Perturbation of the O–U–O Angle in Uranyl by Coordination to a 12-Membered Macrocycle

Reaction of [UO2Cl2(THF)2]2 (THF = tetrahydrofuran) with 2 equiv of HN4 (HN4 = 2,11-diaza­[3,3]­(2,6) pyridinophane) or MeN4 (MeN4 = N,N′-dimethyl-2,11-diaza­[3,3]­(2,6) pyridinophane), in MeCN, results in the formation of UO2Cl2(RN4) (R = H; 1; Me, 2), which were isolated as yellow-orange solids in good yields. Similarly, reaction of UO2(OTf)2­(THF)3 with HN4 in MeCN results in the formation of UO2(OTf)2­(HN4) (3), as an orange powder in 76% yield. Finally, reaction of UO2(OTf)2­(THF)3 with MeN4 in THF results in the formation of [UO2(OTf)­(THF)­(HN4)]­[OTf] (4), as an orange powder in 73% yield. Complexes 1–4 were fully characterized, including characterization by X-ray crystallography. These complexes exhibit the smallest O–U–O bond angles measured to date, ranging from 168.2(3)° (for 2) to 161.7(5)° (for 4), a consequence of an unfavorable steric interaction between the oxo ligands and the macrocycle backbone. A Raman spectroscopic study of 1–4 reveals no correlation between O–U–O angle and the UO νsym mode. However, complexes 1 and 2 do feature lower UO νsym modes than complexes 3 and 4, which can be rationalized by the stronger donor strength of Cl– versus OTf–. This observation suggests that the identity of the equatorial ligands has a greater effect on the UO νsym frequency than does a change in O–U–O angle, at least when the changes in the O–U–O angles are small.