Shilov System Research Articles

Electrocatalytic Shilov chemistry for the oxidation of aliphatic groups

An existing chemistry for the selective oxidation of methane to methanol is the homogeneous platinum(II)-based Shilov system, but limiting its practical implementation is its use of platinum(IV) as the terminal stoichiometric oxidant. However, an examination of the Shilov cycle suggests that electrochemical regeneration of the Pt(IV) oxidant would allow for the system to be catalytic in platinum. We demonstrate that through judicious selection of applied electrical bias to near the initial open circuit potential of the reaction mixture (∼560 mV vs Ag/AgCl), Shilov-type hydroxylation of p-toluenesulfonic acid as a surrogate for methane can be electrocatalytically effected over a homogeneous Pt(II) catalyst, Na2PtCl4 in 0.5 M H2SO4, giving 8% yield of the alcohol product at 4.5 mol% catalyst loading with good Faradaic efficiency and without a co-catalyst or additional redox mediator. Methane can also be converted to methanol electrocatalytically with this system.

Catalytic etherification of alcohols in Shilov system: C[sbnd]O versus C[sbnd]H bond activation

A novel catalytic reaction of alcohol etherification in the system ROH − PtCl42− ‐ PtCl62− was found. Methanol easily transforms into dimethyl ether in the presence of catalytic amounts of PtII chloro complexes at 70°C. Under the same conditions reaction of ethanol affords diethyl ether (catalytic) and π-ethylene PtII complex (stoichiometric). The reactions are accompanied by multiple H/D exchange, which is indicative of intermediacy of corresponding alkyl platinum derivatives. The plausible reaction mechanism involves oxidative addition of alcohol forming intermediate alkyl platinum(IV) derivative followed by decomposition of it via reductive elimination step under the action of alcohol giving the ether and regenerating catalyst. In the case of ethyl alcohol reaction, β-hydrogen abstraction from the intermediate Pt-ethyl species yields π-ethylene platinum(II) complex. Although it seems that the reaction does not involve the initial breaking of CH bonds of an alcohol, this system can be regarded as a model for studying of some peculiarities of Shilov chemistry, in particular, of isotope scrambling mechanisms in Shilov alkane activation.In contrast to reactions of dimethyl and diethyl ethers formation, tert-butyl ethers formation in CD3OH/t-BuOH medium is catalyzed by PtIV chloro complexes also and is not accompanied by isotope scrambling. These observations argue against intermediacy of alkyl platinum derivatives suggesting that acid-catalyzed mechanism operates in tert-butyl alcohol etherification.

ChemInform Abstract: Catalysts for C‐H Functionalization: Platinum, Gold and Even Nanodiamonds

AbstractReview: 194 refs.

ChemInform Abstract: Stable Organoplatinum Complexes as Intermediates and Models in Hydrocarbon Functionalization

AbstractReview: 151 refs.

Switching regioselectivity in the sequential iodoplatination of methyl propiolate C[tbnd]C triple bond by PtIV iodo complexes

Stepwise iodoplatination of methyl propiolate CC triple bond in the system PtII – CH3I – NaI – acetone leads to the formation of methyl derivative of bis-σ-vinyl PtIV complex in which the platinum atom is attached simultaneously to the terminal and internal C(sp2)-atoms. The first iodoplatination step forms chelate metallocycle ▪ (1a) in which platinum atom is bound to the terminal C-atom of the vinyl ligand contrary to the expected for an alkyne with electron withdrawing substituent. The obvious reason for the unusual regioselectivity is more favorable thermodynamics of such mode of the reaction: chelation of the metal center by the oxygen atom of the carboxymethyl group in the σ-vinyl ligand forming metallocycle 1a provides an additional stabilization of the product. In the second iodoplatination step an opposite regioselectivity is observed: iodide ion addition occurs at the terminal carbon atom, and the metal center adds to the internal C-atom in formal agreement with Markovnikov's rule yielding product 2a▪ with different regioselectivity in vinyl ligands. The formed organoplatinum(IV) derivatives contain auxiliary halogenide ligands similar to the key intermediates in the Shilov system and therefore can be considered as compounds modeling some peculiarities of Shilov chemistry.

Using Reduced Catalysts for Oxidation Reactions: Mechanistic Studies of the “Periana-Catalytica” System for CH4 Oxidation

Designing oxidation catalysts based on CH activation with reduced, low oxidation state species is a seeming dilemma given the proclivity for catalyst deactivation by overoxidation. This dilemma has been recognized in the Shilov system where reduced Pt(II) is used to catalyze methane functionalization. Thus, it is generally accepted that key to replacing Pt(IV) in that system with more practical oxidants is ensuring that the oxidant does not over-oxidize the reduced Pt(II) species. The "Periana-Catalytica" system, which utilizes (bpym)Pt(II)Cl2 in concentrated sulfuric acid solvent at 200 °C, is a highly stable catalyst for the selective, high yield oxy-functionalization of methane. In lieu of the over-oxidation dilemma, the high stability and observed rapid oxidation of (bpym)Pt(II)Cl2 to Pt(IV) in the absence of methane would seem to contradict the originally proposed mechanism involving CH activation by a reduced Pt(II) species. Mechanistic studies show that the originally proposed mechanism is incomplete and that while CH activation does proceed with Pt(II) there is a solution to the over-oxidation dilemma. Importantly, contrary to the accepted view to minimize Pt(II) overoxidation, these studies also show that increasing that rate could increase the rate of catalysis and catalyst stability. The mechanistic basis for this counterintuitive prediction could help to guide the design of new catalysts for alkane oxidation that operate by CH activation.

PtII as a proton shuttle during C–H bond activation in the Shilov process

The C-H activation in Shilov's system on cis- and trans-PtCl(2)(H(2)O)(CH(4)) was investigated by ab initio molecular dynamics in water. Simulations revealed an easy C-H bond cleavage forming a transient 5-coordinated species Pt(H)Cl(2)(H(2)O)(CH(3)) that spontaneously releases a proton to the bulk solution.

ChemInform Abstract: The Role of Higher Oxidation State Species in Platinum‐Mediated C—H Bond Activation and Functionalization

AbstractReview: 103 refs.

Selective Oxidation of sp3 C−H Bonds in Water Catalyzed by a Glycinate−Platinum(II) Complex

In aqueous solution, [Pt^(II)(glycinato)Cl_2]^− catalyzes oxidation by [Pt^(IV)Cl_6]^(2−) of the methyl group of p-toluenesulfonate to the corresponding alcohol and aldehyde, with no further oxidation to the carboxylic acid. Both rate and selectivity are improved in comparison to the original Shilov system that employs [Pt^(II)Cl_n(H_2O)_(4−n)]^(2−n) as the catalyst.

The analyticity of abstract parabolic and correct systems

Let A be the matrix associated with an abstract parabolic system in the sense of Shilov or correct system in the sense of Petrovskii. We show that if the spectrum of its symbol is contained in a sector including some negative real axis, then A generates an analytic regularized semigroup. The corresponding result related to numerical range conditions is also showed. Moreover, these results are applied to matrices of partial differential operators on many function spaces

Synthesis, Characterization, and Reactivity of New Platinum(II) Alkyl Alkene Complexes

A new Pt(II) ethyl ethene complex, [Pt(μ-Cl)(CH2CH3)(C2H4)]2, (1), was isolated from the reaction of ethene with K2PtCl4 in water, and the structure was confirmed by X-ray crystallography. The results of oxidation experiments involving 1 indicate that PtII−C2H5 and PtII−(CH2CH2) species are viable intermediates in the conversion of ethane to ethanol and ethane-1,2-diol by the Shilov system.