Diamidocarbene-derived palladium and nickel–sulfur clusters

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.

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Metal clusters are compounds consisting of a small number of closely packed metal atoms stabilized by ligands.¹ These clusters exhibit unique properties owing to the metal–metal and metal–ligand interactions, distinguishing them from individual atoms and bulk metals.² Metal–sulfur clusters, incorporating sulfides and/or thiolates as ligands, are particularly notable for their distinctive reactivity and magnetic properties.³ For example, iron–sulfur clusters are widely used as prosthetic groups in biological systems, facilitating electron transfers and catalyzing multi-electron transformations of small molecules (Fig. 1A).⁴ These clusters often incorporate other transition metals, such as nickel, vanadium, or molybdenum, enhancing their versatility and functionality in biological systems.⁵ In addition, cobalt–sulfur clusters have recently gained significant attention owing to their unique electronic properties, rendering them promising candidates for novel electronic materials.⁶ Other metal–sulfur clusters incorporating different transition and main-group metals have also been extensively studied.⁷

Fig. 1 (A) Typical structure of an iron–sulfur cluster consisting of four iron atoms. (B) Tetrahedral and square-planar coordination geometries.

Exploring new geometries and connectivities in metal–sulfur clusters is essential for developing clusters with novel functions and broadening their applications. Thus, the objective of this study was to expand this scope by using tetravalent metal centers with square-planar coordination geometry, such as Pd(II) and Ni(II), which are less commonly used in metal–sulfur cluster chemistry (Fig. 1B).⁸ Specifically, we synthesized and characterized novel metal–sulfur clusters with square-planar Pd(II) and Ni(II), using a new ligand derived from diamidocarbene (DAC) and carbon disulfide (CS₂).

N-heterocyclic carbenes (NHCs) form stable zwitterionic adducts with CS₂.⁹ These imidazolium dithiocarboxylates are versatile ligands for coordinating various transition metals.¹⁰ We aimed to extend this chemistry to more electrophilic singlet carbene, such as DAC reported by Bielawski et al.¹¹

The DAC–CS₂ adduct (L) was synthesized from the reaction between DAC and CS₂ in diethyl ether (Fig. 2A), yielding a red solid with 88% yield. Crystal structure determination revealed, as expected, a molecular structure consistent with zwitterionic DAC–CS₂, featuring a C_DAC–C_CS2 bond length of 1.499(11) Å and an N–C–C–S torsion angle of 90.7(8)°, indicating the single bond character of C_DAC–C_CS2 (Fig. 2B). These structural parameters were very similar to those of NHC–CS₂ adducts, which typically show a C_NHC–C_CS2 bond length of 1.49 Å and an N–C–C–S torsion angle of 90°.^9f

Fig. 2 (A) Synthesis of DAC–CS₂ adduct L. (B) Solid-state structure of L obtained using single-crystal X-ray crystallography. The thermal ellipsoids are shown at the 50% probability level with carbon (gray), sulfur (yellow), nitrogen (blue), and oxygen (red). Mes substituents are shown as sticks. Hydrogen atoms are omitted for clarity.

After synthesizing the ligand L, we investigated its coordination reactivity with Pd(0) and Ni(0) sources. The reaction of L with tris(dibenzylideneacetone)dipalladium (Pd₂dba₃) in acetonitrile produced black crystals in 53% yield (Fig. 3A). Single-crystal X-ray crystallography revealed that the molecular structure of the product was an unexpected tetrapalladium cluster (L₄Pd₄) with a previously unknown Pd₄S₈ core structure exhibiting pseudo-S₄ symmetry (Fig. 3B and Table S1, ESI†). Interestingly, each palladium center displayed distorted square-planar PdS₄ coordination, consistent with a Pd oxidation state of +2 (sum of S–Pd–S angles for each palladium center was approximately 354°). Unlike the free L, the DAC–CS₂ ligands in the L₄Pd₄ cluster displayed planar geometry with N–C–C–S torsion angles of 9.3(4)–10.7(4)°. The C_DAC–C_CS2 bond lengths for the ligands (1.359(4)–1.361(4) Å) were significantly smaller than that of the free L, indicating double-bond character. These structural parameters suggest that each ligand was reduced by two electrons to L²⁻, completing the charge balance of the cluster. Notably, the analogous reaction of NHC–CS₂ adducts with Pd(0) resulted in a one-electron reduction, forming a radical anionic ligand.¹² This difference clearly illustrates the stronger electron-accepting ability of DAC compared to typical NHCs.

Fig. 3 (A) Syntheses of the tetrapalladium (L₄Pd₄) and tetranickel clusters (L₄Ni₄). [Pd] = Pd₂dba₃, [Ni] = Ni(cod)₂ (B) Solid-state structure of L₄Pd₄ and (C) L₄Ni₄ obtained using single-crystal X-ray crystallography. The thermal ellipsoids are shown at the 50% probability level with palladium (turquoise) and nickel (green). Mes substituents are shown as sticks. Hydrogen atoms, solvent molecules, and disorders are omitted for clarity.

When L was reacted with Ni(cod)₂ as a Ni(0) source, brown crystals were formed and isolated in 64% yield. Single-crystal X-ray crystallography revealed the formation of a tetranickel cluster (L₄Ni₄) with a structure analogous to the L₄Pd₄ cluster (Fig. 3C and Table S2, ESI†). Each nickel center exhibited a distorted square-planar geometry, similar to that in the L₄Pd₄ cluster (sum of S–Ni–S angles was approximately 356°). The N–C–C–S torsion angles ranged from 8.0(4)° to 8.4(4)°, and the C_DAC–C_CS2 bond lengths were 1.357(4)–1.359(4) Å, consistent with the double-bond character of C_DAC–C_CS2 and dianionic nature (L²⁻) of the ligands.

Proton nuclear magnetic resonance (¹H NMR) spectra of L₄Pd₄ and L₄Ni₄ displayed a single set of signals from DAC (Fig. S2 and S3, ESI†), consistent with S₄ symmetry, while the desymmetrization of DAC–CS₂ peaks indicated the formation of a rigid structure that prevented the rotation of C_DAC–C_CS2 bonds, consistent with their double-bond character. The addition of free L to the solution of L₄Pd₄ or L₄Ni₄ did not result in peak broadening in the ¹H NMR spectra, confirming that the clusters retained their structures in solution (Fig. S4 and S5, ESI†).

Interestingly, during the synthesis of L₄Ni₄, a trinickel cluster byproduct (L₄Ni₃) was also observed (Fig. 4A). Although this byproduct could not be isolated, we crystallized the cluster and examined its structure (Fig. 4B and Table S3, ESI†). Similar to the L₄Ni₄ cluster, each nickel center in L₄Ni₃ exhibited distorted square-planar NiS₄ coordination, consistent with the Ni oxidation state of +2. One of the four ligands exhibited different structural parameters. L1–L3 had a geometry similar to those of the ligands in L₄Pd₄ and L₄Ni₄ (C_DAC–C_CS2 bond lengths of 1.338(13)–1.358(13) Å and N–C–C–S torsion angles of 7.8(8)–9.4(7)°), suggestive of a formal dianionic ligand L²⁻. In contrast, L4 had a C_DAC–C_CS2 bond length of 1.523(12) Å and an N–C–C–S torsion angle of 76.5(9)°, resembling the free ligand L. These structural parameters indicate the redox-non-innocence of ligand L, confirming its ability to change oxidation states from 0 to −2 depending on the coordination environment (Fig. 4C).

Fig. 4 (A) Structure of the trinickel cluster (L₄Ni₃). (B) Solid-state structure of L₄Ni₃ obtained using single-crystal X-ray crystallography. The thermal ellipsoids are shown at the 50% probability level with colors as in Fig. 3. Mes substituents are shown as sticks. Hydrogen atoms, solvent molecules, and disorders are omitted for clarity. (C) Ligands in L₄Ni₃ with different structural parameters. C_DAC–C_CS2 bond lengths, N–C–C–S torsion angles, Wiberg bond index (WBI) of C_DAC–C_CS2 bonds, and formal oxidation states (FOSs) are shown.

To further validate our observations and gain insights into the electronic structure of the clusters, density functional theory (DFT) calculations were performed. The optimized geometries (TPSSh/Def2-TZVP) closely matched the experimental structures obtained from X-ray crystallography (Tables S4–S6, ESI†). The ligands in the L₄Pd₄ and L₄Ni₄ clusters showed optimized C_DAC–C_CS2 bond lengths ranging from 1.359 to 1.361 Å and N–C–C–S torsion angles of 2.4° to 3.6°. These values corroborated the crystallographic data indicating the double-bond character of C_DAC–C_CS2 and planarization of the L²⁻ in the cluster environment. In contrast, the L4 ligand of the L₄Ni₃ cluster showed the calculated C_DAC–C_CS2 bond length of 1.480 Å, and N–C–C–S torsion angle of 82.4°, consistent with its zwitterionic electronic structure. Wiberg bond indices (WBIs) were calculated using the natural atomic orbital (NAO) basis set. The C_DAC–C_CS2 bonds exhibited WBIs in the range of 1.59–1.64, aligning with the observed double-bond character. In contrast, the C_DAC–C_CS2 bond in the L4 ligand of L₄Ni₃ cluster had a WBI of 1.0, consistent with the absence of the double-bond character (Fig. 4C).

DFT calculations also reproduced the stretched tetrahedron arrangement of metal atoms in the L₄Pd₄ and L₄Ni₄ clusters (Fig. 5). For example, for the L₄Pd₄ cluster, the Pd1–Pd2 (X-ray: 2.6522(6) Å; DFT: 2.665 Å) and Pd3–Pd4 (X-ray: 2.6526(5) Å; DFT: 2.665 Å) distances were notably smaller than other Pd–Pd distances (X-ray: 3.2845(4)–3.3064(4) Å; DFT: 3.310–3.311 Å). These distances were comparable to the Pd–Pd bond lengths observed in previously reported square-planar Pd(II) dimers (approximately 2.7 Å)¹³ and aligned closely with the Pd–Pd distances in bulk palladium metal (approximately 2.75 Å).¹⁴ Similarly, in the L₄Ni₄ cluster, the calculated Ni1–Ni2 and Ni3–Ni4 distances of 2.402 Å closely matched the experimental values of 2.4214(6) Å (Ni1–Ni2) and 2.4156(5) Å (Ni3–Ni4). Despite these small intermetallic distances, no significant electronic communication was observed between the metal atoms. The calculated WBIs for Pd1–Pd2 (0.18) in the L₄Pd₄ cluster and Ni1–Ni2 (0.16) in the L₄Ni₄ cluster suggest formal metal–metal bond orders close to zero, consistent with the characterization of the metal centers as square planar.

Fig. 5 DFT-optimized structure of (A) L₄Pd₄, (B) L₄Ni₄, and (C) L₄Ni₃. Only metal and sulfur atoms are shown.

In summary, we synthesized and characterized novel palladium and nickel–sulfur clusters, L₄Pd₄ and L₄Ni₄, using a DAC-derived carbon disulfide ligand. These clusters exhibited a previously unknown tetranuclear metal–sulfur cluster geometry, with each metal center displaying square-planar coordination. X-ray crystallography and DFT calculations revealed the redox-active nature of the ligand, capable of adopting formal oxidation states (FOSs) ranging from 0 to −2. While the FOS of the free ligand is 0, the ligands in the L₄Pd₄ and L₄Ni₄ clusters exhibited FOSs of −2. Interestingly, in the L₄Ni₃ cluster, one ligand retained an FOS of 0, while the other three ligands had FOSs of −2. The distinct structural features and redox properties of these palladium and nickel clusters introduce a novel dimension to metal–sulfur cluster chemistry. The potential applications of these clusters in catalysis and materials science are currently under investigation.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT) (RS-2023-00210804). This research was also supported by Global – Learning & Academic research institution for Master's, PhD students, and Postdocs (LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (RS-2023-00301938). Additionally, this work was supported by the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Korea government (Ministry of Science and ICT) (RS-2024-00403266). Finally, this work was also supported by Pusan National University Research Grant, 2022. We gratefully acknowledge Dr Dongwook Kim for his assistance with single crystal X-ray structure determination.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for L, L₄Pd₄, L₄Ni₄, and L₄Ni₃ has been deposited at the CCDC (2381957–2381960†) and can be obtained from https://www.ccdc.cam.ac.uk/.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, NMR spectra, X-ray crystal data, UV-vis spectra, DFT calculation results. CCDC 2381957–2381960. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc04582c

‡ These authors contributed equally.

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