Coordination Chemistry of Uranium (U(IV) and -(VI)) with Bidentate N-donor Ligands, 2,2’-Bipyridine and 1,10-Phenanthroline

Because of their remarkable stability towards a wide variety of transition metals1, the bidentate N-donor ligands 2,2’-bipyridine (bipy) and 1,10-phenanthroline (phen) have attracted considerable attention in the field of coordination chemistry over the last decades. The coordination chemistry of uranium (U) with these N-donor ligands has been also explored primarily for its hexavalent state (U(VI) as UO22+), whilst much less attention has been paid for the lower oxidation states, such as tetravalent (U(IV)). Here we present a systematic study on the coordination chemistry of U(IV) and -(VI) with bipy and phen under different chemical conditions, such as different solvents and changing the metal / ligand ratio.
We succeeded to obtain a series of U(IV) complexes with the U:ligand ratio of 1:1 and 1:2, all showing the eight-fold coordination geometry of the uranium centre. In addition to the ligand, chloro and methanolato ligands are also coordinating to the metal centre for charge compensation. Interestingly, the complexation between U(IV) and the ligand does occur even in protic solvents, in which the ligand is expected to be protonated. We also obtained another series of U(VI) complexes with both bipy and phen, underlining the versatile coordination chemistry of uranyl (UO22+). That is, the coordination between uranyl and the ligand depends strongly on the pH of the solvent used. For instance, as shown in the right of Fig. 1, dinuclear uranyl arrangements with hydroxo-brinding are dominated in the media with higher pH. As illustrated in Fig. 1, bipy and phen are forming isostructural complexes both with U(IV) and- (VI). The electronic structure of the complexes is further studied by quantum chemical calculations.