Reduction and desymmetrisation of the uranyl dication in a macrocyclic framework

The transamination reaction between a Schiff base polypyrrolic macrocycle, H4Ltet/oct, where tet = tetramethyl (C38H36N8), oct = octamethyl (C42H44N8), and [UO2(THF)2{N(SiMe3)2}] results in the sole formation of mono uranyl complexes [UO2(THF)(H2Ltet)], 7, and [UO2(THF)(H2Loct)], 8. The molecular structure of 8 was confirmed by an X-ray diffraction study which shows that the macrocycle folds to form a Pac-man shape. The reaction between 7 and [M{N(SiMe3)2}2], where M = Mn, Fe, Co or Zn, results in the formation heterobimetallic complexes, [UO2(THF)M(THF)(Ltet)], 9, 10, 11, and 13, respectively. The structures of 9 and 11 have been confirmed by X-ray crystallography and show that there is a direct donor bond from one oxo ligand of the uranyl dication to the transition metal, and characterisation by vibrational spectroscopy suggests that the bonding of the uranyl dication has weakened. The double deprotonation of 8 with KN(SiMe2R)2, where R = Me, Ph, and subsequent salt elimination reaction with MX2, where M = Fe, X = I and M = Zn, X = I, Cl, results in the formation of the first discrete reductively functionalised pentavalent uranyl complexes [UO(OSiMe3)(THF)(FeI)2(Loct)], 17, [UO(OSiMe2Ph)(THF)(FeI)2(Loct)], 18, [UO(OSiMe3)(THF)(ZnI)2(Loct)], 19, and [UO(OSiMe3)(THF)(ZnCl)2(Loct)], 20, which contain a covalent Si–O bond to one oxo-group. Complexes 17 to 20 have been fully characterised and the solid state molecular structures of 17 and 19 were determined. Investigation into the mechanism of the functionalisation suggests that the intermediate complex [UO2(THF)(K2L)] is highly oxidising and reactive and promotes the single electron transfer reaction with trimethylsilyl reagents and results in the homolytic cleavage of bonds and concurrent reduction of the uranyl ion. The solution redox properties of 8 have been measured by cyclic voltammetry, and exhibits a single electron reduction at -1.17 V (vs. Fc/Fc+). The reaction of 8 with one equivalent of cobaltocene results in the formation of the pentavalent uranyl complex [CoCp2][UO2(THF)(H2Loct)], 26. The reaction of 8 with two or three equivalents of B(C6F5)3 results in the formation of [UO2(H2Loct)B(C6F5)3], 31, and [UO2(H2Loct)(B{C6F5}3)2], 34, respectively, which are examples of uranyl macrocyclic borane adducts. Reaction of complex 31 with an excess of PMe3 results in the formation of the THF-free uranyl macrocyclic complex [UO2(H2Loct)], 35