Trivalent Actinide Ions Showing Tenfold Coordination in Solution

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)₃]³⁺ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)₃(NO₃)]²⁺. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)₃]³⁺ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)₃(X)]²⁺ was not observed for anions other than nitrate (X^- = NO₂^-, CN^-, or OTf^-). The formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex was studied as a function of NO₃^- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)₃]³⁺ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)₃(NO₃)]²⁺ only forms at nPr-BTP concentrations below 10^-4 mol/L whereas for concentrations greater than 10^-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)₃]³⁺ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)₃]³⁺ complex. Moreover, XPS on [M(nPr-BTP)₃(NO₃)](NO₃)₂ (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C axis of the D₃ symmetrical [Cm(nPr-BTP)₃]³⁺ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.