Water-Soluble Redox-Active Cage Hosting Polyoxometalates for Selective Desulfurization Catalysis

Article abstract of DOI:10.1021/jacs.8b00394

Transformations within container-molecules provide a good alternative between traditional homogeneous and heterogeneous catalysis, as the containers themselves can be regarded as single molecular nanomicelles. We report here the designed-synthesis of a water-soluble redox-active supramolecular Pd₄L₂ cage and its application in the encapsulation of aromatic molecules and polyoxometalates (POMs) catalysts. Compared to the previous known Pd₆L₄ cage, our results show that replacement of two cis-blocked palladium corners with p-xylene bridges through pyridinium bonds formation between the 2,4,6-tri-4-pyridyl-1,3,5-triazine (TPT) ligands not only provides reversible redox-activities for the new Pd₄L₂ cage, but also realizes the expansion and subdivision of its internal cavity. An increased number of guests, including polyaromatics and POMs, can be accommodated inside the Pd₄L₂ cage. Moreover, both conversion and product selectivity (sulfoxide over sulfone) have also been much enhanced in the desulfurization reactions catalyzed by the POMs@Pd₄L₂ host-guest complexes. We expect that further photochromic or photoredox functions are possible taking advantage of this new generation of organo-palladium cage.

Full text of DOI:10.1021/jacs.8b00394

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A Water-Soluble Redox-Active Cage Hosting
Polyoxometalates for Selective Desulfurization Catalysis
Li-Xuan Cai, Shao-Chuan Li, Dan-Ni Yan, Li-Peng Zhou, Fang Guo, and Qing-Fu Sun
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00394 • Publication Date (Web): 13 Mar 2018
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A Water-Soluble Redox-Active Cage Hosting Polyoxometalates for
Selective Desulfurization Catalysis
Li-Xuan Cai,† Shao-Chuan Li,†,‡ Dan-Ni Yan,†,§ Li-Peng Zhou,† Fang Guo,‡ Qing-Fu Sun*†
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†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou 350002, PR China
‡College of Chemistry, Liaoning University, Shenyang 110036, PR China
§University of Chinese Academy of Sciences, Beijing 100049, PR China
KEYWORDS. Self-assembly; Cage; Redox; Water-soluble; Encapsulation; Desulfurization.
ABSTRACT: Transformations within container-molecules provide a good alternative between traditional homogeneous and heter-
ogeneous catalysis, as the containers themselves can be regarded as single molecular nanomicelles. We report here the designed-
synthesis of a water-soluble redox-active supramolecular Pd4L2 cage and its application in the encapsulation of aromatic molecules
and polyoxometalates (POMs) catalysts. Compared to the previous known Pd6L4 cage, our results show that replacement of two cis-
blocked palladium corners with p-xylene bridges through pyridinium bonds formation between the 2,4,6-tri-4-pyridyl-1,3,5-triazine
(TPT) ligands not only provides reversible redox-activities for the new Pd4L2 cage, but also realizes the expansion and subdivision
of its internal cavity. An increased number of guests, including polyaromatics and POMs, can be accommodated inside the Pd4L2
cage. Moreover, both conversion and product selectivity (sulfoxide over sulfone) have also been much enhanced in the desulfuriza-
tion reactions catalyzed by the POMs@Pd4L2 host-guest complexes. We expect that further photochromic and/or photoredox func-
tioins are possible taking advantage of this new generation of organo-palladium cage.
bility of the known container-molecules. POM@cage complexes
have been rarely reported5a,8a,9 and to the best of our knowledge,
catalytic activity of such host-guest complexes has never been
explored.
INTRODUCTION
Development of functional container-molecules has received
increased attention in the past decades, due to the unique proper-
ties and behaviors of the guest molecules confined within the
nanospace.1 Guest encapsulations within the nanocontainer are
mainly through noncovalent interactions, such as electrostatic
attraction, hydrophobic effect, π-π stacking, hydrogen bonding et
al.2 Thus, rational design and modulation on the size, shape, and
the electronic properties of the nano-containers are of key im-
portance in order to realize binding/catalytic transformations of
guest species inside the confined space.2e,3
SCHEME 1. Pd4L2 Nanocage and its near relative Pd6L4 cage.
Pyridinium functionalization has been proven as a promising
and powerful approach in the construction of multi-cationic self-
assembled host.4 Pioneering works by Stoddart and coworders5
have elucidated the vital role of pyridinium moieties in establish-
ing the functions of macrocycles and cages because of their
unique electrostatic and redox properties, like CBPQT4+
，
Herein, we report the self-assembly, photochromic, redox and
host-guest functions of a Pd4L2-type nanocapsule (2) made of four
cis-blocked palladium corners and two pyridinium-functionalized
bis-bidentated ligand (1), which is synthesized from two 2,4,6-
ExnBox4+ ， ExCage6+ ， BlueCage6+. Encapsulation of cy-
clobis(paraquat-p-phenylene) (CBPQT4+) by the Pd12L24 molecu-
lar flask charged with endohedral 1,5-dioxynaphthalene (DNP)
units has been demonstrated, where electrolyte stimulus can act as
a gate leading to emergent binding properties.6 Recently, Yoshi-
zawa et al has reported a Pd2L4 molecular capsule enclosed by
eight redox-active dihydrophenazine panels, which can be con-
verted into a stable tetra-radical-cationic capsule by electrochemi-
cal or chemical oxidation.7
tris(4-pyr-idyl)-1,3,5-triazine (TPT) with
a p-xylene linker
(Scheme 1). As a near relative of the previous Pd6L4 cage (3) re-
ported by Fujita group,2e, 10 the present Pd4L2 cage keeps highly
+12 charged thus is expected to be highly water-soluble. Moreo-
ver, introduction of pyridinium moieties not only enhances the
electron-deficient nature of the TPT panels, but also imparts pho-
tochromic and redox activities into the cage. Meanwhile, the cavi-
ty of 2 is also expanded by insertion of two p-xylene spacers. All
the above characteristics are subsequently used to the guest-
encapsulation studies of 2 toward a series of neutral aromatic
compounds and anionic POM clusters. It is noticed that in case of
POMs@2, the host-guest complexes can still have extra space for
Polyoxometalates (POMs) are a unique family of discrete
metal−clusters and have various applications ranging from cataly-
sis, medicine, electrochemistry, photochromism, to magnetism.8
However, it is still a big challenge to introduce POMs into a su-
pramolecular host, as a result of the small cavities and the insta-
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aromatic thiolester substrates, while no ternary complexes are
cis-capped by bpy ligands. As depicted in Figure 1A, four Pd(II)
ions sitting in the middle the diagonal Pd-Pd distance is 1.93 nm,
which is consistent with the known crystal structures of Pd6L4
cage 3.10c,10d,11 However, the cavity of cage 2 is expanded due to
the introduction of two p-xylene spacers, which could be regarded
as two frusta solid packed together in the bottom-to-bottom man-
ner. The center-to-center distance between the two truncated ben-
zenoid planes is ca. 2.04 nm (Figure S10). Based on such estima-
tion, the cavity of cage 2 is ca. 1.8 times larger than the par-
ent cage 3. Such a great size-expansion, along with the formation
of strongly electron-deficient pyridinium rings on the TPT panels,
are expected to exert dramatic influence on the host-guest proper-
ties of the cage.
observed in case of POMs@3. Desulfurization catalytic properties
of the POMs@2 are then demonstrated, featuring much improved
conversion and product (sulfoxide/sulfone) selectivity.
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RESULTS AND DISCUSSION
Self-assembly and X-ray crystal structure of cage 2. Ligand 1
-
(BF4 salt) was obtained in 87.5% yield by heating 3.5 equiv of
TPT with 1,4-bis(bromo-methyl)benzene at 120 °C for 24 h in
dimethylformamide followed by counter-ion exchange with ex-
cess of NaBF4 (Figure 1A). Molecular formula of ligand 1 has
been confirmed by NMR, ESI-TOF mass spectroscopy (Figures
S1-4).Cage 2 was successfully self-assembled when a suspension
of ligand 1 (12 µmol) was treated with (bpy)Pd(NO3)2 (24 µmol)
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Host-guest studies. Guest encapsulation properties in water for
both cage 2 and cage 3 were firstly compared with a series of
aromatic molecules including polycyclic aromatic hydrocarbons
(PAHs: naphthalene and pyrene), thiolesters (DBT: dibenzothio-
phene; DPS: diphenyl sulfide; MBT: thioanisole). Excess amount
of guest (10-20 equiv) was added as solid to a D2O solution of the
host (2 mM) and the mixture was stirred for 3 h before subjected
to NMR measurement. During stirring, the colorless cage solution
changed gradually from to yellow or orange. Due to poor water-
solubility of these aromatic molecules (except for DPS and MBT),
1
in D2O with vigorous stirring at 70 °C for 12 h. H NMR spectra
confirmed the quantitative formation of 2 and all the proton sig-
1
nals were fully assigned based on a H–1H COSY experiment
(Figures S5, and S8). The downfield-shifting of the pyridyl dou-
blets (9.34 and 9.31 ppm for Ha, 9.17 and 8.85 ppm for Hb) and
the pyridinium doublets (9.55 ppm for Hd and 8.97 ppm for Hc)
compared to that on free ligand 1 indicated the complexation with
palladium. Appearance of only one set of signals for the ligands
indicates the high symmetry of complex 2 (Figure 1B, 1C). Diffu-
sion-ordered spectroscopy (DOSY) NMR spectrum also con-
firmed the formation of a single species with diffusion coefficient
of 1.32×10-10 m2s-1, corresponding to a diameter of 1.9 nm (Fig-
ure 1D).
1
they could hardly be detected by H NMR spectroscopy when
suspended in D2O. However, formations of host-guest complexes
were evidenced by NMR spectra (Table 1, Figure 2 and Figures
S11-21 ). In the presence of the supermolecular cages, clearly
upfield-shifted guest signals were visible, from which guest en-
capsulation was inferred to have taken place inside the hydropho-
bic cavity.12 It is worth to note that in most cases, an increased
number of guest molecules can be accommodated by cage 2 than
cage 3. For example, based on the NMR integration, three naph-
thalene molecules can be encapsulated by cage 2, while cage 3
can hold only up to two such guests (Figure S28). Similarly, one
pyrene molecule can be trapped inside cage 2 but by a clear con-
trast, no binding is observed with cage 3 (Figure S27). In the case
of DBT, a 1:3 host-guest complex of (DBT)3@2 was quantita-
tively formed while no encapsulation of DBT in cage 3 was in-
ferred to take place as neither significant shifts of host nor guest
NMR signals were observed (Figure S27). From these results, we
conclude that the newly designed cage 2 inherently bears more
space and has a greater potential than cage 3 in guest-uptake.
Figure 1 A) Self-assembly of cage 2 form ligand 1, where X-ray
crystal structure of cage 2 is shown (counter ions and solvent
1
molecules are omitted for clarity), H NMR spectra (400 MHz,
D2O, 298 K) of B) ligand 1 and C) cage 2, D) 1H DOSY spectrum
of cage 2.
The structure of cage 2 was then unambiguously determined
by synchrotron X-ray crystallographic analysis. Colorless single
crystals, suitable for X-ray crystallography, were obtained by
vapor diffusion of acetone into an aqueous solution of 2 over one
week. In the crystal structure, two cationic pyridinium ligands are
connected by four square-planar coordinated Pd(II) ions which are
1
Figure 2 H NMR spectra of cage 2 and the host-guest complex-
es (400 Hz, D2O, 298 K). Guest signals are represented as circles:
the hollow circles represent free guests (or external binding) in
solution; solid circles represent guests encapsulated in the cavity.
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ons confirmed that a 1:2 inclusion complex (Mo6O192-)2@2 is
formed (Figure S32).
Table 1. Summary of guests studied and their encapsula-
tion numbers in both cage 2 and 3.
Encapsulation numbersa
1
2
3
4
5
6
7
8
9
As no host-guest complexes are crystallized, we infer that
Guests
close electro-static and anion-π interactions between the pyridini-
um panels and Mo6O19 are the main driving-forces for the host-
guest complex formation. Different from the hydrophobic binding
of neutral aromatic compounds which happens only in aqueous
solution, uptake of POMs by cage 2 proceeds even in organic
solvents, such as CH3CN (Figure S33).
Cage 3
Cage 2
2-
Naphthalene
Pyrene
Dibenzothiophene
Diphenyl sulfide
Thioanisolec
Mo6O19
Mo8O26
PMo12O40
a
2
0b
0
2
-
1
1
0
3
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2
-
2
1
0
We also noted that sizes of the window and the inner cavity
2-
determined the POM binding properties of cage 2. When it comes
4-
2-
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4-
to the Mo8O26 cluster, which is slightly larger than Mo6O19
(with diameters of 8.3 Å and 8.0 Å, correspondingly, Figure S31),
3-
a 1:1 inclusion complex Mo8O264-@2 was formed (Figure S34). It
2-
is worth to mention that 1:1 host-guest complexes for Mo6O19
Guest encapsulation numbers within the cavity were obtained
from NMR titration and integral ratio.bThe pyrene molecule can-
not be encapsulated alone in the cage 3 unless it is co-
and Mo8O264- with cage 3 were observed (Figures S36 and S37).
Meanwhile, strong de-symmetrization of the host NMR signals on
these two host-guest complexes suggests that cavity of cage 3 is
very limited for the theses cluster anions, where the tumbling
motion is restrained on the NMR time-scale. While, the Keggin-
encapsulated with another small molecule: see Reference13.
c
MBT inclusion is in fast equilibration.
The solid state structure of the inclusion complex (DBT)3@2
has also been revealed by X-ray crystallographic analysis (Figure
3). Suitable crystals were obtained by slow evaporation of an
aqueous solution of the host-guest complex. The structure of
(DBT)3@2 shows that three DBT molecules are sitting inside
cage 2 by stabilization of π-π stacking interactions between DBT
molecules and the TPT panels of the cage.
3-
type PMo12O40 with a larger size of 10.5 Å can not be encapsu-
lated in neither cage 2 or cage 3 (Figure S35 and S38). We also
noticed that the larger space offered by cage 2 is beneficial for co-
encapsulation of both organic and inorganic guest molecules (see
discussion below).
IR, UV-Vis spectra, diffuse reflectance, and ESR studies. The
solids of host-guest complexes were obtained by evaporation of
aqueous solution of host-guest complexes. In the IR spectra of
ligand 1 and cage 2, the C=N bond stretching vibrationυ(C=N)
of the pyridinium ring appeared at ∼1644 cm−1. As for the
(Mo6O192-)2@2 and Mo8O264-@2 host-guest complexes, the char-
acteristic peaks of υ(Mo=O) andυ(O-Mo-O) in the region 960-
550 cm-1 confirmed the formation of binary inclusion complexes
(Figure S39). The UV-Vis absorption spectra for the solution of
inclusion complexes with neutral aromatic molecules showed a
new charge transfer (CT) absorption band in the range of 315-400
nm, derived from π-stacking interaction between host and aro-
matic guest (Figures S40-41). Similarly, UV-Vis spectra also
support the formation of host-guest complexes with the POM
clusters, (Mo6O192-)2@2 and Mo8O264-@2, in aqueous solution
(Figure S42), where a new CT band at around 321 nm observed
due to the interaction between the electron-rich POMs and the
electron-deficient pyridinium-based ligands of cage.15
Figure 3 X-ray structure of (DBT)3@2. For clarity, anions and
water molecules have been omitted. Only one set of the disor-
dered DBT (spheres: C, green; H, white; S, yellow) molecules is
shown.
Photochromic behaviors of cage 2 and POMs@2 in the solid
state have been investigated. Only weak color change for cage 2
has been observed after irradiation (Figure S45). Interestingly, the
light-yellow solids of both POMs@2 inclusion complexes turned
pale blue upon irradiation with a xenon lamp in air at room tem-
perature. The diffuse reflectance spectra display new absorption
bands at 407 nm, 718 nm and 1057 nm for (Mo6O192-)2@2, and
406 nm, 715 nm and 1053 nm for Mo8O264-@2 after irradiation
(Figures 4A and S43). The appearance of a shoulder peak around
407 nm also support the existence of stronger intermolecular CT
interactions. The other two long wavelength bands are attributed
to the generation of pyridinium radicals after photo-irradiation,
giving rise to the color change of (Mo6O192-)2@2 and Mo8O264-@2
(Figures 4A inset).
Beside organic guests, three POMs anions with increasing
size, from Mo6O192-, Mo8O264- to PMo12O40 (tetrabutylammoni-
3-
um salts) are also chosen as candidates for host-guest studies, due
to their wide utilization and excellent chemical activity.8d,14 1H
NMR titrations experiments were performed by adding different
ratios of POMs to the solution of cage 2. The DOSY spectra for
the POMs@cage complexes have also been measured (Figures
S22-23), where similar diffusion constants are consistent
with the other host-guest complexes.The new upfield shifted
signals of the cage, especially protons on the cavity surface (Hc,
Hd, He and Hf) rather than the periphery bpy cis-capping ligands
were observed once after the first 0.5 equiv of Mo6O129-2- was add-
ed, indicating the successful inclusion of the Mo6O19 inside the
cavity near the two pyridinium functionalized corners. Strong
binding was indicated by the lack of fast equilibrium, where two
distinct sets of signals assignable to both empty cage and the in-
clusion complex are observed. Further increase of Mo6O192- ani-
Indeed, generation of the radicals was then confirmed by elec-
tron spin resonance spectroscopy (ESR). ESR spectra (solid,
298K) of cage 2 have also been checked, again with slight change
observed after the irradiation. In sharp contrast，ESR studies
before and after irradiation show that light-yellow solid samples
of (Mo6O192-)2@2 and Mo8O264-@2 are almost ESR-silent while
the pale blue ones are ESR-active, giving resonance signals at g
=2.0038 and 2.0039, respectively (Figure 4B). Another broad
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signals at g = 1.9288 and 1.9263 were also observed, which con-
the host-guest complexes (Figure 4C-H). The CVs of ligand 1 and
3-
sists well to the value for ESR signal of Mo6O19 (with a MoV
cage 2 in MeCN containing 0.1 M tetra(n-butyl)ammonium hexa-
fluorophosphate (TBAPF6 electrolyte) are compared in Figures 4C
and 4E with a scan rate of 100 mV/s. Two reversible reduction
waves for ligand 1 were observed at around -0.507 V and -1.175
V vs. Ag/AgCl. The first reduction potential for ligand 1 (-0.507
V) is attributed to the two-electron reduction process on both
pyridinium units to form neutral state 10, which is typical for pyr-
idinium derivative.18 The second more negative one (-1.175 V) is
assigned to the formation of 12- dianion due to the electron accept-
ing triazine unit (Figure 4D).19 In the CV of cage 2, the first re-
duction potential appeared at more negative positions (-0.526 V)
than that of ligand 1, indicating cage 2 formed by two dicationic
ligands maintains the redox characteristics of pyridinium deriva-
tives, but being more difficult to be reduced after coordination to
Pd(II). The second (-1.150 V) and third reduction waves (-1.415
V) are assigned to the two two-electron reduction processes on the
triazine units, particularly the latter is negatively shifted as com-
pared with that of the ligand 10/12- reduction. This observation is
consistent with a previous report: the triazine unit in the cage
skeleton can extend the redox behavior down into the anionic
regime, which also indicates inter-ligand electronic communica-
tions between the triazine panels are taking place through the
coordination bonds.5b It has to be pointed out that without the
pyridinium functional moieties, cage 3 has been proved to be
unstable below -0.8 V as CV showed irreversible redox waves
under similar conditions.20
As for the (Mo6O192-)2@2 host-guest complex, the half wave
potential [E1/2=(Epa+Epc)/2] for the first redox waves is found to
be -0.445V, which is a bit larger than the value of empty cage 2 (-
0.471V). The other two redox waves were shifted to more positive
potentials (Figure 4G) compared to those observed for the empty
cage 2, indicating that encapsulation of POM inside 2 makes the
thiazine units on the ligands much easier to be reduced. It is worth
mentioning that both inclusion complexes (Mo6O192-)2@2 and
Mo8O264-@2 present quasi-reversible or a totally irreversible wave
(Figure S48), which indicates the presence of the electron-rich
POM anions significantly alters the stability of the radical ion
species formed in the reduction process.
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nuclei spin I = 5/2 center, Figure S44).16 Photochromic properties
in anaerobic aqueous solution were also investigated. The same
radical species was generated in the solution of (Mo6O192-)2@2
since the solution also quickly turned blue upon irradiation under
nitrogen atmosphere. The radical species was rather stable as the
colour remain unchanged for more than 3 days at room tempera-
ture. ESR analysis of the blue species in frozen aqueous solution
(100 K) showed splitting signals at g = 2.0110 and g = 1.9273
(Figure S46), which was an obvious evidence that MoV magneti-
cally interacts with the radical species generated by cage 2.17 We
infer that the interaction between the electron-rich POMs donor
and pyridinium acceptor moieties is responsible for the enhanced
photochromic behavior of POMs@cage complexes. We also
found the generation of radical species in the solution of the host-
guest complex (DBT)3@2 (Figure S47), indicative of the main
role of host-guest interaction on the formation of pyridinium radi-
cals.
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We also investigated the CVs of cage 2 and binary complexes
(DBT)3@2 and pyrene@2 in H2O solution containing 30 mM
NaNO3 as electrolyte (Figures S49-51). The concentration of
complexes in aqueous solution and scan rate were the same as that
in MeCN solution. Cage 2 showed only one broad reversible re-
duction peak at -0.452 V, higher than that observed in MeCN
solution. It seems that simultaneous reduction process occurred in
aqueous solution. The half wave potential E1/2 (-0.340 V) is less
anodic, indicating that the solvent medium has an important effect
on electrochemical behavior of cage 2. The CV of (DBT)3@2
shows three irreversible reduction waves at around -0.491 V, -
0.680 V, and -0.861 V. Similar irreversible processes took place
in the CV of pyrene@2, except that the third reduction wave was
observed at -0.933 V, more negatively shifted compared with that
of (DBT)3@2. We infer that the radical species formed in the
reduction process are not stable and have been consumed once
reduced. Further studies are currently underway to address the
electrochemical reaction details for these complexes.
Figure 4 A) UV–vis diffuse reflectance spectra (with insets show-
ing the photos of the sample before and after the photo-irradiation)
and B) ESR spectra for (Mo6O192-)2@2 before (black line) and
after (red line) photo-irradiation (xenon lamp; 200 mW/cm2).
Cyclic voltammograms of C) ligand 1 (1 mM), E) cage 2 (0.5
mM), G) (Mo6O192-)2@2 (0.05 mM, MeCN) with D, F, H) show-
ing the corresponding oxidation states for each reduction waves.
The CVs were recorded in a 0.1 M solution of Bu4NPF6 electro-
lyte at a scan rate of 100 mV/s.
Catalytic properties of POMs@2 host-guest complexes. It is
well known that POMs are preponderant catalysts on oxidation
reactions of alkenes, alkanes, nitrogen- and sulfur-containing
compounds et al.8b,21 Oxidative desulfurization reactions, which
are considered to be promising strategy to remove refractory sul-
fur containing compounds in fuels22 have been chosen as proof-
of-concept model reactions to investigate the catalytic property of
our inclusion complexes of POMs@2.
Cyclic voltammetry. Cyclic voltammetry (CV) experiments have
been carried out to elucidate the redox properties of cage 2 and
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Diphenylsulfide (DPS) was selected as the representative sub-
DPS in the cage cavity. The disappearance of DPS signals and
increasement and downfield-shift of DPSO signals with the reac-
tion time also provided strong evidence to explain turnover and
selectivity (Figture S61).
strate for discussion below. When suspending DPS (1.90 mg, 0.01
mmol) in an D2O/CD3CN (V/V=5:1) solution of the binary com-
1
2
plex Mo8O262-@2 (2 mM, 0.5 mL) at room temperature, H NMR
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spectrum showed that the signals strikingly changed and guest
signals assignable to the DPS substrates were observed with
smaller downfield-shifting compared to the guest chemical shifts
in the (DPS)2@2 complex, which indicates that fast exchanging
ternary guest binding process occurred within the expanded hy-
drophobic space of cage 2. In contrast, no co-encapsulation was
observed when Mo8O262-@3 was tested, where instead replace-
ment of the Mo8O262- guest by DPS happened (Figures S52-57).
As control experiments (see entry 4 and 5 in Table 2), we also
examined the oxidation of DPS with (n-Bu4N)4[Mo8O26] and (n-
Bu4N)2[Mo6O19] as catalysts in water, which gave much lower
conversions (37.6% and 37.0%, respectively) and product selec-
tivity (81.8% and 80.0%, respectively). Direct oxidation of DPS
catalyzed by cage 2 only was also performed, which resulted in
only 10% conversion (entry 6). Sulfoxidation of DPS with none
catalyst was also performed in H2O which only gave trace product
(5% conversion, entry 7). Similarly, other sulfoxidation experi-
ments of DBT and MBT suggest that POMs@2 showed high
conversion and selectivity to form corresponding sulfoxides (entry
8-13 in Table 2, Figures S64-71).
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After removing the excess guest molecules by filtration, the
resulting (DPS+Mo8O262-)@2 solution was heated at 60°C for 9 h
with the addition of an excess amount of t-butyl hydroperoxide
(TBHP, ca. 31 equiv) as the oxidant. After reaction, NMR signals
of DPS molecules disappeared, meanwhile new signals assignable
to the diphenylsulfoxide (DPSO) product appeared (Figure 5C).
Based on NMR and GC-MS analysis (Figure 5D, Figure S72), the
Based on the results above, a plausible mechanism was pro-
posed for the selective desulfurization catalysis: i) the big hydro-
phobic cavity of cage 2 leads to an increased solubility of the
Table 2. Sulfoxidation of sulfides to sulfoxide and sulfones
catalyzed by POMs@cage 2 complexes with TBHPa.
Entry Sulfide Time
Catalyst
Conversion %
(Selectivity %)b
100 (95.0)
100 (75.0)
96.2 (88.9)
37.6 (81.8)
37.0 (80.0)
10.0 (100)
5.0 (71)
(loading equiv.)
Mo8O264-@2 (1) d
(Mo6O192-)2@2 (0.5)d
Mo8O264-@2 (0.1) c
Mo8O264- (0.1) c
Mo6O192- (0.1) c
Cage 2 (0.1) c
1
DPS
DPS
DPS
DPS
DPS
DPS
DPS
DBT
DBT
DBT
MBT
MBT
MBT
9h
2
9h
3
4
5
6
7
8
10h
10h
10h
10h
10h
9h
None
Mo8O264-@2 (0.5)d
(Mo6O192-)2@2 (0.5)d
Mo8O264-@2 (0.1)c
Mo8O264-@2 (0.2)d
(Mo6O192-)2@2 (0.1)d
Mo8O264-@2 (0.1)c
93.8 (85.7)
89.6 (93.7)
51.0 (81.0)
100 (83.6)
100 (92.0)
100 (90)
9
9h
10
11
12
13
16h
9h
9h
1
Figure 5 H NMR spectra (400 MHz, 298 K) of a 1 mM solution
16h
of A) Mo8O264-@2 in D2O/CD3CN(5/1), (B) ternary complex
obtained by adding DPS to the solution of Mo8O264-@2 and C)
after sulfoxidation reaction in D2O/CD3CN (V/V=5/1), D) crude
product extracted to CDCl3. The signals of DPS and DPSO are
represented by (●) and (▼), respectively. The doublet signals at
6.83 ppm and multiple signals at around 6.58 ppm are free DPS
dissolved in D2O, denoted by(○).
aReaction conditions: sulfide (0.01 mmol, 10 equiv) was added to catalysts
(1 equiv) in D2O/CD3CN (V/V=5:1) and then stirred at r.t. for 5 hours,
after which insoluble substrates were removed by filtration. Then TBHP
(4 mg, 70% in H2O) was added and the solution was heated to 60 °C over-
night. bThe conversion of sulfoxidation reactions are determined using
1,3,5- trimethoxybenzene as the internal standard (Figures S72-84). Selec-
1
tivity is calculated as SO/(SO + SO2) and determined by H NMR and
GC-MS. cReaction in water without removal of the excess guest molecules,
d
with 10 mol% catalyst loading. The catalysis loading depend on the en-
substrate was fully consumed and converted mostly into DPSO
(100% conversion, 95.0% chemoseletivity, table 2, entry 1).
When (Mo6O19)2@2 was used as the catalyst, the DPS to DPSO
reaction witnessed high conversion (100%) but slightly lower
(75 %) chemoseletivity (entry 2, Figures S62 and S73).21b
capsulation number of sulfides, see Figures S60-71.
substrate in water; ii) co-encapsulation of POMs and the organic
substrates inside cage 2 enhances the effective local concentration
and forced the bimolecular collision; iii) selectivity enhancement
is realized by the favorable binding of substrates over the sulfox-
ide products.
To investigate the turnover number of POMs@2 catalyst, 10
mol% of Mo8O262-@2 was then tested to catalyze the sulfoxida-
tion of DPS. After 17 hours of stirring under the same condition,
we obtained conversion of 96.2% corresponding to a turn-over
number of 9.6, giving again the sulfoxide as the main product
(88.9% selectivity, entry 3 in Table 2). In order to complete the
catalytic cycle, the inclusion of substrate and exclusion of product
must be fulfilled to ensure the turnover of catalysis.23 Competitive
guests binding between DPS and DPSO with POMs@2 were
studied by NMR titration, which showed that POMs@2 has
weaker binding ability toward DPSO (Figures S58-59). This dif-
ference in binding strength comes from the hydrophilic S=O moi-
ety of product, resulting in the reduced host-guest hydrophobic
interaction to facilitate the replacement of the DPSO product by
CONCLUSIONS
In conclusion, a water-soluble redox-active Pd4L2 cage featur-
ing enlarged pore-opening and internal cavity is obtained. The
expanded hydrophobic cavity of this new generation of organo-
palladium cage can preferentially encapsulate hexamolybdate and
octamolybdate cluster anions, leaving additional room for the co-
encapsulation of sulfides substrates. CV study showed the redox-
activity of the cage and the binary inclusion complexes derived
from the pyridinium moieties on the ligand. Sulfoxidation reac-
tions catalyzed by binary POMs@cage complexes exhibited en-
hanced conversion and chemoselectivity compared to the POMs
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Angew. Chem. Int. Ed. 2017, 56, 3852. (j) Tan, C.; Jiao, J.; Li, Z.; Liu, Y.;
or cage only. The current study provides a general and valuable
strategy for expanding the stability, product selectivity of classical
transition-metal catalysts in water.
Han, X.; Cui, Y. Angew. Chem. Int. Ed. 2018, 57, 2085. (k) Jiao, J.; Tan,
C.; Li, Z.; Liu, Y.; Han, X.; Cui, Y. J. Am. Chem. Soc. 2018, 140, 2251. (l)
Luo, D.; Wang, X.-Z.; Yang, C.; Zhou, X.-P.; Li, D. J. Am. Chem. Soc.
2018, 140, 118. (m) Chen, H.; Huang, Z.; Wu, H.; Xu, J.-F.; Zhang, X.
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ASSOCIATED CONTENT
Full synthetic and structural details, crystallographic data (CIF),
and supplemental figures and tables as described in the text. This
material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
9
Corresponding Author
*qfsun@fjirsm.ac.cn
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Notes
All authors have given approval to the final version of the manu-
script.
ACKNOWLEDGMENT
(6) Bruns, C. J.; Fujita, D.; Hoshino, M.; Sato, S.; Stoddart, J. F.; Fujita,
M. J. Am. Chem. Soc. 2014, 136, 12027.
Dedicated to prof. Xin-Tao Wu on the occasion of his 80th birth-
day. This work was supported by the National Natural Science
Foundation of China (Grant nos. 21521061, 21471150, 21601183),
the Strategic Priority Research Pro-gram of the Chinese Academy
of Sciences (Grant No. XDB20000000), Natural Science Founda-
tion of Fujian Province (Grant nos. 2016J06005, 2016J05051,
2017J05037). We thank the staff of BL17B beamline at the Na-
tional Centre for Protein Sciences Shanghai and Shanghai Syn-
chrotron Radiation Facility, Shanghai, PR China, for assistance
during data collection.
(7) Yazaki, K.; Noda, S.; Tanaka, Y.; Sei, Y.; Akita, M.; Yoshizawa, M.
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