Generation of thiyl radicals in a zinc(ii) porous coordination polymer by light-induced post-synthetic deprotection

Article abstract of DOI:10.1039/c8cc01837e

Kinetic stabilisation of unstable chemical species in nanospace is of potential importance in the field of materials and synthetic chemistry, and porous coordination polymers (PCPs) represent a facile platform to provide such reaction fields. Thiyl radica

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Generation of thiyl radicals in a zinc(II) porous coordination
polymer by light-induced post-synthetic deprotection
Shinpei Kusaka*,a Ryotaro Matsuda*,b,c and Susumu Kitagawa*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Kinetic stabilisation of the unstable chemical species in nanospace independent molecules. In this regard, the confinement of
is of potential importance in the field of material and synthetic thiyl radicals in nanospace could offer a strategy for their
chemistry, and porous coordination polymers (PCPs) represent a kinetic stabilisation by hampering the contact between them
facile platform to provide such reaction fields. Thiyl radicals are as well as the characteristic reactivity for processes such as
important reactive substances that often play a leading role in metal accumulation,6 oxygen trapping, thiol-ene click
organic and bioorganic chemistry. However, their generation in chemistry functionalisation and catalytic activation of olefins.5
nanospace has been barely investigated due to synthetic Nevertheless, the functionalisation of PCPs with thiyl
difficulties. Here, we report
a facile methodology for the substituents is extremely rare, even for a thiol as the more
fuctionalisation of active thiyl substituents on PCP pore surfaces stable derivative,6 which is probably due to their strong
using a post-synthetic protection and deprotection technique. The coordination ability with metal ions or dimerisation to
thiyl radicals were generated inside zinc(II)-based PCP upon the disulfides during the process of construction of PCPs (Figure 1).
deprotection of asymmetric disulfide, which was used as In the current study, we demonstrate
a post-synthetic
protecting group, by ultraviolet light irradiation.
protection and deprotection strategy to introduce thiyl
substituents on the pore surfaces of a zinc(II)-based PCP. The
protection and deprotection technique7 is a post-synthetic
method8 in which PCP is constructed using ligands that contain
protected functional groups, which is followed by their
subsequent deprotection. Although this technique has been
recognized as a powerful tool for the introduction of reactive
Porous coordination polymers (PCPs) or metal organic
frameworks (MOFs) are new types of porous materials that are
constructed using a self-assembling coordination process
between metal ions and organic linkers.1 Because of the
tunability of the pore size, the shape and the environment of
the framework, pores of PCPs are attracting significant
attention as unprecedented reaction fields2 since they exhibit
unique reactivity such as the template effect or size/shape
selectivity.3 In particular, PCPs can kinetically stabilise reactive
chemical species in their nanospace. Furthermore, the high
crystallinity of PCPs renders them to be good platforms to
investigate the reactivity of such chemical species using X-ray
crystallography, which is substantially challenging in the phase
of a homogeneous solution.4 Thiyl radicals are renowned
useful chemical species in material, organic and bioorganic
chemistry.5 One of the problems of these radical species is that
they are prone to undergo dimerisation in solution phase,
thereby limiting the development of various reactions used as
Figure 1 Protection and deprotection strategy for the generation of thiyl radicals inside
nanospace of PCP.
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SEC-1 obtained after soaking with N,N-dimethylformamide
(DMF) to exchange the crystal solvents
Figure 2 (a) Simplified crystal structure of SEC-1 (Zn-O SBUs are represented as purple
polyhedrons and substituents on ligands are omitted for clarity). (b) Zoom-up structure
at around 1D channel pore (carbon: grey, sulfur: yellow, zinc: purple, oxygen: red,
hydrogen atoms are omitted).
Figure 3 PXRD patterns of SEC-1 simulated from SXRD (simulated), DMF exchanged
SEC-1 (before UV irradiation) and SEC-2 (after UV irradiation).
functional groups, such as free amino7 or hydroxide7a,c,d groups
into PCPs, to the best of our knowledge, it has not been
applied for the introduction of sulfur analogue so far.
were well matched with the simulated pattern from SXRD
(Figure 3), although the patterns of the as-synthesized sample
were different (Figure S2), which was probably caused due to
the structural change induced by the mechanical stress and
the loss of guest solvents. It is worth mentioning at this point
that the direct synthesis of a thiol-containing PCP using ligand
Thioethers or thioesters are frequently used protecting groups
for thiols. However, their deprotection requires harsh
conditions, such as hydrolysis or treatment with alkaline metal
in liquid ammonia, that are incompatible with PCPs.9 To
circumvent this problem, we selected an asymmetric disulfide
as a protecting group since it can be mildly eliminated by
ultraviolet(UV)-light irradiation via the homoleptic cleavage of
a S-S bond10. This light-induced post-synthetic modification has
been reported to be successful in a few cases, including
chemically labile zinc(II)-based PCPs.7c,d Furthermore, we
anticipated that the disulfide unit would dissociate
homoleptically, thereby acting as an effective precursor for the
direct generation of thiyl radicals without derivatisation from
thiols.
1
was found to be unsuccessful. Treatment of thiol ligand 1
under the same synthetic condition as SEC-1 produced an
amorphous gel. Other reaction conditions were also
attempted; however, no crystalline material was obtained in
any case.
We further investigated the photo-cleavage deprotection of
disulfides. Crystals of SEC-2 were obtained by UV-light (350
nm) irradiation of a dispersion of single crystals of SEC-1 in
DMF for 48 h. Crystals of SEC-2 maintained their morphology
and transparency, although the colour turned to pale yellow
and the cracks somewhat increased (Figure S3). The reaction
proceeded in a single-crystal-to-single-crystal13 manner and
SXRD analysis was succeeded to reveal that the crystal unit cell
of SEC-2 slightly shrunk from that was observed in SEC-1 (Table
S1), whereas the whole framework structure was unaltered.
We selected 5-mercaptoisophthalic acid 111 as the building
block in which a methylthio group was introduced as the
protecting group by the treatment of
methylmethanethiosulfonate (see the
1
with S-
supporting
information). Disulfide functionalised PCP SEC-1 (sulfur-
embedded crystal) was synthesized from 5-(S-
methylthio)mercaptoisophthalic acid by heating
The PXRD pattern was also consistent with that of SEC-1
,
2
a
which indicated the formation of SEC-2 in the bulk phase
(Figure 3). The electron density corresponding to the –SMe
protecting groups decreased considerably, and the total
deprotection ratio of disulfides was crystallographically
estimated to be 60% (see the supporting information). We
additionally confirmed that the 1H NMR spectrum of the
digested SEC-2 crystals in CD3OD/DCl depicted signals
MeOH/H2O solution at 80 °C using zinc(II) acetate as the metal
source. A single crystal X-ray diffraction (SXRD) analysis (Figure
2) revealed that SEC-1 was developed by isophthalates
connected with Zn6O26 clusters as a secondary building unit
(SBU), which formed a 3D rectangular coordination network
observed in some previously reported PCPs with other
substituents.12 SEC-1 possessed 1D channel pores along the c-
axis in which all the disulfide bonds were exposed, and no
coordination of sulfur atoms to zinc(II) ions was observed.
Notably, the sulfur atoms of the thiophenol units are apart
from each other: The shortest atomic distance between sulfur
atoms was calculated to be 3.58 Å (Figure S1, see the
supporting information), which is much larger than that
depicted by the typical sulfur-sulfur single bond (~2.0 Å). This
would ensure the isolation of all sulfur atoms after
deprotection. The powder X-ray diffraction (PXRD) patterns of
corresponding to the isophthalic acid moiety of thiol
the main deprotected product along with 3,3’,5,5’-
diphenyldisulfidetetracarboxylic acid (22%), and other side
products could be hardly observed (Figure S4, S5). The
presence of ligand further proves the success of the
1 (29%) as
3
1
dissociation of the methylthio protecting groups. Since the
sulfur atoms are sufficiently distant from each other in the
crystal structure of SEC-2, the presence of disulfide
product may be caused due to the oxidation of
3
as minor
1
during the
treatment of SEC-2 and/or partial degradation of the
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framework, allowing the close contact of two sulfur atoms on the sulfur atom, indicating the formation of thiyl radicals.
the mercaptophenyl groups. The deprotection ratio that was These ESR signals were almost saturated within 1 h. Thiyl
determined from 1H NMR (51%) was observed to be radicals in the crystals were found to be thermally unstable at
approximately consistent with the result from SXRD. The peak room temperature and no ESR signal was obtained at room
assignable to the –SMe group was observed only for ligand
(Figure S6), supporting the absence of the dissociated irradiated sample displayed signals that could be attributable
protecting group in the crystal. to ligands and , whereas no free thiol was observed, which
2
temperature. Furthermore, the 1H NMR spectrum of the
2
3
Interestingly, the deprotection ratio was strikingly different indicates that all the dissociated disulfide bonds were
depending on the reaction centres (Figure S7) that ranged recombined. In addition, this result suggests that the diffusion-
from 0% to 100%. The difference in the deprotection ratio can elimination of the protecting group is the rate-determining
be explained in terms of the orientation of disulfide bonds step of the deprotection reaction.
around the pore. For example, S5 and S15 act as “high reactive Moreover, we found that the light-induced cleavage of
sites” and are found to be highly deprotected (71% and 100%, disulfides proceeded in a much cleaner manner inside the PCP
respectively) in SEC-2, on the other hand, S3 and S13 are “low than that is observed in a solution. A pre-degassed solution of
reactive sites” (0% and 37%, respectively) (Figure S8). Thus, ligand
when the –SMe protecting groups are sufficiently exposed to 1H NMR spectrum of the resulting solution indicated that the
the pore, their diffusion elimination and the subsequent main product was thiol However, many unidentified
2 in DMF-d7 was irradiated by 350 nm light for 24 h. The
1
.
deprotection process can take place easily. However, when products were also observed (Figure S13) whose formation
they are located in the opposite direction, the interruption of may be attributable to the reaction of the highly reactive thiyl
the solvent molecules between the two sulfur atoms is radical with the aromatic rings or to the formation of
hindered. Furthermore, since the steric hindrance in the polysulfides. In the case of the dissociation reaction in SEC-1
,
nanospace could prevent free bond rotation, the deprotection the mercaptophenyl groups generated during the reaction
reaction could not proceed efficiently. The high deprotection were spatially separated from each other, and the reactive –
ratio of some sulfur atoms indicates that UV-light transparency SMe protecting groups were eliminated by diffusion from the
is sufficient to dissociate all the disulfide bonds in the crystal. crystal to the solution, thereby preventing undesirable side
The UV-light is likely to penetrate deep inside SEC-1 (Figure S9) reactions. This result demonstrates that the confinement of
because the wavelength of 350 nm is near the absorption edge thiyl radicals in
of ligand . The process of the deprotection reaction was compared to that in the solution state.
monitored by altering the irradiation time (Figure S10). The To summarize, we performed the UV-light-induced
reaction was observed to proceed rapidly during the early dissociation of disulfides inside zinc(II) PCP with the
a nanospace causes unique reactivity
2
a
stage (11% within first 1 h); however, the reaction speed concomitant generation of thiyl radicals on pore surfaces. The
gradually decreased. Finally, the reaction rate was almost distinctive reactivity that was characterised by the
saturated at 55% after 4 days of irradiation. Additionally, 2 confinement of the thiyl radicals in the nanospace was also
days UV-light irradiation using the ground sample of SEC-1 demonstrated. We would also like to highlight that this study is
provided an almost identical result to that obtained from the the first example in which reactive sulfur species are
as-synthesized SEC-1 crystals, indicating that the crystal size introduced in a PCP using chemically labile metal as the node.
has a minor influence on the final product (Figure S10). By We are confident that this work will help the development of
considering these results and the SXRD analysis, it can be new functionality of thiyl radicals as functional materials or
concluded that the insufficient UV-light activation of catalysts to perform various reactions in nanospace.
disulphides cannot be invoked to explain the incomplete This research was supported by the ACCEL (JPMJAC1302) and
deprotection reaction. Instead, the shrinkage of the PRESTO (JPMJPR141C) from the Japan Science and Technology
framework is most likely to be responsible for the suppression Agency (JST), and KAKENHI Grant-in-Aid for Specially Promoted
of the elimination process of the protecting group. We Research (25000007) from the Japan Society of the Promotion
confirmed the permanent porosity of both SEC-1 and SEC-2 by of Science (JSPS).
CO2 adsorption study, although the adsorption amount SEC-2
is lower than SEC-1 due to collapse of the framework upon
Conflict of interest
guest solvent removal (Figure S11). We also note that the
deprotection reaction can proceed under other solvents and
There are no conflicts of interest to declare.
confirmed that the use of methanol has the similar
deprotection product ratio to DMF.
We further conducted an electron spin resonance (ESR) study
Notes and references
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TOC Figure
Generation of highly reactive sulfur species in nanospace is
demonstrated via the photo-dissociation of disulfides.
4 | J. Name., 2012, 00, 1-3
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