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 [(−CH2-)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 η1:η5 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.