Copper(i) complexes of 6,6′-disubstituted 2,2′-bipyridine dicarboxylic acids: New complexes for incorporation into copper-based dye sensitized solar cells (DSCs)

Article abstract of DOI:10.1039/b901346f

Synthetic approaches to 6,6′-disubstituted-2,2′-bipyridine ligands bearing carboxylic acid substituents at the 4,4′ or 5,5′-positions are described. Copper(i) complexes with these ligands have been prepared for use in copper-based dye-sensitized solar cells. The single-crystal structures of 6,6′-dimethyl-2,2′-bipyridine-4, 4′-dicarboxylic acid (H₂4), [H₃6][CF ₃CO₂]·2CF₃CO₂H (H ₂6 = 6,6′-diphenyl-2,2′-bipyridine-4,4′- dicarboxylic acid) and 4[Cu(H₂2)(H2)]·3H₂O (H ₂2 = 6,6′-dimethyl-2,2′-bipyridine-5,5′- dicarboxylic acid) are described, as well as those of the intermediates (1E,5E)-1,6-di(2-furyl)hexa-1,5-diene-3,4-dione and 4,4′-di(2-furyl)-6, 6′-dimethyl-2,2′-bipyridine (3), and the copper(i) complex [Cu(3)₂][PF₆]. The roles that classical and non-classical hydrogen bonding and π-stacking interactions play in determining the solid-state packing in these structures are discussed. Preliminary studies of DSCs with these complexes are reported. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b901346f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Copper(I) complexes of 6,6¢-disubstituted 2,2¢-bipyridine dicarboxylic
acids: new complexes for incorporation into copper-based dye sensitized
solar cells (DSCs)†
Edwin C. Constable,* Ana Hernandez Redondo, Catherine E. Housecroft,* Markus Neuburger and
Silvia Schaffner
Received 21st January 2009, Accepted 2nd June 2009
First published as an Advance Article on the web 7th July 2009
DOI: 10.1039/b901346f
Synthetic approaches to 6,6¢-disubstituted-2,2¢-bipyridine ligands bearing carboxylic acid substituents
at the 4,4¢ or 5,5¢-positions are described. Copper(I) complexes with these ligands have been prepared
for use in copper-based dye-sensitized solar cells. The single-crystal structures of 6,6¢-dimethyl-2,2¢-
bipyridine-4,4¢-dicarboxylic acid (H24), [H36][CF3CO2]·2CF3CO2H (H26 = 6,6¢-diphenyl-2,2¢-
bipyridine-4,4¢-dicarboxylic acid) and 4[Cu(H22)(H2)]·3H2O (H22 = 6,6¢-dimethyl-2,2¢-bipyridine-
5,5¢-dicarboxylic acid) are described, as well as those of the intermediates (1E,5E)-1,6-di(2-furyl)hexa-
1,5-diene-3,4-dione and 4,4¢-di(2-furyl)-6,6¢-dimethyl-2,2¢-bipyridine (3), and the copper(I) complex
[Cu(3)2][PF6]. The roles that classical and non-classical hydrogen bonding and p-stacking interactions
play in determining the solid-state packing in these structures are discussed. Preliminary studies of
DSCs with these complexes are reported.
Introduction
Experimental
Carboxylate-functionalised complexes have become a popular
choice for use in dye-sensitized solar cells (DSCs).1–4 While
ruthenium(II) complexes dominate the dyes studied to date, we
have recently shown that efficient DSCs can be fabricated using
carboxylate-derivatised copper(I) bis(2,2¢-bipyridine) complexes.5
This success has led us to a wider investigation of [CuL2]+
complexes where L is a bpy ligand functionalised in the 4,4¢-
or 5,5¢-positions with carboxylic acid groups. A key structural
requirement for the preparation of air-stable copper(I) complexes
with 2,2¢-bipyridine ligands is the presence of substituents at the
6- and 6¢-positions6 and in this paper we report compounds
with either methyl or phenyl substituents at these positions.
From a structural point of view, these ligands and complexes
are of interest in their own right because of the propensity for
hydrogen bond formation between adjacent CO2H ◊ ◊ ◊ HO2C units.
In addition, it is well established that co-crystallisation of car-
boxylic acids with heterocyclic bases may lead to supramolecular
assemblies based upon O–H ◊ ◊ ◊ N and C–H ◊ ◊ ◊ O interactions.7 In
this paper, we describe the syntheses and copper(I) complexes
of a series of 6,6¢-dimethyl- or 6,6¢-diphenyl-2,2¢-bipyridines
substituted in the 4,4¢- or 5,5¢-positions with carboxylic acid
functionalities, as well as structural details for selected ligands and
complexes.
General
1H and 13C NMR spectra were recorded at room temperature on
Bruker Avance DRX 600 and 500 MHz spectrometers; chemical
shifts are relative to residual solvent peaks with TMS d 0 ppm.
Infrared spectra were recorded on a Shimadzu FTIR-8400S
spectrophotometer with solid samples on a Golden Gate diamond
ATR accessory. Electron impact (EI), electrospray ionisation
(ESI), and MALDI-TOF mass spectra were recorded using
Finnigan MAT 95 and MAT LCQ and PerSeptive Biosystems
Voyager mass spectrometers, respectively. Electronic absorption
spectra were recorded on a Varian-Cary 5000 spectrophotometer.
Electrochemical measurements were performed with an Eco
Chemie Autolab PGSTAT 20 system using glassy carbon working
and platinum auxiliary electrodes with a silver wire as pseudo-
reference electrode; solvents (see Experimental section) were
purified. With the exception of Na3[Cu(6)2], 0.1 M [nBu4N][PF6]
was used as supporting electrolyte, and ferrocene (Fc) was added
at the end of each experiment as an internal reference. For
Na3[Cu(6)2], the supporting electrolyte was 0.1M NaClO4 and
measurements were made relative to Ag/AgCl, then corrected to
be with respect to Fc/Fc+.
Diethyl 6,6¢-dimethyl-2,2¢-bipyridine-5,5¢-dicarboxylate (1)
The method is based on that reported for the synthesis of diethyl
4,4¢,6,6¢-tetramethyl-2,2¢-bipyridine-5,5¢-dicarboxylate.8
Ethyl
2-methylnicotinate (9.34 cm3, 60.5 mmol) and Pd/C (5%, 1.35 g)
were heated at reflux for 10 d under an inert atmosphere. After
cooling the reaction mixture to room temperature, acetone
(20 cm3) was added and the Pd/C was removed by filtration.
The filtrate was evaporated to dryness. After standing overnight
in the refrigerator, the brown–black solid was recrystallised
Department of Chemistry, University of Basel, Spitalstrasse 51, CH 4056
Basel, Switzerland. E-mail: catherine.housecroft@unibas.ch; Fax: +41 61
267 1018; Tel: +41 61 267 1008
† CCDC reference numbers 717559–717565. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b901346f
6634 | Dalton Trans., 2009, 6634–6644
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©

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from MeOH, and 1 was isolated as pale brown needles (0.31 g,
0.93 mmol, 1.5%). H NMR (500 MHz, CDCl3) d/ppm 8.41 (d,
7.49 (d, J = 1.1 Hz, 2H, HA5), 7.12 (d, J = 1.4 Hz, 2H, HB3),
6.56 (dd, J = 3.5, 1.7 Hz, 2H, HB4), 2.75 (s, 6H, HMe). 13C NMR
(126 MHz, CDCl3) d/ppm 158.3 (CA6), 151.5 (CB2), 143.9 (CB5),
139.4 (CA4), 117.6 (CA5), 113.7 (CA3), 112.3 (CB4), 109.9 (CB3), 24.3
(CMe) (CA2 not observed). EI-MS m/z 316.1 [M]+ (calc. 316.1).
IR (n/cm-1) 3115w, 2370w, 1603m, 1574m, 1548s, 1485m, 1410m,
1374m, 1359m, 1218m, 1161m, 1016s, 985m, 930m, 865m, 848m,
813s, 744s, 700s, 669m, 595s. Satisfactory analysis could not be
obtained.
1
J = 8.1 Hz, 2H, HA3), 8.33 (d, J = 8.1 Hz, 2H, HA4), 4.41 (q,
J = 7.1 Hz, 4H, HCH2CH3), 2.92 (s, 6H, HMe), 1.43 (t, J = 7.1 Hz,
6H, HCH2CH3). 13C NMR (125 MHz, CDCl3) d/ppm 166.5 (CC=O),
159.5 (CA6), 156.6 (CA2), 139.5 (CA4), 125.7 (CA5), 118.8 (CA3),
61.3 (CCH2CH3), 25.1 (CMe), 14.3 (CCH2CH3). MALDI-MS m/z 330.1
[M + 2H]+ (calc. 328.1). IR: n/cm-1 2976w, 2933w, 1716s, 1584s,
1547m, 1445m, 1428m, 1390m, 1353m, 1273s, 1249s, 1139m,
1114m, 1078s, 1020m, 986m, 8506s, 773s, 723m. Found: C, 65.89;
H, 6.20; N, 8.54; C18H20N2O4 requires C, 65.84; H, 6.14; N, 8.53%.
6,6¢-Dimethyl-2,2¢-bipyridine-4,4¢-dicarboxylic acid (H24)
Compound H24 was prepared from 3 as previously described, and
6,6¢-Dimethyl-2,2¢-bipyridine-5,5¢-dicarboxylic acid (H22)
1
the H NMR and UV-VIS spectra agreed with literature data.11
Compound 1 (0.27 g, 0.82 mmol) was partially dissolved in a H2O–
EtOH (20 cm3, 1 : 1 vol.) solution which contained KOH (0.46 g,
8.2 mmol). The mixture was heated at reflux for 12 h. After cooling
to room temperature, the solution was partially evaporated under
reduced pressure and the pH was adjusted to 2 with 1 M aqueous
HCl to give a white precipitate. After filtration, H22 was collected
as a white solid (0.221 g, 0.813 mmol, 99%). 1H NMR (500 MHz,
DMSO-d6) d/ppm 8.35 (m, 4H, HA3+A4), 2.82 (s, 6H, HMe). 13C
NMR (126 MHz, DMSO-d6) d/ppm 167.6 (CC=O), 158.6 (CA6),
155.5 (CA2), 139.7 (CA4), 126.4 (CA5), 118.5 (CA3), 24.7 (CMe). EI-MS
m/z 272.1 [M]+ (calc. 272.1). IR (n/cm-1) 3000w, 2893w, 2796w,
2645w, 2527w, 1688s, 1583s, 1544s, 1435m, 1404s, 1379m, 1349
(w), 1285s, 1258s, 1219m, 1140m, 1119m, 1079m, 1033w, 986w,
926m, 851s, 774s, 666s, 641m, 621w. UV-VIS (MeOH) lmax/nm
(e/dm3 mol-1 cm-1) 212 (13000), 250 (13500), 305 (26000). Found:
C, 59.50; H, 4.58; N, 9.82; C14H12N2O4·0.5H2O requires C, 59.78;
H, 4.66; N, 9.96%.
13C NMR (126 MHz, DMSO-d6) d/ppm 166.3 (CC=O), 159.3 (CA6),
155.1 (CA2), 139.4 (CA4), 122.8 (CA5), 116.8 (CA3), 24.2 (CMe). EI-
MS m/z 272.1 [M]+ (calc. 272.1), 228.1 [M - COO]+ (calc. 228.1).
IR (n/cm-1) 3093w, 2916w, 2854w, 2615w, 2545w, 1836w, 1697s,
1566s, 1427s, 1396m, 1296s, 1218m, 1095w, 910m, 763m. Found:
C, 61.17; H 4.90; N, 10.38; C14H12N2O4 requires C, 61.76; H, 4.44;
N, 10.29%.
4,4¢-Di(2-furyl)-6,6¢-diphenyl-2,2¢-bipyridine (5)
1,6-Di(2-furyl)hexa-1,5-diene-3,4-dione (0.500 g, 2.07 mmol), N-
phenacylpyridinium bromide (1.149 g, 4.132 mmol) and am-
monium acetate (1.0 g, 13 mmol) were heated at reflux in
EtOH (20 cm3) for 12 h. The solution was allowed to cool to
room temperature, and the off-white precipitate was collected by
filtration and washed with cold EtOH. 5 was collected as a white
1
solid (0.464 g, 1.05 mmol, 51%). H NMR (500 MHz, CDCl3)
d/ppm 8.82 (d, J = 1.4 Hz, 2H, HA3), 8.23 (d, J = 7.4 Hz, 4H,
HC2), 8.07 (d, J = 1.4 Hz, 2H, HA5), 7.63 (d, J = 1.6 Hz, 2H, HB5),
7.57 (m, 4H, HC3), 7.49 (m, 2H, HC4), 7.11 (d, J = 3.4 Hz, 2H,
HB3), 6.60 (dd, J = 3.4, 1.7 Hz, 2H, HB4). 13C NMR (126 MHz,
CDCl3) d/ppm 157.1 (CA6), 156.1 (CA2), 151.9 (CB2), 143.8 (CB5),
139.4 (CC1), 139.2 (CA4), 129.2 (CC4), 128.8 (CC3), 127.1 (CC2), 114.8
(CA5), 114.2 (CA3), 112.2 (CB4), 109.1 (CB3). EI-MS m/z 440.2 [M]+
(calc. 440.2). IR (n/cm-1) 3040w, 1601m, 1570w, 1543m, 1489w,
1404w, 1366w, 1215w, 1153w, 1007m, 918w, 868m, 814w, 772m,
729m, 687s, 590s, 513s, 486s, 455s, 409s. Found: C 78.87, H 4.42,
N 6.02; C30H20N2O2·H2O requires C 78.59, H 4.84, N 6.11%.
(1E,5E)-1,6-Di(2-furyl)hexa-1,5-diene-3,4-dione
The synthesis followed a literature method,9 although spectro-
scopic data have not previously been reported. (1E,5E)-1,6-
di(2-furyl)hexa-1,5-diene-3,4-dione was isolated as an orange
crystalline solid in 11.7% yield. 1H NMR (500 MHz, CDCl3, ring
B = furan) d/ppm 7.61 (d, J = 15.9 Hz, 2H, H1), 7.57 (dd, J = 1.7,
0.4 Hz, 2H, HB5), 7.31 (d, J = 15.9 Hz, 2H, H2), 6.80 (d, J = 3.5 Hz,
2H, HB3), 6.53 (dd, J = 3.5, 1.8 Hz, 2H, HB4). 13C NMR (126 MHz,
CDCl3) d/ppm 188.7 (CC=O), 151.6 (CB2), 146.2 (CB5), 133.1 (C1),
117.9 (CB3), 117.4 (C2), 113.1 (CB4). EI-MS m/z 242.1 [M]+ (calc.
242.1). IR (n/cm-1) 3123w, 1662m, 1585m, 1543m, 1471m, 1388w,
1289m, 1265m, 1200m, 1068w, 995m, 974s, 926s, 881m, 850m,
831m, 811m, 746s, 637s, 588s, 548s. Found: C, 69.23; H, 4.20;
C14H10O4 requires C, 69.42; H, 4.16%.
6,6¢-Diphenyl-2,2¢-bipyridine-4,4¢-dicarboxylic acid (H26)
KMnO4 (4.95 g, 31.3 mmol) was added to a warmed mixture
t
of 5 (1.06 g, 2.42 mmol), BuOH (145 cm3) and H2O (30 cm3).
After heating at reflux overnight, the mixture was filtered through
Celite. The filtrate was evaporated to ca. 50 cm3, the pH adjusted
to 2 with 2 M HCl, and the white precipitate that formed was
separated by filtration. H26 was isolated as a white solid (0.62 g,
4,4¢-Di(2-furyl)-6,6¢-dimethyl-2,2¢-bipyridine (3)
The preparation of 3 was based on the reported method,10,11
although few characterisation data for 3 were available. (1E,5E)-
1,6-Di(2-furyl)hexa-1,5-diene-3,4-dione (0.500 g, 2.07 mmol), 1-
acetonylpyridinium chloride (0.708 g, 4.132 mmol) and am-
monium acetate (1.0 g, 13 mmol) were heated at reflux in
MeOH (20 cm3) for 12 h. The solution was allowed to cool to
room temperature, and the off-white precipitate was collected by
filtration and washed with cold MeOH. 3 was isolated as a white
1
1.55 mmol, 64%). H NMR (600 MHz, CF3CO2D) d/ppm 9.22
(s, 2H, HA3), 9.00 (s, 2H, HA5), 8.19 (d, J = 7.2 Hz, 4H, HC2),
7.82 (t, 2H, J = 7.3 Hz, HC4), 7.77 (t, J = 7.4 Hz, 4H, HC3). 13C
NMR (151 MHz, CF3CO2D) d/ppm 169.2 (CC=O), 160.3 (CA6),
148.2 (CA2), 146.4 (CA4), 135.6 (CC4), 134.0 (CC1), 132.3 (CC3),
129.4 (CC2), 128 1 (CA5), 123.5 (CA3). ESI-MS m/z 397.2 [M + H]+
(calc. 397.1). IR (n/cm-1) 2978w, 2831w, 2561w, 2361 w), 1690s,
1551s, 1427m, 1389s, 1296m, 1250s, 1142m, 895m, 756s, 679s.
Satisfactory elemental analysis could not be obtained.
1
solid (0.361 g, 1.14 mmol, 55%). H NMR (500 MHz, CDCl3)
d/ppm 8.61 (s, 2H, HA3), 7.58 (dd, J = 1.7, 0.6 Hz, 2H, HB5),
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[Cu(H22)2]Cl
isolated as a dark green solid (336 mg, 0.309 mmol, 68%). 1H NMR
(600 MHz, CD2Cl2 with added K2CO3) d/ppm 8.13 (s, 4H, HA3),
7.77 (d, J = 1.1 Hz, 4H, HB5), 7.75 (s, 4H, HA5), 7.58 (d, J = 7.4 Hz,
8H, HC2), 7.21 (d, J = 3.4 Hz, 4H, HB3), 7.06 (t, J = 7.4 Hz, 4H,
HC4), 6.91 (t, J = 7.6 Hz, 8H, HC3), 6.73 (dd, J 3.3, 1.7 Hz, 4H,
HB4). 13C NMR (151 MHz, CD2Cl2) d/ppm 157.8 (CA6), 153.9
(CA2), 150.9 (CB2), 145.7 (CB5), 139.7 (CA4), 138.8 (CC1), 129.9
(CC4), 128.1 (CC2+C3), 119.2 (CA5), 115.5 (CA3), 113.5 (CB4), 111.5
(CB3). ESI-MS m/z 943.0 [M - PF6]+ (calc. 943.2). IR (n/cm-1)
3122w, 3059w, 1735w, 1608s, 1540m, 1486m, 1454w, 1440w,
1429w, 1407w, 1371w, 1308w, 1244w, 1222m, 1157w, 1076w,
1016m, 937w, 920w, 884m, 833s, 769s, 738s, 692s, 642m, 590m,
554s. UV-VIS (CHCl3) lmax/nm (e/dm3 mol-1 cm-1) 244 (35000),
282 (36000), 313 (48000), 432 (4000), 587 (2400). E◦(MeCN)/V
vs. Fc: +0.27 (reversible). Found: C, 63.63; H, 3.72; N, 4.79;
C60H40CuF6N4O4P·2.4H2O requires C, 63.62; H, 3.99; N, 4.95%.
The procedure was adapted from the literature.8 An aqueous
solution (2 cm3) of H22 (0.090 g, 0.33 mmol) was warmed to
70 ◦C, and 15 drops of 1 M aqueous NaOH were added to the
solution. An aqueous solution (2 cm3) of CuSO4·5H2O (0.042 g,
0.17 mmol) was then added, followed by a further addition of
20 drops of 1 M aqueous NaOH. An aqueous solution (0.3 cm3)
of ascorbic acid (0.044 g, 0.25 mmol) was added to the reaction
mixture and the solution immediately turned dark red. The pH
of the solution was adjusted to 2 by adding 1 M aqueous HCl to
precipitate the product. After filtration, [Cu(H22)2]Cl was isolated
1
as a red solid (0.070 g, 0.11 mmol, 33%). H NMR (500 MHz,
CD3OD) d/ppm 8.54 (s, 8H, HA3+A4), 2.57 (s, 12H, HMe). ESI-MS
m/z 605.1 [M - Cl]+ (calc. 607.1). IR (n/cm-1) 3526w, 2916w,
1712m, 1582m, 1404w, 1381w, 1265m, 1157s, 1095w, 980w, 957w,
926w, 849m, 772s, 664w, 579m, 502m, 463s, 447s. UV-VIS (MeOH)
l
max/nm (e/dm3 mol-1 cm-1) 214 (74400), 274 (47000), 314 (60000),
[Cu(H26)2]Cl
482 (8000). E◦(MeOH)/V vs. Fc: +0.53 (quasi-reversible). Found:
C 52.94, H 4.25, N 8.62; C28H24ClCuN4O8 requires C 52.26, H
3.76, N 8.71%.
The procedure was as for [Cu(H22)2]Cl, using H26 (39.6 mg, 0.100
mmol) and CuSO4·5H2O (12.5 mg, 0.0500 mmol). [Cu(H26)2]Cl
1
was isolated as a green solid (31.1 mg, 0.035 mmol, 35%). H
NMR (see text) (600 MHz, D2O) d/ppm 8.08 (s, 4H, HA3), 6.99
(m, 12H, HA5+C2), 6.49 (m, 4H, HC4), 6.40 (m, 8H, HC3). MS see text.
IR (n/cm-1) 3059w, 2357w, 1716s, 1608w, 1553s, 1456m, 1437m,
1377m, 1296w, 1217s, 1026w, 999w, 901m, 841m, 764s, 737m,
692s, 640m. UV-VIS (see text, H2O): lmax/nm (e/dm3 mol-1 cm-1)
296 (10000), 343 (11000), 437 (2000), 608 (1170). E◦(see text,
H2O)/V vs. Fc: 0.00 (reversible). Found: C, 62.82; H, 3.86; N, 6.05;
C48H31ClCuN4O8·1.7H2O requires C, 62.57; H, 3.76; N, 6.08%.
[Cu(3)2][PF6]
A solution of [Cu(CH3CN)4][PF6] (19 mg, 0.050 mmol) in CH3CN
(2 cm3) was added to a solution of 3 (31.6 mg, 0.100 mmol)
in CHCl3 (5 cm3) resulting in a change from colourless to
red. Upon addition of Et2O to the solution, a red precipitate
formed. After filtration, [Cu(3)2][PF6] was isolated as a red solid
(39 mg, 0.046 mmol, 46%). 1H NMR (500 MHz, CDCl3) d/ppm
8.42 (s, 4H, HA3), 7.68 (m, 8H, HA5+B5), 7.24 (d, J = 3.6 Hz,
4H, HB3), 6.66 (dd, J = 2.9, 1.5 Hz, 4H, HB4), 2.29 (s, 12H,
HMe). ESI-MS m/z 695.8 [M - PF6]+ (calc. 695.2). IR (n/cm-1)
3115w, 2949w, 2912w, 2361w, 2334w, 1610m, 1547m, 1489m,
1441w, 1371w, 1250w, 1221w, 1159w, 1078w, 1020m, 937w,
932w, 885w, 872w, 835s, 739s, 704m, 673w. UV-VIS (CH3CN)
Crystal structure determinations
Data were collected on a Bruker-Nonius Kappa CCD in-
strument; data reduction, solution and refinement used the
programs COLLECT,12 DENZO/SCALEPACK,13 SIR92,14 and
CRYSTALS.15 The positions of the H atoms bonded to O or N
atoms were located in the difference maps. Structures have been
analysed using Mercury v. 1.4.2.16 ORTEP figures were drawn
using Ortep-3 for Windows.17
l
max/nm (e/dm3 mol-1 cm-1) 274 (67000), 289 (71000), 493 (4100).
E◦(MeCN)/V vs. Fc: +0.29 (reversible). Found: C, 55.83; H, 3.77;
N, 6.36; C40H32CuF6N4O4P·H2O requires C, 55.91; H, 3.99; N,
6.52%.
[Cu(H24)2]Cl
(1E,5E)-1,6-Di(2-furyl)hexa-1,5-diene-3,4-dione
The procedure was as for [Cu(H22)2]Cl, using H24 (89.7 mg, 0.33
mmol) and CuSO4·5H2O (0.042 g, 0.17 mmol). [Cu(H24)2]Cl was
isolated as a red solid (58 mg, 0.090 mmol, 27%). 1H NMR
(500 MHz, CD3OD) d/ppm 8.91 (s, 4H, HA3), 8.12 (s, 4H, HA5),
2.32 (s, 12H, HMe). ESI-MS m/z 607.2 [M - Cl]+ (calc. 607.1).
IR (n/cm-1) 3180w, 1712s, 1558s, 1434m, 1388m, 1265m, 1218m,
894m, 763s, 671 m. UV-VIS (MeOH) lmax/nm (e/dm3 mol-1 cm-1)
202 (34300), 252 (27200), 268 (25000), 319 (35400), 483 (9900).
E◦(MeOH)/V vs. Fc +0.42 (quasi-reversible). Found: C, 47.46;
H, 3.99; N, 7.90; C28H24ClCuN4O8·4H2O requires C, 47.00; H,
4.51; N, 7.83%.
C14H10O4, M = 242.23, colourless plate, monoclinic, space
˚
group C2/c, a = 37.426(1), b = 3.7745(2), c = 18.0118(7) A,
◦
3
˚
b = 118.174(2) , U = 2242.96(18) A , Z = 8, Dc = 1.435 Mg
m-3, m(Mo-Ka) = 0.106 mm-1, T = 173 K, 2556 reflections
collected. Refinement of 1651 reflections (164 parameters) with
I > 1.9s(I) converged at final R1 = 0.0470 (R1 all data = 0.0717),
wR2 = 0.0519 (wR2 all data = 0.0648), Rint = 0.026, gof = 1.138.
4,4¢-Di(2-furyl)-6,6¢-dimethyl-2,2¢-bipyridine (3)
C20H16N2O2, M = 316.36, colourless plate, orthorhombic, space
˚
group Pcab, a = 6.5194(1), b = 12.1847(2), c = 19.5707(3) A,
U = 1554.64(4) A , Z = 4, Dc = 1.352 Mg m-3, m(Mo-
3
˚
[Cu(5)2][PF6]
Ka) = 0.089 mm-1, T = 173 K, 1855 reflections collected.
Refinement of 1101 reflections (109 parameters) with I >
2.5s(I) converged at final R1 = 0.0408 (R1 all data = 0.0641),
wR2 = 0.0349 (wR2 all data = 0.0416), Rint = 0.071, gof = 1.121.
The method was as for [Cu(3)2][PF6], starting with
[Cu(CH3CN)4][PF6] (84.6 mg, 0.227 mmol) in CH3CN (3 cm3)
and 5 (200 mg, 0.455 mmol) in CHCl3 (7 cm3). [Cu(5)2][PF6] was
6636 | Dalton Trans., 2009, 6634–6644
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Table 1 Current–voltage characteristic data for DSCs constructed with
copper(I) complexes as photosensitizers. Cells were constructed as de-
scribed in the experimental section
6,6¢-Dimethyl-2,2¢-bipyridine-4,4¢-dicarboxylic acid (H24)
C14H12N2O4, M = 272.26, colourless plate, triclinic, space group
¯
˚
P1, a = 3.8591(6), b = 8.111(1), c = 10.607(2) A, a = 73.344(9),
Compound
(solvent)
Jsc/mA
hglobal
(%)
l
max/nm
(soln)
l
max/nm
(dye)
◦
3
˚
b = 84.983(7), g = 77.881(10) , U = 310.83(8) A , Z = 1,
cm-2
V
oc/mV FF
Dc = 1.454 Mg m-3, m(Mo-Ka) = 0.109 mm-1, T = 173 K,
1468 reflections collected. Refinement of 1461 reflections (91
parameters) with I > 2.0s(I) converged at final R1 = 0.0513
(R1 all data = 0.0834), wR2 = 0.0870 (wR2 all data = 0.1174),
Rint = 0.030, gof = 1.040.
[Cu(H22)2]Cl
(MeOH)
1.21
1.15
0.69
0.46
570
530
484
490
750
0.65 0.45
0.68 0.41
0.63 0.21
0.62 0.14
0.67 5.0
482
480
[Cu(H24)2]Cl
483
492
(MeOH)
[Cu(H26)2]Cla
(DMSO)
437, 608 494, 604
[Cu(3)2][PF6]
(CHCl3)
N719 (MeCN) 11.3
493
534
494
4,4¢-Di(2-furyl)-6,6¢-diphenyl-2,2¢-bipyridine (5)
392, 530
C30H20N2O2, M = 440.50, colourless plate, trigonal, space group
3
¯
˚
˚
R3, a = b = 33.495(2), c = 5.1499(3) A, U = 5003.7(5) A , Z = 9,
Dc = 1.316 Mg m-3, m(Mo-Ka) = 0.083 mm-1, T = 173 K,
2084 reflections collected. Refinement of 2064 reflections (155
parameters) with I > 2.0s(I) converged at final R1 = 0.0375
(R1 all data = 0.0773), wR2 = 0.0554 (wR2 all data = 0.0800),
Rint = 0.050, gof = 1.018.
a Solution UV-VIS data are for the sodium salt, see text.
of parallel adhesive tape 0.5 cm apart to control the thickness
and the area of the TiO2 film (0.9 ¥ 0.5 cm). The colloidal paste
was applied between the tapes by rolling a glass rod on the surface.
The film thickness was measured with a profilometer (Dektak) and
was 6.0 0.5 mm. After annealing at 450 ◦C for 30 min, the slides
were dipped into a solution of the dye in an appropriate solvent
(see Table 1) for several hours. The cells were constructed using
0.5 M LiI, 0.05 M I2, 0.5 M 1-methylbenzimidazole and 0.6 M
1-butyl-3-methylimidazolinium iodide in 3-methoxypropionitrile
as electrolyte. The electrolyte was chosen to give the best possible
comparison to the state-of-the-art optimized systems based on
N719. Cathode electrodes were constructed from FTO glass pieces
of the same dimensions as the anodes platinized by treatment with
5 mM H2[PtCl6] in propan-2-ol followed by heating to 380 ◦C
for 15 min. The anode and cathode were assembled using Surlyn
(Dupont) plastic and the seal made by heating to 110–120 ◦C
while pressing the two together. Measurements were made with
irradiation from the rear using a lamp with intensity 100 mW cm-2
(1 sun). The standard dye N719 was purchased from Solaronix.
4[Cu(H22)(H2)]·3H2O
C112H98Cu4N16O35, M = 2482.28, purple plate, monoclinic, space
˚
group C2/c, a = 19.3405(4), b = 19.6142(4), c = 15.5801(3) A,
◦
3
˚
b = 114.0211(9) , U = 5398.4(2) A , Z = 2, Dc = 1.527 Mg
m-3, m(Mo-Ka) = 0.871 mm-1, T = 173 K, 6190 reflections
collected. Refinement of 4532 reflections (449 parameters) with
I > 2.0s(I) converged at final R1 = 0.0360 (R1 all data = 0.0468),
wR2 = 0.0364 (wR2 all data = 0.0394), Rint = 0.033, gof = 1.044.
[H36][CF3CO2]·2CF3CO2H
C30H19F9N2O10, M = 738.47, colourless needle, triclinic, space
¯
˚
group P1, a = 9.0489(7), b = 9.8221(8), c = 17.542(1) A,
◦
3
˚
a = 93.451(5), b = 98.501(5), g = 95.971(5) , U = 1529.1(2) A ,
Z = 2, Dc = 1.604 Mg m-3, m(Mo-Ka) = 0.155 mm-1, T = 173 K,
9445 reflections collected. Refinement of 4614 reflections (505
parameters) with I > 1.0s(I) converged at final R1 = 0.0833
(R1 all data = 0.1798), wR2 = 0.0638 (wR2 all data = 0.0966),
Rint = 0.075, gof = 1.285.
Results and discussion
Ligand synthesis and characterisation
Entry into the 6,6¢-dimethyl-5,5-dicarboxylic acid series is
achieved through the convenient (but low-yielding) oxida-
tive dimerisation of ethyl 2-methylnicotinate with palladium/
charcoal.8,18,19 Ligand H22 was prepared in near quantitative yield
by the saponification of 18 under basic conditions, and was fully
characterised by routine spectroscopic and mass spectrometric
methods. For H24 and H26, the strategy outlined in Scheme 2 was
employed; this has previously been applied to the synthesis of H24.
[Cu(3)2][PF6]
C40H32CuF6N4O4P, M = 841.23, orange plate, triclinic, space
¯
˚
3
group P1, a = 8.9016(2), b = 10.9570(3), c = 19.0865(4) A,
◦
˚
a = 81.724(1), b = 81.0409(1), g = 88.743(1) , U = 1819.78(8) A ,
Z = 2, Dc = 1.535 Mg m-3, m(Mo-Ka) = 0.724 mm-1, T = 173 K,
13054 reflections collected. Refinement of 7298 reflections (505
parameters) with I > 3.0s(I) converged at final R1 = 0.0378
(R1 all data = 0.0715), wR2 = 0.0408 (wR2 all data = 0.0594),
Rint = 0.051, gof = 1.106.
1
For H24, our H NMR and UV-VIS spectroscopic data agreed
with those published,11 and the 13C NMR spectrum exhibited the
number of resonances expected and was fully assigned by 2D
NMR techniques.
Preparation and evaluation of solar cells
X-Ray quality crystals of the dione precursor shown in Scheme 2
readily formed from an ethanol solution of the compound,
and a search of the CSD16 revealed that its structure had not
previously been reported. The molecular structure of the dione is
shown in Fig. 1. The asymmetric unit contains two independent,
centrosymmetric molecules, and all bond distances and angles
are unexceptional. Each molecule is planar (deviations of atoms
Nanocrystalline titanium dioxide electrodes were prepared by
doctor blading a colloidal TiO2 paste (Solaronix Nanooxide-T,
colloidal anatase) on the conductive transparent surface of a piece
of glass (F-doped SnO2, FTO, Hartford glass company, Tec 8,
8X cm-2) that had been cut to 0.9 ¥ 2.0 cm and cleaned with water
and ethanol. The FTO-coated glass was covered with two layers
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Fig.
1
Structure of one of the independent molecules of
(1E,5E)-1,6-di(2-furyl)hexa-1,5-diene-3,4-dione; ellipsoids are plotted at
˚
the 50% level. Selected bond distances (A): C5–C6 1.336(3), C4–C5
1.429(3), C6–C7 1.467(3), C7–C7a 1.532(4), C7–O2 1.220(2), C1–O1
1.373(2), C4–O1 1.376(2); symmetry code a = 1 - x, 1 - y, -z. Bond
lengths in the second independent molecule are similar.
˚
from the least squares plane through each molecule < 0.2 A),
and the planes through the two independent molecules are almost
parallel (angle subtended = 3.4◦). The molecules are stacked in
an offset manner, and continuation of this stacking arrangement
generates columns. Molecules in adjacent stacks interact with each
other through weak O ◊ ◊ ◊ H–C contacts (C1H11 ◊ ◊ ◊ O4i = 2.68,
i
i
ii
˚
O2 ◊ ◊ ◊ H91 C9 = 2.61, C2H21 ◊ ◊ ◊ O4 = 2.63 A) and C–H ◊ ◊ ◊ palkene
interactions (C3H31 ◊ ◊ ◊ C13iii = 2.89, C3H31 ◊ ◊ ◊ C12iii =3.24 A)
˚
1
2
1
1
(symmetry codes i = x, 1 + y, z; ii = - x, + y, -2 - z; iii = x, –
2
y,–12 + z). Such weak interactions are well established as important
components in solid-state supramolecular assemblies.20–25 The
overall supramolecular assembly is a herring-bone like array
(Fig. 2), with an angle of 58.4◦ between the planes through
molecules in adjacent stacks.
Scheme 1 Ligand structures and ring labelling for NMR spectroscopy.
Fig. 2 Packing of dione molecules (see text).
Ligands H24 and H26 were prepared by permanganate oxidation
of 4,4¢-di(2-furyl)-6,6¢-dimethyl-2,2¢-bipyridine (3) and 4,4¢-di(2-
furyl)-6,6¢-diphenyl-2,2¢-bipyridine (5), respectively.26 The inter-
mediate compounds were fully characterised. Each exhibited a
parent ion in the EI mass spectrum (m/z 316.1 for 3, and m/z 440.2
for 5). The 1H NMR spectra of 3 and 5 are readily distinguished
by the presence of a signal at d 2.75 ppm for the methyl groups
in 3, and diagnostic signals for the phenyl substituents (ring C,
Scheme 1) in the spectrum of 5. The chemical shifts of the reso-
nances assigned to the bipyridine rings (A, Scheme 1) and furan
rings (B) are similar in 3 and 5, with the exception of the signal for
proton HA5 which appears at d 7.49 ppm in 3 and at d 8.07 ppm in 5.
This observation is consistent with the deshielding effect of the 6-
phenyl substituent on HA5. The 13C NMR spectra were assigned us-
ing 2D techniques. Both compounds yielded X-ray quality crystals,
grown by slow evaporation of chloroform solutions of the ligands.
The centrosymmetric structures are shown in Fig. 3(a) and 4(a),
respectively. The bpy unit in each compound adopts the expected
trans-configuration. Both 3 and 5 are close to being planar. In 3,
the angle between the least squares planes of the pyridine and
Fig. 3 (a) Molecular structure of compound 3 with ellipsoids plotted
at the 40% level. Selected bond parameters (A and ◦): C7–C8 1.503(2),
˚
C9–C9i 1.493(3), C4–C5 1.455(2), O1–C1 1.358(2), O1–C4 1.370(2),
C1–C2 1.326(3), C2–C3 1.449(3), C3–C4 1.349(3); C1–O1–C4 106.8(2)
(symmetry code: i = 2 - x, -y, 1 - z). (b) Packing of molecules of 3 is
dominated by p-stacking.
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Fig. 4 (a) Molecular structure of 5, with ellipsoids plotted at the 40% level.
◦
a
˚
Selected bond parameters (A and ): C11–C11 1.486(4), C6–C7 1.490(3),
C9–C12 1.461(3), O1–C12 1.375(2), O1–C15 1.370(3), C12–C13 1.353(3),
C13–C14 1.431(3), C14–C15 1.340(4); C12–O1–C15 106.9(2) (symmetry
code a = 1 - x, 1 - y, 1 - z). (b) Packing of molecules of 5 in the unit cell.
Scheme 2 Synthetic strategy for the formation of ligands H24 and H26.
furan rings is 10.3(1)◦, while in 5 it is 1.0(1)◦. In 5, the phenyl ring
is twisted 10.7(1)◦ out of the plane of the pyridine ring to which it
is bonded. Compounds 3 and 5 crystallise in orthorhombic (Pcab)
and trigonal (R3) space groups, respectively. In 3, the principal
packing forces are p-stacking interactions between bpy units.
Compounds 3 and 5 were oxidised to dicarboxylic acids H2411
and H26 (Scheme 2) using KMnO4. The highest mass peak in
the ESI mass spectrum of H26 appeared at m/z 397.2 and was
assigned to [M + H]+. The NMR spectra of H26 were recorded
in CF3CO2D because the compound is not soluble in common
organic solvents. Although it is difficult to compare the 1H NMR
spectra of 5 in CDCl3 and H26 in CF3CO2D (which may be
protonated, see below), the significant changes in chemical shift
for protons HA3 (d 8.82 to 9.22 ppm) and HA5 (d 8.07 to 9.00
ppm), and the disappearance of signals for the furan ring, confirm
the replacement of the substituent in the 4- and 4¢-positions of
the bpy unit. In the 13C NMR spectrum, a signal at d 169.2 ppm
was assigned to the carboxylic acid carbon atom. Single crystals
of H24 suitable for a X-ray diffraction study were grown from
a methanol solution of the compound, and the centrosymmetric
structure is shown in Fig. 5. The trans-configuration is as expected,
and the molecule is planar (maximum deviation of a framework
¯
Pairs of pyridine rings in adjacent molecules are aligned so as
to generate an offset stack (Fig. 3(b)), the distance between
i
˚
rings containing atoms N1 and N1 being 3.39 A, symmetry
code i = 1 - x, -y, 1 - z). The dominant attractive forces
between molecules of 5 involve weak edge-to-face p-interactions
and Npyridine ◊ ◊ ◊ H–Cfuran non-classical hydrogen bonds, and are in
stark contrast to the p-stacking observed in 3. The edge-to-face
p-interactions feature both phenyl ◊ ◊ ◊ phenyl and furan ◊ ◊ ◊ furan
contacts. The shortest phenyl ◊ ◊ ◊ phenyl contact is C3 ◊ ◊ ◊ H51i–C5i
2
3
1
3
(2.81 A, symmetry code i = + x + y, + x, 113 - z); the distance
˚
i
˚
from the ring centroid to H51 is 3.19 A. Although not directly
aligned, each C13–H131 bond points towards the C14–C15 p-
bond of an adjacent furan ring (C13–H131 ◊ ◊ ◊ C15ii = 3.02, C13–
H131 ◊ ◊ ◊ C14 = 3.46 A, symmetry code ii = 3 - x + y, 23 - x, -3 + z),
and these weak interactions complement the edge-to-face aromatic
interactions. The relative orientations of planar molecules of 5
that follow from the plethora of edge-to-face interactions place
atom from the least squares plane through the molecule is 0.07 A
1
1
ii
˚
˚
for C3). The difference in C5–O1 and C5–O2 bond distances
(see figure caption) is consistent with carboxylic acid (rather
than carboxylate) functionalities. The trans-arrangement of the
two CO2H groups is ideal for the formation of hydrogen-bonded
each nitrogen atom within close proximity of a furan hydrogen
iii
i
i
atom (N1 ◊ ◊ ◊ H151iiiC15iii = 2.68 A, N1 ◊ ◊ ◊ C15 = 3.605(4) A,
polymeric chains (O1H1 ◊ ◊ ◊ O2 = 1.78, O1 ◊ ◊ ◊ O2 = 2.645(2) A,
O1–H1 ◊ ◊ ◊ O2i = 177◦, symmetry code i = 1 - x, 1 - y, 1 -
z), and Fig. 6(a) illustrates that these all run in the same
direction through the lattice. Adjacent chains assemble into
˚
˚
˚
N1 ◊ ◊ ◊ H151iii–C15iii = 170◦; symmetry code iii = + x + y, + x,
2
3
1
3
1
3
¯
- z). The high symmetry that crystallisation in the R3 space
group imparts on packing of molecules of 5 and the dominance of
=
edge-to-face interactions are illustrated in Fig. 4(b).
sheets (Fig. 6(b)) by virtue of C O ◊ ◊ ◊ Hmethyl attractive contacts
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Fig. 5 Molecular structure of H24, with ellipsoids plotted at the 50% level.
Selected bond parameters (A and ◦): C4–C5 1.489(3), O2–C5 1.234(3),
˚
O1–C5 1.306(3), C7–C7a 1.482(4), C1–C2 1.496(3); C4–C5–O1 115.8(2),
C4–C5–O2 121.2(2), O1–C5–O2 122.9(2) (symmetry code a = -x, -y , -z).
Fig. 7 Molecular structure of the [H36]+ cation in [H36][CF3CO2]·
2CF3CO2H; ellipsoids are plotted at the 50% level. Selected bond pa-
◦
˚
rameters (A and ): O1–C11 1.304(3), O2–C11 1.203(3), O3–C12 1.307(3),
O4–C12 1.204(3), N1 ◊ ◊ ◊ N2 2.599(3); O1–C11–O2 125.5(2), O3–C12–O4
123.7(2).
C–O bond distances in each carboxylic acid group (see caption
to Fig. 7) confirms the protonation state. One phenyl substituent
is twisted out of the plane of the bpy unit more than the other
(angles between the least squares planes of rings containing atoms
N1 and C13, and N2 and C19 are 31.3(1) and 7.2(1)◦, respectively).
Although the distance between the least squares planes through
the bpy units of symmetry-related molecules in the triclinic unit
˚
cell is only 3.39 A, there is little overlap of the rings and p-
stacking interactions are therefore negligible. Packing within the
lattice is controlled primarily by hydrogen bonding as illustrated
in Fig. 8. The [CF3CO2]- ion and CF3CO2H solvate molecules
are distinguished by a comparison of the C–O bond distances:
˚
C29–O9 and C29–O10 =1.221(3) and 1.249(3) A, respectively,
˚
C25–O5 and C25–O6 = 1.195(4) and 1.287(4) A, respectively, and
˚
C27–O7 and C27–O8 = 1.189(4) and 1.294(4) A, respectively. The
[CF3CO2]- ion and one of the CF3CO2H molecules are disordered.
In the anion, each F atom position has been modelled over two
sites with 65 and 35% occupancies. In the CF3CO2H molecule,
two of the F atoms are disordered, and each position has been
modelled over two sites with 70 and 30% occupancies.
Fig. 6 (a) Hydrogen-bonded polymeric chains of H24 molecules. (b)
Adjacent chains assemble into 2D sheets.
ii
ii
ii
ii
˚
˚
(O2 ◊ ◊ ◊ H13 C1 = 2.60 A, O2 ◊ ◊ ◊ C1 = 3.428(3) A, O2 ◊ ◊ ◊ H13 –
C1ii = 143◦, symmetry code ii = -1 + x, 1 + y, z). A methyl group
from an adjacent molecule nestles into the cavity defined in Fig. 5
by atoms O2, C6–H61 and N1a.
Crystals of [H36][CF3CO2]·2CF3CO2H were grown over a pe-
riod of two days by dissolving the ligand in CF3CO2H and placing
the vial containing the solution into a sample bottle containing a
small amount of water. Fig. 7 shows the structure of the [H36]+
cation. The bpy unit adopts a cis-conformation, consistent with
protonation at one of the nitrogen atoms and the formation of
an N–H ◊ ◊ ◊ N hydrogen bond (the H atom being located in the
difference map). The angle between the least squares planes of
the two pyridine rings is 1.7(1)◦. The difference between the two
Fig. 8 Hydrogen bonding between cations, anions and solvate molecules
in [H36][CF3CO2]·2CF3CO2H. Symmetry codes: i = 1 - x, 1 - y, 1 - z;
ii = 1 - x, -y, 1 - z; iii = x, 1 + y, z.
6640 | Dalton Trans., 2009, 6634–6644
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Copper(I) complexes
two bpy units = 81.11(5)◦); the bite angle of each bpy ligand
is ª81◦. Two of the furan rings are disordered and have been
modelled with O/C mixed occupancy sites (O3 and C54 75%
occupancy, and C134 and O30 25%; O4 and C58 70% occupancy,
and C138 and O40 30%). Each ligand is close to being planar,
with the maximum deviation being for the furan ring containing
atom O2 (angle between the least squares planes of rings with O2
and N2 = 14.1(1)◦). The packing of the cations is noteworthy.
One near-planar ligand stacks over a ligand of an adjacent cation.
The interplane distances are appropriate for p-stacking (distances
Copper(I) complexes of ligands 3 and 5 were prepared by
treating [Cu(CH3CN)4][PF6] with each ligand, and the hexaflu-
oridophosphate salts of [Cu(3)2]+ and [Cu(5)2]+ were isolated as
red and green solids, respectively, in moderate to good yields.
The electronic absorption spectra of acetonitrile solutions of
[Cu(3)2][PF6] and [Cu(5)2][PF6] exhibited MLCT bands at 493 and
587 nm, respectively, consistent with the observed colours. The
spectra also showed intense, higher energy absorptions assigned
to ligand centred p*← p transitions. Each complex is redox active,
and undergoes a reversible Cu+/Cu2+ process at +0.29 V for
[Cu(3)2][PF6], and +0.27 V for [Cu(5)2][PF6]. The highest mass
peaks in the ESI mass spectra of [Cu(3)2][PF6] and [Cu(5)2][PF6]
corresponded to [M - PF6]+ (m/z 695.8 for [Cu(3)2]+ and 943.0
for [Cu(5)2]+). Coordination of ligand 3 to copper(I) results in
i
i
˚
between planes containing N1/N2 and N1 /N2 = 3.43 A, N3/N4
ii
ii
˚
and N3 /N4 = 3.37 A, symmetry codes: i = 1 - x, 1 - y, 1 - z;
ii = 2 - x, -y, -z). However, the two ligands of each complex
interact with their neighbours in different ways. Stacking between
ligands containing N1/N2 and N1i/N2i involves the whole face
of each ligand. In contrast, half of each N3/N4 ligand stacks
with half of a symmetry-related ligand in an adjacent molecule;
Fig. 10(a) shows the stacking involving pyridine rings N3 ◊ ◊ ◊ N3ii
and illustrates that face-to-face pyridine ◊ ◊ ◊ furan stacking occurs.
The remaining half of the ligand stacks with another cation
(represented as N4 ◊ ◊ ◊ N4iii, symmetry code iii = 1 - x, 1 - y,
-z) leading to the overall evolution of stacked chains of cations
running in parallel directions through the lattice (Fig. 10(b)). The
[PF6]- ions reside in the cavities between the cations, and there are
extensive C–H ◊ ◊ ◊ F–P weak interactions.
1
only small changes to the H NMR spectrum (both in CDCl3).
Similarly, on going from 5 (in CDCl3) to [Cu(5)2]+ (in CD2Cl2),
the chemical shifts of the signals assigned to the bpy and furan
protons undergo small perturbations. However, the signals for the
phenyl protons are significantly shifted to lower frequency (HC2
from d 8.23 to 7.58 ppm, HC3 from d 7.57 to 6.91 ppm, and HC4
from 7.49 to 7.06 ppm). This is a consequence of the shielding
resulting from each phenyl substituent lying over the p-cloud of a
bpy domain of the second ligand, enforced by the near tetrahedral
copper(I) coordination sphere (see structural discussion below).
The electronic spectra of [Cu(3)2][PF6] and [Cu(5)2][PF6] exhibit
intense absorptions in the UV region assigned to ligand-based
p*←p transitions, in addition to absorptions arising from MLCT
transitions (493 nm for [Cu(3)2]+ and 432and 587nm for [Cu(5)2]+).
The introduction of phenyl substituents in the 6- and 6¢-positions
results in the observation of a second MLCT band, extends the
range of the absorption spectrum into the red.27
Crystals of [Cu(3)2][PF6] suitable for X-ray analysis were
grown by slow diffusion of Et2O into a CHCl3 solution of
the complex. The structure of the [Cu(3)2]+ cation is shown in
Fig. 9 and confirms that the two ligands are close to being
orthogonal (angle between the least squares planes through the
Fig. 10 Packing of cations in [Cu(3)2][PF6].
Fig. 9 Molecular structure of the [Cu(3)2]+ cation in [Cu(3)2][PF6];
ellipsoids are plotted at the 40% level and H atoms are omitted. The
The copper(I) complexes of H22, H24 and H26 were prepared
by the addition of copper(II) sulfate to aqueous solutions of
the sodium salts of the acids, followed by reduction with ascor-
bic acid.8 After work-up in aqueous HCl, [Cu(H22)2]Cl and
[Cu(H24)2]Cl were isolated as red solids, and [Cu(H26)2]Cl as a
furan rings containing O3 and O4 are disordered and only the major
◦
˚
occupancy atoms are shown (see text). Selected bond parameters (A and ):
Cu1–N1 2.024(2), Cu1–N2 2.011(2), Cu1–N3 2.000(2), Cu1–N4 2.026(2);
N1–Cu1–N2 81.27(7), N1–Cu1–N3 130.31(7), N2–Cu1–N3 128.77(7),
N1–Cu1–N4 113.72(6), N2–Cu1–N4 126.03(6), N3–Cu1–N4 81.94(6).
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green solid. The cations in [Cu(H22)2]Cl and [Cu(H24)2]Cl are
isomers, and ESI mass spectra showed ions corresponding to
[CuL2]+ at m/z 607. The electronic spectra of methanol solutions
of [Cu(H22)2]Cl and [Cu(H24)2]Cl exhibit MLCT bands at 482
and 483 nm, respectively, in addition to more intense, higher
energy ligand-based absorptions arising from p* ← p transitions
Attempts to obtain the electronic spectrum of a DMSO
solution of [Cu(H26)2]Cl were hampered by the fact that the
complex is not stable in this solvent. Therefore, the spectrum of
an aqueous solution of Na3[Cu(6)2] was recorded. In addition
to high energy absorptions assigned to ligands-centred p*← p
transitions, the spectrum exhibited two MLCT bands at 437 and
608 nm. The spectrum is compared with those of [Cu(H22)2]Cl and
[Cu(H24)2]Cl in Fig. 11, and illustrates the decrease in intensity of
the MLCT bands on going from the complexes with ligands H22
or H24 to [6]-, and, by inference, H26.
1
(Fig. 11). Upon formation of the complexes, signals in the H
NMR spectra undergo minor perturbations, although the change
in solvent from DMSO-d6 to CD3OD makes detailed comparisons
difficult. The elemental analysis of the bulk sample of [Cu(H26)2]Cl
confirmed the formulation, but mass spectrometric and solution
spectroscopic characterisation of [Cu(H26)2]Cl proved to be diffi-
cult. Attempts to obtain an ESI mass spectrum of the complex led
only to the observation of a peak envelope at m/z 396 assigned to
the free ligand. Unlike [Cu(H22)2]Cl and [Cu(H24)2]Cl which are
slightly soluble in MeOH, [Cu(H26)2]Cl is insoluble in water and
Crystals of 4[Cu(H22)(H2)]·3H2O grew after a methanol so-
lution of [Cu(H22)2]Cl had been standing at room temperature
for several weeks. The molecular structure is shown in Fig. 12
and selected bond parameters are given in the caption. The
disordering leads to large estimated standard deviations, but the
bond distances are consistent with the protonation states shown in
the figure. The fully protonated ligand is ordered, while the CO2H
and CO2 units of the second ligand are disordered and have been
modelled (each with the CO2 units constrained to being planar)
over two sites, each with equal occupancy. One methyl group is
also disordered (C28 in Fig. 12) and has been modelled over two
sites of fractional occupancies of 75% and 25%. The compound
crystallises as a hydrate, and one water molecule resides on a two-
fold axis. The copper(I) centre is in the expected pseudo-tetrahedral
environment, the angle between the least squares planes of the
two bpy units being 84.49(7)◦. The packing of [Cu(H22)(H2)]
molecules is worthy of note and involves both p-stacking between
bpy domains and extensive hydrogen bonding between carboxylate
and carboxylic acid groups. The bpy unit containing N1 and N2
stacks over that containing N3i and N4i (symmetry code i = x,
1
common organic solvents with the exception of DMSO. The H
NMR spectrum of a DMSO-d6 solution of [Cu(H26)2]Cl showed
only broad peaks at room temperature. Initially we wondered
if this were due to restricted bond rotation around the Cpy–
Cphenyl bond but the signals did not sharpen upon increasing the
temperature of the solution. Since DMSO is not ideal for crystal
growth at room temperature, we attempted to grow crystals of
[Cu(H26)2]Cl by leaving the crude reaction mixture (still containing
an excess of NaOH) to stand for several days. Single crystals
of Na3[Cu(6)2] (see below) were isolated. The sodium salt was
1
water-soluble, and the H NMR spectrum of a D2O solution of
the complex was well-resolved. We noted earlier that on going
from 5 to [Cu(5)2]+, signals for the phenyl protons shift to lower
frequency, and the same phenomenon is observed on going from
H26 (in CF3CO2D) to [Cu(6)2]3- (in D2O) (HC2 from d 8.19 to
6.99 ppm, HC3 from d 7.77 to 6.40 ppm, and HC4 from d 7.82
to 6.49 ppm). Preliminary structural data for Na3[Cu(6)2].nH2O
confirm that the coordination geometry of the copper(I) centre is
distorted tetrahedral and that each phenyl substituent lies over the
bpy unit of the second ligand, consistent with the shielding effects
noted in the 1H NMR spectrum. Unfortunately, the structure was
severely disordered with respect to the positions of the Na+ ions
and solvate molecules, and attempts to grow better quality crystals
were unsuccessful.
1
2
1 - y, + z) of an adjacent molecule. The stacking is not ideal,
and while the bpy units overlap effectively, their planes are not
parallel. The distance from the centroids of the rings containing
N1 and N2 to the least squares plane through the bpy unit with
Fig. 12 Structure of the [Cu(H22)(H2)] molecule in 4[Cu(H22)(H2)]·
3H2O; H atoms except those in the CO2H groups are omitted, and ellip-
soids are plotted at the 30% level. For disordered atoms, one (if 50 : 50) or
major occupancy (for C28) sites only are shown. Selected bond parameters
◦
˚
(A and ): Cu1–N1 2.057(1), Cu1–N2 2.045(1), Cu1–N3 2.036(1), Cu1–N4
2.052(2), C11–O3 1.272(3), C11–O4 1.228(3), C14–O1 1.311(3), C14–O2
1.211(3), C36–O50 1.25(3), C36–O60 1.29(2), C37–O70 1.208(8), C37–O80
1.283(8); N1–Cu1–N2 81.30(6), N1–Cu1–N3 128.59(6), N2–Cu1–N3
133.40(6), N1–Cu1–N4 121.97(6), N2–Cu1–N4 115.34(6), N3–Cu1–N4
81.12(6), O3–C11–O4 127.3(2), O1–C14–O2 124.1(2), O50–C36–O60
118(2), O70–C37–O80 125.7(6).
Fig. 11 Comparison of part of the electronic spectra of [Cu(H22)2]Cl
(ꢀ, MeOH), [Cu(H24)2]Cl (ꢁ, MeOH) and Na3[Cu(6)2] (ꢀ, H2O, lowest
extinction coefficients).
6642 | Dalton Trans., 2009, 6634–6644
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˚
Like [Cu(H24)2]+,5 complexes [Cu(H22)2]+ and [Cu(H26)2]+ bind
strongly to TiO2 nanoparticles, giving good dye-modified surfaces.
To the best of our knowledge, no dyes bearing furan function-
alities as the anchoring group have previously been reported.
After being immersed in a CHCl3 solution of [Cu(3)2][PF6] for
several hours, the TiO2 nanoparticles on the conducting glass slides
were coloured indicating chemisorption of the dye. The UV-VIS
spectrum exhibited an MLCT band at 494 nm consistent with
the solution spectrum of the complex (Table 1). Although this is
consistent with [Cu(3)2]+ binding to TiO2 nanoparticles through
the furan O-donors, we cannot rule out the possibility of oxidation
of the furan functionalities to carboxylic acids.
i
i
N3 and N4 are 3.24 and 3.47 A, respectively. In contrast, for
the N3/N4 bpy unit, only the ring containing N3 is involved
in significant p-stacking. The interaction occurs with the N3ii
ring across an inversion centre with the distance between least
˚
squares planes through the pyridine rings being 3.46 A. The p-
stacked interactions therefore operate in orthogonal directions to
produce a network that is reinforced by hydrogen bonding between
carboxylates and carboxylic acids. Every CO2 or CO2H group is
involved, as well as the water molecules, resulting in a rigid network
(Fig. 13).
-
The lower performance of the DSCs constructed with H26
compared with H24 is a combination of the lower absorptivity
of the complex and lower adsorption on the oxide surface.
Conclusions
In this paper, we have described systematic approaches to 6,6¢-
disubstituted-2,2¢-bipyridine ligands bearing carboxylic acid sub-
stituents at the 4,4¢- or 5,5¢-positions. Copper(I) complexes with
these ligands have been prepared and a preliminary assessment of
their utilisation as dye-sensitizers in copper-based DSCs has been
made.
Acknowledgements
We thank the University of Basel, the Swiss National Science
Foundation, the Swiss Nanoscience Institute and the EU (HET-
EROMOLMAT) for financial support. Dr Daniel Ha¨ussinger
is thanked for recording the 600 MHz NMR spectra, and
we acknowledge the assistance of Dr Biljana Bozic-Weber in
recording UV-VIS spectroscopic and electrochemical data.
Fig. 13 Crystal packing in 4[Cu(H22)(H2)]·3H2O involves extensive
p-stacking and hydrogen bonding in orthogonal directions.
Performance of DSCs
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