Cu3L2 metal-organic cages for A3-coupling reactions: Reversible coordination interaction triggered homogeneous catalysis and heterogeneous recovery

Article abstract of DOI:10.1039/c8cc07208f

The reported Cu(ii) metal-organic cage (Cu₃L₂, 1) can be a highly active reusable catalyst to homogeneously catalyze the A³-coupling reaction in CHCl₃ and can be heterogeneously recovered by simply adding 1,4-dioxane via formation of the insoluble metal-organic framework (Cu₃L₂(1,4-dioxane)_1.5, 2).

Full text of DOI:10.1039/c8cc07208f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Cu3L2 metal-organic cage for A3-couping reaction: reversible
coordination interaction triggered homogeneous catalysis and
heterogeneous recovery
Received 00th January 20xx,
Accepted 00th January 20xx
Gong-Jun Chen, Chao-Qun Chen, Xue-Tian Li, Hui-Chao Ma, and Yu-Bin Dong*
DOI: 10.1039/x0xx00000x
www.rsc.org/
O
O
Br
Br
The reported Cu(II) metal-organic cage (Cu3L2, 1) can be a highly
active reusable catalyst to homogeneously catalyze the A3-
coupling reaction in CHCl3 and can be heterogeneously recovered
by simply addition of 1,4-dioxane via formation of the insoluble
metal-organic framework (Cu3L2(1,4-dioxane)1.5, 2).
O
O
ethyl acetate
NaH/THF, r.t.
O
Pd(PPh3)4/K2CO3
THF/H2O, reflux
Br
+
O
O
O
O
B
OH
O
HO
A
L
Over the past few decades, metal-organic cages (MOCs) have
attracted considerable attention because of their fascinating
structures and properties.1-6 In comparison to molecular
recognition and separations, the MOCs-based catalysis has
been relatively less studied.7,8 As a discrete class of
supramolecular complexes, they are usually polar organic
solvents soluble without destruction of their structural
1,4-dioxane
CHCl3
Cu(OAc)2
CHCl3
Cu3L2(1,4-dioxane)1.5 (2)
Cu3L2 (1)
Cu(II)-2D network
Cu(II)-MOC
Scheme 1. Synthesis of L, 1 and 2.
As shown in Scheme 1, tripodal ligand
L with terminal
diketone coordination sites was prepared by the coupling of
1,3,5-tribromobenzene with 3-acetylbenzeneboronic acid, and
integrity, so MOCs-based catalysis was normally in
a
homogeneous manner.9-14 However, their recovery and reuse
have been received much less attention. The development of
recyclable and reusable MOC-based catalysts is of great
significance in terms of atomic economy and eco-friendly
issues.
then a condensation reaction of the generated intermediate
and acetacetic ester in the presence of NaH in THF (79% yield,
ESI†). was assembled into Cu3L2 ( ) upon complexation with
A
L
1
Cu(II) in CH2Cl2 at room temperature with a molar ratio of 2:3
(88% yield, ESI†).
Electrospray ionization mass spectrometry (ESI-MS)
provided substantial evidence for the formation of a Cu3L2
species in solution. The MS spectrum (Fig. S3, ESI†) showed
that the peak at m/z 1324.1137 was observed due to the
formation of the [Cu3L2+Na+] species, thereby confirming that
the Cu(II) ions in solution were chelated by three
In this contribution, we report for the first time a discrete
Cu3L2 MOC (
1) by assembly of Cu(OAc)2 and a new tripodal
diketone ligand
L
in solution. 1 contains coordinatively
unsaturated Cu(II) centres and can be a homogeneous catalyst
to highly promote A3-couping reaction in polar organic
medium. Furthermore,
reaction solution by the addition of 1.4-dioxane via formation
of the insoluble Cu(II)-MOF ( , Cu3L2 (1,4-dioxane)1.5) linked via
Cu(II)-1,4-dioxane-Cu(II) linkage. This -to- -to- process is
-based catalysis herein features a coordination
triggered homogeneous catalysis and
1 can be readily recovered from
deprotonated ligand
The molecular structure of
ray single-crystal analysis (CCDC 1848381, Tables S1 and S2,
ESI†). As indicated in Fig. 1, crystallized in the triclinic space
group P-1. The Cu(II) centre in lies in a quasi-square planar
{CuO4} coordinated environment. Two twisted tripodal ligands
link three Cu(II) ions into an elliptical molecular cage, in
L.
1
was eventually determined by X-
2
—
1
2
1
1
reversible, so
interaction
1
1
heterogeneous recovery procedure.
L
which the Cu———Cu distances are in a range of 11.609 (24)-
12.467 (21) Å. The distance between the opposite central
phenyl rings is ca. 7 Å, while the longest distance between two
opposite methyl groups is ca. 2.0 nm (Fig. 1). Besides X-ray
single-crystal analysis, the observation of Cu 2p3/2 peak at
934.3 eV in X-ray photoelectron spectroscopy (XPS) further
confirmed that the copper ions in 1 were bivalent (Fig. S4,
ESI†).15 The existence of co-crystallized CH2Cl2 solvent
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molecules was further confirmed by thermogravimetric polymeric species of
Journal Name
2
crystallized in the monoclinic space
analysis (TGA). The TGA result revealed that the observed group C2/c. As shown in Fig. 1, the Cu3L2 cages were linked
solvent molecule weight loss is 16.5% (calculated 16.5% based together by the involved 1,4-dioxane via three sets of weak
on Cu3L2—3CH2Cl2, Fig. S4, ESI†). Notably, 1 was very soluble in Cu(II)———O bonds (dCu-O = 2.53 (0), 2.62(3) and 2.48(3) Å,
some organic solvents such as CHCl3, CH2Cl2 and THF (Fig. S5, respectively) into a 2D network which contains the rounded
ESI†). In addition, the PXRD pattern performed on the as- pores with crystallographic dimeter of ca. 12 Å. The measured
synthesized sample is the same as that of simulated one, PXRD pattern of
indicating that was generated in a single phase (Fig. 2a). The that of , but is identical to that of simulated one,
formation of Cu3L2 nanocage was further confirmed by demonstrating that
dynamic light scattering (DLS) measurement (Fig. 2b). The The formation of
observed dynamic diameter of
is in good agreement with the TEM image (Fig. 2b) and single measurement (Fig. S6 and S7, ESI†). N2 adsorption at 77 K
crystal analysis. The slight difference in size might be caused by revealed absorption amounts of 87.48 and 125.74 cm3/g by
2 showed that it is distinctly different from
1
1
2
2
was obtained in pure phase (Fig. 2a).
from was further supported by their
1
1
centred at ca. 2.2 nm, which porosity difference based on the gas adsorption−desorpꢀon
1
the solvation effect depending on the different and
measurement.16
2
, respectively. Notably, no copper valence change was
detected in based on the XPS analysis (Fig. S8, ESI†).
It was noteworthy that could dissociate in CHCl3 to
regenerate . Therefore, this -to- -to- can be considered a
2
2
1
1
2
1
reversible coordination interaction triggered dissolution-
precipitation-dissolution process (Fig. 1). On the basis of this
unique coordination-driven solubility, we expected that the 1-
based homogeneous catalysis in CHCl3 and heterogeneous
recovery with the aid of bidentate 1,4-dioxane donor should
be achievable.
Table 1. Optimization of the model A3-coupling reaction a
O
N
1
+
+
N
H
solvent
Fig. 1 Reversible coordination-driven transformation between
1 and 2. The X-ray
single-crystal analysis revealed that
1
and
2
existed as the discrete metallacage
3
T (°C)
25
4
5
6
and Cu(II)-MOF, respectively. The weak coordination bonds between 1,4-dioxane
and Cu(II) centres in are shown. The photographs showed that was CHCl3-
souble and became insoluble upon addition of 1,4-dioxane in CHCl3.
yield (%)b
2
1
entry
1
solvent
CHCl3
CHCl3
CHCl3
THF
1 (mol%)
t (h)
2
3
1
24
24
24
24
24
24
24
2
98
94
84
95
89
57
82
53
81
98
9
2
25
As mentioned above, the obtained Cu3L2 metallacage
contains three square-planar Cu(II) centres which are prone to
bind small molecular donors at the axial coordination to from
square pyramidal or octahedral coordination sphere.17
Therefore, such a coordinated unsaturated metallacage was
expected to be an ideal building block to construct polymeric
network by the incorporation divergent bidentate
coordination molecules such as 1,4-dioxane into polymeric
backbone.18
3
25
0.3
3
4
25
5
25
toluene
CH3OH
CH3CN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
3
6
25
3
7
25
3
8
40
3
9
50
3
2
10
11
12
60
3
2
25
-
2
60
-
2
13
a
Reaction conditions: Under nitrogen, a mixture of benzaldehyde (0.5 mmol, 51
µL), phenylacetylene (0.75mmol, 83 µL), pyrrolidine (0.60mmol, 50 µL) and 1. The
coupling product was determined by 1HNMR and MS spectra (Fig. S9, ESI†) b Yield
was determined by the GC (Fig. S10, ESI†).
We initially examined the catalytic activity of
1 in this way
by the A3-couping reaction, which is known as an important
type of aldehyde-alkyne-amine one-pot multicomponent
reaction. The obtained propargylamines are the important
intermediates for the synthesis of various nitrogen-containing
biologically active compounds.19 Optimization of the reaction
was first performed at different catalyst loading under room
temperature to furnish the desired product in CHCl3 (Table 1,
entries 1−3). The results indicated that this model A3-coupling
Fig. 2 a) The simulated and measured PXRD patterns for
and DLS measurement of
1 and 2. b) TEM image
1.
Following this idea, 1,4-dioxane was added to the CHCl3
solution of and the light green crystalline solids
1,
quantitatively precipitated (Fig. 1), indicating the formation of
polymeric species. The X-ray single-crystal analysis (CCDC
1842443, Tables S1 and S2, ESI†) revealed that the formed
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Compared to the homogeneous case of
rate largely decreased with . As indicated in Fig. S14 (ESI†),
(3 mol%) proceeded very smoothly under room product
was generated in only 79% yield after 2 h at 60oC,
temperature to provide desired product of (Fig. S9, ESI†) in
1, the model reaction
reaction of benzaldehyde
(
3
), pyrrolidine
(
4
)
with
2
phenylacetylene (
of
5) (molar ratio, 1.0:1.2:1.5) in the presence
1
6
6
and a longer time (4.5 h) was required to attain 93% yield. The
difference in catalytic efficiency probably demonstrated the
higher diffusion problem of the larger sized propargylamine to
excellent yield (98%) after 24h (Table 1, entry 1). When the
reaction was conducted with a lower catalyst loading, 1 and
0.3 mol % instead of 3 mol %, the product of
lower yields (Table 1, entries 2 and 3). Furthermore, when
other kind of -soluble solvent such as THF was employed in
the presence of 3 mol % , the coupled product was also
obtained in excellent 95% yield (Table 1, entry 4). In contrast,
the poor soluble solvent such as toluene, MeOH and CH3CN
were employed to give in much lower yields (Table 1, entries
6 was furnished in
leave from the inner surface of the polymeric framework of 2.
This was not surprising if one considered that, for the reaction
to proceed it was necessary that the three kinds of starting
molecules had to encounter each other in the suitable
1
1
6
orientation within the pores of 2.
1
Table 2. A3-coupling reactions among various aldehydes, amines and alkynes catalysed
6
5-7) under the same reaction conditions, demonstrating that by 1a
the efficiency of heterogeneous catalysis was indeed lower
than that of corresponding homogeneous catalysis due to its
inherent nature.
In addition, the reaction temperature appeared to be crucial
to the catalytic efficiency. As shown in Table 1, upon increase
of the reaction temperature from 25 to 60°C, the A3-coupling
reaction rate was significantly enhanced (Fig. S11, ESI†). For
example, 98% yield was achieved when the reaction was
carried out at 60°C for only 2 h (Table 1, entries 8-10), and the
corresponding turnover number (TON) and turnover frequency
(TOF) values for the model reaction are 32.67 and 16.33 h-1,
respectively. Without the aid of
1 (Table 1, entries 11-12),
however, no significant conversion was observed at either
25°C (9% yield) or 60°C (13% yield), clearly denoting the
catalytic activity of 1 herein.
a
Reaction conditions: in nitrogen, aldehyde (0.5 mmol), alkyne (0.75 mmol),
amine (0.60 mmol), 1 (3 mol%), CHCl3 (1 mL), 60oC, 2h. Yields were determined
by the GC (Fig. S15, ESI†).
With the above optimized reaction conditions in hand, we
investigated the scope of this A3-coupling utilizing various
substrates (Table 2). To our delight,
1 could be applicable to
various functional groups such as −CH3, −NO2, −OCH3, −F, −Cl,
and −Br (Table 2, entries 1-8, 93-99% yields) attached
aldehydes. On the other hand, when phenylacetylene was
Fig. 3 a) 1-based catalytic cycle. b) The corresponding PXRD patterns of 2 after
each catalytic run.
Once the reaction was finished, 1 could be readily separated replaced by other substituted aromatic alkynes, the reactions
also proceeded very smoothly under the optimized conditions
(Table 2, entries 9-16, 92-97% yields). In addition, when six-
membered cyclic secondary amines such as piperidine (Table
2, entry 17, 92% yield) and 4-oxopiperidine (Table 2, entry 18,
94% yield) were used instead of pyrrolidine to perform the
reactions, the expected products were also formed in excellent
yields. Notably, the yields correspondingly decreased as the
substrate size increased. As illustrated in Table 2, the coupling
reactions for the desired products based on the naphthyl
(Table 2, entry 19) and fluorenyl (Table 2, entry 20) substituted
aldehydes were in excellent-to-good yields ranging from 97 to
from reaction system by coordination-driven solubility. Upon
addition of 1,4-dioxane, precipitated from reaction solution
via formation of (Fig. S12, ESI†), and the generated could
1
2
2
be resoluble in CHCl3 and regenerated
used for the next catalytic run. Notably,
1
which was directly
could be recycled at
1
least 10 times in this way without loss of its catalytic activity
under the optimized conditions (96 % yield for the tenth
catalytic run, Fig. 3a and Fig. S13, ESI†). After ten catalytic
runs, the PXRD patterns of
integrity and crystallinity of
2
indicated that the structural
was still preserved, reflecting
2
that
1
was stable during the multiple times repeatable A3-
91%, suggesting that the catalytic activity of
1 might result
coupling catalytic process (Fig. 3b). On the other hand,
2
could
from the collaboration of internal and outside surface catalytic
processes.
also be directly used to promote this A3-coupling reaction in a
heterogeneous way but with a lower catalytic efficiency.
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In contrast, the corresponding coupling product generated
from fluorene-2-carboxaldehyde was obtained in only 71%
Acknowledgements
We are grateful for financial support from NSFC (Grant Nos.
21671122, 21475078, 21301109), Taishan scholar’s
construction project and Shandong Provincial Natural Science
Foundation (No. ZR2018MB005).
yield with the aid of
reaction conditions (Fig. S16, ESI†), which further highlighted
the homogeneous catalysis merit of
2 in a heterogeneous way under the same
1.
For understanding the mechanism of this novel Cu-cage
promoted A3-reaction, we performed the ESI-MS
measurement on the model reaction system. As shown in the
Notes and references
spectra (Fig. S17, ESI†), the iminium cation of (I) from
3 and 4
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was detected based on the observed peak at m/z 160.1140
(calcd for (C11H13N) M+ m/z 160.1121). In addition, the peak at
m/z 1404.2037 (calcd for (C80H60Cu3O12) [M+H]+ m/z
1404.2046) indicated formation of the Cu(I)-acetylide adduct
(II) from
5 and Cu3L2 cage. These observations are consistent
with the normally proposed A3-coupling reaction
mechanism,20,21 which involved the in situ generated iminium
cation and the copper-activated Cu(I)-alkyne species. During
the process, the {O4} coordination sphere might account for
adequate electron delocalization to promise the reduction of
Cu(II) to Cu(I), which was further promoted by the redox
potential of Cu(II).22 As shown in Fig. S18 (ESI†), the expected
propargylamine
6
was finally generated, together with water
, by addition of the Cu(I)-acetylide to the
and the regenerated
1
iminium cation. Furthermore, the peak at m/z 1479.2755 for
host-guest species of (C6H5CHO)(C4H8NH)@Cu3L2 (calcd for
(C83H70Cu3NO13) [M+H]+ m/z 1479.2732) was also observed,
implying that the Cu-cage herein served as not only a catalyst
but also a kind of reaction vessel. As shown above, the smaller-
sized substrates which could be readily encapsulated in the
cage container would facilitate the coupling reactions.
Interestingly, the ESI-MS results (Fig. S17, ESI†) also suggested
a product-cage interaction, confirmed by observation of the
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In conclusion, we have reported, as the first of its kind, a
novel Cu(II)-MOC that is a homogeneous catalyst and can
highly promote the A3-coupling reaction to generate
propargylamines with excellent yields and a reasonable scope.
Its catalytic activity is comparable with the reported
homogeneous Cu catalysts,23 but showed a better catalytic
performance than those of copper-based heterogeneous
catalysts (Table S3, ESI†). More importantly, it featured an
interesting coordination-driven solubility and could be
recycled at least ten times without loss its activity, which was
the advantage of solid catalysts such as Cu-MOFs.24 This
reversible coordination interaction triggered homogeneous
catalysis and heterogeneous recovery might open up new
avenues for the design and synthesis of new type of catalysts
with both homo- and heterogeneous catalyst merits.25
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Conflicts of interest
There are no conflicts to declare.
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For table content
A novel Cu3L2 metal-organic cage, which features the coordination interaction triggered solubility, can be a highly active and
reusable catalyst to homogeneously catalyse the one-pot aldehyde-alkyne-amine A3-coupling reaction.
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