The large-scale production of clean energy is one of the major challenges society is currently facing. Molecular hydrogen is envisaged as a key green fuel for the future, but it becomes a sustainable alternative for classical fuels only if it is also produced in a clean fashion. Here, we report a supramolecular biomimetic approach to form a catalyst that produces molecular hydrogen using light as the energy source. It is composed of an assembly of chromophores to a bis(thiolate)-bridged diiron ([2Fe2S]) based hydrogenase catalyst. The supramolecular building block approach introduced in this article enabled the easy formation of a series of complexes, which are all thoroughly characterized, revealing that the photoactivity of the catalyst assembly strongly depends on its nature. The active species, formed from different complexes, appears to be the [Fe 2(-pdt)(CO)4{PPh2(4-py)}2] (3) with 2 different types of porphyrins (5a and 5b) coordinated to it. The modular supramolecular approach was important in this study as with a limited number of building blocks several different complexes were generated.photocatalysis ͉ self-assembly ͉ supramolecular chemistry ͉ metalloporphyrin chromophore ͉ Stern-Volmer plot S upramolecular chemistry, defined by Nobel Prize Laureate Jean-Marie Lehn as the ''chemistry beyond the molecule,'' has changed the way we look at molecules (1). Besides exploring reactivity of molecules, interaction between molecules has become of dominant importance as it provides new means of controlling properties of chemical systems. Supramolecular chemistry has rapidly evolved into a mature field, and the implementation of supramolecular strategies has resulted in breakthroughs in several disciplines (2-4). The reversible character of noncovalent chemistry gives rise to concepts such as adaptation and self-correction, creating fundamentally different system properties compared with traditional covalent strategies. The modular character associated with the building block approach in supramolecular chemistry provides an easy strategy to generate large libraries of analogous structures of nanosize dimension. Such libraries are of interest in research areas where accurate prediction of particular properties of chemical systems is inadequate or impossible. For example, means to predict the selectivity provided by transition metal catalyst are lacking, and therefore high throughput screening of libraries of catalysts is still the most powerful method to find catalyst systems with desired selectivities. Indeed, we and others have introduced supramolecular ways to make transition metal catalysts and used the building block approach to create large libraries of related catalysts, some of which show unrivaled selectivities (5-9).Stimulated by these exciting results, we were wondering whether supramolecular strategies could also provide solutions to other challenges in catalysis. One of the greatest challenges our society is currently facing is the large-scale production of clean energy (10). Molecular hydrogen is e...

The kinetics of the stereoselective semi-hydrogenation of 4-octyne in THF by the highly active catalyst [Pd{(m,m'-(CF(3))(2)C(6)H(3))-bian}(ma)] (2) (bian = bis(imino)acenaphthene; ma = maleic anhydride) has been investigated. The rate law under hydrogen-rich conditions is described by r = k[4-octyne](0.65)[Pd][H(2)], showing first order in palladium and dihydrogen and a broken order in substrate. Parahydrogen studies have shown that a pairwise transfer of hydrogen atoms occurs in the rate-limiting step. In agreement with recent theoretical results, the proposed mechanism consists of the consecutive steps: alkyne coordination, heterolytic dihydrogen activation (hydrogenolysis of one Pd-N bond), subsequent hydro-palladation of the alkyne, followed by addition of N-H to palladium, reductive coupling of vinyl and hydride and, finally, substitution of the product alkene by the alkyne substrate. Under hydrogen-limiting conditions, side reactions occur, that is, formation of catalytically inactive palladacycles by oxidative alkyne coupling. Furthermore, it has been shown that (Z)-oct-4-ene is the primary reaction product, from which the minor product (E)-oct-4-ene is formed by an H(2)-assisted, palladium-catalyzed isomerization reaction.

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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.