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. Metal–nonmetal inverted half-sandwiches, also called inverse metallocenes, have been recently reported. Here the authors report the synthesis of a series of palladium inverse metallocene analogs, identify their fundamental metal building block, characterize their NIR-II photothermal conversion efficiency, and show the potential photothermal applications.