A lithium ethyl­ene­diphos­phon­ate containing lithium chains and symmetric hydrogen bonds

In the title compound, catena-poly[lithium-[mu]₃-ethyl­ene­diphos­phon­ato], [Li(C₂H₇O₆P₂)]_n, the supra­molecular monoclinic (C2/c) structure consists of one-dimensional lithium chains [Li...Li = 2.7036 (8) Å] that are embedded within ethyl­ene­diphosphon­ate anions linked by strong symmetric hydrogen bonds [O...O = 2.473 (3) Å]. The Li atoms and the H atom in the symmetric hydrogen bond reside on twofold rotation axes and there is an inversion center at the mid-point of the C-C bond of the ethylenediphosphonate ligand.

S1. Comment

Lithium ions trapped within cavities of metal oxides, phosphates, and organophosphonates have been sought owing to their potential to serve as electrolyte or electrode materials in lithium batteries. For example, Padhi and co-workers (should there be a reference involving these authors?) discovered the electrochemical properties of LiFePO₄ in 1997, and it is the first cathode material with potentially low cost that could have an impact in electrochemical energy storage (Whittingham, 2004). In general, such Li-containing structures might provide a fundamental base for lithium insertion chemistry. Besides metal phosphates, metal organophosphonates are a class of materials with diverse structures and properties in the field of catalysis, ion exchange and nonlinear optics. The observed structural variations may come from the possibility of incorporation of different organic groups into inorganic matrices composed of multidentate metal phosphonates (Clearfield, 1996). Despite the successful syntheses of large numbers of metal organophosphonates, lithium organophosphonates remain poorly understood and rare.

In this paper, we report the synthesis and structural characterization of a lithium ethylenediphosphonate material, C₂H₇P₂O₆Li, (I), whose structure consists of one-dimensional lithium chains that are embedded within bidentate ethylenediphosphonate anions linked by strong symmetric hydrogen bonds.

Fig. 1 displays the X-ray crystal structure of the title compound. The coordination geometry around the lithium ions is quasi-tetrahedral, with four µ-O atoms from four phosphonate ligands. Atoms Li1 and Li1A are bridged by two µ²-O atoms (O3 and O3A) resulting in a centrosymmetric four-membered planar Li₂O₂ ring (the dihedral angle between alternating four-membered rings A and B is 84.6°) with an Li—Li distance of 2.7036 (8) Å, which is similar to that of Li₄[(MeGa)₆(µ₃-O)₂(t-BuPO₃)₆](THF)₄ (Roesky et al., 1997). In contrast to (I), Li₄[(MeGa)₆(µ₃-O)₂(t-BuPO₃)₆](THF)₄ contains a finite four-atom lithium chain within two 12-membered macrocyclic rings and does not have short-range interactions between Li atoms of neighboring molecules. In (I), the lithium chains are not only trapped in ethylenediphosphonate anions but also extended through symmetric hydrogen bonds. These symmetric double-minimum hydrogen bonds are found between two c-glide symmetry-related O atoms from two phosphonate groups [O1···O1C = 2.473 (3) Å]. According to Alcock (1990), symmetric hydrogen bonds typically display a shorter (about 2.47 Å) O···O distance than do asymmetric hydrogen bonds (about 2.5–3.0 Å).

Two neighboring lithium ions are bridged by a pair of P═O units, from each of two bidentate phosphonate ligands, with a slightly shorter [1.4916 (16) Å] P—O bond for the O atom coordinated to the lithium ion than for the non-coordinated O atoms [1.5384 (16) and 1.5685 (18) Å]. The P═O bond length is similar to those observed previously (Henderson et al., 2003; Roesky et al., 1997).

The alternation of adjacent linked rings A and B gives rise to infinite Li chains along the c axis. Viewed along the c axis, the crystal packing reveals that individual lithium chains are interconnected by ethylenediphosphonate ligands (Fig. 2). Each ethylenediphosphonate ligand in (I) coordinates to two Li atoms through its two phosphonate O atoms, thus acting as a bridging bidentate ligand, further linking the (Li₂O₂)_n sheets into a novel covalent three-dimensional network.

S2. Experimental

Lithium nitrate (0.0344 g, 0.5 mmol) and ethylenediphosphonic acid (0.3791 g, 2.0 mmol) of were dissolved in ethanol (5 ml), and dimethyl carbonate (4 ml, 47.4 mmol) was added. The solution was stirred for several minutes and transferred to an autoclave for hydrothermal synthesis. The temperature was first increased to 393 K then to 473 K at a rate of 120 K h⁻¹. The solution was kept at 473 K for 4 d, and then cooled to 343 K at a rate of 9 K h⁻¹ and naturally cooled to room temperature. Transparent crystals suitable for single X-ray diffraction studies were filtered and washed with ethanol. The crystals were stable for several days to weeks; however, deliquescence was observed after several months.

S3. Refinement

All H-atom parameters were initially refined freely. In the final cycles, the H atoms of CH₂ and OH groups were placed in calculated positions, with C—H = 0.97 Å (CH₂) and O—H = 0.82 Å and refined as riding, with U_iso(H) values set at 1.2 (CH₂) or 1.5 (OH) times U_eq of the parent C or O atoms. The H atom on phosphonate atom O1 was initially refined freely with occupancy factors of 0.6–0.7 and finally fixed with position and occupancy factor of 0.5 to present the state of double-minimum potential. The displacement parameter was set as 1.5 times U_eq(O1).

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SMART; data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: XS [SHELXS97?] (Sheldrick, 1990); program(s) used to refine structure: SHELXTL (Sheldrick, 1997); molecular graphics: XP (Bruker, 1997); software used to prepare material for publication: SHELXTL.

Figures

Fig. 1. A view of C2H7P2O6Li along b axis, showing the atom labeling and displacement ellipsoids at the 20% probability level. Atoms Li1, O3A, Li1A and O3 form Li2O2 ring A, and atoms Li1, O3B, Li1B and O3C form ring B. Two adjacent phosphonate groups are connected by a symmetric hydrogen bond (P1—O1—H4—O1C—P1C). Selected distances (Å): O1—O1C = 2.473 (3); Li1—O3 = 1.961 (3); Li1—O3A = 1.905 (2). Selected angles (°): O3A—Li1—O3 = 91.26 (5); O3B—Li1—O3 = 109.7 (2); O3C—Li1—O3 = 121.82 (7); O3B—Li1—O3A = 121.82 (8); O3C—Li1—O3A = 123.0 (2); O3B—Li1—O3C = 91.26 (5). [Symmetry codes: (A) 1 − x, 1 − y, 1 − z; (B) 1 − x, y, 1/2 − z; (C) x, 1 − y, −1/2 + z; Li1B is at (1 − x, 1 − y, −z); Li1C is at (x, y, −1 + z).]

Fig. 2. The packing of C2H7P2O6Li, viewed along the c direction, showing how each lithium chain is linked to other chains by ethylenediphosphonic acid ligands.

catena-poly[lithium-µ₃-ethylenediphosphonato]

Crystal data

[Li(C2H7O6P2)]	F(000) = 400
Mr = 195.96	Dx = 1.848 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2124 reflections
a = 16.1716 (19) Å	θ = 2.7–27.5°
b = 8.488 (1) Å	µ = 0.59 mm^−1
c = 5.3893 (6) Å	T = 293 K
β = 107.836 (2)°	Plate, colorless
V = 704.20 (14) Å^3	0.42 × 0.24 × 0.20 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	799 independent reflections
Radiation source: fine-focus sealed tube	749 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
ϕ and ω scans	θmax = 27.5°, θmin = 2.7°
Absorption correction: empirical (using intensity measurements) (SAINT; Siemens, 1995 or Bruker, 2000)	h = −19→20
Tmin = 0.824, Tmax = 0.892	k = −11→8
2124 measured reflections	l = −4→7

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.094	H-atom parameters constrained
S = 0.98	w = 1/[σ^2(Fo^2) + (0.0647P)^2 + 1.1079P] where P = (Fo^2 + 2Fc^2)/3
799 reflections	(Δ/σ)max < 0.001
64 parameters	Δρmax = 0.47 e Å^−3
0 restraints	Δρmin = −0.52 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	0.61996 (3)	0.24357 (6)	0.56027 (10)	0.0169 (3)
C1	0.71580 (13)	0.3125 (3)	0.4942 (5)	0.0236 (5)
H1A	0.6988	0.3597	0.3221	0.028*
H1B	0.7419	0.3946	0.6188	0.028*
O3	0.56061 (10)	0.37989 (19)	0.5478 (3)	0.0225 (4)
O1	0.57867 (10)	0.11079 (19)	0.3679 (3)	0.0242 (4)
O2	0.65217 (11)	0.1673 (2)	0.8384 (3)	0.0292 (4)
H2	0.6788	0.2331	0.9443	0.044*
Li1	0.5000	0.5131 (7)	0.2500	0.0221 (11)
H4	0.5000	0.1106	0.2500	0.036*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P1	0.0145 (4)	0.0161 (4)	0.0205 (4)	0.00126 (16)	0.0058 (3)	−0.00044 (18)
C1	0.0191 (11)	0.0180 (11)	0.0371 (13)	0.0007 (8)	0.0137 (9)	0.0010 (10)
O3	0.0230 (8)	0.0213 (8)	0.0255 (8)	0.0073 (6)	0.0107 (6)	0.0012 (6)
O1	0.0190 (8)	0.0212 (8)	0.0290 (9)	0.0020 (6)	0.0021 (6)	−0.0065 (7)
O2	0.0293 (9)	0.0305 (10)	0.0227 (9)	−0.0033 (7)	0.0003 (6)	0.0052 (7)
Li1	0.021 (2)	0.025 (3)	0.022 (2)	0.000	0.010 (2)	0.000

Geometric parameters (Å, º)

P1—O3	1.4916 (16)	O3—Li1	1.962 (4)
P1—O1	1.5384 (16)	O1—H4	1.2358
P1—O2	1.5685 (18)	O2—H2	0.8200
P1—C1	1.791 (2)	Li1—O3^ii	1.905 (3)
C1—C1^i	1.520 (4)	Li1—O3^iii	1.905 (3)
C1—H1A	0.9700	Li1—O3^iv	1.962 (4)
C1—H1B	0.9700	Li1—Li1^v	2.7038 (10)
O3—Li1^ii	1.905 (3)	Li1—Li1^ii	2.7038 (10)
O3—P1—O1	113.42 (9)	P1—O2—H2	109.5
O3—P1—O2	112.60 (10)	O3^ii—Li1—O3^iii	123.0 (3)
O1—P1—O2	106.86 (10)	O3^ii—Li1—O3^iv	121.81 (9)
O3—P1—C1	108.77 (10)	O3^iii—Li1—O3^iv	91.29 (7)
O1—P1—C1	109.19 (10)	O3^ii—Li1—O3	91.29 (7)
O2—P1—C1	105.69 (11)	O3^iii—Li1—O3	121.81 (9)
C1^i—C1—P1	115.3 (2)	O3^iv—Li1—O3	109.6 (3)
C1^i—C1—H1A	108.4	O3^ii—Li1—Li1^v	140.0 (2)
P1—C1—H1A	108.4	O3^iii—Li1—Li1^v	46.52 (9)
C1^i—C1—H1B	108.4	O3^iv—Li1—Li1^v	44.77 (11)
P1—C1—H1B	108.4	O3—Li1—Li1^v	128.0 (3)
H1A—C1—H1B	107.5	O3^ii—Li1—Li1^ii	46.52 (9)
P1—O3—Li1^ii	141.34 (15)	O3^iii—Li1—Li1^ii	140.0 (2)
P1—O3—Li1	129.58 (13)	O3^iv—Li1—Li1^ii	128.0 (3)
Li1^ii—O3—Li1	88.71 (7)	O3—Li1—Li1^ii	44.77 (11)
P1—O1—H4	120.88	Li1^v—Li1—Li1^ii	170.6 (5)
O3—P1—C1—C1^i	178.7 (2)	P1—O3—Li1—O3^ii	174.15 (18)
O1—P1—C1—C1^i	−57.1 (3)	Li1^ii—O3—Li1—O3^ii	0.0
O2—P1—C1—C1^i	57.5 (3)	P1—O3—Li1—O3^iii	−54.9 (3)
O1—P1—O3—Li1^ii	113.8 (2)	Li1^ii—O3—Li1—O3^iii	131.0 (4)
O2—P1—O3—Li1^ii	−7.7 (3)	P1—O3—Li1—O3^iv	49.56 (14)
C1—P1—O3—Li1^ii	−124.5 (2)	Li1^ii—O3—Li1—O3^iv	−124.59 (14)
O1—P1—O3—Li1	−56.8 (2)	P1—O3—Li1—Li1^v	2.2 (4)
O2—P1—O3—Li1	−178.32 (18)	Li1^ii—O3—Li1—Li1^v	−171.9 (4)
C1—P1—O3—Li1	64.9 (2)	P1—O3—Li1—Li1^ii	174.15 (18)

Symmetry codes: (i) −x+3/2, −y+1/2, −z+1; (ii) −x+1, −y+1, −z+1; (iii) x, −y+1, z−1/2; (iv) −x+1, y, −z+1/2; (v) −x+1, −y+1, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H4···O1^iv	1.24	1.24	2.472 (3)	180 (1)

Symmetry code: (iv) −x+1, y, −z+1/2.