Titanium
phosphorus oxides (TiPOs) are promising energy-conversion
materials, but most are of tetravalent titanium (TiIV),
with the trivalent TiIIIPOs less explored because of instability
and obstacles in synthesis. In this study, we used a simple synthetic
strategy and prepared three new TiIIIPOs with different
phosphorus oxoanions: the phosphate Ca2Ti(HPO4)2(PO4)·H2O (1), the phosphite CaTi2(H2O)(HPO3)4·H2O (2), and the hypophosphite
Ti(H2PO2)3 (3). Each
possesses different structures in one, two, and three dimensions,
yet they are related to one another because of their infinite chains.
Compound 1 exhibits proton-coupled electron transfer
(PCET) reactivity in a solid state, losing one proton from its own
HPO4 in oxidation to yield Ca2Ti(HPO4)(PO4)2·H2O (designated as 1
O
), while compound 2 also exhibits PCET reactivity in which the octahedral core [TiIII(H2O)]3+ gives off two protons to
become a titanyl unit [TiIVO]2+ under
oxidation, yielding CaTi2O(HPO3)4·H2O (2
O
). Both 1
O
and 2
O
retain their original frameworks from before oxidation, but
there are some changes in the hydrogen and Ti–O bonds that
affect the IR absorption and powder X-ray diffraction patterns. Compound 3 represents the first titanium hypophosphite, and two polymorphs
were discovered that show structures related to 1 and 2. This work demonstrates a simple strategy that is effective
for preparing titanium(III) compounds in a pure phase; further, new
findings in the pathways of solid-state PCET reactions promote a greater
understanding of the self-sustaining oxidation behavior for TiIIIPO solid materials.
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