ARTICLES
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This experiment indicates that the incorporation of 18O must
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Based on these experiments, we propose a simple working mech-
anism where the observed ruthenium complexes 3, 4, 5 and 6 are
involved as catalytically active species for the conversion of
MeOH and H2O into formic acid and finally to H2 and CO2.
Figure 6 highlights the electronic and chemical ‘non-innocence’ of
the trop2dad/trop2dae ligand pair. In step (a), the hydride
[Ru(H)(trop2dad)]2 (3) reacts with water to form the neutral
[Ru(trop2dad)] complex 4 with the release of 1 equiv. H2. In step
(b), the hydroxyl group of a substrate molecule (alcohol, methandiol
or formic acid) adds to the Ru–N bond to give the assumed inter-
mediate I. The a-C–H bond of the coordinated substrate molecule
is activated in step (c) by H transfer to one of the CH groups in the
trop2dad ligand backbone. In this process, the Ru(II) centre is
reduced to Ru(0) to give a proposed amino imine complex II, and
one molecule of a carbonyl compound (aldehyde, carboxylate,
CO2) is released. The amino imine complex II can react in a
similar manner by addition of the polar O–H bond across the
polar Ru–N bond and subsequent a-C–H bond activation to give
Ru(0) complexes 6 (step (d)) and 5 under release of a second equiv-
alent of a carbonyl compound (step (e)). Both 6 and 5 carry a hydro-
genated trop2dae ligand, which results from the formal uptake of
2 equiv. H2. Finally, Ru(0) complex 5 is converted under the reac-
tion conditions (base, elevated temperatures) to complex 3 (step
(f)) and 1 equiv. of H2, completing the catalytic cycle. This pro-
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Conclusions
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A novel homogeneous catalytic system has been found for the clean
conversion of methanol and water into hydrogen and carbon
dioxide. The process is based on well-defined reaction steps and cir-
cumvents the formation of poisonous CO gas as by-product (avoid-
ing inactivation of the molecular catalyst and contamination of the
hydrogen fuel). During the catalytic cycle, the azadiene ligands
reversibly store molecular hydrogen, and the oxidation states of
the ruthenium centre shuttle between 0 and þ2, facilitated by the
electronically and chemically ‘non-innocent’ coordination environ-
ment. We believe these results may guide future research efforts,
specifically in the immobilization of optimized molecular catalysts
on conducting support materials for the development of electrodes
for methanol-based fuel cells.
Received 3 November 2012; accepted 31 January 2013;
published online 10 March 2013
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5
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