Synthesis and characterization of free-base, copper, and nickel isocorroles

Article abstract of DOI:10.1021/ic100730j

A series of free-base and metalated isocorroles represented as (TT-n-iso-Cor)H₂ and (TT-n-iso-Cor)M^II, where n = 5 or 10 and M = Ni or Cu, were synthesized and characterized by electrochemistry and spectroelectrochemistry in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate. The metalation of the free-base macrocycles with Co^II, Mn^III, or Zn^II was also attempted but was unsuccessful. Six isocorroles were isolated and shown to undergo two stepwise oxidations to give π-cation radicals and dications in CH ₂Cl₂, with the most stable products being obtained in the case of the 10-substituted derivatives. The same isocorroles could also be reduced by one or two electrons, but the initial one-electron addition products are unstable and undergo a rapid chemical reaction giving a reduced corrole or corrole-like product, which could be reoxidized to the corresponding (TTCor)M at a controlled positive potential. This series of reactions effectively illustrates an isocorrole to corrole conversion upon reduction and reoxidation and was monitored by both electrochemistry and thin-layer spectroelectrochemistry.

Full text of DOI:10.1021/ic100730j

5766 Inorg. Chem. 2010, 49, 5766–5774
DOI: 10.1021/ic100730j
Synthesis and Characterization of Free-Base, Copper, and Nickel Isocorroles
Giuseppe Pomarico,† Xiao Xiao,§ Sara Nardis,† Roberto Paolesse,*,† Frank R. Fronczek,‡ Kevin M. Smith,*,‡
Yuanyuan Fang,§,^ Zhongping Ou,§,^ and Karl M. Kadish*,§
†
ꢀ
Dipartimento di Scienze e Tecnologie Chimiche, Universita di Roma Tor Vergata, via della Ricerca Scientifica,
1, 00133 Rome, Italy, ‡Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803,
§Department of Chemistry, University of Houston, Houston, Texas 77204-5003, and ^Department of Applied
Chemistry, Jiangsu University, Zhenjiang, 212013 P. R. China
Received April 15, 2010
A series of free-base and metalated isocorroles represented as (TT-n-iso-Cor)H2 and (TT-n-iso-Cor)MII, wheren = 5 or 10 and
M = Ni or Cu, were synthesized and characterized by electrochemistry and spectroelectrochemistry in CH2Cl2 containing 0.1 M
tetra-n-butylammonium perchlorate. The metalation of the free-base macrocycles with CoII, MnIII, orZnII was also attempted but
was unsuccessful. Six isocorroles were isolated and shown to undergo two stepwise oxidations to give π-cation radicals and
dications in CH2Cl2, with the most stable products being obtained in the case of the 10-substituted derivatives. The same
isocorroles could also be reduced by one or two electrons, but the initial one-electron addition products are unstable and
undergo a rapid chemical reaction giving a reduced corrole or corrole-like product, which could be reoxidized to the
corresponding (TTCor)M at a controlled positive potential. This series of reactions effectively illustrates an isocorrole to corrole
conversion upon reduction and reoxidation and was monitored by both electrochemistry and thin-layer spectroelectrochemistry.
Introduction
their particular behavior, which leads to possible applications in
the area of catalysis or sensors.6-9 Recent advances in the
synthetic chemistry of corroles have also allowed for the pre-
paration of other related analogues, one of which is the isocorrole
which may exist in two isomeric forms, as shown in Figure 2.
The first synthesis of a free-base isocorrole was reported in
2006 by Setsune,10 but the idea was earlier discussed in a
paper by Vogel and collaborators as it related to a Ni
isocorrole derivative.11 More recently, Setsune reported the
synthesis and characterization of several metal isocorrole
complexes.12 It should also be noted that the term “porphy-
rinoid [0.2.0.1]” has been used to indicate an isocorrole.13,14
The isocorrole has also been called a “pseudocorrole”.15
Porphyrins represent the prototypical example of a pyrrolic
macrocycle due to their essential biological functions. Related
families of the so-called “porphyrinoids” are also known and
available from both natural and synthetic sources. These
macrocycles differ from the well-characterized porphyrins by
one or more structural elements, examples being in the number
of pyrrole rings,1 in having one or more of the pyrrole rings
replaced by a furan or thiophene group2 or by the macrocycle
having a different number or position of the meso-bridges as
compared to the porphyrins.3,4 There are also several exam-
ples of porphyrinoid macrocycles where the aromatic system is
interrupted, one example is given by isoporphyrins,5 where the
conjugation pathway is interrupted by the presence of a
saturated meso-carbon as shown in Chart 1.
(6) Gross, Z.; Gray, H. B. Adv. Synth. Catal. 2004, 346, 165–170.
(7) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987–1999.
ꢀ
ꢁ
(8) Barbe, J.-M.; Canard, G.; Brandes, S.; Jerome, F.; Dubois, G.;
Guilard, R. J. Chem. Soc., Dalton Trans. 2004, 1208–1214.
(9) Paolesse, R.; Mandoj, F.; Marini, A.; Di Natale, C. In Encyclopedia of
Nanoscience and Nanotechnology; Nalwa, H., Ed.; American Science Publishers:
Valencia, CA, 2004; Vol. 9, pp 21-42.
Currently, the most frequently studied porphyrin analogues
are the corroles which have acquired a special interest because of
*To whom correspondence should be addressed. roberto.paolesse@
uniroma2.it (R.P.), kmsmith@lsu.edu (K.M.S.), kkadish@uh.edu (K.M.K.).
(1) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. In The Porphyrin Hand-
book; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
2000; Vol. 2, pp 55-124.
(10) Setsune, J. I.; Tsukajima, A.; Watanabe, J. Tetrahedron Lett. 2006,
47, 1817–1820.
(11) Hohlneicher, G.; Bremm, D.; Wytko, J.; Bley-Escrich, J.; Gisselbrecht,
J.-P.; Gross, M.; Mechels, M.; Lex, J.; Vogel, E. Chem.;Eur. J. 2003, 9,
5636–5642.
_ ꢁ
(2) Latos-Grazynski, L. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 2, pp
361-416.
(3) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted and Isomeric
Porphyrins; Pergamon: Oxford, 1997.
(12) Setsune, J. I.; Tsukajima, A.; Okazaki, N. J. Porphyrins Phthalocya-
nines 2009, 13, 256–265.
(13) Will, S.; Rahbar, A.; Schmickler, H.; Lex, J.; Vogel, E. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1390–1393.
(4) Sessler, J. L.; Gebauer, A.; Vogel, E. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000;
Vol. 2, pp 1-54.
(14) Vogel, E.; Binsack, B.; Hellwig, Y.; Erben, C.; Heger, A.; Lex, J.; Wu,
Y.-D. Angew. Chem., Int. Ed. 1997, 36, 2612–2615.
(15) Orlewska, C.; Maes, W.; Toppet, S.; Dehaen, W. Tetrahedron Lett.
2005, 46, 6067–6070.
(5) Woodward, R. B. Ind. Chim. Belge 1962, 27, 1293–1308.
r
pubs.acs.org/IC
Published on Web 05/25/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5767
Chart 1. Structure of Isoporphyrin (left) and Isocorroles (center and
right)
ppm relative to tetramethylsilane (TMS). UV-vis spectra were
measured on a Cary 50 spectrophotometer or with an HP Model
8453 diode array spectrophotometer (Hewlett-Packard Company).
High-resolution mass spectra were recorded on a Voyager DE STR
Biospectrometry workstation in the positive mode, using R-cyano-
4-hydroxycinnamic acid as a matrix (MALDI).
Cyclic voltammetry was carried out at 298 K by using an
EG&G Princeton Applied Research (PAR) 173 potentiostat/
galvanostat. A homemade three-electrode cell was used for
cyclic voltammetric measurements and consisted of a glassy
carbon working electrode, a platinum counter electrode, and a
homemade saturated calomel reference electrode (SCE). The
SCE was separated from the bulk of the solution by a fritted
glass bridge of low porosity which contained the solvent/sup-
porting electrolyte mixture.
Thin-layer UV-vis spectroelectrochemical experiments were
performed with a home-built thin-layer cell which has a light
transparent platinum net working electrode. Potentials were
applied and monitored with an EG&G PAR Model 173 poten-
tiostat. Time-resolved UV-vis spectra were recorded with a HP
Model 8453 diode array spectrophotometer. High-purity N2
from Trigas was used to deoxygenate the solution and was kept
over the solution during each electrochemical and spectroelec-
trochemical experiment.
However, for the sake of consistency with the isoporphyrins,
the term isocorrole will be used to describe the compounds
reported in the present paper.
Our own laboratory has followed a synthetic route differ-
ent from that described in the literature for isocorrole pre-
paration that is based on the readily available meso- triaryl-
corrole as a precursor.16 This analogue was prepared via a
one-step reaction involving oxidation of the starting corrole
with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in
the presence of a nucleophile, usually methanol. The route
provides a simple approach to obtain a wide range of dif-
ferent isocorroles and is important when considering the
remarkable development in new synthetic routes for obtain-
ing corroles bearing aryl substituents.17-20 It is also possible
with this synthetic method to prepare the 5-isomer which
have been only recently been reported.21
The presence of a methoxy or a hydroxy substituent
instead of an alkyl group at the meso-positions of an iso-
corrole makes possible an isocorrole-corrole conversion
upon metal coordination. We observed this effect after bro-
mination of free-base triphenylcorrole, where an oxidation of
the macrocycle led to the corresponding isocorrole species,
which then could be converted back to the parent corrole
during coordination with Co(III).19 This feature prompted us
to further investigate the behavior of the nonaromatic iso-
corrole as a chelate, exploring also its stability when reacted
with different metal ions. This is described in the present
paper where we report the synthesis, electrochemistry, and
spectroelectrochemistry of six free-base or metalated isocor-
roles represented as (TT-n-iso-Cor)H2 and (TT-n-iso-Cor)MII,
where n = 5 or 10 and M = Ni or Cu.
General Procedure for Preparation of 5- and 10-Methoxy-
5,10,15-tritolyl-isocorroles, (TT-5-isoCor)H2 and (TT-10-iso-
Cor)H2. In a 100 mL round-bottomed flask, equipped with a
stirring bar, tritolylcorrole (50 mg, 0.088 mmol) was dissolved
in 30 mL of dichloromethane/methanol mixture (2:1); DDQ
(20 mg, 0.088 mmol) was added, and the mixture was stirred for
about 5 min, monitoring the progress of the reaction by UV-vis
spectroscopy and thin-layer chromatography (TLC). After dis-
appearance of the starting material, the solvent was evaporated
under vacuum, and the crude product dissolved with CH2Cl2
and passed on a short silica gel plug eluted with CH2Cl2. A green
fraction containing the two isomers was collected and crystal-
lized fromCH2Cl2/CH3OH. If separation of mixture was required,
then isomers were purified by preparative layer chromatography
(PLC) (silica gel, CH2Cl2-hexane 2:1 as eluent). The first light-
green fraction corresponds to the isomer 10, the second dark-green
to the 5-isomer, and either crystallized from CH2Cl2/CH3OH.
Overall yield: 75% (40 mg); isomer ratio: H2TT-10-isoCor: 34%
(13.5 mg); (TT-5-isoCor)H2: 66% (26.5 mg). Spectroscopic data
have been previously reported in literature.16
General Procedure for Preparation of Isocorrole Copper Com-
plexes. The mixture of isocorrole was dissolved in CH2Cl2, and a
few mLs of a saturated solution of Cu(OAc)2 in CH3OH were
added. Mixture was stirred to reflux for 45 min, and the course
of the reaction monitored by UV-vis spectroscopy and TLC
(silica gel, CH2Cl2-hexane 2:1) After disappearance of starting
material, solvent was removed under reduced pressure, residue
was purified by PLC (silica gel, CH2Cl2-hexane 2:1). Two frac-
tions were collected, the first corresponding to 10-isomer and the
second to 5-isomer.
Experimental Section
Chemicals. Reagents and solvents (Sigma-Aldrich, Fluka,
and Carlo Erba Reagenti) were of synthetic grade and used
without further purification. Silica gel 60 (70-230 mesh) was
used for chromatography. Dichloromethane (CH2Cl2, 99.8%)
was purchased from EMD Chemicals Inc. and used as received.
Tetra-n-butylammonium perchlorate (TBAP) was purchased
from Sigma Chemical or Fluka Chemika Co., recrystallized
from ethyl alcohol, and dried under vacuum at 40 ꢀC for at least
one week prior to use.
(10-Methoxy-5,10,15-tritolyl-isocorrolato)Cu,(TT-10-isoCor)Cu).
The orange fraction was crystallized from CH2Cl2/CH3OH (yield
27%). Found: C, 74.3; H, 5.1; N, 8.4; C41H32CuN4O required C,
74.6; H, 4.9; N, 8.5. UV-vis: λmax(CH2Cl2), nm 371 (log ε, 4.54),
418 (4.50), 472(4.57), 543(3.75), 749 (3.58); 795 (3.70); 828 (3.97);
HRMS (MALDI): m/z 658.6965, (Mþ), 627.6692, (M-OCH3).
Crystallographic data: C41H32N4O. CHCl3, triclinic space group
Instrumentation. 1H NMR spectra were recorded on a Bruker
AV300 (300 MHz) spectrometer. Chemical shifts are given in
(16) Nardis, S.; Pomarico, G.; Fronczek, F. R.; Vicente, M. G. H.;
Paolesse, R. Tetrahedron Lett. 2007, 48, 8643–8646.
(17) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. 1999, 38,
1427–1429.
(18) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi,
T.; Smith, K. M. Chem. Commun. 1999, 1307–1308.
(19) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. J. Org. Chem. 2001,
66, 550–556.
˚
P-1, a = 7.6453(8), b = 11.2802(10), c = 21.863(2) A, R =
3
˚
79.320(6), β = 85.260(6), γ = 70.830(9)ꢀ, V = 1749.6(3) A , Z =
2, Fcalcd = 1.480 g cm-3, μ(Cu KR) = 3.313 mm-1, Rint = 0.039,
R = 0.050 (4817 data with Fo2 > 2σ(Fo2)), Rw = 0.134 (all 6194
unique data) and 433 refined parameters. A total of 15861 data
was collected at T = 90.0(5) K to θ = 68.7ꢀ with Cu KR radiation
(20) Koszarna, B.; Gryko, D. T. J. Org. Chem. 2006, 71, 3707–3717.
(21) Flint, D. L.; Fowler, R. L.; LeSaulnier, T. D.; Long, A. C.; O’Brien,
A. Y.; Geier, G. R., III J. Org. Chem. 2010, 75, 553-563.
˚
(λ = 1.54178 A) on a Bruker Kappa Apex-II diffractometer,
using an orange lath crystal of dimensions 0.22 ꢀ 0.11 ꢀ 0.02 mm.

5768 Inorganic Chemistry, Vol. 49, No. 12, 2010
Pomarico et al.
Refinement was by full-matrix least-squares using SHELXL,
with hydrogen (H) atoms in idealized positions. The chloroform
is disordered, and its contribution to the structure factors was
removed using SQUEEZE.22 The phenyl group adjacent to me-
thoxy is disordered into two orientations with nearly equal refined
occupancies, 0.529(6)/0.471(6). CCDC-762432 contains the sup-
plementary crystallographic data for this paper. These data can be
obtained online free of charge (or from the Cambridge Crystal-
lographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
UK; fax: (þ44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
(5-Methoxy-5,10,15-tritolyl-isocorrolato)Cu, (TT-5-iso-
Cor)Cu. The brown fraction was crystallized from CH2Cl2/
CH3OH (yield 54%). Found: C, 74.4; H, 5.2; N, 8.3; C41H32Cu-
N4O requires C, 74.6; H, 4.9; N, 8.5. UV-vis: λmax(CH2Cl2), nm
358 (log ε, 4.31), 415 (4.57), 462 (4.30), 541 (3.75), 569 (3.67); 610
(3.45); 820 (3.53); HRMS (MALDI): m/z 658.2563, (Mþ),
627.4931, (M-OCH3).
General Procedure for Preparation of Isocorrole Nickel Com-
plexes. The mixture of isocorroles was dissolved in DMF, and a
three-fold excess of Ni(OAc)2 was added. The mixture was
stirred to reflux for 3 h, and the course of the reaction monitored
by UV-vis spectroscopy and TLC (silica gel, CH2Cl2-hexane
2:1). After the disappearance of starting material, mixture was
left to cool and then precipitated with water and filtered on
paper. Residue was dissolved with CH2Cl2, dried over Na2SO4,
and purified with a silica gel plug eluted with CH2Cl2 and then
purified again by PLC (silica gel, CH2Cl2-hexane 2:1) in order
to separate the isomers. Two fractions were collected, the first
(brown) corresponding to the 10-isomer and the second (green)
to the 5-isomer.
required C, 77.0; H, 5.0; N, 8.8. UV-vis: λmax(CH2Cl2), nm
387 (log ε, 4.56), 445 (4.61), 547 (4.12), 705 (3.85), 867 (3.62).
Results and Discussion
Synthesis and Characterization. The coordination be-
havior of (TT-10-isoCor)H2 and (TT-5-isoCor)H2 was
examined by reaction with the following metal ions: CoII,
MnIII, CuII, NiII, and ZnII. The choice of these metals was
due to their different formal oxidation states when co-
ordinated to a corrole species: cobalt and manganese
usually have formal oxidation states higher than þ2,
while in the case of copper and nickel, an equilibrium
between the þ2 and þ3 oxidation states can occur. A þ2
oxidation state is always observed in the case of zinc.23
Our aim was to investigate the stability of isocorrole
complex against its conversion to the corresponding
corrole derivative; we would expect a metal-to-ligand
electron transfer in the case of metals adopting formal
oxidation states higher than þ2, with a consequent rearo-
matization to the corrole upon reduction of the isocorrole
species.
The first metalation attempt was with cobalt ion, which
has already been demonstrated to induce corrole formation
when coordinated to a free-base isocorrole.19,24 We reacted
a mixture of (TT-10-isoCor)H2 and (TT-5-isoCor)H2 dis-
solved in CH2Cl2 with a saturated solution of Co(OAc)2 and
triphenylphosphine in methanol, obtaining a single reaction
product. UV-vis spectroscopy strongly suggested that the
cobalt ion induced a rearomatization of the macrocycle, this
hypothesis being supported by comparison with a sample of
authentic (TTCor)Co(PPh3).
This result confirmed what was previously observed for
other metal ions (see above), but considering the peculiar
affinity showed by Co ion with the corrole framework, we
decided to study the generality of this reaction using
coordination with Mn ion, which assumes a formal
oxidation state higher than þ2 when coordinated to
corroles.23 This could then provide further support for
the generality of the isocorrole to corrole conversion. The
reaction was carried out in refluxing DMF under an inert
atmosphere using MnCl2 as the metal carrier. The reac-
tion afforded a brown product, which was compared with
a pure sample of (TTCor)Mn, showing identical spectro-
scopic features.
The conversion of an isocorrole to a corrole upon
coordination of a high oxidation-state metal could
be reasonably attributed to the pathway shown in Scheme
1 for the case of the manganese derivative. After metala-
tion of the isocorrole, an oxidation of the central metal
ion is proposed to occur, and this would be accompanied
by reduction at the isocorrole π-ring system to give the
corresponding corrole after rearomatization of the
macrocycle and loss of methoxide.
[10-Methoxy-5,10,15-tritolyl-isocorrolato]Ni, (TT-10-isoCor)Ni.
The brown fraction was crystallized from CH2Cl2/CH3OH (yield
49%). Found: C, 74.9; H, 5.0; N, 8.3; C41H32N4NiO required C,
75.1; H, 4.9; N, 8.5. UV-vis: λmax(CH2Cl2), nm 361 (log ε, 4.30),
409 (4.51), 430 (4.54), 442 (4.59), 531 (3.86); 560 (3.82); 602 (3.70);
818 (3.45); 905 (4.10); 1H NMR δppm(CDCl3, J [Hz]) 7.76 (2 H, d,
J = 8.18, 10-phenyl); 7.32 (4 H, m, 5,15-phenyl); 7.19 (4 H, m,
5,15-phenyl); 7.11 (2 H, d, J = 8.03, 10-phenyl); 6.44 (2 H, d, J =
4.50, β-pyrrole); 6.28 (2 H, d, J = 4.35, β-pyrrole); 6.24 (2 H, d,
J = 4.20, β-pyrrole); 6.15 (2 H, d, J = 4.44, β-pyrrole); 3.40 (3 H,
s, 10-OCH3); 2.42 (6 H, s, 5 and 15-phenyl-CH3); 2.30 (3 H, s,
10-phenyl-CH3). MS (MALDI): m/z 654.0038, (Mþ), 624.0842,
(M-OCH3).
[5-Methoxy-5,10,15-tritolyl-isocorrolato]Ni, (TT-5-isoCor)Ni.
The brown fraction was crystallized from CH2Cl2/CH3OH
(yield 32%). Found: C, 75.0; H, 5.1; N, 8.3; C41H32N4NiO
required C, 75.1; H, 4.9; N, 8.5. UV-vis: λmax(CH2Cl2), nm
402 (log ε, 4.53), 447 (4.66), 677 (3.69), 866 (3.75), 967 (3.90); 1H
NMR δppm(CDCl3, J [Hz]) 7.73 (2 H, d, J = 8.15, 10-phenyl);
7.22 (8 H, m, 5 and 15-phenyl); 7.07 (2 H, d, J = 8.2, 10-phenyl);
6.51 (1 H, d, J = 4.90, β-pyrrole); 6.41 (4 H, m, β-pyrrole); 6.26
(1 H, d, J = 4.54, β-pyrrole); 6.22 (1 H, d, J = 4.54, β-pyrrole);
5.85 (1 H, d, J = 3.94, β-pyrrole); 3.45 (3 H, s, 5-OCH3); 2.42
(3 H, s, phenyl-CH3); 2.40 (3 H, s, phenyl-CH3); 2.30 (3 H, s,
phenyl-CH3). MS (MALDI): m/z 653.8926, (Mþ), 623.7804,
(M-OCH3).
[5,10,15-tritolyl-corrolato]Ni, (TTCor)Ni. Tritolyl-corrole
(30 mg, 0.053 mmol) was dissolved in DMF and Ni(OAc)2
was added. The mixture was stirred under refluxing for 2 h, moni-
toring the course by UV-vis spectroscopy and TLC (silica gel,
CH2Cl2-hexane 2:1). After disappearance of corrole free base,
mixture was left to cool down then precipitated with water
and filtered; residue was dissolved with chloroform, dried over
Na2SO4, and purified by a silica gel column eluted with CH2Cl2.
First (brown) band, corresponding to NiTTC was crystallized by
CH2Cl2/CH3OH, giving 21 mg (0.034 mmol) of brownish crystals
(Yield: 65%). Found: C, 76.8; H, 5.1; N, 8.7; C40H29N4Ni
The second metal ion insertion examined in the present
study was copper, whose electronic state in the corrole is
(23) (a) Paolesse, R. In The Porphyrin Handbook, Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 2, pp 201-232.
(b) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook; Kadish,
K. M., Smith, K., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 2, pp
233-300.
(24) Paolesse, R.; Jaquinod, L.; Senge, M. O.; Smith, K. M. J. Org. Chem.
1997, 62, 6193–6198.
(22) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5769
Scheme 1. Pathway of Conversion an Isocorrole to a Corrole
still a matter of debate, being either Cu(III) or Cu(II)
depending upon the oxidation state of the corrole macro-
cycle.25-30 The equilibrium shown in eq 1 has been
proposed and is supported by the diamagnetic character
of Cu triarylcorrolates at room temperature,28 although it
has been suggested30 that caution should be used for char-
acterization of such a species. Nonetheless a comparison of
the currently investigated compounds with Cu(II) 10-oxa-
corrole, suggests that copper ion in the overall neutral
corrole molecule can be best described as Cu(II).30
II
ðCorÞCuIII / ðCorþ ÞCu
ð1Þ
3
Metalation of the isocorrole with Cu(II) was carried out
in refluxing CH2Cl2 solution using a mixture of isocorrole
regioisomers, to which was added a few milliliters of satu-
rated copper acetate in methanol. Three products were
observed by TLC after disappearance of the starting
material. The first was characterized as (TTCor)Cu, sug-
gesting a similar isocorrole to corrole conversion as when
metalation attempts were made with cobalt or manganese
and gave a (TTCor)Mn or (TTCor)Co product.
Figure 1. UV-vis spectra of (TT-10-isoCor)Cu (solid line) and of (TT-
5-isoCor)Cu (dotted line).
paramagnetic behavior of the copper complexes, as ex-
pected for a species having a Cu(II) ion coordinated
within the isocorrole macrocycle.
We initially assumed that the first-collected orange
product was the 10-isomer and the second red-brown
species the 5-isomer, an assignment based on the fact that
the two complexes have the same Rf value as the corre-
sponding free-base forms of these isomers. To confirm
this hypothesis, we repeated copper insertion experiments
using separately the 5- and 10-isomers. In both cases,
together with a small quantity of (TTCor)Cu, we ob-
tained products identical to those previously isolated,
confirming that the first eluted band corresponds to
(TT-10-isoCor)Cu and the second to (TT-5-isoCor)Cu.
Mass spectra of both the 5- and 10-isocorrole isomers
showed a molecular peak at 660 M/Zþ, corresponding to
the copper isocorrole complex, with a parent peak at
627 M/Zþ, corresponding to the loss of the methoxy
group (see Supporting Information, Figure S1). We were
able to obtain a single crystal of (TT-10-isoCor)Cu
suitable for X-ray characterization by slow diffusion of
methanol into a dichloromethane solution.The molecular
structure is shown in Figure 2, which illustrates only one
of the two disordered MePh orientations. The coordina-
tion geometry is square planar, with the Cu atom lying
A second band in the mixture after reacting the iso-
corrole with copper acetate (TLC, silica gel eluted with
CH2Cl2-hexane 2: 1) was orange in color, while a more
intense third band was red-brown. After separation by
chromatography, the orange fraction was subsequently
identified as the 10-isomer and has three Soret-like bands
existing in the blue-green portion of the visible region
(380-500 nm). The 5-isomer has a single soret band at
415 nm, with a shoulder at a higher wavelength and a
weak absorption shifted toward the blue portion of the
spectrum (Figure 1).
An unambiguous 1H NMR characterization of the
two isocorrole regioisomers was not possible due to the
(25) Luobeznova, I.; Simkhovich, L.; Goldberg, I.; Gross, Z. Eur. J.
Inorg. Chem. 2004, 8, 1724–1732.
(26) Vogel, E.; Will, S.; Tilling, A. S.; Neumann, L.; Lex, J.; Bill, E.;
Trautwein, A. X.; Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1994, 33,
731–735.
(27) Wasbotten, I. H.; Wondimagegn, T.; Ghosh, A. J. Am. Chem. Soc.
2002, 124, 8104–8116.
˚
0.0027(7) A out of the best plane of the four N atoms,
€
~
(28) Bruckner, C.; Brinas, R. P.; Krause Bauer, J. A. Inorg. Chem. 2003,
42, 4495–4497.
˚
which exhibit a maximum deviation of 0.007(3) A from
ꢁ ^
(29) Guilard, R.; Gros, C. P.; Barbe, J. M.; Espinosa, E.; Jerome, F.;
coplanarity. The Cu-N distances in the five-membered
chelate ring are 1.914(2) and 1.915(2) A, while those in
Tabard, A. Inorg. Chem. 2004, 43, 7441–7455.
˚
€
ꢁ
(30) Broring, M.; Bregier, F.; Consul Tejero, E.; Hell, C.; Holthausen,
the six-membered chelate ring containing the sp3
C
M. C. Angew. Chem., Int. Ed. 2007, 46, 445–448.

5770 Inorganic Chemistry, Vol. 49, No. 12, 2010
Pomarico et al.
Figure 3. UV-vis spectra of (TT-10-isoCor)Ni (;) and of (TT-5-
isoCor)Ni (----).
facilitating formation of a stable metalated isocorrole.
Our initial choice was the Zn(II) ion because of its facile
insertion into porphyrins. The same synthetic approach
as described above was used, namely refluxing the mix-
ture of isocorrole regioisomers in CH2Cl2 and adding of
few mLs of saturated Zn(OAc)2 in methanol.
The UV-vis spectral changes which immediately occur
after Zn(II) addition indicate that metalation has taken
place, but all attempts to isolate the Zn isocorrole failed
and the product quickly decomposed during reaction
work up. After that we turned out our attention to Ni(II)
which we hoped would form stable and diamagnetic
compounds, making easier the product characterization.
The isocorrole regiosomers and Ni(OAc)2 were dissolved
in DMF, and the mixture heated to reflux for three hours
while monitoring the course of the reaction by UV-vis
spectroscopy and TLC (silica gel, CH2Cl2-hexane 2:1
as eluent). The reaction gave a brownish product that was
quickly purified by a silica gel plug and eluted with
dichloromethane to separate the desired isocorroles from
the side products. Two fractions were isolated by pre-
parative TLC (silica gel, CH2Cl2-hexane 2:1), and these
eluted in an order suggesting formation of the 10- and
5-isocorrole Ni complexes, respectively. Their UV-vis
spectra are reported in Figure 3.
Figure 2. X-ray molecular structure of (TT-10-isoCor)Cu.
˚
atom are 1.923(3) and 1.929(2) A. These distances are all
longer than typical Cu-N distances in Cu(III) corroles,
˚
which show a mean value of 1.892 A for 44 individual
distances from Cambridge Structural Database31 entries
BERVOK, BINCUX, FATDOU, FATDUA, FATFAI,
GEDQAI, KAGGIJ, QEVQAK, and RINCAS. The
average Cu-N length difference is in accord with
˚
the approximate 0.03 A larger ionic radius of square
planar Cu(II) vs Cu(III). The distances observed in Cu-
(TT-10-isoCor) are, however, slightly shorter than those
seen in Cu(II) porphyrins on account of the contracted ring
of the isocorrole. For example, the Cu-N bond distance in
Cu(tetraphenylporphyrin)32 is 1.995(2) A, identical to the
˚
˚
mean value of 1.996 A seen in several Cu(II) tetraalkyl-
The two regioisomers were identified by 1H NMR spec-
troscopy, an example of which is given in Figure 4. The 1H
NMR spectrum of Ni(TT-10-isoCor) shows a signal pattern
characterized by a higher symmetry than that of Ni(TT-5-
isoCor), and one can note multiple signals of the β-pyrrole
and the presence of three different signals referred to methyl
substituents on the meso-phenyl groups.
We also attempted Ni metalation using the already
separated isocorrole isomers and obtained the same
products as when the reaction was carried out using the
isomeric mixture. No rearrangement of the isomers was
observed upon metalation, thus allowing preparation of
the isocorroles using directly the isomer mixture.
Electrochemistry. Redox properties of the isocorroles
and the related metallocorroles were examined in CH2Cl2
containing 0.1 M TBAP, and examples of cyclic voltam-
mograms are shown in Figure 5a for the three (TT-
isoCor)M derivatives, where M = 2H, Ni(II) or Cu(II).
Each isocorrole derivative undergoes two reversible one
electron oxidations at the conjugated macrocycle and one
or two isocorrole-centered reductions, the latter of which
porphyrins.33 Overall, the 23-atom isocorrole core in Cu-
(TT-10-isoCor) deviates slightly from planarity, with a
3
mean deviation of 0.114 A, the maximum being the sp C
˚
˚
atom, which lies 0.264(5) A from the best plane.
The formation of a metalated isocorrole depends on
both the reaction time and the temperature. Insertion of
copper does not require harsh conditions, but when the
reaction mixture was heated for more than two hours, the
amount of (TTCor)Cu increased significantly. This fea-
ture was confirmed by carrying out the reaction in the
higher boiling DMF solvent using CuCl2 as a metal
carrier. After 2 h in refluxing DMF, (TTCor)Cu was
detected as the main reaction product, while only trace
amounts of copper isocorrole were found in the mixture.
We then attempted insertion of Ni and Zn ions, both
of which would be in the þ2 oxidation state, perhaps
(31) Allen, F. H. Acta Crystallogr. 2002, B58, 380–388.
(32) He, H.-S. Acta Crystallogr. 2007, E63, m976–m977.
(33) Senge, M. O.; Bischoff, I.; Nelson, N. Y.; Smith, K. M. J. Porphyrins
Phthalocyanines 1999, 3, 99–116.

Article
Inorganic Chemistry, Vol. 49, No. 12, 2010 5771
Figure 4. 1H NMR spectra of (TT-10-isoCor)Ni (up) and of (TT-5-isoCor)Ni (down).
are followed by a rapid chemical reaction on the electro-
chemical time scale. Other redox processes are also seen in the
cyclic voltammograms, and these are associated with reac-
tions of a “corrole side product” as determined by compar-
ison with data in the literature for genuine corroles23,34 and
that of (TTCor)Ni or (TTCor)Cu, whose cyclic voltammo-
grams are shown in Figure 5b.
The formal MIII/MII processes of (TTCor)M are located
at þ0.02 V(Ni) or -0.23 V(Cu) (Figure 5b), and electrode
reactions at the same potentials are seen in the Ni and Cu
isocorrole cyclic voltammograms of Figure 5a. Additional
reductions of the corrole beyond the MIII/MII processes are
not observed up to potentials of almost -2.00 V vs SCE34
(see Figure 5b), and thus any major redox processes in this
region of Figure 5a can only be assigned as involving directly
the isocorrole, (TT-10-isoCor)M or (TT-10-isoCor)H2.
The singly and doubly oxidized isocorroles are rela-
tively stable on the cyclic voltammetry time scale, but
this is not the case upon reduction where a chemical
reaction follows electron addition, leading to an irreversible
process, and a product having electrochemical properties
similar to that of the singly reduced corroles, [(TTCor)Ni]-
and [(TTCor)Cu]-. The first irreversible reduction of
(TT-10-isoCor)Ni and (TT-10-isoCor)Cu both occur at
the same potential ofEpc = -0.92 V, and the peak current is
slightly increased as compared to the two oxidations. On the
return sweep, there is no reoxidation associated with the
reduction at -0.92 V, but a reversible oxidation/reduction
process is seen at exactly the same E1/2 value as for reduction
and reoxidation of the corresponding (TTCor)M deriva-
tives, namely -0.03 V for M = Ni and -0.23 V for M =
Cu, thus implying an isocorrole to corrole conversion upon
reduction and reoxidation of these two compounds.
Figure 5. Cyclic voltammograms of (a) (TT-10-isoCor)M and
(b) (TTCor)M in CH2Cl2, 0.1 M TBAP. Reactions within the “boxes”
of Figure 5a are those assigned to the isocorroles, all others being
assigned to oxidation or reduction of a (TTCor)M “side product”.
An electrochemically initiated isocorrole to corrole
conversion is also suggested by cyclic voltammograms
of the other examined isocorroles, as illustrated in Figure
radicals and dianions.35 It should also be noted that the
oxidation peak seen at Ep = -0.06 V after reduction of
the two free-base isocorroles is not obtained if the scan is
terminated prior to the first reduction and is located at a
potential consistent with the [(TTCor)H2]-/(TTCor)H2
redox reactions previously characterized for a variety of
substituted corroles under the same solution conditions.36
6
for (TT-5-isoCor)H2 and (TT-5-isoCor)Ni and
Figure 5a for (TT-10-isoCor)H2. Unlike the Ni(II) and
Cu(II) derivatives in Figure 5a, a well-defined second one-
electron reduction is observed for the other isocorroles,
with the difference in potential between the two one-
electron additions ranging from 240-300 mV for the
free-base derivatives and 410 mV for the Ni isocorrole.
These separations are similar to the ΔE1/2 values observed
between potentials for formation of porphyrin π anion
(35) Kadish, K. M. In The Porphyrin Handbook, Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 8, pp 1-97.
(36) Shen, J.; Shao, J.; Ou, Z.; E, W.; Koszarna, B.; Gryko, D. T.; Kadish,
K. M. Inorg. Chem. 2006, 45, 2251–2265.
(34) Ou, Z.; Shao, J.; Zhao, H.; Ghosh, A.; Kadish, K. M. J. Porphyrins
Phthalocyanines 2004, 8, 1236–1247.

5772 Inorganic Chemistry, Vol. 49, No. 12, 2010
Pomarico et al.
Figure 6. Cyclic voltammograms of (TT-5-isoCor)M, (a) M = 2H and
(b) M = NiII in CH2Cl2, 0.1 M TBAP.
Table 1. Half-Wave Potentials (V vs SCE) of Isocorroles and Related Corroles in
CH2Cl2, 0.1 M TBAP
oxidation
reduction
central
macrocycles metal ΔE2-1 second
first
first second ΔE1-2
TT-10-isoCor 2H
0.34
0.43
0.33
0.70
0.45
0.32
1.16 0.82
-1.00 -1.30a 0.30
-0.92a -1.26a 0.34
-0.92a -1.40a 0.48
-0.84 -1.08a 0.24
-0.69 -1.10a 0.41
-0.69 -1.09a 0.40
NiII
1.14 0.71
1.04 0.71
1.58a 0.88a
1.14 0.69
0.99 0.67
CuII
TT-5-isoCor 2H
NiII
CuII
TTCor
NiIII
CuIII,b
-
0.65
1.57a 0.87c, 0.61 0.02
-
-
-
-
1.35 0.70 -0.23
a Irreversible peak potential at a scan rate of 0.10 V/s. b Data taken
from ref 34. c Split peaks due to dimerization.
Figure 8. UV-vis spectral changes obtained during the reductions of
(a) (TT-10-isoCor)Cu and (b) (TTCor)Cu in CH2Cl2, 0.1 M TBAP. The
bands at 432, 573, and 609 nm are attributed to the anionic corrole, while
the 417 nm band is associated with the unreduced Cu(III) corrole.
Scheme 2. Plausible Reduction Mechanism of (a) Copper Isocorroles
and (b) Free-Base 5- and 10-Isocorroles in CH2Cl2, 0.1 M TBAP
Figure 7. Cyclic voltammograms of (TT-5-isoCor)Ni and (TT-10-iso-
Cor)Ni in PhCN, 0.1 M TBAP.
This suggests an isocorrole to corrole conversion also
occurs in the case of the two free-base derivatives.
figure, (TT-5-isoCor)Ni exhibits a reversiblefirstoxidation
at 0.70 V and an almost reversible first reduction, giving a
highest occupied molecular orbital-lowest unoccupied
molecular orbital (HOMO-LUMO) gap of 1.37 V. This
compares to a gap of ∼1.62 V for the (TT-10-isoCor)Ni in
PhCN or 1.63 V for the same compound in CH2Cl2. The
HOMO-LUMO gap of the isocorroles is in each case
smaller than values for the related corroles, which average
2.20 V under the same solution conditions.
Finally, it should be pointed out that reductions of the
5-isocorroles are easier than the 10-isocorroles by 160 mV
in the case of the two free-base derivatives and by ∼230 mV
in the case of the Ni complex, but almost no differences are
seen in the potentials for oxidation as shown by the data in
Table 1 and Figure 7 which illustrates cyclic voltammo-
grams of the two Ni isocorroles in PhCN. As seen in this

Article
In summary, the cyclic voltammograms of the 5-iso-
Inorganic Chemistry, Vol. 49, No. 12, 2010 5773
CH2Cl2 containing 0.1 M TBAP as supporting electrolyte
(Figure 8a). A potential of -0.50 V was then applied to
the solution in the thin-layer cell, and the isocorrole bands
remain unchanged, with the only difference in the spec-
trum being a loss of the formal Cu(III) corrole band at
417 nm and an increase in the Cu(II) corrole band at
432 nm (see Figure 8a). This contrasts with the much
larger spectral changes which result upon switching to a
controlled reducing potential of -1.10 V. As this reduc-
tion processes, all bands assigned to the isocorrole dis-
appear, and the final spectrum is characterized by an
intense band at 432 and smaller bands at 573 and 609 nm.
An identical spectrum is obtained upon the one-electron
reduction of genuine (TTCor)Cu at -0.50 V (Figure 8b),
thus indicating the same [(TTCor)Cu]- corrole product is
formed upon electroreduction of both compounds.
Although the UV-vis spectra of the unreduced free-
base 5- and 10-isocorrole are quite different in CH2Cl2
(see Experimental Section), exactly the same UV-vis
spectrum is obtained after controlled potential reduction
of the two compounds in the thin layer cell. Furthermore,
the spectrum at the completion of electrolysis is the same
as the spectrum of [(TTCor)H2]- generated by reduction
and proton loss from (TTCor)H3 in PhCN (see Table 2).36
This is consistent with the mechanism proposed in
Scheme 2b and indicates that both isomers of the free-
base isocorroles are converted to the same singly reduced
diprotic corrole upon reduction. The electrogenerated
[(TTCor)H2]- product is relatively stable and can be
reversibly oxidized to give (TTCor•)H2 (λmax = 391 and
corroles are self-consistent in indicating an electrochemi-
cally initiated isocorrole to corrole conversion, and this
was confirmed by the thin-layer spectroelectrochemistry
data. A plausible reduction mechanism is given in Scheme 2
for the copper and free-base derivatives and may also be true
for the nickel derivatives, although some difference in the
intermediate may exist as described shortly.
Spectroelectrochemistry. Thin-layer UV-vis spectro-
electrochemistry was carried out in CH2Cl2, 0.1 M TBAP,
and an example of the spectral changes monitored during
reduction of (TT-10-isoCor)Cu and (TTCor)Cu are illu-
strated in Figure 8. The spectrum of (TT-10-isoCor)Cu in
CH2Cl2 is shown by the solid line in Figure 1. There are
three bands at 372, 474, and 833 nm, all of which are
assignedtothe isocorroleand alsoanother bandat417 nm,
which is due to trace (TTCor)Cu in solution. This spec-
trum is identical to the UV-vis spectrum obtained in
Table 2. UV-vis Data (λmax, nm) of Electrogenerated [(TTCor)H2]- and
(TTCor•)H2
first red product
[(TTCor)H2]-
ox product
(TTCor•)H2
starting
compound
Soret
region
visible
region
Soret
region
Solvent
(TT-10-isoCor)H2 CH2Cl2 424 447 536 595 640
(TT-5-isoCor)H2
391
391
396
431
431
436
CH2Cl2 424 447 536 595 641
PhCN 430 452 536 596 644
a
(TTCor)H3
a Data taken from ref 36.
Figure 9. UV-vis spectral changes obtained upon stepwise controlled potential reduction, reoxidation, and rereduction for (a) (TT-5-isoCor)H2 and
(b) (TT-10-isoCor)H2 in CH2Cl2, 0.1 M TBAP.

5774 Inorganic Chemistry, Vol. 49, No. 12, 2010
Pomarico et al.
Figure 10. Spectral changes during controlled potential reduction of (TT-10-isoCor)Ni and (TT-5-isoCor)Ni in a thin-layer cell (top) with comparison to
the reoxidation product at Eapp = 0.20 V (bottom left) and the genuine (TTCor)Ni in its neutral and singly reduced forms (bottom right).
Scheme 3. Mechanism for Reduction and Reoxidation of (TT-5-iso-
Cor)Ni and (TT-10-isoCor)Ni
shows quantitatively the formation of (TTCor)Ni in each
case, thus again confirming an isocorrole to corrole
conversion also occurs with the copper and free-base
derivatives. This mechanism for the two nickel isocorroles
is shown in Scheme 3.
Conclusions
The reaction of 5- and 10-methoxy-isocorrole derivatives
with different metal ions has beeninvestigated with the aim to
elucidate the coordination behavior of corrole analogs. The
stability of the final isocorrole complex depends upon the
central metal ion; if a formal oxidation state higher than þ2 is
accessible to the coordinated metal ion, such as in the case of
Co and Mn, a metal-to-ligand charge transfer occurs, which
involves a reduction of the isocorrole species with the con-
sequent rearomatization to the corresponding corrole com-
plex. Stable isocorrole complexes have been obtained in the
431 nm) and then rereduced to give [(TTCor)H2]-, again
case of Cu and Ni ions.
under the given experimental conditions.36 The spectral
Electrochemistry and spectroelectrochemistry characteri-
changes which occur at each potential are shown in
zation of free bases, Cu and Ni isocorrole derivatives,
Figure 9, and a summary of the spectral data before and
provides further insight into the redox behavior of such a
after reduction and reoxidation are given in Table 2.
species, confirming the isocorrole to corrole conversion upon
An isocorrole to corrole conversion is also seen for the two
electroreduction.
nickel derivatives, whose cyclic voltammograms are shown
in Figure 7. The UV-vis spectrum of the neutral isocorroles
Acknowledgment. We gratefully acknowledge the sup-
are quite different, but identical spectra are obtained after
port of MIUR, Italy (PRIN project 2007C8RW53), the
controlled potential reduction in the thin-layer cell. This is
International Technology and Science Cooperation Foun-
shown in the top of Figure 10 where the product of reduction
dation (BZ2007058, GJ2007006), the NSF (BK2008226) of
in each case is characterized by a Soret band at 416 nm, a
Jiangsu Province, China, the Robert A. Welch Foundation
Q-band at 588 nm, and a shoulder at 547 nm.
(K.M.K., Grant E-680), and the US National Institutes of
Although the electrogenerated product of the nickel
Health (K.M.S., Grant CA 132861).
isocorrole reductions is the same for each isomer, these
spectra are not identical to the spectrum obtained in the
first reduction of genuine (TTCor)Ni (Figure 10, bottom
right), the main difference being in the Q-band region.
Nonetheless a controlled potential reoxidation of the
electroreduced (TT-5-isoCor)Ni and (TT-10-isoCor)Ni
Supporting Information Available: Mass spectra, cyclic vol-
tammograms, and the UV-vis spectral changes obtained during
controlled potential reductions or oxidations in CH2Cl2, 0.1 M
TBAP and CIF of (TT-10-isoCor)Cu. This material is available
free of charge via the Internet at http://pubs.acs.org.