Small Molecule Activation at the acriPNP Pincer-Supported Nickel Sites

ConspectusNickel pincer systems have recently attracted much attention for applications in various organometallic reactions and catalysis involving small molecule activation. Their exploration is in part motivated by the presence of nickel in natural systems for efficient catalysis. Among such systems, the nickel-containing metalloenzyme carbon monoxide dehydrogenase (CODH) efficiently and reversibly converts CO2 to CO at its active site. The generated CO moves through a channel from the CODH active site and is transported to a dinuclear nickel site of acetyl-coenzyme A synthase (ACS), which catalyzes organometallic C-S and C-C bond forming reactions. An analogous C-S bond activation process is also mediated by the nickel containing enzyme methyl-coenzyme M reductase (MCR). The nickel centers in these systems feature sulfur- and nitrogen-rich environments, and in the particular case of lactate racemase, an organometallic nickel pincer motif revealing a Ni-C bond is observed. These bioinorganic systems inspired the development of several nickel pincer scaffolds not only to mimic enzyme active sites and their reactivity but also to further extend low-valent organonickel chemistry. In this Account, we detail our continuing efforts in the chemistry of nickel complexes supported by acridane-based PNP pincer ligands focusing on our long-standing interest in biomimetic small molecule activation. We have employed a series of diphosphinoamide pincer ligands to prepare various nickel(II/I/0) complexes and to study the conversion of C1 chemicals such as CO and CO2 to value-added products. In the transformation of C1 chemicals, the key C-O bond cleavage and C-E bond (E = C, N, O, or S, etc.) formation steps typically require overcoming high activation barriers. Interestingly, enzymatic systems overcome such difficulties for C1 conversion and operate efficiently under ambient conditions with the use of nickel organometallic chemistry. Furthermore, we have extended our efforts to the conversion of NOx anions to NO via the sequential deoxygenation by nickel mediated carbonylation, which was applied to catalytic C-N coupling to produce industrially important organonitrogen compound oximes as a strategy for NOx conversion and utilization (NCU). Notably, the rigidified acriPNP pincer backbone that enforces a planar geometry at nickel was found to be an important factor for diversifying organometallic transformations including (a) homolysis of various σ-bonds mediated by T-shaped nickel(I) metalloradical species, (b) C-H bond activation mediated by a nickel(0) dinitrogen species, (c) selective CO2 reactivity of nickel(0)-CO species, (d) C-C bond formation at low-valent nickel(I or 0)-CO sites with iodoalkanes, and (e) catalytic deoxygenation of NOx anions and subsequent C-N coupling of a nickel-NO species with alkyl halides for oxime production. Broadly, our results highlight the importance of molecular design and the rich chemistry of organonickel species for diverse small molecule transformations.

Mixed Valence {Ni2+Ni1+} Clusters as Models of Acetyl Coenzyme A Synthase Intermediates

Acetyl coenzyme A synthase (ACS) catalyzes the formation and deconstruction of the key biological metabolite, acetyl coenzyme A (acetyl-CoA). The active site of ACS features a {NiNi} cluster bridged to a [Fe4S4]n+ cubane known as the A-cluster. The mechanism by which the A-cluster functions is debated, with few model complexes able to replicate the oxidation states, coordination features, or reactivity proposed in the catalytic cycle. In this work, we isolate the first bimetallic models of two hypothesized intermediates on the paramagnetic pathway of the ACS function. The heteroligated {Ni2+Ni1+} cluster, [K(12-crown-4)2][1], effectively replicates the coordination number and oxidation state of the proposed "Ared" state of the A-cluster. Addition of carbon monoxide to [1]- allows for isolation of a dinuclear {Ni2+Ni1+(CO)} complex, [K(12-crown-2)n][2] (n = 1-2), which bears similarity to the "ANiFeC" enzyme intermediate. Structural and electronic properties of each cluster are elucidated by X-ray diffraction, nuclear magnetic resonance, cyclic voltammetry, and UV/vis and electron paramagnetic resonance spectroscopies, which are supplemented by density functional theory (DFT) calculations. Calculations indicate that the pseudo-T-shaped geometry of the three-coordinate nickel in [1]- is more stable than the Y-conformation by 22 kcal mol-1, and that binding of CO to Ni1+ is barrierless and exergonic by 6 kcal mol-1. UV/vis absorption spectroscopy on [2]- in conjunction with time-dependent DFT calculations indicates that the square-planar nickel site is involved in electron transfer to the CO π*-orbital. Further, we demonstrate that [2]- promotes thioester synthesis in a reaction analogous to the production of acetyl coenzyme A by ACS.

Reactivity of low-valent nickel carbonyl species supported by acridane based PNP ligands towards iodoalkanes

Nickel monocarbonyl species with Ni(I) and Ni(0) have been synthesized and fully characterized by employing an acriPNP-Ph pincer ligand having a -C(Ph)2- bridge moiety to tether two aromatic rings. To see the effect of the bridge moiety, these complexes were structurally compared with the previously studied nickel complexes supported by PNP and acriPNP-Me ligands and methylation of the nickel carbonyl species was particularly investigated. Since a Ni(I)-CO species is known to be one of the key intermediates during the C-C coupling reaction to give an acetyl species, according to the paramagnetic mechanism of acetyl coenzyme A synthase (ACS), their reactivity toward MeI has been examined. Methylation of a nickel(I)-CO species reveals enhanced C-C coupling when both acriPNP-Me and acriPNP-Ph ligands were used. According to spin density analysis calculated by density functional theory, all Ni(I)-CO species reveal similar spin density at nickel and the carbon atom of CO. X-ray crystallographic data suggest that the corresponding selectivity may be related to the steric influence. For both (acriPNP-Ph)Ni-CO (2) and (acriPNP-Me)Ni-CO (2'), the nickel(I) site is sterically well protected, leading to selective interaction with a methyl radical to give a nickel acyl product. Steric influence was marginally observed when an anionic {(acriPNP-R)Ni-CO}- (R = Me or Ph) species reacted with MeI. The corresponding C-C coupled product was also observed from the methylation of nickel(0)-CO species.

Radicalizing CO by Mononuclear Palladium(I)

A mononuclear, T-shaped palladium(I) d9 metalloradical (3), stabilized by a bulky carbazole-based PNP-ligand, was obtained by reduction of palladium chloride or thermal Pd-C bond homolysis of the corresponding neopentyl complex. Pressurizing with CO gave the Pd(I) carbonyl complex, which was structurally characterized by X-ray diffraction. Delocalization of the unpaired electron to the carbonyl carbon was detected by EPR spectroscopy and theoretically modeled by DFT and ab initio methods. The partially reduced and radicalized CO slowly reacts with di(tert-butyl) disulfide under homolytic S-S cleavage and C-S bond formation to give the corresponding metallathioester.

Electronic isomerism in a heterometallic nickel-iron-sulfur cluster models substrate binding and cyanide inhibition of carbon monoxide dehydrogenase

The nickel-iron carbon monoxide dehydrogenase (CODH) enzyme uses a heterometallic nickel-iron-sulfur ([NiFe4S4]) cluster to catalyze the reversible interconversion of carbon dioxide (CO2) and carbon monoxide (CO). These reactions are essential for maintaining the global carbon cycle and offer a route towards sustainable greenhouse gas conversion but have not been successfully replicated in synthetic models, in part due to a poor understanding of the natural system. Though the general protein architecture of CODH is known, the electronic structure of the active site is not well-understood, and the mechanism of catalysis remains unresolved. To better understand the CODH enzyme, we have developed a protein-based model containing a heterometallic [NiFe3S4] cluster in the Pyrococcus furiosus (Pf) ferredoxin (Fd). This model binds small molecules such as carbon monoxide and cyanide, analogous to CODH. Multiple redox- and ligand-bound states of [NiFe3S4] Fd (NiFd) have been investigated using a suite of spectroscopic techniques, including resonance Raman, Ni and Fe K-edge X-ray absorption spectroscopy, and electron paramagnetic resonance, to resolve charge and spin delocalization across the cluster, site-specific electron density, and ligand activation. The facile movement of charge through the cluster highlights the fluidity of electron density within iron-sulfur clusters and suggests an electronic basis by which CN- inhibits the native system while the CO-bound state continues to elude isolation in CODH. The detailed characterization of isolable states that are accessible in our CODH model system provides valuable insight into unresolved enzymatic intermediates and offers design principles towards developing functional mimics of CODH.

Tridentate NacNac Tames T-Shaped Nickel(I) Radical

The reaction of a nickel(II) chloride complex containing a tridentate β-diketiminato ligand with a picolyl group [2,6-iPr2 -C6 H3 NC(Me)CHC(Me)NH(CH2 py)]Ni(II)Cl (1)] with KSi(SiMe3 )3 conveniently afforded a nickel(I) radical with a T-shaped geometry (2). The compound's metalloradical nature was confirmed through electron paramagnetic resonance (EPR) studies and its reaction with TEMPO, resulting in the formation of a highly unusual three-membered nickeloxaziridine complex (3). When reacted with disulfide and diselenide, the S-S and Se-Se bonds were cleaved, and a coupled product was formed through carbon atom of the pyridine-imine group. The nickel(I) radical activates dihydrogen at room temperature and atmospheric pressure to give the monomeric nickel hydride.

Oxidative Addition and β-Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions

The dinickel(I) complex Ni2 (tBu PONNOPONNO), featuring a planar macrocyclic diphosphoranide ligand tBu PONNOPONNO, offers a unique architectural platform for observing bimetallic elementary reactions. Oxidative addition reactions of alkyl halides produce dinickel(II) complexes of the type Ni2 (μ-R)(μ-X)(tBu PONNOPONNO). However, when R=Et β-hydride elimination is observed to form a dinickel monohydride, with the rate dependent on the nature of X. DFT studies suggest a new mechanism for bimetallic β-hydride elimination, where the rate dependence arises from the steric pressure imposed by the X group on the opposing trans face of the dinickel macrocycle. This work enhances understanding of bimetallic elementary reactions, particularly β-hydride elimination, which have not been well-explored for dinuclear systems.

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Thioester synthesis by a designed nickel enzyme models prebiotic energy conversion

The formation of carbon-carbon bonds from prebiotic precursors such as carbon dioxide represents the foundation of all primordial life processes. In extant organisms, this reaction is carried out by the carbon monoxide dehydrogenase (CODH)/acetyl coenzyme A synthase (ACS) enzyme, which performs the cornerstone reaction in the ancient Wood-Ljungdahl metabolic pathway to synthesize the key biological metabolite, acetyl-CoA. Despite its significance, a fundamental understanding of this transformation is lacking, hampering efforts to harness analogous chemistry. To address these knowledge gaps, we have designed an artificial metalloenzyme within the azurin protein scaffold as a structural, functional, and mechanistic model of ACS. We demonstrate the intermediacy of the Ni^I species and requirement for ordered substrate binding in the bioorganometallic carbon-carbon bond-forming reaction from the one-carbon ACS substrates. The electronic and geometric structures of the nickel-acetyl intermediate have been characterized using time-resolved optical, electron paramagnetic resonance, and X-ray absorption spectroscopy in conjunction with quantum chemical calculations. Moreover, we demonstrate that the nickel-acetyl species is chemically competent for selective acyl transfer upon thiol addition to biosynthesize an activated thioester. Drawing an analogy to the native enzyme, a mechanism for thioester generation by this ACS model has been proposed. The fundamental insight into the enzymatic process provided by this rudimentary ACS model has implications for the evolution of primitive ACS-like proteins. Ultimately, these findings offer strategies for development of highly active catalysts for sustainable generation of liquid fuels from one-carbon substrates, with potential for broad applications across diverse fields ranging from energy storage to environmental remediation.

Nickel-Catalyzed NO Group Transfer Coupled with NOx Conversion

Nitrogen oxide (NO_x) conversion is an important process for balancing the global nitrogen cycle. Distinct from the biological NO_x transformation, we have devised a synthetic approach to this issue by utilizing a bifunctional metal catalyst for producing value-added products from NO_x. Here, we present a novel catalysis based on a Ni pincer system, effectively converting Ni-NO_x to Ni-NO via deoxygenation with CO(g). This is followed by transfer of the in situ generated nitroso group to organic substrates, which favorably occurs at the flattened Ni(I)-NO site via its nucleophilic reaction. Successful catalytic production of oximes from benzyl halides using NaNO₂ is presented with a turnover number of >200 under mild conditions. In a key step of the catalysis, a nickel(I)-^•NO species effectively activates alkyl halides, which is carefully evaluated by both experimental and theoretical methods. Our nickel catalyst effectively fulfills a dual purpose, namely, deoxygenating NO_x anions and catalyzing C-N coupling.

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.

Theoretical Study on the Degree of CO2 Activation in CO2-Coordinated Ni(0) Complexes

The geometrical characteristic and the degree of CO₂ activation of the CO₂-coordinated Ni(0) complexes were investigated computationally by quantum chemical means for bidentate and tridentate ligands of PP, PP^MeP, and PNP, and sometimes with co-complexing Fe(II) to differently coordinate CO₂. We show that the coordination geometry of the central metal is determined by the ligand geometry. The charge and the energy decomposition analyses show that the charge transfer energy through orbital mixing has a strong correlation with CO₂ net charge, while the binding energy cannot due to the lack of the coordination number and the deformation energy of the ligand. Among the examined ligands, PNP with negatively charged secondary amine makes Ni(0) an electron-rich atom, which results in an ∼20% higher CO₂ activation than those of PP and PP^MeP. In particular, Fe(II)-PNP in the CO₂-bridged diatomic complex enhances CO₂ activation by another ∼20%, partly through the inductive effect of Fe(II), which pulls electron density from Ni-PNP across the CO₂-bridge and partly by the backward donation from Fe(II)-PNP. Therefore, the present study encourages us to design a strongly electron-donating ligand and a CO₂-bridged diatomic complex to develop more efficient homogeneous catalyst.

Ligand Field Inversion as a Mechanism to Gate Bioorganometallic Reactivity: Investigating a Biochemical Model of Acetyl CoA Synthase Using Spectroscopy and Computation

The biological global carbon cycle is largely regulated through microbial nickel enzymes, including carbon monoxide dehydrogenase (CODH), acetyl coenzyme A synthase (ACS), and methyl coenzyme M reductase (MCR). These systems are suggested to utilize organometallic intermediates during catalysis, though characterization of these species has remained challenging. We have established a mutant of nickel-substituted azurin as a scaffold upon which to develop protein-based models of enzymatic intermediates, including the organometallic states of ACS. In this work, we report the comprehensive investigation of the S = 1/2 Ni-CO and Ni-CH₃ states using pulsed EPR spectroscopy and computational techniques. While the Ni-CO state shows conventional metal-ligand interactions and a classical ligand field, the Ni-CH₃ hyperfine interactions between the methyl protons and the nickel indicate a closer distance than would be expected for an anionic methyl ligand. Structural analysis instead suggests a near-planar methyl ligand that can be best described as cationic. Consistent with this conclusion, the frontier molecular orbitals of the Ni-CH₃ species indicate a ligand-centered LUMO, with a d⁹ population on the metal center, rather than the d⁷ population expected for a typical metal-alkyl species generated by oxidative addition. Collectively, these data support the presence of an inverted ligand field configuration for the Ni-CH₃ Az species, in which the lowest unoccupied orbital is centered on the ligands rather than the more electropositive metal. These analyses provide the first evidence for an inverted ligand field within a biological system. The functional relevance of the electronic structures of both the Ni-CO and Ni-CH₃ species are discussed in the context of native ACS, and an inverted ligand field is proposed as a mechanism by which to gate reactivity both within ACS and in other thiolate-containing metalloenzymes.

Catalytic hydrogenation of CO2 at a structurally rigidified cobalt center

Catalytic hydrogenation of CO₂ occurs at a cobalt center supported by a rigidified PNP ligand revealing higher catalytic performance.

One metal is enough: a nickel complex reduces nitrate anions to nitrogen gas

A stepwise reduction sequence from nitrate to dinitrogen gas at a single nickel center was discovered. A PNP nickel scaffold (PNP- = N[2-P i Pr2-4-Me-C6H3]2) emerged as a universal platform for the deoxygenation of NO x substrates. In these reactions carbon monoxide acts as the oxygen acceptor and forms CO2 to provide the necessary chemical driving force. Whereas the first two oxygens are removed from the Ni-nitrate and Ni-nitrite complexes with CO, the deoxygenation of NO requires a disproportionation reaction with another NO molecule to form NO2 and N2O. The final deoxygenation of nitrous oxide is accomplished by the Ni-NO complex and generates N2 and Ni-NO2 in a relatively slow, but clean reaction. This sequence of reactions is the first example of the complete denitrification of nitrate at a single metal-site and suggests a new paradigm of connecting CO and NO x as an effective reaction pair for NO x removal.

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.

Carbodiphosphorane-based nickel pincer complexes and their (de)protonated analogues: dimerisation, ligand tautomers and proton affinities

The reactivity patterns of carbodiphosphoranes (CDPs) as ligands are much less explored than those of isoelectronic analogues. In the current manuscript, we investigate the reactivity of the carbodiphosphorane-based PCP nickel(ii) pincer complex [({dppm}2C)NiCl]Cl (1) towards acids and bases, calculate proton affinities, analyse the bonding situation and tautomeric forms with the aim to evaluate whether CDPs can potentially act as cooperative ligands in catalysis (dppm = 1,1-bis(diphenylphosphino)methane). Our investigations show that different tautomeric forms are stable for the coordinated and the uncoordinated ligand. The protonated CDP-based complex 2 represents a rare example of a cationic donor group binding to a cationic metal centre. The continuous arm-deprotonation of 1 leads to the formation of remarkably stable dimers with Ni-C-P-C-metallacycles. In comparison to corresponding boron and amine-based ligands, the coordinated CDP-group exhibits the lowest proton affinity according to DFT calculations, indicating that coordinated CDP ligands can potentially serve as proton relay in cooperative catalysis.

Selective Transformation of CO2 to CO at a Single Nickel Center

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO₂ conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO₂ to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO₂. Our research is inspired by the efficient enzymatic CO₂ catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO₂ are controlled in part by the geometry of a L₃Ni scaffold, we have explored the chemistry of low-valent nickel supported by PP^MeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO₂ adducts, (PP^MeP)Ni(η²-CO₂-κ C) (2) and {Na(12-C-4)₂}{(PNP)Ni(η¹-CO₂-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO₂ and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-μ-CO₂-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO₂ were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO₂ but displays multiple product formation. By incorporation of a -CMe₂- bridging unit, a structurally rigidified ^acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d _{x²- y²} orbital. More importantly, the selective addition of CO₂ to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (^acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO₂ to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO₂ to release the coordinated CO ligand.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)Ni^IX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni^I(Ar) (Ar = Ph, o-tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-ⁱPr₃C₆H₂). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand. The most unstable Ni(I) aryl species are the most active precatalysts for Suzuki-Miyaura reactions because they rapidly decompose to generate the active Ni(0) catalyst. This study shows that Ni(I) aryl species are initially formed in the activation of Ni(I) halide precatalysts for Suzuki-Miyaura reactions and establishes their stoichiometric and catalytic reactivity profile.

Direct CO2 Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO₂ to a low-valent nickel species has been explored with a newly designed ^acriPNP pincer ligand (^acriPNP^- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO₂ conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(^acriPNP)Ni(II)-CO}⁺ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the d_x²-y² orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO₂ binding site. Upon addition of CO₂ to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η¹-CO₂-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO₂. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO₂, Ni-COOH, and Ni-CO.

Mono- and dinuclear Ni(i) products formed upon bromide abstraction from the Ni(i) ring-expanded NHC complex [Ni(6-Mes)(PPh3)Br]

New T- and Y-shaped Ni(i) complexes are reported and analysed by DFT and EPR.

Multielectron Chemistry within a Model Nickel Metalloprotein: Mechanistic Implications for Acetyl-CoA Synthase

The acetyl coenzyme A synthase (ACS) enzyme plays a central role in the metabolism of anaerobic bacteria and archaea, catalyzing the reversible synthesis of acetyl-CoA from CO and a methyl group through a series of nickel-based organometallic intermediates. Owing to the extreme complexity of the native enzyme systems, the mechanism by which this catalysis occurs remains poorly understood. In this work, we have developed a protein-based model for the Ni_P center of acetyl coenzyme A synthase using a nickel-substituted azurin protein (NiAz). NiAz is the first model nickel protein system capable of accessing three (Ni^I/Ni^II/Ni^III) distinct oxidation states within a physiological potential range in aqueous solution, a critical feature for achieving organometallic ACS activity, and binds CO and -CH₃ groups with biologically relevant affinity. Characterization of the Ni^I-CO species through spectroscopic and computational techniques reveals fundamentally similar features between the model NiAz system and the native ACS enzyme, highlighting the potential for related reactivity in this model protein. This work provides insight into the enzymatic process, with implications toward engineering biological catalysts for organometallic processes.

A T-Shaped Nickel(I) Metalloradical Species

A T-shaped Ni^I complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni^I species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

Tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl metal complexes, [TismPriBenz]M: a new class of metallacarbatranes, isomerization to a tris(N-heterocyclic carbene) derivative, and evidence for an inverted ligand field

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPriBenz]Li, [TismPriBenz]MgMe, [TismPriBenz]Cu and [TismPriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ3-coordination, namely [κ3-TismPriBenz]ZnMe, has also been obtained. Furthermore, the [TismPriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPriBenz]Cu to [κ4-C4-TismPriBenz*]Cu. The transannular M-C bond lengths in the atrane compounds are 0.19-0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu-Catrane interactions in [TismPriBenz]Cu and [κ4-C4-TismPriBenz*]Cu are characterized by an "inverted ligand field", in which the occupied antibonding orbitals are localized more on carbon than on copper.

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

Carbon dioxide binding at a Ni/Fe center: synthesis and characterization of Ni(η1-CO2-κC) and Ni-μ-CO2-κC:κ2O,O'-Fe

The degree of CO2 activation can be tuned by incorporating a distinct electronic coordination environment at the nickel center. A mononuclear nickel carboxylate species (Ni-CO2, 3) and a dinuclear nickel-iron carboxylate species (Ni-CO2-Fe, 5) were prepared. The structure of 3 reveals a rare η1-κC binding mode of CO2, while that of 5 shows bridging CO2 binding (μ2-κC:κ2O,O') between the nickel and iron, presented as the first example of a nickel-μ-CO2-iron species. The structural analyses of 3 and 5 based on XRD and DFT data reveal a higher degree of CO2 activation in 5, imparted by the additional interaction with an iron ion.

Activation of heteroallenes by coordinatively unsaturated nickel(ii) alkyl complexes supported by the hydrotris(3-phenyl-5-methyl)pyrazolyl borate (Tp(Ph,Me)) ligand

Activation of sulfur containing heteroallenes by nickel(ii) alkyl complexes supported by the bulky hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand is described. Exposure of Tp(Ph,Me)NiCH2Ph (1a) and Tp(Ph,Me)NiCH2Si(CH3)3 (1b) to CS2 resulted in formation of the insertion products Tp(Ph,Me)Ni(η(2)-CS2)CH2Ph (2a) and Tp(Ph,Me)Ni(η(2)-CS2)CH2Si(CH3)3 (2b) in moderate yields. Reaction of 1a and MeNCS produced two species in a 1 : 1 ratio, identified as Tp(Ph,Me)Ni(η(2)-MeNC)CH2Ph (3) and Tp(Ph,Me)Ni(η(2)-MeNCS)SCH2Ph (4). Isolation of the unexpected insertion product (3) prompted an investigation into the activity of 1a-b in the presence of isocyanides (i.e.(t)BuNC), which resulted in isolation of Tp(Ph,Me)Ni(η(2-t)BuNC)CH2Ph (5a) and Tp(Ph,Me)Ni(η(2-t)BuNC)CH2Si(CH3)3 (5b). Similarly, reaction of 1a with OCS led to the isolation of a rare example of a Ni(i) carbonyl species Tp(Ph,Me)NiCO (6). Alternatively, complex 6 was also formed by exposure of 1a-b to an atmosphere of CO. Isolation of the intermediate species (Tp(Ph,Me)Ni(η(2)-CO)CH2TMS (7b) and Tp(Ph,Me)Ni(CO)(C(O)R, (8a-b) with R = Ph, TMS)) shed light on the formation of such species.

Nickel-centred proton reduction catalysis in a model of [NiFe] hydrogenase

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H₂ evolution (second-order rate constant of 2.5 × 10⁴ M^-1 s^-1; turnover frequency of 250 s^-1 at 10 mM H⁺ concentration) from mildly acidic solutions.

Electronic structure and reactivity of nickel(i) pincer complexes: their aerobic transformation to peroxo species and site selective C-H oxygenation

The study is aimed at a deeper understanding of the electronic structure of the T-shaped nickel(i) complex [LigiPr(iso)Ni] (1b), bearing the iso-PyrrMeBox (bis(oxazolinylmethylidene)pyrrolidinido) pincer ligand, and its CO adduct [LigiPr(iso)Ni(CO)] (2b) as well as to provide insight into the mechanism of autoxidation of the different nickel peroxo species of this ligand type. CO was found to react reversibly with complex 1b resulting in the corresponding CO adduct 2b. The EPR data as well as the results of DFT modeling revealed significant differences in the electronic structure of 1b and 2b. Reaction of [LigPh(iso)Ni] and [LigiPr(iso)Ni] (1a and b) with dioxygen yielded the 1,2-μ-peroxo complexes [Lig(iso)NiO]23a and b which reacted with hydrogen peroxide to give the hydroperoxo complexes [Lig(iso)NiOOH] 5a and b. Thermal aerobic decomposition of the peroxo species 3a and 5a in the presence of O2 led to a C-H activation of the ligand at the benzylic position of the oxazoline ring forming diastereomeric cyclic peroxo complexes 6 and 6'. For the 1,2-μ-peroxo complex 3b the autoxidation of the pincer in the absence of O2 occurred at the tertiary C-H bond of the iPr-group and led to a selective formation of the terminal hydroxo complex [LigiPr(iso)NiOH] 7b and the cyclic alkoxy complex 8 in equimolar quantities, while the corresponding cyclic peroxo species 9 was formed along with 7b in the presence of oxygen. Whether or not O-O bond cleavage occurred in the generation of 9 was established upon performing labeling experiments which indicate that the transformation does not involve an initial O-O bond cleaving step. Based on these observations and a series of stoichiometric transformations a tentative proposal for the processes involved in the anaerobic and aerobic decomposition of 3b has been put forward. Finally, the nickel(ii) methyl complex [LigPh(iso)NiMe] 14 reacted with O2 to give the methylperoxo complex [LigPh(iso)NiOOMe] 15 which slowly converted to a mixture of near equal amounts of the formato and the hydroxo complexes, [LigPh(iso)NiOOCH] 16 and [LigPh(iso)NiOH] 7a, along with half an equivalent of methanol. The formato complex 16 itself decomposed at elevated temperatures to CO2, dihydrogen as well as the nickel(i) species 1a.

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.

Formation of a tetranickel octacarbonyl cluster from the CO2 reaction of a zero-valent nickel monocarbonyl species

The CO₂ reaction of a nickel(0) complex involves multiple reaction pathways including tetranickel cluster formation via reductive disproportionation of CO₂.

N-heterocyclic phosphenium and phosphido nickel complexes supported by a pincer ligand framework

A tridentate ligand framework containing a central N-heterocyclic phosphenium cation (NHP⁺) has been coordinated to nickel. Among the compounds reported is a series of [(PPP)Ni]₂^0/+/2+ dimers in three different redox states.

Reaction of a polydentate cysteine-based ligand and its nickel(ii) complex with electrophilic and nucleophilic methyl-transfer reagents - from S-methylation to acetyl coenzyme A synthase reactivity

The L-cysteine derived N2S2 ligand precursor H2L and its nickel(ii) complex L2Ni2 were investigated with respect to their behaviour in contact with electrophilic and nucleophilic methylation reagents (H2L = (N,N'-dimethyl-(2R,5R)-bis-(sulfanylmethyl)-piperazine). Treatment of deprotonated L(2-) with MeI led to the selective methylation of the thiolate groups thus generating a novel potential ligand, Me2L, which is neutral and contains two thioether donors. The coordinating properties of Me2L were demonstrated by the synthesis of a first nickel(ii) complex: reaction with NiBr2 led to a mononuclear complex 2 where all donor atoms coordinate to the nickel ion, which completes its octahedral coordination sphere by the two bromide ligands. If, however, the complex [LNi]2 (1) is treated with MeI only one thiolate function per ligand moiety is methylated, while the other one remains a thiolate. This leads to [MeLNi](+) complex metal fragments, which trimerize including a μ3-bridging iodide ion to give the compound 3 that was tested with regards to ACS reactivity. While it behaved inert towards CO, attempts to replace the bridging iodide ligand by methyl units in reactions with nucleophilic methylation reagents led to a product, which could not be identified but reacted with CO. Work-up showed that this protocol had converted the thiolate function of MeL(-) into a thioester function, which corresponds to an ACS-like reactivity.