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

Abstract

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, H>sub>2L¹ = N,N′-bis(α-methylsalicylidene)-1,3-propanediamine and H>sub>2}L² = 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.