Anion effects on construction of ZnII compounds with a chelating ligand bis(2-pyridylmethyl)amine and their catalytic activities

Article abstract of DOI:10.1016/j.ica.2010.11.029

Three Zn^II complexes containing bispicam ligands (bispicam = bis(2-pyridylmethyl)amine), [Zn(bispicam)₂](NO₃) ₂·2CH₃OH 4A, [Zn(bispicam)(NO₃) ₂] 4B, and [Zn(bispicam)₂](OTf)₂ 6, were obtained, and their structures were determined by X-ray crystallography. Complexes of the general formulation [Zn(bispicam)₂]X₂ (X = Cl^- (1), Br^- (2), I^- (3), NO₃ ^- (4A), ClO₄^- (5), and OTf^- (6)) show fac geometric isomers (a) or enantiomers (c) and (d) according to anions. Moreover, complexes 4-6 could carry out the catalytic transesterification of a range of esters with methanol under the mild conditions. Importantly, the catalyst 4B with an unsaturated structure has shown better efficiency than the catalysts, 4A, 5, and 6, having saturated structures. To explain this reactivity difference, two different reaction mechanisms have been proposed (metal-based vs. amide N-H-based).

Full text of DOI:10.1016/j.ica.2010.11.029

Inorganica Chimica Acta 366 (2011) 337–343
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Anion effects on construction of ZnII compounds with a chelating ligand
bis(2-pyridylmethyl)amine and their catalytic activities
Youngmee Kim a, Byeong Kwon Park b, Geun Hee Eom b, Soo Hyun Kim b, Hyun Min Park b, Young S. Choi c,
Ho G. Jang c, , Cheal Kim b,
⇑
⇑
a Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, South Korea
b Department of Fine Chemistry, Eco-Product and Materials Education Center, Seoul National University of Science and Technology, Seoul 139-743, South Korea
c Department of Chemistry, Korea University, Seoul 136-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Three ZnII complexes containing bispicam ligands (bispicam = bis(2-pyridylmethyl)amine), [Zn(bispi-
cam)2](NO3)2ꢀ2CH3OH 4A, [Zn(bispicam)(NO3)2] 4B, and [Zn(bispicam)2](OTf)2 6, were obtained, and
their structures were determined by X-ray crystallography. Complexes of the general formulation
[Zn(bispicam)2]X2 (X = Clꢁ (1), Brꢁ (2), Iꢁ (3), NO3ꢁ (4A), ClO4ꢁ (5), and OTfꢁ (6)) show fac geometric iso-
mers (a) or enantiomers (c) and (d) according to anions. Moreover, complexes 4–6 could carry out the
catalytic transesterification of a range of esters with methanol under the mild conditions. Importantly,
the catalyst 4B with an unsaturated structure has shown better efficiency than the catalysts, 4A, 5,
and 6, having saturated structures. To explain this reactivity difference, two different reaction mecha-
nisms have been proposed (metal-based vs. amide N–H-based).
Article history:
Received 22 April 2010
Received in revised form 12 November 2010
Accepted 16 November 2010
Available online 25 November 2010
Keywords:
Geometric isomers
Zinc(II) complexes
Bis(2-pyridylmethyl)amine
Transesterification
Catalyst
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
(amine). Two bispicam ligands can coordinate to metal ion to pro-
vide complexes of the general formulation [M(bispicam)2]X2. For
Self-assembly process for construction of coordination net-
works [1–6] has been affected by the hydrogen bonds [7–13],
p–p stacking [14], and anions [15–19] as well as ligand design
this type of complex system, three potential geometric isomers
are shown in Scheme 1 (a–d). Among them, (b) isomer is the
meridional isomer while the others are facial, and (c) and (d) are
enantiomers [25]. In our previous study, three ZnII complexes
([Zn(bispicam)2]Cl2 1, [Zn(bispicam)2]Br2 2 and [Zn(bispicam)2]I2
3) were obtained [25,26]. Only facial isomer (a) was isolated, and
there were hydrogen bonding interactions between Namine–H and
C–H of [Zn(bispicam)2]2+ cations and free halides, and between
[20] and coordination numbers [21,22]. Among them, anion effect
is a very important role for the self-assembled construction when
anions can be coordinated to the metal centers. For examples, we
have, quite recently, shown the anion effects on construction of ZnII
polymeric compounds containing chelating Hdpa (2,2-dipyridyl-
amine) ligands [23,24]. With coordinating halide, cyanide, acetate,
and benzoate anions, ZnII produced distorted tetrahedral mononu-
clear complexes with two nitrogen donor atoms of Hdpa and two
Owater–H and free halide for construction of molecular packing
and crystal structures [26].
In addition, these hydrogen-bonded complexes 1, 2, and 3
showed, interestingly, to carry out the catalytic transesterification
of a range of esters with methanol at room temperature under the
mild conditions, though all these compounds are saturated with
two bispicam ligands [26]. To explain this unexpected reactivity,
it has been proposed that the hydrogen atom of amine N–H moiety
in the complexes could do the acid-catalyzed transesterification.
To further investigate anion effects on geometrical isomerism in
this type of complexes and on construction of a variety of ZnII com-
plexes containing chelating ligands, Hdpa and bispicam, and to find
efficient catalysts to mediate various catalytic reactions that could
be carried out under mild reaction conditions, three more anions
coordinatꢁing anions. With non-coordinating SO3CF3 (OTfꢁ), BF4
ꢁ
ꢁ
and ClO4 anions, ZnII produced also mononuclear complexes
containing two Hdpa ligands with distorted tetrahedral, flattened
tetrahedral, and six-coordinated geometries, respectively. For the
bridging SO42ꢁ anion, surprisingly, ZnII produced a polymeric com-
pound that shows a heterogeneous catalytic reactivity [23].
Bis(2-pyridylmethyl)amine (bispicam) is a tridentate ligand
where the two terminal N-donor coordination sites (pyridyl) are
identical each other, but different from the central N-donor site
⇑
Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149 (C. Kim).
E-mail addresses: hgjang@korea.ac.kr (H.G. Jang), chealkim@snut.ac.kr (C. Kim).
(NO3ꢁ, C6H5CO2 (benzoate), and OTfꢁ) have been applied to this
ꢁ
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.029

338
Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
benzoate, and zinc perchlorate were purchased from Aldrich and
were used as received. 4-Fluorophenyl acetate and 4-nitrophenyl
benzoate were obtained from Lancaster.
2.2. Instrumentation
Elemental analysis for carbon, nitrogen, and hydrogen was car-
ried out by using an EA1108 (Carlo Erba Instrument, Italy) in the
Organic Chemistry Research Center of Sogang University, South
Korea. IR spectra were measured on a BIO RAD FTS 135 spectrom-
eter as KBr pellets. Product analysis for the transesterification reac-
tion was performed on either a Hewlett–Packard 5890 II Plus gas
chromatograph interfaced with Hewlett–Packard Model 5989B
mass spectrometer or a Donam Systems 6200 gas chromatograph
equipped with
a FID detector using 30-m capillary column
(Hewlett–Packard, HP-1, HP-5, and Ultra 2).
2.3. Syntheses of [Zn(bispicam)2](NO3)2ꢀ2CH3OH (4A)
Zn(NO3)2 (37.9 mg, 0.125 mmol) and ammonium benzoate
(35.5 mg, 0.25 mmol) were dissolved in 4 mL water and carefully
layered by 4 mL methanol solution of bispicam (46.4 mg,
0.25 mmol). Suitable crystals of compound 4A for X-ray analysis
were obtained in a few weeks. The yield was 16.1 mg (20.3%). 1H
NMR (DMSO, 300 MHz): d 7.42–8.55 (m, 16H, aromatic-H), d
4.80–5.00 (br, 2H, N–H), and d 4.13 (s, 8H, CH2). IR (KBr):
Scheme 1. Potential geometric isomers for the [Zn(bispicam)2]2+ cation.
m
(cmꢁ1) = 3459(brs, H2O), 3149(brs, N–H), 2923(m), 2349(m),
1605(s), 1574(m), 1488(s), 1439(s), 1355(w), 1315(w), 1283(m),
1242(w), 1158(w), 1086(s), 1054(m), 1013(s), 915(m), 765(s),
730(w), 639(m), 530(brw), 414(m). Anal. Calc. for C26H34N8O8Zn
(651.98), 4A: C, 47.89; H, 5.27; N, 17.19. Found: C, 48.11; H,
4.98; N, 16.85%.
system. Herein we report the syntheses and crystal structures of
three Zn-containing compounds, [Zn(bispicam)2](NO3)2ꢀ2CH3OH
4A, [Zn(bispicam)(NO3)2] 4B, and [Zn(bispicam)2](OTf)2 6, and
their catalytic activities including [Zn(bispicam)2](ClO4)2 5 [25]
are discussed.
2.4. Syntheses of [Zn(bispicam)(NO3)2] (4B)
2. Experimental section
Zn(NO3)2 (37.9 mg, 0.125 mmol) was dissolved in 4 mL water
and carefully layered by 4 mL methanol solution of bispicam
(46.4 mg, 0.25 mmol). Suitable crystals of compound 4B for X-ray
analysis were obtained in a few weeks. The yield was 15.5 mg
(21.0%). 1H NMR (DMSO, 300 MHz): d 7.41–8.61 (m, 8H, aro-
matic-H), d 4.20–4.40 (br, 1H, N–H), and d 4.00 (s, 4H, CH2). IR
2.1. Materials
Methanol, methylene chloride, para-substituted phenyl acetate,
para-substituted phenyl benzoate, methylacetate, methylbenzoate,
bis(2-pyridylmethyl)amine, zinc nitrate, Zn(OTf)2, ammonium
Table 1
Crystallographic data for compounds 4–6.
4A
4B
5a
6
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C
26H34N8O8Zn
C12H12N5O6Zn
387.64
293(2)
monoclinic
P21/n
7.7048(18)
C24H26Cl2N6O8Zn
662.78
293(2)
monoclinic
C2/c
23.695(3)
C26H26F6N6O6S2Zn
762.02
293(2)
monoclinic
P21/c
20.811(4)
651.98
293(2)
triclinic
ꢀ
P1
9.1790(18)
b (Å)
9.3570(19)
13.167(3)
9.0073(12)
13.114(2)
c (Å)
9.968(2)
14.905(3)
26.057(3)
18.858(3)
a
(°)
84.47(3)
90.00
90.00
90.00
b (°)
72.15(3)
100.221(4)
91.579(2)
113.769(3)
c
(°)
70.89(3)
770.0(3)
90.00
1488.0(6)
90.00
5559.4(13)
90.00
4710.1(14)
Volume (Å3)
Z
1
4
8
6
Absorption coefficient (mmꢁ1
Number of data collected
Number of unique data
R(int)
)
0.857
4300
2949
0.0850
1.693
7924
2915
0.0846
1.135
17251
6572
0.0684
1.001
25313
9115
0.0892
Goodness-of-fit
1.014
0.828
1.020
0.785
Final R indices [I > 2
Final R indices (all data)
r
(I)]
R1 = 0.0632, wR2 = 0.1648
R1 = 0.0735, wR2 = 0.1692
R1 = 0.0398, wR2 = 0.0831
R1 = 0.0843, wR2 = 0.0871
R1 = 0.0606, wR2 = 0.1401
R1 = 0.1348, wR2 = 0.1665
R1 = 0.0733, wR2 = 0.1597
R1 = 0.1595, wR2 = 0.1783
a
The crystal structure of 5 was re-determined, and all the parameters were same with the results reported by Glerup et al. [25].

Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
339
(KBr):
m
(cmꢁ1) = 3399(brs, H2O), 3170(brs, N–H), 2927(m),
disordered in 6. The crystallographic data for compounds 4–6 are
2369(m), 1604(s), 1573(m), 1487(s), 1441(s), 1358(w), 1318(w),
1284(m), 1246(w), 1159(w), 1086(s), 1053(m), 1012(s), 915(m),
765(s), 729(w), 639(m), 553(brw), 413(m). Anal. Calc. for
listed in Table 1.
3. Results and discussion
C12H12N5O6Zn (387.64), 4B: C, 37.18; H, 3.13; N, 18.07. Found: C,
37.53; H, 3.21; N, 17.70%.
In our previous study, we obtained three [Zn(bispicam)2]2+ cat-
ions containing free halides with fac geometry type (a) in Scheme
1) and concluded that hydrogen bonding interactions between Na-
mine–H and C–H of [Zn(bispicam)2]2+ cations and free halides, and
between Owater–H and free halides can play very important roles
for construction of molecular packing and crystal structures [26].
To investigate anion effects on structures of [Zn(bispicam)2]X2
complexes, three more anions (NO3ꢁ, OTfꢁ, and benzoate) were ap-
plied to this system. Complexes of the general formulation
[Zn(bispicam)2]X2 with different anions are shown in Scheme 2.
2.5. Synthesis of [Zn(bispicam)2](OTf)2 (6)
Zn(Otf)2 (29.7 mg, 0.08 mmol) were dissolved in 4 mL water
and carefully layered by 4 mL methanol solution of bispicam
(29.7 mg, 0.16 mmol). Suitable crystals of compound 6 for X-ray
analysis were obtained in a few weeks. The yield was 16.1 mg
(20.3%). 1H NMR (DMSO, 300 MHz): d 7.42–8.55 (m, 16H, aro-
matic-H), d 4.80–5.00 (br, 2H, N–H), and d 4.13 (s, 8H, CH2). IR
(KBr):
m
(cmꢁ1) = 3459(brs, H2O), 3149(brs, N–H), 2923(m),
2349(m), 1605(s), 1574(m), 1488(s), 1439(s), 1355(w), 1315(w),
1283(m), 1242(w), 1158(w), 1086(s), 1054(m), 1013(s), 915(m),
765(s), 730(w), 639(m), 530(brw), 414(m). Anal. Calc. for
3.1. Structure description
In 4A, ammonium benzoate was applied to the reaction of
Zn(NO3)2 and bispicam, but benzoate ligands did not coordinate
C26H26F6N6O6S2Zn (762.02), 6: C, 40.98; H, 3.45; N, 11.03. Found:
C, 41.32; H, 3.40; N, 11.39%.
2.6. Catalytic activity of compounds 4–6
Catalytic reaction conditions: Ester (0.05 mmol) was dissolved
in methanol (1 mL), and the compounds 4–6 (1.0 mg,
1.28 ꢂ 10ꢁ3 mmol for 4A, 1.0 mg, 1.56 ꢂ 10ꢁ3 mmol for 4B,
1.0 mg, 1.28 ꢂ 10ꢁ3 mmol for 5, and 1.0 mg, 1.44 ꢂ 10ꢁ3 mmol
for 6) were added and shaken at 50 °C (450 rpm). Reaction conver-
sion was monitored by GC/Mass analysis of 20 lL aliquots with-
drawn periodically from the reaction mixture. All reactions were
run at least three times and the average conversion yields are pre-
sented. Yields were based on the formation of the products, methy-
lacetate or methylbenzoate.
2.7. X-ray crystallography
The X-ray diffraction data for all four compounds were collected
on a Bruker SMART APEX diffractometer equipped with a mono-
chromater in the Mo Ka (k = 0.71073 Å) incident beam. Each crys-
tal was mounted on a glass fiber. The CCD data were integrated and
scaled using the Bruker-SAINT software package, and the structure
was solved and refined using SHEXTL V6.10 [27]. All hydrogen atoms
were placed in the calculated positions. OTfꢁ anions are highly
Fig. 1. Structure of [Zn(bispicam)2]2+ in 4A.
Scheme 2. Anion effects on construction of [Zn(bispicam)2]X2 complexes.

340
Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
to the ZnII ion, and instead, two bispicam ligands coordinated to a
ZnII ion producing 4A complex. Asymmetric unit contains half of a
molecule, a nitrate anion, and a methanol solvent molecule, and
there is an inversion center on a Zn atom. The structure of the
whole molecule [Zn(bispicam)2](NO3)2ꢀ2CH3OH is shown in
Fig. 1. The coordination geometry around a zinc atom is pseudo-
octahedral, and the isomer isolated is the fac isomer represented
(a) above. The Zn–Namine bond distance of 2.160(3) Å is shorter
than two Zn–Npyridyl bonds of 2.188(3) and 2.200(3) Å (Table 2).
If ammonium benzoate did not applied to this reaction, two nitrate
anions coordinate to the Zn ion providing 4B complex (Fig. 2). The
coordination geometry around a zinc atom is distorted trigonal
bipyramidal (TBP). The Namine is disordered in two nitrogen atoms
with 75 and 25% occupancies. The principle bond distances and
angles of 4B are listed in Table 2.
There are two crystallographically independent ZnII ions in both
5 [25] and 6. Zn(1) atom lies on a site of C2 symmetry providing
enantiomers, and Zn(2) atom lies on an inversion center with Ci
symmetry (Fig. 3). Fig. 4 shows packing diagrams containing fac
isomer, (a) and enantiomers, (c) and (d) in a unit cell in both 5
and 6 complexes. According to the centrosymmetric space group,
there is an equal distribution of the two enantiomers, (c) and (d).
The principle bond distances and angles of 6 are listed in Table 2.
When bispicam ligands react with various ZnII salts (Clꢁ (1)
Fig. 2. Structure of [Zn(bispicam)2(NO3)2] 4B. The open atom represents disordered
N2* atom.
[25], Brꢁ (2), Iꢁ (3) [26], NO3 (4A), ClO4 (5) [25] and OTfꢁ (6)),
ꢁ
ꢁ
Fig. 3. Structure of [Zn(bispicam)2]2+ in 6 containing two independent isomers.
-
Fig. 4. Packing diagrams in a unit cell with fac isomers containing enantiomers representing (a) in black and (c) and (d) in red and blue for 5 (a) and for 6 (b). All ClO4 and
OTfꢁ anions, and all hydrogen atoms were omitted for clarity. (For interpretation of the references in color in this figure legend, the reader is referred to the web version of this
article.)

Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
341
complexes of the general formulation [Zn(bispicam)2]X2 have been
3.2. Catalytic transesterification reactions by the compounds 4–6
obtained, and there were fac geometric isomers (a) or enantiomers
(c) and (d) according to anions. With halides, [Zn(bispicam)2]2+ catꢁ-
ions have only fac geometric isomer (a). With coordinating NO3
anion, a nitrate-coordinated ZnII complex of the formulation
[Zn(bispicam)(NO3)2] was obtained. On the other hand, when the
benzoate was applied with Zn(NO3)2, typical [Zn(bispicam)2]2+ cat-
ion was obtained and showed fac geometric isomer (a). With non-
coordinating ClO4ꢁ and OTf- anions, there were fac isomers (a) and
enantiomers (c) and (d) which are, of course, equal distribution of
these two enantiomers because of the centrosymmetric space
group. These results indicate that anion effects play very important
roles for construction of crystal structures and geometrical isomer-
ism. In addition, it is worthwhile to note that, with bispicam ligand,
only one structure of the general formulation [Zn(bispicam)2]X2
(Clꢁ, Brꢁ, Iꢁ, NO3ꢁ, ClO4ꢁ, and OTfꢁ) was obtained except the
nitrate-coordinated complex of the formulation [Zn(bispicam)-
(NO3)2], while with Hdpa ligand, various structures (tetrahedral,
flattened tetrahedral, six-coordinated mononuclear complexes
and a polymeric compound) were observed according to the
change of the anions.
The most common procedure for transesterification reaction is
to reflux the ester with a catalytic amount of Ti(O-i-Pr)4 in an alco-
hol solvent [28], while the transesterification reaction is important
in organic synthesis in industrial as well as in academic laboratories
[29–31]. Other Lewis acid catalysts such as BuSn(OH)3 and Al(OR)3
are also effective for this conversion [32,33], but require higher
reaction temperature and acidic conditions. These drawbacks drive
us to develop new catalysts that operate under milder conditions.
Therefore, with a need for new transesterification catalysts, we
ꢁ
2ꢁ
have previously reported that Zn(Hdpa)X2 (X = NO3
, SO4 ,
ꢁO2C6H5, Clꢁ, Brꢁ, and Iꢁ) could carry out the catalytic transesteri-
fication of a range of esters with methanol at room temperature
under the mild conditions [23,24]. Interestingly, the complexes
with labile ligands showed efficient catalytic reactivity, while the
complexes with inert ligands or with saturated structures catalyzed
slow conversions for transesterification reactions. In addition, we
have recently shown that the polymeric compounds [Zn(bispi-
cam)2]Cl2 1, [Zn(bispicam)2]Br2 2 and [Zn(bispicam)2]I2 3 with
non-classical C/N–HꢀꢀꢀX hydrogen bonding, interestingly, showed
Table 2
Selected bond lengths [Å] and angles [°] for compounds 4A, 4B, and 6.
4A
Zn(1)–N(2)
Zn(1)–N(3)#1
Zn(1)–N(1)#1
2.160(3)
2.188(3)
2.200(3)
180.0
100.35(12)
79.65(12)
100.24(13)
93.03(12)
79.76(13)
86.97(12)
180.00
Zn(1)–N(2)#1
Zn(1)–N(3)
Zn(1)–N(1)
N(2)–Zn(1)–N(3)#1
N(2)–Zn(1)–N(3)
N(3)#1–Zn(1)–N(3)
N(2)#1–Zn(1)–N(1)#1
N(3)–Zn(1)–N(1)#1
N(2)#1–Zn(1)–N(1)
N(3)–Zn(1)–N(1)
2.160(3)
2.188(3)
2.200(3)
79.65(12)
100.35(12)
180.0
79.76(13)
86.97(12)
100.24(13)
93.03(12)
N(2)–Zn(1)–N(2)#1
N(2)#1–Zn(1)–N(3)#1
N(2)#1–Zn(1)–N(3)
N(2)–Zn(1)–N(1)#1
N(3)#1–Zn(1)–N(1)#1
N(2)–Zn(1)–N(1)
N(3)#1–Zn(1)–N(1)
N(1)#1–Zn(1)–N(1)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1, ꢁy + 1, ꢁz + 1
4B
Zn(1)–O(11)
Zn(1)–O(21)
Zn(1)–N(2)
2.071(3)
2.094(3)
2.160(5)
101.31(12)
99.36(12)
156.49(12)
127.4(3)
146.9(3)
156.1(7)
118.1(8)
28.9(5)
Zn(1)–N(1)
Zn(1)–N(3)
Zn(1)–N(2*)
O(11)–Zn(1)–O(21)
O(11)–Zn(1)–N(3)
O(21)–Zn(1)–N(3)
N(1)–Zn(1)–N(2)
N(3)–Zn(1)–N(2)
N(1)–Zn(1)–N(2*)
N(3)–Zn(1)–N(2*)
2.090(3)
2.100(3)
2.243(14)
85.52(14)
97.47(12)
96.07(11)
79.07(15)
78.32(15)
79.7(3)
O(11)–Zn(1)–N(1)
N(1)–Zn(1)–O(21)
N(1)–Zn(1)–N(3)
O(11)–Zn(1)–N(2)
O(21)–Zn(1)–N(2)
O(11)–Zn(1)–N(2*)
O(21)–Zn(1)–N(2*)
N(2)–Zn(1)–N(2*)
77.4(3)
6
Zn(1)–N(21)
Zn(1)–N(22)
Zn(1)–N(13)
Zn(2)–N(33)#1
Zn(2)–N(32)
Zn(2)–N(31)#1
2.091(6)
2.159(6)
2.224(5)
2.150(5)
2.153(6)
2.180(5)
174.1(2)
100.6(2)
78.9(2)
97.2(2)
175.1(2)
91.3(2)
76.8(2)
107.7(2)
80.2(2)
Zn(1)–N(11)
Zn(1)–N(12)
Zn(1)–N(23)
Zn(2)–N(33)
Zn(2)–N(32)#1
Zn(2)–N(31)
2.150(6)
2.170(6)
2.226(6)
2.151(6)
2.153(6)
2.180(5)
80.4(2)
95.3(2)
96.4(2)
81.2(2)
79.5(2)
94.5(2)
169.6(2)
180.0
99.8(2)
80.2(2)
90.49(19)
100.2(2)
89.51(19)
79.8(2)
180.0
N(21)–Zn(1)–N(11)
N(11)–Zn(1)–N(22)
N(11)–Zn(1)–N(12)
N(21)–Zn(1)–N(13)
N(22)–Zn(1)–N(13)
N(21)–Zn(1)–N(23)
N(22)–Zn(1)–N(23)
N(13)–Zn(1)–N(23)
N(33)#1–Zn(2)–N(32)
N(33)#1–Zn(2)–N(32)#1
N(32)–Zn(2)–N(32)#1
N(33)–Zn(2)–N(31)#1
N(32)#1–Zn(2)–N(31)#1
N(33)–Zn(2)–N(31)
N(32)#1–Zn(2)–N(31)
N(21)–Zn(1)–N(22)
N(21)–Zn(1)–N(12)
N(22)–Zn(1)–N(12)
N(11)–Zn(1)–N(13)
N(12)–Zn(1)–N(13)
N(11)–Zn(1)–N(23)
N(12)–Zn(1)–N(23)
N(33)#1–Zn(2)–N(33)
N(33)–Zn(2)–N(32)
N(33)–Zn(2)–N(32)#1
N(33)#1–Zn(2)–N(31)#1
N(32)–Zn(2)–N(31)#1
N(33)#1–Zn(2)–N(31)
N(32)–Zn(2)–N(31)
N(31)#1–Zn(2)-N(31)
99.8(2)
180.0
89.51(19)
79.8(2)
90.49(19)
100.2(2)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1, ꢁy + 1, ꢁz

342
Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
to carry out the catalytic transesterification of a range of esters with
methanol at room temperature under the mild conditions, though
all these compounds are saturated with two bispicam ligands. To
explain this unexpected reactivity, it has been proposed that the
hydrogen atom of amine N-H moiety in the complexes could do
the acid-catalyzed transesterification.
Therefore, in order to compare the reactivity of the complex 4B
having the unsaturated structure to those of the complexes 4A, 5,
and 6 having the saturated structures, we have examined the
transesterification reaction with the compounds 4–6. Thus, the
compounds 4–6 were treated with 4-nitrophenyl acetate and
methanol at 50 °C under the neutral conditions.
transesterification reaction mechanisms based on the structure of
the complexes 4–6 and our pervious results [24]. With 4B having
the labile ligand NO3ꢁ, the plausible mechanism is shown in
Scheme 3. The substrate phenyl acetate substitutes an anion
(NO3ꢁ) to give the adduct (bispicam)2Zn(NO3)(Sub). Then, the
nucleophile methanol would attack the carbon atom of carbonyl
moiety of the adduct to produce the product methyl acetate.
With complexes, 4A, 5, and 6, having the saturated structures,
we can presume that there is no direct interaction between the
substrate ester and zinc ion of the complexes as Lewis acid, since
they all are saturated with two bispicam ligands. Therefore, we
propose that the hydrogen atom of amine N–H moiety in the
complexes can do the acid-catalyzed transesterification as previ-
ously proposed [23], because the hydrogen atom could be acidic
by coordinating of nitrogen of N–H to zinc ion (Scheme 4). Thus,
this acidic hydrogen interacts with the oxygen atom of carbonyl
of ester, resulting in that the carbonyl is more electrophilic. Then,
the resultant activated carbonyl could be easily attacked by
O
O
catalysts 4-6
CH3COCH3
HOCH3
..
O2N
OCCH3
O2N
+
OH +
ð1Þ
This reaction produced quantitatively the corresponding prod-
uct methyl acetate within 0.25–0.54 days (entry 1 of Table 3),
while a control reaction carried out in absence of the complexes
showed trace amounts of the conversion of the ester to the product
in the same time period. Importantly, the transesterification reac-
tivity shown by the catalyst 4B is very efficient and the second best
among the catalytic efficiencies reported previously with Zn-con-
taining compounds, to our best knowledge [34–39].
The transesterification of other esters including benzoates by
the catalysts 4–6 was also carried out efficiently and the results
are given in Table 3. The substrates with the electron-withdrawing
substituents have undergone faster transesterification, while those
with the electron-donating ones have shown slow reaction. Impor-
tantly, vinyl acetate, which is widely used as a precursor for ester
synthesis [40,41], was also converted efficiently to the product
methyl acetate within 0.13–0.21 days by the catalysts (entry 8),
suggesting that this catalytic system can be useful for preparing
various esters by transesterification. It is worthwhile to note that
the catalysts 4B with an unsaturated structure have shown better
efficiency than the catalysts, 1, 2, 3, 4A, 5, and 6, having saturated
structures, as also previously shown in zinc complexes with Hdpa
ligand [23,24].
L
(bispicam)
Zn
L
O
CH3COPh
-
L= NO3
O
L
Zn
δ−
O
+ L
(bispicam)
CH3COCH3
CH3OH
OPh
δ+
C
CH3
L
O
(bispicam)
Zn
CH3COPh
O
OPh
OCH3
H
C
H3C
Scheme 3. Proposed reaction mechanism with the unsaturated complex 4B.
Though we do not know, at this moment, about the exact reac-
tive species and the reaction mechanism for the transesterification
reaction by the catalysts, we are able to propose two possible
Table 3
Transesterification of esters by methanol in the presence of compounds 4–6 at 50 °C. a
.
Entry Substrate
4A (time/
days)
4B (time/ 5 (time/
6 (time/
b
b
days)b
days)
days)b
1
4-Nitrophenyl
0.54
0.25
0.38
0.31
acetate
2
3
Phenyl acetate
4-Methylphenyl
acetate
4
3
0.92
2
2
3
2
3
4
5
4-Nitrophenyl
benzoate
4-Chlorophenyl
benzoate
9
2
7
5
3
7
2
c
0.71
6
7
Phenyl benzoate
4-Methylphenyl
benzoate
4
2
1.63
0.75
4
4
2
3
8
Vinyl acetate
0.16
0.13
0.17
0.21
a
All esters were completely converted to the corresponding products, methyl
acetate and methyl benzoate. Reaction conditions: esters; 0.05 mmol, catalyst;
1.28 ꢂ 10ꢁ3 mmol for 4A, 1.56 ꢂ 10ꢁ3 mmol for 4B, 1.44 ꢂ 10ꢁ3 mmol for 5, and
1.44 ꢂ 10ꢁ3 mmol for 6, solvent; methanol (1 mL). See the experimental section for
the detailed reaction conditions.
b
Time necessary for the complete conversion of substrate to product.
The solvent was a mixture of CH3OH/CH2Cl2 (1/1) because of low solubility of
c
Scheme 4. Proposed reaction mechanism with the saturated complexes 4A, 5,
and 6.
substrate in CH3OH.

Y. Kim et al. / Inorganica Chimica Acta 366 (2011) 337–343
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Acknowledgements
Financial support from South Korea Ministry Environment
‘‘ET-Human Resource Development Project’’, the Korean Science
Engineering Foundation (R01-2008-000-20704-0 and 2009-
0074066), the Converging Research Center Program through the
National Research Foundation of South Korea (NRF) funded by the
Ministry of Education, Science and Technology (2009-0082832),
and South Korea University Grant is gratefully acknowledged.
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Appendix A. Supplementary material
CCDC 764846, 764847 and 764848 contain the supplementary
crystallographic data for compounds 4A, 4B and 6, respectively.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.ica.2010.11.029.
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