Structure and dynamics of binary and ternary lanthanide(iii) and actinide(iii) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA) – tributylphosphate (TBP) complexes. Part 3, the structure, thermodynamics and reaction mechanisms of 8- and 9-coordinated binary and ternary Y-TTA-TBP complexes studied by quantum chemical methods

Abstract

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)₃(OH₂)₂ and Y(TTA)₃(TBP)(OH₂) and intermolecular exchange between free and coordinated HTTA in Y(TTA)₃(OH₂)(HTTA) and Y(TTA)₃(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol⁻¹. 9-coordinated intermediates play an important role in all reactions. Y(TTA)₃(OH₂)₃ has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol⁻¹, in good agreement with the experimental activation enthalpy of 19.6 kJ mol⁻¹. The mechanism for the intramolecular exchange between structure isomers in Y(TTA)₃(OH₂)₂ involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol⁻¹, respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)₃(HTTA) and Y(TTA)₃(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol⁻¹. The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.