Oxidation of the 14-Membered Macrocycle Dibenzotetramethyltetraaza[14]annulene upon Ligation to the Uranyl Ion.

Reaction of Li(tmtaa) (tmtaaH = dibenzotetramethyltetraaza[14]annulene) with 1 equiv of [UOCl(THF)], in an attempt to form cis-[UO(tmtaa)], affords the bis(uranyl) complex [Li(THF)][Li(THF)][(UOCl)(tmtaa)] (1) as a red-brown crystalline solid in modest yield. Complex 1 can be synthesized rationally by reaction of Li(tmtaa) with 2 equiv of [UOCl(THF)]. Under these conditions, it can be isolated in 44% yield. In the solid state, complex 1 features two [UOCl] fragments that are bridged by a highly puckered (tmtaa) ligand. Both uranyl fragments feature normal uranyl metrical parameters (U-O (av.) = 1.78 Å, O-U-O = 176.8(3)° and 178.0(3)°). The most notable structural feature of 1, however, is the presence of a lithium cation that coordinates to an oxo ligand from each uranyl fragment. In contrast to the Li(tmtaa) reaction, addition of [K(DME)][tmtaa] to 1 equiv of [UOCl(THF)] results in formation of the 2e oxidation products of (tmtaa). Three isomers of CHN (compounds 2, 3, and 4) were isolated as a mixture of orange crystals in 41% combined yield. All three isomers were characterized by X-ray crystallography. We hypothesize that these ligand oxidation products are formed upon decomposition of the unobserved cis uranyl intermediate, cis-[UO(tmtaa)], which undergoes a facile intramolecular redox reaction.