Experimental and Theoretical Investigations of Consequence of

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Experimental and theoretical investigations of consequence of ionic liquid anion on copper (I) catalyzed reaction of aryl iodide and thiols Krishna M. Deshmukh, Rupa S. Madyal, Ziyauddin S. Qureshi, Vilas G. Gaikar, and Bhalchandra M. Bhanage Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie3035338 • Publication Date (Web): 01 Mar 2013 Downloaded from http://pubs.acs.org on March 8, 2013

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Industrial & Engineering Chemistry Research

Experimental and theoretical investigations of consequence of ionic liquid anion on copper (I) catalyzed reaction of aryl iodide and thiols Krishna M. Deshmukha, Rupa S. Madyalb, Ziyauddin S. Qureshia, Vilas .G.Gaikar*b, Bhalchandra M. Bhanage*a a

Department of Chemistry, Institute of Chemical Technology, N. M. Parekh Marg, Matunga, Mumbai-400 019, India.

b

Department of Chemical Engineering, Institute of Chemical Technology, N. M. Parekh Marg, Matunga, Mumbai-400 019, India. E-mail: [email protected] ; [email protected]

ABSTRACT: Diol functionalized ionic liquids (DFILs) with various anions as a ligand have been investigated for the copper (I)-catalyzed C–S coupling reaction. The dependence of reactivity and selectivity of the ionic liquids on the anion was demonstrated by experimental studies and supported by Density Functional Theory (DFT) calculations. The interaction energies obtained from the DFT calculations showed a good agreement with the experimental yields of the products. INTRODUCTION Transition-metal-catalyzed C-S bond formation is a widely utilized reaction in both academic and industrial laboratories, due to the importance of sulfur based derivatives as pharmaceutically active drugs.1-3 The classical Ullmann type reaction for the C-S bond formation involves harsh reaction conditions of elevated temperatures of 200 ºC. The catalyst, copper metal, is also consumed in stoichiometric amounts while use of solvents such as hexamethylphosphoramide

1

ACS Paragon Plus Environment


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(HMPA) significantly adds to subsequent processing cost.4-5 To overcome these difficulties, Migita and co-workers reported a palladium catalyzed C–S cross-coupling reaction that requires reaction temperature of 100°C but the reaction has a disadvantage of longer reaction times.6-7 It is also difficult to apply these reaction conditions at industrial-scale due to the high cost of Pd. High oxophilicity associated with the phosphine ligands and the amine substrate, along with the difficulties associated with removal of Pd residues from the polar reaction products have rendered Pd as an unpopular catalyst. Other transition metals like Nickel,8-10 Iron,

11-12

Cobalt,13 and Copper, 14-23 have emerged as

more attractive metal catalysts of choice. Of these, copper tops the list because it is far cheaper than Pd and also less non-toxic than Ni and Co. In this regard, considerable efforts have been made to increase the efficiency of the reaction with copper salts and several types of ligands such as neocuproine,24 1,1,1-Tris(hydroxymethyl)ethane,

25

ethylene glycol,26-27 β-ketoester,28 1,10-

binaphthyl-2,2’-diamine (BINAM) 29 and 8-hydroxyquinolin-N-oxide.30 Different ligands show, of course, different catalytic activities for the C-S bond formation. Very recently, Font et al monitored catalytic activity aryl CuIII in the aryl heteroatom (S, Se and P) coupled products. This study showed the deprotonation of nucleophile followed by aryl-nucleophile reductive elimination. But, mechanistic step investigation still needs more study.31 There has been an increasing interest in exploiting the potential of ionic liquids to develop green technologies that allow reuse of the catalyst or ligands.32-34 Incorporation of a functional group such as nitrile, 35 phosphinite, 36 and alcohol, 37 in the cation or anion enhances the ligation with the metal complex, thus improving the stability of the catalyst. In addition, a properly designed combination of cation and anion facilitates catalyst recyclability,32-34 facile product isolation, and selectivity of the reaction. 2


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We conducted computational investigations along with experiments to get a better insight for the Cu (I) catalysed reactions in the presence of an ionic liquid. The theoretical calculations involve determination of optimal geometries and relative energetics of the expected complexes. A greater understanding of such reactions is essential to supplement experimental efforts as well as to reduce time consuming experiments and use of chemicals in the trials. An analysis of the optimized structures based on energetics is made to identify distinguishing features that could potentially be helpful for planning experimental investigations. Pidko et al.38 have reported the coordination behavior of copper (II) chlorides in [RMIm][Cl] ionic liquids and their catalytic reactivity toward activation of glucose. The interaction between the ion-pair and the metal chloride was limited to hydrogen bonding with the basic chloride ligands. A computational study by Guo and co-workers39 also has showed the intermediacy of the L-CuI (nucleophile) complexes versus other potential copper species in the coupling of aryl halides with amides. Recently, Wang et al reported DFT studies using B3LYP functional and LanL2DZ basis set on Cu(I) catalysed arylation of aromatic C-H bond in presence of phenanthroline as ligand. This study proposed anionic and neutral complexes of Cu as active species in the Cu-catalysed cross coupling reactions, the formation of which depends upon polarity of solvent.40 An examination of the existing literature reveals very few mechanistic details concerning coupling of an aryl halide with thiophenol.41 We are unaware of any theoretical studies for such reactions. We did not include solvation studies, because we were mainly interested in primary interaction between the aryl halide, thiophenol and the catalyst in ionic liquid, in the absence of competing environment of a solvent. Herewith, we also disclose a new diol functionalized ionic liquid as a ligand for the C-S bond formation. The selectivity of the ligand towards the product formation and / or complexation is 3



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explained on the basis of DFT calculations which are in good agreement with experimental results.

EXPERIMENTAL SECTION: Materials All the chemicals were obtained from Lancaster (Alfa-Aesar) and used without further processing. DFILs having different anions such as chloride (Cl), hexafluorophosphate (PF6), bis(trifluoromethylsulfonyl)amide (Tf2N) were prepared using reported procedures.42-43 Methods The optimized yields of the products were based on Gas chromatography analysis of the reaction mixtures on BP-10 column (30 m x 0.32 mm 1D; 0.25 µm). The products of the coupling reaction were confirmed by GC-MS (Shimadzu QP 2010). The UV–VIS diffuse reflectance spectra were recorded in air at room temperature at 28oC. Infrared spectra of ionic liquids were recorded as neat liquids on NaCl pellets using a Perkin-Elmer Spectrum 100 FT-IR Spectrometer. Electrospray ionisation mass spectrometry (ESI-MS) analysis of ionic compounds was performed by the Advanced Mass Spectrometry (Thermo Fisher Scientific, Finnigan LCQ Advantage MAX, USA) using the following method. For the selected ionic compounds, sample solutions of Cu (I) complexes were prepared by dissolving the samples in deionized water. All analyte samples were readily introduced using a 400 µl continuous auto injector syringe, which was rinsed thoroughly with deionized water between injections. All the ESI-MS experiments were performed on a triple quadrupole mass spectrometer. For all spectral acquisitions, the

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capillary temperature was 275˚C, capillary voltage was 31V and spray voltage was 5kV. The source temperature was maintained at 80 oC, and nitrogen was used as the bath and nebulising gas.

Computational Protocol First DFILs ion pairs were constructed in vacuum using Materials Studio (MS) (ver 4.1, Accelrys Inc., USA). Subsequently, structures of Cu-DFIL complexes, Cu-adducts of reactants with and without DFILs were generated. To obtain reliable initial geometries, geometry optimization was first performed with molecular mechanics (MM) using Universal Force Field (UFF) on each of the structures. In UFF, the bond stretching is described by a harmonic term, angle bending by a three-term Fourier cosine expansion, and torsions and inversions by cosine-Fourier expansion terms. The van der Waals interactions are described by the Lennard-Jones 12-6 potential. The electrostatic interactions are described by atomic monopoles and a screened (distance-dependent) Coulombic term. The UFF includes a parameter generator that calculates the force-field parameters by combining the atomic parameters. The MM optimized structure was taken as the initial geometry for further DFT calculations to calculate interaction energies and other electronic and structural properties. A generalized gradient approximation (GGA) exchange correlation functional suggested by Becke, Lee, Yang and Paar (BLYP) was employed.44-45 The pure gradient-corrected functionals, BP86, BLYP, and PBE, perform as well as the B3LYP hybrid variant.46 We performed the calculations using restricted scalar relativistic all-electron method based on Douglas-Kroll and Koelling 5



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approach.47-48 A double numerical polarization (DNP) basis set, that equals to 6-31G**, was applied to perform the optimization of the structures. All calculations were done without imposing any symmetry to the structures. In order to check the reliability of the optimized structures, we performed frequency calculations on ionic liquids and compared with the experimentally observed frequency values. RESULTS AND DISCUSSION The reaction of 4-iodotoluene with thiophenol in the presence of a base was chosen as a model reaction. Initially, the influence of various copper salts such as Cu(acac)2, Cu(OTf)2, Cu(OAc)2, and CuI was tested. These copper sources, in the absence of a ligand, gave low yields of the product (Table 1; entries 1- 6). Thus, a ligand is essential for accelerating the rates of the reaction. We report here a new diol functionalized ionic liquid (DFIL) as a ligand for the C-S bond formation reaction which increases the yield of the reaction to 93 % (Table 1, entry 7). In order to study the influence of different anions, the reaction of 4-iodotoluene and thiophenol was conducted in three DFILs bearing anions having different coordinating capability such as chloride (Cl), hexafluorophosphate (PF6) and bis(trifluoromethylsulfonyl)amide (Tf2N) (Scheme 1). Initially, it was thought that the diol functionalized ionic liquid containing chloride anion 1(2,3-dihydroxypropyl)-3-methyl-1H-imidazolium chloride ([GlyMIm]Cl) (L1) will hamper the reaction yield by coordinating Cl with copper intermediate to block the reaction cycle.49 But such case was not observed with L1.

1-(2,3-dihydroxypropyl)-3-methyl-1H-imidazolium

bis(trifluoromethylsulfonyl)amide ([GlyMIm]Tf2N) (L3), however, followed the expected trend being

bulky and

non-coordinating

anion

while

1-(2,3-dihydroxypropyl)-3-methyl-1H-

imidazolium hexafluorophosphate ([GlyMIm] PF6) (L2), still gave low yield of the product. We

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had to therefore study these irregular consequences of anions with respect to their coordinating ability by Density Functional Theory calculations. The molecular level insight into the structural and coordination properties of ionic liquids and their complexes with Cu is necessary for further improvement of these catalytic systems. To the best of our knowledge, properties of only a limited number of Cu salts in ionic liquids have been investigated in significant details, whereas, detailed molecular-level information on the coupling reaction in presence of Cu is not available.50 The two sets of results were discussed regarding Cu interaction site, as there is no experimental evidence for the mechanism. 1. Ion Pairs Due to lack of detailed experimental data for such kind of reactions and the structures of the species involved, our study began with the constructing the isolated ion pairs. Though, isolated ion pairs do not represent the valid structure in the solutions, here we discuss the results in term of geometry of the different complexes in solutions and their interaction energies. It would be of interest to know the changes in the structures and energies due to the specific interactions. 2. Ion Pairs of ILs Figure 1(a-c) give the DFT optimized structures of ion pairs of ILs. There are significant number of possible interaction sites for an anion around the cation (both with respect to anion’s position and the side chain orientation). Many studies have been devoted to investigate the cation-anion interaction of the ionic liquid using spectroscopic analysis,50-54 and deuterium isotope effects on the chloride ion NMR signal,52 which reveals that the anion forms hydrogen bonds with the imidazolium proton at position C(2) or C(4/5). Having the diol group in the present ionic liquid favors the frontal position i. e., near the C(2) rather than having C(4/5) positions. We have 7



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considered only one orientation, matching the description given by Hunt,53 where the anion gets arranged at the front side of the imidazolium cation forming hydrogen bond with C(2)-H because of strong electrostatic interactions with the ring structure. The C(2)-H···X (where X = Cl, PF6, Tf2N) bond distance in chloride and hexafluorophosphate is nearly the same, 2.02 and 2.01Å, respectively, whereas, in Tf2N the distance is longer at 2.97Å due to the steric hindrance. In addition, the SO2 group of Tf2N also shows hydrogen bonding with C(2)-H by a distance of 2.13Å. The interaction energy of each ion pair of ionic liquids was calculated using the following equation and the values are reported in Table 2. IE = TE ion-pair – (TE cation + TE anion) Where, TE represents total energy of the ion-pair and that of the corresponding the individual ions. The DFT computed IEs of the ion pairs of ionic liquids suggest that Cl- and Tf2N have energetically lower interactions (-121.4 and -113.1 kcal/mol) than PF6 (-167.9 kcal/mol) with the imidazolium (Im) cation. 3. Frequency Analysis of Ion Pairs Unscaled frequency analysis of all stationary points was performed on the ionic liquids and then compared with the experimental FTIR values. All geometries are fully optimized at the BLYP level and calculated vibrational frequencies are verified to be actual minima with all real frequencies. The low-frequency vibrational spectra of the imidazolium ionic liquids show vibrational bands at about 250 cm-1 that can be assigned to the out-of-plane bending mode of the CH3-(N) group in

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the imidazolium cation of each ionic liquid.51 (S2, ESI†)This assignment is supported by the DFT calculated frequencies in this spectral range with low intensity. The most interesting bands occur below 150 cm-1. The frequencies of differently sized ionic liquids suggest that the band at 120 cm-1 can be attributed to the stretching modes of the C(2)-H bond of imidazolium ion.51 3.1 [GlyMIm] [Cl] The vibrational band at 3100-3300 cm-1 can be assigned to H-C(4)-C(5)-H symmetric vibrations of the imidazolium ring which is in agreement with the values given by Hunt’s,53 using DFTB3LYP/6-31++G(d,p) based studies on [BMIm][Cl]. The computed vibrational mode for C(4/5)H symmetric vibration is at 3250 and 3233 cm-1. The experimental band at 2953 cm-1 is attributed to the stretching mode of the N-methyl group as against the DFT-BLYP computed symmetric vibrational frequency of 2998 cm-1. Methylene symmetric stretching frequency is observed at 3014 cm-1 and the asymmetric stretching at 3060 cm-1. The low frequency analysis of the ionic liquid shows a double peak at 141 and 173 cm-1 that is attributed to intramolecular vibrations of the complex anion. (S1and S2, ESI†). The vibrational bands of lower intensity below 400 cm-1 are attributed to the corresponding bending modes of these hydrogen bonds. 3.2 [GlyMIm][PF6] Santos et al.54 investigated surface orientation of 1-methyl-, 1-ethyl-, and 1-butyl-3methylimidazolium PF6- using the surface sensitive technique sum-frequency generation (SFG) vibrational spectroscopy. But, they have accounted for the cations’ vibrations from 2700-3300 cm-1 and consequently vibrational contribution below 2700 cm-1 and anion interpretations are missing in the spectra of ionic liquids. The computed frequencies at 3176 and 3225 cm-1 represent imidazolium C(2)-H and C(4/5)-H stretching vibrations, respectively. The broad band 9



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in the range of 2800- 3200 cm-1 from the experimental IR data does not clearly distinguish between the two stretching frequencies. But, these results are in agreement with Santos et al.54 for the [EMIm][PF6] ionic liquid. P-F stretching frequency can be assigned to 812 cm-1 by DFT calculations which underestimates the experimental value by 21 cm-1.54 The O-H wagging and rocking is observed between 990-1300 cm-1 by both experiments as well as DFT calculations. 3.3 [GlyMIm][Tf2N] There are four discernible peaks in the 500-800 cm-1 region of the FTIR spectrum. The low frequency vibrational bands, i.e. from 500-1000 cm-1, are missing in the spectra of all the other ionic liquids in the literature, but DFT calculations account for the peaks with four vibrations that are due to the N-S and O-H bending vibration modes from the anion and [GlyMIm]+ cation, respectively. The band obtained experimentally at 3164 cm-1 is due to the imidazolium C(2)-H stretching and ring vibrations while computationally it was observed at 3151 cm-1. There is a 10-25 cm-1 underestimation of the calculated frequencies for the imidazolium ring obtained using the BLYP functional as compared to the experimental frequencies. The computed C-F stretching is at 1055cm-1 while experimentally it is at 1059 cm-1.55 In the low frequency range, the additional bands occur between 150 and 250 cm-1, that are assigned to intramolecular vibrations of the complex anion and agrees better with the DFT calculated values at 135 and 238 cm-1. The double peak between 250 and 287 cm-1 belongs to the wagging modes of O=S=O groups in the Tf2N ion which is consistent with the values reported by Fumino et al.51 4. Cu(I) Pairing with Counter ions of IL First, we checked the strength of ionic interaction between ion pairs of Cu(I) with the counterions of the ionic liquids. (Figure 1d-f) The calculated Cu-Cl distance is 2.10 Å in the 10


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CuCl structure as against a value of 2.06 Å reported for CuCl2 by X-ray diffraction,

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and,

therefore, within the accuracy of the DFT calculations using the given functional and basis set. In the optimized structure of CuPF6, the Cu-F distance is shorter at 2.05Å and the F-Cu-F angle is 71.65°. The P-F bond distance is 1.68 Å and F-P-F bond angle is 89.78°. In Cu-Tf2N, the Cu-N distance is 1.91 Å, much smaller than the other anions while the Cu-O(S) distance is 2.97 Å. The S-N-S and C-S-N bond angles are 125.30 and 103.35°, respectively. In comparison to the distances with counterions of ILs, the Cu-I distance in copper iodide is much longer at 2.43 Å. An increase in the ionic interactions is accompanied by a shortening of the distance between a cation and an anion. The order of increasing strength of ionic interactions is, therefore, as per the metal ion- counterion distances, in the order TF2N- > PF6- > Cl- > I-. 5. Cu(I) Complexing with Ion Pairs of IL The second step of calculations begins with the coordination of the Cu(I) with the ion pair of an ionic liquid. Traditionally when imidazolium-based ionic liquids are used under basic conditions, there is a possibility for the formation of NHCs (N-heterocyclic carbene ligands) by deprotonation at the C2 position of imidazolium. These NHCs further bind to transition metals and forms a complex which has found application as catalyst for carbon-carbon bond formation reaction.57-60 But in the present work, C-NMR analysis did not show the formation of Cu-C bond. Two initial geometry guesses were considered. In one, Cu+ and Cl- were in the dissociated form, with Cl- near the imidazolium cation and Cu(I) coordinating with the two hydroxyl groups of diol of the IL. (Figure 2a-c) In one structure, Cu-Cl was considered coordinating as such with the diol group. In another structure, Cu-Cl was considered coordinating in undissociated form with the diol group as shown in Figure 2d-f.

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The interaction energy (IE) of a Cu complex with the ionic liquid was calculated by subtracting the sum of individual species in the complex from the complex energy. IE = TE complex – (TE ionic liquid + TE copper ion) The simulations reveal that the undissociated Cu(I) –Cl- near hydroxyl groups is more stable by 172.1 kcal/mol than the dissociated from. Whereas, PF6- and Tf2N- anions prefer to remain in the dissociated forms (Tables 2 and 3). That is why their interaction do not differ much with Cu(I) in comparison to undissociated forms. Various molecular and surface orientations have been reported for the 1-alkyl-3-methyl imidazolium based ionic liquids.61-64 Pidko et al.38 have reported energy-minimized ion-pairs giving the metal chlorides sitting on top of the imidazolium ring. Such configuration is not possible in the diol functionalized ILs case as Cu coordinates with two -OH groups of the diol which is also confirmed by the UV studies as described later in the paper. The only scenario in which such complex exists is where the Cu(I) is coordinated to the hydroxyl group(s) of ionic liquid and the anions are in the vicinity of Cu(I) ion as shown in Figure 2a-f. According to the DFT calculations, the coordination modes involving less sterically hindered OH sites can show comparable stability to the O1,O2 binding. 6. CuI Complexing with Ion Pair of IL The effect of ionic liquid on the Cu(I) catalyzed reaction in the absence of an added base was considered further at the computational level. Though in the reaction, iodide is taken up by base, to relate the interaction of iodide in the presence of other anions, the presence of iodide ion is taken into account in the dissociated and associated forms (Figure 3a-f). The interaction energy of the CuI complexing with the ionic liquid was calculated by 12


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IE = TE complex – (TE ionic liquid + TE CuI) The ∆E in [GlyMIm -X][CuI] complex drops markedly , compared to ∆E in the charged [DFILX][Cu]+ complexes. The Cu(I) iodide complexation is comparatively stronger in the case of DFIL-Cl- ion pair by 7 and 11 kcal/mol for Tf2N- and PF6-, respectively. The addition of I- to the metal ion leads to a marked drop in ∆E, by a factor of 5 with Cl-, 9 with PF6- and 6 with Tf2N-. Similarly, in the associated form except chloride case where the drop is by a factor 4. The stabilities and the structural parameters are summarized in Tables 2 and 3. 7. Cu(I) Interaction with PhS- and Ion Pairs of IL To gain a more thorough understanding of the origin of experimental trends, we calculated interaction between Cu(I) and ion pairs of ionic liquid(s) and thiophenolate ion in both viz., dissociated and associated forms (Figure 4a-f). It is expected that the interactions of thiophenolate with [GlyMIm][Cl] would be much stronger due to absence of steric hindrance during the coupling reaction. We investigated the interaction between both, associated and dissociated forms of CuCl with the thiophenolate anion and the imidazolium cation. The dissociated form of CuCl showed for greater interaction for chloride than Tf2N- and PF6- ions by 64.7 and 73.0 kcal/mol, respectively. The associated form also followed the same trend as that of dissociated form shown in Tables 2 and 3. In dissociated CuX (where, X = anion) salt, the Cu-S distances and charge on S are similar for all anions. A noticeable difference is found in Cu-O distances and charges on oxygen. Except in Tf2N-, the Cu-O distances are similar and smaller than the other two anions. This may be due to steric hindrance of the bulky anion Tf2N-. In comparison to chloride (-0.7), PF6- and Tf2Npossess charges of similar magnitude (~ -0.9e) as shown in Table 4. Thereby to maintain 13



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electroneutrality, the charge on oxygen is slightly more in chloride (-1.1e) as compared to other two anions (-1.0e). 8. Cu(I) Interaction with PhS- and Counter Ions of IL The final complexation is considered between Cu(I) and the reactants in the presence of counterions of IL excluding imidazolium ring. The fully optimized structures are shown in Figure (5a-c). The bond lengths of the several dominant parts are drawn in the figure. The attraction arises primarily from the interactions between the Cu(I) and thiophenolate ions. The Cl- containing Cu(I) adduct shows a much stronger interaction (-429.6 kcal/mol) with thiophenolate anion than those of with PF6- (-112.8 kcal/mol) and Tf2N- (-110.4 kcal/mol). A distinct difference between Cl- and other two anions in their ability to coordinate with thiophenolate is apparent from the computed energy values. (Table 5) The difference in the interactions of thiophenolate with Tf2N- and PF6- based ionic liquids is not significant but the trend is in agreement with the experimental findings. This confirms that chloride as a counterion of an IL will facilitate favorable interactions of thiophenolate with Cu(I) and thus may have an positive effect on the reaction rates. A strong interaction is observed in chloride anion with charge transfer from Cu as compared to other anions. 9. Reaction Mechanism In order to have a deeper insight into the reaction mechanism of association and dissociation of DFILs, we carried out ESI-MS and UV-visible spectroscopy analysis. Recently, Cheng et al. 65 reported 1,10-phenanthroline (phen) ligand along with CuI and studied the intermediate formed during the reaction with the help of ESI-MS study. Initially, we considered three major steps in the catalytic coupling reaction. First, formation of an active intermediate complex (A) followed by oxidative addition of aryl halide to give complex (B), the further on nucleophilic addition of 14


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thiol gives the intermediate (C). This regenerates the catalyst to give the desired product through reductive elimination (Scheme 2). With this view in mind, initially we prepared three sets of solutions in IPA. Set-I: CuI (0.5 mmol), L1 (0.5 mmol), Set-II: CuI (0.5 mmol), L1 (0.5 mmol), 4-iodotoluene (0.5 mmol), Set III: CuI (0.5 mmol), L1 (0.5 mmol), 4-iodotoluene (0.5 mmol), thiophenol (0.5 mmol), KOH (0.5 mmol) and subjected to ESI-MS analysis. ESI-MS analysis of the Set- I in positive ion mode shows the peak at m/z 384 and 154.93 indicating the formation of an active intermediate complex [L1][CuI] and radical of the cation [GlyMIm] (Figure 6a). The results of Set-II show the degradation and dimerization of [GlyMIm] cation to give peaks at m/z 246 and 355 corresponding to the K[Gly.CuCl2]+ and K[GlyMIm]2+ respectively, along with active intermediate complex [L1][CuI] of a Set-I (Figure 6b). These results indicate that the oxidative addition of 4-iodotoulene to the set-I is a slow and hence the rate determining step. The results of the Set-III shows the peaks at m/z 157, 191, 348 and 355, corresponding to the regeneration of the [GlyMIm]+, [CuI], [GlyMIm2.Cl]+ and K[GlyMIm]2+. The peaks corresponding to m/z 157, 191 and 348, indicate that the regeneration of the ligand L1 and catalyst CuI (Figure 6c). The negative ion mode results of the first two set gives the peaks at m/z 126, 421 and 440 corresponding to [I]-, [(GlyMIm)2.Cl3]- and [GlyMIm.CuCl2]-. While the absence of peak at m/z 440 in negative ion mode of Set-III further confirms the regeneration of ligand and catalyst. In order to support the ESI-MS results obtained, we measured UV-visible spectra of the three sets. The UV-visible spectra of Set I and Set-II (Figure 7) showed an absorption signal at 356 nm which indicates the coordination of copper to the oxygen of ligand (L1) to form O-Cu-O complex.66 While in the Set-III spectra (Figure 6c) and ionic liquids spectra [GlyMIm][Cl]

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(Figure 6d) the peak at 356 nm was not observed. This further confirms the regeneration of the ionic liquid. The formation on Cu-DFIL and its interactions with anions and theoretical calculations support the experimental results. In ion pairs, the peak at 3150-3200 cm-1 corresponds to the stretching modes of the C(2)-H bond of imidazolium ion. This indicates interaction between C(2)-H and the coordinated anion, which is consistent with the

computational results. The theoretical

calculations showed that associated form is more stable in case of chloride containing system than the dissociated form, which is proved by UV-visible spectra.

CONCLUSION Molecular modeling calculations employing the DFT method on the system provided a useful insight into interactions between ionic liquid and reactants. The Cu complex in the associated form (CuCl with the imidazolium ring) is more stable. The interaction of the Cu with an IL-Cl pair is strongly favorable than PF6- and Tf2N-, and hence the coupling reaction is enhanced in presence of CuI. The interaction energy values suggest that counterions involving chloride ions are less stable than those containing PF6-. Thus, from computational point of view, in the presence of highly coordinating anions such as PF6- and Tf2N-, the affinity of ionic liquid towards copper cation is less strong. Our studies on C-S bond formation have shown that dihydroxyl-ligated Cu(I) chloride anion provides significantly better results than those with bulkier groups. Chloride also shows less interaction with copper and allows to coordinate with the reactants. For comparison, the interactions of CuX with thiophenol were also studied and theoretical results follow the experimental trend. Overall, these studies expand the understanding of structural features of ionic liquids and contribution of the counterion to the catalytic activity. 16


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ACKNOWLEDGEMENT The financial assistance from Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam, India is kindly acknowledged. SUPPORTING INFORMATION Spectral data of characterizations of ionic liquids and Cross-coupled product is available as a electronic supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/. REFERENCES: 1. Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Aryl−Aryl Bond Formation One Century after the Discovery of the Ullmann Reaction. Chem. Rev. 2002, 102, 1359. 2. Beletskaya, I. P.; Cheprakov, A. V. Copper in Cross-Coupling Reactions: The postUllmann Chemistry. Coord. Chem. Rev. 2004, 248, 2337. 3. Gangjee, A.; Zeng,Y.; Talreja, T.; McGuire, J. J.; Kisliuk, R. L.; Queener, S. F. Design and Synthesis of Classical and Nonclassical 6-Arylthio-2,4-diamino-5-ethylpyrrolo[2,3d]pyrimidines as Antifolates. J. Med. Chem. 2007, 50, 3046. 4. Ullmann, F. Ueber eine neue Bildungsweise von Diphenylaminderivaten. Ber. Dtsch. Chem. Ges. 1903, 36, 2382. 5. Wolfe, J. P.; Wagaw, S.; Marcoux, J. F.; Buchwald, S. L. Rational Development of Practical Catalysts for Aromatic Carbon−Nitrogen Bond Formation. Acc. Chem. Res. 1998, 31, 805.

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14. Wu, Y.-J. ; He, H. Copper-Catalyzed Cross-Coupling of Aryl Halides and Thiols Using Microwave Heating, Synlett. 2003, 1789. 15. Bates, C. G.; Saejueng, P.; Doherty, M. Q.; Venkataraman, D. Copper Catalyzed Synthesis of Vinyl Sulfides. Org. Lett. 2004, 6, 5005. 16. Deng, W.; Zou, Y.; Wang, Y.-F. ; Liu, L.; Guo, Q.-X.

CuI-Catalyzed Coupling

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20


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30. Su, K.; Qiu, Y.; Yao, Y.; Zhang, D.; Jiang, S. 8-Hydroxyquinolin-N-oxide-Promoted Copper-Catalyzed C-S Cross-Coupling of Thiols with Aryl Iodides. Synlett 2012, 23, 2853. 31. Font, M.; Parella, T.; Costas, M.; Ribas, X. Catalytic C–S, C–Se, and C–P CrossCoupling Reactions Mediated by a CuI/CuIII Redox Cycle. Organometallics 2012, 31, 7976. 32. Fei, Z.; Zhao, D.; Pieraccini, D.; Ang, W. H.; Geldbach, T. J.; Scopelliti, R.; Chiappe C.; Dyson, P. J. Development of Nitrile-Functionalized Ionic Liquids for C-C Coupling Reactions: Implication of Carbene and Nanoparticle Catalysts. Organometallics 2007, 26, 1588. 33. Iranpoor, N.; Firouzabadi, H.; Azadi, R. An Imidazolium-Based Phosphinite Ionic Liquid (IL-OPPh2) as a Reusable Reaction Medium and PdII Ligand in Heck Reactions of Aryl Halides with Styrene and n-Butyl Acrylate. Eur. J. Org. Chem. 2007, 2197. 34. Wang, L.; Li, H.; Li, P. Task-Specific Ionic Liquid as Base, Ligand and Reaction Medium for the Palladium-Catalyzed Heck Reaction. Tetrahedron 2009, 65, 364. 35. Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. Nitrile-Functionalized Pyridinium Ionic Liquids: Synthesis, Characterization, and Their Application in CarbonCarbon Coupling Reactions. J. Am. Chem. Soc. 2004, 126, 15876. 36. Iranpoor, N.; Firouzabadi, H. ; Azadi, R. Imidazolium-Based Phosphinite Ionic Liquid (IL-OPPh2) as Pd Ligand and Solvent for Selective Dehalogenation or Homocoupling of Aryl Halides. J. Organo Chem. 2008, 693, 2469.

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37. Yan, N.; Yang, X.; Fei, Z.; Li, Y.; Kou, Y.; Dyson, P. J. Solvent-Enhanced Coupling of Sterically Hindered Reagents and Aryl Chlorides using Functionalized Ionic Liquids. Organometallics 2009, 28, 937. 38. Pidko, E. A.; Degirmenci, V.; van Santen, R. A.; Hensen, E. J. M. Coordination Properties of Ionic Liquid-Mediated Chromium(II) and Copper(II) Chlorides and Their Complexes with Glucose. Inorg. Chem. 2010, 49, 10081. 39. Zhang, S-L.; Liu, L.; Fu, Y.; Guo, Q-X. Theoretical Study on Copper(I)-Catalyzed CrossCoupling between Aryl Halides and Amides.Organometallics 2007, 26, 4546. 40. Wang, M.; Fan, T.; Lin, Z. DFT Studies on Copper-Catalyzed Arylation of Aromatic C– H Bonds. Organometallics 2012, 31, 560. 41. Reddy, V. P.; Swapna, K.; Kumar, A. V.; Rama Rao, K. Indium-catalyzed C-S crosscoupling of aryl halides with thiols. J. Org. Chem. 2009, 74, 3189. 42. Bellina, F. ; Bertoli, A. ; Melai, B. ; Scalesse, F. ; Signori, F. ; Chiappe, C. Synthesis and Properties of Glycerylimidazolium Based Ionic Liquids: a Promising Class of TaskSpecific Ionic Liquids. Green Chem. 2009, 11, 622. 43. Cai, Y.; Liu, Y. Efficient Palladium-Catalyzed Heck Reactions Mediated by the DiolFunctionalized Imidazolium Ionic Liquids. Catal. Commun. 2009, 10, 1390. 44. Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A 1988, 38, 3098. 45. Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 786.

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46. Buhl, M.; Chaumont, A.; Schurhammer, R.; Wipff, G.. Ab Initio Molecular Dynamics of Liquid 1,3-Dimethylimidazolium Chloride. J. Phys. Chem. B 2005, 109, 18591. 47. Koelling, D. D.; Harmon, B. N. A Technique for Relativistic Spin-Polarized Calculations. J. Phys. C: Solid State Phys. 1977, 10, 3107. 48. Douglas, M.; Kroll, N. M. Quantum Electrodynamical Corrections to the Fine Structure of Helium. Ann. Phys. (San Diego), 1974, 82, 89. 49. Zhong, R.; Wang, Y-N.; Guo, X-Q.; Hou, X-F. Effect of Counterions and Central Cores of Tripodal Imidazolium Salts on Palladium-Catalyzed Suzuki–Miyaura Cross-Coupling Reactions. J. Organomet. Chem. 2011, 696, 1703. 50. Kim, J. H. ; Palgunadi, J. ; Mukherjee, D. K.; Lee, H. J. ; Kim, H. ; Ahn, B. S. ; Cheong, M.; Kim, H.S. Cu(I)-Containing Room Temperature Ionic Liquids as Selective and Reversible Absorbents for Propyne. Phys. Chem. Chem. Phys. 2010, 12, 14196. 51. Fumino, K.; Wulf, A.; Ludwing, R.; The Cation-Anion Interaction in Ionic Liquids Probed by Far-Infrared Spectroscopy. Angew. Chem. Int. Ed. 2008, 47, 3830. 52. Remsing, R. C.; Wildin, J. L.; Rapp, A. L.; Moyna, G. Hydrogen Bonds in Ionic Liquids Revisited, 35/37Cl NMR Studies of Deuterium Isotope Effects in 1-n-Butyl-3Methylimidazolium Chloride. J. Phys. Chem. B 2007, 111, 11619. 53. Hunt, P. A. Why Does a Reduction in Hydrogen Bonding Lead to an Increase in Viscosity for the 1-Butyl-2,3-Dimethyl-Imidazolium-Based Ionic Liquids? J. Phys. Chem. B 2007, 111, 4844. 54. Santos, C.S.; Baldelli, S. Surface Orientation of 1-Methyl-, 1-Ethyl-, and 1-Butyl-3Methylimidazolium Methyl Sulfate as Probed by Sum-Frequency Generation Vibrational Spectroscopy. J. Phys. Chem. B 2007, 111, 4715.

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55. Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975; Vol. 1. 56. Lorenz, M.; Caspary, N.; Foeller, W.; Agreiter, J.; Smith, A. M.; Bondybey, V. E. Electronic Structure of Triatomic Copper(II) Chloride. Mol. Phys. 1997, 91, 483. 57. Hu, X.; Castro-Rodriguez, I. ; Oslen, K. ; Meyer, K. Group 11 Metal Complexes of NHeterocyclic Carbene Ligands: Nature of the Metal Carbene Bond. Organometallics, 2004, 23,755. 58. Memcsok, D.; Wichmann, K. ; Frenkling, G. The Significance of π Interactions in Group 11 Complexes with N-Heterocyclic Carbenes. Organometallics, 2004, 23, 3640. 59. Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 2000, 100, 39. 60. Jafarpour, L.; Nolan, S.P. Development of Olefin Metathesis Catalyst Precursors Bearing Nucleophilic Carbene Ligands. J. Organometallic Chem. 2001, 617-618, 15. 61. Rivera-Rubero,

S.;

Baldelli,

S.

Surface

Characterization

of

1-Butyl-3-

Methylimidazolium Br-, I-, PF6-, BF4-, (CF3SO2)2N-, SCN-, CH3SO3-, CH3SO4-, and (CN)2N- Ionic Liquids by Sum Frequency Generation. J. Phys. Chem. B 2006, 110, 4756. 62. Lynden-Bell, R. M. Gas-Liquid Interfaces of Room Temperature Ionic Liquids. Mol. Phys. 2003, 101, 2625. 63. Yan, T.; Li, S.; Jiang, W.; Gao, X.; Xiang, B.; Voth, G. A. Structure of the LiquidVacuum Interface of Room-Temperature Ionic Liquids: A Molecular Dynamics Study. J. Phys. Chem. B 2006, 110, 1800.

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64. Bhargava, B. L.; Balasubramanian, S. Layering at an Ionic Liquid-Vapor Interface: A Molecular Dynamics Simulation Study of [bmim][PF6]. J. Am. Chem. Soc. 2006, 128, 10073. 65. Cheng, S-W. ; Tseng, M-C. ; Lii, K-H. ; Lee, C-R.; Shyu, S-G. Intermediates of Copper(I) Catalyzed C-S Cross Coupling of Thiophenol with Aryl Halide by in situ ESIMS study. Chem. Commun. 2011, 47, 5599. 66. Pestryakov, A.N. ; Petranovskii, V.P.; Kryazhov, A. ; Ozhereliev, O. ; Pfeander, N.; Knop-Gericke, A. Study of Copper Nanoparticles Formation on Supports of Different Nature by UV–Vis Diffuse Reflectance Spectroscopy. Chem. Phy. Lett. 2004, 385, 173.

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Page 26 of 41

List of Captions for Scheme, Tables and Figures. Scheme 1. Copper-Catalyzed C-S Cross-Coupling of Thiophenol and 4-Iodotoulene. Scheme 2. Possible reaction mechanism for copper-catalyzed C–S bond formation. Table 1. Cu-Catalyzed C-S Cross-Coupling of Thiophenol and 4-Iodotoulene. Table 2. Energetics of Optimized Ionic Liquids and their Complexes in the Dissociated Form. Table 3. Energetics of Optimized Ionic Liquids and their Complexes in the Associated Form. Table 4. Energetics and Structural Parameters of Im-CuX-PhS. Table 5. Energetics and Structural Parameters of thiophenolate-CuX adduct. Figure 1. Optimized complexes of ionic liquids and Cu(I) salts a) Im-Cl b) Im- TF2N c) Im-PF6 d) CuTF2N e) CuCl f) CuPF6 Figure 2. Optimized complexes of dissociated and undissociated copper complex with imidozolium a) ImCl-Cu b) ImTF2N-Cu c) ImPF6-Cu d) Im-CuCl b) Im-CuTF2N c) Im-CuPF6 Figure 3. Optimized complexes of dissociated and undissociated CuI complex with imidazolium ion a) ImCl-CuI b) Im TF2N-CuI c) ImPF6-CuI d) Im-CuClI e) Im-CuTF2NI f) Im-CuPF6I Figure 4. Optimized complexes of dissociated and undissociated copper salts with thiophenolate ion a) ImCl-Cu-PhS- b) ImTF2N-Cu-PhS- c) Im PF6-Cu-PhS- d) ImCuCl-PhS- e) ImCuTF2N-PhSf) ImCu PF6-PhSFigure 5. Optimized complexes of CuX-thiophenolate a) Cu-TF2N-thiophenolate b) Cu-Clthiophenolate c) Cu- PF6-thiophenolate

26


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Figure 6. ESI-MS analysis of complexes a) LC-Ms spectra of SET-I b) LC-Ms spectra of SET-II c) LC-Ms spectra of SET-III d) LC-Ms spectra of pure ionic liquid [GluMIm][Cl]. Figure 7. Uv-visible spectrum of complex formed

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SH

I

Page 28 of 41

S

CuI, Base, L. Solvent, 12h, 100 oC

L=

N

Cl-

N

N

N

PF6-

N

HO

HO L1

OH

L2

N

Tf2N-

HO OH

L3

OH

Scheme 1:

28


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ClN

ArSR

N ArI

H O O

Cu I

H

Reductive Elimination

Oxidative addition

A

Cl-

ClN

N

N

N H

H O

O Cu

O

Ar

SR

H

Cu

O

Ar

I

H B

C

I-

Base + HSR

Scheme 2.

29



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Page 30 of 41

Table 1 Table 1: Cu-Catalyzed C-S Cross-Coupling of Thiophenol and 4-Iodotoulenea

a

Sr. No.

Catalyst

Ligand

Base

Solvent

Yieldb

1

Cu(acac)2

-

K2CO3

Toluene

30

2

Cu(OTf)2

-

K2CO3

Toluene

Trace

3

Cu(OAc)2

-

K2CO3

Toluene

Trace

4

CuI

-

K2CO3

Toluene

38

5

CuI

K2CO3

IPA

Trace

6

CuI

-

KOH

IPA

43

7

CuI

L1

KOH

IPA

93

8

CuI

L2

KOH

IPA

50

9

CuI

L3

KOH

IPA

89

Reaction condition: Thiophenol (1 mmol), 4-iodotoluene (1 mmol),

Catalyst (5 mol %), Ligand (2 equiv.), Base (2 equiv.), solvent (2 cm3), Temp. 100oC, Time 12h. b

GC yield.

30


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Table 2: Energetics of Optimized Ionic Liquids and their Complexes in the Dissociated Form Interaction energy, kcal/mol Imid:X

Imid:X+Cu(I)

Imid:X+CuI

Imid:X+Cu+PhS

ImidPF6+CuI+PhS

-167.987

-241.301

-27.376

-230.114

ImidTF2N-+CuI+PhS

-113.118

-184.370

-31.026

-238.660

ImidCl+CuI+PhS

-121.391

-192.851

-38.260

-303.424

Table 3: Energetics of Optimized Ionic Liquids and their Complexes in the Associated Form Interaction energy, kcal/mol Imid:X+Cu(I)

Imid:X+CuI

Imid +CuX +PhS

ImidPF6+CuI+PhS

-255.810

-55.724

-269.827

ImidTF2N-+CuI+PhS

-184.370

-31.027

-238.655

ImidCl+CuI+PhS

-364.940

-89.779

-295.034

31



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Page 32 of 41

Table 4: Energetics and Structural Parameters of Im-CuX-PhS

Distance, Å

Cl-Cu-PhS

PF6-Cu-PhS

NTF2-Cu-PhS

Cu-O1

1.98

1.99

2.02

Cu-O2

4.15

3.17

4.15

Cu-S

2.16

2.15

2.16

Charge on Cu

0.18

0.19

0.17

Charge on S

-0.39

-0.35

-0.38

Total charges on O

-1.08

-1.04

-1.03

Total charges on X

-0.69

-0.90

-0.89

Table 5: Energetics and Structural Parameters of Thiophenolate-CuX Adduct

Distance, Å

Cl-Cu-PhS

PF6-Cu-PhS

NTF2-Cu-PhS

Cu-X

2.18

2.00

1.99

Cu-S

2.20

2.17

2.19

Charge on Cu

0.08

0.29

0.24

Charge on S

-0.45

-0.41

-0.42

-429.577

-112.809

-110.439

Interaction energy, kcal/mol

32


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a

b

c

d

e

f

Figure 1

33



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Page 34 of 41

a

b

c

d

e

f

Figure 2

34


Page 35 of 41

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a

b

c

d

e

f

Figure 3

35



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Page 36 of 41

a

b

c

d

e

f

Figure 4

36


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a

b

c

Figure 5

37



set1 #19 RT: 0.22 A V : 1 NL: 2.38E 5 F: + p ES I Full ms [ 150.00-500.00] 304.73 230000 220000 210000

154.93

200000 190000 282.93

180000 170000 160000 150000 140000 Intensity

130000 120000 384.73

110000 100000 90000

422.73

80000

442.60 494.47

70000 212.93

60000

396.93

311.00

237.00

354.93 458.60

50000 330.80

270.87

40000

476.20

412.73

30000 208.93

20000

264.87

172.40 10000 0 200

250

300

350

400

450

500

m/z

Figure 6a SET-2_489_170611 #12 RT: 0.16 F: + p ESI Full ms [ 100.00-600.00]

AV: 1

NL: 1.46E6 246.93

1400000 1300000 1200000 1100000 1000000 900000 355.00 Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

800000 700000 600000 292.80 500000 444.87 400000 270.93 300000

384.53 562.13

200000

410.80 313.00

154.80

502.40

212.80

100000 136.80

519.80

460.33

173.00

598.07

0 150

200

250

300

350 m/z

400

450

500

550

600

Figure 6b

38


Page 39 of 41

SE T-3_456_170611 #12 RT: 0.16 F: + p ES I Full ms [ 100.00-600.00]

AV : 1

NL: 1.22E 6 355.07

1200000 1150000 1100000 1050000 1000000 950000 900000 850000 800000 750000

Intensity

700000 650000 600000 550000

157.00

500000 450000 400000 348.73

350000 300000 250000

321.07

200000 150000

247.07

282.93 372.93

100000

422.87

191.00 204.87

50000

452.93

598.47

480.93

578.47

520.07

138.87

0 150

200

250

300

350 m/z

400

450

500

550

600

Figure 6c

MS_192_170311 #5 RT: 0.16 AV: 1 F: + p ESI Full ms [ 100.00-2000.00] 157.07

NL: 2.07E5

200000 190000 180000 170000 160000 150000 348.93 140000 130000 120000 Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


110000 100000 90000 1504.73

1695.80

80000

1890.93

70000 1600.87

60000 1310.67

50000 40000 30000

1794.27

1408.40 289.93 540.93

1119.13

734.47

20000

924.87

10000

404.80

640.00

859.40

951.87

0 200

400

600

800

1000 m/z

1200

1400

1600

1800

2000

Figure 6d 39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 7

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For Table of Contents Use Only

Experimental and theoretical investigations of consequence of ionic liquid anion on copper (I) catalyzed reaction of aryl iodide and thiols Krishna M. Deshmukha, Rupa S. Madyalb, Ziyauddin S. Qureshia, Vilas G.Gaikar*b, Bhalchandra M. Bhanage*a a

Department of Chemistry, Institute of Chemical Technology, N. M. Parekh Marg, Matunga, Mumbai-400 019, India. b

Department of Chemical Engineering, Institute of Chemical Technology, N. M. Parekh Marg, Matunga, Mumbai-400 019, India. E-mail: [email protected] ; [email protected]

41


Experimental and Theoretical Investigations of Consequence of

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