A pentaerythritol-derived spirodi­phospho­nate

The title spirodi­phospho­nate, 3,9-di­methyl-2,4,8,10-tetraoxa-3λ⁵,9λ⁵-diphospha­spiro­[5.5]­un­decane-3,9-dione, C₇H₁₄O₆P₂, a polymer additive, has crystallographic symmetry 2. At 100 K, its six-membered rings have chair conformations, with endocyclic torsion-angle magnitudes in the range 51.87 (8)–58.93 (9)°. The P=O distance is 1.4749 (8) Å, while the P—C(methyl) distance is 1.7691 (12) Å.

S1. Comment

Many organophosphorus compounds are commercially available for use as polymer additives. The unique diphosphonate, 3,9-dimethyl-2,4,8,10-tetraoxa-3λ⁵,9λ⁵ -diphosphaspiro[5,5]undecane-3,9-dione, (I) (CAS #3001–98-7), has been claimed to be effective in modifying the stability of resins such as polyolefins, polycarbonates and polycarbonate blends (Granzow, 1981; Hardy et al., 1979; Horn, 1979). We determined the structure of (I) as part of an effort towards the design of even more effective organophosphorus additives. \sch

The synthesis of (I) is reported to proceed by the Arbuzov rearrangement of pentaerythritol dimethyldiphosphite, (II) (Mukmenev & Kamai, 1963; Friedman, 1964), the transesterification of pentaerythritol, (III), with diphenyl methylphosphonate (Honig & Weil, 1977) and the reaction of methylphosphonic dichloride, (V), with pentaerythritol in dimethyl methylphosphonate, (IV) (Kiefer, 1983). The latter procedure gave high-purity (I) in good yield, and was our method of choice. The Arbuzov rearrangement of (II) was found to be uncontrollable; heating the reactants above approximately 448 K produced a sudden and violent exotherm resulting in a massive, foaming char. The transesterification approach proceeded, with difficulty, to yield (I). An attempt to substitute a more convenient reagent, dimethyl methylphosphonate, for the diphenyl methylphosphonate was not successful.

The molecule lies on a crystallographic twofold axis, as seen in Fig. 1. The two C—C—C angles which lie across the twofold axis are unequal, with C3—C1—C3ⁱ being 4.91 (14)° larger than C2—C1—C2ⁱ (i = 1 − x, y, 1/2 − z), consistent with the steric difference in this conformation between opposite ends of the molecule in the twofold direction. A similar asymmetry exists in two phosphites having similar spiro ring systems, conformations, and approximate twofold symmetry (Heinemann et al., 1994; Barren et al., 1995). The six-membered rings of (I) have the chair conformation, with endocyclic torsion angles having magnitudes within the range 51.87 (8)–58.93 (9)°. While boat (Day et al., 1984) and half-chair (Drew & Rogers, 1972) conformations have been observed in 2-dioxaphosphorinane rings, the chair seen here is more common (Ul-Haque et al., 1970; Patois et al., 1990; Killean et al., 1971; Browning et al., 1996; Edmundson et al., 1989). The P═O bond has length 1.4749 (8) Å, and is in an axial position of the chair. The P═O bond is more typically in the equatorial position (Ul-Haque et al., 1970; Patois et al., 1990; Killean et al., 1971; Browning et al., 1996; Edmundson et al., 1989). The difference may thus be attributed to the spiro ring system in (I), not present in the other 2-dioxaphosphorinanes. The P—C distance in (I), 1.7691 (12) Å, is similar to those in 5-tert-butyl-2-methyl-2-oxo-1,3,2-dioxaphosphorinane [1.81 (3) Å] (Ul-Haque et al., 1970) and 2,5,5-trimethyl-1,3,2-dioxaphosphorinan-2-one [1.776 (3) and 1.783 (3) Å] (Patois et al., 1990).

S2. Experimental

The compound was prepared by reaction of methylphosphonic dichloride with pentaerythritol in dimethyl methylphosphonate by the method of Kiefer (1983). Crystals were grown from methanol solution.

S3. Refinement

C—H distances fell within the range 0.921 (13)–1.010 (13) Å, and U_iso values for H atoms were within the range 0.019 (4)–0.047 (5) Ų.

Computing details

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SIR (Burla et al., 1989); program(s) used to refine structure: LSFM in MolEN (Fair, 1990); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: CIFGEN in MolEN (Fair, 1990).

Figures

Fig. 1. Ellipsoids at the 50% probability level (i = 1 − x, y, 1/2 − z).

2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]undecane, 3,9,-dimethyl-3,9-dioxide

Crystal data

C7H14O6P2	F(000) = 536
Mr = 256.1	Dx = 1.556 Mg m^−3
Monoclinic, C2/c	Melting point: 519-522 K K
Hall symbol: -C 2yc	Mo Kα radiation, λ = 0.71073 Å
a = 16.4560 (14) Å	Cell parameters from 25 reflections
b = 5.5761 (9) Å	θ = 11.4–18.2°
c = 11.9512 (12) Å	µ = 0.40 mm^−1
β = 94.517 (7)°	T = 100 K
V = 1093.2 (4) Å^3	Fragment, colorless
Z = 4	0.57 × 0.55 × 0.42 mm

Data collection

Enraf Nonius CAD4 (with Oxford Cryostream) diffractometer	1915 reflections with I > 3σ(I)
Radiation source: sealed tube	Rint = 0.025
Graphite monochromator	θmax = 35.0°, θmin = 2.5°
θ/2θ scans	h = −26→22
Absorption correction: ψ scan (North et al., 1968)	k = −8→8
Tmin = 0.828, Tmax = 0.849	l = −19→16
6104 measured reflections	3 standard reflections every 60 min
2232 independent reflections	intensity decay: 0.5%

Refinement

Refinement on F^2	0 constraints
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.031	Weighting scheme based on measured s.u.'s 4Fo^2/[σ^2(Fo^2) + 0.0004Fo^4]
wR(F^2) = 0.047	(Δ/σ)max = 0.008
S = 1.98	Δρmax = 0.64 e Å^−3
2137 reflections	Δρmin = −0.57 e Å^−3
98 parameters	Extinction correction: isotropic (Zachariasen, 1963)
4 restraints	Extinction coefficient: 8 (2) x 10^-7

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

	x	y	z	Uiso*/Ueq
P	0.37384 (2)	0.31415 (5)	0.40865 (2)	0.01415 (5)
O1	0.35845 (5)	0.14492 (8)	0.30174 (7)	0.0182 (3)
O2	0.45337 (4)	0.45944 (13)	0.38308 (6)	0.0132 (3)
O3	0.38079 (5)	0.18275 (15)	0.51608 (7)	0.0196 (3)
C1	0.5	0.1653 (2)	0.25	0.0121 (5)
C2	0.42830 (7)	0.0022 (2)	0.27299 (9)	0.0170 (4)
C3	0.52303 (6)	0.3186 (2)	0.35399 (8)	0.0130 (4)
C4	0.29603 (7)	0.5333 (2)	0.39320 (13)	0.0230 (5)
H2a	0.4082 (9)	−0.091 (3)	0.2078 (14)	0.035 (4)*
H2b	0.4416 (8)	−0.115 (2)	0.3365 (11)	0.020 (4)*
H3a	0.5634 (8)	0.428 (3)	0.3415 (11)	0.019 (4)*
H3b	0.5409 (9)	0.212 (3)	0.4193 (12)	0.023 (4)*
H4a	0.2968 (9)	0.603 (3)	0.3176 (13)	0.047 (5)*
H4b	0.3078 (9)	0.656 (3)	0.4459 (13)	0.032 (4)*
H4c	0.2459 (9)	0.477 (3)	0.4122 (12)	0.032 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P	0.01555 (10)	0.01359 (10)	0.01307 (10)	−0.00223 (8)	−0.00033 (9)	0.00213 (9)
O1	0.0204 (3)	0.0184 (3)	0.0162 (3)	−0.0078 (3)	−0.0009 (3)	−0.0011 (3)
O2	0.0138 (3)	0.0103 (3)	0.0161 (3)	−0.0009 (2)	0.0014 (2)	−0.0017 (2)
O3	0.0236 (3)	0.0220 (4)	0.0145 (3)	−0.0015 (3)	0.0016 (3)	0.0054 (3)
C1	0.0191 (5)	0.0082 (5)	0.0113 (5)	0.0000	−0.0005 (4)	0.0000
C2	0.0308 (5)	0.0103 (4)	0.0155 (4)	−0.0059 (4)	0.0002 (4)	−0.0004 (3)
C3	0.0153 (4)	0.0111 (3)	0.0131 (4)	0.0008 (3)	−0.0009 (3)	−0.0016 (3)
C4	0.0167 (4)	0.0242 (5)	0.0301 (5)	0.0022 (4)	0.0018 (4)	0.0061 (5)

Geometric parameters (Å, º)

P—O1	1.5924 (8)	C2—H2a	0.974 (16)
P—O2	1.5892 (7)	C2—H2b	1.010 (13)
P—O3	1.4749 (8)	C3—H3a	0.921 (13)
P—C4	1.7691 (12)	C3—H3b	1.005 (14)
O1—C2	1.4611 (14)	C4—H4a	0.987 (16)
O2—C3	1.4539 (12)	C4—H4b	0.941 (14)
C1—C2	1.5315 (13)	C4—H4c	0.927 (14)
C1—C3	1.5315 (13)
O1—P—O2	103.25 (4)	C1—C2—H2a	113.0 (9)
O1—P—O3	113.62 (5)	C1—C2—H2b	113.2 (7)
O1—P—C4	104.92 (5)	H2a—C2—H2b	107.2 (12)
O2—P—O3	114.34 (4)	O2—C3—C1	110.26 (7)
O2—P—C4	103.19 (5)	O2—C3—H3a	106.0 (8)
O3—P—C4	116.09 (6)	O2—C3—H3b	108.7 (8)
P—O1—C2	115.67 (6)	C1—C3—H3a	111.8 (8)
P—O2—C3	116.57 (6)	C1—C3—H3b	109.9 (8)
C2—C1—C2^i	107.16 (9)	H3a—C3—H3b	110.1 (11)
C2—C1—C3	109.32 (5)	P—C4—H4a	107.8 (10)
C2—C1—C3^i	109.40 (5)	P—C4—H4b	108.9 (9)
C3—C1—C3^i	112.11 (9)	P—C4—H4c	113.1 (10)
O1—C2—C1	110.50 (8)	H4a—C4—H4b	108.1 (14)
O1—C2—H2a	104.9 (9)	H4a—C4—H4c	115.7 (12)
O1—C2—H2b	107.7 (8)	H4b—C4—H4c	102.8 (13)
O2—P—O1—C2	51.87 (8)	C4—P—O2—C3	−161.09 (7)
O3—P—O1—C2	−72.53 (8)	P—O1—C2—C1	−58.93 (9)
C4—P—O1—C2	159.64 (7)	P—O2—C3—C1	58.50 (9)
O1—P—O2—C3	−52.02 (7)	C3—C1—C2—O1	58.49 (9)
O3—P—O2—C3	71.91 (8)	C2—C1—C3—O2	−57.95 (10)

Symmetry code: (i) −x+1, y, −z+1/2.