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.

合成了一系列具有不同立体和电子性质的甲脒酸锌配合物，其中部分配合物的固态结构通过X射线晶体学得以确定。研究表明，具有对称且无阻碍配体的配合物倾向于通过NNCNN配体骨架的末端N原子与锌中心配位。立体或电子修饰可以克服这种偏好，从而形成固态结构，其中甲脒酸配体通过一个内部N原子与金属中心配位，形成五元环螯合物。在溶液中，这些化合物表现出涉及五元和六元环螯合物的动态平衡。所有化合物均呈现强烈的颜色，且配体取代模式对紫外-可见吸收光谱的影响得到了评估。此外，它们的循环伏安法报告显示，所有化合物均可能通过电化学还原至自由基阴离子（L2Zn–）和双阴离子（L2Zn2–）形式。在配体骨架平面上的无阻碍NAr取代基（Ar = Ph）位于配体骨架平面上，而引入立体需求较大的取代基（Ar = Mes）则有利于垂直取向，使得NMes基团不再与骨架共轭，导致吸收光谱的红移。通过在甲脒酸框架上引入给电子或吸电子取代基，L2Zn化合物系列中的氧化还原电位可以简单地在相对较宽的范围内（∼700 mV）进行调整。