A Proton Magnetic Resonance Study of Ligand Exchange on the Pentakis (N-formylpyrrolidine)dioxouranium (VI) Ion

Abstract

Pentakis(N-formylpyrrolidine)dioxouranium(VI) perchlorate, [UO₂(fpr)₅] (ClO₄)₂, has been isolated from solution. In the presence of excess free fpr the greatly predominant species in CD₂Cl₂ solutions is [UO₂(fpr)₅]²⁺, as shown by ¹H N.M.R. spectroscopy, and fpr exchange is characterized by a two term rate law:

exchange rate = 5(k₁+ k₂[fpr])[UO₂(fpr)₅²⁺]

It is found that k₁(220 K) = 42·6±0·5 s^-1, ΔH^‡ = 30·7±1·4 kJ mol^-1 and ΔS^‡ = -71·6±6·3 J K^-1 mol^-1, and k₂(220 K) = 369±20 dm³ mol^-1 s^-1, ΔH^‡ = 30·5±1·0 kJ mol^-1 and ΔS^‡ = -54·9±5·0 J K^-1 mol^-1. The mechanistic implications of these data are discussed.