PAPER www.rsc.org/crystengcomm | CrystEngComm
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Synthesis and structural comparisons of five new fluorinated metal organicframeworks (F-MOFs)†
Pradip Pachfule, Chandan Dey, Tamas Panda and Rahul Banerjee*
Received 25th August 2009, Accepted 8th December 2009
First published as an Advance Article on the web 14th January 2010
DOI: 10.1039/b917483d
A series of fluorinated metal–organic frameworks (F-MOFs) have been successfully synthesized under
hydrothermal condition using 4,40-(hexafluoroisopropylidene) bis(benzoic acid) (C17H10F6O4,
H2hfbba) as a flexible dicarboxylate building block, Cu+2 and various heterocyclic co-ligands. These
F-MOFs formulated as [Cu(hfbba)(phen)2]$2(H2hfbba)(H2O)(HCO2) (F-MOF-1), [Cu(hfbba)2(2,20-
bipy)2(H2O)] (F-MOF-2), [Cu(hfbba)(4,40dime-2,20-bipy)(HCO2)]$(hfbba)(H2O) (F-MOF-3),
[Cu2(hfbba)2(3-mepy)2]$(DMF)2(3-mepy) (F-MOF-4), and [Cu(hfbba)2(phen)2]$0.5(DMF) (F-MOF-
5) (hfbba ¼ 4,40-(Hexafluoroisopropylidene) bis(benzoate), phen ¼ 1,10-phenanthroline; 2,20-bipy ¼2,20-bipyridine; 4,40dime-2,20-bipy ¼ 4,40dimethyl 2,20-bipyridine; 3-mepy ¼ 3-methyl-pyridine and
DMF ¼ dimethyl formamide) display interesting H-bonded, 1D and 2D structural features. All these
F-MOFs were structurally determined by single-crystal X-ray diffraction. F-MOF-1 and 5 form
a distorted octahedral Cu+2 secondary building unit (SBU). F-MOF-5 forms a one-dimensional
architecture constructed from Cu2+ ions bridged by phen anions and two types of 4 hfbba ligands while
in F-MOF-1, two among the three acid co-ligands are not coordinated with Cu2+ ions. F-MOF-2
displays dinuclear SBUs within the three-dimensional frameworks whereas F-MOF-3 displays
mononuclear SBUs and forms a one-dimensional chain. F-MOF-4 display a interesting 2D framework
with conventional Cu2(CO2)4 paddle wheel SBU. In the crystal structures four hfbba ligands, each
coordinated to a dicopper paddlewheel unit via one of their carboxylate groups. Solid-state properties
such as UV-vis and thermal stability of F-MOF-1–5 have also been studied.
Introduction
Metal–organic frameworks (MOFs) have received considerable
interest in the last two decades.1 The combination of multi-
functional organic linkers with multinuclear metal complexes
acting as nodes has emerged as a modular concept for systematic
design of porous solids with specific applications in separation,
storage, heterogeneous catalysis.2 The work by Yaghi, Long,
Cheetham, Kitagawa, Ferey and others have succeeded in
highlighting the model of carboxylate-based systems in the
generation of highly stable, porous, functionalized crystalline
materials that has surface areas and micropore volumes much
higher than the traditional molecular sieves such as zeolites and
activated carbons.3 Although a great variety of MOFs with
diverse compositions and structures have been reported,
however, to date, the control of product architecture remains
a major challenge in this field due to the fact that the self-
assembly process is frequently influenced by the type of the
ligand, metal-to-ligand ratio, chemical and geometrical
Physical Materials Chemistry Division, National Chemical Laboratory, DrHomi Bhaba Road Pune, 411008, India. E-mail: [email protected];Fax: + 91-20-25902636; Tel: + 91-20-25902535
† Electronic supplementary information (ESI) available: Synthesisprocedures, single-crystal X-ray data collection, structure solution andrefinement procedures, thermal stability and UV of F-MOFs. CCDCreference numbers 745103–745107. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/b917483d
1600 | CrystEngComm, 2010, 12, 1600–1609
preferences of the metal ion, solvent and temperature of synthesis
and other factors.4
Recently researchers have shown interest into the possibility of
synthesizing fluorinated metal–organic frameworks (F-MOFs)
using perfluorinated polycarboxylate ligands.5 These F-MOFs
could emerge as a better H2-storage material as they bind
hydrogen more strongly and efficiently than their non-fluori-
nated counterparts.6 There are only a few reports of MOFs
containing fluorinated carboxylates as perfluorinated carboxyl-
ates7 are significantly more acidic than non-fluorinated carboxy-
lates, and subsequently less soluble in common organic solvents.
Fluorinated acids are also less stable than their non-fluorinated
analogues and it has been found that they often decompose at
temperatures commonly used to form MOFs of higher dimen-
sionality (125–180 �C).
In this contribution, we will describe our recent research on the
synthesis, crystal structures, and properties of a series of
F-MOFs with 4,40-(hexafluoroisopropylidene) bis(benzoic acid)
(C17H10F6O4, H2hfbba) as a flexible dicarboxylate building
block, Cu+2 and various heterocyclic co-ligands (Scheme 1).
These F-MOFs formulated as [Cu(hfbba)(phen)2]$2(H2hfbba)-
(H2O)(HCO2) (F-MOF-1), [Cu(hfbba)2(2,20-bipy)2(H2O)]
(F-MOF-2), [Cu(hfbba)(4,40dime-2,20-bipy)(HCO2)]$(hfb-
ba)(H2O) (F-MOF-3), [Cu2(hfbba)2(3-mepy)2]$(DMF)2(3-mepy)
(F-MOF-4), and [Cu(hfbba)2(phen)2]$0.5(DMF) (F-MOF-5)
(hfbba ¼ 4,40-(Hexafluoroisopropylidene) bis(benzoate), phen ¼1,10-phenanthroline; 2,20-bipy ¼ 2,20-bipyridine; 4,40dime-2,20-
bipy ¼ 4,40dimethyl 2,20-bipyridine; 3-mepy ¼ 3-methyl-pyridine
This journal is ª The Royal Society of Chemistry 2010
Scheme 1 Schematic diagram showing solvent dependence on the
synthesis of F-MOFs.
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and DMF¼ dimethylformamide) display interesting 1D, 2D and
3D structural features. The inclusion of such second links assists
in forming structures containing the fluorinated ligand and also
helps reduce the rather significant increase in density. Thermal
stability, PXRD and UV-vis spectra of these F-MOFs have also
been investigated.
Experimental
Materials and general methods
All reagents and solvents for synthesis and analysis were
commercially available and used as received. The Fourier
transform (FT) IR spectra (KBr pellet) were taken on an PER-
KIN ELMER FT-IR SPECTRUM (Nicolet) spectrometer.
Powder X-ray diffraction (PXRD) patterns were recorded on
a Phillips PNAlytical diffractometer for Cu Ka radiation
(l ¼ 1.5406 �A), with a scan speed of 2� min�1 and a step size of
0.02� in 2q. Thermogravimetric experiments (TGA) were carried
out in the temperature range of 25–700 �C on a SDT Q600
TG-DTA analyzer under N2 atmosphere at a heating rate of
10 �C min�1. Solid-state UV spectra were measured at room
temperature on a PERKIN ELMER Precisely Lambda 650
spectrometer.
Synthesis of F-MOF-1 to 5
[Cu(hfbba)(phen)2]$2(H2hfbba)(H2O)(HCO2) (F-MOF-1).
Hydrothermal reaction of Cu(NO3)2$3H2O (0.029 g, 0.12 mmol)
with 1,10-phenanthroline (0.024 g, 0.12 mmol) and excess
H2hfbba (0.196 g, 0.50 mmol) in a 23 mL acid-digestion bomb
using de-ionized water (7 mL) at 120 �C for 3 d produced blue
coloured crystals of F-MOF-1 in 40% yield (0.0116 mg based on
Cu). Crystals were collected by filtration and dried in air
(10 min).
FT-IR (KBr 4000–400 cm�1): 3655 (s), 3093 (w), 1942 (w), 1701
(s), 1598 (s), 1554 (s), 1395 (s), 1326 (w), 1250 (m), 1211 (w), 1177
(w), 1019 (w), 1291 (w), 929 (w), 843 (m), 778 (m), 744 (m),
723 (m).
This journal is ª The Royal Society of Chemistry 2010
[Cu(hfbba)2(2,20-bipy)2(H2O)] (F-MOF-2). 0.5 mL 2,20-bipyr-
idyl stock solution (0.20 M) and 1.5 mL H2hfbba stock solution
(0.20 M) were mixed in a 5 mL vial. To this solution was added
0.5 mL Cu(NO3)2$3H2O stock solution (0.20 M). The vial was
capped and heated to 85 �C for 96 h. The mother liquor was
decanted and the products were washed with DMF (15 mL) three
times. Blue coloured crystals of F-MOF-2 were collected by
filtration and dried in air (10 min) (yield: 30%; 0.0072 mg based
on Cu).
FT-IR (KBr 4000–400 cm�1): 3040 (s), 2554 (w), 1945 (w), 1817
(w), 1689 (s), 1626 (s), 1557 (m), 1515 (w), 1482 (w), 1390 (m),
1289 (w), 1213 (w), 1175 (m), 1140 (s), 1068 (w), 1020 (m), 946
(w), 846 (w), 779 (m), 749 (m), 727 (s), 589 (w), 541 (w), 496 (w).
[Cu(hfbba)(4,4 0dime-2,2 0-bipy)(HCO2)]$(hfbba)(H2O)
(F-MOF-3). Hydrothermal reaction of Cu(NO3)2$3H2O
(0.029 g, 0.12 mmol) with 4,40-dimethyl-2,20- bipyridine (0.022 g,
0.12 mmol) and excess H2hfbba (0.196 g, 0.50 mmol) in a 25 mL
acid-digestion bomb using de-ionized water (7 mL) at 120 �C for
3 d produced blue coloured crystals of F-MOF-3 in 60% yield
(0.0174 mg based on Cu). Crystals were collected by filtration
and dried in air (10 min).
FT-IR (KBr 4000–400 cm�1): 3695 (br), 3557 (m), 3090 (w),
2496 (w), 2094 (w), 1942 (w), 1824 (w), 1698 (m), 1681 (m), 1591
(w), 1543 (m), 1399 (s), 11326 (w), 1290 (s), 1257 (w), 1172 (w),
1021 (w), 960 (w), 860 (w), 779 (m), 745 (w), 723 (m), 519 (w).
[Cu2(hfbba)2(3-mepy)2]$(DMF)2(3-mepy) (F-MOF-4). 0.5 mL
3-methyl-pyridine stock solution (0.20 M) and 1.5 mL H2hfbba
stock solution (0.20 M) were mixed in a 5 mL vial. To this
solution was added 0.5 mL Cu(NO3)2$3H2O stock solution (0.20
M). The vial was capped and heated to 85 �C for 96 h. The
mother liquor was decanted and the products were washed with
DMF (15 mL) three times. Blue coloured crystals of F-MOF-4
were collected by filtration and dried in air (10 min) (yield: 50%;
0.0120 mg based on Cu).
FT-IR (KBr 4000–400 cm�1): 3676 (br), 3068 (w), 3935 (m),
2657 (w), 2548 (w), 2331 (s), 1944 (m), 1816 (w), 1683 (m), 1632
(w), 1561 (w), 1410 (s), 1291 (w), 1239 (s), 1174 (w), 1090 (w),
1020 (w), 971 (w), 929 (w), 846 (m), 780 (s), 748 (w), 706 (w),
514 (m), 494 (w).
[Cu(hfbba)2(phen)2]$0.5(DMF) (F-MOF-5). 0.5 mL 1,10-phe-
nanthroline stock solution (0.20 M) and 1.5 mL H2hfbba stock
solution (0.20 M) were mixed in a 5 mL vial. To this solution was
added 0.5 mL Cu(NO3)2$3H2O stock solution (0.20 M). The vial
was capped and heated to 85 �C for 96 h. The mother liquor was
decanted and the products were washed with DMF (15 mL) three
times. Blue coloured crystals of F-MOF-4 were collected by filtra-
tion and dried in air (10 min) (yield: 68%; 0.0163 mg based on Cu).
FT-IR (KBr 4000–400 cm�1): 3680 (br), 3056 (s), 2937 (m),
2510 (w), 1941 (m), 1813 (w), 1683 (s), 1599 (m), 1557 (w), 1519
(w), 1411 (s), 1253 (w), 1211 (w), 1172 (w), 1138 (w), 1019 (m), 971
(w), 845 (s), 778 (s), 748 (w), 725 (m), 557 (w), 515 (w), 495 (w).
X-Ray crystallography
All single crystal data were collected on a Bruker SMART APEX
three circle diffractometer equipped with a CCD area detector
CrystEngComm, 2010, 12, 1600–1609 | 1601
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(Bruker Systems Inc., 1999)8 and operated at 1500 W power
(50 kV, 30 mA) to generate Mo Ka radiation (l ¼ 0.71073 �A).
The incident X-ray beam was focused and monochromated using
Bruker Excalibur Gobel mirror optics. Crystals of the F-MOFs
reported in the paper were mounted on nylon CryoLoops
(Hampton Research) with Paraton-N (Hampton Research).
Crystals were flash frozen to 100(2) K in a liquid nitrogen cooled
stream of nitrogen. Data were integrated using Bruker SAINT
software.9 Data were subsequently corrected for absorption by
the program SADABS.10 The space group determinations and
tests for merohedral twinning were carried out using XPREP.11
In all cases, the highest possible space group was chosen. All
structures were solved by direct methods and refined using the
SHELXTL-97 software suite.12 Atoms were located from itera-
tive examination of difference F-maps following least squares
refinements of the earlier models. Hydrogen atoms were placed in
calculated positions and included as riding atoms with isotropic
displacement parameters 1.2–1.5 times Ueq of the attached C
atoms. Data were collected at 298(2) K for F-MOF-5. For the
other four F-MOFs presented in this paper data collection took
place at 100(2) K. This lower temperature was considered to be
optimal for obtaining the best data. All structures were examined
using the Adsym subroutine of PLATON13 to assure that no
additional symmetry could be applied to the models. All ellip-
soids in ORTEP diagrams are displayed at the 50% probability
level unless noted otherwise. The ESI contains a detailed data
collection strategy and crystallographic data for the five F-MOFs
reported in this paper.† Crystal data and details of data collec-
tion, structure solution and refinement are summarized in
Table 1.
Results and discussion
Synthesis and characterization
Direct solution reactions at room temperature give rise to
a microcrystalline precipitate in most MOF syntheses.14 The
hydrothermal method has been demonstrated to be a very
promising technique for growing MOF crystals. Thus, hydro-
thermal methods were applied to obtain larger single crystals
suitable for X-ray diffraction. H2hfbba is sparingly soluble in
water at room temperature. To obtain pure crystals of F-MOF-1
and 3, a strong stirring of the mixture in de- ionized water is
necessary before it is transferred to an acid-digestion bomb for
hydrothermal reaction. In this work, two synthetic pathways
were applied to prepare these F-MOFs. Complexes F-MOF-1
and 3 were obtained by hydrothermal reaction of the ligand with
Cu(NO3)2$3H2O in water in a acid digestion bomb at higher
temperature (120 �C), whereas F-MOF-2, 4 and 5 were synthe-
sized by hydrothermal reaction in a glass vial at lower temper-
ature (85 �C). In each synthetic case, an approximate Cu/
H2hfbba/heterocyclic co-ligands composition of 1/5/1 was found
to produce best yield of the product. All these complexes are
insoluble in common organic solvents. The compositions of
F-MOF-1 to 5 were determined by IR, and single-crystal X-ray
diffraction techniques. The phase purities of the bulk samples
were further identified by powder X-ray diffraction (PXRD)
patterns, which display essential similarity to those of the
calculated ones (see Fig. 7).
1602 | CrystEngComm, 2010, 12, 1600–1609
For the IR spectra, the broad bands centered at ca. 3550 cm�1
indicate the O–H stretching of water for F-MOF-1 and 3.
Notably, in the IR spectra of complexes F-MOF-1 and 3, the
presence of characteristic absorption bands of carboxyl, which
appears at 1700 cm�1 for the free ligand, reveals its presence in
the crystal lattice, whereas the absence of the same absorption
bands for F-MOF-4 and 5 reveals its complete deprotonation.
The strong absorption peak at 1679 cm�1 (a significant shift in
comparison with that of the free ligand) should be ascribed to
metal coordination of the carboxyl group. The coexistence of
both carboxyl and carboxylate groups in F-MOF-1 and 3 can
also be validated by the crystal structure.
Structural description
Crystal structure of [Cu(Hhfbba)(phen)2]$2(H2hfbba)-
(H2O)(HCO2) (F-MOF-1). The asymmetric unit of F-MOF-1
(space group P21/n) consists of one crystallographically inde-
pendent Cu+2 ion, one Hhfbba (i.e. singly protonated H2hfbba),
two free H2hfbba, two phen, one formic acid and one lattice
water molecule. Each Cu+2 ion is surrounded by four nitrogen
atoms from two chelating phen ligands and two oxygen atoms
from one Hhfbba anion, composing a slightly distorted octahe-
dral geometry with Cu–O distances ranging from 1.984(2) to
2.641(2) �A and Cu–N distances ranging from 2.014(3) to 2.272(3)�A (Fig. 2a). One carboxylate of the H2hfbba adopts a bidentate
chelating mode and the other remains un-coordinated. The one-
dimensional structure of F-MOF-1 consists of layers of Cu+2 ions
coordinated to the H2hfbba ligands and phen molecules. Two of
this octahedral Cu(phen)2(Hhfbba) units are attached by
hydrogen bonds between the terminal Hhfbba and uncoordi-
nated H2hfbba groups to form a 1D hydrogen-bonding frame-
work (Fig. 2c and 2d). In complex F-MOF-1, the free carboxylic
acids (four of them) are H-bonded in a catemeric fashion to form
a (COOH)4 cluster (Fig. 2b).15 Hydrogen bonds involving the
lattice water molecules as donors join both kinds of carboxylic
acids to generate a two-dimensional organization in the ab plane.
The average O–H/O distance in this finite hydrogen bonded
network is 1.700(2) �A and the average angle is 172.2 �A (see ESI,
Table S2).† The residual positive charge on the metal ion remains
as it binds to only one formate anion (HCO2�).
Crystal structure of [Cu(hfbba)2(2,20-bipy)2(H2O)] (F-MOF-2).
As shown in Fig. 3a, the asymmetric unit of F-MOF-2 (space
group P21/n) comprises one crystallographically independent
Cu+2 ion, one hfbba ligand, one 2,20-bipy ligand, and a lattice
water molecule. Each Cu+2 ion octahedrally surrounded by two
nitrogen atoms from the bridging 2,20-bipy ligands, three oxygen
atoms from three hfbba ligands and one oxygen atom from the
coordinated water molecule. Two copper atoms are octahedrally
coordinated by eight oxygen atoms, six of them from hfbba
anions, and two trans water molecules and four nitrogen from
two bridging 2,20-bipy ligands creating a dicopper SBU (Fig. 3a).
The Cu–O and Cu–N bonds are in the range of 1.945(2)–2.797(4)
and 2.003(4)–2.005(2) �A, respectively. The H2hfbba ligands
bridge metal centers in two directions, forming a grid-like two
dimensional sheet (Fig. 3b). In complex F-MOF-2, CF3 groups
interacts with each other to form a type II F2C–F/FCF2 syn-
thon (d, 3.251(2) �A; d, 2.748(3) �A; q1, 160.1�, q2, 99.5�) (Fig. 3c).16
This journal is ª The Royal Society of Chemistry 2010
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This journal is ª The Royal Society of Chemistry 2010 CrystEngComm, 2010, 12, 1600–1609 | 1603
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Fig. 1 Different SBU’s discussed in this paper.
Fig. 2 Crystal structure of F-MOF-1. (a) Polyhedral representation of
the SBU showing the coordination environment of Cu(II) Cu (brown), N
(blue), O (red), C (black), F (green). (b) H-bonded (COOH)4 cluster in
F-MOF-1. (c) Hydrogen bonding between hfbba and octahedral
Cu(phen)2(Hhfbba) units. (d) Hydrogen-bond motifs of H2hfbba
between the coordination layers. (e) Packing diagram of F-MOF-1 in
space fill model (view down b axis).
Fig. 3 Crystal structure of F-MOF-2. (a) Polyhedral representation of
the SBU showing the coordination environment of Cu(II). (b) The hfbba
and 2,20-bipy ligands bridge metal centers in two directions, forming
a layered 2D sheet; (c) packing diagram of F-MOF-3 (view down b axis).
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Crystal structure of [Cu(hfbba)(4,40dime-2,20-bipy)(H-
CO2)]$(hfbba)(H2O) (F-MOF-3). The two-dimensional structure
(space group Pccn) in F-MOF-3 consists of one crystallograph-
ically independent Cu+2 ion atom, two hfbba ligands, one
1604 | CrystEngComm, 2010, 12, 1600–1609
4,40dime-2,20-bipy and one lattice water molecule. Each Cu+2 ion
is surrounded by two nitrogen atoms from the bridging 4,40dime-
2,20-bipy ligand, three oxygen atoms from one and half (one full
hfbba ligand and half hfbba ligand is present in the asymmetric
unit) composing a square pyramidal geometry with Cu–O
distances ranging from 1.934(2) to 2.320(2) �A and Cu–N
distances ranging from 2.001(3) to 2.014(3) �A (Fig. 4a). The
hfbba and 4,40dime-2,20-bipy ligands bridge metal centers in two
directions, forming a grid-like one dimensional sheet (Fig. 4b).
Free H2hfbba molecules pack in the void space between these
grid layers. This alternating MOF–organic–MOF layer
arrangement is unprecedented among this series of F-MOFs. The
one-dimensional parallel interpenetrating network (Fig. 4c) is
reinforced by the existence of other non-covalent interactions
like C–H/F hydrogen bonds (d, 3.408(2) �A; d, 2.536(3) �A; q,
152.9�) (Fig. 4d Table 2).17
Crystal structure of [Cu2(hfbba)2(3-mepy)2]$(DMF)2(3-mepy)
(F-MOF-4). Asymmetric unit of F-MOF-4 (space group P21/c)
comprises two crystallographically independent Cu+2 ion, two
This journal is ª The Royal Society of Chemistry 2010
Fig. 4 Crystal structure of F-MOF-3. (a) Polyhedral representation of
the SBU showing the coordination environment of Cu(II). (b) The hfbba
and 4,40dime-2,20-bipy ligands bridge metal centers in two directions,
forming a grid-like 1D sheet. (c) Interpenetration of two grid like 1D
sheet. (d) Packing diagram of F-MOF-3 (view down b axis).
Fig. 5 Crystal structure of F-MOF-4. (a) Ball and Stick representation
of the SBU showing the paddle-wheel motif of Cu(II); Cu (brown), N
(blue), O (red), C (black), F (green). Two 3-methyl-pyridine ligand
occupy the axial positions. (b) 2D (4,4) coordination layer in F-MOF-4
with Cu(II) paddle-wheel binuclear nodes (view down b axis). Hydrogen
atoms, guest molecules and 3-methyl-pyridines are omitted for clarity. (c)
Coordination layer in F-MOF-4 (view down a axis) showing the 5.5 �A
channels, 3-methyl-pyridine guest molecules occupy the voids inside the
channels.
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hfbba and two 3-mepy ligand, one lattice 3-mepy and three
lattice DMF molecules (Fig. 5a). Both Cu+2 centres have almost
the same coordination environments with a nearly ideal square-
pyramidal sphere (s z 0)18 enclosed by two hfbba ligands and
one 3-methyl-pyridine ligand. The dicopper paddlewheel SBU2c
for F-MOF-4 with the Cu/Cu distance of 2.603(1) �A is shown in
Fig. 5a, expanded to show the four hfbba ligands, each coordi-
nated to a dicopper paddlewheel unit via one of their carboxylate
groups. The second carboxylate group of each H2hfbba ligand
coordinates to another paddlewheel unit, generating the
extended corrugated two dimensional sheet, as shown in Fig. 5b,
two 3-methyl-pyridine ligand occupy the axial positions of the
dicopper paddle wheel SBU. In F-MOF-4, the sheets stack along
the y axis affording square -shaped channels from the sheet
cavities that run through the gross structure (Fig. 5c) similarly
sheets stack along the z axis affording hexagonal channels that
run through the gross structure. The distance between equivalent
atoms in neighbouring sheets is approximately 3.5 �A. The –CF3
groups of H2hfbba ligands are directed to the outside of the
larger square channels whereas DMF and 3-methyl-pyridine
guests reside inside the square shaped and hexagonal channels,
respectively. The pore diameter is approximately 5.5 �A across,
based on the largest sphere that could fit into the pore and be in
This journal is ª The Royal Society of Chemistry 2010
contact with the van der Waals surface. Such a large void space
within a framework often leads to molecular entanglement and
two-fold interpenetration. Interestingly we have isolated
a interpenetrated form of F-MOF-4 [Cu2(hfbba)2(3-mepy)2]
(H2O)2(3-mepy) which will be discussed elsewhere.
Crystal structure of [Cu(hfbba)2(phen)2]$0.5(DMF) (F-MOF-
5). The one-dimensional structure of F-MOF-5 (space group
Pbcn) consists of layers of copper ions coordinated to H2hfbba
ligands, phen molecules and disordered DMF ligand. As shown
in Fig. 6a, the asymmetric unit of F-MOF-5 comprises one
crystallographically independent Cu+2 ion, one hfbba ligand, one
phen, and a lattice DMF molecule. Molecular constituents of
F-MOF-5 and 1 are same as they both comprises hfbba ligand
and phen molecule. However, unlike F-MOF-1, each Cu+2 ion is
surrounded by four oxygen atoms from two hfbba anions and
two nitrogen atoms from one chelating phen ligand, composing
a slightly distorted octahedral geometry with Cu–O distances
ranging from 1.953(4) to 2.707(5) �A and Cu–N distances ranging
from 2.002(3) to 2.010(3) �A (Fig. 6a). The structure of F-MOF-5
generate a 1D wavelike chain, extending along the
CrystEngComm, 2010, 12, 1600–1609 | 1605
Fig. 6 Crystal structure of F-MOF-5. (a) Polyhedral representation of
the SBU showing the coordination environment of Cu(II); (b) corrugated
1D coordination layer in F-MOF-5 towards the ab plane.
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crystallographic a directions incorporating Cu+2 ions coordi-
nated by the hfbba ligand and phen molecules. These chains are
reinforced by the C–F/p interaction [4.091(2) �A; 3.036(3) �A;
134.3�] along the c direction leading to the formation of a 2D
layered structure (Fig. 6b).
The structural analysis of these five copper(II) coordination
polymers derived from the 4,40-(hexafluoroisopropylidene)
bis(benzoic acid) (C17H10F6O4, H2hfbba) as a flexible dicarboxy-
late building block, Cu+2 and various heterocyclic co-ligands led
to several interesting structural features. We primarily used 4,40-
(hexafluoroisopropylidene) bis(benzoic acid) because (a) the
twisted conformation of this link may help forming a possible
helical structure, (b) this flexible dicarboxylate building block
tends to vary the structural feature from one-dimensional to two-
dimensional depending on the type of the heterocyclic co-ligands
attached to the metal,7 (c) the different deprotonated degrees of
the ligands under different reaction conditions may result in
variable coordination modes in the products, (d) long molecular
structure of this primary building unit may lead to the formation
of microporous coordination frameworks with channels. The
influence of the solvent (H2O–DMF) on the formation of the
F-MOFs was also reflected in the structures of the materials.
Fig. 1 describes different SBU’s that has been described in this
paper. F-MOF-2 [Cu(hfbba)2(2,20-bipy)2(H2O)] is a 2D structure
where Cu+2 ion, is coordinated with one hfbba, one 2,20-bipy, and
a lattice water molecule. However, the introduction of the two
–CH3 functionality in 4,40dime-2,20-bipy ligand over 2,20-bipy
leads to the formation F-MOF-3, which is a one dimensional grid
structure reinforced by the C–F/F–C interactions.16 The steric
effect of –CH3 functionality of 4,40dime-2,20-bipy probably
hinders the formation of the octahedral SBU and leads to
a square pyramidal SBU. Overall, when the bulky terminal
1606 | CrystEngComm, 2010, 12, 1600–1609
ligands like phen and 4,40dime-2,20-bipy are introduced
H-bonded or 1D structures are obtained. On the other hand,
when the terminal ligands 2,20-bipy and 3-mepy are introduced
into the systems, the resulting F-MOFs show 2D network struc-
tures. A possible reason for F-MOF-1 being H-bonded compared
to the other F-MOFs reported in this paper could be attributed to
the solvent of synthesis. High polarity of water (3 ¼ 80.0 as
compared to DMF’s 3¼ 38.5) acts to stabilize the separated ionic
species of F-MOF-1 and as a result it forms a H-bonded structure.
Thermal stability and PXRD
To examine the architectural and thermal stability of F-MOFs
reported in this paper, we prepared them at the gram scale to
allow detailed investigation of the aforementioned properties.
Thermal gravimetric analysis (TGA) performed on as-synthe-
sized F-MOF-1 to 5 revealed these compounds high thermal
stability (see ESI, Section S3 for all data and interpretations
regarding guest mobility and thermal stability of F-MOF-1 to
5).† The TGA of compound F-MOF-1, 2, 3 and 5 demonstrates
that the compound has an excellent thermal stability as no
strictly clean weight loss step occurred below 300 �C. This indi-
cates that the co-ordinated water molecules leave the framework
at the point of decomposition. A sharp weight loss at 300 �C
indicates decomposition of the host framework. TGA trace for
F-MOF-4 is totally different from the other F-MOFs reported in
this paper as it possess 2D pores (5.5 �A in diameter). The TGA
trace for F-MOF-1 showed a gradual weight-loss step of 20.3%
(25–200 �C), corresponding to escape of all N,N dime-
thylformamide (DMF) and 3-methylpyridine solvent molecules
trapped in the pores (2 DMF; calcd 18.3%) followed by a plateau
(200–300 �C).
In order to confirm the phase purity of the bulk materials,
powder X-ray diffraction (PXRD) experiments were carried out
on all complexes. The PXRD experimental and computer-
simulated patterns of all of them are shown in the ESI (Fig. S1 to
S5).† As shown in Fig. 7, all major peaks of experimental powder
X-ray patterns (PXRD) of compounds F-MOF-2 to 5 matches
well that of simulated PXRD, indicating their reasonable crys-
talline phase purity. The experimental patterns of F-MOFs
1 have a few unindexed diffraction lines and some slightly
broadened in comparison with those simulated patterns. This is
probably due to the loss of H-bonded water molecule from the
lattice produces a different phase than that obtained in the single
crystalline form. Incidentally, F-MOF-1 showed no weight loss
step occurred below 300 �C also indicates that the loss of
H-bonded water before the TGA experiment. The experimental
patterns of F-MOF-2, -3 -4 and -5 show reduced intensity at
higher 2q (> 20�). This could be due to loss of solvent from the
lattice produces during sample preparation and followed by
slight loss of crystallinity at higher 2q. The occurrence of a few
extra peaks for F-MOF-2, -3 -4 and -5 could be attributed as
a different phase (less than 5% of the bulk) than that obtained in
the single crystalline form.
Adsorption properties of F-MOF-4
The sample of F-MOF-4 was prepared by a thorough washing of
crystals with CHCl3 followed by a evacuation at 120 �C for 3 h.
This journal is ª The Royal Society of Chemistry 2010
Fig. 7 Comparison of PXRD patterns of the as-synthesized F-MOFs reported in this paper (red) with the simulated pattern from the single-crystal
structure (black). Overlay of TGA traces of as-synthesized F-MOFs reported in the paper.
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The BET surface area of F-MOF-4 was found to be 196 m2 g�1,
which is quite low and may therefore indicate a slight constric-
tion of the channels after solvent removal. Nitrogen and
hydrogen adsorption isotherms for F-MOF-4 are shown in
Fig. 8. While the volume of hydrogen adsorbed by F-MOF-4 is
rather modest (0.6 wt% H2), but this report once again proves
that hybrid materials containing a per-fluorinated ligand can
emerge as a hydrogen storage materials.
UV-vis properties of F-MOF-1–5
Solid-state luminescent emission spectra of F-MOFs-1 to -5 as
well as the free ligand H2hfbba were recorded at room temper-
ature (Fig. 9). UV-vis absorption at 300–315, 400–405 and at
700–710 nm for compound F-MOF-1–5 can be observed.
The intense broad UV-vis absorption are found at 278 nm and
664 nm for F-MOF-1, at 342 nm and 651 nm for F-MOF-2, at
321 nm and 664 nm for F-MOF-3, at 309 nm and 647 nm for
F-MOF-4, and at 301 nm and 674 nm for F-MOF-5. In order to
understand the nature of the emission band, the photo-
luminescence property of free H2hfbba ligand was measured with
This journal is ª The Royal Society of Chemistry 2010
the observation of one weak absorption at 285 nm. In compar-
ison to the free ligand, most of the emission maxima of
F-MOFs-1 to -5 are changed which could be assigned to a joint
contribution of the intra-ligand transitions (after coordinating to
metal centers, neutral ligands may change their HOMO and
LUMO energy levels) or charge-transfer transitions between the
coordinated ligands and the metal center.
Conclusion
In this contribution we tried to understand the variables and
strategies for producing a particular kind of SBU and attempt to
identify the most likely structure that should result from the
combination of metal carboxylate with various heterocyclic
co-ligands. We successfully synthesized 5 new F-MOF structures
with a flexible dicarboxylate building block, Cu+2 and various
heterocyclic co-ligands. The structural differences between
F-MOF-1 and 5 clearly demonstrate the critical role of condi-
tions in determining the product structure from the reaction
between Cu(NO3)2$3H2O and 4,40-(hexafluoroisopropylidene)
bis(benzoic acid), (C17H10F6O4, H2hfbba) and phenanthroline.
CrystEngComm, 2010, 12, 1600–1609 | 1607
Fig. 8 Nitrogen and hydrogen adsorption isotherms for F-MOF-4.
Fig. 9 UV-vis absorption spectra for F-MOF-1–5.
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The different degree of deprotonation of H2hfbba under
appropriate conditions (i.e. change of solvent from DMF to
water and vice versa) will result in variable coordination and
hydrogen-bonding modes in the resulting crystal structure. This
work shows that the structural characteristics of auxiliary ligands
also play an important role in governing the structures of such
supramolecular complexes.
1608 | CrystEngComm, 2010, 12, 1600–1609
Acknowledgements
PP acknowledges CSIR for a project assistantship (PA-II) from
CSIR’s XIth Five Year Plan Project (NWP0022-H). CD and TP
acknowledge CSIR, New Delhi, India for fellowship support. RB
acknowledges Dr S. Sivaram, Director NCL for start-up grants
and CSIR’s XIth Five Year Plan Project (grant no: NWP0022-H)
for funding and also Dr S. Pal and Dr K. Vijaymohanan, for
their encouragement.
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