Operando XAFS Studies on Rh(CAAC)-Catalyzed Arene Hydrogenation

Rh K-edge X-ray absorption fine structure (XAFS) spectroscopy was used to examine the Rh-catalyzed arene hydrogenation of diphenyl ether by a combination of stoichiometric reactions of [(CAAC(Cy,Dipp))Rh(COD)Cl] (Rh-C1) (CAAC(Cy,Dipp)= cyclic alkyl amino carbene) and operando XAFS kinetics studies. Our results unequivocally show that Rh nanoparticles, generated from the single-site Rh complex Rh-Cl, catalyze the arene hydrogenation. Operando XAFS studies illuminate the role of silver cation on the precatalyst reactivity, the effect of increasing H-2 pressure on increasing the catalytic efficiency, the stabilizing influence of Ph2O on the relative rate of formation of active Rh nanoparticles, and the absence of soluble single-site Rh species that might leach from bulk heterogeneous Rh nanoparticles. We gained insights into the divergent deactivation pathways mediated by substoichiometric benzothiophene and excess (KOBu)-Bu-t toward H-2 activation, which is a key step en route to Rh nanoparticles for arene hydrogenation. Excess KO t Bu leads to the formation of a Rh-(OBu)-Bu-t complex that interferes with H-2 activation, precluding the formation of Rh nanoparticles. Benzothiophene does not interfere with the activation of H-2 at Rh in the CAAC(Cy,Dipp) complex while Rh nanoparticles are formed. Once Rh nanoparticles are formed, however, benzothiophene binds irreversibly to the Rh nanoparticles, preventing the adsorption of H-2 and diphenyl ether for arene hydrogenation.