Photoactive chiral metal-organic frameworks for light-driven asymmetric α-alkylation of aldehydes

Article abstract of DOI:10.1021/ja305367j

Chiral metal-organic frameworks (MOFs) with porous and tunable nature show promise as heterogeneous asymmetric catalysts. Through incorporating the stereoselective organocatalyst l- or d-pyrrolidin-2-ylimidazole (PYI) and a triphenylamine photoredox group into a single framework, we have developed two enantiomeric MOFs, Zn-PYI1 and Zn-PYI2, to prompt the asymmetric α-alkylation of aliphatic aldehydes in a heterogeneous manner. The strong reductive excited state of the triphenylamaine moiety within these MOFs initiated a photoinduced electron transfer, rendering an active intermediate for the α-alkylation. The chiral PYI moieties acted as cooperative organocatalytic active sites to drive the asymmetric catalysis with significant stereoselectivity. Control experiments using the lanthanide-based metal-organic frameworks Ho-TCA and MOF-150, assembled from 4,4′,4″- nitrilotribenzoic acid, as catalysts suggested that both the photosensitizer triphenylamine moiety and the chiral organocatalyst d-/l-PYI moiety were necessary for the light-driven α-alkylation reactions. Further investigations demonstrated that the integration of both photocatalyst and asymmetric organocatalyst into a single MOF makes the enantioselection superior to that of simply mixing the corresponding MOFs with the chiral adduct. The easy availability, excellent stereoselectivity, great separability, and individual components fixed with their well-defined porous and repeating structures make the MOF a versatile platform for a new type of tandem catalyst and cooperative catalyst.

Full text of DOI:10.1021/ja305367j

Article
pubs.acs.org/JACS
Photoactive Chiral Metal−Organic Frameworks for Light-Driven
Asymmetric α‑Alkylation of Aldehydes
Pengyan Wu, Cheng He,* Jian Wang, Xiaojun Peng, Xuezhao Li, Yonglin An, and Chunying Duan*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, People's Republic of China
S
* Supporting Information
ABSTRACT: Chiral metal−organic frameworks (MOFs) with porous and
tunable nature show promise as heterogeneous asymmetric catalysts. Through
incorporating the stereoselective organocatalyst L- or D-pyrrolidin-2-ylimidazole
(PYI) and a triphenylamine photoredox group into a single framework, we
have developed two enantiomeric MOFs, Zn−PYI1 and Zn−PYI2, to prompt
the asymmetric α-alkylation of aliphatic aldehydes in a heterogeneous manner.
The strong reductive excited state of the triphenylamaine moiety within these
MOFs initiated a photoinduced electron transfer, rendering an active
intermediate for the α-alkylation. The chiral PYI moieties acted as cooperative organocatalytic active sites to drive the
asymmetric catalysis with significant stereoselectivity. Control experiments using the lanthanide-based metal−organic frameworks
Ho−TCA and MOF−150, assembled from 4,4′,4″-nitrilotribenzoic acid, as catalysts suggested that both the photosensitizer
triphenylamine moiety and the chiral organocatalyst D-/L-PYI moiety were necessary for the light-driven α-alkylation reactions.
Further investigations demonstrated that the integration of both photocatalyst and asymmetric organocatalyst into a single MOF
makes the enantioselection superior to that of simply mixing the corresponding MOFs with the chiral adduct. The easy
availability, excellent stereoselectivity, great separability, and individual components fixed with their well-defined porous and
repeating structures make the MOF a versatile platform for a new type of tandem catalyst and cooperative catalyst.
molecular sensing, separation, and medical applications.21−24
In particular, chiral porous MOFs are ideally suited for
heterogeneous asymmetric catalytic conversions because they
INTRODUCTION
■
Catalysis is at the heart of chemistry and provides tools for
efficiently and selectively making and breaking chemical bonds
impose size- and shape-selective restrictions through readily
that are crucial for converting basic chemicals into useful
fine-tuned channels and pores25,26 and high enantioselectivity
products for society in a sustainable fashion.1,2 The observation
that light alone could affect unique chemical changes in organic
compounds led early 20th century photochemists to recognize
through the imbedded, regularly ordered chiral functional-
ities.27 The intrinsic crystalline properties also provided precise
knowledge about the pore structure and the nature and
that the sun might represent an inexhaustible source of clean
distribution of catalytically active sites.28−30 In comparison to
chemical potential.3−6 Building on the seminal results by
the heterogeneous catalytic systems that have been examined
employing photoinduced electron transfer processes, a variety
of methods have been developed recently by applying
earlier, the design flexibility and framework tenability resulting
organometallic complexes such as [Ru(bpy)3]2+ and [Ir-
from the huge variations of metal nodes and organic linkers
(ppy)2(dtb-bpy)]+ (bpy = bipyridine, ppy = 2-phenylpyridine,
likely allow the introduction of two independent catalytic units
into one well-defined MOF.31,32 Especially, the incorporation
dtb-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine).7−10 Of particular
of an asymmetric organocatalyst within a photoactive MOF
interest is the cooperative combination of a photocatalyst with
might be an efficient approach to create new synergistic
an organo-catalytic cycle, offering one of the rare catalytic
methods for the α-alkylation of aldehydes.11−13 Like other
precious-metal-catalyzed reactions, it is highly desirable to
develop recyclable and reusable photocatalytic systems based
catalysts combining both photocatalytic and organocatalytic
cycles.
By combining an easily prepared asymmetric organo-catalytic
group, L- or D-pyrrolidine-2-ylimidazole (PYI), and a photo-
on the metal−organic chromophores or more environmentally
friendly organic dyes.14,15 In particular, the ability to recover
active nitrilotribenzoate building block within a single MOF,
herein we report the design and synthesis of two new
and reuse photocatalysts in a heterogeneous manner can not
enantiomorphs of chiral MOFs, Zn−PYI1 and Zn−PYI2
only eliminate the contamination of organic products by trace
(Scheme 1), which can perform light-driven asymmetric α-
amounts of heavy metals but also reduce the processing and
waste disposal costs in large-scale reactions.16,17
alkylation of aliphatic aldehydes in a heterogeneous manner.
We envisioned that the triphenylamine moiety in the MOF
On the other hand, metal−organic frameworks (MOFs) are
hybrid solids with infinite network structures built from organic
bridging ligands and inorganic connecting points.18−20 These
Received: June 3, 2012
materials have shown promise in catalysis, gas storage,
© XXXX American Chemical Society
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Scheme 1. Schematic Representation of the Mirror Image
Structures of Zn−BCIP1 and Zn−BCIP2 and Their
Deprotected Forms Zn−PYI1 and Zn−PYI2, Showing the
Photosensitizer, 4,4′,4″-Nitrilotribenzoic Acid (Red or
Simply Drawn as Block Red Bars), and the Chiral
Organocatalytic Moiety, L- or D-Proline Derivatives (Blue)
Figure 1. Crystal structure of Zn−BCIP1 showing the two-
dimensional sheet constructed by 4,4′,4″-nitrilotribenzoic acid and
zinc ions and the chiral organocatalytic moiety, L-proline derivative,
positioned within the cavities as well as the fragments of the MOF
including the light-harvesting groups (gray), the chiral organocatalyst
unit (blue), the removable Boc moiety (green), and the metal centers
(cyan).
structures further align parallel to each other, completing the
packing pattern and forming 1D channels of 12 × 16 Å2 viewed
along the [100] direction (Scheme 1). L-BCIP molecules locate
within the channels with the potential catalytically active site
N−H of pyrrolidine protected by the tert-butoxycarbonyl (Boc)
group. The use of the thermolabile Boc as the starting material
for the MOF synthesis can protect the pyrrolidine moiety and
preclude framework interpenetration, producing open frame-
works with the vacant space adjacent to the catalytic units.43,44
Thermogravimetric analysis (TGA) of Zn−BCIP1 exhibited
a significant weight loss of 12.9%, approximately corresponding
to one Boc moiety per triphenylamine group in the region
100−200 °C, suggesting that the thermolytic reaction of the
Boc moiety was possibly taking place within this temperature
range. Deprotection of the proline unit was accomplished by
simply heating as-synthesized Zn−BCIP1 in a dry N,N-
dimethylformamide solution using microwave irradiation to
generate the new compound Zn−PYI1 in a postsynthetic
fashion (Figure 2).43−46 The expulsion of the Boc group was
likely worked as an electron donor to initiate a photoinduced
single electron transfer from its excited state to the diethyl 2-
bromomalonate,33−36 a well-known α-alkylation agent, render-
ing an active radical that potentially undergoes σ-bond cleavage
to afford an active intermediate for the α-alkylation of an
aliphatic aldehyde.37 In the meantime, the chiral PYIs would act
as cooperative organocatalytic active sites, where a highly π-
nucleophilic enamine combines with the electron-deficient
radical to forge a crucial reaction center that drives the
heterogeneous catalysis in an asymmetric manner,37−40 as used
in asymmetric aldol reactions and other asymmetric trans-
formations.41
RESULTS AND DISCUSSION
■
The solvothermal reaction of 4,4′,4″-tricarboxyltriphenylamine
(H3TCA) and Zn(NO3)3·6H2O in the presence of L-N-tert-
butoxycarbonyl-2-(imidazole)-1-pyrrolidine (L-BCIP)42 in a
mixed solvent of DMF and ethanol gave the compound Zn−
BCIP1 in a yield of 60%. Elemental analyses along with powder
X-ray analysis (XRD) indicated the pure phase of its bulk
sample. Single-crystal X-ray structural analysis revealed that
Zn−BCIP1 crystallizes in the chiral space group P21. It has a
two-dimensional brick wall layered structure built from three-
connected binuclear zinc nodes and 4,4′,4″-nitrilotribenzoate
bridges (Figure 1). In each binuclear unit, one of the Zn atoms
is coordinated by five oxygen atoms from one bridged hydroxyl
anion, one water molecule, and one bidentate and one
bimonodentate carboxylate group of two different TCA3−
anions, resulting in a distorted-trigonal-bipyramidal geometry.
Another zinc atom is coordinated in a distorted-tetrahedral
geometry with two carboxylate oxygen atoms from different
TCA3− anions, one bridged hydroxyl anion, and one nitrogen
atom from the L-BCIP ligand, respectively. The adjacent layer is
stacked face to face with pairs of L-BCIP located within the
cavities, ensuring the chirality of the whole layer. These bilayer
1
further verified by H NMR of the MOF crystals digested in
DMSO-d6/DCl and IR spectra. The phase purity of Zn−PYI1
was established by powder X-ray diffraction. The observed
diffraction pattern closely matched that of Zn−BCIP1,
evidencing the maintenance of crystallinity.
Dye-uptake studies were carried out by soaking Zn−PYI1 in
a methanol solution containing 2′,7′-dichorofluorescein for 12
h. After the soaked Zn−PYI1 was washed with solvent several
times to remove dye molecules adsorbed on the external
surfaces of the crystals, the dye-loaded Zn−PYI1 was then
digested with sodium ethylenediaminetetraacetic acid. The
amounts of released 2′,7′-dichorofluorescein were quantified by
ultraviolet−visible spectroscopy and demonstrated an uptake
equivalent to as much as 14% of the framework weight.47,48
Since the as-synthesized Zn−BCIP1 did not exhibit any
obvious dye uptake, these results clearly implied the releasing
of channels during the deprotection process. In this case, the
dye uptake experiment also demonstrated conclusively the
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Figure 3. (a) Circular dichroism spectra of bulk crystals of Zn−BCIP1
and Zn−BCIP2. (b) Circular dichroism spectra of bulk crystals of
Zn−PYI1 and Zn−PYI2. (c) Circular dichroism spectra of L-BCIP
and D-BCIP in DMF solution. (d) UV−vis absorption spectra of Zn−
BCIP1 and Zn−PYI1 in the solid state.
Figure 2. Crystal structure of the multifunctional framework Zn−
BCIP1 showing the packing pattern of these layers (top) and the
simulated structure of Zn−PYI1 obtained by thermolytic expulsion of
the Boc moieties (bottom), showing the enlargement of the porous
MOFs within the channels. The cyan, red, blue, gray, and green balls
represent the Zn, O, N, and C atoms and Boc moiety, respectively.
accessibility of the potential actively catalytic sites to the
organic substrates through the open channels.
Figure 4. (a) Solid-state CV of Zn−BCIP1 and Zn−PYI1 with a scan
rate of 50 mV s−1 in the scan range 0.5−1.1 V. (b) Normalized
absorption (black line) and emission spectra (red line) of Zn−PYI1,
excited at 350 nm.
Circular dichroism (CD) spectrum of Zn−BCIP1 exhibited a
positive Cotton effect at about 260 nm and a negative Cotton
effect centered at 330 nm (Figure 3). The spectrum was quite
different from that of L-BCIP. The peak at 330 nm was
assignable to the absorbance difference corresponding to the
π−π* transition of the triphenylamine groups.49−51 This result
might be one of the indicators for the homochirality of the
whole framework. Moreover, CD spectroscopy of Zn−PYI1
showed a negative Cotton effect centered at 345 nm and a
positive Cotton effect at about 245 nm, indicating the
maintenance of chirality from Zn−BCIP1 to Zn−PYI1.
UV−vis absorption spectrum of Zn−PYI1 in the solid state
exhibited an absorption band centered at 350 nm typically
assignable to the π−π* transition of the triphenylamine
group.49−51 Upon excitation at this absorption band, Zn−
PYI1 showed an intense luminescence band at about 450 nm,
as reported in the related compounds.52 Solid-state electro-
chemical measurements exhibited a redox potential at 0.84 V
(vs SCE), assignable to the redox potential of the Zn−PYI1+/
Zn−PYI1 couple (Figure 4). The redox potential of the
excited-state Zn−PYI1+/Zn−PYI1* couple was calculated as
−2.12 V on the basis of a free energy change (E0−0) between
the ground state and the vibrationally related excited state of
2.96 eV.53,54 This potential was comparable to that of the
excited state fac-Ir(ppy)3* in an acetonitrile solution and was
more negative than that of the diethyl 2-bromomalonate (E0 =
−0.49 V).11,37 As a consequence, Zn−PYI1 was considered to
be a candidate for the photocatalysis of the alkylation reaction
of the aldehydes coupled with diethyl 2-bromomalonate.
The pores of Zn−PYI1 showed adsorption ability toward
phenylpropylaldehyde or diethyl 2-bromomalonate, the possi-
ble substrates for asymmetric α-alkylation. As-synthesized Zn−
PYI1 was immersed in CH2Cl2 containing each reagent,
phenylpropylaldehyde or diethyl 2-bromomalonate, respec-
tively. The powder was filtered off and dried in air, and then its
1H NMR and IR spectra were acquired. 1H NMR of Zn−PYI1
adsorbed the substrates digested in DMSO-d6/DCl demon-
strated that Zn−PYI1 could adsorb about 1 equiv of
phenylpropylaldehyde per proline moiety. IR of the catalyst
impregnated with a CH2Cl2 solution of phenylpropylaldehyde
revealed a CO stretching vibration at 1715 cm−1 for
phenylpropylaldehyde included in Zn−PYI1. The red shift
from 1724 cm−1 (free aldehyde) suggested the adsorption and
the activation of the aldehyde in the channels of the MOFs,
through the interactions between Zn−PYI1 and the CO
group of the aldehyde, possibly giving a highly π-nucleophilic
enamine.
An absorption experiment also demonstrated that Zn−PYI1
could also adsorb 1.5 molecular stoichiometric amounts of
diethyl 2-bromomalonate per triphenylamine moiety. Notice-
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ably, the luminescence intensity of Zn−PYI1 was significantly
reduced when it absorbed diethyl 2-bromomalonate molecules
(Figure 5).55 Luminescence decays at 450 nm for Zn−PYI1
redox conditions. Zn−PYI1 thus represents the first example of
an MOF-based heterogeneous asymmetric photocatalyst for
this important reaction, making an MOF a versatile platform for
a new type of tandem catalyst and cooperative catalyst.
A number of control experiments were carried out to unveil
the heterogeneous and photocatalytic nature of the reactions. A
reaction set in the dark yielded negligible amounts of the α-
alkylation product, manifesting the necessity of light for this
reaction. The use of Zn−BCIP1 instead of Zn−PYI1 as the
catalyst could not prompt the reaction, possibly due to either
the poor porosity of Zn−BCIP1 or the low activating ability of
the protected amine moiety toward the substrates. In addition,
the removal of Zn−PYI1 by filtration after 12 h shut down the
reaction, affording only 5% additional conversion upon stirring
for another 12 h. These experiments demonstrate that Zn−
PYI1 is a true heterogeneous catalyst.58 It is also quite
significant that solid Zn−PYI1 can be easily isolated from the
reaction suspension by simple filtration alone and can be reused
at least three times with a slight decrease in its reactivity and
selectivity (yield decreasing from 74% to 70% with the ee value
decreasing from 92% to 88%). The index of XRD patterns of
the Zn−PYI1 bulk sample filtered off from the catalytic
reaction evidence the maintenance of the framework. Also, the
dye uptake study on the recovered catalysts exhibited almost
same uptake ability (13%) for 2′,7′-dichorofluorescein,
comparable with that of its original form.
Figure 5. (a) Solid-state emission spectra of Zn−PYI1 (black line),
Zn−PYI1 upon absorbance of phenylpropylaldehyde (blue line) and
diethyl 2-bromomalonate (red line), excited at 350 nm. (b) Transient
emission spectra of solid Zn−PYI1 and Zn−PYI1 with diethyl 2-
bromomalonate incorporated.
exhibited a biexponential fashion with the lifetime calculated as
2.36 ns. The decrease of fluorescence lifetime of Zn−PYI1
adsorbed diethyl 2-bromomalonate (1.51 ns) suggested the
occurrence of a photoinduced electron transfer process from
Zn−PYI1* to the diethyl 2-bromomalonate molecules.
However, the adsorbance of phenylpropylaldehyde hardly
quenched the luminescence significantly under the same
experimental conditions. These results demonstrated that the
quenching process was typically attributed to the photoinduced
electron process from Zn−PYI1* to diethyl 2-bromomalonate,
enhancing the possibility of occurrence of light-induced α-
alkylation of aldehydes.56,57
The smooth reaction between diethyl 2-bromomalonate and
phenylpropylaldehyde, octaldehyde, or (E)-non-6-enal, respec-
tively, suggested that the window size of the catalyst is large
enough to allow these substrates to pass through (Table 2).
Table 2. Molecular Size of Aliphatic Aldehyde Substrates for
a
α-Alkylation
As shown in Table 1, the transformation was examined
initially by using phenylpropylaldehyde and diethyl 2-
Table 1. Conversions and ee Values (in Parentheses) of the
Photocatalytic α-Alkylation of Aliphatic Aldehydes
a
The assumed structures and the molecular size were calculated by
using the program Chem3D.61,62
However, in the presence of bulky aldehyde 1d,59,60 whose size
is larger than the pore size of Zn−PYI1 as the aldehyde
substrate, the photocatalytic α-alkylation reaction only gave 7%
of product under the same reaction conditions. The size
selectivity of the substrate suggested that the alkylation
reactions occurred mostly in the channel of the catalyst, not
on the external surface. Further adsorption experiments by
immersing Zn−PYI1 into a solution of 1d also confirmed that
the substrate 1d was too large to be adsorbed in the channels of
the MOF.
a
Reaction conditions: diethyl 2-bromomalonate, 0.385 mmol; aliphatic
aldehyde, 0.769 mmol; 2,6-lutidine, 0.769 mmol; catalyst, 1.9 μmol
(0.5 mol %) per triphenylamine moiety; 26 W fluorescent lamp, room
temperature under N2 for 24 h. In the presence of additional chiral
b
D-/L-PYI (76.9 μmol).
bromomalonate as the coupling partners, along with Zn−
PYI1 and a common fluorescent lamp (26 W) as the light
source. The results revealed the successful execution of our
MOF design, showing a high reaction efficiency (74% in yield)
and excellent enantioselectivity (92% ee). Other substrates
(e.g., octaldehyde and (E)-non-6-enal) were also found to be
prone to react with diethyl 2-bromomalonate under these mild
Similarly, the use of D-BCIP resulted in the formation of
another enantiomorph, Zn−BCIP2. Zn−BCIP2 crystallizes in
the chiral space group P21 with cell dimensions similar to those
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of Zn−BCIP1. The CD spectrum of Zn−BCIP2 exhibited
Cotton effects opposite to those of Zn−BCIP1, as expected for
a pair of enantiomers with a mirror-image relationship between
each other.63,64 Also, the photoactive MOF Zn−PYI2 was
obtained by the same synthetic procedure. The solids of the
catalysts with Cotton effects opposite to those of Zn−PYI1
exhibited similar catalytic activities but gave different products
with opposite chiralities in the alkylation reaction of aldehydes
with diethyl 2-bromomalonate. This implies that our approach
could benefit the design and synthesis of efficient asymmetric
catalysts for producing chiral fine chemicals and pharmaceut-
icals, where enantiomorphs with different chiralities manifest
differentiation in their optical properties and biological
activities.65
Ho−TCA was similar to that of the porous metal−organic
framework MOF−150, a MOF assembled from zinc ions and
4,4′,4″-nitrilotribenzoic acid, which was used for control
reactions (vide infra).67 The secondary building unit (SBU)
Ho4(OH)4 cluster is composed of four Ho centers bridged by
six carboxylate groups from six TCA3− ligands, and in turn each
TCA3− ligand connects three Ho4(OH)4 clusters to form a
(6,3)-connected network. Each Ho ion is eight-coordinated by
three μ3-bridged hydroxyl anions, three carboxyl oxygen atoms,
and two water molecules. The 3D network structure of Ho−
TCA can be viewed as a PdF2− net68 or a FeS2− net69 with
sections of 15.0 × 17.8 Å2, which are large enough for the
accommodation and exchange of the corresponding small
guests.
The free volume for fully desolvated Ho−TCA was
estimated to be ∼58.6% by PLATON70 software; TGA
analyses also showed that Ho−TCA exhibited an impressive
solvent weight loss of 10.5 wt % in the temperature range 50−
200 °C, indicative of a stable open framework structure. Dye
uptake studies exhibited a 23% uptake of 2,7-dichlorofluor-
escein corresponding to the framework weight, illustrating the
accessibility of the MOF to small molecules. Solid-state UV−vis
absorption and luminescence spectra of Ho−TCA exhibited the
characteristic absorption band centered at 350 nm and emission
band at about 435 nm. The blue shift of the emission band
leading to higher energy of the excited state of the Ho−TCA is
likely to give it better catalytic ability.
+
Similar to the case for fac-Ir(ppy)3 in the photocatalysis
reaction,37 the excited state of triphenylamine could act as a
reductant in the single electron transfer reaction from Zn−
PYI1* to diethyl 2-bromomalonate that renders a radical anion.
According to a discussion on the mechanism of the
photocatalysis reaction,37 the resulting radical anion rapidly
undergoes σ-bond cleavage to afford the bromide anion and an
electrophilic radical. Within the same time frame, the
condensation of an aldehyde substrate with N−H of a
proline-based catalyst likely gives a highly π-nucleophilic
enamine which then combines with the electron-deficient
radical to forge a crucial reaction center. Consequently, the
rapid oxidation of the resulting amino radical by Zn−PYI1+
would then close the redox cycle and regenerate the
photocatalyst Zn−PYI1.11,66
As-synthesized Ho−TCA was immersed in CH2Cl2 solution
containing diethyl 2-bromomalonate or phenylpropylaldehyde,
respectively, and stirred. The powder was filtered off and dried
To further investigate the catalytic performance of MOFs
containing light-harvesting triphenylamine groups, a new
infinite 3D (6,3)-connected net with μ3-oxo-bridged
Ho4(OH)4 anamorphic cubic clusters as nodes and triphenyl-
amine moieties as three-connected bridges was achieved via a
solvothermal reaction using H3TCA and Ho(NO3)3·6H2O as
the original reactants (Figure 6). Elemental analyses along with
a powder X-ray analysis were indicative of the pure phase of its
1
in air. H NMR spectra demonstrated that Ho−TCA could
include about two molecular stoichiometric amounts of diethyl
2-bromomalonate per triphenylamine moiety or 1 equiv of
phenylpropylaldehyde per holium ion, respectively. The
adsorption of diethyl 2-bromomalonate also quenched the
luminescence of Ho−TCA significantly. Exponential decay
profiles suggested a significant decrease of luminescence
lifetime from 2.26 ns for free Ho−TCA to 1.16 ns for diethyl
2-bromomalonate adsorbed Ho−TCA (Figure 7). The
presence of phenylpropylaldehyde hardly quenched the
luminescence of Ho−TCA, either with or without the
organocatalyst PYI. These results confirmed that the electron
transfer event should occur between the excited state of the
photosensitizer triphenylamine moiety and diethyl 2-bromo-
bulk sample. Ho−TCA crystallizes in the space group P42 c.
̅
1
Single crystal structure analysis revealed that the topology of
Figure 7. (a) Solid-state emission spectra of Ho−TCA upon
adsorption of diethyl 2-bromomalonate (red line) or phenyl-
propylaldehyde and L-PYI (blue line), respectively. (b) Transient
emission spectra of solid Ho−TCA and Ho−TCA upon absorption of
diethyl 2-bromomalonate. The intensities were recorded at 435 nm, on
excitation at 350 nm.
Figure 6. Synthetic approach of Ho−TCA (top) and its crystal
packing along a direction showing the channels with metal ions and
photoactive moieties exposed (bottom).
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malonate, in accord with the proposed catalytic process
mentioned above.
Solid state electrochemical measurements exhibited the redox
potential E1/2(Ho−TCA+/Ho−TCA) of 0.86 V (vs SCE) in
the scan range 0.5−1.1 V (Figure 8). Accordingly, a redox
Ho−TCA adsorbed L-PYI did not show any obvious Cotton
effect peaks corresponding to the triphenylamine groups,
suggesting that its fragments were hardly positioned within a
chiral environment. The immersion of Ho−TCA into a
methanol solution containing a mixture of L-PYI and
phenylpropylaldehyde for 12 h gave the solid of Ho−TCA
containing 1 equiv of L-PYI and 1 equiv of phenyl-
propylaldehyde per holmium ion. This result suggested that
L-PYI was likely to work as a chiral organocatalyst interacting
with the aldehyde, possibly giving a highly π-nucleophilic
enamine.11
Furthermore, the catalytic reactions were also size selective;
replacing the aldehyde with the bulky aldehyde 1d under the
same reaction condition coupled with diethyl 2-bromomalonate
only caused a negligible conversion of the products. The
adsorption experiments also confirmed that the substrate 1d
was too large to be absorbed within the channels of the MOF
Ho−TCA. The results also suggested that the reaction
occurred mostly in the channel of the Ho−TCA. Solid Ho−
TCA can be easily isolated from the reaction suspension by
simple filtration alone and can be reused at least three times
with a slight decrease in its reactivity and selectivity (yield
decreasing from 86% to 84%). Similarly, the use of D-PYI
resulted in the formation of another enantiomorph with similar
conversions. For all the substrates, the obtained yields of the
isolated products of entries 3 and 4 are better than that of entry
1 and entry 2 (Table 1), but the ee values are much lower than
those for Zn−PYIs. The index of XRD patterns of the Ho−
TCA bulk sample filtered off from the catalytic reaction
evidences the maintenance of the framework. Also, the dye
uptake study on the recovered catalysts exhibited almost the
same uptake ability (24%) on 2′,7′-dichlorofluorescein, in
comparison with its original form. These experiments
demonstrate that Ho−TCA is also a heterogeneous catalyst.
The higher transformation efficiencies of Ho−TCA can likely
be attributed to the more negative redox potentials of the
excited states of the triphenylamine moiety. In addition, the
much lower asymmetric selectivity of Ho−TCA can possibly be
attributed to the absence of chiral environments.
Figure 8. (a) Solid-state CV of Ho−TCA and MOF−150 with a scan
rate of 50 mV s−1 in the range 0.5−1.1 V. (b) Normalized absorption
(black line) and emission spectra (red line) of Ho−TCA, excited at
350 nm.
potential of −2.20 V for that of the excited state E1/2(Ho−
TCA+/Ho−TCA*) was calculated according to a E0−0 of 3.06
eV obtained by the electronic spectra.53,54 As mentioned above,
this potential is negative enough to reduce diethyl 2-
bromomalonate and is more negative than that of the redox
potential of the excited state Zn−PYI1+/Zn−PYI1* couple; it
is postulated that Ho−TCA possibly exhibits better catalytic
effects for the α-alkylation of aliphatic aldehyde.
As can be expected, reaction of phenylpropylaldehyde with
diethyl 2-bromomalonate in the presence of 20 mol % of the
secondary amine L-PYI as the organocatalyst with photocatalyst
Ho−TCA afforded the α-alkylation product with a yield of 86%
and ee value of 20%. Importantly, removal of L-PYI from the
standard protocol resulted in negligible alkylation product
during the same time frame. The result demonstrated that L-
PYI played an important role in activating the reaction
substrates and was a necessary organocatalyst for this
reaction.11 1H NMR of Ho−TCA immersed in L-PYI solution
demonstrated that Ho−TCA could adsorb about 1 equiv of L-
PYI per holmium ion (Figure 9); however, CD spectroscopy of
At the same time, the previously reported MOF−150
(Figure 10), containing 4,4′,4″-nitrilotribenzoic acid as the
building blocks, was also chosen as a comparison catalyst for
the light-driven asymmetric α-alkylation of aliphatic aldehydes.
Diethyl 2-bromomalonate also considerably decreased the
luminescence intensities of MOF−150 with a quenching
efficiency of about 26%. Solid-state electrochemical measure-
Figure 9. 1H NMR spectra of Ho−TCA (a), Ho−TCA adsorbed
phenylpropylaldehyde (b), Ho−TCA adsorbed L-PYI (c), and Ho−
TCA adsorbed phenylpropylaldehyde and L-PYI (d). Peaks marked
with red triangles and blue triangles represent signals of phenyl-
propylaldehyde and L-PYI, respectively.
Figure 10. Crystal structure of MOF−15069 along a direction showing
the cavity channels with zinc acetate octahedral SBUs composed of
four ZnO4 tetrahedra sharing a common corner and photoactive
moieties exposed.
F
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Journal of the American Chemical Society
Article
intensity was recorded at 450 nm, and excitation was at 350 nm.
Adding Ho−TCA (52 mg) to a dichloromethane solution containing
substrates and stirring for 12 h afforded new crystalline solids; the
intensity was recorded at 435 nm, and excitation was at 350 nm. Solid-
state cyclic voltammograms were measured by using a carbon-paste
working electrode; a well-ground mixture of each bulk sample and
carbon paste (graphite and mineral oil) was set in the channel of a
glass tube and connected to a copper wire. A platinum-wire counter
electrode and an Ag/AgCl reference electrode were used. Measure-
ments were performed by using a three-electrode system in phosphate-
buffered saline [PBS] at a scan rate of 50 mV s−1, in the range 0.5−1.1
V.
ments exhibited a redox potential at 0.85 V for MOF−150,
from which the redox potential of the related excited state was
calculated as −2.09 V. These potentials are negative enough to
reduce diethyl 2-bromomalonate, so that MOF−150 excited
state might also behave as a reductant in the photoredox cycle.
The reactions catalyzed by MOF−150 in the presence of
additional chiral organo-catalyst L-/D-PYI produced alkylation
products in good yields (62−80%) and low enantioselectivity
(ee 20−22%), as shown in entries 5 and 6 of Table 1.
Both Ho−TCA and MOF−150 are effective photocatalysts
for the α-alkylation reaction between aldehydes and diethyl 2-
bromomalonate, demonstrating that the triphenylamine frag-
ment is the key component for photocatalytic activity. The
different transformation efficiencies of the catalytic reactions
can likely be attributed to the different redox potentials of the
excited states of triphenylamine moieties in different environ-
ments as well as the luminescence quenching efficiencies
related to the adsorbed diethyl 2-bromomalonate substrate.
Interestingly, the ee values using Ho−TCA or MOF−150 as
photocatalyst are much lower than that using Zn−PYIs, despite
the fact that the same chiral adduct L-PYI (or D-PYI) was used
in the reaction. Clearly, the integration of both photocatalyst
and asymmetric organocatalyst into a single MOF makes the
enantioselection superior to that of simply mixing the
corresponding MOFs with the chiral adduct. Most likely this
is due to the restricted movement of the substrates within the
MOF’s interior and multiple chiral inductions as well. The easy
availability, excellent stereoselectivity, and great separability and
also the individual components fixed with their well-defined
porous and repeating structures make the MOF a versatile
platform for a new type of tandem catalyst and cooperative
catalyst.
Synthesis of Zn−BCIP1. A mixture of 4,4′,4″-tricarboxytriphenyl-
amine (H3TCA; 94 mg, 0.25 mmol), Zn(NO3)2·6H2O (292 mg, 1
mmol) and L-BCIP (150 mg, 1 mmol) were dissolved in 15 mL mixed
solvents of DMF and ethanol in a screw-capped vial. The resulting
mixture was kept in an oven at 100 °C for 3 days. Crystals suitable for
1
X-ray structural analysis were obtained after filtration. Yield: 60%. H
NMR (400 MHz, DMSO-d6/DCl): δ = 9.10 (s, 1H, imidazole ring),
7.92, 7.90 (d, 6H, Ph), 7.74−7.65 (t, 2H, imidazole ring), 7.15, 7.13
(d, 6H, Ph), 4.28−4.24 (m, 1H, CH2), 4.14−4.10 (m, 2H, CH2 and
pyrrole ring CH), 3.27−3.25 (m, 2H, pyrrole ring CH2), 1.80−1.64
(m, 4H, 2 × pyrrole ring CH2), 1.42 (s, 9H, tri-tert-butyl). Anal. Calcd
for Zn2H2O(OH)(C21H12NO6)(C13H21N3O2): C, 51.60; H, 4.58; N,
7.08. Found: C, 51.54; H, 4.42; N, 7.10. FTIR (KBr pellet; cm−1):
3416 (w), 2975 (m), 2927 (m), 2893 (m), 1595 (m), 1392 (m), 1315
(m), 1275 (m), 1176 (m), 1090 (s), 1050 (s) cm−1.
Synthesis of Zn−BCIP2. A mixture of 4,4′,4″-tricarboxytriphenyl-
amine (H3TCA; 94 mg, 0.25 mmol), Zn(NO3)2·6H2O (292 mg, 1
mmol), and D-BCIP (150 mg, 1 mmol) were dissolved in 15 mL of
mixed solvents of DMF and ethanol in a screw-capped vial. The
resulting mixture was kept in an oven at 100 °C for 3 days. Crystals
suitable for X-ray structural analysis were obtained after filtration.
1
Yield: 65%. H NMR (400 MHz, DMSO-d6/DCl): δ = 9.14 (s, 1H,
imidazole ring), 7.92, 7.90 (d, 6H, Ph), 7.73−7.69 (t, 2H, imidazole
ring), 7.15, 7.14 (d, 6H, Ph), 4.26−4.23 (m, 1H, CH2), 4.15−4.12 (m,
2H, CH2 and pyrrole ring CH), 3.28−3.25 (m, 2H, pyrrole ring CH2),
1.85−1.66 (m, 4H, 2 × pyrrole ring CH2), 1.43 (s, 9H, tri-tert-butyl).
Anal. Calcd for Zn2H2O(OH)(C21H12NO6)(C13H21N3O2): C, 51.60;
H, 4.58; N, 7.08. Found: C, 51.74; H, 4.50; N, 7.21. FTIR (KBr pellet;
cm−1): 3411 (w), 2974 (m), 2926 (m), 2893 (m), 1595 (m), 1390
(m), 1315 (m), 1282 (m), 1172 (m), 1109 (s), 1053 (s) cm−1.
Synthesis of Zn−PYIs. Zn−BCIP in DMF was heated at 165 °C
CONCLUSION
■
A new approach to obtain a MOF-based asymmetric photo-
catalyst through the cooperative combination of triphenylamine
photocatalysis and proline-based asymmetric organocatalysis
within a single MOF was developed. With their well-defined,
repeating structures, the individual components are fixed in a
structurally controlled manner, and the catalysts have been
successfully applied to prompt the light-driven α-alkylation
reaction with excellent catalytic efficiency and high enantiose-
lection.
1
using microwave irradiation for about 4 h. Yield: > 95%. H NMR
(400 MHz, DMSO-d6/DCl): δ = 9.11 (s, 1H, imidazole ring), 7.93,
7.91 (d, 6H, Ph), 7.74−7.69 (t, 2H, imidazole ring), 7.16, 7.14 (d, 6H,
Ph), 4.25−4.23 (m, 1H, CH2), 4.15−4.13 (m, 2H, CH2 and pyrrole
ring CH), 3.21−3.17 (m, 2H, pyrrole ring CH2), 1.80−1.69 (m, 4H,
pyrrole ring CH2). Anal. Calcd for Zn2H2O(OH)(C21H12NO6)-
(C8H13N3): C, 50.38; H, 4.08; N, 8.10. Found: C, 50.10; H, 4.06;
N, 7.92. FTIR (KBr pellet; cm−1): 3416 (w), 1595 (m), 1560 (m),
1392 (m), 1315 (s), 1275 (m), 1176 (m).
EXPERIMENTAL SECTION
■
Material and Methods. Unless otherwise stated, all chemical
materials were purchased from commercial sources and used without
further purification. 4,4′,4″-Tricarboxytriphenylamine (H3TCA)71 and
L-/D-N-tert-butoxycarbonyl-2-(imidazole)-1-pyrrolidine (L-/D-BCIP)42
Synthesis of Ho−TCA. A mixture of 4,4′,4″-tricarboxytriphenyl-
amine (H3TCA; 94 mg, 0.25 mmol) and Ho(NO3)3·6H2O (459 mg, 1
mmol) were dissolved in 15 mL of mixed solvents of DMF and
ethanol in a screw-capped vial. The resulting mixture was kept in an
oven at 100 °C for 3 days. Yellow block-shaped crystals were obtained
1
were synthesized according to the published procedures. H NMR
spectra were measured on a Varian INOVA 400 M spectrometer.
Elemental analyses were obtained on a Vario EL III Elemental
Analyzer. Powder XRD diffractograms were obtained on a Riguku D/
Max-2400 X-ray diffractometer with Cu sealed tube (λ = 1.541 78 Å).
Thermogravimetric analysis (TGA) was carried out at a ramp rate of 5
°C/min in a nitrogen flow with a Mettler-Toledo TGA/SDTA851
instrument. FT-IR spectra were recorded as KBr pellets on JASCO
FT/IR-430. Solid UV−vis spectra were recorded on a HP 8453
spectrometer. Liquid UV−vis spectra were performed on a TU-1900
spectrophotometer.
The solid fluorescent spectra were measured on a JASCO FP-6500
instrument. The excitation and emission slits were both 3 nm wide.
Adding Zn−PYI1 (50 mg) to a dichloromethane solution containing
substrates and stirring for 12 h afforded new crystalline solids; the
1
after filtration. Yield: 70%. H NMR (400 MHz, DMSO-d6/DCl): δ
7.93, 7.91 (d, 6H, Ph), 7.16−7.14 (m, 6H, Ph). Anal. Calcd for
[Ho4(OH)4(OH)2(C21H12NO6)2(H2O)4]·2H2O: C, 31.17; H, 2.62;
N, 1.73. Found: C, 32.03; H, 1.91; N, 1.89. FTIR (KBr pellet; cm−1):
3442 (w), 1678 (m), 1589 (m), 1538 (m), 1504 (m), 1318 (m), 1273
(m), 1180 (m), 1104 (s), 1016 (s) cm−1.
Single-Crystal X-ray Crystallography. Intensities were collected
on a Bruker SMART APEX CCD diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.710 73 Å) using the SMART
and SAINT programs.72,73 The structure was solved by direct methods
and refined on F2 by full-matrix least-squares methods with SHELXTL
version 5.1.74 Non-hydrogen atoms of the ligand backbones were
refined anisotropically. Hydrogen atoms within the ligand backbones
G
dx.doi.org/10.1021/ja305367j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
and the bridging hydroxyl group were fixed geometrically at calculated
positions and allowed to ride on the parent non-hydrogen atoms.
Crystallographic data for Zn−BCIP1, Zn−BCIP2, and Ho−TCA are
given in Table 3.
complete (as judged by TLC analysis), the reaction mixture was
concentrated in vacuo and the enantiomeric excess of the title
compound was determined by the integration of the two H NMR
signals (both doublets) in CDCl3 arising from the resultant
diastereomeric acetals.
1
Catalysis Reactions with Ho−TCA and MOF-150 as Catalysts. A
mixture of the catalyst (1.9 μmol, 0.005 equiv), 66.0 μL of diethyl
bromomalonate (0.39 mmol, 1.0 equiv), 11.6 mg of L-/D-PYI (76.9
μmol, 0.2 equiv), aldehyde (0.77 mmol, 2.0 equiv), and 90.0 μL of 2,6-
lutidine (0.77 mmol, 2.0 equiv) was degassed for 10 min by bubbling
nitrogen through the reaction medium. After 24 h, the reaction
mixture was subjected to the workup protocol outlined in the general
procedure and purified by flash chromatography using petroleum
ether/Et2O (4/1) as the eluent to afford the title compound as a
colorless oil. The enantiomeric excess was also determined according
to the above method.
Table 3. Crystallographic Data for Zn−BCIP1, Zn−BCIP2,
and Ho−TCA
Zn−BCIP1
Zn−BCIP2
Ho−TCA
empirical
formula
C34H36N4O10Zn2
C34H36N4O10Zn2
C42H42Ho4N2O24
formula wt
T/K
791.45
220
791.45
220
1618.50
298
cryst syst
space group
a/Å
monoclinic
P21
monoclinic
P21
tetragonal
P42 c
̅
1
9.657(1)
22.049(2)
16.376(2)
91.51(1)
3485.7(6)
4
9.789(1)
22.293(3)
16.533(2)
91.75(3)
3605.6(8)
4
12.573(2)
12.573(2)
25.944(8)
b/Å
ASSOCIATED CONTENT
* Supporting Information
■
c/Å
S
β/deg
V/Å3
Figures, tables, text, and CIF files giving crystal data,
experimental details, and the catalysis results. This material is
available free of charge via the Internet at http://pubs.acs.org.
4101.2(2)
2
Z
D
calcd/g
1.504
1.455
1.311
3.869
18 712
μ/mm−1
1.439
1.391
AUTHOR INFORMATION
Corresponding Author
*E-mail: cyduan@dlut.edu.cn.
■
no. of rflns
12 489
29 006
collected
no. of unique 7964
rflns
11 838
3611
Author Contributions
R(int)
0.1123
0.1242
1624
0.1220
1536
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
F(000)
1624
a
R1 (I > 2σ)
0.0760
0.1290
0.0734
0.1544
0.0664
0.1692
wR2 (all
data)
b
Notes
GOF
1.032
1.004
1.017
The authors declare no competing financial interest.
a
b
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = |∑w(|Fo|2 − |Fc|2)|/∑|w(Fo )2|1/2
.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from the
NSFC (Nos. 21171029, 21025102, and 91122031).
■
For Zn−BCIP1 and Zn−BCIP2 crystals, hydrogen atoms of the
water molecules and the hydroxyl groups were found from the
different Fourier map but refined by using the riding model with the
thermal parameter being fixed at 1.2 times of the oxygen atoms to
which they are attached. Several bond distance constraints were used
to aid refinement of the BCIP moiety.
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