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Journal of Organometallic Chemistry 768 (2014) 68e74

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Journal of Organometallic Chemistry

journal homepage: www.elsevier .com/locate/ jorganchem

C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes withbiphenyl backbone. A comparison of bis(1,2,3-triazol-5-ylidene) andbis(imidazol-2-ylidene) ligands

Sandip Guchhait, Keshab Ghosh, Bemineni Sureshbabu, Venkatachalam Ramkumar,Sethuraman Sankararaman*

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

a r t i c l e i n f o

Article history:Received 7 April 2014Received in revised form20 June 2014Accepted 21 June 2014Available online 30 June 2014

Keywords:N-Heterocyclic carbeneNHCePd complexC2-Symmetric ligandTransmetallationNormal NHCMesoionic NHC

* Corresponding author. Tel.: þ91 44 22574210; faxE-mail address: sanka@iitm.ac.in (S. Sankararaman

http://dx.doi.org/10.1016/j.jorganchem.2014.06.0230022-328X/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes (NHCs) derived from 1,10-((1,10-biphenyl)-2,20-diylbis(methylene))bis(3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium) diiodide (5) and 3,30-((1,10-biphenyl)-2,20-diylbis(methylene))bis(1-phenyl-1H-imidazol-3-ium) dibromide (6) were used asligands for the synthesis of the corresponding Pd(II) complexes. 2,20-Disubstituted 1,10-biphenyl moietywas used as the C2-symmetric backbone for the synthesis. 2,20-Bis(bromomethyl)-1,10-biphenyl was usedas a common precursor for the synthesis of both 5 and 6. These salts were treated with Ag2O for the in-situ generation of the corresponding NHCeAg(I) complexes which were transmetallated to the corre-sponding Pd(II) chloro and acetate complexes. The bis(1,2,3-triazol-5-ylidene) derivative gave structur-ally well-defined mono nuclear chelate complexes that were characterized by spectroscopic and singlecrystal XRD data. The bis(imidazol-3-ylidene) derivative gave polymeric complex and not the expectedmono nuclear chelate complex. These complexes were compared for their reactivity in SuzukieMiyauracoupling reaction.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Chelating C2-symmetric ligands are very useful for the synthesisof C2-symmetric metal complexes that find application in metalcatalyzed stereoselective organic synthesis [1,2]. 2,20-Disubstituted1,10-biphenyl [3,4] and 1,10-binaphthyl [5,6] moieties are commonlyused chiral C2-symmetric backbone structures. In recent times N-heterocyclic carbene (NHC) ligands have gained prominence overthe conventional phosphane ligands in transition metal chemistry[7e15]. Their extraordinary stability and superior catalytic activityof NHCs and their metal complexes have been amply demonstrated[7,11,16,17]. Among the various NHCs, imidazol-2-ylidenes are themost widely exploited as organocatalysts and ligands for transitionmetal complexes. C2-Symmetric chelating bis(imidazol-2-ylidene)bearing 1,10-biphenyl and 1,10-binaphthyl moieties are well knownin literature [18e24]. The first chelated chiral bis(imidazol-2-ylidene) complexes were reported by RajanBabu using 2,20-disub-stituted-1,10binaphthyl as the chiral backbone [25]. Subsequently

: þ91 44 2257 0545.).

several reports have appeared on the use of both 1,10-biphenyl and1,10-binaphthyl moieties in imidazolylidene based NHC chemistry[18e24]. However, the chelating C2-symmetric mesoionic carbenes(MICs) such as 1,2,3-triazol-5-ylidenes are relatively still scarce[26,27]. The first example of chelating bis(1,2,3-triazol-5-ylidene)was reported from this laboratory [28]. In continuation of our in-vestigations on NHC-metal complexes [28e32], herein we reportthe synthesis of 1,10-biphenyl based C2-symmetric bis(1,2,3-triazol-5-ylidene) (mesoionic NHC) and bis(imidazol-2-ylidene) (normalNHC) Pd(II) complexes. The structures of bis(1,2,3-triazol-5-ylidene) Pd(II) complexes have been unambiguously establishedby single crystal XRD data. The catalytic activities of the normal andmesoionic NHC complexes were compared for the SuzukieMiyauracoupling reaction.

Results and discussion

Synthesis of precursors to NHC ligands

Diphenic acid (1) was converted to 2,20-bis(bromomethyl)-1,10-biphenyl (2) in three steps according to literature reportedmethods[33,34]. Compound 2 was used as a common precursor for the

Table 1Crystallographic data of compounds 5, 6, 8 and 9.

Parameters 5 6 8 9

Formula C32H30I2N6 C32H34Br2N4O3 C32H28N6Cl2Pd C36H34N6O4PdFormula weight 752.42 682.45 673.90 721.09Radiation l (Å) 0.71073 0.71073 0.71073 0.71073Crystal system Monoclinic Triclinic Orthorhombic MonoclinicSpace group C2/c P-1 Pba2 P2/na (Å) 25.8910(9) 11.0768(4) 14.2758 (12) 9.5296 (5)b (Å) 12.6507(4) 11.2420(4) 9.5037 (7) 12.1826 (8)c (Å) 18.8966(7) 13.6131(5) 12.1910 (7) 14.1960 (9)a� 90 95.452(2) 90 90b� 92.0500(10) 94.943(2) 90 97.505 (3)G� 90 111.390(2) 90 90V (Å3) 6185.4(4) 1557.87(10) 16.54.0 (2) 1633.97 (17)T (K) 298(2) 298(2) 298 (2) 298 (2)Z 8 2 2 2Reflections/ 21,740 18,134 6609 12,201unique/Rint 7565/0.0192 5446/0.0299 3125/0.0248 4368/0.0407m mm�1 2.605 2.640 0.752 0.617F(000) 2960 696 684 740q range 1.79 to 28.27 1.964 to 24.996 1.67 to 27.47 1.67 to 29.70Goodness-

of- fit on F21.015 1.097 1.085 1.012

Final R indices R1 ¼ 0.0324 R1 ¼ 0.0501 R1 ¼ 0.0576 R1 ¼ 0.0564wR2 ¼ 0.0758 wR2 ¼ 0.1314 wR2 ¼ 0.1543 wR2 ¼ 0.1510

R indices(all data)

R1 ¼ 0.0501 R1 ¼ 0.0705 R1 ¼ 0.0818 R1 ¼ 0.0910wR2 ¼ 0.0856 wR2 ¼ 0.1403 wR2 ¼ 0.1722 wR2 ¼ 0.1741

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74 69

synthesis of NHC precursors 5 and 6 (Scheme 1). It was converted tothe corresponding diazide (3) on treatment with sodium azide.Diazide 3 on click reaction with phenylacetylene yielded the cor-responding bis(triazole) 4 in 97% yield. The triazole ring protonsappeared as a singlet at 7.45 ppm and the methylene protonsappeared as an AB quartet centered at 5.23 ppm in the 1H NMRspectrum of 4. Methylation of 4was carried out usingmethyl iodidein an autoclave to yield the corresponding bis(triazolium) diiodide5 in 81% yield, as a crystalline solid. The triazolium ring protonsappeared as a singlet at 8.97 ppm in the 1H NMR spectrum of 5. Themethylene protons of 5 appeared as an AB quartet (5.85 and6.10 ppm) due to their diastereotopic nature in the 1H NMR spec-trum in 1,1,2,2-tetrachloroethane-d2. When the spectrum wasmeasured as a function of temperature from 25 �C to 130 �C, the ABpattern remained unchanged indicating the configurational sta-bility of the axially chiral biphenyl moiety bearing bulky triazoliumsubstituents in the 2,20-positions. This suggests that it might bepossible to resolve 5 using a suitable ion exchange method to theenantiomerically pure form. However, our attempts to resolve 5 asthe corresponding camphor-10-sulfonate salt were unsuccessful.Compound 2 was treated with 2 equivalents of 1-phenyl-1H-imidazole to yield the corresponding bis(imidazolium) dibromide 6as a colorless crystalline solid (Scheme 1). The imidazolium ringprotons appeared as a singlet at 10.53 ppm in the 1H NMR spectrumof 6. The precursor salts 5 and 6 were thoroughly characterized byspectroscopic as well as single crystal XRD data (Table 1 andSupplementary information).

Synthesis of C2-symmetric bis(triazol-5-ylidene) Pd(II) complexes

Bis(triazolium) salt 5 was treated with silver oxide in CH2Cl2 atroom temperature for 24 h to yield the corresponding silver NHCcomplex (7) in 93% yield. Although complex 7 was light sensitiveand decomposed on exposure to room light it could be character-ized by NMR and HRMS data. 1H NMR spectrum indicated the

Scheme 1. Synthesis of NHC precursor

disappearance of the triazolium ring proton at 8.97 ppm. Theisotope distribution pattern obtained for the molecular ion in theESI-MS agreed with that of the calculated. The Pd(II) complexeswere synthesized by transmetallation reaction [35]. Thus treatmentof the silver NHC complex 7 with PdCl2(CH3CN)2 gave the corre-sponding trans-Pd(II) dichloro derivative 8 in 96% and withPd(OAc)2 gave the corresponding trans-Pd(II) diacetate complex 9in 90% yield, respectively (Scheme 2). All these complexes werethoroughly characterized by spectroscopic and single crystal XRD

salts 5 and 6 from diphenic acid.

Scheme 2. Synthesis of C2 symmetric chelating bis(triazol-5-ylidene) Ag(I) and Pd(II) complexes.

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e7470

data. Pd(II) dichloro complex 8 crystallized in orthorhombic systemin Pbc2 space group. Pd center has a distorted square planar transgeometry (Fig 1). The ClePdeCl bond angle is 173.8(2)� and Ccar-

beneePdeCcarbene bond angle is 177.3(5)�. The molecular structurein the crystal has C2 symmetry, the C2 axis passing through thecenter of the biphenyl bond and Pd atom. The CcarbeneePd distance(2.030(5) Å) is in accordance with the earlier reports of Pd-triazol-5-ylidene complexes [28e32] Pd(II) diacetate complex 9

Fig. 1. ORTEP diagram of compound 8 with thermal ellipsoid drawn at 30% probability. SelCl1ePdeCl1 ¼ 173.77(2), Cl1ePd1eC2 ¼ 88.61(19), Cl1ePd1eC2(#1) ¼ 91.26(19), C1eC2e

crystallized in monoclinic system in P2/n space group (Fig 2). Pdcenter has a distorted square planar trans geometry. The OePdeObond angle is 175.5(3)� and CcarbeneePdeCcarbene bond angle is176.0(2)�. Although formation of polymeric metal complexes is adistinct possibility in all of the above reactions, it did not happen. Inall of the above reactions well defined C2-symmetric mono nuclearchelate complex alone was formed as evident from the isolatedhigh yields and 1H NMR spectra of the crude products.

ected bond lengths (Å) and bond angles (�): Pd1eCl1 ¼ 2.341(2), Pd1eC2 ¼ 2.030(5),N3 ¼ 101.62(5). Hydrogen atoms are omitted for clarity.

Fig. 2. ORTEP diagram of compound 9 with thermal ellipsoid drawn at 30% probability. Selected bond lengths (Å) and bond angles (�): Pd1eO1 ¼ 2.042(2), Pd1eC11 ¼ 2.048(4),C11ePd1eC11(#1) ¼ 176.00(2), N1eC11eC7 ¼ 102.38(3). Hydrogen atoms are omitted for clarity.

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74 71

Synthesis of polymeric bis(imidazol-2-ylidene) Pd(II) complexes

The complexation behavior of bis(imidazolylidene ligandderived from bis(imidazolium) dibromide 6 was quite differentfrom that of bis(triazolylidene) ligand derived from bis(triazolium)diiodide 5. When reacted with silver oxide 6 gave the corre-sponding bis(imidazol-2-ylidene) Ag(I) complex as evident fromthe disappearance of the imidazolium protons at 10.53 ppm in the1H NMR spectrum of the reaction mixture. However the resonancesin the 1H NMR spectrum were quite broad and not discernable tothe expected mono nuclear chelate complex. The in-situ generatedAg(I) complex was treated with PdCl2(CH3CN)2 and Pd(OAc)2 toyield the corresponding polymeric bis(imidazol-2-ylidene) Pd(II)dichloride (10) and diacetate (11) complexes, respectively(Scheme 3). Unlike 8 and 9 these complexes were not soluble incommon organic solvents. 1H NMR spectra of the soluble portion(in CDCl3) showed broad peaks. Based on these observations weconclude that unlike the bis(triazolylidene) case which gaveexclusively the chelate complexes 8 and 9 in excellent yields,polymer formation was observed in case of the bis(imidazolyli-dene) derivatives. The corresponding chelating mono nuclearcomplexes, if formed at all, were only in trace amounts. Formation

Scheme 3. Synthesis of polymeric bis(im

of polymeric complexes 10 and 11 instead of monomeric chelatecomplex from 6 is surprising considering several mono nuclearchelate complexes of bis(imidazolylidene) ligands with 1,10-biphenyl and 1,10-binaphthyl backbones have been reported in theliterature [19]. However no structural characterization by singlecrystal XRD has been reported for these complexes.

Comparison of the catalytic activities for SuzukieMiyaura couplingof 1,4-dibromobenzene

NHCePd(II) complexes have been shown wide variety of cata-lytic activities ranging from CeC cross coupling, CeH activation,oxidation, reduction, hydro-carboxylation, hydroarylation etc.[7,10,11,29,36e40]. The catalytic activity is very much pronouncedby the nature of the NHC ligands bonded to Pd atom. In comparisonto normal NHCePd complexes the mesoionic NHCePd complexesshowed higher catalytic activities towards cross coupling reactions[31,41e43].We examined the catalytic activities of complexes 8 and10 towards Suzuki-Miyaura coupling of 1,4-dibromobenzene withphenylboronic acid to yield p-terphenyl (Scheme 4). The dichlorocomplexes 8 and 10 were chosen so as to compare their catalyticefficiencieswith our earlier work in which dichloropalladium

idazol-2-ylidene) Pd(II) complexes.

Scheme 4. Comparative Suzuki-Miyaura coupling using complexes 8 and 10 ascatalysts.

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e7472

derivative bearing mesityl wing-tip groups was used [31]. In bothcases p-terphenyl was obtained in excellent yields. With 2% catalystloading and under otherwise identical conditions the turnovernumber (TON) and turnover frequency (TOF) for catalyst 8 were 88and 12.5 and for catalyst 10 were 75 and 12.5, respectively. Underthe conditions employed for SuzukieMiyaura coupling practicallyno difference in catalytic activity was observed between catalysts 8and 10 as evident from nearly identical TON and TOF. However, theTONs of these catalysts were much lower than our earlier report[31]. These complexes were not effective for the SuzukieMiyauracoupling of chloroarenes [43].

Conclusions

1,10-Biphenyl based C2-symmetric chelating mono nuclearbis(1,2,3-triazol-5-ylidene) Pd(II) complexes (8 and 9) bearingmesoionic NHC ligand were synthesized. The structures ofbis(1,2,3-triazol-5-ylidene) Pd(II) complexes (8 and 9) have beenunambiguously established by single crystal XRD data. In com-plexes 8 and 9 palladium has distorted square planar geometrywith trans stereochemistry. Bis(imidazol-2-ylidene) Pd(II) com-plexes (10 and 11) were polymeric in nature as evident from theirpoor solubility as well as broad lines in the 1H NMR spectra. In thevariable temperature 1H NMR study of the bis(triazolium) salt 5 inthe temperature range of 25e130 �C the AB quartet signal observedfor the diastereomeric methylene protons remained intact indi-cating the configurational stability of the biphenyl backbone. Thissuggested that it might be possible to resolve 5 using a suitable ionexchange method. Finally a direct comparison of the catalytic ac-tivities of complexes 8 and 10 was made in SuzukieMiyauracoupling of 1,4-dibromobenzene with phenylboronic acid to p-terphenyl. The catalytic activities of these two complexes werecomparable as indicated by the nearly same values for the TON andTOF for the above reaction.

Experimental section

Diphenic acid (1) was converted to 2,20-bis(bromomethyl)-1,10-biphenyl (2) in three steps according to literature reportedmethods[33,34] in 79% overall yield. 1-Phenyl-1H-imidazole was preparedfrom bromobenzene and imidazole following literature method[44,19].

Instrumentation

Infrared (IR) spectra were recorded on a JASCO 4100 FT-IRspectrometer. 1H NMR spectra were measured on Bruker AVANCEspectrometers operating at 400 and 500 MHz for proton and 100and 125 MHz for carbon-13 nuclei, respectively. Chemical shiftswere reported in ppm from tetramethylsilane as the internalstandard. 13C NMR spectra were recorded with complete protondecoupling. C-13 chemical shifts were reported in ppm using re-sidual solvent peaks as internal standard. High-resolution massspectra (HRMS) were performed on Micromass ESI Q-TOF micromass spectrometer equipped with a Harvard apparatus syringe

pump. X-ray crystallographic data were collected on a Bruker-AXSKappa CCD-Diffractometer with graphite-monochromator Mo Ka

radiation (l ¼ 0.71073 Å). The structures were solved by directmethods (SHELXS-97) and refined by full-matrix least squarestechniques against F2 (SHELXL-97). Hydrogen atoms were insertedfrom geometry consideration using the HFIX option of the program.For thin layer chromatography (TLC) analysis, E-Merck pre-coatedTLC plates (silica gel 60 F254 grade, 0.25 mm) were used.

Synthesis and characterization

Preparation of silver oxide (Ag2O) [45]Silver oxidewas prepared by the addition of an aqueous solution

of sodium hydroxide (2 g in 20 mL) to an aqueous solution of silvernitrate (1 g in 10 mL). Continuous shaking during the additionensured completion reaction resulting in a brown precipitate. Itwas filtered and washed with 200mL of water and thenwith 50 mLof acetone. Silver oxide (700 mg) thus obtained was dried undervacuum to yield a dark brown solid.

Synthesis of diazide 3 [46]2,20-Bis(bromomethyl)-1,10-biphenyl (2) (5 g, 14.1 mmol) was

dissolved in DMF (15 mL) under N2 atmosphere in a two neckedround bottom flask equipped with a condenser. NaN3 (4.78 g,73.5 mmol) was added and the reaction mixture was heated at85 �C for 12 h. After completion (TLC), the reaction mixture wastaken in a separating funnel, diluted with water (60 mL) andextracted with hexane (3 � 60 mL). The combined organic layerwas dried over Na2SO4 and evaporated under vacuum to give dia-zide 3 in 92% yield (3.58 g) as a colorless liquid, IR (neat): 2097 cm�1

(nazide); 1H NMR (CDCl3, 400 MHz) d: 4.06 and 4.12 (AB quartet,JAB ¼ 13.2 Hz, 4H, CH2), 7.21 (d, J ¼ 7.8 Hz, 2H), 7.37e7.47 (m, 6H);13C NMR (CDCl3, 100 MHz) d: 52.6 (CH2), 128.3, 128.5, 129.4, 130.2,133.7, 139.6.

Synthesis of bis(triazole) 4Diazide 3 (3 g, 11.35 mmol) was treated with phenylacetylene

(2.31 g, 22.7 mmol) in 10 mL degassed DMSO:H2O (9:1, v/v) in a100 mL round bottom flask. CuI (0.43 g, 2.27 mmol) was added andthe reaction mixture was stirred under N2 atmosphere at roomtemperature for 12 h. After completion of the reaction (TLC),100mLof water was added to the reaction mixture. The precipitate formedwas filtered through a sintered funnel. The product was washedwith hexane several times and then dissolved in CH2Cl2. It wasdried over Na2SO4 and then solvent was evaporated under vacuumto obtain pure product in 97% yield (5.16 g) as a grey solid. Mp:154e158 �C; IR(KBr): 1606, 1479, and 1224 cm�1; 1H NMR (CDCl3,500 MHz) d: 5.17 and 5.3 (AB quartet, 4H, JAB ¼ 15.5 Hz, CH2),7.21e7.26 (m, 4H), 7.30 (t, J ¼ 7.5 Hz, 2H), 7.37 (t, J ¼ 7.7 Hz, 4H),7.42e7.46 (m, 6H), 7.71 (d, J ¼ 7.7 Hz, 4H); 13C NMR (CDCl3,125MHz) d: 52.0 (CH2),120.4,125.7,128.3,128.8,128.9,129.0,129.2,130.1, 130.4, 133.4, 138.8, 147.7; HRMS (ESI):m/z: calcd for C30H25N6[M þ H]þ 469.2141, found 469.2145.

Synthesis of bis(triazolium) diiodide 5Bis(triazole) 4 (2 g, 4.26 mmol) was treated with methyl iodide

(7.26 g, 51.2 mmol) in CH3CN (7 mL) in a teflon coated steel auto-clave and heated in an oil bath at 85e90 �C for 48 h. The reactionmixture was evaporated to dryness and the solid product waswashed with hexane and ethyl acetate (5 � 10 mL) Pure triazoliumsalt was obtained as a light brown colour solid in 81% yield (2.6 g).Mp: 184e188 �C; IR (KBr): 3061, 3031, 1610, 1568, 1491, 1456, and1432 cm�1. 1H NMR (DMSO-d6, 500 MHz) d: 4.23 (s, 6H, CH3), 5.65and 5.76 (AB quartet, 4H, JAB ¼ 15.5 Hz, CH2), 7.22 (d, J ¼ 7 Hz, 2H)7.51e7.58 (m, 4H), 7.68e7.70 (m, 12H), 8.97 (s, 2H, triazolium H);

S. Guchhait et al. / Journal of Organometallic Chemistry 768 (2014) 68e74 73

13C NMR (DMSO-d6,125MHz) d: 38.9 (CH3), 54.6 (CH2),122.3,128.3,129.0, 129.1, 129.3, 129.4, 129.5, 130.3, 130.5, 131.5, 138.7, 142.2;HRMS (ESI) m/z: (m ¼ 498.2532, z ¼ 2, dicationic salt) calcd forC16H15N3 [M þ H]þ 249.1266, found 249.1257.

Synthesis of palladium dichloro complex 8Bis(triazolium) diiodide 5 (200 g, 0.27 mmol) was treated with

Ag2O (74 mg, 0.32 mmol) in CH2Cl2 and the mixture was stirred for24 h under N2 atmosphere in the dark. An aliquot of the reactionmixture was evaporated and 1H NMR and ESI mass spectra wererecorded to check the complete conversion of the iodide salt to thecorresponding Ag complex 6. The absence of the triazolium protonresonance at 8.97 ppm in the 1H NMR spectrum clearly indicatedthe complete conversion of 5 to silver complex 7. 1H (NMR, CDCl3,400 MHz) d: 4.19 (s, 6H, CH3), 5.53 and 5.63 (AB quartet, 4H,JAB ¼ 14.4 Hz, CH2), 7.29 (m, 7H), 7.37 (m, 11H); HRMS (ESI) m/z:calcd for C32H28N6Ag [M]þ 603.1426, found 603.1445. After 24 hstirring [PdCl2(CH3CN)2] (75.8 mg, 0.29 mmol) was added to thereaction mixture and stirring was continued for another 24 h. Thereaction mixture was filtered through a sintered crucible and sol-vent was evaporated under vacuum to give complex 8 as a paleyellow solid. It was washed with diethyl ether and ethyl acetateseveral times to obtained pure product (8) in 96% yield (171 mg).Mp: 246e250 �C, IR (KBr): 3448, 3065, 1630, 1478, and 1444 cm�1;1H NMR (CDCl3, 400 MHz) d: 4.07 (s, 6H, CH3), 5.55 and 6.55 (ABquartet, 4H, JAB ¼ 14.4 Hz, CH2), 7.28e7.45 (m, 14H), 7.94 (d,J ¼ 7.4 Hz, 4H); 13C NMR (CDCl3, 100 MHz) d: 37.3 (CH3), 54.6 (CH2),127.6, 127.7, 128.5, 128.7, 129.0, 129.1, 130.1, 131.3, 134.0, 139.4, 145.1,160.7 (carbene C); HRMS (ESI): m/z: [M þ H]þ calcd forC32H29N6Cl2Pd 673.0866, found 673.0882.

Synthesis of palladium diacetate complex 9Bis(triazolium) diiodide 5 (200mg, 0.27mmol) was treatedwith

Ag2O (74 mg, 0.32 mmol) in dichloromethane and the reactionmixture was stirred for 24 h under N2 atmosphere in the dark.Pd(OAc)2 (65 mg, 0.29 mmol) was added in the reaction mixtureand stirring was continued for 24 h. The reaction mixture wasfiltered through a sintered funnel and solvent evaporated to obtainthe crude product as a dark brown colour solid. Upon washing thecrude product with ether and ethyl acetate several times pureproduct was obtained in 90% yield (172 mg). Mp:198e200 �C, IR(KBr): 3430, 3055, 2924,1588,1478, and 1440 cm�1. 1H NMR (CDCl3,500 MHz) d: 1.10 (s, 6H, OCOCH3), 4.50 (s, 6H, CH3), 5.48 and 6.85(AB quartet, 4H, JAB ¼ 14.4 Hz, CH2), 7.32e7.47 (m, 14H), 8.07e8.09(d, 4H); 13C NMR (CDCl3, 125 MHz) d: 22.6 (OCOCH3), 37.2 (CH3),54.2 (CH2), 127.3, 128.2, 128.4, 128.5, 128.6, 128.9, 130.1, 131.3, 134.8,139.8, 145.8, 160.0 (carbene C), 176.1 (CO); ESI-MS: m/z 603 (M-OCOCH3).

Synthesis of bis(imidazolium) dibromide 6Dibromide 2 (1.4 g, 4.1 mmol) and 1-phenyl-1H-imidazole

(1.31 g,1.2mL, 9.1 mmol) were dissolved in dioxane (10mL) and themixture was heated under N2 atmosphere at 85 �C for 14 h. Thereaction mixture was cooled to 0 �C to obtain a white solid pre-cipitate. The solid was collected by filtration and washed withhexane and ether to obtain pure bis(imidazolium) dibromide 6 in98% yield (2.5 g). It was further recrystallized from methanol. Mp:212 �C, IR (KBr): 1552, 1216, 765 cm�1; 1H NMR (CDCl3, 500 MHz) d:5.68 and 5.75 (AB quartet, 4H, J ¼ 15.5 Hz, CH2), 7.10e7.12 (m, 2H),7.29e7.31 (m, 6H), 7.45e7.48 (m, 2H), 7.51e7.54 (m, 4H), 7.66e7.68(m, 4H), 7.67 (s, 2H), 7.90 (s, 2H), 10.53 (s, 2H, imidazolium H); 13CNMR (CDCl3, 125 MHz) d: 138.5, 136.0, 134.3, 131.5, 130.7, 130.4,130.2, 129.7, 129.3129.2, 128.4, 123.8, 121.7, 121.3, 52.03 (CH2);HRMS (ESI) : m/z calcd. for C32H28N4 234.295 (dicationic, z ¼ 2);found 234.1163.

Synthesis of polymeric bis(imidazol-2-ylidene) Pd(II) complexes 10and 11

These complexes were prepared following the same procedureas described in the synthesis of complexes 8 and 9. Bis(imidazo-lium) dibromide 6 (300 mg, 0.48 mmol), Ag2O (133 mg, 0.57 mmol)and Pd(CH3CN)2Cl2 (123 mg, 0.52 mmol) were reacted to obtain250 mg of yellow solid (10) (80%). Similarly the corresponding ac-etate complex (11) was prepared using Pd(OAc)2 (123 mg,0.55 mmol) in 90% yield (300 mg). These complexes were insolublein common organic solvents and they were not processed further.They were used as it is for SuzukieMiyaura coupling reactions.

Suzuki-Miyaura coupling of 1,4-dibromobenzene and phenylboronicacid

A mixture of 1,2-dibromobenzene (100 mg, 0.42 mmol), phe-nylboronic acid (124 mg, 1.02 mmol), triphenylphosphine (5 mg,0.018 mmol), NaOH (68 mg, 1.7 mmol) and Pd-complex 8 (5 mg,2 mol%) were taken in a 50 mL two necked round-bottom flask.Dioxane (4 mL) was added to the mixture and heated at 105 �C.After 6 h the reactionwas complete (TLC). Removal of solvent underreduced pressure followed by aqueous work up gave the crudeproduct which was purified by crystallization from CH2Cl2/hexanemixture to yield pure p-terphenyl as a colorless crystalline solid in88% yield. The reaction was repeated with complex 10 and p-ter-phenyl was obtained in 75%. The product was confirmed by com-parison with authentic sample. 1H NMR (CDCl3, 500 MHz) d:7.35e7.38 (t, 2H, J¼ 7.5 Hz), 7.45e7.48 (t, 4H, J¼ 8 Hz), 7.65e7.66 (d,4H, J ¼ 7.5 Hz), 7.69 (s, 4H); 13C NMR (CDCl3, 125 MHz) d: 127.2,127.5, 127.6, 128.9, 140.2, 140.8.

Acknowledgments

We thank CSIR for fellowship (BS), CSIR and DST, New Delhi forfinancial support (SS) and the Department of Chemistry, IIT Madrasfor infrastructure facilities.

Appendix A. Supplementary data

The following is the supplementary data related to this article:CCDC numbers 960652, 989618, 960653 and 960654 contain the

supplementary crystallographic data for compounds 5, 6, 8 and 9,respectively. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Appendix B. Supplementary data

Supplementary data related to this article can be found athttp://dx.doi.org/10.1016/j.jorganchem.2014.06.023.

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