Ruthenium-Catalyzed Oxidative Homocoupling of Arylboronic Acids in

Jun 8, 2016 - Chinky Binnani , Deepika Tyagi , Rohit K. Rai , Shaikh M. Mobin , Sanjay K. Singh. Chemistry - An Asian Journal 2016 11 (21), 3022-3031 ...
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Ruthenium-Catalyzed Oxidative Homocoupling of Arylboronic Acids in Water: Ligand Tuned Reactivity and Mechanistic Study Deepika Tyagi,† Chinky Binnani,† Rohit K. Rai,† Ambikesh D. Dwivedi,† Kavita Gupta,† Pei-Zhou Li,§ Yanli Zhao,§ and Sanjay K. Singh*,†,‡ †

Discipline of Chemistry, School of Basic Sciences, and ‡Centre of Material Science and Engineering, Indian Institute of Technology (IIT) Indore, Indore 453552, Madhya Pradesh, India § Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore S Supporting Information *

ABSTRACT: 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(σaryl) 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.

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(∼130 °C)10 and in a few cases with additives like ptoluenesulfonyl chloride under N2 atmosphere.12 Moreover, arene-Ru(II) complexes represent an active class of catalysts for several catalytic reactions, such as hydrogenation reactions of unsaturated bonds of carbonyl, imines and alkene bonds, C−N coupling (amidation and hydroamination), C−H bond activation, and oxidative Heck reactions.13,14 Brown et al. reported arene-Ru(II)-catalyzed oxidative Heck reaction using PhB(OH)2 and butyl acrylate for the formation of methylcinnamate in THF with [(η6-p-cymene)Ru(PPh3)Cl2] catalyst and CuCl2 as reoxidant.14a Interestingly, phosphine-free dichloro ruthenium dimer catalyst, [(η6-p-cymene)RuCl2]2 was found to be more active than the [(η6-p-cymene)Ru(PPh3)Cl2], suggesting the inhibitory role of PPh3 in [(η6-pcymene)Ru(PPh3)Cl2] catalyst.14a Studies suggested that the dissociation of ligands (PPh3), chloro-bridge breaking, and coordination properties of the solvents used for the catalytic reactions are few of the several crucial factors influencing these catalytic reactions.14a We have also reported arene-Ru(II) catalysts containing simple nitrogen bound ligand as active catalysts for C−N coupling reaction and hydrogenation/ringopening reaction in water and observed that arene-Ru(II) complexes remain highly active even under aqueous−aerobic

INTRODUCTION

The synthesis of biaryls is important as they are abundantly present in natural products, and extensively applied in pharmaceuticals, polymers, optically active ligands, and various fine chemicals.1 Several methodologies have been developed to synthesize both symmetrical and asymmetrical biaryls.2 Among these, transition metal-catalyzed C−C bond formation for the synthesis of symmetrical biaryls has emerged as an important synthetic methodology.3 Typically, the bottleneck in this process is the key step involving the formation of metal− carbon bond; to overcome this, several nucleophiles such as PhN2BF4, SnBu3Ph, PhMgBr, ArBF3K, and ArB(OH)2 are being widely used as a source of aryl group.4 Among these nucleophiles, arylboronic acid offers several advantages, such as its ready availability, easiness to handle, and less toxicity (particularly over Sn-based reagents).4 For homocoupling of arylboronic acids to biaryls, several catalytic systems based on Cu, Au, Pd, Ni, Cr, Rh, etc., were reported in the past (Scheme 1).5−10 There are several reports using transition metal salts for homocoupling reaction of arylboronic acids under mild conditions, but reports based on homogeneous complexcatalyzed homocoupling reactions are very few (Scheme 1 and Table S1).5−10 Moreover, most of these homogeneous complex-catalyzed reactions were performed in organic solvents (toluene, xylene, DMF, THF)6c,f,11 or at high temperature © XXXX American Chemical Society

Received: May 6, 2016

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DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Catalytic Homocoupling Reaction of Arylboronic Acids Using Various Metal Catalysts

Scheme 2. Synthesis of Arene-Ru(II) Complexes ([Ru]-1−[Ru]-5) with Several Aniline-Based Ligands (L1−L5) Investigated for Homocoupling of Arylboronic Acid

conditions.15 In particular, [(η6-benzene)Ru(aniline)Cl2] complex, [Ru]-1, employed for C−N coupling reaction, displayed superior catalytic reaction than the corresponding dichloro bridged arene-ruthenium precursor dimer catalyst, [(η6benzene)RuCl2]2, under analogous reaction conditions.15a We revealed that being a simple σ-donor ligand, aniline ligands can form a stable mononuclear ruthenium complex, which is a structural analogue of [(η6-arene)Ru(PPh3)Cl2], but in contrary to PPh3, aniline can be easily displaced by incoming coordinating reactant molecule under catalytic reaction conditions.15a These findings were indeed interesting, as electronic and steric properties of the aniline ligand can be easily fine-tuned by applying several substituents on the phenyl ring. Moreover, [Ru]-1 complex advantageously showed high aqueous solubility, and therefore offered opportunity to perform water-based catalytic reaction. Inspired by the high activity of arene-Ru(II) complexes for various C−C coupling reactions, where the reaction of ArB(OH)2 with Ru(II) precursor readily formed Ru−C bond via transmetalation,14 we herein systematically investigated highly active water-soluble arene-Ru(II) catalysts, [(η6benzene)Ru(L)Cl2] (L = aniline or substituted aniline), for catalytic homocoupling of ArB(OH)2 to symmetrical biaryls in water, considering that di(σ-aryl)-Ru species may also form

during the catalytic reaction. While establishing the solid-state structure of the catalyst by single-crystal X-ray diffraction, we have extensively investigated the mode of interaction of the (η6benzene)-Ru(II)aniline catalyst ([Ru]-1) with ArB(OH)2 using mass spectrometry and 1H and 19F NMR. On the basis of the identification of a few of the important intermediates including σ-aryl/di(σ-aryl)-Ru(II) species by mass spectrometry and 19F NMR, we also proposed a considerable mechanistic approach for the catalytic homocoupling of ArB(OH)2 to symmetrical biaryls in water.

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RESULTS AND DISCUSSION Synthesis and Characterization of Water-Soluble Arene-Ru(II)-Based Molecular Catalysts Containing Simple Aniline Ligands. At the outset, we synthesized areneRu(II) complexes with aniline-based ligands in methanol using our previously reported procedure.15a We chose aniline (L1), omethylaniline (L2), 2,6-dimethylaniline (L3), p-methylaniline (L4), and p-chloroaniline (L5) as model ligands, considering the electronic and steric effects due to the different substituents. These ligands (L1−L5) were reacted with [(η6-benzene)RuCl2]2 in methanol under refluxing condition to obtain the corresponding neutral mononuclear arene-Ru(II) complexes, [(η6-benzene)Ru(L)Cl2] (L = L1 ([Ru]-1), L2 ([Ru]-2), L3 B

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Inorganic Chemistry

Figure 1. Crystal structure of the complex [Ru-3]. Ellipsoids are set at 30% probability. All hydrogen atoms, except those on nitrogen, are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ru1−N1 2.181(3), Ru1−Cl1 2.402(11), Ru1−Cl2 2.421(10), Ru1−Cav 1.174, Cl1−Ru1− Cl2 88.91(4), N1−Ru1−Cl1 81.93(9), N1−Ru1−Cl2 82.03(8), C7−N1−Ru1 120.3(2).

the η6-benzene ring. The Ru−nitrogen bond distance (Ru1− N1) for the sterically crowded ligand (L3) is 2.181 Å, which is slightly higher than the Ru−N bond distance (2.158 Å) for our previously reported complex with L1 ligand ([Ru]-1). Ru− chlorine bond distances for Ru1−Cl1 and Ru1−Cl2 are 2.402 and 2.421 Å, respectively. The N1−Ru1−C7 angle (120.3°) indicates the away placement of phenyl ring of ligand (L3) from the metal center. Angles between the legs, Cl1−Ru1−Cl2 (88.91°), N1−Ru1−Cl1 (81.93°), and N1−Ru1−Cl2 (82.03°), and those between the legs and the centroid of η6−benzene ring (Cct) (126.97°−133.26°) for the complex [Ru]-3 are close to those reported earlier.15,16 Detailed crystal refinement data are provided in Table 1, and selected bond lengths and bond angles are given in Tables S2 and S3. We also confirmed the molecular structure of the complex [Ru]-4 by single-crystal Xray diffraction studies, but, unfortunately, we could not retrieve the refined data due to the distortion observed in the structure (Figure S1). Arene-Ru(II)-Catalyzed Homocoupling of Arylboronic Acids to Symmetrical Biaryls. At the outset of our investigations, we chose water as a solvent due to the high aqueous solubility of these complexes, and then examined the effect of various bases on the desired homocoupling reaction using phenylboronic acid (1a) as model substrate in the presence of [Ru]-1 (2.5 mol %) in water at 70 °C (Table 2). Preliminary results indicated that Na2CO3 outperforms with respect to the selectivity (47%) toward the homocoupled product, biphenyl (2a), with >99% conversion of 1a (Table 2, entry 1), among a variety of bases (NaOH, KOH, Na3PO4, K3PO4, NaHCO3, and K2CO3) used for the reaction (Table S4). Notably, in the absence of base, the conversion of 1a decreases drastically to a mere 16% under analogous reaction conditions (Table 2, entry 2), as generally base facilitates the C−B bond scission and assists in subsequent transmetalation step for the aryl insertion into the metal of catalyst. Interestingly, addition of a slight amount of methanol was found to be beneficial, as presumably it enhances the solubility of the substrate for the homocoupling reaction of 1a (Table 2, entry 3). It is worth noting that the reaction could not proceed in the absence of the catalyst (Table 2, entry 4). Further, decreasing the reaction temperature to 60 °C resulted in a decrease in conversion (91%), and at room temperature only

([Ru]-3), L4 ([Ru]-4), and L5 ([Ru]-5), in excellent yields (Scheme 2, please see Experimental Section and Supporting Information for details). The obtained orange to brown colored complexes, [Ru]-2−[Ru]-5 were characterized by 1H NMR, 13 C NMR, elemental analyses, and ESI-mass spectrometry. 1 H NMR spectra of the complexes [Ru]-2−[Ru]-5 displayed that upon coordination to ruthenium, the protons of anilinebased ligands shifted to the downfield region, in comparison to the free ligands.15,16 In complex [Ru]-2, aromatic protons of ligand L2 resonated in the range of 7.17−7.36 ppm in comparison to the resonance observed at 6.72−7.08 ppm for the free ligand L2. Significant downfield shifting in the singlet for methyl protons (o-CH3 2.45 ppm) of Ru coordinated L2 in [Ru]-2 was observed, in comparison to that for free ligand L2 (o-CH3 2.19 ppm). Analogously, for [Ru]-3, aromatic protons corresponding to the ligand L3 resonated in the downfield region (at 6.98(t), 6.77(d), and 2.21(s) ppm) in comparison to those for free ligand L3. For [Ru]-4, two doublets were observed, respectively, at 7.34 and 7.18 ppm. The p-CH3 protons of the ligand L4 in [Ru]-4 appeared as a sharp singlet at 2.36 ppm, in comparison to 2.21 ppm in free ligand L4. Moreover, the Ru bound η6-benezene ring resonated in the range of 5.31−5.95 ppm for the complexes [Ru]-2−[Ru]-5. 1H NMR results clearly inferred a significant downfield shift in the protons corresponding to the coordinated ligands in comparison to the free ligands that further supports the proposed molecular identity of the complexes [Ru]-2−[Ru]-5. The molecular structure of the representative arene-Ru(II) complex, [Ru]-3, was confirmed by the single-crystal X-ray diffraction study using the most appropriate crystals grown from the slow evaporation of methanol−dichloromethane solution of the complex [Ru]-3. The complex [Ru]-3 crystallizes in the trigonal crystal system with R3̅ space group. The solid-state structure of the complex [Ru]-3, as shown in Figure 1, clearly demonstrated that 2,6-dimethylaniline (L3) coordinated to the metal center through the nitrogen atom. Analogous to our previously reported X-ray diffraction study of the complex [Ru]-1,15a the ruthenium center in the complex [Ru]-3 also adopted the pseudo-octahedral geometry, where the η6-benzene ring remains planar as a top and the coordinated ligand (L3) along with two chloro ligands as three legs. The ruthenium center is displaced by 1.661 Å from the centroid of C

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for Complex [Ru]-3 crystal parameter

crystal data

empirical formula formula weight temperature (K) wavelength (Å) crystal system, space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z, calculated density (mg/m3) adsorption coefficient (μ) (mm−1) F (000) crystal size (mm3) θ range for data collection (deg) index ranges

C14H17Cl2NRu 371.26 293(2) 1.54184 trigonal, R3̅ 31.3000(16) 31.3000(16) 8.6405(4) 90 90 120 7330.9(10) 18, 1.514 10.662 3348 0.32 × 0.08 × 0.07 4.89−73.12 −38 ≤ h ≤ 26 −38 ≤ k ≤ 36 −10 ≤ l ≤ 3 5757 3272 [R(int) = 0.0292] 99.7% semiempirical from equivalents 0.474 and 0.413 full-matrix least-squares on F2 3272/0/165 1.039 R1 = 0.0355, wR2 = 0.0840 R1 = 0.0455, wR2 = 0.0883 0.438 and −0.760

reflns collected indep reflns completeness to θ = 67.684° absorption correction max and min transmission refinement method data/restraints/parameters GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e Å−3)

Table 2. Optimization of Reaction Conditions for the Ruthenium-Catalyzed Aqueous−Aerobic Homocoupling of Phenylboronic Acida

sel. (%)j,l entry

base

temp/time (oC/h)

conv (%)j

1 2 3b 4b,c 5b 6b b,d 7 8b,e 9b,f 10b,g 11b,g,h

Na2CO3 without base Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3 Na2CO3

70/4 70/4 70/4 70/4 60/4 rt/4 70/4 70/4 70/4 70/4 70/4

99 16 99 −i 91 6 −i 3 99 99 99

2a

3a

47 traces 63

53 −k 37

30 traces

70 −k

traces 35 80 (41) 80 (67)

−k 65 20 20

a

Reaction conditions: 1a (1.0 mmol), base (2.0 mmol), cat. [Ru]-1 (2.5 mol %), water (5 mL). bWater (4.7 mL) with added methanol (0.3 mL). cWithout catalyst. dWith N2 atmosphere. eCat. [(η6benzene)Ru(PPh3)Cl2] (2.5 mol %). fCat. [{(η6-benzene)RuCl2}2] (1.25 mol %). gWith additive Cu(OAc)2 (1.5 mmol). hCat. [Ru]-1 (5 mol %). iNo reaction. jConversion of phenylboronic acid and selectivity on the basis of 1H NMR and isolated yield in parentheses. k Not detected. lIsolated yield of purified products obtained from column chromatography is given in parentheses.

(67% for biphenyl 2a, Table 2, entry 11). This is presumably at higher catalyst loading when unwanted side reactions (e.g., protodeboronation or polymerization reaction or decomposition of arylboronic acid) considerably subside.5b,18−20 Further, the amount of Cu(OAc)2 was also optimized by evaluating the catalytic reaction carried out with varying amounts of Cu(OAc)2 (0.1−1.5 equiv) (Table S5). Results inferred that a lower content of Cu(OAc)2 resulted in poor yields of biphenyl. Notably, reactions performed without using Cu(OAc)2 or the base (Na2CO3) resulted in very poor yield, 20% and 10%, respectively. Hence, with the best optimized conditions, further experiments were performed in air using 5 mol % Ru catalysts in the presence of 2 equiv of Na2CO3 and 1.5 equiv of Cu(OAc)2. Further, [Ru]-1 catalyst also showed good tolerance for various substituted arylboronic acids (Table 3, entries 1−7). The initial substrate screening experiments indicated that electron-rich p-CH3 (1b)- and p-OCH3 (1c)-substituted phenylboronic acid resulted in higher isolated yield (55% and 68%) for corresponding biaryl products, 2b and 2c, respectively, and thereby presumably suggesting an electronically influenced catalytic reaction where the electron-rich arylboronic acid generates better nucleophiles (Table 3, entries 2 and 3). Conversely, electron-withdrawing substitutes, p-Cl(1d) and p-F- (1e), on phenylboronic acid resulted in relatively lower isolated yield (54% and 36%) for biaryl products, 2d and 2e, respectively (Table 3, entries 4 and 5). Structural analogues of 1b and 1c, but with electron-withdrawing substituents, showed complete conversion with higher yields for corresponding homocoupled products. With p-OCF3-substituted phenylboronic acid (1f), 68% yield for 4,4′-(trifluoromethoxy)biphenyl (2f) was achieved (Table 3, entry 6). Analogously,

6% conversion was observed (Table 2, entries 5 and 6). Moreover, the role of aerobic condition was also found to be essential for the catalytic reaction, as no conversion of 1a was observed for the reaction performed under N2 atmosphere (Table 2, entry 7, and Table S5).17 It is worth mentioning that [(η6-benzene)Ru(PPh3)Cl2] was found to be poorly active (Table 2, entry 8), suggesting the inhibitory role of PPh3.14a Moreover, [{(η6-benzene)RuCl2}2] also resulted in low selectivity for biphenyl (Table 2, entry 9), indicating the crucial role of the aniline ligand to control the selectivity of the products. Please refer to later sections for further discussion on the effect of aniline ligands on the catalytic homocoupling reactions. In our further investigation, we also performed experiments in the optimized reaction condition with an additive, Cu(OAc)2 (Table 2, entry 10). Advantageously, the selectivity toward biphenyl was significantly enhanced to 80%, suggesting the crucial role of Cu(OAc)2 as an oxidant to regenerate the Ru(II) species and retard catalyst decomposition during the catalytic cycle (Scheme S1).4e,5,17 However, complete selectivity for biphenyl could not be achieved, presumably because the protic solvents facilitate phenol formation.5e,6b Despite high conversion of phenylboronic acid with good selectivity for homocoupled product observed with 2.5 mol % Ru catalyst, low isolated yields (41%) were obtained for biphenyl 2a (Table 2, entry 10). However, reaction performed with 5 mol % Ru catalyst resulted in the enhancement of isolated yield of the homocoupled product D

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Table 3. Catalytic Conversion of Various Substituted Arylboronic Acids to Corresponding Symmetrical Biaryls in the Presence of [Ru]-1 Catalysta

a

Reaction conditions: 1 (1.0 mmol), Na2CO3 (2.0 mmol), [Ru]-1 (5 mol %), additive Cu(OAc)2 (1.5 mmol), water (4.7 mL) with added methanol (0.3 mL), T = 70 °C. bConversion of 1a−1g and selectivity for 2a−2g and 3a−3g, respectively, as determined by 1H NMR. cIsolated yield of purified biaryl (2a−2g) products obtained from column chromatography is given in parentheses.

with p-CF3-phenylboronic acid (1g), corresponding homocoupled product, 4,4′-(trifluoromethyl)biphenyl (2g), was achieved in 60% yield (Table 3, entry 7). In contrary to the above, protodeboronation was observed for the reactions with 4-hydroxyphenylboronic acid, 1-naphthylboronic acid, and 2biphenylboronic acid.18−20 After having the best optimized reaction conditions, we investigated the effect of the substituents at the aniline ligands (L1−L5) on the catalytic performance of the resulting areneRu(II) catalysts, [Ru]-1−[Ru]-5, for the homocoupling of 1a to obtain 2a. Among the studied arene-Ru(II) catalysts, anilinecoordinating ruthenium catalyst, [Ru]-1, was found to be highly active, as illustrated by the observed high conversion and yield for 2a (67%). Conversely, arene-Ru(II) catalysts, [Ru]-2, [Ru]-4, and [Ru]-5, bearing ortho-/para-substituted CH3- or Cl-anilines, could not furnish any improvement in the selectivity of the desired biphenyl product (2a). Moreover, [Ru]-3 catalyst, containing sterically hindered 2,6-dimethylaniline ligand, also resulted in lower selectivity for 2a (Figure 2 and Table S6). To investigate further the effect of [Ru]-1− [Ru]-5 catalysts, experiments were performed with all of the arene-Ru(II) catalysts for a range of p-substituted phenylboronic acids containing electron-donating groups, −CH3 (1b) and −OCH3 (1c), and electron-withdrawing groups, −Cl (1d) and −F (1e) (Figure 2 and Table S7). The trend of relative catalytic performance of all of the catalysts observed with 1a also persists for the catalytic homocoupling of p-substituted phenylboronic acid (1b−1e).

Figure 2 displayed biaryl (2a−2e) yields for the catalytic conversion of 1a−1e in the presence of [Ru]-1−[Ru]-5 catalysts. As inferred from Figure 2, [Ru]-3 catalyst containing sterically hindered 2,6-dimethylaniline ligand resulted in lower selectivity for biaryls in comparison to [Ru]-1. Presumably, the sterically hindered 2,6-dimethylaniline retarded the formation of crucial di(σ-aryl)-Ru species. Moreover, reaction with [Ru]5, containing electron-withdrawing p-chloro-substituted aniline ligand, also displayed lower selectivity for biaryls. Catalytic reaction with [Ru]-2−[Ru]-5 also displayed moderately higher yields (36−66%) toward respective biaryl products (2b−2c) of the arylboronic acids with p-substituted electron-donating groups −CH3 (1b) and −OCH3 (1c), in comparison to those with electron-withdrawing substituents, p-Cl (1d) and pF (1e) (25−47%) (Table S7). Encouraged by the results for facile homocoupling reaction of a wide range of arylboronic acids catalyzed by ruthenium catalysts, we further intended to find out the chemoselectivity of coupled products between two different arylboronic acids. Under optimized reaction conditions, reactions were performed using an equimolar ratio of two distinct arylboronic acids (Scheme 3), and selectivities toward the homo- and crosscoupled biaryls were determined by GC−MS (Figures S2−S4). Results inferred that the reaction of 4-methylphenylboronic acid (1b) with 4-methoxyphenylboronic acid (1c) showed higher selectivity for the formation of cross-coupled product (50% for 4-methyl-4′-methoxy-1,1′-biphenyl, 2bc) along with the homocoupled products of both of the arylboronic acids E

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Influence of (5 mol %) arene-Ru(II) catalysts, [Ru]-1−[Ru]-5, on the yield of biaryls (2a−2e) for the catalytic homocoupling of arylboronic acids (1a−1e) at 70 °C.

Scheme 3. Competitive C−C Coupling Reactions of Different Arylboronic Acids As Catalyzed by [Ru]-1 Catalyst

F

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Mass spectral analysis for the identification of σ-aryl-Ru intermediate species of the [Ru]-catalyzed homocoupling reaction of (a) pmethylphenylboronic acid (1b) and (b) p-chlorophenylboronic acid (1d).

after 30 min by ESI−mass spectrometry. In the mass spectra of [Ru]-2 catalyst with p-methylphenylboronic acid (1b), several new species corresponding to the σ-aryl-Ru moieties, [(η6benzene)Ru(σ-C6H4CH3)(CH3CN)2]+ (m/z 353.04), [(η6benzene)Ru(σ-C6H4CH3)(CH3CN)]+ (m/z 312.02), [(η6benzene)Ru(σ-C6H4CH3)]+ (m/z 270.99), were observed in positive mode (Figure 3). The observed mass spectral patterns of various σ-aryl-Ru moieties match well with their simulated mass patterns (Figure S7). A similar mass spectral pattern with σ-aryl-Ru moieties, [(η6-benzene)Ru(σ-C6H4Cl)(CH3CN)2] (m/z 372.99), [(η6-benzene)Ru(σ-C6H4Cl)(CH3CN)]+ (m/z 331.97), [(η6-benzene)Ru(σ-C6H4Cl)]+ (m/z 290.94), was also observed for the reaction of [Ru-2] with p-chlorophenylboronic acid (1d) (Figure 3 and Figure S7). Moreover, in the course of reaction, it was observed that ligand effect significantly controls the selectivity of the homocoupled product. Evidence for the presence of ligand bound σ-aryl-Ru intermediates was further confirmed by mass spectrometry while analyzing the negative mode mass spectra of the above reaction mixture (Figure 4 and Figure S8). For pmethylphenylboronic acid, a fragment observed at m/z = 489.55 was assigned to a ligand bound di(σ-aryl)-Ru species,

(41% for 4,4′-dimethylbiphenyl (2b) and 9% for 4,4′methoxybiphenyl (2c)). Analogously, selectivity toward the cross-coupled product, 4-methyl-4′-methoxybiphenyl (2bd), was found to be higher (47%) for the reaction of 4methylphenylboronic acid (1b) and 4-chlorophenylboronic acid (1d). Relative selectivities for corresponding homocoupled products, 4,4′-dimethylbiphenyl (2b) and 4,4′-dichlorobiphenyl (2d), were 25% and 27%, respectively. Moreover, for the reaction between 4-methylphenylboronic acid (1b) and 4trifluoromethylphenylboronic acid (1g), 43% selectivity for cross-coupled product 4-methyl-4′-(trifluoromethyl)-1,1′-biphenyl (2bg) was achieved along with the respective homocoupled products, 2b and 2g. Stability checks of the catalysts [Ru]-1−[Ru]-5 were performed by thermal gravimetric analysis (TGA) and 1H NMR experiments. TGA results inferred that all of the catalysts exhibit good thermal stability up to ca. 160 °C (Figure S5). Moreover, remarkable stability of these catalysts toward air and water was also observed during the course of the catalytic reactions. To further examine the stability of the active catalysts, [Ru]-1 catalyst heated at 70 °C for an extended period of time (4, 12, and 24 h) was analyzed by 1H NMR. 1H NMR results inferred that the catalyst remains stable even after 24 h of heating at 70 °C in water with no significant decomposition of the catalyst (Figure S6). Mechanistic Studies. Encouraged by the unique catalytic activity of arene-Ru(II)-catalyzed homocoupling of arylboronic acids, we attempted to systematically investigate and identify possible reaction pathway. The identification and characterization of key intermediates of the homocoupling reaction is crucial to gain more understanding for the development of active catalysts. Therefore, the evidence for the formation of σaryl-Ru/di(σ-aryl)-Ru intermediates was confirmed by mass spectrometry and NMR spectroscopy. These σ-aryl organometallic species are regarded as key intermediates in the homocoupling reaction.7f,14 [Ru]-2 catalyst was mixed with arylboronic acid (p-methylphenylboronic acid (1b) or 4chlorobnzeneboronic acid (1d)) in varying catalyst to substrate (C:S) molar ratios of 1:1, 1:2, and 2:1 in acetonitrile with triethylamine added, and the reaction mixture was analyzed

Figure 4. Mass spectral analysis of di(σ-aryl)-Ru species, key intermediates in the catalytic homocoupling of arylboronic acids, with the [Ru]-2 catalyst, obtained in the presence of 2.0 equiv excess of (a) p-methylphenylboronic acid (1b) and (b) p-chlorophenylboronic acid (1d) in acetonitrile. G

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry [{(η6-benzene)Ru(σ-C6H4CH3)2(L2)} + Na − 2H]− (L2 = omethylaniline). Reaction with p-chlorophenylboronic acid also showed analogous di(σ-aryl)-Ru species at m/z = 571.36 corresponding to [{(η6 -benzene)Ru(σ-C6H 4Cl)2 (L2)} + CH3CN + Na − 2H]−. To further extend our study on identification of the possible reaction intermediates, we also performed mass spectral analysis under the optimized catalytic reaction condition in water. Analogous to that observed with the stoichiometric reactions of catalyst with arylboronic acid in acetonitrile (as discussed above), interestingly, we also observed several important σ-aryl-Ru and di(σ-aryl)-Ru intermediate species during the course of the catalytic reaction. As shown in Figure 5, mass spectral analysis of the homocoupling of phenylboronic

with the respective species generated after the loss of the ligand, [(η6-benzene)Ru(σ-C6H4CH3)2] (m/z = 363.09), and, moreover, ligand bound mono σ-aryl-Ru intermediate, [(η6benzene)Ru(σ-C6H4CH3)(L1)(CH3CN)]+ (m/z = 407.11). The observed mass spectral patterns for these intermediate species are in good agreement with their simulated mass spectral patterns (Figure S9). We further explored the mass spectral analysis of the homocoupling of phenylboronic acid (1a) and p-methylphenylboronic acid (1b) in the presence of [Ru]-4 and [Ru]-5 catalysts, to investigate if the analogous ligand bound intermediates were generated during the catalytic reaction, also with these catalysts. Analogous to the mass spectral analysis of [Ru]-1-catalyzed reaction, we also observed similar ligand coordinated di(σ-aryl)-Ru species, [(η6-benzene)Ru(σ− C6H5)2(L4)] (m/z = 441.10) (L4 = p-methylaniline) and mono(σ-aryl)-Ru species [(η6-benzene)Ru(σ-C6H5)(L4)(CH3CN)]+ (m/z = 404.97), for homocoupling reaction of phenylboronic acid (1a) and [(η 6 -benzene)Ru(σC6H5CH3)2(L4)] (m/z = 469.15) and mono(σ-aryl)-Ru species [(η6-benzene)Ru(σ-C6H5CH3)(L4)(CH3CN)]+ (m/z = 419.01), for homocoupling reaction of p-methylphenylboronic acid (1b) with [Ru]-4 catalyst (Figure S10). Analogous intermediates were also observed during the mass spectral analysis for the catalytic homocoupling of 1a and 1b in the presence of [Ru]-5 catalyst (Figure S11). To further strengthen our understanding based on the identification of several σ-aryl-Ru species by mass spectrometry, we aimed to obtain similar σ-aryl-Ru intermediates by 19F NMR spectroscopy using 4-fluorophenylboronic acid (1e) and 4trifluoromethylphenylboronic acid (1g) substrates with the ruthenium catalyst [Ru]-2 in a C:S ratio of 1:2 in CDCl3 solvent. The 19F NMR spectra revealed that peaks corresponding to both reactants were missing, but some new peaks were observed in the range of corresponding σ-aryl Ru species. In the 19 F NMR spectrum for [Ru]-2 catalyst with 4-fluorophenylboronic acid (1e), a new peak observed at δ = −115.799 ppm was attributed to σ-4-fluorophenyl coordinated Ru species, [(η6benzene)Ru(σ-C6H4F)], along with the two resonances that appeared at δ = −123.430 and −125.931 ppm assigned to 4,4′difluorobiphenyl (2e) and 4-fluorophenol (3e), respectively (Figure S12). For [Ru]-2 catalyst with 4-trifluoromethylphenylboronic acid (1g), the 19F NMR spectrum analogously showed a resonance corresponding to σ-4-trifluoromethylphenyl coordinated Ru species, [(η6-benzene)Ru(σ-C6H4CF3)] at δ = −61.859 ppm (Figure 6). The other two resonances at −62.597 and −62.761 ppm were attributed to 4,4′-trifluoromethylbiphenyl and 4-trifluoromethylphenyl, respectively (Figure S13).21 Moreover, we also observed ligand bound σaryl-Ru intermediate, [{(η6-benzene)Ru(σ-C6H4CF3)(Et3N)} + Na]+ (m/z 450.1) in the mass spectral analyses of the acetonitrile solution of [Ru]-2 catalyst and 4-trifluoromethylphenylboronic acid in the presence of trimethylamine (Figure 6). Catalytic reaction of various ruthenium catalysts [Ru]-1− [Ru]-5 studied inferred that there is a significant ligand effect to influence the selectivity of the homocoupled product. Moreover, several ligands coordinated mono/di(σ-aryl)-Ru intermediates have been observed during mass spectral analysis, suggesting that presumably aniline ligands remain coordinated with the ruthenium metal during the catalytic reaction. Comparing the catalytic performance of the (η6-arene)Ru(II)-aniline catalyst ([Ru]-1) to those of the arene-

Figure 5. Mass spectral analysis for the identification of σ-aryl-Ru intermediate species generated for the homocoupling of p-methylphenylboronic acid (1b) in the presence of [Ru]-1 catalyst under catalytic reaction condition.

acid (1a) in the presence of [Ru]-1 catalyst displayed the formation of key di(σ-ryl)-Ru intermediates, [(η6-benzene)Ru(σ-C6H5)2(L1)] (m/z = 427.05), (L1 = aniline) where the aniline ligand remains intact. Appearance of such ligand bound di(σ-aryl)-Ru species during the catalytic reaction (analyzed at 0.5, 1.5, and 3.0 h) further supports the decisive role of the aniline-based ligand to tune the selectivity of biaryls. However, along with the above, ligand bound mono σ-aryl-Ru species, [(η6-benzene)Ru(σ-C6H5)(L1) (CH3CN)]+ (m/z = 390.97) and their respective di(σ-aryl)-Ru species formed by the loss of ligand, [(η6-benzene)Ru(σ-C6H5)2] (m/z = 335.03), were also observed during the mass spectral analysis of the catalytic reaction. We also analyzed the catalytic homocoupling reaction of p-methylphenylboronic acid (1b) by mass spectrometry to identify analogous ligand bound di(σ-aryl)-Ru species during the course of the catalytic reaction. Interestingly, the respective ligand bound di(σ-aryl)-Ru intermediate, [(η6-benzene)Ru(σC6H4CH3)2(L1)] (m/z = 455.23), was also identified, along H

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

effect of the coordinating aniline ligands was observed on the selectivity toward biaryls, where [Ru]-2 containing o-methyl substitution, [Ru]-3 containing sterically hindered 2,6-dimethyl aniline, or [Ru]-5 containing electron-withdrawing p-chloroaniline resulted in the lower selectivity for biaryls, while other complexes containing aniline ([Ru]-1) and 4-methylaniline ([Ru]-4) showed higher selectivity for biaryls. This was further supported by the identification of several ligand bound σ-arylRu and di(σ-aryl)-Ru intermediates by mass spectral and 19F NMR studies of the catalytic reaction. The formation of di(σaryl)-Ru species during the catalytic reaction is crucial and is expected to be a key intermediate of the formation of biaryl by Ru-catalyzed homocoupling of arylboronic acids. We believe that the present study will subsequently contribute in enhancing the mechanistic understanding of Ru-catalyzed homocoupling and other related reactions.

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Figure 6. (a)19F NMR and (b) mass spectral evidence for σ-aryl-Ru species generated during the homocoupling reaction of p-trifluoromethylphenylboronic acid in the presence of [Ru]-2 catalyst.

EXPERIMENTAL SECTION

Materials and Instrumentation. All reactions were performed in aerobic conditions using the high-purity chemicals procured from Sigma-Aldrich and Alfa Aesar. Ruthenium arene precursor, [{(η6benzene)RuCl2}2], was synthesized as per the reported procedure.23 Aniline-based ligands were purchased from Alfa Aesar and used as received for the catalysts synthesis without further purification. Progress of catalytic reactions was monitored using thin layer chromatography (TLC). 1H NMR (400 MHz), 13C NMR (100 MHz), and 19F NMR (376.5 MHz) spectra were recorded at 298 K using CDCl3 and DMSO-d6 as solvents on a Bruker Avance 400 spectrometer. Tetramethylsilane (TMS) was used as an external standard for measuring the chemical shifts (in ppm), which are relative to the center of the singlet at 7.26 ppm for CDCl3 and 2.49 ppm for DMSO-d6 in 1H NMR and to the center of the triplet at 77.0 ppm for CDCl3 and 39.50 ppm for DMSO-d6 in 13C NMR, respectively. Coupling constant (J) values are reported in hertz (Hz), and the splitting patterns are designated as follows: s (singlet); d (doublet); t (triplet); m (multiplet); br (broad). Thermo Scientific FLASH 2000 elemental analyzer aided the elemental analysis of the complex. ESI− mass spectra were recorded on a micrOTF-Q II mass spectrometer. GC−MS analysis was performed on a Shimadzu GCMS-QP2010 Ultra and GC-2010 Plus system in EI (electron impact) mode using Rtx5MS column. Thermal gravimetric analyses (TGA) were performed on the Mettler Toledo thermal analysis system. Single-Crystal X-ray Diffraction Studies. Single-crystal X-ray structure studies of [Ru]-3 and [Ru]-4 were executed on a CCD Agilent Technologies (Oxford Diffraction) SUPER NOVA diffractometer. Using graphite-monochromated Cu Kα radiation (λ = 1.54184 Å)-based diffraction, data were collected at 293(2) K by the standard “phi-omega” scan techniques, and were scaled and reduced using CrysAlisPro RED software. The extracted data were evaluated using the CrysAlisPro CCD software. The structures were solved by direct methods using SHELXS-97, and refined by full matrix leastsquares with SHELXL-97, refining on F2.24 The positions of all of the atoms were determined by direct methods. X-ray data for [Ru]-4 complex were found to be abnormal and could not be refined to attain suitable parameters to report. However, the X-ray data give a clear indication for Ru-coordinated p-methylaniline. All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions, and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. The CCDC number 1442036 contains the supplementary crystallographic data for [Ru]-3. These data are freely available at www.ccdc. cam.ac.uk (or can be procured from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21 EZ, UK; fax: (+44) 1223-336-033; or [email protected]). General Procedure for the Synthesis of Complexes [Ru]-2− [Ru]-5. Treatment of dichloro bridged arene-ruthenium dimer (0.5 mmol) and suitable substituted aniline as a ligand (1.05 mmol) in methanol (100 mL) under refluxing condition for 24 h resulted in the

ruthenium dimer, the positive effect of the ruthenium coordinated aniline ligand on the observed high selectivity for the homocoupled product is apparently visible. Results inferred a significant enhancement in the selectivity of biphenyl product while performing the catalytic reaction with arene-ruthenium dimer with added aniline ligand (Table S8). However, the enhancement was not as par with the corresponding ruthenium-aniline catalyst, further indicating that the preformed ruthenium−aniline bond is a crucial structural requirement in the catalyst to achieve high activity. The catalytic reactions were also performed with ruthenium benzene dimer with added p-methylaniline (L4) and p-chloroaniline (L5), respectively, and the results show the superiority of the respective arene-Ru(II) complexes with coordinated anilinebased ligands, [Ru]-4 and [Ru]-5, over the reaction using arene-ruthenium dimer precursors with added free aniline for synthesis of biaryls. A tentative mechanism for the homocoupling of arylboronic acids to biaryls can therefore be proposed on the basis of several important intermediates, particularly ligand bound di(σaryl)-Ru species (Figures 3−6, and Figures S7 and S9) of the catalytic cycle identified in the reaction mixture using ESI−MS and 19F NMR spectral analysis (Scheme S2). The reactive ruthenium catalysts ([Ru]) reacted with arylboronic acid (1) to form a transmetalated σ-aryl-Ru(II) intermediate ([Ru]-x′). Subsequently, the second molecule of arylboronic acid (1) also reacted with the intermediate ([Ru]-x′) to form another transmetalated di(σ-aryl)-Ru(II) intermediate ([Ru]-xx′).7f Finally, the oxidatively coupled product, biaryl (2), was released by reductive elimination and resulted in the formation of Ru(0) species. This species is further oxidized by an oxidant (aerial O2 and Cu(OAc)2) to regenerate the active ruthenium species Ru(II) (Scheme S1).4e,17,22

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CONCLUSION In summary, we have developed a highly water-soluble molecular catalyst based on arene-Ru(II) complexes, [Ru]-1− [Ru]-5, containing readily available aniline ligands, L1−L5. High aqueous solubility and aqueous−aerobic stability of these molecular catalysts led us to explore their catalytic activity for the homocoupling of arylboronic acids to corresponding biaryls in water under moderate reaction conditions. A remarkable I

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry formation of the orange to brown color solution.15a The solution was filtered through the crucible, and then the solution was dried over a rotary evaporator to obtain brown precipitate. Further, the precipitate was dissolved in a mixture of methanol and dichloromethane (1:1 v/v, 10 mL), and kept at room temperature overnight to obtain the crystalline form of the complex. The obtained crystalline complex was separated from the mother liquor and washed again with hexane and diethyl ether 4−5 times. The identity of resulting complex was assessed using 1H NMR, 13C NMR, mass spectroscopy, CHN elemental analysis, and single-crystal XRD study. Synthesis of [(η6-Benzene)RuCl2(2-methylaniline)] ([Ru]-2). The complex [Ru]-2 was synthesized following the above general procedure, using [{(η6-benzene)RuCl2}2] (0.250 g, 0.5 mmol) as ruthenium precursor, and the ligand 2-methylaniline (112 μL, 1.05 mmol) with 100 mL of methanol as solvent. Orange-brown color, yield: 87% (0.279 g). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.36− 7.28 (m, 3H), 7.17 (t, 1H, J = 8 Hz), 5.31 (s, 6H), 4.79 (br, 2H), 2.45 (s, 3H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 143.78, 131.52, 127.51, 125.93, 120.17, 83.06, 17.74. MS (ESI) m/z calculated for [(η6-benzene)RuCl2(2-methylaniline)]: 321.79 [M − Cl]+, found 322.00 [M − Cl]+. Anal. Calcd for [Ru]-2: C, 43.71; H, 4.23; N, 3.92. Found: C, 43.89; H, 3.99; N, 3.44. Synthesis of Complex [(η6-Benzene)RuCl2(2,6-dimethylaniline)] ([Ru]-3). The complex [Ru]-3 was synthesized following the above general procedure, using [{(η6-benzene)RuCl2}2] (0.250 g, 0.5 mmol) as ruthenium precursor, and the ligand 2,6-dimethylaniline (130 μL, 1.05 mmol) with 100 mL of methanol as solvent. Orange color, yield: 89% (0.267 g). X-ray quality orange colored crystals were grown from the slow evaporation of the solution of [Ru]-3 complex in a mixture of methanol−dichloromethane (1:1 v/v) with diethyl ether added. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 6.77 (d, 2H, J = 8 Hz), 6.38 (t, 1H, J = 8 Hz), 5.95 (s, 6H), 4.44 (br., 2H), 2.05 (s, 6H). 13C NMR (100 MHz, DMSO-d6): δ (ppm) = 127.71, 120.52, 115.78, 87.64, 30.68, 17.77. MS (ESI) m/z calculated for [(η6-benzene)RuCl2(2,6dimethylaniline)]: 301.04 [M − 2Cl]2+, found 301.14 [M − 2Cl]2+. Anal. Calcd for [Ru]-3: C, 45.29; H, 4.62; N, 3.77. Found: C, 45.47; H, 4.46; N, 3.50. Synthesis of Complex [(η6-Benzene)RuCl2(4-methylaniline)]([Ru]4). The complex [Ru]-4 was synthesized following the above general procedure, using [{(η6-C6H6)RuCl2}2] (0.250 g, 0.5 mmol) as ruthenium precursor, and the ligand 4-methylaniline (112 μL, 1.05 mmol) with 100 mL of methanol as solvent. Orange color, yield: 86% (0.276 g). X-ray quality orange colored crystals were grown from the slow evaporation of the solution of [Ru]-4 complex in a mixture of methanol−dichloromethane (1:1 v/v) with diethyl ether added. 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.34 (d, 2H, J = 8 Hz), 7.18 (d, 2H, J = 8 Hz), 5.35 (s, 6H), 4.95 (br, 2H), 2.36 (s, 3H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 143.10, 135.46, 130.19, 119.90, 83.14, 20.91. MS (ESI) m/z calculated for [(η6-benzene)RuCl2(4-methylaniline)]: 321.79 [M − Cl]+, found 322.00 [M − Cl]+. Anal. Calcd for [Ru]-4: C, 43.71; H, 4.23; N, 3.92. Found: C, 43.89; H, 3.99; N, 3.44. Preparation of Complex [(η6-Benzene)RuCl2(4-chloroaniline)] ([Ru]-5). The complex [Ru]-5 was synthesized following the above general procedure, using [{(η6-C6H6)RuCl2}2] (0.250 g, 0.5 mmol) as ruthenium precursor, and the ligand 4-chloroaniline (0.134 g, 1.05 mmol) with 100 mL of methanol as solvent. Orange-brown color, yield: 84% (0.287 g). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.40− 7.32 (m, 4H), 5.38 (s, 6H), 4.83 (br, 2H). MS (ESI) m/z calculated for [(η6-benzene)RuCl2(4-chloroaniline)]: 341.94 [M − Cl]+, found 340.12 [M − Cl]+. Anal. Calcd for [Ru]-5: C, 38.16; H, 3.20; N, 3.71. Found: C, 38.22; H, 3.09; N, 3.49. General Procedure for the Homocoupling of Arylboronic Acids To Form Biaryls. The ruthenium-catalyzed homocoupling of arylboronic acids to obtain symmetrical biaryls was carried out in open atmospheric conditions. For this, arylboronic acid (1 mmol) with sodium carbonate (0.210 g, 2 mmol) and Cu(OAc)2 (0.300 g, 1.5 mmol) was added to a reaction tube containing water (4.7 mL) with methanol (0.3 mL) solution of the [Ru]-catalyst (0.017 g, 5 mol %). The reaction mixture was stirred for the desired reaction time at 70 °C in an oil bath. The progress of the reaction was monitored by thin

layer chromatography (TLC). After the reaction, the reaction mixture was quenched with 1 M solution of HCl (5 mL), and then extracted with ethyl acetate (4 × 10 mL), the organic layer was dried over anhydrous Na2SO4, and further the solvent was evaporated under reduced pressure to get the desired product. The reaction intermediates and products were identified by 1H NMR, 13C NMR, ESI−MS, and GC−MS. The conversion and selectivities of the products were obtained by 1H NMR. Isolated yield was calculated by using column chromatography with n-hexane:ethyl acetate (99:1 or 95:5 v/v on the basis of polarity of desired product) as eluent. General Procedure for the Competitive C−C Coupling Reactions of Different Arylboronic Acids To Form Symmetrical and Asymmetrical Biaryls. The competitive C−C coupling reactions of different arylboronic acids to obtain homocoupled and cross-coupled biaryls were carried out in open atmospheric conditions with a procedure identical to that used for the homocoupling of arylboronic acids. For this, two different arylboronic acids (0.5 mmol each) with sodium carbonate (0.210 g, 2 mmol) and Cu(OAc)2 (0.300 g, 1.5 mmol) were added to a reaction tube containing water (4.7 mL) with methanol (0.3 mL) solution of the [Ru]-1 catalyst (0.017 g, 5 mol %). The reaction mixture was stirred for the desired reaction time at 70 °C in an oil bath. After the reaction, reaction mixture was quenched with 1 M solution of HCl (5 mL), and then extracted with ethyl acetate (4 × 10 mL), organic layer was dried over anhydrous Na2SO4, and further the solvent was evaporated under reduced pressure to get the desired product. The ratio of homo- and crosscoupled products was calculated with the aid of GC−MS analysis. Mass Spectrometric Analysis of Catalytic Homocoupling of a 2 equiv Excess of p-Methylphenylboronic acid (1b) and pChlorophenylboronic Acid (1d) in Acetonitrile. [Ru]-2 was added to p-methylphenylboronic acid (1b) or p-chlorophenylboronic acid (1d) (C:S ratio 1:1, 1:2, and 2:1) in acetonitrile (10 mL) followed by Et3N (2 μL). The reaction mixture was then sonicated for 5 min and analyzed by mass spectrometry. Mass Spectrometric Analysis of Catalytic Homocoupling Reaction of Phenylboronic Acid (1a) and p-Methylphenylboronic Acid (1b), Respectively, in the Presence of [Ru]-1, [Ru]-4, and [Ru]-5 Catalysts under Optimized Reaction Conditions. For this experiment, a 100 μL aliquot was taken out of reaction mixture at the intervals of 0.5, 1.5, and 3 h, for 1a, and 2, 4, and 6 h, for 1b, during the catalytic homocoupling reaction of arylboronicacids with C:S ratio 1:40 in the presence of complexes [Ru]-1, [Ru]-4, and [Ru]-5, respectively, under the optimized reaction conditions. To the aliquot of the reaction mixture was added 2 mL of acetonitrile, and the mixture was centrifuged to obtain the clear solution, which was analyzed by mass spectrometry. 19 F NMR Spectral Analysis of the Catalytic Homocoupling Reaction of 4-Trifluoromethylphenylboronic Acid (1f). [Ru]-2 and 4-trifluoromethylphenylboronic acid (1f) (C:S ratio = 1:2) were added in CDCl3 (500 μL) followed by Et3N (2 μL). The 19F NMR spectrum was recorded for the solution to observe the σ-aryl-Ru intermediate. Mass Spectral Analysis of Catalytic Homocoupling Reaction of 4-Trifluoromethylphenylboronic Acid (1f). [Ru]-2 and 4trifluoromethylphenylboronic acid (1f) (C:S ratio = 1:5) were added in THF (5 mL) followed by Et3N (5 mL), and then stirred for 2 h at 50 °C. The reaction mixture was filtered through a crucible, then the solvent was evaporated, and the obtained solid was analyzed by mass spectrometry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01115. Additional tables, figures, schemes, NMR spectra and data, GC−MS spectra, and references (PDF) X-ray data for compound [Ru]-3 (CIF) J

DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from IIT Indore, CSIR, New Delhi (01(2722)/13/EMR-II), and SERB-DST (SB/FT/CS-028/ 2014), New Delhi, is acknowledged. SIC, IIT Indore, and NTU Singapore are acknowledged for instrumentation facilities. D.T. thanks UGC, New Delhi, and R.K.R. and K.G. thank CSIR, New Delhi, for their SRF grants. C.B. thanks MHRD and A.D.D. thanks CSIR, New Delhi [01(2722)/13/ EMR-II], for their fellowships.

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DOI: 10.1021/acs.inorgchem.6b01115 Inorg. Chem. XXXX, XXX, XXX−XXX