One-Pot versus Sequential Reactions in the Self-Assembly of Gigantic Nanoscale Polyoxotungstates

By using a new type of lacunary tungstoselenite {Se2W29O103} (1), which contains
a
“defect” pentagonal {W­(W)4} unit, we explored
the assembly of clusters using this building block and demonstrate
how this unit can give rise to gigantic nanomolecular species, using
both a “one-pot” and “stepwise” synthetic
assembly approach. Specifically, exploration of the one-pot synthetic
parameter space lead to the discovery of {Co2.5(W3.5O14)­(SeW9O33)­(Se2W30O107)} (2), {CoWO­(H2O)3(Se2W26O85)­(Se3W30O107)2} (3), and
{Ni2W2O2Cl­(H2O)3(Se2W29O103) (Se3W30O107)2} (4), effectively
demonstrating the potential of the {Se2W29}
based building blocks, which was further extended by the isolation
of a range of 3d transition metal doped tetramer family derivatives:
{M2WnOm(H2O)m(Se2W29O102)4} (M = Mn, Co, Ni or Zn, n = 2, m = 4; M = Cu, n = 3, m = 5) (5 - 9).
To contrast the ‘one-pot’ approach, an optimized stepwise
self-assembly investigation utilizing 1 as a precursor
was performed showing that the high nuclearity clusters can condense
in a more controllable way allowing the tetrameric clusters (5 - 8) to be synthesized with higher yield, but
it was also shown that 1 can be used to construct a gigantic
{W174} hexameric-cluster {Cu9Cl3(H2O)18(Se2W29O102)6} (10). Further, 1 can also
dimerize to {(Se2W30O105)2} (11) by addition of extra tungstate under similar
conditions. All the clusters were characterized by single-crystal
X-ray crystallography, chemical analysis, infrared spectroscopy, thermogravimetric
analysis, and electrospray ionization mass spectrometry, which remarkably
showed that all the clusters, even the largest cluster, 10 (∼50 kD), could be observed as the intact cluster demonstrating
the extraordinary potential of this approach to construct robust gigantic
nanoscale polyoxotungstates.