Inverse palladocenes

Metallocene research has influenced the development of organometallic chemistry, and the metal-nonmetal inverted half-sandwich structure (inverse metallocene) was very recently discovered. However, additional instances are required to propose the concept inverse metallocene and it remains uncertain whether metal five-membered ring structures analogous to the cyclopentadienyl anion structure exist. Herein we report the synthesis of a series of palladium analogs Pd₈(PPh)₂(PPh₃)₂(Ph₂P=O)(S-Adm)₅ (Pd₈-P), Pd₆(PPh)(PPh₃)(S-Adm)₆ (Pd₆), Pd₅(PPh)(S-Adm)₄[(Ph₂P)₂O] (Pd₅-O), and Pd₅(PPh)(S-Adm)₄[(Ph₂P)₂CH₂] (Pd₅-C), identify their fundamental metal building block named five-membered Pd aromatic ring, and reveal their conjugation‒photothermy correlation. In particular, we report an average NIR-II photothermal conversion efficiency per metal atom of 14.7% and the stability of the five-membered Pd aromatic ring, as illustrated by the fact that Pd₅-C maintained photothermy performance for >10 heating-cooling cycles even after the ligands were removed. We further demonstrate the great potential of inverse palladocenes in areas such as laser shielding, high-temperature degradation, ignition, and temperature/light control. These results declare the research and application start of a type of materials named inverse palladocenes.