Nickel dithiolenes containing pendant thiophene units: Precursors to dithiolene-polythiophene hybrid materials

Article abstract of DOI:10.1039/b714372a

The synthesis, electrochemical and spectroscopic properties of the new thiophene-substituted metal dithiolene complexes [Ni(b-3ted)₂] (3) and [NBu₄][Ni(b-3ted)₂] ([NBu₄][3]) are described [b-3ted = bis(3-thienyl)-1,2-ethylenedithiolene] and compared with new studies of the known compounds [Ni(b-2ted)₂] (2) and [NBu ₄][Ni(b-2ted)₂] ([NBu₄][2]). X-Ray structures were determined for the neutral complexes 2 and 3 and both showed planar nickel dithiolene units with thiophene groups twisted out of the molecular plane, precluding close molecular π-stacking. Electrochemical investigations revealed two redox processes corresponding to interconversion between neutral, monoanionic and dianionic species for 3, analogous to 2. Oxidation of 0.1 mM [3]^- in CH₃CN was shown to give a green molecular film of 3 which, in contrast to as-prepared crystals of 3, showed significant conductivity. Electrochemical polymerisation of 3 carried out at different concentrations resulted in films; electrochemical, SNIFTIRS and UV-Vis results were consistent with incorporation of the intact metal dithiolene complex in these films. In contrast, electrochemical polymerisation of 2 gave a film charateristic of a polymerised thiophene but with no evidence of incorporation of the intact metal dithiolene complex. Copolymerisation of 3 with thiophene gave a gold-coloured electropolymerised film of significant conductivity that, upon dissolution in CH₂Cl₂, showed the characteristic low energy absorption of the Ni-dithiolene complex, confirming its incorporation. This copolymerisation approach offers a versatile route to the synthesis of metal dithiolene-polythiophene hybrid films. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714372a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Nickel dithiolenes containing pendant thiophene units: precursors to
dithiolene–polythiophene hybrid materials{
a
a
a
b
Tania Anjos, Susan J. Roberts-Bleming, Adam Charlton, Neil Robertson,* Andrew R. Mount,
b
c
Simon J. Coles, Michael B. Hursthouse, Maher Kalaji and Patrick J. Murphy*
c
a
a
Received 18th September 2007, Accepted 19th November 2007
First published as an Advance Article on the web 7th December 2007
DOI: 10.1039/b714372a
The synthesis, electrochemical and spectroscopic properties of the new thiophene-substituted
metal dithiolene complexes [Ni(b-3ted)
b-3ted = bis(3-thienyl)-1,2-ethylenedithiolene] and compared with new studies of the
known compounds [Ni(b-2ted) ] (2) and [NBu ][Ni(b-2ted) ] ([NBu ][2]). X-Ray structures
2 4 2 4
] (3) and [NBu ][Ni(b-3ted) ] ([NBu ][3]) are described
[
2
4
2
4
were determined for the neutral complexes 2 and 3 and both showed planar nickel dithiolene
units with thiophene groups twisted out of the molecular plane, precluding close molecular
p-stacking. Electrochemical investigations revealed two redox processes corresponding to
interconversion between neutral, monoanionic and dianionic species for 3, analogous to 2.
2
Oxidation of 0.1 mM [3] in CH CN was shown to give a green molecular film of 3 which,
3
in contrast to as-prepared crystals of 3, showed significant conductivity. Electrochemical
polymerisation of 3 carried out at different concentrations resulted in films; electrochemical,
SNIFTIRS and UV-Vis results were consistent with incorporation of the intact metal
dithiolene complex in these films. In contrast, electrochemical polymerisation of 2 gave a film
charateristic of a polymerised thiophene but with no evidence of incorporation of the intact
metal dithiolene complex. Copolymerisation of 3 with thiophene gave a gold-coloured
2 2
electropolymerised film of significant conductivity that, upon dissolution in CH Cl , showed
the characteristic low energy absorption of the Ni-dithiolene complex, confirming its
incorporation. This copolymerisation approach offers a versatile route to the synthesis of
metal dithiolene–polythiophene hybrid films.
3
tetrathiafulvalene (TTF) or metal-bis-1,2-dithiolene com-
Introduction
4
,5
plexes.
In contrast to conducting polymers, molecular
Conducting polymers based on the electropolymerisation
of heterocyclic monomers offer the possibility to prepare
versatile, persuasible electronic materials, and as such have
conductors typically show greater structural regularity
allowing improved understanding of structure–property
relationships, and this has led to a variety of materials
showing properties such as metallic conductivity and super-
1
received wide attention over recent years. Within this context,
6
substituted polythiophenes exhibit attractive electronic
properties, such as a low band-gap combined with good
stability and ease of film formation via electrochemical or
conductivity. Furthermore metal-bis-1,2-dithiolene complexes
also show low energy absorption bands in the NIR region
of the spectrum which has led to their study as non-linear
2
7
solution methods.
optical materials.
Conducting molecular/organic materials based on p-stack-
ing of planar redox active molecules have also been
widely studied, involving molecular building blocks such as
From the viewpoint of technological applications however,
processability of brittle crystalline molecular conducting
materials is difficult, and conducting polymers have remained
more prevalent in the development of technological devices,
8
a
such as organic light emitting diodes and photovoltaics. To
Department of Chemistry, University of Wales, Bangor, Gwynedd, UK
LL57 2UW
fully utilise the attractive properties of molecular species such
as metal dithiolene complexes and TTF derivatives it is
important to find ways of increasing the processability of
these materials. In this context, some of us developed over
recent years TTF derivatives bearing thiophene moieties and
b
School of Chemistry, University of Edinburgh, King’s Buildings,
Edinburgh, UK EH9 3JJ. E-mail: neil.robertson@ed.ac.uk
c
School of Chemistry, University of Southampton, Highfield,
Southampton, UK SO17 1BJ
{
Electronic supplementary information (ESI) available: Fig. S1:
Voltammograms of the polymerisation of 3. Fig. S2: Cyclic
voltammogram (CV) of poly-3 in monomer-free solution. Fig. S3: IR
spectrum of 3 in KBr disc. Fig. S4: SNIFTIRs spectra of polymerised
product from 3. Fig. S5: CV of 2. Fig. S6: Voltammograms of the
polymerisation of 2. Fig. S7: CV in monomer-free solution of
polymerised product from 2. Fig. S8: SNIFTIRS spectra of
polymerised product from 2 and discussion of results. Fig. S9: CV
showed that electrochemical polymerisation of these thiophene
9–11
units could be achieved.
This demonstrated the possibility
of preparing polythiophenes bearing TTF side groups with
the potential to develop conducting materials combining the
processability of the conducting polymer with some of the
structural regularity that can be achieved in TTF-based
conductors. The TTF may alter the conductivity properties
3
of 3 in CH CN at a higher scan rate and fuller description of the
formation of a molecular film of 3. See DOI: 10.1039/b714372a
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J. Mater. Chem., 2008, 18, 475–483 | 475

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TTF systems demonstrated that the substitution position of
the thiophene greatly alters the ability of the molecules to
be polymerised. Given that polythiophene consists predomi-
nantly of 2,5-linked thiophenes, copolymerisation of 2 with
thiophenes should result in the incorporation of the metal-bis-
1
,2-dithiolene complex into the polymer backbone, whereas
copolymerisation of the 3-substituted isomer 3 with thiophene
should allow polythiophene to form with the metal complex
attached as a side chain. Thus it is important to explore
the role played by the thiophene substituent position in
determining the electropolymerisation and copolymerisation
capability of these molecules and the properties of the
resultant films.
Scheme 1 Preparation of the bis(2 or 3-thienyl)-1,3-dithiole-2-one
ligand precursors (1a, b). Reagents and conditions: (i) NEt –MeOH (ii)
PPh , CCl –CH Cl , RT (iii) KSC(S)OEt, acetone, RT (iv) HBr–
AcOH, 0 uC.
3
This report therefore concerns a detailed study of complex 3
in comparison with the properties of the previously-studied
complex 2, which differs only in the point of attachment of the
thiophene rings. The synthesis, structural properties, electro-
chemistry and electropolymerisation of both complexes are
reported and a comparison given of their potential for use in
functional materials. Following this, study of the copoly-
merisation of complex 3 with thiophene demonstrates the use
of this precursor in hybrid materials.
3
4
2
2
of the resulting hybrid material either by providing an
alternative conductivity pathway or by modifying the pro-
perties of the polymer by cooperative electronic effects.
It is also of interest to incorporate metal-bis-1,2-dithiolene
complexes into such systems and the precursors we have
developed for the preparation of TTF compounds (Scheme 1)
can also be used as precursors for such dithiolene species.
Indeed previous studies involving electropolymerisation of
metal-bis-1,2-dithiolene complexes with appended thiophene
Results and discussion
The synthesis of the ligand precursors 1a and 1b is shown in
1
2,13
units have been carried out by the groups of Pickup
and
Scheme 1. Full experimental details have been reported by us
10
previously in the context of TTF-based compound syntheses.
1
4
Skabara. This approach offers tremendous opportunities for
variation of the polymer side groups, as an enormous range of
different metal centres and ancillary ligands can be employed
for fine control of the redox, structural and optical properties.
For example, the work of Skabara and co-workers led to the
synthesis of an unusually low band-gap material with a broad
absorption profile demonstrating that the incorporation of
metal dithiolene complexes can prove useful in control of the
optical properties of the polymer. Other advantages that
may be envisaged include introducing a new pathway for
conduction, exercising improved structural regularity or in
introducing new functionality such as receptor groups, long-
wavelength chromophores or catalytic sites.
These ligand precursors were each suspended in methanol
under a dry argon atmosphere. Treatment of 1a or 1b with
sodium methoxide in methanol afforded the soluble disodium
salt of the dithiolate dianion, which was then allowed to react
2 2
with NiCl ?6H O dissolved in methanol. The monoanionic
2 2
salts, [2] and [3] , were precipitated from the dark brown
reaction mixture using tetrabutylammonium bromide
(
Scheme 2). Recrystallisation of the crude complexes from
CH Cl –ethanol, acetone–methanol or acetone–ether yielded
2
2
the pure salts as dark purple crystalline solids.
The neutral complexes 2 and 3 were prepared by reacting the
corresponding monoanion with iodine in acetone. A darkening
of the solution colour was observed, followed by precipitation
of the neutral complex. Alternatively, preparation of neutral 3
In the work by Pickup and co-workers, the electrochemical
polymerisation of 2 (Scheme 2) to achieve a conducting
polymer was described, demonstrating the potential of these
was achieved in higher purity by recrystallisation of [NBu ][3]
4
1
2,13
systems as functional materials.
In addition, our previous
from acetone–ethanol in air. The deep red colour of the salt
work on the electropolymerisation of thiophene-substituted
(
[NBu ][3]) contrasts with the very pale green–brown colour of
4
the final product and is consistent with an oxidation from the
monoanionic to the neutral complex taking place during the
recrystallisation process.
Crystals of 2 show one independent molecular unit (Fig. 1)
4 4
with an approximately planar central NiS C unit (rms
deviation = 0.028) and a centre of inversion located on the
Ni. The thiophene units however, show significant twists from
this plane [12.50(18) and 81.91(12)u for the two independent
rings containing S2 and S4 respectively]. Significant disorder is
apparent within all of the thiophene units, due to free rotation
of the thiophene rings through 180u, which results in a 15%
partial occupation of the sulfur sites by carbon. The molecules
stack along the b-axis with the NiS C plane tilted at
Scheme 2 Preparation of the nickel-bis[bis(2 or 3-thienyl)-1,2 ethyl-
enedithiolene] complexes (2, 3). Reagents and conditions: (i) NaOMe–
4 2 2
MeOH, RT (ii) [NBu ][Br]–MeOH (iii) NiCl ?6H O–MeOH (iv)
4
4
2
I –acetone.
43.15(13)u with respect to this axis. The twist within the
4
76 | J. Mater. Chem., 2008, 18, 475–483
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1
6,17
18
or thioalkyl units. Similar species
core such as TTF units
have also been prepared and studied to explore their
7
,19
optical properties.
Planar or approximately planar species
0
2
]
generally show stacked complexes; examples are [Ni(dmit)
2
2
1
or [Ni(dddt)
2
]
(dmit = 2-thioxo-1,3-dithiole-4,5-dithiol;
dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) which have
˚
˚
interplanar distances of 3.562 A and 3.76 A respectively. In
contrast, the lack of close stacking shown by 2 and 3 is more
2 2 2 2
comparable with species such as [Ni(S C Ph ) ] which has
2
2
Fig. 1 X-Ray structure of [Ni(b-2ted)
2
] (2) showing the molecular
phenyl rings twisted out of the plane.
unit and numbering scheme and with disorder and hydrogen atoms
˚
omitted for clarity. Selected bond lengths/A: Ni1–S1 2.1302(7), Ni1–S3
Typical of nickel-bis-1,2-dithiolene species in the neutral
or monoanionic form, is the intense low energy absorption
around 920–970 nm that all of these new nickel dithiolenes
exhibit, and this has been assigned as the 2b1u A 3b2g
2
1
.1254(6), S3–C6 1.706(2), S1–C1 1.714(2), C6–C7 1.477(3), C1–C2
.464(3), C6–C1 1.399(3).
2
3
transition. The lack of significant bands in the visible
region of the spectrum results in pale colours for the neutral
complexes, and 3 in particular appears almost colourless,
suggesting possible applications for this type of complex
where strong NIR absorption is required without visible
absorption. The presence of this intense low energy band has
proven useful in the analysis of electrochemically-generated
films as discussed below.
thiophene units effectively prevents strong intermolecular
contacts within the stacks and the separation between the
˚
planes within the stacks is 4.31(16) A.
The structure of 3 consists of one independent molecular
unit (Fig. 2) with some rotational disorder present in three of
the four thiophene units leading to partial occupation of S5/C4
(
75/25 ratio), S6/C9 (85/15 ratio) and S8/C19 (55/45 ratio). The
central NiS moiety is essentially planar (rms deviation =
.0552). All of the thiophene units are again twisted out of
this plane [S5 ring = 29.93(6)u, S6 ring = 55.72(7)u, S7 ring =
0.42(8)u and S8 ring = 41.12(8)u] preventing the close
4 4
C
The electrical conductivity of compressed pellets of 3 was
measured using the four-probe method and gave a room
0
2
5
21
temperature value of 1 6 10 Scm . This is as expected for
an undoped molecular material based on a metal-bis-1,2-
dithiolene complex and the relatively low value is likely to also
reflect the lack of p-stacking or strong intermolecular
interactions observed in the X-ray structure.
5
molecular stacking typical for such dithiolene complexes.
Thus the interplanar separation between the NiC S cores of
4
4
˚
adjacent molecular units is 11.773(12) A and the molecules are
slipped along the short molecular axis away from a directly
overlapping stack arrangement.
Electrochemical and spectroelectrochemical studies
Many hundreds of structures have been reported for nickel-
bis-1,2-dithiolene complexes, however the large majority of
these involve dianionic, monoanionic or partially-charged
(
a) Electrochemical studies at low concentration. Cyclic
voltammetry studies were performed on [NBu ][3] (0.2 mmol
in CH CN), with the salt chosen due to the poor solubility of
4
1
5
species. This reflects both the extensive study of these
molecules in the field of conducting/magnetic materials and
also the greater difficulty of crystallising the neutral complexes
which normally have a lower solubility. Thus, only a few tens
of literature reports of the structures of neutral nickel-bis-1,2-
3
the neutral complex 3 in CH
3
CN. The results show two
reversible or quasi-reversible one electron redox processes at
2
1
E
= 20.79 V, DE = 0.09 V and E
= 0.07 V, DE = 0.05 V
1/2
1
/2
(
Fig. 3). By analogy with the electrochemical behaviour
1
5
dithiolenes are recorded. Many of these represent materials
prepared for the study of conducting properties and include
extended, delocalised frameworks attached to the dithiolene
reported for many dithiolene complexes these can be assigned
Fig. 2 X-Ray structure of [Ni(b-3ted)
2
] (3) showing the molecular
unit and numbering scheme and with disorder and hydrogen atoms
˚
omitted for clarity. Selected bond lengths/A: Ni1–S1 2.1275(8), Ni1–S2
2
1
1
.1156(8), Ni1 S3 2.1242(8), Ni1–S4 2.1285(8), S1–C1 1.716(2), S2–C6
.707(2), S3–C11 1.702(2), S4–C16 1.701(2), C1–C2 1.466(3), C6–C7
.474(3), C11–C12 1.470(3), C16–C17 1.468(3), C11–C16 1.401(3),
Fig. 3 Typical voltammograms of a solution of 3 (0.2 mmol) in
21
C1–C6 1.394(3).
3 6
CH CN–0.1 M TBAPF at u = 0.1 V s .
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J. Mater. Chem., 2008, 18, 475–483 | 477

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to the sequential interchange between the neutral, anionic and
1
3,24–28
dianionic states of the nickel dithiolene complex.
The
2
9
separation of the two redox potentials is comparable to
x2
[
Ni(S
2
C
H
2 2
)
2
]
and analogous systems where the metal-bis-
3
,2-dithiolene core is extended with –SR groups. It is
0
1
x2
however significantly larger than in [Ni(dmit)2]
which
indicates a poorer ability of 3 to delocalise the added
charges. This implies that little conjugation onto the
thiophene rings occurs and suggests that the twisted geometry
evidenced in the X-ray structure is also found in solution.
This is in keeping with the typical behaviour of 3-linked
3
1
thiophene species.
Cycling towards positive potential values reveals two
3
4
oxidation peaks at E
= 1.04 V and E
= 1.17 V. The
ox
ox
3
ox
irreversible peak at E
is probably due to the formation of
1
3,32
the cationic state in the dithiolene unit.
The increase in the
4
electrochemical potential up to E ox leads to the deposition of
a polymer on the electrode surface, and therefore this peak can
be assigned to the oxidation of the thiophene units to form
radical cations, which link to form a polymer. Polymerisation
of 3 was achieved by continued potential cycling between 0.3 V
and 1.25 V (Fig. S1, ESI{).
Fig. 4 SNIFTIRS spectra of poly-3 taken from 0.9 to 1.2 V.
Reference spectra collected at 0.3 V. Spectra are shifted for clarity.
spectra (Fig. 4) show an increase in the intensity of the IR
absorbance peaks as the polymer is oxidised from 0.9 to 1.2 V.
21 34,49
The increase in peak currents with potential cycling is
characteristic of electropolymerisation and continuous growth
Solvent bands are seen centred at 3200 and 2300 cm
.
2
The sharp peak at 847 cm , is due to the incorporation of
1
2
,33,34
of a redox active film on the electrode.
The electroactivity
5
hexafluorophosphate ions balancing the charge formed on
0
of this yellow poly-3 film was investigated using CV in
the poly-3 film by the oxidation process.
monomer-free electrolyte solution. This voltammetric response
The presence of the C
b
–H out-of-plane deformation at
(
Fig. S2, ESI{) reveals a redox couple at E1/2 = 0.06 V, DE =
.03 V. The small peak separation, 30 mV, and the variation of
current in direct proportion to scan rate are characteristic of a
2
95 cm suggests that, as previously concluded for the other
1
7
0
polythiophene derivatives investigated, the coupling of thio-
3
5,36
phene units occurs primarily through the a- (2,5) positions. In
surface bound species.
a
keeping with this, IR peaks assigned to the C –H bond are not
This confirms the immobilisation of an active redox couple
at the electrode surface. By comparison with the electro-
chemical behaviour of 3 in solution, this can be attributed to
the redox reaction involving the 21/0 states of the nickel
dithiolene centre, which indicates that the incorporation of
the intact metal complex into the polymer has occurred. The
cyclic voltammogram of poly-3 also shows a second peak at
Eox = 1.17 V that is likely to arise from unlinked thiophenes
in the film.
observed although the presence of the solvent band makes
assignment in this region less reliable. The spectroelectro-
chemical response of poly-3 upon oxidation does not display
the characteristic features usually found in the IR spectra of
oxidised polythiophenes [i.e., IRAVs (Infra-Red Active
Vibrations) and a broad absorbance band in the near infrared]
which further indicates that the degree of conjugation in poly-3
is rather limited. Polymerisation through the four peripheral
thiophene rings in 3 is possible, each one with two a-positions
available for bonding, which would potentially produce a
highly crosslinked polymer. However, the presence of the
sterically bulky nickel dithiolene side groups and the signifi-
cant twist of the thiophene centres would be expected to reduce
the thiophene polymer chain length and the overall conjuga-
tion length in the polymer. This would also explain the lack of
redox peaks attributable to polythiophene.
The spectroelectrochemical investigation of poly-3 was
carried out in a monomer-free electrolyte solution using
the same range of potentials as those applied during the
characterisation of the film by CV. In order to assign the IR
peaks present in the SNIFTIRS spectra of poly-3, the FTIR
spectrum of the neutral dithiolene 3 was initially measured
(
Fig. S3, ESI{). The FTIR spectrum of 3 displays several IR
absorption bands related to the thiophene and dithiolene units.
2
Peaks at 836 and 869 cm are assigned to C–S symmetric and
1
The symmetric CLC stretching vibration of the thiophene
1
1,37
21 34,51
asymmetric stretching vibrations of the thiophene rings.
The C –H vibration and C –H out-of-plane deformation are
observed at 3101 and 779 cm , respectively.
stretching vibrations of the dithiolene group can be found at
ring can be seen at 1511 cm
.
SNIFTIRS peaks at 1400,
2
1337, 1193 and 933 cm can be assigned to the stretching
1
a
b
2
1
38–40
The CLC
vibration of the CLC, C–S and C–S bonds present in the
4
4,47,48
dithiolene unit, respectively.
The vibrations of the Ni–C
2
1 41–44
21
1
456 and 1346 cm
21 45,46
.
The C–C ring vibration is seen at
21
bond occur below the spectral range (at 465 and 435 cm ),
and therefore cannot be seen in the SNIFTIRS spectra.
Analysis of the SNIFTIRS spectra of poly-3 upon reduction
(Fig. S4, ESI{) revealed a shift of the CLC dithiolene
1
378 cm
.
Smaller peaks at 1132 and 945 cm are due to
4
7,48
the CLS and C–S vibrations of the dithiolene unit.
The SNIFTIRS spectra of poly-3 between 4000 and
2
50 cm were collected at successively higher potentials and
1
21
7
vibrations to higher frequencies (from 1400 and 1337 cm
2
to 1442 and 1375 cm , respectively). At the same time, a
1
normalised to the reference spectrum taken at 0.3 V. The
4
78 | J. Mater. Chem., 2008, 18, 475–483
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decrease in the C–S stretching frequency of the dithiolene
electropolymerised film containing 3 with covalently linked
thiophene units. (iii) A copolymer of 3 with thiophene.
2
centre from 933 to 897 cm was also observed. In addition,
1
2
1
the C–S vibration at 1193 cm becomes less intense as the
polymer is reduced. Similar shifts have been reported for a
vast number of metal dithiolenes, and are an indication that
the dithiolene ligands assume a dithiolate character upon
(i) Molecular film of 3. In keeping with the study at lower
concentration, the electrochemistry of [NBu ][3] (1 mM
4
in CH CN) showed redox processes for the 22/21 couple
3
2
6,32,43,52
1
1
2
reduction.
As expected, upon reduction, there is a
(E
= 20.78 V) and the 21/0 couple (E
ox
= +0.12 V)
/2
progressive increase in the intensity of the positive band at
2
(Fig. 5). At fast sweep rates (Fig. S9, ESI{) an associated
2
47 cm , representing the expulsion of PF
1
2
8
6
anions from the
reduction peak for the latter (at E
red
= 0.05 V) was observed
poly-3 as the polymer is electrochemically reduced.
attributable to solution-based reduction. At slower sweep rates
For comparison with 3, we investigated the electrochemistry
of 2 under the same conditions. The CV response (Fig. S5,
ESI{) was found to be generally consistent with that previously
(Fig. 5), a relatively small associated reduction wave was seen
2
near E
red
= 20.04 V, which is due to the aggregation and
deposition of electrogenerated 3 on the electrode surface,
consistent with the low solubility of 3 in CH CN and the
1
2
reported, with similar voltammetry to 3, giving peaks at
3
1
E
E
E
2
= 20.62 V, DE = 0.05 V, E = 0.21 V, DE = 0.04 V,
1/2
1
/2
higher concentration conditions. Holding the potential at or
2
above +0.29 V (above E ox) resulted in the deposition of a
3
2
4
ox = 0.99 V, E ox = 1.23 V. As expected, the separation
1
1/2 2 E 1/2 y 0.8 V is similar to 3. Electropolymerisation of
was achieved by potential scanning between 0.3 and 1.3 V
green film. The oxidation current was independent of film
thickness and, on stirring, the reaction was mass transport
controlled, suggesting significant electrical conductivity in the
deposited film. This is surprising given the low conductivity
observed in a compressed pellet of 3 and the X-ray structure,
which indicated little intermolecular contact between the
molecules of 3. This suggests that the electrochemical
deposition method may result in a different, more conducting
structure within the molecular layer to that seen in crystals of
the material. This is further supported by the green colour of
the film which may suggest enhanced delocalisation compared
with the paler sample from the chemical synthesis. Greater
electronic delocalisation is also suggested by the smaller
(
Fig. S6, ESI{) and a gradual increase in the current intensity
upon consecutive cycles indicated a continuous growth of a
redox active film on the electrode surface. The response of this
red film, investigated by CV in a monomer-free solution
(
Fig. S7, ESI{) and by SNIFTIRS (Fig. S8, ESI{), resembles
that of many polythiophenes; in particular the CV is quite
1
2
different to that described by Kean and Pickup. In our work,
neither electrochemical nor spectroscopic evidence for signifi-
cant incorporation of the intact metal complex into the film
was found (Fig. S7, S8, ESI{). These results are consistent with
decomposition of the metal complex during coupling and
formation of a polythiophene-like film. The difference between
our results and those of Kean and Pickup may be due to the
difference in solvent as the prior literature work was carried
out in CH Cl .
1
2
separation of E and E compared with the solvated molecule.
Holding the potential in the region 20.12 to 20.52 V leads
2
to the quantitative reduction of neutral 3 to [3] and
2
2
dissolution of the green film. This reduction wave in Fig. 5 is
relatively small and the reduction current is insensitive to
Overall, we have shown distinctly different characteristics in
the polymerisation of 3 and 2 under the same experimental
conditions, which reflects the differing linkage postions of the
thiophene. In the case of 3, electropolymerisation can occur
without the need to incorporate the dithiolene units into the
polymer backbone since the thiophenes are unsubstituted in
both the 2 and 5 positions. In contrast, 2 has the 2-position
substituted which would require incorporation of the metal
dithiolene centre into the polymer backbone. Our observation
that electropolymerisation of 3 results in a film incorporating
the intact complex 3 under conditions where we were unable to
obtain a comparable film incorporating 2 is consistent with
results for the analogous 2-thienyl substituted TTF com-
potential, indicating
a kinetically controlled dissolution
process; this is sufficiently slow to enable the film-based 21
to 22 reduction to be seen in the second cycle in Fig. 5
(
the relatively sharp peak labelled b) for the deposited film at
0.75 V, superimposed upon the reduction of solution-based
2
2
[
3] near 20.8 V. A film was generated on the working
electrode by holding the potential at +0.29 V and reduction of
this film at 21.01 V in background electrolyte resulted in a
total reduction charge 2.1 times greater than the oxidation
1
0
pounds, where the suggestion was made that 3-thienyl
substitution in these systems would be more suitable for
polymerisation. These results suggest greater potential for 3 to
be used in electropolymerised films and this is explored further
in the section below.
(
b) Electrochemical studies of 3 at higher concentration:
towards the formation of hybrid films for devices. CV studies of
have been carried out at higher concentrations (1–10 mM in
3
CH CN) giving further insight into the film-forming capability
3
3
Fig. 5 Cyclic voltammogram of [NBu4][3] in CH CN–0.1 M
2
1
of this complex. Three types of film were electrochemically
formed and characterised. (i) An electrodeposited conducting
molecular film comprising the neutral complex 3. (ii) An
6
[TBA][PF ] at 50 mVs . The first cycle (a) is between 20.11 and
21.01 V and shows the reversible couple at 20.78 V. The second cycle
(b) is from 20.11 V to +0.29 V, then 21.01 V and back to 20.11 V.
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 475–483 | 479

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Fig. 7 Cyclic voltammogram of [NBu
4
][3] in CH
3
CN–0.1 M
2 2
Fig. 6 UV-Vis spectrum in CH Cl of films deposited in electro-
2
TBA][PF ] at 50 mVs . The first scan from 20.22 V to +1.39 V
1
[
6
chemical experiments compared with 3. All spectra have been
shows oxidation at +0.12 V then oxidation at +1.39 V. Subsequent
cycles between +1.39 V and 21.02 V showed none of the previously
observed redox processes except further oxidation at around +1.28 V.
normalised to show the same intensity of absorbance at around
920 nm. Films generated by electrochemical oxidation in CH
3
CN were
washed from the electrode using CH Cl after being rinsed in
2
2
electrolyte solution. (a) (Red line) film formed by holding potential
2
at +1.39 V for electrolyte solution containing [3] . (b) (Black line)
conductivity however and the lack of alteration of the
electronic spectrum suggest that the linked thiophenes cannot
complex 3 prepared by chemical synthesis. (c) (Blue line) film formed
2
by holding potential at +0.29 V for electrolyte solution containing [3] .
readily adopt
geometric constraints of the system require a twisted con-
figuration of these units. The complete solubility in CH Cl
a conjugated arrangement and that the
(
d) (Green line) copolymer film formed by holding potential at +1.79 V
2
for electrolyte solution containing [3] and thiophene.
2
2
also suggests limited chain length. The low conductivity and
the limited polythiophene chain length observed are consistent
with the previous results for electropolymerised 3 at lower
concentration (see above).
charge. This is consistent with reduction of [3] from 0 to 22
and confirms the assignment of the 2nd CV reduction peak at
2
0.75 V to the reduction of the 21 state to the 22 state. The
green film was then dissolved in CH Cl and comparison of
2
2
the UV-Vis spectrum with that for the neutral complex 3,
Fig. 6 shows identical spectroscopic features. Thus, a neutral
molecular film, showing significant conductivity and with
kinetically controlled rather than potential dependent re-
reduction, has been formed by oxidation of this nickel
dithiolene complex. To the best of our knowledge the
electrochemical formation and characterisation of such a film
has not previously been reported. Conducting materials
composed of neutral dithiolene molecules are currently of
(
iii) Copolymerisation of 3 and thiophene. In order to enhance
the possibility of hybrid polythiophene–metal dithiolene
complex films with significant polythiophene chain lengths
and enhanced conductivities over poly-3, copolymerisation
was carried out using 10 mM [NBu
4
][3] and 160 mM thiophene
in CH CN electrolyte solution. Using an RDE (at 2 Hz), the
3
potential was held at +1.79 V, sufficient to oxidise both
thiophene and 3 under mass transport control. As expected,
this produced a much-enhanced oxidation current compared
with the oxidation of either 160 mM thiophene or 10 mM
1
6–18
significant interest
and future work will explore the
formation, possible electrocrystallisation and crystal structure
of this film.
[
4
NBu ][3] alone. In contrast to control experiments with
thiophene the current fell gradually with time although less
steeply than for polymerisation of 3 alone. This indicates a
significant, although lower, conductivity for the copolymer
than for polythiophene. A gold-coloured film was formed
contrasting with the black colour observed at these concentra-
tions for a film of electrogenerated polythiophene. The film
was similar in appearance to that observed previously for
(
ii) Electropolymerisation of 3. Scanning the above solution to
a more positive potential revealed an irreversible oxidation
peak at +1.39 V. After this process, cycling back to negative
potentials revealed no significant electroactivity at any of the
potentials where features were previously observed (Fig. 7).
This indicates that the film obtained at +1.39 V is insulating
and prevents further redox processes occurring at the
electrode. A rotating disc electrode (RDE) experiment was
performed with the potential held at +1.39 V and a steadily
declining current was observed, confirming that this oxidation
2
electropolymerisation of [3] at +1.39 V but with a markedly
reduced solubility in CH Cl . The soluble portion was
2
2
analysed by UV-Vis spectroscopy (Fig. 6) and this again
confirmed the presence of the metal dithiolene species as the
characteristic low energy transition could be observed. A
significant difference however, was observed in the higher
energy region with peaks around 220 to 400 nm. Long chain
polythiophene is expected to show absorbance around 500 nm,
but shorter chains show absorption considerably shifted to
2 2
process leads to a film. The film was dissolved in CH Cl and
the UV-Vis spectrum recorded (Fig. 6), however this shows no
significant differences compared with 3. Evidently, some large
chemical change has taken place within the film upon
oxidation at +1.39 V as evidenced by both the passage of
current and the loss of conductivity of the layer.
2
the blue, these bands may therefore be attributed to short
oligomers of polymerised thiophenes, covalently attached
through the pendant thiophenes to 3. Longer oligomers may
also occur in this film but would be expected to be much less
As the electrochemical process does not involve alteration of
the Ni-bis-dithiolene core, it seems likely that covalent linkage
of the thiophene units is occurring at this potential. The low
soluble in CH
2 2
Cl . It is interesting that the portion of the film
4
80 | J. Mater. Chem., 2008, 18, 475–483
This journal is ß The Royal Society of Chemistry 2008

View Online
insoluble in CH
2
Cl
2
was also insoluble in DMF and DMSO
Subtractively normalised interfacial Fourier transform
infrared spectroscopy (SNIFTIRS) measurements were per-
formed using a completely evacuated Bruker IFS 113v
computer controlled FTIR spectrometer. The optics bench
was evacuated prior to any experiment to eliminate inter-
and retained the characteristic golden colour.
These results demonstrate the capability for covalent
incorporation of complex 3 into electropolymerised thiophene
films, which could lead to materials that combine the high
conductivity of polythiophene with the optical characteristics
of a metal bis-1,2-dithiolene complex.
ference from atmospheric CO
2 2
and H O. The spectrometer
operates with a silicon carbide source, which has a range of
21
000–100 cm , a MCT (mercury cadmium telluride) liquid
6
nitrogen cooled detector and a Ge/KBr beam splitter. A silicon
disc was used as the infrared transparent window, separating
the electrochemical cell from the evacuated spectrometer. The
instrument was set-up to allow external reflection by focusing
the IR beam onto the working electrode. The potential was
applied to the working electrode using a potentiostat, HI TEK
type DT2101, connected to a waveform generator (HI-TEK
instruments PP R1). The normalised difference spectra were
obtained by subtracting two reflectance spectra (S22S1)
Conclusions
We have prepared a novel nickel-bis-1,2-dithiolene complex
bearing thiophene groups attached at the 3-position and
structurally characterised it alongside the previously reported
2
-substituted analogue. It was shown that complex 3, with
3
-substituted thiophenes can be electrooxidised to form a
conducting molecular film (without covalent linking) which
seems to be structurally different from the chemically-
synthesised sample and is kinetically stabilised towards re-
reduction. Furthermore, 3 can also be electrochemically linked
at different concentrations to give an insulating film and can
be copolymerised with thiophene to give a film with properties
modified from those of either polythiophene alone or 3
alone. Comparison of these results with electropolymerisation
collected at different potentials (E
dividing by the reflectance spectrum obtained at E
positive and negative bands in these difference spectra indicate
decreased and increased absorbances, respectively at E with
2
and E
1
, respectively) and
1
(S1). The
2
respect to E . Throughout these studies 100 interferograms
1
were averaged for each spectrum.
1
2
studies on 2 under the same experiemental conditions
has shown that the position of linkage of the dithiolene to
the thiophene units is a crucial factor in determining the
electropolymerisation behaviour and the properties of the
resultant films.
[
4 2 4
NBu ][Ni(b-2ted) ] ([NBu ][2])
The 1,3-dithiole-2-one 1a (0.40 g, 1.4 mmol) was added under
N2 to a solution of Na (0.07 g, 3 mmol) in dry methanol
(
20 ml). An immediate colour change to dark yellow occurred
and stirring was continued for 1 h to ensure complete reaction.
TBA][Br] (0.91 g, 2.82 mmol) was added and allowed to
dissolve. NiCl ?6H O (0.16 g, 0.7 mmol) dissolved in 15 ml of
The observed X-ray structures of 2 and 3 showed twisting
of the thiophene units away from the plane of the dithiolene
core, suggesting that the incorporation of 2 and 3 into
conducting polymer chains may be unlikely to play a role in
enhancing conductivity, consistent with our observation of a
non-conducting film of poly-3. In this light, it seems that
our strategy of copolymerisation with thiophene may be
the most promising approach. We have demonstrated the
incorporation of the dithiolene complex 3 into a conducting
polymer while retaining significant conductivity. This has
established the capability of these dithiolene complexes to
lead to new hybrid materials. The synthesis developed for
these species should prove suitable for the preparation of a
wide variety of related bis-dithiolene or mixed-ligand com-
plexes and the capability to copolymerise with known
conducting polymers in a flexible ratio offers the prospect of
varying and tuning the properties of a wide range of new
hybrid materials.
[
2
2
dry methanol was added dropwise over 45 min to the stirred
solution giving a colour change to dark red–brown and a dark
brown precipitate. The flask was allowed to stand in a freezer
for one hour then the precipitate filtered off and washed with
ethanol and dried in air to give 460 mg of crude solid. The solid
was recrystallised from CH
Calc. for C36 Ni, C 53.4, H 1.7, N 5.9, S 31.7; found C
3.4, H 1.5, N 5.9, S 31.9 FABMS m/z = 1050 (MNBu ), 809
2 2
Cl –EtOH to give 345 mg, 47%.
H48NS
8
+
5
4
+
3
21
21
(
MH ). UV-Vis/NIR l
= 950 nm (e = 9 6 10 M cm
)
max
[NBu
4
][Ni(b-3ted)
2
] ([NBu
4
][3])
An analogous procedure was used to that of [NBu ][2] starting
4
with the 1,3-dithiole-2-one 1b. Recrystallisation was from
acetone–methanol. Yield = 62%
8
Calc. for C36H48NS Ni, C 53.4, H 1.7, N 5.9, S 31.7;
Experimental
found C 53.5, H 2.2, N 6.2, S 31.2. FABMS m/z =
+
+
1
4
050 (MNBu ), 809 (MH ). UV-Vis/NIR lmax = 950 nm
All solvents used were dried using standard techniques.
Chemicals used were purchased from Aldrich and used
without further purification. Compounds 1a and 1b were
3
21
21
(
e = 7.5 6 10 M cm
)
[Ni(b-2ted) ] (2)
2
9
–11
prepared as described previously.
Electrochemical and spectroelectrochemical experiments
were performed in dry CH CN using 0.1 M TBAPF
Complex [NBu
4
][2] (100 mg, 0.09 mmol) was dissolved in
(0.05 g,
dichloromethane (10 ml) and to this was added I
3
6
2
supporting electrolyte in a three-electrode cell using a platinum
disc working electrode, a large surface-area platinum counter
0.19 mmol) in dichloromethane (5 ml). The colour changed
immediately to dark green and a precipitate formed. Ethanol
was added and the solution was placed in the freezer. The dark
solid was filtered off, washed with ethanol and dried in air to
+
electrode, and a Ag/Ag reference electrode. All potential
values are quoted against SCE.
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 475–483 | 481

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6
7
Handbook of Organic Conductive Molecules and Polymers, John
Wiley and Sons, Chichester, 1997, vol. 1.
C. A. S. Hill, A. Charlton, A. E. Underhill, K. M. A. Malik,
M. B. Hursthouse, A. I. Karaulov, S. N. Oliver and S. V. Kershaw,
J. Chem. Soc., Dalton Trans., 1995, 587.
yield 50 mg, 80%. Calc. for C H S Ni C 42.3, H 2.1, N 0.0, S
2
0
12 8
4
5.2; found C 43.1, H 2.0, N 0.0, S 43.5. FABMS, m/z = 566.
UV-Vis/NIR lmax = 970 nm.
8
9
F. Wuerthener, Angew. Chem., Int. Ed., 2001, 40, 1037.
A. Charlton, A. E. Underhill, G. Williams, M. Kalaji, P. J. Murphy,
D. E. Hibbs, M. B. Hursthouse and K. M. A. Malik, Chem.
Commun., 1996, 2423.
[
Ni(b-3ted)
2
] (3)
The 1,3-dithiole-2-one 1b (0.16 g, 0.567 mmol) was stirred
in THF (10 ml, distilled) under N
2
and [TMA][OH]?5H
2
O
O
1
0 A. Charlton, A. E. Underhill, G. Williams, M. Kalaji, P. J. Murphy,
K. M. A. Malik and M. B. Hursthouse, J. Org. Chem., 1997, 62,
3098.
(
0.210 g, 1.16 mmol, TMA = tetramethylammonium) added in
0
2
.5 ml of distilled methanol. After 5 min stirring, NiCl ?6H
2
(
0.072 g, 3.0 mmol) in 0.5 ml distilled MeOH was added
11 A. Charlton, M. Kalaji, P. J. Murphy, S. Salmaso, A. E. Underhill,
G. Williams, M. B. Hursthouse and K. M. A. Malik, Synth. Met.,
resulting in an immediate intense red colour. Stirring was
continued overnight with the container left open to air. The
dark precipitate was filtered off and washed with methanol
1
998, 95, 75.
1
1
2 C. L. Kean and P. G. Pickup, Chem. Commun., 2001, 815.
3 C. L. Kean, D. O. Miller and P. G. Pickup, J. Mater. Chem., 2002,
12, 2949.
(
5 ml) giving 0.098 g of dark red solid. This was dissolved in
1
4 C. Pozo-Gonzalo, R. Berridge, P. J. Skabara, E. Cerrada,
M. Laguna, S. J. Coles and M. B. Hursthouse, Chem. Commun.,
acetone (5 ml) and reprecipitated by allowing ethanol (25 ml)
to mix slowly overnight. The resultant green–brown solid was
filtered off and gave 0.047 g (29% yield) of 3. Calc. for
2
5 D. A. Fletcher, R. F. McMeeking and D. Parkin, The United
002, 2408.
1
C
20
H
12
S
8
Ni, C 42.3; H 2.1; N 0.0; found C 42.5, H 2.3, N 0.0.
Kingdom chemical database service, J. Chem. Inf. Comput. Sci.,
996, 36, 746.
16 N. Le Navor, N. Robertson, T. Weyland, J. D. Kilburn,
A. E. Underhill, M. Webster, N. Svenstrup and J. Becher, Chem.
Commun., 1996, 1363.
1
FABMS, m/z = 566. UV-Vis/NIR lmax = 920 nm (e = 3 6
4
0 M cm ).
21
21
1
X-Ray crystallography
17 A. Kobayashi, H. Tanaka, M. Kumasaki, H. Torii, B. Narymbetov
and T. Adachi, J. Am. Chem. Soc., 1999, 121, 10763.
CCDC reference numbers 170534 and 171198. For crystal-
lographic data in CIF or other electronic format see DOI:
18 A. Charlton, C. A. S. Hill, A. E. Underhill, K. M. A. Malik,
M. B. Hursthouse, A. I. Karaulov and J. Moller, J. Mater. Chem.,
1
994, 4, 1861.
1
0.1039/b714372a
˚
8
Ni(b-2-ted) ] 2. C20H12NiS , M = 567.49, l = 0.71073 A,
1
9 J.-F. Bai, J.-L. Zuo, W.-L. Tan, W. Ji, Z. Shen, H.-K. Fun,
K. Chinnakali, I. A. Razak, X.-Z. You and C.-M. Che, J. Mater.
Chem., 1999, 9, 2419.
[
2
˚
/n, a = 12.112(2) A, b =
20 L. Valade, J.-P. Legros, M. Bousseau, P. Cassoux, M. Garbauskas
and L. V. Interrante, J. Chem. Soc., Dalton Trans., 1985, 783.
monoclinic, space group = P2
1
˚
˚
5
1
.4414(11) A, c = 17.026(3) A, b = 110.51(3)u, V =
2
1 H. Kim, A. Kobayashi, Y. Sasaki, R. Kato and H. Kobayashi,
Bull. Chem. Soc. Jpn., 1988, 61, 579.
3
050.9(4) A , Z = 2, m = 1.725 mm , T = 120(2) K, 7607
21
˚
data were collected of which 2391 were independent. The
22 M. Megnamisi-Belombe and B. Nuber, Bull. Chem. Soc. Jpn.,
989, 62, 4092.
2
2
2
1
structure was refined on F to give R1 = 0.0293 (F . 2sF )
and wR2 (all data) = 0.0708. Rotational disorder in thiophene
rings gives partial site occupancy of S2/C5 and S4/C10 in an
2
2
2
2
3 C. S. Winter, S. N. Oliver, R. J. Manning, J. D. Rush, C. A. S. Hill
and A. E. Underhill, J. Mater. Chem., 1992, 2, 443.
4 Y. Ji, J.-L. Zuo, L. Chen, Y.-Q. Tian, Y. Song, Y.-Z. Li and
X.-Z. You, J. Phys. Chem. Solids, 2005, 66, 207.
5 G. A. Bowmaker, P. D. W. Boyd and G. K. Campbell, Inorg.
Chem., 1982, 22, 1208.
6 B. S. Lim, D. V. Fomitchev and R. H. Holm, Inorg. Chem., 2001,
8
5/15 ratio.
˚
2 8
Ni(b-3-ted) ] 3. C20H12NiS , M = 567.49, l = 0.71073 A,
[
˚
triclinic, space group = P-1, a = 9.5217(19) A, b = 9.810(2) A,
˚
4
0, 4257.
˚
c = 11.985(2) A, a = 90.14(3)u, b = 100.37(3)u, c = 93.73(3)u,
27 F. Bigoli, P. Cassoux, P. Deplano, M. L. Mercuri, M. A.
Pellinghelli, G. Pintus, A. Serpe and E. F. Troguc, J. Chem.
Soc., Dalton Trans., 2000, 4639.
3
V = 1098.8(4) A , Z = 2, m = 1.649 mm , T = 150(2) K,
21
˚
1
3 623 data were collected of which 5027 were independent.
2
2
8 W. E. Geiger, Inorg. Chem., 2001, 12, 136.
9 J. A. McCleverty, Prog. Inorg. Chem., 1968, 10, 49.
2
2
2
The structure was refined on F to give R1 = 0.0356 (F . 2sF )
and wR2 (all data) = 0.0977. Rotational disorder in thiophene
rings gives partial site occupancy of S5/C4 (75/25 ratio), S6/C9
30 M. Takahashi, N. Robertson, A. Kobayashi, H. Becker,
R. H. Fiend and A. E. Underhill, J. Mater. Chem., 1998, 8, 319.
3
3
1 S.-A. Chen and J.-M. Ni, Macromolecules, 1992, 25, 6081.
2 F. Bigoli, P. Deplano, F. A. Devillanova, J. R. Ferraro, V. Lippolis,
P. J. Lukes, M. L. Mercuri, M. A. Pellinghelli, E. F. Trogu and
J. M. Williams, Inorg. Chem., 1997, 36, 1218.
(
85/15 ratio) and S8/C19 (55/45 ratio).
Acknowledgements
33 C. Jerome, C. Maertens, M. Mertens, R. Jerome, C. Quattrocchi,
R. Lazzaroni and J. L. Bredas, Synth. Met., 1996, 83, 103.
34 C. Kvarnstrom, H. Neugebauer, S. Blomquist, H. J. Ahonen,
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35 D. Pletcher, Instrumental Methods in Electrochemistry, Halsted
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The authors would like to thank the EPSRC for provision of
crystallographic facilities. We wish to acknowledge the use of
the EPSRC’s Chemical Database Service at Daresbury.
3
3
6 W. Kwan, L. Atanasoska and L. Miller, Langmuir, 1991, 7, 1419.
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