Organometallic Chemistry. Catalysis by nickel in its high oxidation state

The Tiara Nickel Cluster Story from Theory to Catalytic Applications

As a transition material between bulk materials and individual atoms, nickel clusters have interesting electrical and structural characteristics that could make them useful as catalysts. To examine the catalytic efficiency of nickel clusters in different applications, this Review combines experimental techniques with density functional theory (DFT). Researchers have shown that nickel clusters can activate and alter tiny molecules like CO, NO, and H2 through DFT studies that delve deeply into their electronic structures, adsorption mechanisms, and stability. These findings lay the groundwork for the development of effective catalysts for various processes. Nickel clusters considerably improve the hydrogen evolution reaction (HER), indicating their promise for renewable energy conversion. Furthermore, electrocatalysis for the oxygen evolution reaction (OER) showcases the electrochemical performance of nickel clusters, demonstrating their stability and efficiency. The adaptability of nickel clusters is further demonstrated by their use in nitrogen reduction to ammonia. Experimental data confirm that these clusters are good catalysts for this crucial industrial activity. Hydrocarbon reforming and pollutant degradation are two areas in which research has shown that nickel clusters can be useful in thermal reactions. X-ray absorption spectroscopy (XAS) and environmental transmission electron microscopy (ETEM) are examples of in situ characterization techniques that support theoretical predictions by providing real-time insights into the structural alterations and active sites of nickel clusters during these processes. Preparing the way for future research and practical applications in energy and environmental technologies, this thorough study highlights the great potential of nickel clusters in constructing sustainable and efficient catalytic systems.

High-Valent NiIII and NiIV Species Relevant to C-C and C-Heteroatom Cross-Coupling Reactions: State of the Art

Ni catalysis constitutes an active research arena with notable applications in diverse fields. By analogy with its parent element palladium, Ni catalysts provide an appealing entry to build molecular complexity via cross-coupling reactions. While Pd catalysts typically involve a M⁰/M^II redox scenario, in the case of Ni congeners the mechanistic elucidation becomes more challenging due to their innate properties (like enhanced reactivity, propensity to undergo single electron transformations vs. 2e⁻ redox sequences or weaker M–Ligand interaction). In recent years, mechanistic studies have demonstrated the participation of high-valent Ni^III and Ni^IV species in a plethora of cross-coupling events, thus accessing novel synthetic schemes and unprecedented transformations. This comprehensive review collects the main contributions effected within this topic, and focuses on the key role of isolated and/or spectroscopically identified Ni^III and Ni^IV complexes. Amongst other transformations, the resulting Ni^III and Ni^IV compounds have efficiently accomplished: i) C–C and C–heteroatom bond formation; ii) C–H bond functionalization; and iii) N–N and C–N cyclizative couplings to forge heterocycles.

CO, NO, and SO adsorption on Ni nanoclusters: a DFT investigation

Nickel nanoclusters are very promising for catalysis-related applications, especially involving chemical reactions with polluting molecules, such as carbon, nitrogen, and sulfur monoxides, which are directly or indirectly involved in serious environmental pollution problems. Therefore, it is of utmost importance to improve the understanding of the interaction between Ni nanoclusters and diatomic molecules, such as CO, NO, and SO, to provide insights into real subnano catalysts. Thus, here, we report an ab initio investigation based on density functional theory calculations within van der Waals D3 corrections to investigate the adsorption properties of CO, NO, and SO on Ni nanoclusters. From energetic and electronic criteria applied to Nin nanoclusters (n = 2-15), we selected Ni6 (octahedron) and Ni10 (triangular pyramid) nanoclusters as supports. According to our analyses, the molecular adsorption increases the stability of Ni nanoclusters, especially for Ni6 systems. The interaction intensity is larger for SO than for NO and CO in adsorbed systems, and the strong OS-Ni interaction is responsible for the well-known sulfur poisoning on transition-metal systems. The lowest energy adsorption sites are onefold for CO/Ni6, NO/Ni6, and CO/Ni10; twofold for NO/Ni10; and threefold for SO/Ni6 and CO/Ni10, where CO and NO molecules sustain linear and perpendicular geometries, while SO geometry changes to a bent configuration resulting from a sideways adsorption. The equilibrium bond lengths of the molecules expand upon adsorption, from 0.9% (NO/Ni6/10) to 11.3% (SO/Ni6/10), consequently, the internal molecular bond strengths decrease, since there is a reduction in the molecular stretching frequencies. This result occurs most strongly for SO followed by NO and CO systems, which was confirmed by an estimation of the energetic contribution of the distortion after the adsorption process. Thus, the strong S-Ni interaction, given by SO chemisorption on hollow sites with a sideways interaction, implies an energetic decrease and, consequently, a part of the energy gained from the SO-Ni interaction is from the SO and nanocluster distortions. Ultimately, using the energy decomposition analysis (from SAPT0) for XO/Ni6 systems, we improved the understanding of the CO and NO (SO) singlet (doublet) spin multiplicities' interaction with Ni6 nanoclusters.

Alkyliodines in High Oxidation State: Enhanced Synthetic Possibilities and Accelerated Catalyst Turn-Over

In contrast to aryliodine(III) compounds, which have matured into a particularly attractive class of oxidants in modern synthesis, the synthetic potential of related alkyliodine(III) derivatives has remained widely underestimated. This is surprising since several unique synthetic possibilities arise directly from the low stability of their central carbon-iodine bond. In this respect, these high-oxidation-state iodine compounds resemble environmentally benign variants of the prominent metal counterparts such as those derived from palladium, nickel and copper. This Concept article summarizes the general reactivity trends in alkyliodine(III) chemistry and discusses selected examples of their strategic use as highly reactive, transient species in organic synthesis and homogeneous catalysis.

Transient Formation and Reactivity of a High-Valent Nickel(IV) Oxido Complex

A reactive high-valent dinuclear nickel(IV) oxido bridged complex is reported that can be formed at room temperature by reaction of [(L)2Ni(II)2(μ-X)3]X (X = Cl or Br) with NaOCl in methanol or acetonitrile (where L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-μ-oxido-bridged structure as [(L)2Ni(IV)2(μ-O)3]2+. Its structure and its reactivity with organic substrates are demonstrated through a combination of UV-vis absorption, resonance Raman, 1H NMR, EPR, and X-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods. The identification of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidations.

Gas-Phase and Computational Study of Identical Nickel- and Palladium-Mediated Organic Transformations Where Mechanisms Proceeding via M(II) or M(IV) Oxidation States Are Determined by Ancillary Ligands

Gas-phase studies utilizing ion-molecule reactions, supported by computational chemistry, demonstrate that the reaction of the enolate complexes [(CH2CO2-C,O)M(CH3)](-) (M = Ni (5a), Pd (5b)) with allyl acetate proceed via oxidative addition to give M(IV) species [(CH2CO2-C,O)M(CH3)(η(1)-CH2-CH═CH2)(O2CCH3-O,O')](-) (6) that reductively eliminate 1-butene, to form [(CH2CO2-C,O)M(O2CCH3-O,O')](-) (4). The mechanism contrasts with the M(II)-mediated pathway for the analogous reaction of [(phen)M(CH3)](+) (1a,b) (phen = 1,10-phenanthroline). The different pathways demonstrate the marked effect of electron-rich metal centers in enabling higher oxidation state pathways. Due to the presence of two alkyl groups, the metal-occupied d orbitals (particularly dz(2)) in 5 are considerably destabilized, resulting in more facile oxidative addition; the electron transfer from dz(2) to the C═C π* orbital is the key interaction leading to oxidative addition of allyl acetate to M(II). Upon collision-induced dissociation, 4 undergoes decarboxylation to form 5. These results provide support for the current exploration of roles for Ni(IV) and Pd(IV) in organic synthesis.

Organonickel(IV) chemistry: a new catalyst?

With scorpionate ligands finding their way into organonickel chemistry, the state of the art of present-day nickel(IV) chemistry is highlighted. Will rapid CX coupling reactions emerge as a domain of higher-oxidation-state nickel chemistry?