Reversible conversion of valence-tautomeric copper metal-organic frameworks dependent single-crystal-to-single-crystal oxidation/reduction: A redox-switchable catalyst for C-H bonds activation reaction

Article abstract of DOI:10.1039/c5cc02432c

Upon single-crystal-to-single-crystal (SCSC) oxidation/reduction, reversible structural transformations take place between the anionic porous zeolite-like Cu^I framework and a topologically equivalent neutral Cu^ICu^II mixed-valent framework. The unique conversion behavior of the Cu^I framework endowed it as a redox-switchable catalyst for the direct arylation of heterocycle C-H bonds.

Full text of DOI:10.1039/c5cc02432c

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DOI: 10.1039/C5CC02432C
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Reversible Conversion of Valence-Tautomeric Copper Metal-
Organic Frameworks Dependent Single-Crystal-to-Single-Crystal
Oxidation/Reduction: Redox-Switchable Catalyst for C–H Bonds
Activation Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chao Huang, Jie Wu*, Chuanjun Song, Ran Ding, Yan Qiao, Hongwei Hou*, Junbiao Chang* and
Yaoting Fan
Upon single-crystal-to-single-crystal (SCSC) oxidation/reduction, reduction/reoxidation behaviour of RuIII/RuII chiral MOFs was
reversible structural transformations took place between the presented by Lin.7a
anionic porous zeolite-like CuI framework and a topologically
Considering the potential applications in switchable catalysts and
equivalent neutral CuICuII mixed-valent framework. The unique sensors of porous CuI MOFs with tunable SCSC structural
conversion behavior of the CuI framework endowed it as redox- transformation, chemists are keen to design and synthesize redox-
switchable catalyst for direct arylation of heterocycle C–H bonds.
convertible CuI MOF materials capable of reversible SCSC structural
conversions relying on the host-guest chemistry. Although a few CuI
MOFs oxidized to CuII MOFs through a SCSC process have been
presented,7e, 9 the intricate mechanism involved in these central
metal valence tautomerism in a SCSC fashion remains largely
unclear and attempts to oxidize CuI MOFs with oxidants (O2, H2O2, I2,
etc.) always lead to the MOFs dissolving or crystallinity loss. Even
harder, the reduction of CuII in the framework through a SCSC
fashion is still very challenging and has never been observed, not to
mention the reversible SCSC oxidation/reduction structural
transformations between them.
The redox-active metal-organic frameworks (ra-MOFs),1 derived
from direct incorporation of redox active competent building blocks
into the MOFs have been attracting much attention as the more
possible candidates for a new class of multifunctional hybrid
materials, such as, electrode materials,2 electrical conductors,3
selective gas adsorption,4 catalysts,5 ferroelectric and
ferromagneticmaterials,6 etc. In ra-MOFs, an exciting yet little
explored area is that a few ra-MOFs could proceed single-crystal-to-
single-crystal (SCSC) structural transformations, including central
metal valence tautomerism and reassembly of metal-ligand
connectivity and network topology, caused by redox reagents7 or
mechanochemistry.8 During the transformations, their capacity of
altering the oxidation valence states of metal centers, skeleton
structure, and framework charge rewards them peculiar physical
and chemical applications, such as switchable catalyst,7a ion
exchange, and reducing agents for the synthesis of monodispersed
metal nanoparticles.7f-g Besides, so far, a handful of positive-
charged CuI MOFs incorporating free counteranions in the channels
or pores have shown fascinating host-guest dependent SCSC
structural transformation to CuII MOFs driven by oxidation
reactions.9 These examples indicated that it should be practicable
to tune the SCSC redox processes through host-guest interactions.
Notably, the reversible SCSC transformation of ra-MOFs are more
eagerly anticipated for the advancement of switching and sensing
Herein, we report the assembly of
{(H3O)[CuI2(CN)(TTB)0.5]·1.5H2O}n (1) (H4TTB
a
CuI complex
=
1,2,4,5-tetra(2H-
tetrazole-5-yl)-benzene) with an anionic, porous zeolite-like three-
dimensional (3D) framework containing free [H3O]+ ions as guest
molecules. Upon SCSC oxidation/reduction, interesting reversible
structural transformations took place between the anionic CuI
framework of 1 and a topologically equivalent neutral CuICuII mixed-
valent framework of {[CuICuII(CN)(TTB)0.5]·1.5H2O}n (2).
A
mechanism for the reversible transformations was proposed, which
indicated that 1 and 2 showed the first example of host-guest
dependent reversible SCSC oxidation/reduction behavior of
CuI/CuICuII MOFs. In addition, we developed 1 into an effective
heterogeneous catalyst for the direct C–H bond arylation of
heteroarenes, while its oxidized transformer 2 as catalyst under
identical conditions led mainly to the formation of Ullmann
coupling products.
materials, yet only
a single example of reversible SCSC
Imitating the preparation of some charged small pore zeolites,
especially those presenting free counteranions in the channels of
their structure, the introduction of organic structure directing
agents (OSDAs) into the hybrid inorganic-organic matrix have been
applied.10 Reaction of CuCN with H4TTB in dimethylformamide
(DMF)/EtOH/NH3·H2O at 160 °C for
3
days with N,N-
diisopropylethylamine (DIPEA) as OSDA afforded primrose yellow
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Experimentation aimed at evaluating the redox pVrioewpeArrttiicelesOonflin1e
DOI: 10.1039/C5CC02432C
showed that a reversible oxidation/reduction transformation could
be achieved. When the single crystal samples of 1 were left open to
air at ambient temperature for more than three days, slow
conversion into an oxidized complex with retention of single
crystallinity occurred. This could be easily validated by observation
that the primrose yellow crystals gradually turned into black (Figure
2a), and further verified by XPS (Figure 2b).9a, 11 XPS and variable
temperature magnetization measurements on the black samples
supported the change from CuI complex 1 to a mixed-valent CuICuII
complex (Figure S3b). When the primrose yellow single crystals of 1
were heated at 60, 80, and 100 °C, they turned to black within 25,
12, and 5 min, respectively. Conversion was sufficiently complete
inside and outside within 12 h at 100 °C. Remarkably, the crystals
remained intact throughout this transformation, enabling the
structure of the black crystal samples to be determined.
Single-crystal X-ray analysis of the black single crystal revealed a
new crystallographic form {[CuICuII(CN)(TTB)0.5]·1.5H2O}n (2) (Figure
2c). The conversion from 1 to 2 implies a slightly increase of 0.08 Å
of a axis, 0.03 Å of c axis, and is accompanied by an increase of
more than 1.1° of β angle (Table S1). Further studies showed that
the metal-ligand connectivity and network topology of 2 are
consistent with that of 1 though some of their corresponding bond
lengths and angles are different (Table S2).
Theoretical calculations presented the charges on Cu1 and Cu2
being 0.631 and 0.052 e, respectively, which indicated that Cu1 is
oxidized (Figure 2d). Due to half of CuI ions in 1 being oxidized to
CuII ions, the framework of 2 becomes neutral. The oxidation
Figure 1. Crystal structure of 1: (a) Photograph showing the
primrose yellow single crystal of 1. (b) XPS spectrum of 1. (c) View
of the 3D network of 1 along c axis. (Oxygen atoms represented as
[H3O]+ ions). Hydrogen atoms and free water molecule are omitted
for clarity. (d) The 2D layer along a axis. (e) Each layer was
composed of two cyano groups, six tetrazole rings, and six CuΙ
cations. (f) The dimension of the channel with 10.23 × 7.41 Å at the
ab plane.
framework transformation from
1 to 2 with metal-ligand
connectivity and network topology unchanged was crosschecked by
single crystals. Single-crystal X-ray diffraction analysis revealed that
{(H3O)[CuI2(CN)(TTB)0.5]·1.5H2O}n (1) shows a monoclinic C2/c space
group and an anionic, porous zeolite-like 3D framework with free
[H3O]+ ions encapsulated inside the cavity (Figure 1). The univalent
copper centers in 1 were revealed by primrose yellow product and
supported by X-ray photoelectron spectroscopy (XPS).9a, 11
PLATON analysis gave the free void volume ratio of 54.8% in 1,
which suggests a microporous characteristic of 1. As shown in
Figure 1d, CuΙ cations are linked through cyano groups and tetrazole
rings to form a 2D layer containing open cavities with a maximum
inner width of ~7.0 Å (considering van der Waals radii) along a axis
(Figures 1d and e). Such layers are further connected by TTB4-
groups, leading to the formation of a 3D zeolite structure with
dimensions of the channels being 10.23 × 7.41 Å along c axis (Figure
1f). The asymmetric unit of 1 contains half of deprotonated TTB4-
anion, two CuI ions, and one cyano anion (Figure S1a). Overall, the
framework of 1 is anionic and is filled with [H3O]+ ions and guest
water molecules. Given the charge-balance consideration, there
should be one [H3O]+ ion per formula unit.
Figure 2. Crystal structure of 2: (a) Photograph showing the black
single crystal of 2. (b) XPS spectrum of 2. (c) View of the 3D network
of 2 down c axis. Hydrogen atoms and free water molecule are
omitted for clarity. (d) Optimized structure of 2 with UB3LYP
functional and 6-31+G* basis set for C, H, N and 6-311++G** basis
set for Cu. Values in parentheses are the calculated mulliken
charges for selected atoms.
2 | J. Name., 2012, 00, 1-3
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the identical powder X-ray diffraction (PXRD) peaks of 2 compared activity resulting from the surface metal sites of veryVtieinwyArptiaclretOicnlleinse,
DOI: 10.1039/C5CC02432C
to those of 1 (Figure S3d). Besides, in the variable-temperature because big crystals own too few surface metal sites to account for
power X-ray diffraction (VTPXRD; see Figure S2b) patterns of 1, the the catalytic reaction. Remarkably, 1 could maintain the crystallinity
main diffraction peaks remained unchanged at the temperature after the catalytic reaction. Single-crystal X-ray analysis and XPS of
ranging from 25 to 275 °C, though in the high temperature range the single crystal samples after reaction revealed the identical
(i.e. 100~275 °C) the color of samples turned into black and the CuI framework as that of 1. Inductively coupled plasma atomic emission
framework was oxidized into CuICuII framework identified by XPS.
spectrometry (ICP) analysis of the filtrate showed that less than 0.1%
Notably, the transformation from CuI complex into CuICuII mixed- of the copper was leached into solution after 10 hours under the
valent complex is reversible. Treatment of the single crystal samples typical reaction conditions, strongly confirming its heterogeneity. In
of 2 with reducing agent (ascorbic acid) led to a color change from the recyclability experiments, 1 was readily recovered from the
black to primrose yellow, thus suggesting the reduction of CuICuII catalytic reaction via centrifugation, and the recovered catalyst
centers to CuI centers in the MOFs. The result was further showed only slight deterioration after five runs. Moreover, the
confirmed by XPS of the reduced samples. Interestingly, the PXRD patterns of the 1 crystallites after the fifth catalytic reaction
reduction of 2 also took place in a SCSC fashion. Single-crystal X-ray closely matched those of single crystals of 1 and showed no signs of
structure
{(H3O)[CuI2(CN)(TTB)0.5]·2H2O}n (1a) indicated that its framework is framework could remain intact after at least five runs(Figure S9).
essentially identical to that of 1 but with different amounts of Subsequently, this new catalytic system was found to be general
studies
on
the
reduced
sample framework collapse and decomposition, which indicated that the
lattice H2O. The free [H3O]+ ions in 1a could be provided by the for the coupling of heteroarenes with a variety of substituted aryl or
ascorbic acid. PXRD further indicated that 1a retained the single- heteroaryl halides, and the results were summarized in Table S5.
crystal nature and closely matched that of 1 but with different Compared with the homogeneous CuI systems, the coupling of
degrees of crystallinity.
substrates with catalyst 1 required longer reaction times arising
The primrose yellow reduced crystals of CuI complex 1a could be from the slow diffusion of substrates and products through the
reoxidized in air to afford black crystals of CuICuII mixed-valent MOF channels. However, the catalytic system with recyclable
complex 2a in a SCSC fashion, which was further supported by the catalyst 1 in the presence of mild inorganic base not only met the
XPS and X-ray analysis (Figure S5). PXRD and unit-cell preference of modern green chemistry for more environmentally
determinations indicated that 2a remained single crystals with the friendly conditions, but also improved tolerance of functional
same structure as 2. The transformations between CuI complexes (1 groups and thereby extending the scope of possible substrates. The
and 1a) and CuICuII mixed-valent complexes (2 and 2a) in this work high efficiency of the current 1 system is believed to stem from its
showed totally reversible SCSC reduction/oxidation processes.
unique structural features and redox behavior. The porous zeolite-
Further studies indicated that the anionic framework of 1 like framework of 1 has microporous characteristic with a large free
displays distinct redox behavior depending on the nature of the volume and channels along a and c axes, which can facilitate to
guests. With triethylamine (TEA) and diisopropylamine (DIPA) as transport organic substrates and products. In addition, we
OSDAs, CuI complexes {(TEAH)[CuI2(CN)(TTB)0.5]·H2O}n (3) and rationalized that the potential redox activity of 1 is crucial for C–H
{(DIPAH)[CuI2(CN)(TTB)0.5]·0.5H2O}n (4) with the same anionic activation, especially for the oxidative addition step. The catalytic
framework as that of 1 were obtained, respectively, which became reaction is assumed to perform through the base assisted formation
redox inert (Figures S6 and S7).
of heteroarylcopper species formed by the reacting atoms of
Complex 1 could be an ideal CuI MOF-based structural model for deprotonated heterocycles superimposed on the active CuI centers
application in redox-convertible heterogeneous catalysis with C–H of the catalyst, and subsequent reaction of this species with aryl
activation due to the following considerations: (i) the construction halide. The framework of 1 with isolated catalytically active sites
of porous CuI-MOF 1 may provide a convenient heterogeneous self- could stabilize the highly active heteroarylcopper species, prevent
support catalyst with CuI ions acting as both metal nodes and any intermolecular deactivation pathways and facilitate the halide
catalytic sites for the C–H bond activation reaction; (ii) importantly, displacement step.
the totally reversible SCSC oxidation/reduction behaviors of 1 and 2
We then tested the coupling reaction of iodobenzene or p-
make them perfectly capable as redox-convertible catalysts. Herein, iodoanisole with heteroarenes using crystal samples of 2 instead of
1 and 2 were employed in the direct C–H bond arylation of
K2CO3 (200 mol %)
DMF, 140 oC, 10 h
heteroarenes.
N
Ar
The optimization of the reaction conditions for 1-catalyzed C–H
arylation reaction was performed using benzothiazole and
iodobenzene as coupling partners. As shown in Scheme S1, after
extensive screening of solvents, temperature and bases, the
standard conditions were obtained using two equivalent of K2CO3 as
the base in the presence of 2.5 mol % of crystal samples of 1 in DMF
at 140 °C for 10 h, giving the desired arylation product in 90% yield.
1
(2.5 mol %)
Z
C-H activation product
N
Z
Air
Oxidation
H
Ascorbic Acid
Reduction
ArX
+
Z = O, S, NMe Ar = Ph,
1-naphthyl
2
(2.5 mol %)
Ar Ar
biphenyl
heteroaryl
K2CO3 (200 mol %)
DMF, 140 oC, 10 h
Ullmann coupling product
X = I, Br
To ensure that the reaction catalyzed by
1 took place
Scheme 1. Schematic view of redox-convertible catalysts 1 and 2
predominantly within the crystal voids rather than on the external
surface, the C–H arylation reaction was performed with crystals of 1
(~0.3 × 0.3 × 0.2 mm3). It eliminated the possibility of catalytic
for the direct C–H bond arylation of heteroarenes.
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1 as catalyst under identical conditions (Scheme 1). Interestingly, it 4 (a) E. D. Bloch, L. J. Murray, W. L. Queen, S. Chavan, S. N.
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was found that the reactions gave the Ullmann coupling product as
the major product (> 70%), together with a small amount of the C–
H activation product (< 10%). The CuICuII mixed-valent complex 2
mainly catalyzed the Ullmann coupling reactions because the
Maximoff, J. P. Bigi, R. Krishna, V. K. PetDeOrsI:o1n0,.1F0.3G9/rCa5nCdCje0a2n43,2GC.
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[CuI2CuII (CN)2]4+ unit in 2 could be regarded as an active species
5
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Olaiya, M. Head-Gordon, C. J. Chang, F. N. Gastellano, J. R.
2
and enhance the rate of the Ullmann coupling reaction.12 The
coupling product was thus formed by oxidative addition of 2 to the
aryl iodide followed by reductive elimination. 2 also could be
recycled and reused at least five times without significant loss of
catalytic activity and MOF crystallinity (Figure S10).
Long, Chem. Sci. 2013, 4, 3934-3945; (b) O. R. Luca, R. H.
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Therefore, 1 can use as a redox-convertible catalyst for C–H
activation. For the coupling of heteroarenes with aryl and
heteroaryl halides, catalytically active CuI-based catalyst 1 can be
transformed through SCSC oxidation process to CuICuII-based 2,
which could selectively catalyze the Ullmann coupling reaction.
Conversely, the reduction of 2 turns on the catalytic activity for the
direct C–H bond arylation of heteroarenes, and the resulting CuI-
based catalyst 1 is highly active.
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In conclusion, we report the first observation of an interesting
anionic porous zeolite-like CuI complex 1 (primrose yellow) capable
of reversible structural transformation to a topologically equivalent
neutral CuICuII mixed-valent complex
2 (black) upon SCSC
oxidation/reduction. The reversible SCSC redox behavior of 1 can be
ascribed to the host-guest interactions during the redox processes
and its intrinsic features, such as the flexibility of the framework to
allow the deformation and the strong ability of the multinuclear CuI
units to capture O2 molecules. In addition, for the coupling of
heteroarenes with aryl and heteroaryl halides, C–H bond activation
products and Ullmann coupling products were obtained,
respectively, with 1 and 2 as catalyst. Therefore, the unique
structural features and redox behavior of 1 endow it with versatile
characteristics as heterogeneous catalyst such as high catalytic 8 J. Sun, F. Dai, W. Yuan, W. Bi, X. Zhao, W. Sun and D. Sun,
activity, recyclability for reuse, and redox-convertible catalyst for
the C–H bond activation reaction.
Angew. Chem., Int. Ed., 2011, 50, 7061-7064.
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We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21201152, 81330075, and
21371155) and Research Found for the Doctoral Program of Higher
Education of China (20124101110002).
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