new mode of reversible de-aromatization/aromatization reactions of the (pyridine)
pincer-metal platform. This finding opened novel routes to catalytic bond activation
of otherwise unreactive small molecules such as methane, carbon dioxide, and N 2 O.
In much of this chemistry the pincer-metal grouping of the catalytic species forms a
template on which the substrate is activated and subsequently transferred into
product with regeneration of the pincer catalyst. Recent developments are discussed
in the chapter “Recent Advances in the Applications of Metal-Ligand Cooperation
via Dearomatization and Aromatization of Pincer Complexes.”
The exceptional stability of PCP- and PNP-pincer ligands has inspired
researchers in the field to expand the range of central units to more weakly binding
and/or reactive units such as amides (R 2 N–M), phosphide (R 2 P–M), boranes
(R 3 B<–M), side-on bound alkenes, and ketones. In such systems, the strong outer
phosphorus-metal bonds not only stabilize the central units but also keep them in the
coordination sphere of the metal after reactions that cleave or weaken the central
bond. In this way, new reactions can be harvested for the activation of small
molecules and in catalysis.
Recent developments along these lines are presented in the chapters “MetalLigand Cooperation at Acceptor Pincer Ligands” and “Metal-Ligand Cooperativity
of Phosphorus Containing Pincer Systems” of the current volume. In a related vein,
incorporating a carbon-based pincer motif in a porphyrinoid macrocycle can both
increase its robustness and unlock original transformations at the metal-carbon site,
as comprehensively discussed in the chapter “A Pincer Motif Etched into a MetaBenziporphyrin Frame.”
In a distinct but equally fertile approach reviewed in the chapter “Redox-Active
Pincer Ligands,” the kinetic stability of the pincer platform affords compounds, in
which the organic ligand is able to reversibly accept and release electrons in
chemical reactions while preserving the integrity of the molecule. Such redox-active
pincer ligands profoundly alter the reaction manifold accessible to the bound metal
atom by facilitating unusual redox steps. First-row transition metals can become
prone to two-electrons bond-breaking or bond-forming reactions, and noble metals
engage in controlled radical chemistry.
The large range of metal–ligand cooperative processes accessible to pincer
compounds, in combination with their general robustness, is key to the development
of high-performance catalysts. The kinetic stability of these compounds is also wellsuited for earth abundant and environmentally benign first-row transition metals
such as iron and manganese, which tend to easily release ligands of lower denticity.
It is therefore unsurprising that they now play a major role in the development of new
catalytic transformations with improved atom economy and lower environmental
impact. Perhaps the most prominent class of reactions in this respect are those
involving dihydrogen as a reactant or product, which are the topic of two chapters
of the current volume and incidentally appear in others. Chapter “The Role of MetalLigand Cooperation in Manganese(I)-Catalyzed Hydrogenation/Dehydrogenation
Reactions” focuses on the recent emergence of manganese-based catalysts for both
hydrogenation and dehydrogenation reactions, and the chapter “Hydrogenation
vi
Preface
pincer-metal platform. This finding opened novel routes to catalytic bond activation
of otherwise unreactive small molecules such as methane, carbon dioxide, and N 2 O.
In much of this chemistry the pincer-metal grouping of the catalytic species forms a
template on which the substrate is activated and subsequently transferred into
product with regeneration of the pincer catalyst. Recent developments are discussed
in the chapter “Recent Advances in the Applications of Metal-Ligand Cooperation
via Dearomatization and Aromatization of Pincer Complexes.”
The exceptional stability of PCP- and PNP-pincer ligands has inspired
researchers in the field to expand the range of central units to more weakly binding
and/or reactive units such as amides (R 2 N–M), phosphide (R 2 P–M), boranes
(R 3 B<–M), side-on bound alkenes, and ketones. In such systems, the strong outer
phosphorus-metal bonds not only stabilize the central units but also keep them in the
coordination sphere of the metal after reactions that cleave or weaken the central
bond. In this way, new reactions can be harvested for the activation of small
molecules and in catalysis.
Recent developments along these lines are presented in the chapters “MetalLigand Cooperation at Acceptor Pincer Ligands” and “Metal-Ligand Cooperativity
of Phosphorus Containing Pincer Systems” of the current volume. In a related vein,
incorporating a carbon-based pincer motif in a porphyrinoid macrocycle can both
increase its robustness and unlock original transformations at the metal-carbon site,
as comprehensively discussed in the chapter “A Pincer Motif Etched into a MetaBenziporphyrin Frame.”
In a distinct but equally fertile approach reviewed in the chapter “Redox-Active
Pincer Ligands,” the kinetic stability of the pincer platform affords compounds, in
which the organic ligand is able to reversibly accept and release electrons in
chemical reactions while preserving the integrity of the molecule. Such redox-active
pincer ligands profoundly alter the reaction manifold accessible to the bound metal
atom by facilitating unusual redox steps. First-row transition metals can become
prone to two-electrons bond-breaking or bond-forming reactions, and noble metals
engage in controlled radical chemistry.
The large range of metal–ligand cooperative processes accessible to pincer
compounds, in combination with their general robustness, is key to the development
of high-performance catalysts. The kinetic stability of these compounds is also wellsuited for earth abundant and environmentally benign first-row transition metals
such as iron and manganese, which tend to easily release ligands of lower denticity.
It is therefore unsurprising that they now play a major role in the development of new
catalytic transformations with improved atom economy and lower environmental
impact. Perhaps the most prominent class of reactions in this respect are those
involving dihydrogen as a reactant or product, which are the topic of two chapters
of the current volume and incidentally appear in others. Chapter “The Role of MetalLigand Cooperation in Manganese(I)-Catalyzed Hydrogenation/Dehydrogenation
Reactions” focuses on the recent emergence of manganese-based catalysts for both
hydrogenation and dehydrogenation reactions, and the chapter “Hydrogenation
vi
Preface