Nickel Hydride Complexes

Ni-Catalyzed Stereodivergent Synthesis of N-Glycosides

Herein, we describe a stereoselective Ni-catalyzed N-glycosylation of glycals. The reaction is enabled by addition of an in situ generated nickel hydride across an olefin prior to C-N bond-formation. Stereodivergence can be accomplished on kinetic or thermodynamic grounds, thus giving access to either α- or β-N-glycosides with equal ease. The protocol is distinguished by its operational simplicity, generality and exquisite selectivity, thus offering a new gateway to expedite the synthesis of N-glycosides.

Reversible C-CN Bond Cleavage by a Formal Dinickel(I) Hydride Cation

An N-heterocyclic carbene (NHC) ligand supports a stable [Ni2H]+ core, formally dinickel(I). This diamagnetic cation complex features a bent hydride bridge and a Ni···Ni distance, 2.9926(5) Å, larger than two covalent radii. The cation displays weakly protic character, undergoing deprotonation by strong base to form the corresponding (NHC)nickel(0) dimer. Its reaction with aliphatic nitriles results in C-CN bond cleavage. The organic products of this reaction suggest that this bond-breaking step involves reactive nickel alkyl intermediates and occurs reversibly.

Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoIIICp(PPh2NBn2)(CH3CN)]2+ to [HCoIIICp(PPh2NBn2)]+. This complex catalytically converts CO2 to formate under CO2 reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PPh2NBn2 ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the PPh2NBn2 ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pKa values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.

Regio- and Enantioselective Nickel-Catalyzed Ipso- and Remote Hydroamination Utilizing Organic Azides as Amino Sources for the Synthesis of Primary Amines

Primary amines serve as key synthetic precursors to most other N-containing compounds, which are important in organic and medicinal chemistry. Herein, we present a NiH-catalyzed mild ipso- and remote hydroamination technique that utilizes organic azides as deprotectable primary amine sources. This strategy offers a highly flexible platform for the efficient construction of α-chiral branched primary amines, as well as linear primary amines.

Synthesis, Structure, and Bonding of Actinide-Rhenium Polyhydrides

The synthesis of actinide tetrarhenate complexes completes a series of iridate, osmate, and rhenate polyhydrides, allowing for structural and bonding comparisons to be made. Computational studies examine the bonding interactions, particularly between metals, in these complexes. Several factors─including metal oxidation state, coordination number, and dispersion effects─affect metal-metal distances and covalency in these actinide tetrametallates. Related osmium and rhenium octametallic U2M6 clusters are synthesized and described, and subjected to similar structural and electronic analyses.

Selective Monoborylation of Methane by a Mono Bipyridyl-Nickel(II) Hydride Catalyst

We report the development of an earth-abundant metal catalyst for methane C-H borylation. The post-synthetic metalation of bipyridine-functionalized zirconium metal-organic framework (MOF) with NiBr2, followed by treatment with NaEt3BH affords MOF-supported monomeric bipyridyl-nickel(II) dihydride species via active site isolation. The heterogeneous and recyclable nickel catalyst selectively borylates methane at 200 °C using pinacolborane (HBpin) to afford CH3Bpin in 61 % yield with a turnover number (TON) up to 1388. The confinement of the active NiH2-species within the uniformly porous MOF allows selective monoborylation of methane via shape-selective catalysis by preventing the formation of sterically encumbered overborylated products. Unlike MOF-Ni catalyst, its homogeneous control is almost inactive in methane borylation due to its intermolecular decomposition. Our mechanistic investigation, including spectroscopic, kinetic, and control experiments, as well as DFT calculations, revealed that stabilizing mononuclear bipyridyl-nickel dihydride and diboryl species by MOF is crucial for achieving efficient methane borylation via turnover-limiting σ-bond metathesis. This work shows promise in designing MOF-based abundant metal catalysts for the chemoselective functionalization of methane and other inert molecules into valuable chemicals.

Reduction of CS2 to an ethanetetrathiolate by a hydride-bridged uranium-iridium heterobimetallic complex

We report the synthesis of a heterobimetallic U(III)-Ir species which reacts with CS2 to form the novel ethanetetrathiolate fragment via hydride insertion and C-C coupling. Computational studies suggest the formation of a radical intermediate, which may couple with another equivalent to form the final product.

A phosphine-free molecularly-defined Ni(II) complex in catalytic hydrogenation of CO2

The development of base metal catalysts capable of CO2 hydrogenation is a challenge and a necessity to progress from the scarce noble metal catalysts. In this regard, we report herein the first non-phosphine-based Ni complex, supported by a "carbazolato-bis-NHC" pincer ligand framework, for efficient catalytic hydrogenation of CO2 to formate. A tailored combination of the Ni complex as a catalyst, DBU as a base, and Zn(OAc)2 as an additive offered enhanced activity leading to a TON up to 5476 and an excellent yield up to 92% for the formate product from a reaction on ∼27 mmol scale.

Ni-Catalyzed Enantioselective Reductive Cyclization/Amidation and Amination of 1,6-Enynes and 1,7-Enynes

Transition-metal-catalyzed hydroamination of unsaturated hydrocarbons is an appealing synthetic tool for the construction of high value-added chiral amines. Despite significant progress in the asymmetric hydroamination of alkenes, allenes, and 1,3-dienes, asymmetric hydroamination of 1,6-enynes or 1,7-enynes remains rather limited due to the enormous challenges in controlling the chemoselectivity and stereoselectivity of the reaction. Herein, we report a Ni-catalyzed chemo- and enantioselective reductive cyclization/amidation and amination of 1,6-enynes and 1,7-enynes using dioxazolones or anthranils as nitrene-transfer reagents. This mild, modular, and practical protocol provides rapid access to a variety of enantioenriched 2-pyrrolidone and 2-piperidone derivatives bearing an aminomethylene group at the 4-position in good yields (up to 83 %) with excellent enantioselectivities (46 examples, up to 99 % ee). Mechanistic experiments and density functional theory calculations indicate that the reaction is initiated by hydronickelation of alkynes followed by migratory insertion into alkenes, rather than by a [2+2+1] oxidative addition process of nickel to alkenes and alkynes.

Synthesis and characterization of NiAl-hydride heterometallics: perturbing electron density within Al-H-Ni subunits

Heterometallic hydride complexes are of growing interest due to their potential to contribute to highly active insertion-based catalysis; however, methods to modulate electron density within this class of molecules are underexplored. Addition of ancillary ligands to heterotrimetallic NiAl2H2 species (1) results in the formation of heterobimetallic NiAl-hydride complexes with varying phosphine donors (2-(L)2). Incorporation of sigma donating ancillary ligands of increasing strength led to contractions of the Ni-Al distances correlated to a strengthening of a back donation interaction to the Al-H sigma antibonding orbital, most prominently present in 2-(PMe3)2. Demethylation of the aryl ether from 2-(PMe3)2 provides access to a novel anionic nickel-aluminum complex (3) with a maintained bridged hydride moiety between Ni and Al. Increased negative charge in complex 3 results in an elongation of the Ni-Al interaction. Combined crystallographic, spectroscopic, and computational studies support a 3-center interaction within the Al-H-Ni subunits and were used to map the degree of Ni-H character of the series within the Al-H-Ni bonding continuum.

Chromium Complexes with Benzanellated N-Heterocyclic Phosphenium Ligands-Synthesis, Reactivity and Application in Catalytic CO2 Reduction

A chromium complex carrying two benzanellated N-heterocyclic phosphenium (bzNHP) ligands was prepared by a salt metathesis approach. Spectroscopic studies suggest that the anellation enhances the π-acceptor ability of the NHP-units, which is confirmed by the facile electrochemical reduction of the complex to a spectroscopically characterized radical anion. Co-photolysis with H2 allowed extensive conversion into a σ-H2-complex, which shows a diverse reactivity towards donors and isomerizes under H-H bond fission and shift of a hydride to a P-ligand. The product carrying phosphenium, phosphine and hydride ligands was also synthesized independently and reacts reversibly with CO and MeCN to yield bis-phosphine complexes under concomitant Cr-to-P-shift of a hydride. In contrast, CO2 was not only bound but reduced to give an isolable formato complex, which reacted with ammonia borane under partial recovery of the metal hydride and production of formate. Further elaboration of the reactions of the chromium complexes with CO2 and NH3BH3 allowed to demonstrate the feasibility of a Cr-catalyzed transfer hydrogenation of CO2 to methanol. The various complexes described were characterized spectroscopically and in several cases by XRD studies. Further insights in reactivity patterns were provided through (spectro)electrochemical studies and DFT calculations.

Regiodivergent and Enantioselective Synthesis of Cyclic Sulfones via Ligand-Controlled Nickel-Catalyzed Hydroalkylation

Cyclic sulfones have demonstrated important applications in drug discovery. However, the catalytic and enantioselective synthesis of chiral cyclic sulfones remains challenging. Herein, we develop nickel-catalyzed regiodivergent and enantioselective hydroalkylation of sulfolenes to streamline the synthesis of chiral alkyl cyclic sulfones. The method has broad scope and high functional group tolerance. The regioselectivity can be controlled by ligands only. A neutral PYROX ligand favors C3-alkylation whereas an anionic BOX ligand favors C2-alkylation. This control is kinetic in origin as the C2-bound Ni intermediates are always thermodynamically more stable. Reactivity study of a wide range of relevant Ni intermediates reveal a NiI/NiIII catalytic cycle with a NiII-H species as the resting state. The regio- and enantio-determining step is the insertion of this NiII-H species into 2-sulfolene. This work provides an efficient catalytic method for the synthesis of an important class of organic compounds and enhances the mechanistic understanding of Ni-catalyzed stereoselective hydroalkylation.

Nickel Catalyzed Enantioselective 1,4-Hydroamination of 1,3-Dienes

Transition metal-catalyzed enantioselective hydroamination of 1,3-dienes provides a direct methodology for the construction of chiral allylamines. So far, all of the reported examples used nucleophilic amines and proceeded with 3,4-regioselectivity. Herein, we describe the first example of nickel-catalyzed enantioselective 1,4-hydroamination of 1,3-dienes using trimethoxysilane and hydroxylamines with a structurally adaptable aromatic spiroketal based chiral diphosphine (SKP) as the ligand, affording a wide array of α-substituted chiral allylamines in high yields with excellent regio- and enantioselectivities. This operationally simple protocol demonstrated a broad substrate scope and excellent functional group compatibility, significantly expanding the chemical space for chiral allylamines. Experimental and DFT studies were performed to elucidate the mechanism and to rationalize the regio- and enantioselectivities of the reaction.

Photolysis-driven bond activation by thorium and uranium tetraosmate polyhydride complexes

Transition metal multimetallic complexes have seen intense study due to their unique bonding and potential for cooperative reactivity, but actinide-transition metal (An-TM) species are far less understood. We have synthesized uranium- and thorium-osmium heterometallic polyhydride complexes in order to study An-Os bonding and investigate the reactivity of An-Os interactions. Computational studies suggest the presence of a significant bonding interaction between the actinide center and the four coordinated osmium centers supported by bridging hydrides. Upon photolysis, these complexes undergo intramolecular C-H activation with the formation of an Os-Os bond, while the thorium complex may activate an additional C-H bond of the benzene solvent, resulting in a μ-η1,η1 phenyl ligand across one Th-Os interaction.

Nickel-Catalyzed Double Deoxygenative C-N Coupling of Acyloxyamines

A double deoxygenative C-N coupling protocol has been developed by employing acyloxyamines through N-O bond activation. The C-N bond formation under mild reaction conditions, employing NiCl2 as the catalyst and cataCXiumA as a ligand, results in the production of a diverse array of alkylated secondary or tertiary amines, including heterocyclic amines. This method introduces a novel catalytic strategy that emphasizes the versatility of nickel-catalyzed reactions, extending beyond traditional synthetic boundaries.

The Noble Addendum of a Phosphenium Ligand to a Base Metal: Coordination, Activation, and Hydrogenation of Alkenes and Alkynes on a Chromium Complex

The synthesis of a new bis-NHP complex (NHP=N-heterocyclic phosphenium) of chromium via salt metathesis and studies of its reactivity are reported. Photochemical reactions with H2 and selected olefins give rise to non-isolable H2- and π-alkene complexes identified spectroscopically, while internal alkynes react via activation of the triple bond to yield isolable metalla-phospha-cyclobutenes characterized by spectroscopic and XRD data. DFT studies give a preliminary account of the bonding in H2- and alkene-complexes and explain the different reactivity towards alkenes and alkynes as the consequence of kinetic effects. Photolysis of the bis-NHP-complex in the presence of H2 and olefins or alkenes enables the catalytic hydrogenation of the organic substrates, while the π-ethene complex mediates the catalytic hydrogenation of ethene in a dark reaction. The similarities and differences between both catalytic processes are shortly discussed.

Recent advances in earth-abundant transition metal-catalyzed dihydrosilylation of terminal alkynes

Over the past few years, earth-abundant transition metal-catalyzed hydrosilylation has emerged as an ideal strategy for the synthesis of organosilanes. The success in this area of research has expanded to the advancements of alkyne dihydrosilylation reactions, offering broadened synthetic applications through the selective installation of two silyl groups. In particular, catalysts based on Fe, Co, and Ni have engendered enabling platforms for mild transformations with a range of distinct regioselectivity. This mini-review summarizes recent advances in this research field, highlighting the unique features of each system from both synthetic and mechanistic perspectives.

Nickel-Catalyzed α-Alkylation of Arylacetonitriles with Challenging Secondary Alcohols

Nickel(II) complex 1 was utilized as a sustainable catalyst for α-alkylation of arylacetonitriles with challenging secondary alcohols. Arylacetonitriles with a wide range of functional groups were tolerated, and various cyclic and acyclic secondary alcohols were utilized to yield a large number of α-alkylated products. The plausible mechanism involves the base-promoted activation of precatalyst 1 to an active catalyst 2 (dehydrochlorinated product) which activates the O-H and C-H bonds of the secondary alcohol in a dehydrogenative pathway.

Preparation of silyl-terminated branched polyethylenes catalyzed by Brookhart's nickel diimine complex activated with hydrosilane/B(C6F5)3

Brookhart's nickel α-diimine complex [(κ2-N,N-BIAN)NiCl2] (1) (where BIAN = {Ar-NAceN-Ar}, Ace = acenaphthen-1,2-diyl, and Ar = 2,6-(iPr)2-C6H3) activated with a hydrosilane/B(C6F5)3 (SiHB) adduct forms a highly active catalytic system for ethylene polymerization. Under optimal conditions, the activity of the system depends on the nature of hydrosilane and decreases in the order R3SiH > Ph2SiH2 > PhSiH3. The decrease in system activity within the hydrosilane series is correlated with increasing formation of Ni(I) species. In addition to their activation effect, hydrosilanes act as efficient chain termination/chain transfer agents, with the Si/Ni ratio controlling the molecular weight of the resulting polyethylene (PE). The use of Et3SiH generated elastomeric, highly branched polymers with a saturated chain-end, while systems using Ph2SiH2 and PhSiH3 led to branched end-functionalized PEs terminated with the hydrosilyl functionality (i.e. br-PE-SiPh2H or br-PE-SiPhH2).

Nitrous oxide activation by picoline-derived Ni-CNP hydrides

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Nickel-Catalyzed Migratory Cross-Coupling Reactions: New Opportunities for Selective C-H Functionalization

ConspectusMigratory cross-coupling via metal migration is a process of significant academic and industrial interest. It provides an attractive alternative for the selective installation of a functional group at remote C-H positions from simple precursors, thus enabling the direct synthesis of challenging structures not accessible with traditional cross-coupling. In particular, with the merger of 1,n-Ni/H shift and cross-coupling of nickel, the Ni-catalyzed migratory functionalization of simple precursors has undergone particularly intense development and emerged as a valuable field of research in the past few years. This Account will outline the recent progress made in this arena in terms of migration-functionalization modes, diverse functionalizations, and strategies for regio- and stereocontrol. Mechanistic studies and synthetic applications are also discussed.In detail, we systematically categorize our work into two parts based on the migration modes. In the first part, a platform is created for Ni-catalyzed migratory sp3 C-H functionalization of alkenes or alkyl halides via iterative 1,2-Ni/H shift-selective cross-coupling. The key reactive Ni(II)H species for chain-walking could be generated in situ either in a polarity-reversed fashion relying on stoichiometric reductants (X-Ni(II)-H) or in a redox-neutral fashion with the participation of nucleophilic coupling partners (FG-Ni(II)-H). One significant advantage associated with the polarity-reversed NiH system is the use of relatively stable, abundant, and safe olefin surrogates or alkyl halides instead of the sensitive organometallics required in traditional cross-coupling reactions. Another advantage is that diverse functionalizations, including carbonation and more challenging amination and thiolation could be smoothly achieved with suitable electrophiles or their precursors. Finally, to address the challenging multifaceted selectivity and reactivity issues in asymmetric migratory cross-coupling reactions, we have developed a feasible ligand relay catalytic strategy. In this dynamic ligand exchange process, one ligand promotes rapid migration while the other promotes highly regio- and stereoselective coupling. This innovative strategy overcomes the formidable challenge stemming from the difficulty of designing a single ligand to efficiently promote both steps of chain-walking and asymmetric coupling. In the second part, a new platform for Ni-catalyzed migratory sp2 C-H functionalization via 1,4-Ni/H shift-selective cross-coupling has been reported. Starting from readily available aryl or vinyl coupling partners, the in situ-generated aryl- or vinylnickel(II) species could undergo a rapid and reversible 1,4-Ni/H shift along an sp2 backbone, and subsequent selective coupling with various coupling partners would allow regio- and stereoselective access to diverse 1,4-migratory functionalization products. The key to success was the discovery of an appropriate ligand to efficiently promote both migration and subsequent selective cross-coupling. A vinyl-to-aryl 1,4-Ni/H shift successfully enables the modular ipso/ortho difunctionalization of aryl coupling partners, while an aryl-to-vinyl 1,4-Ni/H shift enables regio- and stereoselective access to functionalized trisubstituted alkenes.We hope that this Account will inspire broad interest and future development of migratory cross-coupling reactions. We strongly believe that continued efforts in this fascinating field will overcome many of the remaining challenges, including cutting-edge ligand/catalyst design to enhance reactivity and selectivity, conceptually new migration modes for additional transformations, and in-depth mechanistic studies for rational reaction design.

Reversible Binding of Hydrogen and Styrene Coordination on a Manganese Phosphenium Complex

The reactions of two complexes [(R NHP)Mn(CO)4 ] (R NHP=N-arylated N-heterocyclic phosphenium) with H2 at elevated pressure (≈4 bar) were studied by NMR spectroscopy. Irradiation with UV light initialized in one case (5 a, R=Dipp) the unselective formation of (R NHP-H)MnH(CO)4 ] (6 a) via cooperative addition of H2 across the Mn=P double bond. In the other case (5 b, R=Mes), addition of H2 was unobservable and the reaction proceeded via decarbonylation to a dimeric species [(R NHP)2 Mn2 (CO)7 ] (7 b) that was isolated and identified spectroscopically. Taking into account the outcome of further reaction studies under various conditions in the absence and presence of H2 , both transformations can be explained in the context of a common mechanism involving decarbonylation to 7 a,b as the first step, and the different outcome is attributable to the fact that 7 b is unreactive towards both H2 and CO while 7 a is not. DFT studies relate this divergence to deviations in the molecular constitution and stability arising from a different level of steric congestion. Preliminary studies suggest further that 5 a/H2 as well as 6 a enable the photo-induced hydrogenation of styrene to ethyl benzene, even if the mechanism and possibly catalytic nature of this process remain yet unknown.

Nickel-catalyzed direct stereoselective α-allylation of ketones with non-conjugated dienes

The development of efficient and sustainable methods for the construction of carbon-carbon bonds with the simultaneous stereoselective generation of vicinal stereogenic centers is a longstanding goal in organic chemistry. Low-valent nickel(0) complexes which promote α-functionalization of carbonyls leveraging its pro-nucleophilic character in conjunction with suitable olefin acceptors are scarce. We report a Ni(0)NHC catalyst which selectively converts ketones and non-conjugated dienes to synthetically highly valuable α-allylated products. The catalyst directly activates the α-hydrogen atom of the carbonyl substrate transferring it to the olefin acceptor. The transformation creates adjacent quaternary and tertiary stereogenic centers in a highly diastereoselective and enantioselective manner. Computational studies indicate the ability of the Ni(0)NHC catalyst to trigger a ligand-to-ligand hydrogen transfer process from the ketone α-hydrogen atom to the olefin substrate, setting the selectivity of the process. The shown selective functionalization of the α-C-H bond of carbonyl groups by the Ni(0)NHC catalyst opens up new opportunities to exploit sustainable 3d-metal catalysis for a stereoselective access to valuable chiral building blocks.

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Nickel/Photoredox Dual-Catalyzed Regiodivergent Aminoalkylation of Unactivated Alkyl Halides

A mild and regiodivergent aminoalkylation of unactivated alkyl halides is disclosed via a dual photoredox/nickel catalysis. Bipyridyl-type ligands without an ortho-substituent control the site-selective coupling at the original position, while ortho-disubstituted ligands tune the site-selectivity at a remote, unprefunctionalized position. Mechanistic studies combined with DFT calculations give insight into the mechanism and the origins of the ligand-controlled regioselectivity. Notably, this redox-neutral, regiodivergent alkyl-alkyl coupling features mild conditions, broad substrate scope for both alkyl coupling partners, and excellent site-selectivity and offers a straightforward way for α-alkylation of tertiary amines to synthesize structurally diverse alkylamines and value-added amino acid derivatives.

Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi-Hydrogenation of Alkynes

We report a selectivity-switchable nickel hydride-catalyzed methodology that enables the stereocontrolled semi-reduction of internal alkynes to E- or Z-alkenes under very mild conditions. The proposed transfer semi-hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo-induced generation of nickel hydride, syn-hydronickelation of alkyne, and alkenylnickel isomerization as key steps. Remarkably, mechanistic experiments indicate that the control of the stereoselectivity is not ensuing from a post-reduction alkene photoisomerization under our conditions. Instead, we demonstrate that the stereoselectivity of the reaction is dependent on the rate of a final protonolysis step which can be tuned by adjusting the pKa of an alcohol additive.

Nickel(I)-Phenolate Complexes: The Key to Well-Defined Ni(I) Species

Phosphine-stabilized monovalent nickel complexes play an important role in catalysis, either as catalytically active species or as decomposition products. Most routes to access these complexes are highly ligand specific or rely on strong reducing agents. Our group recently disclosed a path to access nickel(I)-phenolate complexes from bis(1,5-cyclooctadiene)nickel(0) (Ni(cod)2). Herein, we demonstrate this protocol's broad applicability by ligating a wide range of mono- and bidentate phosphine ligands. We further show the versatility of the phenolate fragment as a precursor to nickel(I)-alkyl or aryl species, which are relevant to Ni catalysis or synthetically useful nickel(I)-chloride and hydride complexes. We also demonstrate that the chloride complex can be synthesized in a one-pot procedure starting from Ni(cod)2 in good yield, making this protocol a valuable alternative to current procedures. Single-crystal X-ray diffraction, IR, and EPR (or NMR) spectroscopy were employed to characterize all of the synthesized nickel complexes.

Generation and Stabilization of a Dinickel Catalyst in a Metal-Organic Framework for Selective Hydrogenation Reactions

Although many monometallic active sites have been installed in metal-organic frameworks (MOFs) for catalytic reactions, there are no effective strategies to generate bimetallic catalysts in MOFs. Here we report the synthesis of a robust, efficient, and reusable MOF catalyst, MOF-NiH, by adaptively generating and stabilizing dinickel active sites using the bipyridine groups in MOF-253 with the formula of Al(OH)(2,2'-bipyridine-5,5'-dicarboxylate) for Z-selective semihydrogenation of alkynes and selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. Spectroscopic studies established the dinickel complex (bpy⋅- )NiII (μ2 -H)2 NiII (bpy⋅- ) as the active catalyst. MOF-NiH efficiently catalyzed selective hydrogenation reactions with turnover numbers of up to 192 and could be used in five cycles of hydrogenation reactions without catalyst leaching or significant decrease of catalytic activities. The present work uncovers a synthetic strategy toward solution-inaccessible Earth-abundant bimetallic MOF catalysts for sustainable catalysis.

Nickel-Catalyzed Markovnikov-Selective Hydrodifluoromethylation of Alkynes Using BrCF2 H

A Markovnikov-selective hydrodifluoromethylation of alkynes with BrCF2 H via nickel catalysis is described. This protocol proceeds via a migratory insertion of nickel hydride to alkyne followed by a CF2 H-coupling, enabling straightforward access to diverse branched CF2 H-alkenes with high efficiency and exclusive regioselectivity. The mild condition applies to a wide array of aliphatic and aryl alkynes with good functional group compatibility. Mechanistic studies are presented to support the proposed pathway.

Ni-Catalyzed Enantioselective Intramolecular Mizoroki-Heck Reaction for the Synthesis of Phenanthridinone Derivatives

A Ni-catalyzed enantioselective intramolecular Mizoroki-Heck reaction has been developed to transform symmetrical 1,4-cyclohexadienes with attached aryl halides into phenanthridinone analogues containing quaternary stereocenters. Herein, we report important advances in reaction optimization enabling control of unwanted proto-dehalogenation and alkene reduction side products. Moreover, this approach provides direct access to six-membered ring heterocyclic systems bearing all-carbon quaternary stereocenters, which have been much more challenging to form enantioselectively with nickel-catalyzed Heck reactions. A wide range of substrates were demonstrated to work in good to excellent yields. Good enantioselectivity was demonstrated using a new synthesized chiral iQuinox-type bidentate ligand (L27). The sustainability, low price of nickel catalysts, and significantly faster reaction rate (1 h) versus that of a recently reported palladium-catalyzed reaction (20 h) make this process an attractive alternative.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Origins of Regioselectivity in CuH-Catalyzed Hydrofunctionalization of Alkenes

Factors controlling the regioselectivity in alkene hydrocupration were computationally investigated using energy decomposition analysis. The results demonstrate that the Markovnikov-selective hydrocupration with electronically activated mono-substituted olefins is mostly affected by the destabilizing Pauli repulsion, which is due to the electron delocalization effect. The anti-Markovnikov-selective hydrocupration with 1,1-dialkyl-substituted terminal olefins is dominated by the repulsive electrostatic interactions, which is because of the unequal π electron distribution caused by the induction effect of alkyl substituents.

Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst

Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a Ni^I/III cycle have largely remained elusive. Herein, we report a Ni^II complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s^-1 and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and Ni^I/Ni^III intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of Ni^III intermediates by the independent synthesis and characterization of organometallic Ni^III complexes.

Notable Catalytic Activity of Transition Metal Thiolate Complexes against Hydrosilylation and Hydroboration of Carbon-Heteroatom Bonds

Chemists tend to use transition metal hydride complexes rather than thiolate complexes to catalyse chemical transformations because the hydride complexes possess diverse catalytic reactivity, although most of them are air/moisture-sensitive and difficult to prepare. By comparing the catalytic performances of pincer ligated group 10 metal thiolate and hydride complexes in catalysing the hydroboration and hydrosilylation of C=O and C=N bonds, we demonstrate in this review that transition metal thiolate complexes are much better catalysts than the corresponding hydride complexes in catalysing this type of reactions. Many hydroboration and hydrosilylation reactions catalysed by pincer ligated group 10 metal hydride complexes can also be catalysed by the corresponding thiolate complexes and the thiolate systems are far more active. Therefore, the applications of transition metal thiolate complexes in the catalytic hydroboration and hydrosilylation of unsaturated carbon-heteroatom bonds deserve special attention in future work.

NiH-catalyzed anti-Markovnikov hydroamidation of unactivated alkenes with 1,4,2-dioxazol-5-ones for the direct synthesis of N-alkyl amides

The addition of a nitrogen-based functional group to alkenes via a direct catalytic method is an attractive way of synthesizing value-added amides. The regioselective hydroamidation of unactivated alkenes is considered one of the easiest ways to achieve this goal. Herein, we report the NiH-catalyzed anti-Markovnikov intermolecular hydroamidation of unactivated alkenes enabled by using 2,9-dibutylphenathroline (diBuphen) as the ligand. This protocol provides a platform for the direct synthesis of over 90 structurally diverse N-alkyl amides using dioxazolones, which can be easily derived from abundant carboxylic acid feedstocks. This method succeeds for both terminal and internal unactivated alkenes and some natural products. Mechanistic studies including DFT calculations reveal an initial reversible insertion/elimination of the [NiH] to the alkene, followed by the irreversible amidation to furnish the N-alkyl amides. By crossover experiments and deuterium labeling studies, the observed anti-Markovnikov regioselectivities are suggested to be controlled by the sterical environment of the coupling reaction.

Synthesis and reactivity of a μ-1,2-dinitrogen dinickel(II) complex with a C-H activated silaamidinate pincer ligand

Reaction of the silaamidinate nickel bromide LSi(NAr)₂NiBr₂Li(thf)(OEt₂) (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃, 1) with NaHBEt₃ led to intramolecular C-H activation with the formation of the μ-1,2-dinitrogen dinickel pincer complex [LSi(NAr)(NAr)Ni]₂(μ-1,2-N₂) (Ar = 2-C(CH₃)₂-6-iPrC₆H₃, 2). Single-crystal X-ray diffraction analysis of 2 disclosed a square planar Ni(II) atom bridged by N₂. Reaction of 2 with carbon monoxide and 2,6-dimethylphenyl isocyanide yielded square planar carbonyl and isocyanide complexes 3 and 4 with release of N₂. These results provide new approaches for the coordination of N₂ with nickel(II) species.

NiH-Catalyzed Functionalization of Remote and Proximal Olefins: New Reactions and Innovative Strategies

Transition metal hydride catalyzed functionalization of remote and proximal olefins has many advantages over conventional cross-coupling reactions. It avoids the separate, prior generation of stoichiometric amounts of organometallic reagents and the use of preformed organometallic reagents, which are sometimes hard to access and may compromise functional group compatibility. The migratory insertion of metal hydride complexes generated in situ into readily available alkene starting materials, the hydrometalation process, provides an attractive and straightforward route to alkyl metal intermediates, which can undergo a variety of sequential cross-coupling reactions. In particular, with the synergistic combination of chain-walking and cross-coupling chemistry of nickel, NiH-catalyzed functionalization of remote and proximal olefins has undergone particularly intense development in the past few years. This Account aims to chronicle the progress made in this arena in terms of activation modes, diverse functionalizations, and chemo-, regio-, and enantioselectivity.We first provide a brief introduction to the general reaction mechanisms. Taking remote hydroarylation as an example, the four oxidation states of Ni have allowed us to develop two different reaction strategies to form the final product: a Ni(I)-H/X-Ni(II)-H platform that relies on stoichiometric reductants and a Ni(I/II/III) cycle and a redox-neutral functional group or FG-Ni(II)-H platform that reacts with an alkene substrate and forms the migratory products via a Ni(0/II) pathway. We also demonstrate that diverse functionalization, including general C-C bond-forming reactions and the more challenging C-N/C-S bond-forming reactions could be realized. Moreover, the employment of appropriate chiral ligands has allowed us to successfully realize the corresponding asymmetric hydrofunctionalization reactions of olefins, including hydroalkylation, hydroarylation, hydroalkenylation, hydroalkynylation, and hydroamination. Interestingly, the enantio-determining step could be enantioselective hydronickelation, selective oxidative addition, or selective reductive elimination. To realize more challenging asymmetric migratory hydrofunctionalization, we have developed a general ligand relay catalytic strategy with a combination of two simple ligands, the first for chain-walking and the second for asymmetric coupling. This novel strategy avoids the design of a single, possibly structurally complex chiral ligand to promote both steps of chain-walking and asymmetric coupling. In addition, the success of multicomponent hydrofunctionalization provides a convenient approach to gain simple access to complex molecules. Finally, alkyl halides could be used as olefin precursors to undergo a variety of reductive migratory cross-electrophile coupling reactions. Applications of these remote hydrofunctionalization reactions are also discussed. We hope this Account will inspire future development in the field to overcome key challenges, including conceptually new catalytic strategies, development of high-performance systems with enhanced reactivity and selectivity, cutting-edge catalyst design, and further mechanistic studies.

Use of a PCET Mediator Enables a Ni-HER Electrocatalyst to Act as a Hydride Delivery Agent

The generation of metal hydride intermediates during reductive electrocatalysis in the presence of acid most commonly leads to the hydrogen evolution reaction (HER). Redirecting the reactivity profile of such hydride intermediates toward the reduction of unsaturated substrates is an exciting opportunity in catalysis but presents a challenge in terms of catalyst selectivity. In this study, we demonstrate that a prototypical phosphine-supported Ni-HER catalyst can be repurposed toward the electrocatalytic reduction of a model substrate, methyl phenylpropiolate, via hydride transfer from a Ni^II-H when interfaced with a metallocene-derived proton-coupled electron transfer (PCET) mediator. Key to success is generation of the Ni^II-H at a potential pinned to that of the PCET mediator which is appreciably anodic of the onset of HER. Electrochemical, spectroscopic, and theoretical data point to a working mechanism where a PCET step from the metallocene-derived mediator to Ni^II generates Ni^III-H and is rate-determining; the latter Ni^III-H is then readily reduced to a Ni^II-H, which is competent for substrate reduction. Additional studies show that this tandem PCET-mediated hydride generation can afford high stereoselectivity (e.g., >20:1 Z/E using a phosphine-cobalt precatalyst with ethyl 2-heptynoate) and can also be used for the reduction of α,β-unsaturated ketones.

Tridentate NacNac Stabilized Tin and Nickel Complexes: Access to a Monomeric Nickel Hydride and Its Catalytic Application

The transmetalation reaction of picolyl-supported tridentate nacnac germylene monochloride [2,6-iPr₂-C₆H₃NC(Me)CHC(Me)NH(CH₂py)]GeCl (1) (py = pyridine) with SnCl₂ results in an analogous stannylene chloride (2). The three-coordinated stannylenium cation [{2,6-iPr₂-C₆H₃NC(Me)CHC(Me)NH(CH₂py)}Sn]⁺ with SnCl₃^- as a counteranion (3) has been generated through the abstraction of chloride ligand from 2 using an additional equivalent of SnCl₂. Instead of forming a donor-acceptor complex, 2 undergoes a facile redox transmetalation reaction with Ni(COD)₂ (COD = cyclooctadiene) and CuCl to afford analogous nickel and copper complexes [2,6-iPr₂-C₆H₃NC(Me)CHC(Me)NH(CH₂py)]MCl [M = Ni (4) and Cu (5)]. The reactions of 4 with potassium tri-sec-butylborohydride (commonly known as K-selectride) and AgSbF₆ provide access to monomeric Ni(II) hydride, [2,6-iPr₂-C₆H₃NC(Me)CHC(Me)NH(CH₂py)]NiH (6) and a Ni(II) cation, [{2,6-iPr₂-C₆H₃NC(Me)CHC(Me)NH(CH₂py)}Ni][SbF₆] (7), respectively. 6 was found to be an effective catalyst for the hydroboration of amides.

Remote and Proximal Hydroaminoalkylation of Alkenes Enabled by Photoredox/Nickel Dual Catalysis

A mild and site-selective hydroaminoalkylation of activated and unactivated alkenes via dual photoredox/Ni catalysis is developed. This dual catalytic strategy enables exclusive access to α-selective products, which is complementary to previously reported photocatalytic hydroaminoalkylation of activated alkenes that provides the β-selective products. The chain-walking of a Ni-H intermediate toward a carbonyl allows for the hydroaminoalkylation of unactivated alkenes at remote sp³ C-H sites. This method tolerates a broad substrate scope of both amines and alkenes as well as providing a streamlined synthesis of value-added β-amino acid derivatives from readily available starting materials.

A General and Efficient Solution to Monofluoroalkylation: Divergent Synthesis of Aliphatic Monofluorides with Modular Synthetic Scaffolds

Monofluoroalkanes are important in many pharmaceuticals, agrochemicals and functional materials. However, the lack of easily available and transformable monofluoroalkylating reagents that facilitate a broad array of transformations has hampered the application of monofluoroalkylation. Herein, we report a general and efficient method of preparing diverse aliphatic monofluorides with monofluoroalkyl triflate as the synthetic scaffold. Using both nickel-catalyzed hydromonofluoroalkylation of unactivated alkenes and copper-catalyzed C-C bond formation, the general diversification of the monofluoroalkylating scaffold has been exhibited. The broad utility of this monofluoroalkylating reagent is shown by concise conversion into various conventional fluoroalkylating reagents and construction of monofluoro-alkoxy, -alkylamino motifs with commercially available heteroatom-based coupling partners.

Reactions of an Osmium-Hexahydride Complex with 2-Butyne and 3-Hexyne and Their Performance in the Migratory Hydroboration of Aliphatic Internal Alkynes

Reactions of the hexahydride OsH₆(PⁱPr₃)₂ (1) with 2-butyne and 3-hexyne and the behavior of the resulting species toward pinacolborane (pinBH) have been investigated in the search for new hydroboration processes. Complex 1 reacts with 2-butyne to give 1-butene and the osmacyclopropene OsH₂(η²-C₂Me₂)(PⁱPr₃)₂ (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes into a η⁴-butenediyl form to afford OsH₂(η⁴-CH₂CHCHCH₂)(PⁱPr₃)₂ (3). Isotopic labeling experiments indicate that the isomerization involves Me-to-C_Os hydrogen 1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH₂(η²-C₂Et₂)(PⁱPr₃)₂ (4). Similarly to 2, complex 4 evolves to η⁴-butenediyl derivatives OsH₂(η⁴-CH₂CHCHCHEt)(PⁱPr₃)₂ (5) and OsH₂(η⁴-MeCHCHCHCHMe)(PⁱPr₃)₂ (6). In the presence of pinBH, complex 2 generates 2-pinacolboryl-1-butene and OsH{κ²-H,H-(H₂Bpin)}(η²-HBpin)(PⁱPr₃)₂ (7). According to the formation of the borylated olefin, complex 2 is a catalyst precursor for the migratory hydroboration of 2-butyne and 3-hexyne to 2-pinacolboryl-1-butene and 4-pinacolboryl-1-hexene. During the hydroboration, complex 7 is the main osmium species. The hexahydride 1 also acts as a catalyst precursor, but it requires an induction period that causes the loss of 2 equiv of alkyne per equiv of osmium.

A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation

The nature of transition-metal-olefin bonding has been explained by the Dewar-Chatt-Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal-ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal-bis-ethylene complex forming an unprecedented planar metalla-bis-cyclopropane structure with magnesium-based metalloligands. Treatment of LMgEt (L = [(DippNCMe)₂CH]^-, Dipp = 2,6-ⁱPr₂C₆H₃) with Ni(cod)₂ (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂, which features a linear Mg-Ni-Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni(C₂H₄)₂ moiety as a nickelaspiropentane. The exposure of (LMg)₂Ni(C₂H₄)₂ to 1 bar H₂ at room temperature produced a four-hydride-bridged complex (LMg)₂Ni(μ-H)₄. The profile of H₂ activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex (LMg)₂Ni(C₂H₄)₂ catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.