Complexes based on asymmetrically substituted pyridine-dithiolene ligands [M(4-pedt)2] (M = Au, Cu, Ni; 4-pedt = 1-(pyridin-4-yl)-ethylene-1,2-dithiolate): Synthesis, structure and physical properties

Article abstract of DOI:10.1016/j.poly.2009.01.015

The monoanionic Ni, Au and Cu bisdithiolene complexes based on the 1-(pyridin-4-yl)-ethylene-1,2-dithiolate were prepared and isolated as [PPh₄]⁺, [n-Bu₄N]⁺ and [Na(18C6)]⁺ salts which were characteri

Full text of DOI:10.1016/j.poly.2009.01.015

Polyhedron 28 (2009) 1069–1078
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Complexes based on asymmetrically substituted pyridine–dithiolene ligands
[M(4-pedt)2] (M = Au, Cu, Ni; 4-pedt = 1-(pyridin-4-yl)-ethylene-1,2-dithiolate):
Synthesis, structure and physical properties
Sandra Rabaça a, Ana C. Cerdeira a, Ana I.S. Neves a, Sandra I.G. Dias a, Cécile Mézière b, Isabel C. Santos a,
Laura C.J. Pereira a, Marc Fourmigué c, Rui T. Henriques d, Manuel Almeida a,
*
a Departamento de Química, Instituto Tecnológico e Nuclear/CFMCUL, Estrada Nacional 10, P-2686-953 Sacavém, Portugal
b Laboratoire CIMMA, UMR CNRS 6501, Université d’Angers, Bd. Lavoisier, F-49045 Angers, France
c Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes I, Campus de Beaulieu, F-35042 Rennes, France
d Instituto de Telecomunicações, Pólo de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The monoanionic Ni, Au and Cu bisdithiolene complexes based on the 1-(pyridin-4-yl)-ethylene-1,2-dithi-
olate were prepared and isolated as [PPh4]+, [n-Bu4N]+ and [Na(18C6)]+ salts which were characterised by
single-crystal X-ray diffraction, cyclic voltammetry, EPR and magnetic susceptibility measurements. All
the complexes adopt a square planar coordination geometry with a trans arrangement of the substituent
pyridine rings whose planes are rotated from 13° to 33° relatively to the central MS4 core. The C–C and C–S
bond lengths are consistent with ene-1,2-dithiolate form of the ligand and MIII oxidation state. The Ni
complexes are paramagnetic with an effective magnetic moment of ꢀ1.7 BM.
Received 23 December 2008
Accepted 21 January 2009
Available online 26 February 2009
Keywords:
Gold
Nickel
Ó 2009 Elsevier Ltd. All rights reserved.
Copper
Dithiolene
Crystal structure
Magnetic behaviour
1. Introduction
cused our interest on bis(dithiolene) complexes based on thio-azo
ligands bearing heterocyclic groups and recently described the first
Transition metal bis(dithiolene) complexes have attracted re-
search attention for more than 30 years and, due to their rich redox
behaviour and favourable solid state interactions in their square-
planar geometry, they have been widely used in the preparation
of conducting and magnetic materials [1]. These complexes can
be seen as inorganic analogues of the corresponding TTF type do-
nors where the transition metal replaces the central C@C bond.
The TTF type donors have been at the basis of the large majority
of all known molecular conductors and superconductors, and these
inorganic analogues offer the additional advantage that depending
on the oxidation state and on the transition metal they can be either
diamagnetic or paramagnetic with different magnetic moments.
Recently there has been an increasing focus of attention on the
preparation of donors which, in addition to their electroactive role,
can bind directly to transition metals and some TTF derivatives
substituted with groups containing N coordinating atoms [2] and
their complexes have been recently described [3]. The correspond-
ing inorganic analogues, bis(dithiolene) complexes with ligands
bearing nitrogen coordinating groups have been however a lot less
explored. Having in mind such type of complexes, we recently fo-
neutral Ni bis(dithiolene) complex (formally NiIV) with alkylpyridyl
substituted ligands, Ni(dpesdt)2 (dpesdt = bis(2-pyridyletthylsulfa-
nyl)-,2-dithiolate) [4].
Asymmetrically substituted dithiolenes have been significantly
less studied in spite of providing also interesting properties [5]
namely for magnetic materials [6] and optoelectronics [7]. One
example of bis(dithiolene) complexes bearing heterocyclic groups
are the M(n-pedt)2 complexes (n = 2, 3 or 4), based on the asym-
metrically substituted ligand n-pedt = S–C(H)@C(R)–S; R = 4-pyri-
dyl. Ligand 4-pedt was previously presented in a preliminary
short report [8]. More recently monoanionic gold bisdithiolene
complexes with R = n-pyridyl ligands were also described [9].
In this paper, we describe the preparation and characterisation
of monoanionic [M(4-pedt)2]ꢁ complexes of nickel, copper and
gold obtained as tetrabutylammonium, tetraphenylphosphonium
or sodium 18-crown-6 ether salts.
2. Experimental
2.1. Materials and methods
All manipulations were carried out under anaerobic conditions,
unless stated otherwise. Whenever required, the solvents were
* Corresponding author. Tel.: +351 21 9946171; fax: +351 21 9941455.
E-mail address: malmeida@itn.pt (M. Almeida).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.01.015

1070
S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
dried according to the standard literature procedures [10], freshly
distilled, and saturated with nitrogen prior to use. All starting re-
agents were purchased from commercial sources and used without
further purification or synthesised from published methods. The 4-
(4-pyridyl)-1,3-dithiol-2-one was synthesised according to litera-
ture reports [8,11].
Elemental analyses of the compounds isolated in theses studies
were performed at ITN analytical services using an EA 110 CE
Instruments automatic analyser. Melting points were performed
on a Stuart Scientific SMP2.
2.2.4. Tetrabutylammonium salt of bis[1-(4-pyridyl)-1,2-
ethylenedithiolato]copper, n-Bu4N[Cu(4-pedt)2] (4)
Following the same general procedure used for n-Bu4N[Ni(4-
pedt)2], the oxo compound was added (0.10 g; 0.51 mmol) to a
freshly made sodium methoxide (0.023 g; 1.02 mmol, 3 mL) stirred
until total dissolution, followed by drop wise additio of a solution
of CuCl2 ꢂ 2H2O (0.043 g; 0.25 mmol) in methanol (2 mL). The solu-
tion was stirred for 1 h and afterwards filtered onto an n-Bu4NBr
methanol solution (0.165 g, 0.51 mmol, 3 mL). No precipitate for-
mation was observed. A few H2O drops were slowly added until
start of incipient precipitation and the mixture was left overnight
at 4 °C. The oil formed was separated from the mixture by centri-
fugation, dried under vacuum (P2O5) and recrystallised from
dichloromethane/hexane to be recovered as a microcrystalline pre-
cipitate. 41% yield (0.065 g, 0.10 mmol); m.p. 160–162 °C.
C30H46N3CuS4 (Mol. Wt.: 640.51) Elemental Anal. Calc.: C, 56.26;
H, 7.24; N, 6.56; S, 20.02. Found: C, 55.83; H, 7.01; N, 6.12; S,
19.84%.
2.2. Synthesis
2.2.1. Tetraphenylphosphonium salt of bis[1-(4-pyridin-4-yl)-1,2-
ethylenedithiolato]nickel, PPh4[Ni(4-pedt)2] (1)
The 4-(4-pyridyl)-1,3-dithiol-2-one (0.50 g; 2.56 mmol) was
added to a sodium methoxide solution (0.147 g; 6.40 mmol;
13 mL) After 45 min of stirring, Ni(NO3) ꢂ 6H2O (0.37 g, 1.28 mmol)
was added. One hour later the red solution was filtered to an aque-
ous PPh4Cl solution (0.96 g, 2.56 mmol, 100 mL). A black precipi-
tate appeared immediately from the brown solution. The solid
was recovered by filtration, washed with water and recrystallised
from acetonitrile to afford black crystals of the nickel salt suitable
for X-ray diffraction. 50% yield (0.47 g, 0.64 mmol); m.p. 217–
218 °C. C38H30NiN2PS4 (Mol. Wt.: 732.59). Elemental Anal. Calc.:
C, 62.30; H, 4.13; N, 3.82; S, 17.51. Found: C, 62.09; H, 4.06; N,
3.77; S, 17.31%.
2.2.5. Tetrabutylammonium salt of bis[1-(4-pyridyl)-1,2-
ethylenedithiolato]gold n-Bu4N[Au(4-pedt)2] (5)
Following the same general procedure used for n-Bu4N[Ni(4-
pedt)2], the oxo compound (0.10 g; 0.51 mmol) was dissolved in
a freshly prepared methoxide solution (0.024 mg; 1.02 mmol,
3 mL). The total dissolution of the oxo compound was followed
by drop wise addition of a KAuCl4 ꢂ 4H2O methanol solution
(0.097 g; 0.25 mmol, 2 mL). The brown solution was filtered into
an n-Bu4NBr methanol solution (0.165 g, 0.51 mmol, 3 mL). After
the addition of a few milliliters of water, the reaction vial was kept
for several hours at 4 °C. The brown oil formed was recovered by
centrifugation, dried under vacuum (P2O5) and recrystallised from
dicloromethane/hexane to be recovered as a microcrystalline pre-
cipitate. 60% yield (0.115 g, 0.15 mmol); m.p. 127–129 °C.
C30H46N3AuS4 (Mol. Wt.: 773.93). Elemental Anal. Calc.: C, 46.56;
H, 5.99; N, 5.43; S, 16.57. Found: C, 46.21; H, 5.52; N, 5.25; S,
16.39%.
2.2.2. Tetraphenylphosphonium salt of bis[1-(4-pyridin-4-yl)-1,2-
ethylenedithiolato]copper PPh4[Cu(4-pedt)2] (2)
The oxo compound (0.10 g; 0.51 mmol) was added to a freshly
made sodium methoxide solution (0.023 g; 1.02 mmol, 3 mL) and
stirred at room temperature, until total dissolution, followed by
drop wise addition of
a
solution of CuCl2 ꢂ 2H2O (0.043 g;
0.25 mmol) in methanol (2 mL). The solution was stirred for 1 h
and filtered to a PPh4Br methanol solution (0.214 g, 0.51 mmol,
3 mL). No precipitate formation was observed. The reaction vial
was opened to the air, a few H2O drops were slowly added until
start of incipient precipitation and the mixture was left overnight
at 4 °C. The oil that formed was separated from the mixture by cen-
trifugation, dried under vacuum (P2O5) and recrystallised from
dichloromethane/hexane to be recovered as a microcrystalline pre-
cipitate. 48% yield (0.089 g, 0.12 mmol); m.p. 162–163 °C.
C38H30N2CuS4 (Mol. Wt.:737.44). Elemental Anal. Calc.: C, 61.89;
H, 4.10; N, 3.80; S, 17.39. Found: C, 61.72; H, 4.92; N, 3.84; S,
18.03%.
2.2.6. Crown ether [Na(18C6)] salt of bis[1-(4-pyridyl)-1,2-
ethylenedithiolato]nickel [Na(18C6)][Ni(4-pedt)2] (6)
The oxo compound (0.20 g; 1.03 mmol) was added to a freshly
prepared methoxide solution (0.047 g, 2.05 mmol, 5 mL). After stir-
ring until total dissolution, NiCl2 ꢂ 6H2O methanol solution
(0.122 g; 0.51 mmol, 5 mL) was added. The red solution was stirred
for 1 h and then filtered into an 18-crown-6 methanol solution
(0.271 g, 1.03 mmol, 1 mL). The mixture was left at 4 °C for two
weeks. A mixture of amorphous brown and darker polycrystalline
precipitate was recovered by vacuum filtration. The mixture was
dissolved in dichloromethane and an insoluble precipitate was re-
moved. By slow diffusion of hexane into the dichloromethane solu-
tion, it was possible to recover compound 6 as a dark brown
polycrystalline material. 58% yield (0.197 g; 0.29 mol) m.p. 260–
261 °C. C26H34N2NaNiO6S4 (Mol. Wt.: 680.5). Elemental Anal. Calc.:
C, 45.89; H, 5.04; N, 4.12; S, 18.85. Found: C, 45.43; H, 5.35; N,
4.12; S, 18.30%.
2.2.3. Tetrabutylammonium salt of bis[1-(4-pyridyl)-1,2-
ethylenedithiolato]nickel n-Bu4N[Ni(4-pedt)2] (3)
The procedure was similar to the described above. The oxo com-
pound was added (0.10 g; 0.51 mmol) to a freshly made sodium
methoxide solution (0.023 g; 1.02 mmol, 3 mL) and stirred at room
temperature, until total dissolution, followed by dropwise addition
of a solution of NiCl2 ꢂ 6H2O (0.061 g; 0.25 mmol) in methanol
(2 mL). The red solution was stirred for 1 h and then filtered into
n-Bu4NBr methanol solution (0.165 g, 0.51 mmol, 3 mL). No precip-
itate formation was observed. The reaction vial was opened to the
air, a few H2O drops were slowly added until start of incipient pre-
cipitation and the mixture was left overnight at 4 °C. The reddish
oil formed was separated from the mixture by centrifugation, dried
under vacuum (P2O5) and recrystallised from acetone to be recov-
2.2.7. Crown ether [Na(18C6)] salt of bis[1-(4-pyridyl)-1,2-
ethylenedithiolato] gold [Na(18C6)][Au(4-pedt)2] (7)
The oxo compound (0.10 g, 0.51 mmol) was added to a freshly
prepared sodium methoxide solution (0.0235 g, 1.02 mmol, 5 ml)
and stirred for 45 min. The dark red solution was removed from
the inert atmosphere and solid KAuCl4 ꢂ 4H2O (0.097 g, 0.25 mmol)
was added. The mixture was stirred for 2 h, filtered over Celite to a
solution of 18C6 (0.0677 g, 0.25 mmol) in methanol (1 ml), layered
with pentane and left at 4 °C. A few days later, an amorphous
brown precipitate was recovered by vacuum filtration. From an at-
tempt to recrystallise from dichloromethane/hexane, an oil was
ered as
a microcrystalline precipitate. 65% yield (0.105 g,
0.16 mmol); m.p. 133–134 °C. C30H46N3NiS4 (Mol. Wt.: 635.66)
Elemental Anal. Calc.: C, 56.68; H, 7.29; N, 6.61; S, 20.18. Found:
C, 56.01; H, 7.89; N, 6.12; S, 19.75%.

S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
1071
SHELXL97 [15] program using the WINGX software package [16]. Non-
hydrogen atoms were refined with anisotropic thermal parameters
whereas H-atoms were placed in idealised positions and allowed
to refine riding on the parent C atom. In 3 and 5 one of the butyl
groups on the cation show disorder that was modelled over two
positions with the following occupation factors: 52%:48% and
55%:45%, respectively. The final difference Fourier map revealed
the existence of residual electron densities of: 2.4/ꢁ0.89 e Aꢁ3 for
compound 5, which are located near the Au atom. Molecular
graphics were prepared using ORTEP3 [17] and MERCURY 1.4.2 [18]. A
summary of the crystal data, structure solution and refinement is
listed in Table 1.
EPR spectra were obtained at room temperature on powders
using a conventional X-band spectrometer (Bruker ESP 300E)
equipped with a microwave bridge ER041XK, a rectangular cavity
operating in T102 mode and a field controller ER 032M system.
The modulation amplitude was kept well below the linewidth
and the microwave power well below saturation.
obtained which was dried and characterised. Yield 0.088 g,
0.11 mmol (44%); m.p. 145–146 °C. C26H34O6N2S4AuNa (Mol. Wt.:
818.08). Elemental Anal. Calc.: C, 38.14; H, 4.19; N, 3.42; S, 15.63.
Found: C, 38.38; H, 4.91; N, 3.50; S, 15.28%.
2.3. Physical characterisation
Cyclic voltammetry data were obtained using a BAS C3 Cell
Stand. The measurements were performed at room temperature
in CH2Cl2 solutions, containing [n-Bu4]PF6 (0.1 M) as supporting
electrolyte, with a scan rate of 100 mV/s using platinum wire
working and counter electrodes and a Ag/AgCl reference electrode.
+
+
*
*
The [(Me5C5)2Fe] /[Me5C5)2Fe] ([Fc ] /[Fc ]) couple was used as the
internal reference and the potentials of the redox couples observed
were referenced to that.
Single crystals suitable for X-ray crystallographic analyses were
obtained by recrystallisation in acetonitrile, for compound 1, and
by slow diffusion of hexane into a saturated solution of dichloro-
methane, for compounds 2, 3, 5 and 6. In compound 6 two water
molecules were incorporated in the crystal structure to yield com-
pound 6 ꢂ 2H2O, due to the presence of air humidity.
Static magnetic susceptibility measurements in the range 2–
300 K were performed using a superconducting longitudinal Fara-
day system (Oxford Instruments) under a magnetic field of 5 T and
field gradient of 1 T/m. A polycrystalline sample (ꢀ20 mg) was
placed inside a previous calibrated thin-wall Teflon bucket. The
force was measured with a microbalance (Sartorius S3D-V), apply-
ing forward and reverse field gradients. Magnetisation data were
corrected for contributions due to the core diamagnetism, esti-
The data for compounds 1, 2, 3, 5 and 6 ꢂ 2H2O were collected on
a
Bruker AXS APEX CCD area detector X-ray diffractometer
equipped with an Oxford Cryosystems low-temperature device at
150 K in the and u scans mode. A semi empirical absorption cor-
x
rection was carried out using SADABS [12]. Data collection, cell
refinement and data reduction were done with the SMART and SAINT
programs [13]. The structures were solved by direct methods using
SIR97 [14] and refined by fullmatrix least-squares methods with the
mated from tabulated Pascal constants as ꢁ385 ꢃ 10ꢁ6
,
ꢁ379 ꢃ 10ꢁ6 and ꢁ348 ꢃ 10ꢁ6 emu/mol for 1,
3
and 6,
respectively.
Table 1
Crystallographic data and refinement parameters for complexes Ph4P[Ni(4-pedt)2] (1), Ph4P[Cu(4-pedt)2] (2), n-Bu4N[Ni(4-pedt)2] (3), n-Bu4N[Au(4-pedt)2] (5) and
[Na(18C6)][Ni(4-pedt)2] ꢂ 2H2O (6 ꢂ 2H2O).
Compound
1
2
3
5
6 ꢂ 2H2O
Formula
C38H30N2NiPS4
732.58
0.71073
C38H30CuN2PS4
737.39
0.71073
C30H46N3NiS4
635.65
C30H46AuN3S4
773.90
C26H38N2NaNiO8S4
712.49
0.71073
Formula weight (g molꢁ1
Wavelength (Å)
Crystal system,
space group
a (Å)
)
0.71073
triclinic, P1
0.71073
triclinic, P1
ꢀ
ꢀ
monoclinic, C2/c
monoclinic, C2/c
monoclinic, P21/n
26.8750(5)
17.3528(3)
18.1609(3)
90
127.5650(10)
90
6713.4(2), 8
1.450, 0.906
3032
26.9179(8)
17.3541(6)
18.1969(5)
90
127.3820(10)
90
6754.5(4), 8
1.450, 0.973
3040
9.0033(2)
8.6211(1)
11.7067(14)
9.7243(12)
14.3179(18)
90
96.103(2)
90
1620.7(3), 2
1.468, 0.918
750
b (Å)
c (Å)
11.2258(2)
16.9070(3)
107.7870(10)
90.3960(10)
98.3450(10)
1607.48(5), 2
1.313, 0.887
678
11.1196(2)
17.6962(3)
74.646(1)
88.829(1)
80.336(1)
1612.14(4), 2
1.594, 4.846
780
a
(°)
b (°)
c
(°)
V (Å3), Z
qcalc (Mg/m3),
F(000)
l )
(mmꢁ1
Crystal size (mm)
h range (°)
Collected hkl
0.4 ꢃ 0.3 ꢃ 0.08
2.83–25.03
ꢁ31 ꢄ h ꢄ 31
ꢁ20 ꢄ k ꢄ 20
ꢁ21 ꢄ l ꢄ 21
41666
0.30 ꢃ 0.20 ꢃ 0.16
2.74–25.68
ꢁ32 ꢄ h ꢄ 32
ꢁ21 ꢄ k ꢄ 21
ꢁ20 ꢄ l ꢄ 22
23618
0.28 ꢃ 0.24 ꢃ 0.20
3.21–25.35
ꢁ10 ꢄ h ꢄ 10
ꢁ13 ꢄ k ꢄ 13
ꢁ20 ꢄ l ꢄ 20
13680
0.38 ꢃ 0.30 ꢃ 0.28
2.64–25.68
ꢁ10 ꢄ h ꢄ 10
ꢁ13 ꢄ k ꢄ 13
ꢁ21 ꢄ l ꢄ 21
17908
0.20 ꢃ 0.12 ꢃ 0.10
2.73–25.68
ꢁ14 ꢄ h ꢄ 12
ꢁ11 ꢄ k ꢄ 11
ꢁ17 ꢄ l ꢄ 17
7273
Reflections collected
Independent
5910 [0.0424]
6409 [0.0656]
5821 [0.0422]
6122 [0.0306]
3051 [0.0414]
reflections [Rint
]
Completeness to h (%)
25.03 (99.7)
25.68 (99.7)
25.35 (98.9)
25.68 (99.7)
25.68 (99.3)
Absorption correction
semi-empirical from
equivalents
semi-empirical from
equivalents
semi-empirical from
equivalents
semi-empirical from
equivalents
semi-empirical from
equivalents
Max. and min.
transmission
0.8052 and 0.7133
0.8599 and 0.7590
0.8425 and 0.7893
0.3440 and 0.2603
0.9139 and 0.8378
Refinement method
full-matrix
full-matrix
full-matrix
full-matrix
full-matrix
least-squares on F2
5910/0/418
least-squares on F2
6409/0/418
least-squares on F2
5821/6/370
least-squares on F2
6122/2/370
least-squares on F2
3051/2/201
Data/restraints/parameters
Goodness-of-fit on F2
1.069
1.049
1.079
1.071
1.035
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0276, wR2 = 0.0711
R1 = 0.0341, wR2 = 0.0740
0.416 and ꢁ0.355
R1 = 0.0396, wR2 = 0.0868
R1 = 0.0644, wR2 = 0.0945
0.422 and ꢁ0.473
R1 = 0.0366, wR2 = 0.0899
R1 = 0.0482, wR2 = 0.0947
0.556 and ꢁ0.280
R1 = 0.0254, wR2 = 0.0590
R1 = 0.0401, wR2 = 0.0630
2.348 and ꢁ0.895
R1 = 0.0376, wR2 = 0.0791
R1 = 0.0659, wR2 = 0.0853
0.392 and ꢁ0.502
Largest difference in
peak and hole (e Åꢁ3
)

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S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
3. Results and discussion
3.1. Synthesis
The synthesis of the nickel, copper and gold complexes 1–7
were performed following standard procedures for complexes with
this type of dithiolene ligands (Scheme 1). The dithiolate ligand (L)
was obtained from the corresponding oxo compound by hydrolytic
cleavage with sodium methoxide and without intermediate isola-
tion it was reacted with the selected metal salt to give an anionic
complex which is precipitated as a salt in the presence of the se-
lected cation. The presence of water was necessary to precipitate
the bisdithiolene complex from the solution. In case of 1, a water
solution of the cation was added and the bisdithiolene complex
precipitated immediately as a black precipitate. In the case of com-
pounds 2, 3, 4 and 5 a few drops of water were added at the end of
the reaction until the start of incipient precipitation and after
keeping the reaction mixture at 4 °C it was possible to collect the
product as fine precipitate which tends to form an oil. Compounds
6 and 7 were obtained without adding water after keeping the
solution a few days at 4 °C until complexes precipitated. These
complexes were obtained after recrystallisation from acetonitrile,
acetone or dichloromethane/hexane in overall yields between
41% and 65%.
Complexes 1–7 were characterised by elemental analyses and
cyclic voltammetry studies. The elemental analyses after the
recrystallisations are consistent with a 1:1 cation:anion stoichiom-
etry, in agreement with the X-ray crystal structure determination
of the diffracting single crystals. While this is expected for
M = Au, for Cu and Ni even though starting from a NiIICl2 or CuIICl2
salt, the dianionic bisdithiolene complexes could not be obtained,
even under nitrogen atmosphere and the monoanionic dithiolene
complexes were obtained as evidenced by the X-ray structure
determination or by the electronic paramagnetic resonance (EPR)
spectroscopy. Whereas complexes 1, 2, 3, 5 and 6 ꢂ 2H2O were
unambiguously characterised by single X-ray structure determina-
tion, compounds 4, 6 and 7 could not be studied by single X-ray
diffraction due to their poor crystal quality.
Fig. 1. ORTEP views of the cation and the two independent anion units of Ph4P[Ni(4-
pedt)2], (1), showing the atomic numbering scheme. Displacement ellipsoids are
drawn at the 30% probability level.
H atoms have been omitted for clarity.
[Symmetry codes: (a) 2 ꢁ x, 1 ꢁ y, 1 ꢁ z; (b) 1.5 ꢁ x, 1.5 ꢁ y, 1 ꢁ z].
metric unit comprises two independent half-anion units. Both the
metal atoms located at inversion centres and with one cation unit
located at a general position. The angle between the anionic units
mean planes is between 88.98° and 89.81° for 3 and 5, respectively.
The compound 6 ꢂ 2H2O crystallises in the monoclinic system,
space group P21/n, Z = 2. The asymmetric unit comprises one inde-
pendent half-anion unit and one independent half-cation unit lo-
cated at inversion centres.
3.2. Crystal structure
In all complexes, the centrosymmetric anion [M(4-pedt)2]ꢁ
(M = Ni, Cu or Au) adopts a square-planar coordination geometry
and a trans conformation. The central core of the anions, consti-
tuted by the metal atom, the four sulfur atoms and the four carbon
atoms, is essentially planar (rms deviations of fitted atoms are
0.0091 and 0.0057 Å for compound 1; 0.0182 and 0.0237 Å for
compound 2; 0.0152 and 0.0268 Å for compound 3; 0.0537 and
0.0250 Å for compound 5; 0.0125 Å for compound 6 ꢂ 2H2O).
Important bond lengths within the MS2C2 metallocycle are col-
lected in Table 2. The average M–S bond lengths are in good agree-
ment with values found in other Ni, Cu and Au monoanionic
bisdithiolene complexes, respectively [1c,19–21]. While the cen-
tral core of the complexes is planar, the two pyridine rings are ro-
tated out of the central core plane with dihedral angles between
Compounds 1, 2, 3, 5 and 6 ꢂ 2H2O have been characterised by
single crystal X-ray diffraction technique. The ORTEP views of these
compounds are shown in Figs. 1–5. Selected bond angles and dis-
tances are given in Table 2.
The monoanionic Ph4P salts 1 and 2 are isostructural and crys-
tallise in the monoclinic system, space group C2/c, Z = 8, the asym-
metric unit comprising one cation [PPh4]+ located in a general
position and two independent half-anions [M(4-pedt)2]ꢁ (M = Ni
and Cu for compounds 1 and 2, respectively) both with the metals
located at an inversion centre. The angle between the two [M(4-
pedt)2]ꢁ units mean planes is about 49.47° and 49.52° for com-
pounds 1 and 2, respectively.
The monoanionic n-Bu4N salts 3 and 5 are isostructural and
ꢀ
crystallise in the triclinic system, P1 space group, Z = 2. The asym-
Scheme 1.

S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
1073
Fig. 2. ORTEP views of the cation and the two independent anion units of Ph4P[Cu(4-pedt)2], (2), showing the atomic numbering scheme. Displacement ellipsoids are drawn at
the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (a) 1 ꢁ x, 1 ꢁ y, z; (b) 0.5 ꢁ x, 1.5 ꢁ y, ꢁz].
Fig. 3. ORTEP views of the cation and the two independent anion units of n-Bu4N[Ni(4-pedt)2], (3), showing the atomic numbering scheme. Displacement ellipsoids are drawn
at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (a) ꢁx, ꢁy, ꢁz; (b) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z].
Fig. 4. ORTEP views of the cation and the two independent anion units of n-Bu4N[Au(4-pedt)2], (5), showing the atomic numbering scheme. Displacement ellipsoids are drawn
at the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (a) ꢁx, ꢁy, ꢁz; (b) 1 ꢁ x, 1 ꢁ y, 1 ꢁ z].

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S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
Fig. 5. ORTEP views of the cation and anion units of [Na(18C6)][Ni(4-pedt)2] ꢂ 2H2O, (6 ꢂ 2H2O), showing the atomic numbering scheme. Displacement ellipsoids are drawn at
the 30% probability level. H atoms have been omitted for clarity. [Symmetry codes: (a) ꢁx, ꢁy, ꢁz; (b) 2 ꢁ x, 1 ꢁ y, 1 ꢁ z].
Table 2
Important bond lengths (in Å) within the metallocycle, the torsion angle
x (in °) between the pyridine group plane and the metallocycle plane and d-parameter (defined below)
for compounds 1, 2, 3, 5 and 6 ꢂ 2H2O.
Compound
1
3
6 ꢂ 2H2O
2
5
M = Ni2
2.1293(5)
2.1474(5)
1.737(2)
1.715(2)
1.353(3)
1.478(3)
13.14(6)
1.26
M = Ni1
2.1507(5)
2.1416(5)
1.742(2)
1.715(2)
1.351(3)
1.476(3)
28.50(8)
1.55
M = Ni1
2.1383(5)
2.1463(6)
1.739(2)
1.713(2)
1.364(3)
1.474(3)
29.51(8)
1.49
M = Ni2
M = Ni
M = Cu1
2.1871(7)
2.1797(8)
1.771(3)
1.735(3)
1.335(4)
1.481(4)
28.92(12)
2.03
M = Cu2
2.1626(7)
2.1812(8)
1.765(3)
1.740(3)
1.328(4)
1.483(4)
13.90(8)
1.42
M = Au2
2.3078(10)
2.3053(11)
1.768(4)
1.742(4)
1.345(6)
1.477(5)
21.83(13)
1.47
M = Au1
2.3183(13)
2.3082(11)
1.771(4)
1.735(5)
1.336(6)
1.476(7)
33.44(17)
2.03
M–S (a)
M–S (a0)
S–C (b)
S–C (b0)
C@C (c)
C–C (d)
x
2.1549(6)
2.1478(7)
1.740(3)
1.715(2)
1.351(3)
1.476(3)
28.64(7)
1.44
2.1375(7)
2.1531(7)
1.738(3)
1.714(3)
1.354(4)
1.469(4)
33.56(7)
1.38
d
d = 100(b ꢁ b0)/b.
13° and 33°; depending on the complex, with the majority of them
between 21° and 33°, only one of the anionic units in the Ph4P salts
1 and 2 presents a smaller angle of about 13°. The rotation of the
pyridine rings is induced by steric hindrance between hydrogen
atoms at the 2-position of the pyridine ring and in the metallocycle
carbon but this rotation is also assisted by the interaction with
are coplanar with distances close to the sum of the van der Waals
radii, clearly denoting significant interactions (Fig. S3c).
p–p
A feature related to the unsymmetrical nature of the 4-pedt li-
gand, is the S–C bond length difference within the metallocycle
(Table 2). The shortening of the b0 bond when compared with the
b bond can be evaluated in terms of a parameter d = 100(b ꢁ b0)/
b, as previously done on other unsymmetrical bisdithiolene com-
plexes such as the [Ni(adt)2]ꢁ (adt = acrylonitrile-1,2-dithiolate)
or [Ni(tfadt)2]ꢁ (tfadt = 2-(trifluoromethyl)acrylonitrile-1,2-dithi-
olate) [6,22]. In our case d values are between 1.26% and 2.03% in
the four structures analysed and it can be attributed to the meso-
meric effect of the pyridine group.
neighbouring molecular units in the solid. The salts
5 and
6 ꢂ 2H2O present the higher pyridine rotation values (ꢀ33°) and
this large value can be ascribed to cation interactions (and also an-
ion interactions in the case of complex 5) as denoted by several
short contacts (see Tables S1 and S2 and Figs. S1 and S2).
The smaller pyridine rotations of only ꢀ13°, in 1 and 2, are asso-
ciated with anionic and cationic interactions, including
p–p
inter-
A long debated aspect of the dithiolene complexes is the so
called ‘‘non-innocence” of the ligands, which may lead to formal
ambiguities in the description of the different oxidation sates of
actions between the pyridine groups of the [M(4-pedt)2]ꢁ units:
(see Tables S3 and S4 and Fig. S3). The two pyridine rings involved

S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
1075
Fig. 6. Crystal structure of 1, viewed along the cation stacking axis.
these complexes [19]. In these monoanionic complexes the ligand
may have a significant contribution to the frontier orbitals and can
be viewed as being present in one of two extreme forms: the dian-
ionic ene-1,2-dithiolate or the neutral dithioketone [1a,19]. In this
particular case of the [M(4-pedt)2]ꢁ dithiolenes where the dithiole
ring is not fused to any aromatic ring the lengths of the C–C and C–
S bonds provide a direct indication on the nature of a dithiolene li-
gand. In all complexes studied the C@C (1.32–1.36 Å) and C–S
(1.71–1.77 Å) bond lengths were found in the range typical of sp2
C hybridisation [23] indicative of the ene-1,2-dithiolate form of
the dithiolene and M(III) oxidation state. In the solid state, the
monoanionic [M(4-pedt)2]ꢁ species with M = Ni, Cu and Au,
Fig. 7. Chain of [PPh4]+ cations in compound 1 viewed along b. [Symmetry codes: (a) x, y, z; (b) 1.5 ꢁ x, ½ ꢁ y, 1 ꢁ z; (c) ꢁ1/2 + x, 1/2 ꢁ y; ꢁ½ + z; (d) 1 ꢁ x, y, 1/2 ꢁ z; (e) 2 ꢁ x,
y, 1.5 ꢁ z; (f) 1/2 + x, 1/2 ꢁ y, 1/2 + z].

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S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
present different packing patterns and interactions depending on
the cation type complexes. The compounds 1 and 2 are isostructur-
al with the previously reported (PPh4)[Au(4-pedt)2] [9]. A charac-
teristic of their crystal structure is the occurrence of cation
columns along b + c, isolated by a alveolar anion arrangement
(Fig. 6). As 1 and 2 are isostructural, we will only focus our struc-
tural analysis on the compound 1. In the cation column, it is possi-
ble to observe cation pairs and the only intra column short contact
is H25ꢂꢂꢂC54 (2.785(3) Å) (Fig. 7). Such inversion-centred arrange-
+
ments of PPh4 cations are a recurrent feature among molecules
bearing at least three phenyl rings on the same atoms [24]. In
the alveolar anionic arrangement there are no S–S short contacts,
but there are several other NꢂꢂꢂH, SꢂꢂꢂH, HꢂꢂꢂH short contacts and
p–p interactions (Table S3). Of particular note are the C52–
H52ꢂꢂꢂN2 (3.514(4) Å) and C6–H6ꢂꢂꢂN2 (3.537(3) Å) weak hydrogen
bonding interactions.
The crystal structure of the two tetrabutylammonium salts (3
and 5) can be seen as made from parallel mixed chains constituted
by pairs of cations alternating with an anion. The parallel chains
alternate with sheets of anions (Fig. 8). These two compounds have
several short intermolecular contacts but none of them corre-
sponds to S–S interactions (for compound 5 Table S1).
The crystal structure of compound 6 ꢂ 2H2O can be described in
terms of parallel alternated layers of [Ni(4-pedt)2]ꢁ anions and
Na(18C6)+ cations. The coordination sphere of the sodium cation
is completed by two water molecules, one on each side (Fig. 9).
The angle between the anionic units mean plan is about 44.53.
Only between anions and cations, there are relevant interactions.
Besides several NꢂꢂꢂO, NꢂꢂꢂH, CꢂꢂꢂH, OꢂꢂꢂS, SꢂꢂꢂH short contacts a hydro-
gen bond N1ꢂꢂꢂH4A–O4 (2.916(4) Å) is present, which is the most
important, governing the chain-like arrangement anion–cation–
anion(. . .) as depicted in Fig. 10.
Fig. 9. Crystal structure of 6 ꢂ 2H2O, viewed along the b-axis. The dashed box marks
a layer of [Ni(4-pedt)2]ꢁ anions. [Symmetry codes: (a) x, y, z; (b) ꢁ1 + x, y, z; (c) x, y,
1 + z; (d) 1.5 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z; (e) 1/2 ꢁ x, 1/2 + y, 1/2 ꢁ z; (f) 1.5 ꢁ x, ꢁ1/2 + y,
1.5 ꢁ z; (g) 1 + x, y, z; (h) x, ꢁ1 + y, z; (i) 1 + x, y, 1 + z].
pedt)2]0 (Table 3). The E1/2 potentials for the Ni complexes in these
three compounds are identical. The variation in the scan rate (from
10 to 200 mV sꢁ1) monitoring the wave profile, the separation of
the peak potentials (dE > 59 mV), and the ratio of the cathodic
and anodic currents (-Icp=Ipaꢅ1) indicate that the processes are quasi
reversible. The low values of the first process explain the difficulty
in preparing the dianionic complex, which can be spontaneously
oxidised by minor amounts of oxygen or other oxidising agents.
On the other hand the low value of the second process predicts that
the neutral complex can be prepared. However attempts to isolate
it were so far not successful.
3.3. Redox behaviour
The redox behaviour of the different complexes was studied by
cyclic voltammetry in CH2Cl2 solution containing [n-Bu4N]PF6
(0.1 M) as electrolyte. Each of the compounds 1, 3 and 6 containing
the Ni complex (cyclic voltammogram of compound 6, Supplemen-
tary data, Fig. 4S) exhibit two quasi reversible redox waves, at circa
ꢁ0.52 V and 0.30 V versus [Fc]+/[Fc] which are ascribed to the
couples [Ni(4-pedt)2]2ꢁ/[Ni(4-pedt)2]ꢁ and [Ni(4-pedt)2]ꢁ/[Ni(4-
The compounds 2 and 4 containing the Cu complex (cyclic vol-
tammogram of compound 2, Supplementary data, Fig. 5S) show
also identical cyclic voltammograms with one quasi reversible re-
dox process at ca ꢁ0.38 V, ascribed to the couples [Cu(4-pedt)2]2ꢁ
/
[Cu(4-pedt)2]ꢁ (Table 3). Scans to higher positive potentials
Fig. 8. Crystal structure of 3, viewed along the b-axis. [Symmetry codes: (a) x, y, z; (b) 1 + x, y, z; (c) 1 + x, 1 + y, 1 + z; (d) x, 1 + y, 1 + z; (e) ꢁ1 + x, y, z; (f) ꢁ1 + x, ꢁ1 + y, ꢁ1 + z;
(g) x, ꢁ1 + y, ꢁ1 + z; (h) x, 1 + y, 1 + z; (i) ꢁ1 + x, 1 + y, 1 + z].

S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
1077
Fig. 10. Detail of the mixed layer in 6 ꢂ 2H2O, two adjacent mixed chains. [Symmetry codes: (a) x, y, z; (d) 1.5 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z; (n) 1/2 ꢁ x, ꢁ2.5 + y, 1/2 ꢁ z; (o) 1 + x,
1 + y, z; (p) ꢁ1 + x, ꢁ2 + y, z; (q) 1/2 ꢁ x, ꢁ1.5 + y, 1/2 ꢁ z; (r) 1.5 ꢁ x, 1/2 + y, 1/2 ꢁ z; (s) 2.5 ꢁ x, 1.5 + y, ½ ꢁ z; (t) ꢁ1/2 ꢁ x, ꢁ3.5 + y, ½ ꢁ z].
with cations which are absent in unit Ni2. In spite of the existence
of two different anionic NiIII units in the asymmetric unit of 1, the
two sets of g principal values for this compound are not distin-
guishable within experimental error in agreement with the fact
that the two Ni units present comparable short contacts between
the S atoms.
Table 3
Redox potentials (mV, vs. [Fc]+/[Fc]) of 1–6 at room temperature in CH2Cl2 containing
[n-Bu4N]PF6 (0.1 M) with a 100 mV sꢁ1 scan rate.
Compound
[ML2]2ꢁ/[ML2]ꢁ
[ML2]ꢁ/[ML2]0
[n-Ph4P][Ni(4-pedt)2] (1)
[n-Ph4P][Cu(4-pedt)2] (2)
[n-Bu4N][Ni(4-pedt)2] (3)
[n-Bu4N][Cu(4-pedt)2] (4)
[n-Bu4N][Au(4-pedt)2] (5)
[Na(18C6)][Ni(4-pedt)2] (6)
[Na(18C6)][Au(4-pedt)2] (7)
ꢁ519
ꢁ374
ꢁ523
ꢁ379
297
676
303
771
585
298
588
3.5. Magnetic susceptibility
ꢁ518
The magnetic susceptibility measurements of the compounds 1
and 3 in the range 2–300 K indicate a paramagnetic behaviour
with effective magnetic moments essentially temperature inde-
pendent down to ꢀ15 K with values at room temperature of
1.72 and 1.79 BM, respectively. For compound 6, the room tem-
perature magnetic moment is 1.67 BM, but it slightly increases
upon cooling down to 11 K (1.85 BM), denoting weak ferromag-
netic interactions that are not effective at lower temperature
(confirmed by magnetisation versus magnetic field at 3.1 K – see
Supplementary data). These values are consistent with a S = 1/2
state, due to an unpaired electron of a d7 system in an quadrangu-
lar coordinating geometry. The calculated magnetic moment for
the three compounds assuming a Curie law for S = ½ and g values
gav from EPR is 1.78 BM.
(ꢀ0.6 V versus [Fc]+/[Fc]) reveal the existence of an irreversible
process which should correspond to the couple [Cu(4-pedt)2]ꢁ/
[Cu(4-pedt)2]0.
Compounds 5 and 7 containing the [Au(4-pedt)2]ꢁ complex
(cyclic voltammogram of compound 7, Supplementary data,
Fig. 6S) both exhibit one oxidation and a reduction process, as-
signed to the couples [Au(4-pedt)2]ꢁ/[Au(4-pedt)2]0 (Table 3). For
both gold compounds, the E1/2 potentials are very similar. A varia-
tion in the scan rate (from 10 to 200 mV sꢁ1) monitoring the wave
profile, the separation of the peak potentials (dE > 59 mV), and the
ratio of the cathodic and anodic currents (ꢁIcp=Ipa ¼ 1) indicate that
the oxidation is not electrochemically reversible.
4. Conclusion
It is known that the nature of the R substituent in the com-
plexes Ni(S2C2R2)2 affects the redox potentials and even a small
variation in the electron withdrawing substituent can tune the
redox potential [1a,25]. In this case we observe that the pyri-
dine groups, which are only partially rotated relatively to the
central bisdithiolene plane, thus retaining the possibility of
The new paramagnetic complex, [Ni(4-pedt)2]ꢁ, with the
dithio-azo 4-pedt ligand has been synthesised, and isolated as
the [PPh4]+, [n-Bu4N]+ and Na+(18C6) salts, which were character-
ised by single crystal X-ray diffraction, cyclic voltametry EPR and
magnetic susceptibility. These three compounds present three dis-
tinct crystal structures and similar magnetic and redox behaviour,
characteristic of monoanionic Ni complexes with quadrangular
coordinating geometry and exhibit two quasi reversible redox
waves, attributed to the [Ni(4-pedt)2]2ꢁ/[Ni(4-pedt)2]ꢁ and [Ni(4-
pedt)2]ꢁ/[Ni(4-pedt)2]0 redox processes.
interacting via their
p systems, make the 4-pedt complexes eas-
ier to oxidise than phenyl substituted or even unsubstituted edt
ligands [5].
3.4. EPR measurements
Two other complexes with the same dithio-azo ligand have
been synthesised, [Au(4-pedt)2]ꢁ and [Cu(4-pedt)2]ꢁ, isolated as
the [n-Bu4N]+ and Na+(18C6) salts in case Au and as the [PPh4]+
and [n-Bu4N]+ salts in case Cu, all of them exhibiting diamagnetic
behaviour.
While the monoanionic Cu and Au complexes are diamagnetic
the monoanionic Ni, complexes, as d7 system in a square planar
coordination are expected to be paramagnetic S = 1/2 systems. In-
deed the EPR spectra of 1, 3 and 6 as powders, show a signal typical
of monoanionic Ni dithiolene complexes [1,26,27,28] with rhombic
symmetry, with g1 = 2.1035, g2 = 2.0518, g3 = 2.0128 compound 1
and g1 = 2.1153, g2 = 2.0480, g3 = 2.0087 for the compound 6. In
the case of compound 3 the EPR spectrum denotes the existence
of two Ni sites with different g1 (2.0849 and 2.0834) and g3
(2.0183 and 2.0113) values but with the same (or very close) g2 va-
lue (2.0701). This should be related with the different crystallo-
graphic environments of the two anionic Ni units in the
asymmetric unit of the compound 3. In fact Ni1 unit besides quite
asymmetric Ni–S bonds lengths has several short SꢂꢂꢂH–C contacts
The [n-Bu4N]+ and [PPh4]+ salts of Au and Cu complexes are iso-
structural with the correspondent Ni ones. Both Ni and Cu com-
plexes present
a
similar quasi reversible redox process
corresponding to the couple [ML2]2ꢁ/[ML2]ꢁ but they differ in the
couple [ML2]ꢁ/[ML2]0 which is a quasi reversible process for Ni
and an irreversible one for Cu. For the Au complexes only the cou-
ple [ML2]ꢁ/[ML2]0 is observed.
The ability of the pyridine groups of these complexes to coordi-
nate other metals is presently under study and will be reported
subsequently.

1078
S. Rabaça et al. / Polyhedron 28 (2009) 1069–1078
(i) S.I.G. Dias, A.I.S. Neves, S. Rabaça, I.C. Santos, M. Almeida, Eur. J. Inorg.
Acknowledgements
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L.V. Zakharov, A.L. Rheingold, A.M. Arif, M. Fourmigué, J.S. Miller, Inorg. Chem.
44 (2005) 7530.
This work was partially supported by FCT trough Contract
(PTDC/QUI/64967/2006) and by GRICES-Ministère des Affaires
Etrangères (France) Pessoa No. 07981RL bilateral agreement and
it also benefited from the ESF COST action D35.
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Appendix A. Supplementary data
CCDC 698755, 698756, 698757, 698758 and 698759 contain the
supplementary crystallographic data for 1, 2, 3, 5 and 6 ꢂ 2H2O.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
plementary data associated with this article can be found, in the
online version, at doi:10.1016/j.poly.2009.01.015.
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