Trapping Cyclic Oxocarbon Dianions in the Crystalline State

2 Trapping Cyclic Oxocarbon Dianions in the Crystalline State 

Two independent thiourea molecules C(l) and C(2) are alternately linked together by pairs of N-H- S hydrogen bonds in a shoulder-to-shoulder fashion to form zigzag and puckered ribbons directed along the a axis. With these ribbons arranged side by side, atom S(3) of molecule C(l) forms two acceptor hydrogen bonds with atoms N(2) and N(l) of molecule C(3) belonging to the adjacent ribbon, such that this type of 

The most prominent structural feature of these three channel-type inclusion complexes is their analogous linkage mode of thiourea molecules which leads to the same type of 

The well-ordered tetra-n-butylammonium cations (represented by large shaded spheres for clarity) are arranged in two parallel columns in the [iTO] direction. Half of the phenylurea molecules reach out of the plane of each rhodizonate-phenylurea ribbon, functioning as spacers which effectively separate the double cationic columns. The cross-section of the parallelogram-shaped channel has dimensions of about 11.6 A x 19.2 A. 

This study provides the first reasonably precise molecular dimensions of the rhodi-zonate dianion, which lends support to the aromaticity of this nonbenzenoid cyclic oxocarbon. Notably, the measured C-C bond lengths [1.421(5) — 1.458(5) A] of the rhodizonate in 21, which exhibits approximate D(,h molecular symmetry, are significantly shorter than the corresponding values (1.488 and 1.501 A) in Rb2C60e [46] and the calculated values (1.500 and 1.501 A) for the C2 structure of this dianion [47]. Compound 21 provides yet another example of the use of urea and its derivatives for stabilizing elusive molecular anions such as allophanate [5h] and dihydrogen borate [5c] in a hydrogen-bonded host lattice. 

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