Tricarbonate complex of hexavalent Am with guanidinium: synthesis and structural characterization of [C(NH2)_3]_4[AmO_2(CO_3)_3]·2H_2O,comparison with [C(NH2)_3]_4[AnO_2(CO_3)_3] (An = U, Np, Pu)

摘要

A formation of Am(VI) carbonate complexes has been observed under ozonation of Am(III) in guanidinium carbonate solutions. New [C(NH_2)_3]_4[AmO_2(CO_3)_3]·_2H_2O tricarbonate complex has been isolated and studied by the X-ray analysis. The Am atom has oxygen environment as distorted hexagonal bipyramid with three CO_3~ 2- ions in equatorial plane and oxygen atoms of the AmO_2 group in the apical positions. Averaged Am-OCO_3 bond length is 2.431(13) A, the length of Am=O bond is 1.750(12)A. In crystal hydrogen bonds between [C(NH_2)_3]~+ cations, water molecules and [AmO_2(CO_3)_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(NH_2)_3]_4[AnO_2(CO_3)_3], where An= U(VI), Np(VI) and Pu(VI). In structure of [C(NH_2)_3]_4[AnO_2(CO_3)_3] an actinide contraction appears as a slight shortening of the bond lengths in the actinyl groups AnO_2: 1.80 A for U, 1.78 A for Np 1.76 A Pu. The An-O_(CO3) 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.