Trivalent actinide ions showing tenfold coordination in solution

Trivalent actinides generally exhibit ninefold coordination in solution. 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 the first 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 nm 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. No such shift is observed when adding other anions (NO₂^-, CN^- and OTf^-). The formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex was studied as a function of NO₃^- and nPr-BTP concentrations. Slope analyses confirm 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. For concentrations greater than 10^-4 mol/L only [Cm(nPrBTP)₃]³⁺ is observed. VSBS allows observing vibrations of coordinated functional groups. The vibronic side band of the [Cm(nPr-BTP)₃(NO₃)]²⁺ species exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)₃]³⁺ complex. Moreover, XPS on [Am(nPr-BTP)₃(NO₃)]²⁺ yields signals from both non-coordinated and coordinated nitrate. DFT calculations reveal that the energetically most favoured structure is obtained if the nitrate is positioned on the C₂ 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.