A proton magnetic resonance study of ligand exchange on the hexakis(1,1,3,3-tetramethylurea)scandium(III) ion and its NN-dimethylacetamide analogue

Abstract

Proton n.m.r. spectroscopic studies show that the rate law for tetramethylurea (tmu) exchange on the hexakis-(tetramethylurea)scandium(III) ion is: rate = 6k₁[Sc(tmu)₆³⁺] where, typically, k₁(300 K)= 0.26 ± 0.03 s_–1, ΔH^‡₁= 91.2 ± 2.3 kJ mol^–1 and ΔS^‡₁= 47.8 ± 6.7 J K^–1 mol^–1 in CD₃NO₂ diluent, while k₁(300 K)= 1.08 ± 0.05 s^–1, ΔH^‡₁= 68.6 ± 1.3 kJ mol^–1, and ΔS^‡₁=–15.7 ± 3.8 J K^–1 mol^–1 in CD₃CN diluent. In contrast, the rate law for ligand exchange on the hexakis (NN-dimethylacetamide)scandium(III) ion is: rate = 6(k₁+k₂[dma])[Sc(dma)₆³⁺] in CD₃NO₂ and CD₃CN diluents. For the nitromethane system, k₁(300 K)= 4.6 ± 0.3 s^–1, ΔH^‡₁= 30.3 ± 2.0 kJ mol^–1, and ΔS^‡₁=–132 ± 6 J K^–1 mol^–1 and k₂(300 K)= 112 ± 3 dm³ mol^–1 s^–1, ΔH^‡₂= 26.0 ±0.6 kJ mol^–1, and ΔS^‡₂=–119 ± 2 J K^–1 mol^–1. Similar parameters characterize the acetonitrile solutions. Ligand exchange in solutions of either [Sc(dmf)₆][ClO₄]₃ or [Sc(dmso)₆][ClO₄]₃ lies in the fast-exchange limit of the n.m.r. time scale. The mechanistic implications of these data are discussed and comparisons are made with other tripositively charged metal complex systems.