Synthesis, structure and heterogeneous catalytic activity of a coordination polymer containing tetranuclear Cu(II)-btp units connected by nitrates

Article abstract of DOI:10.1039/b301082a

Synthesis, structure and heterogenous catalytic activity of a coordination polymer containing tetranuclear Cu(II)-btp units connected by nitrates were studied. Blue block-type crystals were obtained from the direct diffusion technique. With acetate counteranions, a discrete Cu(II) molecule was obtained. This polymer is an effective heterogeneous catalyst for regioselective ring opening of epoxide by methanol.

Full text of DOI:10.1039/b301082a

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Synthesis, structure and heterogeneous catalytic activity of a
coordination polymer containing tetranuclear Cu(II)-btp units
connected by nitrates
Sang-Kun Yoo,b Ji Young Ryu,b Jun Yong Lee,b Cheal Kim,*b Sung-Jin Kima and
Youngmee Kim*a
a Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea.
E-mail: ymeekim@ewha.ac.kr
b Department of Fine Chemistry, Seoul National University of Technology, Seoul 139-743,
Korea. E-mail: chealkim@snut.ac.kr
Received 27th January 2003, Accepted 12th March 2003
First published as an Advance Article on the web 25th March 2003
A new coordination polymer containing a tetranuclear
Cu(II)-btp unit is an effective heterogeneous catalyst for the
regioselective ring opening of epoxide by methanol.
compound 1 as a heterogeneous catalyst showed, unexpectedly,
very promising results for alcoholysis of epoxides at ambient
temperature and this novel catalytic system appears to be
very mild, efficient, regioselective, and easily recyclable,
while the discrete Cu() molecule 2 did not catalyze the
alcoholysis.
Self-assembly coordination compounds1,2 have been intensively
synthesized as their porous channels have special properties
such as gas or chemical absorption, ion-exchange, hetero-
geneous catalysis, and so on. Since Cu() has a Jahn–Teller
effect that drives a tetragonally elongated structure and can
produce different types of polymers by changing counter
anions,3 Cu() complexes have often been used as building
blocks for the construction of different polymeric architec-
tures. However, in spite of intensive study of the Cu-
containing coordination polymers, to our knowledge, their use
as heterogeneous catalysts has not been reported yet.4 Herein,
we report that, with btp [2,6-bis(NЈ-1,2,4-triazolyl)pyridine]
ligand, Cu(NO3)2 produces a new btp-bridged tetranuclear
unit connected by nitrate anions to form a polymeric com-
pound whereas Cu(CH3CO2)2 produces a discrete molecule.
Furthermore, preliminary application of the polymeric Cu()
Blue block-type crystals were obtained from the direct diffu-
sion technique.† The structure of 1 was determined by X-ray
crystallography.‡ An asymmetric unit consists of two Cu
atoms, three btp ligands, and four NO3Ϫ anions. Four Cu atoms
are bridged by btp ligands to form a tetranuclear compound.
Chelating nitrate anions to Cu atoms terminate connections for
polymerization. The terminal Cu2 atoms have a monodentate
nitrate anion, a chelating bidentate nitrate anion, and two btp
ligands, and inner Cu1 atoms have four btp ligands and a mono-
dentate nitrate anion. There is a crystallographic center of
inversion at the mid-point of the compound. Fig. 1(a) shows the
complete structure of a tetranuclear compound containing
three bridged rings. Cu1 and Cu2 atoms are also weakly
bridged by nitrate anions [Cu–O(bridging nitrate) 2.482(2) and
Fig. 1 (a) ORTEP9 drawing of the tetranuclear compound containing Cu() ions, btp ligands and nitrates. Axial nitrate anions are omitted
for clarity. Cu–N(btp) 1.978(3)–2.061(3) Å and Cu2–O(chelating nitrate) 2.014(2), 2.042(2) Å. (b) Nitrate bridged polymeric structure.
Cu1–O(monodentate nitrate) 2.359(2) Å and Cu2–(monodentate nitrate) 2.275(2) Å. Cu1–O(bridging nitrate) 2.482(2) Å and Cu2–O(bridging
nitrate) 2.420(2) Å.
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Table 1 Ring opening of epoxides by methanol in the presence of catalyst 1
Entry
Substrate
Time/days
Conversion yielda, b (%)
1
2
3
4
5
6
7
Cyclohexene oxide
6
6
6
60
20
50
3
>99
c
Cyclohexene oxide/Cu(NO3)2
55d
Cyclohexene oxide/complex 2e
Cyclopentene oxide
cis-2,3-Epoxybutane
trans-2,3-Epoxybutane
Styrene oxide
0
>99
>99
>99
>99 (exclusively primary alcohol)
a Catalytic reaction conditions: epoxides (0.05 mmol) were dissolved in methanol (1 mL), and the catalyst 1 (1 mg/FW of 1 ∼1 × 10Ϫ3 mmol) was
added and shaken at room temperature (450 rpm). Reaction conversion was monitored by GC/GC-Mass analysis of 20 µL aliquots withdrawn
periodically from the reaction mixture. All reactions were run at least three times and the average conversion yields are presented. b Based on the
consumption of the starting epoxide. c Reaction conditions were the same as described in a except that 1 × 10Ϫ3 mmol of Cu(NO3)2 salt was used
instead of catalyst 1. d Conversion stopped at 55% and did not proceed even at longer reaction times. e Reaction conditions were the same as
described in a except that 1 × 10Ϫ3 mmol of complex 2 was used instead of catalyst 1.
2.420(2) Å]. The nitrate bridged polymeric structure is shown in
Fig. 1(b). If the weak coordination of nitrate is considered,
the geometry of each Cu atom is tetragonally enlongated octa-
hedral. The Cu1 ؒ ؒ ؒ Cu2 and Cu1 ؒ ؒ ؒ Cu1Ј distances are
9.659(7) and 10.014(7) Å, respectively. Blue needle-type crystals
were obtained by the slow evaporation of a mixture of a
methanol solution of Cu(CH3CO2)2 and a methanol solution of
btp ligand.†,‡ The asymmetric unit cell of mononuclear Cu()
complex 2 contains half a molecule and a water molecule. A
crystallographic center of inversion is located on the Cu atom,
and the complete molecule is shown in Fig. 2. When
Cu(CH3CO2)2 is used in a reaction with btp ligands, btp does
not bridge the Cu atoms. Two acetate ions and two btp ligands
are coordinated to the Cu() ion with O–Cu–O and N–Cu–N
angles of 180.0Њ. Therefore, the geometry of the Cu() ion is
essentially square planar.
geometry of the Cu() ion is also elongated octahedral with
Cu–O(OAc) distances of 2.69(5) Å in compound 2. This result
suggests that both the Jahn–Teller effect of the electronic con-
figuration of Cu() and the counter anion effect are important
for the construction of coordination polymers.
Since copper has recently gained prominence as a catalyst for
numerous transformations6 and heterogeneous catalysts are
easy to handle and separate from the reaction solution,7 the
reactivity of the copper-containing polymer 1 as a hetero-
geneous catalyst was examined in the ring opening of a wide
range of epoxides with methanol.8 The results are summarized
in Table 1.
Cyclohexene oxide was effectively converted to trans-1,2-diol
monomethyl ether (>99% conversion in 6 days), while a control
reaction carried out in the absence of catalyst 1 showed only
trace amounts of the product over the same time period. More
importantly, the catalyst could be easily recovered by a simple
filtration and used repeatedly with about a 5% decrease in
original catalytic activity.§ Furthermore, catalyst 1 has shown
even better catalytic activity than Cu(NO3)2 salt under homo-
geneous conditions (Table 1, entry 2).¶ This novel catalyst 1
was also very active towards acyclic epoxides, as cis- and
trans-2-butene oxides were ring opened to the corresponding
products. All products were determined to have a trans stereo-
chemistry by NMR, GC and GC/MS analysis with com-
parison to authentic samples. Styrene oxide was converted to
2-methoxy-2-phenyl ethanol within 3 days, which was the
fastest, and the alkoxy group was incorporated exclusively at
the benzylic position (α-carbon) instead of the less hindered
β-carbon center to generate primary alcohol. These results
indicate that the regiochemistry of the ring opening by catalyst
1 is dependent on the electronic nature of the substrate rather
than steric factors. In addition, exclusive attack at the benzylic
position of styrene oxide suggests that the more substituted
carbon in the intermediate adduct generated from the substi-
tution of labile bridging nitrate in catalyst 1 by the oxide might
have a significant cationic character [eqn. (1)].
Fig. 2 ORTEP drawing of compound 2, Cu(OAc)2(btp)2. Cu–N(btp)
1.986(2) Å and Cu–O(acetate) 1.943(2) Å.
The btp ligand usually bridges transition metal atoms to
form a polymeric structure as shown in our previous btp
bridged dirhodium(,) compounds,5 but in this study the btp
ligands act as monodentate ligands as well as bidentate bridging
ligands. A Cu() ion having nitrate counteranions produced
a btp-bridged Cu() tetranuclear compound that was also
bridged by nitrate anions to form a polymeric compound. With
acetate instead of nitrate as counteranion, the Cu() ion pro-
duces only a discrete molecule. In the case of compound 1, the
geometry of the Cu() ion is tetragonally elongated octahedral,
while compound 2 has a square planar geometry. Actually, the
This cationic character can be stabilized by the phenyl group
through conjugation, and therefore the nucleophile would
attack the more cationic carbon site to give trans-2-methoxy-2-
phenyl ethanol. To our knowledge, this is the first example that
a Cu-containing self-assembly coordination compound can
carry out the ring opening of epoxide with methanol, while the
(1)
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copper metal during the ring opening reaction. To test for leaching we
filtered the catalyst after the reaction and allowed the filtered catalyst
and the filtrate to react with another aliquot of epoxide. We observed
that the ring opening reaction with the filtered catalyst proceeded at
95% of the original rate, while the filtrate showed less than 5% conver-
sion. On the other hand, the powder X-ray diffraction (XRD) pattern
of the filtered catalyst after the reaction revealed the same pattern as the
original catalyst, suggesting that the original structure of the filtered
catalyst has been kept during the reaction.
discrete Cu() molecule 2 did not catalyze the alcoholysis of
epoxide (Table 1, entry 3).
In summary, we have shown a new structure in coordination
polymer 1 containing btp-bridged tetranuclear Cu() units
weakly connected by nitrate ions. With acetate counteranions
however, a discrete Cu() molecule, 2, was obtained. This result
suggests that both the Jahn–Teller effect of Cu() and the
counteranion effect are important for the construction of
coordination polymers. We have also reported for the first time
that Cu-containing 1 is an efficient heterogeneous catalyst for
regioselective ring opening of epoxide with methanol. This
catalyst system appears to be an efficient, mild, and easily
recyclable method for the alcoholysis of epoxides.
¶ We will discuss a comparison of homogeneous vs. heterogeneous
catalytic activity for the Cu(NO3)2 salt and catalyst 1 elsewhere.
1 S. Noro, R. Kitaura, M. Komdo, S. Kitagawa, T. Ishii, H. Matsuzaka
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Y. Kim acknowledges the Korean Science & Engineering
Foundation (R03-2000-000-00002-0) and C. Kim acknowledges
the Korean Research Foundation (DP0270 and 2002-070-
C00053). We thank Dr Alan Lough from the University of
Toronto, Canada for X-ray data collection.
3 M. Du, X.-H. Bu, Y.-M. Guo and H. Liu, Inorg. Chem., 2002, 41,
4904 and references therein.
Notes and references
† A water solution of Cu(NO3)2 was carefully layered with a methanol
solution of btp ligand. Anal. calc. for C27H21Cu2N25O12, 1: C, 31.96; H,
2.23; N, 34.49. Found: C, 32.95; H, 2.31; N, 34.53%.
For C22H20CuN14O4, 2: C, 43.45; H, 3.32; N, 32.26. Found: C, 43.45;
H, 3.30; N, 32.23%.
4 A few other metal-containing coordination polymers have been
reported to show the catalytic activities: M. Fujita, Y. J. Kwon,
S. Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151;
T. Sawaki, T. Dewa and Y. Aoyama, J. Am. Chem. Soc., 1998, 120,
8539; J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K.
Kim, Nature, 2000, 404, 982.
5 Y. Kim, S.-J. Kim and A. J. Lough, Polyhedron, 2001, 20, 3073.
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Chem. Soc., 1999, 121, 1994; D. A. Evans, S. J. Miller and T. Lactka,
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‡ Crystal data for compound 1: C27H21Cu2N25O12, 0.12 × 0.11 ×
0.10 mm3, FW = 1014.77, triclinic, a = 11.1473(3), b = 13.3356(3),
c = 13.4689(3) Å, α = 78.475(2), β = 70.419(2), γ = 76.033(2)Њ,
V = 1815.12(8) Å3, Z = 2, µ(Mo-Kα) = 1.274 mmϪ1, 23632 measured
reflections [R(int) = 0.073] were used in all calculations, final R = 0.0751
(Rw = 0.1003) with reflections having intensities greater than 2σ,
GOF(F 2) = 1.043. CCDC reference number 196739.
Crystal data for compound 2: C22H24CuN14O6, 0.35 × 0.3 × 0.25
mm3, FW = 644.09, triclinic, a = 6.867(3), b = 9.304(2), c = 10.935(4) Å,
α = 93.40(2), β = 102.42(3), γ = 97.11(2)Њ, V = 674.3(4) Å3, Z = 1,
µ(Mo-Kα) = 0.877 mmϪ1, 2882 measured reflections [R(int) = 0.0152]
were used in all calculations, final R = 0.0405 (Rw = 0.1026) with reflec-
tions having intensities greater than 2σ, GOF(F 2) = 1.117. CCDC refer-
ence number 196738. See http://www.rsc.org/suppdata/dt/b3/b301082a/
for crystallographic data in CIF or other electronic format.
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G. P. Lee, Tetrahedron, 1996, 52, 14361; N. Iranpoor and H. Adibi,
Bull. Chem. Soc. Jpn., 2000, 73, 675.
§ The decrease in the catalytic activity is presumably due to leaching of
9 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
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