Oxidation of aliphatic and aromatic C[sbnd]H bonds by t-BuOOH catalyzed by μ-nitrido diiron phthalocyanine

Article abstract of DOI:10.1016/j.molcata.2016.08.013

Low temperature selective transformation of alkanes to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines in combination with H₂O₂ have been recently identified as powerful oxidation catalysts for these challenging reactions due to the formation of ultra-high valent diiron oxo species PcFe(IV)μNFe(IV)[dbnd]O(Pc⁺[rad]). This very strong two-electron oxidizing species is generated from peroxo complex PcFe(IV)μNFe(III)[sbnd]O[sbnd]O[sbnd]R(Pc) (R[dbnd]H in the case of H₂O₂) via heterolytic O[sbnd]O bond cleavage. Therein we show that the evolution of the peroxo diiron complex depends on the peroxide structure. Using ^tBuOOH we have demonstrated the formation of an one-electron oxidizing PcFe(IV)μNFe(IV)[dbnd]O(Pc) and ^tBuO[rad] radical via homolytic O[sbnd]O cleavage of the peroxocomplex. The reactivity of the μ-nitrido diiron tetra-t-butylphthalocyanine ? ^tBuOOH catalytic system was investigated in the oxidation of different C[sbnd]H bonds in alkanes, olefins, aromatic and alkylaromatic compounds. The main products of cyclohexane oxidation were cyclohexanone and cyclohexanol whereas bicyclohexyl was formed in minor amounts even in the presence of O₂ and ^tBuOOH. Under optimal conditions, the turnover numbers of almost 5300 have been achieved.

Full text of DOI:10.1016/j.molcata.2016.08.013

Accepted Manuscript
Title: Oxidation of aliphatic and aromatic C<ce:glyph
name="sbnd"/>H bonds by t-BuOOH catalyzed by ␮-nitrido
diiron phthalocyanine
Author: Evgeny V. Kudrik Alexander B. Sorokin
PII:
S1381-1169(16)30337-5
DOI:
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Please cite this article as: Evgeny V.Kudrik, Alexander B.Sorokin, Oxidation
of aliphatic and aromatic CH bonds by t-BuOOH catalyzed by ␮-
nitrido diiron phthalocyanine, Journal of Molecular Catalysis A: Chemical
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Oxidation of aliphatic and aromatic C-H bonds by t-BuOOH
catalyzed by µ-nitrido diiron phthalocyanine
Evgeny V. Kudrik,a,b Alexander B. Sorokina*
aInstitut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), UMR 5256,
CNRS-Université Lyon 1, 2, av. A. Einstein, 69626 Villeurbanne Cedex, France
b
Institut of Macroheterocyclic Compounds, Ivanovo State University of Chemical Technology
7, av F. Engels; 153000 Ivanovo, Russia.
e-mail: alexander.sorokin@ircelyon.univ-lyon1.fr
The article is dedicated to Professor Georgii B. Shul’pin on the occasion of his 70th birthday.
1

Graphical Abstract
C H
homolytic cleavage
OtBu
.
OtBu
.
.
C
C
+
O2
Oxidation
products
FeIII
FeIV
FeIII
FeIV
C H
Highlights
 Formation of the diiron peroxo complex Fe(IV)=N–Fe(III)–OOtBu
 Homolytic cleavage of O–O bond leads to two active species reacting with C-H bonds
 Oxidation of cyclohexane, cyclohexene, toluene and benzene
Abstract
Low temperature selective transformation of alkanes to useful products continues to be an
important challenge in chemistry and industry. µ-Nitrido diiron phthalocyanines in combination
with H2O2 have been recently identified as powerful oxidation catalysts for these challenging
reactions due to the formation of ultra-high valent diiron oxo species
PcFe(IV)µNFe(IV)=O(Pc+.). This very strong two-electron oxidizing species is generated from
peroxo complex PcFe(IV)µNFe(IV)-O-O-R(Pc) (R = H in the case of H2O2) via heterolytic O-O
bond cleavage. Therein we show that the evolution of the peroxo diiron complex depends on the
t
peroxide structure. Using BuOOH we have demonstrated the formation of an one-electron
t
oxidizing PcFe(IV)µNFe(IV)=O(Pc) and BuO∙ radical via homolytic O-O cleavage of the
t
peroxocomplex. The reactivity of the µ-nitrido diiron tetra-t-butylphthalocyanine – BuOOH
catalytic system was investigated in the oxidation of different C-H bonds in alkanes, olefins,
aromatic and alkylaromatic compounds. The main products of cyclohexane oxidation were
2

cyclohexanone and cyclohexanol whereas bicyclohexyl was formed in minor amounts even in
the presence of O2 and tBuOOH. Under optimal conditions, the turnover numbers of almost
5300 have been achieved.
Keywords: oxidation, catalysis, alkanes, cyclohexane, phthalocyanine, µ-nitrido dimer,
hydroperoxides.
1. Introduction
Oxidation of aliphatic C-H bonds continues to attract a significant attention because selective
transformation of alkanes to oxygenated products is a direct synthetic approach to a variety of
valuable products and platform molecules. Since aliphatic C-H bonds are among the least
reactive in organic chemistry, their selective oxidation still constitutes a fundamental academic
and industrial challenge. This chemical transformation can be performed in a catalytic fashion
using different catalysts involving transition metal sites [1-19]. Inorganic oxides are widely used
for the activation of C-H bonds [5]. Noble metal species based on Pt, Pd, Au, Ag, Os, Ir, Rh [1,2]
as well as transition metal complexes including vanadium, chromium, iron, manganese, cobalt
and other sites [6-8] have been extensively studied for the C-H activation. Polynuclear copper
complexes and coordination polymers were also applied for the oxidation of alkanes [15]. In the
complimentary approach, bio-inspired catalytic systems [3,9,10] based on porphyrins [12-14],
phthalocyanine [11] and non-heme complexes [15-18] have been developed for the efficient and
selective oxidation of C-H bonds. For instance, aminopyridine iron [20] and manganese
complexes [21] oxidize aliphatic C-H bonds with a high regio- and stereoselectivity.
Although a range of synthetic molecular catalysts for the alkane oxidation has been
developed, there still remains much interest in systems that use abundant, inexpensive and
environmentally compatible metals such as iron in combination with available and cheap ligands.
In this context, phthalocyanine iron complexes are especially attractive because they are efficient
catalysts for many reactions including various oxidations [11,22] and can be readily prepared on
industrial scale. Catalytic studies of iron phthalocyanine complexes led to the discovery of the
remarkable catalytic properties of µ-nitrido diiron phthalocyanine complexes (FePc)2N (Fig. 1),
in which Fe(III) and Fe(IV) sites are connected by the bridging nitrogen atom to form a very
stable delocalized Fe-N-Fe unit with equivalent iron sites in the +3.5 oxidation state [23].
These binuclear complexes are particularly efficient in challenging oxidation reactions
including oxidation of methane [24-27], ethane [28] and even in the oxidative dehalogenation of
C-F, C-Cl and C-Br bonds [29-31]. Such remarkable reactivity has been achieved using
hydrogen peroxide as a terminal oxidant and can be associated with the formation of the ultra-
3

high valent diiron oxo species (L)FeIVNFeIV=O(L+.) (L = macrocyclic ligand, phthalocyanine or
porphyrin) showing the powerful oxidizing properties [23,32,33]. This short-lived species were
prepared using H2O2 or m-chloroperbenzoic acid as oxygen donors and were spectroscopically
characterized at very low temperatures [23]. Such ultra-high valent diiron oxo species are
competent even for the oxidation of the strongest C-H bonds in methane.
It is of great interest to investigate the catalytic properties of µ-nitrido diiron complexes
using other oxidants. Recently, we have shown that the µ-nitrido-bis(tetra-tert-
t
butylphthalocyaninatoiron) complex (FePctBu4)2N in combination with BuOOH in inert
atmosphere mediated the formation of C-C bonds in the hydroacylation of olefins by
acetaldehyde to furnish methyl ketones [34]. Interestingly, the (FePctBu4)2N – H2O2 system
didn’t show this reactivity suggesting different active species generated using H2O2 and
t
tBuOOH. The ability of (FePc)2N – BuOOH system to catalyze the C-C bond formation was
explained by the involvement of one-electron oxidized (Pc)FeIV(µ-N)FeIV(Pc)=O species and
tBuO∙ radical. Both species are capable of abstracting hydrogen atom from C-H bonds which
initiate formation of the substrate radicals and their coupling under anaerobic conditions [34]. In
the presence of air, this system should also exhibit oxidizing properties. In the present work, we
t
have evaluated (FePctBu4)2N in the aerobic oxidation of aliphatic C-H bonds using BuOOH as
the oxidant.
2. Experimental
2.1. Materials
Iron (II) 2,9(10),16(17),23(24)-tetra-tert-butylphthalocyanine was synthesized and purified
according to a published protocol [35]. µ-Nitridobis(tetra-tert-butylphthalocyaninatoiron(III,IV)
[24] was prepared as described previously. tert-Butyl hydroperoxide (70 % aqueous solution)
was purchased from Sigma-Aldrich. All other chemicals were obtained from Alfa Aesar or
Sigma-Aldrich and used as received. Adamantane-1,3-d2 was prepared according to published
procedure [36].
2.2. Instrumentation.
The reaction products were identified and quantified by GC-MS method (Hewlett Packard
5973/6890 system, electron impact ionization at 70 eV, He carrier gas, 30m x 0.25 mm HP-
INNOWax capillary column, polyethylene glycol (0.25 µm coating) and by GC technique
(Agilent 4890D chromatograph equipped with a flame ionization detector and a 30 m x 0.25 mm
VF-5 MS capillary column) using chlorobenzene standard. The UV-vis spectra of
1
phthalocyanine complexes were obtained with Agilent 8453 diode-array spectrophotometer. H
NMR spectra were recorded using a Bruker AM 250 spectrometer.
4

2.3. Catalytic procedures and product analysis
Oxidation of hydrocarbons substrates was performed in a 100 mL glass vessel under dioxygen or
argon atmosphere. Typically, the reactor was charged with 1 or 2 mL either neat substrate or 0.1
M substrate solution in MeCN containing 0.1 mM (FePctBu4)2N. The reactions were initiated by
addition of 70 % aqueous solution of tBuOOH. The reaction mixture was magnetically stirred at
25, 50 or 60°C for desired time. Reaction products were analyzed by GC-MS and GC directly or
using Ph3P method introduced by Shul’pin [37]. Chlorobenzene standard was added as 0.1 M
solution prior to analyses.
3. Results and Discussion
3.1. Oxidation of cyclohexane
Catalytic experiments were conducted in the presence of (FePctBu4)2N and tBuOOH either in the
neat substrates or in 0.1 M substrate solutions in MeCN. In the latter case 0.1 mol% catalyst
loading was applied. The oxidation of neat cyclohexane at 25°C in the presence of 0.1 mM
(FePctBu4)2N and 0.2 M tBuOOH occurred with the turnover frequency of 3.9 min-1. It should be
noted that the corresponding monomeric complex (FePctBu4)Cl was much less efficient in the
oxidation of cyclohexane providing cyclohexanol (Cy-OH) and cyclohexanone (Cy=O) in ~1:1
ratio with turnover number (TON) of 27. In addition, the (FePctBu4)Cl was decomposed after 20
min reaction.
The dependence of the initial rates of the cyclohexane oxidation expressed as total product
concentration after 1 h reaction was examined as a function of the (FePctBu4)2N concentration
(Fig. 2).
This dependence indicates the first order reaction in (FePctBu4)2N. The rate of the cyclohexane
t
oxidation didn’t depend on the BuOOH concentration between 0.05 M and 0.5 M in the
t
presence of 0.05 mM (FePctBu4)2N showing the zero order in BuOOH. At lower oxidant
concentration, [tBuOOH] significantly influenced the reaction rate.
Accumulation of the oxidation products with the reaction time is shown in Fig. 3.
Along with cyclohexanol (Cy-OH) and cyclohexanone (Cy=O) formed in the comparable
amounts, the formation of bicyclohexyl (Cy-Cy) was observed even in the presence of dioxygen
and high concentration of tBuOOH indicating the intermediate formation of cyclohexyl radicals.
The treatment of the reaction mixture with Ph3P according to Shul’pin method [37] didn’t
significantly change the product composition indicating that cyclohexyl hydroperoxide was not
present in the reaction mixture in significant amounts. The product of coupling of cyclohexyl
5

radicals represented 8.0 % of the reaction products while cyclohexanone and cyclohexanol
accounted for 50.4 and 41.6 %, respectively. The total concentration of products attained 79.2
mM after 6 h and 98.4 mM after 24 h. The total turnover number reached 1063 cycles per
catalyst molecule (bycyclohexyl originated from two cyclohexane molecules). The product yield
on tBuOOH was 59 %.
t
The oxidation of cyclohexane was more efficient at 50°C and in the presence of 0.5 M BuOOH
resulting in 1877 turnovers after 14 h reaction. Cy-OH, Cy=O and Cy-Cy were obtained in 72.7,
103.0 and 12.0 mM concentrations, respectively (the product distribution: Cy-OH : Cy=O : Cy-
Cy = 38.7 : 54.9 : 6.4). The product yield on tBuOOH was 67 %. Noteworthy, (FePctBu4)2N was
quite stable under reaction conditions. UV-vis analysis of the final reaction mixture was shown
that only 10 % of the complex was decomposed.
t
In the next experiment, BuOOH was added in three portions each introducing 0.25 M
concentration at the reaction times of 0, 3 h and 6 h. The reaction was further performed for
t
additional 17 h. The portion-wise addition of BuOOH resulted in the higher yields and TON
(Table 1).
The main oxidation product was Cy=O and the product distribution was rather constant during
the reaction. The total TON progressively increased in the course of the reaction to achieve
almost 5300 turnovers. Remarkably, the oxidation of neat cyclohexane resulted in 5.9 %
conversion. The yields of the oxidation products based on BuOOH were 68, 81 and 94 % after 3,
6 and 9 h, respectively, attaining 114 % in the end of the reaction. This result indicates
participation of O2 in the oxidation.
The catalytic performance of (FePctBu4)2N was also evaluated in the oxidation of 0.1 M
t
cyclohexane solution in MeCN. When 2 equiv of BuOOH was used, a 12 % Cy-H conversion
was observed but the TON was moderate attaining 87 turnovers. Cy=O and Cy-OH were
obtained in a 2.6 : 1 ratio.
3.2. Oxidation of toluene
t
The (FePctBu4)2N – BuOOH system mediated oxidation of neat toluene to benzyl alcohol,
benzaldehyde and benzoic acid with 10, 52 and 38 % selectivity, respectively. The total TON
achieved 1216. The trace amounts of ortho- and para-cresols, 2-methylbenzoquinone and
PhCH2–OOtBu coupling product were also formed. Thus, this catalytic system shows a clear
preference for aliphatic C-H oxidation over the oxidation of aromatic C-H bonds. It should be
noted that the oxidation of 0.1 M toluene solution in MeCN was less efficient: benzaldehyde and
benzoic acid were obtained with turnover numbers of 23 and 3, respectively. Taking into account
6

that the oxidation of benzyl alcohol (0.1 M solution in MeCN) provided benzaldehyde with a 28
% yield at 36 % conversion and total TON of 360, one can suggest that the initial cleavage of the
benzylic C-H bond is the limiting step.
3.3. Oxidation of adamantane. Determination of intramolecular kH/kD
Kinetics of the oxidation of 0.1 M adamantane solution in MeCN in the presence of 0.1 mM
t
(FePctBu4)2N and 0.25 M BuOOH (catalyst:substrate:oxidant = 1:1000:2500) at 50°C is shown
in Fig. 4.
Adamantane conversion of 67 % was achieved after 6 h. The oxidation occurred mostly at
tertiary C-H bond with 3°/2° parameter of 17 after correction on the number of C-H bonds. Total
TON attained 670 and 1-adamantanol, 2-adamantanone and 2-adamantanol were obtained with
57, 8 and 2 % yields, respectively.
In order to probe the mechanism of the alkane oxidation, we have investigated the kinetic isotope
effect (kH/kD) on the oxidation of adamantane-1,3-d2. This useful probe contains equivalent
tertiary C-H and C-D bonds and enables determination of intra-molecular kH/kD [36]. The
analysis of the isotopic composition of the adamantanol-1 provides a relatively high kH/kD value
of 3.67 ± 0.11 which didn’t change during 3 h of the reaction (Fig. 5).
3.4. Oxidation of cyclohexene
The oxidation of neat cyclohexene was studied in the presence of 0.2 mM (FePctBu4)2N and 0.25
t
M BuOOH at 60°C. The main oxidation products were 2-cyclohexen-1-ol, 2-cyclohexen-1-one,
3-tert-butylperoxocyclohexene and 3,3’-bicyclohexenyl while the epoxide was obtained only in
minor amount. The yields of the products and the product composition were strongly affected by
the presence or absence of dioxygen (Fig. 6).
When the reaction was performed under Ar, the total concentration of the products decreased
down to 198.9 mM compared with 535.6 mM obtained under air. Under air, allylic alcohol (23.3
%) and ketone (68.7 %) were the major oxidation products whereas 3-tert-
butylperoxocyclohexene and 3,3’-bicyclohexenyl accounted for 6.8 and 1.2 %. Under inert
atmosphere, the main reaction product was allylic peroxide (53.9 %) and the content of allylic
alcohol and ketone decreased to 11.1 and 19.2 %, respectively. Remarkably, the concentration of
2-cyclohexen-1-one dropped from 368 mM under air down to 38.2 mM under Ar. The
significant amount of 3,3’-bicyclohexenyl (15.8 %) arising from coupling of cyclohexenyl
radicals was formed. The total TON dropped from 2709 under air down to 1152 under Ar. The
t
yield of the oxidation products based on the BuOOH amount was 361 %. All these results
indicate the involvement of dioxygen in the oxidation of cyclohexene.
7

3.5. Oxidation of benzene
t
The catalytic activity of the (FePctBu4)2N - BuOOH system toward aromatic C-H bonds was
lower than that of (FePctBu4)2N – H2O2. The (FePctBu4)2N – H2O2 system oxidized benzene to
phenol with the TON of 66 [38]. The oxidation of neat benzene in the presence of 0.5 mM
t
(FePctBu4)2N and 0.35 M BuOOH at 60°C for 24 h resulted in the formation of 14.5 mM of
biphenyl and less than 1.5 mM of phenol. The combined TON achieved 32 turnovers. The
oxidation of a 1 : 1 C6H6 / C6D6 mixture provided biphenol with following isotopic composition:
47.7 % C6H5-C6H5, 45.0 % C6H5-C6D5 and 7.3 % of C6D5-C6D5 after 3 h. This isotopic
composition didn’t change after 24 h reaction. Thus, kH/kD in the oxidation of benzene was
calculated to be 2.36. Noteworthy, kH/kD in the oxidation of the C6H6 / C6D6 mixture by the
(FePctBu4)2N - H2O2 system at 60°C was determined to be 1.16 [38].
3.5. Mechanistic considerations
The present study shows that the (FePctBu4)2N – tBuOOH system efficiently oxidizes C-H bonds
of alkanes, olefins, alkyl aromatic and aromatic compounds in the presence of dioxygen. Under
anaerobic conditions, the same system was particularly efficient in the addition of CH3CHO to a
range of olefins whereas the conventional oxidation products were obtained in very minor
amounts [34]. Importantly, the (FePctBu4)2N – H2O2 system didn’t exhibit such a reactivity.
Several lines of evidence point out on the difference between these systems. Both oxidants are
activated via initial formation of negatively charged diiron peroxo complexes Fe-N-Fe–OOH
(A) and Fe-N-Fe–OOtBu (B) which were detected by ESI-MS technique [29,33] (Fig. 7).
To rationalize this different catalytic behaviour of (FePc)2N – H2O2 (strong oxidizing properties)
t
and (FePc)2N – BuOOH (strong C-C forming ability in aerobic conditions) systems, we have
proposed the different evolution of the corresponding µ-nitrido diiron peroxo species (Fig. 7).
(Pc)FeIV(µ-N)FeIII(Pc)-O-OH undergoes the heterolytic cleavage resulting in (Pc)FeIV(µ-
IV
+
.
N)Fe (Pc )=O transient complex having two redox equivalents above initial Fe(+3.5)Fe(+3.5)
state. This species is a strong two-electron oxidant. Thus, the high reactivity of µ-nitrido diiron
macrocyclic complex – H2O2 system toward strong C-H bonds of the light alkanes was
associated with their ability to form high-valent diiron oxo complex (L)FeIV(µ-N)FeIV=O(L+.), L
= phthalocyanine or porphyrin [24,26]. These highly active oxidizing species obtained using
H2O2 or m-chloroperbenzoic acid as oxygen donors have been characterized at low temperatures
by several spectroscopic techniques and isotopic labeling [23]. DFT calculations have also
indicated particularly strong oxidizing properties of the (L)FeIV(µ-N)FeIV=O(L+.) species and
their high reactivity in the oxidation of methane [23,27,39,40]. We have experimentally
8

evidenced that (L)FeIV(µ-N)FeIV=O(L+.) is competent for methane oxidation and its reactivity is
different from that of hydroxyl radical [24-26].
Contrary to the peroxo complex derived from H2O2, (Pc)FeIV(µ-N)FeIII(Pc)-O-OtBu was
proposed to undergo the homolytic O-O cleavage producing one-electron oxidized (Pc)FeIV(µ-
t
t
N)FeIV(Pc)=O and BuO∙ radical. Under inert atmosphere (FePc)2N – BuOOH system exhibited
a strong tendency for the formation of C-C bonds due to the involvement of FeIV(µ-N)FeIV=O
and tBuO∙ species which attract the hydrogen atom from C-H bonds of the substrates to generate
radicals which then recombine forming coupling products. In the presence of dioxygen, these
radicals are intercepted by O2 forming peroxo radicals which initiate radical oxidation reaction.
The particular feature of this system is the simultaneous generation of two active species capable
of reacting with C-H bonds. This increases the efficiency of the process. The proposed reaction
mechanism is shown in Fig. 8.
Cyclohexenyl hydroperoxyde CyOOH is an important product of this radical oxidation. In turn,
it can be activated by µ-nitrido diiron complex to form (Pc)FeIV(µ-N)FeIII(Pc)-O-OtCy followed
by the formation of (Pc)FeIV(µ-N)FeIV(Pc)=O and CyO∙ radical which initiate further oxidation.
t
Thus, the (FePctBu4)2N - BuOOH system operates via one-electron oxidation pathways
involving free radicals. Following observations are in agreement with this mechanism:
(i) Formation of 3,3’-bicyclohexenyl in notable amounts even in the presence of dioxygen and
t
high concentration of BuOOH. Homolytic O-O cleavage generates two active species,
t
(Pc)FeIV(µ-N)FeIV=O(Pc) and BuO∙ radical which rapidly react with surrounding cyclohexane
molecules to form two cyclohexyl radicals in close proximity to each other. This can explain the
t
formation of Cy-Cy even in the presence of high concentrations of O2 and BuOOH oxidants.
These radicals can recombine to give rise a coupling product Cy-Cy or diffuse out the cage to be
trapped by dioxygen to form Cy-O-O∙ radical. The Cy∙ radicals can also react with other
oxidizing species.
(ii) Formation of Cy-OH and Cy=O in comparable amounts are usually observed when Cy-O-O∙
radicals are involved. Two Cy-O-O∙ molecules interact according to Russell termination
mechanism to form Cy-O-O-O-O-Cy intermediate which generates Cy-OH, Cy=O and O2 upon
degradation.
t
(iii) A 114 % product yield based on BuOOH obtained in the cyclohexane oxidation with
portion-wise addition of tBuOOH suggests the involvement of O2.
(iv) When 0.1 M cyclohexane solution was used, the Cy-Cy coupling product was absent and
Cy=O accounted for 72 % of products despite a moderate TON (87 turnovers). The predominant
formation of Cy-OH could be expected under these conditions. This result can be explained by
9

the fact that parallel formation of two adjacent radicals is disfavoured by the lower substrate
t
concentration. Instead, Cy-H molecule reacts with either (Pc)FeIV(µ-N)FeIV=O(Pc) or BuO∙
radical to form Cy radical which then trapped by adjacent active species to form oxidation
product (Fig. 8).
(v) It is believed that low values of 3°/2° parameter (3°/2° < 6) in adamantane oxidation are
t
indicative of a free-radical mechanism [41] although BuO∙-initiated reactions show the high
values (3°/2° ~ 10) [42]. The 3°/2° values greater than 13-15 are usually associated with a metal-
based mechanism [43]. Cytochrome P450 and heme complexes exhibit the 3°/2° adamantane
regioselectivity as high as 48:1 [44]. Thus, the 3°/2° value of 17 obtained in the oxidation of
adamantane by the (FePctBu4)2N - tBuOOH system is compatible with simultaneous involvement
of both (Pc)FeIV(µ-N)FeIV=O(Pc) and tBuO∙ species.
(vi) Significant kinetic isotope effect of 3.67 observed in the oxidation of adamantane-d2 is
compatible with participation of both (Pc)FeIV(µ-N)FeIV=O(Pc) and tBuO∙ species. The values of
kH/kD in the oxidation of p-xylene and toluene by high-valent iron oxo species of cytochrome P-
450 and by tBuO∙ were nearly identical: 6.4 vs 6.3 and 5.9 vs 6.0, respectively [45].
(vii) Composition of the products of cyclohexene oxidation including allylic oxidation products
and 3,3’-bicyclohexenyl strongly suggests the involvement of radicals and one-electron oxidants.
(viii) Strong dependence of the yield and composition of the products of cyclohexene oxidation
on the presence of O2 (Fig. 6) indicates the participation of O2 in the reaction. A sharp decrease
of 2-cyclohexen-1-ol and 2-cyclohexen-1-one amounts observed in anaerobic conditions, from
125.2 mM and 368 mM down to 22 mM and 38.2 mM, respectively, indicates that these products
should be formed through the reaction pathways involving O2. In turn, a strong increase of 3-tert-
butylperoxocyclohexene and 3,3’-bicyclohexenyl contents under Ar points out that the
concurrent recombination of cyclohexenyl and tBuO∙ radicals becomes a predominate pathway in
the absence of O2.
(ix) While phenol was the principal product of benzene oxidation by the (FePctBu4)2N – H2O2
system operating via two-electron mechanism with the intermediate formation of benzene
t
epoxide [38], biphenyl was obtained with a 91 % selectivity using the (FePctBu4)2N - BuOOH
t
system. Kinetic isotope effects in the oxidation of benzene by BuOOH and H2O2 catalyzed by
t
(FePctBu4)2N are also different: 2.36 and 1.16, respectively. The BuO∙ radical was published to
t
be incompetent in the oxidation of benzene [46]. Decomposition of BuOOH in benzene in the
t
t
presence of BuOOtBu initiator produced acetone, BuOH, CO, CO2 and small amount of O2
without products of benzene oxidation. Consequently, formation of the benzene oxidation
products by the (FePctBu4)2N - tBuOOH system is compatible with the proposed mechanism and
in particular, with the participation of (Pc)FeIV(µ-N)FeIV=O(Pc) in the oxidation by abstracting
10

of hydrogen atom from benzene. Phenyl radicals recombine to form biphenyl since their
recombination with alkoxyl radicals is slow.
4. Conclusions
The results of the current study confirm that the structure of the oxidant determines the
mechanism of its activation by µ-nitrido diiron phthalocyanine resulting in different active
species. Using H2O2, a very strong two-electron oxidizing species PcFe(IV)µNFe(IV)=O(Pc+.) is
generated from the peroxo complex PcFe(IV)µNFe(IV)-OOH(Pc) via heterolytic O-O bond
t
cleavage. In contrast, PcFe(IV)µNFe(IV)-OOtBu(Pc) generated from BuOOH undergoes a
t
homolytic O-O cleavage to form an one-electron oxidizing PcFe(IV)µNFe(IV)=O(Pc) and BuO∙
t
radical showing a different reactivity. The (FePctBu4)2N - BuOOH catalytic system is efficient
in the oxidation of C-H bonds of alkanes, olefins, alkylaromatic and aromatic compounds
furnishing oxidation products with high turnover numbers within several hours at low catalyst
loading. The efficiency of this system can be explained by generation from one peroxide
molecule two active species able to react with different C-H bonds thus opening interesting
possibilities for catalytic oxidation.
In addition, the catalytic system consisting of accessible, earth abundant and non-toxic
iron-based catalyst and industrial environmentally compatible oxidant would be of great practical
interest. N-Bridged diiron phthalocyanine complexes are non-expensive and non-toxic, and can
be available on a large scale. They can be readily immobilized onto different supports. For all
these reasons we believe that this novel approach might be useful for the development of novel
oxidation processes. Radical reactions catalyzed by transition metal catalysts gain in importance
in modern catalysis [47-49]. The further development of this approach might results in useful
synthetic transformations in mild and environmentally benign conditions.
5. Acknowledgements
This work was supported by CNRS, France (PICS project 6295) and RFBR, Russia (project 14-
03-91054).
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13

R
R
R
R
R
R
R
R
Fig. 1. General structure of µ-nitrido diiron phthalocyanine complexes. The complex with
R = tBu was used in this study.
40
35
30
25
20
15
10
5
0
0,0
0,1
0,2
[(FePctBu4)2N], M
Fig. 2. Dependence of the total product yield of cyclohexane oxidation after 1 h on (FePctBu4)2N
concentration. Experimental conditions: 1mL cyclohexane (9.26 M), [tBuOOH] = 0.2 M, 25°C.
14

50
40
30
20
10
0
0
10
20
Reaction time, h
Fig. 3. Accumulation of cyclohexanone (▲), cyclohexanol (●) and bicyclohexyl (■) during
oxidation of neat cyclohexane. Experimental conditions : 1mL cyclohexane (9.26 M),
[(FePctBu4)2N] = 0.1 mM, [tBuOOH] = 0.25 M, 25°C.
100
90
80
70
60
50
40
30
20
10
0
0
2
4
6
Reaction time, h
Fig. 4. Oxidation of adamantane catalyzed by (FePctBu4)2N - tBuOOH system in MeCN at 50°C.
t
Adamantane : (FePctBu4)2N : BuOOH = 0.1 M : 0.1 mM : 0.25 M. ■ - conversion of
adamantane, ● – 1-adamantanol yield, ▲ – 2-adamantanone yield.
15

H
H
kH/kD = 3.67 ± 0.11
Fig. 5. Determination of the intramolecular isotope effect using adamantane-1,3-d2.
OH
0.2 mM(FePctBu4)2N
0.25 M tBuOOH
C6H10 (2 mL)
60°C, 14 h
Air: 125.2 mM
Ar: 22 mM
368 mM
38.2 mM
36.2 mM
107.2 mM
6.2 mM
31.5 mM
t
Fig 6. Oxidation of neat cyclohexene (2mL, 9.87 M) by (FePctBu4)2N - BuOOH system under
air and under inert atmosphere.
16

Heterolytic O-O cleavage
_
_
OH
Fe+3.5
Fe+3.5
OH
FeIII
FeIV
Fe+3.5
Fe+3.5
FeV
FeIV
FeIV
.
H2O2
_
- OH
FeIV
A
Strong oxidizing properties
Homolytic O-O cleavage
_
_
_
OtBu
OtBu
FeIII
FeIV
Formation of two one-electron oxidizing species
Fe+3.5
Fe+3.5
Fe+3.5
Fe+3.5
FeIV
tBuOOH
.
OtBu
FeIV
B
t
Fig. 7. Formation of diiron peroxo complexes in the presence of H2O2 and BuOOH and
their transformation to different diiron oxo species via heterolytic and homolytic O-O bond
cleavage, respectively.
FeIVNFeIII-OH
FeIVNFeIV=O
Cy-H
tBuOOH
recombination
in cage
.
Cy
Cy-Cy
.
.
FeIVNFeIII-O-OtBu
tBuO
Cy
cage
escape
O2
tBuOH
Cy-H
FeIVNFeIII
FeIVNFeIII-O-OCy
CyOO.+.OOCy
CyOOH
.
Cy Cy-H
Cy-OH + Cy=O + O2
.
CyO
FeIVNFeIV=O
O2
Cy-H
CyOH
FeIVNFeIII-OH
Cy-H
t
Fig. 8. Proposed mechanism of the oxidation of cyclohexane by (FePctBu4)2N – BuOOH
system. Phthalocyanine ligands are omitted for clarity. Species reacting with C-H bonds are
depicted in red and principal products are in blue.
17

Table 1. Oxidation of neat cyclohexane by (FePctBu4)2N - tBuOOH system.
Reaction Concentration, mM
Distribution, %
Yield
on Total TON
time, h
Cy-OH:Cy=O:Cy-Cy tBuOOH, %b
Cy-OH Cy=O Cy-Cy
3
48.3
59.4
2.2
44 : 54 : 2
44 : 54 : 2
43 : 55 : 2
36 : 61 : 3
68
1099
2647
4560
5296
6
117.8
195.5
191.2
141.9 5.0
246.5 14.0
323.7 14.7
81
9
94
23
114
a Conditions: 2 mL of cyclohexane (9.26 M), [(FePctBu4)2N] = 0.1 mM, tBuOOH was added in
three portions at 0, 3 and 6 h, total [tBuOOH] = 0.75 M, 50°C. b Yield was based on the amount
of the oxidant tBuOOH added before analysis taking into account that two oxidant equivalents
are consumed to produce Cy=O. The yield of 114 % (last line) is due to the involvement of O2 in
the oxidation reaction.
18