Chiral Metal-Organic Framework Decorated with TEMPO Radicals for Sequential Oxidation/Asymmetric Cyanation Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.8b01630

A chiral porous metal-organic framework (MOF) decorated with radicals has been successfully constructed by cocrystallizing achiral (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-substituted tricarboxylate and enantiopure VO(salen)-derived dipyridine ligands. The chiral MOF can function as an efficient heterogeneous catalyst for the sequential alcohol oxidation/asymmetric cyanation of aldehyde reactions with enhanced activity and enantioselectivity compared to the homogeneous counterpart.

Full text of DOI:10.1021/acs.inorgchem.8b01630

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Chiral Metal−Organic Framework Decorated with TEMPO Radicals
for Sequential Oxidation/Asymmetric Cyanation Catalysis
†
,†
†
,†,‡
Zijian Li, Yan Liu,* Xing Kang, and Yong Cui*
†School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, China
‡
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
*
S Supporting Information
1
2
Scheme 1. Structures of the Organic Linkers H L and VOL
3
ABSTRACT: A chiral porous metal−organic framework
MOF) decorated with radicals has been successfully
(
constructed by cocrystallizing achiral (2,2,6,6-tetramethyl-
piperidin-1-yl)oxyl (TEMPO)-substituted tricarboxylate
and enantiopure VO(salen)-derived dipyridine ligands.
The chiral MOF can function as an efficient heteroge-
neous catalyst for the sequential alcohol oxidation/
asymmetric cyanation of aldehyde reactions with
enhanced activity and enantioselectivity compared to the
homogeneous counterpart.
(
[
MeOH) at 80 °C afforded olive-green crystals of
1 2
Cd (L ) (VOL ) (NO ) (DMA) ]·(DMA) ·(MeOH) ·
4
2
4
3
2
2
2
2
rystalline metal−organic frameworks (MOFs) have been
(H O) (1). The product is stable in air as well as in various
2 5
C
actively studied in recent decades because of their modular
common solvents including water. The formulation of 1 was
confirmed by elemental analysis, thermogravimetric analysis
(TGA), and IR spectroscopy, and powder X-ray diffraction
(PXRD) analysis indicated the pure phase of its bulk sample.
Single-crystal X-ray analysis on 1 reveals that it crystallizes in
the monoclinic chiral space group C2. The four crystallo-
graphically independent Cd atoms all adopt a distorted
pentagonal-bipyramid geometry coordinated by two axial N
atoms of two VOL ligands and five equatorial O atoms from
three L ligands for Cd1 and Cd2 or five O atoms from one L
ligand, one DMA, and one NO
and Cd2 are bridged by a pair of μ
two L ligands into a binuclear {Cd
{Cd } units are doubly interlinked by L ligands to form an
infinite ribbonlike chain (Figure 1a). The L ligand in 1 exhibits
only one bridging mode (one tridentate and two chelating
bidentate carboxylate groups), binding to two {Cd } units in
one ribbon and one Cd3 or Cd4 node via its three carboxylate
groups, respectively. Adjacent ribbons are further linked by the
coordination of VOL ligands, in which each V center lies in a
1
nature, facile tunability, and diverse applications. One of the
current trends in MOF-based catalysis is to emulate nature by
developing multicatalysis systems, wherein multiactive sites are
combined into a single network, for sequential/tandem
2
transformations. The precise incorporation of multiple active
sites at the metal nodes and/or organic linkers can provide an
additional synergistic cooperation effect, thus achieving
improved activities and stereoselectivities over monocatalytic
2
2
,3
1
1
systems, especially for asymmetric catalysis.
However,
−
examples of chiral multifunctional MOF catalysts are still rather
rare, mainly because of the difficulty in controlling the proper
proximity and conformation of active sites to create effective
3
anion for Cd3 and Cd4. Cd1
1
2
-η ,η -carboxylate groups of
} subunit, and neighboring
2
1
2
3
1
cooperative activation.
2
1
Given the widespread importance of stable organic radicals in
optics, electronics, magnetics, and catalysis, the exploration of
radical-immobilized solid materials with extraordinary and
2
4
,5
fascinating properties is highly desirable. Although consid-
erable efforts devoted to this area over the past years have led to
some potentially useful radical materials, only a few examples of
MOFs with radicals as the building units have been reported to
2
square-pyramidal coordination sphere formed by an N O
2
2
2
5
donor set of one L ligand and one double-bond O atom,
generating a 3D framework with the TEMPO radicals appended
regularly in the pores (Figure 1b). 1 possesses open channels
date. Some typical examples for catalysis applications have
recently been presented by Kitagawa and Chmielewski, in which
nitroxide-radical-decorated/appended MOFs were successfully
2
5c,d
with dimensions of approximately 1.73 × 0.86 nm along the a
fabricated for the aerobic oxidation of alcohols. In this work, a
chiral MOF featuring both stable organic radicals and chiral
metallosalen catalysts was synthesized for the first time, which
can be applied to prompt sequential alcohol oxidation/
asymmetric cyanation of aldehyde reactions with excellent
catalytic efficiency and high enantioselectivity.
6
axis and a PLATON-calculated void space of ∼44.9%.
Topological analysis suggests that the framework of 1 can be
described as a (3,3,3,4,4)-c net with the point symbol
2
4
5
4
2
(
4.6.8) (4 .6 .8 .10 )(6 .8) (Figure 1c).
2 2
1
2
Heating a mixture of H L , VOL (Scheme 1), and
Received: June 12, 2018
3
Cd(NO ) ·6H O in dimethylacetamide (DMA)/methanol
3
2
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01630
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
%
1 and 20 mol % TBN in 1, 2-dichloroethane under an O2
atmosphere at 80 °C. The oxidation afforded 46%, 80%, and
9
2% conversions after 1, 4, and 7 h, respectively, and 99%
conversion could be obtained if the reaction was continued for 8
h (Figure S11). TEMPO has been immobilized on a variety of
solid supports to facilitate and further leverage its potential
benefits, and this radical MOF represents one of the best solid
catalysts for aerobic alcohol oxidation reported to date. Upon
the completion of oxidation, the temperature was cooled to 0
5d,9
°
C, and trimethylsilyl cyanide (3.0 equiv) and triphenylphos-
phine oxide (1.0 equiv) were added to the reaction mixture
under a N atmosphere. The desired product 3a was acquired in
2
9
8% conversion and 80% enantiomeric excess (ee) after 48 h.
Consequently, the scope and generality of the sequential
process were studied. As shown in Table 1, benzyl alcohols with
Table 1. Sequential Oxidation/Asymmetric Cyanation
a
Reactions of Alcohols Catalyzed by 1
b
c
entry
Ar
p-Br
p-Cl
3/conv (%)
ee (%)
1
2
a/98 (78)
b/91
80 (65)
75
3
4
H
p-Me
c/94 (85)
d/93
77 (72)
77
5
6
7
8
p-OMe
m-OMe
o-OMe
1-naphthyl
2-naphthyl
e/96 (82)
f/91
g/94 (84)
h/75
83 (52)
81
87 (23)
66
9
i/78
77
1
Figure 1. (a) View of the ribbonlike chain made up of L ligands and
10
m-G
0
j/<5 (45)
n.d. (n.d.)
a
binuclear {Cd } nodes. (b) 3D porous structure of 1. Atoms: Cd, dark
blue; O, red; N, turquiose; C, gray. The VOL ligands are drawn in
bright green for clarity. The O atoms of TEMPO are shown as light-
2
For reaction details, see the Supporting Information; the data in
parentheses are results catalyzed by a mixture of Me L and VOL
the same amount of catalyst loadings as the MOF system).
Determined by H NMR. Determined by HPLC.
2
1
2
3
(
b
1
c
orange balls. (c) Topology of 1.
TGA revealed that the framework is stable up to ∼300 °C.
The circular dichroism (CD) spectra of 1 made from (R)- and
(
electron-withdrawing and -donating groups on the benzene ring
were all well-tolerated, affording the corresponding products in
excellent conversions (91−98%) with good enantioselectivities
(75−87%; Table 1, entries 1−7). The positions of the
substituents on the phenyl ring had a slight effect on the
stereocontrol, and p-, m-, or o-methoxy (OMe)-substituted
benzyl alcohols could give ∼83%, 81%, and 87% ee, respectively.
1- and 2-naphthalenemethanol gave the desired products with
66% and 77% ee, respectively (Table 1, entries 8 and 9). When
secondary alcohols were subjected to the reaction, only ketone
products were obtained because 1 could not promote efficient
cyanation of ketones (Table S5).
2
S)-VOL are mirror images of each other in the solid state,
suggesting their absolute configuration and enantiopurity. The
porosity of 1 was evaluated by CO sorption at 273 K. The
2
typical type I isotherm indicated microporous materials, which
gave an apparent Brunauer−Emmett−Teller (BET) surface area
2
of 179 m /g. Dye-inclusion studies indicated ∼1.5 methyl
orange or 0.8 Rhodamine 6G dye uptake per formula unit for 1,
demonstrating retention of the porous framework structure in
solution. A V 2p3/2 peak was observed at the binding energy of
5
16.4 eV in X-ray photoelectron spectroscopy (XPS) of 1,
7
confirming that V is in a +4 oxidation state.
It has been established that TEMPO can promote the
To examine whether the sequential reaction took place
5d
selective oxidation of alcohols and the VO(salen) complexes
predominantly in the pores or on the surface of the framework, a
8
3
can efficiently catalyze the cyanosilylation of aldehydes. Thus,
bulky substrate bearing a G group (ca. 2.0 × 1.9 × 1.4 nm ) was
0
the catalytic activity of 1 in sequential alcohol oxidation/
asymmetric cyanation of aldehyde was assessed, and the
transformation of p-bromobenzyl alcohol (2a) was selected as
a model reaction. The oxidation process was carried out in a
TEMPO/tert-butyl nitrite (TBN) system, which does not
require any acid or base reagents. After optimization of the
solvent, temperature, and molar ratio of the catalytic
components, the oxidation reaction was conducted with 2 mol
synthesized and subjected to the reaction. MOF 1 gave an
extremely low conversion (<5%) compared to its homogeneous
analogue (45%), indicating that the bulky substrate could not
access the catalytic sites because of its large diameter and
catalysis does occur within the framework (Table 1, entry 10).
The heterogeneous nature of the reaction was demonstrated by
the fact that the supernatant from the sequential reaction after
filtration did not afford any product.
B
DOI: 10.1021/acs.inorgchem.8b01630
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
1
0
The contribution of pore structures of the MOF catalyst was
then evaluated by comparing the catalytic performance of 1 with
that of the corresponding homogeneous system. A 1:2 mixture
of Me3L and VOL2 (the same catalyst loadings as the
heterogeneous system) catalyzed the sequential reactions of p-
bromobenzyl alcohol, benzyl alcohol, p-methoxybenzyl alcohol,
and o-methoxybenzyl alcohol under identical conditions,
affording the corresponding products 3a, 3c, 3e, and 3g in
TEMPO-catalyzed oxidation and/or more distinct confine-
ment effect of the framework catalyst in the sequential process.
After completion of the catalytic reaction, the framework
catalyst can be recovered by simple filtration, which could be
reused without the significant loss of activity and enantiose-
lectivity. The conversions/ee values for the five consecutive runs
are 94/87%, 92/85%, 91/85%, 91/84%, and 89/83%. PXRD
showed that the recycled solid retained high crystallinity. The
1
2
7
2
8%, 85%, 82%, and 84% conversion with 65%, 72%, 52%, and
3% ee, respectively. Importantly, the difference in the catalytic
performances became even more significant as the catalyst
loading decreased (Table 2). At a loading of 1%, 1 and the
sample displayed a decreased BET surface area of 65 m /g,
possibly because the partial framework collapsed. XPS showed
that the recovered vanadium complex retained a +4 oxidation
state.
In conclusion, we have demonstrated the successful synthesis
of a chiral porous radical-decorated MOF. Integration of the
TEMPO radicals and chiral VO(salen) active sites within a
single framework endows the MOF as an effective heteroge-
neous catalyst for consecutive oxidation/asymmetric cyanation
reactions with improved catalytic performance compared with
its homogeneous counterpart. This work may extend the
strategy to assemble multifunctional MOF catalysts, and taking
advantage of this strategy, one can expect more heterogeneous
catalysts with outstanding performance to be discovered.
Table 2. Sequential Oxidation/Asymmetric Cyanation
Reactions of Alcohols Catalyzed by 1 and Related
Homogeneous Catalysts at Different Catalyst Loadings
a
b
1
homo
conv (%)
loading (mol %)
conv (%)
ee (%)
ee (%)
2
1
0
0
98
72
42
21
80
77
72
46
78
51
19
trace
65
39
19
n.d.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01630.
■
*
S
.5
.2
a
For reaction details, see the Supporting Information; conversions
were calculated by H NMR; ee values were determined by HPLC.
Homo = a 1:2 mixture of Me L and VOL (the same catalyst
1
b
1
2
Experimental details and spectral data (PDF)
3
loadings as the heterogeneous system).
Accession Codes
CCDC 1588630 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
1
2
Me L /VOL mixture afforded 72% and 51% conversion of 3a,
3
respectively. When the catalyst loading decreased to 0.5%, the
conversion decreased to 42% for 1 and 19% for the
homogeneous analogue. If the loading was further decreased
to 0.2%, almost no product was detected for the homogeneous
case, whereas 1 could still give 21% conversion. Structural
investigation of 1 reveals that the TEMPO unit is in close
contact with the VO(salen) site, with the shortest V−
O(TEMPO) distance of ∼10.8 Å, thus creating an attractive
scaffold for synergistic catalysis. In the framework-confined
system, the possibility for TEMPO and VO(salen) to meet each
other increases, thereby leading to a dramatic catalytic difference
with the homogeneous counterpart especially at a low catalyst
concentration. In addition, the kinetic curves of the sequential
reaction also revealed that integration of the TEMPO and
VO(salen) species into one framework did enhance their
catalytic activities and enantioselectivities (Figure S11). So, the
framework displays an obvious confinement effect, and the chiral
microenvironment, created by chiral metallosalen scaffolds,
together with TEMPO, phenyl rings, and metal nodes, may be
responsible for the improved catalytic performance.
The asymmetric cyanation reaction of aldehydes was also
conducted with MOF 1. Under similar conditions, p-
bromobenzaldehyde, p-methoxybenzaldehyde, and o-methox-
ybenzaldehyde were successfully converted to the correspond-
ing cyanohydrin silyl ethers in good conversion with 48%, 52%,
and 63% ee, respectively. Obviously, 1-catalyzed sequential
oxidation/asymmetric cyanation reactions offer significantly
higher (24−32%) ee values. The higher asymmetric induction
was probably due to the formation of a tighter vanadium
aldehyde transition state facilitated by the byproducts of
AUTHOR INFORMATION
Corresponding Authors
E-mail: liuy@sjtu.edu.cn.
E-mail: yongcui@sjtu.edu.cn.
■
*
*
ORCID
Yan Liu: 0000-0002-7560-519X
Yong Cui: 0000-0003-1977-0470
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21522104, 21431004, and
■
2
1620102001), the National Key Basic Research Program of
China (2016YFA0203400), the Key Project of Basic Research of
Shanghai (17JC1403100 and 18JC1413200), and the Shanghai
“
Eastern Scholar” Program.
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