O₂ Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate – cat), we report a computational study on the mechanism of a catalytic cycle where (i) O₂ is activated at the metal center of the catecholate complex [Re^V(O)­(cat)₂]⁻ to yield [Re^VII(O)₂(cat)₂]⁻, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e^– buffer. For O₂ homolysis, a series of sequential 1e^– steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O–O linkage. The <i>trans–cis</i> isomerization of <i>trans</i>-[Re^V(O)­(cat)₂]⁻ is the crucial step of O₂ activation, with an absolute free energy barrier of 16.8 kcal mol^–1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O₂ activation are significantly lowered in a polar solvent, thus rendering O₂ homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent

O₂ Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands: A DFT Study of Mechanism

As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate – cat), we report a computational study on the mechanism of a catalytic cycle where (i) O₂ is activated at the metal center of the catecholate complex [Re^V(O)­(cat)₂]⁻ to yield [Re^VII(O)₂(cat)₂]⁻, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e^– buffer. For O₂ homolysis, a series of sequential 1e^– steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O–O linkage. The <i>trans–cis</i> isomerization of <i>trans</i>-[Re^V(O)­(cat)₂]⁻ is the crucial step of O₂ activation, with an absolute free energy barrier of 16.8 kcal mol^–1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O₂ activation are significantly lowered in a polar solvent, thus rendering O₂ homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent

Modeling Catalytic Steps on Extra-Framework Metal Centers in Zeolites. A Case Study on Ethylene Dimerization

In a case study of organometallic catalytic reactions, this work benchmarks density functional theory calculations on zeolite-supported transition metal complexes. Elementary steps of ethylene dimerization and hydrogenation reactions involving the complex [Rh­(C₂H₄)₂(H₂)]⁺, supported on faujasite, were examined by comparing explicit QM (quantum mechanics) cluster models as well as QM/MM (molecular mechanics) embedded models to plane-wave periodic models as reference. Two QM cluster models, 1T and 5T where T refers to tetrahedral units of zeolite, as well as four QM/MM cluster models were explored. For the MM region, the UFF force field was found preferable to the semiempirical method PM6. The embedded cluster models reproduce barriers of C–C and C–H bond formation with deviations from the reference of at most 10 kJ mol^–1. With variations of similar size, the effect of embedding on the energetics of the reactions under study is moderate, likely because of the small nonpolar reactants. For elucidating such catalytic reactions at transition metal species in zeolites, cluster models appear equally well-suited as periodic models but computationally advantageous

Carbon Monoxide Induced Double Cyclometalation at the Iridium Center

Bubbling of CO into a dichloromethane solution of [Ir­(COD)­(CH₃CN)₂]­[BF₄] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated Ir^III complex [Ir­(C^∧N)₂(CO)­(LH)]­[BF₄] (C^∧N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated Ir^III complex

Utricularia crenata

The reactions between [Ir­(COD)­(μ-OAc)]₂ and the functionalized imidazolium salt 1-mesityl-3-(pyrid-2-yl)­imidazolium bromide (MesIPy·HBr) or 1-benzyl-3-(5,7-dimethylnaphthyrid-2-yl)­imidazolium bromide (BnIN·HBr) at room temperature afford the COD-activated Ir^III–N-heterocyclic carbene (NHC) complexes [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br] (<b>1</b>) and [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(κ²<i>C</i>,<i>N</i>-BnIN)­Br] (<b>2</b>), respectively. In contrast, the methoxy analogue [Ir­(COD)­(μ-OMe)]₂ on reaction with MesIPy·HBr under identical conditions affords the Ir^I–NHC complex [Ir­(COD)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br]. Treatment of [Ir­(COD)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br] with sodium acetate does not lead to COD activation. Further, use of 2,2′-bipyridine (bpy) with [Ir­(COD)­(μ-X)]₂ (X = MeO or AcO) in the presence of [ⁿBu₄N]­[BF₄] affords exclusively [Ir­(bpy)­(COD)]­[BF₄] (<b>3</b>). Metal-bound acetate is shown to be an essential promoter for activation of the COD allylic C–H bond. An examination of products reveals the following transformations of the precursor components: cleavage of the imidazolium C₂–H and subsequent NHC metalation, metal oxidation from Ir^I to Ir^III, and 2e reduction of COD, effectively resulting in 1-κ-4,5,6-η-C₈H₁₂ coordination to the metal. Mechanistic investigation at the DFT/B3LYP level of theory strongly suggests that NHC metalation precedes COD allylic C–H activation. Two distinct pathways have been examined which involve initial C₂–H oxidative addition to the metal followed by acetate-assisted allylic C–H activation (path A) and the reverse sequence, i.e., deprotonation of C₂–H by the acetate and subsequent allylic C–H oxidative addition to the metal (path B). The result is an Ir^III–NHC–hydride−κ¹,η²-C₈H₁₁ complex. Subsequent intramolecular insertion of the COD double bond into the metal–hydride bond followed by isomerization gives the final product. An acetate-assisted facile COD allylic C–H bond activation, in comparison to oxidative addition of the same to Ir, makes path A the favored pathway. This work thus raises questions about the innocence of COD, especially when metal acetates are used for the synthesis of NHC complexes from the corresponding imidazolium salts

Carbon Monoxide Induced Double Cyclometalation at the Iridium Center

Bubbling of CO into a dichloromethane solution of [Ir­(COD)­(CH₃CN)₂]­[BF₄] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated Ir^III complex [Ir­(C^∧N)₂(CO)­(LH)]­[BF₄] (C^∧N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated Ir^III complex

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

Asymmetric Reaction of <i>p</i>‑Quinone Diimide: Organocatalyzed Michael Addition of α‑Cyanoacetates

Hitherto unknown catalytic enantioselective transformation of <i>p</i>-quinone diimides is achieved using chiral bifunctional organic molecules. Bifunctional thiourea compounds catalyze the Michael addition of cyanoacetates with excellent yields and enantioselectivities. The initially formed Michael adducts undergo cyclization to yield functionally rich, fused cyclic imidines bearing a quaternary benzylic chiral center. Density functional theory calculations of the competing transition states (TSs) were carried out to explain the observed stereochemical outcome

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

Olefin Oxygenation by Water on an Iridium Center

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H₂O₂ or O₂. DFT studies support a ligand-assisted water activation mechanism