Metal-controlled assembly tuning the topology and dimensionality of coordination polymers of Ag(I), Cd(II) and Zn(II) with the flexible 2-(1H-imidazole-1-yl)acetic acid (Hima)

Article abstract of DOI:10.1016/j.molstruc.2006.07.021

Three new, inorganic-organic coordination polymers based on a versatile linking unit 2-(1H-imidazole-1-yl)acetate (ima) and Ag^I, Cd^II and Zn^II ions, exhibiting one to three dimensionalities and different topology structures, have been prepared in water medium and structurally characterized by single-crystal X-ray diffraction analysis. Reaction of AgNO₃ with Hima afforded a neutral one-dimensional (1-D) chains [Ag(ima)]_n (1) which exhibits a pseudo two-dimensional (2-D) layered architecture through π-π stacking interaction between imidazole rings and intermolecular Ag?Ag interactions. Reaction of CdCl₂ with Hima yielded neutral 2-D coordination polymers [Cd(ima)₂]_n (2) possessing (6, 3) topology structures, which further stack into 3-D supramolecular networks through C-H?O weak interactions. While Zn(NO₃)₂ was used, a non-centric 3-D coordination polymer [Zn(ima)₂]_n (3) featuring a 3-fold interpenetrating diamondoid net was isolated. Among these polymers, the building block ima anion exhibits different coordination modes. These results indicate that the versatile nature of this flexible ligand, together with the coordination preferences of the metal ions, play a critical role in construction of these novel coordination polymers. The spectral, thermal and SHG (second-harmonic generation) properties of these new materials have also been investigated.

Full text of DOI:10.1016/j.molstruc.2006.07.021

Journal of Molecular Structure 831 (2007) 61–68
www.elsevier.com/locate/molstruc
Metal-controlled assembly tuning the topology and dimensionality
of coordination polymers of Ag(I), Cd(II) and Zn(II) with the flexible
2-(1H-imidazole-1-yl)acetic acid (Hima)
*
Yong-Tao Wang , Gui-Mei Tang, Yue Wu, Xu-Yan Qin, Da-Wei Qin
Department of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250100, PR China
Received 25 May 2006; received in revised form 10 July 2006; accepted 17 July 2006
Available online 30 August 2006
Abstract
Three new, inorganic–organic coordination polymers based on a versatile linking unit 2-(1H-imidazole-1-yl)acetate (ima) and AgI,
CdII and ZnII ions, exhibiting one to three dimensionalities and different topology structures, have been prepared in water medium
and structurally characterized by single-crystal X-ray diffraction analysis. Reaction of AgNO3 with Hima afforded a neutral one-dimen-
sional (1-D) chains [Ag(ima)]n (1) which exhibits a pseudo two-dimensional (2-D) layered architecture through p–p stacking interaction
between imidazole rings and intermolecular Agꢀ ꢀ ꢀAg interactions. Reaction of CdCl2 with Hima yielded neutral 2-D coordination poly-
mers [Cd(ima)2]n (2) possessing (6,3) topology structures, which further stack into 3-D supramolecular networks through C–Hꢀ ꢀ ꢀO weak
interactions. While Zn(NO3)2 was used, a non-centric 3-D coordination polymer [Zn(ima)2]n (3) featuring a 3-fold interpenetrating
diamondoid net was isolated. Among these polymers, the building block ima anion exhibits different coordination modes. These results
indicate that the versatile nature of this flexible ligand, together with the coordination preferences of the metal ions, play a critical role in
construction of these novel coordination polymers. The spectral, thermal and SHG (second-harmonic generation) properties of these new
materials have also been investigated.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Coordination polymer; Silver metal; Zinc metal; Cadmium metal; 2-(1H-Imidazole-1-yl) acetic acid; X-ray
1. Introduction
encing the self-assembly process [6] such as the coordina-
tion geometry and the oxidation state of the metal ions
[6], metal-to-ligand ratio [7], nature of the ligands [8], sol-
vents [9] and/or counteranions [10], even the reaction tem-
perature and pH value [11]. In general, the design of
appropriate ligands as ‘‘building blocks’’, together with
the coordination preferences of the metal ions as ‘‘nodes’’,
is undoubtedly the most rational synthetic strategy in
manipulating the MOF topologies and thus modifying
the properties of these materials. In this context, the mul-
ti-functional linkers containing both imidazole and carbox-
ylate groups have been much less used in the construction
of MOFs, since they do not readily fit into predictable pat-
terns due to their structural flexibility and conformational
freedom, although some MOFs have recently been con-
structed by a few related flexible ligands [1a,12,13].
Metal-organic frameworks (MOFs) have been success-
fully constructed and self-assembled by metal ions and
well-designed organic ligands, which have attracted consid-
erable attention currently in the field of supramolecular
chemistry and crystal engineering, due to their unique elec-
tronic, magnetic, catalytic, optical and second-order NLO-
acitve properties [1–5]. So far, a variety of discrete and a
wide range of 1D-3D MOFs have been achieved in the last
decade [1–4], however, the crystal engineering of MOFs
with desired topologies and specific properties still remains
a difficult challenge since there are a variety of factors influ-
*
Corresponding author. Tel./fax: +86 531 82350869.
E-mail address: ceswyt@sohu.com (Y.-T. Wang).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.07.021

62
Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
Scheme 1. Coordination modes of ima in 1–3.
As a sequel work of our on-going investigation on the
Then, a water solution (5 mL) of AgNO3 (17 mg,
design and control of the self-assembly of MOFs based
on asymmetric rigid ligands [8,14], we chose 2-(1H-imidaz-
ole-1yl) acetic acid (Hima) as a flexible multifunctional
building block by considering its following advantages
compared with the usual rigid ligands: (i) it possesses flex-
ibility due to the presence of a –CH2– spacer between the
imidazole and carboxylate groups (see Scheme 1) which
may lead to novel topological architectures upon metal
complexation under appropriate conditions; (ii) it has three
potential binding sites when coordinating to metal ions;
(iii) no metal complex containing this ligand has been doc-
umented to date. We now report herein the preparations
and characterizations of three new MOFs of ima, namely
[Ag(ima)]n (1) [Cd(ima)2]n (2) and [Zn(ima)2]n (3).
0.1 mmol) was added slowly to the above solution. The
resultant colorless solution was filtered. Block colorless sin-
gle-crystals suitable for X-ray diffraction were obtained by
slow evaporation within one week in 65% yield. Anal.
Calcd for C5H5AgN2O2: C, 25.78; H, 2.16; N, 12.02%.
Found: C, 25.71; H, 2.18; N, 12.09%. IR (cmꢁ1): 3109vs,
2977m, 1677m, 1578vs, 1515s, 1384vs, 1309vs, 1249s,
1240s, 1215m, 1190m, 1103vs, 1103vs, 1036vs, 975m,
942m, 923m, 911m, 866m, 830vs, 786s, 774m, 743s,
688m, 654m, 625m, 584m.
2.4. Synthesis of [Cd(ima)2]n (2)
The solution of Hima (25 mg, 0.2 mmol) in water (5 mL)
was added an aqueous solution (5 mL) of CdCl2 (23 mg,
0.1 mmol) followed by dropwise addition of a dilute aque-
ous solution of NaOH, with stirring until the pH was ca. 7.
Further stirring of the solution for ca. 30 min resulted in a
small quantity of colorless precipitate, which was filtered
off. The resultant colorless filtrate was left to stand at room
temperature. Colorless block-like single-crystals suitable
for X-ray diffraction were obtained by slow evaporation
within three days in 75% yield. Anal. Calcd for
C10H10CdN4O4: C, 33.12; H, 2.78; N, 15.45%. Found: C,
33.22; H, 2.75; N, 15.49 %. IR (cmꢁ1): 3137vs, 1593vs,
1510m, 1442m, 1396vs, 1316vs, 1293m, 1235m, 1199m,
1095m, 1039, 982m, 939m, 917m, 877m, 834m, 781m,
693vs, 652m, 631m, 579m.
2. Experimental
2.1. Materials and general methods
All the starting materials and solvents for syntheses were
obtained commercially (ACROS) and used as received.
Elemental analyses were performed on a Perkin-Elmer
240 elemental analyzer. The FT-IR spectra as KBr discs
were recorded pellets in range 400–4000 cmꢁ1 on a Nicolet
5DX spectrometer. Thermogravimetric analysis (TGA)
data were collected with a Perkin-Elmer TGS-2 analyzer
in N2 at a heating rate of 10 ꢁC minꢁ1
.
2.2. Nonlinear optical property measurement
2.5. Synthesis of [Zn(ima)2]n (3)
Kurtz powder SHG measurements [15] were performed
on ground samples of crystalline 1–3 with a synchroscan
streak camera (Hamamatsu Model C1587, 8 ps resolution)
connected to a polychromator as the detector system, and
an optical parametric generator (Spectra Physics, Mod-
el:OPA-800C) pumped by a mode-locked Ti:sapphire laser
oscillator–amplifier system (Spectra Physics, Model: Hurri-
cane) as the pump source. The powder second-harmonic
signals were compared to that of urea to determine the rel-
ative SHG efficiencies of 1–3.
The solution of Hima (25 mg, 0.2 mmol) in water (5 mL)
was slowly added a water solution (5 mL) of Zn(NO3)2
(30 mg, 0.1 mmol) under stirring for ca. 30 min. Then a
dilute aqueous solution of NaOH was added dropwise with
stirring to the above mixture until the pH was ca. 7. The
resultant white precipitate was filtered off and the colorless
solution was left to stand at room temperature. Block col-
orless single-crystals suitable for X-ray diffraction were
obtained by slow evaporation within five days in 72% yield.
Anal. Calcd for C10H10N4O4Zn: C, 38.06; H, 3.19; N, 17.75
%. Found: C, 37.94; H, 3.20; N, 17.82%. IR (cmꢁ1):
3151vs, 3003m, 2921m, 1591vs, 1525m, 1400vs, 1351m,
1319m, 1292m, 1235m, 1113vs, 1091m, 1042m, 983m,
942m, 921m, 868m, 832m, 798m, 764m, 702s, 655s, 629m,
579m.
2.3. Synthesis of [Ag(ima)]n (1)
To a solution of Hima (12.6 mg, 0.1 mmol) in water
(5 mL) a dilute aqueous solution of NaOH was added with
stirring to adjust the pH value of the mixture to ca. 7.

Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
63
2.6. X-ray crystallography
Centre, CCDC Nos. 291,258, 291,261, 291,263 for 1–3,
respectively. Copies of this information may be obtained free
of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.
uk).
Single crystal X-ray diffraction measurements of 1–3
were carried out with a Bruker APEX II CCD diffractom-
eter equipped with a graphite crystal monochromator. The
lattice parameters were obtained by least-squares refine-
ment of the diffraction data. All the measured independent
reflections were used in the structural analysis, and semi-
empirical absorption corrections were applied using the
SADABS program. The program SAINT [16] was used
for integration of the diffraction profiles. All the structures
were solved by direct methods using the SHELXS program
of the SHELXTL package and refined with SHELXL [17].
Metal atoms were located from the E-maps and other non-
hydrogen atoms were located in successive difference
Fourier syntheses. The final refinement was performed by
full-matrix least-squares methods with anisotropic thermal
parameters for all the non-hydrogen atoms based on F2.
All the hydrogen atoms were first found in difference elec-
tron density maps, and then placed in the calculated sites
and included in the final refinement in the riding model
approximation with displacement parameters derived from
the parent atoms to which they were bonded. The absolute
structures for 3 have been determined with the Flack
parameters of 0.032(19) [18]. Crystal data as well as details
of data collection and refinement for the complexes 1–3 are
given in Table 1.
3. Results and discussion
3.1. Preparation of complexes 1–3
All 1–3 were obtained as neutral complexes from aqueous
solutions by reactions of Hima with different metal salts.
Compound 1 could also be isolated when using AgBF4 and
Ag2SO4 as the Ag(I) source, as confirmed by X-ray diffrac-
tion, IR spectra and elemental analyses. Similar cases were
also observed for ZnII and CdII complexes, which indicates
that the final products are independent of the counter-anions
in the metal salts. Furthermore, although 2 and 3 have the
exact 2:1 ligand/metal compositions, our trials showed that
the products do not significantly depend on the ligand-to-
metal ratio, while higher yields were observed when increas-
ing the ligand-to-metal ratio.
3.2. Crystal Structures
3.2.1. Structure analysis of 1
In 1, there is one AgI atom and one ima ligand in each
independent crystallographic unit, as shown in Fig. 1a.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Table 1
Crystal and structure refinement for complexes 1–3
1
2
3
Empirical formula
Formula weight
T/[K]
C5H5AgN2O2
232.98
298
C10H10CdN4O4
362.63
298
C10H10N4O4Zn
315.59
298
˚
Wavelength/[A]
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
Cc
Crystal system
Space group
˚
a/[A]
8.8134(17)
13.460(3)
5.2446(10)
106.399(2)
596.9(2)
4
8.4289(16)
5.1470(10)
12.466(2)
92.667(2)
540.25(17)
2
9.005(5)
16.512(9)
9.172(5)
117.276(7)
1212.2(11)
4
˚
b/[A]
˚
c/[A]
b/[ꢁ]
3
˚
V/[A]
Z
l/[mmꢁ1
]
3.301
2.040
2.042
Crystal size [mm]
q/[g cmꢁ3
0.32 · 0.18 · 0.09
2.593
0.30 · 0.10 · 0.10
2.229
0.30 · 0.20 · 0.20
1.729
]
Reflections collected
Independent reflections
Reflections [I > 2r(I)]
F(000)
3372
1334
1244
448
2878
1197
1072
356
3289
1985
1855
640
Rint
Data/restraints/parameters
h range
0.0352
0.0150
0.0330
1334/0/92
2.41 to 27.96
1.104
0.666/ꢁ0.670
0.0257/0.0614
0.0276/0.0622
1197/0/88
2.98 to 28.01
1.055
0.522/ꢁ0.287
0.0202/0.0513
0.0235/0.0531
1985/2/172
2.47 to 28.15
1.006
0.603/ꢁ0.441
0.0411/0.0859
0.0447/0.0878
Goodness of fit on F2
ꢁ3
˚
Largest peak and hole [e A
]
b
R1a/wR2 [I > 2r(I)]
b
R1a/wR2 (all data)
P
P
a
R1 = iFoj ꢁ jFci/ jFoj.
P
P
2
2 1=2
b
wR2 ¼ ½ ½wðF 2o ꢁ F c2Þ ꢂ= ½wðF 2oÞ ꢂꢂ
.

64
Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
Fig. 1. (a) View of the coordination environment of AgI in 1 with thermal ellipsoids at 30% probability; (b) view of the 1-D chain and pseudo 2-D layer
formed by Agꢀ ꢀ ꢀAg interactions between adjacent chains.
The coordination geometry of the two-coordinated AgI
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for complexes 1–3
center is slightly bent (O–Ag–N 156.30(9)ꢁ). Each ima
anion act as a bridge between two AgI atoms (Scheme
1
Ag(1)–N(2)
Ag(1)–O(1)a
Ag(1)–O(2)c
2.154(2)
2.244(2)
2.651(2)
Ag(1)–Ag(1)b
C(1)–O(1)
3.1489(8)
1.249(3)
1.252(3)
˚
1(a)) with the Ag–Nimidazole bond length of 2.154(2) A nor-
mal for related Ag–N coordination bonds [19] and the
slightly longer Ag–Ocarboxy bond (see Table 2) giving rise
to a zigzag chain.
C(1)–O(2)
O(1)a–Ag(1)–N(2)
O(2)c–Ag(1)–N(2)
156.30(9)
117.18(7)
O(1)a–Ag(1)–O(2)c
85.50(7)
Additionally, short Agꢀ ꢀ ꢀAgi (i = x,ꢁy + 3/2,z + 1/2)
˚
contacts between the adjacent chains is 3.149(3) A result
2
in the formation of pseudo 2D frameworks (Fig. 1b) which
are stacked in an ABAB sequence with all ima ligands in a
same layer arranged along the same plane [20] and exhibit-
ing interlayer hydrogen-bonding interactions (Table 3) and
Cd(1)–N(2)
Cd(1)–O(1)a
Cd(1)–O(2)b
Cd(1)–N(2)c
2.2731(18) Cd(1)–O(2)d
2.3501(16) Cd(1)–O(1)e
2.3487(16) O(1)–C(1)
2.2731(18) O(2)–C(1)
2.3487(16)
2.3501(16)
1.247(3)
1.246(3)
˚
O(1)a–Cd(1)–N(2)
O(2)b–Cd(1)–N(2)
N(2)–Cd(1)–N(2)c
O(2)d–Cd(1)–N(2)
O(1)e–Cd(1)–N(2)
O(1)a–Cd(1)–O(2)b
O(1)a–Cd(1)–N(2)c
O(1)a–Cd(1)–O(2)d
82.53(6)
93.32(6)
180.00
86.68(6)
97.47(6)
91.99(5)
97.47(6)
88.01(5)
O(1)a–Cd(1)–O(1)e
O(2)b–Cd(1)–N(2)c
180.00
86.68(6)
p–p packing interaction between imidazole rings (3.6291 A)
˚
and C(3)–H(3A)ꢀ ꢀ ꢀp (2.72 A) (Fig. 2) [21].
O(2)b–Cd(1)–O(2)d 180.00
O(1)e–Cd(1)–O(2)b
O(2)d–Cd(1)–N(2)c
O(1)e–Cd(1)–N(2)c
O(1)e–Cd(1)–O(2)d
88.01(5)
3.2.2. Structural analysis of 2
93.32(6)
82.53(6)
91.99(5)
Complex 2 is 2-D MOFs. In 2, each metal ion is six-co-
ordinated by six ima ligands using their imidazole nitrogen
or carboxylate oxygen donors. The coordination geometry
of CdII could be described as a nearly ideal octahedron, as
shown in Fig. 3a and Table 2. For each carboxylate group
of ima in 2, two C–O bond distances of the carboxylic
group are almost equivalent, being consistent with its struc-
tural feature (syn–anti coordination) as shown in Scheme
1(b). Thus, Cd atom is coordinated by six distinct ima
ligands and each ima bridges three CdII centers to form a
2-D coordination polymer with (6,3) topology [22] along
the crystallographic ab plane, as illustrated in Fig. 3b. Such
coordination layers are parallel stacked with an ABAB
sequence featuring interlayer hydrogen-bonding interac-
tions (see Table 3 and Fig. 4).
3
Zn(1)–O(1)
Zn(1)–N(4)
Zn(1)–O(3)a
Zn(1)–N(2)b
1.935(4)
2.012(5)
1.962(4)
1.990(5)
O(1)–C(1)
O(2)–C(1)
O(3)–C(6)
O(4)–C(6)
1.262(7)
1.204(7)
1.258(7)
1.229(7)
O(1)–Zn(1)–N(4)
O(1)–Zn(1)–O(3)a
O(1)–Zn(1)–N(2)b
118.2(2)
110.27(16)
109.99(17)
O(3)a–Zn(1)–N(4)
N(2)b–Zn(1)–N(4)
O(3)a–Zn(1)–N(2)b
96.28(19)
105.95(19)
115.86(19)
Symmetry codes: a1 + x, y, 1 + z; bx, 3/2 ꢁ y, ꢁ1/2 + z; c1 + x, 3/2 ꢁ y, 1/
2 + z for 1; a5/2 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z; b5/2 ꢁ x, 1/2 + y, 3/2 ꢁ z; c2 ꢁ x,
ꢁy, 2 ꢁ z; dꢁ1/2 + x, ꢁ1/2 ꢁ y, 1/2 + z; eꢁ1/2 + x, 1/2 ꢁ y, 1/2 + z for 2;
a
b
ꢁ1 + x, ꢁy, ꢁ1/2 + z; ꢁ1/2 + x, 1/2 ꢁ y, 1/2 + z for 3.
3.2.3. Structure analysis of 3
The local coordination geometry around the Zn1 center
can be described as a distorted tetrahedron (Table 2). The
Zn1 center coordinates to two imidazole nitrogen atoms
and two carboxylate oxygen atoms [O(1) and O(3)] of four
different ima ligands (Fig. 5a). Two Zn–Nimidazole bond

Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
65
Table 3
Hydrogen bond geometries in the crystal structure of 1–3
3.3. IR spectra
Complex D–Hꢀ ꢀ ꢀAa
Dꢀ ꢀ ꢀA (A) Hꢀ ꢀ ꢀA (A) D–Hꢀ ꢀ ꢀA (ꢁ)
˚
˚
The IR spectrum of 1 shows characteristic bands of the
dicarboxylate groups in the usual region at 1578 cmꢁ1 for
the asymmetric stretching and at 1384 cmꢁ1 for the sym-
metric stretching (Fig. S1). The difference between these
wave numbers is 194 cmꢁ1, which is attributed to the exis-
tence of monodentate modes of the carboxylate groups
(Scheme 1a) [23]. For 2, the bands are at 1593 cmꢁ1 for
the asymmetric stretching and at 1396 cmꢁ1 for the sym-
metric stretching. The difference value of 203 cmꢁ1 indi-
cates that the bridging modes of ima is present (Scheme
1b). For 3, the bands are at 1591 cmꢁ1 for the asymmetric
stretching and at 1400 cmꢁ1 for the symmetric stretching.
The difference value of 191 cmꢁ1 shows the existence of
monodentate modes of the carboxylate groups (Scheme
1a). The IR spectra indicate that the ima carboxylate
groups function in different coordination modes, being
consistent with the crystal structures of 1–3, respectively.
1
C(2)–H(2A)ꢀ ꢀ ꢀO(1)i
3.453(4)
3.240(3)
2.58
2.59
150
127
C(4)–H(4A)ꢀ ꢀ ꢀO2ii
C(2)–H(2B)ꢀ ꢀ ꢀO(1)i
2
3
3.512(2)
2.57
2.57
2.45
2.53
163
115
144
153
C(3) –H(3A)ꢀ ꢀ ꢀO(1)ii 3.087(2)
C(4)–H(4A)ꢀ ꢀ ꢀO(2)iii 3.242(3)
C(5)–H(5A)ꢀ ꢀ ꢀO(2)iv 3.384(3)
C(2)–H(2A)ꢀ ꢀ ꢀO(2)i
3.447(6)
3.376(6)
2.54
2.42
156
169
C(2)–H(2B)ꢀ ꢀ ꢀO(2)ii
a
i
i
Symmetry codes: x, y, 1 + z; ii1 + x, y, z for 1; 7/2 ꢁ x, ꢁ1/2 + y, 3/
iii
2 ꢁ z; ii5/2 ꢁ x, ꢁ1/2 + y, 3/2 ꢁ z; ꢁ1/2 + x, 1/2 ꢁ y, 1/2 + z; iv7/2 ꢁ x,
1/2 + y, 3/2 ꢁ z for 2; 1/2 ꢁ x, 1/2 + y, z; ii1 ꢁ x, 1/2 + y, 1/2 ꢁ z for 3.
i
lengths are nearly the same, and slightly longer than two
similar Zn–Ocarboxy distances (see Table 2). For the ima
carboxylic groups, the coordinated C–O bond lengths
(1.262(7) and 1.258(7) A) are significantly longer than those
of the uncoordinated bonds (1.204(7) and 1.229(7) A) being
˚
3.4. Second harmonic generation efficiency
˚
similar that in 1.
According to the principles proposed by Kurtz and Per-
ry [15] the strength of second harmonic generation (SHG)
efficiency of the compounds can be estimated by measuring
powder samples. Only 3 is SHG-active, having an SHG
intensities of 0.5 relative to that of urea, which confirm that
it crystallizes in the polar space groups, in consistent with
the structural analyses.
The Zn(1) center is connected to four adjacent Zn
centers through the ima bridges to result in a 3D dia-
mondoid MOF (Fig. 5b) with the adjacent Zn–Zn sepa-
˚
rations of 9.054(3) and 8.784(3) A and the Zn–Zn–Zn
angles in the range of 81.16ꢁ to 124.65ꢁ, being signifi-
cantly deviated from 109.45ꢁ expected for an idealized
diamond network, thus the diamondoid network in 3 is
distorted. With long Zn–Zn separations, large cavities
are formed within each diamondoid net in 3 (Fig. 5b)
which gives rise to the formation of 3-fold interpenetra-
tion. The crystal structure of 3 is very similar to those
reported previously [5a]. Additionally, crystal structure
have been further stabilized by hydrogen-bonding inter-
actions (see Table 3 and Fig. 6).
3.5. Thermogravimetric analysis (TGA)
High thermal stability is an important precondition in
the conversion of coordination frameworks from labora-
tory curiosities to practical materials. Thus, TGA mea-
surements were conducted to determine the thermal
stability of 1–3. For 1, no obvious loss weight was
Fig. 2. Perspective of molecular packing in 1 along c axis. The dashed lines stand for hydrogen bond.

66
Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
Fig. 3. (a) View of coordination environments of the metal atoms in 2 with thermal ellipsoids at 30% probability; (b) View of 2-D layer to (101) plane.
Fig. 4. Perspective of molecular packing in 2 along [010] direction, respectively. The dashed lines stand for hydrogen bond.
observed before at the range of 240 ꢁC. A rapid weight
loss occurred in the 240–284 ꢁC region (peak: 270 ꢁC).
The remaining weight of 49.4% corresponds to the per-
centage (49.7%) of Ag2O. For 2, the TGA shows MOFs
remained intact until two consecutive weight losses occur
in the range of 297–372 ꢁC (peak: 348 ꢁC). The remaining
weight of 35.2% corresponds to the percentage (35.4%)
of CdO. For 3, the TGA indicates that the MOF
remained intact until two consecutive weight losses occur
in the 350–405 ꢁC region (peak: 383 ꢁC). The remaining
weight of 26.0% corresponds to the percentage (25.8%)
of ZnO.

Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
67
Fig. 5. (a) Coordination environment of the metal atom in 3 with thermal ellipsoids at 30% probability; (b) Diamondoid network of 3.
Fig. 6. Perspective of molecular packing in 3 along a axis. The dashed lines stand for hydrogen bond.

68
Y.-T. Wang et al. / Journal of Molecular Structure 831 (2007) 61–68
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Acknowledgements
This work was financially supported by the Starting
Funding of Shandong Institute of Light Industry. We are
indebted to Jian-Ping Ma at Shandong Normal University
for collecting the data of all crystal structures and to Prof.
Xiao-Ming Chen at Sun Yat-Sen University for helpful dis-
cussion. Special thanks are offered to one of the reviewers
who gives a lot of good advice.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be
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