Di- and Tetrameric Molybdenum Sulfide Clusters Activate and Stabilize Dihydrogen as Hydrides

NaY zeolite-encapsulated dimeric (Mo$_2$S$_4$) and tetrameric (Mo$_4$S$_4$) molybdenum sulfide clusters stabilize hydrogen as hydride binding to Mo atoms. Density functional theory (DFT) calculations and adsorption measurements suggest that stabilization of hydrogen as sulfhydryl (SH) groups, as typical for layered MoS$_2$, is thermodynamically disfavored. Competitive adsorption of H$_2$ and ethene on Mo was probed by quantifying adsorbed CO on partly hydrogen and/or ethene covered samples with IR spectroscopy. During hydrogenation, experiment and theory suggest that Mo is covered predominately with ethene and sparsely with hydride. DFT calculations further predict that, under reaction conditions, each MoxSy cluster can activate only one H$_2$, suggesting that the entire cluster (irrespective of its nuclearity) acts as one active site for hydrogenation. The nearly identical turnover frequencies (24.7 ± 3.3 mol$_{ethane}$·h$^{–1}$·mol$_{cluster}$$^{–1}$), apparent activation energies (31–32 kJ·mol$^{–1}$), and reaction orders (∼0.5 in ethene and ∼1.0 in H$_2$) show that the active sites in both clusters are catalytically indistinguishable