Year: 2023

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

Abstract:
The insertion of CO₂ into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO₂ utilization. Here, we synthesize pincer-supported palladium complexes of the type (^tBuPBP)Pd(alkyl) (^tBuPBP = B(NCH₂P^tBu₂)₂C₆H₄⁻; alkyl = CH₂CH₃, CH₂CH₂CH_3, CH₂C₆H₅, and CH₂-4-OMe-C₆H₄) and (^tBuPBP)Pd(C₆H₅) and compare the rates of CO₂ insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO₂ insertion into (^tBuPBP)Pd(CH₂CH₃) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (^tBuPBP)Pd(CH₃), insertion into (^tBuPBP)Pd(CH₂CH₂CH₃) occurs approximately one order of magnitude slower than (^tBuPBP)Pd(CH₃). CO₂ insertion into the benzyl complexes (^tBuPBP)Pd(CH₂C₆H₅) and (^tBuPBP)Pd(CH₂-4-OMe-C₆H₄) is significantly slower than any of the n-alkyl complexes, and CO₂ does not insert into the palladium phenyl bond of (^tBuPBP)Pd(C₆H₅). While (^tBuPBP)Pd(CH₂CH₃) and (^tBuPBP)Pd(CH₂CH₂CH₃) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C–B bonds. This reductive process also occurred for (^tBuPBP)Pd(CH₂C₆H₅) at elevated temperature and a related process involving the formation of a new H–B bond prevented the isolation of (^tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO₂ insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO₂ insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO₂ utilization.

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Title:
Nickel-Catalyzed Carbonylative Coupling of Alkylzinc Reagents and α-Bromo-α,α-difluoroacetamides

Abstract:
We report a nickel-catalyzed carbonylative cross-coupling of alkyl zinc reagents with α,α-difluorobromoacetamides to obtain α,α,-difluoro-β-ketoamides in moderate to good yields. The reaction is catalyzed by a bench-stable nickel(II) pincer complex, in contrast to other reports involving palladium catalysts. The carbonylative reaction is performed in a two-chamber system (COware) in which carbon monoxide (CO) is generated ex situ from the solid precursor SilaCOgen, and then consumed in the adjacent chamber. The reaction operates at low temperatures using near-stoichiometric amounts of CO. Isotopically labeled products can be effortlessly accessed, as demonstrated by using ¹³C-labeled SilaCOgen.

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Title:
Dual Nickel Photocatalysis for O-Aryl Carbamate Synthesis from Carbon Dioxide

Abstract:
We report the use of dual nickel photocatalysis in the synthesis of O-aryl carbamates from aryl iodides or bromides, amines, and carbon dioxide. The reaction proceeded in visible light, at ambient carbon dioxide pressure, and without stoichiometric activating reagents. Mechanistic analysis is consistent with a Ni(I–III) cycle, where the active species is generated by the photocatalyst. The rate-limiting steps were the photocatalyst-mediated reduction of Ni(II) to Ni(I) and subsequent oxidative addition of the aryl halide. The physical properties of the photocatalyst were critical for promoting formation of O-aryl carbamates over various byproducts. Nine new phthalonitrile photocatalysts were synthesized, which exhibited properties that were vital to achieve high selectivity and activity.

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Title:
Alternating Metal-Ligand Coordination Improves Electrocatalytic CO₂ Reduction by a Mononuclear Ru Catalyst

Abstract:
Molecular electrocatalysts for CO₂-to-CO conversion often operate at large overpotentials, due to the large barrier for C−O bond cleavage. Illustrated with ruthenium polypyridyl catalysts, we herein propose a mechanistic route that involves one metal center that acts as both Lewis base and Lewis acid at different stages of the catalytic cycle, by density functional theory in corroboration with experimental FTIR. The nucleophilic character of the Ru center manifests itself in the initial attack on CO₂ to form [Ru-CO₂]⁰, while its electrophilic character allows for the formation of a 5-membered metallacyclic intermediate, [Ru-CO₂CO₂]^0,c, by addition of a second CO₂ molecule and intramolecular cyclization. The calculated activation barrier for C−O bond cleavage via the metallacycle is decreased by 34.9 kcal mol⁻¹ as compared to the non-cyclic adduct in the two electron reduced state of complex 1. Such metallacyclic intermediates in electrocatalytic CO₂ reduction offer a new design feature that can be implemented consciously in future catalyst designs.

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Title:
Exploring the Parameters Controlling Product Selectivity in Electrochemical CO₂ Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes

Abstract:
Selective reduction of CO₂ is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental–computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO₂ to HCOOH, CO, and H₂. In contrast to previous Mn(bpy-R)(CO)₃Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H₂. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)₃ moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H₂ and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO₂ to the metal hydride favoring the formation of H₂ over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO₂ adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

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