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

• Published in: European journal of inorganic chemistry 2012-05, Vol.2012 (14), p.2444-2455
• Main Authors: Klein, Axel, Biewer, Christian, Hamacher, Claudia, Hurkes, Natascha, Perez Outeiral, Jessica, Paniagua, Erik Mora, Schmieder, Ann-Kathrin, Schüren, Andreas O., Burma, Prabhakara Rao, Ciszewski, James T., Vicic, David A.
• Format: Article
• Language: English
• Subjects: Electrochemistry; EPR spectroscopy; Nickel; Organometallics; Spectroelectrochemistry
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201101436

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. The 1,4‐pyrazine bridging unit connecting two organonickel(II) complex fragments in the binuclear organometallic nickel complexes [(μ‐NN){Ni(Mes)Br}2] {NN = 2,5‐bis[1‐(aryl)iminoethyl]pyrazine; Mes = 2,4,6‐trimethylphenyl} provides very low‐energy charge‐transfer absorptions (1.2 to 2 eV) and two rather facile and reversible one‐electron reductions (–0.8 and –1.7 V vs. FeCp20/+).