Bis[3-(trimethyl­silyl)prop­yl] ethyl­enebisphospho­nate, H₂DTMSP[EBP]

The title compound (H₂DTMSP[EBP]), C₁₄H₃₆O₆P₂Si₂, was crystallized by the slow evaporation of a solution in a 20:1 mixture of pentane and acetone. The H₂DTMSP[EBP] mole­cule lies about an inversion center. In the solid state, the mol­ecule exists in an anti configuration, with the mol­ecular backbone C-C bond located on an inversion center. The compound exists in the solid state as hydrogen-bonded infinite sheets in the ab plane, unlike the methyl­ene analogue, which exists as hydrogen-bonded infinite chains, demonstrating an `even-odd' effect of the length of the backbone alkyl chain.

S1. Comment

The title compound, H₂DTMSP[EBP], (I), is the second member of a homologous series of symmetrically substituted alkylenebisphosphonic acids characterized by the P—(CH₂)_n—P molecular backbone (n = 1–6). This series of silyl-substituted partial esters was initially prepared and investigated as potential heavy metal ion extractants using supercritical carbon dioxide, SC—CO₂, as the diluent (Griffith-Dzielawa et al., 2000; McAlister et al., 2001, 2002, 2004; Herlinger et al., 2003). The trimethylsilylpropyl group, TMSP, was shown to be effective in solubilizing bisphosphonic acids in SC—CO₂, with the solubility of the TMSP partial esters showing a hydrocarbon-like even–odd effect that depends upon the number of –CH₂– groups in the alkylene chain. Earlier studies revealed the remarkable effect that the separation between the P atoms in the alkylene chain has on the aggregation, complexation and solvent-extraction properties of symmetrically substituted alkylenebisphosphonic acids (Herlinger et al., 2003; Chiarizia & Herlinger, 2004).

The acid dissociation constants for the first three members (n = 1–3) of this series were determined in a 70:30 w/w methanol–water solvent by potentiometric titration and ³¹P NMR spectroscopy (Zalupski, Jensen et al., 2006; Zalupski, Chiarizia et al., 2006). The acid dissociation constants, K₁ and K₂, for these silyl-substituted bisphosphonic acids were found to follow a distance-dependent order of acid strength, due to the diminished inductive effect of the phosphonic acid groups upon each other as the chain length increases. The first member of the series, P,P'-di[3(trimethylsilyl)-1-propyl]methylenebisphosphonic acid, H₂DTMSP[MBP], was found to be a stronger acid than H₂DTMSP[EBP], but the difference in acidity was not as great as expected (Zalupski, Jensen et al., 2006; Zalupski, Chiarizia et al., 2006). Our current interest in this series of TMSP-containing bisphosphonic acids derives from the effect that the separation between the P atoms should have on their structural chemistry. The compounds could potentially find use in the assembly of extended supramolecular hydrogen-bonded networks, as activators in enzymatic reactions, or as models for enzyme inhibition and mechanism studies.

The title compound, (I), crystallizes with four molecules per unit cell and only one half molecule per asymmetric unit. In the solid state, the molecule exists in an anti configuration (Fig. 1), with the molecular backbone C—C bond located on an inversion centre. The molecule exhibits herringbone packing when viewed along the a axis. Each molecule of H₂DTMSP[EBP] is hydrogen bonded to four other molecules through P═O···H—OP interactions to create infinite sheets in the ab plane. Each of these four hydrogen bonds is symmetry equivalent and the donor–acceptor distance is 2.4986 (16) Å. (Fig. 2)

In contrast, the crystal structure of the first member of this series, H₂DTMSP[MBP], reveals a quite different pattern of hydrogen-bonding (McLauchlan et al., 2004). In the solid state, the molecules of H₂DTMSP[MBP] are stitched together by hydrogen bonds to form an infinite chain along the a axis, with each molecule of H₂DTMSP[MBP] bound by two hydrogen bonds to two neighbouring molecules, with average donor–acceptor distances of 2.506 (3) Å. The P═O, P—OH and P—OR distances and angles are comparable for H₂DTMSP[EBP] and H₂DTMSP[MBP].

In aromatic solvents such as toluene, the silyl-substituted alkylenebisphosphonic acids H₂DTMSP[ABP] (n = 1–6) are strongly aggregated, exhibiting an even–odd effect as the number of –CH₂– bridging groups varies. Results from vapour-phase osmometry suggest that the odd members of the series are strongly hydrogen-bonded dimers whereas the even members of the series are more highly aggregated, existing primarily as strongly hydrogen-bonded hexamers (Fig. 3). This effect is most likely due to the `zigzag' (herringbone-like) pattern adopted by the alkylene chain separating the P atoms. For the odd members of the series, the pattern directs both the P═O and POH groups of the phosphonic acid moieties to the same side of the alkylene chain, whereas for the even members of the series the phosphonic acid groups are on opposite sides of the chain. Thus, these two different orientations control the geometry of the hydrogen-bonded aggregates that can be formed (Chiarizia & Herlinger, 2004). In hydrogen-bonding solvents such as methanol, however, vapour-phase osmometry suggests that both the odd and even members of the series exist primarily as monomers. The even–odd effect appears to be reflected in the solid-state structures of the first two members of the series, though, with H₂DTMSP[MBP] forming infinite chains and H₂DTMSP[EBP] creating infinite sheets. Efforts to see if the same patterns emerge with the heavier members of the series are underway.

Related literature

For related literature, see: Chiarizia & Herlinger (2004); Griffith-Dzielawa, Barrans, McAlister, Dietz & Herlinger (2000); Herlinger et al. (2003); McAlister (2004); McAlister et al. (2001, 2002); McLauchlan et al. (2004); Zalupski, Chiarizia, Jensen & Herlinger (2006); Zalupski, Jensen, Chiarizia, Chiareli & Herlinger (2006).

S2. Experimental

The title compound, which was prepared by a DCC-coupling procedure as previously described, was initially isolated as a colourless viscous oil in 96% yield (Griffith-Dzielawa et al., 2000). X-ray diffraction quality crystals were obtained by very slow evaporation of a 20:1 (v/v) pentane–acetone solution at 253 K. The purity of the compound was established by potentiometric titration, ³¹P NMR spectroscopy and melting point. The equivalent weight was determined by titrating a weighed amount of the compound in a 2:1 (v/v) propan-2-ol–toluene solution with 0.1 M NaOH using an Orion EA 940 pH meter. The ³¹P NMR spectrum was obtained on a VXR 400 MHz s pectrometer using CDCl₃ as the solvent. The melting point was measured using an Arthur H. Thomas, Hoover, capillary melting-point apparatus with a calibrated thermometer [m.p. 364 (1) K (literature value 363–365 K)]. Equivalent weight, calculated: 209 g mol^-1; found: 212 g mol^{-1. 31}P NMR (CDCl₃, versus external 85% H₃PO₄, δ, p.p.m.): 31.44 (s) (literature value: 31.43).

S3. Refinement

H atoms were refined using a riding model with fixed individual displacement parameters [U_iso(H) = 1.2U_iso(C_CH2), 1.5U_iso(C_Me) or 1.5U_iso(O)].

Computing details

Data collection: CrysAlis CCD (Oxford, 2006); cell refinement: CrysAlis RED (Oxford, 2006); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXL97; software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. A plot of one molecule of H2DTMSP[EBP], with the asymmetric unit labelled. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A view of the extended structure of H2DTMSP[EBP], along the c axis. Each molecule is bound by four hydrogen bonds to four adjacent H2DTMSP[EBP] molecules to form an infinite sheet in the ab plane. The bulk of the 3-trimethylsilylpropyl groups have been omitted for clarity. Hydrogen bonds are shown as dashed lines. [Symmetry codes: (i) 2 - x, 1 - y, -z; (ii) 1/2 + x, 3/2 - y, -z; (iii) -1/2 + x, 3/2 - y, -z; (iv) 3/2 - x, -1/2 + y, z; (v) 5/2 - x, -1/2 + y, z.]
	Fig. 3. The proposed hexameric structure of H2DTMSP[EBP] in toluene, R = TMSP = trimethylsilylpropyl. Data suggest a more spherical nature for the hexamer in solution, with the lipophilic TMSP groups extending away from a hydrophilic centre created by the phosphonic acid groups pointing inward, but a flattened projection is shown for clarity.

Bis[3-(trimethylsilyl)propyl] ethylenebisphosphonate

Crystal data

C14H36O6P2Si2	Dx = 1.219 Mg m^−3
Mr = 418.55	Melting point: 91(1) K
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 6010 reflections
a = 7.6865 (3) Å	θ = 3.8–28.5°
b = 11.0294 (8) Å	µ = 0.32 mm^−1
c = 26.8912 (13) Å	T = 100 K
V = 2279.8 (2) Å^3	Prism, colourless
Z = 4	0.39 × 0.24 × 0.21 mm
F(000) = 904

Data collection

Oxford Xcalibur3 CCD area-detector diffractometer	2598 independent reflections
Radiation source: Enhance (Mo) X-ray source	2091 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
Detector resolution: 15.9890 pixels mm^-1	θmax = 27.5°, θmin = 4.0°
ϕ and ω scans	h = −6→9
Absorption correction: numerical Analytical numerical absorption correction using a multifaceted crystal model based on expressions derived by Clark & Reid (1995)	k = −12→14
Tmin = 0.923, Tmax = 0.948	l = −32→34
10013 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)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.087	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0448P)^2 + 0.9918P] where P = (Fo^2 + 2Fc^2)/3
2598 reflections	(Δ/σ)max = 0.001
113 parameters	Δρmax = 0.40 e Å^−3
0 restraints	Δρmin = −0.28 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
C1	0.6663 (2)	0.59694 (16)	0.07682 (6)	0.0228 (4)
H1A	0.6451	0.5477	0.0476	0.027*
H1B	0.5980	0.6706	0.0740	0.027*
C2	0.6139 (2)	0.52865 (16)	0.12270 (6)	0.0230 (4)
H2A	0.6298	0.5800	0.1516	0.028*
H2B	0.6879	0.4580	0.1265	0.028*
C3	0.4235 (2)	0.48836 (15)	0.11960 (6)	0.0207 (3)
H3A	0.4096	0.4374	0.0905	0.025*
H3B	0.3515	0.5597	0.1148	0.025*
C4	0.3418 (2)	0.50535 (16)	0.23040 (6)	0.0275 (4)
H4A	0.2966	0.4625	0.2587	0.041*
H4B	0.2705	0.5750	0.2239	0.041*
H4C	0.4587	0.5310	0.2372	0.041*
C5	0.4840 (3)	0.27110 (16)	0.18826 (7)	0.0286 (4)
H5A	0.6027	0.2977	0.1902	0.043*
H5B	0.4725	0.2125	0.1621	0.043*
H5C	0.4505	0.2349	0.2193	0.043*
C6	0.1140 (2)	0.35234 (18)	0.16084 (7)	0.0301 (4)
H6A	0.1149	0.3023	0.1316	0.045*
H6B	0.0413	0.4218	0.1552	0.045*
H6C	0.0693	0.3067	0.1884	0.045*
C7	0.9617 (2)	0.55842 (14)	−0.01105 (5)	0.0178 (3)
H7A	0.8467	0.5412	−0.0241	0.021*
H7B	1.0341	0.5847	−0.0386	0.021*
O1	0.85763 (14)	0.78640 (10)	0.01125 (4)	0.0218 (3)
O2	1.12876 (14)	0.70338 (11)	0.05460 (4)	0.0207 (3)
H2	1.1988	0.7091	0.0317	0.031*
O3	0.85071 (14)	0.62689 (10)	0.08024 (4)	0.0197 (3)
Si1	0.34062 (6)	0.40350 (4)	0.175058 (16)	0.01931 (13)
P1	0.94606 (5)	0.67886 (4)	0.033276 (15)	0.01598 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0122 (7)	0.0296 (8)	0.0267 (8)	−0.0002 (7)	0.0004 (6)	0.0032 (7)
C2	0.0190 (8)	0.0269 (8)	0.0232 (8)	−0.0020 (7)	−0.0008 (7)	0.0034 (7)
C3	0.0166 (8)	0.0223 (8)	0.0232 (8)	−0.0006 (6)	−0.0007 (6)	0.0022 (7)
C4	0.0214 (9)	0.0302 (9)	0.0310 (9)	−0.0001 (7)	−0.0003 (7)	−0.0047 (7)
C5	0.0339 (10)	0.0248 (9)	0.0270 (9)	0.0032 (8)	−0.0014 (8)	0.0033 (7)
C6	0.0237 (9)	0.0347 (10)	0.0320 (9)	−0.0095 (8)	0.0001 (8)	−0.0020 (8)
C7	0.0163 (8)	0.0207 (8)	0.0166 (7)	−0.0003 (6)	−0.0015 (6)	−0.0008 (6)
O1	0.0170 (6)	0.0204 (5)	0.0279 (6)	0.0013 (5)	−0.0014 (5)	0.0008 (5)
O2	0.0136 (5)	0.0267 (6)	0.0217 (6)	−0.0016 (5)	−0.0013 (5)	−0.0022 (5)
O3	0.0134 (5)	0.0270 (6)	0.0188 (5)	−0.0015 (5)	0.0011 (4)	0.0002 (5)
Si1	0.0166 (2)	0.0202 (2)	0.0212 (2)	−0.00173 (18)	0.00024 (18)	0.00063 (18)
P1	0.0123 (2)	0.0181 (2)	0.0175 (2)	−0.00018 (15)	−0.00055 (15)	−0.00101 (15)

Geometric parameters (Å, º)

C1—O3	1.4587 (18)	C5—H5A	0.9600
C1—C2	1.500 (2)	C5—H5B	0.9600
C1—H1A	0.9700	C5—H5C	0.9600
C1—H1B	0.9700	C6—Si1	1.8708 (18)
C2—C3	1.532 (2)	C6—H6A	0.9600
C2—H2A	0.9700	C6—H6B	0.9600
C2—H2B	0.9700	C6—H6C	0.9600
C3—Si1	1.8722 (16)	C7—C7^i	1.537 (3)
C3—H3A	0.9700	C7—P1	1.7888 (15)
C3—H3B	0.9700	C7—H7A	0.9700
C4—Si1	1.8645 (17)	C7—H7B	0.9700
C4—H4A	0.9600	O1—P1	1.4898 (12)
C4—H4B	0.9600	O2—P1	1.5407 (11)
C4—H4C	0.9600	O2—H2	0.8200
C5—Si1	1.8635 (18)	O3—P1	1.5687 (11)
O3—C1—C2	108.80 (13)	H5A—C5—H5C	109.5
O3—C1—H1A	109.9	H5B—C5—H5C	109.5
C2—C1—H1A	109.9	Si1—C6—H6A	109.5
O3—C1—H1B	109.9	Si1—C6—H6B	109.5
C2—C1—H1B	109.9	H6A—C6—H6B	109.5
H1A—C1—H1B	108.3	Si1—C6—H6C	109.5
C1—C2—C3	110.92 (13)	H6A—C6—H6C	109.5
C1—C2—H2A	109.5	H6B—C6—H6C	109.5
C3—C2—H2A	109.5	C7^i—C7—P1	113.00 (14)
C1—C2—H2B	109.5	C7^i—C7—H7A	109.0
C3—C2—H2B	109.5	P1—C7—H7A	109.0
H2A—C2—H2B	108.0	C7^i—C7—H7B	109.0
C2—C3—Si1	115.26 (11)	P1—C7—H7B	109.0
C2—C3—H3A	108.5	H7A—C7—H7B	107.8
Si1—C3—H3A	108.5	P1—O2—H2	109.5
C2—C3—H3B	108.5	C1—O3—P1	119.08 (10)
Si1—C3—H3B	108.5	C5—Si1—C4	108.50 (8)
H3A—C3—H3B	107.5	C5—Si1—C6	110.68 (9)
Si1—C4—H4A	109.5	C4—Si1—C6	110.44 (8)
Si1—C4—H4B	109.5	C5—Si1—C3	110.02 (8)
H4A—C4—H4B	109.5	C4—Si1—C3	109.45 (8)
Si1—C4—H4C	109.5	C6—Si1—C3	107.74 (8)
H4A—C4—H4C	109.5	O1—P1—O2	115.09 (7)
H4B—C4—H4C	109.5	O1—P1—O3	113.44 (7)
Si1—C5—H5A	109.5	O2—P1—O3	100.98 (6)
Si1—C5—H5B	109.5	O1—P1—C7	110.91 (7)
H5A—C5—H5B	109.5	O2—P1—C7	108.49 (7)
Si1—C5—H5C	109.5	O3—P1—C7	107.25 (7)
O3—C1—C2—C3	−176.65 (13)	C1—O3—P1—O1	53.09 (13)
C1—C2—C3—Si1	−179.29 (12)	C1—O3—P1—O2	176.79 (11)
C2—C1—O3—P1	169.72 (11)	C1—O3—P1—C7	−69.73 (12)
C2—C3—Si1—C5	−54.44 (14)	C7^i—C7—P1—O1	−175.25 (14)
C2—C3—Si1—C4	64.69 (14)	C7^i—C7—P1—O2	57.42 (17)
C2—C3—Si1—C6	−175.19 (12)	C7^i—C7—P1—O3	−50.88 (17)

Symmetry code: (i) −x+2, −y+1, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^ii	0.82	1.68	2.4986 (16)	174

Symmetry code: (ii) x+1/2, −y+3/2, −z.