N-heterocyclic
carbenes (NHCs) are recognized for their ability
to stabilize various main group radicals; however, NHC-derived, sulfur-based
radicals remain rare. In this study, we successfully synthesized and
characterized a series of palladium diradical complexes that featured
new sulfur-based radical ligands from NHC–carbon disulfide
adducts. Spectroscopic and computational characterizations of the
palladium complexes confirmed the open-shell singlet ground state,
which resulted from the antiferromagnetic coupling of two unpaired
electrons on each ligand. Proton nuclear magnetic resonance relaxometry
was used to experimentally confirm the presence of these unpaired
electrons. Moreover, the redox behavior of the complexes was localized
on the ligand center, confirming the redox activity of the ligands.
The discovery of this sulfur-based, redox-active radical ligand underscores
the versatility and significance of NHC-derived radicals, thereby
expanding the repertoire of radical ligands and opening new avenues
for advanced material and catalytic systems.