Crystal structures and Mössbauer studies of NpO +2 complexes with polycarboxylic acids

Abstract

Complexes of neptunyl ( 237NpO + 2 ) with pyromellitic (benzene tetracarboxylic) and mellitic (benzene hexacarboxylic acids were prepared; their formula is: (Na 3NpO 2C 10O 8H 2) 2·11H 2O and Na 4(NpO 2) 2C 12O 12·8H 2O respectively. These solids were studied by X-ray diffraction, Mössbauer spectroscopy ( 237Np) and magnetic susceptibility measurements. ▪ ▪ ▪Fig. 3. Cation-cation complex of NhO 2 + The crystal structure of the Na 3NpO 2 pyromellitate was refined to R = 0,027 and R w = 0.040 for 6793 independent reflexions. Crystal data are: P 1 , a = 14.46(1) Å, b = 9.404(8) Å, c = 6.768(4) Å, α = 95.08°, β ] 103.23°, γ = 87.47°, Z = 1. In this structure (Fig.1), the NpO + 2 ions are bonded to four pyromellitate anions whose carboxyl oxygens form the base of a pentagonal bipyramid surrounding the Np atom. The Mössbauer spectrum recorded at 4.2 K and shown in Fig. 2, exhibited a magnetic hyperfine splitting attributed to paramagnetic relaxation phenomena [1, 2]. The crystal structure of the Na 4(NpO 2) 2 mellitate was solved by classical methods. R = 0.044 and R w = 0.056 for 2372 independent reflexions. a = 12.53(1) Å, b = 11.58(2) Å, c = 17.81(2) Å, β = 105.79°, Z = 4, space group C 2/c. Two NpO + 2 ions are connected to each other and coordinated to four mellitate anions. Figure 3 shows the ‘cationcation’ complex of NpO + 2 . The χ curve for this complex follows the Curie-Weiss law. A magnetic transition occurs at about 10 K. The Mössbauer spectrum ( 237Np) recorded at 4.2 K and then fitted [3] leads to the following hyperfine parameters: H eff = 5490(10) k0e, e 2qQ = 97(2) mm s −1. The isomer shift (δ = −19.2(5) mm s −1 ref. NpAl 2 [4]) corresponds to that of Np(V). Two Np atoms are at a distance shorter than 3.5 Å in this complex and this could explain the magnetic transition observed.