12-crown-3 Type Li+ Complexes - Structures of 3,3-dimethyl-1,5,9-trioxacyclododecane Lithium Perchlorate and 3-methoxymethyl-3-methyl-1,5,9-trioxacyclododecane Lithium Perchlorate Hemihydrate

The crystal structures of the two title Li+ complexes, C-11 H22 O3 . LiClO4 (III) and C-12 H24 O4 . LiClO4 . 0.5H2O (IV), have been determined by X-ray diffractometry. The structure of III was refined to R = 0.080 for 1214 observed reflections, that of IV, with two independent complex entities, to R = 0.057 for 1372 reflections. The conformation of the 12-crown-3 ring is the same in III and IV; both follow, as predicted, the diamond lattice, and this is true also for the side-arm in IV. The cation is tetracoordinated in III but, as expected, pentacoordinated for the side-armed ligand IV. The large differences observed in complex stabilities for III and the previously studied complexes, I (the unsubstituted 12-crown-3) and II (the hexamethyl-substituted 12-crown-3), can be correlated to the gauche torsion angles in the C-C bonds of the ring, the quaternary carbons introducing conformational rigidity.