Structure and dynamics of binary and ternary lanthanide(iii) and actinide(iii) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA) complexes. Part 2, the structure and dynamics of binary and ternary complexes in the Y(iii)/Eu(iii) –TTA – tributylphosphate (TBP) system in chloroform as studied by NMR spectroscopy

Abstract

The stoichiometric reaction mechanisms, rate constants and activation parameters for inter- and intramolecular ligand exchange reactions in the binary Y/Eu(TTA)₃(OH₂)₂–HTTA and the ternary Y/Eu(TTA)₃(OH₂)₂–TBP systems have been studied in chloroform using ¹H and ³¹P NMR methods. Most complexes contain coordinated water that is in very fast exchange with water in the chloroform solvent. The exchange reactions involving TTA/HTTA and TBP are also fast, but can be studied at lower temperature. The rate constant and activation parameters for the intramolecular exchange between two structure isomers in Y(TTA)₃(OH₂)₂ and Y(TTA)₃(TBP)(OH₂) were determined from the line-broadening of the methine protons in coordinated TTA. The rate equations for the intermolecular exchange between coordinated TTA and free HTTA in both complexes are consistent with a two-step mechanism where the first step is a fast complex formation of HTTA, followed by a rate determining step involving proton transfer from coordinated HTTA to TTA. The rate constants for both the inter- and intramolecular exchange reactions are significantly smaller in the TBP system. The same is true for the activation parameters in the Y(TTA)₃(OH₂)₂–HTTA and the ternary Y/Eu(TTA)₃(TBP)(OH₂)–HTTA systems, which are ΔH^≠ = 71.8 ± 2.8 kJ mol⁻¹, ΔS^≠ = 62.4 ± 10.3 J mol⁻¹ K⁻¹ and ΔH^≠ = 38.8 ± 0.6 kJ mol⁻¹, ΔS^≠ = −93.0 ± 3.3 J mol⁻¹ K⁻¹, respectively. The large difference in the activation parameters does not seem to be related to a difference in mechanism as judged by the rate equation; this point will be discussed in a following communication. The rate and mechanism for the exchange between free and coordinated TBP follows a two-step mechanism, involving the formation of Y(TTA)₃(TBP)₂.