Halogen effects on photoluminescence and catalytic properties: A series of spatially arranged trimetallic zinc(ii) complexes

Article abstract of DOI:10.1039/c3dt53137f

Self-assembly of ZnX₂ (X = Cl, Br, and I) with N,N′,N′′-tris(2-pyridinylethyl)-1,3,5-benzenetricarboxamide (L) as a tridentate N-donor ligand yields discrete C₃-symmetric trimetallic zinc(ii) complexes, [Zn₃X₆L(MeOH) ₃]. These form, via π...π interactions and NH...OC hydrogen-bonds, an ensemble constituting a unique columnar stacking structure in an abab... staggered fashion. For this series of complexes, the halogen effects on the photoluminescence, catalysis, and thermal properties were investigated. For [Zn₃Cl₆L(MeOH)₃], a blue luminescence was observed at 462 nm (λ_ex = 369 nm). The transesterification catalysis showed significant halogen effects in the order [Zn₃I₆L(MeOH)₃] > [Zn₃Cl ₆L(MeOH)₃] > [Zn₃Br₆L(MeOH) ₃] in methanol, whereas in a mixture of methanol and acetonitrile, the order was [Zn₃I₆L(MeOH)₃] > [Zn ₃Br₆L(MeOH)₃] > [Zn₃Cl ₆L(MeOH)₃]. Such notable different effects among the three complexes might be explained by the halogens' electronic effects and dissociation properties.

Full text of DOI:10.1039/c3dt53137f

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Halogen effects on photoluminescence and
catalytic properties: a series of spatially arranged
trimetallic zinc(II) complexes†
Cite this: Dalton Trans., 2014, 43,
3842
Haeri Lee, Tae Hwan Noh and Ok-Sang Jung*
Self-assembly of ZnX2 (X = Cl, Br, and I) with N,N’,N’’-tris(2-pyridinylethyl)-1,3,5-benzenetricarboxamide
(L) as a tridentate N-donor ligand yields discrete C3-symmetric trimetallic zinc(II) complexes, [Zn3X6L-
(MeOH)3]. These form, via π⋯π interactions and NH⋯OvC hydrogen-bonds, an ensemble constituting a
unique columnar stacking structure in an abab⋯ staggered fashion. For this series of complexes, the
halogen effects on the photoluminescence, catalysis, and thermal properties were investigated. For
[Zn3Cl6L(MeOH)3], a blue luminescence was observed at 462 nm (λex = 369 nm). The transesterification
catalysis showed significant halogen effects in the order [Zn3I6L(MeOH)3] > [Zn3Cl6L(MeOH)3] > [Zn3Br6L-
(MeOH)3] in methanol, whereas in a mixture of methanol and acetonitrile, the order was [Zn3I6L(MeOH)3]
> [Zn3Br6L(MeOH)3] > [Zn3Cl6L(MeOH)3]. Such notable different effects among the three complexes
might be explained by the halogens’ electronic effects and dissociation properties.
Received 6th November 2013,
Accepted 19th December 2013
DOI: 10.1039/c3dt53137f
www.rsc.org/dalton
trimetallic complexes have been synthesized in recent years,
especially those with a tridentate core.19 Their hyper-branched
Introduction
Diverse polypyridyl N-donor ligands that can coordinate two or structural and chemical features, with their advanced material
more remote metal centers have been used to construct desir- and catalytic properties, have been a good prototype for gene-
able molecular structures.1–4 Current applications of such ration of multifunctional properties.20 Thus, systematic
polynuclear complexes span a wide range of interesting fields C3-symmetric trimetallic complexes offer interesting physico-
such as mixed-valence species, photo-induced electron or chemical properties that have been found to be delicately
energy transfer, magnetic exchange between paramagnetic dependent on the nature of the ligands and co-ligands.
centers, and supramolecular weak interactions.5–10 Specifi-
In this context, in an investigation of the significant
cally, tridentate N-donor building tectonics have been halogen effects of C3-symmetric trimetallic complexes, ZnX2
employed to synthesize cage coordination complexes or C3- (X = Cl, Br, and I) was reacted with C3-symmetric N,N′,N″-tris-
symmetric triangular module metal complexes.11–18 Trimetal- (2-pyridinylethyl)-1,3,5-benzenetricarboxamide (L).16 Further
lic complexes via tridentate ligand metal ions are relatively to this, a systematic series of trimetallic zinc(II) complexes and
rare, unlike the vast majority of dinuclear complexes. Unique their one-dimensional (1D) ensemble crystal structures
tridentate ligands containing pyridyl moieties, for example, formed via intermolecular forces, along with the direct
offer, via the introduction of appropriate metal ions, wider halogen effects on their photoluminescence, transesterification
opportunities for a trigonal metallic array. A number of such catalytic activity, and thermal properties, are reported
herein. Zinc(II) complexes have been extensively examined
for their use in appropriate tetrahedral binding Lewis acidity,
metallo-enzymes, zinc finger proteins, transmetallation, and
Department of Chemistry, Pusan National University, Pusan 609-735, Korea.
homogeneous catalysis.21–28 A variety of zinc(II) coordina-
E-mail: oksjung@pusan.ac.kr; Fax: +82 51 516 7421; Tel: +82 51 510 2591
†Electronic supplementary information (ESI) available: The distances of weak
tion complexes containing aromatic moieties exhibit
unique ligand-to-metal charge transfer luminescent properties
that are strongly dependent on the geometry of the
ligands and counteranions.23,25 Furthermore, zinc(II) com-
pounds with functional organic ligands can potentially be
used in light-emitting-diode and heterogeneous-catalysis
applications.22
inter- and intra-molecular interactions, ORTEP images, solid-state excitation
spectra, photoluminescence spectra in Me2SO of [Zn3Cl6L(MeOH)3], [Zn3Br6L-
(MeOH)3], and [Zn3I6L(MeOH)3]; partial 1H NMR spectra showing catalytic
effects for transesterification of phenyl acetate using [Zn3I6L(MeOH)3]; SEM
images of calcined [Zn3I6L(MeOH)3] at 200 °C for 1 h, 400 °C for 1 h. CCDC
970293–970295. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt53137f
3842 | Dalton Trans., 2014, 43, 3842–3849
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Transesterification catalysis
Experimental section
Phenyl acetate (2.72 g, 2 mmol) was dissolved in methanol
(15 mL) or a mixture of methanol and acetonitrile (15 mL, v/v
= 1 : 1), and the catalysts (0.1 mmol; 34 mg for [Zn3Cl6L-
(MeOH)3]; 43 mg for [Zn3Br6L(MeOH)3]; 52 mg for [Zn3I6L-
(MeOH)3]) were added to the reaction solution. The reaction
solution was stirred at 50 °C for both 4 h and 6 h. The reaction
using [Zn3I6L(MeOH)3] was achieved at reflux temperature. The
conversion yields were monitored with reference to the 1H
NMR spectra. All of the reactions were run at least three times
and the conversion yields are averaged.
Materials and measurements
All chemicals including zinc(II) chloride, zinc(II) bromide,
zinc(II) iodide, and phenyl acetate were purchased from
Aldrich, and used without further purification. N,N′,N″-Tris-
(2-pyridinylethyl)-1,3,5-benzenetricarboxamide (L) was pre-
pared according to the method reported in the literature.16
Elemental microanalyses (C, H, N) were performed on crystal-
line samples at the KBSI Pusan Center using a Vario-EL III.
Thermal analyses were undertaken under a nitrogen atmos-
phere at a scan rate of 10 °C min−1 using a Labsys TGA-DSC
1600. The infrared spectra of samples prepared as KBr pellets
were obtained on a Nicolet 380 FT-IR spectrophotometer. The Crystal structure determination
1H NMR (300 MHz) spectra were recorded on a Varian Mercury
X-ray data were collected on a Bruker SMART automatic diffr-
Plus 300. Scanning electron microscopy (SEM) images were
obtained on a Tescan VEGA 3. Powder X-ray diffraction data
were recorded on a Rigaku RINT/DMAX-2500 diffractometer at
40 kV, 126 mA for Cu Kα. Excitation and emission spectra were
acquired on a FluoroMate FS-2 spectrofluorometer. Fluo-
rescence microscopy images were obtained with an Olympus
BX51 microscope equipped with an AxioCam MRc 5 digital
camera and a U-MWU2 filter set.
actometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) and a CCD detector at −25 °C. Thirty-
six frames of two-dimensional diffraction images were col-
lected and processed to obtain the cell parameters and orien-
tation matrix. The data were corrected for Lorentz and
polarization effects. The absorption effects were corrected
using the multi-scan method (SADABS).29 The structures were
solved using direct methods (SHELXS 97) and refined by full-
matrix least squares techniques (SHELXL 97).30 The non-
hydrogen atoms were refined anisotropically, and the hydrogen
atoms were placed in calculated positions and refined only for
the isotropic thermal factors. The crystal parameters along
with procedural information on the data collection and struc-
ture refinement are listed in Table 1.
Preparation of [Zn3Cl6L(MeOH)3]
A methanol solution (5 mL) of N,N′,N″-tris(2-pyridinylethyl)-
1,3,5-benzenetricarboxamide (L) (78 mg, 0.15 mmol) was
added to a methanol solution (5 mL) of zinc(II) chloride
(61 mg, 0.45 mmol). Slow evaporation of the solvent produced
colorless crystals of [Zn3Cl6L(MeOH)3] in a 76% (117 mg) yield
after 3 days. mp 300 °C (dec.). Found: C, 38.3; H, 4.2; N, 8.1.
Calc. for C33H42Cl6N6O6Zn3: C, 38.6; H, 4.2; N, 8.2%. IR (KBr
pellet, cm−1): 3585 (br), 3506 (br), 3367 (br), 3131 (br), 1635 (s),
1608 (s), 1548 (s), 1488 (s), 1440 (s), 1363 (m), 1307 (s),
1193 (w), 1160 (w), 1064 (w), 1024 (w), 779 (m).
Results and discussion
Synthesis
The reaction of ZnX2 (X = Cl, Br, and I) with N,N′,N″-tris(2-pyri-
dinylethyl)-1,3,5-benzenetricarboxamide (L) as a C3-axis triden-
tate N-donor in methanol at room temperature afforded
discrete C3-symmetric trimetallic zinc(II) complexes, [Zn3X6L-
Preparation of [Zn3Br6L(MeOH)3]
This product was prepared in the same manner as [Zn3Cl6L- (MeOH)3], in high yields, as shown in Scheme 1. The reactions
(MeOH)3], using ZnBr2 instead of ZnCl2 in 73% (141 mg) yield. were initially conducted in the 1 : 1 mole ratio of zinc(II) : L, but
mp 303 °C (dec.). Found: C, 31.0; H, 3.2; N, 6.4. Calc. for the products were not significantly affected by either the mole
C33H42Br6N6O6Zn3: C, 30.6; H, 3.3; N, 6.5%. IR (KBr pellet, ratio or concentrations, which indicated that they were thermo-
cm−1): 3500 (br), 3363 (br), 3077 (br), 1635 (s), 1608 (s), dynamically stable species. These products are remarkable in
1542 (s), 1488 (m), 1440 (m), 1307 (m), 1159 (w), 1064 (w), that there was no evidence for coordination-polymerization,
1024 (w), 775 (m).
not even at high concentrations and despite the lack of protec-
tive groups. Thus, the discrete trimetallic complexes’ for-
mation might be attributable to the intrinsic properties of the
reaction system. The crystalline products were not obtained in
Preparation of [Zn3I6L(MeOH)3]
The product also was prepared in the same manner as ethanol, n-propanol, or i-propanol instead of methanol. The
[Zn3Cl6L(MeOH)3], using ZnI2 instead of ZnCl2 in 72% colorless crystalline products of [Zn3X6L(MeOH)3] were air-
(170 mg) yield. mp 303 °C (dec.). Found: C, 25.2; H, 2.6; N, 5.3. stable and dissociated in methanol, ethanol, N,N-dimethyl-
Calc. for C33H42I6N6O6Zn3: C, 25.1; H, 2.7; N, 5.3%. IR (KBr formamide, and dimethyl sulfoxide, but insoluble in water,
pellet, cm−1): 3509 (br), 3378 (br), 3081 (br), 2798 (w), 1633 (s), n-hexane, dichloromethane, chloroform, and acetone. The
1608 (s), 1542 (s), 1486 (m), 1438 (m), 1307 (m), 1159 (w), results of an elemental analysis and the products’ NMR
1064 (w), 1025 (w), 1002 (m), 759 (m).
spectra were consistent with desirable structures.
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Table 1 Crystallographic data and structure refinement for [Zn3Cl6L(MeOH)3], [Zn3Br6L(MeOH)3], and [Zn3I6L(MeOH)3]
[Zn3Cl6L(MeOH)3]
33H42Cl6N6O6Zn3
1027.54
Hexagonal
P63
32.1306(4)
7.3880(1)
6605.3(2)
6
[Zn3Br6L(MeOH)3]
[Zn3I6L(MeOH)3]
Formula
Mw
Crystal system
Space group
a/Å
C
C33H42Br6N6O6Zn3
1294.30
Hexagonal
P63
31.5420(3)
7.6862(1)
6622.5(1)
6
C33H42I6N6O6Zn3
1576.24
Hexagonal
P63
31.9496(2)
8.0211(1)
7090.8(1)
6
c/Å
V/Å3
Z
ρ/g cm−3
µ/mm−1
F(000)
1.550
2.032
3132
1.947
7.094
3780
2.215
5.473
4428
Rint
0.0824
1.064
0.0448
0.1045
0.0629
1.115
0.0419
0.0890
0.0427
1.188
0.0529
0.0923
GoF on F2
R1 [I > 2σ(I)]a
wR2 (all data)b
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = (∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2])1/2
.
Scheme 1 Synthetic scheme of trimetallic zinc(II) complexes (a). Top
view (b) and side view (c) of columnar stacking structure.
Fig. 1 Top views of single (left) and double (right) molecules of
[Zn3Cl6L(MeOH)3] (a), [Zn3Br6L(MeOH)3] (b), and [Zn3I6L(MeOH)3] (c).
Crystal structures
The crystal structures of [Zn3X6L(MeOH)3] are basically iso-
structural hexagonal P63 space groups, as depicted in Fig. 1
and S1 (see ESI†), and the relevant bond lengths and angles
are listed in Table 2. Their bond lengths and angles around
the zinc(II) ion differ slightly, presumably owing to the
different electronic and steric effects of the halides. The local
geometry around the zinc(II) ion approximates to a typical tetra-
hedral arrangement with two halides (X–Zn–X = 116.29(5)–
127.00(5)°), a nitrogen from L, and an oxygen from the metha-
nol molecule. The L connects three zinc(II) ions to form C3-axis
trimetallic [Zn3X6L(MeOH)3]. The methanol molecule acts as
the fourth ligand rather than a simple solvate molecule. The
intramolecular Zn⋯Zn distances of [Zn3Cl6L(MeOH)3],
[Zn3Br6L(MeOH)3], and [Zn3I6L(MeOH)3] are 11.981(3)–12.0573(8)
Å, 12.304(1)–12.521(1) Å, and 12.509(2)–12.675(2) Å, respect-
ively. Intramolecular interactions of MeOH⋯OvC (1.78–1.79 Å
for [Zn3Cl6L(MeOH)3]; 1.81–1.85
Å for [Zn3Br6L(MeOH)3];
1.74–1.91 Å for [Zn3I6L(MeOH)3]) exist in the solid state
(Fig. 1). All of the products showed an infinite 1D column
ensemble via close intermolecular π⋯π contacts (3.6839(5)–
3844 | Dalton Trans., 2014, 43, 3842–3849
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Table 2 Selected bond distances (Å) and bond angles (°) for [Zn3Cl6L-
(MeOH)3], [Zn3Br6L(MeOH)3], and [Zn3I6L(MeOH)3]
[Zn3Cl6L-
(MeOH)3]
[Zn3Br6L-
(MeOH)3]
[Zn3I6L-
(MeOH)3]
Zn(1)–N(1)
Zn(1)–O(2)
Zn(1)–X(1)
Zn(1)–X(2)
Zn(2)–N(3)
Zn(2)–O(4)
Zn(2)–X(3)
Zn(2)–X(4)
Zn(3)–N(5)
Zn(3)–O(6)
Zn(3)–X(5)
Zn(3)–X(6)
2.061(3)
1.988(3)
2.199(1)
2.229(1)
2.121(4)
2.018(4)
2.225(2)
2.179(2)
2.053(3)
2.015(3)
2.194(1)
2.225(1)
2.043(5)
2.050(5)
2.338(1)
2.368(1)
2.040(4)
2.044(5)
2.3419(8)
2.3729(9)
2.036(4)
2.053(5)
2.3398(8)
2.3769(9)
2.044(7)
2.052(6)
2.569(1)
2.529(1)
2.058(7)
2.032(7)
2.577(1)
2.523(1)
2.042(7)
2.026(6)
2.536(1)
2.578(1)
N(1)–Zn(1)–O(2)
N(1)–Zn(1)–X(1)
N(1)–Zn(1)–X(2)
X(1)–Zn(1)–X(2)
N(3)–Zn(2)–O(4)
N(3)–Zn(2)–X(3)
N(3)–Zn(2)–X(4)
X(3)–Zn(2)–X(4)
N(5)–Zn(3)–O(6)
N(5)–Zn(3)–X(5)
N(5)–Zn(3)–X(6)
X(5)–Zn(3)–X(6)
110.2(1)
110.1(1)
109.0(1)
118.7(5)
107.8(2)
110.7(1)
109.0(1)
120.1(9)
107.8(1)
108.93(9)
110.61(9)
120.84(5)
112.8(2)
107.0(2)
108.5(2)
127.00(5)
106.2(2)
110.32(1)
112.5(1)
118.80(4)
102.5(2)
111.9(1)
112.8(1)
118.59(4)
103.1(3)
112.8(2)
112.5(2)
116.29(5)
111.2(3)
104.9(2)
111.5(2)
124.38(5)
100.2(3)
112.9(2)
114.6(2)
116.90(5)
3.7041(5)
Å for [Zn3Cl6L(MeOH)3]; 3.83(4)–3.86(4) Å for
[Zn3Br6L(MeOH)3]; 3.954(4)–4.011(4) Å for [Zn3I6L(MeOH)3]) and
intermolecular hydrogen-bondings (2.21–2.25 Å or [Zn3Cl6L-
(MeOH)3]; 2.30–2.39 Å for [Zn3Br6L(MeOH)3]; 2.36–2.69 Å for
[Zn3I6L(MeOH)3]) between NH⋯OvC in an abab⋯ layer
(Fig. 2). The π⋯π interactions and hydrogen-bondings were
comparable to the results reported by Valencia et al.31 and Jung
et al.,15 respectively. The columns were arranged in a hexagonal
packing mode, as depicted in Fig. 3. Thus, the slightly different
bond lengths and angles showed significant halogen effects. No
other exceptional features, neither with respect to bond lengths
nor to angles, were observed. The three tetrahedral zinc(II) moi-
eties are in a propeller-like arrangement in the same upside
direction from the central benzene. Accordingly, [Zn3Br6L-
(MeOH)3] and [Zn3I6L(MeOH)3] are chiral crystalline solids con-
sisting of two-column P-helices and one-column M-helices
(where P denotes right-handed helices and M denotes left-
handed helices), whereas [Zn3Cl6L(MeOH)3] is a crystalline solid
consisting of one-column P-helices and two-column M-helices.
All of the crystals in the reaction vessel were a mixture of chiral
crystals and enantiomeric crystals.
Fig. 2 Side view (top) and top view (bottom) of columnar [Zn3Cl6L-
(MeOH)3]. The pink and yellow dashed lines denote the intermolecular
π⋯π interactions and NH⋯OvC hydrogen-bonds, respectively.
factor in the formation of trimetallic complex crystals. What is
the critical driving force behind the formation of the discrete
trimetallic complexes rather than coordination polymers?
A combination of tetrahedral zinc(II) ions and the appropriate
tridentate N-donors seems to be a crucial factor. Specifically,
formation might be attributable to the ortho-pyridyl N-donor
of L. That is, such an N-donor might be an obstacle to the for-
mation of coordination polymers. Moreover, intramolecular
hydrogen-bonding between MeOH⋯OvC perhaps partially
contributes to the formation of discrete trimetallic species.
This would explain the key role of methanol in the self-assembly
system. Another important feature is that both the inter-
molecular NH⋯OvC hydrogen-bonds and the π⋯π interactions
can be attributed to the formation of the thermodynamically
Construction principle
By the reaction of ZnX2 with L in methanol, discrete C3-sym-
metric trimetallic zinc(II) complexes, [Zn3X6L(MeOH)3], not
coordination polymeric species, were obtained in high yields.
The present complexes were favorably formed irrespective of
concentrations or mole ratios, indicating the thermodynamic
stability of their skeletal structures. However, as noted already
in the “Synthesis” section above, a solvent is an important
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466 nm (λex = 401 nm) for [Zn3Br6L(MeOH)3], and 482 nm
(λex = 372 nm) for [Zn3I6L(MeOH)3]. The emission bands of
[Zn3Cl6L(MeOH)3] and [Zn3Br6L(MeOH)3] were blue-shifted
relative to the corresponding ligand L (λem = 480 nm; λex
=
421 nm), while [Zn3I6L(MeOH)3] was red-shifted. The ligand’s
intrinsic characteristics play a pivotal role in the photo-
luminescence mechanism of the present zinc(II) complexes.
These results suggest that the complexes’ emission bands can
be ascribed to the ligand-to-metal charge transfer (LMCT).33,34
The red-shift from [Zn3Cl6L(MeOH)3] to [Zn3I6L(MeOH)3] can
be explained by the electronic effects of halides and the coordi-
nation environments of the zinc(II) ions.35–37 And it is worth-
while, additionally, to mention that [Zn3Cl6L(MeOH)3] showed
much stronger emissions than those of [Zn3Br6L(MeOH)3] and
[Zn3I6L(MeOH)3] at room temperature. Such a significant
difference can be attributed to the effective increase of the
Zn–Cl bond rigidity and the reduction in the loss of energy
through radiation-less decay. However, the quantum yield of
[Zn3Cl6L(MeOH)3] (0.084%) is lower than that of the known
iridium(III) complex, FIrpic (0.42%).38 In order to achieve, via
the inter-molecular NH⋯OvC hydrogen-bonds and the π⋯π
interaction, the ensemble effect on the photoluminescence,
their solution spectra were measured in Me2SO, resulting in
partial dissociation (see ESI†).
Transesterification catalysis
In order to investigate the halogen effects on the catalytic reac-
tions, the [Zn3X6L(MeOH)3] series have been employed as a
homogeneous catalyst of the transesterification reaction of
phenyl acetate with methanol. Various catalysts available for
transesterification of a wide range of esters with alcohol under
mild conditions have been developed.27,28 In the present inves-
tigation, [Zn3X6L(MeOH)3] (X = Cl, Br, and I) (0.1 mmol) sig-
nificantly catalyzed the transesterification of phenyl acetate
Fig. 3 Packing diagrams of [Zn3Cl6L(MeOH)3] (a), [Zn3Br6L(MeOH)3] (b),
and [Zn3I6L(MeOH)3] (c). Red: M-helical column; blue: P-helical column.
stable columnar abab⋯ staggered stacking ensemble. As the
present system did not reveal significant angle strains, we con-
sidered the formation of the overall crystalline structure to be
due in part to the felicitous “weak interactions” rather than to
the characteristics of the “geometry around the zinc(II) ion”.
Photoluminescence
Great attention recently has been paid to d10 metal complexes,
given their variety of applications in chemical sensors, photo-
chemistry, and electroluminescent display.32 The excitation
and emission spectra of the present zinc(II) complexes then,
together with those of L, were measured in the solid state at
room temperature (Fig. 4 and S2, see ESI†). Emissions were
Fig. 4 Solid-state photoluminescence spectra of
[Zn3Cl6L(MeOH)3] (black solid line), [Zn3Br6L(MeOH)3] (red solid line),
and [Zn3I6L(MeOH)3] (blue solid line) at room temperature. Inset: fluo-
L (dashed line),
observed at 462 nm (λex = 421 nm) for [Zn3Cl6L(MeOH)3], rescent-microscopic image of [Zn3Cl6L(MeOH)3].
3846 | Dalton Trans., 2014, 43, 3842–3849
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(2 mmol) with an excess amount of methanol at 50 °C for 4 h Thermal properties
or 6 h, as depicted in Fig. 5. A control reaction without the
The thermogravimetric analysis results for [Zn3X6L(MeOH)3]
are plotted in Fig. 6. As can be seen, the skeletal structure was
stable up to 300 °C before collapsing within the 300–550 °C
range. The high collapse temperature can be attributed to the
angle-strain-free structure. The coordinated methanol mole-
cules began to evaporate at 104 °C (weight loss found: 9.2%;
calcd: 9.4%) for [Zn3Cl6L(MeOH)3], at 162 °C (found: 8.8%;
calcd: 7.4%) for [Zn3Br6L(MeOH)3], and at 171 °C (found:
6.2%; calcd: 6.1%) for [Zn3I6L(MeOH)3]. This triggered
catalysts, meanwhile, showed trace amounts of ester-converted
product. Another catalytic reaction with authentic ZnI2 exhibi-
ted 13.8% conversion yield at 50 °C for 6 h. The catalysis
1
process was monitored with reference to the H NMR spectra
(see ESI†). The trimetallic zinc(II) complexes showed, on cataly-
tic activity in methanol, significant halogen effects in the
order [Zn3I6L(MeOH)3]
> [Zn3Cl6L(MeOH)3] > [Zn3Br6L-
(MeOH)3]. The coordinated (activated) methanol molecule
might be a key factor in transesterification. Of course, the 6 h
catalysis showed a higher conversion than that of 4 h. It seems
that the catalytic effects, as plotted for the present case in
Fig. 5, are strongly dependent on the dissociation of the Zn–X
bonds. It is not easy to elucidate the exact mechanism of cata-
lysis at this stage, though the mechanism of metal-ion-cata-
lyzed transesterification probably involves electrophilic
activation of the carbon center of the carbonyl moiety by
binding of the metal to the carbonyl oxygen.28 If so, the vacant
site and the Lewis acidity of the zinc metal center, accordingly,
play important roles in this catalytic reaction. Thus, the
process of leaving group’s dissociation seems to be one of the
conversion-rate-determining steps. And if this is the case, the
catalysis reaction will show significant solvent effects. There-
fore, in the present study, the catalysis reaction was carried out
in a mixture of methanol and acetonitrile instead of (only)
methanol; the resulting activity order was [Zn3I6L(MeOH)3] >
[Zn3Br6L(MeOH)3]
> [Zn3Cl6L(MeOH)3]. That is, [Zn3I6L-
(MeOH)3] and [Zn3Br6L(MeOH)3] showed high activity in the
mixture solvent whereas [Zn3Cl6L(MeOH)3] demonstrated high
activity in the methanol. This result suggests that Zn–Cl is less
dissociated in the mixture solvent than in methanol whereas
Zn–I and Zn–Br are more dissociated in the mixture solvent
than in methanol. The catalysis showed significant halogen
effects both for the activity with respect to the solvent. With
[Zn3I6L(MeOH)3], the 4 h reflux-temperature catalytic reaction
in the mixture solvent effected complete conversion.
Fig. 5 Transesterification of phenyl acetate by methanol without cata-
lyst (a), in the presence of [Zn3Cl6L(MeOH)3] (b), [Zn3Br6L(MeOH)3] (c),
and [Zn3I6L(MeOH)3] (d) all at 50 °C, and in the presence of [Zn3I6L-
(MeOH)3] at reflux temperature (e). Black column: catalysis in methanol;
red column: mixture of methanol and acetonitrile (v/v = 1 : 1); filled:
catalysis for 4 h; dashed: catalysis for 6 h.
Fig. 6 Top: TGA curves of [Zn3Cl6L(MeOH)3] (black), [Zn3Br6L(MeOH)3]
(red), and [Zn3I6L(MeOH)3] (blue); bottom: morphologies of the residues
of [Zn3Cl6L(MeOH)3] (a), [Zn3Br6L(MeOH)3] (b), and [Zn3I6L(MeOH)3] (c)
after calcination at 600 °C for 4 h. Inset: powder XRD data for zinc(II)
oxide residue (white) and reference pattern (red) from ICDD database
(PDF no. 36-1451).
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significant halogen effects. In order to confirm the mor-
phology change of the crystals during the skeletal collapse, all
of the crystalline samples were monitored by SEM. It was found
that all of the crystals ultimately had changed to zinc(II) oxide
crystals, but that their morphologies differed according to the
halogens. Images of the zinc(II) oxide crystals are provided in
Fig. 6. Calcination for 1, 1, and 4 h at 200, 400, and 600 °C,
respectively, effected contraction via eruption of both co-
ordinated and burned organic molecules, thus producing the
characteristic morphologies (see ESI†). The SEM images revealed
that for [Zn3I6L(MeOH)3], the complex melted after the 200 °C
calcination, and that at 400 °C, the compound changed to zinc
(II) oxide micro-crystals of 1–5 µm size, which fact was confirmed
by SEM-EDS; finally, at 600 °C, differently shaped zinc(II) oxide
submicro-crystals formed. These results established that this
method is an efficient means of morphological control of zinc(II)
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Reaction of zinc(II) halides with a C3-symmetric tridentate
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