Direct and Post-Synthesis Incorporation of Chiral Metallosalen Catalysts into Metal-Organic Frameworks for Asymmetric Organic Transformations

Article abstract of DOI:10.1002/chem.201501486

Two chiral porous metal-organic frameworks (MOFs) were constructed from [VO(salen)]-derived dicarboxylate and dipyridine bridging ligands. After oxidation of V^IV to V^V, they were found to be highly effective, recyclable, and reusable

Full text of DOI:10.1002/chem.201501486

DOI: 10.1002/chem.201501486
Communication
&
Metal–Organic Frameworks |Hot Paper|
Direct and Post-Synthesis Incorporation of Chiral Metallosalen
Catalysts into Metal–Organic Frameworks for Asymmetric Organic
Transformations
Weiqin Xi,[a] Yan Liu,*[a] Qingchun Xia,[a] Zijian Li,[a] and Yong Cui*[a, b]
to access de novo.[5] However, few chiral MOF catalysts that
Abstract: Two chiral porous metal–organic frameworks
(MOFs) were constructed from [VO(salen)]-derived dicar-
boxylate and dipyridine bridging ligands. After oxidation
of VIV to VV, they were found to be highly effective, recy-
clable, and reusable heterogeneous catalysts for the asym-
metric cyanosilylation of aldehydes with up to 95% ee.
Solvent-assisted linker exchange (SALE) treatment of the
pillared-layer MOF with [Cr(salen)Cl]- or [Al(salen)Cl]-de-
rived dipyridine ligands led to the formation of mixed-
linker metallosalen-based frameworks and incorporation
of [Cr(salen)] enabled its use as a heterogeneous catalyst
in the asymmetric epoxide ring-opening reaction.
were fabricated through PSM have been reported, and none
have been obtained through SALE.[2a,b,d,4a,6]
Chiral salen ligands and their metal complexes have been
widely applied in asymmetric synthesis.[7] Our group and
others have reported the synthesis of chiral [Mn/Ru/Co/Fe(sa-
len)]-based MOFs capable of catalyzing enantioselective reac-
tions.[2f,4d–g] Chiral vanadium and chromium salen complexes
are active asymmetric catalysts for various types of organic re-
action.[8] Herein we report the design of two MOFs incorporat-
ing chiral [VO(salen)] units and their use, after oxidation, as effi-
cient heterogeneous catalysts for enantioselective cyanation of
aldehydes. The pillared-layer MOF can function as a platform
to fabricate an isostructural [Cr/VO(salen)] mixed MOF through
SALE for the asymmetric ring-opening of epoxides.
Metalation of salen dicarboxylate ligand H4L1 and salen di-
pyridine ligand H2L2 with VO(acac)2 in MeOH afforded
[VO(H2L1)] and [VOL2], respectively. Heating a mixture of
[VO(H2L1)] and ZnI2 or [VOL2], biphenyl-4,4’-dicarboxylic acid
(H2BPDC), and CdI2 in DMA/MeOH/H2O or DMF/MeOH solvent
mixtures at 858C afforded the MOFs [Zn2(VOL1)2]·DMA·H2O (1)
or [Cd2(VOL2)2(BPDC)2]·4DMF·2H2O (2) as olive-green crystals in
good yields (Scheme 1). The frameworks are stable in air as
well as in various common solvents including water, and the
formulations were confirmed by single-crystal X-ray diffraction,
microanalysis, IR spectroscopy and thermogravimetric analysis
(TGA).
Metal–organic frameworks (MOFs),
a family of crystalline
hybrid solids made from metal ions with organic linkers, have
emerged as a versatile platform to design chiral porous materi-
als for enantioselective processes, such as asymmetric catalysis
and separations that cannot be realized with traditional porous
inorganic materials.[1–3] In particular, given their highly modular
nature and facile tunability, MOFs allow for almost the same
level of structural refinement as molecular homogeneous cata-
lysts, while their large surface area, permanent porosity, and
heterogeneous nature facilitate good catalytic activity and
rapid purification.[1,2] Of the chiral MOFs with highly stereose-
lective catalysis reported to date, the most efficient examples
are all directly crystallized from privileged chiral ligands, such
as BINOL- and salen-based derivatives,[2a,f,4] but it is challenging
for the direct approach to grow large-enough single crystals
for structural studies and to make MOF catalysts with stereose-
lectivities rivaling and even surpassing their homogeneous
counterparts.[1a,d,e] Post-synthesis modification (PSM) and sol-
vent-assisted linker exchange (SALE) have been shown to be
remarkably effective for the synthesis of MOFs that are difficult
X-ray single-crystal analysis on 1 reveals a 2D layered frame-
work that crystallizes in the chiral triclinic space group P1 with
two formula units in the asymmetric unit. Each {VOL1} ligand
[a] W. Xi, Prof. Dr. Y. Liu, Q. Xia, Z. Li, Prof. Dr. Y. Cui
School of Chemistry and Chemical Engineering
and State Key Laboratory of Metal Matrix Composites
Shanghai Jiao Tong University, Shanghai 200240 (P. R. China)
E-mail: liuy@sjtu.edu.cn
yongcui@sjtu.edu.cn
[b] Prof. Dr. Y. Cui
Collaborative Innovation Center of Chemical Science and Engineering
Tianjin 300072 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501486.
Scheme 1. Synthesis of MOFs 1 and 2.
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contains two bidentate carboxylate groups. The V center
adopts a square-pyramidal geometry with the equatorial plane
occupied by the N2O2 donor set of one L1 and the apical posi-
tion by one double-bonded oxygen atom. A pair of ZnII ions
are bridged by four carboxylate groups from four {VOL1} units
to form {Zn2(O2C)4} paddlewheels with the apical positions oc-
cupied by a water or DMF molecule. The dizinc cores are thus
linked by four exo-tetradentate {VOL1} units to four adjacent
dizinc cores to result in a 2D grid in the bc plane. Such 2D
grids stack on top of each other in a slightly staggered ar-
rangement to generate a 3D lamellar structure, with an open
channel of approximately 1.56”0.83 nm2 along the a axis with
{VO(salen)} units (Figure 1a).
2 crystallizes in the chiral orthorhombic space group P2221
with one whole formula unit in the asymmetric unit. Again,
the V center in L2 adopts a square-pyramidal geometry. Each
BPDC ligand contains one bidentate and one monodentate
carboxylate groups. Each of the Cd ions adopts a distorted oc-
tahedral geometry by binding to four carboxylate oxygen
atoms from three BPDC ligands and two pyridyl groups from
two {VOL2} units. A pair of Cd ions related by a two-fold sym-
metry is bridged by two bidentate carboxylate groups of two
BPDC, and each is also coordinated by one chelated carboxyl-
ate group of one BPDC to form a 2D layer structure in the ab
plane. Each Cd ion in the plane further coordinates to two pyr-
idyl groups of two {VOL2} units and connect adjacent 2D
sheets to form a 3D pillared-layer structure with a planar chan-
nel size of approximately 1.52”0.70 nm2 surrounded by chiral
{VOL2} units (Figure 1b and 1c).
The phase purity of bulk samples of 1 and 2 was confirmed
by comparison of their observed and simulated powder X-ray
diffraction (PXRD) patterns. The enantiomeric nature of 1 and
2 formed from R- and S-enantiomers of [VO(H2L1)] and [VOL2]
was demonstrated by solid-state circular dichroism (CD) spec-
tra, which showed almost mirror images of each other. TGA
showed that the frameworks of 1 and 2 are stable up to
around 400 and 3708C, respectively. PLATON calculations indi-
cated that 1 and 2 contain roughly 39.1% and 31.4% void
space available for guest inclusion, respectively.[9] The perma-
nent porosity of 1 and 2 was proven by their N2 adsorption
measurements at 77 K, which show a type I sorption behavior
with Brunauer–Emmett–Teller (BET) surface areas of 382 and
288 m2 gÀ1, respectively. In the X-ray photoelectron (XPS) spec-
trum of 1, the binding energy of V 2p3/2 appeared at approxi-
mately 516.5 eV, suggesting that vanadium is in the +4 oxida-
Figure 1. a) View of the 2D layered framework of 1 along the a axis; b) view
of the pillared-layer structure; c) the 3D structure of 2 along the b axis [Cd
centers are shown as polyhedra in (c)].
As expected, 1a was found to show significant asymmetric
induction in the cyanosilylation reaction. Typically, treatment of
p-bromobenzaldehyde with TMSCN (1.2 equiv) and Ph3PO
(1.0 equiv) in the presence of 1 mol% 1a (relative to the alde-
hyde) in 1,2-dichloroethane at 08C for 36 h gave the cyanohy-
drin silyl ether with 94% conversion and 92% ee. The product
was obtained in 72% and 83% conversions after 8 and 16 h,
respectively, and if the reaction was further continued for 36 h,
94% conversion could be obtained. Notably the enantioselec-
tivity from 1 h to 36 h remained almost constant (91–92%).
The reactions may be catalyzed by a double activation process
occurring through the catalysis of both the Lewis acid [VO(sa-
len)] and the Lewis base PPh3O, which activate aldehyde and
TMSCN, respectively.[11] The absence of PPh3O led to decreased
conversion and enantioselectivity (75% conversion and
tion state.[10] Treating evacuated
1 with (NH4)2Ce(NO3)6
(1.2 equiv) in acetonitrile for 4 h resulted in the formation of
oxidized [VVO(salen)] complex 1a, accompanying with a slightly
color change from dark green to green. The complete oxida-
tion of VIV to VV was evidenced by the V 2p3/2 line appeared at
approximately 518.0 eV in XPS.[10] Although the N2 adsorption
measurement showed only surface adsorption, 1a could readi-
ly adsorb 14.3 wt% methyl orange (MO; ca. 1.47”0.53”
0.53 nm in size) in solution (1 adsorbed 15.3 wt% MO). The
MO-containing solid exhibited almost the same PXRD pattern
as 1. This result indicates that the structural integrity and chan-
nels of 1a are maintained in solution.
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46% ee). Control experiment showed that the parent MOF
1 could also catalyze the reaction, but only moderate conver-
sion and enantioselectivity were achieved (48% conversion
and 65% ee), probably because VIV is a weaker Lewis acid than
VV and is not capable of bonding strongly to the aldehydes.[8c]
Benzaldehyde and a wide range of different benzaldehydes
bearing either electron-withdrawing or electron-donating
groups, as well as different substituted systems, could be suc-
cessfully cyanosilylated to the desired products in good con-
versions with up to 95% ee (Table 1, entries 1–6). Good conver-
sions and enantioselectivities were also obtained for 1- and 2-
naphthaldehyde (Table 1, entries 7 and 8). Aldehydes bearing
heterocyclic groups such as 2-thiophenecarbaldehyde were
and the cyanosilylation reaction indeed occurs inside the
framework. However, further study is needed to elucidate the
reaction mechanism, especially the role of PPh3O.
The confinement effect of the MOF catalyst was also studied
by comparing the catalytic activities of 1a and the homogene-
ous catalyst [VVO(Me2L1)]. 1.0 mol% of [VVO(Me2L1)] (the same
loading of {VVO(salen)} as the heterogeneous reaction) cata-
lyzed the cyanation of p-bromobenzaldehyde, p-methoxybenz-
aldehyde, and 1- and 2-naphthaldehyde after 24 h, affording
the corresponding products with 93% conversion with
63% ee, 91% conversion with 89% ee, 90% conversion with
55% ee and 93% conversion with 63% ee, respectively. Thus
the catalyst 1a gave higher enantioselectivity and comparable
conversion relative to its homogeneous analogue, although it
required longer reaction time for slow mass diffusion in the
porous structure. Interestingly, the difference in catalytic per-
formances became larger at lower C/S ratios (the molar ratio of
{VO(salen)} to substrate). For instance, when the C/S ratio was
decreased to 1:200 and 1:1000, cyanation of p-bromobenzalde-
hyde afforded 72 and 50% conversions of the product with
[VVO(Me2L1)] with 54 and 27% ee, and 81 and 62% conversions
with 1a, with 84 and 72% ee, respectively. The obviously en-
hanced selectivity of 1a might be attributed to the restricted
movement of the substrates and multiple enantioselective in-
ductions in the porous structures.[1d,e]
Table 1. Asymmetric cyanosilylation catalyzed by 1a.[a]
Entry
R
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6[d]
7
8
9
Ph
p-BrC6H4
93
94
90
92
90
88
91
90
92
62
<5
62
92(S)
92(S)
92(S)
95(S)
94(S)
92(R)
92(R)
95(S)
92(S)
35(S)
n.d.
p-MeC6H4
m-MeOC6H4
p-MeOC6H4
p-MeOC6H4
1-naphthyl
2-naphthyl
2-thiophenyl
9-anthryl
In an effort to prove the heterogeneous nature of the above
system, the supernatant of 1a was employed for the cyanosil-
ylation of p-bromobenzaldehyde and no product was detected
at all under otherwise identical conditions. Inductively coupled
plasma atomic mass spectrometry (ICP-AMS) analysis of the
product solution after filtration of the insoluble 1a indicated
almost no leaching of metal ions (ca. 0.0096% for V and
ca. 0.001% for Zn) from the framework, further confirming the
heterogeneous nature of the MOF catalytic system. After the
completion of the reaction, the catalyst 1a could be recovered
by simple filtration and reused for the next run of the cyanosil-
ylation of p-bromobenzaldehyde without significant deteriora-
tion of the catalytic activity (conversion/ee for 1–5 runsꢀ94/
92%, 92/90%, 93/90%, 91/88% and 90/89%, respectively).
PXRD showed that the recycled sample still retained crystallini-
ty and structure intact, albeit slightly distorted. The XPS mea-
surement showed that the recovered vanadium complex re-
tained +5 oxidation state.
10
11
12[e]
coronenyl
coronenyl
n.d.
[a] For reaction details see Experimental Section in the Supporting Infor-
mation; catalyst and Ph3PO loading based on aldehyde; [b] calculated
1
based on H NMR spectroscopy; [c] determined by HPLC (letters in paren-
theses specify the preferred isomer); [d] catalyzed by 1 mol% (S)-1a;
[e] catalyzed by 1 mol% (R)-[VVO(Me2L1)].
also tolerated in this process without loss in efficiency and
enantioselectivity (Table 1, entry 9). The enantiocontrol of prod-
ucts is directed by the intrinsic handedness of the catalyst, as
shown by the (S)-1a-catalyzed cyanosilylation of p-methoxy-
benzaldehyde, which afforded the R enantiomer over the S
enantiomer with 92% ee (Table 1, entry 6).
We next investigated whether the catalysis occurred mainly
within the framework or just on the external surface by using
bulkier substrates as steric probes. When 9-anthraldehyde was
subjected to the cyanation reaction, moderate conversion
(62%) and low ee (35%) were obtained (Table 1, entry 10). The
sterically more hindered substrate coronenyl aldehyde yielded
almost no desired product (Table 1, entry 11), whereas the ho-
mogeneous control reaction with [VVO(Me2L1)] still gave 62%
conversion (Table 1, entry 12). Moreover, the use of a bulkier
tris[1-(2-methoxy)naphthyl]phosphane oxide as additive for the
1a-catalyzed cyanosilylation of p-bromobenzaldehyde afforded
almost no conversion, whereas the homogeneous control with
[VVO(Me2L1)] gave 42.6% conversion with 51% ee. These results
suggest that the MOF catalyst exhibits reagent size selectivity
In a similar way to 1, MOF 2 was oxidized to 2a, which
could adsorb 1.34 methyl orange molecules per formula unit
in solution (2 adsorbed 1.42 MO per formula unit). Under simi-
lar conditions, 2a also demonstrated excellent catalytic activity
and enantioselectivity in cyanation reactions, affording prod-
ucts with different substituents with good conversions and
82–90% ee (see the Supporting Information, Table S4). The ste-
reoselectivities of the confined framework catalyst were slightly
lower than those of 1a, but much higher than those of the ho-
mogeneous catalyst [VOL2] (up to 80% ee). The different selec-
tivities of 1a and 2a suggest that the lamellar [VO(salen)] MOF
might readily swell to facilitate enantioselective interactions
between substrates and catalytic active sites under reac-
tion.[1a,e] Notably, [VO(salen)] complexes have been immobilized
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on diverse supports such as carbon nanotubes, activated
carbon, and mesoporous silica, but these solid catalysts have
usually exhibited lower performances related to their homoge-
neous counterparts.[12] For the asymmetric cyanation reaction,
the enantioselectivity of the immobilized [VO(salen)] catalyst
can only attain as high as 90%.[12a] MOF-catalyzed cyanation re-
actions of aldehydes,[13] including few asymmetric reactions
with moderate to high enantioselectivities have also been re-
ported.[2d,e]
active centers for the ring-opening reaction within 2Cr. After
epoxidation, the recovered 2Cr remained intact, as indicated by
the PXRD pattern (see the Supporting Information, Figure S10).
Moreover, preliminary experiment indicated that 20 mol% of
the {VOL2} units in 2 could also be exchanged by [AlL2Cl]
through SALE, affording a [Al/VO(salen)] mixed MOF that could
not be obtained through direct synthesis. We have thus
shown, for the first time, that chiral MOF catalysts could be
prepared via SALE.[5] Further research on making metallosalen-
based mixed MOFs with different catalytic centers through
SALE for cooperative catalysis or tandem catalysis is in prog-
ress.
Direct solvothermal synthesis of a DMF/MeOH solution con-
taining CdI2, H2BPDC, and [CrL2Cl] or a mixture of [CrL2Cl] and
[VOL2] did not afford isostructural pillared-layer or related
MOFs. Given the high structural similarity of the [CrL2Cl] and
[VOL2] ligands, SALE was employed as a strategy to introduce
{Cr(salen)} functionality into the network of 2 (Scheme 2). Ex-
In conclusion, we have presented two chiral porous [VO(sa-
len)]-based MOFs and their oxidation to generate efficient and
recyclable heterogeneous catalysts for cyanation of aldehydes,
with improved stereoselectivity as compared with their homo-
geneous analogues. The pillared-layer MOF can function as
a robust parent material to fabricate [Cr/VO(salen)] or [Al/VO-
(salen)] mixed MOF catalysts through SALE for asymmetric ep-
oxide ring-opening reaction. This work highlights the ability to
develop metallosalen-based chiral catalytic chemistry that ap-
pears to be difficult or impossible to implement with molecular
catalysts.
Acknowledgements
Scheme 2. Solvent-assisted linker exchange of chiral MOF 2.
This work was supported by the NSFC (21371119, 21431004,
and 21401128), “973” Program (2014CB932102 and
2012CB8217), and SSTC (14YF1401300).
posing 2 to a DMF/MeOH solution of [CrL2Cl] (4 equiv) at 608C
for 8 h led to partial replacement of {VO(salen)} units with
{Cr(salen)Cl} (2Cr). Unfortunately, we failed to obtain high-quali-
ty crystals of 2Cr for single-crystal X-ray structure analysis de-
spite many attempts. ICP-OES suggested that 40 mol% of
{VOL2} was replaced by {CrL2}. The mixed-linker nature of 2Cr
was further supported by XPS, which gave V 2p3/2 and Cr 2p3/2
signals at 515.8 eV and 576.9 eV, respectively.[10,14] No apprecia-
ble difference between parent and daughter networks was ob-
served in the PXRD patterns and the exchanged 2Cr displayed
a slightly decreased BET surface area (256 m2 gÀ1) compared
with 2.
The catalytic applicability of 2Cr in asymmetric epoxide ring-
opening reaction was examined. In the presence of 5 mol% of
2Cr, cis-stilbene oxide reacted with aniline at room temperature
for 24 h in CH2Cl2 to afford the corresponding ring-opening
product in 87% conversion with 76% ee (Scheme 3). Control
experiments showed that [CrL2Cl] could catalyze this reaction,
affording 91% conversion and 93% ee, but [VOL2] and the
parent framework 2 exhibited no activity at all. This further
confirmed that the [Cr(salen)] species had been successfully in-
corporated into the pillared-layer framework and serve as
Keywords: asymmetric catalysis · chirality · linker exchange ·
metal–organic frameworks · vanadium
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Received: April 16, 2015
Published online on July 17, 2015
Chem. Eur. J. 2015, 21, 12581 – 12585
www.chemeurj.org
12585
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