Trapped inorganic phosphate dimer

Article abstract of DOI:10.1039/b713365k

Single crystal X-ray crystallographic signature of the pentafluorophenyl substituted tripodal urea-based receptor shows formation of a pseudo dimeric cage which also encapsulates a phosphate dimer via numerous hydrogen bonding and anion...π interactions.

Full text of DOI:10.1039/b713365k

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Trapped inorganic phosphate dimer{
P. S. Lakshminarayanan,a I. Ravikumar,a E. Sureshb and Pradyut Ghosh*a
Received (in Cambridge, UK) 31st August 2007, Accepted 8th October 2007
First published as an Advance Article on the web 17th October 2007
DOI: 10.1039/b713365k
properties with different anionic guests .7,8 Herein, we report a new
Single crystal X-ray crystallographic signature of the penta-
fluorophenyl substituted tripodal urea-based receptor shows
formation of a pseudo dimeric cage which also encapsulates
a phosphate dimer via numerous hydrogen bonding and
pentafluorophenyl substituted tripodal urea receptor, L, and its
2
H2PO4 complex, 1. Structural analysis shows the formation of
a cage type dimer of L via different non-bonding interactions of
pentafluorophenyl units. Further, two units of C3v symmetric host
L, encapsulate a dimer of [H2PO4]2 as a guest via sixteen hydrogen
…
anion p interactions.
…
The design and syntheses of receptors for recognition and sensing
of inorganic phosphate are of considerable current interest due to
its biological and environmental importance.1 Development of
neutral receptors for this ion have been made by different
laboratories using amide,2 sulfonamide,2a ferrocene amide,2e urea,3
pyrrole,4a–c,f indole,4d and carbazole,4e as recognition elements,
other than ammonium-based,5 and metal-based polyamine6
receptors. For tetrahedral phosphate anion encapsulation, tripodal
receptors, having complementary shape to this anion, were used
to create a C3v symmetric cavity.2e,3a,c,6b In 1995, Mora´n et al.
demonstrated tris(2-aminoethyl)amine based urea receptor, L1
bonding and two anion p interactions.
Tripodal urea-based receptor L is obtained by reaction between
tris(2-aminoethyl)amine and 3 equiv. of pentafluorophenyl iso-
cyanate in dry CH2Cl2 and crystals are grown from dimethyl-
formamide (DMF){. The crystals of complex [L(H2PO4)][N(Bu)4]?
DMF (1) are obtained upon reaction of L with tetrabutylammo-
nium dihydrogen phosphate in DMF/MeCN (1 : 1) upon slow
evaporation{. The structures of L and 1 have been determined by
X-ray crystallography studies.{ The crystal structure of L revealed
that O3 of one arm is involved in two intramolecular N–H
…
O
hydrogen bondings with diamide hydrogen H2C and H3C of
…
1
(Chart 1) for phosphate binding using H-NMR study.3a In this
another arm where H3C is involved in N–H F interaction with
the aryl fluorine atom F11 (Fig. 1).
study the authors proposed 1 : 1 complex formation upon binding
of receptor and dihydrogen phosphate via six hydrogen bonds
inside the cleft of C3v symmetric cavity. Similar binding of
[H2PO4]2 with L2?H+ was also proposed by another group.3c
Unfortunately, X-ray structures of [H2PO4]2 bound to neutral
Further two units of L are held together via intermolecular
…
…
C–F F–C and C–F Cg (Cg1 and Cg1* represent centroid of
–C6F5 units of two molecules) contacts forming a pseudo dimeric
cage in the solid state (Fig. 1). In the dimeric ligand, F13/F13*
…
from one unit of L is involved in a bifurcated, C–F Cg
receptor molecules are limited and none of this structure showed
22
guest encapsulation inside the receptor cavity,3d,4b,d though [SO4
]
…
(F13 Cg1*/F13* Cg1 = 3.027 A) and C–F F–C (F13 F1*/
…
…
…
˚
encapsulation has been structurally demonstrated recently.3e,f
As well as having enjoyed a rich history in bio-inorganic and
medicinal chemistry, recent studies on the receptors containing
fluorine substituted phenyl moiety have shown interesting binding
…
F13* F1 = 2.741 A) interactions, whereas F2 is involved in
˚
…
…
˚
C–F F–C (F2 F9* = 2.753 A) interaction with F9* of the other
L unit and vice versa. Details of these H-bonding and non-bonding
contacts are given in supporting information (Table 2S and 3S){.
Chart 1 Tripodal urea having pentafluorophenyl substitution L,
phenyl substitution L1, and napthyl substitution L2.
aDepartment of Inorganic Chemistry, Indian Association for the
Cultivation of Science, 2A&2B Raja S. C. Mullick Road, Kolkata 700
032, India. E-mail: icpg@iacs.res.in; Fax: (+91) 33-2473-2805
bAnalytical Science Discipline, Central Salt & Marine Chemicals
Research Institute, G. B. Marg, Bhavnagar 364002, India
{ Electronic supplementary information (ESI) available: synthesis and
characterization of L, and complex 1, along with the crystallographic data
and CIF files for Ligand L and complex 1, and 1H-NMR titration details
with anions and L. See DOI: 10.1039/b713365k
… …
Fig. 1 Intramolcular N–H O and N–H F along with intermolecular
…
…
C–F Cg and C–F F–C interactions in L showing the formation of a
pseudo cage structure. The atoms flagged with an asterisk (*) are at
equivalent position (2x, 2 2 y, 1 2 z).
5214 | Chem. Commun., 2007, 5214–5216
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[H2PO4]2 unit and H6D0 of this [H2PO4]2 is further hydrogen
bonded to O7 of the first [H2PO4]2 unit, forming a hydrogen
bonded centrosymmetric dimer of [H2PO4]2, as depicted in Fig. 2b
and 2c.
¯
Complex 1 crystallizes in triclinic system with P1 space group
along with one DMF as solvent of crystallization. It is interesting
to note that the [H2PO4]2 is located inside the cleft of C3v
symmetric L (Fig. 2a). The encapsulated [H2PO4]2 is oriented in
such a way that H6D hydrogen of (O6) forming a strong hydrogen
Fig. 2a shows that the [H2PO4]2 is bound strongly via seven
…
…
bond via O–H O interaction with O70 of the other encapsulated
(N–H O) hydrogen bonds with all three urea moieties of L. Thus
O5 of [H2PO4]2 is involved in four point contacts with the
hydrogen atoms of urea, H2C, H4C, H6C and H7C where as H5C
and H3C make contacts with O7 and O6 of the anion respectively.
Further, H7C which is involved in H-bonding with the O5 makes
…
an additional H-bonding contact with O8 via bifurcated N–H
O
O
…
interaction. Subsequently, O8 also acts as a donor in O–H
interaction with the lone pair of O49 oxygen of the lattice DMF
which further makes a contact with centroid of the tethered C6F5
2
…
˚
arm (O49 Cg2 = 3.474 A). Thus the encapsulated [H2PO4] is
involved in eight hydrogen bonding interactions with L, in
addition to the dimeric association among [H2PO4]2 units. In
biological systems, phosphate-binding proteins provide a classical
example of higher coordination number as observed in 1.9 The
binding of dimer of [H2PO4]2 with the dimer of L (Fig. 2b)
…
involves two additional intermolecular anion p interactions. One
–C6F5 unit of the phosphate encapsulated receptor L is making
relatively closer approach towards O8 of the encapsulated
[H2PO4]2 guest of the other unit, making two intermolecular
…
…
…
H–O Cg contacts (Cg3 O80/Cg30 O8 = 3.420 A) with a
˚
…
˚
shortest distance C8 O8/O80 = 3.002 A where Cg3/Cg30 is the
centroid of the C6F5 ring C4–C9. Therefore, [H2PO4]2 dimer is
involved in altogether eighteen coordination with the dimer of
L?DMF. Various hydrogen bonding interactions for the encap-
sulated dihydrogen phosphate anions with symmetry code are
given in supporting information (Table 5S and 6S{). An earlier
report on the crystal structure of an [H2PO4]2 dimer binding with
urea based ligand showed four coordination outside the receptor
moiety which is the only urea-based structure known to date.3d
Detailed crystallographic analysis on 1 shows two other
…
…
intermolecular C–F Cg (F30 Cg4/F3 Cg40 = 3.545 A with
…
˚
…
…
…
…
,C–F Cg = 124.9u) and two C–F F–C (F5 F100/F50 F10 =
˚
2.872 A) contacts with the associated dimer of L. These
…
intermolecular interactions and intermolecular anion p interac-
tions might have also played a role for dimeric association of
[L(H2PO4)?DMF] in solid state.
2
Solution state binding properties of L with H2PO4 and other
oxyanions such as CH3COO2, NO32, ClO4 are investigated
2
1
by H- NMR titration experiments in DMSO-d6. The addition
2
of aliquots of n-tetrabutyl ammonium salts of H2PO4 and
CH3COO2 to the solutions of the receptor led to the downfield
shift of –NH resonance, whereas there is no considerable shift
observed for nitrate and perchlorate anions. The titration data
gave the best fit for 1 : 1 stoichiometry of host to guest, in
agreement with the Job plots in cases of H2PO42 and CH3COO2
(Fig. 3).
The association constants (log K in M21) of L with H2PO42 and
CH3COO2 are 5.52 and 4.45 M21 respectively. Binding constants
calculated for L1 with H2PO42 by 1H-NMR titration is 4.04 M21
whereas in case of L2 calculated binding constant using
2
Fig. 2 Binding of (a) H2PO4 with L?DMF inside the C3v symmetric
cavity of L, single prime (9) character indicates that this atom is at
equivalent position (21 + x,y,z) (b) H2PO42 dimer inside the pseudo cage
of dimer of L?DMF. DMF is omitted for clarity, the atoms flagged with a
double prime (0) character are at equivalent position (2x,1 2 y,2z) (c)
2
fluorescence titration is 4.60 M21. The receptor L binds H2PO4
more strongly than L1 or L2 in DMSO. The enhanced binding of
2
H2PO4 in the case of L may be attributed to the significantly
2
Space filling presentation of dimeric H2PO4 inside the pseudo cage.
more acidic nature of –NH in L, due to the electron withdrawing
Chem. Commun., 2007, 5214–5216 | 5215
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I . 2 s(I). Crystal data for 1: C46H63F15N9O3P1, Mr = 1186.02, triclinic,
˚
¯
space group P1, a = 13.3363(11), b = 14.0385(11), c = 15.5942(13) A, a =
99.391(2)u, b = 95.841(2)u, c = 108.885(2)u, V = 2687.8(4) A23, Z = 2,
˚
r
calcd = 1.465 g cm23, m = 0.163 mm21, T = 100(2) K, R1 = 0.0640, wR2 =
0.1252, GOF = 1.117 I . 2 s(I). CCDC-655559 (L), and CCDC-655560
(1). For crystallographic data in CIF or other electronic format see DOI:
10.1039/b713365k
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character of the –C6F5 units as well as additional coordination via
…
anion p interaction as observed in the crystal structure. Further,
2
the highest selectivity of L towards the binding of H2PO4 over
other anions like NO32, ClO4 and CH3COO2 is confirmed
2
via a set of 1H-NMR experiments in the presence of different
combinations of these anionic mixtures in solution (supporting
information Fig. 7S{). In the presence of anionic mixtures the
1H-NMR spectra of L closely represent the 1H-NMR spectrum of
2
L in presence of H2PO4
.
In conclusion, this is a new urea based tripodal anion receptor
that shows a significantly high binding constant for dihydrogen
phosphate. To the best of our knowledge, this is the first structural
evidence of H2PO42 encapsulation inside the C3v symmetric cavity
of a neutral receptor. The present mode of encapsulation of
dihydrogen phosphate has not been predicted theoretically nor
previously reported experimentally. We are currently probing this
tripodal urea receptor for encapsulation and binding of other
anionic guests having different shapes and sizes.
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PSL would like to thank CSIR, New Delhi for a Senior
Research Fellowship.
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Notes and references
{ Crystal data for L: C27H18F15N7O3, Mr = 773.48, triclinic, space group
¯
˚
P1, a = 9.2984(10), b = 13.3540(14), c = 13.7161(14) A, a = 118.317(2)u, b =
90.295(2)u, c = 96.329(2)u, V = 1487.0(3) A23, Z = 2, rcalcd = 1.728 g cm23
,
˚
m = 0.179 mm21, T = 100(2) K, R1 = 0.0582, wR2 = 0.1215, GOF = 1.081
5216 | Chem. Commun., 2007, 5214–5216
This journal is ß The Royal Society of Chemistry 2007