Fluoride ion sensing and caging by a preformed molecular D4R zinc phosphate heterocubane

Article abstract of DOI:10.1021/ic500193n

Double-4-ring (D4R) zinc phosphate [Zn(dipp)(DMSO)]₄ (1, dipp = 2,6-di-iso-propylphenylphosphate, DMSO = dimethyl sulfoxide), on treatment with a free fluoride ion source, exhibited ability to sense and capture fluoride ions from a variety of sources, as evidenced by extensive solution ³¹P and ¹⁹F NMR spectral titration studies. The fluoride ion-encapsulated cage [ⁿBu₄N][F@{Zn(dipp)(DMSO)}₄] (2) was isolated in good yield from an equimolar reaction between 1 and ⁿBu₄NF in methanol and characterized by analytical and spectroscopic methods. When 1-methyl-4,4′-bipyridin-1-ium fluoride (MeQ-F) was used as the fluoride ion source a zwitterionic cage [F@{Zn ₄(dipp)₄(MeQ)(DMSO)₃}] (3) was isolated. Crystal structure determination for 3 confirmed not only fluoride incorporation inside the D4R cage but also a weak interaction of the central fluoride ion with all four zinc centers of the cubane, resulting in a trigonal bipyramidal geometry around the zinc centers. To establish the selectivity, cubane 1 was treated with 2 equiv of MeQ-X (X = various anions) under similar conditions to isolate [F@{Zn₄(dipp)₄(MeQ)₂(MeOH) ₂}][X] (X = I 4; BF₄ 5; PF₆ 6) in good yields. The crystal structure determination of 4 and 5 showed that the iodide and tetrafluoroborate anions are found outside the cage while fluoride ion has entered the cavity. The final fluoride encapsulated D4R cage is anionic in 2, neutral in 3, and cationic in 4-6, showing the versatility of the cubane framework to stabilize fluoride ions in all three forms. NMR titrations showed that 1 can sense even 1 ppm level of fluoride ions and sequester them from fluoridated water and toothpaste extract.

Full text of DOI:10.1021/ic500193n

Article
pubs.acs.org/IC
Fluoride Ion Sensing and Caging by a Preformed Molecular D4R Zinc
Phosphate Heterocubane
Alok Ch. Kalita and Ramaswamy Murugavel*
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai-400076, India
S
* Supporting Information
ABSTRACT: Double-4-ring (D4R) zinc phosphate [Zn(dipp)(DMSO)]4
(1, dipp = 2,6-di-iso-propylphenylphosphate, DMSO = dimethyl sulfoxide),
on treatment with a free fluoride ion source, exhibited ability to sense and
capture fluoride ions from a variety of sources, as evidenced by extensive
solution 31P and 19F NMR spectral titration studies. The fluoride ion-
encapsulated cage [nBu4N][F@{Zn(dipp)(DMSO)}4] (2) was isolated in
good yield from an equimolar reaction between 1 and nBu4NF in methanol
and characterized by analytical and spectroscopic methods. When 1-
methyl-4,4′-bipyridin-1-ium fluoride (MeQ-F) was used as the fluoride ion
source a zwitterionic cage [F@{Zn4(dipp)4(MeQ)(DMSO)3}] (3) was
isolated. Crystal structure determination for 3 confirmed not only fluoride
incorporation inside the D4R cage but also a weak interaction of the central
fluoride ion with all four zinc centers of the cubane, resulting in a trigonal
bipyramidal geometry around the zinc centers. To establish the selectivity, cubane 1 was treated with 2 equiv of MeQ-X (X =
various anions) under similar conditions to isolate [F@{Zn4(dipp)4(MeQ)2(MeOH)2}][X] (X = I 4; BF4 5; PF6 6) in good
yields. The crystal structure determination of 4 and 5 showed that the iodide and tetrafluoroborate anions are found outside the
cage while fluoride ion has entered the cavity. The final fluoride encapsulated D4R cage is anionic in 2, neutral in 3, and cationic
in 4−6, showing the versatility of the cubane framework to stabilize fluoride ions in all three forms. NMR titrations showed that 1
can sense even 1 ppm level of fluoride ions and sequester them from fluoridated water and toothpaste extract.
demonstrated recently in a few instances.7−9 As a specific
example, molecular clusters possessing D4R core structure have
INTRODUCTION
■
Chemical sensing and scavenging are two rigorously inves-
been employed to encapsulate atomic species through the
tigated areas in recent times due to environmental and security
isolation of hydrogen-encapsulated T8 cages by γ-irradiation of
concerns.1,2 In particular, development of reliable sensors for
octasilsesquioxanes.7
anions such as F−, NO3 , SO42−, and PO4 inside suitably
designed molecular pockets, cages, and clathrates has been an
active area of research.3 Among various anions, fluoride ion
sensing is of specific interest owing to the high toxicity of
soluble fluorides. Soluble fluorides are commonly found beyond
a certain level in domestic products such as toothpaste
(Na2PO3F), dietary supplements (NaF), glass-etching or
chrome-cleaning agents (NH4HF2), insecticides, rodenticides
(NaF), and also in drinking water (NaF). The lethal dose for
adults is 32−64 mg of elemental fluoride per kilogram of body
weight. The dose that leads to adverse health effects such as
crippling skeletal fluorosis is often roughly about one-fifth of
the lethal dose.4,5 In most cases fluoride poisoning is caused by
drinking water containing more than 2−3 ppm levels of
fluoride.5
−
3−
Bassindale et al. have isolated fluoride ion-encapsulated T8
silsesquioxane cages by carrying out the hydrolysis of trialkoxy
silanes in the presence of a fluoride source.8,9 In a typical
synthesis, RSi(OEt)3 is hydrolyzed in chloroform in the
presence of tetrabutylammonium fluoride (nBu4NF) (1).8
This reaction also works well with germanium to produce
fluoride-encapsulated germanium T8 clusters [RGeO1.5]8.10
Encapsulation of fluoride ion inside a D4R vanadium phosphate
during the cluster formation has also been reported.11 Similarly,
the chemistry of framework solids that has been developing
over the last two decades has seen an increased number of
instances where fluoride ions get incorporated inside the
framework cages during the synthesis of such frameworks,
which often leads to enhanced stability of the framework
solids.12,13
Colorimetry and electrochemistry (ion selective electrodes)
have been utilized to measure the extent of fluoride sensing in
many instances.6 An alternative strategy, to not only sense but
also sequester fluoride ions from solution, is to make use of the
empty space available within the preformed inorganic cages.
Encapsulation of small molecules and ions inside the cluster
cavities during the cage/cluster formation phase been
nBu4NF
[RSi(OEt)3] ⎯⎯⎯⎯⎯⎯→ F@[RSiO1.5]8
CHCl3
(1)
Received: October 28, 2013
Published: March 10, 2014
© 2014 American Chemical Society
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All the above-mentioned examples of fluoride incorporation
take place during the cage formation or synthesis phase, and to
our knowledge there have been no instances in the published
literature where a preformed inorganic molecular cage has been
shown to be a selective fluoride sensor. Our recent success in
synthesizing a large variety of organic-soluble cage-like
molecular phosphates14 and phosphonates15 provided us an
opportunity to investigate the possibility of using small
preformed D4R zinc phosphates to sense, complex, and
capture fluoride ions.
Methanol-soluble D4R phosphate [Zn(dipp)(DMSO)]4 (1)
(dipp = di-iso-propylphenylphosphate) (Figure 1), which can
Figure 2. (left) The 31P NMR spectral titration (CD3OD) of a 1 ×
10−5 M solution of 1 with the following concentrations of nBu4NF: (a)
none; (b) 1 × 10−6 M; (c) 5 × 10−6 M; (d) 1 × 10−5 M. (right) The
n
19F NMR spectrum (CD3OD) of a 5 × 10−6 M solution of Bu4NF
with the following concentrations of 1: (a) none; (b) 2 × 10−6 M; (c)
3 × 10−6 M; (d) 5 × 10−6 M.
(Cl−, Br−, and I−) did not result in any change in the observed
31P NMR spectrum, indicating the selectivity of 1 toward only
fluoride sensing.
A reverse titration was performed to track the movement of
fluoride ions from nBu4NF (δF = −106 ppm; Figure 2, right) to
the center of the cubane. Addition of 1 equiv of 1, in several
steps, leads to the complete disappearance of the −106 ppm
signal, with a new resonance appearing at −113 ppm,
supporting the 31P NMR studies and also confirming
incorporation of fluoride ion inside the cubane (Figure 2,
spectrum d).
Figure 1. Structure of D4R cubane with labile DMSO ligands.
be synthesized in quantitative yield from inexpensive starting
materials such as zinc acetate and a phosphoric acid monoester,
was employed as the host in the present study to sense and
cage fluoride ions. The architecture of cage molecule 1 is such
that the DMSO ligands on the zinc centers point away from the
cubane and hence do not block the access to the cavity.
Synthesis and Spectra of Compounds 2 and 3. To
isolate the fluoride-incorporated cubane, a methanol/DMSO
solution of 1, freshly prepared from the reaction of Zn(OAc)2·
RESULTS AND DISCUSSION
n
■
2H2O with dipp-H2, was treated with Bu4NF at ambient
conditions. [nBu4N][F@{Zn(dipp)(DMSO)}4] (2) (Scheme
1) was isolated as the only product and characterized by
analytical and spectroscopic measurements (Supporting In-
formation, Figure S1 and Table S1).
NMR Titrations Involving [Zn(dipp)(DMSO)]4 (1).
Cluster 1 dissolved in CD3OD exhibits a single sharp resonance
in the 31P NMR spectrum at −4.2 ppm, owing to the highly
symmetric nature of the cluster. It can be envisaged that when
fluoride ion enters the cage, it would be somewhat proximal to
the phosphorus centers and hence should result in a significant
shift of the phosphorus resonance. Hence, to probe the ability
of 1 to encapsulate fluoride ions, 31P NMR spectral titrations
were employed (Figure 2). Spectrum a depicts the observed
spectrum of a CD3OD solution of 1 (1 × 10−5 M) before
addition of F− ions. It can be seen from spectrum b that the
addition of 1 × 10−6 M of nBu4NF produces a new signal at 3.2
ppm, which is roughly 7 ppm downfield-shifted relative to the
signal for pure 1. Successive quantities of nBu4NF added to this
solution increase the intensity of the 3.2 ppm signal to the same
extent as the decrease in intensity of the signal at −4.2 ppm.
After addition of 1.0 × 10−5 M nBu4NF, the signal at −4.2 ppm
completely disappears (spectrum d in Figure 2). These studies
thus provide evidence for a change in the structure of 1 that
affects equally the chemical shift of all four phosphorus atoms
of the D4R structure, which is possible only if the fluoride ion
enters the cubane and occupies the empty space available at the
center of the cubane. Since the center of the cubane to the
phosphorus atoms is much longer for any bonding interactions,
only a weak interaction of the fluoride ion with the four
Scheme 1. Synthesis of 2
The Fourier transform infrared (FT-IR) spectrum exhibits
strong absorptions at 1147, 1037, and 908 cm−1 for
characteristic PO stretching and M−O−P asymmetric and
symmetric stretching vibrations, respectively (Figure 3).14 The
1H NMR spectrum shows well-separated resonances for the
protons of the di-iso-propylphenyl group and tetrabutylammo-
nium cation in a 4:1 ratio. Isopropyl −CH and −CH3 groups
exhibit a septet and a doublet at 3.8 and 1.08 ppm, respectively.
As was observed during the 31P NMR titration studies (vide
supra), the 31P NMR spectrum of the isolated product exhibits
a singlet at 2.5 ppm in DMSO-d6. A single resonance at −113
phosphorus atoms is expected, leading to a small shift in the 31
NMR signal. Similar titrations carried out with other halides
P
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1H and 31P NMR spectral characteristics of 3 are very similar to
2 (Supporting Information, Table S1, Figure S4). A single
resonance at δ = −113 ppm is observed in the 19F NMR
spectrum for the encapsulated fluoride in 3 as in the case of 2.
Molecular Structure of Compound 3. Single-crystal
structure analysis reveals that compound 3 crystallizes in the
orthorhombic noncentrosymmetric P212121 space group. The
asymmetric unit of 3 is featured by the Zn4P4O16 D4R core as
in the case of parent cubane 1, albeit with two major differences
(Figure 5). The first is the presence of fluoride ion at the center
Figure 3. FT-IR spectra of compounds 2−6 diluted in KBr disc.
ppm in 19F NMR spectrum is also consistent with the 19F NMR
titration studies described above.
Additional evidence for the fluoride incorporation comes
from negative ion electrospray ionization (ESI) mass spectral
(MS) studies, which produce MS peaks under mild ionization
conditions with the expected isotopic distribution for [2-
4DMSO]−, [2-3DMSO]−, and [2-2DMSO]− ions. The MS
depicted in Figure 4 confirms the fluoride incorporation
Figure 5. Molecular structure of 3. Selected bond distances [Å]: Zn−
O 1.911(3)−1.948(3) (av 1.928), P−O(Zn) 1.497(3)−1.527(3) (av
1.507), P−O(Ar) 1.606(3)−1.619(3) (av 1.614), Zn−F 2.224(2)−
2.447(2) (av 2.332), P−F 2.910(2)−2.967(2) (av 2.939); bond angles
[deg]: O−Zn−O (equatorial) 116.12(2)−124.94(1) (av119.32), O−
Zn−O (axial) 89.57(1)−93.80(1) (av 91.59), N−Zn−O 88.30(1)−
98.13(2) (av 94.75), O−Zn−F 80.93(1)−88.10(1) (av 85.31), ∠P
103.40(2)−115.21(2) (av 109.22), Zn−O−P 122.47(2)−134.9 (2)
(av127.31).
of cubane, showing significant but weak interactions with all
four zinc centers. The second difference is the change in
coordination geometry around the tetrahedral zinc centers in
Zn4P4 cubanes to distorted trigonal bipyramidal (tbp) in 3
(Figure 5).
Figure 4. Negative ion ESI MS of compound 2.
through the peak appearing at m/z 1305.07, corresponding to
[F@{Zn(dipp)}4]− ion. The host cluster 1 produces a signal at
m/z 1543.03 with the isotopic distribution that corresponds to
[1-DMSO+Na]+ ion in ESI MS (positive ion mode)
(Supporting Information, Figure S2).
Since the MeQ+ ion binds only to one of the four zinc
centers of the cubane, two types of Zn centers are found. Three
of the four zinc centers are surrounded by three phosphate
oxygen atoms at the equatorial positions (av Zn−O 1.928 Å),
one DMSO oxygen at one of the axial positions, and the weakly
bound fluoride ion in the other axial position (av Zn···F 2.332
Å). The fourth and the unique zinc atom is coordinated to
three phosphate oxygen atoms (the Zn(1)−O(9) 1.913(3),
Zn(1)−O(5) 1.923(3), and Zn(1)−O(1) 1.915(3) Å unit) at
the equatorial plane, one axial pyridyl nitrogen of MeQ+
(Zn(1)−N(1) 2.153(3) Å), and the axial fluoride (Zn(1)−
F(1) 2.447 Å).
While it has been clearly established by a combination of
analytical and spectroscopic studies that compound 2
represents the fluoride ion-incorporated version of 1, all
attempts to obtain single crystals of 2 have been unsuccessful,
presumably due to the disorder-prone tetrabutylammonium
countercation. Hence the synthetic strategy was modified by
choosing N-methylviologen hexafluoroantimoniate, [MeQ]-
SbF6, as fluoride ion source, which can also act as a pyridinic
ligand for the cage zinc ions. The pyridinic part of the MeQ
cation can be expected to replace a DMSO on one of the zinc
centers on the cubane to produce a cationic cubane, which can
then cage the fluoride ion to produce an anion-incorporated
cationic cage. As anticipated, treatment of 1 with [MeQ]SbF6
proceeds smoothly under similar reaction conditions to yield
[F@{Zn4(dipp)4(DMSO)3(MeQ)] (3) as single crystals. The
Interestingly the observed Zn···F separations (av 2.332 Å)
are considerably longer than that of a Zn−F covalent bond
(∼1.775 Å). Similarly the P···F distances observed in 3 (av
2.939 Å) are almost twice those of the covalent P−F bonds
(∼1.56 Å as in PF3).16 These values suggest that the zinc
centers of the cubane are far more affected than the phosphorus
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Table 1. Comparison of the Dimensions of Cores of Various D4R Cubanes and the Interaction of Fluoride Ion with Cage
Atoms
13
F@[RSiO1.5]8 (R = vinyl)9f
F@[Ge8O12(OH)8]10
F@[Zn(dipp)(MeQ)(DMSO)3]4 (3)
[Zn(dipp)(collidine)]4
average distances (Å) and angles (deg) average distances (Å) and angles (deg) average distances (Å) and angles (deg) average distances (Å) and angles (deg)
Si···Si (edge), 3.058k
Si···Si (face diagonal), 4.325
Si···Si (body diagonal), 5.301
Si···F, 2.65
∠ O−Si−O (cage), 112.6
∠ C−Si−O, 106.1
Ge···Ge (edge), 3.185
Ge···Ge (face diagonal), 4.467
Ge···Ge (body diagonal), 5.512
Ge···F, 2.760
∠ O−Ge−O (cage), 115.8
∠ O−Ge−O, 102.0
Zn···P (edge), 3.095
Zn···P (edge), 3.196
Zn···Zn (face diagonal), 3.850
P···P (face diagonal), 4.780
Zn···P (body diagonal), 5.277
Zn···F, 2.332; P···F, 2.939
∠ O−Zn−O (cage), 119.3
Zn···Zn (face diagonal), 4.419
P···P (face diagonal), 4.632
Zn···P (body diagonal), 5.529
∠ O−Zn−O (cage), 111.6
centers by the fluoride incorporation (all phosphorus atoms are
left in the original tetrahedral geometry). The entry of fluoride
ion into the cage thus brings about a distortion in the cubane
structure by fluoride drawing zinc centers closer to it without
disturbing the phosphorus centers. As a result the Zn···Zn face
diagonal distance falls to 3.85 Å while the P···P face diagonal
stays at 4.78 Å. In the parent cubane with no fluoride ion
incorporation these values are 4.32 and 4.61 Å, respectively.13
Table 1 depicts a further comparison of the dimensions of
Scheme 2. Synthesis of Neutral (Zwitterionic) 3 and
Cationic 4−6
̀
the cubane core 3 vis-a-vis F@Si8 and F@Ge8 systems reported
in the literature.9,10 The major difference between 3 and the
other two examples is the symmetric nature of the D4R cubane
itself. While the cubane symmetry is more or less preserved in
the Si and Ge cubanes even after the encapsulation of fluoride
anion, the zinc phosphate cubane undergoes a structural change
so as to enhance fluoride to zinc centers interaction. This is
easily reflected in significantly shorter Zn···F distances in 3 (av
2.332 Å) compared to longer Si···F and Ge···F distances (av
2.65 and 2.76 Å, respectively). As a consequence, Si and Ge
centers in F@Si8 and F@Ge8 retain their tetrahedral geometry,
while the zinc centers in 3 adopt tbp geometry after fluoride
incorporation. The unique ability of preformed 1 to encapsulate
fluoride (unlike the incorporation of fluoride in the cage
formation stages in the case of the Si8 and Ge8 systems) can be
attributed to the Lewis acidity of tetrahedral zinc ions.
nBu4NF results in [F@{Zn4(dipp)4(MeQ)2(CH3OH)2}][PF6]
(6). Compounds 4 and 5 have been obtained as single crystals,
and their crystal structures have been determined (Figures 6
and 7). The formation of compounds 4−6 with fluoride ion
Synthesis and Spectra of Compounds 4−6 and the
Test of Selectivity. The use of [MeQ]SbF6 as the source of
fluoride ion as well as N-donor ligand for the synthesis and
structural characterization of 3 impelled the possibility of
introducing a second positive charge on the cubane by using an
extra equivalent of MeQ+ in the reaction. Such an approach
would also offer an opportunity to introduce a second anion,
albeit present outside the cubane. This would in turn allow the
evaluation of the selectivity of D4R cages for the F− anion over
the other anions. To verify this hypothesis (Scheme 2),
compound 1 was treated with 2 equiv of [MeQ]I followed by
Figure 6. Molecular structure of compound 4. Selected bond distances
[Å]: Zn−O 1.905(7)−2.010(6) (av 1.961), P−O(Zn) 1.482(7)−
1.588(7) (av 1.530), P−O(Ar) 1.616(7)−1.646(7) (av 1.631), Zn−F
2.276(5)−2.450(5) (av 2.356), P−F 2.960(6)−3.005(6) (av 2.989);
bond angles [deg]: O−Zn−O (equatorial) 109.20(3)−130.40(3)
(av119.67), O−Zn−O (axial) 92.90(3)−94.70(3) (av 94.03), N−Zn−
O 90.40(3)−97.60(3) (av 93.56), F−Zn−O (equatorial) 84.10(2)−
88.60(2) (av 86.83), N−Zn−F 173.30(2)−174.4(2) (av 173.85), ∠P
103.10(4)−114.0(4) (av109.23), ∠ Zn−O−P 119.50(4)−134.10(4)
(av 127.05).
n
the addition of a fluoride source. The use of Bu4NF as the
fluoride ion source leads to the isolation of [F@{Zn4(dipp)4-
(MeQ)2(CH3OH)2}][I] (4), whereas NaBF4 as the F− ion
source yields [F@{Zn4(dipp)4(MeQ)2(CH3OH)2}][BF4] (5).
The change of viologen to [MeQ][PF6] in the presence of
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the proof (in the solid state) of the ability of the cubane to
encapsulate only fluoride ion and keep other anions such as I−
−
and BF4 outside the cage.
Thermal Analyses. Thermogravimetric analysis (TGA) of
compound 2 under N2 atmosphere reveals a weight loss of
∼67.5% in the range of 100−500 °C, corresponding to the loss
of all organic substituents and water molecules, leading to the
formation of zinc pyrophosphate [Zn2P2O7]0.5 (∼32%,
calculated from molecular formula).20 Compounds 3−6 show
similar thermal behavior, and the remaining ceramic material
obtained in each case corresponds to the zinc pyrophosphate.
Figure 8 shows TGA plots of 2−6 at a heating rate of 10 °C/
min under N2 atmosphere.
Figure 7. Molecular structure of 5. Selected bond distances [Å]: Zn−
O 1.943(6)−1.967(5) (av 1.953), P−O(Zn) 1.495(6)−1.526(6) (av
1.514), P−O(Ar) 1.599(5)−1.641(7) (av 1.619), Zn−F 2.245 (4)−
2.407(4) (av 2.331), P−F 2.977(2)−3.021(2) (av 3.009); bond angles
[deg]: O−Zn−O (equatorial) 109.50(2)−129.30(2) (av119.80), O−
Zn−O (axial) 89.20(2)−93.30(2) (av 90.90), N−Zn−O 89.30(2)−
96.50(3) (av 92.83), F−Zn−O (equatorial) 84.57(2)−89.53(2) (av
87.56), N−Zn−F 173.10(2)−175.7(2) (av 174.40), ∠P 103.50(3)−
115.90 (4) (av109.18), Zn−O−P 119.80(3)−133.40(4) (av 126.14).
inside the cage and the second anion (iodide, tetrafluoroborate,
and hexafluorophosphate, respectively) outside the cage clearly
underscores the selectivity of the D4R cubane to encapsulate
only fluoride and leave any other anions outside the cage.
The cationic fluoride-incorporated cubanes 4−6 were
Figure 8. TGA plots of 2−6 at heating rate of 10 °C/min under N2.
1
characterized by elemental analysis, FT-IR, and H and 31P
NMR spectroscopic methods (Supporting Information, Table
S1). Clusters 4−6 were probed by negative ion ESI mass
spectrometry to confirm fluoride incorporation (Supporting
Information, Figures S5−S7). The observed FT-IR spectra are
similar to that of 2 (Figure 3).14 The 1H and 31P NMR spectra
of 4−6 show spectral behavior similar to 3 (Supporting
Information, Figures S8−S10). For 6, two 31P NMR signals are
observed at 2.5 and −144 ppm; the former corresponds to the
cubane phosphorus center as a singlet, and the latter
corresponds to that center as a septet for the PF6 anion
(Supporting Information, Figure S10). Similar to the spectra of
Possible Applications: Sensing Fluoride Present in
Water and Toothpaste. The ability of compounds 2−6 not
only in encapsulating fluoride ions but also in excluding other
larger anions points to possible applications for the D4R zinc
phosphate cubanes. Since anything in excess of 4 mg/L or 4
ppm of NaF in drinking water can lead to adverse health effects,
cubane 1 was tested for its ability to pick up fluoride ions from
aqueous medium through 19F NMR titration of NaF dissolved
in water against 1 (Figure 9). The fluoride ion in water gives a
single resonance at −144.0 ppm, which on addition of 1 shifts
to −113.5 ppm due to the incorporation of fluoride ions inside
compounds 2 and 3, a single resonance at −113 ppm in the 19
F
NMR spectrum of 4 indicates the presence of only one type of
fluoride (Supporting Information, Figure S8). In the 19F NMR
spectrum of 5, while the signal at −113 ppm is due to D4R-
encapsulated fluoride, two signals with the intensity ratio of 1:5
at −148 and −148.2 ppm are assignable to the 10B- and 11B-
−
coupled fluorine of the BF4 anion (Supporting Information,
Figure S9).17 In the 19F NMR spectrum of 6, apart from the
expected F@D4R resonance at −113 ppm, the doublet
observed at −70 ppm (J = 711 Hz) is assignable to the
−
phosphorus-coupled signal of the PF6 anion (Supporting
Information, Figure S10).18,19
Molecular Structures of Compounds 4 and 5.
Compounds 4 and 5 crystallize in monoclinic P21/c space
group. There are two different types of Zn centers present in
the asymmetric unit of 4 and 5 (Figures 6 and 7; Supporting
Information, Figures S11 and S12). Similar to compound 3, all
Zn centers in 4 and 5 adopt tbp geometry, with one of the axial
positions occupied by fluoride. The bond distances and angles
observed for 4 and 5 are similar to those described above for 3.
The most significant outcome of structural studies on 4 and 5 is
Figure 9. The 19F NMR spectrum (CD3OD) of 3.3 × 10−5 M solution
of NaF dissolved in water with the following concentrations of
compound 1: (a) none; (b) 1.3 × 10−5 M; (c) 2.6 × 10−4 M; (d) 4 ×
10−5 M.
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(Sigma Aldrich), Zn(OAc)2·2H2O (S.d.Fine-Chem.), 4,4′-bipyridine
(Sigma Aldrich), methyl iodide (Sigma Aldrich), tetrabutyl ammonium
fluoride (nBu4NF) (Sigma Aldrich), NaBF4 (Alfa Aesar), AgSbF6
(Sigma Aldrich), and NH4PF6 (Sigma Aldrich) were used as received.
The 2,6-di-iso-propylphenyl phosphate and the bipyridinium methyl
salts, for example, 1-methyl-4,4′-bipyridin-1-ium iodide (MeQI), 1-
methyl-4,4′-bipyridin-1-ium hexafluorophosphate (MeQPF6), and 1-
methyl-4,4′-bipyridin-1-ium hexafluoroantimonate (MeQSbF6), were
synthesized according to the reported methods.22,23
the cage. When all fluoride ions have been incorporated inside
the cage, the resonance at −144.0 ppm completely disappears,
thus giving the exact F− ion concentration in water.
A similar experiment was also performed to determine the
amount of fluoride ion present in commercial toothpastes.
Cubane 1 was treated with DMSO extracts of an anticavity
toothpaste containing Na2PFO3. The 31P NMR spectrum in
this case (Figure 10) clearly shows a new peak at 2.5 ppm,
[nBu4N][F@{Zn(dipp)(DMSO)}4] (2). [Zn(dipp)(DMSO)]4 (1)
(0.025 g, 0.016 mmol) was dissolved in methanol (20 mL), and a
n
saturated solution of Bu4NF in methanol (5 mL) was added, stirred,
and kept for crystallization on benchtop to obtain colorless crystals of
compound 2 after 2 d. mp: >250 °C. Yield: 0.025 g (83%). Anal. Calcd
for C72H128FNO20P4S4Zn4 (Mr = 1860.52) (%): C, 46.48; H, 6.93; S,
6.89. Found: C, 47.06; H, 6.47; S, 6.71. FT-IR (KBr, cm−1): 2961 (vs),
2868 (s), 1652 (br), 1447 (s), 1383 (s), 1256 (s), 1156 (vs), 1037
(vs), 995 (vs), 908 (vs), 769 (vs), and 564 (vs). 1H NMR (DMSO-d6,
3
i
400 MHz): δ 6.9 (m, 12H, Ar), 3.8 (septet, 8H, JHH = 6.8 Hz, Pr−
3
3
CH), 3.1 (t, 8H, JHH = 6.9 Hz, −CH2), 1.5 (m, 8H, JHH = 7.6 Hz,
−CH2), 1.3 (m, 8H, 3JHH = 7.3 Hz, −CH2), 1.05 (d, 48H, 3J = 6.9 Hz,
iPr−CH3), 0.9 (t, 12H, JHH = 7.2 Hz, −CH3) ppm. 31P NMR
3
(DMSO-d6, 161 MHz): δ 3.2 ppm. 19F NMR (DMSO-d6, 400 MHz):
δ −113 (s, 1F, F@D4R) ppm. ESI MS (-ve): calculated for [M-
4DMSO]−, m/z 1305, found 1305.
[F@{Zn4(dipp)4(MeQ)(DMSO)3}] (3). Compound 1 (0.04 g, 0.025
mmol) was dissolved in methanol (20 mL), and a solution of
[MeQ][SbF6] (0.04 g, 0.1 mmol) in methanol (15 mL) was added,
stirred, and kept on benchtop to yield colorless crystals of compound 2
after 4−5 d. mp: >250 °C. Yield: 0.035 g (83%). Anal. Calcd for
C65H97FN2O19P4S3Zn4 (Mr = 1711.14) (%): C, 45.62; H, 5.71; N,
1.64. Found: C, 45.68; H, 6.16; N, 1.48. FT-IR (KBr, cm−1): 3434
(br), 2962 (vs), 2867 (s), 1647 (s), 1613 (s), 1467 (s), 1441 (s), 1337
(s), 1256 (s), 1156 (vs), 996 (vs), 911 (vs), 822 (s), 769 (vs), 743
(vs), and 564 (s). 1H NMR (DMSO-d6, 400 MHz): δ 9.0 (d, 2H, 3J =
Figure 10. 31P NMR spectrum (DMSO-d6) of [Zn(dipp)(DMSO)]4
after adding toothpaste extract containing Na2PFO3.
confirming fluoride encapsulation. This once again demon-
strates the fluoride ion scavenging capabilities of 1 from a
variety of fluoride sources.
Conclusions. In summary, we demonstrated that the
preformed D4R zinc phosphate molecular clusters play host
to fluoride ions in solution and in solid state, with the resultant
F@D4R structures being stabilized in anionic (as in 2), neutral
(as in 3), and cationic (as in 4−6) forms. It was further shown
3
3
6.6 Hz, ortho-N+), 8.8 (d, 2H, J = 6.1 Hz, ortho-N), 8.5 (d, 2H, J =
6.6 Hz, meta-N+), 7.9 (d, 2H, 3J = 6.1 Hz, meta-N+), 6.9 (m, 12H, Ar),
3
i
4.2 (s, 3H, CH3−N+), 3.3 (septet, 8H, JHH = 6.7 Hz, Pr−CH), and
1.06 (d, 48H, JHH = 6.9 Hz, Pr−CH3) ppm. 31P NMR (DMSO-d6,
161 MHz): δ 3.0 ppm. 19F NMR (DMSO-d6, 400 MHz): δ −113 (s,
1F, F@D4R) ppm. ESI MS (-ve): calculated for [M-MeQ-3DMSO]−,
m/z 1305, found 1305.
3
i
n
that fluoride ion source can vary from organic soluble Bu4NF
(or even [MeQ]SbF6) to inorganic fluorides such as NaF (the
source of fluoride ions in drinking water) and Na2PFO3
(commonly found in toothpastes). This is the first report on
the usage of preformed D4R structure for the purposes of anion
sensing and scavenging in a neat fashion under ambient
conditions. A simple technique such as NMR spectroscopy
(exceptional detection limits) can be used to quantitatively
determine the incorporation both in aqueous and organic
media by using either phosphorus or fluorine probes,
complementing other types of fluoride sensors reported in
the literature that use other techniques for sensing action.21
[F@{Zn4(dipp)4(MeQ)2(CH3OH)2}][I]·3CH3OH (4). To a solution
of compound 1 (0.025 g, 0.016 mmol) in methanol (20 mL), [MeQ]I
(0.0360 g, 0.12 mmol) was added. The solution was further diluted
with methanol (80 mL), and a saturated solution of nBu4NF in
methanol (5 mL) was added slowly. The resulting solution was kept
for crystallization. After 5 d, compound 4 was obtained as yellowish
crystal. mp: >250 °C. Yield: 0.026 g (86%). Anal. Calcd for
C75H110FIN4O21P4Zn4 (Mr = 1935.07) (%): C, 46.55; H, 5.73; N,
2.90. Found: C, 46.07; H, 5.20; N, 2.71. FT-IR (KBr, cm−1): 2963
(vs), 2867 (s), 1644 (s), 1615 (s), 1441 (s), 1335 (s), 1256 (s), 1156
1
(vs), 997 (vs), 912 (vs), 772 (s), and 565 (s). H NMR (DMSO-d6,
400 MHz): δ 9.1 (d, 4H, 3JHH = 6.8 Hz, ortho-N+), 8.8 (d, 4H, 3JHH
=
6.2 Hz, ortho-N), 8.5 (d, 4H, 3JHH = 6.9 Hz, meta-N+), 8.0 (d, 4H, 3JHH
EXPERIMENTAL SECTION
■
= 6.2 Hz, meta-N+), 7.0 (m, 12H, Ar), 4.3 (s, 6H, CH3−N+), 3.8
Instruments and Methods. All reactions were carried out under
fume hood in beakers or round-bottom flasks without any special
precautions, if not stated otherwise. All the starting materials and the
products were found to be stable toward moisture and air, and no
specific precaution was taken to rigorously exclude air. The melting
points were measured in glass capillaries. Infrared spectra were
obtained on a Perkin-Elmer Spectrum One FT-IR spectrometer as KBr
diluted discs. Microanalyses were performed on a Thermo Finnigan
(FLASH EA 1112) microanalyzer. NMR spectra were recorded using a
Bruker Advance DPX-400 spectrometer, and the peaks that are not
labeled are always solvent peaks, if not stated otherwise. Thermo
gravimetric analyses were carried out with a Perkin-Elmer thermal
analysis system, under a stream of nitrogen gas, at the heating rate of
10 °C/min. Commercial-grade solvents were purified by employing
standard procedures. Other chemicals such as 2,6-di-iso-propylphenol
3
i
3
(septet, 8H, JHH = 6.8 Hz, Pr−CH), 1.05 (d, 48H, JHH = 6.8 Hz,
iPr−CH3) ppm. 31P NMR (DMSO-d6, 161 MHz): δ 2.5 ppm. 19F
NMR (DMSO-d6, 400 MHz): δ −113 (s, 1F, F@D4R) ppm. ESI MS
(-ve): calculated for [M-2MeQ-5CH3OH-I]−, m/z 1305, found 1305.
[F@{Zn4(dipp)4(MeQ)2(CH3OH)2}][BF4] (5). To a solution of
compound 1 (0.04 g, 0.025 mmol) in methanol (20 mL), [MeQ]I
(0.04 g, 0.12 mmol) was added, and the solution was further diluted
with methanol (150 mL). A saturated solution of NaBF4 in methanol
(5 mL) was added, and the resulting solution was kept for
crystallization. The product was obtained as platelike crystals after
15 d. mp: >250 °C. Yield: 0.038 g (84%). Anal. Calcd for
C72H98BF5N4O18P4Zn4 (Mr = 1798.84) (%): C, 48.07; H, 5.49; N,
3.11. Found: C, 48.04; H, 4.93: N, 3.01. FT-IR (KBr, cm−1): 2963
(vs), 2866 (s), 1647 (s), 1614 (s), 1439 (s), 1417(s), 1335 (s), 1256
3350
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Inorganic Chemistry
Article
Table 2. Crystal Data and Structure Refinement Details for Compounds 3−5
a
compound
3
4
5
identification code
formula
rm359
ak642
ak186mp
C65 H97F N2O19P4S3Zn4
1710.99
C72H98F1I1N4O18P4Zn4
1838.80
C72H98BF5N4O18P4Zn4
1798.71
Fw
temp, [K]
crystal system
space group
a, [Å]
150(2)
150(2)
150(2)
orthorhombic
P212121
monoclinic
P 21/c
monoclinic
P 21/c
15.030(4)
21.704(7)
24.470(6)
90
21.480(2)
15.268(2)
31.930(4)
90
21.118(2)
15.064(1)
31.417(4)
90
b, [Å]
c, [Å]
α, [deg]
β, [deg]
90
97.689(1)
90
97.306(1)
90
γ, [deg]
V, [Å3]
90
7982(4)
4
10378(2)
4
9913(2)
4
Z
D (calcd), [g/cm3]
1.424
1.177
1.205
μ [mm−1
]
1.413
1.326
1.085
crystal size, [mm3]
0.36 × 0.33 × 0.28
3.32 to 25.00
58 068
0.16 × 0.15 × 0.08
1.81 to 25.00
55 298
0.37 × 0.04 × 0.03
2.87 to 25.00
58 910
θ range, [deg]
no. of rflns collected
independent reflns (I0 > 2σ(I0)
GOF
14 011
17 925
16 193
0.819
1.084
0.683
R1(I0 > 2σ(I0)
0.0425
0.1318
0.0631
wR2 (all data)
largest hole and peak [e·Å−3
0.0812
0.3713
0.1463
]
−0.406 and 0.553
−0.975 and 1.857
−0.398 and 0.943
a
Poor quality data; see Experimental Section for details.
1
(s), 1147 (vs), 991 (vs), 905 (vs), 771 (vs), and 609 (s). H NMR
(DMSO-d6, 400 MHz): δ 9.1 (d, 4H, 3JHH = 6.4 Hz, ortho-N+), 8.8 (d,
4H, 3JHH = 6.1 Hz, ortho-N), 8.6 (d, 4H, 3JHH = 6.9 Hz, meta-N+), 8.0
(d, 4H, 3JHH = 6.1 Hz, meta-N+), 7.0 (m, 12H, Ar), 4.38(s, 6H, CH3−
N+), 3.6 (septet, 8H, 3JHH = 7.1 Hz, iPr−CH), 1.1 (d, 48H, 3JHH = 6.9
refinement of the structure was carried out using full matrix least-
squares methods on F2 using SHELXL-97,27 which resulted in the
structure determination of 3. The compound crystallizes in the
orthorhombic P212121 space group. The dipp moiety and the DMSO
solvent show disorder, which has been resolved by dividing the atoms
into two parts. The final refinement converged at the R value of 0.0442
(I > 2σ(I)).
Hz, iPr−CH3) ppm. 31P NMR (DMSO-d6, 161 MHz): δ 2.5 ppm. 19
F
NMR (DMSO-d6, 400 MHz): δ −113 (s, 1F, F@D4R) and −148.1
(two peaks with intensity ratio of 0.22:1 because of the coupling with
10B and 11B, 4F, BF4) ppm. 11B NMR (DMSO-d6, 128 MHz): δ −1.25
ppm. ESI MS (-ve): calculated for [M-2MeQ-2CH3OH-BF4]−, m/z
1305, found 1305.
Crystallization of the crude product of compound 4 from methanol
at room temperature yields crystals of 4 as yellow block-type crystals.
A suitable crystal of size 0.16 × 0.15 × 0.08 mm3 was mounted on a
Rigaku Saturn 724+ ccd diffractometer for unit cell determination and
three-dimensional intensity data collection. 800 frames in total were
collected at 150 K with the exposure time of 24 s per frame. For the
analysis, the detector was kept at a distance of 45 mm from the crystal.
All calculations were carried out using the programs in WinGX
module25 and solved by direct methods (SIR-92).26 The final
refinement of the structure was carried out using full matrix least-
squares methods on F2 using SHELXL-97.27 Unit cell determination
using both high angle and low angle diffraction reveal that the
compound crystallizes with the monoclinic P21/c space group. The
final refinement of the solved structure converged at the R value of
0.1318 (I > 2σ(I)). In the coordinated methanol molecule, the O−H
protons are not added since refinement does not converge to zero
after fixing the protons. Repeated crystallization and data collection
could not give better R values due to highly fragile crystal quality
(Table 2).
[F@{Zn4(dipp)4(MeQ)2(CH3OH)2}] [PF6] (6). To a solution of
compound 1 (0.04 g, 0.025 mmol) in methanol (20 mL), [MeQ]PF6
(0.03 g, 0.1 mmol) was added and diluted with methanol (80 mL). A
saturated solution of nBu4NF in methanol (5 mL) was added, and the
resulting solution was kept for crystallization. After 2 d the product
was obtained as white crystals. mp: >250 °C. Yield: 0.040 g (87%).
Anal. Calcd for C72H98F7N4O18P5Zn4 (Mr = 1857.00) (%): C, 46.57;
H, 5.32; N, 3.02. Found: C, 46.13; H, 4.91; N, 3.13. FT-IR (KBr,
cm−1): 2963 (vs), 2868 (s), 1708 (s), 1615 (s), 1466 (s), 1336 (s),
1223 (s), 1154 (vs), 997 (vs), 909 (vs), 843 (vs), 774 (s), and 558
1
3
(vs). H NMR (DMSO-d6, 250 MHz): δ 9.1 (d, 4H, JHH = 6.7 Hz,
ortho-N+), 8.8 (d, 4H, 3JHH = 6.1 Hz, ortho-N), 8.5 (d, 4H, 3JHH = 6.9
Hz, meta-N+), 7.9 (d, 4H, 3JHH = 6.2 Hz, meta-N+), 6.8 (m, 12H, Ar),
4.2 (s, 6H, CH3−N+), 3.8 (septet, 8H, 3JHH = 6.8 Hz, iPr−CH), 1.0 (d,
48H, 3J = 6.8 Hz, iPr−CH3) ppm. 31P NMR (DMSO-d6, 100 MHz): δ
2.5 (s, 4P, dipp) and −144 (septet, 1P, JPF = 711 Hz, PF6) ppm. 19F
1
Data collection and structure solution and refinement for
compound 5 using a suitable crystal of size 0.37 × 0.04 × 0.03
mm3, carried out in the same way as for compound 3, reveals that this
molecule crystallizes in monoclinic P21/c space group. There were
tetrahedral residual peaks in the molecule, but attempts to assign those
residual peaks to the BF4 anion were not successful. Since other
characterization data was in agreement with the BF4 anion, the
molecule was squeezed and refined. The final refinement of the
structure converged at the R value of 0.0631 (I > 2σ(I). Final
refinement data for compounds 3−5 are given in Table 2.
1
NMR (DMSO-d6, 400 MHz): δ −70 (d, 6F, JPF = 711 Hz, PF6) and
−113 (s, 1F, F@D4R) ppm. ESI MS (-ve): calculated for [M-2MeQ-
2CH3OH-PF6]−, m/z 1305, found 1305.
Single-Crystal X-ray Diffraction Studies. A suitable colorless
crystal of compound 3 (size: 0.36 × 0.33 × 0.28 mm3) obtained
directly from the reaction mixture was mounted on an Oxford Xcalibur
diffractometer equipped with Sapphire-III CCD camera for unit cell
determination and three-dimensional intensity data collection. Data
integration and indexing of 3 was done by using CrysAlisPro.24 All
calculations were carried out using the programs in the WinGX
module25 and solved by direct methods (SIR-92).26 The final
NMR Experiments. In the first experiment, 10−5 mol of
[Zn(dipp)(DMSO)]4 (1) was dissolved in 0.3 mL of CD3OD in an
3351
dx.doi.org/10.1021/ic500193n | Inorg. Chem. 2014, 53, 3345−3353

Inorganic Chemistry
Article
NMR tube, and the 31P NMR spectrum was recorded. This solution
was titrated against a known concentration of [NBu4]F in CD3OD
using a micro pipet (Figure 1). In the second experiment, a 1 × 10−5
M solution of [Zn(dipp)(DMSO)]4 (1) was dissolved in 0.4 mL of
DMSO-d6 in an NMR tube, and the 19F NMR spectrum was recorded.
This solution was titrated against a known concentration of
[Zn(dipp)(DMSO)]4 (1) (Figure 2). In the third experiment,
[Zn(dipp)(DMSO)]4 (1) was dissolved in CD3OD, and to it a
water solution of NaF was added. The solution was subjected to NMR
spectroscopic analysis (Figure 8). In the fourth experiment [Zn-
(dipp)(DMSO)]4 (1) was treated with DMSO extracts of an anticavity
toothpaste (Colgate toothpaste) containing Na2PFO3. 31P NMR
shows encapsulated cubane (Figure 9).
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ASSOCIATED CONTENT
■
S
* Supporting Information
This includes: FT-IR, 1H and 31P NMR spectra, ESI-MS
spectra of 2−6, and TGA plots of 2−6. Crystallographic data of
3−5. CCDC 959362−959364. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: rmv@chem.iitb.ac.in. Phone: +91 22 2576 7163. Fax:
+91 22 25767152.
́
D. Angew. Chem., Int. Ed. 2008, 4, 4137. (h) Bauza, A.; Mooibroek, T.
J.; Frontera, A. Angew. Chem., Int. Ed. 2013, 52, 12317.
(10) Villaescusa, L. A.; Lightfoot, P.; Morris, R. E. Chem. Commun.
Notes
2002, 2220.
The authors declare no competing financial interest.
(11) Riou, D.; Taulelle, F.; Ferey, G. Inorg. Chem. 1996, 35, 6392.
(12) (a) Reinert, P.; Patarin, J.; Loiseau, T.; Ferey, G.; Kessler, H.
Microporous Mesoporous Mater. 1998, 22, 43. (b) Loiseau, T.; Ferey, G.
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ACKNOWLEDGMENTS
■
This work was supported by DST, New Delhi and DAE-BRNS,
Mumbai. R.M. thanks BRNS for a DAE-SRC Outstanding
Investigator Award which enabled the purchase of a single
crystal X-ray diffractometer.
ABBREVIATIONS
■
DippH2, 2,6-di-iso-propylphenyldihydrogenphosphate; s, sharp;
vs, very sharp; w, weak; MeQ, 1-methyl-4,4′-bipyridin-1-ium
cation
(13) Chang, W. K.; Wur, C. S.; Wang, S. L.; Chiang, R. K. Inorg.
Chem. 2006, 45, 6622.
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A. Angew. Chem., Int. Ed. 2006, 45, 5536. (b) Murugavel, R.;
Kuppuswamy, S.; Gogoi, N.; Boomishankar, R.; Steiner, A. Chem.
Eur. J. 2010, 16, 994. (c) Murugavel, R.; Kuppuswamy, S.; Gogoi, N.;
Steiner, A. Inorg. Chem. 2010, 49, 2153. (d) Murugavel, R.;
Walawalkar, M. G.; Pothiraja, R.; Rao, C. N. R.; Choudhury, A.
Chem. Rev. 2008, 108, 3549. (e) Murugavel, R.; Kuppuswamy, S.
Angew. Chem., Int. Ed. 2006, 45, 7022. (f) Murugavel, R.;
Kuppuswamy, S.; Gogoi, N.; Boomishankar, R.; Steiner, A. Chem.
Eur. J. 2008, 14, 3869. (g) Kalita, A. C.; Roch-Marchal, C.; Murugavel,
R. Dalton. Trans 2013, 42, 9755. (h) Murugavel, R.; Kuppuswamy, S.
Inorg. Chem. 2008, 47, 7686. (i) Murugavel, R.; Kuppuswamy, S.;
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