Cationic Nickel Research Articles

Cationic α-Diimine Nickel and Palladium Complexes Incorporating Phenanthrene Substituents: Highly Active Ethylene Polymerization Catalysts and Mechanistic Studies of syn/anti Isomerization

α-Diimine palladium complexes incorporating phenanthryl- and 6,7-dimethylphenanthrylimino groups have been synthesized and characterized. The (diimine)PdMeCl complexes prepared from 2,3-butanedione...

Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers.

We present an extensive study of tetranuclear transition-metal cluster compounds M4(NPtBu3)4 and [M4(NPtBu3)4][B(C6F5)4] (M = Ni, Cu; tBu = tert-butyl), which feature low-coordinate metal centers and direct metal-metal orbital overlap. X-ray diffraction, electrochemical, magnetic, spectroscopic, and computational analysis elucidate the nature of the bonding interactions in these clusters and the impact of these interactions on the electronic and magnetic properties. Direct orbital overlap results in strongly coupled, large-spin ground states in the [Ni4(NPtBu3)4]+/0 clusters and fully delocalized, spin-correlated electrons. Correlated electronic structure calculations confirm the presence of ferromagnetic ground states that arise from direct exchange between magnetic orbitals, and, in the case of the neutral cluster, itinerant electron magnetism similar to that in metallic ferromagnets. The cationic nickel cluster also possesses large magnetic anisotropy exemplified by a large, positive axial zero-field splitting parameter of D = +7.95 or +9.2 cm-1, as determined by magnetometry or electron paramagnetic resonance spectroscopy, respectively. The [Ni4(NPtBu3)4]+ cluster is also the first molecule with easy-plane magnetic anisotropy to exhibit zero-field slow magnetic relaxation, and under a small applied field, it exhibits relaxation exclusively through an Orbach mechanism with a spin relaxation barrier of 16 cm-1. The S = 1/2 complex [Cu4(NPtBu3)4]+ exhibits slow magnetic relaxation via a Raman process on the millisecond time scale, supporting the presence of slow relaxation via an Orbach process in the nickel analogue. Overall, this work highlights the unique electronic and magnetic properties that can be realized in metal clusters featuring direct metal-metal orbital interactions between low-coordinate metal centers.

Cationic Nickel(II)-Catalyzed Hydrosilylation of Alkenes: Role of P, N-Type Ligand Scaffold on Selectivity and Reactivity

Seven structurally similar cationic nickel(II)–alkyl complexes were synthesized by using a series of P, N ligands, and their reactivity was explored in the hydrosilylation of alkenes. More electron...

Spectral, structural, and theoretical investigation of a cationic Nickel(II) complex with N4S2-donor Schiff-base ligand and perchlorate counterions

A reaction between a hexadentate pyridine-based ligand (L) [pyridine-2-ylmethylene][2-({2-[(2-{[pyridine-2-ylmethylene]amino}phenyl)thio]ethyl}thio)phenyl]amine with ) gave the complex of [NiL](ClO4)2.H2O. This synthesized complex has been characterized by a variety of physico-chemical techniques. Based on the structural data, the geometry of NiN4S2 cationic complex is slightly distorted-octahedral. The asymmetric unit consisted of one Schiff-base ligand (L), one Ni(II) ion, two disordered perchlorate counterions, and one water molecule. The crystal packing analysis also showed O—H⋯O and C—H⋯O hydrogen-bonding interactions form a three-dimensional (3D) supramolecular structure. Moreover, Hirshfeld surface analysis ratified the effective role of perchlorate anions and water molecules in the formation and stabilization of the crystal packing. Also, density functional theory calculations were used to analyze the electronic structure of the complex which gave us consistent results with the obtained experimental data for the complex.

Synthesis of highly branched oligoethylenes by air-stable N,N-indazole derivate methallyl Ni(II) complexes

Two new neutral N,N-indazole ligands bearing the carbonitrile functional group (L1 and L2) and two new air-stable cationic nickel methallyl complexes (C1 and C2) were prepared. These compounds were fully characterized by NMR, FT-IR and elemental analysis. In addition, compounds L1, L2 and C2 were analyzed using X-ray diffraction. The reactivity of complexes C1 and C2 toward ethylene was studied by using 5 equivalents of B(C6F5)3. At 12.5 bar of ethylene and 20 °C, complexes C1 and C2 could produce oligoethylenes with a moderate activity ∼ 700 kg of PE (mol of Ni)-1h−1. By increasing the temperature to 60 °C, the catalytic activity increased to ∼ 103 kg of PE (mol of Ni)-1h−1, and oligoethylenes were produced. The microstructure studied by 13C NMR revealed a broad distribution of branches, including methyl, ethyl, propyl, butyl and long branches. Remarkably, complexes C1 and C2 showed the presence sec-butyl branches, which are characteristic of hyperbranched microstructures. The GPC analysis result is consistent with the use of low-molecular-weight materials, and the PD1 was ∼ 1, which may indicate the presence of living behavior.

Synthesis and crystal structure of a 6-chloro-nicotinate salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4'-bi-pyridine.

A 6-chloro-nicotinate (6-Clnic) salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4'-bi-pyridine (4,4'-bpy), namely, catena-poly[[[tetra-aqua-nickel(II)]-μ-4,4'-bi-pyridine-κ2 N:N'] bis-(6-chloro-nicotinate) tetra-hydrate], {[Ni(C10H8N2)(H2O)4](C6H3ClNO2)2·4H2O} n or {[Ni(4,4'-bpy)(H2O)4](6-Clnic)2·4H2O} n , (1), was prepared by the reaction of nickel(II) sulfate hepta-hydrate, 6-chloro-nicotinic acid and 4,4'-bi-pyridine in a mixture of water and ethanol. The mol-ecular structure of 1 comprises a one-dimensional polymeric {[Ni(4,4'-bpy)(H2O)4]2+} n cation, two 6-chloro-nicotinate anions and four water mol-ecules of crystallization per repeating polymeric unit. The nickel(II) ion in the polymeric cation is octa-hedrally coordinated by four water mol-ecule O atoms and by two 4,4'-bi-pyridine N atoms in the trans position. The 4,4'-bi-pyridine ligands act as bridges and, thus, connect the symmetry-related nickel(II) ions into an infinite one-dimensional polymeric chain extending along the b-axis direction. In the extended structure of 1, the polymeric chains of {[Ni(4,4'-bpy)(H2O)4]2+} n , the 6-chloro-nicotinate anions and the water mol-ecules of crystallization are assembled into an infinite three-dimensional hydrogen-bonded network via strong O-H⋯O and O-H⋯N hydrogen bonds, leading to the formation of the representative hydrogen-bonded ring motifs: tetra-meric R 2 4(8) and R 4 4(10) loops, a dimeric R 2 2(8) loop and a penta-meric R 4 5(16) loop.

Oligomerization and polymerization of 5-ethylidene-2-norbornene by cationic palladium and nickel catalysts

Nickel- and palladium-based catalyst systems were developed to convert 5-ethylidene-2-norbornene (ENB) to oligomers and polymers with highly controllable molecular weights.

Efficient alkene hydrosilation with bis(8-quinolyl)phosphine (NPN) nickel catalysts. The dominant role of silyl-over hydrido-nickel catalytic intermediates.

A cationic nickel complex of the bis(8-quinolyl)(3,5-di-tert-butylphenoxy)phosphine (NPN) ligand, [(NPN)NiCl]+, is a precursor to efficient catalysts for the hydrosilation of alkenes with a variety of hydrosilanes under mild conditions and low catalyst loadings. DFT studies reveal the presence of two coupled catalytic cycles based on [(NPN)NiH]+ and [(NPN)NiSiR3]+ active species, with the latter being more efficient for producing the product. The preferred silyl-based catalysis is not due to a more facile insertion of alkene into the Ni–Si (vs. Ni–H) bond, but by consistent and efficient conversions of the hydride to the silyl complex.

Oxidative Formation and Redox Properties of 9‐Methylisocorroles

Cationic nickel(II), palladium(II), and copper(II) isocorrole complexes were obtained from linear tetrapyrrolic 2,2′‐bidipyrrin precursors by metal‐templated oxidative macrocyclization reactions using silver(I) tetrafluoroborate as the oxidant in 40–69 % yield. The new compounds were characterized by optical and resonance methods. Two products from overoxidation and attack of nucleophiles were identified by means of single‐crystal X‐ray diffraction, elucidating possible H atom transfer processes during and after the macrocyclization step. The redox behavior was studied by UV/Vis‐spectroelectrochemistry, and the complexes were found to show different stabilities of the electrogenerated radicals, depending on the nature of the central transition metal ion. A one‐electron reduction step at low potential is common for all species, but reversible only in the case of nickel, and an irreversible reaction of a copper derivative was found to produce copper corrole.

Isolated Cationic Organometallic Nickel(I) Arene Complexes

The NiI salt [Ni(cod)2]+[Al(ORF)4]– {RF = C(CF3)3} is the starting material for ligand exchange reactions with arenes (benzene, mesitylene and hexamethylbenzene) yielding cationic and purely organometallic 19 valence electron [Ni(cod)(arene)]+ complex salts. The reactions are accompanied by fast solution color changes from yellow to intensely green. Single crystal X‐ray structure analysis, EPR, UV/Vis and IR‐spectroscopic investigations confirm the formation of [Ni(cod)(C6Me6)][Al(ORF)4] (2) as well as [Ni(cod)(mes)][Al(ORF)4] (3, mes = mesitylene) with η6‐coordinated arenes and bidentate cod ligands. The reaction with benzene is plausible but remains less characterized. The energetics of these reactions were used to evaluate diverse DFT methods. 16 functionals with and without D3(BJ) dispersion correction failed to describe the reaction energetics correctly. However, correcting the basic reaction energies with a CCSD(T)/TZ→QZ approach and using further isodesmic reactions with inclusion of COSMO‐RS solvation gave reaction energetics in best agreement with all experimental observations.

Acetylene-Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite.

The selective hydrogenation of alkynes to alkenes is an important type of organic transformation with large-scale industrial applications. This transformation requires efficient catalysts with precise selectivity control, and palladium-based metallic catalysts are currently employed. Here we show that four-coordinated cationic nickel(II) confined in zeolite can efficiently catalyze the selective hydrogenation of acetylene to ethylene, a key process for trace acetylene removal prior to the polymerization process. Under optimized conditions, 100% acetylene conversion and an ethylene selectivity up to 97% are simultaneously achieved. This catalyst system also exhibits good stability and recyclability for potential applications. Spectroscopy investigations and density functional theory calculations reveal the heterolytic dissociation of hydrogen molecules and the importance of hydride and protons in the selective hydrogenation of acetylene to ethylene. This work provides an efficient strategy toward active and selective zeolite catalysts by utilizing the local electrostatic field within the zeolite confined space for small-molecule activation and by linking heterogeneous and homogeneous catalysis.

Cationic allyl nickel complexes containing N,O-donor labile ligands: Synthesis and molecular characterisation

Three 2-iminofuran ligands 2-(N-2,6-diisopropylphenylformimino)furan (L1), 2-(N-2,6-dimethylphenylformimino)furan (L2) and 2-(N-4-methoxyphenylformimino)furan (L3) were obtained by condensation reactions of the respective anilines with furfural.The η3-allyl Ni(II) cationic complexes 1–3 have been synthesised by reacting [Ni(η3-C3H5)(μ-Br)]2 with the corresponding ligands L1-L3, in the presence of TlPF6. All complexes have been characterised by FT-IR, NMR spectroscopy and elemental analysis. The molecular structure of complex 2 has been determined by single crystal X-Ray diffraction. Complexes 1 and 2 have displayed a mono bidentate chelation mode of the respective ligand, while complex 3 showed a bis monodentate coordination framework. The respective complexes were studied by DFT calculations, showing a tendency to form the bis monodentate species, the formation of complex 3 being clearly the most favoured one.

Cationic para-benzhydryl substituted α-diimine nickel catalyzed ethylene and 1-decene polymerizations via controllable chain-walking

A series of α-diimine nickel complexes bearing para-benzhydryl groups, {[(2,6-R2-4-CHPh2C6H2N = C)2Nap]NiBr2, Nap: 1,8-naphthdiyl, R = Me (C1); R = Et (C2); R = iPr (C3)}, were synthesized and characterized. Upon activation with AlEt2Cl, these para-benzhydryl substituted catalysts conducted the ethylene and 1-decene polymerizations via controlled chain-walking process. These nickel complexes exhibited good thermal stability and high activities (up to 106 g PE (mol Ni h)−1) towards ethylene polymerization, producing high molecular weight polyethylenes with a relatively low degree of branching and narrow polydispersities (Mw/Mn ≤ 2.01). The catalytic activities [(0.81–2.42) × 106 g PE (mol Ni·h)−1], polyethylene molecular weights [Mn: (7.9–29.3) × 104 g mol−1] and total branching densities (57–102/1000C) can be tuned over a wide range. These controlled branching degrees of the obtained polyethylenes were decreased with increasing the bulkiness of the ligand and decreasing the polymerization temperature. Polymerization of 1-decene using these cationic nickel complexes generated branched poly(1-decene)s (69–87/1000C) with high molecular weights and narrow polydispersities (Mw/Mn = 1.18–1.55).

Pentacoordinated, square pyramidal cationic PCP Ni(II) pincer complexes: ELF and QTAIM topological analyses of nickel–triflate interactions

A previous report introduced a new series of cationic nickel(II) complexes ligated by PCP-type pincer ligands featuring a charge-bearing imidazoliophosphine binding moiety and described their catalytic reactivities in hydroamination of nitriles into amidines. Solid-state characterization of the cationic acetonitrile adducts [(R-PIMIOCOP+)Ni(NCMe)(triflate)]+ (R-PIMIOCOP+ = κP,κC,κP-{2-(R2PO),6-(R2PC4H5N2)C6H3}; R = i-Pr, [1]+; Ph, [2]+) carried out in this follow-up study showed a distorted square pyramidal geometry and a Ni–triflate distance that was shorter than the sum of the Ni and O van der Waals radii, features suggestive of an unusual pentacoordination at the Ni(II) center. In contrast, the related aquo adduct [(i-Pr-PIMIOCOP+)Ni(OH2)(triflate)]+, [3]+, displayed a more conventional square planar geometry. Detailed structural comparisons and theoretical analyses conducted on these and related compounds have allowed a thorough examination of the Ni–triflate interactions in this family of complexes. Thus, topological analysis of the electron localization function (ELF) and quantum theory of atoms in molecules (QTAIM) showed that the Ni–triflate interaction is mostly ionic in nature, but has a weak covalence degree. The monosynaptic V(Ni) subvalence basin of nickel is indeed the ELF signature of the covalence degree of the ionic Ni–O bond, which can be quantified by the negative QTAIM energy density at the Ni–O bond critical point and by the absolute value of the ELF covariance 〈σ2(V(O), C(Ni))〉. The ionic character of the Ni–O bond is also reflected in an energy decomposition analysis, showing that this interaction is mostly electrostatic in nature. The computational analyses carried out on this family of complexes provide valuable insight into the character and relative strengths of various Ni–ligand interactions, and allow a number of useful conclusions, including the following: (1) significant Ni–anion interactions at the apical site are observed only with pincer-type ligands featuring at least one cationic imidazoliophosphine binding moiety; (2) these primarily electrostatic Ni–O interactions gain increasing covalence degree when different pincer backbone, co-ligand L, or counter-anions are introduced to enhance the electron deficiency of the Ni(II) center.

C–H Activation of Methane by Nickel–Methoxide Complexes: A Density Functional Theory Study

Density functional theory is used to analyze methane C–H activation by neutral and cationic nickel–methoxide complexes. This research seeks to identify strategies to reduce high barriers through evaluation of supporting ligand modifications, solvent polarity, overall charge of complex, metal identity, and counterion effects. A Ni–OMe pincer complex was substituted at the para positions of the phenyl and pyridine rings with different electron-donating and -withdrawing groups, and the results showed that resonance effects did not significantly change the ΔG⧧act and ΔGrxn compared to the reference complex, which was confirmed by frontier orbitals plots and Hammett graphs. The effect of solvent polarity is greater upon the thermodynamics more than transition state energies. Among modeled supporting ligands, bipyridine was the most promising. Overall, neutral complexes are calculated to have lower activation barriers than the cationic complexes. For both neutral and cationic complexes, the methane C–H activati...

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes.

The nickel PNP pincer complex (iPrPNP)NiPh (iPrPNP = κP,κN,κP-N(CH2CH2PiPr2)2) was prepared by reacting (iPrPNP)NiBr with PhMgCl or deprotonating [(iPrPNHP)NiPh]Y (iPrPNHP = κP,κN,κP-HN(CH2CH2PiPr2)2; Y = Br, PF6) with KOtBu. The byproducts of the PhMgCl reaction were identified as [(iPrPNHP)NiPh]Br and (iPrPNP′)NiPh (iPrPNP′ = κP,κN,κP-N(CH=CHPiPr2)(CH2CH2PiPr2)). The methyl analog (iPrPNP)NiMe was synthesized from the reaction of (iPrPNP)NiBr with MeLi, although it was contaminated with (iPrPNP′)NiMe due to ligand oxidation. Protonation of (iPrPNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(iPrPNHP)NiX]+ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl–, HO–, or MeO–). Protonation of the Ni–C bond was observed at room temperature with (iPrPNP)NiMe, whereas at 70 °C with (iPrPNP)NiPh, both resulting in [(iPrPNHP)NiCl]Cl as the final product. Protonation of (iPrPNP)NiBr was complicated by site exchange between Br– and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate (iPrPNP)NiPh and (iPrPNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (iPrPNP′)NiBr, (iPrPNP)NiPh, (iPrPNP′)NiPh, (iPrPNP)NiMe, [(iPrPNHP)NiPh]Y (Y = Br, PF6, BPh4), [(iPrPNHP)NiPh]2[NiCl4], [(iPrPNHP)NiMe]Y (Y = Cl, Br, BPh4), [(iPrPNHP)NiBr]Br, and [(iPrPNHP)NiCl]Cl, were characterized by X-ray crystallography.

Acceptorless Dehydrogenation of Hydrocarbons by Noble-Metal-Free Hybrid Catalyst System

A hybrid catalysis that comprises an acridinium photoredox catalyst, a thiophosphate organocatalyst, and a nickel catalyst-enabled acceptorless dehydrogenation of hydrocarbons is reported. The cationic nickel complex played a critical role in the reactivity. This is the first example of acceptorless dehydrogenation of hydrocarbons by base metal catalysis under mild reaction conditions of visible light irradiation at room temperature.

Redox-state dependent activation of silanes and ammonia with reverse polarity (PCcarbeneP)Ni complexes: electrophilic vs. nucleophilic carbenes.

A rigidified PCalkylP ligand allowed for the synthesis and characterization of cationic and radical PCCarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic. Alpha-hydride abstraction from a (PCalkylP)nickel halide complex readily leads to the cationic carbene complex, which furnishes the radical carbene complex by one electron reduction. The reactivity of these reverse polarity carbene complexes towards small molecules (H2, CO, CO2, R3SiH, NH3) reveals different modes of activation when compared to previously reported nucleophilic nickel carbene complexes, and a clear dependence on the redox state of the complex. For H2, CO and CO2, no reaction is observed, but silanes react via hydride transfer and formation of solvated silylium ions. Ammonia is activated in a novel way, wherein it coordinates the carbene carbon and is deprotonated to form a robust C-N bond. This is not only a rare example of ammonia activation by a first row transition metal but also evidence of the intermediacy of group 10 carbenes in direct C-N bond forming reactions.

Probing cluster surface morphology by cryo spectroscopy of N2 on cationic nickel clusters.

We present the cryogenic (26 K) IR spectra of selected [Nin(N2)m]+ (n = 5-20, m = 1 - mmax), which strongly reveal n- and m-dependent features in the N2 stretching region, in conjunction with density functional theory modeling of some of these findings. The observed spectral features allow us to refine the kinetic classification [cf. J. Mohrbach, S. Dillinger, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184304 (2017)] and to define four classes of structure related surface adsorption behavior: Class (1) of Ni6+, Ni13+, and Ni19+ are highly symmetrical clusters with all smooth surfaces of equally coordinated Ni atoms that entertain stepwise N2 adsorption up to stoichiometric N2:Nisurface saturation. Class (2) of Ni12+ and Ni18+ are highly symmetrical clusters minus one. Their relaxed smooth surfaces reorganize by enhanced N2 uptake toward some low coordinated Ni surface atoms with double N2 occupation. Class (3) of Ni5+ and Ni7+ through Ni11+ are small clusters of rough surfaces with low coordinated Ni surface atoms, and some reveal semi-internal Ni atoms of high next-neighbor coordination. Surface reorganization upon N2 uptake turns rough into rough surface by Ni atom migration and turns octahedral based structures into pentagonal bipyramidal structures. Class (4) of Ni14+ through Ni17+ and Ni20+ are large clusters with rough and smooth surface areas. They possess smooth icosahedral surfaces with some proximate capping atom(s) on one hemisphere of the icosahedron with the other one largely unaffected.

Probing cluster surface morphology by cryo kinetics of N2 on cationic nickel clusters.

We present the stepwise N2 adsorption kinetics of size selected Nin+ (n = 5-20) clusters at 26 K as obtained by a hybrid tandem ion trap instrument. Pseudo-first-order kinetic fits confirm consecutive adsorption steps without evidence of cluster isomers and up to adsorption limits, which scale with the cluster size. The reaction rates for the initial N2 adsorption increase smoothly with the cluster size and similar to hard sphere cluster modeling. The isothermal kinetics allow for the tentative elucidation of cluster surface morphologies and for their classification into highly symmetrical clusters with all smooth surfaces, small clusters with rough surfaces, and large clusters with partially rough and smooth surface areas. The parallel characterization of the vibrational spectroscopy of some cluster adsorbate complexes supports and refines the achieved conclusions and is published back to back with this contribution [S. Dillinger, J. Mohrbach, and G. Niedner-Schatteburg, J. Chem. Phys. 147, 184305 (2017)]. These two studies elucidate the adsorbate to cluster interaction, and they confirm and specify the sometimes considerable structural fluxionality of finite and curved metal surfaces in high detail. This work precedes further studies along the present lines of thought.