Binuclear Arylnickel Complexes with Bridging Bis(arylimino)-1,4-pyrazine Ligands

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.