Cis and trans-bis(tetrathiafulvalene-acetylide) platinum(ii) complexes: Syntheses, crystal structures, and influence of the ancillary ligands on their electronic properties

Article abstract of DOI:10.1039/c2dt31686b

A series of four platinum(ii) complexes bearing two tetrathiafulvalene acetylide ligands coordinated either cis or trans to the metal center are reported: cis-Pt(bipy)(CCMe₃TTF)₂, cis-Pt(tBu ₂bipy)(C≡CMe₃TTF)₂, cis-Pt(dppe) (C≡CMe₃TTF)₂ and trans-Pt(PPh₃) ₂(C≡CMe₃TTF)₂. The X-ray diffraction studies of the four complexes are reported and discussed. The electrochemical investigations carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) evidenced different redox behavior as a function of the ancillary ligand. Only for the cis-Pt(dppe)(C≡CMe₃TTF) ₂ complex is the first oxidation wave resolved (ΔE = 70 mV) into two one-electron processes. Spectroelectrochemical investigations performed on the four complexes did not evidence any electronic interactions between the two organic electrophores. The splitting of the first oxidation wave observed in cis-Pt(dppe)(C≡CMe₃TTF)₂ is mainly explained by the non-equivalence of the two TTF moieties induced by the geometrical constraint imposed by the ancillary dppe ligand as found by density functional theory calculations.

Full text of DOI:10.1039/c2dt31686b

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PAPER
Cis and trans-bis(tetrathiafulvalene-acetylide) platinum(II) complexes:
syntheses, crystal structures, and influence of the ancillary ligands on their
electronic properties†
Antoine Vacher, Frédéric Barrière, Franck Camerel,* Jean-François Bergamini, Thierry Roisnel and
Dominique Lorcy*
Received 26th July 2012, Accepted 21st September 2012
DOI: 10.1039/c2dt31686b
A series of four platinum(II) complexes bearing two tetrathiafulvalene acetylide ligands coordinated either
cis or trans to the metal center are reported: cis-Pt(bipy)(CuCMe3TTF)2, cis-Pt(tBu2bipy)-
(CuCMe3TTF)2, cis-Pt(dppe)(CuCMe3TTF)2 and trans-Pt(PPh3)2(CuCMe3TTF)2. The X-ray
diffraction studies of the four complexes are reported and discussed. The electrochemical investigations
carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) evidenced different
redox behavior as a function of the ancillary ligand. Only for the cis-Pt(dppe)(CuCMe3TTF)2 complex is
the first oxidation wave resolved (ΔE = 70 mV) into two one-electron processes. Spectroelectrochemical
investigations performed on the four complexes did not evidence any electronic interactions between the
two organic electrophores. The splitting of the first oxidation wave observed in cis-Pt(dppe)-
(CuCMe3TTF)2 is mainly explained by the non-equivalence of the two TTF moieties induced by the
geometrical constraint imposed by the ancillary dppe ligand as found by density functional theory
calculations.
multistage redox behavior whereas the TTF itself exhibits two
Introduction
reversible one-electron processes.8 It is worth mentioning that
This last decade, the coordination chemistry of tetrathiafulvalene
(TTF) derivatives has focused a lot of attention towards the elab-
oration of electroactive transition metal complexes with original
electronic properties due to interplay between the TTF and the
electron density on the metal.1–5 To study these interactions
between the TTF moiety and the metal center, another approach
has been recently developed which relies on organometallic
chemistry and on the connection of a TTF to the metal center
through a conjugated carbon chain. To date only four examples
of such hybrid organic–inorganic building blocks have been
reported, and all of them involve one or two TTF acetylide
ligands coordinated to a metal center (Chart 1). Interestingly,
electronic coupling was evidenced between the TTF and the
the use of an acetylide–Cr–acetylide organometallic bridge does
not allow an electrochemical detection of interplay between the
two TTFs within [CrCyclam(CuCEDTMeTTF)2]OTf.9 Never-
theless, electrochemical oxidations of this complex lead to two
salts where mixed-valence TTF units were obtained. Among the
various bisacetylide metal complexes, those involving a platinum
center with various ancillary ligands have also proved their
strong propensity to allow electronic interaction by connecting
either organic or organometallic electrophores.10–12
In this context, the aim of this work is to investigate the elec-
tronic properties of mononuclear platinum complexes containing
two TTF acetylide ligands and either diimine or phosphine as
ancillary ligands, namely triphenylphosphine, 1,2-bis(diphenyl-
phosphino)ethane (dppe), 2,2′-dipyridyl (bipy) or 4,4′-di-tert-
butyl-2,2′-dipyridyl (tBu2bipy). Herein, the synthesis, character-
ization and properties of a series of Pt(II) complexes bearing two
monodentate TTF acetylide ligands are reported.
6
metal electrophore within trans-RuCl(CuCMe3TTF)(dppe)2
and Cp*(dppe)Fe(CuCMe3TTF)7 with the strength of the coup-
ling depending on the nature of the metal. Interplay between the
two TTFs along the linker has also been evidenced in the trans-
Ru(CuCMe3TTF)2(dppe)2 through the observation of
Results and discussion
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université
de Rennes 1, Campus de Beaulieu, Bâtiment 10A, 35042 Rennes,
France. E-mail: franck.camerel@univ-rennes1.fr,
Synthesis
dominique.lorcy@univ-rennes1.fr
The synthesis of the target complexes is outlined in Scheme 1.
The coupling reaction,13 catalysed by copper(I) iodide, between
2 equiv. of an alkyne and a bis(chloro)platinum derivative in a
basic medium represents a versatile route towards platinum
†Electronic supplementary information (ESI) available: Computational
details and crystallographic data of new compounds. CCDC
816625–816628. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31686b
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Chart 1 Metal complexes involving TTF acetylide ligand.
cis-Pt(PPh3)2Cl2 complexes were used as starting materials for
the synthesis of complexes 3 and 4 respectively. The substitution
of the chloride ligands on the cis-Pt(PPh3)2Cl2 complex by
bulky alkynyl fragments leads to the formation of a more
thermodynamically stable trans-complex.14 The 31P NMR spec-
trum of 4 shows one signal at 18 ppm indicating the equivalence
of the two phosphorus atoms due to the trans arrangement of the
acetylide TTF ligands around the Pt(II) center.
Molecular structures
Single crystals were obtained for each complex, 1–4, and the
molecular structures have been elucidated by X-ray diffraction
studies. Selected bond lengths and angles are reported in
Table 1. All the complexes, 1–4, display a square planar geome-
try around the platinum atom. Bond lengths analyses of the
central CvC bonds reveal that the TTF moieties are all in a
neutral state for each complex, 1–4. In complex 1, the two TTF
fragments and the bipyridine ligand roughly lie in the same
plane and the long axis of the TTF molecules is parallel to the
long axis of the bipy fragment (Fig. 1). The observed Pt–N bond
lengths as well as the ligand bite angle N1–Pt–N11 (Table 1)
compare well with those usually measured in other bipyPt com-
plexes.15 The observed Pt–C and CuC are identical on the two
TTF fragments (Table 1). These CuC bond lengths are longer
than that found in the Me3TTFCuCH precursor (1.152(8) Å)8
but lie in the same range as that found in trans-RuCl-
(–CuCMe3TTF)(dppe)2 (1.203(3) Å).6 The angle between the
two alkynes in 1 amounts to 88.92(20)°.
Scheme 1 Synthetic routes for the preparation of complexes 1–4.
complexes. Accordingly, we prepared four different platinum
complexes 1–4 bearing two Me3TTF-acetylide ligands
(Scheme 1). The use of the Pt(bipy)Cl2 precursor affords
complex 1 which was found to be insoluble in most of the com-
monly used organic solvents. To improve the solubility of the
target complex, the analogue complex 2 carrying a tBu2bipy
fragment in place of the bipy fragment in 1 was synthesized
using Pt(tBu2bipy)Cl2 as the starting material. In order to study
the influence of the nature of the L-ligand and the cis or trans
configuration of the two acetylide TTF moieties around the
metal on the physicochemical properties of the platinum com-
plexes, the bipy fragment was replaced by either a diphosphine
(1,2-bis(diphenylphosphino)ethane, dppe) ligand or two triphe-
nylphosphine ligands. For that purpose, the cis-Pt(dppe)Cl2 and
The molecular structures of the two crystallographically inde-
pendent complexes 2 are presented in Fig. 2 and Fig. S1.† In the
two complexes, the TTF fragments and the tBu2bipy ligand lie
in the same plane. Interestingly, the orientation of the TTF mole-
cules observed in the two crystallographically independent mole-
cules of complex 2 is different from that observed in the
crystalline structure of the parent complex 1. With complex 2,
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Table 1 Selected bond lengths in Å and angles in °
Compound
X–Pt
Pt–C11
C10uC11
C5vC6
X–Pt–X
C–Pt–C
Me3TTFCuCH
1 TTFA
TTFB
2 TTFA
TTFB
TTFC
TTFD
3 TTFA
TTFB
TTFC
TTFD
4 TTF
—
—
1.152(8)
1.207(8)
1.208(6)
1.184(10)
1.180(9)
1.215(12)
1.219(12)
1.206(8)
1.194(10)
1.207(9)
1.186(11)
1.220(5)
1.349(7)
1.353(7)
1.346(6)
1.322(9)
1.322(9)
1.356(11)
1.343(11)
1.334(9)
1.337(10)
1.343(8)
1.336(9)
1.338(6)
—
79.10(15)
—
88.92(20)
N1: 2.068(4)
N11: 2.062(4)
N1: 2.054(5)
N11: 2.062(5)
N1: 2.034(7)
N11: 2.035(7)
P1: 2.291(2)
P11: 2.296(2)
P2: 2.283(2)
P22: 2.294(2)
P1: 2.305(1)
1.948(8)
1.949(5)
1.957(7)
1.954(6)
1.941(8)
1.952(8)
2.000(6)
2.017(6)
1.995(6)
2.009(7)
1.999(5)
78.15(19)
80.09(27)
85.08(6)
92.21(24)
91.02(34)
88.11(23)
88.23(26)
180.00(3)
85.66(7)
180.00(13)
Fig. 1 ORTEP drawing of the platinum complex 1 with the main num-
bering scheme. Thermal ellipsoids drawn at the 50% probability level.
Fig. 3 ORTEP drawing of one of the two crystallographically indepen-
dent molecules of complex 3 with the main numbering scheme. Thermal
ellipsoids drawn at the 50% probability level.
(91.02(34)° and 92.21(24)°) is observed in the two molecules of
complex 2 and can be explained by the peculiar orientation of
the TTF moieties in this complex.
The molecular structures of the two crystallographically inde-
pendent molecules of complexes 3 are presented in Fig. 3 and
Fig. S2.† The molecular structure confirms that the platinum
center is coordinated by one dppe fragment and two acetylide-
TTF ligands in a cis arrangement. The TTF molecules are
oriented with the long axis of the TTF parallel to the P⋯P axis
of the dppe fragment. The Pt–C bond lengths are longer in the
two molecules of complex 3 than those measured in complexes
1 and 2. This difference is due to a π-back metal to ligand
(bipyridine) donation in complexes 1 and 2. The Pt–P bond
lengths are consistent with those already determined on related
phosphino–alkynyl–platinum derivatives.17 The CuC bond
lengths are similar in the two molecules and compare well with
those measured in complexes 1 and 2. The TTF cores adopt a
boat conformation with the dithiole rings folded along the S⋯S
axis (12/13° TTFA, 24/14° TTFB, 13/13° TTFC and 13/9°
TTFD) contrasting with the planar TTF cores observed for com-
plexes 1 and 2. On the two crystallographically independent
molecules of complex 3, the two TTF fragments do not lie in the
same plane and one TTF clearly appears to be more bent than
the other one. This dissymmetry is induced by steric interactions
and geometrical constraints imposed by the phenyl rings on the
ddpe ligand.
Fig. 2 ORTEP drawing of one of the two crystallographically indepen-
dent molecules of complex 2 with the main numbering scheme. Thermal
ellipsoids drawn at the 50% probability level.
the long axis of the TTF moieties is perpendicular to the long
axis of the bipy fragment (Fig. 2). The bond lengths Pt–C, Pt–N
and CuC are slightly different in the two crystallographically
independent molecules of complex 2 but they remain close to
those discussed above for complex 1 (Table 1) or to the related
Pt(II) diimine acetylide complex.16 A larger C–Pt–C angle
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Fig. 4 X-Ray molecular structure of complex 4: ORTEP drawing of
the complex (top, thermal ellipsoids drawn at the 50% probability level),
side view of the complex (bottom, the PPh3 ligands have been removed
for clarity).
Fig. 5 Evolution of the absorption spectrum of complex 2 in CH2Cl2
at room temperature upon addition of increasing amounts of NOPF6 as
an oxidant.
In complex 4, the two acetylide-TTF ligands are in a trans
arrangement with the platinum atom localized on the inversion
center (Fig. 4). The bond lengths Pt–P, P t –C, CuC compare
well with the bond lengths reported for complex 3 and with
other similar phosphino–alkynyl–platinum complexes.18 The
central C–CuC–Pt–CuC–C spacer is almost linear with angles
at C–Pt–C, Pt–CuC and CuC–C of 180.0(0), 174.4(3),
177.9(3)° respectively. The TTFs adopt a slight boat confor-
mation (Fig. 4).
Table 2 UV-visible and electrochemical dataa in V vs. SCE for
complexes 2–4
Complex
λmax [nm]b(εmax [M−1 cm−1]) E1/21TTF E1/22TTF
2
3
4
229(55 000), 294(60 000),
380(19 000), 483(5000)
228(52 500), 310(41 000),
405(9000)
229(68 700), 326(41 500),
405(11 300)
0.23
0.72
0.19/0.26 0.75
0.21
0.38
0.72
0.88
Me3TTFCuCH 295(15 000), 331(14 000),
386(3000)
Photophysical properties of complex 2
a In CH2Cl2-[NBu4][PF6] 0.1 M at room temperature; scan rate, 100 mV
s−1. E1/2 = (Epa + Epc)/2; Epa and Epc are the anodic peak and the
cathodic peak potentials, respectively. b Electronic absorption data of
complexes 2–4 measured in CH2Cl2 at room temperature (c ∼ 10−5
mol L−1).
Diimine platinum(II) di(acetylide) complexes are known to
display good luminescence properties with bright emission and
long decay lifetimes, usually arising predominantly from the
3MLCT excited state.19 For example, the complex Pt–(tBu2bipy)-
(CuCPh)2 in CH2Cl2 emits at 560 nm with a quantum yield of
0.34 and a luminescence life-time of 1.36 μs.20 Acetylide
ligands including functional units such as redox-active groups
are particularly attractive to obtain redox active optical switches.
In this respect, TTF is an efficient luminescent quencher by elec-
tron-donating effects and oxidation of the TTF usually leads to
the regeneration of the luminescence properties of the lumino-
phore on which this redox unit is attached.21 Hence, the lumines-
cence properties of the soluble complex 2 have been explored in
the neutral state and upon oxidation of the TTF units by adding
successive aliquots of NOPF6 or FeCl3 as the oxidizing agent.
The UV-vis absorption spectrum of complex 2 is shown in
Fig. 5 and the absorption maxima and extinction coefficients are
given in Table 2. The strong bands below 360 nm (ε ∼ 5–6 ×
104 M−1 cm−1) are attributed to intraligand 1π–π* transitions
localized on the tBu2bipy and the alkynyl-TTF fragments. The
(Fig. S3†), the intense high energy bands below 360 nm with
extinction coefficients ε of 4–7 × 105 M−1 cm−1 are assigned to
ligand-localized π–π* transitions whereas the absorption band
localized at 405 nm with a lower extinction coefficient (104 M−1
cm−1) is attributed to a π(CuC) → π*(CuC) transition contain-
ing a slight MLCT contribution.23 The absorption spectra of the
phosphino complexes 3 and 4 appear to be the superimposition
of the absorption spectra of the chloro–phosphino–platinum and
of the Me3TTF–CuC–H precursors and only a hypsochromic
shift of 30 nm of the low energy band, strongly localized on the
acetylide bridge, is observed (Fig. S4†).
At room temperature in CH2Cl2 (either under air or argon),
complex 2 appears to be non-luminescent. Several excitation
wavelengths have been tested and excitation in the MLCT band
localized at 380 nm did not lead to the expected emission in the
range 570–610 nm, usually observed with diimine platinum(II)
di(acetylide) complexes.15a The luminescence quenching is
probably due to photo-induced electron transfer from the TTF
unit toward the excited states localized on the platinum–diimine
fragments.21 Similar electron transfer quenching has already
been reported in a related system bearing electron donor phe-
nothiazine instead of TTF.24
absorption band observed at 380 nm (ε = 1.9 × 104 M−1 cm−1
)
is attributed to the superimposition of the absorption bands cen-
tered on the TTFs and a charge transfer excitation from the d
orbital of the platinum atom to a vacant π* diimine orbital (Pt →
tBu2bpy 1MLCT transition).15a The lowest energy band localized
at 483 nm (ε = 5000 M−1 cm−1) is likely due to a ligand-to-
ligand charge transfer transition (1LL′CT) between the alkynyl-
TTF and the bipy fragments.15a,22 For complexes 3 and 4
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Chemical oxidation of complex 2 upon addition of increasing
amounts of NOPF6 was realized and the evolution of the absorp-
tion spectrum is presented in Fig. 5. The gradual oxidation of the
TTF units leads to the growing of absorption bands localized at
437 nm and 800 nm, characteristic of the formation of the cation
radical species, and a clear isosbestic point is observed at
400 nm. A decrease of the absorption bands below 330 nm,
attributed to π–π* transitions localized on the neutral TTF units,
is also observed. Contrary to our expectations, treatment of the
solutions of complex 2 with 1 and 2 equiv. of an oxidizing agent
such as NOPF6 or FeCl3 did not lead to the formation of a lumi-
nescent complex. This can be ascribed to the fact that the TTF
radical cation displays a strong broad absorption band in the
wavelength range of 580–800 nm.25 Due to spectral overlap
between the absorption of the TTF+˙ radical cation and the emis-
sion of the diimine–platinum–acetylide core, an energy transfer
efficiently takes place. As a consequence, the fluorescence of the
diimine–platinum–acetylide core of complex 2, which would be
increased by oxidation of the TTF units, is quenched by energy
transfer between the diimine–platinum–acetylide fragment and
the TTF+˙ radical units.21e
supporting electrolyte. Due to its insolubility, the redox proper-
ties of complex 1 could not be determined. The cyclic voltam-
mograms (CV) of complexes 2–4 are presented in Fig. 6 and the
electrochemical data are collected in Table 2. Complexes 2, 3
and 4 display two main reversible oxidation waves. Complex 2
displays an additional reduction wave at Ered = −1.32 V vs. SCE
attributed to the one-electron reduction of the tBu2bipy frag-
ment.20 A closer look at the CV pattern of complex 3 reveals
that the first redox system originates from two closely spaced
and resolved oxidation processes at E1/2 = 0.19 V and E1/2
=
0.26 V vs. SCE, respectively corresponding to the successive
generation of the TTF cation radical and the bis(TTF cation
radical). The corresponding first oxidation systems in 2 and 4 are
not resolved. However, they also involve two closely spaced
one-electron transfers assigned to the oxidation of the TTF cores
into the corresponding bis(TTF cation radical) 22(+˙) and 42(+˙).
For all 2, 3 and 4, the second oxidation process is assigned to
the closely spaced and unresolved second oxidation of the bis
(TTF cation radical) 22(+˙), 32(+˙) and 42(+˙) into the bis(TTF
dication) 24+, 34+ and 44+. Note that none of these two-electron
transfers involve inversion of the normal potential ordering.
Rather, they all involve sequential and closely spaced one-elec-
tron transfer. Only the first oxidation system of 3 may be
resolved electrochemically. The oxidation potentials of the TTF
moieties are comparable in the three complexes indicating that
the nature of the coordinating L-ligand (bipyridine or phosphine)
has no significant influence on the electron-donating ability of
the TTF core. Compared with the oxidation potentials of the
Me3TTFCuCH precursor (E1/21 = 0.38 V and E1/22 = 0.88 V vs.
SCE, in CH2Cl2), all the oxidation potentials of the TTF cores
within all these complexes, 2–4 (Table 2), are shifted by 160 mV
towards less anodic potentials. This cathodic shift reveals that
the organometallic fragment, through the acetylide linker,
increases the electron density on the TTF cores and thus high-
lights the electronic interaction between the platinum center and
IR studies
Platinum complexes 1–4 have also been characterized by solid-
state IR spectroscopy. The alkynyl CuC stretching vibration
bands appear at 2090, 2091, 2092 and 2086 cm−1 for complexes 1
to 4 respectively. These values are almost identical in the four com-
plexes and this indicates that there is little effect of either the
L-ligand (bipy for 1 and 2, dppe for 3 and PPh3 for 4) or the
cis/trans geometry on the frequency of the νCuC stretching vibration.
Electrochemical studies
The electrochemical properties of complexes 2–4 were investi-
gated by cyclic voltammetry in CH2Cl2 using [NBu4][PF6] as a
Fig. 6 CV (top) and DPV (bottom) of complexes 2 (left), 3 (middle) and 4 (right) in CH2Cl2-[NBu4][PF6] 0.1 M (E in V vs. SCE, v = 100 mV s−1).
The small wave at −1 V in CVof compound 2 arises from an unassigned impurity.
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UV-vis-NIR spectroelectrochemical investigations
the TTF cores. Such a cathodic shift has only been previously
observed in a few cases such as for (Me2TTF(SiMe2)2)Pt-
(PPh3)2,26 trans-ClRu(CuCMe3TTF)(dppe)2 and trans-Ru
6
In order to get more insight into the interplay between the elec-
troactive TTF through the bisacetylide–Pt bridge, UV-vis-NIR
spectroelectrochemical investigations were carried out on com-
plexes 2, 3 and 4 in a CH2Cl2-[NBu4][PF6] 0.2 M solution
(Fig. 7). The gradual oxidation of the TTF cores only induces
the growth of low energy bands characteristic of radical cation
species at 400 nm and 800 nm. No evidence of intervalence
charge-transfer (IVCT) bands was observed for complexes 2–4
(CuCMe3TTF)2(dppe)2.8 It is worth noting that this shift
depends on the metal linked to the TTF acetylide ligand. Indeed,
in the case of the chromium or iron acetylide-TTF type complex,
namely [CrCyclam(CuCEDTMeTTF)2]OTf9 and Cp*(dppe)-
FeCuC–TTFMe3,7 no modification of the oxidation potentials
of the TTF was observed. In other cases, when interactions exist
between a TTF core connected to a metallic center through co-
ordinating heteroatoms (L-type TTF ligands), an anodic shift is
usually observed.1
The splitting of the first oxidation process of 3 was confirmed
by differential pulse voltammetry (DPV) analysis performed on
the three complexes 2–4 (Fig. 6). The DPV of complex 3 dis-
plays two main oxidation peaks and clearly the first oxidation
system contains two contributions at 0.19 and 0.26 V, corre-
sponding to two monoelectronic oxidation processes. Hence,
each TTF unit in 3 is oxidized to the cation radical sequentially,
with a potential difference of ΔE = 70 mV, whereas the last oxi-
dation process at 0.75 V is unresolved and involves two elec-
trons. The DPV of complex 2 confirms the presence of three
redox processes and integration of the waves confirms that the
reduction process contains one electron whereas each oxidation
peak is unresolved and involves two electrons. Similarly for
complex 4, only two unresolved two-electron oxidation peaks
are effectively observed in the anodic region.
in the measured range from 200–2000 nm (5000–50 000 cm−1
)
(Fig. 7). Although we cannot exclude the presence of an IVCT
band at lower energy, these results suggest a lack of strong elec-
tronic communication between the TTF moieties in the mono-
oxidized complexes in all three complexes (2–4). In a related
trans-ruthenium complex,8 a corresponding potential splitting of
The splitting of the first oxidation process for 3 is independent
of the concentration (10−4 to 10−6 M), indicating that the
sequential oxidation of the TTF cores is due to intramolecular
interactions. This behavior is reminiscent of what was previously
observed with dimeric TTFs where the splitting of the first redox
process was the result of intramolecular interactions either
through space, due to spatial proximity of the TTF cores, or
through bond, due to electronic coupling of the TTF cores along
the linker.27 For related platinum complexes containing two
organometallic electrophores such as ferrocene instead of the
TTF cores, the trans-Pt(PPh2Me)2(CuCFc)2 and the cis-Pt-
(dppe)(CuCFc)2, both ferrocene moieties are oxidized succes-
sively with a potential difference (ΔE) of 80 and 70 mV, respect-
ively.11 The splitting value of 70 mV measured with complex 3
is in the same range and can indicate that weak intramolecular
interactions occur between the two electrophores. However, a
question remains, why this splitting is observed for the cis
isomer 3 and not for the trans isomer 4 as in the case of the
ferrocene complexes.
Regarding spatial interactions, it has been demonstrated that in
TTF dimers linked by one heteroatom, the extent of the spatial
intramolecular interactions increases when the angle between the
two TTF moieties decreases.28 The C–Pt–C angle determined in
the crystalline structure of complex 3 appears smaller than that
measured on the crystalline structure of complex 2. This smaller
angle between the two TTF units in complex 3 can be at the
origin of the splitting of the oxidation waves by increasing the
spatial interactions (not observed with complex 2). However,
measurements in the poorly coordinating electrolyte,29 CH2Cl2-
Na[B(C6H4(CF3)2)4] 0.02 M, do not yield a better resolution of
the first two electron wave for any of the three complexes 2–4,
showing that the TTF cores poorly interact electrostatically.
Fig. 7 UV-vis-NIR monitoring of the electrochemical oxidation of cis-
Pt(tBu2bipy)(CuCMe3TTF)2 2 (a), cis-Pt(dppe)(CuCMe3TTF)2 3 (b)
and trans-Pt(PPh3)2(CuCMe3TTF)2 4 (c) from 0 V to 0.4 V in CH2Cl2-
[NBu4][PF6] 0.2 M.
Dalton Trans.
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110 mV was measured electrochemically and a band assigned as
an IVCT recorded at 1360 nm.
the unresolved first oxidation measured electrochemically
(low ΔE). Results obtained with complexes 2 and 4 are quite
similar regardless of the cis or trans arrangement with even
poorer contribution of the organometallic linker in the spin
density of 2+˙.
DFT computational studies
DFT calculations performed on complex 3 reveal that the
HOMO is dissymmetric with the electronic density localized on
one of the two TTF cores (Fig. 9). Such dissymmetry is consist-
ent with the crystal structure of complex 3 (vide supra). It is
worth mentioning that such dissymmetry has not been evidenced
DFT calculations [Gaussian03, B3LYP/LanL2DZ] were per-
formed on complexes 2, 3 and 4. Full geometry optimization led
to the molecular structure represented in Fig. 8 and 9. The opti-
mized geometries are in good agreement with those obtained by
X-ray diffraction studies concerning the bond lengths and the
bond angles (vide supra). As shown in Fig. 8, the HOMOs
(highest occupied molecular orbitals) of complexes 2 and 4 are
symmetric and have a strong TTF character with a poor contri-
bution of the bisacetylide–Pt spacer and also with poor coeffi-
cients found on the carbon atoms of the distal dithiole rings. The
introduction of the bisacetylide organometallic fragment between
the two TTF moieties induces a dissymmetrization of the
HOMO on the TTF core as the HOMO of the neutral Me3TTF–
CuC–H precursor is symmetric on the TTF core with no contri-
bution from the alkyne.6,8 The SOMOs (highest singly occupied
molecular orbitals) of the oxidized complexes 2+˙ and 4+˙ are
essentially localized on the TTF cores with no contribution from
the platinum center. The calculated spin densities of complexes
2+˙ and 4+˙ appear to be delocalized over the whole molecule
with however weaker contribution of the organometallic bridge.
The poor delocalization through the acetylide–platinum–acety-
lide organometallic linker in the cation radical is consistent with
1
on the H NMR and 31P NMR spectra. The HOMO is centered
on one TTF unit and the HOMO − 1 is localized on the other
TTF unit with energy levels calculated at −4.33 and −4.41 eV
respectively. The dissymmetry is attributed to steric hindrance
due to the bulky dppe ligand and the close proximity of the
phenyl groups which force the TTF units to be in two distinct
environments. With the rigid and symmetric tBu2bipy fragment
in complex 2, the two TTF fragments have the same environ-
ment and are equivalent, leading to a closely spaced and un-
resolved first oxidation potential. In complex 4, the TTF cores
are also equivalent and the conformation of the TTF cores is not
constrained by the presence of PPh3 groups. Interestingly, the
SOMO of complex 3+˙ is perfectly symmetric with a main con-
tribution of two equivalent TTF cores. The spin density in the
mono-oxidized species 3+˙ is essentially localized on the TTF
fragments with a slight delocalization on the organometallic
bridge.
Fig. 8 Graphical representations of the HOMO and SOMO of complexes 2 and 430 shown with a cut-off of 0.04 [e bohr−3 1/2
]
and spin density of
the mono-oxidized complexes 2+˙ and 4+˙ shown with a cut-off of 0.001 e bohr−3
.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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Fig. 9 Graphical representations of the HOMO and SOMO of complexes 3 and 3+˙ shown with a cut-off of 0.04 [e bohr−3 1/2
]
and spin density of
the mono-oxidized complexes 3+˙ shown with a cut-off of 0.001 e bohr−3
.
Contrary to complexes 2 and 4, complex 3 presents two
resolved sequential oxidations. This peculiar electrochemical be-
havior is likely due to geometrical constraints and interactions
with the phenyl groups of the structurally blocked and distorted
dppe ligand. For all complexes, the lack of effect of the poorly
coordinating electrolyte29 on the splitting of the first wave and
the lack of spectroscopic evidence for intervalence charge trans-
fer excludes a strong interaction between the two TTF moieties
and lends more support to the proposed explanation based on the
demonstrated structural non-equivalence of the two organic
electrophores.
unresolved potentials. No evidence of electronic communi-
cations between the two TTF units through the organometallic
fragment was found in these complexes, especially no interva-
lence charge-transfer (IVCT) bands are observed during the
spectroelectrochemical studies. Based on these preliminary
measurements, we conclude that alkynyl–platinum fragments do
not allow efficient electronic communications between the two
organic electrophores in contrast to alkynyl–ruthenium frag-
ments. DFT calculations show a dissymmetry of the two TTF
ligands in complex 3 also observed in the crystal structure. This
is proposed to account for the more pronounced splitting of the
first oxidation wave, as strong electronic coupling was ruled out
by spectroelectrochemical investigations.
Conclusions
Experimental section
In summary, we have presented a series of cis and trans platinum
complexes bearing two TTF-ligands. Four complexes, cis-Pt-
(bipy)(CuCMe3TTF)2 (1), cis-Pt(tBu2bipy)(CuCMe3TTF)2
(2), cis-Pt(dppe)(CuCMe3TTF)2 (3) and trans-Pt(PPh3)2-
(CuCMe3TTF)2 (4), were readily synthesized and characterized
in order to study the influence of the nature of the L-ligands and
the cis/trans geometry of the acetylide ligands on the electronic
properties of the complexes. The luminescence of the bipy–plati-
num–alkynyl fragment is quenched in the neutral state by intra-
molecular photoinduced electron transfer from TTF donors.
However, chemical oxidation of the TTF moieties by NOPF6 or
FeCl3 did not restore the luminescence properties of the
diimine–platinum–acetylide complex. The luminescence of the
oxidized complex is most likely quenched by energy transfer
toward the formed TTF+˙ radical cation. Electrochemical
measurements and DFT calculations have revealed that in com-
plexes 2 and 4, the two TTF units on each complex are equival-
ent and are oxidized to the radical cation at closely spaced and
General
NMR spectra were recorded on a Bruker AV300III spectrometer
at room temperature using perdeuterated solvents. Chemical
shifts are reported in ppm referenced to TMS for H NMR, 13C
1
NMR and to H3PO4 for 31P NMR. FT-IR spectra were recorded
using a Varian-640 FT-IR spectrometer equipped with a PIKE
ATR apparatus. Fluorescence spectra were recorded on a Fluoro-
log-3 fluorescence spectrometer (FL3-22, Horiba Jobin Yvon)
with 1 cm quartz cells. Mass spectra were recorded with a
Bruker MicrOTOF-Q II instrument for complexes 1, 3 and 4 and
with a Micromass ZABSpecoaTOF instrument for complex 2 by
the Centre Régional de Mesures Physiques de l’Ouest, Rennes.
CVs were carried out on a 10−3 M solution of the complex in
CH2Cl2-[NBu4][PF6] 0.1 M and in CH2Cl2-Na[B(C6H4(CF3)2)4]
0.02 M. Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate,
Na[B(C6H4(CF3)2)4],
was
purchased
from
Aldrich.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

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Complex
3
cis-Pt(dppe)(CuCMe3TTF)2. cis-Pt(dppe)Cl2
Spectroelectrochemical experiments were carried out in a 1 mm
length quartz cell. The electrochemical set-up was constituted by
a micro-perforated platinum–iridium foil as a working electrode,
a platinum wire as a counter electrode and an SCE as a reference
electrode. A Model 362 scanning potentiostat from EG&G
Instruments was used to set the applied potential and a Cary 5
spectrophotometer was employed to record the UV-vis-NIR
spectra. All experiments were performed in CH2Cl2-[NBu4][PF6]
0.2 M. CVs were recorded on a Model 362 scanning potentiostat
from EG&G Instruments at 0.1 V s−1 on a platinum disk elec-
trode (1 mm2). Potentials were measured versus a KCl saturated
calomel electrode (SCE). Anhydrous CH2Cl2 and NEt3 were
obtained by distillation over P2O5 and CaH2 respectively. All
synthetic manipulations were performed under an inert and dry
nitrogen atmosphere using standard Schlenk techniques. Silica
gel used in chromatographic separations was obtained from
Acros Organics (Silica Gel, ultra pure, 40–60 μm). The ethynyl-
trimethyl-TTF (HCuCMe3TTF) was prepared according to a
previously published procedure.6,8 cis-Pt(PPh3)2Cl2 was pre-
pared by the Jensen method.31 The complex cis-Pt(dppe)Cl2 was
prepared by following the literature methods.32 All other
reagents and materials from commercial sources were used
without further purification.
(100 mg, 0.15 mmol) was introduced in a Schlenk tube with
HCuCMe3TTF (81 mg, 0.30 mmol) in distilled CH2Cl2
(10 mL) in the presence of freshly-distilled NEt3 (5 mL) and CuI
(6 mg, 0.030 mmol) as a catalyst. The reaction mixture was
stirred under an inert nitrogen atmosphere at room temperature
for 12 h. The solvent was evaporated and the residue treated
with water and extracted with CH2Cl2. The organic layer was
washed with water and dried over MgSO4. Complex 3 was preci-
pitated by slow diffusion of pentane into a CH2Cl2 solution of
the crude product. The complex was finally crystallized by slow
evaporation of a concentrated CH2Cl2 solution (reddish-brown
crystals, m = 45 mg, yield = 30%). 1H NMR (CDCl3) δ 1.91 (br,
18H, CH3), 2.40 (m, 4H, CH2), 7.44 (m, 12H, CH), 7.82 (m,
8H, CH); 13C NMR (CDCl3) δ 13.6, 15.5, 122.4, 128.8, 129.0,
131.4, 133.3, 133.5; 31P NMR (CDCl3) δ 41.2 (s, 2P, JP–Pt
=
2298 Hz); IR (ATR): 2092 cm−1 (νCuC); HRMS (ESI): m/z
calcd for C48H42P2S8194Pt 1130.0148. Found: 1130.0148.
Complex 4 trans-Pt(PPh3)2(CuCMe3TTF)2. cis-Pt(PPh3)2Cl2
(100 mg, 0.13 mmol) and HCuCMe3TTF (68 mg, 0.26 mmol)
were dissolved in a mixture of dry CH2Cl2 (10 mL) and distilled
NEt3 (5 mL) inside a Schlenk tube. The solution was degassed
and CuI (5 mg, 0.026 mmol) was added. After stirring for 48 h
at room temperature, a red precipitate appeared. After filtration,
the red precipitate was washed with cold CH2Cl2. The material
was dissolved in CH3CN and red crystals were grown by slow
diffusion of diethyl ether (m = 90 mg, 57% yield). Due to its
low solubility complex 4 could not be characterized by 13C
NMR. 1H NMR (CDCl3) δ 1.90–1.95 (br, 18H, CH3), 7.39–7.73
(m, 30H, CH); IR (ATR): 2086 cm−1 (νCuC); 31P NMR (CDCl3)
Complex 1 cis-Pt(bipy)(CuCMe3TTF)2. Pt(bipy)Cl2(47 mg,
0.11 mmol) was reacted with HCuCMe3TTF (60 mg,
0.22 mmol) in distilled CH2Cl2 (10 mL) in the presence of
freshly-distilled NEt3 (5 mL) and CuI (4 mg, 0.022 mmol) as a
catalyst. The reaction mixture was stirred under an inert nitrogen
atmosphere at room temperature for 24 h. A black precipitate
was filtered off and washed with methanol and CH2Cl2. After
drying, 65 mg of black powder were isolated (yield = 65%).
Single crystals were harvested by slow cooling of a clear hot
DMF solution of 1. Due to its low solubility complex 1 could
δ
18 (s). HRMS (ESI): m/z calcd for C58H48P2S8194Pt
1256.06183. Found: 1256.0618.
1
not be characterized by 13C NMR. H NMR (hot D6-DMSO)
X-Ray crystallography
δ (ppm) 1.94 (s, 12H, CH3), 2.26 (S, 6H, CH3), 7.91 (m, 2H,
CH), 8.42 (m, 2H, CH), 8.67 (m, 2H, CH), 9.30 (m, 2H, CH)).
IR (ATR): 2090 cm−1 (νCuC); HRMS (ESI): m/z calcd for
C32H26N2S8194Pt: 887.9483. Found: 887.9483.
Details of the structural analyses for 1, 2, 3 and 4 are summar-
ized in Table 3. X-Ray crystal structure determinations were per-
formed on an APEXII Bruker-AXS diffractometer equipped with
a CCD camera and a graphite-monochromated Mo Kα radiation
source (λ = 0.71073 Å), from the Center de Diffractométrie
(CDFIX), Université de Rennes 1, France. Structures were
solved by direct methods using the SIR97 program,33 and then
refined with full-matrix least-squares methods based on F2
(SHELXL-97)34 with the aid of the WINGX program.35 For com-
plexes 2 and 3, the contribution of the disordered solvents to the
calculated structure factors was estimated following the BYPASS
algorithm,36 implemented as the SQUEEZE option in
PLATON.37 A new data set, free of solvent contribution, was
then used in the final refinement. All non-hydrogen atoms were
refined with anisotropic atomic displacement parameters. H
atoms were finally included in their calculated positions.
Complex 2 cis-Pt(tBu2bipy)(CuCMe3TTF)2. Pt(tBu2bipy)Cl2
(99 mg, 0.185 mmol) and HCuCMe3TTF (100 mg, 0.37 mmol)
were dissolved in distilled CH2Cl2 (10 mL) in a Schlenk tube.
Nitrogen was bubbled through the mixture for 30 min, after
which CuI (7 mg, 0.037 mmol) and freshly-distilled NEt3
(5 mL) were added and the mixture stirred at room temperature
for 24 h. The solvent was rotary evaporated and the residue
treated with water and extracted with CH2Cl2. The organic layer
was washed with water and dried over MgSO4. The residue was
purified by column chromatography on silica gel with CH2Cl2 as
the eluent. The product was isolated as a red powder in 61%
yield (0.10 g). Single crystals were obtained by slow evaporation
of a concentrated CH2Cl2 solution of 2. 1H NMR (CDCl3)
δ 1.47 (s, 18H, CH3), 1.95 (s, 12H, CH3), 2.21 (s, 6H, CH3),
7.54 (m, 2H, CH), 7.99 (m, 2H, CH), 9.34 (m, 2H, CH); 13C
NMR (CDCl3) δ 12.7, 29.2, 34.8, 93.7, 118.1, 121.5, 121.7,
123.6, 149.7, 155.1, 162.5; IR (ATR): 2091 cm−1 (νCuC);
HRMS (LSIMS): m/z calcd for C40H42N2S8194Pt: 1000.0740.
Found: 1007.0740.
Computational details
Density functional theory38 calculations were performed with the
hybrid Becke-3 parameter exchange functional39 and the Lee–
Yang–Parr nonlocal correlation functional40 (B3LYP)
implemented in the Gaussian 03 (revision D.02) program suite41
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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Table 3 Crystallographic data
Compound
1
2
3
4
Formula
C32H26N2PtS8
890.12
2(C40H42N2PtS8)
2004.65
2(C48H42P2PtS8)
2264.65
C58H48P2PtS8, 2(CH3CN)
1340.58
FW (g mol−1
)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
Monoclinic
P21/c
12.2927(4)
20.7303(7)
13.1062(4)
90
106.7260(10)
90
3198.57(18)
100(2)
Orthorhombic
Pcmn
17.6121(12)
21.4397(14)
38.038(2)
90
Orthorhombic
P212121
12.5211(4)
23.7040(8)
33.0267(11)
90
90
90
9802.3(6)
150(2)
Triclinic
P1
ˉ
9.2106(9)
11.4614(9)
15.8523(13)
103.405(4)
102.101(4)
105.756(5)
1498.8(2)
150(2)
90
90
γ (°)
V (Å3)
T (K)
14 363.1(16)
150(2)
Z
D
4
6
4
1
calc (g cm−3
)
1.848
4.937
21 234
1.391
3.307
70 212
1.535
3.302
48 559
1.485
2.713
22 108
μ (mm−1
Total refls.
)
Uniq. refls. (Rint
Unique refls.
R1, wR2
R1, wR2 (all data)
GoF
)
7265 (0.0465)
5233 (I > 2σ(I))
0.0352, 0.0678
0.0665, 0.0776
1.04
16 876 (0.0646)
9669 (I > 2σ(I))
0.0523, 0.1174
0.101, 0.1297
0.977
20 527 (0.0477)
16 284 (I > 2σ(I))
0.0433, 0.0905
0.0621, 0.0959
1.001
6734 (0.0402)
6454 (I > 2σ(I))
0.0318, 0.0735
0.0346, 0.0749
1.072
using the LANL2DZ basis set42 with the default convergence
criteria implemented in the program. The figures were generated
with Molekel 4.3.43 Computational details are provided as ESI.†
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Dalton Trans.