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Electrochemical Solid-State Studies on Oligomeric p-Phenylenes as Model Compounds for Conductive Polymers.

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C(31
Fig. 3. Structure of 16 in the crystal (without H atoms). Selected bond lengths
[pm] and angles ["I (standard deviations in brackets): Asl-Cl 195.2(2), A S K 2 1
194.6(2), As1 -C24 191.8(4); C1-As1-C21 101.3(1), C1-As1 -C24 105.1 (11, C21Asl-C24 86.0(1).
way as that of 16 as a result of a tandem ring expansion/cyclization.
[l] a) P. Gillespie, P. Hoffmann, H. Klusacek, D Marquarding, S Pfohl, F.
Ramirez. E. A. Tsolis. I. Ugi, Angew Clzem. 83 (1971) 691 ; A n p w Chern.
In!. Ed. Engl. lO(1971) 687; b) W. S. Sheldrick, Top. Curr. Chem. 73 (1978)
1. c ) D. Hellwinkel. ibid. 109 (1979) 1 ; d) P. Lemmen, R. Baumgartner, 1.
Ugi, F. Ramirez. Chem. Srr. 28 (1988) 451.
[2] D. Hellwinkel. W Blazcher, W. Krapp, W. S. Sheldrick, Chem. Ber. 113
(1980) 1406.
[31 H. L. Carell, H. M. Bermann. J. S. Ricci, W. S. Hamilton, F. Ramirez. J. F.
Marecek, L. Krdmer, 1. Ugi, J. Am. Chem. Soc 97 (1975) 38.
[4] For analogous systems with nitrogen and carbon centers see: D. Hellwinkel. G. Aulmich, M. Melan. Chem. Ber. 114 (1981) 86; D. Hellwinkel,
M. Meldn, ihid. 107 (1974) 616.
[5] S. Samann in Houhen- Wevl-Miiller: Merhnden der organrschen Chemie. Bd.
XlllhY. Thieme, Stuttgart 1978, p. 250.
161 10a has already been prepared by Chen el. al. via a completely different,
less effective route' C. H. Chen, J. J. Doney, J. L. Fox, H. R. Russ, J Org.
Chem. SO (198s) 2914.
[71 The same effects were also observed by Chen et. al.: see [6] and literature
cited therein.
181 J. J. Ddiy. J. Chrm. Sot.. 1964, 3799.
[91 A. N. Sobolev, V. K . Belsky, N. Yu. Chernikova. F. Yu. Akhamadulina, J.
Orgummet. Chem. 244 (1983) 129.
[lo] Crystallographic data. 5 a : monoclinic, space group P2,/n, CI = 12.498(3).
b = 9.961(2), c = 21.027(5) A, {f = 102.83(2)", 2 = 4. R = 0.052, R,, =
0.045 for 3240 unique reflections. 56: monoclinic space group P2,lu. a =
11.838(3),h=11.557(2),c= 1 4 . 7 9 8 ( 3 ) ~ , ~ = 9 0 . 1 3 ( 2 ) " , 2 = 4 , R = 0 . 0 4 ,
R , = 0.032 for 3556 unique reflections. 12. monoclinic, space group P2,/c,
u = 10.918(7). b = 28.32(1). c = 11.036(5) A, B = 110.73(4)' 2 = 4, R =
0.043, R, = 0.037 for 3561 unique reflections. 16: triclinic, space group P7,
(1 = 8.339(4), h = 8.595(7), c = 13.865(9). s = 99.90(6). B = 99.80(5), p =
105.39(5),. Z = 2, R = 0.045, R, = 0.04 for 3937 unique reflections. Further details of the crystal structure investigations are available on request
from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-54475, the names oi"
the authors, and the Journal citation.
I l l ] J. A. Aeschlimann. N. D. Lees. N. P. McCleland. G. N. Nicklin, J Chrm
So< 127 (1925) 66.
[12] Cf. A. Tzschdch,J. Heinicke: Arsenhererocjden, VEB Deutscher Verlag f u r
Grundstoffindustrie. Leipzig 1978.
(131 V. 1. Gavrilov. V. N. Khlebnikov, A. A. Komleva, B. D. Chernokal'skii,
Zh. Obshch. Khim. 44 (1974) 2506; Chem. Abstr. 82 (1975) 140 268m.
Electrochemical Solid-state Studies
on Oligomeric p-Phenylenes as Model Compounds
for Conductive Polymers **
19
By Klaus Meerholz and Jiirgen Heinze *
21
20
We are presently investigating whether this novel ring expansion in the form of a 1,2-phenylenemethylene moiety can
also be extended to analogous derivatives of other main
group elements (in particular N, P, Sb, Si, C).
Received: January 31, 1990 [Z 3771/3772 IE]
German version: Angew. Chem. 102 (1990) 677
CAS Registry numbers:
3a, 127445-64-1; 3b, 127445-65-2; 4, 127445-66-3; 5a, 127445-61-8; Sb,
127445-62-9; 6, 127445-67-4, ?a, 127445-68-5: 7b, 127445-69-6; 8a, 12744570-9;8b, 127445-71-0;9, 127445-72-1;10a. 96897-82-4; lob, 127445-73-2; l l a .
127445-79-8; l l b , 127445-80-1; 12, 127472-31-5; 13, 2866-58-2; 16, 12744563-0; 17, 127445-77-6; 18, 127445-74-3; 19, 127445-75-4; 20, 127445-78-7; 21,
127445-76-5; o-chloranil, 2435-53-2; 10-chloro-1OH-phenoxarsine, 2865-20-5.
692
Cl
VCH Yerlugsgesellvchaft mhH, 0-6940 Weinheim. 1990
The characteristic current plateau in the cyclovoltammograms of conductive polymers has been the subject of
intensive discussion for some time. On the one hand, capacitive charges in the sense of a molecuIar condenser are assumed as the reason for such plateaus,['-31 while on the
other there are increasing indications that capacitance effects
play only a subordinate role and that the plateau current can
largely be attributed to faradaic redox processes.[4- 71
Our voltammetric investigation of soluble oligomers of
defined structure from the p-phenylenevinylene and p-phenylene
has shown that the number of possible
redox states increases with increasing chain length, and that
the energy gap strongly increases between the lowest and
highest redox states.
We have now investigated for the first time voltammetrically the redox behavior of defined oligomers of the pphenylene series in the solid state. For comparison, Figure
1 a shows a typical cyclic voltammogram for the reduction of
[*] Prof. Dr. J. Heinze, DipLChem. K Meerholz
[**I
lnstitut fur Physikalische Chemie der Universitlt
Alberstrasse 21. D-7800 Freiburg (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and Bayer AG (gifts of chemicals).
0570-0833i90jo606~0692$03.50+ 2510
Angen. Chem. I n [ . Ed. Engl. 29 (1990) No. 6
p-quaterphenyl in
The system is apparently reversibly reduced to the dianion in two separate redox steps at
- 2.28 and - 2.45 V. Surprisingly, in the solid-state experiment (Fig. 1 b), irrespective of the chosen scan rate only one
a)
b)
-3.5
-3.0
-2.5
-2.0
E LVI
-1.5
Fig. 1. Cyclovoltammograms in Me,NH/O.l M Bu,NBr of a) p-quaterphenyl
(saturated solution; T = 10°C. = 100 mVs-'), b)p-quaterphenyl (thin layer
on Pt; T = -75"C, v = 10 mVs-'), and c)p-sexiphenyl (thin layer on Pt;
T = -75.C. 1' = 10 mVs-I); potentials calibrated with Cp,Co@/Cp,Co vs
Ag/AgCI.
~1
cathodic and one anodic wave are observed (at - 2.54 and
- 2.25 V, resp.), both of which correspond to the transfer of
two electrons per quaterphenyl molecule (see Experimental).
In our opinion the different redox transitions in the two
voltammograms (Fig. 1a, 1 b) are closely coupled with the
structural properties of quaterphenyl in solution and in the
solid state. In solution the phenyl rings can rotate freely
about the linked bonds and thus also assume the conformation with the conjugation between the phenylene subunits
that is optimum for the redox process. In the solid state, on
the other hand, this free rotation is strongly hindered by
intermolecular interactions. The molecules are present in a
twisted conformation with a lesser degree of conjugation
than is actually possible. Therefore, the first reduction in the
solid state takes place a t a more negative potential than in
solution. In the monoionic state the molecules stabilize
through a change in geometry from the twisted benzoid
structure into a partially planar quinoid-like structure with
better conjugation between the rings. Similar structural
changes have been observed with substituted 5,6-dihydrobenzo[c]cinnolines in solution[''I and with tetrathiafulvalene in a Nafion matrix.'".1z1 As a consequence of this
change in structure at the stage of the monoanion the redox
potential for dianion formation shifts to more positive potentials and results in a formal two-electron transfer. At the
dianion stage a further stabilizing change of structure takes
place. During the reoxidation, which, because of the stabilization, takes place at more positive potentials than the reduction, a formal two-electron transfer is again observed,
which is now attributable to the relaxation of the system into
the initially twisted structure.
In a further solid state experiment we have investigated the
electrochemical reduction of p-sexiphenyl voltammetrically
(Fig. Ic). In both scan directions three waves are observed,
between which the current drops to almost zero. Coulometric measurements indicate transfer of two electrons for the
first wave, of one electron for the second and up to one for
the third (Tables 1 b-d). p-Sexiphenyl can accordingly be
reversibly charged to the tetraanion. This finding is consistent with the rule that the number of possible redox states
increases with increasing chain length of the oligomer. Particularly striking is the fact that the reoxidation waves lie at
very much more positive potentials than the corresponding
reduction waves.
In our opinion these effects clearly show that p-sexiphenyl
molecules also stabilize through the transition from a benzoid into a quinoid-like structure upon uptake of electrons.
This geometric stabilization still continues even after formation of the dianion. However, the extent of the additional
stabilization is significantly less after the fourth electron
transfer.
Table 1. Coulometric data for the reduction and oxidation of p-quaterphenyl and p-sexiphenyl. n
number of a monomer unit, Q,,, = anodic and cathodic charge, respectively.
pQuaterpheny1
d
1
2.16 (2.00)
0
1S O
- 2.55
0.50(2.00)
0.75 (3.00)
0.94 (3.76)[e]
0
0
0
1.66
1.66
1.66
0
3.20 (099)
0
8.72(2.73)
0
0
1.66
h
}
iJ
j
p-Sexiphenyl
1.65
1.76
1.16
2 10
2.10
1 .I6
1
20 rci
..
1
4 lcl
1
1
number of charging and discharging cycles,f= average binding
- 3.22
-2.95
- 3.25
fl
=
6.44(2.00)
140.20 (43.8)
6.85 (2.14)
54 00 (16.88)
11.32(3.54)
7.26(2.27)
137.00 (42.8)
45.28 (14 15)
1.83
2.29
2.29
[a] The experiments b-d and e-j were carried out successively on the same sample. [b] Charges after subtraction of background currents. The charge equivalents
transferred per mole of educt are quoted in brackets (see Experimental). [c] Solid-state polymerization. [d] Q, = - Q, except in the case off and h. [el It was not possible
to measure the last reduction wave entirely because the reduction of the electrolyte system starts at - 3.25 V.
Angen.. Chem. /nt. Ed EngI. 29 (1990) No. 6
C>
VCH Verlugsgesellsrhafi mbH, 0-6940 Weinheim. 1990
0570-OR33190jO606-0693 S 03.50+ -2510
693
In principle, similar results are obtained in the anodic
oxidation of a p-sexiphenyl film as in the reduction. In a
low-temperature experiment, reversible generation of the trication is possible. Upon further anodic charging the tetracation is formed, which, however, is not indefinitely stable
and undergoes a coupled solid state reaction. The reactivity
of the system increases when the temperature is raised, so
that the p-sexiphenyl trication already reacts at room tempera t ure.
If formation of the trication is completely excluded
(Ex= 1.65 V), p-sexiphenyl can be reversibly converted into
the dication more than 500 times without loss of activity
(Fig. 2 a, Table 1 e). Particularly striking are the extremely
narrow waves and the large potential difference between the
anodic and the cathodic peak potential of AEp = 410 mV,
which indicates a strong energetic stabilization of the
charged system.
a)
.2 uA
1
I
I
0.5
1.0
1.5
E
rvi
-
t
2.0
Fig. 2. Cyclovoltammograms In CH,CI,/O.i M Bu,NPF, of a) p-sexiphenyl
(thin layer on Pt), b)p-sexiphenyl in the multisweep experiment with
€, = 1.75 V, c ) the dodecaphenyl isomer, d) polyphenylene; potentials calibrated with Cp,Fe/Cp,Fe* vs AgIAgCI, T = 1 O T , v = 20 r n V s - ' .
If the switching potential in the multisweep experiment is
set in the ascent of the anodic trication wave (El = 1.76 V),
two new, gradually increasing waves appear, whilst the original signals decrease (Fig. 2 b). The resulting isopotential
point confirms that p-sexiphenyl reacts to give a new electroactive species without side reactions. This reaction is
694
(i
VCH ~ r l a ~ g e . s e l l . ~ c hmhH.
a f t D-6940 Weinheim. 1990
quantitative. In addition, at high scan rates, broad waves as
are typical for the reduction of protons are observed at low
potentials in the cathodic reverse scan. The coulometric
analysis of the voltammograms shows that one charge is lost
per molecule (by proton cleavage) in the condensation reaction (Table 1 f). The material formed is, in contrast to p-sexiphenyl, completely insoluble in all conventional solvents,
and its voltammetric peak potentials lie negative to those of
p-sexiphenyl. Furthermore, the halfwidths of the waves are
still very small (Fig. 2c), and the peak-potential difference
(AEn = 430 mV) resembles that of p-sexiphenyl. These are
clear indications that a material of definite composition is
formed. From all these observations we conclude that p-sexiphenyl dimerizes quantitatively in this solid state reaction to
give a dodecaphenyl isomer whose redox steps each correspond to the transfer of four electrons per molecule.
If still higher electrode potentials (e.g. El = 2.1 V) are applied in the multisweep experiment, further polymerization
steps are induced. Depending upon the oxidation potential,
2-4 charge equivalents are thereby lost by cleavage of protons (Table 1 h). The waves are once again shifted to more
negative potentials, but are now also significantly broader,
and the difference in peak potential decreases (AE, =
350 mV). An isopotential point is once again observed. The
voltammogram, with its characteristic current plateau, is
now typical for conductive polymers (Fig. 2d).
The amount of charge consumed in a pure charging and
discharging cycle remains almost constant for all the materials investigated (Fig. 2a, c, d ; Table I e, g, j); on average,
every third phenyl unit becomes charged. (The slight difference in charge of 8 % between the experiments g and j can be
attributed to the fact that the redox potentials in experiment
j are shifted negative relative to those in g.) We can deduce
from this that no material is lost or decomposed during the
experiment. This is additionally supported by the appearance of isopotential points during the polymerization. The
characteristic plateau region can thus be attributed unequivocally to faradaic redox processes.
Since the amount of charge that is additionally consumed
for the solid-state polymerization is greater than that which
is theoretically required for the complete linkage of all
chains, the average binding number ,f' of a monomeric unit
must be greater than the limiting v a l u e f = 2 for chains of
indefinite length (see Table 1 i, j). A value o f f > 2, however,
signifies that coupling reactions leading to a network take
place in addition to chain-lengthening steps. The chains are
coupled via the ortho and meta positions of the phenyl units.
Such a formation of defects has already been detected by
FTIR-ATR measurements on polythiophene.''31
The network formed exclusively by solid-state polymerization is apparently made up of numerous different structural
segments. As a consequence the individual charge transitions, other than in ideal, linear chains, no longer take place
in distinctly separate potential steps but are distributed over
a large number of closely adjacent redox states. The typical
voltammogram of conductive polymers corresponds to the
superposition of all redox waves. Due to formation of the
network, the mobility of the subunits formed in the polymer
matrix is reduced. This might explain why a smaller peak-potential difference is observed for the polymer than for the
oligomers.
Our investigations show that polyphenylene not only
contains long, linear, p-coupled chains, as was previously
assumed, but a polymer network. Since polypyrrole
and polythiophene exhibit similar voltammograms, we
presume that a network is partially formed also in those
cases.
0570-0833/90/0606-0694E 03.50i ,2510
Angeu Chem. h t . Ed. Engl. 29 (1990) No. 6
Esper h e n tal
For the solid state investigations, the oligomer to be investigated was deposited
onto a Pt-disk electrode (@ = 1 mm) by sublimation o r by dipcoating the
respective oligomer in dichloromethane. In this way, fully chargeable (i v )
homogenous layers of differing thickness could be prepared.
The amount of material employed was determined from the integral under
the redox waves ( = charge). In the reduction of p-sexiphenyl the charge numbers of the redox steps could be derived unambiguously from the observed
charge ratio of 2.1 for the first two redox steps (Table 1 b,c). I n the case of
quaterphenyl an experiment was carried out with a large electrode and a weighable amount of material; this gave a value of 2 for the charge number. A similar
experiment with sexiphenyl confirmed the above described direct method for
the quantitative measurement.
By reduction and oxidation of one and the same sample ofp-sexiphenyl it was
confirmed that the integral under the first reduction wave is identical with that
under the first oxidation wave (Ei = 1.65 V), i.e., two electrons per molecule are
transferred in both cases.
-
particularly electrophilic alkenes.['I We report here on the
first example for the other non-concerted limiting case which
is to be expected in the cycloaddition of strongly electrophilic
1,3-dipoles to particularly nucleophilic alkenes. In a comparable Diels-Alder reaction with inverse electron demand a
zwitterion was recently isolated and identified as intermedi1,3-Dipolar cycloaddition of alkyl and aryl azides RN, to
5-alkylidenedihydrotetrazoles 2 leads to spirocycles of type
4.[41O n the other hand, in reactions with the strongly electrophilic azides 1 a-c in toluene the zwitterions 3 or the 5iminotetra-hydro-l,2,3,4-tetrazines
5 - together with molecular nitrogen - are obtained. The latter are formed from the
corresponding spirocycles 4, which, however, are not ob-
Received: January 24, 1990;
supplemented: April 2,1990 [Z 3755 IE]
German version: Angew. Chem. 102 (1990) 695
[l] S W. Feldberg, J. ,4177. Chem. Soc. 106 (1984) 4671.
[2] a) J Tanguy, N. Mermilliod. M. Hoclet, J. Electrochem. Soc. 134 (1987)
795: b) J. Tanguy, N . Mermilliod, Synth. M e t 21 (1987) 129.
[3] R. C. M. Jakobs. L. J. J. Janssen, E. Barendrecht, R e d . Trav. Chim Pays50.5 103 (1984) 275.
[4] a ) J Heinze, M. Dietrich, J. Mortensen, Makromol. Chrm. Makromol.
S i m p . 8 (1987) 73; b) J. Heinze, R. Bilger, K . Meerholz, Ber. Bunsenges.
Ph1.s. Chem. 92 (1988) 1266: c) J. Heinze, n ) p . Curr. Chem. 152 (1990) 1
[5] S. Bruckenstein. J. W. Sharkey, J. EIectroanal. Chem. 241 (1988) 211
[6] R S. Hutton, M. Kalaji, L. M. Peter, J. Electroanal. Chem. 270(1989)429.
[7] J. Heinze. M. Storzbach, J. Mortensen. Ber. Bunsengrs. P / ? w Chem 91
( 1 9x7) 960.
[8] J. Heinze, J. Mortensen, K. Mullen, R. Schenk. J. Chem. Soc Chem. Commini. 1987, 701.
[9] K . Meerholz. J. Heinze, J. Am. Chem. Soc. 111 (1989) 2325.
[lo] M. Dietrich, J. Heinze, H. Fischer, F. A. Neugebauer, A n g e u . Chem. 98
(1986) 999: Angen. Chem. Inr. Ed. Engl. 25 (1986) 1021.
[l l ] T. P. Henning. A. J. Bard, J. Elcrtrochem. Soc 130 (1983) 613.
[12] M. D. Ward, J.Electrounal. Chem. 273 (1989) 79.
I131 H. Neugebauer. A. Neckel. N. Brinda-Konopik in H. Kuzmany, M.
Mehring, S. Roth (Eds.): Electronic Properties of Polvmers and Relaled
Cotrrpoutds, Springer, Berlin 1985, p. 221.
+
Me
2
Me
3
R = Alkyl,
Aryl
1
N=N
4
5
la: R = 2,4,6-(N02)3-C,H2;
lb: R = 4-Me-C6H,-S02; IC: R = Me-SO,
2a R' = R2 = Me; 2b: R' = H, R2 = rBu
Zwitterions as Intermediates of the 1,3-Dipolar
Cycloaddition of Electrophilic Azides to
5-Alkylidenedihydrotetrazoles - the Other
Non-Concerted Limiting Case **
served directly. The ring expansion of the tetrazole to the
tetrahydro-I ,2,3,4-tetrazine ring i s an example of the rare
I ,Zshift of a nitrogen atom.r51
By Helmut Quast,* Dieter Regnat, Eva-Maria Peters,
Karl Peters and Hans Georg von Schnering
Table 1. Substituents, yields and melting points of the zwitterions 3 and of the
5-iminotetrahydro-l.2,3,4-tetrazines
5.
Dedicated to Professor Rolf Huisgen
on the occasion o f his 70th birthday
The perturbational treatment of 1,3-dipolar cycloadditions"' predicts for extreme HOMO-LUMO energy differences two limiting cases of a non-concerted two-step mechanism in which zwitterions occur as intermediates. Huisgen
et al. realized one of the non-concerted limiting cases by
reaction of strongly nucleophilic thiocarbonyl ylides with
[*] Prof Dr. H. Quast, DipLChem. D. Regnat
Institut fur Organische Chemie der Universitdt
Am Hubland. D-8700 Wiirzburg (FRG)
E.-M. Peters, Dr. K . Peters, Prof. Dr. H. G. von Schnering
Max-Planck-Institut fur Festkorperforschung
HeisrnbergstraUe 1. D-7000 Stuttgart 80 (FRG)
[**I This work was supported by the Fonds der Chemischen Industrie. D.R
thanks the Fonds der Chemischen Industrie for a postgraduate grant.
Angow. C%em.Int. Ed. EngI. 29 (1990) No. 6
Cpd.
R'
R2
R3
Yield ["A]
M p. [ C ]
3a
3b
3c
5c
3d
Sd
3e
Sf
Me
H
Me
Me
H
H
H
Me
Me
tBu
Me
Me
rBu
tBu
fBu
Me
2,4,6-(NO,)&H,
2,4,6-(NOI),-C,H,
4-Me-C,H,-S02
4-Me-C,H,-S02
4-Me-C,H4-S0,
4-Me-C6H,-SO,
Me-SO,
Me-SO,
90
37
quant.
73
quant.
68
52
86
115-126
115
172- 173
139.- I40
107- 108
1 1 1 -112
106-107
The zwitterions 3a, d, e (Table 1 ) can be recrystallized
from boiling 2-propanol. The structure of the zwitterion 3 e
(Fig. 1) was confirmed by X-ray diffraction analysis.[61 It
shows some remarkable features in the crystal which may
c) VCH Verlagsgesellschaf~mhH, 0-6940 Weinheini, 1990
+
0570-083319010606-0695$03.50 ,2510
695
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