Catalytic Carbonylative Rearrangement of Norbornadiene via Dinuclear Carbon–Carbon Oxidative Addition

Escape from Palladium: Nickel-Catalyzed Catellani Annulation

While Catellani reactions have become increasingly important for arene functionalizations, they have been solely catalyzed by palladium. Here we report the first nickel-catalyzed Catellani-type annulation of aryl triflates and chlorides to form various benzocyclobutene-fused norbornanes in high efficiency. Mechanistic studies reveal a surprising outer-sphere concerted metalation/deprotonation pathway during the formation of the nickelacycle, as well as the essential roles of the base and the triflate anion. The reaction shows a broad functional group tolerance and enhanced regioselectivity compared to the corresponding palladium catalysis.

NHC-BIAN-Cu(I)-Catalyzed Friedländer-Type Annulation of 2-Amino-3-(per)fluoroacetylpyridines with Alkynes on Water

The direct catalytic alkynylation/dehydrative cyclization of 2-amino-3-trifluoroacetyl-pyridines on water was developed for the efficient synthesis of a broad range of fluorinated 1,8-naphthyridines from terminal alkynes. A novel N-heterocyclic carbene (NHC) ligand system that combines a π-extended acenaphthylene backbone with sterically bulky pentiptycene pendant groups was successfully utilized in a copper- or silver-mediated cyclization. Computational analysis of the reaction pathway supports our explanation of the different experimental conversions and yields for the set of copper and silver catalysts. The impact of steric hindrance at the metal center and the flexibility of substituents on the imidazole ring of the NHC on catalytic performance are also discussed.

Dinickel Active Sites Supported by Redox-Active Ligands

Redox reactions that take place in enzymes and on the surfaces of heterogeneous catalysts often require active sites that contain multiple metals. By contrast, there are very few homogeneous catalysts with multinuclear active sites, and the field of organometallic chemistry continues to be dominated by the study of single metal systems. Multinuclear catalysts have the potential to display unique properties owing to their ability to cooperatively engage substrates. Furthermore, direct metal-to-metal covalent bonding can give rise to new electronic configurations that dramatically impact substrate binding and reactivity. In order to effectively capitalize on these features, it is necessary to consider strategies to avoid the dissociation of fragile metal-metal bonds in the course of a catalytic cycle. This Account describes one approach to accomplishing this goal using binucleating redox-active ligands.In 2006, Chirik showed that pyridine-diimines (PDI) have sufficiently low-lying π* levels that they can be redox-noninnocent in low-valent iron complexes. Extending this concept, we investigated a series of dinickel complexes supported by naphthyridine-diimine (NDI) ligands. These complexes can promote a broad range of two-electron redox processes in which the NDI ligand manages electron equivalents while the metals remain in a Ni(I)-Ni(I) state.Using (NDI)Ni₂ catalysts, we have uncovered cases where having two metals in the active site addresses a problem in catalysis that had not been adequately solved using single-metal systems. For example, mononickel complexes are capable of stoichiometrically dimerizing aryl azides to form azoarenes but do not turn over due to strong product inhibition. By contrast, dinickel complexes are effective catalysts for this reaction and avoid this thermodynamic sink by binding to azoarenes in their higher-energy cis form.Dinickel complexes can also activate strong bonds through the cooperative action of both metals. Norbornadiene has a ring-strain energy that is similar to that of cyclopropane but is not prone to undergoing C-C oxidative addition with monometallic complexes. Using an (NDI)Ni₂ complex, norbornadiene undergoes rapid ring opening by the oxidative addition of the vinyl and bridgehead carbons. An inspection of the resulting metallacycle reveals that it is stabilized through a network of secondary Ni-π interactions. This reactivity enabled the development of a catalytic carbonylative rearrangement to form fused bicyclic dienones.These vignettes and others described in this Account highlight some of the implications of metal-metal bonding in promoting a challenging step in a catalytic cycle or adjusting the thermodynamic landscape of key intermediates. Given that our studies have focused nearly exclusively on the (NDI)Ni₂ system, we anticipate that many more such cases are left to be discovered as other transition-metal combinations and ligand classes are explored.

Well-Defined NHC-Ni Complexes as Catalysts: Preparation, Structures and Mechanistic Studies in Cross-Coupling Reactions

Developmental studies are ongoing to discover a way to utilise new N-heterocyclic carbene (NHC)-Ni complexes as catalysts. Using a bulky NHC ligand, it is possible to synthesise an NHC/phosphine-mixed heteroleptic Ni(II) complex, which can serve as an excellent catalyst for various cross-coupling reactions. During the study of the reaction mechanisms using these Ni complexes, NHC-Ni(I) complexes were accidentally discovered, and it was observed that they exhibit excellent catalytic activity for cross-coupling reactions. The possibility of the presence of NHC-Ni(I) intermediates in these catalytic reaction pathways has been experimentally demonstrated. Depending on the type of reaction, dinuclear Ni(I) and mononuclear Ni(I) complexes can function as intermediates. The results of the investigation of each reaction mechanism are summarised, and the prospects are described.

Tuning metal-metal interactions for cooperative small molecule activation

Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small molecule activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small molecule activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliphatic linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small molecule substrates. The union of these design elements appears to be a powerful combination for analogizing critical aspects of heterogeneous and metalloenzyme catalysts.

Ruthenium-Catalyzed [2 + 2] versus Homo Diels-Alder [2 + 2 + 2] Cycloadditions of Norbornadiene and Disubstituted Alkynes: A DFT Study

The ruthenium-catalyzed [2 + 2] and homo Diels-Alder [2 + 2 + 2] cycloadditions of norbornadiene with disubstituted alkynes are investigated using density functional theory (DFT). These DFT calculations provide a mechanistic explanation for observed reactivity trends with different functional groups. Alkynyl phosphonates and norbornadiene form the [2 + 2 + 2] cycloadduct, while other functionalized alkynes afford the respective [2 + 2] cycloadduct, in excellent agreement with experimental results. The computational studies on the potential energy profiles of the cycloadditions show that the rate-determining step for the [2 + 2] cycloaddition is the final reductive elimination step, but the overall rate for the [2 + 2 + 2] cycloaddition is controlled by the initial oxidative cyclization. Two distinct mechanistic pathways for the [2 + 2 + 2] cycloaddition, cationic and neutral, are characterized and reveal that Cp*RuCl(COD) energetically prefers the cationic pathway.

Competing H2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H₂ upon substrate binding. Here we present a pyrazolate-based bis(β-diketiminato) ligand [L^Ph]^3- with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [L^Ph(Ni^II-H)₂]^- (3) are prone to intramolecular reductive H₂ elimination, and an equilibrium between 3 and orthometalated dinickel(II) monohydride complexes 2 is evidenced. 2 is shown to form via intramolecular metal-metal cooperative phenyl group C(sp²)-H oxidative addition to the dinickel(I) intermediate [L^PhNi^I₂]^- (4). While Ni^I species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at Ni^I complexes has rarely been described. The reversible H₂ and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3 and also for subsequent thermally driven formation of a twice orthometalated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for the unmasking of a highly reactive [L^PhNi^I₂]^- intermediate 4 either via elimination of H₂ from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H₂ byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.

Recent advances of dinuclear nickel- and palladium-complexes in homogeneous catalysis

In this highlight, we provide a current perspective of synthetic methodology development catalyzed by dinuclear Ni- and Pd-complexes in the past decade. The new catalytic reactivities of dinuclear Ni- and Pd-complexes are discussed.

Cooperative H2 Activation on Dicopper(I) Facilitated by Reversible Dearomatization of an "Expanded PNNP Pincer" Ligand

A naphthyridine-derived expanded pincer ligand is described that can host two copper(I) centers. The proton-responsive ligand can undergo reversible partial and full dearomatization of the naphthyridine core, which enables cooperative activation of H2 giving an unusual butterfly-shaped Cu4 H2 complex.

Multi-electron Reduction Capacity and Multiple Binding Pockets in Metal-Organic Redox Assembly at Surfaces

Metal-ligand complexation at surfaces utilizing redox-active ligands has been demonstrated to produce uniform single-site metals centers in regular coordination networks. Two key design considerations are the electron storage capacity of the ligand and the metal-coordinating pockets on the ligand. In an effort to move toward greater complexity in the systems, particularly dinuclear metal centers, we designed and synthesized tetraethyltetra-aza-anthraquinone, TAAQ, which has superior electron storage capabilities and four ligating pockets in a diverging geometry. Cyclic voltammetry studies of the free ligand demonstrate its ability to undergo up to a four-electron reduction. Solution-based studies with an analogous ligand, diethyldi-aza-anthraquinone, demonstrate these redox capabilities in a molecular environment. Surface studies conducted on the Au(111) surface demonstrate TAAQ's ability to complex with Fe. This complexation can be observed at different stoichiometric ratios of Fe:TAAQ as Fe 2p core level shifts in X-ray photoelectron spectroscopy. Scanning tunneling microscopy experiments confirmed the formation of metal-organic coordination structures. The striking feature of these structures is their irregularity, which indicates the presence of multiple local binding motifs. Density functional theory calculations confirm several energetically accessible Fe:TAAQ isomers, which accounts for the non-uniformity of the chains.

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹ P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M-L bonding to the π-ligand. The degree of backbonding correlates with ² J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.

Cyclophanes as Platforms for Reactive Multimetallic Complexes

Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N₂, O₂, CO₂) with prominent roles in geochemical element cycles or detoxification. Notable examples include the iron-molybdenum cofactor of the molybdenum-dependent nitrogenases, which catalyze N₂ fixation, and the NiFe₄S₄ cluster and the Mo(O)SCu site in various carbon monoxide dehydrogenases. The prevailing proposed reaction mechanisms for these multimetallic cofactors relies on a cooperative pathway, in which the oxidation state changes are distributed over the aggregate coupled with orbital overlap between the substrate and more than one metal ion within the cluster. Such cooperativity has also been proposed for chemical transformations at the surfaces of heterogeneous catalysts. However, the design details that afford cooperative effects and allow such reactivity to be harnessed effectively in homogeneous synthetic systems remain unclear. Relatedly, hydride donors ligated to these metal cluster cofactors are suggested as precursors to the state that reacts with substrates; here too, however, the reactivity of hydride-decorated clusters supported by weak-field ligands is underexplored. Inspired by the reactivity potential of multimetallic assemblies evidenced in biological systems, approaches to design, synthesize, and evaluate reactivity of polynuclear metal compounds have been actively explored. In a similar vein to the templating function afforded by enzyme active sites, a carefully engineered organic ligand can be employed to control metal nuclearity of the complex and the local coordination environment of each metal center. This Account presents our efforts within this field, beginning with ligand design considerations followed by a survey of observed small molecule activation by trimetallic cyclophanates. We highlight the distinct reactivity outcomes accessed by multimetallic compounds as compared to aggregates that assemble in reaction mixtures from monometallic precursors. Contributing to the opportunity for programmed cooperativity in these designed multimetallic compounds, the cyclophane also dictates the orientation of substrate binding and metal-substrate interactions, which has a prominent influence on reactivity. For example, the dinitrogen-tricopper(I) cyclophanate reacts with dioxygen with markedly different results as compared to monocopper compounds. As an unexpected outcome, one series of tricopper compounds were discovered to be competent catalysts for carbon dioxide reduction to oxalate-a formally one-electron process-hinting at an inherently broader reaction scope for weak-field clusters at lowering the barrier for one-electron pathways as well as multielectron redox transformations. Further reflecting the role of the ligand in tuning reactivity, the trimetallic trihydride cluster compounds, [M₃(μ-H)₃]³⁺ (M = Fe^II, Co^II, Zn^II), demonstrate substrate specificity for CO₂ over various other unsaturated molecules and surprising stability toward water. This series reflects the role of the local environment of a shallow ligand pocket to control substrate access. Summed together, the systems described here evidence the anticipated cooperative reactivity accessed in designed multimetallic species vs self-assembled monometallic systems (e.g., O₂ activation and O atom transfer) as well as control of substrate access by seemingly subtle structural effects. Indeed, future efforts aim to interrogate the limits of cooperativity in these systems as well as the role of ligand dynamics and sterics on reactivity.

Ni(i)–Ni(iii) cycle in Buchwald–Hartwig amination of aryl bromide mediated by NHC-ligated Ni(i) complexes

NHC-ligated Ni(i) intermediates in Buchwald–Hartwig amination of aryl halides were isolated and determined. The presence of a Ni(iii) intermediate was also indicated at low temperature.

Iron-catalyzed oxidative C–C(vinyl) σ-bond cleavage of allylarenes to aryl aldehydes at room temperature with ambient air

The iron-catalyzed C−C single bond cleavage and oxidation of allylarenes without the assistance of heteroatoms/directing groups to produce aryl aldehydes is disclosed.