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

EPR simulation method together with pH-potentiometry combined with UV–Vis spectrophotometry were used for the study of the ternary system 4-fuorosalicylic acid (HA)–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−1AB2] and [CuH−1A] was detected by all methods at neutral pH. At lower pH values, [CuA2B2] and [CuB] become dominant, and this fact is in good agreement with [CuA2B2(H2O)2] crystals obtained from similar solutions. The structural unit of the [CuA2B2(H2O)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.

Graphical abstractSystems 4-fuorosalicylic acid (HA)–N,N-diethylnicotinamide (B)–copper(II) in aqueous solution have been studied by EPR, pH-potentiometry and UV–Vis spectra. The mixed–ligand complexes with 4-fluorosalicylate anions and N,N-diethylnicotinamide 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−1AB2] and [CuH−1A] was detected by all methods at neutral pH. At lower pH values, [CuA2B2] and [CuB] become dominant, and this is in good agreement with [CuA2B2(H2O)2] crystals obtained from similar solutions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► 4-Fluorosalicylic acid (HA) – N,N-diethylnicotinamide (B) – copper(II) equilibrium. ► The stability constants of the species including detected deprotonated ones. ► At lower pH values, [CuA2B2] and [CuB] are dominant. ► The [CuA2B2(H2O)2] crystal structure.