Thiacalix[4]arene-Protected Titanium-Oxo Clusters: Influence of Ligand Conformation and Ti-S Coordination on the Visible-Light Photocatalytic Hydrogen Production

p-tert-Butylthiacalix[4]arene (H₄TC4A)-decorated tetranuclear (Ti₄) and hexanuclear (Ti₆) polyoxo-titanium clusters (PTCs) were synthesized in isopropyl alcohol by a low-temperature solvothermal method. The structures were determined by single-crystal X-ray diffraction, in which the ligands adopt 1,2-alternative and cone-shaped conformations, respectively. Both PTCs show UV-vis absorptions up to 600 nm with band gaps at 2.19 eV (Ti₄) and 2.24 eV (Ti₆), respectively. Visible-light photocatalytic H₂ production for Ti₄ was measured to be 73.56 μmol/(g h), which is almost 5 times of that of Ti₆ (15.16 μmol/(g h)). Results of density functional theory (DFT) calculations suggested that the different Ti-S coordination modes originating from the conformational disparity of the ligands are primarily responsible for the observed different photocatalytic activities. Not only does this work provides a better understanding of the structure-property relationships in using titanium-oxo clusters for photocatalytic H₂ production but it also offers a means of tuning the band gap of the catalysts by varying the conformations of the ligands.