Acid-Controlled Synthesis of Carboxylate-Stabilized Ti44 -Oxo Clusters: Scaling up Preparation, Exchangeable Protecting Ligands, and Photophysical Properties.

A range of polyoxotitanium clusters (PTCs) was constructed by tuning the type of acid (inorganic and organic) in alcoholic solvents, from Ti4 , Ti6 , Ti9 , Ti11 , to Ti16 . After removing the tBuOH solvent, giant carboxylate-stabilized Ti44 -oxo clusters in which propionic acid serves as both ligand and solvent were ultimately obtained. The four labile sites in the Ti44 cluster core can be occupied by two formate and two propionate anions (PTC-165) or a pair of glutarate (PTC-166) or 3-methylglutarate anions (PTC-167). According to the synthesis of PTC-155 to PTC-167, the propionic acid solvent plays a crucial role in the construction of giant Ti oxo clusters. Their one-pot yields, which readily reached up to 8.75 g for PTC-165 and 9.96 g for PTC-166, proved the feasibility of large-scale preparation. More interestingly, the obtained Ti44 -oxo clusters are almost completely surrounded by carboxylate ligands, which allow them to retain crystalline stability in air for about three weeks and in either acidic or basic aqueous solution over a wide pH range for at least 6 h. In addition, PTC-165 and PTC-166 exhibit excellent UV photocurrent response and reversible photochromic effect. This work provides a systematic approach for constructing high-nuclearity and stable PTCs on a large scale, which is significant for future applications of PTC-based photochemical devices.