Tricarbonate complex of hexavalent Am with guanidinium: synthesis and structural characterization of [C(NH2)3]4[AmO2(CO3)3]·2H2O, comparison with [C(NH2)3]4[AnO2(CO3)3] (An=U, Np, Pu)

Abstract

Abstract A formation of Am(VI) carbonate complexes has been observed under ozonation of Am(III) in guanidinium carbonate solutions. New [C(NH2)3]4[AmO2(CO3)3]·2H2O tricarbonate complex has been isolated and studied by the X-ray analysis. The Am atom has oxygen environment as distorted hexagonal bipyramid with three CO3 2− ions in equatorial plane and oxygen atoms of the AmO2 group in the apical positions. Averaged Am–OCO3 bond length is 2.431(13) Å, the length of Am=O bond is 1.750(12) Å. In crystal hydrogen bonds between [C(NH2)3]+ cations, water molecules and [AmO2(CO3)3]4− complex anions link the fragments of structure into the three-dimensional network. To compare the peculiarities of the structure of tricarbonate complexes it was prepared [C(NH2)3]4[AnO2(CO3)3], where An = U(VI), Np(VI) and Pu(VI). In structure of [C(NH2)3]4[AnO2(CO3)3] an actinide contraction appears as a slight shortening of the bond lengths in the actinyl groups AnO2: 1.80 Å for U, 1.78 Å for Np 1.76 Å Pu. The An–OCO3 bond length in the equatorial plane of hexagonal bipyramids remains almost unchanged in the series from U to Am. An analysis of the structures showed that hydrogen bonding played an important role in crystal formation.