Activation and selective oxy-functionalization of alkanes with metal complexes: Shilov reaction and some new aspects

Accepted Manuscript Title: Activation and selective oxy-functionalization of alkanes with metal complexes: Shilov reaction and some new aspects Author: Albert A. Shteinman PII: DOI: Reference:

S1381-1169(16)30344-2 http://dx.doi.org/doi:10.1016/j.molcata.2016.08.020 MOLCAA 10007

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Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

7-6-2016 18-8-2016 21-8-2016

Please cite this article as: Albert A.Shteinman, Activation and selective oxy-functionalization of alkanes with metal complexes: Shilov reaction and some new aspects, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.08.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Activation and selective oxy-functionalization of alkanes with metal complexes: Shilov reaction and some new aspects Аlbert А. Shteinman Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia fax: +7 (496) 522-3507; e-mail: [email protected]

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Highlights   

Shilov chemistry is paradigm of C-H bond activation involving metal alkyls. The quest for new ideas in the field is actual now and is discussed in the review. New routes for activation and oxygenation of saturated hydrocarbons are discussed.

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Abstract The quest of selective catalytic reactions for direct conversion of alkanes into valued products remains to be the most important task objective of modern chemistry and metal complex catalysis. Nowadays it is adopted that the formation of metal–alkyl intermediates (M–R) is a necessary condition for activation and functionalization of alkanes on metal complexes but the mechanism of subsequent reactions of metal alkyls remain obscure, so that effective catalytic systems of this kind are still rare and uncommon. Although it is widely adopted that alkane σ-complexes (M·RH) most frequently are primary hydrocarbon intermediates in these processes, low profile in the literature is given to their reactivity and these are often considered simply just as some ‘collision complexes’. Nevertheless, theoretical and experimental studies provide more and more evidence that the С–Н bonds in such complexes may be markedly weakened and/or polarized, thus opening wide horizons for occurrence of subsequent direct homolytic or heterolytic reactions of alkanes. This review addresses the discussion of new routes for activation and oxygenation of saturated hydrocarbons, including those via alkane σ-complexes, without formation of metal– alkyl intermediates. Keywords: metal complex catalysis, oxidation of alkanes, alkyl intermediates, alkane σ-complexes Dedicated to Professor Georgiy B. Shul’pin on the occasion of his 70th birthday 1. Introduction Catalytic oxygenation of С–Н bond is an exceedingly important reaction in both living nature and chemical industry. The quest of selective catalytic processes for direct conversion of methane into methanol has long been a key problem in chemistry. Despite great efforts made by scientists and technologists, the problem remains to be far from its resolution [1-4]. The metal-complex activation of methane and ethane—discovered in the end of the 60s in studies on the H–D exchange of these molecules with deuterated water/acetic acid solutions of Pt(II) salts [2a]—gave impetus to the onrush of a new area of homogeneous catalysis and organometallic chemistry, metallocomplex activation of C–H bond in alkanes and other organic molecules. The occurrence of the Н–D exchange between alkanes and environment in these systems was indicative for reversible detachment of proton to yield alkyl complex of PtII, R–PtII, whose reactivity is markedly higher than that of starting hydrocarbons. In the presence of oxidant, the Н–D exchange was accompanied by mild oxidation of alkane [2b]. Later the range of saturated hydrocarbons was expanded [3], which afforded to disclose an unusual bond selectivity of their reactions with PtII: the rate of Н–D exchange and oxidation were found to grow in the order prim-С–Н > sec-С–Н > tert-С–Н, that is,

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in the order opposite to the selectivity of radical reactions and the strength of С–Н bonds. But such selectivity correlates with the strength of M–R bonds and the steric hindrance for intrusion of alkane into the coordination sphere of metal. Similar behavior of selectivity was observed for numerous reactions of alkanes with other metal complexes and can be regarded as a test on the coordination activation of alkanes. For over the past five decades, the metal-complex activation of C–H bond has turned into the vast area of homogeneous metallocomplex catalysis as evidenced by a number of excellent reviews and monographs [4]. Along with the use of heavier transition metals and redox systems of the Shilov chemistry (PtII–PtIV–AcOH–H2O)[5], some new directions of research have sprung for over the past years, such as: (1) ever growing interest in more abundant in Earth crust (and less expensive) 3d transition metals [6] some of which are present in the active centers of enzymes and/or successfully used in a number of chemical processes; (2) the quest and exploration of non-redox catalysts and reactions proceeding without a change in the oxidation level of metal [6b–d, 7], in contrast to conventional redox systems; (3) the use of inexpensive and environment-friendly oxidants (dioxygen and hydrogen peroxide) and combining the activation of RH and O2 in a single process [8]; and (4) the design of catalytic systems based on reactions of alkane σ-complexes [9]. Whereas the catalytic activation of alkanes with Pt was later carried out with other transition metals, the Shilov alkane–platinum chemistry remains a paradigm of the metallocomplex activation of С–Н bond with involvement of metal alkyls. This is because the activation mechanism for the RH–PtII–PtIV system has been studied to a greater extent and the involvement of Pt–R intermediates reliably proven. For numerous similar systems based on other metals, a key role of metal alkyls intermediates has been either established or assumed. As a result, it has become widely adopted that the formation of metal–alkyl intermediates is a necessary condition for homogeneous activation of C–H bond with metal complexes. Moreover, the formation of M–R complexes is normally identified as activation, while the subsequent reactions are termed as functionalization of alkanes. Meanwhile, the formation of metal alkyls is a functionalization per se, and such a reaction has long been known in chemistry as metallation. Now we have many examples for activation of alkanes with metal complexes but a limited number of catalytic systems for their functionalization. In other words, the problem of functionalization becomes prevailing over the problem of activation. Although it is admitted that alkane σ-complexes [9] most frequently act as primary hydrocarbon intermediates, little attention has been given to characterization of their reactivity and they are considered simply as some ‘collision complexes’. However, relevant theoretical and experimental studies clearly demonstrate that in such complexes C–H bonds may be markedly weakened and/or polarized, thus opening new horizons for subsequent homolytic or heterolytic reactions. 2. Oxidative functionalization of С–Н bonds with metal complexes: Catalytic systems and mechanisms

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To date, the catalytic systems for oxidative functionalization of C–H bonds can be subdivided into the following groups. (1) Systems for direct activation of С–Н bonds by metal complexes with formation of metal–alkyl intermediates [4b]. (2) Chemical systems involving metal oxo-complexes—analogs of natural oxygenases—acting via the activation of dioxygen [10]. (3) Chain-radical processes with involvement of molecular oxygen and regulated by metal complexes [11]. At low temperatures, such processes may become rather selective. In the literature, the splitting of С–Н bond is normally considered as its activation, which in systems (2) and (3) proceeds via formation of alkyl radicals R, in contrast to systems (1) where fragment R is immobilized at metal. The oxygenation process involves the formation of С–О bond at the stage of functionalization; in case of interaction of R·with O2, it is the formation of RO2 radicals and their subsequent transformations. Generally, the reaction of R·with metal complex yields RX compounds, where X = metal ligand. When the mobility of radical R·formed in system (2) through the reaction M=O + HR → MOHR is restricted by the cage effect or outer-sphere interactions (in biological systems), the formation of alcohols takes place at high selectivity. This review is confined to the analysis of catalytic systems (1) where the activation of alkanes proceeds by the mechanism of oxidative addition, electrophilic substitution or 1,2-addition [7a] with formation of M–R bond; while oxyfunctionalization, by the mechanism of reductive elimination, reductive nucleophilic substitution or Bayer–Villiger (BV) reaction [12] without change in the oxidation state of metal, the so-called organometallic BV (OMBV) reaction. In the latter case, the coordination of oxidant YO (such as PhIO, IO4-, H2O2) with metal is followed by the transfer of alkyl residue to atom O with a loss of Y [13]. The value of isotopic effect (kH/kD = 3 ± 0.5, 100) for activation of alkanes with Pt(II) complexes in CH4–D2O and CD4–H2O systems suggests that rupture of the C– H bond is a limiting stage for the H–D exchange in alkanes. The fact that the reactivity grows in the order C2H6 > CH4 > CH3COOH implies that, in reaction with С–Н bond, Pt(II) acts as electron acceptor (electrophile) [4a, 14a]. The occurrence of the reaction in water suggests that the interaction of PtII with alkanes proceeds by the ‘mild acid–mild base’ scheme. Rate of such a reaction is defined by the vertical ionization potential which is a measure of mildness or polarizability, lower for alkanes and higher for water molecules. Within the range 60–100C, the activation energy for methane oxidation is 26 kcal/mol. As could be expected, the reactivity of Pt complexes in reaction with alkanes depends on the coordination environment of Pt. Rate constants for reactions of twelve PtII complexes with different ligands and

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alkanes were found [14b] to vary within three orders of magnitude and follow the reverse trans-effect row of ligands in square PtII complexes: CN  CNS < PPh3 < DMSO  Py < NO2 < I < Br < NH3 < Cl < H2O  F  SO4 (1) Since the above effect correlates with the amplitude of Pt–С and Pt–Н stretching vibrations (i.e. bond strength) in the presence of various ligands, it has been concluded that, for reaction of alkanes with PtII, a motive force is the strength of Pt– C and Pt–H bonds. Important information about reaction character can be derived from studies on the effect of substituents in methane derivatives. The behavior of rate constants for H–D exchange in H–CXYZ (X, Y, Z = H, Cl, F, CH3, CF3, OH, COOH, Ph) was found to obey the two-parametric Taft equation log(k/k0) = ρ*σ* + nψ, where ρ* = –1.4 [15a]. It follows that PtII acts as a mild acceptor and that the rupture of С–Н bond upon insertion of PtII proceeds homolytically. The above equation is applicable to other homolytic processes such as elimination of H atoms by radicals or thermal dissociation of С–Н bond [15b]. It has been shown that PtIV does not activate alkanes effectively; the latter reduce PtIV to PtII and the process proceeds auto-catalytically with a key role of PtII as a catalyst [16]. Such concepts for the type of interaction between alkanes and PtII and reaction mechanism have been further developed in subsequent theoretical and experimental studies [4f, 17]. To date, the mechanism for activation of methane with platinum salts (Shilov reaction) can be represented as follows (Scheme 1): Scheme 1. The mechanism of Shilov reaction.

It is believed that at the first stage a methane molecule enters the coordination sphere of metal complex and forms the σ-complex PtII·СH4. For the first time, this pioneering idea was expressed in [3], by analogy with the formation of π-complexes of aromatic compounds also undergoing H–D exchange in similar systems. The authors [3] also assumed that proton detachment takes place directly in the PtII -

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alkane complex: PtII + СH4 → PtII·СH4 → PtII–СH3 + Н+, without formation of alkyl hydride, i.e. just as electrophilic substitution. In 1982, direct evidence for the insertion of the 16-electron coordinationunsaturated complex Cp*IrL (Cp* = C5Me5, L = CO, PMe3) into С–Н bonds of alkanes or arenes in aprotic non-polar media to form relatively stable metal alkyl(aryl)hydrides was presented for the first time [18]: (2)

This finding confirmed the validity of the earlier hypothesis about the possibility of PtII insertion into С–Н bond to yield alkyl hydrides [4a]. Just as in Shilov’s reaction, the insertion of Ir complex into the primary С–Н bond of alkanes is more preferable then into the secondary C–H bond, despite another type of reaction medium: (3)

Protonolysis of PtII–alkyl complexes containing Me2NCH2CH2NMe2 or PEt3 ligands in some cases afforded to record the NMR spectra of PtIV alkyl hydrides formed at low temperatures [19]; upon heating, the latter ones decomposed to PtII complexes and alkanes. In deuterated acids, decomposition of the above intermediates gave rise to multiple H–D exchanges in newly formed alkane and unreacted alkyl groups. These results imply that, in conditions under consideration, reversible transformations of alkyl, alkyl hydride, and alkane σ-complex can be readily expected to result in multiple H–D exchange. Long-lasting discussion on whether the interaction of PtII with alkanes can be regarded as electrophilic substitution or oxidative addition has led to a conclusion that the activation of alkane gets started as oxidative addition and accomplished as synchronous proton elimination yielding alkyl platinum [4c]. Recently, this idea got support from quantum-chemical calculations: easy splitting of C–H bond in the primary σ-complex [PtCl2(H2O)(CH4)] giving five-coordinated methyl-hydride intermediate [Pt(H)Cl2(H2O)(CH3)] was found to result next in immediate release of proton into solution [20]. Most likely mechanisms for multiple H–D exchange could be: (a) alkyl–carbene (AC) mechanism involving Pt–CH3 and Pt=CH2 species or (b) alkane–alkyl (AA) one with participation of Pt·CH4 and Pt–CH3 intermediates. Studies on multiple exchanges in cyclohexane, in conditions of competition between H–D exchange and oxidation, have led to unequivocal choice in favor of mechanism AA and to a

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statement that a first intermediate in the activation of alkanes indeed is the PtII– alkane σ-complex (Scheme 2) [21]. Scheme 2. The detailed mechanism of H-D exchange in Shilov reaction.

This was later confirmed by a number of experiments and calculations (see below). An activated at PtII alkane can be then converted into functional derivatives through catalytic reactions by using other terminal oxidants such as quinones, polyoxo metallates (hetero polyacids), CuCl2, Н2О2 or О2 [22,23,17c], rather than impractical [PtCl6]2–. The use of CuCl2 afforded to improve the catalytic efficiency of the Shilov system in selective oxidation of methane to methanol up to TON = 100 [17c]. The complex of CuI formed in the reaction with PtII is rapidly oxidized by dioxygen. Similar systems were used for effective oxidation of mono-functional alkane derivatives such as propionic acid, methane, ethane, and propane sulfoacids at terminal methyl groups [23]. Recently, the C(sp3)–H selective oxidation of protonated aliphatic amines in the PtII/CuII system was carried out at 120–150C at minimal catalyst loading (to 1 mol %) and in high yield (to 97%) and selectivity (>95%) [24]. Among PtII systems for oxidation of methane and other alkanes, most effective and close to industrial implementation seems to be the bipyrimidine complex of platinum, PtCl2(bipim), in sulfuric acid [25]: (4)

In this system (4), the reactivity of methane, methanol, and methyl ester of sulfuric acid at 220 are related as 100 : 1000 : 1. Due to this, high selectivity (80%) can be attained at high extent of conversion (90%). The hydrolysis of thus formed methyl bisulfate regenerates a final product, methanol. Initially, SO3 was regarded as an oxidant for PtII–alkyl complex. However, more detailed studies revealed the involvement of more rapid oxidation reaction of PtII–alkyl intermediate with the

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PtIV(Hbipim) complex accumulating in the system [25b]. Therefore, the above system can be regarded as a modified Shilov system. High activity of PtCl2(bipim) complex and its acid resistance can be associated with the protonation of the ligand (Hbipim+) [25a]. Similar catalytic systems in H2SO4 are also known for complexes of other transition metals (Au, Pd, Hg), but higher catalytic activity was exhibited by platinum complexes [26]. The system with [PtCl2(Me-bipim)]+[HPV2Mo10O40]– complex containing the anion of redox-active hetero polyacid as a counter ion and with dioxygen or air as an oxidant was found effective in much milder conditions (Н2О, 50С) [27]. Among oxidation products there are methanol, formaldehyde, and acetaldehyde; at a selectivity of 50% with respect to methanol and TON = 60. PdII complexes in trifluoroacetic acid also turned capable of activating and oxidizing С–Н bonds in alkanes [28].

Fig. 1. Complex PdII(NHC)Br2 with bis-N-heterocyclic carbene ligand. For instance, the complex PdII (NHC)Br2 (Fig. 1) with a chelate bis-Nheterocyclic carbene ligand (NHC) catalyzes the conversion of methane to methyltrifluoro acetate in the presence of Br2 as an oxidant at TON = 60; with K2S2O8 as an oxidant, one can reach TON = 500 [28]. By analogy with platinum, these reactions were found [28] to proceed via PdII–Me and PdIV–Me intermediates. The system containing Ir complexes in trifluoroacetic acid and catalyzing the H–D exchange in methane at 105–135C was described in [29]. In this system, methane is converted, in the presence of KIO4, to methyltrifluoro acetate at 150–180C (TON = 6.3). Several interesting catalytic systems were suggested on the basis of RhCl3 in polar media [4h, 8a, 30]. In the presence of СО and О2, methane is converted to acetic acid in these systems. Reaction mechanism includes the formation of Rh–Me intermediate, subsequent insertion of СO to give RhCOMe intermediate, and hydrolysis of the latter with water to yield acetic acid [30a]. In the presence of O2 and CO, long-in-focus catalytic systems RhCl3–CuO–NaCl convert methane to methyltrifluoro acetate (methanol) and acetic acid at a selectivity of 83% (by methanol) and TOF = 144 h–1 [30b]. Designed are the catalytic systems based on Au complexes with bioflavonoids (rutin and quercetin) that in aqueous solution at normal conditions can oxidize methane to methanol both by dioxygen and in anaerobic conditions with K3[Fe(CN)6] as an oxidant (TON = 35, TOF = 18 h–1) [31]. Just as in case of Rh and Au, calculations predict the involvement of metal– alkyl intermediate [32].

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In summary, we have to underline the existence of some similarity between the mechanism of catalysis in the above systems and that in the Shilov system. In this context, all of them can be termed as redox systems. Functionalization of С–Н bond in redox systems involves the two-electron oxidation of metal–alkyl intermediate followed by reductive elimination of RX or reductive nucleophilic attack of X– on R–PtIV. A proper choice of oxidant for M–R encounters some difficulty since it must be sufficiently strong in order to oxidize metal–alkyl intermediate but simultaneously to retain a catalyst in a low oxidation level necessary for activation of alkane [25c]. These circumstances (as well as some others) markedly restrict the development of effective catalytic systems of this group for practical implementation. As an alternative to redox systems, suggested was the activation of С–Н bond via 1,2-addition of alkane (R–H) along the M–OR bond to form M–R with subsequent release of free alcohol without change in the oxidation level of metal [7a, 7b, 33]. In the above systems, the catalytic cycle includes the delivery and insertion of О atom into the M–R bond and regeneration of O carrier (Scheme 3). Scheme 3. Non-redox mechanism of alkane oxygenation.

Although the cycle also involves the formation of M–R intermediate, a distinctive feature is that the latter is formed via 1,2-addition along M–OR bond but not upon oxidative addition or electrophilic substitution as in the redox systems. Still another approach to oxygenation of alkanes can be as follows. Generally the functionalization of alkanes by metal complexes proceeds in two stages: (i) the activation of alkane yielding alkyl complex M–R and (ii) the functionalization per se yielding e.g. R–Х (where Х is some functional group); in other words, alkane is regarded as getting activated only at the stage of M–R formation. This stage is thought to be a necessary condition for the functionalization. Meanwhile, the formation of metal alkyl (i.e. metallation of alkane) is a functionalization per se; whereas the activation of alkane gets started at an earlier stage, just at its coordination with metal. The reactivity of the formed -complexes and a possible new mechanisms for activation and oxy-functionalization of alkanes on this basis will be discussed in Sections 6 and 7. 3. Alkyl intermediates

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Convincing evidences for formation of metal-alkyl intermediates in the catalytic systems under study were provided by kinetic, spectral, isotope-exchange studies as well as by quantum-chemical calculations. The kinetics of oxidative chlorination of C–H bond in acetic acid in the presence of PtII complex (yielding ClCH2COOH as a product) was first studied in [16]. Within the range 80–100C, the rate constant and activation energy for chlorination coincided with those for H–D exchange in the methyl group, thus pointing out the identity of the limiting stage, activation of C–H bond (5), in both processes (6) and (7): PtCl2 + RH → RPtCl

(5)

RPtCl + D2O → RD + Pt(OD)Cl

(6)

RPtCl + PtCl62– → RCl + 2 PtCl42–

(7)

On the other hand, within the range 100–130C the activation energy was much lower and the reaction rate depended on PtIV concentration, that is, rate-controlling is the stage of functionalization (7). In this reaction as well as in oxidation of cyclohexane and benzene, the observed deficit between the consumption of PtIV and formation of products was due to accumulation in solution of relatively stable PtIVCH2COOH intermediate, thus providing indirect evidence for its involvement in oxidation reaction. Later, the above conclusions were confirmed by detailed studies on oxidation of methane in the PtCl42––H2PtCl6–CF3COOH–H2O system at 90– 120C [34]. An important result of this study was direct observation of the kinetics of formation and decomposition of CH3–PtIV, one of key intermediates in this reaction. Joint investigation of the kinetics for Н–D exchange and oxidation of cyclohexane [35] revealed that, when [H2PtCl6] concentration increased and [D+] decreased, the H–D exchange rate was getting down and the oxidation rate was getting up, and vice versa, but the sum of the above rates at all conditions remains constant and apparently equal to the rate of alkane activation—thus confirming the presence of the same alkyl–PtII intermediate in both processes (8):

(8) Since multiple H–D exchange in methane is only possible due to reversible formation of CH3- PtII intermediate, the occurrence of multiple H–D exchange already points to the involvement of this intermediate in reaction mechanism and can be regarded as strong evidence for the very fact of metallocomplex activation of alkane. Complex [CH3PtIICl3]2– has been identified in the NMR spectra of solid

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products prepared by reduction of [CH3PtIVCl5]2– in aprotic organic solvent [36]. Treatment of this powder with aqueous [PtCl6]2– solution results in regeneration of stable in these conditions [CH3PtIVCl5]2– complex along with some amount of methane formed upon protonolysis of [CH3PtIICl3]2– in water. In fact, complex CH3PtII is highly unstable in acidic media and to date have not been detected in conditions of Shilov reaction. In this context, the rather exact coincidence of the distributions of deuterated methanes in direct H–D exchange of CH4 (A) and formed at the methylation of PtIICl2 with CH3HgBr in the K2PtCl4/CH3COOD/D2O system (B) (Scheme 4) has given a quite convincing proof for participation of Me-PtII in the activation of methane [37]. Scheme 4. Evidence for CH3–PtII involvement in Shilov reaction

Much more stable intermediate CH3–PtIV was prepared by independent synthesis [38] and characterized by NMR spectra to get direct evidence (see Fig. 2) for its participation in the oxidation of methane with H2PtCl6 catalyzed by K2PtCl4 in H2O– CF3COOH mixture. There is entire consentaneity of the 1Н NMR spectra of the intermediate complex formed in reaction (Fig. 2,а) and of that prepared by independent synthesis from CH3I and [PtCl4]2- (Fig. 2,b).

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Fig. 2. Evidence for participation of CH3–PtIV intermediate in the oxidation of methane. Involvement of alkyl intermediates in catalytic reactions of Pt complexes was also confirmed by numerous theoretical studies on reaction mechanism in the systems under consideration [4f, 39, 40]. Alkyl intermediates in the systems containing other metals have been studied to a lesser extent. The formation of CH3–HgOSO3H intermediate during electrophilic activation and functionalization of methane with mercury sulfate in sulfuric acid was trustfully proved by the methods of 13С и 199 Hg NMR spectroscopy [26]. The formation of Zn–CH3 during the activation of methane on Zn-substituted hetero polyacids at 25C was proved by 13С SS NMR spectra [41], as well as over Zn2+substituted ZSM-5 zeolites [42].The formation of AuI–CH3 and AuIII– CH3 [43], PdII–CH3 and PdIV–CH3 [44], Rh–CH3 [45], etc. during functionalization of methane and other alkanes was established in kinetic and isotopic studies and also by DFT calculations. 4. Reactivity of alkyl–metal complexes at the stage of functionalization As already mentioned, the reactivity of metal–alkyl complexes is markedly higher than that of starting alkanes. In this section, we will consider the most important reactions for functionalization of М–С bond: (a) reductive functionalization (nucleophilic substitution and reductive elimination), (b) insertion of functional

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group into M–R bond, and (c) the migration of R to the oxo group on metal or to the O atom of oxidant coordinated with metal. Reductive functionalization of R-Mn+2 bond is the key stage in Shilov-type redox-active systems. In typically acidic media, a competing process in functionalization is the liberation of alkane (reverse process) [46]. Catalytic oxidation of methane in the PtII–PtIV–AcOH–H2O system yields CH3Cl and CH3OH that are formed upon reductive decomposition of CH3–PtIV intermediate or upon its reductive nucleophilic attack by Cl– ion or water molecule [34]. Heating synthesized CH3PtIVCl3 in water leads to formation of methyl chloride and methanol, i.e. the same products that are formed in the system upon oxidation of methane; at this, the rate of thermal decomposition of CH3–PtIV is strictly the same as that for oxidation of methane and proportional to concentration of CH3–PtIV intermediate. In contrast to CH3-PtII thermal decomposition of CH3–PtIV in water does not yield methane, thus confirming the irreversibility of the reaction CH3-PtII → CH3–PtIV. It has been found [35] that the rate constant of RPtII with PtIV in water at 98C is greater than that for protonolysis by a factor of 104 [35]. Reductive functionalization of CH3-RhIII is favored by electron-donating ligands and also by an iodide added as co-catalyst [47]. The attack of nucleophile X- on – CH3-RhIII in acidic medium yielding CH3X—a key stage of methane functionalization on RhI/RhIII catalysts—is accompanied by partial liberation of methane, that is, it leads to formation of СН4/СН3Х mixture (Х = ОН, ОАс or trifloro acetate). Selectivity to CH3Х (depending on the presence/absence of halide) attains a value of 60%. Interestingly, oxidative functionalization of CH3-PtIV is not accompanied by in-parallel protonation [34]. In the quest of an optimal Rh complex for activation/functionalization of methane, DFT calculations were performed for a series of RhIII bis-(quinolinyl)benzene complexes with different ligands [48]. The introduction of electron-donating Ме group into ligand was found to lower a potential barrier for activation of methane, while the incorporation of electronaccepting F atoms decreases a barrier for functionalization of CH3-RhIII. A better accommodation between activation and functionalization was attained in case of unsubstituted ligand. Theoretical study [6d] on reductive functionalization of 3d-complexes M–Me with hydroxide ion has shown that, for designing new catalysts, most promising are transition metals CrII, MnII, and CuII. Quantum-chemical calculations for [M(diimin)2(Me)(Cl)] complexes [6c] suggest that earlier 3d transition metals are most convenient for regulating the height of potential barrier for reductive functionalization of M–Me bond via the nucleophilic attack of НО– resulting in formation of methanol. For complexes of the above metals, highly exergonic ways with a very low SN2 barrier have been found. For 3d transition metals, ligand properties are more important for a proper choice of optimal catalyst than those of metal [6c]. Discovered was the stoichiometric aerobic splitting of PtII–Me bond in aqueous solutions to yield methanol and PtII–OH complex [49]. Di-(2-pyridyl)methane sulfonate ligand (dpms) was found to ensure easy aerobic oxidation of PtII–Me

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complex to [PtIVMe(OH)2]+ and reductive elimination of methanol in acidic or basic aqueous solutions. Aerobic photooxidation of [(dpms)PdIIMe(OH)2]- in water at рН 6–14 and 21C gave a mixture of MeOH, C2H6, and MeOOH in a high overall yield. Upon variation in рН and complex concentration, the reaction can be made selective with respect to ethane (up to 94% selectivity) or methanol (up to 54%). The yield of MeOOH could be varied between 0 and 40%. As demonstrated by SS NMR spectroscopy, the Zn–Me complexes formed in Znsubstituted HZSM-5 zeolites under the action of methane are capable of inserting СО, СО2 or О2 molecules into C–H bond [50]. In the latter case, the reaction proceeds at room temperature via the stage Zn–OO–Me →Zn–O–Me to yield (along with methanol) smaller amounts of other oxygen-containing products. Upon room-temperature photolysis in acetonitrile, complexes with tripyridyl ligands NNN—such as (NNN)PtII–Me and (NNN)PdII–Me—with bulky substituents in the vicinity of the methyl group were found to react with О2 to give peroxide complexes М–ОО–Ме [51]. Easy insertion of О2 is achieved due to marked weakening of М–С bond upon sterically hindered interaction with ligand. According to a suggested mechanism, dimers of the complexes in their excited state react with 3 О2 to give superoxo and peroxo intermediates. Reported was the selective electrocatalytic oxidation of Re–Me complex to methanol (87% yield by current) catalyzed by immobilized polypyridyl Ru compound [52]. Tentatively, the reaction mechanism involves the intrusion of RuII(OH) into Re–Me bond to form RuII– (MeOH)–ReO3 intermediate. This is the first example of electrocatalytic approach to the problem of insertion of O atom into M–C bond and an interesting model for anaerobic oxidation of methane. UV photolysis of ThO and СН4 in solid Ar at 4 K was found (by IR spectroscopy) to yield CH3Th(O)H, along with methanol formed via CH3Th(O)H intermediate, that is, through migration of CH3 radical to the O atom of the oxo group [53]. Interaction of Me–ReO3 with Н2О2 was found to generate methanol at 20C in basic medium [13]. Similar reactions take place upon interaction with other oxidants such as PhIO, PyO, and IO4- . Their activity was rationalized in terms of modified Bayer–Villiger mechanism that was termed organometallic BV (OMBV) mechanism. DFT calculations show that, in OMBV mechanism, rate-controlling is the migration of Ме group to the O atom of coordinated with metal oxidant to form MeО–ReO3. The possibility of such a non-redox mechanism is related to the electrophility of metal center and to the absence of donating d-orbital which, in case of heavier transition metals (e.g. PtIV, IrV), is thought to be a prerequisite for redox mechanism of functionalization. It is a recent finding [54] that the transfer of atom O of dioxygen to Me–ReO3 can be catalyzed by flavins (F) with a 600-fold gain in the rate of the reaction M–R + FOOH  M–OR + F–OH. The reaction of Me–ReO3 with IO4– yields IO3– and MeO–ReO3, and the latter can be readily hydrolyzed to methanol. Similarly, the complex [Cp*W(O2)R] (Cp* = c-C5Me5, R = CH2SiMe3) reacts with O donors (Н2О2, PhIO, IO4–) in THF/water mixtures to give ROH [7c].

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Oxy-functionalization reactions by OMBV mechanism [55] look exceedingly attractive, although it still remains unclear whether or not they can be accommodated into an effective catalytic cycle for functionalization of alkanes. 5. σ-Complexes as primary hydrocarbon intermediates in metal complex activation of alkanes The ever growing number of publications in the field convincingly urge that the insertion of metal complexes into С–Н bond always is preceded by the formation of short-living σ-adducts of alkane to metal complex. It is now clear that the oxidative addition of alkanes proceeds in two stages: formation of σ-complex and rupture of С–Н bond. The participation of σ-intermediates in activation of alkanes was supported by plotting entire potential energy surfaces for each stage of alkane activation in photodecomposition of Cp*Rh(CO)2 in liquid Kr or Xe [56]. σ-Complexes of alkanes and alkyl hydrides were detected at low temperature by time-resolved IR and NMR (1H, 13C) spectroscopy. The strength of methane bonding in metal complexes (at the same set of ligands) increases in the order: W < Re < Rh < Ir < Pd < Pt [57]. For coordination of methane and other alkanes with a metal center, an analog is the so-called agostic bond [58] that arises upon intramolecular interaction between С–Н bond of ligand and coordination-unsaturated metal center. Due to the chelate effect, this intramolecular σ-bonding is stronger than the intermolecular σ-bond and for this reason has been studied much better. The coordination of С–Н bond with metal (of η1-С,Н or η2-С,Н kind) in agostic complexes is also typical of alkane σcomplexes (Fig. 3).

η1-H

η2-H,H

η2-C,H

Fig.3. Types of -CH4·M complexes. Another analog of alkane σ-adducts to metal complexes is dihydrogen for which the formation of σ-complexes is well established [59]. -Complexes of alkanes and other saturated compounds (H2, SiH4) is an extention of the classical Werner complexes in which ligand donates to metal either its non-bonding electron pair (amines, phosphines) or the bonding pair of π-electrons (olefins). A higher occupied molecular orbital (НОМО) of alkane acts as a donor while its lower unoccupied molecular σ*-orbital (LUМО), as an acceptor of metal electrons (reverse donating), thus making the bong stronger (Fig. 4) [59a].

17

Fig. 4. Orbital interactions in -CH4·M complexes. As a result, we are dealing with an unconventional two-electron three-center bond. The reverse donating not only stabilizes σ-complex but also loosens С–Н bond by pumping electrons into it until complete rupture [59c]. Electron-deficient d6 complexes, such as Cr(CO)5, are capable (without rupture of the C–H bond) of forming only weak adducts to alkanes, whereas electron-rich complexes, such as Cp*Rh(CO)2, favor oxidative addition. Compared to H2 a close contact with metal atom for methane (and especially higher alkanes) is sterically hindered, which also hampers the reverse donation. Calculations show that the donation from σСН is important for formation of adduct, while for splitting the C–H bond strong reverse donation to anti-bonding σСН*-orbital is necessary. In some cases, alkane σcomplexes are in equilibrium with alkyl hydrides. In the presence of intramolecular interaction or lipophilic support, the stability of σ-complexes can be markedly improved [60]. The first σ-complex of methane, RhI·СH4, synthesized by protonation of RhI–Me

(9) complex (9) in СDFCl2 solution at –110C, had a sufficiently long lifetime, and could be characterized by NMR [61]. Calculations have shown that σ-complex IrI·СH4 is situated only by 5 kcal/mol above the ground state of Н–Ir–СH3 alkyl hydride [62]. In contrast to IrI·СH4, the σ-complex of Rh with methane showed higher stability (τ1/2 = 83 min at –87C, ΔG‡ = 14.5 kcal mol–1) compared to respective RhIII methyl hydride. Theoretical study [63] suggested that the ground singlet state of metal complex is more favorable for strong bonding of alkane; while the ground triplet state or the

18

lower excited triplet state exhibit a low potential barrier for oxidative addition. Since the electronic configuration of metal complex is defined by a type of metal and ligand, we can in principle organize (a) a situation with a low barrier for oxidative addition without strong alkane bonding and conversely or (b) strong bonding of alkane making the adduct sufficiently stable, provided that the barrier for oxidative addition is high. In reaction MO + CH4 of the direct oxidation of methane in an Ar matrix, the formation of alkane complexes was confirmed by IR spectra and DFT calculations [64]. The formation of Zn–CH4 upon adsorption of methane molecule to ZnHZSM-5 zeolite at room temperature was manifested [65] by a shift in the frequency (ν1) of symmetric stretching mode of С–Н bond, as well as by 1H MAS NMR spectra. Despite numerous spectral and even X-ray evidences [66] for formation of alkane σ-complexes, they are normally explored by theoretical methods [67] because of their poor stability at normal conditions. The averaged energy of methane bonding in the complexes of heavier transition metals (Rh, Pd, Ir, Pt) is 15 kcal/mol [68]. 6. Reactivity of σ-complexes CH4–M. Coordination-assisted activation of alkanes The results of quantum-chemical studies imply that, already upon coordination with a metal center, the C–H bond in alkane may get activated, by analogy with the C–H bonds in the neighborhood of an activating group (Ph, CO, OH, etc.) in organic compounds. As a rule, coordination of alkane with metal weaken the С–Н bond; this is in line with the calculated length of activated C–H bonds (Table 1) in various complexes. Table 1. Bond length r in -CH4·M complexes M PdII RhI PtII cis PtII trans IrI ReI AuI

r(M–C) 2.388 2.380 2.49 2.25 2.365 2.60 2.287

r(M–H) 1.910 1.869 – 1.782 1.92 1.825

r(C–H) 1.133 1.137 1.17 1.27 1.172 1.15 1.189

Refs. 68 61 20 69 57 32

Weak С–Н bond favors the occurrence of homolytic reactions typical of alkanes. Coordination of alkanes is often accompanied by polarization of С–Н bond, which must facilitate acidic dissociation in polar media or proton elimination by respective base. Apparently, the insertion of metal complex into С–Н bond is also markedly simplified by preliminary coordination of alkane with metal. Therefore, with full confidence we may talk about coordination-assisted activation of alkanes, by analogy with that of H2 and other small molecules. Probably, the range of reactions with

19

alkane σ-complexes can be widened up at the expense of reactions with radicals, carbenes, and also with radicaloid and carbenoid reagents. The influence of ligands on reactivity of σ-complexes seems convenient to consider on the example of better studied σ-complexes of H2 molecule [59] or agostic complexes [58]. Upon coordination with metal, the length of the Н–Н bond (r) may vary between 0.82 and 1.5 Å; for comparison, r = 0.74 Å in non-coordinated Н2 molecule and r > 1.55 Å in dihydrides. Activation of H2 is sensitive to a type of metal, ligand, and a charge of complex. Strong electron-donating ligands, third-row transition metals, electroneutral charge of complex facilitate the elongation of Н–Н bond until its rupture; whereas electron-accepting ligands, first-row transition metals, and positive charge of complex all strengthen the Н2 molecule (lower r values). Larger r values decrease a potential barrier for homolytic reactions. It is safe to assume that the above features hold true and for σ-complexes of alkanes. Another criterion for the extent of activation of С–Н bond in М(η2-СН) is a ratio of 1JСН for coordinated C–H bond to that of non-coordinated one in NMR spectra. The longer (more activated) С–Н bond, the smaller a 1JСН value [70]. For free alkane, 1JСН  130 Hz, and its decrease is indicative of strong interaction with a metal center. For low-temperature coordination of cyclopentane in [CpRe(CO)2(C5H10)] 1JСН = 113 Hz while in case of [CpRe(CO)(PF3)(C5H10)] complex, 1JСН = 75 Hz, which is due to strong activating action of PF3 ligand [70, 71]. The mean value of 1JСН = 80 Hz in methyl group can be regarded as an upper limit for alkyl hydride and 1JСН = 100 Hz, as that for non-activated alkane σcomplex. The values in between correspond to a stretched (activated) С–Н bond [71]. The NMR results (especially in combination with theoretical calculations) afford estimating the length and strength of bonding in σ-complexes of alkanes, as well as some electronic parameters for evaluating their reactivity [68]. The reactivity of σ-complexes of cis- and trans-[PtCl2(H2O)(CH4)](Fig. 5) formed upon entering methane in [PtCl2(H2O)2] was studied, relative to their transformation

Fig. 5. Isomers of [PtCl2(H2O)(CH4)]. into CH3–PtII, by ab initio simulation of molecular dynamics [20]. The trans-isomer shows high reactivity and its reaction lasts for 2.5 ps. In less reactive cis-isomer Cl-, situated trans to methane, is stronger ligand than trans-H2O and do not allow a close approach of СН4 to PtII: r(Pt–C) was found to have a value of 2.49 Å against r = 2.25 Å in trans-isomer. It means that the С–Н bond is less activated in this case: r(C–Н) = 1.17Å as compared to 1.27Å in trans-isomer.

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Upon coordination of methane, its hydrogen atoms become more acidic; in this case, the coordinated methane may exhibit a higher kinetic acidity (smaller рKа) than methyl hydride, because its deprotonation is not accompanied by change in coordination number or in oxidation level of metal complex. According to calculation [20], during the conversion of methane, via σ-PtII·CH4, into Me–PtII–Н its рKа value changes from 40–50 for starting molecule to рKа = –5.2 in alkyl hydride. In case of H2 (рKа around 35), its coordination with electrophilic cationic metal complex increases its acidity by forty orders of magnitude (to рK around –6) [59c]. Alkane complexes may undergo heterolysis not only due to liberation of proton to solution bulk but also via intramolecular transfer to ligand or through intermolecular transfer to external base. So, triethylamine can detach proton from the agostic Ru complex in CD2CI2 at room temperature [72]. Intramolecular proton transfer from agostic complex to acetate ligand via a six-membered transition state was demonstrated for Pd acetate [73]. Scheme 5. Heterolytic and homolytic reactivity of -CH4·M complexes

All already known and possible types of reactions of -CH4·M complexes with heterolytic or homolytic rupture of C–H bond are classified in Scheme 5. Reactions of the first group include the release of acidic proton to solution bulk (1), proton elimination by external base (2), and proton transfer to ligand within a complex (3). Most important are homolytic reactions of coordinated alkane. Among these, best studied is the reaction of metal insertion into С–Н bond (4) (formally, oxidative addition of С–Н bond to metal complex) that proceeds via a three-centered transition state and reactions of alkane metathesis (5) proceeding via a four-membered transition state. Intermolecular homolytic reaction of alkane σ-complexes have been studied inadequately. The reactions with radicals (6) or carbenes have not been reported so far. For the sake of functionalization, most interesting might seem (a) the

21

elimination of H atom from coordinated alkane or (b) the transfer of O atom to the activated C–H bond—both with involvement of superoxo or oxo metal complexes. Since -complexes with the methyl group of alkane are thermodynamically more stable than the complexes with methylene group (because of steric factors), in this way we can expect for terminal selectivity from normal and iso-alkanes and for higher reactivity of methane compared to that of higher alkanes. Quite probable homolytic reaction of alkane σ-complexes is the activation of methane with porphyrin complexes of Rh [74] which likely proceeds in two stages: (a) preliminary coordination of methane with Rh complex to form σ-RhI·CH4 and (b) subsequent abstraction of H atom by another radicaloid complex of Rh (7). As will be shown below, the oxidation of methane with enzymatic systems of anaerobic bacteria also may proceed via interaction of methane σ-complex with radical species. Anaerobic oxidation of methane. In essence, homogeneous activation of methane with transition metal complexes in solution is a functional chemical model for anaerobic oxidation of methane in nature. The active center of anaerobic methanotrophs contains the complex of Ni with tetrapyrrole macrocyclic ligand— corphin, also called Cofactor F430 (Fig. 6)—which is included in methyl coenzyme M reductase (MCR) and functions along with two co-enzymes, coM and coB [75, 76]. (Here and later we used coM and coB instead of usually accepted CoM and CoB in order to exclude some confusion with metal Co).

Fig. 6. Cofactor F430. Enzyme MCR takes part in both anaerobic oxidation of methane and its biosynthesis at the last stage of metabolism of methanogenic microorganisms. At this stage, МеS–coM (CH3S–CH2CH2SO3–) and coB–SH (HS(CH2)6CONHCH(CO2– )CH(CH3)OPO32–) transform to hetero disulfide of coenzymes В and М, coB–S–S– coM, and methane. Reliable evidences for the reversibility of methanogenesis have been presented only recently [77]. Different mechanisms for methane oxidation with enzyme systems of sulfate methanotrophs have been suggested in the literature [78].

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For a long time, the electrophilic metallation of methane upon interaction with highvalence NiIII complex of MCR was thought to be most likely [79]. However, DFT calculations have shown that the formation of Ме–Ni–F430 in reaction of NiII(III)– F430 with methane is thermodynamically impossible [80]. In view of this, an alternative mechanism involving the abstraction of H atom from methane by thiyl radical was suggested [80]. Indeed, the generation of thiyl radicals RS· for activation of C–H bonds in anaerobic conditions is widely spread in nature [81], but only for C–H bonds with a bond strength of below 98 kcal/mol; so that such a hypothesis is inapplicable for methane with bond strengths about 106 kcal/mol. In my opinion, the most probable mechanism for the reaction in question might be the concerted cleavage of C–H bond in Ni·CH4 with participation of thiyl radical of coM in the enzyme-substrate complex (10) [coM-S· H−CH3·NiII] → [coM-SH + CH3−NiIII],

(10)

which is similar to the activation of methane with radicaloid complexes of RhI [74]. This new mechanism can be described by the following sequence of elementary stages: NiI + coB–S–S–coM → coB–S·+ coM–S– + NiII

(11)

NiII·CH3–H + ·S–coB → NiIII–CH3 + HS–coB

(12)

NiIII–CH3 + S––coM → NiI + CH3S–coM

(13)

Stage (11)—reminding the interaction of FeII with H2O2 (Fenton system)— generates thiyl radical and d8 complex of NiII. The latter is isoelectronic with PtII and capable of coordinating methane with formation of NiII·CH4 σ-complex in which the С–Н bond is weakened to an extent sufficient for the abstraction of H atom by thiyl radical, stage (12). Nucleophilic attack of newly formed methyl complex by the anion of coenzyme М accomplishes the catalytic cycle, stage (13). The involvement of NiIII–CH3 and NiII·CH4 intermediates is confirmed by observation [82] of multiple H–D exchange between formed methane and D2O medium during reaction of purified MCR with methanogenic coenzymes CH3S–coM and coB–SH, which once more confirms the reaction reversibility. 7. Outlook The discovery of homogeneous activation of alkanes in the late 60s turned a starting point for the onset of new line of research that attracts ever growing attention of researchers in our days. Despite huge efforts, the catalytic redox systems suggested so far exhibit important drawbacks that restrict their industrial-scale implementation. Nowadays, there is an exigency for elaboration of new approaches to selective oxyfunctionalization of alkanes, including the quest of new catalytic systems and

23

mechanisms. A search for catalytic systems for direct and selective oxidation of alkanes is complicated by the lack of reliable information about the nature of active intermediates and on the mechanism of M–C bond functionalization, and also by the multiplicity and complexity of the factors affecting the elementary stages of the above processes. Presently, there is a tendency, on one side, to theoretical calculations oriented on catalysts optimization and, on the other, to detailed studies of the elementary reactions of stable alkyl–metal complexes aiming their conversion into functional alkane derivatives. New approaches to the activation of alkanes via M–OR complexes, especially with involvement of first-raw transition metals, and oxy-functionalization through insertion into М=О bond or by OMBV mechanism seem rather attractive, although the accommodation of these mechanisms into effective catalytic cycles still looks problematic. Over the past years, some information has been accumulated which gives grounds for believing that the alkane coordinated with metal atom may turn activated to an extent sufficient for occurrence of subsequent reactions and thus to become involved in chemical transformations taking place before formation of methyl–alkyl intermediate. Along with indirect kinetic and isotopic evidences for participation of alkane σ-complexes in activation and functionalization of С–Н bond on metal complexes, important information about the structure, lifetime, dynamics, and relative stability of such unique compounds was obtained by NMR spectroscopy, time-resolved IR spectroscopy, and XRD. Time is now ripe for more detailed investigation, both theoretical and experimental, on the reactivity of alkane σcomplexes. On this way, we can expect for discovery of numerous new reactions and finding out new routes for functionalization of methane and other alkanes. In the quest of new catalytic processes for selective oxidation of alkanes, it may turn interesting to combine the activation of C–H bonds with the activation of molecular oxygen, in a two in one process. One of possible solutions (already realized in some systems) may be the design of catalytic chains with О2 as a terminal oxidant. For example, methane is oxidized to methanol by PtCl2(bipim)/SO3 system in H2SO4 followed by regeneration of SO3 with dioxygen. Another solution may be direct reaction of alkane -complexes with such oxidants as oxo-complexes of transition metals, N-oxides, hydrogen peroxide, and О2. Some examples of such an approach can be found in the literature. For oxidation of methane with dioxygen in the presence of Au–bioflavonoid complexes, DFT calculations [32] predict that the reaction with О2 yields the intermediate simultaneously containing methyl, hydroxide, and hydroperoxide groups (14): I

I

I

+ O2

III

– CH3OH

I

+ CH4

Au → Au ·CH4 → Au –CH3 → Au (CH3)(OH)(OOH) → Au -OOH → CH3OH (14) In this case, the first methanol molecule is formed upon reductive elimination of CH3 and OH while the second one, via of σ-CH4·AuI–OOH or СН3–AuI–OOH

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intermediates in which the O atom of hydroperoxide group is inserting into the C–H bond of methane by OMBV mechanism. In this system, the catalyst can be regenerated by substitution of methanol for methane. The idea of elaborating a relatively simple catalytic cycle for selective oxidation of alkanes in direct reaction of R–M σ-complex with oxidant is rather reasonable and attractive, but it requires additional theoretical examination by quantum-chemical modeling. Of especial significance can be the activation of RH and О2 molecules upon their joint coordination on the same metal center, thus inducing the shift of electron density from alkane to dioxygen via the metal and the formation of R–M–OOH intermediate. In such an intermediate, the oxy-functionalization of the R–M bond may proceed by OMBV mechanism. For improving the selectivity in the processes of methane oxidation, it seems promising to use sulfur or disulfides that are milder oxidants in comparison with dioxygen or hydrogen peroxide [83, 84]. Direct single-step oxidation of methane remains to be the most popular line of research in catalysis [85]. Presumably, better perspectives are behind the reactions of Shilov chemistry type operative in aqueous solution or the processes imitating the oxidation of alkanes by anaerobic bacteria, and may be the catalytic processes involving the first raw of transitions metals. The information covered by the present review implies that the aims of numerous experimental and theoretical studies in the field are the processes of direct oxidation of methane to methanol, with special emphasis on the reactivity of alkyl intermediates. It is becoming more and more recognized [85c] that methanol is a product of chemical industry that is of key importance not only as valued raw material for organic synthesis but also as an effective means for storage, conversion, and transportation of energy. Further advances in studies on coordination-assisted activation of methane can be expected to result in some breakthrough in chemical industry and thus to accelerate solution of various material, energetic, and ecological problems. References. 1. (a) A.M. Kirillov, G.B. Shul’pin, Coord. Chem. Rev. 257 (2013) 732-754. (b) A.M. Kirillov, M.V. Kirillova, A.J.L. Pombeiro, Adv. Inorg. Chem. 65 (2013) 1-31. 2. (a) N.F. Gol'dshleger, M.B. Tyabin, A.E. Shilov, A.A. Shteinman, Russ. J. Phys. Chem. (Engl. Transl.) 43 (1969) 2174-2175. (b) N.F. Gol'dshleger, V.V. Es'kova, A.E. Shilov, A.A. Shteinman, Russ. J. Phys. Chem. (Engl. Transl.) 46 (1972) 13531354. 3. (a) R.J. Hodges, D.E. Webster, P.B. Wells, Chem. Commun. (1971) 462-3. (b) R.J. Hodges, D.E. Webster, P.B. Wells, J. Chem. Soc. (A) (1971) 3230- 3238. 4. (a) A.E. Shilov, A.A. Shteinman, Coord. Chem. Rev. 24 (1977) 97- 143. (b) A.E. Shilov, A.A. Shteinman, Russ. Chem. Rev. 81 (2012) 291- 316. (c) A.E. Shilov, G.B. Shul'pin, Chem. Rev. 97 (1997) 2879-2932 . (d) A.E. Shilov, G.B. Shul'pin, Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer Academic, Dordrecht, 2000. (e) J.A. Labinger, in: P.J.

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