Real-time Probing the Formation of [M(2,2-bipyridine)2(NO3)3] (M = Ce, La, Tb) Complexes and Influence of Synthesis Parameters

Despite the strong technological importance of lanthanide complexes, their formation processes are rarely investigated. This work is dedicated to determining the influence of synthesis parameters on the formation of [Ce(bipy)(2)(NO3)(3)] as well as Ce3+- and Tb3+-substituted [La(bipy)(2)(NO3)(3)] (bipy = 2,2 '-bipyridine) complexes. To this end, we performed in situ luminescence measurements, synchrotron-based X-ray diffraction (XRD) analysis, infrared spectroscopy (IR), and measured pH value and/or ion conductivity during their synthesis process under real reaction conditions. For the [Ce(bipy)(2)(NO3)(3)] complex, the in situ luminescence measurements initially presented a broad emission band at 490 nm, assigned to the 5d -> 4f Ce3+ ions within the ethanolic solvation shell. Upon the addition of bipy, a red shift to 700 nm was observed. This shift was attributed to the changes in the environment of the Ce3+ ions, indicating their desolvation and incorporation into the [Ce(bipy)(2)(NO3)(3)] complex. The induction time was reduced from 8 to 3.5 min, by increasing the reactant concentration by threefold. In contrast, [La(bipy)(2)(NO3)(3)] crystallized within days instead of minutes, unless influenced by high Ce3+ and Tb3+ concentrations. Monitoring and controlling the influence of the reaction parameters on the structure of emissive complexes is important for the development of rational synthesis approaches and optimization of their structure-related properties like luminescence.