Further Evidence for the Stabilization of U(V) within a Tetraoxo Core

Two complex layered uranyl borates,
K10[(UO2)16(B2O5)2(BO3)6O8]·7H2O (1)
and K13[(UO2)19(UO4)­(B2O5)2(BO3)6(OH)2O5]·H2O (2), were
isolated from supercritical water reactions. Within these compounds,
borate exists only as BO3 units and is found as either
isolated BO3 triangles or B2O5 dimers,
the latter being formed from corner sharing of two BO3 units.
These anions, along with oxide and hydroxide, bridge between uranyl
centers to create the complex layers in these compounds. U­(VI) cations
are found within uranyl, UO22+ units, that are
bound by four or five oxygen atoms to create tetragonal and pentagonal
bipyramidal environments. The most striking feature in this system
is found in 2, where a [UO4(OH)2] unit exists that contains U­(V) within a tetraoxo core with trans
hydroxide anions; therefore, this compound is a mixed-valent U­(VI)/U­(V)
borate. The presence of a 5f1 uranium
site within 2 leads to unusual optical properties.