Coordination polymers with free Bronsted acid sites for selective catalysis

Article abstract of DOI:10.1039/c4nj01898b

By employing the 5-nitro-1,2,3-benzenetricarboxylate ligand, two Cu(ii) coordination polymers with free -COOH groups as Bronsted acid sites are synthesized, and both compounds show high activity and regioselectivity in catalyzing epoxide ring-opening reac

Full text of DOI:10.1039/c4nj01898b

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New Journal of Chemistry
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DOI: 10.1039/C4NJ01898B
The Cu(II) coordination polymers with free –COOH groups as Brønsted acid sites show high
activity and region-selectivity on catalyzing epoxide ring-opening reaction.

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DOI: 10.1039/C4NJ01898B
Journal Name
RSCPublishing
COMMUNICATION
Coordination Polymers with free Brønsted acid Sites
for Selective Catalysis
Cite this: DOI: 10.1039/x0xx00000x
LuꢀFang Maa,b, ZhenꢀZhen Shic, FeiꢀFei Lic, Jian Zhang*a, and LiꢀYa Wang*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
By employing the 5-nitro-1,2,3-benzenetricarboxylate ligand,
two Cu(II) coordination polymers with free –COOH groups
as Brønsted acid sites are synthesized, and both compounds
show high activity and region-selectivity on catalyzing
epoxide ring-opening reaction.
Rational design and construction of new metalꢀorganic
frameworks (MOFs) or coordination polymers (CPs) with attractive
potential application is an active research field in recent years.1ꢀ4 CPꢀ
based catalysts have recently gained prominence for epoxide ringꢀ
opening reactions, because these reactions generally occur in the
presence of an acid catalyst and a nucleophile (e.g., alcohols, amines)
under fairly mild conditions.5 Many homogeneous catalysts are
appealing platforms for epoxide ringꢀopenings, including metal salen
complexes (e.g., CrIII, CoII, MnII) and metal salts with or without
additives.6 In contrast, the CPꢀbased heterogeneous catalysts for such
a reaction are rarely reported.7 One of the biggest challenges in these
ringꢀopening reactions is regionꢀselective control, as the nucleophile
can attack either position of the epoxide ring.
In this work, we present two CuꢀCPs with high efficiency on
epoxide ringꢀopening reaction. Remarkably, both CuꢀCPs as
heterogeneous catalysts show very strict regionꢀselectivity on control
trans stereochemistry of the final product. Two CuꢀCPs with
formula of [Cu(Hnbta)(btb)] (1) and [Cu2(Hnbta)2(btx)2]·2H2O (2)
(H3nbta
=
5ꢀnitroꢀ1,2,3ꢀbenzenetricarboxylic acid, btb
= 1,4ꢀ
bis(1,2,4ꢀtriazolꢀ1ꢀyl)butane,
btx 1,4ꢀbis(1,2,4ꢀtriazolꢀ1ꢀ
=
ylmethyl)benzene) are hydrothermally synthesized and structurally
characterized by singleꢀcrystal Xꢀray diffraction.† It is notable that
the Hnbta ligand in the structure of compound 1 (or 2) is not fully
deprotonated, and the presence of the naked –COOH groups
contributes to the catalysis and regionꢀselectivity.
Figure 1. (a) The coordination environment in compound 1; (b) the
layer in 1, showing naked –COOH groups (H atoms are highlighted
as purple balls, NO2 groups are omitted for clarify.).
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New Journal of Chemistry
VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4NJ01898B
Table 1. Ringꢀopening reactions of epoxides with aniline and oꢀ
substituted anilines using compounds 1 and 2 at room temperature.a
entry
epoxide
amine (R)
catalyst
yield %b
1
2
styrene oxide
styrene oxide
ꢀH
ꢀH
1
95,93c,92d
Figure 2. The chain structure in 2, showing naked –COOH groups (H
atoms are highlighted as purple balls.).
2
93,91c,90d
3
styrene oxide
ꢀCH3
ꢀCl
1
92
90
Compound 1 features a layer structure containing dinuclear Cu
units linked by the Hnbta and btb ligands. Each dinuclear Cu unit is
surrounded by four Hnbta ligands and four btb ligands (Figure 1a),
and there are no possible unsaturated sites around this unit. The 1ꢀ
and 3ꢀCOOH groups in the Hnbta ligand are deprotonated and
coordinated to three Cu(II) centers, while the 2ꢀCOOH group is free.
Each btb ligand connects two Cu(II) centers from two adjacent
dinuclear Cu units. The Hnbta ligands link the Cu(II) centers into a
layer (Figure 1b), and the btb ligands further cover two sides of the
layer. Those free 2ꢀCOOH groups are safely protected inside the
layer. No obvious interactions are observed between the adjacent
layers.
4
styrene oxide
1
5
cyclohexene oxide
cyclohexene oxide
cyclohexene oxide
cyclopentene oxide
cyclopentene oxide
cyclopentene oxide
styrene oxide
ꢀH
1
93
6
ꢀCH3
ꢀCl
1
92
7
1
90
8
ꢀH
1
93
9
ꢀCH3
ꢀCl
1
1
92
10
11
90
Similar dinuclear Cu units and free ꢀCOOH groups are presented
in the structure of compound 2 (Figure 2). Unlike 1, the Hnbta
ligands link the Cu(II) centers into a chain in 2, which is further
surrounded by the ꢁ2ꢀbtx ligands. It is interesting to note that two
free ꢀCOOH groups from two opposite Hnbta ligands forms two Oꢀ
H…O hydrogen bonds.
ꢀH
Cu(NO3)2
trace
a Conditions: catalyst – 5 mol%, 4h stirring. bIsolated yield. cThird and
dfourth run with reused catalyst.
The presence of such fascinating free ꢀCOOH groups in both
structures of 1 and 2 makes them promising Brønsted acid catalysts
for epoxide ringꢀopening reaction. Both of them were examined for
their performance as heterogeneous catalysts for aminolysis reaction
of epoxides under solventꢀfree conditions. The results are collected
in Table 1.
Initial control reactions without CuꢀCPs showed that epoxide ringꢀ
opening did not occur at all. Likewise, when the catalyst was filtered
off, the reaction was no longer promoted. These control experiments
strongly suggest
a heterogeneous catalytic process by using
compound or 2. Further, another control reactions with
1
Cu(OAc)2/Cu(NO3)2 and H3nbta ligands were also set up to
determine if trace amounts of the metal source and ligands could act
as homogeneous catalysts. Despite using 10 mol% loadings, no
significant reactivity was seen from any of the controls, indicating
that Cu(OAc)2/Cu(NO3)2 metal source alone is a poor catalyst for
epoxide ringꢀopening (Table 1,entry 11).
In a typical reaction, 5 mol% sample of 1 was used for the
conversion of an equimolar mixture of styrene oxide (0.4 mmol) and
aniline (0.4 mmol) at room temperature. After 4 h, the reaction
leaded to a 95% conversion of styrene oxide, a 100% selectivity of
the corresponding βꢀamino alcohol. The same reaction conditions
were used for testing the catalytic activity of 2 (5 mol%). Here
compound 1 was used for a more comprehensive study. Aiming to
determine the potential applicability of the protocol, the substrate
scope of the aminolysis reaction was then investigated with the use
of two different amines. The results are summarized in Table 1
(entries 2, 3). The structural effect on the yield of aminolysis was
similar to that observed in the reported literatures.7 Aminolysis of
methylꢀ and chloroꢀ aniline was slow (Table 1, entries 2, 3). The
difference among the reactivity of aniline, methyl aniline and chloro
aniline illustrates the effect of steric bulkiness on the reaction yield.
Notably, several tests were also performed to understand the
chemical stability of both CuꢀCPs. Overall, both CuꢀCPs can be
recovered by simple filtration and reused several times (tested four
consecutive times) without much loss of activity (~3% drop in the
fourth run). The yields of each repetition of catalytic reaction were
shown in Figure S1 (Supporting Information). Thus, the recovered
compounds remained stable with complete retention of reactivity,
which is further confirmed by powder Xꢀray diffraction (PXRD)
analysis (Figure S2, Supporting Information). The recovered
material shows no obvious PXRD change, compared to the origin
structure of the catalyst. However, the intensity of PXRD decreased
To prove the heterogeneous mechanism of the aminolysis reaction,
several control experiments were performed to confirm that these
CuꢀCPs were indeed the catalytic source for epoxide ringꢀopening.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4NJ01898B
b College of Chemistry and Chemical Engineering, Luoyang Normal
with repetition of catalytic reactions (Figure S3, Supporting
Information).
University, Luoyang 471022, P. R. China
c Department of Physics and Chemistry, Henan Polytechnic University,
Jiaozuo, 454000, P. R. China
Importantly, all products were determined to have a trans
stereochemistry by NMR, with comparison to the reported literature
values (Table 1, entries 1–9).8 Accordingly, in the case of
cyclohexene oxide or cyclopentene oxide, the respective stereoꢀ
selective trans products were exclusively generated without any
observation of the cis products (Table 1, entries 3–9). Moreover,
with styrene oxide as a model asymmetrical substrate, the regionꢀ
selectivity was observed (Table 1, entries 1–3). Such selectivity
leads to only single product in all cases. Considering the structural
features of 1, the Brønsted acid sites (ꢀCOOH groups) is inside the
layer and the surface of the layer is those btb ligands. When the
epoxide rings are close to the ꢀCOOH sites through possible
hydrogen bonding interactions, the close packing between epoxide
rings and btb ligands might generate steric hindrance. Thus, further
nucleophilic attack should occur at a less hindered site of the
epoxide ring. In all cases reported here, the nucleophile would attack
the more cationic carbon site to give transꢀamino alcohol.
† Crystal data for
12.3286(12) Å,
= 296(2) K, space group
3589 independent reflections (Rint = 0.0243). The final R1 value was
0.0288 ( > 2 )). The final wR > 2 )). The
2) value was 0.0772 (
final R1 value was 0.0360 (all data). The final wR
2) value was 0.0830
(all data). Crystal data for = 1149.91, triclinic,
1
:
M
= 508.90, monoclinic,
= 13.1324(13) Å, = 105.5440(10)°,
)/n = 4, 14340 reflections measured,
a
= 12.4021(12) Å,
b =
c
β
V
= 1934.5(3) Å3,
T
P
2
(1
, Z
I
σ(I
(F
I
σ(I
(F
2
:
M
a
= 10.272(3) Å,
b
γ
=
=
15.184(4) Å,
108.045(3)°,
c
= 16.200(4) Å,
= 2299.2(10) Å3,
α
T
= 100.009(3)°, β = 99.610(3)°,
V
= 296(2) K, space group Pꢀ1, Z = 2,
17015 reflections measured, 8454 independent reflections (Rint = 0.0453).
The final R1 value was 0.0546 ( > 2 )). The final wR
2) value was
0.1336 ( > 2 )). The final R1 value was 0.0942 (all data). The final
2) value was 0.1587 (all data).
I
σ(I
(F
I
σ(I
wR
‡
(F
Electronic supplementary information (ESI) available: [Powder XRD
patterns, NMR data and cif file (CCDCꢀ960798, 960800]. See
DOI: 10.1039/b000000x/
In summary, through a deliberate coordination of the 5ꢀnitroꢀ
1,2,3ꢀbenzenetricarboxylate ligand, the remaining free –COOH
groups in two CuꢀCPs can act as Brønsted acid sites for catalyzing
epoxide ringꢀopening reaction. Especially, the btb and bth coꢀligands
as surfaces of these layer and chain structures in two CuꢀCPs might
have synergistic effect on control the trans stereochemistry of the
final product. This work revealed a successful strategy on designing
functional CPs with Brønsted acidꢀtype catalytic sites.
We thank the support of this work by 973 program
(2012CB821705), Program for New Century Excellent Talents
in University (NCETꢀ11ꢀ0947) and NSFC (21221001,
21371089, 21271098).
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Experimental details:
Synthesis of [Cu(Hnbta)(btb)] (1). A mixture of H3nbta (0.1
mmol, 25.5 mg), btb (0.1 mmol, 19.2mg), Cu(OAc)2·H2O (0.1
mmol, 19.9 mg) and H2O 12 mL was placed in a Teflonꢀlined
stainless steel vessel, heated to 130 C for 4 days, and then
o
cooled to room temperature over 24 h. Blue block crystals of
1
were obtained. Elemental analysis (%): calcd for
C17H15CuN7O8 C 40.12, H 2.97, N 19.27; found C 40.19, H
2.92, N 19.20. IR (cmꢀ1): 3124 m, 1714 m, 1621 s, 1603 s, 1566
s, 1533 s, 1421 s, 1352 s, 1237 s, 1132 s, 1068 m, 999 s, 825 s,
714 s, 704 s, 653 m.
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Synthesis of [Cu2(Hnbta)2(btx)2]·2H2O (
synthesized in the similar way as that described for
that btb was replaced by btx (0.1 mmol, 23.0 mg). Blue block
crystals of were obtained. Elemental analysis (%): calcd for
2
). Compound
2 was
, except
1
2
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C42H34Cu2N14O18 C 43.87, H 2.98, N 17.05; found C 43.82, H
2.91, N 17.11. IR (cmꢀ1): 3134 m, 1701 m, 1639 m, 1620 s,
1581 m, 1529 s, 1397 s, 1342 s, 1330 s, 1289 s, 1128 s, 1003 m,
825 s, 727 s, 709 s, 669 s.
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Notes and references
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian
350002, P. R. China. Eꢀmail: zhj@fjirsm.ac.cn
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