Structure and catalytic activity of a hexarhenium cluster-supported copper cubanes [{Cu4(μ-OH)4(NH3)7}2{Re6Se8(CN)6}][Re6Se8(CN)6] · 2H2O

Article abstract of DOI:10.1016/j.inoche.2007.02.005

The first hexarhenium cluster-supported Cu₄(OH)₄ cubanes had been synthesized. The complex displayed a belt-like one-dimensional structure with dangling Cu₄ cubanes. As a heterogeneous catalyst, the complex showed a high catalytic activity in the transesterification of a range of esters with methanol under the mild conditions. Moreover, it can be reused without any loss of activity through five runs with ester, which suggests that this catalyst is an efficient, mild, and easily recyclable one for the alcoholysis of esters.

Full text of DOI:10.1016/j.inoche.2007.02.005

Inorganic Chemistry Communications 10 (2007) 586–589
www.elsevier.com/locate/inoche
Structure and catalytic activity of a hexarhenium
cluster-supported copper cubanes
[{Cu4(l-OH)4(NH3)7}2{Re6Se8(CN)6}][Re6Se8(CN)6] Æ 2H2O
b
b,
c
c,
*
Li Xu a,b, Youngmee Kim , Sung-Jin Kim *, Hee Jin Kim , Cheal Kim
a
Information College of Science and Technology, Nanjing Forestry University, Nanjing 210037, People’s Republic of China
b
Department of Chemistry and Division of Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea
c
Department of Fine Chemistry, Seoul National University of Technology, Seoul 139-743, Republic of Korea
Received 28 December 2006; accepted 2 February 2007
Available online 20 February 2007
Abstract
The first hexarhenium cluster-supported Cu4(OH)4 cubanes had been synthesized. The complex displayed a belt-like one-dimensional
structure with dangling Cu4 cubanes. As a heterogeneous catalyst, the complex showed a high catalytic activity in the transesterification
of a range of esters with methanol under the mild conditions. Moreover, it can be reused without any loss of activity through five runs
with ester, which suggests that this catalyst is an efficient, mild, and easily recyclable one for the alcoholysis of esters.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Cyano-bridged hexarhenium cluster; Crystal structure; Transesterification; Catalytic activity
The soluble cyano-hexarhenium clusters [Re6Q8(CN)6]4ꢀ
(Q = S, Se, Te) have been used as molecular building
blocks in crystal engineering or developing supporting
materials for catalysts [1]. In solid frameworks, metal cat-
ions bridge the Re6 clusters via six cyanide groups forming
large cubical cavities which can include plenty of small
solvent molecules as well as metal ions [1]. We have previ-
ously prepared polymeric materials: 2D framework
[Mn(salen)]4n[Re6Te8(CN)6]n [2] and 3D framework
Na[Mn(salen)3][Re6Se8(CN)6] [3], and also reported Re6
cluster-supported manganese(III) porphyrin: [Mn(OEP)]4-
[Re6Te8(CN)6] (OEP = octaethylporphinato dianion),
[Mn(TPP)]4[Re6Te8(CN)6] (TPP = tetraphenylporphinato
dianion) [4], in which, [Mn(TPP)]4[Re6Te8(CN)6] showed
a high catalytic activity in the epoxidation of olefins by
iodosylbenzene, and Re6 cluster-supported iridium(I) com-
pound [Re6Te8{CNIr(CO)(PPh3)2}6], which is effective in
catalyzing hydrogenation [5].
Since copper has recently gained prominence as a cata-
lyst for numerous transformations [6], we have planned
to attach cubane units [Cu4(OH)4(NH3)7]4+ to cyano-hexa-
rhenium clusters [Re6Se8(CN)6]4ꢀ to produce a new hexa-
rhenium cluster-supported catalyst for the heterogeneous
catalytic reactions. Here we report first Re6 cluster-sup-
ported copper cubanes: [{Cu4(l-OH)4(NH3)7}2{Re6-
Se8(CN)6}][Re6Se8(CN)6] Æ 2H2O (1), where cupper units
Cu4(l-OH)4(NH3)7 are linked to the cyano-rhenium
clusters to form a polymeric belt-like inorganic extended
framework with novel catalytic transesterification
activity.
*
Corresponding authors. Tel.: +82 2 3277 2350; fax: +82 2 3277 2384
[{Cu4(OH)4(NH3)7}2{Re6Se8(CN)6}][Re6Se8(CN)6] Æ
2H2O (1) was prepared by a direct diffusion technique in
which an aqua solution of [Cu(NH3)4SO4] was carefully
(S.-J. Kim), tel.: +82 2 970 6693; fax: +82 2 973 9149 (C. Kim).
E-mail addresses: sjkim@ewha.ac.kr (S.-J. Kim), chealkim@snut.ac.kr
(C. Kim).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.02.005

L. Xu et al. / Inorganic Chemistry Communications 10 (2007) 586–589
587
Fig. 1. The structure of compound 1.
layered with an aqua solution of K4[Re6Se8(CN)6] cluster.1
The structure of the compound was solved by X-ray single-
crystal analysis.2
The asymmetric unit contains two half of a Re cluster,
a cubane unit [Cu4(OH)4(NH3)7]4+, and a water solvent
molecule. The complete structure is generated by the sym-
metry operations (ꢀx + 2, ꢀ y + 1, ꢀ z + 1) and (ꢀx + 2,
ꢀ y + 1, ꢀ z). Two cyano groups of the cluster in trans
˚
directions (C–N distance of 1.15(2) A) are connected to
two copper atoms from two different cubane units
[Cu4(OH)4(NH3)7]4+, and the other four cyano groups with
˚
C–N distances of 1.12(2) and 1.18(2) A remain dangling
(Fig. 1). Another isolated Re6 cluster keeps the charge bal-
ance. The IR spectrum of 1 had two C ” N stretching peaks
at 2109 and 2151 cmꢀ1, which shows two different types of
cyano groups: bridging and terminal ones. This result is
consistent with the X-ray crystallographic data. In cubane
units, four Cu(II) atoms and four hydroxyl oxygen atoms
occupy the alternative vertices of the cubane. One of four
Cu(II) ions in a cubane is linked by a cyano group of the
Re6 cluster, and also coordinated by one ammonia and
Fig. 2. The belt-like one-dimensional structure with intermolecular
˚
Cu. . .N interactions (the distance of Cu–N(x, ꢀ 1 + y, z) is 2.697(2) A)
shown in dotted lines.
three hydroxyl groups. The other three Cu(II) ions are
coordinated by three hydroxyl groups and two ammonia
molecules. The coordination geometry of each Cu(II) ion
is best described as a distorted square pyramid with two
hydroxyl oxygen and two ammonia nitrogen atoms (Cu–
1
Preparation of single crystals of 1: the single crystals of 1 were
prepared by a direct diffusion technique in which H2O solution of
[Cu(NH3)4SO4] was carefully layered with
a H2O solution of
˚
O/N mean distance of 2.0 A) in the plane and one hydroxyl
K4[Re6Se8(CN)6] complex. During preparing [Cu(NH3)4SO4], the stoichi-
ometric amount of ammonia was added dropwise to a stirred solution of
CuSO4 Æ 5H2O until pale blue precipitation disappeared. The bulk material
of 1 was tried to be prepared by mixing a H2O solution of [Cu(NH3)4SO4]
and H2O solution of K4[Re6Se8(CN)6], and the result was confirmed by
powder X-ray diffraction data. Anal. Calc. for C12H54Cu8N26O10Re12Se16
(4728.9): C, 3.05; H, 1.15, N, 7.70. Found: C, 3.11; H, 1.09; N, 7.68%.
˚
oxygen atom (Cu–O mean distance of 2.2 A) on the apical
position. An extended belt-like structure (Fig. 2) is formed
through intermolecular Cuꢁ ꢁ ꢁN interactions (The distance
˚
of Cuꢁ ꢁ ꢁN(x, ꢀ 1 + y,z) is 2.697(2) A).
In our compound 1, Cu4(l-OH)4 was coordinated to the
octahedral metal-chalcogenide cluster [Re6Se8(CN)6]4ꢀ
that is the first hexarhenium cluster-supported Cu4
cubanes. The Cu4(l-OH)4 core in compound 1 is similar
to that in the previously reported compound [Cu4-
(l-OH)4][Re4Te4(CN)12] (2). The geometrical environment
of the cluster anion [Re6Se8(CN)6]4ꢀ is similar to that pre-
viously reported [7]. The cubane-like Cu4(l-OH)4 fragment
2
Crystal data for [{Cu4(OH)4(NH3)7}2{Re6Se8(CN)6}][Re6Se8-
(CN)6] Æ 2H2O (1): C12H54Cu8N26O10Re12Se16 (M = 4728.9), triclinic
ꢀ
˚
˚
˚
space group, P1, a = 9.367(4) A, b = 10.025(4) A, c = 19.910(7) A,
3
˚
a = 93.298(7), b = 100.742(7), c = 97.518(7), V = 1814.7(12) A , Z = 1,
dcalc = 4.327 g/cm3, 10 136 reflections measured, 6941 unique
(Rint = 0.0369) which were used in all calculations, final R = 0.0479
(Rw = 0.0974) with reflections having intensities greater than 2r,
GOF = 0.916.

588
L. Xu et al. / Inorganic Chemistry Communications 10 (2007) 586–589
O
O
was reported previously in three complexes [8], but only in
one compound [Cu4(l-OH)4][Re4Te4(CN)12] 2, Cu4-
(l-OH)4 core was connected to the cluster. In this previ-
ously reported compound 2, Cu4(l-OH)4 was linked to a
metal-chalcogen cubane cluster [Re4Te4(CN)12]4ꢀ through
cyanide groups. The coordination numbers of Cu(II) in 1
and 2 are all 5, and the distortions of Cu4(l-OH)4 from
ideal cubic geometry in 1 and 2 are almost the same (The
catalyst 1
HOCH3
..
CH3COCH3
OH +
O2N
OCCH3
O2N
+
ð1Þ
The potential benefits of the heterogeneous catalyst
include facilitation of catalyst separation from reagents
and reaction products, and simplification of methods for
catalyst recycle [9]. For a truly effective heterogeneous cat-
alyst, therefore, it is critical that recovery could be simple
and efficient, and that the recovered catalyst could retain
its original reactivity through multiple cycles. As the poly-
meric compound 1 showed adequate activity, we tested for
its recovery and reuse. After the reaction completed, the
catalyst was recovered by filtration and thoroughly washed
with methanol for the consecutive runs. The recovered cat-
alyst was used for a new reaction batch of p-nitrophenyl
acetate. The catalyst 1 showed excellent reusability for 5
times without showing any significant deterioration of cat-
alytic activity (see Table S1 of the supplementary
information).
Then, we have carried out another control reaction
before further reactivity study with other substrates,
because in the catalytic process the leached metal species
might catalyze efficiently the transesterification reaction
instead of the heterogeneous catalysts [10]. To examine this
possibility we have filtered the catalyst 1 after the transeste-
rification reaction of p-nitrophenyl acetate and allowed the
filtered catalyst and the filtrate to react with another ali-
quot of p-nitrophenyl acetate, respectively, as shown in
the previous study [10]. We have observed that the transe-
sterification reaction with the filtered catalyst proceeded at
the original rate, while the filtrate did show about 20% con-
version within the same time interval. This result strongly
suggests that the dominant reactive species is the heteroge-
neous catalyst 1, not the other species such as the leached
metal. However, about 20% conversion by the filtrate indi-
cate that trace amounts of Cu metal ions might be leached
during the catalysis. 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. Based on these results,
we have concluded that the heterogeneous catalyst 1 could
be recycled multiple times without a significant loss of
activity.
˚
˚
mean band distances of Cuꢁ ꢁ ꢁCu are 3.054 A and 3.257 A
˚
˚
in 1, and those in 2 are 2.99 A and 3.18 A). The coordina-
tion geometry of Cu(II) ion of our compound 1 is a dis-
torted square pyramid similar to the previously reported
compound 2. In our compound 1, Cu(II) ions are linked
by ammonia molecules besides cyano groups, but Cu(II)
ions in compound 2 are coordinated only by cyano groups.
The main difference between our compound 1 and the pre-
viously reported compound 2 is that compound 1 show a
one-dimensional belt-like structure, and the compound 2
show a two-dimensional layered structure.
Since copper has recently gained prominence as a cata-
lyst for numerous transformations [6] and heterogeneous
catalysts are easy to handle and separate from the reaction
solution [9], we have examined the reactivity of the com-
pound 1 as a potential heterogeneous catalyst for the
transesterification reaction under mild conditions. The
compound 1, which was ground well into appropriate sizes
for high surface area but not too small for a convenient fil-
tration [10], was treated with p-nitrophenyl acetate and
methanol in the presence of 1 at 50 ꢁC under the neutral
conditions (Eq. (1)) [11]. This reaction, surprisingly, pro-
duced quantitatively the corresponding product methyl
acetate within 3 days (entry 1 of Table 1; see the supple-
mentary information for the detailed reaction conditions),
while a control reaction carried out in absence of the cata-
lyst 1 showed trace amounts of the conversion of the ester
to the product in the same time period. To the best of our
knowledge, this is the first report on the use of the hexarhe-
nium cluster-supported copper compound for the heteroge-
neous transesterification reaction.
Table 1
Transesterfication of esters by methanol in the presence of the compound
1 at 50 ꢁCa
Entry
Substrate
1 (Time/day)
Yieldb
1
2
3
4
5
6
7
a
4-Nitrophenyl acetate
4-Fluorophenyl acetate
Phenyl acetate
3
24
13
10
10
10
2
100
100
100
100
100
100
100
The transesterification of other esters by the catalyst 1
was also carried out efficiently and the results are given in
Table 1. Phenyl acetate and p-fluorophenyl acetate have dis-
played slow reactivity by the catalyst 1 (entries 2 and 3), but
they were completely converted to the corresponding prod-
ucts, respectively. Also, the novel catalyst 1 was active to
benzoates with an electron-donating or – withdrawing
group that underwent the transesterification with 10 days
(entries 4–6). Importantly, vinyl acetate, that is widely used
as a precursor for ester synthesis, was also converted effi-
ciently to the product methyl acetate within 2 days (entry
4-Nitrophenyl benzoatec
4-Chlorophenyl benzoate
Phenyl benzoate
Vinyl acetate
All esters were completely converted to the corresponding product by
methanol. See the supplementary information for the detailed reaction
conditions.
b
Based upon the products, methyl acetate or methyl benzoate.
The solvent was a mixture of CH3OH/CH3CN (1/1) because of low
c
solubility of substrate in CH3OH.

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In summary, for the first time, we synthesized hexarhe-
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belt-like one-dimensional structure with dangling Cu4
cubanes. Moreover, we found that the compound 1 could
carry out the heterogeneous catalytic transesterification
of a range of esters with methanol under the mild condi-
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recyclable method for the alcoholysis of esters.
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Program, the Korean Science & Engineering Foundation
(R01-2005-000-10490-0(2005)), the SRC program of
MOST/KOSEF through the Center for Intelligent Nano-
Bio Materials at Ewha Womans University (R11-2005-
008-00000-0), and Research Foundation for Talent of
Nanjing Forestry University of PR China is gratefully
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Appendix A. Supplementary data
CCDC 293042 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
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