Dimensionality Alteration and Intra- versus Inter-SBU Void Encapsulation in Zinc Phosphate Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.5b02949

4,4′-Bipyridine-N-oxide (BIPYMO, 1), a less commonly employed coordination polymer linker, has been used as a ditopic spacer to bridge double-four-ring (D4R) zinc phosphate clusters to form novel framework coordination polymers. Zinc phosphate framework compounds [Zn₄(X-dipp)₄(BIPYMO)₂]_n·2MeOH [X = H (2), Cl (3), Br (4), I (5); dipp = 2,6-diisopropylphenyl phosphate] have been obtained by treating a methanol solution of zinc acetate with X-dippH₂ and BIPYMO (in a 1:1:1 molar ratio) at ambient conditions. Framework phosphates 2-5 can also be obtained by treating the preformed D4R cubanes [Zn(X-dipp)(DMSO)]₄ with required quantities of BIPYMO in methanol. Single-crystal X-ray diffraction studies reveal that these framework solids are two-dimensional (2D) networks as opposed to the diamondoid networks obtained when the parent unoxidized 4,4′-bipyridine is used as the linker (Inorg. Chem. 2014, 53, 8959). The two types of voids (viz., smaller intra-D4R and larger inter-D4R) present in these framework solids can be utilized for different types of encapsulation processes. For example, the in situ generated 2D framework 2 encapsulates fluoride ions accompanied by a change in the dimensionality of the framework to yield {[(nC₄H₉)₄N][F(Zn₄(dipp)₄(BIPYMO)₂)]}_n (6). The three-dimensional framework 6 represents the first structurally characterized example of a fluoride-ion-encapsulated polymeric coordination compound or a metal-organic framework. The possibility of utilizing inter-D4R voids as hosts for small organic molecules has been explored by treating in situ generated 2 with a series of organic molecules of appropriate size. Framework 2 has been found to be a selective host for benzil and not for other structurally similar molecules such as benzoquinone, benzidine, anthracene, naphthalene, α-pyridoin, etc. The benzil-occluded isolated framework [benzil@{Zn₄(dipp)₄(BIPYMO)₂}]_n (7) has been isolated as single crystals, and its crystal structure determination revealed the binding of benzil molecules to the framework through strong π-π interactions.

Full text of DOI:10.1021/acs.inorgchem.5b02949

Article
pubs.acs.org/IC
Dimensionality Alteration and Intra- versus Inter-SBU Void
Encapsulation in Zinc Phosphate Frameworks†
Aijaz A. Dar,‡ Gulzar A. Bhat, and Ramaswamy Murugavel*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: 4,4′-Bipyridine-N-oxide (BIPYMO, 1), a less
commonly employed coordination polymer linker, has been
used as a ditopic spacer to bridge double-four-ring (D4R) zinc
phosphate clusters to form novel framework coordination
polymers. Zinc phosphate framework compounds [Zn4(X-
dipp)4(BIPYMO)2]n·2MeOH [X = H (2), Cl (3), Br (4), I
(5); dipp = 2,6-diisopropylphenyl phosphate] have been
obtained by treating a methanol solution of zinc acetate with
X-dippH2 and BIPYMO (in a 1:1:1 molar ratio) at ambient
conditions. Framework phosphates 2−5 can also be obtained
by treating the preformed D4R cubanes [Zn(X-dipp)-
(DMSO)]4 with required quantities of BIPYMO in methanol.
Single-crystal X-ray diffraction studies reveal that these framework solids are two-dimensional (2D) networks as opposed to the
diamondoid networks obtained when the parent unoxidized 4,4′-bipyridine is used as the linker (Inorg. Chem. 2014, 53, 8959).
The two types of voids (viz., smaller intra-D4R and larger inter-D4R) present in these framework solids can be utilized for
different types of encapsulation processes. For example, the in situ generated 2D framework 2 encapsulates fluoride ions
accompanied by a change in the dimensionality of the framework to yield {[(nC4H9)4N][F@(Zn4(dipp)4(BIPYMO)2)]}n (6).
The three-dimensional framework 6 represents the first structurally characterized example of a fluoride-ion-encapsulated
polymeric coordination compound or a metal−organic framework. The possibility of utilizing inter-D4R voids as hosts for small
organic molecules has been explored by treating in situ generated 2 with a series of organic molecules of appropriate size.
Framework 2 has been found to be a selective host for benzil and not for other structurally similar molecules such as
benzoquinone, benzidine, anthracene, naphthalene, α-pyridoin, etc. The benzil-occluded isolated framework [ben-
zil@{Zn4(dipp)4(BIPYMO)2}]n (7) has been isolated as single crystals, and its crystal structure determination revealed the
binding of benzil molecules to the framework through strong π−π interactions.
We have been investigating metallophosphate chemistry in
recent times in great detail from structural, mechanistic, and
possible applications points of view.10 In this regard, we have
recently reported on the usage of discrete D4R metallophosphate
cages for the sensing and scavenging of fluoride ions.11 A few
examples of fluoride-ion-encapsulated D4R cages that are not
phosphate-based have also been realized in the past.12 It has
further been shown that D4R zinc phosphates can be linked by
N,N′-ditopic linkers of varying length into three-dimensional
(3D) assemblies, resulting in framework solids with significant
porosity and gas adsorption properties.13 While the D4R cubanes
themselves are capable of encapsulating fluoride ions, the
framework solids synthesized by using 4,4′-bipyridine (BIPY)
as the ditopic linker are incapable of hosting smaller ions and
molecules.13 In an attempt to overcome this deficiency and
synthesize D4R zinc phosphate based MOFs, which can be
employed as host lattices for small molecular species, the N,O-
ditopic ligand 4,4′-bipyridine-N-oxide (BIPYMO) has been used
1. INTRODUCTION
Porous materials such as zeolites, metal−organic frameworks
(MOFs), and covalent organic frameworks represent one of the
extensively investigated classes of compounds in view of their
utility for a variety of applications. Besides the principal utility in
the areas of gas storage,1 catalysis,2 and drug delivery,3 these
solids have been employed for chemical separation4 and
molecular sensing5 as well. More recently, the encapsulation of
diverse guest molecules by these porous materials, ranging from
biomolecules or drug molecules to simple inorganic ions, has
been reported, albeit at times with limited success.6 The pore
aperture properties play a crucial role in exhibiting selectivity
toward a specific class of molecules, which may navigate through
or stay within the cavities/voids of these extended solids.7 The
pore size has been successfully modulated by the choice of
organic linkers used in the case of MOFs, although beyond a
certain length of the linker, interpenetration is a common
phenomenon, leading to diminished porosity.8 Besides the size,
the shape and dimensionality of the pores within these solids also
change their properties and utilities.9
Received: December 21, 2015
© XXXX American Chemical Society
A
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as the linker in the present investigation. Details of this study for
sensing purposes, describing the synthesis of two-dimensional
(2D) or 3D porous coordination polymers and their ability to
selectively host fluoride ions or benzil molecules inside the intra-
or intercubane cavities, respectively, are described in this
contribution.
2.2. Synthesis and Spectral Characterization of [Zn4(X-
dipp)4(BIPYMO)2]·2MeOH [X = H (2), Cl (3), Br (4), I (5)]. A
one-pot reaction involving [Zn(OAc)2·2H2O], X-dippH2, and
BIPYMO in a 1:1:1 ratio in methanol at room temperature for a
few hours, followed by crystallization of the products by slow
evaporation of the solvent, yields products 2−5 as single crystals
(Scheme 1, path A). Frameworks 2−5 can also be prepared by
treating the preformed D4R zinc phosphates [Zn(X-dipp)-
(DMSO)]413,17 with BIPYMO in methanol (Scheme 1, path B).
The new products have been characterized by both analytical and
spectroscopic methods. The composition of the new compounds
could be easily derived from their elemental analysis values. The
phase purity of all of the new compounds has been investigated
by powder X-ray diffraction (PXRD) of the vacuum-dried bulk
samples. It appears that, because of the rapid loss of lattice
solvent molecules from compounds 3 and 4, there is a change in
the crystal structure, and hence some discrepancy has been
observed between the simulated and experimental PXRD
patterns. Hence, PXRD measurements of 3, 4, and 7 were
repeated using freshly crystallized samples that are only air-dried.
These measurements yield diffraction patterns that match the
simulated patterns. In the case of 6, however, the crystal removed
from the mother liquor leads to immediate amorphization,
yielding very poor PXRD results (Figures S3−S6).
2. RESULTS AND DISCUSSION
2.1. BIPYMO as the Ditopic Spacer. The use of BIPY as a
bridging ligand to form coordination polymers is well established
in the literature through more than 6500 crystal structures
recorded in the CSD based on BIPY (Figure 1).14 The oxidized
The Fourier transform infrared (FT-IR) spectra of 2−5
recorded as KBr-diluted thin disks show strong absorption bands
at 1174, 1016, and 919 cm−1 for 2, 1178, 1017, and 921 cm−1 for
3, 1178, 1017, and 918 cm−1 for 4, and 1182, 1018, and 913 cm−1
for 5, corresponding to the O−P−O stretching and M−O−P
asymmetric and symmetric vibrations, respectively. The absence
of any absorption around 2350 cm−1 implies that there are no
free P−OH groups on the aryl phosphate ligands in 2−5 (Figure
S7). Frameworks 2−5 are insoluble in most organic solvents
except in dimethyl sulfoxide (DMSO). Hence, NMR character-
ization of the products was carried out in a DMSO solution. It
should, however, be noted that zinc phosphate frameworks upon
dissolution in DMSO decompose back to cubanes that contain
DMSO ligands on the zinc centers (A) and free ditopic spacer
Figure 1. BIPY, BIPYDO, and BIPYMO binding modes displaying the
deviation of oxygen-bound metal ions from the N−N axis of the ligand
and the relative occurrence of these compounds as ligands in the CSD.
and more flexible 4,4′-bipyridine-N,N′-dioxide (BIPYDO),
capable of forming angular coordination bonds because ligation
is through oxygen, has emerged as a versatile supramolecular
linker in recent times (∼300 hits in the CSD).15 The third form,
BIPYMO (1), which combines ligation by a nitrogen sp2 center
and an oxygen center, that is capable of displaying angles from
110 to 160° on the two ends of the same ligand, has been
investigated only recently.16 Its potential as a supramolecular
synthon has not yet been explored in detail (Figure 1).16
The use of this ligand system in zinc phosphate chemistry
would especially be interesting based on the hypothesis that the
nitrogen and oxygen terminals would behave quite differently not
only in terms of the electronic effects but also in terms of the
bond angles possible at the ligating atoms. This can, in turn, lead
to the isolation of newer types of framework structures that are
different from those isolated using BIPY as the linker.13
ligands10a (Scheme 1, path C). The H NMR spectra of 2−5
1
exhibit well-separated resonances for all of the protons of
phosphate and BIPYMO ligands with a peak integral ratio of 2:1,
affirming the role of BIPYMO as the bridging ligand in 2−5
(Figure S8). The presence of four doublets in the region 7.8−8.7
ppm corresponds to the aryl protons of BIPYMO, and the singlet
observed around 7.0 ppm corresponds to aryl protons of the
phosphate ligands. In the case of 2, multiplets are observed in the
same region. Further, a multiplet and a doublet observed at
around 3.5 and 1.0 ppm correspond to the methine and methyl
protons of the phosphate ligands, respectively. A single
resonance is observed at −5.20, −5.38, −5.47, and −5.52 ppm
in the 31P NMR spectra of 2−5 in DMSO, respectively (Figure
S9), corresponding to the formation of species A in a DMSO-d6
solution (Scheme 1).
2.3. Molecular Structures of 3 and 4. Single crystals of 2−
5 have been isolated directly from the reaction mixtures by the
slow evaporation of solvent at room temperature. While the
crystals obtained for 3 and 4 have been found to be suitable for X-
ray diffraction, crystals of 2 and 5 did not diffract well. Hence, the
structures of 3 and 4 have been determined as representative
examples for this series of zinc phosphates. Compounds 3 and 4
are isomorphous and crystallize in the tetragonal space group
P41212. A perspective view of the molecular structure of the
The BIPYMO ligand was prepared by a protocol that is slightly
different from an earlier published synthetic procedure where m-
chloroperoxybenzoic acid was used as the oxidant.16a In the
present study, the oxidation of BIPY has been carried out by 35%
H2O2 in glacial acetic acid, followed by extraction with water and
recrystallization in an ethyl acetate/methanol (9:1, v/v) solution.
The crystalline off-white solid 1 (30% yield) has been
characterized by analytical and spectroscopic methods. Four
1
doublets in H NMR with equal peak integrals, at δ 7.60, 7.80,
8.30, and 8.50 ppm, correspond to four sets of aromatic protons
of 1 (Figure S1). The peak at m/z 173.07 in the mass spectrum in
the Supporting Information corresponds to the [M + H]+
molecular ion (Figure S2).
B
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Scheme 1. Synthesis of BIPYMO-Based Zinc Phosphate 2D and 3D Frameworks 2−7
repeating unit of these framework compounds is shown in Figure
2.
body diagonal (5.475 Å in 3 and 5.505 Å in 4). The cage
dimensions of 3 and 4 are comparable to those of other reported
D4R zinc phosphates (Tables 1 and 2).17
The D4R zinc phosphate secondary binding units (SBUs) in 3
and 4 consist of zinc and phosphorus vertices that are bridged by
a μ2-oxygen atom. Both zinc and phosphorus centers adopt a
nearly tetrahedral geometry with average bond angles of 109.54°
and 109.46°, respectively. The average P−O(Zn) bond lengths
in 3 and 4 are 1.491 and 1.512 Å, respectively, which are
significantly shorter than a formal P−O single bond (1.60 Å) but
considerably longer than PO double-bond distances (1.45−
1.46 Å).19 Each of the six faces of the D4R SBUs is the eight-
membered Zn2O4P2 S4R rings that adopt a distorted pseudo-C4
crown conformation, with the Zn−O−P angles in the ring
ranging from 128.32(2) to 155.22(5)° in 3 and from 125.94(1)
to 158.53(1)° in 4. The dimensions of the D4R SBUs can best be
understood from the lengths of the Zn···P edge (3.153 Å in 3 and
3.176 Å in 4), P···P face diagonal (4.668 Å in 3 and 4.659 Å in 4),
Zn···Zn face diagonal (4.329 Å in 3 and 4.363 Å in 4), and Zn···P
In contrast to the 3D diamondoid frameworks obtained for
BIPY-bridged zinc phosphate framework solids,13 the solid-state
structures of 3 and 4 reveal that these compounds are 2D
coordination frameworks bridged by BIPYMO linkers (Figures 3
and S14 and S15). The ditopic BIPYMO linkers bridge the zinc
centers of two neighboring D4R zinc phosphate cubanes by
coordinating through the oxygen atom on the one end and the
nitrogen atom on the other end. The Zn−O−N bond angle of
around 120° implies that the oxygen atom of BIPYMO is in sp2
hybridization, as has also been reported in the case of some N-
oxide coordination compounds.18 These bent Zn−O−N angles
in 3 and 4 restrict them to form only a 2D framework, in contrast
to the 3D frameworks formed when BIPY is used as the spacer
with the same zinc phosphate building blocks.13 The 8.379-Å-
C
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Figure 2. Molecular structure diagrams of 3 and 4 (isopropyl groups, hydrogen atoms, and lattice solvent molecules are omitted for the sake of clarity).
Table 1. Comparison of the Dimensions of Unfilled D4R Cubanes 3, 4, and 7 with Fluoride-Encapsulated 6
Table 2. Comparison of the Structural Parameters of 3, 4, 6, and 7 with Those Found for Earlier Reported D4R Zinc Phosphates
face diagonal
Zn−O
Zn−N
Zn···P
Zn−O−P
Zn···Zn
P···P
body diameter
[Å]
Zn−O−N
formula
10
[Å]
[Å]
[Å]
[deg]
[Å]
[Å]
[deg]
[Zn(dippH2)(collidine)]4
1.916
1.923
1.921
1.933
1.916
1.925
1.955
3.196
3.179
3.197
3.175
3.175
3.195
3.063
4.419
4.347
4.471
4.447
4.328
4.371
3.795
4.632
4.598
4.527
4.541
4.618
4.629
4.767
5.529
5.477
5.536
5.544
5.485
5.495
5.255
[Zn(dippH2)(2-apy)]4·2MeOH10
[Zn(Cl-dippH2)(collidine)]4
15
[Zn(Cl-dippH2)(2apy)]4·2MeOH15
[Zn4(Cl-dipp)4(BIPYMO)2]n·2H2O (3)
[Zn4(Br-dipp)4(BIPYMO)2]n·2H2O (4)
2.043
2.042
2.131
134.22
135.28
126.51
119.76
123.29
119.36
[{(C4H9)4N}+{F@Zn4(dipp)4(BIPYMO)2}]n
(6)
[benzil@{Zn4(dipp)4(BIPYMO)2}]n (7)
1.895
2.055
3.213
125.25
4.292
4.589
5.455
108.63, 120.62
long BIPYMO linker separates two bridged zinc centers of the
neighboring D4R SBUs by 11.601 Å.
The gas adsorption measurements have been carried out at 77
K. As-synthesized materials were dried under vacuum at 70−80
°C for 24 h to remove adsorbed solvent molecules, after
appropriate solvent exchange. Brunauer−Emmett−Teller sur-
face area values for these compounds were found to be
insignificant, ca. 30, 3.4, 2.4, and 1.7 m2/g for 2−5, respectively
(Figures S10−S13), suggesting that the intercubane voids in
these 2D frameworks are inaccessible, probably because of the
bulky aryloxide ligands on the phosphorus. Hence, the possibility
D
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Figure 3. (a) Section of 3 showing linking of D4R SBUs by BIPYMO into 2D networks (down the c axis). (b) Possible intracubane (orange spheres) and
intercubane (yellow spheres) voids in 3 (down the c axis).
Figure 4. 1H NMR (DMSO-d6, 500 MHz) spectra of (left) fluoride-free framework 2 and (right) fluoride-ion-encapsulated framework 6.
of intracubane voids (orange spheres in Figure 3b), rather than
intercubane voids, for encapsulation has been investigated.
2.4. Fluoride-Encapsulated Phosphate Framework,
{[(nC4H9)4N][F@(Zn4(dipp)4(BIPYMO)2)]}n (6). Discrete D4R
cages are analogues of single-pore zeolitic entities, and hence
several types of D4R cages have been investigated for the
encapsulation of fluoride ions during the formation stages of
these clusters,12 including our recent demonstration that D4R
zinc phosphates can selectively sense and encapsulate fluoride
ions from various sources.11 Because framework compounds 2−
5 are 2D networks of D4R zinc phosphates, it will be interesting
to probe the utility of these compounds for fluoride-ion sensing/
encapsulation. Being soluble only in DMSO, the postsynthetic
utility of 2−5 for the encapsulation of fluoride ions in solution is
ruled out because it has been established recently that in DMSO
such framework compounds do not remain intact but
decompose to DMSO-decorated cubanes [Zn(dipp)(DMSO)]4
and free ditopic spacer ligands.10a Therefore, the in situ
generated frameworks 2−5 have been examined for fluoride-
ion recognition/encapsulation studies. A one-pot reaction of zinc
acetate, X-dippH2 (X = H), and BIPYMO with tetrabutylammo-
nium fluoride [1 M solution in tetrahydrofuran (THF)] in
methanol at ambient conditions yielded fluoride-encapsulated
framework 6 as a crystalline product after allowing the reaction
mixture to stand undisturbed for 1 week at room temperature
(Scheme 1, path D). The same reaction carried out using X-
dippH2 (X = Cl, Br, I) as the phosphate source did not result in
fluoride-ion incorporation. The parahalogens of these phosphate
ligands seem to sterically disfavor the fluoride encapsulation
owing to the bulky tetrabutylammonium cations competing with
the halogens for the same intercubane voids available in the
resultant frameworks. Fluoride inclusion also does not take place
when alkali- or alkaline-earth-metal fluorides are used, probably
because of the poor solubility of these halides in organic solvents.
The FT-IR spectrum of 6 does not exhibit any absorption at
around 2350 cm−1, indicating the absence of any free P−OH
groups of the phosphate ligands. The strong absorptions
observed at around 1157, 1045, and 910 cm−1 arise due to the
O−P−O stretching and M−O−P asymmetric and symmetric
1
stretching vibrations, respectively (Figure S16). The H NMR
spectrum shows well-separated signals for all of the protons of
BIPYMO, phosphate ligand, and counterion [(C4H9)4N]+ in a
1:2:4 ratio, as suggested by the peak integral ratio of the NMR
signals (Figure 4). The 31P NMR spectrum in DMSO-d6 (the
only solvent in which 6 dissolves through DMSO complexation,
releasing BIPYMO in solution) exhibits a single resonance at
2.97 ppm, which represents a downfield shift of about 8 ppm,
compared to the chemical shift of the empty framework 2 (Figure
S17). The 19F NMR spectrum also exhibits a single resonance at
−113 ppm (Figure S18). The 31P and 19F chemical shift values
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are characterictic for fluoride encapsulated within the D4R zinc
phosphate cage.11
2.5. Molecular Structure of 6. Single crystals of 6 were
obtained by slow evaporation of the solvent. Single-crystal X-ray
structure determination reveals that 6 crystallizes in the
empty frameworks 3 and 4, which are 2D sheets. The increase in
the dimensionality of 6 can be attributed to possible steric
hindrance/congestion, resulting from bulky [(C4H9)4N]+ ions
and a slight change of the angles around the zinc centers of the
cubane (Figure S19).
2.6. Synthesis and Characterization of [ben-
zil@(Zn4(dipp)4(BIPYMO)2)]n (7). The larger intercubane
voids in the 2D framework compounds 2−5 have been probed
for recognition of small organic molecules. The host framework
of 2, generated in situ by the reaction of [Zn(OAc)2·2H2O], X-
dippH2 (X = H), and BIPYMO in methanol, when treated with a
mixture of benzene, toluene, aniline, benzoic acid, naphthalene,
anthracene, benzidine, benzophenone, benzil, α-pyridoin, 1,2-
diphenylethane-1,2-diamine, etc., yielded the benzil-encapsu-
lated framework 7 as the only product.
tetragonal space group I42d. The crystal structure of 6 (Figures
5 and 6) displays two major differences compared to the parent
̅
Moreover, when treated independently with these organic
molecules, the host framework was found to be selective for
benzil, and when other small molecules were used, only empty
frameworks were obtained. Compound 7, the benzil-included 2,
has been characterized by analytical as well as spectroscopic
methods.
The FT-IR spectral data for 7 is similar to 2−6, with a
prominent additional CO stretching absorption at 1661 cm−1
(Figure S20). The 31P NMR spectrum shows a single resonance
at −5.23 ppm (Figure S21). The 1H NMR spectrum shows well-
separated peaks for dipp, BIPYMO, and benzil moieties and
suggests the presence of these molecules in a 4:2:1 ratio (Figure
S22).
Figure 5. Molecular structure of compound 6 (hydrogen atoms, the
isopropyl of the aryl ring, and counterion [(C4H9)4N]+ are omitted for
clarity).
compounds 2−5: (i) the intra-D4R cubane voids in 6 are filled
with fluoride anions, and the tetra-n-butylammonium cations
occupy the intercubane voids; (ii) the framework structure of 6 is
3D, in contrast to the 2D structures of 2−5. Thus, compound 6 is
a 3D network of fluoride-ion-encapsulated D4R zinc phosphate
cubanes [F@D4R]−, which are bridged by BIPYMO linkers.
Each of the anionic [F@D4R]− cages consists of four zinc
atoms, four phosphate ligands, and an encapsulated fluoride ion.
The fluoride ion is placed inside the D4R cage closer to the Zn1
centers [Zn1−F 2.258(1) Å] than to the Zn2 centers [Zn2−F
2.373(1) Å]. The observed Zn−F distances (Zn−F covalent
bond ∼1.775 Å) imply weak but significant interaction between
the zinc and fluoride centers, leading to a change in the geometry
about zinc from tetrahedral in the empty D4R zinc phosphates to
trigonal-bipyramidal. The distance between the fluoride ion and
phosphorus centers is, however, longer [P1−F1 2.931(1) Å and
P2−F1 2.946(1) Å] and is almost twice that of the P−F distance
in PF5 (∼1.56 Å as in PF3),20 suggesting no significant interaction
between the phosphorus and encapsulated fluoride ions. This is
also reflected in only a small shift in the 31P chemical shift value of
the encapsulated cubanes.10a
The phosphorus centers are in tetrahedral geometry with an
average O−P−O bond angle of 109.24°. The presence of a
fluoride ion distorts the D4R cages, drawing the zinc vertices
inside and shortening the average Zn···Zn face diagonal to 3.795
Å, while the average P···P face diagonal is slightly elongated
(4.767 Å). The distortion is further evident by comparable face
diagonals (Zn···Zn ∼ 4.450 Å and P···P ∼ 4.610 Å). Selected
bond parameters of 6 are compared with those of 3 and 4 and
other earlier reported D4R zinc phosphates in Tables 1 and 2.
The bulky organic counterions [(nC4H9)4N]+, which occupy
the intercubane voids, are highly disordered and could not be
modeled. Because they are only physically held within the
framework voids of 6 and do not involve any chemical
interactions, they have been squeezed during refinement. It is
worth noting that, in 6, the BIPYMO groups bridge the anionic
[F@D4R]− cages to form a 3D network (Figure 6), unlike the
2.7. Molecular Structure of 7. A single-crystal X-ray
diffraction study reveals that 7 crystallizes in the triclinic space
group P1. A perspective view of the molecular structure and the
̅
packing diagram of 7 are shown in Figures 7 and 8, respectively
(also see Figure S23). Unlike fluoride-encapsulated 6, compound
7 retains the 2D skeleton of the host framework. The host
framework of 7 is similar to empty frameworks of 3 and 4,
consisting of D4R zinc phosphates bridged by BIPYMO linkers.
Various bond parameters of 7 are compared with the empty and
fluoride-encapsulated compounds in Tables 1 and 2.
Like free benzil, the benzil molecules occluded within 7 also
exist in the trans configuration (Figures 9 and S24) because the
cis form is energetically not favored because of the steric
hindrance by aryl rings.19 The torsion angles between two aryl
and carbonyl groups are C78−C77−C76−C69 172.36(7)°,
C82−C77−C76−C69 1.09(1)°, C77−C76−C69−C70
−86.84(2)°, and O19−C76−C69−O20 −86.88(1)°. The
bond parameters observed for the encapsulated benzil molecule
do not deviate significantly from those of the free molecule.21
Structural analysis of 7 reveals two possible reasons for the
observed selective encapsulation of benzil molecules by in situ
formed host molecule 2, over other closely related analogues: (a)
the shape selectivity of the host and (b) both of the aryl rings of
benzil are involved in significant strong π−π interactions with the
aryl rings of the framework BIPYMO linkers (Figure 9).
2.8. Thermal Analysis. As has been described in section 2.2,
the vacuum-dried samples lose lattice solvent molecules readily
and, hence, thermogravimetric analyses of 2−7 have been carried
under a N2 atmosphere at a heating rate of 10 °C/min for both
air-dried and vacuum-dried samples. The weight loss of ∼60% in
the temperature range of 100−550 °C corresponds to the loss of
organic moieties, i.e., BIPYMO and 2,6-diisopropylphenyl
groups, to produce [ZnO3P(OH)] in 2−5, while in the guest-
encapsulated framework compounds 6 and 7, the first weight loss
(beginning at 200 and 250 °C, respectively) can be attributed to
F
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Figure 6. Framework structure of 6. The yellow spheres represent fluoride ions inside D4R cages (down the b axis).
formation of pyrophosphate Zn2P2O7. The air-dried samples also
behave in a similar manner but for a small weight loss observed at
temperatures of less than 200 °C, corresponding to the amount
of lattice solvent present in the system (Figures S25 and S26).
3. CONCLUSIONS
Zinc phosphate based extended coordination polymers have
been designed and synthesized. Their utility as versatile
molecular hosts has also been demonstrated. 2D networks 2−5
have been made accessible via both in situ and postsynthetic
methods employing an uncommon ditopic linker, BIPYMO.
Interestingly, compared to 3D frameworks of D4R zinc
phosphates reported with BIPY,13 the partially oxidized form
BIPYMO integrates the D4R zinc phosphates into 2D sheets.
The structural analyses of 3 and 4 reveal the presence of two
types of voids (intra- and intercubane), which have been shown
to be selective for fluoride ions and benzil molecules,
respectively. The framework 6 represents a very rare example
of a structurally characterized coordination polymer/MOF with
encapsulated fluoride ions. The intercubane voids are highly
selective for benzil molecules, owing to shape selectivity as well as
π−π interactions with the host framework.
Figure 7. Molecular structure diagram of 7 (along the b axis; hydrogen
atoms and isopropyl groups of the aryl ring of the phosphate ligand are
omitted for clarity).
the loss of loosely bound tetrabutylammonium and benzil
molecules. Upon heating to 1000 °C, condensation of [ZnO3P-
(OH)] with the loss of water molecules takes place, leading to the
G
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Figure 8. 2D framework illustration of 7 (viewed down the c axis).
ethane-1,2-diamine, and [Zn(OAc)2·2H2O] (s.d. Fine-Chem) were
used as procured. 2,6-Diisopropylphenyl phosphate (dippH2), 4-chloro-
2,6-diisopropylphenyl phosphate (Cl-dippH2), 4-bromo-2,6-diisopro-
pylphenyl phosphate (Br-dippH2), and 4-iodo-2,6-diisopropylphenyl
phosphate (I-dippH2) were synthesized as described previously.21
4.2. Synthesis of 1. To a solution of BIPY (16.5 mmol, 2.60 g) in 15
mL of glacial acetic acid was added 1.25 cm3 of 35% aqueous H2O2, and
the mixture was heated at 70−75 °C. After 3 h, another 0.17 cm3 of 35%
H2O2 was added, and the reaction was continued for additional 9 h at the
same temperature. The solvent was removed on a rotary evaporator, and
the residue was made alkaline with a saturated aqueous solution of
sodium carbonate and washed with chloroform. The extracted aqueous
phase yielded an off-white crystalline compound by the slow evaporation
of solvent at room temperature after 3 days, which upon recrystallization
in an ethyl acetate/methanol (9:1 v/v) solution yielded 1 as a crystalline
solid. Yield: 0.86 g (30%). Mp: 135 °C. 1H NMR (D2O, 400 MHz): δ
7.60 (d, 3JHH = 4.1 Hz, 2H, m-NO), 7.80 (d, 3JHH = 7.0 Hz, 2H, m-N),
3
8.30 (d, JHH = 7.0 Hz, 2H, o-NO), 8.50 (d, 3JHH = 4.2 Hz, 2H, o-N).
HRMS (ESI). Calcd for C10H8N2O: m/z 172.06. Found: m/z 173.07
([M + H]+).
Figure 9. Trans-configurational shape and π−π interactions between
the aryl rings of benzil and the BIPYMO linkers of the framework
stabilizing the guest molecules in 7 (down the c axis).
4.3. Synthesis of 2−5. To a stirred solution of [Zn(OAc)2·2H2O]
(0.25 mmol, 0.055 g) and X-dippH2(0.25 mmol; X = H, 0.062 g; X = Cl,
0.073 g; X = Br, 0.084 g; X = I, 0.096 g) in methanol was added a solution
of BIPYMO (0.13 mmol, 0.022 g) in methanol at 25 °C. The filtered
solution yielded 2−5 as single crystals after 1 week. The framework
compounds were also synthesized by the reaction of [Zn(X-dipp)-
(DMSO)]4 (0.25 mmol) in methanol with BIYPYMO (0.13 mmol) at
room temperature.
4. EXPERIMENTAL SECTION
4.1. Methods and Materials. All reactions were carried out at
ambient reaction conditions. Melting points were measured in glass
capillaries and are reported uncorrected. FT-IR spectra were obtained
on a PerkinElmer Spectrum One spectrometer as KBr-diluted disks.
Microanalyses were performed on a Thermo Finnigan (FLASH EA
2. Yield: 0.63 g (60%). Mp: >250 °C. Elem anal. Calcd for
C68H88N4O20P4Zn4 (Mr = 1666.2): C, 49.01; H, 5.32; N, 3.36. Found: C,
49.52; H, 5.12; N, 3.41. IR (KBr, cm−1): 2964 (s), 2867 (w), 1616 (s),
1481 (w), 1336 (s), 1174 (br), 1016 (s), 919 (w), 770 (w), 691 (w), 599
(w), 545 (w). 1H NMR (DMSO-d6, 500 MHz): δ 8.61 (d, 4H, bipy−H,
3JHH = 6.2 Hz), 8.35 (d, 4H, bipy−H,3JHH = 7.4 Hz), 7.96 (d, 4H, bipy−
H, 3JHH = 7.4 Hz), 7.84 (d, 4H, bipy−H, 3JHH = 6.2 Hz), 7.02 (d, 8H,
Ar−H, 3JHH = 1.9 Hz), 6.96 (t, 4H, Ar−H, 3JHH = 6.2 Hz), 3.66 (septet,
1
1112) microanalyzer. The H (Me4Si internal standard), 31P (85%
H3PO4 external standard), and 19F NMR spectra were recorded on a
Bruker Advance DPX-500 spectrometer. Thermogravimetric analyses
were carried out on a PerkinElmer Pyris thermal analysis system, under a
stream of nitrogen gas at a heating rate of 10 °C/min. Solvents were
purified according to standard procedures and distilled prior to use.20
Solid-state absorption studies were carried out on a Varian Cary Eclipse
spectrofluorimeter. Commercially available starting materials such as
4,4′-bipyridine (BIPY; Sigma-Aldrich), tetra-n-butylammonium fluoride
(TBAF; 1 M THF, Sigma-Aldrich), benzil (Merck), toluene (Merck),
aniline (Merck), benzoic acid (Merck), naphthalene (Merck),
anthracene (Sigma-Aldrich), benzidine (Sigma-Aldrich), benzophenone
(Sigma-Aldrich), benzil (Sigma-Aldrich), α-pyridoin,1,2-diphenyl-
i
3
3
8H, Pr−CH, JHH = 6.8 Hz), 1.07 (d, 48H, CH3, JHH = 6.8 Hz). 31P
NMR (DMSO-d6, 202 MHz): δ −5.2. TGA [temp range, °C (weight
loss, %)]: 30−331 (∼3.0), 331−373 (∼33.0), 373−511 (∼26.0), 511−
900 (∼2.5), 900−1082 (∼19.5).
3. Yield: 0.068 g (60%). Mp: >250 °C. Elem anal. Calcd for
C68H84Cl4N4O20P4Zn4 (Mr = 1804.7): C, 45.25; H, 4.69; N, 3.10.
H
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Table 3. Crystal Data for Compounds 3, 4, 6, and 7
a
3
4
6
7
CCDC no.
empirical formula
fw
1407144
1407145
1407146
1407147
C36H48Cl2N2O11P2Zn2
944.06
C36H48Br2N2O11P2Zn2
1037.34
C136H168FN8O36P8Zn8
3280.66
C82H89N4O20P4Zn4
1836.02
temp [K]
cryst syst
space group
a [Å]
293(2)
150(2)
150(2)
150(2)
tetragonal
P41212
tetragonal
tetragonal
triclinic
P41212
I42d
̅
P1
̅
15.7496(4)
15.7496(4)
35.8673(21)
15.7636(17)
15.7636(17)
36.843(4)
27.581(5)
27.581(5)
27.581(5)
15.121(7)
15.009(7)
21.587(9)
102.690(6)
102.475(2)
109.882(6)
4263.0(3)
2
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
V [Å3]
8896.8(6)
8
9155(2)
8
33067(1)
4
Z
D(calcd) [Mg/cm3]
1.320
1.493
2.916
2.81−25.00
68147
8059
0.659
1.430
μ [mm−1
]
1.317
0.644
1.258
Θ range [deg]
3.36−25.00
68638
7815
2.62−25.00
124089
14558
1.141
2.59−25.00
31359
no. of reflns collected
indep reflns [I0 > 2σ(I0)]
GOF
14780
1.052
1.149
0.053
0.152
1.170
R1 [I0 > 2σ(I0)]
wR2 (all data)
0.051
0.096
0.0770
0.149
0.291
0.314
a
Tetrabutylammonium cations [TBA]+ in the channels are highly disordered and, hence, could not be modeled appropriately. Hence, they were
squeezed during refinement.
Found: C, 45.45; H, 4.38; N, 3.21. IR (KBr, cm−1): 3457 (br), 2965 (s),
2868 (w), 1616 (s), 1466 (b), 1336 (s), 1178 (br), 1017 (s), 921 (w),
827 (s), 697 (w), 599 (w), 547 (w). 1H NMR (DMSO-d6, 500 MHz): δ
8.62 (d, 4H, 3JHH = 6.2 Hz, bipy−H), 8.35 (d, 4H, 3JHH = 7.3 Hz, bipy−
H), 7.94 (d, 4H, 3JHH = 7.3 Hz, bipy−H), 7.84 (d, 4H, 3JHH = 6.2 Hz,
bipy−H), 7.02 (s, 8H, Ar−H), 4.10 (q, 3JHH = 5.2 Hz), 3.63 (septet, 8H,
3JHH = 6.8 Hz, iPr−CH), 3.15 (d, 3JHH = 5.2 Hz), 1.09 (d, 48H, 3JHH = 6.8
Hz, CH3). 31P NMR (DMSO-d6, 202 MHz): δ −5.4. TGA [temp range,
°C (weight loss, %)]: 30−320 (∼2.5), 320−470 (∼56.5), 470−860
(∼4.0), 860−1100 (∼28.0).
>250 °C. Elem anal. Calcd for C84H120FN5O18P4Zn4 (Mr = 1892.1): C,
53.31; H, 6.39; N, 3.70. Found: C, 53.34; H, 7.17; N, 4.56. IR (KBr,
cm−1): 3436 (br), 2962 (s), 2867 (m), 1613 (s), 1476 (s), 1256 (m),
1
1157 (s), 1045 (m), 996 (s), 910 (s), 771 (s), 565 (s). H NMR
(DMSO-d6, 500 MHz): δ 8.65 (d, 3JHH = 5.19 Hz), 8.33 (d, 3JHH = 5.40
Hz), 7.94 (d, 3JHH = 5.42 Hz), 7.82 (d, 3JHH = 4.59 Hz), 6.94 (d, 3JHH
=
7.0 Hz), 6.87 (t, 3JHH = 6.72 Hz), 5.74 (s), 3.84 (septet, 3JHH = 6.8 Hz),
3.15 (m, 3JHH = 3.08 Hz), 1.54 (m, 3JHH = 7.32 Hz), 1.30 (m, 3JHH = 7.37
3
3
Hz), 1.06 (d, JHH = 6.79 Hz), 0.91 (t, JHH = 7.39 Hz). 31P NMR
(DMSO-d6, 202 MHz): δ 2.6. 19F NMR (DMSO-d6, 470 MHz): δ −113.
TGA [temp range, °C (weight loss, %)]: 30−340 (∼9.5), 340−511
(∼44.0), 511−896 (∼5.5).
4. Yield: 0.064 g (52%). Mp: >250 °C. Elem anal. Calcd for
C68H84Br4N4O20P4Zn4 (Mr = 1982.5): C, 41.20; H, 4.27; N, 2.83.
Found: C, 41.67; H, 3.74; N, 2.88. IR (KBr, cm−1): 3449 (br), 2964 (s),
2868 (w), 1617 (s), 1465 (b), 1336 (s), 1178 (br), 1017 (s), 918 (w),
803 (w), 691 (w), 599 (w), 545 (w). 1H NMR (DMSO-d6, 500 MHz): δ
8.61 (d, 4H, bipy−H, 3JHH = 6.2 Hz), 8.35 (d, 4H, bipy−H, 3JHH = 7.4
4.5. Synthesis of 7. To a solution of [Zn(OAc)2·2H2O] (0.40
mmol, 0.086 g) and dippH2 (0.40 mmol, 0.102 g) in methanol (30 mL)
was slowly added a solution of BIPYMO (0.40 mmol, 0.069 g) in
methanol. A solution of benzil (0.40 mmol) in methanol was added
slowly to yield a clear solution, which yielded 7 as single crystals after 1
week. Compound 7 was obtained as the only product even when a
solution of a mixture of organic molecules was added to the reaction
mixture (viz., benzene, toluene, aniline, benzoic acid, naphthalene,
anthracene, benzidine, benzophenone, benzil, α-pyridoin, 1,2-diphenyl-
ethane-1,2-diamine, etc.). Yield: 0.100 g (55%). Mp: >250 °C. Elem
anal. Calcd for C82H94N4O20P4Zn4 (Mr = 1834.2): C, 53.49; H, 5.15; N,
3.04. Found: C, 52.45; H, 4.98; N, 3.04. IR (KBr, cm−1): 3445 (br), 2964
(s), 2867 (m), 1661 (s), 1614 (s), 1450 (s), 1257 (m), 1170 (w), 1074
(s), 1012 (s), 915 (s), 770 (s), 529 (s). 1H NMR (DMSO-d6, 500 MHz):
δ 8.64 (d, 3JHH = 5.76 Hz), 8.33 (d, 3JHH = 7.24 Hz), 7.93 (d, 3JHH = 7.17
Hz), 7.96 (d, 4H, bipy−H, 3JHH = 7.4 Hz), 7.84 (d, 4H, bipy−H, 3JHH
=
3
6.2 Hz), 7.15 (s, 8H, Ar−H), 4.10 (q, CH3OH, JHH = 5.2 Hz), 3.63
(septet, 8H, iPr−CH, 3JHH = 6.8 Hz), 3.15 (d, CH3OH, 3JHH = 5.2 Hz),
1.09 (d, 48H, CH3, 3JHH = 6.8 Hz). 31P NMR (DMSO-d6, 202 MHz): δ
−5.5. TGA [temp range, °C (weight loss, %)]: 30−320 (∼2.5), 320−
510 (∼60.0), 510−850 (∼4.5), 850−1050 (∼27.5).
5. Yield: 0.054 g (40%). Mp: >250 °C. Elem anal. Calcd for
C68H84I4N4O20P4Zn4 (Mr = 2170.5): C, 37.63; H, 3.90; N, 2.58. Found:
C, 37.58; H, 3.73; N, 3.18. IR (KBr, cm−1): 3406 (br), 2962 (s), 2866
(w), 1617 (s), 1441 (b), 1334 (s), 1182 (br), 1018 (s), 913 (w), 793 (w),
681 (w), 593 (w), 541 (w). 1H NMR (DMSO-d6, 500 MHz): δ 8.65 (d,
4H, bipy−H, 3JHH = 6.2 Hz), 8.32 (d, 4H, bipy−H, 3JHH = 7.4 Hz), 7.92
(d, 4H, bipy−H, 3JHH = 7.4 Hz), 7.80 (d, 4H, bipy−H, 3JHH = 6.2 Hz),
7.28 (s, 8H, Ar−H), 3.56 (septet, 8H, iPr−CH, 3JHH = 6.8 Hz), 1.06 (d,
Hz), 7.92 (d, 3JHH = 7.00 Hz), 7.81 (t, 3JHH = 6.00 Hz), 7.78 (d, 3JHH
7.74 Hz), 7.61 (t, 3JHH = 8.04 Hz), 7.01 (d, 3JHH = 8.25 Hz), 7.96 (t, 3JHH
= 6.32 Hz), 3.65 (septet, 3JHH = 6.77 Hz), 1.07 (d, 3JHH = 6.85 Hz). 31
=
P
3
48H, CH3, JHH = 6.8 Hz). 31P NMR (DMSO-d6, 202 MHz): δ −5.5.
NMR (DMSO-d6, 202 MHz): δ −5.2. TGA [temp range, °C (weight
loss, %)]: 30−220 (∼5.0), 220−700 (∼64.0).
TGA [temp range, °C (weight loss, %)]: 30−320 (∼2.5), 320−510
(∼60.0), 510−875 (∼4.5), 875−1090 (∼27.52).
4.6. Single-Crystal X-ray Diffraction Studies. Single crystals of
compounds 2−7 were directly obtained from the reaction mixture by
slow evaporation of the solvent. The determination of the unit cell
parameters and data collection were performed with Mo Kα radiation (λ
= 0.7107 Å). Diffraction intensities were collected on a Rigaku Saturn
724+ CCD diffractometer, and data integration and indexing were
carried out using CrystalClear-SM Expert. Various calculations were
4.4. Synthesis of 6. To a solution of [Zn(OAc)2·2H2O] (0.40
mmol, 0.086 g) and dippH2 (0.40 mmol, 0.102 g) in methanol (30 mL)
was slowly added a solution of BIPYMO (0.20 mmol, 0.035 g) in
methanol. A solution of TBAF (1 M THF; 0.40 mmol, 0.40 mL) in
methanol was slowly added to the above reaction mixture, and 6 was
obtained as a crystalline product after 1 week. Yield: 0.085 g (45%). Mp:
I
DOI: 10.1021/acs.inorgchem.5b02949
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Article
carried out using WinGX.22 The structures were solved by direct
methods (SIR-92),23 and the final refinement was carried out using full
least-squares methods on F2 using SHELXL-97.24 Although multiple
attempts were made, poor-quality crystals of 6 did not yield good
diffraction data. The structure of 6 contains highly disordered [Bu4N]+
counterions that could not be precisely modeled and refined
anisotropically. The lattice solvent molecules in 4 are disordered, and
hence associated hydrogen atoms have not been identified or fixed.
Details of the structure determination are reported in Table 3.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02949.
Synthesis, crystallographic details, additional figures, and
spectral characterization (PDF)
X-ray crystallographic data in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*Tel: +91 22 2576 7163. Fax: +91 22 2572 3480. E-mail: rmv@
chem.iitb.ac.in.
■
Present Address
‡Department of Chemistry (Inorganic Branch), University of
Kashmir, Hazratbal, Srinagar, J&K, 190006, India
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by SERB and DST Nanomission, New
Delhi, and DAE (BRNS), Mumbai. R.M. thanks BRNS for an
DAE-SRC Outstanding Investigator Award, which enabled the
purchase of a single-crystal CCD diffractometer. A.A.D. thanks
UGC for a research fellowship.
DEDICATION
■
†This paper is dedicated to the memory of Professor P.
Natarajan.
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