Room-Temperature Formate Ester Transfer Hydrogenation Enables an Electrochemical/Thermal Organometallic Cascade for Methanol Synthesis from CO.

The reduction of CO to synthetic fuels is a valuable strategy for energy storage. However, the formation of energy-dense liquid fuels such as methanol remains rare, particularly under low-temperature and low-pressure conditions that can be coupled to renewable electricity sources via electrochemistry. Here, a multicatalyst system pairing an electrocatalyst with a thermal organometallic catalyst is introduced, which enables the reduction of CO to methanol at ambient temperature and pressure.

Linear Free Energy Relationships Associated with Hydride Transfer From [(6,6'-R-bpy)Re(CO)H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects in the Secondary Coordination Sphere.

Six rhenium hydride complexes, [(6,6'-R-bpy)Re(CO)H] (bpy = 2,2'-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO to form rhenium formate complexes of the type [(6,6'-R-bpy)Re(CO){OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted.

Kinetic Studies of CO Insertion into Metal-Element σ-Bonds.

ConspectusDespite the plethora of metal catalyzed reactions for CO utilization that have been developed in academic laboratories, practical systems remain elusive. The understanding of the elementary steps in catalysis is a proven method to improve catalytic performance. In many catalytic cycles for CO utilization, the insertion of CO into a metal-element σ-bond, such as hydrides, alkyls, amides, or hydroxides, is a crucial step.

Homogeneous Organic Reductant Based on 4,4'-Bu-2,2'-Bipyridine for Cross-Electrophile Coupling.

The synthesis of a new homogeneous reductant based on 4,4'-Bu-2,2'-bipyridine, Bu-OED, is reported. Bu-OED was prepared on a multigram scale in two steps from inexpensive and commercially available starting materials, with no chromatography required for purification. Bu-OED has a reduction potential of -1.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling.

A family of 4,4'-Bu-2,2'-bipyridine (bpy) ligands with substituents in either the 6-position, 4,4'-Bu-6-Me-bpy (bpy), or 6 and 6'-positions, 4,4'-Bu-6,6'-R-bpy (bpy; R = Me, Pr, Bu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the bpy ligand impact the properties of the Ni complexes.

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt-porphyrin complexes.

A Co-porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (, Li, Na, K, Ca, Sr, and Ba) bind to the aza-crown ether group of 1. The binding constant for Li is comparable to that observed for a free aza-crown ether.

Photoelectrochemical CO Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon.

A high-surface-area -type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol.

Bulky, electron-rich, renewable: analogues of Beller's phosphine for cross-couplings.

In recent years, considerable progress has been made in the conversion of biomass into renewable chemicals, yet the range of value-added products that can be formed from biomass remains relatively small. Herein, we demonstrate that molecules available from biomass serve as viable starting materials for the synthesis of phosphine ligands, which can be used in homogeneous catalysis. Specifically, we prepared renewable analogues of Beller's ligand (di(1-adamantyl)--butylphosphine, cataCXium® A), which is widely used in homogeneous catalysis.

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

The insertion of CO into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO utilization. Here, we synthesize pincer-supported palladium complexes of the type (PBP)Pd(alkyl) (PBP = B(NCHPBu)CH; alkyl = CHCH, CHCHCH CHCH, and CH-4-OMe-CH) and (PBP)Pd(CH) and compare the rates of CO insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO insertion into (PBP)Pd(CHCH) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (PBP)Pd(CH), insertion into (PBP)Pd(CHCHCH) occurs approximately one order of magnitude slower than (PBP)Pd(CH).

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands.

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)Cl to form complexes of the type Re(bpy)(CO)Cl, which are related to known catalysts for CO reduction.

Synthesis of Triarylmethanes via Palladium-Catalyzed Suzuki-Miyaura Reactions of Diarylmethyl Esters.

The synthesis of triarylmethanes via Pd-catalyzed Suzuki-Miyaura reactions between diarylmethyl 2,3,4,5,6-pentafluorobenzoates and aryl boronic acids is described. The system operates at mild conditions and has a broad substrate scope, including the coupling of diphenylmethanol derivatives that do not contain extended aromatic substituents. This is significant as these substrates, which result in the types of triarylmethane products that are prevalent in pharmaceuticals, have not previously been compatible with systems for diarylmethyl ester coupling.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations.

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn or Mn, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry.

Compact Super Electron-Donor to Monolayer MoS.

The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS, achieving a carrier density of 1.10 ± 0.

Ligand and solvent effects on CO insertion into group 10 metal alkyl bonds.

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (PBP)M(CH) (PBP = B(NCHPR)CH ; R = Cy or Bu; M = Ni or Pd) to generate κ-acetate complexes of the form (PBP)M{OC(O)CH} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual PBP ligand, which features a central boryl donor that exerts a strong -influence, and the identification of a new decomposition pathway that results in C-B bond formation.

Control of Catalyst Isomers Using an -Phenyl-Substituted RN(CHCHPPr) Pincer Ligand in CO Hydrogenation and Formic Acid Dehydrogenation.

A novel pincer ligand, PNP [PhN(CHCHPPr)], which is an analogue of the versatile MACHO ligand, PNP [HN(CHCHPPr)], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by PNP and PNP [CHN(CHCHPPr)] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered.

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling.

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.

Chemical Reduction of Ni Cyclam and Characterization of Isolated Ni Cyclam with Cryogenic Vibrational Spectroscopy and Inert-Gas-Mediated High-Resolution Mass Spectrometry.

Ni cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) is an efficient catalyst for the selective reduction of CO to CO. A crucial elementary step in the proposed catalytic cycle is the coordination of CO to a Ni cyclam intermediate. Isolation and spectroscopic characterization of this labile Ni species without solvent has proven to be challenging, however, and only partial IR spectra have previously been reported using multiple photon fragmentation of ions generated by gas-phase electron transfer to the Ni cyclam dication at 300 K.

A Widely Applicable Dual Catalytic System for Cross-Electrophile Coupling Enabled by Mechanistic Studies.

A dual catalytic system for cross-electrophile coupling reactions between aryl halides and alkyl halides that features a Ni catalyst, a Co cocatalyst, and a mild homogeneous reductant is described. Mechanistic studies indicate that the Ni catalyst activates the aryl halide, while the Co cocatalyst activates the alkyl halide. This allows the system to be rationally optimized for a variety of substrate classes by simply modifying the loadings of the Ni and Co catalysts based on the reaction product profile.

Differences in the Performance of Allyl Based Palladium Precatalysts for Suzuki-Miyaura Reactions.

Palladium(II) precatalysts are used extensively to facilitate cross-coupling reactions because they are bench stable and give high activity. As a result, precatalysts such as Buchwald's palladacycles, Organ's PEPPSI species, Nolan's allyl-based complexes, and Yale's 1--butylindenyl containing complexes, are all commercially available. Comparing the performance of the different classes of precatalysts is challenging because they are typically used under different conditions, in part because they are reduced to the active species via different pathways.

Thermodynamic and kinetic hydricity of transition metal hydrides.

The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal-hydride bond to release a free hydride ion, H, as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis.

Bis(dialkylphosphino)ferrocene-Ligated Nickel(II) Precatalysts for Suzuki-Miyaura Reactions of Aryl Carbonates.

Aryl carbonates, a common protecting group in synthetic organic chemistry, are potentially valuable electrophiles in cross-coupling reactions. Here, after performing a thorough evaluation of different precatalysts, we demonstrate that (dcypf)Ni(2-ethylphenyl)(Br) (dcypf = 1,1-bis-(dicyclohexylphosphino)ferrocene) is an efficient precatalyst for Suzuki-Miyaura reactions using a variety of aryl carbonates as substrates. Mechanistic studies indicate that (dcypf)Ni(2-ethylphenyl)(Br), which contains a bidentate phosphine that binds in a trans geometry, is an effective precatalyst for these reactions for two reasons: (i) it rapidly forms the Ni(O) active species and (ii) it minimizes comproportionation reactions between the Ni(O) active species and both the unactivated Ni(II) precatalyst and on-cycle Ni(II) complexes to form catalytically inactive Ni(I) species.

Ni(I)-Alkyl Complexes Bearing Phenanthroline Ligands: Experimental Evidence for CO Insertion at Ni(I) Centers.

Although the catalytic carboxylation of unactivated alkyl electrophiles has reached remarkable levels of sophistication, the intermediacy of (phenanthroline)Ni(I)-alkyl species-complexes proposed in numerous Ni-catalyzed reductive cross-coupling reactions-has been subject to speculation. Herein we report the synthesis of such elusive (phenanthroline)Ni(I) species and their reactivity with CO, allowing us to address a long-standing question related to Ni-catalyzed carboxylation reactions.