Redox bifunctionality in a Pt(ii) dithiolene complex of a tetrathiafulvalene diphosphine ligand

Article abstract of DOI:10.1039/b812797b

The Pt(ii) dithiolene complex of a tetrathiafulvalene diphosphine ligand exhibits two reversible redox systems at close potentials, localized on the weakly interacting TTF (tetrathiafulvalene) and Pt(dmit) moieties.

Full text of DOI:10.1039/b812797b

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Redox bifunctionality in a Pt(II) dithiolene complex of a tetrathiafulvalene
diphosphine ligand†
Kyong-Soon Shin,a YounJung Jung,a Su-Kyung Lee,a Marc Fourmigue´,*b Fre´de´ric Barrie`re,b
Jean-Franc¸ois Bergaminib and Dong-Youn Noh*a
Received 25th July 2008, Accepted 11th September 2008
First published as an Advance Article on the web 24th September 2008
DOI: 10.1039/b812797b
The Pt(II) dithiolene complex of a tetrathiafulvalene diphos-
phine ligand exhibits two reversible redox systems at close
potentials, localized on the weakly interacting TTF (tetrathi-
afulvalene) and Pt(dmit) moieties.
at 0.59 V vs. Ag/AgCl, that is ª 0.54 V vs. SCE. On the other
hand, we also found that the recently described platinum complex5
(P2)PtCl2 incorporating the TTF redox centre oxidizes reversibly
at 0.60 and 1.07 V vs. SCE. Since the two redox centres in (P2)PtCl2
and (dppe)Pt(dmit) oxidize at about the same potential (0.54–
0.60 V vs. SCE), it was anticipated that a combination of both
would lead to an interesting competition between the two possible
localization sites of the cation radical, as described below in the
original (P2)Pt(dmit) complex.
Following the discovery of the first organic metal TTF-TCNQ
(TTF = tetrathiafulvalene, TCNQ = tetracyanoquinodimethane)
in the 1970’s and the first organic superconductors in the
1980’s, an enormous amount of creative research, still ongoing
today, has led to the development of a very rich physics of
low-dimensional materials.1 Beyond the quest for novel donor
molecules, new strategies are also being developed to build multi-
functional organic–inorganic molecular solids where an admixture
of several motifs translates into materials with different properties
(conductivity, ferro- or ferrimagnetism) with possible synergistic
effects.2 In that respect, the functionalization of the TTF core with
substituents endowed with coordination ability toward metallic—
eventually para-magnetic—centres has been, and still is, one of the
major routes towards such hybrid materials. In such derivatives,
the metal complex itself can also be electroactive but, for the
elaboration of conducting solids, it is mandatory that the TTF
core oxidizes before the metal complex, in order to allow for the
formation of TTF radical cations and their solid state organization
in conducting stacks.3 One single example has been described so
far where the metallic centre oxidizes before the TTF core. Indeed,
among the various complexes of the chelating 3,4-dimethyl-3¢,4¢-
bis(diphenylphosphino)tetrathiafulvalene (P2),4 such as mono-
chelate complexes (P2)MCl2 (M = Ni,4 Pd and Pt5), and
(P2)M(CO)n (M = Mo, W, Re6,7 or M = Fe, Ru8) and bis-chelate
complexes [(P2)2M]n+ (M = Rh, Ag, Cu, Pt, Pd, Fe, Co and
Ni),9–11 only (P2)Fe(CO)3 was shown to oxidize first into the Fe(I)
species.8 In that respect, the situation where both the TTF core
and the metallic fragment would oxidize at the same potential
offers attractive possibilities for non-symmetrical mixed-valence
systems,12 at variance with the prototypical symmetrical mixed-
valence systems.13,14
(P2)PtCl2 was prepared from P2 and PtCl2(CH3CN)2, by a
modification of the described procedure.5 As the displacement of
the chloride anions of (P2)PtCl2 with, for example, thallium triflate
had been shown to be problematic,5 it was reacted directly with a
dibutyltin derivative of the dmit2- dithiolate, that is Bu2Sn(dmit)
to afford the title compound in good yield.‡ Such tin derivatives
have proved indeed to be very efficient dithiolate transfer agents.18
(P2)Pt(dmit) was purified by chromatography and recrystallised
from CHCl3–MeOH. Its molecular structure was determined from
single crystal X-ray diffraction§ (Fig. 1).
As part of serial studies on the heteroleptic P2PtS2 system,15,16we
noted that (dppe)Pt(dmit), (dmit: 4,5-dimercapto-1,3-dithiole-2-
thione) was reported to oxidise reversibly to the cation state17
aDepartment of Chemistry, Seoul Women’s University, Seoul, 139-774,
Korea. E-mail: dynoh@swu.ac.kr; Fax: +82-2-970-5972
Fig. 1 Molecular structure and atom numbering in (P2)Pt(dmit).
bSciences Chimiques de Rennes, Universite´ de Rennes 1, UMR CNRS 6226,
Rennes, 35042, France. E-mail: marc.fourmigue@univ-rennes1.fr; Fax: +33
2 23 23 67 32
The PtII centre adopts an almost perfect square-planar geometry
with a dihedral angle between the P2Pt and PtS2 mean planes
of 1.96(8)◦. On the other hand, strong deviations from planarity
are observed in the dithiole rings of the TTF core, 19.07(7)◦ for
folding along S3 ◊ ◊ ◊ S4 and 23.2(1)◦ for folding along S1 ◊ ◊ ◊ S2.
† Electronic supplementary information (ESI) available: Results of DFT
calculations on (P2)Pt(dmit). CCDC reference number 696367. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b812797b
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Note that these distortions of the TTF core in its neutral
state are also observed on the dithiole ring bearing the –PPh2
groups, in the optimized geometry deduced from theoretical
calculations performed at the DFT level (B3LYP/LanL2DZ). The
corresponding frontier orbitals for (P2)Pt(dmit) are represented in
Fig. 2, where it appears that the HOMO is strictly localized on the
Pt(dmit) moiety while the HOMO - 1 is localized on the TTF
core. These preliminary calculations might thus indicate that an
oxidation process would first affect the Pt dithiolene part of the
molecule rather than the TTF core.
Compared with the free P2 ligand, the coordination of PtCl2
in (P2)PtCl2 induces a strong anodic shift of 0.26 and 0.36 V for
1
the two TTF-centred redox waves E1/2 and E1/23, of the order of
magnitude of that observed in most P2 metal complexes.4–11 On the
other hand, the (P2)Pt(dmit) exhibits one additional redox peak
E1/22, at 150 mV above the first oxidation peak of (P2)Pt(dmit). The
presence of three waves was confirmed from DPV measurements
while bulk electrolysis indicates that one-electron transfer is asso-
ciated with each redox process. Considering the close proximity
of the first two redox waves in (P2)Pt(dmit) and the calculations
described above, the question then arises of the localisation of these
two processes, together with the degree of interaction between the
two redox centres.
During the bulk electrolysis experiments, vivid colour changes
of the solutions are observed. For (P2)PtCl2, the orange coloured
solution of the neutral complex turned into a dark green colour
solution when the first oxidation was completed (l = 624 nm),
∑
characteristic of the [TTF]+ state.19 In the (P2)Pt(dmit) dithiolene
complex, the two first oxidation waves are strongly overlapping,
preventing the isolation of the intermediate mono-cationic state.
The solution was observed to turn dark-green at the beginning
of the electrolysis while a deep-blue colour solution was finally
obtained in the dicationic state. Note that a blue-violet colour was
indeed reported for the oxidation product of (dppe)Pt(dmit),17
and we confirmed that this compound indeed exhibits indeed an
absorption band at 530–540 nm upon oxidation.
Fig. 2 Molecular orbitals calculated for (P2)Pt(dmit).
Spectroelectrochemical investigations on (P2)Pt(dmit) (Fig. 4)
show, upon oxidation, a strong band centred around 625 nm,
characteristic of the TTF cation radical, as already observed in the
(P2)PtCl2 oxidation product. Shoulders are also noticed on both
sides of this band, and a decomposition into Gaussian curves of the
low-energy part of the spectrum (Fig. 5) shows that bands at 530
and 733 nm grow simultaneously, with the first band at 530 nm thus
clearly associated with the oxidation of the (diphosphine)Pt(dmit)
system, by analogy with the oxidation of (dppe)Pt(dmit).
The electrochemical properties of the Pt(II) complex were inves-
tigated by cyclic voltammetry (CV), differential pulse voltammetry
(DPV) and bulk electrolysis (BE) in CH2Cl2 (Fig. 3 and Table 1).
Fig. 3 Cyclic voltammogram of (P2)Pt(dmit).
Table 1 E1/2 values for the reversible, one-electron redox processes
identified in P2, (P2)PtCl2 and (P2)Pt(dmit)a
E1/21/V
E1/22/V
E1/23/V
P2
0.34
0.60
0.59
—
—
0.74
0.71
1.07
1.08
Fig. 4 Spectro-electrochemistry of (P2)Pt(dmit).
(P2)PtCl2
(P2)Pt(dmit)
This behaviour indicates that the two redox systems involved
in (P2)Pt(dmit), that is, the TTF core on the one hand and the
Pt(dmit) moiety on the other, oxidize almost simultaneously. At
a In CH2Cl2 solution with 0.1 M n-Bu4N·BF4 at 500 mV s-1 with SCE as
reference electrode.
5870 | Dalton Trans., 2008, 5869–5871
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Synthesis of (P2)Pt(dmit). Bu2Sn(dmit)18 (0.5 mmol, 220 mg) and
(P2)PtCl2(0.5 mmol, 430 mg) were dissolved in acetone (20 mL) with
stirring for 3 h. The red solution was evaporated and separated by column
chromatography (SiO2, CHCl3). The red band was collected, dried under
reduced pressure and crystallized from CHCl3–MeOH. Yield 75% (370
mg). MALDI-TOF (m/z): 992.0402 (M+ + 1). FT-IR (KBr, cm-1) 3056
=
(Ar C–H), 2984 (–CH3), 1627 (C C), 1466, 1435 (Ar ip str), 1098 (P–Ph
str), 1056 (C S), 743 (oop CH def), 690, 548, 520 (oop ring def). UV-Vis
=
(CH3CN, nm, emax ¥ 104 M-1 cm-1) 196 (1.96), 228 (sh, 0.64), 272 (sh, 0.23),
1
326 (sh, 0.06), 452 (sh, 0.02), 460 (0.02). H NMR (CDCl3, d ppm) 7.77
(8H, m) 7.53 (12H, m) 1.90 (6H, s). 31P{ H} NMR (CDCl3, H3PO4, d ppm)
1
-33.86 (1JPt–P = 2839 Hz).
§ X-Ray data collections were performed on an APEXII, Bruker-AXS
diffractometer with graphite-monochromated Mo-Ka radiation (l =
˚
0.71073 A) at 100 K. Structures were solved by direct methods (SHELXS-
97) and refined by full matrix least-squares methods (SHELXL-97).
Absorption correction was applied. Hydrogen atoms were introduced at
calculated positions (riding model), included in structure factor calcula-
tions, and not refined. Crystal data for (P2)Pt(dmit): C35H26P2PtS9·CHCl3,
M = 1111.5 g mol-1, crystal dimensions 0.58 ¥ 0.11 ¥ 0.06 mm, monoclinic,
˚
space group P21/n, a = 10.2717(6), b = 26.2907(17), c = 15.3070(11) A,
◦
3
b = 96.911(3) , V = 4103.6(5) A , Z = 4, rcalcd = 1.799 g cm-3, F(000) =
˚
Fig. 5 Decomposition of the experimental absorption spectra.
2184, m = 4.179 mm-1, T = 100 K, 2qmax = 55.14. Final results (for 490
parameters) were R1 = 0.0302, wR2 = 0.0636 and S = 1.015 for 8019
reflections with I > 2s(I).
the present stage of our investigations, it is difficult to assess
if the small potential difference between the two first oxidation
waves is only a consequence of the inherent dissymmetry of the
system or if it is attributable to an electrostatic interaction between
the two redox centres. We can already anticipate a rather weak
interaction between the radical species since: (i) the two p systems
are connected through the orthogonal P→Pt s bonds and (ii) the
radical cation on the TTF side is preferentially localized on the
outer 4,5-dimethyl-1,3-dithiole ring, as shown by the asymmetry of
the HOMO - 1 coefficients on the TTF core (Fig. 2), a behaviour
1 P. Batail, Chem. Rev., 2004, 104, 4887.
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∑
already observed for example in oxidized [(P2)M(CO)4]+ (M =
Mo, W) complexes.8
We are therefore in the presence of a rare example of a TTF
metal complex where both redox active moieties oxidise almost
simultaneously and independently, a behaviour which is also
related to non-symmetrical mixed-valence systems.12,20 Isolation
of the cationic and dicationic species is certainly now needed to
gain a deeper understanding of this versatile system. Indeed, the
dmit ligand can also be easily replaced by other dithiolene ligands
of different electron donating ability, allowing for a fine tuning of
the relative weight of the TTF and Pt(dithiolene) moieties in the
oxidation process.
9 M. Fourmigue´, C. E. Uzelmeier, K. Boubekeur, S. L. Bartley and K. R.
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This work was initiated and supported by the Korean-French
Science and Technology Amicable Research Program (STAR)
2006–07 (no. 12974RC) and 2008–09 (no. 19036SM). D.-Y. N.
acknowledges financial supports from Korea Science and Engi-
neering Foundation (KOSEF R01-2003-000-10470-0) and Korea
Research Foundation (KRF-2003-070-C00029). We thank the
CINES (Montpellier) for computing time.
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Notes and references
‡ Experimental data: Synthesis of (P2)PtCl2. A chloroform solution
(20 mL) of PtCl2(CH3CN)2 (0.72 mmol, 250 mg) and P24 (0.72 mmol,
432 mg) was refluxed overnight with stirring. The red solution was cooled
to room temperature, evaporated under reduced pressure and washed with
18 S. M. S. V. Doidge-Harrison, J. T. S. Irvine, A. Khan, G. M. Spencer,
J. L. Wardell and J. H. Aupers, J. Organomet. Chem., 1996, 516, 199;
M. Nomura and M. Fourmigue´, Inorg. Chem., 2007, 47, 1301.
19 V. Khodorkovsky, L. Shapiro, P. Krief, A. Shames, G. Mabon, A.
Gorgues and M. Giffard, Chem. Commun., 2001, 2736; S. V. Rosokha
and J. K. Kochi, J. Am. Chem. Soc., 2007, 129, 828.
20 N. Dowling, P. M. Henry, N. A. Lewis and H. Taube, Inorg. Chem.,
1981, 20, 2345; Z. Li, A. M. Beatty and T. P. Fehlner, Inorg. Chem.,
2003, 42, 5707; M. C. B. Colbert, S. L. Ingham, J. Lewis, N. J. Long and
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diethyl ether. Yield 97% (603 mg). MALDI-TOF (m/z): 866.1093 (M+
+
1), 831.1475 (M+ - Cl + 1). FT-IR (KBr, cm-1) 3054 (Ar C–H), 2916,
=
2835 (–CH3), 1629 (C C), 1481, 1436 (Ar ip str), 1101 (P–Ph str), 743
(oop CH def), 690, 561, 524, 485 (oop ring def). UV-Vis (CH3CN, nm,
emax ¥ 104 M-1 cm-1) 196 (2.22), 234 (0.72), 270 (sh, 0.34), 280 (sh, 0.30),
316 (0.29), 452 (0.01). 1H NMR (CDCl3, d ppm) 7.85 (8H, m) 7.57 (12H,
1
m) 1.90 (6H s). 31P{ H} NMR (CDCl3, H3PO4, d ppm) -27.22 (1JPt–P
=
3640 Hz).
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 5869–5871 | 5871
©