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L. Yang et al. / Inorganica Chimica Acta 406 (2013) 307–314
through harmonizing the subtle relationship between ‘‘robustness’’
and ‘‘flexibility’’ in these systems has attracted much recent inter-
est. As we know, flexible imidazole ligands with (–CH2–)n (n = 1, 2,
3) spacers as a critically important family of building blocks are
reliable candidates for assembly of diverse supramolecular com-
plexes with fascinating topologies (e.g., one-dimensional (1D)
chain or channel, two-dimensional (2D) network and three-dimen-
sional (3D) architecture) and peculiar physicochemical properties
owing to the excellent coordination ability of the imidazole ring
[16]. On the other hand, the flexible nature of the alkyl (–CH2–)n
spacer allows the ligands to bend and rotate freely so as to be used
as ‘‘organic clips’’ to bind transition metal ions into neutral bime-
tallic macrorings or cages [16d,17].
(1.54056 Å). The simulated PXRD spectra from single-crystal struc-
tures were carried out using Mercury (version 1.4.2, 2002) and
were compared to confirm the composition of the crystal with
the experimental PXRD pattern.
Unless otherwise noted, all reagents were obtained from com-
mercial suppliers and used without further purification. Tetrahy-
drofuran (THF) was dried by heating at reflux for at least 8 h
over sodium/benzophenone and freshly distilled prior to use. Li-
gands 1, 2, and 3 were synthesized according to our previously re-
ported method [21].
2.2. Characterization of ligands 1–3
Recently, we have designed and synthesized a series of bent re-
stricted bidentate pyrazole bridging spacers coordinated with sil-
ver salts for construction of dinuclear metallocyclic complexes
[18]. As part of our ongoing interests in the N-heterocyclic chemis-
try [19,20], we have also used the flexible bismethylimidazolyl
derivatives self-assembling with Pd(OAc)2 to generate a switching
of metal–organic vesicles to globular networks depending on
whether the ligands bear the hydroxyl groups [21]. Considering
the bent geometry of such spacers, we wondered if these ligands
could bind metal ions to give new self-assembly architectures
and topologies such as dinuclear metallocycles. In this study, we
select the flexible exo-bidentate imidazole-containing ligands
1–3 (Scheme 1) and different divalent palladium salts to synthesize
three new metal–organic assemblies, namely, {[Pd2(1)4](NO3)4-
Á2DMSOÁ2H2O}n (4), [Pd2(2)2Br4]n (5), and {[Pd2(3)2Cl4]Á4DMSO}n
(6). The new compounds have been characterized by 1H NMR spec-
troscopy, elemental analysis and X-ray crystallography. The results
show that all of these supramolecular complexes are composed of
the discrete dinuclear metallocyclic units. Besides the highly
essential metal–ligand interactions that are basically responsible
for coordination assemblies, there exist a variety of other second-
2.2.1. 1,3-Bis(imidazol-1-ylmethyl)benzene (1)
Pale yellow solid. Yield: 56.3%. M.p. 53–55 °C. 1H NMR
(400 MHz, CDCl3): d 5.07 (s, 4H), 6.85 (dd, J = 1.2 Hz, 1.2 Hz, 2H),
6.90 (s, 1H), 7.06–7.09 (m, 4H), 7.30 (t, J = 5.9 Hz, 1H), 7.50 (s,
2H) ppm. 13C NMR (100 MHz, CDCl3): d 50.4, 119.3, 125.9, 127.1,
129.7, 129.8, 137.2, 137.4 ppm. MS (ESI) m/z: 239 [M+H]+.
2.2.2. 1,3-Bis(imidazol-1-ylmethyl)-5-methylbenzene (2)
Pale white solid. Yield: 67.9%. M.p. 105–106 °C. 1H NMR
(400 MHz, CDCl3): d 2.27 (s, 3H), 5.02 (s, 4H), 6.71 (s, 1H), 6.85
(dd, J = 1.2 Hz, 1.6 Hz, 2H), 6.86–6.87 (m, 2H), 7.05 (dd, J = 1.2 Hz,
1.2 Hz, 2H), 7.50 (s, 2H) ppm. 13C NMR (100 MHz, CDCl3): d 21.2,
50.4, 119.3, 123.2, 127.8, 129.7, 137.1, 137.3, 139.8 ppm. MS (ESI)
m/z: 253 [M + H]+.
2.2.3. 2,6-Bis(imidazol-1-ylmethyl)-4-tert-butylphenol (3)
Pale yellow solid. Yield: 42.7%. M.p. 158–160 °C. 1H NMR
(400 MHz, DMSO-d6): d 1.15 (s, 9H), 5.17 (s, 4H), 6.87 (s, 2H),
7.02 (s, 2H), 7.14 (s, 2H), 7.69 (s, 2H), 9.23 (s, 1H) ppm. 13C NMR
(100 MHz, DMSO-d6): d 31.6, 34.2, 45.9, 120.1, 125.6, 126.6,
128.7, 137.9, 142.9, 150.5 ppm. MS (ESI) m/z: 310 [M]+.
ary interactions such as
p p stacking interactions, hydrogen
Á Á Á
bonds (e.g., O–HÁ Á ÁO, C–HÁ Á ÁO, C–HÁ Á ÁBr), which also play signifi-
cant roles in the aspect of extended arrays of these metallocyclic
building motifs into high-dimensional supramolecular frame-
works. More interestingly, the structural versatility of resulting
aggregates strongly depends on the presence of additional func-
tional sites in the aryl ring of chosen ligands.
2.3. Synthesis of {[Pd2(1)4](NO3)4Á2DMSOÁ2H2O}n (4)
A DMSO solution (2 mL) of Pd(NO3)2Á2H2O (26.6 mg, 0.1 mmol)
was added slowly with constant stirring to a DMSO solution (2 mL)
of 1 (23.8 mg, 0.1 mmol) to give an orange solution. The reaction
mixture was stirred for 30 min and was then filtered to remove a
trace amount of undissolved substance. Light yellow block
crystals suitable for X-ray analysis were obtained by slow diffusion
of acetone into the corresponding DMSO solution at ambient tem-
perature after several days. Yield: 48%. Anal. Calc. for C60H72N20
O16Pd2S2 (4): C, 44.86; H, 4.52; N, 17.44. Found: C, 44.50; H,
4.37; N, 18.07%.
2. Experimental
2.1. General remarks
1H NMR spectra were obtained with
a Bruker AV-400
(400 MHz) spectrometer, while 13C NMR spectra were recorded
with a Bruker AV-400 (100 MHz) spectrometer. Mass spectra were
obtained on a Finnigan-LCQDECA instrument. Elemental analyses
were performed with a CARLO ERBA1106 instrument. Melting
points were determined with XRC-1 and are uncorrected. Thermal
analyses were performed on a TA Q500 from room temperature to
750 °C with a heating rate of 10 °C minÀ1 under a N2 atmosphere.
Powder X-ray diffraction (PXRD) patterns were collected on a Phi-
2.4. Synthesis of [Pd2(2)2Br4]n (5)
The title compound was prepared by the same procedure as
compound 4 using 2 (25.2 mg, 0.1 mmol) instead of 1 and PdBr2(-
CH3CN)2 (34.8 mg, 0.1 mmol) instead of Pd(NO3)2Á2H2O. Orange
block crystals were obtained. Yield: 75%. Anal. Calc. for C30H32N8-
Pd2Br4 (5)Á2DMSO: C, 34.22; H, 3.72; N, 9.39. Found: C, 33.46; H,
3.68; N, 9.27%.
lips X’PERT Pro MPDX powder diffractometer with Cu Ka radiation
CH3
t-butyl
N
N
N
N
N
N
N
N
N
N
N
N
OH
1
2
3
Scheme 1. Chemical structures of ligands 1–3.