Unusual metal / ligand interactions supported by donor buttresses – MeLiBo

As expected, the project involves strong and fruitfull interaction between experiment and theory. The synthetic and characterisation work in the lab very much benefit from the computational studies, carried out to help design the most promising targets and rationalize the obtained results (bonding modes, spectroscopic data, reactivity)

- The variety of complexes featuring M -> Z interactions has been extended to Indium. Triphosphine-indance complexes have been prepared and charcaterized with gold and palladium.
- All phosphine-borane complexes known to date feature an organic linker between the two sites. We have shown that phosphino-boranes, with direct P-B linkage, can also behave as ambiphilic ligands. A Pt complex has been prepared and its bonding situation has been thoroughly analyzed. It is side-on coordinated, an alkene-like behavior, but electronically dissymetric.
- The strategy consisting in using phosphorus buttresses has been extrapolated to the activation of P-C bonds. Starting from a diphosphine-phosphine oxyde ligand, we have obtained original pincer complexes, via oxydative addition of P-C bond at Pd.
- The same approach also proved very fruitfull to study the interaction of disilanes with coinage metals. With copper, it is possible to caracterize a sigma complex of disilane, while gold undergoes spontaneous oxidative addition. These behaviors / elementary steps were so far believed to be strongly disfavored.

The obtained results suggest that ambiphilic ligands and complexes of the coinage metals may display very different behavior than other systems. The understanding of all underlying factors will require important fundamental studies. Interesting perspectives in catalyse are also offered

The first stage of this project has lead to 5 major publications, including 2 JACS and 1 Angewandte. The visibility of the results is substantiated by the important number of conferences / oral communications (20 including 18 in international events).

Thanks to the progress achieved over the last few years with group 13 (B, Al, Ga…) and group 14 (Si, Sn) elements, the ability of Lewis acids to coordinate as Z-type ligands (two-electron sigma-acceptors) to transition metals appears much more general than believed initially. This concept opens very interesting perspectives, but our knowledge of sigma-acceptor ligands still remains very superficial. In particular, we have only a very rough idea of the variety of M->Z interactions, and do not know that much either about their influence on the properties of metal complexes. In this proposal, we seek to determine to which extent the Lewis acidity can be decreased while retaining the ability to coordinate as sigma-acceptor ligand (phosphonium complexes) and we will study the influence of Z-type ligands on the optical properties of complexes (is it possible to modulate the photophysical properties of complexes by varying the nature or even only the substituents of sigma-acceptor ligands?).

In addition, we will extend the strategy that consists in using donor buttresses (typically phosphine moieties) to support other types of unusual metal/ligand interactions than M->Z. In this respect, we will explore the sigma coordination of diboranes and disilanes. Only one complex of this type is known to date, although they are key intermediates in catalytic diboration and bis-silylation reactions. The characterization of such sigma-diborane and sigma-disilane complexes would provide valuable information regarding the bonding situation (donation/backdonation, sigma-coordination/oxidative addition). Furthermore, the oxidative addition of SiSi single bonds could give access to original high-oxidation state complexes [Pd(IV), Au(III)].

Another important objective of this proposal is to develop theoretical methods to describe the spectroscopic properties of complexes. This concerns not only the UV-visible absorption and emission spectra (via the optimisation of the triplet and singlet excited-states), but also the NMR characteristics of heteronuclei (31P, 11B, 29Si) for complexes featuring direct metal/heteroelement contacts.

Lastly, we seek to develop and evaluate dynamic methods that allow to describe accurately the reactivity of complexes, and especially the energetic data (reaction energies and activation barriers). For this purpose, the activation of SiSi single bonds [from sigma-disilane to bis(silyl) complexes] represents a good model. The use of Effective Group Potentials will be investigated and compared with QM/MM methods.