Direct Base-Assisted C‒H Cyclonickelation of 6-Phenyl-2,2'-bipyridine

The organonickel complexes [Ni(Phbpy)X] (X = Br, OAc, CN) were obtained for the first time in a direct base-assisted arene C(sp2)-H cyclometalation reaction from the rather unreactive precursor materials NiX2 and HPhbpy (6-phenyl-2,2'-bipyridine) or from the versatile precursor [Ni(HPhbpy)Br2]2. Different from previously necessary C‒Br oxidative addition at Ni(0), an extended scan of reaction conditions allowed quantitative access to the title compound from Ni(II) on synthetically useful timescales through base-assisted C‒H activation in nonpolar media at elevated temperature. Optimisation of the reaction conditions (various bases, solvents, methods) identified 1:2 mixtures of acetate and carbonate as unrivalled synergetic base pairs in the optimum protocol that holds promise as a readily usable and easily tuneable access to a wide range of direct nickelation products. While for the base-assisted C‒H metalation of the noble metals Ru, Ir, Rh, or Pd, this acetate/carbonate method has been established for a few years, our study represents the leap into the world of the base metals of the 3d series

Synthesis, Structure and Reactivity of Cyclometalated Nickel(II) Complexes: A Review and Perspective

The importance of cyclometalated Ni(II) complex species assumed in catalytic processes related to C-X or C-H activation and functionalisation e.g. in C-C cross coupling reactions stands in stark contrast to the small number of investigations on the synthesis, reactivity, electrochemical and spectroscopic properties of cyclometalated complexes to model catalytically relevant species. This review reveals that the methods to synthesise cyclometalated Ni(II) complexes are rather weakly developed compared with corresponding Pd or Pt derivatives. Furthermore, cyclometalated ligands with their strongly sigma-donating carbanionic functions, their chelate effect and often good pi-accepting properties are also interesting from the general viewpoint of organometallic coordination chemistry and applications beyond catalysis. The recently reported cyclometalated complexes [Ni(Phbpy)(R)] with the carbanionic Phbpy(-) = 6-(phenyl-2-ide)-2,2'-bipyridine ligand (R = Br, CH3, CF3) were described in detail to demonstrate some of the specific properties of such cyclometalated complexes. Further ligand design might allow to synthesise cyclometalated Ni(II) complexes with very interesting properties such as efficient triplet luminescence. What has been achieved in this field with Pt(II) and Pd(II) might be transferred to Ni(II) in the future and some ideas are presented

Redox Series of Cyclometalated Nickel Complexes [Ni((R)Ph(R ')bpy)Br](+/0/-2-) (H-(R)Ph(R ')bpy = Substituted 6-Phenyl-2,2 '-bipyridine)

New organonickel complexes [Ni((R)Ph(R')bpy)Br] carrying various substituted derivatives of the tridentate-(CNN)-N-Lambda-N-Lambda ligand 6-(phenyl-2-ide)-2,2'-bipyridine (-Phbpy) were synthesized from the precursor [Ni(COD)(2)] (COD = 1,5-cyclooctadiene) and the protoligands (ligand precursors) Br-(R)Ph(R')bpy. Several synthetic routes for the protoligands were studied and compared. All new compounds have been analyzed and spectroscopically characterized. From several complexes crystal and molecular structures Br were obtained from XRD experiments. UV-vis absorption spectroscopy and detailed electrochemical measurements reveal the impact of the various substituents on the electronic structure of the complexes. Quantum chemical DFT calculations illustrate the composition of highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) and support the assignment of the single-electron reduction and oxidation products as bpy-localized ligand radical species and transient nickel(III) intermediates, respectively

Controlling Nuclearity and Stereochemistry in Vanadyl(V) and Mixed Valent V-IV/V-V Complexes of Oxido-Pincer Pyridine-2,6-dimethanol Ligands

The coordination chemistry of three oxido-pincer ligands 2,6-(HOCR2)(2)(pyridine) (H2L) based on 2,6-pyridinedimethanol [R = H (H(2)pydim), Me (H(2)pydip), Ph (H(2)pyphen)] towards vanadium(V) was explored. Reaction of NH4VO3 with the protoligands H2L gave the dinuclear complexes [(L)OV(mu-O)VO(L)]. Mononuclear anionic species [VO2(L)](-), which were isolated as alkaline metal salts were obtained from reactions of [VO(acac)(2)] (acac(-) = acetylacetonate) and H2L under basic conditions and addition of HCl to these species allowed to isolate the unprecedented oxido chlorido complexes [VOCl(L)] for pydip and pyphen. Cyclic voltammograms of the dinuclear [V2O3(L)(2)] and mononuclear [VOCl(L)] complexes show reversible V-V/V-IV reduction waves, while corresponding waves of the anionic [VO2(L)](-) are completely irreversible. The mixed-valent V-IV/V-V species [V2O3(L)(2)](center dot-) were characterized by EPR and UV/Vis spectroelectrochemistry revealing a delocalized system with a 15 line EPR spectrum and an intervalence charge transfer (IVCT) band for the bulky pyphen ligand but localized radicals in case of the pydim and pydip derivatives (8 line EPR, no IVCT). DFT calculated structures of the three derivatives show an V-O-V arrangement for [V2O3(pyphen)(2)](center dot-) of about 145 degrees ideally suited for delocalization, whereas for [V2O3(pydip)(2)](center dot-) an angle of 128 degrees was found

Nitrogen-Nitrogen Bond Formation via a Substrate-Bound Anion at a Mononuclear Nickel Platform

The nickel-C4F8 fragment coordinates an aminoaryl N-H ketimine to form a stable complex, which upon treatment with base and oxidant leads to an N-N bond forming reaction and the release of indazole product. A key and previously unidentified intermediate in the formation of the indazole was a diimine complex of nickel bearing significant charge on the aryl ring that initially contained the amine substituent. The C4F8 coligand was key for the redox transformation and for stabilization of the intermediate for characterization

Pyridine-2,6-Dicarboxylic Acid Esters (pydicR(2)) as O,N,O-Pincer Ligands in Cu-II Complexes

The pyridine-2,6-carboxylic esters pydicR(2) with R = Me or Ph form the unprecedented mononuclear Cu-II complexes [Cu(pydicR(2))Cl-3](-) in one-pot reactions starting from pyridine-2,6-carboxychloride pydicCl(2), Cu-II chloride, and NEt3 in MeOH or PhOH solution under non-aqueous conditions. The triethylammonium salts (HNEt3)[Cu(pydicR(2))Cl-3] were isolated. The methyl derivative could be crystallized to allow a XRD structure determination. Both structures were optimized using DFT calculations in various surroundings ranging from gas phase and the non-coordinating solvent CH2Cl2 to the weakly coordinating acetone and well-coordinating solvents acetonitrile (MeCN) or dimethylformamide (DMF), while detailed calculation showed the charge distribution, dipole moments, and HOMO-LUMO gap energies changing upon solvation. According to these calculations, the ion pairs and the anionic Cu-II complexes were stable, which shows only Cu-Cl bond elongation and weakening of the charge transfer between the anionic complex and the cation as solvents become polar. Synthesis attempts in the presence of water yielded the Cu-II complexes [Cu(pydic)(OH2)(2)](n) and [Cu(OH2)(6)][{Cu(pydic)}(2)(mu-Cl)(2)], which results from pydicCl(2) hydrolysis. Alternatively, the new pydic(IPh)(2) (IPh = 2-iodo-phenyl) ester ligand was synthesized and reacted with anhydrous CuCl2, which yields the new binuclear complex [{Cu(pydic(IPh)(2))Cl}(2)(mu-Cl)(2)]. EPR spectroscopy of the solid compounds reveals typical axial spectra in line with the observed and DFT calculated geometries. Cyclic voltammetry and UV-vis absorption spectroscopy in solution are in line with un-dissociated complex species [Cu(pydicR(2))Cl-3](-)

Heteroleptic Complexes of the Tridentate Pyridine-2,6-di-tetrazolate Ligand

The synthesis and spectroscopic characterisation of a series of heteroleptic complexes of the tridentate pyridine-2,6-di-(5-tetrazolate) (pydtz(2-)) with Co(II), Ni(II), Cu(II) and Zn(II) and pyridine (Py) and H2O as coligands is reported. Single crystal XRD data reveals the formation of octahedrally configured complexes [Co(pydtz)(H2O)(Py)(2)], [Co(pydtz)(H2O)(2)(Py)]2H(2)O, [Ni(pydtz)(Py)(3)]2Py, and [Zn(pydtz)(H2O)(Py)(2)]Py, and a markedly Jahn-Teller distorted octahedral structure for [Cu(pydtz)(H2O)(2)(Py)]. Magnetic measurements reveal a S = 3/2 high-spin configuration for the Co(II) complex [Co(pydtz)(H2O)(Py)(2)], an S = 1 ground state for the Ni(II) derivative and S = 1/2 Cu(II) ions for [Cu(pydtz)(H2O)(2)(Py)], the latter is supported by electron-paramagnetic-resonance spectroscopy. The crystalline materials are subject to severe corrosion as revealed by powder XRD and DSC-TG experiments. H2O and pyridine co-crystallisates and coligands are easily cleaved. At the same time DSC-TG reveal very high stability of the [M(pydtz)] fragments with exothermic decomposition at T > 300 degrees C. As a consequence, in pyridine solution in all cases the species [M(pydtz)(Py)(3)] are observed. Ultraviolet-visible light absorption spectroscopy reveals a strong ligand field procured by the pydtz(2-) ligand and electrochemical measurements suggest very strong sigma-donation but only weak -accepting abilities of pydtz(2-) compared to the isostructural terpy (2,2';6',2''-terpyridine) ligand

Nitrogen-Nitrogen Bond Formation via a Substrate-Bound Anion at a Mononuclear Nickel Platform

The nickel-C4F8 fragment coordinates an aminoaryl N-H ketimine to form a stable complex, which upon treatment with base and oxidant leads to an N-N bond forming reaction and the release of indazole product. A key and previously unidentified intermediate in the formation of the indazole was a diimine complex of nickel bearing significant charge on the aryl ring that initially contained the amine substituent. The C4F8 coligand was key for the redox transformation and for stabilization of the intermediate for characterization

[2 x 2] Molecular Grids of Ni(II) and Zn(II) with Redox-Active 1,4-Pyrazine-Bis(thiosemicarbazone) Ligands

Tetranuclear complexes [M-4(L-R)(4)] with M = Ni(II) or Zn(II), with a [2 x 2] grid-type structure, were assembled in good yields and purity from the easily accessible but unprecedented pyrazine-bridged bis(thiosemicarbazone) protoligands (ligand precursors) H2LR (1,4-pyrazine-2,5-bis(R-carbaldehyde-thiosemicarbazone); R = Me, Et, Pr-i, or Ph). The complexes were characterised in solution by NMR, MS, IR, and UV-Vis absorption spectroscopy and (spectro)electrochemical methods. HR-MS spectra unequivocally reveal that the tetranuclear species are very stable in solution and any measurements represent these species. Only at higher temperatures (fragmentation in solution: MS and in the solid: TG-DTA) or upon the addition of protons (acidic UV-Vis titrations) can the tetrameric entities be decomposed. Single crystal XRD measurement remained preliminary. Rapid loss of co-crystallised solvent molecules within the [2 x 2] grid-type structures resulted in crystals of very poor quality, but the results were qualitatively in line with spectroscopy, electrochemistry, and quantum chemical (DFT) calculations. IR and NMR spectroscopy point clearly to a thiolate coordination of dianionic (deprotonated) ligands. The electrochemistry reveals four electronically coupled and reversible one-electron reductions centred largely at the pyrazine bridges. EPR and UV-Vis spectroelectrochemical measurements in combination with DFT calculation support the assignment