Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

Leveraging ligand-based proton and electron transfer for aerobic reactivity and catalysis

While O2 is an abundant, benign, and thermodynamically potent oxidant, it is also kinetically inert. This frequently limits its use in synthetic transformations. Correspondingly, direct aerobic reactivity with O2 often requires comparatively harsh or forcing conditions to overcome this kinetic barrier. Forcing conditions limit product selectivity and can lead to over oxidation. Alternatively, O2 can be activated by a catalyst to facilitate oxidative reactivity, and there are a variety of sophisticated examples where transition metal catalysts facilitate aerobic reactivity. Many efforts have focused on using metal-ligand cooperativity to facilitate the movement of protons and electrons for O2 activation. This approach is inspired by enzyme active sites, which frequently use the secondary sphere to facilitate both the activation of O2 and the oxidation of substrates. However, there has only recently been a focus on harnessing metal-ligand cooperativity for aerobic reactivity and, especially, catalysis. This perspective will discuss recent efforts to channel metal-ligand cooperativity for the activation of O2, the generation and stabilization of reactive metal-oxygen intermediates, and oxidative reactivity and catalysis. While significant progress has been made in this area, there are still challenges to overcome and opportunities for the development of efficient catalysts which leverage this biomimetic strategy.

Synthesis of a potassium capped terminal cobalt-oxido complex

An unusual example of a potassium capped terminal cobalt-oxido complex has been isolated and crystallographically characterized. The synthesis of [tBu,TolDHP]CoOK proceeds from a previously reported parent compound, [tBu,TolDHP]CoOH, via deprotonation with KOtBu. Structural and electronic characterization suggest a weakly coupled dimer in a distinct seesaw geometry with a Co(III) oxidation state and a non-innocent radical ligand.

Experimental and theoretical insights for designing Zn2+ complexes to trigger chemo-selective hetero-coupling of alcohols

Designing well-defined Zn-complexes for sustainable dehydrogenative catalysis overcoming the difficulties associated with activating Zn2+(d10)-metal species is considered paramount goal in catalysis. Herein, we explore the plausibility of β-alkylation of secondary alcohols with primary alcohols by well-defined 3d10 Zn-complexes. Detailed organometallic and catalytic investigations, in conjunction with computational analyses, were conducted to ascertain the potential involvement of the catalyst at various stages of the catalytic process.

Metal-ligand cooperativity in chemical electrosynthesis

Electrochemistry has been an increasingly useful tool for organic synthesis, as it can selectively generate reactive intermediates under mild conditions using an applied potential. Concurrently, synergistic activity of a metal and a ligand has been used in thermal catalysis and electrocatalytic renewable fuel generation for substrate selectivity and improved catalyst activity. Combining these synthetic strategies is an attractive approach for mild, selective, and sustainable electrosynthesis. This perspective discusses examples of metal-ligand synergistic catalysis in electrochemical applications in organic and organometallic synthesis. The range of reactions and ligand design principles illustrates many opportunities for further discovery in this area and the potential for far-reaching synthetic benefits.

Selective Cobalt-Mediated Formation of Hydrogen Peroxide from Water under Mild Conditions via Ligand Redox Non-Innocence

Despite the broad importance of hydrogen peroxide (H2O2) in oxidative transformations, there are comparatively few viable routes for its production. The majority of commercial H2O2 is currently produced by the stepwise reduction of dioxygen (O2) via the anthraquinone process, but direct electrochemical formation from water (H2O) would have several advantages─namely, avoiding flammable gases or stepwise separations. However, the selective oxidation of H2O to form H2O2 over the thermodynamically favored product of O2 is a difficult synthetic challenge. Here, we present a molecular H2O oxidation system with excellent selectivity for H2O2 that functions both stoichiometrically and catalytically. We observe high efficiency for electrocatalytic H2O2 production at low overpotential with no O2 observed under any conditions. Mechanistic studies with both calculations and kinetic analyses from isolated intermediates suggest that H2O2 formation occurs in a bimolecular fashion via a dinuclear H2O2-bridged intermediate with an important role for a redox non-innocent ligand. This system showcases the ability of metal-ligand cooperativity and strategic design of the secondary coordination sphere to promote kinetically and thermodynamically challenging selectivity in oxidative catalysis.

Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons

Alkyne semihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e-) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes. Mechanistic studies reveal that the transformation is concerted and Z-selective. Calculations support a ligand-based hydrogen-atom transfer pathway instead of a hydride mechanism, which is commonly invoked for transition metal hydrogenation catalysts. The synthesis of the proposed intermediates demonstrates that the catalytic mechanism proceeds through a reduced formal Ni(I) species. The high yields for terminal alkene products without over-reduction or oligomerization are among the best reported for any homogeneous catalyst. Furthermore, the metal-ligand cooperative hydrogen transfer enabled with this system directs the efficient flow of H atom equivalents toward alkyne reduction rather than hydrogen evolution, providing a blueprint for applying similar strategies toward a wide range of electroreductive transformations.

Synthesis and catalytic activity of well-defined Co(I) complexes based on NHC-phosphane pincer ligands

A new methodology for the preparation of Co(I)-NHC (NHC = N-heterocyclic carbene) complexes, namely, [Co(PCNHCP)(CO)2][Co(CO)4] (1) and [Co(PCNHCP)(CO)2]BF4 (2), has been developed (PCNHCP = 1,3-bis(2-(diphenylphosphanyl)ethyl)-imidazol-2-ylidene). Both complexes can be straightforwardly prepared by direct reaction of their parent imidazolium salts with the Co(0) complex Co2(CO)8. Complex 1 efficiently catalyses the reductive amination of furfural and levulinic acid employing silanes as reducing agents under mild conditions. Furfural has been converted into a variety of secondary and tertiary amines employing dimethyl carbonate as the solvent, while levulinic acid has been converted into pyrrolidines under solventless conditions. Dehydrocoupling of the silane to give polysilanes has been observed to occur as a side reaction of the hydrosilylation process.

Cobalt-Catalyzed Enantioselective Hydrogenation of Diaryl Ketones with Ferrocene-Based Secondary Phosphine Oxide Ligands

A new class of cobalt catalytic system for asymmetric hydrogenation of ketones was herein reported, involving the development of novel ferrocene-based secondary phosphine oxide ligands. An unusual P-O bidentate coordination pattern with cobalt was confirmed by an X-ray diffraction study. The bichelating tetrahedral cobalt(II) complexes afforded high reactivities (up to 99% yield) and good to excellent enantioselectivities (up to 92% ee) in the AH of various ortho-substituted diaryl ketones. In addition, the diferrocenyl cobalt complex was characterized with intriguing UV-vis absorption and electrochemical properties.

Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel-Bipyridine Complexes

Hydrogenation reactions of carbon-carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C-C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.

A cobalt adduct of an N-hydroxy-piperidinium cation

Cooperativity between organic ligands and transition metals in H-atom (proton/electron) transfer catalysis has been an important recent area of investigation. Tetramethylpiperidine-N-oxyl (TEMPO) radicals feature prominently in this area, prompting us to examine cooperativity between its hydrogenated congener, TEMPOH, and Co centers ligated by dihydrazonopyrrole ligands which have previously been shown to also store H-atom equivalents. Addition of TEMPOH to ( tBu,TolDHP)CoOTf results in formation of an unusual Co-adduct of 1-hydroxy-2,2,6,6-tetramethylpiperidin-1-ium (TEMPOH2 +) which has been characterized with IR spectroscopy and single crystal X-ray diffraction. This adduct is thermally unstable, and decomposes, putatively via N-O homolysis, to generate 2,2,6,6-tetramethylpiperidine and the Co-hydroxide complex [( tBu,TolDHP)CoOH][OTf]. Computational investigations suggest a proton-coupled electron transfer step to generate the TEMPOH2 + adduct where the Co center serves as an electron acceptor. Despite the prevalence of aminoxyl reagents in catalysis, particularly in aerobic transformations, metal complexes of differently hydrogenated congeners of TEMPO are rare. The isolation of a TEMPOH2 + adduct and investigations into its formation shed light on related transformations that may occur during metal-aminoxyl cooperative catalysis.