2-Ethyl-2-(hydroxy­methyl)-1,3-propane­diol at 120 K: three hydrogen bonds generate a three-dimensional structure

Molecules of the title compound, C₂H₅C(CH₂OH)₃, are linked by three O—H...O hydrogen bonds [H...O, 1.87–1.90 Å; O...O 2.704 (2)–2.719 (2) Å; O—H...O, 164–174°] into a single three-dimensional framework. This framework is readily analysed in terms of parallel molecular ladders, generated by two of the hydrogen bonds, linked together by the third hydrogen bond.

Key indicators

• Single-crystal X-ray study
• T = 120 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.036
• wR factor = 0.100
• Data-to-parameter ratio = 19.2

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

We recently reported the structure of N,N'-bis[tris(hydroxymethyl)methyl]ethanediamide, (HOCH₂)₃CNHCOCONHC(CH₂OH)₃, (I), where despite the presence of six independent O—H···O hydrogen bonds, the supramolecular structure is only two-dimensional (Ross et al., 2001). A simpler analogue of (I) is the title compound, C₂H₅C(CH₂OH)₃, (II), whose phase relationships were studied a number of years ago (Gowda et al., 1982). When a melt of (II) was cooled to below the melting temperature (333 K) a disordered cubic phase, having space group Fm3m and Z = 4, was first obtained: when this phase was cooled to 273 K or allowed to stand at ambient temperature it was transformed into an ordered monoclinic phase with space group P2₁/n and Z = 4. Here we report a low-temperature, 120 (2) K, study of the monoclinic phase of (II), as grown directly from solution rather than obtained indirectly from the melt, and we give a detailed analysis of the hydrogen bonding. Although there are only three independent O—H···O hydrogen bonds in (II) (Table 2), the supramolecular structure is three-dimensional, in contrast to that of (I).

Polyhydroxy compounds such as (I) and (II) have very limited solubility in non-hydrogen-bonding solvents: this, in combination with the very hygroscopic nature of (II) makes the growth from solution of solvent-free crystals of (II) a troublesome matter. After many unsuccessful attempts using a wide range of solvents, we were able to grow satisfactory solvent-free crystals from a solution in ethyl benzoylacetate, PhCOCH₂COOEt: the molecules of this compound contain sufficient hydrogen-bond acceptor sites to render it an effective solvent for (II), while the molecular size and the irregular molecular shape effectively preclude the inclusion of solvent molecules in the resulting crystals of (II). When crystallized in this way, (II) forms the same monoclinic phase as was obtained earlier from the metastable cubic phase.

The supramolecular structure of (II) is most readily analysed using the sub-structure approach (Gregson et al., 2000): a simple combination of two of the hydrogen bonds generates a molecular ladder, and all of the molecular ladders are linked into a single three-dimensional framework by the action of the third hydrogen bond. Hydroxyl O2 (Fig. 1) at (x, y, z) acts as hydrogen-bond donor to O3 at (-x, 1 - y, 1 - z), while O2 at (-x, 1 - y, 1 - z) acts as donor to O3 at (x, y, z) so generating a centrosymmetric R²₂(12) motif (Fig. 2). Similarly, O3 at (x, y, z) acts as hydrogen-bond donor to O4 at (1 - x, 1 - y, 1 - z), so generating a second centrosymmetric R²₂(12) ring (Fig. 2). The combination and propagation of these two hydrogen bonds generates a molecular ladder running parallel to the [100] direction (Fig. 2). The uprights of the ladder are an antiparallel pair of C²₂(8) chains, and the C1—C3—O3 portion of the molecule acts as the rungs of the ladder. Between the runs are the R²₂(12) rings, with the rings containing O2 centred at (n, 1/2, 1/2) (n = zero or integer) and those containing O4 centred at (n+0.5, 1/2, 1/2) (n = zero or integer).

This ladder lies along the line (x, 1/2, 1/2) and there are symmetry-related ladders running along the cell edges, on the lines (x, 0, 0), (x, 0, 1) and so on: hence two ladders pass through each unit cell, and the ladders are linked by the third hydrogen bond. Hydroxyl O4 at (x, y, z) acts as hydrogen-bond donor to O2 at (0.5 + x, 0.5 - y, -0.5 + z), while O4 at (0.5 + x, 0.5 - y, -0.5 + z) in turn acts as donor to O2 at (1 + x, y, -1 + z), so producing a C(6) chain running parallel to the [101] direction, and generated by the n-glide plane at y = 0.25 (Fig. 3).

The hydroxyl atoms O4 at (x, y, z) and (1 - x, 1 - y, 1 - z) are both part of the ladder which runs along the line (x, 0, 0): these O atoms act as hydrogen-bond donors to atoms O2 at (0.5 + x, 0.5 - y, -0.5 + z) and (0.5 - x, 0.5 + y, 1.5 - z) respectively, which lie in the ladders along (x, 0, 0) and (x, 1, 1) respectively. The atoms O2 at (x, y, z) and (1 - x, 1 - y, 1 - z) in the (x, 1/2, 1/2) ladder act as hydrogen-bond acceptors from atoms O4 at (-0.5 + x, 0.5 - y, 0.5 + z) and (1.5 - x, 0.5 + y, 0.5 - z) respectively, which themselves lie in the ladders along (x, 0, 1) and (x, 1, 0) respectively. Hence, the ladder along (x, 1/2, 1/2) is linked directly to the four ladders along the unit cell edges (Fig. 3), and hence all the ladders in the structure are linked into a single three-dimensional framework.

While the bond lengths and angles in (II) present no unusual features, the molecular conformation (Table 1) is of interest. There is almost perfect staggering about all of the C—C bonds, but the –C(CH₂OH)₃ fragment does not exhibit the idealized threefold local symmetry: instead the molecule as a whole has a conformation close to C_s (m) molecular symmetry (Fig. 1, and Table 1)

S2. Experimental

A sample of (II) was purchased from Aldrich. Solvent-free crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of a solution in ethyl benzoylacetate with rigorous exclusion of moisture.

S3. Refinement

Compound (II) crystallized in the monoclinic system; space group P2₁/n was uniquely assigned from the systematic absences. H atoms were treated as riding atoms with C—H distances of 0.98 (CH₃) or 0.99 Å (CH₂) and an O—H distance of 0.84 Å. Two data sets were collected at 120 (2) K from crystals obtained in two separate crystallizations; the results reported here are based on the dataset which gave marginally the lower R value (0.0355 versus 0.0366).

Computing details

Data collection: KappaCCD 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: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2001); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).

Figures

	Fig. 1. The molecule of (II) showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.
	Fig. 2. Part of the crystal structure of (II) showing formation of a molecular ladder along [100]. For the sake of clarity, H atoms bonded to C are omitted. The atoms marked with a star (*), hash (#) or dollar sign ($) are at the symmetry positions (1 + x, y, z), (1 - x, 1 - y, 1 - z) and (-x, 1 - y, 1 - z), respectively.
	Fig. 3. Part of the crystal structure of (II) showing the linking of the [100] molecular ladders into a three-dimensional framework. For the sake of clarity, H atoms bonded to C are omitted. The atoms marked with a dollar sign ($), single or double stars (*), or single or double hashes (#) are at the symmetry positions (1 - x, 1 - y, 1 - z), (0.5 + x, 0.5 - y, -0.5 + z), (-0.5 + x, 0.5 - y, 0.5 + z), (1.5 - x, 0.5 + y, 0.5 - z) and (0.5 - x, 0.5 + y, 1.5 - z) respectively.

2-Ethyl-2-(hydroxymethyl)-1,3-propanediol

Crystal data

C6H14O3	F(000) = 296
Mr = 134.17	Dx = 1.231 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 8.3806 (2) Å	Cell parameters from 1647 reflections
b = 9.5768 (3) Å	θ = 3.1–27.5°
c = 9.1466 (3) Å	µ = 0.10 mm^−1
β = 99.4790 (13)°	T = 120 K
V = 724.08 (4) Å^3	Lath, colourless
Z = 4	0.22 × 0.14 × 0.10 mm

Data collection

KappaCCD diffractometer	1647 independent reflections
Radiation source: fine-focus sealed X-ray tube	1348 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
ϕ scans, and ω scans with κ offsets	θmax = 27.5°, θmin = 3.1°
Absorption correction: multi-scan (DENZO-SMN; Otwinowski & Minor, 1997)	h = −10→10
Tmin = 0.979, Tmax = 0.990	k = −12→12
6745 measured reflections	l = −11→11

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)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.100	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0494P)^2 + 0.1161P] where P = (Fo^2 + 2Fc^2)/3
1647 reflections	(Δ/σ)max < 0.001
86 parameters	Δρmax = 0.24 e Å^−3
0 restraints	Δρmin = −0.21 e Å^−3

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

	x	y	z	Uiso*/Ueq
C1	0.25514 (12)	0.29486 (11)	0.49169 (11)	0.0171 (2)
C2	0.07057 (12)	0.30862 (11)	0.45423 (11)	0.0191 (2)
H2A	0.0426	0.3879	0.3854	0.023*
H2B	0.0247	0.2226	0.4039	0.023*
O2	0.00135 (9)	0.33079 (9)	0.58505 (8)	0.0243 (2)
H2	−0.0893	0.3695	0.5623	0.036*
C3	0.32643 (12)	0.41849 (11)	0.58661 (11)	0.0198 (3)
H3A	0.2827	0.4191	0.6808	0.024*
H3B	0.4454	0.4079	0.6108	0.024*
O3	0.28844 (9)	0.54800 (8)	0.51066 (8)	0.0241 (2)
H3	0.3555	0.6091	0.5465	0.036*
C4	0.31413 (12)	0.29497 (12)	0.34259 (11)	0.0212 (3)
H4A	0.2555	0.2223	0.2779	0.025*
H4B	0.2901	0.3866	0.2938	0.025*
O4	0.48463 (9)	0.26867 (9)	0.36054 (8)	0.0229 (2)
H4	0.5088	0.2420	0.2795	0.034*
C5	0.30529 (13)	0.16102 (12)	0.58129 (11)	0.0217 (3)
H5A	0.4237	0.1639	0.6153	0.026*
H5B	0.2528	0.1616	0.6706	0.026*
C6	0.26376 (16)	0.02405 (13)	0.49933 (13)	0.0325 (3)
H6A	0.1468	0.0194	0.4648	0.049*
H6B	0.2970	−0.0543	0.5664	0.049*
H6C	0.3207	0.0189	0.4141	0.049*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0144 (5)	0.0243 (6)	0.0132 (5)	0.0012 (4)	0.0038 (4)	0.0000 (4)
C2	0.0156 (5)	0.0265 (6)	0.0155 (5)	0.0006 (4)	0.0038 (4)	0.0001 (4)
O2	0.0161 (4)	0.0380 (5)	0.0202 (4)	0.0068 (3)	0.0072 (3)	0.0060 (3)
C3	0.0156 (5)	0.0270 (6)	0.0169 (5)	0.0018 (4)	0.0028 (4)	−0.0003 (4)
O3	0.0161 (4)	0.0232 (4)	0.0321 (4)	−0.0011 (3)	0.0015 (3)	0.0003 (3)
C4	0.0164 (5)	0.0319 (6)	0.0158 (5)	0.0030 (5)	0.0042 (4)	0.0004 (4)
O4	0.0165 (4)	0.0338 (5)	0.0199 (4)	0.0003 (3)	0.0076 (3)	−0.0059 (3)
C5	0.0218 (5)	0.0267 (6)	0.0168 (5)	0.0050 (5)	0.0039 (4)	0.0024 (4)
C6	0.0362 (7)	0.0259 (6)	0.0348 (7)	0.0027 (5)	0.0039 (5)	0.0007 (5)

Geometric parameters (Å, º)

C1—C4	1.525 (2)	O3—H3	0.84
C1—C3	1.530 (2)	C4—O4	1.434 (2)
C1—C2	1.534 (2)	C4—H4A	0.99
C1—C5	1.542 (2)	C4—H4B	0.99
C2—O2	1.429 (2)	O4—H4	0.84
C2—H2A	0.99	C5—C6	1.522 (2)
C2—H2B	0.99	C5—H5A	0.99
O2—H2	0.84	C5—H5B	0.99
C3—O3	1.432 (2)	C6—H6A	0.98
C3—H3A	0.99	C6—H6B	0.98
C3—H3B	0.99	C6—H6C	0.98
C4—C1—C3	110.67 (8)	O4—C4—C1	111.18 (8)
C4—C1—C2	105.29 (8)	O4—C4—H4A	109.4
C3—C1—C2	110.21 (8)	C1—C4—H4A	109.4
C4—C1—C5	112.07 (8)	O4—C4—H4B	109.4
C3—C1—C5	107.15 (8)	C1—C4—H4B	109.4
C2—C1—C5	111.51 (8)	H4A—C4—H4B	108.0
O2—C2—C1	111.05 (8)	C4—O4—H4	109.5
O2—C2—H2A	109.4	C6—C5—C1	115.74 (9)
C1—C2—H2A	109.4	C6—C5—H5A	108.3
O2—C2—H2B	109.4	C1—C5—H5A	108.3
C1—C2—H2B	109.4	C6—C5—H5B	108.3
H2A—C2—H2B	108.0	C1—C5—H5B	108.3
C2—O2—H2	109.5	H5A—C5—H5B	107.4
O3—C3—C1	111.11 (8)	C5—C6—H6A	109.5
O3—C3—H3A	109.4	C5—C6—H6B	109.5
C1—C3—H3A	109.4	H6A—C6—H6B	109.5
O3—C3—H3B	109.4	C5—C6—H6C	109.5
C1—C3—H3B	109.4	H6A—C6—H6C	109.5
H3A—C3—H3B	108.0	H6B—C6—H6C	109.5
C3—O3—H3	109.5
C6—C5—C1—C2	66.4 (2)	C4—C1—C2—O2	−171.33 (8)
C6—C5—C1—C3	−172.93 (9)	C3—C1—C2—O2	−51.9 (2)
C6—C5—C1—C4	−51.4 (2)	C4—C1—C3—O3	56.4 (2)
C5—C1—C2—O2	66.9 (2)	C2—C1—C3—O3	−59.6 (2)
C5—C1—C3—O3	178.88 (8)	C3—C1—C4—O4	66.7 (2)
C5—C1—C4—O4	−52.8 (2)	C2—C1—C4—O4	−174.24 (8)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O3^i	0.84	1.87	2.704 (2)	174
O3—H3···O4^ii	0.84	1.87	2.711 (2)	174
O4—H4···O2^iii	0.84	1.90	2.719 (2)	164

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