Recent advances in trivalent f-element borate chemistry

Abstract

The reactions of LnCl₃ · 6 H₂O with molten boric acid result in the formation of three different products depending on the identity of the lanthanide metal. The analogous reactions using AnCl₃ · 6 H₂O (An = Pu(III), Am(III), and Cm(III) with molten boric acid under the same conditions do not yield the same results as those of the 4f series or of the other 5f elements. For the most part, the actinide products obtained have no lanthanide analogues making them unique compounds. Furthermore, the reactions of LnX₃ · yH₂O (X = Br, I) (Ln = La-Nd) with molten boric acid results in the formation of two products. The analagous set of reactions was performed with PuX₃ · yH₂O (X = Br, I) which, yet again, gave rise to completely novel compounds. All of these compounds are three-dimensional framework structures comprised of sheets. These sheets are made up of both BO₃ and BO₄ units which are arranged in such a maner that creates triangular holes in which the lanthanide or actinide sites reside. The borate units of the triangular holes provide six oxygen atoms that coordinate in the equatorial region resulting in unique nine- and/or ten-coordinate geometries. The similarities and differences between the 4f and 5f borate compounds are discussed with particular emphasis on the periodic trends of this system.