Synthesis and reactions of nickel(iii) complexes

Ligand-Dominated Activation of CO2 and CS2 by the Putative Nickel Phosphiniminato Intermediates

Room-temperature photoactivation of the first- and second-generation PN3P-pincer nickel azido complexes 1a and 1b in the presence of CO2 or CS2 afforded N-bound carbamates, dithiocarbamates, and isothiocyanates, providing insights into CO2 and CS2 activation and demonstrating how a seemingly small difference in the ligand structure significantly influences the reactivity. Theoretical calculations disclosed that the charge of the phosphorus atom plays a critical role in determining the nitrogen atom transfer to form a plausible nickel phosphiniminato intermediate.

Determination of thiram residues in fresh water using flow injection diperiodatonickelate(IV)-quinine chemiluminescence detection

This study developed a simple flow injection (FI) method based on diperiodatonickelate(IV)-sulfuric acid reaction using chemiluminescence (CL) detection for the determination of thiram (THI) fungicide in fresh water using quinine as the sensitizer. The possible mechanism of the CL reaction was described using UV-Vis. absorption and CL spectra. Experimental variables were optimized by applying a univariate approach, and a linear calibration curve was obtained in the range of 1.0 × 10^-3 -2.0 mg L^-1 (R²  = 0.9994, n = 9) with a limit of detection of 5.0 × 10^-4  mg L^-1 (S/N = 3) and an injection throughput of 200 h^-1 . This approach was successfully applied to determine THI in fresh water by using solid-phase extraction and achieved a good recovery rate of 94%-110% with a relative standard deviation of 1.9%-3.7% (n = 4). The results obtained were compared with the reported FI-CL and high-performance liquid chromatography-ultraviolet methods, and the three methods did not differ significantly at the 95% confidence limit.

C-H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e^- oxidation of its Ni(III) derivatives (1-OH and 1-NO₃). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong π-covalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C-H bond strength in the range of 76-92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp³ C-H bonds.

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

Preparation and Characterisation of a Bis-μ-Hydroxo-NiIII 2 Complex

Hydroxide-bridged high-valent oxidants have been implicated as the active oxidants in methane monooxygenases and other oxidases that employ bimetallic clusters in their active site. To understand the properties of such species, bis-μ-hydroxo-Ni^II ₂ complex (1) supported by a new dicarboxamidate ligand (N,N'-bis(2,6-dimethyl-phenyl)-2,2-dimethylmalonamide) was prepared. Complex 1 contained a diamond core made up of two Ni^II ions and two bridging hydroxide ligands. Titration of the 1 e^- oxidant (NH₄ )₂ [Ce^IV (NO₃ )₆ ] with 1 at -45 °C showed the formation of the high-valent species 2 and 3, containing Ni^II Ni^III and Ni^III ₂ diamond cores, respectively, maintaining the bis-μ-hydroxide core. Both complexes were characterised using electron paramagnetic resonance, X-ray absorption, and electronic absorption spectroscopies. Density functional theory computations supported the spectroscopic assignments. Oxidation reactivity studies showed that bis-μ-hydroxide-Ni^III ₂ 3 was capable of oxidizing substrates at -45 °C at rates greater than that of the most reactive bis-μ-oxo-Ni^III complexes reported to date.

Reactivity difference in the oxidative nucleophilic reaction of peroxonickel(iii) intermediates with open-chain and macrocyclic systems

The open-chain peroxonickel(iii) intermediate is much more reactive than the macrocyclic analogue in aldehyde deformylation.

Room Temperature Ni(II) Catalyzed Hydrophosphination and Cyclotrimerization of Alkynes

The catalytic activity of nickel complexes in hydrophosphination involving secondary phosphines is not a commonly studied transformation. Beyond a small number of stand-out examples, many reports in the literature focus on the use of simple nickel salts. β-Diketiminates have been proven to be incredibly effective ligands for catalysis using a range of metal centers. This synthetic study investigates the catalytic ability of a Ni(II) β-diketiminate complex in the hydrophosphination of alkenes and alkynes, with a serendipitous discovery of its ability to effect alkyne cyclotrimerization and phosphine dehydrocoupling.

A Ni(iii) complex stabilized by silica nanoparticles as an efficient nanoheterogeneous catalyst for oxidative C-H fluoroalkylation

We have developed Ni(III)-doped silica nanoparticles ([(bpy)xNi(III)]@SiO2) as a recyclable, low-leaching, and efficient oxidative functionalization nanocatalyst for aromatic C-H bonds. The catalyst is obtained by doping the complex [(bpy)3Ni(II)] on silica nanoparticles along with its subsequent electrooxidation to [(bpy)xNi(III)] without an additional oxidant. The coupling reaction of arenes with perfluoroheptanoic acid occurs with 100% conversion of reactants in a single step at room temperature under nanoheterogeneous conditions. The catalyst content is only 1% with respect to the substrates under electrochemical regeneration conditions. The catalyst can be easily separated from the reaction mixture and reused a minimum of five times. The results emphasize immobilization on the silica support and the electrochemical regeneration of Ni(III) complexes as a facile route for developing an efficient nanocatalyst for oxidative functionalization.

Tuning the Reactivity of Terminal Nickel(III)-Oxygen Adducts for C-H Bond Activation

Two metastable Ni^III complexes, [Ni^III(OAc)(L)] and [Ni^III(ONO₂)(L)] (L = N,N'-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate, OAc = acetate), were prepared, adding to the previously prepared [Ni^III(OCO₂H)(L)], with the purpose of probing the properties of terminal late-transition metal oxidants. These high-valent oxidants were prepared by the one-electron oxidation of their Ni^II precursors ([Ni^II(OAc)(L)]^- and [Ni^II(ONO₂)(L)]^-) with tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Fascinatingly, the reaction between any [Ni^II(X)(L)]^- and NaOCl/acetic acid (AcOH) or cerium ammonium nitrate ((NH₄)₂[Ce^IV(NO₃)₆], CAN), yielded [Ni^III(OAc)(L)] and [Ni^III(ONO₂)(L)], respectively. An array of spectroscopic characterizations (electronic absorption, electron paramagnetic resonance, X-ray absorption spectroscopies), electrochemical methods, and computational predictions (density functional theory) have been used to determine the structural, electronic, and magnetic properties of these highly reactive metastable oxidants. The Ni^III-oxidants proved competent in the oxidation of phenols (weak O-H bonds) and a series of hydrocarbon substrates (some with strong C-H bonds). Kinetic investigation of the reactions with di-tert-butylphenols showed a 15-fold enhanced reaction rate for [Ni^III(ONO₂)(L)] compared to [Ni^III(OCO₂H)(L)] and [Ni^III(OAc)(L)], demonstrating the effect of electron-deficiency of the O-ligand on oxidizing power. The oxidation of a series of hydrocarbons by [Ni^III(OAc)(L)] was further examined. A linear correlation between the rate constant and the bond dissociation energy of the C-H bonds in the substrates was indicative of a hydrogen atom transfer mechanism. The reaction rate with dihydroanthracene (k₂ = 8.1 M^-1 s^-1) compared favorably with the most reactive high-valent metal-oxidants, and showcases the exceptional reactivity of late transition metal-oxygen adducts.

Characterization and reactivity of a terminal nickel(III)-oxygen adduct

High-valent terminal metal-oxygen adducts are hypothesized to be the potent oxidizing reactants in late transition metal oxidation catalysis. In particular, examples of high-valent terminal nickel-oxygen adducts are scarce, meaning there is a dearth in the understanding of such oxidants. A monoanionic Ni(II)-bicarbonate complex has been found to react in a 1:1 ratio with the one-electron oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate, yielding a thermally unstable intermediate in high yield (ca. 95%). Electronic absorption, electronic paramagnetic resonance, and X-ray absorption spectroscopies and density functional theory calculations confirm its description as a low-spin (S = 1/2), square planar Ni(III)-oxygen adduct. This rare example of a high-valent terminal nickel-oxygen complex performs oxidations of organic substrates, including 2,6-di-tert-butylphenol and triphenylphosphine, which are indicative of hydrogen atom abstraction and oxygen atom transfer reactivity, respectively.

Mononuclear nickel(II)-superoxo and nickel(III)-peroxo complexes bearing a common macrocyclic TMC ligand

Mononuclear metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are generated as key intermediates in the catalytic cycles of dioxygen activation by heme and non-heme metalloenzymes. We have shown recently that the geometric and electronic structure of the Ni-O2 core in [Ni(n-TMC)(O2)]+ (n = 12 and 14) varies depending on the ring size of the supporting TMC ligand. In this study, mononuclear Ni(II)-superoxo and Ni(III)-peroxo complexes bearing a common macrocylic 13-TMC ligand, such as [NiII(13-TMC)(O2)]+ and [NiIII(13-TMC)(O2)]+, were synthesized in the reaction of [NiII(13-TMC)(CH3CN)]2+ and H2O2 in the presence of tetramethylammonium hydroxide (TMAH) and triethylamine (TEA), respectively. The Ni(II)-superoxo and Ni(III)-peroxo complexes bearing the common 13-TMC ligand were successfully characterized by various spectroscopic methods, X-ray crystallography, and DFT calculations. Based on the combined experimental and theoretical studies, we conclude that the superoxo ligand in [NiII(13-TMC)(O2)]+ is bound in an end-on fashion to the nickel(II) center, whereas the peroxo ligand in [NiIII(13-TMC)(O2)]+ is bound in a side-on fashion to the nickel(III) center. Reactivity studies performed with the Ni(II)-superoxo and Ni(III)-peroxo complexes toward organic substrates reveal that the former possesses an electrophilic character, whereas the latter is an active oxidant in nucleophilic reaction.

Shuttling of nickel oxidation states in N4S2 coordination geometry versus donor strength of tridentate N2S donor ligands

Seven bis-Ni(II) complexes of a N(2)S donor set ligand have been synthesized and examined for their ability to stabilize Ni(0), Ni(I), Ni(II) and Ni(III) oxidation states. Compounds 1-5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine ((X)L1), where X = 6-H, 6-Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine form (L2); compound 7 is the amide analog (L3). The compounds are perchlorate salts except for 7, which is neutral. Complexes 1 and 3-7 have been structurally characterized. Their coordination geometry is distorted octahedral. In the case of 6, the tridentate ligand coordinates in a facial manner, whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of (X)L1, the Ni-N(py) distances for 1~5 < 3 < 4 increase and UV-vis λ(max) values corresponding to the (3)A(2g)(F)→(3)T(2g)(F) transition show an increasing trend 1~5 < 2 < 3 < 4. Cyclic voltammetry of 1-5 reveals two quasi-reversible reduction waves that correspond to Ni(II)→Ni(I) and Ni(I)→Ni(0) reduction. The E(1/2) for the Ni(II)/Ni(I) couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1 by amine 6 or amide 7 N donors reveals that complex 6 in CH(3)CN exhibits an irreversible reductive response at E(pc) = -1.28 V, E(pa) = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E(1/2) = +0.84 V (ΔE(p) = 60 mV) that corresponds to Ni(II)→Ni(III). The electrochemically generated Ni(III) species, [(L3)(2)Ni(III)](+) is stable, showing a new UV-vis band at 470 nm. EPR measurements have also been carried out.

Geometric and electronic structure and reactivity of a mononuclear "side-on" nickel(III)-peroxo complex

Metal-dioxygen adducts, such as metal-superoxo and -peroxo species, are key intermediates often detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. The synthesis and spectroscopic characterization of an end-on nickel(II)-superoxo complex with a 14-membered macrocyclic ligand was reported previously. Here we report the isolation, spectroscopic characterization, and high-resolution crystal structure of a mononuclear side-on nickel(III)-peroxo complex with a 12-membered macrocyclic ligand, [Ni(12-TMC)(O(2))](+) (1) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Different from the end-on Ni(II)-superoxo complex, the Ni(III)-peroxo complex is not reactive in electrophilic reactions, but is capable of conducting nucleophilic reactions. The Ni(III)-peroxo complex transfers the bound dioxygen to manganese(II) complexes, thus affording the corresponding nickel(II) and manganese(III)-peroxo complexes. The present results demonstrate the significance of supporting ligands in tuning the geometric and electronic structures and reactivities of metal-O(2) intermediates that have been shown to have biological as well as synthetic usefulness in biomimetic reactions.

Electron exchange columns through entrapment of a nickel cyclam in a sol-gel matrix

An electron exchange column (analogous to ion exchange columns) was developed using the unique redox properties of the nickel-tetraazamacrocyclic complexes (nickel cyclam [Ni(II)L(1)](2+)) and nickel-trans-III-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, ([Ni(II)L(2)](2+)), and the physical and chemical stability of the ceramic materials using the sol-gel process to entrap the complexes. The entrapment by the biphasic sol-gel method is based on non-covalent bonds between the matrix and the complex; therefore the main problem was leaching. Parameters controlling the leaching were investigated. Redox cycles with the reducing agent ascorbic acid, and persulfate as the oxidizing agent were performed.

Electronic structure of the members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) and [NiL(X)](n) (X = Cl, CO, P(OCH(3))(3)) species containing a tetradentate, redox-noninnocent, Schiff base macrocyclic ligand L: an experimental and density functional theoretical study

The electronic structure of the four members of the electron transfer series [NiL](z) (z = 3+, 2+, 1+, 0) have been established experimentally (EPR spectroscopy and X-ray crystallography) and by density functional theoretical (DFT) calculations using the B3LYP functional in conjunction with a conductor-like screening model (COSMO) for acetonitrile solvent effects. L represents a generic designation of the tetradentate macrocycle 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]-heptadeca-1(17),2,11,13,15-pentane where the true oxidation level is not specified; (L(Ox))(0) represents its neutral form, (L )(1-) is the one-electron reduced pi radical anion, and (L(Red))(2-) is the singlet (or triplet) diradical dianion of this ligand. It is shown that the above series consists of square planar [Ni(III)(L(Ox))](3+) (S = 1/2), [Ni(II)(L(Ox))](2+) (S = 0), [Ni(II)(L )](1+) (S = 1/2), [Ni(II)(L(Red))](0) (S = 0). The structure of [Ni(II)(L(Red))](0) has been determined by X-ray crystallography. The electrochemistry of [Ni(II)(L(Ox))](PF(6))(2) in the presence of hard chloride anions shows the presence of trans-[Ni(III)(L(Ox))Cl(2)](+), the EPR spectrum of which has been recorded and calculated, and of trans-[Ni(II)(L(Ox))Cl(2)](0) (S = 1). Upon further reduction the coordinated Cl(-) ligands dissociate and [Ni(II)(L )](1+) and [Ni(II)(L(Red))](0) are successively generated. Similarly, in the presence of good pi-acceptor ligands such as CO or P(OCH(3))(3) the following five-coordinate, square base pyramidal species are found to be stable: [Ni(I)(L(Ox))(X)](1+) (S = 1/2), [Ni(I)(L )(X)](0) (S = 0, 1) (X = CO, P(OCH(3))(3)). As shown by EPR spectroscopy in the work of J. Lewis and M. Schröder, J. Chem. Soc., Dalton Trans., 1982, 1085, the monocations consist of a central nickel(i) ion (d(9), S(Ni) = 1/2). These spectra have been faithfully reproduced by the calculations. The neutral complexes [Ni(I)(L )(X)](0) are singlet or triplet diradicals comprising a central nickel(i) ion and a pi radical anion (L )(1-). Interestingly, six-coordinate species trans-[Ni(L)(X)(2)](n) (n = 2+, 1+, 0) are computationally not stable in the gas phase or in solution. No experimental evidence has been found for their existence.

Homoleptic organoderivatives of high-valent nickel(III)

Homoleptic perhalophenyl derivatives of divalent nickel complexes with the general formula [NBu(4)](2)[Ni(II)(C(6)X(5))(4)] [X=F (1), Cl (2)] have been prepared by low-temperature treatment of the halo-complex precursor [NBu(4)](2)[NiBr(4)] with the corresponding organolithium reagent LiC(6)X(5). Compounds 1 and 2 are electrochemically related by reversible one-electron exchange processes with the corresponding organometallate(III) compounds [NBu(4)][Ni(III)(C(6)X(5))(4)] [X=F (3), Cl (4)]. The potentials of the [Ni(III)(C(6)X(5))(4)](-)/[Ni(II)(C(6)X(5))(4)](2-) couples are +0.07 and -0.11 V for X=F or Cl, respectively. Compounds 3 and 4 have also been prepared and isolated in good yield by chemical oxidation of 1 or 2 with bromine or the amminium salt [N(C(6)H(4)Br-4)(3)][SbCl(6)]. The [Ni(III)(C(6)X(5))(4)](-) species have SP-4 structures in the salts 3 and 4, as established by single-crystal X-ray diffraction methods. The [Ni(II)(C(6)F(5))(4)](2-) ion in the parent compound 1 has also been found to exhibit a rather similar SP-4 structure. According to their SP-4 geometry, the Ni(III) compounds (d(7)) behave as S=1/2 systems both at microscopic (EPR) and macroscopic levels (ac and dc magnetization measurements). The spin Hamiltonian parameters obtained from the analysis of the magnetic behavior of 3 and 4 within the framework of ligand field theory show that the unpaired electron is centered mainly on the metal atom, with >97 % estimated d(z(2) ) contribution. Thermal decomposition of 3 and 4 proceeds with formation of the corresponding C(6)X(5)--C(6)X(5) coupling compounds.