Heterogeneous catalyzed aryl-nitrogen bond formations using a valine derivative bridged metal-organic coordination polymer

Article abstract of DOI:10.1039/b823335g

A new 1D macrocyclic copper(ii) coordination polymer, [CuL₂] (1, L = N-(4-pyridyl)-d,l-valine), based on a valine derived ligand was synthesized and characterized by single-crystal X-ray diffraction studies. The catalytic experiments showed tha

Full text of DOI:10.1039/b823335g

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www.rsc.org/dalton | Dalton Transactions
Heterogeneous catalyzed aryl–nitrogen bond formations using a valine
derivative bridged metal–organic coordination polymer†
Chuan-De Wu,* Li Li and Lian-Xu Shi
Received 5th January 2009, Accepted 15th May 2009
First published as an Advance Article on the web 7th July 2009
DOI: 10.1039/b823335g
A new 1D macrocyclic copper(II) coordination polymer, [CuL2] (1, L = N-(4-pyridyl)-D,L-valine), based
on a valine derived ligand was synthesized and characterized by single-crystal X-ray diffraction studies.
The catalytic experiments showed that compound 1 was an efficient catalyst for the cross-coupling
reaction of arylboronic acids with imidazole in the absence of additional additives. The crystal structure
analysis of 1 suggested that the vacant axial coordination position of the copper(II) atoms played a very
important role in the efficient catalytic transformation process.
Introduction
Experimental details
In the past few years, there has been a great increase in the
interest and generation of functional metal–organic frameworks
(MOFs), owing to their various topological architectures and
applicable properties in diverse fields such as catalysis, gas storage,
separation and ion-exchange, etc.1–6 Since amino acid residues pro-
vide functional properties and highly selective substrate-binding
abilities in many extended biological structures, the preparation
of bio-analogous materials comprising amino acid residues as
building blocks has attracted a great deal of attention.7,8 There
is much current interest in the crystal engineering of coordination
networks by combining amino acids and metal nodes with suitable
geometries, which can help develop new technologically useful
materials for a variety of applications.9
Materials and methods
The N-(4-pyridyl)-D,L-valine (L) ligand was synthesized according
to the literature method,10 while all the other chemicals were
purchased and used without further purification. The IR spectrum
was recorded using KBr pellets on a FTS-40 spectrophotometer.
Powder X-ray diffraction data (PXRD) were recorded on a
˚
RIGAKU D/MAX 2550/PC using Cu Ka (l = 1.5406 A)
radiation. Thermogravimetric analysis (TGA) was carried out in
air on a NETZSCH STA 409 PC/PG instrument at a heating rate
◦
1
of 4 C min-1. H NMR spectra were recorded on a 400 MHz
spectrometer in CDCl3 solution and the chemical shifts were
reported relative to the internal standard TMS (0 ppm).
We are particularly interested in the synthesis of coordination
solids based on multitopic linkers derived from amino acids
and the exploration their catalytic properties in heterogeneous
processes. According to the coordination preferences of transition
metal atoms, we have modified the amino acids with pyridyl
groups to generate a series of effective bridging linkers (Scheme 1).
Basically, the carboxylic acid and pyridyl groups within each
ligand can link up metal nodes for the construction of extended
solid-state framework structures, while the remaining unit within
the ligand can be used for attachment of functional groups. Herein
we wish to report the synthesis, crystal structure and highly
efficient catalytic properties of a new 1D coordination polymer,
[CuL2] (1, L = N-(4-pyridyl)-D,L-valine).
Synthesis of [CuL2] (1). Heating a mixture of Cu(NO3)2·3H2O
(12.1 mg, 0.05 mmol) and L (9.7 mg, 0.05 mmol) in a mixed solvent
of EtOH (3 mL), H2O (1.5 mL) and DMF (0.1 mL) in a capped vial
at 90 ◦C afforded purple crystals after five days, which were filtered,
washed with EtOH and Et2O, and dried at room temperature
(yield: 52% based on L). Anal. calcd for C20H26CuN4O4 (%): H,
5.83; C, 53.44; N, 12.47. Found: H, 5.76; C, 52.96; N, 12.51. IR
(KBr pellet, n/cm-1): 1622 s, 1534 s, 1468 w, 1411 w, 1400 s, 1344 m,
1299 m, 1252 m, 1209 s, 1162 m, 1091 m, 1065 m, 1024 s, 819 s,
747 m, 665 w, 597 w, 530 m.
Representative procedure for the Ar–N coupling of arylboronic
acids with imidazole using solid catalyst 1. A mixture of
0.12 mmol phenylboronic acid, 0.1 mmol of imidazole and
0.01 mmol of solid 1 in 2 mL MeOH was vigorously stirred in air
at room temperature for 12 h. The reaction mixture was filtered,
washed, evaporated and extracted with CH2Cl2. The organic
phase was combined, dried over anhydrous MgSO4, filtered and
evaporated. Subsequently, the organic phase was separated and
passed through a short silica gel column to remove the surplus
phenylboronic acid (hexane : ethyl acetate, 50 : 50 volume ratio).
An aliquot was analyzed by gas chromatography (GC) to give the
conversion value, calculated by using a pure sample as standard.
1H NMR (500 MHz, CDCl3): d 7.84 (s, 1H), 7.43–7.47 (m, 2H),
7.32–7.37 (m, 3H), 7.26 (s, 1H), 7.19 (s, 1H) ppm; MS (ESI) m/z:
145.1 ([M + H]+).
Scheme 1
Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R.
China. E-mail: cdwu@zju.edu.cn
† CCDC reference number 695224. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b823335g
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Table 1 Crystal data and structural refinements for compound 1
X-Ray data collection and structure determination
Data collection and the determination of the unit cell for com-
pound 1 were performed on a Rigaku R-AXIS RAPID. The data
were collected using graphite-monochromated Mo Ka radiation
Compound
[CuL2]
Formula
C20H26CuN4O4
449.99
Purple
Monoclinic
C2/c
Formula weight
Crystal color
Crystal system
Space group
˚
(l = 0.71073 A) at 293 K. Empirical absorption corrections were
applied using the ABSCOR program.11 The structure was solved
by direct methods and refined using a full-matrix least-squares
method with the SHELXTL-97 program package.12 All non-
hydrogen atoms were located successfully from Fourier maps and
were refined anisotropically. The positional disorder in compound
1 was evidenced by the presence of two positions for each copper
atom despite collecting the crystal data several times. We have
tried using low symmetry space groups, such as monoclinic space
˚
Unit cell dimensions
a = 15.833(3) A
˚
b = 9.330(2) A
˚
c = 15.152(3) A
b = 114.57(3)◦
2035.4(7)
4
3
˚
Volume/A
Z
Calculated density/g cm-3
F(000)
1.468
940
1.107
3.13 to 23.00
5910 (Rint = 0.1518)
1344/138
1.062
0.0907 (0.1294)
0.297 and -0.254
¯
Absorption coefficient/mm-1
q for data collection/◦
Reflections collected
Data/parameters
Goodness-of-fit on F2
R1 (wR2) [I > 2s(I)]
groups Cc and C2, and triclinic space groups P1 and P1, to attempt
to solve the positional disorder problem. However, the problem
remained. Thus, we attempted to solve the crystal structure by
refining each copper atom as occupying two disordered positions.
Initially, the occupancies were refined using free variables, but
in the final cycles of refinement, these occupancies were held
as fixed values near the refined values of 75% for Cu(1) and
25% for Cu(2). Since copper atoms are sited on a two-fold axis,
the occupancy for Cu(1) should be 0.375, while Cu(2) is 0.125.
Although the final results (R1 = 0.0907, wR2 = 0.1294) are
slightly large, the molecular structure is well behaved. The poor
crystal data suggested that only the gross connectivity of the
structure was obtained. Hence, we will not discuss the structural
characteristics in detail. CCDC 695224.† The data collection
parameters, crystallographic data and final agreement factors are
collected in Table 1.
-3
˚
Largest diff. peak and hole/e A
ꢀ
ꢀ
ꢀ
ꢀ
R1 = (|Fo| - |Fc|)/ |Fo|, wR2 = [ w(Fo - Fc2)2/ w(Fo2)2]0.5
.
2
Results and discussion
Purple crystals of 1 were synthesized by the reaction of
Cu(NO3)2·3H2O with the L ligand, in a mixed solvent of H2O,
EtOH and DMF, at 90 ◦C for five days. Single-crystal X-ray
diffraction analysis had revealed that compound 1 crystallizes
in the monoclinic space group C2/c. There is half a Cu(II)
atom and one L ligand in the asymmetric unit (Fig. 1). Each
copper(II) atom is tetra-coordinated to two carboxyl oxygen
Fig. 1 ORTEP representation of the symmetry expanded local structure
for 1 (30% probability ellipsoids).
It is worth noting that there is no significant supermolecular
interaction other than van der Waals contacts between the 1D
macrocycle chains (Fig. 3). By careful inspection of the crystal
structure, we found that there are only weak hydrogen bonds
between the parallel polymeric chains, which arise from the
amino groups and deprotonated carboxyl oxygen atoms (N–O =
˚
atoms (Cu(1)–O = 1.961(7) A) and two pyridyl groups (Cu(1)–
˚
N = 1.948(7) A) of four L ligands in a distorted square-planar
geometry, with axial positions left uncoordinated. Thus, two L
ligands are bidentate, trans-linking two Cu(II) centers to form an
˚
2.918 A).
˚
18-membered macrocyclic ring 8.517 A in diameter that lies in
The TGA graph showed that there was no weight loss until
the ac plane, which is further extended into a one-dimensional
polymeric chain running along the (101) direction (Fig. 2).
160 ◦C for compound 1 (Fig. 4). Above 160 ◦C, the product began
to lose coordinated L ligands, resulting in decomposition, which
Fig. 2 The 1D polymeric macrocyclic chain in 1 (cyan, red, blue and deep grey colors represent Cu, O, N and C atoms, respectively).
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diffraction pattern similar to that of the unheated sample. This
result indicates that the neutral framework of 1 was maintained
and stable up to 120 ◦C, indicating it can be used for further
catalytic application studies.
Since the copper(II) atom is in a distorted square-planar
coordination geometry and the axial positions are vacant in 1,
we speculated that the copper atoms can coordinate to suitable
organic substrates, utilizing the axial coordination sites to gen-
erate catalytic intermediates. These interesting structural features
promoted us to study the catalytic activities of compound 1.
The development of efficient methods for the synthesis of
N-arylimidazoles under mild conditions has recently gained
considerable attention in synthetic chemistry because of the com-
pounds’ biological activities.13 And, as it is well established that,
in homogeneous and heterogeneous systems, Cu(II) complexes are
active catalysts for a range of cross-coupling reactions of aryl-
boronic acids with organic amines,14–17 we attempted to employ our
complex in the cross-coupling reaction of arylboronic acids with
imidazole to verify our hypothesis. The treatment in air of 1 with
phenylboronic acid and imidazole in methanol for 12 h generated
the anticipated N-phenylimidazole in very high yield (Table 2,
entry 1). Subsequently, a number of structurally and electronically
diverse substrates were employed for the catalytic coupling reac-
tion (Table 2, entries 2 to 6). When p-tolylboronic acid was used,
the corresponding N-arylation product was obtained in good yield
(Table 2, entry 2). Upon treatment of p-chlorophenylboronic acid
with imidazole, a chlorinated N-phenylimidazole was formed in
94% yield (Table 2, entry 4). However, the cross-coupling yield
was lower for o-cyanophenylboronic acid (Table 2, entry 6). When
the cross-coupling reaction time was prolonged, the conversion
was slightly improved (Table 2, entry 3). The catalyst can be
separated by simple filtration from the reaction mixture, which
was then reused in the successive run. The slightly decreased
yield (85%) may be due to the small amount of catalyst lost
through filtration. PXRD for the recovered material showed that
the structural integrity of the catalyst was maintained during the
catalytic reaction (Fig. 5). The average particle size of recovered
solid 1 was 45 nm in diameter, deduced from Scherrer’s formula
Fig. 3 A view of the packing diagram of 1 (green, red, blue, deep grey
and light grey colors represent Cu, O, N, C and H atoms, respectively).
Fig. 4 TGA plot of 1.
was consistent with the X-ray crystal structural analysis result. The
PXRD of a bulk sample of 1 showed a sharp diffraction pattern
similar to that of the simulated, which confirmed the purity of the
sample (Fig. 5). The PXRD spectrum of a sample of 1 that had
◦
been ground and heated at 120 C for 5 h in air showed a sharp
¯
using the (114) diffraction peak. We can not observe trace cross-
coupling product from a mixture of phenylboronic acid and
imidazole in the presence of the filtrate of 1 in methanol under
Table 2 The cross-coupling of arylboronic acids with imidazole and
compound 1 catalysta
Entry
Ar
Time/h
Yield (%)b
1
2
3
4
5
6
Ph
12
12
24
12
12
12
97
85
94
94
84
55
4-CH3–Ph
4-CH3–Ph
4-Cl–Ph
3-CH3–Ph
4-CN–Ph
a Arylboronic acid (0.12 mmol), imidazole (0.1 mmol) and solid 1
(0.01 mmol) were stirred in MeOH (2 mL) at room temperature. b Yields
(%) were determined by GC on a SE-54 column.
Fig. 5 Powder X-ray diffraction patterns for compound 1.
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otherwise identical conditions. These experiments demonstrated
the heterogeneous nature of the present catalytic system. However,
as there is no channel or cavity in solid 1, the catalytic reaction
might take place on the solid surface. Most interestingly, the
surface promoted catalytic reaction can make the transformation
process highly efficient without any additional additives.
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There have been many reports on attempts to optimize the
copper-promoted N-arylation of arylboronic acids with imidazole
to furnish N-arylimidazoles.15–17 Comparing with these reports,
the catalytic activity of 1 used at room temperature was better
than that of homogenous and heterogeneous catalysts used under
similar reaction conditions, and it even rivals them when used at
evaluated reaction temperatures. The striking catalytic activity of 1
prompted us to investigate further its catalytic properties. We tried
different solvents, such as DMF, THF, CH2Cl2 and H2O. However,
almost no product can be observed when using any of these, except
for trace amounts of the desired product obtained when using
DMF. Obviously, it can be concluded that the solvent effect was
one of the most important factors in the coupling of arylboronic
acids with imidazole in the case of compound.1 Together with
the crystal structure analysis result, we believe that the distinct
catalytic activity of 1 can be attributed to the coordinatively vacant
axial positions of copper atoms, which generated dramatically
different results compared to the aforementioned systems.
Collman et al. have proposed that the coupling reaction
mechanism to involve binuclear Cu(II) catalyzed C–N bond
formation.15 However, our structurally characterized catalyst
suggests a mononuclear process because all of the copper(II) atoms
are fixed by the coordinating ligands during the catalytic process.
Conclusions
In conclusion, we have successfully synthesized a new bridged
copper(II) coordination polymer with valine derived ligands, which
acted as an efficient catalyst for the cross-coupling of arylboronic
acids with imidazole in methanol. The attractive features of the
catalytic system are: (1) no need for additional additives, (2) can
be used at room temperature, (3) affords good to excellent yields,
(4) reusability of the catalyst. The crystal structure analysis,
together with the catalytic results, suggests that the coordinatively
vacant axial positions in the copper(II) atoms plays a very impor-
tant role in the efficient catalytic activity of 1. Most significantly
of all, the structurally characterized catalyst is very important for
further study of the catalytic mechanism.
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Acknowledgements
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We are grateful for the financial support of the NSF of China
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12 G. M. Sheldrick, Program for Structure Refinement, University of
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