Molecular interactions of zinc(II) cyclams with polycarboxylato ligands

Article abstract of DOI:10.1016/j.ica.2004.10.025

Four new zinc(II) cyclams of the composition {Zn(L)(tp^2-) ? H₂O}_n (1), {Zn(L)(H₂bta^2-) ? 2H₂O}_n (2), [Zn₂(L)₂(ox ^2-)] 2ClO₄ ? 2DMF (3), and Zn(L)(H ₂btc^-)₂ ? 2DMF (4), where L = cyclam, tp^2- = 1,4-benzenedicarboxylate ion, H₂bta^2- = 1,2,4,5-benzenetetracarboxylate ion, ox^2- = oxalate ion, DMF = N,N-dimethylformamide, and H₂btc^- = 1,3,5- benzenetricarboxylate ion, have been synthesized and structurally characterized by a combination of analytical, spectroscopic and crystallographic methods. The carboxylato ligands in the complexes 1-4 show strong coordination tendencies toward zinc(II) cyclams with hydrogen bonding interactions between the pre-organized N-H groups of the macrocycle and oxygen atoms of the carboxylato ligands. The macrocycles in 1, 2, and 4 adopt trans-III configurations with the appropriate R,R,S,S arrangement of the four chiral nitrogen centers, respectively. However, the complex 3 shows an unusual cis V conformation with the R,R,R,R nitrogen configuration. The finding of strong interactions between the carboxylato ligands and the zinc(II) ions may provide additional knowledge for the improved design of receptor-targeted zinc(II) cyclams in anti-HIV agents.

Full text of DOI:10.1016/j.ica.2004.10.025

Inorganica Chimica Acta 358 (2005) 1274–1278
www.elsevier.com/locate/ica
Note
Molecular interactions of zinc(II) cyclams with
polycarboxylato ligands
a
Hyejeong Jo , Alan J. Lough , Ju Chang Kim
b
a,
*
a
Department of Chemistry, Pukyong National University, 599-1 Daeyon-dong, Nam-gu, Pusan 608-737, Republic of Korea
b
Department of Chemistry, University of Toronto, Toronto, Ont., Canada M5S 3H6
Received 15 July 2004; accepted 17 October 2004
Available online 19 November 2004
Abstract
Four new zinc(II) cyclams of the composition {Zn(L)(tp2ꢀ) Æ H2O}n (1), {Zn(L)(H2bta2ꢀ) Æ 2H2O}n (2), [Zn2(L)2(ox2ꢀ)]
2ClO4 Æ 2DMF (3), and Zn(L)(H2btcꢀ)2 Æ 2DMF (4), where L = cyclam, tp2ꢀ = 1,4-benzenedicarboxylate ion, H2bta2ꢀ = 1,2,4,5-
benzenetetracarboxylate ion, ox2ꢀ = oxalate ion, DMF = N,N-dimethylformamide, and H2btcꢀ = 1,3,5-benzenetricarboxylate ion,
have been synthesized and structurally characterized by a combination of analytical, spectroscopic and crystallographic methods.
The carboxylato ligands in the complexes 1–4 show strong coordination tendencies toward zinc(II) cyclams with hydrogen bonding
interactions between the pre-organized N–H groups of the macrocycle and oxygen atoms of the carboxylato ligands. The macro-
cycles in 1, 2, and 4 adopt trans-III configurations with the appropriate R,R,S,S arrangement of the four chiral nitrogen centers,
respectively. However, the complex 3 shows an unusual cis V conformation with the R,R,R,R nitrogen configuration. The finding
of strong interactions between the carboxylato ligands and the zinc(II) ions may provide additional knowledge for the improved
design of receptor-targeted zinc(II) cyclams in anti-HIV agents.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Zinc(II) complex; Crystal structure; Coordination polymer; Cyclam
1. Introduction
cyclams. It has been suggested that the carboxylate
groups of aspartate residues Asp 171 and Asp 262 of
the CXCR4 co-receptor play key roles in the recognition
of cyclams [7]. Therefore, the understanding of interac-
tions between carboxylates and zinc(II) cyclams is signif-
icant in the design and development of more effective
anti-HIV agents. Recently, several zinc complexes such
as [Zn(L)(H2O)2](OAc)2 [8], {Zn(L)(phthalate)}n Æ (CH3-
OH)2n [8], [Zn(Bz-cyclam)Cl]Cl Æ 2.5CDCl3 [4], and
[Zn2(Xyl-bicyclam)(OAc)2] (OAc)2 Æ 2CH3OH [4] have
been prepared and structurally characterized to eluci-
date a selective recognition of zinc(II) cyclams by carb-
oxylates. However, the zinc(II) cyclams having
carboxylato ligands are still rare. During our efforts to
expand such examples, which enable us to offer the
knowledge of the nature of interactions between zinc(II)
cyclams and carboxylates, we successfully prepared and
Cyclams and bicyclams exhibit anti-HIV activity
[1,2]. Particularly, the bicyclams in which the cyclam
moieties tethered via aliphatic or aromatic linkers are
potent and selective inhibitors of HIV-1 and HIV-2
[3,4]. The phenylenebismethylene bridging bicyclam
(AMD 3100) has recently been on clinical trials for the
treatment of AIDS and also shows an ability to mobilize
stem cells [5,6]. The cyclams block the entry of the virus
into the white cells by specific binding to the co-receptor
CXCR4, and co-receptor binding strength and anti-HIV
activity enhance on the complexation of zinc(II) ions to
*
Corresponding author. Tel.: +82 51 620 6382; fax: +82 51 628
8147.
E-mail address: kimjc@pknu.ac.kr (J.C. Kim).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.10.025

H. Jo et al. / Inorganica Chimica Acta 358 (2005) 1274–1278
1275
structurally characterized new carboxylato zinc(II) cycl-
ams. In this report, we describe the synthesis, structures
and properties of zinc(II) complexes 1, 2, 3, and 4 ob-
tained from the reactions between Zn(L)(ClO4)2 and
various organic polycarboxylates.
(10 mL) of 1,2,4,5-benzenetetracarboxylic acid (91 mg,
0.357 mmol). The solution was allowed to stand at room
temperature for several days until transparent plates
formed. Anal Calc. for C20H32N4O10Zn (2): C, 43.33;
H, 5.78; N, 10.11. Found: C, 43.74; H, 5.81; N,
10.09%. IR (Nujol, cmꢀ1): 3449 (OH), 3224, 3164
(NH), 1693, 1572, 1340, 1310 (COO).
NH
N
HN
HN
NH
NH
N
2.2.3. [Zn2(L)2(ox2ꢀ)]2ClO4 Æ 2DMF (3)
NH
NH
HN
HN
To a DMF (10 mL) solution of Zn(L)(ClO4)2 (166
mg, 0.357 mmol) was added dropwise an aqueous solu-
tion (10 mL) of potassium oxalate monohydrate (66 mg,
0.357 mmol). The solution was allowed to stand at room
temperature. Transparent blocks were obtained after
several days. Anal. Calc. for C28H62N10O14Cl2Zn2 (3):
C, 34.84; H, 6.42; N, 14.51. Found: C, 35.20; H, 6.44;
N, 14.44%. IR (Nujol, cmꢀ1): 3270, 3250 (NH), 1633,
1313, 790 (COO), 1095 (ClO4).
HN
L
AMD 3100
2. Experimental
2.1. Materials, methods and apparatus
2.2.4. Zn(L)(H2btcꢀ)2 Æ 2DMF (4)
To a DMF (10 mL) solution of Zn(L)(ClO4)2 (166
mg, 0.357 mmol) was added dropwise a DMF solution
(10 mL) of 1,3,5-benzenetricarboxylic acid (75 mg,
0.357 mmol). The solution was allowed to stand at room
temperature for several days until transparent blocks
formed. Anal. Calc. for C34H48N6O14Zn (4): C, 49.15;
H, 5.78; N, 10.12. Found: C, 49.07; H, 5.78; N,
10.03%. IR (Nujol, cmꢀ1): 3450 (OH), 3248, 3169
(NH), 1703, 1622, 1557 (COO).
All chemicals used in this work were of reagent grade
and were used without further purification. Infrared
spectra were measured as Nujol mulls between KBr
plates using a Perkin–Elmer Paragon 1000 FT-IR spec-
trophotometer over the range 4000 and 400 cmꢀ1. Ele-
mental analyses were performed by the Korea
Research Institute of Chemical Technology, Taejon,
Korea. Suitable crystals of 1–4 for X-ray diffraction
studies and subsequent spectroscopic measurements
were manually collected under the microscope.
2.3. X-ray crystallography
A summary of selected crystallographic data for 1–4
is given in Table 1. X-ray data were collected on a Non-
ius KappaCCD diffractometer, using graphite-mono-
2.2. Synthesis
˚
chromated Mo Ka radiation (k = 0.71073 A). A
The starting material Zn(L)(ClO4)2 was synthesized
according to the published procedures [7].
Caution: The perchlorate salts used in this study are
potentially explosive and should be handled in small
quantities.
combination of 1° / and x (with kappa offsets) scans
was used to collect sufficient data. The data frames were
integrated and scaled using the Denzo-SMN package
[9]. The structures were solved and refined, using the
SHELXL PC V5.1 package [10]. Refinement was per-
n
formed by full-matrix least-squares on F2, using all data
(negative intensities included). Hydrogen atoms were in-
cluded in calculated positions, except for those involving
hydrogen bonding specifically for the hydrogen atoms of
the carboxyl groups and those bonded to the nitrogen
atoms, which were refined with isotropic thermal
parameters.
2.2.1. Synthesis of {Zn(L)(tp2ꢀ) Æ H2O}n (1)
To a DMF (10 mL) solution of Zn(L)(ClO4)2 (166
mg, 0.357 mmol) was added dropwise an aqueous solu-
tion (10 mL) of 1,4-benzenedicarboxylic acid disodium
salt (75 mg, 0.357 mmol). The solution was allowed to
stand at room temperature for several days until trans-
parent blocks formed. Anal. Calc. for C18H30N4O5Zn
(1): C, 48.23; H, 6.70; N, 12.50. Found: C, 48.38; H,
6.83; N, 12.45%. IR (Nujol, cmꢀ1): 3391 (OH), 3234,
3173 (NH), 1573, 1370 (COO).
3. Results and discussion
The crystals 1–4 retain their transparency for several
weeks in a refrigerator, but they lose solvent molecules
in a couple of days upon exposure to the atmosphere
as evidenced by elemental analyses. The infrared spectra
2.2.2. Synthesis of {Zn(L)(H2bta2ꢀ) Æ 2H2O}n (2)
To a DMF (10 mL) solution of Zn(L)(ClO4)2 (166
mg, 0.357 mmol) was added dropwise a DMF solution

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H. Jo et al. / Inorganica Chimica Acta 358 (2005) 1274–1278
Table 1
Crystal data and details of structural determinations for 1–4
1
2
3
4
Empirical formula
Formula weight
Temperature (K)
C18H30N4O5Zn
447.83
150(1)
C20H32N4O10Zn
553.87
150(2)
C28H62Cl2N10O14Zn2
964.52
150(1)
C34H48N6O14Zn
830.15
150(1)
˚
Wavelength (A)
0.71073
monoclinic
C2/c
0.71073
triclinic
0.71073
monoclinic
P2(1)/n
0.71073
monoclinic
P2(1)/n
Crystal system
Space group
Unit cell dimensions
ꢀ
P1
˚
a (A)
16.3442(5)
16.2092(6)
8.9481(3)
8.5540(3)
8.7040(3)
13.2202(2)
12.0175(2)
13.3587(2)
8.4170(10)
20.7650(4)
11.3520(2)
˚
b (A)
˚
c (A)
9.4100(4)
64.992(12)
a (°)
b (°)
c (°)
119.435(2)
82.664(13)
75.796(14)
103.819(10)
110.7621(10)
˚
Volume (A)
2064.58(12)
4
615.29(4)
1
2060.92(6)
2
1855.24(5)
2
Z
Final R indices [I > 2r(I)]
R indices (all data)
R1 = 0.0307, wR2 = 0.0777
R1 = 0.0414, wR2 = 0.0838
R1 = 0.0375, wR2 = 0.0875
R1 = 0.0487, wR2 = 0.0959
R1 = 0.0287, wR2 = 0.0706
R1 = 0.0343, wR2 = 0.0737
R1 = 0.0383, wR2 = 0.0903
R1 = 0.0545, wR2 = 0.0974
P
P
P
P
2
2 1=2
R1 = iFo ꢀ Fci/ |Fo| and wR2 ¼ ½ ½wðF 2o ꢀ F c2Þ ꢂ= ½wðF o2Þ ꢂꢂ
.
and microanalyses for 1–4 clearly support the structures
determined by the X-ray diffraction studies as described
in Section 2. Addition of 1,4-benzenedicarboxylate ion
to a DMF solution of Zn(L)(ClO4)2 produced the com-
plex 1. The structure of 1 consists of a one-dimensional
coordination polymer with a basic Zn(L)(tp2ꢀ) unit
(Fig. 1). The macrocycle adopts trans III configuration
in the solid state. The coordination environment around
the central zinc(II) ion shows a slightly distorted octahe-
dron in the chain direction with four Zn–N and two Zn–
O bonds. The Zn atom sits on an inversion center. The
four Zn–N distances are in the range 2.1083(15)–
slightly shorter than that found in {Zn(L)(phtha-
˚
late)}n(CH3OH)2n; Zn–O=2.22 A) [8]. One of the pertinent
structural features found in 1 is the pre-organization
of N–H groups of the macrocycle with its role in profa-
cial selection of tp2ꢀ binding through hydrogen bond-
˚
ing interactions (N1–H1ꢁ ꢁ ꢁO2 = 2.971(2) A). Similarly
to other macrocyclic-tp2ꢀ systems [11], there are two dif-
ferent series of one-dimensional coordination polymers
running toward different directions in 1. The two
different chains are interconnected by water molecules
through hydrogen bonding interactions (O1W–
˚
H1Wꢁ ꢁ ꢁO2 = 2.857(2) A), resulting in the formation of
˚
˚
2.1114(15) A. The Zn–O distance of 2.1559(12) A is
an ultimate three-dimensional network.
In 2, the basic coordination pattern of the anion to-
ward the zinc(II) is very similar to that found in 1.
The polymeric one-dimensional chain is formed toward
the axial direction of the Zn(L) (Fig. 2). The Zn atom is
located on an inversion center. The Zn–O distance of
˚
2.2839(14) A is even longer than that observed in 1. It
seems that the increased steric hindrance by the
introduction of two more carboxyl groups in the
H2bta2ꢀ prevents the anion from approaching closely
to the zinc(II) macrocycle. Apart from the Zn–O bonds,
the hydrogen bond between the pre-organized N–H
groups of the macrocycle and the COOꢀ of the H2bta2ꢀ
contributes to the formation of a polymer (N1–
˚
H1ꢁ ꢁ ꢁO2#1 = 2.876(2) A, symmetry codes: #1 ꢀx + 1,
ꢀy + 1, ꢀz + 1). The complex adopts trans III, as does
complex 1. The salient structural feature of 2 is the
observation of only a single series of one-dimensional
coordination polymers running toward the crystallo-
graphic c directions. Water molecules mediate in inter-
connecting each polymeric chain through hydrogen
bonding interactions to form two-dimensional plane
Fig. 1. Molecular structure of 1 with atom-labeling scheme. Hydrogen
atoms other than those participating in hydrogen bonding are omitted
˚
for clarity. Selected bond distances (A) and angles (°). Zn1–N1,
2.1083(15); Zn1–N2, 2.1114(15); Zn1–O(1), 2.1559(12); N1–Zn1–N2,
85.35(6); N1–Zn1–O1, 92.14(6); N2–Zn1–O1, 92.60(5).
˚
(O4–H4Oꢁ ꢁ ꢁO1W = 2.574(2) A, O1W–H2Wꢁ ꢁ ꢁO2#4 =

H. Jo et al. / Inorganica Chimica Acta 358 (2005) 1274–1278
1277
Fig. 3. Molecular structure of 3 with atom-labeling scheme. Hydrogen
atoms other than those participating in hydrogen bonding are omitted
˚
for clarity. Selected bond distances (A) and angles (°). Zn1–N1,
2.1598(14); Zn1–N2, 2.1490(13); Zn1–N3, 2.1429(14); Zn1–N4,
2.1658(14); Zn1–O1, 2.1175(11); Zn1–O2, 2.1397(11); O1–Zn1–O2,
78.74(4); O1–Zn1–N3, 92.43(5); O2–Zn1–N3, 95.50(5); O1–Zn1–N2,
166.71(5); O2–Zn1–N2, 88.31(5); N3–Zn1–N2, 91.78(5); O1–Zn1–N1,
95.20(5); O2–Zn1–N1, 92.86(5); N3–Zn1–N1, 169.64(5).
Fig. 2. Molecular structure of 2 with atom-labeling scheme. Hydrogen
atoms other than those participating in hydrogen bonding are omitted
˚
for clarity. Selected bond distances (A) and angles (°). Zn1–N1,
2.0971(19); Zn1–N2, 2.0978(19); Zn1–O1, 2.2839(14); N1–Zn1–N2,
85.47(8); N1–Zn1–O1, 87.54(6); N2–Zn1–O1, 90.78(7).
showing the R,R,R,R set of nitrogen configurations.
The two Zn–O distances of 2.1175(11) and 2.1397(11)
˚
A are comparable to those found in a related macrocy-
clic zinc(II) system [Zn2(Xyl-bicyclam)(OAc)2](OAc)2 Æ
˚
˚
˚
2CH3OH (Zn–O = 2.089(2), 2.407(2) A) [4]. Apart from
2.644(2) A, O1W–H1Wꢁ ꢁ ꢁO3#3 = 2.738(2) A ; symme-
try codes: #3 ꢀx + 1, ꢀy, ꢀz + 2, #4 x, y ꢀ 1, z). The
shape of the H2bta2ꢀ in 2 resembles that found in
[Ni(L1)](H2bta2ꢀ) Æ (H4bta) {L1 = 3,10-bis(2-hydroxy-
ethyl)-1,3,5,8,10,12-hexaazacyclotetradecane} [12]. As
are often the examples [13–15], the carboxylato groups
and carboxylic acid groups of the present H2bta2ꢀ do
not lie in the phenyl ring plane, since the dihedral angle
between the carboxylato and the aromatic ring (O1 C9
C6 C7) is 31.8° and the dihedral angle between the car-
boxylic acid and the aromatic ring (O3 C10 C7 C6) is
54.2°. In the recent studies, we have reported various
shapes of the H4bta, H3btaꢀ, and H2bta2ꢀ, where the
shapes vary with the degree of deprotonation and the
types of hydrogen bonding systems [16].
ꢀ
the bridging oxalato ligand, two ClO4 anions play a
role in mediating the two macrocycles by weak hydro-
gen bonds. The four N–H groups in 3 are also pre-orga-
nized, however, the patterns of the pre-organization
observed in 3 are different from those observed in 1, 2
and 4 as the macrocycles are folded. The two hydrogen
atoms on nitrogen atoms N1 and N3 as hydrogen bond
donors face the oxygen atoms of perchlorate anions, and
the remaining two hydrogen atoms on N2 and N4 face
the oxygen atoms of solvent DMF molecules, assisting
in the formation of an ultimate dimeric compound 3.
The structure of 4 consists of Zn(L), two H2btcꢀ li-
gands, and two solvent DMF molecules. The coordina-
tion geometry around the zinc(II) ion reveals a six-
coordinated axially distorted octahedron with four
nitrogen atoms from the macrocycle and two oxygen
atoms from the carboxylato ligands of H2btcꢀ. An
inversion center exists on the central zinc(II) ion. Fig.
4 shows a drawing of 4 with the atom labeling scheme.
The structure of
3
consists of binuclear
[Zn2(L)2(ox2ꢀ)]2+ cations, which are hydrogen bonded
to perchlorate anions and solvent DMF molecules.
There is a crystallographic inversion center at the middle
of point of the C11–C11#1 bond of the bridging oxalato
ligand. The oxalato ligand coordinates to the zinc(II)
ion by a bis-bidentate mode. A view of the molecule is
shown in Fig. 3. The coordination environment around
the zinc(II) ion is described as a distorted octahedron
with four nitrogen atoms from the macrocycle and two
oxygen atoms from the bridging oxalato ligand. In 3,
the macrocyclic ligand skeleton adopts an unusual cis
V conformation in a manner observed in [Zn2(Xyl-bicy-
˚
The Zn–O distance of 2.2222(12) A and the Zn–N dis-
˚
tances of 2.0923(16)–2.1128(15) A are normal and fall
on those found in complexes 1–3. The H2btcꢀ is always
involved in the formation of the complex 4 although an
acid form H3btc is used during the synthesis. Two sets of
N–H are observed in 4 and each set participates in the
formation of hydrogen bonding interactions as hydro-
˚
gen bond donors (N1–H1ꢁ ꢁ ꢁO2 = 2.9287(19) A, N2–
˚
clam)(OAc)2](OAc)2 Æ 2CH3OH [4],
ox2ꢀ)]2NO3 [17] and [(NiL1)2(l-ox2ꢀ)]2ClO4 [18],
cis-[Ni2(L)2(l-
H2ꢁ ꢁ ꢁO5#3 = 3.0674(19) A; symmetry code: #3 x ꢀ 1,
y, z ꢀ 1).

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H. Jo et al. / Inorganica Chimica Acta 358 (2005) 1274–1278
Appendix A. Supplementary data
Crystallographic data have been deposited at the
Cambridge Crystallographic Data Center (CCDC),
CCDC Nos. 208587–208590 for 1–4. Supplementary
data associated with this article can be found, in the on-
line version at doi:10.1016/j.ica.2004.10.025.
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In conclusion, we have prepared and fully character-
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coordination tendency toward the zinc(II) cyclams with
hydrogen bonding interactions between the pre-orga-
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of the carboxylato ligands, respectively. The complexes
1, 2, and 4 adopt trans-III configurations with the
appropriate R,R,S,S arrangement of the four chiral
nitrogen centers. However, the complex 3 adopts an
unusual cis V conformation with the R,R,R,R nitrogen
configuration. This understanding may provide a new
basis for the improved design of receptor-targeted zin-
c(II) cyclams.
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