Complexes of Uranium(IV) with Acyclic Polyphenoxides Resulting from Cleavage of Homooxacalixarenes

Reaction of UCl4 with homooxacalixarenes in pyridine results in cleavage of the ether bridges upon nucleophilic attack by pyridine, likely assisted by the U(IV) ion. The zwitterionic diphenoxide species bis[5-tert-butyl-2-oxido-3-(1-pyridiniomethyl)phenyl]methane L1 formed from p-tert-butyltetrahomodioxacalix[4]arene, p-tert-butylhexahomotrioxacalix[6]arene or p-tert-butyloctahomotetraoxacalix[8]arene and the zwitterionic triphenoxide species 4-tert-butyl-2,6-bis[5-tert-butyl-2-oxido-3-(1-pyridiniomethyl)-benzyl]phenolate (L2)- formed from p-tert-butyltetrahomodioxacalix[6]arene give polyphenoxide complexes of U(IV). Coordination of L1 to UCl4 yields the neutral complex [UCl4(L1)] and, on further heating, the cationic [UCl2(L1)2]2+ species, whereas (L2)- gives only the neutral compd. [UCl3(L2)]. In these three complexes, the metal atom is in slightly distorted octahedral environments, with bonding to all phenoxide groups. These are the 1st instances of U(IV) complexes with polyphenoxide ligands, resulting from unprecedented homooxacalixarene cleavage.