High-throughput study of the Cu(CH3COO)2· H2O-5-nitroisophthalic acid-heterocyclic ligand system: Synthesis, structure, magnetic, and heterogeneous catalytic studies of three copper nitroisophthalates

Article abstract of DOI:10.1021/cg1015934

A high-throughput screening was employed to identify new compounds in Cu(CH₃COO)₂·H₂O-NIPA-heterocyclic ligand systems. Of the compounds identified, three compounds, [Cu₃{(NO ₂)-C₆H₃

Full text of DOI:10.1021/cg1015934

ARTICLE
pubs.acs.org/crystal
High-Throughput Study of the Cu(CH3COO)2 H2O-5-Nitroisophthalic
3
Acid-Heterocyclic Ligand System: Synthesis, Structure, Magnetic,
and Heterogeneous Catalytic Studies of Three Copper
Nitroisophthalates
Debajit Sarma,† K. V. Ramanujachary,‡ Norbert Stock,§ and Srinivasan Natarajan*,†
†Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India
‡Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey, 08028, United States
§Institute of Inorganic Chemistry, Christian Albrechts University, Otto-Hahn-Platz 6/7, D 24118, Kiel, Germany
S Supporting Information
b
ABSTRACT: A high-throughput screening was employed to
identify new compounds in Cu(CH3COO)2 H2O-NIPA-
3
heterocyclic ligand systems. Of the compounds identified, three
compounds, [Cu3{(NO2)-C6H3-(COO)2}3(C3N6H6)] (I),
[Cu2(μ3-OH)(H2O){(NO2)-C6H3-(COO)2}(CN4H)] -
3
(H2O) (II),and[Cu2(μ3-OH)(H2O){(NO2)-C6H3-(COO)2}-
(CN5H2)] 2(H2O) (III), have been isolated as good quality
3
single crystals by employing conventional hydrothermal meth-
ods. Three other compounds, Cu2{(NO2)-C6H3-(COO)2}-
(CN4H)(H2O) (IIa), Cu2{(NO2)-C6H3-(COO)2}(CN5H2)
(IIIa), and Cu2{(NO2)-C6H3-(COO)2}{(CN5H2)2}2H2O
(IIIb), were identified by a combination of elemental analysis,
thermogravimetric analysis (TGA), and IR spectroscopic studies,
although their structures are yet to be determined. The single
crystalline compounds were also characterized by elemental
analysis, TGA, IR, UV-vis, magnetic, and catalytic studies. The structures of the compounds have paddle wheel (I) and infinite Cu-
O(H)-Cu chains (II and III) connected with NIPA and heterocyclic ligands forming two- (II) and three-dimensional (I and III)
structures. The bound and lattice water molecules in II and III could be reversibly removed/inserted without affecting the structure. In the
case of II, the removal of water gives rise to a structural transition, but the dehydrated phase reverts back to the original phase on prolonged
exposure to atmospheric conditions. Magnetic studies indicate an overall antiferromagnetism in all of the compounds. Lewis acid catalytic
studies indicate that compounds II and III are active for cyanosilylation of imines.
’ INTRODUCTION
compounds having metal clusters or extended -M-O-M-
linkages.5
The design and synthesis of inorganic coordination polymers
or metal organic frameworks (MOFs) continue to be of interest
for their potential as well as actual use in gas storage, separation,
and catalysis.1 MOFs provide unique opportunities to control the
formation of new structures by fine-tuning the organic and the
inorganic component.2 In addition, the well-established princi-
ples of supramolecular assembly can be employed fruitfully in the
preparation of important targeted MOF structures.3 The control
achieved in the design and the synthesis of MOFs creates
opportunities to investigate wide-ranging properties, which are
being pursued vigorously. One of the interest in transition metal-
based MOFs is to study and correlate the structure with the
magnetic behavior.4 Long-range magnetic interactions and or-
dering can be achieved through significant orbital overlap
between the participating transition metal center and the ligands.
Strong magnetic interactions are generally associated with
Of the many MOFs that have been prepared and character-
ized, those belonging to terepthalic acid (1,4-BDC) and trimesic
acid (1,3,5-BTC) appear to be the dominant ones. The combina-
tion of 1,4-BDC and transition elements has given rise to many
new compounds, which exhibit interesting magnetic behavior.6
Similarly, the 1,3,5-BTC has also given rise to important com-
pounds, most notably the copper compound, [Cu3(1,3,5-
BTC)2(H2O)3]n (HKUST-1).7 The angular disposition be-
tween the three carboxylic acid groups in the trimesic acid
(120°) along with the rigid aromatic backbone has been
exploited by many research groups, to form new compounds,
establishing the versatility of trimesic acid as a good linker.8 Some
Received: November 30, 2010
Revised:
January 29, 2011
Published: March 07, 2011
r
2011 American Chemical Society
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Crystal Growth & Design
ARTICLE
of the metal-trimesate compounds also exhibit interesting
magnetic properties.9
(0.038 g, 0.3 mM) were added under continuous stirring. The mixture
was homogenized for 30 min at room temperature. The final mixture was
transferred, sealed in a 23 mL PTFE-lined stainless steel autoclave, and
heated at 125 °C for 3 days under autogenous pressure. For the
Substitution of one of the carboxylic acid groups in trimesic
acid not only would break the symmetry but also give rise to
differences in its binding with suitable metal centers. One such
organic linker would be 5-nitroisophthalic acid. The nitro group
of the 5-nitroisophthalic acid, on the other hand, could provide
useful variations by performing the role of (i) a possible hydro-
gen bond acceptor, (ii) an electron-donating group, and (iii)
improving the reactivity. In spite of the advantages offered by the
5-nitroisophthalic acid, the usefulness of the acid as a possible
linker in the assembly of MOFs has not been exploited in detail.10
In synthesizing MOFs, there have been many different
approaches, which are broadly based on the trial and error
methodology. Recent research has established that the high-
throughput (HT) approach is very versatile and paves the way
for the systematic screening of a multicomponent system.11
Yaghi and co-workers have exploited this method for the
identification and study of a family of ZIFs (zeolitic imidazo-
late frameworks).12
preparation of II, Cu(CH3COO)2 H2O (0.118 g, 0.6 mM), 5-nitro-
3
isophthalic acid (0.127 g, 0.6 mM), and tetrazole (0.05 mL of 30%
tetrazole in acetonitrile) were used. For the preparation of III, Cu-
(CH3COO)2 H2O (0.079 g, 0.4 mM), 5-nitroisophthalic acid (0.063 g,
3
0.3 mM), and aminotetrazole (0.026 g, 0.3 mM) were employed. During
the preparation of compounds II and III, all of the other synthetic
conditions were kept similar to that followed for I. The final products
contained large quantities of green block (I), blue rod (II), and blue rod
(III) type single crystals, were filtered under vacuum, were washed with
deionized water, and were dried at ambient conditions.
Initial characterizations were carried out by elemental analysis,
powder XRD, thermogravimetric analysis (TGA), and infrared (IR)
spectroscopic studies. Powder XRD patterns were recorded in the 2θ
range 5-50° using Cu KR radiation (Philips X'pert) (ESI, Supporting
Information, Figures S1-S3). The IR spectra for the compounds were
recorded as KBr pellets (Perkin-Elmer, Spectrum 1000). The IR spectra
exhibited typical peaks corresponding to the hydroxyl group, amino
group, nitro group, and carboxylate groups of the compounds (ESI,
Supporting Information, Figure S4). IR (KBr): γas(O-H) =
We have been interested in exploring the reactivity of 5-
nitroisophthalic acid with copper salts. To this end, we have
employed the HT approach. Our efforts have resulted in the
identification of three distinct phases: [Cu3{(NO2)-C6H3-
(COO)2}3(C3N6H6)] (I), [Cu2(μ3-OH)(H2O){(NO2)-C6H3-
3460-3597 cm-1, γs(N-H) = 3152-3455 cm-1, γs(C-H)aromatic
2927-3085 cm-1, γs(CdO) = 1597-1618 cm-1, δ (H2O) = 1540-
1569 cm-1, δ(COO) = 1432-1438 cm-1, γs(N-O) = 1352-1390 cm-1
γs(C-C)skeletal (CN)skeletal 904-
1074-1082 cm-1
926 cm-1, and δ (CHaromatic out of plane = 767-788 cm-1 (Support-
=
,
(COO)2}(CN4H)] (H2O) (II), and [Cu2(μ3-OH)(H2O)
=
,
δ
=
3
{(NO2)-C6H3-(COO)2}(CN5H2)] 2(H2O) (III). Here, we
)
3
describe the synthesis, structure, magnetic, and preliminary studies
on heterogeneous catalytic properties of the three copper nitro-
isophthalate compounds.
ing Information, Table S1). Elemental analysis: Anal. calcd for I: C,
34.32; H, 1.59; N, 13.35. Found: C, 34.18; H, 1.95; N, 13.89. Anal. calcd
for II: C, 22.70; H, 1.89; N, 14.73. Found: C, 23.08; H, 2.11; N, 14.22. Anal.
calcd for III: C, 22.11; H, 2.25; N, 17.20. Found: C, 22.68; H, 2.73; N, 16.89.
Anal. calcd for IIa: C, 25.54; H, 1.43; N, 16.55. Found: C, 26.08; H, 2.31; N,
16.89. Anal. calcd for IIIa: C, 25.72; H, 1.20; N, 19.99. Found: C, 26.16; H,
2.4; N, 28.07. Anal. calcd for IIIb: C, 22.21; H, 2.05; N, 28.52. Found: C,
22.93; H, 2.71; N, 28.07.
’ EXPERIMENTAL SECTION
Synthesis and Initial Characterization. The initial HT screen-
ing was carried out in a reactor block with 24 cavities containing Teflon
linears (4 ꢀ 6 array). The volume of the individual Teflon reactors was
2 mL, and the reactor block was sealed inside a stainless steel autoclave.
For the HT reactions, the reagent concentrations were as follows:
Single Crystal Structure Determination. A suitable single
crystal of each compound was carefully selected under a polarizing
microscope and glued to a thin glass fiber. The single crystal data were
collected on a Bruker AXS smart Apex CCD diffractometer at 293(2) K.
The X-ray generator was operated at 50 kV and 35 mA using Mo KR (λ =
0.71073 Å) radiation. Data were collected with ω scan width of 0.3°. A
total of 606 frames were collected in three different setting of j (0, 90,
and 180°) keeping the sample-to-detector distance fixed at 6.03 cm and
the detector position (2θ) fixed at -25°. The data were reduced using
SAINTPLUS,13 and an empirical absorption correction was applied
using the SADABS program.14 The structure was solved and refined
using SHELXL9715 present in the WinGx suit of programs (Version
1.63.04a).16 The hydrogen position of the μ3-OH groups and the water
molecules in compounds II and III could not be located from the
difference Fourier maps. The possible hydrogen positions were arrived
based on the bond valence sum (BVS) calculations for the μ3-OH group.17
All of the other hydrogen positions were initially located from the difference
Fourier maps, and for the final refinement, the hydrogen atoms were placed
in geometrically ideal positions and refined in the riding mode. Final
refinement included atomic positions for all of the atoms, anisotropic
thermal parameters for all of the nonhydrogen atoms, and isotropic thermal
parameters for all of the hydrogen atoms. Full-matrix least-squares refine-
ment against |F2| was carried out using the WinGx package of programs.15
Details of the structure solution and final refinements for the compounds
are given in Table 1. CCDC 792141-792143 contains the crystallographic
data for the compounds. These data can be obtained free of charge from
The Cambridge Crystallographic Data Center (CCDC) via www.ccdc.cam.
ac.uk/data_request/cif.
Cu(CH3COO)2 H2O, 0.02-0.06 mmol; 5-nitroisophthalate (NIPA),
3
0.01-0.06 mmol; and heterocyclic ligand, 0.01-0.08 mmol. Typically,
prepared were the following solutions: 0.2 M solution of Cu-
(CH3COO)2 H2O in water and 0.2 M solution of NIPA, melamine,
3
and aminotetrazole in methanol. The terazole was used as received (30%
in acetonitrile). In a model reaction mixture, Cu(CH3COO)2 H2O-
3
NIPA-melamine were taken in the mole ratio 0.4:0.3:0.3, and the final
volume for the reaction was made up to 1.5 mL by the addition of water.
The reaction mixtures were stirred for 30 min for homogenization prior
to the experiment. The sealed multiclave was placed in a preheated oven
at 125 °C for 3 days. After the reaction, the products were recovered by
filtration and washed with water under vacuum. The products were
analyzed by powder X-ray diffraction (XRD).
The powder XRD patterns of the products indicated the possibility of
obtaining hitherto unknown compounds. To optimize the conditions
and to synthesize larger quantities of the sample, required for the full
characterization, the conventional hydrothermal method was subse-
quently employed. The hydrothermal method, indeed, was successful
and yielded good quality single crystalline products, which are desig-
nated as compounds I, II, and III. The additional products, identified
during the HT experiments, designated IIa, IIIa, and IIIb, could be
obtained only as polycrystalline fine powders.
For the conventional hydrothermal preparation of I, Cu(CH3C-
OO)2 H2O (0.079 g, 0.4 mM) was dissolved in 10 mL of water.
3
To this, 5-nitroisophthalic acid (0.084 g, 0.4 mM) and melamine
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Crystal Growth & Design
ARTICLE
Table 1. Crystal Data and Structure Refinement Parameters for Compounds I-IIIa
I
II
III
[Cu3{(NO2)-C6H3-
[Cu2(μ3-OH)(H2O){(NO2)-
[Cu2(μ3-OH)(H2O){(NO2)-
empirical formula
formula weight
(COO)2}3(C3N6H6)]
944.10
C6H3-(COO)2}(CN4H)] (H2O)
C6H3-(COO)2}(CN5H2)] 2(H2O)
3
3
476.26
488.27
crystal system
space group
a (Å)
trigonal
monoclinic
orthorhombic
R3
P2(1)/m
P2(1)2(1)2(1)
16.3706(8)
10.0134(2)
6.4094(15)
b (Å)
16.3706(8)
6.5861(2)
14.857(4)
c (Å)
22.5362(14)
11.4842(3)
16.066(5)
R (°)
90
90
90
β (°)
90
90.144(2)
90
γ (°)
volume (Å3)
Z
120
90
90
5230.5(5)
757.37(3)
1529.8(7)
18
4
4
T (K)
293(2)
293(2)
293(2)
F
calcd (g cm-3
μ (mm-1
θ range (deg)
)
1.798
2.062
2.103
)
1.907
2.876
2.852
2.49-25.99
2.70-25.98
1.87-24.59
λ (Mo KR) (Å)
Rint
0.71073
0.71073
0.71073
0.0326
0.0308
0.0506
reflection collected
unique reflections
no. of parameters
R indices [I > 2σ(I)]
R indices (all data)
11645
4640
6264
2277
1619
2533
172
151
244
R1 = 0.0278, wR2 = 0.0837
R1 = 0.0330, wR2 = 0.0850
R1 = 0.0499, wR2 = 0.1183
R1 = 0.0490, wR2 = 0.1113
R1 = 0.0591, wR2 = 0.1204
R1 = 0.0656, wR2 = 0.1216
a R1 = Σ||Fo| - |Fc||/Σ|Fo|; wR2 = {Σ[w(Fo2 - Fc2)]/Σ[w(Fo ) ]}1/2; w = 1/[F2(Fo)2 þ (aP)2 þ bP]; P = [max(Fo, O) þ 2(Fc)2]/3, where a = 0.0565
2 2
and b = 0.0000 for I, where a = 0.0239 and b = 3.9139 for II, and where a = 0.0516 and b = 3.5074 for III.
Heterogeneous Catalytic Studies. The reagents required for
the heterogeneous catalytic cyanosilylation studies, N-benzilidine
aniline and trimethylsilyl cyanide (Aldrich), were used as received.
The reagents were taken in a 100 mL round-bottom flask with the
freshly distilled dichloromethane (DCM) solvent. After the addition
of the solid catalyst (copper nitroisophthalate), the mixture was
stirred for 6 h at 0 °C in N2 atmosphere. The product was filtered
through Millipore membrane filters to remove the catalyst particles.
The supernatant liquid was dried under vacuum to remove the
solvent. The product was analyzed and evaluated for the conversion
of the reactants.
The compound II, whose structure is determined as part of this study,
appears to form only in a narrow compositional range, and a single
phase of II can be isolated only at a low NIPA concentration at 125 °C.
The compound IIa forms only as microcrystalline powder throughout
the scanned compositional range (ESI, Supporting Information,
Figure S5). The reactions with aminotetrazole as the ligand indicated
the formation of three distinct phases. Of these, compound III
appears to be the stable phase and can be isolated as a single crystalline
compound over a wide compositional range at 125 °C. At low NIPA
concentration, compound IIIa forms as a microcrystalline powder at
75 and 125 °C. A new phase, compound IIIb, appears to form at
150 °C as a microcrystalline powder (ESI, Supporting Information,
Figure S6). The microcrystalline phases IIa, IIIa, and IIIb were
indexed, and the unit cell parameters were obtained from the powder
XRD patterns by the Le Bail method18 (ESI, Supporting Information,
Figures S7-S9 and Table S3). The possible compositions of these
were obtained by correlating a variety of techniques. Thus, elemental
analysis, TGA, and IR spectroscopic studies were employed to arrive
at the molecular formula for the compounds IIa, IIIa, and IIIb. The
possible molecular formulas were found to be Cu2{(NO2)-C6H3-
(COO)2}(CN4H)(H2O) for IIa, Cu2{(NO2)-C6H3-(COO)2}
{(CN5H2) for IIIa, and Cu2{(NO2)-C6H3-(COO)2}{(CN5H2)2}
2H2O for IIIb. Although the molecular formulas of IIa, IIIa, and
IIIb are comparable to the present compounds II and III, the
bonding between the metal centers and the ligands (NIPA and
tetrazole/aminotetrazole) is still not known, and one could possi-
bly consider them as polymorphic/related phases. Our efforts to
isolate these compounds as single crystalline products were not
successful.
HT Studies. The system, Cu(CH3COO)2 H2O-NIPA-hetero-
3
cyclic ligand (melamine/tetrazole/aminotetrazole)-H2O, was ex-
plored employing a HT screening method. The products were
characterized by powder XRD, which suggested the possibility of
the formation of six new phases. Of these, three compounds (I-III)
were subsequently prepared as single crystalline product by employ-
ing the conventional hydrothermal method. The other three com-
pounds IIa, IIIa, and IIIb were isolated only as microcrystalline
products and were characterized in detail using a variety of techniques.
The possible lattice partameters for compounds IIa, IIIa, and IIIb
were obtained by the Le Bail fit of the observed powder XRD pattern.
The following are the trends observed with regard to the composition
and temperature of the reactions under the HT conditions. The HT
study of the melamine system indicated that the compound appears to
form as a single phase. An unknown phase, however, appears to form
both at high and at low melamine concentrations. The stability of
compound I decreased with either the increase or the decrease in the
temperature of the reaction (Figure 1). The reaction involving the
tetrazole ligand indicated that the most stable phase is compound IIa.
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Crystal Growth & Design
ARTICLE
Figure 1. (a) Summary of the HT screening of the Cu/NIPA/melamine system as a function of reactant concentration and temperature. The results
presented are based on the analysis of the PXRD studies.
Table 2. Selected Observed Bond Distances in the Compounds I-IIIa
bond
distances (Å)
bond
distances (Å)
bond
distances (Å)
1.9683(16)
I
Cu(1)-O(1)#1
Cu(1)-O(4)
1.9619(16)
1.9762(16)
Cu(1)-O(2)#2
Cu(1)-N(1)
1.9621(16)
2.2560(18)
Cu(1)-O(3)#3
II
Cu(1)-O(1)#1
Cu(1)-O(2)
Cu(1)-O(3)
Cu(1)-O(4)
1.903(6)
2.000(5)
2.206(6)
2.279(7)
Cu(1)-N(1)#2
Cu(1)-N(1)
Cu(2)-O(3)#3
Cu(2)-O(3)
2.196(5)
2.196(5)
1.951(3)
1.951(3)
Cu(2)-O(8)#4
Cu(2)-O(8)
Cu(2)-N(2)
Cu(2)-N(2)#3
2.479(8)
2.479(8)
1.992(5)
1.992(5)
III
Cu(1)-O(1)
Cu(1)-O(2)#1
Cu(1)-O(3)
Cu(1)-O(4)#1
1.911(5)
1.993(5)
2.313(5)
2.433(5)
Cu(1)-N(2)#2
Cu(1)-N(1)
Cu(2)-O(3)#3
Cu(2)-O(3)
2.093(8)
2.104(7)
1.935(7)
1.938(7)
Cu(2)-O(5)
Cu(2)-N(3)#4
Cu(2)-N(4)
2.239(7)
1.981(8)
2.001(8)
a Symmetry transformations used to generate equivalent atoms: I: #1, -x, -y þ 1, -z; #2, -y þ 2/3, x - y þ 4/3, z þ 1/3; #3, y - 2/3, -x þ y - 1/3,
-z - 1/3. II: #1, x þ 1, y, z; #2, x, -y þ 5/2, z; #3, -x þ 2, -y þ 2, -z þ 2; #4, x, y, z. III: #1, -x þ 1/2, -y þ 1, z þ 1/2; #2, x þ 1, y, z; #3, x - 1/2, -y
þ 1/2, -z þ 1; #4, x þ 1/2, -y þ 1/2, -z þ 1.
’ RESULTS
1.962(2)-1.976(2)Å (average, 1.967 Å), and the Cu-N bond
distance is 2.256(2) Å for I (Table 2).
Structure of [Cu3{(NO2)-C6H3-(COO)2}3(C3N6H6)] (I). The
asymmetric unit of [Cu3{(NO2)-C6H3-(COO)2}3(C3N6H6)]
(I) consists of 19 nonhydrogen atoms. One Cu2þ ion, one NIPA,
and one-third of a melamine molecule contribute to the asymmetric
unit (ESI, Supporting Information, Figure S10). The Cu2þ ions are
coordinated with four carboxylate oxygen atoms and one nitrogen
atom from melamine, forming a distorted square pyramidal geo-
metry (CuO4N). The Cu-O bonds have distances in the range of
The structure of I consists of connectivity between the copper
centers, the melamine, and the nitroisophthalate. Each of the
carboxyalte oxygens of the 5-nitroisophthalic acid is bonded to
one copper atom and has monodentate connectivity (ESI,
Supporting Information, Figure S11). The nitro group does
not participate in bonding in the structure. The connectivity
between the copper and the carboxylate group gives rise to the
paddle wheel unit (ESI, Supporting Information, Figure S12).
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Crystal Growth & Design
ARTICLE
3.342 Å and the N-H
O bond angles of 157 and 151°
3 3 3
observed in I suggest that these interactions are not very strong.20
Structure of [Cu2(μ3-OH)(H2O){(NO2)-C6H3-(COO)2}-
(CN4H)] (H2O) (II). The asymmetric unit of [Cu2(μ3-OH)-
3
(H2O){(NO2)-C6H3-(COO)2}(CN4H)] (H2O)
(II)
3
consists of 25 nonhydrogen atoms. Two crystallographically
independent Cu2þ ions along with one NIPA anion, one
tetrazolate, one μ3-hydroxyl group, one coordinated water
molecule, and one lattice water molecule contribute to the
asymmetric unit (ESI, Supporting Information, Figure S14).
Both of the Cu2þ ions have distorted octahedral coordination;
Cu(1) is coordinated by one μ3 hydroxyl ion [O(3)], three
carboxylate oxygens, and two nitrogens from the tetrazolate
ligand, forming Cu(OH)O3N2 octahedra, and Cu(2) is coordi-
nated by two μ3-hydroxyl ion [O(3) and O(3)#], two aqua
oxygens, and two triazolate nitrogens to form a Cu(OH)2-
(H2O)2N2 octahedra. The Cu-O bonds have distances in the
range 1.903(6)-2.479(8) Å (average, 2.156 Å), and the Cu-N
bonds have distances in the range 1.992(5)-2.196(5) Å
(average, 2.094 Å) for II (Table 2).
The structure of II can be understood by considering the
independent connectivity between the metal centers with tetra-
zole and NIPA. The two carboxylate groups of the NIPA connect
to two copper centers with one mondentate and one bis-
bidentate coordination mode (ESI, Supporting Information,
Figure S15). The connectivity between the NIPA and the copper
centers results in the formation of a one-dimensional chain
structure (Figure 3a). The copper atoms themselves are con-
nected through μ3-hydroxyl group forming an infinite one-
dimensional zigzag Cu-O(H)-Cu chain, which lies along the
21 axis. The tetrazolate units occupy the grooves in the zigzag
(helical) chain and bond with four copper centers (ESI, Support-
ing Information, Figure S16), giving rise to a one-dimensional
ladderlike structure. The copper atoms are arranged in a trian-
gular manner connected through the corners (Figure 3b). The
copper-tetrazolate ladders and the copper-nitroisophthalate
chains are connected together in such way that they are mutually
orthogonal to each other forming a two-dimensional bilayer
arrangement. The layers are arranged in such a way to give rise to
a pseudo three-dimensional structure encompassing a one-
dimensional channel of width 6.8 Å ꢀ 8.9 Å (shortest atom-
atom contact distances, not including the van der Waals radii)
(Figure 3c). The coordinated water molecules from the copper
center project into the middle of the channels. The bilayers are
Figure 2. (a) Figure shows the connectivity between the paddle
wheel copper centers and the nitroisophthalate (NIPA) forming the
three-dimensional structure in [Cu3{(NO2)-C6H3-(COO)2}3-
(C3N6H6)] (I). (b) View of the two-dimensional layer formed by
the connectivity between the copper paddle wheels and the mela-
mine. (c) The overall view of three-dimensional structure of I. Note
that the melamine units occupy the middle of the channels formed by
Cu-NIPA connectivity.
arranged in a AAA
fashion (Figure 3d).
3 3 3
Structure of [Cu2(μ3-OH)(H2O){(NO2)-C6H3-(COO)2}(CN5-
The paddle wheel is one of the fundamental building units in the
family of MOFs and has been routinely observed in compounds
containing copper.19 The connectivity between the copper
paddle wheels and the acid gives rise to a three-dimensional
structure with one-dimensional channels along the c-axis
(Figure 2a). The melamine unit also connects three such paddle
wheels through the Cu-N bonds (ESI, Supporting Information,
Figure S13), forming a two-dimensional layer in the ab plane
(Figure 2b). It may be noted that the Cu-NIPA as well as the
Cu-melamine connectivity exhibit the 3-fold symmetry, which
is typical of the space group (R3). Interconnectivity between the
Cu-NIPA and the Cu-melamine units is such that the mela-
mine units occupy the middle of the one-dimensional channels,
which renders the channels inaccessible (Figure 2c). The term-
inal amino group of the melamine molecules participates in N-
H2)] 2(H2O) (III). The asymmetric unit of [Cu2(μ3-OH)(H2O)
3
{(NO2)-C6H3-(COO)2}(CN5H2)] 2(H2O) (III) consists of
3
27 nonhydrogen atoms. Two crystallographically independent Cu2þ
ions, one NIPA anion, one aminotetrazolate ligand, one μ3-hydroxyl
group, one coordinated water molecule, and two lattice water
molecules constitute the asymmetric unit (ESI, Supporting Informa-
tion, Figure S17). Of the two copper atoms, Cu(1) is octahedrally
coordinated by one μ3 hydroxyl ion [O(3)], three carboxylate
oxygens, and two nitrogens from the aminotetrazolate, forming a
Cu(OH)O3N2 unit, and Cu(2) is coordinated by two μ3-hydroxyl
ion [O(3) and O(3)#], two nitrogens from the aminotetrazole
molecule, and one aqua oxygen to form a Cu(OH)2(H2O)N2
distorted trigonal bipyramidal geometry. The Cu-O bonds have
distances in the range 1.911(5)-2.433(5) Å (average, 2.109 Å) and
the Cu-N bond distances in the range of 1.981(8)-2.104(7) Å
(average, 2.045 Å) (Table 2).
H
O hydrogen bonds. The N O distances of 2.822 and
3 3 3
3 3 3
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Figure 4. (a) One-dimensional zigzag chain formed by the connectivity
between copper and NIPA in [Cu2(μ3-OH)(H2O){(NO2)-
C6H3-(COO)2}(CN5H2)] 2(H2O) (III). (b) View of the one-dimen-
3
sional ladderlike structure formed by the connectivity between copper
and aminotetrazolate. Note the similarity with the structure in II
(Figure 3b). (c) View of the three-dimensional strucure of III formed
by the connectivity between the copper-NIPA chains and copper-
aminotetrazolate ladders. Note the orthogonal arrangement of the
copper-aminotetrazole ladders (see the text).
The structure of III is closely related to the structure of II.
The two carboxylate groups of the NIPA unit connect with two
copper centers with one mondentate and one bis-bidentate
coordination mode (ESI, Supporting Information, Figure S18).
The connectivity between the NIPA with the copper center
results in a one-dimensional structure, which is similar to that
observed in II (Figure 4a). The μ3-hydroxyl groups connect
the copper atoms, forming an infinite one-dimensional zigzag
Cu-O(H)-Cu chain as in II, with a corner shared triangular
arrangement. The aminotetrazole unit occupies the spaces in
between the copper atoms, forming a one-dimensional ladderlike
arrangement (Figure 4b). The ladders are positioned in such a way
that they are orthogonal to each other (Figure 4c). The copper-
nitroisophthalalte chains and the copper-aminotetrazolate ladders
are connected together, forming the three-dimensional structure
(Figure 4c).
Figure 3. (a) One-dimensional chain formed by the connectivity
between copper and NIPA in [Cu2(μ3-OH)(H2O){(NO2)-
C6H3-(COO)2}(CN4H)] (H2O) (II). (b) View of the one-dimen-
3
sional ladderlike structure formed by the connectivity between copper
and tetrazolate. (c) View of the two-dimensional layer in the ac plane
formed by the connectivity between the copper-NIPA chains and the
copper-tetrazolate ladders. Note the orthogonal arrangement of the
two one-dimensional units (see the text). (d) View of the arrangement of
the layers in II. Note the formation of the three-dimensional-like
structure through supramolecular interactions.
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’ DISCUSSION
A HT screening method has been employed for the identifica-
tion of the formation of new phases in Cu-NIPA-heterocyclic
ligand systems. We have been able to obtain one single crystalline
product with melamine as the ligand (compound I). From the
phase diagram (Figure1), it is clear that this phase forms over a
wide compositional range at 125 °C, whereas the formation of I is
limited at low (75 °C) as well as at higher temperature (150 °C).
In the case of the tetrazolate ligand, the identified compound (II)
forms in a very narrow concentration range, and the stability of II
also appears to be sensitive to the variation in the temperature.
The phase diagram of the reaction mixture involving aminote-
trazolate as the ligand appears to be even more complex. We have
identified three phases of which one was isolated as a single
crystalline phase (III), whereas two other phases (designated as
IIIa and IIIb) form as a microcrystalline powder. We observed
that the stabilities of the phase III appear to be low at low (75 °C)
as well as at high (150 °C) temperatures.
From the structural point of view, the three compounds I-III
are formed through the coordination of nitroisophthalate and
heterocyclic ligand with the copper atoms. Compound I is
different from II and III as it is formed with a copper paddle wheel
structure, whereas II and III have similar one-dimensional -Cu-
O(H)-Cu- connectivity. The structural formula and the asym-
metric unit of compound II and III show that they differ only by one
lattice water molecule. The connectivity between the tetrazole (II)
and the aminotetrazole (III) units with the copper centers is the
same, and also, the connectivity between the carboxylate groups and
the copper is similar between the two structures II and III. However,
both of the structures differ in the overall dimensionality. While
compound II has a two-dimensional layer structure, compound III
has a three-dimensional structure. This can be understood by a
closer look at the connectivity of the one-dimensional ladders
formed between the copper and the azaheterocycles (tetrazole
and aminotetrazole) with the NIPA. In structure II, the NIPA links
the adjacent ladders, forming the two-dimensional layer structure;
the NIPA and the copper tetrazole ladders are similar, being stacked
one above the other (Figure 5a). In III, the connectivity between the
NIPA and the copper aminotetrazole is different. The Cu-amino-
tetrazole ladders are not arranged parallel to each other but are
arranged orthogonal to each other (Figure 4c). This subtle differ-
ence in the connectivity between the structures of II and III is
responsible for the dimensionality differences. The NIPA units,
thus, connect the ladders in such a way that it forms a three-
dimensional structure (Figure 5b). When the connectivity of all of
the nodes is considered, one finds that both compounds II and III
have comparable nodal connectivity: the μ3-hydroxyl group (three-
connected node), heterocycle (four-connected node), and both of
the copper ions (five-connected node) remains the same. Hence,
the Schlafli symbols for both of the structures are also the same
(42.6.84.102.12)(43.62.8)(43)(44.62)(8).
Figure 5. (a) Schematic representation of the connectivity observed in
II. (b) Schematic representation of the connectivity observed in III.
Note the differences in the connectivity, which leads to a 3D structure
(see the text).
interaction between the benzene rings were 1.66 kcal mol-1. The
values observed are similar to the π-π interaction energies
observed before.23 The nitroisophthalate units in the structure of
III are far apart (∼6.5 Å) from each other to have any meaningful
π-π /CH π interactions.
3 3 3
Thermogravimetric Studies. TGA on the compounds has
been carried out in flowing air (flow rate = 20 mL min-1) in the
temperature range 30-800 °C (heating rate = 5 °C min-1). The
TGA studies indicate that compound I exhibits a single-step
weight loss. The total observed weight loss of 74% in the
temperature range 280-350 °C corresponds to the loss of all
of the carboxylate and the melamine units (calcd, 74.7%).
Compound II exhibits an initial weight loss of ∼7.5% below
100 °C, which may be due to the loss of the lattice and the
coordinated water molecules (calcd, 9.2%) and the second
weight loss of 57% in the temperature range 290-310 °C, which
would correspond to the loss of the carboxylate and the tetra-
zolate moieties. The total observed weight loss of 64.5% com-
pares well with the loss of all of the water molecules (coordi-
nated and lattice), the tetrazolate, and the carboxylate units
(calcd, 66.6%). Compound III also exhibits a similar behavior
with losses of ∼8.5% up to 130 °C and 61% in the range 260-
275 °C. The total observed weight loss of 69.5% corresponds
well with the loss of the carboxylate, the aminotetrazolate, and
all of the water molecules (calcd, 67.4%) (ESI, Supporting
Information, Figure S19). The final calcined products in all of
the cases was found to be crystalline and corresponds to CuO
(JCPDS: 80-1916).
The staking of the two-dimensional layers of compound II is
also stabilized by the π-π interaction between the nitro-
isophthalate anions. The role of π-π interactions in the forma-
tion of the supramolecular compounds is a topic of interest and is
well documented.21 In compound II, we find significant intra-
layer π-π /CH π interactions between the aromatic rings
3 3 3
of the nitroisophthalate anions (Figure 6). To understand the
nature and the energies involved in these interactions, we per-
formed calculations using Gaussian98 software package at the
B3LYP/6-31þG(d,p) level.22 The calculated energies for the
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Figure 6. Stacking of the layers in II. The π-π interactions are shown in the inset.
We have also carried out the thermal studies on the micro-
crystalline powdered samples IIa, IIIa, and IIIb to understand
the thermal stabilities. The TGA studies of compound IIa
indicate an initial weight loss of ∼3.7% up to 150 °C, which
may be due to the loss of a water molecule (calcd, 4.3%). The
second weight loss of ∼57.8% near the temperature range of
230-290 °C may be due to the loss of the craboxylate and the
tetrazolate units. The total observed weight loss of 67% nearly
compares with the loss of water molecule and the organic
moieties (calcd, 67%). Compound IIIa exhibits a single-step
weight loss with the observed weight loss of ∼64% in the
temperature range 245-280 °C, which corresponds to the loss
of all of the carboxylate and the aminotetrazolate units (calcd,
61.2%). Compound IIIb shows an initial weight loss of ∼9% up
to 125 °C and ∼61% in the range 200-280 °C. The total
observed weight loss of ∼70% corresponds well with the loss of
the carboxylate, the aminotetrazolate, and all of the water mole-
cules (calcd, 69.8%) (ESI, Supporting Information, Figure S20).
The final calcined product in all of the cases was found to be
crystalline by powder XRD and corresponds to CuO (JCPDS:
80-1916). We have employed TGA, IR, and the elemental
analysis as a means of identifying the probable molecular formula
in the case of IIa, IIIa, and IIIb. As can be noted, the weight loss
in these compounds (IIa and IIIb) is also comparable with the
losses observed for compounds II and III. It is likely that the
structures could also be comparable. The Le Bail fit of the PXRD
patterns of the compounds IIa, IIIa, and IIIb gave lattice
parameters (ESI, Supporting Information, Table S3), which are
also comparable to the present compounds. We believe that
the unknown compounds are likely to have structures that are
comparable to the ones reported here.
UV-Visible Spectroscopic Studies. All three compounds
isolated in single crystalline form in the present study are colored.
The diffuse reflectance UV-vis spectra were measured at room
temperature for the as-synthesized compounds along with the
NIPA anion (Na salt of NIPA), melamine, and aminotetrazole
(ESI, Supporting Information, Figures S21 and S22). The Na-
NIPA exhibited two main absorption peaks centered at ∼330
and ∼390 nm, melamine has two main peaks at ∼285 and
∼315 nm, and aminotetrazole has peaks at ∼200, ∼285, and
∼320 nm. The UV-vis spectrum of the tetrazole (30% in
acetonitrile) was recorded in the solution phase, which exhibits
a main peak at ∼225 nm. All of these peaks are due to the
intraligand charge transfer transitions and appear as a broad peak
in all of the compounds. In addition, a broad absorption band in
the range 500-1000 nm was also observed in all three com-
pounds. This band may be due to the d-d transition of the Cu2þ
ions. Similar absorption bands have been observed earlier.24
Dynamics of the Water Molecules. Thermal Studies. Com-
pounds II and III have coordinated as well as lattice water
molecules. The TGA studies clearly indicated that the water
molecules are lost below ∼150 °C, which prompted us to
examine the possible removal and readsorption of the water
molecules. To investigate the reversible adsorption behavior, a
modified TGA set up with a port for introducing the water vapor
was employed. A sample of each compound (II and III) was
taken in the TGA crucible and heated to the temperature of
complete dehydration in an atmosphere of flowing dry nitrogen
(20 mL/min) (125 °C for II and 140 °C for III) using a heating
rate of 5 °C/min. The samples were cooled slowly to room
temperature with a cooling rate of 2 °C/min. To study the
rehydration behavior of the compounds, saturated water vapor
was introduced at room temperature into the system for 60 min.
During the rehydration step, we observed that both compounds
II and III reabsorb the water molecules quickly and reach almost
the original weights taken at the start of the dehydration cycle.
Thus, 97.5 and 99% of the initial weight were observed from the
dehydrated weights of 91.7 and 90% for II and III, respectively.
We also noted that both of the compounds reabsorbed fully
(∼100%) when kept exposed to atmospheric conditions for a
longer duration. Although this trend may be expected, we wanted
to examine the recyclability of the dehydration-rehydration
behavior. To check the full reversibility, the dehydration and
rehydration cycles were repeated twice. We observed that both of
the compounds, II and III (ESI, Supporting Information, Figure
S23, and Figure 7), exhibit good recyclability. This observation
suggests that the dehydration-rehydration behavior is fully
reversible. The powder XRD pattern recorded on the fully
rehydrated sample also confirms that the integrity of the original
sample is retained (ESI, Supporting Information, Figure S24).
Powder XRD Studies. As the dehydration-hydration cycle is
fully reversible, we sought to investigate the structure of the
dehydrated phase employing in situ single crystal studies. Our
efforts were not successful as the dehydrated phase appears to
lose the single crystalline nature and did not diffract well under
the in situ conditions. To probe the crystallinity changes during
the dehydration-rehydration cycles, we carried out ex situ
temperature-dependent powder XRD studies on both com-
pounds II and III. Powdered samples of II and III were heated
at different temperatures and then examined by powder XRD.
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could be replaced by the D2O molecules. Similar studies carried out
on compound II were not successful.
Optical Studies. We wanted to examine the changes that
would accompany the removal of water in II and III using
UV-vis studies. Structurally, there are two distinct Cu species in
III, of which only one has a coordinated water molecule, the
removal of which is expected to lead to differences in the primary
coordination around the copper. In addition, it is likely that the
coordination changes in copper can also lead to changes in color.
On dehydration, we observed that the color indeed changes from
blue to green. The blue color, of course, returns back after
exposure to atmospheric conditions, as expected (ESI, Support-
ing Information, Figure S29). In addition, the PXRD studies also
indicated that the water removal from compound III does not
disrupt the structural integrity.
To probe this change of color further, compound III was
heated under vacuum in a glass tube for 5 h at 150 °C, and the
green-colored samples were carefully transferred to a quartz
sealed cuvette inside a glovebox. The UV-vis spectrum of the
dehydrated sample, then, was recorded at room temperature.
The UV-vis spectra of the dehydrated sample of compound III
show two peaks centered at ∼265 and ∼350 nm, which are due
to the intraligand transitions of aminotetrazolate and nitroi-
sophthalate. In addition, a broad absorption band in the range
of 500-1000 nm was also observed. The broad absorption band
exhibits a maximum at around ∼850 nm. To investigate the
changes that accompany these bands during the rehydration
behavior, the UV-vis spectra of the dehydrated sample were
studied by exposing the sample to the atmospheric conditions.
During the rehydration, the intraligand transition bands did not
exhibit any appreciable changes, whereas the nature of the broad
absorption bands starts to change as a function of time (ESI,
Supporting Information, Figure S30). The center of the broad
absorption shifts to ∼780 nm along with a shoulder at ∼650 nm.
From the available literature on copper-containing compounds,
it appears that the octahedral compounds of Cu2þ exhibit
absorption bands near ∼625 nm (16000 cm-1), which often
has a broad tail into the near-IR region. The broad band has
Figure 7. TGA histogram showing the reversible water uptake in
compound III. The blue curve represents the heating and the
cooling cycle, and the red curve represents the weight loss and
weight gain.
The powder XRD pattern of the fully dehydrated phase of II
(heated at 125 °C for 5 h) indicates a new phase. The original
phase reappears when the sample was kept exposed to the
atmospheric condition for about 3 h (ESI, Supporting Informa-
tion, Figure S25). This suggests that the removal of the water
molecules (coordinated and lattice) from II may be accompanied
by a structural reorganization. The powder XRD pattern of III
heated at 150 °C/5 h indicates a shift of the main peak from 2θ =
8.1-8.25°. The main peak reverts back toward the original peak
position of 2θ = 8.1°, when the sample was exposed to the
atmospheric condition for 1 h (ESI, Supporting Information,
Figure S26). The dehydrated sample of the compound III could
be fitted using the Le Bail method, which gives the cell
parameters for the dehydrated phase (ESI, Supporting Informa-
tion, Figure S27 and Table S4). As can be noted, the a and b
lattice parameters exhibit a small expansion, whereas c exhibits
contraction. Similar changes in the lattice parameters during the
dehydration have been encountered before.25
2
generally been assigned to the transions from dxy2, dz and dxz, dyz
pairs to the σ antibonding and half-filled dx -y2 level. The
trigonal bipyramidal compounds exhibit peaks centered around
980 (10200 cm-1) and 1220 nm (8200 cm-1). The four-
coordinated Cu2þ are generally rare and exist in square pyrami-
dal geometry, which gives rise to peaks at 565 (17700 cm-1), 675
(14750 cm-1), and 775 nm (12900 cm-1).27 The complete
assignments for all of the observed absorption bands were not
possible in compound III as both octahedral and trigonal
bipyramidal Cu2þ centers exist. It is likely that the trigonal
bipyramidal Cu2þ center changes to a four-coordinated geome-
try upon dehydration in III. The observed resultant band is a
mixture of the known transitions of Cu2þ in the various
coordination environments. This study was also helpful in
confirming the reversibility of the water molecules in III as the
UV-vis bands regenerate during the dehydration-
rehydration cycle.
IR Spectroscopic Studies. To understand the reversible beha-
vior as well as to examine the possibility of ligand exchanges in
III, which appears to retain the structural integrity during
dehydration, we have carried out IR spectroscopic studies
(Perkin-Elmer, SPECTRUM 1000). As part of the ligand ex-
change studies, we exchanged D2O in place of H2O. The IR
spectroscopic studies exhibited the normal band for H2O at
∼3500 cm-1. On exchange with D2O, we observed the appear-
EPR and Magnetic Studies. The X-band EPR spectra of the
compounds in the solid state were recorded at room temperature
(ESI, Supporting Information, Figure S31-S33). All of the
compounds exhibit a single resonance peak centered at g =
2.125 (I), g = 2.178 (II), and g = 2.216 (III), respectively. The
EPR signals exhibit a small anisotropy and hyperfine splitting,
which are generally expected for this type of solid. The EPR study
ance of two new bands centered at ∼2600 and ∼2470 cm-1
,
which would correspond to the coordinated and lattice D2O
stretching vibrations (ESI, Supporting Information, Figure S28) .26
This study confirms that the water molecules in III are labile and
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Figure 9. Temperature variation of the χMT for II. The inset shows the
corresponding 1/χM vs T plots.
Figure 8. Temperature variation of the χMT for I. The inset shows the
corresponding 1/χM vs T plots.
of 0.026 emu mol-l K at 2 K (Figure 10). The molar magnetic
susceptibility, χM, of II increases very slowly from 0.0024 emu mol-l
at room temperature to a value of 0.0137 at 14 K, after which it
exhibits a sharp rise (Supporting Information, Figure S35). A similar
behavior was observed for III where the χM value increases from
0.0025 emu mol-l at room temperature to a value of 0.0086 at 14 K,
after which it exhibits a sharp increase (Supporting Information,
Figure S36). The field-cooled (FC) and zero-field-cooled (ZFC)
studies on both of the compounds did not exhibit any appreciable
differences in the χM behavior up to 2 K (ESI, Supporting
Information, Figures S37 and S38). The magnetization studies also
did not reveal any possible hysteresis behavior (ESI, Supporting
Information, Figures S39 and S40). The present studies indicate that
there are no long-range ferromagnetic correlations between the
Cu2þ ions in both of the compounds, II and III. The 1/χM vs T data
for compounds II and III can be fitted in the temperature range 50-
300 K for the Curie-Weiss behavior, which gave a value for C of
0.848 emu/mol and θP of -55 K for II and a value of 1.076 emu/mol
for C and θP = -116.8 K for III. The very high negative values of
θP obtained for both II and III again indicate that the magnetic
exchanges between the copper centers through the oxygen are
strong. Similar large negative θP values have been observed in other
copper-containing MOF compounds.30
Heterogeneous Catalytic Studies. Single site heterogeneous
catalysis is important as it facilitates the correlation of catalytic
activity with the crystal structure.31 MOFs provide useful oppor-
tunities for investigating such single site catalysis, as the metal
centers are connected through aromatic carboxylates.32 In addition,
many MOFs have labile ligands such as the solvent molecules that
are loosely bound with the metal centers, which can be removed
to create coordinatively unsaturated active catalytic center. The
observed catalytic activity, then, can be related to the availability of
desolvated metal centers or clusters.33 Of the many catalytic
confirms that the copper ions in all of the compounds are present
in the þ2 oxidation state.
Temperature-dependent magnetic susceptibility studies for all
three compounds were performed on powdered samples using
a SQUID magnetometer (Quantum Design Inc., United States).
The χMT value of I decreases slowly from an initial value of
0.80 emu mol-l K at room temperature to reach a value of
0.023 emu mol-l K at 2 K (Figure 8). The molar susceptibility,
χM, value of I increases slowly to reach a maxima at ∼16 K, which
suggests that the onset of long-range order is achieved. The value of
χM decreases upon cooling further up to 7 K, beyond which it
increases sharply again. The sharp increase below 7 K indicates the
presence of some paramagnetic impurity. A similar behavior has been
observed in compounds containing copper and are known as a Curie
tail (Supporting Information, Figure S34).28 A plot of 1/χM vs T for
the compound I in the temperature range 50-300 K can be fitted to
the Curie-Weiss behavior with a value for C of 0.95 emu/mol and
θP of -76.6 K. The large negative values of θP indicate that the
dominant exchanges between the copper ions are antiferromagnetic.
The large Weiss constant (θP) along with high ordering temperature
suggests that there is a good overlap between the metal and the
oxygen orbitals, leading to strengthened superexchange interactions.
The presence of infinite M-O(H)-M bonds in the com-
pounds II and III along with the triangular arrangement of the
copper atoms is expected to give interesting magnetic behavior.29
At room temperature, the χMT values of II and III are 0.71 and
0.77 emu mol-l K for two Cu2þ ions (χMT = 0.375 emu mol-l
K
for a S = 1/2 system, single Cu2þ ions), which are close to the
value expected for two noncoupled Cu2þ ions. The χMT value of
II decreases slowly and reaches a minimum of 0.15 emu mol-l K
at 5 K after which it increases marginally (Figure 9). For compound
III, the χMT value decreases continuously and reaches a minimum
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quantitative formation of the aminonitrile (∼95%). Compound
III exhibited a conversion of ∼53%. The catalytic behavior of the
three compounds may be correlated with the structure. As can be
noted, in compound I, the copper centers are fully coordinated
by the ligands, and the bulky aromatic units probably inhibit the
approach of the imine to the metal center (Lewis site) to have any
meaningful catalytic behavior. Compounds II and III, on the
other hand, have a coordinated water molecule, which are labile
and can be removed to provide the necessary Lewis acid center
for the catalytic reaction. The higher yield in the case of
compound II could be due to the presence of two coordinated
water molecules. This would impart higher Lewis acidic character
in II. In III, of the two copper centers, only one has a coordinated
water molecule, which on dehydration gives rise to the Lewis acid
center. It is not surprising that III exhibits a reasonable catalytic
activity but less than that of II. The spent catalysts were found to
be stable as they exhibited the same XRD pattern as that of the
fresh catalyst. This suggests that the nitroisophthalate com-
pounds are stable under the experimental conditions. We have
also repeated the cyanosilylation studies on the used catalyst,
which indicated that the catalytic conversion of imines was
comparable to the studies performed using the fresh catalyst.
This study suggests that the copper nitroisophthalate phases can
be used as a Lewis acid catalyst in recyclable manner.
’ CONCLUSIONS
Figure 10. Temperature variation of the χMT for III. The inset shows
the corresponding 1/χM vs T plots.
The present study is an attempt to employ HT screening as a
possible means of identifying new compounds along with the
stability ranges as a function of temperature, time, and composition
during the formation of MOFs from a complex multicomponent
mixture of chemicals. The studies have given rise to six new
compounds in the mixture involving copper salts, aromatic acid,
and heterocyclic ligand. Of these, three compounds, [Cu3{(NO2)-
C6H3-(COO)2}3(C3N6H6)] (I), [Cu2(μ3-OH)(H2O){(NO2)-
reactions, cyanosilylation of imines is used frequently to demon-
strate the presence of Lewis acid centers in MOFs. Fujita and co-
workers were one of the earliest researchers to exploit the usefulness
of the Lewis acid centers by employing the cyanosilylation of
carbonyl compounds.34 During the present study, we have isolated
three copper-containing compounds, of which two (II and III) have
coordinated water molecules. The coordinated water can be
removed easily to produce coordinatively unsaturated copper
centers, which can act as the Lewis acid. In a typical experiment,
activated powdered catalyst (0.03 g of I, 0.033 mmol) was
suspended in freshly distilled CH2Cl2 solution (10 mL) of imine
(A, 0.09 g, 0.5 mmol). Trimethylsilyl cyanide (0.075 g, 0.75 mmol)
was added at 0 °C, and the reaction mixture was stirred for 6 h.
Similar catalytic conditions and concentrations of the reactants were
employed for compounds II and III.
C6H3-(COO)2}(CN4H)] (H2O) (II), and [Cu2(μ3-OH)-
3
(H2O){(NO2)-C6H3-(COO)2}(CN5H2)] 2(H2O) (III) have
3
been successfully synthesized in single crystalline form by the use of
conventional hydrothermal approaches. The HT study clearly
indicated the stability trends in the formation of three new phases.
The compounds Cu2{(NO2)-C6H3-(COO)2}(CN4H)(H2O)
(IIa), Cu2{(NO2)-C6H3-(COO)2}{(CN5H2) (IIIa), and Cu2-
{(NO2)-C6H3-(COO)2}{(CN5H2)2}2H2O (IIIb) have been
isolated as microcrystalline products. The structural studies indicate
that the connectivity between the copper and the NIPA as well as
copper and heterocyclic ligand form identifiable units, which are
linked together, forming the final two- (II) and three-dimensional (I
and III) structures. The bound and lattice water molecules in II and
III could be reversibly removed. The dehydrated phase of III has a
comparable structure as the parent hydrated phase, although the
single crystalline nature appears to be lost upon dehydration.
Magnetic studies indicate a predominant antiferromagneic behavior
with a possible canting of the copper spins. Lewis acid catalytic
studies suggest that the dehydrated compounds (II and III) are
good catalysts for the cyanosilylation of imines. Further work is
required to employ the HT method fruitfully in the identification
and isolation of newer compounds.
The product, aminonitrile (B), was isolated and analyzed. The
control experiments in the absence of the catalyst (copper
nitroisophthalate) compounds have also been carried out to
ensure that the reaction proceeds catalytically. To make further
comparison, we have also carried out the same catalytic reaction
in the presence of Cu(CH3COO)2 H2O as the catalyst, which
3
exhibited a conversion of ∼5-10% to nitrile. The catalytic
’ ASSOCIATED CONTENT
studies on compound I also exhibited a poor conversion ∼7%
S
only, which is comparable to that of the Cu(CH3COO)2 H2O.
Supporting Information. Simulated and experimental
b
3
Compound II exhibited the maximum conversion with a near
powder XRD patterns, TGA curves, IR spectra, topological
1367
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Crystal Growth & Design
ARTICLE
analysis, EPR spectra, bond angles and magnetic plots of the
compounds, cyclic TGA of compound III, and the Le Bail fit for
compounds IIa, IIIa, and IIIb and dehydrated compound III.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Corresponding Author
*E-mail: snatarajan@sscu.iisc.ernet.in.
’ ACKNOWLEDGMENT
S.N. thanks the Department of Science and Technology
(DST), Government of India, for the award of a research grant
under the DST-DAAD program. The Council of Scientific and
Industrial Research (CSIR), Government of India, is thanked for
the award of a fellowship (D.S.). S.N. also thanks the Department
of Science and Technology, Government of India, for the award
of the RAMANNA fellowship.
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