Robust Mn(iii): N -pyridylporphyrin-based biomimetic catalysts for hydrocarbon oxidations: heterogenization on non-functionalized silica gel versus chloropropyl-functionalized silica gel

Article abstract of DOI:10.1039/d0dt01383h

Two classes of heterogenized biomimetic catalysts were prepared and characterized for hydrocarbon oxidations: (1) by covalent anchorage of the three Mn(iii) meso-tetrakis(2-, 3-, or 4-pyridyl)porphyrin isomers by in situ alkylation with chloropropyl-functionalized silica gel (Sil-Cl) to yield Sil-Cl/MnPY (Y = 1, 2, 3) materials, and (2) by electrostatic immobilization of the three Mn(iii) meso-tetrakis(N-methylpyridinium-2, 3, or 4-yl)porphyrin isomers (MnPY, Y = 4, 5, 6) on non-modified silica gel (SiO2) to yield SiO2/MnPY (Y = 4, 5, 6) materials. Silica gel used was of column chromatography grade and Mn porphyrin loadings were deliberately kept at a low level (0.3% w/w). These resulting materials were explored as catalysts for iodosylbenzene (PhIO) oxidation of cyclohexane, n-heptane, and adamantane to yield the corresponding alcohols and ketones; the oxidation of cyclohexanol to cyclohexanone was also investigated. The heterogenized catalysts exhibited higher efficiency and selectivity than the corresponding Mn porphyrins under homogeneous conditions. Recycling studies were consistent with low leaching/destruction of the supported Mn porphyrins. The Sil-Cl/MnPY catalysts were more efficient and more selective than SiO2/MnPY ones; alcohol selectivity may be associated with hydrophobic silica surface modification reminiscent of biological cytochrome P450 oxidations. The use of widespread, column chromatography, amorphous silica yielded Sil-Cl/MnPY or SiO2/MnPY catalysts considerably more efficient than the corresponding, previously reported materials with mesoporous Santa Barbara Amorphous No 15 (SBA-15) silica. Among the materials studied, in situ derivatization of Mn(iii) 2-N-pyridylporphyrin by covalent immobilization on Sil-Cl to yield Sil-Cl/MnP1 showed the best catalytic performance with high stability against oxidative destruction and reusability/recyclability.

Full text of DOI:10.1039/d0dt01383h

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N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
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DOI: 10.1039/D0DT01383H
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Robust Mn(III) N-pyridylporphyrin-based biomimetic catalysts for
hydrocarbon oxidations: Heterogenization on non-functionalized silica gel
versus chloropropyl-functionalized silica gel
Victor Hugo A. Pinto, Nathália K. S. M. Falcão, Bárbara Mariz-Silva, Maria
Gardennia da Fonseca, Júlio S. Rebouças*
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Department of Chemistry, CCEN, Universidade Federal da Paraíba, João
Pessoa, PB, 58051-900, Brazil.
*Author for correspondence: Dr. Júlio Santos Rebouças, Universidade Federal
da Paraíba, Departamento de Química, João Pessoa/PB, 58051-900, Brazil,
Tel./Fax: +55-83-3216-7591, E-mail address: jsreboucas@quimica.ufpb.br (Júlio
S. Rebouças); ORCID: 0000-0003-3818-029X

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Abstract
Two classes of heterogenized biomimetic catalysts were prepared and
characterized for hydrocarbon oxidations: (1) by covalent anchorage of the three
Mn(III) meso-tetrakis(2-, 3-, or 4-pyridyl)porphyrin isomers by in situ alkylation
with chloropropyl-functionalized silica gel (Sil-Cl) to yield Sil-Cl/MnPY (Y = 1, 2,
3) materials, and (2) by electrostatic immobilization of the three Mn(III) meso-
tetrakis(N-methylpyridinium-2, 3, or 4-yl)porphyrin isomers (MnPY, Y = 4, 5, 6)
on non-modified silica gel (SiO ) to yield SiO /MnPY (Y = 4, 5, 6) materials. Silica
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gel used was of column chromatography grade and Mn porphyrin loadings were
deliberately kept at a low level (0.3% w/w). These resulting materials were
explored as catalysts for iodosylbenzene (PhIO) oxidation of cyclohexane, n-
heptane, and adamantane to yield the corresponding alcohols and ketones; the
oxidation of cyclohexanol to cyclohexanone was also investigated. The
heterogenized catalysts exhibited higher efficiency and selectivity than the
corresponding Mn porphyrins under homogeneous conditions. Recycling studies
were consistent with low leaching/destruction of the supported Mn porphyrins.
The Sil-Cl/MnPY catalysts were more efficient and more selective than
SiO /MnPY ones; alcohol selectivity may be associated with hydrophobic silica
2
surface modification reminiscent of biological cytochrome P450 oxidations. The
use of widespread, column chromatography, amorphous silica yielded Sil-
Cl/MnPY or SiO /MnPY catalysts considerably more efficient than the
2
corresponding, previously reported materials with mesoporous Santa Barbara
Amorphous No 15 (SBA-15) silica. Among the materials studied, in situ
derivatization of Mn(III) 2-N-pyridylporphyrin by covalent immobilization on Sil-Cl
to yield Sil-Cl/MnP1 showed the best catalytic performance with high stability
against oxidative destruction and reusability/recycle.
Keywords: porphyrin, silica gel, immobilization, oxyfunctionalization, alkane,
biomimetic catalysis

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Abbreviations
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6
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6
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6
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7
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7
MnP, Mn porphyrin; MnPs, Mn porphyrins; CPTS, (3-chlopropyl)trimethoxysilane;
Sil-Cl, 3-chloropropyl-functionalized silica gel; (1), Mn(III) meso-tetrakis(2-
pyridyl)porphyrin chloride (also known in the literature as MnT-2-PyPCl); (2),
Mn(III) meso-tetrakis(3-pyridyl)porphyrin chloride (also known in the literature as
MnT-3-PyPCl), (3), Mn(III) meso-tetrakis(4-pyridyl)porphyrin chloride (also known
in the literature as MnT-4-PyPCl); MnP4, Mn(III) meso-tetrakis(N-
methylpyridinium-2-yl)porphyrin chloride (also known in the literature as MnTM-
2-PyPCl5); MnP5, Mn(III) meso-tetrakis(N-methylpyridinium-3-yl)porphyrin
chloride (also known in the literature as MnTM-3-PyPCl ); MnP6, Mn(III) meso-
5
tetrakis(N-methylpyridinium-4-yl)porphyrin chloride (also known in the literature
as MnTM-4-PyPCl ); MCM-41, mesoporous silica Mobil Composition Matter No
5
41; SBA-15, mesoporous silica Santa Barbara Amorphous No 15; DR-UV/VIS,
diffuse reflectance UV/VIS spectroscopy; Cy-H, cyclohexane; Cy-ol,
cyclohexanol; Cy-one, cyclohexanone; Adm, adamantane; Adm-1-ol, 1-
adamantanol; Adm-2-ol, 2-adamantanol; 1-ol, 1-heptanol; 2-ol, 2-heptanol; 3-ol,
3-heptanol; 4-ol, 4-heptanol; 2-one, 2-heptanone; 3-one, 3-heptanone; 4-one, 4-
heptanone.

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Introduction
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The three neutral Mn(III) meso-tetrakis(2-, 3-, or 4-pyridyl)porphyrin
isomers [(1), (2), (3)] and their three methylated, cationic derivatives Mn(III)
meso-tetrakis(N-methylpyridinium-2, 3, or 4-yl)porphyrins (MnPY, Y = 4, 5, 6)
(Fig. 1) comprise a class of Mn porphyrins (MnPs) greatly studied as biomimetic
models for oxidoreductase enzymes, such as, cytochromes P450, peroxidases,
catalases, and superoxide dismutases, under homogeneous conditions.1-11
Although these Mn porphyrins possess structural features, such as dangling
pyridyl moieties on (1), (2), and (3) (suited for quaternization reactions) or
permanent cationic N-methylpyridinium moieties on MnPY (Y = 4, 5, 6) (suited
for electrostatic interactions), they remain little explored as heterogenized
oxidation catalysts as compared to regular meso-tetraphenylporphyrin
derivatives.12,13 Among the isomers of these Mn porphyrins, most of the
heterogenization work is centered on (3) and MnP6, given their traditionally
easier preparation routes and commercial availability.
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1
1
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Mn(III) N-pyridylporphyrins
N
Cl
Ar =
Ar =
(
1)
Cl
Cl
N
HO
Cl
N
Ar
Ar
Ar
N
N
Mn
N
N
O
O Si
O
Si
O
N
N
Cl
(2)
(3)
O
Si
N
Mn
N
Cl
N
OH
Si
HO
Cl
N
OH
Si
N
Si
O
Sil-Cl
Cl
Ar
Ar =
N
Cl
Si
O
Si
O
H O, 
2
O
Si
Si
O O
O
OH
MeOTs
Sil-Cl/MnP3
Mn(III) N-methylpyridiniumporphyrins
HO
N
N
OH
Si
OH OH
Si
N
Ar =
MnP4
MnP5
MnP6
N
Ar
Ar
Ar
Ar
HO
M
n
N
O
Si
N
N
OH
Si
N
OH
N
N
Ar =
Ar =
HO
N
O
O
OH
HO
N Mn N
HO
Si
HO
Si
OH
Si
N
SiO2
H O, 
Si
HO
N
2
SiO /MnP6
2
1
1
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Figure 1. Schematic representation of the two classes of catalysts prepared and
used in this work: in situ derivatization of neutral Mn porphyrin (3) by 3-
chloropropyl-functionalized silica gel (Sil-Cl) to yield Sil-Cl/MnP3 (related (1) and
(2) isomers were anchored similarly); methyl tosylate (MeOTs) derivatization of
(3) to yield cationic Mn porphyrin MnP6 followed by immobilization on silica gel
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(SiO ) to yield SiO /MnP6 (related MnP4 and MnP5 isomers were anchored
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similarly).
Neutral Mn(III) meso-tetrakis(2-, 3-, or 4-pyridyl)porphyrins (1), (2), (3) are
simple first-generation Mn porphyrins amenable to immobilization on haloalkane-
functionalized materials through covalent bonding between the porphyrin pyridyl
moieties and the support-based pendant organic halides, generating a cationic
N-alkylpyridinium moiety of a MnPY-type structure with halide as counter-
ion.1,14,15 Thus, (3) has been successfully anchored on iodopropyl- and
chloropropyl-functionalized silica gel (Sil-I and Sil-Cl, respectively)14,15 or
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chloropropyl silica-coated magnetic nanoparticles for alkene epoxidations and
alkane hydroxylations. (3) supported on chloromethylated polystyrene yielded
suitable catalysts for alkene epoxidation, alkane oxidation, sulfides oxidation, and
carboxylic acid decarboxylation.17-19 Of note, the Fe(III) analogue of (3) was
immobilized on Sil-Cl and chloropropyl-functionalized Santa Barbara Amorphous
No 15 mesoporous silica (SBA-15Cl) for the oxidative degradation of
pentabromophenol.20
Mn(III) N-methylpyridiniumporphyrins (MnPY, Y = 4, 5, 6) are the simplest
second-generation Mn porphyrins derived from (1), (2), and (3), respectively (Fig.
1). They are a class of water-soluble cationic MnPs considerably more studied in
catalysis than their neutral precursors (1), (2), (3). Their immobilization on inert
supports may be readily achieved by electrostatic interactions between the MnPY
permanent positive pyridinium charges and a negative support surface.3,12,21,
MnPY-based oxidative heterogenized catalysts have been prepared by
anchoring theses cationic MnPs on supports, such as, non-modified amorphous
silica gel,12,22 imidazole- or- sulfonato-functionalized amorphous silica , Mobil
Composition Matter No 41 (MCM-41) and Santa Barbara Amorphous No 15
(SBA-15) mesoporous silica,1,3,24 amorphous- or hexagonal mesoporous silica
(HMS)-coated magnetic nanoparticles,3,25 and clays.21,26 As compared to the
corresponding homogenous systems, heterogenization of MnP6 yielded
generally better oxidation catalysts.12,24,26
23
1
Recently, a joint effort between our group and Nakagaki’s group resulted
in 2 classes of mesoporous SBA-15-based materials exploiting these Mn
porphyrins, i.e., (1), (2), and (3), covalently derivatized by and heterogenized on

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chloropropyl-functionalized mesoporous SBA-15 silica (SBA-15Cl/MnPY) and
DOI: 10.1039/D0DT01383H
MnPY isomers electrostatically immobilized on non-functionalized mesoporous
SBA-15 silica (SBA-15/MnPY). These materials were able to catalyze the
cyclohexane (Cy-H) oxidation to cyclohexanol (Cy-ol) and cyclohexanone (Cy-
one) by iodosylbenzene (PhIO) with moderate yields, good selectivity toward
alcohol, and excellent recyclability. Cyclohexane and cyclohexanol are well-
studied substrates for model oxidation reactions and of industrial and economic
relevance27-30 to the production of adipic acid and caprolactam, which are, in turn,
used for the manufacture of nylons (nylon-66 and nylon-6).27-29
Herein we simplified and extended the scope of these heterogenized
catalytic systems by replacing SBA-15-type silicas with cheaper, ordinary,
column chromatography grade, amorphous silica gel as inert supports, to yield
the corresponding classes of materials containing covalently bound cationic
complexes (Sil-Cl/MnPY, Y = 1, 2, 3) and electrostatically immobilized cationic
MnPY complexes (SiO /MnPY, Y = 4, 5, 6) (Fig. 1). Given the low cost and
2
widespread availability of chromatographic silica in chemistry laboratories, these
new Sil-Cl/MnPY and SiO /MnPY materials should be of broader interest and
2
greater accessibility for oxidation catalysis than our previously reported SBA-15-
based systems. Neutral (1), (2), and (3) and cationic MnPYs are the Mn porphyrin
sources for the preparation of these heterogenized catalysts. Despite their
different chemical nature prior to immobilization, the final porphyrinoid
compounds supported on both Sil-Cl and SiO surfaces are all MnPY-type,
2
cationic Mn(III) N-alkylpyridylporphyrins (Sil-Cl/MnPY and SiO /MnPY, Fig.1).
2
All six solids were investigated as catalysts for PhIO-oxidation of cyclic
alkanes (cyclohexane and adamantane) and a linear alkane (n-heptane). The
oxidation of cyclohexanol to cyclohexanone was also investigated as a control
reaction. The impact of the materials on catalytic efficiency, selectivity, and
recyclability were also addressed with respect to: 1) the effect of MnP
heterogenization (vs. homogenous systems); 2) the nature of the immobilized Mn
porphyrin isomers, 3) catalyst oxidative stability; and 4) some hints on the effects
of the support (SiO vs. Sil-Cl). Additionally, direct comparisons on the Cy-H
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oxidation data with the corresponding literature mesoporous SBA-15 silica
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systems were undertaken.

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Experimental
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General Information
All reagents and solvents were of commercial, high purity grade and
detailed in the Electronic Supplementary Information (ESI, Section S1). Silica gel
for column chromatography (SiO , high-purity grade, particle size of 70 230
2
mesh, median pore diameter 60 Å, Aldrich) was activated as described
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elsewhere. Sil-Cl was prepared using a slightly modified literature procedure
and characterized3,12,16-23,25,26,31,33-36 as described in ESI (Sections S2 and S3).
Complexes (1), (2), (3), and MnPY (Y = 4, 5, 6) were prepared by literature
routes.4,37,38-40 The isolated complexes in the chloride form were
chromatographically and spectroscopically identical to the samples used in our
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4,5,41,38,42-44
–
previous study to those reported elsewhere.
Tosylate (OTs ) salts of
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MnPY (Y = 4, 5, 6) were prepared by ion exchange chromatography. Details on
MnP synthesis and characterization are given in ESI (Section S4 and S5).
Iodosylbenzene (PhIO) was prepared from (diacetoxyiodo)benzene (PhI(OAc)2,
Aldrich) as reported elsewhere.46 PhIO was stored under refrigeration and its
active oxygen content was determined periodically by iodometry. All equipment
for analytical and physical measurements are given in ESI (Section S1).
Preparation of Sil-Cl/MnPY (Y = 1, 2, 3) materials
The detailed preparations of these materials are given in ESI (Section S6).
-1
Briefly, 19.55 µmol of (1) (as a 0.405 mmol L aqueous solution) and 5.00 g of
Sil-Cl were refluxed under mechanical stirring for 24 h until a full discoloration of
the supernatant and a darkening of the solids were observed. The suspension
was filtered and the solid was washed with small portions of H O until the
2
presence of MnP in the washings was not detected by UV/VIS spectroscopy; the
washing procedure was repeated with MeOH, EtOH, and CHCl (in this order).
3
All washings were collected to allow for the spectrophotometric determination of
non-immobilized (1). The resulting solid was oven dried at 80 ⁰C for 24 h to yield
4.86 g of Sil-Cl/MnP1 as a light orange material. The amount of Mn porphyrin
immobilized onto Sil-Cl was determined indirectly by analyzing
spetrophotometrically the recovered Mn porphyrin in reaction supernatant and
washing solutions as reported elsewhere.1-3 Mn porphyrin loading (in µmol MnP

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per g of Sil-Cl) was calculated as the ratio between the immobilized amount of
DOI: 10.1039/D0DT01383H
MnP and the starting mass of Sil-Cl. The immobilization yield was defined as the
ratio between the amounts of immobilized versus starting MnP. Preparations of
Sil-Cl/MnP2 and Sil-Cl/MnP3 as light green solids were carried out similarly (see
ESI, Section S6 for details).
These reactions were repeated by different coauthors, using
independently prepared samples, to check for reproducibility. No apparent
differences were observed between corresponding Sil-Cl/MnPY (Y = 1, 2, 3)
batches.
Preparation of SiO /MnPY (Y = 4, 5, 6) materials
2
The synthesis and workup procedures for the electrostatic
heterogenization of the chloride salts of cationic MnPY isomers (Y = 4, 5, 6) onto
SiO were similar to that described for the Sil-Cl/MnPY materials, except that the
2
nature of the starting reagents were changed and the reaction time was
shortened to 3 h (see ESI, Section S7, for details). MnPY loadings and
immobilization yields were determined as described above.
Oxidation reactions
A detailed description of the methodology used for the oxidation reactions
is given in ESI (Sections S8 and S9). Briefly, 2 mL vials were charged with
predefined amounts of PhIO, catalyst, 1:1 MeCN:CHCl solvent, and substrate,
3
and sealed with a teflon/silicone septum and a screw cap. The oxidation reactions
were carried out at room temperature (ca. 26 ºC), under air, in the absence of
light, with magnetic stirring. The reactions were quenched at 90 min with 50 µL
of a saturated sodium tetraborate and sodium sulfite aqueous solution (see ESI,
-1
Section S9). Then, 50 µL of a 28 mmol L solution of a GC internal standard in
MeCN:CHCl (1:1, v/v) was added. The products of the reactions were quantified
3
by gas chromatography (GC). Bromobenzene (PhBr) was used as internal
standard for the reactions with cyclohexane, cyclohexanol, and adamantane as
substrates, while 1-octanol was used in the reactions of n-heptane oxidation.The
reported yields for all products represent an average of least three replicates and
were calculated based on the initial amount of PhIO (limiting reactant),
considering the following stoichiometry: 1 mol of PhIO per mol of alcohol and 2

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mols of PhIO per mol of aldehyde or ketones.1,4,12 For the homogeneous systems
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(using non-immobilized MnP), MnP destruction (bleaching) was determined
spectrophotometrically after quenching the reaction.
For the recycling studies, the solid catalysts were recovered by
centrifugation, washed with CHCl , EtOH, MeOH, and H O (in this order), dried
3
2
at 80 ºC for 6 h, and, then, reused in a new oxidation run. The reaction media
and collected washings were analyzed by UV/VIS spectroscopy to verify whether
there was leaching of MnPs from the solid catalysts.
The control reactions were carried out in the absence of catalysts but
containing (i) PhIO+substrate+solvent, (ii) support+substrate+solvent (labeled
Sil-Cl or SiO entries), (iii) supports+PhIO+substrate+solvent (labeled SiO /PhIO
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or Sil-Cl/PhIO entries).
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72 Results and discussion
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Characterization of Sil-Cl and SiO supports
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Sil-Cl was synthesized by chemical functionalization of chromatographic
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grade amorphous silica gel (SiO ) with CPTS by the heterogeneous route. The
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amount of choropropyl moieties on the silica surface was estimated by elemental
analysis of carbon (3.72%) and chlorine (2.91%), which is consistent with a
-1
functionalization of 930 µmol g , considering the a stoichiometry of three carbon
atoms per chlorine atom. Both Sil-Cl and SiO were also characterized by fourier-
2
transform infrared spectroscopy, thermogravimetric analysis, textural analyses,
scanning electron microscopy, transmission electron microscopy, and 29Si and
13C nuclear magnetic resonance spectroscopies. The results agree with literature
data and the corresponding discussions are presented in ESI (Section S3).
Preparation of Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6).
2
Two classes of heterogenized catalysts were prepared either by in situ
derivatization and covalent immobilization of (1), (2), or (3) on Sil-Cl to yield
Sil-Cl/MnPY catalysts (Fig. 1), or by electrostatic immobilization of MnPY (Y = 4,
5, 6) on SiO to yield SiO /MnPY catalysts (Fig. 1). The immobilization of MnPX
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by the surface chloropropyl groups of Sil-Cl, mimicking, thus, a regular, organic
DOI: 10.1039/D0DT01383H
halide-based alkylating agent (Fig. 1). Conversely, for the immobilization process
of MnPY onto SiO , MnPX was initially quaternized via N-pyridyl methylation to
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yield MnPY,38,40 which was then immobilized onto SiO by exploring the cationic
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nature of the MnPY N-methylpyridinium moieties and the anionic nature of SiO2
surface.1,2,12,13
The MnP immobilization yields and the MnP loadings on Sil-Cl/MnPY (Y
= 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) were calculated by the difference between
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the initial amount of MnP used in the reaction and the amount of unreacted MnP
recovered in the reaction supernatant and washings (Table 1). The amount of
unreacted, recovered MnP in the various washings was determined
spectrophotometrically, as in related systems.1-3 Attempts to quantify MnP in the
solids by UV/VIS (by destroying the silica matrix with aqueous NaOH) or directly
by diffuse reflectance UV/VIS (DR-UV/VIS) were unsuccessful. Attempts to
quantify Mn content by atomic absorption spectroscopy gave unreliable results in
our hands so far, most likely due to unoptimized method for sample treatment.
Table 1. Isolation yields, MnP immobilization yields and MnP loadings related to
the Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) solids.
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Solids
Isolation yield /% Immobilization yield /%a MnP Loading /µmol g-1 a
Sil-Cl/MnP1
Sil-Cl/MnP2
Sil-Cl/MnP3
97
98
92±5
90±3
97±2
~100
~100
4.3±0.9
4.1±0.1
5.9±0.9
5.2
98
SiO /MnP4
~100
~100
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SiO /MnP5
4.6
2
99
SiO /MnP6
~100
5.0
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mean and standard deviation.
In situ derivatization and covalent immobilization yield of MnPs onto Sil-Cl
were on average greater than 90%, whereas electrostatic immobilization of MnPY
onto SiO was essentially quantitative (Table 1), which is in direct contrast with
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the related SBA-15 and HMS systems where immobilization yields were ~60%
and ~28% on average, respectively, for similar MnP loadings.1-3 The

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indicates that higher loads of MnP on SiO may be achieved, but this was not
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investigated herein; materials of high metalloporphyrin loads have not resulted in
better oxidation catalysts in previous studies.13
Immobilization of neutral (1), (2), (3) onto non-modified silica gel (SiO2),
which lacks the possibility for in situ covalent derivatization of these Mn
porphyrins, yielded MnP-containing materials of little interest, as weakly
adsorbed (1), (2), or (3) were essentially fully leached with a variety of organic
solvents (MeOH, EtOH, MeCN, and mixtures with CH Cl or CHCl ); this is
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consistent with the reported behavior of these neutral MnPs toward regular
chromatographic SiO and related data on mesoporous SBA-15 silica.1,4
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Conversely, all attempts to release MnP from Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6) materials via extraction with either water or organic
2
solvents (MeOH, MeCN, EtOH, CHCl ) were unsuccessful, revealing the stability
3
of these materials against MnP leaching. Release of MnP from these materials
was achieved only under drastic conditions via silica matrix dissolution with
concentrated aqueous NaOH. The present study was focused, thus, on the
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) materials.
2
The presence of MnPs on Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y =
2
4, 5, 6) was confirmed by diffuse reflectance UV/VIS (DR-UV/VIS) of the
materials, given the high molar absorptivity of MnPs (see ESI, Section S4 and
9
S5) and their characteristic spectral features. Other routine characterization
techniques as elemental analysis, infrared spectroscopy, and thermoanalysis
were not able to detect the anchored MnPs due to their low loadings of about
0.3% (w/w). These limitations are consistent with previous heterogenized MnP
studies.2,13,21
The DR-UV/VIS spectra of the three Sil-Cl/MnPY (Y = 1, 2, 3) solids share
similar spectral profiles with the corresponding non-immobilized (1), (2), (3)
isomers (Fig. 2), which indicates that the structure of the porphyrin ring was
preserved during immobilization. Indeed, alkylation reactions of the peripheral
pyridyl moieties of Mn(III) N-pyridylporphyrins are accompanied by little spectral
profile changes (see ESI, Section S4). This behavior was also observed in our
1
previous work with mesoporous SBA-15 silica as support. Only Sil-Cl/MnP3,
which incorporates the para isomer, showed a redshift of the Soret band (Fig.

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2e), possibly due to a perturbation of the π-electrons of the porphyrin macrocycle
DOI: 10.1039/D0DT01383H
provoked by the easier interaction with the support.21,47,48
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Figure 2. DR-UV/VIS spectra of the (a) Sil-Cl/MnP1, (b) (1), (c) Sil-Cl/MnP2, (d)
(2), (e) Sil-Cl/MnP3, and (f) (3). The spectra of non-immobilized MnPs were
recorded as solids dispersed on barium sulfate.
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The DR-UV/VIS spectra of the three SiO /MnPY (Y = 4, 5, 6) solids are
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showed in Fig. 3. Again, the Soret band and the Q bands on these spectra were
similar to those of the corresponding non-supported MnPY, indicating that the
overall electronic structure of these systems is little affected by the electrostatic
immobilization. This is consistent with previous observations on mesoporous
SBA-15 and HMS systems.1-3
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Figure 3. DR-UV/VIS spectra of (a) SiO /MnP4, (b) MnP4, (c) SiO /MnP5, (d)
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MnP5, (e) SiO /MnP6, and (f) MnP6. The spectra of non-immobilized MnPs were
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Catalytic biomimetic oxidations
The oxyfunctionalization of alkanes by C-H bond activation and/or
selective oxidation under mild conditions remains a challenge in academia and
industry and a much sought-after subject in catalysis.28-30,49-52 The efficiency of
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) materials as catalysts for
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C–H activation via alkane hydroxylation was evaluated in typical
metalloporphyrin-based model reactions using cyclohexane, n-heptane, and
adamantane as substrates and iodosylbenzene (PhIO) as oxidant (Fig. 4).
Oxidation of cyclohexanol was also carried out to uncover some features of
ketone formation in these systems. In all cases, reactions with the corresponding
non-immobilized MnPs ((1), (2), (3) or MnPY) under homogeneous conditions
were carried out to evaluate the role played by the support on catalytic efficiency
and reaction selectivity. Chloride salts of the cationic MnPY (X = 4, 5, 6) isomers
are, however, poorly soluble in 1:1 (v/v) MeCN:CHCl solvent mixture used in the
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oxidation reactions. Thus, the corresponding tosylate salts of MnPY, which are
soluble in MeCN:CHCl solvent mixtures, were used in the homogeneous
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reactions. Herein, the product yields for alkane oxidations were calculated based
on the initial amount of PhIO (limiting reactant), considering a 1:1 stoichiometry
for alcohols and 2:1 stoichiometry for aldehyde and ketones.1,4,12
O
PhIO
OH
MnP
Cy-H
Cy-ol
Cy-ol
Cy-one
O
PhIO
MnP
OH
Cy-one
OH
O
PhIO
MnP
OH
Adm
Adm-1-ol
Adm-2-ol
Adm-2-one
O
O
O
O
1-heptanal
2-one
OH
3-ol
OH
4-one
OH
PhIO
MnP
n-heptane
HO
1
-ol
2-ol
3-ol
4-ol
2
2
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Figure 4. Reaction schemes for the oxidation of cyclohexane (Cy-H),
cyclohexanol (Cy-ol), n-heptane, and adamantane (Adm) catalyzed by
homogeneous and immobilized MnPs studied in this work.
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Cyclohexane as model substrate
Cyclohexanol (Cy-ol) and cyclohexanone (Cy-one) yields and Cy-ol selectivity for
cyclohexane (Cy-H) oxidation catalyzed by the heterogenized materials Sil-
Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) for three independent catalytic
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cycles are presented in Figures 5 and 6, respectively. The data for all control
reactions, including the homogeneous systems with the non-immobilized Mn
porphyrins, are given in ESI (Section 10, Fig. S15). All reactions were carried out
using a MnP/PhIO/Cy-H molar ratio of 1:10:4628. In the control reactions of Cy-
H with PhIO alone, SiO /PhIO, or Sil-Cl/PhIO, a low total yield (Cy-ol + Cy-one)
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of about 2% was observed, indicating that the supports SiO or Sil-Cl were unable
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to catalyze efficiently the PhIO-oxidation of cyclohexane in the absence of Mn
porphyrin (ESI, Section 10, Fig. S15).
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Figure 5. First (cycle 1), second (cycle 2), and third (cycle 3) cycles of
cyclohexane oxidation reactions by PhIO catalyzed by the supported materials
Sil-Cl/MnPY (Y = 1, 2, 3). After completion of each cycle, the catalysts were
recovery and reused. Reactions conditions: MnP/PhIO/Cy-H molar ratio of
1:10:4628 (0.2 µmol of MnP, 2.0 µmol of PhIO, 930.0 µmol of Cy-H),
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MeCN:CHCl mixture (1:1, v/v) as solvent, magnetic stirring, 26 ºC, 90 min, and
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air atmosphere. Yields calculated based on the starting PhIO. 2 mol of PhIO were
considered for the formation of 1 mol of Cy-one. Total yield = Cy-ol + Cy-one.
Selectivity = 100 x [Cy-ol/(Cy-ol + Cy-one)].

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Figure 6. First (cycle 1), second (cycle 2), and third (cycle 3) cycles of
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cyclohexane oxidation reactions by PhIO catalyzed by the supported materials
SiO /MnPY (Y = 4, 5, 6). After completion of each cycle, the catalysts were
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recovery and reused. The reaction conditions were identical to those of Figure 5.
Yields calculated based on the starting PhIO. 2 mol of PhIO were considered for
the formation of 1 mol of Cy-one. Total yield = Cy-ol + Cy-one. Selectivity = 100
x [Cy-ol/(Cy-ol + Cy-one)].
A comparison between the supported MnPs systems (on either Sil-Cl or
SiO ) (Figs. 5 and 6) with their corresponding non-immobilized MnP counterparts
2
(ESI, Fig. S15) reveals a considerable increase in the total oxidation yield (Cy-ol
+ Cy-one) for all heterogenized systems Sil-Cl/MnPY or SiO /MnPY. A likely
2
explanation, which is supported by recycling experiments (see below) is related
to the increased oxidative stability of MnPs upon immobilization on either Sil-
Cl/MnPY (Y = 1, 2, 3) or SiO /MnPY (Y = 4, 5, 6) materials as compared with
2
their corresponding homogeneous systems. Of note, (1), (2), (3) and MnPY (Y =
4, 5, 6) in homogeneous systems showed oxidative destruction (bleaching) of
about 70%, as determined spectrophotometrically in the end of the reactions. The
role of inert supports in the increased oxidative stability of MnPs is well
documented.12,26,53,54

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The highest Cy-ol selectivities were observed in the Sil-Cl/MnPY (Y = 1,
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2, 3) solids and this may also be related to a role played by the nature of the
support. Sil-Cl/MnPY solids (Fig. 5, entries 1, 4, and 7) showed an overall 10%
higher selectivity and 20% higher total yield than SiO /MnPY solids (Fig. 6,
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entries 1, 4, and 7). As suggested in the SBA-15 related systems, this may
tentatively be related to the lipophilic character of Sil-Cl (given the excess surface
chloropropyl moieties) as compared to the more polar nature of unmodified SiO2.
This may facilitate the access of apolar Cy-H substrate) and the replacement of
polar Cy-ol product from the catalytic center, resembling the protein role in
cytochromes P450.11 Proper experiments need, however, to be designed and
carried out to unambiguously assess this surface lipophilic effects on catalysis.
Sil-Cl/MnPY (Y= 1, 2, 3) catalysts based on cheap, ordinary,
chromatographic silica were more efficient and equally selective for cyclohexane
1
oxidation than the more elaborate SBA-15Cl-based catalyst counterparts. The
total oxidation yields achieved with Sil-Cl/MnP1, Sil-Cl/MnP2, and Sil-Cl/MnP3
were of 89, 91, and 93%, respectively (Fig. 5, entries 1, 4, and 7), which contrasts
with those of 54, 62, and 72% reported for SBA-15Cl-based catalysts SBA-
15Cl/MnP1, SBA-15Cl/MnP2, and SBA-15Cl/MnP3, respectively.1 This
difference in overall efficiency between Sil-Cl/MnPY (Y = 1, 2, 3) and
SBA-15Cl/MnPY (Y = 1, 2, 3) may be related to the localization of the MnPs on
the supports. Lower yields with mesoporous SBA-15Cl/MnPY (Y = 1, 2, 3)
materials have suggested that MnP is probably buried inside the mesoporous,
1
hindering access of substrate and oxidant. Conversely, for Sil-Cl/MnPY
catalysts, we speculate that MnPs are predominantly exposed on the material
surface, favoring the catalytic reaction.55-57 A similar behavior was observed with
the SiO /MnPY catalysts when compared to their analogous SBA-15/MnPY
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materials: total yields achieved with SiO /MnPY solids (aprox. 74%, Fig. 6,
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entries 1, 4, and 7) were remarkably higher than those with SBA-15/MnPY
materials (aprox. 45%).1
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) were evaluated in
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recycling reactions (Fig. 5 and 6). All immobilized catalysts maintained a high
overall yield, particularly in cycle 2, with some loss in efficiency on cycle 3. The
Cy-ol selectivity of the reactions were essentially unaffected, at least during 3

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cycles. These results are in direct contrast to those of SBA-15 systems, where
DOI: 10.1039/D0DT01383H
overall yields were reduced by aprox. 50% in the second cycle already.1
Sil-Cl/MnP1 material was the most robust catalyst among all supported
MnPs, showing an efficiency loss of only 10% (Fig. 5, entries 1-3) during the three
oxidation reaction cycles. SiO /MnP4 and SiO /MnP6 catalysts had an efficiency
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loss of 18% and 16%, respectively (Fig. 6, entries 4-6 and 7-9). Conversely,
SiO /MnP5 catalyst showed the largest loss (ca. 29%) in efficiency by the end of
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the third reaction cycle (Fig. 6, entries 1-3). The decrease in catalytic efficiency
throughout the recycling reactions was ascribed to oxidative destruction of
supported MnP (bleaching) along the oxidation cycles, as in other related
systems.58,59 MnP leaching was considered of lower impact (if any) as no sign of
MnP in the reaction supernatant or catalyst washing solvents has been detected
by UV/VIS spectroscopy.
The oxidative stability of the heterogenized catalysts (Sil-Cl/MnPY and
SiO /MnPY) was evaluated using a more drastic condition with a PhIO/MnP
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molar ratio of 100 in the first reaction cycle (cycle A) followed by the standard
reaction condition with a PhIO/MnP molar ratio of 10 in the second reaction cycle
1
(cycle B), as similarly carried out in the literature SBA-15-based systems. The
non-immobilized catalysts were also studied under homogeneous conditions with
PhIO/MnP molar ratio of 100 (cycle A) for comparison with the corresponding
heterogenized systems. All data for these reactions are presented in ESI (Section
11, Figs. S16 and S17). The PhIO, Sil-Cl/PhIO, and SiO /PhIO control systems
2
(without MnP) showed less than 1% Cy-ol and Cy-one formation.
All catalysts showed low catalytic efficiency with a high PhIO/MnP molar
ratio of 100 (Cycle A; ESI Fig. S16). Upon catalyst reuse under standard
PhIO/MnP ratio of 10 (Cycle B; ESI Fig. S17), all catalysts recovered, at least
partially, the efficiency recorded in Figs. 5 and 6. A more general discussion and
some hypotheses1,21,54,60-63 on these results are presented in ESI (Section 11).
Briefly, they indicate that (a) the materials are to some extent reasonably stable
against oxidative degradation under large excess PhIO; (b) SiO2-based materials
were more resist than the corresponding Sil-Cl ones, in agreement with previous
1
data on the SBA-15-based catalysts; (c) the remarkable stability of Sil-Cl/MnP1
and SiO /MnP4 may be associated with the recurring porphyrin-based
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biomimetic feature known as “the ortho effect”,4,38,54,64-66 in which the ortho,

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attack to vulnerable porphyrin meso positions.
Cyclohexanol as model substrate
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) were evaluated as
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catalysts in the oxidation of Cy-ol to Cy-one by PhIO as a means to verify whether
Cy-one produced during the cyclohexane oxidation could result from Cy-ol
product being further oxidized. In these studies using Cy-ol as substrate, the
amount of initial Cy-ol was established considering the maximum theoretical
concentration of Cy-ol corresponding to a 100% yield in the cyclohexane
oxidations, that is, a stoichiometric molar Cy-ol/MnP of 10:1. Cy-one yields were
based on the initial PhIO (limiting reactant), considering 1 mol of PhIO for the
formation of 1 mol of Cy-one.
All materials were able to catalyze the Cy-ol oxidation to Cy-one to some
extent (Fig. 7), indicating, thus, that the ketone formed in the cyclohexane
oxidation may be derived from Cy-ol product reoxidation. Metalloporphyrin-
catalyzed oxidation of alcohol to ketones has been addressed.67-69 However, the
formation of ketone in alkane oxidations via alternative mechanisms, involving
radical reactions55,70,71 cannot be completely ruled out.72 Regardless of the
mechanism(s), in the Cy-ol oxidations catalyzed by Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6), the Cy-one yields were relatively high and comparable
2
to those of the cyclohexane oxidation (Fig. 5). Among these systems, Sil-
Cl/MnP1 and Sil-Cl/MnP3 showed slightly better catalytic performances than the
others (Fig. 7, entry 4 and 6).

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Figure 7. Cyclohexanol oxidation by PhIO catalyzed by supported materials
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6), including the control
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reactions (PhIO, Sil-Cl/PhIO, and SiO /PhIO). The reaction conditions were
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identical to those of Figure 5 except that Cy-ol replaced cyclohexane and the
MnP/PhIO/Cy-ol molar ratio was of 1:10:10. Yields calculated based on the
starting PhIO. 1 mol of PhIO was considered for the formation of 1 mol of Cy-one.
Total yield = Cy-ol + Cy-one.
These preliminary results on Cy-ol oxidation indicate that the
heterogenized MnP materials Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4,
2
5, 6) are potentially effective catalysts for alcohol oxidation and deserve, thus,
further investigation.73,74
n-Heptane as model substrate
n-Heptane is a fairly stable acyclic alkane difficult to be oxyfunctionalized
due to the inertness of its C–H bonds, whose energies are higher for the primary
−
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carbons (101±1.8 kcal mol ) than for the secondary ones (98±1.8 kcal mol ).
The selective oxidation of linear alkanes at the primary carbons is highly sought
and remains a challenge.
The heterogenized catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y
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products (Fig. 4) were detected and independently quantified in all reactions: 1-
heptanol (1-ol), 2-heptanol (2-ol), 3-heptanol (3-ol), 4-heptanol (4-ol), 1-heptanal,
2-heptanone (2-one), 3-heptanone (3-one), and 4-heptanone (4-one). No
attempts were undertaken to resolve the stereochemistry of 2-ol and 3-ol. This
range of oxidation product has been partially reported with related immobilized
metalloporphyrin-based catalytic systems.2,76-78 The n-heptane oxidations were
evaluated for Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) catalytic
2
efficiency, reaction chemo- and regioselectivity, and catalyst recycling. The
product yields for three oxidation cycles with Sil-Cl/MnP1 and SiO /MnP4, as
2
representatives of each catalyst class, is presented in Figure 8. Data for the other
materials along with the homogeneous systems are in ESI (Section 12, Figs. S18
– S20).
In the first cycle of n-heptane oxidation reactions, the control systems
(PhIO, SiO /PhIO, and Sil-Cl/PhIO) led to total product yield lower than 5% (data
2
not shown). Conversely, total oxidation yields in the 66 – 95% range with the
supported catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) under
2
heterogeneous conditions (Fig. 8 and ESI Figs. S18 and S19) were unusually
high for n-heptane oxidation catalyzed by metalloporphyrin-based materials.5,25
The high catalytic efficiency of these material is also in direct contrast with the 33
– 68% total yield range found with the non-immobilized catalysts under
homogeneous conditions (ESI, Fig. S20). This substantial increase in total
oxidation yield with the heterogenized catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6) is likely associated with the remarkable impact of the
2
support on stabilizing the anchored MnPs against oxidative degradation, as
observed in the cyclohexane systems. Of note, non-immobilized MnPs were
bleached by approx. 40% [for (1), (2), (3)] and 50% (for MnP4, MnP5, MnP6),
respectively, during n-heptane oxidation.

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4
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Figure 8. First cycle (cycle 1), second (cycle 2), and third (cycle 3) cycles of n-
heptane oxidation reactions by PhIO catalyzed by the supported materials Sil-
4
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Cl/MnP1 and SiO /MnP4. After completion of each cycle, the catalysts were
2
recovery and reused. Reactions conditions: MnP/PhIO/n-heptane molar ratio of
1:10:5000 (0.2 µmol of MnP, 2.0 µmol of PhIO, 1000.0 µmol of n-heptane),
MeCN:CHCl mixture (1:1, v/v) as solvent, magnetic stirring, 26 ºC, 90 min, and
3
air atmosphere. Yields calculated based on the starting PhIO. 2 mol of PhIO were
considered for the formation of 1 mol of ketone or aldehyde. Total yield = 1-ol +
2-ol + 3-ol + 4-ol + 1-heptanal + 2-one + 3-one + 4-one.
The overall superior efficiency of Sil-Cl/MnPY (Y = 1, 2, 3) catalysts over
SiO /MnPY (Y = 4, 5, 6) observed in Cy-H oxidations (Figs. 5 and 6) was more
2
pronounced on the n-heptane systems, except that SiO /MnP4 was as highly
2
efficient as the Sil-Cl/MnPY (Y = 1, 2, 3) catalysts (Fig. 8 and ESI Figs. S18 and
S19). Another relevant feature of these materials revealed by the n-heptane
oxidations relates to the remarkable change in chemoselectivity within these two
classes of catalysts (Fig. 8 and Table 2). Whereas the SiO /MnPY (Y = 4, 5, 6)
2
materials were more selective toward the carbonyl products (ketones and
aldehyde), Sil-Cl/MnPY (Y = 1, 2, 3) catalysts were more selective toward n-
heptanols, corroborating once more the Cy-H data on the role played by the
surface chloropropyl chain moieties of Sil-Cl support in the oxidation reactions.
Additionally, it is worth noting that non-immobilized (1), (2), and (3) shows
chemoselectivity toward carbonyl products (Table 2 and ESI Fig. S20), indicating

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that a full inversion in n-heptane oxidation chemoselectivity is observed upon
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MnP immobilization on the Sil-Cl support to yield Sil-Cl/MnPY (Y = 1, 2, 3)
catalysts. As described for the Cy-H oxidation reactions, it is suggested that the
chloropropyl chains on Sil-Cl renders Sil-Cl/MnPY (Y = 1, 2, 3) catalysts likely
more lipophilic, facilitating the approach of n-heptane to the materials surface and
the access to the MnP catalytic sites, thus resulting in a high chemoselectivity for
alcohol. The possible role of surface chloropropyl chains in repealing n-heptanols
from the catalyst active sites, preventing, thus, further oxidation to carbonyl
products cannot be fully assessed or ruled out at the moment without further
investigation.
Table 2. Chemoselectivity and Regioselectivity of n-heptane PhIO-oxidations
catalyzed by Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) under
2
heterogeneous conditions or by non-immobilized MnPs under homogeneous
conditions. The reaction conditions are indicated in Figure 8.
g
Regioselectivity /%
Total yield /%
c
Entry
Catalyst
Chemoselect. /%
ω-1d ω-2e ω-3f
a
b
C-OH C=O
1
2
3
4
5
6
7
8
9
Sil-Cl/MnP1
Sil-Cl/MnP2
Sil-Cl/MnP3
50
51
51
36
24
24
16
23
19
11
11
15
36
40
44
48
42
42
46
44
42
15
20
18
58
56
54
43
36
36
26
35
31
43
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33
32
36
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26
21
22
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32
32
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33
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32
34
36
41
46
44
33
43
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SiO /MnP4
2
SiO /MnP5
2
SiO /MnP6
2
(1)
(2)
(3)
10
11
12
MnP4
MnP5
MnP6
45
a
b
5
5
5
5
22
23
24
25
C-OH (alcohols) = 1-ol + 2-ol + 3-ol + 4-ol. C=O (ketones + aldehyde) = 1-heptanal +
c
d
2-one + 3-one + 4-one. Chemoselectivity = (C-OH)/[C-OH + C=O). ω-1: sum of 2-ol
e
f
g
and 2-one, ω-2: sum of 3-ol and 3-one, ω-3: sum of 4-ol and 4-one. Normalized
regioselectivity (ω-1) = 100 x [(2-ol + 2-one)/2]/{[(1-ol+1-heptanal)/3] + [(2-ol + 2-one)/2]

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+ [(3-ol + 3-one)/2] + (4-ol + 4-one)}, normalized regioselectivity (ω-2) and normalized
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regioselectivity (ω-3) were calculated analogously.
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n-Heptane is a substrate that allows also for the evaluation of
regioselectivity associated with the catalyzed oxidations. Considering that n-
heptane primary carbons ω (C1 and C7 positions) have a total of 6 C-H bonds,
the secondary carbons ω-1 (C2 and C6 positions) and ω-2 (C3 and C5 positions)
have a total of 8 C-H bonds, and central secondary carbon ω-3 (C4 position) has
only 2 C-H bonds, all regioselectivities associated with ω, ω-1, ω-2, and ω-3
carbons were normalized to account for the statistical probability of n-heptane
oxidation on each position (Table 2). The regioselectivity of oxidations catalyzed
by the heterogenized catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4,
2
5, 6) was under thermodynamic control, that is, oxyfunctionalization of the primary
carbons was very low (ω regioselectivity was lower than 2% in all cases) and ω-
1 (sum of 2-ol and 2-one), ω-2 (sum of 3-ol and 3-one), and ω-3 (sum of 4-ol and
4-one) regioselectivities were nearly the same, indicating that there was no
oxidation preference among the secondary carbons. Thus, the silica support or
surface organic chain of Sil-Cl did not exert any significant influence (if any) in
guiding the orientation of n-heptane toward the catalytic center.
After the first oxidation cycle, the supported catalysts Sil-Cl/MnPY (Y = 1,
2, 3) and SiO /MnPY (Y = 4, 5, 6) were centrifuged, washed with CHCl , EtOH,
2
3
MeOH, and H O, dried at 80 ºC for 24 h, and reused (cycle 2) under the same
2
reaction condition as those of the first cycle (Fig. 8 and ESI Figs. S18 and S19).
This process was repeated once more, giving rise to a third cycle (cycle 3) (Fig.
8 and ESI Figs. S18 and S19). The robustness of Sil-Cl/MnP1 and Sil-Cl/MnP3
as oxidation catalysts, as observed earlier in the cyclohexane systems, was also
confirmed in the n-heptane reactions; the total oxidation yields associated with
these catalysts were very high (in the 82 – 95% range) and just slightly affected
by reuse for at least three oxidation cycles. The corresponding meta MnP isomer-
based heterogenized catalysts Sil-Cl/MnP2 and SiO /MnP5, however, showed
2
considerable loss of efficiency upon recycling (ESI Fig. S18), as also observed in
the cyclohexane oxidations. In general, the Sil-Cl/MnPY (Y = 1, 2, 3) catalysts
were more resistant against oxidative deactivation than the corresponding
SiO /MnPY (Y = 4, 5, 6) materials.
2

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Despite the catalytic efficiency of Sil-Cl/MnPY (Y = 1, 2, 3) being little
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affected by catalyst recycle in n-heptane oxidations, the chemoselectivity of these
st
reactions were compromised along the three reuses, particularly from the 1 to
the 2nd cycles (Table 2, entries 1–3; and ESI Tables S4 and S5, entries 1–3).
Such a selectivity decrease was not observed in cyclohexane oxidation catalyzed
by these Sil-Cl/MnPY (Y = 1, 2, 3) materials. Given the closer structural similarity
of the surface chloropropyl moieties to n-heptane rather than to cyclohexane, it
is tempting to suggest that some partial destruction of these surface chloropropyl
groups (as a somewhat competing substrate during n-heptane oxidation), may
be likely responsible to decrease the lipophilicity of Sil-Cl surface. This, hence,
would facilitate the access of n-heptanols to the MnP catalytic center, promoting
their further oxidation to the corresponding carbonyl derivatives and, finally,
compromising chemoselectivity for alcohols upon catalyst reuse. Whereas the
putative cumulative oxidation of surface chloropropyl moieties would decrease
chemoselectivity, it may also represent a sacrificial means to protect surface MnP
against oxidative destruction, which is, thus, consistent with little effect on Sil-
Cl/MnPY (Y = 1, 2, 3) catalyst efficiency upon recycling. Conversely, it is worth
noting that the chemoselectivity of the n-hexane oxidations catalyzed by
SiO /MnPY (Y = 4, 5, 6) remains essentially unchanged upon recycling; SiO2
2
support, as opposed to Sil-Cl, is not prone to PhIO modification and the decrease
in catalyst efficiency observed with the SiO /MnPY (Y = 4, 5, 6) systems may be
2
a result of direct surface MnPY degradation during reuse; this is consistent with
a direct effect on efficiency, but of little impact on chemoselectivity.
Catalyst recycling exert little effect on regioselectivity of n-heptane
oxidation catalyzed by Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6)
2
(Table 2 and ESI Tables S4, and S5).
Adamantane as model substrate
The adamantane is a substrate bulkier and conformationally more rigid
than cyclohexane and n-heptane. The oxidation in its tertiary carbons results in
1-adamantanol (Adm-1-ol), whereas oxidation in its secondary carbons results in
either 2-adamantanol (Adm-2-ol) or adamantanone (Adm-2-one), as depicted in
Figure 4.54,79-82

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Figure 9 summarizes the data obtained on adamantane oxidation
DOI: 10.1039/D0DT01383H
catalyzed by the heterogenized catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6) and their homogeneous MnP counterparts. Control
2
experiments (PhIO, Sil-Cl/PhIO, and SiO /PhIO) in absence of catalysts showed
2
total oxidation yields as little as 6%. Under homogeneous conditions, (1), (2), and
(3) isomers were equally efficient among themselves (45% on average; Fig. 9
entries 1-3) and more efficient than homogeneous MnPY (Y = 4, 5, 6) (25% on
average, Fig. 9, entries 7-9). A considerable increase in catalytic efficiency was
achieved upon heterogenization, as observed previously in the cyclohexane and
n-heptane systems; total oxidation yields for adamantane oxidations catalyzed by
Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) ranged from 55% to 81%
2
(Fig. 9, entries 4-6, 10-12). Such an efficiency improvement may be attributed to
the role played by the support in increasing the oxidative stability of immobilized
MnPs. Total oxidation yields achieved with Sil-Cl/MnPY (Y = 1, 2, 3) catalysts
(77% on average, Fig. 9, entries 4-6) were approx. 15% greater than those with
SiO /MnPY (Y = 4, 5, 6) catalysts (62% on average, Fig. 9, entries 10-12), which
2
may tentatively be related to the lipophilicity of Sil-Cl surface as discussed for the
cyclohexane and n-heptane oxidations.
In all systems, hydroxylation was the main reaction observed giving rise to
Adm-1-ol and Adm-2-ol. Except in the SiO /MnPY (Y = 4, 5, 6) systems, in which
2
further Adm-2-ol oxidation led to Adm-2-one in yields as low as 5% (Fig. 9, entries
10-12), the formation of ketone in all other systems was lower than 1%, if any.
Whereas such small but measurable effect may perhaps be related to the
hydrophilic nature of SiO support in SiO /MnPY (Y = 4, 5, 6) catalysts, analyses
2
2
of chemoselectivity were of little relevance in these adamantane-based model
reactions.
Regarding reaction regioselectivity, all catalysts studied herein were
selective toward hydroxylation of adamantane, chiefly at its tertiary carbon to yield
Adm-1-ol, following a similar behavior reported to other non-supported and
supported metalloporphyrin-based systems.54,79-81 The high preference toward
Adm-1-ol is ascribed to the lower C–H bond strength of adamantane tertiary
carbon, which results in its higher reactivity.79,82-84 In all systems, regioselectivity
was normalized with respect to the statistical probability of the oxidation associate

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with the number of C–H bonds, i.e., 4 tertiary C-H bonds to yield Adm-1-ol, and
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12 secondary C-H bonds to yield Adm-2-ol and Adm-2-one.
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Figure 9. First cycle (cycle 1) of adamantane oxidation by PhIO catalyzed by the
non-immobilized MnPs and the supported materials Sil-Cl/MnPY (Y = 1, 2, 3)
and SiO /MnPY (Y = 4, 5, 6). Reactions conditions: MnP/PhIO/Adm molar ratio
2
of 1:10:500 (0.2 µmol of MnP, 2.0 µmol of PhIO, 100.0 µmol of Adm),
MeCN:CHCl mixture (1:1, v/v) as solvent, magnetic stirring, 26 ºC, 90 min, and
3
air atmosphere. Total yield = Adm-1-ol + Adm-2-ol + Adm-2-one. Yields
calculated based on the starting PhIO. 2 mol of PhIO were considered for the
formation of 1 mol of Adm-2-one. Normalized regioselectivity = 100 x (Adm-1-
ol)/[(Adm-1-ol) + (Adm-2-ol)/3 + (Adm-2-one)/3].
The total yield and regioselectivity of Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6) catalysts were not changed over the three reaction
2
cycles (Fig. 9 and ESI, Section 13, Figs. S21 and S22).
Reuse of the supported catalysts Sil-Cl/MnPY (Y = 1, 2, 3) and
SiO /MnPY (Y = 4, 5, 6) for two more cycles (cycles 2 and 3, ESI Figs. S21 and
2
S22) revealed that the reaction yields were little affected by catalyst recycling,
corroborating the results with the other model substrates that these materials are
particularly catalytically robust and resistant against oxidative degradation. The
small changes observed in product distribution upon catalyst recycling were
essentially limited to the Sil-Cl/MnPY (Y = 1, 2, 3) systems, in which Adm-2-one

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yields increased from less than 1% in the first cycle to ca 4% in the third cycle;
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this was, however, of little impact on chemoselectivity as 1- and 2- adamantanol
account for more than 95% of total oxidation yields. Finally, it is worth noting that
the high efficiency and robustness observed for the Sil-Cl/MnP1 and
Sil-Cl/MnP4 catalysts in cyclohexane and n-heptane oxidations were also
observed in the adamantane oxidation reactions.
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Chloropropyl-functionalized silica (Sil-Cl) was used as support for direct
derivatization and covalent immobilization (quaternization reaction) of the three
Mn(III) N-pyridylporphyrin isomers in aqueous medium to yield Sil-Cl/MnPX (X=
1, 2, 3). Additionally, SiO /MnPY (Y= 4, 5, 6) solids were prepared by the
2
electrostatic
immobilization
of
the
three
cationic
Mn(III)
N-
methylpyridiniumporphyrin isomers on non-modified SiO2.
The materials were evaluated as PhIO-oxidation catalysts for alkane
functionalization using cyclohexane, n-heptane, and adamantane as model
substrates. The strong bonding of MnPs with the inorganic matrices (Sil-Cl and
SiO ) resulted in heterogenized materials highly stable against leaching.
2
The supported materials Sil-Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4,
2
5, 6) showed good to excellent catalytic efficiency in all model substrate
oxidations. Sil-Cl/MnPY (Y = 1, 2, 3) catalysts based on cheap, ordinary,
chromatographic silica were more efficient and equally selective for cyclohexane
oxidation than related literature SBA-15-based catalysts. The Sil-Cl/MnPY (Y =
1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) systems were also able to catalyze
2
cyclohexanol oxidation indicating that they are promising catalysts for alcohol
oxidation.
The supports played important roles in the catalytic efficiency, chemo- and
regioselectivity, and oxidative stability of heterogenized MnPs on the Sil-
Cl/MnPY (Y = 1, 2, 3) and SiO /MnPY (Y = 4, 5, 6) materials. Although regular
2
chromatographic silicas are not expected to yield regioselectively noteworthy
catalysts, the study on the oxidation of n-heptane was particularly revealing on
the impact the surface modification exerted on catalyst properties. In these n-

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heptane oxidations, Sil-Cl/MnPY (Y = 1, 2, 3) catalysts showed chemoselectivity
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for alcohols, whereas SiO /MnPY (Y = 4, 5, 6) catalysts promoted
2
chemoselectivity for carbonyl compounds (ketones and aldehyde).
Overall, Sil-Cl/MnP1 stood out as particularly resistant against oxidative
destruction and catalytically robust in cyclohexane, n-heptane, and adamantane
model-substrate oxidations, without significant loss of either efficiency or
selectivity for at least three oxidation cycles.
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92 Conflicts of interest
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There are no conflicts to declare.
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We are gratefully thankful to Prof. Ynara M. Idemori for the access to her
laboratory facilities at Universidade Federal de Minas Gerais (Belo Horizonte,
MG, Brazil), where our first catalytic studies where carried out, and for her
continuous encouragement and stimulating discussions. We thank Dr. Ramon K.
S. Almeida (University of Campinas, Brazil) for the 13C and 29Si NMR spectra.
Electron Microscopy Center at University Federal of Paraná (CME-UFPR, Brazil)
is gratefully acknowledged for SEM/TEM analyses. Financial support from
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil),
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES,
Brazil), and Financiadora de Estudos e Projetos (FINEP, Brazil) is acknowledged.
V.H.A. Pinto thanks CAPES for a PhD scholarship. N.K.S.M. Falcão and B. Mariz-
Silva thank CNPq and CAPES, respectively, for MSc scholarships.