Macrocyclic receptor molecules for guanidinium cations. 2. Synthesis and x-ray structures of 2,6-pyrido crown ethers and their guanidinium complexes

2,6-Pyrido crown ethers I (n = 0-6) were synthesized and their complexation with guanidinium salts studied by using two phase extn. expts. The crystal and mol. structures of the complexes and the free 2,6-pyrido-24-crown-8 were detd. by x-ray crystallog. In the free and protonated crown ethers, the cavity is occupied by the arom. ring of the crown ether. In the complexes with guanidinium salts the guest mol. is encapsulated within the cavity, using all its hydrogen atoms for H-bonding between host and guest. The best fit for the guanidinium ion is found for the 27-membered macrocycle with a complementary host-guest H-bonding scheme. Crown ethers with 18 ring atoms form perching complexes.