The exchange of dimethyl methylphosphonate on Pentakis(dimethyl methylphosphonate)dioxouranium(VI) ion

Abstract

The greatly predominant dioxouranium(V1) species existing in the presence of perchlorate anions in dilute dimethyl methylphosphonate (dmmp) solutions in CH₂Cl₂ diluent is shown from ¹H N.M.R. studies to be [U0₂(dmmp)₅]²⁺. Lineshape analyses of exchange-modified ¹H N.M.R. lineshapes show the rate of dmmp exchange to be consistent with the rate law: k_ex5[U0₂(dmmp)₅²⁺] over a 9.7-fold variation of [dmmp]. A typical set of rate parameters is k_ex(268 K) = 82 ± 5 s^-1, ΔH? = 57.3 ± 1.0 kJ mol^-1 and ΔS? = 6.3 ± 3.6 J k^-1 mol^-1 for a solution in which [U0₂(dmmp)₅²⁺], [dmmp] and [CH₂Cl₂] are respectively 0.104, 0.515 and 13.59 mol dm^-3. The mechanism of the exchange process, which is considered to be predominantly dissociative in nature, is discussed.