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

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journal contribution
posted on 15.06.2016, 17:20 authored by Ambikesh D. Dwivedi, Chinky Binnani, Deepika Tyagi, Kuber S. Rawat, Pei-Zhou Li, Yanli Zhao, Shaikh M. Mobin, Biswarup Pathak, Sanjay K. Singh
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 η6-arene and the troponate/aminotroponate ligand drives the formation of cyclometalated 2-phenylpyridine Ru–arene species, [(η6-arene)­Ru­(κ2-C,N-phenylpyridine) (OH2)]+ by elimination of troponate/aminotroponate ligands and retaining η6-arene, while cyclometalated 2-phenylpyridine Ru–troponate/aminotroponate species [(κ 2-troponate/aminotroponate)­Ru­(κ2-C,N-phenylpyridine)­(OH2)2] was generated by decoordination of η6-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.