In situ generation of functionality in a reactive binicotinic-acid-based ligand for the design of multi-functional copper(ii) complexes: Syntheses, structures and properties

Article abstract of DOI:10.1039/c4ce02204a

Under hydrothermal conditions, including tuning the reaction ratio and reaction temperature, three-dimensional (3D) porous {[Cu₃(hbpdc)(OH)₂(H₂O)]·2H₂O} (1), two-dimensional (2D) sheet {Cu₂(hbpdc)(H

Full text of DOI:10.1039/c4ce02204a

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In situ generation of functionality in a reactive
binicotinic-acid-based ligand for the design of
multi-functional copperIJII) complexes: syntheses,
structures and properties†
Cite this: CrystEngComm, 2015, 17,
1871
a
a
a
Dongsheng Deng,a Hui Guo,a Guohui Kang,ab Lufang Ma, Xu He and Baoming Ji
*
Under hydrothermal conditions, including tuning the reaction ratio and reaction temperature, three-
dimensional (3D) porous {[Cu3IJhbpdc)IJOH)2IJH2O)]Ĵ2H2O} (1), two-dimensional (2D) sheet {Cu2IJhbpdc)-
IJH2O)2}n (2), dinuclear ijCuIJhbpdc)0.5IJH2O)2] (3), dimer ijCu2IJmbpdc)2IJpy)2]Ĵ9H2O (4), and mononuclear
ijCuIJhbpdcH2)IJH2O)2] (5) have been synthesized via in situ ligand transformation reaction, in which 3,3′-
dimethoxy-2,2-bipyridine-6,6′dicarboxylic acid (mbpdcH2) undergoes demethylation to 3,3′-dihydroxy-2,2′
bipyridine-6,6′-dicarboxylic acid (hbpdcH4). The detailed coordination patterns of the hbpdcH4 ligand have
been revealed by single-crystal X-ray diffraction. The catalytic results demonstrate that polymers 1 and 2
can function as heterogeneous and reusable catalysts for the Strecker reaction of various imines. In addi-
tion, magnetic susceptibility measurements of complexes 1, 2 and 4 reveal antiferromagnetic coupling
between the copperIJII) ions. Complex 1 follows the Curie–Weiss law, while 2 and 4 obey the Bleaney–
Bowers dinuclear model.
Received 6th November 2014,
Accepted 9th January 2015
DOI: 10.1039/c4ce02204a
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achieved, the rational design and synthesis of MOHMs with
unique structure and function still remain a significant chal-
Introduction
As a flourishing research field, metal–organic hybrid mate-
rials (MOHMs), containing paramagnetic metal ions incorpo-
rated into coordination polymers to fabricate extended
structures, are currently of significant interest in materials
chemistry, not only because of their fascinating structural
diversities, but also their useful properties and promising
application in the fields of gas storage, separation, magnetism,
and catalysis.1 In general, such MOHMs are typically composed
of organic ligands as linkers and metal ions or metal ion
clusters as nodes.2 The assembly of reaction components
from solution to the solid state can be achieved through vari-
ous means, and each of these pathways can afford unique
materials. In this respect, much effort has been directed
toward developing the ability to control the reactions so that
the products can be engineered to have the desired proper-
ties.3 Although remarkable progress in this area has been
lenge. Many factors, such as the nature of the metal ion,4
structural features of the organic ligand,5 counterions,6
temperature,7 reagent ratio,8 pH value,9 etc., can exert great
impact on the final architectures. With respect to tempera-
ture, its effect on the formation of the coordination polymers
has not been intensively explored, although some reports
described how the reaction temperature affects the generation
of different coordination polymers.7,10
Currently, multidenate N- and O-donor bridging ligands
have been widely used to construct MOHMs, due to their
wide range of binding modes that provide superexchange
pathways for magnetic coupling among paramagnetic metal
centers.11,12 Thus, inorganic–organic hybrid materials formed
with pyridine-, pyrazine-, pyrimidine-, imidazole-carboxylic
acid, etc. as linkers are known in the literature.13 In this
regard, binicotinic acid and its derivatives are excellent candi-
dates for the construction of novel intriguing structures and
topologies, because they have multiple coordination sites and
can adopt more versatile conformations.14,15
a College of Chemistry and Chemical Engineering, Luoyang Normal University,
Luoyang 471022, PR China
b College of Chemistry and Chemical Engineering, Zhengzhou University,
Zhengzhou 450052, PR China
In addition to magnetic properties, inorganic–organic
hybrid materials have provided a tunable platform for the
design of heterogeneous catalysts due to their high surface
areas inside the pores and uniform catalytic sites (both metal
ions and linkers can act as catalytic sites).1c,16 Some MOHMs
† Electronic supplementary information (ESI) available: X-ray crystallographic
data for complexes 1–5 in CIF format, PXRD patterns, TGA plots, tables for
hydrogen bonds, and NMR spectra. CCDC 968444–968449. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4ce02204a
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have open metal sites which exhibit Lewis acidity and could
catalyze organic reactions such as cyanosilylation of alde-
hydes or ketones, Mukaiyama aldol reaction, ring-opening of
epoxides, Friedel–Crafts alkylation reaction, etc.17
IR spectra were recorded as KBr pellets on a Nicolet Avatar-
360 spectrometer in the 4000–400 cm−1 range. TG-DSC mea-
surements were performed by heating the crystalline sample
from 25 to 900 °C at a rate of 2 °C min−1 in a N2 atmosphere
using a SDTQ600 differential thermal analyzer. Powder X-ray
diffraction was performed with a Bruker D8-ADVANCE X-ray
diffratometer operated using a 0.02° step scan from 5 to
50 °C in 2θ and 0.2 s preset time. 1H NMR spectra were
recorded on a Bruker DPX-400 spectrometer in CDCl3 with
TMS as an internal standard. Variable-temperature magnetic
susceptibilities were measured with an MPMS-7 SQUID mag-
netometer. Diamagnetic corrections were performed with
Pascal's constants for all constituent atoms.
In light of the abovementioned facts, we are here to focus
our attention on using 3,3′-dimethoxy-2,2-bipyridine-6,6′dicar-
boxylic acid as a multidentate ligand to construct functional
materials, based on the following considerations: 1) the two
carboxylic hydrogen atoms can be easily deprotonated,
allowing the four carboxylate oxygen atoms to bind to two or
three or four metal centers, as observed in other binicotinic
acid ligands;14,15 and 2) when two methoxy groups hydro-
lyzed to hydroxyl groups through an in situ ligand transforma-
tion reaction under hydrothermal conditions, the resulting
3,3′-dihydroxy-2,2′bipyridine-6,6′-dicarboxylic acid ligand
displays multidenate coordination modes, as depicted in
Scheme 1a–f. The carboxylate oxygen can form a chelating
bond to a metal center with the adjacent hydroxyl oxygen and
pyridyl nitrogen. Consequently, the hbpdc4− ligand, with
multifunctional coordination sites, is an effective component
in the design of a diverse range of functional coordination
complexes. On the basis of the above analysis, we first syn-
thesized a 3D porous MOF, {[Cu3IJhbpdc)IJOH)2IJH2O)]Ĵ2H2O}
(1).14d Next, we synthesized another four complexes
Synthesis of polymer 1 IJ{[Cu3IJhbpdc)IJOH)2IJH2O)]Ĵ2H2O}n).
Polymer
1
was prepared according to the literature
method.14d A total of 0.2 mmol (63.5 mg) of mbpdcH2 and
0.4 mmol (79.9 mg) of CuIJCH3COO)2Ĵ4H2O in a molar ratio
of 1 : 2 were combined with 13 mL of deionized water in a
15 mL stainless steel bomb. This was sealed and heated at
160 °C for 4 days, then cooled to room temperature at a rate
of 5 °C h−1. Blue block crystals suitable for X-ray single crystal
analysis were obtained in 81% yield (based on Cu). These
crystals were separated, washed with water, and dried under
ambient conditions.
{Cu2IJhbpdc)IJH2O)2}n
(2),
ijCuIJhbpdc)0.5IJH2O)2]
(3),
Synthesis of polymer 2 IJ{Cu2IJhbpdc)IJH2O)2}n). Polymer 2
was obtained by the same method as that of 1 except that the
reaction temperature was changed from 160 to 140 °C. After
being cooled to room temperature, blue crystals were
obtained in 85% yield. Anal. calcd. for C12H8Cu2N2O8: C,
33.11; H, 1.85; N, 6.44%. Found: C, 33.18; H, 1.96; N, 6.49%.
IR (KBr, cm−1): 3416(s), 3138(m), 1651(m), 1576(s), 1424(m),
1362(s), 1318(m), 1120(m), 877(m), 863(m), 797(s), 699(s).
Synthesis of complex 3 IJ[CuIJhbpdc)0.5IJH2O)2]). Complex 2
was synthesized by the same procedure used for preparing 1
except that the metal-to-ligand ratio was altered from 1 : 2 to
2 : 1. After being cooled to room temperature, blue crystals
were obtained in 20% yield. Anal. calcd. for C6H6CuNO5: C,
30.58; H, 2.57; N, 5.94%. Found: C, 30.66; H, 2.69; N, 5.99%.
IR (KBr, cm−1): 3418(m), 3041(s), 2950(m), 1697(s), 1634(s),
1567(m), 1492(m), 1422(s), 1359(s), 1289(s), 1242(m), 1172(m),
912(s), 863(s), 851(m), 790(s), 715(m).
ijCu2IJmbpdc)2IJpy)2]Ĵ9H2O (4), and ijCuIJhbpdcH2)IJH2O)2] (5) by
using the mbpdcH2 ligand with the help of different reaction
conditions including reaction temperature and reactant ratio.
The crystal structures and coordination modes of hbpdcH4,
along with the regulatory effect of reaction conditions on
these complexes, have been analyzed. The magnetic proper-
ties of complexes 1, 2 and 4 have been investigated. In addi-
tion, the heterogeneous catalytic activities of complexes 1
and 2 for the Strecker reactions of various imines in aqueous
media have also been investigated.
Experimental section
Materials and general methods
The reagents needed for synthesis were used as received. 3,3′-
Dimethoxy-2,2-bipyridine-6,6′dicarboxylic acid was prepared
according to the literature method.14d C, H, and N microanal-
yses were carried out using a Flash 2000 elemental analyzer.
Synthesis of complex 4 IJ[Cu2IJmbpdc)2IJpy)2]Ĵ9H2O). Complex
4 was prepared by a similar procedure to that for 1 except
Scheme 1 The coordination modes of the hbpdcH4 ligand in complexes 1–5.
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that the metal-to-ligand ratio was altered from 1 : 2 to 1 : 1,
and pyridine was used as the additive. After being cooled to
room temperature, blue crystals were obtained in 70% yield.
Anal. calcd. for C38H48Cu2N6O21: C, 43.39; H, 4.60; N, 7.99%.
Found: C, 43.52; H, 4.72; N, 8.15%. IR (KBr, cm−1): 3453(m),
2965(w), 2933(w), 2863(w), 1639(s), 1568(m), 1429(s), 1389(s),
1337(m), 1225(m), 1124(m), 893(m), 854(m), 788(m), 709(m).
Synthesis of complex 5 IJ[CuIJhbpdcH2)IJH2O)2]). The prepa-
ration of 5 was similar to that of 1 except that the metal-to-
ligand ratio was altered from 1 : 2 to 1 : 1. After being cooled
to room temperature, blue crystals were obtained in 70%
yield. Anal. calcd. for C12H10CuN2O8: C, 38.56; H, 2.70; N,
7.49%. Found: C, 38.69; H, 2.86; N, 7.62%. IR (KBr, cm−1):
3418(w), 3041(s), 2950(m), 1697(s), 1634(s), 1567(m), 1492(m),
1422(s), 1359(s), 1289(s), 1242(m), 1172(m), 912(s), 863(s),
851(m), 790(s), 715(m).
Catalysis experiments
Polymers 1 and 2 were activated at 120 °C for 8 h under vac-
uum before the reaction was carried out. A typical Strecker
reaction procedure was performed as follows. A quantity of
24 mg (0.005 mmol of 1) of the catalyst was suspended in dry
CH2Cl2 (2 mL) followed by the addition of imines (0.1 mmol)
and trimethylsilyl cyanide (0.15 mmol). The reaction mixtures
were stirred for 4 h at 298 K under N2. Catalytic recyclability
was checked four times with the same batch of the catalyst.
The recovered catalysts were characterized by X-ray powder
diffraction and showed identical results to those of the fresh
samples.
Results and discussion
Synthesis and spectroscopic characterization
As it is well known, in situ ligand transformation reaction
may occur during hydrothermal reaction, which is currently
regarded as an effective strategy to obtain novel MOHMs that
are inaccessible or not easily achieved by conventional
methods.20 However, the controlled synthesis of MOHMs is
still a major challenge.21 Small changes in one or more of the
variables of hydrothermal reaction, such as temperature,
metal-to-ligand ratio, and pH value, have a profound influence
on the final crystallization outcome.22 As far as these factors
are concerned, complexes 1–5 were successfully obtained by
the in situ reaction of mbpdcH2 with CuIJII) acetate through
tuning hydrothermal temperature, metal-to-ligand ratio and
additive. The self-assembly strategies are given in Scheme 2.
On the basis of identical reactants, the structures of 1 and 2
differed in the case of altered reaction temperature. Complexes
3 and 5 were obtained by changing the metal-to-ligand ratio.
Finally, complex 4 can be obtained by altering the metal-to-
ligand ratio in the addition of pyridine.
Single-crystal structure determination
A suitable single crystal was carefully selected under a polar-
izing microscope and glued carefully to a thin glass fiber.
Crystallographic data for 1–5 were collected on a Bruker
Smart Apex-II CCD area detector equipped with graphite-
monochromatic Mo Kα radiation (λ = 0.71073 Å) at room
temperature. Data were reduced using SAINTPLUS, and an
empirical absorption correction was applied using the
SADABS program. The structures were solved with direct
methods and refined with full-matrix least-squares tech-
niques on F2 using the SHELXTL program package.18,19 The
non-hydrogen atoms were placed in geometrically ideal posi-
tions and refined as riding atoms with a common isotropic
thermal parameter. Details of the structure solution and final
refinements are given in Table 1. CCDC 96844 (1), 968446 (2),
968447 (3), 968448 (4) and 968449 (5) contain the supplemen-
tary crystallographic data.
Table 1 Crystallographic data and structure refinement summary for complexes 1–5
Complexes
1
2
3
4
5
Chemical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
C12H10Cu3N2O10
532.84
Monoclinic
C2/c
16.892(3)
15.722(2)
5.8595(9)
90
106.112(2)
90
1495.0(4)
4
C12H8Cu2N2O8
435.28
Monoclinic
P21/c
18.211(3)
5.1283(10)
13.915(3)
90
96.803(2)
90
1290.4(4)
4
C6H6CuNO5
235.66
C38H48Cu2N6O21
1051.90
C12H10CuN2O8
373.76
Triclinic
Triclinic
Triclinic
¯
¯
¯
P1
P1
P1
6.2937(16)
6.8393(17)
9.545(4)
99.689(4)
90.764(4)
116.946(3)
359.14(19)
1
12.1868IJ13)
12.4222IJ13)
15.4307IJ16)
77.399(10)
88.583(10)
85.884(10)
2273.8(4)
2
4.785(7)
9.330(14)
14.53(2)
89.807(16)
81.067(16)
77.496(16)
625.4(16)
2
γ/°
Volume/Å3
Z
Dcalc/g cm−3
2.367
4.294
2.241
3.348
2.179
3.027
1.536
2.274
1.985
1.798
μ/mm−1
T/K
293(2)
5230
1388
0.0272
1.029
0.0244
0.0611
294(2)
8900
2395
0.0295
1.022
0.0296
0.0724
293(2)
2762
1333
0.0215
1.073
291(2)
17 105
8423
0.0265
1.030
0.0395
0.0981
296(2)
4758
2309
0.0203
1.058
Reflns. collected
Unique reflns
Rint
Goodness-of-fit
R1 (I > 2σ)
wR2 (I > 2σ)
0.0338
0.0813
0.0282
0.0750
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Scheme 2 Self-assembly syntheses of 1–5.
Description of crystal structures
different dinuclear copper building blocks, resulting in one
dimensional chains along the a axis with a Cu1⋯Cu2
distance of 4.698 Å, as depicted in Fig. 1b. Additionally, it
is noted that the latter hbptc4− ligands in the bidentate
IJk1)-IJk1)-μ2 bridging mode (Scheme 1c) link the two adjacent
ijCu2N2O2IJCO2R)2] building blocks, resulting in one dimen-
sional chains along the b axis with a Cu2⋯Cu2 distance of
5.128 Å, as depicted in Fig. 1c. The two kinds of chains are
not parallel but form angles about 80° with each other.
Therefore, the resulting two-dimensional sheet in the ab
plane is formed by two kinds of chains sharing the Cu2 ions,
as shown in Fig. 1d.
In addition, through other hydrogen-bonding interactions
originating from the coordinated water hydrogen atom and
the carboxyl oxygen atom between the adjacent sheets (O⋯O,
2.689(3) Å, 162.1°; 2.731(3) Å, 165.9°; 2.855(3) Å, 169.3°;
2.861(3) Å, 172.0°), the 2D sheet is further linked into a 3D
supramolecular network, as shown in Fig. 1e. Details of hydro-
gen bond interactions are listed in Table S2 (see the ESI†).
Crystal structure of 3. X-ray single crystal analysis of 3
indicates a discrete centrosymmetric dinuclear complex, as
shown in Fig. 2a. The asymmetric unit consists of one crystal-
lographically unique CuIJII) ion, half hbpdc4− ligand, and two
coordinated water molecules. The CuIJII) ion adopts a five-
coordinated square-pyramidal geometry, formed by one nitro-
gen donor and two carboxylate oxygen atoms from one hbptc4−
ligand and two aqua ligands. The Cu–N and Cu–O bond
distances lie in the range of 1.980 Å and 1.861IJ3)–2.291IJ3) Å
(Table S1, in the ESI†), being comparable to those found for
similar complexes in the literature.23 For each hbpdc4−
ligand, two carboxylate groups take the same chelating
coordination mode to join two CuIJII) ions (see Scheme 1d).
As a result, each hbpdc4− ligand links two CuIJII) ions to form
Crystal structure of 1. Under 160 °C hydrothermal condi-
tions, the reaction of CuIJII) acetate with the mbpdcH2 ligand
can afford polymer 1 with a metal-to-ligand ratio of 2 : 1. Poly-
mer 1 features a highly ordered 3D nanoporous structure
encapsulating a zigzag water chain. The detailed structural
discussion of 1 has been reported in our previous paper.14d
Crystal structure of 2. The asymmetric unit consists of two
crystallographically unique CuIJII) ions, two halves of hbpdc4−
ligands, and two coordinated water molecules. The Cu1 and
Cu2 ions lie on general positions while the two halves of
hbpdc4− lie on another independent inversion center. The
coordinated water molecules also occupy general positions.
As shown in Fig. 1a, the coordination geometry of the Cu1
ion can be described as a distorted square-pyramidal geometry,
being bonded to two carboxylate oxygen atoms from one
hbpdc4− ligand (Cu–O 1.865(2) and 1.941(2) Å), one nitrogen
atom of the hbpdc4− ligand (Cu–N 1.985(3) Å) and two aqua
ligands (Cu–O 1.973(2) and 2.273(3) Å) (Table S1, in the ESI†).
The Cu2 ion adopts a six-coordinated octahedral geometry.
The equatorial plane is composed of two chelating carboxyl-
ate oxygen atoms, one monodentate carboxylate oxygen atom,
and the monodentate nitrogen atom of two hbpdc4− ligands.
The apical positions are occupied by the monodentate car-
boxylate oxygen atoms of the hbpdc4− ligand. The O–Cu–O
and O–Cu–N angles in the equatorial plane and the O–Cu–O
angle along the apical direction are in the range of 85.62IJ10)–
173.41IJ11)° (Table S1, in the ESI†), indicating that the octahe-
dron is very distorted.
In the crystal structure, one hbpdc4− ligand in a bis-IJtridentate)
chelating mode connects Cu1 and Cu1A, giving a dinuclear
copper building block ijCu2N2O2IJCO2R)2] with short separa-
tions of Cu1⋯Cu1 (6.212 Å), while the other hbptc4− ligand
also in the bis-IJtridentate) chelating mode links Cu2 and
Cu2A, leading to the other dinuclear copper building block
ijCu2N2O2IJCO2R)2] with separations of Cu2⋯Cu2 (6.238 Å).
The former hbptc4− ligands in a bidentate IJk1-k1)-IJk1-k1)-μ2
bridging mode (Scheme 1b) connect the two adjacent but
a
dinuclear unit. Within this unit, the neighboring
Cu1A⋯Cu1B separations are 3.854 Å.
In the crystal structure of 3, the dinuclear units are
connected to each other through the formation of hydrogen
bonds between the adjacent coordinated water molecules,
resulting in a 1D chain along the c axis. The adjacent chains
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are further connected to each other through the formation of
hydrogen bonds between water molecules and carboxylate
oxygen atoms, resulting in a 3D supramolecular network, as
shown in Fig. 2b. Details of hydrogen bond interactions are
listed in Table S2 (see the ESI†).
Crystal structure of 4. In contrast to the above coordina-
tion polymers 1 and 2, complex 4 is a discrete double-
stranded dimer, which crystallizes in the monoclinic space
¯
group P1, and each asymmetric unit consists of two CuIJII)
ions, two mhbptc2−, and nine lattice water molecules. As
depicted in Fig. 3a, the structure of 4 is built up of dinuclear
units, in which the two copper ions have the same coordina-
tion modes with Cu⋯Cu = 4.500(1) Å. Each CuIJII) ion shows
a five-coordinated square-pyramidal geometry, formed by two
nitrogen donors, two carboxylate oxygen atoms from two
mhbpdc2− ligands and one pyridine nitrogen atom, in which
the Cu–O bond distances range from 2.033(3) to 2.150(3) Å,
and Cu–N bond distances lie in the normal range of
2.196IJ3)–2.212IJ3) Å (Table S1, in the ESI†). The two mhbpdc2−
adopt bisIJdidentate) chelating fashions (Scheme 1e) to link
two copper ions with the twist angles between two 2,2′-
bipyridyl rings being 63.6(3) and 67.0IJ2)°, which are signifi-
cantly larger than those found in complexes 1–3.
Considering that there are nine lattice water molecules
within the structure, the packing of complex 4 in the crystal
lattice is worth mentioning. In the crystal structure of 4, the
crystalline water molecules and carboxylate oxygen atoms are
the main part of the hydrogen bonded motifs. The lattice
water molecules contribute to bridge the adjacent double-
stranded dimers to a 3D network with hydrogen bonds, as
shown in Fig. 3b. The water molecules are in the interlayer
space and linked to each other by hydrogen bonds to form
IJH2O)14 clusters. Fig. 3c shows the five molecules (O4, O5,
O7, O8 and O9) and their symmetric equivalents that form a
ten-membered ring, which is further hydrogen bonded to a
water tetramer (O2, O3, O1, O6), giving a IJH2O)14 cluster, with
a non-bonded O⋯O distance of 2.901 Å. Similar to that
observed in the reported complexes,24 both water–MOF and
water–water interactions can be important for the stability of
the overall 3D structure of 4. Details of hydrogen bond inter-
actions are listed in Table S2 (see the ESI†).
Crystal structure of 5. By reducing the ligand-to-metal
ratio from 1 : 2 to 1 : 1, the reaction of copper ions with
mbpdcH2 produced the mononuclear complex 5. It is held
together through the formation of hydrogen bonds among
mononuclear (rather than dinuclear) copper-bpdcH2 units.
The asymmetric unit consists of one crystallographically
2−
unique CuIJII) center, one hbpdcH2 ion, and two coordi-
nated water molecules, as depicted in Fig. 4a. In 5, the CuIJII)
ion is also five-coordinated. Every CuIJII) ion binds simulta-
neously to one carboxylate oxygen, one hydroxyl oxgen and
Fig. 1 (a) The coordination environment of the CuIJII) center in 2 with
the ellipsoids drawn at the 50% probability level. The hydrogen atoms
are omitted for clarity (the axial long Cu–O bonds are shown as broken
lines). Symmetry operators: (A) −x + 1, −y, −z + 1; (B) −x + 2, −y + 1,
−z + 1. (b) and (c) View of the 1-D chain in 2 (the axial long Cu–O
bonds are shown as broken lines). (d) View of the 2-D sheet in the
ab plane. (e) View of the 3-D supramolecular network. The extensive
hydrogen bonds are colored red.
2−
one pyridyl nitrogen from one hbpdcH2 ion and two aqua
ligands.
2−
In the crystal structure of 5, the hbpdcH2 ion adopts a
tridentate chelating fashion (Scheme 1f) to bind a CuIJII) ion
through adjacent one carboxylate O6, one hydroxyl O1 and
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Fig. 2 (a) The coordination environment of the CuIJII) center in 3 with the ellipsoids drawn at the 50% probability level. The hydrogen atoms are
omitted for clarity (the axial long Cu–O bonds are shown as broken lines). Symmetry operators: (A) −x, −y + 1, −z + 2. (b) View of the 3-D supra-
molecular network. The extensive hydrogen bonds are colored red.
one pyridine N2. The second carboxylic acid and hydroxyl
functions remain uncoordinated and are unprotonated, thus
neutralizing the overall positive charge of the CuIJII) ions. In
the packing motif, each mononuclear unit is assembled via
hydrogen bonding between carboxylic hydrogen and carboxylic
oxygen together with the coordinated water molecules stabiliz-
ing the mononuclears which completely results in the overall
3D supramolecular structure of 5, as shown in Fig. 4b. Details
of hydrogen bond interactions are listed in Table S2 (see ESI†).
The TGA curve of 4 exhibits an initial weight loss from
room temperature to 105 °C, with an observed weight loss of
15.7% corresponding to the release of lattice water molecules
(calcd. 15.4%). After that an additional weight loss of 9.2%
up to 293 °C may be attributed to the gradual release of coor-
dinated pyridine molecules.
Catalytic activity
Earlier work revealed that some Cu-containing polymeric
compounds were found to be good heterogeneous catalysts
that offer the practical advantages of simplifying the separa-
tion and isolation of products and of the potential for cata-
lyst recycling.25 These results have prompted us to design
new Cu-containing polymeric compounds as potential hetero-
geneous catalysts. The crystal structure analysis reveals that
there are two structural novelties in Cu-MOFs, 1 and 2. For
example, both networks contain coordinatively unsaturated
metal centers. In addition, the other unique feature is the
observation that CuIJII) ions are coordinated by a large
number of labile water molecules. These structural features
encouraged us to explore possible organic transformations.
As it is well known, the Strecker reaction is one of the pre-
eminent multicomponent reactions useful for the synthesis
of α-amino acids via the intermediacy of α-amino nitriles.26
Thermal stability and powder X-Ray diffraction
The pure phases of complexes 1, 2, 4 and 5 were confirmed by
PXRD measurements, and the results are shown in Fig. S1.†
Meanwhile, each PXRD pattern of the as-synthesized sample
is consistent with the simulated one. This result shows the
phase purity of the bulk samples.
The thermal stabilities of 2, 4 and 5 were examined by
thermogravimetric analysis (TGA), and the results are shown
in Fig. S2.†
Complexes 2 and 5 undergo weight losses of 8.79% and
9.76%, respectively, which can be assigned to the loss of
coordinated water molecules (calcd. 8.27% for 2 and 9.63%
for 5), and the frameworks of 2 and 5 remain stable up to
approximately 402 °C.
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Fig. 3 (a) Coordination environment of the Cu ion in 4 with the ellipsoids drawn at the 50% probability level. The hydrogen atoms and lattice
water are omitted for clarity. (b) View of the 3-D supramolecular network. The extensive hydrogen bonds are indicated as dotted lines. (c) View of
the IJH2O)14 cluster in 4 (hydrogen bonds are indicated as dotted lines).
Therefore, we examined their performance as heterogeneous
catalysts for the Strecker reaction of imines under mild
conditions.
identified by 1H NMR analysis. Under these conditions, a
smooth reaction resulted that produced the corresponding
α-amino nitrile in 95% yield. The same reaction conditions
were used for testing the catalytic activity of complex 2. The
results are collected in Table 2.
As can be seen from Table 2, both 1 and 2 could promote
the cyanosilylation reaction between trimethylsilyl cyanide
with N-benzylidenebenzenamine (entry 1) and substituted
imine (entries 2–10). In the absence of networks 1 and 2, the
reaction did not proceed at all, thereby supporting the possi-
ble Lewis acid catalyzed activity of the networks. Likewise,
when the catalyst was filtered off, the reaction was no longer
promoted. Further, a control experiment using CuIJAc)2 as the
catalyst did not result in any product formation. These con-
trol experiments strongly suggest that the soluble and catalyt-
ically active species are not leached out from both networks.
Thus, the reaction was promoted by heterogeneous catalysis
of complexes 1 and 2.
To evaluate the catalytic activity of complexes 1 and 2,
these complexes were activated at 120 °C for 8 h under
vacuum, to remove coordinated H2O molecules before the
reaction. Next, we used the trimethylsilyl cyanide addition to
N-benzylidenebenzenamine as a test reaction (Table 2, entry 1)
and afforded the corresponding α-amino nitrile as the princi-
pal product. In a typical reaction, a 24 mg (0.005 mmol) sample
of 1 was used for the conversion of 0.1 mmol of imine in
dry CH2Cl2 at room temperature. An excess of 50% trimethyl-
silyl cyanide was employed with respect to the amount of
imine. The progress of the reaction was monitored by the thin
layer chromatography. After the required reaction time, the
resulting solution was filtered to remove the catalyst. The
resulting solvents were evaporated in vacuum, and aqueous
NaHCO3 solution (1%, 5 mL) was added into it. The organic
components were extracted into ethyl acetate, which was then
dried over anhydrous Na2SO4. Evaporation of the solvent
gave the crude product. The isolated yield was obtained by
flash column chromatography on silica gel with petroleum
ether/ethyl acetate = 12/1 as the eluent. The product was
Notably, the heterogeneous catalysts can be conveniently
recovered from the reaction mixture by simple filtration and
used several times (tested four consecutive times; entry 1,
Table 2) without much loss in the catalytic efficiency
(4% drop in isolated yield in the fourth run). Moreover, the
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Table 2 Strecker reaction of trimethylsilyl cyanide with various imines
Yielda (%)
Entry
1
Imine
1
2
95, 93,b 91c
93, 91,b 89c
2
3
95
93
93
91
4
5
6
7
92
94
93
92
91
92
90
90
Fig. 4 (a) The coordination environment of the CuIJII) center in 5 with
the ellipsoids drawn at the 50% probability level. The hydrogen atoms
are omitted for clarity. (b) View of the 3D supramolecular network. The
extensive hydrogen bonds are colored yellow.
8
9
92
94
91
92
recovered MOFs remained stable with complete retention of
reactivity, which is further confirmed by PXRD analysis. As
shown in Fig. S3 and S4,† the recovered material shows no
change in the structure of the catalyst.
10
94
93
Magnetic properties
a
b
Yield based on column chromatography. Isolated yield after the
third and fourth run. c Isolated yield after the third and fourth run.
The magnetic susceptibility of powdered samples of com-
plexes 1, 2 and 4 was measured from 2 to 300 K under a con-
stant magnetic field of 0.1 T.
Polymer 1 displays a strong antiferromagnetic interaction.
The detailed discussion is reported in our previous paper.14d
The magnetic susceptibilities of 2 were measured in the
2–300 K temperature range, and shown as χMT and χM versus
T plots in Fig. 5. The experimental χMT value of 2 at room
temperature is 0.76 cm3 K mol−1, which is close to that
expected for two uncoupled CuIJII) ions (0.75 cm3 K mol−1, S =
1/2). The temperature dependence of the reciprocal suscepti-
bilities IJ1/χM) of 2 obeys the Curie–Weiss law above 30 K with
θ = −24.57 K, C = 0.89 cm3 K mol−1. As temperature is lowered
to 2 K, the χMT values decrease first slowly and then rapidly.
The behavior suggests that antiferromagnetic interactions are
operative in 2.
According to the structure of 2, it could be presumed that
the main magnetic interactions between the metal centers
might happen between two carboxylate bridged CuIJII) ions
(namely, between Cu1 and Cu2), whereas the superexchange
interactions between CuIJII) ions (Cu2 and Cu2) through the
hbptc4− ligand bridges can be ignored because of the
long length of the hbpdc4− ligands. Therefore, the analysis
of the susceptibility curves was performed using the
Bleaney–Bowers formula27 with the exchange Hamiltonian
(H = −JS1S2, S1 = S2 = 1/2)
χM = (2Nβ2g2/kT)[3 + exp(−J/kT)]−1(1 − ρ) + (Nβ2g2/2kT)ρ + Nα
Least-squares fitting of the experimental data led to J =
−13.46 cm−1, g = 2.04, ρ = 0.020, and R = 2.2 × 10−3 with
P
P
R defined as ( μobsd − μcalcd)2/ IJ μobsd)2, where ρ is the per-
centage of the paramagnetic impurities, Nα the temperature-
independent paramagnetism (120 × 10−6 cm3 mol−1), and
N, β and k have their usual meanings. The curve calculated
with these parameters gave a reasonable fit to the experimen-
tal data as shown by the solid line in Fig. 5.
The magnetic properties of complex 4 under the form of the
χMT and χM versus T plot are shown in Fig. 6. The experimental
χMT value of 4 at room temperature is 0.77 cm3 K mol−1,
which is close to that expected for two uncoupled CuIJII) ions
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hydrothermal conditions to assemble five multi-functional
copperIJII) complexes. By fine tuning the reactant ratio and
reaction temperature, 3D, 2D, and 0D complexes can be
acquired. Such slight modifications in the reaction condi-
tions affect strongly the composition and structure of the
obtained complexes.
Complex 1 exhibits very strong antiferromagnetic inter-
actions, while complexes 2 and 4 display weak antiferro-
magnetic interactions. In 2, the relative configuration of the
hbpdcH4 ligand with respect to the magnetic orbitals offers a
poor overlap between them, while in complex 4 it is due to
the lack of a magnetic exchange pathway, which can be
clearly seen from the structural plot.
The catalytic results reveal that complexes 1 and 2, featuring
Lewis acid-type catalytic sites, can function as heterogeneous,
very efficient and reusable catalysts for the Strecker reaction of
imines.
Fig. 5 Temperature dependence of χMT and χM for 2. Open points are
the experimental data, and the solid line represents the best fit
obtained from the Hamiltonian given in the text.
Acknowledgements
We gratefully acknowledge financial support from the Natural
Science Foundation of China (grant no. 21372112 and
21272109) and the Technology Innovation Team Support
Programs of Henan Province University (no. 2012IRTSTHN019
and 14IRTSTHN008).
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