Synthesis and properties of a Cu4(SCN)4 cubane cluster-based coordination polymer with a diamond net

Article abstract of DOI:10.1016/j.inoche.2011.01.024

A triply-interpenetrating diamondoid coordination polymer [Cu ₄(SCN)₄(tpom)]·2H₂O (1, tpom = tetrakis(4-pyridyloxymethylene)methane) was prepared, which is built from an unprecedented pseudohalide cubane cluster Cu₄(SCN)₄ and tetrahedral tpom ligand. 1 exhibits high thermal stability and temperature-dependent photoluminescence behaviors resembling those of Cu ₄Cl₄ complexes.

Full text of DOI:10.1016/j.inoche.2011.01.024

Inorganic Chemistry Communications 14 (2011) 558–561
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and properties of a Cu (SCN) cubane cluster-based coordination polymer
4
4
with a diamond net
a,b
a
a
a
a
a,
a
Shi-Bin Ren , Le Zhou , Jun Zhang , Cheng-Hui Li , Yi-Zhi Li , Hong-Bin Du ⁎, Xiao-Zeng You
a
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 317000, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
4 4 2
A triply-interpenetrating diamondoid coordination polymer [Cu (SCN) (tpom)]·2H O (1, tpom=tetrakis
Received 20 December 2010
Accepted 26 January 2011
Available online 3 February 2011
(
4-pyridyloxymethylene)methane) was prepared, which is built from an unprecedented pseudohalide
cubane cluster Cu (SCN) and tetrahedral tpom ligand. 1 exhibits high thermal stability and temperature-
dependent photoluminescence behaviors resembling those of Cu
4
4
4
Cl
4
complexes.
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Coordination polymer
CuSCN
Cubane cluster
Diamond net
Photoluminescence
3 4
PCuSCN]
based on 13C NMR and IR
There has been considerable current interest in varieties of
polynuclear complexes of transition metals with filled-shell d10
electronic configuration. These compounds generally possess long-
lived excited states, leading to strong emissions that would allow for a
better comprehension of fundamental phenomena and potential
applications in sensors, light emitting diode devices, solar light
harvesting and conversion [1]. Particularly interesting among these
are the derivatives of Cu(I) that are often brightly luminescent even at
RT. Cu(I) tends to form a variety of coordination compounds with
halides ranging from zero-dimensional (0D) complexes to 3D frame-
existence has been claimed in [nBu
spectral data [8]. In fact, there is only one metal thiocyanate cubane
cluster compound, i.e. [(CH PtSCN] [9], known to date [10]. We [11],
among others [2–4,6,7], have been interested in coordination polymers
formed by Cu(I) halides or Cu(I) thiocyanates. We herein report the
)
3 3
4
synthesis of a new 3D microporous coordination polymer [Cu
(tpom)]·2H O (tpom=tetrakis(4-pyridyloxymethylene)methane), 1,
which is, to the best of our knowledge, the first structurally
characterized compound consisting of an unprecedented Cu (SCN)
cubane cluster.
Complex 1 was synthesized by the reaction of CuSCN, tpom and
excess NH SCN in the mixed-solvent system of CH CN and H
under solvothermal conditions [12]. Attempts to prepare similar Cu
(SCN) cluster-based complexes as in 1 using pyridine (py),
4 4
(SCN)
2
4
4
2 2
works with structural motifs such as rhomboid Cu X dimers, cubane
Cu , and step and ladder-like (CuX) chains (X=Cl, Br, I) [2–4].
4
X
4
n
4
3
2
O
Notably, these polyhedral Cu(I) halide clusters have been employed as
building blocks for construction of novel 3D coordination polymers
with intriguing structures and photoluminescent properties.
4
4
4-hydropyridine and 4-cyanopyridine instead of tpom under similar
conditions have so far met with failures. Complex 1 is stable in air
and is insoluble in common solvents such as methanol, ethanol,
acetone, acetonitrile, ether, DMF and water. The phase purity of
complex 1 was confirmed by elemental analysis, IR and X-ray
powder diffraction. FT-IR spectrum of 1 showed a strong peak at
−
Thiocyanate (SCN ) is a known pseudohalide, which also shows
strong affinity to Cu(I) to form a number of cuprous thiocyanate
complexes. In contrast with halides, however, thiocyanate is a highly
ambidentate ligand, and exhibits theoretically as many as 13 coordina-
tion modes binding to metal atoms through either the nitrogen or the
sulfur atom, or both (Figure S1 in the Supporting Information) [5]. As a
result, there are a limited number of structural motifs known for Cu
−
1
2112 cm , characteristic of the C≡N vibration from the thiocya-
nate. The ν(C≡N) vibration in 1 is higher than that of Cu(SCN) (py)
(bidentate bridge, 2074 cm ) but lower than that of β-CuSCN
2
2
−
1
(
2 2
SCN), most of which are polymeric, including rhomboid Cu (SCN)
−
1
dimers, 1D zig-zag chains, 2D sheets, and 3D networks as in α- and
β-forms of CuSCN [6,7]. The thiocyanate analogues of the well-known
Cu X cubane clusters have not been reported so far, though its
4 4
(tetradentate bridge, 2173 cm ), indicating the presence of the
tridentate thiocyanate bridge Cu−NCS=Cu [5,7].
X-ray single-crystal diffraction analysis [13] revealed that the
structure of 1 resembles that of the recently reported [Cu
tpom)]·H O [11b], and crystallizes in the tetragonal space group
I–4. Complex 1 possesses a 3D neutral cluster-based open framework
composed of the unprecedented Cu (SCN) cubane cluster and
2
4 4
I
(
2
⁎
Corresponding author. Tel.: +86 25 83686581; fax: +86 25 83314502.
E-mail address: hbdu@nju.edu.cn (H.-B. Du).
4
4
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.01.024

S.-B. Ren et al. / Inorganic Chemistry Communications 14 (2011) 558–561
559
tetrahedral ligand tpom (Fig. 1). Both units have an exact S
4
internal
symmetry. In the cubane Cu (SCN) clusters, each Cu atom adopts a
4
4
distorted tetrahedral coordination geometry through bonding to two
S atoms from two thiocyanates and two N atoms from a tpom ligand
and a thiocyanate, respectively. The Cu–S and Cu–N distances range
2
.415(1)–2.508(1) and 1.964(5)–2.000(4) Å, respectively, in accor-
dance with those of other CuSCN complexes [5,6]. The bond angles
around Cu are within 102.4(1)–124.8(2)°, which confirms a distorted
tetrahedral coordination geometry. Notably, the shortest diagonal
Cu…Cu distance within the cubane cluster is 3.012(1) Å, longer than
4 4 4 4
those of Cu I complexes [2] but comparable to those of Cu Cl
clusters [14]. The bond valence sum calculations [15] for Cu (0.96)
confirm that Cu is monovalent. It is noted that the thiocyanate in 1
acts as a tridentate bridge, and a set of four thiocyanates in parallel
2 3
coordinates through both their μ -N and μ -S ends with four Cu atoms
to form a cubane cluster. The Cu–N–CS angle is 149.2(4)°, much lower
than those of other metal thiocyanates (between 160 and 180°). The
Cu–S–CN angles vary from 96.7(2) to 97.4(2)°, which are within the
normal range of 90–120°. The C―S and C―N bond distances are 1.642
(
5) and 1.167(7) Å, respectively, similar to those of other metal
thiocyanates [5].
The 3D framework of 1 is formed by coordination of each
tetrahedral quadridentate ligand tpom via its N donor atoms to four
respective Cu (SCN) cubane clusters (Fig. 2). From the topological
4 4
point of view, complex 1 possesses a sphalerite (ZnS) structure, an AB
6
derivative of the diamond net (short vertex symbol 6 ) [16], with the
4 4
tetrahedral Cu (SCN) and quadridentate ligand tpom units as the A
and B 4-connecting nodes, respectively. The structure is triply-
interpenetrated, leading to elimination of almost all the free space
in 1. The rest solvent-accessible voids, in which lie disordered guest
water molecules, only comprise 7.3% of the total crystal volume as
calculated using PLATON [17] based on the crystal structure. The open
channels in 1 are too small to allow adsorption for N
but just enough to let H pass through (0.5 H per unit cell at 77 K and
atm) as indicated by hysteresis between the H adsorption and
2
gas molecules,
2
2
1
2
desorption isotherms (Figure S2 in the Supporting Information).
Thermogravimetric analysis of complex 1 showed that there was
about 5.7% of weight loss below 250 °C (Figure S3 in the Supporting
Information), corresponding to the release of the surface and occluded
water molecules (calculated 3.7%). Above 300 °C, complex 1 began to
lose its organic ligand (observed 41.4%, calculated 46.7%), followed by
the decomposition and vaporization of the residue above 535 °C. The
thermal stability of 1 was confirmed by X-ray powder diffraction
(
XRPD) analyses. The XRPD patterns of the synthesized samples in
general match those calculated on the basis of the single-crystal
structure in peak positions. The differences in reflection intensities
between the calculated and experimental patterns are due to the
variation in crystal orientation for the powder samples. The XRPD
6
Fig. 2. Representations of (a) a single and (b) a triply-interpenetrated 6 -dia net of
4 4
complex 1. The H atoms and solvent molecules are omitted for clarity. Both Cu (SCN)
patterns of the samples heated at 150 and 250 °C in N
2
for 2 h,
and tpom in (b) are represented as 4-connected nodes.
respectively, exhibit negligible changes in comparison with that of the
pristine one (Figure S4 in the Supporting Information). However, 1
transformed into a black powder of Cu S upon heating at 300 °C in N
2 2
for 2 h. The results indicate that the structural skeleton of 1 remains
stable up to 250 °C in nitrogen atmosphere.
The UV–Visible diffuse-reflectance (DR) spectrum of 1 at RT shows
an absorption onset at about 400 nm (Fig. 3), indicating that it is a
wide-gap semiconductor with a band gap of 3.10 eV. Similar to its
4 4
Cu X analogues, complex 1 shows temperature-dependent emissive
behaviors. Upon excitation at 77 K, 1 exhibits an intense, broad and
asymmetric emission band centered at 470 nm. However, this
emission is extremely weak at RT. In fact, there is no other
measureable emission in the whole visible region at RT, unlike
4 4 2 4 4
[Cu I (tpom)]·H O [11b] and other Cu I complexes [1] that usually
Fig. 1. Coordination environments of Cu
4 4
(SCN) cubane cluster and tetrahedral ligand
tpom. The H atoms are omitted for clarity.
show a strong low energy emission around 600 nm and a weak higher

560
S.-B. Ren et al. / Inorganic Chemistry Communications 14 (2011) 558–561
dependent luminescence behaviors, which upon excitation emits at RT a
very weak but at 77 K a very strong blue light attributed to a 3XLCT
exited state. The weak Cu…Cu interactions in 1 result in no
photoemission owing to the cluster-centered excited state that is
4 4
often observed in various Cu I complexes.
Acknowledgements
We are grateful for financial support from the National Basic
Research Program (2011CB808704 and 2007CB925101), and the
National Natural Science Foundation of China (20931004 and
2
1071075).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2011.01.024.
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