Gayeong Lim, Dongmin Kang, Jeongcheol Shin, Youngsuk Kim*
Cite this article as: ACS Omega 2025, 10, 29754.
DOI: https://doi.org/10.1021/acsomega.5c04023
This study introduces a novel cyclic(alkyl)(amino)carbene (CAAC)-derived CS₂ adduct as a highly effective radical ligand, enabling square planar Pd(II) and Ni(II) complexes to adopt unique multiconfigurational open-shell singlet ground states with completely reversible, multi-electron redox activity.
Synthesis of Open-Shell Complexes: Two novel d⁸ square planar complexes, 1–Pd and 1–Ni, were successfully synthesized using a CAAC–CS₂ adduct. Structural data confirmed the ligands possess partial double-bond character (C–C bond ≈ 1.40 Å) and a planar geometry, functioning as radical anions.
Proton T₁ NMR Relaxometry: Spin-lattice relaxation time (T₁) measurements provided direct experimental evidence of ligand-centered unpaired electrons. Notably, 1–Pd exhibited significantly shorter T₁ values (e.g., 187–211 ms) compared to 1–Ni, proving that the Pd complex possesses a much stronger open-shell character.
Multiconfigurational Ground State: Density functional theory (DFT) calculations revealed an open-shell singlet ground state (S = 0) driven by antiferromagnetic coupling of two radical ligands. The small energy gaps to the triplet and closed-shell singlet states facilitate a highly multiconfigurational electronic landscape.
Extensive Redox Activity: Cyclic voltammetry demonstrated that these complexes are highly redox-active. 1–Pd displayed four well-defined, reversible redox processes (from –1.585 V to 0.282 V vs. Fc/Fc⁺), proving the ligand's robust capacity for multi-electron transfers.
Radical ligands play a crucial role in modifying the electronic structures and reactivity of transition-metal complexes, often endowing them with unique properties. In this study, we demonstrate that a cyclic(alkyl)(amino)carbene (CAAC)-derived carbon disulfide adduct functions as a radical ligand, supporting Pd(II) and Ni(II) complexes with multiconfigurational electronic structures. Single-crystal X-ray crystallography confirms that both 1–Pd and 1–Ni adopt a square planar geometry with structural parameters consistent with the presence of ligand-centered unpaired electrons. Computational studies indicate that both complexes favor an open-shell singlet ground state, with small energy gaps to the triplet and closed-shell singlet states, which could likely facilitate electronic state mixing. This work highlights the effectiveness of CAAC-derived radical ligands in stabilizing multiconfigurational electronic states in metal complexes, opening avenues for potential applications in catalysis and materials science.
Incorporating radical ligands into transition-metal complexes is a highly sought-after strategy to alter electronic structures and unlock unique reactivities, such as metal-ligand cooperative catalysis or single-molecule magnetism. However, traditional organic radicals are notoriously unstable, severely limiting the structural diversity of radical ligands available to synthetic chemists.
Building on the recent success of N-heterocyclic carbene (NHC) radicals, we designed a new class of radical ligands based on cyclic(alkyl)(amino)carbenes (CAACs). Thanks to the exceptional π-accepting ability and steric bulk of the CAAC framework, the CAAC–CS₂ adduct effectively stabilizes radical species. By coordinating these ligands to redox-innocent Pd(II) and Ni(II) centers, we successfully enforced open-shell electronic structures in traditionally stable, square planar d⁸ configurations.
A major highlight of this research is the perfect alignment between experimental NMR relaxometry and quantum mechanical calculations. While X-ray crystallography confirmed the geometric identity of the radical anionic ligands (L•–), measuring the T₁ relaxation times allowed us to observe the actual distribution of unpaired electron density. The smaller exchange coupling constant (J) calculated via broken-symmetry DFT perfectly explained why 1–Pd exhibited a stronger open-shell nature (shorter T₁) than 1–Ni. This comprehensive approach proves that CAAC-derived radical ligands can actively manipulate the electronic configuration of transition metals, paving the way for next-generation catalysts and magnetic materials.
