The Shilov system is a classic example of

catalytic Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

C-H bond activation and

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partial oxidation. C. I. Herrerias, X. Yao, Z. Li, C.-J. Li, Reactions of C-H Bonds in Water, ''Chem. Rev.''; 2007; 107(6); 2546-2562.

Overview

The Shilov system was discovered by Alexander E. Shilov in 1969-1972 while investigating H/D exchange between isotopologues of CH₄ and H₂O catalyzed simple

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

coordination complexes A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

. The Shilov cycle is the partial oxidation of a hydrocarbon to an

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

or alcohol precursor (RCl) catalyzed by Pt^IICl₂ in an aqueous solution with t^IVCl₆sup>2− acting as the ultimate oxidant. The cycle consists of three major steps, the

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...

activation of the C-H bond, oxidation of the complex, and the

nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

oxidation of the

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...

substrate. An equivalent transformation is performed industrially by

steam reforming Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen product ...

methane to

syngas Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, in various ratios. The gas often contains some carbon dioxide and methane. It is principly used for producing ammonia or methanol. Syngas is combustible and can be used as ...

then reducing the

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...

. The transformation can also performed biologically by

methane monooxygenase Methane monooxygenase (MMO) is an enzyme capable of oxidizing the C-H bond in methane as well as other alkanes. Methane monooxygenase belongs to the class of oxidoreductase enzymes (). There are two forms of MMO: the well-studied soluble form (s ...

. Overall Transformation RH + H₂O + tCl₆sup>2− → ROH + 2H⁺ + PtCl₂ + 4Cl⁻

Major steps

The initial and rate limiting step involving the electrophilic activation of RH₂C-H by a Pt^II center to produce a Pt^II-CH₂R species and a proton. The mechanism of this activation is debated. One possibility is the

oxidative addition Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...

of a sigma coordinated C-H bond followed by the reductive removal of the proton. Another is a

sigma-bond metathesis In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium(III) ...

involving the formation of the M-C bond and a H-Cl or H-O bond. Regardless it is this step that kinetically imparts the chemoselectivity to the overall transformation. Stronger, more electron-rich bonds are activated preferentially over weaker, more electron-poor bonds of species that have already been partially oxidized. This avoids a problem that plagues many partial oxidation processes, namely, the over-oxidation of substrate to thermodynamic sinks such as H₂O and CO₂. In the next step the Pt^II-CH₂R complex is oxidized by t^IVCl₆sup>2− to a Pt^IV-CH₂R complex. There have been multiple studies to find a replacement oxidant that is less expensive than t^IVCl₆sup>2− or a method to regenerate t^IVCl₆sup>2−. It would be most advantageous to develop an electron train which would use oxygen as the ultimate oxidant. It is important that the oxidant preferentially oxidizes the Pt^II-CH₂R species over the initial Pt^II species since Pt^IV complexes will not electrophilically activate a C-H bond of the alkane (although Pt^IV complexes electrophilically substitute hydrogens in aromatics - see refs. ^{/sup> and ^{/sup> ). Such premature oxidation shuts down the catalysis.

Finally the Pt^IV-CH₂R undergoes nucleophilic attack by OH⁻ or Cl⁻ with the departure of Pt^II complex to regenerate the catalyst.

References

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Organometallic chemistry
Catalysis}}