Biomimetic alkane oxidation by iodosylbenzene and iodobenzene diacetate catalyzed by a new manganese porphyrin: Water effect

Article abstract of DOI:10.1016/j.apcata.2015.03.022

This work describes the synthesis and characterization of the novel third-generation catalyst 5,10-(3,5-bromo,4-aminophenyl)-15,20-(phenyl)-2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III) chloride, cis-[Mn^IIIBr₁₂DAPDPP]Cl, and compares the catalytic activity of this compound with the catalytic activity of the first- and second-generation manganese porphyrins [Mn^IIITPP]Cl and cis-[Mn^IIIDAPDPP]Cl, respectively, in cyclohexane, adamantane and n-hexane, oxidation by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)₂). This work also investigates how addition of water and imidazole influences the catalytic systems in the adamantane and cyclohexane oxidation. In the absence of water and imidazole, cis-[Mn^IIIBr₁₂DAPDPP]Cl leads to higher product yields as compared with [Mn^IIITPP]Cl and cis-[Mn^IIIDAPDPP]Cl in cyclohexane oxidation. The third-generation (β-octabrominated) cis-[Mn^IIIBr₁₂DAPDPP]Cl was not fully destroyed in reactions with PhI(OAc)₂ as oxidant. In the presence of imidazole, [Mn^IIITPP]Cl and cis-[Mn^IIIDAPDPP]Cl give superior cyclohexanol yields as compared with cis-[Mn^IIIBr₁₂DAPDPP]Cl. Addition of water during adamantane oxidation by PhI(OAc)₂ increases 1-adamantanol yield. As for cyclohexane oxidation by PhIO or PhI(OAc)₂, the presence of water raises product yields and diminishes catalyst destruction, especially in the case of cis-[Mn^IIIDAPDPP]Cl. The presence of water in systems employing PhI(OAc)₂ as oxidant affords higher product yields as compared with systems that use PhIO as oxidant.

Full text of DOI:10.1016/j.apcata.2015.03.022

Accepted Manuscript
Title: Biomimetic Alkane Oxidation by Iodosylbenzene and
Iodobenzene Diacetate Catalyzed by a New Manganese
Porphyrin: Water Effect
´
Author: Vinıcius Santos da Silva Ynara Marina Idemori
Gilson DeFreitas-Silva
PII:
S0926-860X(15)00180-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.03.022
APCATA 15312
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
6-1-2015
16-3-2015
18-3-2015
Please cite this article as: V.S. Silva, Y.M. Idemori, G. DeFreitas-Silva, Biomimetic
Alkane Oxidation by Iodosylbenzene and Iodobenzene Diacetate Catalyzed by a
New Manganese Porphyrin: Water Effect, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.03.022
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*Graphical Abstract (for review)
Page 1 of 35

*Highlights (for review)
Biomimetic Alkane Oxidation by Iodosylbenzene and Iodobenzene
Diacetate Catalyzed by a New Manganese Porphyrin: Water Effect
Highlights
 Synthesis of a novel ß-brominated catalyst derived from a non-symmetric
porphyrin.
 Water improve product yield in the cyclohexane oxidation by PhIO or
PhI(OAc)2.
 Stability of ß-brominated catalyst toward oxidative degradation in PhI(OAc)2
systems.
Page 2 of 35

*Manuscript
Biomimetic Alkane Oxidation by Iodosylbenzene and Iodobenzene
Diacetate Catalyzed by a New Manganese Porphyrin: Water Effect
Vinícius Santos da Silva1, Ynara Marina Idemori1, Gilson DeFreitas-Silva1
*gilsonufmg@ufmg.br
1 Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de
Minas Gerais, 31270-901. Belo Horizonte, MG, Brazil
*Corresponding author:
Prof. Gilson de Freitas Silva (G. DeFreitas-Silva)
Departamento de Química – Instituto de Ciências Exatas – Universidade Federal de
Minas Gerais
Belo Horizonte, MG, 31270-901, BRAZIL
Tel.: +55 31 3409 5772; fax: +55 31 3409 5700.
E-mail address: gilsonufmg@ufmg.br
Page 3 of 35

Abstract: This work describes the synthesis and characterization of the novel third-
generation catalyst 5,10-(3,5-bromo,4-aminophenyl)-15,20-(phenyl)-
2,3,7,8,12,13,17,18-octabromoporphyrinatomanganese(III) chloride, cis-
[MnIIIBr12DAPDPP]Cl, and compares the catalytic activity of this compound with the
catalytic activity of the first- and second-generation manganese porphyrins [MnIIITPP]Cl
and cis-[MnIIIDAPDPP]Cl, respectively, in cyclohexane, n-hexane, and adamantane
oxidation by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2). This work
also investigates how addition of water and imidazole influences the catalytic systems
in the adamantane and cyclohexane oxidation. In the absence of water and imidazole,
cis-[MnIIIBr12DAPDPP]Cl leads to higher product yields as compared with [MnIIITPP]Cl
and
cis-[MnIIIDAPDPP]Cl.
The
third-generation
(ß-octabrominated)
cis-
[MnIIIBr12DAPDPP]Cl was not fully destroyed in reactions with PhI(OAc)2 as oxidant. In
the presence of imidazole, [MnIIITPP]Cl and cis-[MnIIIDAPDPP]Cl give superior
cyclohexanol yields as compared with cis-[MnIIIBr12DAPDPP]Cl. Addition of water
during adamantane oxidation by PhI(OAc)2 increases 1-adamantanol yield. As for
cyclohexane oxidation by PhIO or PhI(OAc)2, the presence of water raises product
yields and diminishes catalyst destruction, especially in the case of cis-
[MnIIIDAPDPP]Cl. The presence of water in systems employing PhI(OAc)2 as oxidant
affords higher product yields as compared with systems that use PhIO as oxidant.
Keywords: manganese porphyrins; alkanes oxidation; water as additive; iodobenzene
diacetate.
Page 4 of 35

1. Introduction
Over the last decades, practical and economic issues have made investigations into
the selective oxidation of organic molecules under mild conditions stand out in the
scientific field [1]. In this sense, the oxidation of poorly reactive alkanes is noteworthy
[2].
Since the work developed by Groves et al. in 1979 [3], when the authors employed a
first-generation metalloporphyrin to catalyze alkane hydroxylation and alkene
epoxidation, synthetic metalloporphyrins have become the object of many studies on
the oxidation of organic substrates by a number of oxygen donors, such as
iodosylbenzene (PhIO), periodate (IO4-), hypochlorite (ClO-), and iodobenzene
diacetate (PhI(OAc)2) [4, 5]. Despite researchers’ efforts, only in 2009 was the first
industrial-scale process tested - iron and cobalt porphyrins catalyzed cyclohexane
oxidation [6]. Therefore, metalloporphyrin-catalyzed alkane oxidation constitutes a
promising area in chemistry, which has stimulated new works with potential future
applications.
An interesting strategy to improve the catalytic efficiency of metalloporphyrins is to
introduce bulky electron-withdrawing groups into the β-pyrrole positions of the
macrocycle. In the particular case of manganese porphyrins (MnP), the substituents
make the catalyst more reactive: they destabilize the active species MnV(O), which
facilitates oxygen atom transfer from this species to the substrate and increases
product yield [7-10].
Another approach that enhances catalyst efficiency during MnP-catalyzed oxidation
reactions is to add ligands, like imidazole. The ligand influences product yield and
selectivity and diminishes catalyst destruction [11, 12] because it can coordinate to the
metal ion and weaken the Mn=O bond, making the active species MnV(O)P more
reactive toward the organic substrate [13]. Moreover, the added ligand can prevent
displacement of the metal ion from the porphyrin macrocycle plane, thereby reducing
catalyst destruction [11, 13].
Page 5 of 35

Addition of water to such systems can also improve catalyst efficiency; however, there
still is no consensus on how water acts during metalloporphyrin-catalyzed biomimetic
oxidation. A theoretical study by Barcelles et al. [14] has shown that water may act as
an axial ligand. On the other hand, In et al. [15] have reported that water hydrolyzes
PhI(OAc)2, to generate PhIO in situ. Silva et al. [16] have verified that water enhances
catalyst efficiency during cyclohexane oxidation by PhIO and PhI(OAc)2, whereas
CarvalhoDa-Silva et al. [17] have not detected any rise in product yield upon addition of
water to systems that employ PhIO as oxidant.
In this context, we have synthesized and characterized a novel β-octabrominated MnP,
5,10-(3,5-bromo-4-aminophenyl)-15,20-(phenyl)-2,3,7,8,12,13,17,18-octabromoporphy-
rinatomanganese(III) chloride (cis-[MnIIIBr12DAPDPP]Cl, Figure 1) derived from 5,10-(4-
aminophenyl)-15,20-(phenyl)porphyrin (cis-H2DAPDPP, Figure 1) and applied it as a
biomimetic catalyst. We used cis-[MnIIIBr12DAPDPP]Cl and its non-brominated
counterpart cis-[MnIIIDAPDPP]Cl (Figure 1) in alkane (cyclohexane, adamantane, or n-
hexane) oxidation by iodosylbenzene (PhIO) or iodobenzene diacetate (PhI(OAc)2).
Knowing that addition of ligands to the reaction medium significantly improves the
catalytic efficiency of MnPs [28, 31, 33], we also conducted oxidation reactions in the
presence of imidazole or water, to verify how they affect the oxidative process.
Figure 1
2. Experimental
2.1. Materials and Methods
2.1.1. Reagents
5,10,15,20-tetraphenylporphyrin (H2TPP, Figure 1) was purchased from Aldrich
Chemical Co and was purified on an alumina column; CH2Cl2 was used as solvent.
H2TPP: UV–vis (CHCl3), λmax, nm (log ε): 421 (5.48), 516 (4.13), 551 (3.92), 591 (3.81),
Page 6 of 35

and 645 (3.74) [17]. The first-generation MnP complex [MnIIITPP]Cl (Figure 1) was
synthesized as reported in the literature [18]: UV–vis (CH3CN), λmax (nm), (log ε): 373
(4.65), 400 (4.57), 475 (4.96), 528 (3.64), 581 (3.87), 618 (3.96), 763 (2.56). Analytical
grade CH3CN, CH3OH, CH2Cl2, N,N-dimethylformamide (DMF), and CHCl3 were
obtained from Aldrich Chemical Co and distilled prior to use. PhIO was prepared
according to a literature procedure [19], stored at −20o C in a freezer, and assayed
periodically by iodometric titrations. All the other reagents and solvents were of
analytical grade and were used without further purification, unless stated otherwise.
2.1.2. Equipment
UV–vis spectra (190–1100 nm) were recorded on an HP-8453A diode-array
spectrophotometer. Infrared (IR) spectra were registered on a Perkin Elmer
spectrometer model BXFTIR; the samples were prepared in KBr pellets. Room-
o
1
temperature (25 C) H NMR spectra were obtained in CDCl3 on a Bruker DPX-200
Advance spectrometer operating at 200 MHz; tetramethylsilane (TMS) was the internal
standard. Gas chromatography was conducted on a Shimadzu GC-17A chromatograph
equipped with a flame ionization detector and a Carbowax capillary column (measuring
30.0 m × 0.32 mm, with a film thickness of 0.25 µm). The ultrasound equipment
Unique® MaxiClean 1400, 40 kHz, was also employed in the experiments. Cyclic
voltammetry was carried out on an Eco Chemie l-Autolab potentiostat; the GPES
software was used. The electrochemical cell contained a homemade glassy carbon
working electrode, a platinum wire counter electrode, and a homemade Ag/AgCl
reference electrode. Before each measurement, the working electrode surface was
polished to a mirror finish with alumina, rinsed thoroughly with water, cleaned in an
ultrasonic bath with water, and rinsed with water and ethanol. The electrochemical
measurements were accomplished under N2 in dry CH2Cl2 solutions containing 0.1 mol
L-1 N-tetrabutylammonium tetrafluoroborate (TBABF4, Aldrich, 99%) as supporting
electrolyte, MnP 0.5 mmol L-1, and ferrocene as internal standard. Half-wave potentials
Page 7 of 35

(E1/2) are reported versus the Fc+/Fc couple, as recommended by IUPAC for
measurements in non-aqueous solvents [20]. Three to five voltammograms were run
for each manganese porphyrin at any given scan rate (10, 25, 50, 100, 200, and 500
mV s-1). The ESI-MS analyses were conducted on an LCQFleet (ThermoScientific, San
Jose, CA, USA) mass spectrometer equipped with electrospray ionization (ESI) source
operating in the positive ion mode; CH3OH was used as solvent.
2.2. Metalloporphyrin catalyst synthesis
2.2.1. 5,10-(4-nitrophenyl)-15,20-diphenylporphyrin – cis-H2DNPDPP and 5,15-(4-
nitrophenyl)-10,20-diphenylporphyrin – trans-H2DNPDPP
The cis/trans-H2DNPDPP isomers originated as byproducts of H2TPP nitration [16].
The isomers mixture was purified by liquid chromatography on silica (Sigma-Aldrich,
130-270 mesh) column with CHCl3 as eluent. UV-vis electron absorption spectroscopy,
infrared vibrational spectroscopy, and 1H NMR aided product characterization.
2.2.2. 5,10-(4-aminophenyl)-15,20-diphenylporphyrin – cis-H2DAPDPP and 5,15-(4-
aminophenyl)-10,20-diphenylporphyrin – trans-H2DAPDPP
The mixture containing the dinitrophenylporphyrin isomers (cis- and trans-H2DNPDPP,
198.08 mg, 0.28111 mmol) resulting from H2TPP nitration was reduced according to an
adaptation of the procedure proposed by Hasegawa et al. [21]. Concentrated
hydrochloric acid (20 mL) and tin(II) chloride dihydrate (368.18 mg, 1.6318 mmol) were
used in the process, to afford cis-H2DAPDPP and trans-H2DAPDPP. These porphyrins
were purified by liquid chromatography on silica (Sigma-Aldrich, 130-270 mesh)
column; CHCl3 was the eluent. This procedure separated the dinitrophenylporphyrins
cis- and trans-H2DNPDPP, which eluted first, from the diaminophenylporphyrins cis-
and trans-H2DAPDPP. The fractions corresponding to the aminophenylporphyrins were
dried in the rotary evaporator. The mixture of cis- and trans-H2DAPDPP isomers was
solubilized in a small chloroform volume and separated by thin layer chromatography
Page 8 of 35

(TLC) on silica (Sigma-Aldrich, 2-25 µm) as the stationary phase. The TLC plate was
0.5 mm thick, 22 cm high, and 20 cm large. The solution containing the mixture of
porphyrins (70 µL) was applied on the TLC plate and eluted with
a
chloroform/hexane/ethyl acetate (2:1:1) mixture. The fractions corresponding to the cis-
(Rf. 0.21) and trans- (Rf. 0.43) isomers were removed with a spatula. The
silica/porphyrin fractions of each isomer were extracted with CHCl3, which was then
removed in the rotary evaporator. The product was placed in a desiccator containing
SiO2. UV-vis electron absorption spectroscopy, infrared vibrational spectroscopy, and
1H NMR aided characterization of the porphyrins.
Reaction yield: trans-H2DAPDPP: 21.40 mg, 0.03319 mmol, 12 %.
UV–vis (CHCl3) λmax, nm (log ε): 424 (5.25), 522 (4.16), 557 (4.05), 594 (3.78), and 650
1
(3.71). H NMR CDCl3 δ: 8.95 (m, 4H), 8.85 (m, 4H), 8.24 (m, 4H), 8.03 (s, 4H), 7.78
(m, 6H), 7.10 (m, 4H), 4.05 (4H), and -2.73 (s, 2 H).
Reaction yield: cis-H2DAPDPP: 100.86 mg, 0.15643 mmol, 56 %.
UV–vis (CHCl3) λmax, nm (log ε): 423 (5.44), 520 (4.13), 557 (3.98), 593 (3.63), and 650
(3.66). 1H NMR CDCl3 δ: 8.94 (s, 4H), 8.83 (s, 4H), 8.24 (m, 4H), 8.03 (s, 4H), 7.78 (m,
6H), 7.10 (m, 4H), 4.04 (4H), and -2.73 (s, 2 H).
2.2.3. 5,10-(4-aminophenyl)-15,20-(phenyl)porphyrinatomanganese(III) chloride (cis-
[MnIIIDAPDPP]Cl)
The metalloporphyrin cis-[MnIIIDAPDPP]Cl (Figure 1) was obtained according to the
protocol described by Silva et al. [16], which employed the free-base porphyrin (30.51
mg, 0.04732 mmol) and Mn(OAc)2.4H2O (125 mg). Yield 88 % (30.66 mg, 0.04182
mmol). UV–vis (CHCl3), λmax (nm) (log ε): 381(4.25), 481(4.56), 586(3.56), and
624(3.70). FTIR in KBr pellets (cm-1): (1620) δ NH2, (1294) δ porphyrin skeleton, (1010)
δ Mn-N. ESI-MS in CH3OH [cis-MnIIIDAPDPP]+ m/z (relative intensity) 697 (100 %).
Page 9 of 35

2.2.4. 5,10-(3,5-bromo-4-aminophenyl)-15,20- (phenyl)-2,3,7,8,12,13,17,18-octabromo-
porphyrinatomanganese(III) chloride (cis-[MnIIIBr12DAPDPP]Cl)
The metalloporphyrin cis-[MnIIIBr12DAPDPP]Cl (Figure 1) was synthesized as described
by Silva et al. [16]; cis-[MnIIIDAPDPP]Cl (25.75 mg, 0.03512 mmol) and liquid bromine
(~0.3 mL, 5.4 mmol, 100-fold molar excess) were employed. The reaction lasted 1 h.
The yield was 64 % (37.64 mg, 0.02241 mmol). UV–vis (CHCl3), λmax (nm) (log ε): 394
(4.64), 508 (4.85), 622 (3.98), and 664 (4.08). FTIR in KBr (cm-1): (1313) δ porphyrin
skeleton, (1227) ν Cβ-Br, (1042) δ Mn-N. ESI-MS in the positive mode, in CH3OH: cis-
[MnIIIBr12DAPDPP - Cl-] m/z (relative intensity) 1644 (56%); cis-[MnIIIBr12DAPDPP – Br-
and Cl-] m/z (relative intensity) 1566.2 (97.2%), cis-[MnIIIBr12DAPDPP – 2 Br- and Cl-]
m/z (relative intensity) 1486.3 (100%)
2.3. Alkane oxidation reactions
All the catalytic reactions were performed in 2-mL Wheaton® vials sealed with teflon-
faced silicon septa. Reactions were conducted under magnetic stirring, at 25 °C, for 90
min, according to procedures adapted from de Sousa et al. [22]. Oxidation was carried
out in air; either PhI(OAc)2 or PhIO was used as oxygen donor. Reaction mixtures
comprised 2.0 × 10−4 mmol of the catalyst (MnP); 2.0 × 10−3 mmol of the oxidant (PhIO
or PhI(OAc)2); 100 µL of cyclohexane (0.93 mmol), or 100 µL of n-hexane (0.76 mmol),
or 100 µL of adamantane solution in CH2Cl2 (0.2 mol L-1; 0.02 mmol); and 200 µL of
CH2Cl2. The catalyst/oxidant/substrate molar ratio was 1:10:4650 for cyclohexane, and
1:10:3800 for n-hexane. As for adamantane, the catalyst/oxidant/substrate molar ratio
was 1:10:100, because adamantane was poorly soluble in CH2Cl2. When deemed
necessary, the reaction was quenched by addition of sulfite and borax solution [22].
The reaction mixtures were analyzed by capillary gas chromatography; bromobenzene
was the internal standard. The retention times of the products were confirmed by
comparison with the retention times of authentic product samples [23]. The yields were
based on either initial PhIO or PhI(OAc)2. Each reaction was accomplished at least
Page 10 of 35

three times, and the reported data represent the average of the results of these
reactions. Errors in yields and selectivity were calculated on the basis of the
reproducibility of the reactions. The degree of MnP destruction (bleaching) was
determined by UV–vis spectroscopy at the end of the catalytic run. Control reactions
were conducted in the absence of the catalyst. The effect of imidazole was studied by
adding a 10-µL aliquot of an imidazole (Im) 1.0 × 10−2 M solution in CH2Cl2 to the
reaction medium. To test the effect of water on adamantane oxidation, 0.5 µL of water
was added to the reaction mixture. As for cyclohexane oxidation, different volumes of
water were added - 0.5 µL, 1.0 µL, 2.5 µL, and 5.0 µL.
3. Results and Discussion
3.1. Synthesis
The method proposed by Luguya et al. [24] leads to low H2NPTPP yields.
Nevertheless, the mixture of cis- and trans-H2DNPDPP isomers is the main product,
and reduction of these isomers affords the diaminophenylporphyrins cis- and trans-
H2DAPDPP.
The UV-vis spectra of the cis- and trans-isomers (Supplementary material) agree with
literature data [24] and are not significantly different. The spectral profile results from
the presence of an amino group (-NH2) at two para-mesoaryl positions of the
macrocycle. Because these groups and the porphyrin macrocycle are arranged in
different planes, steric hindrance prevents them from interacting with the π-electron
cloud of the porphyrin ring [25].
The vibrational spectra (Supplementary Material) indicate reduction of the nitro group
to amino - the band at 1620 cm-1 refers to symmetric angular deformation of the plane
corresponding to the amino group N-H bond.
The methodology described by Silva et al. [16] yields the second-generation cis-
[MnIIIDAPDPP]Cl and the third-generation cis-[MnIIIBr12DAPDPP]Cl. UV-vis absorption
Page 11 of 35

spectroscopy, infrared vibrational spectroscopy, and mass spectrometry aided
characterization of these metalloporphyrins.
The UV-vis spectra (Figure 2) confirm metal insertion into the porphyrin ring. The
spectra of the MnP display an LMCT band at 385 nm, typical of these complexes, and
fewer bands in the visible region. The infrared vibrational spectra attest to metal
insertion into the porphyrin ring, too - the band due to N-H (pyrrole) bond deformation is
absent, and a new band due to axial deformation of the Mn-Npyrrole bond arises [26].
Figure 2
UV-vis spectroscopy helped to monitor the bromination reaction. Macrocycle
polybromination occurs within only one hour. This result suggests that the presence of
the amino group, which donates electron density, increases the reactivity of the β-
pyrrole and ortho-mesoaryl positions of the macrocycle. Figure 2 reveals a
bathochromic shift in the Soret band of cis-[MnIIIBr12DAPDPP]Cl as compared with the
Soret band of cis-[MnIIIDAPDPP]Cl. This shift usually occurs upon bromination of MnP,
as discussed in the literature [17, 27-29].
The mass spectra of cis-[MnIIIDAPDPP]Cl and cis-[MnIIIBr12DAPDPP]Cl
(Supplementary Material) present a peak at m/z 697.2 and 1644.1, respectively,
associated with loss of the chloride ion. The fact that the amino groups activate and
drive substitutions at the ortho- and para-mesoaryl positions of the porphyrin
macrocycle justifies the introduction of bromine atoms into the mesoaryl rings. Because
the para-mesoaryl position relative to the amino substituent contain the porphyrin
macrocycle, substitutions with bromine only occur in the ortho-mesoaryl positions
relative to –NH2.
3.2. Electrochemistry
Page 12 of 35

The redox potentials associated with the macrocycle and the metal center directly
affect formation of the high-valent catalytically active species that oxidizes organic
substrates [30].
Table 1 contains the data relative to the redox process centered on the metal ion (MnIII
+ e ⇌ MnII). The studied systems are quasi-reversible, because they do not obey
some of the typical reversibility parameters described by Matsuda and Ayabe [52]. The
MnIII/MnII reduction potential of cis-[MnIIIBr12DAPDPP]Cl undergoes a 506-mV anodic
shift as compared with cis-[MnIIIDAPDPP]Cl – the bromine atoms withdraw electron
density from the macrocycle and the metal center, thereby stabilizing the energy of the
frontier orbitals HOMO and LUMO and favoring metal reduction [31, 32]. Additionally,
the steric hindrance conferred by bromine distorts the macrocycle and destabilizes the
energy levels of the HOMO, but not of the LUMO orbitals. The difference between the
energies of the HOMO and LUMO orbitals decreases, culminating in the anodic shift of
the metal center redox potential [28, 33].
Table 1
3.3. Catalytic studies
Investigation into the oxidation of poorly reactive substrates such as cyclohexane, n-
hexane and adamantane (Fig. 3) enables comparison of catalyst efficiency during
oxidation of linear and cyclic alkanes [5].
This work used two distinct oxygen donors in the MnP-catalyzed alkane oxidation
reactions: PhIO and PhI(OAc)2. It also examined how addition of water and imidazole
affects cyclohexane and adamantane oxidation.
Figure 3
Page 13 of 35

3.3.1. MnP-catalyzed n-Hexane oxidation by PhIO or PhI(OAc)2
Linear alkane oxidation is a highly complex process. For this reason, this chemical
transformation has attracted the attention of the scientific community [5, 34, 35].
Because the literature contains few papers on metalloporphyrin-catalyzed n-hexane
oxidation [36, 37], this work aimed to evaluate the efficiency of the synthesized MnP
catalysts in the oxidation of n-hexane by PhIO and PhI(OAc)2.
Fig. 4 reveals that the second-generation cis-[MnIIIDAPDPP]Cl catalyzes n-hexane
oxidation by PhIO less efficiently than the third-generation cis-[MnIIIBr12DAPDPP]Cl, as
judged from the product yields. Moreover, cis-[MnIIIDAPDPP]Cl undergoes more
extensive degradation as compared with cis-[MnIIIBr12DAPDPP]Cl, which is also true
when PhI(OAc)2 is the oxidant. Comparison between the systems that use PhIO and
PhI(OAc)2 shows that the oxidation product yields remain virtually the same in the case
of cis-[MnIIIDAPDPP]Cl, but PhI(OAc)2 destroys the catalyst to a larger extent. As for
cis-[MnIIIBr12DAPDPP]Cl, product yields are slightly better when PhI(OAc)2 is the
oxidant.
Figure 4
An interesting aspect of linear alkane oxidation is selectivity in terms of the oxidation of
primary, secondary, and tertiary carbons, which present distinct bond dissociation
energies: 435.4, 396.1, and 391.1 kJ mol-1, respectively [38]. These energies explain
why none of the reactions afford 1-hexanol; they are also the reason why 3-hexanol is
the preferential product.
The third-generation cis-[MnIIIBr12DAPDPP]Cl gives the highest 2-ol/3-ol ratio. The use
of PhI(OAc)2 as oxidant results in 2-ol/3-ol ratios of ~1. Comparison between alcohol
and ketone product yields reveals that the MnPs are more selective for alcohol
products when PhIO is the oxidant. Because MnP-catalyzed n-hexane oxidation affords
Page 14 of 35

low product yields, we decide to not evaluate the effects of water and imidazole
addition to the catalytic systems.
3.3.2. MnP-catalyzed adamantane oxidation by PhIO
Adamantane is a cyclic alkane that is more reactive than linear alkanes. It can undergo
oxidation at the tertiary carbon, to give 1-adamantanol (1-Adol), or at the secondary
carbon, to furnish 2-adamantanol (2-Adol) and 2-adamantanone (2-Adone). Fig. 5
summarizes the results from adamantane oxidation catalyzed by cis-[MnIIIDAPDPP]Cl
and cis-[MnIIIBr12DAPDPP]Cl.
Figure 5
As in the case of n-hexane, the third-generation cis-[MnIIIBr12DAPDPP]Cl catalyzes
adamantane oxidation more efficiently than the second-generation cis-
[MnIIIDAPDPP]Cl, providing larger 1-Adol and 2-Adol yields as well as lower degree of
catalyst destruction. Both cis-[MnIIIDAPDPP]Cl and cis-[MnIIIBr12DAPDPP]Cl are over
95 % regioselective for 1-Adol. The ketone product (2-Adone) emerges in smaller
amounts (~ 0.5 %).
Comparison of the efficiency of the MnPs prepared herein with the efficiency of some
catalysts described in the literature shows that the second-generation cis-
[MnIIIDAPDPP]Cl leads to product yields similar to those reported by Groves et al. [3],
whereas the third-generation cis-[MnIIIBr12DAPDPP]Cl is more efficient. In the latter
case, the bromine atoms introduced into the β-pyrrole positions of the porphyrin ring
withdraw electron density from the metal center, to destabilize the high-valent active
species. In addition, these same bromine atoms distort the metalloporphyrin
macrocycle, which acquires a saddle-type conformation [39] that diminishes self-
oxidation and consequent catalyst degradation [9, 17, 33]. However, comparison of the
efficiency of the second-generation cis-[MnIIIDAPDPP]Cl with the efficiency of the
Page 15 of 35

second-generation [MnIII(PFTDCPP)]Cl obtained by Doro et al. [40] demonstrates that
the latter catalyst is much more efficient. This might be associated with the presence of
electron-withdrawing substituents (fluorine and chlorine) in the ortho-, meta-, and para-
mesoaryl positions of the porphyrin ring in [MnIII(PFTDCPP)]Cl, which make the high-
valent active species more reactive and less prone to self-oxidation [4]. Besides that,
Doro et al. [40] used a much higher MnP/PhIO ratio (1:100) than the one employed
herein (1:10).
Addition of imidazole to the system based on [MnIIIDAPDPP]Cl almost does not alter 1-
Adol and 2-Adol yields, but it decreases catalyst degradation. Doro et al. [40] reported
similar results for a higher MnP/Im molar ratio (1:20 versus 1:0.5 in our case). As for
the third-generation cis-[MnIIIBr12DAPDPP]Cl, the 1-Adol yield drops slightly, and
addition of imidazole does not change the degree of catalyst destruction. When Doro et
al. [40] used the third-generation [MnII(PFTDCCl8PP)] to catalyze adamantane
oxidation by PhIO in the presence of imidazole, they achieved superior yields of
hydroxylated products as compared with cis-[MnIIIBr12DAPDPP]Cl. This difference may
be associated with the MnP/Im molar ratio: Doro et al. [40] used 1:30, whilst in this
work the ratio is 1:0.5. Moreover, [MnII(PFTDCCl8PP)] bears electron-withdrawing
substituents (Cl and F) in the ortho-, meta-, and para-mesoaryl positions of the
porphyrin macrocycle, which enhance the reactivity of the high-valent active species.
Addition of water diminishes product yields for both the second-generation cis-
[MnIIIDAPDPP]Cl and the third-generation cis-[MnIIIBr12DAPDPP]Cl, but catalyst
degradation decreases in the former case. The lesser degradation of cis-
[MnIIIDAPDPP]Cl might stem from water coordination to the metal center, which
prevents displacement of the metal ion from the porphyrin macrocycle and avoids
catalyst self-oxidation.
3.3.3. MnP-catalyzed adamantane oxidation by PhI(OAc)2
Page 16 of 35

PhI(OAc)2 constitutes an alternative to PhIO. During MnP-catalyzed adamantane
oxidation by PhI(OAc)2, the second-generation cis-[MnIIIDAPDPP]Cl affords lower 1-
Adol yield and catalyst degradation is higher as compared with the third-generation cis-
[MnIIIBr12DAPDPP]Cl. Interestingly, despite the smaller product yield, cis-
[MnIIIDAPDPP]Cl does not generate 2-Adol, which makes the system 100%
regioselective. Comparison between PhI(OAc)2 and PhIO reveals that cis-
[MnIIIDAPDPP]Cl yields less 1-Adol and undergoes more extensive destruction in the
presence of the former oxidant. Nevertheless, the system is 100% regioselective for 1-
Adol. Regarding cis-[MnIIIBr12DAPDPP]Cl, PhI(OAc)2 gives better product yields as
compared with PhIO. Li and Xia [41] have already reported on the use of PhI(OAc)2 to
oxidize adamantane in the presence of MnPs. These authors achieved 32 and 1 %
yield of 1-Adol and 2-Adol, respectively, when they employed [MnIIITPP]Cl as catalyst
in the presence of ionic liquids. Their results resembled the results obtained for cis-
[MnIIIBr12DAPDPP]Cl. It is noteworthy that Li and Xia used a MnP/PhI(OAc)2 ratio of
1:225 versus the MnP/PhI(OAc)2 ratio of 1:10 employed here. When these authors
used the second-generation [MnIIITPPS4], the 1-Adol and 2-Adol yields were 52 and
2%, respectively [41].
Addition of imidazole to the catalytic systems at a MnP/Imidazole ratio of 1:0.5 does not
alter 1-Adol yields in the case of the second-generation cis-[MnIIIDAPDPP]Cl, but
catalyst destruction increases. As for the third-generation cis-[MnIIIBr12DAPDPP]Cl,
addition of imidazole does not augment the 1-Adol yield. Therefore, the use of
imidazole at a MnP/Imidazole ratio of 1:0.5 is not advantageous in these systems. Li
and Xia [41] added imidazole to adamantane oxidation reactions by PhI(OAc)2
catalyzed by the first-generation [MnIIITPP]Cl and the second-generation [MnIIITPPS4]
in ionic liquids. They verified that 1-Adol yields increased at a MnP/Imidazole ratio of
1:50. We believe that the MnP/Im ratio used herein is low, so further studies are
necessary to obtain more efficient catalytic systems.
Page 17 of 35

Addition of water to the systems that use PhI(OAc)2 as oxidant raises 1-Adol yields
slightly for both cis-[MnIIIDAPDPP]Cl and cis-[MnIIIBr12DAPDPP]Cl. These results
reinforce the proposal of In et al., who suggested that PhI(OAc)2 hydrolysis generates
PhIO in situ [15]. On the other hand, addition of water to the systems in which PhIO is
the oxidant affects the yield of hydroxylated products very little.
3.3.4. MnP-catalyzed cyclohexane oxidation by PhIO
Cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) are the main products of
cyclohexane oxidation catalyzed by the investigated MnPs. Fig. 6 shows that, when
PhIO is the oxidant, the second-generation cis-[MnIIIDAPDPP]Cl gives better results
and is more selective toward Cy-ol as compared with the first-generation [MnIIITPP]Cl.
This result is probably associated with the presence of the amino group in the para-
mesoaryl positions of the porphyrin macrocycle - this group may coordinate to the
metal center of another MnP, to destabilize the high-valent MnV(O)P species and make
it more reactive [16, 42, 43].
The system based on the third-generation cis-[MnIIIBr12DAPDPP]Cl is extremely
selective and affords significantly higher Cy-ol yield as compared with the systems
based on the first-generation [MnIIITPP]Cl and on the second-generation cis-
[MnIIIDAPDPP]Cl. This evidences the importance of introducing bromine substituents
into the β-pyrrole positions of the porphyrin macrocycle.
Figure 6
The results depicted in Fig. 6 show that addition of imidazole to the system based on
the [MnIITPP]Cl significantly improves product yield and selectivity toward Cy-ol, which
agrees with literature reports [16, 22]. Indeed, imidazole can coordinate to the metal ion
and weaken the Mn=O bond, making the active species MnV(O)P more reactive toward
the organic substrate. As for cis-[MnIIIDAPDPP]Cl, addition of imidazole does not affect
Page 18 of 35

product yield significantly, because the amino group in the MnP may coordinate to the
metal center of another MnP and diminish the effects of imidazole [43].
Addition of imidazole at a MnP/Im 1:0.5 ratio to the system based on the third-
generation cis-[MnIIIBr12DAPDPP]Cl reduces product yield. Repulsive interactions
between the pentacoordinated MnP (MnP + imidazole and/or MnP + MnP) and the
oxidant PhIO might take place, because the saddle-shaped β-octabrominated
porphyrin macrocycle [39] favors repulsion between the bromine atoms and PhIO. As a
consequence, formation of the high-valent active species becomes more difficult.
3.3.5. MnP-catalyzed cyclohexane oxidation by PhI(OAc)2
Comparison of the efficiency of [MnIIITPP]Cl, cis-[MnIIIDAPDPP]Cl, and cis-
[MnIIIBr12DAPDPP]Cl in cyclohexane oxidation by PhI(OAc)2 reveals that cis-
[MnIIIBr12DAPDPP]Cl is the most efficient, but this third-generation catalyst is less
resistant to self-oxidation. Comparison between the oxidants PhIO and PhI(OAc)2
shows that the product yields vary very little when [MnIIITPP]Cl and cis-
[MnIIIDAPDPP]Cl are the catalysts. In turn, the yields markedly increase in the
presence of cis-[MnIIIBr12DAPDPP]Cl and PhI(OAc)2, but the system becomes less
selective and catalyst destruction augments.
Addition of imidazole (at a MnP/Im ratio of 1:0.5) to the systems based on PhI(OAc)2
(as oxidant) significantly increases Cy-ol in the case of [MnIIITPP]Cl and cis-
[MnIIIDAPDPP]Cl, while product selectivity remains unaltered. This might be due to
axial coordination of imidazole to the MnP metal center, which enhances the reactivity
of the high-valent active catalytic species. As for the third-generation cis-
[MnIIIBr12DAPDPP]Cl, Cy-ol falls significantly, as a result of the steric hindrance
conferred by the saddle-shaped conformation of the catalyst.
Comparison of catalyst efficiency in the presence of the oxidants PhI(OAc)2 and PhIO
during cyclohexane oxidation with added imidazole shows that the former oxidant gives
Page 19 of 35

rise to more efficient systems. Hence, PhI(OAc)2 is an attractive alternative to PhIO in
systems with added imidazole.
Few works have described the use of PhI(OAc)2 in metalloporphyrin-catalyzed
oxidation in the presence of additives [16, 41, 44]. Only Silva et al. and Li and Xia have
reported increased metalloporphyrin catalyst efficiency in the presence of PhI(OAc)2
and imidazole [16, 41].
3.3.6. MnP-catalyzed cyclohexane oxidation by PhIO or PhI(OAc)2 with addition of
water
[MnIIITPP]Cl and cis-[MnIIIDAPDPP]Cl-catalyzed cyclohexane oxidation by PhIO in the
presence of 0.5 µL of water (Fig. 7) gives lower product yield when compared with
systems without water addition. This result contrasts with the report by Silva et al. [16]
and the comparison of the catalysts, cis-[MnIIIDAPDPP]Cl (with two –NH2 group, Figure
1) and [MnIIIAPTPP]Cl (with one –NH2 group) from the work by Silva et al. [16], shows
that the introduction of only one amino group (-NH2) into the para-mesoaryl position of
the porphyrin ring impacts on the catalytic behavior of this catalysts. These findings
suggest that the presence of water may make intermolecular coordination of the amino
groups to the metal center more difficult – hydrogen-bonding may occur between the
amino group and water, which should decrease product yield and selectivity for Cy-ol.
CarvalhoDa-Silva et al. [17] achieved similar results when they used metalloporphyrins
bearing (-COOCH3) groups in the para-mesoaryl positions of the porphyrin macrocycle,
which may also form hydrogen bonds with water. Therefore, we decided to verify how
addition of larger amounts of water to the systems based on PhIO as oxidant affects
product yield (Fig. 7).
Figure 7
Page 20 of 35

Concerning the second-generation cis-[MnIIIDAPDPP]Cl, addition of 1.0 µL of water
increases product yields (Cy-ol + Cy-one) and lowers the degree of catalyst
destruction, probably because water coordinates to the metal center. As proposed by
Barcells et al. [14], the reactivity of the high-valent catalytic active species increases,
which diminishes catalyst destruction by avoiding displacement of the metal ion from
the
porphyrin
macrocycle
plane.
Regarding
the
third-generation
cis-
[MnIIIBr12DAPDPP]Cl, addition of 1.0 µL of water also elevates product yield, but
catalyst degradation remains unaffected. Addition of larger amounts of water shows
that maximum product yield emerges after addition of 2.5 and 1.0 µL of water to the
system based on cis-[MnIIIDAPDPP]Cl and cis-[MnIIIBr12DAPDPP]Cl, respectively.
Interestingly, higher amounts of this ligand slightly reduce product yield, but the yield is
still superior to the yield obtained without addition of water.
When PhI(OAc)2 is the oxidant, addition of 0.5 µL of water elicits different changes as
compared with PhIO as oxidant. For the second-generation cis-[MnIIIDAPDPP]Cl, the
product (Cy-ol + Cy-one) yield rises, whereas the yield remains virtually the same in
the case of the third-generation cis-[MnIIIBr12DAPDPP]Cl. For both cis-[MnIIIDAPDPP]Cl
and cis-[MnIIIBr12DAPDPP]Cl, amounts of water larger than 1.0 µL do not significantly
modify product yield, which are always superior to the product yield achieved in the
absence of water. Larger yields in the presence of PhI(OAc)2 and water suggest that
PhI(OAc)2 hydrolyzes and generates PhIO in situ, as proposed by In et al. [15].
Furthermore, water can also act as an axial ligand, as proposed by Barcells et al. [14].
Together, these two phenomena justify larger product yields in the presence of water
and PhI(OAc)2.
4. Conclusion
The brominated third-generation cis-[MnIIIBr12DAPDPP]Cl complex results from direct
bromination of cis-[MnIIIDAPDPP]Cl, which comprises only two steps. This type of
bromination reaction meets the requirements of green chemistry, but the scientific
Page 21 of 35

community does not employ it very often [27]. Application of the synthesized second-
generation cis-[MnIIIDAPDPP]Cl and the third-generation cis-[MnIIIBr12DAPDPP]Cl as
catalysts for the oxidation of poorly reactive organic substrates by PhIO or PhI(OAc)2
reveals some interesting features. The yield of n-hexane oxidation is low, which attests
to the high stability of this linear alkane. As for the oxidation of adamantane, a cyclic
alkane, both MnPs are regioselective for 1-Adol. Regioselectivity is as high as 100% in
the presence of the second-generation cis-[MnIIIDAPDPP]Cl and PhI(OAc)2. The third-
generation cis-[MnIIIBr12DAPDPP]Cl gives large product yields during cyclohexane
oxidation by either PhIO or PhI(OAc)2.
Addition of 0.5 µL of water during cyclohexane oxidation by PhIO reduces yields.
Larger amounts of water improve product yield in the case of both MnPs, 2.5 and 1.0
µL of water for cis-[MnIIIDAPDPP]Cl and cis-[MnIIIBr12DAPDPP]Cl, respectively. Further
addition of water lowers product yield, which might be a consequence of MnP
hexacoordination – this type of coordination saturates the catalyst coordination sphere
and makes the MnP unavailable for reaction with the oxidant. The addition of water
during cyclohexane oxidation by PhI(OAc)2 increases product yield, especially in the
case of cis-[MnIIIDAPDPP]Cl. However, a more detailed investigation into how water
influences organic substrate oxidation is necessary to establish the role that this ligand
plays in the reactions.
This work has verified that the third-generation cis-[MnIIIBr12DAPDPP]Cl is a more
efficient catalyst than the second-generation cis-[MnIIIDAPDPP]Cl, irrespective of the
oxidant. PhI(OAc)2 can replace PhIO as an oxidant; indeed, sometimes the former
oxidant is even more efficient than the latter.
Acknowledgements
Financial support from CNPq, and FAPEMIG is gratefully acknowledged. We are
grateful to Prof. Dr. Paulo Jorge Sanches Barbeira for use of the potentiostat Eco
Chemie l-Autolab during cyclic voltammetry analysis.
Page 22 of 35

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Page 25 of 35

Figure Captions
Figure 1. Structural representation of the porphyrin and manganese porphyrin
complexes employed in this work.
Figure 2. UV-vis absorption spectra of cis-H2DAPDPP (5.11 x 10-6 mol L-1), cis-
[MnIIIDAPDPP]Cl (1.79 x 10-5 mol L-1) and cis-[MnIIIBr12DAPDPP]Cl (1.06 x 10-5 mol L-1)
in CH2Cl2.
Figure 3. Structural representation of cyclohexane, n-hexane, and adamantane
oxidation reactions.
Figure 4. Oxidation of n-hexane by PhIO or PhI(OAc)2 catalyzed by MnP in CH2Cl2: 1-
hexanol (1-ol), 2-hexanol (2-ol), 3-hexanol (3-ol), 2-hexanone (2-one), and 3-
hexanone(3-one) yields and degree of catalyst destruction. Reactions in the absence of
catalyst did not yield any significant amount of the products. Reaction conditions: [MnP]
= 5 × 10−4 mol L−1, [oxidant] = 5 × 10−3 mol L−1, MnP/oxidant/n-hexane/CH2Cl2 molar
ratio = 1:10:3800:15550, 25 °C, magnetic stirring.
Figure 5. Oxidation of adamantane by PhIO or PhI(OAc)2 catalyzed by MnP in CH2Cl2:
1-Adamantanol (1-Adol), 2-Adamantanol (2-Adol), and 2-Adamantanone (2-Adone)
yields, regioselectivity toward 1-Adol, and degree of catalyst destruction. Reactions in
the absence of catalyst did not yield any significant amount of the products. Reaction
conditions: [MnP]
=
5
×
10−4 mol L−1, [oxidant]
=
5
×
10−3 mol L−1,
MnP/oxidant/adamantane/CH2Cl2 molar ratio = 1:10:100:15550, 25 °C, magnetic
stirring. Regioselectivity for 1-Adol [=100%. 1-Adol /(1-Adol + (2-Adol + 2-Adone)/3)].
Page 26 of 35

Figure 6. Cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) yields and selectivity
toward cyclohexanol obtained in the oxidation of cyclohexane by PhIO, PhIO in the
presence of imidazole, PhI(OAc)2, and PhI(OAc)2 in the presence of imidazole,
catalyzed by MnP in CH2Cl2. Reactions in the absence of catalyst did not yield any
significant amount of the products. Reaction conditions: [MnP] = 5 × 10−4 mol L−1,
[oxidant] = 5 × 10−3 mol L−1, MnP/oxidant/cyclohexane/CH2Cl2 molar ratio =
1:10:4650:15550, 25 °C, magnetic stirring. Selectiv ity Cy-ol [=100%. Cy-ol/(Cy-ol + Cy-
one)].
Figure 7. Oxidation of cyclohexane by PhIO or PhI(OAc)2 catalyzed by MnP in CH2Cl2:
cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) yields, selectivity toward
cyclohexanol, and degree of catalyst destruction. Reactions in the absence of catalyst
did not yield any significant amount of the products. Reaction conditions: [MnIIIP] = 5 ×
10−4 mol L−1, [oxidant] = 5 × 10−3 mol L−1, MnP/oxidant/cyclohexane/CH2Cl2 molar ratio =
1:10:4650:15550, 25 °C, magnetic stirring. Selectiv ity Cy-ol [=100%. Cy-ol/(Cy-ol + Cy-
one)].
Page 27 of 35

Table
Table 1. Potentials of the anodic peak (Epa) and cathodic peak (Epc) and of the half-wave
potential (E1/2) vs Fc+/Fc for MnIII/MnII in manganese porphyrins (in CH2Cl2, TBABF4 0.1 mol L-1,
100 mV s-1).
E (mV)
E (mV)
E
(mV)
ΔE (mV)
Complex
pa
pc
1/2
1/2
cis-[MnIIIDAPDPP]Cl
cis-[MnIIIBr12DAPDPP]Cl
- 765
- 273
- 892
- 373
- 829
- 323
-
506
Page 28 of 35

Figure
Page 29 of 35