Electron transfer mechanism of organocobalt porphyrins. Site of electron transfer, migration of organic groups, and cobalt-carbon bond energies in different oxidation states

Article abstract of DOI:10.1021/ja973257e

The chemical or electrochemical oxidations and coupled chemical reactions of (TPP)Co(R) and (TPP)Co(R)(L) where R = Bu, Et, Me, or Ph, L = a substituted pyridine, and TPP = the dianion of tetraphenylporphyrin were investigated in acetonitrile or dichloromethane. The homogeneous one- or two electron oxidation of the Co(III) σ-bonded complexes was accomplished using [Fe(phen)₃]³⁺ (phen = 1,10 phenanthroline) as an oxidant. The products of the initial oxidation as well as that of the subsequent R group migration from (TPP)Co(R) or (TPP)Co(R)(L) to give the N-aryl or N-alkyl Co(II) porphyrins were characterized by ESR and UV-vis spectroscopies, while the rates of migration were determined using stopped flow kinetics, The rate constants of R group migration from the metal to nitrogen in [(TPP)Co(R)]⁺ were found to vary by 6 orders of magnitude upon going from (TPP)Co(Ph) (1.3 x 10^-3 s^-1) to (TPP)Co(Bu) (1.2 x 10³ s^-1) at 298 K in acetonitrile, but no significant differences were observed between E(1/2) for the first oxidation of (TPP)Co(Ph) or (TPP)Co(Bu). ESR spectra of the transient singly oxidized porphyrins indicate that the six-coordinated derivatives, represented as [(TPP)Co(R)(MeCN)]⁺ or [(TPP)Co(R)(L)]⁺ (R = Ph and Me), have a significant d⁵ cobalt(IV) character in acetonitrile, while the five- coordinate compounds, [(TPP)Co(R)]⁺, in dichloromethane can be formulated as Co(III) porphyrin π cation radicals. The formation constants for conversion of (TPP)Co(R) to (TPP)Co(R)(L) were calculated spectroscopically and the logarithmic values vary linearly with the pK(a) of the sixth axial ligand. The E(1/2) values for the first oxidation of (TPP)Co(R)(L) in dichloromethane and the migration rate constants for the R group of [(TPP)Co(R)(L)]⁺ in acetonitrile are also related to the ligand pK(a). Cobalt-carbon bond dissociation enthalpies and entropies were determined for (TPP)Co(R) and [(TPP), Co(R)]⁺, and a comparison of these two sets of data reveals that the Co(IV)-carbon bond in the singly oxidized species is significantly weaker than the Co(III)-carbon bond in the neutral complex.

Full text of DOI:10.1021/ja973257e

2880
J. Am. Chem. Soc. 1998, 120, 2880-2889
Electron Transfer Mechanism of Organocobalt Porphyrins. Site of
Electron Transfer, Migration of Organic Groups, and Cobalt-Carbon
Bond Energies in Different Oxidation States
Shunichi Fukuzumi,*,† Kenichi Miyamoto,† Tomoyoshi Suenobu,†
Eric Van Caemelbecke,‡ and Karl M. Kadish*,‡
Contribution from the Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity,
Osaka 565-0871, Japan, and Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641
ReceiVed September 16, 1997
Abstract: The chemical or electrochemical oxidations and coupled chemical reactions of (TPP)Co(R) and
(TPP)Co(R)(L) where R ) Bu, Et, Me, or Ph, L ) a substituted pyridine, and TPP ) the dianion of
tetraphenylporphyrin were investigated in acetonitrile or dichloromethane. The homogeneous one- or two-
electron oxidation of the Co(III) σ-bonded complexes was accomplished using [Fe(phen)3]3+ (phen ) 1,10-
phenanthroline) as an oxidant. The products of the initial oxidation as well as that of the subsequent R group
migration from (TPP)Co(R) or (TPP)Co(R)(L) to give the N-aryl or N-alkyl Co(II) porphyrins were characterized
by ESR and UV-vis spectroscopies, while the rates of migration were determined using stopped flow kinetics.
The rate constants of R group migration from the metal to nitrogen in [(TPP)Co(R)]+ were found to vary by
6 orders of magnitude upon going from (TPP)Co(Ph) (1.3 × 10-3 s-1) to (TPP)Co(Bu) (1.2 × 103 s-1) at 298
K in acetonitrile, but no significant differences were observed between E1/2 for the first oxidation of (TPP)-
Co(Ph) or (TPP)Co(Bu). ESR spectra of the transient singly oxidized porphyrins indicate that the six-coordinated
derivatives, represented as [(TPP)Co(R)(MeCN)]+ or [(TPP)Co(R)(L)]+ (R ) Ph and Me), have a significant
d5 cobalt(IV) character in acetonitrile, while the five-coordinate compounds, [(TPP)Co(R)]+, in dichloromethane
can be formulated as Co(III) porphyrin π cation radicals. The formation constants for conversion of (TPP)-
Co(R) to (TPP)Co(R)(L) were calculated spectroscopically and the logarithmic values vary linearly with the
pKa of the sixth axial ligand. The E1/2 values for the first oxidation of (TPP)Co(R)(L) in dichloromethane and
the migration rate constants for the R group of [(TPP)Co(R)(L)]+ in acetonitrile are also related to the ligand
pKa. Cobalt-carbon bond dissociation enthalpies and entropies were determined for (TPP)Co(R) and [(TPP)-
Co(R)]+, and a comparison of these two sets of data reveals that the Co(IV)-carbon bond in the singly oxidized
species is significantly weaker than the Co(III)-carbon bond in the neutral complex.
Introduction
diglyoximes7-9 and cobalt(III) complexes of strongly donating
macrocyclic ligands,10 but Co(IV) porphyrins have yet to be
reported in the literature.
The first one-electron oxidation of σ-bonded organocobalt-
(III) porphyrins is often followed by a metal to nitrogen
migration of the σ-bonded organic ligand.1-4 Singly oxidized
σ-bonded cobalt porphyrins have been characterized as contain-
ing Co(III) π cation radicals2,3 rather than Co(IV), and this
assignment contrasts with assignments for singly oxidized
σ-bonded iron porphyrins1,5,6 which were proposed to contain
Fe(IV) for compounds containing aryl axial ligands. Non-
porphyrin complexes containing a cobalt(IV) oxidation state
have been obtained upon oxidation of σ-bonded cobalt(III)
The first one-electron oxidation of σ-bonded organocobalt-
(III) porphyrins can involve the metal center, the conjugated π
ring system, or the σ-bonded axial ligand with the specific site
of electron transfer being related, in large part, to the solution
conditions and donor properties of the σ-bonded axial ligand.
This is investigated in the present work which utilizes electro-
chemistry, UV-vis and ESR spectroscopy, and stopped flow
(7) Halpern, J.; Topich, J.; Zamaraev, K. I. Inorg. Chim. Acta 1976, 20,
L21.
† Osaka University.
(8) (a) Halpern, J.; Chan, M. S.; Hanson, J.; Roche, T. S.; Topich, J. A.
J. Am. Chem. Soc. 1975, 97, 1606. (b) Magnuson, R. H.; Halpern, J.; Levitin,
I. Ya.; Vol′pin, M. E. J. Chem. Soc., Chem. Commun. 1978, 44. (c) Abley,
P.; Dockal, E. R.; Halpern, J. J. Am. Chem. Soc. 1972, 94, 659. (d) Topich,
J.; Halpern, J. Inorg. Chem. 1979, 18, 1339. (e) Vol′pin, M. E.; Levitin, I.
Ya.; Sigan, A. L.: Nikitaev, A. T. J. Organomet. Chem. 1985, 279, 263.
(9) (a) Anderson, S. N.; Ballard, D. H.; Chrzastowski, J. Z.; Dodd, D.;
Johnson, M. D. J. Chem. Soc., Chem. Commun. 1972, 685. (b) Anderson,
S. N.; Ballard, D. H.; Johnson, M. D. J. Chem. Soc., Perkin Trans. II 1972,
311. (c) Chopra, M.; Hun, T. S. M.; Leung, W.-H.; Yu, N.-T. Inorg. Chem.
1995, 34, 5973.
‡ University of Houston.
(1) Guilard, R.; Kadish, K. M. Chem. ReV. 1988, 88, 1121.
(2) Dolphin, D.; Halko, D. J.; Johnson, E. Inorg. Chem. 1981, 20, 4348.
(3) (a) Callot, H. J.; Cromer, R.; Louati, R. A.; Gross, M. NouV. J. Chem.
1984, 8, 765. (b) Callot, H. J.; Metz, F. J. Chem. Soc., Chem. Commun.
1982, 947. (c) Callot, H. J.; Cromer, R. Tetrahedron Lett. 1985, 26, 3357.
(4) Kadish, K. M.; Han, B. C.; Endo, A. Inorg. Chem. 1991, 30, 4502.
(5) (a) Ortiz de Montellano, P. R.; Kunze, K. L.; Augusto, O. J. Am.
Chem. Soc. 1982, 104, 3545. (b) Battioni, P.; Mahy, J. P.; Gillet, G.; Mansuy,
D. J. Am. Chem. Soc. 1983, 105, 1399.
(6) (a) Lanc¸on, D.; Cocolios, P.; Guilard, R.; Kadish, K. M. J. Am. Chem.
Soc. 1984, 106, 4472. (b) Balch, A. L.; Renner, M, W. J. Am. Chem. Soc.
1986, 108, 2603. (c) Guilard, R.; Boisselier-Cocolios, B.; Tabard, A.;
Cocolios, P.; Simonet, B.; Kadish, K. M. Inorg. Chem. 1985, 24, 2509.
(10) (a) Anson, F. C.; Collins, T. J.; Coots, R. J.; Gipson, S. L.:
Richmond, T. G. J. Am. Chem. Soc. 1984, 106, 5037. (b) Collins, T. J.;
Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J. Am. Chem. Soc. 1991,
113, 8419.
S0002-7863(97)03257-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/11/1998

Electron Transfer Mechanism of Organocobalt Porphyrins
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2881
SO-TOP, CEA, France. Trifluoroacetic acid was also obtained com-
mercially. Tetrabutylammonium perchlorate (TBAP), obtained from
Fluka Fine Chemical, was recrystallized from ethanol and dried in vacuo
prior to use.
kinetics to investigate the reactivity and spectroscopic properties
of singly oxidized organometallic porphyrins of the type [(TPP)-
Co(R)]+, [(TPP)Co(MeCN)]+, and [(TPP)Co(R)(L)]+ where R
) Bu, Et, Me or Ph, MeCN ) acetonitrile, L ) a substituted
pyridine, and TPP ) the dianion of tetraphenylporphyrin. The
results of this study indicate that a Co(IV) oxidation state may
be accessed upon oxidation of (TPP)Co(R) or (TPP)Co(R)(L)
under certain experimental conditions, and the high valence state
of the cobalt ion in the singly oxidized cobalt porphyrins appears
to directly or indirectly influence the rate of migration of the
σ-bonded axial ligand. This study also reports the cobalt-
carbon bond dissociation enthalpies and entropies of both (TPP)-
CoIII(R) and [(TPP)CoIV(R)]+.
Product Analysis. One equiv of [Fe(phen)3](ClO4)3 (1.0 × 10-2
M) was added to an NMR tube that contained (TPP)Co(R) (1.0 × 10-2
M) in CD3CN/CDCl3 (1:1 v/v, 0.60 cm3) under 1.0 atm of argon. The
addition of excess CF3COOH to the solution resulted in demetalation
and the porphyrin product was identified as (N-RTPP)H on the basis
1
of its H NMR spectrum.12 1H NMR measurements were performed
with a JNM-GSX-400 (400 MHz) NMR spectrometer. 1H NMR (CD3-
CN, 298 K); δ(Me4Si, ppm): (N-MeTPP)H, δ -4.08 (s, 3H, N-CH3),
0.07 (s, 1H), 7.9-8.8 (m, 8H), 9.87 (s, 1H); (N-EtTPP)H, δ 3.48 (s,
3H), 4.74 (s, 3H), 7.9-8.9 (m, 8H). The yield in each case was
determined as 100 ( 10% by comparison of the integrated signal area
with those of an internal standard (dioxane).
Experimental Section
The Evans NMR method18a was used to determine the solution
magnetic susceptibility of the paramagnetic Co(II) complex, [(N-
MeTPP)Co]+. The NMR spectrometer used in this measurement was
equipped with a high magnetic field superconducting magnet. Cor-
rection has been made to the original Evans equation18a according to
the literature.18b A capillary tube containing the solvent (CD3CN/CDCl3,
1:1 v/v, 0.2 mL), [Fe(phen)3]2+ (1.0 × 10-2 M), and tetramethylsilane
was inserted into an NMR tube which contained (TPP)Co(Me) (1.0 ×
10-2 M), [Fe(phen)3]3+ (1.0 × 10-2 M), and tetramethylsilane in CD3-
CN/CDCl3 (1:1 v/v, 0.6 mL). A diamagnetic correction for the TPP
ligand was made based on the reported value.19 The change of solvent
density with temperature was also taken into account in determination
of the solution magnetic susceptibility.20
One equiv of [Fe(phen)3](ClO4)3 (4.0 × 10-3 M) in deaerated MeCN
(0.5 mL) was added with a syringe to a sample tube sealed with a
rubber septum that contained (TPP)Co(Me) (4.0 × 10-2 M) in deaerated
CHCl3 (0.5 mL). After completion of the reaction, the gaseous products
were analyzed by GC using a Unibeads 1-S column. A commercial
standard gas from GL Science Co. Ltd, Japan, was used as a reference
and contained methane (0.99%), ethane (1.02%), propane (1.01%),
isobutane (1.00%), and butane (1.00%). The determination of the yields
were made using sample tubes of the same size after equilibrium of
the reference gas.
Spectral and Kinetic Measurements. Typically, a 10 µL aliquot
of (TPP)Co(R) (1.5 × 10-3 M) in CHCl3 or CH2Cl2 was added to a
quartz cuvette (10 mm i.d.) which contained [Fe(phen)3](ClO4)3 (5.0
× 10-6 M) in deaerated MeCN (3.0 mL). This led to an electron
transfer from (TPP)Co(R) to [Fe(phen)3](ClO4)3. UV-vis spectral
changes associated with this electron transfer were monitored using a
Shimadzu UV-2200 spectrophotometer, a Hewlett-Packard 8452A diode
array spectrophotometer, or a Hewlett-Packard 8453 diode array
spectrophotometer. All experiments were carried out in a dark cell
compartment using deaerated solutions. It was confirmed that the
thermal rates were not affected by the monitoring light.
Kinetic measurements of the electron transfer from (TPP)Co(R) to
[Fe(phen)3](ClO4)3 were carried out using a Union RA-103 stopped-
flow spectrophotometer. Deaerated MeCN solutions of (TPP)Co(R)
and [Fe(phen)3](ClO4)3 were transferred to the spectrophotometric cells
by means of a glass syringe which had earlier been purged with a stream
of argon. All kinetic measurements were carried out under deaerated
conditions. The rates of the migration reactions were followed by
spectrally monitoring the increase in absorption band intensity due to
the oxidized porphyrin product under pseudo-first-order conditions
where the concentration of [Fe(phen)3]3+ was maintained at more than
10-fold excess of the (TPP)Co(R) concentration. Pseudo-first-order
rate constants were determined by a least-squares curve fit using an
NEC microcomputer. The first-order plots of ln(A∞ - A) vs time (A∞
and A are the final absorbance and the absorbance at the reaction time,
respectively) were linear for three or more half-lives with the correlation
coefficient, F > 0.99.
Materials. Cobalt(II) tetraphenylporphyrin, (TPP)Co, was prepared
as described in the literature11 and then oxidized by oxygen in methanol
containing HCl to obtain tetraphenyl porphyrinatocobalt(III) chloride,
(TPP)CoCl, which was further purified by recrystallization from
methanol. Methylcobalt(III) tetraphenylporphyrin, (TPP)Co(Me), was
prepared from a reaction between methylhydrazine and (TPP)CoCl
followed by oxidation of the resulting intermediate by oxygen.12
Reactions involving (TPP)CoCl were performed in chloroform/aceto-
nitrile (5:1 v/v) since the solubility of (TPP)CoCl is larger in this mixed
solvent than in neat chloroform or acetonitrile. (TPP)Co(Ph) was
prepared according to literature procedures.13 (TPP)Co(Et) and (TPP)-
Co(Bu) were prepared from the reaction of (TPP)CoCl with Et4Sn or
Bu4Sn as described previously.14 Et4Sn (4.0 × 10-2 M) was added to
a chloroform/acetonitrile (5:1 v/v) solution of (TPP)CoCl (1.0 × 10-2
M), which had been deaerated with argon gas and kept in the dark.
The solution was stirred for a few hours, and the (TPP)Co(R) product
was recrystallized from methanol.
The purity of (TPP)Co(R) thus obtained was checked by elemental
1
analysis and H NMR spectroscopy. Anal. Calcd for C45H31N4Co,
(TPP)Co(Me): C, 78.71; H, 4.55; N, 8.16. Found: C, 78.42; H, 4.43;
N, 8.14. Anal. Calcd for C46H33N4Co, (TPP)Co(Et): C, 78.85; H,
4.75; N, 8.00. Found: C, 78.67; H, 4.96; N, 7.92. Anal. Calcd for
C48H37N4Co, (TPP)Co(Bu): C, 79.11; H, 5.12; N, 7.69. Found: C,
79.03; H, 5.19; N, 7.66. Anal. Calcd for C50H33N4Co, (TPP)Co(Ph):
C, 80.20; H, 4.44; N, 7.48. Found: C, 79.95; H, 4.43; N, 7.62. 1H
NMR (CDCl3/CD3CN 5:1 v/v, 298 K); δ(Me4Si, ppm) of coordinated
alkyl ligand: (TPP)Co(Me), -4.41 (s, 3H); (TPP)Co(Et), -4.93 (t,
3H, J ) 7.4 Hz), -3.46 (q, 2H, J ) 7.4 Hz); (TPP)Co(Bu), -4.64 (m,
2H, J ) 7.7 Hz), -3.54 (t, 2H, J ) 7.8 Hz), -1.36 (m, 2H, J ) 7.3
Hz), -0.73 (t, 3H, J ) 7.3 Hz); (TPP)Co(Ph), 0.37 (d, 2H, J ) 8.3
Hz), 4.63 (dd, 2H, J ) 8.3, 6.8 Hz), 5.26 (t, 1H, J ) 6.8 Hz). Proton
resonances of the porphyrin ligand were found at 7.71 (m, 12H), 8.06
(m, 8H), 8.76 (s, 8H). Since the (TPP)Co(R) derivatives are light
sensitive,15 the compounds were kept in the dark.
Tris(1,10-phenanthroline)iron(III) perchlorate, [Fe(phen)3](ClO4)3,
was prepared by oxidizing the iron(II) complex with ceric sulfate in
aqueous H2SO4.16 Acetonitrile and dichloromethane, used as solvents,
were purified and dried with CaH2 according to standard procedures.17
Spectral grade chloroform was obtained from Wako Pure Chemicals.
Pyridine and substituted pyridines (3,5-dichloropyridine, 4-cyanopy-
ridine, 3-chloropyridine, 3-picoline, 3,4-lutidine, and 4-(dimethylamino)-
pyridine) were obtained commercially and purified using standard
methods.17 Acetonitrile-d3 and chloroform-d3 were obtained from EURI
(11) Shirazi, A.; Goff, H. M. Inorg. Chem. 1982, 21, 3420.
(12) Mansuy, D.; Battioni, J.-P.; Dupre`, D.; Sartori, E.; Chottard, G. J.
Am. Chem. Soc. 1982, 104, 6159.
(13) Callot, H. J.; Metz, F.; Cromer, R. NouV. J. Chem. 1984, 8, 759.
(14) Fukuzumi, S.; Kitano, T. Inorg. Chem. 1990, 29, 2558.
(15) (a) Kendrick, M. J.; Al-Akhdar, W. Inorg. Chem. 1987, 26, 3972.
(b) Perree-Fauvet, M.; Gaudemer, A.; Boucly, P.; Devynck, J. J. Organomet.
Chem. 1976, 120, 439.
(18) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Schubert, E. M. J.
Chem. Educ. 1992, 69, 62 and references therein.
(16) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593.
(17) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Elmsford, 1966.
(19) Eaton, S. S.; Eaton, G. R. Inorg. Chem. 1980, 19, 1095.
(20) (a) Hanson, E. S. Ind. Eng. Chem. 1949, 41, 99. (b) Ostfeld, D.;
Cohen, I. A. J. Chem. Educ. 1972, 49, 829.

2882 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Fukuzumi et al.
The coordination of substituted pyridine as a sixth axial ligand to
(TPP)Co(R) in MeCN was monitored by measuring the UV-vis
spectral changes as a function of the ligand concentration.
ESR Measurements. The ESR spectra of chemically generated
[(TPP)Co(R)]+, [(TPP)Co(R)(MeCN)]+, and [(TPP)Co(R)(L)]+ were
measured at both room temperature, 203 K, and 77 K (in frozen
solutions) with a JEOL X-band spectrometer (JES-ME-LX or JES-
RE1XE). Typically a CHCl3 solution of 10 mM (TPP)Co(R) was
deaerated in an ESR tube with argon bubbling, and after which it was
placed in the cavity that was thermostated with a JEOLDVD Unit (ES-
DVT3). The g values were calibrated with a Mn2+ marker, and the
hyperfine coupling constants were determined by computer simulation
using a Calleo ESRa II program coded by Calleo Scientific Software
Publishers. The areas of ESR signals are compared between [(TPP)-
Co(Me)(MeCN)]+ and [(TPP)Co]2+ by correcting the difference in the
g values.
Cyclic Voltammetry. Redox potentials of (TPP)Co(R) in MeCN
or CH2Cl2 containing 0.10 M TBAP as supporting electrolyte were
determined at room temperature or 203 K by cyclic voltammetry under
deaerated conditions in the dark using a three electrode system and a
BAS 100B electrochemical analyzer. The working and counter
electrodes were platinum, while Ag/AgNO3 (0.01M) was used as the
reference electrode. All potentials are reported as V vs SCE. The
E1/2 value of ferrocene used as a standard is 0.37 V vs SCE in MeCN
under our solution conditions.
Figure 1. Visible absorption spectra of [(TPP)Co(R)]+ (R ) Et, Me,
and Ph) observed upon mixing (TPP)Co(R) (5.0 × 10-6 M) and [Fe-
(phen)3](ClO4)3 (5.0 × 10-6 M) at 233 K for (TPP)Co(Et) and at 298
K for (TPP)Co(R) (R ) Me and Ph) in MeCN. All measurements were
made 1 s after mixing.
Results and Discussion
Electron Transfer Oxidation of (TPP)Co(R) by [Fe-
(phen)3]3+. The electron transfer from (TPP)Co(R) (R ) Me,
Et, Bu, or Ph) to [Fe(phen)3]3+ is rapid in MeCN at 298 K as
indicated by the almost instantaneous formation of [Fe(phen)3]2+
(λmax ) 508 nm). When 1 equiv of [Fe(phen)3]3+ is added to
an MeCN solution of (TPP)Co(Ph), the Soret band of (TPP)-
Co(Ph) at 408 nm disappears as [(TPP)Co(Ph)]+ (λmax ) 434,
410, 380 nm) is generated. This rapid reaction is then followed
by a slow migration of the phenyl group from cobalt to one of
the four nitrogens of the macrocycle to yield [(N-PhTPP)Co]+
(λmax ) 436 nm) as a final porphyrin product. The UV-visible
spectrum of [(TPP)Co(Ph)]+ (λmax ) 434, 410 nm) is similar
to the UV-visible spectrum of [(TPP)Co]2+ (λmax ) 433, 414
nm) which is generated by the two-electron oxidation of (TPP)-
Co with [Fe(phen)3]3+. Since [(TPP)Co]2+ is known to be a π
radical cation,21,22 the similar absorption spectra of [(TPP)Co-
(Ph)]+ and [(TPP)Co]2+ show a significant contribution of TPP
π radical cation in [(TPP)Co(Ph)]+. In the case of [(TPP)Co-
(Me)]+ and [(TPP)Co(Et)]+, however, the 406 nm absorption
band of the singly oxidized product is significantly stronger than
the shoulder band at 410 nm for [(TPP)Co(Ph)]+ (see Figure 1,
where the spectrum of [(TPP)Co(Et)]+ was measured at 233 K
to slow the migration). Such a difference in the absorption
spectra suggests that the abstraction of an electron from [(TPP)-
Co(R)] (R ) Me and Et) may involve the metal rather than the
conjugated porphyrin ligand to yield the Co(IV) oxidation state.
The final migration product is the doubly oxidized, [(N-
RTPP)Co]2+, when 2 equiv of [Fe(phen)3]3+ are reacted with
(TPP)Co(R). The final porphyrin product obtained after de-
metalation was identified as (N-RTPP)H by its 1H NMR
spectrum (see Experimental Section). Methane, formed via
homolytic cleavage of the Co-Me bond in [(TPP)Co(Me)]+,
was also detected as a minor product (5% yield) in the one-
electron oxidation of (TPP)Co(Me) (see Experimental Section).
Kinetics of Migration. Migration rates of the σ-bonded
ligand of [(TPP)Co(R)]+ (R ) Ph and Me) were followed by
Figure 2. Plot of kobs vs [Fe3+] for electron transfer from (TPP)Co-
(Me) (5.0 × 10-6 M) to [Fe(phen)3]3+ in deaerated MeCN at different
temperatures.
monitoring the product absorption band as [(N-RTPP)Co]+ is
produced after addition of 1 equiv of [Fe(phen)3]3+ to (TPP)-
Co(R) in MeCN at 298 K. The migration rates of [(TPP)Co-
(Me)]+ were also obtained by monitoring the change in
absorption as [(N-MeTPP)CoIII]2+ is produced by a two-electron
oxidation of (TPP)Co(Me) using more than 2 equiv of [Fe-
(phen)3]3+ in MeCN at 298 K. The rates for the two-electron
oxidation of (TPP)Co(Me) obey first-order kinetics.
The dependence of the observed first-order rate constant (kobs
)
on the [Fe(phen)3]3+ concentration was determined at different
temperatures, and a summary of the data (Figure 2) shows that
the kobs value is independent of changes in the [Fe(phen)3]3+
concentration, which is represented as [Fe3+]. This result
suggests that the Me group migration is the rate-determining
step in the two-electron oxidation of (TPP)Co(Me).
The R group migration of (TPP)Co(Et) and (TPP)Co(Bu) is
too fast to be followed by conventional UV-visible spectros-
copy, and stopped-flow methods were therefore utilized to
follow the migration. The rate of [(N-RTPP)Co]2+ formation
during the two-electron oxidation of [(TPP)Co(R)] (R ) Et or
Bu) with a large excess [Fe(phen)3]3+ also obeys first-order
kinetics in MeCN at 298 K. The dependence of the observed
first-order rate constant (kobs) on the [Fe(phen)3]3+ concentration
at different temperatures is shown in Figure 3 for R ) Et. A
similar result was obtained for R ) Bu. In each case the kobs
value increases with increase in the [Fe(phen)3]3+ concentration
to approach a constant value which corresponds to the migration
rate constant (kmig). Such a dependence of kobs on the [Fe-
(21) (a) Ohya-Nishiguchi, H.; Khono, M.; Yamamoto, K. Bull. Chem.
Soc. Jpn. 1981, 54, 1923. (b) Ichimori, K.; Ohya-Nishiguchi, H.; Hirota,
N.; Yamamoto, K. Bull. Chem. Soc. Jpn. 1985, 58, 623.
(22) Dolphin, D.; Felton, R. H. Acc. Chem. Res. 1974, 7, 26.

Electron Transfer Mechanism of Organocobalt Porphyrins
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2883
Figure 3. Plot of kobs vs [Fe3+] for electron transfer from (TPP)Co-
(Et) (5.0 × 10-5 M) to [Fe(phen)3]3+ in deaerated MeCN at different
temperatures.
Figure 4. Plot of kobs vs [Fe3+
]
for electron transfer from (TPP)-
-1
-1
Co(Bu) (5.0 × 10-5 M) to [Fe(phen)3]3+ in deaerated MeCN at different
temperatures.
Scheme 1
(phen)3]3+ concentration can be explained by the sequence of
steps shown in Scheme 1, where k1, kmig, and k2 represent rate
constants of the initial electron transfer from (TPP)Co(R) to
[Fe(phen)3]3+, the R group migration from [(TPP)Co(R)]+ to
[(N-RTPP)Co]+, and the following electron transfer from [(N-
RTPP)Co]+ to [Fe(phen)3]3+, respectively.
By applying the steady-state approximation to the intermedi-
ates in Scheme 1, the dependence of kobs on the concentration
of [Fe(phen)3]3+ can be derived as shown in eq 1, where [Fe-
(phen)3]3+ is represented as Fe3+ (for the derivation, see
Supporting Information). When the initial electron transfer in
Scheme 1 is the rate-determining step, i.e., k1 , kmig and k2, eq
1 is reduced to kobs = k1[Fe3+]. On the other hand, when the
migration is the rate-determining step, i.e., kmig , k1 and k2, eq
2 is reduced to kobs = kmig. Equation 1 can be rewritten in the
Figure 5. (a) Experimental ESR spectrum and (b) computer simulation
spectrum of [(TPP)Co(Me)]+ (1.0 × 10-3 M) generated by addition of
[Fe(phen)3]3+ (1.0 × 10-3 M) to (TPP)Co(Me) in MeCN/CHCl3 (v:v
1/1) at 77 K.
-1
form of eq 2 which predicts a linear correlation between kobs
and [Fe3+ -1
]
.
ESR Spectra of Oxidized Complexes of (TPP)Co(R). The
Me group migration rate constant of [(TPP)Co(Me)]+ in MeCN
ranges from 1.7 × 10-3 s-1 at 278 K to 5.2 × 10-2 s-1 at 318
K (see Table 1), and the rate is slow enough at 298 K so that
the ESR spectrum of [(TPP)Co(Me)]+ could be measured at
77 K just after the one-electron-transfer oxidation of (TPP)Co-
(Me) (2.0 × 10-3 M) by Fe(phen)33+ (2.0 × 10-3 M) in MeCN/
CHCl3 (1:1 v/v). This spectrum is shown in Figure 5a. The
spectrum exhibits an anisotropic signal with a broad line which
reflects coupling of the unpaired electron with the nuclear spin
of 59Co (I ) 7/2), but the hyperfine structure is not well resolved.
The computer simulation spectrum is shown in Figure 5b and
gives the ESR parameters g1 ) 2.067, g2 ) 2.009, g3 ) 2.005,
A1Co ) 11.6 G, A2Co ) 3.8 G, A3Co ) 3.0 G.
kobs ) k1kmigk2[Fe3+]/{kmig(k1 + k2) + k1k2[Fe3+]} (1)
kobs-1 ) {k1k2[Fe3+]/(k1 + k2)}-1 + kmig
(2)
-1
Although the steady-state approximation to derive eq 2 may
not be exactly valid when kmig ≈ k1, eq 2 can be used to
-1
extrapolate the kmig value as shown by the linear plot of kobs
vs [Fe3+ -1
]
in Figure 4 for R ) Bu. A linear plot of kobs-1 vs
for R ) Et was also obtained for the data in Figure 3.
[Fe3+ -1
]
The kmig values at different temperatures obtained from the
intercepts of the linear plots are listed in Table 1.
The migration rate constants vary by 6 orders of magnitude
between the different [(TPP)Co(R)]+ complexes and increase
with increase in the electron donor ability of R from R ) Ph
(1.3 × 10-3 s-1) to R ) Bu (1.2 × 103 s-1) in MeCN at
298 K.23
The ESR spectrum of [(TPP)Co(Me)]+ could also be mea-
sured in solution at 298 K and gives isotropic g (2.027 ( 0.001)
(23) Despite the large variation in the kmig values, they were determined
as those independent of [Fe3+] when the migration is always the rate-
determining step.

2884 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Fukuzumi et al.
Table 1. Rate Constants for Migration of R Group of [(TPP)Co(R)]+ in MeCN at Various Temperatures
kmig, s-1 a
R
278 K
283 K
288 K
298 K
303 K
318 K
323 K
Bu
Et
Me
Ph
b
5.2 × 102
6.9 × 102
1.2 × 103
2.6 × 102
1.4 × 10-2
1.3 × 10-3
c
b
b
c
b
c
b
b
8.5 × 101
1.7 × 10-3
b
b
b
b
1.7 × 102
b
b
5.2 × 10-2
7.1 × 10-3
2.0 × 10-3
1.0 × 10-2
a The experimentl error is (5%. b Not measured. c Too fast to be determined accurately.
Table 2. Isotropic g and ACo Values of [(TPP)Co(R)]+ at 298 K or
203 K in a Mixed MeCN/CHCl3 or EtCN/CHCl3 Solvent (1:1 v/v)
The effective magnetic moment, µeff, of [(N-MeTPP)Co]+ in
CD3CN/CDCl3 (1:1 v/v) is 4.4 µB at 298 K based on solution
magnetic susceptibility measurements using the Evans NMR
method (see Experimental Section). This magnetic moment
indicates an S ) 3/2 spin state which is consistent with a high
spin d7 Co(II) complex.27 An ESR signal could not be detected
for [(N-MeTPP)CoII]+ at the liquid He temperature, and this is
probably due to the fast relaxation time of Co(II).28
g (ACo, G)
R
MeCN/CHCl3
EtCN/CHCl3
Bu
Et
Me
Ph
2.034 (19.0)a
2.033 (15.2)a
2.026 (7.0)
2.015 (6.2)
2.027 (7.0)
2.015 (5.3)
The oxidation of [(N-MeTPP)CoII]+ with excess Fe(phen)3
3+
a Measured in liquid solutions at 203 K.
leads to a diamagnetic Co(III) complex. The ESR spectrum of
the product obtained after the migration and subsequent electron
transfer when (TPP)Co(Me) (1.0 × 10-3 M) is mixed with 2
and ACo (7.0 G) values which agree within experimental error
with the values obtained as the average of the anisotropic g
values (2.027 ( 0.002) and ACo values (6.1 ( 1.0 G). The
ESR spectra of [(TPP)Co(Et)]+ and [(TPP)Co(Bu)]+ could also
be measured in propionitrile (EtCN) and CHCl3 (1:1 v/v) at
203 K, and the g and ACo values under these experimental
conditions are listed in Table 2. Propionitrile was used in place
of acetonitrile in order to avoid freezing of the solvent. In
neither case, additional ligand superhyperfine structure attributed
to interaction of the unpaired electron with the nitrogen nuclei
of the porphyrin ligand could be observed.
equiv of Fe(phen)3 (2.0 × 10-3 M) in CH3CN/CHCl3 (1:1
3+
v/v) shows a signal at g ) 2.004 with ACo ) 9.0 G, which is
assigned to [(TPP)Co]2+. However, a comparison of the ESR
signal intensity obtained by double integration of the first
derivative signal for [(TPP)Co(Me)]+ and [(TPP)Co]2+ indicates
that only 5 ( 1% of the initial (TPP)Co(Me) complex is
converted to [(TPP)Co]2+
. This agrees with the fact that
methane was detected as a minor product (5 ( 0.5% yield) via
homolytic cleavage of the Co-Me bond in [(TPP)Co(Me)]+
(Vide supra). Thus, the overall stoichiometry of the reaction is
given by eq 3, where the hydrogen source of methane is not
included.
A comparison of the g and ACo values of [(TPP)Co(R)]+ (R
) Me, Et, Bu) to those of the [(TPP)CoIII]2+ porphyrin π radical
cation (g ) 2.003, ACo ) 6.0 G)21 and several non-porphyrin
Co(IV) complexes (g ) 2.020-2.033, ACo ) 12.3-15.5 G)7
suggests that [(TPP)Co(R)]+ can best be described as a d5 cobalt-
(IV) complex rather than as a cobalt(III) π radical cation. The
absence of superhyperfine splitting due to nitrogen nuclei of
the porphyrin ligand suggests that the unpaired electron is
predominantly localized in the π-type dxy rather than the σ-type
(TPP)Co(Me) + 2[Fe(phen)3]3+
f
0.95[(N-MeTPP)Co]2+ + 0.05[(TPP)Co]2+ + 0.05CH4 +
2[Fe(phen)3]2+ (3)
2
2
dx -y orbital. In contrast, an appreciable hyperfine due to
nitrogen nuclei (AN ) 2.8 G) is observed in the (TPP)CoCl2 π
radical cation, and there is a linear correlation between ACo and
AN when the counter anions are changed.21 In this case, the
Co superhyperfine is induced by spin polarization of the
σ-electron spin density in the Co-N bonds.21 In the case of a
tetrahedral tetrakis(1-norbonyl)cobalt(IV) complex,24 however,
the unpaired electron may be localized in a σ-type orbital, and
the ESR spectrum shows a large anisotropy (g ) 1.984, g| )
2.036) with large Co hyperfine splitting (|A | ) 120 G, |A|| )
40 G).25
This part of the mechanism is shown in Scheme 2 for the
case of (TPP)Co(Me).
The ESR spectra indicate that the initial electron-transfer
oxidation of (TPP)Co(Me) leads to [(TPP)Co(Me)]+ which has
a significant d5 cobalt(IV) character rather than to a Co(III)
porphyrin π radical cation. The formation of methane as one
of the oxidation products indicates that a homolytic cleavage
of the Co-Me bond of [(TPP)Co(Me)]+ occurs to give methyl
radicals, a minor part of which leads to the formation of methane
by hydrogen abstraction from the solvent.29 However, a major
part of the methyl radicals is “trapped” by a nitrogen of the
porphyrin ring to yield the final migrated product, [(N-MeTPP)-
Co]+. A radical addition to the nitrogen of the porphyrin ring
The ESR spectrum of [(TPP)Co(Ph)]+ exhibits hyperfine
structure due to 59Co, both at 77 and 203 K, and the determined
g and ACo values are listed in Table 2. The g (2.015 ( 0.001)
and ACo (6.2 ( 0.5 G) values of [(TPP)Co(Ph)]+ are significantly
smaller than those of the other [(TPP)Co(R)]+ derivatives (R
) Me, Et, and Bu). This indicates a significant contribution of
the limiting formal representation in [(TPP)Co(Ph)]+ as a d6
cobalt(III) porphyrin π cation radical as opposed to the other
[(TPP)Co(R)]+ derivatives (R ) Me, Et, and Bu) which have a
large amount of Co(IV) character.26
(26) A similar conclusion was made for a phenyl σ-bonded cobalt corrole,
(OEC)Co(Ph), where OEC is the trianion of 2,3,7,8-12,13,17,18-octaeth-
ylcorrole; (OEC)Co(Ph) can best be represented as a resonance hybrid
between a cobalt(III) π radical cation and a cobalt(IV) corrole, see: Will,
S.; Lex, J.; Vogel, E.; Adamian, V. A.; Van Caemelbecke, E.; Kadish, K.
M. Inorg. Chem. 1996, 35, 5577.
(27) The spin value for the d7 spin sate (S ) 3/2) of a high spin CoCl4
2-
is 4.59 µB, see: Cotton, A. F.; Wilkinson, G. AdVanced Inorganic Chemistry;
John Wiley & Sons: New York, 1980; p 772.
(28) The ESR spectra of low spin cobalt(II) complexes have been well-
characterized in frozen media but cannot be detected in solution because
of their short spin-lattice relaxation time, see: Walker, F. A. J. Am. Chem.
Soc. 1970, 92, 4235. Wayland, B. B.; Minkiewicz, J. V.; Abd-Elmageed,
M. E. J. Am. Chem. Soc. 1974, 96, 2795. Wayland, B. B.; Abd-Elmageed,
M. E. J. Am. Chem. Soc. 1974, 96, 4809.
(24) (a) Byrne, E. K.; Theopold, K. H. J. Am. Chem. Soc. 1989, 111,
3887. (b) Byrne, E. K.; Theopold, K. H. J. Am. Chem. Soc. 1987, 109,
1282. (c) Bower, B. K.; Tennent, H. G. J. Am. Chem. Soc. 1972, 94, 2512.
(25) Bower, B. K.; Findlay, M.; Chien, J. C. W. Inorg. Chem. 1974, 13,
759.

Electron Transfer Mechanism of Organocobalt Porphyrins
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2885
Scheme 2
results in cleavage of one nitrogen-cobalt bond, accompanied
by a reduction of Co(III) to give [(N-MeTPP)CoII]+ as a major
porphyrin product (Scheme 2).
Figure 6. Cyclic voltammograms measured at a scan rate of 0.10 V
s-1 for 1.0 mM (TPP)Co(R) where (a) R ) Ph or Me, T ) 298 K and
the solvent is MeCN/CHCl3 and (b) R ) Et, T ) 298 or 203 K and the
solvent is CH2Cl2.
The R group migration rate increases with increase in the
electron donor ability of R (in the order Ph < Me < Et < Bu,
see Table 1), and this trend suggests that an intramolecular
electron transfer from the R group to the Co(IV) metal of
[(TPP)Co(R)]+ is the rate-determining step in the homolytic
cleavage of the CoIV-R bond. In such a case, the higher
oxidation state of the cobalt center will facilitate an intramo-
lecular electron transfer from the R group to the metal center.
Thus, the stronger the donor ability of R, the more the d5 cobalt-
(IV) character in [(TPP)Co(R)]+ and the faster will be the
migration rate, as is experimentally observed in this study.
Further support for the mechanism shown in Scheme 2 is given
on the following pages.
Cyclic Voltammetry. Slow scan cyclic voltammograms of
(TPP)Co(R) (R ) Ph and Me) in MeCN/CHCl3 exhibit two
reversible oxidations as shown in Figure 6a. Both processes
are reversible at room temperature, and this is expected due to
the slow migration rates of [(TPP)Co(Ph)]+ and [(TPP)Co(Me)]+
(see Table 1). In contrast, the cyclic voltammograms of (TPP)-
Co(Et) and (TPP)Co(Bu) at 298 K show a well defined anodic
peak for the first oxidation but no coupled cathodic peak. This
is consistent with the facile migration of the alkyl group in the
singly oxidized species and is shown in Figure 6b for the case
of (TPP)Co(Et) at 298 K. The voltammograms of the Et and
Bu derivatives become reversible at 203 K as shown for (TPP)-
Co(Et) in Figure 6b, since the migration rates are significantly
slower at this temperature. The one-electron oxidation potentials
of (TPP)Co(R) in CH2Cl2 or MeCN/CHCl3 at 298 and 203 K
are listed in Table 3.
Table 3. Oxidation Potentials (V vs SCE) of (TPP)Co(R) in
CH2Cl2 and MeCN/CHCl3 Mixed Solvent Containing 0.1 M TBAP
at 298 and 203 K
E1/2, V vs SCE
R
298 K
203 K
n-Bu
Et
Me
Ph
0.96a (0.84a,b
0.95a (0.83a,b
0.95 (0.84c)
0.98 (0.79b)
)
)
0.81
0.84
0.85
0.89
a Anodic peak potential, Epa at 0.1 V s-1
.
b Measured in MeCN/
CHCl3 (4:1 v/v). c Measured in MeCN/CHCl3 (1:4 v/v).
electron donor ability of the σ-bonded ligand upon going from
R ) Bu to R ) Ph (Table 3). This lack of a dependence on
the (TPP)Co(R) oxidation potential by the σ-bonded ligand is
consistent with an assignment of the HOMO orbital of (TPP)-
Co(R) as a π-type dxy orbital which is orthogonal to the axial
σ-orbital (Vide supra). The oxidation potentials of (TPP)Co-
(R) in CH2Cl2 at 298 K are significantly more positive than the
potentials in MeCN/CHCl3 (shown in parentheses in Table 3).
Such a difference in potentials between the two solvents
indicates that MeCN acts as a sixth axial ligand to give [(TPP)-
Co(R)(MeCN)]+ and the electron donation of the ligand results
in a negative shift of the oxidation potential for (TPP)Co(R) in
MeCN/CHCl3 as compared to the five-coordinate compound
in CH2Cl2. In this case, the sixth axial ligand may act as a
π-donor.
Effects of Base on Oxidation Potentials of (TPP)Co(R).
The addition of pyridine to an MeCN solution of (TPP)Co(Et)
results in a significant change in the UV-vis spectrum. From
these changes, the formation constants for axial ligand binding
of the six substituted pyridines to (TPP)Co(R) were determined
in MeCN depending on the pKa values of the substituted
pyridines which vary from 0.67 for 3,5-Cl2Py to 9.71 for
The first oxidation potential of (TPP)Co(R) is reversible in
CH2Cl2 at 203 K and changes only slightly with decrease in
(29) The formation of methane as a minor product in the electron-transfer
oxidation of (TPP)Co(Me) is reminiscent of minor radical products produced
in dealkylation of alkylcobalt(III) complexes with iodine, which occurs via
electron transfer from alkylcobalt(III) complexes to iodine, followed by
the facile homolytic cleavage for the cobalt-carbon bond of the alkylcobalt-
(IV) complexes, see: Ishikawa, K.; Fukuzumi, S.; Tanaka, T. Inorg. Chem.
1989, 28, 1661.

2886 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Fukuzumi et al.
Table 4. Halfwave Potentials (V vs SCE) for the Oxidation of
(TPP)Co(R)(L) at 298 K in CH2Cl2 or MeCN/CHCl3 Containing 1.0
× 10-2 M Substituted Pyridine (L) and 0.1 M TBAP
Table 5. Rate Constants (logkmig) for the Metal to Nitrogen R
Group Migration of [(TPP)Co(R)]+ and [(TPP)Co(R)(L)]+ at 298 K
in MeCN Containing 2.0 × 10-2 M Substituted Pyridine
sixth axial ligand, L
type pKa
none
σ-bonded ligand, R
sixth axial ligand, L
σ-bonded ligand, R logkmig
no.
Et
Me
0.95 (0.84b)
0.91
Ph
no.
type
pKaa
Me
Ph
0.84a
0.77a
0.74a
0.72a
0.68a
0.69a
0.65a
0.63a
0.98 (0.79c)
0.91 (0.79c)
0.89 (0.78c)
0.86 (0.75c)
0.78 (0.75c)
0.78 (0.75c)
0.76 (0.73c)
0.72 (0.69c)
none
-1.6
-1.8
-1.6
-1.5
-2.0
-1.6
-2.5
b
-2.9
-2.7
-3.0
-2.8
-2.3
1
2
3
4
5
6
7
3,5-Cl2Py
4-CNPy
3-ClPy
0.67
1.86
2.81
5.28
5.79
6.46
9.71
1
2
3
4
5
6
7
3,5-Cl2Py
4-CNPy
3-ClPy
Py
3-MePh
3,4-Me2Py
4-NMe2Py
0.67
1.86
2.81
5.28
5.79
6.46
9.71
0.88
0.84
0.76
0.76
0.73
0.66
Py
3-MePy
3,4-Me2Py
4-NMe2Py
-1.5
b
a At 203 K. b Measured in MeCN/CHCl3 (1:4 v/v). c Measured in
MeCN/CHCl3 (4:1 v/v).
a Taken from ref 30. b The oxidation of (TPP)Co(R)(L) by
[Fe(phen)3]3+ does not occur in the presence of 4-NMe2Py (no. 7) which
may be due to an oxidation of the substituted pyridine instead of
(TPP)Co(R)(L).
4-NMe2Py.30 A summary of the values of formation constants
is given in Supporting Information. The E1/2 values for
oxidation of (TPP)Co(R) in CH2Cl2 containing a substituted
pyridine (L) are shifted negatively as compared to the half-
wave potentials for oxidation of the same compound in neat
CH2Cl2. The difference in E1/2 between the five- and six-
coordinate complexes (∆E1/2) is related to the formation constant
as well as to the ligand concentration in solution, [L]. This is
shown in eq 4, where Kred and Kox are the formation constants
of (TPP)Co(R)(L) and [(TPP)Co(R)(L)]+ (eqs 5 and 6, respec-
tively).31
∆E1/2 ) (2.3RT/F)log{(1 + Kox[L])/(1 + Kred[L])} (4)
Kred
(TPP)Co(R) + L y z (TPP)Co(R)(L)
(5)
(6)
Kox
[(TPP)Co(R)]+ + L y z [(TPP)Co(R)(L)]+
The E1/2 values for oxidation of (TPP)Co(R) (R ) Et, Me,
and Ph) in CH2Cl2 and CH2Cl2 containing L (1.0 × 10-2 M)
are summarized in Table 4. The E1/2 values of (TPP)Co(Ph) in
the presence of L (1.0 × 10-2 M) were also determined in
MeCN/CHCl3 (4:1 v/v) and are listed in parentheses in Table
4.
Effects of Base on Migration Rate. Migration rates of
[(TPP)Co(R)(L)]+ (R ) Ph and Me) were determined at 298 K
by monitoring the increase in absorbance at λmax due to [(N-
RTPP)Co(L)]+ in MeCN containing L (2.0 × 10-2 M). The
migration rate constants (kmig) are listed in Table 5 along with
pKa values of the substituted pyridines. As shown in Figure
7a, the kmig values of [(TPP)Co(Ph)(L)]+ increase by 1-2 orders
of magnitude with increase in the ligand pKa.
In contrast to the above linear relationship, the variations in
logkmig values of [(TPP)Co(Me)(L)]+ with ligand pKa show a
nonlinear relationship, i.e., logkmig first increases slightly, reaches
a maximum for 3-chloropyridine (no. 3 in Table 5), and then
decreases for all substituted pyridines with pKa values larger
than that of 3-chloropyridine (see Figure 8a).
Correlations of E1/2 with the pKa of the axial ligand are shown
in Figures 7b and 8b for the oxidation of (TPP)Co(Ph)(L) and
(TPP)Co(Me)(L), respectively. In each case, the E1/2 values
become more negative with increase in pKa of the substituted
pyridine. Thus, the axial ligand coordination on cobalt results
in an increased electron density on the metal center from which
Figure 7. Correlation between the pKa of the substituted pyridine and
(a) the migration rate constant of [(TPP)Co(Ph)(L)]+ in MeCN at 298
K, (b) the oxidation potential of (TPP)Co(Ph)(L) in CH2Cl2, and (c)
the formation constant of (TPP)Co(Ph) in MeCN.
an electron is removed. The formation constants (logK) of
(TPP)Co(R)(L) (R ) Ph and Me) also correlate with the pKa
of the axial ligand, and these relationships are shown in Figures
7c and 8c. Both logK values increase linearly with increase in
the ligand pKa.
(30) Shoefield, K. S. Hetero-Aromatic Nitrogen Compounds; Plenum
Press: New York, 1967; p 146.
(31) For similar analysis, see: (a) Fukuzumi, S.; Kondo, Y.; Tanaka, T.
J. Chem. Soc., Chem. Commun. 1985, 1053. (b) Fukuzumi, S.; Kondo, Y.;
Mochizuki, S.; Tanaka, T. J. Chem. Soc., Perkin Trans. 2 1989, 1753.
Effects of Base on ESR Spectra of Singly Oxidized
Complex. The ESR spectra of [(TPP)Co(Ph)]+ and [(TPP)-
Co(Me)]+ are identical in CH2Cl2 and have the same g (2.008)
and ACo (5.0 G) values (Table 6). The g and ACo values are

Electron Transfer Mechanism of Organocobalt Porphyrins
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2887
Figure 9. Experimental ESR spectra and computer simulation spectra
of (a) [(TPP)Co(Ph)(3-MePy)]+ (5.0 mM) and (b) [(TPP)Co(Me)(3-
MePy)]+ (5.0 mM) generated by addition of 5.0 mM [Fe(phen)]33+ to
(TPP)Co(Ph) and (TPP)Co(Me) in MeCN/CHCl3 (v:v 1/1) containing
0.40 M 3-MePy at 208 K.
containing a bound MeCN molecule, [(TPP)Co(Me)(MeCN)]+
than in [(TPP)Co(Ph)(MeCN)]+ (see Table 6). However, when
a strong base (L) is added to the MeCN/CHCl3 solution, the
resulting [(TPP)Co(Ph)(L)]+ and [(TPP)Co(Me)(L)]+ species
again have almost identical ESR spectra. The striking similarity
in the ESR spectra of [(TPP)Co(Ph)(L)]+ and [(TPP)Co(Me)-
(L)]+ is shown in Figure 9 for the case of L ) 3-MePy. Figure
9 also illustrates the computer-simulated spectra of the two
complexes.
Figure 8. Correlation between the pKa of the substituted pyridine and
(a) the migration rate constant of [(TPP)Co(Me)(L)]+ in MeCN at 298
K, (b) the oxidation potential of (TPP)Co(Me)(L) in CH2Cl2, and (c)
the formation constant of (TPP)Co(Me) in MeCN.
Table 6. Isotropic g and ACo Values of [(TPP)Co(R)]+ in CH2Cl2
and (TPP)Co(R)(L in MeCN/CHCl3 (1:1 v/v) Containing 0.40 M
Ligands at 298 Ka
The ESR parameters of [(TPP)Co(Ph)(L)]+ and [(TPP)Co-
(Me)(L)]+ are summarized in Table 6. The g value of [(TPP)-
Co(Ph)(L)]+ increases with increase in the ligand pKa to reach
a maximum value which is consistent with formation of a
genuine Co(IV) complex.7 In the case of the Me derivative,
the coordination of MeCN is strong enough to change the
oxidation state from Co(III) for [(TPP)Co(Me)]+ in CH2Cl2 to
Co(IV) for [(TPP)Co(Me)(MeCN)]+ in MeCN/CHCl3. In
addition, the coordination of a base (L) which is stronger than
MeCN results in no further significant changes in either the g
or ACo values of [(TPP)Co(R)(L)]+ (R ) Ph and Me). The fact
that the g and ACo values do not vary upon changing the pKa of
the ligand also confirms that [(TPP)Co(R)(L)]+ can best be
described as a d5 cobalt(IV) complex.
sixth axial ligand, L
type pKa
noneb
R ) Ph
R ) Me
no.
g
ACo, G
g
ACo, G
2.008
2.015
2.016
2.016
2.016
2.017
2.023
2.023
2.025
5.0
5.3
7.9
8.2
8.4
2.008
2.027
2.020
2.021
2.021
2.023
2.023
2.023
2.020
5.0
7.0
9.0
MeCN
1
2
3
4
5
6
7
3,5-Cl2Py
4-CNPy
3-ClPy
0.67
1.86
2.81
5.28
5.79
6.46
9.71
9.9
11.5
12.6
12.6
12.6
12.2
Py
8.6
3-MePy
3,4-Me2Py
4-NMe2Py
12.1
12.1
11.7
a Cationic species was produced in an ESR cell by oxidation with
[Fe(phen)3](ClO4)3 and measurements made immediately afterward.
b Measured in CH2Cl2.
Site of Electron Transfer Prior to Migration. The above
results can be explained by the reactions shown in Scheme 3
for (TPP)Co(Me) and (TPP)Co(Ph) prior to the migration step.
A transient [(TPP)CoIII(R)]+ species is invariably formed upon
oxidation in CH2Cl2 but this is not the case in MeCN where
either [(TPP)CoIII(R)(MeCN)]+ or [(TPP)CoIV(R)(MeCN)]+ are
formed depending on the specific σ-bonded axial ligand.
The higher the oxidation state of the singly oxidized
compound, i.e., the more its Co(IV) character, the faster the
migration rate. This result is best seen in the case of the Ph
derivative where the Co(IV) character first increases upon going
similar to those of the [(TPP)CoIII]2+ porphyrin π radical cation
(g ) 2.003, ACo ) 6.0 G),21 suggesting that both [(TPP)Co-
(Ph)]+ and [(TPP)Co(Me)]+ in a noncoordinating solvent such
as CH2Cl2 can be best described as a cobalt(III) π radical cation.
On the other hand, when the ESR spectra of [(TPP)Co(Ph)]+
and [(TPP)Co(Me)]+ are measured in MeCN/CHCl3, the
isotropic g (2.027 ( 0.001) and ACo (7.0 G) values of [(TPP)-
Co(Me)(MeCN)]+ are different from those of [(TPP)Co(Ph)-
(MeCN)]+ (g ) 2.015 and ACo ) 5.3 G), reflecting more Co(IV)
character in the singly oxidized σ-bonded methyl derivative

2888 J. Am. Chem. Soc., Vol. 120, No. 12, 1998
Fukuzumi et al.
Scheme 3
Figure 10. Plot of kobs vs [TEMPO] for the anaerobic thermolysis of
(TPP)Co(Bu) (5.0 × 10-6 M) in MeCN at various temperatures.
from [(TPP)Co(Ph)]+ to [(TPP)Co(Ph)(L)]+ and then further
increases with increase in the ligand pKa, thus resulting in an
acceleration of the migration rate. In contrast, [(TPP)Co(Me)-
(MeCN)]+ already contains a Co(IV) metal center and a
conversion of [(TPP)CoIV(Me)(MeCN)]+ to [(TPP)CoIV(Me)-
(L)]+ by axial ligand coordination thus increases the electron
density on the cobalt, resulting in a slower migration rate. Thus,
the cleavage of the cobalt-carbon bond in the migration reaction
occurs via a rate-determining intramolecular electron transfer
from R to Co, which results in a metal to nitrogen migration of
the R group.
Comparison of Bond Energies for Co(III)- and Co(IV)-
Carbon Bonds. Cobalt-carbon bond dissociation energies of
organocobalt(III) complexes have previously been determined
using the kinetic analysis of homolytic cleavage of cobalt-
carbon bond.32,33 The cobalt-carbon bond dissociation energies
of (TPP)Co(R) can be determined in a similar fashion and
compared with the bond energies for the singly oxidized [(TPP)-
Co(R)]+ complexes. The rates of homolytic cleavage of the
cobalt-carbon bond in (TPP)Co(R) were determined by moni-
toring both the decrease of the 406 nm band and the increase
of bands due to (TPP)Co formation in MeCN containing a
TEMPO radical trap.32-34 The rates obey first-order kinetics
with respect to the (TPP)Co(R) concentration (eq 7). The
observed first-order rate constants increase with the TEMPO
concentration and approach a constant value as shown in Figure
10 for the case of (TPP)Co(Bu). Such a saturated dependence
Figure 11. Plot of kobs-1 vs [TEMPO]-1 for the anaerobic thermolysis
of (TPP)Co(Bu) (5.0 × 10-6 M) in MeCN at various temperatures.
Scheme 4
to this scheme, eq 8 can be derived and will show a linear
correlation between kobs and [TEMPO]-1. Such linear plots
-1
can be experimentally observed as shown in Figure 11.35
d[(TPP)Co(R)]
k-1[(TPP)Co]
-
) kobs[(TPP)Co(R)]
(7)
1
1
dt
)
+
(8)
kobs k1
k1k2[TEMPO]
of the rate constant on the TEMPO concentration indicates that
radicals generated by the homolytic cleavage of the cobalt-
carbon bond are trapped by the TEMPO radical before they
recombine with the Co(II) center (see Scheme 4). According
The k1 values for homolytic cleavage of the cobalt-carbon
bond in (TPP)Co(R) are obtained from the intercepts. The
activation parameters (∆Hq and ∆Sq) are obtained from Eyring
plots and these values are listed in Table 7. The activation
enthalpies (∆Hq) and entropies (∆Sq) for the R group migration
(32) (a) Halpern, J. Acc. Chem. Res. 1982, 15, 238. (b) Halpern, J. Pure
Appl. Chem. 1983, 55, 1059.
of [(TPP)Co(R)]+ are obtained from the Eyring plots of kmig
,
(33) (a) Tsou, T.-T.; Loots, M.; Halpern, J. J. Am. Chem. Soc. 1982,
104, 623. (b) Ng, F. T. T.; Rempel, G. L.; Halpern, J. J. Am. Chem. Soc.
1982, 104, 621. (c) Geno, M. K.; Halpern, J. J. Am. Chem. Soc. 1987, 109,
1238. (d) Finke, R. G.; Smith, B. L.; Mayer, B. J.; Molinero, A. A. Inorg.
Chem. 1983, 22, 3677. (e) Riordan, C. G.; Halpern, J. Inorg. Chim. Acta
1996, 243, 19.
(34) Koenig, T.; Finke, R. G. J. Am. Chem. Soc. 1988, 110, 2657; Garr,
C. D.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 10440; Garr, C. D.; Finke,
R. G. Inorg. Chem. 1993, 32, 4414.
and these values are also listed in Table 7. The ∆Sq values of
(TPP)Co(R) and [(TPP)Co(R)(MeCN]+ do not vary significantly
with the different R groups (see Table 7), and the difference in
reactivity between the neutral and oxidized complexes is
(35) The [Co(II)]/[TEMPO] dependence in eq 7 has been well established.
See refs 32-34.

Electron Transfer Mechanism of Organocobalt Porphyrins
J. Am. Chem. Soc., Vol. 120, No. 12, 1998 2889
Table 7. Activation Enthalpies (∆Hq) and Entropies (∆Sq) for
Homolytic Cleavage of the Co-R Bond of (TPP)Co(R) and for R
Group Migration of [(TPP)Co(R)]+
Co(R) are similar to those for other organocobalt(III) complexes
with the same σ-bonded organic groups31,32 but are significantly
larger than those for the corresponding Co(IV) complexes. Thus,
the cobalt-carbon bond in the high oxidation state Co(IV)
complex becomes significantly weaker as compared to that in
the lower oxidation state Co(III) complex.37
∆Hq, kcal mol-1 a
∆Sq, cal K-1 mol-1 a
R
(TPP)Co(R) [(TPP)Co(R)]+ (TPP)Co(R) [(TPP)Co(R)]+
Bu
Et
Me
Ph
19.3
19.8
19.7
8.4
8.7
14.5
15.0
-19.7
-19.1
-20.0
-16.3
-18.1
-18.8
-21.7
Acknowledgment. This work was partially supported by an
International Scientific Research Program (No. 08044083) from
the Ministry of Education, Science, Culture and Sports, Japan.
K.M.K. also acknowledges support from the Robert A Welch
Foundation (Grant E-680).
a The experimental error is (5%.
therefore ascribed to differences in the ∆Hq values. It is
important to note that the ∆Sq value for homolytic cleavage of
the cobalt-carbon bond in (TPP)CoIII(Me) (-20.0 cal K-1
mol-1) agrees well with the ∆Sq value for the R group migration
of [(TPP)CoIV(Me)]+ (-18.8 cal K-1 mol-1). Such an agree-
ment between the two values also supports the migration
mechanism shown in Scheme 2, where the homolytic cleavage
of the Co-R bond in [(TPP)CoIV(R)]+ is the rate-determining
step for the R migration.
Supporting Information Available: Derivation of eq 1, a
typical kinetic figure, tables of spectral data for neutral and
oxidized (TPP)Co(R) and R-migrated products, figures of the
spectral changes observed in the absence and presence of various
substituted pyridines, table of formation constants, and a figure
of spectral changes for the complexation of (TPP)Co(R) with
substituted pyridines (9 pages). See any current masthead page
for ordering and Web access instructions.
The ∆Hq values can be converted to the Co(III)-R dissocia-
tion energy (DCo-R) by using eq 9, where Fc is the ratio of cage
recombination to the sum of all competing cage processes and
JA973257E
(36) Koenig, T. W.; Hay, B. P.; Finke, R. G. Polyhedron 1988, 7, 1499.
(37) The facile homolytic cleavage for the cobalt-carbon bonds of
alkylcobalt(IV) complexes has been well-documented as compared to the
slow cleavage of the corresponding alkylcobalt(III) complexes, which
requires thermal or photochemical activation, see: Ishikawa, K.; Fukuzumi,
S.; Goto, T.; Tanaka, T. J. Am. Chem. Soc. 1990, 112, 1577. Fukuzumi, S.;
Ishikawa, K.; Tanaka, T. Organometallics 1987, 6, 358. Ishikawa, K.;
Fukuzumi, S.; Tanaka, T. Bull. Chem. Soc. Jpn. 1987, 60, 563. Fukuzumi,
S.; Kitano, T.; Ishikawa, M.; Matsuda, Y. Chem. Phys. 1993, 176, 337.
DCo-R ) ∆Hq - Fc∆Hq
(9)
η
∆Hq is the activation enthalpy for viscous flow.34,36 Since 0
η
< Fc < 1 and 1 < ∆Hq < 2 kcal mol-1, the Fc∆Hq value
η
η
may be taken as 1 ( 1 kcal mol-1. The DCo-R values for (TPP)-