Troponate/Aminotroponate Ruthenium-Arene Complexes: Synthesis, Structure, and Ligand-Tuned Mechanistic Pathway for Direct C-H Bond Arylation with Aryl Chlorides in Water

A series of water-soluble troponate/aminotroponate ruthenium(II)-arene complexes were synthesized, where O,O and N,O chelating troponate/aminotroponate ligands stabilized the piano-stool mononuclear ruthenium-arene complexes. Structural identities for two of the representating complexes were also established by single-crystal X-ray diffraction studies. These newly synthesized troponate/aminotroponate ruthenium-arene complexes enable efficient C-H bond arylation of arylpyridine in water. The unique structure-activity relationship in these complexes is the key to achieve efficient direct C-H bond arylation of arylpyridine. Moreover, the steric bulkiness of the carboxylate additives systematically directs the selectivity toward mono- versus diarylation of arylpyridines. Detailed mechanistic studies were performed using mass-spectral studies including identification of several key cyclometalated intermediates. These studies provided strong support for an initial cycloruthenation driven by carbonate-assisted deprotonation of 2-phenylpyridine, where the relative strength of eta(6)-arene and the troponate/aminotroponate ligand drives the formation of cyclometalated 2-phenylpyridine Ru-arene species, [(eta(6)-arene)Ru(kappa(2)-C,N-phenylpyridine) (OH2)](+) by elimination of troponate/aminotroponate ligands and retaining eta(6)-arene, while cyclometalated 2-phenylpyridine Ru troponate/aminotroponate species [(kappa (2)-troponate/aminotroponate)Ru(kappa(2)-C,N-phenylpyridine)(OH2)(2)] was generated by decoordination of eta(6)-arene ring during initial, C-H bond activation of 2-phenylpyridine. Along with the experimental mass-spectral evidence, density functional theory calculation also supports the formation of such species for these complexes. Subsequently, these cycloruthenated products activate aryl chloride by facile oxidative addition to generate C-H arylated products.