Structure-Dependent Electrochemical Behavior of Thienylplatinum(II) Complexes of N,N-Heterocycles

Article abstract of DOI:10.1002/ejic.200300320

trans- [Pt(MeCN)(PPh₃)₂(2-thienyl)]BF₄ (1) serves as a convenient precursor to bifunctional mononuclear trans-[Pt(PPh ₃)₂(η¹-N-N)(2-thienyl)]BF₄ [N-N = pyrazine (2); 2-chloropyrazine, (3)] and dinuclear trans,trans-[Pt ₂(PPh₃)₄(μ-N-N)(2-thienyl) ₂](BF₄)₂ [(N-N = 4,4′-bipyridine (4); 4,4′-vinylenedipyridine (5)] complexes. The nuclear selectivity is conveniently controlled by the choice of the heterocyclic ligands or spacers. Both structural types 3 and 5 were confirmed by single-crystal X-ray crystallographic analyses. Their solution identities were established by positive-ion Electrospray Mass Spectrometry (ESMS). The electroactivities of these complexes were studied by cyclic voltammetry (CV). Continuous CV scans of 4 and 5 revealed variations in the redox waves with the number of scans. While the initial oxidative scan exhibited only a broad, irreversible wave, further cycling showed the growth of two additional redox couples up to about the tenth cycle. The peak currents of these redox couples began to decay with prolonged potential cycling beyond the tenth cycle. These findings are consistent with the formation of electroactive oligomers/polymers, and this conclusion is supported by visible thin film formation on the electrodes. In contrast, the mononuclear complexes (2 and 3) do not show such behavior. The films formed were further studied by repetitive potential cycling and XPS. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200300320

FULL PAPER
Structure-Dependent Electrochemical Behavior of Thienylplatinum(II
)
Complexes of N,N-Heterocycles
Feng Zhao,[a] Xingling Xu,[a] Soo Beng Khoo,*[a] and T. S. Andy Hor*[a]
Keywords: Platinum / N ligands / Cyclic voltammetry / Thin films / Heterocycles
trans-[Pt(MeCN)(PPh3)2(2-thienyl)]BF4 (1) serves as a con-
venient precursor to bifunctional mononuclear trans-
[Pt(PPh3)2(η1-N-N)(2-thienyl)]BF4 [N-N = pyrazine (2); 2-
chloropyrazine, (3)] and dinuclear trans,trans-[Pt2(PPh3)4(µ-
with the number of scans. While the initial oxidative scan
exhibited only a broad, irreversible wave, further cycling
showed the growth of two additional redox couples up to
about the tenth cycle. The peak currents of these redox
N-N)(2-thienyl)2](BF4)2 [(N-N = 4,4Ј-bipyridine (4); 4,4Ј-viny- couples began to decay with prolonged potential cycling
lenedipyridine (5)] complexes. The nuclear selectivity is con-
veniently controlled by the choice of the heterocyclic ligands
or spacers. Both structural types 3 and 5 were confirmed by
single-crystal X-ray crystallographic analyses. Their solution
identities were established by positive-ion Electrospray Mass
Spectrometry (ESMS). The electroactivities of these com-
plexes were studied by cyclic voltammetry (CV). Continuous
beyond the tenth cycle. These findings are consistent with
the formation of electroactive oligomers/polymers, and this
conclusion is supported by visible thin film formation on the
electrodes. In contrast, the mononuclear complexes (2 and 3)
do not show such behavior. The films formed were further
studied by repetitive potential cycling and XPS.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
CV scans of 4 and 5 revealed variations in the redox waves Germany, 2004)
Introduction
ing heterocyclic ligands (depicted below) in the assembly
of multimeric bifunctional d8 networks based on a readily
synthesized precursor. Introduction of different N-ligands,
viz. pyridine or pyrazine-based heterocycles enables us to
tune the spatial and structural features that have a direct
impact on the electrochemical responses.
The preparation of macro- and supramolecular assembl-
ies with functionalised properties is currently a major chal-
lenge in synthetic chemistry.[1] A careful selection of a tran-
sition metal and a conjugated ligand could result in not
only targeted molecular sizes and shapes,[2] but also ex-
pected electrochemical, magnetic and optical properties. In-
corporation of transition metals into the conjugated back-
bone enhances metal dπ-pπ* back-bonding, which is ex-
pected to contribute significantly to the π-electron delocal-
ization.[3] The combination of conjugated ligands, electron-
rich metal centers, and the high degree of covalence in-
herent in soft-soft bonding can promote low-energy elec-
tronic interactions between the metal and the ligand. Our
recent interest has been focused on novel electroactive me-
tal-organic complexes.[4] Our current interest in d8 thienyl
complexes[5] stems from their geometric directionality and
conductive potential[6] upon polymerization. Recently, we
have extended this study to the construction of N- and P-
bridged transition metal thienyl complexes. The simplest
way to achieve this is through the introduction of a bridging
co-ligand, or spacer. A non-thienyl spacer would bring an
extra dimension of functionality to the complex. This paper
describes a combined use of thienyl and nitrogen-contain-
Results and Discussion
(1) Synthesis
[a]
Department of Chemistry, Faculty of Science, National
University of Singapore,
The reaction in acetonitrile solution between equimolar
trans-[PtBr(PPh3)2(2-thienyl)][7,8] and AgBF4, gives trans-
[Pt(MeCN)(PPh3)2(2-thienyl)]BF4, (1). Its quantitative yield
3 Science Drive 3, Singapore 117543, Singapore
Fax: (internat.) ϩ65-6779-1691
E-mail: andyhor@nus.edu.sg
Eur. J. Inorg. Chem. 2004, 69Ϫ77
DOI: 10.1002/ejic.200300320
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69

F. Zhao, X. Xu, S. B. Khoo, T. S. A. Hor
FULL PAPER
Scheme 1. Formation pathway of complexes 1Ϫ5
and the presence of a labile MeCN solvent molecule make complexes showed the expected trans-phosphanes [typically
it a suitable precursor to bifunctional complexes. Although 1JPt-P (ca. 3480 Hz)] that are equivalent. As expected, both
1 can be isolated, it is more convenient to use it in situ. the shifts and couplings are fairly insensitive to subtle elec-
Upon removal of AgBr by filtration from the product tronic changes in the cis N-ligands.
mixture, addition of 4,4Ј-bipyridine or 4,4Ј-vinylenedi-
(2) Solid State Structures
pyridine gives dinuclear trans,trans-[Pt2(PPh3)4(µ-N-N)(2-
thienyl)2][BF4]2 [N-N ϭ 4,4Ј-bipyridine (4), 4,4Ј-vinylenedi-
pyridine (5)] respectively. However, use of pyrazine and 2-
chloropyrazine as substrates gives only mononuclear trans-
[Pt(PPh3)2(η1-N-N)(2-thienyl)][BF4] [N-N ϭ pyrazine, (2),
2-chloropyrazine, (3)] as yellow solids in high yields, despite
Complexes 3 (Figure 1) and 5 (Figure 2) were chosen as
models for X-ray crystallographic elucidation of the solid-
state structures. The refinement data and bond/angle par-
ameters are collected in Table 1 and Table 2, respectively.
Complex 3 is mononuclear, comprising two trans-oriented
phosphanes [P(2)ϪPt(1)ϪP(2) 176.28(6)°] and a thienyl
the presence of
a
trans-disposed N-functionality
(Scheme 1).
These illustrate an effective nuclear selectivity through a
simple control of the N-spacer. Heterocycles in which
charge delocalization is efficient, such as 2 and 3, would
have a lower nucleophilicity of the second nitrogen to an
extent that it prefers to be pendant rather than bridging.
The presence of the bulky PPh3 ligand could also impede
the approach of the neighboring PtII moiety towards a less
protruded heterocyclic arm. Construction of cationic metal-
based molecular boxes have met with a similar fate.[9,10]
NMR analysis suggested that these cationic complexes are
nonlabile. The 1H coordination shifts (upfield) of the α- and
β- pyrazine or pyridine hydrogen atoms (possibly due to
shielding by the PPh3 ligand) are indicative of coordinative
nonlability. The α- and β-pyridine hydrogen atoms of 2 are
similarly shifted upfield to δ ϭ 8.21, 7.96 ppm, relative to
free pyrazine (δH ϭ 8.58), and, similarly for 5, to δ ϭ 7.81,
7.06 ppm, relative to free 4,4Ј-vinylenedipyridine (Hα δ ϭ
Figure 1. Thermal ellipsoids (50% probability) of 3 with the atomic
8.62, Hβ δ ϭ 8.39 ppm). The 31P NMR spectra of all the numbering scheme
70
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Thienylplatinum(II) Complexes of N,N-Heterocycles
FULL PAPER
Figure 2. Thermal ellipsoids (50% probability) of 5 with the atomic numbering scheme
Table 1. Crystallographic data and structure refinement details for 3 and 5
Complex
3
5
Empirical formula
Formula mass
Color
C45H38BCl3F4N2P2PtS
1089.02
pale yellow
0.26 ϫ 0.14 ϫ 0.10
tetragonal
P4(3)
295(2)
10.6753(5)
C92H80B2F8N2O3P4Pt2S2·CH2Cl2
2100.33
yellow
0.28 ϫ 0.15 ϫ 0. 06
monoclinic
P2/c
223(2)
22.654(3)
11.2330(13)
20.113(2)
Crystal size (mm)
Crystal system
Space group
T (K)
˚
a (A)
˚
b (A)
10.6753(5)
38.654(3)
˚
c (A)
α (°)
β (°)
γ (°)
90
90
90
2156
90
111.021(2)
90
2084
4777.5(10)
F(000)
3
˚
V (A )
4405.1(4)
Z
4
2
Absorption coefficient (mmϪ1
)
3.540
1.644
3.154
1.459
Density (g·cmϪ3
Index ranges
)
Ϫ12 Յ h Յ 10, Ϫ12 Յ k Յ 12, Ϫ45 Յ l Յ 45
25879
7761(Rint ϭ 0.0636)
R1 ϭ 0.0423, wR2 ϭ 0.0548
R1 ϭ 0.0546, wR2 ϭ 0.0565
0.995
Ϫ26 Յ h Յ 26, Ϫ12 Յ k Յ 13, Ϫ23 Յ l Յ 23
26547
8416 (Rint ϭ 0.0515)
R1 ϭ 0.0563, wR2 ϭ 0.1570
R1 ϭ 0.0818, wR2 ϭ 0.1683
1.043
Reflections collected
Independent reflections
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
Goodness-of-fit on F2
Ϫ3
˚
Largest diff. peak and hole (e·A
)
1.141 and Ϫ0.889
3.154 and Ϫ1.813
group opposite the chloropyrazine ring [N(1)ϪPt(1)ϪC(1) vinylenedipyridine bridging ligand, and the thienyl groups
179.4(5)°]. The nitrogen atom meta to the chloro func- are disordered.
[10]
All PtϪN[11] [2.082(4)-2.107(4) A] and PtϪP
lengths
˚
tionality is preferentially coordinated whereas the ortho
˚
counterpart is pendant. The thienyl and chloropyrazine [2.3212(19)-2.323(3) A] are normal although the PtϪN
rings are tilted at an angle of 29.5° to each other. The prox- bonds of 5 are somewhat longer (in 1.2%) than in 3. There
imity of the phenyl rings makes the pendant nitrogen atom is no evidence of perturbance to the olefinic [C(10)ϪC(10A)
˚
less susceptible to attack. Complex 5 is dinuclear with a 1.352 A] moiety at the center of the bridge.
4,4Ј-vinylenedipyridine group bridging two PtII centers, op-
posite two thienyl terminal ligands [N(1)ϪPt(1)ϪC(1)
(3) ESMS Characterization
168.9(2)°]. The two pyridyl rings are strictly coplanar (θ,
0°).
The development of electrospray mass spectrometry
(ESMS) by Fenn[12] and others[13] has provided a powerful
Complex 3 has a noncrystallographic twofold symmetry soft technique to study multiply-charged ions, primarily
along the N(1)ϪPt(1)ϪC(1) axis. Both substituents at the large biomolecules, in solution. Recently, we have extended
Pt(1) center, namely thienyl and 2-chloropyrazine groups this development as a combinatorial-like technique to probe
are disordered along this axis. Complex 5 has a center of the chemistry of [Pt2(µ-X)2(PPh3)4] (X ϭ S, Se).[14] In this
inversion between the methylene carbon atoms of the 4,4Ј- paper we use ESMS to investigate the fragmentation of the
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71

F. Zhao, X. Xu, S. B. Khoo, T. S. A. Hor
FULL PAPER
tation pathway is illustrated in Scheme 2. The presence of
the intact, doubly charged molecular ion Peak A (m/z ϭ
892.7, 100%), [M2(4,4Ј-vinylenedipyridine)]2ϩ lends support
to the suggestion that the solution and solid state structures
are in agreement. Fragmentation peaks B (m/z ϭ 802.0,
10%), D (m/z ϭ 843.8, 4%), and C (m/z ϭ 983.8, 32%) were
also detected, corresponding to Mϩ, [M(MeCN)]ϩ and
[M(4,4Ј-vinylenedipyridine)]ϩ, respectively. Similarly, in the
positive-ion ES mass spectrum of complex 4, the intact,
doubly charged molecular ion peak at m/z ϭ 880.1 was ob-
served, together with the fragmental molecular ion peaks at
m/z ϭ 957.0 {[M(4,4Ј-bipyridine)]ϩ}, 802.0 and 843.8.
Their formation can be explained by bridge cleavage by
the residual MeCN molecule, thereby generating two cat-
ionic mononuclear complexes in which one bears a pendant
heterocyclic ligand and the other captures the solvent li-
gand (Scheme 2). The ligand replacement or bridge cleavage
by the strong coordination solvent MeCN has been re-
ported before.[15] However, we should add that this takes
place only when voltage is applied under spectrometric con-
ditions. The cleavage invariably occurs at the heterocyclic
junction, indicating that this is the weakest link in this series
of bifunctional networks. The strong σ-donating and π-ac-
cepting character of the phosphanes as well as the anionic
nature of thienyl, which electrostatically strengthens the
MϪC bond, make them relatively immune to such attack.
Table 2. Selected bond lengths and angles for 3 and 5
3
˚
Bond lengths (A)
Pt(1)ϪC(1)
Pt(1)ϪN(1)
C(4)ϪS(1)
C(8)ϪN(1)
2.005(10) Pt(1)ϪP(2)
2.082(4) Pt(1)ϪP(1)
1.657(14) C(1)ϪS(1)
1.316(11) C(5)ϪN(1)
2.323(2)
2.323(2)
1.705(12)
1.346(10)
Bond angles (°)
C(1Ј)ϪPt(1)ϪC(1)
C(1)ϪPt(1)ϪN(1) 179.4(5)
0.8(19)
C(1)ϪPt(1)ϪP(2) 88.2(8)
C(1)ϪPt(1)ϪP(1) 88.1(8)
N(1)ϪPt(1)ϪP(2)
91.60(19)
N(1)ϪPt(1)ϪP(1) 92.12(19)
P(2)ϪPt(1)ϪP(1) 176.28(6)
5
˚
Bond lengths (A)
Pt(1)ϪC(1)
Pt(1)ϪN(1)
C(4)ϪS(1)
C(9)ϪN(1)
C(10)-C(10A)
2.038(7)
2.107(4)
1.7997(10) C(1)ϪS(1)
1.377(8)
1.352
Pt(1)ϪP(2)
Pt(1)ϪP(1)
2.3221(19)
2.3212(19)
1.7995(10)
1.348(9)
C(5)ϪN(1)
Bond angles (°)
C(1Ј)ϪPt(1)ϪC(1) 11.03(17)
C(1)ϪPt(1)ϪN(1) 168.9(2)
C(1)ϪPt(1)ϪP(2) 94.1(2)
C(1)ϪPt(1)ϪP(1) 87.4(2)
N(1)ϪPt(1)ϪP(1) 90.10(16)
N(1)ϪPt(1)ϪP(2)
89.23(16)
P(2)ϪPt(1)ϪP(1) 175.73(7)
(4) Electrochemical Behavior
Cyclic voltammetric (CV) studies were performed on
complexes 2, 3, 4 and 5 (1 m) in CH2Cl2 by using
Bu4NBF4 (0.10 ) as the supporting electrolyte. All com-
plexes show one irreversible oxidation wave in the first scan.
Complexes 4 and 5 show different electrochemical behavior
from that of 2 and 3. The repetitive cycling profiles for 5
are illustrated in Figure 4.
parent ions in solution. Analysis of the mononuclear com-
plexes in CH2Cl2 in the positive ion mode gave a [M(N-
N)]ϩ peak for 2 (m/z ϭ 881.7) and 3 (m/z ϭ 915.7) [M ϭ
Pt(PPh3)2(2-thienyl); N-N ϭ pyrazine, (2), 2-chloropyrazine,
(3)]. In these complexes, there is a common peak associated
with the replacement of the pyrazine ligand by MeCN viz.
[M(MeCN)]ϩ (m/z ϭ 843.8), which was observed as the
main peak for complex 1. These data suggested that the
solid-state identity of these complexes is maintained in solu-
tion.
During the first cycle, the forward (anodic) scan gave a
complex oxidation wave O1, which could be assigned to
thiophene oxidation,[16] with at least two unresolved peaks.
On reversing the scan, the reduction peaks R3 and R2 were
observed. During the second cycle, new oxidation peaks O2
and O3 emerged, which were not observed on the first scan.
Associated with these, the earlier oxidation wave O1
dropped significantly in the second anodic scan, and disap-
peared completely in subsequent cycles. Conversely, peaks
O2/R2 and O3/R3 continued to increase in size to the tenth
cycle, after which they began to decay with further cycles.
This behavior is consistent with the formation of electroac-
tive or conductive polymers or high oligomers.[17] Since
these species are less readily soluble in the analyte solution,
their formation would alter the redox pattern of the sample,
and eventually render the analyte featureless in the CV
scans. Polymerization is likely to take place at the thienyl
site since electropolymerization of thiophene, bithiophene
and substituted thiophenes is common.[18] When the elec-
trode was removed from solution at the conclusion of the
scanning experiments, rinsed with CH2Cl2 and dried, a
greenish thin film was visible on the electrode surface. The
The positive-ion ES mass spectrum of complex 5 dis-
solved in CH2Cl2 is shown in Figure 3. A possible fragmen-
Figure 3. Positive-ion ES mass spectrum of 5 in CH2Cl2 solvent.
The insets show the (a) calculated and (b) observed isotope distri-
bution pattern for the intact dinuclear parent ion
72
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Thienylplatinum(II) Complexes of N,N-Heterocycles
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Scheme 2. Possible fragmentation pathway of 5 depicting the species observed in the positive ion ES mass spectrum
the oxidation wave O1 with no other obvious peak forma-
tion. The relevant electrochemical parameters for the com-
plexes are summarized in Table 3. The CV profiles thus
serve as supporting finger-printing diagnosis for these com-
plexes.
In an attempt to understand this disparity, we carried out
further electrochemical investigations of the films. After
film formation (typically 10 scans), the potential cycling in
the complex solution was terminated, and the film-coated
electrode was washed with CH2Cl2. The electrode was then
re-immersed in the blank electrolyte solution and cycled
again between ϩ0.00 V and ϩ1.60 V. The peak heights de-
creased slowly with repetitive potential cycling, suggesting
that the films formed from 4 and 5 are not electrochemically
stable. The CV responses of these films upon treatment are
also different from those seen in the O2/R2 and O3/R3 co-
Figure 4. Continuous cyclic scanning between 0.00 to ϩ1.60 V of
5 (0.1 mol·dmϪ3) supported by Bu4NBF4 in CH2Cl2
material properties of this film are under investigation in uples when the films were first formed. As seen in Table 3,
our laboratory.
after the tenth cycle, the anodic peak current in O2 is 2.79
O2
Cycling experiments on 4 revealed a similar decay ϫ 10Ϫ5 A (ipcR2/ipa ϭ 0.96 in this cycle) while that in O3
phenomenon, with slightly different redox potentials. This is 3.68 ϫ 10Ϫ5 A (ipcR3/ipa ϭ 1.02). This gives an O2/O3
O3
suggests that the CV behavior is structurally related, and peak height ratio of 0.76. Similar analysis of the isolated
the redox potentials are sensitive to the bridging ligands. film (Figure 5) shows 9.57 ϫ 10Ϫ6 A for the anodic peak
Accordingly, 2 and 3, which are isostructural, show similar current in O2 and 1.58 ϫ 10Ϫ5 A for that in O3, thus yield-
O2
O3
initial oxidative behavior to that of 4 and 5 (i.e. only a ing a ratio of 0.60 (ipcR2/ipa ϭ 0.86 and ipcR3/ipa
ϭ
broad, irreversible oxidation wave) but no corresponding 0.73). The current after film formation and isolation is un-
film formation is observed with continuous cycling. Cycling usual but can be understood. Firstly, the electrogenerated
of these two analyte solutions resulted in the decrease of cation could be a very reactive electrophilic PtIII, which is
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F. Zhao, X. Xu, S. B. Khoo, T. S. A. Hor
FULL PAPER
Table 3. Electrochemical data for complexes 2Ϫ5 and films formed at a glassy carbon electrode
Complex[a]
Epa
Epc
Epa
∆Epp(3)
ipcR3/ipa
Epc
Epc
∆Epp(2)
ipcR2/ipa
O1[b]
R3[c]
O3
[d]
O3[e]
R2
O2
O2
2
1.26
1.20
1.16
1.28
Ϫ
Ϫ
Ϫ
0.91
0.90
0.86
0.82
Ϫ
Ϫ
Ϫ
Ϫ
1.11
1.12
Ϫ
Ϫ
Ϫ
Ϫ
0.25
0.30
Ϫ
Ϫ
Ϫ
Ϫ
0.95
1.02
Ϫ
Ϫ
0.33
0.31
0.26
0.25
Ϫ
Ϫ
Ϫ
Ϫ
0.52
0.51
Ϫ
Ϫ
Ϫ
Ϫ
0.26
0.26
Ϫ
Ϫ
Ϫ
Ϫ
0.94
0.96
3
4
5
Film (4)[f]
Film (5)[g]
Ϫ
[a]
Electrochemical data for complexes 2Ϫ5 from cyclic voltammetry in CH2Cl2/0.1  Bu4NBF4: potential in V vs. Ag/AgNO3, scan rate
100 mV sϪ1
.
Anodic peak potential for irreversible oxidation. Cathodic peak potential for reduction.
Peak-to-peak separation
[b]
[c]
[d]
[e]
[f]
[g]
∆Epp ϭ Epa Ϫ Epc (V). The ratio of reduction current to oxidation current. Film (4) was derived from 10 cycles of 4. Film (5)
was derived from 10 cycles of complex 5.
Figure 6. The XPS spectra of the film formed from 5
Figure 5. Continuous cyclic scanning between 0.00 to ϩ1.60 V of
the film formed from 5 in blank electrolyte solution containing 0.1
 Bu4NBF4 in CH2Cl2
solution was terminated, the film-coated electrode was re-
trieved and washed with CH2Cl2, and the film analyzed by
bound to decay rapidly at room temperature.[19] Secondly, XPS, the Pt(4f) spectrum showed the spin-orbit compo-
the stability of the film at the potentials at which it is nents separated by 3.25 eV (Figure 6). The Pt(4f) binding
formed could be poor, which is known as the ‘‘thiophene energies, which compare well with those of phosphane-
paradox’’[20] in the synthesis of polythiophene conducting bearing platinum,[22] are indicative of PtII. Further S(2p),
polymers via electrooxidation.
N(1s) and P(2p) spectral analyses revealed fairly typical
To gain some understanding of the elemental compo- spectra for thienyl, bipyridyl and phosphane complexes.
sition of the film, we carried out an XPS analysis[21] of the Further studies of the film properties and characteristics,
film formed from 5. This film was formed typically after which are outside the present scope, will be a subject of our
about 10 scans. When the potential cycling of the complex future reports.
Scheme 3. Possible dimerization pathway for the mononuclear species 2 and 3 under applied potential in CV experiments
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Thienylplatinum(II) Complexes of N,N-Heterocycles
FULL PAPER
Scheme 4. Possible polymerization process for 4 and 5 that leads to film formation on the surface of the electrode from continuous
scanning
It is not our primary interest to study the details of the and 3, dimerization would lead to a Pt2 complex bridged
electropolymerization mechanism, but we can propose a through bithiophenyl groups and with terminal NϪN
reasonable pathway, involving repetitive electrogeneration groups. There is no α-thienyl end and hence no further
of a cation radical, followed by α-α coupling, that is thienylϪthienyl coupling. These electrogenerated dinuclear
grounded in the literature[23Ϫ25] and allows us to explain the complexes are soluble and they do not lead to further poly-
different behavior of the mono- (Scheme 3) and dinuclear merization or film formation.
complexes (Scheme 4). At the first cycle of electropromoted
coupling, the dinuclear precursors 4 and 5 yield bithio-
phene-bridged Pt4 complexes that have two free ends of α-
thienyl group. The latter could undergo further coupling
Conclusion
using the free α functionality by a similar mechanism as
Using ESMS as a guide, we have successfully constructed
above. This would transform a Pt4 into a Pt6 chain, and so novel mononuclear and dinuclear thienylplatinum com-
on. As there is no facile chain-termination step, a film plexes with different bridging coligands. The nuclear selec-
would be the obvious product on ending the electropoly- tivity can be easily controlled by the choice of the N-spacer.
merization cycles. However, for mononuclear complexes 2 The electrochemical experiments led to promising metal-
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F. Zhao, X. Xu, S. B. Khoo, T. S. A. Hor
FULL PAPER
(1004.13): calcd. C 52.63, H 3.61, N 2.79, S 3.19; found C 52.28,
H 3.12, N 2.55, S 3.58. H NMR (CDCl3): δ ϭ 7.2Ϫ7.5 (m, 60 H,
phenyl), 8.69 (d, 1 H, pyrazine), 7.93 (d, 1 H, pyrazine), 7.61 (s, 1
H, pyrazine), 6.94 (d, 1 H, thiophene), 6.39 (t, 1 H, thiophene), 5.96
(d, 1 H, thiophene) ppm. 31P NMR (CDCl3): δ ϭ 18.12 (s) ppm.
containing thin films from electrocycling of dinuclear com-
plexes with a thiophene function at either end. The poten-
tial for film formation from structurally characterizable ma-
terials and the possibility for adjusting the film properties
through the use of different spacers would add a new di-
mension to our current research.
1
trans,trans-[Pt2(µ-4,4Ј-bipyridine)(PPh3)4(2-thienyl)2][BF4]2 (4): The
synthetic procedure is similar to that of 2. 4,4Ј-Bipyridine (7.8 mg,
0.05 mmol) and complex [trans-PtBr(2-thienyl)(PPh3)2] (88 mg,
0.1 mmol) gave a pale yellow solid residue of complex 4 (78.5 mg,
80%). C90H74B2F8N2P4Pt2S2 (1935.38): calcd. C 55.85, H 3.85, N
1.45, S 3.31; found C 55.40, H 3.79, N 1.38, S 3.83. 1H NMR
(MeCN): δ ϭ 7.2Ϫ7.5 (m, 60 H, phenyl), 8.48 (d, 4 H, pyridine),
6.89 (d, 4 H, pyridine), 7.03 (d, 2 H, thiophene), 6.35 (t, 2 H, thio-
phene), 5.37 (d, 2 H, thiophene) ppm. 31P NMR (MeCN): δ ϭ
18.46 (s) ppm.
Experimental Section
All reactions were routinely performed under purified nitrogen
using standard Schlenk techniques. Solvents used were of reagent
grade and were freshly distilled and degassed under purified nitro-
gen before use. Elemental analyses were carried out in the Chemis-
try Department of National University of Singapore (NUS). 1H
and 31P NMR spectra were recorded on a Bruker ACF 300 MHz
spectrometer at ca. 300 K. 1H and 31P NMR chemical shifts are
quoted in ppm downfield of Me4Si and externally referenced to
85% H3PO4, respectively.
trans,trans-[Pt2(PPh3)4(2-thienyl)2(µ-4,4Ј-vinylenedipyridine)][BF4]2
(5): The procedure is similar to that of 2. 4,4Ј-Vinylenedipyridine
(9.1 mg, 0.05 mmol) and complex [trans-PtBr(2-thienyl)(PPh3)2]
(88 mg, 0.1 mmol) gave a yellow solid residue of complex 5
(80.3 mg, 82%). Recrystallization from CH2Cl2/hexane gave pale
yellow crystals. C92H76B2F8N2P4Pt2S2 (1961.42): calcd. C 56.34, H
3.91, N 1.43, S 3.27; found C 56.49, H 3.65, N 1.50, S 3.16. 1H
NMR (CDCl3): δ ϭ 7.2Ϫ7.5 (m, 60 H, phenyl), 7.81 (d, 4 H, pyri-
dine), 7.06 (d, 4 H, pyridine), 7.07 (s, 2 H, pyridine), 6.96 (d, 2 H,
thiophene), 6.41 (t, 2 H, thiophene), 5.97 (d, 2 H, thiophene) ppm.
31P NMR (CDCl3): δ ϭ 18.59 (s) ppm.
Electrospray Mass Spectrometry (ESMS): The electrospray mass
spectra were recorded in positive-ion mode on a Finningan LCQ
spectrometer. The spray voltage was 4.5 kV, and the capillary tem-
perature was 40 °C. The peaks in the ESMS are identified by the
most intense m/z value within the isotopic mass distribution. Iso-
tope patterns were recorded under high-resolution conditions for
all major ions and compared with theoretical patterns obtained
using the Isotope program.[26] In all cases there was good agree-
ment between the experimental and calculated isotopic mass distri-
butions.
Crystallography: The diffraction experiments for complexes 3 and
5 were carried out on a Bruker AXS CCD diffractometer with a
Mo-Kα sealed tube. The software SMART[27] was used for col-
lecting frames of data, indexing reflections, and determination of
lattice parameters, SAINT[27] for integration of intensity of reflec-
tions and scaling, SADABS[28] for empirical absorption correction,
and SHELXTL[29] for space group and structure determination,
refinements, graphics, and structure reporting. In the crystal struc-
tures of 3 and 5, the thienyl groups were disordered. Two indepen-
dent models were refined in each case. The chlorine atom of the
chloropyrazine ligand in 3 was also found to be disordered. The
asymmetric unit in 5 has half a dichloromethane solvate molecule
and 1.5 mol of disordered water molecules. The positive residual
electron densities in 5 were associated with the Pt atom. A sum-
mary of crystallographic parameters for the data collections and
refinements is given in Table 1. CCDC-182455 and CCDC-182456
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; Fax:
(internat.) ϩ 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
Cyclic Voltammetry (CV): Containers (glassware, polyethylene
bottles, etc.) were soaked overnight in 10% HNO3 prior to use.
Electrochemical experiments were performed with an Autolab
PGSTAT 30 electrochemical system (Eco Chemie, Netherlands). A
locally made three-electrode glass cell of approximately 5 mL ca-
pacity was used for all electrochemical experiments. The reference
electrode (Ag|AgNO3, 0.01  in MeCN) was placed in a compart-
ment containing the supporting electrolyte solution separated from
the working electrode compartment by a 4 mm diameter Vycor frit.
Working electrodes were 3 mm diameter glassy carbon disks.
trans-[Pt(PPh3)2(pyrazine)(2-thienyl)][BF4] (2): To a stirred solution
of AgBF4 (19.5 mg, 0.1 mmol) in MeCN (2 mL) was added a solu-
tion of [trans-PtBr(PPh3)2(2-thienyl)] [88.3 mg, 0.1 mmol in CH2Cl2
(10 mL)]. The mixture was stirred, shielded from light, for 1 h and
AgBr was removed by filtration. A solution of pyrazine (8.0 g,
0.1 mmol) in CH2Cl2 (2 mL) was then added to the colorless fil-
trate. The mixture was stirred at room temperature for 4 h. The
clear pale yellow solution was condensed to about 2 mL under
vacuum, and the pale yellow solid residue of complex 2 (92.0 mg,
95%) was collected by adding Et2O. The product was purified by
recrystallization from CH2Cl2/hexane to give pale yellow crystals.
C44H37BF4N2P2PtS (969.69): calcd. C 54.5, H 3.85, N 2.89, S 3.30;
found C 54.38, H 3.59, N 2.85, S 3.51. 1H NMR (CDCl3): δ ϭ
7.2Ϫ7.5 (m, 60 H, phenyl), 8.21 (d, 1 H, pyrazine), 7.96 (d, 1 H,
pyrazine), 6.93 (d, 1 H, thiophene), 6.37 (t, 1 H, thienyl), 5.94 (d,
1 H, thienyl) ppm. 31P NMR (CDCl3): δ ϭ 18.34 (s) ppm.
Acknowledgments
The authors acknowledge the National University of Singapore
(NUS) for financial support and the technical staff for supporting
services, in particular, J. J. Vittal and G. K. Tan for the valuable
help in crystallographic analysis and interest in the project,
L. J. Chen for spectroscopic assistance, and H. Xu for XPS (NUS
Physics) assistance. F. Z. thanks NUS for the research scholarship.
X. Xu thanks NUS for a postdoctoral fellowship. We also thank
W. Henderson (Waikato) for helpful discussions and advice, and
P. Teo for some experimental assistance.
trans-[Pt(2-chloropyrazine)(PPh3)2(2-thienyl)][BF4] (3): The syn-
thesis follows that of 2. 2-Chloropyrazine (11.5 mg, 0.1 mmol) and
[trans-PtBr(2-thienyl)(PPh3)2] (88 mg, 0.1 mmol) gave a light yellow
powder of complex 3 (98.8 mg, 94%). Recrystallization from
CH2Cl2/Et2O gave light yellow crystals. C44H36BClF4N2P2PtS
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Eur. J. Inorg. Chem. 2004, 69Ϫ77
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