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

Abstract

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 [TiIVO]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.