Luminescent microporous metal-metallosalen frameworks with the primitive cubic net

Article abstract of DOI:10.1039/c1dt11584g

Two microporous three-dimensional metal-metallosalen frameworks are constructed from a dicarboxyl-functionalized Schiff base ligand and their H ₂, CO₂ and CH₄ adsorption capacities and fluorescence quenching behavior to th

Full text of DOI:10.1039/c1dt11584g

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COMMUNICATION
Luminescent microporous metal–metallosalen frameworks with the primitive
cubic net†
Chengfeng Zhu, Weimin Xuan and Yong Cui*
Received 22nd August 2011, Accepted 4th October 2011
DOI: 10.1039/c1dt11584g
Two microporous three-dimensional metal–metallosalen
frameworks are constructed from a dicarboxyl-functional-
ized Schiff base ligand and their H2, CO2 and CH4 adsorp-
tion capacities and fluorescence quenching behavior to the
vapor of nitroaromatic compounds are studied.
Metal–organic frameworks (MOFs) have attracted growing inter-
est because of their intriguing architectures and topologies and
promising applications in diverse areas such as gas storage, cata-
lysis and separation.1 Diversity of organic ligands and metal
clusters such as dinuclear, trinuclear and tetranuclear and even
higher-nuclear clusters have been utilized as secondary building
blocks (SBUs) to make robust and porous MOFs.2 In particular,
combinations of 6-connected [Zn4(μ4-O)(O2CR)6] SBUs and 2-
connected linear dicarboxylate bridging ligands of different
lengths have produced a family of well-established isoreticular
MOFs, which exhibited tunable pore and/or channel sizes,
shapes and functionalities. Nevertheless, it remains a great chal-
lenge to further functionalize the Zn4O-based isoreticular porous
network materials.3
Metallosalen complexes [salen = N,N′-ethylenebis(salicylidene
aminato)] have potential applications in homogenous catalysis
and separation,4 but less attention has been given to assembling
metallosalens into crystalline infinite solids.5 Recently, we
reported the self-assembly of porous metal–organic crystalline
materials from pyridyl/carboxyl-functionalized metallosalen
compexes and showed their ability to selectively adsorb small
organic molecules.4,6 We report here the synthesis of a dicar-
boxyl-functionalized Schiff base ligand to build two 3D micro-
porous metal–metallosalen frameworks and their H2, CO2 and
CH4 adsorption capacities and fluorescence quenching behavior.
As outlined in Scheme 1, the dicarboxyl-functionalized salen
ligand 1,2-phenylendiamine-N,N′-bis(3-tert-butyl-5-(carboxyl)-
salicylide-ene (H4L) was synthesized in good yield by the 1 : 1
condensation of o-phenylendiamine with 3-tert-butyl-5-formyl-
4-hydroxybenzoic acid. The ligand was characterized by 1H
Scheme 1 Synthesis of H4L, 1 and 2.
and 13C NMR spectroscopies and ESI mass spectrometry
(Fig. S12†). Heating Zn(OAc)2·4H2O and H4L (2 : 1 molar ratio)
in dimethyl sulfoxide (DMSO) and water at 80 °C afforded
yellow block-like crystals [Zn4O(ZnL)3(DMSO)5]·3DMSO·
5H2O (1) in good yield.‡ The products are all stable in air and
insoluble in water and common organic solvents. The formula of
1 was determined by single-crystal X-ray diffraction studies,
elemental analysis, IR spectroscopy and thermogravimetric
analysis (TGA).
A single-crystal X-ray diffraction study performed on 1 reveals
a neutral 3D open metal–organic network with the primitive cubic
net.§ 1 crystallizes in the monoclinic space group P2(1)/n and the
asymmetric unit contains one formula unit. In the ZnL ligand,
each Zn center adopts a square-pyramidal geometry with the
equatorial plane occupied by the N2O2 donors of one L ligand
and the apical position by one oxygen atom of a DMSO molecule
(Fig. S1†). The Zn–N and Zn–O bond lengths range from 2.045
(6) to 2.123(6) Å and from 1.944(6) to 1.993(4) Å, respectively,
while the zinc centers are located 0.451, 0.437 and 0.433 Å above
the plane formed by N2O2 coordination toward the axial direc-
tion. Four Zn2+ ions form a tetranuclear Zn4O cluster around a
μ4-O atom (Fig. 1). Of the four metal ions, three adopt a distorted
tetrahedral geometry by coordinating to a μ4-O atom and three
carboxylate oxygen atoms from three ZnL metalloligands and the
fourth adopts a distorted octahedral geometry by coordinating to
a μ4-O atom, three carboxylate oxygen atoms and two DMSO
molecules. The Zn–O bond distances vary from 1.913(2) to 2.006
(2) Å, and the adjacent Zn⋯Zn distances range from 3.082 to
3.281 Å, which are in the usual range for similar clusters.3
School of Chemistry and Chemical Technology and State Key
Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,
Shanghai, 200240, China. E-mail: yongcui@sjtu.edu.cn; Fax: (+86) 21-
5474-1297
†Electronic supplementary information (ESI) available: Details of the
synthetic procedures, spectra data and crystallographic data. CCDC
reference numbers 787816 for 1 and 840526 for 2, respectively. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11584g
Each Zn4O cluster in 1, acting as a 6-connecting octahedral
building unit, is thus linked by six ZnL units and each ZnL unit
is linked to two Zn4O units to create a 3D structure and the top-
ology of 1 can be best described as a primitive cubic (α-Po) net
(Fig. 2a, 2b). The single network possesses a large parallelogram
channel with diagonal distances of ∼19.2 × 31.2 Å along the
3928 | Dalton Trans., 2012, 41, 3928–3932
This journal is © The Royal Society of Chemistry 2012

Fig. 2 Structure of 1 showing (a) the coordination geometry of the
tetranuclear Zn4O(CO2)6 building unit (H atoms are omitted for clarity);
(b) two interpenetrated primitive cubic nets, and space-filling represen-
tations of (c) a 1D channel of 5.1 × 3.9 Å; (d) schematic illustration of
the doubly interpenetrating frameworks in purple and green (the Zn4O
cluster is simplified as a node).
Fig. 1 View of the tetranuclear [Zn4O6(DMSO)2] (top) and dinuclear
Zn2O(CO2)4 (down) clusters in 1 and 2, respectively. Atoms labelled
gray, carbon; cerulean, zinc; red, oxygen; yellow, sulphur; blue,
nitrogen.
b-axis. Eight Zn4O clusters are engaged in a distorted porous
cube with a cavity dimension of 19.8 × 20.6 × 20.9 Å (Fig. 2b).
The overall structure of 1 is a pair of identical α-Po nets of
[Zn4O(ZnL)3], which are mutually interpenetrated with each
other to form a doubly interpenetrating 3D framework. The
pores of 1 along the b direction are fully filled with ZnL units
from another α-Po net, while the pores are significantly reduced
by double interpenetration of the 3D frameworks. Overall, there
exists a 1D channel of 5.1 × 3.9 Å which are filled with guest
molecules (Fig. 2c). Calculations using the PLATON program
indicate that 1 has 51.6% of total volume that is accessible for
solvent molecules.7
A solvothermal reaction of Zn(ClO4)2·6H2O, 4,4′-bipyridine
(bpy) and H4L (a 2 : 1 : 1 molar ratio) in a mixture of dimethyl
acetamide (DMA), water and triethylamine at 80 °C for 8 h
afforded yellow strip-like crystals [Zn2O(ZnL)2(bpy) (DMA)
(H2O)]·4DMA·4H2O (2).‡ The products are all stable in air and
insoluble in water and common organic solvents. The formula of
2 was determined by single-crystal X-ray diffraction studies,§
elemental analysis, IR and TGA.
Fig. 3 Structure of 2 showing (a) the coordination geometry of the
dinuclear Zn2O(CO2)4 building unit (H atoms are omitted for clarity);
(b) two interpenetrated primitive cubic nets, and space-filling represen-
tations of (c) a 1D channel of 13.4 × 12.1 Å; (d) schematic illustration of
the doubly interpenetrating frameworks in purple and green (center of
the dinuclear Zn2O(CO2)4 as node).
The structure of 2 is constructed of dinuclear Zn2(μ2-O)
(CO2)4 building blocks bridged by four ZnL units and bpy pillar
linkers to form a 3D framework of a primitive cubic net (Fig. 3a,
3b). 2 also crystallizes in the monoclinic space group P2(1)/n and
the asymmetric unit contains one formula unit. Each Zn center
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3928–3932 | 3929

in the two crystallographically independent ZnL units adopts a
similar square-pyramidal geometry to that in 1. In the dinuclear
Zn2O motif, the Zn1 adopts a distorted octahedron by coordi-
nation to a μ2-O atom, four carboxylate oxygen atoms from four
metallosalens and one nitrogen atom from bpy molecule, while
Zn2 adops a distorted tetrahedron by coordination to a μ2-O
atom, two carboxylate oxygen atoms from two metallosalens
and one nitrogen atom from the bpy molecule (Fig. 1). Thus, the
dinuclear Zn2O motif is linked by four ZnL units o form a 2D
sheet that are further pillared by 4,4′-bipyridine to create a 3D
structure with a primitive cubic net, which possesses a large par-
allelogram channel with diagonal distances of ∼23.0 × 31.2 Å
similar to 1 along the b-axis, but with shorter diagonal distances
of ∼21.3 × 27.6 Å along the a-axis due to the shorter bpy pillar
compared with the dicarboxylate ZnL unit. Although the overall
structure of 2 is a pair of identical α-Po nets of [Zn2O(ZnL)2-
(bpy)], which are mutually interpenetrated with each other to
form a doubly interpenetrating 3D framework (Fig. 3b), there
still exists a larger 1D channel of 13.4 × 12.1 Å along the b axis
(Fig. 3c). Calculations using the PLATON program indicate that
2 has 43.7% of the total volume that is accessible for solvent
molecules.
The phase purity of both 1 and 2 were supported by the
powder X-ray diffraction (XRD) pattern of the bulk sample,
which are consistent with the calculated patterns (Fig. S11†).
TGA show that 1 loses 11.3% of its total weight upon heating to
200 °C, which corresponds to the loss of five water and three
DMSO guest molecules per formula unit (calcd 11.8%) and the
framework is stable up to 400 °C (Fig. S9†). While 2 loses 8.4%
of its total weight upon heating to 175 °C, corresponding to four
waters and one DMA molecule per formula (calcd 21.4%). The
weight losses of 2 estimated from the compositions obtained
from the single crystal X-ray diffraction are different from those
obtained by TGA, which might be due to some of the adsorbed
guest molecules having already been washed by CH2Cl2 and
evaporated during the drying process before TGA analysis. The
TGA result shows that 2 is stable up to 425 °C. Powder XRD
experiments indicate that the evacuated frameworks of 1 and 2
retain crystallinity, although several diffraction peaks of 1 shifted
a little compared with the pristine sample owing to slightly struc-
tural distortion after evacuation.
The permanent porosity of 1 is confirmed by its N2 adsorption
isotherm at 77 K. After desolvation, 1 exhibits a Type-I sorption
behavior, with a BET surface area of 112 m2 g−1 (Fig. S15†).
Whereas 2 only exhibits surface adsorption with a BET surface
area of 10 m2 g−1. High-pressure gas adsorption measurements
were performed to investigate the H2, CH4 and CO2 uptake
capacities of 1 and 2 (Fig. 4). After activation at 100 °C under
vacuum, 1 can take up 1.75 wt% (195 cm3 g−1 STP) of hydro-
gen at 10 bar and 77 K, while 2 has a higher adsorption capacity
and can take up 2.7 wt% (302 cm3 g−1 STP) of hydrogen under
the same conditions. The hydrogen sorption profiles show a type
I isotherm curve with a steep uptake in the low-pressure region,
indicative of a strong affinity of H2 molecules toward pore sur-
faces.8 The H2 adsorption property is comparable to other MOFs
of moderate H2 adsorption.9 At 273 K and 10 bar, the adsorption
capacities of 1 for CO2 and CH4 are 4.4 wt% (22.6 cm3 g−1
STP) and 1.2 wt% (16.6 cm3 g−1 STP), respectively. And 2 can
also take up 9.6 wt% (49.0 cm3 g−1 STP) of CO2 and 1.9 wt%
Fig. 4 The H2, CO2 and CH4 adsorption isotherms of 1 and 2 (H2:
77 K; CO2: 273 K; CH4: 273 K; closed symbols, adsorption; open
symbols, desorption).
(25.9 cm3 g−1 STP) of CH4, respectively. The marked different
uptake capacities for H2, CH4 and CO2 indicate that they may
hold potential for gas separation.10
(Salen)Zn complexes have been shown to exhibit fluorescence
properties and accurately discriminated nitroaromatics.11 This
stimulates us to explore the utility of MOFs assembled from
salen(Zn) in sensoring nitroaromatics compounds that are highly
explosive and environmentally deleterious. Fluorescence spectra
were recorded on powder samples in thin-layer form, and it was
found that compounds 1 and 2 emit strongly at 525 nm and
535 nm, respectively, upon excitation at 420 nm. To explore the
applicability of the two frameworks for explosives detection, the
fluorescence spectra of thin-layers were monitored, before and
after exposing them to the vapors of nitroaromatics for varied
periods of time (see Fig. 5). As expected, all nitroaromatics act
as fluorescence quenchers for 1 and 2. Among them, the most
effective quencher is nitrobenzene (NB) compared with nitro-
toluene (NT) and m-nitrotoluene (mNT). NB exhibits the stron-
gest quenching effect not only because of its strongly electron-
withdrawing -NO2 group and the highest vapor pressure but also
the absence of the electron-donating -CH3 group.12 Within ten
seconds, NB quenches the emission by 44% and 67%, respect-
ively, for 1 and 2. After 4 min, the quenching percentages up to
58% and 75%, respectively. The quenching percentage (%) was
simply estimated using the formula (Io − I)/Io × 100%, where Io
is the original peak maximum intensity of the frameworks, I is
the maximum intensity after exposure to the analyte. The thin
3930 | Dalton Trans., 2012, 41, 3928–3932
This journal is © The Royal Society of Chemistry 2012

Notes and references
‡Preparation of 1: A mixture of Zn(OAc)2·4H2O (20.3 mg, 0.08 mmol)
and H4L (20.6 mg, 0.04 mmol) was placed in a small vial containing
DMSO (5 mL) and H2O (0.5 mL). The vial was sealed and heated at
80 °C for one day; yellow block-like crystals were collected, washed
with methanol and acetone and dried in air. Yield: 19.8 mg (73.9%
based on Zn). Elemental analysis (%) Calcd for C106H160N6O32S8Zn7: C
46.39, H 5.88, N 3.06, S 9.35. Found: C 46.25, H 5.84, N 3.03, S 9.40.
IR (KBr): 3412(w), 3001(w), 2947(m), 2909(w), 1606(s), 1583(s), 1520
(m), 1392(s), 1340(s), 1197(m), 1174(m), 1140 (m), 1021(w), 800(w),
790(w), 753(w), 726(w), 706(w), 542(w) cm−1. Preparation of 2: A
mixture of Zn(ClO4)2·6H2O (29.6 mg, 0.08 mmol), 4, 4′-bipyridine
(7.68 mg, 0.04 mmol) and H4L (20.6 mg, 0.04 mmol) was placed in a
small vial containing DMA (5 mL), H2O (0.5 mL) and Et3N(one drop).
The vial was sealed and heated at 80 °C for 8 h; yellow strip-like crystals
were collected, washed with dichloromethane and dried in air. Yield:
33.1 mg (85.0% based on Zn). Elemental analysis (%) Calcd for
C
90H128N11O21Zn4: C 55.11, H 6.58, N 7.85. Found: C 55.20, H 6.53,
N 7.82. IR (KBr): 3430(w), 3003(w), 2946(m), 2906(w), 1708(w), 1605
(s), 1584(s), 1523(m), 1437(m), 1390(s), 1333(s), 1196(m), 1174(m),
1133(m), 1109(m) 1020(w), 934(w), 860(w), 790(w), 751(w), 726(w),
703(w), 639(w) 544(w), 500(w) cm−1
.
§Crystallographic data for 1: M = 2401.86, monoclinic, space group
P2(1)/n, a = 22.286(2) Å, b = 19.845(2) Å, c = 34.290(4) Å, β = 90.968
(3)°, V = 15163(3) Å3, Z = 4, μ = 1.208 mm−1, Dc = 1.052 Mg m−3, F
(000) = 4952, 34348 unique (Rint = 0.1591), R1 = 0.0752, wR2 = 0.1597
[I > 2σ(I)], GOF = 1.028. The intensity data were collected on a Bruker
Smart Apex II CCD diffractometer with graphite-monochromated Mo-
Kα radiation (λ = 0.71073 Å) at 123 K. All absorption corrections were
performed by using the SADABS program. The structure was solved by
direct methods, and all non-H atoms were subjected to anisotropic
refinement by full-matrix program. Contributions to scattering due to
these highly disordered solvent molecules were removed using the
SQUEEZE routine of PLATON;7 structures were then refined again
using the data generated.
Fig. 5 Time-dependent fluorescence quenching by nitroaromatics
before and after exposing 1 (left) and 2 (right) to the vapors of nitro-
aromatics for varied periods of time.
Crystallographic data for 2: M = 1961.07, monoclinic, space group
P2(1)/n, a = 23.0013(6) Å, b = 14.0578(4) Å, c = 31.2380(8) Å, β =
90.881(2)°, V = 10099.5(5) Å3, Z = 4, μ = 1.648 mm−1, Dc = 1.277 Mg
m
−3, F(000) = 4056, 10141 unique (Rint = 0.0420), R1 = 0.1060, wR2 =
layer of 2 showed higher sensitivity toward nitroaromatics
vapors than 1 films again, ascribed to the distinguished channels
in 1 and 2. We postulate that the larger open channel in 2 is more
beneficial for the quenchers to diffuse into and to interact
between the analyte and ZnL units. Fluorescence quenching is
undergoing a mixed quenching pathway for ZnL. In the static
process (ground-state process), a nonfluorescent QZnL adduct is
forming in which the quencher binds to the ground-state of ZnL.
And the dynamic quenching (excited-state process) comes from
the quencher colliding with the excited state ZnL* through a
bimolecular electron transfer from the phenolate ring of ZnL* to
the nitroaromatics.13 However, no peak shift was observed in the
quenching process, this means that the nitroaromatics do not
interrupt the ligand-to-ligand charge transfers of the interpene-
trating nets as observed in the crystal dynamic MOFs.14
0.2829 [I > 2σ(I)], GOF = 0.993. The intensity data were collected on a
Bruker Smart Apex II CCD diffractometer with graphite-monochromated
Cu-Kα radiation (λ = 1.54178 Å) at 123 K. All absorption corrections
were performed by using the SADABS program. The structure was
solved by direct methods, and all non-H atoms were subjected to an-
isotropic refinement by full-matrix program.
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Acknowledgements
This work was supported by NSFC-20971085 and 21025103,
“973” Programs (2007CB209701 and 2009CB930403),
STCSM-10DJ1400100 and the key project of MOE.
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This journal is © The Royal Society of Chemistry 2012