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.