Communication
ture by iodometric titration measurements (Figure 4 inset) con- shown that complex 2 can reversibly accept up to four elec-
firmed the formation of H2O2 in 30 % yield relative to 2. Thus trons in three consecutive electrochemical steps. Consequently,
2 performs a two-electron oxidation of water to hydrogen-per- 2 is a more efficient electron-transfer agent than any of the
oxide. Notably, a related pentanuclear iron complex,[23] which naturally occurring or synthetic iron-sulfur clusters (where maxi-
is stable over six redox states, has been reported as an efficient mum one or two electron transfers can occur).[24,25] The ques-
catalyst for the oxidation of water to dioxygen. The O–O bond tion that arises then is “why Nature has ignored such a simple
formation step is suggested to be mediated by high-valent iron- and efficient redox catalyst in favor of its sulfur analogues?” The
centers. In contrast for 2, the formation of H2O2 as the only answer may lie in the demonstrated ability of 2 in oxidizing
product of water-oxidation, may suggest the involvement of water to hydrogen peroxide with the concomitant change in
hydroxyl radicals in the O–O bond formation step. A detailed the core structure of 2. Thus, in addition to their electron trans-
mechanistic investigation of the water oxidation reaction medi- fer properties, the Fe8(μ4-O)4 clusters also have strong oxidizing
ated by 2 is ongoing in our laboratory, and will be reported in capabilities, which may lead to several unwanted side reactions
a future study.
under physiological conditions in case they were used as bio-
logical electron-transfer reagents. Notably, water splitting at a
magnetite surface[26] has also been recently observed, although
no oxidation products could be identified. Thus, the Fe8(μ4-O)4
unit in 2 not only resembles the building units in magnetite,
it also models the surface chemistry of these oxides, which is
dominated by interactions with water and solvated ions.
Supporting Information (see footnote on the first page of this
article): X-ray structure of 1, NMR spectrum and cyclic voltammo-
gram of 2, and crystallographic tables.
CCDC 1825492 (for 1) and 1825494 (for 2) contain the supplemen-
Acknowledgments
Figure 4. UV/Vis absorption spectra of a 0.2 m
M
solution of 2 (black line) and
Financial support from the Deutsche Forschungsgemeinschaft
(Cluster of Excellence “Unifying Concepts in Catalysis” EXC 314-
2; Collaborative Research Centre – CRC 1109; and the Heisen-
berg-Professorship to K. R.) is gratefully acknowledged. T. C.
thanks the Alexander von Humboldt Foundation for a postdoc-
toral grant.
its decay product in the presence of 400 equiv. of water (red line) in a solvent
mixture of acetone/CH2Cl2 (95:5) at 25 °C under Ar. The inset shows the UV/
Vis absorption spectral changes observed upon treating the reaction mixture
of 2 (0.2 mM) and 400 equiv. of water with 1500 equiv. of NaI in the presence
of 500 equiv. of TFA in a solvent mixture of acetone/CH2Cl2 (95:5) at 25 °C
under Ar.
Keywords: Cluster compounds · Water oxidation · Dioxygen
activation · Iron oxides · Magnetic properties
Conclusions
A family of octanuclear [Fe8(μ4-O)4(μ-4-R-pz)12X4] (Fe8) clusters
(R = CH3, H, and Cl; X = Cl, Br; pz = pyrazolate) has been previ-
ously synthesized and intensively characterized as structural
models for the Fe8(μ4-O)4 units present in the all ferric minerals
ferrihydrite (Fe5HO8·4H2O), maghemite (γ-Fe2O3), as well as in
mixed-valent magnetite (Fe3O4).[10–13] We have now extended
this study to report the synthesis, X-ray structure, spectroscopic
properties, and water reactivity of a new Fe8 cluster 2 involving
the 4-tBu-pz ligand. Mechanistic investigations reveal that 2 is
generated by dioxygen activation [plausibly at an iron(II)-
pyrazolate adduct] rather than the previously suggested mech-
anism of water activation by iron(III)-pyrazolate centers. The in-
troduction of tBu substituent at the pzH 4-position is shown to
have a negligible impact on the geometric structure of the Fe8
clusters. Nevertheless, the electronic structure of 2 is signifi-
cantly different than the [Fe8(μ4-O)4(μ-pz)12Cl4] complex 3, as
evidenced by the near-doubling of the Mössbauer ΔEQ parame-
ter for the Feo centers in 2 relative to 3, and an approximately
300 mV shift of redox potentials to more positive values in 2.
By CV and UV/Vis/NIR spectroelectrochemistry it has been
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