posted on 2019-10-10, 12:03authored byJames
A. Sommers, Danielle C. Hutchison, Nicolas P. Martin, Karoly Kozma, Douglas A. Keszler, May Nyman
Zr/Hf aqueous-acid clusters are relevant
to inorganic nanolithography,
metal–organic frameworks (MOFs), catalysis, and nuclear fuel
reprocessing, but only two topologies have been identified. The (Zr4) polyoxocation is the ubiquitous
square aqueous Zr/Hf-oxysalt of all halides (except fluoride), and
prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate
solution leads to a striking modification of Zr4, yielding two structures identified by single-crystal
X-ray diffraction. Zr25, isolated
from a reaction solution of 1:1 peroxide/Zr, is fully formulated [Zr25O10(OH)50(O2)5(H2O)40](ClO4)10·xH2O. Zr25 is a pentagonal assembly of 25 Zr-oxy/peroxo/hydroxyl
polyhedra and is the largest Zr/Hf cluster topology identified to
date. Yet it is completely soluble in common organic solvents. ZrTd, an oxo-centered tetrahedron
fully formulated [Zr4(OH)4(μ-O2)2(μ4-O)(H2O)12](ClO4)6·xH2O, is isolated from a 10:1 peroxide/Zr reaction solution.
The formation pathways of ZrTd and Zr25 in water were described
by small-angle X-ray scattering (SAXS), pair distribution function
(PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr4 undergoes disassembly by 1 equiv
of peroxide (per Zr) to yield small oligomers of Zr25 that assemble predominantly in the solid state,
an unusual crystal growth mechanism. The self-buffering acidity of
the Zr-center prevents Zr25 from
remaining intact in water. Identical species distribution and cluster
fragments are observed in the assembly of Zr25 and upon redissolution of Zr25. On the other hand, the 10:1 peroxide/Zr ratio of
the ZrTd reaction solution yields
larger prenucleation clusters before undergoing peroxide-promote disassembly
into smaller fragments. Neither these larger cluster intermediates
of ZrTd nor the smaller intermediates
of Zr25 have yet been isolated
and structurally characterized, and they represent an opportunity
to expand this new class of group IV polycations, obtained by peroxide
reactivity and ligation.