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

Reaction of Li2(tmtaa) (tmtaaH2 = dibenzotetramethyltetraaza[14]annulene) with 1 equiv of [UO2Cl2(THF)3], in an attempt to form cis-[UO2(tmtaa)], affords the bis(uranyl) complex [Li(THF)3][Li(THF)2][(UO2Cl2)2(tmtaa)] (1) as a red-brown crystalline solid in modest yield. Complex 1 can be synthesized rationally by reaction of Li2(tmtaa) with 2 equiv of [UO2Cl2(THF)3]. Under these conditions, it can be isolated in 44% yield. In the solid state, complex 1 features two [UO2Cl2] fragments that are bridged by a highly puckered (tmtaa)2- 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 Li2(tmtaa) reaction, addition of [K(DME)]2[tmtaa] to 1 equiv of [UO2Cl2(THF)3] results in formation of the 2e- oxidation products of (tmtaa)2-. Three isomers of C22H22N4 (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-[UO2(tmtaa)], which undergoes a facile intramolecular redox reaction.