Transitional metal complexes methane activation

The metal-methyl mean bond dissociation enthalpies in the main-group homoleptic molecules, MMe , vary from 131 kJmoF in ZnMc2 to 283kJmoF in AlMc3. Thus, the above value for Z) (M-Me) in a transition metal complex is not particularly high, suggesting that many cootdinatively unsaturated transition metal complexes would activate methane. 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

The activation of C-H bonds for direct C C bond formation reactions has the potential to become very important especially if it can be accomplished for sp C-H bonds, in methane or alkanes as these are the major feedstocks available. In addition, C-H bond activation of functionalized organic compounds for selective C-C bond formation has been and will continue to be a very important goal of organometallic catalysis. So far the use of transition metal complexes has led to interesting results which however are not yet industrially relevant. 

In the gas phase reaction of Rh with c-CaH, among other processes, elimination of C2H4 has been observed and the resulting metal ion was shown to have the structure of a methylidene-rhodium complex (238) instead of a hydrido-methylidene species (239) . RhCHa (238) reacts readily with H2 and CH4 and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase. Reactions of both Rh = CH2 and its analogues Fe-CH2 and C0-CH2 with cyclic hydrocarbons were studied, and it is assumed that in the initial step metallacycloalkanes are generated (Scheme 35). 

The ability of a reaction intermediate such as Cr(CO)5 to bind methane may be counterintuitive but commands attention for its significance in relation to C-H bond activation, a teasing but vital issue in the context of the chemical industry (118). Numerous complexes of alkanes with unsaturated transition-metal fragments (including atoms and ions) have now been detected in both the gas and condensed phases. It is not surprising that all of them are unstable at room temperature. Experiments in which alkanes undergo oxidative addition to, or reductive elimination from, transition-metal complexes have revealed, nonetheless, the intermediacy of alkane complexes. 

The effects of ligands in the activation of C-H bonds by transition metal complexes were discussed in certain works. It has been suggested [66a] that a combination of hydride and lone-pair ligands with a minimum of 7c-bonding should be an optimal combination for the reaction between methane and some model Rh(I) and Ru(II) complexes. For example, it should be advantageous to... 

The study of gas-phase activation of H-H, C-H and C-C bonds of the hydrogen molecule and saturated hydrocarbons, respectively, by bare transition metal atoms and cations is very attractive for getting insight to the mechanisms and factors (such as nature of metal atoms and their lower-lying electronic states) controlHng catalytic activities of transition metal complexes. Such studies, which are free from the ligand and solvent effects, have been subject of many experimental [2] and theoretical [3] papers in the past 10-15 years. Experimental studies indicate that reaction of some transition metal cations (such as Fe+, Co+, and Rh ) with methane exclusively leads to the ion-molecule complex M+(CH4), while others (such as Sc+ and Ir ) pro-... 

C-H o-bond activation of hydrocarbons by transition metal complexes is of considerable importance in modern organometallic chemistry and catalytic chemistry by transition-metal complexes [1], because a functional group can be introduced into alkanes and aromatic compounds through C-H o-bond activation. For instance, Fujiwara and Moritani previously reported synthesis of styrene derivatives from benzene and alkene via C-H o-bond activation of benzene by palladium(ll) acetate [2]. Recently, Periana and his collaborators succeeded to activate the C-H o-bond of methane by the platinum(ll) complex in sulfuric acid to synthesize methanol [3], Both are good examples of the reaction including the C-H o-bond activation. 

Although the oxidative addition of the C-H o-bond to the transition metal complex is difficult, the oxidative addition of methane derivative to a palla-dium(O) complex was experimentally proposed [19], where a chelate phosphine was employed as a ligand and two electron-withdrawing substituents were introduced to the sp C atom. The C-H o-bond activation of methane by palladium(O) and platinum(O) chelate phosphine complexes was theoretically investigated [8e, 20], as shown in Fig. 3. When monodentate phosphine was employed, the C-H bond is lengthened very much in the transition state and the geometry of the transition state is similar to that of the product. When the chelate phosphine was employed, the Pd-C, Pd-H, and C-H distances in the... 

Using TAfS° (68) rs 14 kJ mol, A,G (68) rs —49 kJ mol is finally obtained, that is, the energetics of reactions (66) and (68) are comparable. A possible reason for the different reactivity of arenes and alkanes is that an arene may have a kinetic advantage over an alkane by forming a strong T/ -bond with the metal. The electron backdonation from the metal to the antibonding tt orbitals of the arene weakens the C(sp )-H bond and favors the formation of the aryl hydride. While this kinetic explanation may account, by itself, for the preference of arene addition, it is observed in Table 1 that for late transition metal complexes the differences DH° (M-Ph) — DH°(M-Me) are substantially higher than Z)//°(Ph-H) — Z)/7 (Me-H). This trend will, of course, imply that benzene activation is thermodynamically favorable, relative to methane activation. 

Among the earliest reports of alkane activation by a transition metal complex were the articles by Shilov in which Pt(n) served as a catalyst for methane oxidation and Pt(iv) served as a stoichiometric oxidant." The mechanism of C-H activation was termed electrophilic, as the cationic metal was postulated to interact with the electrons of the C-H bond which then lost a proton, forming a metal-carbon bond without a change in oxidation state. Oxidation of the complex by two electrons was then followed by nucleophilic attack at carbon, giving a functionalized hydrocarbon (Scheme 1). 

Thus, the activation barrier of the reaction between methane and such coor-dinatively unsaturated intermediates is very low. A similar methane activation has also been reported using other transition-metal complexes as shown in equations (2) and (3) (4-6). 

The other example to be discussed in this context comes from Pettit s group. Simultaneous treatment of the iron complex (/u.-CH2)[Fe(CO)4]2 (35) with hydrogen and ethylene gives both methane (66%) and propylene (6%), the expected products from the two separate reactions. In addition, ethane (—600%) is formed, with the actual hydrogenation catalyst still to be determined (72). Because simple diazoalkanes provide the cleanest method to metal-attached alkylidenes, and with the expectation that dissociative chemisorption of diazomethane to absorbed CH2 and free N2 would occur, the reactions of CH2N2 with and without H2 over various transition metals were examined in a careful study with regard to the product ratio (73). It was found, that gas-phase decomposition of the parent diazoalkane upon passage over active Ni, Pd, Fe, Co, Ru, or Cu-... 

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