Subin Park,‡ Jeong-Yoon Hwang,‡ Jeongcheol Shin,* Youngsuk Kim*
Cite this article as: J. Am. Chem. Soc. 2024, 146, 28508.
DOI: https://doi.org/10.1021/jacs.4c11082
This study successfully synthesizes rare palladium diradical complexes using novel sulfur-based NHC–CS₂ radical ligands, demonstrating an open-shell singlet ground state and highly reversible, ligand-centered redox activity that expands the toolkit for metal-ligand cooperative catalysis.
Synthesis of Diradical Complexes: Stable square-planar palladium complexes (2a••–2d••) were successfully synthesized using bulky NHC–CS₂ adducts and Pd(0) or Pd(II) sources.
Open-Shell Singlet State: Density functional theory (DFT) and experimental UV-Vis-NIR spectroscopy (LLCT transition at ~1015 nm) confirmed that the complexes possess an open-shell singlet ground state driven by antiferromagnetic coupling of two unpaired electrons.
Proton NMR Relaxometry Mapping: Spin-lattice relaxation time (T₁) measurements experimentally mapped the spin density; protons closest to the imidazole ring exhibited remarkably short T₁ values (89–94 ms), precisely matching the calculated spin-dipolar coupling constants.
Reversible Ligand-Centered Redox Activity: The neutral diradical complexes can be oxidized reversibly. Double oxidation yields a closed-shell diamagnetic dication ([2a2+][OTf–]2), while single oxidation generates a mixed-valent radical cation ([2a•+][OTf–]) exhibiting an intervalence charge transfer (IV-CT) band.
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.
Radical ligands are crucial in transition-metal coordination chemistry, offering unique electronic structures and enabling metal-ligand cooperative catalysis. While N-heterocyclic carbenes (NHCs) are well known for stabilizing main-group radicals, NHC-derived sulfur-based radicals have remained exceedingly rare and underexplored due to their intrinsic instability. This study challenges that limitation by utilizing highly stable NHC–CS₂ adducts as a new platform for radical ligands.
A major challenge in radical chemistry is precisely locating the unpaired electrons. Instead of relying solely on computational models or EPR, we employed proton NMR relaxometry to experimentally "see" the spin distribution. By measuring the spin-lattice relaxation times (T₁), we proved that the unpaired electrons reside strictly on the ligands, not the palladium center. The rapid relaxation (short T₁) of protons near the imidazole ring provided direct experimental evidence of the ligand-centered radical character.
The palladium diradical complexes we synthesized act as non-innocent ligands. The cyclic voltammetry and chemical oxidation experiments demonstrated that the oxidation state of the palladium center (+2) remains unchanged, while the ligands undergo clean, reversible single and double electron oxidations. This highly localized redox activity opens new avenues for developing advanced catalytic systems and smart materials where the ligand actively stores and releases electrons during chemical transformations.
2024-jacs-Pd.pdf
