Inorganic Chemistry, Vol.57, No.14, 8317-8324, 2018
Oxidation of the 14-Membered Macrocycle Dibenzotetramethyltetraaza[14]annulene upon Ligation to the Uranyl Ion
Reaction of Li-2(tmtaa) (tmtaaH(2) = 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 Li-2(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 angstrom, O-U-O = 176.8(3)degrees and 178.0(3)degrees). 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-2(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.