acceptor ligand interaction. Besides the different symmetry of the accepting orbital, a
significant difference between σ- and π-acceptors is the necessary presence, in the
latter, of a (weakly) donating π orbital that contributes to the bonding to the transition
metal. As a consequence, σ-acceptor motifs are likely more electron-withdrawing
than π-acceptors and hence may have a stronger impact on the electronic properties
(e.g., Lewis acidity) of the transition metal. On the other hand, π-acceptors open a
wide spectrum of opportunities for metal-ligand cooperation due to the accessibility
of related structures such as vinyl- (C¼C) and carbene species (C ¼ E, E ¼ C, N, O)
and to the presence of additional lone pairs (π-ketone and π-imine).
In recent years, the field of acceptor pincer ligands has expanded rapidly from
reports on unique coordination behavior and studies on stoichiometric cooperative
reactivity to catalytic examples demonstrating the value and potential of the different
classes of cooperative acceptor ligands for homogeneous catalysis. The “inverted”
polarity of the metal-acceptor interaction manifests itself in two main classes of
cooperative processes.
First, the accessible empty orbital can reversibly accept electron density
(hemilability), increasing the range of electronic structures accessible to a given
transition metal center. This adaptive coordination can be expanded further with the
participation of neighboring groups, such as boron-bound aromatic residues or with
alternative (donor) binding modes such as the η
1 (N) mode for imines. This flexibility
allows both σ- and π-acceptor ligands to stabilize transition metal centers in a range
of formal oxidation states and possibly facilitate (formal) redox processes. In
particular, the hemilabile coordination behavior of a π-acceptor ketone motif
(L3) has an accelerating effect in the alkyne cyclotrimerization reaction catalyzed
by L3Ni
0 (BPI) [100, 101].
Second, the empty orbital can reversibly accept a nucleophilic fragment, which is
frequently a hydride (bifunctional activity). Distinct reactivity pathways occur upon
hydride uptake by either σ- or π-acceptor ligands: the former often acts as hydride
acceptor by hydride insertion to form a bridging L1-H-TM motif, while the latter
undergoes β-hydride insertion to reduce the π-bond. Both of these processes have
been observed in the stoichiometric activation of H–H and E–H (E ¼ Si, C, N, O,
etc.) bonds. Furthermore, such hydride insertions are a crucial step in two examples
of cooperative catalysis discussed in this chapter, namely, the catalytic
hydrodechlorination of (hetero)aryl chlorides by the σ-acceptor pincer complex
L1Pd
0 (PPh 3 ) [70] and reductive deoxygenation of amine and pyridine N-oxides
catalyzed by the π-acceptor complex [L3Rh
I (PPh 3 )][BAr
F
4 ] [107]. In this way,
both cooperative hydride uptake mechanisms have their specific impact. Future work
exploiting the ability of acceptor ligands to transiently accept a hydride or other
nucleophiles for substrate activation and catalysis is eagerly awaited.
In general, the correlation between acceptor pincer ligand coordination and
cooperative (catalytic) reactivity of the metal complex constitutes an exciting area
for discovery of bond activation processes and catalytic reactions using metal-ligand
cooperation.
Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands
63
significant difference between σ- and π-acceptors is the necessary presence, in the
latter, of a (weakly) donating π orbital that contributes to the bonding to the transition
metal. As a consequence, σ-acceptor motifs are likely more electron-withdrawing
than π-acceptors and hence may have a stronger impact on the electronic properties
(e.g., Lewis acidity) of the transition metal. On the other hand, π-acceptors open a
wide spectrum of opportunities for metal-ligand cooperation due to the accessibility
of related structures such as vinyl- (C¼C) and carbene species (C ¼ E, E ¼ C, N, O)
and to the presence of additional lone pairs (π-ketone and π-imine).
In recent years, the field of acceptor pincer ligands has expanded rapidly from
reports on unique coordination behavior and studies on stoichiometric cooperative
reactivity to catalytic examples demonstrating the value and potential of the different
classes of cooperative acceptor ligands for homogeneous catalysis. The “inverted”
polarity of the metal-acceptor interaction manifests itself in two main classes of
cooperative processes.
First, the accessible empty orbital can reversibly accept electron density
(hemilability), increasing the range of electronic structures accessible to a given
transition metal center. This adaptive coordination can be expanded further with the
participation of neighboring groups, such as boron-bound aromatic residues or with
alternative (donor) binding modes such as the η
1 (N) mode for imines. This flexibility
allows both σ- and π-acceptor ligands to stabilize transition metal centers in a range
of formal oxidation states and possibly facilitate (formal) redox processes. In
particular, the hemilabile coordination behavior of a π-acceptor ketone motif
(L3) has an accelerating effect in the alkyne cyclotrimerization reaction catalyzed
by L3Ni
0 (BPI) [100, 101].
Second, the empty orbital can reversibly accept a nucleophilic fragment, which is
frequently a hydride (bifunctional activity). Distinct reactivity pathways occur upon
hydride uptake by either σ- or π-acceptor ligands: the former often acts as hydride
acceptor by hydride insertion to form a bridging L1-H-TM motif, while the latter
undergoes β-hydride insertion to reduce the π-bond. Both of these processes have
been observed in the stoichiometric activation of H–H and E–H (E ¼ Si, C, N, O,
etc.) bonds. Furthermore, such hydride insertions are a crucial step in two examples
of cooperative catalysis discussed in this chapter, namely, the catalytic
hydrodechlorination of (hetero)aryl chlorides by the σ-acceptor pincer complex
L1Pd
0 (PPh 3 ) [70] and reductive deoxygenation of amine and pyridine N-oxides
catalyzed by the π-acceptor complex [L3Rh
I (PPh 3 )][BAr
F
4 ] [107]. In this way,
both cooperative hydride uptake mechanisms have their specific impact. Future work
exploiting the ability of acceptor ligands to transiently accept a hydride or other
nucleophiles for substrate activation and catalysis is eagerly awaited.
In general, the correlation between acceptor pincer ligand coordination and
cooperative (catalytic) reactivity of the metal complex constitutes an exciting area
for discovery of bond activation processes and catalytic reactions using metal-ligand
cooperation.
Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands
63