Stereoselective alkane oxidation with meta-chloroperoxybenzoic acid (MCPBA) catalyzed by organometallic cobalt complexes

Article abstract of DOI:10.3390/molecules21111593

Cobalt pi-complexes, previously described in the literature and specially synthesized and characterized in this work, were used as catalysts in homogeneous oxidation of organic compounds with peroxides. These complexes contain pi-butadienyl and pi-cyclopentadienyl ligands: [(tetramethylcyclobutadiene)(benzene)cobalt] hexafluorophosphate, [(C₄Me₄)Co(C₆H₆)]PF₆ (1); diiodo(carbonyl)(pentamethylcyclopentadienyl)cobalt, Cp?Co(CO)I₂ (2); diiodo(carbonyl) (cyclopentadienyl)cobalt, CpCo(CO)I₂ (3); (tetramethylcyclobutadiene)(dicarbonyl)(iodo) cobalt, (C₄Me₄)Co(CO)₂I (4); [(tetramethylcyclobutadiene)(acetonitrile)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [(C₄Me₄)Co(bipy)(MeCN)]PF₆ (5); bis[dicarbonyl(B-cyclohexylborole)]cobalt, [(C₄H₄BCy)Co(CO)₂]₂ (6); [(pentamethylcyclopentadienyl)(iodo)(1,10-phenanthroline)cobalt] hexafluorophosphate, [Cp?Co(phen)I]PF₆ (7); diiodo(cyclopentadienyl)cobalt, [CpCoI₂]₂ (8); [(cyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [CpCo(bipy)I]PF₆ (9); and [(pentamethylcyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [Cp?Co(bipy)I]PF₆ (10). Complexes 1 and 2 catalyze very efficient and stereoselective oxygenation of tertiary C-H bonds in isomeric dimethylcyclohexanes with MCBA: cyclohexanols are produced in 39 and 53% yields and with the trans/cis ratio (of isomers with mutual trans- or cis-configuration of two methyl groups) 0.05 and 0.06, respectively. Addition of nitric acid as co-catalyst dramatically enhances both the yield of oxygenates and stereoselectivity parameter. In contrast to compounds 1 and 2, complexes 9 and 10 turned out to be very poor catalysts (the yields of oxygenates in the reaction with cis-1,2-dimethylcyclohexane were only 5%-7% and trans/cis ratio 0.8 indicated that the oxidation is not stereoselective). The chromatograms of the reaction mixture obtained before and after reduction with PPh₃ are very similar, which testifies that alkyl hydroperoxides are not formed in this oxidation. It can be thus concluded that the interaction of the alkanes with MCPBA occurs without the formation of free radicals. The complexes catalyze oxidation of alcohols with tert-butylhydroperoxide (TBHP). For example, tert-BuOOH efficiently oxidizes 1-phenylethanol to acetophenone in 98% yield if compound 1 is used as a catalyst.

Full text of DOI:10.3390/molecules21111593

molecules
Article
Stereoselective Alkane Oxidation with
meta-Chloroperoxybenzoic Acid (MCPBA) Catalyzed
by Organometallic Cobalt Complexes †
Georgiy B. Shul’pin 1 *, Dmitriy A. Loginov , Lidia S. Shul’pina , Nikolay S. Ikonnikov ,
,2,
3
3
3
3
3
3
3
Vladislav O. Idrisov , Mikhail M. Vinogradov , Sergey N. Osipov , Yulia V. Nelyubina and
Polina M. Tyubaeva 2
1
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina, dom 4,
Moscow 119991, Russia
2
Chair of Chemistry and Physics, Plekhanov Russian University of Economics, Stremyannyi pereulok,
dom 36, Moscow 117997, Russia; popov.AnA@rea.ru
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ulitsaVavilova,
3
dom 28, Moscow 119991, Russia; dloginov@ineos.ac.ru (D.A.L.); shulpina@ineos.ac.ru (L.S.S.);
ikonns@ineos.ac.ru (N.S.I.); vladchem32@gmail.com (V.O.I.); mmvngrdv@gmail.com (M.M.V.);
osipov@ineos.ac.ru (S.N.O.); unelya@xrlab.ineos.ac.ru (Y.V.N.)
*
†
Correspondence: shulpin@chph.ras.ru; Tel.: +7-495-939-7317; Fax: +7-495-651-2191
This work was partly presented at the 16th International Congress on Catalysis, Beijing, China, 2016,
lecture OF24.
Academic Editor: Derek J. McPhee
Received: 23 October 2016; Accepted: 16 November 2016; Published: 22 November 2016
Abstract: Cobalt pi-complexes, previously described in the literature and specially synthesized
and characterized in this work, were used as catalysts in homogeneous oxidation of organic
compounds with peroxides. These complexes contain pi-butadienyl and pi-cyclopentadienyl
ligands: [(tetramethylcyclobutadiene)(benzene)cobalt] hexafluorophosphate, [(C Me )Co(C H )]PF
6
4
4
6
6
(
(
1); diiodo(carbonyl)(pentamethylcyclopentadienyl)cobalt, Cp*Co(CO)I2 (2); diiodo(carbonyl)
cyclopentadienyl)cobalt, CpCo(CO)I2
(3
);
(tetramethylcyclobutadiene)(dicarbonyl)(iodo)
0
cobalt, (C Me )Co(CO) I (
4
); [(tetramethylcyclobutadiene)(acetonitrile)(2,2 -bipyridyl)cobalt]
4
4
2
hexafluorophosphate, [(C Me )Co(bipy)(MeCN)]PF (
5
); bis[dicarbonyl(B-cyclohexylborole)]cobalt,
(6); [(pentamethylcyclopentadienyl)(iodo)(1,10-phenanthroline)cobalt]
4
4
6
[
(C H BCy)Co(CO) ]
4
4
2 2
hexafluorophosphate, [Cp*Co(phen)I]PF6
(7); diiodo(cyclopentadienyl)cobalt, [CpCoI2]2
0
(
(
8
); [(cyclopentadienyl)(iodo)(2,2 -bipyridyl)cobalt] hexafluorophosphate, [CpCo(bipy)I]PF6
0
9);
and [(pentamethylcyclopentadienyl)(iodo)(2,2 -bipyridyl)cobalt] hexafluorophosphate,
and catalyze very efficient and stereoselective oxygenation of
[
Cp*Co(bipy)I]PF6 (10). Complexes
1
2
tertiary C–H bonds in isomeric dimethylcyclohexanes with MCBA: cyclohexanols are produced in 39
and 53% yields and with the trans/cis ratio (of isomers with mutual trans- or cis-configuration of
two methyl groups) 0.05 and 0.06, respectively. Addition of nitric acid as co-catalyst dramatically
enhances both the yield of oxygenates and stereoselectivity parameter. In contrast to compounds
1
and 2, complexes 9 and 10 turned out to be very poor catalysts (the yields of oxygenates in the
reaction with cis-1,2-dimethylcyclohexane were only 5%–7% and trans/cis ratio 0.8 indicated that
the oxidation is not stereoselective). The chromatograms of the reaction mixture obtained before
and after reduction with PPh are very similar, which testifies that alkyl hydroperoxides are not
3
formed in this oxidation. It can be thus concluded that the interaction of the alkanes with MCPBA
occurs without the formation of free radicals. The complexes catalyze oxidation of alcohols with
tert-butylhydroperoxide (TBHP). For example, tert-BuOOH efficiently oxidizes 1-phenylethanol to
acetophenone in 98% yield if compound 1 is used as a catalyst.
Molecules 2016, 21, 1593; doi:10.3390/molecules21111593
www.mdpi.com/journal/molecules

Molecules 2016, 21, 1593
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Keywords: alkanes; alcohols; metal complexes; X-ray analysis; meta-chloroperoxybenzoic acid
MCPBA); stereoselective oxidation
(
1
. Introduction
Selective functionalization of C–H bonds in hydrocarbons and other compounds is a very real aim
of contemporary catalytic science [
catalysts in oxygenation of hydrocarbons as well as alcohols [
to catalyze various transformations of organic compounds [15
1
–
4
]. Numerous transition metal complexes play roles of efficient
14]. Derivatives of cobalt are known
24]. Molecular oxygen, hydrogen
5–
–
peroxide, tert-butyl hydroperoxides, and peroxyacids were used as oxidants in these oxygenations.
Certain complexes of transition metals have been previously reported to oxidize organic compounds
including alkanes with meta-chloroperoxybenzoicacid (MCPBA [25]) [25
complexes of cobalt [21 24 38], manganese [26 27 34 36 44], iron [28], nickel [30
vanadium [40]). Earlier, Nam [15 16] reported that alkane oxidation with MCPBA catalyzed by some
cobalt compounds proceeds stereoselectively. It is interesting to check the oxidation various cobalt
organometallic complexes bearing ligands of different types, -cyclobutadienyl, cyclopentadienyl,
-arene, carbonyl, amine, etc. derivatives, and compare both their activity in the reaction and
–
44] (see, for example,
,
,
,
,
,
,
–
33 35 37 39 42 43] and
,
,
,
,
,
,
π
π
stereoselectivity. It is also attractive to find any dependence of the product yield and stereoselectivity
parameter on the structure of the particular complex. Complexes containing various ligands in different
compositions have been chosen.
In the present work, we studied oxidation of alkanes and alcohols with peroxides catalyzed
by certain organometallic derivatives of cobalt. Many of the complexes have been synthesized and
characterized in this work for the first time.
2
. Results and Discussion
2
.1. Catalysts, Substrates and Oxidants
We used various cyclobutadienyl and cyclopentadienyl derivatives of cobalt as catalysts for
oxidation of organic compounds with peroxides. Structural formulae of the catalysts are depicted in
Scheme 1.
2
.2. Syntheses of Catalysts
Syntheses of new complexes obtained in this work are described in Section 3.2. Complex
[
CpCo(bipy)I]PF (
9
) was prepared in high yield by the described in the literature method but
with the usage of the improved counterion-exchange procedure (see Section 3.2). The related
pentamethylated complexes with N,N-ligands, [Cp*Co(phen)I]PF ( ) and [Cp*Co(bipy)I]PF (10),
6
7
6
6
were synthesized in similar way by reactions of Cp*Co(CO)I (
2
) with 1,10-phenanthroline and
2
0
2
,2 -bipyridyl. The cyclobutadiene complex [(C Me )Co(bipy)(MeCN)]PF (
5
) was prepared by
4
4
6
0
photochemical replacement of benzene ligand in [(C Me )Co(C H )]PF (
1
) by 2,2 -bipyridyl in
4
4
6
6
6
acetonitrile solution. All cationic complexes described here were isolated as salts with the
−
PF6 anion. All catalysts are indefinitely stable in air in solid state.
2
.3. Structures of Catalysts
Complexes , and 10 were investigated by X-ray diffraction. Their molecular structures, bond
7, 9
lengths and angles as well as crystallographic data and structure refinement parameters are given in
Figures 1–3 as well as in Section 3.3.
In the case of
All complexes have a piano-stool geometry. The bi-pyridine ligand in
the dihedral angle formed by the intersection of the planes defined by the pyridyl rings is equal to
9, the symmetry-independent part of the unit cell contains two formula units.
9
and 10 is almost planar;

Molecules 2016, 21, 1593
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◦
5
and
.83 (6.55) and 8.78 , respectively. The Co···C distance in pentamethylated complexes 10 (1.691 Å)
5
7
(1.687 Å) is somewhat longer than the corresponding distance in the unsubstituted derivative 1
(
1.667 and 1.683 Å, average 1.675 Å); this can be explained by sterical effect of five methyl groups. The
length of the Co–I bond in the three complexes 7, 9 and 10 varies in the range 2.5718(13)–2.5798(4) Å.
CH
3
PF
CH
3
6
CH
3
H
3
C
H
CH
3
CH
3
CH
3
3
C
I
CH
3
Co
2
Co
3
Co
1
I
CO
CO
I
I
PF
CH3
6
CH
3
CH
3
CH3
CH
3
CH
3
CH3
CH
3
Co
4
Co
NCCH
3
I
CO
OC
N
N
5
CH
3
PF
6
H
3
C
H
CH
3
3
C
I
CH
N
3
Co
N
B
Co
8
Co
6
2
I
I
2
OC
CO
7
CH
3
PF
6
PF
6
H
3
C
H
CH
3
3
C
I
CH
3
Co
Co
10
I
N
N
N
N
9
Scheme 1. Structural formulae of the catalysts used in this work.
−
Figure 1. Structure of cation [Cp*Co(phen)I]
(
7
). Atoms are represented by 50% thermal ellipsoids.
◦
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [ ]: Co1—C11 2.059(3),
Co1—C12 2.079(3), Co1—C13 2.061(3), Co1—C14 2.099(3), Co1—C15 2.097(3), Co1—N1 1.958(2),
Co1—N2 1.968(2), Co1—I1 2.5798(4), and N1—Co1—N2 82.85(9).

Molecules 2016, 21, 1593
4 of 17
−
(9). Atoms are represented by 50% thermal ellipsoids.
Figure 2. Structure of cation [CpCo(bipy)I]
◦
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [ ] for the first/second
symmetry-independent species: Co1—C11 2.046(17)/2.049(17), Co1—C12 2.06(2)/2.050(16),
Co1—C13 2.056(17)/2.063(17), Co1—C14 2.127(17)/2.061(16), Co1—C15 2.072(19)/2.045(17), Co1—N1
1.932(14)/1.936(15), Co1—N2 1.966(13)/1.956(17), Co1—I1 2.574(3)/2.575(2), and N1—Co1—N2
8
2.2(5)/82.7(6).
−
Figure 3. Structure of cation [Cp*Co(bipy)I]
(
10). Atoms are represented by 50% thermal ellipsoids.
◦
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [ ]: Co1—C11 2.048(10),
Co1—C12 2.066(10), Co1—C13 2.074(12), Co1—C14 2.095(10), Co1—C15 2.065(11), Co1—N1 1.965(8),
Co1—N2 1.954(8), Co1—I1 2.5718(13), and N1—Co1—N2 81.6(3).
The main geometric parameters of 10 resembles those in a recently characterized chloride analog
◦
[
Cp*Co(bipy)Cl]PF6 [45]. For example, the dihedral angle between the Cp and bipy planes in 10 (38.2 )
◦
◦
◦
is close to that in [Cp*Co(bipy)Cl]PF (41.5 ) but smaller than in 9 (average 50.1 ) and 7 (49.2 ).
6
2
.4. Determination of Alkyl Hydroperoxides by GC before and after Reduction with PPh3
Earlier, one of us developed a convenient method for the analysis of mixtures obtained in alkane
oxidation with molecular oxygen or peroxides [46
oxidation leads to the formation of three main products (alkyl hydroperoxide, alcohol and ketone).
If a reaction solution is injected directly (see, for example, [57 62]) to the chromatograph, the alkyl
–55] (the Shul’pin method [56]). In many cases,
–
hydroperoxide ROOH (if present) decomposes in the injector and/or column to produce the alcohol
and ketone in approximately equal amounts.
However, if a sample of the reaction mixture is reduced by an excess of solid PPh (or thiourea)
3
during 10–20 min, the alkyl hydroperoxide is quantitatively transformed into the corresponding
alcohol. Comparing concentrations of the alcohol and ketone before and after treatment with PPh3,
we can qualitatively conclude on existence or non-existence of ROOH in the solution [63
the real concentrations of the alkyl hydroperoxide as well as of alcohol and ketone can be calculated
estimated) using the data obtained before and after reduction [72 81]. Selectivity of the oxidation
reaction can be characterized by the parameter [ROOH + A]/[K] where [ROOH + A] and [K] are
–71]. Moreover,
(
–

Molecules 2016, 21, 1593
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concentrations of alcohol and ketone after reduction with PPh , respectively [82
,
83]. However, this
3
parameter gives us absolutely no information on existence or non-existence of ROOH in the solution.
We must emphasize that, although treatment with PPh can be used in order to only remove from
3
the reaction mixtures peroxides (starting H O , tert-BuOOH, produced ROOH etc.) [84–91], the main
2
2
aim of the method under discussion is estimation of real concentrations of ROOH, alcohol and ketone
formed in the reaction. Chromatograms obtained only after reduction of the reaction mixture cannot be
used for determination of real concentration of each oxygenate. Nevertheless, such chromatograms
give valuable information on the total concentration of produced oxygenates.
2
.5. Oxidation of Alkanes and Alcohols with Peroxides
We have tested the catalytic effect of compound 1–10 in the reactions of alkanes and alcohols with
various oxidants. These reactions are summarized in Scheme 2.
Maximum
TON = 6.6
TOF = 3.3 h
+
2 2 2
H O /O
Catalyst
OOH
OH
−
1
2
−H O
COOOH
Catalyst
COOH
Maximum
Yield = 23%
TON = 106
+
Cl
−
1
TOF = 53 h
Cl
CH
3
CH
3
COOOH
Catalyst
COOH
CH
3 3
CH
Maximum
Yield = 53%
TON = 150
+
Cl
OH
−
1
TOF = 75 h
Cl
c-Dimethyl alcohol
OH CH
CH
CH
3
COOOH
Catalyst
COOH
3
Maximum
Yield = 27%
TON = 38
+
CH
3
Cl
3
−1
TOF = 14h
Cl
t-Dimethyl alcohol
3
H C
Maximum
3
H C
Yield = 98%
TON = 650
TOF = 50 h
CH OH
t-BuOOH/O
t-BuOH
C
O
−
1
+
2
Catalyst
−
Scheme 2. Examples of oxidation reactions studied in this work.
All complexes exhibited low activity in the oxidations with hydrogen peroxide. Some examples
are presented in Table 1. The oxidation affords predominantly cyclohexyl hydroperoxide because the
treatment of the reaction solution with PPh leads to the decrease of the ketone peak and increase of
3
the alcohol peak in GC [55] (compare entries 1 and 2 in Table 1).
Table 1. Oxidation of cyclohexane with hydrogen peroxide catalyzed by cobalt complexes 1.
Entry
Cat
1
Time (min)
Reduction with PPh3
Cyclohexanone (mM)
Cyclohexanol (mM)
TON TOF (h 1)
−
1
2
3
4
5
6
7
8
9
30
30
120
120
60
180
120
60
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
0.05
0
0.05
0
0.1
0.5
0
0.2
1.0
0.07
0.3
0.08
0.7
0.6
1.0
1.4
1.0
2.3
0.24
0.6
0.48
1.2
5 2
7
2
7
120
6.6
3.3
1
−4
◦
Conditions. Concentrations [Cat
]
0
= 5
×
10 M, [cyclohexane]
0
= 0.46 M, [H
2
O
2
]
0
= 1.5 M, 50 C. Solvent
2
MeCN, total volume of the reaction solution was 5 mL; Nitric acid (0.05 M) was added.

Molecules 2016, 21, 1593
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Hydrogen peroxide is not a good oxidizing agent for alcohols in our case of cobalt catalysts.
◦
For example, 1-phenylethanol (0.33 M) after 180 min at 60 C (other conditions are the same as in
Table 1) gave 5.0 mM of acetophenone (yield was only 1.5%) when the reaction was catalyzed by
complex
1. In contrast, tert-BuOOH efficiently oxidizes (yield 98% after 13 h) 1-phenylethanol if
compound 1 is used as a catalyst (Figure 4).
A
B
C
0.3
0.15
0.2
5
2
4
0
0
.2
.1
0.10
0.05
0.1
1
3
6
0
0
0
0
5
10
15
0
1
2
3
4
0
5
10
15
Time (h)
Time (h)
Time (h)
Figure 4. The 1-phenylethanol (0.33 M) oxidation with tert-butyl hydroperoxide (TBHP; 70%, aqueous,
.65 M): ( ) Accumulation of acetophenone with time in in the absence (curve 1) and in the presence
1
A
− −1
4
(curve 2) of compound
1
(5
×
10 M). The reaction gave TON 650 and TOF 50 h . Temperature was
◦
7
0 C, solvent was acetonitrile (total volume of the reaction solution was 5 mL); (
B
) The same oxidation
◦
catalyzed by complex
catalyzed by complexes 9 (curve 5) and 10 (curve 6) at 70 C.
4
at 50 (curve 3) and 70 C (curve 4); (
C
) The same oxidation ([TBHP] = 0.9 M)
0
◦
2
.6. Stereoselective Oxidation with meta-Chloroperoxybenzoic Acid (MCPBA)
It turned out that the third oxidant, MCPBA, is a weak oxidant in the reaction with
1
-phenylethanol. Thus, the oxidation catalyzed by complex 1 in the presence of HNO (for the
3
conditions, see Table 1) afforded after 3 h acetophenone in only 18% yield. The oxidation of cyclohexane
was also inefficient (Table 2). It should be noted that the chromatogram made before and after
reduction of samples with triphenylphosphine as well as the ketone/alcohol ratio is not changed
in the chromatograms. This indicates that cyclohexyl hydroperoxide is not formed in the course
of the oxidation (for this simple method, see References [46
presented here for comparison, demonstrates that cyclohexyl hydroperoxide, CyOOH, is produced in
the oxidation with MCPBA catalyzed by the salt Mn(ClO ) . In the course of the reaction, CyOOH
–56]). In contrast, Figure 5, which is
4
2
gradually decomposes to afford cyclohexanone and cyclohexanol.
Table 2. Oxidation of cyclohexane with meta-chloroperoxybeanzoic acid (MCPBA) catalyzed by
cobalt complexes 1.
Entry
Cat
Time (min)
HNO3 (mM)
Cyclohexanone (mM)
Cyclohexanol (mM)
Total Yield (%)
1
2
3
4
5
1
5
120
120
120
60
50
0
50
50
50
18
26
20
0.7
1.9
5
6
4.5
2.6
5.3
16
23
18
3
6
180
5
1
−4
Conditions. Concentrations [Cat
]
0
= 5
×
10 M, [cyclohexane] = 0.23 M, [MCPBA] = 0.26 M. Solvent MeCN,
0
0
total volume of the reaction solution was 5 mL. For entry 2, TON = 106, TOF = 53 h 1.
−

Molecules 2016, 21, 1593
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80
+
4
new portion
+
25
+
60
+
20
+
+
1
+
4
0
1
5
+
+
+
+
+
2
+
1
0
5
0
+
+
+
2
0
+
+
3
0
0
1 24
Time (h)
25
Figure 5. Accumulation of cyclohexanol (curve 1), cyclohexanone (curve 2), and cyclohexyl
hydroperoxide (curve 3), as well as TON (curve 4) in the reaction of cyclohexane (0.46 M) with MCPBA
◦
−4
(
0.025 M) in MeCN at 25 C catalyzed by Mn(ClO ) (5
×
10 M). Concentrations of cyclohexanone
and cyclohexanol were measured by GC twice: before and after the reduction of the reaction mixture
with PPh [46 56]. At the time moment denoted by an arrow, an additional amount (0.025 M) of
4
2
–
3
MCPBA was introduced into the reaction solution. After 25 h, TON was 76, and the yield of all
oxygenates based on MCPBA was 76%.
The most impressive results have been obtained in oxidations of alkanes containing tertiary C–H
bonds. Thus, cobalt complexes under consideration exhibited relatively high activity in the oxidation of
isomers of dimethylcyclohexane with MCPBA (Table 3). The chromatograms of the reaction solutions
made before and after reduction of samples with triphenylphosphine are very similar (see Table 3,
run 6) and the ketone/alcohol ratio is not changed in the chromatograms. It should be noted that
tertiary alkyl hydroperoxides easily decompose in hot injector with splitting C–H bonds and formation
of carbonyl derivatives [2]. If tertiary alkyl hydroperoxide is produced the chromatograms before and
after reduction with PPh should be different which is not our case. Thus, alkyl hydroperoxides are
3
not formed in the course of the oxidation (for the method, see References [46
alcohols attained 53% based on MCPBA (Table 3, run 22). In many cases shown in Table 3 the reaction
proceeds stereoselectively, the highest parameter trans/cis = 0.05–0.07 was attained for catalysts
and . The lowest ratios trans/cis have been achieved when nitric acid in low concentration has been
–56]). The yield of tertiary
1, 2
3
added to the reaction solution.
Table 3. Oxygenation of isomeric dimethylcyclohexanes with MCPBA catalyzed by cobalt complexes 1.
[
HNO3]
Time
(min)
t-Dimethyl-Alcohol c-Dimethyl-Alcohol
Yield
(%)
Ratio
trans/cis
Run
Cat
Substrate
(
mM)
(mM)
(mM)
1
2
3
4
5
6
7
8
9
None
None
None
1
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
trans-1,2-DMCH
trans-1,2-DMCH
trans-1,2-DMCH
0
0
50
0
0
50
50
50
50
0
0
30
120
30
0.13
2.9
0.07
3
4
2.4
3
5
8
19
19
26
0.17
4.1
0.5
23
29
52
11
19
12
8
0.2
5
0.76
0.74
0.14
0.13
0.14
0.05
0.30
0.26
0.70
2.4
0.4
19
24
39
10
17
14
19
19
19
30
120
120
30
120
30
30
120
30
2
3
3
4
10
11
12
8
1
2.4
19
50

Molecules 2016, 21, 1593
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Table 3. Cont.
[
HNO3]
Time
(min)
t-Dimethyl-Alcohol c-Dimethyl-Alcohol
Yield
(%)
Ratio
trans/cis
Run
Cat
Substrate
(
mM)
(mM)
(mM)
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
trans-1,2-DMCH
trans-1,2-DMCH
trans-1,2-DMCH
cis-1,4-DMCH
cis-1,4-DMCH
trans-1,4-DMCH
trans-1,4-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
trans-1,4-DMCH
trans-1,4-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
cis-1,2-DMCH
50
50
50
50
50
50
50
0
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
120
30
120
30
120
30
120
120
30
120
30
120
30
120
30
120
90
180
30
120
300
30
120
30
120
30
36
13
14
3
14
30
30
10
3.5
4
2
8
9
48
9
3
27
15
16
37
43
23
23
30
34
53
35
46
26
32
17
26
16
10
30
30
6
21
1.6
1.5
0.07
0.11
12
4
2
18
2
32
44
69
50
61
37
44
1
0.28
0.08
0.06
0.07
0.07
0.08
0.07
27
34
0.64
0.4
0.08
0.07
0.5
0.3
0.3
0.82
0.82
0.84
0.84
3
4
3.5
4
3
3
24
35
9
4
3
1
5
6
14
10
39
40
6
54
57
6
3
3
7
8
17
17
4.5
4.5
3
50
53
7
7
5
9
6
4
4
10
120
3
5
1
Reactions were performed in MeCN (total volume 2.5 mL). Concentrations: [Substrate]
0
= 140 mM,
◦
[MCPBA]
0
= 260 mM, catalyst [Cat
]
0
= 0.5 mM, 40 C. Samples were analyzed by GC after reduction with
3
PPh . t-Dimethyl and c-dimethyl alcohol are tertiary alcohols with trans- and cis-mutual orientation of methyl
groups, respectively. The ratio trans/cis is the ratio of these two isomers. Parameters for entries 22 and 35 were
−
1 2
TON = 150 and TOF = 75 h
PPh (t-dimethyl alcohol, 2.5 mM; t-dimethyl alcohol, 52 mM) did not practically distinguish from the data
obtained for the reduced sample; Trifluoroacetic acid (50 mM) was used instead of nitric acid; Salt LiNO
50 mM) was used instead of nitric acid.
; The chromatogram obtained for the reaction solution before reduction with
3
3
4
3
(
The data collected in Table 3 allow us to compare activities and stereoselectivity parameters
of different cobalt complexes. Table 3 indicates sufficiently different catalytic activities
and stereoselectivity parameters in the oxidations catalyzed by different cobalt complexes.
Addition of nitric acid greatly enhances the efficiency in all cases. In the hydroxylation of
cis-1,2-dimethylcyclohexane complexes
after 2 h). However, the oxidation catalyzed by complex
that in the presence of compound
1 and 8 exhibit the highest activity (yields are 39% and 53%
1
is much more selective (trans/cis = 0.05) than
0
8
(trans/cis = 0.3). Complexes
9
and 10 containing bulky 1,1 -dipyridil
ligands are very poor and non-stereoselective catalysts which is apparently due to sterical hindrance
around the reaction center. Previously, Nam et al. [16] in the oxidation catalyzed by Co(ClO ) used
4
2
large excess of cis-1,2-dimethylcyclohexane (1 mmol) over MCPBA (0.02 mmol) and obtained the
oxygenates in 80% yield based in the oxidant. We introduced into the reaction low amount of the
expensive substrate (140 mM) and larger concentration of MCPBA (260 mM). In our case, hydroxylated
products were obtained in high yield: 53% based on the substrate.
2
.7. Studies of Obtained Complexes by Electrospray Ionization/Mass Spectrometry (ESI-MS) Method
In order to determine possible organo-cobalt spices responsible for the catalytic oxidation of
alkanes by MCPBA, we investigated the reaction of the obtained cobalt complexes with HNO3
by the ESI-MS method, which is a common tool to gain mechanistic insights into homogeneous
metal-catalyzed reactions [92]. The primary reaction of containing bidentate nitrogen ligands iodide
complexes
7, 9, and 10 with HNO in acetonitrile leads to the replacement of iodide with either a nitrate
3
anion or an acetonitrile molecule. Thus, the ESI-MS spectrum of the reaction mixture with complex 7
2
+
(
Figure 6) reveals two main peaks corresponding to ionic fragments [Cp*Co(phen)(CH CN)]
3

Molecules 2016, 21, 1593
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+
(
m/z = 207.8) and [Cp*Co(phen)(NO )] (m/z = 436.0). It can be concluded on the basis of Table 3
3
(runs 33, 36–39) that complexes
7
,
9
, and 10 containing phenanthroline or dipyridine ligands are the
poorest catalysts in the alkane oxygenation.
Figure 6. The ESI-MS spectrum (positive-ion mode) of the reaction mixture of complex
7
and HNO3
◦
−4
10 M, [HNO ] = 0.04 M.
3 0
in acetonitrile solution at 55 C after 1 h. Concentrations: [
7]0 = 6.5
×
Total volume of the reaction mixture was 3 mL.
In contrast, interaction of HNO under the same reaction conditions with complexes
3
,
6
, and
results in complete decomposition of the complexes to form solvated Co salts detected by the
appearance of two characteristic peaks in the ESI-MS spectra with m/z = 152.3 and 243.5 (Figure 7)
3
2+
8
attributed to species [Co(MeCN)6]2 and [Co(MeCN) (NO )] , respectively. The catalysts
+
+
3
–
6
, and
exhibited in the oxidation moderate yield and stereoselectivity (see Table 3). Noteworthy, although
complexes 6, and heated with HNO exhibit the same patterns in the ESI-MS spectra they have
different behavior in the catalytic oxidation of cis-1,2-DMCH. The oxidation stereoselectivity in the
case of complex is lower than that for complexes and . It can be due to lower bulkiness of active
species formed from 8 in comparison with the case of pre-catalysts 3 and 6.
8
3
3
3,
8
3
8
3
6
Figure 7. The ESI-MS spectrum (positive-ion mode) of the reaction mixture of complex
6
and HNO3
◦
−4
in acetonitrile solution at 55 C after 1 h. Concentrations: [
6]0 = 6.5
×
10 M, [HNO ] = 0.04 M.
3 0
Total volume of the reaction mixture was 3 mL.
It is interesting that efficient in stereoselective oxidation complex
1
(Table 3, runs 4–19) was
◦
significantly more stable toward HNO . After heating in acetonitrile during 1 h at 55 C the ESI-MS
3
+
spectrum reveals the sole signal (m/z = 245) corresponding to the starting cation [Cb*Co(C H )] .
6
6
However, addition of MCPBA and prolonged heating for 1 h leads to appearance of extra signals in the

Molecules 2016, 21, 1593
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spectrum (Figure 8), which clearly indicates the interaction of
1
with MCPBA leading to the activation
+
of pre-catalyst [Cb*Co(C H )] .
6
6
Figure 8. The ESI-MS spectrum (positive-ion mode) of the reaction mixture of complex
1 with
◦
−4
HNO and MCPBA in acetonitrile solution at 55 C after 1 h. Concentrations: [
1]0 = 6.5
×
10 M,
3
[
HNO ] = 0.04 M, [MCPBA] = 0.015 M. Total volume of the reaction mixture was 3 mL.
3 0 0
2
.8. On the Mechanism of Catalyzed C–H Bond Oxidation with MCPBA
We demonstrated in Section 2.5 that the catalyzed alkane oxidation with H O affords
2
2
predominantly alkyl hydroperoxides because the treatment of the reaction solution with PPh leads to
3
the decrease of the ketone peak and increase of the alcohol peak in GC [55] (compare entries 1 and 2
in Table 1). The cyclohexane oxidation with MCPBA catalyzed by the manganese salt affords some
amount of cyclohexyl hydroperoxide (Figure 5). On the contrary, catalysis by cobalt complexes of the
oxidation of dimethycyclohexane with MCPBA led to the identical results for the product content
measured before and after reduction with PPh (see Table 3, run 6). Thus, we may conclude that alkyl
3
hydroperoxides are not formed in the oxidation with MCPBA and that free radicals are not involved
into the process. The reaction most probably proceeds via the concerted mechanism with peroxide
oxygen insertion into the alkane C–H bond [37,42,61].
3
. Materials and Methods
3
.1. General
The syntheses were carried out under an inert atmosphere in dry solvents. The isolation of
products was conducted in air. Hydrogen peroxide, TBHP, and MCPBA were used as oxidants.
1
Acetonitrile was employed as solvent in all oxidations. H-NMR spectra (
δ
in ppm) were recorded
on a Bruker Avance-400 spectrometer (Moscow, Russia) (400.13 MHz) relative to residual protons of
the solvents.
3
.2. Syntheses of Complexes
The isolation of products was conducted in air. Catalysts [(C Me )Co(C H )]PF (
1
) [93],
) [96],
) [97] have been described previously in the literature. Complex [CpCo(bipy)I]PF6 (
4
4
6
6
6
Cp*Co(CO)I2 (2) [94], CpCo(CO)I2 (3) [95], (C Me )Co(CO) I (4) [93], [(C H BCy)Co(CO) ] (6
4 4 2 4 4 2 2
and [CpCoI2]
n
(8
9)
was prepared by King’s method [95] in high yield, with the use of the improved counterion-exchange
procedure described below.
Compound [(C Me )Co(bipy)(MeCN)]PF (5). A solution of the complex [(C Me )Co(C H )]PF (1)
4 4 6 6 6
4
4
6
(
2
100 mg, 0.26 mmol) in acetonitrile (3 mL) was irradiated using mercury luminescent lamp (400 W) for
h with the use of a running water cooler for 2 h. 2,2 -Bipyridyl (44 mg, 0.28 mmol) was added to the
0
obtained solution. The reaction mixture was stirred overnight. The solvent was removed in vacuo.

Molecules 2016, 21, 1593
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The residue was extracted with CH Cl and ether was added. The precipitate was filtered off, washed
2
2
with ether, and dried in vacuo. Yield of compound [(C Me )Co(bipy)(MeCN)]PF (
5) obtained as
4
4
6
an orange solid was 63 mg (48%). Anal. Calc. for C H N CoF P:C, 47.16; H, 4.56; N, 8.25%. Found:
2
0
23
3
6
1
C, 46.31; H, 4.59; N, 8.52%. H-NMR (acetone-d ):
δ
= 8.99 (d, J = 5.2 Hz, 2 H, bipy), 8.66 (d, J = 8.0 Hz,
6
2
H, bipy), 8.29 (m, 2 H, bipy), 7.83 (m, 2 H, bipy), 1.31 (s, 12 H, C Me ).
4
4
Compound [Cp*Co(phen)I]PF6 (7). Benzene (5 mL) was added to a mixture of complex Cp*Co(CO)I2
(3
, 120 mg, 0.24 mmol) and phen (56 mg, 0.36 mmol). The reaction mixture was stirred overnight.
The solvent was removed in vacuo. The complex [Cp*Co(phen)I]I was extracted from the residue with
water. Then an excess of an aqueous KPF solution was added. The black precipitate that formed was
6
filtered off, washed with water, and dried in vacuo. Reprecipitation from acetonitrile with ether gave
complex [Cp*Co(phen)I]PF6 (
H, 3.59; N, 4.33%. Found: C, 41.03; H, 3.44; N, 4.51%. H-NMR (acetonitrile-d ):
7, 70 mg, 45%) as a black solid. Anal. Calc. for C H N CoIF P:C, 40.89;
22 23 2 6
1
δ
= 9.71 (d, J = 5.2 Hz,
3
2
H, phen), 8.72 (d, J = 8.0 Hz, 2 H, phen), 8.14 (m, 4 H, phen), 1.60 (s, 15 H, Cp*).
Compound [CpCo(bipy)I]PF ( ). Complex [CpCo(bipy)I]PF was prepared by King’s method [95
9
]
6
6
in high yield, with the use of the improved counterion-exchange procedure given below. Benzene
5 mL) was added to a mixture of complex CpCo(CO)I2 [95] ( ,120 mg, 0.28 mmol) and bipy (73 mg,
.55 mmol). The reaction mixture was stirred overnight. The precipitate of [CpCo(bipy)I]I was filtered
off, washed with benzene and ether, and dried in vacuo. The precipitate was extracted with water.
(
3
0
Then an excess of an aqueous KPF solution was added. The dark violet precipitate that formed was
6
filtered off, washed with water, and dried in vacuo. Reprecipitation from acetone with ether gave
1
complex [CpCo(bipy)I]PF6 (
9
, 126 mg, 86%) as a dark violet solid. H-NMR (acetone-d ):
δ
= 10.07
6
(
(
d, J = 5.2 Hz, 2 H, bipy), 8.54 (d, J = 8.0 Hz, 2 H, bipy), 8.33 (m, 2 H, bipy), 7.87 (m, 2 H, bipy), 6.18
s, 5 H, Cp).
Compound [Cp*Co(bipy)I]PF6 (10). Benzene (5 mL) was added to a mixture of complex Cp*Co(CO)I2
, 120 mg, 0.24 mmol) and bipy (47 mg, 0.36 mmol). The reaction mixture was stirred overnight.
The solvent was removed in vacuo. The complex [Cp*Co(bipy)I]I was extracted from the residue with
(2
water and methanol. Then an excess of an aqueous KPF solution was added. The black precipitate
6
that formed was filtered off, washed with water, and dried in vacuo. Reprecipitation from acetone
with ether gave complex [Cp*Co(bipy)I]PF (10, 47 mg, 32%) as a black solid. Anal. Calc. for
6
1
C H N CoIF P:C, 38.62; H, 3.70; N, 4.50%. Found: C, 38.69; H, 3.68; N, 4.46%. H-NMR (acetone-d ):
20
23
2
6
6
δ
= 9.61 (d, J = 5.6 Hz, 2 H, bipy), 8.58 (d, J = 8.0 Hz, 2 H, bipy), 8.35 (m, 2 H, bipy), 7.95 (m, 2 H, bipy),
1
.68 (s, 15H, Cp*).
3
.3. X-ray Diffraction Study
Crystals were grown by slow diffusion in a two-layer system of ether/petroleum ether mixture
with solution of complex in acetonitrile (for [Cp*Co(phen)I]PF6 (
) and [Cp*Co(bipy)I]PF6 (10)). X-ray diffraction data for 10 were collected with Bruker Apex2 DUO
diffractometer (Moscow, Russia) at 100 K and those for and with Bruker Apex 2 diffractometer
at 120 and 100 K, respectively, using graphite monochromated Mo-K radiation ( = 0.71073 Å,
-scans). The structures were solved by the direct method and refined by the full-matrix least-squares
7)) or acetone (for [CpCo(bipy)I]PF6
(9
7
9
α
λ
ω
2
against F in anisotropic approximation for non-hydrogen atoms. The positions of hydrogen atoms
were calculated, and they were refined in isotropic approximation in riding model. Crystal data
and structure refinement parameters for
performed using the SHELXTL PLUS 5.0 [98]. CCDC 1504665, 1504667 and 1504666 contain the
supplementary crystallographic data for , and 10, respectively. These data can be obtained free of
7, 9, and 10 are collected in Table 4. All calculations were
7, 9
charge via www.ccdc.cam.ac.uk/data_request/cif.
The molecular structures, bond lengths and angles as well as crystallographic data and structure
refinement parameters of the three catalysts
above and Table 4 below.
7, 9, and 10 obtained in this work are given in Figures 1–3

Molecules 2016, 21, 1593
12 of 17
Table 4. Crystallographic data and structure refinement parameters for 7, 9, and 10.
Compound
7
9
10
C20H23CoF IN P
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C22H23CoF IN P
C15H13CoF IN P
6 2
552.07
Orthorhombic
6
2
6
2
646.22
Triclinic
P-1
622.20
Monoclinic
Pna21
16.7546(14)
8.4636(7)
24.681(2)
P2 /n
1
8.2684(9)
11.4505(13)
12.4520(14)
87.800(2)
84.628(2)
77.714(2)
1146.7(2)
1.872
9.0966(18)
26.448(5)
9.2018(17)
-
90.574(4)
-
2213.7(7)
1.867
23.02
b (Å)
c (Å)
◦
α ( )
-
-
◦
β ( )
◦
γ ( )
-
3499.9(5)
2.095
3
V (Å )
Dcalc (g·cm 3)
−
Linear absorption, µ (cm 3)
−
22.27
28.98
F(000)
636
56
2128
52
1224
54
◦
2θmax,
Reflections measured
Independent reflections
Observed reflections (I > 2σ(I))
Parameters
20,367
5529 (Rint = 0.0416)
31,839
6888 (Rint = 0.0642)
28,418
4819 (Rint = 0.1797)
4730
303
6398
484
4320
307
R1
wR2
0.0301
0.0800
0.0620
0.1721
0.0739
0.2145
Goodness-of-fit
1.017
1.44 and −0.72
1.271
2.49 and −1.51
1.477
2.34 and −1.92
Largest diff. peak and hole (e Å 3)
−
3
.4. Catalytic Oxidation of Alkanes and 1-Phenylethanol
Typically, catalyst and the co-catalyst (nitric or trifluoroacetic acid) were introduced into the
reaction mixture in the form of stock solutions in acetonitrile. The reactions of alcohols and
hydrocarbons were carried out in air in thermostated Pyrex cylindrical vessels with vigorous stirring
and using MeCN as solvent. The substrate (alcohol or hydrocarbon) was then added and the reaction
started when the oxidant was introduced in one portion. (CAUTION: The combination of air or
molecular oxygen and peroxides with organic compounds at elevated temperatures may be explosive!).
1
The reactions with 1-phenylethanol were analyzed by H-NMR method (solutions in acetone-d ;
6
“
Bruker AMX-400” instrument, 400 MHz). Areas of methyl group signals were measured to quantify
oxygenates formed in oxidations of 1-phenylethanol. As we made previously, the samples obtained in
the alkane oxidation were typically analyzed twice (before and after their treatment with PPh ) by GC.
3
This method (comparison of chromatograms of the same sample obtained before and after addition of
PPh ) was proposed by Shul’pin earlier [46–56] and allows us to estimate real concentration of alkyl
3
hydroperoxide, ketone (aldehyde and alcohol) present in the reaction solution. Samples of the reaction
mixture were analyzed by GC (the chromatograph-3700; fused silica capillary column FFAP/OV-101
2
0/80 w/w, 30 m
×
0.2
µ
m
×
0.3 µm; helium as a carrier gas). Attribution of peaks was made by
comparison with chromatograms of authentic samples and by GC–MS (INEOS, Moscow, Russia).
Acknowledgments: The authors gratefully acknowledge support of the Russian Foundation for Basic Research
(projects 16-03-00254, and 16-29-10672).
Author Contributions: G.B.S. and D.A.L. conceived and designed experiments on synthesis and catalytic
application of cobalt compounds; D.A.L., V.O.I. and S.N.O. made synthesis of the complexes; L.S.S., N.S.I.,
M.M.V. and P.M.T. performed experiments on oxidations and analysis of products by GC and ESI-MS; Y.V.N.
carried out X-ray experiments and their analysis; and G.B.S. and D.A.L. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.

Molecules 2016, 21, 1593
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