A pcu-type metal-organic framework with spindle [Zn7(OH) 8]6+ cluster as secondary building units

Article abstract of DOI:10.1039/b615675d

The in situ solvothermal reaction of 9,10-dicyanoanthracene and ZnCl ₂/NaN₃ gave the complex, {[Zn₇(OH) ₈(DTA)₃]·H₂O}_n (1) (DTA ^2- = 9,10-ditetrazolateanthracene), which presents a pcu-type topological framework formed by DTA^2- bridging unprecedented heptanuclear spindle [Zn₇(OH)₈]⁶⁺ clusters as SBUs, and exhibits strong luminescent emission with long lifetime. The Royal Society of Chemistry.

Full text of DOI:10.1039/b615675d

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A pcu-type metal–organic framework with spindle [Zn7(OH)8]6+ cluster
as secondary building units{
Jian-Rong Li, Ying Tao, Qun Yu and Xian-He Bu*
Received (in Cambridge, UK) 30th October 2006, Accepted 17th January 2007
First published as an Advance Article on the web 6th February 2007
DOI: 10.1039/b615675d
(DTA22, an analog of DTB22, Chart 1S, ESI{) bridging
The in situ solvothermal reaction of 9,10-dicyanoanthracene
unprecedented heptanuclear spindle [Zn7(OH)8]6+ cationic clusters.
and ZnCl2/NaN3 gave the complex, {[Zn7(OH)8(DTA)3]?H2O}n
(1) (DTA22 = 9,10-ditetrazolateanthracene), which presents a
We selected H2DTA as the ligand, in view of its bulky skeleton
pcu-type topological framework formed by DTA22 bridging
unprecedented heptanuclear spindle [Zn7(OH)8]6+ clusters as
spacer (anthracene ring) and the rich coordination chemistry of
tetrazolates, attempting to construct MOFs with unique structure
types and associated properties. It is worth noting that many
SBUs, and exhibits strong luminescent emission with long
MOFs are constructed from tetrahedral, square, triangular,
lifetime.
octahedral, or trigonal-prismatic building blocks, however, to the
best of our knowledge, MOFs based on the assembly of such a
spindle heptanuclear building units have not been reported.
Maple prism crystals of 1 were obtained by the in situ
solvothermal reaction of 9,10-dicyanoanthracene (DCA) with
ZnCl2 and NaN3 (Fig. 1(a)),{ which has high thermal stability
(decomp. ca. 380 uC, Fig. 3S, ESI{). Its purity was further
confirmed by XRPD (Fig. 1S, ESI{). It should be pointed out that
in the solvothermal process the two nitriles of DCA have been
converted to tetrazolate groups, giving the ligand DTA22, which
has been observed several times previously.7
The rational design of metal–organic frameworks (MOFs) has
attracted considerable attention in supramolecular and materials
chemistry due to their enormous variety of interesting structural
topologies and wide potential applications as functional materials.1
The major task of the synthesis for such MOFs is to choose
appropriate metal-connecting nodes and/or organic bridging
ligands to control the formation of these complexes with required
structures and properties. In this case, metal cluster entities as
secondary building units (SBUs) have been proved as an effective
and powerful synthetic strategy in constructing new MOFs,2 in
which neutral zinc carboxylate clusters are most commonly used,
usually comprising of two, three or four zinc centers. Further,
some novel MOFs with penta-, hexa-, octa-, undeca-nuclear (and
even higher) SBUs have also been constructed in recent years.3,4
Depending on their architecture, MOFs based on polynuclear
metallic units offer potential for unique properties, such as
luminescence, magnetism and catalysis. Moreover, with expansion
of such cluster SBUs, it is also believed that new MOFs might
exhibit characteristics (such as optoelectronic property) similar to
those of nanosized metal oxide semiconducting materials.4 The
synthetic challenge is to increase and control the degree of
nuclearity of the inorganic ‘‘clusters’’ that are mainly responsible
for the physical properties.
X-Ray crystal structure analysis§ revealed that 1 crystallizes in
¯
the trigonal space group R3c, and shows a structure in which
nanosized heptanuclear spindle cationic [Zn7(OH)8]6+ clusters
(Fig. 1(b) and (d)) are linked via DTA22 ligands to produce a
neutral three-dimensional (3D) framework. All of the ZnII ions in
the cluster have a tetrahedral coordination geometry, but lie in
three types of coordination environments: each of two ZnII ions
(Zn1), located on a three-fold axis and related to each other by the
On the other hand, in such reported MOF materials, most
organic bridging moieties are di- or multi-carboxylate ligands. As
the analog of carboxylic acid, although a few tetrazolate-based
coordination solids have been reported,5 the coordination
chemistry of multidentate tetrazolate-based ligands remains largely
unexplored. Recently, Long et al. have synthesized several micro-
porous MOF materials by using 1,4-ditetrazolatebenzene (DTB22
)
as an organic linkage, some of which exhibit good hydrogen
storage properties.5j Herein, we report a novel MOF architecture,
{[Zn7(OH)8(DTA)3]?H2O}n (1) with a pcu-type (primitive cubic
lattice net) topology,6 constructed by 9,10-ditetrazolateanthracene
Department of Chemistry, Nankai University, Tianjin 300071, China.
E-mail: buxh@nankai.edu.cn; Fax: +86-22-23502458
{ Electronic supplementary information (ESI) available: Synthesis of
DCA; IR, UV, TG-DTA and XRPD and detailed crystallographic data of
1 and DCA. See DOI: 10.1039/b615675d
Fig. 1 (a) Preparation of 1. (b) The spindle [Zn7(OH)8]6+ clusters
showing the local coordination environments of ZnII. (c) The coordination
mode of the ligand DTA22. ZnII are shown in purple, O in red, N in blue,
and C in grey. (d) Artistic spindles (from www.mixedmediasculpture.com).
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symmetry operation ‘y 2 1/3, x + 1/3, 2z 2 1/6’, is tetrahedrally
coordinated by four hydroxy O atoms (Zn1–O1 1.959(4), Zn1–O2
˚
1.927(7) A); the Zn2 atom lies on a two-fold axis and three such
equatorial ZnII ions are related to each other by the symmetry
operations ‘1 2 y, 1 + x 2 y, z’ and ‘y 2 x, 1 2 x, z’, with each
˚
coordinating to two hydroxy O (Zn2–O1 1.925(4) A) and two
˚
tetrazolate N atoms (Zn2–N1 2.035(4) A). The remaining two axial
ZnII (Zn3) ions are also on a three-fold axis and are related to each
other by the symmetry operation ‘y, x, 2z 2 1/2’, and each Zn3
˚
ion is coordinated to one hydroxy O (Zn3–O2 1.886(7) A) and
˚
three tetrazolate N atoms (Zn3–N4 2.005(4) A). Eight hydroxyl
groups also perform two types of bridging modes with different
Zn–O–Zn angles and Zn–Zn distances: six bent (O1, on a normal
˚
position, 133.7(2)u and 3.569(3) A) and two quasi-linear (O2, on a
˚
three-fold axis, y180u and 3.813(2) A). The former is normal, but
for the latter, the flat disk-like thermal ellipsoid of the O2 atom
with large Ueq [101(4) nm2], along with the largest diff. peak
nearby reveals the bent characteristic of Zn–O2–Zn. The thermal
ellipsoid has its long axes (U11 and U22) perpendicular, while the
short one (U33) is parallel to the Zn–Zn vector (ratio U11/U33 ca.
8.4 (CIF file of ESI{)). This indicates that the bent Zn–O–Zn is
located randomly or rotates around the Zn–Zn vector. A similar
situation has been observed in a reported complex.8 Thus, eight
OH2 bond seven ZnII atoms to form a spindle cationic cluster with
higher symmetry, different from any previously reported Zn7
cluster.9 To the best of our knowledge, this unusual heptanuclear
cluster has never been observed either in the solid state or in
Fig. 2 (a) The capsule formed by six ligands bonding one [Zn7(OH)8]6+
.
(b) 3D structure of 1 viewing from the c direction. (c) 3D framework
structure with the anthracene groups omitted and each tetrazolate shown
as a green stick, ZnII is shown as purple, O as red. (d) Schematic
representation of the pcu network in 1 with ligands represented by purple
sticks and [Zn7(OH)8]6+ by blue spheres.
˚
˚
coordination chemistry. The cluster is 11.8 A in length and 5.6 A in
width at the spindle collar. Furthermore, each Zn7 cluster is
contacted to six DTA22 ligands by twelve Zn–N bonds, with each
ligand coordinating one equatorial (Zn2) and one axial ZnII (Zn3)
atoms through its two tetrazolate groups. The Zn7 cluster thus
resides in the trigonal-bipyramid capsule formed by the six DTA22
ligands (Fig. 2(a)). Each DTA22 ligand lies in an inversion center
with the dihedral angle between the tetrazolate plane and
anthracene ring being 78.6u, and coordinates to two Zn7 clusters
using its four N donors from two tetrazolate groups in a side-trans
bridging fashion. Notably, each tetrazolate ring is attached to two
ZnII ions via its N1 and N4 atoms (Fig. 1(c)), in an uncommon
‘‘opposite-on’’ coordination mode, although observed in some
complexes.5g,h This type of ligand usually adopts a N2 and N3
bridging mode in its metal complexes. The unusual coordination
mode in 1 is probably due to the bulky anthracene group between
two tetrazolate groups. This explanation is also likely in other
tetrazolate systems adopting this coordination mode, in which the
5-substituents are always bulky.5g,h Such a bridging coordination
mode of DTA22 in 1 is different from those in related DTB22
complexes,5d,j obviously owing to the different size of spacer
groups.
no need to consider it in the topological analysis, only as a linker.
On the basis of this simplification, the structure of 1 can be
described as a six-connected 3D network with a pcu-type (or
strictly, decorated pcu-type) topology (Fig. 2(d) and 3).6 From a
geometric stacking point of view, the coordinated Zn7 cluster has a
trigonal-biyramid configuration with the six DTA22 as edges
(Fig. 3). Thus, such trigonal-biyramid blocks accumulate by
sharing six edges with each other to form the 3D structure.
Encouraged by the single-crystal diffraction result, which reveals
the presence of heptanuclear OH-bridging ZnII clusters, being
further linked by bulky conjugated aromatic organic moieties in
the framework, optical diffuse reflectance, absorption and
photoluminescence spectra of 1 in the solid state were measured
at room temperature, to investigate its electric and optical
properties. The reflectance result shows the presence of an optical
gap, Eg # 2.9 eV (Fig. 4S, ESI{), which suggests that 1 may have
semiconductor property. However, no semicircles, but rather
scattered data points were observed in impedance spectra
measured between 220 and 220 uC, indicating that 1 is actually
an insulator with extremely large resistance (.109 ohm at r.t.)
These heptanuclear clusters, which behave as SBUs, are
interconnected through the tetrazolate groups of DTA22 cations
to generate an extended 3D framework (Fig. 2(b), (c)). A better
insight into the structure of 1 can be obtained by the standard
procedure of reducing multidimensional structures to simple node-
and-linker reference nets, known as the topological approach.10
Herein, each heptanuclear SBU in 1 is connected to six adjacent
SBUs through six DTA22 moieties along six directions (Fig. 2(a)),
therefore, each SBU can be defined as a six-connected node. As
DTA22 only acts as a bridging ligand (viz. double bridge), there is
Fig. 3 Schematic representation for the formation of the pcu net in 1.
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to room temp. over 24 h. Maple crystals were collected and washed with
water and acetone. Yield: y20% based on DCA. Anal. Calc. for
C48H34Zn7N24O9 (1548.58): C 37.23, H 2.21, N 21.71%; found: C 37.51,
H 2.02, N 21.95%.
§ Crystal data for 1: C48H34Zn7N24O9, Mr = 1548.58; trigonal; space group
3
¯
˚
˚
R3c; a = b = 13.934(2), c = 48.052(10) A; V = 8080(2) A ; Z = 6; Dc =
1.910 g cm23; T = 293 K; reflections collected/unique = 24129/2065; R1 =
0.0507, wR2 = 0.1109 (I . 2s(I)); R1 = 0.0785, wR2 = 0.1231 (all data) and
GOF = 1.044. CCDC 625275. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b615675d
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(Fig. 5S, ESI{). In the absorption spectra of 1, there are two clear
absorption peaks centered at 236 and 372 nm, corresponding to
the K- and B-band, respectively, which essentially matched the
absorptions of DCA (Fig. 6S, ESI{), although narrowed. Thus,
the characteristic B-band absorption of 1 should be assigned as the
intra-ligand p A p* transition in the anthracene ring of the ligand
DTA22. In addition, 1 exhibits intense blue photoluminescence
with an emission maximum at 483 nm upon excitation at 396 nm
(Fig. 4), which is different (and stronger) from that of DCA in
emission position (Fig. 7S, ESI{). This emission is assignable as a
pL* A pL transition since the heteroatoms of the heterocyclic
aromatic ligand will have decreased p and p* orbital energies (the
HOMO and LUMO, as well as orbitals with energies close to
these, may not be significantly contributed by the d10 metal atoms,
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enhancement of luminescence may be attributed to the ligand
chelation to the metal center, which effectively increases the rigidity
and asymmetry of the ligand and reduces the loss of energy by
radiationless decay. The lifetime of the 483 nm peak was measured
as 4.042 ns (Fig. 4), suggesting that this compound may be an
excellent candidate for potential photoactive materials.
In summary, we have obtained a 3D ZnII complex by using a
di-tetrazolate ligand with a bulky spacer. It shows a pcu-type
network topology with an unprecedented heptanuclear spindle
[Zn7(OH)8]6+ cluster as secondary building units, and has strong
luminescent emission with long lifetime. Further work is being
focused on using other tetrazole ligands to construct MOFs.
We thank Dr Xi Liu and Prof. Guo-Cong Guo for measuring
the optical diffuse reflectance, Dr Qiang Wu and Prof. Li-Juan
Zhao for measuring the luminescent lifetime, and Prof. Guang-She
Li for measuring the transport properties of 1. This work was
supported by the NSFC (No. 20225101, 20373028 and 20531040).
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Notes and references
{ Synthesis of {[Zn7(OH)8(DTA)3]?H2O}n (1): A mixture of DCA (23 mg,
0.1 mmol), NaN3 (26 mg, 0.4 mmol) and ZnCl2 (55 mg, 0.4 mmol) in 10 mL
of H2O–EtOH (2 : 1) was sealed in a Teflon-lined stainless autoclave and
heated to 160 uC for 12 h, held at this temperature for 36 h, and then cooled
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 1527–1529 | 1529