Shilov system
Encyclopedia
The Shilov system is a classic example of catalytic C-H bond activation
and oxidation which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partial oxidation.
in 1969-1972 while investigating H/D exchange between isotopologues of CH4 and H2O catalyzed simple transition metal
coordination complexes. The Shilov cycle is the partial oxidation of a hydrocarbon to an alcohol
or alcohol precursor (RCl) catalyzed by PtIICl2
in an aqueous solution with [PtIVCl6]2-
acting as the ultimate oxidant. The cycle consists of three major steps, the electrophilic activation of the C-H bond, oxidation of the complex, and the nucleophilic oxidation of the alkane
substrate. A equivalent transformation is performed industrially by steam reforming
methane to syngas
then reducing the carbon monoxide
to methanol
. The transformation can also performed biologically by methane monooxygenase
.
Overall Transformation
RH + H2O + [PtCl6]2- → ROH + 2H+ + PtCl2 + 4Cl-
of a sigma coordinated C-H bond followed by the reductive removal of a the proton. Another is a sigma bond metathesis 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 PtII-CH2R complex is oxidized by [PtIVCl6]2- to a PtIV-CH2R complex. There have been multiple studies to find a replacement oxidant that is less expensive than [PtIVCl6]2- or a method to regenerate [PtIVCl6]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 PtII-CH2R species over the initial PtII species since PtIV complexes will not electrophilically activate a C-H bond of the alkane (although PtIV complexes electrophilically substitute hydrogens in aromatics - see refs. [1] and [2] ). Such premature oxidation shuts down the catalysis.
Finally the PtIV-CH2R undergoes nucleophilic attack by OH- or Cl- with the departure of PtII complex to regenerate the catalyst.
C-H bond activation
Carbon–hydrogen bond activation or C−H activation may be defined as a reaction that cleaves a carbon–hydrogen bond. Often the term is restricted to reactions involving organometallic complexes and proceeding by coordination of a hydrocarbon to the inner-sphere of metal, either via an...
and oxidation which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partial oxidation.
Overview
The Shilov system was discovered by Alexander E. ShilovAlexander E. Shilov
Alexander E. Shilov was born in Ivanovo, Russia. He studied Chemistry in Kiev and received his diploma degee in 1952 from Kiev State University. In 1952-1955 he was working with Nobel Laureate Nikolay Semyonov toward his Ph.D. at the Academy of Sciences in Moscow...
in 1969-1972 while investigating H/D exchange between isotopologues of CH4 and H2O catalyzed simple transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
coordination complexes. The Shilov cycle is the partial oxidation of a hydrocarbon to an alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
or alcohol precursor (RCl) catalyzed by PtIICl2
Platinum(II) chloride
Platinum chloride is the chemical compound PtCl2. It is an important precursor used in the preparation of other platinum compounds. It exists in two crystalline forms, but the main properties are somewhat similar: dark brown, insoluble in water, diamagnetic, and odorless.-Structure:The...
in an aqueous solution with [PtIVCl6]2-
Platinum(IV) chloride
Platinum chloride is the inorganic compound of platinum and chlorine with the empirical formula PtCl4. This brown solid features platinum in the 4+ oxidation state.-Structure:...
acting as the ultimate oxidant. The cycle consists of three major steps, the electrophilic activation of the C-H bond, oxidation of the complex, and the nucleophilic oxidation of the alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
substrate. A equivalent transformation is performed industrially by steam reforming
Steam reforming
Fossil fuel reforming is a method of producing hydrogen or other useful products from fossil fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to...
methane to syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
then reducing the carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
to methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
. The transformation can also performed biologically by methane monooxygenase
Methane monooxygenase
Methane monooxygenase, or 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 ....
.
Overall Transformation
RH + H2O + [PtCl6]2- → ROH + 2H+ + PtCl2 + 4Cl-
Major steps
The initial and rate limiting step involving the electrophilic activation of RH2C-H by a PtII center to produce a PtII-CH2R species and a proton. The mechanism of this activation is debated. One possibility is the oxidative additionOxidative 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...
of a sigma coordinated C-H bond followed by the reductive removal of a the proton. Another is a sigma bond metathesis 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
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
.
In the next step the PtII-CH2R complex is oxidized by [PtIVCl6]2- to a PtIV-CH2R complex. There have been multiple studies to find a replacement oxidant that is less expensive than [PtIVCl6]2- or a method to regenerate [PtIVCl6]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 PtII-CH2R species over the initial PtII species since PtIV complexes will not electrophilically activate a C-H bond of the alkane (although PtIV complexes electrophilically substitute hydrogens in aromatics - see refs. [1] and [2] ). Such premature oxidation shuts down the catalysis.
Finally the PtIV-CH2R undergoes nucleophilic attack by OH- or Cl- with the departure of PtII complex to regenerate the catalyst.