An Example of O2 Binding in a Cobalt(II) Corrole System and High-Valent Cobalt-Cyano and Cobalt-Alkynyl Complexes

Article abstract of DOI:10.1021/ja036983s

The novel cobalt corrolazine (Cz) complexes (TBP)₈CzCoCN (1) and (TBP)₈CzCo(CCSiPh₃) (2) have been synthesized and examined in light of the recent intense interest regarding the role of corrole ligands in stabilizing high oxidation states. In the case of 2, the molecular structure has been determined by X-ray crystallography, revealing a short Co-C distance of 1.831(4) A and an intermolecular π-stacking interaction between Cz ring planes, and this structure has been analyzed in regards to the electronic configuration. By a combination of spectroscopic techniques it has been shown that 1 is best described as a cobalt(III)-π-cation-radical complex, whereas 2 is likely best represented as the resonance hybrid (Cz)Co^IV(CCSiPh₃)?(Cz^+.)Co ^III(CCSiPh₃). The reduced cobalt(II) complex, [(TBP) ₈CzCo^II(py)]^-, has been generated in situ and shown to bind dioxygen at low temperature to give [(TBP)₈CzCo ^III(py)(O₂)]^-. For the reduced complex [(TBP)₈CzCo^II(py)]^-, the EPR spectrum in frozen solution is indicative of a low-spin cobalt(II) complex with a d _z2 ground state. Exposure of [(TBP)₈CzCo ^III(py)]^- to O₂ leads to the reversible formation of the cobalt(III)-superoxo complex [(TBP)₈CzCo ^III(py)(O₂)]^-, which has been characterized by EPR spectroscopy. VT-EPR measurements show that the dioxygen adduct is stable up to T ≈ 240 K. This work is the first observation, to our knowledge, of O₂ binding to a cobalt(II) corrole.

Full text of DOI:10.1021/ja036983s

Published on Web 02/06/2004
An Example of O2 Binding in a Cobalt(II) Corrole System and
High-Valent Cobalt-Cyano and Cobalt-Alkynyl Complexes
Bobby Ramdhanie,† Joshua Telser,‡ Andrea Caneschi,§ Lev N. Zakharov,|
Arnold L. Rheingold,| and David P. Goldberg*,†
Contribution from the Department of Chemistry, Johns Hopkins UniVersity, 3400 North Charles
Street, Baltimore, Maryland 21218, Chemistry Program, RooseVelt UniVersity,
Chicago, Illinois 60605, Dipartimento di Chimica e UdR INSTM di Firenze, Polo Scientifico,
Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy, and Department of Chemistry and
Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe,
La Jolla, California 92093
Received June 30, 2003; E-mail: dpg@jhu.edu
Abstract: The novel cobalt corrolazine (Cz) complexes (TBP)8CzCoCN (1) and (TBP)8CzCo(CCSiPh3) (2)
have been synthesized and examined in light of the recent intense interest regarding the role of corrole
ligands in stabilizing high oxidation states. In the case of 2, the molecular structure has been determined
by X-ray crystallography, revealing a short Co-C distance of 1.831(4) Å and an intermolecular π-stacking
interaction between Cz ring planes, and this structure has been analyzed in regards to the electronic
configuration. By a combination of spectroscopic techniques it has been shown that 1 is best described as
a cobalt(III)-π-cation-radical complex, whereas 2 is likely best represented as the resonance hybrid
(Cz)CoIV(CCSiPh3) T (Cz+•)CoIII(CCSiPh3). The reduced cobalt(II) complex, [(TBP)8CzCoII(py)]-, has been
generated in situ and shown to bind dioxygen at low temperature to give [(TBP)8CzCoIII(py)(O2)]-. For the
reduced complex [(TBP)8CzCoII(py)]-, the EPR spectrum in frozen solution is indicative of a low-spin cobalt(II)
II
complex with a dz ground state. Exposure of [(TBP)8CzCo (py)]- to O2 leads to the reversible formation of
2
the cobalt(III)-superoxo complex [(TBP)8CzCoIII(py)(O2)]-, which has been characterized by EPR
spectroscopy. VT-EPR measurements show that the dioxygen adduct is stable up to T ≈ 240 K. This work
is the first observation, to our knowledge, of O2 binding to a cobalt(II) corrole.
Introduction
many new discoveries in the synthesis, physical properties, and
reactivity of transition metal-corrole complexes. A wide variety
The synthesis of corroles has a long history, but only in the
past few years has there been an increase of interest in this class
of porphyrinoid compounds.1-4 Corroles are ring-contracted
analogues of porphyrins that retain the tetrapyrrolic, 18-π-
electron aromatic porphyrinoid nucleus. The stabilization of high
oxidation states in transition metals is one of their most
interesting features. The renewed interest in corroles has been
spurred by important developments in the improved syntheses
of meso-aryl-substituted corroles,5-14 which in turn has led to
of intriguing high-valent transition metal complexes have been
synthesized with corrole ligands, and certain metallocorroles
have demonstrated catalytic activity (epoxidation, cyclopropa-
nation).15-19 Moreover, there has been a long-standing interest
in corroles for their relevance to the chemistry of heme proteins
as well as to the structurally related corrin cofactor found in
vitamin B-12.
We have recently reported the synthesis of the first triaza-
corrole, in which the meso carbon atoms have been replaced
by nitrogen atoms, and we have given the trivial name
“corrolazine (Cz)” to this new class of porphyrinoid com-
† Johns Hopkins University.
‡ Roosevelt University.
§ Dipartimento di Chimica e UdR INSTM di Firenze, Polo Scientifico.
| University of California.
(1) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted, & Isomeric Porphyrins;
Elsevier Science Inc.: New York, 1997; Vol. 15.
(2) Paolesse, R. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 2, pp 201-
232.
(9) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem. 2001,
66, 550-556.
(10) Gryko, D. T. Chem. Commun. 2000, 2243-2244.
(11) Gryko, D. T.; Jadach, K. J. Org. Chem. 2001, 66, 4267-4275.
(12) Brin˜as, R. P.; Bru¨ckner, C. Synlett 2001, 442-444.
(13) Asokan, C. V.; Smeets, S.; Dehaen, W. Tetrahedron Lett. 2001, 42, 4483-
4485.
(3) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000;
Vol. 2, pp 233-300.
(14) Collman, J. P.; Decre´au, R. A. Tetrahedron Lett. 2003, 44, 1207-1210.
(15) Gross, Z.; Simkhovich, L.; Galili, N. Chem. Commun. 1999, 599-600.
(16) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed. 2000,
39, 4045-4047.
(4) Gryko, D. T. Eur. J. Org. Chem. 2002, 1735-1743.
(5) Paolesse, R.; Licoccia, S.; Bandoli, G.; Dolmella, A.; Boschi, T. Inorg.
Chem. 1994, 33, 1171-1176.
(6) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. 1999, 38, 1427-
1429.
(17) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Chem.-Eur. J.
2001, 7, 1041-1055.
(7) Gross, Z.; Galili, N.; Simkhovich, L.; Saltsman, I.; Botoshansky, M.; Bla¨ser,
D.; Boese, R.; Goldberg, I. Org. Lett. 1999, 1, 599-602.
(8) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi, T.;
Smith, K. M. Chem. Commun. 1999, 1307-1308.
(18) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.;
DiBilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132-2134.
(19) Mahammed, A.; Gray, H. B.; Meier-Callahan, A. E.; Gross, Z. J. Am. Chem.
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9
10.1021/ja036983s CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 2515-2525
2515

A R T I C L E S
Ramdhanie et al.
pounds.20 The synthesis of the metal-free triazacorrole (TBP)8-
CzH3 (TBP ) 4-tert-butylphenyl) is easily accomplished in a
few steps from commercially available reagents (six steps from
4-tert-butylbenzyl bromide) and is therefore of practical use as
a ligand for transition metal chemistry. A series of cobalt
complexes was recently prepared from (TBP)8CzH3.21 The
feasibility of stabilizing high-valent states with a corrolazine
ligand has been demonstrated with the isolation and spectro-
scopic characterization of a stable manganese(V)-oxo complex,
(TBP)8CzMnVtO.22,23 The analogous high-valent manganese
corrole complexes (tpfc)MnVtO16 and [(tpfc)MnVtN]-18 with
5,10,15-tris(pentafluorophenyl)corrole have been described pre-
viously, and another MnV-oxo complex has recently been
prepared with a perfluorinated corrole.24 In addition, the high-
valent MnV-imido corrole complexes (tpfc)MnVdNR (R )
2,4,6-trimethylphenyl; 2,4,6-trichlorophenyl) have recently been
synthesized25 and are the first examples of isolable MnV-
terminal imido complexes. High-valent corrole complexes of
other metals such as iron have been reported although,
particularly with Fe, there is some debate about the noninno-
cence of the corrole and the true nature of the electronic
configuration (e.g. (corrole)FeIVL vs (corrole+•)FeIIIL).26-33 In
cobalt-corrole chemistry, the only examples reported to date
of formally high-valent systems are the σ-phenyl complexes
(OEC)Co(Ph) and [(OEC)Co(Ph)]ClO4 (OEC ) octaethylcor-
role) described by Kadish and Vogel.34 Herein we describe the
synthesis of two novel, formally high-valent, cobalt triazacorrole
complexes with σ-carbon-bonded axial ligands, (TBP)8CzCo-
(CN) and (TBP)8CzCo(CCSiPh3). These complexes have been
examined for their structural and spectroscopic properties, and
their electronic configurations are discussed in light of these
data.
edented.37 We have established that [(TBP)8CzCoII(py)]- ex-
hibits reversible O2 binding to give a CoIII-superoxo adduct at
low temperature. In addition, it is demonstrated that the same
complex under conditions of excess O2 can generate the free
superoxide anion.
Experimental Section
General Remarks. The starting material octa-tert-butylphenyl
cobalt(III) corrolazine ((TBP)8CzCoIII), was prepared as reported
previously.21 Reagents and solvents were purchased from commercial
sources and were of reagent-grade quality. Tetrahydrofuran was distilled
from Na/benzophenone under a nitrogen atmosphere. Preparation and
handling of air-sensitive materials were carried out under an argon
atmosphere using standard Schlenk techniques or in a Vacuum
Atmospheres Company VAC-AV-3 inert atmosphere (<1 ppm O2)
drybox under nitrogen atmosphere. Solvents and solutions were
deoxygenated by either repeated freeze-pump-thaw cycles or by direct
bubbling of argon through the solution for 20-30 min. Dioxygen gas
(ultrapure grade, 99.994%) was passed through a column of Drierite
before being used. NMR spectra were measured on a Varian Unity
FT-NMR instrument at 400 MHz (1H) and 79 MHz (29Si). All spectra
were recorded in 5-mm o.d. NMR tubes, and chemical shifts were
reported as δ values from standard solvent peaks. UV-vis spectral
studies were carried out with a Hewlett-Packard 8453 diode array
spectrometer equipped with HPChemstation software. Mass spectrom-
etry was carried out using a Kratos SEQ MALDI-TOF mass spectrom-
eter. IR spectra were obtained on a Perkin-Elmer RX I FT-IR
Spectrometer. Electron paramagnetic resonance (EPR) spectra were
obtained on a Bruker EMX EPR spectrometer controlled with a Bruker
ER 041 X G microwave bridge. The EPR spectrometer was equipped
with a continuous-flow liquid helium cryostat and ITC503 temperature
controller made by Oxford Instruments, Inc. The field/frequency was
calibrated by measuring the g value of DPPH. Elemental analyses were
performed by Atlantic Microlab, Inc., Atlanta, GA.
Octa(tert-butylphenyl) Cobalt Corrolazine Cyanide (TBP)8CzCo-
(CN) (1): Method A. To a stirring solution of octa(tert-butylphenyl)
cobalt corrolazine (250 mg, 0.177 mmol) in 50 mL of distilled pyridine
was added an aqueous solution of sodium cyanide (4.0 g, 8.2 mmol).
The solution was allowed to stir for 30 min at room temperature. The
volatiles were removed under vacuum, and the resulting brown solid
was redissolved in methylene chloride. Purification by silica gel
chromatography (neat methylene chloride on silica) afforded a brown
solid (180 mg 71%). Rf ) 0.4. 1H NMR (400 MHz, CD2Cl2): δ (ppm)
8.60-7.00 (br, 32H), 1.60-1.40 (br, 72H). UV-vis (CH2Cl2) λmax [nm]
(ꢀ × 10-4) 445 (4.6), 680 (1.6), 738 (1.5). IR: ν(CN) region (KBr)
2198, 2135 cm-1. MALDI-TOF (negative ion mode): m/z (% inten-
sity): 1440 (100, M + 1). Anal. Calcd for C97H104N8Co‚H2O: C, 79.86;
H, 7.32; N, 7.68. Found: C, 79.60; H, 7.75; N, 7.90.
In addition to our interest in high oxidation state corrole
complexes, we are also interested in the biomimetic chemistry
of corroles and how they compare to their porphyrin counter-
parts. As part of these efforts we describe here the formation
of a reduced cobalt(II) triazacorrole and its reactivity with
dioxygen. It has been long established that cobalt(II) porphyrins
will reversibly bind dioxygen in a manner similar to heme
proteins (hemoglobin/myoglobin),35,36 but to our knowledge O2
binding to a cobalt(II) corrole complex of any type is unprec-
(20) Ramdhanie, B.; Stern, C. L.; Goldberg, D. P. J. Am. Chem. Soc. 2001,
123, 9447-9448.
(21) Ramdhanie, B.; Zakharov, L. N.; Rheingold, A. L.; Goldberg, D. P. Inorg.
Chem. 2002, 41, 4105-4107.
(22) Mandimutsira, B. S.; Ramdhanie, B.; Todd, R. C.; Wang, H. L.; Zareba,
A. A.; Czernuszewicz, R. S.; Goldberg, D. P. J. Am. Chem. Soc. 2002,
124, 15170-15171.
Method B. To a stirring solution of octa(tert-butylphenyl) cobalt
corrolazine (250 mg, 0.177 mmol) in 50 mL of pyridine was added
(NH4)2Ce(NO3)6 (1.0 g, 1.8 mmol) in EtOH (50 mL). The solution was
allowed to stir for 30 min at room temperature. A solution of sodium
cyanide (0.85 g, 17.3 mmol) in water (50 mL) was then added to the
reaction mixture. After stirring for an additional 30 min, the solvents
were removed under vacuum, and the resulting brown solid was
redissolved into 100 mL of methylene chloride. The methylene chloride
layer was washed with water (3 × 100 mL), dried over magnesium
sulfate, and concentrated under vacuum. The brown solid was then
redissolved into a minimal amount of methylene chloride and purified
by silica gel chromatography (100 mg, 40.0%).
(23) For a recent theoretical study on high-valent corrolazines, see: Tangen,
E.; Ghosh, A. J. Am. Chem. Soc. 2002, 124, 8117-8121.
(24) Liu, H. Y.; Lai, T. S.; Yeung, L. L.; Chang, C. K. Org. Lett. 2003, 5,
617-620.
(25) Eikey, R. A.; Khan, S. I.; Abu-Omar, M. M. Angew. Chem., Int. Ed. 2002,
41, 3592-3595.
(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) Van Caemelbecke, E.; Will, S.; Autret, M.; Adamian, V. A.; Lex, J.;
Gisselbrecht, J.-P.; Gross, M.; Vogel, E.; Kadish, K. M. Inorg. Chem. 1996,
35, 184-192.
(28) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z. Inorg. Chem.
2000, 39, 2704-2705.
(29) Cai, S.; Walker, F. A.; Licoccia, S. Inorg. Chem. 2000, 39, 3466-3478.
(30) Gross, Z. J. Biol. Inorg. Chem. 2001, 6, 733-738.
(31) Ghosh, A.; Steene, E. J. Inorg. Biochem. 2002, 91, 423-436.
(32) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41, 5433-
5439.
(33) Zakharieva, O.; Schunemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker,
F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636-6648.
(34) Will, S.; Lex, J.; Vogel, E.; Adamian, V. A.; Van Caemelbecke, E.; Kadish,
K. M. Inorg. Chem. 1996, 35, 5577-5583.
(35) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. ReV. 1979, 79, 139-
179.
(36) Smith, T. D.; Pilbrow, J. R. Coord. Chem. ReV. 1981, 39, 295-383.
(37) A cofacial bis(cobalt) porphyrin-corrole dimer has been reported to bind
dioxygen with the cobalt corrole in the +3 oxidation state: Guilard, R.;
Je´roˆme, F.; Gros, C. P.; Barbe, J.-M.; Ou, Z. P.; Shao, J. G.; Kadish, K.
M. C. R. Acad. Sci. 2001, 4, 245-254.
9
2516 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

Synthesis of Cobalt Corrolazine Complexes
A R T I C L E S
Octa(tert-butylphenyl) Cobalt Corrolazine Triphenylsilylacetylide
(TBP)8CzCo(CCSiPh3) (2). To a stirring solution of (TBP)8CzCoIII
(100 mg, 0.071 mmol) in 50 mL of distilled THF was added
triphenylsilylacetylene (201 mg, 0.71 mmol). Triethylamine was added
(715 mg, 0.71 mmol, 1.0 mL), and the solution was allowed to stir at
room temperature for 16 h. The THF and triethylamine were removed
under vacuum. The remaining brown residue was then redissolved in
a minimal amount of methylene chloride. Purification by silica gel
chromatography (neat methylene chloride) afforded a brown solid (70
mg, 58%) Rf ) 0.60 (methylene chloride on silica). Free-flowing
powdered samples for elemental analysis and SQUID measurements
were obtained by precipitation of 2 from toluene/MeOH. 1H NMR (400
MHz, CD2Cl2): δ (ppm) 26.4 (br), 21.5 (br), 17.7 (br), 12.5 (br), 10.1
(br), 9.5 (br), 8.6-8.3 (m), 8.1-7.4 (m), 7.2-6.7 (m), 6.8 (m), 6.2
(m), 4.9 (br), 3.6 (br), 2.3 (br), 1.5 (br), -14.0 (br), -20.5 (br). 29Si
NMR (79 MHz, CD2Cl2): δ (ppm) -28.9. UV-vis (CH2Cl2) λmax [nm]
(ꢀ × 10-4) 450 (4.0), 619 (1.3), 673 (1.6), 778 (0.8). IR: ν(CtC) region
(KBr) 2062 cm-1. LDI-TOF (negative ion mode): m/z (% intensity):
1698 (60, M + 1), 1415 (100, M-CCSi(Ph)3). Anal. Calcd for
system with S ) 1/2 coupled to a single nucleus, which is 59Co (100%,
I ) 7/2) in the simulations described here. Additional hyperfine-coupled
nuclei are treated using a perturbation theory model. The orientation
of the hyperfine coupling matrix can be rotated with respect to the
electronic coordinate system (g matrix) by Euler angles.41 For the
simulations involving [(TBP)8CzCoII(py)]-, the g and A(59Co) matrixes
were assumed to be collinear, as is standard absent detailed single-
crystal EPR studies. However, for the dioxygen-bound complex
[(TBP)8CzCoIII(py)(O2)]-, such an assumption is not valid because the
unpaired spin is predominantly located on the resulting superoxo ligand,
and thus the Co complex is effectively a “superhyperfine ligand” to
the dioxygenic paramagnet, rather than the normal situation in which
a diamagnetic ligand is “superhyperfine” coupled to a paramagnetic
metal ion. Single-crystal EPR studies are needed to determine the exact
relation between the electronic (g matrix) and nuclear (A(59Co) matrix)
coordinate systems. Such a study has been carried out on the related
oxygen adduct of vitamin B12 (B12rO2) by Jo¨rin et al.,42 and thus we
used this study as a starting point to obtain a reasonable simulation of
the observed frozen solution EPR spectrum for [(TBP)8CzCoIII(py)(O2)]-,
despite the large parameter space: three 59Co hyperfine matrix
components and three Euler angles (even assuming exact knowledge
of the g matrix). Following Jo¨rin et al., we assume that the largest g
value for [(TBP)8CzCoIII(py)(O2)]- (g3 ) 2.075; 2.079 for B12rO2)
corresponds to the O-O vector and that the largest A value (A3 ) 60
MHz; 63 MHz for B12rO2) lies in the Cz plane, roughly bisecting the
N-Co-N angle. The remaining g matrix components also correspond
quite closely to those of B12rO2: g1 ) 1.996; 1.994 for B12rO2and g2 )
2.011; 2.012 for B12rO2. The remaining A matrix components are in
the range 20-40 MHz and thus are also similar to those determined
exactly for B12rO2: A1 ) 21 MHz, A2 ) 27 MHz.
To reproduce the observed EPR spectrum, it is necessary both to
“twist” the O-O π-system (defined by g1 and g2) with respect to the
Cz plane and to “tilt” the O-O σ-bond (defined by g3 ) gz) away
from the Cz plane. This is accomplished by rotation about the Euler
angles R ) 15° and â ) 25°, respectively. The uncertainty in these
rotation angles is roughly 5°. An additional rotation (by Euler angle γ)
would define the projection of the O-O vector in the Cz plane, but
our frozen solution data do not warrant this refinement.
SQUID Measurements. The magnetic susceptibility of a powder
sample of compounds 1 and 2 were measured between 4 and 300 K
with applied magnetic fields of 0.1, 1, and 2 T using a Cryogenic Ltd.
S600 SQUID magnetometer. The samples were wrapped in Teflon tape.
The data were corrected for the magnetism of the sample holder, which
was independently determined at the same temperature range and field.
The underlying diamagnetism of the samples was estimated from
Pascal’s constants.43 Magnetization measurements were performed on
the same sample at 2.5 K with an applied field up to 6 T.
C116H119N7Si1Co‚H2O‚2CH3OH: C, 79.61; H, 7.31; N, 5.51. Found:
C, 79.21; H, 7.31; N, 6.11.
Formation of [(TBP)8CzCoII(py)]-. UV-Vis Titration. For the
reduction of (TBP)8CzCoIII(py)2 to [(TBP)8CzCoIII(py)]-, it is important
to keep the solution scrupulously air-free, and thus a 250-mL Schlenk
flask was specially modified to contain a 1-cm path-length quartz cell
(total volume ≈ 3 mL) fused to a sidearm assembly which could then
be filled by simply tilting the flask and allowing the solution to drain
into the sidearm attachment. This setup allowed for the convenient
injection of aliquots of reducing agent followed by subsequent stirring
of the reaction mixture while minimizing the possibility of exposure
to air. The specially modified Schlenk flask was loaded with a stir bar
and charged with 10 mL of a green-brown solution of (TBP)8CzCoIII-
(py)2 (9.56 × 10-5 M) in pyridine/EtOH, 1/1 (v/v)). The solution was
degassed by sparging with argon, and then 100-µL aliquots of a 0.145
M solution of NaBH4 in ethanol were added in succession, producing
a gradual color change from a dark brown-green to a blue-green. After
each addition, the reaction mixture was allowed to stir for 5 min to
reach equilibrium, and then a UV-vis measurement was taken. The
NaBH4 solution was added until no further change was observed in
the UV-vis spectrum (total NaBH4 added: 1.1 mL, 0.16 mmol, 166
equiv).
Formation of [(TBP)8CzCoII(py)]- (3) and [(TBP)8CzCoIII(py)(O2)]-
for EPR Spectroscopy. To 100 µL of a solution of (TBP)8CzCoIII-
(py)2 (8.4 × 10-3 M) in pyridine/EtOH (1/1, v/v) was added 100 µL
of a solution of NaBH4 (0.145 M) in ethanol in an inert-atmosphere
drybox. The solution immediately changed from the brown-green color
of (TBP)8CzCoIII(py)2 to the blue-green color of [(TBP)8CzCoII(py)]-.
The solution was transferred to a quartz EPR tube (4 mm i.d.), sealed
with septum/Parafilm, and removed from the glovebox for EPR
measurements or further reaction with dioxygen. To generate the
cobalt-superoxo complex [(TBP)8CzCoIII(py)(O2)]-, an EPR tube
containing a freshly prepared solution of [(TBP)8CzCoII(py)]- was
cooled in dry ice (-78 °C) and then bubbled with dry O2 gas for ∼20
s. There was an immediate color change from blue-green to dark brown,
indicating the formation of [(TBP)8CzCoIII(py)(O2)]-. The oxygenated
solution was then quick-frozen in an N2(l) bath (-196 °C) and stored
at that temperature until it was transferred to the EPR instrument. The
spectrum of [(TBP)8CzCoII(py)]- can be regenerated by thawing the
solution to room temperature and bubbling with Ar(g) for ∼10 s. The
oxygenation/deoxygenation steps can be cycled ∼3 times, after which
the EPR signal for [(TBP)8CzCoII(py)]- cannot be regenerated unless
more sodium borohydride is added to the solution.
X-ray Crystallography. Crystal data for 2‚(C7H8) at 150(2) K:
123H127CoN7Si, Mr ) 1790.34, triclinic, P1h, a ) 15.7889(11) Å, b )
C
18.3915(12) Å, c ) 19.4754(13) Å, R ) 71.026(1)°, â ) 73.109(1)°,
γ ) 89.800(1)°, V ) 5091.4(6) Å3, Z ) 2, Fcalcd ) 1.168 g cm-3, F(000)
) 1910, µ(Mo KR) ) 0.234 mm-1. X-ray diffraction intensities were
collected on a Bruker SMART APEX CCD diffractometer (T ) 150(2)
K, Mo KR radiation (λ ) 0.71073 Å), crystal dimensions: 0.30 ×
0.25 × 0.10 mm-3, 2θmax ) 56.5°, total number of reflections 31903,
independent reflections 22217 [Rint ) 0.0236]). SADABS44 absorption
correction was applied (Tmin/Tmax ) 0.858). Structures were solved using
direct methods and completed by subsequent difference Fourier
(38) Nilges, M. J. Ph.D. Thesis, University of Illinois, Urbana, IL, 1979.
(39) Belford, R. L.; Nilges, M. J. In EPR Symposium, 21st Rocky Mountain
Conference; Denver, CO, 1979.
(40) Maurice, A. M. Ph.D. Thesis, University of Illinois, Urbana, IL, 1980.
(41) McGavin, D. G. J. Magn. Reson. 1987, 74, 19-55.
(42) Jo¨rin, E.; Schweiger, A.; Gu¨nthard, H. H. Chem. Phys. Lett. 1979, 61, 228-
232.
(43) Carlin, R. L. Magnetochemistry; Springer-Verlag: New York, 1986.
(44) Sheldrick, G. M. SADABS (2.01), Bruker/Siemens Area Detector Absorption
Correction Program, Bruker AXS, Madison, Wisconsin, 1998.
Simulations of EPR Spectra. EPR spectra were simulated using
the program QPOWA, originally written by R. L. Belford and
co-workers38-40 and subsequently modified by J. Telser. The program
uses a standard spin Hamiltonian with matrix diagonalization of a
9
J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2517

A R T I C L E S
Ramdhanie et al.
Scheme 1
Scheme 2
syntheses and refined by full matrix least-squares procedures on F2.
Besides the main molecule 2 in the crystal structure there is a solvate
toluene molecule. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. The H atoms were placed at calculated
positions and refined in a riding group model. Number of refinement
parameters is 1187. The final R factors [I > 2σ(I)]: R1 ) 0.1108,
wR2 ) 0.2650, GoF ) 1.126. All software and sources of the scattering
factors are contained in the SHELXTL (5.10) program package (G.
Sheldrick, Bruker XRD, Madison, WI).
When ceric ion is used in place of air as the oxidant, a second
product is formed which has been identified as the bis-cyano
complex, (TBP)8CzCo(CN)2. This complex was initially identi-
fied during chromatography as a brown band (silica gel, eluent
CH2Cl2), which eluted after the mono-cyano product. The bis-
cyano complex has been identified by MALDI-MS (M+1, 1467)
and IR spectroscopy. The yield for this compound was quite
low and difficult to reproduce, and therefore its synthesis was
not further pursued, although we were able to confirm the basic
connectivity of this complex by X-ray crystallography.45
Interestingly, there is no evidence for the formation of the bis-
cyano complex in the absence of ceric ion. These observations
are consistent with the formulation of a higher oxidation level
for the bis-cyano species as compared to the mono-cyano
complex. The overall oxidation level for 1 is the same as that
for the σ-phenyl-corrole complex (OEC)Co(C6H5) reported by
Vogel and Kadish,34 which could also be oxidized further by
Results and Discussion
Synthesis of (TBP)8CzCo(CN). The synthesis of (TBP)8CzCo-
(CN) was established according to Scheme 1. Addition of excess
sodium cyanide dissolved in water to a pyridine solution of
(TBP)8CzCoIII in the presence of air leads to an immediate color
change (within 1 min) from green to brown. The initial green
color is typical for (TBP)8CzCoIII(py)2, which is formed in situ
at the beginning of the reaction. In the absence of air no color
change is noted, even after stirring for 16 h at room temperature.
If the anaerobic reaction mixture is then exposed to air, the
expected green-to-brown color change takes place. These results
point to dioxygen as the necessary oxidant for the formation of
the mono-cyano complex and are consistent with 1 being one
oxidation level above (TBP)8CzCoIII(py)2. The electronic
configuration of 1 can be formulated by two extremes:
(TBP)8CzCoIV(CN), in which the cobalt is in the +4 oxidation
state, or (TBP)8Cz+•CoIII(CN), in which the “hole” resides on
the macrocycle giving a π-cation-radical complex, and it is this
latter form that is strongly suggested by its EPR spectrum (vide
infra). The crude product was easily purified by standard
chromatographic methods (silica gel, eluent methylene chloride)
to give 1 in good yield (71%). This complex is stable in both
the solid state and in solution at room temperature in the air
for several weeks.
-
treatment with FeIII(ClO4)3 to give [(OEC)Co(C6H5)]+ClO4
(OEC ) â-octaethylcorrolate). The latter complex was described
as a doubly oxidized corrole with a central Co(III) ion.34,46 Given
this precedent, we suggest that the bis-cyano complex is likely
doubly oxidized on the ring.
FT-IR Spectroscopy of the Cyano Complex. The FT-IR
spectrum of a sample of 1 prepared as a KBr pellet or in solution
(CH2Cl2) shows two peaks at 2135 and 2198 cm-1, which are
clearly different from free CtN- (2080 cm-1 in aqueous
solution) and can be assigned to two different CtN stretches.
It is well-established that the CtN stretch in general increases
upon going from a terminal (M-CtN) to a bridging (M-Ct
N-M) coordination mode in metal-cyanide compounds.47 The
(45) The X-ray structure of this complex was of low quality because the crystal
was weakly diffracting, but the Co(CN)2 structure was confirmed. The
quality of the structure, however, was not good enough to provide definitive
information regarding the oxidation state.
The mono-cyano complex can also be prepared by treating
the (TBP)8CzCoIII starting material with the oxidizing agent
(NH4)2CeIV(NO3)6 followed by reaction with NaCN in water.
(46) Harmer, J.; Van Doorslaer, S.; Gromov, I.; Bro¨ring, M.; Jeschke, G.;
Schweiger, A. J. Phys. Chem. B 2002, 106, 2801-2811.
(47) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 5th ed.; John Wiley & Sons: New York, 1997.
9
2518 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

Synthesis of Cobalt Corrolazine Complexes
A R T I C L E S
no observable hyperfine coupling from the cobalt ion and no
significant g anisotropy at low temperature. However, the line
width at low temperature is slightly larger than a typical
porphyrin51,52 or phthalocyanine53,54 π-cation-radical and may
indicate a small amount of delocalization on to the cobalt center.
The magnetic susceptibility of 1 was measured between 4 and
300 K by a SQUID magnetometer and revealed diamagnetic
behavior throughout the temperature range. This behavior is
consistent with the presence of strong antiferromagnetic (AF)
coupling between molecules. Although the structure of this
material has not been characterized by X-ray crystallography,
the solid-state IR data discussed above show the presence of
cyanide bridges, which are expected to provide a pathway for
strong antiferromagnetic coupling. In addition, porphyrin π-cation-
radicals are well known to aggregate in both the solution- and
solid state through π-stacking interactions and as a consequence
to exhibit direct spin-pairing via π-π overlap.55 This kind of
direct spin-pairing mechanism may also be responsible for the
quenching of the magnetic moment.
Figure 1. ORTEP diagram of (TBP)8CzCo(CCSiPh3) (2). Thermal el-
lipsoids are drawn at a 50% probability level, and hydrogen atoms have
been omitted for clarity.
peak at 2135 cm-1 is similar to the ν(CtN) stretch (2130 cm-1
)
Synthesis of (TBP)8CzCo(CCSiPh3). We were unable to
grow X-ray quality crystals of the mono-cyano complex. In our
previous experience we have had much better success in growing
X-ray quality crystals of (TBP)8CzMLn complexes that contain
large axial ligands, and thus we decided to prepare the
triphenylsilylacetylide complex 2, with the knowledge that a
terminal alkynyl ligand is isoelectronic with CN-. The synthesis
of 2 is shown in Scheme 2. The cobalt complex (TBP)8CzCoIII
was combined with 10 equiv of triphenylsilylacetylene and
triethylamine in THF under aerobic conditions. The reaction
mixture was stirred for 16 h at room temperature to give crude
(TBP)8CzCo(CCSiPh3), which was purified by chromatography
(silica gel, methylene chloride) to afford the cobalt acetylide
complex in good yield (58%). This complex was then crystal-
lized from toluene/MeOH to give crystals of X-ray quality. If
air is excluded from the reaction mixture, no product is formed
after stirring for 16 h at room temperature. The presence of air
is essential for the reaction to proceed, indicating O2 is the active
oxidant, as found for the mono-cyano complex.
for the monomeric phthalocyanine (Pc) compound [PcCoIII-
(CN)2]-,48 which contains terminal cyanide ligands, whereas
the peak at 2198 cm-1 corresponds to a bridging M-CtN-M
stretch; compare ν(CtN) ) 2158 cm-1 for polymeric [PcCoIII-
(CN)]n, in which the cyanide ligand is bridging between cobalt
centers.48 The higher frequency stretch at 2198 cm-1 for 1 is
also quite close to that reported for [(OEP)Co(CN)]n (OEP )
octaethylporphyrin), which has ν(CtN) ) 2186 cm-1 and is
described as having a bridged M-CtN-M polymeric struc-
ture.49 Thus (TBP)8CzCo(CN) likely exists in both monomeric
and cyanide-bridged forms. Interestingly, the IR spectrum for
the bis-cyano complex (TBP)8CzCo(CN)2 in KBr exhibits only
one peak at 2189 cm-1. This stretching frequency may indicate
bridging cyanides, although the 2:1 (CN:Co) formula would be
more consistent with terminal cyanide ligands. Other factors,
such as oxidation state and electronegativity of the metal center,
can also affect the stretching frequency. In general, an increase
in oxidation state or electronegativity causes stronger σ-donation
from CN-, which results in a higher frequency because electrons
are being removed from an antibonding (with respect to the
C-N bond) orbital.47 Thus, the high CtN stretch for the bis-
cyano complex may be reflective of the higher oxidation state
for Co in this complex, rather than a bridging mode for CN-.50
1H NMR, EPR, and Magnetic Susceptibility of (TBP)8-
CzCo(CN). The 1H NMR spectrum of 1 shows broad resonances
in the aromatic region corresponding to the peripheral phenyl
substituents, and a broad singlet between 1.4 and 1.20 ppm
arising from the tert-butyl groups. The broadness of the peaks
is likely due to the presence of the paramagnetic, monomeric
form. Further information concerning the electronic configura-
tion was obtained by EPR spectroscopy.
Structure and Bonding in the Acetylide Complex. An
ORTEP diagram of (TBP)8CzCo(CCSiPh3) is shown in Figure
1, and selected bond distances and angles are presented in Table
1 together with those for (TBP)8CzCoIII(PPh3) and (TBP)8-
CzCoIII(py)2 for comparison. The macrocycle is relatively planar,
with the largest deviation from the average plane of the 23 core
atoms being 0.187(3) Å and the mean deviation being 0.107 Å.
The cobalt ion sits almost directly in the plane (d(Co-plane) )
-0.076(1) Å) and is 0.092(2) Å above the plane formed by the
four pyrrole nitrogen atoms, in contrast to the other five-
coordinate complex (TBP)8CzCoIII(PPh3), which shows the
cobalt ion sitting significantly out of the plane toward the axial
ligand. Interestingly, there is an umbrella shape to 2, with the
acetylide forming the “handle” and the macrocycle domed
slightly toward the -CCSiPh3 unit.
The EPR spectrum at 298 K for the mono-cyano complex
dissolved in toluene reveals a singlet centered at g ) 2.0021
with a peak-to-trough separation of 12 G. A similar spectrum
is obtained at low temperature in frozen solution (25 K), with
g ) 2.0038 and 26 G peak-to-trough separation. This signal is
indicative of a π-cation-radical-type compound, since there is
(51) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem.
Soc. 1970, 92, 3451-3459.
(52) Kalsbeck, W. A.; Seth, J.; Bocian, D. F. Inorg. Chem. 1996, 35, 7935-
7937.
(53) Myers, J. F.; Canham, G. W. R.; Lever, A. B. P. Inorg. Chem. 1975, 14,
461-468.
(54) Ough, E.; Gasyna, Z.; Stillman, M. J. Inorg. Chem. 1991, 30, 2301-2310.
(55) Fuhrhop, J. H.; Wasser, P.; Riesner, D.; Mauzerall, D. J. Am. Chem. Soc.
1972, 94, 7996.
(48) Hanack, M. Mol. Cryst. Liq. Cryst. 1984, 105, 133-149.
(49) Fahmy, N.; Leverenz, A.; Hanack, M. Synth. Met. 1991, 41-43, 2615-
2619.
(50) Geiss, A.; Vahrenkamp, H. Inorg. Chem. 2000, 39, 4029-4036.
9
J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2519

A R T I C L E S
Ramdhanie et al.
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for (TBP)8CzCo(CCSiPh3), (TBP)8CzCoIII(PPh3), and (TBP)8CzCoIII(py)2
III
III
(TBP)8CzCo(CCSiPh3)
(TBP)8CzCo (PPh3)a
(TBP)8CzCo (py)2a
Co-N(1)
1.834(3)
1.827(3)
1.815(3)
1.812(3)
1.831(4)
0.092(2)
-0.076(1)
1.373(12)
1.456(12)
1.502(6)
3.640(5)
3.691(5)
1.194(7)
93.84(15)
91.49(14)
82.97(14)
91.13(14)
94.51(18)
94.70(17)
91.06(17)
91.56(17)
1.838(3)
1.852(3)
1.826(3)
1.834(3)
2.1747(12)
0.288
1.838(3)
1.850(4)
1.830(3)
1.834(3)
2.000(4), 1.970(4)
0.006
Co-N(2)
Co-N(3)
Co-N(4)
Co-Lb
Co-(N(1)-N(4))plane
Co-(23-atom)core
Câ-Câ (av)
0.442
0.084
1.381(11)
1.446(16)
1.492(6)
3.638
1.390(12)
1.448(17)
1.459(6)
3.678
CR-Câ (av)
CR-CR
N(1)‚‚‚N(3)
N(2)‚‚‚N(4)
C(97)tC(98)
N(1)-Co-N(2)
N(2)-Co-N(3)
N(3)-Co-N(4)
N(4)-Co-N(1)
N(1)-Co-La
N(2)-Co-La
N(3)-Co-La
N(4)-Co-La
3.612
3.663
92.29(13)
89.44(14)
82.10(14)
90.53(14)
105.49(10)
99.31(10)
92.89(10)
97.56(10)
94.51(15)
91.39(15)
83.06(13)
91.06(15)
89.83(16), 86.60(16)
89.34(16), 91.05(16)
91.58(16), 91.95(16)
90.07(15), 89.88(16)
a See ref 21. b L ) axial ligand.
The Co-N(av) distance of 1.82(1) Å for 2 is slightly smaller
than the Co-N(av) distance of 1.838(9) Å found for both
(TBP)8CzCoIII(PPh3) and (TBP)8CzCoIII(py)2. This may be a
reflection of the higher oxidation level of the cobalt center in
the acetylide complex. Interestingly, the Co-C bond length in
2, 1.831(4) Å, is among the shortest known for any M-CtCR
complex.56,57 A search of the Cambridge Structural Database58
shows that there are no other Co-CtCR porphyrinoid com-
plexes for direct comparison, and thus one of the closest
analogues is the Co(III) complex Co(DO)(DOH)pn(CtCSiMe3)-
(I) ((DO)(DOH)pn ) 1,3-bis(diacetylmonoximeimino)propane),
which has a planar array of N4 donor atoms from an oxime-
based ligand and a trimethylsilylacetylide axial ligand.59 The
Co-C distance of 1.948(5) Å for this complex is significantly
longer than that found for 2, although it should be pointed out
that in the former complex there is an iodide ligand coordinated
trans to the acetylide group. The only other known corrole
Co-C bond distances come from (OEC)Co(Ph) and [(OEC)Co-
(Ph)]+ClO4-, which have Co-C distances of 1.937(3) and
1.970(7) Å, respectively.34 The short Co-C distance in 2 can
be attributed to either π-back-bonding or to a strong σ-type
interaction resulting from a high oxidation state cobalt ion.57 If
M-CCR π-interactions were significant, a lengthening of the
-CtCR bond would be expected; however, this distance in 2
(1.194(7) Å) is close to the mean value (1.201(16) Å) of known
MCtCR bond lengths.57,60 In addition, the stretching frequency
ν(CtC) would be expected to weaken in the presence of
M-CCR π-back-bonding. By comparing ν(CtC) for 2 (2062
Figure 2. Fragment of the crystal structure of 2‚(C7H8) showing the
π-stacking interaction. Solvent molecule and all hydrogen atoms and
peripheral 4-tert-butylphenyl substituents in 2 have been omitted for clarity.
the M-C bond.56 Taken together, the structural and vibrational
data are in favor of a strong M-C σ-interaction due to a high
oxidation state cobalt ion.
As shown in the packing diagram of Figure 2, the acetylide
complex forms a π-stacked dimer pair in the solid state. The
π-stacking interaction is characterized by the geometric param-
eters given in Table 2, which have been developed previously
for the classification of porphyrin π-π dimers.62 In general,
the π-interactions are sensitive to the level of ring oxidation,
and the mean-plane-separation (mps), lateral shift (l.s.) and slip
angle (θ) follow the trend: π-cation-radical < neutral porphyrin;
an oxidized ring system typically has a tighter π-π interaction
and much greater cofacial overlap. For the few corrole examples
cm-1) with that of triphenylsilylacetylene (2036 cm-1 61
or
)
Co(DO)(DOH)pn(CtCSiMe3)(I) (2040 cm-1), it is clear that
there is a small strengthening in ν(CtC) for 2. A slight increase
in ν(CtC) for 2 correlates with a strong σ-type interaction for
(56) Nast, R. Coord. Chem. ReV. 1982, 47, 89-124.
(57) Manna, J.; John, K. D.; Hopkins, M. D. AdV. Organomet. Chem. 1995, 38,
79-154.
(58) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380-388.
(59) Giese, B.; Zehnder, M.; Neuburger, M.; Trach, F. J. Organomet. Chem.
1991, 412, 415-423.
(60) Vaid, T. P.; Veige, A. S.; Lobkovsky, E. B.; Glassey, W. V.; Wolczanski,
P. T.; Liable-Sands, L. M.; Rheingold, A. L.; Cundari, T. R. J. Am. Chem.
Soc. 1998, 120, 10067-10079.
(62) Scheidt, W. R.; Brancato-Buentello, K. E.; Song, H.; Reddy, K. V.; Cheng,
(61) Naka, A.; Ishikawa, M. J. Organomet. Chem. 2000, 611, 248-255.
B. Inorg. Chem. 1996, 35, 7500-7507.
9
2520 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

Synthesis of Cobalt Corrolazine Complexes
A R T I C L E S
Table 2. Comparison of Geometric Parameters for π-π Dimers in Porphyrins, Corroles, and 2a
2
Porph+• 62,b
Porph62,b
(OEC)Co(Ph)34
[(OEC)Co(Ph)]+ 34
(OEC)Fe(NO)]+ 63
M-M
4.351
4.457
3.47
2.712
37.5
-
-
5.33
-
3.50
3.67f
46.0f
4.13
-
3.45
1.66f
25.39f
3.96
-
Ct-Ctc
mpsd
3.26(6)
3.25(6)
0.1(6)
4.94(24)
3.55(14)
3.48(47)
43.8(3.6)
3.21
-
l.s.e
slip angle (θ)
1.74(1.11)
-
a Distances in Å, angles in deg. b Porph ) neutral porphyrin. c Ct ) centroid of the porphyrinoid core. d mps ) mean-plane-separation. e l.s. ) lateral
shift. f Calculated from data obtained from the Cambridge Structural Database (see ref 58).
Table 3. EPR Parameters for [(TBP)8CzCoII(py)]- and CoII-Corrin, - Porphyrin, -Phthalocyanine, and -Corrole Complexesa
g
x
g
y
g
z
A Co,b
A Co,b
A Co,b
A N,b
ref
x
y
z
z
[(TBP)8CzCoII(py)]-
B12r
2.40
2.310
2.44
2.20
2.190
2.16
1.98
2.004
2.001
40
65 ( 12
114e
40
340
60c
this work
80
75
70
74
84
85
87
87
d
80 ( 12
302 ( 10
53 ( 2
(TCTDH)Co(py)
56e
277e
42e
(OEP)CoII(py)
g ) 2.26; g| ) 2.025
g ) 2.32; g| ) 2.005
A
A
e 25; A| ) 231
A| ) 44
A| ) 45e
-
(Pc)CoII(4-Mepy)
[(DHC)CoII]-
) 42e; A| ) 274e
2.776
3.349
3.236
3.242
3.223
3.225
2.321
2.187
2.263
2.214
2.206
2.147
1.966
1.903
1.847
1.848
1.848
1.854
392e
286e
132e
-
[(OMTPC)CoII]-
[(MEC)CoII]-,g
[(PMC)CoII]-,g
[(MEC)CoII(py)]-,g
[(PMC)CoII(py)]-,g
507e,f
507e,f
507e,f
504e,f
482e,f
213e,f
291e,f
302e,f
316e,f
274e,f
-
-
-
-
-
-
48e,h
49e,h
87
87
-
a Abbreviations: B12r ) five-coordinate cob(II)alamin; TCTDH ) 5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecahexaene; DHC ) 8,12-
diethyl-2,3,7,13,17,18-hexamethyl corrole; OMTPC ) 5,10,15-triphenyl-2,3,7,8,12,13,17,18-octamethylcorrole; MEC ) 2,3,17,18-tetramethyl-7,8,12,13-
tetraethylcorrole; PMC ) 8,12-bis[2-(ethoxycarbonyl)ethyl]-2,3,7,13,17,18-hexamethylcorrole. b A values in MHz. c AxN ) AyN ) 10 MHz. d Combined X-band/
Q-band data on a single-crystal of B12r doped in B12b. B12b ) hydroxocob(III)alamin. e Converted from cm-1 units using 1 × 10-4 cm-1 ) 2.8 MHz. f A
Co
N
values given as A1Co, A2Co, and A3
. .
g g values given as g1, g2, and g3. h Listed as A2
available, a similar trend seems to hold: the π-stacking
interaction in (OEC)Co(Ph) is weaker than that for (OEC)Co-
(Ph)+ as shown by the slightly smaller M-M distance and
considerably smaller lateral shift for the latter complex. A similar
tight stacking interaction is seen for the Fe(III) π-cation-radical
complex [(OEC)Fe(NO)]+.63 From this standpoint it is clear that
the structural parameters for 2 are more aligned with the
properties of a neutral ring system than that of an oxidized
π-cation-radical.
Magnetic Susceptibility and EPR Spectroscopy for
(TBP)8CzCo(CCSiPh3). The magnetic susceptibility of a
powder sample of 2 was measured in the range 4-300 K by a
SQUID magnetometer. As found for the cyanide complex, the
data reveal diamagnetic behavior for 2, which can be attributed
to spin-spin coupling mediated by π-stacking interactions.
Definitive structural evidence for the presence of such π-stacking
was lacking for the cyanide complex, but the X-ray structure
of 2 provides concrete proof that close π-π overlap is favored
for this complex (Figure 2). Interestingly, the closely related
corrole complex (OEC)Co(Ph) exhibits a similarly strong
intermolecular spin-spin interaction which is mediated by a
π-stacking interaction (Table 2) and gives rise to a singlet
ground state.34
The EPR spectrum of 2 in toluene at 56 K is shown in Figure
3. This spectrum is dramatically different from the simple
π-cation-radical-type spectrum seen for the isoelectronic mono-
cyano complex. The spectrum for 2 has a full width of ∼150
G, and the central line is located at g ) 2.0. There appears to
be hyperfine splitting on both the low-field and high-field sides
of the spectrum, with the features to the high-field side more
clearly resolved. At first glance these features appear to be
hyperfine splitting from the cobalt nucleus. Initial attempts to
Figure 3. EPR spectrum of (TBP)8CzCo(CCSiPh3) in toluene at 56 K.
Conditions: frequency, 9.478 GHz; incident microwave power, 4.72 mW;
modulation frequency, 100 kHz; modulation amplitude 1.0 G; receiver gain,
8.93 × 104.
simulate this spectrum as a cobalt(IV) complex (i.e., (TBP)8-
CzCoIV(CCSiPh3)) by using a combination of g anisotropy and
59Co hyperfine coupling did not give satisfactory results. We
then attempted to simulate the spectrum by assuming the
electronic configuration was best described as a π-cation-radical,
(Cz+•)CoIII(CCSiPh)3, with the observed fine structure due to
splitting from the four equivalent 14N nuclei of the pyrrole rings;
such splitting from equivalent N-pyrrole atoms has been seen
for certain porphyrin π-cation-radicals (e.g. Zn(TPP+•).51,64
These simulations also did not fully reproduce the experimental
spectrum, nor did simulations in which different combinations
of 14N and 59Co hyperfine interactions were included.65
From the EPR data and structural analysis of 2 we conclude
that this complex is clearly not well represented as a pure
(63) Autret, M.; Will, S.; Van Caemelbecke, E.; Lex, J.; Gisselbrecht, J.-P.;
Gross, M.; Vogel, E.; Kadish, K. M. J. Am. Chem. Soc. 1994, 116, 9141-
9149.
(64) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academy
Press: New York, 1979; Vol. IV, p 197.
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J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2521

A R T I C L E S
Ramdhanie et al.
of pyridine/EtOH (50/50 v/v) resulted in the generation of the
reduced complex [(TBP)8CzCoII(py)]-, as shown in Scheme 3.
This reaction was monitored by UV-vis spectroscopy, and the
data are shown in Figure 5. The initial spectrum prior to the
addition of any NaBH4 corresponds to that previously reported
for (TBP)8CzCoIII(py)2, with λmax 445, 670 nm.21 As successive
amounts of NaBH4 are added, there is a dramatic change in the
UV-vis spectrum. The Q-band at 670 nm rapidly decreases,
and two new peaks at 615 and 769 nm appear. In addition, the
Soret band at 445 nm increases slightly along with a shoulder
at 472 nm. There is tight isosbestic behavior throughout the
reduction, and no evidence for other intermediates or products
other than the reduced [(TBP)8CzCoII(py)]-. A large excess of
NaBH4 (166 equiv) is needed to drive the reaction to completion.
Further additions of NaBH4 had no effect on the spectrum. The
final spectrum is stable for many hours, as long as air is strictly
excluded. If the solution is exposed to air, the spectrum
immediately reverts back to the original spectrum of the cobalt-
(III) bis-pyridine complex. The data clearly demonstrate that
the UV-vis spectrum is quite sensitive to a change in cobalt
oxidation state, and show that the cobalt(III) complex is cleanly
and quantitatively reduced by NaBH4 to the cobalt(II) product.
[(TBP)8CzCoII(py)]-. EPR Spectroscopy. The reduced
complex [(TBP)8CzCoII(py)]- was characterized by EPR spec-
troscopy, as shown in Figure 6. There is a wealth of information
available on the characteristic EPR spectra of cobalt(II)
porphyrins,35,36,66-71 phthalocyanines,72-74 and other tetradentate
ligands (e.g., Schiff’s base systems),35,75,76 and the spectrum in
Figure 6 is similar to the spectra in many of these earlier studies.
In general, the EPR spectrum of 3 is typical of a five-coordinate,
low-spin d7 Co(II) complex, in which the unpaired electron lies
Figure 4. UV-vis spectra of 1.38 × 10-4 M (TBP)8CzCoIII (blue), 2.11
× 10-5 M, (TBP)8CzCoCN (red), and 2.29 × 10-5 M (TBP)8CzCo-
(CCSiPh3) (black) in CH2Cl2.
cobalt(IV) complex, nor is it fully described by a pure π-cation-
radical configuration. A similar conclusion was reached by
Kadish, Vogel, and co-workers for the analogous corrole
complex (OEC)Co(Ph), for which EPR and other spectroscopic
measurements point toward the best description of the electronic
configuration as the resonance hybrid (OEC)CoIV(Ph) T
(OEC+•)CoIII(Ph).34 A recent study of the same complex
involving detailed pulsed EPR/ENDOR measurements and DFT
calculations by Schweiger and co-workers yields a description
of the electronic configuration with 65% spin density residing
on the macrocycle and 35% spin density located in a cobalt dyz
orbital.46 We believe an analogous situation holds for 2, in which
the electronic configuration is best represented by the resonance
hybrid (Cz)CoIV(CCSiPh)3 T (Cz+•)CoIII(CCSiPh)3, although
the spin distribution may be quite different from that calculated
for (OEC)Co(Ph).
2
in an orbital predominantly dz in character (vide infra). The
spectrum exhibits rhombic symmetry, and there is a clear
splitting in the high-field region due to hyperfine coupling
7
between the unpaired electron and the 59Co nucleus (I ) /2).
There are six broad peaks visible for the expected eight-line
splitting pattern from the cobalt nucleus, with the remaining
two components obscured by overlap with the features to lower
field. A hyperfine coupling of AzCo ) 340 MHz is easily
obtained by inspection of the spectrum. In addition, a more
closely spaced three-line splitting pattern is seen for some of
the peaks on the high-field side. Such a pattern is typical for a
hyperfine interaction with a single nitrogen atom (14N, I ) 1,
AzN ) 60 MHz), indicating that the cobalt(II) corrolazine species
has one coordinated pyridine ligand in an axial position. The
cobalt and nitrogen hyperfine interactions are similar to the other
five-coordinate complexes listed in Table 3.
UV-Vis Spectra of (TBP)8CzCo(CCSiPh3), (TBP)8CzCo-
(CN), and (TBP)8CzCoIII. The UV-vis spectra of the three
cobalt complexes (TBP)8CzCo, 1 and 2 are shown in Figure 4.
All three complexes exhibit an intense Soret band near 445 nm,
but there are distinct differences in the Q-band region (600-
900 nm). For the mono-cyano and acetylide complexes there is
an obvious red-shift in the Q-band region as compared to the
(TBP)8CzCoIII starting material. A similar effect was noted for
(TBP)8CzCoIII(PPh3) and (TBP)8CzCoIII(py)2; the binding of
axial PPh3 or pyridine ligands to (TBP)8CzCoIII caused a red-
shift in the Q-band from 600 to 670 nm.21 For 1 and 2 the spectra
are complicated by the fact that these complexes not only contain
strong axial ligands, but are also oxidized by one electron.
Interestingly, 1 and 2 show rather different UV-vis spectra even
though they are isoelectronic. The discrepancies between the
spectra for these compounds are likely more of a reflection of
the electronic configuration (π-cation-radical for 1 vs the
resonance hybrid description for 2) than a reflection of the
identity of the axial ligands.
The best simulation of this spectrum (red line) is also given
in Figure 6, and the g and A values used for the simulated
(66) Walker, F. A. J. Magn. Reson. 1974, 15, 201-218.
(67) Walker, F. A. J. Am. Chem. Soc. 1970, 92, 4235-4244.
(68) Basolo, F.; Hoffman, B. M.; Ibers, J. A. Acc. Chem. Res. 1975, 8, 384-
392.
(69) Van Doorslaer, S.; Schweiger, A. Phys. Chem. Chem. Phys. 2001, 3, 159-
166.
(70) Baumgarten, M.; Winscom, C. J.; Lubitz, W. Appl. Magn. Reson. 2001,
20, 35-70.
Formation of [(TBP)8CzCoII(py)]- (3). UV-vis Studies.
The addition of excess NaBH4(aq) to (TBP)8CzCoIII in a mixture
(71) Ozarowski, A.; Lee, H. M.; Balch, A. L. J. Am. Chem. Soc. 2003, 125,
12606-12614.
(72) Assour, J. M. J. Am. Chem. Soc. 1965, 87, 4701-4706.
(73) Assour, J. M.; Kahn, W. K. J. Am. Chem. Soc. 1965, 87, 207.
(74) Cariati, F.; Galizzioli, D.; Morazzoni, F.; Busetto, C. J. Chem. Soc., Dalton
Trans. 1975, 556-561.
(65) An insightful reviewer suggested that an alternative configuration may arise
from a ground state that involves spin density which is localized on the
three meso nitrogen atoms. Indeed, simulations involving 3 × 14N nuclei
were quite similar to 4 × 14N simulations but were not much of an
improvement on the whole.
(75) Pezeshk, A.; Greenaway, F. T.; Dabrowiak, J. C.; Vincow, G. Inorg. Chem.
1978, 17, 1717-1725.
(76) Pezeshk, A. Inorg. Chem. 1992, 31, 2282-2284.
9
2522 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

Synthesis of Cobalt Corrolazine Complexes
A R T I C L E S
Scheme 3
Indeed, under the right conditions, similar rhombic EPR
parameters have been observed for vitamin B12 (B12r in Table
3).80 Along the same lines, many Schiff’s base complexes, which
also have reduced in-plane symmetry, exhibit rhombic EPR
spectra.35,75,76 For example, the TCTDH complex in Table 3
has been studied in detail by EPR spectroscopy and exhibits g
and A values quite similar to those of the corrolazine complex.
The rhombic splitting for this complex was attributed to the
alternating five- and six-membered chelate rings of the Schiff’s
base ligand.75
Theoretical treatments for the spin Hamiltonian parameters
of low-spin (d7) cobalt(II) complexes have been developed by
several workers,81-83 in which the g and A values can be related
to the ground-state electronic configuration and the energy
separations of low-lying doublet and quartet excited states. We
have not attempted to apply these equations here, but are
nonetheless able to benefit from the previous application of these
equations to come to the conclusion that the ground state for 3
Figure 5. UV-vis titration for the reduction of (TBP)8CzCoIII(py)2 (9.56
× 10-5 M in pyridine) with NaBH4 (0.145 M in EtOH) to give
[(TBP)8CzCoII(py)]- under argon at room temperature.
2
is best described as dz based on the observed g and A values
for this complex. For example, the g and A parameters for the
TCTDH complex (Table 3) are similar to those for the
corrolazine complex, and a detailed theoretical treatment by the
authors in this case led to the conclusion that the ground state
6
is clearly dz ((dx -y dxzdyz) dz 1).75
2
2
2
2
Interestingly, the few examples of CoII-corroles that have
been reported exhibit dramatically different EPR spectra as
compared to the other complexes in Table 3, in part because
they have a marked tendency to remain four-coordinate and not
bind axial ligands. The previous [(corrole)CoII]- anionic
complexes were generated by chemical reduction in a fashion
similar to 3. Hush and Woolsey described the generation of
[(DHC)CoII]- (no axial ligands) by reduction of (DHC)CoIII with
sodium film in THF; however, attempts to form a five-
coordinate pyridine adduct were unsuccessful.84 Boschi and
Kadish produced [(OMTPC)CoII]- via bulk electroreduction of
(OMTPC)CoIII(PPh3), finding that the PPh3 axial ligand appar-
ently dissociates after reduction to generate a four-coordinate
species.85 Only Murakami and co-workers have observed the
binding of axial ligands to cobalt(II) corroles.86,87 The EPR
signals for both four-coordinate [(MEC)CoII]- and [(PMC)CoII]-
(obtained by reduction with NaBH4) and five-coordinate
[(MEC)CoII(py)]- and [(PMC)CoII(py)]- complexes (14N hy-
Figure 6. Experimental (blue) and simulated (red) EPR spectrum of
[(TBP)8CzCoII(py)]- in pyridine/EtOH (50/50 v/v) at 77 K. Experimental
conditions: frequency, 9.304 GHz; incident microwave power, 12.69 mW;
modulation frequency, 100 kHz; modulation amplitude, 1.0 G; receiver gain,
1.59 × 104. Simulation parameters: g ) [1.985, 2.200, 2.400], A(59Co) )
[40.0, 40.0, 340.0] MHz, A(14N) ) [10.0, 10.0, 60.0] MHz, single-crystal
Gaussian line width (hwhm) matrix, W ) [60, 60, 30] MHz.
spectrum are listed in Table 3 together with the parameters for
other analogous Co(II) complexes. The majority of CoII por-
phyrins and phthalocyanines, such as (OEP)CoII(py)70 and (Pc)-
CoII(4-Mepy)74 listed in Table 3, exhibit an EPR spectrum with
axial symmetry. In contrast, a rhombic spectrum is seen for the
corrolazine complex, which can be explained by the lower in-
(80) Jo¨rin, E.; Schweiger, A.; Gu¨nthard, H. H. J. Am. Chem. Soc. 1983, 105,
4277-4286.
(81) Lin, W. C. Inorg. Chem. 1976, 15, 1114-1118.
plane symmetry of the contracted corrolazine nucleus. The
(82) Lin, W. C.; Lau, P. W. J. Am. Chem. Soc. 1976, 98, 1447-1450.
(83) McGarvey, B. R. Can. J. Chem. 1975, 53, 2498-2511.
(84) Hush, N. S.; Woolsey, I. S. J. Chem. Soc., Dalton Trans. 1974, 24-34.
(85) Adamian, V. A.; D’Souza, F.; Licoccia, S.; Di Vona, M. L.; Tassoni, E.;
Paolesse, R.; Boschi, T.; Kadish, K. M. Inorg. Chem. 1995, 34, 532-540.
(86) Murakami, Y.; Yamada, S.; Matsuda, Y.; Sakata, K. Bull. Chem. Soc. Jpn.
1978, 51, 123-129.
77-80
cobalt(II) corrin from vitamin B12
is more appropriate for
comparison in this regard because of its contracted ring structure.
(77) Schrauzer, G. N.; Lee, L.-P. J. Am. Chem. Soc. 1968, 90, 6541-6543.
(78) Schrauzer, G. N.; Lee, L.-P. J. Am. Chem. Soc. 1970, 92, 1551-1556.
(79) Hamilton, J. A.; Yamada, R.; Blakley, R. L.; Hogenkamp, H. P. C.; Looney,
F. D.; Winfield, M. E. Biochemistry 1971, 10, 347-355.
(87) Murakami, Y.; Matsuda, Y.; Sakata, K.; Yamada, S.; Tanaka, Y.; Aoyama,
Y. Bull. Chem. Soc. Jpn. 1981, 54, 163-169.
9
J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2523

A R T I C L E S
Ramdhanie et al.
perfine splittings were observed) were recorded.86,87 For all of
the corrole complexes in Table 3, a rhombic spectrum with an
unusually high maximum g value (g1) is observed. The corrole
g values have some similarity with the g values reported for a
series of (TPP)CoII complexes in the absence of axial bases (e.g.
g| ) 3.322, g ) 1.798 for (TPP)CoII diluted in a solid matrix
of TPPH2).69 The latter complexes were analyzed with a simple
first-order equation adopted from McGarvey,83 which shows g
) 2.0023 + 6c1, with c1 ) λ/∆E(xz,yzfz2). Thus, the absence
2
of strong axial ligation along the z axis keeps the dz orbital
relatively low in energy and ∆E(xz,yzfz2) relatively small,
which in turn leads to a large g value. However, the presumed
axially ligated [(MEC)CoII(py)]- and [(PMC)CoII(py)]- com-
plexes still exhibit a large g1 (g|) value, with parameters that
are quite similar to the four-coordinate species [(MEC)CoII]-
and [(PMC)CoII]-. These data suggest that the pyridine must
bind very weakly to the cobalt center in these complexes,
keeping ∆E(xz,yzfz2) small. These observations are in concert
with a previous hypothesis from Hush and Woolsey that the
negative charge of a CoII-corrole complex prevented the
binding of axial ligands through electronic repulsion,84 in
contrast to the strong binding seen for neutral cobalt(II)
porphyrins and phthalocyanines.
Figure 7. Experimental (black) and simulated (blue) EPR spectrum of
[(TBP)8CzCoIII(py)(O2)]- in pyridine/EtOH (50/50 v/v) at 56 K. Experi-
mental conditions: frequency, 9.477 GHz; incident microwave power, 4.66
mW; modulation frequency, 100 kHz; modulation amplitude, 1.0 G; receiver
gain, 1.59 × 104. Simulation parameters: g ) [1.996, 2.011, 2.075], A(59Co)
) [20.0, 40.0, 60.0] MHz; A(59Co) rotated about g matrix by Euler angles:
R ) 15°, â ) 25°, γ ) 0; a single-crystal Gaussian line width (hwhm)
matrix, W ) [12, 14, 20] MHz was also used.
From the EPR data it is clear that 3 binds pyridine much
more tightly than the conventional CoII-corroles. The difference
in the affinity for axial ligands between cobalt(II) corroles versus
corrolazines can be rationalized by the higher electronegativity
of the meso N atoms in a corrolazine framework as compared
to the meso carbon atoms of the conventional corroles. The meso
N atoms make the cobalt center a stronger Lewis acid through
electron-withdrawing effects. A similar argument has been used
to explain the much tighter binding of pyridine to (TBP)8CzCoIII
as compared to cobalt(III) corroles.21 We made several attempts
to produce a four-coordinate [(Cz)CoII]- complex by reduction
in noncoordinating solvents (toluene, CH2Cl2), but no EPR
signal was observed, suggesting that the reduction does not take
place under these conditions.
Binding of O2. To our knowledge there are no reports
describing the binding of O2 to a CoII-corrole, despite the large
precedent that exists for such a reaction with CoII-porphyrinoid
and Schiff’s base complexes.35,36 Given that our CoII-corro-
lazine complex exhibits EPR and pyridine binding properties
similar to those of CoII-porphyrins and corrins, it seemed
reasonable to speculate that we might observe O2 binding as
well. Bubbling of O2 through a freshly prepared solution of
[(TBP)8CzCoII(py)]- in an EPR tube at -78 °C followed by
rapid freezing in liquid nitrogen leads to the EPR spectrum
shown in Figure 7. The EPR signal of [(TBP)8CzCoII(py)]- has
disappeared and a spectrum attributable to a cobalt-dioxygen
adduct is present. This spectrum has a much narrower spread
in g values and smaller cobalt hyperfine interaction than the
CoII starting material, indicative of an organic radical-type EPR
Figure 8. Variable-temperature EPR spectra of [(TBP)8CzCoIII(py)(O2)]-
in fluid solution (pyridine/EtOH (50/50 v/v)). Temperature range from T
) 160 to 239 K. Conditions: frequency, 9.464 GHz; incident microwave
power, 0.938 mW; modulation frequency, 100.0 kHz; modulation amplitude,
10.0 G; receiver gain, 1.59 × 104.
Taken together, the overall features of the experimental spectrum
and simulation lead to the conclusion that the reduced cobalt-
corrolazine is capable of binding O2 at low temperature to give
a cobalt(III)-superoxo species.
Variable-Temperature Behavior of the Cobalt(III)-Su-
peroxo Species. The overall stability and temperature-dependent
O2 binding characteristics of various LnCoIIIO2 species have
-
been of particular interest because of the importance in
understanding the electronic and structural factors that control
this process. Thus, the EPR spectrum of the cobalt-superoxo
complex was measured at various temperatures from 160 to 239
K, as shown in Figure 8. At temperatures above ∼200 K, a
clear eight-line pattern is obtained, and the experimental
spectrum at 228 K has been simulated to give Aiso(59Co) ) 40
MHz and giso ) 2.026. Note that these values are an exact
average of the individual A(59Co) and g components seen in
frozen solution (Figure 7), confirming the retention of the
spectrum, as expected for a CoIII-O2 species. A good
-
simulation of this spectrum is also shown in Figure 7, and it
was calculated by considering the g and A tensors noncoincident
following the methods used for simulating numerous other (L)-
CoIII-O2- spectra (especially Jo¨rin et al.,42 as described in the
Experimental Section). The g (1.996, 2.011, 2.075) and A (20,
40, 60 MHz) principal values obtained from the simulation
match those of many other cobalt(III)-superoxo species.35,36
9
2524 J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004

Synthesis of Cobalt Corrolazine Complexes
A R T I C L E S
superoxo complex in fluid solution. Similar spectra and simula-
tions have been obtained for a number of LnCoIIIO2- complexes
in isotropic media.88-92 The EPR spectrum disappears at T >
240 K, indicative of a shift to the right in the equilibrium
[(TBP)8CzCoII(py)]- causes the release of superoxide, but these
results demonstrate that free O2 is easily detectable by EPR
under our conditions and support our previous conclusion that
[(TBP)8CzCoII(py)]- exhibits reversible O2 binding behavior
under relatively low concentrations of O2.
-
-
LnCoIIIO2 / (Ln)CoII + O2. A shift of this type is typical of
a cobalt-dioxygen adduct. Thawing and refreezing of the
oxygenated sample can be cycled several times before decay
of the sample occurs. We speculate that the binding of O2 is
successful in the case of a triazacorrole and not the conventional
Conclusions
We have synthesized two new cobalt corrolazine complexes
bearing the isoelectronic, σ-bonded axial ligands CN- and
CtCSiPh3-. These complexes are rare examples of formally
high oxidation state cobalt corroles. From a combination of
synthetic observations, FTIR, EPR, SQUID, UV-vis, and in
the case of the acetylide complex X-ray crystallography, we
suggest their electronic configurations are best formulated as
(Cz+•)CoIII(CN) and (Cz)CoIV(CCSiPh)3 T (Cz+•)CoIII(CCSiPh)3,
respectively. Thus, in both cases it appears as though there is
significant “noninnocent” character to the corrolazine ligand,
although more detailed spectroscopic work (e.g. ESEEM/
ENDOR studies46) is needed to make quantitative spin density
assignments for the corrolazine ligand versus the cobalt metal
ion. We have also generated the reduced cobalt(II) complex
[(TBP)8(CzCoII(py)]- and have shown that it has a typical low-
2
corroles because of two main reasons: the dz ground state
exhibited by the cobalt(II)-triazacorrole is essential for O2
coordination, and the meso N atoms are electron-withdrawing
compared to carbon, which helps to stabilize the overall anionic
charge of the CzCoII unit and favor binding of O2.
Generation of Free Superoxide Anion. Interestingly, an
EPR spectrum for free superoxide anion can be observed if the
CoII solution is exposed to a large excess of dioxygen. If the
solution of [(TBP)8CzCoII(py)]- is bubbled with oxygen for
greater than ∼30 s at room temperature and then frozen to 77
K, neither the EPR spectrum of the CoII complex nor that of
the CoIII-O2 is seen, but rather a new spectrum which
-
corresponds to free superoxide anion is observed g ) [2.10,
2.00, 1.98]. A separate sample of KO2 was dissolved in the
same solvent mixture (pyridine/EtOH 50/50) and revealed an
identical EPR spectrum at 77 K. There are a few well-
documented examples of the production of free O2- by similar
metal complexes; CoII-tetrasulfophthalocyanine was shown to
release O2- in the presence of CN- or OH-,93 and the porphyrin
complexes (OEP)RhCl94 and (TPP)Co95 have been shown to
II
2
spin Co dz ground state by EPR spectroscopy, in contrast to
the few earlier examples of cobalt(II) corroles. In addition, we
have demonstrated that the cobalt(II) complex will reversibly
bind dioxygen, which is a reaction well established in porphyrin
chemistry but previously unknown for cobalt corroles. The
oxygen adduct was characterized by EPR spectroscopy as a
typical cobalt(III)-superoxo adduct, and shown to be stable at
temperatures below ∼240 K by VT-EPR measurements.
generate free O2 via a putative, short-lived MIII-O2 inter-
-
-
mediate. It is unclear at this time why further oxygenation of
Acknowledgment. This work was supported by the NSF
(Grants CHE0094095 and CHE0089168 to D.P.G., CHE0091968
to A.L.R.). D.P.G. is also grateful for an Alfred P. Sloan
Research Fellowship (B.R.-4153). A.C. is grateful for the Italian
MIUR fund.
(88) Hoffman, B. M.; Diemente, D. L.; Basolo, F. J. Am. Chem. Soc. 1970, 92,
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Supporting Information Available: X-ray crystallographic
file for compound 2 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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JA036983S
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J. AM. CHEM. SOC. VOL. 126, NO. 8, 2004 2525