Chlorinated corroles

Article abstract of DOI:10.1039/c2dt31261a

Fully β-pyrrole chlorinated corrole was obtained via reaction of the cobalt(iii) complex of 5,10,15-tris(pentafluorophenyl)corrole with chlorine. The structural and electronic effects of chlorination are reported, together with the coordination chemistry

Full text of DOI:10.1039/c2dt31261a

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COMMUNICATION
Chlorinated corroles†
Atif Mahammed, Mark Botoshansky and Zeev Gross*
Received 12th June 2012, Accepted 1st August 2012
DOI: 10.1039/c2dt31261a
spectroscopy revealed no meaningful resonances. While these
phenomena suggest bleaching or complete destruction of the
macrocycle, addition of pyridine and NaBH4 to the reaction
mixture caused an immediate color change from yellow to green
and the appearance of the characteristic electronic spectrum
of hexa-coordinated cobalt(III) corroles. In fact, complex 2 was
obtained in about 90% yield after simple work-up.12 Full charac-
terization was achieved by spectroscopy (19F and 1H NMR,
UV/vis, HR-MS), electrochemistry, and X-ray diffraction. The
combined data served to demonstrate that 2 is a diamagnetic six-
coordinate cobalt(III) corrole of high symmetry with two pyridine
axial ligands.
Addition of trifluoroacetic acid (TFA) to a benzene solution of
2 induced remarkable spectral changes in the electronic spectrum
(Fig. 1B). This may be appreciated by the remarkably different
λmax of the near-UV (Soret) band: 444 nm with no TFA, 400 nm
with 7 mM TFA, and 410 nm with 200 mM TFA. To understand
these phenomena and characterize the new complexes, the
Fully β-pyrrole chlorinated corrole was obtained via reaction
of the cobalt(III) complex of 5,10,15-tris(pentafluorophenyl)-
corrole with chlorine. The structural and electronic effects of
chlorination are reported, together with the coordination
chemistry of this complex.
Halogenation of the eight β-pyrrole carbon atoms of porphyrins
and corroles leads to dramatic changes in their spectroscopic,
electrochemical and catalytic properties.1 The characteristic near-
UV (Soret) and near-IR (Q) bands in the absorption spectra of
the halogenated macrocycles are strongly red-shifted,1a,2 the oxi-
dation and reduction potentials are shifted to much more positive
potentials,3 they are much more robust,4 and orders of magnitude
more active oxidation catalysts than the non-halogenated
counterparts.5 Synthetic access to octafluoro derivatives still
relies on pyrrole condensation with 2,3-difluoropyrrole,6 octa-
bromo- and octachloro-substituted porphyrins may be obtained
via bromination/chlorination of the metal-protected macrocycle,7
and no octaiodo porphyrins or corroles have been reported yet.
Consistent with the much larger π-density of corroles relative to
porphyrins, bromination of metallocorroles is an extremely facile
synthetic pathway to octabromo derivatives8 and iodination
allowed access to a tetraiodinated corrole with unique photophy-
sical properties.9 Early evidence suggests that chlorinated metal-
locorroles should also be accessible,10a but this was realized in
only one example so far – the octachlorinated copper(III) corrole
reported by Dehaen et al.10b We became interested in obtaining
octachlorinated cobalt corroles because the cobalt(III) complex of
β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)corrole is a
superior catalyst for oxygen reduction in terms of kinetics, onset
potentials and selectivity for four-electron reduction to water.11
These advantages are clearly due to the effect of the electron-
withdrawing bromides on the CoII/CoIII redox potential. We now
present the synthesis of a fully β-pyrrole-chlorinated cobalt(III)
corrole (2, Scheme 1), its full characterization and some of its
coordination chemistry.‡
Scheme 1 Synthesis of chlorinated corrole.
Reaction of the cobalt(III) corrole 1 with excess Cl2 in
benzene solution induced a color change from green to yellow, a
strong intensity-decrease of the Soret band and complete dis-
appearance of the Q-bands. The new absorption bands (Fig. 1A)
are not typical of corroles, and examination by 19F and 1H-NMR
Fig. 1 (A) UV-vis spectra of the reaction mixture of 1 and Cl2 in
benzene before (gray) and after the successive addition of pyridine and
NaBH4 (black). (B) UV-vis spectra of 2 (1 μM) in benzene in the
absence (dotted) and in the presence of 7 mM (gray), and 200 mM
(black) of TFA.
Schulich Faculty of Chemistry, Technion-Israel Institute of Technology,
Haifa 32000, Israel. E-mail: chr10zg@tx.technion.ac.il
†Electronic supplementary information (ESI) available. CCDC 877907
for 2. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2dt31261a
10938 | Dalton Trans., 2012, 41, 10938–10940
This journal is © The Royal Society of Chemistry 2012

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Scheme 2 The chemical structures of 3 and 4, obtained via gradual
addition of TFA to a benzene solution of 2.
Fig. 2 19F NMR spectra of 15 mM 2 in benzene-d6 in the absence (A)
and in the presence of 50 mM (B) and 500 mM (C) TFA.
Fig. 3 Molecular structure of 2. Co–N(pyrrole) and Co–N(pyridine)
19F NMR spectra in deuterated benzene were examined. The
spectrum of 2 without TFA (Fig. 2A) displays the same pattern
as the non-chlorinated analogue 1, consistent with an axially
symmetric coordination structure (C2v symmetry) with two
trans-coordinated pyridine molecules. Addition of a small
amount of TFA revealed the formation of a new complex, which
according to its spectrum is diamagnetic (Fig. 2B, narrow reso-
nances with regular chemical shifts) and of Cs symmetry (widely
separated ortho-F atoms). This is consistent with the absence of
axial symmetry due to only one axial pyridine ligand, i.e., com-
pound 3 in Scheme 2. The spectrum obtained in the presence of
excess TFA is also well-resolved (Fig. 2C), but all resonances
display very large paramagnetic shifts that are clearly inconsis-
tent with low spin cobalt(III). The o-, p-, m-F atoms of the mag-
netically equivalent C5- and C15-C6F5 groups are at −90.8,
−141.5, and −149.1 ppm, respectively, while the corresponding
resonances of the unique C10-C6F5 are at −144.7, −153.4 and
−167.2 ppm. Relative to the diamagnetic 2, the o-, m-, and p-F
resonances of each ring are shifted in the same direction and
monotonically less, indicating that the origin of the paramagnetic
shifts is dominantly due to dipolar (i.e., through space and not
bonds) interactions.13 It is also interesting that the resonances of
the C6F5 rings on C-5 and C-15 are shifted much more and in
opposite directions than those of the C10-C6F5 group, thus point-
ing toward opposite spin density on these carbon atoms. All
these data and the fact that S = 1 ground states with large an-
isotropies are reasonably common for square planar cobalt(III)
complexes14 lead to the conclusion that the new complex (4 in
Scheme 2) consists of an intermediate spin cobalt(III) complex
with no axial ligands. All together, the absorption spectra in
Fig. 1B may safely be assigned to complexes 2 (λmax = 444 nm),
3 (λmax = 400 nm) and 4 (λmax = 410 nm) of Scheme 2.
bond lengths are 1.872(4)–1.912(4)
Å and 1.984(4)–2.004(5) Å,
respectively.
bonds are about 0.1 Å longer than the in plane Co–N equatorial
bonds, as might be expected. The structure of 2 is distinctly
different from those of analogous tetraarylporphyrins, where
chlorine or bromine atoms on the pyrrole β-carbons induce
severe distortion, reducing the molecular symmetry from D4h to
D2, because the pairs of β-halogen atoms are located alternately
above and below the average porphyrin plane.16
The cyclic voltammogram (CV) of 2 reveals that its first
oxidation (E1/2 = 1.1 V vs. Ag/AgCl in CH3CN) and second
reduction (E1/2 = −0.8 V) processes are reversible, while the first
reduction is not (Fig. 4). The latter observation is analogous to a
previous finding with pyridine-coordinated cobalt(III) complexes
of other corroles.15 Based on the analysis of previous work we
assign the first reduction of 2 to a CoIII/CoII couple and the ir-
reversibility of the process to dissociation of pyridine from the
cobalt(II) complex due to a reduced affinity for binding axial
ligands. Addition of TFA to a solution of 2 induced dissociation
of the pyridine axial ligands to produce complex 4, which has a
reversible and well-defined voltammogram (E1/2 = 0.36 V). The
absence of pyridine as axial ligands in complex 4 positively
shifted the Epc for the reduction of CoIII to CoII by about
850 mV. This finding is very important for the oxygen reduction
reaction: cobalt(II) is known to be a good electrocatalyst for
these processes and more positive onset potentials are a highly
desired feature.11 Comparison between the CVof 2 and 1 reveals
that all redox couples of 2 shifted positively as a result of the
substitution of eight chlorides on β-pyrrole positions. The most
important effect regards the E1/2 for the second reduction:
590 mV more positive for 2 relative to 1. Cobalt(I) complexes
are known as very potent catalysts for a variety of reactions and
the possibility of performing electrocatalysis at reasonable poten-
tials is crucial for practical aspects.17 It should be mentioned that
analogous metalloporphyrins are much less affected by halo-
genation and in some extreme cases the first oxidation potentials
of β-pyrrole halogenated metalloporphyrins do in fact not shift
The molecular structure of 2 (Fig. 3) revealed that its macro-
cyclic framework is isostructural with 1, with the cobalt atom
located in the plane of an essentially flat corrole.15 The deviation
of the N atoms from the mean plane of the corrole macrocycle is
≤0.001 Å (comparable with ≤0.002 Å for 1), and the deviation
of the Co atom from that plane is 0.001 Å. The Co–N axial
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(c) K. M. Kadish, F. D’Souza, A. Villard, M. Autret, E. Van Caemel-
becke, P. Bianco, A. Antonini and P. Tagliatesta, Inorg. Chem., 1994, 33,
5169.
4 (a) P. Hoffmann, G. Labat, A. Robert and B. Meunier, Tetrahedron Lett.,
1990, 31, 1991; (b) M. W. Grinstaff, M. G. Hill, J. A. Labinger and
H. B. Gray, Science, 1994, 264, 1311.
5 (a) A. Kumar, I. Goldberg, M. Botoshansky, Y. Buchman and Z. Gross,
J. Am. Chem. Soc., 2010, 132, 15233; (b) M. W. Grinstaff, M. G. Hill,
E. R. Birnbaum, W. P. Schaefer, J. A. Labinger and H. B. Gray, Inorg.
Chem., 1995, 34, 4896; (c) B. Meunier, Chem. Rev., 1992, 92, 1411.
6 (a) E. Steene, A. Dey and A. Ghosh, J. Am. Chem. Soc., 2003, 125,
16300; (b) H.-Y. Liu, T.-S. Lai, L.-L. Yeung and C. K. Chang, Org. Lett.,
2003, 5, 617; (c) D. K. Dogutan, J. R. McGuire and D. G. Nocera, J. Am.
Chem. Soc., 2011, 133, 9178; (d) J. Leroy, A. Bondon, L. Toupet and
C. Rolando, Chem.–Eur. J., 1997, 3, 1890; (e) E. K. Woller and
S. G. DiMagno, J. Org. Chem., 1997, 62, 1588.
7 (a) D. Mandon, P. Ochenbein, J. Fischer, R. Weiss, K. Jayaraj,
R. N. Austin, A. Gold, P. S. White and O. Brigaud, Inorg. Chem., 1992,
31, 2044; (b) E. R. Birnbaum, J. A. Hodge, M. W. Grinstaff,
W. P. Schaefer, L. Henling, J. A. Labinger, J. E. Bercaw and H. B. Gray,
Inorg. Chem., 1995, 34, 3625.
8 (a) L. Wagnert, R. Rubin, A. Berg, A. Mahammed, Z. Gross and
H. Levanon, J. Phys. Chem. B, 2010, 114, 14303; (b) L. Wagnert,
A. Berg, E. Stavitski, T. Berthold, G. Kothe, I. Goldberg, A. Mahammed,
L. Simkhovich, Z. Gross and H. Levanon, Appl. Magn. Reson., 2006, 30,
591; (c) J. H. Palmer, A. C. Durrell, Z. Gross, J. R. Winkler and
H. B. Gray, J. Am. Chem. Soc., 2010, 132, 9230.
9 J. Vestfrid, M. Botoshansky, J. H. Palmer, A. C. Durrell, H. B. Gray and
Z. Gross, J. Am. Chem. Soc., 2011, 133, 12899.
10 (a) G. Golubkov, J. Bendix, H. B. Gray, A. Mahammed, I. Goldberg,
A. J. DiBilio and Z. Gross, Angew. Chem., Int. Ed., 2001, 40, 2132;
(b) T. H. Ngo, W. Van Rossom, W. Dehaen and W. Maes, Org. Biomol.
Chem., 2009, 7, 439.
Fig. 4 CV traces in CH3CN of 2 (2 mM) in the absence (gray) and
presence of 60 mM TFA (black). Inset: Color changes of silica-adsorbed
2, by holding it over aqueous solutions of HCl and ammonia.
toward the expected anodic direction.1 These results can be
explained by contrasting the X-ray structures of β-pyrrole halo-
genated corroles vs. β-pyrrole halogenated porphyrins. The
X-ray structures of all β-pyrrole brominated metallocorroles
reported to date (including the chlorinated complex 2) show that
the corroles retain planar conformation.1d,2a,5a,18 This pheno-
menon is completely different from the corresponding porphyrin
complexes, where β-pyrrole halogenation induces large distor-
tion of the macrocycle.3b, 5b,7 While this is signalled by dramatic
red shifts in UV-vis absorptions, it also leads to a very significant
attenuation of the expected effect of the electron-withdrawing
halogen substituents on redox potentials.2
11 A. Schechter, M. Stanevsky, A. Mahammed and Z. Gross, Inorg. Chem.,
2012, 51, 22.
12 Synthesis of 2: Concentrated HCl (37%, 8 mL) was added through a
dropping funnel, with a pressure equalizing tube, to a round bottomed
flask containing KMnO4 (1.5 g). Yellow chlorine gas was librated during
2 min and bubbled through a benzene solution (40 mL) of 1 (65 mg,
64.3 μmol) in a round bottomed flask. The reaction flask was stoppered
and the benzene solution was stirred for 30 min, after which the benzene
and excess chlorine were removed under vacuum (a water solution of
sodium thiosulfate was added into the evaporator receiving flask in order
to destroy excess of Cl2). The residue was dissolved in 10 mL benzene.
Pyridine (400 μM) and an ethanol solution of NaBH4 (100 mg in 5 mL)
were added successively and the reaction mixture was stirred for 5 min.
The solvents were removed under vacuum. Complex 2 was isolated as the
major product (first fraction) by column chromatography on silica gel 60
(eluent: hexanes/EtOAc 15 : 1). Solvent evaporation and recrystallization
from CH2Cl2/n-hexane mixtures resulted in 65 mg (58.3 μmol, 91%
yield) of 2 as dark green crystals. 19F NMR (188 MHz, C6D6): δ, ppm =
−138.7 (dd, 3J(F,F) = 24.8 Hz, 4J(F,F) = 8.1 Hz, 2F; ortho-F), −139.3
We report the first β-pyrrole chlorinated cobalt corrole,
obtained by direct chlorination under very mild reaction con-
ditions. The molecular structure of this complex reveals that the
cobalt atom is located in the plane of an essentially flat corrole.
Addition of acid to this complex induces dramatic changes in
electrochemical and spectroscopic properties due to the dis-
sociation of the axial ligands. The pronounced color changes
may be used for sensing gaseous molecules, as demonstrated for
ammonia and HCl vapors in the inset of Fig. 4.
We gratefully acknowledge the financial support of this
research by a fund from the RBNI.
Notes and references
3
4
3
(dd, J(F,F) = 24.4 Hz, J(F,F) = 7.9 Hz, 4F; ortho-F), −151.4 (t, J(F,F)
3
‡Crystal data for 2: C51H10Cl8CoF15N6O0.5, Fw = 1342.18, triclinic,
= 21.4 Hz, 2F; para-F), 152.1 (t, J(F,F) = 21.4 Hz, 1F; para-F), −163.2
ˉ
P1, a = 11.610(2), b = 12.280(2), c = 19.439(4) Å, α = 88.83(2), β =
(m, 4F; meta-F), −163.9 (m, 2F; meta-F). 1H NMR (200 MHz, C6D6):
δ, ppm = 4.78 (t, 7.6 Hz, 2H; para-H of pyridine), 3.96 (t, 7.0 Hz, 4H;
meta-H of pyridine), 1.10 (d, 5.6 Hz, 4H; ortho-H of pyridine). UV/Vis
(benzene): λmax, nm (ε, M−1 cm−1) = 444 (1.22 × 106), 590 (3.05 × 105),
606 (2.82 × 105). HR(ESI)-MS in negative ion mode (M−) (M-2pyridine
= C37N4F15Cl8Co): calcd for m/z = 1127.6664, obsd 1127.6714 (100%).
X-ray quality crystals of 2 were obtained by slow recrystallization from
mixtures of benzene/n-heptane (1 : 1).
82.82(2), γ = 87.01(2)°, V = 2745.7(9) Å3, Z = 2, Dx = 1.623 g cm−3, μ
= 0.796 mm−1, θ min/max = 2.78/24.75°, reflections collected/unique
23 539/9257 (Rint = 0.094), R1/wR2 [I > 2σ(I)] = 0.0662/0.1930, S =
0.847. Largest diff. peak and hole = 0.686 and −0.309 e Å−3. CCDC
877907.
1 (a) A. Mahammed, B. Tumanskii and Z. Gross, J. Porphyrins Phthalo-
cyanines, 2011, 15, 1275; (b) Z. Ou, J. Shao, F. D’Souza, P. Tagliatesta
and K. M. Kadish, J. Porphyrins Phthalocyanines, 2004, 8, 201;
(c) P. Bhyrappa, B. Purushothaman and J. J. Vittal, J. Porphyrins Phtha-
locyanines, 2003, 7, 682; (d) E. Rabinovich, I. Goldberg and Z. Gross,
Chem.–Eur. J., 2011, 17, 12294.
2 (a) T. Takeuchi, H. B. Gray and W. A. Goddard III, J. Am. Chem. Soc.,
1994, 116, 9730; (b) J. H. Palmer, M. W. Day, A. D. Wilson,
L. M. Henling, Z. Gross and H. B. Gray, J. Am. Chem. Soc., 2008, 130,
7786; (c) N. Y. Nelson, C. J. Medforth, D. J. Nurco, S.-L. Jia,
J. A. Shelnutt and K. M. Smith, Chem. Commun., 1999, 2071.
13 Z. Gross and C. M. Barzilay, Chem. Commun., 1998, 1105.
14 (a) J. C. Brewer, T. J. Collins, M. R. Smith and B. D. Santarsiero, J. Am.
Chem. Soc., 1988, 110, 423; (b) C. H. Cho, T. Y. Chien, J. H. Chen,
S. S. Wang and J. Y. Tung, Dalton Trans., 2010, 39, 2609.
15 A. Mahammed, I. Giladi and Z. Gross, Eur. J. Chem., 2001, 7, 4259.
16 E. R. Birnbaum, J. A. Hodge, M. W. Grinstaff, W. P. Schaefer,
L. Henling, J. A. Labinger, J. E. Bercaw and H. B. Gray, Inorg. Chem.,
1995, 34, 3625.
17 V. Artero, M. Chavarot-Kerlidou and M. Fontecave, Angew. Chem., Int.
Ed., 2011, 50, 7238.
3 (a) F. D’Souza, M. E. Zandler, P. Tagliatesta, Z. Ou, J. Shao, E. Van
Caemelbecke and K. M. Kadish, Inorg. Chem., 1998, 37, 4567;
(b) J. A. Hodge, M. G. Hill and H. B. Gray, Inorg. Chem., 1995, 34, 809;
18 (a) I. Aviv-Harel and Z. Gross, Coord. Chem. Rev., 2011, 255, 717;
(b) R. Paolesse, S. Nardis, F. Sagone and R. G. Khoury, J. Org. Chem.,
2001, 66, 550.
10940 | Dalton Trans., 2012, 41, 10938–10940
This journal is © The Royal Society of Chemistry 2012