ALBRECHT RESEARCH GROUP

We are fascinated by the diversity of effects that ligands can have to instigate new properties in transition metal complexes and we are therefore strongly interested in designing and developing new ligands that impart specific properties to the metal center. One of these ligands are triazolylidenes, a mesoionic carbene that we discovered in 2008. This ligand offers various opportunities for catalysis, including strong electron donor properties, a high synthetic flexibility that allows for a wide variety of functional and chelating groups to be incorporated, and a high polarity, which imparts solubility in water and other polar media. We have obtained evidence in a variety of catalytic reactions like nickel-catalysed hydrosilylation, ruthenium-catalyzed alcohol dehydrogenation, or iridium-catalyzed water oxidation, that the triazolylidene ligand scaffold is controlling the reactivity of the coordinated metal center. Because of the synthetic versatility and the easy accessibility of the ligand, tailoring of the ligand properties is straightforward and we have disclosed improved catalytic activity by ligand tuning. More recently, we have discovered a straightforward approach to functionalize the ligand on the complex as opposed to the more traditional approach of preparing a functional ligand and only then install the metal center. This approach gives rapid access to a large diversity of functionalized ligands in a very last step, and to ligand scaffolds that are not accessible by traditional routes.

Our efforts to improve catalytic activities of homogeneous transition metal-based catalysts focuses currently in particular on redox reactions, including transfer hydrogenation, hydrosilylation, olefin oxidation, alcohol and water oxidation. A common feature of most of these redox processes is the challenge to manage both the electron transfer (the actual redox event) and also the concomitant proton transfer. Efficient coupling of these two processes sets specific requirements to the catalyst design. Our investigations — strongly benefiting from stimulating collaborations with experts in spectroscopy and computational chemistry — are targeted in particular towards a better understanding of the mechanism of the catalytic cycle as a rational approach to improve the catalytic activity. For example, we have developed very robust water oxidation catalysts based on a triazolylidene iridium scaffold. Much of this work is performed in inspiring collaborations (S. Bernhard, CMU; A. Llobet, ICIQ; A. Macchioni, U Perugia; E. Clot, U. Montpellier; and others) More recently, we have put a major focus on replacing precious metal complexes that are typically used for such redox processes with Earth-abundant metals such as iron, nickel, and cobalt (collaborations within the ITN NoNoMeCat; J. McMaster, U Nottingham).

Inspired by the tautomeric relationship of imidazole and N-heterocyclic carbenes, we have been intrigued about the implications of such carbenes in metalloenzymes. Enzymes are the machinery of all biological transformations, and provide inspiration for designing artificial catalysts. New insights into the mode of action of enzymes is therefore instrumental for developing more competent synthetic catalysts as well as for better understanding natural systems. In collaboration with F. Paradisi (U Nottingham), we have begun to explore carbene binding to metalloenzymes. We have been focusing on blue copper proteins such as azurin and specific mutants of azurin to investigate the possibility of carbene bonding to the copper center, and to evaluate the effect that such bonding entails. While azurin is predominantly involved in electron transfer processes through reversible copper(II) reduction and oxidation, other blue copper proteins are catalytically active in redox transformations of substrates. Hence, these enzymes provide an attractive probe to explore the relevance of carbene bonding on the catalytic performance of the enzyme.

We are exploring avenues to organize functional molecular entities into larger ensembles by using self-assembly and coordination polymers. For example, we have investigated the electron transfer behaviour through a ditopic carbene and have assembled such materials into one-dimensional polymers that can, in principal, serve as molecular wires for shuttling electrons. In an alternative approach, self-assembly in solution has been utilized to arrange complexes with a spin-labile iron center into larger aggregates, which induces cooperative spin transitions in solution. The transition temperature is predictable based on the molecular recognition units that are available for self-assembly. In a next step, we aim at transferring these aggregates onto supports, for example by using Langmuir-Blodgett techniques (collaborations with S. Brooker, Otago U; G. Morgan, UCD; T. Gunnlaugsson, TCD).