One, two, and three methylene phosphonic acid groups (-CH 2PO3H2) on a mesitylene ring: Synthesis, characterization and aspects of supramolecular aggregation

Article abstract of DOI:10.1039/b9nj00691e

The ease of substitution of -CH₂Br group on the mesitylene ring in a stepwise manner has been exploited to prepare mesitylene mono-, bis-, and tris-methylene phosphonic acids C₆H₂Me₃(CH ₂PO₃

Full text of DOI:10.1039/b9nj00691e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
One, two, and three methylene phosphonic acid groups (–CH2PO3H2)
on a mesitylene ring: synthesis, characterization and aspects
of supramolecular aggregationw
Ramaswamy Murugavel*ab and Mayank Pratap Singha
Received (in Victoria, Australia) 19th November 2009, Accepted 17th March 2010
DOI: 10.1039/b9nj00691e
The ease of substitution of –CH2Br group on the mesitylene ring in a stepwise manner
has been exploited to prepare mesitylene mono-, bis-, and tris-methylene phosphonic
acids C6H2Me3(CH2PO3H2) (1), C6HMe3(CH2PO3H2)2 (2), and C6Me3(CH2PO3H2)3 (3) in
80–90% yield, through a Michaelis–Arbuzov reaction followed by acid hydrolysis. Compounds
1–4 have been characterized by analytical and spectroscopic (IR, NMR, and MS) techniques.
The X-ray structural investigations on all the three compounds reveal that the central
mesitylene ring is a robust platform for hosting multiple methylene phosphonic acid
groups. Mesitylene mono-methylene phosphonic acid 1, which can be considered as the sterically
encumbered modification of benzyl phosphonic acid, interestingly associates as a one-dimensional
tubular structure through extensive O–HÁ Á ÁO hydrogen bonding between adjacent –PO3H2
groups, albeit with no such interactions between the tubes due to the highly hydrophobic
nature of the surface. The X-ray structure of mesitylene-1,3-diphosphonic acid 2 reveals that the
molecules exhibit a syn-orientation of the –CH2PO3H2, which are involved in extensive
P–OHÁ Á ÁO hydrogen bonding to result in a two-dimensional layered structure. The presence of
three –PO3H2 units on the same side of the mesitylene ring in 3 results in a three-dimensional
framework solid. The present studies clearly reveal a linear relationship between the
dimensionality of the supramolecular structure and the number of phosphonic acid moieties on
the mesitylene ring.
Introduction
A major focus in the ever-expanding field of supramolecular
chemistry lies in the design of simple molecules in which
multiple ‘adhesive’ functional groups (e.g. tectons such as
–COOH) are judiciously placed on a rigid central building
block (synthons such as aromatic rings).1–3 An elegant example
of such an approach is the demonstration of the formation
of the hydrogen-bonded sheet structure of benzene-1,3,5-
tricarboxylic acid (trimesic acid), often referred to as a
honeycomb or chicken-wire network.3a While trimesic acid
represents one of the early examples of poly(carboxylic)
aromatic acids amenable for supramolecular aggregation, later
research showed that several similar compounds with more
than one carboxylic function on an aryl ring, such as those
shown in Scheme 1 (A–D),4 could be employed for the
generation of architectures in the solid-state through extensive
hydrogen bonding.
Phosphonic acids have long been known for their ability to
bind many metal ions (per acid group) and thus they generate
a Department of Chemistry, Indian Institute of Technology – Bombay,
Powai, Mumbai-400076, India. E-mail: rmv@chem.iitb.ac.in;
Fax: +91-22-2572 3480
b Centre for Research in Nanotechnology and Science, Indian Institute
of Technology – Bombay, Powai, Mumbai-400076, India
w Electronic supplementary information (ESI) available: Details of
spectroscopic investigations of compounds 1–3. CCDC reference
numbers 755660–755662. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/b9nj00691e
Scheme 1 Arene-based polyacids used for the preparation of supra-
molecular assemblies and metal–organic frameworks.
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highly polymeric inorganic–organic networks.5,6 Among
these, notable are the layered metal phosphonates discovered
in the 1970s, and shown to have interesting applications.6 This
abilities of these groups of compounds was exploited by
placing two or more phosphonic acid moieties on a rigid aryl
platform, as shown in Scheme 1 (E–H).7 This has been well
demonstrated in the case of diphosphonic acids (used to
prepare pillared solids) and triphosphinic acids (used to
prepare three-dimensional structures).6 Apart from their use
as polydentate ligands,6 these bis- and tris-phosphonic acids
can themselves self-assemble both in solution and in the solid
state through extensive (and strong) intra- and intermolecular
P–O–HÁ Á ÁO–P hydrogen bonds, as in the case of trimesic acid
described above.3a Notable among such hydrogen-bonded
structures of phosphonic acids are 4-methyl-2,6-bis(phos-
phonomethyl)phenol,8 o-, m-, and p-fluorobenzylphosphonic
acid,9 (R)-1-phenyl-2-carboxyethylphosphonic acid,10 rac-
(hydroxy(phenyl)methylene)diphosphonic acid monohydrate,
(hydroxy(4-nitrophenyl)methylene)diphosphonic acid,11 and
(4-vinylbenzyl)phosphonic acid,12 which have been characterized
by single-crystal X-ray diffraction studies and shown to have
interesting association properties.
the respective phosphonic acids 1–3 (Scheme 2). All the three
new compounds have been characterized by elemental analysis,
IR spectroscopy, NMR spectroscopy (1H and 31P), ESI-MS,
diffuse-reflectance UV-vis spectroscopy, and fluorescence
spectroscopy. All three compounds yielded satisfactory elemental
analysis. In the case of compound 2, although the single
crystals contained lattice ethanol, the vacuum-dried sample
did not show the presence of lattice ethanol in the sample. The
lattice water present in the single crystals of 3, however, was
retained even after prolonged drying, and hence its elemental
analysis corresponds to the formulation as found for the single
crystals.
The infrared spectra (recorded as KBr discs) displayed an
intense absorption at 2260 for 1, 2239 for 2, and 2331 cmÀ1 for
3, corresponding to the presence of P–OH groups. The
31P NMR spectra recorded in deuterated methanol show a single
resonance for all the three samples (d 28.2 for 1, 27.14 for 2,
and 22.2 ppm for 3). The UV-vis spectra in methanol show
absorption maxima at 208–212 nm for all three compounds;
likewise, there is an emission for each compound at 301–312 nm
in the UV region. The solid-state UV spectrum was also
studied for all the compounds in the reflectance mode.
As a part of our ongoing studies on metal phosphonates13
and phosphates14 in order to achieve new types of supra-
molecular architectures, we have designed and synthesized
three new benzyl phosphonic acids. The aromatic rings in
the new acids have been substituted at all six positions by
additional alkyl groups to enhance the solubility. Thus, while
the 1-, 3-, and 5-positions of the aryl ring have methyl
substituents, the 2-, 4-, and 6-positions host a methylene group
to which the –PO3H2 moiety is attached. Unlike compound H
(Scheme 1), where the mesityl ring and the phosphonate
groups are in the same plane, the introduction of the flexible
–CH2 linker between the aromatic ring and –PO3H2 moiety in
the present case could lead to different relative dispositions for
the phosphonic acids on either side of the central ring. Thus
for a given molecule, depending on the relative dispositions of
the phosphonic acids moieties (cis or trans, for example), it is
possible to recognize different supramolecular arrays. The
present investigation addresses these questions, and reports
the strategies used to synthesize these molecules, as well as
their spectral characterization and crystal structure determi-
nation, to showcase the structural variety these molecules can
offer in forming supramolecular arrays.
ESI-MS studies
Phosphonic acids, due to the presence of polar hydrophilic
–PO3H2 terminus, are known to aggregate in solution and
produce discrete clusters.16 Hence, the association behavior of
all the three acids in the gas phase was examined with the help
of ESI-MS studies in methanol under positive ionization
mode. Interestingly, the spectra obtained not only exhibit an
(M + 1)+ ion peak in each case, but also contain a peak
corresponding to the formation of H-bonded aggregates in
solution. For example, the mesitylene mono-methylene
phosphonic acid 1 displays peaks at m/z 214, 426, 642, 856
and 1070 corresponding to the aggregates (M + 1)+, (2M + 1)+,
(3M + 1)+, (4M + 1)+ and (5M + 1)+, respectively. Similar
observations were also observed for 2 (309 (M + 1)+, 616
(2M + 1)+, 924 (3M + 1)+) and 3 (1232 (4M + 1)+. 403
(M + 1)+, 804 (2M + 1)+ and 1206 (3M + 1)+). The spectra
are shown in Fig. 1.
Determination of molecular structures
While the ESI-MS studies clearly shed light on the formation
of discrete aggregates in the gas phase, we wanted to study this
phenomenon further, and to this end the association behavior
of these title compounds in the solid-state was explored by
single-crystal X-ray diffraction studies (Table 1). The single
crystals for all the three acids were grown by slow evaporation
of the solvent from an appropriate solution. While the crystall-
ization of products 1 and 3 was straightforward, compound 2
yielded poor-quality single crystals even after several attempts.
It was also necessary to add small amounts of aniline to the
solution of 2 during crystallization.
Results and discussion
Synthesis and characterization
To synthesize the three phosphonic acids, a general synthetic
procedure was employed. Treatment of 33% HBr in
CH3COOH with mesitylene in different stoichiometric ratios
under varying reaction conditions (such as duration and
temperature) led to the formation of the respective benzyl
bromides in a high yield.15 The bromomesityl derivatives were
subjected to the Michaelis–Arbuzov reaction with triethyl-
phosphite in xylene. The removal of excess xylene and triethyl-
phosphite, followed by the hydrolysis of the product by
concentrated hydrochloric acid, resulted in the formation of
Crystal structure of 1
Compound 1 crystallizes in the monoclinic space group P21/c.
The asymmetric part of the unit cell contains one molecule of 1
with no other lattice solvents. While all the methyl groups on
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Scheme 2 Synthesis of phosphonic acids using the Michaelis–Arbuzov reaction.
Fig. 1 Mass spectra of 1, 2, and 3 (the asterisks represent the aggregated molecular ion peaks).
the mesitylene ring lie in the plane of the aromatic ring, the
solitary –PO3H2 group is situated above the plane of the ring
(as shown in Fig. 2) due to the C–C–P angle of 116.991 at
the methylene carbon. The P–C bond length found in 1
(1.758 A) falls in the range observed for earlier reported
alkylphosphonic acids.8–12 The phosphorus atom exhibits
tetrahedral geometry, with the angles varying in the range
107.4 to 114.51. There are clearly two types of P–O distances
in the molecule. While the larger distances (P1–O2 1.545
and P1–O3 1.553 A) are associated with the P–OH bonds,
the shorter one (P1–O1 1.496 A) corresponds to the
PQO phosphoryl moiety. It is pertinent to state that the
acidic protons in the molecule were readily located from the
difference maps and refined for the convergence of their
positional parameters. Thus, in the solid state, clearly both
P–O and PQO bonds exist with no significant delocalization
of the protons between the three oxygen atoms on the
phosphorus.
The presence of PQO and P–OH groups in 1, like the CQO
and C–OH groups in carboxylic acids, leads to an interesting
hydrogen-bonded network (Table 3). For example, extensive
intermolecular hydrogen bonding between the individual
molecules of 1 results in the formation of a ribbon-like
structure, as shown in Fig. 3. In this ribbon-like association,
two rows of molecules of 1 are arranged in a head-to-head
fashion, where the –PO3H2 groups come together to form a
locked zipper-like structure (Fig. 3), and the three oxygen
atoms of each phosphonic acid are involved in four different
H-bonds (of which only two are unique O1Á Á ÁO2 2.568 A,
O1Á Á ÁO3 2.589 A). While the phosphoryl oxygen O1 (acceptor)
is involved in two hydrogen bonds, the O2 and O3 oxygen
atoms (donors) form only one hydrogen bond each.
Further, the mesitylene methyl (Me3C6H2CH2) groups on the
phosphorus also associate themselves in such a way that the
adjacent aryl rings along the ribbon chain are held together by
a p–p stacking interaction, as shown in Fig. 3. The distance
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Table 1 Crystal data and details of structure refinement for 1–3
1
2a
3
Empirical formula
Formula weight
Crystal system
C10H15O3P
214.19
Monoclinic
P21/c
150(2)
0.71073
a = 14.055(4) A
b = 4.6342(11) A
c = 16.179(4) A
C13H24O7P2
354.26
Orthorhombic
P212121
150(2)
0.71073
a = 13.687(5) A
b = 21.666(5) A
C12H23O10P3
420.21
Monoclinic
P21/c
150(2)
Space group
Temperature (K)
Wavelength (A)
Unit cell dimensions
0.71073
a = 9.7623(7) A
b = 7.876(2) A
c = 10.4978(8) A
b = 107.936(9)
1743.0(3)
c = 7.258(5) A
—
2152(2)
b = 95.38(2)1
1049.2(4)
Volume (A3)
Z
4
1.356
0.241
456
4
1.093
0.225
752
4
1.601
0.392
880
Density (calculated) (Mg mÀ3
)
Absorption coefficient (mmÀ3
F(000)
)
Crystal size (mm3)
0.32 Â 0.28 Â 0.24
0.31 Â 0.26 Â 0.22
0.40 Â 0.37 Â 0.31
y range for data collection
Reflections collected
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
3.03–24.91
8445
0.673
2.96–25.001
17 950
0.756
3.06–24.991
5382
1.136
R1 = 0.0566, wR2 = 0.1112
R1 = 0.1591, wR2 = 0.1255
0.310 and À0.312
R1 = 0.0977, wR2 = 0.2464
R1 = 0.1866, wR2 = 0.2652
0.508 and À0.377
R1 = 0.0368, wR2 = 0.1009
R1 = 0.0465, wR2 = 0.1055
0.411 and À0.462
Largest diff. peak and hole (e AÀ3
)
a
Compound 2 forms very poorly diffracting crystals.
Fig. 2 Molecular structure of 1.
Table 2 Selected bond lengths (A) and angles (1) in 1–3
1
P(1)–O(1)
P(1)–O(2)
P(1)–O(3)
P(1)–C(10)
1.496(4)
1.545(4)
1.553(4)
1.758(5)
C(1)–C(10)–P(1)
O(1)–P(1)–O(2)
O(1)–P(1)–O(3)
O(2)–P(1)–O(3)
118.6(4)
114.5(2)
110.5(2)
107.4(2)
Fig. 3 Tubular association of molecules of 1 in the crystal.
structures of ortho- and meta-fluorobenzylphosphonic acid
have been determined by single-crystal X-ray diffraction
studies.9 Both the isomers of fluorobenzylphosphonic acid
organize themselves in a supramolecular architecture through
weak hydrogen bonding in a pattern that is similar to that
found in 1 (i.e. head-to-head, but with different orientations of
phenyl rings). In case of fluorobenzylphosphonic acids, the
adjacent phenyl rings are not involved in the side-on overlap
leading to p–p interactions (as in the case of 1), probably
owing to strong F–F repulsions (in order to minimize the
F–F repulsions, the adjacent rings in the lattice lie almost
perpendicular to each other).9 Similarly, the 4-vinylphosphonic
acid also has weak hydrogen-bonding interactions between the
phosphonic acid moieties in a head-to-head fashion, with
phenyl rings not participating in any p–p interactions.9 On
the other hand, other aliphatic phosphonic acids organize
themselves in a cluster or polymeric form through a weak
hydrogen-bonding network. For example, cyclohexylphosphonic
2
P(1)–O(1)
P(1)–O(2)
P(1)–O(3)
1.500(7)
1.523(7)
1.520(6)
P(2)–O(4)
P(2)–O(5)
P(2)–O(6)
1.488(7)
1.562(6)
1.493(6)
3
O(1)–P(1)
O(3)–P(1)
O(5)–P(2)
O(7)–P(3)
O(9)–P(3)
1.571(2)
1.501(2)
1.537(2)
1.540(2)
1.502(2)
O(2)–P(1)
O(4)–P(2)
O(6)–P(2)
O(8)–P(3)
C(7)–C(1)–P(1)
1.536(2)
1.575(2)
1.490(2)
1.553(2)
110.6(2)
between any two adjacent mesitylene rings along the chain was
found to be 4.63 A, suggesting fairly strong p–p interactions
between these aromatic rings.
It would be of further interest to compare the structure of 1
with similar alkyl phosphonic acids reported in the literature.
Although the parent benzyl phosphonic acid (C6H5CH2PO3H2)17
has not yet been structurally characterized, the solid-state
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Table 3 Possible O–HÁ Á ÁO hydrogen bonds in 1–3
D–HÁ Á ÁA
Compound 1
DÁ Á ÁA (A) HÁ Á ÁA (A) D–H (A) D–HÁ Á ÁA (1)
O2–H(2W)Á Á ÁO1a
2.564(4) 1.88
2.595(5) 1.77
0.87
0.87
134
156
O3–H(3W)Á Á ÁO1b
Compound 2i
O(1)Á Á ÁO(5)c
O(2)Á Á ÁO(4)d
O(3)Á Á ÁO(6)c
2.51(1)
2.49(1)
2.52(1)
—
—
—
—
—
—
—
—
—
Fig. 4 Molecular structure of 2.
Compound 3
The three oxygen atoms and two protons on each of the
phosphorus termini are further used as ‘glue’ to connect the
one-dimensional wave-like chains to a two-dimensional sheet
with the original wave-like surface (Fig. 5). This type of stepwise
of aggregation in 2 generated from atomic coordinates is shown
in Fig. 6. The schematic representation at the bottom of Fig. 6
further illustrates that the two-dimensional wave-like sheets
are stacked in such as way that there is strong aromatic p–p
interactions between the mesitylene rings of adjacent layers.
A closer look at the types of interactions responsible for the
supramolecular 2-D sheet formation reveals that there are
three unique hydrogen bonds originating from each molecule
of 2 (O1Á Á ÁO5 2.52 A, O2Á Á ÁO4 2.49 A and O3Á Á ÁO6 2.52 A)
(Fig. 5). Due to the poor quality of the diffraction data and the
absence of any marked difference between the three P–O
distances on each of the two phosphonic acid groups, no
attempt has been made either to identify the P–OH hydrogen
atoms from the difference map or geometrically fix them.
Hence, the hydrogen bonding pattern has been derived from
DÁ Á ÁA (OÁ Á ÁO) distances only. Unlike in 1, each oxygen atom
in 2 is involved in only one hydrogen bond with the neighboring
acceptor atom. In the final structure, each molecule of 2 is
surrounded by six other neighbors, with the aryl–aryl ring
distances varying from 7.258 A to 11.246 A.
O(5)–H(5)Á Á ÁO(10)
O(2)–H(1)Á Á ÁO(9)e
O(4)–H(4)Á Á ÁO(1)e
O(1)–H(2)Á Á ÁO(3)e
O(8)–H(8)Á Á ÁO(3)e
O(7)–H(7)Á Á ÁO(6)f
2.507(2) 1.72(3)
2.511(2) 1.68(3)
2.797(3) 1.90(3)
2.601(3) 1.95(5)
2.614(2) 1.75(3)
2.557(2) 1.75(3)
0.79(3) 172.64(4)
0.86(4) 160.72(4)
0.92(3) 165.18(3)
0.65(5) 171.98(6)
0.86(3) 174.34(4)
0.84(3) 162.62(3)
0.95(5) 176.60(4)
0.88(3) 171.46(3)
O(10)–H(10A)Á Á ÁO(6)g 2.707(3) 1.75(5)
O(10)–H(10B)Á Á ÁO(9)h 2.729(3) 1.86(3)
Symmetry of the acceptor atoms:a 2 À x, + y, À z. x, 1 + y, z.
b
1
2
3
2
c
d
e
f
1
2
1
x À , 32 À y, 1 À z. x À , 23 À y, À z. 2 À x, À y, 1 À z. 1 + x, 12 À y,
2
g
h
i
1
12 + z. x, 12 À y, z À . x À 1, 12 À y, z À . Hydrogen atoms attached
1
2
to PO3 groups were not fixed.
2
acid18 forms a zig-zag polymeric chain like structure while
tert-butylphosphonic acid16 forms a cluster-like assembly,
although a polymeric association is also known.
Crystal structure of 2
Growing single crystals of mesitylene-bis(methylene)phosphonic
acid 2 proved far more difficult. In a successful attempt,
compound 2 was crystallized by dissolving the white powder
of 2 in dimethylformamide and carefully layering the resultant
solution with aniline. After two months, colorless prisms of 2
were obtained, which were later found to have crystallized in
the orthorhombic non-centrosymmetric P212121 space group
with considerable twinning. The asymmetric part of the unit
cell contains one molecule of 2 along with a molecule of
ethanol. In the monomeric form of compound 2, the two
methylene phosphonic acid moieties are situated on the same
side of the aromatic ring in such a way that the –PO3H2 groups
(syn) and the central aromatic ring are perpendicular to each
other (Fig. 4). The cis arrangement of the two phosphonic acid
groups is somewhat surprising due to the fact that in the anti
arrangement the system could have avoided the steric crowding.
However, it appears that the supramolecular structure
formation (vide infra) is the driving force for the formation
of syn configuration. These two phosphonic acid arms
make slightly different angles at the methylene carbon atoms
(C(6)–C(11)–P(1) 115.31; C(4)–C(9)–P(2) 115.91) and are
almost parallel to each other (Fig. 4) (torsion angle of 3.331
between the two –PO3H2 arms) (Table 2). The phosphorus
atom is in a slightly distorted tetrahedral geometry, with
angles of 110 to 113.71.
The syn and parallel orientations of the two –PO3H2 groups
on the mesitylene ring renders an approximate ‘ ’ shape for 2,
with P–O termini at the open ends. Arranging these ‘ ’-shaped
objects in the normal and inverted forms alternately with a
periodic shift as shown in Fig. 5 (top) leads to the formation of
a wavelike hydrogen-bonded one-dimensional chain (Table 3).
Fig. 5 Schematic representation of the supramolecular assembly in 2.
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Scheme 3 Possible conformations for the diacid and triacid.
torsional angles further testify to the bowl shape of the molecule.
In contrast, in the case of 1,3,5-benzene-triphosphonic acid, all
the three –PO3H2 groups and the benzene ring lie in the same
plane.16,18 Further, unlike in the monoacid 1 and the diacid 2,
in 3 there is no effective p–p stacking between the mesitylene
rings (the shortest distance between any two adjacent aryl
rings is 10.4 A).
The presence of six P–OH protons and three PQO groups
on the rim of the bowl-shaped 3 leads to an extensive association
of the molecules through a H-bonding network. The construction
of the final observed supramolecular structure in 3 is shown in
Fig. 8. The polar heads at the rim of two molecules of 3 come
face-to-face, as shown in Fig. 8b, to form a dimeric structure,
in which all six phosphonic acid groups interact with each
other through short O–HÁ Á ÁO hydrogen bonds. These dimeric
structures are connected to each other by the lattice water
molecules as well as additional O–HÁÁÁO interactions involving
phosphonic acid moieties, leading to successive formation of a
1-D polymeric chain (Fig. 8c), a two-dimensional sheet
(Fig. 8d) and finally a three-dimensional framework structure
(Fig. 8e). In the entire structure, the adjacent molecules of 3
are inverted with respect to each other so that the polar groups
can aggregate together by hydrogen-bonding interactions,
whereas the non-polar aromatic rings can stack one over other
with the aid of p-interactions. The structure of 3 consists of
seven unique hydrogen bonds, five of which involve phosphoric
moieties alone as donor and acceptors. The lattice water takes
part in the additional two hydrogen bonds. One can thus
conclude that the preference for the bowl-shaped cis,cis,cis-
conformation for 3 (the W-form) over the other isomeric
forms depicted in Scheme 3 is primarily due to the ability of
the triacid to engage itself in the maximum number of supra-
molecular interactions in this conformation (thirteen hydrogen
bonds emanating from each molecule of 3 and three hydrogen
bonds emanating from each lattice water molecule) (Table 3).
Fig. 6 Evolution of face-on stacked 2-D sheets arrangement in the
crystals of 2.
Crystal structure of 3
Suitable crystals for single-crystal X-ray analysis were grown
by dissolving the crude sample of 3 in methanol, carefully
layering diethyl ether on it, and keeping the solution at 5 1C.
After ten days, well formed crystals started to separate from
mother liquor. Compound 3 crystallizes in the monoclinic
crystal system in the P21/c space group as the monohydrate.
The final refined structure is depicted in Fig. 7.
The three –PO3H2 groups on 3 are placed on the same side
of the mesitylene ring, and hence the molecular structure of 3
resembles that of an open bowl (which can also be described as
a W-form or cis–cis–cis form) (Scheme 3).19–23 This arrangement,
where the functional hydrophobic PO3H2 groups constitute
the rim of bowl and the aryl ring forms the flat base, is the
preferred structural form for a mesitylene ring bearing three
–CH2X groups (X = SO3H,19 1-phenyl-1H-tetrazol-5-ylsulfanyl,20
OH,21 2-isopropylphenol,22 and Br23) on the 2-, 4-, and
6-positions of the aryl ring. The tetrahedral C(7)–C(1)–P(1),
C(9)–C(3)–P(2), and C(11)–C(5)–P(3) angles (110.641, 115.101
and 114.301) at the methylene carbon atoms and the associated
Conclusions
We have shown in this contribution that even a simple
aromatic hydrocarbon such as mesitylene can be used a
platform for hosting multiple phosphonic acid groups through
a series of simple organic transformations (Scheme 1). The
mono-, di-, and trimethylene bromides on the mesitylene
readily undergo Michaelis–Arbuzov reactions to yield the
respective phosphonate esters in high yields, and the acid
hydrolysis yields the free phosphonic acids in each case.
Fig. 7 Molecular structure of 3.
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conventional procedures and were distilled prior to their
use.24 Commercially available starting materials such as
33% HBr in acetic acid (Spectrochem), paraformaldehyde
(S.d. Fine), Xylene (S.d. Fine), mesitylene (Aldrich) and
triethylphosphite (Aldrich) were used as received. Mono-
(bromomethyl)mesitylene, di(bromomethyl)mesitylene and
tri(bromomethyl)mesitylene) were synthesized using a previously
described literature method.15 The conversion of bromo
derivatives to the corresponding diethyl phosphonate esters
was carried out as described previously by a Michaelis–Arbuzov
reaction involving the alkyl bromide and triethyl phosphite.25
Infrared spectra were obtained on a Perkin Elmer Spectrum
One FT-IR spectrometer as KBr diluted discs. The 1H and
31P NMR spectra were recorded on a Varian 300 and 400 MHz
instrument using Me4Si as an internal reference for the
1H NMR spectra. The melting points were measured in glass
capillaries and are reported uncorrected. The ESI mass spectra
were recorded on Micromass Q-Tof spectrometer. Microanalyses
were performed on a Thermo Finnigan (FLASH EA 1112)
microanalyzer. UV-vis spectra (both in solid and solution)
were obtained on a Shimadzu UV-260 spectrophotometer and
the fluorescence spectral measurements were carried out on a
Perkin Elmer LS-55 luminescence spectrometer.
Synthesis of mesityl-1-methylenephosphonic acid (1)
Mesityl-1-methylene-bis(ethylphosphonate) was heated under
reflux in excess concentrated hydrochloric acid for 24 h.
During this time a white powder started to precipitate, which
was filtered off, and washed with cold water followed by
petroleum ether, and air-dried. Recrystallization from methanol
and diffusion with dichloromethane resulted in colorless crystals.
Yield 90%. M.p. >250 1C. Anal. Calcd. for C10H15O3P: C,
56.07; H, 7.06. Found: C, 56.30; H, 7.08. Selected IR frequencies
(as KBr diluted disc): 3416(s), 2922(s), 2660 (w), 1614(s),
Fig. 8 Evolution of the 3-D supramolecular structure in 3.
Despite difficulties, it was possible to isolate the mono-, di-,
and triphosphonic acids as single crystals. The structure
determination in the solid-state for all the three derivatives
has revealed valuable information regarding the conforma-
tional preferences of these molecules, as well as their ability to
associate in the solid state to form supramolecular structural
motifs. Interestingly, when only one –CH2PO3H2 group is
present on the aryl ring, a one dimensional tubular aggregation
of molecules of 1 is observed. The presence of two –CH2PO3H2
groups in 2 leads to the expansion of the structural motif to the
second dimension, yielding a wave-like association. The bowl-
shaped molecule 3, which contains three –CH2PO3H2 groups on
the aryl ring, maximizes the number of H-bonding interactions in
the system and yields a perfect three-dimensional supramolecular
structure. Thus, the present study clearly shows a direct
correlation between the dimensionality and the number of acid
groups present. Finally, apart from the interest in these molecules
as supramolecular synthons to construct new structural motifs,
the presence of a large number of acidic protons makes them
ideal candidates as ligands in metal phosphonate chemistry
to realize newer cages and layered solids. We are currently
investigating these aspects.
1259(s), 1046(s), 859(s), 581(m), 489(s). m/z
= 215.0
(M + 1)+. 1H NMR (400 MHz, d/ppm, MeOH-d4): 6.72
(s, 2H, Ar–H), 3.1 (d, 2H, –CH2–PO3H2, J = 21.84 Hz), 2.25
(s, 6H, Ar–CH3), 3.5 (s, 3H, Ar–CH3). 31P NMR (400 MHz,
d/ppm, MeOH-d4): 28.20. UV-vis (methanol, e/nm): 208.6
(20 860). DR-UV: 276.5, 471.7. Fluorescence (nm): 212.6
(lexcitation), 301.3 (lemission).
Synthesis of mesityl-1,3-bis(methylenephosphonic acid) (2)
Mesityl-1,3-bis(methylene-bis(ethylphosphonate)) was boiled
with aqueous concentrated hydrochloric acid for 15 h under
reflux. During this period a white solid separated out, and this
was filtered off, washed with water, and air-dried. A small
fraction of the compound was recrystallized from dimethyl-
formamide. Yield 83%. M.p. >240 1C. Anal. Calcd. for
C11H18O6P2: C, 42.87; H, 5.89. Found: C, 42.21; H, 5.12.
Selected IR frequencies (as KBr diluted disc): 3419(s), 2239(s),
1540(s), 1452(s), 1232(s), 1052(s), 931(s), 804(s), 720(s), 620(s),
Experimental section
1
544(s). m/z = 309.0 (M + 1)+. H NMR (400 MHz, d/ppm,
Instruments and methods
MeOH-d4): 7.0 (s, 1H, Ar–H), 3.3 (d, 4H, –CH2–PO3H2,
J = 21.84 Hz), 2.36 (s, 3H, Ar–CH3), 2.31 (s, 3H, Ar–CH3).
31P NMR (400 MHz, d/ppm, MeOH-d4): 27.14. UV-vis
(methanol, e/nm): 212 (21 200). DR-UV: 274.8, 461.8.
Fluorescence (nm): 271.5 (lexcitation), 312.4 (lemission).
All the starting materials and the products were found to be
stable towards moisture and air. Hence, no specific precautions
were taken to rigorously exclude air during the manipulation
of the compounds. Solvents were purified by employing
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Synthesis of mesityl-1,3,5-tris(methylenephosphonic acid) (3)
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Z. Wang, M. Okamura, A. Medina and S. M. Cohen, Chem.
Commun., 2008, 3366.
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Struct. Crystallogr. Cryst. Chem., 1979, 35, 1614; (b) O. Ermer,
J. Am. Chem. Soc., 1988, 110, 3747; (c) S. V. Kolotuchin,
E. E. Fenlon, S. R. Wilson, C. J. Loweth and S. C. Zimmerman,
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A mixture of phosphonate ester and excess concentrated
hydrochloric acid was heated under reflux for 30 h, and the
solution cooled to room temperature. Removal of solvent
under vacuum resulted in an off-white solid, which was
recrystallized from methanol by layering of diethyl ether.
Colorless crystals formed after 5 days, and these were collected
by filtration and washed with cooled methanol. Yield 79%.
M.p. >280 1C. Anal. Calcd. for C12H21O9P3: C, 35.83; H,
5.26. Found: C, 34.35; H, 5.25. Selected IR frequencies (as
KBr diluted disc): 3300(s), 2331(s), 1421(s), 1262(s), 1190(s),
1137(s), 993(s), 958(s), 747(s), 520(s), 552(s). m/z = 403.0
(M + 1)+. 1H NMR (400 MHz, d/ppm, DMSO-d6): 3.26
(d, 6H, –CH2–PO3H2, J = 21.06 Hz), 2.51 (s, 9H, Ar–CH3).
31P NMR (400 MHz, d/ppm, DMSO-d6): 22.2. UV-vis
(methanol, e/nm): 208.4 (208 400). DR-UV: 278.6, 513.7.
Fluorescence (nm): 213.2 (lexcitation), 308 (lemission).
4 (a) Y. Ogata, M. Tsuchida and A. Muramot, J. Am. Chem. Soc.,
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Single-crystal X-ray diffraction studies
Intensity data for 1–3 were collected on an Oxford Xcalibur
CCD diffractometer operating at 150 K. All calculations were
carried out using the programs in WinGX module.26
The structure was solved in each case by direct methods
(SIR-92).27 The final refinement of the structure was
carried out using full least-squares methods on F2 using
SHELXL-97.28 The hydrogen atoms in case of 1 and 2 were
placed in geometrically calculated positions (obtaining the
torsional angles from electron density) and were refined using
a riding model. No attempts were made fix the hydrogen
atoms attached to oxygen atoms in 2 due to the poor quality
of the data and the inability to distinguish PQO from P–OH
groups on P1 and P2. In the case of 3, all the hydrogen atoms
attached to carbon atoms were fixed as above. However, the
hydrogen atoms attached to oxygen centres in 3 were located
from the difference maps and refined with individual isotropic
displacement parameters.
7 (a) G. Prochniak, J. Zon, M. Daszkiewicz, A. Pietraszko and
´
A. Videnova-Adrabinska, Acta Crystallogr., Sect. C: Cryst. Struct.
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(c) D. Kong, J. Zon, J. McBee and A. Clearfield, Inorg. Chem.,
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Cryst. Growth Des., 2008, 8, 3271.
8 G. Ferguson, J. F. Gallagher, W. Vogt, J. Phillips and G. J.
B. Williams, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
1993, 49, 1024.
9 K. J. Langley, P. J. Squattrito, F. Adani and E. Montoneri, Inorg.
Chim. Acta, 1996, 253, 77.
10 S. Gardner, M. Motevalli, K. Shastri, A. C. Sullivan and
P. B. Wyatt, New J. Chem., 2002, 26, 433.
In the case of 2, the quality of the data obtained was very
poor (even with repeated data collection from different
crystals) which was due to the poor quality of the single
crystals themselves. Hence the refinement of the structure of
2 did not converge to low R or wR2 values. Selected crystal
data are presented in Table 1 and detailed crystallographic
data for all the structures are provided in the ESIw.
11 M. Lecouvey, C. Barbey, A. Navaza, A. Neuman and T. Prange,
Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2002, 58, o521.
12 R. Frantz, J.-O. Durand, F. Carre, G. F. Lanneau, J. L. Bideau,
B. Alonso and D. Massiot, Chem. Eur. J., 2003, 9, 770.
13 (a) R. Murugavel and S. Shanmugan, Chem. Commun., 2007, 1257;
(b) R. Murugavel and S. Shanmugan, Dalton Trans., 2008, 5358;
(c) R. Murugavel and M. P. Singh, Inorg. Chem., 2006, 45, 9154.
14 (a) R. Murugavel and S. Kuppuswamy, Angew. Chem., Int. Ed.,
2006, 45, 7022; (b) R. Murugavel, S. Kuppuswamy,
R. Boomishankar and A. Steiner, Angew. Chem., Int. Ed., 2006,
45, 5536; (c) R. Murugavel and S. Kuppuswamy, Chem. Eur. J.,
2008, 14, 3869; (d) R. Murugavel, S. Shanmugan and
S. Kuppuswamy, Eur. J. Inorg. Chem., 2008, 1508;
(e) R. Murugavel and S. Shanmugan, Organometallics, 2008, 27,
2784; (f) R. Murugavel, S. Kuppuswamy and S. Randoll, Inorg.
Chem., 2008, 47, 6028; (g) R. Murugavel and S. Kuppuswamy,
Inorg. Chem., 2008, 47, 7686.
Acknowledgements
This work was supported by DST, New Delhi. M.P.S. thanks
UGC, New Delhi, for a research fellowship. We thank the
DST-funded National Single Crystal Diffraction Facility
at IIT-Bombay for the diffraction data and the SAIF,
IIT-Bombay for the spectral data.
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¨
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