Diverse zinc(ii) coordination assemblies built on divergent 4,2′:6′,4′′-terpyridine derivatives: Syntheses, structures and catalytic properties

Article abstract of DOI:10.1039/c4ra16441e

A series of metal-organic architectures (compounds 1-5) based on zinc salts and four 4′-substituted 4,2′:6′,4′′-tpys including two new ligands have been synthesized and structurally characterized. The ligands L1-L4 with various 4′-substituents on 4,2′:6′,4′′-tpy react with Zn(OAc)₂·2H₂O to yield assemblies 1-4 containing either 1-D polymeric chains (1-3) or a discrete dinuclear complex (4). X-ray structural analysis revealed that although similar 1-D polymeric chains were observed in both 1 and 3, the 3-D packing modes were essentially different. The chains in 1 were stacked to form a network structure with microporous channels that were not present in 3. In contrast, 4 was a discrete dinuclear complex containing a paddle-wheel {Zn₂(μ-OAc)₄} motif, although another minor component possibly coexists in the bulk sample as revealed by the PXRD studies. Crystals of 5 were prepared from the reaction between L4 and ZnI₂ and its X-ray structure revealed a 1-D polymeric chain with a wave-like structure. Phase-pure compounds 1-3 and 5 were tested for the catalytic transesterification of phenyl acetate with alcohols, and the results indicated that 1 was the most active catalyst for this reaction, affording the new ester product in 95% yield at 50 °C under neat conditions, while other catalysts also catalysed the reaction with modest yields. Several different alcohols were examined as substrates for 1-catalysed transesterification and it was found that the size of substrates has important influence on the catalytic efficiency. In addition, amine additives were found to remarkably promote the catalytic efficiency of the less active catalyst 3. The structure-catalytic activity relationship was discussed in detail based on the catalytic data obtained. This journal is

Full text of DOI:10.1039/c4ra16441e

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Diverse zinc(II) coordination assemblies built on
0
0
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divergent 4,2 :6 ,4 -terpyridine derivatives:
Cite this: RSC Adv., 2015, 5, 15870
syntheses, structures and catalytic properties†
a
b
c
ad
a
Guoqi Zhang,* Yi-Xia Jia, Wenbo Chen,* Wen-Feng Lo, Nyeisha Brathwaite,
e
e
James A. Golen and Arnold L. Rheingold
0
A series of metal–organic architectures (compounds 1–5) based on zinc salts and four 4 -substituted
0 0 00
4
,2 :6 ,4 -tpys including two new ligands have been synthesized and structurally characterized. The
0
0
0
00
2 2
ligands L1–L4 with various 4 -substituents on 4,2 :6 ,4 -tpy react with Zn(OAc) $2H O to yield
assemblies 1–4 containing either 1-D polymeric chains (1–3) or a discrete dinuclear complex (4). X-ray
structural analysis revealed that although similar 1-D polymeric chains were observed in both 1 and 3,
the 3-D packing modes were essentially different. The chains in 1 were stacked to form a network
structure with microporous channels that were not present in 3. In contrast, 4 was a discrete dinuclear
complex containing a paddle-wheel {Zn
coexists in the bulk sample as revealed by the PXRD studies. Crystals of 5 were prepared from the
reaction between L4 and ZnI and its X-ray structure revealed a 1-D polymeric chain with a wave-like
2 4
(m-OAc) } motif, although another minor component possibly
2
structure. Phase-pure compounds 1–3 and 5 were tested for the catalytic transesterification of phenyl
acetate with alcohols, and the results indicated that 1 was the most active catalyst for this reaction,
ꢀ
affording the new ester product in 95% yield at 50 C under neat conditions, while other catalysts also
catalysed the reaction with modest yields. Several different alcohols were examined as substrates for
Received 15th December 2014
Accepted 26th January 2015
1-catalysed transesterification and it was found that the size of substrates has important influence on the
catalytic efficiency. In addition, amine additives were found to remarkably promote the catalytic
efficiency of the less active catalyst 3. The structure–catalytic activity relationship was discussed in detail
based on the catalytic data obtained.
DOI: 10.1039/c4ra16441e
www.rsc.org/advances
electronic properties of the ligands as well as their complexes.
Consequently, novel metal–organic architectures including
,2 :6 ,4 -Terpyridine (4,2 :6 ,4 -tpy), a divergent analogue of the metallocycles, polycatenated metallocapsule, one-dimensional
Introduction
0 0 00 0 0 00
4
0
0
00
0
0
00
classic chelating 2,2 :6 ,2 -terpyridine (2,2 :6 ,2 -tpy) ligand, has (1-D) chains, two- or three-dimensional (2-D, 3-D) networks
0
0
00
been attracting considerable attention from chemists in recent have been obtained, resulting from the subtle choice of 4,2 :6 ,4 -
1,10–20
0
years,
versatile coordination polymers and networks upon reacting metal ions of different coordination geometries. However, the
due to its facile synthesis and its ability to form tpy ligands with different substituents on the 4 -position and
1–23
1
0
0
0
00
with metal ions. Typically, various 4 -substituted 4,2 :6 ,4 -tpy fact that other factors including reaction conditions (solvents,
ligands can be readily obtained by using a one-pot synthetic temperature and stoichiometry etc.), crystallization methods and
procedure and choosing suitable synthetic precursors; this counter anions also have great inuence on the structures of the
allows the facile tuning of the chemical structures and resulting assemblies indicates that we are still far from under-
standing and controlling the assembly process.
A general nding from all of the known supramolecular
a
Department of Sciences, John Jay College and the Graduate Center of the City
0
0
00
structures involving 4,2 :6 ,4 -tpy derivatives is that the central
University of New York, New York, NY 10019, USA. E-mail: guzhang@jjay.cuny.edu
b
0 0 00
pyridine ring of the 4,2 :6 ,4 -tpy motif remains non-coordi-
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou,
1
–23
Zhejiang 310014, China
c
nated,
and this has inspired simple applications of related
II
II
College of Environmental and Chemical Engineering, Shanghai University of Electric Zn - or Mn -coordination polymers as acid/base colorimetric
Power, Shanghai 200090, China. E-mail: wbchen2007@126.com
d
6,7
and uorescent sensors, and the attempted N-alkylation on a
-D Zn -coordination polymer. However, to the best of our
Department of Chemistry, Yeshiva University, New York, NY 10033, USA
II
7
1
e
Department of Chemistry, University of California San Diego, La Jolla, CA 92093, USA
knowledge, the catalytic applications of metal complexes or
†
Electronic supplementary information (ESI) available. CCDC 1017546–1017550.
0
0
00
coordination polymers built from 4,2 :6 ,4 -tpy derivatives are
far unexplored, in contrast to the well-known capability of metal
For ESI and crystallographic data in CIF or other electronic format. See DOI:
1
0.1039/c4ra16441e
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ambient conditions (no inert atmosphere). Solution electronic
absorption spectra were recorded on a Shimadzu UV-1800
spectrophotometer, and FT-IR spectra were measured on a
Shimadzu 8400S instrument with solid samples using a Golden
1
13
Gate ATR accessory. H and C NMR spectra were obtained at
room temperature on a Bruker III 400 MHz spectrometer with
TMS as an internal standard. FAB-MS spectra were recorded on
a JOEL HX110 mass spectrometer using FAB (fast atom
bombardment) technique. GC-MS analysis was carried out on a
Shimadzu GCMS-QP2010S gas chromatograph mass spectrom-
eter. Elemental Analyses were performed by Midwest Microlab
LLC in Indianapolis. Powder X-ray diffraction (PXRD) was per-
formed with a Bruker D8 Discover microdiffractometer with the
General Area Detector Diffraction System (GADDS) equipped
˚
with a VANTEC-2000 2D detector. The X-ray beam was mono-
˚
chromated with a graphite crystal (l Cu-Ka ¼ 1.54178 A) and
collimated with a 0.5 mm capillary (MONOCAP). The sample-to-
detector distance was 150 mm. The step times 300 s for 1–3 and
6
00 s for 4 were applied. The integrated 1D pattern was analyzed
plus
33
by the soware DIFFRAC
pattern was calculated by the soware Mercury v. 2.4. Ligands
L1 and L3 were synthesized according to previously published
EVA. The simulated powder
34
Scheme 1 The molecular structures of ligands L1–4 studied in this
work.
8,9
procedures.
0
0
00
complexes derived from 2,2 :6 ,2 -tpy ligands in homogeneous
24–30
Ligand L2
catalysis.
We envisioned that in the metal–organic assembles of
a 4,2 :6 ,4 -tpy ligand, the metal-binding sites would offer a Lewis In a 250 cm round-bottom ask, 4-acetylpyridine (3.63 g,
0
0
00
3
acidic reaction centre and the non-bound central pyridine rings 30.0 mmol) was added to a solution of pyridine-2-carbaldehyde
3
provide an additional basic site, and thus acid/base bifunctional (1.61 g, 15.0 mmol) in EtOH (50 cm ). KOH pellets (1.68 g,
catalytic activity can be anticipated. Indeed, bifunctional catalysis 30 mmol) were then added, followed by aqueous NH (25%,
3
3
has proven to be a powerful tool in numerous organic trans- 80 cm ). The resulting orange-brown solution was stirred at
31,32
formations.
In addition, the ease of synthesis and modication room temperature for 25 h, during which time a grey suspen-
0
0
0
00
on the 4 -position of a 4,2 :6 ,4 -tpy backbone makes the catalytic sion formed. The solid was collected by ltration, washed with
reactivity of the corresponding metal assembles readily tunable.
H O and EtOH and dried in vacuo over P O . L2 was isolated as a
To further explore the self-assembly behaviour and their white solid (2.03 g, 43.7%). H NMR (400 MHz, DMSO-d ) d 8.83
2
2 5
1
6
0
0
00
material applications, we wish to extend the existing 4,2 :6 ,4 -tpy (d, J ¼ 4.5 Hz, 1H), 8.81 (s, 2H), 8.79 (d, J ¼ 6.0 Hz, 4H), 8.49 (d,
ligand family and investigate their potential in heterogeneous J ¼ 8.0 Hz, 1H), 8.31 (d, J ¼ 6.0 Hz, 4H), 8.07 (td, J ¼ 7.8, 1.5 Hz,
1
3
catalysis while combined with various metal salts. Therefore, in 1H), 7.57 (dd, J ¼ 7.2, 4.9 Hz, 1H). C NMR (101 MHz, DMSO-d )
6
0
this work we report the syntheses of two new 4 -substituted d 154.6, 153.0, 150.5 (4C), 150.0, 148.8, 145.2, 137.7, 124.8,
0
0
00
ꢂ5
4
,2 :6 ,4 -tpy ligands (L2 and L4, Scheme 1) and the reactions of 122.0, 121.1 (4C), 118.3 ppm. UV-vis l /nm (3.05 ꢁ 10 mol
max
ꢂ3
3
3
ꢂ1
ꢂ1
ligands L1–4 with Zn(OAc)2 and ZnI2 salts. As a result, the dm , CH Cl –MeOH) 248 (3/10 dm mol cm 24.0), 310
2
2
II
ꢂ1
synthesis and crystal structures of ve new Zn -organic assem- (5.93). FT-IR (solid, cm ): 3025m, 1587s, 1557s, 1539s, 1507s,
bles, namely, [{Zn (L1)(OAc) $1.5MeOH} ] (1), [{Zn(L2)(OAc)
1473s, 1444w, 1417w, 1394s, 1311w, 1298w, 1213w, 1167w,
2), [{Zn (L3)(OAc) ] (3), [Zn (L4) (OAc) ] (4) and [{Zn(L4)I
2
4
n
2 n
} ]
(
2
4
}
n
2
2
4
2
}
n
] (5) 1060m, 996s, 823s, 778s, 731s, 669m, 635s, 613m. FAB-MS: m/z
+
were described and their structures were discussed in compar- 310.28 [M] (calc. 310.12). Anal. calcd for C H N : C 77.40, H
2
0
14 4
1
ison with previously reported results. We also report our efforts 4.55, N 18.05%. Found C 77.46, H 4.51, N 18.09%.
for the rst time on the catalytic applications of these new
assembles for the transesterication of phenyl acetate with
alcohols under heterogeneous conditions, in order to understand
Ligand L4
Ligand L4 was prepared by the same procedure as that for 2
except that piperonal (2.25 g, 15.0 mmol) was used instead of
the structure–catalytic activity relationship in metal assembles of
0 0 00
,2 :6 ,4 -tpys.
4
pyridine-2-carbaldehyde. L4 was isolated as a white solid (3.36 g,
1
6
3.4%). H NMR (400 MHz, DMSO-d
6
) d 8.85–8.63 (m, 4H), 8.43
(
8
s, 2H), 8.38–8.29 (m, 4H), 7.80 (d, J ¼ 1.9 Hz, 1H), 7.69 (dd, J ¼
Experimental section
General
CH
2
13
.2, 1.9 Hz, 1H), 7.13 (d, J ¼ 8.1 Hz, 1H), 6.15 (s, 2H, H ).
C
NMR (101 MHz, DMSO-d ) d 154.3, 150.3 (4C), 149.7, 148.7,
6
Solvents and reagents were purchased from Fisher or Sigma- 148.3, 145.3, 130.7, 121.8, 121.2 (4C), 118.4, 108.8, 107.7,
ꢂ5
ꢂ3
Aldrich in the US. All reactions were performed under 101.6 ppm. UV-vis lmax/nm (2.95 ꢁ 10 mol dm , CH
2 2
Cl –
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MeOH) 237 (3/10 dm mol cm 46.4), 271 (34.1), 311 (22.4). [{Zn(L4)I } ] (5)
Paper
3
3
ꢂ1
ꢂ1
2
n
ꢂ1
FT-IR (solid, cm ): 2871w, 2777w, 1593s, 1558m, 1505s, 1488s,
The synthetic procedure is similar to that of 1, except that L4
35.3 mg, 0.100 mmol) and ZnI (31.8 mg, 0.100 mmol) were
used. Colourless crystals of 5 were collected by decanting the
1
9
447m, 1419w, 1400s, 1250s, 1219m, 1112w, 1050s, 998w,
49m, 848s, 828m, 809s, 669w, 630s. FAB-MS: m/z 353.25 [M]
(
2
+
(calc. 353.12). Anal. calcd for C22
H
15
N
3
O
2
: C 74.78, H 4.28, N
solvent aer two weeks, which were washed with MeOH and
11.89%. Found C 74.93, H 4.27, N 11.97%.
ꢂ1
dried in the air. Yield: 45.6 mg (68.0%). FT-IR (solid, cm
)
2872w, 1609s, 1597s, 1558w, 1534m, 1500s, 1489s, 1446w,
[{Zn
2 4 n
(L1)(OAc) $1.5MeOH} ] (1)
1410s, 1249s, 1217s, 1062m, 1043m, 1021s, 937m, 917m, 861m,
8
31s, 801s, 741w, 689m, 645s. Anal. calcd for C H I N O -
22 15 2 3 2
A solution of L1 (31.0 mg, 0.100 mmol) in MeOH–C
6
H
5
Cl
3
Zn$0.2H
6.21%.
O: C 39.29, H 2.25, N 6.25%. Found C 39.08, H 2.30, N
2
(
10 cm , 1 : 4, v/v) was placed in a test tube. A mixture of MeOH
3
6 5
and C H Cl (5 cm , 1 : 1, v/v) was layered on the top of this
solution, followed by a solution of Zn(OAc) $2H O (87.2 mg,
2
2
3
General procedure for catalytic transesterication
0.400 mmol) in MeOH (10 cm ). The tube was sealed and
allowed to stand at room temperature for a week, during which Under typical conditions, the reactions were performed in asks
time X-ray quality colourless crystals grew on the bottom of the tted with water circulated condensors. 1.0 mmol of phenyl
II
tube. The crystals were collected by decanting the solvent and acetate, 0.01 mmol of Zn catalysts (1 mol% based on the
3
were washed with MeOH and dried in air. Yield: 20.5 mg (29.0% ligands) were placed in the ask, to which 4.0 cm of 1-butanol
ꢂ1
ꢀ
based on L1). FT-IR (solid, cm ) 1638s, 1617s, 1591s, 1426s, was added. The reaction was heated to 50 C for 18 h under
223m, 1067m, 1027m, 850s, 825s, 665s, 650m, 619s, 522s. rigorous stirring, aer which the reaction mixture was ltered
Anal. calcd for C H N O Zn $1.5CH OH: C 48.85, H 4.45, N and the ltrate diluted with dichloromethane and then
1
2
8
26
4
8
2
3
7.72%. Found C 48.32, H 4.15, N 7.76%.
analyzed by GC-MS. The yield for each reaction was obtained by
using the internal standard method.
[
{Zn(L2)(OAc)
2 n
} ] (2)
Crystal structure determinations
A solution of Zn(OAc) $2H O (43.6 mg, 0.200 mmol) in MeOH
(
CH Cl –MeOH (10 cm , v/v, 3 : 1). The reaction mixture was
2 2
stirred at room temperature for 10 min, and was then ltered,
the clear ltrate was le to stand at room temperature for 3 days
upon slow evaporation, colourless crystals were collected by
2
2
3
Suitable crystals of 1–5 were mounted on Cryoloops with
Paratone-N oil. Data were collected at 100 K with a Bruker APEX
II CCD using Mo-Ka radiation and corrected for absorption with
SADABS and structures solved by direct methods. All non-
hydrogen atoms were rened anisotropically by full-matrix
5 cm ) was added to a solution of L2 (31.0 mg, 0.100 mmol) in
3
2
least squares on F . Hydrogen atoms were found from Fourier

ltration, washed with MeOH, and dried in air. Yield: 35.6 mg,
ꢂ1
difference maps and rened isotropically, otherwise they were
placed in calculated positions with appropriate riding param-
eters. For structure 1 diffused solvent was treated using Platon
7
1
9
C
4
2.4% based on L2. FT-IR (solid, cm ) 1616s, 1558m, 1540m,
507w, 1472m, 1457w, 1378s, 1326s, 1221m, 1066m, 1027w,
26w, 847s, 782s, 729m, 669s, 649s, 617m, 515s. Anal. calcd for
35
˚
3
program SQUEEZE, and a void volume of 285 A with electron
count of 108 was treated as six molecules of methanol; Unit
cards of instruction les were adjusted to address changes in
chemical formula, molecular mass, density and F000 values.
ORTEP and molecular packing gures were drawn with the
24
H
20
N
4
O
4
Zn: C 58.37, H 4.08, N 11.35%. Found C 58.01, H
.06, N 11.32%.
[
2 4 n
{Zn (L3)(OAc) } ] (3)
34
program Mercury v. 2.4. Renement details are summarized in
Table 1.†
The synthetic procedure is similar to that of 1, except that the
ligand was L3 (35.2 mg, 0.100 mmol). Orange crystals of 3 was
obtained aer two weeks. Yield: 62.3 mg (86.7% based on L3).
ꢂ1
FT-IR (solid, cm ) 1639s, 1618s, 1590s, 1530m, 1427s, 1210w, Results and discussion
1
067s, 1029m, 847s, 813s, 664s, 652w, 620s, 521m. Anal. calcd
for C31 Zn : C 51.76, H 4.48, N 7.79%. Found C 51.85, H
.52, N 7.76%.
Synthesis and characterization
H
32
N
4
O
8
2
Ligands L2 and L4 were prepared in moderate or good yields by
4
0
0
00
a procedure similar to that for other 4,2 :6 ,4 -tpy ligands
1,5–8
reported in the literature.
with pyridine-2-carbaldehyde or piperonal in the presence of
The synthetic procedure is similar to that of 2, except that the KOH followed by addition of aqueous NH afforded L2 and L4
The reactions of 4-acetylpyridine
2 2 4
[Zn (L4) (OAc) ] (4)
3
ligand was L4 (35.3 mg, 0.100 mmol). Colourless blocks of 4 was as white solids aer isolation by ltration, respectively. Mass
obtained aer a week. Yield: 42.2 mg (78.9% based on L4). FT-IR spectroscopy revealed peak envelops at m/z 353.26 and 310.28
ꢂ1
1
13
(
solid, cm ) 2988br, 1615s, 1597s, 1558m, 1539m, 1505s, that matched with those calculated. H and C NMR spectra of
1
8
449m, 1417s, 1375s, 1324s, 1248s, 1118w, 1063m, 1026s, 928w, both L2 and L4 were recorded in DMSO-d and are consistent
6
49m, 838s, 814s, 731w, 681s, 669s, 648s, 617m, 563s. Anal. with the molecular structures as illustrated in Scheme 1. In the
1
calcd for C52
H
42
N
6
O
12Zn
2
$CH
3
OH: C 57.57, H 4.19, N 7.60%.
2
H NMR spectrum of L4, The peak for CH protons on the
Found C 57.07, H 4.03, N 7.54%.
benzodioxole ring was observed at 6.15 ppm as a singlet.
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Table 1 Crystallographic data and structure refinement for compounds 1–5
Compound
1
2
3
4
5
Formula
14
C H
13
N
2
O
4
Zn$0.75CH
3
OH
C
24
H
20
N
4
O
4
Zn
C
31
H
32
N
4
O
8
Zn
2
C
52
H
42
N
6
O12Zn
2
22 15 2 3 2
C H I N O Zn
Formula weight
Crystal system
Space group
362.67
Monoclinic
C2/c
26.0536(19)
14.9913(11)
8.1639(6)
90
107.153(2)
90
493.81
719.35
Monoclinic
C2/c
26.7024(17)
14.8976(10)
8.0841(5)
90
106.816(2)
90
1073.66
Triclinic
P1ꢀ
672.54
Monoclinic
P2(1)/n
8.2156(10)
19.161(2)
13.7259(18)
90
103.651(4)
90
2099.7(4)
1.562
Monoclinic
P2(1)/c
10.0987(6)
11.3461(5)
18.6487(10)
90
103.791(2)
90
2075.18(19)
2.153
˚
a/A
10.8145(15)
13.5094(19)
16.680(2)
102.930(4)
93.543(4)
102.850(4)
2299.6(6)
1.551
˚
b/A
˚
c/A
ꢀ
ꢀ
a/
b/
ꢀ
g/
3
˚
U/A
3046.8(4)
1.581
8
3078.4(3)
1.552
4
ꢂ3
D
Z
c
/Mg m
4
2
4
ꢂ1
m/mm
1.637
1.211
1.616
1.118
4.184
T/K
100(2)
100(2)
100(2)
100(2)
100(2)
Reections/unique
Parameters
3214/2404
194
0.0464, 0.1117
0.0698, 0.1204
1.070
4314/3653
300
0.0421, 0.1103
0.0507, 0.1162
1.048
3168/2569
209
0.0386, 0.0772
0.0551, 0.0833
1.065
9477/5930
653
0.0508, 0.1062
0.1028, 0.1261
1.000
4280/3908
271
0.0326, 0.0956
0.0367, 0.0985
1.092
a
a
b
b
R
R
1
, wR
, wR
2
[I > 2s(I)]
(all data)
1
2
GOF
a
b
2
o
2 2
2 1/2
R
1
¼ F
o
ꢂ F
c
/F .
o
wR
2
¼ [w(F
ꢂ F
c
) /w(F
o
) ]
.
Reacting ligands L1–4 with Zn(OAc) $2H O by using either molecular formula of {Zn (L1)(OAc) $1.5MeOH} (Table 1).
2
2
2
4
n
solution reactions or layering technique gave crystals of 1–4 that However, the quality of the structure prevented the solvent
were suitable for X-ray structural analysis, while good-quality molecules from being accurately modelled, and so the data
35
single crystals of 5 involving ZnI
by the layering technique. The bulk samples of 1–5 are insoluble sentation of the repeat unit is depicted in Fig. 1a. In the
in water and common organic solvents including alcohols, repeat structural unit, a paddle-wheel {Zn (m-OAc) } motif is
except that 4 is slightly soluble in dichloromethane or observed, which links the ligand L1 with two of its terminal
chloroform. pyridine-N atoms through its apical positions to form an innite
All compounds were characterized by FT-IR spectroscopy zig-zag chain (Fig. 1b), reminiscent of those formed between
2
were obtained in good yield were treated using the program SQUEEZE. An ORTEP repre-
2
4
0 0 0 00 0
and elemental analysis, and structurally determined by X-ray Zn(OAc)
crystallography. The PXRD patterns of 1–3 revealed the bulk phenyl)-4,2 :6 ,4 -terpyridine, or 4 -(4-methylthiophenyl)-4,2 :6 ,4 -
2 2
$2H O and 4 -phenyl-4,2 :6 ,4 -terpyridine, 4 -(4-bromo-
0
0
00
0
0
0
00
6,7
crystalline samples were single phase, in agreement with the terpyridine. All bond lengths and angles are similar to those
6,7
results from single-crystal structural analysis (see ESI†). reported and listed in the caption of Fig. 1. The ligand
ii
2
However, PXRD prole of the bulk sample of crystal 4 contains a displays a C -symmetry and the atom N1 is generated by the
few peaks from another phase, indicating the coexistence of a symmetry code: ꢂx + 1, y, ꢂz + 3/2. The C -axis runs through
2
minor component with a structure different from 4. The crystals atoms N2, C9 and N3, although the pyridine ring with N3 is
of 5 experienced decomposition upon exposure to the air and no slightly out of the plane formed by the almost coplanar
0
0
00
II
signicant PXRD peaks were observed. The crystal renement 4,2 :6 ,4 -tpy domain. Each Zn centre is bound to four oxygen
results for 1–5 are summarized in Table 1.
atoms of acetate groups and one pyridine-N atom of ligand L1,
and is in a square pyramidal coordination environment. The
˚
Zn–Zn distance of 2.9046(8) A in the {Zn
2
(m-OAc)
4
} unit is close
Structural description
to those for known compounds with the same structural
motif. It is, however, worth noting that only two of the three
terminal pyridine-N atoms are involved in the coordination with
metal centres, remaining the 4-pyridyl group on the 4 -position
non-coordinated, and so was the central pyridine. According to
previous work, the tripodal ligand L1 tends to coordinate with
2 2
metal salts such as ZnCl or ZnI by all three terminal pyridine N
atoms, allowing the formation of metallocapsules or cage-like
Although Zn(OAc) -containing coordination polymers of
6,7
2
0 0 00
4
,2 :6 ,4 -tpy ligands were exclusively synthesized from the
1,6,7,21–23
solution reactions in the literature,
attempts to obtain
$2H O in both
2
–MeOH solutions were unsuccessful.
0
single crystals of 1 by reacting L1 with Zn(OAc)
CHCl –MeOH and CH Cl
2
2
3
2
Instead, X-ray quality crystals of 1 were harvested by the layering
technique, in spite of low yield. Thus, careful layering a meth-
anol solution of Zn(OAc) $2H O onto a solution of L1 in MeOH–
9
2
2
structures. While the observation that in 1 atom N3 remains
non-coordinated is unexpected, this increases the possibility of
C H Cl, separated by a MeOH–C H Cl solvent mixture afforded
6
5
6
5
colourless crystals of 1. 1 crystallizes in the monoclinic space
group C2/c as a one-dimensional coordination polymer with a
1
to have an extra basic site for catalysis as we assumed
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structural analysis conrmed unambiguously the formation of
one-dimensional coordination polymer {Zn(L2)(OAc)2}n with
the repeat structural unit shown in Fig. 2a. Unlike 1, only
mononuclear Zn centres are observed in the polymeric chain of
2
and each Zn atom is in a distorted tetrahedral coordination
environment, being bonded to two ligands and two acetate ions.
˚
The Zn–O distances (1.919(2) ad 1.9318(19) A) are remarkably
shorter than those in 1. The tpy domain is essentially planar and
the torsion angle between the least square planes of pairs of
ꢀ
adjacent pyridine rings is 4.12 . The 2-pyridyl group on the
0
0 0 00
4
-position of 4,2 :6 ,4 -tpy remains unoccupied. The torsion
angle between the least square planes of 2-pyridine and the
ꢀ
central pyridine ring is 9.59 , rather smaller than that found
between the non-coordinated 4-pyridine and the central pyri-
ꢀ
dine ring in 1 (30.15 ). Although the {Zn
2
(m-OAc)
4
} cluster was
Fig. 1 (a) The repeat unit in 1 with ellipsoids plotted at the 40%
probability level, and H atoms omitted for clarity. Selected bond
i
parameters: Zn1–O2 ¼ 2.028(3), Zn1–O1 ¼ 2.037(3), Zn1–O3 ¼
i
i
2
2
.043(3), Zn1–N1 ¼ 2.045(3), Zn1–O4 ¼ 2.067(2), Zn1–Zn1
¼
i
i
.9046(8) A˚ ; N1–Zn1–Zn1 ¼ 176.14(8), O2–Zn1–O1 ¼ 160.59(11),
i
i
O3–Zn1–O4 ¼ 160.52(10), O2 –Zn1–N1 ¼ 100.85(11), O1–Zn1–N1 ¼
ꢀ
9
8.50(11), O3–Zn1–N1 ¼ 98.37(10) . Symmetry code: i ¼ ꢂx + 1/2,
ꢂy + 3/2, ꢂz; ii ¼ ꢂx + 1, y, ꢂz + 3/2. (b) The 3-D space-filling mode of
through p/p stacking of tpy domains showing a microporous
framework viewed down the b-axis.
1
0
previously (vice infra). The fact that the 4 -(4-pyridyl) group of L1
remained uncoordinated could be tentatively attributed to the
existence of acetate counter anions, which favoured the
formation of 1-D coordination chains or discrete complexes,
rather than more complex network structures in all related
6,7,20
examples.
The 3-D packing mode exhibits ordered
arrangements of 1-D polymeric chains through p-stacking of tpy
domains along the crystallographic c-axis. As a result, a micro-
porous network was formed with non-coordinated pyridine-N
pointing to the inner channels that have a dimension of
2
˚
approximately 6.0 ꢁ 7.0 A and are mainly lled with the solvent
methanol molecules (Fig. 1b).
Fig. 2 (a) The repeat unit in 2 with ellipsoids plotted at the 40%
probability level, and H atoms omitted for clarity. Selected bond
parameters: Zn1–O3 ¼ 1.919(2), Zn1–O1 ¼ 1.9318(19), Zn1–N1 ¼
Reacting Zn(OAc) $2H O with ligand L2 in a CH Cl –MeOH
2
2
2
2
solution aer evaporation of the solution for 3 days afforded
colorless crystals of 2 that are suitable for X-ray diffraction
i
i
2
.058(2), Zn1–N4 ¼ 2.051(2) A˚ ; N1–Zn1–N4 ¼ 106.59(9), N1–Zn1–O3
ꢀ
¼
110.18(9), N1–Zn1–O1 ¼ 98.46(8), O1–Zn1–O3 ¼ 127.04(9) .
analysis. Elemental analysis data is consistent with an empirical Symmetry code: a ¼ ꢂx + 3/2, y + 1/2, ꢂz + 1/2. (b) The structure of a
formula of Zn(L2)(OAc)
for 1, in which a 2 : 1 metal–ligand ratio was found. X-ray
2
. This is obviously different from that helical chain with M-chirality observed in 2. (c) The 3-D space-filling
representation along the crystallographic a axis, showing the compact
packing in the crystals.
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oen observed in Zn -coordination polymers with 4,2 :6 ,4 -tpy
RSC Advances
II
0
0
00
6,7
derivatives,
it was reported that
a
conversion from
0
[
Zn (L)(OAc) ] to [Zn(L)(OAc)2]n (L represents 4 -phenyl-
2
4 n
0 0 00
4,2 :6 ,4 -terpyridine) had occurred during the process of crys-
tallization, and the extended structure of 2 is mimicking that of
Zn(L)(OAc) , possessing a helical twist as shown in Fig. 2b.
[
2 n
]
Helical chains in the crystals with both P- and M-chirality were
found to arrange alternately through p-stacking interactions
between the pyridine rings, resulting in a quite condensed
molecular packing mode as observed in the 3-D space-lling
diagram of the crystal structure (Fig. 2c).
Reaction of L3 with Zn(OAc) $2H O by the layering tech-
2
2
nique similar to that for crystal growth of 1 gave crystals of 3 in
high yield. Elemental analysis of the crystalline sample of 3
revealed an empirical formula of Zn (L3)(OAc) and X-ray struc-
2 4
tural analysis conrmed that 3 is isomorphic to 1 that was
0
described previously, although the 4 -substituent (4-dimethyl-
0
0
00
aminophenyl group) on 4,2 :6 ,4 -tpy in ligand L3 is obviously
different from that of L1 (4-pyridyl group). An ORTEP structure
of the repeat unit in 3 is shown in Fig. 3a and partial bond
parameters are given in the caption. The 1-D polymeric zigzag
chains in 3 run along the crystallographic a-axis and pack
through p-stacking interactions between the aromatic rings,
thereby mimicking those observed in 1. However, the presence
of 4-dimethylaminophenyl group on ligand L3 has important
consequences on the 3-D packing mode of polymeric chains in
3. As seen in Fig. 3b, extra p-stacking interactions were found
between the 4-dimethylaminophenyl units of 1-D chains from
0
0
00
adjacent layers except for those between the 4,2 :6 ,4 -tpy
domains, which resulted in a signicantly different 3-D
arrangement of chains in 3 than that in 1 as viewed down the
crystallographic b-axis (Fig. 1b and 3b). No co-crystallized
solvent molecules were found in the crystal packing, owing to
the more condensed network structure in 3 (Fig. 3c).
Slow evaporation of
a reaction mixture of L4 with
Zn(OAc) $2H O in CH Cl –MeOH solution over a period of one
2
2
2
2
week gave colorless blocks that matched with the formula of
Zn(L4)(OAc) based on elemental analysis. X-ray structural
2
analysis of a good-quality single crystal picked from the bulk
sample revealed a discrete dinuclear molecule with a compo-
nent of Zn (L4) (OAc) (4), different from the polymeric struc-
2 2 4
tures found in 1–3 (Fig. 4). However, PXRD measurement of the Fig. 3 (a) The repeat unit in 3 with ellipsoids plotted at the 40%
probability level, and H atoms omitted for clarity. Selected bond
bulk sample indicated that the crystalline solid possibly
contains another minor component and we were unable to
determine other possible single-crystal structures in the solid
lengths: Zn1–N1 ¼ 2.025(2), Zn1–O1 ¼ 2.045(2), Zn1–O2 ¼ 2.046(2),
i
Zn1–O3 ¼ 2.049(2), Zn1–O4 ¼ 2.043(2), Zn1–Zn1 ¼ 2.8924(7) A˚ .
Symmetry code: i ¼ ꢂx + 1/2, ꢂy + 3/2, ꢂz. (b) The 3-D packing in 3
due to the difficulty of separation. The presence of another viewed down the b-axis highlighting the p-stacking between the 4-
possible phase in this sample is not surprising, as it was dimethylaminophenyl units with red and blue spheres. (c) The space-
filling representation showing the compact packing mode in the
crystals.
recently discussed in detail that the competition between
0
0
00
polymers and discrete complexes based on 4,2 :6 ,4 -tpy ligands
11
and zinc(II) acetate oen occurs. Therefore, a coordination
polymer with the same metal–ligand composition (1 : 1 ratio) 3. Molecules of 4 further assemble into 2-D sheets through
might have formed with a small amount during the crystalli- p-stacking interactions between the tpy domains along the
zation of 4. In 4, the paddle-wheel type {Zn
observed again and it links two ligands with one of the terminal for the central pyridine-N atom, one of terminal N atoms is non-
coordinated and L4 behaves in fact as a monodentate ligand.
than that in polymer 1, while all Zn–O bond lengths fall in the The bulk sample of 4 is poorly soluble in common organic
range of values for similar {Zn (m-OAc)
} motifs found in 1 and solvent, except in DMF or DMSO.
2 4
(m-OAc)
} motif is crystallographic c-axis (Fig. 4b). It is unexpected that in 4, except
˚
N atoms. The Zn–Zn distance (2.9258(6) A) is slightly longer
2
4
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Fig. 5 (a) The repeat unit in 5 with ellipsoids plotted at the 40%
probability level, and H atoms omitted for clarity. Selected bond
i
parameters: Zn1–N1 ¼ 2.080(4), Zn1–N3 ¼ 2.084(4), Zn1–I1 ¼
2
.5338(7), Zn1–I2 ¼ 2.5608(7), C21–O1 ¼ 1.386(6), C20–O1 ¼ 1.424(7),
i
C20–O2 ¼ 1.414(7), C19–O2 ¼ 1.396(6) A˚ ; N1–Zn1–N3 ¼ 99.26(16),
i
N1–Zn1–I1 ¼ 107.80(12), N1–Zn1–I2 ¼ 106.81(12), N3 –Zn1–I1 ¼
i
ꢀ
123.87(2) .
Fig. 4 (a) The ORTEP structure of the dinuclear complex 4 with 110.22(12), N3 –Zn1–I2
¼
106.09(11), I1–Zn1–I2
¼
ellipsoids plotted at the 40% probability level, and H atoms omitted for Symmetry code: a ¼ x, ꢂy + 1/2, z ꢂ 1/2. (b) The packing mode in 5
clarity. Selected bond parameters: Zn1–O5 ¼ 2.014(2), Zn1–O9 ¼ revealing the pairs of helical chains pack through p-stacking between
2
.026(2), Zn1–O3 ¼ 2.029(3), Zn1–N1 ¼ 2.032(3), Zn1–O7 ¼ 2.087(3), the tpy domains.
Zn1–Zn2 ¼ 2.9258(6), Zn2–O8 ¼ 2.020(3), Zn2–O6 ¼ 2.028(2), Zn2–
O10 ¼ 2.034(2), Zn2–O4 ¼ 2.052(3), Zn2–N4 ¼ 2.034(3) A˚ ; O5–Zn1–
O9 ¼ 160.08(10), O5–Zn1–O3 ¼ 90.39(12), O3–Zn1–O9 ¼ 90.16(11),
ꢀ
remarkable exception from those observed for related zinc
dihalide and acetate assembles with other 4 -arene-substituted
rings shown as filled spheres. (c) The 3-D space-filling representation 4,2 :6 ,4 -tpys, in which chains are exclusively racemic or
O5–Zn1–N1 ¼ 103.59(10), O3–Zn1–N1 ¼ 101.69(11) . (b) The packing
0
mode in the unit cell showing the p-stacking between the pyridine
0
0
00
showing compact molecular packing.
1–4
homochiral helical structures.
Catalytic transesterication
Given the uncommon assembly of ligand L4 with
Zn(OAc)
iodide. Layering a methanolic solution of ZnI
MeOH solution of L4 resulted in the formation of colourless chemical structures.
crystals of 5 that were suitable for single-crystal X-ray diffrac- complexes have proven to be active catalysts for
2
$2H
2
O, we next examined the reaction of L4 with zinc Catalytic transesterication is an important, atom-economy
2
onto the CH Cl transformation for generating diverse esters with different
2
2
–
3
6–38
Although a variety of transition metal
3
9,40
tion. Structural analysis revealed that the structure of 5 is a 1-D transesterication under homogeneous conditions,
the
coordination polymer with the formula of {Zn(L4)I } , in which recently reported m -oxo-tetranuclear zinc cluster complex
2
n
4
2
4 3 6
each ZnI unit is bound to two distinct ligands with terminal Zn (OCOCF ) O serves as one of the most efficient catalyst for
4
0
II
pyridine-N atoms (Fig. 5). Each Zn atom is in a slightly distorted this transformation. In addition, Zn -based coordination
tetrahedral coordination environment and the bond lengths polymers have also been extensively explored for the hetero-
4
1–44
and angles are unexceptional (see the caption of Fig. 5). The geneous transesterication of a variety of esters,
high-
coordination polymeric chains were found to be wave-like and lighting the potential of zinc(II) assembles in carrying out this
0
run through the crystallographic c-axis, with all the 4 -position important catalytic transformation. However, zinc(II) coordi-
0
0
00
substituents of the ligand pointing to the same direction. The nation polymers derived from 4,2 :6 ,4 -tpy ligands were not
Zn–Zn distance between the closest peaks of the wave is studied yet for this type of catalysis. We were therefore
˚
1
8.649 A (Fig. 5b). Again, p-stacking interactions between the interested in ascertaining whether or not the zinc(II) assem-
tpy domains of the chains assemble 1-D chains into a compact bles of 1–3 and 5 were active catalysts for the trans-
network, similar to those observed in structures 2 and 3 (Fig. 3c esterication of ester substrates. 4 was not tested because of
and 4c). It is noteworthy that the wave-like chain in 5 is a its phase impurity. Solid samples of zinc(II) compounds were
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a
used to catalyse the tranesterication of phenyl acetate with Table 2 Catalytic transesterification of phenyl acetate with alcohols
1
-butanol and several other alcohols (Scheme 2). All reactions
b
Entry
Catalyst
Solvent
Alcohols
Yield [%]
were performed on a 1.0 mmol scale using 1 mol% of the solid
catalysts (calculated based on the ligands) over a period of 18
h under conditions as indicated in Table 2. The yields of new
ester products were determined by GC-MS analysis while
using 1,2,4-trimethylbenzene as an internal standard, and the
catalytic results are summarized in Table 2. Initially, when
compound 1 was used to catalyse the reaction of phenyl
acetate with 1-butanol at room temperature, only trace
product of 1-butyl acetate was detected aer a reaction period
of 18 h (entry 1, Table 2). However, we were pleased to nd
c
1
1
1
2
3
5
3
3
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
CH CN
3
n-Hexane
THF
Neat
Neat
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
1-Butanol
Methanol
1-Hexanol
2-Propanol
Benzylic alcohol
tert-Butanol
1-Butanol
2
95
66
33
13
99
98
6
5
25
4
56
39
99
92
30
15
5
2
3
4
5
d
6
e
7
f
8
DMAP
None
Zn(OAc)
9
10
11
12
13
2
1
1
1
1
1
1
1
1
ꢀ
that heating the reaction to 50 C leads to a drastic improve-
ment of the reactivity and 95% yield was obtained through the
GC analysis (entry 2, Table 2), which is comparable to those 14
II
results for other Zn -containing catalysts reported under 15
3
7–40
16
Neat
Neat
Neat
Neat
similar conditions in the literature.
Accordingly, other
17
18
19
compounds were also examined for this reaction under the
same condition. It was found that other compounds gave
inferior results than 1, catalysing the transesterication of
phenyl acetate in 13–66% yields (entries 3–5, Table 2). In
g
L1/Zn(OAc)
2
35
a
Condition: 1 mmol of phenyl acetate, 0.01 mmol (1 mol%) of catalyst
and 4 mL of alcohols under neat conditions, 50 C, 18 h. Yields based
ꢀ
b
c
d
contrast, the previously reported analogue [Zn
with a phenyl group at the 4 -position of terpyridine showed
also modest reactivity under the same conditions. The
2
(L)(OAc)
4
]
n
on GC-MS analysis. Reaction run at room temperature. DMAP
e f
0
(5 mol%) added. Triethylamine (5 mol%) added. DMAP (5 mol%)
g
2 2
was used as a catalyst. 2 mol% L1 and Zn(OAc) $2H O were employed.
controlled reactions without a catalyst or using Zn(OAc)
catalyst gave only trace amount or low conversion of the ester-
converted product (entries 9 and 10, Table 2). Low conver- drastically increased the catalytic activity of a Zn
2
as a
4 3 6
(OCOCF ) O
40,45
sions were also obtained when the reaction was conducted in cluster for transesterifcation.
Indeed, when 5 mol% DMAP or
solvents such as acetonitrile, n-hexane or tetrahydrofuran triethylamine was added to the reaction mixture containing
than that under neat conditions (entries 11–13, Table 2). catalyst 3, almost quantitative conversions were observed (entries
Methanol and 1-hexanol were also found to be good 6 and 7, Table 2). Controlled experiment showed that DMAP
substrates for the transesterication catalysed by 1, while 2- alone was not an effective catalyst for this reaction (entry 8,
propanol showed poorer reactivity (entries 14–16, Table 2). Table 2). Moreover, compared to 2 the higher catalytic activity of
The larger alcohol, benzylic alcohol was found to be even less 1 could be attributed to the microporous channels (ꢃ6.0 ꢁ 7.0
2
˚
reactive under neat condition, and the bulky tert-butanol gave A ) present in the network of 1, which allowed substrate mole-
almost no conversion of corresponding ester product (entries cules to access internal catalytically active sites. This is also
1
7 and 18, Table 2).
It is worth stating that the different reactivity of Zn -con- inferior catalytic reactivity (entries 14–18, Table 2). However, it
consistent with the fact that substrates of larger size showed
II
taining assembles is relevant to their diverse solid-state struc- was unclear whether the reactions were run heterogeneously,
tures as we discussed above. The catalytic results appeared albeit the poor solubility of these assembles in alcohols. The UV-
relatively better while using catalysts 1 and 2 (95% and 66% vis spectra of ltered solutions of crystalline samples dispersed
yields, respectively) than using 3 and 5 (33% and 13% yields). in n-butanol at room temperature showed only negligible
We have conrmed that in the structures of compounds 1 and 2, absorption peaks in the region of 200–800 nm, while a saturated
ꢀ
additional non-coordinating terminal pyridine rings were n-butanol solution of 1 at 50 C revealed obvious absorption
present except for the central pyridine ring, and these provided bands identical to the free ligand absorption (see ESI†), indi-
extra basic sites in the catalysts, in addition to the Lewis acidic cating the partial dissociation of the coordination polymer under
metal centres and could have assisted to activate the substrates the catalytic conditions. Thus, a dissociation–reassembly process
during the catalysis. It was recently reported that the basic might have taken place during the catalytic reactions. Neverthe-
additives, 4-dimethylaminopyridine (DMAP) or other amines less, the structure of 1 should still play an important role in
promoting catalytic reactivity, as
a controlled experiment
employing a L1/Zn(OAc) (2 mol%) mixture in n-butanol was found
2
to be much less efficient than using 1 (entry 2 vs. 19, Table 2).
Conclusions
0
0
0
00
Two new 4 -substituted 4,2 :6 ,4 -tpy ligands L2 and L4 have
Scheme 2 Transesterification of phenyl acetate with 1-butanol cata-
lysed by zinc(II) compounds.
been synthesized and characterized. The assembly between
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these ligands along with another two known ligand analogs and
5 E. C. Constable, G. Zhang, C. E. Housecro, M. Neuburger
and J. A. Zampese, CrystEngComm, 2009, 11, 2279.
6 E. C. Constable, G. Zhang, E. Coronado, C. E. Housecro and
M. Neuburger, CrystEngComm, 2010, 12, 2139.
7 E. C. Constable, G. Zhang, C. E. Housecro, M. Neuburger
and J. A. Zampese, CrystEngComm, 2010, 12, 2146.
8 E. C. Constable, G. Zhang, C. E. Housecro, M. Neuburger
and J. A. Zampese, CrystEngComm, 2010, 12, 3733.
9 E. C. Constable, G. Zhang, C. E. Housecro and
J. A. Zampese, CrystEngComm, 2011, 13, 6864.
zinc salts (Zn(OAc) and ZnI ) afforded four coordination poly-
2
2
meric networks and one discrete dimeric complex (compounds
–5). X-ray structural determination conrmed the structural
diversity in the resulting metal assembles. Although the same
metal source Zn(OAc) was used for the syntheses of 1–4,
different coordination assembles were revealed. In the struc-
tures of 1, 3 and 4, the paddle-wheel {Zn (m-OAc) } motif was
1
2
2
4
exclusively observed. The ligands in 1 and 3 adopt similar
coordination mode and form 1-D polymeric chains. However,
the 3-D packing structures in the crystals differ remarkably from 10 E. C. Constable, C. E. Housecro, S. Vujovic and
0
each other, due to the different 4 -substituted groups on
4
J. A. Zampese, CrystEngComm, 2014, 16, 3494.
0
0
00
,2 :6 ,4 -tpy. A microporous network was found in 1, while 11 Y. M. Klein, E. C. Constable, C. E. Housecro, J. A. Zampese
chains stack compactly together through p/p interactions in 3 and A. Crochet, CrystEngComm, 2014, 16, 9915.
without porosity. The structure of 4 contains a discrete dinu- 12 Y. M. Klein, E. C. Constable, C. E. Housecro and
clear complex, in contrast to those of 1 and 3. The mononuclear A. Prescimone, Inorg. Chem. Commun., 2014, 49, 41.
Zn(OAc) } motif observed in 2 connects to ligand L2 to result in 13 E. C. Constable, C. E. Housecro, A. Prescimone, S. Vujovic
the 1-D helical chain. This is in contrast to the structure as seen and J. A. Zampese, CrystEngComm, 2014, 16, 8691.
in 5, in which the {ZnI } units assemble ligand L4 into a wave- 14 Y. M. Klein, E. C. Constable, C. E. Housecro and
{
2
2
like polymeric chain with no helical twisting.
All zinc(II) coordination assembles were evaluated as cata- 15 F. Yuan, X. Wang, H.-M. Hu, S.-S. Shen, R. An and G.-L. Xue,
lysts for the transesterication of phenyl acetate with alcohols. Inorg. Chem. Commun., 2014, 48, 26.
The results indicated that compound 1 was the most active 16 E. C. Constable, C. E. Housecro, S. Vujovic and
catalyst, affording the converted ester product in high yield, J. A. Zampese, CrystEngComm, 2014, 16, 328.
while much lower conversions were observed with all other 17 X.-L. Yang, Y.-Q. Shangguan, H.-M. Hu, B. Xu, B.-C. wang,
J. A. Zampese, Polyhedron, 2014, 81, 98.
compounds. We have tentatively attributed this to the fact that
the solid-state structure of 1 contains both non-coordinated
J. Xie, F. Yuan, M.-L. Yang, F.-X. Dong and G.-L. Xue, J.
Solid State Chem., 2014, 216, 13.
terminal pyridine that could act as an internal base for catal- 18 B. Xu, J. Xie, H.-M. Hu, X.-L. Yang, F.-X. Dong, M.-L. Yang
ysis and microporous channels that allow substrates with suit- and G.-L. Xue, Cryst. Growth Des., 2014, 14, 1629.
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0
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ChemCatChem, 2011, 3, 1384.
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