Oxygen and proton reduction by decamethylferrocene in non-aqueous acidic media

Article abstract of DOI:10.1039/b926963k

Experimental studies and density functional theory (DFT) computations suggest that oxygen and proton reduction by decamethylferrocene (DMFc) in 1,2-dichloroethane involves protonated DMFc, DMFcH⁺, as an active intermediate species, producing hy

Full text of DOI:10.1039/b926963k

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Oxygen and proton reduction by decamethylferrocene in non-aqueous
acidic mediaw
a
Bin Su,z Imren Hatay,ab Pei Yu Ge,a Manuel Mendez,a Clemence Corminboeuf,c
Zdenek Samec,d Mustafa Ersozb and Hubert H. Girault*a
Received (in Cambridge, UK) 18th January 2010, Accepted 3rd March 2010
First published as an Advance Article on the web 18th March 2010
DOI: 10.1039/b926963k
Experimental studies and density functional theory (DFT)
computations suggest that oxygen and proton reduction by
decamethylferrocene (DMFc) in 1,2-dichloroethane involves
protonated DMFc, DMFcH+, as an active intermediate species,
producing hydrogen peroxide and hydrogen in aerobic and
anaerobic conditions, respectively.
yellow, to which addition of an acid, hydrogen tetrakis(penta-
fluorophenyl)borate (HTB),y led to an immediate colour
change to bright green. In the UV-visible spectrum, the
absorption band of DMFc centered at 425 nm is replaced by
a strong band of DMFc+ at 779 nm. Moreover, by shaking
the green organic solution with pure water and then
by titrating this aqueous phase with sodium iodide (NaI),
triiodide was detected with a spectroscopic signature at 286 nm
and 330 nm. Triiodide was generated by oxidation of iodide by
H2O2.7 These results suggest the occurrence of oxygen
reduction to H2O2 by DMFc in DCE in the presence of the
organic acid:
O2 reduction by ferrocene and its derivatives in organic media
in the presence of an acid, such as trichloroacetic and trifluoro-
acetic acids and perchloric acid, has been known for many
years.1–5 Recently, O2 reduction has also been studied in
biphasic systems composed of an organic solvent containing
a ferrocene derivative in contact with an aqueous inorganic
acid solution containing a very lipophilic anion.6–8 At such
liquid–liquid interfaces, O2 reduction takes place by involving
the lipophilic electron donors i.e. ferrocene derivatives located
in the organic phase and aqueous protons. Moreover, it has
been found that in the same biphasic system the electron-rich
decamethylferrocene (DMFc) can also reduce aqueous
protons under anaerobic conditions, leading to the evolution
of hydrogen.9 In both cases, the reaction has been proposed to
proceed in two steps: first a heterogeneous proton transfer
facilitated by DMFc from the aqueous to the organic phase as
observed by voltammetry experiments at liquid–liquid inter-
faces, followed by a homogenous proton/oxygen reduction in
the organic phase, the mechanism of which being yet unresolved.
In this communication, we present experimental results for
oxygen/proton reductions by DMFc in bulk 1,2-dichloro-
ethane (DCE) in the presence of organic acids. In the case of
oxygen reduction, DFT computations support a reaction
pathway involving protonated DMFc, DMFcH+, as an
intermediate species, which reacts with oxygen to produce
hydrogen peroxide.
2DMFc + O2 + 2H+TBꢀ - 2DMFc+TBꢀ + H2O2 (1)
However, only 0.03 mM of H2O2 was detected, an amount
which is about five times less than that of DMFc+ produced
(1.5 mM). This indicates a much lower yield than the stoichio-
metric value for H2O2. One possible reason is the further
reduction of H2O2 by DMFc to finally produce water. Indeed,
in biphasic systems, the yield of H2O2 can reach 40% as its
extraction to water competes with further reduction.7 Another
possible reason for this low yield is the competing proton
reduction. The latter hypothesis was investigated by performing
the same experiment under anaerobic conditions.
Fig. 2 shows gas chromatograms obtained on a Perkin-
Elmer gas chromatograph (Clarus 400) equipped with packed
5 A molecular sieves, 80/100 mesh, using a TCD detector and
argon as the carrier gas. In anaerobic conditions (Fig. 2a) a
clear H2 signal can be observed confirming similar results to
Fig. 1a illustrates an experiment performed in DCE under
aerobic conditions. A dilute solution of DMFc appeared
a Laboratoire d’Electrochimie Physique et Analytique,
´
´
Ecole Polytechnique Federale de Lausanne (EPFL), Station 6,
CH-1015 Lausanne, Switzerland. E-mail: hubert.girault@epfl.ch;
Fax: +41 21 6933667; Tel: +41 21 6933145
b Department of Chemistry, Selcuk University, 42031 Konya, Turkey
c Laboratory for Computational Molecular Design, EPFL, BCH 5312,
CH-1015 Lausanne, Switzerland
Fig. 1 (a) Colour change of a DCE solution containing 2 mM DMFc
before (left) and after (right) addition of 2 mM HTB under aerobic
conditions for 30 min; (b) UV-Visible spectra of DCE solutions shown
in Fig. 1a (black line: left flask, red line: right flask) and pure water
after being shaken with the organic solution in the right flask and
addition of sodium iodide (blue line).
d J. Heyrovsky Institute of Physical Chemistry of ASCR,
v.v.i, Dolejskova 3, 182 23 Prague 8, Czech Republic
w Electronic supplementary information (ESI) available: Computational
details. See DOI: 10.1039/b926963k
z Present address: Institute of Microanalytical Systems, Department
of Chemistry, Zhejiang University, Hangzhou 310058, P. R. China.
ꢁc
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2918 | Chem. Commun., 2010, 46, 2918–2919

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Fig. 2 Gas chromatogram of the head space after 3 h in anaerobic (a)
and aerobic conditions (b) for a DCE solution containing 5 mM
DMFc and 10 mM HTB.
those obtained in biphasic systems,9 whereas some small
amount of H2 can still be observed in aerobic conditions
(Fig. 2b) suggesting that proton and oxygen reductions do
compete, but that hydrogen production cannot account for the
low H2O2 generation yield. It must be stressed that the
solution turns green within minutes in the presence of oxygen
whereas it takes a couple of hours in anaerobic conditions.
Previous studies at liquid–liquid interfaces suggested that in
both cases the initial step is the protonation of DMFc leading
to the formation of DMFcH+. The protonation of ferrocene
was reported to result in a tilting of the rings thus facilitating
the binding of O2 at the Fe atom directly, which is then
followed by oxygen reduction.2 Considering that insertion of
triplet O2 is spin-forbidden, Fomin proposed the formation of
an hydrogen-bond intermediate, O2 being sandwiched between
two protonated ferrocenes followed by the generation of H2O2
by concerted breakdown of two Fe–H and formation of two
H–O bonds.10 The present DFT computations do not support
the hypothesis that triplet molecular oxygen O2 coordinates to
the iron atom through a spin-forbidden mechanism or inserts
into the Fe–H bond. Instead, the present computations show
that O2 can approach the activated hydride directly via a
delocalized triplet (diradical) transition state [DMFcꢂꢂꢂHꢂꢂꢂOO]+
(Scheme 1) with an activation barrier of 15 kcal molꢀ1 in the
gas phase and 14.6 kcal molꢀ1 in the solvent. This process
leads to the formation of decamethylferrocenium DMFc+ and
a hydrogen peroxyl radical. The generation of H2O2 from the
latter is then expected to proceed rapidly. Note that other
possible mechanistic routes occurring via either a superoxoiron
[DMFc–O2] (i.e. protonation last, spin-forbidden) or a super-
oxide intermediate [DMFc–O2H]+ (i.e. insertion into the
Fe–H bond) were found to have considerably higher activation
energy barriers than the mechanism proposed above (see
details in Supplementary Dataw).
Scheme 1 Proposed mechanism for the reduction of O2 into HO2 by
DMFc. Gas phase zero-point corrected energies computed at the
BP86/DZP level are given in red. COSMO corrected energies
(e = 10.36) computed at the same level are given in blue. The unpaired
spin density is displayed in green. The spin quantum number (i.e. s) is
given as a superscript for each compound. Further details can be
found in the Supplementary Data.w
that protonated metallocenes are highly reactive in reduction
reactions useful to energy research such as hydrogen
production, oxygen or even carbon dioxide reduction.
This work was supported by EPFL, the Swiss National
Science Foundation (FNRS 200020-116588) and the COST
Action, D36/007/06. C. C. acknowledges the Sandoz Family
Foundation. I. H. also gratefully acknowledges the Scientific
and Technological Research Council of Turkey (TUBITAK)
under the 2212-PhD Scholarship Programme. Z. S. is also
grateful to the Grant Agency of the Czech Republic (grant no.
203/07/1257). We also thank Prof. C. Amatore (ENS, Paris)
for helpful discussions.
Notes and references
y HTB was prepared by shaking x mM LiTB and 10 mM HCl in the
water phase with pure DCE for 1 h. The concentration of HTB
extracted to DCE was approximately x mM, as determined by the
pH change of the water phase before and after shaking.
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In the case of proton reduction, hydrogen evolution can
proceed via a proton attack on DMFcH+ or by a bimolecular
route involving two DMFcH+. The present experimental
results confirm that hydrogen production is a much slower
process than oxygen reduction, which could be explained by
the bimolecular pathway.
In conclusion, oxygen/proton reductions by DMFc in
1,2-dichloroethane have been observed experimentally. DFT
computations suggest a reaction pathway involving protonated
DMFc, DMFcH+, as an intermediate species, which reacts
with oxygen to produce hydrogen peroxide. This study shows
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