Stereospecific sp3 C–H oxidation with m-CPBA: A CoIII Schiff base complex as pre-catalyst vs. its CoIIICdII heterometallic derivative

Article abstract of DOI:10.1016/j.apcata.2018.05.004

The mono- and binuclear Schiff base complexes [CoL₃]·DMF (1) and [CoCdL₃Cl₂]·0.5H₂O (2) were facilely synthesized using zerovalent cobalt and cadmium chloride (for 2) as starting materials and the pre-formed pro-ligand HL (2-methoxy-6-[(methylimino)methyl]phenol, the product of condensation of o-vanillin and methylamine) in air. The compounds were characterized by single crystal X-ray diffraction analysis and spectroscopic methods in solution and in the solid state. Both complexes demonstrate a profound catalytic activity in the stereoselective oxidation of cis-1,2-dimethylcyclohexane (model substrate) with m-CPBA (m-chloroperbenzoic acid) under mild conditions in the presence of promoters of various acidity (HNO₃, TFA and HOAc). The heterometallic binuclear Co^IIICd^II pre-catalyst (2) was more active than the mononuclear Co^III one (1), exhibiting higher products yields up to 51% and excellent stereospecificity (up to 99.2% retention of stereoconfiguration). This result could be associated with a synergistic effect of two different metals in 2. Based on the large obtained kinetic isotope effect and H₂¹⁸O labeling studies, the overall reaction mechanism was proposed to proceed without the participation of free alkyl radicals. The acidity of the promoter was shown to influence catalytic parameters for both 1 and 2 so that the better parameters are achieved with the acid possessing lower pK_a values (a stronger acid). The comparison of the catalytic behaviours of 1 and 2 is discussed in detail considering relevant examples from the literature.

Full text of DOI:10.1016/j.apcata.2018.05.004

Accepted Manuscript
Title: Stereospecific sp3 C–H Oxidation with m-CPBA: a
CoIII Schiff Base Complex as Pre-catalyst vs. its CoIIICdII
Heterometallic Derivative
Authors: Oksana V. Nesterova, Katerina V. Kasyanova,
Valeriya G. Makhankova, Vladimir N. Kokozay, Olga Yu.
Vassilyeva, Brian W. Skelton, Dmytro S. Nesterov, Armando
J.L. Pombeiro
PII:
S0926-860X(18)30219-9
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.05.004
APCATA 16653
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
6-3-2018
30-4-2018
2-5-2018
Please cite this article as: Nesterova OV, Kasyanova KV, Makhankova VG, Kokozay
VN, Vassilyeva OY, Skelton BW, Nesterov DS, Pombeiro AJL, Stereospecific
sp3 C–H Oxidation with m-CPBA: a CoIII Schiff Base Complex as Pre-catalyst
vs. its CoIIICdII Heterometallic Derivative, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.05.004
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Stereospecific sp3 C–H Oxidation with m-CPBA: a CoIII
Schiff Base Complex as Pre-catalyst vs. its CoIIICdII
Heterometallic Derivative
Oksana V. Nesterovaa,*, Katerina V. Kasyanovab, Valeriya G. Makhankovab, Vladimir N. Kokozayb,
Olga Yu. Vassilyevab,*, Brian W. Skeltonc, Dmytro S. Nesterova, Armando J. L. Pombeiroa
aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049-001 Lisboa, Portugal; e-mail: oksana.nesterova@tecnico.ulisboa.pt
bDepartment of Chemistry, Taras Shevchenko National University of Kyiv, 64/13 Volodymyrska
str., Kyiv 01601, Ukraine E-mail: vassilyeva@univ.kiev.ua
cSchool of Molecular Sciences, M310, University of Western Australia, Perth, WA 6009, Australia
Graphical Abstract
1

Highlights
 CoIII and CoIII/CdII Schiff base complexes catalyse stereospecific hydroxylation of
cis-1,2-dimethylcyclohexane with m-CPBA in the presence of an acidic promoter.
 The heterometallic CoIII/CdII complex is considerably more active than the
homometallic CoIII one exhibiting higher yields of products and excellent
stereospecificity.
 The acidity of the promoter is crucial for achievement of high catalytic activity.
 Catalytic systems have been studied by isotopic labelling, kinetic isotope effects and
radical trap experiments.
Abstract
The mono- and binuclear Schiff base complexes [CoL3]·DMF (1) and [CoCdL3Cl2]·0.5H2O (2)
were facilely synthesized using zerovalent cobalt and cadmium chloride (for 2) as starting materials
and the pre-formed pro-ligand HL (2-methoxy-6-[(methylimino)methyl]phenol, the product of
condensation of o-vanillin and methylamine) in air. The compounds were characterized by single
crystal X-ray diffraction analysis and spectroscopic methods in solution and in the solid state. Both
complexes demonstrate a profound catalytic activity in the stereoselective oxidation of cis-1,2-
dimethylcyclohexane (model substrate) with m-CPBA (m-chloroperbenzoic acid) under mild
conditions in the presence of promoters of various acidity (HNO3, TFA and HOAc). The
heterometallic binuclear CoIIICdII pre-catalyst (2) was more active than the mononuclear CoIII one
(1), exhibiting higher products yields up to 51% and excellent stereospecificity (up to 99.2%
retention of stereoconfiguration). This result could be associated with a synergistic effect of two
18
different metals in 2. Based on the large obtained kinetic isotope effect and H2 O labelling studies,
the overall reaction mechanism was proposed to proceed without the participation of free alkyl
radicals. The acidity of the promoter was shown to influence catalytic parameters for both 1 and 2
so that the better parameters are achieved with the acid possessing lower pKa values (a stronger
acid). The comparison of the catalytic behaviours of 1 and 2 is discussed in detail considering
relevant examples from the literature.
Keywords: homogeneous catalysis, stereoselective C-H oxidation, m-chloroperbenzoic acid, metal
complex catalysis, 18O isotopic labelling
2

1. Introduction
Oxidation of inactivated sp3 C–H bonds that is routinely carried out in biology remains a formidable
challenge in chemistry [1, 2]. In spite of impressive efficiency of natural enzymes such as
cytochrome P450 [3, 4] Rieske dioxygenase [5] or methane monooxygenase (MMO) [6], which
have ability to insert oxygen into C–H bonds with exciting selectivity under ambient conditions [5,
7], examples of man-made sp3 C–H oxidation catalytic systems with selectivities comparable to
those of the natural systems are limited [8-10]. The explanation is that strong activators (e.g.
oxidizing agents) and specific conditions are needed to overcome the inert character of very strong,
nonpolar bonds. The application of such methods often requires laborious isolation procedures with
subsequent purification of desired isomers and shows low regio-, stereo- and/or chemoselectivities.
Therefore, catalysts, which are able to perform selective functionalization of alkanes C–H bonds
under mild reaction conditions, are highly desired.
Complexes of transition metals proved to be efficient catalysts for a broad range of organic
reactions, including direct C–H functionalization [11, 12]. Further, the use of catalysts comprising
dissimilar metal ions (heterometallic complexes) may offer advantages over the homometallic
catalytic systems [13, 14]. Metals of different kinds may participate in the same catalytic cycle, thus
enhancing the catalytic activity (synergistic effect) [13, 14]. The second metal can also play an
indirect role by influencing the redox potential of the main catalytic centre or changing its
coordination preferences, as it happens, for instance, in the Ca/Mn active centre of the photosystem
II and its models [15, 16].
While considerable efforts have been made towards alkane oxidation using homometallic
complexes as catalysts, the catalytic properties of heterometallic compounds in these reactions
received significantly less attention. Ligands having N,O-donor sites, such as aminoalcohols, are
attractive for the construction of polynuclear heterometallic compounds [12, 17] and, among them,
Schiff bases occupy special place due to their rich structural diversity [18]. It has been shown that
Schiff base complexes serve as efficient catalysts in a wide range of reactions [12, 18, 19]. Earlier
we reported a pronounced catalytic activity of monometallic Schiff base complexes of copper,
manganese and iron in the mild alkanes C–H bonds oxidation [20-22]. The catalytic activity of
heterometallic Cu/Fe and Co/Fe Schiff base complexes in the same reaction was found to be
considerably higher [23, 24] and, moreover, in the latter case the activity was associated to
heterometallic species [Co2Fe(Sae)4]+ (H2Sae = salicylidene-2-ethanolamine), illustrating a
synergistic effect of two different metals [24].
3

Recently, we reported the catalytic activity of an isoindole-based Co complex in the mild oxidation
of alkanes that proceeds with pronounced stereoselectivity when m-chloroperbenzoic acid (m-
CPBA) is used as an oxidant [25]. In the search for understanding of the origin of stereoselectivity
of the C–H bonds oxidation governed by the catalytically active metal complex, we have now
explored the catalytic behaviour of two related CoIII complexes, [CoL3]·DMF (1) and
[CoCdL3Cl2]·0.5H2O
(2)
with
a
Schiff
base
ligand
(HL
–
2-methoxy-6-
[(methylimino)methyl]phenol, Scheme 1; DMF – N,N-dimethylformamide) in alkanes oxidation
reactions. It should be noted that heterometallic systems for alkane oxidation with m-CPBA are
virtually unknown and the effects of acidic or basic promoters on the catalytic performance have
not been studied yet.
The title compounds 1 and 2 have been synthesized as part of our ongoing research on transition
metal complexes of Schiff base ligands aimed at preparing compounds of various nuclearity and
structures with diverse potential applications. In these studies, we successfully apply direct
synthesis of coordination compounds based on a spontaneous self-assembly, in which one of the
metals is introduced as a powder (zerovalent state) and oxidized by aerial dioxygen during the
synthesis [23, 26-30]. In continuing our research line, herein we report the synthesis and catalytic
activity of mononuclear complex 1 and binuclear heterometallic compound 2, in the stereoselective
hydroxylation of cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) with m-CPBA as oxidant using
different acidic promoting agents under mild conditions.
2. Experimental
2-Hydroxy-3-methoxy-benzaldehyde (o-Vanillin) was purchased from Aldrich and used as
received. All other chemicals were purchased from local suppliers and used without additional
treatment. All synthetic experiments were carried out in air. Elemental analyses for CHN were
provided by the Microanalytical Service of the Instituto Superior Técnico. Infrared spectra (4000–
1
400 cm−1) were recorded using a BX-FT IR Perkin Elmer instrument in KBr pellets. The H NMR
spectra of 1, 2 and HL in DMSO-d6 were measured at room temperature using a Mercury 400
Varian spectrometer at 400 MHz. The chemical shift (δ) values in ppm are referenced to internal
Me4S; the coupling constants (J) are given in Hz. ESI–MS(±) spectra were run on a LCQ Fleet mass
spectrometer equipped with an electrospray (ESI) ion source (Thermo Scientifc), using ca. 10–5 M
solutions of 1 and 2 in acetonitrile. The pro-ligand HL was synthesized according to a slightly
modified reported procedure [31]. In the syntheses, the condensation reaction between o-vanillin
and CH3NH2·HCl was utilized without isolation of the resulting Schiff base.
4

2.1. Synthesis of CoL3·DMF (1)
o-Vanillin (0.3 g, 2.0 mmol) in methanol (10 mL) was stirred with CH3NH2·HCl (0.14 g, 2.0 mmol)
and dimethylaminoethanol (0.1 mL) in a 50 mL conical flask at 333 K for 30 min. SnCl2·2H2O
(0.23 g, 1.0 mmol) dissolved in 10 mL DMF and Co powder (0.06 g, 1.0 mmol) were added to the
resulting yellow solution of the Schiff base, and the mixture gradually turned green-brown as it was
stirred magnetically at 333 K until dissolution of the cobalt powder was observed (1.5 h). (Total
dissolution of the metal powder was not achieved due to adhesion of a small fraction of the metal
particles to the stirring bar.) The resultant brown solution was filtered and left to stand at room
temperature. Dark brown needles of 1 were formed by the following day. They were filtered off,
washed with methanol and dried in air. Yield (based on Co): 74%. Anal. Calcd for C30H37CoN4O7
(624.56): С 57.69, H 5.97, N 8.97%. Found: C 57.45, H 5.72, N 8.93%.
2.2. Synthesis of CoCdL3Cl2·0.5H2O (2)
o-Vanillin (0.23 g, 1.5 mmol) in 10 mL ethanol was stirred with CH3NH2·HCl (0.10 g, 1.5 mmol)
and dimethylaminoethanol (0.1 mL) in a 50 mL conical flask at 333 K for 30 min. CdCl2·2.5H2O
(0.11 g, 0.5 mmol) dissolved in 5 mL ethanol was added to the resulting yellow solution of the
Schiff base, and the solution was stirred magnetically at 333 K for 30 min. After that, Co powder
(0.03 g, 0.5 mmol) was added to the flask and the mixture that gradually turned brown was stirred
until dissolution of the cobalt powder was observed (1.3 h). (Total dissolution of the metal powder
was not achieved due to adhesion of a small fraction of the metal particles to the stirring bar.) The
resultant brown solution was filtered and left to stand at room temperature. Dark brown-green
prisms of 2 were formed by the following day. They were filtered off, washed with ethanol and
dried in air. More product was obtained from the mother liquor upon slow evaporation in air for
several days. Yield: 63%. Anal. Calcd for C27H31CdCl2CoN3O6.50 (743.78): С 43.60, H 4.20, N
5.65%. Found: C 43.31, H 4.26, N 5.34%.
2.3. Single-crystal X-ray diffraction
Crystallographic data for 1 and 2 were collected at 100(2) K on an Oxford Diffraction Gemini
diffractometer fitted with Cu Kα radiation. Following analytical absorption corrections and solution
by direct methods, the structures were refined against F2 with full-matrix least-squares using the
program SHELXL-2014 [32]. The non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms in both structures were introduced at calculated positions and
refined using of a riding model with isotropic displacement parameters based on those of the parent
atom. The site occupancy of the solvent water molecule in 2 was constrained to 0.5 after trial
5

refinement. CCDC 1566104 (1) and 1566105 (2) contain the supplementary crystallographic data
for this paper. These data are provided free of charge by the Cambridge Crystallographic Data
Centre.
Crystal data for 1: C30H37CoN4O7, M = 624.56, orthorhombic, Pna21, a = 13.5398(7) Å, b =
10.8076(6) Å, c = 20.0218(11) Å, V = 2929.8(3) Å3, T = 100(2) K, Z = 4, 26041 reflections
measured, 4304 independent reflections (Rint = 0.1116), R1 = 0.0500, wR2 = 0.1130 [I > 2σ(I)], GoF
= 1.029, absolute structure parameter -0.038(5).
Crystal data for 2: C27H31CdCl2CoN3O6.50, M = 743.78, monoclinic, P21/c, a = 14.090(2) Å, b =
16.887(2) Å, c = 13.179(2) Å, β = 110.804(2)°, V = 2931.3(7) Å3, T = 100(2) K, Z = 4, 14806
reflections measured, 5209 independent reflections (Rint = 0.0793), R1 = 0.0654, wR2 = 0.1529 [I >
2σ(I)], GoF = 1.060.
2.4. Alkanes catalytic oxidation
Typically, the reactions were carried out in thermostated cylindrical vials under vigorous stirring in
open air. At first, 0.9–2.1 mg of solid pre-catalyst 1 or 2 was weighed into the reaction flask, then
3.4 or 3.5 mL of CH3CN, 0.5 mL of CH3NO2 stock solution (internal standard; 1 mL of CH3NO2
mixed with 9 mL of CH3CN), 2.1–50 μL of promoting agent (HNO3, HOAc and TFA were used as
aqueous solutions or used in a form of stock solutions in acetonitrile) and 70 μL of cis-1,2-
dimethylcyclohexane (cis-1,2-DMCH) were added. When solid adamantane (68 mg) was used as
substrate, it was added into the vial before the addition of CH3CN. Then, the solution of m-
chloroperbenzoic acid (m-CPBA) in CH3CN (30 mg in 1 mL of acetonitrile) was prepared and it
was dropwise added to a warm (40 °C) solution of other components during 10 seconds under
vigorous stirring. (CAUTION. The combination of organic compounds with m-chloroperbenzoic
acid at elevated temperatures may be explosive!). The total volume of the reaction solution was 5
mL. According to developed methods [33], obtained samples were quenched at r. t. with excess of
solid PPh3 and directly analyzed by GC and GC-MS. The index of the retention of
stereoconfiguration (RC) was calculated considering that the commercial substrate, cis-1,2-
dimethylcyclohexane, contained 0.86% of trans-isomer.
2.5. Gas Chromatography
A Perkin-Elmer Clarus 600 gas chromatograph equipped with two non-polar capillary columns
(SGE BPX5; 30 m × 0.32 mm × 25 μm), one having an EI-MS (electron impact) detector and the
other one with a FID detector, was used for analyses of the reaction mixtures. The following GC
sequence has been used: 50 °C (3 min), 50–120 °C (8 degrees per minute), 120–300 °C (35 degrees
6

per minute), 300 °C (3.11 min), 20 min total run time; 200 °C injector temperature. For analysis of
oxidation products of highly-boiling compounds, such as adamantane, different conditioning was
employed: 50 °C (3 min), 50–150 °C (30 degrees per minute), 150–300 °C (14 degrees per minute),
300 °C (2.95 min), 20 min total run time; 200 °C injector temperature. Helium was used as the
carrier gas with a constant 1 mL per minute flow. All EI–MS spectra were recorded with 70 eV
energy.
2.6. GC–MS
The identification of peaks at chromatograms was made by comparison of respective EI mass-
spectra with those from the NIST v.2.2 database. The mass-spectra patterns of some products
(tertiary cis- and trans-1,2-dimethylcyclohexanoles) were not found in the NIST database, and
identification of these products was made by comparing with the reported previously mass-spectra
[34].
3. Results and discussion
3.1. Synthesis and IR spectra
The reaction of o-vanillin and CH3NH2·HCl in ethanol in the presence of dimethylaminoethanol
produces a yellow coloured solution of the Schiff base HL. The heterometallic complex 2 was
readily prepared by interaction of cobalt powder, cadmium chloride and ethanol solution of the pre-
formed HL in the Co0 : CdCl2 : HL = 1 : 1 : 3 mole ratio. The dissolution of the metal powder is
easily achieved under moderate heading and stirring of the reaction mixture in open air. Earlier, we
have shown that oxidation of zerovalent metal in ligand solution and formation of its coordination
compound are stipulated by the presence of a proton-donating species and O2 from the air, which is
reduced to give H2O [35]. This approach of using zerovalent metal as an initial reagent was further
developed into a synthetic scheme to prepare heterometallic compounds [26]. However, in the
attempt to obtain a Co/Sn heterometallic complex, the pure Co(III) compound 1 was isolated
instead.
The infrared spectra of 1 and 2 are very informative and show similar patterns (Figure S1). The
presence of adsorbed or lattice water molecules can be detected by the intense broad bands centred
at about 3440 cm−1 in both spectra. Several bands observed above and below 3000 cm–1 are
assigned to aromatic =C–H and alkyl –C–H stretching, respectively. The characteristic ν(C=N)
absorption of the Schiff base which appears at 1634 cm–1 as a strong intense band in the IR
spectrum of HL (Figure S1), is detected at 1632 (1) and 1628 (2) cm–1. A number of sharp and
intense peaks are found in the aromatic ring stretching (1600–1400 cm–1) and C–H out-of-plane
7

bending regions (800–700 cm–1) of both complexes. A sharp intense band at 1664 cm–1 in the IR
spectrum of 1 corresponds to the C=O stretching frequency of DMF.
3.2. Structural Description
The cobalt(III) complex 1 is built of discrete CoL3 molecules and DMF molecules of crystallization.
The molecule of 1 is chiral and has no crystallographically imposed symmetry (Figure 1). There are
two pairs of enantiomers in every crystal cell. The ligand moieties are deprotonated at the phenol O
atom and octahedrally coordinate the Co(III) atom through the three azomethine N and three
phenolate O atoms in a mer configuration. As can be seen in Figure 1 viewed along the pseudo-3-
fold axis, two of the ligands are pointed ‘up’ and one ‘down’. Three crystallographically non-
equivalent salicylaldimine ligands have Co–O and Co–N bond lengths in the ranges 1.881(4)–
1.913(4) and 1.932(5)–1.936(5), respectively, with average Co–O and Co–N bond lengths being of
1.893 and 1.934 Å, respectively (Table S1). The trans angles at the metal atom vary from 172.0(2)
to 176.7(2), the cis angles are equal to 84.9(2)–94.1(2)° (Table S1). The geometrical parameters of
1 are similar to those of isomorphous CoL3·THF (CSD refcode WEQTES) with the same ligand
[36]. In the solid state, CoL3 molecules form layers in the ab plane divided by the DMF molecules
(Figure S2). Interactions between complex molecules are weak, the closest Co···Co intermolecular
separation exceeds 7.08 Å. No – stacking is observed.
In the heterometallic molecule of 2, the Co atom retains its octahedral coordination by three
deprotonated Schiff base ligands with almost the same geometric parameters: Co–O and Co–N
bond lengths are in the ranges 1.876(6)–1.921(6) and 1.938(7)–1.948(7) Å, respectively; the trans
angles at the metal atom vary from 172.0(2) to 176.7(2), the cis angles are equal to 84.9(2)–94.1(2)°
(Figure 2, Table S2). The cadmium atom has two quite short bonds with the phenolato oxygen
atoms, O(11) and O(31) [2.291(6), 2.218(6) Å], of the two deprotonated Schiff bases and two
longer bonding distances to the chlorine atoms [Cl1: 2.417(2), Cl2: 2.391(2) Å] in a distorted
tetrahedral geometry. The angles at the metal atom vary from 66.4(2) to 130.29(16) (Table S2). In
addition, Cd(1) is weakly bonded to the oxygen atom O(12) at 2.724(7) Å, which implies that the
Cd(1) coordination sphere approximates an irregular square pyramid with Cl(1) atom in the apex.
The -phenolato-bridges between metal centers result in the cobalt-cadmium separation of 3.315(2)
Å.
The dinuclear units show no significant intermolecular interactions in the solid state. Numerous C–
H···Cl contacts in the crystal lattice of 2 with the H···Cl distances of above 2.68 Å should arise
8

from van der Waals close packing. Oxygen atom of the water solvent molecule, O1, appears to act
as a donor to Cl2 atom as evidenced by the O···Cl distance of about 3.20(1) Å.
3.3. Characterization of 1 and 2 in solution by NMR and ESI Mass Spectrometry
The diamagnetic nature of Co(III) complexes is helpful in their characterization by NMR
1
spectroscopy. The data on the room temperature H NMR spectra of 1, 2 and HL in DMSO-d6
(Figure S3) are summarized in Table 1 with proton labels as shown in Scheme 1. Assignments of
the signals in the spectra of 1 and 2 were made by comparison with the respective signals in the
spectrum of the pro-ligand interpreted before [31]. The mer arrangement of the three deprotonated
bidentate Schiff bases around the cobalt(III) centre in 1 causes asymmetry of all three ligands. The
1H NMR spectrum of complex 1 shows the presence of three different ligand environments, thus
confirming retention of the mer-octahedral geometry of Co(III) in DMSO solution. The –CH=N–
protons of the deprotonated Schiff base ligands appear as three singlets in a 1 : 1 : 1 ratio at  7.94,
7.79 and 7.46 ppm in its spectrum. Nine CH3O and nine CH3 protons are seen at two different
positions each at  3.62, 3.54 and 3.21, 3.19 ppm, respectively, in a 1 : 2 ratio. Nine protons of the
rings are observed as four doublets, a triplet and a multiplet in the range 6.91–6.24 ppm due to three
chemically different aromatic rings. The spectrum of 2 is very similar to that of 1 and contains the
same set of signals in the 8 to 2.5 ppm region. A definite conclusion on possible dissociation of the
heterometallic molecule in DMSO cannot be made due to the fact that cadmium coordination to
phenolato O atoms has a negligible effect on the remaining protons of the aromatic rings while the
metal contact to the methoxo oxygen atom O12 is too long.
Electrospray ionization mass spectrometry (ESI-MS) was used as a powerful tool for speciation
analysis of 1 and 2 in solution. The investigation of diluted (ca. 1 × 10−5 M) solutions of the
complexes in acetonitrile showed that the behaviours of the mono- (1) and binuclear (2) compounds
clearly differ. The most intense peak in the spectra of both compounds was recognized as
[(CoIIIL3)2Na]+ at m/z = 1125. Two other intense peaks in the spectrum of 2 correspond to [CoIIIL2]+
and [CoIIIL3]+ species at m/z = 387 and 556, respectively. The same species are found in the
spectrum of 1, however, with considerably lower intensities. [CoIIIL3Na]+ is present in both spectra
at 383 m/z. A transformation process for 1 proceeds further revealing decomposition of the initial
Schiff base ligand and possible reduction of CoIII ions: [CoIII(o-Van-H)3CoII(o-Van-H)(o-Van)]+ at
m/z = 934 and [CoII(o-Van-H)2Na]+ at 383 m/z (o-Van is o-vanillin; o-Van-H is deprotonated o-
vanillin) show 30 and 20% peak intensities, respectively. Three species at m/z = 700, 1308 and 1434
in the spectrum of 2 were assigned to heterometallic particles [CoIIIL3CdCl]+, [(CoIIIL3)2CdCl2Na]+
9

and [(CoIIIL3)2Cd2Cl3]+, respectively. Not seen for 1, the species at 166 and 937 m/z in the spectrum
of 2 were identified as the free ligand, [(HL)H]+, and [CoIII L5]+, respectively. Both spectra also
2
show numerous peaks within a 100–160 m/z range with low intensities. The data obtained support
existence of the mono- and binuclear cores of 1 and 2 in CH3CN solution and also reveal the
presence of remarkable aggregation processes. Formation of larger particles is particularly evident
in the case of heterometallic 2.
It should be stressed, however, that metal ions and metal complexes might undergo gas-phase
ligand replacement and intramolecular charge transfer reactions under the ESI-MS conditions.
Therefore, the structure may be altered during the ESI process and the original species information
is lost [37, 38].
3.4. Alkanes catalytic oxidation with m-chloroperbenzoic acid
3.4.1. General observations.
Oxidation of cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) is a catalytic process which can be
successfully used as a model reaction to study the bond and stereoselectivities of the catalytic C–H
activation systems [12, 39-41]. Compounds 1 and 2 were investigated as pre-catalysts for the
stereospecific oxidation of cis-1,2-dimethylcyclohexane with m-chloroperbenzoic acid using
different acids as promoting agents (Scheme 2). To reduce the overoxidation effects we used a 3.7
excess of alkane substrate relative to the oxidant in all the tests. No alkane oxidation products (or
only traces) were obtained in the absence of the pre-catalysts. The main observed reaction product
of the studied reaction was the tertiary 3° cis-alcohol (tertiary alcohol having the methyl groups in
cis position to each other). The by-products, tertiary 3° trans-alcohol and ketones that appeared
from the activation of 2° C–H bonds of cis-1,2-DMCH, were also detected. As known in similar
situations [25, 42], ketones usually result from overoxidation of secondary alcohols. Hence, it is
assumed that in the present case the formation of the ketones occurs via the subsequent oxidation
of alcohols and not by direct C–H bonds oxidation of the alkane (this assumption is in agreement
with cyclohexane oxidation curves, see below).
Promoting agents can have a strong influence on the oxidation process [12]. The role of a strong
protic acid, such as nitric acid, can be associated to the stabilization of transition states of metal
catalysts or to the creation of an unsaturated coordination environment around the metal. Nitric and
trifluoroacetic (TFA) acids are recognized promoters for efficient oxidations of alkanes with H2O2
[12]. A weaker carboxylic acid, such as acetic acid (HOAc), can serve as a co-catalyst in the same
type of reactions which can proceed with high stereoselectivity. In these cases, HOAc facilitates the
10

HVMO (high-valent metal-oxo) species formation under the carboxylic-acid-assisted mechanism
[5, 43]. Therefore, in this study the influence of three typical acid promoters of various acidities,
namely nitric, acetic and trifluoroacetic acids, was investigated.
The accumulations of the tertiary cis-alcohols with time upon cis-1,2-DMCH oxidation with m-
chloroperbenzoic acid in the presence of pre-catalysts 1 and 2 and acidic promoters, are shown in
Figures S4–S6. For nitric acid, two concentrations of the pre-catalysts have been tested. For 1, the
maximum observed initial reaction rates W0 are almost equal in both studied concentrations of 1,
being of 8.7 × 10–5 and 8.9 × 10–5 M s–1 for [1]0 of 2.9 × 10–4 and 5.4 × 10–4 M, respectively,
providing the TOF (turnover frequency) values of 1080 and 593 h–1. For 2, the following
parameters have been obtained: W0 = 3.8 × 10–5 M s–1 (TOF = 507 h–1) for [2]0 = 2.7 × 10–4 M and
W0 = 1.5 × 10–4 M s–1 (TOF = 964 h–1) for [2]0 = 5.6 × 10–4 M. Considering the observed yields and
stereoselectivities (see the Supporting Info), the lower concentrations of the pre-catalysts showed
better performance and were used through all the following tests.
Nitric acid is the most efficient promoter (Figure 3), affording the cis/trans ratios of 59 and 78 for 1
and 2, respectively (after 1 h). Notably, the latter corresponds to 99.2% retention of
stereoconfiguration (RC index, see the Experimental part for details), accounting for an almost
absolute stereospecificity of the 2/HNO3 heterometallic catalytic system. The products yields are
also higher when the pre-catalyst 2 is used (36 and 51% for 1 and 2, respectively). In both systems
with 1 and 2 the yields of chlorobenzene by-product do not exceed 0.6% (Figure 3).
The catalytic systems containing TFA and HOAc promoters appear to be much less efficient
(Figures 3, S7 and S8), showing the highest yield up to 26% (2/HOAc system) with the maximum
cis/trans ratio of 9 (1/TFA system). Moreover, these systems yield elevated amounts of
chlorobenzene by-product up to 14% for 2/HOAc (compare with 0.4 and 0.6% for 1/HNO3 and
2/HNO3 systems, respectively).
The highest normalized bond selectivity 3° : 2° of 40 was observed for the 2/HNO3 system (Table
2). When using TFA and HOAc, the bond selectivity becomes lower with the lowest value of 25 for
1/HOAc catalytic system.
The catalytic systems containing TFA and HOAc promoters appear to be much less efficient
(Figures 3, S7 and S8), showing the highest yield up to 26% (2/HOAc system) with the highest
cis/trans ratio of 9 (1/TFA system). At the same time, these systems yield elevated amounts of
chlorobenzene by-product up to 14% for 2/HOAc (compare with 0.4 and 0.6% for 1/HNO3 and
2/HNO3 systems, respectively).
11

The highest normalized bond selectivity 3° : 2° of 40 was observed for the 2/HNO3 system (Table
2). When using TFA and HOAc, the bond selectivity becomes lower with the lowest value of 25 for
1/HOAc catalytic system.
3.4.2. Chlorobenzene formation kinetics during cis-1,2-DMCH oxidation.
Chlorobenzene is a side product of oxidations with m-CPBA that results from the decarboxylation
of the m-chlorobenzoate radical, a typical intermediate [44, 45]. The rate of this decarboxylation is
known to be high (ca. 106 s–1) and comparable to those for H atom abstraction processes [44, 46].
The m-chlorobenzoate radical is formed through homolytic m-CPBA O–O bond splitting and is
related to a free radical process, which is less selective than metal-mediated ones [8].
Chlorobenzene was considered an indicator for a contribution of low-selective reaction mechanisms
when using m-CPBA [25, 47-49]. Similar oxidants, such as peroxyphenylacetic acid produce
toluene instead of chlorobenzene in the same radical decarboxylation [50, 51]. Hence, we were
interested in finding possible correlations between accumulation of chlorobenzene and overall
selectivity of the catalytic system.
In the reactions of oxidation of cycloalkanes with m-chloroperbenzoic acid catalysed by compounds
of cobalt [52], nickel [48, 49, 53] and ruthenium [54] with pyridylalkylamine ligands, the high
amounts of formed chlorobenzene (over 40% based on the oxidant) suggested significant
involvement of the homolytic O–O splitting pathway in the respective oxidation processes, what is
reflected by a low ratio of tertiary : secondary bond selectivity of 5 : 1 (for methylcyclohexane)
[48]. Much lower amounts of chlorobenzene (up to 3.3%) were observed when using cobalt
isoindole complex as a catalyst, in the absence of any promoter [25]. In the latter case, pronounced
stereo- and bond selectivities were observed.
The accumulations of chlorobenzene (Figures 3, S10 and S11) for the case of HNO3 promoter show
similar amounts of chlorobenzene (up to 0.6%) for both 1 and 2, with the respective reaction rates
W0 PhCl of 4.3 × 10−7 and 6.0 × 10−7 M s−1, respectively. Considering the yield of tertiary cis-
alcohols of up to 51% and assuming that the principal pathway of the reaction occurs through the
heterolysis of the m-CPBA O–O bond, one can estimate the ratio between heterolysis and
homolysis processes as ca. 85 : 1. For TFA and HOAc promoters, much higher concentrations of
chlorobenzene were observed (Figure 3). The initial reaction rates W0 PhCl corresponding to the
systems 1/TFA, 1/HOAc, 2/TFA and 2/HOAc, i.e. 3.2 × 10−6, 9.5 × 10−6, 3.7 × 10−6 and 5.7 × 10−6
M s–1, respectively, are higher than those for 1/HNO3 and 2/HNO3.
12

The plot of the cis/trans ratio vs. yield of chlorobenzene in cis-1,2-DMCH oxidation, observed after
90 min, is depicted in Figure 4, top. For each pre-catalyst, the higher chlorobenzene amounts
correspond to lower stereoselectivity. In general, although pre-catalyst 2 produces larger amounts of
chlorobenzene with TFA and HOAc promoters with the same cis/trans ratio as 1, the nitric acid
promoter is more efficient with the heterometallic complex 2. The W0 PhCl values built as a function
of the acidity parameter pKa (Figure 4, bottom; 8.80, 12.65 and 23.51 pKa values [55] were used for
HNO3, TFA and HOAc, respectively) show a linear dependence for complex 1 and a non-linear for
2. The nearly equal initial formation rates W0 PhCl for 1 and 2 when using HNO3 or TFA promoters
suggest that the chlorobenzene formation kinetics depends more on the acidity of the promoter,
rather than on the nature of the pre-catalyst. Moreover, a higher concentration of the pre-catalyst
affords a higher reaction rate of chlorobenzene formation (Figure S12). The latter observation
points out that chlorobenzene forms through metal-catalysed processes, rather than in the metal-free
transformation of m-CPBA.
3.4.3. Adamantane oxidation.
Both 1/HNO3 and 2/HNO3 systems afford higher yields of products in the adamantane oxidation
with m-chloroperbenzoic acid than those for cis-1,2-DMCH, namely 68 (W0 = 7.9 × 10−5 M s−1) and
65% (4.2 × 10−5 M s−1) for 1 and 2, respectively (after 1 h) (Figure S9). 1-Adamantanol is the main
product, as illustrated by the 3° : 2° bond normalized selectivities of 22 : 1 (1) and 23 : 1 (2). These
values are close to those obtained for cis-1,2-dimethylcyclohexane oxidation (Table 2).
Chlorobenzene amounts are ca. twice lower than those found in systems with cis-1,2-DMCH,
showing 0.3% yields for both 1 and 2. For secondary products, low alcohol/ketone (A/K) ratios
varying from 0.5 to 1.6 are observed for both pre-catalysts.
Formation of N-(1-adamantyl)acetamide by-product was revealed in adamantane oxidation (Scheme
3). The amounts of this by-product are comparable to those of adamantane-1,3-diol (Figure S15).
Earlier, the formation of low quantities of N-(1-adamantyl)acetamide was detected in the
adamantane oxidation catalysed by the isoindole cobalt complex and using m-CPBA as an oxidant
in the absence of promoting agent [25]. N-(1-adamantyl)acetamide can be prepared through the
interaction of hydroxylated or halogenated adamantane with acetonitrile in the presence of acids,
nitrosium salts and some other reagents [56]. Direct formation of N-(1-adamantyl)acetamide from
alkane (adamantane) is known, but typically requires harsh conditions [56] and/or expensive
reagents [57, 58]. We presume that in our case the formation of N-(1-adamantyl)acetamide proceeds
via the Ritter type reaction and may involve 1-adamantanol as an intermediate. The absence of
13

amidated products in the adamantane oxidation with H2O2 under similar conditions (a copper
complex as catalyst, HNO3 promoter, acetonitrile solvent) [21] suggests that the nature of the
catalyst and the application of m-CPBA as oxidizing agent are essential.
3.4.4. Kinetic isotope effect and 18O labelling tests.
To gain a greater insight into the performance of 1/HNO3 and 2/HNO3 catalytic systems, the
competitive oxidation reactions with equimolar mixtures of deuterated and normal cyclohexanes
(0.05 M each) in the presence of 18O labelled water (1 M) was performed. The accumulations of all
four products (normal and deuterated cyclohexanols and cyclohexanones) are depicted in Figure 5.
An overall shape of the accumulation curves is typical for the consecutive reactions C6H12
→
C6H11OH → C6H10=O (A→B→C type). The initial periods of kinetic curves, where total
accumulations (C6H11OH + C6H10=O) obey a linear law, were fitted to equations [59]
where B(t) and C(t) are concentrations of cyclohexanol and cyclohexanone after reaction time t, and
A0 is the initial concentration of cyclohexane (constants k1 and k2 correspond to A → B and B → C
reactions, respectively). It was found, that in all cases alcohols are oxidized much faster than initial
alkanes, as evidenced by k2/k1 ratios varying from 34 to 145. This explains low alcohol/ketone
(A/K) ratios between 0.27 and 1.44, observed at 90 min time. Thus, a correct value of KIE (kinetic
isotope effect) should be determined from the ratio of k1H/k1D constants, but not from simple
measuring of C6H11OH and C6D11OH concentrations at some point. Using the k1H/k1D method, the
KIE values were found to be 5.7(3) and 6.2(2) for 1 and 2, respectively. Considering the estimated
standard deviations, one may assume that the KIE values for both complexes are equal. Such high
values of KIE are comparable to those observed for the systems operating via iron and ruthenium
high-valent metal-oxo (HVMO) C–H attacking species [60], and suggest that the H atom
abstraction could be among the rate determining steps [61].
18
The mean incorporations of oxygen from H2 O in the normal and deuterated cyclohexanols are
2.0(1) and 2.9(1.4)% for 1 and 3.5(2) and 2.9(3)% for 2 for C6H11OH and C6D11OH, respectively.
18
The main difference between the pre-catalysts lies in the O incorporation into normal alcohol,
which is almost twice higher for 2, giving larger amounts of 18O-labelled normal alcohol (Figure 6).
As the reaction samples were quenched with Ph3P, which readily reacts with ROOH peroxides [33]
and HVMO species [62] to form Ph3P=O in the following ways:
14

ROOH + Ph3P → ROH + Ph3P=O
Mn=O + Ph3P → Mn–2 + Ph3P=O
it was interesting to see whether 18O oxygen appears in Ph3P=O. The phosphine oxide contained up
18
18
to 53 and 42% of O for 1 and 2, respectively (at 90 min), showing gradual accumulations of O
18
with time (Figure S13). A blank test revealed that Ph3P=O does not exchange oxygen with H2 O in
acetonitrile when HNO3 promoter was used. Formation of large portions of Ph3P=18O is expected
for catalytic systems that function via a free radicals pathway, when Ph3P reduces labelled
hydroperoxide, such as cyclohexyl hydroperoxide C6H1118O18OH [63-67]. However, the free radical
18
18
systems are not expected to transfer O from H2 O to alkyl hydroperoxides and, moreover, no
traces of C6H11OOH were detected in the GCMS chromatograms in the present case [68].
A gradual accumulation of Ph3P=18O over time (Figure S13) accounts for the existing of some 18O-
containing species, which concentration increases with time. This can be due to formation of
18
catalytic O-labelled intermediates as well as a non-catalytic oxygen exchange. Considering the
18
growth of O incorporation into Ph3P=O during the time period up to 90 min, while the oxidation
reaction stops after 20 mins, the version of a non-catalytic oxygen exchange looks more reasonable.
However, the nature of this exchange and the species responsible for Ph3P=O labelling were not
18
identified. No O-labelled m-chlorobenzoic acid was detected, while peracids are known [69] not
to exchange oxygen with water (a blank test confirmed the absence of such an oxygen exchange).
Finally, a different character of the 18O accumulations into the alcohols (low level, nearly constant)
18
and Ph3P=O (high level, continuous growth) implies different pathways of O appearance in the
respective products.
Formation of the chlorocyclohexane by-product was observed for both 1/HNO3 and 2/HNO3
catalytic systems, both for deuterated and normal cyclohexane substrates (Figure 7). The amount of
chlorinated normal cyclohexane (C6H11Cl) is sufficient for its detection as a regular peak in the
chromatogram, while that of C6D11Cl is considerably lower and can be detected only by a targeted
search for its mass-spectrum (Figures 7, S16). From the fact that both deuterated and normal
chlorination by-products were detected, one may rule out the reduction of chlorobenzene as a
source of chlorocyclohexane. Further, as chlorocyclohexane was detected for complex 1, which
does not contain chloride anions in its structure, m-CPBA (or its derivatives, m-chlorobenzoic acid
and chlorobenzene) is the single origin of chlorine in the chlorocyclohexanes. Similar observations
were made earlier [25]. A possible explanation could be that the C–Cl bond splits at the stage of
15

chlorobenzene radical formation (see below; Scheme 4) releasing the chlorine radical Cl• which
further attacks cyclohexane or its short-lived alkyl radical to give chlorocyclohexane.
The KIE values found for 1 and 2 lie in the range observed for other catalysts in reactions of
alkanes oxidation with m-CPBA (Table 3), being close to porphyrin complexes of iron (4b; at low
temperature), and porphyrin (5) and isoindole (3) complexes of cobalt. Cobalt perchlorate 8 also
exhibits a rather high KIE of 8 (Table 3), although with low yields of products (ca. 1%) based on
the substrates [42]. Compounds of iron are proposed mostly to oxidize alkanes through high-valent
metal-oxo (HVMO) intermediates of Fe(IV) or Fe(V), showing the KIE with values around 3.5
(Table 3). Complexes of ruthenium 9 and 10, for which participation of Ru(IV)=O intermediates is
proposed, also show KIE of the same magnitude. There are, however, exceptions such as the Fe(IV)
complex 22, which exhibits KIE of 25 during the stoichiometric oxidation (Table 3). Remarkably,
18
the incorporations of oxygen from H2 O into the hydroxylation products span over a wider range.
3.4.5. Radical trap experiments.
To establish the involvement of the alkyl radicals into cis-1,2-DMCH oxidation, we used a radical
trap, CBrCl3, to see how it influences the catalytic activity. The catalytic system 2/HNO3 that
showed the highest stereoselectivity among the systems tested (cis/trans = 87 at 90 min, Figure S5),
was chosen for this test. When the reaction was initiated with addition of 0.1 M of CBrCl3 (1 equiv.
relative to substrate), the cis/trans ratio dropped to 24 (after 90 min) and the yield of tertiary
alcohols decreased to 10% based on the cis-1,2-DMCH (compare to 53% of 3° alcohols in the
system without addition of CBrCl3). Several by-products were observed, some of them presumably
being chlorinated and brominated cis-1,2-DMCH according to their mass-spectra. The peaks I and
II (Figure 8) can be tentatively assigned to Cl and ClBr derivatives of the alkane substrate based on
the characteristic peaks near 146 and 111 m/z in the respective EI mass spectra (Figures S17 and
S18). The strong peaks III–V with similar mass spectra (Figures S19–S21) were identified as
brominated derivatives of cis-1,2-dimethylcyclohexane [25]. Peak VI also looks like a Cl-derivative
(see a typical pattern at 145 m/z, Figure S22). Peak VII is recognized as 8-hydroxy-octanone, and
although the reference spectrum reveals differences from that of VII (Figure S23), one could assign
this peak to a halogen-free product. Detection of 1-bromo-3-chlorobenzene is in agreement with the
presence of m-chlorobenzyl radicals, formed through the decarboxylation of m-chlorobenzoate ones
[44]. However, the CBrCl3 trap was found to react with PPh3 to form CHCl3 and CHBrCl2 (Figure
8, inset), where hydrogen probably comes from the alkane. Both CHCl3 and CHBrCl2 were found
only at trace levels when the samples were injected before PPh3 addition. The possibility of
spontaneous reaction between PPh3 and CBrCl3 while passing through a hot (200 °C) GC injector is
16

ruled out because CBrCl3 reacts with PPh3 rather slowly requiring at least 20 min of treatment at
room temperature for complete disappearance of the CBrCl3 peak (Figure S25).
The observation that CBrCl3 is able to induce a metal-free oxidation and halogenation of cis-1,2-
DMCH (in open air) was unexpected. The test performed in the absence of 2 and m-CPBA revealed
notable amounts of tertiary alcohols and Br-derivatives at the beginning of the reaction, then
leading to Cl- and Br-derivatives as the main products (Figure 8). If the cyclohexane substrate was
used, no alkane functionalization products were detected. It is well known that the energy of C–Br
bond in CBrCl3 is rather low (55.3 kcal mol–1) [85] and the bond undergoes homolysis under
sunlight or heating to form highly reactive Br• and Cl3C• radicals [85]. The latter easily react with
alkyl radicals and alkanes themselves. We assume that these processes take place in the metal-free
cis-1,2-DMCH/HNO3/CBrCl3/O2 system. Considering the great similarity between the patterns of
halogenated products I–VI in the cases of metal/oxidant-free and 2/m-CPBA systems (Figure 8), we
believe that in the latter case the halogenated products are formed through a metal-free pathway,
while the tertiary cis-alcohol and 1-bromo-3-chlorobenzene come from the metal-catalysed route.
Hence, the radical trap (CBrCl3) competes with the main pathway of the reaction in the 2/HNO3/m-
CPBA system rather than interferes with it. Nevertheless, it can be suggested that the alkyl and m-
chlorobenzyl radicals are present in the catalytic mixtures.
3.4.6. Discussion.
Coordination of m-CPBA or peroxyphenylacetic acid (PPAA) to a coordination compound is
generally described as a process which may lead to a) oxygenation without elimination of HVMO
species, b) homolytic or c) heterolytic splitting of the O–O bond (Scheme 4) [8]. The two latter
pathways afford HVMO species or respective M–O• radicals. The mechanism of this type was
studied for complexes of manganese [51], iron [72, 76], cobalt [25, 50, 86], nickel [48, 49, 53, 87-
91], and ruthenium [92], and is in accordance with DFT, catalytic and spectroscopic data. We
accepted this general pathway as a basis and attempted to recognize the dominant route for the
systems with pre-catalysts 1 and 2. As the starting cobalt oxidation state in 1 and 2 is 3+, the
heterolysis pathway should lead to Co(V) species. The existence of such intermediates, while
possible, is rather doubtful considering the very high activity even of Co(IV) compounds (see
below). The mechanisms of similar cobalt-catalysed oxidations with m-CPBA are presumed to
involve HVMO cobalt intermediates (for instance, 3 and 5 in Table 3) [25, 50, 71, 93, 94],
particularly as a source of stereoselectivity. However, in contrast to numerous Fe(IV)=O and
Fe(V)=O HVMO complexes, which possess rich chemistry and sufficient stability to make their
17

isolation and study possible [95, 96], the respective compounds of Co(IV) and Co(V) remain
elusive. The high reactivity of HVMO cobalt species causes their very short lifetimes and,
therefore, difficulties in reliable characterization. There are reports by Nam and co-workers that
Co(IV)=O species can be stabilized within heterobimetallic Co/M assemblies, where M is a redox-
inactive metal such as Sc(III), Ce(III), Y(III) or Zn(II) [97, 98]. For example, the existence of a
Co(IV)–O–Sc(III) core was proposed (one may see that such a core resembles the Co/Cd core in 2)
[97, 98]. Later, however, some of these data were re-investigated and the possibility of their
interpretation as Co(III) species was suggested [99]. Recently a Co(IV) complex was trapped by
spectroscopic methods, where it was obtained by photo- or chemical oxidation of a Co(II) complex
with 1,4,7,10- tetramethyl-1,4,7,10-tetraazacyclotridecane ligand [100]. Although this HVMO
cobalt compound was not isolated (it decayed within 4 min), it was shown to possess strong
oxidative properties, particularly in the C–H bonds oxidation [100].
Based on the above discussions, we presume that the main oxidation pathway in 1/promoter/m-
CPBA and 2/promoter/m-CPBA systems likely involves Co(II) and/or Co(III) species and the
“concerted” route (Scheme 4), although participation of Co(IV) ones cannot be excluded. The
expected short lifetime of Co(IV) intermediates suggests that their potential oxygen exchange with
18
18
H2 O (expected for HVMO compounds) cannot be a sufficient explanation of O incorporation
into a hydroxylated product. Hence, as m-CPBA did not reveal notable 18O enrichment when treated
18
with H2 O, one may suggest that water is able to exchange its oxygen with one of the Co/m-CPBA
intermediates.
The main pathway of the reaction of m-CPBA with the cobalt centre is believed not to lead to the
elimination of free aroyloxyl ArC(O)O• or aroylperoxyl ArC(O)OO• radicals (homolytic m-CPBA
splitting, Scheme 4). Elimination of the ArC(O)O• radical from the cobalt-based intermediate
(Scheme 5) should be considered as a minor process, strongly influenced by the acidity of the
promoter. Such a scheme explains a pronounced dependence of chlorobenzene formation both on
the pre-catalyst concentration and pKa of the acid promoter. The free aroyloxyl radical initiates a
series of metal-free reactions between radical species, m-CPBA and alkane (Scheme 5) [44, 45].
The aroylperoxyl ArC(O)OO• radical, known to result from the reaction of aroyloxyl radical with
m-chloroperbenzoic acid, was stated to hydroxylate alkanes via a concerted mechanism with
retention of initial stereoconfiguration of the alkane substrate [44]. The aryl radical, formed through
the decarboxylation of the aroyloxyl radical, abstracts H atom from the tertiary bond of alkane,
producing chlorobenzene as well as a free alkyl radical [8]. The latter affords a mixture of cis- and
18

trans-isomers [33, 41] before reacting with O2 [33], finally yielding both stereoisomers of tertiary
alcohols (Scheme 5).
Although the details of this mechanism are to be established, the analysis of the combined catalytic
parameters (yields and stereoselectivities, Tables 2 and 4) clearly shows advantages of the Co/Cd
heterometallic system (2) over the Co mononuclear pre-catalyst (1). Moreover, the 18O
incorporation parameters in cyclohexane oxidation were also found to be different for pre-catalysts
1 and 2. These observations can be understood in terms of the synergistic catalytic effect of different
metals, already recognized for various combinations of metals in diverse catalytic processes,
including alkane oxidation [12, 13]. It is known that redox-inactive metals, such as cadmium, can
catalyse peroxidative oxidation where the catalytic effect occurs via interaction of two peroxide
molecules coordinated to the same metal centre [101-104]. Further, the properties (and, therefore,
catalytic activity) of a redox-active metal centre can be influenced by another redox-inactive one,
located at close proximity [15, 16, 105]. When each kind of metal participates specifically on the
overall reaction mechanism, a cooperative effect of the two metals (often without an obvious
understanding) can occur [13]. In the Scheme 5, we propose that the most probable role of the
cadmium centre in the pre-catalyst 2 is in influencing the coordination geometry and redox
properties of the cobalt atom, facilitating, e.g., its interaction with m-CPBA.
There are some catalytic systems capable of oxidizing cis-1,2-dimethylcyclohexane with m-CPBA
under conditions relevant to the present study (Table 4). The cis/trans ratio shown by the 2/HNO3
system is among the highest ones, also disclosing one of the highest 3° : 2° selectivities. The
heterometallic Co/Fe complex with Schiff base ligand 3 exhibits the cis/trans ratio of 17. Good
products yields are also provided by a series of organometallic complexes of cobalt [67]. Complex 4
from the series that bears tetramethylcyclobutadiene ligand is given in Table 4 as a representative
example. Complexes 5 and 6 with silsesquioxane ligands show moderate stereoselectivity in
6/HNO3 system only, while in the absence of the nitric acid promoter both complexes act as non-
selective catalysts. The isoindole cobalt complex 7 is reported to catalyse stereoselective reactions
in the absence of any promoter (Table 3) with yields based on the substrate up to 15% [25]. A
number of complexes with porphyrin ligands and various transition metals were tested towards
stereoselective oxidation under promoter-free conditions (8–13) and showed pronounced cis/trans
ratios and yields based on the oxidant (Table 4). Perchlorate salts of the same metals (14–16) were
also shown to catalyse the stereoselective reaction. Although significant yields based on the oxidant
were achieved in the catalytic systems 8–16, the obtained products yields based on cis-1,2-DMCH
19

did not exceed 1%. In contrast, the catalytic systems 1/HNO3/m-CPBA and 2/HNO3/m-CPBA are
able to reach 15% yield based on the substrate. The negligible stereoselectivity of iron(III) chloride
(17) can be slightly improved using an 8-fold excess of the polydentate amine ligand L6 relative to
the iron salt (18).
4. Conclusions
We have explored the catalytic properties of the mononuclear Co (1) and heterometallic binuclear
CoIIICdII (2) Schiff base complexes in the stereoselective reaction of cis-1,2-dimethylcyclohexane
oxidation with m-CPBA oxidant and promoters of different acidity (HNO3, TFA and HOAc) under
mild conditions. Both complexes reveal a notable activity, with yields of the products up to 15%
and 51% based on 1,2-DMCH and m-CPBA, respectively, and retention of stereoconfiguration of
alkane substrate up to 99.2%. The heterometallic CoIIICdII complex (2) is more active than the
mononuclear CoIII complex (1), exhibiting higher products yields and a higher stereoselectivity.
This finding can be understood in terms of the synergistic catalytic effect of different metals, so that
the cadmium atom in the pre-catalyst 2 may play a role in suppressing the minor non-selective
radical mechanism or in stabilizing stereoselective intermediates.
We have investigated the influence of the nature of an acidic promoter and found that the best
catalytic parameters can be achieved with the acid possessing the lowest pKa value (strongest acid).
The chlorobenzene side product formation, associated with a non-selective radical mechanism,
depends on the pKa of the promoter. From the pronounced stereo- and bond selectivities, high
kinetic isotope effects in the cyclohexane oxidation (5.7 and 6.2 for 1 and 2, respectively) and
18
incorporation of oxygen from H2 O into the cyclohexanol, we presume that the mechanism of
studied reaction proceeds via a non-free-radical pathway, with possibility of involvement of Co(IV)
high-valent metal-oxo species.
The present work provides novel insights into the m-CPBA chemistry, as well as into the C–H
catalytic oxidation using this oxidant. The use of heterometallic pre-catalyst 2 is shown to have
advantages over the mononuclear 1 and other known homometallic catalysts in terms of yields,
bond- and stereoselectivity.
20

Acknowledgements
This work was supported by the Foundation for Science and Technology (FCT), Portugal (projects
PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013; fellowship SFRH/BPD/99533/2014). The
authors acknowledge the IST Node of the Portuguese Network of Mass-spectrometry for the ESI-
MS measurements. Research support by the COST Action CM1305 ECOSTBio (Explicit Control
Over Spin-States in Technology and Biochemistry) is gratefully acknowledged.
21

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Figure 1. Molecular structure of complex 1, showing the principal atom numbering scheme. Non-H
atoms are shown with 50% probability displacement ellipsoids.
Figure 2. Molecular structure of complex 2, showing the principal atom numbering scheme. Non-H
atoms are shown with 50% probability displacement ellipsoids.
28

Figure 3. The activities of the catalytic systems comprising the substrate, cis-1,2-DMCH (0.1 M),
oxidant, m-CPBA (0.027 M), pre-catalyst ([1]0 = 2.9 × 10–4 M; [2]0 = 2.7 × 10–4 M) and acidic
promoter (5.5 × 10–3 M), in acetonitrile at 40 °C. Data are shown for the probe taken 1 h after the
reaction start. The cis/trans ratio is the ratio of cis- to trans-tertiary alcohol products. The yield of
oxygenates refers to the sum of all tertiary and secondary alcohols and ketones. Both oxygenates
and chlorobenzene yields are based on the oxidant.
29