1 General Introduction
Traditionally, when considering the design of a metal complex for a specific desired
(stoichiometric or catalytic) transformation, the metal centre (with an oxidation state
commonly ranging from 0 to VI for 1st and 2nd row mid-to-late transition metals) is
the locus for chemical bond activation and follow-up reactivity, with one or more
ligands bound to this metal centre acting as spectators that stabilize, tune and/or
sterically restrict the coordination sphere around it in order to invoke selective
substrate coordination, activation and transformation. Redox-chemistry in these
complexes is typically confined to the metal centre, because the energy input
required to induce the transfer of an electron to (reduction) or from (oxidation) a
spectator, or ‘redox-inert’, ligand is much higher than the cost associated with
changing the oxidation state of the metal. Often, the ability of a metal to change
its oxidation state whilst mediating a chemical conversion is essential for catalytic
processes. Generally speaking, catalytic processes can be distinguished in reductive,
oxidative or redox-neutral processes. Many (industrially relevant) reactions, even
when formally redox-neutral, involve two-electron redox steps and are thus linked
to, e.g. 2nd and 3rd row (noble) transition metals. Base metals generally prefer to
undergo one-electron redox events, but controlled (‘metal-mediated’) odd-electron
pathways are still less commonly encountered, although there is a great interest in the
use of abundant, cheap and non-toxic materials. Moreover, the development of
unprecedented types of reactivity – which could facilitate or significantly shorten
the synthesis of highly desired molecules – is also sought after. Certain types of
ligands have shown to be able to do both by working in synergy with the metal and
thereby expanding upon a metal’s ‘common’ reactivity. Apart from ligand platforms
that actively participate in bond breaking or making processes (coined reactive
ligands or metal-ligand cooperative systems) [1–5], a second class of ligands can
be defined that show reversible redox-chemistry. Organic ligands that feature energetically well-accessible, low-lying π-donor (relevant for ‘oxidation’) or π*-acceptor
(key during ‘reduction’) orbitals can undergo reversible shuttling between at least
two well-defined redox states whilst being bound to a transition metal or main group
element (and commonly also in free form) [6–12]. Conjugation generally
strengthens the overall extend of this phenomenon, but this is not necessarily
required. When this redox-shuttling takes place without changes to the oxidation
state of the metal(loid), the ligand is termed redox-active (Fig. 1). In cases where the
redox event leads to a more diffuse and ambiguous overall electronic structure, due
to strong electronic coupling between a ligand and a metal centre, the term redox
noninnocence is generally deemed more appropriate [13, 14], although relatively
extensive crossover usage of the terms in the literature is encountered. Several types
of metalloenzymes exploit ligand-centred redox activity as a means to break down
multi-electron reactions into several single-electron steps, thereby avoiding high
energetic penalties (large overpotentials) and enabling the overall transformation
to occur near thermodynamic potential.
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J. I. van der Vlugt
Traditionally, when considering the design of a metal complex for a specific desired
(stoichiometric or catalytic) transformation, the metal centre (with an oxidation state
commonly ranging from 0 to VI for 1st and 2nd row mid-to-late transition metals) is
the locus for chemical bond activation and follow-up reactivity, with one or more
ligands bound to this metal centre acting as spectators that stabilize, tune and/or
sterically restrict the coordination sphere around it in order to invoke selective
substrate coordination, activation and transformation. Redox-chemistry in these
complexes is typically confined to the metal centre, because the energy input
required to induce the transfer of an electron to (reduction) or from (oxidation) a
spectator, or ‘redox-inert’, ligand is much higher than the cost associated with
changing the oxidation state of the metal. Often, the ability of a metal to change
its oxidation state whilst mediating a chemical conversion is essential for catalytic
processes. Generally speaking, catalytic processes can be distinguished in reductive,
oxidative or redox-neutral processes. Many (industrially relevant) reactions, even
when formally redox-neutral, involve two-electron redox steps and are thus linked
to, e.g. 2nd and 3rd row (noble) transition metals. Base metals generally prefer to
undergo one-electron redox events, but controlled (‘metal-mediated’) odd-electron
pathways are still less commonly encountered, although there is a great interest in the
use of abundant, cheap and non-toxic materials. Moreover, the development of
unprecedented types of reactivity – which could facilitate or significantly shorten
the synthesis of highly desired molecules – is also sought after. Certain types of
ligands have shown to be able to do both by working in synergy with the metal and
thereby expanding upon a metal’s ‘common’ reactivity. Apart from ligand platforms
that actively participate in bond breaking or making processes (coined reactive
ligands or metal-ligand cooperative systems) [1–5], a second class of ligands can
be defined that show reversible redox-chemistry. Organic ligands that feature energetically well-accessible, low-lying π-donor (relevant for ‘oxidation’) or π*-acceptor
(key during ‘reduction’) orbitals can undergo reversible shuttling between at least
two well-defined redox states whilst being bound to a transition metal or main group
element (and commonly also in free form) [6–12]. Conjugation generally
strengthens the overall extend of this phenomenon, but this is not necessarily
required. When this redox-shuttling takes place without changes to the oxidation
state of the metal(loid), the ligand is termed redox-active (Fig. 1). In cases where the
redox event leads to a more diffuse and ambiguous overall electronic structure, due
to strong electronic coupling between a ligand and a metal centre, the term redox
noninnocence is generally deemed more appropriate [13, 14], although relatively
extensive crossover usage of the terms in the literature is encountered. Several types
of metalloenzymes exploit ligand-centred redox activity as a means to break down
multi-electron reactions into several single-electron steps, thereby avoiding high
energetic penalties (large overpotentials) and enabling the overall transformation
to occur near thermodynamic potential.
136
J. I. van der Vlugt