Zinc(II) polymeric compounds with a chelating ligand bis(2-pyridylmethyl)amine (bispicam) directed by intermolecular C/N/O-H?X (X = Cl, Br, I) interactions: Catalytic activities

Article abstract of DOI:10.1016/j.molstruc.2008.03.052

[Zn(bispicam)₂]²⁺ as a hydrogen-bond donor and three free halides (Cl^-, Br^- and I^-) as hydrogen-bond acceptors were employed to investigate the role of non-classical C/N-H?X hydrogen bonding for construction of polymeric compounds. Three zinc salts (ZnCl₂, ZnBr₂ and ZnI₂) were reacted with bis(2-pyridylmethyl)amine (bispicam) to produce [Zn(bispicam)₂]Cl₂ 1, [Zn(bispicam)₂]Br₂ 2 and [Zn(bispicam)₂]I₂ 3. Hydrogen bonding interactions between N_amine-H and C-H of [Zn(bispicam)₂]²⁺ cations and free halides, and between O_water-H and free halides can play very important roles for construction of molecular packing and crystal structures. In addition, these hydrogen-bonded complexes 1, 2 and 3 were, interestingly, shown to carry out the catalytic transesterification of a range of esters with methanol at 50 °C under the mild conditions, though all these compounds are saturated with two bispicam ligands. To explain this unusual reactivity, it has been proposed that the hydrogen atom of amine N-H moiety in the complexes could do the acid-catalyzed transesterification.

Full text of DOI:10.1016/j.molstruc.2008.03.052

Journal of Molecular Structure 890 (2008) 123–129
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Zinc(II) polymeric compounds with a chelating ligand
bis(2-pyridylmethyl)amine (bispicam) directed by intermolecular
C/N/O–Hꢀ ꢀ ꢀX (X = Cl, Br, I) interactions: Catalytic activities
Byeong Kwon Park a, Sun Hwa Lee a, Eun Yong Lee a, Han Kwak a, Young Min Lee a, Yu Jin Lee a,
b
b,
*
Je Yeol Jun a, Cheal Kim a, , Sung-Jin Kim , Youngmee Kim
*
a Department of Fine Chemistry, and Eco-Product and Materials Education Center, Seoul National University of Technology, Seoul 139-743, Republic of Korea
b Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
[Zn(bispicam)2]2+ as a hydrogen-bond donor and three free halides (Clꢁ, Brꢁ and Iꢁ) as hydrogen-bond
acceptors were employed to investigate the role of non-classical C/N–Hꢀ ꢀ ꢀX hydrogen bonding for con-
struction of polymeric compounds. Three zinc salts (ZnCl2, ZnBr2 and ZnI2) were reacted with bis(2-pyr-
idylmethyl)amine (bispicam) to produce [Zn(bispicam)2]Cl2 1, [Zn(bispicam)2]Br2 2 and [Zn(bispicam)2]I2
3. Hydrogen bonding interactions between Namine–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 molec-
ular packing and crystal structures. In addition, these hydrogen-bonded complexes 1, 2 and 3 were, inter-
estingly, shown to carry out the catalytic transesterification of a range of esters with methanol at 50 °C
under the mild conditions, though all these compounds are saturated with two bispicam ligands. To
explain this unusual reactivity, it has been proposed that the hydrogen atom of amine N–H moiety in
the complexes could do the acid-catalyzed transesterification.
Article history:
Received 8 January 2008
Received in revised form 29 March 2008
Accepted 31 March 2008
Available online 10 April 2008
Keywords:
Zinc(II) coordination polymer
Transesterification
Catalyst
Bis(2-pyridylmethyl)amine
Hydrogen bond
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Zn(Hdpa)I2, have shown, surprisingly, unusual heterogeneous cata-
lytic activities on transesterification reaction of esters.
Classical and non-classical hydrogen bonding provides a good
structural motif for construction of a polymeric compound from a
simple building block [1]. Much attention has been paid to the role
played by non-classical C–Hꢀ ꢀ ꢀO/N hydrogen bonding in construc-
tion of molecular packing and crystal structures [2]. Weak non-clas-
sical C/N–Hꢀ ꢀ ꢀX (X = halide) hydrogen bonding can also be a
structural motif for construction of polymeric compounds and can
stabilize the crystal structures even though they have often been
considered as van der Waals interactions, especially when X charac-
ters are halide anions or M–X bonds [3,4]. We and others have previ-
ously shown that a chelating ligand 2,20-dipyridylamine (Hdpa) has
mostly formed mononuclear Zn(II) complexes by the reaction with
zincsalts [5], andthentheseZn(II) complexescanbe usedasbuilding
blocks for construction of polymeric compounds through weak non-
classical C/N–Hꢀ ꢀ ꢀX and C/N–Hꢀ ꢀ ꢀO hydrogen bonding [6]. With ha-
lide anions among them, especially, the intermolecular N–Hꢀ ꢀ ꢀX and
C–Hꢀ ꢀ ꢀX (X = Cl, Br and I) hydrogen bond interactions played impor-
tant roles to form a 2-dimensional framework [6]. Moreover, these
hydrogen-bonded polymers, Zn(Hdpa)Cl2, Zn(Hdpa)Br2 and
Bis(2-pyridylmethyl)amine (bispicam)isanotherchelatingtriden-
tate ligand that is similar to the chelating ligand Hdpa with N–H moi-
ety. Therefore it is expected that the N–H moiety could help construct
polymeric compoundsthroughweaknon-classical N–Hꢀ ꢀ ꢀX hydrogen
bonding as shown in [Zn(Hdpa)X2] complexes (X = Clꢁ, Brꢁ and Iꢁ).
To further investigate the role of non-classical C/N–Hꢀ ꢀ ꢀX
hydrogen bonding for construction of polymeric compounds and
to find efficient catalysts to mediate various catalytic reactions that
could be carried out under mild reaction conditions, we have em-
ployed [Zn(bispicam)2]2+ as a hydrogen-bond donor and three zinc
halides (ZnCl2, ZnBr2 and ZnI2) as hydrogen-bond acceptors. Herein
we report the synthesis, crystal structures and reactivities of three
Zn-containing compounds, [Zn(bispicam)2]Cl2 1 [7], [Zn(bispi-
cam)2]Br2 2 and [Zn(bispicam)2]I2 3, formed by the reaction of
the chelating bispicam ligand and three zinc halides.
2. Experimental section
2.1. Materials
Methanol, methylene chloride, para-substituted phenyl acetate,
para-substituted phenyl benzoate, methylacetate, methylbenzoate,
bis(2-pyridylmethyl)amine and zinc halides were purchased from
* Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149 (C. Kim),
tel.: +82 2 3277 3589; fax: +82 2 3277 2384 (Y. Kim).
E-mail addresses: chealkim@snut.ac.kr (C. Kim), ymeekim@ewha.ac.kr (Y. Kim).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.03.052

124
B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
Aldrich and were used as received. 4-Fluorophenyl acetate and 4-
nitrophenyl benzoate were obtained from Lancaster.
1086(s), 1054(m), 1013(s), 915(m), 765(s), 730(w), 639(m),
530(brw), 414(m). Anal. Calcd. for C24H34Br2N6O4Zn (695.76), 2:
C, 41.43; H, 4.94; N, 12.08. Found: C, 41.52; H, 4.58; N, 12.10%.
2.2. Instrumentation
2.5. Synthesis of [Zn(bispicam)2]I2 (3)
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, Korea.
IR spectra were measured on a BIO RAD FTS 135 spectrometer as
KBr pellets. Product analysis for the transesterification reaction
was performed on either a Hewlett-Packard 5890 II Plus gas chro-
matograph interfaced with Hewlett-Packard Model 5989B mass
Bis(2-pyridylmethyl)amine (29.68 mg, 0.16 mmol) was dis-
solved in 4 mL methylene chloride and carefully layered by 4 mL
methanol solution of 26.05 mg (0.08 mmol) of ZnI2. Suitable crys-
tals of compound 3 for X-ray analysis were obtained in a few
weeks. The yield was 10.6 mg (11.1%). 1H NMR (DMSO,
300 MHz): d 7.41–8.44 (m, 16H, aromatic-H), d 5.48-5.62 (brs,
2H, N–H), d 4.27 (s, 8H, CH2) and d 3.37 (s, H2O). IR (KBr):
m(cmꢁ1) = 3471(brs, H2O), 3116(brs, N–H), 2917(brs), 2362(m),
1605(s), 1573(s), 1486(s), 1438(s), 1354(w), 1314(w), 1283(m),
1249(w), 1154(m), 1089(m), 1054(m), 1014(s), 917(m), 761(s),
729(w), 639(m), 475(w), 413(m). Anal. Calcd. for C26H34I2N6O2Zn
(781.76), 3: C, 39.94; H, 4.39; N, 10.75. Found: C, 40.44; H, 4.12;
N, 10.99%.
spectrometer or
a Donam Systems 6200 gas chromatograph
equipped with a FID detector using 30-m capillary column (Hew-
lett-Packard, HP-1, HP-5 and Ultra 2).
2.3. Synthesis of [Zn(bispicam)2]Cl2 (1)
A total of 17.38 mg (0.125 mmol) of ZnCl2 were dissolved in
4 mL water and carefully layered by 4 mL methanol solution of
bis(2-pyridylmethyl)amine (46.38 lL, 0.25 mmol). Suitable crystals
of compound 1 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, 16H, aromatic-H), d 4.20–4.40 (br, 2H, N–H), d 4.00
(s, 8H, CH2) and d 3.37 (s, H2O). IR (KBr): m(cmꢁ1) = 3399(brs,
H2O), 3170(brs, N–H), 2927(m), 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. Calcd. for C24H38Cl2N6O6Zn (642.87), 1:
C, 44.84; H, 5.97; N, 13.08. Found: C, 44.35; H, 6.01; N, 13.36%.
2.6. Catalytic activity of compounds 1–3
Catalytic reaction conditions: ester (0.05 mmol) was dissolved
in an appropriate solvent (1 mL), and the compounds 1–3
(1.0 mg, 1.56 ꢂ 10ꢁ3 mmol for 1, 1.0 mg, 1.44 ꢂ 10ꢁ3 mmol for 2
and 1.0 mg, 1.28 ꢂ 10ꢁ3 mmol for 3) were added and shaken at
50 °C (450 rpm). Reaction conversion was monitored by GC/Mass
analysis of 20 lL aliquots withdrawn periodically from the reaction
mixture. All reactions were run at least three times and the average
conversion yields are presented. Yields were based on the forma-
tion of the products, methylacetate or methylbenzoate.
2.4. Synthesis of [Zn(bispicam)2]Br2 (2)
2.7. X-ray crystallography
A total of 18.38 mg (0.08 mmol) of ZnBr2 were dissolved in 4 mL
water and carefully layered by 4 mL methanol solution of bis(2-
pyridylmethyl)amine (29.68 mg, 0.16 mmol). Suitable crystals of
compound 2 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), d 4.13
(s, 8H, CH2) and d 3.37 (s, H2O). 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),
The X-ray diffraction data for all three compounds were col-
lected on a Bruker SMART APX diffractometer equipped with a
monochromater in the Mo Ka (k = 0.71073 Å) incident beam.
Each crystal 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 V5.10
[8]. All hydrogen atoms were placed in the calculated positions.
The crystallographic data for compounds 1–3 are listed in
Table 1
Crystallographic data for compounds 1–3
1a
2
3
Empirical formula
Formula weight
Temp. (K)
C24H38Cl2N6O6Zn
642.87
293(2)
C24H34Br2N6O4Zn
695.76
293(2)
C26H34I2N6O2Zn
781.76
293(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c (no. 14)
9.1805(13)
10.0775(14)
16.448(2)
90.00
94.553(2)
90.00
1516.9(4)
2
Monoclinic
P21/c (no. 14)
10.118(2)
18.560(4)
15.967(3)
90.00
102.12(3)
90.00
2931.6(10)
4
Triclinic
P1 (no. 2)
ꢀ
9.0649(8)
9.4629(9)
9.8747(9)
76.3630(10)
82.965(2)
68.6040(10)
765.82(12)
1
2.850
4313
2934
0.0609
a (°)
b (°)
c (°)
Volume (Å3)
Z
Absorption coefficient (mmꢁ1
No. of data collected
No. of unique data
R(int.)
)
1.032
7540
2921
0.0598
3.603
16,226
5741
0.0380
Goodness-of-fit
Final R indices [I > 2r(I)]
Final R indices (all data)
0.848
1.011
1.080
R1 = 0.0428, wR2 = 0.0982
R1 = 0.0971, wR2 = 0.1358
R1 = 0.0373, wR2 = 0.0758
R1 = 0.0743, wR2 = 0.0866
R1 = 0.0365, wR2 = 0.0953
R1 = 0.0381, wR2 = 0.0963
a
[Zn(bispicam)2]Cl2 1 was reported previously [7], but we ourselves prepared it again to investigate the role of non-classical C/N–Hꢀ ꢀ ꢀX hydrogen bonding for construction
of polymeric compounds containing [Zn(bispicam)2]2+ units.

B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
125
Table 1. Structural information was deposited at the Cambridge
3. Results and discussion
Crystallographic Data Center (CCDC reference numbers 672358
for 2, 672359 for 3).
3.1. Synthesis
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
(amine). For a 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 [7]. In this study three zinc salts (ZnCl2, ZnBr2 and
ZnI2) were reacted with bispicam to produce [Zn(bispicam)2]Cl2
1, [Zn(bispicam)2]Br2 2 and [Zn(bispicam)2]I2 3. Only (a) facial iso-
mers are isolated in our system. [Zn(bispicam)2]Cl2 1 was reported
previously [7], but we ourselves prepared it again to investigate
the role of non-classical C/N–Hꢀ ꢀ ꢀX hydrogen bonding for construc-
tion of polymeric compounds containing [Zn(bispicam)2]2+ units.
Table 2
Selected bond lengths [Å] and angles [°] for compound 1
Zn(1)–N(2)#1
Zn(1)–N(3)
Zn(1)–N(1)#1
2.153(3)
2.166(3)
2.202(4)
Zn(1)–N(2)
Zn(1)–N(3)#1
Zn(1)–N(1)
2.153(3)
2.166(3)
2.202(4)
N(2)#1–Zn(1)–N(2)
N(2)–Zn(1)–N(3)
180.0
N(2)#1–Zn(1)–N(3)
N(2)#1–Zn(1)–N(3)#1
N(3)–Zn(1)–N(3)#1
N(2)–Zn(1)–N(1)#1
N(3)#1–Zn(1)–N(1)#1
N(2)–Zn(1)–N(1)
79.76(13)
100.24(13)
180.0
100.06(13)
95.21(12)
79.94(13)
84.79(12)
100.25(13)
79.75(13)
79.94(13)
84.79(12)
100.06(13)
95.22(12)
180.0
N(2)–Zn(1)–N(3)#1
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)
N(3)#1–Zn(1)–N(1)
N(1)#1–Zn(1)–N(1)
Scheme 1.
Symmetry transformations used to generate equivalent atoms: #1 ꢁx, ꢁy, ꢁz.
Fig. 1. (a) Structure of the cation of compound 1. Displacement ellipsoids are shown at the 30% probability level. (b) Hydrogen bond interactions in a one-dimensional
polymeric compound. H atoms not involved in intermolecular interactions have been omitted for clarity. Green dotted lines represent Namine–Hꢀ ꢀ ꢀCl and Owater–Hꢀ ꢀ ꢀCl
hydrogen bonds.

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B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
Table 3
Table 4
Hydrogen bonds for compound 1 (Å and °)
Selected bond lengths [Å] and angles [°] for compound 2
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
h(DHA)
Zn(1)–N(12)
Zn(1)–N(23)
Zn(1)–N(11)
2.157(3)
2.174(3)
2.184(3)
Zn(1)–N(13)
Zn(1)–N(22)
Zn(1)–N(21)
2.169(3)
2.177(3)
2.192(3)
N(2)–H(2N). . .Cl(1)#2
O(2S)–H(2S1). . .Cl(1)
O(1S)–H(1S2). . .Cl(1)#3
O(1S)–H(1S1). . .Cl(1)#4
0.88(4)
2.36(4)
2.228(8)
2.26(2)
2.291(6)
3.234(4)
3.185(5)
3.168(4)
3.249(4)
172(3)
174(5)
157(5)
175(5)
0.960(2)
0.960(2)
0.960(2)
N(12)–Zn(1)–N(13)
N(13)–Zn(1)–N(23)
N(13)–Zn(1)–N(22)
N(12)–Zn(1)–N(11)
N(23)–Zn(1)–N(11)
N(12)–Zn(1)–N(21)
N(23)–Zn(1)–N(21)
N(11)–Zn(1)–N(21)
79.62(11)
179.69(10)
100.57(11)
79.67(11)
95.98(10)
100.71(10)
83.61(10)
179.48(10)
N(12)–Zn(1)–N(23)
N(12)–Zn(1)–N(22)
N(23)–Zn(1)–N(22)
N(13)–Zn(1)–N(11)
N(22)–Zn(1)–N(11)
N(13)–Zn(1)–N(21)
N(22)–Zn(1)–N(21)
100.13(11)
179.57(12)
79.68(11)
84.16(10)
100.73(11)
96.25(10)
78.89(10)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx, ꢁy, ꢁz; #2
x ꢁ 1, y, z ꢁ 1; #3 x, y, z ꢁ 1; #4 ꢁx + 1, ꢁy + 1, ꢁz + 1.
Fig. 2. (a) Structure of the cation of compound 2. Displacement ellipsoids are shown at the 30% probability level. (b) Hydrogen bond interactions in a two-dimensional
polymeric compound. H atoms not involved in intermolecular interactions have been omitted for clarity. Green dotted lines represent Namine–Hꢀ ꢀ ꢀBr, Owater–Hꢀ ꢀ ꢀOwater and
Owater–Hꢀ ꢀ ꢀBr hydrogen bonds.(For interpretation of color mentioned in this figure the reader is referred to the web version of the article.)

B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
127
Table 5
3.2. Structure description
Hydrogen bonds for compound 2 (Å and °)
The geometry at the zinc atom of all three complexes is pseudo-
octahedral, as expected, and three Zn(II) complexes with tridentate
bispicam ligand produced fac isomers represented (a) above
among three potential geometrical isomers ((a) facial isomer, (b)
meridional isomer, and facial (c) and (d) isomers which are enanti-
omers to each other) [7].
The zinc atom in 1 lies on an inversion center giving the Ci iso-
mer as shown in Fig. 1a. The geometry at zinc atom is pseudo-octa-
hedral, and the isomer isolated is the fac isomer represented (a)
above. The Zn–Namine bond of length 2.153(3) Å is shorter than
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
h(DHA)
O(1S)–H(1B). . .O(2S)
0.930(2)
0.930(2)
0.86(3)
0.930(2)
0.930(2)
0.930(2)
0.84(3)
1.809(14)
2.442(6)
2.56(3)
2.394(9)
2.451(13)
2.445(11)
2.57(3)
2.717(5)
3.369(4)
3.372(3)
3.314(3)
3.363(4)
3.359(3)
3.382(3)
165(5)
175(5)
158(3)
170(4)
167(4)
167(4)
164(3)
O(3S)–H(3B). . .Br(2)#1
N(12)–H(12N). . .Br(1)#2
O(2S)–H(2A). . .Br(1)#3
O(2S)–H(2B). . .Br(1)#4
O(1S)–H(1A). . .Br(2)#5
N(22)–H(22N). . .Br(2)#6
Symmetry transformations used to generate equivalent atoms: #1 x,ꢁy + 1/2, z + 1/
2; #2 ꢁx + 1, ꢁy + 1, ꢁz + 2; #3 ꢁx + 1,ꢁy + 2, ꢁz + 1; #4 x, y, z ꢁ1; #5 x, y + 1, z; #6
x + 1, ꢁy + 1/2, z + 1/2.
Fig. 3. (a) Structure of the cation of compound 3. Displacement ellipsoids are shown at the 30% probability level. (b) Hydrogen bond interactions in a two-dimensional
polymeric compound. H atoms not involved in intermolecular interactions have been omitted for clarity. Green dotted lines represent Namine–Hꢀ ꢀ ꢀI and C–Hꢀ ꢀ ꢀI hydrogen
bonds..(For interpretation of color mentioned in this figure the reader is referred to the web version of the article.)

128
B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
Table 6
The zinc atom in 3 also lies on an inversion center giving the Ci
Selected bond lengths [Å] and angles [°] for compound 3
isomer as shown in Fig. 3a. The structure of 3 is also very similar to
that of 1 with the fac isomer represented (a) above, and the princi-
ple bond distances and angles of 2 are listed in Table 6. With iodide
anions, Namine–Hꢀ ꢀ ꢀI and C–Hꢀ ꢀ ꢀI interactions between free iodide
anions and [Zn(bispicam)2]2+ units generate 6-membered Cl2C2N2
rings to form a two-dimensional sheet as shown in Fig. 3b (Table
7).
To our knowledge, the system containing C/N/O–Hꢀ ꢀ ꢀX (X = Cl,
Br and I) hydrogen bonding between free (full negative charged)
halides and Zn(II) complex cations is unprecedented even though
metal-assisted hydrogen bonding (C–Hꢀ ꢀ ꢀX–M) is well docu-
mented [3,4]. With chloride anions, Owater–Hꢀ ꢀ ꢀCl interactions gen-
erate flat 4-membered Cl2O2 rings that are connected to
[Zn(bispicam)2]2+ units through Namine–Hꢀ ꢀ ꢀCl interactions to form
an one-dimensional polymeric compound. With bromide anions,
Namine–Hꢀ ꢀ ꢀBr and Owater–Hꢀ ꢀ ꢀBr interactions generate a two-
dimensional sheet. With iodide anions, Namine–Hꢀ ꢀ ꢀI and C–Hꢀ ꢀ ꢀI
interactions generate 6-membered Cl2C2N2 rings to form a two-
dimensional sheet. These results indicate that hydrogen bonding
interactions between Namine–H and C–H of [Zn(bispicam)2]2+ cat-
ions and free halides, and between Owater–H and free halides play
very important roles for construction of molecular packing and
crystal structures.
Zn(1)–N(2)#1
Zn(1)–N(1)
2.159(3)
2.172(3)
Zn(1)–N(2)
Zn(1)–N(1)#1
2.159(3)
2.172(3)
Zn(1)–N(3)#1
N(2)#1–Zn(1)–N(2)
N(2)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)#1
N(2)#1–Zn(1)–N(3)#1
N(1)–Zn(1)–N(3)#1
N(2)#1–Zn(1)–N(3)
N(1)–Zn(1)–N(3)
2.186(3)
180.0
Zn(1)–N(3)
2.186(3)
99.92(11)
80.08(11)
180.0
79.39(12)
93.68(11)
100.61(12)
86.32(11)
N(2)#1–Zn(1)–N(1)
N(2)#1–Zn(1)–N(1)#1
N(1)–Zn(1)–N(1)#1
N(2)–Zn(1)–N(3)#1
N(1)#1–Zn(1)–N(3)#1
N(2)–Zn(1)–N(3)
80.08(11)
99.92(11)
100.61(12)
86.32(11)
79.39(12)
93.68(11)
180.0
N(1)#1–Zn(1)–N(3)
N(3)#1–Zn(1)–N(3)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1,ꢁy,
ꢁz + 2.
Table 7
Hydrogen bonds for compound 3 (Å and °)
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
h(DHA)
165.66(1)
153.95(1)
171.72(1)
C(4)–H(4). . .I(1)#2
C(7)–H(7A). . .I(1)
N(2)–H(2N). . .I(1)#3
0.93
0.97
0.81(4)
3.059(2)
3.076(1)
2.786(1)
3.967(4)
3.970(4)
3.585(3)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1, ꢁy,
ꢁz + 2; #2 x ꢁ 1, y, z; #3 ꢁx + 1,ꢁy + 1,ꢁz + 2.
3.3. Catalytic transesterification reactions by the compounds 1–3
two Zn–Npyridylbonds of 2.166(3) and 2.203(4) Å (Table 2). With
chloride anions, Owater–Hꢀ ꢀ ꢀCl interactions generate flat 4-mem-
bered Cl2O2 rings, and these 4-membered Cl2O2 rings are con-
nected to [Zn(bispicam)2]2+ units through Namine–Hꢀ ꢀ ꢀCl
interactions to form an one-dimensional polymeric compound as
shown in Fig. 1b (Table 3). The 4-membered rings are also con-
nected to other water molecules through Owater–Hꢀ ꢀ ꢀCl interac-
tions. To the best our knowledge, such an arrangement has not
been experimentally observed so far although cyclic (H2O)4 units
have been reported [9].
We have recently reported that hydrogen-bonded polymers,
Zn(Hdpa)Cl2, Zn(Hdpa)Br2 and Zn(Hdpa)I2, formed from the reaction
of ZnX2 (X = Clꢁ, Brꢁ and Iꢁ) with a chelating ligand Hdpa have
shown, surprisingly, unusual heterogeneous catalytic activities on
transesterification reaction of a range of esters with methanol at
room temperature under the mild conditions [6]. This result led us
to attempt the transesterification reaction using the compounds 1,
2, and 3. Treatment of 4-nitrophenyl acetate and methanol in the
presence of the catalysts at 50 °C produced methyl acetate quantita-
tively under the neutral conditions within 1 day (Eq. (1); see entry 1
of Table 8), respectively, while trace or a small amount of transeste-
rification occurs without each catalyst (the sixth column) [10].
The asymmetric unit of 2 contains a whole molecule, and the
structure of 2 is very similar to that of 1 with the fac isomer repre-
sented (a) above (Fig. 2a). The principle bond distances and angles
of 2 are listed in Table 4. The Zn–N distances range from 2.157(3)
to 2.184(3) Å. With bromide anions, Owater–Hꢀ ꢀ ꢀBr interactions
generate 4-membered Br2O2 rings. These Br2O2 rings are connected
to [Zn(bispicam)2]2+ units through Namine–Hꢀ ꢀ ꢀBr interactions, and
O
catalyst 1
HOCH3
..
O2N
OCCH3
+
ð1Þ
they are also connected to other water molecules through Owater
–
O
Hꢀ ꢀ ꢀOwater interactions. The connected water molecules are con-
nected to free bromide anions through Owater–Hꢀ ꢀ ꢀBr interactions.
The free bromide anions are connected to other [Zn(bispicam)2]2+
units through Namine–Hꢀ ꢀ ꢀBr interactions. These Owater–Hꢀ ꢀ ꢀBr and
Namine–Hꢀ ꢀ ꢀBr interactions produce 28-membered rings to form a
two-dimensional sheet as shown in Fig. 2b (Table 5).
CH3COCH3
O2N
OH +
Based on these results, we have examined the catalytic activity
of the hydrogen-bonded polymeric compounds 1, 2, and 3 for
transesterification of various esters. The results are also given in
Table 8
Transesterification of esters by methanol in the presence of compounds 1, 2, and 3 at 50 °Ca
Entry
Substrate
1 (time/days)
2 (time/days)
3 (time/days)
Controlb (time/days)
1
2
3
4
5
6
7
4-Nitrophenyl acetate
Phenyl acetate
1
4
11
4
13
4
1
4
3
4
7
7
0.08
1
2
6
2
4
3
0.08
1 (10%)
4 (5%)
4-Nitrophenyl benzoatec
4-Chlorophenyl benzoate
Phenyl benzoate
11 (8%)
4 (30%)
13 (25%)
7 (15%)
1 (10%)
4-Methylphenyl benzoate
Vinyl acetate
0.21
a
All esters were completely converted to the corresponding products, methyl acetate and methyl benzoate. Reaction conditions: esters; 0.05 mmol, catalyst;
1.56 ꢂ 10ꢁ3 mmol for 1, 1.44 ꢂ 10ꢁ3 mmol for 2, and 1.28 ꢂ 10ꢁ3 mmol for 3, solvent; methanol (1 mL). See the experimental section for the detailed reaction conditions.
b
The transesterification reactions of esters were carried out without the catalysts. Numbers in parentheses indicate the conversion yields of esters to products within a
given time.
c
The solvent was a mixture of CH3OH/CH2Cl2 (1/1) because of low solubility of substrate in CH3OH.

B.K. Park et al. / Journal of Molecular Structure 890 (2008) 123–129
129
free halides and Zn(II) complex cations is unprecedented even
though metal-assisted hydrogen bonding (C–Hꢀ ꢀ ꢀX–M) is well
documented. We have also reported that Zn-containing hydro-
gen-bonded polymers 1, 2 and 3 could carry out the catalytic
transesterification of a range of esters with methanol under
the mild conditions. Though all these compounds are saturated
with two bispicam ligands, interestingly, they have shown the
efficient transesterification reactions. To explain this unusual
reactivity, it has been proposed that the hydrogen atom of amine
N–H moiety in the complexes could do the acid-catalyzed
transesterification. Further explorations into the uses of this cat-
alyst family in organic transformations as well as mechanistic
investigations are under study.
Acknowledgements
Financial support from the Korea Research Foundation (2006-
312-C00569 and 2007-314-C00159), THE SEOUL R & BD Program,
Environmental Technology Educational Innovation Program
(2006) of the Ministry of Environment, and the SRC program of
the Korea Science and Engineering Foundation (KOSEF) through
the Center for Intelligent Nano-Bio Materials at Ewha Womans
Scheme 2.
University
(Grant:
R11-2005-008-03001-0)
is
gratefully
acknowledged.
Table 8. Phenyl acetate have displayed slower reactivities than p-
nitrophenyl acetate by these catalysts with 4, 4 and 2 day reaction
time, respectively (entry 2). These catalysts 1–3 were also active to
benzoates with an electron-donating or -withdrawing group that
underwent the transesterification with 2–11 days (entries 3–6).
The substrates with the electron-withdrawing substituents have
undergone faster transesterification (entries 3 and 4), while those
with the electron-donating ones have shown slow reaction (entry
6). Moreover, vinyl acetate, that is widely used as a precursor for
ester synthesis [11], was also converted efficiently to the product
methyl acetate by the catalysts 1–3 within 0.21–0.38 day (entry
7), suggesting that this catalytic system can be useful for preparing
various esters by transesterification.
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 the possible
transesterification reaction mechanism based on the structure of
the complexes 1, 2 and 3. We can presume that there is no direct
interaction between the substrate ester and zinc ion of the com-
plexes 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 transeste-
rification, because the hydrogen atom could be acidic by coordinat-
ing of nitrogen of N–H to zinc ion (Scheme 2). Thus, this acidic
hydrogen interacts with the oxygen atom of carbonyl of ester,
resulting in that the carbonyl is more electrophilic. Then, the resul-
tant activated carbonyl could be easily attacked by methanol to
produce the product methyl ester as shown in Scheme 2. Detailed
mechanistic studies are currently under investigation.
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