Oxygen and methanol mediated irreversible coordination polymer structural transformation from a 3D Cu(i)-framework to a 1D Cu(ii)-chain

Article abstract of DOI:10.1039/c4cc00857j

An interesting irreversible structural transformation visible to the naked-eye occurs when a 3D Cu(i)-polymeric complex Cu₂L(NO ₃)₂(DMF)_0.4 (1) is suspended in CH ₃OH in air to produce a 1D-Cu(ii) polymeric complex Cu(μ-OCH ₃)(L)(NO₃) (2) (L = 1,2-bis[4-(pyrimidin-4-yl)phenoxy] ethane). The transformation mechanism from 1 to 2 was also investigated. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc00857j

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Oxygen and methanol mediated irreversible
coordination polymer structural transformation
from a 3D Cu(I)-framework to a 1D Cu(II)-chain†
Cite this: Chem. Commun., 2014,
50, 4434
Received 31st January 2014,
Accepted 4th March 2014
Jing-Yuan Ge, Jian-Cheng Wang, Jun-Yan Cheng, Peng Wang, Jian-Ping Ma,
Qi-Kui Liu and Yu-Bin Dong*
DOI: 10.1039/c4cc00857j
www.rsc.org/chemcomm
An interesting irreversible structural transformation visible to the naked- are flexible. For example, the polymeric coordination patterns could
eye occurs when a 3D Cu(I)–polymeric complex Cu2L(NO3)2(DMF)0.4 (1) be tuned by the temperature based on the ligand conformation
is suspended in CH3OH in air to produce a 1D-Cu(II) polymeric complex variation.8 Motivated by our interest in temperature-dependent
Cu(l-OCH3)(L)(NO3) (2) (L = 1,2-bis[4-(pyrimidin-4-yl)phenoxy]ethane). synthesis of coordination polymers, in this contribution, we report
The transformation mechanism from 1 to 2 was also investigated.
two new coordination polymers Cu2(L)(NO3)2(DMF)0.4 (1, orange)
and Cu(m-OCH3)(L)(NO3) (2, green-blue) based on a new open-chain
In coordination polymers,1 structural transformation,2 which is polyether-bridged organic spacer L at different temperatures
caused by some external stimuli such as solvent, ionic templates (Scheme S1, ESI†). The flexible ligand L respectively adopts
and ligand exchange,3 light,4 temperature variation5 and some- trans- and cis-conformations at high (90 1C) and low temperatures
times mechanochemical forces,6 often occurs. In some cases, (r.t.), which is in good agreement with our previous observation.8
structural transformations based on the coordination polymers Notably, 1 undergoes an oxygen and methanol triggered structural
are accompanied with the dimensionality and color changes transformation to 2, visible to the naked-eye and the structural
resulting from the variation of the central metal valence and feature changed from a 3D porous framework (1) to a 1D double
metal coordination mode. Such a stimuli-responsive dynamic chain motif (2), and the valence of the copper ion changed
structural transformation is important, which is very useful in from univalent to bivalent correspondingly. The dioxygen was
the preparation of new switching or sensing materials.
Among the various metal-complexes structural transformations, anion during the structural transformation process.
the transformation of copper species is more interesting and As shown in Scheme 1, compound 1 was isolated as an orange
supposedly transformed to H2O2 via the superoxide radical
valuable. As we know, copper proteins in biological systems can crystalline solid by the combination of L (molar ratio L:Cu(II) = 1:2)
provide various functions including dioxygen transport by hemo- in a mixed-solvent system (DMF/MeCN) at 90 1C in good yield (65%).
cyanin, oxygenation and dehydrogenation of substrates combined
with the transformation of O2 to either H2O2 or water.7 For under-
standing and mimicking such biological oxygenation behaviors,
the synthesis of new Cu(I)-coordinating systems and the investiga-
tion of their spontaneous oxygenation are necessary. Such effort
will generate a sufficiently large database from which the specific
artificial bionic enzymes could be sieved out.
Our previous studies showed that the coordination polymers
generated from open-chain polyether-bridged organic linkers
College of Chemistry, Chemical Engineering and Materials Science,
Collaborative Innovation Center of Functionalized Probes for Chemical Imaging,
Key Laboratory of Molecular and Nano Probes, Ministry of Education,
Shandong Normal University, Jinan 250014, P. R. China.
Scheme 1 Representation of the synthesis of 1–2 and the structural
transformation from 1 to 2. Crystals of 1, on being kept in MeOH in the
air at the ambient temperature for about 36 h, undergo a visual irreversible
structural transformation to yield a green-blue product 2 (the color
change is shown in the figure with powdered samples of 1 and 2).
E-mail: yubindong@sdnu.edu.cn
† Electronic supplementary information (ESI) available: Synthesis and character-
ization data for all compounds, including figures for ORTEP, XPS, XRPD, 1H NMR,
IR, CIF files and crystal data. CCDC 984134 and 984135. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c4cc00857j
4434 | Chem. Commun., 2014, 50, 4434--4437
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Fig. 1 The Cu(I) nodes in 1 are connected to each other by the tetradentate
L into Àa 2D net. These 2D nets are further interlocked together through
m-NO3 (Cu(1)–O(3)–N(1)–O(2)–Cu(1) linkage, basically perpendicular to
the net plane) into a 3D frameworkÀcontaining square-like channels, in
which the disordered DMF and NO3 are located. The disordered DMF
Fig. 3 1D chain of 2 composed of {Cu2(m-OCH3)2} cluster cores and L in
the solid state. The interchain p–p interactions exist in 2.
À
molecules are omitted for clarity in left figure, and the DMF and NO3 in
right figure are shown in space-filling model.
Single-crystal analysis (ESI†) revealed that compound 1 crystallizes
in the monoclinic C2/c space group.
As shown in Fig. 1, each Cu(I) center in 1 lies in a tetrahedral
{CuN2O2} coordination sphere. The two N-donors come from
two terminal pyÀrimidyl groups and two O-donors come from a
coordinated NO3 anion and a coordinated DMF solvent molecule,
respectively. In the solid state, Cu(I) nodes are linked together by the
trans-tetradentate L linkers into a 2D net extended in the crystallo-
graphic ab plane (Fig. 1). These 2D nets are further interlocked
Fig. 4 XRPD pattern (left) and XPS spectrum (right) of 2.
details are comparable with other reported methoxide-bridged
dinuclear Cu(II) compounds.10 These {Cu2(m-OCH3)2} cluster cores
are bridged by the cis-bidentate ligand L into an infinite 1D double
chain along the crystallographic c axis (Fig. 3). The interchain p–p
interactions drive the 1D chains into a 2D net in the crystallographic
bc plane. Again, the XRPD pattern indicates that the as-synthesized
compound 2 is pure (Fig. 4). It is different from 1, XPS (Fig. 4) and
electron paramagnetic resonance (EPR) measurements (Fig. S3,
ESI†) of 2 demonstrate that the copper cation is bivalent.9
The spontaneous structural transformation from 1 to 2 was
accidentally discovered when we explored the solvent-exchange
(DMF by MeOH) based on 1 under ambient conditions. The
transformation started once compound 1 was suspended in MeOH
in air along with a visual color change. As shown in Fig. 5, the color
of 1 changed gradually from orange to green-blue (2) during the
transformation process. Besides the color change visible to the
naked eye, the structural transformation process was monitored
À
together by the m-NO3 to form a 3D framework containing square-
like open channels (dimensions B 10 Â 10 Å) running along the
crystallographic c axis. The shortest interlayer Cu(I)ÁÁÁCu(I) distance
À
is 5.440 Å. The disordered NO3 anions and DMF molecules are
located in the channels (Fig. 1). XRPD patterns indicated that the
obtained sample is pure, and the X-ray photoelectron spectroscopy
(XPS) measurements indicate that the copper center in 1 is univalent
(Fig. 2),9 which is well in agreement with the single-crystal X-ray
diffraction analysis (ESI†).
Compound 2 was obtained as a green-blue crystalline solid
in a DMF–MeOH mixed-solvent system at room temperature. It
is different from 1, the ligand L in 2 adopts cis-conformation,
and as a bidentate linker to bind copper nodes. Single-crystal
X-ray diffraction studies (ESI†) revealed that 2 crystallizes in the
monoclinic space group P2(1)/n. Each Cu(II) node locates in a square
pyramidal coordination sphere {CuN2O3}, which is composed of two
pyrimidyl N-donors (dCu–N bond lengths are 2.024(3) and 2.333(3) Å,
respectively), three O atoms from one monodentate nitrate (dCu–O
=
2.160(11) Å) and two bridging methoxide groups (Cu–O bond lengths
range from 1.934(2) to 2.030(2) Å) (Scheme 1). The methoxide groups
are co-bridging two Cu(II) nodes to form a {Cu2(m-OCH3)2} cluster
core. The dCu–Cu in {Cu2(m-OCH3)2} unit is 3.0359(10) Å, while the
oCu–O–Cu angles vary from 76.73(8) to 103.26(2)1. These geometric
Fig. 5 Left: XRPD patterns of 1, 1-1a (in CH3OH 6 h), 1-1b (in CH3OH
12 h), 1-1c (in CH3OH 24 h), 1-1d (in CH3OH 36 h) and 2. Photographs
showing the color change of the samples of 1 in CH3OH in air at different
time. Right: XPS spectrum of 1-1d.
Fig. 2 XRPD pattern (left) and XPS spectrum (right) of 1.
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by X-ray powder diffraction. As shown in Fig. 5, XRPD patterns
show that the peaks corresponding to 2 gradually appeared while
the characteristic peaks of 1 disappeared by degrees as time went
on. In 36 h, the XRPD pattern of 1-1d is fully coincident to that of 2,
indicating the structural transformation was finished (from C2/c
to P2(1)/n). In addition, the IR spectrum performed on 1-1d
shows that the band associated with 4CQO at 1655 cmÀ1 has
disappeared, indicating that the coordinated DMF molecules are
out of the copper coordination sphere (Fig. S4, ESI†).
Besides XRPD patterns, the observation of Cu 2p3/2 peak at
934.91 eV in XPS (Fig. 5) of 1-1d also demonstrated the oxidization
from Cu(I) in 1 to Cu(II) in 2. The binding energy peak shift of
1.99 eV is consistent with the reported difference of binding energy
peak between Cu(I) and Cu(II).9a Accordingly, the EPR spectrum of
1-1d is the same as that of 2 (Fig. S3, ESI†). Furthermore, ICP
(Inductively Coupled Plasma) measurements (Fig. S5, ESI†) indicate
that the amount of copper in 1-1d increases from 18.1% (calcd
18.2% in 1) to 12.1%, which is identical to that of 2 (calcd 12.1%).
The elemental analysis also indicated that composition of 1-1d is
Fig. 6 Left: ESR spectrum of 1 in MeOH with DMPO. The siÀx observed
ꢀ
characteristic peaks confirm the formation of the DMPO–O2 adducts,
and the signal intensity change of ESR spectra after addition of SOD to it.
Right: the suggested transformation mechanism.
the same as that of 2 (Table S1, ESI†). In addition, the process of formation of a Cu(II)-superoxo species generated from the super-
structural transformation can be accelerated with the increase of oxide radical anions.12 So the ESR spin-trap technique (with
the reaction temperature (Table S2, ESI†). For example, the trans- 5,5-dimethyl-1-pyrroline N-oxide, DMPO) was used to detect the
À
ꢀ
formation from 1 to 2 at 60 1C (26 h) is much faster than that at O2
radical species. When DMPO was added to the MeOH
À
ꢀ
room temperature (36 h), indicating that this structural transforma- solution of 1, the ESR spectra clearly indicate that the O2 was
tion is a dynamic controlled process. Unfortunately, the crystals of 1 trapped by DMPÀO based on the six observed characteristic peaks
did not diffract the X-ray beams due to the loss of the single of the DMPO–O2 ꢀ adducts (Fig. 6).13 Reasonably, no color change
À
ꢀ
crystallinity during the transformation process.
was observed once the O2 radicals were trapped by DMPO,
We assume herein that the oxygen from air acts as the oxidant in indicating that structural transformation from 1 to 2 did not
the process of structural transformation. To confirm such an occur. When superoxide dismutase SOD (2000 U mLÀ1) was added
À
ꢀ
assumption, several parallel experiments were carried out. When to the above solution, the signal intensities of the O2 radical
compound 1 was suspended in CH3OH under an N2 atmosphere at species were significantly weakened as time went on, which
À
ꢀ
room temperature, the corresponding XRPD pattern indicated that further confirmed the formation of O2 radical species during
no structural transformation occurred and the framework of 1 the process. According to the above observation, a possible
remained intact (Fig. S6, ESI†). Secondly, the structural transforma- mechanism for this structural transformation was suggested. As
tion in MeOH is much faster under a pure oxygen atmosphere (22 h) shown in Fig. 6, the Cu(I) ion in 1 was oxidized to the Cu(II) ion in
À
than that in air (36 h) (Fig. S6, ESI†). Besides oxygen, the MeOH 2 by O2 to generate a Cu(II)–(O2 ꢀ) superoxo species which further
medium is also essential for this transformation. As mentioned linked one more Cu(I) ion to produce a Cu2O2 peroxo species.14
above, the coordinated DMF molecules in 1 are replaced by MeOH The peroxo intermediate was anticipated to react with two MeOH
in 2. When 1 was immersed in other kinds of ROH solvents such as to generate a binuclear di-m-methoxocopper(II) complex and
EtOH or i-PrOH, no structural transformation was observed based release the H2O2.11e,15
on the XRPD patterns (Fig. S7, ESI†); the coordinated DMF mole-
In summary, we have synthesized two coordination polymers
cules in 1, however, are partly exchanged by the corresponding ROH generated from a new ethylene glycol ether-bridged dipyrimidyl
solvent molecules. For different ROH solvents, the water template ligand. The different reaction conditions resulted in different metal
drives compound 1 to an unknown structural motif based on the valences and ligand conformations, and consequently, different
X-ray powder diffraction (Fig. S8, ESI†). So, CH3OH might be a coordination patterns. In addition, an interesting irreversible struc-
unique co-ligand to facilitate the formation of {Cu2O2} cluster node tural transformation from a 3D Cu(I) complex (orange) to a 1D Cu(II)
during this transformation process, whereas EtOH, i-PrOH and H2O chain (green-blue) detectable by the naked eye was observed. The O2
cannot trigger the transformation of 1 into 2. It is noteworthy that no and CH3OH medium are the iÀndispensable factors to induce this
1
ꢀ
ligand was detected in the mother liquor based on the H NMR transformation. The formed O2 radical anion, an essential active
spectrum (Fig. S9, ESI†), indicating that it might be a solid-state species for the structural transformation, was trapped by DMPO and
structural transformation.
SOD, which provided solid evidence for the Cu(I) spontaneous
The isolation of a similar bivalent Cu2(OMe)2 species from the oxygenation in air, and furthermore, a useful O2 transformation
reaction of Cu(I) precursor bearing cyclic and noncyclic ligands mode based on Cu(I) coordination species.
with dioxygen has been reported by some other groups,11 but the
We are grateful for financial support from the 973 Program
formation process is still not understood. The earlier studies (Grant No. 2012CB821705 and 2013CB933800), NSFC (Grant
suggest that oxygenation of Cu(I) complexes proceed via the No. 21271120 and 21101099) and ‘‘PCSIRT’’.
4436 | Chem. Commun., 2014, 50, 4434--4437
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