Use of Metalloligands [CuL] (H₂L = Salen Type Di-Schiff Bases) in the Formation of Heterobimetallic Copper(II)-Uranyl Complexes: Photophysical Investigations, Structural Variations, and Theoretical Calculations

Five heterobimetallic copper­(II)–uranium­(VI) complexes [(CuL¹)­UO₂(NO₃)₂] (1), [{CuL¹(CH₃CN)}­UO₂(NO₃)₂] (2), [{CuL¹(CH₃COCH₃)}­UO₂(NO₃)₂] (3), [{CuL²(CH₃CN)}­UO₂(NO₃)₂]­(4), and [{CuL²(CH₃COCH₃)}­UO₂(NO₃)₂]­[{CuL²}­UO₂(NO₃)₂] (5) have been synthesized by reacting the Cu­(II)-derived metalloligands [CuL¹] and [CuL²] (where, H₂L¹ = N,N′-bis­(α-methylsalicylidene)-1,3-propanediamine and H₂L² = N,N′-bis­(salicylidene)-1,3-propanediamine) with UO₂(NO₃)₂·6H₂O in 1:1 ratio by varying the reaction temperature and solvents. Absorption and fluorescence quenching experiments (steady-state and time-resolved) indicate the formation of 1:1 ground-state charge transfer copper­(II)–uranium­(VI) complexes in solution. X-ray single-crystal structure reveals that each complex contains diphenoxido bridged Cu­(II)–U­(VI) dinuclear core with two chelated nitrato coligands. The complexes are solvated (acetonitrile or acetone) in the axial position of the Cu­(II) in different manner or desolvated. The supramolecular interactions that depend upon the co-ordinating metalloligands seem to control the solvation. In complexes 2 and 3 a rare NO₃^–···NO₃^– weak interaction plays an important role in forming supramolecular network whereas an uncommon UO···NO₃^– weak interaction helps to self-assemble heterobinuclear units in complex 5. The significance of the noncovalent interactions in terms of energies and geometries has been analyzed using theoretical calculations.