High current density electrodeposition of silver from silver-containing liquid metal salts with pyridine-N-oxide ligands

Article abstract of DOI:10.1039/c3dt52416g

New cationic silver-containing ionic liquids were synthesized and used as non-aqueous electrolytes for the electrodeposition of silver layers. In the liquid state of these ionic liquids, a silver (i) cation is coordinated by pyridine-N-oxide (py-O) ligands in a 1:3 metal-to-ligand ratio, although in some cases a different stoichiometry of the silver center crystallized out. As anions, bis(trifluoromethanesulfonyl)imide (Tf₂N), trifluoromethanesulfonate (OTf), methanesulfonate (OMs) and nitrate were used, yielding compounds with the formulae [Ag(py-O)₃][Tf₂N], [Ag(py-O)₃][OTf], [Ag(py-O)₃][OMs] and [Ag(py-O) ₃][NO₃], respectively. The compounds were characterized by CHN analysis, FTIR, NMR, DSC, TGA and the electrodeposition of silver was investigated by cyclic voltammetry, linear potential scans, scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). With the exception of [Ag(py-O)₃][Tf₂N], which melts at 108 °C, all the silver(i) compounds have a melting point below 80 °C and were tested as electrolytes for silver electrodeposition. Interestingly, very high current densities were observed at a potential of -0.5 V vs. Ag/Ag⁺ for the compounds with fluorine-free anions, i.e. [Ag(py-O)₃][NO ₃] (current density of -10 A dm^-2) and [Ag(py-O) ₃][OMs] (-6.5 A dm^-2). The maximum current density of the compound with the fluorinated anion trifluoromethanesulfonate, [Ag(py-O) ₃][OTf], was much lower: -2.5 A dm^-2 at -0.5 V vs. Ag/Ag⁺. Addition of an excess of ligand to [Ag(py-O) ₃][OTf] resulted in the formation of the room-temperature ionic liquid [Ag(py-O)₆][OTf]. A current density of -5 A dm^-2 was observed at -0.5 V vs. Ag/Ag⁺ for this low viscous silver salt. The crystal structures of several silver complexes could be determined by X-ray diffraction, and it was found that several of them had a stoichiometry different from the 1:3 metal-to-ligand ratio used in their synthesis. This indicates that the compounds form crystals with a composition different from that of the molten state. The electrochemical properties were measured in the liquid state, where the metal-to-ligand ratio was 1:3. Single crystal X-ray diffraction measurements showed that silver(i) is six coordinate in [Ag(py-O) ₃][Tf₂N] and [Ag(py-O)₃][OTf], while it is five coordinate in the other complexes. In [Ag₃(py-O)₈][OTf] ₃, there are two different coordination environments for silver ions: six coordinate central silver ions and five coordinate for the outer silver ions. In some of the silver(i) complexes, silver-silver interactions were observed in the solid state. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3dt52416g

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PAPER
High current density electrodeposition of silver
from silver-containing liquid metal salts with
pyridine-N-oxide ligands†
Cite this: Dalton Trans., 2014, 43,
1589
Jeroen Sniekers,a Neil R. Brooks,a Stijn Schaltin,b Luc Van Meervelt,a Jan Fransaerb
and Koen Binnemans*a
New cationic silver-containing ionic liquids were synthesized and used as non-aqueous electrolytes for the
electrodeposition of silver layers. In the liquid state of these ionic liquids, a silver (I) cation is coordinated by
pyridine-N-oxide (py-O) ligands in a 1 : 3 metal-to-ligand ratio, although in some cases a different
stoichiometry of the silver center crystallized out. As anions, bis(trifluoromethanesulfonyl)imide (Tf2N),
trifluoromethanesulfonate (OTf), methanesulfonate (OMs) and nitrate were used, yielding compounds
with the formulae [Ag(py-O)3][Tf2N], [Ag(py-O)3][OTf], [Ag(py-O)3][OMs] and [Ag(py-O)3][NO3], respecti-
vely. The compounds were characterized by CHN analysis, FTIR, NMR, DSC, TGA and the electrodeposi-
tion of silver was investigated by cyclic voltammetry, linear potential scans, scanning electron microscopy
(SEM) and energy-dispersive X-ray spectrometry (EDX). With the exception of [Ag(py-O)3][Tf2N], which
melts at 108 °C, all the silver(I) compounds have a melting point below 80 °C and were tested as electro-
lytes for silver electrodeposition. Interestingly, very high current densities were observed at a potential of
−0.5 V vs. Ag/Ag+ for the compounds with fluorine-free anions, i.e. [Ag(py-O)3][NO3] (current density of
−10 A dm−2) and [Ag(py-O)3][OMs] (−6.5 A dm−2). The maximum current density of the compound with
the fluorinated anion trifluoromethanesulfonate, [Ag(py-O)3][OTf], was much lower: −2.5 A dm−2 at
−0.5 V vs. Ag/Ag+. Addition of an excess of ligand to [Ag(py-O)3][OTf] resulted in the formation of the
room-temperature ionic liquid [Ag(py-O)6][OTf]. A current density of −5 A dm−2 was observed at −0.5 V
vs. Ag/Ag+ for this low viscous silver salt. The crystal structures of several silver complexes could be deter-
mined by X-ray diffraction, and it was found that several of them had a stoichiometry different from the
1 : 3 metal-to-ligand ratio used in their synthesis. This indicates that the compounds form crystals with a
composition different from that of the molten state. The electrochemical properties were measured in
the liquid state, where the metal-to-ligand ratio was 1 : 3. Single crystal X-ray diffraction measurements
showed that silver(I) is six coordinate in [Ag(py-O)3][Tf2N] and [Ag(py-O)3][OTf], while it is five coordinate
in the other complexes. In [Ag3(py-O)8][OTf]3, there are two different coordination environments for
silver ions: six coordinate central silver ions and five coordinate for the outer silver ions. In some of the
silver(I) complexes, silver–silver interactions were observed in the solid state.
Received 2nd September 2013,
Accepted 21st October 2013
DOI: 10.1039/c3dt52416g
www.rsc.org/dalton
Introduction
Ionic liquids are solvents that consist entirely of ions.1 They
are very useful for electrochemical applications because of
aKU Leuven, Department of Chemistry, Celestijnenlaan 200F – P.O. Box 2404, B-3001
their wide electrochemical window and the presence of intrin-
sic ionic charge carriers.2 Disadvantages include their usually
high viscosity and low solubility for metal ions.3 By incorporat-
ing a metal ion into the molecular structure of the ionic
liquid, high metal concentrations can be achieved. If this
metal is incorporated into the cation, it will electromigrate to
the cathode during electrodeposition, allowing a better mass
transport to counteract the higher viscosity of the ionic liquid.
The cathodic reaction of these ionic liquids is the deposition
Heverlee, Belgium. E-mail: Koen.Binnemans@chem.kuleuven.be
bKU Leuven, Department of Metallurgy and Materials Engineering, Kasteelpark
Arenberg 44 – P.O. Box 2450, B-3001 Heverlee, Belgium
†Electronic supplementary information (ESI) available: Crystallographic data in
CIF format, additional figures of the crystal structures of α-[Ag(py-O)3][Tf2N],
β-[Ag(py-O)3][Tf2N], [Ag3(py-O)8][OTf]3, [Ag(py-O)3][OTf] and [Ag(py-O)2][NO3]
and tables with crystallographic information about interatomic distances
(Table S1) and π–π interactions (Table S2). CCDC 958484–958489. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52416g
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of metal, so that there are no issues with the cathodic homogenous sample preparation for CHN analysis challen-
decomposition of the ionic liquid components. Only a few ging. When the samples were heated until above their melting
examples of these “liquid metal salts” can be found in the temperature, silver CHN cups could be filled with the appro-
literature,4–7 and their use for electrochemical purposes has priate amount of the compound while maintaining
a
only been rather scarcely investigated.8–14 Our research group 1 : 3 metal-to-ligand ratio. Since sample preparation for CHN
recently indicated the potential of these liquid metal salts as requires very precise mass measurements, this requires some
electrolytes for the high-rate electrodeposition of copper15–17 time during which the water soluble compounds [Ag(py-O)3]-
and silver.18–20 Current densities up to −25 A dm−2 at a poten- [OMs] and [Ag(py-O)3][NO3] absorbed atmospheric moisture.
tial of −10 V vs. Ag/Ag+ (without compensation for the iR drop) The other compounds were not water soluble and also no
were achieved for [Cu(MeCN)4][Tf2N]15 and current densities traces of water were found in their CHN results. From the com-
up to −20 A dm−2 at −10 V (without iR drop compensation) for pound [Ag(py-O)3][OTf], the compound [Ag3(py-O)8][OTf]3 crys-
[Ag(MeCN)4]2[Ag(Tf2N)3].18 Metal deposits of a good quality tallized out, which was also found in the CHN analysis of
could be obtained, even without the use of additives, although those crystals. For [Ag(py-O)3][Tf2N], CHN sample preparation
the smoothness of the deposited layer significantly increased was more straightforward since this compound crystallizes in
upon the addition of thiourea or 1-H-benzotriazole to the the same metal-to-ligand ratio. Since the water soluble com-
electrolyte in the case of [Ag(MeCN)4]2[Ag(Tf2N)3].18
pounds were immediately brought into the inert atmosphere
In this paper, we describe new silver-containing liquid of the glove box after synthesis and drying, the water that
metal salts with pyridine-N-oxide ligands (py-O) and several was absorbed during sample preparation for the CHN
different anions: bis(trifluoromethanesulfonyl)imide (bistrifl- measurements was not present during the electrochemical
imide, Tf2N), trifluoromethanesulfonate (triflate, OTf) and the experiments.
non-fluorinated anions methanesulfonate (mesylate, OMs) and
nitrate (NO3). The metal-to-ligand ratios were in general 1 : 3,
Electrochemistry
although in some cases compounds with different stoichio- Gold-covered silicon wafers were used as the working electrode
metries crystallized, as evidenced by single crystal X-ray diffr- for electrodeposition. In order to make the current density
action studies. The electrochemistry of the silver-containing uniform across the whole electrode area, the electrode was
liquid metal salts was studied and the compounds were used recessed by placing it inside a PTFE sample holder, which
for the electrodeposition of silver coatings. Also, the electro- decreased the tendency for dendritic growth and defines the
chemical characteristics of a compound of stoichiometry 1 : 6, area of the working electrode. The presence of dendrites
[Ag(py-O)6][OTf], were investigated.
increases the real surface area, which means that the real
current density is lower than the value based on the geometri-
cal surface area of the electrode. A complete description of the
electrochemical cell is given elsewhere.15 Before use, the sub-
strates were cleaned by rinsing with acetone and dried. After
the deposition of a silver layer, the deposit was rinsed with
Experimental
General
All chemicals were purchased from IoLiTec (Heilbronn, water and acetone and dried. The electrolyte was contained in
Germany) or Sigma-Aldrich (Bornem, Belgium) and were of a platinum crucible, and it was not stirred during the experi-
reagent grade and used as received. The infrared spectra were ments. The electrochemical experiments were conducted at
recorded at a resolution of 4 cm−1, using an attenuated total 90 °C. The experiments were performed using a Solartron
reflectance (ATR) technique on a Bruker Vertex 70 FTIR Instruments SI 1287 electrochemical interface controlled by a
spectrometer, with a Platinum ATR extension. Assignment of computer with the Corrware software. All potential values in
the peaks was based on the available literature.21 Melting this paper are relative to a silver wire directly immersed in
points were determined on a Mettler-Toledo 822 DSC instru- solution (pseudo-reference electrode). The potential values
ment at a heating rate of 10 °C min−1 under a helium atmos- were corrected for the iR drop in real time by using the current
phere. TGA studies were performed on a TA instruments Q600 interrupt technique as was also described elsewhere.15 A silver
thermogravimeter from room temperature up to 400 °C at a coil was used as a counter electrode. It is known that ionic
heating rate of 5 °C min−1. The morphology and elemental liquids, even the most hydrophobic ones, can take up atmos-
composition of the silver deposits were determined by scan- pheric moisture which might alter the deposit morphology,
ning electron microscopy (SEM) and energy-dispersive X-ray depending on the amount of absorbed water.22 Therefore, all
spectrometry (EDX) (Philips XL 30 FEG). The viscosities were electrochemical experiments were performed in an argon-
measured on a Brookfield cone plate viscometer (LVDV-II+ pro- filled glove box (with oxygen and water concentrations below
grammable viscometer) with a cone spindle CPE-40. The 1 ppm).
sample holder was heated to 80 °C by a circulating water bath.
Elemental analyses (C, H, N) were carried out using a CE
Crystallography
Instruments EA-1110 elemental analyzer. For most compounds Crystals that were grown from the compounds with the general
however, a different metal-to-ligand stoichiometry crystallized formulae [Ag(py-O)3][Tf2N], [Ag(py-O)3][OTf], [Ag(py-O)3][OMs]
out than the 1 : 3 ratio that was synthesized. This made and [Ag(py-O)3][NO3] which were suitable for single crystal
1590 | Dalton Trans., 2014, 43, 1589–1598
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X-ray diffraction were mounted on a nylon loop attached to a (0.94 g, 9.90 mmol) and stirred for 1 hour at room tempera-
copper pin and placed on an Agilent SuperNova diffractometer ture. Excess acetonitrile was removed on a rotary evapor-
into the cold stream of an Oxford Cryostream 700 at 100(2) K ator and the product was dried overnight on a high vacuum
using Mo Kα radiation (λ = 0.71073 Å). The absorption correc- line, yielding a light brown solid at room temperature. Yield:
tions were applied using CrysAlisPro.23 All structures were 1.93 g (99%). CHN found (calculated for C11H13AgN2O5S):
solved using direct methods and refined by the full-matrix C 33.37 (33.60)%, H 3.21 (3.33)%, N 6.87 (7.12)%. IR (ATR,
least-squares procedure in SHELXL.24 For refinement and νmax/cm−1): 3101 (CH), 3073 (CH), 3016 (CH), 2939 (CH), 1463
making pictures, the program OLEX2 was used.25 All hydrogen (ring), 1208 (NO), 1189, 1164 (CH in plane), 1072 (CH in
atoms were placed in calculated positions and refined using a plane), 1035, 1016 (ring breathing), 821 (NO), 781, 760 (CH out
riding model. For the crystal structure of the α-polymorph of of plane), 673, 547, 524, 491, 473; NMR: δH (400 MHz, DMSO-
[Ag(py-O)3][Tf2N], the bistriflimide anions were disordered d6): 8.24 (m), 7.31 (m), 1.99 (s); δC (100 MHz, DMSO-d6): 138.85
over two positions. For one of the anions, the major disorder (s), 126.71 (s), 125.61 (s), 77.32 (s), 77.01 (s), 76.67 (s); Melting
part comprised 72.4(5)% of the total and for the other it was point: 86 °C.
78.9(1)%. For the β-polymorph, one of the bis(trifluoromethane-
[Ag(py-O)3][OMs]. To silver(I) methanesulfonate (10.0 g,
sulfonyl)imide anions was also disordered over two posi- 49.3 mmol) in acetonitrile (25 mL) was added pyridine-N-oxide
tions. The major disorder part here comprised 65.6(7)% of the (14.06 g, 147.8 mmol) and stirred for 1 hour at room tempera-
total occupancy. Appropriate restraints on bond distances and ture. Excess acetonitrile was removed on a rotary evapor-
thermal parameters were used. CCDC 958484–958489 contains ator and the product was dried overnight on a high vacuum
the supplementary crystallographic data for this paper.
line, yielding a light brown liquid at room temperature. Yield:
23.59 g (98%). After a few days, the product slowly solidifies to
a glassy solid. CHN found (calculated for C16H18AgN3O6S·H2O):
Synthesis
AgOTf. To silver(I) oxide (1.00 g, 4.38 mmol) in tetrahydro- C 38.08 (37.96)%, H 4.35 (3.98)%, N 8.29 (8.30)%. IR (ATR,
furan (25 mL) was added trifluoromethanesulfonic acid νmax/cm−1): 3101 (CH), 3071 (CH), 3015 (CH), 2938 (CH), 1606
(1.32 g, 8.77 mmol) and stirred for 1 hour at room tempera- (ring), 1461 (ring), 1419 (SO2), 1325 (SO2), 1209 (NO), 1166 (CH
ture. Solvent was removed on a rotary evaporator and the in plane), 1071 (CH in plane), 1035, 1015 (ring breathing), 931,
product was dried overnight on a high vacuum line, yielding a 915, 832 (NO), 761 (CH out of plane), 673, 546, 523, 491, 468;
white solid. Yield: 2.145 g (95%). CHN found (calculated for NMR: δH (300 MHz, CDCl3): 8.22 (m), 7.28 (m), 1.95 (s); δC
AgCF3SO3): C 4.42 (4.67)%, H 0.00 (0.00)%, N 0.00 (0.00)%. IR (75 MHz, CDCl3): 139.43 (s), 126.16 (s), 125.91 (s), 77.47 (s),
(ATR, νmax/cm−1): 1233 (CF3), 1194 (CF3), 1005, 616, 578 (CF3), 77.04 (s), 76.62 (s); Melting point: 77 °C; Viscosity: 22 mPa s
574 (CF3). Melting point: >200 °C.
AgOMs. To silver(I) oxide (1.00 g, 4.38 mmol) in acetonitrile
(80 °C).
[Ag(py-O)2][NO3]. To silver(I) nitrate (1.00 g, 5.89 mmol) in
(25 mL) was added methanesulfonic acid (0.84 g, 8.77 mmol) acetonitrile (25 mL) was added pyridine-N-oxide (1.12 g,
and stirred for 1 hour at room temperature. Solvent was 11.78 mmol) and stirred for 1 hour at room temperature.
removed on a rotary evaporator and the product was dried Excess acetonitrile was removed on a rotary evaporator and
overnight on a high vacuum line, yielding a white solid. Yield: the product was dried overnight on a high vacuum line, yield-
1.696 g (95%). CHN found (calculated for AgCH3SO3): C 6.13 ing a light brown solid at room temperature. Yield: 2.06 g
(5.92)%, H 1.98 (2.21)%, N: 0.00 (0.00)%. IR (ATR, νmax/cm−1): (97%). CHN found (calculated for C10H10AgN3O5S): C 33.10
1700, 1338, 1116, 1016, 770, 514. Melting point: >200 °C.
(33.35)%, H 2.28 (2.79)%, N 11.42 (11.67)%. IR (ATR, νmax/
[Ag(py-O)3][OTf]. To silver(I) triflate (10.0 g, 38.9 mmol) in cm−1): 3103 (CH), 3074 (CH), 3054 (CH), 1463 (ring), 1409 (NO
acetonitrile (25 mL) was added pyridine-N-oxide (11.1 g, from anion), 1290 (NO from anion), 1207 (NO from py-O), 1174
116.8 mmol) and stirred for 1 hour at room temperature. (CH in plane), 1147 (CH in plane), 1072 (CH in plane), 1033
Excess acetonitrile was removed on a rotary evaporator and (ring breathing), 1018 (ring breathing), 925, 831 (NO from py-
the product was dried overnight on a high vacuum line, yield- O), 819, 760 (CH out of plane), 671, 551, 510, 492, 475; NMR:
ing a brown solid. Yield: 20.88 g (99%). CHN found (calculated δH (400 MHz, DMSO-d6): 8.21 (m), 7.37 (m), 1.91 (s); δC
for C43H40Ag3F9N8O14S3): C 34.73 (34.81)%, H 3.10 (2.72)%, N (100 MHz, DMSO-d6): 138.81 (s), 126.69 (s), 124.91 (s); Melting
7.32 (7.55)%. IR (ATR, νmax/cm−1): 3115 (CH), 3077 (CH), 3021 point: 44 °C.
(CH), 2941 (CH), 2863 (CH), 1607 (ring), 1464 (ring), 1262,
[Ag(py-O)3][NO3]. To silver(I) nitrate (3.00 g, 17.7 mmol) in
1213 (NO)/(CF3), 1168 (CH in plane), 1152 (CH in plane), 1070 acetonitrile (25 mL) was added pyridine-N-oxide (5.04 g,
(CH in plane), 1026 (ring breathing), 1018 (ring breathing), 53.1 mmol) and stirred for 1 hour at room temperature.
915, 830 (NO), 763 (CH out of plane), 699, 631, 572 (CF3), 549, Excess acetonitrile was removed on a rotary evaporator and
514, 487, 476; NMR: δH (400 MHz, DMSO-d6): 8.39 (m), 7.37 the product was dried overnight on a high vacuum line, yield-
(m), 2.20 (s); δC (100 MHz, DMSO-d6): 140.12 (s), 127.83 (s), ing a light brown liquid at room temperature. Yield: 7.976 g
126.49 (s), 120.39 (q), 35.97 (m); Melting point: 76 °C; (99%). After a few days, the product slowly crystallizes. CHN
Viscosity: 35 mPa s (80 °C).
found (calculated for C15H15AgN4O6·H2O): C 37.89 (38.07)%, H
[Ag(py-O)2][OMs]. To silver(I) methanesulfonate (1.00 g, 3.92 (3.62)%, N 11.72 (11.84)%. IR (ATR, νmax/cm−1): 3380
4.93 mmol) in acetonitrile (25 mL) was added pyridine-N-oxide (H2O), 3104 (CH), 3074 (CH), 3054 (CH), 3018 (CH), 1607
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(ring), 1463 (ring), 1405 (NO from anion), 1291 (NO from
anion), 1208 (NO from py-O), 1173 (CH in plane), 1147 (CH in
plane), 1072 (CH in plane), 1033 (ring breathing), 1017 (ring
breathing), 925, 831 (NO from py-O), 820, 761 (CH out of
plane), 671, 550, 509, 491, 475; NMR: δH (300 MHz, CDCl3):
8.22 (m), 7.28 (m), 1.93 (s); δC (75 MHz, CDCl3): 139.44 (s),
126.17 (s), 125.95 (s), 77.49 (s), 77.06 (s), 76.64 (s); Melting
point: 67 °C; Viscosity: 18 mPa s (80 °C).
Table 1 Melting points and viscosity of the complexes
Compound
m.p. (°C)
Viscosity (mPa s) (80 °C)
[Ag(py-O)3][Tf2N]
[Ag(py-O)3][OTf]
[Ag(py-O)6][OTf]
[Ag(py-O)3][OMs]
[Ag(py-O)3][NO3]
108
76
n.d.b
35
8
22
18
<RTa
77
67
a Room temperature ionic liquid, but [Ag(py-O)3][OTf] crystallized out
over time. b n.d.: not determined, because of high melting point.
[Ag(py-O)6][OTf]. To silver(I) triflate (10.0 g, 38.9 mmol) in
acetonitrile (25 mL) was added pyridine-N-oxide (22.2 g,
233.4 mmol) and stirred for 1 hour at room temperature.
Excess acetonitrile was removed on a rotary evaporator and nitrate was used as a starting reagent and the py-O ligand was
the product was dried overnight on a high vacuum line, yield- added in a 1 : 3 stoichiometry. All products were originally
ing a brown liquid at room temperature. Yield: 30.26 g (94%). obtained as supercooled liquids, but they crystallized spon-
After a few days, small crystals submersed in this liquid arise. taneously after a few days in the storage vessel. Only the first
Single crystal X-ray diffraction characterized the crystals to be crystallization required a longer time. In some cases, the crys-
[Ag(py-O)3][OTf]. CHN found (calculated for C31H30AgF3- tals were suitable to be studied by single crystal diffraction and
N6O9S): C 44.87 (44.99)%, H 4.01 (3.65)%, N 9.98 (10.15)%. IR are described below. In most cases the compound that crystal-
(ATR, νmax/cm−1): 3115 (CH), 3077 (CH), 3052 (CH), 3021 (CH), lized from the melt had a different ligand-to-metal stoichio-
1606 (ring), 1463 (ring), 1262, 1214 (NO)/(CF3), 1169 (CH in metry in the crystalline state than in the liquid state. The
plane), 1153 (CH in plane), 1026 (ring breathing), 1017 (ring melting temperatures of most of the compounds were between
breathing), 830 (NO), 763 (CH out of plane), 670, 631, 572 65 °C and 80 °C. Therefore, electrochemical measurements
(CF3), 548, 514, 477; NMR: δH (400 MHz, DMSO-d6): 8.36 (m), were performed at 90 °C. Only [Ag(py-O)6][OTf] was a liquid at
7.34 (m), 2.19 (s); δC (100 MHz, DMSO-d6): 139.71 (s), 127.51 room temperature, but this also formed some small crystals
(s), 126.19 (s), 120.24 (q), 36.01 (m); Melting point: <RT.; submersed in the liquid after a few days. [Ag(py-O)3][Tf2N] had
Viscosity: 8 mPa s (80 °C).
a melting temperature of 108 °C (Table 1). Interestingly, the
[Ag(py-O)3][Tf2N]. To [Ag(MeCN)][Tf2N], which was syn- compound with the bis(trifluoromethanesulfonyl)imide anion,
thesized as described earlier16 (5 g, 11.6 mmol) in acetonitrile [Ag(py-O)3][Tf2N], was the only compound which melts above
(25 mL), was added pyridine-N-oxide (3.31 g, 34.8 mmol) 100 °C. This was unexpected, because the bis(trifluoromethane-
and stirred for 1 hour at room temperature. Excess aceto- sulfonyl)imide anion yields in general ionic liquids with low
nitrile was removed on a rotary evaporator and the product was melting points and low viscosities. The electrochemical pro-
dried overnight on a high vacuum line, yielding a light brown perties of [Ag(py-O)3][Tf2N] were therefore not investigated,
solid at room temperature. Yield: 15.56 g (99%). CHN found due to the compound’s high melting point.
(calculated for C17AgF6N4O7S2H15): C 30.23 (30.33)%, H 2.39
(2.25)%, N 8.13 (8.32)%. IR (ATR, νmax/cm−1): 3111 (CH), 3059
(CH), 3025 (CH), 1466 (ring), 1350, 1332, 1172 (CH in plane),
Crystal structures
1135 (SO2), 1050 (SNS), 1017 (ring breathing), 830 (NO), 762 The crystal structures of all crystalline silver(I) complexes with
(CH out of plane), 739 (CF3), 672, 602 (SO2), 569, 550, 508, 476. pyridine-N-oxide ligands were determined by single-crystal
NMR: δH (400 MHz, DMSO-d6): 8.29 (m), 7.22 (m), 2.08 (s); δC X-ray diffraction. The crystallographic data are summarized in
(100 MHz, DMSO-d6): 139.32 (s), 127.10 (s), 126.01 (s), 119.96 Table 2. The crystallographic asymmetric unit of [Ag(py-O)3]-
(q), 39.98 (m); Melting point: 108 °C.
[Tf2N] contains six silver atoms, six pyridine-N-oxide ligands
and two bis(trifluoromethanesulfonyl)imide anions in the highly
symmetric space group P31c. All six of the silver atoms lie on a
threefold rotation axis with the py-O molecules and Tf2N
anions occupying general positions. Each py-O ligand is biden-
tate and coordinates to two adjacent silver atoms and each
Results and discussion
Synthesis
Starting reagents AgOTf, AgOMs and [Ag(MeCN)][Tf2N] were silver atom is coordinated by six py-O ligands creating linear
prepared for the synthesis of the liquid metal salts described one-dimensional chains (Fig. 1). Adjacent Ag atoms along the
in this manuscript. To these starting reagents, pyridine-N-oxide chain are close enough to each other so that they are involved
(py-O) was added in a 1 : 3 metal-to-ligand ratio. In one case a in Ag⋯Ag interactions with Ag⋯Ag distances varying from
1 : 6 ratio was used for [Ag(py-O)6][OTf] to test the influence of 3.0244(7) Å to 3.247(1) Å (Table S1†). As a consequence of the
an excess of ligand. Acetonitrile was used as the solvent and space group symmetry and the location of the silver centers in
was removed in vacuo after the reaction. The product was dried the unit cell, there are actually two crystallographically unique
overnight on a Schlenk line at room temperature until the one-dimensional chains, as can be seen in the packing
acetonitrile solvent no longer appeared on the FTIR or NMR diagram (Fig. S1†). Both chains run parallel to the crystallo-
spectra. For [Ag(py-O)3][NO3], commercially available silver(I) graphic c axis, but those which are at the unit cell corners are
1592 | Dalton Trans., 2014, 43, 1589–1598
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Table 2 Crystal data and structure refinement for the structures of α-[Ag(py-O)3][Tf2N], β-[Ag(py-O)3][Tf2N], [Ag3(py-O)8][OTf]3, [Ag(py-O)3][OTf],
[Ag(py-O)2(OMs)] and [Ag(py-O)2(NO3)]
[Ag(py-O)3][Tf2N]
α-Polymorph
[Ag(py-O)3][Tf2N]
β-Polymorph
[Ag3(py-O)8][OTf]3
[Ag(py-O)3][OTf]
[Ag(py-O)2(OMs)] [Ag(py-O)2(NO3)]
Empirical formula
Formula weight
Temperature/K
Color
C
17AgF6N4O7S2H15 C17AgF6N4O7S2H15 C43H40Ag3F9N8O14S3 C16H15AgF3N3O6S C11H13AgN2O5S
C10H10AgN3O5
360.07
100
Colorless
0.14 × 0.10 ×
0.03
673.32
100
Colorless
0.36 × 0.20 ×
0.12
673.32
100
1483.62
100
542.24
100
393.17
100
Colorless
0.40 × 0.20 ×
0.10
Colorless
0.27 × 0.19 ×
0.11
Colorless
0.40 × 0.08 ×
0.07
Colorless
0.28 × 0.24 ×
0.12
Size/mm3
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
Trigonal
P31c (no. 159)
25.4018(11)
25.4018(11)
12.772(5)
90
90
120
7028.2(5)
12
1.909
Rhombohedral
R3 (no. 148)
25.5554(4)
25.5554(4)
56.4189(11)
90
90
120
31 909.5(9)
18
1.892
Orthorhombic
P21 (no. 4)
7.0245(2)
25.8560(8)
14.6711(4)
90
90.513(3)
90
2665.49(2)
2
1.908
1.318
0.71073
9891
9405
Tetragonal
I41/a (no. 88)
25.9649(5)
25.9649(5)
11.8154(3)
90
90
90
7965.6(3)
16
1.809
Orthorhombic
P21/c (no. 14)
11.6001(2)
6.90176(1)
17.1007(4)
90
104.866(2)
90
1323.28(4)
4
1.973
1.701
0.71073
3055
2809
Triclinic
P1 (no. 2)
ˉ
ˉ
8.2360(4)
8.7680(5)
8.7895(4)
77.874(5)
80.894(4)
72.726(5)
589.44(5)
2
2.029
1.731
0.71073
2664
2531
β/°
γ/°
Volume/Å3
Z
ρcalc Mg/m3
μ/mm−1
Wavelength/Å
1.134
0.71073
1.124
1.184
0.71073
4888
0.71073
16 793
14 641
Reflections collected 8083
Independent
reflections
6650
4419
R(int)
0.0460
0.0324
0.0659
0.736/−1.033
0.0446
0.0378
0.0922
0.787/−0.583
0.0361
0.0333
0.0644
0.821/−0.736
0.0265
0.0221
0.0544
0.563/−0.445
0.0231
0.0201
0.0469
0.426/−0.393
0.0180
0.0166
0.0433
0.527/−0.333
R1 [I ≥ 2σ(I)]
wR2 [all data]
Δρmax,min/e Å−3
down the chain, whilst the latter chains contain four silver
centers and there are 12 Ag–O bonds visible. The bis(trifluoro-
methanesulfonyl)imide anions occupy the positions between
the chains (Fig. S1†). There are also π–π interactions between
the py-O ligands, as summarized in Table S2.†
A different polymorph was also crystallized upon addition
of an excess of py-O ligand to [Ag(py-O)3][Tf2N]. The molecular
structure is essentially the same as the polymorph that was
just described, with silver centers linked by three bridging
py-O ligands to create one dimensional chains of six coordinate
silver centers augmented by Ag⋯Ag interactions (Fig. S2†).
This polymorph also possesses high symmetry and is in the
ˉ
rhombohedral space group R3, but in this instance there are
ten independent silver centers (all on symmetry elements), nine
independent py-O ligands and three independent bis(trifluoro-
methanesulfonyl)imide anions. Looking perpendicularly to
the chains, it is possible to see the many different orientations
of py-O ligands around the central axis (Fig. 2). The Ag–O dis-
tances range from 2.4492(2) Å to 2.513(2) Å. The Ag⋯Ag dis-
tances range from 3.0417(4) Å to 3.2239(4) Å (Table S1†). The
π–π interactions are presented in Table S2.† [Ag(py-O)3][Tf2N]
has a high melting temperature of 108 °C, which makes it less
interesting for electrochemical characterization. Due to the high
melting point of [Ag(py-O)3][Tf2N], different anions were tested
to see if they resulted in compounds with lower melting points.
The solid state structure of [Ag3(py-O)8][OTf]3 was crystal-
lized from [Ag(py-O)3][OTf], showing that the compound that
crystallizes does not always have the same composition as the
bulk solution. The asymmetric unit of [Ag3(py-O)8][OTf]3 was
found to have three silver atoms, eight py-O ligands and three
Fig. 1 View of the Ag⋯Ag interactions and their py-O ligands in the
α-polymorph of [Ag(py-O)3][Tf2N]. Disorder of the anions is omitted for
clarity. The red labels A and B refer to planes defined in Table S2.
different from those at positions (1/3, 2/3, c) and (2/3, 1/3, c).
The former chains have two crystallographically distinct silver
centers and it is possible to observe nine Ag–O bonds looking
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Fig. 2 View along the c axis of the packing in the crystal structure of a
the β-polymorph of [Ag(py-O)3][Tf2N]. Disorder of the anions is omitted
for clarity.
Fig. 3 View of the one-dimensional chain structure of the [Ag3(py-O)8]∞
chains in the structure of [Ag3(py-O)8][OTf]3.
trifluoromethanesulfonate anions in the crystallographic
asymmetric unit (Fig. S3†). The [Ag3(py-O)8] cationic unit is a
Single crystals of [Ag(py-O)3][OTf] were grown from a liquid
pseudo-centrosymmetric linear moiety with the central silver with the composition [Ag(py-O)6][OTf]. The crystallographic
atom being six coordinate and the outer two silver atoms are asymmetric unit of [Ag(py-O)3][OTf] was found to contain one
five coordinate. The range of Ag–O bond distances for the six silver atom coordinated by three py-O ligands and one non-
coordinate silver atoms are 2.413(3) Å to 2.559(3) Å and for the coordinating trifluoromethanesulfonate anion. Ag–O distances
two five coordinate silver atoms are 2.270(3) to 2.598(3) Å and vary between 2.3652(1) Å and 2.5117(1) Å (Table S1†). This
2.249(3) Å to 2.632(3) Å (Table S1†). Although the Ag–O dis- longest distance is significantly larger than the sum of the
tance of 2.632(3) Å is longer than the sum of the bonding radii atomic radii of silver and oxygen so this interaction cannot be
for Ag and O, it is still considered as a bond because it is only considered a true bond, but it is considerably shorter than the
ca. 0.05 Å longer than the next longest bond. There are also sum of the van der Waals radii. Thus each silver center can be
long Ag⋯Ag interactions (Ag–Ag = 3.2916(5) Å and 3.3210(5) Å) considered as having a [5 + 1] coordination environment. Each
within the [Ag3(py-O)8] moiety. Of the eight py-O molecules, silver center is connected to another through three bridging
two (those containing O1 and O50) coordinate in a monoden- py-O ligands giving rise to [Ag(py-O)3]∞ moieties that form a
tate fashion, four (O8, O22, O36 and O43) are bidentate brid- one-dimensional chain structure parallel to the crystallo-
ging two separate silver atoms and two (O15 and O29) are graphic c axis (Fig. S5 and S6†). There is also a Ag⋯Ag inter-
tridentate bridging three separate atoms. The central six co- action along the chain with a distance of 3.0190(2) Å. The π–π
ordinate silver atoms of the [Ag3(py-O)8] moiety is connected to interactions are presented in Table S2.† The trifluoromethane-
each of its two silver neighbors through three py-O ligands, in sulfonate anions occupy positions in between the [Ag(py-O)3]∞
a similar way to that seen in the structures of [Ag(py-O)3][OTf] chains. This is rather similar to what is found when looking at
and [Ag(py-O)3][Tf2N]. The outer five coordinate silver atoms the c axis in the packing of the crystal structure of both poly-
are connected to silver atoms of symmetry related [Ag3(py-O)8] morphic forms of [Ag(py-O)3][Tf2N], only the angle between
moieties through only two py-O ligands (Fig. 3), but these are three consecutive silver centers is not linear but 163.197(5)°.
the two py-O ligands that are tridentate, also coordinating one
From the compound [Ag(py-O)3][OMs], single crystals of [Ag-
of the central silver atoms of the [Ag3(py-O)8] moiety. Through (py-O)2][OMs] were obtained. The asymmetric unit of [Ag(py-
these two py-O ligands, the [Ag3(py-O)8] moieties form a one- O)2][OMs] contains one silver atom, coordinated to two py-O
dimensional chain structure parallel to the crystallographic a ligands and one methanesulfonate anion through one of its
axis (Fig. 3). The Ag⋯Ag distance between two different oxygens. All Ag–O distances vary between 2.3389(1) Å and
[Ag3(py-O)8] moieties is 3.4800(5) Å, which is longer than the 2.5820(1) Å (Table S1†). Each py-O ligand is bidentate and con-
Ag⋯Ag distances within the [Ag3(py-O)8] unit and probably too nects two [Ag(py-O)2(OMs)] units making each silver-atom five
long to be a Ag⋯Ag interaction. The trifluoromethanesulfonate coordinate (Fig. 4). Due to the bridging py-O ligands, chains of
anions are totally non-coordinating and occupy positions in [Ag(py-O)2(OMs)]∞ are formed parallel to the crystallographic
the crystal lattice between the [Ag3(py-O)8]∞ chains (Fig. S4†). b axis. The π–π interactions between the py-O ligands in the
The packing diagram viewed along the crystallographic a axis asymmetric unit are presented in Table S2.†
(Fig. S4†) shows that there are π–π interactions between the
Single crystals of [Ag(py-O)2][NO3] crystallized from [Ag-
pyridine rings along the [Ag3(py-O)8]∞ chains. The π–π inter- (py-O)3][NO3]. The asymmetric unit contains one silver atom,
actions are relatively strong with plane-centroid distances as coordinated by two py-O ligands and one nitrate anion that
low as 3.187 Å (see Table S2† for details).
1594 | Dalton Trans., 2014, 43, 1589–1598
coordinates in a bidentate fashion. Two silver centers are
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Fig. 4 View of the [Ag(py-O)2][OMs]∞ chain moieties in the structure of [Ag(py-O)2][OMs]. The red labels A and B refer to planes defined in Table S2.
connected to one another through two bidentate bridging py-O 120 °C, after which its mass started decreasing. At 400 °C, the
ligands to form a [Ag2(py-O)4(NO3)2] moiety (Fig. S7†). The compound with the trifluoromethanesulfonate anion had
other py-O ligands are monodentate. Overall, each silver atom about 45% of its mass remaining, indicating that three py-O
is five coordinate as shown in Fig. S7.† All Ag–O distances vary ligands had evaporated and that the residue was AgOTf. Both
between 2.2958(1) Å and 2.4253(1) Å, but the distances compounds appeared to be stable at 90 °C and electrochemical
between the silver and the oxygen from the py-O ligands is measurements were performed at this temperature. However,
shorter than the distance between the silver and the oxygen of [Ag(py-O)3][OMs] started to degrade at temperatures as low as
the nitrate anion (Table S1†). In contrast with the previously 50 °C. Around 250 °C, there was 41% of its initial mass
described crystal structures there is no infinite chain structure, remaining, corresponding to AgOMs. Above 320 °C, the methane-
due to the bidentate nature of the NO3 coordination. The sulfonate anion also degraded. Also [Ag(py-O)6][OTf] starts
[Ag2(py-O)4(NO3)2]-moieties are aligned in sheets parallel to losing mass at temperatures slightly below 100 °C. This is
ˉ
the [1 0 1] direction as shown in Fig. S8.† For more infor- possibly because, due to the excess of ligands, not all ligands
mation about the π–π interactions, see Table S2.†
are coordinated to the silver center and evaporate more easily.
At 400 °C, about 30% of its initial mass is remaining, corres-
ponding to a loss of 6 ligands and AgOTf remaining. To ensure
comparability of the electrochemical experiments, the
measurements on all compounds were performed at 90 °C,
even the ones which showed some small mass loss at this
temperature, but care was taken to keep the compound for the
shortest possible period at this elevated temperature.
Thermal stability
TGA traces were measured for [Ag(py-O)3][OTf], [Ag(py-O)6]-
[OTf], [Ag(py-O)3][OMs] and [Ag(py-O)3][NO3] (Fig. 5). The
nitrate complex started losing py-O ligands at 120 °C and it
had about 37% of its mass remaining at 240 °C, corresponding
to the loss of three py-O ligands and AgNO3 remaining. At even
higher temperatures, the nitrate anion degraded as well.
[Ag(py-O)3][OTf] was also found to be thermally stable up to
Electrochemistry
Cyclic voltammograms of [Ag(py-O)3][NO3], [Ag(py-O)3][OMs]
and [Ag(py-O)3][OTf] are shown in Fig. 6. The cathode was a
gold-sputtered silicon substrate, a silver coil was used as a
sacrificial anode, and a silver wire served as a pseudo reference
electrode. The measurements were performed at a scan rate of
50 mV s−1 and a temperature of 90 °C. Since at this tempera-
ture all the tested compounds are in their liquid state, the
1 : 3 metal-to-ligand stoichiometry is present, independent of
what crystals might form at lower temperatures. All cyclic vol-
tammograms were measured in a custom-made PTFE sample
holder with a narrow channel in front of the cathode, in order
to make the current density uniform and to have a well-
defined surface area.15 Over this channel, an iR drop arises. All
cyclic voltammograms were corrected for this iR drop, using a
current interrupt technique as described elsewhere.15 The
cyclic voltammograms show the typical deposition/stripping
behavior, with silver deposition at negative potentials vs. the
Ag/Ag+ pseudo-reference electrode and stripping of silver at
positive potentials. At potentials more positive than +0.7 V vs.
Fig. 5 TGA trace of [Ag(py-O)3][NO3] (solid line), [Ag(py-O)3][OMs]
(dashed line), [Ag(py-O)3][OTf] (dotted line) and [Ag(py-O)6][OTf]
(dashed-dotted line) in an argon atmosphere. Heating rate: 5 °C min−1
.
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Fig. 6 Cyclic voltammogram of [Ag(py-O)3][NO3] (solid line), [Ag(py-
O)3][OMs] (dashed line) and [Ag(py-O)3][OTf] (dotted line) vs. Ag/Ag+
reference electrode. Scan rate: 50 mV s−1, temperature 90 °C, on a
gold-sputtered silicon electrode.
Fig. 7 Linear potential scan of [Ag(py-O)3][NO3] (solid line), [Ag(py-O)3]-
[OMs] (dashed line) and [Ag(py-O)3][OTf] (dotted line) vs. Ag/Ag+ refer-
ence electrode. Scan rate: 50 mV s−1, temperature 90 °C, on a gold-
sputtered silicon electrode.
Ag/Ag+, oxidation of the pyridine-N-oxide ligands was observed.
The liquid metal salts do not show a cathodic decomposition
limit, since the cathodic reaction is the deposition of silver. At
the anode, silver is regenerated into solution by oxidation of
silver metal to silver(I) ions. Interestingly, the compounds with
fluorine-free anions, [Ag(py-O)3][NO3] and [Ag(py-O)3][OMs],
showed the highest current densities of −10 and −6.5 A dm−2
at −0.5 V vs. Ag/Ag+, respectively, while the lowest current den-
sities (−2.5 A dm−2) were observed for the compound with the
trifluoromethanesulfonate anion, [Ag(py-O)3][OTf]. This can
be explained by differences in viscosity: the lower the viscosity
of a compound, the faster the transport of silver ions through
the electrolyte and the higher the current density. [Ag(py-O)3]-
[NO3] has the lowest viscosity and also showed the highest
current densities. The compound with trifluoromethane-
sulfonate anions has the highest viscosity, so the lowest
current densities were observed.
Fig. 8 Cyclic voltammogram of [Ag(py-O)6][OTf] vs. Ag/Ag+ reference
electrode. Scan rate: 50 mV s−1, temperature 90 °C, on a gold-sputtered
silicon electrode.
In agreement with earlier studies15–20 on copper- and silver-
containing liquid metal salts, no cathodic peak was observed
(no limiting current), indicating that the electrochemical
process never enters a diffusion-controlled regime. Therefore,
the system was pushed to very negative potential values and a
linear potential scan to potential values as negative as −10 V
vs. Ag/Ag+ was measured (Fig. 7). [Ag(py-O)3][NO3] shows a
remarkably high current density of −22.5 A dm−2, while
[Ag(py-O)3][OMs] and [Ag(py-O)3][OTf] have a current density
of −16 and −9 A dm−2, respectively. These results were not cor-
rected for the iR drop, since at high potentials this technique
becomes unstable.
Since silver is six coordinate in [Ag3(py-O)8][OTf]3, [Ag-
(py-O)3][OTf] and [Ag(py-O)3][Tf2N], due to the bidentate brid-
ging coordination of the py-O ligands, the liquid metal salt
with a 1 : 6 metal-to-ligand ratio [Ag(py-O)6][OTf] was syn-
thesized to check whether all six coordination positions of
silver could be filled by monodentate py-O ligands. The result
was a room temperature liquid with a low viscosity, which
formed crystals of [Ag(py-O)3][OTf] submersed in an excess of
py-O after standing at room temperature for a few days. To test
the influence of this excess of ligands on electrochemical pro-
perties,
a cyclic voltammogram of [Ag(py-O)6][OTf] was
measured at 90 °C, a temperature at which the crystals were
melted. The cyclic voltammogram (Fig. 8) shows an overall
shape equal to those in Fig. 6, but the current density at −0.50
V vs. Ag/Ag+ is a factor two higher than for [Ag(py-O)3][OTf].
This is due to the much lower viscosity of [Ag(py-O)6][OTf]
(only 8 mPa s at 80 °C) compared to that of [Ag(py-O)3][OTf]
(35 mPa s at 80 °C). However, the current density of [Ag(py-O)6]-
[OTf] is not as high as that of [Ag(py-O)3][NO3], while its vis-
cosity is lower. This is possibly due to the lower silver content
of [Ag(py-O)6][OTf] (13.03 wt% Ag) versus that of [Ag(py-O)3]-
[NO3] (23.70 wt%).
1596 | Dalton Trans., 2014, 43, 1589–1598
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Fig. 9 SEM images of silver deposits at −1.0 A dm−2 from [Ag(py-O)3][NO3] (a), [Ag(py-O)3][OMs] (b) and [Ag(py-O)3][OTf] (c), and deposits at
−5.0 A dm−2 from [Ag(py-O)3][NO3] (d), [Ag(py-O)3][OMs] (e) and [Ag(py-O)3][OTf] (f).
Scanning electron microscopy (SEM) images were made for deposits of good quality, even without the use of additives. For
all silver films, deposited at −1.0 A dm−2 (thickness: 0.65 µm) the non-fluorinated liquid metal salt [Ag(py-O)3][NO3], the
and −5.0 A dm−2 (thickness: 3.22 µm) for 60 seconds (Fig. 9). highest current density of −10 A dm−2 at −0.5 V vs. Ag/Ag+ was
At higher current densities, the silver coatings were rougher observed. Crystal structures of [Ag(py-O)3][Tf2N], [Ag(py-O)3]-
for all the tested silver-containing liquid metal salts. Nonethe- [OTf], [Ag3(py-O)8][OTf]3, [Ag(py-O)2][OMs] and [Ag(py-O)2]-
less, the silver deposits of all silver complexes showed a good [NO3] were described and showed the variable coordination of
coverage of the surface. The silver deposits of [Ag(py-O)3][OMs] the silver centers, the ligands and the anions. This paper
at −5.0 A dm−2 showed the strongest tendency to grow dendri- shows that silver-containing ionic liquids with non-fluorinated
tically, even though a special sample holder is used to reduce anions can be successfully used for the electrodeposition of
dendritic growth. EDX measurements showed that the deposits high quality silver coatings. This is a significant progress com-
were pure silver (Fig. S9†). Since no gold peak was found in the pared to earlier results, because ionic liquids with non-fluori-
EDX spectra, the surface was completely covered.
nated anions are much cheaper than their fluorinated
counterparts. Moreover, no issues with fluorinated decompo-
sition products need to be taken into account.
Conclusion
Several new silver-containing liquid metal salts with pyridine-
N-oxide ligands were synthesized, characterized and tested as
Acknowledgements
electrolytes for electrodeposition of silver. All ionic liquids The authors acknowledge financial support by the
showed remarkably high current densities and produced silver FWO-Flanders (research project research G.0B9613.N and
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research community “Ionic Liquids”) and by the IWT-Flanders
(SBO project IWT 80031 “MAPIL”). We thank Danny Winant
for his assistance with TGA measurements and Dirk Henot for
performing the CHN analyses. Support by IoLiTec (Heilbronn,
S. Kawakami, E. Syouno, H. Er and E. Taguchi, J. Colloid
Interface Sci., 2011, 356, 630–638.
9 A. P. Abbott, J. C. Barron, K. S. Ryder and D. Wilson,
Chem.–Eur. J., 2007, 13, 6495–6501.
Germany) and The Dow Chemical Company is greatly appreci- 10 A. Cingolani, Effendy, F. Marchetti, C. Pettinari,
ated. The authors also thank the Hercules Foundation for
supporting the purchase of an X-ray diffractometer through
project AKUL/09/0035.
B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans.,
1999, 4047–4055.
11 J. F. Huang, H. M. Luo and S. Dai, J. Electrochem. Soc.,
2006, 153, J9–J13.
12 A. P. Abbott, G. Capper, D. L. Davies and R. K. Rasheed,
Chem.–Eur. J., 2004, 10, 3769–3774.
13 H. M. A. Abood, A. P. Abbott, A. D. Ballantyne and
K. S. Ryder, Chem. Commun., 2011, 47, 3523–3525.
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