This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 431--438 431

Cite this: NewJ.Chem.,2013,37, 431

Carbazole functionalized luminescent silver(I), gold(I)and gold(III)–N-heterocyclic carbene complexes: a newsynthetic disproportionation approach towardsAu(I)–NHC to provide Au(III)–NHC†

Joydev Dinda,*a Sirsendu Das Adhikary,a Saikat Kumar Sethb andAmbikesh Mahapatrac

Synthetic, structural and photophysical properties of carbazole functionalized novel procarbenic species

1-(methyl/picolyl)-3-(N-ethylcarbazolyl)-1H benzimidazoliumhexaflurophosphate (1 and 2) and their

N-heterocyclic carbene (NHC) complexes of Ag(I) (3 and 4), Au(I) (5 and 6) and Au(III) (7 and 8) are

described. According to the solid-state structures, a linear coordination geometry of Ag(I)–NHC (4) and

Au(I)–NHC (6) has been established by X-ray diffraction studies; whereas the Au(III)–NHC complex (7)

adopts the square planar geometry. The Au(III)–NHC (7 and 8) complexes have been synthesized from

Au(I)–NHC (5 and 6) by capitalizing on a novel disproportionation protocol. Furthermore, the

Au–Ccarbene bonds in complexes 7 and 8 are inert towards changes in the oxidation state of the metal

atom. All the proligands (1 and 2) and complexes (3–8) are luminescent at room temperature.

Introduction

N-heterocyclic carbenes (NHCs) are defined as singlet carbeneswhich contain a carbene carbon incorporated in nitrogen-containing heterocycles. Ofele and Wanzlick in 1960 firstreported the existence of NHCs and in 1991 Arduengo andhis co-workers successfully first isolated the free NHC bydeprotonation of its imidazolium precursor.1 After that,N-heterocyclic carbenes (NHCs) became universally acceptedligands in organometallic and inorganic chemistry.2 The facileaccess to NHCs and remarkable stability of the carbene com-plexes against air, moisture, heat and oxidizing conditions arecomplemented by their superior s-donor properties comparedto the most basic phosphines.3 Because of their ‘push–pull’mechanism, they can bind with a broad range of transitionmetals which participate in catalysis.4 A large number ofN-heterocyclic carbene complexes have also been synthesized

and structurally characterized and their potential use as med-icine5 and luminescent materials6 has also been studied. Alarge portion of all known NHC complexes contain a group oftransition metals as the metal atom.4 Coinage metal–NHCs areextensively studied for their intriguing structural propertiesand numerous applications. Ag(I)–NHCs are the most studiedamong coinage metal–NHCs due to their straightforward pre-paration via the Ag2O route and sources of other metal–NHCsthrough transmetallation.7 In addition, their diverse propertiesin bonding, structures and potential applications in medici-ne,4a,8 nanomaterials,9 liquid crystals9 and organic catalysis4a

are also grounds of attraction. The interest toward Au(I)–NHCshas surged in the past two decades because of their easypreparation via the Ag–carbene transmetallation route andtheir various applications. These developments support therecent resurgence of interest in gold catalyzed reactions,10,11

which in turn has initiated a search for new gold–NHC com-plexes.10 Among the wide range of applications of gold andgold(I) complexes, organo-gold(III) complexes are rarelydescribed. The inertness of the Au–CNHC bond during substitu-tion reactions at linear [AuX(NHC)] complexes was noted byWang and Lin et al. earlier in 1999.7b It has also been observedthat the Au–Ccarbene bond is not affected if gold(I)–NHC com-plexes bearing unsaturated imidazolin-2-ylidene or saturatedimidazolidin-2-ylidene ligands are oxidized with halogens likebromine or chlorine to produce gold(III)–NHC complexes of the

a School of Applied Science, Applied Synthetic Chemical Research Lab., Haldia

Institute of Technology, Haldia 721657, Purba Medinipur, West Bengal, India.

E-mail: dindajoy@yahoo.comb Department of Physics, M. G. Mahavidyalaya, Bhupatinagar, Purba Medinipur,

West Bengal 721425, Indiac Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700032,

West Bengal, India

† CCDC 880920–880922. For crystallographic data in CIF or other electronicformat see DOI: 10.1039/c2nj40740j

Received (in Victoria, Australia)19th August 2012,Accepted 24th October 2012

DOI: 10.1039/c2nj40740j

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432 New J. Chem., 2013, 37, 431--438 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

type [AuX3(L)] (L = imidazolin-2-ylidene or imidazolidin-2-ylidene; X = Br, Cl).4f This protocol has been followed for along time to synthesize Au(III)–NHC.4f,12f In the past few years,Au(I) and Au(III)–NHCs have shown potential for applications inmedicine, especially in anticancer,12 antiarthritis13 and anti-bacterial activities.14

‘Argentophilicity’ and ‘aurophilicity’,4a,b interactions withmagnitude similar to hydrogen bonding, provide additionalstability to silver and gold super structures and sometimesplays an important role in intriguing luminescence properties.6

On the other hand, fluorescent ligands without d10–d10 inter-actions may also be luminescent. Coinage metal–NHCs showvery interesting luminescence properties and are potentiallyuseful precursors for optical sensors.

Taking into account the strong s-donating abilities of NHCsand luminescence of carbazole, we have designed carbazole func-tionalized NHCs. In the literature, limited examples of luminescentbehaviours of carbazole incorporated metal–NHCs are available.6f

Here, we report the preparation, structural characterization andluminescence of novel silver(I), gold(I) and gold(III)–NHC complexesof 1-(methyl/picolyl)-3-(N-ethylcarbazolyl) benzimidazolidine bear-ing flexible spacers. Due to the coordination of the carbene ligands,the photophysical property is altered compared to the parentproligand systems, rendering these complexes potentially usefulactive sites for photovoltaic applications. On the other hand, for thefirst time capitalizing the principle of disproportionationgold(III)–NHC complexes have been synthesized fromgold(I)–NHCs avoiding the noxious use of Cl2 (g) or Br2 (l) etc.15

Result and discussionSynthesis and characterization

Two benzimidazolium salts 1-methyl-3-(N-ethylcarbazolyl)-benzimidazoliumhexafluorophosphate (1) and 1-picolyl-3-(N-ethylcarbazolyl)-benzimidazoliumhexaflurophosphate (2) havebeen prepared from N-chloroethylcarbazole and 1-methyl-benzimidazole or 1-picolylbenzimidazole in dioxane under refluxand by subsequent treatment of the resultant benzimidazoliumchloride salts with NH4PF6 in aqueous media. They were isolated ascolourless solids. 1-Picolylbenzimidazole16a and N-chloroethyl-carbazole16b have been synthesized following reported procedures.Formation of the benzimidazolium salts 1 and 2 was confirmed bythe appearance of a CH benzimidazolium proton at 9.37 ppm and9.72 ppm, respectively, in 1H NMR spectra and also at 142.8 ppmand 152.7 ppm in 13C NMR spectra. As shown in Scheme 1,deprotonation of the benzimidazolium salts with Ag2O in acetoni-trile at room temperature yielded 1-methyl-3-(N-ethylcarbazolyl)ben-zimidazolylidenesilver(I)hexafluorophosphate (3) and 1-picolyl-3-(N-ethylcarbazolyl) benzimidazolylidenesilver(I)hexafluorophos-phate (4). 1-Methyl-3-(N-ethylcarbazolyl)benzimidazolylidenegol-d(I)hexafluorophosphate (5) and 1-picolyl-3-(N-ethylcarbazolyl)-benzimidazolylidenegold(I)hexafluorophosphate (6) were synthe-sized from 3 and 4 in acetonitrile by transmetallation at roomtemperature. 1-Methyl-3-(N-ethylcarbazolyl)benzimidazolylidene-gold(III)chloride (7) and 1-picolyl-3-(N-ethylcarbazolyl)benzimidazo-lylidenegold(III)chloride (8) were prepared from Au(I)-complexes

(5 and 6), by capitalizing the synthetic protocol of the disproportio-nation reaction15 (shown in Scheme 1). Surprisingly, when a clear,colourless acetonitrile solution of gold(I)–NHC (5 and 6) andAu(SMe2)Cl was stirred at room temperature for 5–6 hours, thecolourless solution was changed to orange yellow with slightprecipitation. After separation and purification, the yellow complexwas obtained in B30–32% yield and characterized as Au(III)–NHC(7 and 8).

Formation of the Ag–NHCs from the proligands (1 and 2)was confirmed by the disappearance of a benzimidazolium C2proton at 9.37 ppm and 9.72 ppm and a downfield shift of allaromatic protons compared to 1 and 2 respectively. The1H NMR and 13C NMR spectra and elemental analytical dataare also consistent with the proposed formula. The 13C NMRsignals of Ccarbene of 3 and 4 shifted to 179.2 ppm and182.4 ppm, respectively, where these same signals appearedat 142.8 and 152.2 ppm, respectively, for proligands 1 and 2.The NCN resonance in the 13C NMR spectra of 5 and 6 areobserved at 172.5 and 176.4 ppm, respectively, and these valuesare consistent with the similar biscarbene Ag(I) and Au(I)–NHCsystems.16d We further characterized all the new complexes 3–6by FAB+ mass spectroscopy to confirm if the species in solutionare the same as seen in the solid-state structures. In the case of3, the highest signal at m/z of 758.7 corresponds to thebinuclear species, [Ag(1-H)2]+, followed by m/z of 1001.9, whichcorresponds to [Au(2-H)2]+ in complex 6. The 1H NMR spectra ofcomplexes 7 and 8 are similar to those of complexes 5 and 6.The 13C NMR signals of carbenic carbon in gold(III)–NHCcomplexes 7 and 8 show the resonances in the range of d =145.6–147.8 ppm. These values represent an upfield shift (Dd =26.9–28.6 ppm) of the Ccarbene resonance upon oxidation ofgold(I) to gold(III). This strong upfield shift is surprising when itis compared to the downfield shift (Dd = 1–2 ppm) of thearomatic ring carbon upon gold(I) - gold(III) oxidation. Theresonances, observed for Ccarbene in 7 and 8, however, resemblethe values found for the NCHN resonances in the parentbenzimidazolium salts (d = 142.8–152.2 ppm). The upfield shiftof the Ccarbene resonance upon gold(I) - gold(III) oxidation wasobserved for related [AuX3(NHC)] complexes bearing imidazo-lin-2-ylidene or imidazolidin-2-ylidene ligands4f,11 and someauthors have explained this observation with the higher Lewisacidity of gold(III) in comparison to gold(I) as a known phenom-enon.4f,11 The FAB+ mass spectra showed the highest signal atm/z = 522.4 and 599.5 for complexes 7 and 8 respectively, andrevealed the labile nature of chloride ligands. The absorptionmaxima (lmax) of the Ag(I), Au(I) and Au(III)–NHC complexes(3–8) vary from 290–340 nm, whereas the same emit in the380–450 nm region due to their distinct intramolecular chargetransfer character.

Single crystal X-ray structural description of 4, 6 and 7

Complexes 4, 6 and 7 have been further characterized by X-raydiffraction analysis. All the crystals were developed by the slowdiffusion of diethyl ether into a saturated acetonitrile solutionof the respective complexes. As shown in Fig. 1, complex 4is mononuclear and crystallized as triclinic, P%1 space group.

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The crystallographic parameters are listed in Table 1. Thecentral silver (Ag1) atom lies on an inversion centre and isbicoordinated by two benzimidazolylidenes in a linear fashion[bond angle C(7)–Ag(1)–C(7) =180.01]. The selected bond para-meters of complex 4 are listed in Table 2. The pyridine groupsdo not interact with the central Ag(I) atom. The Ag(1)–C(7) bonddistance of 2.084(3) Å is consistent with the other similar

Ag–benzimidazolylidenes, e.g. N,N0-dibutylbenzimidazolylidenesilver chloride (Ag–Ccarb = 2.096(4)–2.091(4),16d,17 less than thesum of the covalent radii of carbon and silver [sum of covalentradii of Ag(I) and carbon is 2.111 A1]). Another structural feature ofthe compound is that the two picolyl groups are directed towardsthe opposite side of the coordination axis (Ccarb–Ag–Ccarb) i.e.adopt the anti configuration.

The molecular structure of complex 6 is shown in Fig. 2 andcrystallographic data are listed in Table 1. X-ray crystallographyreveals that complex 6 is isomorphous to 4. The central gold(Au1) atom lies on an inversion centre and is bicoordinated bytwo benzimidazolylidenes in a linear topology similar to that of4 (bond angle C(7)–Au(1)–C(7) = 180.01). The selected bondparameters of complex 6 are listed in Table 2.

The Au–Ccarbene distance of 2.008(8) Å is slightly longer thanthat of the picolyl substituted benzimidazoline gold(I) complex[1.999(8) Å]12e but within the van der Waals radii of gold andcarbon. The average Ccarbene–N separation of 1.365(5) Å isslightly longer than the corresponding value (1.328(3) Å)observed in 4, indicating little back bonding from the metalto the NHC ligands.

The molecular structure of complex 7 is established by theX-ray diffraction studies and is depicted in Fig. 3. The moleculecrystallizes as monoclinic, P21 space group.

The Au(III) atoms exhibit a distorted square planar environ-ment defined by the carbene C and three Cl (maximum devia-tions less than 0.02 Å from square planar geometry).

The Au(III) centre adopts a pseudo square planar geometry.The Au(III)–Ccarbene bond length is 1.998(6) Å, slightly shorter

Scheme 1 Synthesis of proligands, Ag(I), Au(I) and Au(III)–NHC complexes.

Fig. 1 ORTEP diagram and atom numbering scheme of complex 4 with dis-placement ellipsoids at the 40% probability level. C7i denotes the equivalentposition of C7 where i = 2 � x, 1 � y, �z (H and PF6 have been removed forclarity).

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than the Au(I)–Ccarbene bond distance of 2.008(8) Å observedin 4. Very similar results are obtained in the case of bromo-[1,3-diethyl-4,5-bis(4-fluorophenyl)imidazol-2-ylidene]gold(I):[Au(1)–C(1) = 1.988(9) Å] and tribromo[1,3-diethyl-4,5-bis(4-fluorophenyl)imidazol-2-ylidene] gold(III): [Au(1)–C(1)2.005(8) Å].12f The oxidation state of the gold atom apparentlybears no significance for the Au–C bond lengths.12f Due tothe trans effect of the carbene donor the Au–Cl(1) trans bondsin 7 [2.3082(18) Å] are longer than the Au–Cl (2/3) cisbonds [2.2751(19)–2.2557(19) Å]. These parameters match theequivalent values observed for related gold(III) monocarbenecomplexes.4f,18 The N–Ccarbene–N bond angle [109.7(5)1] in 7 islarger than the equivalent angle [104.2(7)1] in the gold(I)complex 6. The solid state structure of 7 shows p–p stackinginteractions as shown in Fig. 4.

Emission properties of precursors 1 and 2 and complexes 3–8

The emission spectra of 1 and 2 and 3–8 are shown in Fig. 5.The acetonitrile solution of proligands 1 and 2 shows emissionat 460 and 465 nm, respectively, after excitation at 350 nm. Thefluorescence quenching and blue shift are observed in the caseof complexes 3–8. The lmax emission of 3–8 is observed at380–450 nm. In the case of complexes 3–6, emission is observed at380–445 nm; whereas complexes 7 and 8 emit in the 440–450 nmrange. This can be attributed to the electron-withdrawing effect of

Table 1 Crystal data and structure refinement parameters for the title complexes 4, 6 and 7

Complex (4) Complex (6) Complex (7)

Empirical formula C54H44N8PF6Ag C54H44N8PF6Au C22H19N3Cl3AuFormula weight 1057.81 1146.91 628.72Temperature 293(2) K 293(2) K 293(2) KCryst. system, space group Triclinic, P%1 Triclinic, P%1 Monoclinic, P21

Unit cell parameters a = 10.382(2) Åb = 11.335(3) Åc = 12.001(5) Åa = 113.852(8)1b = 99.102(8)1g = 108.116(6)1

a = 10.107(2) Åb = 11.4690(10) Åc = 12.087(2) Åa = 113.2440(10)1b = 100.451(3)1g = 106.5210(10)1

a = 7.6394(9) Åb = 15.8473(18) Åc = 8.8524(11) Åa = 90.001b = 95.036(3)1g = 90.001

Volume/�3 1160.2(6) 1163.6(3) 1067.6(2)Z, calculated density 1, 1.514 Mg m�3 1, 1.637 Mg m�3 2, 1.956 Mg m�3

Absorption coeff. 0.542 mm�1 3.268 mm�1 7.280 mm�1

Crystal size/mm 0.21 � 0.18 � 0.11 0.18 � 0.11 � 0.05 0.28 � 0.19 � 0.09y-range/1 1.96 to 24.26 1.94 to 22.01 2.31 to 25.00Limiting indices �11 r h r 12,

�12 r k r 13,�13 r l r 13

�9 r h r 10,�12 r k r 12,�12 r l r 12

�9 r h r 9,�18 r k r 18,�10 r l r 10

Reflections collected/unique 10 390/3744 [R(int) = 0.0383] 6996/2820 [R(int) = 0.0458] 9840/3745 [R(int) = 0.0396]Completeness to y (%) 99.7 98.3 100Flack parameter — — 0.012(8)Max. and min. transm. 0.94 and 0.89 0.85 and 0.65 0.52 and 0.20Data/restraints/parameters 3744/0/319 2820/0/319 3745/1/263Goodness-of-fit on F2 1.029 1.069 1.017Final R indices [I 4 2s(I)] R1 = 0.0377,

wR2 = 0.0789R1 = 0.0441,wR2 = 0.1040

R1 = 0.0256,wR2 = 0.0583

R indices (all data) R1 = 0.0479,wR2 = 0.0837

R1 = 0.0468,wR2 = 0.1061

R1 = 0.0280,wR2 = 0.0593

Largest diff. peak and hole/e �3 0.603 and �0.301 1.946 and �1.110 1.113 and �0.704

Table 2 Selected bond parameters of 4 and 6

(4) (6)

Bond length/ÅMa–C(7) 2.084(3) 2.008(8)C(7)–N(2) 1.356(4) 1.374(10)C(7)–N(3) 1.351(3) 1.355(9)N(3)–C(8) 1.460(4) 1.463(10)Bond angles/1C(7)–M(1)–C(7)i 180.0 180.0N(3)–C(7)–N(2) 105.4(2) 104.2(7)N(3)–C(7)–Ag(1) 129.4(2) 129.5(6)N(2)–C(7)–Ag(1) 125.2(2) 126.2(5)N(3)–C(8)–C(9) 112.6(3) 113.0(7)N(4)–C(9)–C(8) 112.5(3) 112.6(7)

a M = Ag = 4; M = Au = 6.

Fig. 2 ORTEP diagram and atom numbering scheme of complex 6 with dis-placement ellipsoids at the 40% probability level. C7i denotes the equivalentposition of C7 where i = �x, �y, 1 � z (H and PF6 have been removed for clarity).

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silver(I) and gold(I)/gold(III), which inhibits the photo inducedelectron transfer (PET) process of the lone-pair electron of thenitrogen atom to the carbazole ring.19 In the solution state, thered-shift for fluorescence peaks of 7 and 8 from their correspondinggold(I) complexes 5 and 6 may be ascribed to an excimer formationarising from p–p stacking interactions20 as shown in Fig. 4.

Conclusion

In conclusion, here we report on a convenient syntheticapproach toward NHC silver(I) and gold(I) complexes compris-ing carbazole wingtip. The Au(III)–NHC complexes have beensynthesized from Au(I)–NHC by capitalizing on a novel dispro-portionation protocol. This new methodology to synthesizegold(III)–NHC complexes is developed for the first time in our

laboratory and hopefully will be attractive to others. Due to thecoordination of the carbene ligand, the photophysical proper-ties of NHC silver(I), gold(I) and Au(III) complexes are altered ascompared to the precursors, rendering these complexes poten-tially useful for photovoltaic applications; more detailedstudies will be carried out in the near future.

Experimental sectionGeneral procedures

All the reagents 1-methylbenzimidazole, 2-picolylchloridehydrochloride, Ag2O, carbazole, and 1,2-dichloroethane werepurchased from Sigma-Aldrich, UK, and used without furtherpurification. Au(SMe2)Cl and N-ethylcarbazole were preparedaccording to the literature.16 All manipulations were carried outunder an open atmosphere unless otherwise stated. All thesolvents were distilled over appropriate drying agents and N2-saturated prior to use. 1H NMR and 1H COSY spectra weremeasured on Bruker 400 and 13C NMR on 300 MHz spectro-meters at 25 1C using tetramethylsilane as an internal standard.Electronic and emission spectra of the complexes wereobtained on a Shimadzu UV-1601 and Perkin-Elmer-LS-55luminescence spectrometer, respectively.

General synthesis

1-Methyl-3-(N-ethylcarbazolyl)-1H benzimidazoliumhexafluoro-phosphate (1). 1-Methylebenzimidazole (500 mg, 3.79 mmol) andN-chloroethylcarbazole (869 mg, 3.786 mmol) were taken indioxane and refluxed for 48 hours at 100 1C. Then the solventwas removed and the gummy mass was suspended in dichloro-methane, and the colourless solid was extracted in dichloro-methane and recrystallized from dichloromethane and Et2O toget the solid mass. The solid mass was redissolved in minimumwater and aqueous saturated KPF6 solution was added, immediatewhite precipitate was observed, which was filtered, washed with

Fig. 3 ORTEP diagram and atom numbering scheme of complex 7 with dis-placement ellipsoids at the 40% probability level (H has been removed forclarity). [Pertinent bond lengths (Å) and bond angle (1): Au(I)–C(7) = 1.998(6),Au(1)–Cl(3) = 2.2557(19), Au(1)–Cl(2) = 2.2751(19), Au(1)–Cl(1) = 2.3082(18),N(3)–C(7) = 1.346(8), N(2)–C(7) = 1.324(7) and N(2)–C(7)–N(3) = 109.7(5),N(2)–C(7)–Au(1) = 124.3(4), N(3)–C(7)–Au(1) = 125.7(5), C(7)–Au(1)–Cl(3) =90.19(18), C(7)–Au(1)–Cl(2) = 86.38(18), C(7)–Au(1)–Cl(1) = 178.16(18),Cl(3)–Au(1)–Cl(1) = 91.31(8), Cl(2)–Au(1)–Cl(1) = 92.13(7), Cl(3)–Au(1)–Cl(2) =176.56(7)].

Fig. 4 Packing view of 7, one dimensional polymer through p–p stackinginteractions (benzimidazole ring = blue, carbazole ring = golden yellow).

Fig. 5 Emission (right) and excitation (left) spectra of ligands 1 and 2 andcomplexes 3–8.

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water and dried over vacuum. Yield was 1036 mg, 2.20 mmol,58%. 1H NMR (400 MHz, DMSO-d6): d = 9.37 (s, 1H, Ha); 8.08(d, J = 11.7 Hz, 2H, Hd,d0); 7.85 (d, J = 12.0 Hz, 1H, Hb,b0); 7.65(m, 2H, Hc,c0); 7.44 (t, J = 11.3 Hz, 2H, He,e0); 7.30 (d, J = 4.8 Hz, 2H,Hg,g0); 7.18 (t, J = 5.85 Hz, 2H, Hf,f0); 4.94 (s, 4H, –N–CH2–CH2–N–);3.33 (s, 3H, N–CH3) ppm. 13C NMR (300 MHz, DMSO-d6): d 142.8,141.2, 126.6, 126.2, 123.8, 120.9, 119.5, 117.2, 114.0, 113.4, 111.3,110.2, 109.6, 43.8, 34.2, 33.6 ppm. MS (FAB+): m/z 325.4 (L-PF6)+.M.p.: 212 � 2 1C. Anal. calc. for C22H20N3PF6: C, 56.05; H, 4.25; N,8.92%. Found: C, 55.87; H, 4.20; N, 8.85%.

1-Picolyl-3-(N-ethylcarbazolyl)-1H benzimidazolium hexa-fluorophosphate (2). A THF solution (25 mL) of benzimidazole(2.000 g, 17.0 mmol) was added to a suspension of oil-freesodium hydride (0.488 g, 20.4 mmol) in THF (25 mL) andstirred for 1 h at 60 1C. Then 20 mL THF solution of 2-chloro-methylpyridinehydrochloride (2.363 g, 18.6 mmol) was addeddropwise to the above solution. The resultant mixture wasstirred continuously for another 48 h at 60 1C and a yellowsolution was obtained. The solvent was removed in a rotaryevaporator and H2O (30 mL) was added to the residue. Thesolution was extracted in CH2Cl2 (3 � 20 mL) and the extractingsolution was dried over anhydrous MgSO4. After removingCH2Cl2, a red brown liquid, 1-picolylbenzimidazole, wasobtained. Yield: 2.832 g (92%).

1-Picolylbenzimidazole (500 mg, 2.39 mmol) and N-chloro-ethylcarbazole (550 mg, 2.40 mmol) were taken in dioxane andrefluxed for 48 hours at 100 1C. Then the solvent was removedand the gummy mass was suspended in dichloromethane(20 mL), and the colourless solid was extracted in dichloro-methane (3 � 15 mL) and recrystallized from dichloromethaneand Et2O to get the solid mass. The solid mass was redissolvedin minimum water and aqueous saturated KPF6 solution wasadded, immediate precipitate was observed, which was filtered,washed with water and dried over vacuum. Yield was 783.64mg, 1.43 mmol, 60%.1H NMR (400 MHz, DMSO-d6): d = 9.72(s, 1H, Ha); 8.45 (d, J = 4.46 Hz, 1H, Hh); 8.06 (d, J = 7.4 Hz, 2H,Hd,d0); 7.83–7.75 (m, 4H, Hi,j); 7.48 (d, J = 8.3 Hz, 2H, Hc,c0); 7.44(t, J = 11.1 Hz, 4H, He,e0); 7.29 (d, J = 8.1 Hz, 1H, Hk); 7.21 (t, J =7.1 Hz, 2H, Hg,g0); 7.16 (t, J = 7.7 Hz, 4H, Hf,f0); 5.72 (s, 2H); 5.05(s, 4H) ppm. 13C NMR (300 MHz, DMSO-d6): d 152.7, 149.6,143.2, 139.6, 137.6, 131.0, 130.9, 126.8, 126.5, 125.7, 123.7,122.5, 122.3, 120.4, 119.4, 113.5, 112.9, 108.5, 50.8, 45.8,42.1 ppm. MS (FAB+): m/z 402.5 (L-PF6)+. M.p.: 232 � 2 1C.Anal. calc. for C27H23N4PF6: C, 59.12; H, 4.20; N, 10.22%.Found: C, 58.90; H, 4.14; N, 10.15%.

1-Methyl-3-(N-ethylcarbazolyl)benzimidazolidene silver(I)hexafluorophosphate (3). Proligand 1 (80 mg, 0.17 mmol) andAg2O (20 mg, 0.09 mmol) were taken in dry acetonitrile and thecolourless solution was stirred for 6 hours in a dark place. Thenthe solution was filtered through celite to remove excess Ag2O.The solvent was removed and the white silver complex wasrecrystallized from acetonitrile and diethyl ether. Yield was55 mg, 0.061 mmol, 68%. 1H NMR (400 MHz, DMSO-d6): 8.05(d, J = 7.3 Hz, 2H, Hd,d0); 7.73 (d, J = 7.5 Hz, 2H, Hb,b0); 7.66(m, 2H, Hc,c0); 6.90 (t, J = 7.5 Hz, 2H, He,e0); 6.77 (d, J = 7.9 Hz,2H, Hg,g0); 6.53 (t, J = 7.4 Hz, 2H, Hf,f0); 4.68 (s, 2H); 4.51(s, 2H);

3.32 (s, 3H, N–CH3) ppm. 13C NMR (300 MHz, DMSO-d6): d179.2, 141.5, 127.4, 126.2, 124.2, 121.3, 120.2, 117.6, 114.7,113.9, 111.3, 110.2, 109.6, 44.6, 34.6, 33.7 ppm. MS(FAB+) m/z(%): 758.7 (AgL2-PF6)+. M.p.: 264 � 2 1C (decomposed). Anal.calc. for C44H38N6AgPF6: C, 58.47; H, 4.21; N, 9.30%. Found: C,58.17; H, 4.16; N, 9.23%.

1-Picolyle-3-(N-ethylcarbazolyl)benzimidazolidene silver(I) hexa-fluorophosphate (4). Proligand 2 (70 mg, 0.13 mmol) and Ag2O(15 mg, 0.065 mmol) were taken in dry acetonitrile and thecolourless solution was stirred for 6 hours in a dark place thenthe solution was filtered through celite to remove excess Ag2O andthe solvent was removed to get silver complex as a solid mass.Yield was 48 mg, 0.046 mmol, 70%. 1H NMR (400 MHz, DMSO-d6):d = 8.10 (d, J = 7.8 Hz, 1H, Hh); 8.07 (d, J = 4.5 Hz, 2H, Hd,d0); 7.76(d, J = 7.8 Hz, 2H, Hb,b0); 7.59 (t, J = 8.4 Hz, 2H, Hj); 7.42 (t, J =7.8 Hz, 2H, Hi); 7.36 (d, J = 8.1 Hz, 2H, Hc,c0); 6.90 (t, J = 10.3 Hz, 2H,He,e0); 6.87 (d, J = 7.3 Hz, 1H, Hk); 6.81 (d, J = 7.2 Hz, 2H, Hg,g0); 6.48(t, J = 7.3 Hz, 2H, Hf,f0); 5.06 (s, 2H); 5.01 (s, 2H) ppm.13C NMR (300MHz, DMSO-d6): d 182.4, 152.4, 144.8, 140.2, 139.7, 133.8, 131.8,127.4, 126.8, 125.5, 124.6, 123.7, 121.7, 119.7, 114.8, 113.2, 110.4,51.2, 46.4, 43.6 ppm. MS (FAB+) m/z (%): 912.8 (AgL2-PF6)+. M.P.:280 � 2 1C (decomposed). Anal. calc. for C54H44N8AgPF6: C, 61.31;H, 4.16; N, 10.60%. Found: C, 61.12; H, 4.12; N, 10.51%.

1-Methyl-3-(N-ethylcarbazolyl)benzimidazolidene gold(I) hexa-fluorophosphate (5). Proligand 1 (80 mg, 0.17 mmol) and Ag2O(20 mg, 0.09 mmol) were taken in dry acetonitrile and the colour-less solution was stirred for 6 hours in a dark place. Then thesolution was filtered through celite to remove excess Ag2O. Into theclear filtrate, Au(SMe2)Cl (25 mg, 0.085 mmol) was added dropwiseand immediate white precipitate was observed, and the mixturewas stirred for 1.5 hours in a dark place and filtered through celite.The solvent was removed and the white mass was recrystallizedfrom acetonitrile and diethyl ether. Yield was 56 mg, 0.056 mmol,66%. 1H NMR (400 MHz, DMSO-d6): 8.14 (d, J = 7.5 Hz, 2H, Hd,d0);8.02 (d, J = 7.8 Hz, 2H, Hb,b0); 7.78 (m, 2H, Hc,c0); 7.36 (t, J = 7.6 Hz,2H, He,e0); 6.85 (d, J = 8.2 Hz, 2H, Hg,g0); 6.62 (t, J = 8.7 Hz, 2H, Hf,f0);5.02 (s, 2H); 3.36(s, 3H, N–CH3) ppm. 13C NMR (300 MHz, DMSO-d6): d 172.5, 142.1, 127.6, 126.4, 124.3, 121.5, 120.3, 117.8, 114.9,114.1, 111.5, 110.3, 109.7, 44.8, 34.8, 33.9 ppm. MS (FAB+) m/z:847.8 (AuL2-PF6)+. M.P.: 278 � 2 1C (decomposed). Anal. calc. forC44H38N6AuPF6: C, 53.23; H, 3.83; N, 8.47%. Found: C, 52.96; H,3.79; N, 8.38%.

1-Picolyle-3-(N-ethylcarbazolyl)benzimidazolidene gold(I) hexa-fluorophosphate (6). Proligand 2 (70 mg, 0.13 mmol) and Ag2O(15 mg, 0.065 mmol) were taken in dry acetonitrile and thecolourless solution was stirred for 6 hours in a dark place thenthe solution was filtered through celite to remove excess Ag2O. Intothe clear filtrate, Au(SMe2)Cl (20 mg, 0.068 mmol) was addeddropwise and immediate white precipitate was observed, themixture was stirred for 1.5 h in a dark place and filtered throughcelite. The solvent was removed and the white mass was recrystal-lized from acetonitrile and diethyl ether. Yield was 54 mg,0.047 mmol, 69%. 1H NMR (400 MHz, DMSO-d6): d = 8.19 (d, J =8.1 Hz, 1H, Hh); 8.10 (d, J = 4.0 Hz, 2H, Hd,d0); 7.85 (d, J = 7.6 Hz, 2H,Hb,b0); 7.67(t, J = 7.6 Hz, 2H, Hj); 7.53 (t, J = 7.5 Hz, 2H, Hi);7.45 (d, J = 8.1 Hz, 2H, Hc,c0); 6.99 (t, J = 5.7 Hz, 2H, He,e0); 6.94

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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 431--438 437

(d, J = 9.8 Hz, 1H, Hk); 6.89 (d, J = 7.9 Hz, 2H, Hg,g0); 6.51 (t, J =7.1 Hz, 2H, Hf,f0); 5.12 (s, 2H); 5.11 (s, 2H) ppm. 13C NMR(300 MHz, DMSO-d6): d 176.4, 153.1, 144.2, 141.6, 139.5, 133.6,132.3, 127.8, 126.8, 125.4, 124.7, 123.2, 122.8, 121.4, 120.5,115.5, 113.9, 110.5, 51.8, 46.8, 42.7 ppm. MS (FAB+): m/z1001.9 (AuL2-PF6)+. M.p.: 300 � 2 1C (decomposed). Anal. calc.for C54H44N8AuPF6: C, 56.55; H, 3.84; N, 9.77%. Found: C,56.37; H, 3.79; N, 9.68%.

1-Methyl-3-(N-ethylcarbazolyl)benzimidazolidene gold(III)chloride (7). Proligand 1 (80 mg, 0.17 mmol) and Ag2O (20 mg,0.09 mmol) were taken in dry acetonitrile and the colourlesssolution was stirred for 6 hours in a dark place. Then thesolution was filtered through celite to remove excess Ag2O. Intothe clear filtrate, Au(SMe2)Cl (25 mg, 0.085 mmol) was addeddropwise and immediate white precipitate was observed; themixture was stirred for 1.5 hours in a dark place, then threeequivalent Au(SMe2)Cl was added into the solution and stirredfor 6 to 8 hours and filtered. The solvent was removed and thereddish mass was recrystallized from acetonitrile and diethylether. Yield was 16.98 mg, 0.027 mmol, 32%. 1H NMR (400 MHz,DMSO-d6): 8.06 (d, J = 7.8 Hz, 2H, Hd,d0); 8.03 (d, J = 7.7 Hz, 2H,Hb,b0); 7.47 (m, 4H, Hc,c0); 7.36 (t, J = 6.7 Hz, 4H, He,e0); 7.29 (d, J =5.3 Hz, 2H, Hg,g0); 7.11 (t, J = 4.3 Hz, 4H, Hf,f0); 5.08 (s, 4H); 3.48 (s,3H, N–CH3) ppm. 13C NMR (300 MHz, DMSO-d6): d 145.6, 127.9,126.6, 124.4, 121.7, 120.4, 117.9, 115.2, 114.1, 111.5, 110.4, 109.9,45.3, 35.1, 34.1 ppm. MS (FAB+): m/z 522.4 (AuL-2Cl)+. M.p.: 246� 2 1C (decomposed). Anal. calc. for C22H19N3Cl3Au: C, 42.01; H,3.02; N, 6.68%. Found: C, 41.83; H, 2.99; N, 6.60%.

1-Picolyle-3-(N-ethylcarbazolyl)benzimidazolidene gold(III)chloride (8). Proligand 2 (80 mg, 0.146 mmol) and Ag2O (17 mg,0.073 mmol) were taken in dry acetonitrile and the colourlesssolution was stirred for 6 hours in a dark place then the solutionwas filtered through celite to remove excess Ag2O. Into the clearfiltrate, Au(SMe2)Cl (22 mg, 0.074 mmol) was added dropwise andimmediate white precipitate was observed, the mixture was stirredfor 1.5 hours in a dark place, then three equivalent of Au(SMe2)Clwas added into the solution and stirred for 6 to 8 hours and filtered.The solvent was removed and the reddish mass was recrystallizedfrom acetonitrile and diethyl ether. Yield was 15.52 mg, 0.022 mmol,30%. 1H NMR (400 MHz, DMSO-d6): d = 8.24 (d, J = 7.5 Hz, 1H, Hh);8.11 (d, J = 4.1 Hz, 2H, Hd,d0); 7.87(d, J = 7.8 Hz, 2H, Hb,b0); 7.70 (t, J =8.2 Hz, 2H, Hj); 7.53(t, J = 7.6 Hz, 2H, Hi); 7.42 (d, J = 8.2 Hz, 2H,Hc,c0); 7.05 (t, J = 6.4 Hz, 2H, He,e0); 6.97 (d, J = 9.5 Hz, 1H, Hk); 6.90(d, J = 7.7 Hz, 2H, Hg,g0); 6.53(t, J = 7.4 Hz, 2H, Hf,f0); 5.16 (s, 2H); 5.13(s, 2H) ppm.13C NMR (300 MHz, DMSO-d6): d 147.8, 145.4, 141.4,134.2, 127.6, 126.4, 125.3, 117.8, 115.1, 113.8, 112.1, 110.7, 110.5,53.8, 44.4, 34.9 ppm. (FAB+) MS: m/z 599.5 (AuL-2Cl)+. M.p.: 270 �272 1C (decomposed). Anal. calc. for C26H21N4AuCl3: C, 45.05; H,3.03; N, 8.09%. Found: C, 44.89; H, 2.99; N, 8.01%.

Crystal structures determination

Single crystals suitable for data collection were grown fromslow diffusion of diethyl ether into a saturated acetonitrilesolution of 4, 6 and 7. The crystal data and details of the datacollection of 4, 6 and 7 are given in Table 1. X-ray data werecollected on a CCD diffractometer (graphite monochromated

MoKa radiation, l = 0.71073 A1) using o scans. The structureswere refined by the Full-matrix least squares method onSHELX-97.21 An empirical absorption correction SADABS wasapplied. The nonhydrogen atoms were refined anisotropically.Hydrogen atoms that were not bound to imidazolium-C2 atomswere placed in calculated positions and assigned to an isotropicdisplacement parameter of 0.08 A1. CCDC 880920–880922 forcompounds 4, 6 and 7 respectively.†

Acknowledgements

J.D. is grateful to DST, India, for financial support under‘Young Scientist Scheme’ (SR/FT/CS-046/2009) and Prof. SubhasChandra Bhattacharya, JU, Kolkata, for his constantencouragement.

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