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

Reaction of [UO2Cl,(THF)(2)](2) (THF = tetrahydrofuran) with 2 equiv of (H)N4 ((H)N4 = 2,11-diaza[3,3](2,6) pyridinophane) or (Me)N4 ((Me)N4 = N,N'-dimethyl-2,11-diaza[3,3] (2,6) pyridinophane), in MeCN, results in the formation of UO2Cl2((R)N4) (R = H; 1; Me, 2), which were isolated as yellow-orange solids in good yields. Similarly, reaction of UO2(OTf)(2)(THF)(3) with (H)N4 in MeCN results in the formation of UO2(OTf)(2)((H)N4) (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) ((H)N4)] [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)degrees (for 2) to 161.7(5)degrees (for 4), a consequence of an unfavorable steric interaction between the oxo ligands and the macrocyde backbone. A Raman spectroscopic study of 1-4 reveals no correlation between O-U-O angle and the U=O nu(sym) mode. However, complexes 1 and 2 do feature lower U=O nu(sym) 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 nu(sym) frequency than does a change in O-U-O angle, at least when the changes in the O-U-O angles are small.