Ruthenium-Catalyzed Oxidative Homocoupling of Arylboronic Acids in Water: Ligand Tuned Reactivity and Mechanistic Study

Molecular catalysts based on water-soluble arene-Ru­(II) complexes ([Ru]-1–[Ru]-5) containing aniline (L1), 2-methylaniline (L2), 2,6-dimethylaniline (L3), 4-methylaniline (L4), and 4-chloroaniline (L5) were designed for the homocoupling of arylboronic acids in water. These complexes were fully characterized by 1H, 13C NMR, mass spectrometry, and elemental analyses. Structural geometry for two of the representative arene-Ru­(II) complexes [Ru]-3 and [Ru]-4 was established by single-crystal X-ray diffraction studies. Our studies showed that the selectivity toward biaryls products is influenced by the position and the electronic behavior of various substituents of aniline ligand coordinated to ruthenium. Extensive investigations using 1H NMR, 19F NMR, and mass spectral studies provided insights into the mechanistic pathway of homocoupling of arylboronic acids, where the identification of important organometallic intermediates, such as σ-aryl/di­(σ-aryl) coordinated arene-Ru­(II) species, suggested that the reaction proceeds through the formation of crucial di­(σ-aryl)-Ru intermediates by the interaction of arylboronic acid with Ru-catalyst to yield biaryl products.