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_O), 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_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^IIIPO solid materials.

中文翻译：

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Ca2[Ti(HPO4)2(PO4)]·H2O、Ca[Ti2(H2O)(HPO3)4]·H2O 和 Ti(H2PO2)3：通过质子耦合电子转移进行固态氧化

钛磷氧化物 (TiPOs) 是很有前途的能量转换材料，但大多数是四价钛 (Ti ^IV )，三价 Ti ^III POs 由于合成过程中的不稳定性和障碍而较少探索。在本研究中，我们采用简单的合成策略制备了三种具有不同磷氧阴离子的新型 Ti ^III PO：磷酸盐 Ca ₂ Ti(HPO ₄ ) ₂ (PO ₄ )·H ₂ O ( 1 )、亚磷酸盐 CaTi ₂ (H ₂ O)(HPO ₃ ) ₄ ·H ₂ O ( 2 ) 和次磷酸盐 Ti(H₂ PO ₂ ) ₃ ( 3 )。一、二、三维度各有不同的结构，却又因无穷的链条而相互关联。化合物1在固态表现出质子耦合电子转移(PCET)反应性，在氧化过程中从其自身的HPO _{4失去一个质子，生成Ca 2} Ti(HPO ₄ )(PO ₄ ) ₂ ·H ₂ O（记为1 _O )，而化合物2也表现出 PCET 反应性，其中八面体核心 [Ti ^III (H ₂ O)] ³⁺在氧化作用下放出两个质子成为钛氧基单元[Ti ^IV =O] ²⁺，生成CaTi ₂ O(HPO ₃ ) ₄ ·H ₂ O ( 2 _O )。1 _O和2 _O都保留了氧化前的原始骨架，但氢键和 Ti-O 键发生了一些变化，这些变化会影响 IR 吸收和粉末 X 射线衍射图谱。化合物3代表了第一个次磷酸钛，并且发现了两个多晶型物，它们显示出与1和2相关的结构. 这项工作展示了一种简单的策略，可有效制备纯相中的钛 (III) 化合物；^{此外，固态PCET反应途径的新发现促进了对Ti III} PO固体材料自持氧化行为的更好理解。