Changes in transition metal oxidation state, often formal 2-electron redox changes, are common during homogenous catalytic processes. For example, many homogeneous catalytic reactions use oxidative addition to break bonds and later reductive elimination to form new bonds. Thus, lowering the energy barriers of the oxidative addition and reductive elimination is important for the catalyst design. Recently, our group has synthesized a series of rhodium complexes with capping arene ligands, including 8,8′-(1,2-phenylene)diquinoline (6-FP), 8,8′-(1,2-naphthalene)diquinoline (6-NPFP), 1,2-bis(N-7-azaindolyl)benzene (5-FP), and 1,2-bis(N-7-azaindolyl)naphthalene (5-NPFP)}. We have hypothesized that the capping arene ligands can be used to tune the relative energetics of different redox states during catalytic processes. This presentation will focus on studies of catalytic olefin hydrogenation, which generally has well-defined reaction pathways, to compare the ligand effect of four (FP)Rh(η2-C2H4)(Cl) (FP = 6-FP, 6-NPFP, 5-FP or 5-NPFP) complexes. The rates of olefin hydrogenation show a trend that 6-NPFP &gt; 5-NPFP &gt; 6-FP &gt; 5-FP. Using combined experimental and computational studies, we have attempted to understand the impact of the capping arene ligand on the Rh catalyzed styrene hydrogenation. For the (6-FP)Rh(η2-C2H4)(Cl) catalyzed styrene hydrogenation, the ΔH‡ and ΔS‡ are 14.2(6) kcal●mol-1 and -18(2) cal●K-1●mol-1, respectively. H/D scrambling between D2 and styrene is observed in which deuterium is more selectively incorporated into the β position of styrene. With the help of DFT calculations, we have proposed a plausible mechanism for this catalyzed styrene hydrogenation.

Investigation of capping arene ligated rhodium complexes for catalysis: A mechanistic study of olefin hydrogenation

ACS Spring 2022

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