Probing the Framework Metal Dependent Properties of Actinide-Centered Polyoxoalkoxide Sandwich-Type Complexes

Development of a
simple and scalable synthesis of (TBA)3[W5O18MoNO] provides for the formation
of the
mixed-metal lacunary polyoxoalkoxide, (TBA)2[W4O13(OMe)4MoNO][Na(MeOH)]. This complex was
used to synthesize a series of polyoxoalkoxide sandwich-type complexes
with the general formula (TBA)2[M{W4O13(OMe)4MoNO}2], where M = Zr(IV), Hf(IV), Th(IV),
U(IV), and Np(IV). Compared to the analogous all-molybdenum complexes,
the series have drastically different optical and redox properties.
The results indicate that framework metal substitution acts as a tool
for “orbital engineering”, with Density Functional Theory
(DFT) calculations revealing that the major consequence of incorporation
of tungsten into the complexes is localization of LUMO and LUMO+1
on the molybdenum centers remaining in the molecule. The change in
the distribution of the frontier orbitals translates to discrepancies
in the electronic properties of the series. Given the rarity of polyoxometalate
complexes featuring a U(V) ion, one electron oxidation of (TBA)2[U(IV){W4O13(OMe)4MoNO}2] was pursued. Isolation of the corresponding U(V) centered
sandwich-type complex is reported, only the second example of U(V)-polyoxometalate
complex described to date.