1,3:16,18-Bis­(xylyl)-30-crown-8

The title crown ether, C₂₈H₄₀O₈, crystallizes in an ortho­rhombic cell with the full mol­ecule generated from crystallographic inversion symmetry. The ring consists of 30 atoms which could potentially influence the size of the ring cavity and the conformational flexibility. Unusual C-O-C-C and O-C-C-O torsion-angle geometries, deviating by as much as 30° from their ideal values, have been observed.

S1. Comment

Macrocyclic compounds such as crown ethers can show selectivity, via the oxygen atoms, in complexing with a diverse range of neutral, polar and cationic substrates. The ligand properties can also be modified by altering the number of oxygen atoms, replacing oxygen with other heteroatoms or changing the length of aliphatic chains and/or aromatic groups between the oxygen atoms.

The crown ether molecule, (I), (Figure 1) crystallizes in an orthorhombic unit cell, with the full molecule generated by an inversion operation. Selected geometric parameters are presented in Table 1. Bond distances and bond angles are within normal ranges; C_sp3—O between 1.4149 (18)–1.4302 (18) Å and C—O—C angles from 110.54 (11) to 113.87 (11)°, these values being comparable with other crown ether complexes based on benzo-annelated systems such as dibenzo-30-crown-10 (Bush & Truter, 1972). \sch

Torsion angle analysis around C—O and aliphatic C—C bonds within the crown ring, can provide an insight into the puckering of the ligand. In general the C—O—C—C torsion angles are expected to lie close to 180° (trans) whereas the O—C—C—O torsion angles tend to cluster around 60° (gauche). In this molecule, two C—O—C—C torsion angles possess unusual geometries; C8—O1—C7—C6 deviates significantly from the ideal trans value, 148.79 (13)°, and C13—O4—C14—C2 is gauche, 76.52 (15)°, whilst all other C—O—C—C values group around ±180°. Two O—C—C—O torsion angles adopt their expected gauche-type geometries while the third (belonging to O2—C9—C8—O1) is trans, 175.94 (11)°. Smaller rings such as dibenzo-26-crown-8 also display unusual torsion angle measurements (Buchanan et al., 1997).

The absence of interannular contacts, illustrated by non-bonding distances between O1···C8ⁱ of 3.835 (2) and C1···O3ⁱ of 3.529 (2) Å, [symmetry code: (i) -x, -y, 1 - z], results in a small central ring cavity. There seems to be no correlation between the number of atoms in the ring and the size of the cavity, other 30 atom crown species show appreciably larger central cavities (Anelli et al., 1988). Overall the crown ether is flat, the largest deviation from the mean plane, of the 30 ring atoms, being 0.890 (2) Å, with slight puckering at the xylyl moiety. The packing efficiency of the compound was anaylsed, using the CALC VOID option in PLATON (Spek, 1999). The packing index (Kitajgorodskij, 1973) of this crown is 71%, which is comparable to the value obtained for dibenzo-30-crown-10 (Bush & Truter, 1972), 68%, and 30-crown-10 (Bheda et al., 1994) of 67%.

The crown ether ring has rearranged on complexation with a diphenyl magnesium substrate (Markies et al., 1994), leading to a change in the torsion angle descriptors and hence the ring conformation. Only three of the four oxygen atoms ligate to the magnesium ions; presumably either steric interactions between diphenyl groups on the magnesium and the 1,3-xylyl units, or a relatively long O1···O2 distance of 3.577 (2) Å, prevents full participation. It would be interesting to see if higher coordination numbers could be achieved with other cationic species such as K⁺ or Na⁺ (Mercer & Truter, 1973; Owen & Truter, 1979).

S2. Experimental

The dimeric compound was formed as a side product in the synthesis of the monomer 1,3-xylyl-15-crown-4 reported by Gray et al. (1976, 1977). Isolation and purification from the oligomeric fraction, followed by repeated crystallization of the crude product from ethanol, produced crystals suitable for an X-ray experiment.

S3. Refinement

All hydrogen atoms were constrained to ride on their parent carbon atom with U_iso(H) = 1.2U_eq(C); C—H distances refined to 0.95 or 0.99 Å for aromatic CH and CH₂ groups, respectively.

Computing details

Data collection: Locally modified CAD4-Version 5 Software (Enraf-Nonius, 1989); cell refinement: SET4 (de Boer & Duisenberg, 1984); data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: PLATON.

Figures

Fig. 1. Displacement ellipsoids plot (50% probability) of the ligand with atom labels. Symmetry operation to generate equivalent atoms: -x, -y, 1 - z.

(I)

Crystal data

C28H40O8	F(000) = 1088
Mr = 504.60	Dx = 1.294 Mg m^−3
Orthorhombic, Pccn	Mo Kα (Zr filtered) radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 25 reflections
a = 16.9096 (17) Å	θ = 14.0–17.9°
b = 18.351 (2) Å	µ = 0.09 mm^−1
c = 8.350 (2) Å	T = 100 K
V = 2591.0 (8) Å^3	Block, colourless
Z = 4	0.58 × 0.45 × 0.42 mm

Data collection

Enraf Nonius CAD-4F diffractometer	Rint = 0.028
Radiation source: rotating anode	θmax = 27.5°, θmin = 1.6°
Graphite monochromator	h = 0→21
ω/2θ scans	k = −23→23
5946 measured reflections	l = 0→10
2976 independent reflections	2 standard reflections every 60 min
2337 reflections with I > 2σ(I)	intensity decay: 3.5%

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)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.109	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0469P)^2 + 1.2483P] where P = (Fo^2 + 2Fc^2)/3
2976 reflections	(Δ/σ)max = 0.001
163 parameters	Δρmax = 0.56 e Å^−3
0 restraints	Δρmin = −0.22 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
O1	−0.03830 (6)	0.09811 (7)	0.61773 (13)	0.0269 (3)
O2	0.08263 (6)	0.15216 (5)	0.28697 (12)	0.0192 (2)
O3	0.15828 (6)	0.12745 (6)	−0.01500 (13)	0.0223 (2)
O4	0.18470 (6)	0.02467 (6)	−0.26293 (12)	0.0210 (2)
C1	−0.11489 (8)	0.06066 (8)	1.01751 (17)	0.0176 (3)
H1	−0.1009	0.0108	1.0284	0.021*
C2	−0.16272 (8)	0.09322 (8)	1.13273 (17)	0.0183 (3)
C3	−0.18354 (9)	0.16589 (8)	1.11454 (19)	0.0219 (3)
H3	−0.2166	0.1886	1.1918	0.026*
C4	−0.15638 (9)	0.20573 (8)	0.98418 (19)	0.0233 (3)
H4	−0.1704	0.2556	0.9735	0.028*
C5	−0.10900 (8)	0.17296 (8)	0.86989 (18)	0.0205 (3)
H5	−0.0912	0.2001	0.7800	0.025*
C6	−0.08736 (8)	0.10007 (8)	0.88669 (17)	0.0180 (3)
C7	−0.03018 (9)	0.06551 (9)	0.77082 (18)	0.0232 (3)
H7A	0.0246	0.0721	0.8100	0.028*
H7B	−0.0409	0.0126	0.7631	0.028*
C8	0.03278 (8)	0.10001 (8)	0.52810 (17)	0.0197 (3)
H8A	0.0501	0.0501	0.4999	0.024*
H8B	0.0754	0.1242	0.5897	0.024*
C9	0.01299 (9)	0.14305 (9)	0.37931 (18)	0.0209 (3)
H9A	−0.0275	0.1169	0.3158	0.025*
H9B	−0.0087	0.1913	0.4095	0.025*
C10	0.06613 (9)	0.19243 (8)	0.14525 (17)	0.0208 (3)
H10A	0.0479	0.2420	0.1739	0.025*
H10B	0.0235	0.1681	0.0840	0.025*
C11	0.13910 (9)	0.19735 (8)	0.04474 (18)	0.0213 (3)
H11A	0.1301	0.2313	−0.0455	0.026*
H11B	0.1835	0.2163	0.1099	0.026*
C12	0.22508 (9)	0.12963 (9)	−0.11801 (19)	0.0241 (3)
H12A	0.2701	0.1529	−0.0622	0.029*
H12B	0.2126	0.1588	−0.2145	0.029*
C13	0.24675 (9)	0.05381 (8)	−0.16576 (18)	0.0215 (3)
H13A	0.2970	0.0542	−0.2266	0.026*
H13B	0.2540	0.0232	−0.0692	0.026*
C14	−0.19017 (9)	0.05190 (8)	1.27946 (18)	0.0211 (3)
H14A	−0.1579	0.0673	1.3725	0.025*
H14B	−0.2458	0.0651	1.3021	0.025*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0193 (5)	0.0425 (7)	0.0188 (5)	0.0038 (5)	0.0031 (4)	0.0065 (5)
O2	0.0172 (5)	0.0227 (5)	0.0176 (5)	0.0016 (4)	0.0003 (4)	0.0035 (4)
O3	0.0230 (5)	0.0198 (5)	0.0242 (5)	−0.0023 (4)	0.0065 (4)	0.0011 (4)
O4	0.0206 (5)	0.0236 (5)	0.0189 (5)	−0.0015 (4)	−0.0019 (4)	0.0035 (4)
C1	0.0160 (6)	0.0177 (7)	0.0192 (7)	−0.0012 (5)	−0.0018 (5)	0.0011 (6)
C2	0.0152 (6)	0.0231 (7)	0.0167 (7)	−0.0030 (5)	−0.0015 (5)	−0.0002 (6)
C3	0.0212 (7)	0.0229 (8)	0.0215 (7)	0.0028 (6)	0.0008 (6)	−0.0039 (6)
C4	0.0238 (7)	0.0189 (7)	0.0271 (8)	0.0010 (6)	−0.0019 (6)	0.0013 (6)
C5	0.0185 (7)	0.0215 (7)	0.0214 (7)	−0.0037 (6)	−0.0017 (6)	0.0043 (6)
C6	0.0146 (6)	0.0227 (7)	0.0167 (7)	−0.0013 (5)	−0.0024 (5)	−0.0006 (6)
C7	0.0237 (7)	0.0269 (8)	0.0191 (7)	0.0033 (6)	0.0044 (6)	0.0030 (6)
C8	0.0177 (7)	0.0218 (7)	0.0195 (7)	0.0001 (5)	0.0036 (6)	0.0010 (6)
C9	0.0172 (7)	0.0247 (8)	0.0208 (7)	0.0006 (6)	0.0006 (6)	−0.0002 (6)
C10	0.0214 (7)	0.0205 (7)	0.0204 (7)	0.0026 (6)	−0.0030 (6)	0.0032 (6)
C11	0.0244 (7)	0.0194 (7)	0.0201 (7)	−0.0018 (6)	−0.0009 (6)	0.0043 (6)
C12	0.0224 (7)	0.0255 (8)	0.0245 (8)	−0.0056 (6)	0.0059 (6)	0.0024 (6)
C13	0.0187 (6)	0.0251 (7)	0.0208 (7)	−0.0033 (6)	0.0001 (6)	0.0015 (6)
C14	0.0211 (7)	0.0235 (7)	0.0187 (7)	−0.0015 (6)	0.0020 (6)	−0.0003 (6)

Geometric parameters (Å, º)

O1—C8	1.4163 (17)	C7—H7A	0.9900
O1—C7	1.4180 (18)	C7—H7B	0.9900
O2—C9	1.4175 (17)	C8—C9	1.510 (2)
O2—C10	1.4228 (17)	C8—H8A	0.9900
O3—C11	1.4140 (18)	C8—H8B	0.9900
O3—C12	1.4203 (17)	C9—H9A	0.9900
O4—C14^i	1.4149 (18)	C9—H9B	0.9900
O4—C13	1.4302 (18)	C10—C11	1.495 (2)
C1—C6	1.390 (2)	C10—H10A	0.9900
C1—C2	1.392 (2)	C10—H10B	0.9900
C1—H1	0.9500	C11—H11A	0.9900
C2—C3	1.388 (2)	C11—H11B	0.9900
C2—C14	1.514 (2)	C12—C13	1.493 (2)
C3—C4	1.389 (2)	C12—H12A	0.9900
C3—H3	0.9500	C12—H12B	0.9900
C4—C5	1.384 (2)	C13—H13A	0.9900
C4—H4	0.9500	C13—H13B	0.9900
C5—C6	1.394 (2)	C14—O4^i	1.4150 (18)
C5—H5	0.9500	C14—H14A	0.9900
C6—C7	1.508 (2)	C14—H14B	0.9900
C8—O1—C7	113.87 (11)	O2—C9—H9A	109.9
C9—O2—C10	110.54 (11)	C8—C9—H9A	109.9
C11—O3—C12	111.74 (11)	O2—C9—H9B	109.9
C14^i—O4—C13	112.25 (11)	C8—C9—H9B	109.9
C6—C1—C2	120.96 (13)	H9A—C9—H9B	108.3
C6—C1—H1	119.5	O2—C10—C11	109.67 (11)
C2—C1—H1	119.5	O2—C10—H10A	109.7
C3—C2—C1	118.97 (13)	C11—C10—H10A	109.7
C3—C2—C14	119.48 (13)	O2—C10—H10B	109.7
C1—C2—C14	121.52 (13)	C11—C10—H10B	109.7
C2—C3—C4	120.51 (14)	H10A—C10—H10B	108.2
C2—C3—H3	119.7	O3—C11—C10	109.43 (12)
C4—C3—H3	119.7	O3—C11—H11A	109.8
C5—C4—C3	120.21 (14)	C10—C11—H11A	109.8
C5—C4—H4	119.9	O3—C11—H11B	109.8
C3—C4—H4	119.9	C10—C11—H11B	109.8
C4—C5—C6	119.98 (14)	H11A—C11—H11B	108.2
C4—C5—H5	120.0	O3—C12—C13	109.31 (12)
C6—C5—H5	120.0	O3—C12—H12A	109.8
C1—C6—C5	119.37 (13)	C13—C12—H12A	109.8
C1—C6—C7	120.01 (13)	O3—C12—H12B	109.8
C5—C6—C7	120.49 (13)	C13—C12—H12B	109.8
O1—C7—C6	109.81 (12)	H12A—C12—H12B	108.3
O1—C7—H7A	109.7	O4—C13—C12	108.65 (13)
C6—C7—H7A	109.7	O4—C13—H13A	110.0
O1—C7—H7B	109.7	C12—C13—H13A	110.0
C6—C7—H7B	109.7	O4—C13—H13B	110.0
H7A—C7—H7B	108.2	C12—C13—H13B	110.0
O1—C8—C9	105.05 (11)	H13A—C13—H13B	108.3
O1—C8—H8A	110.7	O4^i—C14—C2	113.47 (12)
C9—C8—H8A	110.7	O4^i—C14—H14A	108.9
O1—C8—H8B	110.7	C2—C14—H14A	108.9
C9—C8—H8B	110.7	O4^i—C14—H14B	108.9
H8A—C8—H8B	108.8	C2—C14—H14B	108.9
O2—C9—C8	108.98 (11)	H14A—C14—H14B	107.7
C6—C1—C2—C3	0.7 (2)	C7—O1—C8—C9	−174.42 (12)
C6—C1—C2—C14	−177.28 (13)	C10—O2—C9—C8	179.85 (12)
C1—C2—C3—C4	−0.6 (2)	O1—C8—C9—O2	175.94 (11)
C14—C2—C3—C4	177.38 (14)	C9—O2—C10—C11	−176.43 (12)
C2—C3—C4—C5	0.8 (2)	C12—O3—C11—C10	177.09 (12)
C3—C4—C5—C6	−1.0 (2)	O2—C10—C11—O3	67.86 (15)
C2—C1—C6—C5	−0.9 (2)	C11—O3—C12—C13	175.10 (12)
C2—C1—C6—C7	174.90 (13)	C14^i—O4—C13—C12	−164.85 (12)
C4—C5—C6—C1	1.1 (2)	O3—C12—C13—O4	66.53 (15)
C4—C5—C6—C7	−174.74 (13)	C13—O4—C14^i—C2^i	76.52 (15)
C8—O1—C7—C6	148.79 (13)	C3—C2—C14—O4^i	163.19 (13)
C1—C6—C7—O1	152.98 (13)	C1—C2—C14—O4^i	−18.83 (19)
C5—C6—C7—O1	−31.26 (19)

Symmetry code: (i) −x, −y, −z+1.