Homogeneous oxidation of alkanes: Role of rhodium–alkyl complexes

Article abstract of DOI:10.1016/j.molcata.2016.07.026

Catalytic systems RhCl₃–KI–NaCl and RhCl₃–Cu(OAc_f)₂–NaCl in aqueous perfluorinated carboxylic acids (CF₃COOH, C₃F₇COOH) are effective in coupled oxidation of alkanes and carbon monoxide with dioxygen. In their presence, predominant is the outer-sphere oxidation of alkanes into respective esters (alcohols) with involvement of peroxo rhodium species as an oxidant (mechanism A). The process occurs partially by the inner-sphere mechanism B involving Rh–alkyl intermediates. Mechanism B is supported by (a) formation of alkyl chlorides, (b) synthesis of acetic acid in conversion of methane, and (c) positional selectivity in oxidation of propane.

Full text of DOI:10.1016/j.molcata.2016.07.026

Accepted Manuscript
Title: Homogeneous oxidation of alkanes: Role of
rhodium–alkyl complexes
Author: E.G. Chepaikin A.P. Bezruchenko G.N. Menchikova
A.E. Gekhman
PII:
S1381-1169(16)30276-X
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http://dx.doi.org/doi:10.1016/j.molcata.2016.07.026
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Journal of Molecular Catalysis A: Chemical
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11-7-2016
14-7-2016
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Dedicated to the 70th Anniversary of Prof. G.B. Shulpin
Homogeneous oxidation of alkanes: Role of rhodium–alkyl complexes
E. G. Chepaikin1, A. P. Bezruchenko1, G. N. Menchikova1, А. Е. Gekhman2
1Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
2Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of
Sciences, Moscow, 119991 Russia
e-mail: echep@ism.ac.ru
Keywords: alkanes, oxidation, homogeneous catalysis, Rh–alkyl bond

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Graphical Abstract

3
Highlights
 Induction period in oxidation of CH4 and C3H8
 Formation of CH3Cl, n-C3H7Cl, i-C3H7Cl, and C4H9Br
 Positional selectivity in oxidation of propane
 Formation of Rh–alkyl intermediates
 Synthesis of acetic acid from CH4, CO, and O2

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ABSTRACT
Catalytic systems RhCl3–KI–NaCl and RhCl3–Cu(OAcf)2–NaCl in aqueous
perfluorinated carboxylic acids (CF3COOH, C3F7COOH) are effective in coupled
oxidation of alkanes and carbon monoxide with dioxygen. In their presence,
predominant is the outer-sphere oxidation of alkanes into respective esters (alcohols)
with involvement of peroxo rhodium species as an oxidant (mechanism A). The
process occurs partially by the inner-sphere mechanism B involving Rh–alkyl
intermediates. Mechanism B is supported by (a) formation of alkyl chlorides, (b)
synthesis of acetic acid in conversion of methane, and (c) positional selectivity in
oxidation of propane.

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1. Introduction
Oxidation of lower alkanes on homogeneous metal-complex catalysts seems
promising for use in reprocessing natural and associated petroleum gas [1]. The
problem has been a subject of several monographs [2–5] and numerous reviews [6–
10]. It is widely adopted that the activation of C–H bonds proceeds largely by the
following two mechanisms (Scheme 1).
Scheme 1. Outer-sphere (A) and inner-sphere (B) mechanisms of C-H bond
activation in alkanes.
The outer-sphere mechanism (А) postulates that the activation of alkanes takes place
with participation of oxygen atoms in oxo or peroxo metal complexes [2].
Mechanism A is also applicable to the action of oxygenases and related compounds.
For such a mechanism, the kinetic isotopic effect, kH/kD, is above 3.5 (and about 5.0
for methane monooxygenase). The inner-sphere mechanism (B) involving the
formation of metal–alkyl bond is effective in other catalytic systems such as those
developed by Shilov [3] and Periana [8]. For the Shilov system, mechanism B is
supported the isotopic exchange between methane (CH4) and deuterated media and
2–
also by the value of kH/kD ≈ 3 at 100С. Chloride-containing systems (e.g. PtCl4 –
2–
PtCl6 ) catalyze the formation of not only alcohols but also alkyl chlorides. Metal–
alkyl complexes were obtained by independent synthesis from e.g. H2PtCl6 and alkyl
iodides. Prof. Shul’pin was the first who managed to synthesize aryl–Pt(IV)
compounds directly from arenes [11]. Some catalytic systems can operate via
combined action of mechanisms A and B. As an example, we can mention the
catalytic systems based on Rh and Pd complexes and I-, Cu-, and Fe-containing

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compounds acting as co-catalysts [9, 10e, 12–15]. Such systems catalyze coupled
oxidation of alkanes and CO with dioxygen.
Scheme 2. Reactions of methane in AcfOH/H2O medium in the presence of Rh–I–Cl,
Rh–Cu–Cl, and Rh–Fe–Cl catalytic systems.
Rh–I–Cl, Rh–Cu–Cl, and Rh–Fe–Cl catalytic systems, just as the other ones [16–20],
are most effective in aqueous solution of trifluoroacetic acid (AcfOH) and
heptafluorobutyric acid (PrfCOOH).
As shown in Scheme 2, the reactions of methane in AcfOH/H2O media may yield
methanol (ester), acetic acid (upon oxidative carbonylation of methane), and formic
acid (deep oxidation of methane).
There are grounds to assume that the oxidation of methane to methanol (methyl
trifluoroacetate) in the presence of rhodium–iodide–chloride (Rh–I–Cl) and
rhodium–copper–chloride (Rh–Cu–Cl) catalytic systems in aqueous trifluoroacetic
acid proceeds largely by mechanism А [12–14] while the concomitant oxidative
carbonylation of methane to acetic acid, by mechanism А is hard to explain.
In this communication, we will overview the relevant experimental evidences for
formation of Rh–alkyl intermediates under the action of the above Rh-containing
catalytic systems.
2. Experimental
The following chemicals were used in our experiments: RhCl3(H2O)x (34.5 wt %
Rh), NaCl and CuO (reagent grade), H2O (bidistillate), AcfOH (Aldrich, extra pure),
PrfCOOH (Aldrich, extra pure) and pyridine (reagent grade, distilled). Dioxane
(reagent grade) was boiled over Na for 2 h and then distilled. Alkyl trifluoroacetates
and heptafluorobutyrates (used in chromatographic calibrations) were prepared by
esterification and distilled. Reaction products were identified by gas chromatography
and gas chromatography/mass spectrometry. Copper trifluoroacetate Cu(OAcf)2 was
obtained by dissolution of CuO in boiling AcfOH. Filtered-out Cu(OAcf)2 crystals
were dried in vacuum and stored in an exsiccator. Gaseous reagents—CO (99.9%
pure), CH4 (99.8%), C3H8 (99.8%), O2 (99.9%), and H2 (brand A)—were used
without further purification. Our experimental and analytical procedures are
described in detail elsewhere [12–15].
3. Results and discussion

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According to mechanism A, the activation and oxidation of methane takes place
without formation of intermediate complexes with an Rh–methyl bond. However, the
formation of acetic acid (Scheme 2) requires the carbonylation of Rh–methyl
intermediate.
In [18–20] it is assumed that mechanism B is in effect and dioxygen only plays a
role of reoxidant for Pd(0) and Pd(II), despite the fact that Pd(II) compounds are
incapable of oxidizing alkanes by thermodynamic considerations. According to
literature data and our previous results [9, 10e, 12–14], the role of oxidant can be
readily played by hydrogen peroxide or its equivalents such as hypoiodic acid and
hydroperoxides of Cu (II) and Fe(III) formed under the action of reducing agents
(H2, CO) on dioxygen. In the presence of Rh and Pd complexes, the formation of
active oxidants takes place with involvement of co-catalysts (compounds of I, Cu,
and Fe).
The important role of Rh–alkyl intermediates in realization of mechanism B is
supported by the following observations: (i) formation of alkyl chlorides; (ii)
induction period at elevated concentration of Cl–; (iii) formation of acetic acid upon
oxidative carbonylation of methane, and (iv) positional selectivity in oxidation of
propane.
In case of Rh and Pd catalysts, the process takes place with involvement of co-
catalysts (I-, Cu-, and Fe-containing compounds). Catalytic oxidation of alkanes to
alkyl chlorides (RCl) by mechanism B (reaction 1) may proceeds only via metal–
alkyl intermediate followed by reductive elimination of RCl from the inner
coordination sphere [2–4].
LnM(Cl)R → LnM + RCl
(1)
The oxidation of alkanes was found to yield alkyl chlorides in the presence of
Rh–I–Cl and Rh–Cu–Cl catalytic systems, whereas the oxidation of n-butane in the
presence of Rh–Cu–Br catalytic system gave butyl bromides. The yield of target
products in oxidation of methane in AcfOH/H2O attained a maximum at [Cl–] = 0.02
g-ion L–1 and then gradually decreased with increasing [Cl–] [13]. For catalytic
system Rh–Cu–Cl in aqueous heptafluoro butyric acid a maximal yield of propane
oxidation products is observed at [Cl–] ≈ 0.06 g-ion L–1 .The oxidation of propane
was almost completely suppressed at [Cl–] = 0.105 g-ion L–1 (Fig. 1).

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Fig. 1. Yield of oxidation products [P] —(∆) n-Pr-OC(O)Prf, (●) i-PrOC(O)Prf, and
(■) acetone—vs. concentration [Cl–] of chlorine ions: [RhCl3] = 5  10–3 М,
[Cu(OAcf)2] = 0.1 M, C3F7COOH/H2O = 4/1 (v/v), T = 80C, 12 mmol propane, 8
atm О2, 16 atm СО, 40 atm He.
At some certain Cl– concentrations, the kinetic curves exhibit an induction
period in the oxidative functionalization of both methane (Fig. 2) and propane (Fig.
3). At [Cl–] = 0.03 g-ion L–1, the oxidation of propane proceeds without induction
period (Fig. 4). The presence of induction period can be associated with reprocessing
of excessive amount of Cl– ions into respective alkyl chlorides via reaction (1). The
induction period is most pronounced at [Cl–] = 0.2 g-ion L–1 (curves 3, 4 in Fig. 2).

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Fig. 2. Yield of oxidation products as a function of time: [RhCl3] = 5  10–3 М,
[Cu(OAcf)2] = 0.1 M, C3F7COOH/H2O = 4/1 (v/v), T = 90C, 60 atm methane, 5.6
atm О2, 18.4 atm СО. Curves 1, 2: [Cl–] = 0.10 g-ion L–1, (○) CH3OC(O)Prf, (□)
AcOH; curves 3,4: [Cl–] = 0.20 g-ion L–1, () CH3OC(O)Prf, (■) AcOH.
Our previous results [12] give grounds to assume that, in Rh–I–Cl catalytic
system, the role of active oxidant is played by the HOI intermediate formed in
reaction (2).
O2 + 2HI → 2HOI
(2)
(3)
(4)
½O2 + HI → I2 + H2O
I2 + CO + H2O → 2HI + CO2
In the above scheme, reaction (3) is a side one, while reaction (4) of HI
regeneration is catalyzed by Rh complexes. A most likely catalytic cycle for
formation of acetic acid in this system is illustrated in Scheme 3.

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Scheme 3. Оxidative carbonylation of methane in the presence of catalytic system
Rh–I–Cl; X = Cl–, I–.
In the Rh–Cu–Cl system, one of active two-electron oxidants is copper
hydroperoxide [13] formed through reaction (5).
Cu2Cl2 + ½O2 + H2O → 2Cu(OOH)Cl
(5)
In the process of coupled oxidation of alkane and CO, copper hydroperoxide is
transformed into CuCl2. Copper(I) is regenerated from CuCl2 via reaction (6)
catalyzed by Rh complexes. A tentative catalytic cycle of the process is represented
in Scheme 4.
2CuCl2 + CO + H2O → Cu2Cl2 + CO2 + 2HCl
(6)

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Scheme 4. Oxidative carbonylation of methane in the presence of catalytic system
Rh–Cu–Cl.
The stages of reductive elimination of CH3C(O)Cl from intermediate 6 and its
hydrolysis are given by analogy with the Monsanto scheme for synthesis of acetic
acid [21, 22]. Alternative route is also possible: hydrolysis of intermediate 6 to
compound 7 followed by reductive elimination yielding CH3COOH. Intermediate
acetyl chloride cannot be detected because of rapid hydrolysis.
In Schemes 3, 4, compounds 1 and 4 are the -complexes in which methane gets
activated, while compounds 2 and 5 are the methyl complexes of Rh. Final product
(CH3COOH) is formed via intermediate acyl complexes 3 and 6, 7.

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Fig. 3. Yield of oxidation products [P]—(∆) n-Pr-OC(O)Prf, (●) i-PrOC(O)Prf, and
(■) acetone—as a function of time: [RhCl3] = 5  10–3 М, [Cu(OAcf)2] = 0.1 M,
[NaCl] = 0.08 M, [Cl–] = 0.095 g-ion L-1, C3F7COOH/H2O = 4/1 (v/v), T = 80C, 12
mmol propane, 8 atm О2, 16 atm СО, 40 atm He.
In contrast to the catalysts active by mechanism B, the above systems do not
catalyze the H/D exchange between alkanes and medium. At the first glance, this is
in contradiction with the formation of Rh–alkyl intermediates per se. However, this
contradiction can be resolved if we take into account some specific features of
methyl carbonyl Rh complexes. According to [21, 22], in the synthesis of acetic acid
in the presence of Rh(CO)2I2–HI system in AcfOH/HI/H2O, the equilibrium constant
for formation of acetyl complex is 3.2  103 at 35C, but heating was found to yield
methyl iodide rather than methane. For this reason, the Rh–methyl complex in our
systems can be expected to transform into Rh–acyl and then into acetic acid rather
than get subjected to hydrolysis (or acidolysis) followed by the formation of
methane.

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Fig. 4. Yield of oxidation products [P]—(∆) n-PrBuf, (●) i-PrBuf, and (■) acetone—
as a function of time: [RhCl3] = 5  10-3 М, [Cu(OAcf)2] = 0.1 M, [NaCl] = 0.015 М,
[Cl–] = 0.03 g-ion L-1, C3F7COOH/H2O = 4/1 (v/v), T = 80C, 12 mmol propane, 8
atm О2, 16 atm СО, 40 atm He.
Upon replacement of methane by methanol or methyl iodide in catalytic systems
Rh–I–Cl and Rh–Cu–Cl, no formation of acetic acid is observed; therefore the initial
stages of the oxidative carbonylation of methane differ from those of methanol by
Monsanto method [21, 22]. Also there was no isotopic exchange between propane
(C3H8) and PrfCOOD/D2O medium. This can hardly be explained by the oxidative
carbonylation of propane. In the presence of Rh–I–Cl и Rh–Cu–Cl catalytic systems,
occurrence of this reaction was observed neither in AcfOH/H2O nor in PrfCOOH–
H2O media [12, 13, 17]. It can be assumed that during oxidation of С2–С4 alkanes the
alkyl intermediates (even if formed) will sooner transform into alcohols (esters)
rather than yield alkanes.
For oxidation of propane, it seems urgent to analyze positional selectivity PS for
the catalytic systems under consideration. For oxidation of propane in the Shilov
system, the ratio of rates for formation of n-propanol to i-propanol is rn /ri = 3 [3].
Position selectivity (PS) taking into account the number of terminal and inner C–H
bonds can be writes as
(7)

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(8)
For the Shilov system, we obtain PSn/PSi = 1. Therefore, in the Shilov system
(mechanism B), all C–H bonds in propane are equivalent. Similar calculations for
oxo complex of Fe(IV) with polydentate nitrogen-containing ligands (oxygenase
model, mechanism A) gave PSn/PSi = 0.021 [23]. For mechanism A, steric factors
seem to be of lesser importance compared to the thermodynamic ones. In view of
this, propane reacts at weaker methylene C–H bonds. Our systems occupy some
intermediate position: PSn/PSi = 0.27. Note also that in the oxidation in the Shilov
system of n-alkanes with C ≥ 5, the reactivity of terminal C–H bonds become higher
than that of inner C–H bonds. It is assumed that the inner C–H bonds become less
accessible because of convolution of n-alkanes in globules [3]. Steric accessibility of
C–H bonds in methylene groups of alkanes was also considered in [24].
From a relationship (R) between the rates for formation of acetic acid and
methanol at the oxidative carbonylation and oxidation of methane, we can estimate a
contribution from mechanism B into the activation of methane. For the Rh–Cu–Cl
system in AcfOH/H2O R ≈ 0.2 [13], whereas for the Rh–I–Cl system R ≈ 0.17 [12]
and for that in PrfCOOH/H2O R ≈ 0.13 [17]. A contribution from mechanism B for
the oxidation of propane is impossible to evaluate so far. But judging from the
following sequence of PSn/PSi: for the systems—Pt(II)–Pt(IV) (PSn/PSi = 1) > Rh–
Cu(I)–Cl (PSn/PSi = 0.27) > LnFe=O (PSn/PSi =0.021)—mechanism B seems
operative at least partially.
4. Conclusions
The activation and oxidative functionalization of alkanes in the presence of
homogeneous catalytic systems containing rhodium compounds and redox catalysts
in aqueous perfluorinated acids proceed largely by outer-sphere mechanism А to give
esters (alcohols). However, some reported results—formation of acetic acid via
oxidative carbonylation of methane, formation of alkyl chlorides, kinetic data—
provide evidence in favor of partial realization of inner-sphere mechanism B.
It looks likely that the inhibition of mechanism A may in principle make
predominant mechanism B with selective synthesis of acetic acid from methane and
carbon monoxide. This can be achieved by varying synthesis conditions and using
hard-oxidizable ligands to stabilize intermediates with an Rh–Me bond.

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