Lewis acid enhanced switching of the 1,1-dicyanodihydroazulene/ vinylheptafulvene photo/thermoswitch

Article abstract of DOI:10.1039/c1cc10804b

Mild Lewis acids enhance the rate of the thermal conversion of vinylheptafulvene (VHF) to dihydroazulene (DHA). In the absence of light, stronger Lewis acids promote the otherwise photoinduced DHA to VHF conversion.

Full text of DOI:10.1039/c1cc10804b

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COMMUNICATION
Lewis acid enhanced switching of the 1,1-dicyanodihydroazulene/
vinylheptafulvene photo/thermoswitchwz
Christian Richard Parker, Christian Gregers Tortzen, Søren Lindbæk Broman,
Magnus Schau-Magnussen, Kristine Kilsa and Mogens Brøndsted Nielsen*
˚
Received 11th February 2011, Accepted 1st April 2011
DOI: 10.1039/c1cc10804b
Mild Lewis acids enhance the rate of the thermal conversion of
vinylheptafulvene (VHF) to dihydroazulene (DHA). In the
absence of light, stronger Lewis acids promote the otherwise
photoinduced DHA to VHF conversion.
complexation to the available lone pairs of the cyano groups
in 1 and 2. It is well-known that nitriles can act as ligands for
soft metal ions, and this has previously been employed for
3
assembly of several complexes and three-dimensional arrays.
The usefulness of the nitrile group in supramolecular chemistry
was further emphasised by the ability of nitrile-functionalised
4
calixarenes to act as sensors for soft metal ions. As the
Photoswitches have been used extensively for controlling the
state of supramolecular systems and assemblies by means of
1
light. The most prevalent of these have been the azobenzenes
conversion of VHF to DHA is thought to proceed via a
2c
and dithienylethenes, which can be interconverted between their
two states by light of different wavelengths. Another system that
has been less explored is the dihydroazulene/vinylheptafulvene
zwitterionic structure, in which the positive charge is localised
on the seven-membered ring and the negative charge is stabilised
by the two cyano groups, coordination of LA to the cyano
groups of 2 is particularly attractive. Here we have studied the
influence of zinc and silver ions, in addition to aluminium and
boron halides, and counter-balancing effects of Lewis basic
solvents. We note that LAs were previously used for triggering
2
photo/thermoswitch. DHA 1 undergoes a photochemically
induced ring-opening to VHF 2, which reverts back to the
DHA via a thermally induced ring-closure (Scheme 1). Previous
work has focused on how functional groups directly attached to
the carbon atoms in the five- and seven-membered rings of
5
a photochromic spirooxazine switch and photoisomerisation
6
of cinnamic esters.
The first objective was to study the effect of the LA on the
VHF to DHA thermal back-reaction (TBR) in different solvents
2
DHA influence the switching events.
We became interested to elucidate the possibility of further
tuning of the switching event via a reversible Lewis acid (LA)
at 35 1C (Table S1, ESIz). ZnCl was chosen as the LA and it was
2
brought into solution in different solvents as its etherate complex,
ZnCl ꢀ(Et O) in Et O. First, 10 molar eq. of ZnCl ꢀ(Et O) was
2
2
2
2
2
2
2
added to the solution of DHA 1, which was then irradiated at its
absorption maximum (ca. 357 nm depending on the solvent),
resulting in quantitative conversion to VHF 2. The TBR could be
approximated to an exponential decay, thus the rate of the TBR
could be defined by an apparent half-life (t1/2) of compound 2. In
chlorinated solvents, CH
two-to-three fold increase in the rate of the TBR was observed
when ZnCl O) was present. In contrast, solvents which
2 2 3 2 2
Cl , CHCl , CH ClCH Cl (DCE), a
2
ꢀ(Et
2
2
contained atoms with accessible lone pairs (MeCN, Me CO,
2
Scheme 1 Conversions between dihydroazulene (DHA 1) and vinyl-
EtOAc, EtOH) showed no increase in the rate of the reaction
heptafulvene (VHF 2). YX
3
= AlCl
3
, BF , BBr
3
3
.
(Table S1, ESIz) as such Lewis bases outcompete the CN groups
in coordinating to the ZnCl (Scheme 2).
2
A second screening was conducted with different LAs,
Brønsted acids, and salts in DCE at 25 1C (Table S2, ESIz).
Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
E-mail: mbn@kiku.dk
w This article is part of a ChemComm ‘Supramolecular Chemistry’
web-based themed issue marking the International Year of Chemistry
The addition of acetic acid, HBF
4
ꢀEt
2
O, or an ionising salt
(
[NBu
ZnCl
ꢀ(Et
added altered t1/2. As expected, higher ZnCl
Et O) increased the conversion rate; however, too little Et
4
]PF
6
) had little effect on t1/2. Next, the influence of
2011.
2
2
O)
2
was investigated to see if the amount of Et
2
O
z Electronic supplementary information (ESI) available: Experimental
procedures for the synthesis of compounds 2 to 5. Copies of NMR and
UV-Vis spectra and X-ray data CCDC 804840 (5). For ESI and
crystallographic data in CIF or other electronic format see DOI:
concentration (less
2
2
2
O
caused ZnCl to precipitate out of solution (Table 1). With 10 eq.
2
10.1039/c1cc10804b
of ZnCl ꢀ(Et O) (199 : 1, DCE : Et O) a t of only 21 min was
2
2
2
2
1/2
6
102 Chem. Commun., 2011, 47, 6102–6104
This journal is c The Royal Society of Chemistry 2011

Scheme 2 Lewis acid induced enhancement of the thermal back
reaction. LA = Lewis acid, S: = Lewis basic solvent.
ꢁ5
Fig. 1 UV-Vis absorption spectra (2.63 ꢂ 10 M) in DCE showing
red-shift in VHF lmax with increase of AgOTfꢀ(Et O)
Table 1 Apparent t1/2 (min) of the TBR (2 - 1, 2.63 ꢂ 10ꢁ M) with
5
2
3
.
ZnCl
2
ꢀ(Et O)
2 2
at 25 1C
b
making a greater fraction of the VHFs convert following the
fast pathway. Although two parallel decays interwoven by the
equilibrium should be in play, the data can be approximated to
Lewis acid
Eq.
t
1/2/min
2
DCE : Et O
—
—
10
10
10
25
0.1
597
598
316
98
1 : 0
49 : 1
27 : 1
147 : 1
597 : 1
240 : 1
59 700 : 1
+
a
a single exponential decay, where t1/2 of the VHF/[VHF–LA]
ZnCl
ZnCl
ZnCl
ZnCl
ZnCl
2
2
2
2
2
ꢀ(Et
ꢀ(Et
ꢀ(Et
ꢀ(Et
ꢀ(Et
2
O)
2
O)
2
O)
2
O)
2
O)
2
2
2
2
2
(9 : 1)
(49 : 1)
a
decreases as the LA concentration increases (Tables 1 and 2).
a
(199 : 1)a
(199 : 1)a
(199 : 1)
21
25
The AlCl
TBR (possibly due to the high affinity of aluminium for the
oxygen of THF). Yet, in the absence of THF ligands, AlCl
was found to react with the DHA (vide infra). BF O did
ꢀEt
3
ꢀTHF complex induced no effect on the rate of the
352
3
a
Ratio of CH ClCH
Solvent ratio in a cuvette.
2
2
Cl (DCE) : Et O in the ZnCl
2
2
ꢀ(Et
2 2
O) solution.
b
3
2
not react with DHA but reacted quickly with the red coloured
VHF 2 (lmax 474 nm) to form a colourless solution (lmax 288 nm,
Fig. 2). We anticipate formation of a tropylium species 4
by protonation of the suspected intermediate 3 (Scheme 1)
on the g carbon, relative to the two cyano groups. It was
noticed that upon addition of ethanol, compound 4 reverted
back to the red-coloured VHF, which then underwent the usual
thermal conversion to the original DHA. NMR studies seemed
to confirm formation of 4 with the characteristic downfield
obtained. An increased content of the Lewis base Et
2
O promoted
the right hand pathway shown in Scheme 2.
Silver ions were then examined. The addition of dry AgOTf to
a solution of 1 induced a chemical reaction upon irradiation of a
solution at 357 nm (1 was not regenerated) and gave a precipitate
(no chemical reaction was observed with 1 or 2 in the absence
of light). The hydrated AgOTf gave cloudy solutions in DCE,
7
while the etherate, presumably [Ag(Et O) ]OTf, was found not
2
3
signals of the tropylium CH at d
H
C
9.09–8.92 and d 156.80,
to cause a reaction or precipitation upon irradiation of 1 and 2;
+
also it seemed stable to air. However, the [Ag(Et O) ] gave
1
56.10, 154.98 (CD CN). This is the suspected uncharacterised
3
2
3
cationic intermediate that is to be formed in the synthesis
2e
of DHA, by hydride abstraction from 2-(cyclohepta-2,4,6-
an increase to the rate of TBR (Table 2), reducing t1/2 from 597
to 71 min with 10 eq. and to as low as 9 min with 250 eq.
trienyl-1-phenylethylidene) malononitrile. The NMR experiment
showed that the reaction was faster in CD CN than in CD Cl .
Compound 4 was stable in an NMR tube (CD CN, CD Cl ) for
Interestingly, the addition of 10 eq. [Ag(Et
(Et O) in DCE : Et O 49 : 1 only showed a minor increase in
O) ]OTf or ZnCl
2 3
2
ꢀ
3
2
2
2
2
2
3
2
2
the speed of the photoinduced ring-opening of 1 (Table S3,
ESIz).
at least two weeks and was deprotonated to VHF 2 by Brønsted
basic solvents (H O, EtOH, NEt ).
Unexpectedly, treatment of a solution of DHA in CH
with strong LAs, AlCl or BBr , resulted in direct ring-opening
in the absence of light within minutes. The AlCl gave a
+
The absorption is redshifted for the [VHF–Ag] complex
2
3
2
Cl
2
compared to VHF (lmax shifts from 475 to 519 nm, Fig. 1) while
a similar shift was not observed for the DHA absorption.
Investigating the spectral shift as a function of increasing
3
3
3
+
3
ꢁ1
[
Ag ] yielded an association constant in the order of 10 M
(
Fig. S4, ESIz).
Analysis of the rate of the TBR showed that the ZnCl
Et O) or [Ag(Et O) ]OTf acted in a catalytic fashion. The
2
ꢀ
(
2
2
2
3
equilibrium (Pathway I in Scheme 2) is formed much faster
than either of the conversion rates. Thus, as the fast conversion
+
of [VHF–LA] to DHA proceeds, LA is released resulting in
+
more [VHF–LA] formed as the equilibrium adjusts, thereby
ꢁ5
Table 2 Apparent t2
1
VHF lmax (nm) with eq. AgOTfꢀ(Et
(min) of the TBR (2 - 1, 2.63 ꢂ 10 M) and
O) at 25 1C
2
3
Eq. Ag(I)
Half-life
0
0.1
0.5
1
10
71
25
33
100 150 250
13 10
ꢁ5
Fig. 2 UV-Vis absorption spectra (3.28 ꢂ 10 M) in DCE for
various LAs added. Regeneration of VHF (red curve) occurs by
addition of EtOH (also causing dilution of the sample).
597 556 406 317
9
VHF lmax 474 475 475 475 478 483 508 514 519
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6102–6104 6103

The compound was sensitive to acidic conditions and silica gel
needed to be neutralised prior to chromatography. Irradiation
resulted in conversion of the ‘‘DHA’’ (5) to a ‘‘VHF’’ form
(lmax shifts from 353 to 447 nm in cyclohexane) but the initial
compound was not regenerated thermally. However, the ‘‘VHF’’
form decayed at a rate about 30 times faster than VHF 2 in
cyclohexane.
Fig. 3 Conversion of red 2 to yellow 1 coloured crystals (77–84 1C,
starting temperature 65 1C; ramp rate 1 1C min ).
ꢁ1
In conclusion, the rate of the thermal back reaction of VHF
quantitative conversion of 1 to 2, after treatment of the pink-
coloured intermediate formed, presumably 3 (Scheme 1,
Fig. 2), with water or ethanol; it is stoichiometric and not
to DHA was controlled by LAs such as ZnCl
[Ag(Et O) ]OTf. The effect was counter-balanced by a Lewis
2
ꢀ(Et
2 2
O) and
2
3
basic solvent. The reversibility of the LA complexation to the
cyano groups is particularly attractive for the future explora-
tion of the DHA/VHF system in advanced multistate switches
and supramolecular sensors with colour changes as read-out.
Stronger LAs such as BBr and AlCl were found to chemically
3
catalytic in its reactivity. It was higher yielding than BBr . In a
control UV-Vis experiment, VHF gave similarly coloured
reaction products (lmax 524 and 533 nm for AlCl and BBr ,
3
3
respectively, Fig. 2) which supports the ring-opening theory.
The pink colour of the solutions faded with time. The reaction
3
3
induce ring-opening of the DHA molecule in the absence of
light, and subjecting DHA to PhCOCl and AlCl provided a
between DHA and AlCl
which indicated the presence of a tropylium ring in support of
structure 3. We found that treatment of DHA with AlCl
3 3
or BBr was also followed by NMR,
3
carbimidoyl chloride derivative. This functionality may be a
scaffold for further functionalization and tuning of the system.
It is also attractive to subject the cyano group(s) to even stronger
coordinating metal-centres for a further enhancement of the TBR
and rendering the system more resistant to Lewis basic solvents.
This would potentially lead to more sensitive thermoswitches.
The Villum Kann Rasmussen Foundation is gratefully
acknowledged for their support. In addition, we acknowledge
The Danish Research Council for Independent Research |
Natural Sciences (grants #09-066663 and #272-08-0540).
3
followed by water is a convenient way of converting DHA to
VHF on a preparative scale in the absence of light. Although
VHF derivatives have been isolated previously, it required
prolonged and powerful irradiation of a saturated DHA
2f
solution in hexane (in which the TBR is slow). The VHF
crystals lose their dark red colour as they degraded back to
1
DHA at rt (40% in about one month; determined by
H
NMR). Heating the crystals caused a change in their colour
from dark red (2) to yellow (1) (Fig. 3).
The observation that the g carbon of 3 gets protonated
suggests that other electrophiles could be used to attack
Notes and references
1
(a) M. Irie, Chem. Rev., 2000, 100, 1685; (b) B. L. Feringa, Acc.
Chem. Res., 2001, 34, 504; (c) F. M. Raymo and M. Tomasulo,
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3
this position. Yet, treatment with PhCOCl/AlCl gave an
unexpected compound resulting from the addition of PhCOCl
to one of the CN groups forming a carbimidoyl chloride (5)
(
Scheme 3). This reaction did not occur in the absence of AlCl
rt, 1 week) or when PhCOCl was used in excess relative to
3
(
AlCl . The X-ray crystal structure of this compound (Fig. 4)
2
(a) J. Daub, T. Kno
Ed. Engl., 1984, 23, 960; (b) H. Go
J. Daub, J. Phys. Chem., 1993, 97, 4110; (c) S. L. Broman, M.
A. Petersen, C. G. Tortzen, A. Kadziola, K. Kilsa
˚
M. B. Nielsen, J. Am. Chem. Soc., 2010, 132, 9165; (d) M.
¨
chel and A. Mannschreck, Angew. Chem., Int.
3
¨
rner, C. Fischer, S. Gierisch and
concurs with the other spectroscopic and microanalytical data.
While this functional group is known in a few compounds in
8
the literature, to the best of our knowledge this appears to
˚
and
˚
A. Petersen, S. L. Broman, K. Kilsa, A. Kadziola and
˚
M. B. Nielsen, Eur. J. Org. Chem., 2011, 1033; (e) S. L. Broman,
be the first crystal structure of a carbimidoyl chloride group.
˚
S. L. Brand, C. R. Parker, M. A. Petersen, A. Kadziola and
M. B. Nielsen, ARKIVOC, in press; (f) J. Daub, S. Gierisch,
U. Klement, T. Kno
986, 119, 2631.
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1
3
1
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Scheme 3 Conversion of one CN to a carbimidoyl chloride.
7
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8
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Fig. 4 Ortep diagram of carbimidoyl chloride compound 5. Selected
bond lengths are shown in Table S7 (ESIz).
6
104 Chem. Commun., 2011, 47, 6102–6104
This journal is c The Royal Society of Chemistry 2011