Hemilabile ligands, combining strong and weak donor groups, have become essential tools for catalysis. The sulfonyl group, due to its weak donor character, remains unexplored for the stabilization of group 14 elements or transition metals. The sulfoxide group, on the other hand, can allow coordination by the sulfur atom or by the oxygen atom and has a stereogenic sulfur atom for applications in asymmetric catalysis. We have exploited the coordination properties of these two sulfur functions for the stabilization of metallylenes by pincer-ligands or of transition metals by mixed metallylene-sulfoxide ligands.
Metallylenes stabilized by pincer-type ligands
Pincer-type ligands are major tools for the stabilization of transition metal complexes. On the other hand, their use for the stabilization of metallylenes remains sporadic. We have developed a new type of pincer- ligands involving sulfonyl groups (SO2), making it possible to consider a hemilabile nature of the ligand linked to the presence of two oxygen atoms available on each sulfonyl function for the stabilization of group 14 elements (Angew. 2015, 54, 805). The hemilabile nature of the ligands has been demonstrated by the change in geometry (trans -> cis) of the two sulfonyl groups during the coordination of metallylenes (Ge, Sn) with transition metals (W, Fe) (Chem. Eur. J. 2016, 22, 1349 and Eur. J. Inorg. Chem. 2017, 29, 4214). This new type of ligand shows an adjustable character, allowing an electronic and geometric modification of the metallylene involved in coordination chemistry.
This new method of stabilization of metallylenes was completed by the development of bis-sulfoxide (SO) ligands and mixed sulfone-sulfoxide (SO2-SO) ligands, making it possible to demonstrate the prevalence of stabilization of the group 14 element by the sulfoxide group in comparison with the sulfone function, and allowing the insertion of a stereogenic information within the structure via the sulfoxide function (Dalton Trans. 2019, 48, 2399).
Mixed metallylene-sulfoxide ligands
Metallylenes exhibit a Lewis base character allowing their use as ligands for transition metals. However, examples of the use of these ligands in catalysis are rare compared to carbenes and have never been described in asymmetric catalysis. We have envisioned new mixed metallylene-sulfoxide ligands making it possible to insert stereogenic information within the structure of the transition complex, as well as a hemilabile nature of the ligand by associating a strongly s-donor germylene center and a weakly donor sulfoxide group. (Dalton Trans. 2018, 47, 15751). A library of various mixed germylene-sulfoxide ligands was prepared by varying the nature of the spacer group between the two units and their coordination properties were studied with different transition metals, showing complexation of the metallic center either by the sulfur atom or by the oxygen atom of the sulfoxide function. The hemilabile nature of these new ligands has been demonstrated through the reactivity of a nickel(0) complex under carbon monoxide atmosphere via the decoordination of the sulfoxide ligand from the metal center.