Dinuclear Trivalent and Mixed-Valence Uranium [(−CH2−)5]4-calix[4]tetrapyrrole Complexes with Short Intermetallic Distances

A dinuclear trivalent uranium complex ( 1 ) and a mixed-valence U(III)/U(IV) complex ( 2 ) have been prepared by using the [(−CH 2 -) 5 ] 4 -calix[4]tetrapyrrole tetraanion. Both compounds display unusually short U−U distances with two metals held together by the bridging tetraanion, where each pyrrole ring adopts the η 1:η 5 bonding mode. Both magnetic measurements at variable temperature and DFT calculations carried out on the trivalent 1 indicate the existence of antiferromagnetic coupling between the two metal centers as the only significant U−U interaction.