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Nickel(II) Complexes with Tetra- and Pentadentate Aminopyridine Ligands:  Synthesis, Structure, Electrochemistry, and Reduction to Nickel(I) Species

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journal contribution
posted on 29.01.2002, 00:00 by Sergey V. Kryatov, B. Sharmila Mohanraj, Vladislav V. Tarasov, Olga P. Kryatova, Elena V. Rybak-Akimova, Bharathi Nuthakki, James F. Rusling, Richard J. Staples, Alexander Y. Nazarenko
A series of nickel(II) complexes with polydentate aminopyridine ligands N,N,N-tris-[2-(2‘-pyridyl)ethyl]ethane-1,2-diamine (L1), N,N,N-tris-[2-(2‘-pyridyl)ethyl]-N‘-methylethane-1,2-diamine (L2), and N,N-bis-[2-(2‘-pyridyl)ethyl]-N,N‘-dimethylethane-1,2-diamine (L3) were synthesized and characterized by elemental analysis and spectroscopic methods. Single-crystal X-ray diffraction studies showed that the Ni(II) ions have five-coordinate square-pyramidal geometry in [NiL2](ClO4)2, similar to that previously found in [NiL1](ClO4)2·CH3NO2 (Hoskins, B. F.; Whillans, F. D. J. Chem. Soc., Dalton Trans. 1975, 657), and square-planar geometry in [NiL3](ClO4)2. All three nickel(II) complexes are reduced by sodium borohydride or sodium amalgam in organic solvents to nickel(I) species, which were identified by highly anisotropic EPR spectra at 100 K:  g1 = 2.239, g2 = 2.199, and g3 = 2.025 for [NiL1]+; g = 2.324 and g = 2.079 for [NiL2]+ and [NiL3]+. Cyclic voltammetry of the nickel(II) complexes in acetonitrile exhibited reversible reduction waves at −1.01 V for [NiL1]2+, −0.91 V for [NiL2]2+, and −0.83 V for [NiL3]2+ versus SCE, potentials which are significantly less negative than those of most previously characterized Ni(II) complexes with nitrogen-only donor atoms. Complexes [NiL1]2+ and [NiL2]2+ showed high catalytic activity in the electroreduction of 1,2-trans-dibromocyclohexane to cyclohexene.