Influence of Fluorine Substitution on the Unusual Solid-State [2 + 2] Photo-Cycloaddition Reaction between an Olefin and an Aromatic Ring

Article abstract of DOI:10.1021/acs.cgd.5b00664

Solid-state [2 + 2] photo-cycloaddition reactions observed so far were exclusively between a pair of olefin bonds. Usually when the phenyl-olefin bonds have been closely aligned, they were found to be either photoinert or sliding of molecules takes place

Full text of DOI:10.1021/acs.cgd.5b00664

Article
pubs.acs.org/crystal
Influence of Fluorine Substitution on the Unusual Solid-State [2 + 2]
Photo-Cycloaddition Reaction between an Olefin and an Aromatic
Ring
Raghavender Medishetty, Zhaozhi Bai, Hui Yang, Ming Wah Wong,* and Jagadese J. Vittal*
Department of Chemistry, National University of Singapore, 3, Science Drive 3, 117543, Singapore
S
* Supporting Information
ABSTRACT: Solid-state [2 + 2] photo-cycloaddition reactions observed so far were
exclusively between a pair of olefin bonds. Usually when the phenyl−olefin bonds have
been closely aligned, they were found to be either photoinert or sliding of molecules takes
place for [2 + 2] cycloaddition reaction between olefins in the solid state, although
intramolecular phenyl−olefin reactions are well-known in solution. In the crystal structure
of [Zn2(ptol)4(4spy)2] (ptol = para-toluate), the neighboring 4-styrylpyridine (4spy)
ligands are organized in a head-to-tail manner. On one side of the complex in the crystal
structure, the olefin bonds in the 4spy pairs are perfectly aligned to undergo cycloaddition
reaction, but on the other side, the olefin bond pairs are slightly offset and found to be
photoinert at 223 K forming only a dimer in single crystals. The sliding of 4spy groups has
been restricted by the steric hindrance of the adjacent methyl group of the ptol ligands. A
similar packing of 2-fluoro-4′-styrylpyridine (2F-4spy) pairs was found in [Zn2(ptol)4(2F-
4spy)2]. Again, normal cycloaddition reaction occurs on one side of the 2F-4spy ligand
pairs, whereas the second offset 2F-4spy ligand pairs undergo a rare [2 + 2] cycloaddition
reaction between the fluorophenyl group and olefin bond resulting in the formation of a one-dimensional coordination polymers
containing a bicyclic product in a quantitative yield. The bicyclic ring in the photoproduct can be thermally cleaved back to olefin
1
and phenyl groups. These observations have been confirmed by single-crystal X-ray crystallography, H NMR, and 19F NMR
studies. Density functional theory calculations were performed to elucidate the nature of the interactions between the
fluorophenyl and olefin groups. The greater reduction of aromaticity of 2F-4spy in the excited singlet state compared to the 4spy
system may explain the observed reactivity difference between the two systems. The improved reactivity in 2F-4spy may also be
attributed to the fact that the olefin−phenyl distance is shorter in 2F-4spy than in 4spy (3.63 versus 3.69 Å). This solid state
phenyl−olefin photodimerization helps to pave the way for making new bicyclic derivatives.
Scheme 1. (a) Normal [2 + 2] Cycloaddition between a Pair
of Olefin Bonds after Sliding of Molecules. (b)
Intramolecular Photodimerization between Phenyl and
Olefin Bonds Usually Observed in Solution
INTRODUCTION
■
The solid-state [2 + 2] cycloaddition reaction dates back to the
work of Lieberman in 1889 on the photodimerization of
cinnamic acid.1 The formation of α-truxillic and β-truxinic acids
from two types of cinnamic acid crystals was subsequently
interpreted as crystal lattice controlled reaction by Bernstein
and Quimby in 1943.2 Later, Schmidt and his co-workers
correlated the structures of the cinnamic acids and several other
enone derivatives with the steric configurations of their
photoproducts and introduced the topochemical princi-
ples.3−5 Since then several hundreds of photo-cycloaddition
reactions have been reported to take place exclusively between
a pair of olefin bonds.6−9 In a number of cases where Schmidt’s
topochemical criteria were not met, the olefin pairs were either
found to be photostable or underwent sliding/pedal motion to
meet the criteria in the solid state as shown in Scheme 1.10−13
A simple Cambridge Structural Database search for the
intermolecular alignment between olefin and phenyl bonds
within 3.5−4.2 Å distance criterion generates more than
100 000 hits.14 Despite this, solid-state photo-cycloaddition
reaction between olefin bond and phenyl ring has rarely been
reported, where it has been found to occur up to only 66%, as
characterized by NMR spectroscopy.15 In contrast, intra-
molecular cyclization between phenyl group and olefin bond
have been extensively studied in solution (Scheme 1b).16−18
Received: May 13, 2015
© XXXX American Chemical Society
A
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(4H, pyridyl protons of rctt-ppcb), 7.83 ppm (d, 8H, J = 7.4 Hz, ptol
protons), 7.0−7.4 ppm (m, 14H, aromatic protons of rctt-ppcb), 7.22
ppm (d, 8H, J = 7.8 Hz, p-toluate protons), 4.61 (s, 4H, cyclobutane
proton, rctt-ppcb), 2.34 ppm (s, 12H, methyl protons of p-tol). IR
data. 1700 (m), 1610 (s), 1561 (s), 1508 (m), 1414 (s), 1224 (m),
1176 (m), 1112 (m), 1070 (m), 1032 (m), 972 (m), 850 (m), 825
(m), 766 (s), 700 (s), 625 (m), 544, 473 (m), 438 (m).
During the course of our investigation we stumbled upon this
interesting asymmetric photo-cycloaddition between a fluo-
rophenyl ring and an olefin bond driven by the solid-state
packing of the molecules. Although the phenyl ring and an
olefin bond are closely aligned between the neighboring 4-
styrylpyridine (4spy) ligands of a Zn(II) complex, it was almost
photostable. The reaction readily occurs under UV light when
4spy was replaced by 2-fluoro-4′-styrylpyridine (2F-4spy) in the
Zn(II) complex in a single-crystal-to-single-crystal (SCSC)
If the single crystals of 1 were irradiated under UV light for 120 h,
1
the H NMR has more peaks in addition to 2 along with shift in
pyridyl peak from δ 8.55 ppm could not be completely assigned. See
Figure S13 in Supporting Information.
1
manner and further supported by 19F and H NMR spectral
Synthesis of [Zn2(ptol)4(2F-4spy)2] 4. Colorless block single
crystals were obtained from the slow evaporation of the aqueous
ethanolic solution of Zn(OAc)2·2H20 (11 mg, 0.05 mmol), p-toluic
acid (13.6 mg, 0.1 mmol), and 2F-4spy (10 mg, 0.05 mmol) and the
addition of three drops of 0.75 M NaOH. The crystals were dried at
room temperature (yield: 15 mg (56%)). The elemental analysis (%)
Calculated for C29H24FNO4Zn (534.89): C, 65.12; H, 4.52; N, 2.62.
data. We have shown that in addition to the alignment of
phenyl−olefin bonds, fluorine substitution influences the
quantitative dimerization reaction in the solid state. The details
are discussed below.
EXPERIMENTAL SECTION
■
1
Found: C, 65.00; H, 4.24; N, 2.64. H NMR (DMSO-d6, 300 MHz,
Materials and Methods. All the chemicals and solvents were of
reagent grade and purchased from different commercial resources and
were used without further purification. 4spy and 2F-4spy were
synthesized according to the literature.19,20 Powder X-ray diffraction
(PXRD) data were recorded on a D5005 Siemens X-ray diffractometer
with graphite monochromatized Cu Kα radiation (λ = 1.54056 Å) at
room temperature (298 K). NMR spectra were recorded on a 300 or
400 MHz FT-NMR spectrometer with residual solvent as the
298 K): δ 8.58 ppm (d, 4H, J = 7.3 Hz, pyridyl protons of 2F-4spy),
7.84 ppm (d, 4H, J = 8.1 Hz, pyridyl protons of 2F-4spy), 7.2−7.7
ppm (m, 12H, aromatic protons of 2F-4spy), 7.21 ppm (d, 8H, J = 7.9
Hz, p-toluate protons), 2.34 ppm (s, 12H, methyl protons of p-tol). IR
(KBr pellet, cm−1) 1609 (s), 1562 (s), 1505 (s), 1411 (s), 1223 (m),
1178 (m), 1110, 1028 (m), 975, 855 (m), 818 (m), 762 (s), 692 (m),
629 (m), 554, 518, 477, 425.
1
Synthesis of [Zn2(ptol)4(2F-ppcb)0.5(L1F)0.5] 5. Compound 4
reference in DMSO-d6 solution for H NMR and trifluoroacetic acid
1
as a standard for 19F-NMR chemical shifts. Thermogravimetric analysis
(TGA) was performed under nitrogen atmosphere with a heating rate
of 5 °C min−1 on a TA Instruments SDT-2960. The C, H, N analyses
were carried using Elementar Vario Micro Cube instrument at the
Elemental Analysis Lab, CMMAC, Department of Chemistry, National
University of Singapore.
was subjected to UV-irradiation for 2 h. H NMR (DMSO-d6, 300
MHz, 298 K): δ 8.58 ppm (d, 2H, J = 7.5 Hz, pyridyl protons of L1F),
8.48 ppm (d, 2H, J = 7.6 Hz, pyridyl protons of L1F), 8.35 ppm (d,
4H, J = 7.5 Hz, pyridyl protons of rctt-2F-ppcb), 7.83 (d, 8H, J = 9.2
Hz, aromatic protons of p-toluate), 7.2 ppm (d, 8H, J = 9.1 Hz,
aromatic protons of ptol), 6.9−7.7 ppm (m, 8H, aromatic protons of
rctt-2F-ppcb, 6H, L1F), 6.2−6.6, 5.2−5.5, 3.66 (m, 8H, olefinic
protons of L1F), 4.72 ppm (s, 4H, cyclobutane protons of rctt-2F-
ppcb), 2.34 (s, 12H, methyl protons of p-toluate). IR (KBr pellet,
cm−1): 1600 (s), 1563 (m), 1510, 1414 (s), 1230, 1178, 1110, 1071,
1034, 851, 764 (s), 694, 627, 567(m), 476, 444.
X-ray Crystallography. Crystal data of all these crystals were
collected on a Bruker APEX diffractometer attached with a CCD
detector and graphite-monochromated Mo Kα radiation (λ = 0.71073
Å) using a sealed tube. Absorption corrections were made with the
program SADABS,25 and the crystallographic package SHELXTL26,27
was used for all calculations. Crystallographic data and structural
refinements for 1, 2, 4, and 5 are listed in Supplementary Table 1.
CCDC 1007520−1007523 (1, 2, 4, and 5) contains the
supplementary crystallographic data of this paper.
All the UV irradiation experiments were conducted in a LUZCHEM
UV reactor. In the case of crystalline powders, the ground single
crystals were packed in between the glass slides placed in the UV
reactor. These glass slides were flipped back at regular intervals of time
to maintain the uniform exposure of UV irradiation. The SCSC
transformation of 1 to 2 was performed on the diffractometer at 223 K
using Asahi spectra MAX 150, and in the case of 4 to 5, all the small
crystals or platy crystals cracked into pieces under UV irradiation.
However, the big block-shaped crystals survived the photoreaction
after exposing the crystals for 3 h UV using an Asahi spectra MAX 150
instrument.21−24
Synthesis of [Zn2(ptol)4(4spy)2] 1. Pale yellow block-shaped
single crystals were obtained from the slow evaporation of the aqueous
methanol solution of Zn(OAc)2·2H20 (11 mg, 0.05 mmol), p-toluic
acid (13.6 mg, 0.1 mmol), and 4spy (9 mg, 0.05 mmol) and the
addition of three drops of 0.75 M NaOH. The crystals were dried at
RESULTS AND DISCUSSION
1
■
room temperature (yield: 16 mg (62%)). H NMR (DMSO-d6, 300
MHz, 298 K): δ 8.55 ppm (s, 4H, pyridyl protons of 4spy), 7.83 ppm
(d, J = 8.1 Hz, 8H, ptol protons), 7.2−7.7 ppm (m, 18H, aromatic
protons of 4spy), 7.21 ppm (d, 8H, J = 7.8 Hz, p-toluate protons), 2.34
ppm (s, 12H, methyl protons of p-tol). The elemental analysis (%)
Calculated for C29H25NO4Zn (516.90): C, 67.38; H, 4.87; N, 2.71.
Found: C, 67.00; H, 4.45; N, 2.64. IR (KBr pellet, cm−1) 1610 (s),
1564 (s), 1509 (m), 1413 (s), 1212 (m), 1177 (s), 1030 (m), 972 (s),
860 (m), 818 (s), 762 (s), 688 (s), 625 (m), 543 (s).
Pale yellow single crystals of [Zn2(ptol)4(4spy)2] (1) (ptol =
para-toluate) were obtained by slow evaporation of the aqueous
ethanol solution of Zn(OAc)2, p-toluic acid, and 4spy in a 1:2:1
molar ratio with three drops of 0.75 M NaOH solution. This
compound crystallized in the monoclinic space group P21/c
with Z = 4, and the asymmetric unit contains a formula unit.
Each Zn(II) atom is bridged by two ptol ligands and
asymmetrically chelated by a ptol as shown in Figure 1.
Hence, each Zn(II) atom adopts a distorted square pyramidal
geometry with a pyridyl nitrogen atom occupying the apical
position. Both the chelated ptol ligands are almost parallel but
slip-stacked. The coordination sphere observed here is very rare
and different from the well-known paddle-wheel structure. A
CSD search showed only one example for this geometry among
various discrete metal complexes.28
Synthesis of [Zn4(ptol)8(rctt-ppcb)(4spy)2] 2. Single crystals of
2 were obtained by keeping the single crystals of 1 under UV light for
1
1−2 h at −50 °C. H NMR (DMSO-d6, 300 MHz, 298 K): δ 8.55
ppm (s, 4H, pyridyl protons of 4spy), δ 8.33 ppm (d, 4H, J = 7.1 Hz,
pyridyl protons of rctt-ppcb), 7.83 ppm (d, 16H, J = 7.1 Hz, ptol
protons), 7.0−7.8 ppm (m, 32H, aromatic protons of rctt-ppcb and
4spy), 7.21 ppm (d, 16H, J = 7.8 Hz, p-toluate protons), 4.61 (s, 4H,
cyclobutane proton, rctt-ppcb), 2.34 ppm (s, 24H, methyl protons of
p-tol).
Crystallographic inversion centers are present on the both
sides of the molecule close to the middle of the neighboring
4spy ligands, resulting in the neighboring 4spy molecules to
Synthesis of [Zn2(ptol)4(rctt-ppcb)] 3. Single crystals of 1 were
ground finely and packed between the slides and exposed to UV for
more than 80 h. 1H NMR (DMSO-d6, 300 MHz, 298 K): δ 8.33 ppm
B
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support to our conclusion (Figure S19 in Supporting
Information).
On the contrary, a number of new peaks started appearing in
1
the H NMR spectrum of the single crystals of 1 irradiated for
120 h at room temperature indicating that some other side
reaction has also taken place in addition to normal [2 + 2]
cycloaddition reaction. However, the intensities of these new
peaks are too low and masked by the intense peaks of rctt-ppcb
(Figure S13, Supporting Information). These additional
chemical shifts could arise from the reaction of CC bond
with other parts of the adjacent 4spy ligand, isomerization or
cyclization. Attempts have been made to synthesize an
isostructural compound with different substituents on 4spy
ligand and fluoro substitution on the ortho position of the
phenyl group, namely, trans-2-fluoro-4′-styrylpyridine (2F-
4spy) resulted in very similar packing.
Colorless block shaped single crystals of [Zn2(ptol)4(2F-
4spy)2] (4) were synthesized by a procedure similar to 1, and
the crystal structure is also isotypical to 1. However, one of the
Zn(II) has slightly different coordination sphere. Both Zn(II)
atoms are bridged by two ptol ligands and chelated by a ptol
anion. But an oxygen atom (O5) of one of the chelated ptol
ligands is also bridging the other Zn(II) atom (Zn2−O5, 2.504
Å) as shown in Figure 3. This is also reflected in the twisting
Figure 1. Ball and stick model of compound 1. The hydrogen atoms
are omitted for clarity.
align in a head-to-tail fashion. Because of the asymmetrical
building unit, different types of alignments of 4spy pairs occur
on both sides of 1. The pyridyl and phenyl groups of the 4spy
ligands bonded to Zn1 atoms are aligned face to face with a
center-to-center separation of 3.77 Å, while the well-aligned
olefin groups are separated by 3.73 Å, which is well within the
Schmidt’s topochemical criteria.4,5,29 Hence, this CC pair is
expected to be photoreactive under UV light and likely to
furnish the anticipated cyclobutane derivative quantitatively. On
the contrary, the 4spy ligand pairs bonded to the Zn2 atoms are
also aligned parallel but slip-stacked. The middle of each CC
bond is closer to the center of the neighboring phenyl ring
(3.55 Å). This olefin pair is expected to be photoinert unless
the ligands slide during the photo reaction under UV light.4,5,7
The single crystal of 1 after data collection was irradiated
under UV light for 1 h at 223 K at the diffractometer. The X-ray
crystallography reveals the formation of a new compound,
[Zn4(ptol)8(rctt-ppcb)(4spy)2], 2 (where rctt-ppcb = regio-cis,
trans, trans-1,3-bis(4′-pyridyl)-2,4-bis(phenyl)cyclobutane) as
shown in Figure 2.
Figure 3. A view of 4 with selected labeling of atoms. The hydrogen
atoms are not shown for clarity.
angle of 24° between the plane containing Zn2, O5, O6, and
C43 atoms and toluene ring. Hence, Zn2 adopts a distorted
octahedral geometry, while Zn1 has a distorted square-
pyramidal coordination geometry with the apical position
coordinated by a 2F-4spy ligand. Further, the 2F-4spy bonded
to Zn1 is disordered. Two independent orientations were
modeled with the occupancy refined to 0.818(5) and 0.182(5).
One of the bridging ptol ligands was also disordered.
Figure 2. Ball and stick model of compound 2 and showing
cyclobutane ring. Symmetry code (A): 2 − x, 2 − y, 2 − z. The
hydrogen atoms are not shown.
The Zn(II) complex 2 has a crystallographic inversion center
at the center of the cyclobutane ring. Overall, the packing in 2 is
very similar to that of 1, and the center of the adjacent phenyl
ring is closer to the middle of olefin bond by 3.55 Å. The
methyl groups from the neighboring ptol ligands from the b-
direction mutually hinder sliding of the adjacent 4spy ligands.
However, grinding of 1 or 2 and exposure to UV light yielded
quantitative photoproduct, [Zn2(ptol)4(rctt-ppcb)] (3), as
determined from 1H NMR spectral data recorded after
dissolving in DMSO-d6 (see Figure S14 in Supporting
Information). It is likely that the mechanical grinding changed
the crystal packing, and hence the alignment of the adjacent
4spy ligands to account for the complete disappearance of
olefin protons upon UV exposure. Further, the powder X-ray
diffraction (PXRD) pattern of the powdered 2 is different from
the simulated pattern of the single crystal data of 2, lending
A crystallographic center of inversion is also present on both
sides of the molecule close to the middle of the neighboring 2F-
4spy ligands as in 1. The olefin groups of the 2F-4spy ligands
bonded to Zn1 are aligned parallel, separated by 3.74 Å and
expected to be photoreactive under UV light. Whereas the
adjacent 2F-4spy ligands bonded to the Zn2 atoms are also
aligned parallel, they are slip-stacked as in 1. The CC bond
pairs are separated by 4.92 Å (Figure 4), and hence these pairs
are expected to behave very similar to 1.
1
The photoreactivity was tested under UV light. The H
NMR spectrum was recorded after the powdered 4 was
irradiated under UV light for 2 h and dissolved in DMSO-d6.
The chemicals shifts at 8.35 ppm (pyridyl protons) and 4.72
ppm (cyclobutane protons) show the formation of 50%
photoreaction of 2F-4spy in a head-to-tail fashion, which
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Figure 5. Olefin−phenyl interactions in 4 and the possible
photoproducts of UV irradiation experiment of 4. For clarity the
hydrogen atoms are not shown.
Figure 4. A perspective view of the packing of 4 in the solid state
showing how the neighboring 2F-4spy ligands are aligned. The
hydrogen atoms are not shown for clarity.
and compared with 4 and the rctt-2F-ppcb in a metal complex
(Figure 6a). The 19F NMR spectra of 4 and rctt-2F-ppcb show
peaks at −117.7 ppm and −115.9 ppm, respectively, whereas 5
exhibits three peaks, at −110.5 ppm, −114.5 ppm, and −115.9
ppm. The absence of NMR peak at −117.7 ppm confirms that
there is no unreacted 2F-4spy ligand in 5, and the presence of a
peak at −115.9 ppm shows the formation of rctt-2F-ppcb.
Hence, the chemical shifts at −110.5 ppm and −114.5 ppm are
results in the formation of rctt-2F-ppcb (rctt-2F-ppcb = rctt-1,3-
bis(4-pyridyl)-2,4-bis(2′-fluorophenyl)cyclo butane), and likely
to have originated from the reaction of the ligand bonded to
1
Zn1 atom. In addition to the above peaks, the H NMR
spectrum exhibits several new peaks similar to those of UV
irradiated powder 1, which do not belong to either rctt-2F-ppcb
or 2F-4spy ligands, suggesting that there might be some other
photoreaction occurring instead (see below). However, they
were poorly resolved to deduce any structural information
clearly.
In order to determine the solid-state structure of the unusual
photoproduct (5) unequivocally by single crystal X-ray
diffraction methods, we attempted to retain the single crystals
at the end of the UV experiments and eventually succeeded. By
virtue of this SCSC transformation, complete solid-state
structural elucidation of 5 was possible, and the results are
discussed below.
Even after the [2 + 2] photo-cycloaddition reaction,
compound 5 exists in the space group P21/c as 4. During the
photoreaction, the volume of the cell was reduced by ∼2% with
changes in the lengths of a- and c-axes (Supplementary Table
1). As expected from the precursor structure 4, the aligned
CC bond pairs in 2F-4spy ligand coordinated to Zn1 show
quantitative [2 + 2] cycloaddition reaction and resulted in the
formation of rctt-2F-ppcb. The crystallographic inversion center
is still preserved in the photo product on both sides of
Zn2(ptol)4 core. However, the photo-cycloaddition of 2F-4spy
pair which is bonded to Zn2 (where the olefin groups are
separated by 4.92 Å) showed disorder marred by the presence
of inversion symmetry. Upon further comparison with the
structure of 4, it was realized that the olefin bonds are aligned
parallel to the aromatic C−C bonds with a distance of 3.63 Å in
a complementary manner as shown in Figure 4.
The disorders of the photoproduct between the Zn2 atoms
in 5 could be modeled successfully in two ways in the least-
squares refinement cycles: (i) reaction between one olefin bond
and a phenyl group yielding bicyclo[4.2.0]octa-2,4-diene
derivative (here after called as L1F, see Figure 5) which
possess a very highly strained bicyclic ring containing four
stereocenters in a quantitative yield. The ligand L1F is
disordered due to crystallographic inversion symmetry. (ii)
Both the olefin bonds from the neighboring 2F-4spy ligands
reacted with the adjacent phenyl groups to furnish ladderane
like structure (here after mentioned as L2F, see Figure 5) with
five rings fused together along with 50% of the unreacted 2F-
4spy ligands.
Figure 6. 19F NMR spectra of 4, 5 and rctt-2F-ppcb in a Zn(II)
complex (from bottom to top) in DMSO-d6 (above). The H NMR
spectrum of 5 showing the formation of L1F (below).
1
To determine the correct structure unambiguously between
these two possibilities, 19F NMR spectrum of 5 was recorded
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due to either L1F or L2F. Two different chemical shifts are
expected from two different F atoms in L1F but not in L2F.
Further, X-ray structure analysis of 4 indicates that the
formation of L2F should be accompanied by 50% of the
unreacted 2F-4spy ligand. Hence, 19F NMR data clearly
confirm the formation of L1F over L2F. In addition, 1H
NMR spectrum of the photoproduct 5 correlated well with L1F
structure, but not with L2F (Figure 6b, Figure S16 in
Supporting Information).
pair geometries in the crystal (Figure S25 in Supporting
Information). These results clearly indicate that the π−π
stacking interaction between 4spy moieties plays an essential
role in the crystal packing of 1 and 4. The enhanced stability of
the 2F-4spy dimers is attributed to the direct electrostatic
interaction between the fluoro substituent and the adjacent
phenyl ring.35
The experimental and theoretical studies of benzene/phenyl
ring activation in solution using UV light to S1 excited state has
been reported in the literature.18,36−38 We calculated the
HOMO of the lowest excited state using the TD-DFT
method39,40 at the M06-2X/6-31+G* level, to further
investigate this novel solid-state photoreaction. As expected,
the HOMO of the excited states has a major contribution from
the ethylene moiety (Figure 7), which can react with another
The combination of the crystallographic data along with 19F
1
and H NMR spectral results corroborate the formation of a
novel bicyclic product L1F through an unprecedented [2 + 2]
cycloaddition reaction between an olefin bond and a phenyl
ring. The photoreaction of 4 had led to the formation of a 1D
coordination polymeric structure, 5 containing both of rctt-2F-
ppcb and L1F alternately between the Zn2(ptol)4 cores.
To understand the stability of these highly strained rings, the
powdered 5 was heated at 175 °C for 22 h, and the product was
characterized by 1H NMR spectroscopy (Figure S17 in
Supporting Information). Interestingly, it has been found that
the L1F dissociated back to 2F-4spy, but rctt-2F-ppcb was still
intact during heating. This suggests that the bicyclic ring L1F is
relatively unstable and can be thermally cleaved reversibly.
Solid-state [2 + 2] photo-cycloaddition reactions have been
investigated extensively, and they occur exclusively between a
pair of olefin bonds. Although phenyl−olefin bonds have been
found to be closely aligned in the solid state structures, they
have been found to be either photoinert or sliding of molecules
occurs to favor [2 + 2] cycloaddition reaction between olefins.
Such a [2 + 2] cycloaddition reaction between the phenyl and
olefin groups encountered here is scarce in the solid state. One
such phenyl−olefin reaction has been reported in 66%
conversion as characterized by NMR spectroscopy.15 In this
work, although nicely aligned, the reaction between phenyl and
olefin bond is very sluggish between 4spy pairs, whereas the
reaction is facile when a fluorine is introduced in 4spy. This
unusual phenyl−olefin cycloaddition reaction results in the
formation of a highly strained bicyclic product at the expense of
the aromaticity of the phenyl ring in the solid state. The
importance of fluorinated benzene on the dynamics and
reactivity in the solid state has also been realized recently in
azobenzene isomerization reaction.30,31 A pair of pyridyl rings
in 2-hydroxyquinoline-4-carboxylate ligand has been found to
undergo dimerization during the hydrothermal synthesis of
metal−organic frameworks.32 The solution reaction is likely to
occur between a pair of olefin bonds in the tautomeric forms of
the ligand with the loss of aromaticity, rather than between two
aromatic pyridyl groups.
To shed light on the observed unusual [2 + 2] phenyl−olefin
photo-cycloaddition reaction, density functional theory (DFT)
calculations were performed with the M06-2X functional,33
using the Gaussian 09 program.34 To elucidate the π−π
stacking interaction between two 4spy ligands in crystals 1 and
4, the binding energies of a pair of 4spy and a pair of 2F-4spy in
both X-ray structures were evaluated at M06-2X/6-311+G-
(2df,p) + BSSE level. The π-stacking stabilization energies of
both types of dimer in two different geometries of stacking
arrangement, namely, face-to-face and displaced parallel, are
fairly large, between −29 and −35 kJ mol−1 (Figure S24 in
Supporting Information). In both cases, fluoro substitution
leads to a slight increase in interaction energy, by 3−5 kJ mol−1.
Interestingly, full geometry optimizations of a pair of free 2F-
4spy dimers yield geometries fairly close to the observed ligand
Figure 7. (a) HOMO of the lowest excited singlet state of 2F-4spy.
(b) HOMO of the ground state of 2F-4spy. Note the change of
HOMO orbital symmetry from symmetric in the ground state to
antisymmetric in the excited state for the two carbon atoms of
fluorophenyl moiety that forms cyclobutane of L1F.
alkene to give cyclobutane when the two olefin units are aligned
properly within the Schmidt’s radius. Furthermore, there is a
smaller but significant orbital contribution from the phenyl
moiety (Figure 7). Most importantly, the required antisym-
metric symmetry is satisfied at the C−C bond indicated for [2
+ 2] phenyl−olefin cycloaddition. Thus, our MO calculations
confirm that the phenyl−olefin photo-cycloaddition is
symmetry allowed.
Structural analysis of the excited singlet state geometries of
4spy and 2F-4spy indicates that the aromaticity of the phenyl
ring is reduced in both cases and to a greater extent in 2F-4spy,
as evidenced from the larger deviation of C−C bond lengths in
2F-4spy (Figure S26 in Supporting Information). This is
evidently a result of the participation of the pz orbital of the 2-
fluoro substituent in the π conjugation. As a consequence of the
loss of aromaticity of the phenyl ring by photo excitation, the
excited system is more susceptible to cycloaddition with olefins,
when the desired overlap at the ethylene moiety with another
ground state double bond is not available in the solid state. The
greater reduction of aromaticity of 2F-4spy in the excited
singlet state compared to the 4spy system may explain the
observed reactivity difference between the two systems. The
improved reactivity in 2F-4spy may also be attributed to the
fact that the phenyl-olefin distance is shorter in 2F-4spy than
4spy (3.63 versus 3.69 Å).
Dewar benzene is an interesting isomer of benzene that has
two highly strained nonconjugated double bonds.41 Hence, a
conceivable alternate reaction pathway for the formation of L1F
would involve one 2F-4spy isomerizing to a Dewar-benzene
isomer, followed by [2 + 2] cycloaddition to the ethylene
moiety of another 2F-4spy. For 2F-4spy, such a Dewar-benzene
isomer (Figure S27 in Supporting Information) is substantially
higher in energy, by 307.2 kJ mol−1, and the optimized
E
DOI: 10.1021/acs.cgd.5b00664
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
geometry is strongly distorted from the planar geometry
(Figure S27 in Supporting Information). Therefore, based on
reaction thermodynamics and crystal packing considerations, it
is less likely that the observed [2 + 2] cycloaddition occurs via a
Dewar-benzene intermediate.
charge on the ACS Publications website at DOI: 10.1021/
acs.cgd.5b00664.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: chmwmw@nus.edu.sg.
*E-mail: chmjjv@nus.edu.sg.
■
It is intriguing to observe that a second [2 + 2] phenyl−
olefin cycloaddition from L1F to yield L2F does not occur.
This can be explained by the fact that the conjugation which
enables activation of phenyl in 2F-4spy no longer exists upon
formation of L1F together with the increase in the distance
between the phenyl and olefin group after cyclobutane
formation. The nonconjugated fluorophenyl group is expected
to undergo photo excitation at wavelength significantly less
than 320−360 nm. This hypothesis is readily supported by TD-
DFT calculations of L1F and 2F-4spy (Supplementary Table
2). The lowest UV absorption wavelength with significant
oscillator strength of L1F is shorter by 32 nm compared to that
of 2F-4spy.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Education, Singapore for financial
support through NUS FRC Grant Nos. R-143-000-562-112 and
R-143-000-604-112, (Late) Prof. Lip Lin Koh, Ms. Geok Kheng
Tan and Ms. Yimain Hong for their help in X-ray data
collection.
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