Affinity of An(VI) for N4-Tetradentate Donor Ligands: Complexation of the Actinyl(VI) Ions with N4-Tetradentate Ligands

In this report the affinity of four N₄-tetradentate ligands that incorporate the 2-methylpyridyl functionality with hexavalent actinides $(mathrm{AnO}_{2}^{2+})$ has been investigated in methanol solution. The ligands studied include N,N??-bis(2-methylpyridyl)diaminoethane (BPMDAE), N,N??-bis(2-methylpyridyl)-1,3-diaminopropane (BPMDAP), N,N??-bis(2-pyridylmethyl)piperazine (BPMPIP), and trans-N,N-bis(2-pyridylmethyl)-1,2-diaminocyclohexane (BPMDAC). Conditional stability constants describing the strength of the interaction were determined by UV?Cvisible spectrophotometry. The log₁₀ K ₁₀₁ values for both U(VI) and Pu(VI) are comparable and show the same trend of stability with ligand structure. Dinuclear complexes are also indicated as being important. The log₁₀ K ₂₀₁ values for Pu(VI) complexation with the N₄-ligands are identical for the four ligands (within experimental error), indicating that the structure of the ligand backbone has little effect on the stability of the (PuO₂)₂L²⁺ complex. The exception to this trend is the behavior of N,N??-bis(2-pyridylmethyl)piperazine (BPMPIP) with Pu(VI). This ligand displays a tendency to reduce Pu(VI) within the experimental time frame of 45 minutes. BPMPIP is the only ligand tested that contains tertiary amines in the ligand backbone. The decomposition of BPMPIP by Pu(VI) suggests a susceptibility of tertiary amines to oxidative degradation.