A nuclear magnetic resonance study of trimethyl phosphate exchange on the hexakis(trimethyl phosphate)scandium(III) ion

Abstract

¹ H N.m.r. lineshape analysis methods have been used to investigate the exchange of tmp (trimethyl phosphate) on [Sc(tmp)₆]³⁺ in CD₃CN, sym-C₂H₂Cl₄, and CD₃NO₂ solvents. In the first two solvents the rate of tmp exchange on [Sc(tmp)₆]³⁺ is virtually independent of [tmp] and, typically, k_ex(300 K)= exchange rate/6[Sc(tmp)₆³⁺]= 65.7 ± 2.6 s^–1, ΔH^‡= 29.8 ± 0.4 kJ mol^–1, and ΔS^‡=–111 ± 2 J K^–1 mol^–1 when [Sc(tmp)₆³⁺], [tmp], and [CD₃CN] are 0.110 0, 0.693, and 15.0 mol dm^–3 respectively. In CD₃NO₂ solvent the exchange rate = 6k′_ex[Sc(tmp)₆³⁺][tmp] and k′_ex(300 K)= 51.3 ± 1.8 dm³ mol^–1 s^–1, ΔH^‡= 26.0 ± 0.9 kJ mol^–1, and ΔS^‡=–126 ± 3 J K^–1 mol^–1. The mechanistic implications of these rate laws are discussed.