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um complex resulted in the generation of an unusual tetranu-
clear palladium complex, in which the cod ligand retains its
ÀH
charge and coordination mode. On the other hand, the diplati-
num complex can be reversibly protonated at the cod li-
ÀH
gands, whereby those ligands undergo a s/p rearrangement
and change their coordination mode from an allyl/alkene to an
alkene/alkene mode. Additionally, we have shown that the cod
ligands can also be activated by reaction of the protonated di-
platinum complex with NaOMe through nucleophilic attack,
which leads to the formation of a new CÀOMe bond at the
cod ligands and a change in their coordination mode from
alkene/alkene to alkyl/alkene. For the diplatinum complexes,
À
all protonation reactions, as well as the reaction with OMe ,
have been shown to be completely reversible. Reversible elec-
tron transfers have been observed at the bridging quinone li-
gands for all the complexes. The locus of the electron-transfer
steps at the bridging quinone ligands has been unequivocally
established by using UV/Vis and EPR spectroelectrochemical
methods. We have also shown that the protonation of the
Scheme 7. Reaction cycle displaying transfer of two protons and two elec-
trons to a substrate.
3,5-di-tert-butyl-catechol was probed by using [8] as an elec-
remote cod ligands leads to a shift of the bridging quino-
ÀH
tron and proton source (Scheme 7).
noid ligand reduction by ꢀ500 mV/proton. Thus, the protonat-
ed complex displays reduction potentials that are more than
one Volt positively shifted compared to its non-protonated
counterpart. Taking advantage of this low reduction potential,
we have managed to chemically isolate the first example of
a dinuclear complex containing an aromatic bridge of a qui-
none-derived ligand with a [O,N,O,N] donor set. The doubly
protonated and doubly reduced diplatinum complex has been
utilized in a three-way cooperative process to transfer two pro-
tons from two different cod ligands and two electrons from
the bridging quinone ligand to reduce 3,5-di-tert-butyl-benzo-
quinone to 3,5-di-tert-butyl-catechol (Scheme 7). The protona-
tion at the remote cod ligands helps to control reduction po-
tentials of electron-transfer steps occurring at the site-decou-
pled quinone bridge. It is the achievement of low reduction
potentials that finally makes electron and proton transfer to
the substrate possible.
To perform this reaction, [5] was reduced in situ with cobal-
tocene to [8], and the conversion was followed by UV/Vis
spectroscopy. This conversion led to a color change from
green to violet. Once [8] was formed, the substrate 3,5-di-tert-
butyl-benzoquinone was added to the reaction mixture, which
led to an immediate color change to orange. The UV/Vis spec-
trum of the mixture changed accordingly (Figure S9 in the
Supporting Information). After stirring for 24 h, the reaction
mixture was worked up (see the Experimental Section in the
Supporting Information), and the isolated solid was subjected
to NMR spectroscopy. The NMR spectrum of the isolated solid
in CD Cl showed signals that correspond to [1], 3,5-di-tert-
2
2
butyl-catechol and cobaltocenium (Figure S10 in the Support-
ing Information). Thus, compound [8] transfers two electrons
and two protons to 3,5-di-tert-butyl-benzoquinone leading to
its conversion to the catechol form. In the process, [8] gets
converted to [1]. The cobaltocenium signal observed in the
NMR spectrum arises from the oxidized form of cobaltocene,
which is used for the in situ reduction of [5] to [8]. The isolat-
ed compound also displays a UV/Vis spectrum that matches
with the main transition of [1] (Figure S9 in the Supporting In-
formation). Compound [1] can now be protonated to convert
it to [5], which closes the cycle (Scheme 7).
Thus, it is seen that two protons, one each from a different
cod ligand, and two electrons from the bridging quinone
ligand in [8] can be transferred to a substrate leading to its
double protonation and double reduction. It is intriguing that
such site-decoupled proton and electron sources can work in
tandem to transfer electrons and protons to a particular sub-
strate in a three-way cooperative process.
Control of redox potentials through remote protonation
steps is common in biological systems, and biocatalysis usually
works through transfer of electrons and protons. Most man-
made homogeneous catalysts on the other hand take advant-
age of oxidative addition and reductive elimination steps for
substrate activation and transformation. However, in recent
years, examples of substrate activation by consecutive proton-
and electron-transfer steps have appeared with man-made cat-
[2c]
alysts. We believe that the proof of concept that we have
delivered in this work of site decoupled proton and electron
transfer sites that can work in tandem for performing substrate
bond activation will further contribute to develop concepts for
new kinds of catalysis. Additionally, we have also shown that
spectroelectrochemistry is a powerful technique to determine
spectroscopic signatures of reactive intermediates that appear
in bond-activation processes. These spectroscopic signatures
can then be utilized to elucidate the pathway of substrate acti-
vation in such complexes containing non-innocent ligands. We
note that all proton- and electron-transfer steps observed in
these complexes are ligand based.
Conclusion
We have presented herein diplatinum and dipalladium com-
plexes that contain anionic cod stopper ligands and a redox-
ÀH
active quinonoid bridging ligand. Protonation of the dipalladi-
Chem. Eur. J. 2014, 20, 15178 – 15187
15186
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