The ?Nickel Effect?

Alkali-metal nickelates: catalytic cross-coupling, clusters and coordination complexes

Alkali-metal nickelates are a class of highly reactive heterobimetallic complexes derived from Ni(0)-olefins and polar organo-alkali-metal reagents. First reported over 50 years ago, it is only in recent years that these overlooked complexes have found formidable roles in sustainable catalysis and beyond. In this article, we will showcase the emerging catalytic applications of lithium nickelates and discuss the mechanisms by which these heterobimetallic complexes facilitate challenging cross-coupling reactions. We will also review the unique structure and bonding of alkali-metal nickelates, as interrogated by X-ray crystallography and complementary bonding analysis, and finally explore the diverse coordination and co-complexation chemistry of these heterobimetallic complexes.

Stoichiometric and Catalytic Lithium Nickelate-Mediated C-F Bond Alkynylation of Fluoroarenes

Low-valent nickelates have recently been shown to be key intermediates that facilitate challenging cross-coupling reactions under mild conditions. Expanding the synthetic potential of these heterobimetallic complexes, herein we report the success of trilithium nickelate Li3(TMEDA)3Ni(C≡C-Ph)3 in promoting stoichiometric C-F activation of assorted aryl fluorides furnishing novel mixed Li/Ni(0) or Li/Ni(II) species depending on the substrate and conditions employed. These stoichiometric successes can be upgraded to catalytic regimes to enable the atom-efficient alkynylation of aryl fluorides and polyfluoroarenes with lithium acetylides and precatalyst Ni(COD)2, which operates without the intervention of external ligands, Cu cocatalysts, or additives.

Quantum chemical calculations for reaction prediction in the development of synthetic methodologies

Quantum chemical calculations have been used in the development of synthetic methodologies to analyze the reaction mechanisms of the developed reactions. Their ability to estimate chemical reaction pathways, including transition state energies and connected equilibria, has led researchers to embrace their use in predicting unknown reactions. This perspective highlights strategies that leverage quantum chemical calculations for the prediction of reactions in the discovery of new methodologies. Selected examples demonstrate how computation has driven the development of unknown reactions, catalyst design, and the exploration of synthetic routes to complex molecules prior to often laborious, costly, and time-consuming experimental investigations.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Catalytic Properties of Two Complexes of chromium(III) and cobalt(II) with Nitrilotriacetate, Dipicolinate, and 4-Acetylpyridine

In this paper, a synthesis of two innovative coordination compounds, based on chromium(III) and cobalt(II) ions with N,O-donor ligands (nitrilotriacetate, dipicolinate) and 4-acetylpyridine, is reported. The obtained metal-organic compounds were structurally characterized using the single-crystal X-ray diffraction (XRD) method. The well-defined chromium(III) and cobalt(II) complexes were used as precatalysts in the oligomerization reaction of 2-chloro-2-propen-1-ol and 2-propen-1-ol with methylaluminoxane (MMAO) as an activator. The products of the oligomerization reaction were subjected to full physicochemical characteristics, i.e., time-of-flight mass spectrometry (MALDI-TOF-MS), TGA, and differential scanning calorimetry (DSC) methods. The catalytic activity of the precatalysts in both reactions was calculated and compared with other catalysts known in the literature.

Diphenylacetylene stabilised alkali-metal nickelates: synthesis, structure and catalytic applications

Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li₄(solv)_n(Ar)₄Ni₂{μ₂:η²,η²-Ph-CC-Ph} (where solv = Et₂O, THF; Ar = Ph, o-Tol, naphthyl, 4-^tBu-C₆H₄) which can be accessed through the combination of Ni(COD)₂, Ph-CC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMO^tBu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li₂(solv)_n(Ar)₂Ni{η²-Ph-CC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.

Ethylene Dimerization and Oligomerization Using Bis(phosphino)boryl Supported Ni Complexes

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (^RPBP)NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol_ethylene·mol_Ni^-1·s^-1 for the formation of butenes with 41(1)% selectivity for 1-butene using (^PhPBP)NiBr, and 68 mol_ethylene·mol_Ni^-1·s^-1 for butenes production with 87.2(3)% selectivity for 1-butene using (^tBuPBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and (^tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C₄ through C₂₀ products was achieved, while the use of (^PhPBP)NiBr as the pre-catalyst retained selectivity for C₄ products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni-B unit of the (^RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Influence of N- and P-substituents in N-aryl-phosphinoglycine ligands on the selectivity of Ni-catalysed ethylene oligomerization

Quantum-chemical calculations were performed to rationalize the experimental molecular weight distribution of α-olefin products, revealing the main mechanistic models of the process.

Supported σ-Complexes of Li-C Bonds from Coordination of Monomeric Molecules of LiCH3, LiCH2CH3 and LiC6H5 to MoMo Bonds

LiCH₃ and LiCH₂CH₃ react with the complex [Mo₂(H)₂(μ‐Ad^Dipp2)₂(thf)₂] (1⋅thf) with coordination of two molecules of LiCH₂R (R=H, CH₃) and formation of complexes [Mo₂{μ‐HLi(thf)CH₂R}₂(Ad^Dipp2)₂], 5⋅LiCH₃ and 5⋅LiCH₂CH₃, respectively (Ad^Dipp2=HC(NDipp)₂; Dipp=2,6‐ⁱPr₂C₆H₃; thf=C₄H₈O). Due to steric hindrance, only one molecule of LiC₆H₅ adds to 1⋅thf generating the complex [Mo₂(H){μ‐HLi(thf)C₆H₅}(μ‐Ad^Dipp2)₂], (4⋅LiC₆H₅). Computational studies disclose the existence of five‐center six‐electron bonding within the H−Mo≣Mo−C−Li metallacycles, with a mostly covalent H−Mo≣Mo−C group and predominantly ionic Li−C and Li−H interactions. However, the latter bonds exhibit non‐negligible covalency, as indicated by X‐ray, computational data and the large one‐bond ^6,7Li,¹H and ^6,7Li,¹³C NMR coupling constants found for the three‐atom H−Li−C chains. By contrast, the phenyl group in 4⋅LiC₆H₅ coordinates in an η² fashion to the lithium atom through the ipso and one of the ortho carbon atoms.

The Anionic Pathway in the Nickel-Catalysed Cross-Coupling of Aryl Ethers

The Ni‐catalysed cross‐coupling of aryl ethers is a powerful method to forge new C−C and C−heteroatom bonds. However, the inert C(sp²)−O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni⁰ centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron‐rich hetero‐bimetallic nickelates by adding organometallic nucleophiles to a Ni⁰ centre. Assessing the rich co‐complexation chemistry between Ni(COD)₂ and PhLi has led to the structures and solution‐state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni⁰‐ate complexes plays a key role in the catalytic cycle.

Coordination of LiH Molecules to Mo≣Mo Bonds: Experimental and Computational Studies on Mo2LiH2, Mo2Li2H4, and Mo6Li9H18 Clusters

The reactions of LiAlH₄ as the source of LiH with complexes that contain (H)Mo≣Mo and (H)Mo≣Mo(H) cores stabilized by the coordination of bulky Ad^Dipp2 ligands result in the respective coordination of one and two molecules of (thf)LiH, with the generation of complexes exhibiting one and two HLi(thf)H ligands extending across the Mo≣Mo bond (Ad^Dipp2 = HC(NDipp)₂; Dipp = 2,6-ⁱPr₂C₆H₃; thf = tetrahydrofuran, C₄H₈O). A theoretical study reveals the formation of Mo–H–Li three-center–two-electron bonds, supplemented by the coordination of the Mo≣Mo bond to the Li ion. Attempts to construct a [Mo₂{HLi(thf)H}₃(Ad^Dipp2)] molecular architecture led to spontaneous trimerization and the formation of a chiral, hydride-rich Mo₆Li₉H₁₈ supramolecular organization that is robust enough to withstand the substitution of lithium-solvating molecules of tetrahydrofuran by pyridine or 4-dimethylaminopyridine.

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Nickel Catalyzed Olefin Oligomerization and Dimerization

Brought to life more than half a century ago and successfully applied for high-value petrochemical intermediates production, nickel-catalyzed olefin oligomerization is still a very dynamic topic, with many fundamental questions to address and industrial challenges to overcome. The unique and versatile reactivity of nickel enables the oligomerization of ethylene, propylene, and butenes into a wide range of oligomers that are highly sought-after in numerous fields to be controlled. Interestingly, both homogeneous and heterogeneous nickel catalysts have been scrutinized and employed to do this. This rare specificity encouraged us to interlink them in this review so as to open up opportunities for further catalyst development and innovation. An in-depth understanding of the reaction mechanisms in play is essential to being able to fine-tune the selectivity and achieve efficiency in the rational design of novel catalytic systems. This review thus provides a complete overview of the subject, compiling the main fundamental/industrial milestones and remaining challenges facing homogeneous/heterogeneous approaches as well as emerging catalytic concepts, with a focus on the last 10 years.

Ni-Based Complexes in Selective Ethylene Oligomerization Processes

Linear alpha-olefins are widely used in the petrochemical industry and the world demand for these compounds increases annually. At present, the main method for producing linear alpha-olefins is the homogeneous catalytic ethylene oligomerization. This review presents modern nickel catalysts for this process, mainly systems for obtaining of one of the most demanded oligomer—1-butene—which is used for the production of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). The dependence of the catalytic performance on the composition and the structure of the used activated complexes, the electronic and coordination states of the nickel center was considered.

Mechanistic insights into alkene chain growth reactions catalyzed by nickel active sites on ordered microporous and mesoporous supports

This mini-review discusses the mechanistic details underlying the activation and deactivation behavior, and the kinetics and selectivity among alkene isomer products, observed on Ni-based ordered porous materials during light alkene oligomerization.

Towards highly active and stable nickel-based metal–organic frameworks as ethylene oligomerization catalysts

The crucial impact of metal coordination on selectivity and leaching is elucidated by comparing MOFs constructed from different clusters and linkers.

A broadly tunable synthesis of linear α-olefins

The catalytic synthesis of linear α-olefins from ethylene is a technologically highly important reaction. A synthesis concept allowing the formation of selective products and various linear α-olefin product distributions with one catalyst system is highly desirable. Here, we describe a trimetallic catalyst system (Y–Al–Ni) consisting of a rare earth metal polymerization catalyst which can mediate coordinative chain transfer to triethylaluminum combined with a simultaneously operating nickel β-hydride elimination/transfer catalyst. This nickel catalyst displaces the grown alkyl chains forming linear α-olefins and recycles the aluminum-based chain transfer agent. With one catalyst system, we can synthesize product spectra ranging from selective 1-butene formation to α-olefin distributions centered at 850 gmol⁻¹ with a low polydispersity. The key to this highly flexible linear α-olefin synthesis is the easy tuning of the rates of the Y and Ni catalysis independently of each other. The reaction is substoichiometric or formally catalytic regarding the chain transfer agent.

Linear α-olefins are important bulk chemicals annually produced in megaton scale. Here, the authors report a trimetallic catalyst system for the synthesis of 1-butene and a broad range of α-olefins and achieve control over chain length by tuning the reaction rates of the nickel and yttrium catalysts.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Bifunctional hybrid catalysts derived from Cu/Zn-based nanoparticles for single-step dimethyl ether synthesis

Model kit for bifunctional catalysts: colloidal Cu/Zn-based nanoparticles were synthesized and used as building blocks in syngas to dimethyl ether (STD) catalysts.

α-Diimine nickel complexes of ethylene and related alkenes

A series of α-diimine-supported nickel complexes with a coordinated alkene, [LNi(alkene)], were synthesized and their structure and bonding were studied.

The electronic structures of small Ni(n) (n=2-4) clusters and their interactions with ethylene and triplet oxygen: a theoretical study

Density functional theory (DFT) calculations of small nickel clusters and their interacting systems are carried out using the BLYP and B97-2 methods, after DFT calibration. All bare nickel clusters in this study have high multiplicities and are paramagnetic. Our results for the interactions between ethylene and oxygen with Ni(n) (n=2-4) clusters at different adsorption modes show that for ethylene, π-orientation is preferred, and that oxygen adsorption in a bridge mode is stronger than on-top coordination. Vibrational frequency analysis reveals that the vibrational modes of ethylene π-coordinated to nickel clusters converge toward the corresponding value for surface-bound ethylene, as the cluster size increases from two to four, showing that finite clusters can be used as localized models for ligand adsorption on nickel surfaces. We also calculate DFT global reactivity descriptors, chemical potential and hardness, and use these to predict the relative stability and reactivity of each bare cluster.