Novel dithiolene complexes incorporating conjugated electroactive ligands

Article abstract of DOI:10.1039/b801187g

A series of new metal (M) dithiolene complexes bearing terthiophene (3, 12, M = Ni; 4, M = Pd; 5, 6, M = Au) and 2,5-bis(para-methoxyphenyl)thiophene units (14, M = Ni; 15, 16, M = Au; 17, M = Pd) have been synthesised in 38-99% yield. The electrochemical properties of the materials have been characterised by cyclic voltammetry and UV-vis spectroelectrochemistry. The nickel complexes possess low oxidation potentials (-0.12 to -0.25 V vs Ag/AgCl) due to the electron-rich dithiolene centres and all complexes display ligand-based redox activity. The terthiophene derivatives have been polymerised by electrochemical oxidation to give stable films with, in the case of poly(3), broad absorption characteristics. Charge transfer materials have been isolated from 14 and 16 with conductivities in the range 9 × 10^-6 to 7 × 10 ^-8 S cm^-1. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b801187g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Novel dithiolene complexes incorporating conjugated electroactive ligands†
Peter J. Skabara,*a Cristina Pozo-Gonzalo,‡b Nora Lardie´s Miazza,c Mariano Laguna,c Elena Cerrada,c
d
Asun Luquin,c Blanca Gonza´lez,§ Simon J. Coles,e Michael B. Hursthouse,e Ross W. Harrington f and
William Clegg f
Received 22nd January 2008, Accepted 25th March 2008
First published as an Advance Article on the web 7th May 2008
DOI: 10.1039/b801187g
A series of new metal (M) dithiolene complexes bearing terthiophene (3, 12, M = Ni; 4, M = Pd; 5,
6, M = Au) and 2,5-bis(para-methoxyphenyl)thiophene units (14, M = Ni; 15, 16, M = Au; 17, M =
Pd) have been synthesised in 38–99% yield. The electrochemical properties of the materials have been
characterised by cyclic voltammetry and UV-vis spectroelectrochemistry. The nickel complexes possess
low oxidation potentials (−0.12 to −0.25 V vs Ag/AgCl) due to the electron-rich dithiolene centres and
all complexes display ligand-based redox activity. The terthiophene derivatives have been polymerised
by electrochemical oxidation to give stable films with, in the case of poly(3), broad absorption
characteristics. Charge transfer materials have been isolated from 14 and 16 with conductivities in the
range 9 × 10−6 to 7 × 10−8 S cm−1.
IR liquid crystal devices.16 As part of our studies into 3,4-
Introduction
dithio-substituted terthiophene species,17–20 we have reported the
The role of conjugated macromolecules with the structural or
synthesis, electropolymerisation and characterisation of a new
functional capacity for coordinating to metals is diverse.1,2 The
cross-linked polythiophene bearing nickel bis(dithiolene) units
fused to the main chain.21 In this full paper, we compare
the electrochemical and absorption characteristics of an end-
blocked bis(terthiophene) analogue and present the synthesis and
properties of a new series of electroactive dithiolene complexes
and corresponding charge transfer materials.
incorporation of the metal can be accomplished at the monomer
stage or post-polymerisation and can influence the electronic
properties of the polymer by perturbation of conductivity,
structure, and optical and electronic properties. Consequently,
these materials have varied applications. For example, metal-free
polymers can behave as selective and highly responsive sensors
towards metal-containing analytes,3–5 whereas metallopolymers
can be used in catalysis6,7 or as low band gap materials.8
Results and discussion
Electrogenerated metallopolymers have received particular at-
tention over recent years, with pyrrole and thiophene derivatives as
frequent monomers of choice.7,9–12 Very recently, conjugated poly-
mers have been prepared electrochemically, in which [Ru(terpy)2]13
and [Ni(dithiolene)2]8 units are present in the main chain of
a polythiophene-based material. Metal dithiolene complexes in
particular provide a rich history of materials properties14 with
applications such as air-stable field effect transistors15 and near-
Synthesis
The strategy behind the synthesis of polymerisable dithiolene
monomers is shown in Scheme 1. Terthiophene 119 was dissolved
in THF and treated with 2.2 equivalents of sodium ethoxide
under reflux to generate the dithiolate intermediate 2. Addition of
aWestCHEM, Department of Pure and Applied Chemistry, University of
Strathclyde, Glasgow, UK G1 1XL. E-mail: peter.skabara@strath.ac.uk
bSchool of Chemistry, University of Manchester, Oxford Road, Manchester,
UK M13 9PL
cDepartamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales
de Arago´n, Universidad de Zaragoza-C.S.I.C., 50009, Zaragoza, Spain
dDepartamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad
Auto´noma de Madrid, Cantoblanco, 28049, Madrid, Spain
eDepartment of Chemistry, University of Southampton, Highfield,
Southampton, UK SO17 1BJ
f
School of Natural Sciences (Chemistry), Newcastle University, Newcastle
upon Tyne, UK NE1 7RU
† CCDC reference numbers 675707–675709. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b801187g
‡ Present address: CIDETEC, Paseo Miramo´n, 196, E20009 San
Sebastia´n, Spain.
§ Present address: Departamento de Qu´ımica Inorga´nica y Bioinorga´nica,
Facultad de Farmacia, Universidad Complutense de Madrid, Ciudad
Universitaria, Madrid 28040, Spain.
Scheme 1 Reagents and conditions: (i) THF, NaOEt, reflux, 15 min;
(ii) metal salt (and R4NBr for 3, 4 and 6).
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NiCl2 and n-Bu4NBr gave the corresponding nickel bis(dithiolene)
complex 3 (50% yield), after precipitation with petroleum ether
(boiling range 40–60 ◦C). Similarly, the Pd(II) analogue (4) was
obtained from 2 by addition of Na2[PdCl4] and Et4NBr (49%
yield). Gold(III) complexes, 5 and 6, were isolated from the reaction
of 2 with PPN[AuCl4] (38% yield) and n-Bu4N[AuBr4] (55% yield),
respectively (PPN = N(PPh3)2).
was obtained by treatment of 11 with sodium ethoxide (generating
the dithiolate intermediate, as in Scheme 1), followed by the
addition of tetrabutylammonium bromide and nickel(II) chloride.
The product was obtained as a dark red solid in 81% yield.
A second type of electroactive ligand was used to prepare
analogous complexes (Chart 1). Compound 1322 was subjected
to the same procedure outlined in Scheme 1 to generate 14–17 in
near quantitative yield (apart for 15, which was isolated in 42%
yield).
The synthesis of a derivative of 3, end-capped with four methyl
groups, is illustrated in Scheme 2. The route follows the same
strategy adopted for the preparation of terthiophene 1,20 which
uses vinylene trithiocarbonate 7 as the starting material. Mono-
lithiation of 7 with one equivalent of lithium diisopropylamide
(LDA) was followed by the addition of 5-methylthiophene-2-
carboxaldehyde and the whole procedure was repeated a second
time. The resulting diol (8) was obtained after aqueous work-
up and reacted immediately without purification with MnO2
to afford diketone 9 (67% yield from 7). Conversion of the
trithiocarbonyl functionality to the dithiooxone (10, 72%) was
accomplished by the reaction of mercuric acetate. Phosphorus
pentasulfide was used to cyclise compound 10 and the resulting
end-capped terthiophene (11) was isolated in 52% yield. Finally, 12
Chart 1
X-Ray crystallography
The structure of 3 has been reported previously.21 Green needles
of compound 5 were grown from a THF–diethyl ether solution by
slow evaporation. The asymmetric unit of 5 is shown in Fig. 1 and
consists of two independent half-units with inversion centres at the
gold atoms of the dithiolene units. The 5-membered rings within
the dithiolene unit represented by Au(2)–S(8)–C(18)–C(19)–S(9)
are essentially planar with a maximum deviation from the plane
˚
of 0.004(4) A. In contrast, the equivalent unit, Au(1)–S(2)–C(6)–
˚
C(7)–S(3), is puckered with a maximum deviation of 0.108(4) A.
The terthiophene units adopt anti–syn conformations, in which
the peripheral anti thiophene rings have short S · · · S contacts
˚
with neighbouring thiolate sulfurs of S(3) · · · S(5) = 3.358(3) A
˚
and S(6) · · · S(8) = 3.149(3) A. The shorter contact is associated
with closer coplanarity between adjacent thiophene units and
could be due to a higher degree of p-delocalisation; the torsion
angles for C(7)–C(8)–C(9)–C(10) and C(15)–C(16)–C(17)–C(18)
are 146.5(6)◦ and 161.24(6)◦, respectively. The bithiophene units
arranged in a syn conformation are also twisted, with torsion
angles of C(3)–C(4)–C(5)–C(6) = 33.6(9)◦ and C(19)–C(20)–
C(21)–C(22) = 39.3(9)◦. The only significant intermolecular short
contact is found for S(2) · · · S(10) (x + 1, y, z), in which the distance
˚
is 3.455(3) A.
Dark red crystals of compound 12 were obtained by slow
diffusion of diethyl ether or cyclohexane into a THF solution
of the compound. In the case of this nickel complex, the
terthiophene is composed of an all-anti conformation and in this
case, the asymmetric unit contains half an anion, with an inversion
centre at the metal atom (Fig. 2). Intramolecular short contacts
˚
within adjacent thiophenes exist for S(1) · · · S(4) (3.159(1) A) and
˚
Scheme 2 Reagents and conditions: (i) LDA, −78 ◦C, THF, 5-methylthio-
phene-2-carboxaldehyde, then repeat; (ii) MnO2, CH2Cl2; (iii) Hg(OAc)2,
CH2Cl2, AcOH; (iv) P2S5, NaHCO3, 1,4-dioxane, heat; (v) NaOEt, THF,
reflux, 15 min, then (n-Bu)4NBr and NiCl2·6H2O.
S(2) · · · S(5) (3.198(2) A). The torsion angles between terminal
thiophenes and the central heterocycle are 164.2(2)◦ for C(4)–
C(5)–C(6)–C(7) and 168.0(2)◦ for C(8)–C(9)–C(10)–C(11), whilst
the maximum deviation from the plane for the central dithiolene
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˚
rings, Ni–S(1)–C(7)–C(8)–S(2), is 0.015 A. Overall, these features
portray a more nearly planar structure for 12, compared to
compound 5.
Similar to that of 5, the crystal structure of 16 (Fig. 3) also
shows two independent half-units with inversion centres at the gold
atoms. One methoxy functionality (O(4)–C(25)) shows disorder
in its alignment with its adjacent benzene ring. The phenylene
rings in both molecules are twisted in the same range as the
thiophene units in 5, with maximum torsion angles of 164.8(9)◦
(C(10)–C(4)–C(3)–C(2)) and 141.2(9)◦ (C(1)–C(11)–C(12)–C(18))
for the Au(1) molecule and 144.0(7)◦ (S(6)–C(29)–C(30)–C(31))
and 137.2(8)◦ (S(6)–C(20)–C(21)–C(22)) for the Au(2) molecule.
Au(1) and Au(2) lie out of the plane formed by S(2)–Au(1)–S(1)–
S(1ꢀ)–S(2ꢀ) and S(4ꢀ)–S(5ꢀ)–S(4)–S(5)–Au(2) by 0.8545(11) and
˚
0.6517(10) A, respectively (primes denote symmetry-equivalent
atoms). Examining the planarity of the 5-membered rings in the
dithiolene units for each of the complexes (5, 12 and 16), the
folding vectors along S · · · S show significant deviations in the Au
complexes. Whereas the value for the nickel complex is 2.32(4)◦,
the relative folding vectors in 16 have angles of 10.8(2)◦ (Au(1) unit)
and 12.8(2)◦ (Au(2) unit). Although the S · · · S folding vector in
the Au(1) molecule of 5 is 24.02(12)◦, the second molecule has a far
smaller value, 2.25(12)◦, demonstrating that a reasonably planar
conformation is not exclusive to the Ni dithiolene unit.
Electrochemistry
End-capped oligothiophenes are ideal model structures for the
electrochemical characterisation of unsubstituted analogues, since
the blocking groups inhibit chemical coupling processes during
oxidation/reduction processes, enabling the observation of well-
defined (and in most cases reversible) redox waves.23 In this work,
compound 12 has been used to evaluate the electroactivity of
compound 3. The cyclic voltammograms of both compounds
are shown in Fig. 4 (see also Table 1 for data on all monomers
and polymers). In our previous communication,21 the redox wave
centred at −0.13 V for compound 3 was attributed to the oxidation
of the nickel dithiolene unit (32− ꢀ 3−) and the position of this
wave remains essentially unaltered in compound 12. The reduction
of the terthiophene units in 3 is shifted in 12 to a more negative
potential (by 400 mV) as a result of the inductive effects of the
four terminal methyl groups. The greatest difference between the
electroactivity of the two compounds concerns the oxidation of
the terthiophene units. Whereas 3 gives an irreversible peak at
+0.46 V (representing the onset of polymerisation), compound 12
displays three quasi-reversible oxidation waves at +0.30, +0.70 and
+1.11 V. The first of these oxidations represents the simultaneous
oxidation of both terthiophene units and these processes can be
resolved by square wave voltammetry (see inset in Fig. 4). A higher
build up of positive charge and subsequent coulombic repulsion
is responsible for the large difference in the third and fourth
oxidation processes of the complex, which could be assigned to
either the terthiophene units or the second oxidation of the nickel
dithiolene.
Fig. 1 The structure of 5 with H atoms and PPN counterion omitted.
Symmetry operations for the second half of each anion: 1 − x, 1 − y, 1 −
z for Au(1); −x, −y, −z for Au(2).
The electrondonatingability of the dithiolene systems is reduced
in the palladium (4) and gold analogues (5 and 6). As for 3,
compound 4 displays two sequential oxidation processes but the
potentials are raised by 0.26 and 0.16 V, respectively. The greater
degree of p-delocalisation within the nickel complex is represented
Fig. 2 The structure of complex 12 with H atoms omitted. Symmetry
operation for the second half of the anion: 1 − x, 1 −y, 1 − z.
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Fig. 3 The structure of 16 with H atoms and PPN counterion omitted. Symmetry operations for the second half of each anion: 1 − x, − y, 1 − z for
Au(1); 1 − x, 1 − y, −z for Au(2).
by the reversibility and low potential for the first oxidation process.
As stated above, the first reversible oxidation wave for compound
3 originates from the nickel dithiolene unit, but the irreversibility
of the two oxidation processes in 4 should be due, in part, to the
removal of electrons from at least one of the terthiophene units,
since polymerisation takes place (see later). In the gold complexes
5 and 6, which only vary in the nature of the more delocalised
fragment which encompasses the anion, only one irreversible
oxidation wave is observed at roughly the same potential and this is
confidently assigned to the terthiophene ligand. For all monomer
complexes, a single irreversible reduction wave is observed in the
region −1.16 to −1.40 V.
Polymers of compounds 3–6 were grown electrochemically from
solutions of the corresponding monomers (ca. 10−3 M in CH2Cl2)
by repetitive cycling over the range 0.0 to +1.3 V on gold or
indium tin oxide (ITO) glass working electrodes (representative
Fig. 4 Cyclic voltammograms of compounds 3 (solid line) and 12 (dashed
traces are shown in Fig. 5). In each case, the growth of a
line) using glassy carbon working electrode, Ag/AgCl reference electrode
fresh oxidation wave represents the emerging electrochemical
and Pt counter electrode, in CH2Cl2 (substrate ca. 10−3 M) at a scan rate of
signature of the new polymeric material. Once the polymer films
were established, the coated working electrodes were removed
from the monomer solution, washed with dichloromethane and
placed in a monomer-free acetonitrile solution containing 0.1 M
100 mV s−1. The inset shows the square wave voltammogram for compound
12 between −0.4 and +0.6 V.
Table 1 Electrochemicala and absorption spectroscopy datab for compounds 3–6, 12, poly(3), poly(4), poly(5) and poly(6)
Compound
Eox/V
E
red/V
kmax/nm
3
−0.13 (70)c, +0.46d
−1.16d
−1.40d
−1.25d
−1.25d
−1.56d
−1.04, −1.26
—
340, 391, 480, 530
300, 321(sh), 440, 477(sh)
266, 306, 391
4
+0.13d, +0.30d
5
6
12
poly(3)
poly(4)
poly(5)
poly(6)
+0.57d
+0.60d
259, 308, 390
−0.12 (120)c, +0.30 (190)c, +0.70 (240)c, +1.11 (80)c
+0.04d, +0.66 (10)c, +0.97 (80)c
+0.12 (60)c, +0.27 (100)c
+0.32 (250)c, +0.85 (160)c
+0.30 (90)c, +0.80 (50)c
342, 394, 478, 527
492, 908
514 (717)e
—
—
405, 494 (647)e
412, 497 (665)e
a Data obtained from cyclic voltammetry experiments conducted in CH2Cl2 vs Ag/AgCl for monomers and on ITO glass in acetonitrile for polymers.
b Experiments carried out in CH3CN solution for molecular species and solid state on ITO glass for the polymers. c Figures in parentheses represent
DEpa−pc in mV. d Irreversible peak. e Onset of longest wavelength absorption band.
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exclusively to the polythiophene chains, since there is no metal-
centred electroactivity evident in monomers 4–6 and none is
expected in the related polymer structures. Poly(4), poly(5) and
poly(6) have poor stability under n-doping and the polymers are
lost from the electrode at negative potentials (less than −1.0 V) as
a result of chemical degradation or dissolution from the electrode
surface.
The electroactivity of poly(3) is surprising in that the reversibil-
ity of the nickel dithiolene-centred oxidation is lost and the process
is raised by 170 mV (compared to the monomer).21 Indeed, the
oxidation processes associated with the polythiophene chains
are higher than those for the palladium- and gold-containing
polymers; this is due to the increase of positive charge located at
the oxidised metal centres in poly(3), which will lower the electron
density of the polythiophene ligand.
Electronic absorption spectroscopy
Absorption data and, in particular, UV-vis spectroelectrochem-
istry, provide a deeper insight into the electronic properties of
the polymers. The electronic spectrum of the polymer grown on
ITO glass, along with spectra of 3 at −0.40 and +0.25 V in
MeCN, are shown in Fig. 7. A fresh solution of the monomer
gives an identical spectrum to that obtained from a solution of
3 at −0.4 V (spectroelectrochemistry was conducted using an
optically transparent thin-layer electrochemical (OTTLE) cell).
The complex oxidises after several days under ambient conditions,
giving a spectrum that can be reproduced spectroelectrochemically
at +0.25 V and which features an additional absorption peak at
917 nm, corresponding to the oxidised nickel dithiolene monoan-
ion species.8 In both oxidation states, the peaks at 335 and 342 nm
correspond to a p–p* transition within the triaryl units, as seen for
similar terthiophene systems.6 For the polymer film, this transition
is shifted bathochromically by ca. 150 nm, indicative of an increase
in conjugation and providing further proof for the generation of a
polymeric material. The peak for the monoanion dithiolene system
remains at 908 nm, indicating that, under the electrochemical
conditions employed, this is the preferred ground state of the
polymer. However, it should be noted that a neutral dithiolene may
also show an absorption in this part of the spectrum. The most
remarkable feature of poly(3) is the extremely broad absorption of
the material between 400 and 1000 nm. Conjugated polymers that
cover such a vast range are extremely rare; since the maximum
Fig. 5 Electrochemical growth of poly(4) (top) and poly(5) (bottom) in
CH2Cl2; monomer concentration ca. 10−3 M, 100 mV s−1, vs Ag/AgCl.
NBu4PF6. The cyclic voltammograms of each of the polymers of
compounds 4–6 gave two reversible or quasi-reversible oxidation
waves, in contrast to the single irreversible oxidation peaks of
the corresponding monomers (Fig. 6). In poly(4), the oxidation
peaks were approximately 0.2 and 0.6 V lower than the gold-
containing polymers. For the Pd- and Au-containing materials, the
two reversible/quasi-reversible oxidation processes are assigned
Fig. 6 Cyclic voltammograms of poly(4), poly(5) and poly(6) films on Au
in monomer-free acetonitrile solution containing 0.1 M n-Bu4PF6, scan
rate 100 mV s−1, vs Ag/AgCl.
Fig. 7 Electronic absorption spectra for 3 in MeCN and poly(3) as a thin
film on ITO glass.
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of the photon flux of the sun peaks around 700 nm (just out of
the range of most conjugated polymers such as simple PPVs and
polythiophenes), low band-gap polymers such as poly(3) could
be extremely useful materials as light-harvesting components in
plastic photovoltaic devices.9
The spectroelectrochemical plot of 12 is shown in Fig. 8 and
should be considered alongside the electrochemical data depicted
in Fig. 4 and summarised in Table 1. A dramatic change in
absorption characteristics is observed once the potential exceeds
0.0 V, coinciding with the first oxidation process of the compound
(the spectroelectrochemical experiment was conducted on the
material as a thin film, hence the small difference in potentials).
Peaks at 478 and 527 nm diminish rapidly and strong fresh peaks
emerge at 404 and 434 nm. Concurrently, the signature of the
dithiolene monoanion arises at 914 nm. Above +0.8 V the film of
compound 12 begins to fall away from the electrode, due to the high
solubility of the multicharged intermediate. Since the complication
of coupling reactions between oxidised units (i.e. polymerisation)
has been eliminated from compound 12 by the end-capping
methyl groups, we benefit from well-defined absorption spectra
that can be ascribed irrefutably to the characteristics of stable
charged intermediates. From the spectroelectrochemical plot and
the discussion above, we can conclude that the oxidised state of
monomer 3, which persists in solution after several days, is in
fact the multi-oxidised species with electrons removed from the Ni
dithiolene unit and the terthiophene segments (Fig. 7). During the
oxidation of 3, repetitive cycling (0.0 to +1.3 V) is required for the
cation radical intermediates to trap and dimerise to form poly(3).
Fig. 9 UV-vis spectroelectrochemical plot for poly(3) (top) and poly(5)
(bottom) deposited on ITO glass in dichloromethane solution.
changes in absorption characteristics are similar to related
poly(terthiophene)s,17,18,24 hence we can conclude that there is
no electronic contribution from the metal centres in poly(4),
poly(5) and poly(6). Fig. 9 (top) shows the spectroelectrochemical
spectrum for poly(3) and there is negligible change until the second
oxidation wave is encountered at ca. +0.3 V. The emerging peak at
850 nm is lost at higher potential, specifically between the two
reversible oxidation peaks (+0.66 and +0.97 V, Table 1), and
a new broad band forms at 800 nm. Interestingly, the main p–
p* band of the polythiophene chain is unchanged upon doping,
indicating that the charged species are more localised within the
dithiolene unit than the polymer main chain. The behaviour
of poly(3) is very different from the electronic characteristics
displayed by poly(5) upon oxidation and it is difficult to assign
the electronic processes accurately because of the complex redox
behaviour of poly(3). Indeed, compound 12 undergoes at least
Fig. 8 UV-vis spectroelectroelectrochemical plot for compound 12 de-
posited on ITO glass in dichloromethane solution.
The spectroelectrochemical plot for poly(5) is shown in Fig. 9
(bottom) and represents the general absorption characteristics
of the Pd- and Au-containing polymers. In these materials,
the longest wavelength absorption peaks diminish over the
first oxidation process and at higher potentials (>+0.4 V) the
emergence of a band at ca. 1050 nm represents the genera-
tion of polarons/bipolarons in the polythiophene chains. The
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four oxidation processes up to +1.2 V, providing a good model for
the redox activity observed in the corresponding polymer. Thus,
it can be assumed that the spectroelectrochemical behaviour of
poly(3) arises from the oxidation of both nickel dithiolene and
thiophene units, with the charged intermediates delocalised mainly
over the metal dithiolene segments.
Experimental
General procedures and materials
All reactions and product manipulations were performed under
an atmosphere of argon using standard Schlenk techniques.
THF was dried over and distilled from Na. Melting points were
taken using Gallenkamp Melting Point apparatus. H and 13C
1
NMR spectra were collected on Varian UNITY-300 or Bruker
ARX-300 spectrometers (chemical shifts are in ppm, coupling
constants are in Hz), IR spectra were measured with a Perkin-
Elmer 883 or Perkin-Elmer FT-IR Espectrum One using KBr
pellets, UV-vis spectra of 0.1 mM solutions in a 1 cm cuvette
were measured with a UNICAM UV 300 and mass spectra were
collected with V.G. Autoespec, Micromass Trio 2000 or Kratos
concept 1S instruments. Elemental analyses were obtained on a
Perkin Elmer 240B/2400B or a Carlo Erba Instruments EA1108
elemental analyser. NBu4[AuBr2],28 PPN[AuCl2],28 Na2[PdCl4],29
Charge transfer materials
Dithiolenes 14–17 were isolated as the monoanionic species via the
spontaneous oxidation of the dianion intermediates. The stability
of the dianions are therefore very different from dithiolenes 3–
6, demonstrating the increased electron-donating properties of
the methoxyphenylene-based ligands. This type of spontaneous
oxidation has been observed in other Ni dithiolene complexes.25–27
In an attempt to isolate crystalline molecular charge transfer
materials, compounds 14–17 were oxidised under electrochemical
and chemical conditions. Cyclic voltammetry showed that the
redox properties of compounds 14–17 varied greatly. The nickel
derivative gives the expected Ni-centred oxidation at −0.25 V
(quasi-reversible), and is accompanied by two further irreversible
oxidations (+0.15 V and +0.44 V). Compared to complex 3, the
lower oxidation potential of 14 is attributed to the increased
electron-donating properties of the p-methoxyphenylene groups,
compared with the thiophenes in 3. The gold complexes are
reversibly oxidised at higher potentials (ca. +0.6 V) and undergo
reduction at ca. −1.1 V (irreversible). In contrast, the Pd analogue
was not oxidised within the potential window applied (up to
+1.2 V), but underwent two irreversible reductions at −0.25 V
and −0.73 V. In view of the above, it was not surprising that
only the nickel complex (14) gave charge transfer materials
upon chemical oxidation. Chemical oxidation was achieved by
mixing 14 with a solution of the tetrathiafulvalene-based salt
(TTF)3(BF4)2. Elemental analysis of the black product gave the
formula (TTF)4[Ni{S2C4S(SC6H4OCH3)2}2]3 and the material had
a conductivity of 9 × 10−6 S cm−1.
30
(TTF)3(BF4)2 were synthesized by literature procedures. Con-
ductivity measurements were carried out using the compressed
pellet, two-probe method.31
X-Ray crystallography
The data collection and refinement parameters of structures 5,
12 and 16 are presented in Table 2. X-Ray diffraction data for
compounds 5 and 16 were collected on a Bruker-Nonius Kap-
paCCD area detector situated at the window of a rotating anode
˚
[k(Mo-Ka) = 0.71073 A]. Data for compound 12 were collected
˚
at Daresbury SRS station 9.8 (k = 0.6868 A). The structures
were solved by direct methods and refined using programs of the
SHELX family.32 Hydrogen atoms were included in the refinement,
but their displacement parameters and positions were constrained
to ride on the atoms to which they are bonded. The data were
corrected for absorption effects using SADABS.33 The PLATON
SQUEEZE34 algorithm was applied to 16 to model the diffuse
contribution from a highly disordered solvent of crystallisation to
the electron density.
CCDC reference numbers 675707–675709. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b801187g
Electrocrystallisation from solutions of 14 and 16 in acetonitrile
gave black microcrystalline powders. Elemental analyses indicated
that there was very little countercation (NEt4 or PPN) present in
the solids. The conductivities of the electrochemically oxidised
materials were found to be 2 × 10−7 S cm−1 for the product of 14
and 7 × 10−8 S cm−1 for that of oxidised 16.
Cyclic voltammetry
Measurements were performed on a CH Instruments 660A
Electrochemical Workstation with iR compensation, using anhy-
drous dichloromethane as the solvent, aqueous Ag/AgCl as the
reference electrode and platinum wire and gold disk as the counter
and working electrodes, respectively. All solutions were degassed
(Ar) and contained the substrate in concentrations of ca. 10−3 M,
together with n-Bu4NPF6 (0.1 M) as the supporting electrolyte.
Conclusion
Novel and stable metal dithiolene complexes have been prepared
and isolated in good yields. The structures incorporate redox-
active units fused to the ligands and terthiophene analogues can
be readily polymerised electrochemically to form films of the corre-
sponding cross-linked poly(thiophenes). Electrochemical and UV-
vis spectroelectrochemical experiments have been conducted on
the polymers and the model compound 12. The nickel dithiolenes
show complex electrochemical behaviour involving both the
metal dithiolene fragment and the terthiophene/poly(thiophene)
chains. This is not the case for Pd and Au analogues, in which
electroactivity is confined to the polymer backbone.
Syntheses
[NBu4]2[Ni{S2C4S(SC4H3)2}2] 3. To a solution of 1 (0.1 g,
0.3 mmol) in dry THF (30 ml) under N2 was added NaOEt (0.5 M,
2.4 ml, 1.2 mmol) and the reaction was refluxed for 15 min. Bu4NBr
(0.1 g, 0.3 mmol) was then added followed by NiCl2·6H2O (0.04 g,
0.15 mmol). The mixture was stirred overnight. The solvent was
reduced in vacuo (5 ml), and diethyl ether was used to precipitate 3
as a dark red solid (0.17 g, 50%). Dark red crystals were obtained
from a hexane–THF solution. Found: C, 56.37; H, 7.00; N, 2.31%.
3076 | Dalton Trans., 2008, 3070–3079
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Table 2 The data collection and refinement parameters of complexes 5, 12 and 16
5
12
16
Empirical formula
Formula weight
Crystal system
Space group
C60H42AuNP2S10
1356.45
Triclinic
¯
P1
14.2162(1)
14.6583(2)
15.0674(2)
76.383(1)
80.037(1)
65.190(1)
2760.10(6)
2
C60H92N2NiS10
1220.67
Monoclinic
P21/c
8.7090(7)
21.2066(16)
17.7517(13)
C72H58AuNO4P2S6
1452.46
Triclinic
¯
P1
11.101(3)
15.888(4)
19.945(5)
105.418(4)
100.449(4)
99.858(4)
3244.5(13)
2
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
103.521(2)
3
˚
V/A
3187.7(4)
2
0.670
Z
l/mm−1
T/K
3.143
120
2.560
120
120
Crystal size/mm
0.42 × 0.10 × 0.04
0.10 × 0.08 × 0.05
0.38 × 0.18 × 0.11
◦
hmax
/
27.45
23.81
23.30
Reflections collected
Independent reflections
Rint
38561
12263
0.0442
671
0.0434
0.1178
1.027
2.21, −2.55
22238
5371
0.0183
343
0.0306
0.0805
1.044
14816
9255
0.0331
785
0.0483
0.1120
0.945
2.66, −1.36
Refined parameters
R (F, F2 > 2r)
Rw (F2, all data)
Goodness of fit (F2)
−3
˚
Difference map extremes/e A
0.69, −0.25
C56H84NiN2S10 requires: C, 57.78; H, 7.22; N, 2.40%. dH (CDCl3):
7.25 (m, 4H), 6.98 (br, 4H), 6.88 (br, 4H), 3.6 (br, 16H), 1.7 (m,
16H), 1.35 (m, 16H), 0.83 (t, 24H, J = 6.87). m/z (FAB−) 678.
dH (CDCl3): 7.4 (dd, 4H, J = 5.4 and 1.2), 7.3 (dd, 4H, J = 3.8
and 1.2), 7.1 (dd, 4H, J = 5.4 and 3.8), 3.4 (m, 8H,), 1.8 (m, 8H),
1.4 (m, 8H), 0.9 (t, 12H, J = 7.2). Mp 227–229 ◦C. m/z (FAB−)
816.
[NEt4]2[Pd{S2C4S(SC4H3)2}2] 4. To a solution of 1 (0.1 g,
0.3 mmol) in dry THF (30 ml) under N2 was added NaOEt (0.5 M,
2.4 ml, 1.2 mmol) and the reaction was refluxed for 15 min. Et4NBr
(0.06 g, 0.3 mmol) was then added, followed by Na2[PdCl4] (0.04 g,
0.15 mmol). The mixture was stirred overnight. The compound
was collected by filtration to afford 4 as a brown solid and washed
with copious amounts of water to eliminate excess Et4NBr (0.16 g,
49%). Found: C, 44.79; H, 4.61; N, 2.38%. C32H32PdN2S10 requires:
C, 44.90; H, 3.72; N, 1.64%. dH (DMSO): 7.5 (q, 8H, J = 9.1 and
5.1), 7.0 (t, 4H, J = 4.4), 3.2 (q, 12H, J = 9.1 and 6.9), 1.2 (t, 8H,
J = 6.9). Mp 230–232 ◦C. m/z (FAB−) 725.
[PPN][Au{S2C4S(SC4H3)2}2] 5. To a solution of 1 (0.1 g,
0.3 mmol) in dry THF (30 ml) under N2 was added NaOEt
(0.5 M, 2.4 ml, 1.2 mmol) and the reaction was refluxed for 15 min.
PPN[AuCl4] (0.13 g, 0.15 mmol) was then added and the mixture
was stirred overnight. The solvent was removed under reduced
pressure to afford 5 as a green solid, which was purified from a
THF–diethyl ether solution (0.15 g, 38%). Found: C, 52.27; H,
2.79; N, 0.98%. C60H42AuNP2S10 requires: C, 53.11; H, 3.10; N,
1.03%. dH (CDCl3): 7.52 (m, 10H), 7.35 (m, 20H), 7.24 (dd, 4H,
J = 1.2 and 3.6), 7.11 (dd, 4H, J = 1.2 and 5.4), 6.94 (dd, 4H, J =
3.6 and 5.4). Mp 143–145 ◦C. m/z (FAB−) 816.
4,5-Bis(5-methyl-2-thienylhydroxymethyl)-1,3-dithiole-2-thione 8
and 4,5-bis(5-methyl-2-thienoyl)-1,3-dithiole-2-thione 9. Under
an inert atmosphere of N2, vinylenetrithiocarbonate 7 (2.50 g,
18.66 mmol) was dissolved in dry THF (75 ml) and cooled to
−78 ◦C. To this solution, lithium diisopropylamide monotetrahy-
drofuran (LDA·THF) (1.5 M solution in cyclohexane, 13.7 ml,
20.53 mmol) was added and the reaction left to stir for ap-
proximately 20 min. Then 5-methyl-2-thiophenecarboxaldehyde
(2.21 ml, 20.53 mmol) was added and the mixture stirred for
10 min. LDA·THF was added again (13.7 ml, 20.53 mmol) and
stirring continued for 15 min, which was then followed by the ad-
dition of another portion of 5-methyl-2-thiophenecarboxaldehyde
(2.21 ml, 20.53 mmol) and left to stand for 5 min before being
allowed to warm to room temperature. The reaction mixture
was poured into saturated sodium bicarbonate solution (until
the precipitate was dissolved) and the organic layer separated.
The aqueous phase was then extracted with dichloromethane, the
organic extracts combined and dried over MgSO4, and the solvent
was removed under reduced pressure. The residue was purified by
column chromatography (silica, dichloromethane and then ethyl
acetate) to afford the mono- and the diol 8 respectively in order
of elution. The diol was obtained as an unstable red oil, which
was reacted immediately (5.61 g). The diol (5.61 g) was dissolved
in dichloromethane (125 ml) and to this a ten-fold excess (w/w)
of manganese(IV) oxide (56 g) was added portionwise and the
mixture stirred for 2 min at room temperature and immediately
filtered through a layer of silica (ca. 10 cm thick) in a fritted filter
funnel, eluting with dichloromethane. The solvent was removed
under reduced pressure to afford compound 9 as an orange tar,
[NBu4][Au{S2C4S(SC4H3)2}2] 6. To a solution of 1 (0.1 g,
0.3 mmol) in dry THF (30 ml) under N2 was added NaOEt (0.5 M,
2.4 ml, 1.2 mmol) and the reaction was refluxed for 15 min. n-
Bu4N[AuBr4] (0.08 g, 0.15 mmol) was then added and the mixture
was stirred overnight. The solvent was removed under reduced
pressure to afford 6 as a green solid, which was purified from
a THF–diethyl ether solution (0.14 g, 55%). Found: C, 40.79; H,
3.88; N, 1.13%. C40H48AuNS10 requires C, 45.31; H, 4.56; N, 1.32%.
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which gradually solidified upon standing (4.77 g, 67% from 7).
dH (CDCl3): 7.54 (2H, d, J = 3.9), 6.78 (2H, d, J = 3.9), 2.51
(6H, s). dC (CDCl3): 207.6, 175.1, 154.2, 144.0, 139.9, 136.3, 127.5,
16.2.m/z (CI) 383 [M + H]+.
[NEt4][Ni{S2C4S(p-OCH3C6H4)2}2] 14. To a solution of 4,6-
bis(4-methoxyphenyl)-thieno[3,4-d]-1,3-dithiol-2-one20 13 (0.1 g,
0.25 mmol) in dry THF (25 ml) under argon was added 7.5 ml
of NaEtO (0.1 M). The reaction was stirred until the solid was
dissolved (no more than 40 min because the ketone starting
material precipitates back). Then 0.125 mmol (26 mg) of [NEt4]Br
followed by 0.125 mmol (30 mg) of NiCl2·6H2O were added. After
24 h an orange solid was filtered and washed with water (to remove
the [NEt4]Br excess) (99%). Found: C, 58.97; H, 5.76; N, 1.70; S,
4,5-Bis(5-methyl-2-thienoyl)-1,3-dithiole-2-one 10. To a solu-
tion of 9 (2.84 g, 7.43 mmol) in a dichloromethane–glacial acetic
acid mixture (3 : 1, v/v, 80 ml) was added mercury(II) acetate
(3.31 g, 10.40 mmol) and the mixture stirred at room temperature
overnight. The mixture was filtered through a layer of silica, eluting
with dichloromethane. The filtrate was washed with water and a
saturated sodium bicarbonate solution. The organic extracts were
dried over MgSO4 and the solvent was removed under reduced
pressure to give a crude yellow solid. The solid was dissolved
in a minimal amount of dichloromethane with heating and then
petroleum ether (boiling range 40–60 ◦C) was added to precipitate
the compound. A shiny yellow solid was obtained (1.95 g, 72%).
dH (CDCl3): 7.51 (2H, d, J = 3.9), 6.77 (2H, d, J = 3.9), 2.51 (6H,
s). dC (CDCl3): 187.3, 176.8, 153.9, 140.2, 136.3, 135.1, 127.4, 16.2.
m/z (APCI+) 367 [M + H]+, 339 [M − CO + H]+. m/z (APCI−)
338 [M − CO + e]−.
20.84%. C44H48NNiO4S6 requires: C, 58.33; H, 5.30; N, 1.55; S,
−1
=
21.21%. mmax/cm : 1605 (C C) (KBr). dH (CDCl3): 7.52 (8H, d,
J = 7.8 Hz), 6.99 (8H, d, J = 7.5 Hz), 3.87 (12H, s), 3.34 (8H, m),
1.30 (12H, m). m/z (LSIMS−) 774 ([M]–, 38%), 458 (100%).
(Q)[Au{S2C4S(p-OCH3C6H4)2}2] (Q = NBu4 15, PPN 16).
Complexes 15 and 16 were prepared by a procedure similar to
14 using NBu4[AuBr4] (0.125 mmol, 95 mg) and PPN[AuCl4]
(0.125 mmol, 110 mg) respectively. For compound 15, a blue solid
was filtered off after 24 h (42%). For compound 16, the solution
was evaporated in vacuo to the point of crystallisation, whereupon
diethyl ether (20 ml) was added to precipitate the product as a
violet solid (99%).
4,6-Bis(5-methyl-2-thienyl)thieno[3,4-d]-1,3-dithiole-2-thione 11.
A mixture of compound 10 (1.60 g, 4.37 mmol), phosphorus
pentasulfide (9.71 g, 21.85 mmol) and sodium hydrogencarbonate
(1.84 g, 21.85 mmol) in 1,4-dioxane (80 ml) was heated to 90 ◦C for
3 h. The reaction mixture was allowed to cool to room temperature
before adding water portionwise (10 × 10 ml) (Caution! H2S and
CO2 evolution). The mixture was stirred overnight. The precipitate
was filtered off and washed with hot water before dissolving in
chloroform and drying over MgSO4. After filtration, the solution
was stirred with decolourising charcoal for 30 min and then filtered
through a layer of silica. The solvent was removed under reduced
pressure to give terthiophene 11 as a yellow solid (1.28 g, 80%).
Recrystallization was achieved by dissolving the compound in a
minimal amount of chloroform with heating and then petroleum
ether (boiling range 40–60 ◦C) was added. The suspension was
placed in an ice bath and the precipitate was filtered and washed
with petroleum ether. A yellow solid was obtained (0.83 g, 52%).
Found: C, 48.83; H, 2.28; S, 44.49%. C15H10OS5 requires: C, 49.19;
H, 2.73; S, 43.73%. dH (CDCl3): 6.99 (2H, d, J = 3.6), 6.73 (2H,
dd, J = 3.6 and 1.1), 2.52 (6H, br s). dC (CDCl3): 192.5, 141.2,
132.2, 126.4, 125.5, 125.1, 123.4, 15.4. m/z (CI) 367 [M + H]+.
15. Found: C, 54.44; H, 5.03; N, 1.63; S, 17.10%.
C52H64AuNO4S6 requires: C, 54.00; H, 5.54; N, 1.21; S, 16.62%.
mmax/cm−1 1606 (C C) (KBr). dH (CDCl3): 7.72 (8H, d, J = 8.7 Hz),
=
6.88 (8H, d, J = 8.4 Hz), 3.75 (12H, s), 3.02 (8H, m), 1.40 (8H, m),
1.17 (8H, m), 0.79 (12H, m). m/z (LSIMS−) 913 ([M]−, 100%),
459 (72).
16. Found: C, 59.05; H, 3.55; N, 0.80; S, 12.82%.
C72H58AuNO4P2S6 requires: C, 59.51; H, 4.00; N, 0.96; S, 13.22%.
−1
=
mmax/cm 1606 (C C) (KBr). dH (CDCl3): 16: 7.67 (8H, d, J = 9.1
Hz), 7.49 (8H, d, J = 5.5 Hz), 7.35 (22H, m), 6.81 (8H, d, J = 8.8
Hz), 3.78 (12H, s). m/z (LSIMS−) 914 ([M]−, 100%), 556 (32).
[NEt4][Pd{S2C4S(p-OCH3C6H4)2}2] 17. Complex 17 was pre-
pared by a procedure similar to 14 using [NEt4]Br (17 mg,
0.125 mmol) and Na2[PdCl4]. (37 mg, 0.125 mmol). The solvent
was reduced in vacuo (5 ml) and diethyl ether was used to
precipitate 17 (82%) as a brown solid which was washed with water.
Found: C, 55.04; H, 5.22; N, 1.81; S, 19.80%. C44H48NO4PdS6
requires: C, 55.44; H, 5.04; N, 1.47; S, 20.16%. mmax/cm−1 1605
=
(C C) (KBr). dH (DMSO): 7.98 (8H, d, J = 8.7 Hz), 6.88 (8H, d,
J = 8.8 Hz), 3.76 (12H, s), 3.15–3.18 (8H, m), 1.00–1.12 (12H, m).
m/z (LSIMS−) 822 ([M]−, 5%), 764 (4), 459 (20).
[NBu4]2[Ni{S2C4S(SC5H5)2}2] 12. To a solution of the methyl-
capped terthiophene 11 (0.25 g, 0.68 mmol) in dry tetrahydrofuran
(50 ml) under N2 was added NaOEt (0.5 M ethanol solution,
5.44 ml, 2.72 mmol) and the mixture was refluxed for 15 min.
n-Bu4NBr (0.22 g, 0.68 mmol) was then added, followed by
NiCl2·6H2O (0.081 g, 0.34 mmol). The reaction was stirred
overnight. The mixture was filtered and the filtrate was reduced in
volume, then petroleum ether (boiling range 40–60 ◦C) was added
to precipitate compound 12 as a dark red solid (0.337 g, 81%).
Dark red crystals were obtained by slow diffusion of diethyl ether
or cyclohexane into a THF solution of the compound. Found:
C, 58.06; H, 7.29; S, 25.84; N, 2.26%. C60H92N2NiS10 requires: C,
59.10; H, 7.55; S, 26.26; N, 2.30%. dH (CDCl3): 6.82 (4H, br s),
6.59 (4H, br s), 2.74 (12H, br s), 3.58 (16H, br s), 1.63 (16H, br s),
1.28 (16H, br s), 0.80 (24H, t, J = 6.4). m/z (APCI−) 734 [M −
2n-Bu4N + e]−.
Oxidation reactions of dithiolene complexes
(A) Chemical oxidation with (TTF)3(BF4)2. To a solution of the
corresponding dithiolene complex (0.05 mmol) in MeCN (15 ml)
was added 0.065 mmol (51 mg) of (TTF)3(BF4)2 (1 : 1.3 ratio).
The corresponding mixtures were stirred for 15 h and then the
product was collected by filtration; in some cases it was necessary
to concentrate the solution under reduced pressure.
(TTF)4[Ni{S2C4S(SC6H4OCH3)2}2]3 14T. Found: C, 50.40;
H, 3.20; S, 29.87%. (C6H4S4)4(C36H28O4S6Ni)3 requires: C, 50.37;
H, 2.62; S, 29.05%. Conductivity r: 9 × 10−6 S cm−1.
(B) Electrocrystallisation. To one arm of the “H-shaped”
electrolysis cell was added 0.04 mmol of the dithiolene (14, 16),
and a solution of [NEt4]ClO4 (0.1 mmol) (14) or ClPPN (0.1 mmol)
(16) in 10 ml of dry acetonitrile was added to both arms of the cell.
3078 | Dalton Trans., 2008, 3070–3079
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The device was left under argon and protected from light. After
standing for 19 d with a constant current of 1.2 lA, a feathered
material was deposited on the platinum wire. The material was
removed from the electrode and left to dry.
14 R. Kato, Chem. Rev., 2004, 104, 5319–5346.
15 T. D. Anthopoulos, S. Setayesh, E. Smits, M. Colle, E. Cantatore, B.
de Boer, P. W. M. Blom and D. M. de Leeuw, Adv. Mater., 2006, 18,
1900–1904.
16 K. L. Marshall, G. Painter, K. Lotito, A. G. Noto and P. Chang, Mol.
Cryst. Liq. Cryst., 2006, 454, 47–79.
(Q)[M{S2C4S(SC5H5)2}2] (M = Ni, Q = NEt4 14-elec; M =
Au, Q = PPN 16-elec). 14-elec. Found: C, 54.21; H, 3.28; S,
24.14%. (C8H20N)x(C36H28NiO4S6)y (x : y = 0 : 1) requires: C, 55.74;
H, 3.63; S, 24.79%. Conductivity r: 2 × 10−7 S cm−1;
17 R. Berridge, P. J. Skabara, C. Pozo-Gonzalo, A. Kanibolotsky, J. Lohr,
J. J. W. McDouall, E. J. L. McInnes, J. Wolowska, C. Winder, N. S.
Sariciftci, R. W. Harrington and W. Clegg, J. Phys. Chem. B, 2006, 110,
3140–3152.
18 R. Berridge, S. P. Wright, P. J. Skabara, A. Dyer, T. Steckler, A. A.
Argun, J. R. Reynolds, W. H. Ross and W. Clegg, J. Mater. Chem.,
2007, 17, 225–231.
19 C. Pozo-Gonzalo, T. Khan, J. J. W. McDouall, P. J. Skabara, D. M.
Roberts, M. E. Light, S. J. Coles, M. B. Hursthouse, H. Neugebauer,
A. Cravino and N. S. Sariciftci, J. Mater. Chem., 2002, 12, 500–
510.
20 P. J. Skabara, I. M. Serebryakov, D. M. Roberts, I. F. Perepichka, S. J.
Coles and M. B. Hursthouse, J. Org. Chem., 1999, 64, 6418–6424.
21 C. Pozo-Gonzalo, R. Berridge, P. J. Skabara, E. Cerrada, M. Laguna,
S. J. Coles and M. B. Hursthouse, Chem. Commun., 2002, 2408–
2409.
16-elec. Found: C, 47.92; H, 2.93; S, 20.83%. (C36H30NP2)x
(C36H28AuO4S6)y (x : y = 0 : 1) requires: C, 47.31; H, 3.08; S,
21.04%. Conductivity r: 7 × 10−8 S cm−1.
Acknowledgements
The authors thank EPSRC for funding the UK National Crystal-
lography Service, and STFC for access to synchrotron facilities.
22 T. Khan, J. J. W. McDouall, E. J. L. McInnes, P. J. Skabara, P. Fre`re,
S. J. Coles and M. B. Hursthouse, J. Mater. Chem., 2003, 13, 2490–
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23 C. R. Mason, P. J. Skabara, D. Cupertino, J. Schofield, F. Meghdadi,
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24 P. J. Skabara, R. Berridge, I. M. Serebryakov, A. L. Kanibolotsky, L.
Kanibolotskaya, S. Gordeyev, I. F. Perepichka, N. S. Sariciftci and C.
Winder, J. Mater. Chem., 2007, 17, 1055–1062.
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Hibbs and K. M. A. Malik, J. Mater. Chem., 1995, 5, 1731–1734.
26 A. Charlton, A. E. Underhill, A. Kobayashi and H. Kobayashi,
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