Diamidocarbene-derived palladium and nickel–sulfur clusters

Minji Lee†, Hyunju Noh†, Youngsuk Kim* 

Cite this article as: Chem. Commun. 2024, 60, 13867.
DOI: https://doi.org/10.1039/D4CC04582C

Short Summary

This study expands the boundaries of metal–sulfur cluster chemistry by synthesizing novel tetranuclear palladium and nickel clusters (L₄Pd₄ and L₄Ni₄) that feature unusual square-planar metal centers and highly redox-active diamidocarbene-CS₂ ligands capable of two-electron reduction.

Key Findings

• Synthesis of Novel Clusters: Successfully isolated L₄Pd₄ and L₄Ni₄ clusters containing a previously unknown M₄S₈ core structure with pseudo-S₄ symmetry, synthesized from a diamidocarbene-carbon disulfide (DAC-CS₂) adduct and Pd(0)/Ni(0) sources.

• Unusual Square-Planar Geometry: Unlike typical biological iron-sulfur clusters that adopt tetrahedral geometries, each Pd(II) and Ni(II) center in these novel clusters maintains a distorted square-planar coordination (sum of S–M–S angles ≈ 354–356°).

• Extreme Redox Non-Innocence: Crystallographic data and DFT calculations confirm that the DAC-CS₂ ligand is highly redox-active, dynamically accommodating formal oxidation states ranging from 0 (in the free ligand or the L₄Ni₃ byproduct) to -2 (in the L₄Pd₄ and L₄Ni₄ clusters).

• Metal–Metal Distances vs. Bonding: Despite exhibiting very short intermetallic distances (e.g., Pd–Pd distance of 2.65 Å, comparable to bulk palladium metal), density functional theory (DFT) reveals Wiberg bond indices close to zero, indicating no significant electronic communication between the metal centers.

Abstract

Novel palladium and nickel–sulfur clusters were synthesized using a diamidocarbene-derived carbon disulfide ligand. Structural characterization revealed a tetranuclear metal–sulfur cluster geometry with each metal center exhibiting square-planar coordination. The ligand was redox-active, accommodating oxidation states ranging from 0 to –2.

Beyond Traditional Biological Clusters

Metal–sulfur clusters, such as the well-known Fe-S clusters, are fundamental to biological systems for facilitating electron transfer. However, exploring new geometries and connectivities—specifically utilizing tetravalent metal centers with square-planar coordination like Pd(II) and Ni(II)—has remained a significant challenge. By successfully incorporating these less common metals into a well-defined M₄S₈ core, this study broadens the fundamental geometric repertoire of metal–sulfur cluster chemistry.

The Power of Diamidocarbenes (DACs)

A key innovation in this study is the use of an extremely electrophilic singlet carbene, diamidocarbene (DAC). While traditional N-heterocyclic carbene (NHC)–CS₂ adducts typically undergo only a one-electron reduction to form radical anions upon coordination, the DAC–CS₂ ligand demonstrates a superior electron-accepting ability. It readily accepts two electrons, planarizing completely to act as a dianionic (L²⁻) ligand.

Capturing the Intermediate State

The redox non-innocence of the ligand was elegantly captured in the fortuitous isolation of a trinickel (L₄Ni₃) byproduct. In this single molecule, one ligand retains a formal oxidation state of 0 (zwitterionic), while the other three are fully reduced to -2. This rare structural snapshot directly proves the ligand's ability to act as an electron reservoir, hinting at profound potential applications for these clusters in multi-electron catalytic transformations and the development of novel electronic materials.

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