Organometallic alkane CH activation

Infrared Spectroscopic Characterization of Early 4d Transition Metal Carbene Cations, ZrCH2+ and NbCH2. J Phys Chem A 2024;128:6658-6667. [PMID: 39083656 DOI: 10.1021/acs.jpca.4c03553] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

IR multiple-photon dissociation (IRMPD) action spectroscopy is combined with quantum chemical calculations to examine the [M,C,2H]+ species for the early 4d metals, M = Zr and Nb. These ions were formed by reacting laser ablated M+ ions with cyclopropane (c-C3H6) in a molecular beam apparatus. Both IRMPD spectra exhibit one major band near 700 cm-1 and a second weaker band at about twice that wavenumber, more evident when irradiated in focus. The [Nb,C,2H]+ species also has a sharp band at 800 cm-1. Comparison with B3LYP calculations allow assignment of the [M,C,2H]+ structures to agostic carbenes, which is similar to the structures found for the 5d analogues, WCH2+ and TaCH2+. A molecular orbital analysis traces the reasons for the agostic deformation from a classic C2v symmetric carbene.

Effect of Substituted Pyridine Co-Ligands and (Diacetoxyiodo)benzene Oxidants on the Fe(III)-OIPh-Mediated Triphenylmethane Hydroxylation Reaction

Iodosilarene derivatives (PhIO, PhI(OAc)2) constitute an important class of oxygen atom transfer reagents in organic synthesis and are often used together with iron-based catalysts. Since the factors controlling the ability of iron centers to catalyze alkane hydroxylation are not yet fully understood, the aim of this report is to develop bioinspired non-heme iron catalysts in combination with PhI(OAc)2, which are suitable for performing C-H activation. Overall, this study provides insight into the iron-based ([FeII(PBI)3(CF3SO3)2] (1), where PBI = 2-(2-pyridyl)benzimidazole) catalytic and stoichiometric hydroxylation of triphenylmethane using PhI(OAc)2, highlighting the importance of reaction conditions including the effect of the co-ligands (para-substituted pyridines) and oxidants (para-substituted iodosylbenzene diacetates) on product yields and reaction kinetics. A number of mechanistic studies have been carried out on the mechanism of triphenylmethane hydroxylation, including C-H activation, supporting the reactive intermediate, and investigating the effects of equatorial co-ligands and coordinated oxidants. Strong evidence for the electrophilic nature of the reaction was observed based on competitive experiments, which included a Hammett correlation between the relative reaction rate (logkrel) and the σp (4R-Py and 4R'-PhI(OAc)2) parameters in both stoichiometric (ρ = +0.87 and +0.92) and catalytic (ρ = +0.97 and +0.77) reactions. The presence of [(PBI)2(4R-Py)FeIIIOIPh-4R']3+ intermediates, as well as the effect of co-ligands and coordinated oxidants, was supported by their spectral (UV-visible) and redox properties. It has been proven that the electrophilic nature of iron(III)-iodozilarene complexes is crucial in the oxidation reaction of triphenylmethane. The hydroxylation rates showed a linear correlation with the FeIII/FeII redox potentials (in the range of -350 mV and -524 mV), which suggests that the Lewis acidity and redox properties of the metal centers greatly influence the reactivity of the reactive intermediates.

IR spectroscopic characterization of [M,C,2H]+ (M = Ru and Rh) products formed by reacting 4d transition metal cations with oxirane: Spectroscopic evidence for multireference character in RhCH2. Phys Chem Chem Phys 2024;26:11445-11458. [PMID: 38572552 PMCID: PMC11022548 DOI: 10.1039/d4cp00012a]

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Ru and Rh) species. These ions were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room-temperature ion trap. IRMPD spectra for the Ru species exhibit one major band and two side bands, whereas spectra for the Rh species contain more distinct bands. Comparison with density functional theory (DFT), coupled-cluster (CCSD), and equation-of-motion spin-flip CCSD (EOM-SF-CCSD) calculations allows assignment of the [M,C,2H]+ structures. For the spectrum of [Ru,C,2H]+, a combination of HRuCH+ and RuCH2+ structures reproduces the observed spectrum at all levels of theory. The well-resolved spectrum of [Rh,C,2H]+ could not be assigned unambiguously to any calculated structure using DFT approaches. The EOM-SF-CCSD calculations showed that the ground-state surface has multireference electronic character, and symmetric carbenes in both the 1A1 and 3A2 states are needed to reproduce the observed spectrum.

IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches

A combination of IR multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Fe and Co) species. These were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room temperature ion trap. IRMPD spectra for the Fe and Co species are very similar and exhibit one major band. Comparison with density functional theory (DFT) and coupled cluster with single and double excitations (CCSD) calculations allows assignment of the spectra to MCH2+ carbene structures. For these 3d transition metal systems, experimental IRMPD spectra compare relatively poorly with DFT calculated IR spectra, but CCSD calculated spectra are a much better match primarily because the M-C stretch gains significant intensity. The origins of this behavior are explored in some detail. The present results are also compared to previous results for the 4d and 5d congeners and the periodic trends in these structures are evaluated.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

An osmium(II) methane complex: Elucidation of the methane coordination mode

The activation of inert C─H bonds by transition metals is of considerable industrial and academic interest, but important gaps remain in our understanding of this reaction. We report the first experimental determination of the structure of the simplest hydrocarbon, methane, when bound as a ligand to a homogenous transition metal species. We find that methane binds to the metal center in this system through a single M···H-C bridge; changes in the ¹J_CH coupling constants indicate clearly that the structure of the methane ligand is significantly perturbed relative to the free molecule. These results are relevant to the development of better C─H functionalization catalysts.

Bringing Selectivity in H/D Exchange Reactions Catalyzed by Metal Nanoparticles through Modulation of the Metal and the Ligand Shell

Ru and Rh nanoparticles catalyze the selective H/D exchange in phosphines using D₂ as the deuterium source. The position of the deuterium incorporation is determined by the structure of the P-based substrates, while activity depends on the nature of the metal, the properties of the stabilizing agents, and the type of the substituent on phosphorus. The appropriate catalyst can thus be selected either for the exclusive H/D exchange in aromatic rings or also for alkyl substituents. The selectivity observed in each case provides relevant information on the coordination mode of the ligand. Density functional theory calculations provide insights into the H/D exchange mechanism and reveal a strong influence of the phosphine structure on the selectivity. The isotope exchange proceeds via C-H bond activation at nanoparticle edges. Phosphines with strong coordination through the phosphorus atom such as PPh₃ or PPh₂Me show preferred deuteration at ortho positions of aromatic rings and at the methyl substituents. This selectivity is observed because the corresponding C-H moieties can interact with the nanoparticle surface while the phosphine is P-coordinated, and the C-H activation results in stable metallacyclic intermediates. For weakly coordinating phosphines such as P(o-tolyl)₃, the interaction with the nanoparticle can occur directly through phosphine substituents, and then, other deuteration patterns are observed.

Diyne-steered switchable regioselectivity in cobalt(II)-catalysed C(sp2)-H activation of amides with unsymmetrical 1,3-diynes

The regiochemical outcome of a cobalt(II) catalysed C-H activation reaction of aminoquinoline benzamides with unsymmetrical 1,3-diynes under relatively mild reaction conditions can be steered through the choice of diyne. The choice of diyne provides access to either 3- or 4-hydroxyalkyl isoquinolinones, paving the way for the synthesis of more highly elaborate isoquinolines.

Palladium-catalyzed activation of HnA–AHn bonds (AHn = CH3, NH2, OH, F)

We have quantum chemically studied activation of HnA–AHn bonds (AHn = CH3, NH2, OH, F) by PdLn catalysts with Ln = no ligand, PH3, (PH3)2, using relativistic density functional theory at ZORA-BLYP/TZ2P. The activation energy associated with the oxidative addition step decreases from H3C–CH3 to H2N–NH2 to HO–OH to F–F, where the activation of the F–F bond is barrierless. Activation strain and Kohn–Sham molecular orbital analyses reveal that the enhanced reactivity along this series of substrates originates from a combination of (i) reduced activation strain due to a weaker HnA–AHn bond; (ii) decreased Pauli repulsion as a result of a difference in steric shielding of the HnA–AHn bond; and (iii) enhanced backbonding interaction between the occupied 4d atomic orbitals of the palladium catalyst and σ* acceptor orbital of the substrate.

C-X Bond Activation by Palladium: Steric Shielding versus Steric Attraction

The C-X bond activation (X = H, C) of a series of substituted C(n°)-H and C(n°)-C(m°) bonds with C(n°) and C(m°) = H3 C- (methyl, 0°), CH3 H2 C- (primary, 1°), (CH3 )2 HC- (secondary, 2°), (CH3 )3 C- (tertiary, 3°) by palladium were investigated using relativistic dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. The effect of the stepwise introduction of substituents was pinpointed at the C-X bond on the bond activation process. The C(n°)-X bonds become substantially weaker going from C(0°)-X, to C(1°)-X, to C(2°)-X, to C(3°)-X because of the increasing steric repulsion between the C(n°)- and X-group. Interestingly, this often does not lead to a lower barrier for the C(n°)-X bond activation. The C-H activation barrier, for example, decreases from C(0°)-X, to C(1°)-X, to C(2°)-X and then increases again for the very crowded C(3°)-X bond. For the more congested C-C bond, in contrast, the activation barrier always increases as the degree of substitution is increased. Our activation strain and matching energy decomposition analyses reveal that these differences in C-H and C-C bond activation can be traced back to the opposing interplay between steric repulsion across the C-X bond versus that between the catalyst and substrate.

C-H Bond Activation and C-C Coupling of Methane on a Single Cationic Platinum Center: A Spectroscopic and Theoretical Study

We spectroscopically investigated the activation products resulting from reacting one and multiple methane molecules with Pt⁺ ions. Pt⁺ ions were formed by laser ablation of a metal target and were cooled to the electronic ground state in a supersonic expansion. The ions were then transferred to a room temperature ion trap, where they were reacted with methane at various partial pressures in an argon buffer gas. Product masses observed were [Pt,C,2H]⁺, [Pt,2C,4H]⁺, [Pt,4C,8H]⁺, and [Pt,2C,O,6H]⁺, which were mass-isolated and characterized using infrared multiple-photon dissociation (IRMPD) spectroscopy employing the free electron laser for intra-cavity experiments (FELICE). The spectra for [Pt,2C,4H]⁺ and [Pt,4C,8H]⁺ have several well-defined bands and, when compared to density functional theory-calculated spectra for several possible product structures, lead to unambiguous assignments to species with ethene ligands, proving Pt⁺-mediated C–C coupling involving up to four methane molecules. These findings contrast with earlier experiments where Pt⁺ ions were reacted in a flow-tube type reaction channel at significantly higher pressures of helium buffer gas, resulting in the formation of a Pt(CH₃)₂⁺ product. Our DFT calculations show a reaction barrier of +0.16 eV relative to the PtCH₂⁺ + CH₄ reactants that are required for C–C coupling. The different outcomes in the two experiments suggest that the higher pressure in the earlier work could kinetically trap the dimethyl product, whereas the lower pressure and longer residence times in the ion trap permit the reaction to proceed, resulting in ethene formation and dihydrogen elimination.

Methane is reacted with gas-phase atomic platinum cations in a room temperature ion trap, and the resulting product ions are characterized using IR light of the Free Electron Laser for Intra-Cavity Experiments (FELICE). We observed the formation of PtCH₂⁺, Pt(ethene)⁺, and Pt(ethene)₂⁺. These findings demonstrate the C−H bond activation and C−C coupling of methane molecules.

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand

The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimca^C5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh₃)₃Cl₂]. In contrast, Pd(OAc)₂ reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle.

Vibrational Motions Make Significant Contributions to Sequential Methyl C-H Activations in an Organometallic Complex

[(Pentamethylcyclopentadienyl)Rh(III)(bipyridine)(chloride)]⁺ (Cp*Rh-Cl) undergoes sequential deuteriation of its 15 Cp* CH groups in polar deuterated solvents. Vibrational spectra of H₁₄-Cp*Rh-Cl and D₁₄-Cp*Rh-Cl were captured via inelastic neutron spectroscopy (INS) and assigned using density functional theory (DFT) phonon calculations. These calculations were precisely weighted to the spectrometer's neutronic response. The Cp* ring behaves as a moving carousel, bringing each CH₃ close to the Rh-OH/D center where proton abstraction occurs. Vibrations relevant for carousel movement and proximal positioning for H transfer were identified. DFT modeling uncovered changes in vibrations along the reaction path, involving a Rh(I)-fulvene intermediate. Vibronic energy contributions are large across the entire transition. Remarkably, they amount to over a 400-fold increase in the proton transfer rate. The inclusion of vibrational degrees of freedom could be applied more widely to catalysts and molecular machines to harness the energetics of these vibrations and increase their effective rates of operation.

Characterization of Pseudomonas sp. TMB2 produced rhamnolipids for ex-situ microbial enhanced oil recovery

The present study describes the ex-situ production of a biosurfactant by Pseudomonas sp. TMB2 for its potential application in enhancing oil recovery. The physicochemical parameters such as temperature and pH were optimized as 30 °C and 7.2, respectively, for their maximum laboratory scale production in mineral salt medium containing glucose and sodium nitrate as best carbon and nitrogen sources. The surface activity of the resulting culture broth was declined from 71.9 to 33.4 mN/m having the highest emulsification activity against kerosene oil. The extracted biosurfactant was characterized chemically as glycolipid by Fourier-transform infrared spectroscopy and 1H and 13C nuclear magnetic resonance spectroscopy analyses. The presence of mono-rhamnolipids (Rha-C8:2, Rha-C10, Rha-C10-C10, and Rha-C10-C12:1) and di-rhamnolipids (Rha-Rha-C12-C10, Rha-Rha-C10-C10, and Rha-Rha-C10-C12:1) congeners were determined by liquid chromatography-mass spectroscopy analysis. The thermostability and degradation pattern of the candidate biosurfactant were tested by thermogravimetry assay and differential scanning calorimetry studies for its suitability in ex-situ oil recovery technology. The rhamnolipid based slug, prepared in 4000 ppm brine solution reduced the interfacial tension between liquid paraffin oil and aqueous solution to 0.8 mN/m from 39.1 mN/m at critical micelle concentration of 120 mg/L. The flooding test was performed using conventional core plugs belonging to oil producing horizons of Upper Assam Basin and recovered 16.7% of original oil in place after secondary brine flooding with microscopic displacement efficiency of 27.11%.

The unique catalytic role of water in aromatic C–H activation at neutral pH: a combined NMR and DFT study of polyphenolic compounds

NMR and DFT studies demonstrate the unique catalytic role of H₂O in aromatic C–H activation which results in a reduction of ΔG^# values by a factor of 20–30 kcal mol⁻¹.

Cyclic (alkyl)(amino)carbenes in organic and organometallic methane C–H activation: a DFT and MCSCF study

Density functional theory (DFT) and multi-configurational self-consistent field (MCSCF) calculations are used to investigate the electronic and steric properties of cyclic (alkyl)(amino)carbenes (CAACs).

Platinum(II) complexes of pyridine–amine ligands with phenol substituents: isotactic supramolecular polymers

The platinum(II) complexes [PtCl(SMe2)(κ2-N,N′-L)]Cl and [PtMe(SMe2)(κ2-N,N′-L)]Cl, L = 2-C5H4NCH2NH-x-C6H4OH (x = 2, 3, or 4), have been prepared and structurally characterized. In all cases, the complexes form supramolecular polymers in the solid state by NH··Cl and OH··Cl hydrogen bonding to the chloride anion. The ligands are chiral at the amine nitrogen atom, and in all cases, the polymers are isotactic, formed by self-recognition or narcissistic self-assembly. The structures in the crystalline state all have the Me2S ligand trans to pyridyl, but in solution, the methylplatinum(II) complexes isomerise slowly to give an equilibrium with the isomers having the methyl group trans to the pyridyl donor.

Catalyst-controlled positional-selectivity in C–H functionalizations

C–H bonds are ubiquitous in organic molecules and typically these bonds are chemically indistinct from each other and it would be highly advantageous for a synthetic chemist to have the ability to choose which C–H bond is functionalized in a given molecule.

Multiple activations of CH bonds in arenes and heteroarenes

Re₂(CO)₈(μ-C₆H₅)(μ-H) reacts with anthracene four times to yield the quadruply CH activated complex [Re₂(CO)₈(μ-H)]₄(μ-η²-1,2-μ-η²-3,4-μ-η²-5,6-μ-η²-7,8-C₁₄H₆).

Dehydrogenative coupling of 4-substituted pyridines mediated by a zirconium(ii) synthon: reaction pathways and dead ends

The mechanism of the reductive homocoupling of pyridine derivatives mediated by the ZrII synthon [(PNP)Zr(η6-toluene)Cl] (1) has been investigated. Selective transformation into three different types of product complexes has been observed, depending on the N-heterocyclic substrate employed: the bipyridyl complexes 3-R (R = Me, Et, t Bu, Bn, Ph, CHCHPh), which are the homocoupling products, the η2-((4-dimethylamino)pyridyl) complex 4 as well as the bis(isoquinolinyl) complex 5. By deuterium labelling experiments the participation of the ligand backbone in the pyridine coupling reaction via potential cyclometallation steps was ruled out. Based on DFT modelling of the possible reaction sequences a reaction mechanism for the coupling sequence could be identified. The latter is initiated by a reductive syn C-C coupling rather than based on an initial C-H activation of the pyridine substrate.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

In recent years, osmium complexes have found applications not only in thecis-hydroxylation of olefins but also very efficient in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides.