Ligand-Centered Redox in Nickel(II) Complexes of 2‑(Arylazo)pyridine and Isolation of 2‑Pyridyl-Substituted Triaryl Hydrazines via Catalytic N‑Arylation of Azo-Function

A series of nickel complexes of 2-(arylazo)­pyridine
have been synthesized,
and the precise structure and stoichiometry of the complexes are controlled
by the use of different metal precursors. Molecular and electronic
structures of the isolated complexes are scrutinized thoroughly by
various spectroscopic techniques, single crystal X-ray crystallography,
and density functional theory (DFT). Two different classes of Ni­(II)
complexes are identified where the ligands bind as neutral or anion
radicals in the respective metal complexes. These are shown to be
chemically interconvertible, and their characterization confirmed
that the redox series is entirely ligand-centered without affecting
the bivalent oxidation state of the metal ion. An efficient method
of Ni­(II) catalyzed N-arylation of 2-(arylazo)­pyridine substrates
has been elaborated. The chemical reactions have led to isolation
of strongly fluorescent 2-pyridyl-substituted hydrazine derivatives,
which have been characterized thoroughly. Three-dimensional X-ray
structure of a hydrazine molecule, 2-(2-(naphthalen-1-yl)-2-phenylhydrazinyl)­pyridine,
is reported. Isolated hydrazines satisfy all the prerequisites of
an ideal dye with moderate absorptive property, large Stokes shift,
high quantum yields, and high photostability.