Relativistic density functional theory has been applied to the uranyl(VI) and uranyl(V) complexes of unsubstituted (1) and dodeca-alkyl-substituted (2) isoamethyrin (hexaphyrin(18.104.22.168.0.0)). The experimentally observed bent conformation in the uranyl(VI) complex of 2 (Sessler, J. L. et al. Angew. Chem., Int. Ed. 2001, 40, 591) is reproduced accurately by the calculations. It is entirely due to the external alkyl substitutents; the unsubstituted complexes of 1 are planar. Complex geometry and stability are seen to be the result of two competing factors; aromatic stabilization favors a planar conformation of the macrocycle whereas the bending affords a much better fit between the cavity and the uranyl cation. The uranyl(VI) complex of 2 is more stable than that of 1 as a result; the trend is reversed for the larger uranyl(V) cation. An energy decomposition analysis shows that the differences between U(VI) and U(V) originate in the different capabilities of these cations for covalent and/or polarization interactions with the ligands rather than in sterical factors.
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