Joost N. H. Reek

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Two major energy-related problems confront the world in the next 50 years. First, increased worldwide competition for gradually depleting fossil fuel reserves (derived from past photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Transition Metal Catalysis Using Functionalized Dendrimers

Dendrimers are well-defined hyperbranched macromolecules with characteristic globular structures for the larger systems. These novel polymers have inspired many chemists to develop new materials and several applications have been explored, catalysis being one of them. The recent impressive strides in synthetic procedures increased the accessibility of functionalized dendrimers, resulting in a rapid development of dendrimer chemistry. The position of the catalytic site(s) as well as the spatial separation of the catalysts appears to be of crucial importance. Dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes, their efficient natural counterparts, whereas the surface-functionalized systems have been proposed to fill the gap between homogeneous and heterogeneous catalysis. This might yield superior catalysts with novel properties, that is, special reactivity or stability. Both the core and periphery strategies lead to catalysts that are sufficiently larger than most substrates and products, thus separation by modern membrane separation techniques can be applied. These novel homogeneous catalysts can be used in continuous membrane reactors, which will have major advantages particularly for reactions that benefit from low substrate concentrations or suffer from side reactions of the product. Here we review the recent progress and breakthroughs made with these promising novel transition metal functionalized dendrimers that are used as catalysts, and we will discuss the architectural concepts that have been applied.

Ligands that store and release electrons during catalysis

First-row transition metals can be given a noble character by redox-active ligands, thus enabling two-electron oxidative addition and reductive elimination steps (see scheme). A recently reported cobalt-mediated Negishi-type cross-coupling reaction provides an illustrative example of this concept and reveals its potential to develop new catalytic reactions with cheap, abundant metals.

Chiral Induction Effects in Ruthenium(II) Amino Alcohol Catalysed Asymmetric Transfer Hydrogenation of Ketones: An Experimental and Theoretical Approach

The enantioselective outcome of transfer hydrogenation reactions that are catalysed by ruthenium(II) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substituent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reaction to a large extent: enantioselectivities (ee values) of up to 95% were obtained for the reduction of acetophenone. The catalytic cycle of ruthenium(II) amino alcohol catalysed transfer hydrogenation was examined at the density functional theory level. The formation of a hydrogen bond between the carbonyl functionality of the substrate and the amine proton of the ligand, as well as the formation of an intramolecular H...H bond and a planar H-Ru-N-H moiety are crucially important for the reaction mechanism. The enantioselective outcome of the reaction can be illustrated with the aid of molecular modelling by the visualisation of the steric interactions between the ketone and the ligand backbone in the ruthenium(II) catalysts.

Co@NH2-MIL-125(Ti): cobaloxime-derived metal–organic framework-based composite for light-driven H2 production

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.

New directions in supramolecular transition metal catalysis

Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organometallic chemistry and transition metal catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and transition metal catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possibility of novel conversions induced by metal centres that are in unusual environments. This perspective describes new approaches to transition metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer transition metal catalysts for future applications.

Me2–NHC based robust Ir catalyst for efficient water oxidation

Me(2)-NHC proved to be a valuable ligand in iridium catalyzed water oxidation reactions, both when carried out electrochemically as well as upon oxidation with cerium ammonium nitrate. Mechanistic data suggest that water oxidation occurs efficiently at a well defined iridium species via a mononuclear pathway.

Mononuclear Water Oxidation Catalysts

Recently, several mononuclear water oxidation catalysts have been reported, a breakthrough considering the dogma that at least two metal sites were required to oxidize water efficiently. In this Review various mononuclear catalysts which have been reported in the last five years are reviewed, as well as their implementation in prototype devices that allow dioxygen formation to be coupled to dihydrogen production will be discussed.

Assembly of Encapsulated Transition Metal Catalysts

Enforced ligand dissociation as a result of steric interactions between ZnII porphyrin units and the N atoms of pyridylphosphane ligands determines the catalytic properties of the encapsulated transition metal complexes. These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-catalyzed hydroformylation. M=transition metal catalyst.

Supramolecular control on chemo- and regioselectivity via encapsulation of (NHC)-Au catalyst within a hexameric self-assembled host

The encapsulation of a Au(I) catalyst within a self-assembled, hydrogen bonded, hexameric capsule dramatically changes its catalytic activity, leading to unusual products due to the steric requirements of the hosts cavity.