Enantiomeric two-fold interpenetrated 3D zinc(ii) coordination networks as a catalytic platform: significant difference between water within the cage and trace water in transesterification

Article abstract of DOI:10.1039/c7dt00217c

Self-assembly of Zn(ClO₄)₂ with 1,1,2,2-tetramethyl-1,2-di(pyridin-3-yl)disilane (L) as a bidentate N-donor gives rise to 3D coordination networks, [Zn(μ-OH)(L)]₃(ClO₄)₃·5H₂O (1·5H₂O), of unique, 10³-a srs net topology. An important feature is that two enantiomeric 3D frameworks, 4¹- and 4³-[Zn(μ-OH)(L)]₃(ClO₄)₃·5H₂O, are interpenetrated to form a racemic two-fold 3D network with cages occupied by two water molecules. Another structural characteristic is a C₃-symmetric planar Zn₃(μ-OH)₃ 6-membered ring with tetrahedral Zn(ii) ions. The steric hindrance of substrates and trace water effects on transesterification catalysis using the network have been scrutinized. The coordination network acts as a remarkable heterogeneous transesterification catalytic system that shows both the significant steric effects of substrate alcohols and momentous water effects. The substrate activity is in the order ethanol > n-propanol > n-butanol > iso-propanol > 2-butanol > tert-butanol. For the reaction system, solvate water molecules within the cages of the interpenetrated 3D frameworks do not decrease the transesterification activity, whereas the trace water molecules in the substrate alcohols act as obvious obstacles to the reaction.

Full text of DOI:10.1039/c7dt00217c

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Enantiomeric two-fold interpenetrated 3D zinc(II)
coordination networks as a catalytic platform:
significant difference between water within the
cage and trace water in transesterification†
Cite this: DOI: 10.1039/c7dt00217c
Eunkyung Choi, Minjoo Ryu, Haeri Lee and Ok-Sang Jung*
Self-assembly of Zn(ClO4)2 with 1,1,2,2-tetramethyl-1,2-di(pyridin-3-yl)disilane (L) as a bidentate
N-donor gives rise to 3D coordination networks, [Zn(μ-OH)(L)]3(ClO4)3·5H2O (1·5H2O), of unique, 103-a
srs net topology. An important feature is that two enantiomeric 3D frameworks, 41- and 43-[Zn(μ-OH)
(L)]3(ClO4)3·5H2O, are interpenetrated to form a racemic two-fold 3D network with cages occupied by
two water molecules. Another structural characteristic is a C3-symmetric planar Zn3(μ-OH)3 6-mem-
bered ring with tetrahedral Zn(II) ions. The steric hindrance of substrates and trace water effects on
transesterification catalysis using the network have been scrutinized. The coordination network acts as
a remarkable heterogeneous transesterification catalytic system that shows both the significant steric
effects of substrate alcohols and momentous water effects. The substrate activity is in the order ethanol
> n-propanol > n-butanol > iso-propanol > 2-butanol > tert-butanol. For the reaction system, solvate
water molecules within the cages of the interpenetrated 3D frameworks do not decrease the transester-
ification activity, whereas the trace water molecules in the substrate alcohols act as obvious obstacles to
the reaction.
Received 18th January 2017,
Accepted 6th March 2017
DOI: 10.1039/c7dt00217c
rsc.li/dalton
from any fat or soybean oil; and indeed, the synthesis of bio-
diesel from renewable biological sources has advantages both
Introduction
An intriguing issue in the domain of coordination networks is environmentally and with respect to sustainable production.33,34
the construction of an attractive topology, preferably with prac- Furthermore, such a transesterification is more efficient for the
tical task-specific roles.1–9 Some functional coordination net- formation of esters than esterification reaction from carboxylic
works have been widely synthesized by self-assembly of metal acids and alcohols, presumably owing to solubility and mole-
ions and polydentate ligands in consideration of various cular structure behaviors. General transesterifications often
factors such as anions, mole ratios, reaction temperature, and have been catalyzed, and thus are sensitive to catalysts, moist-
reaction solvents.10–14 Among diverse coordination networks, ure, pressure, and other factors.32–34 Zn(II) species such as zinc
three-connected open-skeletons are promising candidates as (II) oxide and zinc(II) coordination polymers have been used
structural models for scaffolding-like functional materials15–19 effectively as esterification and transesterification catalysts.35,36
that can serve as alternatives to four-connected diamonoid
In this context, systematic research on the specific influ-
skeletons.20,21 Thus, some metal coordination networks have ence on transesterification via a unique racemic crystal of two
been used as practical solid catalysts for organic reactions.22–31 enantiomeric two-fold 3D zinc(II) networks, [Zn(μ-OH)
The transesterification reaction, for example, is a basic, funda- (L)]3(ClO4)3·5H2O, as a solid catalyst, has been carried out. The
mentally important process for mass-production of bio-diesel two aims of this research were to explore the steric effects of
substrate alcohols as well as the trace water effects on solid
transesterification catalysis. Notably, significantly different
effects between trace water molecules in reactant alcohols and
Department of Chemistry, Pusan National University, Pusan 46241, Korea.
isolated water molecules within the cages of the network are
reported herein. Generally, zinc(II) complexes have been inves-
tigated and deemed to be appropriate for tetrahedral-binding
Lewis acidity, metallo-enzymes, zinc finger proteins, transme-
tallation, and homogeneous catalysis37–43 in the transesterifi-
cation of a wide range of esters with alcohols.35,36
E-mail: oksjung@pusan.ac.kr
†Electronic supplementary information (ESI) available: TGA and DSC curves,
1H NMR spectra, FT-IR spectra, ICP-OES data and powder X-ray diffraction patterns
of the present zinc(II) compounds, ORTEP drawing of [Zn(μ-OH)(L)](ClO4), crystal
structures of [ZnBr2L], and 1H NMR spectra for the procedure of transesterifica-
tion using [Zn(μ-OH)(L)](ClO4). CCDC 1522055 and 1522056. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c7dt00217c
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Table 1 Crystallographic data for [Zn(μ-OH)(L)]3(ClO4)3·5H2O (1·5H2O)
and [ZnBr2(L)] (2)
Experimental
Materials and methods
1·5H2O
2
All chemicals including zinc(II) bromide, zinc(II) perchlorate
hexahydrate, n-butyllithium, 3-bromopyridine, and 1,2-
dichlorotetramethyldisilane were purchased from Sigma-
Aldrich and used without further purification. 1,1,2,2-
Tetramethyl-1,2-di(pyridin-3-yl)disilane (L) was prepared by
our previously reported method.44 Elemental analyses (C, H, N)
were performed on crystalline samples at the KBSI Pusan
Center using a Vario-EL III analyzer. 1H (300 MHz) and 13C
(75 MHz) NMR spectra were obtained on a Varian Mercury
Plus 300. Infrared spectra were recorded on a Nicolet 380
FT-IR spectrophotometer using samples prepared as KBr
pellets. Thermal analyses were undertaken under a nitrogen
atmosphere at a scan rate of 10 °C min−1 using a Labsys
Formula
Mw (g mol−1
Crystal sys.
Space group
a (Å)
C42H73N6O20Si6Zn3Cl3
1453.06
C14H20N2Si2ZnBr2
497.69
Orthorhombic
Pnna
13.6038(4)
25.6130(7)
11.8630(3)
4133.5(2)
1.599
)
Cubic
ˉ
Ia3d
30.7561(2)
30.7561(2)
30.7561(2)
29 093.4(6)
1.327
16
1.252
12 032
0.0771
100.0
1.228
0.0940
0.2630
b (Å)
c (Å)
V (Å3)
σ (g cm−3
)
Z
8
µ (mm−1
F(000)
Rint
)
5.166
1968
0.0443
100.0
1.095
0.0298
0.0710
Completeness (%)
GoF on F2
R1 [I > 2σ(I)]a
wR2 (all data)b
TGA-DSC 1600. ICP-OES data were recorded using
PerkinElmer Optima 8300 model.
a
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = (∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2])1/2
.
Syntheses
[Zn(μ-OH)(L)]3(ClO4)3·5H2O (1·5H2O). An acetonitrile solu-
tion (3 mL) of zinc(II) perchlorate hexahydrate (3.7 mg,
0.01 mmol) was carefully layered onto a tetrahydrofuran solu-
tion (3 mL) of L (2.7 mg, 0.01 mmol). The reaction mixture was
slowly evaporated to obtain yellow crystals suitable for X-ray
single crystallography in 7 days (7.4 mg, 52%). Mp 294 °C
(dec.). Elemental Anal. Calcd Found: C, 34.60; H, 5.12; N, 5.84.
Calc. for C42H61N6O15Si6Zn3Cl3·5H2O: C, 34.71; H, 5.06; N,
5.78%. FT-IR νmax/cm−1: 1595s, 1411m, 1253m, 1108s, 1061s,
1030w, 825w, 800s, 769w, 705m, 622m, 585m.
the cell parameters and orientation matrix. The data were cor-
rected for Lorentz and polarization effects. The absorption
effects were corrected using the multi-scan method
(SADABS).45 The structures were resolved using direct methods
(SHELXS 2013/1) and refined by full-matrix least squares tech-
niques (SHELXL 2014/7).46 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 and procedural information
corresponding to the data collection and structure refinement
are listed in Table 1.
[ZnBr2(L)] (2). An acetone solution (3 mL) of zinc(II) bromide
(3.4 mg, 0.015 mmol) was carefully layered onto an acetonitrile
solution (1 mL) of L (4.1 mg, 0.015 mmol), resulting in the for-
mation of yellow crystals suitable for X-ray single crystallo-
graphy in 4 days (4.9 mg, 65%). Mp 281 °C (dec.). Elemental
Anal. Found: C, 33.50; H, 4.19; N, 5.55. Calc. for
Results and discussion
Synthesis
C14H20N2Si2ZnBr2: C, 33.78; H, 4.05; N, 5.63%. FT-IR νmax/cm−1
:
1587s, 1471w, 1402m, 1333w, 1252m, 1201w, 1128w, 1057m, The overall synthetic procedure is summarized in Scheme 1.
835m, 789s, 769s, 704s, 656m.
Self-assembly of Zn(ClO4)2·6H2O with 1,1,2,2-tetramethyl-1,2-di
(pyridin-3-yl)disilane (L) afforded two-fold interpenetrated 3D
coordination frameworks of unusual topology, namely the two
Transesterification catalysis
Phenyl acetate (0.13 mL, 1 mmol) was dissolved in each of the enantiomers 41- and 43-[Zn(μ-OH)(L)]3(ClO4)3·5H2O, in the
alcohols (7.5 mL), and to each the catalyst (0.05 mmol, solvent system (Scheme 1). The structure consists of 6-mem-
45.4 mg 1·5H2O) was added. All of the catalysis reactions were bered hydrolyzed Zn3(μ-OH)3 moieties. The formation of the
stirred at 50 °C. In the case of the ethanolysis, the catalytic Zn3(μ-OH)3 skeletal ring can be explained similarly to the
reactions were carried out according to the quantity of water in account that is available in the literature.47 The reaction of
ethanol (commercially available 99.9%, 98%, and 95% ZnBr2 with L as an N-donor produced zigzag 1D coordination
1
ethanol). Conversion yields were monitored by using H NMR polymers, [ZnBr2(L)], which subsequently were used for the
spectra. All of the reactions were run at least three times, and catalysis comparison. These reactions were originally treated at
their average conversion yields were recorded.
the 1 : 1 mole ratio of Zn(II) : L, but the products were not sig-
nificantly affected by either the mole ratio or the concen-
trations, which indicated their thermodynamic stability. The
Crystal structure determination
X-ray data were collected on a Bruker SMART automatic diffr- colorless crystalline products, [Zn(μ-OH)(L)]3(ClO4)3·5H2O
actometer with graphite-monochromated Mo Kα radiation (λ = (1·5H2O) and [ZnBr2(L)] (2), are air-stable and insoluble in
0.71073 Å) and a CCD detector at −25 °C. Thirty-six frames of water, chloroform, and acetone. The products are easily disso-
2D diffraction images were collected and processed to obtain ciated in N,N-dimethylformamide and dimethylsulfoxide
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zinc(II) ion (Zn–N = 2.027(7) Å; Zn–O = 1.924(3) Å), and thus
forms a three-connected 3D network of the 103-a topology
type. The distortion of the tetrahedral Zn(II) ion was confirmed
by the O(1)–Zn(1)–N(1) bond angles, which ranged from 106.9(2)–
111.5(2)°. The C3-symmetric Zn3(μ-OH)3 ring adopts
a
plane (Zn–O–Zn = 130.8(4)°; O–Zn–O = 109.2(4)°) (Fig. S4†),
which is in contrast to the known Zn3(μ-OH)3 species.47 A sig-
nificant feature is that the two enantiomeric (41 and 43) 3D
frameworks interpenetrate to form racemic crystals (Fig. 1).
The interpenetration of the two enantiomeric bowl parts forms
unique cages (3.3 × 3.3 × 5.0 Å3) that are occupied by two H2O
molecules (Fig. 2). The volume of 1·5H2O occupying solvate
molecules was 19.7% (5740.9/29 093.4 Å) as detected on the
basis of PLATON/SOLV calculation.48 The perchlorate anions
act as counteranions rather than ligands.
The structure of 2 is, for catalytic comparison, a 1D sinusoi-
dal coordination polymer (Zn–N = 2.035(2)–2.056(2) Å) for 2
Scheme 1 Synthetic scheme for 41- and 43-[Zn(μ-OH)(L)]3(ClO4)3·5H2O
and catalytic effects.
(Fig. S1†); 1·5H2O is slightly soluble even in methanol. They
were characterized in this study by microanalyses, IR, thermal
analyses, and X-ray single crystallography, as will be explained
in detail. The decomposition temperature of 2 was 281 °C, and
that of 1·5H2O was 294 °C (Fig. S2†). Their IR spectra were
determined to be consistent with their structures (Fig. S3†).
Crystal structures
The relevant bond lengths and angles are listed in Table 2.
1·5H2O has a bridged bidentate L, a 6-membered cyclic
Zn3(μ-OH)3 motif (Zn⋯Zn = 3.50(1) Å), and the tetrahedral
Table 2 Selected bond distances and bond angles for [Zn(μ-OH)
(L)]3(ClO4)3·5H2O (1·5H2O) and [ZnBr2(L)] (2)
1·5H2O
2
Zn(1)–O(1)
1.924(3)
1.924(3)
1.924(3)
2.027(7)
2.027(7)
109.2(4)
111.5(2)
106.9(2)
106.9(2)
111.5(2)
110.9(4)
Zn(1)–N(1)
Zn(1)–Br(1)
Zn(2)–N(2)
Zn(2)–Br(2)
2.056(2)
2.3464(4)
2.035(2)
2.3475(4)
Zn(1)–O(1)#3
Zn(1)#5–O(1)
Zn(1)–N(1)
Zn(1)–N(1)#4
O(1)–Zn(1)–O(1)#3
O(1)–Zn(1)–N(1)
O(1)#3–Zn(1)–N(1)
O(1)–Zn(1)–N(1)#4
O(1)#3–Zn(1)–N(1)#4
N(1)–Zn(1)–N(2)#4
N(1)–Zn(1)–N(1)#1
Br(1)–Zn(1)–Br(1)#1
N(2)–Zn(2)–N(2) #2
Br(2)–Zn(2)–Br(2)#2
97.0(1)
124.63(3)
99.5(1)
118.46(3)
Fig. 1 Packing structures of 1·5H2O in a (100) direction (top), and in a
(111) direction with highlighted cage moieties (bottom) (red and green
indicate interpenetrated 3D coordination polymers, curved arrows indi-
cate the chirality).
#1−x + 1/2, −y − 1, z, #2x, −y + 1/2, −z − 1/2, #3z, x, y, #4−z + 1/4,
−y + 1/4, −x + 1/4, #5−y + 1/4, −x + 1/4, −z + 1/4.
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Transesterification catalytic efficiency
First of all, in order to elucidate the steric effects of the sub-
strates on the heterogeneous catalytic activity, 1·5H2O was
employed in a transesterification reaction of phenyl acetate
with various alcohols51 because, as stated in the introduction,
not only are transesterifications carried out without side reac-
tions, but also the reactions are very useful in the bio-fuels
molecular system.33 1·5H2O (0.05 mmol) significantly cata-
lyzed the reactions of phenyl acetate (1 mmol) with excess alco-
hols. The catalysis process was monitored by using 1H NMR
spectra (Fig. S6†). The catalytic yields at 60 min are shown in
Tables S1–S3.† A control reaction without the catalysts, mean-
while, showed trace amounts of ester-converted product in 5%
yield even for 3 days. The catalyst showed drastic alcohol
related steric effects in the order EtOH > n-PrOH > n-BuOH >
iso-PrOH > 2-BuOH > tert-BuOH, indicating, as depicted in
Fig. 3, a strong dependency of the reaction rate on the bulki-
ness of alcohols. For the catalysis reaction of phenyl acetate
with EtOH, comparable catalytic systems were compared,
resulting in the order 1·5H2O > a mixture of Zn(ClO4)2 and L >
2 (Fig. 4), which indicated that the present catalytic system,
1·5H2O, is a superior catalyst to the others. The catalytic reac-
tion using a simple mixture of ZnClO4 and L instead of the
catalyst proceeded at a much slow rate, suggesting that the
detailed structure of 1·5H2O is another important factor. The
reaction rate of 1·5H2O > 2 can be explained in terms of the
difference in the nature of the OH and bulky Br moiety around
the tetrahedral zinc(II) ion. This hydroxyl effect is consistent
with both the esterification and transesterification activities of
zinc hydroxy nitrate (Zn5(OH)8(NO3)2·2H2O).9,10 Accordingly,
the trace water molecules in each of the alcohols drastically
decreased the reaction rates. For example, the reaction in
general EtOH (95%) was much slower than that in absolute
EtOH. The same reaction rate in 98% EtOH is the median
value (Fig. 5), indicating that trace water is very sensitive to
Fig. 2 Top views (top) and side view (bottom) for bowl-like cage moi-
eties of [Zn(μ-OH)(L)]3.
(Fig. S5†). Each L connects two zinc(II) ions in a μ2-fashion.
The geometry of zinc(II) is tetrahedral (N–Zn–N = 97.0(1)–
99.5(1)°; Br–Zn–Br = 118.46(3)–124.63(3)°). A period of sinusoi-
dal coordination polymers consists of four zinc(II) halides and
four ligands.
Construction principle
The reaction of Zn(ClO4)2 with L afforded three-connected 3D
coordination polymeric structures. The bridged-hydroxo
zinc(II) network, 1·5H2O, was characterized by X-ray crystallo-
graphy as consisting of enantiomeric (41 and 43) two-fold 3D
coordination polymers; the Zn3(μ-OH)3 moieties possess a C3
symmetry, with each hydroxo group bridging to zinc centers.
What is the critical driving force behind the formation of
bridged-hydroxo species? The bridged-hydroxo moiety prob-
ably is formed through hydrolysis of trace water molecules in
the reaction system.47 For instance, in the present study, the
addition of trace NaOH in the reaction system accelerated the
formation of the product. The structure was favorably self-
assembled irrespective of concentrations and mole ratios, indi-
cating that the skeleton is thermodynamically stable. The
appropriate torsion angles of the Py-Si-Si-Py ligand might play
a significant role in the construction of the topological
network. The size of the halide or perchlorate anion seems to
partially contribute to the formation of the skeletal network.
Rational construction of the desirable, interpenetrated topo-
logy would be a challenging issue. In fact, for the 103 srs net
topology, the two enantiomers can elegantly interpenetrate
(Fig. 1), as was already known to the Ciani group.17,49,50
Fig. 3 Plot showing catalytic yields of transesterification of phenyl
acetate using 1·5H2O in EtOH (red line), n-propanol (blue line),
n-butanol (green line), iso-propanol (purple line), 2-butanol (orange
line), and tert-butanol (pink line).
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Fig. 4 Plot showing catalytic yields of transesterification of phenyl
acetate in 99.9% EtOH using 1·5H2O (solid red line), ZnClO4 + L (solid
blue line), and 2 (solid green line) under the same conditions described
in the Experimental section.
Fig. 6 Proposed mechanism for transesterification reaction.
20 min. Even though it is not easy to elucidate the exact mech-
anism of the catalysis at this stage, the mechanism of Zn(II)-
cation-catalyzed transesterification probably involves electro-
philic activation of the carbon center of the carbonyl moiety by
binding of the Zn(II) to the carbonyl oxygen.52,53 Thus, the
vacant site and Lewis acidity of the zinc(II) center play impor-
tant roles in this catalytic transesterification. Additionally, the
skeletal structural collapse was prohibited by the chelate effect
of tridentate ligands. Based on this idea, a possible mechan-
ism for transesterification is shown in Fig. 6. No zinc(II)
species was detected after the catalysis in absolute ethanol as
shown in the ICP data (Table S4†).54 When the catalyst amount
was increased twice, the catalytic rates increased significantly.
Such a fact suggests that the structures, including the surface
properties of the present 3D porous materials, play an impor-
tant role in catalysis. The recycled catalyst shows a similar
efficiency even though the crystallinity of 1·5H2O decreases
after catalytic reaction (Fig. S7 and S8†).
Fig. 5 Plot showing catalytic yields of transesterification of phenyl
acetate using 1·5H2O in 99.9% EtOH (solid red line), 98% EtOH (solid
green line), and 95% EtOH (solid purple line), and 1 in 99.9% EtOH
(dashed blue line) under the same conditions described in the
Experimental section.
transesterification. The most significant result, however, was
that the water solvate molecules within cages do not play any
significant role in the reaction, indicating that they are safely
Conclusions
nestled. In comparison of catalytic efficiency, the water-de- Self-assembly of zinc(II) perchlorate with 1,1,2,2-tetramethyl-
solvate species 1, which is prepared by heating at 50 °C under 1,2-di(pyridin-3-yl)disilane (L) afforded two enantiomeric two-
vacuum for 2 days, showed a similar result. The perchlorate fold 3D coordination networks, 41- and 43-1·5H2O, of unique
groups were located in the vacant sites, and were not involved 103-a srs net topology. This system is a proof-of-experiment
in the coordination of the Zn(II) cations. The alcohol substrate- example of the development of 3D networks as a solid catalyst,
approaching frequency to the Zn(II) center might be a key demonstrating the direct structural effects, substituent bulki-
factor in transesterification. The solubility of 1·5H2O is an ness, and trace water effects. The significantly different effects
additional significant factor relevant to catalytic efficiency. For between the water molecules inside and outside of the oval-
example, in MeOH, the catalyst was soluble, and thus, the reac- shaped cage represent an important conceptual advance in the
tion rate was very fast, and the reaction was carried out within development of such a catalytic system. More systematic
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studies, for example on the synthesis of related ligands, are in 20 A. M. Kirillov, S. W. Wieczorek, A. Lis, M. F. C. Guedes da
progress. Further investigation of this series of coordination
frameworks, including a new topology based on the N-donor
tridentate ligand, also is underway.
Silva, M. Florek, J. Krol, Z. Staroniewicz, P. Smolenski and
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This work was supported by the National Research Foundation
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