Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)–Plutonium(III) Oxalates

The mixed-actinide uranium­(IV)–plutonium­(III) oxalate single crystals (NH₄)_0.5[Pu^III_0.5U^IV_0.5(C₂O₄)₂·H₂O]·<i>n</i>H₂O (<b>1</b>) and (NH₄)_2.7Pu^III_0.7U^IV_1.3(C₂O₄)₅·<i>n</i>H₂O (<b>2</b>) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV–vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U^IV and Pu^III, and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of <b>1</b> and an average structure of <b>2</b> were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds <b>1</b> and <b>2</b> are the first mixed uranium­(IV)–plutonium­(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of <b>1</b>, space group <i>P</i>4/<i>n</i>, <i>a</i> = 8.8558(3) Å, <i>b</i> = 7.8963(2) Å, <i>Z</i> = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound <b>2</b>, an average structure has been determined in space group <i>P</i>6/<i>mmm</i> with <i>a</i> = 11.158(2) Å and <i>c</i> = 6.400(1) Å. The honeycomb-like framework [Pu^III_0.7U^IV_1.3(C₂O₄)₅]^2.7‑ results from a three-dimensional arrangement of mixed (U_0.65Pu_0.35)­O₁₀ polyhedra connected by five bis-bidentate μ²-oxalate ions in a trigonal-bipyramidal configuration