Formazanate Ligands as Structurally Versatile, Redox-Active Analogues of β‑Diketiminates in Zinc Chemistry

A range
of tetrahedral bis­(formazanate)zinc complexes with different steric
and electronic properties of the formazanate ligands were synthesized.
The solid-state structures for several of these were determined by
X-ray crystallography, which showed that complexes with symmetrical,
unhindered ligands prefer coordination to the zinc center via the
terminal N atoms of the NNCNN ligand backbone. Steric or electronic
modifications can override this preference and give rise to solid-state
structures in which the formazanate ligand forms a 5-membered chelate
by binding to the metal center via an internal N atom. In solution,
these compounds show dynamic equilibria that involve both 5- and 6-membered
chelates. All compounds are intensely colored, and the effect of the
ligand substitution pattern on the UV–vis absorption spectra
was evaluated. In addition, their cyclic voltammetry is reported,
which shows that all compounds may be electrochemically reduced to
radical anionic (L2Zn–) and dianionic
(L2Zn2–) forms. While unhindered NAr
substituents lie in the plane of the ligand backbone (Ar = Ph), the
introduction of sterically demanding substituents (Ar = Mes) favors
a perpendicular orientation in which the NMes group is no longer in
conjugation with the backbone, resulting in hypsochromic shifts in
the absorption spectra. The redox potentials in the series of L2Zn compounds may be altered in a straightforward manner over
a relatively wide range (∼700 mV) via the introduction of electron-donating
or -withdrawing substituents on the formazanate framework.