A novel reaction of 7,7,8,8-tetracyanoquinodimethane (TCNQ): Charge-transfer chromophores by [2 + 2] cycloaddition with alkynes

Article abstract of DOI:10.1039/b713683h

A series of donor-acceptor molecules, featuring intense low-energy intramolecular charge-transfer bands, was prepared by regioselective [2 + 2] cycloaddition between 7,7,8,8-tetracyanoquinodimethane (TCNQ) and N,N-dialkylanilino-substituted (DAA-substitut

Full text of DOI:10.1039/b713683h

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A novel reaction of 7,7,8,8-tetracyanoquinodimethane (TCNQ): charge-
transfer chromophores by [2 + 2] cycloaddition with alkynes{
Milan Kivala,a Corinne Boudon,b Jean-Paul Gisselbrecht,b Paul Seiler,a Maurice Grossb and
Franc¸ois Diederich*a
Received (in Cambridge, UK) 6th September 2007, Accepted 15th October 2007
First published as an Advance Article on the web 24th October 2007
DOI: 10.1039/b713683h
been ignored. Namely, it possesses two types of strongly electron-
deficient CC double bonds that could, in analogy to tetracya-
noethylene (TCNE),6 undergo thermal [2 + 2] cycloaddition with
electron-rich alkynes. Here, we describe such a reaction between
TCNQ and N,N-dialkylanilino-substituted (DAA-substituted)
alkynes, giving access to a new family of non-planar CT
chromophores, such as 1–9. To the best of our knowledge, this
is the very first example of a thermal [2 + 2] cycloaddition to
TCNQ.
A series of donor–acceptor molecules, featuring intense low-
energy intramolecular charge-transfer bands, was prepared by
regioselective [2 + 2] cycloaddition between 7,7,8,8-tetracyano-
quinodimethane (TCNQ) and N,N-dialkylanilino-substituted
(DAA-substituted) alkynes, followed by ring opening of the
initially formed cyclobutenes.
7,7,8,8-Tetracyanoquinodimethane (TCNQ) was reported in the
early 1960s,1 and its chemical and physicochemical properties have
been thoroughly investigated since then.2,3 Due to its powerful
electron-accepting properties, TCNQ forms intermolecular charge-
transfer (CT) complexes with various organic and inorganic
electron donors, some of them exhibiting high electric conductivity
or interesting magnetic behaviour.4,5 These properties make
TCNQ still an attractive subject of contemporary advanced
materials research.
A variety of electron-rich, DAA-substituted alkynes (10–17)
were prepared (ESI{) and subsequently subjected to the reaction
with TCNQ to probe their reactivity. All acetylenic precursors
reacted in a uniform manner to give products 1–7 in high to
quantitative yields (except for 5, Scheme 1).{ Thus, terminally
deprotected alkyne 10 reacted at 20 uC in CH2Cl2 to give adduct 1
(81%) as a black metallic solid. Chromophore 1 was also isolated
in 42% yield starting from Me3Si-protected 11, since silyl-
deprotection took place during chromatographic purification on
SiO2 (CH2Cl2–EtOAc 97 : 3). The use of the less labile (iPr)3Si
protecting group unexpectedly led to complete decomposition
during attempted chromatography. Oligomeric 8 and 9 (Fig. 1)
were also prepared in high yields from the corresponding alkyne
precursors.6 For example, the threefold addition of TCNQ to
produce oligomeric 9 proceeded in 66% yield (that is, 87% yield for
each addition step). All compounds 1–9 are dark metallic solids
that are stable at ambient temperature under air.
However, while the reactivity of TCNQ has been explored,
some of the structural features of this molecule have obviously
aLaboratorium fu¨r Organische Chemie, ETH Zu¨rich, Ho¨nggerberg,
HCI, CH-8093 Zu¨rich, Switzerland.
E-mail: diederich@org.chem.ethz.ch; Fax: (+41) 44 632 1109
bLaboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
Institut de Chimie – LC3 – UMR 7177, C.N.R.S., Universite´ Louis
Pasteur, 4, rue Blaise Pascal, F-67000 Strasbourg, France
{ Electronic supplementary information (ESI) available: Syntheses and
UV/Vis spectra for new compounds and crystal structures for 1 and 3. See
DOI: 10.1039/b713683h
Scheme 1 Synthesis of donor-substituted TCNQ derivatives: i, CH2Cl2 or toluene or 1,2-dichloroethane, 20 uC or 80 uC; 81% (1), 100% (2), 93% (3),
100% (4), 33% (5), 72% (6), 78% (7).
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Fig. 1 Oligomeric TCNQ-based chromophores 8 and 9.
Fig. 2 (a) ORTEP plot of 1, arbitrary numbering. Atomic displacement
parameters at 220 K are drawn at the 30% probability level. (b)
Arrangement of neighbouring molecules in the crystal packing of 1
showing short intermolecular contacts. Molecules A (x,y,z) and A* (2 2 x,
2y,1 2 z) are related by an inversion centre. For selected bond lengths and
angles, see Fig. S1 (ESI{). Quinoid character: dr = (((a + a9)/2 2 (b + b9)/2)
+ ((c + c9)/2 2 (b + b9)/2))/2.9
We assume the reaction proceeds by means of thermal [2 + 2]
cycloaddition between the exocyclic CC double bond of TCNQ
and the triple bond of the alkyne, followed by ring opening of the
intermediately formed strained cyclobutene to give the observed
product. The cycloaddition step proceeds presumably via a
zwitterionic or a biradical intermediate, as a concerted [2 + 2]
mechanism involving the HOMO–LUMO interaction is symmetry
forbidden.7 The reaction is completely regioselective with respect
to TCNQ and proceeds exclusively at one of the dicyanovinyl
moieties and not at the endocyclic double bonds.8{ No multiple
additions were observed, even in the presence of an excess of
alkyne at elevated temperature (up to 80 uC). The constitution of
the products was unambiguously confirmed by NMR spectro-
scopy (HSQC and HMBC) and, as shown below, by X-ray
analysis.
3-diene (TCBD) derivatives.6 The dr value for the DMA ring in 3
could not be estimated due to the reduced accuracy.
The UV/Vis spectra of chromophores 1–9 are dominated by
intense, broad CT bands with end absorptions reaching into the
near infrared region (Fig. 3 and Figs. S3 and S4, Table S1 (ESI{)).
Upon introduction of a second DMA donor, the intensity of the
CT band increases strongly (compare the spectra of 3 and 4 or 6
and 7, Fig. 3). All molecules show a pronounced solvatochromism
in CH2Cl2–hexane mixtures (see Table S2 (ESI{)). The largest
solvent effect was observed for 2, with the CT band shifting from
Single crystals of 1 and 3, suitable for X-ray structure analysis,
were grown by slow diffusion of hexane into 1,1,2,2-tetrachloro-
ethane solutions of the compounds at 20 uC.§ The X-ray analysis
(Fig. 2) nicely confirmed the regioselective addition of the alkyne at
one exocyclic CC double bond of TCNQ and proved the
constitution of the formed non-planar donor–acceptor chromo-
phores (see Figs. S1 and S2 (ESI{)). In the crystal packing of 1
(Fig. 2b), two molecules related by an inversion centre show short
lmax = 559 nm (1.63 eV) in hexane to lmax = 655 nm (1.89 eV) in
CH2Cl2 (see Figs. S5 and S6, Table S2 (ESI{)).
The redox properties of 1–9 were studied by cyclic voltammetry
(CV) and rotating-disc voltammetry (RDV) in CH2Cl2 (+ 0.1 M
6
…
multipolar CN CN interactions, as already previously observed.
Particularly interesting are the short contacts between nitrogen
atoms that are polarised in an opposite way through the
intramolecular charge-transfer interactions. The negatively
polarised N-atoms of a CN moiety in one molecule interact at
van der Waals distance with the positively polarised N-atom of the
N,N-dimethylanilino (DMA) moiety of an adjacent one. The
efficiency of the CT from the donor to the acceptor moieties can be
expressed as the quinoid character (dr) of the DMA ring (for its
definition,9 see caption to Fig. 2; for bond lengths, see Figs. S1 and
S2 (ESI{)). In benzene, the dr value equals 0, whereas values
between 0.08 and 0.10 are found in fully quinoid rings. The DMA
ring in 1 exhibits a high dr value of 0.046 comparable to the highest
value observed for DMA-substituted 1,1,4,4-tetracyanobuta-1,
Fig. 3 Electronic absorption spectra of chromophores 1, 3, 4, 6 and 7 in
CH2Cl2 at 298 K.
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with respect to alkyne addition is unambiguous for 1–6, 8 and 9. Spectral
similarity suggests for 7 the constitution shown in Scheme 1.
§ Crystal data of 1 at 220(2) K: C22H15N5, Mr = 349.39: triclinic, space
nBu4NPF6) (see Table S3 (ESI{)). Each DAA moiety in
chromophores 1–9 undergoes a one-electron oxidation step which
is irreversible, except for 5. While the two DMA moieties in 4 and
7 are oxidised in two separated one-electron steps, all DAA
moieties in oligomeric 8 and 9 are oxidised in a single, irreversible
multielectron step, denoting no electrostatic interactions between
the redox centres.
group P1 (no. 2), Dc = 1.241 g cm23, Z = 2, a = 8.8680(13), b = 9.2552(14),
¯
˚
c = 11.7022(14) A, a = 101.406(2), b = 93.315(12), c = 94.983(2)u, V =
3
935.2(2) A . Bruker-Nonius Kappa-CCD diffractometer, MoKa radiation,
˚
l = 0.7107 A, m = 0.077 mm21. Crystal dimensions ca. 0.25 6 0.23 6
˚
0.20 mm. The numbers of measured and unique reflections are 6407 and
3755, respectively. (Rint = 0.035). Final R(F) = 0.045, wR(F2) = 0.119 for
247 parameters and 3099 reflections with I . 2s(I) and 3.03 , h , 26.32u
(corresponding R-values based on all 3755 reflections are 0.057 and 0.128,
respectively). Crystal data of 3 at 173(2) K: 3(C28H19N5), 1.5(C2H2Cl4),
Monomeric chromophores 1–7 remain potent electron-accep-
tors, and display two reversible one-electron reduction steps,
centred on the two dicyanovinyl moieties. Except for 1, the
observed potential difference is rather small (ranging from 90 mV
for 4 to 150 mV for 5 and 6). The difference is indeed much smaller
than previously observed for DAA-substituted TCBD derivatives
(230–570 mV).6 This is readily explained by the larger distance
between the two dicyanovinyl groups in 2–7 as a result of insertion
of the cyclohexa-2,5-diene-1,4-diylidene moiety. However, in the
case of 1, the potential difference is somewhat larger (260 mV).
With its small H-atom substituent, 1 may adopt a more planar
structure in solution, thereby making the conjugation between the
dicyanovinyl moieties and the DAA donor more efficient than in
the other cases. As a result of higher planarity, the electrostatic
repulsion for the electrogenerated dianion is stronger in 1. The
stepwise reduction of 8 and 9, with the latter undergoing six
reversible one-electron reductions in the narrow potential range
between 20.51 and 21.14 V, is a good indication that the
dicyanovinyl moieties in these chromophores are not independent
redox-active centres, as observed previously.10
Mr = 1528.20: triclinic, space group P1 (no. 2), Dc = 1.283 g cm23, Z = 2,
¯
˚
a = 11.1429(14), b = 16.3191(15), c = 22.2110(17) A, a = 82.970(11), b =
3
˚
85.747(12), c = 81.416(11)u, V = 3957.2(7) A . Bruker-Nonius Kappa-CCD
diffractometer, MoKa radiation, l = 0.7107 A, m = 0.273 mm21. Crystal
˚
dimensions ca. 0.11 6 0.10 6 0.09 mm. The numbers of measured and
unique reflections are 18424 and 10703, respectively. (Rint = 0.054). Final
R(F) = 0.111, wR(F2) = 0.288 for 980 parameters and 7198 reflections with
I . 2s(I) and 2.95 , h , 22.92u (corresponding R-values based on all
10703 reflections are 0.151 and 0.326, respectively). CCDC 658467–658468.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b713683h. As a note of caution, exposure to 1,1,2,2-tetrachloro-
ethane should be avoided due to its high toxicity.
1 D. S. Acker, R. J. Harder, W. R. Hertler, W. Mahler, L. R. Melby,
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In summary, we have described a novel, completely regio-
selective [2 + 2] cycloaddition of TCNQ with DAA-substituted
alkynes, followed by ring opening of the initially formed
cyclobutene derivative to yield new CT chromophores with
appealing optoelectronic and redox properties. The generality of
this generally high-yielding, atom-economic transformation was
demonstrated by running the reaction with a series of acetylenic
substrates. The exploration of the optical nonlinearities of these
new TCNQ derivatives and their application to the preparation of
organic-based magnets and conductive materials are currently
under investigation.
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Support by the ETH Research Council and the NCCR
‘‘Nanoscale Science’’, Basel is gratefully acknowledged. We thank
Prof. Dr Bernhard Jaun (ETHZ) for the interpretation of NMR
data of the compounds described in this paper.
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Notes and references
{ All new compounds were fully characterised by IR, UV/Vis, 1H and 13
NMR, mass spectrometry and/or elemental analysis. The regioselectivity
C
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Chem. Commun., 2007, 4731–4733 | 4733