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

Unsymmetrical <i>N</i>‑Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2′-Bipyridine and <i>N</i>,<i>N</i>‑Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)­Ni­(Mes)­X] [R-PyMA = <i>N</i>-aryl-1-(pyridin-2-yl)­methanimine; aryl = phenyl, 2,6-Me₂-, 3,5-Me₂-, 2,4,6-Me₃-, 2,6-^<i>i</i>Pr₂-, 3,5-(OMe)₂-, 2-NO₂-4-Me-, 4-NO₂-, 2-CF₃-, and 2-CF₃-6-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 <i>trans</i>-[(PPh₃)₂Ni­(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_Py^N_methanimine 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_Py^N_Py) and <i>N</i>,<i>N</i>-diaryl-substituted aliphatic α-diimines (N_methanimine^N_methanimine) such as the diaza-1,3-butadienes (DAB). First attempts to use these complexes in Negishi-type cross-coupling reactions were successful