ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The selective transformation of ubiquitous but inert C–H bonds to other functional groups has far-reaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. The past twenty years have seen many examples of C–H bond activation at transition-metal centres, often under remarkably mild conditions and with high selectivity. Although profitable practical applications have not yet been developed, our understanding of how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion are, has progressed considerably. In fact, the recent development of promising catalytic systems highlights the potential of organometallic chemistry for useful C–H bond activation strategies that will ultimately allow us to exploit Earth's alkane resources more efficiently and cleanly.

Understanding and exploiting C–H bond activation

Two decades have passed since the discovery in liver microsomes of a haemprotein that forms a reduced-CO complex with the absorptive maximum of the Soret at 450 nm (Klingenberg, 1958; Garfinkel, 1958) and the identification of this protein as a new cytochrome: pigment cytochrome, P-450 (Omura and Sato, 1962, 1964a). In the intervening years, the study of cytochrome P-450 dependent monoxygenases has expanded exponentially. From the first crude attempts to solubilise a P-450 (Omura and Sato, 1963, 1964b) to the determination of the primary, secondary, and tertiary structure of cytochrome P-450cam by amino acid sequencing (Haniu et al., 1982a,b) and x-ray crystallography (Poulos et al., 1984) our understanding of this unique family of proteins has been advancing on all fronts. Since, perhaps, the greatest understanding of the structure and mechanism of P-450s has come from concentrated study of P-450cam of the Pseudomonas putida camphor-5-exo-hydroxylase, this review will concentrate on findings with P-450cam; attention will be drawn to parallel and contrasting examples from other P-450s as appropriate.

Structure and Chemistry of Cytochrome P450

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Recent Advances in Polyoxometalate-Catalyzed Reactions

Reports on nitrogen fixation in solution are reviewed. The optimum catalyst is the polynuclear complex. The reaction proceeds as a multielectron process, and the limiting step involves the electron transfer from a reducing agent.

Catalytic reduction of molecular nitrogen in solutions

The reactions of saturated hydrocarbons and compounds modelling them with metal complexes, leading to the cleavage of the C–H and C–C bonds, are examined. In particular, processes which result in the formation of organometallic derivatives are described and the mechanism of the oxidation of alkanes by enzymes and their chemical models is discussed. The bibliography includes 355 references.

https://iopscience.iop.org/article/10.1070/RC1987v056n05ABEH003282/pdf

Activation and Catalytic Reactions of Alkanes in Solutions of Metal Complexes

The review presents the results of studies in the last decade on the reduction of molecular nitrogen in aprotic and protic media. The products, intermediate complexes, and mechanisms of these reactions are considered. The relation between the mechanisms of the N2 reduction reactions discovered recently and the mechanism of biological nitrogen fixation is discussed. The bibliography includes 161 references.

http://iopscience.iop.org/article/10.1070/RC1974v043n05ABEH001811/pdf

Fixation of Nitrogen in Solution in the Presence of Transition Metal Complexes

Reactions involving the cleavage of C–H bonds by metal complexes in saturated and aromatic hydrocarbons and also in other compounds are examined. Some of these processes occur with formation of a carbon–metal bond, whilst in others the interaction of the complexes with the hydrocarbon takes place without direct contact between the metal atom and the C–H bonds. Metal compounds are widely used as initiators of the liquid-phase oxidation of hydrocarbons at relatively low temperatures. There is a prospect of creating new technologies for the chemical processing of petroleum and gas hydrocarbons, whereby they can be converted into valuable products, for example, into alcohols, ketones, and carboxylic acids, on the basis of processes involving metal complexes. The study of the metal complex activation of the C–H bond also makes it possible to understand and model the metalloenzyme-catalysed hydrocarbon oxidation reactions in the living cell. The bibliography includes 340 references.

http://iopscience.iop.org/article/10.1070/RC1990v059n09ABEH003560/pdf

Activation of the C–H bond by metal complexes

The review addresses direct methane oxidation — an important fundamental problem, which has attracted much attention of researchers in recent years. Analysis of the available results on biomimetic and bio-inspired methane oxygenation has demonstrated that assimilating of the experience of Nature on oxidation of methane and other alkanes significantly enriches the arsenal of chemistry and can radically change the character of the entire chemical production, as well as enables the solution of many material, energetic and environmental problems. The bibliography includes 310 references.

http://iopscience.iop.org/article/10.1070/RC2012v081n04ABEH004271/pdf

Aleksandr E Shilov

Papers

Catalytic reduction of molecular nitrogen in solutions

Activation and Catalytic Reactions of Alkanes in Solutions of Metal Complexes

Fixation of Nitrogen in Solution in the Presence of Transition Metal Complexes

Activation of the C–H bond by metal complexes

Methane hydroxylation: a biomimetic approach