Intriguing structural chemistry of neutral and anionic layered monoalkylphosphates: Single-source precursors for high-yield ceramic phosphates

Article abstract of DOI:10.1039/c7ce01066d

Building up on an available synthetic methodology, phosphate monoesters ROPO₃H₂ have been synthesized in good yields. The synthetic procedure employed features acetic anhydride mediated activation of phosphoric acid in the presence of alcohols, leading to the formation of phosphate monoesters. The products have been isolated as their cyclohexyl amine salts, [CyNH₃]₂[(MeO)PO₃]·3H₂O (1) and [CyNH₃][(RO)PO₃H] (Cy = cyclohexyl; R = Et (2), iPr (3), or tBu (4)). Neutralization of 1-4 by readily available inexpensive ion exchange resin Amberlite produces monoalkylphosphates (RO)P(O)(OH)₂ (R = Me (5), Et (6), iPr (7), or tBu (8)). Thermally labile 1-4 and 7 have been structurally characterized by single crystal X-ray diffraction studies. Due to their intrinsic thermal instability due to β-H elimination, these compounds can be used as ligands for the preparation of single-source precursors for ceramic phosphates by reacting them with suitable metals ions. It is also possible to isolate co-crystals of the anionic and neutral forms of these phosphates as it has been demonstrated in the isolation and structural characterization of [(iPrO)PO₃H₂]·{[CyNH₃][(iPrO)PO₃H]} (9). To demonstrate the utility of these monoalkylphosphates in the low-temperature synthesis of metal phosphate bioceramics, isopropyl phosphate 7 has been employed to prepare calcium phosphate [{Ca((iPrO)PO₃)(OH₂)}·H₂O]_n (10), which undergoes neat thermal decomposition in two stages to lose water and propene to yield β-Ca₂P₂O₇ at low temperatures (280 °C).

Full text of DOI:10.1039/c7ce01066d

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Intriguing Structural Chemistry of Neutral and Anionic Layered
Monoalkylphosphates: Single-Source Precursors for High-Yield
Ceramic Phosphates
Received 00th January 20xx,
Accepted 00th January 20xx
Gulzar A. Bhat, Alok Ch. Kalita† and Ramaswamy Murugavel*
DOI: 10.1039/x0xx00000x
www.rsc.org/
ABSTRACT: Building up on an available synthetic methodology, phosphate monoesters ROPO3H2 have been synthesized in
good yields. The synthetic procedure employed features acetic anhydride mediated activation of phosphoric acid in the
presence of alcohols, leading to the formation of phosphate monoesters. The products have been isolated as their
cyclohexyl amine salts, [CyNH3]2[(MeO)PO3]‧3H2O (1) and [CyNH3][(RO)PO3H] (Cy = cyclohexyl; R = Et (2), iPr (3), or tBu
(4)). Neutralization of 1-4 by readily available inexpensive ion exchange resin Amberlite produces monoalkyl phosphates
(RO)P(O)(OH)2 (R = Me (5); Et (6), iPr (7), or tBu (8)). Thermally labile 1-4 and 7 have been structurally characterized by
single crystal X-ray diffraction studies. Due to intrinsic thermal instability owing to β-H elimination, these compounds can
be used as ligands for the preparation of single-source precursors for ceramic phosphates by reacting them with suitable
metals ions. It is also possible to isolate co-crystals of the anionic and neutral forms of these phosphates as it has been
demonstrated in the isolation and structural characterization of [(iPrO)PO3H2]⋅{[CyNH3] [(iPrO)PO3H]} (9). To demonstrate
the utility of monoalkylphosphates in the low-temperature synthesis of metal phosphate bioceramics, isopropyl phosphate
7 has been employed to prepare calcium phosphate [{Ca((iPrO)PO3)(OH2)}⋅H2O]n (10), which undergoes neat thermal
decomposition in two stages to lose water and propene to yield β- Ca2P2O7 at low temperatures (280 oC)
These issues have however been overcome in recent times to
a certain extent.10, 12
Introduction
Apart from the above-mentioned importance in biological
Phosphate esters are vital constituents of various
biologically active compounds such as nucleotides, glycolipids,
nucleic acids, proteins and steroids.1-3 These monoesters are
actively involved in cell signaling, regulation pathways; they
chemistry, phosphate esters are also useful in materials
chemistry. Metal phosphate chemistry has greatly expanded
since the initial discovery of aluminophosphates (ALPOs) in
1981.13 Much of this frameworks chemistry has been
developed under hydrothermal conditions starting from
phosphoric acid as the phosphate source. In contrary, there
has been only limited efforts to employ alkyl esters of
phosphoric acid to synthesize soluble metal phosphate
find potential applications in medicinal chemistry as water
soluble pro drugs.4,
5
Phosphorylation is carried out in
biological systems by ATP in the presence of kinases as
catalysts; on the other hand, phosphatases catalyzes the
removal of phosphate group in the biological systems.6-8
Therefore, in order to understand the biological pathways and
develop therapeutic routes, laboratory methods for
synthesizing analogous compounds are mandatory. The mono-
substituted phosphate esters derived from alcohols are not
commercially available due to their intrinsic instability owing
to various decomposition pathways.9,10 Although several
laboratory methods for the preparation of these monoesters
are known, many of these routes are inundated by side
reactions resulting in poor selectivity and formation of di and
tri-substituted esters or the corresponding pyrophosphates.11
compounds. Chart
1 depicts a correlation of physical
properties vis-à-vis their usefulness in
Chart 1. Phosphoric acid and its alkyl esters
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building framework structures for phosphoric acid and its adopted by us in the present study, involves the formation of
esters. Thus, while H3PO4 forms framework structures, the use acetyl phosphate, a high energy biochemical intermediate,
of mono- and diesters, (RO)P(O)(OH)2 and (RO)2P(O)(OH), can which is capable of phosphorylating alcohols and other
enhance the chance of isolating soluble model compounds nucleophiles. In a typical synthesis, phosphoric acid was
which can act as single source precursors for metal phosphate suspended in pyridine-alcohol mixture and treated with
ceramic materials.14, 15
triethylamine (2 equiv) and acetic anhydride (2 equiv)
o
Toward this strategy, research groups of Tilley16-23 and successively. The resultant mixture was heated at 90 C for 60
Murugavel24-29 apart from others,30,31 have utilized an minutes followed by addition of water and subsequent heating
hydrolytically
and
thermally
unstable
diester, at 90 oC for another 60 minutes. The resultant mixture was
[(tBuO)2P(O)(OH)] as starting material for the preparation of a extracted with diethyl ether and the aqueous layer was
large number of s, p and d-block element / metal phosphates treated with CyNH2 and cooled to low temperature to yield the
that serve as single-source precursors for fine particle ceramic corresponding cyclohexylamine salts as crude product. These
materials. As illustrated in Chart 1, monoesters (RO)P(O)(OH)2 salts were recrystallized from hot ethanol to obtain
have hardly been used for this purpose due to the absence of
established synthetic protocols to stabilize these unstable
species.32,33 Stabilization of monoalkyl phosphates in multi-
gram quantities will pave way for their chemistry to be
explored in detail and allow the isolation of metal phosphate
molecules and clusters that contain M:P ratio of 1:1. While
monoalkyl phosphates have been known since as early as
1920s, they have invariably been isolated only as their
ammonium salts or barium salts due to their inherent
instability towards hydrolysis.34,35 Use of ammonium salts of
phosphates in the reaction with metal precursors produces
undesired side products that are difficult to completely
remove before the resultant metal phosphate molecules can
be subjected to thermal decomposition to produce ceramic
phosphates. Hence it is desirable to employ free monoalkyl
phosphates in anhydrous state for their applications in
materials chemistry.
[CyNH3]2[(MeO)PO3]
cyclohexyl; R = Et ( ), iPr (
In order to obtain amine-free alkyl monoester phosphates,
ion exchange treatment was carried out on using strongly
⋅
3H2O (
1
) and [CyNH3][(RO)PO3H] (Cy =
2
3), or tBu (
4)).
1-4
acidic Amberlite IR-120, instead of the more expensive Dowex
50 WX2 used earlier to prepare the free ethyl phosphate.32
Mixing of adequate amounts of Amberlite IR-120 resin with 1-4
in methanol and stirring for 24 hours results in the complete
exchange of cyclohexyl ammonium cations by protons to yield
neutral mono alkyl phosphates (RO)P(O)(OH)2 (R = Me (
), iPr ( ), or tBu ( )) as low melting solids or viscous liquids.
Phosphates have been characterized by analytical and
spectroscopic methods. While the spectral data for are
presented in the ESI (Figures S1-S10), those of free acids 5-8
5); Et
(
6
7
8
1-8
1-4
are discussed here in detail. The presence of peaks around
3000-2850 cm-1 in the FT-IR spectra correspond to C-H
stretching of alkyl groups and broad peaks around 2350 cm-1
correspond to the P-OH groups (Figure S11). The 1H NMR
spectra of all the four compounds show well-resolved signals.
The present investigation addresses this issue by building
upon the earlier report of Pascal et al.10 to produce multi-gram
quantities of free phosphate esters, (RO)P(O)(OH)2 (R = Me, Et,
iPr and tBu) and establish their molecular structures both in
free acid form and as cyclohexyl ammonium salts. These
compounds represent rare examples of organic layered-solids
which are stabilized by extensive supramolecular hydrogen
bonding networks. Further, these phosphate monoesters have
been used to prepare thermally labile layered calcium
In the 1H NMR spectrum of
3.16 ppm as a singlet while the P-OH resonances appear as a
broad singlet at δ 7.5 ppm. In , the methylene and the methyl
5, the methyl protons resonate at δ
6
protons resonate at δ 4.00 ppm (as a quartet) and at δ 1.2 ppm
(as a triplet), respectively. The protons of the isopropyl
substituent in 7 appear at δ 4.5 ppm (septet, methine) and at δ
1.28 ppm (doublet, methyl) while the P-OH protons resonate
1
as a broad singlet at δ 8.4 ppm. In the H NMR spectrum of
organophosphate
that
undergoes
facile
thermal
decomposition to produce
β
ceramic material) in high ceramic yields.
-calcium pyrophosphate (a bio- compound 8 the tertiary butyl protons resonate as a singlet at
δ 1.74 ppm while the P-OH proton resonates at δ 8.95 ppm
(Figure S12-S15). These compounds have further been
characterized by
Results and discussion
RO
OR
P
OH
a, b, c
d
P
O
ROH
+
+
HO
NH3
HO
P
OH
O
HO
O
OH
N
O
R= Me (1); Et (2);
iPr (3); tBu (4)
R= Me (5); Et (6);
iPr (7); tBu (8)
(a) Et3N, acetic anhydride, reflux, 3h; (b) H2O, reflux, 1 h; (c) CyNH2 in acetone/H2O; (d)
Amberlite, stirring for 24 h in methanol at room temperature.
Scheme 1. Synthesis of mono-alkyl phosphates and their salts 1-8.
Synthesis. Pascal et al. reported the synthesis of mono-alkyl
phosphates through acetic anhydride mediated activation of
inorganic phosphate or phosphoric acid.10 This method,
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Figure 1. 31P NMR spectra of compounds 5-8 in (DMSO-d6).
783.94 and 896.92, corresponding to the monomeric to heptameric
forms, respectively, Similarly, the peaks appearing at m/z, 126.98,
253.00, 379.01, 505.02, 631.03, and 757.03 correspond to the
monomeric to hexameric forms of 6, respectively (Figure 2). Similar
aggregation behaviour is also observed for 7 in the negative ion
mode (see ESI).
Molecular structures. X-ray diffraction quality single crystals of
cyclohexyl ammonium alkylphosphates 1-4 were grown directly
from an ethanol solution of the respective compound stored at -4
˚C. Single crystals were obtained from these solutions typically
within a day. The four compounds crystallize in the monoclinic
crystal system in P21/c, C2/c, P21/n and I2/a space groups,
respectively. A ball and stick model of the contents of the
Figure 2. ESI-MS (positive ion mode) spectrum of 6 showing association behaviour in asymmetric part of the unit cell for 1-4 are shown in Figures 3. The
acetonitrile solution; peaks at m/z 126.98, 253.00, 379.01, 505.02, 631.03 and 757.03
structural determination reveals that
1
crystallizes as
a
correspond to M+1, 2M+1, 3M+1, 4M+1 , 5M+1 and 6M+1 of 6 respectively.
bis(cyclohexylammonium) salt of methylphosphate while other
compounds 2-4 crystallize as mono(cyclohexyl-ammonium) salt of
the respective alkylphosphate. The bond lengths and angles
observed for 1-4 (Table 1) are similar to those observed for other
monoaryl phosphates described in the literature.36-39 The crystal
structure analysis further reveals strong hydrogen bonding
interactions between the phosphate groups of alkyl phosphates and
the cyclohexyl ammonium –NH3 groups, resulting in extensive solid
state aggregation, similar to those reported in the literature for
various silanols, monoaryl phosphates and phosphonic acids, which
can act as both hydrogen bond donors as well as acceptors due to
the presence of Si–OH, P–OH, and P=O groups.40, 41
13C NMR spectroscopy (Figure S16-S18). The most convincing
evidence for the formation of mono-alkyl phosphate esters 5-8
comes from the observation of a single resonance in the 31P NMR
spectra at δ -0.07, -1.65, -1.86 and -0.68 ppm for 5-8, respectively
(Figure 1).
While cyclohexylammonium salts 1-4 do not yield
molecular ions in their ESI-MS, the neutral phosphates (barring 8)
display an interesting mass spectral pattern showing the aggregated
species. For example, the positive ion mode ESI-MS of 5 contains
predominant peaks at m/z 112.96, 224.96, 448.95, 560.94, 672.94
Figure 3. Molecular structures of compounds 1-4. Selected bond parameters are listed in Table 1.
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Table 1. Selected structural parameters for monoalkyl phosphates and their salts
Compound
Space
group
P21/c
C2/c
Charge on
phosphate
dianion
P1-O1(R)
P1=O2
P1-O3(H)
P1-O4
1.653(3)
1.508(3)*
1.496(1)
1.497(5)
1.495(2)
1.493(1)
1.486(2)
1.508(2)
1.495(3)*
1.567(2)
1.559(5)
1.559(2)
1.547(1)
1.542(2)
1.566(2)
1.496(3)*
1.511(1)
1.503(5)
1.501(2)
1.545(1)
1.537(2)
1.494(2)
[CyNH3]2[(MeO)PO3]⋅3H2O (1)
[CyNH3][(EtO)PO3H] (2)
monoanion 1.596(2)
monoanion 1.590(5)
monoanion 1.582(2)
[CyNH3][(iPrO)PO3H] (3)
P21/n
I2/a
[CyNH3][(tBuO)PO3H] (4)
[(iPrO)PO3H2] (7)
P42/n
P21/n
neutral
neutral
1.557(1)
1.564(2)
[(iPrO)PO3H2]⋅{[CyNH3] [(iPrO)PO3H]} (9)
monoanion 1.574(2)
*
bond distances found in the dianion
Structure of 1.
also show similar P-O distances. An interesting aspect of the
structures of 2-4 is the formation of a hydrogen bonded –POOH
dimer, as found in carboxylic acid dimers (Figure 5; left).42 These
phosphate dimers interact with six different CyNH3 cations around
them. Each of these cations in turn are involved in three N-H⋅⋅⋅O
interactions. As a result of each cation or each anion interacts with
three anions or three cations, respectively, to form a tightly woven
2-D sheet (Figure 5; right). These sheets further self-assemble in the
form of stacks, a feature that is also observed in the case of
compounds 3 and 4 (Figure 6). Consistent with the size of the alkyl
substituent, the observed inter-layer distances in 2-4 are 9.15,
12.16, and 12.00 Å, respectively.
The asymmetric unit of 1 contains a doubly deprotonated methyl
phosphate dianion (MeO)PO32-, two cyclohexyl ammonium cations,
and three lattice water molecules (Figure 3a). The ability of each
one of these constituents to participate in strong H-bonding results
in a multitude of secondary interactions (Figure 4; see also ESI).
Among the four P-O bonds in the anionic part, P1-O1 (1.653(3)) is
longer than the rest of the three P-O linkages (1.508(3), 1.495(3),
and 1.496(3), respectively). This is consistent with the presence of a
methyl group on O1, while free O2, O3, and O4 share the di-
negative charge (Table 1). It is interesting to note that each of the
two –CyNH3 groups acts as H-bond donor for the formation of three
N-H⋅⋅⋅O hydrogen bonds. Similarly, the PO3 unit is involved in as
many as nine H-bonds (either O⋅⋅⋅H-O or O⋅⋅⋅H-N interactions).
Consequently, a 2-D sheet is formed due to these interactions
(Figure 4). These 2-D sheets further stack to form layered structure
(Figure 4; right), where no significant secondary interactions were
observed between the layers.
Molecular structure of 7. Not withstanding the ease with which
one could convert cyclohexylammonium salts 1-4 to the respective
free acids 5-8, the process of obtaining X-ray diffraction quality
single crystals proved to be difficult. The fact that these compounds
are low melting solids or liquids at room temperature precluded
any high quality diffraction study. The isopropyl derivative 7
however proved to be the best choice for this purpose as its
colorless single crystals could be obtained by cooling the low
melting viscous solid (physical state at room temperature) in the
absence of any solvent at -33˚C. The crystals thus obtained melt
immediately on warming the flask to room temperature. Low
Structures of 2-4. The asymmetric part of the unit cells of 2-4
contain one (RO)PO3H- anion and a cyclohexylammonium cation.
Due to the mono deprotonation of the phosphate group, there are
three types of P-O distances found in these crystal structures. For
example, in 2, the longest distance is associated with the P-O(C)
linkage (1.596(2) Å) while that of P-OH linkage is somewhat shorter
(1.596(2) Å). The two P-O distances of the delocalized POO- anion
are even shorter (1.496(1) Å and 1.511(1) Å). Compounds 3 and 4
temperature single crystal structure analysis reveals that
7
crystallizes in the tetragonal P42/n space group with one molecule
of the free acid and no lattice solvent molecules (Figure 7a; Table
1), thus limiting supramolecular aggregation to only
Figure 4. Structure evolution in 1, leading to the formation of a layered solid made of 2-H bonded sheets
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Figure 5. Structure evolution in 2; compounds 3 and 4 also exhibit similar behavior.
Figure 6. Layered structures of monoalkyl phosphate salts, 2-4.
complementary H-bonds formed by the –PO3H2 group with its
neighbors. In the absence of additional H-bond donors/
acceptors, it can be assumed that the association of 7 would
closely be related to the structures found for organosilanetriols
or phosphonic acids, where either a double-layer or tubular form
of association dominates.15 Closer inspection of the H-bonding
pattern in 7 (Figure 7b) reveals that the often encountered
carboxylic dimer association of phosphorus acids is completely
absent in the present case. Instead the interaction of each
molecule of 7 with three more molecules through as many as
four P=O···H-O-P hydrogen bonds (O-H···O) is observed. The
phosphoryl oxygen O2 acts as acceptor for two hydrogen bonds
while each of the two (P)OH moiety assumes the role of a
hydrogen bond donor. The fourth oxygen in the molecule, O1, is
not involved in any secondary interactions. The preferred
manifestation of four hydrogen bonds emanating from three
different oxygen atoms of a tetrahedral phosphorus is unlikely to
be 2-D sheets as observed for 3 (vide supra). As it can be seen in
Figure 8a, the restrictive H-bonding in 7 leads to formation of
hollow tubular columns which are arranged in the lattice
adjacent to each other, with no interaction between the
individual columns (Figure 8b).
free acid [(iPrO)PO3H2] (7) and [CyNH3] [(iPrO)PO3H] (3).
Compound 9 crystallizes in the monoclinic crystal system with
P21/n space group, with one neutral isopropyl phosphate, one
anionic isopropyl phosphate and a cyclohexylammonium cation
occupying the asymmetric part of the unit cell (Figure 9a). The P-
O bond distances in the [(iPrO)PO3H2] part of 9 (i.e. around P2)
are very similar to those found in 7, while those in the
[(iPrO)PO3H] unit (i.e around P1) are similar to those observed
for 3 (Table 1), suggesting that there is no transfer of proton
during the co-crystal formation. The hydrogen atom of the
cyclohexyl amine shows intermolecular hydrogen bonding with
the
hydroxyl
group
of
Isolation and molecular structure of 9. During one of the
attempts to covert the salt 3 to the free acid 7, small quantities
of single crystals of a second species, [(iPrO)PO3H2]⋅{[CyNH3]
[(iPrO)PO3H]} (9) was obtained. Single crystal X-ray diffraction
study established this species as 1:1 mixture or co-crystal of the
Figure 7. (a) Assembly of free phosphate moieties in 7, Phosphorus centers are in
tetrahedral geometry, average bond parameters: P=O (1.4942(12) Å), P-OH
(1.5450(12) Å), and P-OiPr (1.5574(11) Å). (b) Intermolecular hydrogen bonding
interactions in 7.
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of the cyclohexyl moiety occurs leading to the formation of
H3PO4. Increasing the temperature beyond 300 oC, the
phosphoric acid undergoes self-condensation to yield
pyrophosphoric acid and then finally P2O5.
As it can be seen from Figure 11, thermal decomposition of
free acid
7
is quite different from that of its
cyclohexylammonium salt 3. The decomposition of the –OiPr
group of 7 as propene sets in already at ca. 110 oC while the
o
decomposition for 3 begins only at 220 C. Further, the ceramic
yield of decomposition of 7 is more than twice that of 3. These
two observations suggest that the free acids 5-8 result in better
ceramic yields by burning less organic matter. Hence these
compounds can be employed as starting materials for the
preparation of single source precursors for metal phosphate
nanomaterials.
Figure 8. (a) Tubular column formed by free organophosphate 7; (b) perpendicular
view showing the arrangement of several such parallel tubes in the lattice.
isopropylphosphate and anionic oxygen atom of the mono
deprotonated isopropyl phosphate resulting in the formation of
2D supramolecular sheets (Figure 9b). These sheets are arranged
as a layered stack as shown in Figure S27 (ESI).
Figure 10. TGA traces for compounds 1-4.
Figure 9. (a) Ball and stick representation of constituents of the asymmetric part of
the unit cell of 9; (b) hydrogen bonding network; average bond parameters: P=O
(1.488(12) Å), P-OH (1.538(12) Å), and P-OiPr (1.564(11) Å).
Thermal decomposition studies. It has been earlier shown that
the metal di-tert-butyl phosphate (M-dtbp) complexes undergo
facile low-temperature decomposition due to the availability of
inherent decomposition pathways (β-elimination) leading to the
formation of pure ceramic phosphates.29, 43 This decomposition
produces iso-butene gas and water only as the other thermolysis
products. It is expected that compounds 5-8 should be more
amenable as single source precursors since they contain only one
alkyl substituent as against the two tert-butyl groups on dtbp.
This would not only reduce the organic matter that needs to be
decomposed but also would result in higher ceramic yields.
Hence the thermal behaviour of the monoalkyl phosphates has
been investigated for their cyclohexylammonium salts 1-4 rather
than the room temperature liquids 5-8 (Figure 10). The TGA of 7,
which was obtained as single crystals at low temperature, has
also been studied (vide infra; Figure 11).
Figure 11. Comparison of thermogravimetric analysis of compound 3 and 7
Utility of monoalkylphosphate as single source precursors:
Chemistry of [{Ca((iPrO)PO3)(OH2)}⋅H2O]n (10). Although all the
four monoalkyl phosphates 5-8 can in principle be employed for
the preparation of single source precursors, due to the ease of
handling and high purity, iso-propylphosphate 7 was chosen as
the candidate for the metalation studies. In a typical experiment,
addition of a methanol solution of 7 to an aqueous solution of
Ca(OAc)2⋅H2O (1:1 molar ratio) at room temperature results in
Thermal decomposition of 1-4 carried under a flow of dry
nitrogen at a heating rate of 10 °C min−1 revealed facile
decomposition at low temperatures with the elimination of the
corresponding alkenes (Figure 10). On further heating, removal
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the formation of calcium phosphate [{Ca((iPrO)PO3)(OH2)}⋅H2O]n
(10) as microcrystalline white precipitate within a few minutes of
mixing of the two solutions (Scheme 2). Separation of the
precipitated solid by filtration followed by repeated washing with
methanol resulted in analytically pure 10, which has been
characterized by elemental analysis, IR spectroscopy, thermal
analysis, and PXRD studies, apart from further characterization
by solid-state (13C and 31P) CP MAS NMR spectroscopy and
electron microscopy (SEM and EDAX). The insoluble nature of the
compound points to the formation of polymeric species than
discrete molecular assemblies.
O
MeOH / H2O
[{Ca((RO)PO3)(OH2)}.H2O]n
10
+
OH
Ca(OAc)2.H2O
P
RO
HO
Scheme 2. Synthesis of complex calcium phosphate 10
stir, room temp.
R = iPr
.
In the FT-IR spectrum of 10 (see ESI), the absence of any
residual (P)O-H peaks around 2300 cm-1 clearly indicates
complete deprotonation of the isopropyl phosphate ligand. The
presence of alkyl groups is confirmed by two sharp peaks at
around 2900-3000 cm-1(C-H stretching). The bands appearing at
1123, 1080 and 1033 cm-1 are assigned to P=O and P-O-M
stretching (antisymmetric and symmetric) frequencies,
respectively, while the presence of water molecule is confirmed
by a broad peak at3400 cm-1 (see ESI). The CP MAS 31P NMR
spectrum shows a fairly sharp resonance at δ 3.42 ppm,
suggesting the presence of only one type of phosphorus center in
10 (Figure 12a). Similarly, the solid state 13C-CP MAS spectrum
Figure 12: (a) CP-MAS 31P NMR spectrum of 10 (b) CP-MAS 13C NMR
spectrum of 10.
exhibits two resonances at
δ 67.16 and 23.95 ppm,
corresponding to the methine and methyl carbons of the
isopropyl group (Figure 12b).
Definitive proof to establish the polymeric nature of 10
comes from the combined PXRD and SEM studies (Figure 13). The
powder XRD spectral pattern of 10 (Figure 13a) clearly
establishes the layered structure for this type of compounds, as
has been well documented in divalent metal phosphonate
chemistry.44 The calculated interlayer spacing of 13.28 Å for 10
compares well with the d-spacing observed in calcium
Figure 13. (a) PXRD pattern of as synthesized 10; (b) SEM micrograph of 10 at 500x
magnification.
The combined advantage that the organic content in
[{Ca((iPrO)PO3)(OH2)}⋅H2O]n (10) is low and that the iso-
propoxide can undergo a facile thermolysis at low temperatures
(as seen for 3 and 7) prompted a detailed investigation of the
thermal behavior of 10. The combined TGA/DTA and DSC study
establishes a multistage thermolysis, leading to the formation of
the ceramic phosphate. The TGA profile of 10 (Figure 15) reveals
that the loss of two water molecules amounting to 16 weight %
(theoretical value 16.80 %) takes place in two stages in the
phosphonates with similar
R groups for which the X-ray
structures have been deduced.45, 46 The SEM of 10 further reveals
lamellar type of structure (Figure 13b).
o
temperature range 50-220 C, suggesting that one of the water
molecules is bound to calcium ion while the other is present as
lattice solvent. Between 250 and 280 oC, the decomposition of
the isopropxide take place as evidenced by a sharp weight loss
amounting to 23 weight % (theoretical value 26 %) that
corresponds to the loss of propene due to β-elimination.
Ca(HPO4) formed at this stage (62.5 % and 63.5 % experimental
and theoretical yields) further loses water on heating beyond 300
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oC to form the final ceramic phosphate ½Ca2P2O7 in 59.2 % yield
(theoretical yield 59.3 % ). To the best of our knowledge, the
thermolysis of 10 represents a new standard among single
source precursors for ceramic materials, by producing a ceramic
phosphate at around 300 oC in about 60% ceramic yield.
Figure 15. PXRD pattern of pyrolysis product of 10 (obtained by heating to 800
°C) and the JCPDS data (14313-ICSD) for -Ca2P2O7
β
Figure 14. TGA trace for the decomposition of 10.
Figure 16. SEM micrographs of β-Ca2P2O7 at 30000x.
50-220 o
- 2H2O
C
[Ca((iPrO)PO3)]
84 % (85 %)
[{Ca(iPrO)PO3)(OH2)}.H2O]n
(16%)
250-280 o
-propene, 23 %)
C
300-580 o
C
[Ca(HOPO3)]
62.5 % ( 63.4 %)
1/2 Ca2P2O7
- H2O (3%)
59.2 % (59.3 %)
Scheme 3. Thermal decomposition pathway for 10
.
In order to further characterize the decomposition product(s)
formed by 10, bulk thermolysis was carried out on a tubular
furnace by heating a sample of 10 at 800 oC. Although the
thermolysis is complete even at even lower temperatures, in
order to obtain crystalline phase we chose to heat the sample at
800 oC for at least 12 hours. The white powder produced was
examined by PXRD (Figure 16) and found to match the diffraction
pattern of an authentic sample of β−Ca2P2O7.50, 51 Microscopic
studies reveal that at elevated temperatures the ceramic
phosphate undergoes fusion producing large crystals in which
the microcrystal domains can still be seen (Figure 16 ). It should
be noted that the beta-form of the calcium pyrophosphate has
been implicated as an important bio-ceramic material for bone
curing. The efficacy of the material depends on its nano-sized
form. Hence the low temperature synthesis described here
would be suitable for the synthesis of carbon or nitrogen free
Figure 17. EDX spectrum of β-Ca2P2O7 with elemental mapping.
Conclusions
Multigram quantities of unstable and normally inaccessible
phosphate monoesters ROPO3H2 (R = M, Et, iPr and tBu)
have been synthesized by modifying an existing synthetic
protocol known for the preparation of more stable ethyl
phosphate. The methyl, ethyl, iso-propyl and tert-butyl
monoesters were initially isolated as their corresponding
cyclohexylamine salts and were characterized by single-
crystal X-ray diffraction to establish the layered natured of
these organic ionic phosphates. The cyclohexyl ammonium
cations in 1-4 could be readily ion-exchanged by an
inexpensive resin to produce the corresponding free acids 5-
fine particle β-Ca2P2O7.
8
in good yields. These monoesters are thermally labile and
undergo facile thermolysis at low temperatures through β
elimination (other than the methyl derivative), guaranteeing
their utility as ideal starting materials for synthesizing single-
source precursors (SSPs) for the eventual preparation of high
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purity ceramic metal phosphates. As a proof of concept, we
have shown that starting from free acid , it is possible to
produce a layered calcium phosphate which undergoes
the aqueous phase was washed three times with diethyl ether
(125 mL) and concentrated. The oily liquid residue obtained was
dissolved in acetone/water (9:1) and then cyclohexylamine (10.5
mL, 153 mmol) was added. The clear solution obtained was
7
o
decomposition below 300 C to produce
β-Ca2P2O7 in about
o
cooled at 4 C for 12 h. The white solid formed was collected by
60% ceramic yield.
filtration and dried. The solid was heated in ethanol and the
insoluble residue was filtered off, and the filtrate was cooled for
12 h at 4 oC. Colourless crystals of 1-4 obtained were filtered,
washed with ethanol, and dried under vacuum.
The demonstration of successful synthesis of mono-alkyl
phosphates in synthetically useful quantities in the present
study opens up access for exploring their metal chemistry to
be explored across the periodic table. We continue to
endeavor studies on these directions to produce fine-
particle metal phosphate materials that find applications in
catalysis, intercalation, adsorption, and ion-exchange
chemistries.
1: Mp: 200 °C, Yield: 1.6 g (50%), FT-IR (as KBr pellet, cm-1):
1
2939(vs), 28612(s), 2213 (br), 1160 (vs), 1051(s), 763(s). H NMR
(D2O, 500 MHz, ppm): 3.33(s, 3H), 3.02 (m, 2H), 1.88 (m, 4H),
1.69 (m, 4H), 1.54 (m, 2H), 1.09-1.25 (m, 13H). 31P NMR (D2O, 202
MHz, ppm): δ 4.56. 13C NMR (D2O, 125 MHz, ppm): δ 51.2, 50.0,
30.2, 24.2, 23.7.
Experimental section
2: Mp: 202 °C, Yield: 1.5 g (65 %), FT-IR (as KBr pellet, cm-1):
2926(vs), 2861(s), 2211(br) 1053(s), 763(s).1H NMR (D2O, 400
MHz, ppm): 3.80 (q, 2H), 3.25 (m, 1H), 2.05 (m, 2H), 1.78 (m, 2H),
1.65 (m, 1H), 1.10-1.14 (m, 5H), 1.25 (t, 3H). 31P NMR (D2O, 160
MHz, ppm): δ 3.38. 13C NMR (D2O, 100 MHz, ppm) δ 60.6, 50.2,
30.3, 24.2, 23.7, 15.8 .
Methods and materials. Infrared spectra were obtained on a
Perkin–Elmer Spectrum One FT-IR spectrometer as thin films.
NMR studies were performed on Bruker Avance DPX-400 and
500 MHz spectrometers. Thermogravimetric analyses were
carried out on a Perkin–Elmer Pyris thermal analysis system
o
under a stream of nitrogen gas at a heating rate of 10 C/min.
Elemental analyses were performed on a Thermo Finnigan
(FLASH EA 1112) microanalyzer. The ESI-MS studies were carried
out on Bruker MaXis impact mass spectrometer. Powder XRD
data were collected on a PANalytical Empyrean diffractometer
equipped with Cu-Ka radiation ((λ =1.54056 Å).
3: Mp: 196 °C, Yield: 1.4 g (57 %), FT-IR (as KBr pellet, cm-1):
2935(s), 2857(w), 2227(s), 1057(s), 774(s) .1H NMR
(D2O,400MHz): δ 4.50 (septet, 1H), 3.15 (m, 1H), 2.00 (m, 2H),
1.75 (m, 2H), 1.60 (m, 1H), 1.10-1.40 (m, 5H), 1.30 (d, 6H). 31P
NMR (D2O, 160 MHz, ppm): δ 2.61 ppm. 13C NMR (D2O, 100 MHz,
ppm): δ 69.7, 50.3, 30.3, 24.2, 23.7, 23.1.
The morphologies of 10 and its decomposition product
were studied by a JEOL model JSM-7600F field-emission gun-
scanning electron microscope operating at an accelerating
voltage of 0.1 to 30 kV. The samples were prepared by drop-
casting the powdered samples onto the carbon substrate. The
samples were sputtered with platinum prior to the imaging
4: Mp: 190 °C, Yield: 1.5 g (60 %), FT-IR (as KBr pellet, cm-1):
2926(vs), 2861(s), 2211(br) 1053(s), 763(s).1H NMR (D2O, 400
MHz, ppm): δ 3.06 (m, 1H), 1.91 (m, 2H), 1.73 (m, 2H), 1.58 (m,
1H), 1.08-1.29 (m, 5H), 1.32 (s, 9H). 31P NMR (D2O, 200 MHz,
ppm): δ -1.81, 13C NMR (D2O, 125 MHz, ppm): δ 76.8, 50.1, 30.29,
29.32, 24.2, 23.7.
Solvents were purified according to standard
procedures prior to use.52 Commercially available chemicals such
as crystalline phosphoric acid (Sigma Aldrich), Amberlite ion
exchanger IR-120 (s.d. Fine), Pyridine (Alfa Aesar), triethyl amine
(Alfa Aesar) and acetic anhydride (Lancaster) were used as
procured.
Synthesis and characterization of 5-8. Methanol (100 mL)
solutions of compounds 1-4 (8 g) were stirred in the presence of
Amberlite IR-120 cationic exchanger (20 g) overnight and the
resulting solution was filtered and washed several times with
methanol. The filtrate was evaporated under reduced pressure
to obtain 5-8 as viscous liquids. In the case of 7, single crystals
formed from the oil when stored undisturbed for several days. In
a few instances, attempts to crystallise 7 resulted in the isolation
of [(iPrO)PO3H2]⋅{[CyNH3] [(iPrO)PO3H]} (9), which is formed
through incomplete ion exchange of 3 .
Synthesis and characterization of 1-4. Compounds 1-4 were
synthesized by simplifying a reported literature method.10 A two
necked round-bottom flask was charged with crystalline H3PO4
(5.0 g, 51 mmol) and pyridine (20.75 mL, 255 mmol) under N2
atmosphere. Afterwards alcohol (510 mmol) and then
triethylamine (14 mL, 102 mmol) were added via a dropping
funnel one after another. After complete dissolution, acetic
anhydride (9.65 mL, 102 mmol) was added dropwise. The
reaction mixture was boiled under reflux for 2 h and then cooled
to room temperature. After addition of water (125 mL), the
reaction mixture was heated at 90 oC for 1 h and cooled to room
temperature. The solution was diluted with water (60 mL) and
5: Yield: 1.5 g (62 %). FT-IR (liquid film, cm-1): 2977(s), 2861(vs),
1
2312 (br), 1645(br), 1190 (s) 1143(s), 793 (m). H NMR (DMSO-
d6, 400 MHz): δ 3.5 (d, 3H), 7.6 (s, 2H). 31P NMR (DMSO-d6, 202
MHz, ppm): δ -1.65, 13C NMR (DMSO-d6, 125 MHz, ppm): δ 52.91
(CH3). ESI-MS: m/z, 112 (M+ 1).
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6: Yield: 2.4 g (54 %). FT-IR (Liquid film, cm-1): 2926(s), 2861(vs),
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Temp. Range C (% Weight loss): 27-185 (16 %, 2H2O); 161-290
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This work was supported by (1) DST Nanomission (SR/NM/NS-
1119/2011), (2) SERB, New Delhi (SB/S1/IC-48/2013) and (3) IIT-
Bombay Bridge Funding. R. M. thanks SERB (SB/S2/JCB-85/2014),
New Delhi for a J. C. Bose Fellowship, G.A.B thanks UGC New
Delhi for a research fellowship.
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Table 2. Characterization data of compounds 1-8
Compound
Yield
(%)
Physical 1H NMR
31P NMR ESI-MS
Space group
state at
25 oC
Solid
[CyNH3]2[(MeO)PO3] (1).3H2O
[CyNH3][(EtO)PO3] (2)
[CyNH3][(iPrO)PO3] (3)
[CyNH3][(tBuO)PO3] (4)
[(MeO)P(O)(OH)2] (5)
[(EtO)P(O)(OH)2] (6)
51
53
60
63
57
60
61
45
3.33 (3H, s)
4.58
3.38
-
Monoclinic, P21/c,
Solid
3.82 (2H, q), 1.22 (3H, t)
4.40(1H,sep), 1.25(6H, d)
1.31 (9H, s)
-
Monoclinic, C2/c,
Solid
2.61
-
Monoclinic, P21/n
Solid
-1.81
-0.07
-1.65
-1.82
-0.68
-
Monoclinic, I2/a
Liquid
Liquid
Liquid
Liquid
3.16 (3H, s), 7.5(2H, s)
3.95 (2H, q), 1.19, (3H,t)
4.5 (1H, sep), 1.28(6H, d)
1.74 (9H, s), 8.95 (2H, s)
112.96,(M+1),
126.98(M+1)
139(M-1)1
-
-
-
[(iPrO)P(O)(OH)2] (7)
[(tBuO)P(O)(OH)2] (8)
1negative ion mode
Tetragonal, P42/n
-
Table 3. Crystallographic refinement details of compounds 1-4, and 7 and 9.
Compound
Identification code
Empirical formula
FW
1
2
3
4
7
9
GB-701
C13H37N2O7P
364.41
GB-200
C8H20NO4P
225.22
150(2)
Monoclinic
C2/c
GB-309
C9H22NO4P
239.25
150(2)
GB-732
C10H24NO4P
253.27
298(2)
Monoclinic
I2/a
GB-309
C3H9O4P
140.07
150(2)
Tetragonal
P42/n
14.6655(2)
14.6655(2)
5.8852(10)
90
GB-318
C12H31NO8P2
379.32
150(2)
Temp, [K]
Crystal system
Space group
a, [Å]
298(2)
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
P21/n
12.6511(5)
6.4515(2)
24.2892(7)
90
13.596(7)
5.9912(3)
18.3149(7)
90
13.455(1)
6.139(4)
16.694(1)
90
17.6894(18)
6.6145(7)
23.997(3)
90
13.596(7)
10.066(5)
15.921(9)
90
b, [Å]
c, [Å]
α, [°]
β
, [°]
98.918(3)
90
96.199(4)
90
107.778(8)
90
100.392
90
90
111.966(9)
90
γ
, [°]
90
V, [Å3]
1958.48(11)
4
2489.09(19)
8
1313.15(2)
4
2761.8(5)
8
1265.77(4)
8
2020.7(18)
4
Z
D(calcd), [mg/cm3]
µ [mm-1]
Ө range, [°]
1.236
1.202
1.210
1.218
1.470
1.247
0.173
0.213
0.206
0.200
0.367
0.249
1.697 to 25.00
2.71 to 25.00
8941
3.18 to 25.00
13962
2.341 to 24.996 2.77 to 24.97
2.58 to 24.99
13969
No.
of
reflns. 14288
9844
9065
collected
Independent reflns
GOF
3448
2187
2307
2440
1110
3526
1.052
0.0801
0.2458
1.050
0.0464
0.1249
1.077
0.0913
0.2653
1.044
0.0596
0.1705
1.021
0.0308
0.0992
1.167
0.1264
0.1468
R1(I0>2σ(I0)
wR2 (all data)
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 12
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Page 13 of 13
CrystEngComm
View Article Online
DOI: 10.1039/C7CE01066D
A simple protocol for multi-gram synthesis of unstable and normally
inaccessible phosphate monoesters ROPO3H2 is reported, apart from
demonstration of their thermal instability and utility as starting materials for
metal phosphate single source precursors.