KO
t Bu, albeit not necessary, improves the overall yield. Furthermore, zinc dust is
essential for the reaction to proceed, which implies a preactivation of the
Ni
II (Cl) 2 (NNN) system to generate a ligand-centred radical within one of the
azo-fragments. Alcohol deprotonation and alcoholate coordination set up for ratelimiting intramolecular H atom transfer from the substrate to the azo radical side arm
to release the oxidized aldehyde or ketone product, aided by single-electron transfer
from the ketyl radical substrate to the other azo side arm. This latter fragment
becomes reoxidized by an equiv of O 2 , releasing superoxide. A second equiv of
substrate then undergoes proton transfer to generate one fully hydrogenated
hydrazido side arm in the ligand and another Ni-bound alcoholate. Ligand
reoxidation with external dioxygen completes the cycle. Hence, the Ni only serves
to bring the substrate in close proximity to the ligand-centred locus of reactivity. The
temperature-dependent KIE of 12–17 measured between 283 and 300 K implies a
contribution from quantum mechanical tunnelling.
Given that the nickel merely serves as substrate docking station, the
corresponding Zn complex also proved catalytically active for the same reaction,
with the ligand even able to act as four-electron-four-proton reservoir [99]. The Ni
derivative was also reported as a catalyst for the tandem aerobic synthesis of azines
from aromatic alcohols and hydrazine, again with ligand redox controlling the
reaction [100]. Using Ru analogs with this bis(arylazo)pyridine NNN pincer and
bearing different kinds of ancillary ligands, ranging from electron-donating to
π-accepting, Goswami and co-workers determined that these have profound effect
on the responsivity of the azo moiety and thus on the reactivity thereof [101].
Iron complexes of the strongly related phenanthroline-derived monoazo-NNN
systems were also developed, their electronic structure determined and their activity
for alcohol oxidation established. However, due to the absence of a second redoxactive azo-arm, Fe
II /F
I redox was implied as part of the mechanism for ketone release
in this particular situation [102]. This same ligand scaffold was also recently used for
Ni by the group of Paul, demonstrating redox-induced interconversion and
hemilability in homoleptic complexes [103] and for Co-catalysed coupling of
alcohols and 2-aminobenzamide to furnish quinazolin – 4(3H)-ones [104]. Crabtree
and co-workers reported on a Ni system with bis(naphthyridinyl)pyridine as pincer
ligand, incorporating two pendant bases, for electrocatalytic CO 2 reduction
[105]. No precise information on the precise electronic structure is provided,
although the authors speculate about ligand-centred redox.
When the flanking imino groups within the 1,4-azadiene arrangement are
included within a ring, several additional motifs become available. The group of
Caulton investigated the electrochemistry of bis(tetrazinyl)pyridine (btzp)-based
complexes of chromium and molybdenum [106]. Reaction of the precursor species
M(CO) 3 (NCMe) 3 with free ligand resulted in formation of homoleptic M(btzp) 2
complexes. A combined spectroscopic, X-ray crystallographic and computational
study revealed that the electronic structures of the respective complexes was best
described as M
2+ with two-ligand radical anions, by virtue of the energetically
accessible π
* -orbitals within the tetrazine fragments. As such, extensive metal-toligand electron transfer occurs upon coordination. However, this in itself is not
Redox-Active Pincer Ligands
151
t Bu, albeit not necessary, improves the overall yield. Furthermore, zinc dust is
essential for the reaction to proceed, which implies a preactivation of the
Ni
II (Cl) 2 (NNN) system to generate a ligand-centred radical within one of the
azo-fragments. Alcohol deprotonation and alcoholate coordination set up for ratelimiting intramolecular H atom transfer from the substrate to the azo radical side arm
to release the oxidized aldehyde or ketone product, aided by single-electron transfer
from the ketyl radical substrate to the other azo side arm. This latter fragment
becomes reoxidized by an equiv of O 2 , releasing superoxide. A second equiv of
substrate then undergoes proton transfer to generate one fully hydrogenated
hydrazido side arm in the ligand and another Ni-bound alcoholate. Ligand
reoxidation with external dioxygen completes the cycle. Hence, the Ni only serves
to bring the substrate in close proximity to the ligand-centred locus of reactivity. The
temperature-dependent KIE of 12–17 measured between 283 and 300 K implies a
contribution from quantum mechanical tunnelling.
Given that the nickel merely serves as substrate docking station, the
corresponding Zn complex also proved catalytically active for the same reaction,
with the ligand even able to act as four-electron-four-proton reservoir [99]. The Ni
derivative was also reported as a catalyst for the tandem aerobic synthesis of azines
from aromatic alcohols and hydrazine, again with ligand redox controlling the
reaction [100]. Using Ru analogs with this bis(arylazo)pyridine NNN pincer and
bearing different kinds of ancillary ligands, ranging from electron-donating to
π-accepting, Goswami and co-workers determined that these have profound effect
on the responsivity of the azo moiety and thus on the reactivity thereof [101].
Iron complexes of the strongly related phenanthroline-derived monoazo-NNN
systems were also developed, their electronic structure determined and their activity
for alcohol oxidation established. However, due to the absence of a second redoxactive azo-arm, Fe
II /F
I redox was implied as part of the mechanism for ketone release
in this particular situation [102]. This same ligand scaffold was also recently used for
Ni by the group of Paul, demonstrating redox-induced interconversion and
hemilability in homoleptic complexes [103] and for Co-catalysed coupling of
alcohols and 2-aminobenzamide to furnish quinazolin – 4(3H)-ones [104]. Crabtree
and co-workers reported on a Ni system with bis(naphthyridinyl)pyridine as pincer
ligand, incorporating two pendant bases, for electrocatalytic CO 2 reduction
[105]. No precise information on the precise electronic structure is provided,
although the authors speculate about ligand-centred redox.
When the flanking imino groups within the 1,4-azadiene arrangement are
included within a ring, several additional motifs become available. The group of
Caulton investigated the electrochemistry of bis(tetrazinyl)pyridine (btzp)-based
complexes of chromium and molybdenum [106]. Reaction of the precursor species
M(CO) 3 (NCMe) 3 with free ligand resulted in formation of homoleptic M(btzp) 2
complexes. A combined spectroscopic, X-ray crystallographic and computational
study revealed that the electronic structures of the respective complexes was best
described as M
2+ with two-ligand radical anions, by virtue of the energetically
accessible π
* -orbitals within the tetrazine fragments. As such, extensive metal-toligand electron transfer occurs upon coordination. However, this in itself is not
Redox-Active Pincer Ligands
151