Ni(II), Pd(II) and Cu(II) coordination complexes of a π-extended dianionic N^N−^N−^N ligand: Synthesis, structures and electronic absorption spectroscopy

Two new tetradentate, dianionic N^N-^N-^N ligands combining amido and -extended phenanthridine (benzo[c]quinoline) donors are presented. The coordination chemistry of the ligands was explored via construction of complexes of divalent transition metal Ni(II), Cu(II), and Pd(II) ions. Single-crystal X-ray diffraction confirmed the expected distorted square-planar geometry for examples of each metal, consistent with both ligand-field expectations and the geometric restrictions of the ligand framework. The complexes absorb strongly in the visible region of the electromagnetic spectrum, with steady-state electronic absorption spectra containing a prominent low energy band attributed by time-dependent density functional theory (TD-DFT) to transitions of mixed ILCT/MLCT character, with contributions from dd transitions. Cyclic voltammetry and spectroelectrochemical experiments were used to probe the ability of the new ligand frameworks to support electron-transfer.