Nesting complexation of C-60 with large, rigid D-2 symmetrical macrocycles

A series of four chiral D-2 symmetrical macrocycles, in which two 3,3'-disubstituted Binol units are bridged by conjugated organic spacers of differing lengths and/or electronic properties, have been synthesized and characterized. The four different bridges consist of either ether or ester linkages in combination with either short biphenyl spacers or long diethynylphenyl spacers. NMR, CD spectroscopy, and molecular modeling help rationalize the shape of the cyclic scaffolds and even subtle modifications in the bridging units lead to drastic changes in conformation. The three macrocycles with longer bridging units and/or ester linkages form stable 1 : 1 complexes with C-60 in toluene. The one with a short spacer and ether linkage does not. The binding constants have been determined with a high degree of accuracy via equilibrium-restricted factor analysis; with long spacers and ester linkages log K-a = 4.37(2); with short spacers and ester linkages log K-a = 3.498(4); with long spacers and ether linkages log K-a = 3.509(2).