Solid state photodimerization of trans-2-(4-pyridyl)-4-vinylbenzoic acid via salt formation and isomerisation of cyclobutane compounds in solution

Article abstract of DOI:10.1039/c2ce26086g

Several salts of trans-2-(4-pyridyl)-4-vinylbenzoic acid (HPVBA) obtained by reacting with inorganic acids and organic diamines are discussed in the context of their solid state packing and photoreactivity. The acids CF ₃CO₂H, HClO

Full text of DOI:10.1039/c2ce26086g

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PAPER
Solid state photodimerization of trans-2-(4-pyridyl)-4-vinylbenzoic acid via
salt formation and isomerisation of cyclobutane compounds in solution{
Goutam Kumar Kole, Geok Kheng Tan and Jagadese J. Vittal*
Received 7th July 2012, Accepted 14th August 2012
DOI: 10.1039/c2ce26086g
Several salts of trans-2-(4-pyridyl)-4-vinylbenzoic acid (HPVBA) obtained by reacting with inorganic
acids and organic diamines are discussed in the context of their solid state packing and
photoreactivity. The acids CF3CO2H, HClO4, HNO3, H2SO4 produced salts where H2PVBA+ cations
are oriented parallel in head-to-tail (HT) fashion, however, the trifluoroacetate salt did not undergo
photodimerization. The head-to-tail (HT) parallel orientation was inversed to head-to-head (HH)
orientation by incorporating excess H2SO4 during salt formation. Thus two stereoisomers viz. HT-
and HH-dimers were obtained from salts derived from same salt-former. Salts were also made
exploiting carboxylic acid functionality by employing ethylenediamine, 1,3-diaminopropane, 1,4-
diaminobutane and it was found that only the salt obtained with 1,4-diaminobutane undergo
photodimerization. In an analogy, it is shown that in spite of poor predictability, the strategy of salt
formation works well and has significant impact in the synthesis of functional cyclobutane
derivatives.
trans-cinnamic acid and related compounds, mainly symmetric
Introduction
olefin compounds have been engineered in the crystalline state.
Research on combining two photoreactive CLC bonds into a
cyclobutane ring has become a focused area in crystal engineer-
However, unsymmetric olefins are rather more interesting and
challenging where two possible orientations viz. head-to-head
ing, organic photochemistry and solid state chemistry.1
(HH) and head-to-tail (HT) can be controlled by applying crystal
engineering principles.4h,4j,8 Recently, MacGillivray et al. have
Although there are some exceptions, it is an accepted fact that
the photodimerization reaction happens between two CLC
reported that two unsymmetric olefins namely trans-3-(49-pyr-
˚
bonds aligned parallel and separated within 4.2 A (Schmidt’s
idyl) acryclic acid (4-PA) and HPVBA were difficult to co-
crystallize with resorcinol-derived templates due to synthon
competition in the presence of carboxylic acid groups, a
competitor of phenols for hydrogen bonding with pyridyl
groups.9 In addition, only HH-dimer of an unsymmetric olefin
can be obtained from co-crystals using resorcinol-type templates.
In our early contribution, we have reported the strategy of salt
formation at pyridyl site with strong inorganic acids for the
synthesis of both HH- and HT-dimers of 4-PA.4h,4j The parallel
orientation of pyridyl cations in HT-fashion is stable due to
cation2p interaction,4d–f,10 whereas, the HH-arrangement is
quite unstable due to cation–cation repulsion which could be
stabilised by the presence sulphate bi-anion. Unsymmetrical
pyridyl carboxylic acids like 4-PA and HPVBA have another
advantage of forming salts with strong amines, in addition to
with inorganic acids, utilizing their carboxylic acid functionality.
In this study, we have extrapolated the strategy of salt
formation with HPVBA, a relatively longer olefin has been
chosen to allocate hetero-functionality to the cyclobutane ring,
in order to obtain both the HH- and HT-dimer via photodimer-
ization reaction of its salts. We have exploited both the pyridyl
and carboxylic acid functionality to make its salts with strong
inorganic acids as well as with organic amines, respectively.
criteria).2 The contact geometry of two reacting bonds is crucial
for the formation of the right isomer of the product. The
understanding of crystal engineering principles and their proper
utilizations have resulted in parallel arrangements of CLC bonds
suitable for photodimerization reaction. In this regard, several
co-crystals,3 organic salts,4 discrete5 and polymeric6 coordina-
tion compounds have extensively been studied. Another impor-
tant aspect of studying these systems, apart from their solid state
packing, is accessing novel cyclobutane compounds with specific
geometry and desired functional groups which can be utilized as
potential ligands for making metal–organic materials.4g,7
Among the hydrogen-bonded co-crystals studied so far,
resorcinol is the mostly employed template for aligning CLC
bonds of olefin containing pyridyl groups.3a–f The CLC bonds of
certain dicarboxylic acids have also been aligned parallel using
template containing pyridyl groups and vice versa.3g–i Other than
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, -117543, Singapore. E-mail: chmjjv@nus.edu.sg;
Fax: +65 67791691; Tel: +65 65162975
{ Electronic supplementary information (ESI) available: Experimental
details, TGA plots, NMR spectra, UV-Vis, and PL spectra. CCDC
reference numbers 888316–888319. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2ce26086g
7438 | CrystEngComm, 2012, 14, 7438–7443
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Perchlorate salt of HPVBA (2)
Results and discussion
Single crystal X-ray analysis revealed that the salt of composi-
HPVBA was allowed to react with various strong acids and
amines. All the molecular salts were synthesized from aqueous
media and the diffraction quality single crystals were grown by
slowly evaporating their alcoholic solutions.{ The yields of these
crystallization processes vary in the ranges of 90–95%.
tion (H2PVBA)ClO4?MeOH (2) crystallizes in the triclinic space
¯
group, P1. Various types of hydrogen bonding interactions
…
…
…
including O–H O, N–H O, C–H O play crucial roles to
construct the solid state structure. The pyridinium cations form
2
charge assisted hydrogen bonding with one ClO4 anion in
2
2
R2 (7) fashion and interact with another ClO4 ion in C(2)
fashion via weak C(a)-H bonds.11 The carboxylic acid groups are
involved in hydrogen bonding with methanol molecules, in
Trifluoroacetate salt of HPVBA (1)
The single crystals of anhydrous salt of composition
(H2PVBA)(CF3CO2) (1) were grown by evaporating methanolic
solution slowly and the structure was refined in triclinic space
4
R4 (12) fashion to form a carboxylic acid dimer synthon (via
methanol). A thorough perusal of the crystal structure reveals
that the H2PVBA+ cations are aligned parallel in head-to-tail
¯
group, P1. As expected, the pyridyl group of HPVBA was found to
be protonated. Various kinds of charge assisted hydrogen bonding
˚
fashion with a distance of 3.837 A between two CLC bonds
(Fig. 3). Upon irradiation under UV light for 40 h, the single
…
like N–H O, O–H O, C–H O play important roles to construct
…
…
the solid-state structure as shown in Fig. 1. One oxygen atom of the
+
TFA anions form N–H O hydrogen bonds to a H2PVBA cation
…
and another oxygen form O–H O hydrogen bonds to carboxylic
acid group of another H2PVBA+ cation and therefore, clipping
crystals of this salt were found to furnish only 80% photo-
1
dimerization as monitored by H NMR spectroscopy from the
…
appearance of a multiplet peak for cyclobutane protons to d
5.01 ppm. The incomplete conversion might be due to the change
in the crystal structure accompanied by the loss of MeOH
solvent molecules in the process of dimerization. When the salt
of the dimer compound was neutralised with aqueous NaOH, the
peak for cyclobutane protons shifts at d 4.69 (m) ppm but two
doublet of doublet peaks were not observed as they have been
generally observed for rctt-HT-4,4-BPCD.4h,4j This may be due
to the fact that the cyclobutane protons with 4-pyridyl and
4-carboxyphenyl substituent are almost degenerate (see ESI{).
The photoreaction of this salt can be employed to synthesize a
new cyclobutane derived ligand viz. rctt-1,3-bis(49-carboxyphe-
nyl)-2,4-bis(499-pyridyl)cyclobutane or rctt-HT-4,4-BCBPCB.
them in parallel orientation in a head-to-tail fashion (Fig. 2). The
˚
distance between the CLC bonds is found to be 4.235 A and the salt
was found photostable under UV light although the distance is
almost at the borderline of Schmidt’s distance criteria.2
Fig. 1 Various kinds of supramolecular interactions in 1.
Fig. 3 The head-to-tail parallel arrangement of H2PVBA+ cations in 2.
The C–H hydrogen atoms are omitted.
Fig. 2 The head-to-tail parallel orientation of H2PVBA+ clipped by
TFA anions. The C–H hydrogen atoms are omitted.
Another interesting point to note is that methanol molecules
present in the structure have a significant role in the packing of
H2PVBA+ cations. The same salt when crystallized from ethanol
and 1,4-dioxane separately were both found not to undergo any
photodimerization. The single crystal obtained from ethanol was
too small for the intensity data collection. The rapid loss of 1,4-
dioxane from single crystals when taken out of the mother liquor
also prevented us from determining its structure. However, both
of their compositions were determined by elemental analysis and
TGA to be (H2PVBA)ClO4 (3) (as 1,4-dioxane is lost from the
system). The photostability of this anhydrous salt (obtained
from ethanol) and the photoreactivity of the solvated salt
(containing methanol) clearly indicate that methanol played
some template effect during crystallization which led to the
parallel arrangement of H2PVBA+ cations. Therefore, the partial
{ Crystal data for 1 at 100(2) K: C16H12F3NO4, M = 339.27, triclinic, space
¯
˚
group P1, a = 7.2580(9), b = 8.4271(10), c = 12.6094(15) A, a = 77.860(2)u, b =
75.529(2)u, c = 72.829(3)u, V = 705.67(15) A , Z = 2, Dc = 1.597 g cm23, m =
3
˚
0.140 mm21, Goof on F2 = 1.091, final R1 = 0.0578, wR2 = 0.1395 [for 2895
data I > 2s(I)]. Crystal data for 2 at 223(2) K: C15H16ClNO7, M = 357.74,
¯
˚
triclinic, space group P1, a = 8.6037(11), b = 9.5432(12), c = 10.2702(13) A, a
3
˚
= 93.248(2)u, b = 101.615(2)u, c = 107.129(2)u, V = 783.27(17) A , Z = 2, Dc =
1.517 g cm23, m = 0.283 mm21, Goof on F2 = 1.221, final R1 = 0.0784 wR2 =
0.2203 [for 3008 data I > 2s(I)]. Crystal data for 4 at 223(2) K:
¯
C56H52N4O21S3, M = 1213.20, triclinic, space group P1, a = 11.1151(9), b =
˚
11.8964(9), c = 22.1546(18) A, a = 91.414(2)u, b = 98.911(2)u, c = 110.246(2)u,
V = 2705.6(4) A , Z = 2, Dc = 1.489 g cm23, m = 0.224, mm21, Goof on F2 =
3
˚
1.035, final R1 = 0.0652, wR2 = 0.1352 [for 8465 data I > 2s(I)]. Crystal data
for 8 at 100(2) K: C40.75H39N4O6.75, M = 692.76, monoclinic, space group
˚
P21/n, a = 10.1643(11), b = 10.1819(10), c = 34.305(4) A, b = 95.067(3)u, V =
3
3536.4(6) A , Z = 4, Dc = 1.301 g cm23, m = 0.089 mm21, Goof on F2 = 1.072,
˚
final R1 0.0861 =, wR2 = 0.2053 [for 3999 data I > 2s(I)].
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conversion (80%) can be accounted to the loss of MeOH solvent
from the single crystals during the process of photodimerization.
light for 50 h, we observed only about 25% photodimerization in
1H NMR spectroscopy leading to the formation of HT-BCBPCB
(Fig. 5).
Sulphate–bisulphate salt of HPVBA (4)
When HPVBA was reacted with an excess of H2SO4 in water,
an intense yellow coloured solid was obtained (5). Surprisingly,
this yellow solid furnished 80% photodimerization in head-to-
head fashion upon irradiation under UV light for 50 h, which
was confirmed by 1H NMR spectroscopy (Fig. 5). The formation
of HH-dimer 1,2-bis(49-carboxyphenyl)-3,4-bis(499-pyridyl)cy-
clobutane or rctt-HH-4,4-BCBPCB was confirmed by the
presence of two doublet peaks at d 4.93 and 4.83 ppm, which
HPVBA was also reacted with H2SO4 in methanol and the single
crystals obtained were tested for their photoreactivity. The
hydrated salt of composition (H2PVBA)4(HSO4)2(SO4)?H2O (4)
¯
crystallizes in the triclinic space group, P1. The asymmetric unit
contains one formula unit namely, four H2PVBA+ cations, two
bisulphate anion, one sulphate anion and one water molecule. As
expected, various types of neutral and charge assisted hydrogen
1
can be correlated with the H NMR spectrum of rctt-HH-4,4-
…
…
bonding like N–H O and O–H O are present in the solid-state
structure as shown in Fig. 4. The carboxylic acid groups of
H2PVBA+ cations form carboxylic acid dimer homosynthon
BPCD discussed in our previous papers.4h,4j When this UV
irradiated product was neutralised with NaOH (aq.), these two
doublet peaks coalesce to a single doublet peak at d 4.68 ppm.
The composition of this precipitated yellow powder was
determined by TGA and elemental analysis to be
(H2PVBA)3(HSO4)3(H2SO4)(H2O)6.5 (see experimental section
for details{). Therefore, the amount of H2SO4 added during salt
formation is very crucial and led to a different packing of
H2PVBA+ cations and entirely opposite photoreactivity. The
role of extra H2SO4 in the packing of H2PVBA+ cations is not
clearly understood due to the lack of a single crystal structure.
To the best of our knowledge this type of control over the
orientation of the CLC bonds and the resultant stereochemistry
of the products in photodimerization reaction just by changing
the composition of the salt was not known before.
2
(R2 (8)). The water molecule present in this salt also donates
hydrogen bonding to sulphate and bisulphate anions and accepts
from the protonated pyridyl group (H–N) of H2PVBA+ cation
4
and forms an H-bonded ring with graph set R6 (16). The C(a)–H
of H2PVBA+ cations also interact with the neighbouring
sulphate/bisulphate anions but are not shown in the Fig. 4 for
clarity.
Now if we look at the relative orientations of H2PVBA+
cations, the combination of both the head-to-head and head-to-
tail manner are observed. All four H2PVBA+ cations in the
asymmetric unit, labelled as A–D in Fig. 4, are oriented in HH-
fashion with distances between CLC bonds of 4.617, 4.772 and
˚
5.304 A. All these distances are not favourable for photodimer-
ization reaction. However, when we analyze the relative
orientations of H2PVBA+ units which are related by a centre
of inversion, we see that they (A and the one below in Fig. 4) are
orientated parallel in an HT-fashion. The distance between CLC
Photoreactivity of salts of HPVBA with HNO3 and HCl
HPVBA was also reacted with HNO3 and HCl and the resulting
yellow crystalline samples were studied for their photoreactivity.
We could not grow diffraction quality single crystals for both but
the composition of these salts were determined by elemental
analysis and TGA. An anhydrous salt of composition
(H2PVBA)NO3 (6) was synthesized with HNO3, which under-
went quantitative photodimerization upon irradiation under UV
˚
bonds is 3.772 A, which is well within the range of Schmidt
criteria. Therefore, among the four H2PVBA+ cations in the
asymmetric unit one is expected to be photoreactive and the
compound is supposed to show 25% photodimerization reaction
in HT-fashion. When this compound was irradiated under UV
Fig. 4 The crystal structure of sulphate–bisulphate salt of HPVBA, 4. The C–H hydrogen atoms are omitted.
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Fig. 5 1H NMR spectra of 4 before (a) and after (b) UV irradiation; (c) for 5 after UV irradiation and recrystallization.
light for 30 h. The absence of peaks for ethylenic protons and
two carboxylate–olefins into a parallel arrangement. However,
we are unable to verify this experimentally.
the presence of a multiplet peak for cyclobutane protons at d
1
5.04 ppm in H NMR spectrum confirm the formation of HT-
A comparative discussion
dimer in quantitative yield (see ESI{). On the other hand, the
anhydrous salt obtained by reacting with HPVBA and HCl in
methanol was found to be photostable.
Significant success has been achieved in the design of photo-
reactive co-crystals. However, there has been a little progress in
understanding the nature of supramolecular interactions and
solid state packing of the salts. Among the salts described above,
the photoreactivity was observed randomly in an unpredictable
manner in two ways. The first priority is to make the
photoreactive salt and then the second concern is controlling
Photoreactivity of salts of HPVBA with diamines
The utilisation of –CO2H groups in various photoreactive olefins
for making molecular salts and their photoreactivity is also
reported in literature by us and others.4a–c,4g,4i,4k Here, we
attempted to exploit the carboxylic acid functionality of HPVBA
by deprotonating the acidic proton by ethylenediamine, 1,3-
diaminopropane and 1,4-diaminobutane. The flaky crystals
obtained from these three diamines were studied for their
photoreactivity and we found only the salt with 1,4-diaminobu-
tane (7) underwent about 75% photodimerization in HT-fashion
after UV irradiation for 40 h.
2
the orientation i.e. whether in HH- or HT-fashion. In CF3CO2
salt, two H2PVBA+ cations were clipped nicely, however the salt
2
was photostable. On the other hand, ClO4 did not clip two
H2PVBA+ cations, but the parallel arrangement was observed
without any clipping and it was photoreactive in HT-fashion.
The presence of methanol solvents in perchlorate salt has also
been shown to have very crucial role for this salt to be
photoreactive. It was reported earlier that the hydrochloride
salts were photoreactive in HT-fashion for 4-PA and 4-styr-
ylpyridine.4f,4j However, this was not the case for HPVBA. The
nitrate salt of 4-PA was photostable but the same salts of
4,49-bpe4j,12 as well as HPVBA were found to be photoreactive.
All these observations clearly indicate that it is quite a
formidable task to predict the photoreactivity in the salts of
pyridyl derivatives containing olefins with inorganic acids. Not
only the pyridyl salts of inorganic acids but also the carboxylate
salts of organic amines are difficult to be predicted or designed as
we can see the results for ethylenediamine, 1,3-diaminopropane,
1,4-diaminobutane etc. It is also important to note that in many
cases the salts are hydrated (or solvated) and the solvents have a
crucial role in the photoreactivity. The reason behind this can
Ethylenediamine is known to template trans-cinnamic acid
analogues that undergo photodimerization.4a We also have
shown that 1,3-diaminopropane can template H2SDC for the
same purpose but not ethylenediamine or 1,4-diaminobutane.4g,4i
Here, we observe that for HPVBA, 1,4-diaminobutane serves as
a template but not the other two amines in the series. To our
disappointment, all the crystals isolated were flaky and not
suitable for intensity data collection to solve the structure by
X-ray crystallography. However, these observations clearly
indicate that there is definitely some correlation of crystal
packing with the chain length of diamines and olefinic spacers.
+
Any two hydrogen atoms in the –NH3 groups can donate
hydrogen bonding to two carboxylate groups, thereby clipping
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stem from the fact that the charge assisted hydrogen bonding in
salts is less directional in nature compared to neutral hydrogen
bonding in co-crystals. In addition to hydrogen bonding interac-
tions, electrostatic interactions also influence solid state packing of
salts. Therefore, the bonding interaction can form within a wide
range of solid angles which causes them to be less predictable.13
One might argue that aligning two photoreactive CLC bonds
in a parallel orientation in a salt is nothing but screening (trial
and error) and may not be achieved by design. We expected HH-
dimer from sulphate–bisulphate salt of HPVBA, however it
surprisingly resulted in HT-dimer in this case; yet the HH-dimer
was obtained serendipitously by incorporating excess H2SO4
during crystallization. Moreover, both the HH- and HT-dimers
were obtained from the series of these salts by tuning the salt
former and solvent of crystallization. Although, the predict-
ability of solid state structure of salts is poor, salts have several
advantages such as a) robust structure, b) higher thermal
stability, c) better water solubility and d) ease of separation of
the products after photodimerization just by simple acid–base
work up. In addition, the pyridyl containing cyclobutane
compounds undergo isomerization from rctt- to rcct- isomers
in solution providing access to more cyclobutane compounds as
ligands. Therefore, organic salts deserve more attention espe-
cially in the solid state photodimerization and isomerisation
reactions in order to assess their usefulness in green synthesis.
employed to make co-crystals with dipyridyl spacer compounds,
however we were only successful in obtaining the single crystals
of (HT-4,4-BPBCCB)(4,49-bpe) from the evaporation of an
equimolar solution of 4,49-bpe and HT-4,4-BPBCCB in 1 : 4
DMF and methanol mixture. Single crystal X-ray diffraction
analysis revealed that HT-4,4-BPBCCB adopted rctt-conforma-
tion in the co-crystal (HT-4,4-BPBCCB)(4,49-bpe)(MeOH)0.75
(H2O)2 (8). From the observed values of CLO distances for two
carboxylic acid groups [CLO distances are 1.23(5) and 1.21(5)
˚
A, respectively, and both the corresponding C–OH distances
˚
are 1.32(5) A], it can certainly be called a co-crystal and not a
salt. The two carboxylic acid groups donate H-bonding to two
pyridyl groups in R22(7) and C(2) fashion as shown in Fig. 6.
The keto oxygen of the carboxylic acid group interacts with
…
˚
methanol molecules with O O distance of 2.946 A but with
…
O–H O angle of 115.6u, this may not be significant for
H-bonding. But, one pyridyl group accepts H-bonding from a
…
methanol molecule with N O distance of 2.77 A and N H–
…
˚
O angle of 137.2u. Meanwhile the other pyridyl group has
…
interaction with water molecule maintaining N O distance of
˚
2.86 A, but these interactions may not be significant as the
…
N
H–O angles are 87.7u and 75.9u. Similarly, the distance
…
˚
between two water molecules (O O distance of 2.88 A)
suggest bonding interaction, but the positions of H atoms do
not. Among two water molecules present, the hydrogen atoms
of one could not be located. As the growth of H-bonded
network is interrupted by the solvent molecule, the overall
structure of this co-crystal was determined to be one
dimensional.
Co-crystal of rctt-HT-4,4-BPCD with 4,49-bpe
For crystallographic characterization, both these cyclobutane
compounds rctt-HH-4,4-BPBCCB and HT-4,4-BPBCCB were
Fig. 6 One dimensional hydrogen bonded chain of 8. The C–H hydrogen atoms are omitted.
Scheme 1 The structures of the rctt- and rcct-isomers of HT and HH-4,4-BCBPCB.
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ˇ ´
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Isomerisation of HH-BCBPCB and HT-BCBPCB
The acid catalysed isomerisation of rctt-4,4-tpcb to rtct-tpcb and
the isomerisation of rctt-HH-4,4-BPCD and rctt-HT-4,4-BPCD
to their corresponding rcct-isomers have been reported from our
laboratory4j,7b,14 and by Bricen˜o.5e,15 The above two cyclobutane
compounds were also observed to undergo similar isomerisation
from rctt- to rcct- isomers under acidic pH, which was observed
to be faster upon heating. The 1H NMR spectrum confirming the
isomerisation for HH-dimer is shown in ESI{ and the structures
of the corresponding rcct-isomers are shown in Scheme 1. Two
doublet peaks for cyclobutane protons for rctt-HH-4,4-BCBPCB
converts to four triplet peaks upon heating, which indicate that
the degeneracy of the cyclobutane protons is lifted due to
isomerisation to rcct-isomer (symmetry reduced). We also
observed that the isomerisation is not quantitative because the
product rcct-isomer is less stable than rctt-isomers as there are
three substituents on the same side of the cyclobutane ring
resulting in steric hindrance. The rctt-HT-4,4-BCBPCB also
undergoes similar isomerisation under similar conditions as was
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ˇ ´
1
observed in H NMR spectroscopy (see ESI{).
Conclusion
We have discussed several salts of HPVBA obtained by reacting
with inorganic acids and organic diamines, and they were
investigated for their photoreactivity. Both the HH- and HT-
dimers of this unsymmetric olefin were obtained stereoselec-
tively. It has been shown that the solvents present in the salts
play a crucial role in their photoreactivity. More interestingly,
both the HT- and HH-dimers could be obtained from same salt
former, H2SO4, just by changing its concentration during
crystallization. Such flipping of orientation from HT- to HH-
arrangement by single salt former was not known before.
Although the predictability of solid state structure of salts is
poor, it is shown that the strategy of salt formation works well in
organic synthesis and it warrants much more attention to
understand how two ionic counterparts combine and construct
the solid state packing in salts to gain credibility in crystal
engineering.
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Acknowledgements
We gratefully acknowledge the Ministry of Education,
Singapore, for financial support through NUS FRC Grant
R-143-000-439-112. We sincerely thank Hong Yimian for her
kind help with the X-ray crystallography.
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