Synthesis, characterization, and cycloaddition reaction studies of zinc(II) acetate complexes containing 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine ligands

Article abstract of DOI:10.1016/j.poly.2016.10.005

The complexes [Py(Pz)₂]Zn(OC([dbnd]O)Me)₂(1) and [Py(Me₂Pz)₂]Zn(OC([dbnd]O)Me)₂(2), where ligands Py(Pz)₂and Py(Me₂Pz)₂are tridentate 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine, respectively, have been synthesized and characterized. The single crystal X-ray diffraction analysis confirmed compound 2 to be monomeric with six-coordinate zinc center. In addition to tridentate ligand Py(Me₂Pz)₂, both κ¹-acetate and κ²-acetate ligands are ligated to zinc metal atom in 2. The synthesized complexes 1 and 2 were used as effective catalysts for the cycloaddition between CO₂and epoxides in the presence of various kinds of cocatalysts such as n-Bu₄PBr, n-Bu₄NI, n-Bu₄NBr, n-Bu₄NCl, PPNCl, and DMAP under the condition of 75 °C, 10 bar CO₂pressure, 0.1 mol% catalyst loading, and 24 h. The reaction temperature, CO₂pressure, and catalyst loading ratio applied in this study are somewhat milder condition than those for other reported zinc-based catalysts. In addition, 1/n-Bu₄PBr system showed the best catalytic activity for the cycloaddition of CO₂to propylene oxide, which showed the highest reactivity among seven other kinds of epoxides.

Full text of DOI:10.1016/j.poly.2016.10.005

Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, and cycloaddition reaction studies of zinc(II)
acetate complexes containing 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis
(
3,5-dimethylpyrazol-1-yl)pyridine ligands
Min Seok Shina, a,1, Beom Jin Oh a,1, Ji Yeon Ryu , Myung Hwan Park , Min Kim , Junseong Lee
b
c
a
b,
⇑
,
⇑
Youngjo Kim
a
Department of Chemistry and BK21+ Program Research Team, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea
Department of Chemistry, Chonnam National University, Gwangju 61186, Republic of Korea
Department of Chemistry Education, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
2 2 2 2 2
The complexes [Py(Pz) ]Zn(OC(@O)Me) (1) and [Py(Me Pz) ]Zn(OC(@O)Me) (2), where ligands Py(Pz)2
and Py(Me Pz) are tridentate 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,5-dimethylpyrazol-1-yl)pyri-
2 2
dine, respectively, have been synthesized and characterized. The single crystal X-ray diffraction analysis
confirmed compound 2 to be monomeric with six-coordinate zinc center. In addition to tridentate ligand
Received 9 July 2016
Accepted 4 October 2016
Available online xxxx
1
2
Py(Me
complexes 1 and 2 were used as effective catalysts for the cycloaddition between CO
presence of various kinds of cocatalysts such as n-Bu PBr, n-Bu NI, n-Bu NBr, n-Bu NCl, PPNCl, and DMAP
under the condition of 75 °C, 10 bar CO pressure, 0.1 mol% catalyst loading, and 24 h. The reaction tem-
perature, CO pressure, and catalyst loading ratio applied in this study are somewhat milder condition
than those for other reported zinc-based catalysts. In addition, 1/n-Bu PBr system showed the best cat-
2
Pz)
2
, both
j
-acetate and
j
-acetate ligands are ligated to zinc metal atom in 2. The synthesized
Keywords:
Zinc
Tridentate ligand
Cycloaddition
2
and epoxides in the
4
4
4
4
2
CO
Epoxide
2
2
4
alytic activity for the cycloaddition of CO
among seven other kinds of epoxides.
2
to propylene oxide, which showed the highest reactivity
Ó 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
and poisonous problem, one of possible candidates would be zinc
complexes bearing chelating ligands.
The chemical transformation of carbon dioxide into valuable
There are many kinds of reported zinc compounds as catalysts
for aliphatic cyclic carbonates. Examples include Zn(II) complexes
chelated by monodentate pyridinium alkoxy ligand [13,14], biden-
chemicals is currently one of significant themes in both academic
and industrial fields [1]. Since aliphatic cyclic carbonates, which
have broad applications for aprotic solvents [2], electrolytes for
secondary batteries [3], and intermediates for polycarbonates [4],
were commercialized in 1950s [5], a wide range of catalytic sys-
tems including transition metal complexes [6,7], quaternary
ammonium salt [8,9], ionic liquids [10–12], and so on have been
tested to make cyclic carbonates from carbon dioxide and epox-
ides. Despite the fact that some excellent initiators have been
found among these systems for aliphatic cyclic carbonates, the
search for new catalysts with eco-friendly and low toxic properties
is still remain of interest. In the aspect of solving environmental
0
tate imine-benzotriazole phenolate [15], bidentate 2,2 -bipyridine
derivatives [16], bidentate acetylacetonato [17], bidentate pyridine
N-oxide [18], bidentate carboxylate [19], tridentate bis(arylimi-
nomethyl)pyrrole derivatives [20], tridentate pyridino-imino-phe-
nolato derivatives [21], tetradentate salen derivatives [22,23],
tetradentate salphen derivatives [24–30], and tetradentate
porphyrin derivatives [31]. Most reports of zinc catalysts for
cyclic carbonates [13–31] have focused on compounds chelated
by bidentate [15–19] or tetradentate ligands [22–31]; much
less attention has been directed towards complexes containing
tridentate ligands [20,21].
2
Tridentate 2,6-bis(pyrazol-1-yl)pyridine (PyPz ) ligand [32] and
its derivatives have been extensively studied as ligands in
coordination chemistry for 25 years [33,34]. A variety of their
metal-based inorganic systems, including those with Cr [35],
Fe [36–40], Co [39–41], Ni [41,42], Cu [43,44], Ag [45], Re [46],
⇑
Corresponding authors. Fax: +82 43 267 2279.
E-mail addresses: leespy@chonnam.ac.kr (J. Lee), ykim@chungbuk.ac.kr (Y. Kim).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.poly.2016.10.005
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0
Please cite this article in press as: M.S. Shin et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.10.005

2
M.S. Shin et al. / Polyhedron xxx (2016) xxx–xxx
Ru [47–50], Pd [50], Pt [51], Eu [52], and Zn [53–56] have been
developed. They have some applications in the field of polymeriza-
tion catalysts [35,36,39], spin-crossover material [40], cross-
coupling catalysts [42], dye-sensitized solar cells [49], and
luminescent materials [51,52]. Interestingly, chromium- and vana-
dium-based catalysts containing 2,6-bis(3,5-dimethylpyrazol-1-
water was added to the reaction vessel. Organic portion was sepa-
rated and the aqueous layer was extracted with diethyl ether
(3 ꢀ 20 mL). The combined organic portions were dried with
4
MgSO , filtered, and concentrated. After column chromatography,
2
desired product PyPz was obtained as a colorless powder
(42.0%, 1.83 g).
1
ylmethyl)pyridine, similar to PyPz
for ethylene polymerization [57,58]. Unlike other metal complexes,
zinc-based complexes with PyPz ligand are not widely studied.
Even though the molecular structure of zinc(II) chloride complex
containing PyPz ligand was determined [53], to our best knowl-
edge, any zinc compound with 2,6-bis(3,5-dimethylpyrazol-1-yl)
pyridine (Py(Me Pz) ) ligand has never been reported in the liter-
2
, have good catalytic activity
3
H NMR (CD OD, 400.13 MHz): d 8.73 (dd, J = 2 Hz and 0.4 Hz,
2H), 8.04 (t, J = 6.4 Hz, 1H), 7.80 (d, J = 6.4 Hz, 2H), 7.77 (d,
J = 0.8 Hz, 2H), 6.56 (dd, J = 2.4 Hz and 1.6 Hz, 2H).
2
1
3
C NMR (CD
110.3, 109.2.
3
OD, 100.61 MHz): d 151.4, 143.7, 143.1, 128.8,
2
2
2
2.3.2. Synthesis of 2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine (Py
(Me Pz)
In a manner analogous to that used in the procedure for PyPz
the desired product Py(Me Pz) as a colorless powder was pre-
ature. Good ethylene polymerization behavior for chromium and
vanadium-based catalysts containing its related ligand such as
2
2
)
2
,
2
,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine prompted us to
2
2
search for new catalysts.
pared from 3,5-dimethylpyrazole (1.92 g, 20.0 mmol), potassium
(0.78 g, 20.0 mmol), and 2,6-dibromopyridine (2.37 g, 10.0 mmol)
This work reports the synthesis and their catalysis of the
cycloaddition reaction between CO
acetate complexes with PyPz and Py(Me
solid state structure of zinc(II) acetate complex chelated by Py
Me Pz) was confirmed by single crystal X-ray diffraction.
2
and epoxides using new zinc
in a yield of 55.7 % (1.49 g).
1
2
2
Pz) . In addition, the
2
3
H NMR (CD OD, 400.13 MHz): d 8.03 (t, J = 8.0 Hz, 1H), 7.64 (d,
J = 8.0 Hz, 2H), 6.08 (s, 2H), 2.53 (s, 6H), 2.25 (s, 6H).
1
3
(
2
2
3
C NMR (CDCl , 100.61 MHz): d 152.5, 151.7, 142.8, 142.3,
1
15.5, 110.0, 14.01, 13.37.
2
. Experimental
2
.3.3. Synthesis of (PyPz
mixture of 0.211 g (1.00 mmol) of PyPz
1.00 mmol) of zinc acetate dihydrate was suspended in 30 mL of
2 2
)Zn(OC(=O)Me) (1)
A
2
and 0.219 g
2.1. General Procedure
(
methanol at room temperature and heated to 80 °C. After 24 h stir-
ring, all volatiles were removed under vacuo and the obtained col-
orless solid was washed with hexane. The desired product 1 was
All manipulations were carried out under an atmosphere of
dinitrogen by using standard Schlenk-type glassware on a dual
manifold Schlenk line in a glove box [59]. Dinitrogen was deoxy-
genated using activated Cu catalyst and dried with drierite [60].
All chemicals including 2,6-dibromopyridine, 3,5-dimethylpyra-
zole, pyrazole, zinc acetate dihydrate and epoxides were purchased
from Aldrich and used as supplied unless otherwise indicated. Car-
bon dioxide (99.999%) was used as received without further purifi-
obtained as a colorless powder in a yield of 69.8 % (0.275 g).
1
3
H NMR (CD OD, 400.13 MHz): d 8.77 (d, J = 2.4 Hz, 2H), 8.24 (t,
J = 6.4 Hz, 1H), 7.97 (s, 2H), 7.91 (d, J = 6.4 Hz, 2H), 6.70 (s, 2H), 1.94
s, 6H).
13C NMR (CD
29.7, 111.0, 110.0, 22.69.
Elemental Analysis Calc. for C15
7.74. Found: C, 45.53; H, 3.98; N, 17.70.
(
3
OD, 100.61 MHz): d 180.8, 148.7, 145.7, 143.9,
1
1
2
cation. CD
sieves and used after vacuum transfer to a Schlenk tube equipped
with a J. Young valve [60]. CD OD and CDCl were dried over 4 Å
3 3
OD and CDCl were dried over 4 Å activated molecular
15 5 4
H N O Zn: C, 45.65; H, 3.83; N,
3
3
activated molecular sieves and used after vacuum transfer to a Sch-
lenk tube equipped with a J. Young valve [60].
.3.4. Synthesis of (Py(Me
In a manner analogous to that used in the procedure for 1, the
desired product 2 as a colorless solid was prepared from Py(Me
Pz) (0.267 g, 1.00 mmol) and zinc acetate dihydrate (0.219 g,
.00 mmol) in a yield of 67.9 % (0.306 g).
2 2 2
Pz) )Zn(OC(@O)Me) (2)
2
-
2
.2. Measurements
2
1
1H and 13C NMR spectra were recorded at ambient temperature
on a 400 MHz NMR spectrometer using standard parameters. All
1
H NMR (CD
3
OD, 400.13 MHz): d 8.28 (t, J = 8 Hz, 1H), 7.73 (d,
J = 7.2 Hz, 2H), 6.34 (s, 2H), 2.71 (s, 6H), 2.35 (s, 6H), 1.88 (s, 6H).
chemical shifts are reported in d units with reference to the peaks
13
C NMR (CD
44.1, 113.5, 110.4, 22.89, 14.74, 12.99.
23 5 4
Elemental Analysis Calc. for C19H N O Zn: C, 50.62; H, 5.14; N,
3
OD, 100.61 MHz): d 180.5, 153.3, 147.6, 146.5,
1
13
of residual CDCl
3
(d 7.24, H NMR; d 77.0, CNMR) or CD
3
OD (d
.30, H NMR; d 49.0, C NMR) [61]. Elemental analyses was per-
formed with EA 1110-FISONS analyzer.
1
1
2
1
13
3
5.54. Found: C, 50.88; H, 5.04; N, 15.38.
2
.3. Synthesis
.3.5. X-ray structure determination of 2
Crystallographic assessment of 2 was performed at ambient
temperature using a Bruker APEXII CCD area detector diffractome-
ter with graphite-monochromated Mo K (k = 0.71073 Å) radia-
2 2 2
Compounds PyPz and Py(Me Pz) were prepared according to
literature method reported previously [32]; however, they were
achieved in a slightly modified way and with different results
including yields and spectroscopic data.
a
tion. A single crystal of suitable size and quality was selected and
mounted on a glass capillary using ParatoneÒ oil and centered in
the X-ray beam using a video camera. Multi-scan reflection data
were collected with a frame width of 0.5° in w and h and 5 s expo-
sures per frame. Cell parameters were determined and refined by
SMART [62], while data reduction was performed using SAINT soft-
ware [63]. Data were corrected for Lorentz and polarization effects.
Empirical absorption correction was applied using SADABS [64]. The
structures of the compounds were solved by direct methods and
refined by the full matrix least-squares method, using the SHELXTL
program package and applying anisotropic thermal parameters
2
.3.1. Synthesis of 2,6-bis(pyrazol-1-yl)pyridine (PyPz
To a stirred solution of pyrazole (2.72 g, 40.0 mmol) in 20.0 mL
of dimethoxyethane (DME) were added sliced pieces of potassium
1.56 g, 40.0 mmol) at room temperature. After refluxing for 3 h,
,6-dibromopyridine (4.74 g, 20.0 mmol) in 20 mL of DME was
2
)
(
2
slowly added to reaction vessel by cannula at 70 °C. The mixture
was then heated to 110 °C and stirred for 4 days. The reaction mix-
ture was allowed to cool to room temperature and then 50 mL of
Please cite this article in press as: M.S. Shin et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.10.005

M.S. Shin et al. / Polyhedron xxx (2016) xxx–xxx
3
cocatalyst (10
lmol), into a stainless steel pressure reactor (10 mL
2
inner volume). Then, CO was charged in the reactor and the pres-
sure was adjusted to desired pressure at appropriate temperature.
The reactor was maintained for the desired time. After then, the
pressure reactor was cooled to ambient temperature, and the
excess CO
H NMR analysis.
2
was vented. A small sample of mixture was taken for
1
3
. Results and discussion
The aim of this work is the synthesis, characterization, and the
comparison of cycloaddition reaction behaviors of novel zinc com-
plexes with N,N,N-type tridentate ligands such as PyPz
2
and Py
. As shown in Scheme 1, the preparation of PyPz and
was achieved by the reaction of potassium and corre-
(
Me Pz)
2 2
2
Py(Me Pz)
2 2
sponding pyrazole in dimethoxyethane, followed by the addition of
,6-dibromopyridine. Suitable workup gave PyPz and Py(Me Pz)
as colorless powder in 42.0 % and 55.7%, respectively. Zinc acetate
complexes 1 and 2 were obtained from reactions of Zn(OAc)
(OH with PyPz and Py(Me Pz) in methanol, respectively.
2
2
2
2
Fig. 1. X-ray molecular structure of compound 2 with 50% thermal ellipsoids. H
atoms are omitted for clarity.
2
-
ꢁ
2
)
2
2
2
2
Table 1
The colorless crystalline solids 1 and 2 are freely soluble in metha-
Crystallographic data and parameters for compound 2.
nol and methylene chloride and insoluble in hexane and toluene.
1
13
All compounds were fully characterized by H and C NMR
spectroscopy. As given in Table 3, aromatic signals of tridentate
2
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
‘C19 H23 N5 O4.50 Zn’
458.79
monoclinic
1
13
ligands bound to zinc metal in the H and C NMR spectra for 1
and 2 are shifted downfield compared to the signals for corre-
P2
1
/c
sponding free ligands, indicating that PyPz
-donor ligands were strongly bound and chelated to zinc metal.
In addition, methyl protons of two acetate ligands in both 1 and
showed a single peak at about 1.9 ppm. This observation means
2 2 2
and Py(Me Pz) as
p
10.5679(3)
b (Å)
12.7067(3)
2
c (Å)
15.5972(4)
a
(°)
b (°)
(°)
90
two acetate ligands in solution structure may have the same chem-
ical environments in NMR time scale. Thus, solid state structure
and solution structure for 2 may be different because two acetates
showed different bind modes to zinc metal as shown in Fig. 1. The
NMR signals were sharp and variable-temperature NMR spectrum
did not change.
Recrystallization from methanol at ꢂ20 °C gave 2 as a colorless
crystalline solid suitable for X-ray diffraction analysis, which
assessed its solid-state structure (Fig. 1). Tables 1 and 2 list
detailed crystallographic data and selected interatomic distances
and angles, respectively. The structure was confirmed as the mono-
94.3689(13)
90
2088.35(9)
4
1.459
1.214
952
c
3
V (Å )
Z
D
l
F(000)
Index ranges
3
calc (g/cm )
(mm
ꢂ1
)
ꢂ14 6 h 6 14, ꢂ15 6 k 6 15,
ꢂ18 6 l 6 18
1.933–25.402
27434
h range (deg)
Reflections collected
Independent reflections
Number of observed reflections
3840
3026
1
meric complex, and crystallized in the P2 /c space group. The
(
I > 2
Number of parameters refined
GOF(I > 2 (I))
(all data)
r(I))
ligand Py(Me Pz) acts in a tridentate manner through its three
2
2
546
1.048
r
N-donors. The maximum value of the dihedral angle between the
planes of the aromatic rings is about 9°, which suggests the high
planarity of Py(Me Pz) in complex 2. The Zn–N bond lengths of
2 2
R
R
wR
wR
1
1
0.0682
0.0532
0.1190
0.1107
0.464/ꢂ0.370
a
(I > 2
r
(I))
2
(all data)
a
2
(I > 2r(I))
Largest diff. peak and hole
Table 2
a
2
2
2
2)2]}1/2.
Selected interatomic distances (Å) and angles (°) of compound 2.
R = R||Fo| – |F ||/R |F | and wR = {R[w(F – F ) ]/R[w(F
1 c o 2 o c o
Distances
Zn–O1
Zn–O2
Zn–O3
Zn–N1
Zn–N3
Zn–N5
N1–N2
N4–N5
2.105(14)
2.488(15)
2.067(10)
2.182(20)
2.152(16)
2.119(13)
1.330(26)
1.379(18)
O1–C16
O2–C16
O3–C18
O4–C18
C16–C17
C18–C19
N3–C10
N3–C6
1.118(24)
1.142(27)
1.324(13)
1.178(13)
1.430(15)
1.551(15)
1.331(18)
1.355(25)
for all non-hydrogen atoms [65]. Due to the disorder of acetate
parts, several restraints (SADI for Zn–O1 and Zna–O1a, C18–O3
and C18–O4; Zn–O3 and Zna–O3a; ISOR for C2, C16, C5, O4A,
C10, C18) were applied in the structure refinement process. The
molecular structure of 2 (Fig. 1) was drawn using the program DIA-
MOND [66]. Details of the crystallographic parameters and selective
bond length and angles are listed in Tables 1 and 2, respectively.
Angles
O1–Zn–O2
O1–Zn–O3
O2–Zn–O3
O1–Zn–N1
O1–Zn–N3
O1–Zn–N5
O2–Zn–N1
O2–Zn–N3
50.12(56)
94.95(56)
140.41(46)
107.56(60)
138.92(54)
98.24(50)
81.92(65)
90.47(55)
O2–Zn–N5
O3–Zn–N1
O3–Zn–N3
O3–Zn–N5
N1–Zn–N3
N1–Zn–N5
N3–Zn–N5
99.04(52)
94.67(57)
126.12(46)
104.78(43)
71.93(67)
146.12(58)
74.19(47)
2
.4. Representative procedures for the cycloaddition between epoxide
and CO
2
The cycloaddition reaction of CO
2
to epoxide was carried out by
mol), and
charging a stirring bar, epoxide (10 mmol), catalyst (10
l
Please cite this article in press as: M.S. Shin et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.10.005

4
M.S. Shin et al. / Polyhedron xxx (2016) xxx–xxx
R
R
R
R
R
1
) K /DME
R
R
R
N
N
N
N
N
2 2
Zn(OAc)2 ' (OH )
N
NH
N
N
N
N
N
2
Zn
2
)
R
MeOH
O
R
O
O
Br
N
Br
R = H; Py(Pz)
R = Me; Py(Me
2
O
R = H;
R = Me;
1
2
2
Pz)
2
Scheme 1. Synthetic routes to 1 and 2.
Table 3
Table 4
Cocatalyst screening for the cycloaddition of CO
2
to epoxides using complexes 1 and 2.a
The scope of epoxides on the cycloaddition reaction of epoxide and CO
catalyst 1.
2
using
a
O
0
.1 mol% 1 or 2
O
O
+
CO2
O
O
0.1 mol% 1
O
0
.1 mol% cocatalyst
+
CO2
O
O
R
4
0.1 mol% n-Bu PBr
TONb
Selectivity(%)c
R
Entry
Cat.
Cocatalyst
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
n-Bu
n-Bu
n-Bu
n-Bu
PPNCl
DMAP
n-Bu
n-Bu
n-Bu
n-Bu
PPNCl
DMAP
n-Bu
n-Bu
n-Bu
n-Bu
PPNCl
DMAP
4
4
4
4
PBr
NI
NBr
NCl
783
430
617
578
411
225
774
578
646
744
578
147
40
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
–
Entry
1
Product
TONb
783
Selectivity (%)c
>99
O
O
O
O
O
O
O
2
3
O
O
482
381
>99
>99
2
4
4
4
4
PBr
NI
NBr
NCl
0
1
2
3
4
5
6
7
8
None
4
4
4
4
PBr
NI
NBr
NCl
90
50
30
50
–
–
–
–
4
5
O
O
586
367
>99
>99
O
O
O
O
0
–
Cl
O
a
Cycloaddition conditions: [propylene oxide] = 10 mmol, [Zn] = 0.01 mmol,
cocatalyst] = 0.01 mmol, [propylene oxide]/[Zn] = 1000/1, CO = 10 bar,
Temperature = 75 °C, time = 24 h.
[
2
b
But
Turnover number (TON) = (mol of propylene oxide consumed)/(mol of zinc).
c
Calculated by 1H NMR spectral integration.
6
7
O
O
484
103
>99
>99
O
O
O
O
OPh
2
.18–2.12 Å is within the normal range of previously reported Zn–
N bond lengths [53–56]. Interestingly, two acetate anions chelated
to zinc metal have different binding mode. Since bond lengths of
Zn–O1, Zn–O2, Zn–O3, and Zn–O4 are 2.105(14) Å, 2.488(15) Å,
a
Cycloaddition conditions; [epoxide] = 10 mmol, [1] = 0.01 mmol, [n-Bu
= 0.01 mmol, [epoxide]/[1] = 1000/1, CO = 10 bar, Temperature = 75 °C.
4
PBr]
2
j
.067(10) Å, and 3.004(9) Å, respectively, one acetate anion has
2
1
2
-type binding mode and the other has
j
-mode. Furthermore,
b
Turnover number (TON) = (mol of propylene oxide consumed)/(mol of catalyst).
Calculated by 1H NMR spectral integration.
O–C bonds in acetate ion have relatively three short and double
bond of O1–C16 (1.118(24) Å), O2–C16 (1.142(27) Å), and O4–
C18 (1.178(13) Å) and one long and single bond of O3–C18
c
(
1.324(13) Å). Thus, the solid state structure of 2 could be defined
reported zinc catalysts [13–21,24,27,30,31]. Harsh conditions such
as more elevated temperature [13–16,18,21,24], higher CO
3
2
1
as [
j
-2,6-bis(3,5-dimethylpyrazol-1-yl)pyridine](
j
-acetato)(
j
-
2
acetato)zinc(II). The coordination geometry of 2 around zinc metal
is close to pseudo-octahedral. The N1–Zn-N5 provided most wide
angle (146.1(5)°) and could be considered as trans position of octa-
hedral structure.
pressure [13–18,21,24,27,30], and larger amount of catalyst load-
ing [13–21,27,30,21] than our systems were applied in the litera-
ture. However, longer reaction time of 24 h for our system was
needed.
The cycloaddition of CO
2
to propylene oxide was performed
To check the cocatalyst effect on the catalytic activity, we used
without solvent using the zinc compounds 1 and 2 as catalysts in
the presence of cocatalyst. The cycloaddition results are summa-
rized in Table 3. We carried out the reaction at 75 °C and 10 bar
six different kinds of cocatalysts, such as n-Bu
NBr, n-Bu NCl, bis(triphenylphosphoranylidene)ammonium chlo-
ride (PPNCl), and 4-dimethylaminopyridine (DMAP). Cocatalyst
n-Bu PBr for catalysts 1 and 2 is superior compared with the five
others (Table 3, entries 1–12). In addition, catalyst 1 showed the
best catalytic activity in the presence of n-Bu PBr (entry 1). Com-
4 4 4
PBr, n-Bu NI, n-Bu -
4
CO
2
pressure under the condition of a fixed 0.1 mol% catalyst load-
4
ing. Propylene oxide was easily converted into the cyclic carbonate
with high selectivity (99%) without any polymerized products.
Interestingly, our reaction conditions applied for cycloaddition of
4
pounds 1 and 2 showed marginal difference in catalytic activity
(entries 1–12). Both 1 and 2 with non-salt-type cocatalyst DMAP
2
CO to propylene oxide are relatively milder than those for
Please cite this article in press as: M.S. Shin et al., Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.10.005

M.S. Shin et al. / Polyhedron xxx (2016) xxx–xxx
5
O
Acknowledgments
O
O
Br
+
O
Y. K. thanks the National Research Foundation of Korea (NRF),
the Korean Ministry of Education (MOE) through the Creative
Human Resource Training Project for Regional Innovation
N
R
N
N
R
Zn
RO
OR
1
(
grant number 2014H1C1A1066874) and Basic Science Research
Program (grant number 2015R1D1A1A01061043) for financial
support. J. L. acknowledges financial supports by the BRL Program
(2015R1A4A1041036) through the National Research Foundation
of Korea (NRF) funded by the Ministry of Science, ICT & Future
Planning.
N
N
N
N
N
N
Zn
RO
OR
Zn
RO
OR
O
O
O
Br
O
R
R
N
N
N
Appendix A. Supplementary data
Zn
RO
OR
O
Br
CCDC 1490123 contains the supplementary crystallographic
data for 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2016.10.005.
CO
2
Br
R
Scheme 2. Plausible mechanism for the cycloaddition of CO
by 1/n-Bu PBr.
2
to epoxides catalyzed
4
showed very low activities (entries 6 and 12). Interestingly, six
cocatalysts without 1 and 2 showed low conversion in the TON
range of 0–90 (entries 13–18). Thus the coupling of catalyst and
cocatalyst showed the synergistic activity for this cycloaddition
reaction.
With the optimized condition in hands, we next moved to
investigate the scope of substrates (Table 4) using 1 as a catalyst
4
and n-Bu PBr as a cocatalyst. Substrates include various kinds of
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