The
synthesis, characterization, and electrochemical studies of
the dinuclear complex [(MeOH)Fe(Hbbpya)-μ-O-(Hbbpya)Fe(MeOH)](OTf)4 (1) (with Hbbpya = N,N-bis(2,2′-bipyrid-6-yl)amine)
are described. With the help of online electrochemical mass spectrometry,
the complex is demonstrated to be active as a water oxidation catalyst.
Comparing the results obtained for different electrode materials shows
a clear substrate influence of the electrode, as the complex shows
a significantly lower catalytic overpotential on graphitic working
electrodes in comparison to other electrode materials. Cyclic voltammetry
experiments provide evidence that the structure of complex 1 undergoes reversible changes under high-potential conditions, regenerating
the original structure of complex 1 upon returning to
lower potentials. Results from electrochemical quartz crystal microbalance
experiments rule out that catalysis proceeds via deposition of catalytically
active material on the electrode surface.
Using on-line mass spectrometry in combination with classical electroanalytical techniques makes it possible to reliably determine onset potentials and to distinguish between competing reactions such as oxygen evolution and carbon dioxide formation. Using these on-line MS methods, catalytic water oxidation activity was demonstrated for cis-[Fe(cyclam)Cl2]Cl (1) and [Fe(cyclamacetate)Cl] (2).
Here we showcase the synthesis and catalytic response of the anionic iridium(III) complex [IrCl 3 (pic)(MeOH)] − ([1] − , pic = picolinate) toward the evolution of oxygen. Online electrochemical mass spectrometry experiments illustrate that an initial burst of CO 2 due to catalyst degradation is expelled before the oxygen evolution reaction commences. Electrochemical features and XPS analysis illustrate the presence of iridium oxide, which is the true active species.
The complex α-[Fe(mcp)(OTf)2] (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)-cyclohexane-1,2-diamine
and OTf
= trifluoromethanesulfonate anion) was reported in 2011 by some of
us as an active water oxidation (WO) catalyst in the presence of sacrificial
oxidants. However, because chemical oxidants are likely to take part
in the reaction mechanism, mechanistic electrochemical studies are
critical in establishing to what extent previous studies with sacrificial
reagents have actually been meaningful. In this study, the complex
α-[Fe(mcp)(OTf)2] and its analogues were investigated
electrochemically under both acidic and neutral conditions. All the
systems under investigation proved to be electrochemically active
toward the WO reaction, with no major differences in activity despite
the structural changes. Our findings show that WO-catalyzed by mcp–iron
complexes proceeds via homogeneous species, whereas the analogous
manganese complex forms a heterogeneous deposit on the electrode surface.
Mechanistic studies show that the reaction proceeds with a different
rate-determining step (rds) than what was previously proposed in the
presence of chemical oxidants. Moreover, the different kinetic isotope
effect (KIE) values obtained electrochemically at pH 7 (KIE ∼
10) and at pH 1 (KIE = 1) show that the reaction conditions have a
remarkable effect on the rds and on the mechanism. We suggest a proton-coupled
electron transfer (PCET) as the rds under neutral conditions, whereas
at pH 1 the rds is most likely an electron transfer (ET).
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