Hydrogenation of N-Heteroarenes Using Rhodium Precatalysts: Reductive Elimination Leads to Formation of Multimetallic Clusters

A rhodium-catalyzed method for the hydrogenation of N-heteroarenes is described. A diverse array of unsubstituted N-heteroarenes including pyridine, pyrrole, and pyrazine, traditionally challenging substrates for hydrogenation, were successfully hydrogenated using the organometallic precatalysts, [(η⁵-C₅Me₅)Rh(N-C)H] (N-C = 2-phenylpyridinyl (ppy) or benzo[h]quinolinyl (bq)). In addition, the hydrogenation of polyaromatic N-heteroarenes exhibited uncommon chemoselectivity. Studies into catalyst activation revealed that photochemical or thermal activation of [(η⁵-C₅Me₅)Rh(bq)H] induced C(sp²)-H reductive elimination and generated the bimetallic complex, [(η⁵-C₅Me₅)Rh(μ₂,η²-bq)Rh(η⁵-C₅Me₅)H]. In the presence of H₂, both of the [(η⁵-C₅Me₅)Rh(N-C)H] precursors and [(η⁵-C₅Me₅)Rh(μ₂,η²-bq)Rh(η⁵-C₅Me₅)H] converted to a pentametallic rhodium hydride cluster, [(η⁵-C₅Me₅)₄Rh₅H₇], the structure of which was established by NMR spectroscopy, X-ray diffraction, and neutron diffraction. Kinetic studies on pyridine hydrogenation were conducted with each of the isolated rhodium complexes to identify catalytically relevant species. The data are most consistent with hydrogenation catalysis prompted by an unobserved multimetallic cluster with formation of [(η⁵-C₅Me₅)₄Rh₅H₇] serving as a deactivation pathway.