Solvent-soluble coordination polymer that reconstructs cyclic frameworks that trap a kinetically labile [Cu(CO3)2]2- unit

Article abstract of DOI:10.1021/ic400315g

A new 2D coordination polymer consisting of Cu^II ions, bis-imidazole-type ligands, and CO₃^2- was prepared. This compound converted to new pentanuclear complexes with cyclic frameworks, which trapped a kinetically labile [Cu(CO₃)₂]^2- unit, by recrystallization from MeOH/Et₂O or EtOH/Et₂O.

Full text of DOI:10.1021/ic400315g

Communication
pubs.acs.org/IC
Solvent-Soluble Coordination Polymer That Reconstructs Cyclic
Frameworks That Trap a Kinetically Labile [Cu(CO3)2]2− Unit
Tatsunari Inoue,† Katsunori Yamanishi,‡ and Mitsuru Kondo*,§
†Department of Chemistry, Faculty of Science, ‡Graduate School of Science and Technology, and §Center for Instrumental Analysis,
Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
S
* Supporting Information
Complex 1 was obtained by the reaction of bitb with basic
ABSTRACT: A new 2D coordination polymer consisting
copper(II) carbonate, CuCO3·Cu(OH)2. This complex was also
obtained using copper(II) hydroxide, Cu(OH)2, instead of basic
copper(II) carbonate. The addition of an NH3 aqueous solution
of the copper salt to an EtOH solution of bitb produced a purple
solid. Recrystallization of the crude product from MeOH/H2O
gave 1 as purple crystals, which were suitable for single-crystal X-
ray analysis. The observed powder X-ray diffraction (PXRD)
pattern of the crude product was consistent with the simulated
PXRD pattern based on the solved structure (Figure S11 in the
Supporting Information, SI), indicating that 1 is soluble in
MeOH and can be recrystallized from MeOH/H2O.
2−
of CuII ions, bis-imidazole-type ligands, and CO3 was
prepared. This compound converted to new pentanuclear
complexes with cyclic frameworks, which trapped a
kinetically labile [Cu(CO3)2]2− unit, by recrystallization
from MeOH/Et2O or EtOH/Et2O.
ncapsulation of discrete molecules in the coordination cages
E
is a unique method for the isolation of labile or unstable
molecules that are difficult to isolate under ambient con-
ditions.1−4 As a typical example, Fujita and co-workers have
successfully isolated cis forms of azobenzene and stilbene1 and a
mixed-valence dimer, [(TTF)2]+· (TTF = tetrathiafulvalene).2
These guest molecules are difficult to isolate without
deformation at ambient conditions. Raymond and co-workers
have shown that the iminium ion, which is easily hydrolyzed by
H2O, can be isolated in aqueous media by encapsulation in
coordination cages.3
Structural characterization demonstrated that 1 has a network
structure, as shown in Figure 2b. Each CuII center is based on a
Neutral multi-imidazole-type ligands5 have been used for the
construction of various molecular capsules.6 We have focused on
the self-assembled construction of coordination cages by CuII
ions and 1,4-bis(imidazol-1-ylmethyl)-2,3,5,6-tetramethylben-
zene (bitb) ligands (Figure 1); this ligand forms an M2L4-type
Figure 2. Coordination environment around the CuII center (a) and the
2D structure of 1 (b). Disorders of carbonate ions, solvent molecules,
and hydrogen atoms are not shown for clarity.
Figure 1. Syn and anti forms of bitb.
square-planar coordination geometry with two imidazole
nitrogen atoms and two carbonate oxygen atoms (Figure 2a).
The carbonate ion was disordered at the two positions by the
imposed crystallographic C2 axis, which runs through the CuII
centers along the b axis. The Cu−O bonds [1.942(8) Å and
1.889(8) Å] are slightly shorter than the Cu−N bonds [2.007(6)
Å] in the coordination environment.
−
molecular capsule for ClO4− and BF4 , responding to the guest
anions.7 In continuing studies of the self-assembled system of
CuII ions with bitb, we have synthesized a new coordination
polymer constructed by Cu2+ ions, bitb, and CO3 and have
2−
found that this coordination polymer reconstructs pentanuclear
complexes, which have new cyclic frameworks including a
kinetically labile [Cu(CO3)2]2− unit in the cage. This paper
describes the syntheses and structures of a new CuIIbitb
coordination polymer, {[Cu(CO3)(bitb)]·2H2O}n (1), and the
two reconstructed CuIIbitb complexes [Cu(CO3)2⊂Cu4(μ-
OMe)4(bitb)4]CO3·7.5MeOH·3H2O (2a) and [Cu-
(CO3)2⊂Cu4(μ-OEt)4(bitb)4]CO3·3Et2O·3EtOH (2b).
Each CuII center is bridged by bitb to yield 1D chains along the
a−c vector. For the two different conformations, syn and anti,
which the ligand can adopt because of the two methylene groups
Received: February 6, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic400315g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(Figure 1), the anti form is observed for bitb in the network
framework of 1. The carbonate ion bridges two CuII centers in
the adjacent chains along the b axis, yielding a 2D structure with a
grid motif.
During the single-crystal growth experiment for 1, we found
that diffusion of the MeOH solution of 1 into Et2O affords a new
pentanuclear complex, 2a. When EtOH was used instead of
MeOH, its ethoxy derivative, 2b, was obtained. Their structures
were clarified by single-crystal X-ray analysis studies.
Figure 3 shows the molecular structure of 2a. The two
imidazole nitrogen atoms and two MeO− oxygen atoms bind to
isolation of the monomeric complex has not been established.
Although Rb2[Cu(CO3)2]·H2O was structurally characterized by
single-crystal X-ray analysis, this compound was shown to have a
chain structure with carbonate bridges between the CuII centers.9
To study the effect of the cyclic cages of 2a and 2b on the
isolation of a [Cu(CO3)2]2− unit, we also attempted a crystal
structure determination of another CuIICO32− complex, reported
as Na2[Cu(CO3)2]·3H2O.10 Although the preparation and cell
parameters of this compound were mentioned, the crystal
structure was not characterized.8
This complex crystallizes as {Na2[Cu(CO3)2(H2O)]·2H2O}n,
which has a chain structure, as shown in Figure 4b. The CuII
Figure 3. Side view (a) and top view (b) of the crystal structure of 2a.
Counteranions, solvent molecules, and hydrogen atoms are not shown
for clarity. Color code: blue, copper; red, oxygen; cyan, nitrogen; black,
carbon.
Figure 4. Coordination environment around the CuII center (a) and the
chain structure (b) of {Na2[Cu(CO3)2(H2O)]·2H2O}n. Hydrogen
atoms are not shown for clarity.
the CuII center at the cis positions. Two bitb with syn form
connect two CuII centers, forming a [Cu2(bitb)2]4+ unit. The
four MeO− groups connect the two [Cu2(bitb)2]4+ units to form
a cyclic framework [Cu4(μ-OMe)4(bitb)4]4+. A crystallographic
inversion center exists in the center of the cyclic framework. The
cyclic framework creates a cage with a size of approximately 11.0
× 5.0 × 4.0 Å3.
center is based on a distorted square pyramid with four carbonate
oxygen atoms and a water molecule. For the two carbonate ions,
one bridges two CuII ions to yield a chain structure. It is apparent
that isolation of the monomeric [Cu(CO3)2]2− unit is difficult
2−
because of the infinite structure formation with CuII−CO3
−
CuII bridges. This result means that the cyclic cages of M4L4(μ-
OR)4 just trap the kinetically labile [Cu(CO3)2]2− unit as a
discrete anion.
The cyclic framework traps a [Cu(CO3)2]2− unit in the cage by
the four weak Cu−O interactions [2.311(4) and 2.256(4) Å] to
the four carbonate oxygen atoms. A distorted square-pyramidal
geometry around the CuII center in the cyclic framework is
formed by association of the oxygen atom of the [Cu(CO3)2]2−
unit at the apical site. The Cu−O bond distances for carbonate
oxygen are longer than those of the other Cu−N (av. 1.977 Å)
and Cu−O (av. 1.942 Å) bond distances because of the Jahn−
Teller effect.
This CuIIbitb-CO32− system yielded different products, 1, 2a,
and 2b, depending on the solvent system used for recrystalliza-
tion. To obtain insight into the effects of the recrystallization
solvents, the structures present in the solutions were
characterized.
The solid-state reflection spectra of 1, 2a, and 2b show d−d
bands at λmax = 567, 631, and 639 nm, respectively (Figures S20−
S22 in the SI). On the other hand, the solution spectra of these
three complexes show similar d−d bands that depend on the
solvents used for the measurements; they are at about 630 and
634 nm in MeOH and EtOH, respectively (Figures S24 and S25
in the SI).
Complex 2a showed a similar spectrum pattern in its reflection
spectrum in the solid state (λmax = 631 nm) and absorption
spectrum in MeOH (λmax = 630 nm). The electrospray ionization
time-of-flight (ESI-TOF) mass spectrometry (MS) spectrum of
2a showed a parent ion peak at m/z 869.7 ascribed to
[Cu(CO3)2⊂Cu4(μ-OMe)4(bitb)4]2+ (Figure S28 in the SI).
These results indicate that the cyclic framework of 2a is retained
in MeOH.
Complex 1 showed different d−d absorptions in the solid state
(λmax = 567 nm) and solution state (λmax = 630 and 634 nm in
MeOH and EtOH, respectively). Their absorption patterns are
rather similar to those of 2a in MeOH and 2b in EtOH.
Interestingly, the ESI-TOF MS spectrum of 1 showed an intense
peak at m/z 869.7 assigned to [Cu(CO3)2⊂Cu4(μ-OMe)4-
(bitb)4]2+ (Figure S26 in the SI). That is, 1 reconstructs the cyclic
framework that traps a kinetically labile [Cu(CO3)2]2− unit in
MeOH.
For the [Cu(CO3)2]2− unit in the cage, each CO32− binds to
the CuII center by four-membered chelation. The two C−O
bond distances [1.333(7) and 1.322(5) Å] in the chelation ring
are significantly longer than the other C−O bond distance
[1.200(7) Å], showing localization of the two negative charges at
the two coordinating oxygen atoms. This is in contrast to the case
of the CO32− ion, which has three similar C−O bond distances
(Figure S7 in the SI) because of the delocalized negative charges,
located outside the cyclic framework.
The structure of 2b is shown in Figure S5 in the SI. Although
the structural manner is the same as that of 2a, the tilt of the
[Cu(CO3)2]2− unit in the cage is significantly different. When the
tilt angle is estimated from the plane−plane angle between the
[Cu(CO3)2]2− unit and the Cu4 plane that is defined by four CuII
atoms in the cyclic framework, they are about 10° and 0° for 2a
and 2b, respectively. This difference is connected with the longer
Cu−Cu distance (6.711 Å) bridged by bitb in 2b than in 2a
(6.666 Å).
Although the [Cu(CO3)2(H2O)2]2− species has been
proposed as a plausible structure, which forms in an aqueous
solution of Cu2+ ion and CO32− at pH 8,8 direct observation or
B
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Inorganic Chemistry
Communication
Science 2009, 324, 1697−1699. (f) Pinacho Crisos
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state (λmax = 639 nm) and absorption spectrum in EtOH (λmax
=
634 nm) are similar, the ESI-TOF MS spectrum did not show a
main peak for [Cu(CO3)2⊂Cu4(μ-OEt)4(bitb)4]2+ but for
[Cu(CO3)2⊂Cu4(μ-OMe)4(bitb)4]2+ (m/z 869.7; Figure S30
in the SI) when MeOH was used as a mobile phase. Similarly,
exchanges of MeO− in the cyclic framework of 2a with EtO−
were observed when EtOH was used as a mobile phase (Figure
S34 in the SI). Apparently, EtO− groups in 2b were exchanged
with MeO−, which come from MeOH that was used as a mobile
phase for the ESI-TOF MS spectrum measurement.
When the same amount of water was added to a MeOH
solution of 1, the intense peak at m/z 869.7 for the ESI-TOF MS
spectrum was drastically decreased (Figures S26 and S27 in the
SI). This result shows that the cyclic framework is decomposed
by H2O, and thus isolation of 1 by recrystallization from MeOH/
H2O is due to this process.
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In summary, a new 2D coordination polymer, 1, consisting of
CuII ions, bitb, and CO3 was prepared and structurally
2−
characterized. This coordination polymer was soluble in
MeOH and EtOH. Diffusion of the MeOH or EtOH solution
of 1 into Et2O gave new pentanuclear complexes (2a and 2b)
with cyclic frameworks, which trapped a kinetically labile
[Cu(CO3)2]2− unit. Systematic studies using ESI-TOF MS
spectra indicate that 1 reconstructs the cyclic framework for 2a,
[Cu(CO3)2⊂Cu4(μ-OMe)4(bitb)4]2+, in MeOH. Although this
framework is decomposed by water, recrystallization from
MeOH/H2O yielded 1, while recrystallization from MeOH/
Et2O afforded 2a. The further construction of Cu-bitb self-
assembled complexes is in progress.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, crystallographic data in CIF format,
crystal structures, UV−vis and MS spectra, IR spectra, and PXRD
patterns of the compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: scmkond@ipc.shizuoka.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Challenging
Exploratory Research (Grant 24655047) from the Japan Society
for the Promotion of Science, Japan. We thank Y. Yamamoto of
the Center for Instrumental Analysis for assistance in elemental
analysis measurements.
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