Mechanistic proposals for nickel-catalyzed coupling reactions often invoke five-coordinate alkyl- or aryl-bound Ni(II) and/or high-valent nickel(III) species, but because of their reactive nature, they have been difficult to study and fingerprint. In this work, we invoked the stabilizing properties of fluoroalkyl ligands to access such nickel species bearing ligands that are commonplace in organic coupling reactions. We show that five-coordinate Ni(II) complexes containing nickel-carbon bonds can readily be prepared given the appropriate precursor, and we also present evidence for the formation of Ni(III) species upon chemical and electrochemical oxidation of the five-coordinate complexes.

Binuclear organometallic nickel complexes of the type [(μ‐NN){Ni(Mes)Br}2] {NN = α‐diimine chelate ligand of the type 2,5‐bis[1‐(aryl)iminoethyl]pyrazine; Mes = mesityl = 2,4,6‐trimethylphenyl} have been prepared and characterised electrochemically and spectroscopically in detail. A combination of NMR spectroscopy and quantum chemical calculations allowed the assignment of stereoisomers and their relative stability. The long‐wavelength absorptions (600–1000 nm) assignable to charge‐transfer transitions reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low‐lying π* orbitals. The reversible reductive electrochemistry yields stable radical anionic complexes with mainly ligand‐centred spin density as shown by electron paramagnetic resonance (EPR) spectroscopy and UV/Vis spectroelectrochemistry of the free ligands and their nickel complexes in combination with DFT calculations. Preliminary investigations of the complexes as catalysts in Negishi cross‐coupling reactions gave promising results.

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Oxidation of the nickel complexes [(BOXAM)-Ni(R)] (HBOXAM = bis((4-isopropyl-4,5-dihydrooxazol-2-yl)phenyl)amine) with R = CF 3 , CH 3 , Cl leads to the corresponding radical cationic complexes, which rapidly undergo homolytic Ni−R bond splitting. The corresponding radicals R • can be generated by either anodic (electrochemical) or aerial oxidation and were detected by spin trapping experiments using N-tert-butyl-α-phenylnitrone (PBN). The splitting reaction was also monitored by UV−vis−near-IR absorption spectroelectrochemistry, showing the formation of the "byproduct" complex [(BOXAM)Ni(THF)] + in THF solution. Additionally, the undissociated complex radicals [(BOXAM)Ni(R)] •+ were studied by EPR and UV−vis−near-IR spectroelectrochemistry, revealing inter alia largely ligand-centered oxidation. DFT calculations on ground and oxidized states support this finding.

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me-, 3,5-Me-, 2,4,6-Me-, 2,6-Pr-, 3,5-(OMe)-, 2-NO-4-Me-, 4-NO-, 2-CF-, and 2-CF-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh)Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers. Possible intermediates for the isomerization were investigated by DFT calculations. All complexes were studied by multiple spectroscopic means, electrochemistry, and spectroelectrochemistry (for the reduction processes). The long-wavelength metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with the R substituent of the ligand and the cathodic electrochemical potentials to a far smaller degree. Both are almost invariable upon variation of X. All of this is in line with Ni-based and π*-based lowest unoccupied molecular orbitals (LUMOs). In line with the unsymmetric character of the N^N ligand, electrochemistry and MLCT transitions seem to not correspond to the same type of π* LUMO, making these PyMA ligands more interesting than the symmetric heteroaromatic polypyridine ligands such as 2,2'-bipyridine (bpy; N^N) and N,N-diaryl-substituted aliphatic α-diimines (N^N) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful.