5,7,13,15-Tetraoxo-2,2,10,10-tetrakis(trifluoromethyl)-4,8,12,16-tetraoxa-1(1,4),3(1,4),6(1,2),9(1,4),11(1,4),14(1,2)-hexabenzenahexadecaphane tetrahydrofuran monosolvate

The title compound, C46H24F12O8·C4H8O, consists of a cyclic aryl ester dimer and a tetrahydrofuran molecule. In the structure of the cyclic dimer, one carbonyl group stretches above the cavity and the other below.


Qing-Zhong Guo and Yi Du Comment
Ring-opening polymerization (ROP) reactions constitute an important class of polymerization reactions. The advantages of using ROP of cyclic aryl oligomers to prepare high performance aromatic thermoplastics, such as polycarbonate and poly(aryl ester)s, have been widely recognized in recent years (Brunelle et al., 1990;Brunelle, 2008;Chan et al., 1995;Jiang et al. 1997a). It is generally believed that the ROP of aromatic cyclic oligomers is essentially thermoneutral and driven by entropy changes as the cyclic oligomers have big size with little or no ring strain. In order to obtain decisive evidence of the macrocyclic structure and investigate the nature of ROP, the single-crystal X-ray structure of cyclic ester dimer, the title compound, was determined.
The structure of cyclic dimer, cyclo-Bis[1,4-phenylene(hexafluoroisopropylidene)-phthalate (shown in Fig. 1) exhibits two types of conformation about ester groups. One of the carbonyl groups stretch above the cavity of the cyclic structure and the others stretch beneath the cavity. The interplanar dihedral angle of the phenyls attached to the hexafluoroisopropylidene is 69.67°. The distance between C(14) and its symmetrical carbon atom is 1.0729 nm. The bond angles at C7 -O1-C8 of 119.6° and O3-C23(O4)-C6 i of 111.0° are all close to the idealized values of 118. 8° and 111.7°, respectively (Jiang et al., 1997b;Coulter & Windle, 1989). The phenyl rings in cyclic dimer have a good planarity (root mean square deviations from the planarity of the phenyl planes are 0.00043, 0.00069 and 0.00053 nm, respectively).
Overall, X-ray analysis indicates that the cyclic dimer is constructed without severe internal strain. This result indicates that the ROP of cyclic aryl ester dimer is driven by entropy changes and provides a base for the study on the mechanism of ROP reaction and the relationship between the cyclic nature and ROP reaction.

Experimental
The cyclization reaction was conducted in a 500 ml threeneck round-bottom flask charged with 150 ml dichloromethane, 30 ml distilled water and 0.16 g cetyltrimethylammonium bromide at room temperature. A solution of phthaloyl dichloride (1.014 g, 5 mmol) in 50 ml dichloromethane and a solution of disodium salt of 4,4′-(hexafluoroisopropylidene) diphenol (1.682 g, 5 mmol) in 50 ml distilled water were delivered into the mechanically stirred flask in an equimolar fashion over an 8 h period. After the addition, the mixture was stirred for another 2 h to ensure complete reaction. The organic phase was separated by a separating funnel and extracted with distilled water three times and then evaporated to dryness. The colorless cyclic dimer was obtained by recrystallization from tetrahydrofuran (THF). The isolated yield of cyclic dimer was 1.3 g (54.7% yield). Colorless block crystals suitable for X-ray analysis were obtained by slow evaporation from a THF solution at room temperature for about one week.

Refinement
The H atoms were placed in idealized positions and allowed to ride on the relevant carbon atoms, with C-H = 0.93Å and Uiso(H) = 1.0Ueq(C) except for in THF, where C-H = 0.97 Å.

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq Occ. (