Structure and bonding investigation of plutonium peroxocarbonate complexes using cerium surrogates and electronic structure modeling

Herein, we report the synthesis and structural characterization of K₈ [(CO₃)3Pu]₂ (μ-η²-η²-O₂)₂12H₂O. This is the second Pu-containing addition to the previously studied alkali-metal peroxocarbonate series M₈ [(CO₃)₃A] ₂ (μ-η²-η²- O₂)₂xH₂O (M = alkali metal; A = Ce or Pu; x = 8, 10, 12, or 18), for which only the M = Na analogue has been previously reported when A = Pu. The previously reported crystal structure for Na₈ [(CO₃)₃Pu] ₂ (μ-η²-η²-O₂)₂12H₂O is not isomorphous with its known Ce analogue. However, a new synthetic route to these M₈ [(CO₃)₃A] ₂ (μ-η²-η²-O₂)₂12H₂O complexes, described below, has produced crystals of Na₈ [(CO₃)₃Ce] ₂ (μ-η²-η²-O₂)₂12H₂O that are isomorphous with the previously reported Pu analogue. Via this synthetic method, the M = Na, K, Rb, and Cs salts of M₈ [(CO₃)₃Ce] ₂ (μ-η²-η²-O₂)₂xH₂O have also been synthesized for a systematic structural comparison with each other and the available Pu analogues using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory calculations. The Ce salts, in particular, demonstrate subtle differences in the peroxide bond lengths, which correlate with Raman shifts for the peroxide O_p-O_p stretch (O_p = O atoms of the peroxide bridges) with each of the cations studied: Na+ [1.492(3) Å/847 cm^-1], Rb+ [1.471(1) Å/854 cm^-1], Cs+ [1.474(1) Å/859 cm^-1], and K+ [1.468(6) Å/ 870 cm^-1]. The trends observed in the O_p-O_p bond distances appear to relate to supermolecular interactions between the neighboring cations.