Cuboidal Mo3S4 Clusters as a Platform for Exploring Catalysis: A Three-Center Sulfur Mechanism for Alkyne Semihydrogenation

Abstract

We report a trinuclear Mo3S4 diamino cluster that promotes the semihydrogenation of alkynes. Based on experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the cluster act as the active site for this transformation. In the first step, two of these mu-S ligands react with the alkyne to form a dithiolene adduct; this process is formally analogous to the olefin adsorption on MoS2 surfaces. Then, H-2 activation occurs in an unprecedented way that involves the third mu-S center, in cooperation with one of the dithiolene carbon atoms. Notably, this step does not imply any direct interaction between H-2 and the metal centers, and directly results in the formation of an intermediate featuring one (mu-S)-H and one C-H bond. Finally, such half-hydrogenated intermediate can either undergo a reductive elimination step that results in the Z-alkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates the E-alkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of Z- and E-stilbene. The results herein could have significant implications on the understanding of the catalytic properties of MoS2-based materials.