Dinuclear Trivalent and Mixed-Valence Uranium [(−CH<INF>2</INF>−)<INF>5</INF>]<INF>4</INF>-calix[4]tetrapyrrole Complexes with Short Intermetallic Distances

A dinuclear trivalent uranium complex (<BO>1</BO>) and a mixed-valence U(III)/U(IV) complex (<BO>2</BO>) have been prepared by using the [(−CH<INF>2</INF>-)<INF>5</INF>]<INF>4</INF>-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 <BO>1</BO> indicate the existence of antiferromagnetic coupling between the two metal centers as the only significant U−U interaction.