Base-induced self-assembly for one-dimensional coordination polymers via chiral pendant-armed schiff base mononuclear Pb(II) macrocycles

Article abstract of DOI:10.1021/ic5008846

A pair of 18-membered [1 + 1] chiral pendant-armed Schiff base macrocyclic mononuclear Pb(II) complexes with an unusual N₁O₂ coordination mode, synthesized from two chiral isomeric dialdehyde components, can be further self-assembled to one-dimensional ribbon coordination polymers by adding NaOH as a base to remove two phenolic protons.

Full text of DOI:10.1021/ic5008846

Communication
pubs.acs.org/IC
Base-Induced Self-Assembly for One-Dimensional Coordination
Polymers via Chiral Pendant-Armed Schiff Base Mononuclear Pb(II)
Macrocycles
Kun Zhang,† Chao Jin,‡ Yuchen Sun,† Feifan Chang,† and Wei Huang*,†
†State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
‡College of Sciences, Nanjing Tech University, Nanjing, Jiangsu Province 210009, P. R. China
S
* Supporting Information
template. As a result, two 18-membered [1 + 1] mononuclear
Schiff base macrocyclic Pb(II) complexes (R-1 and S-1) have
been obtained (Scheme 1). X-ray single-crystal diffraction
ABSTRACT: A pair of 18-membered [1 + 1] chiral
pendant-armed Schiff base macrocyclic mononuclear
Pb(II) complexes with an unusual N1O2 coordination
mode, synthesized from two chiral isomeric dialdehyde
components, can be further self-assembled to one-
dimensional ribbon coordination polymers by adding
NaOH as a base to remove two phenolic protons.
Scheme 1. Schematic Illustration for the Synthesis of [1 + 1]
Chiral Macrocyclic Mononuclear Pb(II) Complexes and
Their Relative 1D Ribbon Coordination Polymers
rystal engineering of coordination polymers has been
Cwidely investigated because of its importance for material
science.1 With the simplest topological type of coordination
array,2 one-dimensional (1D) coordination polymers and their
ease of formation by self-assembly allow researchers to
incorporate functions at the metal centers or in the backbone
of the organic linkers very easily, in order to develop functional
polymeric materials with interesting magnetic, electrical,
mechanical, and optical properties.3 Until now, chemists have
achieved many kinds of 1D coordination polymers such as
linear, zigzag, ribbon, helical, ladder, stair, and rotaxane
structural architectures.4
However, since the chelating effects of macrocyclic back-
bones originated from their multiple N and O donors, self-
assembly based on the metallomacrocyclic building blocks is
very challenging.5 A practical strategy is using rigid and planar
metallomacrocycles with weak-coordinated apical ligands as
building blocks, introducing different strong-coordinated link-
ers to replace weak-coordinated apical ligands, and assembling
further into coordination polymers without altering the basal
coordination configuration of metal centers. In our previous
studies, Schiff base dinuclear macrocyclic zinc(II) complexes
with weak-coordinated solvent molecules and counterions at
the axial positions have been used as building blocks to
assemble a series of 1D coordination polymers without
changing their rigid basal Zn2N4O2 coordination plane.5e,f
Recently, we have tried to extend 2,6-diformyl-4-chlorophe-
nol to chiral pendant-armed dialdehydes and used them to react
with diamines to prepare chiral pendant-armed Schiff base
macrocyclic complexes in the presence of certain metal ion
template. Herein we design and prepare a pair of new extended
dialdehydes (R-H2pabb and S-H2pabb) with chiral pendant
arms (Schemes SI1 and SI2, Supporting Information), and they
are reacted with 1,3-propanediamine by the Pb(II) ion
analysis of R-1 reveals that an unusual N1O2 coordination mode
for the pendant-armed dialdehyde component has been found
for the central Pb(II) ion, where two strong intramolecular O−
H···N hydrogen bonds are observed between the phenolic
protons and the Schiff base nitrogen atoms to block the
coordination of Schiff base units.
Considering the unsaturated coordination of macrocyclic
donors and the presence of two weak-coordinated nitrate
anions, we think it is possible to destroy the O−H···N
hydrogen bonds by removing the phenolic protons and nitrate
anions accordingly based on the electroneutrality principle. As a
result, several coordination sites of the Pb(II) ion can be spared
and the Schiff base units can be coordinated at the same time,
so further self-assembly could be fulfilled just by varying the
coordination geometry without adding additional bridging
linkers. By following this strategy, a pair of chiral 1D
coordination polymers R-2 and S-2 has been successfully
produced by base-induced linker-free self-assembly, where
chiral Pb(II) macrocycles R-1 and S-1 serve as building blocks,
together with the significant alteration of flexible pendant arms
stemmed from the cleavage of Pb−N coordinative bond.
The two chiral N-modificatory dialdehydes (R-H2pabb and S-
H2pabb) were prepared by the reaction between R- or S-1-
Received: April 15, 2014
Published: July 24, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Communication
phenylethylamine and 5-chloro-3-(chloromethyl)-2-hydroxy-
benzaldehyde involving continuous two-step nucleophilic
substitutions.6 The construction of Schiff base macrocycle
was carried out by the 1:1 condensation between R- or S-
H2pabb and certain diamine in the participancy of certain metal
ion template.7 It is found that the combination of Pb(II) ion
and 1,3-propanediamine is somewhat a “perfect match” in the
effective formation of [1 + 1] mononuclear Schiff base
metallomacrocycles for this pair of chiral pendant-armed
dialdehydes. This means that the size, charge, and coordination
geometry of Pb(II) ion fits well with the flexibility of dialdehyde
and diamine precursors to organize the final Schiff base
macrocyclic ligands H2LR and H2Ls. In fact, some other metal
ions (Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, and Pd) and diamines
[1,2-cyclohexanediamine, 1,2-bis(2-aminoethoxy)ethane, and
1,2-benzenediamine]8 with different sizes and flexibility have
been used under the same experimental conditions, but the
products were very complicated and difficult to isolate and
purify. As we have mentioned before, two phenolic protons and
nitrate anions in R-1 and S-1 can be removed by adding a base
like NaOH to adjust the pH value in the range 8−9,9 and the
final products R-2 and S-2 have been obtained, accompanied by
a color change of the reaction solution from red to yellow
green. The single crystals of macrocyclic complexes R-1 and R-
2 suitable for X-ray diffraction analysis were grown from the
acetonitrile−methanol mixture by slow evaporation in air at
room temperature for 2 weeks. However, attempts to obtain
high-quality single crystals of S-1 and S-2 for X-ray diffraction
determination have not been successful.
A strong FT-IR absorption peak is found to be 1666 and
1668 cm−1 in R-H2pabb and S-H2pabb, corresponding to the
presence of aldehyde groups in their molecular structures. In
contrast, this absorption peak disappears and a new absorption
peak is observed at 1640 and 1638 cm−1 in chiral macrocyclic
complexes R-1 and S-1, respectively, indicating the trans-
formation from the aldehyde groups to the Schiff base CN
units. Moreover, the characteristic Schiff base CN absorption
peak is significantly shifted to the same 1631 cm−1 in 1D
coordination polymers R-2 and S-2, suggesting its variation
from uncoordinated to coordinated mode, which can be further
verified by the following X-ray single-crystal structures. In
addition, the CD spectra of two chiral dialdehydes and four
Pb(II) complexes, which can reflect and verify the different
chirality of compounds, have been recorded for comparison.
The results reveal that the R isomers give Cotton effects that
are mirror images of their respective S isomers (Figure SI2,
Supporting Information), agreeing well with the enantiomeric
nature of these chiral compounds.
Figure 1. (a) ORTEP drawing of R-1 with the atom-numbering
scheme. Displacement ellipsoids are drawn at the 30% probability
level, and only the phenolic protons are shown. (b) View of molecular
packing of R-1 down the b-axis. Hydrogen atoms are omitted for
clarity.
are much shorter than the other three (2.624(5), 2.666(9), and
2.817(11) Å). The dihedral angles between pendant-armed
phenol ring and two phenolic rings are 82.2(7) and 76.8(8)°,
respectively. It is worth pointing out that the two Schiff base
nitrogen atoms are uncoordinated, which is different from most
of the reported Schiff base macrocylic complexes with the
common N2O2 coordination mode. Instead, two strong
intramolecular O−H···N hydrogen bonds are observed
between the Schiff base nitrogen atoms and the phenolic
protons (Table SI2, Supporting Information). It is suggested
that the formation of hydrogen bonds will stabilize the dynamic
imine bonds and block the coordination of Schiff base units
simultaneously. Moreover, intramolecuar π−π interactions are
observed between two parallel phenolic rings with a centroid-
to-centroid separation of 3.503(9) Å (Figure 1b).
On the other hand, R-2 is a 1D coordination polymer which
is self-assembled from R-1 via the addition of NaOH.
Accompanied by the deprotonation process, two nitrates have
been eliminated according to the electroneutrality principle,
and two imine nitrogen atoms take part in the coordination
with the central Pb(II) ion. As a result, the conventional and
stable N2O2 coordination mode is formed in R-2, where the
above-mentioned three longer coordinative bonds in R-1 have
been cleaved. A dinuclear repeating unit is formed in 1D
coordination polymer R-2, where two central Pb(II) ions
display distinguishable coordination environment (Figure 2a).
Pb1 is twisted six-coordinate octahedronal, while Pb2 is
distorted five-coordinate trigonal bipyramidal (τ = 0.975).
The dihedral angles between the pendant-armed phenol ring
and two phenolic rings are 36.7(8) and 47.9(9)°, respectively.
Three of four phenolic oxygen atoms (O1, O3, and O4) serve
From the X-ray structure analyses of R-1 and R-2 derived
from dialdehyde R-H2pabb,10 it is known that they have the
same 18-membered [1 + 1] Schiff base macrocyclic ligand
except that two phenolic protons are present in R-1.
Nevertheless, there are extraordinary differences between
them. On the one hand, R-1 is a mononuclear macrocyclic
Pb(II) complex, which is countered by two weak-coordinated
nitrate anions, displaying an unusual coordination fashion
(Figure 1a). The central Pb(II) is five-coordinated by two
phenolic oxygen atoms (O1 and O2), a tertiary nitrogen atom
(N3), and two oxygen atoms (O4 and O6) from two nitrate
anions. The coordination configuration of the Pb(II) center in
R-1 can be described as a distorted pyramid with a τ value of
0.077.11 Two bond lengths between the phenolic oxygen atoms
and the central Pb(II) ion are 2.346(8) and 2.294(7) Å, which
Figure 2. (a) ORTEP drawing of R-2 with the atom-numbering
scheme. Displacement ellipsoids are drawn at the 30% probability
level. Hydrogen atoms are omitted for clarity. (b) View of packing
structure of 1D ribbon coordination polymer R-2 down the b-axis.
Hydrogen atoms are omitted for clarity.
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graphic b axis (Figure 2b). Additionally, intramolecuar π−π
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Schiff base macrocyclic Pb(II) complexes (R-1 and S-1). It is
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(10) Crystal data for R-1: C27H27Cl2N5O8Pb, M = 827.64,
monoclinic, space group P21, a = 10.568(1) Å, b = 12.453(1) Å, c =
10.693(1) Å, α = γ = 90°, β = 92.773(1)°, V = 1405.5(2) Å3, T =
291(2) K, Z = 2, 9414 reflections measured, 5377 independent
reflections (Rint = 0.030), Flack parameter, 0.022(9), the final R1 =
0.0357 [I > 2σ(I)] and 0.0489 (all data), and the final wR2 = 0.0774 [I
> 2σ(I)] and 0.0823 (all data). CCDC 996990. Crystal data for R-2:
C54H50Cl4N6O4Pb2, M = 1403.20, monoclinic, space group C2, a =
26.095(3) Å, b = 7.460(1) Å, c = 28.384(3) Å, α = γ = 90°, β =
111.599(2)°, V = 5137.7(9) Å3, T = 291(2) K, Z = 4, 17252 reflections
measured, 7016 independent reflections (Rint = 0.077), Flack
parameter, 0.096(14), the final R1 = 0.0627 [I > 2σ(I)] and 0.0691
(all data), and the final wR2 = 0.1692 [I > 2σ(I)] and 0.1749 (all data).
CCDC 996991.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details, characterization data, crystallographic data
(CIF). CCDC 996990−996991. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: whuang@nju.edu.cn.
■
(11) Addison, A. W.; Rao, T. N.; Reedijk, J.; Van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Major State Basic
Research Development Programs (Nos. 2013CB922101 and
2011CB933300), the National Natural Science Foundation of
China (No. 21171088), and the Natural Science Foundation of
Jiangsu Province (Grant No. BK20130054).
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