Crystal structure, thermal stability and theoretical investigation on four 1,3-bis(1,2,4-triazol-1-yl)propane-based copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2009.10.014

Self-assembly of flexible 1,3-bis(1,2,4-triazol-1-yl)propane (btp), inorganic Cu(II) salt and rigid benzene-based carboxylate coligand generates four complexes, {[Cu(btp)₂(CH₃OH)(H₂O)]·H₂O·2ClO₄}

Full text of DOI:10.1016/j.ica.2009.10.014

Inorganica Chimica Acta 363 (2010) 308–316
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Crystal structure, thermal stability and theoretical investigation
on four 1,3-bis(1,2,4-triazol-1-yl)propane-based copper(II) complexes
*
*
En-Cui Yang , Wen Feng, Jing-Yi Wang, Xiao-Jun Zhao
College of Chemistry and Life Science, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2009
Received in revised form 14 October 2009
Accepted 21 October 2009
Available online 26 October 2009
Self-assembly of flexible 1,3-bis(1,2,4-triazol-1-yl)propane (btp), inorganic Cu(II) salt and rigid benzene-
based carboxylate coligand generates four complexes, {[Cu(btp)2(CH3OH)(H2O)]ꢀH2Oꢀ2ClO4}n (1),
{[Cu(btp)(Hbtc)2]ꢀ0.5H2O}n (2), [Cu(btp)2(H3btea)2]n (3), and [Cu(btp)(nb)2] (4) (H3btc = 1,3,5-benzenetri-
carboxylic acid, H4btea = 1,2,4,5-benzenetetracarboxylic acid, Hnb = p-nitrobenzoic acid), which are fully
structural characterized by single-crystal X-ray crystallography, elemental analysis, IR, and TG–DTA tech-
niques. Structural determinations reveal that the polymeric two-dimensional (2D) Cu-btp grid-like layer
for 1, 1D linear single- and double-stranded chains for 2 and 3, as well as the discrete binuclear structure
for 4, are jointly directed by the coordination polyhedrons of the Cu(II) ion and the exo-bidentate bridging
btp core ligand with various conformations. The theoretical calculations suggest that the trans–trans btp
is the most stable conformation, and the metal binding site is collectively determined by the electron
density of N donors and the spatial orientation of the btp ligand. Unexpectedly, the polycarboxylate
anions in 1–4 can only act as terminal coligands not popular bridging connectors. The thermal stability
of the resulting complexes is also compared.
Keywords:
Flexible ligand
1,3-Bis(1,2,4-triazol-1-yl)propane
Theoretical calculation
Thermal stability
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
influence the structures of the resulting complex to some differ-
ent extent, in addition to balancing the charges of a cationic
During the past decades, intense interest has been focused on
the design and synthesis of flexible ligand-based complexes be-
cause of their intriguing framework connectivity and potential
applications in the fields of magnetic, ion exchange, porous,
luminescence and optical devices [1–6]. Indeed, flexible ligands
with five-membered polyazole rings separated by aliphatic chain,
such as bis(imidazol-1-yl)alkanes (n = 2, 4, 6) [7–10], bis(1,2,4-
triazol-1-yl)alkanes (n = 1, 2, 3, 4, 6) [11–20] and bis(tetrazol-
1-yl)alkanes (n = 2, 4, 6) [21–23], have demonstrated to be very
effective upon the formation of extended transition-metal poly-
mers. Obviously, the adaptable conformation of the flexible li-
gands is easy to meet the coordination geometrical
requirement of metal ions and significantly tune the formation
of the resulting coordination polymers [8,16,24–31]. On the
other hand, it is well known that the coligands with different
skeleton structure and electronic nature play essential roles on
complex. However, to the best of our knowledge, the 1,3-
bis(1,2,4-triazol-1-yl)propane (btp)-based complex with rigid
aromatic acid as coligand has limited documented by far [17].
Therefore, in our continuing efforts to systematically investigate
the self assembly behavior of the 1,2,4-triazole and its deriva-
tives [33,34], herein, btp was chosen as a core ligand to react
with inorganic Cu(II) salt and 1,3,5-benzenetricarboxylic acid
(H3btc), 1,2,4,5-benzenetetracarboxylic acid (H4btea) and p-nitro-
benzoic acid (Hnb), respectively. As a result, four coordination
polymers, {[Cu(btp)2(CH3OH)(H2O)]ꢀH2Oꢀ2ClO4}n (1), {[Cu(btp)-
(Hbtc)2]ꢀ0.5H2O}n (2), [Cu(btp)2(H3btea)2]n (3) and [Cu(btp)(nb)2]
(4), were isolated and structurally characterized by single-crystal
X-ray diffraction, elemental analysis, FT-IR spectra, and TG–DTA
techniques. Additionally, the relative stability of the various
btp conformations and its electron density on heterocyclic N do-
nors, have been theoretical calculated and compared. On the
other hand, interestingly, rather than multiple dentate bridging
ligands, the three aromatic polycarboxylate coligands can merely
act as terminal ligands to complete the metal coordination
sphere. Thus, the resulting four complexes are significantly
determined by the exo-bidentate btp ligands, in which the struc-
tural diversity ranged from two-dimensional (2D) grid-like layer
to discrete binuclear structure, is also closely related to the coor-
dination geometry of the Cu(II) ion.
the structures and functions of the target complexes with mixed
ꢁ
ligands. Recently, various anions such as ClO4ꢁ, NO3ꢁ, BF4ꢁ, PF6
,
NCSꢁ, and Wells–Dawson polyoxoanions are successively intro-
duced into the metal-bis(1,2,4-triazol-1-yl)alkanes (n = 3, 4) sys-
tem [17–20,32], revealing that the shape and size of the anions
* Corresponding authors. Tel./fax: +86 22 23766556 (X.-J. Zhao).
E-mail address: xiaojun_zhao15@yahoo.com.cn (X.-J. Zhao).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.10.014

E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
309
a 52% yield (based on Cu(II) salt). Anal. Calc. for C21H18CuN8O8: C,
43.95; H, 3.16; N, 19.52. Found: C, 43.81; H, 3.22; N, 19.68%. FT-
IR (KBr, cmꢁ1): 3154w, 3124w, 1631s, 1584vs, 1522s, 1374s,
1345vs, 1285m, 1201w, 1124m, 999w, 878w, 829m, 722m,
670w, 571w, 530w.
2. Experimental
2.1. Reagents and general methods
The btp ligand was synthesized according to the literature meth-
od [18]. And other starting materials and solvents were of reagent
grade obtained from commercial sources and used as received
without further purification. Fourier transform (FT) IR spectra
(KBr pellets) were taken on an Avatar-370 (Nicolet) spectrometer.
Elemental analyses for C, H, and N were performed on a CE-440
(Leemanlabs) analyzer. TG–DTA experiments were carried out on
a Shimadzu simultaneous DTG-60A thermal analysis instrument
with a heating rate of 8 °Cꢀminꢁ1 from room temperature to
800 °C under a nitrogen atmosphere (flow rate 10 mL minꢁ1).
2.6. X-ray crystallography
X-ray single-crystal diffraction intensities for 1–4 were col-
lected on a computer controlled Bruker APEX-II CCD diffractometer
at 296(2) K with Mo K
a
radiation (0.71073 Å) by using a
x
ꢁu scan
technique. There was no evidence of crystal decay during data col-
lection. The program SAINT [35] was used for integration of the dif-
fraction profiles. Semi-empirical absorption corrections were
applied using SADABS [36] program. These structures were solved
by direct methods and refined with the full-matrix least-squares
technique using the SHELXS-97 and SHELXL-97 programs [37,38].
Anisotropic thermal parameters were assigned to all non-hydrogen
atoms. The organic hydrogen atoms were generated geometrically.
The starting positions for water H atoms were found in difference
syntheses and then fixed in the given positions. For 1, the perchlo-
rate anion was found disordered. Two sets of tetrahedral oxygen
atoms (O2, O3, O4, O5; O20, O30, O40, and O50) were refined with
geometric constraints with occupancies of 59.3% and 40.7%,
respectively. In 2, the lattice water molecule (O2) was disordered
with occupancy of 50.0%. No attempts have been performed to lo-
cate hydrogen atoms of the disordered water molecule. Further
crystallographic data and structural refinement details are summa-
rized in Table 1. The selected bond distances and angles for 1–4 are
listed in Table 2, respectively. Hydrogen bond geometries are in-
cluded in Table 3.
2.2. Syntheses of {[Cu(btp)2(CH3OH)(H2O)]ꢀH2Oꢀ2ClO4}n (1)
To an aqueous solution (10 mL) of Cu(ClO4)2ꢀ6H2O (26.3 mg,
0.1 mmol) was slowly added a CH3OH solution (10 mL) containing
btp (17.8 mg, 0.1 mmol) with constant stirring. The resulting grass-
green mixture was further stirred for 1 h and then filtered. Upon
slow evaporation of the filtrate at room temperature, blue block-
shaped crystals suitable for X-ray diffraction were collected within
10 days. Yield: ꢂ45% (based on Cu(II) salt). Anal. Calc. for
C15H28Cl2CuN12O11: C, 26.23; H, 4.11; N, 24.47. Found: C, 26.34;
H, 3.92; N, 24.61%. FT-IR (KBr, cmꢁ1): 3429br, 3141s, 1630m,
1538s, 1290m, 1220w, 1131vs, 1005m, 672m, 625s.
Caution: Perchlorate salts of transition-metal complexes with
organic ligands are potentially explosive. Only a small amount of
material should be prepared and handled with caution.
2.3. Syntheses of {[Cu(btp)(Hbtc)2]ꢀ0.5H2O}n (2)
2.7. Computational details
Btp (17.8 mg, 0.1 mmol), H3btc (21.0 mg, 0.1 mmol), Cu-
(NO3)2ꢀ4H2O (30.8 mg, 0.1 mmol) and water (15 mL) were sealed
in a 23 mL stainless steel vessel and was heated at 160 °C for
72 h. After the mixture was cooled to room temperature at a rate
of 2 °C hꢁ1, blue block-shaped crystals suitable for single-crystal
X-ray diffraction analysis were isolated directly, washed by ethanol
and dried in air. Yield: ꢂ55% (based on Cu(II) salt). Anal. Calc. for
C25H19CuN6O12.5: C, 45.02; H, 2.87; N, 12.60. Found: C, 45.10; H,
2.92; N, 12.86%. FT-IR (KBr, cmꢁ1): 3444br, 3138m, 1707vs,
1617m, 1561s, 1447w, 1358m, 1288m, 1222s, 1123m, 744w,
678w.
The initial geometry of the btp ligand for the theoretical calcu-
lation was obtained from the crystal structure determined by sin-
gle-crystal X-ray diffraction. All the conformations were fully
optimized at the B3LYP [39,40] method with the 6-311+G** basis
set without imposing any symmetry constraint. Frequency calcula-
tions were also performed to confirm that the stationary points
from the geometry optimization calculations were in real minima.
All the calculations were performed using the GAUSSIAN 03 software
package [41] on a Dell Precision 490 computer.
3. Results and discussion
2.4. Syntheses of [Cu(btp)2(H3btea)2]n (3)
3.1. Synthesis and FT-IR spectra of the complexes 1–4
The same synthetic procedure as 2 was used for the preparation
of 3, except that H3btc was replaced by H4btea (25.4 mg,
0.1 mmol). Green block-shaped crystals were obtained in a 50%
yield (based on Cu(II) salt). Anal. Calc. for C17H15Cu0.5N6O8: C,
44.09; H, 3.26; N, 18.15. Found: C, 43.99; H, 3.18; N, 18.21%. FT-
IR (KBr, cmꢁ1): 3433br, 3177w, 3106w, 1709vs, 1603s, 1541s,
1472m, 1370s, 1283vs, 1236s, 1129vs, 1017s, 903m, 846w,
770w, 660m, 585w.
Complex 1 is obtained by conventional evaporation method be-
cause of the considerable solubility of the reactant mixture. In con-
trast, complexes 2–4 are isolated by popular hydrothermal
method. Notably, the pH value of the medium containing aromatic
poly-carboxylic acid is not controlled in order to obtain different
deprotonated species.
In the FT-IR spectra, the broad absorption at 3429 cmꢁ1 for 1
and 3444 cmꢁ1 for 2 is attributed to the O–H stretching vibrations
of the water and/or methanol molecule in the both complexes,
respectively. A band appeared at 1707 cmꢁ1 for 2 and 1709 cmꢁ1
for 3, respectively suggests the coexistence of –COOH and –COOꢁ
[42,43]. The asymmetric and symmetric stretching vibrations of
the carboxylate groups locate at 1617, 1561, 1358 cmꢁ1 for 2 and
1603, 1541, 1370 cmꢁ1 for 3. And their relatively larger separations
(>95 cmꢁ1) indicate the monodentate binding mode of carboxylate
group [44,45]. In contrast, the characteristic bands at 1584, 1522,
1374, and 1345 cmꢁ1 in 4 imply the mixed coordination modes
2.5. Syntheses of [Cu(btp)(nb)2]n (4)
A mixture of btp (8.9 mg, 0.05 mmol), Hnb (16.7 mg, 0.1 mmol),
and Cu(NO3)2ꢀ4H2O (46.2 mg, 0.15 mmol) was dissolved in H2O
(15 mL) and the pH value of the mixture was carefully adjusted
to ca. 5 by slow addition of 0.2 mol Lꢁ1 NaOH solution. The mixture
was then transferred into a 23 mL stainless steel vessel, which was
heated at 120 °C for 48 h, and slowly cooled to room temperature
at a rate of 2.0 °C hꢁ1. Blue block-shaped crystals were obtained in

310
E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
Table 1
Crystallographic data and structure refinement summary for complexes 1–4.
1
2
3
4
Formula
C15H28Cl2CuN12O11
686.93
0.24 ꢃ 0.23 ꢃ 0.22
orthorhombic
Pna21
18.6717(8)
12.5822(5)
12.0263(5)
90
90
90
2825.4(2)
4
1.615
C25H19CuN6O12.5
667.00
0.22 ꢃ 0.18 ꢃ 0.16
monoclinic
C2/c
6.5702(12)
24.339(4)
17.082(3)
90
97.915(3)
90
C17H15Cu0.50N6O8
463.12
C21H18CuN8O8
573.97
Formula weight (g molꢁ1
Crystal size (mm)
Crystal system
Space group
a (Å)
)
0.24 ꢃ 0.23 ꢃ 0.22
0.25 ꢃ 0.23 ꢃ 0.22
triclinic
triclinic
ꢀ
ꢀ
P1
P1
9.4815(6)
10.1256(6)
10.9280(6)
116.1150(10)
95.0230(10)
100.3950(10)
909.87(9)
2
8.7125(4)
11.7000(6)
11.7989(5)
85.2960(10)
88.0140(10)
74.0500(10)
1152.45(9)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
2705.6(9)
4
1.637
Z
Dcalc (g cmꢁ3
)
1.690
0.697
1.654
1.015
l
(mmꢁ1
)
1.036
0.888
F(0 0 0)
1412
1360
475
586
Rint
0.0230
1.040
0.0162
1.043
0.0086
1.034
0.0102
1.040
Goodness-of-fit (GOF) on F2
Residuals (e Åꢁ3
h/k/l
)
0.703, ꢁ0.525
ꢁ22, 21/ꢁ13, 14/ꢁ14, 12
13 931/4452
1.95–25.01
4452/107/409
0.0483/0.1376
0.0529/0.1422
0.7960/0.7800
0.386, ꢁ0.303
ꢁ7, 7/ꢁ24, 28/ꢁ17, 20
6876/2388
1.67–25.02
2388/0/207
0.0278/0.0799
0.0335/0.0843
0.8680/0.8250
0.297, ꢁ0.317
ꢁ10, 11/ꢁ12, 11/ꢁ12, 7
4492/3163
2.11–25.00
3163/0/289
0.0327/0.0836
0.0373/0.0866
0.8617/0.8505
0.239, ꢁ0.363
ꢁ10, 10/ꢁ13, 13/ꢁ6, 14
5889/4039
1.82–25.01
4039/0/343
0.0270/0.0725
0.0296/0.0743
0.8000/0.7760
Reflections collected/unique
h Range for data collection
Data/restraints/parameters
R1, wR2 [I > 2r
(I)]a
R1, wR2 (all data)a
Maximum and minimum transmission
P
P
P
P
(||Fo|ꢁ|Fc||)/ |Fo|; wR2 = [ w(|Fo|2ꢁ|Fc|2)2/ w(Fo2)]1/2
.
a
R1
=
Table 3
Table 2
Hydrogen-bonding parameters (Å, °) for 1–4.
Selected bond distances (Å) and angles (°) for complexes 1–4.
Donor–Hꢀꢀꢀacceptor
D–H
HꢀꢀꢀA
DꢀꢀꢀA
D–HꢀꢀꢀA
1
Cu(1)–N(12)a
2.002(4)
2.015(5)
2.316(4)
89.45(19)
90.33(19)
177.74(19)
96.24(18)
95.83(18)
Cu(1)–N(3)
2.006(5)
2.015(5)
2.690(6)
167.93(18)
90.60(18)
89.15(18)
92.2(2)
1
Cu(1)–N(9)
Cu(1)–O(1)
Cu(1)–N(6)b
O1–H1AꢀꢀꢀO10a
O10–H10BꢀꢀꢀO9b
O11–H11BꢀꢀꢀO7d
O11–H11AꢀꢀꢀO4c
0.88
0.85
0.85
0.85
1.964
2.206
2.342
2.501
2.713(1)
2.958(1)
2.984(1)
3.121(1)
142
147
133
130
Cu(1)–O(11)
N(12)a–Cu(1)–N(3)
N(3)–Cu(1)–N(9)
N(3)–Cu(1)–N(6)b
N(12)a–Cu(1)–O(1)
N(9)–Cu(1)–O(1)
N(12)a–Cu(1)–N(9)
N(12)a–Cu(1)–N(6)b
N(9)–Cu(1)–N(6)b
N(3)–Cu(1)–O(1)
N(6)b–Cu(1)–O(1)
2
O3–H3ꢀꢀꢀO2a
0.82
1.873
2.648(3)
157
90.1(2)
3
2
O4–H4ꢀꢀꢀN5b
O2–H2ꢀꢀꢀO5a
O2–H2ꢀꢀꢀO6a
0.82
0.82
0.82
2.004
1.681
2.594
2.822(1)
2.488(2)
3.193(1)
176
167
131
Cu(1)–O(1)
O(1)–Cu(1)–O(1)a
O(1)–Cu(1)–N(1)
1.9624(14)
180.00(9)
89.31(7)
Cu(1)–N(1)
1.9796(18)
90.69(7)
180.00(9)
O(1)–Cu(1)–N(1)a
N(1)a–Cu(1)–N(1)
b
c
Symmetry codes for 1: a1/2ꢁx, 1/2 + y, ꢁ1/2 + z; ꢁx, ꢁy, 1/2 + z; ꢁx, 1ꢁy, 1/2 + z;
3
Cu(1)–N(6)a
2.0117(17)
2.0179(18)
179.999(1)
90.09(7)
97.223(2)
82.777(2)
180.0
Cu(1)–O(1)
2.5229(1)
ꢁx, 1ꢁy, ꢁ1/2 + z. For 2: ꢁx + 3, ꢁy + 1, ꢁz. For 3: ax + 1, y, z; ꢁx + 2, ꢁy + 1,
d
a
b
Cu(1)–N(3)
ꢁz + 1.
N(6)a–Cu(1)–N(6)b
N(6)b–Cu(1)–N(3)c
O(1)–Cu(1)–N(3)c
O(1)–Cu(1)–N(3)
O(1)–Cu(1)–O(1)d
N(6)a–Cu(1)–N(3)c
N(3)c–Cu(1)–N(3)
O(1)–Cu(1)–N(6)a
O(1)–Cu(1)–N(6)b
89.91(7)
180.0
93.413(2)
86.578(2)
stretch vibrations of the carboxylate group. Therefore, only the
characteristic bands at 1288, 1283 and 1285 cmꢁ1 for 2–4 were ob-
served. The perchlorate anion in 1 presents the characteristic
bands at 1131 and 625 cmꢁ1, which are similar to those found in
ionic perchlorate species [48,49].
4
Cu(1)–O(6)
1.9701(13)
1.9863(17)
2.3299(14)
153.67(6)
91.52(6)
91.32(6)
119.27(5)
87.34(6)
Cu(1)–O(1)
Cu(1)–N(3)
1.9803(13)
1.9959(17)
Cu(1)–N(6)a
Cu(1)–O(5)a
O(6)–Cu(1)–O(1)
O(1)–Cu(1)–N(6)a
O(1)–Cu(1)–N(3)
O(6)–Cu(1)–O(5)a
N(6)a–Cu(1)–O(5)a
O(6)–Cu(1)–N(6)a
O(6)–Cu(1)–N(3)
N(6)a–Cu(1)–N(3)
O(1)–Cu(1)–O(5)a
N(3)–Cu(1)–O(5)a
92.66(6)
88.97(6)
170.12(6)
86.88(5)
83.37(6)
3.2. Structural analysis of {[Cu(btp)2(CH3OH)(H2O)]ꢀH2Oꢀ2ClO4}n (1)
Single-crystal X-ray analysis reveals that complex 1 is an inter-
esting Cu-btp two-dimensional (2D) grid layer (Fig. 1a), exhibiting
a well-known CuCl4-like (4, 4) topology [18]. The asymmetric unit
of 1 contains one Cu(II) atom, two neutral btp ligands, one terminal
CH3OH, two water molecule (one is coordinateꢁd and the other one
is lattice water molecule), and two free ClO4 anions for charge
compensation. Each Cu(II) atom is in a distorted octahedron, sur-
rounded by four N atoms from two pairs of head-to-tail arranged
btp ligands in the equatorial plane, one coordinated CH3OH and
one water molecule in the axial positions. The equatorial Cu–N
bonds are generally shorter than those of axial Cu–O bonds (see
Symmetry codes for 1: ax, yꢁ1, z; x, y, zꢁ1. For 2: ꢁx + 5/2, ꢁy + 1/2, ꢁz. For 3:
b
a
ꢁx + 1, ꢁy, ꢁz + 1; bx, y + 1, z; cꢁx + 1, ꢁy + 1, ꢁz + 1; dꢁx + 3, ꢁy + 1, ꢁz + 1. For 4: a
a
ꢁx + 1, ꢁy + 1, ꢁz.
of carboxylate groups in the monodentate and bridging bidentate
fashions [46,47]. Additionally, the stretching vibrations of the btp
triazole ring were observed in complexes 1–4. For complex 1, they
locate at 1538 and 1290 cmꢁ1. However, the strong band at ca.
1561–1537 for triazole in 2–4 is overlapped with the asymmetric

E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
311
Fig. 1. (a) 2D layer of 1 with atomic labels in the asymmetric unit (hydrogen atoms are omitted for clarity); (b) a dense ‘‘double layer” formed by OꢁHꢀꢀꢀO hydrogen-bonding
interactions.
Table 2). Two crystallographically independent btp ligands obvi-
ously adopt two conformations. The first btp ligand containing
N1–N6 exhibits a trans–gauche conformation, in which the dihe-
dral angle of two triazole rings is 69.573° and the longest NꢀꢀꢀN sep-
aration from the two triazole rings is 8.2065(3) Å. In contrast, the
second btp molecule containing N7–N12 is in a trans–trans, the
dihedral angle and the longest NꢀꢀꢀN separation of the two triazole
rings are 86.385° and 8.7607(3) Å, respectively.
As shown in Fig. 1a, two pairs of head-to-tail arranged btp li-
gands connect the neighboring Cu(II) ions in a exo-bidentate bridg-
ing mode. Thus an undulated 2D layer containing distorted 40-
membered [Cu4btp4] metallomacrocycles is formed with the
CuꢀꢀꢀCu distances of 12.58 and 12.02 Å corresponding, respectively
to the crystallographic b and c axes. Notably, such the 2D layer mo-
tif is isostructural to the early reported complexes {[Cu(btp)2-
(CH3CN)(H2O)]ꢀ2CF3SO3}n and {[Cu(btp)2(CH3CN)2]ꢀ2ClO4}n [18].
Furthermore, two adjacent layers are connected into a dense ‘‘dou-
ble layer” by the hydrogen-bonding interactions between the per-
chlorate anions/lattice water molecules and coordinated
methanol/water molecules (Fig. 1b). And the distance between
the adjacent layers is 1.77 Å calculated from the center of a square
grid generated by four Cu(II) atoms in one layer to the nearest
Cu(II) atom in the adjacent layer, which is smaller than the re-
ported complexes ({[Cu(btp)2(CH3CN)(H2O)]ꢀ2CF3SO3}n and
{[Cu(btp)2(CH3CN)2]ꢀ2ClO4}n: 2.12 and 1.98 Å) [18]. Thus, in addi-
tion to charging compensation, ClO4ꢁ anions in 1 are helpful to ex-
tend the higher dimensional ordered supramolecular architecture.
the central atom and the rotational freedom of the propane linker
in the btp ligand. Unexpectedly, rather than a versatile bridging li-
gand, the H3btc in 2 is doubly deprotonated and adopts a mono-
dentate fashion to coordinate terminally with Cu(II) atom by one
of the deprotonated carboxylate group. Instead, the other carboxyl-
ate O atom and carboxylic group can alternately act as the H-bond
acceptor and donor, assembling the 1D infinite chains into a 3D
interdigitated supramolecular framework by classic O–HꢀꢀꢀO
hydrogen-bonding interaction (Fig. 2b and Table 3). Topologically,
when the Cu(II) ions are considered as nodes, bidentate btp ligand
and H-bonded Hbtc2ꢁ dimer are as different linkers, the whole
structure of 2 can be simplified as a CdS-type network showing a
four-connected (65 8) topology [50] (Fig. 2b).
3.4. Structural description of [Cu(btp)2(H3btea)2]n (3)
Complex 3 forms a 1D double-stranded chain running parallel
to [0 0 1]. Each Cu(II) ion is in a distorted octahedral coordination
environment and coordinates to four symmetry-related triazole
groups (Cu–N = 2.0116(17) and 2.0180(18) Å) that occupy the ver-
tices of an almost ideal square plane (see Table 2 and Fig. 3a). The
apical positions of the octahedron are occupied by carboxylate O
atoms (Cu–O = 2.0180(18) Å) of H3bteaꢁ anion and the Cu–O bond
forms an angle of 81.99° with respect to the square plane occupied
by the triazole nitrogen atoms. Two btp molecules are wrapped
around each other and are held together by Cu(II) atoms, forming
a
1D double-stranded chain with the CuꢀꢀꢀCu separation of
10.12(6) Å (Fig. 3b). The sole btp ligand in 3 adopts trans–gauche
conformation with the dihedral angle of the two triazole rings of
52.82° and the N3ꢀꢀꢀN6 separation of 7.4897(4) Å. The aromatic col-
igand in 3 is only monodeprotonated, coordinating with Cu(II)
atom in a monodentate fashion to complete the metal coordination
sphere. And the remained –COOH and –COOꢁ groups can act as O–
HꢀꢀꢀO hydrogen-bonding donor and acceptor to assemble and stabi-
lize the double-stranded chain into layer structure (see Fig. 3c and
Table 3).
3.3. Structural description of {[Cu(btp)(Hbtc)2] 0.5H2O} (2)
Complex 2 crystallizes in the monoclinic space group C2/c,
exhibiting an infinite 1D chain motif bridged by btp ligands. As
shown in Fig. 2a, the sole Cu(II) ion, locating at a crystallographic
inversion center, is four-coordinated to two carboxylate oxygen
atoms from two terminal Hbtc2ꢁ anions, and two N atoms from
two symmetry-related btp ligands, displaying a square-planar
geometry. The adjacent Cu(II) atoms are also joined together by
bridging exo-bidentate btp ligands along the a-axis into an infi-
nitely linear chain. The nearest CuꢀꢀꢀCu separation is 12.12 (16) Å,
which is slightly shorter than that (12.58 Å) in 1 due to the flexibil-
ity of the btp ligand. Interestingly, the flexible btp with a two-fold
rotation axis exhibits a trans–trans conformation, which is some-
what similar to the first btp ligand in 1. However, it should be
noted that the two triazole rings form a dihedral angle of 87.139°
and the N1ꢀꢀꢀN1B separation is 8.1615(11) Å. Obviously, these dif-
ferent structures arise from different coordination geometry of
3.5. Structural description of [Cu(btp)(nb)2] (4)
Rather than a polymeric nature, complex 4 is a centrosymmetric
binuclear structure (Fig. 4a). The unique Cu(II) ion is in a distorted
trigonal-bipiramidal coordination environment. Thus, the common
coordination polyhedrons of the Cu(II) ion are all observed in the
four complexes. In 4, the vertices of the triangle consist of three
carboxylate O atoms with the distance of Cu–O ranged from
1.9701(13) to 2.3299(13) Å, and the apices of the pyramid contain

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E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
Fig. 2. (a) 1D chain of 2 with atomic labels in the asymmetric unit (hydrogen atoms are omitted for clarity); (b) 3D supramolecular network of 2 formed by OꢁHꢀꢀꢀO
hydrogen-bonding interactions and its topological structure (btp and one of the carboxylate group of Hbtc2ꢁ were omitted for clarity).
two triazole
N
atoms (N3, N6A) (Cu–N = 1.9863(17) and
plexes, we performed the thermogravimetric study of the
complexes 2–4 at an inert atmosphere from the room tempera-
ture to 800 °C (Fig. 5). Notably, complex 1 is not included, be-
cause transition-metal perchlorate is potentially explosive.
Polymeric 2 of the three complexes is the most thermally stable
up to 300 °C. The disordered lattice water molecule, mixed li-
gands of btp and Hbtc2ꢁ in 2 are decomposed from 300 to
540 °C. The final residue is calculated to be CuO (obsd. 14.6%,
calcd. 13.9%). In contrast, 3 displays considerable thermal insta-
bility (ꢂ90 °C) than 2. The weight loss of 90.8% in 3 between
210 and 530 °C is corresponding to the decomposition of organic
ligands in continuous fashion (calcd. 91.7%). Binuclear 4 is more
stable than 1D double-stranded 3 by 30 °C. The mixed ligands
of btp and nbꢁ in 4 are decomposed from 240 to 590 °C, accom-
panying the weight loss of 87.2% (calcd. 88.0%). The remaining
weight of 12.8% should be CuO, which is in agreement with the
calculated value of 12.0%. Thus, the introduction of terminal aro-
matic coligands remarkably changes the thermal stability of the
resulting complexes.
1.9959(17) Å). The btp ligand exhibits an approximate C-shaped
structure, which conveniently coordinates with two symmetry-re-
lated Cu(II) ions with the separation of CuꢀꢀꢀCu of 4.2829(1) Å and is
much shorter than the reported btp-based complex [17–20,32].
Additionally, the btp ligand adopts a gauche–gauche conformation
with the dihedral angle and the longest NꢀꢀꢀN separation of the
two triazole rings of being 22.99° and 3.9688(1) Å. To our best
knowledge, such the btp ligand in 4 exhibits the largest torsion
upon connecting two adjacent Cu(II) atoms. On the other hand,
the carboxylate groups of nbꢁ adopt two coordination modes.
One is in a monodentate fashion to complete the metal coordina-
tion sphere and the other one exhibits a bis-monodentate mode
to join the other symmetry-related Cu1A to form a discrete binu-
clear structure.
CꢁHꢀꢀꢀ
p interactions between the carbon atoms of aromatic ring
(C16–H16) and the triazole ring of btp assemble the discrete binu-
clear structures into a 1D chain (Fig. 4b). The distances of H atom
d(HꢀꢀꢀCg) and C–donor D(CꢀꢀꢀCg) to the center of the triazole ring
(Cg) are 2.6886 and 3.5808(3) Å, respectively, and the angle h(C–
HꢀꢀꢀCg) is 161.0°. Furthermore, strong offset face-to-face
p–p stack-
3.7. Conformations of the flexible btp ligands and their stability
ing interactions can also be observed, which make the 1D chain
stack into a 2D supramolecular layer. The centroid-to-centroid dis-
tance of the packed benzene is 3.7845 Å. And the dihedral angle
between the rings is 0.00°.
Due to the free rotation of the C–C single bond in the propane
linker, btp ligand is expectable to show various conformations,
which can further induce the formation of the intriguing com-
plexes. Indeed, the flexible btp ligand in complexes 1–4 has exhib-
ited three popular conformations (trans–trans, trans–gauche, and
gauche–gauche), although it adopts only one exo-bidentate bridg-
ing binding mode. Herein, such the three conformations are further
distinguished by the dihedral angle and the longest NꢀꢀꢀN separa-
3.6. Thermal stability of complexes 2–4
To investigate the contribution of the rigid benzene-based car-
boxylate coligand on the thermal stability of the resulting com-

E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
313
Fig. 3. (a) Local coordination environment of Cu(II) atom in 3 with the atom labels in the asymmetric unit (hydrogen atoms are omitted for clarity); (b) 1D double-stranded
chain of 3 with the intrachain O4–H4ꢀꢀꢀN5 hydrogen-bonding interaction; (c) 2D network of 3 formed by classic OꢁHꢀꢀꢀO hydrogen-bonding interactions.
tion of the two triazole rings. As a result, five specific structures of
btp ligand are obtained. Undoubtedly, the variable conformations
of btp can be substantially influenced by the coordination sphere
of the metal ion and the presence of the coligands. For example,
btp ligands in complexes 1 and 3 locate at the equatorial plane
of the six-coordinated Cu(II) ion, and exhibit three different confor-
mations, which are probably due to the presence of the relatively
large coligands (H3bteaꢁ versus H2O/CH3OH). However, in 2–4
with different aromatic coligands, the spatial orientation and the
conformation of the btp ligand are also different from each other,
which may be resulted from the variable coordination polyhedrons
of the Cu(II) atoms.
To explore the relative stability of these btp structures as well
as to obtain the electron density of the heterocyclic N donors,
full geometry optimizations of the five btp structures and the
unsubstituted triazole for comparison were performed by density
functional theory (DFT) based on the geometrical parameters ob-
tained from X-ray crystal structure determinations. The
calculated electron density distributions for N donors in free
1,2,4-triazole and the five btp ligands were shown in Fig. 6.
And the electronic and the total energy of the btp are summa-
rized in Table 4. As shown in Table 4, both the electronic energy
(ꢁ601.4065203 a.u.) and the total energy (ꢁ601.221341 a.u.) of
the btp-1-II are the lowest among the five structures. Thus, if

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E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
Fig. 4. (a) Binuclear structure of 4; (b) 1D supramolecular chain of 4 formed by CꢁHꢀꢀꢀ
p weak interactions; (c) 2D layer of 4 produced by pꢀꢀꢀp interactions.
we consider btp-1-II as the energy reference, the stability order
of the five structures follows btp-1-II < btp-2 < btp-1-I < btp-
4 < btp-3, suggesting that the trans–trans conformation is much
stable than trans–gauche or gauche–gauche conformation. On
the other hand, the electron density distributions of the three
N
sites in unsubstituted triazole follows N4 > N1 > N2.
However, after the two triazole molecules were coupled by pro-
pane linker, the electron density distributions of the N donors
are changed significantly (Fig. 6). Except for btp-3, all the N1
donors of the btp ligands carry positive charge ranged from
0.0606 to 0.2127 e, which is in a sharp contrast to the N1 site
of the triazole (ꢁ0.2304 e). It should be noted that the electron
density of N4 donors in btp-1-II and btp-4 are much lower than
that N2 sites, although the N4 donors of the btp are consistently
coordinated to Cu(II) in the complexes 1–4, which may be as-
cribed to the steric hindrance of the btp ring in 1 and 4. Addi-
tionally, the electron density distributions of the N donors in
btp molecule are somewhat symmetry-dependent. Thus, it can
be concluded that the metal binding site of the btp ligand is
jointly determined by the electron density of N donors and the
steric hindrance of the ligand.
Fig. 5. TG curves for complexes 2–4.

E.-C. Yang et al. / Inorganica Chimica Acta 363 (2010) 308–316
315
Fig. 6. The optimized structures for 1,2,4-triazole and the btp ligands in 1–4 showing the calculated electron density distributions for N donors (H atoms are omitted for
clarity).
Table 4
The electronic and total energy of the five btp structures in complexes 1–4.*
Energy
btp-1-I
btp-1-II
btp-2
btp-3
btp-4
Ee (a.u.)
Ezero-point (a.u.)
Etotal (a.u.)
ꢁ601.4058575
0.185165
ꢁ601.220693
0.000648
ꢁ601.4065203
0.185180
ꢁ601.221341
ꢁ601.4063264
0.185438
ꢁ601.220888
0.000453
ꢁ601.3936903
0.185298
ꢁ601.4026716
0.185152
ꢁ601.208393
ꢁ601.217520
D
D
E (a.u.)
0.012948
0.003821
E (kJ molꢁ1
)
1.70
1.19
34.00
10.030
*
Etotal = Ee + Ezero-point
.
4. Conclusions
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