A Nuclear Magnetic Resonance Study on Ligand Exchange Reactions in La( m ), Nd( m ), and U( VI ) Nitrato Complexes with n-Octyl(phenyl)-N,N-Diisobutylcarbamoylmethylphosphine Oxide in Non-aqueous Solvents.

Abstract

The exchange reactions of n-octyl(pheny1)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in La(III), Nd(III), and U(VI) nitrate complexes with CMPO (La(III)-, Nd(III)-, and U(VI)-CMPO complexes) have been studied in CD3COCD3 by means of 31P NMR method. The number of CMPO coordinated to the first coordination sphere of La(III) ion was directly determined to be 3 by the area integrations of 31P NMR signals of free and coordinated CMPO molecules. The same coordination number of 3 was also obtained for the U(VI)-CMPO complex. The coordination number was not determined for the Nd(III)-CMPO complex, because of its paramagnetic behavior. The exchange rate constants of CMPO in La(III)- and U(VI)- CMPO complexes were obtained by the two-site exchange model. Paramagnetic line broadening was observed in the Nd(III)-CMPO complex and the rate constant for the exchange of CMPO was determined by the line-broadening method. The exchange rates of CMPO in La(III)- and Nd(III)-CMPO complexes depend on the free CMPO concentration ([CMPO]), while that in U(VI)-CMPO complex is independent of [CMPO]. The dissociative (D) and dissociative interchange (Id ) mechanisms were proposed for the exchange reactions in the La(III)- and Nd(III)-CMPO complexes, and dissociative (D) or Id mechanism was proposed for the U(VI)-CMPO complex. The dissociative rate constants (s−1) at 25°C and activation parameters ΔH# (kJ·mol−1) and ΔS# (J·K−1·mol−1) are 4.76x103, 28.7±0.1, −78.4±0.2 for La(III)-CMPO complex, 4.72x103, 42.6±0.4, −31.7±1.3 for Nd(III)-CMPO complex, and 3.20x103, 46.9±0.6, −20.5±2.2 for U(VI)-CMPO complex, respectively.