Ligand Field Sensitive Spin Acceleration in the Iron-Catalyzed [2 + 2] Cycloaddition of Unactivated Alkenes and Dienes.

Redox-active pyridine(diimine) (PDI) iron catalysts promote the reversible [2 + 2] cycloaddition of alkenes and dienes to cyclobutane derivatives that have applications ranging from fuels to chemically recyclable polymers. Metallacycles were identified as key intermediates, and spin crossover from the singlet to the triplet surface was calculated to facilitate the reductive coupling step responsible for the formation of the four-membered ring. In this work, a series of sterically and electronically differentiated PDI ligands was studied for the [2 + 2] cycloaddition of ethylene and butadiene to vinylcyclobutane. Kinetic studies revealed that the fastest and slowest turnover were observed with equally electron-deficient supporting ligands that either feature phenyl-substituted imine carbon atoms (MeBPDI) or a pyrazine core (MePZDI). While the oxidative cyclization was comparatively slow for both catalysts, the rate of reductive coupling─determined by stoichiometric 13C2H4 labeling studies─correlated with the turnover frequencies. Two-state density functional theory studies and the distinct electronic structures of related (iPrBPDI) and (iPrPZDI) iron methyl complexes revealed significantly different ligand field strengths due to either diminished ligand σ-donation (MeBPDI) or promoted metal π-backbonding (MePZDI). Spin acceleration, leading to fast reductive coupling and catalytic turnover, was promoted in the case of the weaker ligand field and depends on both the nature and position of the electron-withdrawing group. This study provides strong evidence for the role of two-state reactivity in C(sp3)-C(sp3) bond formation and insights on how ligand design either promotes or inhibits spin acceleration in earth-abundant metal catalysis.

Theory-guided development of homogeneous catalysts for the reduction of CO2 to formate, formaldehyde, and methanol derivatives.

The stepwise catalytic reduction of carbon dioxide (CO2) to formic acid, formaldehyde, and methanol opens non-fossil pathways to important platform chemicals. The present article aims at identifying molecular control parameters to steer the selectivity to the three distinct reduction levels using organometallic catalysts of earth-abundant first-row metals. A linear scaling relationship was developed to map the intrinsic reactivity of 3d transition metal pincer complexes to their activity and selectivity in CO2 hydrosilylation. The hydride affinity of the catalysts was used as a descriptor to predict activity/selectivity trends in a composite volcano picture, and the outstanding properties of cobalt complexes bearing bis(phosphino)triazine PNP-type pincer ligands to reach the three reduction levels selectively under different reaction conditions could thus be rationalized. The implications of the composite volcano picture were successfully experimentally validated with selected catalysts, and the challenging intermediate level of formaldehyde could be accessed in over 80% yield with the cobalt complex 6. The results underpin the potential of tandem computational-experimental approaches to propel catalyst design for CO2-based chemical transformations.

An Adaptive Rhodium Catalyst to Control the Hydrogenation Network of Nitroarenes.

An adaptive catalytic system that provides control over the nitroarene hydrogenation network to prepare a wide range of aniline and hydroxylamine derivatives is presented. This system takes advantage of a delicate interplay between a rhodium(III) center and a Lewis acidic borane introduced in the secondary coordination sphere of the metal. The high chemoselectivity of the catalyst in the presence of various potentially vulnerable functional groups and its readiness to be deployed at a preparative scale illustrate its practicality. Mechanistic studies and density functional theory (DFT) methods were used to shed light on the mode of functioning of the catalyst and elucidate the origin of adaptivity. The competition for interaction with boron between a solvent molecule and a substrate was found crucial for adaptivity. When operating in THF, the reduction network stops at the hydroxylamine platform, whereas the reaction can be directed to the aniline platform in toluene.

Cobalt-Catalyzed Hydrosilylation of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Level-How to Control the Catalytic Network?

The selective hydrosilylation of carbon dioxide (CO2) to either the formic acid, formaldehyde, or methanol level using a molecular cobalt(II) triazine complex can be controlled based on reaction parameters such as temperature, CO2 pressure, and concentration. Here, we rationalize the catalytic mechanism that enables the selective arrival at each product platform. Key reactive intermediates were prepared and spectroscopically characterized, while the catalytic mechanism and the energy profile were analyzed with density functional theory (DFT) methods and microkinetic modeling. It transpired that the stepwise reduction of CO2 involves three consecutive catalytic cycles, including the same cobalt(I) triazine hydride complex as the active species. The increasing kinetic barriers associated with each reduction step and the competing hydride transfer steps in the three cycles corroborate the strong influence of the catalyst environment on the product selectivity. The fundamental mechanistic insights provide a consistent description of the catalytic system and rationalize, in particular, the experimentally verified opportunity to steer the reaction toward the formaldehyde product as the chemically most challenging reduction level.

Controlling the Product Platform of Carbon Dioxide Reduction: Adaptive Catalytic Hydrosilylation of CO2 Using a Molecular Cobalt(II) Triazine Complex.

The catalytic reduction of carbon dioxide (CO2 ) is considered a major pillar of future sustainable energy systems and chemical industries based on renewable energy and raw materials. Typically, catalysts and catalytic systems are transforming CO2 preferentially or even exclusively to one of the possible reduction levels and are then optimized for this specific product. Here, we report a cobalt-based catalytic system that enables the adaptive and highly selective transformation of carbon dioxide individually to either the formic acid, the formaldehyde, or the methanol level, demonstrating the possibility of molecular control over the desired product platform.

A µ-Phosphido Diiron Dumbbell in Multiple Oxidation States.

The reaction of the ferrous complex [LFe(NCMe)2 ](OTf)2 (1), which contains a macrocyclic tetracarbene as ligand (L), with Na(OCP) generates the OCP- -ligated complex [LFe(PCO)(CO)]OTf (2) together with the dinuclear µ-phosphido complex [(LFe)2 P](OTf)3 (3), which features an unprecedented linear Fe-(µ-P)-Fe motif and a "naked" P-atom bridge that appears at δ=+1480 ppm in the 31 P NMR spectrum. 3 exhibits rich redox chemistry, and both the singly and doubly oxidized species 4 and 5 could be isolated and fully characterized. X-ray crystallography, spectroscopic studies, in combination with DFT computations provide a comprehensive electronic structure description and show that the Fe-(µ-P)-Fe core is highly covalent and structurally invariant over the series of oxidation states that are formally described as ranging from FeIII FeIII to FeIV FeIV . 3-5 now add a higher homologue set of complexes to the many systems with Fe-(µ-O)-Fe and Fe-(µ-N)-Fe core structures that are prominent in bioinorganic chemistry and catalysis.

Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction.

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C6F5)3) to give the conventional B-Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.