Chemical Papers 66 (8) 733–740 (2012)DOI: 10.2478/s11696-012-0196-5

ORIGINAL PAPER

Synthesis of new dendritic antenna-like polypyridine ligands

Maciej Zalas*, B�lazej Gierczyk, Micha�l Ceg�lowski, Grzegorz Schroeder

Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland

Received 17 January 2012; Revised 26 March 2012; Accepted 27 March 2012

An efficient synthesis of multidentate polypyridine ligands, 3,5-bis(2,2′-bipyridin-4-ylethynyl)ben-zoic acid and 3,5-bis(2,5-bis(2-pyridyl)-pyridin-4-ylethynyl)benzoic acid, with potential applicationin the production of ruthenium dyes for dye-sensitised solar cells was developed. Isolation of inter-mediate products and final compounds is simple and the yields are very high. The ligands obtainedcan be used in the synthesis of dendritic analogues of well known and very efficient N3 dye and“black dye”.c© 2012 Institute of Chemistry, Slovak Academy of Sciences

Keywords: polypyridine ligands, dendritic polypyridine ligands, ruthenium dyes, solar energy, dye-sensitised solar cells

Introduction

Monodentate pyridine and multidentate polypyri-dine ligands have been frequently used in the synthe-sis of transition metals complexes (Kalinowska-Lis etal., 2011; Klein et al., 2011). Some of the most in-tensively studied polypyridine complexes are those in-cluding ruthenium, because of their unique combina-tion of spectroscopic, photophysical, photochemical,and electrochemical properties (Dutta et al., 2011).Polypyridine ruthenium complexes have found wideapplication in many areas of technology such as solarenergy conversion (O’Regan & Grätzel, 1991; Hagfeldtet al., 2010; Hagfeldt & Grätzel, 1995, 2000; Vou-gioukalakis et al., 2011), photoinduced water oxida-tion (Puntoriero et al., 2011), molecular electronic de-vices (Balzani et al., 2001; Campagna et al., 2002;Puntoriero et al., 2008), and photoactive DNA cleav-age for therapeutic purposes (Dutta et al., 2011; Tanet al., 2008).The solar energy conversion properties of ruthe-

nium polypyridine dyes used as sensitisers in dye-sensitised solar cells (DSSCs) have been widely stud-ied over the last two decades. Of the wide varietyof organic, inorganic, and metaloorganic dyes appliedas DSSCs sensitisers, Ru-complexes have shown thebest performance (Hagfeldt et al., 2010). Complexes

Ru(4,4′-dicarboxy-2,2′-bipyridine)2(NCS)2 (N3 dye)and [Ru(4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine)(NCS)3](tetra-n-butylammonium)3 (black dye) areknown as the most efficient ones (Hagfeldt et al., 2010;Li et al., 2006; Gunes & Sariciftci, 2008).Dyes for DSSC applications should be charac-

terised with a wide range of absorption from the so-lar spectrum and high molecular extinction coefficient,moreover, the increase of the adsorption abilities ofthe dye on the surface of the working electrode in thesolar cell should lead to more efficient light harvest-ing (Hagfeldt et al., 2010; Vougioukalakis et al., 2011;Funaki et al., 2009). There are two possibilities of thedevelopment of new and more efficient ruthenium dyesi.e. synthesis of new polypyridine ligands and chemi-cal modification of the already known ruthenium dyes(Funaki et al., 2009).Multinuclear dendritic ruthenium complexes in-

cluding polypyridine ligands have been previouslystudied as an example of chiral metallodendrimers(Bodige et al., 1997; Alston et al., 2010) but, to thebest of our knowledge, dendritic ruthenium complexeshave never been applied as DSSC’s sensitisers.Here, synthetic procedures leading to two new den-

dritic bi- and terpyridine ligands, which can be usedto synthesize N3 dye and “black dye” analogues withtwo metallic centres in a single complex molecule, are

*Corresponding author, e-mail: maciej.zalas@amu.edu.pl

734 M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012)

presented. These analogues can have a large potentialof application as efficient sensitisers for DSSCs.

Experimental

Reagents were used as supplied by Sigma–AldrichCo. (USA) and Merck KGaA (Germany). Solventswere purified by standard procedures. 1H (400 MHz)and 13C (101 MHz) NMR spectra were recorded on aVarian VNMRS-400 spectrometer (Varian NMR Inc.,USA) with standard acquisition parameters. Carbonsignal assignments were made on basis of 2D HSQCand HMBC experiments. Mass spectra were recordedon AMD Intectra Mass AMD 402 (EI-MS) (AMD In-tectra GmBH, Germany) and Waters/Micromass Q-tof Premier (ESI-MS) (Waters Inc., USA) spectrome-ters. Column chromatography was performed on silicagel 60 (70–230 mesh, Merck), while the TLC analy-sis was done on silica gel 60 TLC plates with a flu-orescence indicator (Merck KGaA, Germany). Melt-ing points were determined on a Boethius apparatus(Boethius HMK, Germany) and are uncorrected.To estimate the solubility of the synthesised lig-

ands (IX, XIV) and their esters (VIII, XIII), a mix-ture of the studied substance (10 mg) and the solvent(0.1 mL) was sonicated in a screw vial at 20◦C for5 min. The addition of constant aliquots of the sol-vent and the sonication procedure were repeated untilthe disappearance of solid material in the vial was ob-served. If a homogenous mixture was not obtained af-ter the addition of 5 mL of the solvent, the compoundwas treated as an insoluble one.

2,2′-Bipyridine-1-oxide (I)

2,2′-Bipyridine (20.0 g, 128 mmol) was dissolvedin 100 mL of trifluoroacetic acid and cooled to 10◦C;then, a 30 mass % hydrogen peroxide solution (20 mL,0.155 mol) was added and the mixture obtained wasstirred at ambient temperature for 3 h. After this time,the absence of unreacted 2,2′-bipyridine was confirmedby TLC (chloroform/methanol, ϕr = 9 : 1). The solu-tion was mixed with chloroform (250 mL) and washedwith 3 M NaOH (3 × 200 mL). Water phase was reex-tracted twice with chloroform (2 × 100 mL), and thecombined organic phases were dried with Na2SO4, fil-tered, and evaporated under diminished pressure. Theproduct was obtained as pale yellow oil which crys-tallises after some time into off-white solid (22.0 g).Purity of the product was higher than 99 % (by GCand NMR) and it did not contain any traces of 2,2′-bipyridine as well as its 1,1′-dioxide.

4-Nitro-2,2′-bipyridine-1-oxide (II)

A mixture of I (22.0 g, 128 mmol) and potassiumnitrate (70.0 g, 686 mmol) in 170 mL of concentratedsulphuric acid was stirred at 80◦C for 30 h. Then, the

mixture was poured onto ice (500 g) and neutralisedwith 25 mass % NaOH to pH 9.0. The precipitateformed was filtered off, washed thoroughly with coldwater, dried, and dissolved in dichloromethane. Theinsoluble material was filtered off and the solvent wasevaporated, and the solid residue was crystallised froma mixture of dichloromethane/hexane (ϕr = 2 : 1) togive 25.1 g of yellow crystals.

4-Bromo-2,2′-bipyridine-1-oxide (III)

Compound II (25.0 g, 115 mmol) was dissolved in350 mL of glacial acetic acid. After heating the mix-ture to 60◦C, acetyl bromide (75 mL, 1.015 mol) wasadded and the solution was stirred at this temperaturefor 2 h. The mixture was poured onto ice (250 g), neu-tralised with Na2CO3 and the product was extractedwith dichloromethane (3 × 150 mL). The combinedorganic layers were washed with cold water (100 mL),dried with Na2SO4 and evaporated to give 27.13 g ofwhite solid. Product of the purity higher than 98 %(by GC and NMR) was obtained and it was used inthe next step without purification.

4-Bromo-2,2′-bipyridine (IV)

Stirred suspension of III (27.0 g, 108 mmol) in500 mL of chloroform was cooled to 0◦C and phos-phorus tribromide (50 mL, 526 mmol) was addeddropwise. The mixture obtained was refluxed for 2 h,poured onto ice, and neutralised with 20 mass %NaOH solution. The organic phase was separated andthe water layer was extracted with dichloromethane(3 × 100 mL). The combined organic extracts werewashed with cold water (100 mL), dried with Na2SO4,and evaporated. The crude product was crystallisedfrom ethanol, to give 20.9 g of white crystals.

Ethyl 3,5-dibromobenzoate (V)

A solution of 3,5-dibromobenzoic acid (5.0 g,17.85 mmol) in 100 mL of anhydrous ethanol washeated under reflux and thionyl chloride (5 mL) wasadded dropwise. Heating was continued for 2 h andthe solvent was evaporated. The residue was puri-fied by column chromatography on silica gel usingdichloromethane as the eluent. After solvent evapo-ration, the product (5.34 g) was obtained as whitecrystals.

Ethyl 3,5-bis(trimethylsilylethynyl)benzoate(VI)

Compounds VI and VII were obtained accordingto method given by Matsuda et al. (2002). A solu-tion of V (5.3 g, 17.21 mmol) in dry triethylamine(60 mL) was deoxygenated with argon and Pd2(dba)3(150 mg), CuI (100 mg), and triphenylphosphine

M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012) 735

(250 mg) were added. The mixture was heated up to70◦C and ethynyltrimethylsilane (10 mL, 0.15 mol)was added. The solution was allowed to react for24 h at this temperature, the solvent was evaporatedand the solid, a brownish-black residue, was extractedwith chloroform (2 × 100 mL). The combined organicextracts were evaporated with silica gel (20 g), theresidue was loaded onto a column of silica gel and theproduct was eluted with hexane/ether (ϕr = 98 : 2).After solvent evaporation, VI (5.57 g) was obtained asyellowish needles.

Ethyl 3,5-diethynylbenzoate (VII)

Trimethylsilyl derivative VI (5.5 g, 16.08 mmol)was dissolved in 300 mL of freshly distilled THF (theuse of not freshly distilled THF caused the reactioncompletion in less than 5 min, however, polymerisa-tion of the product may occur during the solvent evap-oration, which can decrease the yield dramatically),then glacial acetic acid (1.85 mL, 30.37 mmol) anda solution of tetrabutylammonium fluoride trihydrate(10.5 g, 33.33 mmol) in 20 mL of THF were added.The mixture was allowed to react for 0.5 h, the solventwas evaporated, and the product was purified by col-umn chromatography on silica gel with hexane/diethylether (ϕr = 45 : 5) as the eluent. After solvent evap-oration, VII (2.93 g) was obtained as cream colouredflocky crystals.

Ethyl 3,5-bis(2,2′-bipyridin-4-ylethynyl)benzoate (VIII), ethyl 3,5-bis(2,5-bis(2-pyridyl)-pyridin-4-ylethynyl)benzoate (XIII)

Compound VII (0.5 g, 2.5 mmol) and 4-bromo-2,2′-bipyridine (1.5 g, 6.38 mmol) were dissolved in 50 mLof triethylamine. After deoxygenation of the solutionwith argon, Pd2(dba)3 (50 mg), CuI (30 mg), andtriphenylphosphine (150 mg) were added. The mix-ture was heated at 70◦C under stirring for 48 h. Then,the solvent was evaporated and the black residue wassuspended in chloroform (100 mL) followed by an ad-dition of 15 g of silica gel and solvent evaporation.The residue was loaded onto a column of silica gel andeluted with a mixture of chloroform/methanol (ϕr =4 : 1). After solvent evaporation, compound VIII (1.08g) was obtained as yellowish solid.Analogously, 1.5 g of compound XIII (pale yellow

powder) was obtained starting from compounds VII(0.5 g, 2.5 mmol) and XII (1.74 g, 5.6 mmol). In thiscase, a mixture of chloroform/methanol (ϕr = 7 : 3)was used as the eluent for column chromatography.

3,5-Bis(2,2′-bipyridin-4-ylethynyl)benzoic acid(IX), 3,5-bis(2,5-bis(2-pyridyl)-pyridin-4-ylethynyl)benzoic acid (XIV)

Bipyridine derivative VIII (1.0 g, 1.97 mmol) was

dissolved in a mixture of THF (150 mL) and methanol(100 mL) followed by an addition of lithium hydrox-ide (0.5 g, 20.8 mmol) in 25 mL of water. The mix-ture was stirred overnight at ambient temperature, thesolvent was evaporated, and the solid residue was sus-pended in water (50 mL). The suspension was neu-tralised with 10 mass % HCl and the mixture wasstirred overnight to give a yellow precipitate. This wasfiltered off, washed with water, and dried. The product(0.94 g) was obtained as yellow powder.Analogously, compound XIV (0.96 g) was obtained

(as yellow powder) starting from compound XIII(1.0 g, 1.51 mmol).

1,5-Bis(2-pyridyl)pentane-1,3,5-trione (X)

To a stirred suspension of sodium hydride (5.4 g,225 mmol) in dry THF (125 mL), a solution of acetone(3.53 g, 61 mmol) and ethyl 2-pyridinecarboxylate(27.98 g, 185 mmol) in dry THF (125 mL) was addeddropwise over 5 h. The solution was refluxed for 5 hand the solvent was evaporated. The obtained residuewas dissolved in water, filtered, and acidified withacetic acid to pH 7. The precipitate formed was filteredoff and recrystallised from ethanol, to obtain 9.0 g ofyellow solid.

2,6-Bis(2-pyridyl)-4-(1H)-pyridinone (XI)

A mixture of X (9.0 g, 33 mmol) and ammoniumacetate (20.0 g, 260 mmol) in ethanol (200 mL) washeated under reflux for 24 h. Then, the volume ofthe mixture was reduced to 100 mL and the obtainedbrown solution was cooled. The formed precipitatewas filtered off, washed with diethyl ether, and re-crystallised from ethanol. The product (5.4 g) was ob-tained as off-white crystals.

4-Bromo-2,6-bis(2-pyridyl)pyridine (XII)

The mixture of XI (5.4 g, 22 mmol), phospho-rus pentabromide (14.5 g, 34 mmol), and phosphorusoxybromide (65.9 g, 230 mmol) was heated at 100◦Cfor 12 h. The brownish-black residue was mixed withcrushed ice and the solution obtained was neutralisedwith potassium carbonate. The product was extractedtwice with dichloromethane, the solvent was evapo-rated, and the solid obtained was purified by columnchromatography on neutral alumina with a mixture ofdichloromethane/hexane (ϕr = 2 : 1) as the eluent. Awhite crystalline product (5.3 g) was obtained aftersolvent evaporation.

Results and discussion

The synthesis was divided into three main parts in-cluding the preparation of polypyridine group precur-sors, compounds IV and XII, preparation of the den-

736 M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012)

Fig. 1. Synthesis of 4-bromo-2,2′-bipyridine (dendritic bis-bipyridine precursor). Reaction conditions: i) H2O2, TFA; ii) KNO3,H2SO4; iii) AcBr; iv) PBr3.

Fig. 2. Synthesis of ethyl 3,5-diethynylbenzoate (clipping group). Reaction conditions: i) SOCl2, EtOH; ii) Et3N, ethynyltrimethyl-silane, Pd2(dba)3, CuI, triphenylphosphine; iii) AcOH, tetrabutylammonium fluoride.

Fig. 3. Synthesis of 4-bromo-2,6-bis(2-pyridyl)pyridine (dendritic bis-terpyridine precursor). Reaction conditions: i) NaH, acetone;ii) ammonium acetate; iii) PBr5, POBr3.

Fig. 4. Synthesis of dendritic polypyridine ligands. Reaction conditions: i) VII, Et3N, Pd2(dba)3, CuI, triphenylphosphine; ii) XII,Et3N, Pd2(dba)3, CuI, triphenylphosphine; iii) LiOH, THF, MeOH.

drite clipping group, compound VII, and finally bond-ing of the precursors and the clipper to obtain den-dritic polypyridine ligands, compounds IX and XIV.Fig. 1 presents the synthesis route for 4-bromo-2,2′-bipyridine (IV ) preparation from commercially avail-

able 2,2′-bipyridine. The three-step synthesis leads tothe final product IV with very good yield and high pu-rity. Schematic synthetic procedure of clipping grouppreparation – ethyl 3,5-diethynylbenzoate (VII ) ispresented in Fig. 2. The synthesis is based on the pro-

M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012) 737

Table 1. Characterisation data of prepared compounds

wi(calc.)/mass %wi(found)/mass % Yield M.p.

Compound Formula Mr

C H N % ◦C

I C10H8N2O 172.06 69.7669.70

4.684.71

16.2716.29

100 53–54

II C10H7N3O3 217.05 55.3055.41

3.253.17

19.3519.20

90 181.5–182

III C10H7BrN2O 249.97 47.8448.02

2.812.94

11.1611.30

94 108–109.5

IV C10H7BrN2 233.98 51.0951.22

3.002.81

11.9211.97

83 53–55

V C9H8Br2O2 307.89 35.1035.27

2.612.71

––

97 57–59

VI C19H26O2Si2 342.15 66.6166.54

7.657.77

––

95 48–49

VII C13H10O2 198.07 78.7778.81

5.095.13

––

92 95–96

VIII C33H22N4O2 506.18 78.2578.12

4.384.42

11.0611.19

85 225–227

IX C31H18N4O2 478.15 77.8177.66

3.793.59

11.7111.53

100 > 250 (dec.)

X C15H12N2O3 268.09 67.1667.08

4.514.65

10.4410.39

55 106–107

XI C15H11N3O 249.10 72.2872.35

4.454.58

16.8616.70

66 166–166.5

XII C15H10N3Br 311.01 57.7157.61

3.233.27

13.4613.20

77 137–138

XIII C43H28N6O2 660.24 78.1778.10

4.274.17

12.7212.58

90 249–252

XIV C41H24N6O2 632.20 77.8477.71

3.824.09

13.2813.07

100 > 270 (dec.)

Table 2. Spectral data of prepared compounds

Compound Spectral data

I 1H NMR (CDCl3), δ: 7.28 (ddd, 1H, H-5), 7.36 (m, 2H, H-4,5′), 7.83 (td, 1H, H-4′), 8.18 (dd, 1H, H-3), 8.31 (ddd,1H, H-6), 8.72 (ddd, 1H, H-3′), 8.89 (dt, 1H, H-6′)13C NMR (CDCl3), δ: 124.27 (C-5), 125.22 (C-5′a), 125.37 (C-3a), 125.80 (C-3′a), 127.74 (C-4), 136.11 (C-6), 140.46(C-4′), 147.19 (C-2′), 149.23 (C-6′), 149.46 (C-2)HRMS (EI), m/z (found/calc.): 172.0612/172.0637 (M+, C10H8N2O)

II 1H NMR (CDCl3), δ: 7.45 (ddd, 1H, H-5′), 7.88 (td, 1H, H-4′), 8.07 (dd, 1H, H-5), 8.37 (d, 1H, H-6), 8.79 (ddd,1H, H-3′), 8.88 (dt, 1H, H-6′), 9.16 (d, 1H, H-3)13C NMR (CDCl3), δ: 118.85 (C-3), 122.61 (C-5), 125.10 (C-3′), 125.35 (C-5′), 136.67 (C-4′), 141.92 (C-6), 142.46(C-4), 147.55 (C-2′), 148.24 (C-2), 149.82 (C-6′)HRMS (EI), m/z (found/calc.): 217.0482/217.0487 (M+, C10H7N3O3)

III 1H NMR (CDCl3), δ: 7.30 (m, 2H, H-5,5′), 7.78 (td, 1H, H-4′), 8.08 (d, 1H, H-6), 8.32 (d, 1H, H-3), 8.65 (ddd, 1H,H-3′), 8.87 (dt, 1H; H-6′)13C NMR (CDCl3), δ: 119.01 (C-4), 124.72 (C-5′), 125.44 (C-3′), 128.2 (C-5), 130.58 (C-3), 136.40 (C-4′), 141.42(C-6), 147.83 (C-2), 148.27 (C-2′), 149.38 (C-6′)HRMS (EI), m/z (found/calc.): 249.9760/249.9742 (M+, C10H7BrN2)

IV 1H NMR (CDCl3), δ: 7.32 (ddd, 1H, H-5′), 7.46 (dd, 1H, H-5), 7.81 (td, 1H, H-4′), 8.38 (dt, 1H, H-6′), 8.47 (d, 1H,H-6), 8.61 (d, 1H, H-3), 8.67 (ddd, 1H, H-3′)13C NMR (CDCl3), δ: 121.32 (C-3′), 124.27 (C-5′), 124.47 (C-5), 126.85 (C-3), 133.93 (C-4), 137.04 (C-4′), 149.21(C-6′), 149.81 (C-6), 154.70 (C-2′), 157.33 (C-2)HRMS (EI), m/z (found/calc.): 233.9788/233.9793 (M+, C10H7BrN2)

V 1H NMR (CDCl3), δ: 1.38 (t, 3H, CH3), 4.39 (q, 2H, CH2), 7.82 (d, 2H, H-2,6), 8.07 (t, 1H, H-4)13C NMR (CDCl3), δ: 14.22 (CH3), 61.85 (CH2), 122.93 (C-3,5), 131.27 (C-2,6), 133.60 (C-1), 138.09 (C-4), 164.02(C——O)HRMS (EI), m/z (found/calc.): 307.8862/307.8871 (M+, C9H8Br2O2)

738 M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012)

Table 2. (continued)

Compound Spectral data

VI 1H NMR (CDCl3), δ: 0.33 (s, 18H, CH3Si), 1.43 (t, 3H, CH3CH2), 4.42 (q, 2H, CH3CH2), 7.78 (t, 1H, H-4), 8.10(d, 2H, H-2,6)13C NMR (CDCl3), δ: 0.20 (CH3Si), 14.28 (CH3), 61.41 (CH2), 96.04 (———C—Si), 102.95 (———C—C), 123.75 (C-3,5),130.82 (C-1), 132.58 (C-2,6), 138.97 (C-4), 165.18 (C——O)HRMS (EI), m/z (found/calc.): 341.1499/341.1471 (M+, C19H26O2Si2)

VII 1H NMR (CDCl3), δ: 1.41 (t, 3H, CH3CH2), 3.17 (s, 2H, ———CH), 4.41 (q, 2H, CH3CH2), 7.79 (t, 1H, H-4), 8.17 (d,2H, H-2,6)13C NMR (CDCl3), δ: 29.70 (CH3), 61.55 (CH2), 78.85 (———CH), 82.44 (———C—), 122.88 (C-3,5), 131.11 (C-1), 133.21(C-2,6), 139.20 (C-4), 164.96 (C——O)HRMS (EI), m/z (found/calc.): 198.0660/198.0681 (M+, C13H10O2)

VIII 1H NMR (CDCl3), δ: 1.43 (t, 3H, CH3CH2), 4.42 (q, 2H, CH3CH2), 7.36 (ddd, 2H, H-5′′), 7.42 (d, 2H, H-6′), 7.43(dd, 2H, H-5′), 7.85 (td, 2H, H-4′′), 7.92 (t, 1H; H-4), 8.24 (d, 2H, H-2,6), 8.58 (d, 2H, H-3′), 8.71 (m, 2H, H-3′′,6′′)13C NMR (CDCl3), δ: 14.31 (CH3CH2), 61.68 (CH3CH2), 88.57 (———C), 91.59 (———C), 121.16 (C-3

′′), 123.27 (C-5′′),123.32 (C-3,5), 124.10 (C-5′), 125.23 (C-3′), 131.53 (C-1), 131.67 (C-4′), 133.21 (C-2,6), 137.04 (C-4′′), 138.49 (C-4),149.29 (C-6′a), 149.30 (C-6′′a), 155.41 (C-2′), 156.39 (C-2′′), 164.91 (C——O)HRMS (ESI+), m/z (found/calc.): 507.1799/507.1821 ([M + H]+, C33H23N4O2)

IX 1H NMR (DMSO-d6), δ: 7.51 (ddd, 2H, H-5′′), 7.66 (dd, 2H, H-5′), 8.00 (td, 2H, H-4′′), 8.17 (t, 1H, H-4), 8.22 (bs,2H, H-2,6), 8.42 (d, 2H, H-6′), 8.55 (bd, 2H, H-3′), 8.73 (ddd, 2H, H-6′′), 8.79 (m, 2H, H-3′′), 12.2 (bs, 1H, COOH)13C NMR (DMSO-d6), δ: 88.11 (———C), 91.82 (———C), 120.63 (C-3

′′), 122.11 (C-5′′), 122.32 (C-3,5), 124.72 (C-5′),125.53 (C-3′), 129.46 (C-1), 130.88 (C-2,6), 132.07 (C-4′), 133.15 (C-1), 137.54 (C-4′′), 137.70 (C-4), 149.44 (C-6′),149.90 (C-6′′), 154.34 (C-2′), 155.69 (C-2′′), 165.72 (COOH)HRMS (ESI+), m/z (found/calc.): 479.1535/479.1508 ([M + H]+, C31H19N4O2)

Xb 1H NMR (CDCl3), δ: 4.42 (s, 2H, CH2), 6.98 (s, 1H, ——CH), 7.41 (ddd, 1H, H-5), 7.50 (ddd, 1H, H-5′), 7.83 (td,1H, H-4), 7.86 (td, 1H, H-4′), 8.17 (dt, 1H, H-3), 8.20 (dt, 1H, H-3′), 8.66 (d, 1H, H-6), 8.69 (d, 1H, H-6′), 15.35(bs, 1H, OH)HRMS (ESI+), m/z (found/calc.): 269.0970/269.0926 ([M + H]+, C15H13N2O3)

Xc 1H NMR (CDCl3), δ: 6.80 (s, 2H, ——CH), 7.40 (ddd, 2H, H-5), 7.83 (dt, 2H, H-4), 8.02 (dt, 2H, H-3), 8.68 (d, 2H,H-6), 14.62 (bs, 2H, OH)

XI 1H NMR (CDCl3), δ: 7.31 (s, 2H, H-3′,5′), 7.46 (H-5′,5′′), 7.73 (dd, 2H, H-4′,4′′), 7.92 (d, 2H, H-3′,3′′), 8.76 (d,2H, C-6′,6′′), 9.34 (bs, 1H, NH)HRMS (ESI+), m/z (found/calc.): 250.0901/250.0980 ([M + H]+, C15H12N3O)

XII 1H NMR (CDCl3), δ: 7.56 (ddd, 2H, H-5′,5′′), 7.87 (td, 2H, H-4′,4′′), 8.58 (dt, 2H, H-3′,3′′), 8.65 (s, 2H, H-3,5),8.70 (d, 2H, H-6′,6′′)13C NMR (CDCl3), δ: 121.36 (C-3′,3′′), 124.10 (C-5′,5′′), 127.27 (C-3,5), 135.05 (C-4), 136.95 (C-4′,4′′), 149.17(C-6′,6′′), 154.82 (C-2′,2′′), 156.37 (C-2,6)HRMS (EI), m/z (found/calc.): 311.0007/311.0058 (M+, C15H10N3Br)

XIII 1H NMR (CDCl3), δ: 1.42 (t, 3H, CH3CH2), 4.44 (q, 2H, CH3CH2), 7.37 (ddd, 4H, H-5′′,5′′′), 7.90 (t, 1H, H-4),7.91 (td, 4H, H-4′′,4′′′), 8.23 (d, 2H, H-2,6), 8.60 (s, 4H, H-3′,5′), 8.62 (d, 4H, H-3′′,3′′′), 8.74 (d, 4H, H-6′′,6′′′)13C NMR (CDCl3), δ: 14.33 (CH3CH2), 61.65 (CH3CH2), 88.62 (———C), 91.68 (———C), 121.22 (C-3

′′,3′′′), 123.32(C-3,5), 123.90 (C-3′,5′), 124.34 (C-5′′,5′′′), 131.53 (C-1), 132.45 (C-4′), 133.21 (C-2,6), 136.73 (C-4′′,4′′′), 138.49(C-4), 149.06 (C-6′′,6′′′), 155.12 (C-2′′,2′′′), 155.33 (C-2′,6′), 164.95 (C——O)HRMS (ESI+), m/z (found/calc.): 661.2414/661.2353 ([M + H]+, C43H29N6O2)

XIV 1H NMR (DMSO-d6), δ: 7.49 (ddd, 4H, H-5′′,5′′′), 8.04 (td, 4H, H-4′′,4′′′), 8.12 (t, 1H, H-4), 8.24 (d, 2H, H-2,6),8.61 (s, 4H, H-3′,5′), 8.68 (d, 4H, H-3′′,3′′′), 8.72 (d, 4H, H-6′′,6′′′), 12.9 (bs, 1H, COOH)13C NMR (DMSO-d6), δ: 88.09 (———C), 91.99 (———C), 120.86 (C-3

′′,3′′′), 122.29 (C-3,5), 123.60 (C-5′′,5′′′), 124.11(C-3′,5′), 129.67 (C-1), 131.00 (C-2,6), 133.13 (C-4′), 136.84 (C-4′′,4′′′), 137.87 (C-4), 149.74 (C-6′′,6′′′), 154.50(C-2′,6′), 154.92 (C-2′′,2′′′), 164.95 (C——O)HRMS (ESI+), m/z (found/calc.): 633.19849/633.20390 ([M + H]+, C41H25N6O2)

a) Uncertain assignments; b) mono-enolic form, minor component; c) bis-enolic form, major component.

cedure presented by Matsuda et al. (2002). Carboxylicgroup of the “clipper” molecule plays the role of ananchoring group during the adsorption of the poten-tial dye, prepared with the use of the ligands stud-ied, on the working electrode of DSSC. Preparationof 4-bromo-2,6-bis(2-pyridyl)pyridine (XII ), which isa precursor of the “black dye” ligand dendritic ana-logue, is shown in Fig. 3. The synthesis starts from2-picolinic acid and it is a modification of the proce-

dures presented by Huang and Han (2006) and Muroand Castellano (2007). Final synthesis of the dendriticligands (IX and XIV ) is presented in Fig. 4. Char-acterisation and spectral data of the prepared com-pounds are given in Tables 1 and 2.It was found (see Table 3) that the obtained com-

pounds are practically insoluble in water or hex-ane. Ethyl esters of the obtained polypyridine lig-ands were moderately soluble in polar solvents, such

M. Zalas et al./Chemical Papers 66 (8) 733–740 (2012) 739

Table 3. Solubility of studied ligands and their ethyl esters

Solubility/(mg cm−3)Solvent

VIII IX XIII XIV

Water insoluble insoluble insoluble insolubleMethanol 20 7 20 5Acetonitrile 25 20 11 7Acetone 25 25 5 4DMSO 25 20 20 20DMF 20 20 9 7Benzene 25 17 insoluble insolubleDichloromethane 50 33 insoluble insolubleHexane insoluble insoluble insoluble insoluble

as dichloromethane (DCM) or benzene and methanol,and well soluble in polar, aprotic solvents. The stud-ied ligands in their acid form are less soluble in allsolvents. They are insoluble in benzene and DCM,but they were dissolved in DMSO, DMF, acetonitrile,or methanol (solvents usually used in the preparationof ruthenium complexes of bipyridines or terpyridines(Spiccia et al., 2004)).

Conclusions

The synthesis procedure presented is a good wayto obtain multidentate dendritic polypyridine ligandswith very good yields. Two proposed ligands in bothacid and ethyl ester forms are well soluble in typicalsolvents used in the synthesis of ruthenium polypyri-dine complexes. This result shows that the ligands pre-sented have a potential application in the preparationof ruthenium dyes, which can finally be used as sensi-tisers in DSSCs.

Acknowledgements. This work was supported by the fundsof the Polish National Science Centre, grant No. NN204023538.

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