Structure, Phase Transitions, and Isotope Effects in [(CH₃)₄N]₂PuCl₆

Abstract

The single-crystal X-ray diffraction structure of [(CH3)4N]2PuCl6 is presented for the first time, resolving long-standing confusion and speculation regarding the structure of this compound in the literature. A temperature-dependent study of this compound shows that the structure of [(CH3)4N]2PuCl6 undergoes no fewer than two phase transitions between 100 and 360 K. The phase of [(CH3)4N]2PuCl6 at room temperature is Fd3̅c a = 26.012(3) Å. At 360 K, the structure is in space group Fm3̅m, with a = 13.088(1) Å. The plutonium octahedra and tetramethylammonium cations undergo a rotative displacement, and the degree of rotation varies with temperature, giving rise to the phase transition from Fm3̅m to Fd3̅c as the crystal is cooled. Synthesis and structural studies of the deuterated salt [(CD3)4N]2PuCl6 suggest that there is an isotopic effect associated with this phase transition, as revealed by a changing transition temperature in the deuterated versus protonated compound, indicating that the donor-acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are driving the phase transformation.

Phase Transitions in Tetramethylammonium Hexachlorometalate Compounds (TMA)2MCl6 (M = U, Np, Pt, Sn, Hf, Zr)

“…Diffraction data collected at 300 K can be satisfactorily indexed to a face‐centered cubic lattice. As previously observed for [(CH 3 ) 4 N] 2 PuCl 6 and other [(CH 3 ) 4 N] 2 MCl 6 compounds (with M = Pt IV and Sn IV ), weak reflections detected in our data support a larger unit cell (Table ) than previously reported structures Additional data collected at variable temperatures showed a change in structure with temperature. The structure was solved and refined in Fd 3 c using the larger lattice parameter for all complexes at 300 K. Selected metrical data regarding the structure and refinement are presented in the Supporting Information (Tables S1 to S12).…”

“…Diffraction data collected at 300 K can be satisfactorily indexed to a face-centered cubic lattice. As previously observed for [(CH 3 ) 4 N] 2 PuCl 6 [8] and other [(CH 3 ) 4 N] 2 MCl 6 compounds (with M = Pt IV and Sn IV ), [10] weak reflections detected in our data support a larger unit cell (Table 1) than previously reported structures. [11] Additional data collected at variable temperatures showed a change in structure with temperature.…”

“…M-Cl distances for Pu IV from ref. [8] For the actinides and the other transition metal complexes, a linear relationship between the M-Cl bond and the metal ionic radii is observed as expected from previous studies on hexachloro compounds. [13] For the platinum complex, the Pt-Cl bond length is consistent with various tetravalent hexachloroplatinate compounds.…”

“…[6] Others have correlated the phase transition temperatures in these compounds with the d-electron configuration of the metal ion in the [MX 6 ] 2anion. [7] Our recent study of the structure of the hexachloroplutonate compound [(CH 3 ) 4 N] 2 PuCl 6 highlighted a rotative-displacive phase transition resulting in a change in structure between Fd3c and Fm3m around room temperature along with evidence for a first-order phase transition below 120 K. [8] Because of the earlier correlation of the transition temperatures in the transition metal hexametalates with electronic properties of the metal ions, we were interested to determine if the transition temperatures in the actinide hexametalates correlated with the f-electron configuration of the actinide ion, especially because of the periodic changes in actinide-ligand bonding as one moves across the early actinide series. [9] To investigate the influence of the f-electron configuration of the metal in the actinide complexes, tetramethylammonium hexachloroactinate compounds of U IV , and Np IV were synthesized and studied for comparison with the previously reported Pu IV complex.…”

“…Our recent study of the structure of the hexachloroplutonate compound [(CH 3 ) 4 N] 2 PuCl 6 highlighted a rotative–displacive phase transition resulting in a change in structure between Fd 3 c and Fm 3 m around room temperature along with evidence for a first‐order phase transition below 120 K . Because of the earlier correlation of the transition temperatures in the transition metal hexametalates with electronic properties of the metal ions, we were interested to determine if the transition temperatures in the actinide hexametalates correlated with the f‐electron configuration of the actinide ion, especially because of the periodic changes in actinide–ligand bonding as one moves across the early actinide series .…”