Solvent-polarity-controlled assembly of polynuclear titanium-calixarene complexes: constructing isolated {}, {}, {} and {} clusters

Three Ti-O clusters and a Tb-Ti-O cluster capped by p-tert -butylcalix[8]arene (H 8 C8A), namely, [Ti 7 O 8 (C8A)(CH 3 O) 4 (CH 3 OH) 4 ] ( CIAC-262 ), [Ti 13 O 15 (C8A) 2 (CH 3 O) 6 (CH 3 OH) 3 (H 2 O) 2 ] ( CIAC-263 ), [Ti 24 O 28 (OH) 8 (C8A) 4 (DMF) 2 (H 2 O) 2 ] ( CIAC-264 ) and [Tb 2 Ti 13 O 15 (OH) 4 (H 3 C8A)(C8A) 2 (HCOO) 3 (DMF) 4 (H 2 O)] ( CIAC-265 ), were prepared by controlling the amount of methanol in the reaction system under solvothermal conditions. The amount of methanol is reduced from 4 mL to 2 mL and then to 0 mL, resulting in a { Ti 7 } cluster with Ti 7 -C8A unit, a { Ti 13 } cluster constructed by two perpendicular tail-to-tail Ti 7 -C8A units with a vertex-shared mode, and a { Ti 24 } cluster assembled by linking two Ti 10 -(C8A) 2 units generated by the symmetric operation with a Ti 4 entity, respectively. To replace the Ti 4 linker in the { Ti 24 } cluster, Tb ions were incorporated in the methanol-free system. Then a { Tb 2 Ti 13 } cluster constructed by linking a Ti 10 -(C8A) 2 unit and a Ti 2 -C8A entity with a Tb 2 Ti linker was obtained. Catalytic sulfoxidation experiments were performed to evaluate the structure-property relationships in these calixarene-based clusters. Driven by Ti atoms and active Ti sites with higher steric hindrance, compound CIAC-264 efficiently catalyzes the selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide (96.00% for conversion and 98.28% for selectivity) using 30% H 2 O 2 under mild conditions (298 K). Calixarene molecules in CIAC-264 protect the Ti-O cluster from decomposition during the catalytic reaction, which could be directly observed from MALDI-TOF MS spectra. This work not only opens a new insight for preparing high-nuclearity Ti-calixarene clusters with interesting assemblies but also provides new stable titanium oxide candidates for selective oxidation of organic sulfides. The assembly of polynuclear titanium-calixarene complexes is based on Ti 4 -C8A SBUs.