Pyridyl-N-Heterocyclic Carbene-Containing cis-[Ni(C₆F₅)₂(N^C)] Complexes─UV–Vis Absorption Spectroscopy and Electrochemistry

Two Ni(II) complexes, cis-[Ni(C6F5)2(IPy)] (1) and cis-[Ni(C6F5)2(PyImMe)] (2), containing the N^C coordinating pyridyl-N-heterocyclic carbene (NHC) ligands, IPy = 1,3-di(pyridin-2-yl)-imidazole-2-ylidene and PyImMe = 1-(pyridin-2-yl)-3-methyl-imidazole-2-ylidene, were synthesized and analyzed through elemental analysis, 1H and 19F NMR, and HR-ESI-MS(+) mass spectrometry. The diamagnetic complexes show square planar configurations around Ni(II) as shown by single-crystal X-ray diffraction. The geometries were used to benchmark density functional theory (DFT) calculated geometries, which were used for single point and time-dependent DFT (TD-DFT) calculations using the TPSSh functional. The DFT-calculated energy gaps between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of about 3 eV agree quite well with the experimental data from cyclic voltammetry with reductions at around -2.1 V and oxidations at around 0.7 V. The TD-DFT calculations model very well the observed UV-vis absorption spectra with characteristic long-wavelength absorptions in the range of 330-450 nm. Comparison with the previously reported [Ni(C6F5)2(bpy)] (bpy = 2,2'-bipyridine) reveals higher LUMO energies, blue-shifted absorptions, a far higher reactivity after one-electron reduction, and a similar reactivity after one-electron oxidation for the complexes 1 and 2 as can be expected from the character of the NHC moiety.