Structural characterization of metal containing catalysts is one of the key challenges facing modern chemistry (and biology). The detailed understanding of the structure of the active site of heterogeneous catalysts is a key element for controlling these complex systems and designing improved sites in a rational way, to modulate their activity and to progress to the conception of new and better catalysts as efficiently as possible. X-ray diffraction is the most widely used and reliable technique to study structural aspects in catalysis. However, today it is more and more often the case that single crystals cannot be produced, or more generally that for heterogeneous systems that there is not even the concept of a single crystal. This is the case for modern heterogeneous catalysts or for the large protein assemblies encountered in biology, and for which atomic level characterization is an extremely difficult challenge. Nevertheless, atomic level characterization is essential to further progress, and is currently the principle obstacle to development of better molecular catalysts.
In collaboration with the group of J.-M. Basset, C. Copéret and their co-workers at CPE Lyon, and R.R. Schrock at MIT, our goal since 2001 has been to address the question of how to study the structure and dynamics of a wide range of well-defined solid-state organometallic compounds on (e.g. silica) surfaces. The answer lies in advanced NMR methods.
This quest has led to considerable progress.
We have shown how it is possible to use sophisticated multi-dimensional NMR techniques to fully characterise catalytic species on silica surfaces, and subsequently to obtain structural and dynamic parameters related to their reactivity. To do this in a broad range of systems we have developed or implemented multi-dimensional techniques for 1H-13C or 1H-15N shift correlation, to measure 1H-13C scalar couplings, to observe double and triple quantum correlations, to obtain high-resolution proton spectra, or to measure residual dipolar interactions or chemical shift.
In particular, in landmark publications, we have recently shown (using triple-quantum experiments) how a single tungsten atom on a silica surface can activate molecular nitrogen, or how (using 1H-13C hetcor spectra) we can determine the reaction intermediates in alkene metathesis. We have also investigated the dynamics of a wide range of alkylidene complexes, by combining the measurements of time-averaged NMR parameters and DFT calculations (in collaboration with O. Einsenstein). This allowed us to introduce the highly original hypothesis of a dynamics-reactivity relationship for surface catalysis, similar to that well established for proteins.
