Temperature-resolved study of three [M(M′O₄) ₄(TBPO)₄] complexes (MM′ = URe, ThRe, ThTc)

The crystal structures of the title complexes were measured at several temperatures between room temperature and 100 K. Each sample shows reversible crystal-to-crystal phase transitions as the temperature is varied. The behaviour of [U(ReO₄)₄(TBPO)₄] (I) and [Th(ReO ₄)₄(TBPO)₄] (II) (TBPO = tri-n-butylphosphine oxide) is very similar; at room temperature, crystals of (I) and (II) are isostructural, with space group I42m, and reducing the temperature to 100 K causes a lowering of the space-group symmetry to C-centred cells, space groups Cc for (I) and Cmc2₁ for (II). The variation of lattice symmetry of [Th(TcO₄)₄(TBPO)₄] (III) was found to be somewhat different, with the body-centred cubic space group, I43m, occurring at 293 K, a reduction of symmetry at 230 K to the C-centred orthorhombic space group, Cmc2₁, and a further transition to the primitive orthorhombic space group, Pbc2₁, below 215 K. Elucidation of the correct space-group symmetry and the subsequent refinement was complicated in some cases by the twinning by pseudo-merohedry that arises from the lowering of the space-group symmetry, occurring as the temperature is reduced. All three of the crystal structures determined at room temperature have high atomic displacement parameters, particularly of the ⁿBu groups, and (III) shows disorder of some of the O atoms. The structures in the space group Cmc2₁ show some disorder of ⁿBu groups, but are otherwise reasonably well ordered; the structures of (I) in Cc and (III) in Pbc2₁ are ordered, even to the ends of the alkyl chains. Inter-comparison of the structures measured below 293 K, using the program OFIT from the SHELXTL package, showed that generally, they are remarkably alike, with weighted r.m.s. deviations of the M, M′ and P atoms of less than 0.1 Å, as are the 293 K structures of (I) and (II) with their low-temperature counterparts. However, the structure of (III) measured in the space group Cmc2₁ is significantly different from both the structure of (III) at 293 K and that found below 215 K, with weighted r.m.s. deviations of the Th, Tc and P atoms of 0.40 and 0.37 Å, respectively. An extensive network of weak intra- and intermolecular C-H⋯O hydrogen bonds found between the atoms of the ⁿBu and [M′O₄] groups probably influences the packing and the overall geometry of the molecules.