Self-assembly, concomitant photochemical processes, and improvement of the yield of [2 + 2] photoreactions from supramolecular arrays via mechanochemical assistance

Article abstract of DOI:10.1039/c3ce26752k

Novel examples of self-assembly, structural transformations and concomitant photochemical processes from supramolecular ternary assemblies directed by charge-assisted hydrogen bonds are shown. In addition, mechanochemical assistance is also shown as a potential tool in order to improve the yield of [2 + 2] photoreactions in the solid state.

Full text of DOI:10.1039/c3ce26752k

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Self-assembly, concomitant photochemical processes,
and improvement of the yield of [2 + 2]
photoreactions from supramolecular arrays via
mechanochemical assistance3
Cite this: CrystEngComm, 2013, 15, 2795
Received 24th October 2012,
Accepted 5th February 2013
DOI: 10.1039/c3ce26752k
Alexander Bricen˜o,*a Dayana Leal,ab Gabriela Ortega,a Graciela D´ıaz de Delgado,b
Edgar Ocandoa and Liz Cubillana
www.rsc.org/crystengcomm
Novel examples of self-assembly, structural transformations and
concomitant photochemical processes from supramolecular tern-
ary assemblies directed by charge-assisted hydrogen bonds are
shown. In addition, mechanochemical assistance is also shown as
ionic ternary assemblies based on carboxylic acids and stilbazole
or stilbene derivatives: (2Cl-HStb+)(Hmal2)(H2Fu)0.5 (1) and
(2,29-H2bpe2+)(Fu22)(H2Fu) (2) (where HStb+: stilbazolium;
H2bpe2+: stilbenium; Hmal2: hydrogen maleate; Fu22: fumarate
and H2Fu: fumaric acid). Both compounds are photoreactive upon
UV-irradiation. In addition, the reinvestigation of two other
photoreactive assemblies, (2,29-bpe)(H2Fu)5a (3) and trans-3-(3-
pyridyl)acrylic acid6 (4), was also undertaken. As UV-irradiation of
1 quantitatively yielded the expected photodimer, the photoreac-
tion of 2, 3 and 4 were improved until obtaining nearly
quantitative yields of all the products via sequential grinding-
irradiation steps.{
Crystals of 1 and 2 were obtained by mixing trans-2-chloro-
stilbazole (2-Cl-Stb) with maleic acid (H2Mal) and 1,2-bis(2-
pyridyl)ethylene (2,29-bpe) with fumaric acid (H2Fu) in molar ratio
1 : 1 and 1 : 2 in methanol, respectively. These compounds can
also be obtained as crystalline single phases by direct grinding of
the starting compounds in short periods of time (30–60 min) via
liquid-assisted grinding7 in the presence of a few drops of
a
potential tool in order to improve the yield of [2
+ 2]
photoreactions in the solid state.
Chemical synthesis performed in the solid state is gaining interest
as an important green alternative for the preparation of new and
conventional compounds.1,2 A particular case of solid state
reactions is represented by topochemical [2 + 2] cycloaddition of
olefins. In recent years, distinctive supramolecular approaches
have been exploited in order to direct stereo- and regiocontrolled
photoproducts.2,3 On the other hand, mechanochemical activation
methods have received significant attention in the last years due to
attractive advantages such as: shorter reaction times, reduction of
costs, energy and waste combined with high yields in comparison
with traditional synthesis in liquid-phase.1 This alternative
provides the possibility of affording a greater level of complexity
and sophistication in the study of solid state transformations,
which could involve sequential synthesis steps from molecular
activation in order to obtain complex molecular compounds to the
preparation of functional supramolecular arrangements self-
assembled via mechanochemistry. Recently, such approach has
been exploited to carry-out stereocontrolled synthesis of cyclobu-
tane derivatives from topochemical [2 + 2] cycloaddition reac-
tions.3,4 In this context, as part of ongoing efforts on the
preparation and design of photoreactive solids5 we have reported
unprecedented examples of concomitant [2 + 2] cycloaddition
solid state reactions from co-crystals self-assembled via mechan-
ochemistry (Scheme 1).5a Herein, we have prepared two novel
methanol (see ESI, Fig. S1).
3
The combination of H2Mal with 2-Cl-Stb in solution produces
an unexpected ternary crystal: (Hmal2)(2Cl-HStb+)(H2Fu)0.5 (1).
Formation of H2Fu is due to the partial cis–trans isomerisation of
H2Mal to H2Fu at RT (Scheme 2). Such isomerisation has been
observed previously by Rao and co-workers in the presence of
other nitrogen-containing heterocycles and polar solvents.8 This
process was monitored by 1H-NMR in order to evaluate the degree
of cis–trans conversion in solution as a function of time in the
presence of DMSO and CH3OH at RT. This analysis revealed an
a
´
Instituto Venezolano de Investigaciones Cientıficas (IVIC), Apartado 21827, Caracas
1020-A, Venezuela. E-mail: abriceno@ivic.gob.ve; Fax: +58-212-5041350;
Tel: +58-212-5041320
b
´
Universidad de Los Andes (ULA), Apartado 40, La Hechicera, Merida 5251,
Venezuela
Electronic supplementary information (ESI) available: Experimental details. 1H-
3
NMR spectra and PXRD patterns. CCDC 900648 and 900649. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce26752k
Scheme 1
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Scheme 2
Fig. 2 (a) View of the 2D H-bonded network in the structure of 2, showing
hydrogen bond interactions. (b) Interactions between double bonds of
2,29-H2bpe2+ (c) Fu22 anions and carboxylic acid molecules.
important effect of the solvent used on the rate of isomerisation.
The reaction is slower in DMSO (93 days) than in CH3OH (30 days)
molecule, suitable for [2 + 2] cycloaddition in the solid state9
(Fig. 1(b, c)). In particular, this last one is organized in a crisscross
manner. Thus, such an arrangement may lead to a novel example
of a concomitant solid state reactivity pattern upon UV-irradia-
tion.5a
to achieve quantitative conversion to trans-H Fu (Fig. S2, ESI ).
3
2
Given this dynamic behaviour of maleic acid in solution in the
presence of 2-Cl-Stb, the solid formed was never obtained as a
crystalline single phase with a reasonable chemical purity. The
self-assembly of 1 was only successfully achieved via direct
grinding from the three components in the appropriate molar
ratio (Scheme 2).
Compound 2 consists of a novel self-assembly between 2,29-bpe
and H2Fu. The asymmetric unit of 2 contains one half stilbenium
dication (2,29-H2bpe)2+, one half fumarate anion (Fu22) and one
half H2Fu. The crystal structure of 2 consists of a novel self-
assembly between cations and anions via charge-assisted carbox-
The asymmetric unit of 1 contains one hydrogen maleate anion
(Hmal2), one stilbazolium cation (2Cl-HStb+) and half molecule of
H2Fu, which occupies a special position. The crystal structure of 1
consists of a self-assembly of anions and cations linked by charge-
assisted carboxylate–pyridinium supramolecular synthons [N1–
…
ylate–pyridinium synthons [N1–H1N O3: 2.607(3) Å]. This supra-
molecular motif is extended along the b-axis to generate H-bonded
…
H1N O4: 2.681(6) Å] (Fig. 1(a)). This ionic moiety forms dimeric
zig-zag ribbons (Fig. 2(a)). These ribbons are linked through H2Fu
…
units via self-complementary hydrogen bonding interactions
between HMal2 anions [C15–H15 O2: 3.483(7) Å]. Adjacent
…
via self-complementary lateral C–H O bonding interactions
between carboxylic and C–H bond from adjacent organic cations.
Additionally, the proton of the acidic groups in H2Fu molecule
adopts an antiplanar configuration forming H-bonded chains with
supramolecular units are connected through fumaric acid
molecules via H-bonding interactions between Hmal2 anions
Fu22 anions via O–H O charge-assisted hydrogen bonds [O1–
…
…
and H2Fu molecules [O5–H5OH O4: 2.601(5) Å], leading to a 2D
…
network. The 3D structure is achieved by p–p interactions directed
by organic cations. The relative orientation of the molecules allows
the existence of close contacts between the double bonds either in
pairs of 2Cl-HStb+ cations or between an Hmal2 anion and a H2Fu
H1OH O4: 2.594(3) Å], which are intercalated between 2,29-bpe
molecules. These interactions lead to 2D H-bonded sheets parallel
to the bc-plane. The 3D structure is achieved by p–p interactions
between the sheets related by simple translation. The relative
orientation of the molecules allows the presence of close contacts
between the double bonds of the same kind of molecules suitable
for [2 + 2] cycloaddition. This structure displays structural features
similar to the (2,29-bpe)(H2Fu) co-crystal with stoichiometry ratio
(1 : 1) previously reported by us,5a which produces unprecedented
concomitant topochemical [2 + 2] cycloaddition reactions in a
single crystal. Thus, such arrangement also anticipates the
possibility of a similar behaviour in the solid state upon
irradiation.
Encouraged by the formation of different stoichiometric
variations of H2Fu and 2,29-bpe via mechanochemistry,5a we
evaluated the possibility of interconversion between both phases.10
Thus, we studied the transformation of 2 to 3 as a function of
grinding time. Analysis of the PXRD patterns of the equimolar
mixture of 2 with 2,29-bpe after 15 min revealed the incipient
formation of 3 and the complete disappearance of 2 (molar ratio
1 : 1). Likewise, compound 3 can be interconverted to 2, by
addition of stoichiometric amounts of H2Fu via mechanochem-
Fig. 1 (a) View of the 2D H-bonded network in the structure of 1, showing
hydrogen bond interactions. (b) Interactions between double bonds of
stilbazolium cations and carboxylic acid molecules (c).
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Fig. 4 Comparative view of H-bonded layers found for the structures of 2 and 3.
rctt-1,2,3,4-cyclobutanetretracarboxylic acid (H4Cbtc) with ca. 78%
and 27% of yield (see Scheme 3), respectively (Fig. S4(a), ESI ). The
3
reactivity observed for 2 is similar to that found previously in the
(2,29-bpe)(H2Fu) co-crystal (3) with (1 : 1) stoichiometry, which
produces a mixture of the photodimers 2,29-tpcb and H4Cbtc.5a
Apparently, the existence of a greater number of H-bonding
interactions in the structure of 2 reduces the mobility of the acid
molecules enabling a higher yield of the product.
Toda and co-workers in 1987 were, perhaps, the first to report a
catalytic effect of mixing upon the stereospecificity of
a
photoreaction. In this case, a diol was used as host compound
in the photoreaction of a chalcone derivative.11a Different host–
guest ratios were used and the photoreactions were carried out in
cycles of irradiation and shaking. Thus, the combination of solid-
state host–guest assembly, irradiation, and mixing in the solid
state, drove the stereoselective photoreaction of the chalcone.
More recently, MacGillivray and co-workers have re-explored a
similar approach called Mechano-assisted supramolecular cataly-
sis11b in order to drive solid state [2 + 2] photoreactions directed by
a linear template. Although these results provide interesting
applications for the study of solid state photoreactions, its scope is
limited due to the difficulty in the prediction of solid state crystal
packing, together with extremely limited molecular mobility in the
solid state to promote the self-assembly or disassembly of the pair
substrate/catalyst (or template). Nevertheless, we have recognized
that such approach could be exploited in order to improve the
yield of any photoreaction until near quantitative yield, simply by
the combination of multisteps number of grinding-irradiation
cycles of the resulting molecular solid state solution (photopro-
duct, remaining photoactive target and possible template). Taking
advantage of this approach, we re-evaluated the photoreactivity of
2 and of two other known organic assemblies reported by us:
(2,29-bpe)(H2Fu) (3) and trans-3-(3-pyridyl)acrylic acid (4). A second
grinding cycle was applied to compound 2 irradiated at 302 nm for
Fig. 3 XRPD patterns showing the conversion from compound 2 to 3 (a) and
viceversa 3 to 2 (b), as a function of grinding time via mechanochemistry.
istry after 15 min (Fig. 3). Complete transformations of both
phases are achieved after 30 min of liquid-assisted grinding in
methanol, along with an increase in crystallinity, which is reflected
by the decrease of background radiation of the patterns and the
augmentation of intensity. It is noteworthy that these transforma-
tions involve an interesting proton transfer process between the
components, transforming an ionic assembly 2 into a neutral co-
crystal 3 and viceversa. Structural differences between both
H-bonded supramolecular arrays are shown in Fig. 4.
1
The H-NMR characterisation of the solids obtained after UV
irradiation of 1 and 2 for 3–4 days confirms the cycloaddition
reaction in both cases. The spectrum of the crude solid obtained
from 1 at 350 nm reveals the quantitative formation of rctt-1,3-
bis(4-pyridyl)-2,4-bis(2-chlorophenyl)cyclobutane head to tail iso-
mer (2-Cl-dpcb; yield: 100%, see Scheme 3) together with the
partial isomerisation of H Mal to H Fu (55 : 45) (Fig. S3, ESI ). The
3
2
2
yield of the photodimer remains consistent when 1 is irradiated at
a shorter wavelength (302 nm). Furthermore, cis–trans isomerisa-
tion was not observed (Fig. S3, ESI ).
3
As expected, the 1H-NMR spectrum of the irradiated solid 2 at
302 nm showed the formation of two concomitant photoproducts
characteristic of rctt-tetrakis(2-pyridyl)cyclobutane (2,29-tpcb) and
Scheme 3 Molecular diagram of photoproducts obtained.
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and irradiation steps provides an interesting green route to high
yields of photoproducts. This approach opens a window of
opportunities to develop efficient routes for the preparation and/or
improvement of the yield of new and conventional cyclobutane-
like stereoisomers from stoichiometric solids.5d Such results may
be improved using the method described recently by MacGillivray
and co-workers based on the use of a vortex mixer that permits
automated grinding and simultaneous UV- irradiation of the solid
sample.12 This alternative can be very helpful in order to overcome
the limitations imposed by the topochemical postulates in solid
state photoreactions. In these reactions, the yield of the product is
seriously affected by important changes on distances and relative
orientations between the pairs of photoactive species due to large
molecular movements or structural rearrangements during the
course of the reaction. Further studies on improving [2 + 2]
photoreactions from supramolecular arrays via mechanochemical
assistance are in progress.
Fig. 5 View of the interactions between double bonds found in the crystal
structures of 3 and 4.
30 min and after exposed to UV-irradiation during 2 days again.
The new 1H-NMR spectrum of the solid obtained after the second
irradiation showed an increase in yields for both photoproducts:
96% for 2,29-tpcb whereas the yield of H4Cbtc only rose to 35%
(Fig. S4(a), ESI ). Thus, a third grinding-irradiation cycle was tried.
3
In this attempt, ca. 40% fresh 2,29-bpe was added to the solid and
co-ground for 30 min. A subsequent third exposure to UV-
irradiation for 2 days also was carried-out. The spectrum of the
solid obtained showed the presence of the same signals together
Acknowledgements
with an additional complex set of signals (Fig. S4(b), ESI ), which
3
suggests the formation of an additional unidentified photopro-
duct. Apparently, the presence of both cyclobutanes compromises
the topochemical control of the initial reaction. Only a small
increase on the yield of H4Cbtc was observed (45%).
We thank FONACIT (grant LAB-97000821) for financial
support and M. A. Ramos and R. Atencio (INZIT) for technical
assistance.
In order to illustrate the origin of the photoreactivity of 3 and 4,
short contacts between the double bonds of pairs of molecules are
shown in Fig. 5.
References
¯
{ Crystal data for 1: C19H16ClNO6, Mt = 389.78, triclinic, space group P1, T =
293(2) K, a = 8.687(2), b = 8.900(2), c = 13.039(3) Å, a = 104.06(3), b= 97.67(3),
c = 105.15(3)u, U = 922.8(3) Å3, Z = 2, m(Mo Ka) = 0.243 mm21, rcalcd = 1.403
g cm23. Rint = 0.058, R1(F2) = 0.096, wR(F2) = 0.21, S = 1.12 for 2296
independent reflections (I > 2s(I)). CCDC 900648.
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second grinding-irradiation cycle was then carried-out. The
¯
Crystal data for 2: C20H18N2O8, Mt = 414. 36, monoclinic, space group P1,
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