The synthesis, structure, topology and catalytic application of a novel cubane-based copper(ii) metal-organic framework derived from a flexible amido tripodal acid

Article abstract of DOI:10.1039/c4dt03087g

A novel chiral metal-organic framework, [Cu₄(HL)₂(H₂O)₄(MeO)₄]_n (1), has been successfully synthesized from a tripodal flexible ligand (2S,2′S,2"S)-2,2′,2"-(benzenetricarbonyltris(azanediyl

Full text of DOI:10.1039/c4dt03087g

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Synthesis, Structure, Topology and Catalytic Application of a Novel
Cubane-based Copper(II) Metal–Organic Framework derived from a
Flexible Amido Tripodal Acid
Anirban Karmakar,*a,b Clive L. Oliver*c, Somnath Royb and Lars Öhrströma
aChemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology, Kemivägen 10,
Gothenburg, Sweden. E-mail: ohrstrom@chalmers.se
bCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001,
Lisbon, Portugal. Email: anirbanchem@gmail.com
cCentre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town,
Rondebosch, Cape Town, 7701, South Africa. Email: Clive.Oliver@uct.ac.za
1. Abstract
A novel chiral metal-organic framework, [Cu4(HL)2(H2O)4(MeO)4]n (1), have been successfully synthesized from a
tripodal flexible ligand (2S,2'S,2''S)-2,2',2''-(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L).
Compound 1 was characterized by IR, and X-ray powder diffraction analysis. The structure was determined
from X-ray single crystal diffraction analysis revealing that 1 possesses a 3D network, featuring a tetranuclear
cubane-type secondary building block [Cu4(MeO)4]4+, formed via the connection of four metal ions to four
methoxide ions. These secondary building blocks are linked by four different HL2- ligands to construct a porous
three dimensional framework of the dia topology with one-dimensional channels. Compound 1 also acts as a
heterogeneous catalyst for the diastereoselective nitroaldol (Henry) reaction, providing high yields (up to 91%)
and good diastereoselectivities at ambient conditions. This catalyst can be recycled without significant loss of
activity.
2. Introduction
Porous metal–organic frameworks (MOFs)1 have attracted much interest in recent years due to their intriguing
topologies1j-m along with potential applications in the fields of gas sorption,2 magnetism,3 nonlinear optics,4
chiral separation5 and catalysis.6 The judicious selection of appropriate linkers can give rise to the desired pore
shape and size upon coordination and consequently contribute significantly to the structural design of MOFs,
as well as their properties. Highly symmetrical multitopic ligands with dicarboxylate,7 tricarboxylate8 and
tetracarboxylate 9 functionalities may bridge mono- or multi-nuclear metal nodes, leading to stable MOFs with
permanent porosity,10 while flexible ligands with additional functional groups may adopt more versatile
conformations and coordination modes according to the geometric requirements of different metal ions, which
may lead to unpredictable, yet interesting topologies and properties. However, only a few examples with
flexible di- and tripodal ligands have been reported and the investigation of metal-organic frameworks based
on them is still a great challenge.11
The Henry or nitroaldol reaction is known as one of the most powerful and atom-economic reactions for C–C
bond formations with various functionalized structural motifs.12 Usually this reaction is performed with
homogeneous basic catalysts, such as alkali metal hydroxides, alkoxides or amines, with a rather good
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efficiency.13 The development of new catalysts and procedures for the Henry reaction has been constantly
elaborated in view of the reduction of toxic by-products and the increase of yield and diastereoselectivity. In
the last two decades many other asymmetric catalysts have been developed to convert aldehydes or α-keto
esters into the corresponding nitroalkanol with good enantio- and diastereoselectivity.14 Even though high
yields were achieved with reactions performed with homogeneous catalysts, the achievement of a high
stereoselectivity is still challenging, and only a scant number of examples are known15 using heterogeneous
catalysts. Some copper and zinc -containing metal organic frameworks16 have been reported, which actively
catalyze the nitroaldol reaction. However, there is still a demand to develop new types of catalysts based on
cheap and environmentally tolerable metal organic framework, that could be easily recyclable (hence forming a
heterogeneous system) and show a high efficiency under mild conditions.
Recently, a systematic investigation on a series of flexible di- and tripodal acid ligands containing various amino
acid groups have been reported by us, which led to the interesting hydrogen bonding networks.17 We have
extended our study by synthesizing a higher dimensional motif (metal organic framework) with an easily
accessible tripodal linker. Thus, the two main objectives of this study were as follows: (i) to synthesize a Cu(II)-
MOF using a tripodal acid linker; (ii) to apply the synthesized framework as a heterogeneous catalyst in the
nitroaldol combination of nitroethane with various aldehydes. In that context, we chose (2S,2'S,2''S)-2,2',2''-
(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L) [scheme 1] which is more flexible than benzene
tricarboxylic acid as the carboxylic acid groups are reasonably free to rotate whilst the amide functional groups
may potentially be involved in hydrogen bonding for further stabilization of the resulting structure.
O
Metal coordinating
group
OH
Hydrogen bonding
O
NH
group
OH
H
H
N
N
O
O
O
HO
O
(2S,2'S,2''S)-2,2',2''-(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L)
Scheme 1
Hence, we report herein the synthesis and characterization of a new chiral copper(II) metal organic framework,
prepared under hydrothermal condition, using a flexible tripodal acid linker H3L [(2S,2'S,2''S)-2,2',2''-
(benzenetricarbonyltris(azanediyl))tripropanoic acid]. The structural features of the obtained Cu-MOF
[Cu4(HL)2(H2O)4(MeO)4]n (1), could be established by single crystal X-ray diffraction analysis which revealed a
cubane-type metal-oxygen cluster not yet observed for a metal-organic framework. The structure was also
subjected to a topological study.
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The catalytic performance of this framework as a heterogeneous catalyst, in terms of activity, heterogeneity,
and recyclability, was successfully tested for the nitroaldol (Henry) reaction of nitroethane with various
aldehydes in a methanolic medium.
3. Results and Discussion
3a. Syntheses and Characterization
The hydrothermal reaction of H3L with copper(II) nitrate trihydrate in the presence of dimethyl formamide and
methanol lead to the formation of [Cu4(HL)2(H2O)4(MeO)4]n (1) [L
=
(2S,2'S,2''S)-2,2',2''-
(benzenetricarbonyltris(azanediyl))tripropanoate].
In the IR spectra, the characteristic strong bands of coordinated carboxylate groups of 1 appear at 1530 cm-1
and 1360 cm-1 for the asymmetric and the symmetric stretching, respectively. C–O stretching of coordinated
carboxylate group is observed in 1270 cm-1. A strong band in 1637 cm-1 is due to ν(CO) of an uncoordinated
carboxylate groups,18 whereas ν(OH) of water molecules is in the 3303 cm-1 region. The band at 1580 cm-1 is
attributed to C=O stretching vibration of the amide group. Metal organic framework 1 was also characterized
by single crystal X-ray diffraction and powder X-ray diffraction analysis.
Thermal gravimetric analysis was also performed on framework 1 (Figure S5, supplementary information).
Framework 1 shows a weight loss of 5.30% between 71 -141˚C, corresponding to the loss of two molecules of
water (calcd: 5.60%). Upon further heating, framework 1 exhibits a weight loss of 9.40 % in the 180-239˚C
temperature range, which accounts for the total removal of two coordinated methoxy molecules (calcd:
9.60%). The remaining material then starts to decompose from 241˚C until 492˚C.
3b. Crystal structure analysis
Single-crystal X-ray diffraction studies reveal that [Cu4(HL)2(H2O)4(MeO)4]n (1) crystallizes in a chiral space group
(the tetragonal space group P43212), which is expected due to chirality of the ligand. The asymmetric unit
contains two copper(II) ions, one HL2- ligand, two water and two methoxide ions (Figure 1). The formulation of
this framework is based on the typical Jahn-Teller distortion of Cu(II) (vide infra) and the Cu-O distances
[1.945(4)- 1.951(4) Å], which correspond closer to those observed for Cu-OMe (1.94 Å) than for Cu-O(H)Me
(1.99 Å).19a In addition, the EPR results below confirm that the copper ions are in the 2+ oxidation state.
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Figure 1 Asymmetric unit of 1 with partial atom labeling scheme.
Both copper(II) centres have square pyramidal geometry (τ5= 0.06 for Cu1 and τ5= 0.04 for Cu2) and the
coordination sphere of each copper(II) center is occupied by one carboxylate oxygen atom of a HL2- unit [Cu1–
O2, 1.969(4) Å; Cu2–O5, 1.977(4) Å], three methoxide O-atoms and a water molecule. The carboxylate oxygen,
two methoxide O-atoms and the water molecule occupy the equatorial sites and the axial site coordinated by
the remaining methoxide ion. As expected and due to Jahn–Teller distortion19b-e the axial Cu-O bond distance is
slightly longer [Cu1‒O1M1, 2.424(4) Å; Cu2‒O1M2, 2.405(4) Å] than the equatorial ones. The equatorial Cu‒O
bond lengths vary from 1.945(4) to 2.004(5) Å (Table S2). Selected bond distances and angles of 1 are listed in
Table S2 (supporting information).
A
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B
Figure 2 (A) Coordination scheme around the tetranuclear copper(II) core in 1. (B) Structure of the tetranuclear
cubane-type clusters of [Cu4(MeO)4]4+ unit showing the metal coordination spheres.
In this framework tetranuclear cubane-type clusters of [Cu4(MeO)4]4+ (Figure 2B) serve as secondary building
block units (SBU’s) and are located about 2-fold rotation axes. In the literature very few copper(II) complexes
having methoxo-bridged20a-f and oxygen-bridged20g-j cubane-type tetranuclear clusters are reported, but to our
knowledge, no metal-organic frameworks involving any type of oxygen bridge with the same cubane motif as
observed for 1 have been reported, albeit that it has been observed for discrete clusters.20d
Every tetranuclear cluster is associated with four HL2- ligands, four molecules of water and four µ3-methoxy
ions. Two of the carboxylate groups of ligand HL2- associate with the two neighbouring tetranuclear clusters,
while the carboxylic acid group remains uncoordinated and orients itself towards the open channels.
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Figure 3 A representation of the 3D network of 1
The packing view of 1 is characterized by open channels along the crystallographic c axis (Figure 3) with
approximate dimension of 6 x 7 Å2 and a void space of 16.2% per unit cell according to PLATON.34 The structure
of 1 is also stabilized by hydrogen bonding interactions, three amide hydrogen atoms of ligand HL2- are
hydrogen bonded with three amide oxygen atoms of neighboring HL2- ligands via N1-H1·····O1#1 2.9581(2) Å,
N2-H2·····O7#2 2.9302(2) Å and N3A-H3A·····O4 #3 3.0067(2) Å, #1 1/2-y,1/2+x,-1/4+z; #2 y,x,1-z; #3 --
1/2+y,1/2-x,1/4+z; interactions. Hydrogen bonding interactions between carboxylate oxygen atoms O2 and
O0AA (as acceptors) and the OW2 hydrogen atoms of water molecule (as bifurcated donors) are also observed
[OW2-H1W2·····O2#4 3.1165(2) Å, OW2-H2W2·····O3#5 2.9566(2) Å, #4 1/2+x,1/2-y,1/4-z; #5 1-y,-x,1/2-z]. In
addition, intermolecular C−H⋅⋅⋅O contacts are relevant and help to stabilize the structure (Table S3, supporting
information).
The topology of 1 was determined by considering the Cu4O4 unit as vertices and the ligand as a linear connector
and analyzing the network with SYSTRE. 21a-b This operation reveals the ubiquitous diamond or dia topology, the
most common of all four connected nets21b and the structure with an overlaid network is shown in Figure 4. It
is noteworthy that although 1 crystallizes in an enantiomorphous space group the dia net is not one of the
inherently chiral network topologies (qtz is the most common of these that are four-connected). As observed
in the literature the topology of the net is not necessarily chiral even if the space group is chiral.22
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b
a
c
Figure 4 Structure of compound 1 with the underlying dia topology show in purple. Solvent waters not shown.
4. Catalytic activity of the 1 in the Henry reaction
We have tested the catalytic activity the copper MOF 1 as a solid heterogeneous catalyst in the nitroaldol (or
Henry) reaction of nitroethane with various aldehydes. In a typical reaction, a mixture of aldehyde (0.50 mmol),
nitroethane (2.50 mmol), 3 mol % of Cu-MOF catalyst and methanol contained in a capped glass vessel was
stirred at 70 ˚C for 24 h. The mixture was then centrifuged to remove the solid catalyst. The evaporation of the
solvent gave the crude product which was a mixture of the β-nitroalkanol diastereoisomers (syn and anti
forms, with predominance of the former) which were analyzed by 1H NMR.
The optimization of the reaction conditions (temperature, reaction time, amount of catalyst, solvent) was
carried out in a model nitroethane–benzaldehyde system with 1 as the catalyst (Scheme 2 with typical reaction
conditions; Table 1).
HO
HO
CHO
R
Cu-MOF 1
CH3CH2NO2
NO2
NO2
+
MeOH, 70oC, 24h
R
syn
R
anti
Scheme 2 Nitroaldol (Henry) reaction
When 3 mol % of solid complex 1 is used as catalyst at 70˚C, a conversion of 89 % (syn/anti: 75:25) of
benzaldehyde into β-nitroalkanol is reached (entry 6, Table 1) after 24 h. Upon extending the reaction time to
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48 h, the reaction yield increase slightly (91 %, syn/anti: 76:24). The plot of yield versus time for the Henry
reaction of benzaldehyde and nitroethane with complex 1 is presented in figure 5A.
A
B
Figure 5 A) Plots of β-nitroalkanol yield vs. time for the Henry reaction of benzaldehyde and nitroethane in
presence of compound 1 as catalyst. B) Kinetic profiles in four consecutive reaction cycles employing 1 as
catalyst.
A blank reaction was performed (absence of any metal source; Table 1, entry 18) using benzaldehyde as
substrate, at 70 ºC and in methanol. No β-nitroalkanol was detected after a reaction time of 24 h. The
nitroaldol reaction also did not take place when using compound H3L instead of catalyst 1 (Table 1, entry 20).
We have also checked the reactivity of Cu(NO3)2.3H2O in methanol medium and the obtained reaction yield
was much lower, i.e., ca. 24% (entry 19, Table 1).
Table 1Optimization of the parameters of the Henry nitroaldol reaction between benzaldehyde and nitroethane with
1 as the catalysta
Entry
Catalyst
Time (h)
Amount of T (˚C)
Catalyst
Solvent
Yield (%)b
Selectivityc
TONd
(syn/anti)
(mol%)
1
2
3
4
1
1
1
1
2
4
6
8
3.0
3.0
3.0
3.0
70
70
70
70
MeOH
MeOH
MeOH
MeOH
40
58
65
72
77:23
76:24
75:25
75:25
13.3
19.3
21.6
24.0
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5
6
7
1
1
1
10
24
48
3.0
3.0
3.0
70
70
70
MeOH
MeOH
MeOH
78
89
91
75:25
75:25
76:24
26.0
29.6
30.3
8
1
1
1
24
24
24
1.0
5.0
7.0
70
70
70
MeOH
MeOH
MeOH
58
79
81
77:23
79:21
78:22
58.0
15.8
11.5
9
10
11
12
13
1
1
1
24
24
24
3.0
3.0
3.0
70
70
70
CH3CN
THF
12
45
84
79:21
80:20
80:20
4.0
15.0
28.0
Water
14
15
16
17
1
1
1
1
24
24
24
24
3.0
3.0
3.0
3.0
20
30
MeOH
MeOH
MeOH
MeOH
5
79:21
77:23
78:22
69:31
1.6
35
71
63
11.7
23.7
21.0
50
110
18
Blank
24
24
24
24
24
-
70
70
70
70
70
MeOH
MeOH
MeOH
MeOH
MeOH
-
-
-
8.0
-
Cu(NO3)2.3H2O
19
3.0
-
24
-
82:18
20
H3L
1
-
-
21e
22f
3.0
87
23
29.0
1
3.0
91:9
7.7
a Reaction condition: 3.0 mol% of catalyst 1, benzaldehyde (0.50 mmol), nitroethane (2.50 mmol) and methanol
b
c
1
d
(2 mL). Number of moles of β-nitroalkanol per mole of aldehyde X 100. Calculated by H NMR. Number of
e
f
moles of β-nitroalkanol per mole of catalyst (2 Cu per mole). Nitromethane was used as substrate;
Nitropropane was used as substrate
We have also tested the effects of temperature, catalyst amount and solvents. The increase in the amount of
catalyst 1 from 1.0 to 3.0 mol% enhances the product yield from 58 to 89%, but further increase amount of
catalyst leads to a slight decrease of the reaction yield (entries 8–10, Table 1).
To select the most suitable solvent, experiments with various solvents (CH3CN, THF, MeOH and water) have
been carried out with 1 and the results (Table 1, entries 6 and 11–13) indicate that methanol (conversion of
89%) is the best polar solvent, whereas the worst one is CH3CN (12% conversion). In water and THF, yields of
84% or 45%, respectively, were obtained (Table 1, entries 12 and 13).
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Varying the temperature from 20 to 70˚C improved the yield of β-nitroalkanol from 5 to 89% but a further
temperature increase had a negative effect (entries 14–17, Table 1). The systems exhibit diastereoselectivity
towards the syn isomer, typically leading to syn/anti molar ratios in the 82:18 to 78:22 range using nitroethane
as substrate. The selectivity in syn and anti products is an important issue in the Henry reaction and efforts
have been focused on the development of catalytic diastereo- or enantio-selective processes.23 The size of the
nitroalkane chain also affects the yields; e.g. with nitropropane the conversion was only 23% (entry 22, Table
1), whilst with nitroethane or nitromethane conversion values of 89% or 87%, respectively, were obtained
(entries 6 and 21, Table 1).
We also examined the catalytic activity of 1 with different types of substituted aromatic and aliphatic
aldehydes in the reaction with nitroethane. The results are summarized in Table 2. p-Nitrobenzaldehyde
produced the maximum yield (95%), while the lowest yield (27%) was obtained for p-methoxybenzaldehyde.
The yields for two aliphatic aldehydes were good (84–85%) but lower than that for benzaldehyde.
Table 2: Henry reaction of various aldehydes and nitroethane with catalyst 1a
Aldehyde
Yield(%)b
Selectivityc
(syn/anti)
78:22
TONd
95
27
31.7
9.0
76: 24
71:29
79:21
77:23
81
34
47
27.0
11.3
15.7
52
87:13
17.3
CH3CHO
CH3CH2CHO
85
84
78:12
75:24
28.3
28.0
a Reaction conditions: 3.0 mol% of catalyst 1, aldehyde (0.50 mmol), nitroethane (2.5 mmol) and methanol (2
1
mL). b Number of moles of β-nitroalkanol per mole of aldehyde X 100. c Calculated by H NMR. d Number of
moles of β-nitroalkanol per mole of catalyst.
Competition reactions involving equal molar amounts of p-nitrobenzaldehyde and acetaldehyde were
conducted to probe whether catalysis occurs primarily on the surface of the MOF or within the pores. If
catalysis were occurring to a significant extent within pores, we might well expect the 3-nitrobutan-2-ol
product to be formed preferentially over the 2-nitro-1-(4-nitrophenyl)propan-1-ol, since the internal cavities of
framework 1 would have selectively adsorbed small molecules (acetaldehyde) over large molecules (p-nitro
benzaldehyde) due to its smaller pore size (6 Å x7 Å channels). But we have observed p-nitro benzaldehyde
produce more yield than acetaldehyde. Also we have used various sizes of different aldehydes but we didn’t
observe any selectivity for particular aldehyde, the aryl aldehydes bearing electron-withdrawing groups exhibit
higher reactivities as compared to those having electron-donating moieties, which may be related to an
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increase in the electrophilicity of the substrate in the former case. Thus, it may be possible that catalysis occurs
mainly on the surface of the MOF and not on the pores of framework 1.
In order to examine the stability of 1 in the Henry reaction, it was recycled in four consecutive experiments,
and it was observed that its activity remained almost the same (Figure 5B). FT-IR and powder X-ray diffraction
of catalyst 1 taken before and after the reaction indicated that the structure of the solid was retained (Figure
S1). Additionally, the filtrated solution, after the separation of the catalyst, was evaporated and the amount of
copper determined, being only 0.012% of the amount used in the reaction, thus ruling out any significant
leaching of the catalyst. This test also indicates the significant amount of stability of framework 1 during the
catalytic reaction.
Although there are some reports on coordination polymers16 which are catalytically active for this kind of
reaction, the yields and selectivity are usually higher for our compound as compared to other metal organic
frameworks. Recently, Chuan-De Wu et. al. reported a 3D porous metal–organic framework constructed from
two kinds of pyridine carboxylates and copper(II) ions, in the reaction of 4-nitrobenzaldehyde and nitroethane,
leads to an overall yield of only 78 % after 36 h reaction time (in our case 95% in 24h).16c Moreover, our catalyst
1 exhibits a marked selectivity towards the syn diastereoisomer (Table 2, syn:anti of 78:22) which was not
reported in other cases. In comparison with other reported heterogeneous catalysts for the Henry reaction, our
catalyst has the advantages of being relatively cheap and easy-to-prepare.
5. EPR analysis
The EPR spectrum of 1 was recorded in solid state, Fig. 6 (a), at room temperature and (b) in at 77 K. The X-
band EPR spectrum of 1 depicts a broad band centred at g = 2.15 (3171 G for ν = 9.526 GHz) which corresponds
to the transition ΔMS = ±1. No clearly detectable zero field splitting or half-field signals were observed. The
frozen solution spectrum, Fig. 6 (b), shows a band centred at g = 2.14 which also corresponds to the transition
ΔMS = ±1. By simulation of the spectra, the following g values were obtained: g1 = 2.05, g2 = 2.17 and g3 = 2.23.
A
B
Figure 6 First derivative EPR spectra of 1 at room temperature (a) and at 77 K (b).
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The geometric parameter G, which is a measure of the exchange interaction between the copper centres in the
polycrystalline compound, is calculated using the equation: for rhombic spectra G = (g3 ˗ 2.003)/(g⊥ ˗ 2.003),
where g⊥ = (g1 + g2)/2. If G < 4.0, a considerable exchange interaction is indicated in the solid complexes.24 The
G value for compound 1 in the polycrystalline state at 77 K is 2.12. For complexes of this type, the parameter R
2
can be indicative of the predominance of the dz2 or dx2˗y orbital in the ground state, where R = (gz ˗ gy)/(gy ˗ gx).
If R > 1, the greater contribution to the ground state arises from the dz2 orbital and the structure is closer to a
trigonal bipyramid than to a square pyramid. Instead, if R < 1, a greater contribution to the ground state arises
2
from the dx2˗y orbital and the structure is closer to a square pyramid than to a trigonal bipyramid. In the
present case, R = 0.5, being compatible with the X-ray diffraction data. The solution spectrum could not be
obtained due to insolubility of the framework 1 in common organic solvent. This result unequivocally confirms
the oxidation state as Cu(II) as inferred in section 3b.
5. Conclusion
We successfully isolated a copper(II) metal organic framework derived from a tripodal flexible ligand
(2S,2'S,2''S)-2,2',2''-(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L) under hydrothermal condition.
The single-crystal X-ray diffraction experiment revealed that this framework consists of tetranuclear cubane-
type clusters of [Cu4(MeO)4]4+ acting as secondary building blocks. The tetranuclear cubane of copper(II) is
formed via the connection of four metal ions by four µ3- bridged methoxide ions. This metal-oxygen motif,
although found for discrete metal-clusters, have not been observed as part of a metal-organic framework
hitherto. The framework has one dimensional channels along the c axis with approximate dimension of 6 x 7 Å2
and a void space of 16.2% per unit cell volume.
Framework 1 effectively catalyzes the Henry reaction of nitroethane with various aldehydes producing the
corresponding β-nitroalkanols in high yields and with a significant stereoselectivity towards the syn
diastereomer. We have also proved the stability and recyclability of the catalysts. The syn/anti ratio of the
nitroaldol products depends on various factors such as the amount of catalyst, the electrophilicity of the
substrates and the reaction conditions.
The above observations provide further evidence that simple Cu(II) complexes can be utilized as effective
heterogeneous catalysts in the important type of reactions of this study. Further explorations of the uses of
this catalyst family in other organic transformations, as well as mechanistic investigations, are ongoing.
6. Experimental
All the chemicals were obtained from commercial sources and used as received. (2S,2'S,2''S)-2,2',2''-
(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L) was prepared according to the literature [17]. The
infrared spectra (4000–400 cm-1) were recorded on a Bruker IFS-125 spectrometer in KBr pellets; abbreviations:
s = strong, m = medium, w = weak, bs = broad and strong, mb = medium and broad. Powder X-ray diffraction
(PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry) theta-2theta diffractometer,
with copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary monochromator, operated at 40 kV and 40 mA.
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Flat plate configuration was used and the typical data collection range was between 5˚ and 35˚ 2θ.
Thermogravimetric analysis was carried out with a Perkin-Elmer Instrument system (STA6000) at a heating rate
of 10˚C min-1 under a dinitrogen atmosphere, in the range of room temperature to ca. 600˚C. The X-band EPR
spectra were measured either at room temperature (RT) or at liquid nitrogen (LN) temperature (77 K) on a
Bruker ESP 300E spectrometer. The ESP 300E spectrometer was operated at ~9.51 GHz with a frequency
modulation of 100 kHz. While keeping the resolution either at 2048 points or at 4096 points, the microwave
power was adjusted to 20 dB attenuations and the receiver gain was set to either 3.0×105 or 4.0×105. To
improve the signal to noise ratio, 10 scans of each sample were accumulated. The spectral acquisition
parameters were constant for each experiment. All measurements were done using 3 mm quartz tubes
(Wilmad 707-SQ-250M). The EPR spectra were simulated using an EPR simulation program.25
6a. Synthesis of 1
A mixture of H3L (42 mg, 0.1 mmol) and Cu(NO3)2. 3H2O (24 mg, 0.1 mmol) was dissolved in 4 mL of MeOH :
DMF (1:0.1 v/v) and placed in a capped glass vessel. Then 0.1 ml of 1(M) KOH is added into it and mix
thoroughly, blue precipitate appear immediately. The precipitate was dissolve by adding 0.5 ml of NH4OH drop-
wise until solution was turn deep blue. The resulting mixture was sealed in an 8 mL glass vessel and heated at
80 ˚C for 48 h. After cooling to room temperature the reaction mixture was kept for slow evaporation. After 7
days deep blue block crystals appeared. Yield 34% (based on Cu), which is calculated based on crystalline
material obtained. Lower reaction yield of Cu(II) frameworks also observe in some previously reported
literature.26 FT-IR (KBr, cm-1): 3303 (bs), 1637 (s), 1530 (s), 1450 (w), 1407 (m), 1360 (s), 1270 (w), 1090 (w), 724
(s).
6b. Procedure for the nitroaldol (Henry) reaction catalyzed by 1
In a typical reaction, a mixture of aldehyde (0.50 mmol), nitroethane (2.50 mmol) and 1 catalyst (9.70 mg, 3
mol %) was placed in a capped glass vessel, and then 2.0 mL methanol were added into it. The mixture was
heated at 70 ˚C for 24 h, and subsequently quenched by centrifugation and filtration at room temperature. The
filtrate was extracted with dichloromethane. The organic extracts were collected over anhydrous sodium
sulfate; subsequent evaporation of the solvent gave the crude product. The product was dissolved in DMSO-d6
1
and analyzed by H NMR. The yield of the β-nitroalkanol product (relatively to the aldehyde) was established
1
typically by taking into consideration the relative amounts of these compounds, as given by H NMR and
previously reported.16d The ratio between the syn and anti isomers was also determined by 1H NMR
1
spectroscopy. In the H NMR spectra, the values of vicinal coupling constants (for the β-nitroalkanol products)
between the α-N–C–H and the α-O–C–H protons identify the isomers, being J = 7–9 or 3.2–4 Hz for the syn or
anti isomers, respectively.27 The 1H-NMR spectra and the calculation of the yield and selectivity for framework 1
in the Henry reaction is presented in supporting information (Figure S4).
In order to perform the recycling experiment, first we washed the used catalyst (separated by centrifugation
and filtration of the supernatant solution) with methanol and dried it in air. It was then reused for the
nitroaldol reaction as described above.
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6c. X-ray crystal structure determination
A suitable crystal of 1 was immersed in cryo-oil, mounted on a Nylon loop, and measured at a temperature of
100(2) K using an Oxford Cryostream 700. The X-ray diffraction data were collected on a Bruker DUO APEX II
CCD diffractometer using graphite monochromated Mo Ka radiation (λ = 0.71073 Å). The data reduction and
cell refinement were performed using SAINT-Plus28 and scaled for absorption effects by multi-scan using
SADABS.29 The space group was determined from systematic absences by XPREP30 and confirmed using the
program Layer.31The structures were solved by direct methods using SHELXS-9732 and subsequently refined by
full-matrix least-squares methods, based on F2 values against all reflections.33 Non-hydrogen atoms were
refined with anisotropic displacement parameters,, whilst hydrogen atoms were placed geometrically, except
for water hydrogen atoms which were placed from difference Fourier maps. Thermal parameters of hydrogen
atoms were constrained to Uiso = 1.2Ueq(parent atom). Excess electron density in the channels that could not be
modelled was ‘cleaned’ using the SQUEEZE routine in PLATON34. CCDC 1024691 for compound 1, contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
7. Acknowledgements
AK express his gratitude to the Fundação para a Ciência e a Tecnologia for post-doctoral fellowships (Ref. No.
SFRH/BPD/76192/2011), LÖ and CLO thank the Swedish Research Links program for Southern Africa, LÖ is also
grateful for funding from the Swedish Science Council and Chalmers Tekniska Högskola and CLO thanks the
National Research Foundation of South Africa for funding.
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Graphical Abstract
Synthesis, Structure, Topology and Catalytic Application of a Novel
Cubane-based Copper(II) Metal–Organic Framework derived from a
Flexible Amido Tripodal Acid
Anirban Karmakar,*a,b Clive L. Oliver*c, Somnath Royb and Lars Öhrströma
aChemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology, Kemivägen
10, Gothenburg, Sweden. E-mail: ohrstrom@chalmers.se
bCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001, Lisbon, Portugal. Email: anirbanchem@gmail.com
cCentre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town,
Rondebosch, Cape Town, 7701, South Africa. Email: Clive.Oliver@uct.ac.za