Synthesis, crystallographic characterization and homogeneous catalytic activity of novel unsymmetric porphyrins

Article abstract of DOI:10.1039/c7ra08734a

Unsymmetric porphyrins, containing both pentafluorophenyl (PFP = A) and 3,4-dimethoxyphenyl (DMP = B) substituents at the meso positions, were prepared using Lindsey's methodology. The isomeric trans-A₂B₂ (P1) and cis-A₂B<

Full text of DOI:10.1039/c7ra08734a

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PAPER
Synthesis, crystallographic characterization and
homogeneous catalytic activity of novel
unsymmetric porphyrins†
Cite this: RSC Adv., 2017, 7, 50610
a
a
A. R. Antonangelo,
K. C. M. Westrup,
L. A. Burt,b C. Grazia Bezzu,b
T. Malewschik, G. S. Machado, F. S. Nunes, N. B. McKeown b and S. Nakagaki
a
c
a
a
*
*
Unsymmetric porphyrins, containing both pentafluorophenyl (PFP ¼ A) and 3,4-dimethoxyphenyl (DMP ¼
B) substituents at the meso positions, were prepared using Lindsey's methodology. The isomeric trans-A2B2
(P1) and cis-A2B2 (P2) porphyrins together with the tris(pentafluorophenyl)porphyrin A3B (P3) were isolated
using chromatography. The porphyrins were characterized by UV-VIS, 1H NMR spectroscopy, mass
spectrometry, elemental analysis (C, H, N) and cyclic voltammetry (CV), and their molecular structures
were confirmed by single crystal XRD. Their manganese complexes, MnP1, MnP2 and MnP3, were also
synthesised and used as catalysts in cyclooctene and cyclohexane oxidation reactions under
homogeneous conditions. The catalytic studies were supported by electrochemical measurements and
showed that the number of electron-withdrawing substituents on the porphyrins rings influences the
catalytic activity. These porphyrins may be used as precursors for the design of new materials, such as
Polymers of Intrinsic Microporosity (PIMs).
Received 7th August 2017
Accepted 23rd October 2017
DOI: 10.1039/c7ra08734a
rsc.li/rsc-advances
species (metal-oxo porphyrin) and also protects the ring, to
some extent, against auto oxidative destruction.6,7 In this
Introduction
context,
the
free
base
ligand
[5,10,15,20-tetrakis
Porphyrins are macrocyclic compounds with a large extended
conjugated p-system and they are oen called “the pigments of
life”, as they play an important role in many biological systems.1
These compounds and their metallated complexes exhibit
unique properties2 and can be synthesized through various
methodologies.3 The macrocycles also show great exibility
towards structural modication which makes them very inter-
esting molecules in a number of different elds.4
Metalloporphyrins, especially those containing Fe(III) and
Mn(III) cations, have been extensively used as catalysts in
oxidation reactions of a range of organic compounds using
different oxidants. Studies have shown that the introduction of
electron-withdrawing and/or bulky substituents at the ortho
position of the meso-phenyl porphyrins (so called second
generation porphyrins) can enhance their catalytic activity in
oxidation reactions.5–7 This performance stems from the elec-
tronic effects that the substituents exert on the porphyrin ring,
which enhances the electrophilicity of the active catalytic
(pentauorophenyl)porphyrin], [H2(TPFPP)], represents an
important compound.6–8 The uorine atoms on the phenyl ring
at the four meso positions increases the stability and catalytic
activity. In addition, the uorine at para positions can be readily
substituted by nucleophiles, which makes this compound
a versatile platform for the design of new materials.8,9 Several
materials have been recently prepared, which utilize the
[H2(TPFPP)] porphyrin in a heterogeneous catalytic system.8,10–13
McKeown and co-workers used [H2(TPFPP)] porphyrin as
a monomer in a polymerization reaction with 5,50,6,60-tetrahy-
droxy-3,3,30,30-tetramethyl-1,10-spirobisindane (biscatechol) to
prepare a porphyrin network PIM (so called polymer of intrinsic
microporosity).12 The high surface area (up to 1000 m2 gꢀ1) and
the chemical stability of this material encourages the synthesis
of new network PIMs and their application in heterogeneous
oxidation catalysis.12–14
Meso substituted A2B2 trans-porphyrins bearing different
functional groups are useful building blocks for the design and
synthesis of new materials with a wide range of applications,
such as biomimetic systems and molecular materials.15–17 The
trans pattern of substitution in these kinds of porphyrins can be
used to achieve new materials with well-dened structures. For
example, the same polymerization reaction previously used to
synthesize the porphyrin network PIM, could be employed
utilizing one meso–trans A2B2 porphyrin as a preformed mono-
mer in order to achieve new soluble ladder PIMs. Moreover,
a
´
ˆ
´
´
Laboratorio de Bioinorganica e Catalise, Departamento de Quımica, Universidade
´
Federal do Parana, Curitiba, 81531-990, Brazil. E-mail: shirleyn@ufpr.br
bEastChem School of Chemistry, University of Edinburgh, Edinburgh, Scotland, EH9
3FJ, UK. E-mail: neil.mckeown@ed.ac.uk
c
´
´
Centro de Estudos do Mar, Pontal do Parana-PR, Universidade Federal do Parana
(UFPR), Brazil
† Electronic supplementary information (ESI) available. CCDC 1483009, 1536642
and 1536643. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c7ra08734a
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transition metal ions may also be coordinated to the porphyrin ether : ethylacetate : triethylamine (80 : 20 : 1). The solid (pale
units for future investigation in catalytic membranes.14 Thus, yellow) was then triturated with hexane and ltered, yielding
we undertook the synthesis of a group of new unsymmetric a colourless solid (1.29 g, 63.2% yield). Molecular formula:
porphyrins with either pentauorophenyl (PFP ¼ A) or 3,4-
C
17H18N2O2. 1H NMR data (500 MHz, CDCl3, 298 K) d ppm: 7.95
dimethoxyphenyl (DMP ¼ B) at the four meso positions.
(s, 2H, NH pyrrole), 6.85–6.84 (d, 1H, J ¼ 8.1 Hz, ArH), 6.79–6.76
(m, 2H, ArH), 6.73–6.72 (m, 2H, CH pyrrole), 6.20–6.18 (q, 2H, J
¼ 2.8 Hz, CH pyrrole), 5.98–5.96 (m, 2H, CH pyrrole); 5.46 (s, 1H,
CH meso), 3.90 (s, 3H, –OCH3), 3.83 (s, 3H, –OCH3). 13C NMR
data (125 MHz, CDCl3, 298 K) d ppm: 149.1, 148.1, 134.6, 132.7,
120.3, 117.1, 111.8, 111.2, 108.5, 107.1, 56.0, 55.9, 43.6. LRMS
(EI): m/z 282.1 (M+), according to the literature data.21
Preparation of free base porphyrins P1, P2 and P3, and their
manganese complexes MnP1, MnP2 and MnP3. The free base
porphyrins were synthesized by Lindsey methodology22 using 5-
(3,4-dimethoxyphenyl)dipyrromethane (1.29 g; 4.57 mmol) and
pentauorobenzaldehyde (0.9 g, 4.57 mmol) dissolved in dry
CH2Cl2 (460 mL) followed by the addition of TFA (0.6 mL; 8.13
mmol). The solution was stirred under N2 atmosphere at room
temperature for 50 min. Then, 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (1.34 g, 5.90 mmol) was added and the resul-
tant solution was stirred further for 1 h. The resulting crude
product was puried over two successive chromatography
columns (silica, 6 cm diameter ꢂ 30 cm height, CH2Cl2), where
it was possible to isolate the three porphyrins: P1 (Rf ¼ 0.75), P2
(Rf ¼ 0.61) and P3 (Rf ¼ 0.84).
Experimental
Materials
All chemicals used in this study were purchased from Sigma-
Aldrich, Merck or Fluka and were of analytical grade. Iodo-
sylbenzene (PhIO) was synthesized according to the literature.18
The ꢁsolid was carefully dried under reduced pressure and kept
at 5 C; its purity was assessed by iodometric titration.19
1H and 13C NMR spectra were recorded on Bruker Advance
III 500 MHz spectrometers, using deuterated chloroform as
solvent. Chemical shis are reported in ppm (d) and coupling
constants (J) are given in Hz. The multiplicities of the signals in
the 1H NMR spectra are abbreviated by s (singlet), d (doublet), q
(quartet) and m (multiplet). Mass spectra were acquired on
a Bruker Ultraex MALDI-TOF or Thermo Electron MAT900
mass spectrometers (MALDI or EI). Electronic spectra (UV-VIS)
were obtained from dichloromethane solutions on a Hewlett-
Packard HP 8452A diode array spectrophotometer in the 200–
800 nm range. Analysis were accomplished with a 1 cm path
length cell. Fourier Transform Infrared (FTIR) spectra were
measured on a Biorad 3500 GX spectrophotometer in the 400–
4000 cmꢀ1 range with a spectral resolution of 4 cmꢀ1. Elemental
analysis were obtained from a Series II 2400 Perkin Elmer
Elemental Analyzer. Electron paramagnetic resonance (EPR)
measurements of the powdered new manganese porphyrins
were conducted on an EPR Bruker EMX MicroX spectrometer
(frequency X, band 9.5 GHz) at room temperature and 77 K
(liquid N2), by using perpendicular microwave polarization X-
band.
Porphyrin P1. Purple solid (0.134 g, 6.4% yield). Elemental
analysis (%) calcd. for C48H28F10N4O4: C 63.03, H 3.09, N 6.12.
Found: C 62.97, H 3.18, N 5.73.1H NMR data (500 MHz, CDCl3,
298 K) d ppm ¼ 9.04–9.03 (d, 4H, J ¼ 4.8 Hz, Hb-pyrrolic), 8.82–
8.81 (d, 4H, J ¼ 4.8 Hz, Hb-pyrrolic), 7.80–7.78 (m, 4H, ArH),
7.32–7.30 (d, 2H, J ¼ 7.9 Hz, ArH); 4.22 (s, 6H, –OCH3), 4.03 (s,
6H, –OCH3), ꢀ2.78 (s, 2H, NH). UV-VIS in CH2Cl2 at 298 K, lmax
(log 3/M ꢀ 1 cmꢀ1): 420 (5.38), 512 (4.25), 548 (3.77), 588 (3.76)
and 644 (3.51). FT-IR data (cmꢀ1, KBr pellet): 3321, 3116, 2933,
1517, 1496, 1479, 1465, 1440, 1406, 1353, 1346, 1319, 1257,
1237, 1166, 1139, 1076, 1041, 1027, 987, 923, 919, 813, 802, 779,
756, 734. MALDI-TOF-MS (m/z): 914.1 [MH]+.
Porphyrin P2. Purple solid (0.055 g, 2.6% yield). Elemental
analysis (%) calcd. for C48H28F10N4O4: C 63.03, H 3.09, N 6.12,
found: C 63.13, H 3.18, N 5.72.1H NMR data (500 MHz, CDCl3,
298 K) d ppm: 9.04–9.03 (d, 2H, J ¼ 4.8 Hz, Hb-pyrrolic), 8.95 (s,
2H, Hb-pyrrolic), 8.87 (s, 2H, Hb-pyrrolic), 8.79–8.78 (d, 2H, J ¼
4.8 Hz, Hb-pyrrolic), 7.80–7.76 (m, 4H, ArH), 7.31–7.29 (d, 2H, J
¼ 8.1 Hz, ArH), 4.21 (s, 6H, –OCH3); 4.03 (s, 6H, –OCH3), ꢀ2.70
(s, 2H, NH). UV-VIS in CH2Cl2 at 298 K, lmax (log 3/M ꢀ 1 cmꢀ1):
421 (5.39), 514 (4.24), 550 (3.74), 588 (3.78) and 642 (3.22). FT-IR
data (cmꢀ1, KBr pellet): 3321, 3105, 2931, 1515, 1498, 1479,
1463, 1440, 1404, 1350, 1321, 1257, 1238, 1166, 1139, 1120,
1081, 1041, 1027, 987, 918, 865, 802, 761, 742. MALDI-TOF-MS
Cyclic voltammetry (CV) was carried out with an IVIUM
CompactStat potentiostat/galvanostat. A glassy carbon electrode
was employed for the measurements at I ¼ 0.1 mol Lꢀ1 kept
constant
with
TBAPF6
(tetrabutylammonium
hexa-
uorophosphate). A silver wire and a platinum wire were used
as pseudo-reference and auxiliary electrodes, respectively.
Ferrocene was used as an internal standard reference and
potentials are reported versus the standard hydrogen electrode,
SHE. Experiments were conducted with a 1.0 ꢂ 10ꢀ3 mol Lꢀ1
complex concentration in dichloromethane solutions at
ambient temperature and under argon atmosphere.
Synthesis and characterization
Preparation of 5-(3,4-dimethoxyphenyl)dipyrromethane. The (m/z): 914.1 [MH]+.
dipyrromethane was synthesized by Lindsey's methodology20
Porphyrin P3. Purple solid (0.01 g, 0.5% yield). Elemental
using a solution of 3,4-dimethoxybenzaldehyde (1.2 g, 7.22 analysis (%) calcd. for C46H19F15N4O2: C 58.49, H 2.03, N 5.93,
mmol) and freshly distilled pyrrole (20 mL, 288 mmol) followed found: C 57.06, H 1.84, N 5.68.1H NMR data (500 MHz, CDCl3,
by the addition of triuoroacetic acid (TFA) (55 mL, 0.72 mmol). 298 K) d ppm: 9.06–9.05 (d, 2H, J ¼ 4.8 Hz, Hb-pyrrolic), 8.90 (m,
The product was puried by chromatography column (6 cm 4H, Hb-pyrrolic), 8.83–8.82 (d, 2H, J ¼ 4.8 Hz, Hb-pyrrolic), 7.79–
˚
diameter ꢂ 20 cm height, silica, pore size: 60 A, 40–63 mm) 7.77 (m, 2H, ArH); 7.32–7.30 (d, 1H, J ¼ 8.0, ArH), 4.22 (s, 3H,
using
a
mixture
of
solvents:
petroleum –OCH3), 4.03 (s, 3H, –OCH3), ꢀ2.79 (s, 2H, NH). UV-VIS in
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CH2Cl2 at 298 K, lmax (log 3/M ꢀ 1 cmꢀ1):415 (5.41), 510 (4.31), argon for 15 min. Then, the mixture of solvents (ACN : DCM,
544 (3.71), 586 (3.90) and 638 (3.30). FT-IR data (cmꢀ1, KBr 1 : 1) was added to the reactions asks, followed by addition of
pellet): 3321, 3107, 2933, 1517, 1498, 1483, 1467, 1438, 1402, the substrates (Z)-cyclooctene or cyclohexane. The molar ratio
1352, 1342, 1319, 1257, 1240, 1166, 1143, 1078, 1058, 1045, (catalyst/PhIO/substrate) used was 1 : 10 : 1000 which was
1027, 989, 925, 918, 802, 756, 736. MALDI-TOF-MS (m/z): 944.1 calculated based on the molecular mass of the metal-
[MH]+.
loporphyrins (about 0.5 mg of metalloporphyrins were used for
The free base porphyrins P1, P2 and P3 were metallated by each reaction). The oxidation reactions were performed under
a methodology described in the literature23 using manganese(II) magnetic stirring for 1 h, at room temperature, in the absence of
acetate as metal salt. The process was monitored by TLC and light. Aer 1 h, the excess of iodosylbenzene was consumed by
UV-VIS techniques. The complexes were puried by column addition of sodium sulte and the reaction mixture was trans-
(CH2Cl2 and MeOH) and the excess of salt was removed by ferred to a volumetric ask (2 mL). The resulting solution con-
washing the metalloporphyrins solids with plenty of warm taining the reaction products was analyzed by gas
water. The new metalloporphyrins, designated MnP1, MnP2 chromatography using bromobenzene or 1-pentanol as internal
and MnP3, respectively, were characterized by UV-VIS spec- standards. The products from the oxidation reactions were
troscopy (ESI: Fig. S7†) and EPR technique was used to conrm quantied with an Agilent 6850 gas chromatograph (ame
the oxidation state of manganese and also to conrm the ionization detector) equipped with a 30 m long DB-WAX capil-
absence of manganese(II) acetate (ESI: Fig. S8†).
lary column with 0.25 mm internal diameter (J&W Scientic).
Product yields were based on the quantity of PhIO added to each
reaction.
Single crystal X-ray diffraction
Single crystal X-ray diffraction data were collected on either
a i19-FFD-air (xed Chi) Pilatus M2 at the Diamond Light source
Beam line I19-1 (P1) or a Rigaku Oxford Diffraction Super Nova
Results and discussion
Porphyrin synthesis
˚
(P2 and P3) diffractometer using synchrotron (0.6889 A) or Cu-
˚
Ka (1.5418 A) radiation, respectively. The crystal temperature
was controlled using an Oxford Cryo systems Cryo stream 700+
low temperature device for all of the samples. All data were
processed using Crysalis Pro. Crystal structure solutions were
achieved using intrinsic phasing with ShelXT and rened using
least squares minimization with the ShelXL renement package
interfaced with Olex2.24,25 In structures P2 and P3 N-bound
hydrogens were identied from a difference Fourier map and
rened with appropriate geometric restraints. C-bound H atoms
in all structures were placed in calculated geometric positions
and rened using the riding model. Crystallographic data for
structures P1, P2 and P3 have been deposited at the Cambridge
Crystallographic Data Center. CCDC: 1483009, 1536642 and
1536643. Further crystallographic data can be found in the ESI.†
It was anticipated that the porphyrin meso trans-A2B2(P1) would
be synthesized using Lindsey's methodology,20,22 between 5-(3,4-
dimethoxyphenyl)dipyrromethane and pentauorobenzalde-
hyde (Scheme 1). Analysis of the crude product mixture revealed
that the desired porphyrin P1 was prepared along with the cis-
A2B2 isomer P2 and the tris-pentauorophenylporphyrin A3B
(P3) (Scheme 1). The formation of P2 and P3 is probably asso-
ciated with scrambling of the phenyl substituents due to acid-
olysis and recombination of dipyrromethanes.26,27 The yield for
the desired porphyrin (P1) was low (6.4%), but respectable in
comparison with that of the isomeric unsymmetric porphyrin
5,15-bis(3,5-dimethoxyphenyl)-10,20-bis(pentauorophenyl)
(3.8%), prepared using the mixed aldehyde methodology.28
Catalytic studies of MnP1, MnP2 and MnP3 using (Z)-
cyclooctene and cyclohexane as substrates
The catalytic activity and efficiency of the new metal-
loporphyrins was tested in the oxidation reactions of (Z)-cyclo-
octene (previously puried on alumina column) and
cyclohexane with iodosylbenzene (PhIO). Furthermore, the two
associated symmetric metalloporphyrins, (5,10,15,20-tetrakis-
pentauorophenyl)porphyrin manganese(III), MnTPFPP, and
5,10,15,20-tetrakis-(3,4-dimethoxyphenyl)porphyrin
man-
ganese(III), MnTDMPP, were also used as catalysts for
comparison.
The catalytic reactions were performed in a 1.5 mL glass ask
equipped with a magnetic stirrer, in a dark chamber. The
solvent mixture (acetonitrile/dichloromethane, ACN : DCM
1 : 1, v/v) and the substrates were purged with argon for 15 min.
The catalysts (MnP1, MnP2, MnP3, MnTPFPP and MnTDMPP)
and the oxidant PhIO (catalyst/PhIO at a molar ratio of 1 : 10)
Scheme 1 Schematic representation of the free-base porphyrins
were added in the reaction ask and they were also purged with preparation.
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1
The H NMR spectroscopy was fundamental for differentia- Mn(III) ions has four unpaired d electrons with S ¼ 2, typically
tion between the trans-A2B2 (P1) and cis-A2B2 (P2) isomers featuring a pronounced Jahn–Teller distortion which results in
(Table 1 and ESI: Fig. S3–S5†). The D2h symmetry of trans-A2B2 substantial spin–orbit coupling.38 Thereby, the coupling and
molecule (P1) gives specic signal patterns with two doublets rapid spin relaxation process results in absence of signal in
for the b-pyrrole protons at 9.0 and 8.8 ppm, while the lower mononuclear Mn(III) EPR spectra even at 77 K (under perpen-
symmetry in the cis-A2B2 (P2) compared to the trans P1 isomer29 dicular microwave polarization).5,38
gives rise to two singlets at 8.9 and 8.8 ppm and two doublets at
Furthermore, the EPR conrms that the excess of man-
9.0 and 8.7 ppm. In the tris-A3B molecule (P3) the b-pyrrole ganese(II) acetate, which shows a signal in g ¼ 2 (ESI: Fig. S8†),
protons are split up into two doublets at 9.0 and 8.8 ppm and was removed during the purication of the new metal-
one multiplet at 8.9 ppm. These results are in agreement with loporphyrins (through column and washing with warm water).
the basic patterns of splitting of the b-pyrrole protons in
unsymmetric porphyrins as reported for Meng and co-
workers.29
Single crystal X-ray diffraction
Each one of the free base porphyrins P1, P2 and P3 was
successfully characterized by single crystal XRD that allowed
unambiguous conrmation of each structure. Suitable crystals
were grown via slow vapour diffusion of methanol into chloro-
form solution. Crystal data, data collection and structure
renement can be found in the ESI.† Selected crystallographic
and geometric parameters for each structure are also shown in
Table 2. Crystals of P1, P2 and P3 form monoclinic unit cells
belonging to the space groups P21/c, I2/a and Cc, respectively.
The ORTEP39 and molecular crystal packing diagrams for each
porphyrin are shown in Fig. 1.
The UV-VIS spectra of the porphyrin macrocycle are consis-
tent with the D2h micro-symmetry according to the Four-Orbital
Model developed by Gouterman (ESI: Fig. 6†).30 The spectra of
the free base porphyrins shows an intense absorption near
400 nm (Soret band) and four less intense bands (Q bands)
labeled as IV, III, II and I, between 450 and 700 nm. In general,
electron-donating substituents lead to red shis for both the
Soret and Q bands.30–35 The electronic coupling between
electron-donating groups and the porphyrin core in P1–P3
decreases the HOMO–LUMO energy gaps and causes a red shi
in its spectra compared to the free base porphyrin H2TPFPP
(ESI: Fig. 6†).
Furthermore, it is well established that the relative intensity
of the four Q bands is dependent on the substituent groups at
the macrocycle periphery. According to the relative intensities,
the four visible Q bands are classied as etio, rhodo, oxorhodo,
and phyllo.30,36 The porphyrins P1–P3 give a phyllo spectra (with
intensities IV > II > III > I), which is in agreement with meso-
phenyl porphyrins spectra. The reduction in the intensities of
bands III and I observed in the spectrum of the porphyrin P3
may be attributed to the restricted rotation of the phenyl groups
resulting from steric interaction of the ortho-uorine substit-
uent and the b-pyrrole hydrogens.36
The unsymmetric units of P1 and P2 contain one-half of each
molecule, along with one molecule of chloroform. The
porphyrin rings are both relatively at, with the largest devia-
˚
˚
tion from planarity being 0.04 A and 0.09 A for P1 and P2
respectively, demonstrating little difference for the cis or trans-
isomer. The dihedral angle between the DMP substituents and
the porphyrin ring appear to be relatively similar for both P1
and P2, varying from 66.4 to 67.9ꢁ, with the PFP groups of P1
also within this range. However, the PFP groups of P2 adopt
a close to perpendicular orientation to the macrocycle, with an
angle of 86.0ꢁ.
The porphyrin macrocycles in the crystal structures of P1 and
P2 are arranged in a herring bone and off-set stacked crystal
packing motif, respectively (Fig. 1b and d). Analysis of the
packing in the crystal of P1 reveals a variety of close intermo-
lecular interactions between each of the macrocycles, of which
C/H, C/O, H/N, F/H, F/C and F/F interactions are
present. Despite the similarities in the chemical composition of
P1 and P2, fewer types of interactions could be observed in the
crystal structure of P2, attributed to the relative positions of the
substituted phenyl substituents. The crystal structure of P2
contains C/F, F/H, F/C and O/H close interactions, with
The metalation process of the free base porphyrins P1, P2
and P3 with manganese(II) acetate was monitored by recording
the electronic spectra at different reaction times. The bath-
ochromic shi observed in the Soret band is characteristic of
Mn(III) porphyrins, which conrms the formation of MnP1,
MnP2 and MnP337 (ESI: Fig. S7†). These new metalloporphyrins
were also characterized by solid state EPR (ESI: Fig. S8†) and no
signal was observed at room temperature or 77 K, in agreement
with presence of Mn(III) in the porphyrin core.5,38 Indeed, the
Table 1 1H Chemical shift data (d, ppm) for trans-A2B2 (P1), cis-A2B2 (P2) and tris-A3B (P3) in CDCl3. Multiplicity and integration in parentheses
H-b-pyrrolic
ArH
–OCH3
NH
Trans-A2B2 (P1)
Cis-A2B2 (P2)
Tris-A3B (P3)
9.04–9.03 (d, 4H) 8.82–8.81
(d, 4H)
9.04–9.03 (d, 2H) 8.95 (s, 2H)
8.87 (s, 2H) 8.79–8.78 (d, 2H)
9.06–9.05 (d, 2H) 8.9 (m, 4H)
8.83–8.82 (d, 2H)
7.80–7.78 (m, 4H) 7.32–7.30
(d, 2H)
7.80–7.76 (m, 4H) 7.31–7.29
(d, 2H)
7.79–7.77 (m, 2H) 7.32–7.30
(d, 1H)
4.22 (s, 6H) 4.03 (s, 6H)
4.21 (s, 6H) 4.03 (s, 6H)
4.22 (s, 3H) 4.03 (s, 3H)
ꢀ2,78 (s, 2H)
ꢀ2,70 (s, 2H)
ꢀ2,79 (s, 2H)
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Table 2 Selected crystallographic and geometric parameters for the crystal structure of P1, P2 and P3
P1
P2
P3
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C
50H30Cl6F10N4O4
C50H30Cl6F10N4O4
1153.48
200
Monoclinic
I2/a
12.0821(2)
17.9808(2)
22.7546(3)
96.0320(10)
4915.97(12)
4
C47H22F15N4O3
975.68
120
Monoclinic
Cc
11.5360(2)
26.8989(6)
15.0809(3)
109.961(2)
4398.56(16)
4
1153.48
100
Monoclinic
P21/c
13.948(3)
15.6771(17)
11.2006(16)
109.177(18)
2313.2(7)
2
˚
a/A
˚
b/A
˚
c/A
b/ꢁ
Volume/A
3
˚
Z
2Q range for data collection/ꢁ 2.996 to 40.268
6.278 to 152.246
39 378
6.240 to 152.16
35 755
Reections collected
8341
Independent reections
Final R indexes [I $ 2s(I)]
Final R indexes [all data]
CCDC deposition number
2340 [Rint ¼ 0.0857, Rsigma ¼ 0.0726] 5127 [Rint ¼ 0.0661, Rsigma ¼ 0.0289] 7617 [Rint ¼ 0.0660, Rsigma ¼ 0.0400]
R1 ¼ 0.0725, wR2 ¼ 0.1897
R1 ¼ 0.1160, wR2 ¼ 0.2565
1483009
R1 ¼ 0.0659, wR2 ¼ 0.1874
R1 ¼ 0.0708, wR2 ¼ 0.1943
1536642
R1 ¼ 0.1043, wR2 ¼ 0.2897
R1 ¼ 0.1079, wR2 ¼ 0.3001
1536643
demonstrates a much larger deviation of planarity compared to
˚
P1 and P2, with a maximum deviation from planarity of 0.351 A.
The two PFP substituents both exhibit signicant disorder that
could be modelled over two positions but still demonstrate
a large amount of libration as shown on the thermal ellipsoids.
The disorder appears to coincide with the presence of small
voids around the PFP substituents, that when combined
3
˚
account for 5.7% (251 A ) of the unit cell, as calculated by
PLATON VOID.40
Further analysis with PLATON SQUEEZE40 calculated
a residual electron count of 47 eꢀ that corresponds to another
possible 0.6 molecules of methanol per molecule of P3.
However, their scattering contribution to the overall structure
renement was not removed. The dihedral angle between the
substituted phenyl groups varies from 68.4ꢁ to 86.3ꢁ, with the
maximum dihedral angles observed for the two disordered PFP
rings. The macrocycles within the crystal structure of P3 are
arranged in a slip-stacked formation (Fig. 1f), which is arranged
via a number of O/H, C/H, F/H, F/N, F/C and N/H close
interactions and, as expected, a large number of the intermo-
lecular interactions observed involve uorine.
Cyclic voltammetry of H2P
Fig. 2 shows the cyclic voltammograms (CV) of the free base
porphyrins in CH2Cl2 solutions. H2Ps showed two quasi-
reversible oxidations and two quasi-reversible reductions. The
oxidations exhibited E1/2 potentials that are referred to the SHE
as follows: +1.671 V and +1.836 V for P1, +1.632 V and +1.866 V
for P2 and +1.829 V and +2.023 V for P3. Reductions were
observed at E1/2: ꢀ0.942 V and ꢀ0.536 V for P1, ꢀ0.972 V and
ꢀ0.608 V for P2 and ꢀ0.853 V and ꢀ0.447 V for P3.
Fig. 1 ORTEP molecular diagrams of (a) P1, (c) P2 and (e) P3 where the
solvent molecules and hydrogens have been omitted for clarity.
Thermal ellipsoids are also shown at 50% probability for P1 and P2 and
35% probability for P3. Crystal packing diagrams for (b) P1, (d) P2 and (f)
P3 where the solvent molecules and hydrogen, oxygen and fluorine
atoms have been omitted for clarity. Thermal ellipsoids are also shown
at 50% probability for P1 and P2 and 35% probability for P3.
These values clearly show a similar redox behavior for P1 and
P2, which is expected since they have the same substituents,
the O/H close contacts generated by the association between and differ only on their geometric conguration, cis or trans,
the methoxy substituents of each porphyrin (Fig. S12-ESI†). thus offering the same overall outcome to the electronic density
The unsymmetric unit of P3 contains a complete porphyrin of the porphyrin. In contrast, the unsymmetric porphyrin P3
macrocycle with one molecule of methanol. The ring exhibited a signicant resistance to oxidation, in agreement
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Cyclic voltammetry of MnP
The cyclic voltammograms (CV) of manganese(III) porphyrins
showed irreversible oxidations starting at +1.8 V, which were
assigned to the electrogenerated cation radicals and the dica-
tions of the macrocycle ring (Fig. 2). An irreversible reduction of
MnP, probably related to the irreversible oxidations, was seen in
the range of ꢀ0.35 V to ꢀ0.48 V along with quasi-reversible
reduction processes between ꢀ0.926 V and ꢀ0.999 V to
produce the monoanion radical and dianions porphyrins, in
that order. Anodic peak potentials of the quasi-reversible elec-
tron transfer centered in the manganese ion were observed at
+0.440 V, +0.413 V and +0.558 V, for MnP1, MnP2 and MnP3,
respectively. The overall electron transfer sequence can be
summarized in the following steps: [MnIIP]2ꢀ / [MnIIPc]ꢀ
/
[MnIIP] / [MnIIIP]+ / [MnIIIPc]+ / [MnIIIP]2+
.
44,45
Ucoski and co-workers46 reported the electrochemistry of the
5,10,15,20-tetrakis(1,3-benzodioxole)-MnIII
porphyrin in
dichloromethane and observed a E1/2 ¼ ꢀ0.61 V vs. SHE for the
MnIII/MnII couple. This result agrees with the donor character of
the dioxole substituent, compared to the substituents reported
in this work, making the oxidation of the manganese center an
easier process in that case. Also for comparison, E1/2 of MnIII/
MnII(TPFPP) and of MnIV/MnIII(TPFPP) were observed in
acetonitrile at +0.05 and at +1.75 V vs. SHE, repectively.47
Fig. 2 Cyclic voltammograms of H2P and MnP in dichloromethane
containing 0.1 mol Lꢀ1 TBAPF6. Scan rate at 100 mV sꢀ1. Initial anodic
scan.
with the presence of a higher number of uoro-atoms, which
causes a strong inductive electron withdrawing effect. The mean
potential difference between the two oxidations and between
the two reductions for H2Ps are 0.198 V and 0.392 V, respec-
tively. This result is in accordance with analogue cases in the
literature for successive monoelectronic ring-centered
processes, explained by the formation of porphyrin p-cation/
anion radicals and dications/dianions as depicted in
Scheme 2.41,42 Furthermore, the potential difference between
the rst oxidation and the rst reduction, DE1/2(oxi–red), is a good
estimate of the band gap between the frontier orbitals HOMO
and LUMO. It was found the following order of decreasing
DE1/2(oxi–red): P3 (2.276 V) > P2 (2.240 V) > P1 (2.207 V), in
accordance with the higher resistance of P3 towards oxidation.
The values for P3 are slightly higher than the 2.23 V observed for
the meso-tetraphenylporphyrin [H2(TPP)],43 and lower than
Catalytic studies of MnP1, MnP2 and MnP3 using (Z)-
cyclooctene and cyclohexane as substrates
The efficiency of the porphyrins MnP1, MnP2 and MnP3 and the
model compounds MnTPFPP and MnTDMPP as catalysts in
homogeneous oxidation reactions was tested using the
substrates (Z)-cyclooctene and cyclohexane and iodosylbenzene
(PhIO) as an oxidant (Fig. 3). It has been previously shown that
electron-withdrawing substituents at the ortho position of the
meso-phenyl substituents in the porphyrin ring can enhance the
catalytic activity of these compounds for oxidation reactions.6,7
Hence, MnTPFPP was used as a model catalyst because it is
recognized as a highly efficient catalyst in homogeneous
oxidation reactions for various substrates.48,49 Its enhanced
performance can be attributed mainly to its greater stability
under oxidative conditions, resulting from the electron-
withdrawing nature of the PFP substituent, which protects, to
some extent, the self-destruction of the porphyrin ring. On the
other hand, MnTDMPP was used as an opposing model catalyst
for comparison because it contains only the highly electron-
donating DMP substituents.
2.34
V for 5,10,15,20-tetrakis(pentauorophenyl)porphyrin
[H2(TPFPP)]. All electrochemical data are collected in Table 3.
The molecule (Z)-cyclooctene is a useful diagnostic substrate
as it is easily oxidized in the presence of metalloporphyrins,
with (Z)-cycloocteneoxide usually being the sole product of
reaction.48–50
The catalytic activity of the new manganese(III) porphyrins
MnP1, MnP2 and MnP3 using (Z)-cyclooctene as a substrate
resulted in similar product yields. The new unsymmetric man-
ganese(III) porphyrins also showed analogous catalytic activity
to that observed for the completely symmetric porphyrin
MnTPFPP (ꢃ86%) (Fig. 4). This result suggests that the presence
of two (MnP1 and MnP2) or one (MnP3) 3,4-dimethoxy groups
Scheme 2 Schematic representation of monoelectronic process in
the formation of porphyrin p-cation/anion radicals and dications/
dianions.
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Table 3 Half-wave potentials (V vs. SHE) measured at 100 mV sꢀ1 of free base porphyrins and metaloporphyrins in CH2Cl2, 0.1 M TBAPF6
d
Eox1
Eox2
DEox
Ered1
Ered2
DEred
(Eox1 ꢀ Ered1
)
Ea (MnIII/MnII)
Ref.
H2TPP
H2TPFPP
H2TDMPP
P1
P2
P3
MnP1
MnP2
MnP3
+0.93
+1.26
+2.123a
+1.433
+1.836
+1.866
+2.023
n.o.
0.33
0.285
0.134
0.165
0.234
0.194
—
ꢀ1.29
ꢀ1.62
0.33
2.22
2.340c
2.264
2.207
2.240
2.276
2.3
—
—
41
tw
tw
tw
tw
tw
tw
tw
tw
+1.838a
+1.299
+1.671
+1.632
+1.829
+1.8a
ꢀ0.543
ꢀ0.965
ꢀ0.536
ꢀ0.608
ꢀ0.447
ꢀ0.48b
ꢀ0.35b
n.o.
ꢀ0.952
ꢀ1.267
ꢀ0.942
ꢀ0.972
ꢀ0.853
ꢀ0.926
ꢀ0.882
ꢀ0.999
0.409
0.302
0.406
0.364
0.406
0.45
—
—
—
+0.440a
+0413a
+0.558a
+1.8a
n.o.
n.o.
—
—
0.53
—
2.2
—
+1.8a
a
b
c
d
Epa
.
Epc
.
(Epa1 ꢀ Epc1). HOMO–LUMO gap; n.o. ¼ not observed; tw ¼ this work.
in the structure of the new synthesized metalloporphyrins does of the meso-phenyl macrocycle, which is in agreement with
not seem to interfere in their catalytic activity for the oxidation results obtained previously using
of (Z)-cyclooctene. However, the catalytic activity of the structure.50
a
similar porphyrin
completely symmetric MnTDMPP was lower (ꢃ73%), which can
be attributed to the absence of electron-withdrawing substit- V), MnP2 (+0.413 V) and MnP3 (0.558 V) (Table 3) correlates well
uent in this porphyrin.50
with their increasing catalytic activity, in that order, towards the
The oxidation peak potentials (Epa) values of MnP1 (+0.440
Cyclohexane is a less reactive substrate than (Z)-cyclooctene oxidation of cyclohexane (Fig. 5). The similarity of Epa for MnP1
in oxidation reactions and when metalloporphyrins are used as and MnP2 agrees with the close yields (ꢃ40%) observed for the
catalysts, both cyclohexanol and cyclohexanone are obtained as cyclohexanol production, while MnP3 showed a 49% yield for
major products.10,48–50 The novel metalloporphyrins reported the alcohol. The higher reduction potential of MnP3 compared
here have shown alcohol selectivity in the cyclohexane oxidation to the other metalloporphyrins herein reported means that the
(Fig. 5), which is in agreement with reported results of homo- active intermediate containing the high valent manganese51–53
geneous catalysis using other metalloporphyrins.48–50
IV or V Mn](O)P3 is a better oxidant agent due to the increased
For the cyclohexane reactions it was possible to observe number of pentauorophenyl groups in the porphyrin compo-
a trend in the catalytic activity of the uorine-substituted met- sition, thus, in the course of the reaction, it favors the oxygen
alloporphyrins which increased with an increasing number of transfer to the substrate. Furthermore, the pentauorophenyl
pentauorophenyl substituents present in the porphyrin: MnP1 substituents protect the porphyrin from oxidative damage,
ꢃ MnP2 (ꢃ40% yield for alcohol) < MnP3 (ꢃ49%) < MnTPFPP increasing the turnover numbers and helps increase the yields
(ꢃ54%). The low catalytic activity of the MnTDMPP (ꢃ14%) of the oxidation reactions.
under the reaction conditions is associated with the absence of
bulky or electron-withdrawing substituents at the ortho position
Fig. 4 (Z)-Cyclooctene oxidation reaction using PhIO catalyzed by
MnP1, MnP2, MnP3, MnTPFPP and MnTDMPP. Catalyst/PhIO/
Fig. 3 Structures of porphyrins MnP1, MnP2 and MnP3, and the model Substrate molar ratio ¼ 1 : 10 : 1000. The product yields were calcu-
compounds MnTPFPP, MnTDMPP used as catalysts for (Z)-cyclo- lated based on the amount of PhIO used in the reactions. Results
octene and cyclohexane oxidation reactions.
represent reactions performed in duplicate.
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21045). The authors are also grateful to The University of
Edinburgh and acknowledge beam line I19 of the Diamond
Light Source (DLS), UK, for assistance in obtaining crystallo-
graphic data.
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