by strain $(\ensuremath{\sim}5%)$ or symmetric interface design (10%). Therefore, interface reconstructions lead to unusual electronic properties, opening space for the advancement of oxide electronics.

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Tridentate NPN Ligands with a Central Secondary Phosphine Oxide Donor and their Corresponding Metal Complexes

AbstractThree new Ru complexes with N‐P(OH)‐N donor ligands were synthesized and characterized. A ruthenium complex with a ligand with quinolinyl side arms exhibited monodentate coordination through the phosphorus atom with an intramolecular POH⋅⋅⋅N(quinolinyl) hydrogen bond but with no interaction between nitrogen donors and the ruthenium center. Two pentamethylcyclopentadienyl ruthenium complexes with a ligand with pyridyl side arms exhibited fac‐κ3‐NPN’ tridentate coordination to the metal center, one with an interionic POH⋅⋅⋅FPF5 hydrogen bond in the crystal. A paramagnetic nickel [Ni3(N‐P(O)‐N)4] cluster was also characterized; the crystal structure shows three nickel centers in two different environments. The central nickel is coordinated to four deprotonated ligands, two are bidentate κ2‐NO, two are monodentate κ‐O and is connected to the other two nickel centers by two sets of bridging μ2‐O,P moieties. Non‐central nickel centers are each coordinated to one bidentate ligand κ2‐PN and one monodentate ligand κ‐P.

Benzosuberyl Substituents as a “Sandwich-like” Function in Olefin Polymerization Catalysis

For the rational design of metal catalyst in olefin polymerization catalysis, various strategies were applied to suppress the chain transfer by bulking up the axial positions of the metal center, among which the “sandwich” type turned out to be an efficient category in achieving high molecular weight polyolefin. In the α-diimine system, the “sandwich” type catalysts were built using the typical 8-aryl-naphthyl framework. In this contribution, by introducing the rotationally restrained benzosuberyl substituent into the ortho-position of N-aryl rings, a new class of “sandwich-like” α-diimine nickel catalysts was constructed and fully identified. The rotationally restrained benzosuberyl substituents played a “sandwich-like” function by capping the nickel center from two axial sites. Compared to the nickel catalyst Ni1 bearing freely rotated benzhydryl substituent, Ni2 featuring benzosuberyl substituent enabled the increase (8 times) of polymer molecular weights from 8 kDa to 65 kDa in the polymerization of ethylene. By further increasing the steric bulk of another ortho-site of the N-aryl ring, the polymer molecular weight even reached an ultrahigh level of 833 kDa (Mw=1857 kDa) using the optimized Ni3. Notably, these nickel catalysts could also mediate the copolymerization of ethylene with methyl 10-undecenoate, with Ni3 giving the highest copolymer molecular weight (88 kDa) and the highest incorporation of comonmer (2.0 mol%), along with high activity of up to 105 g·mol−1·h−1.

Nickel supported ZIF-8.PEG modified catalyst: A designed active catalyst with high H2 productivity in steam reforming of ethanol at moderate temperature

Steam reforming of renewable ethanol at moderate temperatures is an attractive alternative for hydrogen production than conventional steam reforming of natural gas which is only feasible at very high temperatures (>650 ℃). Formulating a catalyst for this reaction with ideally dispersed metallic active phase can reduce the activation energy and enhance the activity at moderate temperatures. With this purpose in mind, ZIF-8 with high specific surface area has been nominated as catalytic support in this survey for the dispersion of nickel nanoparticles. The support compositional and interfacial properties were further modified by polyethylene glycol (PEG) to prepare an ideal support with high activity at moderate temperatures. Pristine and modified samples were comparatively characterized by XRD, TGA, BET, FTIR, TEM, FESEM, EDS, AAS, H2-TPR and NH3-TPD analyses and catalytically evaluated at 400–450 ℃. TPR profiles of catalysts have demonstrated notable differences in the reduction temperature and quantity of metallic centers which have directly affected the catalytic activity. Lower reduction temperature and higher number of metallic centers were observed for Ni/ZIF-8. PEG which has originated from the tuned interface i.e. high accessibility of nickel centers with highly exposed flower-like structure and lower strength of support/active phase interaction. Thus, Ni/ZIF-8. PEG has revealed considerable improvement in terms of ethanol conversion and H2 yield. At 450 °C, the average H2 yield employing Ni/ZIF-8. PEG catalyst reached 52.6% which was about 16% and 46% higher than those observed by Ni/ZIF-8 and conventional Ni/Al2O3, respectively. Moderate acidity of the ZIF-8. PEG, higher reducibility of active phase and suitable interfacial interactions have resulted in better performance of this catalyst.

Apoptotic induction in K562 cell line by new water-soluble complexes of nickel(II) and zinc(II)

New water-soluble Zn(II) and Ni(II) complexes with three thiosemicarbazone ligands, modified by substitution (H, Me, Ph), H2L1Cl, H2L2Cl and H2L3Cl were synthesized through the reaction of the ligands with chloride salts of metals. All complexes were characterized by physico-chemical techniques. Elemental analysis and X-ray crystallography confirmed that metal to ligand ratio 1:2 in nickel(II) complexes and 1:1 in zinc(II) complexes. In the mononuclear Ni-H2L1Cl complex, the nickel(II) centre is surrounded by two ligands in distorted octahedral geometry. Both ligands are tridentate, nitrogen, sulfur, and oxygen donors (NSO), in thione form, with oxygen atom acting as phenolate in one of the ligands while in the other it coordinates as phenol. In the Zn-H2L1Cl complex, the ligand acts as monodentate through sulfur atom in thione form. Geometry around the zinc(II) is tetrahedral, with remaining positions occupied by three chloride anions, resulting in Cl3S environment around the metal. All of the complexes showed good solubility and stability in aqueous media. The cytotoxic effect of the complexes was measured using the MTT assay on MCF-7 and K562 cell lines, and the results showed that Zn-H2L2Cl had lower IC50 values compared to the other compounds. Flow cytometry analysis on the K562 cells showed the apoptosis cell death after treating with the Zn-H2L2Cl. Cell cycle studies confirmed the cell cycle arrest in sub-G1 phase for Zn-H2L2Cl.

Magnetic Anisotropy in a Cubane-like Ni4 Complex: An Ab Initio Perspective.

Magnetic anisotropy, in the absence of an external magnetic field, relates to the degeneracy lift of energy levels. In the standard case of transition metal complexes, this property is usually modeled by an anisotropic spin Hamiltonian and one speaks of "zero-field splitting" (ZFS) of spin states. While the case of mononuclear complexes has been extensively described by means of ab initio quantum mechanical calculations, the literature on polynuclear complexes studied with these methodologies is rather scarce. In this work, advanced multiconfigurational wave function theory methods are applied to compute the ZFS of the ground S = 4 state of an actual tetranickel(II) complex, displaying a magnet behavior below 0.5 K. First, the isotropic couplings are computed in the absence of the spin-orbit coupling operator, in the full complex and also in clusters with only two active nickel(II) centers, confirming the occurrence of weak ferromagnetic couplings in this system. Second, the single-site magnetic anisotropies are computed on a cluster bearing only one active nickel(II) site, showing that the single-site anisotropy axes are not oriented in an optimal fashion for generating a large uniaxial molecular anisotropy. Furthermore, the possibility for involving only a few local orbital excited states in the calculation is assessed, actually opening the way for a consistent and manageable treatment of the ZFS of the ground S = 4 state. Third, multiconfigurational calculations are performed on the full complex, confirming the weak uniaxial anisotropy occurring for this state and also, interestingly, revealing a significant contribution of the lowest-lying orbitally excited S = 3 states. Overall, by comparison with the experiment, the reported results question the common habit of using only one structure, in particular derived from a crystallography experiment, to compute magnetic anisotropy parameters.

Nickel catalysts promoted with lanthanum for ethanol steam reforming: Influence of support and treatment on activity

The effect of support nature on the catalytic performance of Ni-based catalysts for ethanol steam reforming (ESR) was investigated. Nickel catalysts supported on lanthanum-modified Al2O3, TiO2, Al2O3-TiO2 and Al-pillared bentonite (denoted as Clay) were prepared by impregnation, characterized by X-ray diffraction (XRD), N2 adsoption-desorption, IR spectroscopy of acetonitrile adsorption and thermogravimetric analysis (TGA), and evaluated at 500 °C under atmospheric pressure. Both Ni/La-Al2O3 and Ni/La-TiO2-Al2O3 exhibited the best stability, whereas Ni/La-TiO2 and Ni/La-Clay deactivated quickly but with similar or higher H2 production. Catalyst deactivation was caused by carbon deposition, which formation was related to strong Brønsted sites. Efficiency of lanthanum as inhibitor of carbon deposition depended on the support nature.Oxidative regeneration allowed recovering initial activity of all used catalysts and even improving the performance of the latter two catalysts. This unexpected improvement can be due to change of nickel centres nature, caused by local overheating during the deposited carbon combustion.

Synthesis of 1,2-bis(imidazolidin-2-imine)benzene nickel complexes and their application for norbornene (co)polymerization with styrene

A series of 1,2-bis(imidazolidin-2-imine)benzene nickel complexes with different substituents on the imidazolidin-2-imine or benzene ring was synthesized and characterized. The complexes exhibited a tetrahedral configuration, and Ni2 bearing a phenyl substituent on the imine ligand provided highest steric hinderance around nickel center with a buried volume percentage of 53.1%. The nickel complexes in the presence methylaluminoxane (MAO) were highly active for the addition polymerization of norbornene and exhibited good thermal stability, where Ni3 bearing fluorine substituent on the benzene ligand backbone showed highest activity of 1.68 × 106 g·mol−1·h−1 at 100 °C to produce high molecular weight polynorbornene (1.2 × 105 g·mol−1). Most importantly, the catalysts promoted the copolymerization of norbornene and styrene with high activity (～105 g·mol−1·h−1), high molecular weight (~104 g·mol−1), and tunable styrene content (up to 17 mol%) and glass transition temperature. The linear relationship between the glass transition temperature and styrene content indicated the formation of the uniform copolymers. A preference for the norbornene insertion in each catalytic system was investigated by the evaluation of the monomer reactivity ratios.

Transformation of Formazanate at Nickel(II) Centers to Give a Singly Reduced Nickel Complex with Azoiminate Radical Ligands and Its Reactivity toward Dioxygen.

The heteroleptic (formazanato)nickel bromide complex LNi(μ-Br)2NiL [LH = Mes-NH-N═C(p-tol)-N═N-Mes] has been prepared by deprotonation of LH with NaH followed by reaction with NiBr2(dme). Treatment of this complex with KC8 led to transformation of the formazanate into azoiminate ligands via N-N bond cleavage and the simultaneous release of aniline. At the same time, the potentially resulting intermediate complex L'2Ni [L' = HN═C(p-tol)-N═N-Mes] was reduced by one additional electron, which is delocalized across the π system and the metal center. The resulting reduced complex [L'2Ni]K(18-c-6) has a S = 1/2 ground state and a square-planar structure. It reacts with dioxygen via one-electron oxidation to give the complex L'2Ni, and the formation of superoxide was detected spectroscopically. If oxidizable substrates are present during this process, these are oxygenated/oxidized. Triphenylphosphine is converted to phosphine oxide, and hydrogen atoms are abstracted from TEMPO-H and phenols. In the case of cyclohexene, autoxidations are triggered, leading to the typical radical-chain-derived products of cyclohexene.