Transition metal catalysis plays an important role in both industry and in academia where selectivity, activity and stability are crucial parameters to control. Next to changing the structure of the ligand, introducing a confined space as a second coordination sphere around a metal catalyst has recently been shown to be a viable method to induce new selectivity and activity in transition metal catalysis. In this review we focus on supramolecular strategies to encapsulate transition metal complexes with the aim of controlling the selectivity via the second coordination sphere. As we will discuss, catalyst confinement can result in selective processes that are impossible or difficult to achieve by traditional methods. We will describe the template-ligand approach as well as the host–guest approach to arrive at such supramolecular systems and discuss how the performance of the catalyst is enhanced by confining it in a molecular container.

Confined nanospaces in which reactions can take place, have been created by various approaches such as molecular capsules, zeolites and micelles. In this tutorial review we focus on the application of self-assembled nanocapsules with well-defined cavities as nanoreactors for organic and metal catalysed transformations. The self-assembly of nanocapsules based on noncovalent bonds such as hydrogen bonds and metal–ligand interactions is discussed to introduce the properties of the building blocks and capsules thereof. We will elaborate on the encapsulation effects that can be expected when reactions are carried out in a capsule-protected environment. Subsequently, literature examples will be described in which self-assembled nanocapsules are applied as nanoreactors, for various types of organic and metal catalysed reactions.

We present a synthetic strategy for the efficient encapsulation of a derivative of a well-defined cobaloxime proton reduction catalyst within a photoresponsive metal–organic framework (NH₂-MIL-125(Ti)). The resulting hybrid system Co@MOF is demonstrated to be a robust heterogeneous composite material. Furthermore, Co@MOF is an efficient and fully recyclable noble metal-free catalyst system for light-driven hydrogen evolution from water under visible light illumination.

Solar energy driven processes with H₂O and CO₂ as basic feedstocks can produce “solar fuels” that could substitute their fossil based counterparts. This article summarizes the main findings of a techno-economic analysis of systems that can generate different types of fuels with renewable energy as starting point. These “renewable fuels” could potentially play a key role in future energy systems, both as a storage medium in the power sector and as an energy carrier in e.g. the transport sector, or deliver fundamental building blocks for the chemical industry. We determine whether, how, and when renewable fuels might become competitive alternatives for fossil fuels. The technologies required to produce renewable fuels are analyzed by the application of learning curves associated with individual system components. We thereby make projections for possible decreases in investment costs and reductions in fuel production costs. In an optimistic scenario we find that competitivity could be reached between 2025 and 2048 for all renewable energy production pathways that we investigate, for hydrogen, syngas, methanol, and diesel. Two techniques yield break-even costs before 2050 even in a conservative base case scenario: H₂ production through electrolysis and diesel production by Fischer–Tropsch synthesis. Both processes use solid oxide electrolysis, which profits from rapid cost reductions and high efficiency.

Transition metal catalysis is of utmost importance for the development of sustainable processes in academia and industry. The activity and selectivity of metal complexes are typically the result of the interplay between ligand and metal properties. As the ligand can be chemically altered, a large research focus has been on ligand development. More recently, it has been recognized that further control over activity and selectivity can be achieved by using the "second coordination sphere", which can be seen as the region beyond the direct coordination sphere of the metal center. Hydrogen bonds appear to be very useful interactions in this context as they typically have sufficient strength and directionality to exert control of the second coordination sphere, yet hydrogen bonds are typically very dynamic, allowing fast turnover. In this review we have highlighted several key features of hydrogen bonding interactions and have summarized the use of hydrogen bonding to program the second coordination sphere. Such control can be achieved by bridging two ligands that are coordinated to a metal center to effectively lead to supramolecular bidentate ligands. In addition, hydrogen bonding can be used to preorganize a substrate that is coordinated to the metal center. Both strategies lead to catalysts with superior properties in a variety of metal catalyzed transformations, including (asymmetric) hydrogenation, hydroformylation, C-H activation, oxidation, radical-type transformations, and photochemical reactions.