Synthesis and Electrochemical Reactivity of σ-Bonded and N-Substituted Cobalt Porphycenes

Article abstract of DOI:10.1021/ic971534c

The first synthesis and characterization of σ-bonded and N-substituted cobalt porphycenes is reported. The investigated compounds are represented as (Pc)Co(R) and (N-CH₃OEPc)CoCl, where R is CH₃ or C₆H₅, Pc is the dianion of 2,3,6,7,12,13,16,17-octaethylporphycene (OEPc), 2,7,12,17-tetrapropylporphycene (TPrPc), or 2,7,12,17-tetraethyl-3,6,13,16-tetramethylporphycene (EtioPc), N-CH₃OEPc is the monoanion of N-methyl-2,3,6,7,12,13,16,17-octaethylporphycene. Each σ-bonded (Pc)Co(R) derivative can be reversibly reduced or oxidized by two electrons, but a slow migration of the σ-bonded R group occurs following electrogeneration of [(Pc)Co(R)]^+. leading, as a final product, to an N-substituted cobalt(II) porphycene which is also electroactive and undergoes two reductions in PhCN. The singly reduced product of this reaction is formulated as a Co(II) π-anion radical which undergoes a slow back-migration of the CH₃ group to regenerate (OEPc)Co(CH₃).

Full text of DOI:10.1021/ic971534c

Inorg. Chem. 1998, 37, 2693-2700
2693
Synthesis and Electrochemical Reactivity of σ-Bonded and N-Substituted Cobalt
Porphycenes
K. M. Kadish,*,† P. L. Boulas,† M. Kisters,‡ E. Vogel,*,‡ A. M. Aukauloo,§ F. D’Souza,†,| and
R. Guilard*,§
Department of Chemistry, University of Houston, Houston, Texas 77204-5641, Institut fu¨r Organische
Chemie, Universita¨t zu Ko¨ln, Greinstrasse 4, 50939 Ko¨ln, Germany, and Laboratoire d’Inge´nierie
Mole´culaire pour la Se´paration et les Applications des Gaz Associe´ au CNRS (LIMSAG), UMR 5633,
Faculte´ des Sciences Gabriel, Universite´ de Bourgogne, 6 Boulevard Gabriel, 21100 Dijon, France
ReceiVed December 5, 1997
The first synthesis and characterization of σ-bonded and N-substituted cobalt porphycenes is reported. The
investigated compounds are represented as (Pc)Co(R) and (N-CH3OEPc)CoCl, where R is CH3 or C6H5, Pc is the
dianion of 2,3,6,7,12,13,16,17-octaethylporphycene (OEPc), 2,7,12,17-tetrapropylporphycene (TPrPc), or 2,7,-
12,17-tetraethyl-3,6,13,16-tetramethylporphycene (EtioPc), N-CH3OEPc is the monoanion of N-methyl-2,3,6,7,-
12,13,16,17-octaethylporphycene. Each σ-bonded (Pc)Co(R) derivative can be reversibly reduced or oxidized
by two electrons, but a slow migration of the σ-bonded R group occurs following electrogeneration of [(Pc)Co-
(R)]+• leading, as a final product, to an N-substituted cobalt(II) porphycene which is also electroactive and undergoes
two reductions in PhCN. The singly reduced product of this reaction is formulated as a Co(II) π-anion radical
which undergoes a slow “back-migration” of the CH3 group to regenerate (OEPc)Co(CH3).
Introduction
of new conducting materials or in the synthesis of complexes
possessing metal-metal interactions that can be used as catalysts
σ-Bonded cobalt(III) porphyrins1-3 along with other orga-
nocobalt complexes4-9 have been used as model compounds
to study the release of adenosyl or methyl radicals by homolysis
of the cobalt-carbon bond of adenosylcobalamin (coenzyme
B12, AdoB12) or methylcobalamin (MeB12).10,11 Cobalt por-
phyrins have also been studied as precursors in the synthesis
for the reduction of dioxygen such as the face-to-face dipor-
phyrin complexes.12-15 Interestingly, some organocobalt por-
phyrin complexes were also recently found to efficiently initiate
the polymerization of acrylates by homolysis of the cobalt-
carbon bond.16
Porphycenes,17,18 like their parent porphyrin isomers, can
coordinate a variety of metal ions in different oxidation
states.17-23 A number of spectral and electrochemical studies
* To whom correspondence should be addressed.
† University of Houston.
‡ Universita¨t zu Ko¨ln.
§ Universite´ de Bourgogne.
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| Present address: Department of Chemistry, The Wichita State Uni-
versity, 1845 Fairmount, Wichita, KS 67260-0051.
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S0020-1669(97)01534-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/09/1998

2694 Inorganic Chemistry, Vol. 37, No. 11, 1998
Kadish et al.
have been carried out on metalloporphycenes,21-23 but only two
derivatives containing a cobalt central metal ion have, to date,
been described in the literature.21a,h,23i These are (OEPc)CoII
and (TPrPc)CoII, where OEPc and TPrPc are the dianions of
2,3,6,7,12,13,16,17-octaethylporphycene and 2,7,12,17-tetra-
propylporphycene, respectively. This present paper reports the
first synthesis and electrochemical characterization of cobalt
porphycenes with metal-carbon and nitrogen-carbon bonds.
The investigated compounds are represented by (Pc)Co(R) and
(N-CH3OEPc)CoCl, where Pc is the dianion of 2,3,6,7,12,13,-
16,17-octaethylporphycene (OEPc), 2,7,12,17-tetrapropylpor-
phycene (TPrPc), or 2,7,12,17-tetraethyl-3,6,13,16-tetrameth-
ylporphycene (EtioPc), R is CH3 or C6H5, and N-CH3OEPc is
the monoanion of N-methyl-2,3,6,7,12,13,16,17-octaethylpor-
phycene.
quite planar,19,21 and they thus represent compounds whose
electrochemical behavior can be compared to that of the less
planar porphyrin macrocycles. This study of (Pc)Co(R) and
[(N-ROEPc)Co]+ thus provides an opportunity to investigate
how differences in the physicochemical properties of σ-bonded
and cobalt N-alkylated or arylated porphycenes and porphyrins
can be related to each other and also to their porphyrin ana-
logues. It was of special interest to determine if the cobalt-
carbon bond is affected during reduction or oxidation of the
σ-bonded cobalt porphycene and specifically if a cleavage of
the cobalt-carbon bond and migration of the axial ligand occur
after oxidation to give the corresponding N-arylated or N-
alkylated cobalt(II) compound.1,32-34 It was also of interest to
investigate the reduction of [(N-ROEPc)Co]+ since, in the case
of porphyrins, the singly reduced N-substituted Co(II) porphyrin
is unstable and undergoes a “back-migration” to re-form the
initial σ-bonded complex.32,34a,b,35,36 N-substituted porphyrins
are able to stabilize metal ions in low oxidation states35,37-40
but porphycenes do not show such ability due, in large part, to
the smaller size of the central cavity. Thus, it was also of
interest to monitor the site of electron transfer along with the
possible electrochemically initiated migration of (N-CH3OEPc)-
CoCl to (OEPc)Co(CH3).
A great deal of effort has been directed toward under-
standing how the nonplanarity of a given tetrapyrrole macro-
cyclic compounds can be related to their chemical and physi-
cal properties24-31 and especially their redox properties.24a,28-31
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σ-Bonded and N-Substituted Cobalt Porphycenes
Inorganic Chemistry, Vol. 37, No. 11, 1998 2695
to give violet lustrous needles. Yield: 34 mg (50%). Mp: 274-275
°C (dec, CH2Cl2/hexane). 1H NMR (CDCl3): δ ) 9.47 (s, 4H, H-9,
10, 19, 20), 5.17 (t, 1H, para-H-phenyl)), 4.57 (m, 2H, meta-H-phenyl),
3.97 (m, 8H, H-3a, 6a, 13a, 16a), 3.91 (m, 8H, H-2a, 7a, 12a, 17a),
1.67 (t, 12H, H-3b, 6b, 13b, 16b), 1.77 (t, 12H, H-2b, 7b, 12b, 17b),
0.05 (d of d, 2H, ortho-H-phenyl). 13C NMR (CDCl3): δ ) 149.8,
147.3, 146.3, 137.8, 133.3, 121.9, 120.7, 107.0, 21.0, 20.3, 18.5, 18.1.
MS [m/z (%)]: 668 (43) [M+], 591 (100) [M - C6H5]. IR: ν ) 3052,
2964, 2930, 2870, 1509, 1467, 1302, 1063, 1013, 952, 728 cm-1. UV-
vis (CH2Cl2) [λ, nm (ꢀ)]: 302 (26 000), 361sh (37 000), 390 (71 000),
591 (31 300), 612sh (19 900).
Experimental Section.
All common solvents were purified in an appropriate manner and
distilled under argon prior to use. Pyridine, for the electrochemical
studies, was of the highest available purity (Aldrich Chemical Co.,
packaged under nitrogen in Sure/Seal bottles) and was used without
further purification. Tetra-n-butylammonium perchlorate (TBAP)
(Eastman Kodak Co.) was recrystallized from ethanol and then dried
under reduced pressure at 40 °C prior to use. Tetra-n-butylammonium
chloride (TBACl) (Eastman Kodak Co.) was used as received.
(OEPc)CoCl. (OEPc)Co was synthesized as described in the
literature.21a To a suspension of (OEPc)Co (120 mg, 0.2 mmol) in
methanol (140 mL) was added 4.8 mL of concentrated HCl. After
vigorous stirring for 15 h, the solution was filtered and concentrated
to about 30 mL by vacuum. CH2Cl2 (100 mL) was added and the
resulting solution washed twice with water and then dried over Na2-
SO4 before removal of the solvent by vacuum. The residue was
dissolved in CH2Cl2 and hexane added which led to precipitation of a
black powder. Yield: 60 mg (48%). Mp: >300 °C (CH2Cl2/hexane).
1H NMR (CD3OD): δ ) 10.25 (s, 4H, H-9, 10, 19, 20), 4.45 (q, 8H,
H-3a, 6a, 13a, 16a), 4.32 (q, 8H, H-2a, 7a, 12a, 17a), 1.98 (t, 12H,
H-2b, 7b, 12b, 17b), 1.90 (t, 12H, H-3b, 6b, 13b, 16b). 13C NMR
(CD3OD): δ ) 153.8, 149.3, 147.7, 141.3, 108.2, 22.2, 21.5, 19.5,
19.3. MS [m/z (%)]: 626 (11) [M+], 591 (100) [M-Cl]. IR: ν ) 2967,
2931, 2871, 1508, 1450, 1406, 1375, 1302, 1056, 1012, 948 cm-1. UV-
vis (CH2Cl2) [λ, nm (ꢀ)]: 391 (62 000), 582sh (9900), 618 (23 100).
(OEPc)Co(CH3). Methylmagnesium iodide in diethyl ether was
added dropwise to a solution of (OEPc)CoCl (60 mg, 0.1 mmol) in 12
mL of absolute 1,2-dimethoxyethane under an argon atmosphere at 0
°C. The progress of the reaction was monitored by thin-layer
chromatography. After no more starting material could be detected,
the solution was then poured into a cold mixture containing 50 mL of
CH2Cl2 and 10 mL of saturated NH4Cl. The organic phase was
separated, washed three times with water (20 mL), and then dried over
Na2SO4. The solvent was removed under vacuum and the residue
crystallized twice from CH2Cl2/hexane to yield violet prisms. Yield:
28 mg (45%). Mp: 178-179 °C (dec, CH2Cl2/hexane). 1H NMR
(C6D6): δ ) 9.34 (s, 4H, H-9, 10, 19, 20), 3.96 (m, 8H, H-3a, 6a, 13a,
16a), 3.79 (q, 8H, H-2a, 7a, 12a, 17a), 1.74 (t, 12H, H-2b, 7b, 12b,
(N-CH3OEPc)CoCl. A 10 mL chloroform solution containing 53
mg (0.1 mM) of (OEPc)H2, 0.5 g of sodium formate, and 0.5 mL (8
mM) of iodomethane was refluxed for 15 h. After evaporation of the
solvent, the residue was dissolved in chloroform, washed with water,
and chromatographed on aluminum oxide (activity III, chloroform).
The second fraction was collected and diluted with chloroform to a
volume of 20 mL. Cobalt(II) chloride hexahydrate was added and the
mixture refluxed for 3 h. The organic layer was then washed twice
with water and chromatographed on silica (3:1 chloroform/ethyl
acetate). Crystallization from 1:1 chloroform/hexane yields 42.6 mg
(71%) of green needles. Mp: 194-196 °C. MS [m/z (%)]: 641 (5)
[M+], 606 (30) [M - Cl]+, 591 (100) [M - CH3 - Co - Cl]+. IR:
ν ) 2966, 2933, 2873, 1488, 1453, 1128, 1014, 1002, 952, 304 cm-1
.
UV-vis (CH2Cl2) [λ (ꢀ)]: 243sh (17 000), 335sh (20 000), 402 (39 100),
625sh (14 300), 666 nm (19 900).
(N-CH3OEPc)H2+ClO4-. A 10 mL chloroform solution containing
53 mg (0.1 mM) of (OEPc)H2, 0.5 g of sodium formate, and 0.5 mL
(8 mM) of methyl iodide was refluxed for 15 h. The solvent was then
removed by vacuum and the residue dissolved in chloroform, washed
with water, and chromatographed on basic alumina with chloroform.
The second, blue fraction was collected and shaken twice with 10%
perchloric acid. Crystallization from CH2Cl2/hexane yielded 51.4 mg
(80%) of blue needles. Mp: 165-167 °C (dec). 1H NMR (CDCl3):
δ ) 9.72 (d, 1H, H-19), 9.68-9.61 (m, 2H, H-8, 10), 9.61 (d, 1H,
H-20), 4.11-3.67 (m, 12H, H-6a, 12a, 13a, 16a, 17a), 3.45-3.09 (m,
4H, H-2a, 3a), 1.98 (br s, 1H, NH), 1.81-1.35 (m, 18H, H-6b, 7b,
12b, 13b, 16b, 17b), 1.31-1.18 (m, 6H, H-2b, 3b), -3.57 (s, 3H,
N-CH3), -3.32 (br, s, 1H, NH). 13C NMR (CDCl3): δ ) 153.66,
151.14, 150.68, 147.41, 146.95, 141.62, 141.14, 140.62, 140.27, 139.92,
136.44, 136.13, 134.19, 132.44, 132.14, 130.78, 116.58, 115.51, 115.03,
111.73, 30.39, 21.22, 21.04, 20.87, 20.49, 19.95, 19.71, 19.63, 17.98,
17.75, 17.70, 17.50, 17.41, 16.06, 15.27. MS [m/z (%)]: 549 (100)
[M]+, 550 (100) M+H+. IR: ν ) 2975, 2937, 2877, 1539, 1492, 1455,
1297, 1241, 1112, 1057, 1017, 996, 957, 915, 662, 458 cm-1. UV-
vis [λ, nm (ꢀ)] ) 274 (9900), 327sh (15 700), 394 (82 400), 421sh
(39 700), 608 (19 700), 683 (25 000).
17b), 1.68 (t, 12H, H-3b, 6b, 13b, 16b), -4.29 (s, 3H, H-methyl). 13
C
NMR (C6D6): δ ) 149.5, 148.1, 146.8, 137.5, 106.9, 21.3, 20.5, 18.8,
18.3. MS [m/z (%)]: 606 (28) [M+], 591 (100) [M - CH3]. IR: ν )
2964, 2931, 2869, 1510, 1300, 1135, 1057, 1013, 952, 760 cm-1. UV-
vis (CHCl3) [λ (ꢀ)]: 296 (32 100), 364sh (39 500), 389 (81 000), 566sh
(18 000), 596 (33 700), 6712sh nm (28 600). Anal. Calcd for C37H47N4-
Co: C, 73.25; H, 7.81; N, 9.23. Found: C, 73.09; H, 7.52; N, 9.20.
(TPrPc)Co(CH3). Methylmagnesium iodide in diethyl ether was
added dropwise to a solution containing 60 mg (0.1 mM) of (2,7,12,-
17- tetrapropylporphycenato)cobalt(III) chloride18,21b in 12 mL of
absolute 1,2- dimethoxyethane under argon atmosphere at 0 °C. The
progress of the reaction was monitored via TLC (alumina, dichlo-
romethane). After the starting material could no longer be detected,
the solution was poured into a mixture of 50 mL of dichloromethane
and 10 mL of saturated aqueous ammonium chloride. The organic
layer was washed twice with water, dried over sodium sulfate, and
then evaporated by vacuum. The residue was crystallized twice from
CH2Cl2/hexane to give violet prisms. Yield: 28 mg (50%). Mp: dec,
178-179 °C. 1H NMR (CDCl3): δ ) 9.44 (s, 4H, H-3, 6, 13, 16),
9.09 (s, 4H, H-9, 10, 19, 20), 3.99 (m, 8H, R-CH2), 2.38 (m, 8H, â-CH2),
1.35 (t, 12H, CH3), -4.90 (s, 3H, Co-CH3). 13C NMR (CDCl3): δ )
151.4, 149.1, 146.1, 121.0, 106.9, 31.2, 25.0, 14.6, -20.1. MS [m/z
(%)]: 550 (70) [M+], 535 (100) [M - CH3+]. IR: ν ) 2954, 2925,
2867, 1864, 1608, 1553, 1528, 1485, 1455, 1376, 1307, 1238, 1134,
1101, 1049, 970, 931, 913, 809, 738, 529, 436, 362, 321 cm-1. UV-
vis (CH2Cl2) [λ (ꢀ)]: 289 (29 100), 365sh (53 700), 385 (91 900), 560sh
(18 000), 592 (40 400), 610sh nm (3700). Anal. Calcd for C33H31N4-
Co: C, 71.98; H, 7.14; N, 10.18. Found: C, 71.60; H, 7.14; N, 9.98.
(OEPc)Co(C6H5). The phenylcobalt(III) complex was synthesized
from (OEPc)CoCl and phenylmagnesium iodide in diethyl ether using
a procedure similar to the one reported above for (OEPc)Co(CH3). After
workup, the title compound was crystallized twice from CH2Cl2/hexane
(EtioPc)Co(C6H5). The synthesis of (EtioPc)Co(C6H5) paralleled
the procedure given above for (OEPc)Co(C6H5) and was carried out
using (EtioPc)CoCl and phenylmagnesium bromide in THF. After
conventional workup, the target compound was crystallized from a
mixture of dichloromethane and heptane to give violet needles (yield
50%). Mp: 282 °C. 1H NMR (CDCl3): δ ) 9.64 (s, 4H, H-9, 10,
19, 20), 5.17 (dd, 1H, para-H-phenyl), 4.54 (t, 1H, meta-H-phenyl),
3.93 (m, 4H, CH2-2,7,12,17), 3.54 (s, 4H, CH3-3,6,13,16), 1.75 (t, 4H,
CH3-2,7,12,17), -0.29 (d, 1H, ortho-H-phenyl). 13C NMR (CDCl3):
δ ) 151.2, 147.8, 146.0, 138.1, 134.1, 123.0, 121.3, 107.4, 22.0, 18.2,
17.8. MS [m/z (%)]: 612 (40) (M), 535 (100) (M - C6H5). IR: ν )
2977, 2938, 2877, 1541, 1490, 1450, 1296, 1240, 1110, 1056, 998,
956, 916, 660, 460 cm-1. UV-vis [λ, nm (ꢀ)]: 328 (26 000), 387
(73 000), 557 (17 000), 595 (31 000).
Instrumentation. Cyclic voltammetry was carried out with a
Princeton Applied Research model 173 potentiostat coupled to a model
175 universal programmer, a model EC 225 2A voltammetric analyzer
(IBM Instruments Inc), or a BAS-100 electrochemical analyzer (Bio-
analytical Systems). Current-voltage curves were recorded on a model
RE0151 X-Y recorder or a DMP-40 digital plotter (Houston Instru-
ments). A three-electrode system was used and consisted of a platinum
button or a glassy carbon disk working electrode, a platinum wire
counter electrode, and a saturated calomel electrode (SCE) as reference.
All potentials listed in this manuscript are given as V vs SCE. The

2696 Inorganic Chemistry, Vol. 37, No. 11, 1998
Kadish et al.
Table 1. Half-Wave Potentials (V vs SCE) for the Reversible
Electrode Reactions of Cobalt Porphycenes in PhCN, 0.1 M TBAP
oxidn redn
2nd 1st M(III)/M(II)
macrocycle
OEPc
R
2nd
1st
none
C6H5
CH3
C6H5
none
CH3
1.26 1.13
1.05 0.81
0.99 0.75
1.09 0.80
1.22 0.99
1.14 0.83
0.47a
-1.07 -1.43
-1.09 -1.46
-1.13 -1.51
-1.13 -1.46
-0.99 -1.36
-1.08 -1.44
b
b
EtioPc
TPrPc
0.67a
b
a Epa at 0.1 V/s. b Additional redox processes are observed following
migration of the axial ligand. See text for details.
reductions at almost identical potentials (see Table 1). The four-
coordinate (OEPc)CoII species is reduced at -1.07 and -1.43
V, while the two (OEPc)Co(R) complexes are reduced at -1.09
and -1.46 V (R ) C6H5) or -1.13 and -1.51 V (R ) CH3).
Previous studies of (OEPc)Co have shown that electroreduction
leads to formation of a porphycene π-anion radical and
dianion,21a,h,23i and this is also the case for the σ-bonded
complexes investigated in the present study, all of which show
separations between half-wave potentials of 360-380 mV that
are diagnostic of electron addition to the conjugated porphycene
macrocycle.21a,23i
Figure 1. Cyclic voltammograms showing the reduction and oxidation
of (a) (OEPc)Co, (b) (OEPc)Co(C6H5) and (c) (OEPc)Co(CH3) in PhCN
containing 0.1 M TBAP. Scan rate ) 0.1 V/s.
reference electrode was separated from the bulk of the solution by a
fritted-glass bridge filled with the solvent and supporting electrolyte.
Solutions were deoxygenated by nitrogen purging.
Spectroelectrochemical experiments and thin-layer coulometry were
performed at a platinum mesh thin-layer electrode.41 Potentials were
monitored with a Princeton Applied Research model 173 potentiostat
coupled to a model 175 universal programmer. Time-resolved UV-
visible spectra were recorded with a Tracor Northern model TN-6500
rapid scan spectrophotometer/multichannel analyzer. UV-visible
spectra of the neutral complexes were recorded with an IBM Instruments
Inc. model 9430 UV-visible spectrophotometer.
Distinctly different redox behavior is seen for the oxidation
of (OEPc)Co, (OEPc)Co(CH3), and (OEPc)Co(C6H5). The
(OEPc)Co complex undergoes an irreversible Co(II)/Co(III)
process in PhCN which is located at Ep ) 0.47 V for a scan
rate of 0.1 V/s, and this is followed by two reversible one-
electron macrocycle oxidations located at E1/2 ) 1.13 and 1.26
V.21a,h,23i Two reversible one-electron oxidations are also seen
for (OEPc)Co(C6H5) and (OEP)Co(CH3) at potential scan rates
higher than 50 mV/s. These processes are located at E1/2
)
Results and Discussion
0.81 and 1.05 V for (OEPc)Co(C6H5) and at E1/2 ) 0.75 and
0.99 V for (OEPc)Co(CH3). Similar redox behavior is seen
for (EtioPc)Co(C6H5) and (TPrPc)Co(C6H5), and a summary
of the reversible half-wave potentials for these compounds is
given in Table 1.
Sensitivity of (Pc)Co(R) toward Light and Oxygen. All
spectroscopic and electrochemical studies on σ-bonded cobalt
porphycenes were carried out under an N2 atmosphere since
the presence of oxygen and ambient light may lead to
decomposition of the σ-bonded complexes by photolysis of the
metal-carbon bond.42-44 Two types of decomposition path-
ways are known to occur. One is via insertion of dioxygen
into the metal-carbon bond,42,43 while the other involves a
cleavage of the cobalt-carbon bond leading to a cobalt(II)
complex.44 Both types of photochemical reactions were exam-
ined as to their possible occurrence with (Pc)Co(R) under
oxygen. To evaluate the light and oxygen sensitivity of the
σ-bonded cobalt porphycenes, a solution of (OEPc)Co(CH3) was
exposed to air under ambient light and its UV-visible spectrum
recorded at regular intervals of time. No spectral changes were
observed for more than 2 h suggesting that a photodecompo-
sition of the (OEPc)Co(R) derivatives does not occur during
the time scale of the electrochemical and spectroscopic mea-
surements.
The absolute potential difference between the two oxidations
of (OEPc)Co(R) (∆E1/2) is 240 mV for both σ-bonded deriva-
tives, while the ∆E1/2 values between the two oxidations of
(EtioPc)Co(C6H5) and (TPrPc)Co(CH3) are 290 and 310 mV,
respectively. These ∆E1/2 values obtained for the two successive
oxidations compare with a 130 mV potential difference between
the two ring-centered oxidations of (OEPc)Co. Also, the
potential separation between the first oxidation and first
reduction of the four investigated σ-bonded compounds (see
Table 1) agrees well with the HOMO-LUMO gap of 1.88-
1.90 V reported for the free-base and zinc metal complexes of
OEPc or TPrPc macrocycles.21a,h These results clearly suggest
that all oxidations and reductions of these compounds are located
at sites involving the conjugated macrocycle.
The singly and doubly reduced (OEPc)Co(R) species are
stable on the cyclic voltammetry time scale, and solutions of
these anionic porphycenes could also be prepared by controlled
potential electrolysis. This is not the case for singly oxidized
(OEPc)Co(CH3), which undergoes, in a matter of minutes, a
chemical reaction that leads to formation of new electroactive
species. This chemical transformation is proposed to involve
a migration of the CH3 group from the cobalt center to one of
the four nitrogens of the porphycene ring to give (N-CH3OEPc)-
CoClO4 as a final product of the combined electrochemical/
chemical process. Evidence for this assignment is given in
following sections of this article.
Electrochemistry of (Pc)Co(R). Cyclic voltammograms
illustrating the reduction and oxidation of (OEPc)Co(CH3) and
(OEPc)Co(C6H5) in PhCN, 0.1 M TBAP are shown in Figure
1, which also displays, for comparison, the reduction and
oxidation of (OEPc)Co under the same solution conditions. All
three OEPc derivatives undergo two reversible one-electron
(41) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
(42) Kendrick, M. J.; Al-Akhdar, W. Inorg. Chem. 1987, 26, 3971.
(43) Pe´rre´e-Fauvet, M.; Gaudemer, A.; Boucly, P.; Devynck, J. J. Orga-
nomet. Chem. 1976, 120, 439.
(44) Clarke, D. A.; Grigg, R.; Johnson, A. W.; Pinnock, H. A. J. Chem.
Soc., Chem. Commun. 1967, 309.

σ-Bonded and N-Substituted Cobalt Porphycenes
Inorganic Chemistry, Vol. 37, No. 11, 1998 2697
The second electroreduction of (OEPc)Co(C6H5) leads to a
bleaching of all absorption bands in the UV region of the
spectrum and, by comparison with results previously reported,21a,23i
suggests formation of a porphycene dianion, i.e., a ring-centered
electroreduction. Similar types of spectral changes are observed
during the first and second reductions of (OEPc)Co(CH3),
suggesting similar sites of electron transfer during reduction of
both σ-bonded derivatives.
The products of (OEPc)Co(C6H5) oxidation are sufficiently
stable on the spectroelectrochemical time scale (10-30 s) so
that the UV-visible spectrum of [(OEPc)Co(C6H5)]•+ (Figure
3) and [(OEPc)Co(C6H5)]2+ (not shown) could be obtained in
THF just after their electrogeneration in the thin-layer cell. These
spectra are characteristic of a porphycene π-cation radical and
dication, respectively,21a and indicate the absence of a metal-
centered reaction under these experimental conditions. Unfor-
tunately, similar assignments could not be made in the case of
(OEPc)Co(CH3) due to the fact that the methyl migration rate
is too rapid to spectrally observe the singly oxidized porphycene
in the absence of a final migration product (see following
section).
Axial Ligand Migration. A migration of the σ-bonded axial
ligand in [(OEPc)Co(CH3)]+ and [(OEPc)Co(C6H5)]•+ was
electrochemically and spectrally monitored, and the electro-
chemical results for (OEPc)Co(CH3) are shown in Figure 4. Two
new reversible reductions are observed at -0.52 and -0.94 V
during a cathodic potential scan which was made after holding
the applied potential for 60 or more seconds at a value more
positive than E1/2 for the first oxidation in PhCN, 0.1 M TBAP
(see Figure 4a). These new redox processes compare well with
processes observed in the cyclic voltammogram of (N-CH3-
OEPc)CoClO4 (E1/2 ) -0.54 and -0.94 V) which was recorded
in the same solvent (Figure 4b) and lend strong support to the
migration process schematically shown in Scheme 1.
Figure 2. Thin-layer spectral changes observed during the first and
second controlled-potential reductions of (OEPc)Co(C6H5) in THF
containing 0.2 M TBAP. The applied potentials were -1.3 and -1.7
V, respectively, for the two reductions.
The migration of the methyl group in singly oxidized (OEPc)-
Co(CH3) is qualitatively faster than that of the phenyl group in
singly oxidized (OEPc)Co(C6H5) (as easily ascertained by the
cyclic voltammetric data), and this agrees with quantitative
results in the tetraphenylporphyrin (TPP) series where first-order
migration rate constants vary by 1 full order of magnitude upon
going from singly oxidized (TPP)Co(CH3) (kmig ) 1.4 × 10-2
s-1) to singly oxidized (TPP)Co(C6H5) (kmig ) 1.3 × 10-3 s-1).45
In a similar manner, Dolphin et al.32 have shown that electro-
chemically generated [(TPP)Co(C2H5)]•+ is unstable in CH2-
Cl2 at room temperature and rapidly undergoes a metal to
nitrogen migration of the C2H5 group, whereas [(TPP)Co-
(COOC2H5)]•+ is relatively stable under the same experimental
conditions. The differences in stability of the singly oxidized
species might simply be interpreted in terms of differences in
the electron-donating ability of the axial ligand. The more
electron donating the axial ligand, the weaker will be the cobalt-
carbon bond and the faster will be the migration. However, as
earlier demonstrated,45 the rates of migration also relate directly
to the degree of Co(IV) character in the singly oxidized species
and this would imply a greater Co(IV) character in the case of
[(OEPc)Co(CH3)]+.
Figure 3. Thin-layer spectral changes observed during the first
controlled potential oxidation of (OEPc)Co(C6H5) in THF containing
0.2 M TBAP.
Spectral Characterization of [(OEPc)Co(R)]n (n ) -2 to
+2). The reduction and oxidation of each porphycene was
investigated by thin-layer UV-visible spectroelectrochemistry
in both PhCN and THF, and examples of the resulting data in
THF are given in Figures 2 and 3 for (OEPc)Co(C6H5). The
UV-visible spectrum of neutral (OEPc)Co(C6H5) in PhCN is
similar to that of other non-σ-bonded metalloporphycenes con-
taining a M(II) central metal ion.18,20,21a,b,e-g,22,23i The compound
before reduction has four absorption bands located at 361,
395, 593, and 618 nm, while the singly reduced compound in
PhCN, 0.2 M TBAP has absorption bands at 352, 381, 550,
738, and 872 nm. The 550 nm band is characteristic of a
porphycene π-anion radical as are the two broad bands at 738
and 872 nm.21a,c,i,23i These spectral data are given in Table 2,
which also summarizes the UV-visible spectral properties of
each electrogenerated [(OEPc)Co(C6H5)]n (n ) -2 to +2)
complex in PhCN.
1
The H NMR chemical shifts of the protons located on the
ethylenic bridge of the porphycene macrocycle (H-ethylenic)
can, in theory, be used to probe the electron-donating capacity
of the σ-bonded alkyl or aryl groups because of the through-
(45) (a) Miyamoto, K.; Suenobu, T.; Fukuzumi, S. Presented at 42nd
Symposium on Organometallic Chemistry, Higashi-Hiroshima, Japan,
1995; Abstr. 166. (b) Fukuzumi, S.; Miyamoto, K.; Suenobu, T.; Van
Caemelbecke, E.; Kadish, K. M. J. Am. Chem. Soc. 1998, 120, 2880.

2698 Inorganic Chemistry, Vol. 37, No. 11, 1998
Kadish et al.
Table 2. UV-Visible Data for [(OEPc)Co(R)]n (n ) 0, -1, -2, +1, +2) in PhCN, 0.2 M TBAP, Where R ) CH3 and C6H5
a
λ
max (10-5ꢀ)
complex
(OEPc)Co(C6H5)
361sh (0.52)
352 (0.62)
346 (0.48)
395 (1.03)
380 (0.67)
390 (0.39)
390 (0.79)
381 (0.41)
394 (1.21)
593 (0.45)
618 (0.41)
[(OEPc)Co(C6H5)]•-
[(OEPc)Co(C6H5)]2-
[(OEPc)Co(C6H5)]•+
[(OEPc)Co(C6H5)]2+
(OEPc)Co(CH3)
549 (0.22)
737bb (0.21)
742bb (0.12)
862bb (0.15)
825 (0.16)
545sh (0.05)
516sh (0.09)
427sh (0.26)
563sh (0.26)
496sh (0.13)
593 (0.03)
596bb (0.10)
597bb (0.07)
610 (0.60)
549 (0.30)
352 (0.52)
375 (0.88)
645sh (0.05)
673sh (0.17)
[(OEPc)Co(CH3)]•-
[(OEPc)Co(CH3)]2-
737 (0.23)
850 (0.17)
413 (0.41)
a sh ) shoulder; bb ) broad band.
Figure 5. Cyclic voltammograms of (a) (N-CH3OEPc)CoCl in PhCN
containing 0.1 M TBAP, b) (N-CH3OEPc)CoCl containing 0.1 M
(THA)Cl, and (c) (N-CH3OEP)CoCl containing 0.1 M TBAP and
AgClO4 (1.5 equiv). Scan rate ) 0.1 V/s.
Figure 4. Cyclic voltammograms of (a) (OEPc)Co(CH3) and (b) (N-
CH3OEPc)CoClO4 in PhCN 0.1 M TBAP.
associated with the original compound and two of which are
Scheme 1
due to (N-CH3OEPc)CoClO4 which is generated by the ex-
change of Cl- with ClO4 from the supporting electrolyte as
-
shown in eq 1.
(N-CH3OEPc)CoCl + ClO4- a
(N-CH3OEPc)CoClO4 + Cl- (1)
The above ligand exchange reaction was confirmed by cyclic
voltammetric measurements made before and after addition of
0.1 M tetrahexylammonium chloride ((THA)Cl) or silver
perchlorate to solutions of (N-CH3OEPc)CoCl in PhCN, 0.1 M
TBAP. The addition of a Cl- salt generates (N-CH3OEPc)-
CoCl by shifting the equilibrium to the left, while the addition
metal interaction between the axial ligand and the macrocycle,
as has been described in the literature for porphyrin derivatives.
The more electron donating the axial ligand, the higher the field
at which the H-ethylenic resonances appear. In the case of
(OEPc)Co(CH3), the ethylenic proton resonances appear at 9.34
ppm, whereas, in the case of (OEPc)Co(C6H5), they appear at
9.47 ppm. This indicates a higher electron-donating ability of
the methyl than the phenyl axial ligand and is consistent with
the higher reactivity observed for the one-electron oxidized
product of (OEPc)Co(CH3).
-
of ClO4 in the form of AgClO4 next shifts the equilibrium to
the right and leads to the formation of (N-CH3OEPc)CoClO4.
Voltammograms under the above three experimental condi-
tions are shown in Figure 5 and indicate that (N-CH3OEPc)-
CoCl is reduced via two one-electron-transfer steps, the first of
which is reversible and located at E1/2 ) -0.76 V. The second
reduction is quasireversible and located at Epc ) -1.20 V and
Epa ) -0.97 V for a potential scan rate of 0.1 V/s. (N-CH3-
OEPc)CoClO4 is also reduced by two successive one-electron
Electrochemistry of (N-CH3OEPc)CoCl. (N-CH3OEPc)-
CoCl exhibits four reductions in PhCN, two of which are
reductions, both of which are reversible and located at E1/2
)

σ-Bonded and N-Substituted Cobalt Porphycenes
Inorganic Chemistry, Vol. 37, No. 11, 1998 2699
Table 3. Half-Wave Potentials (V vs SCE) for Reduction of
(N-RPc)CoX Complexes in PhCN, 0.1 M TBAP
redn
Pc
R
X
1st
2nd
OEPc
CH3
Cl-
ClO4
ClO4
ClO4
-0.76
-0.54
-0.49
-0.52
-1.20a
-0.94
-0.73c
-0.92
-
-
-
b
C6H5
CH3
TPrPc
a Epc at 0.1 V/s. b Electrogenerated species after oxidation of
c
(OEPc)Co(C6H5) in PhCN, 0.1 M TBAP. Epc at 0.1 V/s.
-0.54 and -0.94 V in PhCN (see Figure 5c). These data are
summarized in Table 3.
The porphycene species electrogenerated after electroreduc-
tion of (N-CH3OEPc)CoCl or (N-CH3OEPc)CoClO4 in PhCN
are stable on the cyclic voltammetric time scale but not under
bulk electrolysis conditions where the first reduction of (N-CH3-
OEPc)CoCl leads to formation of a new species which is
reduced in two reversible one-electron transfers located at E1/2
) -1.14 and -1.49 V. No demetalation of (N-CH3OEPc)-
CoCl occurs, as ascertained by the fact that reductions associated
with free-base (N-CH3OEPc)H2+ClO4- are not detected (these
electrode reactions occur at E1/2 ) 0.20 and -0.20 V). Two
additional porphycene species can also be generated in the
bulk electroreduction of (N-CH3OEPc)CoCl. The first is
(OEPc)Co, and the second, (OEPc)Co(CH3). A (OEPc)Co
product would result from cleavage of the nitrogen-carbon
bond on (N-CH3OEPc)CoCl while (OEPc)Co(CH3) could be
formed in a “reverse migration”, as in the case of chemically
or electrochemically reduced N-alkyl- or arylcobalt porphy-
rins.32,34a,b,35,36,45 The similarity of reduction potentials for
(OEPc)Co and (OEPc)Co(CH3) in PhCN does not allow for a
differentiation of these two possible chemical products based
solely on the cathodic electrochemistry. However, the anodic
electrochemistry of the [(N-CH3OEPc)CoCl]- “decomposition
product” is uniquely consistent with the formation of (OEPc)-
Co(CH3) since the oxidation potentials and spectra (see follow-
ing section) match up perfectly and no process is seen at Ep
close to 0.47 V, a potential where the Co(II)/Co(III) reaction
of (OEPc)Co is known to occur (see Figure 1 and Table 1).
Spectral Characterization of [(N-CH3OEPc)CoCl]n, Where
n ) 0, -1, and -2. The UV-vis spectrum of (N-CH3OEPc)-
CoCl in PhCN containing 0.2 M Ph4PCl shows strong absorption
bands at 421 and 669 nm and two weaker bands that appear as
shoulders at ca. 400 and ca. 620 nm. All four absorption bands
decrease in intensity and two new broad bands appear at 550
and 870 nm during the first reduction (see Figure 6). The 550
nm band is the key marker band for a porphycene π anion
radical, and the data in Figure 6 thus suggest that the electrore-
duction of (N-CH3OEPc)CoCl occurs at the conjugated mac-
rocycle rather than at the metal center.
Figure 6. Thin-layer spectral changes seen during the first and second
controlled-potential reductions of (N-CH3OEPc)CoCl in PhCN, 0.2 M
Ph4PCl. The applied potentials were -0.85 and -1.25 V, respectively.
The second reduction of (N-CH3OEPc)CoCl is associated with
an overall reduction of intensity in the absorption bands, and
the final spectrum of the doubly reduced species can be assigned
to a Co(II) N-CH3 porphycene dianion. However, as mentioned
above, [(N-CH3OEPc)CoCl]•- undergoes a chemical transfor-
mation to give (OEPc)Co(CH3) on longer time scales under an
applied potential, and the resulting spectral changes which lend
support to this assignment are shown in Figure 7. The results
in Figure 7 are consistent with the electrochemical data and
demonstrate that the CH3 group migrates from the porphycene
core of electrogenerated [(N-CH3OEPc)Co]•- to give, as a
final porphycene product, the σ-bonded Co(III) derivative,
(OEPc)Co(CH3).
Figure 7. UV-visible spectra of (N-CH3OEPc)CoCl in PhCN, 0.2 M
TBAP (top), the mixture of species generated when the potential is
held for 15 min after the first reduction of (N-CH3OEPc)CoCl and
stepped back to 0 V (middle), and (OEPc)Co(CH3) (bottom).
Conclusion. The electrochemical and spectroelectrochemical
results lead to the conclusion that the first oxidations of (OEPc)-
Co(C6H5) and (OEPc)Co(CH3) are followed by an axial ligand

2700 Inorganic Chemistry, Vol. 37, No. 11, 1998
Kadish et al.
migration to ultimately generate an N-substituted cobalt(II)
porphycene. A similar electrochemically initiated migration of
the axial ligand is known in the case of σ-bonded cobalt por-
phyrins, (P)Co(R),1,32-34 but the reaction qualitatively appears
to be much slower in the case of porphycenes. The singly and
doubly reduced (OEPc)Co(R) complexes are stable, which is
not the case for many related (Por)Co(R) complexes where a
reduction by one electron is often followed by a cleavage of
the cobalt-carbon bond.12a,34a,43,46,47 This suggests a stronger
cobalt-carbon bond for the σ-bonded cobalt porphycenes than
for the σ-bonded porphyrin derivatives.
The first and second reductions of (N-CH3OEPc)CoCl are
ring centered as shown by UV-visible spectroelectrochemistry
and thus do not lead to formation of a cobalt(I) complex. This
contrasts markedly with results for N-substituted metallopor-
phyrins that are known to stabilize metal ions in their low
oxidation states.35,37-40 For example, electrogenerated N-methyl
copper(I)38 and cobalt(I)35,37,40 porphyrins are stable but a metal-
centered Co(II)/Co(I) reaction seems not to occur in the
porphycene series.
Surprisingly, despite the difference in electrochemical be-
havior between (N-CH3OEPc)CoCl and N-substituted Co(II)
metalloporphyrins, both types of complexes undergo a migration
of the N-substituent from the pyrrole nitrogen to the metal ion
after reduction by one electron. To our knowledge, there are
only two cases where a nitrogen to metal migration occurs after
reduction of an N-alkyl porphyrin at the conjugated π-ring
system of the macrocycle. These involve the cobalt(II) and
nickel(II) complexes of N-methyltetrakis(4-sulfonatophenyl)-
porphyrins.48
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the CNRS (R.G., UMR
5633) is gratefully acknowledged.
IC971534C
(46) Maiya, G. B.; Han, B. C.; Kadish, K. M. Langmuir 1989, 5, 645.
(47) Lexa, D.; Save´ant, J.-M.; Soufflet, J. P. J. Electroanal. Chem.
Interfacial Electrochem. 1979, 100, 159.
(48) Guldi, D. M.; Neta, P.; Hambright, P.; Rahimi, R. Inorg. Chem. 1992,
31, 4849.