the catecholate fragment to generate the corresponding quinone that showed
η
2 C-C arene coordination to Mo. Follow-up work revealed that the redox-active
hydroquinone-type motif Mo(CO) 3 (Parene
OH2 P) can be transformed into the quinone derivative Parene
O2
P through loss of two protons and four electrons to form
Mo(CO)(Cl) 2 (Parene
O2 P) in a series of steps [210]. Proton-coupled electron transfer
reactivity for this system was also established and thermochemical analysis of the
–OH bonds established that Mo coordination weakens the bond dissociation free
energy (BDFE) relative to the free ligand. The coordination chemistry and reactivity
of the closely related 1,4-catechol-based pincer ligand design was explored with Pd
[211]. Reaction of the free, fully protonated ligand with Pd
II Cl 2 (cod) and subsequent
halide abstraction provided cationic [PdCl(Parene
OH2 P)]OTf with a Pd-η
2 -arene
interaction. In situ reduction of the Pd-dichlorido analog with a Ni
0 species led to
Pd
0 buttressed by the phosphine donors only and no arene binding observable.
However, reaction with the palladium precursor Pd acetate led to formation of the
1,4-quinone derivative bound in an all-carbon η
4 -fashion to Pd
0 . The interconversion
from the former to the latter species was achieved by reaction with O 2 , which was
studied in detail using time-resolved and temperature-dependent UV-vis spectroscopy and kinetics, as well as NO and N 2 O.
The same group also reported on the related syn-9,10-anthracenediyl-linked
bisphenol that supported a Zr
IV (bisphenyl) complex that underwent photoinduced
release of bibenzyl [212]. Metric data obtained from X-ray crystallography indicated
that the anthracene unit has a lower degree of aromaticity in the central ring
consistent with a two-electron reduction, thus resulting in η
4 bonding of the central
ring to Zr. Although formally considered a Zr
IV complex, this system has two
electrons stored within the anthracene unit, which sets it up for anthracene-based
redox-chemistry. Analogous to known Zr
II reactivity, the oxidative cyclometalation
of alkynes was observed by coupling of two equiv of diphenyl acetylene or phenyl
acetylene, giving zirconacyclopentadiene species. These complexes were able to
insert CO into the metallacycle to obtain tetraphenylcyclopentadienone. Stoichiometric heterocoupling between phenyl acetylene and p-tolunitrile was also observed.
This latter reaction could also be performed in a catalytic fashion using 5 mol%
complex at 105
C to produce pyrimidines (i.e. two TolCN react with one PhCCH),
suggesting that some reductive elimination step from the formed azazirconacyclic
intermediate is also feasible to regenerate the ‘masked Zr
II
’ species. Mechanistic
studies were employed to shed light on the likely order of events leading to the
pyrimidine product.
4 Concluding Remarks
In summary, ligand-centred redox activity has emerged as a recent addition to the
portfolio when considering not only the design but also the desired reactivity
inferred upon or by a pincer ligand, in connection with a (main group or) transition
metal in its binding pocket. Several types of designs feature prominently in this
172
J. I. van der Vlugt
η
2 C-C arene coordination to Mo. Follow-up work revealed that the redox-active
hydroquinone-type motif Mo(CO) 3 (Parene
OH2 P) can be transformed into the quinone derivative Parene
O2
P through loss of two protons and four electrons to form
Mo(CO)(Cl) 2 (Parene
O2 P) in a series of steps [210]. Proton-coupled electron transfer
reactivity for this system was also established and thermochemical analysis of the
–OH bonds established that Mo coordination weakens the bond dissociation free
energy (BDFE) relative to the free ligand. The coordination chemistry and reactivity
of the closely related 1,4-catechol-based pincer ligand design was explored with Pd
[211]. Reaction of the free, fully protonated ligand with Pd
II Cl 2 (cod) and subsequent
halide abstraction provided cationic [PdCl(Parene
OH2 P)]OTf with a Pd-η
2 -arene
interaction. In situ reduction of the Pd-dichlorido analog with a Ni
0 species led to
Pd
0 buttressed by the phosphine donors only and no arene binding observable.
However, reaction with the palladium precursor Pd acetate led to formation of the
1,4-quinone derivative bound in an all-carbon η
4 -fashion to Pd
0 . The interconversion
from the former to the latter species was achieved by reaction with O 2 , which was
studied in detail using time-resolved and temperature-dependent UV-vis spectroscopy and kinetics, as well as NO and N 2 O.
The same group also reported on the related syn-9,10-anthracenediyl-linked
bisphenol that supported a Zr
IV (bisphenyl) complex that underwent photoinduced
release of bibenzyl [212]. Metric data obtained from X-ray crystallography indicated
that the anthracene unit has a lower degree of aromaticity in the central ring
consistent with a two-electron reduction, thus resulting in η
4 bonding of the central
ring to Zr. Although formally considered a Zr
IV complex, this system has two
electrons stored within the anthracene unit, which sets it up for anthracene-based
redox-chemistry. Analogous to known Zr
II reactivity, the oxidative cyclometalation
of alkynes was observed by coupling of two equiv of diphenyl acetylene or phenyl
acetylene, giving zirconacyclopentadiene species. These complexes were able to
insert CO into the metallacycle to obtain tetraphenylcyclopentadienone. Stoichiometric heterocoupling between phenyl acetylene and p-tolunitrile was also observed.
This latter reaction could also be performed in a catalytic fashion using 5 mol%
complex at 105
C to produce pyrimidines (i.e. two TolCN react with one PhCCH),
suggesting that some reductive elimination step from the formed azazirconacyclic
intermediate is also feasible to regenerate the ‘masked Zr
II
’ species. Mechanistic
studies were employed to shed light on the likely order of events leading to the
pyrimidine product.
4 Concluding Remarks
In summary, ligand-centred redox activity has emerged as a recent addition to the
portfolio when considering not only the design but also the desired reactivity
inferred upon or by a pincer ligand, in connection with a (main group or) transition
metal in its binding pocket. Several types of designs feature prominently in this
172
J. I. van der Vlugt