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. The work established that the DAC–CS₂ framework can access both the neutral (L⁰) and two-electron-reduced dianionic (L²⁻) states. However, the intermediate one-electron-reduced monoanionic state (L⁻) could not be isolated or experimentally characterized, leaving access to this oxidation state as an important unresolved challenge.

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°).

• Two-Electron Redox Flexibility: Crystallographic data and DFT calculations demonstrate that the DAC–CS₂ ligand is strongly redox-active. The study experimentally established both the neutral L⁰ state and the fully two-electron-reduced L²⁻ state, illustrating the ability of the ligand to accommodate substantial changes in electron count.

• An Unresolved One-Electron State: Despite establishing the L⁰/L²⁻ redox range, no complex containing a well-defined monoanionic DAC–CS₂ ligand (L⁻) was obtained. Thus, whether the one-electron-reduced state could be stabilized as an isolable coordination compound remained an open question.

• 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 direct metal–metal bonding interaction.

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 feature of this chemistry is the use of the highly electrophilic singlet carbene diamidocarbene (DAC). Whereas conventional N-heterocyclic carbene (NHC)–CS₂ adducts commonly support one-electron-reduced radical-anion states, the DAC–CS₂ framework exhibits a particularly strong capacity for further reduction. In the Pd and Ni clusters, the ligand accepts two electrons and adopts a planarized dianionic (L²⁻) electronic structure. This strong preference for the two-electron-reduced state also raised an important question: can the intermediate one-electron-reduced DAC–CS₂ monoanion (L⁻) be stabilized independently? The present cluster chemistry did not provide such a species, making controlled access to the L⁻ oxidation state a key target for subsequent studies.

Evidence for Multiple Accessible Redox States

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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