Advances in Organometallic Chemistry and Catalysis: The Silver / Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book

Part I

Activation and Functionalization of Carbon Single Bonds and of Small Molecules

Chapter 1

Organometallic Complexes as Catalysts in Oxidation of C–H Compounds

Georgiy B. Shul'pin

Department of Kinetics and Catalysis, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia

1.1 Introduction

Organometallic (i.e., containing π or σ metal–carbon bonds) derivatives of transition metals are known as excellent catalysts in reactions that do not involve the insertion of oxygen atoms [1]. They are used in (selected examples of recent publications are given) hydrogen/deuterium (H/D) exchange [2a], dehydrogenation [2b–e], homogeneous syngas conversion [2f], hydrosilylation [2g], carbonylation [2h], and homogeneous water gas shift reaction [2i]. In other recent works, complex [ c1-math-0007 ], where c1-math-0008 and c1-math-0009 ,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene, was used as a catalyst in the racemization of chiral alcohols [2j], neutral c1-math-0010 -arene ruthenium complexes with monodentate P-donor ligands found to catalyze the transfer hydrogenation reaction [2k].

Organometallic catalysts were employed in oxidation reactions of some organic compounds. Thus, complex c1-math-0011 is a precatalyst for olefin epoxidation [3a], heterodinuclear ruthenium–iron complexes showed high activity for the catalytic oxidation of secondary alcohols with tert-butyl hydroperoxide to give ketones in aqueous media [3b]. In contrast, organometallic complexes were very rarely used as (pre)catalysts in oxygenation reactions of aromatic and saturated hydrocarbons [4a–e] (the latter can be called noble gases of organic chemistry because of their known inertness).

In various C–H oxygenation reactions, organometallic complexes can play the role of precatalyst. Compounds bearing carbon–metal bonds can also be some of intermediate compounds in the catalytic cycles. In this chapter, we discuss reactions in which an organometallic complex catalyzes the insertion of oxygen atoms into C–H bonds of hydrocarbons or other organic compounds. The focus will be made on the author's own works.

1.2 Oxygenation Reactions with Oxidants other than Peroxides

The first example of a metal-catalyzed oxygen atom insertion into the C–H bond was the reaction found by Shilov and Shteinman and their coworkers in 1972 (for reviews, see References 1h and 5). These authors demonstrated that c1-math-0012 ion could catalyze H/D exchange in methane in a c1-math-0013 COOD solution and, if c1-math-0014 is added, the latter oxidizes methane to methanol (Shilov chemistry). The catalytic cycle in which c1-math-0015 -methyl complexes of platinum(II) and platinum(IV) are involved is shown in Fig. 1.1.

Figure 1.1 The catalytic cycle proposed for the methane oxidation to methanol by c1-math-0016 catalyzed by c1-math-0017 .

Later, Periana and coworkers proposed (2,2′-bipyrimidyl)platinum(II)dichloride as a catalyst (Periana system; see a recent review [4d]). Fuming sulfuric acid is the oxidant in this case. A simplified scheme of the catalytic cycle is shown in Fig. 1.2. It can be seen that some intermediates contain c1-math-0018 -methyl-platinum bonds.

Figure 1.2 The simplified catalytic cycle for the methane oxidation by the Periana system.

Adapted from Reference 4d.

Complexes containing the fragment c1-math-0019 ( c1-math-0020 is pentamethylcyclopentadienyl) are active precatalysts in the C−H oxidation of cis-decalin and cyclooctane. Ceric ammonium nitrate was a sacrificial oxidant and water was the oxygen source (Fig. 1.3). Calculations using the Density functional theory (DFT) method showed that the C–H oxidation of cis-decalin by c1-math-0021 (ppy)(Cl) ( c1-math-0022 -phenylpyridine) follows a direct oxygen insertion mechanism on the singlet potential energy surface [6]. The authors proposed that some of intermediate species contain the c1-math-0023 ring coordinated to the iridium ion. The authors also made a general conclusion: oxidation catalysis by organometallic species can be hard to interpret because of the possibility that the real catalyst is an oxidation product of the precursor.

Figure 1.3 Stereospecific oxygenation of cis-decalin catalyzed by the Ir organometallic derivative [6].

Indeed, organometallic precatalysts can be transformed during an induction period into catalytically active species that do not contain metal–carbon bonds. For example, molybdenum [7a] and tungsten [7b] carbonyls catalyze aerobic photooxygenation of cyclohexane to cyclohexyl hydroperoxide (primary product) and cyclohexanol and cyclohexanone (Fig. 1.4). The proposed mechanism is shown in Fig. 1.5. It includes the formation during the induction period of an oxo derivative. Complexes CpFe(π-PhH) c1-math-0024 and (π-durene) c1-math-0025 also catalyzed the aerobic alkane photooxygenation [7c]. The mechanism has not been studied.

Figure 1.4 Oxidation of cyclohexane (CyH, 0.46 M) to cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone with air under irradiation with full light of high pressure Hg arc (1000 W) in MeCN c1-math-0026 . Photocatalysts c1-math-0027 (where c1-math-0028 , graph a [7a] and W, graph b [7b]) and complexes CpFe(π-PhH) c1-math-0029 (graph c) and (π-durene) c1-math-0030 (graph d) [7c] are shown.

Figure 1.5 Mechanism proposed for the photooxygenation of alkanes, RH, in the presence of Mo or W carbonyls. (See insert for color representation of the figure.)

1.3 Oxygenation of C–H Bonds with Peroxides

In the course of our systematic studies of hydrocarbon oxidation with peroxides, we have found a few organometallic catalysts and systems based on organometallic compounds. In some cases, these systems turned out to be extremely efficient, much more efficient than systems containing simple salts of transition metals.

Recently, we have discovered [8], for the first time, that ferrocene (catalyst 1.1) is an efficient (pre)catalyst for several types of oxidative transformations, namely, the oxidation of alkanes and benzene by c1-math-0031 or tert-butyl hydroperoxide. The oxidation of gaseous and liquid alkanes to alkyl hydroperoxides by c1-math-0032 proceeds in MeCN at c1-math-0033 . An obligatory cocatalyst is pyrazine-2-carboxylic acid (PCA, or Hpca, where H is a proton and pca is the anion of PCA). In the cyclohexane oxidation, the yield and TON after 1.5 h attained 32% and 1200, respectively. In the ethane oxidation, TON reached 970. Maximum yield (58% based on the alkane) was obtained for the c1-math-0034 -butane oxidation after 4 h.

The simplest kinetic scheme of the alkane oxygenation based on the kinetic data was proposed. In the first stage, ferrocene c1-math-0035 is oxidized to ferricenium cation c1-math-0036 , which is in turn transformed into species Fe that is a fragment containing one iron ion.

These are the fast stages of the generation from c1-math-0039 and c1-math-0040 the main species, which is active in the catalytic process. Produced fragment Fe interacts with a PCA molecule to form the complex Fe(PCA):

K1 c1-math-0041

Here (PCA) is a PCA fragment (possibly pyrazinecarboxylate, pca). The formed complex can react with the second PCA molecule yielding in this case an adduct containing two PCA fragments per one Fe ion:

K2 c1-math-0042

Two adducts Fe(PCA) can dimerize to afford the dinuclear complex c1-math-0043 :

K3 c1-math-0044

Complex c1-math-0045 is a catalytically active species that produces hydroxyl radicals from c1-math-0046 :

k4 c1-math-0047

Hydroxyl radicals react in parallel routes with solvent (acetonitrile) and substrate (cyclohexane, RH):

k5 c1-math-0048

k6 c1-math-0049

The last reaction is the rate-limiting step in the sequence of alkane transformations into cyclohexyl hydroperoxide. If we assume that the concentration of c1-math-0050 is quasi-stationary and concentrations of all iron complexes are in quasi-equilibrium and take into account conditions c1-math-0051 and c1-math-0052 , we obtain the equation for the initial reaction rate as follows:

where

and

We can rewrite the equation for the initial reaction rate in the following form:

where

and

The following values c1-math-0059 and c1-math-0060 have been calculated for the conditions of our experiments. Using these values for parameters c1-math-0061 and c1-math-0062 , the initial reaction rates (presented by a dotted curve) have been calculated at different concentrations of PCA under conditions described in the caption of Fig. 1.6b.

Figure 1.6 (a) Dependence of the initial rate c1-math-0063 of oxygenate accumulation in the cyclohexane oxidation with c1-math-0064 catalyzed by ferrocene 1.1 in MeCN on the initial concentration of ferrocene (curve 1). Conditions: c1-math-0065 , c1-math-0066 . Curve 2: linearization of curve 1 in coordinates c1-math-0067 . (b) Dependence of the initial rate c1-math-0068 of oxygenate accumulation in the cyclohexane oxidation with c1-math-0069 catalyzed by 1.1 in MeCN on the initial concentration of PCA (in the intervals c1-math-0070 and c1-math-0071 ). Conditions:

, c1-math-0073 . Dotted curves present the simulated dependences.

tert-Butyl hydroperoxide (0.58 M; 70% aqueous) oxidizes cyclohexane (0.92 M) in MeCN at 50 °C in the presence of 1.1 c1-math-0074 and PCA c1-math-0075 , affording (after reduction with c1-math-0076 ) cyclohexanol (0.02 M) and cyclohexanone (0.001 M) after 4.5 h. Heating a solution of benzene (0.58 M) with c1-math-0077 (1.28 M) in MeCN at 50 °C in the presence of 1.1 c1-math-0078 and PCA c1-math-0079 gave phenol (0.038 M after 1 h). In the presence of 2,2′-bipyridine c1-math-0080 instead of PCA, 1.1 c1-math-0081 catalyzes the oxygenation with c1-math-0082 (1.28 M) of benzene (0.58 M) to phenol (0.05 M after 3 h) with a long induction period.

We also found recently [8] the first example of alkane hydrocarboxylation in aqueous acetonitrile with the c1-math-0083 system catalyzed by an iron complex, that is, ferrocene (Table 1.1). For example, the reaction of propane (1 atm) with CO (10 atm) at 60 °C during 4 h gave isomeric butyric acids in 60% total yield.

Table 1.1 Hydrocarboxylation of Alkanes by the 1.1/ c1-math-0084 Systema

Another metallocene, namely, decamethylosmocene, c1-math-0093 (catalyst 1.2), turned out to be a good precatalyst in a very efficient oxidation of alkanes with hydrogen peroxide in acetonitrile at c1-math-0094 [9]. The reaction proceeds with a substantial lag period that can be reduced by the addition of pyridine in a small concentration. Alkanes, RH, are oxidized primarily to the corresponding alkyl hydroperoxides, ROOH. TONs attain 51,000 in the case of cyclohexane (maximum turnover frequency was c1-math-0095 ) and 3600 in the case of ethane. The oxidation of benzene and styrene afforded phenol and benzaldehyde, respectively. A kinetic study of cyclohexane oxidation catalyzed by 1.2 and selectivity parameters (measured in the oxidation of c1-math-0096 -heptane, methylcyclohexane, isooctane, cis-dimethylcyclohexane, and trans-dimethylcyclohexane) indicated that the oxidation of saturated, olefinic, and aromatic hydrocarbons proceeds with the participation of hydroxyl radicals.

We discovered [10] that triosmium dodecacarbonyl (compound 1.3, Fig. 1.7) catalyzes a very efficient oxidation of alkanes by c1-math-0097 in MeCN to afford alkyl hydroperoxides (primary products) as well as alcohols and ketones (aldehydes) at c1-math-0098 if pyridine is added in a low concentration. TONs attained 60,000 (Fig. 1.8a) and turnover frequencies were up to c1-math-0099 . A plateau in the dependence of c1-math-0100 on initial concentration of cyclooctane, [RH] (Fig. 1.8b), indicates that there is a competition between RH and another component of the reaction mixture for a transient oxidizing species. Indeed, at high concentration of the hydrocarbon, all oxidizing species are accepted by RH and the maximum possible oxidation rate is attained. This concurrence can be described by the following kinetic scheme:

i c1-math-0101

1 c1-math-0102

2 c1-math-0103

3 c1-math-0104

4 c1-math-0105

where c1-math-0106 is the rate of generation of oxidizing species X. The analysis of this scheme in a quasi-stationary approximation relative to species X leads to the following equation:

Figure 1.7 Efficient oxidation catalysts based on osmium carbonyls.

Figure 1.8 (a) Kinetic curves of accumulation of cyclooctyl hydroperoxide (curve 1), cyclooctanone (curve 2), and cyclooctanol (curve 3) in the cyclooctane (0.5 M) oxidation with c1-math-0108 catalyzed by c1-math-0109 (1.3) in MeCN at c1-math-0110 . Concentrations of the three products were measured using a simple method, previously developed by us [10–12] with the reduction of samples with c1-math-0111 . (b) Dependence of c1-math-0112 on the initial concentration of cyclooctane ( c1-math-0113 ). (c) Linearization of dependence shown in (b) using coordinates c1-math-0114 /[cyclooctane]0.

Adapted from Reference 10(a).

In accord with the last equation, we can see the linear dependence of the experimentally measured reciprocal parameter c1-math-0115 on reciprocal concentration c1-math-0116 (Fig. 1.8c). The tangent of this straight line slope angle corresponds to the value c1-math-0117 . The segment that is cut off by the line on Y-axis is equal to c1-math-0118 . Thus, we can calculate the following value:

At our conditions c1-math-0120 , and c1-math-0121 , we can calculate the following parameters c1-math-0122 for different data found in the literature:

, and c1-math-0124 . It follows from this estimation that the most probable competitors of cyclooctane for hydroxyl radicals are pyridine and acetonitrile. Rate constants c1-math-0125 can be calculated as follows: c1-math-0126 in the case of pyridine and c1-math-0127 in the case of acetonitrile. These values are typical for the reactions of hydroxyl radicals with alkanes: c1-math-0128 for cyclopentane, c1-math-0129 for cyclohexane, and c1-math-0130 for cycloheptane in aqueous solution. It can be seen that the experimentally found competition is in good agreement with the assumption that the oxidizing species in our system is hydroxyl radical. Radical c1-math-0131 attacks the hydrocarbon RH to generate alkyl radical c1-math-0132 , which very rapidly reacts with molecular oxygen.

Similar trinuclear carbonyl hydride cluster, c1-math-0133 (compound 1.4), catalyzes the oxidation of cyclooctane to cyclooctyl hydroperoxide by hydrogen peroxide in acetonitrile solution [12]. Selectivity parameters obtained in oxidations of various linear and branched alkanes as well as kinetic features of the reaction indicated that the alkane oxidation occurs with the participation of hydroxyl radicals. A similar mechanism operates in the transformation of benzene into phenol and styrene into benzaldehyde. The system also oxidizes 1-phenylethanol to acetophenone. The kinetic study led to a conclusion that the oxidation of alcohols does not involve hydroxyl radicals as the main oxidizing species and apparently proceeds with the participation of osmyl species, c1-math-0134 . Finally, a carbonyl osmium(0) complex with c1-math-0135 -coordinated olefin, ( c1-math-0136 ,4-diphenylbut-2-en-1,4-dione)undecacarbonyl triangulotriosmium (1.5, Fig. 1.7), catalyzes the oxygenation of alkanes (cyclohexane, cyclooctane, c1-math-0137 -heptane, isooctane, etc.) with hydrogen peroxide, as well as with tert-butyl hydroperoxide and meta-chloroperoxybenzoic acid in acetonitrile solution [13]. Simple osmium salts c1-math-0138 also catalyze (especially in the presence of pyridine or other N-bases) alkane hydroperoxidation with c1-math-0139 in acetonitrile [14a] or water [14b], but these reactions are less efficient in comparison with processes catalyzed by organoosmium compounds.

Hexanuclear rhodium carbonyl cluster, c1-math-0140 (compound 1.6, Fig. 1.9), catalyzes benzene hydroxylation with hydrogen peroxide in acetonitrile solution [15a]. Phenol and quinone (in less concentration) are formed with the maximum attained total yield and TON of 17% and 683, respectively. It is noteworthy that certain other rhodium carbonyl complexes, containing cyclopentadienyl ligands, c1-math-0141 (1.7) and c1-math-0142 (1.8), are less efficient catalysts, whereas cyclopentadienyl derivatives of rhodium, which do not contain the carbonyl ligands, c1-math-0143 (1.9), RhCp(cyclooctatetraene) (1.10) and c1-math-0144 (cyclooctatetraene) (1.11), turned out to be absolutely inactive in the benzene hydroxylation. In the presence of compound 1.6, styrene is transformed into benzaldehyde and (in less concentration) acetophenone and 1-phenylethanol. Addition of acids is known to accelerate some metal-catalyzed oxidation reactions. In our case, when trifluoroacetic acid was added to the reaction solution catalyzed by cluster 1.6, the initial reaction rate was approximately three times higher. It should be emphasized that no oxygenated products have been detected when alkanes were used as substrates in the 1.6-catalyzed oxidation. Ethyl groups in ethylbenzene were also not oxygenated. It has been tentatively assumed that the interaction of cluster 1.6 with hydrogen peroxide leads to splitting Rh–Rh and Rh–CO bonds to form vacant sites that coordinate benzene molecules. Possibly, the c1-math-0145 –CO fragment is oxidized in the initial period of the reaction to afford c1-math-0146 –C(O)OH and Rh–C(O)OOH species. The catalytic cycle presented in Fig. 1.10 was proposed for the oxidation reaction. In the initial period, a rhodium complex under the action of hydrogen peroxide and water is transformed into a hydroxy derivative A. The interaction of species A with hydrogen peroxide affords a hydroperoxo derivative B. The latter forms a c1-math-0147 -arene complex C. Species C can be converted into rhodadioxolane D, which decomposes further to produce phenol and initial catalytically active species A.

Figure 1.9 Compound 1.6 is an efficient catalyst for the benzene oxidation, compounds 1.7 and 1.8 are less efficient, and compounds 1.9, 1.10, and 1.11 are inactive.

Figure 1.10 A catalytic cycle proposed for the benzene hydroxylation catalyzed by a rhodium complex ({Rh} is a Rh-containing fragment).

Adapted from Reference 15a.

Cyclopentadienylbenzeneiridium(III) tetrafluoroborate c1-math-0148 (complex 1.12) was completely inactive in oxidation with hydrogen peroxide and tert-butyl hydroperoxide but exhibited a moderate activity in oxidation with c1-math-0149 -chloroperoxybenzoic acid at room temperature [15b]. The c1-math-0150 –1.12 system showed a moderate activity in the oxidation of secondary alcohols. For example, cyclohexanol was oxidized at room temperature to cyclohexanone (30% yield for 6 h) when a fourfold excess of PCA as a cocatalyst was added to the reaction solution.

It has been shown recently that cyclopentadienyl vanadium complexes catalyze the oxidation of benzylic groups by tert-BuOOH [16]. Compound c1-math-0151 (1.12) catalyzes benzylic C–H oxidation selectively and effectively, giving no aromatic oxidation products. The authors assume that intermediate catalytically active species contain Cp rings (Fig. 1.11).

Figure 1.11 A catalytic cycle proposed for the benzyl hydroxylation catalyzed by vanadium complex 1.12.

Adapted from Reference 16.

1.4 Conclusions and Outlook

It is clearly seen from this chapter that organometallic complexes are not leading catalysts for various reactions that afford valuable oxygenates from hydrocarbons and other C–H compounds. Such complexes are usually expensive and their synthesis is often not simple. However, in some cases, organometallics outrival commercially available inorganic salts in activity and selectivity. One can expect that the research on the application of organometallic catalysts in oxidation reactions will continue in the future.

Acknowledgment

This work was supported by the Russian Foundation for Basic Research (Grant 12-03-00084-a).

References

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Chapter 2

Toward Functionalization of Alkanes Under Environmentally Benign Conditions

Armando J. L. Pombeiro

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

2.1 Introduction

Alkanes are very rich carbon compounds but their use as raw materials for organic synthesis has been hampered by their high inertness. Their conversion into organic products with an added value (for reviews, see e.g., [1–15]), is a challenge in modern chemistry, and alkanes are used generally as fuels (full oxidation to c2-math-0001 ), with complete loss of carbon to the atmosphere and resulting noxious environmental effects associated with carbon dioxide accumulation therein.

The development of mild and green processes for their partial oxidation and functionalization would promote the potential of their application toward alternative raw materials for organic synthesis. Single-pot methods to achieve functionalized products, such as carboxylic acids, would be highly advantageous also in terms of simplicity, in comparison with the current multistage and energy-demanding processes used in industry.

Of particular significance to achieve environmentally benign systems would be the use of water as a solvent, but this is also challenging in view of the lack of solubility of the alkanes and, commonly, also of the metal catalysts. The approach followed in the author's group often involves the use of hydrosoluble ligands at appropriate metal centers, which can lead to the formation of water-soluble catalyst precursors. Examples are indicated in the following sections.

2.2 Peroxidative Oxidations of Alkanes to Alcohols and Ketones, Catalyzed by Transition Metal Complexes

Both cyclic and acyclic alkanes undergo partial oxidation to alcohols and ketones, with hydrogen peroxide and under mild conditions (Scheme 2.1a) in the catalytic systems discussed herein, but the oxidation of cyclohexane to cyclohexanol and cyclohexanone (Scheme 2.1b) has been typically used as the model reaction, in view, for example, of its simplicity (cyclohexane bears only one type of carbon atoms) and its industrial significance (the mixture of cyclohexanol and cyclohexanone obtained by such a reaction is used for the preparation of adipic acid, a key intermediate for the production of nylon-6,6).

Scheme 2.1 Peroxidative oxidation of an alkane (a) and, in particular, of cyclohexane (b) to the corresponding alcohol and ketone, with aqueous c2-math-0002 , catalyzed by a transition metal catalyst (Cat.), under typical mild reaction conditions of this work.

2.2.1 Scorpionate Complexes as Catalyst Precursors

In contrast to boron-based scorpionates, such as tris(pyrazolyl)borate and derivatives, those based on carbon, namely tris(pyrazolyl)methane, c2-math-0003 , and hydrosoluble-derived ones (Scheme 2.2a), are still underexplored, in spite of their potential, when suitably functionalized, to form water-soluble complexes. A good example is the sulfonate derivative, that is, tris(pyrazolyl)methane sulfonate (Tpms), which is hydrolytically stable over a wide pH range and leads to sandwich and/or half-sandwich complexes with various transition metals. The half-sandwich compounds of iron, vanadium, and copper, bearing this or a related scorpionate ligand (Scheme 2.3a) [16–20], act as good catalyst precursors for the partial oxidation of alkanes to the corresponding alcohols and ketones, in acetonitrile, with aqueous hydrogen peroxide as an oxidant, under mild conditions (e.g., 20–40 °C), typically in acidic medium.

Scheme 2.2 Examples of types of ligands (or their precursors) in metal catalysts for the peroxidative oxidation of alkanes: (a) C-based scorpionates, (b) azoderivatives or arylhydrazones of β-diketones, (c) aminopolyalcohols, and (d) benzene polycarboxylic acids.

Scheme 2.3 Examples of transition metal catalyst precursors for the peroxidative oxidation of alkanes, bearing (a) a C-scorpionate [16–20], (b) an azoderivative of a β-diketone [21], (c) an aminopolyalcohol derivative ( c2-math-0004 deprotonated form of triethanolamine, c2-math-0005 tea) [33, 34], and (d) a benzene dicarboxylate [33, 34] ligand.

This is illustrated for the peroxidative oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone (Scheme 2.1b), a reaction with industrial application (see above). The conditions required in the industrial process (quite higher temperatures) are much harsher than ours, and the conversions are rather low in order to achieve a good selectivity. In our systems based on a half-sandwich complex, turnover numbers (TONs, moles of product per mole of catalyst precursor) up to 690 and yields up to 25% have been achieved with the Fe precatalyst [ c2-math-0006 (Tpms)] [20].

In a few cases (with Fe or Cu hydrosoluble precatalysts), the system can operate in the absence of any added organic solvent (even acetonitrile) [20], a feature that is noteworthy toward the development of a green catalytic system. However, our catalysts are not effective when using air (or dioxygen) instead of hydrogen peroxide as the oxidant, a disadvantage relative to the industrial process.

A broader discussion on the use of scorpionate complexes in catalysis is given in Chapter 22.

2.2.2 Azoderivatives of β-Diketone Complexes as Catalyst Precursors

Suitably functionalized azoderivatives of β-diketones (ADB or arylhydrazones of β-diketones, AHBD) (Scheme 2.2b) are also convenient hydrosoluble species toward water-soluble catalyst precursors in this field and of particular interest are those bearing an acid substituent (carboxylic or sulfonic group) [21], which can operate without requiring the common presence of an added acid promoter. The acidic moiety conceivably has two main roles: provides water solubility and acts as the acid promoter. Hence, the complexes c2-math-0007 (Scheme 2.3b) and

substituted (2-hydroxy-phenylhydrazone)pentane-2,4-dione] appear to behave as dual-role catalyst precursors, in acid-free medium, combining, in each molecule, an active copper center and an acid site (TONs and yields up to 163 and 14%, respectively, are achieved for the model oxidation of cyclohexane, in NCMe/aqueous c2-math-0009 , at c2-math-0010 C) [21].

2.2.3 Multinuclear Complexes as Catalyst Precursors

Although the mononuclear Cu half-sandwich scorpionate complexes (see above) are commonly less active than the Fe ones, copper complexes can be more effective in multinuclear assemblies (including coordination polymers) and our approach for these species concerns their self-assembly synthesis by using a suitable combination of a metal source, a main chelating ligand, a spacer or linker, apart from a pH regulator. A wide discussion on the types of self-assembled multicopper complexes and their use for alkane functionlization is presented in Chapter 3, and only a very brief overall view is outlined here.

Typical examples of chelating ligands and spacers applied for self-assembly of multicopper coordination compounds and polymers include (see above) AHBD [21–24], aminopolyalcohols, and benzenepolycarboxylic acids [25–34] (Scheme 2.2b–d).

A diversity of 1D, 2D, or 3D copper assemblies can be obtained, including coordination polymers, as well as multi- or mononuclear species with discrete molecules. Among the latter, the tetranuclear c2-math-0011 -oxo complex derived from triethanolamine c2-math-0012 (Scheme 2.3c) is particularly active (TONs or yields up to 380 or 39%, respectively, for the conversion of cyclohexane to cyclohexanol and cyclohexanone) [33, 34]. It is also active (although less effectively, by one order of magnitude) for the oxidation of methane and ethane to methanol and ethanol, respectively [34].

Heterometallic species can be conveniently obtained, in particular by direct self-assembly, from a metal powder, and they can exhibit a metal synergic effect with a remarkable catalytic activity. Hence, the hexanuclear heterodimetallic c2-math-0013 Schiff base complex c2-math-0014 c2-math-0015 (Scheme 2.4), without copper, self-assembled from Co powder, c2-math-0016 , and c2-math-0017 under air, exhibits an outstanding TON of c2-math-0018 (corresponding to a turnover frequency of c2-math-0019 ), for the oxidation of cylohexane in NCMe/aqueous c2-math-0020 , at room temperature [35].

Scheme 2.4 Direct self-assembled heterometallic catalyst precursors for the peroxidative oxidation of alkanes: (a) [Co4Fe2O(Sae)8] (H2Sae = salicylidene-2-ethanolamine) [35] and (b)

[36].

A good activity with a synergic effect is also achieved (TON and yield up to 100 and 25%, respectively) by the heterotrimetallic c2-math-0022 complex

[36].

2.2.4 Role of Water

The use of water as a solvent, even when mixed with an organic one, is a positive feature of a catalytic system, which aims to be of environmental significance.

However, the role of water can lie beyond that of a mere solvent, as suggested by the observation, in some cases, that water promotes the catalytic activity, and attested by theoretical density functional theory (DFT) calculations [37–40]. This was studied in detail for the aqueous c2-math-0024 –NCMe systems based on the oxo–Re complex c2-math-0025 (methyl trioxo-rhenium, MTO) [37] and on the vanadate c2-math-0026 or vanadatrane c2-math-0027 [38] catalyst precursors, which are effective for the peroxidative oxidation of alkanes to the corresponding alcohols and ketones. The vanadate system was initially established by Shul'pin et al. [41] and included pyrazinecarboxylic acid (PCAH) as a promoter.

As indicated by radical trap experiments, various types of selectivity (regio-, bond-, and stereo-selectivity), and kinetic and theoretical studies [18–24, 27, 29, 33–40], these peroxidative oxidations of alkanes occur via radical mechanisms. They are believed to proceed via free hydroxyl radical c2-math-0028 that acts as an H-abstractor from the alkane RH to yield the corresponding alkyl radical c2-math-0029 . Fast reaction of c2-math-0030 with c2-math-0031 generates the alkylperoxyl radical c2-math-0032 , which, following known pathways [38 39, and references therein], lead to the formation of the final alcohol (ROH) and the corresponding ketone via the alkylperoxide ROOH.

The hydroxyl radical is generated on metal-assisted reduction of c2-math-0033 (see Scheme 2.5, for a c2-math-0034 catalytic system) [38, 40–42].

Scheme 2.5 Overall reactions involved in a vanadium(V/IV)-assisted generation of hydroxyl radical c2-math-0035 from hydrogen peroxide. The c2-math-0036 products can stand for c2-math-0037 .

The concerned overall c2-math-0038 reactions involve proton-transfer steps, for example, from ligated c2-math-0039 to an oxo ligand, which are promoted by water on bridging both ligands with the formation of six-membered transition states (TSs) that are thus stabilized [37–40]. This is exemplified by c2-math-0040 , the TS involved in such a proton transfer in the c2-math-0041 –pyrazinecarboxylate (PCA) system (Scheme 2.6) [38]. The assistance of water, which acts as a catalyst, lowers (Scheme 2.6a) the activation barrier by 7–11 kcal/mol, in comparison with the four- or five-membered TSs that would form if the proton transfer would occur [41–43] with the assistance of the PCA ligand (Shul'pin robot-type mechanism, Scheme 2.6b) or directly from the c2-math-0042 ligand to an oxo ligand. Hence, water, in a controlled amount, can be more effective for this purpose than the more complex PCA ligand.

Scheme 2.6 Examples of transition states (TSs) involved in a proton-transfer step from a ligated c2-math-0043 to an oxo ligand on the way to generate the hydroxyl radical: (a) six-membered TS (water-assisted c2-math-0044 -transfer) at a PCA-V catalyst (PCA=pyrazine carboxylate) [38]; (b) five- or four-membered TSs (PCA-assisted c2-math-0045 -transfer, robot's arm mechanism) at a PCA-V catalyst [41–43]; and (c) six-membered oxo-divanadium TS at a divanadate-type model [40]. (See insert for color representation of the figure.)

A similar effect of water has been proposed for other types of reactions without involving alkanes, for example, olefin epoxidations catalyzed by cyclopentadienyl–Mo systems [44], and thus it can be of a considerable generality.

It is noteworthy to mention that the involvement of a second metal center can also promote the proton transfer, as believed to occur in di- or oligovanadate catalysts, which exhibit a higher activity than monovanadate [40]. In such systems, six-membered oxo–divanadium TSs (Scheme 2.6c) can be formed, lowering the energy barrier by circa 4.2 kcal/mol relatively to the proton transfer at a monovanadate center [40].

Furthermore, water can have an even deeper role in alkane functionalizaton, as a hydroxylating reagent, which will be discussed in Section 4.

2.2.5 Nontransition Metal Catalyzed Alkane Oxidation

When thinking on green catalysis, one is encouraged to try to avoid the use of any transition metal catalyst that commonly has an environmentally nonbenign character (although, in some cases, namely with Fe, Cu, or V catalysts, they can be tolerated). Of significance toward this aim is the recognition by Mandelli and Shul'pin [45] that aluminum, a nontransition metal, can replace a transition metal catalyst, as shown by the c2-math-0046 system, which catalyzes the oxidation of octane and heptane to the corresponding alcohols and ketones.

This is particularly interesting in the view that a redox-inactive metal is replacing a redox-active metal, in oxidation catalysis, and was investigated by DFT calculations [46].

These theoretical studies indicate the crucial role played by the intermediate c2-math-0047 , bearing (i) an highly activated hydrogen peroxide ligand with a dramatically decreased HO–OH bond energy (6.1 kcal/mol) in comparison with free c2-math-0048 (39.4 kcal/mol), and (ii) a ligated monodeprotonated form c2-math-0049 . At this intermediate, this latter ligand reduces (intramolecular redox process) hydrogen peroxide to hydroxide c2-math-0050 and hydroxyl c2-math-0051 , being itself oxidized to the hydroperoxyl radical c2-math-0052 , a labile ligand that liberates from the metal (Scheme 2.7) [46].

Scheme 2.7 Key hydrogen peroxide intermediate c2-math-0053 in the hydroxyl radical formation, at the c2-math-0054 -catalyzed peroxidative oxidation of alkanes [46].

Therefore, the transition metal is avoided in this c2-math-0055 –Al catalytic system on account of a suitable redox-active co-ligand c2-math-0056 that can play the redox role of the transition metal, by acting as a reducing agent of c2-math-0057 toward the generation of the hydroxyl radical. The generality of such an interesting behavior is worth to be investigated.

2.3 Metal-Free Alkane Hydrocarboxylation and Related Carboxylation

Because, as shown above, in the oxidation catalysis of alkanes, one can replace a transition metal catalyst by a redox-inactive nontransition metal catalyst, the question arises whether it would be possible to go even further and eliminate completely the use of any metal catalyst, thus establishing a metal-free system capable of oxidizing alkanes under mild conditions.

This has been achieved in the hydrocarboxylation of alkanes in water–acetonitrile (2 : 1–1 : 2 volume ratio range) medium, with CO and peroxydisulfate c2-math-0058 (Eq. 2.1). [25, 26, 28, 47–51].

2.1

The conditions are rather mild (30–60 °C), they do not require any acid addition, and yields up to 72% are obtained. Water behaves as a hydroxylating agent as demonstrated by using c2-math-0060 , which leads to the c2-math-0061 -labeled acid c2-math-0062 as the major product.

The system is active for both liquid and gaseous alkanes, but with a rather low activity for methane. Although operating under metal-free conditions, it is also metal-promoted namely by some copper complexes that act as catalyst precursors for the oxidation of alkanes with hydrogen peroxide, typically the abovementioned tetranuclear c2-math-0063 -oxo triethanolaminate complex c2-math-0064

The mechanism is also radical, as indicated by the suppression of acid (RCOOH) formation by a radical trap and the preferable carbonylation at a secondary carbon relative to a primary one. According to DFT calculations [51], it proceeds mainly as shown in Scheme 2.8.

Scheme 2.8 Main radical mechanism of the hydrocarboxylation of alkanes with peroxydisulfate, CO, and water, in aqueous c2-math-0065 medium [51]. The minor 7 (or 7a) to 8 alternative pathway does not concern water as the hydroxylating agent. (See insert for color representation of the figure.)

Peroxydisulfate acts as a radical source and as an oxidant (a third minor role as a hydroxylating agent is mentioned below). The first role concerns its homolysis that leads to the sulfate radical c2-math-0066 , which abstracts hydrogen from the alkane (RH) forming the alkyl radical c2-math-0067 (step 1, Scheme 2.8). This is carbonylated by CO to give the acyl radical c2-math-0068 (step 2), which is oxidized either by peroxydisulfate (its second role) with coupling to sulfate to give the acyl sulfate c2-math-0069 (step 3, metal-free pathway) or by the metal promoter to form the acyl cation c2-math-0070 (step 5, metal-promoted route) [51].

Nucleophilic attack of water, either at the acyl sulfate or at the acyl cation (step 4 or 6, respectively), leads to the formation of the carboxylic acid RCOOH, as the final product [51]. Hence, apart from being a solvent, water also plays a fundamental role as a stoichiometric nucleophilic reagent toward alkane-derived acyl species.

When using c2-math-0071 , a minor amount of nonlabeled RCOOH is also obtained, which can be accounted for by steps 7 (or 7a) and 8, where c2-math-0072 (derived from peroxydisulfate), instead of water, acts as the hydroxylating agent [51].

This alkane-hydrocarboxylating system can be considered as a development toward the green direction of a previous carboxylation system based on the use of peroxydisulfate in trifluoroacetic acid (TFA) at 80 °C (Scheme 2.9a), which was pioneered by Fujwara [12, 15], and further improved by the author's group [52–60] by finding more active and convenient metal catalysts, and establishing the mechanism of the catalysis. In fact, the water–acetonitrile mixture, in the above alkane-hydrocarboxylating system (Eq. 2.1, Scheme 2.8), has replaced successfully the noxious TFA as a solvent in the latter carboxylation system, the operating conditions became much milder and environmentally tolerable, and a role of water as a reagent was found. However, peroxydisulfate could not be replaced by a greener oxidant, such as c2-math-0073 .

Scheme 2.9 Alkane carboxylation with CO and c2-math-0074 , in TFA: (a) General reaction [12, 15, 52–60] and (b) proposed mechanism for an oxo-vanadium catalyst [52, 53]. (See insert for color representation of the figure.)

In the c2-math-0075 /TFA system, vanadium catalysts were found to be the most active, in particular amavadin (also spelled amavadine) and its models [52–54, 57, 59, 61]. Amavadin is the nonoxo–vanadium complex c2-math-0076 [ c2-math-0077 deprotonated basic form of N-(hydroxyimino)dipropionic acid] (Scheme 2.10) that is present in some toadstools (amanita muscaria), but its biological role still remains undiscovered [61]. Its catalytic activity, and those of its models, such as [V(HIDA)2]²− [HIDA = deprotonated basic form of N-(hydroxyimino)diacetic acid] and related vanadatrane c2-math-0078 , are so high (TONs up to over c2-math-0079 or carboxylic acids yields up to over 90%, for the most inert alkanes, methane, or ethane) [52, 53] that amavadin and the toadstools where it is found have been considered by this author as a kind of magic (inspired on a well-known Queen band song) and the latter were called elsewhere [62] as magic mushrooms, new catalysts from Nature.

Scheme 2.10 Amavadin complex [V(HIDPA)2]²− [HIDPA = deprotonated basic form of N-(hydroxyimino)dipropionic acid] with its natural source, amanita muscaria.

The initial steps of the mechanism of the alkane (RH) carboxylation (Scheme 2.9b) [52, 53] are identical to those of the hydrocarboxylation, discussed above, with c2-math-0080 as the source of the sulfate radical, which acts as H-abstractor from the alkane to give the alkyl radical c2-math-0081 , and CO as the carbonylating agent of this radical to yield the acyl radical c2-math-0082 .

However, peroxydisulfate, instead of water, provides the source of the hydroxyl oxygen as substantiated by DFT calculations [52, 53], which indicate the plausible addition of the acyl radical to a peroxo-V intermediate to form a percarboxylate ligand, RC(O)OO, which, on O–O bond homolysis, generates the carboxyl radical c2-math-0083 that, via H-abstraction from the alkane or from TFA, yields the acid RCOOH.

It is noteworthy to mention that both the hydrocarboxylation and the carboxylation of alkanes processes discussed herein provide single-pot routes to carboxylic acids that are much simpler and operate under much milder conditions in comparison with the industrial processes. This is well illustrated for the case of the industrial production of acetic acid [63, 64], which involves three distinct stages under energy-demanding and environmentally nontolerable conditions: high temperature catalytic steam reforming of methane or coal to CO and dihydrogen, high temperature catalytic conversion of synthesis gas into methanol, and carbonylation of methanol with expensive Rh or Ir catalysts (Monsanto or Cativa process, respectively), still at a considerably elevated temperature.

Moreover, amavadin also catalyses the abovementioned peroxidative oxidation of alkanes, as well as their peroxidative halogenations [65], although with a much lower activity.

2.4 Final Remarks

Steps have been taken toward eco-friendly catalytic systems active in alkane functionalization under mild conditions, preferably in aqueous media, by using hydrosoluble catalysts obtained from ligands that are water soluble. Systems can operate usually with a green oxidant (aqueous hydrogen peroxide) and in partially aqueous media.

Moreover, water can play a fundamental role beyond that of a mere solvent, acting as a promoter of the catalytic activity (by favoring proton-transfer steps) or even as a reagent, that is, the hydroxylating reagent in the alkane functionalization.

A transition metal can be avoided, by using a nontransition metal catalyst, provided a suitable redox-active co-ligand is present.

A metal-free system was already established for the carboxylation of alkanes with CO and water, operating in water–acetonitrile and under acid-free conditions. The latter feature (no added acid) is common to a few other catalytic systems active for alkane hydroxylation, namely by taking advantage of a ligand bearing an acid group (dual-role catalyst).

The alkane functionalization reactions proceed via radical mechanisms, with a high chemoselectivity, although with low regio-, bond-, and stereo-selectivities as expected for the involvement of the hydroxyl radical, features that were not discussed in this chapter.

Theoretical DFT studies allow to disclose conceivable reaction mechanisms.

The systems exhibit a high simplicity and provide the oxidized and carboxylated (carboxylic acids) products in a single-pot process, thus contrasting with the higher complexity of the industrial synthetic processes for the same products.

In addition, a number of the developed catalysts are also active, under different experimental conditions, in oxidations of other substrates, in particular some of the copper catalysts for alcohol aerobic (TEMPO-mediated) or peroxidative oxidations [21, 22]. The reactions may be microwave-assisted (details are given in Chapter 18).

It is also worth mentioning the biological significance of these studies and the inspiration of biology on their development. In fact, pMMO (particulate methane monooxygenase), a main enzyme in the metabolic pathway of methanotrophs, is a membrane multicopper enzyme that catalyzes the oxidation of alkanes to the corresponding alcohols [66–68], which is mimicked by the multinuclear copper systems.

Moreover, amavadin, a water-soluble natural vanadium complex, still with an undisclosed biological role in the mushrooms where it is accumulated, has been successfully applied as a remarkably effective catalyst precursor in the field of alkane carboxylation, although under conditions that are not found in biological systems. Concerning the peroxydisulfate/TFA system for alkane carboxylation, this acid solvent has already been replaced by a mixture of water/acetonitrile, operating under milder conditions, but a cheaper and less noxious oxidant (hydrogen peroxide or dioxygen) conceivably has yet to be found before the process gains a widespread use.

Acknowledgment

The co-authors of the cited references are gratefully acknowledged. Dr. M. F. C. Guedes da Silva is further acknowledged for the assistance in the preparation of the schemes. The work has been supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal (projects PTDC/QUI-QUI/102150/2008 and PEst-OE/QUI/UI0100/2013).

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Chapter 3

Self-Assembled Multicopper Complexes and Coordination Polymers for Oxidation and Hydrocarboxylation of Alkanes

Alexander M. Kirillov*, Marina V. Kirillova*, and Armando J. L. Pombeiro*

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

3.1 Introduction

The selective and atom-efficient oxidative functionalization of alkanes under mild conditions, toward the synthesis of various added-value organic products (alcohols, ketones, aldehydes, and carboxylic acids), constitutes a challenge to modern chemistry because of the exceptionally high inertness of these hydrocarbons [1, 2]. An important research direction is toward the search for new bioinspired catalytic systems [1a–3] that are capable of converting alkanes into different oxidation products. Given the recognized biological function of copper and its presence in the active sites of many oxidation enzymes [3, 4], including the multicopper particulate methane monooxygenase (pMMO) [5], the development of new bioinspired multicopper catalysts and efficient alkane functionalization protocols thereof constitutes a subject of high importance.

Recently, we have developed a versatile aqueous medium self-assembly method for the generation of diverse multicopper(II) complexes and coordination polymers derived from cheap and commercially available ligands such as aminoalcohols and benzenecarboxylates [6–15]. The obtained compounds were applied as highly efficient and versatile catalysts or catalyst precursors in two different alkane functionalization reactions. These include the mild oxidation of alkanes (typically cyclohexane as a model substrate) by hydrogen peroxide into alkyl hydroperoxides, alcohols, and ketones [6–9, 11, 16, 17], as well as the hydrocarboxylation of gaseous and liquid c3-math-0008 alkanes, by carbon monoxide, water, and potassium peroxodisulfate into the corresponding c3-math-0009 carboxylic acids [12–15, 18–22].

Hence, in this chapter, we describe the principle of aqueous medium self-assembly synthesis, the selected self-assembled aminoalcoholate multicopper(II) complexes and coordination polymers, and their catalytic application in homogeneous oxidative functionalization of alkanes.

3.2 Self-Assembly Synthesis in Aqueous Medium

From the environmental and economical viewpoints, water is the ideal green solvent for both the synthesis of coordination compounds and the catalytic transformations of organic molecules including the oxidative functionalization of alkanes [23]. However, the performance of catalytic reactions in aqueous medium typically requires the use of hydrosoluble catalysts that often mimic the functions of enzymes. Although various bioinspired multicopper complexes were synthesized as models of pMMO and related copper-based enzymes [3–5a], those catalysts were often not soluble in water, exhibited modest activities, or were almost not tested in oxidative transformations wherein alkanes are used as substrates.

Bearing the above-mentioned points in mind, we have developed a simple and versatile self-assembly protocol for the synthesis of diverse multicopper(II) complexes and coordination polymers in aqueous medium, under ambient conditions, and using simple and commercially available chemicals (Scheme 3.1). This self-assembly method is based on a combination with water, at room temperature (rt) and in air, of copper source, main chelating ligand, pH-regulator, and supporting ligand or spacer, followed by crystallization [10–15]. As a metal source, simple copper salts such as copper(II) nitrate or acetate were used, whereas triethanolamine (H3tea) and closely related aminoalcohols [ c3-math-0010 -ethyl- and c3-math-0011 -butyldiethanolamine (H2edea, H2bdea), c3-math-0012 , c3-math-0013 -bis(2-hydroxyethyl)-2-aminoethanesulfonic acid ( c3-math-0014 bes), or bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane ( c3-math-0015 bts)] were applied as main chelating ligands (Scheme 3.2). Alkali or alkaline earth metal hydroxides [NaOH, LiOH, or c3-math-0016 ] were utilized as pH-regulators, while the supporting ligands were selected from benzenecarboxylates (e.g., benzoic, terephthalic, or pyromellitic acids), simple inorganic anions (azide, thiocyanate, or tetrafluoroborate), or cyanometallates (e.g., c3-math-0017 ) [6, 8–15, 24].

Scheme 3.1 General scheme of self-assembly synthesis. Adapted from Reference 10.

Scheme 3.2 Aminoalcohols applied in self-assembly synthesis of multicopper(II) compounds.

By modifying the type of main chelating ligand, pH-regulator or supporting ligand, we have synthesized a considerable number of aminoalcoholate multicopper compounds, ranging from discrete di-, tri-, and tetracopper(II) complexes, to 1D, 2D, and 3D coordination polymers [6–15, 24]. Many of the obtained compounds are water soluble and have also been tested as catalysts or catalyst precursors in the oxidative functionalization of alkanes. Although some parent compounds are not soluble in water, they can also act as catalyst precursors of active hydrosoluble species on treatment with an acid promoter and/or oxidant [7, 9, 15, 24]. The representative examples of highly active di-, tri-, tetra-, and polynuclear copper