Polyterthiophene-bearing pendant organomolybdenum complexes: Electropolymerization of erythro-[Mo2(μ-C5H5)2(CO) 4{μ-η2:η2-C(R)≡C[C 4HS(C4H3S-2)<s

Article abstract of DOI:10.1039/a800390d

Terthiophenes bearing pendant organomolybdenum complexes, erythro-[Mo₂(η-C₅H₅)₂(CO) ₄{μ-η²:η²-C(R)≡ C[C₄HS(C₄H₃S-2)₂-2,5]} (R = H 1a o

Full text of DOI:10.1039/a800390d

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Polyterthiophene-bearing pendant organomolybdenum complexes:
2
2
electropolymerization of erythro-[Mo (ì-C H ) (CO) {ì-ç :ç -
2
5
5 2
4
C(R)᎐C[C HS(C H S-2) -2,5]}]
4
4
3
2
a
a
a
a
a
b
Dong Hyun Kim, Bong Soo Kang, Soo Min Lim, Ki-Min Bark, Bong Gon Kim, Motoo Shiro,
c
,a
Yoon-Bo Shim and Sung Chul Shin*
a
Department of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Chinju, 660-701, Korea
b
Rigaku Corporation X-Ray Research Laboratory, Matsubara-cho, Akishima-shi, Tokyo, 196,
Japan
c
Department of Chemistry, Pusan National University, Pusan, 609-735, Korea
2
2
Terthiophenes bearing pendant organomolybdenum complexes, erythro-[Mo (η-C H ) (CO) {µ-η :η -C(R)᎐
᎐
2
5
5
2
4
C[C HS(C H S-2) -2,5]} (R = H 1a or Ph 1b) were prepared by reaction of [Mo (η-C H ) (CO) ] with 3Ј-alkynyl-
4
4
3
2
2
5
5
2
4
2
,2Ј:5Ј,2Љ-terthiophenes. The molecular structures of 1a and 1b were identified by spectroscopic methods and
X-ray diffraction. Cyclic voltammetry of 1a and 1b showed deposition of electroactive polymeric films on the
electrode surface. The absorption spectra of 1a, 1b and their polymer films are presented.
2
2
Great interest has been focused upon rigid π-polymers bearing
carbon-rich organometallics. These organometallic polymers
are important assemblies for the study of electron-transfer pro-
size of the Mo (C H ) (CO) [µ-η :η -C(R)᎐C] (R = H or Ph)
2 5 5 2 4
᎐
moiety. To the best of our knowledge, the present report is the
first example of electrochemical polymerization of thiophene
appended a polynuclear organometallic moiety.
1
2
cesses, the preparation of organometallic liquid crystals, the
3
construction of nanostructured materials and molecular
4
devices.
Experimental
Materials
Grafting of specific functionalities on the individual rings of
heterol conducting polymers can tune their physical properties
in a unique way. Although organotransition-metal complexes
can modify the properties of conducting polymers in a specific
way that may be impossible via classical organic functionalities,
studies have been restricted to ferrocene derivatives and only a
few are able to form electroactive polymers by direct anodic
Solvents were distilled under nitrogen, from appropriate drying
agents, and degassed prior to use. 3Ј-Bromoterthiophene and
Mo (η-C H ) (CO) ] were prepared by literature methods.
All other chemicals were from commercial suppliers and used
without further purification. Column chromatography was
performed on Kieselgel 60 (230–400 mesh).
9
10
[
2
5
5
2
4
5
electropolymerization. Most involve their copolymerization
with unsubstituted pyrrole.
Herein, we report the synthesis and the electrochemical
Measurements
2
2
2
[Mo (η-C H ) (CO) {µ -η :η -C(R)᎐
᎐
polymerization
of
2 5 5 2 4
1
3
C[C HS(C H S-2) -2,5]}] 1, in which the Mo (C H ) (CO) [µ-
Melting points are uncorrected. Proton and C NMR spectra
4
4
3
2
2
5
5
2
4
2
2
η :η -C(R)᎐C] (R = H or Ph) moiety is appended to the 3Ј-
position of terthiophene, and the optophysical properties of the
resulting polymers. The molybdenum binuclear complex moiety
bridged by an unsaturated hydrocarbon has been chosen as an
organometallic unit since considerable interest has been focused
upon such organometallic complexes as models for studying the
interaction of organic molecules with metal surfaces during the
in CDCl were recorded on a Bruker DRX 500 spectrometer
3
using SiMe4 as an internal standard. Infrared spectra were
taken on a Hitachi 270–50 spectrometer with KBr discs, UV/
VIS absorption spectra of the monomers on a Gilford
RESPONSE spectrophotometer. Elemental analyses were
performed by the Central Laboratory at Gyeongsang National
University. The electrochemical and spectroelectrochemical
experiments were performed in CH Cl as the solvent, which
6
course of synthetic organic transformations. In present system,
2
2
two chiral centers are generated at the construction stage of the
was purified and dried by distillation from CaH prior to use.
2
moiety from [Mo (η-C H ) (CO) ] and unsymmetric alkynes.
Tetrabutylammonium hexafluorophosphate (0.1 ) was used as
the supporting electrolyte; it was recrystallized twice from
methanol and dried in vacuo at 100 ЊC for several days. The
concentration of the solutions of monomers 1a and 1b was
2
5
5
2
4
Conjugated polymers substituted by chiral groups have poten-
tial technological applications as materials for stereoselectivity
7
modified electrodes and/or microwave absorbers. Conducting
Ϫ4
polymers having catalytic functions are also important in the
ca. 10 . Cyclic voltammetry was performed with a computer-
field of heterogeneous catalysis concerned with electrochemical
processes, which has shown rapid development recently. Con-
controlled EG&G PAR 273 potentiostat. Platinum discs (1 mm
outside diameter) were used as the working electrodes, a
platinum wire as the counter electrode, and an Ag–AgCl wire as
the reference electrode; this was internally calibrated vs. the
chemical redox couple ferrocene–ferrocenium. Spectroelectro-
chemical measurements of the polymer films were carried out in
a thin-layer cell incorporating ITO glasses as the optical trans-
parent electrodes, a platinum sheet as the counter electrode, and
an Ag–AgCl wire as the reference electrode. The cell was placed
8
ducting polymers grafted by chiral organotransition-metal
complexes may provide new research models for the hetero-
geneous chiral catalysts in electrochemical processes. In order
to relieve steric hindrance caused by the bulky complex unit in
the polymerization process, terthiophene has been chosen as a
monomeric unit of polymerization; the distance between the
two terminal α-carbon atoms of terthiophene is longer than the
J. Chem. Soc., Dalton Trans., 1998, Pages 1893–1898
1893

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in the probe beam of a Perkin-Elmer Lambda 900 spectro-
photometer and the applied potential controlled by a EG&G
model 264A polarographic analyzer.
OH
Br
Crystallography
S
S
(i)
S
S
S
S
All measurements were made on a Rigaku RAXIS-IV imaging-
plate area detector. Experimental details are included in Table
I
II
1
. The structure was solved using TEXSAN crystallographic
11 12
software by direct methods and expanded using Fourier
techniques. Non-hydrogen atoms were refined anisotropically,
hydrogen atoms were included but not refined. Neutral atom
scattering factors and anomalous dispersion effects were
taken from the literature.
(iii)
(ii)
13
14
15
Ph
S
CCDC Reference number 186/959.
S
S
S
Syntheses
S
S
3
Ј-(3-Hydroxy-3-methylbut-1-ynyl)-2,2Ј:5Ј,2Љ-terthiophene
II. To a mixture of 3Ј-bromoterthiophene (2 g, 6.1 mmol),
Pd(dppf)Cl ] (0.05 g, 1 mol %), and CuI (0.035 g, 3 mol %) in
IIIb
IIIa
(iv)
[
(iv)
2
3
degassed diisopropylamine (30 cm ) was added 2-methylbut-3-
yn-2-ol (0.57 g, 6.8 mmol). The reaction mixture was refluxed
with stirring for 24 h under a nitrogen atmosphere. After con-
firmation of the disappearance of 3Ј-bromoterthiophene by
TLC, the mixture was washed with saturated NaHCO and
brine, and extracted with CH Cl . The extract was dried over
CO
OC
CO
OC
Mo
3
Mo
2
2
R
magnesium sulfate, and the solvent was removed under reduced
pressure. The residue was chromatographed on a silica gel col-
umn using a mixed eluent of n-hexane–ethyl acetate (5:1) to
give a pale yellow solid II (1.62 g, 80%), m.p. 73–74 ЊC (Found:
C, 61.7; H, 4.30; S, 29.0. C H OS requires C, 61.8; H, 4.27; S,
S
S
S
1
a; R = H
1b; R = Ph
17
14
3
Ϫ1
2
9.1%); ν
˜ max/cm 2219 and 3454. δH 1.70 (s, 6 H, CH ), 2.20
3
Scheme 1 (i) 2-Methylbut-3-yn-2-ol, [Pd(dppf)Cl ], CuI, diisopropyl-
amine, reflux, 24 h; (ii) KOH, toluene–methanol (1:1), reflux, 20 h; (iii)
phenylacetylene, [Pd(dppf)Cl ], CuI, diisopropylamine, reflux, 24 h; (iv)
[Mo (η-C H ) (CO) ], toluene, 1 h
2
(
s, 1 H, OH), 7.04–7.46 (m, 7 H, aryl). δ 31.6, 66.3, 78.2, 98.8,
C
1
1
17.6, 124.6, 125.4, 125.9, 126.0, 127.6, 128.4, 134.5, 136.0,
36.6 and 138.8. EI mass spectrum: m/z 330 (M ).
2
ϩ
2
5
5
2
4
3
Ј-(Ethynyl)-2,2Ј:5Ј,2Љ-terthiophene IIIa. To a solution of
a reddish brown solid of 1a (0.53 g, 75%). X-Ray-quality
crystals could be obtained from n-hexane–toluene at Ϫ20 ЊC,
compound II (2 g, 6.1 mmol) in degassed toluene–methanol
1:1, 50 cm ) was added KOH (1.7 g, 30 mmol). The mixture
was refluxed under a nitrogen atmosphere for 20 h. After con-
firmation of the disappearance of the starting II by TLC, the
solvent was evaporated under reduced pressure. Distilled water
was poured onto the residue and extracted with CH Cl . The
3
(
m.p. 146 ЊC (decomp.) (Found: C, 47.8; H, 2.55; S, 13.5.
Ϫ1
C H Mo O S requires C, 47.6; H, 2.57; S, 13.6%); ν˜ max/cm
28 18 2 4 3
1837, 1905 and 1995. δH 4.73 (s, 1 H, ᎐CH), 5.26 (s, 10 H,
᎐
2C H ) and 7.02–7.39 (m, 7 H, aryl). δ 59.0, 73.9, 91.9, 123.7,
5
5
C
2
2
124.6, 126.2, 126.7, 127.3, 127.9, 128.5, 128.9, 135.0, 136.5,
ϩ
extract was washed with saturated NH Cl and dried over mag-
136.9, 143.8, 229.4 and 230.9. EI mass spectrum: m/z 705 (M ),
4
ϩ
ϩ
nesium sulfate, and the solvent removed under reduced pres-
sure. The residue was chromatographed on a silica gel column
using n-hexane as an eluent to give a yellow solid of IIIa (1.2 g,
622 (M Ϫ 3CO) and 594 (M Ϫ 4CO).
2
2
[
Mo (ç-C H ) (CO) {ì-ç :ç -C(Ph)᎐C[C HS(C H S-2) -
2 5 5 2 4 4 4 3 2
7
4%), m.p. 58–59 ЊC (Found: C, 61.7, H, 2.97; S, 35.2. C H S
14 8 3
Ϫ1
2,5]}] 1b. The same procedure as above was performed from
compound IIIb (0.42 g, 1.2 mmol) and [Mo (η-C H ) (CO) ]
˜
requires C, 61.7; H, 2.96; S, 35.3%); ν max
/cm 2047 and 3297.
2
5
5
2
4
δ 3.38 (s, 1 H, ᎐CH) and 7.02–7.55 (m, 7 H, aryl). δ 77.4, 80.6,
H
C
(0.43 g, 1 mmol) at 80 ЊC, affording reddish brown solids of 1b
0.2 g, 25%), m.p. 143–146 ЊC (Found: C, 52.1; H, 2.91; S, 12.4.
1
1
15.0, 122.7, 123.5, 124.2, 124.3, 125.9, 126.0, 126.4, 132.7,
33.7, 134.4 and 137.9. EI mass spectrum: m/z 272 (M ).
(
ϩ
Ϫ1
C H Mo O S requires C, 52.2; H, 2.83; S, 12.3%); ν
˜
max/cm
34
22
2
4 3
1
845, 1930 and 2000. δH 5.08 (s, 10 H, 2C H ) and 6.74–7.22
5
5
3
Ј-(Phenylethynyl)-2,2Ј:5Ј,2Љ-terthiophene IIIb. The same
procedure used for II was performed from 3Ј-bromothiophene
3.27 g, 10 mmol) and phenylacetylene (1.2 g, 12 mmol), afford-
(
m, 12 H, aryl). δC 91.3, 95.5, 95.8, 126.0, 126.9, 128.0, 129.4,
1
1
7
29.5, 129.7, 130.3, 130.4, 131.7, 132.0, 132.8, 136.7, 137.2,
39.2, 145.8, 148.3, 232.5 and 233.1. EI mass spectrum: m/z
26 (M Ϫ 2CO) and 671 (M Ϫ 4CO).
(
ing a yellow solid of IIIb (2.58 g, 74%), m.p. 99–101 ЊC (Found:
C, 68.8; H, 3.49; S, 27.5. C H S requires C, 68.9; H, 3.47; S,
7.6%); ν˜ max/cm 2172. δH 7.03–7.60 (m, 12 H, aryl). δ 84.94,
4.06, 117.6, 123.0, 124.0, 124.9, 125.4, 125.6, 126.9, 127.8,
28.3, 131.3, 134.0, 136.1, 136.5 and 138.2. EI mass spectrum:
m/z 348 (M ).
ϩ
ϩ
20
12 3
Ϫ1
2
9
1
C
Results and Discussion
Syntheses
ϩ
The monomeric complexes 1a and 1b have prepared as shown in
Scheme 1. 3Ј-Bromoterthiophene I was treated with 2-methyl-
but-3-yn-2-ol in diisopropylamine in the presence of [Pd(dppf)2-
2
2
[
Mo (ç-C H ) (CO) {ì-ç :ç -C(H)᎐[C HS(C H S-2) -2,5]}]
2 5 5 2 4 4 4 3 2
᎐
1
a. To a solution of compound IIIa (0.33 g, 1.2 mmol) in tolu-
ene (20 cm ) was added freshly prepared [Mo (η-C H ) (CO) ]
3
Cl ] at reflux temperature to give 3Ј-(3-hydroxy-3-methylbut-1-
2
5
5
2
4
2
(
0.43 g, 1 mmol). The mixture was stirred at room temperature
ynyl)terthiophene II in 80% yield. Compound II was hydrolysed
by KOH in toluene–methanol (1:1) at reflux temperature to
afford 3Ј-(ethynyl)terthiophene IIIa in 74% yield. 3Ј-(Phenyl-
ethynyl)terthiophene IIIb was obtained in 74% yield from the
coupling of I with phenylacetylene under the same conditions
under a nitrogen atmosphere for 1 h. After confirmation of the
disappearance of the starting IIIa by TLC, the solvent was
evaporated under reduced pressure. The residue was chromato-
graphed on silica gel column using n-hexane as an eluent to give
1
894
J. Chem. Soc., Dalton Trans., 1998, Pages 1893–1898

View Article Online
Table 1 Crystallographic data for complex 1a
Table 2 Selected distances (Å), angles (Њ) and torsion angles (Њ) of
complex 1a
Empirical formula
C H Mo O S
28 18 2 4 3
M
706.51
Mo(1)᎐Mo(2)
Mo(1)᎐C(16)
Mo(2)᎐C(15)
C(15)᎐C(16)
C(17)᎐C(18)
C(19)᎐C(25)
S(3)᎐C(28)
2.986(1)
2.274(8)
2.224(10)
1.36(1)
1.42(1)
1.51(1)
1.732(6)
1.47(1)
1.729(6)
Mo(1)᎐C(15)
Mo(2)᎐C(13)
Mo(2)᎐C(16)
C(16)᎐C(17)
C(18)᎐C(19)
C(25)᎐S(3)
2.211(9)
1.99(1)
2.187(9)
1.49(1)
1.38(1)
1.749(6)
1.40(1)
1.730(6)
Crystal colour, habit
Crystal dimensions/mm
Crystal system
Space group
a/Å
b/Å
c/Å
β/Њ
Reddish brown
0.32 × 0.17 × 0.04
Monoclinic
C2 (no. 5)
31.815(6)
7.640(1)
11.483(6)
104.884(3)
2697
C(17)᎐C(20)
C(21)᎐S(2)
C(20)᎐C(21)
S(2)᎐C(24)
3
U/Å
Z
4
Mo(1)᎐Mo(2)᎐C(15)
Mo(2)᎐Mo(1)᎐C(15)
Mo(1)᎐C(16)᎐Mo(2)
Mo(1)᎐C(16)᎐C(15)
Mo(2)᎐C(15)᎐C(16)
Mo(2)᎐C(16)᎐C(17)
C(16)᎐C(17)᎐C(18)
47.5(2)
47.9(3)
84.0(3)
69.9(5)
70.6(6)
131.3(6)
121.7(8)
Mo(1)᎐Mo(2)᎐C(16)
Mo(2)᎐Mo(1)᎐C(16)
Mo(1)᎐C(15)᎐C(16)
Mo(1)᎐C(16)᎐C(17)
Mo(2)᎐C(16)᎐C(15)
C(15)᎐C(16)᎐C(17)
C(16)᎐C(17)᎐C(20)
49.2(2)
46.8(2)
74.9(5)
131.7(6)
73.6(5)
141.8(9)
127.1(8)
Ϫ3
D /g cm
F(000)
µ(Mo-Kα)/cm
T/K
1.740
1400
11.93
296
50.1
2431
c
Ϫ1
2
θmax/Њ
No. reflections collected
Observed reflections [I > 1.00σ(I)]
2419
0.045
0.056
1.80 and Ϫ0.63
a
R(Fo)
RЈ(Fo)
Torsion angles
b
Mo(1)᎐C(15)᎐C(16)᎐C(17) Ϫ132(1)
Mo(1)᎐C(16)᎐C(17)᎐C(20) Ϫ135.9(8)
Ϫ3
Maximum, minimum ∆ρ/e Å
a
b
2
2 ¹
R = Σ||F | Ϫ |F ||/Σ|F |. RЈ = [Σw(|F | Ϫ |F |) /Σw|F | ] �² .
Mo(2)᎐C(16)᎐C(17)᎐C(18)
C(15)᎐C(16)᎐C(17)᎐C(18)
S(2)᎐C(21)᎐C(20)᎐C(17)
Mo(1)᎐C(16)᎐C(17)᎐C(18)
Mo(2)᎐C(15)᎐C(16)᎐C(17)
Mo(2)᎐C(16)᎐C(17)᎐C(20)
C(15)᎐C(16)᎐C(17)᎐C(20)
S(3)᎐C(25)᎐C(19)᎐C(18)
Ϫ79(1)
159(1)
Ϫ25(1)
46(1)
1347(1)
97(1)
o
c
o
o
c
o
Ϫ23(1)
6(1)
Ϫ1
13
1
845 cm for 1b). In the C NMR spectra the carbonyl carbon
resonances are centered at δ 229.4 and 230.9 for 1a and 232.5
and 233.1 for 1b. These spectral data fall in the ranges of those
16,17
previously reported for dimolybdenum carbonyl complexes.
While the parent ion appears in the mass spectrum of 1a, the
spectrum of 1b shows a strong M Ϫ 2CO peak instead of the
molecular ion. Both complexes exhibit sequential loss of
carbonyls with strong ions corresponding to the loss of four
carbonyls (M Ϫ 4CO). Such successive CO losses and the bare
metal species as strong peaks are often observed for structurally
18
related complexes.
The crystal structure of complex 1a is depicted in Fig. 1, and
the key molecular parameters are given in Table 2. Both of the
π bonds, Mo(1)᎐C(16) and Mo(2)᎐C(16), on the bimetallic
complex unit depart significantly from coplanarity to the
middle thiophene ring [torsion angle Mo(1)᎐C(16)᎐C(17)᎐
C(18) 46(1), Mo(2)᎐C(16)᎐C(17)᎐C(20) 97(1)Њ]. Such out-of-
coplanarity results from the internal crowding in the molecule,
which is commonly observable for the perpendicular bimetallic
16
acetylene complexes. The C(16)᎐C(17) bond length is 1.49(1)
Å, which falls into the range for a single bond. The present
structural characteristics with respect to the coplanarity and the
bond length indicate that there is no π-type interaction between
the terthiophene and the complex unit.
Fig. 1 Crystal structure of complex 1a. The thermal ellipsoids are
drawn at the 30% probability level
used with II. The stereogenic reaction of compound IIIa with
[
Mo (η-C H ) (CO) ] in toluene at room temperature furnished
2 5 5 2 4
Electrochemistry
the complex erythro-1a with C symmetry in 75% yield. The
1
Ϫ1
complex erythro-1b was obtained in 25% yield from coupling
with 3Ј-(phenylethynyl)terthiophene IIIb at 80 ЊC. The latter
reaction at room temperature gave 1b in only poor yield (<5%)
due to the low reactivity of the phenyl-substituted carbon–
carbon triple bond. Generally [Mo (η-C H ) (CO) ] shows low
A cyclic voltammogram (ν = 100 mV s ) of complex 1a in
CH
Cl exhibits three discrete, highly irreversible electrode pro-
cesses [Fig. 2(a)]. By comparison with the redox properties of
2
2
19
terthiophene as a reference compound, the process having
pa = 1.43 V can be assigned to the terthiophene moiety. The
cyclic voltammograms of heterol monomers generally show
highly irreversible redox waves due to the deposition of electro-
chemically active polymer films on the electrode surface. Two
irreversible processes at more negative potentials, Epa = 0.50
and 0.84 V, are associated with the oxidation of the complex
E
2
5
5
2
4
16
reactivity toward diaryl-substituted acetylenes. Complexes 1a
and 1b are soluble in CHCl and toluene, and moderately sol-
3
uble in hexane. In the solid state they appear stable on exposure
to moisture and air, but in solution are slowly decomposed.
Spectroscopic data for complexes 1a and 1b are given in the
Experimental section. Their IR spectra exhibit two terminal
ϩ
2ϩ
2
core to the radical cation, Mo
᝽ and dication, Mo
, respect-
2
Ϫ1
carbonyl stretching bands (1995, 1905 cm for 1a; 2000, 1930
ively. The cyclic voltammogram of 1b also exhibits three redox
Ϫ1
Ϫ1
cm for 1b) and one semibridging carbonyl (1837 cm for 1a,
waves [Fig. 2(b)]. The irreversible process at Epa = 1.27 V is
J. Chem. Soc., Dalton Trans., 1998, Pages 1893–1898
1895

View Article Online
Fig. 2 Single-sweep cyclic voltammograms of (a) complex 1a and (b)
Ϫ1
1
b. Scan rate: 100 mV s
Fig. 3 (a) Electropolymerization of complex 1a by 20 repeated poten-
tial scans and (b) cyclic voltammogram of an electrode modified by the
polymer films of 1a. Scan rate: 100 mV s
assigned to the terthiophene. The redox processes (E = 0.47 V
Ϫ1
pa
ϩ
2ϩ
for Mo2 , 0.86 V for Mo2 ) are electrochemically more revers-
᝽
ible than the corresponding electrode processes of 1a. In con-
repetitive scans the modified electrodes were thoroughly rinsed
trast to the highly irreversible processes of the complex core of
with CH Cl , dried under vacuum, and studied by cyclic vol-
2
2
1
a, the observed more reversible behavior of the core of 1b is
tammetry. As shown in Fig. 3(b), a single-sweep cyclic voltam-
attributed to the higher stability from the dispersion of the
extra charge into the incorporated phenyl ring. Although the
peak separations for the core-based processes are character-
istically larger than the theoretical value, 59 mV, for a reversible
process (∆E = 120 mV for Mo2 , 170 mV for Mo2 ), the pro-
cess may be essentially reversible in that a large deviation from
the theoretical value is generally observed in cluster electro-
chemistry and has previously been attributed to uncompen-
sated junction potentials, poisoning of the electrode and/or
a slow rate of electron transfer on Pt. Connelly’s group
investigated the redox properties of some similar bimetallic
Ϫ1
mogram (ν = 100 mV s ) of the polymer films exhibits totally
irreversible processes due to their high instability. Similarly pre-
pared, polymer films of 1b reveal significantly more reversible,
electrochemical behavior than that of 1a (Fig. 4). The redox
ϩ
2ϩ
p
᝽
0
2
ϩ
ϩ
2ϩ
waves of the Mo –Mo
and Mo2 –Mo2 couples are cen-
2
᝽
᝽
tered at 0.48 and 0.83 V, respectively. The observed difference
between the two films is also attributed to the more effective
charge dispersion of the core bound by the phenyl ring com-
pared to the unbound as observed in the cyclic voltammograms
of the monomers. In the cyclic voltammograms of the polymer
films grown from 1b the wave positions of the core show no
substantial shifts from the corresponding process of the
monomers. This indicates that not only the structure of the
monomers is essentially kept intact, but also there is no
coplanarity of the core with the π-conjugated backbone and
thus no π-type electronic interaction between the components.
The coplanarity of the core with the backbone must cause a
large shift of the redox waves of the core as a result of the
20
21
22
5
2
2
1
2
complexes, [Mo (η -L) (CO) {µ-η :η -C(R )᎐C(R )}] (L =
2
2
4
C H or C Me ), by cyclic voltammetry. They reported that the
5
5
5
5
peak position and reversibility of the electrode process was
variable depending on the substituents. The complexes of the
C Me5 system exhibit two reversible redox peaks which are
5
corresponding to the formation of the stable monocation and
dication. In the case of C H -substituted complexes the oxid-
5
5
ation process corresponding to the formation of the dication
23
extensive charge delocalization over the system. The out-of-
planarity of the bimetallic core with the terthiophene backbone
is further supported by the X-ray diffraction data.
was irreversible. However, in the present study, 1b having C H5
5
shows two reversible redox peaks indicating the formation of a
stable monocation and dication. To the best of our knowledge,
1
b is the first example of a C H5 complex of this type (as
5
Absorption spectra
opposed to C Me ) to show two reversible redox chemistry.
5
5
Electrochemically active, polymer films of complex 1a could
be grown on the platinum electrode surface by repetitive cycling
over the potential range from 0.80 to 1.50 V [Fig. 3(a)]. After 20
The absorption spectral data of complexes 1a, b and their
polymeric films deposited on ITO glass are shown in Fig. 5,
together with those of IIIa and IIIb for comparison. The
1
896
J. Chem. Soc., Dalton Trans., 1998, Pages 1893–1898

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Fig. 4 (a) Electropolymerization of complex 1b by 20 repeated poten-
tial scans and (b) cyclic voltammogram of an electrode modified by the
Ϫ1
polymer films of 1b. Scan rate: 100 mV s
absorption spectrum of IIIa in CHCl exhibits a moderately
3
intense, high energy transition at 269 nm and an intense, lower
energy transition at 369 nm, The high energy band is attributed
2
4
to a π–π* localized excitation of the thiophene ring while the
lower energy band is attributed to a π–π* transition in the con-
Ϫ5
Fig. 5 Absorption spectra of 3.0 × 10  solutions of (a) compounds
...... ......
) in CHCl , and
3
IIIa (–––) and IIIb (
), (b) 1a (–––) and 1b (
25
jugated π system. These two bands are generally observed for
(c) polymer films of 1a (——) and 1b (......) on ITO glasses
oligothiophenes. The absorption spectrum of IIIb in CHCl also
3
exhibits two bands; an intense, high energy band at 309 nm and
a moderately intense low energy band at 376 nm. In the absorp-
electrochemically deposited on ITO glass from 1a and 1b the
π–π* transition of the conjugated π system is red-shifted ca. 30
nm from those of the monomers as a result of the elongation of
the conjugation by the polymerization. The new broad, strong
intensity band (ranging from 350 to 600 nm for the polymer
from 1a, from 400 to 600 nm for that from 1b) cannot be clearly
assigned at present. One plausible origin may be intermolecular
charge-transfer transitions in the closely contacted state of
the polymer molecules but a clear-cut identification must await
further research.
tion spectrum of 1a in CHCl the π–π* localized excitation of
3
the thiophene ring shifted to the high energy UV region below
2
50 nm while the π–π* transition of the conjugated π system is
blue-shifted by ca. 60 nm from that of IIIa. This effect is
ascribed to the distortion of the terthiophene π system caused
by steric hindrance between the bulky complex unit at the 3Ј
position of the central thiophene and sulfur atoms of the
adjacent two thiophenes in their anti conformation. A new
band at 345 and a lower energy, weak band at 538 nm have been
previously assigned to the σ → σ* transition for the molyb-
denum–acetylenic carbon bond and the d → d transition
Conclusion
π
π*
6a,26
for the Mo᎐Mo bond, respectively.
Binding the complex
Terthiophenes bearing pendant organotransition metal com-
2
2
unit to IIIb results in the aspects similar to 1a in the absorption
spectrum. The π–π* localized excitation of the thiophene ring
shifts to the higher energy UV region below 250 nm while the
π–π* transition of the conjugated π system is blue-shifted by
ca. 70 nm from that of IIIb. The σ → σ* transition is buried
in the low energy tail of the π–π* transition of the terthiophene
π system and the d → d transition is centered at 538 nm.
plexes, erythro-[Mo (η-C H ) (CO) {µ-η :η -C(R)᎐C[C HS-
2 5 5 2 4 4
(C H S-2) -2,5]}] (R = H, 1a or Ph 1b) were prepared by a reac-
4
3
2
tion of [Mo (η-C H ) (CO) ] with 3Ј-alkynylterthiophenes. The
2
5
5
2
2
Mo᎐Cacetylenic π bonds of the complex unit in 1a significantly
depart from coplanarity to the middle thiophene ring as a result
of the internal crowding in the molecule. Electrochemically
active, polymer films of 1a could be grown on a platinum
electrode surface by repetitive cycling over the potential range
π
π*
In each optimized absorption spectrum of the polymer films
J. Chem. Soc., Dalton Trans., 1998, Pages 1893–1898
1897

View Article Online
from 0.80 to 1.50 V. While 1a shows a totally irreversible
electrode process, 1b reveals a more reversible process which is
attributed to the higher stability arising from the dispersion of
the extra charge of the complex core into the incorporated
phenyl ring. A similar trend is also observed in their polymer
films. The absorption spectra of 1a and 1b exhibit the σ → σ*
transition of the molybdenum–acetylenic carbon bond and the
dπ → dπ* transition of the Mo᎐Mo bond, respectively. The
absorption spectra of the corresponding polymer films reveal a
π → π* transition of the polymer backbone and a new broad,
strong intensity band, respectively.
(b) N. E. Schore, C. S. Ilenda, M. A. White, H. E. Bryndza, M. G.
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c) V. C. Gibson, G. Parkin and J. E. Bercaw, Organometallics, 1991,
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0 R. J. Klinger, W. Butler and M. D. Curtis, J. Am. Chem. Soc., 1975,
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1 TEXSAN, Structure Analysis Package, Molecular Structure
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Acknowledgements
This work was supported by the NON DIRECTED
RESEARCH FUND, Korea Research Foundation, 1996.
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Received 14th January 1998; Paper 8/00390D
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