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 η(6)-arene and the troponate/aminotroponate ligand drives the formation of cyclometalated 2-phenylpyridine Ru-arene species, [(η(6)-arene)Ru(κ(2)-C,N-phenylpyridine) (OH2)](+) by elimination of troponate/aminotroponate ligands and retaining η(6)-arene, while cyclometalated 2-phenylpyridine Ru-troponate/aminotroponate species [(κ (2)-troponate/aminotroponate)Ru(κ(2)-C,N-phenylpyridine)(OH2)2] was generated by decoordination of η(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.

Water-soluble arene-ruthenium complexes coordinated with readily available aniline-based ligands were successfully employed as highly active catalysts in the C-H bond activation and arylation of 2-phenylpyridine with aryl halides in water. A variety of (hetero)aryl halides were also used for the ortho-C-H bond arylation of 2-phenylpyridine to afford the corresponding ortho- monoarylated products as major products in moderate to good yields. Our investigations, including time-scaled NMR spectroscopy and mass spectrometry studies, evidenced that the coordinating aniline-based ligands, having varying electronic and steric properties, had a significant influence on the catalytic activity of the resulting arene-ruthenium-aniline-based complexes. Moreover, mass spectrometry identification of the cycloruthenated species, {(η -arene)Ru(κ -C,N-phenylpyridine)} , and several ligand-coordinated cycloruthenated species, such as [(η -arene)Ru(4-methylaniline)(κ -C,N-phenylpyridine)] , found during the reaction of 2-phenylpyridine with the arene-ruthenium-aniline complexes further authenticated the crucial roles of these species in the observed highly active and tuned catalyst. At last, the structures of a few of the active catalysts were also confirmed by single-crystal X-ray diffraction studies.

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 (1)H, (13)C 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 (1)H NMR, (19)F 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.

Water-soluble ruthenium(II)-arene complexes [(η 6arene)Ru(κ 2 -L)] n+ (n = 0, 1) ([Ru]-1-[Ru]-10) containing pyridine-based bischelating N,O/N,N donor ligands (L1-L5) are synthesized and employed for the catalytic C-H bond activation/ arylation of a wide range of 2-phenylpyridines and aryl halides in water, affording the corresponding mono-and biarylated products. Exploring the reactivity of the synthesized complexes, our investigations, including time-dependent 1 H NMR spectroscopic studies with ruthenium-arene catalysts, demonstrate a remarkable structure-activity relationship for the ligand-tuned C-H activation/arylation of 2-phenylpyridine, where the complexes with bischelating N,O donor-based ligands (acteylpyridine and picolinate) outperform those with N,N donor ligands [a] ditives, driven by steric bulkiness, in tuning the selectivity of C-H arylation products. [13] Later, we also explored aniline-based ruthenium-arene complexes for C-H bond arylation in water, where the strength of the aniline-ruthenium bond and substitution on the aniline ligand were found to be crucial factors for tuning the selectivity of mono-versus biarylated products during the reaction. [14] Further, in our deliberate efforts to develop an efficient catalytic system for C-H activation/functionalization reactions, [13,14] herein we have synthesized a series of ruthenium(II)-arene complexes containing pyridine-based N,O and N,N donor ligands, and we have systematically investigated their catalytic performance for ortho C-H bond activation/arylation of 2-phenylpyridine with several aryl halides in water. The synthesized ruthenium-arene complexes have been well-characterized using NMR spectroscopy and mass spectrometric analysis, and the molecular structure of a few of the representative complexes have been authenticated by single-crystal X-ray diffraction studies. Attempts have also been made to establish the relationship between the pattern of bonding of the ligands to the ruthenium metal center and the observed catalytic activity of these complexes for ortho C-H bond-activation/arylation reactions using time-scaled 1 H NMR spectroscopy. Results and Discussion Synthesis of Water-Soluble Ruthenium(II)-Arene Complexes [Ru]-1 to [Ru]-10Water-soluble ruthenium-arene complexes ([Ru]-1-[Ru]-10) containing pyridine-based N,O and N,N donor ligands were synthesized in good yield by reacting the respective ruthenium(II)arene dimer with the readily available ortho-substituted pyridine-based N,O/N,N donor bidentate ligands, with substituents ranging from carboxylic [pyridine-2-carboxylic acid (L1)], acetyl [2-acetylpyridine (L2)], ester [2-methylpicolinate (L3)], and imino groups [N-benzyl-pyridylimine (L4) and N-butyl-pyridylimine (L5)], as shown in Scheme 1. Scheme 1. Synthesis of ruthenium-arene complexes [Ru]-1-[Ru]-10 containing pyridine-based N,O and N,N donor ligands.

A variety of ligands are being explored extensively to achieve enhanced performance for ligand assisted C-H bond activation/functionalization reactions. We explored here several readily available biomass-derived ligands as effective additives to significantly enhance the catalytic activity of arene-Ru(II) dimer for ortho C-H bond arylation in a water-based catalytic reaction. We achieved almost 7-fold enhancement in the catalytic activity with [(η 6 -p-cymene)RuCl 2 ] 2 catalyst in the presence [a] At an outset, we evaluated the ligands L1-L7 (2 mol-%) for C-H arylation of 2-phenylpyridine (1a) with 4-chloroanisole (2a), as model substrates, catalyzed by [(η 6 -p-cymene)RuCl 2 ] 2 (1 mol-%) catalyst at 80°C in water/ethanol (9:1 v/v) solution. After evaluating various biomass-derived ligands (L1-L7), we found that levulinic acid (L1), greatly accelerated the catalytic activity with 70 % yield of monoarylated product (3a) in 4 h (3a/4a selectivity = 93:7) (Table 1, entry 2). Yield for 3a was further improved to 87 % by extending the reaction for 8 h under analogous reaction condition. The remarkable activity shown by Ru(II) catalyst in the presence of the acyclic ligand L1, can be Eur.