Syntheses, structures, and photoluminescent properties of a series of metal-organic frameworks constructed by 5,5′-bis(1H-imidazol-1-yl)-2, 2′-bithiophene and various carboxylate ligands

Article abstract of DOI:10.1039/c3ce41966e

A series of new metal-organic frameworks (MOFs), namely, {[Co(BIBP)(hfipbb)]}_n (1), {[Co(BIBP)(Hbtc)]}_n (2), {[Zn(BIBP)(5-OH-bdc)]}_n (3), {[Cd(BIBP)(bdc)]}_n (4), {[Zn₂(BIBP)_0.5(hfipbb)₂]·H ₂O}_n (5), {[Zn₄(BIBP)₄(bdc) ₄]}_n (6), {[Zn(BIBP)(dl-ca)]·H₂O} _n (7), (BIBP = 5,5′-bis(1H-imidazol-1-yl)-2,2′- bithiophene, H₂hfipbb = 4,4′-(hexafluoroisopropylidene) bis(benzoic acid), H₃btc = 1,3,5-benzenetricarboxylic acid, H ₂bdc = isophthalic acid, 5-OH-H₂bdc = 5-hydroxyisophthalic acid, d-H₂ca = d-camphor acid), have been hydrothermally synthesized. These compounds were structurally characterized by IR spectroscopy, elemental analysis and X-ray single-crystal diffraction. Compound 1 exhibits a 2D → 3D framework with an unusual parallel polycatenation of corrugated 2D (4,4) nets. Compounds 2-4 are structurally similar and display 2D layer structures, in which 2 and 3 extend into 3D supramolecular frameworks through interlayer O-H...O hydrogen-bonding interactions. Compound 5 displays a 3D 2-fold interpenetrating framework with a rare {4⁴·6 ⁶} sqp topology, containing an interesting ...LRLR... double helical layer structure. Compound 6 is a 2D network with {4 ⁴·6²}-sql topology and further extends via C-H...O hydrogen bonds into the 3D supramolecular framework. Compound 7 possesses a rare 3D non-interpenetrated cds-type framework with the Schlaefli symbol {6⁵·8}. Thermal stabilities for 1-7 and photoluminescence properties of the compounds 3-7 have been examined in the solid state at room temperature. Furthermore, solid-state UV-vis spectroscopy experiments show that compounds 1-7 exhibit optical band gaps which are characteristic for optical semiconductors, with band gaps of 2.41, 2.44, 2.48, 2.46, 2.83, 2.68, and 2.72 eV, respectively.

Full text of DOI:10.1039/c3ce41966e

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Syntheses, structures, and photoluminescent
properties of a series of metal–organic
frameworks constructed by
Cite this: CrystEngComm, 2014, 16,
900
5,5′-bis(1H-imidazol-1-yl)-2,2′-bithiophene
and various carboxylate ligands†
*
Zhi-Qiang Shi, Yi-Zhi Li, Zi-Jian Guo and He-Gen Zheng
A series of new metal–organic frameworks (MOFs), namely, {[Co(BIBP)(hfipbb)]}n (1), {[Co(BIBP)(Hbtc)]}n (2),
{[Zn(BIBP)(5-OH-bdc)]}n (3), {[Cd(BIBP)(bdc)]}n (4), {[Zn2(BIBP)0.5(hfipbb)2]·H2O}n (5), {[Zn4(BIBP)4(bdc)4]}n
(6), {[Zn(BIBP)(DL-ca)]·H2O}n (7), (BIBP
= 5,5′-bis(1H-imidazol-1-yl)-2,2′-bithiophene, H2hfipbb =
4,4′-(hexafluoroisopropylidene)bis(benzoic acid), H3btc = 1,3,5-benzenetricarboxylic acid, H2bdc =
isophthalic acid, 5-OH-H2bdc = 5-hydroxyisophthalic acid, D-H2ca = D-camphor acid), have been
hydrothermally synthesized. These compounds were structurally characterized by IR spectroscopy,
elemental analysis and X-ray single-crystal diffraction. Compound 1 exhibits a 2D → 3D framework
with an unusual parallel polycatenation of corrugated 2D (4,4) nets. Compounds 2–4 are structurally
similar and display 2D layer structures, in which 2 and 3 extend into 3D supramolecular frameworks
through interlayer O–H⋯O hydrogen-bonding interactions. Compound 5 displays a 3D 2-fold inter-
penetrating framework with a rare {44·66} sqp topology, containing an interesting ⋯LRLR⋯ double
helical layer structure. Compound 6 is a 2D network with {44·62}-sql topology and further extends via
C–H⋯O hydrogen bonds into the 3D supramolecular framework. Compound 7 possesses a rare 3D
non-interpenetrated cds-type framework with the Schläfli symbol {65·8}. Thermal stabilities for 1–7 and
photoluminescence properties of the compounds 3–7 have been examined in the solid state at room
temperature. Furthermore, solid-state UV–vis spectroscopy experiments show that compounds 1–7
exhibit optical band gaps which are characteristic for optical semiconductors, with band gaps of 2.41,
2.44, 2.48, 2.46, 2.83, 2.68, and 2.72 eV, respectively.
Received 28th September 2013,
Accepted 29th October 2013
DOI: 10.1039/c3ce41966e
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influence of metal centers, organic ligands, the ratio of reagents,
temperature, reaction solvents, pH value, counterions, and
Introduction
In recent years, the rational design and synthesis of metal–organic
frameworks (MOFs) or porous coordination polymers (PCPs)
has been of considerable interest not only owing to their
intriguing diverse architectures,1 but also because of their
variety of potential applications in magnetism,2 gas storage
and separation,3 luminescence,4 catalysis,5 sensor,6 and so
forth. However, it is still a challenge to construct predicted
structures with desired properties because of the combined
so on.7 Therefore, the target MOFs with specific structures
and especial functions can be obtained by careful regulation
of the synthesis conditions and designation of the organic
ligands. It is worth noting that, in comparison with tradi-
tional properties of MOFs mentioned previously, this study
has received relatively little attention for the photoelectric
properties of MOFs.8 So, MOFs possessing photoelectric
properties have high theoretical and potential practical
value. In our previous work,9 we adopted 5,5′-bis(4-pyridyl)-
2,2′-bithiophene (BPBP) which contains a conjugated electron-
donating bithiophene group combined with dicarboxylate
ligands and successfully obtained six complexes with diverse
interesting structures and topologies as well as semicon-
ductor properties. Taking inspiration from previous work,
another conjugated electron-donating bithiophene ligand
5,5′-bis(1H-imidazol-1-yl)-2,2′-bithiophene (BIBP) was selected
to construct a coordination architecture. Compared with BPBP,
State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing National Laboratory of Microstructures,
Nanjing University, Nanjing 210093, PR China. E-mail: zhenghg@nju.edu.cn;
Fax: +86 25 83314502
† Electronic supplementary information (ESI) available: Additional figures, FT-IR
spectra, TGA curves, UV–vis absorption spectra, patterns of photochemistry,
selected bond distances and hydrogen bond parameters for compounds 1–7.
CCDC 963191–963197. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ce41966e
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at room temperature using BaSO4 as a standard with 100%
reflectance.
Syntheses of the compounds 1–7
Synthesis of {[Co(BIBP)(hfipbb)]}n (1).
A mixture of
Co(NO3)2·6H2O (14.6 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (19.6 mg,
0.05 mmol), DMF (1 mL) and H2O (5 mL) was placed in a
Parr Teflon-lined stainless steel vessel (15 mL) under autog-
enous pressure and heated at 85 °C for 3 days. Purple block
crystals of 7.1 mg (yield of 19% based on Co) were collected.
Anal. calcd. for C31H18CoF6N4O4S2: C, 49.81%; H, 2.43%;
N, 7.49%. Found: C, 49.77%; H, 2.48%; N, 7.54%. IR
(KBr, cm−1): 3443(w), 1599(s), 1556(vs), 1522(w), 1499(w),
1381(s), 1293(s), 1255(s), 1172(s), 1067(w), 970(w), 851(w),
782(m), 748(m), 459(w).
Scheme 1 N-containing BIBP ligand and carboxylate ligands.
the prominent characteristic of BIBP ligand is that the nitrogen
coordination atoms can adjust their coordination orientation
through rotation of C–N single bonds between thiophene and
imidazole rings.10 So the BIBP has a more flexible coordination
fashion. Up to now, only one MOFs containing BIBP have been
reported.11 But, the mixed-ligand MOFs containing BIBP have
not been reported. With the aforementioned consideration,
our recent study has been mainly focused on the assemblage
BIBP to react with divalent transitional-metal ions in the
presence of various carboxylate ligands to get new MOFs
with rich structural characteristics and encouraging photo-
luminescence properties.
Herein, we adopted five carboxylate ligands with different
rigidity, namely 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)
(H2hfipbb),12 1,3,5-benzenetricarboxylic acid (H3btc),13 isophthalic
acid(H2bdc),14 5-hydroxyisophthalic acid (5-OH-H2bdc),15
and D-camphor acid (D-H2ca)16 (Scheme 1), as co-ligands to
react with the BIBP ligand and different bivalent metal salts.
Then, seven new MOFs were obtained under hydrothermal
conditions, namely {[Co(BIBP)(hfipbb)]}n (1), {[Co(BIBP)(Hbtc)]}n
(2), {[Zn(BIBP)(5-OH-bdc)]}n (3), {[Cd(BIBP)(bdc)]}n (4),
{[Zn2(BIBP)0.5(hfipbb)2]·H2O}n (5), {[Zn4(BIBP)4(bdc)4]}n (6),
{[Zn(BIBP)(DL-ca)]·H2O}n (7). Their syntheses, crystal struc-
tures, optical band gaps, photoluminescent properties, and
thermal stabilities are reported in this article.
Synthesis of {[Co(BIBP)(Hbtc)]}n (2).
A
mixture of
Co(NO3)2·6H2O (14.6 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
1,3,5-benzenetricarboxylic acid (10.5 mg, 0.05 mmol), CH3CN
(4.5 mL) and H2O (1.5 mL) was placed in a Parr Teflon-lined
stainless steel vessel (15 mL) under autogenous pressure and
heated at 105 °C for 3 days. Purple block crystals of 6.8 mg
(yield of 24% based on Co) were collected. Anal. calcd. for
C23H14CoN4O6S2: C, 48.85%; H, 2.50%; N, 9.91%. Found:
C, 48.81%; H, 2.55%; N, 9.96%. IR (KBr, cm−1): 3441(w),
1722(s), 1622(vs), 1590(m), 1556(s), 1490(w), 1455(m), 1383(s),
1235(m), 1181(m), 1040(m), 935(w), 829(w), 791(w), 649(w).
Synthesis of {[Zn(BIBP)(5-OH-bdc)]}n (3). A mixture of
Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
5-hydroxyisophthalic acid (10.5 mg, 0.05 mmol), CH3CN
(1.5 mL) and H2O (4.5 mL) was placed in a Parr Teflon-lined
stainless steel vessel (15 mL) under autogenous pressure
and heated at 90 °C for 3 days. Yellow block crystals of
6.8 mg (yield of 25% based on Zn) were collected. Anal.
calcd. for C22H14N4O5S2Zn: C, 48.58%; H, 2.59%; N, 10.30%.
Found: C, 48.52%; H, 2.64%; N, 10.34%. IR (KBr, cm−1):
3450(w), 1621(m), 1573(vs), 1520(w), 1496(w), 1417(s), 1381(s),
1294(w), 1278(w), 1238(w), 1205(w), 1039(w), 983(m), 789(m),
731(w), 502(w).
Synthesis of {[Cd(BIBP)(bdc)]}n (4).
A
mixture of
Experimental section
Materials and general methods
Cd(NO3)2·4H2O (15.4 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
isophthalic acid (10.5 mg, 0.05 mmol), DMF (1.5 mL) and
H2O (4.5 mL) was placed in a Parr Teflon-lined stainless
steel vessel (15 mL) under autogenous pressure and heated
at 85 °C for 3 days. Yellow block crystals of 8.6 mg (yield
of 30% based on Cd) were collected. Anal. calcd. for
BIBP ligand was synthesized as reported in the literature,11
and other reagents and solvents were of analytical grade and
used without further purification. Elemental analyses for C,
H and N were conducted on a Perkin Elmer 240C elemental
analyzer. The FT-IR spectra were recorded on a Nicolet Impact
410 spectrometer with KBr pellets in the range of 4000–400 cm−1.
Thermogravimetric analysis (TGA) was performed on a
Perkin Elmer thermogravimetric analyzer under a nitrogen
atmosphere with a heating rate of 10 °C min−1. Solid-state
luminescent spectra were recorded with a SHIMADZU VF-320
X-ray fluorescence spectrophotometer at room temperature.
C
22H14CdN4O4S2: C, 45.96%; H, 2.45%; N, 9.75%. Found:
C, 45.91%; H, 2.51%; N, 9.79%. IR (KBr, cm−1): 3442(w),
1609(vs), 1557(vs), 1519(s), 1493(m), 1431(w), 1374(vs), 1295(w),
1293(w), 1238(w), 1116(w), 1070(w), 1037(s), 932(m), 817(w),
787(s), 763(s), 643(w).
Synthesis of {[Zn2(BIBP)0.5(hfipbb)2]·H2O}n (5). A mixture
of Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol), BIBP (7.45 mg,
0.025 mmol), 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)
(39.2 mg, 0.1 mmol), CH3CN (3 mL) and H2O (3 mL) was placed
UV–vis diffuse reflectance spectra were measured on
Shimadzu UV-3600 double monochromator spectrophotometer
a
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in a Parr Teflon-lined stainless steel vessel (15 mL) under
autogenous pressure and heated at 120 °C for 3 days. Yellow
block crystals of 8.6 mg (yield of 16% based on Zn) were
collected. Anal. calcd. for C41H23F12N2O9SZn2: C, 45.66%;
H, 2.15%; N, 2.60%. Found: C, 45.61%; H, 2.19%; N, 2.65%.
IR (KBr, cm−1): 3451(w), 1609(s), 1557(s), 1523(w), 1382(s),
1293(w), 1255(w), 1213(m), 1172(s), 1067(w), 945(w), 928(w),
846(w), 726(m), 512(w).
of 1–7 are listed in Table 1. The selected bond lengths and
angles and hydrogen bond parameters are given in Tables
S1 and S2 (ESI†). CCDC numbers are 963191–963197 for 1–7,
respectively. Topological analyses were performed using the
program package TOPOS.20
Description of the crystal structures
Synthesis of {[Zn4(BIBP)4(bdc)4]}n (6).
A
mixture of
{[Co(BIBP)(hfipbb)]}n (1). Single crystal X-ray analysis reveals
that compound 1 is a parallel polycatenated 2D + 2D → 3D
framework based on an undulated 44·62-sql square layer. It
Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
isophthalic acid (10.5 mg, 0.05 mmol), DMA (3 mL) and
H2O (3 mL) was placed in a Parr Teflon-lined stainless steel
vessel (15 mL) under autogenous pressure and heated at
100 °C for 3 days. Yellow needle crystals of 5.8 mg (yield
of 22% based on Zn) were collected. Anal. calcd. for
¯
crystallizes in the triclinic crystal system, space group P1.
A unique portion of the structure 1 consists of one Co(II)
cation, two halves of a BIBP ligand and one deprotonated
4,4′-(hexafluoroisopropylidene)bis(benzoic acid). As depicted
in Fig. 1a, the Co(II) cation adopts a distorted tetrahedral
geometry defined by two oxygen atoms [Co–O = 1.908(7)–
1.989(2) Å] from two symmetrical hfipbb ligands and two
N atoms [Co–N = 2.036(3)–2.049(3) Å] from two BIBP ligands.
The Co–O/N bond lengths are all normal. As shown in Fig. 1b,
the BIBP ligands link Co(II) cations to form one infinite
1D zig-zag chain with the Co⋯Co distance of 16.090 Å and
Co–Co–Co angle of 127.78°. Then, the other infinite 1D
linear chain with the Co⋯Co distance of 14.509 Å and
Co–Co–Co angle of 180.00° is also generated through the
coordination between the carboxylate groups and the Co(II)
cations. Finally, these two kinds of chains are cross-linked
by sharing the Co(II) cations into a highly undulating 2D
network. In this network, each 54-membered macrocycle is
formed by four Co(II) cations, two BIBP ligands, and two
hfipbb ligands with a dimension of 26.442 Å × 15.604 Å
(diagonal lengths). From a topological viewpoint, the Co(II)
cations can be considered as 4-connected nodes, and hfipbb
and BIBP ligands are considered as linkers; thus, the
2D network can be simplified to a puckered sql network,
which contains a window of 14.509 Å × 16.090 Å (Fig. 1c).
Such large windows of the net and high undulation of the
single sql sheet allows two nearest neighbouring sheets
to penetrate each other in a parallel mode leading to a 3D
polycatenated network (Fig. 1d). For interpenetrated 2D net-
works, the 2D → 2D parallel interpenetration or 2D → 3D
inclined polycatenation are common examples,21 but the 2D → 3D
parallel polycatenation is relatively rare. As far as we know,
only a limited number of examples of 2D → 3D polycatenation
structures have been reported in a parallel mode.22
C
88H54N16O16S8Zn4: C, 50.05%; H, 2.67%; N, 10.61%. Found:
C, 50.01%; H, 2.73%; N, 10.66%. IR (KBr, cm−1): 3451(w),
1619(vs), 1558(s), 1529(m), 1505(w), 1381(m), 1344(s), 1301(w),
1248(w), 1110(w), 1068(w), 1041(w), 947(w), 788(w), 762(w),
642(w).
Synthesis of {[Zn(BIBP)(DL-ca)]·H2O}n (7). A mixture of
Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), BIBP (14.9 mg, 0.05 mmol),
D-camphor acid (10.0 mg, 0.05 mmol), DMF (1.5 mL) and
H2O (4.5 mL) was placed in a Parr Teflon-lined stainless
steel vessel (15 mL) under autogenous pressure and heated
at 95 °C for 3 days. Yellow block crystals of 3.8 mg (yield
of 13% based on Zn) were collected. Anal. calcd. for
C
24H26N4O5S2Zn: C, 49.70%; H, 4.52%; N, 9.66%. Found:
C, 49.66%; H, 4.57%; N, 9.71%. IR (KBr, cm−1): 3444(w),
1597(s), 1536(vs), 1461(m), 1412(s), 1373(m), 1324(w), 1297(w),
1177(w), 1130(w), 1052(w), 1006(w), 810(w), 502(w).
Single crystal structure determination and refinement
Diffraction measurements were performed at room tempe-
rature on a Bruker Smart Apex II CCD diffractometer using
Mo Kα radiation (λ = 0.71073 Å). The structures were solved
by direct methods and refined by full-matrix least-squares
procedures based on F2 using the SHELXTL17 program. All
non-hydrogen atoms were located from the trial structures
and then refined anisotropically. The hydrogen atoms were
placed at the calculation positions and allowed to ride on the
parent atoms. The distribution of peaks in the channels of 5
and 7 were chemically featureless to refine using conven-
tional discrete-atom models. To resolve this issue, the contri-
bution of the electron density by the remaining water
molecules in 5 and 7 were removed by the SQUEEZE routine
in PLATON.18 The number of solvent molecules were
obtained by elemental analyses. A semiempirical absorption
correction was applied using the SADABS program.19 In
compound 1, the phenylene and carboxylate groups are dis-
ordered over two general positions with the site occupancy
of 0.5 for C9–C15/C9′–C15′, O3/O3′ and O4/O4′. In compound
7, the disordered C atom of camphorate ligand was refined
isotropically using C atom split over two sites C20–C21 with
a site occupancy of 0.50. The detailed crystallographic data
{[Co(BIBP)(Hbtc)]}n (2), {[Zn(BIBP)(5-OH-bdc)]}n (3) and
{[Cd(BIBP)(bdc)]}n (4). The structures of 2–4 are very similar,
therefore, only the structure of 2 is described here in detail.
The structures of 3 and 4 are shown in Fig. S8 and S9 (ESI†).
A single crystal X-ray structural study shows that 2 crystallizes
¯
in the triclinic crystal system with P1 space group. The
unique portion of structure 2 consists of one crystallographi-
cally independent Co(II) cation, one BIBP ligand and one partial
deprotonated Hbtc2− ligand. The two deprotonated carboxylate
groups adopt different coordination modes, one carboxylate
group takes the bismonodentate coordination mode to
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sixteen-membered rings. Then, such chains are further linked
by BIBP ligands to form a 2D network (Fig. 2c). If the SBUs
[Co2(CO2)2] are considered as 4-connected nodes, and all crys-
tallographically independent ligands are considered as linkers;
thus, the 2D network can be simplified to an sql net with
Schläfli symbol {44·62}. Furthermore, the adjacent 2D networks
are packed parallel in a ⋯AAAA⋯ fashion, as depicted in
Fig. 2d. It is noted that protonated carboxylate oxygen O3 and
coordinated carboxylate oxygen O2 between these adjacent
2D networks could interact with each other and form strong
O3–H3A⋯O2#4 (symmetry code: #4 = 2 − x, 1 − y, 1 − z) hydrogen
bonds (Table S2, ESI†), which further link the 2D networks
into a 3D supramolecular architecture (Fig. 2e).
Fig. 1 (a) Coordination environment of the Co(II) cation in 1 (30%
probability displacement ellipsoids). The hydrogen atoms are omitted
for clarity. Symmetry codes: #1 = x, y, −1 + z; #2 = 3 − x, 2 − y, −z;
#3 = 2 − x, −y, 1 − z. (b) Left: an infinite 1D zig-zag chain formed by
Co(II) cations and BIBP ligands. Middle: an infinite 1D chain generated
through the carboxylate groups of hfipbb ligands and the Co(II) cations.
Right: undulating 2D network of 1. (c) View of the single puckered sql
network of 1. (d) Schematic representation of the 3D framework of
two interpenetrating 2D frameworks with parallel polycatenation
structure of 1.
{[Zn2(BIBP)0.5(hfipbb)2]·H2O}n (5). X-ray analysis reveals
that compound 5 crystallizes in the monoclinic space group
P2/c. As shown in Fig. 3a, the unique portion of structure 5
consists of two crystallographically independent Zn(II) cations,
half of a BIBP ligand and two completely deprotonated
4,4′-(hexafluoroisopropylidene)bis(benzoic acids). Both Zn(II)
cations are four-coordinated with distorted tetrahedral
geometry, but their environments are different. Zn1 is coor-
dinated by three bidentate carboxylate oxygen atoms from
three hfipbb ligands and one nitrogen atom from the BIBP
ligand, while Zn2 is coordinated by three bidentate carboxylate
oxygen atoms from three hfipbb ligands and one monodentate
carboxylate oxygen atom from one hfipbb ligand. The Zn–N
and Zn–O bond lengths fall in the normal range of 1.988(2)
and 1.902(2)–1.953(3) Å, respectively. In compound 5, the
left- and right-handed helical chains are formed through
the coordination between the hfipbb ligands and the Zn(II)
cations (Fig. 3d). These adjacent single-handed helical chains
are further interconnected into a 2D chiral layer by sharing
the Zn(II) cations (Fig. 3c). The most interesting is that the two
types of chiral layers adopt a rare arrangement23 by alternate
bridging by the BIBP ligands to generate a 3D framework
(Fig. 3b) with tubular channels along the b axis. Therefore, the
amounts of left- and right-helices are equal in compound 5,
giving a racemic structure. Topologically, each dinuclear Zn(II)
unit can be considered as a 5-connected node, so the resulting
structure can be regarded as a 5-connected sqp network with
point symbol {44·66}, which is different from the recent publica-
tion revealing a rare example of uninodal 5-coordinated net
with a point Schläfli symbol {44·66} (three-letter notation noz).24
To the best of our knowledge, only a few MOFs with sqp
topology especially constructed by mixed organic ligands
have been reported.25 Furthermore, open void space within
the single 3D framework is minimized by the inter-
penetration of another identical 3D framework to maintain
the stability of the whole structure, which leads directly to
the formation of a 2-fold interpenetrated final framework
(Fig. 3e). PLATON analysis gives the free void volume ratio
of 15.0% in compound 5.
bridge two Co centers while the other carboxylate group
adopts chelating in a bidentate mode. As illustrated in Fig. 2a,
the Co(II) center is six-coordinated by four carboxylic oxygen
atoms from three symmetrical Hbtc2− ligands at the equatorial
positions and two nitrogen atoms from two symmetrical BIBP
ligands at the axial position. This gives a slightly distorted
{CoN2O4} octahedral coordination geometry. The Co–N bond
lengths are in the range of 2.1187(15) Å–2.1423(15) Å, and the
Co–O bond lengths are 2.0303(12)–2.2257(12) Å. As shown
in Fig. 2b, pairs of symmetry-related Hbtc2− ligands adopt a
bridging mode joining adjacent Co(II) cations to form an
infinite 1D double chain, which contains eight-membered and
Fig. 2 (a) Coordination environment of the Co(II) cation in 2 (30%
probability displacement ellipsoids). The hydrogen atoms are omitted
for clarity. Symmetry codes: #1 = x, 1 + y, z; #2 = 2 − x, 1 − y, −z;
#3 = 1 + x, −1 + y, −1 + z. (b) 1D double chain formed by Co(II) cations
and HBTC2− ligands. (c) Polyhedral view of the 2D coordination network
of 2. (d) Packing diagram showing the ⋯AAAA⋯ type offset stacks of
2D sheets in 2. (e) Schematic representation the 3D supramolecular
framework of 2 via the hydrogen bonds (green dashed line).
{[Zn4(BIBP)4(bdc)4]}n (6). Compound 6 crystallizes in the
monoclinic crystal system, space group P21/c and exhibits a
2D layer structure. The unique portion of structure 6 consists
of four unique Zn(II) cations, four BIBP ligands and four
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Fig. 4 (a) Coordination environment of the Zn(II) cation in 6 (30%
probability displacement ellipsoids). The hydrogen atoms are omitted
for clarity. Symmetry codes: #1 = 1 + x, y, z; #2 = 1 + x, y, 1 + z.
(b) Polyhedral view of the 2D coordination network of 6 along the
a axis. (c) View of 3D supramolecular framework of 6 incorporating
hydrogen bonds (purple dashed line).
Fig. 3 (a) Coordination environment of the Zn(II) cations in 5. The
hydrogen atoms are omitted for clarity (30% ellipsoid probability).
Symmetry codes: #1 = 1 − x, 1 + y, 1.5 − z; #2 = −x, −1 + y, 1.5 − z;
#3 = 1 − x, −2 − y, 2 − z. (b) View of 3D single framework of 5. (c) A
perspective of the left-handed and right-handed layers in 5. (d) Space-
filling diagram of the helical chains in the 2D helical layers. (e) Schematic
representation of the 2-fold interpenetrated sqp topology of 5.
codes: #3 = 1 − x, 1 − y, −z; #4 = 1 − x, 1/2 + y, 1/2 − z) (Table S2,
ESI†) as illustrated in Fig. 4c.
{[Zn(BIBP)(DL-ca)]·H2O}n (7). Compound 7 exhibits a rare
3D non-interpenetrated framework with cds topology. Single
crystal X-ray analysis reveals that compound 7 crystallizes in
the monoclinic crystal system, space group P21/c. The unique
portion of the structure consists of one independent Zn(II)
cation, one deprotonated camphor acid and two halves of a
BIBP ligand. It is noted that compound 7 is a racemate,
although the chiral ligand of D-H2Ca was used as starting
material. So, the in situ racemisation of the D-H2Ca was
caused by the hydrothermal conditions during the synthesis
of compound 7.26 As shown in Fig. 5a, the Zn(II) cation is
located in a distorted tetrahedral geometry {ZnN2O2}, com-
pleted by two N atoms belonging to two BIBP ligands and
two O atoms from two DL-ca2− ligands. The Zn–N and Zn–O
bond lengths are in the ranges of 2.032(4)–2.041(3) and
1.966(3)–1.980(3) Å. The bond angles around the Zn(II) cation
range from 95.18(13)° to 127.40(12)°. In 7, the ca2− ligands
bridge the adjacent Zn(II) cations in a monodentate coordina-
tion mode to give rise to a 1D infinite chain (Zn(II)–DL-ca2−
chain) with Zn⋯Zn separation of 9.328 Å (Fig. 5b). The 1D
zig-zag chain (Zn(II)–BIBP chain) is also formed by the BIBP
deprotonated isophthalic acids (Fig. 4a). The four Zn(II)
cations display two different coordination geometries. As
shown in Fig. 4a, Zn1, Zn2, and Zn4 are in a distorted {ZnN2O2}
tetrahedron derived from two nitrogen atoms of two BIBP
ligands and two carboxylate oxygen atoms from two bdc2−
ligands, while Zn3 cation adopts a typical five-coordinated
{ZnN2O3} square-pyramidal geometry with two nitrogen
atoms of two BIBP ligands and three carboxylate oxygen
atoms from two bdc2− ligands. The Zn–O lengths are in the
range of 1.964(3)–2.337(3) Å, while the Zn–N bond lengths
are in the range of 2.002(3)–2.089(2) Å, which are in the normal
ranges. It is noticed that the four deprotonated isophthalic
acids adopt different coordination modes. One adopts bis-
monodentate mode, while the other three show monodentate
modes. The single 2D layer (Fig. 4b) could be simplified to a
highly undulated {44·62}-sql network. In addition, the adjacent
2D layers are further extended into an intriguing 3D supra-
molecular framework through weak intermolecular hydrogen
bonds C12–H12⋯O5#3 and C(17)–H(17)⋯O(3)#4 (symmetry
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and 224 nm (D-H2ca) are shown in Fig. S10a,† which can be
ascribed to π → π* and n → π* transitions of the ligands, respec-
tively. The absorption peaks at 222, 325, 592 nm (compound 1),
219, 328, 521 nm (compound 2), 210, 327 nm (compound 3),
214, 331 nm (compound 4), 215, 338 nm (compound 5),
226, 332 nm (compound 6), and 213, 335 nm (compound 7)
are also observed in the respective spectra. In all the spectra,
the lower energy bands are assigned as d–d transitions of
Co(II) cations28 (1 and 2) and ligand-to-metal charge transfer
transitions29 of the other five Zn(II) and Cd(II) compounds,
and the higher energy bands are considered as intraligand
transitions. To investigate the semiconductivity, the diffuse
reflectance data is transformed into the Kubelka–Munk func-
tion to obtain their band gaps (Eg). As shown in Fig. S10b,†
the band gap of BIBP is approximately 2.55 eV, which exhibits
a semiconductive nature. The Eg values assessed from the
steep absorption edge are 2.41 eV for , 2.44 eV for 2, 2.48 eV
for 3, 2.46 eV for , 2.83 eV for 5, 2.68 eV for 6, and 2.72 eV
for 7. So, these compounds exhibit optical band gaps which
are characteristic for optical semiconductors.
Luminescent properties
Luminescent MOFs are of great interest due to their various
applications in photochemistry, chemical sensors and light-
emitting diodes. Therefore, the solid-state luminescence of
free ligands BIBP, H2hfipbb, H2bdc, 5-OH-H2bdc, as well as
compounds 3–7 are investigated at room temperature, as
depicted in Fig. S1 and Table S3 (ESI†). The main emission
peaks of the BIBP, H2hfipbb, H2bdc, and 5-OH-H2bdc were
observed at 496 nm (λex = 354 nm), 325 nm (λex = 290 nm),
380 nm (λex = 276 nm), and 368 nm (λex = 289 nm), which
can be assigned to the π* → n or π* → π transitions.30 The
emission peaks are observed at 508 nm (λex = 394 nm) in 3,
513 nm (λex = 397 nm) in 4, 526 nm (λex = 392 nm) in 5,
524 nm (λex = 393 nm) in 6, and 485 nm (λex = 390 nm) in 7.
Compounds 3–7 exhibit their emission characters similar to
that of the free BIBP ligand. Therefore, the emission bands
of the compounds 3–7 can be mainly attributed to the intra-
ligand (BIBP) emissions. Compared to the free BIBP ligand,
the emission band of 3, 4, 5 and 6 is 12 nm, 17 nm, 30 nm,
and 28 nm, blue-shifted and the emission band of 7 is 11 nm
red-shifted, respectively.
Fig. 5 (a) Coordination environment of the Zn(II) cations in 7. The
hydrogen atoms are omitted for clarity (30% ellipsoid probability).
Symmetry codes: #1 = −1 + x, 1.5 − y, −0.5 + z; #2 = −x, 1 − y, −z;
#3 = 1 − x, 2 − y, 1 − z. (b) Left: view of the chain formed by Zn(II)
cations and D-ca2− ligands. Middle: view of the zig-zag chain formed
by Zn(II) cations and BIBP ligands. Right: view of 3D framework of 7.
(c) Schematic view of the cds topology of structure 7.
ligands and Zn(II) cations with a Zn⋯Zn distance of 16.328 Å
(Fig. 5b). The combination of 1D chains of Zn(II)–DL-ca2− and
Zn(II)–BIBP generates the 3D structure of 7 (Fig. 5b). The
desolvated framework shows 16.2% void space to the total
crystal volume as calculated by PLATON. A better insight
into the nature of this intricate framework is provided by a
topology analysis. From the viewpoint of structural topology,
the Zn(II) cations can be regarded as 4-connected nodes with
all crystallographically independent BIBP ligands and DL-ca2−
as linkers; thus, the 3D framework of 7 can be simplified as
an uninodal 4-connected network with CdSO4 (cds) topology
(with the Schläfli symbol {65·8}), as displayed in Fig. 5c. It is
noteworthy that almost all cds-type MOFs are interpenetrated
due to the self-duality of the cds net,27 compound 7 is a rare
example of a non-interpenetrated cds-type framework.
Thermal analysis
Thermogravimetric analysis (TGA) was conducted to investi-
gate the thermal stability of compounds 1–7 (Fig. S12, ESI†).
Compounds 5 and 7 show a weight loss of 1.58% in 158–202 °C
and 2.95% in 162–220 °C assigned to the release of lattice
water molecules (calcd. 1.67% for 5 and 3.10% for 7). The
remaining frameworks are thermally stable up to 394 and
452 °C, respectively. After that, the organic components
start to decompose. Compounds 1–4 and 6 exhibit similar
thermal behaviors. They do not contain guest molecules, so
there is no obvious weight loss before 392, 360, 371, 364,
Optical absorption properties
The solid-state UV–vis spectra of the free organic ligands and
compounds 1–7 are displayed in Fig. S10 (ESI†). The absorp-
tion peaks at 230, 320 nm (BIBP), 228, 283 nm (H2hfipbb),
225, 301 nm (H3btc), 308 nm (5-OH-H2bdc), 292 nm (H2bdc),
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and 369 °C for 1–4 and 6, respectively. Then the frameworks
begin to collapse, accompanying the loss of organic ligands.
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Conclusions
In summary, seven new coordination polymers have been suc-
cessfully synthesized under hydrothermal conditions by the
reaction of BIBP and a series of carboxylate ligands together
with metal salts. Compound 1 features a fascinating 2D +
2D → 3D framework with parallel polycatenation of undulated
44·62-sql layers. Compounds 2–4 display 2D layer structures
with {44·62}-sql topology. Compound 5 exhibits 2-fold inter-
penetrating 3D frameworks with rare {44·66} sqp topology,
containing a fascinating ⋯LRLR⋯ double helical layer struc-
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and further extended into a 3D supramolecular framework via
C–H⋯O hydrogen bonds. Compound possesses 7 a rare 3D
non-interpenetrated cds-type framework with {65·8} topology.
Thermal stabilities for 1–7 and photoluminescence properties
of compounds 3–7 have been investigated. Furthermore, the
UV–vis absorption spectra and optical energy gaps of 1–7
imply that these compounds are potential wide gap semi-
conductor materials.
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Acknowledgements
We gratefully acknowledge the financial support by the Natural
Science Foundation of China (21371092; 20971065; 91022011;
21021062), and National Basic Research Program of China
(2010CB923303).
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