Understanding the Early Stages of Nickel Sulfide Nanocluster Growth: Isolation of Ni3 , Ni4 , Ni5 , and Ni8 Intermediates

A homologous series of macrocyclic Ni clusters: synthesis, structures, and catalytic properties

Due to their intriguing ring structures and promising applications, nickel-thiolate clusters, such as [Nin(SR)2n] (n = 4-6), have attracted tremendous interest. However, investigation of the synthesis, structures, and properties of macrocyclic Nin clusters (n > 8) has been seriously impeded. In this work, a homologous series of macrocyclic nickel clusters, Nin(4MPT)2n (n = 9-12), was fabricated by using 4-methylphenthiophenol (4MPT) as the ligand. The structures and compositions of the clusters were determined by single-crystal X-ray diffraction (SXRD) in combination with electrospray ionization mass spectrometry (ESI-MS). Experimental results and theoretical calculations show that the electronic structures of the clusters do not change significantly with the increase of Ni atoms. The coordination interactions between Ni and S atoms in [NiS4] subunits are proved to play a crucial rule in the remarkable stability of Ni clusters. Finally, these clusters display excellent catalytic activity towards the reduction of p-nitrophenol, and a linear correlation between catalytic activity and ring size was revealed. The study provides a facile approach to macrocyclic homoleptic nickel clusters, and contributes to an in-depth understanding of the structure-property correlations of nickel clusters at the atomic level.

Size-Dependent Electrocatalytic Water Oxidation Activity for a Series of Atomically Precise Nickel-Thiolate Clusters

The development of renewable energy technologies is critical for reducing global carbon emissions. Water splitting offers a promising renewable energy mechanism by converting water into H₂ and O₂ gas, which can directly power fuel cells or be utilized as chemical feedstocks. To increase the efficiency of water splitting, catalysts must be developed for the water reduction and water oxidation half-reactions. To promote rational catalyst design, atomically precise metal clusters (APMCs) with earth-abundant metals provide a framework for developing both structure-activity relationships and cost-effective catalysts. Previous reports on the water oxidation activity of nickel-thiolate clusters [Ni_n(SR)_2n] have not developed a systematic description of a possible size-activity relationship. Utilizing recent advancements in preparative chromatography for isolating APMCs, we have synthesized a series of Ni_n(SR)_2n (n = 4, 5, or 6) clusters as electrocatalysts for the oxygen evolution reaction. We discovered a clear size-activity and size-stability trend, with intrinsic activity and stability increasing with cluster size. Using density functional theory, we found that intrinsic activity is inversely correlated to intermediate binding energy, and by extension the oxidation potential of each cluster. Our work demonstrates the ability of APMCs to uncover previously unknown structure-activity relationships that can guide future catalyst design.

[Ni30S16(PEt3)11]: An Open-shell Nickel Sulfide Nanocluster with a “Metal-like” Core

Reaction of [Ni(1,5-cod)₂] (30 equiv.) with PEt₃ (46 equiv.) and S₈ (1.9 equiv.) in toluene, followed by heating at 115 °C for 16 h, results in the formation of the atomically precise nanocluster (APNC), [Ni₃₀S₁₆(PEt₃)₁₁] (1), in 14% isolated yield. Complex 1 represents the largest open-shell Ni APNC yet isolated. In the solid state, 1 features a compact “metal-like” core indicative of a high degree of Ni–Ni bonding. Additionally, SQUID magnetometry suggests that 1 possesses a manifold of closely-spaced electronic states near the HOMO–LUMO gap. In situ monitoring by ESI-MS and ³¹P{¹H} NMR spectroscopy reveal that 1 forms via the intermediacy of smaller APNCs, including [Ni₈S₅(PEt₃)₇] and [Ni₂₆S₁₄(PEt₃)₁₀] (2). The latter APNC was also characterized by X-ray crystallography and features a nearly identical core structure to that found in 1. This work demonstrates that large APNCs with a high degree of metal–metal bonding are isolable for nickel, and not just the noble metals.

The atomically-precise nanocluster, [Ni₃₀S₁₆(PEt₃)₁₁], features a compact “metal-like” core indicative of a high degree of Ni–Ni bonding, along with an open-shell ground state.

[Ni23Se12(PEt3)13] Revisited: Isolation and Characterization of [Ni23Se12Cl3(PEt3)10]

The reaction of [Ni(1,5-COD)₂] (1.0 equiv), PEt₃ (0.04 equiv), SePEt₃ (0.52 equiv), and [NiCl₂(PEt₃)₂] (0.07 equiv) in a mixture of toluene and THF results in the formation of [Ni₂₃Se₁₂Cl₃(PEt₃)₁₀] (1), which can be isolated in moderate yield after workup. Complex 1 was characterized by NMR spectroscopy, ESI-MS, and X-ray crystallography. This open-shell nanocluster features a central [Ni₁₃]⁷⁺ anticuboctahedral kernel, which is encapsulated by a [Ni₁₀(μ-Se)₉Cl₃]^- shell, along with ten PEt₃ ligands and three (μ₄-Se)^2- ligands. On the basis of our spectroscopic and crystallographic analysis, coupled with in situ spectroscopic monitoring, we believe that the previously reported nanocluster, [Ni₂₃Se₁₂(PEt₃)₁₃], is actually better formulated as [Ni₂₃Se₁₂Cl₃(PEt₃)₁₀].

[Ni8(CNtBu)12][Cl]: A nickel isocyanide nanocluster with a folded nanosheet structure

The reaction of 1.75 equiv of ^tBuNC with Ni(1,5-COD)₂, followed by crystallization from benzene/pentane, resulted in the isolation of [Ni₈(CN^tBu)₁₂][Cl] (2) in low yields. Similarly, the reaction of Ni(1,5-COD)₂ with 0.6 equiv of [Ni(CN^tBu)₄], followed by addition of 0.08 equiv of I₂, resulted in the formation of [Ni₈(CN^tBu)₁₂][I] (3), which could be isolated in 52% yield after work-up. Both 2 and 3 adopt folded nanosheet structures in the solid state, characterized by two symmetry-related planar Ni₄ arrays, six terminally bound ^tBuNC ligands, and six ^tBuNC ligands that adopt bridging coordination modes. The metrical parameters of the six bridging ^tBuNC ligands suggest that they have been reduced to their [^tBuNC]^2- form. In contrast to the nanosheet structures observed for 2 and 3, gas phase Ni₈ is predicted to feature a compact bisdisphenoid ground state structure. The strikingly different structural outcomes reveal the profound structural changes that can occur upon addition of ligands to bare metal clusters. Ultimately, the characterization of 2 and 3 will enable more accurate structural predictions of ligand-protected nanoclusters in the future.