Oxygen Insertion into Metal Carbon Bonds: Formation of Methylperoxo Pd(II) and Pt(II) Complexes via Photogenerated Dinuclear Intermediates

Facile Cleavage of Activated Ketones: An Access to Thioethers via In Situ Generation of Anhydrides by Pummerer-Type Rearrangement

Herein, we report an oxygen insertion in activated ketones from simple inorganic carbonates for the synthesis of symmetric aromatic anhydrides. For the first time, Li2CO3 acts as an oxygen source and the in situ generated symmetric aromatic anhydrides undergo Pummerer-type rearrangement to access α-benzoyloxy-thioethers. Attractively, this protocol occurs under metal-, ligand-, and oxidant-free conditions and is compatible with a wide range of substrates. Control experiments reveal the reaction pathway.

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O₂ or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent Ni^III and structurally characterized mixed-valent Ni^II-Ni^IV intermediates and radical intermediates, resembling O₂ activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF₃)₂ complexes resulting in the formation of a stable Ni^III product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.

Formate-driven catalysis and mechanism of an iridium-copper complex for selective aerobic oxidation of aromatic olefins in water

A hetero-dinuclear IrIII-CuII complex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate in water. An IrIII-H intermediate, generated through formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species, and the process was promoted by a hydrophobic [IrIII-H]-[substrate aromatic ring] interaction in water. The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was promoted by the presence of the CuII moiety under acidic conditions. The IrIII-CuII complex exhibited cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as evidenced by no reactivities observed from the corresponding mononuclear IrIII and CuII complexes, as the individual components of the IrIII-CuII complex. The reaction mechanism afforded by the IrIII-CuII complex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction intermediates, kinetic analysis, and theoretical calculations.

Secondary phosphine oxide-triggered selective oxygenation of a benzyl ligand on palladium

The oxygenation of a benzyl ligand in [PdBnCl(cod)] was dramatically accelerated by using secondary phosphine oxides (SPOs), selectively affording either BnOOH or BnOH, depending on the concentration of O2. The SPOs coordinate to palladium in the form of phosphinous acids, operating as Brønsted acids to facilitate further reaction with O2.

Computational study on epoxidation of propylene by dioxygen using the silanol-functionalized polyoxometalate-supported osmium oxide catalyst

The silanol-functionalized POM-supported single-site Os oxide catalyst has been theoretically considered for epoxidation of propylene in the presence of dioxygen based on density functional theory calculations.

A DFT-based mechanistic proposal for the light-driven insertion of dioxygen into Pt(ii)-C bonds

The photocatalyzed insertion of dioxygen into the Pt(ii)-methyl bond in terpyridine platinum complexes has been shown to proceed efficiently, but its mechanism remains a challenge. In particular, there are serious counter-intuitive differences in the reactivity of structurally similar complexes. M06 calculations in solvent with a valence double-ζ basis set supplemented by polarization and diffusion shells (benchmarked against ωB97x-D calculations with a larger basis set) are able to provide a satisfactory mechanistic answer. The proposed mechanism starts with the absorption of a photon by the metal complex, which then evolves into a triplet state that reacts with the triplet dioxygen fragment. A variety of possible reaction paths have been identified, some leading to the methylperoxo product and others reverting to the reactants, and the validity of some of these paths has been confirmed by additional experiments. The balance between the barriers towards productive and unproductive paths reproduces the diverging experimental behavior of similar complexes and provides a general mechanistic picture for these processes.

Unraveling the Role of a Flexible Tetradentate Ligand in the Aerobic Oxidative Carbon-Carbon Bond Formation with Palladium Complexes: A Computational Mechanistic Study

Mechanistic details of the aerobic oxidative coupling of methyl groups by a novel (^MeL)Pd^II(Me)₂ complex with the tetradentate ligand, ^MeL = N, N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane, has been explored by density functional theory calculations. The calculated mechanism sheds light on the role of this ligand's flexibility in several stages of the reaction, especially as the oxidation state of the Pd changes. Ligand flexibility leads to diverse axial coordination modes, and it controls the availability of electrons by modulating the energies of high-lying molecular orbitals, particularly those with major d _z² character. Solvent molecules, particularly water, appear essential in the aerobic oxidation of Pd^II by lowering the energy of the oxygen molecule's unoccupied molecular orbital and stabilizing the Pd^X-O₂ complex. Ligand flexibility and solvent coordination to oxygen are essential to the required spin-crossover for the transformation of high-valent Pd^X-O₂ complexes. A methyl cation pathway has been predicted by our calculations in transmetalation between Pd^II and Pd^IV intermediates to be preferred over methyl radical or methyl anion pathways. Combining an axial and equatorial methyl group is preferred in the reductive elimination pathway where roles are played by the ligand's flexibility and the fluxionality of trimethyl groups.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Pd-Catalyzed Aerobic Oxidation Reactions: Strategies To Increase Catalyst Lifetimes

The palladium complex [(neocuproine)Pd(μ-OAc)]₂[OTf]₂ (1, neocuproine = 2,9-dimethyl-1,10-phenanthroline) is an effective catalyst precursor for the selective oxidation of primary and secondary alcohols, vicinal diols, polyols, and carbohydrates. Both air and benzoquinone can be used as terminal oxidants, but aerobic oxidations are accompanied by oxidative degradation of the neocuproine ligand, thus necessitating high Pd loadings. Several strategies to improve aerobic catalyst lifetimes were devised, guided by mechanistic studies of catalyst deactivation. These studies implicate a radical autoxidation mechanism initiated by H atom abstraction from the neocuproine ligand. Ligand modifications designed to retard H atom abstractions as well as the addition of sacrificial H atom donors increase catalyst lifetimes and lead to higher turnover numbers (TON) under aerobic conditions. Additional investigations revealed that the addition of benzylic hydroperoxides or styrene leads to significant increases in TON as well. Mechanistic studies suggest that benzylic hydroperoxides function as H atom donors and that styrene is effective at intercepting Pd hydrides. These strategies enabled the selective aerobic oxidation of polyols on preparative scales using as little as 0.25 mol % of Pd, a major improvement over previous work.

Mechanism of Me-Re Bond Addition to Platinum(II) and Dioxygen Activation by the Resulting Pt-Re Bimetallic Center

Unusual cis-oxidative addition of methyltrioxorhenium (MTO) to [PtMe₂(bpy)], (bpy = 2,2'-bipyridine) (1) is described. Addition of MTO to 1 first gives the Lewis acid-base adduct [(bpy)Me₂Pt-Re(Me)(O)₃] (2) and subsequently affords the oxidative addition product [(bpy)Me₃PtReO₃] (3). All complexes 1, MTO, 2, and 3 are in equilibrium in solution. The structure of 2 was confirmed by X-ray crystallography, and its dissociation constant in solution is 0.87 M. The structure of 3 was confirmed by extended X-ray absorption fine structure and X-ray absorption near-edge structure in tandem with one- and two-dimensional NMR spectroscopy augmented by deuterium and ¹³C isotope-labeling studies. Kinetics of formation of compound 3 revealed saturation kinetics dependence on [MTO] and first-order in [Pt], complying with prior equilibrium formation of 2 with oxidative addition of Me-Re being the rate-determining step. Exposure of 3 to molecular oxygen or air resulted in the insertion of an oxygen atom into the platinum-rhenium bond forming [(bpy)Me₃PtOReO₃] (4) as final product. Density functional theory analysis on oxygen insertion pathways leading to complex 4, merited on the basis of Russell oxidation pathway, revealed the involvement of rhenium peroxo species.

Divergent reactivity of platinum(ii) and palladium(ii) methylperoxo complexes and the formation of an unusual hemi-aminal complex

The 6,6''-diaminoterpyridine palladium(ii) methylperoxo complex eliminates methyl hydroperoxide and reacts with acetone to form a novel hemi-aminal palladium complex, whereas the analogous platinum(ii) complex generates formaldehyde and a platinum(ii) hydroxo complex.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square-Planar Copper(I) Complex

6,6''-Bis(2,4,6-trimethylanilido)terpyridine (H2Tpy(NMes)) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H2 Tpy(NMes))copper(I)-halide (Cl, Br and I) complexes is dictated by the strength of the NH-halide hydrogen bond. The Cu(I)Cl and Cu(II)Cl complexes are nearly isostructural, the former presenting a highly unusual square-planar geometry about Cu(I) . The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid Cu(I)/Cu(II) electron-transfer self-exchange rates. Cu(H2 Tpy(NMes))Cl shows similar fast electron transfer (≈10(5)  m(-1)  s(-1)) which is the same order of magnitude as biological systems.

Oxidation of a Monomethylpalladium(II) Complex with O2 in Water: Tuning Reaction Selectivity to Form Ethane, Methanol, or Methylhydroperoxide

Photochemical aerobic oxidation of n-Pr4N[(dpms)Pd(II)Me(OH)] (5) and (dpms)Pd(II)Me(OH2) (8) (dpms = di(2-pyridyl)methanesulfonate) in water in the pH range of 6-14 at 21 °C was studied and found to produce, in combined high yield, a mixture of MeOH, C2H6, and MeOOH along with water-soluble n-Pr4N[(dpms)Pd(II)(OH)2] (9). By changing the reaction pH and concentration of the substrate, the oxidation reaction can be directed toward selective production of ethane (up to 94% selectivity) or methanol (up to 54% selective); the yield of MeOOH can be varied in the range of 0-40%. The source of ethane was found to be an unstable dimethyl Pd(IV) complex (dpms)Pd(IV)Me2(OH) (7), which could be generated from 5 and MeI. For shedding light on the role of MeOOH in the aerobic reaction, oxidation of 5 and 8 with a range of hydroperoxo compounds, including MeOOH, t-BuOOH, and H2O2, was carried out. The proposed mechanism of aerobic oxidation of 5 or 8 involves predominant direct reaction of excited methylpalladium(II) species with O2 to produce a highly electrophilic monomethyl Pd(IV) transient that is involved in subsequent transfer of its methyl group to 5 or 8, H2O, and other nucleophilic components of the reaction mixture.

Mechanistic examination of aerobic Pt oxidation: insertion of molecular oxygen into Pt–H bonds through a radical chain mechanism

DFT calculations were performed in an effort to evaluate the mechanism of O₂ insertion into the Pt–H bond of Tp^Me2Pt^IVMe₂H catalyzed by AIBN or light.

Oxygenation of a benzyl ligand in SNS-palladium complexes with O2: acceleration by anions or Brønsted acids

n-Bu₄NX or HX accelerated the oxygenation of an SNS-benzylpalladium complex, and the product selectivity was regulated primarily by a proton.

Oxidation of Half-Lantern Pt2(II,II) Compounds by Halocarbons. Evidence of Dioxygen Insertion into a Pt(III)-CH3 Bond

The half-lantern compound [{Pt(bzq)(μ-N^S)}2] (1) [bzq = benzo[h]quinoline, HN^S = 2-mercaptopyrimidine (C4H3N2HS)] reacts with CH3I and haloforms CHX3 (X = Cl, Br, I) to give the corresponding oxidized diplatinum(III) derivatives [{Pt(bzq)(μ-N^S)X}2] (X = Cl 2a, Br 2b, I 2c). These compounds exhibit half-lantern structures with short intermetallic distances (∼2.6 Å) due to Pt-Pt bond formation. The halogen abstraction mechanisms from the halocarbon molecules by the Pt2(II,II) compound 1 were investigated. NMR spectroscopic evidence using labeled reagents support that in the case of (13)CH3I the reaction initiates with an oxidative addition through an SN2 mechanism giving rise to the intermediate species [I(bzq)Pt(μ-N^S)2Pt(bzq)((13)CH3)}]. However, with haloforms the reactions proceed through a radical-like mechanism, thermally (CHBr3, CHI3) or photochemically (CHCl3) activated, giving rise to mixtures of species [X(bzq)Pt(μ-N^S)2Pt(bzq)R] (3a-c) and [X(bzq)Pt(μ-N^S)2Pt(bzq)X] (2a-c). In these cases the presence of O2 favors the formation of species 2 over 3. Transformation of 3 into 2 was possible upon irradiation with UV light. In the case of [I(bzq)Pt(μ-N^S)2Pt(bzq)((13)CH3)}] (3d), in the presence of O2 the formation of the unusual methylperoxo derivative [I(bzq)Pt(μ-N^S)2Pt(bzq)(O-O(13)CH3)}] (4d) was detected, which in the presence of (13)CH3I rendered the final product [{Pt(bzq)(μ-N^S)I}2] (2c) and (13)CH3OH.

Alkane C-H functionalization and oxidation with molecular oxygen

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C-H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C-H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

Oxyfunctionalization with Cp*Ir(III)(NHC)(Me)(Cl) with O₂: identification of a rare bimetallic Ir(IV) μ-oxo intermediate

Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(μ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the μ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the μ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.