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characterized (see Supplementary Information). Alcohol addition
product I can be formed either via regeneration of 3 and addition
of an ROH moiety (that is, formation of 11 as a side equilibrium),
or via substitution of RO2 for HO2. The next step may involve
hydrogen transfer from the ligand arm (intermediate II) and
subsequent H2 evolution to form complex III. Subsequently,
hydride elimination from the coordinated RO2 occurs to form a
dearomatized complex with an associated aldehyde complex IV.
The aldehyde can dissociate and react with another equivalent of
complex 3, or alternatively intermediate IV can react with water to
give coordinated gem–diolate complex V. Loss of H2 from the
gem–diolate complex leads to the carboxylic acid product and
regenerates the catalyst; this last step may happen without metal
catalysis13. Addition of carboxylic acid across the dearomatized
complex can be argued to be highly thermodynamically favourable,
leading to 9 in preference over 11 and I. Thus, one of the most
important roles for the stoichiometric base equivalent may be to
trap the product as a salt, allowing for regeneration of the active
catalyst. The liberation of H2 during the experiment was
qualitatively analysed by gas chromatography. In aqueous basic
solutions, complex 11 was seen after heating the alcohol and
complex 3 inside a closed NMR tube, suggesting it is the resting
state of the complex during catalysis in a large excess of water;
production of carboxylic acid salts was not seen inside aclosed system.
To interrogate the plausibility of the mechanism outlined in Fig. 3,
we calculated, by density functional theory (DFT), the minimized
energy structures of all the intermediates presented in the figure.
According to calculations performed by the groups of Hall23 and
Yoshizawa24 on our original water-splitting system26, one of the
most endergonic steps of the process is the transfer of a proton
from the methylene arm of the ligand to the hydride located on the
metal centre. According to the reports of Yoshizawa and Hall,
where the barrier for H2 release is suggested to be exaggerated, this
is also the rate-determining step of the water-splitting reaction. A
high barrier is also suggested from previous computational studies
carried out in our group25,26. As well as calculating the enthalpies of
all the proposed intermediates of the system, which show that the pro-
posed catalytic cycle is plausible, we looked at the step of hydrogen
transfer in particular, which may very well be the rate-determining
step for the catalytic cycle. The results and the change in enthalpies
with respect to 3 are presented in Fig. 3. The transfer of hydrogen
from the methylene arm to the metal centre has a barrier of
ꢀ28.3 kcal mol21. From the Eyring equation, the expected highest
barrier value to account for the turn over numbers (TONs) observed
would have an upper limit of ꢀ26 kcal mol21, which compares well
with the calculated value (see DFT section of the Supplementary
Information for further discussion).
26. Kohl, S. W. et al. Consecutive thermal H2 and light-induced O2 evolution from
water promoted by a metal complex. Science 324, 74–77 (2009).
Acknowledgements
This research was supported by the European Research Council (ERC) under the FP7
framework (no. 246837) and by the Kimmel Center for Molecular Design. D.M. is the Israel
Matz Professorial Chair of Organic Chemistry. E.K. would like to thank D. Laikov for
providing access to the Priroda DFT software.
Received 18 June 2012; accepted 20 November 2012;
published online 6 January 2013
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Author contributions
E.B. made the initial discovery, carried out catalytic experiments and wrote the manuscript.
E.K carried out catalytic experiments, stoichiometric experiments, DFT calculations,
synthesis and crystallization of complex 10, and wrote the manuscript. G.L. performed the
X-ray structural study of complex 10. D.M. designed and directed the project and wrote
the manuscript.
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Additional information
Supplementary information and chemical compound information are available in
the online version of the paper. Reprints and permission information is available online at
addressed to D.M.
6. Bianchini, C. & Shen, P. K. Palladium-based electrocatalysts for alcohol
oxidation in half cells and in direct alcohol fuel cells. Chem. Rev. 109,
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Competing financial interests
The authors declare no competing financial interests.
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