Uranyl Complexes of Related Tripodal 1,2,3-Triazole-Containing Ligands on the Ph3P(O) Platform. Structural Features in Solutions

The study addresses the effect of the structures of two related tripodal ligands differing in the binding mode of the triazole moiety and the linker length, L1 = {2-[(1-Ph-1,2,3-triazol-4-yl)CH2O]C6H4}3P(O) and L2 = {2-[(4-Ph-1,2,3-triazol-1-yl)CH2CH2O]C6H4}3P(O), on their coordination and extraction properties towards [UO2]2+. The structure of complexes [UO2(NO3)2L1] (I) and [UO2(NO3)2L2] (II) was studied in the solid state (elemental analysis, IR and Raman spectroscopy) and in solution (IR and multinuclear (1H, 13C, 31H) NMR spectroscopy). According to the body of spectral and quantum chemical data, both ligands are coordinated in the bidentate fashion in solid uranyl complexes, with L1 being P(O),N3-coordinated and L2 being P(O),N2 coordinated. The possibility of equilibria in solutions of I and II was evaluated at the ZORA-PBE0/ZORA-Def2-TZVP, ZORA-SARC-TZVP level of theory using the CPCM solvation model (MeCN). According to experimental and calculated data, the major components of complex I in CD3CN and CDCl3 are neutral species with the P(O),N3-coordinated ligand, while minor components are the neutral complex with the P(O)-coordinated ligand and ionic complexes. The equilibrium shifts when the solvent is replaced. In a solution of II in CD3CN, neutral species with P(O)-coordinated ligand occur in equilibrium with ionic complexes (as contact ion pairs), with the latter predominating. In CDCl3, the equilibrium is more complicated: apart from the above species, new ionic complexes appear in the equilibrium as solvent-separated and partly dissociated ion pairs, with their total content being more than a half. In acetonitrile solutions, the formation of intramolecular Ctr–H…OU, C–H…OU, and Ctr–H…ONO3 H-bonds was established in complexes I and II. The calculated energy of H-bonds is in good agreement with published data. In CDCl3 solutions, no such H-bonds were observed due to specific solvation. The difference between the requirements of the coordination polyhedra of lanthanide and uranyl cations to the structures of L1 and L2 for complex formation is analyzed. Data on the structures of complexes I and II in acetonitrile were correlated with the efficiency of U(VI) extraction with L1 and L2 from aqueous phase into 1,2-dichloroethane. Similar data for lanthanum complexes III and IV and the extraction efficiency of Eu(III) were used for comparison. Due to specific solvation, chloroform was not used as a reference solvent.