Synthesis, crystal structures and catalytic activity of Cu(II) and Mn(III) Schiff base complexes: Influence of additives on the oxidation catalysis of cyclohexane and 1-phenylehanol

Article abstract of DOI:10.1016/j.molcata.2016.09.005

The complexes of copper [Cu(κONN’-HL)(NO₃)(DMF)](NO₃)?H₂O (1) and [Cu(κONN’-HL)Cl₂]??DMSO (2), and of manganese [Mn(κON-HL)₂Cl₂]Cl (3) and [Mn(κON-HL)₂(NO₃)₂](NO₃)?H₂O (4) were synthesized by reactions of the respective chloride or nitrate salt with a non-aqueous solutions of the Schiff base aminoalcohol HL (product of condensation of salicylic aldehyde and aminoethylpiperazine) and characterized by X-ray diffraction analysis. The catalytic investigations disclosed a prominent activity of the copper compounds 1 and 2 towards oxidation of cyclohexane with hydrogen peroxide in the presence of various promoters (nitric, hydrochloric, oxalic acids and pyridine), under mild conditions. The unusual promoting effect of pyridine on the catalytic activity of the copper catalysts allowed to achieve yields up to 21% based on cyclohexane. Chromatographic studies revealed that cyclohexyl hydroperoxide is a main reaction product and chlorocyclohexane (in the presence of HCl as promoter) was also detected, suggesting a free radical reaction pathway with hydroxyl radicals as attacking species. Complexes 1 and 2 act also as catalysts in the oxidation of 1-phenylethanol with tert-butylhydroperoxide, showing acetophenone yields up to 62% and TON (turnover numbers) up to 620 in the presence of the K₂CO₃ promoter.

Full text of DOI:10.1016/j.molcata.2016.09.005

Accepted Manuscript
Title: Synthesis, crystal structures and catalytic activity of
Cu(II) and Mn(III) Schiff base complexes: influence of
additives on the oxidation catalysis of cyclohexane and
1-phenylehanol
Author: Oksana V. Nesterova Dmytro S. Nesterov Agnieszka
´
Krogul M. Fatima C. Guedes da Silva Armando J.L. Pombeiro
PII:
S1381-1169(16)30377-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.09.005
MOLCAA 10029
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Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
29-4-2016
7-8-2016
2-9-2016
Please cite this article as: Oksana V.Nesterova, Dmytro S.Nesterov, Agnieszka Krogul,
´
M.Fatima C.Guedes da Silva, Armando J.L.Pombeiro, Synthesis, crystal structures and
catalytic activity of Cu(II) and Mn(III) Schiff base complexes: influence of additives
on the oxidation catalysis of cyclohexane and 1-phenylehanol, Journal of Molecular
Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.09.005
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Synthesis, crystal structures and catalytic activity of Cu(II)
and Mn(III) Schiff base complexes: influence of additives on
the oxidation catalysis of cyclohexane and 1-phenylehanol
Oksana V. Nesterova,aDmytro S. Nesterov,a* Agnieszka Krogul,b M. Fátima C. Guedes da Silvaa* and
Armando J. L. Pombeiroa*
a Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.Rovisco Pais,
1049-001 Lisboa, Portugal; e-mails: dmytro.nesterov@tecnico.ulisboa.pt;
fatima.guedes@tecnico.ulisboa.pt; pombeiro@tecnico.ulisboa.pt
b Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland.
Dedicated to Prof. Georgiy B. Shul’pin on the occasion of his 70th birthday and as a recognition for his
scientific achievements.
1

Graphical Abstract
2

Highlights
. Oxidation of cyclohexane with H2O2 catalysed by copper and manganese complexes.
. Pronounced promoting effect of pyridine.
. Direct GCMS observation of cyclohexyl hydroperoxide as a main reaction product.
. The formation of chlorocyclohexane in the presence of HCl promoter.
. Solvent-free oxidation of phenylethanol with t-BuOOH.
Abstract
The complexes of copper [Cu(ONN’-HL)(NO3)(DMF)](NO3)·H2O (1) and[Cu(ONN’-
HL)Cl2]·½DMSO (2), and of manganese [Mn(ON-HL)2Cl2]Cl (3) and [Mn(ON-
HL)2(NO3)2](NO3)·H2O (4) were synthesized by reactions of the respective chloride or nitrate salt with
a non-aqueous solutions of the Schiff base aminoalcohol HL (product of condensation of salicylic
aldehyde and aminoethylpiperazine) and characterized by X-ray diffraction analysis. The catalytic
investigations disclosed a prominent activity of the copper compounds 1 and 2 towards oxidation of
cyclohexane with hydrogen peroxide in the presence of various promoters (nitric, hydrochloric, oxalic
acids and pyridine), under mild conditions. The unusual promoting effect of pyridine on the catalytic
activity of the copper catalysts allowed to achieve yields up to 21% based on cyclohexane.
Chromatographic studies revealed that cyclohexyl hydroperoxide is a main reaction product and
chlorocyclohexane (in the presence of HCl as promoter) was also detected, suggesting a free radical
reaction pathway with hydroxyl radicals as attacking species. Complexes 1 and 2 act also as catalysts in
the oxidation of 1-phenylethanol with tert-butylhydroperoxide, showing acetophenone yields up to
62% and TON (turnover numbers) up to 620 in the presence of the K2CO3 promoter.
Keywords: alkane oxidation; alcohol oxidation; hydrogen peroxide; alkyl radical; metal complex
catalysis.
3

1. Introduction
Complexes of copper and manganese attract special attention, namely due to their ability to show
phenomena such as molecular magnetism[1] and magnetic refrigerating,[2] bioactivity,[3]etc.
Furthermore, copper and manganese are found in active centres of many bioenzymes,[4, 5] for example
in superoxide dismutase (dismutation of superoxide),[6]catalase (decomposition of hydrogen
peroxide),[7]particulate methane monooxygenase(methane oxidation)[8]or photosystem II(water
photooxidation).[9, 10]This encourages the study of those metals for simple catalytic models of
enzymatic systems, in attempting to approach their activity and explain their mechanisms of action.
Complexes of copper and manganese with polydentate amine and aminoalcohol ligands are also
recognized as catalysts in oxidative catalysis, particularly in C–H functionalization of various
substrates.[11, 12]The activation of alkanes and alcohols under mild conditions is a principal step
towards their conversion into valuable chemicals,[13-15]providing a strong motivation for further
investigation of N,O-donor copper and manganese complexes.
Schiff bases are useful tools in the construction of novel ligand systems by means of in situ
condensation of a variety of readily available aldehydes and amines.[16-18] Recently, we described the
iron complex [FeIII(HL)Cl2(DMF)]Cl·DMF with the Schiff base ligand HL formed from salicylic
aldehyde and aminoethylpiperazine.[19] That complex exhibited an excellent catalytic activity in
cyclohexane oxidation with H2O2 in the presence of nitric acid as a promoter.[19]The presence of the
promoter was crucial, and only a weak activity was found in its absence or in the presence of another
promoting agent(acetic acid). The importance of an acid promoter (nitric acid) was also observed for
the heterometallic complex [Co4Fe2O(L1)8]·4DMF·H2O with a Schiff base ligand (H2L1 = salicylidene-
2-ethanolamine), which is an exceptionally active catalyst in the mild oxidation of cycloalkanes with
hydrogen peroxide in the presence of that acid,[20] as well as for other Schiff base and aminoalcohol
catalysts.[21-25]Based on those results, we have now synthesized four novel complexes of copper and
manganese with the ligand HL and tested their catalytic activity in two radical reactions, oxidation of
cyclohexane and 1-phenylethanol in the presence of various promoting agents.
2. Experimental
4

All chemicals were of reagent grade and used as received. All experiments were carried out in air.
Elemental analyses for CHNS were provided by the Microanalytical Service of the Instituto Superior
Técnico. Infrared spectra (4000–400 cm−1) were recorded using a BX-FT IR “Perkin Elmer” instrument
in KBr pellets.
2.1 Synthesis of [Cu(ONN’-HL)(NO3)(DMF)](NO3)·H2O (1):
Salicylaldehyde (0.53 mL, 5 mmol) and 1-(2-aminoethyl)piperazine (0.66 mL, 5 mmol) were dissolved
in DMF (20 mL), forming a light-yellow solution which was magnetically stirred at 50–60 °C (30 min).
Then, a solution of Cu(NO3)2·2.5H2O (1.16 g, 5 mmol) in 5 mL of DMF was added dropwise and the
resulting green mixture was magnetically stirred for 30 min. Green crystals suitable for X-ray
crystallographic study were formed in one month after addition of iPrOH and Et2O. Yield: 0.85 g, 34%.
Anal. calc. for C16H28CuN6O9 (M = 511.98): C, 37.5; N, 16.4; H, 5.5%. Found: C, 37.8; N, 16.5; H,
5.3%. The compound is sparingly soluble in DMSO, DMF, water and CH3CN.
2.2 Synthesis of [Cu(ONN’-HL)Cl2]·½DMSO(2):
Salicylaldehyde (0.53 mL, 5 mmol) and 1-(2-aminoethyl)piperazine (0.66 mL, 5 mmol) were dissolved
in DMSO (20 mL), forming a light-yellow solution which was magnetically stirred at 50–60 °C (30
min). Then, a solution of CuCl2·2H2O (1.16 g, 5 mmol) in 5 mL of DMSO was added dropwise and the
resulting green mixture was magnetically stirred for 30 min. Green crystals suitable for X-ray
crystallographic study were formed in one day after standing at room temperature. Yield: 0.44 g, 10%.
Anal. calc. for C28H44Cl4Cu2N6O3S (M = 813.63): C, 41.3; N, 10.3; H, 5.5%. Found: C, 38.5; N, 8.5; H,
5.7% (the difference with calculated values is due to the presence of DMSO molecules, which were not
accounted by the X-ray analysis). The compound is sparingly soluble in DMSO, DMF, water and
CH3CN.
2.3 Synthesis of [Mn(ON-HL)2Cl2]Cl (3):
5

Salicylaldehyde (0.53 mL, 5 mmol) and 1-(2-aminoethyl)piperazine (0.66 mL, 5 mmol) were dissolved
in CH3OH (25 mL), forming a light-yellow solution which was magnetically stirred at 50–60 °C (30
min). Then, a solution of MnCl2·4H2O (0.98 g, 5 mmol) in 5 mL of CH3OH was added dropwise and
the resulting yellow-brown mixture was magnetically stirred for 30 min. Dark-brown crystals suitable
for X-ray crystallographic study were formed in three days after addition of iPrOH. Yield: 1.15 g, 37%.
Anal. calc. for C26H38Cl3MnN6O2 (M = 627.91): C, 49.7; N, 13.4; H, 6.1%. Found: C, 50.1; N, 13.9; H,
5.6%. The compound is sparingly soluble in DMSO, DMF and CH3CN.
2.4 Synthesis of [Mn(ON-HL)2(NO3)2](NO3)·H2O (4):
Salicylaldehyde (0.27 mL, 2.5 mmol) and 1-(2-aminoethyl)piperazine (0.33 mL, 2.5 mmol) were
dissolved in CH3OH (20 mL), forming a light-yellow solution which was magnetically stirred at 50–60
°C (30 min). Then, a solution of Mn(NO3)2·4H2O (0.63 g, 2.5 mmol) in 5 mL of CH3OH was added
dropwise and the resulting green-brown mixture was magnetically stirred for 30 min. Brown-violet
crystals suitable for X-ray crystallographic study were formed in one month after addition of iPrOH and
Et2O. Yield: 0.78 g, 44%. Anal. calc. for C26H40MnN9O12 (M = 725.61): C, 43.0; N, 17.4; H, 5.6%.
Found: C, 42.6; N, 17.2; H, 5.8%. The compound is sparingly soluble in DMSO, DMF and CH3CN.
2.5 Crystallography
The X-ray diffraction data (Table 1) were collected using a Bruker AXS KAPPA APEX II
diffractometer with graphite-monochromated Mo-Kα radiation. Data were collected using omega scans
of 0.5° per frame, and a full sphere of data was obtained. Cell parameters were retrieved using Bruker
SMART software and refined using Bruker SAINT[26] on all the observed reflections. Absorption
corrections were applied using SADABS.[27] The structures of were solved by direct methods using
SIR-97[28] and refined with SHELXL-2014/7.[29] Calculations were performed using the WinGX
System-Version 2014.1.[30]
For all the non-hydrogen atoms, least square refinements with anisotropic thermal motion parameters
were employed and isotropic for the remaining atoms. The hydrogen atoms bonded to carbon were
included at geometrically calculated positions and refined using a riding model. Uiso(H) were defined as
6

1.2Ueq of the parent carbon atoms for phenyl and methylene residues and 1.5Ueq of the parent carbon
atoms for the methyl groups. The hydrogen atoms of the ammonium groups were not located from the
difference Fourier map and thus were included in the final refinement at positions calculated from the
geometry of the molecules, with distances and angles restrains. There were disordered molecules in the
structure of 2 which could not be modelled and the electron density was removed by using
Platon/Squeeze routine. The total number of electrons (289) and void volume (772 Å3) suggest the
presence of six DMSO molecules (42 electrons and a volume of 129 Å3 per molecule) which were not
included in the final refinement. CCDC 1474857 (1), 1474858 (3), 1474859 (4), 1474860 (2a) and
1474861(2) contain the supplementary crystallographic data for this paper.
2.6 Catalytic oxidation of cyclohexane
To5 µmol of solid catalyst weighed into the reaction flask, 4.4 mLCH3CN, 50 µmol of promoter (HCl
and HNO3 were used as 37 and 65% aqueous solutions, respectively; solid oxalic acid was weighted in
the same flask prior addition of solvent), 0.5 mL of CH3NO2stock solution (internal standard; 1 mL of
CH3NO2 mixed with 9 mL of CH3CN), 108 µL (1 mmol) of cyclohexane and 0.28 mL (5 mmol; 50%
aqueous) of H2O2were added in this order at 50 °C under vigorous stirring (CAUTION: the
combination of air or molecular oxygen and H2O2 with organic compounds at elevated temperatures
may be explosive!). Aliquots (ca. 0.5 mL) of reaction mixture were transferred, upon cooling, into a
vial containing an excess (ca. 150 mg) of solid Ph3P. A Perkin-Elmer Clarus500 gas chromatograph
with a BP-20 capillary column (SGE,30 m × 0.32 mm × 25 μm) and a Perkin-Elmer Clarus 600 gas
chromatograph, equipped with a Perkin-Elmer Clarus 600 C mass-spectrometer (electron impact), with
a BPX5 capillary column (SGE, the same dimensions) and helium carrier gas were used for
quantitative analyses of the reaction mixtures.
2.7 Catalytic oxidation of 1-phenylethanol
In a typical experiment, 1-phenylethanol (5.00 mmol), TBHP (70% aqueous solution, 10.0 mmol) and
catalyst precursor 1–4 (2.5 – 5 μmol, 0.05 – 0.1 mol% vs. substrate) were introduced to a cylindrical
glass tube, which was then placed in a hot oil bath. The system was stirred for 2 h at 80 °C. After the
7

reaction was completed, the mixture was allowed to cool down to room temperature. 300 μL of
benzaldehyde (internal standard) and 5 mL of acetonitrile (to extract the substrate and the organic
products from the reaction mixture) were added. The obtained mixture was stirred for 10 min and then
a sample (1 μL) was taken from the organic phase and analysed by GC (or GC–MS) using the internal
standard method.
3. Results and discussion
3.1 Synthesis, spectroscopic analysis
The complexes 1–4 were obtained using a two steps synthetic strategy(Scheme 1): 1) in situ formation
of a Schiff base ligand and 2) its complexation reaction with a metal precursor. The ligand HL was
obtained by in situ condensation of the salicylaldehyde and 1-(2-aminoethyl)piperazine. The choosing
synthetic method is widely used as an efficient tool for the preparation of Schiff base containing
coordination complexes. Applying of such an approach allows to make the immediate utilization of the
Schiff base ligand formed, since its formation is a reversible process and its elaboration can be
problematic due to the low yields and laborious separation procedures. The interaction of metal
chlorides or nitrates with a non-aqueous solutions of HL using a molar ratio of MXn : HL = 1 : 1,
resulted in green or brown solutions (for Cu and Mn, respectively) obtained at the end of the reactions.
The reactions were initiated and brought to completion by heating and stirring in open air.
Microcrystals of the complexes 1–4 were formed in ca.1 month after successive addition of iPrOH and
diethyl ether into the resulting solutions (1 and 4) or after standing at room temperature within one day
(2) or in three days after addition of iPrOH (3).
The IR spectra of 1–4 in the 4000–400 cm–1 range confirmed the presence of the Schiff base ligand.
The broad medium intensity bands in the 3400–3550 cm–1 region were attributed to (O–H) vibrations
of HL. The very strong bands at 1643 (2), 1623 (3) and 1617 cm–1 (4) were assigned to ν(C=N)
stretching vibrations of the Schiff bases and the peak at 1637 (1) was attributed to the overlapped
(C=N) and (CO) vibrations of both Schiff base ligand and DMF. The presence of the nitrate ligand in
1 and 4 can be identified by the strong ν(NO)absorption peaks at 1384 cm–1 in the spectra of both
compounds. The presence of DMSO solvate molecule in 2 can be identified by the peak at 1017 cm–1
that corresponds to the (S=O) vibration.
8

3.2 Crystal structures
–
The asymmetric unit of 1 consists of one [Cu(ONN’-HL)(NO3)(DMF)]+ cation, one NO3 anion and a
water molecule, which joined into supramolecular one-dimensional chains assisted by hydrogen bonds
(Figs. 1, right and S1).The Schiff base HL in 1 shows a tridentate chelation mode with the binding
atoms occupying three of the basal metal coordination sites; the remaining equatorial position is
engaged with the ODMF atom and the axial location with the Onitrate thus rendering the metal cation
with a distorted square-pyramidal (5 = 0.24)[31] O3N2coordination environment (Fig. 1, left). The
Cu–O bond lengths assume values of 1.9066(15), 1.9560(15) and 2.455(2) Å, the larger values
involving the DMF and nitrate O-atoms, in this order (Table S1). Concerning the Cu–N bond
distances, the one trans to the Ofenolate is larger than that opposite to ODMF (2.0954(16) against
1.9259(17) Å) thus revealing a larger trans effect of the anionic group as compared to the neutral
one. The O–Cu–Ntrans angles are 163.25(7) and 177.61(7)º. The nearest Cu···Cu non-bonded
separations within the supramolecular chain are 7.159(1) Å.
The asymmetric unit of 2 contains two crystallographically independent molecules of [Cu(ONN’-
HL)Cl2], revealing close geometrical parameters (Table S2, Fig. 2, left), and one DMSO molecule.
The copper atoms in 2 have distorted square-pyramidal coordination environments (5 = 0.28 and
0.31)[31] with ON2Cl2 donor sets formed by the donor atoms of the Schiff base and two chloride
anions. The basal Cu–X (X = O, N, Cl) bond lengths in 2 range from 1.926(12) to 2.255(5) Å, while
the apical ones are of 2.719(6) Å (Cu1–Cl1) and 2.806(6) Å (Cu2–Cl3). The O(Cl)–Cu–Ntrans angles
assume values between 157.2(6) to 175.7(5)º. The molecules of 2 are packed into 2D supramolecular
layers by means of strong hydrogen bonding between the nitrogen atoms of HL and the chloride anions
(Fig. 2, right). The nearest Cu···Cu distance in 2 is of 4.817(3) Å.
An interesting feature of complex 2 is the ability to recrystallize from the perfluoropolyetheroil
“Fomblin Y”, to give complex [Cu(ONN’-HL)Cl2]·2.5DMSO (2a) (Table S2, Fig. 3). The effect has
been observed during the single crystal X-ray experiment (this oil is commonly used to fix the crystal
in the loop), where new crystals grew on the sample plate after 24h. While the original formulae of the
molecule of 2 remains unchanged, the crystal structure undergo drastic changes in the geometrical
parameters (Table S2), in the cell symmetry (P212121 in 2 but P–1 in 2a) and in the number of
crystallization DMSO molecules. The metal complexes in 2a are associated into 2D layers by means of
NH···Cl and NH···OHL interactions (Figs. S2 and S3) and ultimately give rise to a 3D network upon
9

longer range contacts with the DMSO molecules. The closest Cu···Cu distance in the structure of 2a
(6.518(2) Å) is larger than that in 2 (see above). Since the perfluoropolyetheroil is known as a highly
inert compound and a poor solvent, the present effect of recrystallization of the coordination compound
with the “classical” N,O-donor ligands is a rare effect with deserves further investigation.
The manganese complexes 3 and 4 contain the HL ligand coordinated in a N,O-chelating fashion (Figs.
4 and 5), with the coordination polyhedrons around the Mn centres in both structures adopting O2N2Cl2
(in 3) or O4N2 (in 4) octahedral geometries. The X–Mn–Xtrans (X = O, N, Cl) angles are of 180.0º
(Tables S3 and S4) and the compounds crystallized with uncoordinated chloride (in 3) or nitrate (in 4)
anions. The molecules of 3 and of 4 are packed into a 3D supramolecular network by means of H-
bonding interactions (Figs. S4 and S5).The nearest Mn···Mn distance is of 8.704(0) (in 3) and 8.397(19)
Å (in 4).
A striking feature of the structure of 4 concern the relative orientation of the nitrate ligands in the two
crystallographically independent molecules present in the structure, since in one of them (that of Mn2)
the O-atoms establish short range contact interactions with the delocalized -systems of the MnOC3N
metallacycles [N20O23···centroid 2.936(6) Å].
3.3 Catalytic oxidation of cyclohexane
Complexes 1–4 were studied as catalysts in the oxidation of cyclohexane with H2O2 in acetonitrile, in
the presence of various co-catalysts (promoters). Cycloalkanes, particularly cyclohexane, are useful
model substrates towards establishing the C–H functionalization activity of novel catalytic systems.
The C–H bond dissociation energy of cyclohexane (97 kcal/mol)[32] is high enough to prevent its
catalyst-free oxidation with organic peroxides under mild conditions and low concentrations. Further,
the mild oxidation of cyclohexane typically gives just two main products, cyclohexanol and
cyclohexanone, which can be easily quantified, thus facilitating the catalytic studies. Promoting agents
can play a complex role in the oxidation process. Strong protic acids (e.g. nitric, hydrochloric,
trifluoroacetic acids) may create an unsaturated coordination environment around the metal cation upon
ligand protonation and also hamper undesired decomposition of hydrogen peroxide (catalase-like
10

activity).[19-21, 24, 33]Carboxylic acids (e.g. acetic, oxalic, pyrazinecarboxylic acids) can serve as
agents stabilizing transition states of metal catalysts.[34-36] Basic promoters (e.g. triethylamine,
pyridine) are known to interact with metal catalysts converting them into active species.[37-40]
It has been shown that the catalytic activity of copper compounds (such as 1 and 2) in the mild
cyclohexane oxidation is typically enhanced by the presence of a protic acid as a promoter,[24, 25, 41]
while the activity of manganese compounds (such as 3 and 4) is sensitive to carboxylic acids.[35, 42-
44]Thus, we decided to study the influence of these typical acid promoters, as well as of a basic one
(pyridine, for comparative purpose), on the catalytic activity of complexes 1–4(Scheme 2).
The yields of products after 3 h reaction time are shown in the Table 2. The copper complexes1 and
2act as catalysts in all the cases, except of oxalic acid, where no products were detected even at trace
levels. Despite having very close structures, 1 and 2 reveal different catalytic activities upon treating
with the same promoting agents. The initial reaction rates W0of cyclohexane oxidation with HNO3
promoter were estimated as 9 × 10–7 and 3.1 × 10–5 and M s–1for 1 and 2, respectively. Interestingly, the
reaction rate exhibited by the complex 2 after decay of the initial activity becomes close to that
observed for 1 (Fig. 6a).In contrast to nitric acid, the hydrochloric acid promoter leads to very close
yields (Table 2) and reaction rates (7.7 × 10–6 M s–1) for 1 and 2.No alkane oxidation products were
obtained in the absence of the catalyst.
The accumulation dependences are also similar (Fig. 6b), pointing out that the catalytic behaviours of 1
and 2in the presence of HCl are identical. Pyridine provides the highest yields (Table 2) and reaction
rates (W0> 2.6 × 10–5 M s–1) for complexes 1 and 2 within the range of conditions studied. As it can be
seen (Fig. 6c), the catalytic system based on the complex 2 reaches a plateau upon 20 min after the
reaction starts, with no further change in the yield, whereas the compound 1 does not lose its activity
until 2 h reaction time.
The manganese complexes 3 and 4 were found to be almost inactive under all conditions studied.
Pyridine was the best promoting agent, but even in its presence the yields of products based on the
cyclohexane did not exceed 0.3% (Table 2). The initial reaction rate W0 (calculated for complex 3) was
found to be just 6 × 10–7 M s–1, which, among the 1 – 4 systems with detectable activity, is the lowest
one. In the presence of promoters other than pyridine, the complexes 3 and 4 were found to be
11

completely inactive: no even traces of products were detected, even employing mass-spectral analysis
of reaction products.
The reaction mechanisms are believed to proceed via formation of free carbon-centred radicals as main
C–H bond attacking species. Such pathway is expected[45] for transition metal complexes interacting
with a large excess (e.g., 1000 equiv.) of hydrogen peroxide. The principal processes of this mechanism
are as follows. Reduction of H2O2 by a [Mn] leads to the formation of a hydroxyl radical:
[Mn] + H2O2 → [Mn+1] + HO– + HO•
The latter is a strong hydrogen abstracting agent, able to split even cyclohexane C–H bond:
HO• + R–H → R• + H2O
The alkyl radical reacts with dioxygen to form alkyl peroxide radical, which then transforms into alkyl
hydroperoxide[45, 46] via a set of complex reactions:
R• + O2 → ROO•
ROO• + [Mn] → [Mn+1] + ROO–
ROO– + H+ → ROOH
[Mn+1] + H2O2 → [Mn] + H+ + HOO•
ROO• + HOO• → ROOH + O2
The main product from this mechanism is alkyl hydroperoxide (ROOH), not alcohol or ketone. No
alkyl hydroperoxide is formed when the alkane is oxidized by high-valent metal-oxo species without
participation of long-living alkyl radicals (oxygen rebound and related mechanisms).[47, 48]
Cyclohexyl hydroperoxide is a relatively unstable compound and its decomposition route depends on
reaction and/or analytical conditions.[45] While in a typical catalytic mixture it gradually decomposes
to cyclohexanol and cyclohexanone, an attempt to analyse the reaction samples using conventional gas
chromatography (GC) technique may lead to other products due to catalytic decomposition of
hydroperoxide in the hot injector and/or GC column. Bearing this in mind, reaction samples are
typically quenched with a strong reducing agent (Ph3P) before injection into the GC, according to the
following reaction:
12

ROOH + Ph3P → ROH + Ph3P=O
This method, developed by Shul’pin,[45]quantitatively transforms organo-hydroperoxides to the
respective alcohols and in this way allows a correct estimate of products, avoiding spontaneous
decomposition of RCOOH. Also, the comparison of chromatograms taken before and after addition of
Ph3P may confirm the presence or absence of ROOH in the reaction mixture.[45] For a selected
catalytic system (complex 2 and nitric acid), the concentrations of cyclohexanone and cyclohexanol
were measured before and after addition of Ph3P, giving A/K (alcohol/ketone) ratios of 1 and 19,
respectively. The large prevalence of cyclohexanol after addition of Ph3P is a strong evidence for the
presence of cyclohexyl hydroperoxide.
It has been shown that cyclohexyl hydroperoxide can be directly detected[49] using proper conditions
of GC measurements (particularly, clean injector and non-polar capillary column) and preferably
employing GC-MS techniques.[50-52] Hence, all samples of catalytic reactions taken at 1h reaction
time were subjected for GC-MS tests (see experimental part for details) before and after addition of
Ph3P. All active tests revealed the presence of cyclohexyl hydroperoxide, according to mass-
spectrometry. The example chromatograms of the cyclohexane oxidation catalysed by complex 1 in the
presence of HCl is shown at Fig. 7.
The large peak of ROOH completely disappears after addition of Ph3P to the reaction sample.
Formation of cyclohexyl hydroperoxide is a direct evidence of a free-radical reaction mechanism, as
described above, where the main attacking species is hydroxyl radical. Finally, careful investigation of
the chromatograms disclosed the presence of chlorinated products, namely derivatives of cyclohexane
and acetonitrile (Fig. 6b).
The presence of hydroxyl radicals as the main attacking species was additionally confirmed by a bond
selectivity test. Oxidation of methylcyclohexane revealed the formation of the products (alcohols and
ketones) with the normalized 1° : 2° : 3° ratios shown in Table 3 (Fig. S6). The bond selectivities
exhibited by the catalysts 1–4 are close to those observed for other catalytic systems known to generate
hydroxyl radicals (Table 3). For the copper catalysts 1 and 2 the ratio between primary and secondary
oxygenated C–H bonds was surprisingly low (1° : 2° = 1 : 2) (Table 3), while this parameter typically
ranges from 1 : 3 to 1 : 10. The overall yields of products for 1 and 2 were estimated as 15.2 and 9.9%,
respectively, after 1 hour reaction time. This is slightly lower than those observed for oxidation of
cyclohexane (18.3 and 15.1%, respectively, Fig. 6c).The chromatograms recorded before addition of
13

PPh3 to the reaction samples revealed a group of peaks (Fig. S7, 13.9–14.2 mins retention time) which
disappear after the addition of the solid PPh3. The mass-spectra of this group show peaks up to 155 m/z
(Fig. S8). This m/z value is higher than that expected (130 m/z.) for the methylcyclohexyl
hydroperoxide molecular ion. Therefore, the above group of peaks possibly belongs to derivatives
formed upon decomposition of alkyl hydroperoxides in the GC injector or column. No
methylcyclohexyl hydroperoxide was detected, in contrast to the cyclohexyl hydroperoxide (Fig. 7).
Pyridine was found to be the most efficient promoter in cyclohexane oxidation, giving the best yields in
combination with copper complexes (Table 2). The promoting effect of pyridine has been recognized in
the acceleration of formation of catalytically active species[39] and in hydrogen atom transfer
mediating.[55] Pyridine may also compete with the alkane substrate in the reaction with the hydroxyl
radical,[40, 56] in this way influencing the reaction pathway. The presence of pyridine is somewhat
related to Gif chemistry,[46, 57] where metal (iron or copper) catalysts are used in this solvent or its
mixture with acetic acid. However, in our case pyridine is applied as a promoter, being in a small
concentration compared to that of the solvent. Further, no pyridine derivatives were detected among the
reaction products, suggesting that it does not serve as a hydroxyl radical acceptor under the conditions
used (probably due to 20 fold excess of CyH). Pyridine (and species incorporating it, such as
bipyridine) are known to form stable coordination compounds with copper. Hence, in our case pyridine
presumably acts (i) by coordinating the metal centre, (ii) facilitating proton-transfer steps, e.g. in the
conversion of hydrogen peroxide into the hydroxyl radical[53, 58], and/or (iii) favouring the oxidation
of Cu(I) to Cu(II), as observed for Fe/Py catalytic systems.[46]In the case of pyridine, the difference
between the activities of complexes 1 and 2 can conceivably be explained by higher lability of the
monodentate chloride anions in 2 compared to the nitrate anion in 1 (Scheme 1). It is known that nitrate
anion is able to bind copper ions in a bidentate-chelate mode.[59] The DMF molecule in 1 may
decoordinate in solution, with rearrangement of nitrate to form a chelate, thus interfering with the
coordination of hydrogen peroxide and/or of pyridine promoter, resulting in a lower reaction rate (Fig.
6c).
In the presence of hydrochloric acid promoter both copper complexes 1 and 2 revealed similar
activities (Table 2, Fig. 6b), suggesting the substitution of nitrate anion in the coordination sphere of 1
for chloride anions. The opposite replacement (of chloride in 2 by nitrate), when the reaction is
performed with HNO3 promoter, is slower and may explain the initial period of high reaction rate of the
2/HNO3 system (Fig. 6a, curve 2). The complete inhibition of the catalytic activity in the presence of
14

oxalic acid can be associated to its irreversible coordination to metal centres. Formation of binuclear
species, where oxalic acid serves as a bridge, is also a possibility. In both cases no sufficient vacant
positions remain in the coordination spheres, suppressing the catalytic activity.
The formation of R–Cl products (chlorocyclohexane, Fig. 6b) resembles the peroxidative halogenation
process, catalysed by chloroperoxidases[60] and model metal complexes[61]:
H2O2 + Cl– + H++ R–H → R–Cl + 2H2O
The mechanism of such a process may proceed via metal-promoted oxidation by H2O2of chloride to
HOCl with the subsequent halogenation of the hydrocarbon[60, 62]:
H2O2 + Cl– + H+ → HOCl + H2O
HOCl + R–H → R–Cl + H2O
Alternatively, the metal-catalysed oxidation of chloride with H2O2 in acidic medium can lead to the
chloro radical[63] which, upon reaction with the alkyl radical, forms the halogenated product
(chlorocyclohexane).
3.4 Catalytic oxidation of 1-phenylethanol
Complexes 1–4 were also tested as catalysts for the peroxidative oxidation of 1-phenylethanol to
acetophenone using aqueous tert-butylhydroperoxide (TBHP) as oxidizing agent (Scheme 3), in a
solvent-free medium under typical conditions (see Table 4, footnote a). The choice of this model
reaction was justified by its importance in chemical industry.[64]
The effects of various factors (presence of additives, amount of catalyst and reaction temperature) on
the yield of acetophenone and selectivity of the catalyst were investigated (the results are summarized
in Table 4, and Fig. 9). The reaction carried out under typical additive-free conditions and in the
presence of a low amount of catalyst (0.05 mol% vs. substrate) leads to a rather low (10–20%) yield of
acetophenone and TON (moles of product per mol of catalyst precursor) of 200-500 for all the
investigated complexes (entries 1, 9, 17, 25 in Table 4).
15

The next series of reactions were performed in the presence of the catalyst (1–4) and different additives
(Fig. 9a). The addition of either TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, a free radical) or NHPI
(precursor of phthalimido-N-oxyl radical) increases the yield and TON for 1 and 3 (entries 2, 5, 18,21
in Table 4), whereas no significant influence of these additives is observed for 2 and 4 (entries 10, 13,
26,29 in Table 4).The presence of HNO3 hampers the reaction (entries 4, 12, 20,28 in Table 4). Such
inhibitory effect is observed for all investigated complexes and similar observation can be found in the
literature.[65-68] In contrast, the addition of K2CO3 significantly increases the reaction rate. Such
results are in agreement with the previously recognized promoting effect of basic additives.[69, 70]
The effect of the amount of catalyst was also studied (Fig. 9b, Table 4). A two-fold increase of this
amount causes almost at wo-fold increase of the acetophenone yield in the case of 1 (entry 6 in Table
4); however, if a double amount of 2–4 is used, the yield remains almost unchanged, leading to the
corresponding TON decrease (compare e.g. entries 19 and 22 in Table 4). Blank tests in the absence of
the catalyst precursor, performed under the typical reaction conditions, reveal very low yields (entries
33–35 in Table 4). Moreover, the reaction strongly depends on the temperature (Fig. 8) and we observe
a different influence of temperature on the activity of Mn(III)-based catalyst (3 and 4) comparing to the
Cu(II)-based ones (1 and 2). The highest catalytic activity of Mn(III) complexes was noticed when the
reaction was conducted at 80 °C, whereas the activity of the Cu(II) complexes increases with
increasing temperature and the highest yield and TON values were obtained at 100 °C (Figs. 8, 9c).
The lower yield in the case of the Mn(III) complexes 3 and 4 can be related to their decomposition
under the used reaction conditions in contrast to 1 and 2 which are conceivably more stable.
Attempts to perform the oxidation of 1-phenylethanol at 50 °C resulted in a marked drop in the yield of
acetophenone relative to that obtained at 80 °C (for each complex, Fig. 8). Furthermore, we see that the
nature of the coordinated anion has prominent influence on the activity of the catalyst. Probably, the
behaviours of nitrate and chloride anions should be similar to those for cyclohexane oxidation (see
above), with chloride anions being more labile. The oxidation of 1-phenylethanol at 80 °C without any
additives, in the presence of the same molar amount of 1, led to a much lower yield of acetophenone
than that obtained for 2 (entries 1 and 9 in Table 4) and the same relation is observed for Mn(III)-based
catalyst 3 and 4 (compare entries 17 and 25 in Table 4). The dependences of the yield on the
temperature for complexes 1 and 2 are similar to those reported for aminoalcohol complexes of
16

copper.[71] In all experiments, a high selectivity towards the formation of the ketone was found since
no traces of by-products were detected by GC and GC–MS analyses of the final reaction mixtures (only
the unreacted alcohol and the ketone product were found).
4. Conclusions
We have synthesized novel coordination compounds of Cu(II) and Mn(III) with a polydentate Schiff
base ligand, which were characterized by single-crystal X-ray diffraction. One of the copper
compounds (2) was found to exhibit a rare effect of spontaneous recrystallization from the Fomblin
perfluoropolyether oil. Complexes 1–4 were tested as catalysts in the reaction of mild oxidation of
cyclohexane with hydrogen peroxide in the presence of acidic (nitric, hydrochloric and oxalic acids)
and basic (pyridine) promoters. It was found that the most active systems comprise the copper
complexes 1 and 2 with pyridine as promoter, showing high yields of products (cyclohexanol and
cyclohexanone) up to 21 % based on the substrate. To date, pyridine was known to promote the
catalytic activity of osmium and vanadium complexes,[39, 55] while copper-based systems typically
benefit from an acidic promoter.[25] Herein we demonstrate that pyridine can be successfully applied
as a promoter in the catalytic oxidation of cyclohexane with hydrogen peroxide catalysed by copper
species.
In all the active systems the presence of cyclohexyl hydroperoxide was detected by GC-MS techniques,
pointing out a free radical pathway with the hydroxyl radical as the main C–H attacking species. In
addition, chlorocyclohexane was also detected (in the reaction performed in the presence of HCl), what
constitutes a rare observation and is of significance for achieving alkane chlorination.
In the studies of 1-phenylethanol oxidation with TBHP (tert-butylhydroperoxide) the highest activity
was exhibited by the copper complex 1, whereas the other compound of copper (2), being also active,
showed a slightly lower activity. These copper complexes can also be applied as effective catalyst
precursors in the solvent-free oxidation of 1-phenylethanol.However, the coordination compounds of
manganese (3 and 4) were found to possess a lower activity under all the conditions studied.
17

Acknowledgements
This work was supported by Fundação para a Ciência e Tecnologia (FCT), Portugal (projects
PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013; fellowships SFRH/BPD/63710/2009 and
SFRH/BPD/99533/2014).
Supplementary material
Selected bond lengths and angles for crystal structures of 1–4 (Tables S1–S4), description and figures
of their supramolecular structures, chromatograms and mass-spectra of the reaction products in the
methylcyclohexane oxidation.
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21

Figure 1. Left: the crystal structure of 1with 50% probability thermal ellipsoids,showing the atom
numbering. H atoms are omitted for clarity. Right: The packing of the H-bonded chains in 1 viewed
down the c axis.
Figure 2. Left: the crystal structure of 2 with 50% probability thermal ellipsoids, showing the atom
numbering. H atoms are omitted for clarity. Right: a fragment of the supramolecular 2D layer in 2,
viewed down the crystallographic c axis. DMSO molecules are omitted for clarity.
22

Figure 3.The crystal structure of 2awith 50% probability thermal ellipsoids, showing the atom
numbering. H atoms are omitted for clarity.
Figure 4.The crystal structure of 3with 50% probability thermal ellipsoids, showing the atom
numbering.
23

Figure 5.The crystal structure of 4with 30% probability thermal ellipsoids, showing the atom
numbering.
24

a
b
c
25

Figure 6. Accumulations of oxygenates (sums of cyclohexanol and cyclohexanone) with the time in
cyclohexane (0.2 M) oxidation with H2O2 (1 M), catalysed by complexes 1 or 2 (1 × 10–3 M) in the
presence of HNO3 (0.01 M) (a), HCl (0.01 M) (b) and pyridine (0.01 M) (c), in acetonitrile (total
volume of the reaction solution was 5 mL) at 50 °C.
Figure 7.Fragments of the chromatograms (see Fig. 6b caption for reaction conditions), showing the
main reaction products (recorded before and after addition of solid Ph3P, top and bottom,
respectively).The large peak of cyclohexyl hydroperoxide, seen at 10.22 min (top chromatogram),
completely disappears after addition of Ph3P (bottom chromatogram). The weak peak of salicylic
aldehyde appears from the decomposition of the ligand H2L in the GC injector.
26

Figure 8. Dependences of the acetophenone yield on the reaction temperature, in the presence of
K2CO3 additive.
a
27

b
c
Figure 9. Influence of different additives (TEMPO, K2CO3, HNO3, NHPI) (a), amount of catalyst (b)
and temperature (c) on the yield of acetophenone in oxidation of 1-phenylethanol.
28

Scheme 1. Synthesis of the ligand HL and molecular structures of the coordination compounds
obtained.
Scheme 2. Catalytic oxidation of cyclohexane with H2O2, catalysed by complexes 1–4.
Scheme 3.Catalytic oxidation of 1-phenylethanol with TBPH (aqueous tert-butylhydroperoxide),
catalysed by complexes 1–4.
29