Hydrogen-bonded network in biphenyl-4,4′-diphospho­nic acid

The crystal structure of the title compound, C₁₂H₁₂O₆P₂, displays two different regions alternating along the a axis: a hydrogen-bonded region encompassing the end-positioned phospho­nic acid groups and a hydro­phobic region formed by the aromatic spacers. The asymmetric unit contains only half of the biphenyl-4,4′-diphospho­nic acid (4,4′-bpdp) mol­ecule, which is symmetric with an inversion centre imposed at the mid-point between the two aromatic rings. The periodic organization of the mol­ecules is controlled by two strong O—H...O inter­actions between the phospho­nic acid sites. Weak C—H...π inter­actions are established in the aromatic regions.

S1. Comment

Studies aimed at building predictable structures span from hydrogen-bonded supramolecular networks to hybrid metal–organic frameworks (MOF). The establishment of porosity in coordination polymer structures has been a challenging but central goal in solid-state chemistry, since by analogy with zeolites, it opens up possibilities for chemical separation, gas sorption, ion exchange, sensing and catalysis. The most important factors determining MOFs are the chemical and geometrical preferences of the metal ion and the specificity of the bridging polydentate ligand. Therefore, variable metal–organic phosphonates have been studied with respect to their potential applications in many of the above areas (Kong et al., 2006; Sharma et al., 2000; Cabeza et al., 2002). Bisphosphonates can bind with different coordination modes (from η¹µ₁ up to η⁶µ₆) dependent upon the level of deprotonation and the stereo accessibility of the donating lone pairs from one side, and the geometry of the metal ion 'vacant' sites from the other side. This allows for the formation of variable coordination arrays (Matczak-Jon & Videnova-Adrabińska, 2005). The use of a rigid spacer between the end-functional sites in diphosphonates reduces the orientational freedom of the ligand and makes the frameworks more predictable (Cao et al., 2004). A distance of 10.665 Å between the two phosphonate groups in biphenyl-4,4'-diphosphonic acid allows for porosity of organic–inorganic hybrid networks (Zhang et al., 1998; Poojary et al., 1996). Despite the usefulness of arenediphosphonate moieties as ligands, the crystal structures of the corresponding acids are not known. Here, we present the crystal organization and the supramolecular network of biphenyl-4,4'-diphosphonic acid, (I).

The acid molecule is symmetric with an inversion centre imposed in the midline between the two phenyl rings (Fig. 1) and, thus, the asymmetric unit of the crystal comprises only one-half of the molecule. The two phenyl rings are coplanar and slightly deformed. The P—O bonds of the phosphonic group are bisectional and the P═O bond is axial with respect to the mean molecular plane. Therefore, the spatial orientations of the hydrogen-bond donor and acceptor sites in 4,4'-bpdp are not appropriate for the formation of the R₂²(8) motifs (Bernstein et al., 1995) observed in the ribbon extensions of dicarboxylic acids, and the studied compound displays a three-dimensional supramolecular network.

A search of the Cambridge Structural Database (CSD, Version?; Allen, 2002) revealed only a single diphosphonic structure, the triclinic polymorph of 1,4-butanediphosphonic acid, which forms ribbons via R₂²(8) (Mahmoudkhani & Langer, 2002). Two different hydrogen-bond interactions control the organization of the molecules in the crystal structure. One of them, assigned as O1—HO1···O2 [2.546 (2) Å], is used to connect the reflection-related molecules in order to form thick molecular monolayers (bc). The phosphonic groups are arranged outward and the biphenyl rings inward the monolayers [author query - meaning unclear]. The second hydrogen bond, O3—HO3···O2 [2.568 (2) Å], is established between rotation-related molecules belonging to neighbouring layers, and therefore, joins the monolayers. The overall crystal structure displays two different regions alternating along the a axis: the hydrophilic regions where the phosphonic groups are arranged and the hydrogen-bond interactions take place; and the hydrophobic regions, where the aromatic spacers reside. The biphenyl rings in the latter region are arranged with c-translation distance in order to form two differently oriented stacks alternating along the b axis. C—H···π interactions are established between adjacent (reflection-related) stacks in the monolayer (Fig. 2). However, with regard to the basic forces responsible for the solid-state organization of the compound and the observed supramolecular network, it is reasonable to describe the structure in terms of pillared hydrogen-bonded monolayers. Thus, each phosphonic acid group serves to bridge four neighbouring phosphonic acid groups in order to form a two-dimensional hydrogen-bonded network parallel to the (bc) crystallographic plane (Fig. 3). The (P1—)O3—HO3···O2(═ P1) bond extends the screw-related sites into formal chains along the b axis and (P1—)O1—HO1···O2(═ P1) links the chains into a puckered two-dimensional network. Two centrosymmetric motifs R₂⁴(12) [or R₄²(12) author which is correct?] and R₄⁴(16) are generated between the c glide chains and propagate along the b axis. The biphenyl rings, aligned from both sides of the monolayers, act as linkers (pillars) between them and follow the symmetry demands of the hydrogen-bonded network.

Only four structures of unsubstituted diphosphonic acids with functional sites not located on the same C atom were found in the CSD and they all belong to the α,ω aliphatic family with the number of C atoms between 2 and 4. The crystal packing of the even-numbered diacids displays some similarity to that of 4,4'-bpdp. The phosphonic sites in them are also arranged in hydrogen-bonded monolayers, but the connection patterns and symmetry relations, both inside the monolayers and between them, are different. Due to the shortness and flexibility of the aliphatic linker, the pillaring effect in them is not well expressed. The triclinic polymorph of butanediphosphonic acid demonstrates a two-dimensional interpenetration of two symmetry-independent hydrogen-bonded [via R₂²(8)] ribbons. Numerous hydrogen bonds cross-link two symmetry-independent molecular units of propanediphosphonic acid to extend them into a three-dimensional network.

Related literature

For related literature, see: Allen (2002); Bernstein et al. (1995); Cabeza et al. (2002); Cao et al. (2004); Kong et al. (2006); Mahmoudkhani & Langer (2002); Matczak-Jon & Videnova-Adrabińska (2005); Poojary et al. (1996); Sharma & Clearfield (2000); Wang et al. (2003); Zhang et al. (1998).

S2. Experimental

The synthesis of the compound was carried out in two steps starting from 4,4'-dibromobiphenyl (6.27 g, 0.020 mol) and following a known procedure (Wang et al., 2003). However, the method of isolation of tetraethyl biphenyl-4,4'-diphosphonate was modified and 1,3-diisopropylbenzene and other volatile components of the reaction mixture were removed under reduced pressure. The black residue obtained was extracted with cyclohexane (3 × 50 ml), filtered, and left for crystallization. The crystals were further filtered off and dried in air in order to obtain the crude product (7.08 g, 83%), which was recrystallized from cyclohexane (30 ml) to obtain pure tetraethyl biphenyl-4,4'-diphosphonate (5.73 g, 67%, m.p. 354–356 K). ¹H NMR (300 MHz, CDCl₃, δ, p.p.m.): 1.29 (t, J=7.1 Hz, 12H, CH₃), 3.96–4.18 (m, 8H, OCH₂), 7.58–7.65 (m, 4H, C₆H₄), 7.78–7.88 (m, 4H, C₆H₄). ³¹P{H} NMR (121 MHz, CDCl₃, δ, p.p.m.): 20.05 (s). IR (KBr, ν_max, cm^-1): 2986, 2904, 1134, 1249, 1055, 1025, 960, 795, 768, 563, 536. Biphenyl-4,4'-diphosphonic acid was obtained by refluxing tetraethyl biphenyl-4,4'-diphosphonate (4.09 g, 0.0959 mol) in concentrated HCl (40 ml) and water (40 ml) and stirring for 12 h. It was then filtered off, washed with water (5 ml) and dried in air to yield the final product, (I) (2.08 g, 93%). ¹H NMR (300 MHz, D₂O-NaOD, δ, p.p.m.): 7.65–7.85 (m, C₆H₄). ³¹P{H} NMR (121 MHz, D₂O-NaOD, δ, p.p.m.): 12.83 (s). IR (KBr, ν_max, cm^-1): 2929, 2710, 2269, 2203, 1601, 1140, 1047, 1001, 547, 525. Crystals suitable for X-ray measurements were obtained from water–methanol mixture (3:1 v/v). The solution was heated and stirred for 2 h. The clear solution was then sealed in small glass containers and heated to 348 K for 2 d. Small plate-shaped crystals were obtained upon slow cooling (velocity of the temperature decline 1 K h^-1). The m.p. was measured on an Electrothermal Engineering Ltd Melting Point Apparatus IA9100. Five independent measurements show that the crystals melt in the temperature range 632–637 K.

S3. Refinement

The hydrogen atoms were visible on the difference maps and were refined with isotropic factors correlated with the anisotropic factors of the atoms to which they are bonded.

Computing details

Data collection: CrysAlis (Oxford Diffraction, 2003); cell refinement: CrysAlis; data reduction: CrysAlis; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek 2003); software used to prepare material for publication: publCIF.

Figures

	Fig. 1. The molecular structure of biphenyl-4,4'-diphosphonic acid showing the atom-labelling scheme. The displacement ellipsoids are drawn at the 50% probability level. Symmetry code: (a) = -x,1 - y,1 - z.
	Fig. 2. Presentation of the two-dimensional hydrogen-bonded network and the ring motifs generated in there. The hydrogen bonds are shown as dashed lines.
	Fig. 3. A view of the pillared crystal structure demonstrating the relationships between the differently oriented aromatic stacks. The hydrogen bonds are presented as dashed lines.

biphenyl-4,4'-diphosphonic acid

Crystal data

C12H12O6P2	F(000) = 324
Mr = 314.16	Dx = 1.694 Mg m^−3
Monoclinic, P21/c	Melting point: 361(3) K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 13.0552 (18) Å	Cell parameters from 879 reflections
b = 7.0852 (10) Å	θ = 4.2–26.4°
c = 6.7287 (13) Å	µ = 0.38 mm^−1
β = 98.176 (14)°	T = 293 K
V = 616.07 (17) Å^3	Scaly habit, colourless
Z = 2	0.53 × 0.35 × 0.02 mm

Data collection

Kuma KM-4 with an area CCD detector diffractometer	1246 independent reflections
Radiation source: fine-focus sealed tube	879 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.044
Detector resolution: 1024x1024 with blocks 2x2, 33.133pixel/mm pixels mm^-1	θmax = 26.4°, θmin = 4.2°
ω scans	h = −15→16
Absorption correction: numerical X-SHAPE (Stoe & Cie, 1998)	k = −8→8
Tmin = 0.873, Tmax = 0.988	l = −8→6
6361 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.088	Only H-atom coordinates refined
S = 0.98	w = 1/[σ^2(Fo^2) + (0.0485P)^2] where P = (Fo^2 + 2Fc^2)/3
1246 reflections	(Δ/σ)max < 0.001
109 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.49 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
P1	0.36197 (4)	0.45827 (8)	0.22194 (9)	0.0216 (2)
O1	0.34195 (13)	0.4822 (2)	−0.0067 (2)	0.0334 (5)
HO1	0.371 (2)	0.404 (4)	−0.072 (4)	0.050*
O2	0.41069 (11)	0.2720 (2)	0.2797 (2)	0.0294 (4)
O3	0.42202 (13)	0.6294 (3)	0.3150 (3)	0.0403 (5)
HO3	0.476 (2)	0.666 (4)	0.274 (4)	0.060*
C1	0.05050 (15)	0.4975 (3)	0.4597 (3)	0.0201 (5)
C2	0.14089 (16)	0.5623 (3)	0.5719 (4)	0.0264 (5)
H2	0.1353 (17)	0.608 (3)	0.702 (4)	0.032*
C3	0.23479 (17)	0.5544 (3)	0.4996 (3)	0.0261 (5)
H3	0.2952 (18)	0.594 (3)	0.581 (4)	0.031*
C4	0.24097 (16)	0.4811 (3)	0.3116 (3)	0.0199 (5)
C5	0.15061 (17)	0.4189 (3)	0.1963 (4)	0.0276 (6)
H5	0.1538 (17)	0.370 (3)	0.065 (3)	0.033*
C6	0.05748 (17)	0.4278 (3)	0.2703 (4)	0.0275 (6)
H6	−0.0022 (18)	0.387 (3)	0.191 (3)	0.033*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P1	0.0169 (3)	0.0238 (4)	0.0256 (4)	−0.0006 (2)	0.0083 (2)	−0.0010 (3)
O1	0.0419 (11)	0.0349 (11)	0.0257 (10)	0.0090 (8)	0.0129 (8)	0.0024 (7)
O2	0.0229 (9)	0.0347 (10)	0.0333 (9)	0.0108 (7)	0.0129 (7)	0.0083 (7)
O3	0.0290 (10)	0.0455 (12)	0.0512 (12)	−0.0187 (8)	0.0225 (9)	−0.0201 (9)
C1	0.0158 (11)	0.0201 (12)	0.0249 (13)	0.0005 (8)	0.0050 (10)	−0.0008 (9)
C2	0.0201 (11)	0.0360 (15)	0.0245 (13)	−0.0026 (10)	0.0078 (10)	−0.0089 (11)
C3	0.0176 (11)	0.0328 (14)	0.0279 (14)	−0.0054 (10)	0.0036 (10)	−0.0059 (11)
C4	0.0159 (10)	0.0213 (12)	0.0232 (12)	0.0012 (9)	0.0055 (9)	0.0010 (9)
C5	0.0224 (12)	0.0350 (14)	0.0268 (13)	−0.0017 (10)	0.0078 (11)	−0.0073 (11)
C6	0.0167 (11)	0.0382 (15)	0.0274 (14)	−0.0048 (10)	0.0021 (10)	−0.0092 (11)

Geometric parameters (Å, º)

P1—O2	1.4920 (15)	C2—C3	1.383 (3)
P1—O3	1.5287 (17)	C2—H2	0.95 (2)
P1—O1	1.5329 (17)	C3—C4	1.380 (3)
P1—C4	1.777 (2)	C3—H3	0.94 (2)
O1—HO1	0.83 (3)	C4—C5	1.388 (3)
O3—HO3	0.83 (3)	C5—C6	1.379 (3)
C1—C6	1.382 (3)	C5—H5	0.96 (2)
C1—C2	1.386 (3)	C6—H6	0.93 (2)
C1—C1^i	1.496 (4)
O2—P1—O3	114.83 (10)	C1—C2—H2	116.5 (13)
O2—P1—O1	111.38 (9)	C4—C3—C2	120.6 (2)
O3—P1—O1	109.12 (10)	C4—C3—H3	119.7 (14)
O2—P1—C4	111.03 (9)	C2—C3—H3	119.7 (14)
O3—P1—C4	102.57 (9)	C3—C4—C5	118.41 (19)
O1—P1—C4	107.33 (10)	C3—C4—P1	121.08 (17)
P1—O1—HO1	115.6 (19)	C5—C4—P1	120.47 (17)
P1—O3—HO3	121.5 (19)	C6—C5—C4	120.4 (2)
C6—C1—C2	117.28 (19)	C6—C5—H5	120.4 (13)
C6—C1—C1^i	121.3 (2)	C4—C5—H5	119.1 (13)
C2—C1—C1^i	121.4 (2)	C5—C6—C1	121.8 (2)
C3—C2—C1	121.5 (2)	C5—C6—H6	119.4 (14)
C3—C2—H2	121.9 (14)	C1—C6—H6	118.9 (14)
C6—C1—C2—C3	−1.3 (3)	O3—P1—C4—C5	148.30 (19)
C1^i—C1—C2—C3	178.7 (2)	O1—P1—C4—C5	33.4 (2)
C1—C2—C3—C4	0.1 (4)	C3—C4—C5—C6	−1.1 (4)
C2—C3—C4—C5	1.1 (3)	P1—C4—C5—C6	176.65 (18)
C2—C3—C4—P1	−176.60 (18)	C4—C5—C6—C1	−0.2 (4)
O2—P1—C4—C3	89.1 (2)	C2—C1—C6—C5	1.3 (3)
O3—P1—C4—C3	−34.0 (2)	C1^i—C1—C6—C5	−178.6 (2)
O1—P1—C4—C3	−148.96 (19)	C2—C1—C1^i—C6^i	0.1 (3)
O2—P1—C4—C5	−88.6 (2)	C2—C1—C1^i—C2^i	180.0 (2)

Symmetry code: (i) −x, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—HO1···O2^ii	0.83 (3)	1.72 (3)	2.546 (2)	171 (3)
O3—HO3···O2^iii	0.83 (3)	1.74 (3)	2.567 (2)	170 (3)

Symmetry codes: (ii) x, −y+1/2, z−1/2; (iii) −x+1, y+1/2, −z+1/2.