Crystal structure of 4-methyl-2,6,7-trioxa-1-phosphabi-cyclo-[2.2.2]octa-ne.

The title mol-ecule, C5H9O3P, has a bi-cyclo-[2.2.2] structure with the P atom at the prow and the bridge-head C atom, with the bonded methyl group, at the stern. The three six-membered rings in the bi-cyclo-[2.2.2] structure have essentially identical good boat conformations.

Chemical context

Phospho­rus-based ligands bind strongly to transition metals and these complexes offer a wide range of properties due to the high volume of accessible substituents (Downing & Smith, 2004 ▸; Tolman, 1977 ▸; Joslin et al., 2012 ▸). Complexation experiments with these ligands can yield mono- or bi-nuclear complexes (van den Beuken et al., 1997 ▸). Phospho­rus-based complexes are an important class of compounds in homogeneous catalysis and coordination chemistry (Downing & Smith, 2004 ▸; Kühl, 2005 ▸). In particular, we have noted inter­esting studies comparing the donor ability of bicyclic phosphites and the related acyclic phosphites; the phospho­rus atom in the former shows more positive charge than in the acyclic phosphites and, hence, the donor ability of bicyclic phosphites is lower than that of the related acyclic phosphites (Vande Griend et al., 1977 ▸; Joslin et al., 2012 ▸). The present work is a continuation of an investigation into the synthesis and study of bi- and tri-cyclic, penta- and hexa-coordinated phospho­ranes to form anionic, neutral and zwitterionic compounds (Said et al. 1996 ▸; Timosheva, et al. 2006 ▸; Kumara Swamy & Satish Kumar, 2006 ▸). In this paper, we report the synthesis, clean isolation and crystal structure of 4-methyl-2,6,7-trioxa-1-phosphabi­cyclo­[2.2.2]octane (Tolman, 1977 ▸; Joslin et al., 2012 ▸).

Structural commentary

The mol­ecular structure of the title compound, Fig. 1 ▸, shows a bi­cyclo­[2.2.2] structure with the phospho­rus atom as one bridge-head atom and C3, with the bonded methyl group, as the other. The three six-membered rings in the bi­cyclo­[2.2.2] structure have essentially identical, good boat conformations. The P—O bond lengths are very similar, lying in the range 1.613 (2)–1.616 (2) Å; the O—P—O angles at the prow have angles in the range 100.17 (9)–101.34 (10)°, whereas the C—P—C angles at the stern lie in the range 107.99 (17)–109.08 (18)°.

Figure 1

A view of a mol­ecule of bi­cyclo-P(OCH2)3CMe, indicating the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.

A comparison between acyclic and bicyclic phosphites based on the ‘hinge’ effect has shown (Vande Griend et al., 1977 ▸; Joslin et al., 2012 ▸) that the O—P—O and P—O—C angles, a and b in Scheme 1, change upon ligation with a metal. Due to the steric profile of the bicyclic phosphite, the changes here in a, a′ and b, b′ upon metal ligation are less than in acyclic phosphites. Verkade had pointed out earlier that the p-orbital overlap between P and O in bicyclic phosphites is less than in acyclic phosphites, making P more positive and therefore reducing the basicity of P relative to that in acyclic phosphites (Vande Griend et al., 1977 ▸); hence, the coordination ability of acyclic phosphites is higher than that of bicyclic phosphites (Verkade, 1972 ▸). A variety of multi-cyclic phospho­rus compounds including their coordination to various metals has been studied. Based on the trends found in basicity, it is expected that the title compound would show a donating ability to metal centres very similar to that of the more commonly studied bicyclic phosphite P(OCH2)3CEt (Verkade, 1972 ▸). The average of O—P—O bond angle (a, Scheme 1) in our study is 100.7o, whereas the average O—P—O bond angle in coordinated phosphites (a′, Scheme 1) is larger, e.g. in Ru{P(OCH2)3CEt}Cl2, it is 102.5o (Joslin et al., 2012 ▸), the same as in [Rh2I2(C6H5N2O2)2(COMe)2{P(OCH2)3CMe}2] (Venter et al., 2009 ▸); this suggests a slightly larger Tolman angle (Tolman et al., 1977 ▸) after metal ligation. In another study, the enhanced π-accepting ability of the bicyclic phosphite ligand compared to the PPh3 and other phosphine ligands was demonstrated clearly in the shorter M—P bond distances in the bicyclic phosphite complexes (Erasmus et al., 1998 ▸).

Supra­molecular features

Contacts between mol­ecules are at normal van der Waals distances, the shortest of which is H4B⋯O6′, at 2.58 Å (Table 1 ▸). The nearest neighbours of the phospho­rus atom are hydrogen atoms at distances of at least 3.09 Å. A view of the packing along the b axis is shown in Fig. 2 ▸.

Table 1

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C4—H4B⋯O6i	0.97	2.58	3.495 (4)	158

Symmetry code: (i) .

Figure 2

A view along the crystallographic b axis.

Database survey

From a selection of crystal structure results for bicyclic phosphites from the Cambridge Structural Database (Groom et al., 2016 ▸), we note that the P—O bond distances:

1) are shortest in phospho­nium ions, as in [Ph3C{P(OCH2)3CMe}]+ (Fang et al., 2000 ▸), at ca 1.552 Å,

2) in the phosphates, as O=P(OCH2)3CR, (e.g. Nimrod et al., 1968 ▸; Santarsiero, 1992 ▸) are ca 1.57 Å,

3) in the metal-coordinated phosphites, M–{P(OCH2)3CR} (e.g. Aroney et al., 1994 ▸; Venter et al., 2009 ▸; Davis & Verkade, 1990 ▸; Predvoditelev et al., 2009 ▸; Basson et al., 1992 ▸; Erasmus et al., 1998 ▸; Joslin et al., 2012 ▸; Albright et al., 1977 ▸) are ca 1.59 Å, and

4) in our results, correlate with those of other unsubstituted phosphites, (e.g. Wojczykowski & Jutzi, 2006 ▸; Milbrath et al., 1976 ▸; Predvoditelev et al., 2009 ▸) with P—O bond lengths of ca 1.62 Å.

Within each group, there is very little variation in the P—O distances. The bond angles in the bicyclic structure are quite constrained, but we do note a trend, down the four groups of increasing P—O distances, of a corresponding decrease in O—P—O angles from ca 107 to 100°.

Synthesis and crystallization

To 4.26 g (35.46 mmol) of 2-(hy­droxy­meth­yl)-2-methyl­propane-1,3-diol in 70 mL of dry benzene at RT was added 4.26 g (106.38 mmoles in mineral oil 60%) of NaH in small portions over a period of 20 minutes. The mixture was stirred for 3h before 4.87 g (35.46 mmol) of PCl3 were added dropwise over a period of 20 mins in benzene (10 mL) using a dropping funnel. The reaction mixture was stirred overnight before NaCl was removed by filtration under nitro­gen cover. Benzene was removed completely under low pressure. 5 mL of diethyl ether was added, followed by 3 mL of n-hexane. The mixture was placed in deep freeze to afford the title compound as a white solid (yield 4.52 g, 86%; m.p. 369–373 K). The product was purified further by sublimation at 393 K/0.5 mm to yield crystals. 1H NMR (CDCl3, 400 MHz): 0.73 (s, 3H, CH3), 3.94 (s, 6H, CH2). 13C NMR (CDCl3, 400 MHz): 16.60 (s, 1C, CH3), 31.98 [d, 1C, C(CH3)3], 71.80 (s, 3C, CH2). 31P NMR (CDCl3, 400 MHz): 91.45 p.p.m. IR cm−1: 2950, 1380. Elemental analysis: calculated: C, 40.55; H, 6.13; found: C, 40.83; H, 6.19.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸.

Table 2

Experimental details

Crystal data
Chemical formula	C5H9O3P
M r	148.09
Crystal system, space group	Orthorhombic, P n a21
Temperature (K)	140
a, b, c (Å)	10.4408 (6), 6.2129 (5), 10.5052 (5)
V (Å3)	681.45 (7)
Z	4
Radiation type	Mo Kα
μ (mm−1)	0.34
Crystal size (mm)	0.65 × 0.17 × 0.07
Data collection
Diffractometer	Oxford Diffraction Xcalibur 3/Sapphire3 CCD
Absorption correction	Multi-scan (CrysAlis PRO; Agilent, 2013 ▸)
T min, T max	0.684, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	10309, 1561, 1405
R int	0.043
(sin θ/λ)max (Å−1)	0.649
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.033, 0.084, 1.11
No. of reflections	1561
No. of parameters	82
No. of restraints	1
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	0.24, −0.12
Absolute structure	Flack x determined using 605 quotients [(I +)−(I −)]/[(I +)+(I −)] (Parsons et al., 2013 ▸)
Absolute structure parameter	0.07 (6)

Computer programs: CrysAlis PRO (Agilent, 2013 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸), ORTEPIII (Johnson, 1976 ▸) and ORTEP-3 for Windows (Farrugia, 2012 ▸) and WinGX (Farrugia, 2012 ▸).

The H atoms were included in idealized positions and treated as riding atoms: C—H = 0.93– 0.97 Å with U iso(H) = 1.5Ueq(C) for methyl H atoms and = 1.2U eq(C) for methyl­ene H atoms.

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989016009993/lh5816sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989016009993/lh5816Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989016009993/lh5816Isup3.cdx

Click here for additional data file.

Supporting information file. DOI: 10.1107/S2056989016009993/lh5816Isup4.cml

CCDC reference: 1486648

Additional supporting information: crystallographic information; 3D view; checkCIF report

C5H9O3P	Dx = 1.443 Mg m−3
Mr = 148.09	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pna21	Cell parameters from 2685 reflections
a = 10.4408 (6) Å	θ = 3.3–32.3°
b = 6.2129 (5) Å	µ = 0.34 mm−1
c = 10.5052 (5) Å	T = 140 K
V = 681.45 (7) Å3	Prism, colourless
Z = 4	0.65 × 0.17 × 0.07 mm
F(000) = 312

Oxford Diffraction Xcalibur 3/Sapphire3 CCD diffractometer	1561 independent reflections
Radiation source: Enhance (Mo) X-ray Source	1405 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.043
Detector resolution: 16.0050 pixels mm-1	θmax = 27.5°, θmin = 3.8°
Thin slice φ and ω scans	h = −13→13
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2013)	k = −8→8
Tmin = 0.684, Tmax = 1.000	l = −13→13
10309 measured reflections

Refinement on F2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.033	H-atom parameters constrained
wR(F2) = 0.084	w = 1/[σ2(Fo2) + (0.0459P)2 + 0.0252P] where P = (Fo2 + 2Fc2)/3
S = 1.11	(Δ/σ)max < 0.001
1561 reflections	Δρmax = 0.24 e Å−3
82 parameters	Δρmin = −0.12 e Å−3
1 restraint	Absolute structure: Flack x determined using 605 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.07 (6)

Experimental. CrysAlisPro, Agilent Technologies, Version 1.171.36.21 Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.

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

	x	y	z	Uiso*/Ueq
P1	0.87893 (7)	0.45449 (13)	0.75670 (8)	0.0380 (2)
O1	0.9057 (2)	0.5737 (4)	0.6228 (3)	0.0470 (6)
C2	0.8029 (3)	0.5722 (6)	0.5302 (3)	0.0373 (7)
H2A	0.8308	0.4974	0.4541	0.045*
H2B	0.7816	0.7189	0.5068	0.045*
C3	0.6847 (2)	0.4614 (5)	0.5841 (2)	0.0253 (5)
C4	0.7208 (3)	0.2300 (4)	0.6164 (3)	0.0349 (6)
H4A	0.6473	0.1566	0.6524	0.042*
H4B	0.7456	0.1549	0.5393	0.042*
O5	0.8263 (2)	0.2254 (4)	0.7070 (2)	0.0399 (5)
O6	0.7460 (2)	0.5660 (4)	0.7998 (2)	0.0457 (7)
C7	0.6445 (3)	0.5749 (6)	0.7058 (3)	0.0371 (7)
H7A	0.6242	0.7240	0.6871	0.045*
H7B	0.5681	0.5070	0.7397	0.045*
C8	0.5752 (3)	0.4642 (6)	0.4878 (3)	0.0370 (7)
H8A	0.6017	0.3921	0.4113	0.055*
H8B	0.5021	0.3919	0.5231	0.055*
H8C	0.5530	0.6105	0.4683	0.055*

	U11	U22	U33	U12	U13	U23
P1	0.0322 (4)	0.0504 (4)	0.0314 (4)	0.0023 (3)	−0.0076 (4)	−0.0037 (5)
O1	0.0297 (11)	0.0630 (15)	0.0483 (14)	−0.0148 (10)	−0.0050 (10)	0.0112 (12)
C2	0.0313 (15)	0.047 (2)	0.0340 (16)	−0.0046 (13)	−0.0002 (13)	0.0088 (13)
C3	0.0232 (12)	0.0319 (13)	0.0209 (13)	−0.0003 (11)	−0.0002 (10)	−0.0004 (11)
C4	0.0391 (16)	0.0332 (15)	0.0323 (14)	0.0001 (13)	−0.0036 (12)	−0.0026 (13)
O5	0.0448 (12)	0.0399 (12)	0.0350 (11)	0.0094 (10)	−0.0096 (9)	0.0025 (9)
O6	0.0455 (13)	0.0625 (17)	0.0292 (11)	0.0128 (11)	−0.0079 (9)	−0.0209 (10)
C7	0.0295 (16)	0.0508 (18)	0.0310 (14)	0.0082 (13)	−0.0008 (12)	−0.0080 (13)
C8	0.0305 (16)	0.053 (2)	0.0274 (15)	−0.0001 (14)	−0.0049 (12)	0.0014 (13)

P1—O5	1.613 (2)	C4—O5	1.456 (4)
P1—O1	1.614 (3)	C4—H4A	0.9700
P1—O6	1.616 (2)	C4—H4B	0.9700
O1—C2	1.449 (4)	O6—C7	1.450 (4)
C2—C3	1.522 (4)	C7—H7A	0.9700
C2—H2A	0.9700	C7—H7B	0.9700
C2—H2B	0.9700	C8—H8A	0.9600
C3—C7	1.519 (4)	C8—H8B	0.9600
C3—C4	1.524 (4)	C8—H8C	0.9600
C3—C8	1.527 (4)
O5—P1—O1	100.46 (13)	C3—C4—H4A	109.5
O5—P1—O6	100.17 (13)	O5—C4—H4B	109.5
O1—P1—O6	101.34 (14)	C3—C4—H4B	109.5
C2—O1—P1	117.00 (18)	H4A—C4—H4B	108.1
O1—C2—C3	110.7 (2)	C4—O5—P1	116.88 (18)
O1—C2—H2A	109.5	C7—O6—P1	116.95 (18)
C3—C2—H2A	109.5	O6—C7—C3	110.7 (2)
O1—C2—H2B	109.5	O6—C7—H7A	109.5
C3—C2—H2B	109.5	C3—C7—H7A	109.5
H2A—C2—H2B	108.1	O6—C7—H7B	109.5
C7—C3—C2	109.1 (3)	C3—C7—H7B	109.5
C7—C3—C4	108.6 (2)	H7A—C7—H7B	108.1
C2—C3—C4	108.0 (2)	C3—C8—H8A	109.5
C7—C3—C8	110.2 (2)	C3—C8—H8B	109.5
C2—C3—C8	110.8 (2)	H8A—C8—H8B	109.5
C4—C3—C8	110.1 (2)	C3—C8—H8C	109.5
O5—C4—C3	110.5 (2)	H8A—C8—H8C	109.5
O5—C4—H4A	109.5	H8B—C8—H8C	109.5
O5—P1—O1—C2	50.4 (3)	C3—C4—O5—P1	1.8 (3)
O6—P1—O1—C2	−52.3 (3)	O1—P1—O5—C4	−52.8 (2)
P1—O1—C2—C3	2.4 (4)	O6—P1—O5—C4	50.8 (2)
O1—C2—C3—C7	57.2 (3)	O5—P1—O6—C7	−54.0 (3)
O1—C2—C3—C4	−60.7 (3)	O1—P1—O6—C7	49.0 (3)
O1—C2—C3—C8	178.7 (3)	P1—O6—C7—C3	3.3 (4)
C7—C3—C4—O5	−60.0 (3)	C2—C3—C7—O6	−60.4 (3)
C2—C3—C4—O5	58.2 (3)	C4—C3—C7—O6	57.1 (3)
C8—C3—C4—O5	179.2 (2)	C8—C3—C7—O6	177.8 (3)

D—H···A	D—H	H···A	D···A	D—H···A
C4—H4B···O6i	0.97	2.58	3.495 (4)	158