Sterically congested uranyl complexes with seven-coordination of the UO2 unit:: the peculiar ligation mode of nitrate in [UO2(NO3)2(Rbtp)] complexes

Addition of 1 or 2 molar equiv of Rbtp [Rbtp = 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine; R = Me, Pr-n to UO2(OTf)(2) in anhydrous acetonitrile gave the neutral compounds [UO2(OTf)2(Rbtp)] [R = Me (1), nPr (2)] and the cationic complexes [UO2(Rbtp)(2)][OTf](2) [R = Me (3), Pr-n (4)], respectively. No equilibrium between the mono and bis(Rbtp) complexes or between [UO2(RbtP)(2)][OTf](2) and free Rbtp in acetonitrile was detected by NMR spectroscopy. The crystal structures of 1 and 3 resemble those of their terpyridine analogues, and 3 is another example of a uranyl complex with the uranium atom in the unusual rhombohedral environment. In the presence of 1 molar equiv of Rbtp in acetonitrile, UO2(NO3)(2) was in equilibrium with [UO2(NO3)(2)(Rbtp)] and the formation of the bis adduct was not observed, even with an excess of Rbtp. The X-ray crystal structures Of [UO2(NO3)(2)(Rbtp)] [R = Me (5), Prn (6)] reveal a particular coordination geometry with seven coordinating atoms around the UO2 fragment. The large steric crowding in the equatorial girdle forces the bidentate nitrate ligands to be almost perpendicular to the mean equatorial plane, inducing bending of the UO2 fragment. The dinuclear oxo compound [U(CyMe(4)btbP)(2)(mu-O)UO2(NO3)(3)][OTf] (7), which was obtained fortuitously from a 1:21 mixture of U(OTf)(4), CyMe(4)btbp, and UO2(NO3)(2) [CyMe(4)btbp = 6,6'-bis-(3,3,6,6-tetramethyl-cyclohexane-1,2,4-triazin-3-yl)-2,2'-bipyridine] is a very rare example of a mixed valence complex involving covalently bound U-IV and U-VI ions; its crystal structure also exhibits a seven coordinate uranyl moiety, with one bidentate nitrate group almost parallel to the UO2 fragment. The distinct structural features of [UO2(k(2)-NO3)(2)(Mebtp)], with its high coordination number and a noticeable bending of the UO2 fragment, and of [UO2(K-2-NO3)(k(1)-NO3)(terpy)], which displays a classical geometry, were analyzed by Density Functional Theory, considering the bonding energy components and the molecular orbitals involved in the interaction between the uranyl, nitrate, and Mebtp or terpy moieties. The unusual geometry of the Mebtp derivative with the seven coordinating atoms around the U02 fragment was found very stable. In both the Mebtp and terpy complexes, the origin of the interaction appears to be primarily steric (Pauli repulsion and electrostatic); this term represents 62-63% of the total bonding energy while the orbital term contributes to about 37-38%.