A chiral porous metallosalan-organic framework containing titanium-oxo clusters for enantioselective catalytic sulfoxidation

Article abstract of DOI:10.1039/c3sc50487e

A chiral porous zeolite-like metal-organic framework is constructed by using mixed ligands of dipyridyl-functionalized chiral Ti(salan) and biphenyl-4,4′-dicarboxylate. The framework containing salan-bound Ti ₄O₆ clusters consists of both hydrophobic and amphiphilic mesocages and is shown to be an efficient and recyclable heterogeneous catalyst for the oxidation of thioethers to sulfoxides by aqueous H₂O ₂ (up to 82% ee), displaying markedly enhanced enantioselectivity over the homogeneous catalyst by providing a cavity confinement effect. This work highlights the potential of significantly improving enantioselectivity of homogeneous catalysts by using MOFs as support structures.

Full text of DOI:10.1039/c3sc50487e

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A chiral porous metallosalan-organic framework
containing titanium-oxo clusters for enantioselective
Cite this: DOI: 10.1039/c3sc50487e
catalytic sulfoxidation†
*
Weimin Xuan, Chengcheng Ye, Mengni Zhang, Zhijie Chen and Yong Cui
A chiral porous zeolite-like metal-organic framework is constructed by using mixed ligands of dipyridyl-
functionalized chiral Ti(salan) and biphenyl-4,40-dicarboxylate. The framework containing salan-bound
Ti4O6 clusters consists of both hydrophobic and amphiphilic mesocages and is shown to be an efficient
and recyclable heterogeneous catalyst for the oxidation of thioethers to sulfoxides by aqueous H2O2 (up
to 82% ee), displaying markedly enhanced enantioselectivity over the homogeneous catalyst by
providing a cavity confinement effect. This work highlights the potential of significantly improving
enantioselectivity of homogeneous catalysts by using MOFs as support structures.
Received 20th February 2013
Accepted 14th May 2013
DOI: 10.1039/c3sc50487e
www.rsc.org/chemicalscience
asymmetric synthesis,8,12 the reduced forms (salan) that have
increased ligand exibility and stronger nitrogen donors are
Introduction
less studied.13,14 We have recently reported the assembly of
twisted chiral metallosalans into helical cages for enantiose-
lective recognition of chiral molecules with amino, hydroxyl and
carboxylate functionalities that can form hydrogen bonds with
the two amino groups of the salan ligand.14 Chiral Ti(salan)
complexes have been demonstrated to be highly effective in
catalyzing oxidation of alkenes and suldes.15 We report here
the synthesis of a dipyridyl-functionalized chiral Ti(salan)
ligand to make a porous MOF containing interesting Ti4O6
cluster cores and show that the framework is an efficient
heterogeneous catalyst for the asymmetric oxidation of thio-
ethers with aqueous H2O2 as oxidant (Scheme 1).10
Metal-organic frameworks (MOFs) are hybrid crystalline solids
composed of organic struts and inorganic nodes and have
emerged as a novel type of highly porous materials.1,2 These
materials feature structural diversity and amenability to be
designed with specic functionality at the molecular level.
Owing to their tunable but uniform pore sizes and functio-
nalizable pore walls, MOFs are attractive solid-state scaffolds
for molecule transformations relevant to chemical feed-
stocks,3,4 energy conversions5 and environmental pollution.6
They offer advantages over other immobilized catalyst systems
as crystalline structures, with high catalyst loadings, more
uniformity, and isolated and accessible active sites. Of
particular relevance here are those MOFs constructed by
incorporating well-known and efficient homogeneous cata-
lysts such as metallosalens, metalloporphyrins and Ti–BINO-
Late complexes.7–9 The resulted catalysts are, however,
typically less effective than their homogeneous analogs.4 So,
structurally controlled integration of individual components
into designed architectures remains a big challenge.3,4 In fact,
examples of catalytic MOFs, especially those for asymmetric
catalysis with a framework connement effect on incorporated
molecular catalysts are less explored.4,10,11
Although chiral salen ligands and their metal complexes
have received much attention for their applications in
School of Chemistry and Chemical Technology and State Key Laboratory of Metal
Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail: yongcui@sjtu.edu.cn; Fax: +86 21-5474-1297
† Electronic supplementary information (ESI) available: Experimental details,
crystallographic data, additional structural gures and spectroscopic data.
CCDC 899470. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3sc50487e
Scheme 1 Synthesis of the ligand H2L and MOF 1.
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Results and discussion
The chiral salan ligand H2L was prepared by the Schiff base
condensation of 3-tert-butyl-5-(4-pyridyl)salicylaldehyde and
enantiopure 1,2-diaminocyclohexane followed by reduction
with NaBH4. Metalation of H2L with Ti(OBu)4 (1 : 1 molar ratio)
in THF afforded TiL(OBu)2. Heating TiL(OBu)2, H2BPDC
(biphenyl-4,40-dicarboxylic acid) and CdI2$4H2O (4 : 3 : 3 molar
ratio) in DMF at 80 ꢀC afforded yellow rod-like crystals of
[H2NMe2]2[Cd3{TiO6(TiL)3}(BPDC)3(H2O)3]$16H2O (1). The
product is stable in air and insoluble in water and common
organic solvents, and was formulated on the basis of elemental
analysis, IR and thermogravimetric analysis (TGA). The phase
purity of the bulk sample was established by comparison of
its observed and simulated powder X-ray diffraction
(PXRD) patterns.
(R)-1 crystallizes in the chiral hexagonal P622 space group,
with a crystallographic C3 axis passing through the center and
so has one third of one formula unit in the asymmetric unit.17
The basic building unit is a salan-bound Ti4O6L3 cluster, in
which four Ti atoms lie in the plane of an equilateral triangle.
Three C3-symmetry related Ti1 atoms lie at the triangle vertices
and Ti2 at the triangle center. Ti1 and Ti2 are connected by a
Fig. 1 (a) View of the structure of the Ti4O6L3 cluster in 1. (b) A mesoporous
cage encapsulated by two Ti4O6L3 units (part of the cavity is highlighted by a
colored sphere). (c) Parallel association of six 1D chains constructed by Cd2+ and
BPDC into a tubule that is shown as a yellow column. (d) The 3D porous structures
built of interconnected cages viewed along the c-axis. Green Ti, purple Cd, blue N,
red O, gray C in (a), (b) and (d).
˚
pair of m-oxo atoms with a Ti/Ti distance of 2.94(2) A. The
central Ti2 atom is thus octahedrally coordinated by six m-oxo
˚
atoms with bond lengths of 1.896(9) and 2.072(9) A, typical for
an octahedral Ti4+ ion in oxo clusters.18 The peripheral Ti1
atoms also adopt a octahedral geometry with a D conguration
by binding to two cis oxo atoms and four ONNO atoms of L,
which features a trans conguration of the phenolate groups
and thus wraps around the titanium in the desired fac–fac mode
typical of the salan ligands. The six pyridyl groups are coplanar
with the Ti4O6 core and orient towards three different direc-
tions, pointing away from units with a separation distance of
along the a-axis and b-axises (Fig. S3, ESI†). To our knowledge, 1
is the rst example of a zeolite-like MOF equipped with chiral
NH functionalities,2c which are benecial for enantioselective
guest recognition and inclusion.
Solid-state CD spectra of 1 made from R- and S-enantiomers
of L are mirror images of each other, indicating their enantio-
meric nature. Calculations using PLATON show that 1 has about
ꢂ54.3% of the total volume available for guest inclusion.19 TGA
˚
17.517(1) A (the average terminal N/N distance) to minimize
ꢀ
steric repulsion.
revealed that guest molecules could be removed at 80–150 C.
The Ti4O6L3 cluster thus acts as a hexadentate ligand
binding to six Cd ions via its six pyridyl groups. The Cd atom
adopts a pentagonal bipyramidal geometry by binding to four
bidentate carboxylate oxygen atoms from two BPDC, two pyridyl
groups from two Ti4O6L3 and one water. Adjacent Cd centers are
thus bridged by BPDC in a trans fashion forming an innite zig-
zag chain along the c-axis. Six 1D chains associate in parallel
forming the wall of a hexagonal tubule with an opening size of
1.0 nm ꢁ 1.0 nm. As shown in Fig. 1, the framework consists of
box-shaped coordination cages, encapsulated by two Ti4O6L3
clusters, six rod-like BPDC pillars and twelve metal hinges that
are related by C3 symmetry. With one crystallographic C3 axis
running through a pair of Ti2 atoms and three crystallographic
C2 axes that bisect three pairs of opposite BPDC edges, the cage
possesses D3 point group symmetry. Of the adjacent cages
sharing one Ti4O6L3 core, one has a hydrophobic cavity con-
taining tert-butyl groups and another has an amphiphilic cavity
containing NH and cyclohexane groups. The hydrophobic or
amphiphilic cage shares its quadrilateral faces with three
neighboring amphiphilic/hydrophobic cages, leading to a 3D
porous structure with 1D square channels of 0.5 nm ꢁ 1.5 nm
Aer desolvation, the N2 sorption measurement of the apohost
sample at 77 K showed only low adsorption (7.6 m2 gꢃ1),
although the theoretical value is expected to be 1231.9 m2 gꢃ1
.
The discrepancy between the calculated and experimental
surface areas probably results from the framework distortion on
solvent removal. Powder XRD indicated that the framework
retained its structural integrity and crystallinity upon guest
removal, but a structural distortion occurred. Aer soaking in
DMF/MeOH for 1 day, the evacuated 1 regained the PXRD
pattern of the pristine sample, which is consistent with frame-
work distortion during solvent removal. The ‘breathing effect’ is
quite common among porous MOFs, particularly for those
containing exible ligands.20 Examination of the chemical
stability of 1 was performed by heating the sample in boiling
H2O or 30% aqueous H2O2 for 3–6 days. PXRD patterns of these
heated materials indicated retained crystallinity aer 3 days but
an almost total loss of crystallinity aer 6 days. Interestingly, 1
could readily adsorb 1.8 methyl orange (MO) (ꢂ1.47 nm ꢁ
0.53 nm ꢁ 0.53 nm in size) per formula unit in solution,
causing the initially pale yellow crystals to become dark orange,
and the inclusion solids exhibited the same PXRD patterns as
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the pristine sample (Fig. S4, ESI†). This result indicates that the
High enantioselectivity was attained with the bulky substrate
structural integrity and open channels of 1 are maintained in 2-naphthyl benzyl sulde, affording 59% conversion and 64%
solution. ee (Table 1, entry 9). Under the standard experimental condi-
The titanium oxo-clusters [TinOm(OR)x(L)y],18 as a model of tion, when this sulde was completely oxidized by using
bulk nanoscale titanium oxides, offer the opportunity to 1.5 equiv. 30% H2O2, the optical purity of the sulfoxide
understand the information of the bulk nanoscale oxides in increased to 82% ee (Table 1, entry 10). The resulting high
terms of both atomic structure and chemical reactivity and have enantioselectivity might be attributable to the kinetic resolution
been shown to be versatile building blocks for the assembly of in the further oxidation of sulfoxide to sulfone, which may
advanced hybrid materials. But, there are only two Ti-based enrich the enantiopurity of the primary oxidation product.23
MOFs having thus far been reported, namely, the isostructural Note that a further increase in steric bulk of the benzyl group in
Ti8O8(OH)4(BDC)6 [MIL-125(Ti)] and Ti8O8(OH)4(BDC-NH2)6 the substrate resulted in an obvious decrease in the conversion
[MIL-125(Ti)-NH2] (BDC ¼ 1,4-benzenedicarboxylate).21 Both of (23%; Table 1, entry 12). The remarkable size-selectivity implies
them were obtained in microcrystalline form and the crystal that the reaction primarily occurs inside the uniform pores of
structures were solved by ab initio on the sample using PXRD the solid 1 and, therefore, attests to the heterogeneous nature of
data by a direct method. Therefore, 1 represents the rst Ti- the catalysis.
based MOF that has been characterized by single-crystal XRD
To probe whether the oxidation of the suldes occurs
and is a rare example of a MOF containing titanium-oxo clus- predominantly within the channel-containing material or
ters. The diffuse reectance spectrum shows the absorption instead on the external surface, a sterically more demanding
onset at about 462 nm, indicating that 1 is a semiconductor substrate naphthyl dendritic sulde was synthesized and sub-
with a band gap of 2.84 eV and underlining its potential as a jected to the oxidation condition (Table 1, entries 13 and 14).
photocatalyst.16
Less than 2% conversion was observed aer 72 h, which was
The presence of TiL units in 1 prompted us to explore its much lower than the 70% conversion obtained by the homo-
utilization as a solid catalyst for oxidation of thioethers. geneous TiL(OBu)2 catalyst (24 mol% loading). This result
Optically active sulfoxides are important chiral auxiliaries in suggests that this bulky substrate can not access the catalytic
organic synthesis and valuable intermediates for many sites in the porous structure due to its large diameter and so the
pharmaceuticals and biologically active compounds.22a The oxidation reaction is indeed occurring within the MOF. This
oxidation reactions were conducted in CH2Cl2 at room point is also suggested by the fact that ground and unground
temperature by using aqueous 30% H2O2 (1.2 equiv.) as the particles of 1 (ꢂ200 nm in size) exhibited similar catalytic
oxidant aer initial optimization of reaction conditions. 8 activities in oxidation of PhSCH2Ph aer 24 h.24
mol% loading of the activated (R)-1 afforded 35–82%
To study the connement effect of the MOF on the catalyst,
conversions and 36–64% ee of the target (R)-sulfoxides with the activity of TiL(OBu)2 was examined. Thioanisole, phenyl
80–89% chemoselectivities aer 72 h for the suldes bearing isopropyl sulde, phenyl benzyl sulde and 2-naphthyl benzyl
both electron-rich and electron-poor species (Table 1). With were oxidized by aqueous H2O2 in the presence of 24 mol%
lower catalyst loadings, lower transformations and enantio- TiL(OBu)2 (the same TiL loading as the heterogeneous system).
selectivities were achieved (e.g. 24 and 18% conversions and The conversions reached those of the corresponding heteroge-
17 and 7% ee for 0.1 and 1 mol% 1 aer 72 h, respectively). neous systems aer 28–36 h, the reactions afforded 5, 8, 11 and
When the –Me group in PhSMe was substituted for a bulkier 13% ee of the sulfoxides with 80, 67, 81 and 92% chemo-
group like isopropyl or benzyl, the enantioselectivity selectivities, respectively (Table 1, entries 2, 4, 6 and 11).
increased from 36 to 53% ee and the conversion decreased Reducing the catalyst loading of TiL(OBu)2 from 24 to 8 mol%
from 77 to 53%, with no signicant change in the chemo- lead to much poorer chemo- and enantioselectivities (Table S4,
selectivity. The decrease in conversions is consistent with the ESI†). It is likely that the Ti(salan)-framework displayed mark-
larger substrate generally corresponding to slower diffusion, edly enhanced enantioselectivities and comparable chemo-
while the increase in enantioselectivity may be due to the selectivities with respect to the homogeneous catalyst, although
bulky reactants that were encapsulated in chiral pores the heterogeneous reactions are slower than the homogeneous
causing increased geometric constraint and steric hindrance ones, due to slow mass-diffusion in the porous structure.
to the intermediates that favor enantioselectivity during the
In general, the heterogeneous catalysts used for sulde
reaction.8f Introduction of an electron-donating –Me group oxidation are immobilized homogeneous catalysts.22 Although
into the phenyl ring of the substrate led to slightly higher substantial progress has been made, in most cases such hybrid
enantioselection (Table 1, entry 7). In the oxidation of the catalysts resulted in a signicant lowering of their activity and
substrate with a strong electron-withdrawing –NO2 group with selectivity: ee values, with a few exceptions, did not exceed 30–
H2O2 (Table 1, entry 8), a lower conversion of 20% was 40%.22b For example, Zhu et al. reported the use of chiral salan-
obtained, whereas the addition of urea into the reaction led to WO3 solid and H2O2 for the catalytic oxidation of methyl aryl
90% conversion and 57% ee of the sulfoxide. Note that the suldes, affording sulfoxides in 65–86% yield with 27–67%
chiral nature of the sulfoxides is determined by the handed- ee.25a Kim et al. investigated the oxidation of methyl aryl suldes
ness of the catalyst, as further evidenced by the oxidation of by H2O2 catalyzed by a chiral Zn-MOF of L-lactate, giving the
PhSCH2Ph by 8 mol% (S)-1 affording the S enantiomer over product in high yield but with no enantioselectivity.25b The
the R enantiomer (51% ee).
present enhanced enantioselectivity may arise from geometrical
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Table 1 Catalytic Oxidation of sulfides by 1a
Entry
Substrate
Catalystb
Conversionc (%)
eed (%)
Selectivitye (%)
Sulfone yield f (%)
1
2
3
4
5
6
3a
3a
3b
3b
3c
3c
3d
3e
3f
3f
3f
3g
3h
3h
R-1
77
77
75
75
53
53
57
90
59
72
72
23
<2
70
36
5
42
8
87
80
84
67
80
81
87
80
81
74
92
89
n.d
94
10
15
12
25
11
10
7
18
11
19
6
R-TiLf
R-1
R-TiLf
R-1
53
11
39
57
64
82
13
59
n.d
13
R-TiLf
R-1
7
8g
9
R-1
R-1
10h
11
12
13
14
R-1
R-TiLf
R-1
3
n.d
4
R-1
R-TiL
a
b
c
For reaction details see Experimental section. 8 mol% loading of 1 and 24 mol% loading of TiL [¼ TiL(OBu)2]. Calculated as ([sulfone] +
d
e
[sulfoxide])/([sulde] + [sulfoxide] + [sulfone]). ee determined by HPLC; the (R)-isomers are preferred in all cases. Calculated as [sulfoxide]/
f
([sulfoxide] + [sulfone]). Reaction time: 24, 24, 28 and 30 h for entries 2, 4, 6 and 11, respectively. Calculated as [sulfone]/([sulde] + [sulfoxide]
+ [sulfone]). g Oxidant: H2O2/urea (UHP). h 1.5 equiv. aqueous H2O2.
constrains imposed by chiral rigid microenvironments around (ꢂ0.0021% for Cd and 0.0014% for Ti) from the structure per
the site-isolated Ti centers, which may effectively restrict cycle, either as molecular species or as particles too small to be
molecular motion, leading to efficient enantiodiscrimination.
We also examined the heterogeneity of the MOF catalyst. The
supernatant from the oxidation of PhSCH2Ph aer ltration
through a regular lter did not afford any additional oxidation
removed by ltration through celite.
Conclusions
product. To evaluate the stability of the solid catalysts, we We have presented the assembly of a homochiral porous
investigated recycled and reused in the oxidation of MOF containing titanium-oxo clusters based on a dipyridyl-
1
PhSCH2Ph. Upon completion of the reaction over 72 h, the functionalized Ti(salan) ligand. The framework was shown to be
catalyst 1 could be recovered in nearly quantitative yield by an efficient heterogeneous catalyst for the asymmetric oxidation
simple ltration. A comparison of catalytic conversions and of thioethers to sulfoxides, with improved enantioselectivity
chemo- and enantioselectivities versus time of the pristine and relative to the monomeric catalyst. The ready tunability of such
recycled samples showed that 1 could be used repeatedly a modular approach based on chiral metallosalans promises to
without signicant loss of catalytic performance aer three lead to a variety of framework materials with enantioselective
cycles (53, 80 and 53%, 51, 81 and 50%, and 50, 80 and 49% of functions.
conversions, chemo- and enantioselectivities for 1–3 runs,
respectively). Consecutive PXRD of the reused MOF material
showed the solid catalyst remained crystalline aer three cycles
and but that the structure became seriously distorted. The XPS
Experimental section
Synthesis of 1
spectrum measurement showed that the recovered titanium A mixture of CdI2$4H2O (0.03 mmol), TiL (0.04 mmol), H2BPDC
complex retained its +4 oxidation state. Inductively coupled (0.03 mmol) and DMF (1.0 mL) in a capped vial was heated at
plasma-atomic mass spectrometry (ICP-AMS) analysis of the 80 ꢀC. Aer 4 days, yellow rod-like crystals of 1 were ltered,
product solution indicated little loss of the metal ions washed with THF, and dried at room temperature. Yield:
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5.41 mg (42.1%). Anal (%). calcd for C160H210Cd3N14O43Ti4: C,
54.19; H, 5.97; N, 5.53. Found: C, 53.98; H, 5.91; N, 5.50. Ti
exhibiting a +4 oxidation state was conrmed by the XPS
spectrum.
Z. Su, W. Li and E. Wang, Angew. Chem., 2012, 51, 7985; (g)
X. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu and Z. Li,
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Oxidation of suldes
The activation of 1 was accomplished by exchanging guest
solvent molecules with anhydrous methanol, followed by evac-
uation under vacuum at an optimized temperature of 80 ꢀC. The
activated 1 (0.020 mmol) and the related sulde (0.25 mmol)
were combined in CH2Cl2 (1 mL) and stirred for 20 min at r.t,
followed by adding 1.2 equiv. 30% H2O2 (or urea-H2O2). The
mixture was further stirred for 72 h. The ee values of the sulf-
oxides were determined by HPLC using Chiralcel columns and
1
the conversions were determined by H NMR.
Acknowledgements
This work was supported by NSFC-21025103, the “973” Program
(2009CB930403), and the Shanghai Science and Technology
Committee (10DJ1400100).
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K. Gedrich, M. Heitbaum, A. Notzon, I. Senkovska,
a ¼ 32.0273(3) A, b ¼ 32.0273(3) A, c ¼ 29.1427(7) A, V ¼
3
˚
25 888.1(7) A , Z ¼ 4, T ¼ 123(2) K, F(000) ¼ 6736, rcalcd
¼
0.836 g cmꢃ3. m(Cu Ka) ¼ 3.295 mmꢃ1 (l ¼ 1.54178 A),
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¨
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