Tuning electronic structures of uranyl fluorides via increasing equatorial pyridyl number and extending pyridyl conjugation

Uranyl complexes, [(UO2)(py)F4]2– (1a, py = pyridine), [(UO2)(py)3(cis-F)2] (1a′), [(UO2)(py)5]2+ (1a″), [(UO2)(bpy)F3]– (2a, bpy = 2,2′-bipyridine), [(UO2)(tpy)F2] (3a, tpy = 2,2′:6′2″-terpyridine) and [(UO2)(qpy)F]+ (4a, qpy = 2,2′:6′,2″:6″,2‴-quaterpyridine) have been examined using scalar relativistic density functional theory (DFT). It is shown that both increasing the monopyridyl number (from 1a to 1a′ and 1a″) and extending the pyridyl conjugation (from 1a to 2a, 3a and 4a) are capable of tuning electronic structures of uranyl complexes. Unlike those of 1a, for instance, 4a is featured with π(qpy) character of HOMO and HOMO-1, and its σ(UO) bond is greatly stabilized to form HOMO-2; and more π∗(qpy)-type orbitals insert between U(f)-based and π∗(UO) unfilled orbitals. For comparison, fourfold uranyl complexes with one less equatorial fluorine ligand ([(UO2)(py)F3]– (1b), [(UO2)(bpy)F2] (2b), [(UO2)(tpy)F]+ (3b) and [(UO2)(qpy)]2+ (4b)) were calculated. Both thermodynamic and geometrical results suggest that polypyridyl (such as bpy, tpy and qpy) dioxouranium complexes favor five-coordinated mode in the equatorial plane, whereas fourfold is preferred by the single-pyridyl complex.