For the last few decades, coordination chemists have
seen many
ligands whose role pervades far beyond being a supporting ancillary.
The ambiguity in their electronic structure description and challenges
to the precise determination of a metal’s oxidation state,
when such ligands are coordinated with a metal, have sparked intense
debate. Owing to this issue, these ligands have been examined with
multiple spectroscopic techniques aided by high-level theoretical
calculations. Typically, difficulty in accurate electronic structure
determination stems from significant metal–ligand covalency
and hence strong electronic coupling between a metal and a ligand.
Such properties of a molecule lay the ideal groundwork for developing
catalysts that can be built on the mutual cooperation of both ligand
and associated metal in an active manner. In the last several years
the momentum has shifted to the application of such redox-active backbones
in catalysis. Redox-active ligands have had a tremendous and continually
growing impact on catalysis research. They can behave as a redox reservoir,
or they impact the process by changing the basicity of the metal by
effective substrate activation. Their utility spans over a number
of areas including small molecule activation, homogeneous catalysis,
carbon dioxide reduction, and hydrogen evolution reactions. Herein
we briefly review the progress of ligand-based redox reactions over
the last decade and highlight recent applications in catalysis research.
Some of the chosen examples fascinatingly demonstrate the prowess
of redox-active ligands in driving the chemistry in a preponderant
manner. Some of the challenges and future aspects are also discussed.
A class of potent zinc-based photocatalyst have been developed in this report, whose reducing properties are dependent on the ligand redox of the beta-dikitiminate backbone. Two of the molecules have...
Earth-abundant
and cheaper zinc-based organometallic molecules
as luminophores are drawing significant research attention for solid-state
lighting devices. In this paper, we report two air-stable zinc complexes,
where the zinc is coordinated to two sterically encumbered β-diketiminate
ligands in a tetrahedral geometry. In such a geometry, eight phenyl/aryl
rings from the ligand backbones are oriented in a propeller shape,
augmenting the restricted rotation of the putative rings. Such an
architecture harnesses aggregation-induced emission behavior with
an excellent solid-state emission property. The rigidity of these
molecules reduces the possibility of non-radiative transitions and
makes them excellent fluorescence emitters. Both molecules exhibit
electroluminescence (EL) in the yellowish-green region of the visible
spectrum. We have utilized these molecules as emitters to fabricate
multilayered organic light-emitting diode (OLED) devices. The emitter Zn-I in host m-MTDATA exhibits EL with a
maximum external quantum efficiency of 4.4%. Among the handful of
zinc-based OLEDs, the performance of this emitter is very commendable
with power and current efficacies of 15.2 lm W–1 and 12.1 cd A–1, respectively, along with a brightness
of 2426 cd m–2.
A photoactive Zinc β-diketiminate complex spans a wide redox window of 3.97 V at its excited state. Having a highly reducing excited-state potential, it generates electrophilic trifluoromethyl radical by the...
The formazan ligands have been investigated as redox noninnocent backbone for a long time. Despite its well-established behaviour as redox reservoir, demonstration of catalytic efficiency governed by redox noninnocence remains...
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