A copper(I)/copper(II)-salen coordination polymer as a bimetallic catalyst for three-component Strecker reactions and degradation of organic dyes

Article abstract of DOI:10.1039/c3cc47996j

A copper(I)/copper(II)-salen coordination polymer prepared by solvothermal reactions shows prominent bimetallic catalytic activities towards three-component Strecker reactions and photodegradation of organic dyes under visible-light illumination. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc47996j

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A copper(I)/copper(II)–salen coordination polymer
as a bimetallic catalyst for three-component
Strecker reactions and degradation of organic dyes†
Cite this: Chem. Commun., 2014,
50, 2295
Received 17th October 2013,
Accepted 31st December 2013
Yun-Long Hou, Raymond Wai-Yin Sun, Xiao-Ping Zhou, Jun-Hao Wang and
Dan Li*
DOI: 10.1039/c3cc47996j
www.rsc.org/chemcomm
A copper(I)/copper(II)–salen coordination polymer prepared by
Metalloligands have been widely employed as linkers for the
solvothermal reactions shows prominent bimetallic catalytic activ- preparation of MOFs/CPs,9 since these ligands could feature in the
ities towards three-component Strecker reactions and photo- straightforward immobilization of CUSs under solvothermal syn-
degradation of organic dyes under visible-light illumination.
thesis.6,9c–e A notable example of metalloligands is metallosalens,
and the reported metallosalen-based MOF catalysts usually contain
Inspired by the fact that several multi-functional enzymes contain a single CUS.4b,10 With the ease in structural modification, metallo-
multiple metal-based catalytic units,1 worldwide efforts have been salens indeed could be readily tuned to display a wide range of
made to employ different materials as novel classes of bi-/ catalytic activity towards a number of organic transformations.
multi-metallic2 and multi-functional catalysts.3 Metal–organic
In the literature, several CuI2I2 clusters have shown prominent
frameworks (MOFs), a subcategory of coordination polymers catalytic activity in a multi-component reaction.11 Moreover, various
(CPs) having structural flexibility to bear more than one catalytic copper(I)-based CPs including these copper(I) clusters which may
unit resulting from the huge variations of metal nodes and organic have potential to be used as effective heterogeneous catalysts have
linkers,4 have recently been demonstrated to achieve bifunc- been structurally characterized and reviewed by others12 and by us.13
tional catalytic activities.5 For instance, some site-isolated Lewis As an effort in exploring the catalytic applications of different kinds
acid–Brønsted base MOFs have been employed in aldol- of polymeric materials,3 we aim to design a novel bimetallic CP
condensations,5a Knoevenagel reactions5b and one-pot tandem which could present multi-functional catalytic activities by combin-
reactions,5c–e with coordinatively unsaturated metal sites (CUSs)4b,6 ing catalytically active units of copper(I) and metallosalens. In
and organic ligands serving as acidic and basic catalytic sites, this work, we develop a straightforward solvothermal approach
respectively. Moreover, various MOFs/CPs are insoluble and stable for the preparation of a bimetallic CP, {[CuII(SalImCy)](CuII)2ꢀDMF}n
in common organic solvents or aqueous solutions. These intrinsic (1, wherein SalImCy
=
N,N0-bis-[(imidazol-4-yl)methylene]cyclo-
properties thus render MOFs/CPs to be potentially used as green hexane-1,2-diamine, Fig. 1a), which contains copper(II)–salen-based
heterogeneous catalysts which could readily be reused. Nevertheless, catalysts CuII(SalImCy) and copper(I) iodide clusters. Its bimetallic
preparation of bimetallic MOFs/CPs via integration of one type of catalytic activities toward three-component coupling reactions and
CUS with another type of metal-based catalytic moiety remains a visible-light driven degradation of organic dyes have been examined.
formidable challenge.7 Limited examples include a bimetallic
The copper(II)–salen ligand [CuII(SalHImCy)](NO3)2 was pre-
heterogeneous catalyst palladium(II)–copper(II) MOF which could pared by a one-pot reaction of in situ-formed SalHImCy with
be synthesized via post-synthetic modification (PSM)8 with potent CuII(NO3)2ꢀ3H2O in a molar ratio of 1 : 1 (see ESI†). This ligand
catalytic activity in a multi-component reaction.7a The reported PSM exhibits a good solubility (>10 mg mLꢁ1) in dimethylformamide
approach in preparing MOFs, however, is sometimes limited by (DMF), CH3CN and C2H5OH and is stable in air at 298 K, which
its sophisticated modification steps, as well as the chemical and in turn could be used as a precursor for subsequent solvo-
physical instabilities of the intermediates/products.8
thermal reactions.
CP 1 in the form of red cuboid-like crystals was obtained by a
solvothermal reaction of [CuII(SalHImCy)](NO3)2 with CuII in a molar
ratio of 1 : 2 in a DMF/C2H5OH mixture (v/v, 2 : 1) for 24 h (Fig. 1a). It
has been formulated and characterized on the basis of elemental
analysis, IR spectroscopy, and thermogravimetric and single-crystal
X-ray diffraction analyses. The experimental details are given in the
ESI† (Table S1 and Fig. S1). CP 1 can be readily prepared by either
Department of Chemistry and Research Institute for Biomedical and Advanced
Materials, Shantou University, Guangdong 515063, P. R. China.
E-mail: dli@stu.edu.cn; Tel: +86-754-86502741
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of 1, experimental details for catalysis and dye degradation reactions, and
other physical measurements. CCDC 966731. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3cc47996j
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Table 1 Asymmetric three-component Strecker reaction catalyzed by 1a
Entry Substrate (R)
tb (h) Conversionc (%) a : bd
TOFe (hꢁ1
)
1
2
3
4
5
6
7
H
1
2
2
2
3
1
1
99
99
99
93
86
99f
95g
67 : 33 198
4-Methyl
4-tert-Butyl
3,5-Di-tert-butyl
4-Phenyl
H
68 : 32
70 : 30
71 : 29
67 : 33
99
99
92
60
67 : 33 198
66 : 34 191
H
a
Fig. 1 (a) Schematic drawing of solvothermal synthesis of
1 from
Aromatic aldehydes (1 mmol), (R)-(+)-1-phenylethylamine (1 mmol),
CuII(SalImCy) and CuII. (b) A representative zigzag chain and the asym-
TMSCN (1.5 mmol), CD3CN (2 mL), and 1 (0.5 mol%), sealed in a screw-
cap vial were stirred at 283 K. Reaction time t, hour. % conversions
b
c
metric unit of 1 viewed along the a-axis.
d
were determined by GC. Diastereoselectivity was determined by H-NMR.
e
TOF = turnover frequency (moles of reactants converted per moles of
active sides per unit time), hꢁ1
. The second cycle. The third cycle.
f
g
one of the four different forms of the 1,2-cyclohexanediamine (i.e.,
1S, 2S (+)-1,2-cyclohexanediamine); while all of them render 1 to
%
form crystal lattices in the triclinic centro-symmetric P1 space group 93% and 86%, respectively. The effect of the change of reaction
(Table S1, ESI†). The phase purity of the bulk sample has been temperature from 283 K to 273 K (Table S2, ESI†) and 298 K
established by comparing its observed and simulated powder X-ray (Table S3, ESI†) has also been examined. We found that these
diffraction (PXRD) patterns (Fig. S2, ESI†). CP 1 is found to be highly temperature changes did not significantly affect the catalytic activ-
stable in air as well as in solvents including H2O, DMF, C2H5OH and ities of 1 in the Strecker reaction in terms of the percentage
CH3CN at 298 K.
conversion (86–99%) and diastereoselectivity (2–2.4) (Fig. S4, ESI†).
Single-crystal X-ray diffraction analysis reveals that prominent Since changes in temperature as well as the size of the aldehyde
structural features of 1 include the square-planar 4-coordinate CuII in substrate did not render a significant impact on the catalytic activity
the metallosalen units (Fig. 1b, yellow) and the 3-coordinate CuI in the of 1 in terms of percentage conversions and diastereoselectivity, we
rhomboid CuI2I2 clusters (Fig. 1b, purple). The CuII ion, which reckon that the catalytically active metal centers on the solid exterior
embraces the CUS, is chelated by two imines [CuII–Nimine
1.9571(12)–1.9613(14) Å] and two imidazols [CuII–Nimidazol
,
,
surface may be highly accessible, as evidenced by reports on efficient
salen-based 1D CP catalysts.15
1.9630(13)–1.9631(13) Å] (Fig. 1b). The copper(II)–salen linker,
For comparison, the catalytic activities of two reference com-
with an angle of 112.51 between two deprotonated imidazol N pounds CuII and [CuII(SalHImCy)](NO3)2 have also been examined
atoms, is bridged by two CuI2I2 nodes to form a zigzag chain under similar experimental conditions. Both of them were found to
along the a-axis. Along the b-axis, 1D zigzag chains stack with display a much lower catalytic activity than 1, in terms of the amount
each other to form tubular channels which could be used to trap employed (10 mol% for CuII and 0.5 mol% for [CuII(SalHImCy)]-
DMF molecules (Fig. S3, ESI†).
(NO3)2) and the reaction time required (3 h), as well as the
Given that various metallosalens and CuII demonstrated potent percentage conversions obtained (82% for CuII and 87% for
catalytic activities on three-component Strecker reactions,11,14 we first [CuII(SalHImCy)](NO3)2) for the a-aminonitrile products (Table S4,
examined the catalytic activity of 1 in these reactions using aldehydes, ESI†). The superior catalytic activity of 1 indicates that a cooperative
trimethylsilyl cyanide and asymmetric amines as substrates. For catalytic event is achieved by two different catalytically active sites
instance, the reaction was carried out by mixing benzaldehyde, (i.e., CuI and CuII) of 1. In this reaction, the CuII–Im moieties of 1
(R)-(+)-1-phenylethylamine and trimethylsilyl cyanide (TMSCN) in may function as Brønsted bases to activate the cyanides, while CuII
the presence of 1 at 0.5 mol% in CD3CN at 283 K for 1 h. Two ions act as Lewis acids to activate the imine intermediates in the
a-aminonitriles (R,R)-(+)-a-pheny1-a-[(l-phenylethyl)amino]-acetonitrile Strecker reaction.7a,16 Furthermore, the inductively-coupled-plasma
and (R,S)-(+)-a-pheny1-a-[(l-phenylethyl)amino]-acetonitrile were the spectroscopic analysis demonstrates the heterogeneity of 1 with
reaction products and were found in the mixture with a 99% less than 0.02% of copper content in the reaction mixture.9b,17
conversion as determined by 1H-NMR spectrometry (Table 1, recycling test with three consecutive runs shows that 1 can be
entry 1). The turnover frequency (TOF) of 1 was found to be 198 hꢁ1
re-used without a significant loss of catalytic activity (the con-
A
.
Apart from benzaldehyde, four other aromatic aldehydes have also versions can reach from 95 to 99%, see Table 1, entries 6 and 7).
been employed as substrates in the 1-catalyzed Strecker reaction. Solid residues of 1, which were isolated from the reaction
Similar to that of 1, up to 99% conversion can be obtained by using mixture via centrifugation, displayed the same PXRD pattern
aromatic aldehydes substituted with a 4-methyl (entry 2) or a 4-tert- as that of the pristine solid of 1 (Fig. S2, ESI†). These results
butyl (entry 3) group. Increasing the bulkiness by using 3,5-di-tert- support that 1 has an adequate stability and recoverability to be
butyl (entry 4) or 4-phenyl (entry 5) substitution may result in slightly used as an efficient bimetallic heterogeneous catalyst for the
changing the percentage conversions of a-aminonitriles from 99% to three-component reactions.
2296 | Chem. Commun., 2014, 50, 2295--2297
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three-component Strecker reactions. Meanwhile, 1 has also been
demonstrated to display promising visible-light-driven bimetallic
catalytic activity in degrading various organic dyes. This work paves
the way to the solvothermal synthesis of efficient heterogeneous
multifunctional materials which could catalyze transformations
through a bimetallic pathway.
We acknowledge the National Basic Research Program of China
(973 Programs, 2012CB821706 & 2013CB834803), the National
Natural Science Foundation for Distinguished Young Scholars of
China (20825102), the National Natural Science Foundation of
China (91222202, 21171114, 21101103) and Shantou University.
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Chem. Commun., 2014, 50, 2295--2297 | 2297