Solvent-polarity-controlled assembly of polynuclear titanium-calixarene complexes: constructing isolated {Ti7}, {Ti13}, {Ti24} and {Tb2Ti13} clusters

Abstract

Three Ti–O clusters and a Tb–Ti–O cluster capped by p-tert-butylcalix[8]arene (H₈C8A), namely, [Ti₇O₈(C8A)(CH₃O)₄(CH₃OH)₄] (CIAC-262), [Ti₁₃O₁₅(C8A)₂(CH₃O)₆(CH₃OH)₃(H₂O)₂] (CIAC-263), [Ti₂₄O₂₈(OH)₈(C8A)₄(DMF)₂(H₂O)₂] (CIAC-264) and [Tb₂Ti₁₃O₁₅(OH)₄(H₃C8A)(C8A)₂(HCOO)₃(DMF)₄(H₂O)] (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 {Ti₇} cluster with Ti₇–C8A unit, a {Ti₁₃} cluster constructed by two perpendicular tail-to-tail Ti₇–C8A units with a vertex-shared mode, and a {Ti₂₄} cluster assembled by linking two Ti₁₀–(C8A)₂ units generated by the symmetric operation with a Ti₄ entity, respectively. To replace the Ti₄ linker in the {Ti₂₄} cluster, Tb ions were incorporated in the methanol-free system. Then a {Tb₂Ti₁₃} cluster constructed by linking a Ti₁₀–(C8A)₂ unit and a Ti₂–C8A entity with a Tb₂Ti 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% H₂O₂ 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.