Synthesis and characterization of Cu(II) paddlewheel complexes possessing fluorinated carboxylate ligands

The goal of this study was to establish the relationship between the 19F NMR line broadening and the varying distance between the 19F nucleus and copper(II) ion, with the aim of gathering data that can be used to interpret 19F NMR spectra of subsequent fluorine-labeled, copper-binding proteins. Fluorinated alkyl and aryl copper(II) carboxylates were synthesized from fluorinated carboxylic acids and Cu(OH)2. The copper(II) carboxylates were characterized using 19F NMR, IR, and single crystal X-ray diffraction. In the alkyl carboxylate compounds, the line broadening and chemical shift lessened with increased distance between the fluorine atom and the copper ions; however, in the aryl carboxylate derivatives, increased distance was not a factor in the amount of line broadening or change in chemical shift between the acid and metal salt. The compound, bis(3-(trifluoromethyl)butyrate) copper(II) (5) was found to possess the optimum combination of decreased line broadening and increased chemical shift sensitivity in 19F NMR. The crystal structures obtained for compounds 1, 2, 4, and 6 were analogous to previous copper(II) carboxylate complexes, though it is noted that compound 6, bis(5,5,5-trifluoropentanoate) copper(II) assumes a tetrameric structure lacking apical ligands, and thus enables the formation of an extended network of near-neighbor copper(II) ions.