Proacetylenic reactivity of a push-pull buta-1,2,3-triene: New chromophores and supramolecular systems

Article abstract of DOI:10.1002/asia.201100997

Proacetylenic: An aniline-based donor-acceptor-substituted butatriene exhibits not only cumulene-like dimerization to give a [4]radialene (λ_max=756 nm), but also acetylene-like (proacetylenic) reactivity towards tetracyanoethene, affording via [2+2] cycloaddition- cycloreversion a NIR-absorbing zwitterionic chromophore (λ_max= 825 nm). The cationic charge on the imminium-type nitrogen in the zwitterion is evidenced by host-guest complexation with a tetraphosphonate cavitand. Copyright

Full text of DOI:10.1002/asia.201100997

DOI: 10.1002/asia.201100997
Proacetylenic Reactivity of a Push–Pull Buta-1,2,3-triene: New
Chromophores and Supramolecular Systems
Yi-Lin Wu,[a] Francesca Tancini,[a] W. Bernd Schweizer,[a] Daniela Paunescu,[a]
Corinne Boudon,[b] Jean-Paul Gisselbrecht,[b] Peter D. Jarowski,[c] Enrico Dalcanale,[d] and
FranÅois Diederich*[a]
Dedicated to the 150th anniversary of German-Japanese relations
Proaromaticity is an efficient concept for reducing
HOMO–LUMO gaps in push–pull chromophores.[1–3] Exper-
imental electrochemical and optical gaps of donor–acceptor-
substituted (push–pull) quinoids (Figure 1) are much lower
in energy, compared to acyclic or nonquinoid cyclic analogs,
as a result of the significant charge-separated character of
their ground state. The increase in aromaticity of quinoids
induced by electronic asymmetry has been evidenced in ex-
tensive physical and theoretical studies[2] and the concept
successfully applied to the development of chromophores
with high optical nonlinearities.[3]
Structural analysis of push–pull [3]cumulenes, such as 1
and 2 (Figure 1), revealed that the central cumulenic double
ꢀ
bond is very short, similar in length to the C C bond in al-
kynes. The X-ray crystal struc-
ture of 1,1-bis(dimethylamino)-
4,4-dicyanobutatriene
(1a)[4]
ꢁ
shows a short central C2 C3
bond of 1.201 ꢀ, thus implying
acetylenic character. Addition-
ally, the 13C NMR signal at
22.9 ppm (CDCl3) indicates a
highly shielded environment at
the dicyano-substituted C1 in
1a and suggests that this mole-
cule might be best described by
its charge-separated acetylenic
form, rather than the [3]cumu-
lenic form. Similarly, an upfield
resonance at 19.0 ppm was
Figure 1. Left: Proaromatic donor–acceptor-substituted quinoids (D=electron-donating group, A=electron-
accepting group). Right: Known donor–acceptor-substituted [3]cumulenes 1a–c[4,8] and 2[5] (R was not clearly
defined in Ref. [5]).
found for the diaminocyclopropenylidene derivative 2.[5]
Based on topological resonance energy (TRE) analyses of
annulenes and cyclynes, Chauvin and co-workers suggested
that acetylene is “… ca. three times more topologically aro-
matic than … benzene”,[6] which should also favor the acety-
lenic charge-separated state of donor–acceptor-substituted
[3]cumulenes. Despite these experimental and theoretical
findings, a proacetylenic, i.e. acetylene-like, reactivity of the
central double bond in push–pull [3]cumulenes has not yet
been observed. Only polar additions of nucleophiles/electro-
philes to 1a, converting the butatriene into buta-1,3-dienes
have been reported.[4] Herein, we describe the synthesis of a
new donor–acceptor-substituted butatriene and demonstrate
the chameleonic reactivity of its central CC bond with re-
spect to cycloadditions, undergoing both olefin-type and
acetylene-type transformations. The opto-electronic and mo-
lecular recognition properties of the resulting new chromo-
phores are discussed. It is noticeable that donor–acceptor-
substituted cumulenes have been predicted to be even more
[a] Y.-L. Wu, Dr. F. Tancini, Dr. W. B. Schweizer, D. Paunescu,
Prof. Dr. F. Diederich
Laboratorium fꢁr Organische Chemie
ETH Zꢁrich
Hçnggerberg, HCI, 8093 Zꢁrich (Switzerland)
Fax : (+41)44-632-1109
E-mail: diederich@org.chem.ethz.ch
[b] Prof. Dr. C. Boudon, Dr. J.-P. Gisselbrecht
Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide,
UMR 7177
Universitꢂ de Strasbourg, C.N.R.S.
4 rue Blaise Pascal, 67081 Strasbourg Cedex (France)
[c] Dr. P. D. Jarowski
Advanced Technology Institute and Department of Physics
University of Surrey
Stag Hill, Guidlford GU2 7XH, Surrey (United Kingdom)
[d] Prof. Dr. E. Dalcanale
Dipartimento di Chimica Organica e Industriale and Unitꢃ INSTM
UdR Parma, Universitꢃ di Parma
Parco Area delle Scienze 17/a, 43124 Parma (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100997.
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COMMUNICATION
In comparison to 1a, which
features a longest-wavelength
absorption maximum at lmax
=
356 nm,[4] the additional phen-
ylene rings in 3 induce a bath-
ochromic shift of the absorp-
tions. In the purple CH2Cl2 so-
lution of 3, maxima at lmax
=
588 nm
and
(e=54700mꢁ1 cmꢁ1)
541 nm (e=
48500mꢁ1 cmꢁ1) were observed,
and
their
intramolecular
charge-transfer (CT) character
was suggested by the disap-
pearance of these bands upon
protonation with trifluroacetic
acid (TFA). The longest-wave-
length maximum of 3 is batho-
chromically shifted by 90 nm
(0.38 eV) relative to the analo-
gous buta-1,3-diene 5[10] (lmax
=
498 nm, Figure 2). According
to rotating disk voltammetry
(half-wave potentials E1/2 vs.
Fc+/Fc, in CH2Cl2 containing
0.1m nBu4NPF6), this reduc-
tion in optical gap is mainly
due to the stabilization of the
LUMO level of 3 (Ered =ꢁ1.35
for 3 and ꢁ1.71 V for 5) and
Scheme 1. Synthesis of push–pull butatriene 3 and chromophores 6 and 7. Conditions: i) Et3N, MeCN, 08C!
r.t., 2.5 h, 97%; ii) quinuclidine, THF, reflux, 17 h, 80%; iii) [Ni(CO)
48%; iv) tetracyanoethene, CH2Cl2 or MeCN, r.t., 30 min, 99%.
2ACHTGNURTEN(NUNG PPh3)2], PPh3, benzene, reflux, 4.5 h,
potent non-linear optical chromophores than comparable
push–pull oligo-enes and oligo-ynes.[7]
slightly due to a variation in the HOMO energy (Eox =
+0.57 for 3 and +0.52 V for 5).
We synthesized 1,1-dicyano-4,4-bis(N,N-dimethylanilino)-
butatriene 3 from the vinyl chloride precursor 4, following
protocols of Viehe and co-workers[4] and Gompper and
Wolf[8] with a slight modification (Scheme 1). The X-ray dif-
fraction analysis (see the Supporting Information) of 3 gave
Thermal dimerization of 3 was not observed upon heating
in MeCN at reflux for 2 days.[11] However, by applying the
protocol developed by Iyoda and co-workers[12] for cyclooli-
gomerization of cumulenes, 3 underwent a Ni(0)-mediated
[2+2] cyclodimerization to give [4]radialene 6 in 48% yield
(Scheme 1). The reaction is highly regioselective: only the
ꢁ
an average central C2 C3 bond length for the two symme-
ꢁ
try-independent molecules in the crystal of 1.23(2) ꢀ, short-
er than in symmetric butatrienes[4a,9] but slightly longer than
in 1a. The 13C NMR signal at 40 ppm (100 MHz, CDCl3) im-
plies a significant electron density at dicyano-substituted C1.
This signal is, however, relatively downfield as compared to
the comparable resonance in 1a and 2, which suggests that
the charge-separated mesomeric contribution for 3 is small-
er. This is in line with the weaker electron-donating power
of a N,N-dimethylanilino (DMA) as compared to a N,N-di-
compound from head-to-tail dimerization at the central C2
methylamino (in 1a) or
a diaminocyclopropenylidene
moiety (in 2). The strong IR absorption at 2044 cmꢁ1 for the
central CC vibration is at the borderline between the posi-
tions of acetylene and cumulene bands, while the moderate-
ly strong absorption at 2213 cmꢁ1 for the CN stretch is at
higher energy than those reported for 1a (2178 cmꢁ1)[4] and
proaromatic dicyano compounds (2168 and 2183 cmꢁ1).[2]
Taken together, these data suggest that 3 might behave as a
hybrid of cumulene and acetylene.
Figure 2. Electronic absorption spectra of 3 and 5–7 in CH2Cl2 at 258C
(ca. 10ꢁ5 m).
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Push–Pull Buta-1,2,3-triene
C3 bond was isolated. In principle, this Ni(0)-mediated reac-
tion could yield a [6]radialene[12d] that might be benzene-like
due to three-fold push–pull interactions. However, such a
compound was not observed in the crude mixture by mass
spectrometry.
pulsion between the exocyclic substituents. Additionally, C1
and C1’ show a 0.24 ꢀ deviation from the plane set by the
central four-membered cycle, whereas C4 and C4’ are essen-
tially located in this plane.
[4]Radialene 6 has a metallic copper luster in the solid-
state and is blue-colored in solution. The UV/Vis spectrum
shows two strong intramolecular CT bands at lmax =589 nm
(e=42700mꢁ1 cmꢁ1) and 756 nm (e=113300mꢁ1 cmꢁ1) in
CH2Cl2 (Figure 2). Although it is a structural dimer of 5, the
absorption profiles of the two
The X-ray crystal structure of 6 revealed two symmetry-
independent, practically superimposable molecules in the
ꢀ
unit cell (P1 space group, Figure 3a). The nearly C2h sym-
metric conformer found in the crystal is more stable than
chromophores have no similari-
ty. The sharp, intense absorp-
tion of 6 in the deep-red region
is reminiscent of those of
squaraines, a class of squaric
acid-derived, zwitterionic cya-
nine dyes.[17] Electrochemically,
four oxidation events at +0.31,
+0.64 (2eꢁ), and +0.89 V and
two reductions at ꢁ1.22 and
ꢁ1.97 V were recorded, consis-
tent with the apparent number
of electrochemically active
moieties (four aniline and two
dicyanovinyl
groups).
The
unique p-system of 6 is further
evidenced by the fact that its
first oxidation and reduction
events are more favorable by
200 and 500 mV, respectively,
as compared to 5. The two re-
duction steps in 6, on the other
hand, are expectedly much less
favorable than in octacya-
no[4]radialene which features
positive potential values for the
reduction to the dianion.[18]
Figure 3. a) ORTEP representation of 6, T=273 K, anisotropic displacement ellipsoids are shown at the 50%
probability level. Only one of two almost superimposable, symmetry-unrelated molecules is shown; see the
Supporting Information for the other one and their relative orientation in the crystal. Selected bond lengths
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
[ꢀ] and angles [8]: C2 C3=C2’ C3’ 1.474(3), C2 C3’=C2’ C3 1.473(3), C2 C3 C2’=C2 C3’ C2’ 89.19(17),
[16]
ꢁ
ꢁ
ꢁ
ꢁ
C3 C2’ C3’=C3’ C2 C3 90.81(17). The quinoid character dr is calculated as shown, from the bond lengths
in the DMA moieties (see the Supporting Information); average dr [ꢀ] for all four DMA rings=0.041 (in ben-
zene, dr=0; in fully quinoid rings, dr would be on the order of 0.10–0.12 ꢀ). b) ORTEP representation of 7,
T=100 K, anisotropic displacement ellipsoids are shown at the 50% probability level. Selected bond lengths
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
[ꢀ] and dr [ꢀ]: C1 C2 1.399(3), C2 C3 1.497(4), C3 C4 1.468(3), C2 C17 1.401(3), C3 C18 1.369(3), C5 C6
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1.409(4), C5 C10 1.417(4), C6 C7 1.365(4), C7 C8 1.420(5), C8 C9 1.417(4), C9 C10 1.368(4), C11 C12
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1.427(4), C11 C16 1.434(3), C12 C13 1.350(4), C13 C14 1.426(4), C14 C15 1.436(4), C15 C16 1.350(4), dr
(ring A) 0.050, dr (ring B) 0.081.
Introduction of tetracyanoe-
thene (TCNE) to a solution of
the other possible D2-symmetric head-to-tail dimer by ap-
3 in CH2Cl2 or MeCN at 258C resulted in an immediate
color change from purple to brownish orange. A stable and
highly polar molecule was isolated by column chromatogra-
phy on SiO2 in nearly quantitative yield (99%). X-ray dif-
fraction analysis revealed a zwitterionic molecule (7, Fig-
ure 3b), which presumably forms by a formal [2+2] cycload-
dition–cycloreversion process via a cyclobutene intermediate
(Scheme 1).[19] Since this reaction is known to occur between
cyanoolefins and strongly polarized electron-rich acetylenes,
the isolation of 7 signifies that the central CC bond of push–
pull butatriene 3 may have a reactivity similar to a triple
bond, corroborating the structural results described above.
The charges are permanently separated by the central,
cross-conjugated dicyanoethenylidene moiety in the zwitter-
ionic molecule. The positive terminus is stabilized by two
electron-donating DMA moieties and the negative terminus
by a tetracyanoallyl system. According to NMR studies, the
two DMA rings are magnetically identical in solution and
proximately 5 kJmolꢁ1 according to density functional
theory calculations (B3LYP/6-31G+ACTHNUTRGNEUNG(d,p) in the gas phase;
see the Supporting Information for the geometry of the D2-
symmetric conformer).[13] With the centroid of each mole-
cule locating on a crystallographic center of symmetry and
no disorder, according to the isotropic atomic displacement
parameters, the four-membered ring of 6 is in effect a per-
fect square. By contrast, most reported [4]radialenes feature
a puckered ring caused by steric repulsion between exocyclic
substituents.[14] There is significant pyramidalization about
the two cyano-substituted carbon atoms C1 and C1’ (POAV
(p-orbital axis vector) angle[15] ~48; for numbering, see
Figure 3). The pyramidalization is indicative of negative
charge density at these atoms and is consistent with the qui-
noid character dr[16] of 0.04 ꢀ determined for the DMA
donor rings (see Figure 3 for definition and determination
of dr). This pyramidalization partially relieves the steric re-
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FranÅois Diederich et al.
COMMUNICATION
the two terminal carbons (C1 and C17, Figure 3b) of the tet-
racyanoallyl group are equivalent as well. These observa-
tions agree with X-ray data that indicate single-bond charac-
moved upfield and broadened until disappearance upon
complexation by 8, suggesting that the DMA units experi-
ence a shielding effect from the cavity of the host, and that
their rotational freedom becomes reduced by inclusion.
From the signals of 8, we noticed upfield shifts of the
ArCH3 (Dd=ꢁ0.10 ppm) and Ph-Hortho signals (Dd=
ꢁ0.09 ppm), indicative of their interaction with a bound cat-
ionic DMA moiety. By contrast, the signals from the Hdown
protons (Dd=0.14 ppm) and the nearby ArCHCH2 protons
exhibited downfield shifts, suggesting the presence of an
anionic moiety at the bottom of 8. A complete disassembly
of the 7·8 aggregate was achieved by addition of N-octylam-
monium bromide (9), which has a higher affinity to 8 due to
additional ionic H-bonding with the P=O groups.[22,23] The
brownish-orange color characteristic for unbound 7 was im-
mediately restored, and a 31P signal at 10.86 ppm was re-
corded, diagnostic for the binding of the octylammonium
guest (see the Supporting Information for these NMR spec-
tra).
ꢁ
ꢁ
ter in the C2 C3 (1.497(4) ꢀ) and C3 C4 (1.468(3) ꢀ)
bonds, which allows rapid rotation. On the other hand, this
equivalency implies that the charges are delocalized. In the
ꢁ
ꢁ
solid-state, the C1 C2 and C2 C17 bonds have the same
length within experimental precision; however, the two
DMA rings show different degrees of quinoid character dr
of 0.050 and 0.081 ꢀ; the latter number approaches that of
7,7,8,8-tetracyanoquinodimethane (TCNQ, dr=0.102).[20]
The electrochemistry (E1/2 vs. Fc+/Fc) of zwitterion 7 dif-
fers substantially from that of other neutral push–pull chro-
mophores containing DMA donors and cyanoolefin accept-
ors.[21] Two oxidation and two reduction processes were ob-
served. The first oxidation was observed at a high potential
of 0.85 V and the first reduction at ꢁ0.26 V. The small elec-
trochemical gap (1.11 eV) correlates well to the low-energy
absorption band in the near-IR (NIR) region (lmax =825 nm;
absorption onset ~1150 nm, or 1.08 eV; Figure 2). The ab-
sorption band became less intense upon addition of TFA;
similar spectral changes have been found for other DMA-
based chromophores, but the protonation site in the present
case is more likely to be on the allyl anion moiety. During
the course of neutralizing the
The binding mode deduced from NMR studies is corrobo-
rated by the X-ray crystal structure of the chain-like 7·8 as-
sembly (Figure 4), which highlights the four P=O groups in
an all-inward configuration,[23] establishing ion-dipole inter-
actions with a positively charged imminium-type center of 7
added acid with Et3N, we ob-
served that the absorption pro-
file of 7 was first restored, and
then irreversibly changed with
an excess of base.
We investigated the capabili-
ty of the positively polarized
DMA moieties to interact with
supramolecular receptors that
strongly bind cationic species.
We selected tetraphosphonate
cavitand
8 (Figure 4) as the
host,[22] for its well-known ten-
dency to engulf positively
charged molecules within the
upper cavity.[22,23]
By addition of
7
to
8
(3.3 mm) in CDCl3, a 31P NMR
signal at 9.72 ppm (cf. 6.81 ppm
for free 8) was recorded for the
resulting deep-green solution.[24]
Such a downfield shift is consis-
tent with ion-dipole interactions
between a strongly positively
polarized, nearly cationic DMA
moiety of 7 and the P=O di-
poles in 8. A similar 31P NMR
shift was reported for tetra-
phosphonate cavitands encapsu-
lating other positively charged
species.[22,23] In the 1H NMR
Figure 4. Solid-state geometry of the complex between zwitterion 7 and cavitand 8 based on X-ray diffraction
analysis, T=100 K. Included solvent molecules and hydrogen atoms are omitted for clarity. Color code: gray=
carbon (darker for 7, lighter for 8), blue=nitrogen, red=oxygen, and orange=phosphor. a) The 1:1 complex
7·8. The short P=O···NACHTNUGTRNEUNG(CH3)2 distances of 2.9–3.0 ꢀ are shown by the dotted green lines. b) Supramolecular
spectra, the DMA signals of 7 co-polymeric arrangement of 7·8; the attractive electrostatic interactions are shown by the dotted purple lines.
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Push–Pull Buta-1,2,3-triene
ˇ
[2] Y.-L. Wu, F. Bures, P. D. Jarowski, W. B. Schweizer, C. Boudon, J.-P.
(short heavy-atom distances of P=O···NACTHNUTGRNEG(UN CH3)2 =2.9–3.0 ꢀ
Gisselbrecht, F. Diederich, Chem. Eur. J. 2010, 16, 9592–9605.
[3] a) R. Andreu, M. J. Blesa, L. Carrasquer, J. Garꢆn, J. Orduna, B. Vil-
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rete, M. Allain, J. Am. Chem. Soc. 2005, 127, 8835–8845; b) R.
Andreu, L. Carrasquer, S. Franco, J. Garꢆn, J. Orduna, N. M. de Bar-
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6647–6657; c) R. Andreu, E. Galꢇn, J. Orduna, B. Villacampa, R.
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(C(CN)2) of a neighboring zwitterion 7 is positioned at the
lower rim of 8, possibly due to the electrostatic attraction
toward the nearby, bound cationic imminium center
(CN···NMe2 distance: 6.9 ꢀ) and C-H···anion interactions
with the propyl legs of the cavitand. This arrangement leads
to the formation of linear, AB-type co-polymeric arrays in
the solid state (Figure 4b).
The association constant for a 1:1 host-guest complex 7·8,
303K
ass
K
=(2.13ꢂ0.04)ꢅ104 mꢁ1, was determined by isothermal
titration calorimetry (ITC) in nitrobenzene for reasons of
solubility.[25] Interestingly, the process is both enthalpically
and entropically driven, thus highlighting the key role
played by solvation[22] (DH=ꢁ17.6ꢂ0.5 kJmolꢁ1, TDS=
+7.6ꢂ0.6 kJmolꢁ1, DG=ꢁ25.1ꢂ0.1 kJmolꢁ1).
In summary, we demonstrated the acetylene-like (proace-
tylenic) reactivity of a push–pull butatriene 3 in a formal
[2+2] cycloaddition–cycloreversion reaction to give zwitter-
ion 7, in which charges are separated through cross-conjuga-
tion and stabilized by DMA rings (cation) and cyano groups
(allyl anion). The positive charge on the DMA rings, and in
particular their NMe2 centers, is evidenced by complexation
with a tetraphosphonate cavitand. The startling and unex-
pected optical similarity between squaraines and 6, formed
via a cumulene-like cyclodimerization of 3, indicates the po-
tential application of push–pull [4]radialenes as a new class
of dyes with absorptions close to or in the NIR, but with
lower polarity and higher solubility. The synthesis of higher,
donor–acceptor-substituted [2n+1]cumulenes and the inves-
tigation of their reactivity and optoelectronic properties are
currently being pursued. Additionally, the visible color
changes of 7 upon complexation with 8 suggest the employ-
ment of this host system for the colorimetric detection of
other highly polarized push–pull guest chromophores.
ˇ
[10] F. Bures, W. B. Schweizer, J. C. May, C. Boudon, J.-P. Gisselbrecht,
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[11] Direct heating of solid 3 (thermally stable up to ca. 1908C) at 2308C
resulted in the formation of very minor amounts of 6, accompanied
by mostly insoluble black material of, presumably, polymeric nature,
which remained solid up to 4008C.
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Acknowledgements
This research was supported by the European Research Council (ERC)
Advanced Grant No. 246637 “OPTELOMAC”. We are grateful for
access to the Competence Center for Computational Chemistry (C4)
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Keywords: proacetylenic
radialene · self-assembly · zwitterions
·
push–pull chromophore
·
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FranÅois Diederich et al.
COMMUNICATION
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Received: December 9, 2011
Published online: && &&, 0000
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COMMUNICATION
Push–Pull Chromophore
Yi-Lin Wu, Francesca Tancini,
W. Bernd Schweizer, Daniela Paunescu,
Corinne Boudon,
Jean-Paul Gisselbrecht,
Peter D. Jarowski, Enrico Dalcanale,
FranÅois Diederich*
&&&& — &&&&
Proacetylenic Reactivity of a Push–
Pull Buta-1,2,3-triene: New Chromo-
phores and Supramolecular Systems
Proacetylenic: An aniline-based
cycloaddition–cycloreversion a NIR-
absorbing zwitterionic chromophore
(lmax =825 nm). The cationic charge on
the imminium-type nitrogen in the
zwitterion is evidenced by host–guest
complexation with a tetraphosphonate
cavitand.
donor–acceptor-substituted butatriene
exhibits not only cumulene-like dime-
rization to give a [4]radialene
(lmax =756 nm), but also acetylene-like
(proacetylenic) reactivity towards tet-
racyanoethene, affording via [2+2]
Chem. Asian J. 2012, 00, 0 – 0
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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