GoonerWong

The reaction of [Cp*Ir(bzpy)NO₃] (1; bzpy=2-benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H₂O₂, NaIO₄, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η³ interaction with iridium (2 a). Formal addition of H₂O to 2 a or H₂O₂ to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A η³ interaction between the three non-oxygenated carbons of “ex-Cp*” and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a–c species were tested in water-oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min⁻¹).

Signs of three: Three intermediates from the oxidative transformation of a Cp*–iridium water-oxidation catalyst have been intercepted and characterized by using NMR spectroscopy, ESI-MS and, in one case, X-ray crystallography. Progressive oxidation of Cp* has been observed, whereas the benzoylpyridine ancillary ligand remains intact. Isolated intermediates and their mixture are still active in water-oxidation catalysis (see scheme).

This essay is based on research leading to the identification of catalysts capable of the selective oxidation of water to molecular oxygen. The real need for such materials relates to the large-scale photochemical dissociation of water into its constituent elements, as opposed, for example, to the electrochemical decomposition of water at macroscopic electrodes. Combining the catalyst with the essential components needed for efficacious photochemistry brings special challenges, as does the ultimate need to scale up the system by a massive amount. Nature has developed a highly successful process for O₂ evolution under ambient illumination that makes use of a cubane tetramanganese cluster having a closely associated calcium cation in attendance. This catalyst is surprisingly delicate and it is debatable as to whether we could adapt such a system for use with artificial photosystems. Historically the latter have used colloidal metal oxides, and consideration is given here as to which materials might offer the most promising catalytic performance. Moving towards heterogeneous systems has more practical meaning, but the same materials come to mind. Recent attention to cobalt-based catalysts is highlighted as a possible breakthrough that might lead to interesting electrochemical systems when combined with wind turbines. Molecular catalysts provide interesting opportunities for photochemical O₂ evolution but suffer from problems of scale-up. This field has witnessed the most important progress over the past decade or so but still needs urgent attention if advanced materials are to be identified in a timely manner. Finally, consideration is given to the actual status of the field in specific terms of developing an effective artificial photosynthetic apparatus. Moving progressively from using sacrificial redox agents as a simple means to isolate the oxidative photochemical cycle towards full water cleavage will stimulate the development of demonstration models suitable for public display. The reward should be increased investment.

Mimicking natural photosynthesis that leads to CO₂ fixation and concomitant O₂ liberation from water remains an elusive goal. New (photo)anodes and studies of mechanisms for water oxidation continue to be reported, but there is still no sign of a viable artificial photosynthetic system worthy of scaling up to industrial proportions. This essay explores the underlying reasons for this but seeks to reassure investors that there is both a market and an outstanding chance for profitable returns.

An overview of the papers in this cluster issue is presented.

“...in recent years, quite some progress has been made in our understanding of the catalytic water oxidation processes. In order to further boost the momentum of the field and to provide tools for new researchers entering the field of catalytic water oxidation, we have guest-edited this Cluster Issue of EurJIC on water oxidation.”

Read more by the Guest Editors on p. 571.

An overview of the papers in this cluster issue is presented.

“...in recent years, quite some progress has been made in our understanding of the catalytic water oxidation processes. In order to further boost the momentum of the field and to provide tools for new researchers entering the field of catalytic water oxidation, we have guest-edited this Cluster Issue of EurJIC on water oxidation.”

Read more by the Guest Editors on p. 571.

EurJIC brings you a fine collection of studies into the dynamic, stimulating and highly topical field of Water Oxidation. Guest Editors Dennis Hetterscheid and Licheng Sun enthusiastically provided their broad expertise on this topic to compile an issue containing many different points of view on this very important subject.

EurJIC brings you a fine collection of studies into the dynamic, stimulating and highly topical field of Water Oxidation. Guest Editors Dennis Hetterscheid and Licheng Sun enthusiastically provided their broad expertise on this topic to compile an issue containing many different points of view on this very important subject.