A controlled approach to well-defined oligothiophenes via oxidatively induced reductive elimination of stable Pt(II) oligothienyl complexes

Article abstract of DOI:10.1021/ja070083k

Stable Pt(II) oligothienyl complexes have been synthesized, and their geometric and electronic structure has been fully characterized. Upon activation by chemical or electrochemical oxidation, reductive elimination of the Pt(II) complexes resulted in oligothiophenes with double length, representing a controlled homocoupling method. Copyright

Full text of DOI:10.1021/ja070083k

Published on Web 02/23/2007
A Controlled Approach to Well-Defined Oligothiophenes via Oxidatively
Induced Reductive Elimination of Stable Pt(II) Oligothienyl Complexes
Fan Zhang and Peter Ba¨uerle*
Institute of Organic Chemistry II and AdVanced Materials, UniVersity of Ulm, Albert-Einstein-Allee 11,
D-89081 Ulm, Germany
Received January 10, 2007; E-mail: peter.baeuerle@uni-ulm.de
Scheme 1
The synthesis and characterization of Pt(II) oligothienyl com-
plexes L2Pt(3T)2 2a,b as stable models for catalytically active
corresponding Ni and Pd complexes in transition metal-catalyzed
cross-coupling reactions are described. Oxidatively induced reduc-
tive elimination of L2Pt(3T)2 2a,b to sexithiophene (6T) 3
constitutes a novel homocoupling method which starts from
terthiophene 1.
Oligothiophenes (OT) are among the most frequently investigated
conjugated systems and organic semiconductors because of their
outstanding properties and their successful application in organic
and molecular electronics.1 Pd- and Ni-catalyzed cross-coupling
revealing molecular ions ([M+H]+) at m/z ) 1468.2668 (2a) and
1326.3158 (2b), respectively.
reactions with the sequence (1) oxidative addition, (2) transmetal-
lation, and (3) reductive elimination became most important for
the synthesis of oligomers with well-defined chain and conjugation
length.2 Very recently, Pd-catalyzed (homo)couplings of thiophenes
by C-H activation have been demonstrated.3 Corresponding Pt
complexes are only useful in exceptional cases owing to their higher
stability,4 which on the other hand allows the use of stable cis-Pt-
(RR′) complexes as models to investigate reductive elimination
under formation of the C-C coupling product R-R′.5 In this
respect, it was recently shown how electronic effects influence the
kinetics of thermally activated reductive eliminations of cis-Pt
(ArAr′) complexes,6 but only a few examples through oxidative
stimulation were reported.7
A more detailed structural characterization of dppfPt(3T)2
complex 2a was possible by X-ray structure analysis of single
crystals which could be obtained by slow diffusion of n-hexane
into a solution of 2a in THF. The view of an individual molecule
(Figure 1) shows a pseudo-square-planar geometry of the Pt center
including a bite angle of 100.2° (P1-Pt-P2) and 87.3° for C35-
Pt-C54. Bond lengths of 2.31 and 2.32 Å are found for Pt-P
distances and of 2.05 and 2.02 Å for Pt-C35 and Pt-C54,
respectively. Some rotational disorder is noticed for the terminal
thiophene units and one butyl side chain, which is quite typical for
structures of oligothiophenes.15
We recently explored cis-Pt-(σ-acetylide-OT)2 complexes and
their reductive elimination to OT-substituted diacetylenes which
can be further transformed to thiophenes by sulfide anions. Using
this “metal template approach”, we synthesized linear OTs8 and
more complex conjugated structures and topologies, such as
macrocycles9 or [2]catenanes10 which were not accessible by other
coupling methods. A much more direct approach would be realized
by cis-Pt(II)-oligothienyl complexes which is the topic of this study.
Only a very limited number of Pt(II) complexes with thienyl ligands
are known.11
While cis-Pt(ArAr′) complexes allow the thermal elimination of
C-C coupling products,6 our L2Pt(3T)2 complexes 2a,b form
undefined polymeric products upon heating in toluene at 110 °C.
However, the use of 2 equiv of a one-electron oxidant such as silver
triflate at room temperature cleanly furnished the homocoupling
product sexithiophene 316 in 66% yield from 2a and 56% yield
from 2b (Scheme 1). These yields are comparable or superior to
other homocoupling reactions of oligothiophenes, such as the Pd-
catalyzed C-H activation3 or the Cu(II)-oxidative homocoupling
of lithiated derivatives.1,17
As shown in Scheme 1, lithiation of 3′,4′-dibutyl-2,2′:5′,2′′-
terthiophene 112 with 1 equiv of n-BuLi, followed by reaction with
Investigation of the optical and redox properties of the Pt
complexes 2a,b provides insight into their electronic structure and
the interplay of conjugated π-system and metal fragment. The
absorption spectra of the complexes show one prominent band with
a maximum at 367 nm which is shifted to significantly lower energy
and double intensity in comparison to ligand 1 (λmax ) 335 nm).
The absorption of the complexes is dominated by the π-π*
transition of the 3T ligands. The degree of π conjugation is
increased by additional charge transfer from the Pt center to the
ligands via dπ f pπ interaction. However, this backbonding effect
does not lead to full conjugation as can be seen by the further red-
shift of absorption maximum of product 3 (λmax ) 413 nm).
Cyclic voltammograms (CV) of Pt complexes 2a,b are shown
in Figure 2. For the L2Pt(3T)2 complexes we find one irreversible
oxidation wave at relatively low potential (2a: Ep1 ) 0.12 V. 2b:
Ep1 ) 0.09 V) and two reversible one-electron redox processes at
13
14
dppfPtCl2 or dpppPtCl2 in a molar ratio of 2:1 provided
terthienyl-based Pt complexes 2a and 2b as yellow microcrystalline
solids in 74% and 56% yield, respectively. The novel complexes
1
have been fully characterized by H, 31P NMR, and mass spectra
which clearly confirm their structure. In the aromatic part of the
1H NMR spectra of 2a,b signals belonging to the phenyl protons
of the dppf and dppp (7.28-7.72 ppm) and terthiophene ligands
are visible, and from which the â-protons of the thiophene adjacent
to the Pt are considerably upfield shifted (2a: 6.03, 6.55 ppm. 2b:
6.12, 6.56 ppm). 31P NMR spectra of 2a,b exhibit a single peak
with a set of 195Pt satellites (2a: 14.33 ppm, 1JPt-P ) 2082 Hz. 2b:
-0.48 ppm; 1JPt-P ) 1951 Hz) confirming that the two OT-ligands
are oriented trans to the dppf and dppp ligand.11c Ultimate proof of
the structure of 2a,b comes from high-resolution mass spectra
9
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J. AM. CHEM. SOC. 2007, 129, 3090-3091
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C O M M U N I C A T I O N S
active Ni- or Pd-intermediates and to investigate their geometric
and electronic structure. Chemical or electrochemical oxidation of
the central Pt(II) to Pt(IV) induces reductive elimination under C-C
coupling and formation of the homocoupling product, sexithiophene
3. The two-step reaction constitutes a novel homocoupling method
of oligothiophenes. Studies aiming at the template-directed synthesis
of more complicated linear and cyclic structures are in progress.
Acknowledgment. We acknowledge financial support by the
German Research Foundation (DFG, SFB 569), help in X-ray
crystallography (B. Mu¨ller, Univ. Ulm; Dr. J. W. Bats, Univ.
Frankfurt), and HR mass spectra (Prof. C.A. Schalley, FU Berlin;
A. Rang, Univ. Bonn).
Supporting Information Available: Experimental details, analyti-
cal data absorption spectra, and X-ray structure analysis; crystal-
lographic information (CIF) for complex 2a. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Figure 1. Molecular structure of dppfPt(3T)2 complex 2a.
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Figure 2. Cyclic voltammograms of L2Pt(3T)2 complexes 2a,b and
sexithiophene 3 (2a: c ) 0.87 × 10-3 mol L-1. 2b: 1.03 × 10-3 mol L-1
.
,
3: 1.73 × 10-3 mol L-1 in CH2Cl2/0.1 M Bu4NPF6, 295 K, V ) 100 mVs-1
potentials vs the ferrocene/ferrocenium (Fc/Fc+) couple).
higher, identical potentials (2a: E°2 ) 0.34, E°3 ) 0.49 V. 2b:
E°2 ) 0.33, E°3 ) 0.48 V). A fourth reversible wave can be
identified for dppfPt(3T)2 complex 2a at E°4 ) 0.77 V. The
assignment of the various redox processes can be rationalized by
comparison to the CV of homocoupling product 3 which as expected
shows two reversible one-electron waves (E°1 ) 0.33, E°2
)
0.49 V) concomitant with the formation of stable radical cations
and dications.1b Both waves perfectly coincide with the second and
third redox wave obtained for the L2Pt(3T)2 complexes. Therefore,
we ascribe the first irreversible oxidation wave in the CVs of the
complexes to the two-electron oxidation of the central Pt(II) to Pt-
(IV).18 As for the chemical oxidation of complexes 2a,b, the CV
reductive elimination is initiated by the increase of the oxidation
state of the central Pt(II) moiety which then releases homocoupling
product 3 and reflects a typical ECE electrode mechanism.19 The
fourth reversible wave of dppfPt(3T)2 complex 2a is ascribed to
the oxidation of the dppf-ligand which compared to dppfPtCl2 and
uncoordinated dppf is anodically shifted because of Pt f P
backbonding.20
In the same line, potentiostatic electrolysis of complexes 2a or
2b at 0.67 V versus Ag/AgCl (∼0.2 V vs Fc/Fc+) gave sexithiophene
3 in 67% and 63% yield, respectively, after workup, which are in
the same range as those from alternative chemical oxidation. This
electrochemical behavior is in full accordance with various cis-Pt-
(σ-acetylide-OT)2 complexes which we already investigated.8-10
In conclusion, we were able to synthesize Pt(II) oligothienyl
complexes 2a,b as stable models of corresponding catalytically
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