Molecular and Electronic Structures of Complexes Containing 1‑(2-pyridylazo)-2-phenanthrol (PAPL): Revisiting a Redox-Active Ligand

Herein
we report the molecular structures and electronic properties of neutral,
homoleptic, six-coordinate complexes of the general formula M­(PAPL)2, where PAPL is the monoanion of 1-(2-pyridylazo)-2-phenanthrol
(M = Mn, Ni, Zn). Although, the coordination chemistry of PAPL has
been investigated in a few previous reports in the 1970s and 1980s,
there are, to our knowledge, no reported single crystal X-ray diffraction
studies of any complexes, or any other electronic property or computational
studies of complexes containing the PAPL anion until now. The electronic
structures of the complexes are probed with a combination of cyclic
voltammetry, UV–vis spectroscopy, and spectroelectrochemical
measurements. Density functional theory calculations support the redox-active
nature of the PAPL ligand. In all complexes we observe two reversible
ligand-centered reduction processes, suggesting it may be possible
to access the open-shell radical-anionic state of the ligand.