Calorimetric titration and hydrogen-1 nuclear magnetic resonance studies of adduct formation between cobalt(III)β-diketonates and a lanthanide shift reagent

Abstract

Titration calorimetry has been used to study the reaction of the lanthanide shift reagent [Eu(fod)₃], tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionato)europium(III), with tris(β-diketonato)cobalt(III) complexes of pentane-2,4-dione (Hacac), benzoylacetone (Hbzac), and dibenzoylmethane (Hdbm) in benzene. Complementary ¹H n.m.r. studies have also been performed. In accord with previous results, symmetrical 1 : 1 adduct formation involving three oxygen bridges to [Eu(fod)₃] occurs for the first two complexes. For [Co(acac)₃], a ΔH^⊖ value of –30.5 ± 1.2 kJ mol^–1 and a K value of 10⁴ dm³ mol^–1 were obtained. For fac-[Co(bzac)₃], the ΔH^⊖ value is approximately zero with K 10³ dm³ mol^–1; hence the aryl functions are quite effective in reducing the donor capacity of the bridging oxygens in this case. In contrast, the calorimetric and n.m.r. studies indicate that there is no adduct formation by [Co(dbm)₃], a result which apparently reflects steric shielding by the aryl groups of the oxygen-donor atoms of this complex.