A mononuclear Mn2+ complex based on a novel tris-(ethyl acetate) pendant-armed tetraazamacrocycle: Effect of pyridine on self-assembly and weak interactions

Article abstract of DOI:10.1016/j.inoche.2012.03.042

A novel pyridine containing tetraazamacrocycle 3,6,9,15-tetraazabicyclo[9. 3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid ethyl ester (1) and its mononuclear Mn²⁺ complex (2) have been prepared. Pyridine induced the π-π stacking of [Mn1]²⁺ and provided donors for nonclassical CH...O hydrogen bonding as well. In the solid state of 2, the combination of π-π interactions and hydrogen bonding constructed a 3D crystal structure and thus formed pairs of interacting mononuclear complexes that mediated weak magnetic exchanges between the Mn²⁺ ions. Meanwhile, self-assembly of 2 dissociated in the aqueous solution, and the effect of pyridine on the nature of DNA binding with 2 have also been investigated.

Full text of DOI:10.1016/j.inoche.2012.03.042

Inorganic Chemistry Communications 21 (2012) 16–20
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A mononuclear Mn2+ complex based on a novel tris-(ethyl acetate) pendant-armed
tetraazamacrocycle: Effect of pyridine on self-assembly and weak interactions
⁎
Jinghan Wen, Zhirong Geng, Yuxin Yin, Zhilin Wang
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel pyridine containing tetraazamacrocycle 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,
6,9-triacetic acid ethyl ester (1) and its mononuclear Mn2+ complex (2) have been prepared. Pyridine induced
the π–π stacking of [Mn1]2+ and provided donors for nonclassical C\H…O hydrogen bonding as well. In the
solid state of 2, the combination of π–π interactions and hydrogen bonding constructed a 3D crystal structure
and thus formed pairs of interacting mononuclear complexes that mediated weak magnetic exchanges between
the Mn2+ ions. Meanwhile, self-assembly of 2 dissociated in the aqueous solution, and the effect of pyridine on
the nature of DNA binding with 2 have also been investigated.
Received 20 February 2012
Accepted 30 March 2012
Available online 12 April 2012
Keywords:
Macrocyclic ligand
Metal complex
Pyridine
© 2012 Elsevier B.V. All rights reserved.
Magnetism
DNA binding
Manganese is an essential biogenic element that participates in a
number of enzymes (manganese superoxide dismutase, arginase,
glutamine synthetase) as a cofactor [1]. Besides, Mn2+ can substitute
Ca2+ in many biological systems and processes owing to their similar
effective ionic radii, which enables the detection of calcium influx in
central nervous system [2]. On the other hand, it is well known that
tetraazamacrocycles are efficient chelators due to the formation of
high stability complexes. In particular, finely tuning the coordination
environment by adding pendant arms to unsaturated nitrogen donor
atoms has expanded this area through functionalization [3]. Recently,
pyridine has been introduced in the macrocycles that were lacking
conjugate systems, which was expected to increase the thermodynamic
stabilities [4] of the metal complexes and also assist to functionalize the
original macrocycles by providing a suitable coordinating site. In addi-
tion, pyridine in the macrocycle is inclined to form aromatic–aromatic
stacking, which is a powerful organizing force in self-assembly and mo-
lecular recognition processes [5]. Mononuclear complexes containing
aromatic ligands can be assembled into different types of supramolecu-
lar architectures in the crystal design by the π–π stacking between
aromatic rings, which has also been recognized as one of the key factors
in dominating magnetic coupling [6].
synergically coordinated with Mn2+ and yielded a mononuclear Mn2+
complex (2). In the solid state, pyridine induced the self-assembly of
the separate unit [Mn1]2+, carbon atoms of pyridine also functioned as
the donors of the nonconventional C\H…O hydrogen bonding. The
π–π stacking and hydrogen bonding thus formed not only cooperatively
strengthened the three dimensional structure of 2, but also gave pairs of
interacting mononuclear complexes that mediated weak magnetic ex-
changes between Mn2+ ions. The results were verified by both magnetic
susceptibility measurements and EPR spectroscopy. Furthermore, self-
assembly of 2 dissociated in the aqueous solution. Considering that the
propensity to self-associate through π–π interactions is closely related
to the ability to intercalate with DNA [7], binding nature of 2 with DNA
has also been examined and demonstrated to be associated with
the effect of pyridine.
Synthetic route for the preparation of the macrocycle ligand 1 and its
complex 2 is outlined in Scheme 1. L was synthesized referring to a slight
modification of previously reported methods [8]. The attempt of obtain-
ing 2,6-bis(bromomethyl) pyridine (3) by brominating 2,6-dimethyl
pyridine was unsatisfactory due to the generation of both mono- and
bis-brominated products during the reaction. Therefore, 3 was produced
with relatively high yield by reacting 2,6-bis(hydroxymethyl) pyridine
with HBr in condensed H2SO4. Reaction of 3 with all N-protected com-
pound 4 afforded the protected macrocycle 5, which then underwent a
de-protection reaction together with 33% HBr in AcOH to give the moth-
er ring L. The target ligand 1 was eventually obtained through the reac-
tion of L and ethyl bromoacetate (molar ration 1:3.3). Preparation and
crystal growth of 2 are summarized in Reference [9].
Thereby motivated, we herein describe a novel pyridine containing
tetraazamacrocycle ligand 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-
1(15),11,13-triene-3,6,9-triacetic acid ethyl ester (1). Nitrogen atoms
of the mother ring and oxygen atoms of the three pendant arms
⁎
Corresponding author at: State Key Laboratory of Coordination Chemistry, Coordi-
The mononuclear complex 2 is built up from an asymmetric unit
composed of [Mn1]2+, one H2O and two ClO4− counterions. Molecular
structure of the divalent [Mn1]2+ cation is shown in Fig. 1, in which
nation Chemistry Institute, School of Chemistry and Chemical Engineering, Nanjing
University, Hankou Road 22, Nanjing 210093, PR China. Tel./fax: +86 25 83686082.
E-mail address: wangzl@nju.edu.cn (Z. Wang).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.03.042

J. Wen et al. / Inorganic Chemistry Communications 21 (2012) 16–20
17
Scheme 1. Preparation of 1 and 2.
Mn2+ is hepta-coordinated and adopts a distorted pentagonal bipyra-
midal geometry defined by four nitrogen donors [N(1), N(2), N(3),
N(4)] from L and three oxygen atoms [O(1), O(3), O(5)] from three
pendant arms. The four Mn–N distances range from 2.165 (3) to
2.422 (3) Å, and the shortest Mn–N distance corresponds to the
Mn\pyridyl nitrogen bond. Selected bond distances and angles are
given in Table S2.
An analysis of the unit cell packing diagram (Fig. 2) shows that
centrosymmetric pairs of [Mn1]2+ cations are stacked through inter-
molecular π–π interactions. The angle between the planes of pyridine
fragments is 3.1°, and the distance between the centroids is 3.87 Å, in-
dicating the occurrence of relatively strong π–π interactions [10]. An
intricate combination of π–π interactions and hydrogen bonding gov-
erns the three-dimensional crystal structure of 2. There are two types
of hydrogen bonding in 2 (Table S3), the oxygen atom of the water
molecule [O(15)] forms classical strong O\H…O hydrogen bonds
with the ClO4− oxygen atoms [O(9), O(12), O(13)]. Besides, carbon
atoms of [Mn1]2+ cations [C(2), C(4), C(12)] form nonclassical
weak C\H…O hydrogen bonds with oxygen atoms belonged to
ClO4− [O(10), O(13)] and H2O [O(15)], respectively. Hydrogen bond-
ing connects each mononuclear unit that forms a 2D structure prop-
agating along the bc plane. Meanwhile, π–π interactions together
with hydrogen bonding further stabilize the plane along the a axis
and finally fulfill the 3D structure.
The above results have verified that metal centers in 2 are linked by
π–π stacking and hydrogen bonding, which have been known to be able
to transmit magnetic interactions [11]. Therefore, magnetic susceptibil-
ity of complex 2 was measured in the temperature range of 1.8–300 K in
a 2000 Oe applied magnetic field to investigate Mn2+–Mn2+ interac-
tions (Fig. 3). The value of the χMT product at room temperature
(3.94 cm3 mol−1 K) is apparently lower than expected for one isolated
Mn2+ ion (4.37 cm3 mol−1 K), which can be attributed to the antiferro-
magnetic coupling between Mn2+ ions [12]. The χMT product main-
tained constant upon cooling to 70 K then sharply decreased to
3.77 cm3 K mol−1 at 1.8 K, indicating the existence of antiferromagnetic
interactions and/or zero-field splitting at low temperature.
Moreover, a nonlinear fitting via Eq. (1) over the whole tempera-
ture range (Fig. 3) reveals a Curie–Weiss behavior with the Curie con-
stant C=4.20 cm3 mol−1 K, the Weiss constant θ=−1.18 K and the
background susceptibility χ0 =−8.29×10−4 cm3 mol−1, respective-
ly. The small negative value of θ clearly suggests that the antiferro-
magnetic interaction between Mn2+ ions in 2 is not significant,
which is probably ascribed to the long distance between two Mn2+
ions (7.32 Å, Fig. 2). Meanwhile, zJex is calculated as −0.57 cm−1 uti-
lizing Eq. (2) and S=5/2 for Mn2+, and Jex can then be readily
obtained as −0.29 cm−1 adopting z=2 for the packing diagram.
χM ¼ C=ðT−θÞ þ χ0
θ ¼ ½zJexSðS þ 1Þꢀ=3k
ð1Þ
ð2Þ
Mn2+ ions in the mononuclear complex 2 have also been demon-
strated to interact by the EPR spectrum. Fig. 4 shows the EPR X-band
powder spectrum of 2 at room temperature, which consists of an in-
tense single signal centered at g=2.37. No Mn hyperfine structure,
Fig. 2. 3D structure of 2 composed by hydrogen bonding and dimers through the inter-
molecular π–π stacking of [Mn1]2+. Black dashed line: O\H…O hydrogen bonds; red
dashed line: C\H…O hydrogen bonds.
Fig. 1. Cation complex [Mn1]2+ structure of the asymmetric unit of 2. All hydrogen
atoms were omitted for clarity.

18
J. Wen et al. / Inorganic Chemistry Communications 21 (2012) 16–20
Fig. 5. Plots of emission intensity I/I0 vs. [2]/[DNA]. The solid line is the best fit line. [2]=
0–2 μM, [DNA]=[EB]=20 μM. λex =520 nm.
Fig. 3. Temperature dependence of χM and χMT for 2 with the solid line representing
the best fit of the curve.
Finally, a strong stacking interaction between an aromatic chro-
mophore and the base pairs of DNA is accompanied by a hypochro-
mism and a red shift in the UV absorption, and the extent of
hypochromism is commonly consistent with the strength of interca-
lative interaction [16]a. Thus, UV–vis spectra were measured, and
the binding mode of 2 with CT-DNA was investigated by the absor-
bance decay with increasing concentrations of DNA in the buffer
(50 mM Tris–HCl, pH 7.2). Fig. 6 shows the DNA binding isotherm
for 2 with DNA base pairs: [DNA]/[2] ratio ranging from 0 to 2.7.
Complex 2 exhibits two intense absorption bands at 210 and
270 nm, which are both attributed to the π–π* transition absorption
of pyridine. The hypochromism of ca. 18.9% for the band at 270 nm
for 2 with increasing concentrations of DNA suggests that the com-
plex strongly binds to DNA. Meanwhile, the absorption band at
210 nm exhibits a hypochromism and a bathochromism of approxi-
mately 47.2% and 7 nm, respectively. Binding constant (Kb) of 2
with DNA was calculated as 1.33×104 M−1 (binding site size s=
0.61) utilizing the Bard and Thorp model for non-cooperative and
non-specific binding [20] (Eq. (S2)), which is lower than those ob-
served for typical classical intercalators [21]. Structurally, the ligand
of 2 provides an aromatic moiety that probably overlaps with the
stacking base pairs of the DNA helix to facilitate intercalation,
whereas the fluorescent and UV results both prove that 2 binds to
DNA by partial intercalation.
which is issued from the hyperfine interaction of I=5/2 Mn nuclear
spin with S=5/2 electronic spin, could be evidenced. The lack of hy-
perfine structure indicates that Mn2+ ions in 2 are indeed magneti-
cally interacting [13], which is also in agreement with the crystal
structure and magnetic studies.
Complex 2 dissolves in water readily. Electrospray ionization mass
spectrum (ESI-MS) of 2 demonstrates that ligand 1 still binds to Mn2+
in a 1:1 stoichiometry in the aqueous solution (Fig. S6), and the sta-
bility constant (Ks) of 2 was determined as 9.2×104 M−1 by the
Benesi–Hildebrand method [14] utilizing UV–vis spectra (data not
shown). In other word, self-assembly of 2 in the solid state dissoci-
ated in water. It has been widely recognized that π–π stacking be-
tween aromatic rings is essential in the intercalation of both
inorganic [15,16] and organic [16,17] molecules with DNA. There-
fore, interaction of 2 with DNA was first evaluated by EB–DNA sys-
tem, which can be used to distinguish intercalating and non-
intercalating compounds [18]. Decrease of EB fluorescence (Fig. 5)
suggests the competition of 2 with EB in binding to DNA. Consider-
ing that competitive binding of other intercalators would lead to a
loss of fluorescence due to the depletion of EB–DNA complex, it
can be concluded that 2 actually intercalated into DNA. Meanwhile,
quenching constant (Ksv) was obtained as 1.14 by linearly fitting to
the Stern–Volmer equation (Eq. (S1)), indicating the intercalation
is moderate [19].
Fig. 4. X-Band powder EPR spectra of 2 obtained at 300 K, microwave frequency is
9.485 GHz.
Fig. 6. UV–vis studies of DNA binding of 2. From top to bottom: [DNA]/[2]=0–2.7. [2]=
100 μM.

J. Wen et al. / Inorganic Chemistry Communications 21 (2012) 16–20
19
In summary, we have developed a pyridine containing Mn2+
tetraazamacrocycle complex 2, and have demonstrated that two
types of hydrogen bonding and the intermolecular π–π stacking
of the pyridine rings belonged to each mononuclear unit were ca-
pable of mediating weak magnetic coupling between Mn2+ ions
at the supramolecular level. Particularly, the self-assembly of 2 dis-
sociated in the aqueous solution, which showed moderate binding
strength with CT-DNA by most probably partial intercalation
owing to the π–π interaction of pyridine with DNA base pairs.
Work is still underway in our lab to develop various pyridine con-
taining ligands and corresponding complexes.
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We thank the National Natural Science Foundation of China
(21021062, 21027013, 21075064 and 90813020) and the National
Basic Research Program of China (2007CB925102) for the financial
support.
Appendix A. Supplementary data
[9] Preparation and crystal growth of 2 is as follows. Ligand 1 (0.232 g, 0.50 mmol)
was dissolved in EtOH (10 mL). Accompanied by vigorous stirring, Mn(ClO4)2
(0.127 g, 0.50 mmol) was added to the above solution. The mixture was gently
refluxed for 1 h and then cooled to room temperature, after which the solvent
was partially removed to approximately 5 mL and the resulting solid was isolated
by filtration, and air-dried to afford the product as a pale yellow solid. Yield: 65%.
ESI-MS (MeOH–H2O, v/v 1/999), m/z (%): 536.25 [1+Mn+H2O]+, 550.25 [1+
Mn+MeOH]+; Anal. Calcd. for C23H38N4O15MnCl2: C, 37.48; H, 5.16; N, 7.60.
Found: C, 37.35; H, 4.81; N, 7.49; IR: 1742, 1703, 1637, 1618, 1401, 1144, 1120,
1089, 627 cm−1. Small pale yellow bulk crystals suitable for X-ray crystallogra-
phy were grown via slow evaporation of the EtOH filtrate at room temperature.
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nitrogen-containing ligands, J. Chem. Soc., Dalton Trans. (2000) 3885–3896.
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Supplementary data (Experimental details, characterizations of
the compounds, crystallographic data and selected bond lengths (Å)
and angles (°) for 2, and equations referred in the manuscript.
Figs. S1–S6) to this article can be found online at http://dx.doi.org/
10.1016/j.inoche.2012.03.042. CCDC number for 2: 826816.
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