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 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.