Effect of pH and Reaction Time on the Structures of Early Lanthanide(III) Borate Perchlorates

Reactions of LnCl3·6H2O (Ln
= La–Nd,
Sm, Eu), concentrated (11 M) perchloric acid, and molten boric acid
result in the formation of four different compounds. These compounds
are Ln­[B8O10(OH)6(H2O)­(ClO4)]·0.5H2O (Ln = La–Nd, Sm), Pr­[B8O11(OH)4(H2O)­(ClO4)], Ln­[B7O11(OH)­(H2O)2(ClO4)] (Ln = Pr, Nd, Sm, and Eu), and Ce­[B8O11(OH)4(H2O)­(ClO4)].
All Ln­(III) cations are ten-coordinate with a capped triangular cupola
geometry and contain an inner-sphere, monodentate perchlorate moiety.
This geometry is obtained because of the coordination of the oxygen
donors within the polyborate sheet which create triangular holes and
provide residence for the lanthanide metal centers. Aside from Ln­[B8O10(OH)6(H2O)­(ClO4)]·0.5H2O (Ln = La–Nd, Sm), which are two-dimensional
sheet structures, all other compounds are three-dimensional frameworks
in which the layers are tethered together by BO3 units
found roughly perpendicular to the sheets. Furthermore, a change in
product is observed depending on the reaction duration while holding
all other synthetic variables constant. This report also demonstrates
that lanthanide borates can be prepared in extreme acidic conditions.