Synthesis and spectroscopic characterization of actinyl(VI) tetrahalide coordination compounds containing 2, 2′-bipyridine

A series of three new actinide tetrahalide coordination compounds containing 2,2'-bipyridinium (H(2)bpy) have been synthesized by room temperature evaporation and characterized via single crystal X-ray diffraction, Raman spectroscopy, and infrared spectroscopy. The solid-state compounds (Np1, U1, and U2) contain the AnO(2)(2+) cation coordinated to four halide (chloride or bromide) atoms to form the anionic tetrahalide molecular unit [AnO(2)X(4)](2-) (X = Cl, Br) and are charge balanced by the 2,2'-bipyridinium cations that engage in electrostatic interactions with the actinyl complex. Np1 (N2C10H10)[NpO2Cl4] contains 2,2'-bipyridine in the cis conformation where both the nitrogen atoms are directed towards the [NpO2O4](2-) monomer. Within U1 (N2C10H9)[UO2Cl4], the 2,2'-bipyridine is in a trans conformation, and found in the same orientation in the isomorphous structure U2(N2C10H9)[UO2Br4]. The compounds were further characterized by vibrational spectroscopy with specific interest in the actinyl cation. Raman spectroscopy provided information on the actinyl symmetric stretch (v(1)) whereas IR spectroscopy is used to determine the location of the asymmetric stretch (v(3)). Spectral similarities are observed between the two isomorphous structures, but slight variations across the series of structures are found to be associated with bipyridine bands in the various conformations. Additional analysis of these vibrational features in U1 and U2 provided information on the actinyl force constants, allowing insight into the relative uranyl bond strength that was compared to similar tetrahalide compounds.