A nuclear magnetic resonance study on ligand exchange reactions in La(III), Nd(III), and U(VI) nitrato complexes with n-octyl(phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide in non-aqueous solvents

Abstract

The exchange reactions of n-octyl(phenyl)-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 of31P 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 of31P 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 ∆H≠(kJ mol-1) and ∆S≠(JK-1 mol-1) are 4.76X103, 28.7±0.1, -78.4±0.2 for La(III)-CMPO complex, 4.72xl03, 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. © 1995 Taylor & Francis Group, LLC.