Thermodynamics of metal-ligand bond formation. XI. Adducts of heterocyclic bases with copper(II) complexes of fluorinated β-diketones

Abstract

Thermodynamic data have been obtained for the reaction in benzene solution of copper(11) complexes of hexafluoropentanedione and three trifluorinated α-diketones with several unidentate and bidentate heterocyclic bases and terpyridine. With pyridine the hexafluoro  complex forms very stable 1 : 1 and 1 : 2 adducts. The enthalpy of adduct formation (-ΔH) is about 39 kJ mol^-1 for the addition of each molecule of pyridine, about 10 kJ mol^-1 more than for the formation of the 1 : 1 adduct with the acetylacetone complex. The trifluoro complexes form only 1 : 1 adducts with enthalpy of formation about 31 kJ mol^-1 ; these are much more stable than Cu(acac)₂(py), but the extra stability is mainly due to entropy factors. With bipyridine and phenanthroline all four fluoro complexes form very stable 1 : 1 adducts with enthalpies of formation about twice those for addition of one molecule of pyridine; this shows that two Cu-N bonds are formed. 2,9-Dimethyl-1,l0-phenanthroline forms a similar adduct, but the enthalpy of formation is much less, due to steric interference. Terpyridine forms a 1 : 1 adduct of much lower stability, probably behaving as an ortho-substituted bidentate ligand. Infrared spectra of the adducts of Cu(tfaa)₂ and Cu(hfaa)₂ support molecular structures in which one of the Cu-O bonds linking each β-diketone is lengthened to allow the formation of a strong pair of Cu-N bonds.