A structural hierarchy in the hydrogen-bonded adduct ethane-1,2-di­phospho­nic acid-4,4'-bipyridyl-water (1/1/2): an N-component N-­dimensional structure (N = 3) with substructures having N = 1 and 2

The title compound, 4,4'-bipyridinium ethane-1,2-diyl-1,2-di­phospho­nate dihydrate, is a hydrated salt, C₁₀H₁₀N₂²⁺·­C₂H₆O₆P₂^2-·2H₂O, in which the components are linked by extensive hydrogen bonding. The cations and anions lie on inversion centres and with the water mol­ecules each form separate one-component one-dimensional chains along [100]: the anions and the water mol­ecules form a two-component two-dimensional substructure, (001) sheets, while the cations and anions form a second two-component two-dimensional substructure, (011) sheets. All three components combine to form a three-dimensional framework.

S1. Comment

Phenylphosphonic acid, PhPO(OH)₂, forms hydrogen-bonded adducts with a range of diamines, in which the supramolecular architecture can be one-, two-, or three-dimensional (Ferguson et al., 1998). A common feature of these adducts is the transfer of protons from the acid to the amine, so forming the anion [PhPO₂(OH)]⁻: the combination of ion formation, on the one hand, and on the other the ability of this anion to act as both hydrogen-bond donor and hydrogen-bond acceptor, leads to the formation of a rich diversity of strong, ionic hydrogen bonds.

Developing this theme, we have now investigated the bis-phosphonic acid ethane-1,2-diphosphonic acid, (HO)₂P(O)CH₂CH₂P(O)(OH)₂, and we report here the structure of the hydrated 1:1 adduct formed with 4,4'-bipyridyl, (I). The structure of (I) consists of a three-dimensional hydrogen-bonded framework, which can be most conveniently described by means of the substructure approach (Gregson et al., 2000). It is possible to identify in the three-dimensional structure of (I) three distinct one-component, one-dimensional substructures and two distinct two-component, two-dimensional substructures. \sch

The constitution of the adduct (I) is that of a salt HNC₅H₄C₅H₄NH²⁺·(HO)PO₂CH₂CH₂PO₂(OH)²⁻·2H₂O: the cation and anion both lie across centres of inversion, so that the asymmetric unit consists of one half of each ion, together with one water molecule. In the water molecule one of the H atoms is disordered over two sites having equal occupancy, but all the other H atoms in (I) are fully ordered (Fig. 1). Each of the individual molecular components forms chains running parallel to the [100] direction. The anions form molecular ladders in which C(4) hydrogen-bonded chains form the uprights and the P—C—C—P backbones of the anions form the treads: O1 at (x, y, z) acts as hydrogen-bond donor to O3 at (1 + x, y, z), while the symmetry-related O1 in the same anion, which is at (2 − x, −y, 1 − z) acts as donor to O3 at (1 − x, −y, 1 − z). In this manner a ladder is formed in which the anions centred at (n, 0, 1/2) (n = zero or integer) enclose R²₂(14) rings centred at (n + 1/2, 0, 1/2) (n = zero or integer) (Fig. 2).

A second [100] chain is formed by the water molecules (Fig. 2): the half-occupancy H atoms H42 and H43 at (x, y, z) form hydrogen bonds with the water O4 atoms at (1 − x, −1 − y, 1 − z) and (2 − x, −1 − y, 1 − z) respectively, so forming a series R²₂(4) rings centred at (n + 1/2, −0.5, 1/2) and (n, −0.5, 1/2) respectively (n = zero or integer). Similar continuous chains of partially disordered water molecules have been observed also in meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane– terephthalic acid–water (1/1/4) (Lough et al., 2000).

It may be assumed that at the local level only, one of the H sites between each pair of O atoms is occupied at any instant: simultaneous occupation of both would not only disrupt the hydrogen bonding, but would require the two H atoms concerned to be within covalent bonding distance of one another. Instantaneous occupation of either H site between a given pair of O atoms thus precludes occupation of the other. If it is further assumed that the O atom in each water molecule forms, at any time, two and only two covalent bonds, then occupation of any one of the disordered sites within a particular chain necessarily defines the occupancy (unity or zero) of all other such sites within that chain. Hence, on this basis, the 0.5 occupancy modelled from the X-ray diffraction data can be regarded as resulting from spacial averaging of the H site occupancy. It is also possible that there is temporal averaging: if the potential energy function describing the motion of a single H atom along the line between a pair of water O atoms exhibits two well defined equivalent minima, separated by a barrier insufficiently high to prevent tunnelling, then temporal averaging of the H sites will again reproduce the observed 0.5 occupancy. Both spacial and temporal averaging mechanisms may be operating concurrently: in any event, at the local level, the centres of inversion embedded within these chains must be a reflection of the overall averaging process.

These two components, the anions and the water molecules, thus independently generate strings parallel to the [100] direction and alternating along the [010] direction: the anion ladders run along the lines (x, n, 1/2) (n = zero or integer) while the water chains run along (x, n + 1/2, 1/2).

The third one-dimensional substructure is formed by the cations: aromatic π···π stacking interactions between adjacent cations generate a third type of [100] chain. The pyridine ring at (x, y, z), part of the cation centred at (0, 1/2, 0), interacts with that at (1 − x, 1 − y, −z) which is part of the cation centred at (1, 1/2, 0): the perpendicular spacing between neighbouring rings is 3.467 (4) Å and the centroid offset is 1.055 (4) Å. Propagation of these interactions by the action of the inversion centres means that each cation forms π···π stacking interactions with its two neighbours along [100].

Two distinct two-component, two-dimensional substructures can be identified, resulting from two distinct pairwise combinations of the one-dimensional chains. The anions chains, uniquely, are directly linked to both of the other types of chain. The chains built from anions and from water molecules are linked by further O—H···O hydrogen bonds to form cation-free sheets parallel to (001) built from R⁴₅(12) and R²₂(14) rings (Fig. 2). Atom O4 at (x, y, z) acts as hydrogen-bond donor, via the fully ordered H41 to O3 also at (x, y, z), and propagation of this interaction by means of the centres of inversion links together the water and anion chains into a cation-free two-dimensional substructure. This sheet lies in the domain 0.20 < z < 0.80, while the cation stacks lie in the domain −0.28 < z < 0.28 when H atoms are included, or −0.20 < z < 0.20 when H atoms are excluded. A second two-component sheet is built from the cations and anions: the cation centred at (0, 1/2, 0) is directly linked to the two anions centred at (1, 0, 1/2) and (−1, 1, −0.5) which form parts of two distinct anion ladders. In this manner, purely-ionic sheets are formed parallel to (011) (Fig. 3).

The formation of the single three-dimensional framework can be regarded either as the linking of the cation-free (001) sheets by the cations, or as the linking of the purely ionic (011) sheets by the water molecules (Fig. 3). Thus N11 at (x, y, z) is donor to O2 also at (x, y, z), a component of the sheet centred at z = 0.5: the symmetry related N11 in the same cation, which is centred at (0, 1/2, 0), acts as donor to O2 at (-x, 1 − y, −z), a component of the sheet centred at z = −0.5. Again propagation of this interaction by the centres of inversion links each (001) sheet to its two immediate neighbours, thus generating a single three-dimensional framework. The connections between neighbouring (001) sheets are further reinforced by C—H···O hydrogen bonds in which the donors are C12 and C16, the two C atoms adjacent to the protonated N: C12—H12 and C16—H16 are thus expected to be the most acidic C—H bonds in the structure. C12 at (x, y, z) is donor to O4 at (1 − x, −y, 1 − z), a component of the z = 0.5 sheet, while C16 at (x, y, z) is donor to O2 at (2 − x, −y, −z), a component of the z = −0.5 sheet.

The N—H···O hydrogen bond involving cationic N and anionic O is particularly short for its type: of the O—H···O hydrogen bonds, the shortest by far is that between anions. These two hydrogen bonds are thus expected to be particularly strong, although none of the hydrogen bonds in this structure can be classified as weak. Not only are the D···A distances all short or very short for their types, but the D—H···A angles all exceed 160° (Table 2).

There are no examples of the [(HO)PO₂CH₂CH₂PO₂(OH)]²⁻ anion recorded in the Cambridge Structural Database (Allen & Kennard, 1993): however, the structure of the parent acid (HO)₂P(O)CH₂CH₂P(O)(OH)₂ has been reported (Peterson et al., 1977). The presence of the centre of inversion in the anion means that the P—C—C—P backbone necessarily has a trans planar conformation, but there are no symmetry constraints on the orientation of the PO₂(OH) fragment. This group in fact exhibits a slight rotation from perfect staggering with the adjacent CH₂ group, with one of the anionic O atoms antiperiplanar to the C—C bond (Table 1). In the parent acid, also centrosymmetric, one of the hydroxyl groups is antiperiplanar to C—C. The P—O bond lengths in the anion show a clear distinction between the P—O(H) bond and those in the PO₂⁻ group (Table 1). Within the PO₂⁻ fragment, the O—P—O angle is significantly larger than tetrahedral: this may be compared with similar wide angles in the O—S—O unit is sulfones and in the CO₂⁻ unit in carboxylate anions. In all these systems there is a significant net negative charge on the paired O atoms. The other bond lengths and angles within the anion have values similar to those in the neutral parent acid (Peterson et al., 1977). In the cation the C—N—C angle is greater than 120°, as typically found in protonated pyridines, whereas in unprotonated pyridines this angle is generally significantly less than 120°: the other internal dimensions of the cation are unexceptional.

S2. Experimental

Equimolar quantities of ethane-1,2-diphosphonic acid and of 4,4'-bipyridyl were separately dissolved in methanol: the solutions were mixed and the mixture was set aside to crystallize, exposed to air, producing analytically pure crystals of the adduct (I). Analysis: found C 37.8, H 5.6, N 7.1%: C₁₂H₂₀N₂O₈P₂ requires C 37.7, H 5.5, N 7.3%. Crystals suitable for single-crystal X-ray diffraction were selected directly from the analytical sample.

S3. Refinement

Compound (I) crystallized in the triclinic system; space group P-1 was assumed and confirmed by the analysis. H atoms were treated as riding atoms with C—H 0.95 and 0.99, N—H 0.88, hydroxy O—H 0.82 Å. It was apparent from difference maps that one of water H atoms was disordered over two sites with equal occupancy and the three water H atoms (H41, H42, H43) were included in the refinement with a DFIX free-variable restraint [which refined to 0.86 (2) Å] for the three O—H distances. Examination of the structure with PLATON (Spek, 1999) showed that there were no solvent accessible voids in the crystal lattice.

Computing details

Data collection: Kappa-CCD server software (Nonius, 1997); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: NRCVAX96 (Gabe et al., 1989) and SHELXL97 (Sheldrick, 1997); molecular graphics: NRCVAX96, ORTEP (Johnson, 1976) and PLATON (Spek, 1999); software used to prepare material for publication: NRCVAX96, SHELXL97 and WORDPERFECT macro PRPKAPPA (Ferguson, 1999).

Figures

	Fig. 1. The molecular components of (I) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. Atoms H42 and H43 each have site occupation factor 0.5.
	Fig. 2. Part of the crystal structure of (I) showing the cation-free (001) net built from anion ladders and water chains.
	Fig. 3. Part of the crystal structure of (I) in projection along the a direction showing the formation of the three-dimensional framework from the two-component (001) and (011) sheets.

Ethane-1,2-diphosphonic acid–4,4'-bipyridyl–water (1/1/2)

Crystal data

C10H10N2^2+·C2H6O6P2^2^−·2H2O	Z = 1
Mr = 382.24	F(000) = 200
Triclinic, P1	Dx = 1.538 Mg m^−3
a = 4.8297 (2) Å	Mo Kα radiation, λ = 0.71073 Å
b = 7.9967 (5) Å	Cell parameters from 7018 reflections
c = 11.0644 (6) Å	θ = 3.2–30.0°
α = 89.022 (3)°	µ = 0.31 mm^−1
β = 80.294 (3)°	T = 100 K
γ = 78.500 (3)°	Needle, pale yellow
V = 412.69 (4) Å^3	0.28 × 0.12 × 0.08 mm

Data collection

Kappa-CCD diffractometer	2364 independent reflections
Radiation source: fine-focus sealed X-ray tube	1893 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
ϕ scans and ω scans with κ offsets	θmax = 30.0°, θmin = 3.2°
Absorption correction: multi-scan DENZO-SMN (Otwinowski & Minor, 1997)	h = 0→6
Tmin = 0.919, Tmax = 0.976	k = −10→11
7018 measured reflections	l = −15→15

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)] = 0.046	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.125	H-atom parameters constrained
S = 1.10	w = 1/[σ^2(Fo^2) + (0.0343P)^2 + 0.3686P] where P = (Fo^2 + 2Fc^2)/3
2364 reflections	(Δ/σ)max < 0.001
121 parameters	Δρmax = 0.36 e Å^−3
3 restraints	Δρmin = −0.56 e Å^−3

Special details

Experimental. The program DENZO-SMN (Otwinowski & Minor, 1997) uses a scaling algorithm [Fox, G·C. & Holmes, K·C. (1966), Acta Cryst. 20, 886–891] which effectively corrects for absorption effects. High redundancy data were used in the scaling program hence the 'multi-scan' code word was used. No transmission coefficients are available from the program (only scale factors for each frame). The scale factors in the experimental table are calculated from the 'size' command in the SHELXL97 input file.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
N11	0.5453 (4)	0.2342 (2)	0.12677 (16)	0.0163 (4)
C12	0.3553 (4)	0.3518 (3)	0.1977 (2)	0.0187 (4)
C13	0.1374 (4)	0.4583 (3)	0.15081 (19)	0.0173 (4)
C14	0.1131 (4)	0.4423 (3)	0.02790 (18)	0.0139 (4)
C15	0.3118 (4)	0.3149 (3)	−0.0426 (2)	0.0189 (4)
C16	0.5265 (4)	0.2139 (3)	0.00914 (19)	0.0190 (4)
P1	0.96759 (10)	−0.06718 (7)	0.31005 (5)	0.01250 (16)
O1	1.2443 (3)	−0.21237 (18)	0.29072 (14)	0.0158 (3)
O2	0.9716 (3)	0.04811 (19)	0.19920 (13)	0.0172 (3)
O3	0.7152 (3)	−0.1549 (2)	0.33540 (14)	0.0183 (3)
C1	0.9893 (4)	0.0541 (3)	0.44229 (18)	0.0161 (4)
O4	0.7177 (3)	−0.4145 (2)	0.50614 (16)	0.0245 (4)
H11	0.6847	0.1692	0.1579	0.020*
H12	0.3700	0.3626	0.2816	0.022*
H13	0.0046	0.5421	0.2022	0.021*
H15	0.2990	0.2977	−0.1261	0.023*
H16	0.6629	0.1288	−0.0397	0.023*
H1	1.3871	−0.1711	0.2988	0.024*
H1A	1.1593	0.1077	0.4239	0.019*
H1B	0.8174	0.1467	0.4590	0.019*
H41	0.732 (7)	−0.339 (4)	0.450 (3)	0.044 (7)*
H42	0.573 (10)	−0.461 (8)	0.502 (7)	0.044 (7)*	0.50
H43	0.891 (7)	−0.473 (8)	0.499 (6)	0.044 (7)*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N11	0.0162 (8)	0.0165 (9)	0.0170 (8)	−0.0040 (6)	−0.0044 (6)	0.0039 (7)
C12	0.0216 (10)	0.0188 (11)	0.0174 (10)	−0.0049 (8)	−0.0072 (8)	0.0000 (8)
C13	0.0191 (9)	0.0165 (10)	0.0162 (10)	−0.0021 (8)	−0.0039 (8)	−0.0037 (8)
C14	0.0133 (8)	0.0156 (10)	0.0136 (9)	−0.0052 (7)	−0.0022 (7)	−0.0003 (8)
C15	0.0177 (9)	0.0230 (11)	0.0153 (10)	−0.0012 (8)	−0.0037 (8)	−0.0031 (8)
C16	0.0152 (9)	0.0218 (11)	0.0177 (10)	0.0005 (8)	−0.0011 (8)	−0.0022 (9)
P1	0.0101 (2)	0.0149 (3)	0.0125 (3)	−0.00176 (18)	−0.00282 (17)	−0.00054 (19)
O1	0.0110 (6)	0.0169 (8)	0.0193 (7)	−0.0007 (5)	−0.0041 (5)	−0.0045 (6)
O2	0.0169 (7)	0.0201 (8)	0.0140 (7)	−0.0013 (6)	−0.0033 (5)	0.0003 (6)
O3	0.0118 (6)	0.0241 (8)	0.0211 (8)	−0.0068 (6)	−0.0053 (6)	0.0013 (6)
C1	0.0170 (9)	0.0148 (10)	0.0155 (10)	−0.0001 (7)	−0.0034 (7)	−0.0033 (8)
O4	0.0243 (8)	0.0267 (9)	0.0260 (9)	−0.0106 (7)	−0.0086 (7)	0.0064 (7)

Geometric parameters (Å, º)

N11—C12	1.334 (3)	P1—O1	1.573 (2)
N11—H11	0.8800	P1—O2	1.522 (2)
C12—C13	1.383 (3)	P1—O3	1.509 (2)
C12—H12	0.9500	P1—C1	1.798 (2)
C13—C14	1.395 (3)	O1—H1	0.8400
C13—H13	0.9500	C1—C1^ii	1.537 (4)
C14—C15	1.397 (3)	C1—H1A	0.9900
C15—C16	1.381 (3)	C1—H1B	0.9900
C16—N11	1.335 (3)	O4—H41	0.86 (2)
C14—C14^i	1.494 (4)	O4—H42	0.86 (2)
C15—H15	0.9500	O4—H43	0.86 (2)
C16—H16	0.9500
C12—N11—C16	120.8 (2)	C15—C16—H16	119.5
C12—N11—H11	119.6	O1—P1—O2	109.46 (8)
C16—N11—H11	119.6	O2—P1—O3	114.98 (9)
N11—C12—C13	121.0 (2)	O3—P1—O1	106.53 (8)
N11—C12—H12	119.5	C1^ii—C1—P1	113.2 (2)
C13—C12—H12	119.5	O1—P1—C1	105.83 (9)
C12—C13—C14	119.85 (19)	O2—P1—C1	108.97 (9)
C12—C13—H13	120.1	O3—P1—C1	110.65 (9)
C14—C13—H13	120.1	P1—O1—H1	109.5
C13—C14—C15	117.46 (19)	C1^ii—C1—H1A	108.9
C13—C14—C14^i	121.8 (2)	P1—C1—H1A	108.9
C15—C14—C14^i	120.7 (2)	C1^ii—C1—H1B	108.9
C16—C15—C14	119.9 (2)	P1—C1—H1B	108.9
C16—C15—H15	120.0	H1A—C1—H1B	107.7
C14—C15—H15	120.0	H41—O4—H42	110 (5)
N11—C16—C15	120.94 (19)	H41—O4—H43	103 (5)
N11—C16—H16	119.5	H42—O4—H43	123 (7)
C16—N11—C12—C13	−1.3 (3)	C12—N11—C16—C15	0.5 (3)
N11—C12—C13—C14	0.5 (3)	C14—C15—C16—N11	1.1 (3)
C12—C13—C14—C15	1.1 (3)	O1—P1—C1—C1^ii	−57.8 (2)
C12—C13—C14—C14^i	−178.1 (2)	O2—P1—C1—C1^ii	−175.5 (2)
C13—C14—C15—C16	−1.8 (3)	O3—P1—C1—C1^ii	57.2 (2)
C14^i—C14—C15—C16	177.4 (2)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N11—H11···O2	0.88	1.65	2.528 (2)	173
O1—H1···O3^iii	0.84	1.73	2.536 (2)	160
O4—H41···O3	0.86	1.92	2.783 (2)	173
O4—H42···O4^iv	0.86	1.89	2.749 (3)	174
O4—H43···O4^v	0.86	1.92	2.778 (3)	174
C12—H12···O4^vi	0.95	2.34	3.273 (3)	166
C16—H16···O2^vii	0.95	2.50	3.440 (3)	168

Symmetry codes: (iii) x+1, y, z; (iv) −x+1, −y−1, −z+1; (v) −x+2, −y−1, −z+1; (vi) −x+1, −y, −z+1; (vii) −x+2, −y, −z.