nucleophilic fragment (e.g., hydride) from substrate molecules. Applications of
acceptor pincer ligands as cooperative catalysts are discussed.
Keywords Acceptor ligands · Ambiphilic ligands · Bond activation · Cooperative
catalysis · Metal-ligand cooperation · Pincer · π-Ligands
1 Introduction
The great successes of homogeneous catalysts in terms of stability, activity, and
selectivity can be attributed to one’s ability to precisely tune the properties of a
transition metal (TM) center by means of ligand design. Traditionally, supporting
ligands have been thought of as spectator ligands whose role was to tune the
properties of a transition metal and thereby facilitate metal-centered bond activation
of substrates. This paradigm is currently challenged by systems displaying metalligand cooperative reactivity, including (1) ligands facilitating bifunctional substrate
activation [1–6], (2) redox-active ligands [7–14], and (3) ligands showing hemilabile
coordination behavior [15–20]. Here, bond activation and/or bond-forming events
involve strong interplay of the metal center and the cooperative ligand, facilitating
reaction pathways that would be less accessible by using conventional homogenous
catalyst. A prominent early example of catalysts incorporating bifunctional ligands is
the BINAP/diamine-Ru system, where the amine ligand functions as a proton relay
in the hydrogenation of ketones [21]. Since then, the metal amide/metal-amine
interconversion has become one of the preeminent concepts for metal-ligand cooperative systems and has led to many catalytic applications [22, 23]. More broadly,
ligands featuring a donor functional group that can transiently accept a proton or
another electrophilic fragment now occupy a place of choice in the toolbox of
synthetic chemists (and in the present volume).
More recently, cooperative ligands featuring an acceptor site for metal-ligand
cooperation are emerging as a fertile area of investigations [24–31]. Whereas the
classical description of coordination (Werner-type) and organometallic complexes
involves ligands donating electron density to the metal, it had long been recognized
that the bonding of many ligands (CO, olefins, and other π-ligands) could only be
accurately described by including a secondary interaction involving electrons
flowing from the metal to the ligand (π-backbonding). In the case of acceptor
ligands, this inverse electron flow becomes the dominant bonding interaction: they
feature an accessible empty orbital which forms a – generally weak – metal-ligand
interaction by effectively withdrawing electron density from the transition metal. On
the basis of the symmetry of the accepting orbital, a distinction between σ- or
π-acceptor ligands can be made.
Acceptor ligands offer opportunities for unusual, cooperative bond activation
pathways (Fig. 1). For instance, the accessible empty orbital of an acceptor ligand
can act as a hydride relay in the bifunctional activation of E–H bonds or, more
26
M. R. Tiddens and M.-E. Moret
acceptor pincer ligands as cooperative catalysts are discussed.
Keywords Acceptor ligands · Ambiphilic ligands · Bond activation · Cooperative
catalysis · Metal-ligand cooperation · Pincer · π-Ligands
1 Introduction
The great successes of homogeneous catalysts in terms of stability, activity, and
selectivity can be attributed to one’s ability to precisely tune the properties of a
transition metal (TM) center by means of ligand design. Traditionally, supporting
ligands have been thought of as spectator ligands whose role was to tune the
properties of a transition metal and thereby facilitate metal-centered bond activation
of substrates. This paradigm is currently challenged by systems displaying metalligand cooperative reactivity, including (1) ligands facilitating bifunctional substrate
activation [1–6], (2) redox-active ligands [7–14], and (3) ligands showing hemilabile
coordination behavior [15–20]. Here, bond activation and/or bond-forming events
involve strong interplay of the metal center and the cooperative ligand, facilitating
reaction pathways that would be less accessible by using conventional homogenous
catalyst. A prominent early example of catalysts incorporating bifunctional ligands is
the BINAP/diamine-Ru system, where the amine ligand functions as a proton relay
in the hydrogenation of ketones [21]. Since then, the metal amide/metal-amine
interconversion has become one of the preeminent concepts for metal-ligand cooperative systems and has led to many catalytic applications [22, 23]. More broadly,
ligands featuring a donor functional group that can transiently accept a proton or
another electrophilic fragment now occupy a place of choice in the toolbox of
synthetic chemists (and in the present volume).
More recently, cooperative ligands featuring an acceptor site for metal-ligand
cooperation are emerging as a fertile area of investigations [24–31]. Whereas the
classical description of coordination (Werner-type) and organometallic complexes
involves ligands donating electron density to the metal, it had long been recognized
that the bonding of many ligands (CO, olefins, and other π-ligands) could only be
accurately described by including a secondary interaction involving electrons
flowing from the metal to the ligand (π-backbonding). In the case of acceptor
ligands, this inverse electron flow becomes the dominant bonding interaction: they
feature an accessible empty orbital which forms a – generally weak – metal-ligand
interaction by effectively withdrawing electron density from the transition metal. On
the basis of the symmetry of the accepting orbital, a distinction between σ- or
π-acceptor ligands can be made.
Acceptor ligands offer opportunities for unusual, cooperative bond activation
pathways (Fig. 1). For instance, the accessible empty orbital of an acceptor ligand
can act as a hydride relay in the bifunctional activation of E–H bonds or, more
26
M. R. Tiddens and M.-E. Moret