Ca 2 [Ti(HPO 4 ) 2 (PO 4 )]·H 2 O, Ca[Ti 2 (H 2 O)(HPO 3 ) 4 ]·H 2 O, and Ti(H 2 PO 2 ) 3 : Solid-State Oxidation via Proton-Coupled Electron Transfer.

Titanium phosphorus oxides (TiPOs) are promising energy-conversion materials, but most are of tetravalent titanium (Ti IV ), with the trivalent Ti III POs less explored because of instability and obstacles in synthesis. In this study, we used a simple synthetic strategy and prepared three new Ti III POs with different phosphorus oxoanions: the phosphate Ca 2 Ti(HPO 4 ) 2 (PO 4 )·H 2 O ( 1 ), the phosphite CaTi 2 (H 2 O)(HPO 3 ) 4 ·H 2 O ( 2 ), and the hypophosphite Ti(H 2 PO 2 ) 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 HPO 4 in oxidation to yield Ca 2 Ti(HPO 4 )(PO 4 ) 2 ·H 2 O (designated as 1 O ), while compound 2 also exhibits PCET reactivity in which the octahedral core [Ti III (H 2 O)] 3+ gives off two protons to become a titanyl unit [Ti IV ═O] 2+ under oxidation, yielding CaTi 2 O(HPO 3 ) 4 ·H 2 O ( 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 Ti III PO solid materials.