Solid state double [2 + 2] photochemical reactions in the co-crystal forms of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one: Establishing mechanism using single crystal X-ray, UV and 1H NMR

Article abstract of DOI:10.1039/c0ce00612b

Double [2 + 2] photochemical reaction of 1,5-bis(4-pyridyl)-1,4-pentadiene- 3-one (1P) was observed with 100% yield in four co-crystal forms of 1P with the hydrogen bonding template molecule phloroglucinol (PG) or 5-methoxy resorcinol (MR). The crystal st

Full text of DOI:10.1039/c0ce00612b

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PAPER
Solid state double [2 + 2] photochemical reactions in the co-crystal forms of
1,5-bis(4-pyridyl)-1,4-pentadiene-3-one: establishing mechanism using single
1
crystal X-ray, UV and H NMR†
*
Ramkinkar Santra and Kumar Biradha
Received 7th September 2010, Accepted 26th January 2011
DOI: 10.1039/c0ce00612b
Double [2 + 2] photochemical reaction of 1,5-bis(4-pyridyl)-1,4-pentadiene-3-one (1P) was observed
with 100% yield in four co-crystal forms of 1P with the hydrogen bonding template molecule
phloroglucinol (PG) or 5-methoxy resorcinol (MR). The crystal structure of compound 1P does not
have the required geometry to produce the double [2 + 2] reaction product, but it gives a single [2 + 2]
reaction product with 15% yield upon time restricted irradiation. All the reactions in co-crystals of 1P
resulted in the stereospecific exo–exo tetrapyridyl tricyclo[6.2.0.03,6]-decane. 1H NMR and the UV-vis
spectroscopic studies clearly established a stepwise mechanism for this reaction through the formation
of monocyclobutane intermediate. One co-crystal form of 1P with PG and another co-crystal form of
1P with resorcinol were found to be unreactive due to different reasons. Furthermore the
mechanochemical grinding products of 1P with RN or MR have exhibited similar structural and
reactivity correlations with those crystallized from solution.
the double bonds to reactive geometries. Some of the interactions
Introduction
which are in use for this purpose are strong hydrogen bonds,9
coordination bonds,10 halogen bonds11 and metal/metal inter-
actions.12 In particular the approach of MacGillivray et al. for
bringing two bis-pyridyl ethylene molecules within the reactive
distance by clipping them with various resorcinol derivatives via
O–H/N hydrogen bonds has gained a special attention.9 Also
the resultant tetra-pyridyl products from such reactions are of
importance for constructing various functional MOFs.13 There-
fore [2 + 2] reactions with the simplest bis-pyridyl olefin, i.e. 1,2-
bis(4-pyridyl)ethylene, have been reported by several groups
using different crystal engineering strategies.
The chemical reactions in the solid state offer stereo- and regio-
chemical control as the environment around the reactive func-
tional groups is well defined due to the restricted molecular
movements.1,2 Various photochemical reactions such as [2 + 2]
dimerization or polymerization, Norish/Yang-type I or II3 and
di-p-methane rearrangements4 are some of the popular reactions
in solid state chemistry.5 Among these, [2 + 2] reactions are well
studied by Schmidt and he postulated a set of rules by system-
atically exploring several substituted olefins.1 However, many
examples of olefins have been reported which violate Schmidt’s
topochemical principles of [2 + 2] reactions.6 More often such
violations were attributed to the varied degree of molecular
motions within the crystal lattice. The newly gained knowledge
on various intermolecular interactions and the identification of
a variety of supramolecular synthons7 had given an impetus to
re-examine the photochemical reactions of olefins which have
unreactive geometries.8 Therefore, the syntheses of co-crystals or
metal complexes of olefins were seen as a means for re-orienting
Various substituted dienes are also of importance for [2 + 2]
reactions due to the possibility of formation of tricyclic deriva-
tives in stereo-selective manner which are otherwise difficult to
synthesize. Symmetrically substituted pentadiene-3-one is one
such example which contains two olefins separated by a carbonyl
group. In this paper we aim to study the photochemical [2 + 2]
reactions of 1 with R ¼ 4-pyridyl (1P). Molecule 1 can exhibit
three stable conformations (Scheme 1), therefore in principle it is
possible to obtain different stereo- and regio-selective dimeric
products by arresting various conformations in the co-crystals of
1P with di-hydroxy and tri-hydroxy benzenes. We note here that
previously the photochemical reactions of 1,4-pentadiene-3-one
moiety with R ¼ –COOH, 3,4-dichlorophenyl or –COOMe have
been studied and found to produce the exo–exo tri-
cyclo[6.2.0.03,6]-decane (exo–exo TCD) ring system upon
photochemical irradiation, while 1 with R ¼ phenyl, that is
dibenzylidene acetone, was shown to be photochemically
Department of Chemistry, Indian Institute of Technology, Kharagpur,
721302, India. E-mail: kbiradha@chem.iitkgp.ernet.in; Fax: +91-3222-
282252; Tel: +91-3222-283346
† Electronic supplementary information (ESI) available: Other
geometrical parameters for all the reactive double bonds in the
co-crystals, IR spectra, 1H NMR spectra, 13C NMR spectra, XRPD
patterns, and crystallographic data for compounds 4–8 in CIF. CCDC
reference numbers 688735 and 793077–793080. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0ce00612b
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Scheme 1 Three possible conformations of 1 and respective (TCD) products upon double [2 + 2] photochemical reaction.
unreactive.14 Further while studying the photochemical reaction
of 1 with R ¼ 3,4-dichlorophenyl, Green and Schmidt have
proposed two possible pathways for this reaction and tried to
establish the mechanism by irradiating the compound at different
wavelengths (Scheme 2). Their efforts to isolate or to identify the
formation of single photochemical reaction product (mono-
cyclobutane, MCB) were unsuccessful; therefore they have
concluded that the reaction may be proceeding through the other
root in which both the double bonds react simultaneously.
In contrast to the above observations, our study on the
photochemical reactivity of 1P in the co-crystal of 1P with
phloroglucinol indicates that the reaction proceeds through
stepwise mechanism.15 This aspect was proven by us using single
procedure.16 The crystals of 1P were obtained from toluene and
found that the double bonds are not aligned in the required
fashion for double [2 + 2] photochemical reaction.15 Therefore
the compound 1P has been co-crystallized with resorcinol (RN),
phloroglucinol (PG) and 5-methoxy resorcinol (MR) from
various solvents and mixtures of solvents. Only one crystalline
form (1P$RN, 2) was obtained for the co-crystals of 1P with
resorcinol in spite of several crystallization trials from different
solvents. In contrast, various crystallization reactions with PG
and 1P resulted in
a total of four crystalline forms:
[1P$PG$CH3CN], 3; [4(1P)$3PG$4H2O], 4; [2(1P)$PG$4H2O], 5
and [1P$PG], 6. Similar to RN, MR also forms only one crys-
talline form [2(1P)$2MR$H2O], 7. The crystal structures of 1P
and co-crystals 2–7 were determined and analyzed in terms of
their double bond geometries, hydrogen bonding interactions
and photochemical reactivity. Pertinent crystallographic details,
hydrogen bonding interactions, and geometrical parameters for
the alignment of double bonds with the photochemical reaction
details are given in Tables 1–3 respectively.
1
crystal X-ray diffraction and H NMR spectra. In this manu-
script we aim to see the generality of such step-wise mechanism in
the various related co-crystals of 1P and also aim to explore the
possibility of obtaining different stereo- and regio-isomeric
dimers. Accordingly, we describe here our studies on top-
ochemical reactions in four more co-crystals of 1P. Further the
double [2 + 2] reactions in all these co-crystals have been moni-
tored by 1H NMR and UV-vis spectroscopy to establish the
mechanism. The possibility to synthesize these co-crystals of 1P
via mechanochemical grinding and thereby the photochemical
reactivity of such co-grounded solids will also be addressed in
this article.
Topochemical reactivity of 1P
Among the three possible isomers of 1P, the transoid–transoid
isomer deviates from linearity and also it is least stable due to
steric interactions between the b-hydrogens. The CSD search on
the aromatic substituted compounds with the 1,4-pentadienone
moiety shows that ten out of fourteen structures exhibit cisoid–
cisoid conformation, three exhibit cisoid–transoid and only one
exhibits transoid–transoid conformation. These statistics
Results and discussion
Compound 1P was synthesized from acetone-1,3-dicarboxylic
acid and 4-pyridinecarboxaldehyde following the literature
Scheme 2 Schematic representation for the possible intermediates of double [2 + 2] reaction of 1.
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Table 1 Crystallographic parameters for the crystal structures of 4–8a
Compound
4
5
6
7
8
Formula
MW
T/K
C78H74N8O17
1395.45
293(2)
C36H38N4O9
670.70
293(2)
C21H18N2O4
362.37
293(2)
C22H21N2O4.25
381.41
293(2)
C30H24N4O2
472.53
293(2)
System
Space group
Monoclinic
P2(1)/c
19.3013(16)
15.3054(13)
23.9379(19)
90.00
122.935(4)
90.00
6971.4(10)
4
1.330
Monoclinic
P2(1)/c
12.873(2)
15.947(3)
16.881(3)
90.00
90.206(5)
90.00
3465.4(10)
4
1.286
Triclinic
ꢀ
P1
Monoclinic
C2/c
12.091(2)
14.993(3)
22.115(4)
90.00
97.250(5)
90.00
3977.2(11)
8
1.287
Monoclinic
P2(1)/n
15.178(5)
6.090(2)
26.470(8)
90.00)
102.280(10)
90.00
2390.9(13)
4
1.313
ꢁ
a/A
8.6268(9)
9.6019(10)
12.1252(13)
76.149(3)
88.030(3)
67.701(3)
900.46(16)
2
ꢁ
b/A
ꢁ
c/A
a/ꢀ
b/ꢀ
g/ꢀ
3
ꢁ
Vol./A
Z
Dcalc/Mg mꢁ3
R1 (I > 2s(I))
wR2 (on F2, all data)
1.337
0.0408
0.1416
0.0592
0.1901
0.0637
0.1919
0.0808
0.2245
0.0761
0.2028
a
Crystallographic parameters for crystal structures of 1P, 2, 3 and 9–11 are published in our previous communication.
indicate that even in the crystal structures the probability of
exhibiting least stable isomer is very less while the probability for
the most stable isomer is significantly high.
Notably none of these compounds were reported to exhibit
conformational polymorphs. Recently a somewhat structurally
related diene molecule, 1,3-bisstyrylbenzene dicarboxylic acid,
Table 2 Some significant hydrogen bond interactions in the structures of 1P and 2–11a
D–H/A/ꢀ
ꢁ
ꢁ
Comp.
Interaction
H/A/A
D/A/A
1P
2
C(12)–H(12)/O(32)a
C(26)–H(26)/N(21)b
O(2A)–H(2A)O/N(11)c
O(1A)–H(1A)O/N(21)
O(1)–H(1)O/N(21)d
O(3)–H(3)O/N(31)e
O(5)–H(5)O/N(100)
O(1E)–H(1)/N(11B)f
O(3E)–H(2)/N(11A)g
O(5E)–H(3)/N(21D)g
O(1F)–H(4)/N(11C)h
O(3F)–H(5)/O(3)W
O(5F)–H(6)/N(21B)h
O(1G)–H(7)/N(21A)i
O(3G)–H(8)/O(1)W
O(5G)–H(9)/N(11D)i
O(1)–H(1)O/O(1)W
O(2)–H(2)O/N(31A)j
O(3)–H(3)O/N(21B)k
O(1)–H(1)O/O(10)
2.50
2.61
3.2405(17)
3.4703(17)
2.753(2)
2.759(2)
2.761(3)
2.803(3)
2.828(3)
2.648(3)
2.672(3)
2.631(3)
2.715(3)
2.641(3)
2.735(3)
2.816(3)
2.680(3)
2.751(3)
2.800(3)
2.833(3)
2.705(3)
2.888(10)
2.684(11)
2.7186(19)
2.7729(19)
2.815(4)
2.735(4)
3.248(5)
3.354(5)
3.455(6)
2.755(4)
2.765(4)
2.810(5)
2.811(7)
2.747(6)
2.772(6)
3.272(4)
3.384(6)
137
154
1.829(19)
1.87(3)
1.86(4)
1.94(3)
2.03(3)
1.68
1.62
1.57
1.82
1.71
1.87
1.77
1.61
1.88
1.94
2.02
1.85
2.04
1.81
1.76
1.86
1.94
2.03
2.37
2.47
2.58
1.80(4)
1.76(4)
1.94(5)
1.92(6)
1.76(6)
1.74(8)
2.44
174.4(17)
161(3)
177(3)
171(3)
162(3)
154
177
179
178
175
174
172
175
178
170
176
175
160
166
170
175
170
170
157
158
156
170(3)
176(3)
164(4)
153(4)
172(5)
168(7)
142
3
4
5
6
O(1)–H(1)O/O(10A)
O(2)–H(2)O/N(11)l
O(3)–H(3)O/N(21)m
O(31)–H(31)O/N(11)n
O(33)–H(33)O/N(21)o
C(12B)–H(12B)/O(1B)p
C(14A)–H(14A)/O(1A)q
C(25A)–H(25A)/N(11B)r
O(1A)–H(1A)O/N(21)
O(3A)–H(3A)O/N(11)s
O(2A)–H(2A)O/N(100)
O(1A)–H(1A)O/N(100)t
O(3A)–H(3A)O/N(11)u
O(2A)–H(2A)O/N(21)v
C(13B)–H(13B)/N(21A)w
C(22B)–H(22B)/O(100)x
7
8
9
10
11
2.54
152
a
Symmetry code: aꢁ1/2 + x, 1/2 ꢁ y, ꢁ1/2 + z; bꢁ1/2 + x, ꢁ1/2 ꢁ y, ꢁ1/2 + z; c2 + x, 1/2 ꢁ y, ꢁ1/2 + z; dꢁx, 1 ꢁ y, ꢁz; eꢁ1 + x, 1 ꢁ y, ꢁ1/2 + z; fꢁ1 + x, 1/2
ꢁ y, ꢁ1/2 + z; g1 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z; h-x, 1 ꢁ y, ꢁz; i1 + x, 1/2 ꢁ y, 1/2 + z; j1 + x, 1/2 ꢁ y, 1/2 + z; kx, 1/2 ꢁ y, 1/2 + z; lꢁx, 1 ꢁ y, ꢁz; m2 ꢁ x, ꢁy, 1 ꢁ z;
n
ꢁ1/2 ꢁ x, 1/2 ꢁ y, ꢁz; oꢁ1/2 + x, 1/2 ꢁ y, ꢁ1/2 + z; px, ꢁ1 + y, z; qx, 1 + y, z; r3/2 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z; s2 ꢁ x, y, 3/2 ꢁ z; t1 ꢁ x, ꢁy, 1 ꢁ z; ux, 1 ꢁ y, ꢁ1/2
v
w
x
+ z; x, 1 ꢁ y, ꢁ1/2 + z; ꢁ1/2 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z; ꢁ1/2 + x, ꢁ1/2 ꢁ y, 1/2 + z.
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was shown to exhibit such conformation related polymorphic
forms and thereby results in various isomeric dimers or polymers
upon irradiation.17
(Fig. 1). The pentadienone moiety exhibits near planarity while
the pyridyl rings make angles of 21.51ꢀ and 11.34ꢀ with the plane
of double bonds. In the lattice the molecules join via C–H/N
and C–H/O hydrogen bonds to form a two-dimensional layer.
The molecules stack on each other such that each one of the
molecule has two types of interactions: one side of the molecule
has offset arrangement in which cisoid double bonds are parallel
and satisfy Schmidt’s criteria for [2 + 2] topochemical reaction
(Table 3) while the other side of the molecule interacts with
inversion related molecule such that the double bonds have
crisscross alignments with the centroid-to-centroid distance of
Compound 1P has been crystallized from a variety of solvents
and solvent mixtures in anticipation of polymorphs or various
solvates. However, it always resulted in a unique crystalline form
in which the molecule 1P exhibits cisoid–transoid conformation
˚
3.84 A (Fig. 1b–d).
The grinded crystalline material of 1P was irradiated in
sunlight by placing the material between two thin glass plates.
The 1H NMR spectra after one day of irradiation in the
sunlight show peaks corresponding to cyclobutane derivative 8
along with several other uncharacterized peaks that are
presumably corresponding to oligomers or polymers. Further
irradiation of this mixture resulted in only oligomers and
polymers but not dimeric compound 8. Therefore, after irra-
diation of 1P for a day, the compound 8 was separated in 15%
yield as a white solid by crystallizing the reaction mixture from
acetone–methanol. Suitable single crystals of 8 for X-ray
diffraction were grown from methanol. Crystal structure
analysis shows that 8 contains a dimer with the stereochemistry
expected from offset arrangement (Fig. 2). In the crystal
structure of 8, the molecules form a 1D chain through a bifur-
cated C–H/O hydrogen bonds of the cyclobutane C–H and
b-H of pyridine with the carbonyl oxygen. These chains interact
with each other via weak C–H/N interactions to yield a three-
dimensional packing.
Unreactive co-crystals
The co-crystals 2 and 6 were found to be unreactive for different
reasons. In the co-crystal 2 the RN moieties do not act as a clip to
bring the two units of 1P within the photochemical reactive
distance. The RN exhibits divergent geometry with two –OH
groups pointing away from each other.
As a result it forms 1D-chains via O–H/N hydrogen bonds
(Fig. 3). These chains pack through weak interactions such that
the double bonds do not contain reactive alignments.
The co-crystal 6 (1P$PG) is one of the four co-crystals of PG
with a unique quality of not having any solvent in the crystal
lattice. The crystal structure analysis of 6 reveals that PG
exhibits the required conformation to act as a template but it
Fig. 1 Illustrations for the crystal structure of 1P: (a) 2D layer through
weak C–H/O and C–H/N hydrogen bonds; (b) two types of p-stacks
of 1P; (c) offset and (d) crisscross arrangement (top view).
Table 3 Geometrical parameters for the alignment of double bonds and photochemical reaction details for compound 1P and 3–7
Comp.
C]C/C]C torsion
Irradiation time in sunlight
Max. yield of product
distances
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
3.88 A, 3.84 A, 4.00 A
1P
3
4
ꢂ0ꢀ, ꢂ61.5ꢀ
ꢂ3.63ꢀ, ꢂ3.97ꢀ, ꢂ5.42ꢀ, ꢂ2.52ꢀ
1 day
3–4 h
2 days
ꢂ15% of 8
3.60 A, 3.85 A
3.62 A, 3.71 A, 3.63 A, 3.72 A, 3.63
ꢂ3.65ꢀ
ꢂ100% TCD
ꢂ100% TCD
ꢁ ꢁ ꢁ ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
A, 3.66 A, 3.52 A, 3.73 A
ꢂ0.66ꢀ, ꢂ3.16ꢀ
1 day
3 days
5–6 h
ꢂ100% TCD
No reaction
ꢂ100% TCD
ꢁ
ꢁ
ꢁ
3.63 A, 3.74 A, 3.64 A, 3.76 A
ꢁ
5
6
7
ꢁ
3.61 A
ꢂ0ꢀ
ꢂ15.36ꢀ
ꢁ
3.69 A
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Fig. 4 Illustrations for the crystal structure of 6: (a) 1D chain through
O–H/N and O–H/O (O]C) hydrogen bonds; (b) alignment of cisoid
double bonds of 1P in such 1D chain.
Double [2 + 2] reaction in co-crystals of 1P with phloroglucinol
The contents of asymmetric units in the co-crystals of 3–5 are as
follows: 3 contains one each of acetonitrile, PG, and 1P; 4
contains four molecules of 1P, three molecules of PG and four
molecules of water; 5 contains two molecules of 1P, one molecule
of PG and four molecules of water. The molecule 1P exhibits
cisoid–cisoid conformation in the co-crystals of 3 and 4, while it
exhibits cisoid–transoid conformation in the co-crystals of 5
(Fig. 5). In 3, two –OH groups of PG clip two molecules of 1P via
O–H/N hydrogen bonds as anticipated and the third –OH
group of PG interacts with CH3CN via O–H/N hydrogen
bonds. While in 4, two types of clipping were found: the first type
is identical to the one observed in 3 and in the second type of
clipping one side of the two p-stacked moieties of 1P is clipped
by PG whereas the second side is clipped by PG and two water
molecules. In 5, the two p-stacked units of 1P are clipped in
required fashion but one side (cisoid side) by the PG and the
other side (transoid side) by the three water molecules. In all
three co-crystals the double bonds satisfy the required
topochemical criteria for [2 + 2] photochemical reaction
(Table 3). From the geometries of the supramolecular p-stacks of
1P in the co-crystals, it was anticipated that 3 and 4 should yield
exo–exo TCD and 5 should yield exo–endo TCD. In contrast the
photochemical irradiation of all three co-crystals yielded exo–exo
TCD. Also, the reactivity of these compounds was found to
differ significantly. The compounds 3 and 5 react even in room
light while the compound 4 reacts only either under sunlight or
UV light. Further, the crystals of 3 were found to be intact during
the entire process of the reaction. In contrast the crystals of 4 and
5 loose the crystalline nature during the irradiation, may be due
to the loss of lattice water. The loss of water in 5 may also explain
the non-correlation of reactant geometry with the product
geometry. As mentioned before one end (transoid) of the p-
stacked dimers of 1P was tied by the H2O molecules. The loss of
these H2O molecules triggers the rearrangement of double bond
to cisoid conformation, which can be termed as a pedal motion,
resulting in exo–exo TCD. Generally, pedal motion causes
orientation of crisscrossed double bonds into the parallel align-
ments or an isomerization of double bonds. In the co-crystal 5,
the driving force for such pedal motion might be the conversion
of unstable conformer into the stable conformer of 1P.
Fig. 2 Illustrations for the crystal structure of 8: (a) 1D chain through
bifurcated C–H/O hydrogen bonds; (b) packing of molecules.
Fig. 3 Illustrations for the crystal structure of 2: (a) 1D chain through
O–H/N hydrogen bonds; (b) packing of two such 1D chains showing no
alignments of double bonds.
does not clip the molecule 1P in a required fashion to
give the double [2 + 2] reaction (Fig. 4). The molecule 1P
exhibits cisoid–transoid and cisoid–cisoid geometry with 50% of
occupancy each. In the crystal structure, two O–H/N
hydrogen bonds and one O–H/O]C hydrogen bond between
1P and PG lead to the formation of a double chain in which the
cisoid (non-disordered) double bonds satisfy the Schmidt’s
criteria to yield the photodimer 8. However, it was found
that the co-crystal 6 is photostable and no reaction occurs
even after irradiation for many days. Although double
bonds satisfy the required geometrical criteria (Table 3) the
lack of required molecular motions and disorder within the
crystal lattice could be a possible reason for the observed
unreactivity.
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Fig. 5 Illustrations for the crystal structures of 3, 4, and 5: (a) supramolecular templated unit in 3 through O–H/N hydrogen bond; CH3CN forms O–
H/N hydrogen bond with the third –OH group of PG and (b) side view; (c) supramolecular unit showing the dimeric stacking of 1P in 4 and (d) side
view; (e) supramolecular dimeric stacking of 1P in 5 through the H-bonding by PG and water molecules and (f) side view; notice the difference in
geometry of 1P.
Double [2 + 2] reaction in the co-crystal of 1P with methoxy
resorcinol
form [2(1P)$2MR$H2O], 7. The asymmetric unit of 7 contains
one each of 1P and MR and half unit of water. The molecule MR
exhibits the required clipping geometry and clips two units of 1P
via O–H/N hydrogen bonds (Fig. 6). The alignments of double
bonds satisfy the required geometrical criteria for topochemical
reactivity (Table 3). The irradiation of the material resulted in the
The failed clipping of 1P units by resorcinol in 2 and the
unreactivity of the co-crystal 6 prompted us to investigate the
reaction of 1P in another system by considering 5-methoxy
resorcinol. It is important to note here that MacGillivray et al.
have demonstrated that the substitution at C-5 position of
resorcinol increases the probability of resorcinol to exhibit ideal
clipping geometry. Therefore we have considered 5-methoxy
resorcinol (MR) to complex with 1P. Our results suggest that this
complexation reaction unlike PG results in only one crystalline
1
100% conversion of 1P into TCD as suggested by H NMR.
Single crystal-to-single crystal transformations
The co-crystals of 3 and 7 were found to undergo [2 + 2] reactions
retaining the crystalline nature.18 However, among these only the
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only single orientation representing those of the product and the
un-reacted component. As a consequence of the single orienta-
tion of the pyridyl groups, some distortion occurred in the
C(Py)–C(Py)–olefinic or cyclobutane angles. However from this
structure, it is difficult to conclude whether the observed disorder
is due to the presence of the products of single [2 + 2] and/or
double [2 + 2] and/or unreacted components.
The crystal structure analysis of fully reacted crystals (10)
reveals that the crystal packing mostly remains intact. However,
huge geometrical differences were observed between the supra-
molecular dimer observed in 3 and covalent dimer (TCD) of 1P
(Fig. 7c) observed in 10. The newly formed C–C bonds in TCD
ꢁ
have the bond lengths of 1.619(7) and 1.607(7) A. The C-atoms
ꢁ
of carbonyl carbons are much closer (2.663 A) in TCD than in
ꢁ
the supramolecular dimer (3.631 A). The distance between the N-
Fig. 6 Illustrations for the crystal structure of 7: (a) supramolecular
templated unit of 1P by MR through O–H/N hydrogen bonds and (b)
side view.
atoms of the pyridine rings which are hydrogen bonded to same
PG molecule slightly increases in 10 compared to that in 3 (4.504
ꢁ
vs. 4.128 A). Further the interplanar angles between these pyridyl
rings also differ significantly, they are more parallel in TCD
(7.87ꢀ) than those in supramolecular dimer (33.7ꢀ). These
parameters indicate that the significant changes occurred in the
crystal lattice during the transformation. Despite such huge
differences between supramolecular dimer and TCD, the
hydrogen bonds remain same in both the structures. More
interestingly, solvated CH3CN remains in the crystal lattice
during the entire reaction process.
co-crystals of 3 were found to be intact as a single crystal without
breaking into small pieces during the entire process. The single
crystal X-ray analyses of the irradiated crystals of 3 were per-
formed after partial irradiation (9) and also after complete
irradiation (10). The yellow crystals of 3 turn to colorless crystals
(10) after the complete conversion to TCD. The cell parameters
of 9 and 10 are almost similar to those of 3 with some minor
differences.15 The crystal structure of 9 reveals that 60% of the
double bonds are reacted and 40% of the double bonds are
unreacted (Fig. 7). Therefore 9 exhibits a disorder in the form of
two sets of coordinates: one set corresponds to an unreacted
double bond with 40% occupancy while the other set corresponds
to cyclobutane with 60% occupancy. The pyridyl groups exhibit
1
Stepwise dimerization, evidenced by monitoring through H
NMR
Our preliminary investigation on the photochemical reaction of 3
1
by H NMR at various stages of reaction clearly indicated that
Fig. 7 Illustrations for the crystal-to-crystal conversion of 1P to TCD: (a) supramolecular dimer of six components in 3 (b) after partial reaction, two
sets of atom positions are identified for both olefin (gray) and cyclobutane (yellow) in 9; (c) after complete conversion, TCD molecule retains the
hydrogen bonding with PG (10) in spite of significant structural changes.
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the reaction is indeed stepwise.15 Similarly, here we have moni-
tored photochemical irradiation of co-crystals 4, 5, and 7 by 1H
NMR spectroscopy in anticipation of identifying the interme-
double dimer appears. After a total of 15 days, the reaction
mixture accounts for 81% of the intermediate MCB and 19% of
the product TCD of double [2 + 2] reaction. With regard to the
other co-crystals, crystals of 7 were found to be reactive in room
light but relatively slower than those of 3. The co-crystals of 5
were found to react even more slowly while those of 4 are almost
unreactive. Therefore we have monitored the reactions of co-
crystals 5 and 7 in the room light by recording 1H NMR spectra
at various stages of irradiation (Fig. 8 and 9). In both cases,
initially the intermediate MCB was found to be a major product
which gradually converts into TCD as reaction progresses.
However in the case of 5, the concentration of MCB was found to
be less compared to those of 3 and 7. The maximum concen-
tration of MCB was observed up to 81%, 34% and 57% for 3, 5
and 7 respectively.
1
diate MCB. The H NMR spectra after partial irradiation of
these co-crystals showed two new doublets (d 4.295 and 4.784) in
the cyclobutane proton region in addition with the anticipated
doublets for cyclobutane ring corresponding to TCD (d 4.429
and 4.642). These two new peaks were attributed to the cyclo-
butane protons of MCB. Further, two new doublets (d 7.554 and
7.121) were observed in the upfield region (d 7.778 and 7.559 for
1P) corresponding to the double bonds of the intermediate MCB.
The peaks corresponding to pyridine -aH and -bH were observed
as four different doublets in new positions when compared to
two doublets for pyridyl proton of 1P and TCD. The splitting
patterns and coupling constants of these new peaks clearly
demonstrate the stepwise mechanism of the reaction with
formation of intermediate MCB, and also excludes the formation
of other radical intermediate. However, when the irradiation was
carried out in the sunlight the major constituents at the various
stages of the reaction are 1P and/or TCD. Therefore at all stages
the intermediate MCB were found to be present as a very minor
component. This observation suggests that the formation of
TCD from MCB is faster (sunlight) than the formation of MCB
from 1P. Therefore we have monitored the reaction by keeping in
the room light. The exposure of the crystals of 3 on watch glass in
the room light leads to the initial formation of small amount of
single dimer (MCB), 47% was converted into MCB after first two
days of exposure which was observed to increase further to 62%
after two more days. In these four days, no trace of a double
dimer was found, but after two more days of exposure a trace of
Stepwise dimerization, evidenced by monitoring through UV
The reactions of 3, 5 and 7 by exposing to the room light were
also monitored using the UV-vis spectroscopy. The conjugation
of double bond with the pyridyl group decreases from monomer
1P to intermediate MCB to the final product TCD, therefore
a gradual blue shift in their absorption maxima (lmax) values is
expected as the reaction progresses. The absorption spectra were
recorded by dissolving 0.0403 mg, 0.0311 mg and 0.0376 mg of
co-crystals 3, 5 and 7 respectively in 10 mL of methanol such that
all the solutions have unique concentration (1 ꢃ 10ꢁ5 M) with
respect to the monomer 1P as the initial % of the reaction is zero.
In a similar way the UV-vis absorption was recorded at different
stages of the reaction of 3, 5 and 7 by exposing them to room
Fig. 8 1H NMR spectra in d6 DMSO recorded at various stages of
reaction in co-crystal 5: (a) before irradiation; the gradual changes as the
reaction progresses shown in (b) to (d); (e) after 100% reaction occurs: :
(Py–H of 1P), C (Py–H of MCB), B (Py–H of TCD), , (alkene–H of
1P), - (alkene–H of MCB), A (cyclobutane–H of MCB), > (cyclo-
butane–H of TCD).
Fig. 9 1H NMR spectra in d6 DMSO at various stages of the reaction in
co-crystal 7: (a) before irradiation; the gradual changes as the reaction
progresses shown in (b) to (e); (f) after 100% reaction occurs: symbols
represent various proton positions as mentioned in Fig. 8.
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Fig. 10 UV-vis absorption spectra recorded for co-crystal 3 at various
stages of the reaction. The curve (a) represents reactants and (h) repre-
sents products.
Fig. 12 UV-vis absorption spectra recorded for co-crystal 7 at various
stages of the reaction. The curve (a) represents reactants and (f) repre-
sents products.
light. The initial spectrum of 3 (i.e. at 0% of reaction) exhibits
absorption maxima (lmax) at 206 nm (PG) and 293 nm (styryl
pyridine) (Fig. 10). The UV-vis spectrum of the product with
100% TCD does not exhibit lmax at 293 nm but exhibits new lmax
at 259 nm due to hypsochromic and hypochromic shifts. Initially
(curves b and c) the molar absorptivity (3) at the 267 nm increases
and gradually decreases as the reaction progresses (curves c–i).
Finally it disappears after the complete conversion of 1P to
TCD. These observations clearly indicate that the absorption
maximum at 267 nm corresponds to the product MCB. The peak
position and the molar absorptivity at 206 nm remain unaltered
throughout the reaction as it corresponds to molecule PG. For
the sake of comparison we have also measured the UV-vis
spectrum for the product (8) obtained from the topochemical
reaction of 1P which has the similar chromophore as the MCB.
The dimer 8 exhibits lmax at 274 nm which is closer to that of
MCB (267 nm). The UV-vis absorption for the other two reactive
co-crystals 5 and 7 has also exhibited similar features as those
observed for 3 (Fig. 11 and 12).
Photochemical reactions of the solvent free co-grounded solids of
1P with RN, PG and MR
In recent days mechanochemical grinding of two components
was recognized as very efficient and environment friendly root to
obtain materials which are chemically equivalent to the ones that
are synthesized via co-crystallizing the components by conven-
tional solvent evaporation.19,20 We have analyzed the potentiality
of the templates RN, PG and MR to form complexes with 1P by
mechanical grinding and compared the XRPD patterns of
grounded samples with those of conventionally prepared
Fig. 13 XRPD patterns for co-grounded samples and respective co-
crystals (calculated): (a) co-grounded sample of 1P and RN; (b) co-
crystal 2; (c) co-grounded sample of 1P and PG; (d) co-crystal 6, and (e);
co-grounded sample of 1P and MR; (f) co-crystal 7.
Fig. 11 UV-vis absorption spectra recorded for co-crystal 5 at various
stages of the reaction. The curve (a) represents reactants and (e) repre-
sents products.
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observed only in one of the reactive forms. The lack of corre-
spondence between geometry of reactants and the geometry of
the product in 5 and the unreactivity in 6 despite satisfying the
Schmidt’s topochemical criteria are two anomalies observed in
this paper. Although several examples with such anomalies are
known the reasons are not so clear. The pedal motion was
attributed for the observed difference in 5, while the restricted
molecular motion was attributed to the lack of reactivity in 6. We
note here that out of seven crystal structures containing 1P four
have the cisoid–cisoid conformation of 1P while two have the
cisoid–transoid conformation of 1P and one has both confor-
mations with 50% of occupancy each (disordered). Currently we
are exploring coordination polymers of TCD and 8 with various
metal salts.
Experimental
FTIR spectra were recorded with a Perkin Elmer Instrument
Spectrum Rx Serial No. 73713. Powder XRD patterns were
Fig. 14 Hydrogen bonded 2D layer in the crystal structure of 11.
1
recorded with a PHILIPS Holland PW-1710 diffractometer. H
NMR and 13C NMR (200/400 MHz) spectra were recorded on
a BRUKER-AC 200/400 MHz spectrometer. UV-vis absorption
was recorded using a Shimadzu UV-1601 spectrophotometer.
Elemental analyses were carried out by a Perkin Elmer Series II
2400 and melting points were recorded using a Fisher Scientific
melting point apparatus cat. No. 12-144-1.
materials. It was found that the co-grinding of RN or MR with
1P (in 1 : 1 ratio) produces the materials which are equivalent to
2 or 7 respectively (Fig. 13). The XRPD patterns of these co-
grounded samples match totally with those of 2 and 7. The
photochemical reactivity of these samples also remains same as
that of 2 and 7, that is in the case of RN the co-grounded material
is unreactive whereas in the case of MR the material is reactive
with 100% yield of TCD. Further, the XRPD patterns of co-
grounded samples of PG with 1P do not match with those of 3, 4,
5 or 6. Upon irradiation of this co-grounded solid resulted in the
mixture of uncharacterized products. The difference in XRPD
patterns and photochemical behavior from those of 3–6 indicates
that the co-grounded sample PG and 1P could be a new phase of
co-crystalline material of the components.
Preparation of co-crystals 2–3, the photochemical reaction
details for 3 and generation of co-crystals 9–10 have been
described in our previous communication. Co-grounded samples
of 1P with RN, PG or MR in 1 : 1 ratio were prepared by using
mortar and pestle with about 1 hour continuous grinding.
Photochemical reaction of crystalline 1P and separation of
product 8
Rod-like light yellow colored crystals of 1P (4 g) were obtained
by crystallization from toluene which were then exposed to
sunlight for a day. A maximum conversion to the product 8
occurs at this stage of irradiation (as checked by 1H NMR
spectra and TLC) which otherwise go for polymerization on
further irradiation. The partially irradiated material was then
dissolved in methanol (15 mL) to produce a reddish color solu-
tion. A white solid gradually settles down from this solution
which was then again recrystallized from methanol. The solid
compound was characterized as the single [2 + 2] reaction
product 8 (yield: 0.609 g, 15%). Mp 215–220 ꢀC; (Found C,
76.73; H, 5.04; N, 11.51. Calc. for C30H24N4O2: C, 74.25; H, 5.12;
N, 11.86%); lmax(MeOH)/nm 203, 274; nmax/cmꢁ1 3200, 1617,
1535, 1498, 1420, 1298, 1160, 1009, 815, 784, 732; dH(400 MHz;
D6-DMSO) 4.639/4.752 (AA’BB0, 4H, Cy-butane proton), 6.806
(d, J ¼ 16.8 Hz, 2H, olefin proton); 7.370 (d, J ¼ 6 Hz, 4H, Py–
bH), 7.405 (d, J ¼ 16.8 Hz, 2H, olefin proton); 7.528 (d, J ¼ 6 Hz,
4H, Py–bH), 8.496 (d, J ¼ 5.6 Hz, 4H, Py–aH), 8.593 (d, J ¼ 6
Hz, 4H, Py–aH); dC(200 MHz; D6-DMSO) 198.06 (2C,
carbonyl); 150.85 (4C, vinyl Py C2/6), 149.89 (4C, cyclobutyl Py
C2/6), 148.30 (2C, Cyclobutyl Py C4), 141.97 (2C, vinyl Py C4),
140.80 (2C, alkene C b to carbonyl), 130.15 (2C, alkene C a to
carbonyl), 123.79 (4C, cyclobutyl Py C3/5), 122.53 (4C, vinyl Py
C3/5), 49.70 (2C, cyclobutane).
Isolation of TCD
Finally the product TCD was isolated in almost quantitative
yield from any of the 100% reacted co-crystals of 3–5 or 7 by
acid–base work-up. Suitable single crystals of [TCD$2MeOH]
(11) were grown by crystallizing the isolated TCD from
methanol. Crystal structure analysis shows that the TCD is not
as symmetrical as the one observed in 10 (Fig. 14). In the lattice
two TCDs are paired via the C–H/O hydrogen bond synthons
between carbonyl oxygen and cyclobutane C–H. These pairs are
further connected into 2D sheets through herringbone type of
arrangement involving C–H/N hydrogen bonds and methanol
molecules remain hydrogen bonded to pyridyl nitrogens.
Conclusions
In conclusion we have demonstrated the synthesis of double [2 +
2] photodimerized product (TCD) from a new bis-pyridyl olefin
(1P) in four co-crystal forms utilizing the hydrogen bonding
interactions. Our studies also reveal that a quick and environ-
mental friendly synthesis of the TCD is possible by a (1 : 1)
mechanochemical grinding of 1P and MR. The results presented
here clearly support the stepwise mechanism for double [2 + 2]
reaction of 1P. Single crystal-to-single crystal conversion was
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Preparation of co-crystals 4
3H, methoxy), 5.760 (d, J ¼ 2 Hz, 2H, aromatic H of MR), 5.797
(d, J ¼ 2 Hz, 1H, aromatic H of MR), 7.550/7.781 (AB quartet,
4H, alkene proton), 7.727 (d, J ¼ 6 Hz, 4H, Py–bH), 8.666 (d, J ¼
6 Hz, 4H, Py–aH), 9.157 (s, 2H, MR–OH).
A 1 : 1 mixture of 1P (0.05 g, 0.212 mmol) and PG $2H2O
(0.0343 g) was dissolved in methanol (8 mL) and left at room
temperature in the dark for slow evaporation. Light yellow
colored block shaped crystals of 4 were obtained in 57% yield
with respect to 1P after 3–4 days. Single crystals completely loose
Separation of TCD from the reactive co-crystals 3–5 and 7
ꢀ
ꢀ
water at 120 C then started gradual blackening above 180 C
and no clear melting up to 300 ꢀC; (Found C, 67.47; H, 5.04; N,
About 1 g of 100% photochemically reaction products in each
case was dissolved in 50 mL water by adding HCl (1 N) dropwise
(until all material dissolved). The solution was extracted with
50 mL ethyl acetate 3–4 times. The aqueous part was then
neutralized by dropwise addition of dilute aqueous tri-ethyl-
amine solution. The white precipitation obtained was filtered and
washed with water and dried. The TCD obtained in almost
quantitative yield. Mp 210–215 ꢀC; (Found C, 75.07; H, 5.27; N,
11.15. Calc. for C30H24N4O2: C, 76.25; H, 5.12; N, 11.86%);
lmax(MeOH)/nm 202, 259; nmax/cmꢁ1 3044, 2615, 1718, 1607,
1559, 1490, 1422, 1320, 1151, 1067, 1004, 857, 821, 688; dH(200
MHz; D6-DMSO) 4.398 (d, J ¼ 5.2 Hz, 4H, Cy–butane proton);
4.616 (d, J ¼ 5.2 Hz, 4H, Cy–butane proton), 7.051 (d, J ¼ 5.6
Hz, 8H, Py–bH), 8.263 (d, J ¼ 5.6 Hz, 8H, Py–aH); dC(200 MHz;
D6-DMSO) 209.87 (2C, carbonyl), 149.67 (8C, Py C2/6), 147.85
(4C, Py C4), 123.68 (8C, Py C3/5), 49.94 (4C, cyclobutane C a to
carbonyl), 44.70 (4C, cyclobutane C b to carbonyl).
7.82. Calc. for C78H74N8O17: C, 67.13; H, 5.34; N, 8.03%); nmax
/
cmꢁ1 3260, 3055, 2902, 2686, 1673, 1651, 1631, 1600, 1550, 1490,
1421, 1348, 1326, 1302, 1162, 1149, 1113, 1002, 984, 822, 692,
555; dH(200 MHz; D6-DMSO) 5.635 (s, 9H, aromatic H of PG),
7.552/7.783 (AB quartet, 16H, alkene proton), 7.728 (d, J ¼ 5.6
Hz, 16H, Py–bH), 8.667 (d, J ¼ 5.6 Hz, 16H, Py–aH), 8.922
(s, 9H, PG–OH).
Preparation of co-crystal 5
A 1 : 1 mixture of 1P (0.05 g, 0.212 mmol) and PG$2H2O (0.0343
g) was dissolved in acetonitrile (10 mL) and water (5 mL), and
kept at room temperature in the dark for slow evaporation.
Brownish rhombic shaped crystals of 5 were obtained in 85%
yield with respect to 1P afteꢀr 2–3 days. The crystal of 5 found to
ꢀ
loose water at around 120 C and finally melts at 177–180 C;
(Found C, 68.11; H, 5.78; N, 8.56. Calc. for C36H38N4O6: C,
Crystal structure determination
69.44; H, 6.15; N, 9.00%); lmax(MeOH)/nm 204 and 296; nmax
/
cmꢁ1 3202, 2626, 1662, 598, 1552, 1498, 1414, 1346, 1329, 1302,
1184, 1164, 1152, 1002, 984, 820, 563; dH(200 MHz; D6DMSO)
5.637 (s, 3H, aromatic H of PG), 7.552/7.784 (AB quartet, 8H,
alkene proton), 7.728 (d, J ¼ 6 Hz, 8H, Py–bH), 8.667 (d, J ¼ 6
Hz, 8H, Py–aH), 8.928 (s, 3H, PG–OH).
All the single crystal data were collected on a Bruker-APEX-II
CCD X-ray diffractometer that uses graphite monochromated
ꢁ
Mo Ka radiation (m ¼ 0.71073 A) at room temperature (293 K)
by hemisphere method. The structures were solved by direct
methods and refined by least square methods on F2 using
SHELX-97.21 Non-hydrogen atoms were refined anisotropically
and hydrogen atoms were fixed at calculated positions and
refined using a riding model. The H-atoms attached to O-atom
are located wherever is possible and refined using the ridging
model. The single crystal data and refinement details for 1P, 2, 3,
9, 10 and 11 were communicated in our previous communication.
Pertinent crystallographic (4–8) and hydrogen bonding para-
meters (1P and 2–11) are given Tables 1 and 2 respectively. The
1P molecules in 6 are disordered such that one of the double
bond has both cisoid and transoid geometries with 50% occu-
pancies. Due to this disorder two of the pyridine C-atoms and
CO group of 1P also exhibit two positions with 50% occupancy
each (Fig. 15). The water molecule in 7 exhibits very high thermal
motion indicating possible disorder or partial loss of water.
Therefore platon squeeze option was used in the final refinement
and corresponded with the removal of the contributions of 1/4 of
a water molecule per asymmetric unit.22
Preparation of co-crystal 6
A 1 : 1 mixture of 1P (0.05 g, 0.212 mmol) and PG $2H2O
(0.0343 g) was dissolved in toluene (10 mL) and methanol (5 mL)
and left at room temperature in the dark for slow evaporation.
Orange colored plate like crystals of 6 were obtained in 86% yield
after 10–12 hours. The crystals started gꢀradual blackening above
ꢀ
180 C and no clear melting up to 300 C; (Found C, 68.73; H,
4.84; N, 7.36%. Calc. for C21H18N2O4: C, 69.60; H, 5.01; N,
7.73); nmax/cmꢁ1 3518, 3033, 2912, 2794, 2633, 1680, 1654, 1626,
1599, 1552, 1499, 1417, 1344, 1318, 1163, 1146, 1104, 1002, 985,
821, 689, 556; dH(400 MHz; D6-DMSO) 5.633 (s, 3H, aromatic H
of PG), 7.559/7.779 (AB quartet, 4H, alkene proton), 7.730 (d,
J ¼ 5.6 Hz, 4H, Py–bH), 8.668 (d, J ¼ 5.6 Hz, 4H, Py–aH), 8.925
(s, 3H, PG–OH).
Preparation of co-crystal 7
A 1 : 1 mixture of 1P (0.05 g, 0.212 mmol) and MR (0.0296 g)
was dissolved in acetonitrile (10 mL) and methanol (2 mL) and
left at room temperature in the dark. Light yellow colored block
shaped crystalꢀs of 7 were obtained in 64% yield after 2–3 days.
Mp: 180–182 C; (Found C, 68.94; H, 5.01; N, 7.16. Calc. for
C22H20N2O4: C, 70.20; H, 5.36; N, 7.44%); lmax(MeOH)/nm 205
and 296; nmax/cmꢁ1 3033–2580, 1684, 1662, 1630, 1601, 1552,
1499, 1441, 1419, 1382, 1343, 1320, 1210, 1187, 1171, 1154, 1104,
1090, 1005, 985, 838, 821, 554; dH(200 MHz; D6DMSO) 3.594 (s,
Fig. 15 ORTEP drawing of 1P illustrating the disorder in 6.
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Acknowledgements
We gratefully acknowledge financial support from DST, DST-
FIST for single crystal X-ray facility. RS thanks UGC for
research fellowship
~
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This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 3246–3257 | 3257