ESR and pH-potentiometric study of the mixed–ligand complex formation in the copper(II)–4-fluorosalicylic acid–N,N-diethylnicotinamide system: Structure and spectral properties of [Cu(4-fluorosalicylate)2(N,N-diethylnicotinamide)2(H2O)2] complex

Abstract EPR simulation method together with pH-potentiometry combined with UV–Vis spectrophotometry were used for the study of the ternary system 4-fuorosalicylic acid (H A )– N , N -diethylnicotinamide ( B )–copper(II) in aqueous solution. The N , N -diethylnicotinamide ligand is a weak donor, its mixed–ligand complexes with 4-fluorosalicylate anions are more favoured. The number of coordinated N , N -diethylnicotinamide molecules increases with decreasing temperature: up to four ones were detected in the coordination sphere of copper(II) in frozen solutions. The formation of [CuH −1 AB 2 ] and [CuH −1 A ] was detected by all methods at neutral pH. At lower pH values, [Cu A 2 B 2 ] and [Cu B ] become dominant, and this fact is in good agreement with [Cu A 2 B 2 (H 2 O) 2 ] crystals obtained from similar solutions. The structural unit of the [Cu A 2 B 2 (H 2 O) 2 ] complex consists of a copper(II) ion, which is monodentately coordinated by a pair of 4-fluorosalicylate anions and by a pair of N , N -diethylnicotinamide in trans positions in the basal plane, and by two water molecules in the axial positions of a tetragonal bipyramid.