Ca₂[Ti(HPO₄)₂(PO₄)]·H₂O, Ca[Ti₂(H₂O)(HPO₃)₄]·H₂O, and Ti(H₂PO₂)₃: Solid-State Oxidation via Proton-Coupled Electron...

Titanium phosphorus oxides (TiPOs) are promising energy-conversion materials, but most are of tetravalent titanium (Ti^IV), with the trivalent Ti^IIIPOs less explored because of instability and obstacles in synthesis. In this study, we used a simple synthetic strategy and prepared three new Ti^IIIPOs 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), while compound 2 also exhibits PCET reactivity in which the octahedral core [Ti^III(H₂O)]³⁺ gives off two protons to become a titanyl unit [Ti^IV═O]²⁺ under oxidation, yielding CaTi₂O(HPO₃)₄·H₂O (2). Both 1 and 2 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 Ti^IIIPO solid materials.