Magnesium Pincer Complexes and Their Applications in Catalytic Semihydrogenation of Alkynes and Hydrogenation of Alkenes: Evidence for Metal-Ligand Cooperation

PCP and POCOP complexes of calcium

The first synthesis of PCP and POCOP complexes of calcium are described. Ca(N(SiMe3)2)2 is reacted with Li[tBuPCP] or Li[tBuPOCOP] to form (tBuPCP)Ca(N(SiMe3)2) (1a) and (tBuPOCOP)Ca(N(SiMe3)2) (1b). Reaction of 1b or Ca(N(SiMe3)2)2with excess Li[tBuPOCOP] leads to a homoleptic complex (tBuPOCOP)2Ca, 2. The (iPrPOCOP) ligand exclusively forms the homoleptic complex (iPrPOCOP)2Ca, 3.

Stable Mg2+ Dication Weakly Stabilized/Coordinated in Solution: Synthesis, Structure, Reactivity, and Use in Catalysis

The first soluble and stable Mg2+ dication stabilized only by weakly coordinating and chemically robust carborate anions [HexCB11Cl11]- is described. Mg[HexCB11Cl11]2 (1), prepared by reaction of Mg(nBu)2 with 2 equiv of [Ph3C][HexCB11Cl11], consists, in the solid state, of a central Mg2+ surrounded by two [HexCB11Cl11]- anions. In solution, experimental and classical molecular dynamics simulations (cMD) agree with cation/anion association being retained, reflecting the high electrophilicity of the Mg center. Yet, reflecting only weak anion/cation interactions, species 1 polymerizes 1-hexene and coordinates alkynes. However, 1 displays no reaction with HSiEt3 at room temperature, consistent with a low hydridicity of the hard (HSAB) Mg2+ center. Contrasting with 1 (FIA = 264 kJ mol-1; FIA: Fluoride Ion Affinity), salt Mg[(nBu)3NB12H4Cl7]2 (2), incorporating the more basic ammoniododecaborate [(nBu)3NB12H4Cl7]- anion, is significantly less Lewis acidic (FIA = 214.7 kJ mol-1) and unreactive with alkenes and alkynes. Salt 1 effectively catalyzes alkene/alkyne hydrosilylation via an initial alkene/alkyne coordination/initiation, as suggested by experimental and computational data. It also efficiently catalyzes (with a catalyst loading down to 0.1 mol%) the hydrosilylation of CO2 to CH4 in the presence of HSiEt3. Salt 1 smoothly promotes the catalytic transfer hydrogenation of 1,1-diphenylethylene, and it is also an active imine hydrogenation catalyst in the presence of H2.

Unexpected activity of MgO nanoclusters for the reductive-coupling synthesis of organonitrogen chemicals with C = N bonds

Reductive-coupling of nitro compounds and alcohols is a sustainable route for constructing C = N bonds in organonitrogen chemicals, yet challenging due to the inertness of α-Csp3-H bond in alcohols and the vulnerability of C = N bonds towards hydrogenation. Here, we report the surprising catalytic activity of ultrafine alkaline-earth metal oxide MgO nanoclusters (0.9 ± 0.3 nm) that efficiently activate α-Csp3-H bonds, facilitating the transfer hydrogenation and synthesis of value-added chemicals bearing C = N bonds with high to excellent yields (86-99%). Controlled experiments and characterizations showed the crucial role of oxygen vacancies (Ov) and local Mg environment (Mg-O bond) in MgO for substrate adsorption and activation via electronic interactions between substrate's negatively charged oxygen atoms and Ov sites in MgO nanoclusters. Theoretical calculation further confirmed that Ov significantly lowered the energy barrier of the hydrogen atom transfer from α-Csp3-H in ethanol to the nitro group in nitrobenzene (29.3 vs. 52.9 kcal/mol), which is the rate-determining step with the highest energy barrier in reductive-coupling reactions. Our method not only provides an efficient and sustainable pathway for synthesizing organonitrogen chemicals with C = N bonds but also inspires the exploration of main group element catalysts as alternatives to transition metal and noble metal catalysts for organic transformations.

Aryl-Radical-Initiated Z-Olefin Synthesis via Synergistic Palladium and Photocatalysis

We report a novel photocatalyzed selective functionalization of terminal olefins for the synthesis of Z-olefins and cyclobutanes under mild conditions in excellent yields. A series of Z-cinnamic acid derivatives, enamines, and amide-containing cyclobutanes were efficiently synthesized using this method. Mechanistic studies indicated that the success of this reaction is based on the triple radical relay strategy, anion-π interaction for aryl halide activation and aryl radical generation, palladium for controlling chemoselectivity of the aryl radical, and photocatalyst for controlling stereoselectivity via diradical species, respectively.

Facile, Reversible Hydrogen Activation by Low-Coordinate Magnesium Oxide Complexes

New approaches to achieve facile and reversible dihydrogen activation are of importance for synthesis, catalysis, and hydrogen storage. Here we show that low-coordinate magnesium oxide complexes [{(RDipnacnac)Mg}2(μ-O)] 1, with RDipnacnac = HC(RCNDip)2, Dip = 2,6-iPr2C6H3, R = Me (1a), Et (1b), iPr (1c), readily react with dihydrogen under mild conditions to afford mixed hydride-hydroxide complexes [{(RDipnacnac)Mg}2(μ-H)(μ-OH)] 4. Dehydrogenation of complexes 4 is strongly dependent on remote ligand substitution and can be achieved by simple vacuum-degassing of 4c (R = iPr) to regain 1c. Donor addition to complexes 4 also releases hydrogen and affords donor adducts of magnesium oxide complexes. Computational studies suggest that the hydrogen activation mechanism involves nucleophilic attack of an oxide lone pair at a weakly bound H2···Mg complex in an SN2-like manner that induces a heterolytic dihydrogen cleavage to yield an MgOH and an MgH unit. Alternative synthetic routes into complex 4b from a magnesium hydride complex have been investigated and the ability of complexes 1 or 4 to act as catalysts for the hydrogenation of 1,1-diphenylethene (DPE) has been tested.

Photocatalytic Hydrogenation of Alkenes Using Water as Both the Reductant and the Proton Source

Utilization of clean and low-cost water as the reductant to enable hydrogenation of alkenes is highly attractive in green chemistry. However, this research subject is considerably challenging due to the sluggish kinetics of the water oxidation half-reaction. It is also very difficult to avoid the undesired oxidation of alkenes because that this oxidation is far easier to occur than the desired oxidation of water from thermodynamic standpoint. Herein, this challenge is overcome by applying a cooperative catalysis where HCl is used as the cocatalyst to accelerate Pt/g-C3N4-catalyzed water oxidation and suppress the undesired oxidation of the alkene. This provides an example for using water as the reductant and the proton source to enable the photocatalytic hydrogenation of alkenes. The present method exhibits broad substrate applicability, and allows various arylethenes and aliphatic alkenes to undergo the hydrogenation smoothly.

Synthesis and reactivity of a heteroleptic magnesium hydride on a dearomatized picolyl-based NNN-chelator

Heteroleptic magnesium hydrides are important for their stoichiometric and catalytic reduction chemistry. Their primary nucleophilic site is typically the hydride, while the ancillary ligands commonly used are mostly spectators. Chemically non-innocent ligands in comparison are rarely applied on MgH as their reactivity can be complex. Milstein et al. have recently reported Mg-mediated alkyne hydrogenation by using their metal-ligand cooperation (MLC) concept on a dearomatized picolyl-based PNP pincer that is non-innocent with a nucleophilic nature. A '(PNP)MgH' is noted as the active catalyst in hydrogenation but without structural validation. Inspired by the same, we report herein a novel NNN-chelator (MesL) with a dearomatized picolyl moiety and its well-defined MgH. Having two prominent nucleophilic sites, the present MgH shows metal-ligand competition while reacting with certain electrophiles. It also distinguishes nonpolar alkynes and polar carbonyls by cleanly inserting itself into the former but not the latter. The nucleophilicities of the two sites are also probed by DFT methods and compared with Milstein's (PNP)MgH. Although the present system shows no MLC-type H2 activation, the addition of a CS2 molecule in that way is realized.

Diverse Functionality of Molecular Germanium: Emerging Opportunities as Catalysts

Fundamental research on germanium as the central element in compounds for bond activation chemistry and catalysis has achieved significant feats over the last two decades. Designing strategies for small molecule activations and the ultimate catalysts established capitalize on the orbital modalities of germanium, apparently imitating the transition-metal frontier orbitals. There is a growing body of examples in contemporary research implicating the tunability of the frontier orbitals through avant-garde approaches such as geometric constrained empowered reactivity, bimetallic orbital complementarity, cooperative reactivity, etc. The goal of this Perspective is to provide readers with an overview of the emerging opportunities in the field of germanium-based catalysis by perceiving the underlying key principles. This will help to convert the discrete set of findings into a more systematic vision for catalyst designs. Critical exposition on the germanium's frontier orbitals participations evokes the key challenges involved in innovative catalyst designs, wherein viewpoints are provided. We close by addressing the forward-looking directions for germanium-based catalytic manifold development. We hope that this Perspective will be motivational for applied research on germanium as a constituent of pragmatic catalysts.

Heteroleptic Magnesium n-Butyl on a Chemically Non-innocent 2-Anilidomethylpyridine Ligand Leading to Diverse Magnesium Hydrides

Molecular magnesium hydrides and hydride-rich clusters are of significant interest for applications ranging from catalysis and small molecule activation to hydrogen storage. Here, we investigate the 2-anilidomethylpyridine framework NNL as an ancillary support for magnesium organometallics with a special emphasis on hydrides. The proligand NNLH (N-[2,6-bis(1-methylethyl)phenyl]-α,6-diphenyl-2-pyridinemethanamine) gives [(NNL)Mg(nBu)(thf)] (1) by nbutane elimination from Mg(nBu)2(thf)n. A stronger donor such as DMAP replaces the THF from 1 to give [(NNL)Mg(nBu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(NNL)Mg(μ-OnBu)]2 (32). The homoleptic [(NNL)2Mg] (8) is made from 1 and a second equiv of NNLH. 1's terminal nBu group is selectively protonated by HN(SiMe3)2 to give [(NNL)MgHMDS] (4; HMDS = N(SiMe3)2), whereas Ph3SiOH partially protonates the backbone anilide as well to give a mixture of [(NNL)Mg(OSiPh3)(thf)] (5) and free NNLH. Like HN(SiMe3)2, aprotic MeOTf also reacts by selectively abstracting the nBu group from 1 to give [(NNL)Mg(μ:κ2-O,O'-OTf)(thf)]2 (62). Interestingly, screening the common synthetic routes for magnesium hydrides leads to diverse outcomes upon varying the Mg precursors and hydride sources. 1 and PhSiH3 give the hydride cluster [{(NNL)2Mg2(μ-H)}2(μ-H)4Mg] (7), whereas 2 and PhSiH3 give the molecular complex [(NNLde)Mg(dmap)2] (9) with a dearomatized pyridyl backbone. 1 and HBpin (pinacolborane) give a product mixture, from which a different hydride cluster [(NNL)2Mg2(μ-H)}2(μ:κ2-O,O'-O2C2Me4)] (10) is identified, showing a rare instance of complete deborylation of a HBpin molecule. 1 and HBcat (catecholborane) also give a product mixture, one of which is the borylated ligand [(NNL)Bcat] (11). HBpin with 4 as the Mg precursor takes the ligand borylation route more selectively to give [(NNL)Bpin] (12). Last, 1 reacts with iPrNH2BH3 to give [(NNL)Mg{NH(iPr)BH3}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(NNL)2Mg2(μ-H){(μ-NH(iPr)BH3}] (14) by partial β-hydride elimination. In comparison, [(NNL)Mg(iPrNHBH3)(dmap)] (15) arising from the DMAP-bound 2 and iPrNH2BH3 is stable toward such elimination.

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

Syntheses and exploration of the catalytic activities of organotin(IV) compounds

Six organotin(IV) compounds (1-6) have been synthesized by reaction of the polydentate pro-ligands H3L and H2L, respectively, with the corresponding diorganotin chlorides. All of the compounds were characterized by FT-IR spectroscopy, 1H, 13C{1H}, and 119Sn (1H) NMR spectroscopy, HRMS spectrometry, and single-crystal X-ray diffraction. The solid-state structures show that all of the compounds are monomeric (except compound 3) and contain a penta-coordinated tin atom. Compound 3 is a dimer with two hexa-coordinated tin atoms. Compounds 1-3 contain a non-coordinated hydroxymethyl group. All of the compounds have been screened for their catalytic efficacy in the synthesis of 1,2 disubstituted benzimidazoles using o-phenylenediamine and aldehyde derivatives. It has been observed that both the Lewis acidic Sn(IV) centre and the hydroxymethyl group (hydrogen bond donor) catalyse the reactions with a product yield of up to 92%.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Different Reaction Modes Operating in ansa-Half-Sandwich Magnesium Catalysts

Magnesium-based catalysts are becoming popular for hydroelementation reactions specially using p-block reagents. Based on the seminal report from Schäfer's group (ChemCatChem 2022, 14, e202201007), our study demonstrates that the reaction mechanisms exhibit a far greater degree of complexity than originally presumed. Magnesium has a variety of coordination modes (and access to different hybridizations) which allows this electron-deficient centre to modulate its catalytic power depending on the σ-donor properties of the reagent. DFT calculations demonstrate several reaction channels closely operating in these versatile catalysts. In addition, variations in limiting energy barriers resulting from catalyst modifications were examined as a function of the Hammett constant, thereby predicting enhanced efficiency in reaction conversions.

A Phosphine-Free Air-Stable Mn(II)-Catalyst for Sustainable Synthesis of Quinazolin-4(3H)-ones, Quinolines, and Quinoxalines in Water

The synthesis, characterization, and catalytic application of a new phosphine-free, well-defined, water-soluble, and air-stable Mn(II)-catalyst [Mn(L)(H2O)2Cl](Cl) ([1]Cl) featuring a 1,10-phenanthroline based tridentate pincer ligand, 2-(1H-pyrazol-1-yl)-1,10-phenanthroline (L), in dehydrogenative functionalization of alcohols to various N-heterocycles such as quinazolin-4(3H)-ones, quinolines, and quinoxalines are reported here. A wide array of multisubstituted quinazolin-4(3H)-ones were prepared in water under air following two pathways via the dehydrogenative coupling of alcohols with 2-aminobenzamides and 2-aminobenzonitriles, respectively. 2-Aminobenzyl alcohol and ketones bearing active methylene group were used as coupling partners for synthesizing quinoline derivatives, and various quinoxaline derivatives were prepared by coupling vicinal diols and 1,2-diamines. In all cases, the reaction proceeded smoothly using our Mn(II)-catalyst [1]Cl in water under air, affording the desired N-heterocycles in satisfactory yields starting from cheap and readily accessible precursors. Gram-scale synthesis of the compounds indicates the industrial relevance of our synthetic strategy. Control experiments were performed to understand and unveil the plausible reaction mechanism.

Unlocking the potential of metal ligand cooperation for enantioselective transformations

Metal-ligand cooperation, in which both the metal and the ligand of a transition metal complex actively participate in chemical transformations leading to enhanced reactivity or selectivity in chemical reactions, has emerged as a powerful and versatile concept in catalysis. This Viewpoint discusses the development trajectory of transition metal-based complexes as catalysts in (de)hydrogenative processes, in particular those cases where metal-ligand cooperation has been invoked to rationalise the observed high reactivities and excellent selectivities. The historical context, mechanistic aspects and current applications are discussed with the suggestion to explore the potential of the MLC mode of action of such catalysts in enantioselective transformations beyond (de)hydrogenative processes.

Hydrogenation of Alkynes and Olefins Catalyzed by Quaternary Ammonium Salts

Catalytic hydrogenation of unsaturated hydrocarbons to alkenes and alkanes using molecular hydrogen is one of the most fundamental transformations in organic synthesis. While methodologies involving transition metals as catalysts in homogeneous and heterogeneous processes have been well developed, metal-free catalytic hydrogenation offers an ideal approach for future chemistry. Herein, the common and inexpensive quaternary ammonium salts are first introduced as catalysts in the catalytic hydrogenation system for the transformations from alkynes or olefins into the corresponding olefins or alkanes. Interestingly, the hydrogenation process of alkynes can be controlled to selectively produce alkenes or alkanes under different conditions. Moreover, the possible mechanism is discussed in new insights into the catalytic behavior of quaternary ammonium salts.

Reductive Metallation of Dendralenes and Myrcene Using Dimagnesium(I) Compounds: A Facile Route to Unsaturated Organomagnesium Compounds

The two-electron reduction of 2,3-dimethylbuta-1,3-diene (DMB) with β-diketiminate and guanidinate substituted dimagnesium(I) compounds has given complexes in which two bidentate amido-magnesium fragments are bridged through the π-system of the DMB dianion, viz. [(LMg)2 (μ-DMB)] (L=Xyl Nacnac, [HC(MeCNXyl)2 ]- , Xyl=2,6-xylyl; or Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl). Similar double reductions of [4]dendralene (4dend) have afforded the complexes, [(LMg)2 (μ-4dend)] (L=Ar Nacnac, Ar=Xyl or mesityl (Mes); or Priso) in which the 4dend dianion is π-coordinated to the bidentate amido-magnesium fragments. Treatment of several such complexes with THF leads to Z- to E-isomerization of the dendralene fragment, and formation of purely σ-bonded Mg-C interactions in the THF coordinated products [{(Ar Nacnac)(THF)Mg}2 (μ-4dend)] (Ar=Xyl, Mes or Dip). Reaction of myrcene (Myr) with [{(Xyl Nacnac)Mg}2 ] proceeds via reductive coupling of Myr to give a previously unknown acyclic, branched C20 tetra-olefin dianion complex [{(Xyl Nacnac)(THF)Mg}2 (μ-Myr)2 ]. Preliminary reactions of [(LMg)2 (μ-DMB)] with H2 and/or CO yielded a series of products, including novel magnesium hydride compounds, products derived from couplings of CO with the reduced DMB fragment (viz. magnesium dimethylcyclohexadienediolates), and one magnesium cyclopropanetriolate complex from the magnesium(I) induced coupling of DMB with H2 and CO.

Lanthanide Mimicking by Magnesium for Oxazolidinone Synthesis

In the last decade, magnesium complexes have emerged as a viable alternative to transition-metal catalysts for the hydrofunctionalization of unsaturated bonds. However, their potential for advanced catalytic reactions has not been thoroughly investigated. To address this gap, we have developed a novel magnesium amide compound (3) using a PNP framework that is both bulky and flexible. Our research demonstrates that compound 3 can effectively catalyze the synthesis of biologically significant oxazolidinone derivatives. This synthesis involves a tandem reaction of hydroalkoxylation and cyclohydroamination of isocyanate using propargyl alcohol. Furthermore, we conducted comprehensive theoretical calculations to gain insights into the reaction mechanism. It is important to note that these types of transformations have not been reported for magnesium and would significantly enhance the catalytic portfolio of the 7th most abundant element.

Spin-Crossing in the (Z)-Selective Alkyne Semihydrogenation Mechanism Catalyzed by Mo3S4 Clusters: A Density Functional Theory Exploration

Semihydrogenation of internal alkynes catalyzed by the air-stable imidazolyl amino [Mo3S4Cl3(ImNH2)3]+ cluster selectively affords the (Z)-alkene under soft conditions in excellent yields. Experimental results suggest a sulfur-based mechanism with the formation of a dithiolene adduct through interaction of the alkyne with the bridging sulfur atoms. However, computational studies indicate that this mechanism is unable to explain the experimental outcome: mild reaction conditions, excellent selectivity toward the (Z)-isomer, and complete deuteration of the vinylic positions in the presence of CD3OD and CH3OD. An alternative mechanism that explains the experimental results is proposed. The reaction begins with the hydrogenation of two of the Mo3(μ3-S)(μ-S)3 bridging sulfurs to yield a bis(hydrosulfide) intermediate that performs two sequential hydrogen atom transfers (HAT) from the S-H groups to the alkyne. The first HAT occurs with a spin change from singlet to triplet. After the second HAT, the singlet state is recovered. Although the dithiolene adduct is more stable than the hydrosulfide species, the large energy required for the subsequent H2 addition makes the system evolve via the second alternative pathway to selectively render the (Z)-alkene with a lower overall activation barrier.

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Designing New Magnesium Pincer Complexes for Catalytic Hydrogenation of Imines and N-Heteroarenes: H2 and N-H Activation by Metal-Ligand Cooperation as Key Steps

Utilization of main-group metals as alternatives to transition metals in homogeneous catalysis has become a hot research area in recent years. However, their application in catalytic hydrogenation is less common due to the difficulty in heterolytic cleavage of the H-H bond. Employing aromatization/de-aromatization metal-ligand cooperation (MLC) highly enhances the H₂ activation process, offering an efficient approach for the hydrogenation of unsaturated molecules catalyzed by main-group metals. Herein, we report a series of new magnesium pincer complexes prepared using PNNH-type pincer ligands. The complexes were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ and N-H bonds by MLC employing these pincer complexes was developed. Using the new magnesium complexes, homogeneously catalyzed hydrogenation of aldimines and ketimines was achieved, affording secondary amines in excellent yields. Control experiments and DFT studies reveal that a pathway involving MLC is favorable for the hydrogenation reactions. Moreover, the efficient catalysis was extended to the selective hydrogenation of quinolines and other N-heteroarenes, presenting the first example of hydrogenation of N-heteroarenes homogeneously catalyzed by early main-group metal complexes. This study provides a new strategy for hydrogenation of C═N bonds catalyzed by magnesium compounds and enriches the research of main-group metal catalysis.

Catalytic, Z-Selective, Semi-Hydrogenation of Alkynes with a Zinc-Anilide Complex

The reversible activation of dihydrogen with a molecular zinc anilide complex is reported. The mechanism of this reaction has been probed through stoichiometric experiments and density functional theory (DFT) calculations. The combined evidence suggests that H₂ activation occurs by addition across the Zn-N bond via a four-membered transition state in which the Zn and N atoms play a dual role of Lewis acid and Lewis base. The zinc hydride complex that results from H₂ addition has been shown to be remarkably effective for the hydrozincation of C═C bonds at modest temperatures. The scope of hydrozincation includes alkynes, alkenes, and a 1,3-butadiyne. For alkynes, the hydrozincation step is stereospecific leading exclusively to the syn-isomer. Competition experiments show that the hydrozincation of alkynes is faster than the equivalent alkene substrates. These new discoveries have been used to develop a catalytic system for the semi-hydrogenation of alkynes. The catalytic scope includes both aryl- and alkyl-substituted internal alkynes and proceeds with high alkene: alkane, Z:E ratios, and modest functional group tolerance. This work offers a first example of selective hydrogenation catalysis using zinc complexes.

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt₂)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr₃(SMe₂)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC₈) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C₆F₅)₃), forming a frustrated radical pair [(SiNSi)B]^•+[B(C₆F₅)₃]^•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C₆F₅)₃] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.