TOWARDS PARTIAL OXIDATION OF METHANE TO METHANOL: OXIDATION OF MONOMETHYL MII-CH3 (M=Pt, Pd) COMPLEXES WITH O2 IN WATER

C H Functionalization of methane, catalyzed by PtII compounds with H2PtCl6 as stoichiometric oxidant, has been reported by Shilov et al. in the 1970's. Since then a number of attempts have been made to utilize atmospheric oxygen instead of expensive H2PtCl6. The key to a success is to achieve fast and selective PtIIMe - to - PtIVMe oxidation with O2. Previously our group has reported di(2-pyridyl)methanesulfonate (dpms) ligand - enabled aerobic oxidation of PtIIMe to produce PtIVMe intermediates and methanol. In this work, factors affecting the rate and selectivity of aerobic oxidation of aqueous (dpms)MIIMe(OH2) complexes (M = Pt, Pd) are studied in detail with special attention paid to the effect of additives and the solution pH. We found that oxidation of (dpms)PtIIMe(OH2) is fastest at pH 8.0 and formation of a Pt-to-Pt methyl group transfer product, a C1 - symmetric (dpms)PtIVMe2(OH) complex, occurs at pH > 10. The latter becomes the major product at pH 14. Results of a kinetics study, isotopic labeling experiments and DFT calculations (collaboration with Prof. W.A. Goddard) are reported and the mechanism of the oxidation reactions is discussed. Compared to (dpms)PtIIMe(OH2), (dpms)PtIIMe(OAc) complex is less reactive towards O2, whereas (dpms)PtIIMe(I)- complex reacts at a faster rate than (dpms)PtIIMe(OH2) at pH 6.5; chloro- and bromo-analogs are unreactive. Reactivity of PdII complexes containing the same auxiliary dpms ligand is more diverse compared to the PtII analogs. For example, neutral (dpms)PtIIPh(DMSO) is inert towards O2, while (dpms)PdIIMe(SMe2) undergoes aerobic functionalization to form methanol, among other products, already at room temperature. Oxidation of (dpms)PdIIMe(X) , X = I, OH with O2 in water results in formation of methanol and ethane under milder conditions compared to (dpms)PtIIMe(OH2). Palladium complexes have been submitted to oxidation with I2 and peroxides in aqueous solution, kinetics studies, and model reacions. Results show that when O2 is used as the oxidant, photochemical oxidation leads to both ethane and methanol in high combined yield under ambient light and temperature. Reaction selectivity towards MeOH can be modulated by adjusting the reaction pH. The mechanism of these oxidation reactions is proposed, and is different from the mechanism of oxidation of analogous Pt complexes

Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O_2 in Water: Pt^(II)Me-to-Pt^(IV)Me and Pt^(II)Me-to-Pt^(IV)Me_2 Reactivity

The mechanism of oxidation by O_2 of (dpms)Pt^(II)Me(OH_2) (1) and (dpms)Pt^(II)Me(OH)^− (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4–14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C_1-symmetric monomethyl Pt^(IV) complex (dpms)Pt^(IV)Me(OH)_2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt^(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O_2 activation at the Pt^(II) center to form a Pt^(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O_2 than neutral complex 1 (pK_a = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH^– ions; the reaction order in [Pt^(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt^(IV) hydroperoxide 4. At pH ≥ 12, formation of a C_1-symmetric dimethyl complex 6, (dpms)Pt^(IV)Me_2(OH), along with [(dpms)Pt^(II)(OH)_2]^− (7) becomes the dominant reaction pathway (50–70% selectivity). This change in the product distribution is explained by the formation of a C_s-symmetric intermediate (dpms)Pt^(IV)Me(OH)_2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; k_H/k_D = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A S_N2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile

Mechanism of O_2 Activation and Methanol Production by (Di(2- pyridyl)methanesulfonate)Pt^(II)Me(OH_n)^((2−n)−) Complex from Theory with Validation from Experiment

The mechanism of the (dpms)Pt^(II)Me(OH_n)^((2–n)−) oxidation in water to form (dpms)Pt^(IV)Me(OH)_2 and (dpms)Pt_(IV)Me_2(OH) complexes was analyzed using DFT calculations. At pH 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt(IV)–OOH species with the methyl group trans to the sulfonate. This methyl Pt(IV)–OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure–reactivity relationship is also observed for the S_N2 functionalization to form methanol by two isomeric (dpms)Pt^(IV)Me(OH)_2 complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH_3 group trans to the sulfonate group is 2–9 kcal/mol lower. The possibility of the involvement of Pt(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt(II) oxidation both enthalpically and entropically

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-CH 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-CH 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.Ministerio de Ciencia e Innovación Projects CTQ2010–15833, CTQ2013-45011 - P and Consolider - Ingenio 2010 CSD2007 - 00006Junta de Andalucía FQM - 119, Projects P09 - FQM - 5117 and FQM - 2126EU 7th Framework Program, Marie Skłodowska - Curie actions C OFUND – Agreement nº 26722

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

Photochemical aerobic oxidation of <i>n</i>-Pr₄N­[(dpms)­Pd^IIMe­(OH)] (<b>5</b>) and (dpms)­Pd^IIMe­(OH₂) (<b>8</b>) (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 <i>n</i>-Pr₄N­[(dpms)­Pd^II(OH)₂] (<b>9</b>). 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^IVMe₂(OH) (<b>7</b>), which could be generated from <b>5</b> and MeI. For shedding light on the role of MeOOH in the aerobic reaction, oxidation of <b>5</b> and <b>8</b> with a range of hydroperoxo compounds, including MeOOH, <i>t</i>-BuOOH, and H₂O₂, was carried out. The proposed mechanism of aerobic oxidation of <b>5</b> or <b>8</b> 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 <b>5</b> or <b>8</b>, H₂O, and other nucleophilic components of the reaction mixture

First principle study on the mechanism of O2 activation by Pt(II) monomethyl complex

The Shilov system is the first successful case to activate methane C-H bonds and convert it to Me derivs. In this system, the kinetics of the oxidn. of Pt(II) is important because the oxidn. competes with the protonolysis of Pt(II)-Me complex, the reverse reaction of C-H activation. Several oxidants, such as Pt(IV), chlorine, hydroperoxide and Cu(I) are effective. However, the direct utilization of dioxygen as stoichiometric oxidant without electron-transfer reagent is not achieved yet. Vedernikov et al demonstrated that Pt(II) monomethyl complex with facially chelating ligands such as di(2-pyridine)methanesulfonate ligand (dpms) contg. the semilabile sulfonate donor facilitates the kinetics of oxygen activation, making it an attractive route to achieve aerobic oxidn. in C-H activation.We carried out DFT calcns. with B3LYP functional to study the mechanism of Pt(II) oxidn. Starting with Pt(II) and O2(gas), the reaction first goes through the triplet-singlet crossing point to form Pt(IV)-OOH with 25.4 kcal/mol barrier at pH=7. Because the semilabile sulfonate ligand is ready to coordinate on the metal, the free energy cost to transit from 4-coordinate Pt(II) to 6-coorinate Pt(IV) is reduced, making the kinetics fast. The next step is to cleave the O-O bond in Pt(IV)-peroxide through a bimol. reaction which oxidizes a second Pt(II). Lower pH accelerates this step because the formation of Pt(IV)-OOH takes one proton from solvent. At pH=7 the barrier height is calcd. as 27.1 kcal/mol. We also examd. the methyl-transfer between Pt(II) and Pt(IV) obsd. exptl. at higher pH, where the oxidn. is hindered for the scarcity of proton. Pt(II) complex first isomerizes to make the Me group trans to the sulfonate ligand and then forming Pt(IV)-OOH. With a better leaving group trans to Me group, the nucleophilic substitution by Pt(II) has much lower barrier (17.0 kcal/mol v.s. 32.5 kcal/mol for the unisomerized case)

Mechanism of O₂ Activation and Methanol Production by (Di(2-pyridyl)methanesulfonate)Pt^IIMe(OH_<i>n</i>)^{(2–<i>n</i>)–} Complex from Theory with Validation from Experiment

The mechanism of the (dpms)­Pt^IIMe­(OH_<i>n</i>)^{(2–<i>n</i>)<i>−</i>} oxidation in water to form (dpms)­Pt^IVMe­(OH)₂ and (dpms)­Pt^IVMe₂(OH) complexes was analyzed using DFT calculations. At pH < 10, (dpms)­Pt^IIMe­(OH_<i>n</i>)^{(2–<i>n</i>)–} reacts with O₂ to form a methyl Pt­(IV)–OOH species with the methyl group trans to the pyridine nitrogen, which then reacts with (dpms)­Pt^IIMe­(OH_<i>n</i>)^{(2–<i>n</i>)–} to form 2 equiv of (dpms)­Pt^IVMe­(OH)₂, the major oxidation product. Both the O₂ activation and the O–O bond cleavage are pH dependent. At higher pH, O–O cleavage is inhibited whereas the Pt-to-Pt methyl transfer is not slowed down, so making the latter reaction predominant at pH > 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt­(IV)–OOH species with the methyl group trans to the sulfonate. This methyl Pt­(IV)–OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure–reactivity relationship is also observed for the S_N2 functionalization to form methanol by two isomeric (dpms)­Pt^IVMe­(OH)₂ complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH₃ group trans to the sulfonate group is 2–9 kcal/mol lower. The possibility of the involvement of Pt­(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt­(II) oxidation both enthalpically and entropically

Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O₂ in Water: Pt^IIMe-to-Pt^IVMe and Pt^IIMe-to-Pt^IVMe₂ Reactivity

The mechanism of oxidation by O₂ of (dpms)­Pt^IIMe­(OH₂) (<b>1</b>) and (dpms)­Pt^IIMe­(OH)⁻ (<b>2</b>) [dpms = di­(2-pyridyl)­methanesulfonate] in water in the pH range of 4–14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a <i>C</i>₁-symmetric monomethyl Pt^IV complex (dpms)­Pt^IVMe­(OH)₂ (<b>5</b>) with high selectivity ≥97%; the reaction rate is first-order in [Pt^IIMe] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O₂ activation at the Pt^II center to form a Pt^IV hydroperoxo species <b>4</b> is the reaction rate-limiting step and (ii) the anionic complex <b>2</b> is more reactive toward O₂ than neutral complex <b>1</b> (p<i>K</i>_a = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH^– ions; the reaction order in [Pt^IIMe] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex <b>2</b> by Pt^IV hydroperoxide <b>4</b>. At pH ≥ 12, formation of a <i>C</i>₁-symmetric dimethyl complex <b>6</b>, (dpms)­Pt^IVMe₂(OH), along with [(dpms)­Pt^II(OH)₂]⁻ (<b>7</b>) becomes the dominant reaction pathway (50–70% selectivity). This change in the product distribution is explained by the formation of a <i>C</i>_<i>s</i>-symmetric intermediate (dpms)­Pt^IVMe­(OH)₂ (<b>8</b>), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex <b>6</b> is negligible; <i>k</i>_H/<i>k</i>_D = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A S_N2 mechanism was proposed for the formation of complex <b>6</b> that involves complex <b>2</b> as a nucleophile and intermediate <b>8</b> as an electrophile