rely on fossil fuels is of great importance. Renewable energy sources, such as wind, solar, and tidal energy constitute arguably the most promising of these clean energy solutions, but suffer from the fact that they are intermittent. [6] Direct power supply from these sources therefore cannot be relied upon to satisfy instantaneous energy demands. [7] A means of storing the energy generated by these renewable sources is therefore essential if we are to depend more heavily on renewably generated power. [8] Hydrogen (H 2) is often proposed in this context as a promising "carbon neutral" energy carrier (i.e., fuel). In such a system, renewably generated electricity is used to electrolyze water to generate hydrogen and oxygen. The oxygen may be vented to the atmosphere whilst the hydrogen is stored as a fuel. This hydrogen is then subsequently oxidized (either by combustion or in a fuel cell) to regenerate water and to release energy. Hydrogen is not a perfect fuel but it does have a number of attractive properties such as its low toxicity, ability to be transported safely over long distances via pipeline, [9] and its high energy density per unit mass (three times greater than that of gasoline). [10] Moreover, sustainably sourced hydrogen could be used to reduce CO 2 or N 2 from the atmosphere to generate carbon-neutral fuels and commodity chemicals such as hydrocarbons and ammonia. In many ways then, hydrogen can be viewed as the key to a sustainable energy cycle. The process of water electrolysis can be considered in terms of its two half-reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). These half-equations differ somewhat depending on the pH at which the electrolysis is carried out. At low pH, the HER and OER proceed as follows (all potentials are vs the standard hydrogen electrode, SHE)
Modifying the reactivity of substrates by encapsulation is a fundamental principle of capsule catalysis. Here we show an alternative strategy, wherein catalytic activation of otherwise inactive quinone "co-factors" by a simple Pd 2 L 4 capsule promotes a range of bulk-phase, radical-cation cycloadditions. Solution electron-transfer experiments and cyclic voltammetry show that the cage anodically shifts the redox potential of the encapsulated quinone by a significant 1 V. Moreover, the capsule also protects the reduced semiquinone from protonation, thus transforming the role of quinones from stoichiometric oxidants into catalytic single-electron acceptors. We envisage that the host−guest-induced release of an "electron hole" will translate to various forms of non-encapsulated catalysis that involve other difficult-to-handle, highly reactive species.
We report a series of rotaxane-based anion-π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in > 60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.
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