Electrochemistry and spectroelectrochemistry β, β′-fused quinoxalinoporphyrins and related extended bis-porphyrins with co(III), Co(II), and Co(I) central metal ions

Article abstract of DOI:10.1021/ic901851u

A series of cobalt(II) and cobalt(III) porphyrins with fused quinoxaline rings at one or more β,/β′-pyrrolic units of the macrocycle were synthesized and characterized as to their electrochemical properties in nonaqueous media. Their UV - visible spectra were also measured before and during oxidation or reduction in a thin-layer cell. The investigated quinoxalinoporphyrins are represented as (PQ)Co, (QPQ)CoCl, (PQ ₂)CoCl, Co(P)-TA-(P)Co, and Co(PQ)-(QP)Co, where PQ = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-quinoxalino[2,3-b′]porphyrin, QPQ = the dianion of the corresponding linear bisquinoxalino[2,3-b′:12, 13-b″]porphyrinI PQ₂ = the dianion of the corresponding corner bisquinoxalino[2,3-b′:7,8-b″]porphyrin, and (P)-TA-(P) = the tetraanion of the bis-porphyrin with 5,10,15,20-tetrakis-(3,5-di-tert- butylphenyl)porphyrins fused at opposite ends of tetraazaanthracene. (P)Co and (P)CoCI were also characterized where P = the dianion of 5,10,15,20-tetrakis(3, 5-di-tert-butylphenyl)porphyrin. Each compound could be cycled between their Co(III), Co(II), and Co(I) forms under the application of a given oxidizing or reducing potential, although a one-electron reduction of the Co(II) quinoxalinoporphyrins led to a product with mixed Co(I) and porphyrin π-anion radical character followed by generation of a pure Co(I) π-anion radical species at more negative potentials. The effect of the position and number of quinoxaline groups on the redox potentials and mechanisms of each electron transfer were elucidated, and comparisons made to structurally similar compounds containing both redox active and redox inactive central metal ions. Surprisingly, the position and number of quinoxaline groups on the macrocycle has little or no effect on the redox potentials for the Co(II) → Co(III) or Co(III) → Co(II) processes, but this is not the case for other electron transfer reactions where significant differences are seen between the examined compounds. Significant interactions are also observed between the two porphyrin macrocycles of the laterally bridged dicobalt(II) bis-porphyrin dyad Co(P)-TA-(P)Co in its singly and doubly reduced form, but only weak interactions are seen between the two Co(PQ) units of the single bond biquinolalinyl-bridged dicobalt(II) bis-porphyrin dyad Co(PQ)-(QP)Co.

Full text of DOI:10.1021/ic901851u

Inorg. Chem. 2010, 49, 1027–1038 1027
DOI: 10.1021/ic901851u
Electrochemistry and Spectroelectrochemistry of β,β0-Fused
Quinoxalinoporphyrins and Related Extended Bis-porphyrins with Co(III), Co(II),
and Co(I) Central Metal Ions
Weihua Zhu,†,§ Maxine Sintic,‡ Zhongping Ou,*,†,§ Paul J. Sintic,‡ James A. McDonald,‡ Peter R. Brotherhood,‡
Maxwell J. Crossley,*,‡ and Karl M. Kadish*,†
†Department of Chemistry, University of Houston, Houston, Texas 77204-5003, ‡School of Chemistry, The University
of Sydney, NSW 2006, Australia, and §Department of Applied Chemistry, Jiangsu University, Zhenjiang, 212013, China
Received September 18, 2009
A series of cobalt(II) and cobalt(III) porphyrins with fused quinoxaline rings at one or more β,β0-pyrrolic units of the
macrocycle were synthesized and characterized as to their electrochemical properties in nonaqueous media. Their
UV-visible spectra were also measured before and during oxidation or reduction in a thin-layer cell. The investigated
quinoxalinoporphyrins are represented as (PQ)Co, (QPQ)CoCl, (PQ2)CoCl, Co(P)-TA-(P)Co, and Co(PQ)-(QP)Co,
where PQ = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-quinoxalino[2,3-b0]porphyrin, QPQ = the dianion
of the corresponding linear bisquinoxalino[2,3-b0:12,13-b00]porphyrin, PQ2 = the dianion of the corresponding corner
bisquinoxalino[2,3-b0:7,8-b00]porphyrin, and (P)-TA-(P) = the tetraanion of the bis-porphyrin with 5,10,15,20-tetrakis-
(3,5-di-tert-butylphenyl)porphyrins fused at opposite ends of tetraazaanthracene. (P)Co and (P)CoCl were also
characterized where P = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin. Each compound could be
cycled between their Co(III), Co(II), and Co(I) forms under the application of a given oxidizing or reducing potential,
although a one-electron reduction of the Co(II) quinoxalinoporphyrins led to a product with mixed Co(I) and porphyrin π-
anion radical character followed by generation of a pure Co(I) π-anion radical species at more negative potentials. The
effect ofthe position and number ofquinoxaline groups onthe redox potentials and mechanisms of each electron transfer
were elucidated, and comparisons made to structurally similar compounds containing both redox active and redox
inactive central metal ions. Surprisingly, the position and number of quinoxaline groups on the macrocycle has little or no
effect on the redox potentials for the Co(II) f Co(III) or Co(III) f Co(II) processes, but this is not the case for other
electron transfer reactions where significant differences are seen between the examined compounds. Significant
interactions are also observed between the two porphyrin macrocycles of the laterally bridged dicobalt(II) bis-porphyrin
dyad Co(P)-TA-(P)Co in its singly and doubly reduced form, but only weak interactions are seen between the two
Co(PQ) units of the single bond biquinolalinyl-bridged dicobalt(II) bis-porphyrin dyad Co(PQ)-(QP)Co.
Introduction
devices.4,6,10,15-18 Fusion of one or more quinoxaline groups
to the β,β0-pyrrolic units of 5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)porphyrin has only a minimal effect on the
Numerous quinoxalinoporphyrins have been electro-
chemically examined in recent years1-14 as models for the
development of porphyrin-based molecular wires and
(7) Ohkubo, K.; Sintic, P. J.; Tkachenko, N. V.; Lemmetyinen, H.; E, W.;
Ou, Z.; Shao, J.; Kadish, K. M.; Crossley, M. J.; Fukuzumi, S. Chem. Phys.
2006, 326, 3–14.
(8) Ou, Z.; E, W.; Shao, J.; Burn, P. L.; Sheehan, C. S.; Walton, R.; Kadish,
K. M.; Crossley, M. J. J. Porphyrins and Phthalocyanines 2005, 9, 142–151.
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Fukuzumi, S.; Crossley, M. J. Inorg. Chem. 2004, 43, 2078–2086.
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Chem. 2003, 1, 2777–2787.
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(12) Kadish, K. M.; E, W.; Ou, Z.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Chem. Commun. 2002, 356–357.
(13) Fukuzumi, S.; Ohkubo, K.; E, W.; Ou, Z.; Shao, J.; Kadish, K. M.;
Hutchison, J. A.; Ghiggino, K. P.; Sintic, P. J.; Crossley, M. J. J. Am. Chem.
Soc. 2003, 125, 14984–14985.
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Z.; Shao, J.; Ohkubo, K.; Reimers, J. R.; Fukuzumi, S.; Crossley, M. J. J.
Phys. Chem. A 2008, 112, 556–570.
*To whom correspondence should be addressed. E-mail: kkadish@
uh.edu.
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Kadish, K. M.; Crossley, M. J.; Reimers, J. R. Phys. Chem. Chem. Phys.
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r
2009 American Chemical Society
Published on Web 12/22/2009
pubs.acs.org/IC

1028 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
oxidation potentials,3-5,8,9 and this contrasts with the reduc-
tions which are positively shifted in potential by 90-190 mV
per each fused quinoxaline group, with the exact magnitude
of the shift depending upon the solvent, type, and oxidation
state of the central metal ion and number of fused quinoxa-
line groups on theporphyrin macrocycle whichcan vary from
1 to 4.19 The site of electron transfer in free-base quinoxali-
noporphyrins and quinoxalinoporphyrins containing
non-redox active transition metal ions involves primarily
the conjugated π-ring system of the porphyrin macrocycle
but partial electron density is located on the quinoxaline part
of the molecule after reduction,3,4,8 especially in solutions
containing added protons5 or scandium(III) ion,20 both of
which will bind or ion pair with the quinoxaline nitrogen
atoms.
AgII/AgI process3 as opposed to ring-centered reduction for
the other investigated compounds. The site of electron
transfer should be important in evaluating the redox beha-
vior of quinoxalinoporphyrins, but the oxidation state of the
central metal ion in the initial compounds should also be
considered. We earlier examined Au(III) and Au(II) quinox-
alinoporphyrins9,12 and now turn our attention to the cobalt
derivatives which can exist in Co(III), Co(II), and Co(I)
oxidation states under the appropriate application of an
oxidizing or reducing potential. Numerous synthetic cobalt
porphyrins with a variety of meso and β-pyrrolic substituents
have been studied as to their spectroscopic and electroche-
mical properties under different solution conditions,21-40 but
cobalt quinoxalinoporphyrins have never before been char-
acterized.
Understanding the most important factors which affect the
site of electron transfer in quinoxalinoporphyrins is of vital
interest to tune and direct the chemical reactivity of larger
arrays containing the quinoxalinoporphyrin unit.10 This is
especially true in the case of redox active transition metal
derivatives where the porphyrin central metal ion can be
stepwise cycled between its MI, MII, MIII or even MIV and
MV forms as a function of the applied potential and varia-
tions in the type and number of axial ligands.15 It is well-
documented in the literature that the site of electron transfer
in metalloporphyrins can be tuned by appropriate selection
of solvent, axial ligand binding, and/or porphyrin ring
substituents,9,21,22 and this should also be the case for
quinoxalinoporphyrins which have been synthesized to func-
tion as integral parts of molecular wires.15-18
Our laboratories have studied in detail the redox reactivity
of (P)M, (PQ), (QPQ)M, and (PQ2)M in their neutral,
oxidized, and reduced forms where PQ = the dianion of
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-
b0]porphyrin, QPQ = the dianion of the corresonding linear
bisquinoxalino[2,3-b0:12,13-b00]porphyrin, PQ2 = the dia-
nion of the corresponding corner bisquinoxalino[2,3-b0:7,8-
b00]porphyrin, and M = Zn(II), Ni(II), Cu(II), or Pd(II). The
ease of reductions follows the order: (P)M < (PQ)M <
(QPQ)M with the difference in E1/2 being 160-170 mV
between (P)M and (PQ)M or (PQ)M and (QPQ)M, inde-
pendent of thecentralmetal ion.5,8 A linear correlation is thus
seen between E1/2 of the first reduction and the number of
fused quinoxaline groups on the macrocycle for these three
series of compounds. The (PQ2)M derivatives do not fit the
trend since the π-system of both Q groups are not fully
conjugated with the porphyrin macrocycle.14
The site of reduction or oxidation in cobalt quinoxalino-
porphyrins can, in principle, involve the cobalt central metal
ion, the conjugated π-ring system of the porphyrin macro-
cycle, or the quinoxaline part of the molecule, all of which are
redox active.3,8,21 This is examined in the present study where
seven new compounds were synthesized and characterized as
to their electrochemical and spectroscopic properties after
oxidation and reduction in PhCN containing 0.1 M tetra-n-
butylammonium perchlorate (TBAP). The examined com-
pounds are shown in Chart 1 and represented as (P)Co,
(P)CoCl, (PQ)Co, (QPQ)CoCl, (PQ2)CoCl, Co(P)-TA-(P)-
Co, and Co(PQ)-(QP)Co, where TA = tetraazaanthracene.
The magnitude of electronic coupling between the two
porphyrin macrocycles of a laterally bridged dicobalt(II)
bis-porphyrin dyad, Co(P)-TA-(P)Co 6 and the single bond
biquinolalinyl-bridged dicobalt(II) bis-porphyrin dyad dyad,
Co(PQ)-(QP)Co 7, are also explored.
Experimental Section
Instrumentation. Melting points were recorded on a Reichert
melting point stage and are uncorrected. Microanalyses were
performed at the Microanalytical Unit, The University of New
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A similar linear relationship is observed for the reduction
of Ag(II) porphyrins with P, PQ, or QPQ macrocycles despite
the fact that these compounds undergo a metal-centered
(30) Ziegelbauer, J. M.; Olson, T. S.; Pylypenko, S.; Alamgir, F.; Jaye, C.;
Atanassov, P.; Mukerjee, S. J. Phys. Chem. C 2008, 112, 8839–8849.
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Tagliatesta, P. J. Chem. Soc., Dalton Trans.: Inorg. Chem. 1999, 1595–1602.
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D.; Tagliatesta, P. J. Porphyrins Phthalocyanines 1998, 2, 439–450.
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Van Caemelbecke, E.; Guo, N.; Kadish, K. M. Inorg. Chem. 1996, 35, 6530–
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K. M. J. Chem. Soc., Dalton Trans.: Inorg. Chem. 1996, 2793–2797.
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T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042–4048.
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N. S.; Reimers, J. R. J. Am. Chem. Soc. 2002, 124, 9299–9309.
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Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1029
Chart 1. Structures of Investigated Cobalt Porphyrins Where R = 3,5-Di-tert-butylphenyl
South Wales. Ultraviolet-visible spectra were routinely re-
corded on a Hitachi 150-20 spectrophotometer in chloroform
that was deacidified by filtration through an alumina column.
1H NMR spectra were recorded on a Bruker DPX-400 (400
MHz) spectrometer as stated. Deuteriochloroform was used as
the solvent with tetramethylsilane (TMS) as an internal stan-
dard. Signals are recorded in terms of chemical shift (in ppm),
multiplicity, coupling constants (in Hz), and assignment, in that
order. The following abbreviations for multiplicity are used: s,
singlet; d, doublet; dd, doublet of doublets; t, triplet; tt, triplet of
triplets; m, multiplet; br, broad; ABq, AB quartet. Electrospray
ionization (ESI) mass spectra were recorded on a ThermoQuest
Finnigan LCQ DECA instrument. Mass spectra were obtained
as an envelope of the isotope peaks of the molecular ion. The
mass corresponding to the envelope’s maxima is reported and
was compared with the maxima of a simulated spectrum. High
resolution electrospray ionization Fourier transform ion cyclo-
tron resonance spectrometry (HR-ESI-FT/ICR) were recorded
on a Bruker Daltonics BioAPEX II FT/ICR mass spectrometer
equipped with a 4.7 T MAGNEX superconducting magnet
and an Analytical external ESI source. Column chromatogra-
phy was routinely carried out using the gravity feed column
technique on Merck silica gel Type 9385 (230-400 mesh).
Analytical thin layer chromatography (TLC) analyses were
performed on Merck silica gel 60 F254 precoated sheets
(0.2 mm). Alumina refers to Merck aluminum oxide 90 active
neutral I, type 1077.
fritted glass bridge of low porosity which contained the solvent/
supporting electrolyte mixture.
Thin-layer UV-visible spectroelectrochemical experiments
were performed with a home-built thin-layer cell which has a
light transparent platinum net working electrode. Potentials
were applied and monitored with an EG&G PAR Model 173
potentiostat. Time-resolved UV-visible spectra were recorded
with a Hewlett-Packard Model 8453 diode array spectrophot-
ometer. High purity N2 from Trigas was used to deoxygenate the
solution and kept over the solution during each electrochemical
and spectroelectrochemical experiment.
Chemicals. All solvents used were routinely distilled prior to
use, unless otherwise stated. Light petroleum refers to the
fraction of bp 60-80 °C. Ether refers to diethyl ether and was
distilled over crushed calcium chloride and stored over sodium
wire. Ethanol-free chloroform was obtained by distillation from
calcium chloride and passage through alumina. Merck AR
grade methanol was used. Where solvent mixtures were used,
the proportions are given by volume.
Benzonitrile (PhCN) was obtained from Aldrich Co. and
distilled over P2O5 under vacuum prior to use. Dichloromethane
(CH2Cl2) was obtained from Aldrich Co. and used as received
for electrochemistry and spectroelectrochemistry experiments.
Tetra-n-butylammonium perchlorate (TBAP) was purchased
from Sigma Chemical or Fluka Chemika Co., recrystallized
from ethyl alcohol, and dried under vacuum at 40 °C for at least
1 week prior to use.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)porphyrinato}-
cobalt(II) 1. 5,10,15,20- Tetrakis(3,5-di-tert-butylphenyl)por-
phyrin (400 mg, 0.376 mmol) was dissolved in chloroform (20
mL), and a solution of cobalt(II) acetate tetrahydrate (234 mg,
0.940 mmol) in methanol (10 mL) was added. The mixture was
heated to reflux with stirring, and TLC indicated immediate
completion of the reaction. The solvents were removed, and the
Cyclic voltammetry was carried out at 298 K using an EG&G
Princeton Applied Research (PAR) 173 potentiostat/galvano-
stat. A homemade three-electrode cell was used for cyclic
voltammetric measurements and consisted of a platinum button
or glassy carbon working electrode, a platinum counter elec-
trode, and a homemade saturated calomel reference electrode
(SCE). The SCE was separated from the bulk of the solution by a

1030 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
residue purified by chromatography over silica (dichloro-
methane-light petroleum 1:4). The front running red band was
collected, evaporated to dryness, and the resultant residue was
recrystallized from dichloromethane-acetonitrile (1:1) to yield
{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)- porphyrinato}co-
balt(II) 1 (365 mg, 87%) as a dark red-purple microcrystalline
solid, mp > 300 °C. MS (ESI): 1120.5 (Mþ requires 1120.5). IR
(CHCl3): 3063 w, 3022 m, 2964 s, 2905 m, 2868 m, 1593 s, 1477
m, 1464 m, 1427 m, 1394 m, 1364 s, 1352 m, 1296 m, 1248 m,
1221 s, 1207 s, 1076 w, 1005 m, 930 m cm-1. UV-vis (CHCl3):
413 (log ε 5.46), 530 (4.24) nm. MS (HR-ESI-FT/ICR): m/z
1119.6638, calcd for [C72H92N4Co]þ: 1119.6649.
temperature for 1.5 h. Dichloromethane (30 mL) was added,
and the organic layer washed with water (4 ꢀ 50 mL). The
organic layer was dried over anhydrous sodium sulfate, and the
solvent removed to afford chloro{5,10,15,20-tetrakis(3,5-di-
tert-butylphenyl)bisquinoxalino[2,3-b0:12,13-b00]porphyrinato}
cobalt(III) 4 (0.016 g, 67%) as a dark green solid, mp. >300 °C.
MS (ESI): 1324.73 ([M - Cl]þ required 1358.68). IR (CHCl3):
3043 w, 2989 w, 1551 w, 1240 s, 1194 s cm-1. UV-vis (CHCl3):
356 (log ε 4.57), 417 (4.45), 486 (4.86), 566 (3.70), 615 (3.70), 668
(4.08) nm. MS (HR-ESI-FT/ICR): m/z 1323.70719 [M - Cl]þ,
calcd for [C88H96N8Co]þ: 1323.70845.
Chloro{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquino-
xalino[2,3-b0:7,8-b00]por-phyrinato}cobalt(III) 5. A mixture of
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquino-xalino[2,
3-b0:7,8-b00]porphyrin (0.036 g, 0.028 mmol), cobalt(II) acetate
tetrahydrate (0.141 g, 0.566 mmol), and anhydrous sodium
acetate (0.049 g, 0.591 mmol) in toluene (10 mL) and glacial
acetic acid (10 mL) was heated at reflux under nitrogen for 48 h.
The solution was allowed to cool, then dichloromethane (50
mL) added, and the organic layer washed with deionized water
(2 ꢀ 75 mL), saturated sodium hydrogen carbonate solution
(2 ꢀ 75 mL), and deionized water (2 ꢀ 75 mL) again. The organic
layer was dried over anhydrous sodium sulfate, and the solvent
removed. The residue was dissolved in chloroform (10 mL), and
concentrated hydrochloric acid (10 M, 1 mL) was added drop-
wise. The reaction mixture was then stirred at room temperature
for 1 h. Dichloromethane (30 mL) was added, and the organic
layer washed with water (4 ꢀ 50 mL). The organic layer was
dried over anhydrous sodium sulfate, and the solvent removed
to give chloro{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bis-
quinoxalino[2,3-b0:7,8-b00]porphyrinato}cobalt(III) 5 (0.015 g,
74%) as a dark brown-green solid, mp. >300 °C. MS (ESI):
1324.87 ([M - Cl]þ required 1358.68). IR (CHCl3): 2964 w, 2392
w, 1263 w, 1229 s, 1213 s, 1202 s cm-1. UV-vis (CHCl3): 351
(log ε 4.65), 413 sh (4.46), 440 (4.56), 483sh (4.52), 507 (4.71), 609
(4.12), 643sh (2.81) nm. MS (HR-ESI-FT/ICR): m/z 1323.70691
[M - Cl]þ, calcd for [C88H96N8Co]þ: 1323.70845.
Chloro{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}-
cobalt(III) 2. {5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-por-
phyrinato}cobalt(II) 1 (200 mg, 0.179 mmol) was dissolved in
chloroform (100 mL), and methanol (100 mL) and hydrochloric
acid (32% w/v, 3 mL) was added. The mixture was stirred for 1 h
at which point chloroform (100 mL) was added and the mixture
washed with deionized water (3 ꢀ 100 mL), dried over anhy-
drous sodium sulfate, filtered, and the solvent removed. The
residue was recrystallized from dichloromethane-acetonitrile
(1:1) to yield chloro{5,10,15,20-tetrakis(3,5-di-tert-butylphe-
nyl)- porphyrinato}cobalt(III) 2 (194 mg, 94%) as lustrous
purple crystals, mp > 300 °C. MS (ESI): 1120.7 ([M-Cl]þ
requires 1120.5). IR (CHCl3): 3062 w, 3022 s, 2966 s, 2905 m,
2870 m, 1593 m, 1477 m, 1466 m, 1427 w, 1394 w, 1364 m, 1352
m, 1296 m, 1248 m, 1225 m, 1213 m, 1200 w, 1080 w, 1005 m, 935
w, 928 w cm-1. UV-vis (CHCl3): 412 (log ε 4.99), 544 (4.03) nm.
1H NMR (400 MHz, CDCl3) δ 1.50 (72 H, br s, t-butyl H), 7.80
(4 H, app t, J = 1.8 Hz, p-Ar H), 8.04 (4 H, br s, o-Ar H), 8.15 (4
H, br s, o-Ar H), 8.66 (8 H, s, β-pyrrolic H). 1H NMR (400 MHz,
CDCl3-1% d5-pyridine) δ 1.42 (72 H, s, t-butyl H), 7.55 (8 H, d,
J = 1.8 Hz, o-Ar H), 7.74 (4 H, app t, J = 1.8 Hz, p-Ar H), 9.11
(8 H, s, β-pyrrolic H). MS (HR-ESI-FT/ICR): m/z 1119.6653 [M
- Cl]þ, calcd for [C72H92N4Co]þ: 1119.6649.
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-
b0]porphyrinato}cobalt(II) 3. A mixture of 5,10,15,20-tetrakis-
(3,5-di-tert-butylphenyl)quinoxalino[2,3-b0]porphyrin (0.020 g,
0.017 mmol) and cobalt(II) acetate tetrahydrate (0.037 g, 0.147
mmol) in chloroform-methanol (2:1) (21 mL) was heated at
reflux under nitrogen for 24 h. The solution was allowed to cool,
then filtered through a plug of alumina, washed with dichlor-
omethane, and the solvent removed. Dichloromethane (50 mL)
was added to the residue, and the solution washed with deionized
water (3 ꢀ 50 mL). The organic layers were dried over anhydrous
sodium sulfate, and the solvent removed to give a brown-red solid
(0.025 g). Recrystallization from dichloromethane-acetonitrile
gave {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b0]porphyrinato}cobalt(II) 3 (20 mg, 94%) as a dark red-
brown solid, mp. >300 °C. MS (ESI): 1221.85 (Mþ required
1221.69). IR (CHCl3): 3001 m, 1607 w, 1246 w, 1240 m, 1236 s,
1194 s, 1184 w cm-1. UV-vis (CHCl3): 403 (log ε 5.13), 435
(5.00), 555 (4.52), 598 (4.24) 598 nm. MS (HR-ESI-FT/ICR): m/z
1221.68640 [M - Cl]þ, calcd for [C82H94N6Co]þ: 1221.68665.
Chloro{5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquin-
oxalino[2,3-b0:12,13-b00]pophyrinato}cobalt(III) 4. A mixture of
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-bisquinoxalino[2,3-
b0:12,13-b00]porphyrin (0.019 g, 0.015 mmol), cobalt(II) acetate
tetrahydrate (0.078 g, 0.314 mmol) and anhydrous sodium
acetate (0.030 g, 0.362 mmol) in toluene (10 mL) and glacial
acetic acid (10 mL) was heated at reflux under nitrogen for 18 h.
The solution was allowed to cool, then dichloromethane (50
mL) added and the organic layer washed with deionized water
(4 ꢀ 50 mL), saturated sodium hydrogen carbonate solution
(3 ꢀ 50 mL), and deionized water (4 ꢀ 50 mL) again. The or-
ganic layer was dried over anhydrous sodium sulfate, and the
solvent removed. The residue was dissolved in chloroform (10
mL), and concentrated hydrochloric acid (10 M, 1 mL) was
added dropwise. The reaction mixture was then stirred at room
{5,10,16,21,26,31,37,42-Octakis(3,5-di-tert-butylphenyl)-44H,
46H,48H,50H-benzo[100,200:5,6;400,500:50,60]dipyrazine[2,3-b:20,30-
b0]bisporphyrinato}cobalt(II) 6. A solution of 5,10,16,21,26,
31,37,42-octakis(3,5-di-tert-butylphenyl)-44H,46H,48H,50H-
benzo[100,200:5,6;400,500:50,60]dipyrazine[2,3-b:20,30-b0]bisporphy-
rin (225 mg, 0.0999 mmol) in chloroform (140 mL) and metha-
nol (15 mL) was heated at reflux with cobalt(II) acetate tetra-
hydrate (350 mg, 1.405 mmol) for 72 h. Dichloromethane (100
mL) was then added, and the organic phase was washed with
water (2 ꢀ 100 mL), dried over anhydrous sodium sulfate, and
filtered. The filtrate was removed under vacuum, and the residue
purified by chromatography over silica (dichloromethane-light
petroleum, 1:3). The major band was collected, and the solvent
evaporated to dryness to give {5,10,16,21,26,31,37,42-Octakis-
(3,5-di-tert-butylphenyl)-44H,46H,48H,50H-benzo[100,200:5,6;
400,500:50,60]dipyrazine[2,3-b:20,30-b0]bisporphyinato}cobalt(II)
6
(118 mg, 50%) as a brown solid, mp >300 °C. UV-vis
(CHCl3): 422 (log ε 5.35), 449 (5.28), 523 (4.84), 587 (4.28),
662 (4.34) nm. Anal. Calcd for C158H182N12Co2: C, 80.17; H,
7.75; N, 7.1. Found: C, 80.14; H, 7.81; N, 6.8%.
600,6000-{5,50,10,100,15,150,20,200-Octakis(3,5-di-tert-butylphe-
nyl)quinoxalino[2,3-b:20,30-b0}bisporphyrinato}cobalt(II) 7.
A
solution of free-base biquinoxalinyl bis-porphyrin (33 mg,
0.0142 mmol) in chloroform (10 mL) and methanol (15 mL)
was heated at reflux with cobalt(II) acetate tetrahydrate (32 mg,
0.146 mmol) for 48 h. Chloroform (100 mL) was then added,
and the organic phase was washed with water (2 ꢀ 100 mL),
dried over anhydrous sodium sulfate, and filtered. The fil-
trate was removed under vacuum, and the residue recrystal-
lized from a dichloromethane-methanol mixture to afford
600,6000-{5,50,10,100,15,150,20,200-Octakis- (3,5-di-tert-butylphenyl)-
quinoxalino[2,3-b:20, 30-b0}bisporphyrinato}cobalt(II) 7 (32 mg,

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1031
Table 1. Half-Wave Potentials (V vs SCE) of Investigated Compounds in PhCN Containing 0.1 M TBAP
ring oxidation (E1/2
second first
1.35 1.13
)
CoIII/II
reduction
a
b
#cpd
compound
(P)Co
(P)CoCl
Epa
Epc
first
second
third
1
2
3
4
5
6
7
0.64
0.64
0.75
0.64
0.66
0.75
0.67
-0.04
-0.26
-0.01
-0.24
-0.36
-0.05
0.10
-0.87
1.27
1.19c
1.08
1.10
1.30
1.18c
0.90
1.19c
0.90
0.85
1.10
1.18c
-0.87
(PQ)Co
-0.70
-1.82d
-1.61
-1.65
-1.42
-1.69
(QPQ)CoCl
(PQ2)CoCl
Co(P)-TA-(P)Co
Co(PQ)-(QP)Co
-0.63
-1.91d
-1.88d
-1.91d
-1.84d
-0.64
-0.59,-0.73
-0.69
a Oxidation of CoII to CoIII
scan rate of 0.10 V/s.
.
b Reduction of CoIII to CoII. c Overlapping reactions for generation of π-cation radical and dication. d Peak potential at a
92%) as a brown microcrystalline powder, mp >300 °C. UV-vis
(CHCl3): 413 (log ε 5.34), 453sh (5.17), 520sh (4.44), 562 (4.53),
603 (4.43) nm. MS (HR-ESI-FT/ICR): m/z 1221.685799 (M2þ),
calcd for [C164H186N12Co2]2þ: 1221.68435.
1H NMR analysis of 5 gave broad spectra as expected for
a cobalt(III) porphyrin.42,43
(P)CoIIICl 2 was prepared in high yield from its corre-
sponding cobalt(II) analogue by stirring a chloroform
solution of 1 in the presence of concentrated hydrochloric
acid for 1 h. The Q-band is red-shifted from 529 nm in 1 to
544 nm 2, confirmed axial chelation. Furthermore, to
minimize the presence of a high spin state cobalt(III)
which would complicate NMR analyses, 1% d5-pyridine
was added to deuterated chloroform. Under these solu-
tion conditions the chloride ligand in 2 was displaced by
pyridine, leading to a well-resolved spectrum which could
be fully assigned, thus confirming the structural integrity
and oxidation state of this compound.
Results and Discussion
Synthesis. Synthesis of the cobalt porphyrins 1, 3, 6,
and 7 were carried out using standard metalation proce-
dures, that is, refluxing a solution of the corresponding
free-base macrocycle and cobalt(II) acetate tetrahydrate
in chloroform and methanol, solvents that solubilize both
reagents. IR spectra of the metalated products indicated
the absence of inner NH free-base porphyrin stretches,
and mass spectral analysis showed the appropriate mo-
lecular ion of the cobalt porphyrin. UV-vis spectra were
characterized by two major Q bands, also indicating
cobalt(II) chelation. In addition, sharp 1H NMR spectra
were not obtained, consistent with the paramagnetic
character of these compounds.
The cobalt(II) metalation procedure described above
failed to work for the free-base bisquinoxalinoporphyrins
4 and 5, and a solution containing toluene and glacial acetic
acid with excess sodium acetate salt was used to facilitate
metalation in these cases. During the course of the reaction
the color of the mixture turned dark green as Co(III) was
generated.41 This oxidation is feasible as ligands, in parti-
cular those bearing oxygen groups (e.g., CO, NO),21can
facilitate conversion to Co(III) in the presence of oxygen.
However, to be certain of obtaining a pure Co(III) porphyr-
in material after workup, the reaction products were directly
converted to their corresponding cobalt(III) chloride form
by refluxing the chelate in a solution containing concen-
trated HCl giving 4 and 5 as the final products. UV-visible
spectroscopy of 4 suggests both the metal-centered oxida-
tion and the presence of a chloride axial ligand. The Q-band
at 609 nm and the shoulder at 643 nm in 5 are both red-
shifted in comparison to other metal(II) complexes. For
example, the corner copper(II) bisquinoxalinoporphyrin,
(PQ2)Cu, has Q bands at 561 and 602 nm.14 The UV-visible
spectrum of 5 also has three peaks in the Q-band region
located at 668, 615, and 566 nm, and this differs from the
spectrum of the oxidized copper(II) analogue where elec-
tron abstraction occurs from the conjugated macrocycle.14
Electrochemistry. The electrochemistry of each com-
pound was carried out in PhCN containing 0.1 M TBAP.
The reduction and oxidation potentials are summarized
in Table 1 which is divided into three groups: (i) the Co(II)
and Co(III) tetrakis(3,5-di-tert-butylphenyl)porphyrins 1
and 2; (ii) the mono and bis-quinoxalinoporphyrin deri-
vatives 3, 4, and 5, and (iii) the linked bis-cobalt quinox-
alinoporphyrins 6 and 7. The redox behavior of these
three groups is described below.
Tetrakis(3,5-di-tert-butylphenyl)porphyrins 1 and 2.
The electrochemistry of synthetic Co(II) and Co(III)
porphyrins has been reported under numerous solution
conditions.21-40 Up to five redox reactions can be ob-
served depending upon the positive and negative poten-
tial limits of the utilized solvent/supporting electrolyte
system. Most Co(II) porphyrins in nonaqueous media
undergo one or two reductions which are virtually in-
dependent of the solvent and supporting electrolyte,
consistent with a relatively weak axial ligand binding to
Co(II) and the lack of any solvent or ligand binding to
Co(I).21 Derivatives with highly electron-withdrawing
substituents may also undergo three reductions, one at
the cobalt center and two at the macrocycle.44 Three
oxidations may also be seen for many synthetic Co(II)
porphyrins in nonaqueous media with a large positive
potential range such as CH2Cl2 or PhCN. The first
oxidation occurs at a potential which is highly dependent
upon both the solvent/supporting electrolyte mixture and
the type of axial ligand on the singly oxidized species.21
The first electron abstraction from Co(II) porphyrins has
in almost all cases been assigned as leading to formation
of a Co(III) complex but in rigorously dry CH2Cl2 with
(41) Fox, M. A.; Grant, J. V.; Melamed, D.; Torimoto, T.; Liu, C.-Y.;
Bard, A. J. Chem. Mater. 1998, 10, 1771–1776.
(42) Chapman, R. D.; Fleischer, E. B. J. Chem. Soc., Chem. Commun.
1981, 332–333.
(43) Chapman, R. D.; Fleischer, E. B. J. Am. Chem. Soc. 1982, 104, 1582–
1587.
(44) Lin, X. Q.; Boisselier-Cocolios, B.; Kadish, K. M. Inorg. Chem. 1986,
25, 3242–3248.

1032 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
Figure 1. Cyclic voltammograms of (P)CoII 1, (P)CoIIICl 2, (PQ)CoII 3, (QPQ)CoIIICl 4, and (PQ2)CoIIICl 5 in PhCN, 0.1 M TBAP.
weakly coordination axial ligands, a Co(II) porphyrin
π-cation radical has been shown to exist. Whatever the
site of electron transfer in the first one-electron oxidation,
two additional one-electron abstractions will occur to give
the doubly and triply oxidized porphyrins, the latter of
which has been assigned as a Co(III) porphyrin dication.21
The electrochemistry of (P)CoII examined in the pre-
sent study is similar to that of the well-characterized
(TPP)CoII under the same solution conditions.21,22,39
As shown in Figure 1b and Scheme 1, the electrogen-
erated (P)CoIIIClO4 undergoes two oxidations at E1/2
1.13 and 1.35 V, both of which are reversible to stepwise
=
give a Co(III) π-cation radical and dication.
The first one-electron reduction and re-oxidation of
(P)CoIIICl 2 in PhCN (Figure 1a) can also be described
by a box mechanism with two non-coupled redox reactions,
each involving an EC mechanism as shown in Scheme 1.
The neutral Co(III) porphyrin is characterized by a one-
electron reduction at Epc = -0.26 V to give [(P)CoIICl]-
while the one-electron re-oxidation proceeds via (P)CoII
after a rapid loss of Cl- from the reduction product and is
characterized by an anodic peak at Epa = 0.64 V on the
reverse sweep for a scan rate of 0.1 V/s (also marked with an
asterisk in Figure 1a). Two reversible one-electron oxida-
tionsof(P)CoIIICl are located at E1/2 = 0.90 and 1.27 V and
are shifted to less positive potentials as compared to the
same redox processes of electrogenerated (P)CoIIIClO4
(E1/2 = 1.13 and 1.35 V) in 0.1 M TBAP (Table 1 and
Figure 1). This negative shift in E1/2 for the two ring-
centered oxidations of the chloride bound species amounts
to 230 and 80 mV (see Figures 1a,b) and results from an
increased anion binding constant for the higher oxidized
forms of the porphyrin as compared to its reduced form.
The overall combined mechanism for the oxidation and
reduction of (P)CoIIICl and (P)CoII is shown in Scheme 1
where redox processes of the ClO4- coordinated porphyr-
in are illustrated in the top row of the scheme and the
The Co(II)/Co(I) process of (P)CoII occurs at E1/2
=
-0.87 V in PhCN, but the expected second reduction is
located beyond the negative potential limit of the solvent
and is not observed at room temperature. Like in the case
of (TPP)CoII, the Co(II)/Co(III) reaction of (P)CoII is
irreversible, consistent with two non-coupled redox pro-
cesses. One is an oxidation of (P)CoII followed by ClO4
-
binding to the product (an electrochemical EC mechan-
ism) and the other a re-reduction of (P)CoIIIClO4 fol-
-
lowed by loss of ClO4 (also an EC mechanism). The
anodic peak for oxidation of (P)CoII in PhCN is located
at Epa = 0.64 V for a scan rate of 0.1 V/s while the
cathodic peak for re-reduction of (P)CoIIIClO4 is located
at Epc = -0.04 V. Both processes are indicated by an as-
terisk (*) in Figure 1b. Similar reactions have been
reported for numerous other cobalt porphyrins where a
classic box mechanism has been proposed,21,22 and, as
described later in this manuscript, it is also observed for
all seven compounds investigated in the current study.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1033
Scheme 1. Proposed Mechanism for Oxidation and Reduction of (P)CoII and (P)CoIIICl in PhCN, 0.1 M TBAP
literature. As seen from the table, the peak maxima for
(P)CoCl 2 in its neutral and reduced forms are quite
similar to peak maxima for the TPP derivatives in the
same oxidation states and solvent.35 Neither singly re-
duced (P)CoII nor singly reduced (TPP)CoII has a band in
the near-IR region of the spectrum, thus ruling out
assignment of a Co(II) π-anion radical as the reduction
product.
Mono and Bis-quinoxalinoporphyrins 3-5. The redox
behavior of the three monocobalt quinoxalinoporphyr-
ins, (PQ)CoII, (QPQ)CoIIICl, and (PQ2)CoIIICl, parallels
in part what was previously described for quinoxalino-
porphyrins with other central metal ions,3,5,8,14 but sig-
nificant differences are observed for the currently
investigated compounds as described below.
Comparison of (PQ)CoII with (P)CoII. Several major
differences are seen in the cyclic voltammograms of
(PQ)CoII 3 and (P)CoII 1 (Figure 1b,c). The first reduc-
tion of (PQ)CoII occurs at -0.70 V as compared to -0.87
V for (P)CoII and this 170 mV positive shift in E1/2 is
similar to what was previously reported for upon going
from (P)MII to (PQ)MII (M = Zn, Cu, Ni, or Pd) where
the average ΔE1/2 was also 170 mV.8 However, the
spectroelectrochemical data suggest a different site of
electron transfer for the first reduction of the two cobalt
porphyrins, being metal-centered in the case of (P)CoII
and a mixture of metal and ring-centered in singly re-
duced (PQ)CoII 3. A second one-electron reduction of
(PQ)CoII can also be seen in PhCN and is assigned to a
metal-centered reduction on the basis of the spectroelec-
trochemical data. This reaction is not seen for (P)CoII in
Figure 2. UV-visible spectral changes of (P)CoIIICl 2 during controlled
potential(a) reductionat-0.60 V (CoIII f CoII), (b) reductionat -1.20 V
(CoII f CoI), and (c) oxidation at 1.10 V in PhCN containing 0.1 M
TBAP.
the current study but has been reported to occur at E1/2 =
-1.97 V for (TPP)Co under similar solution conditions.
Unlike the 170 mV difference in E1/2 for the first
reduction of (P)CoII and (PQ)CoII, much smaller differ-
ences in potential are observed in the non-coupled CoII f
CoIII and CoIII f CoII reactions of the same two com-
pounds (Figure 1). The second and third one-electron
oxidations of (PQ)CoII are overlapped in potential, and
the peak current for this process at E1/2 = 1.19 V is
approximately double that for the reduction at -0.70 V
under the same experimental conditions. This indicates
an electrochemical mechanism involving two overlapping
one-electron oxidations (an electrochemical EE mechan-
ism) as opposed to a global two-electron abstraction
where the diffusion controlled peak current would be
proportional to n3/2 or 2.77 times larger than a one-
electron transition.45 An overlap of oxidation potentials
Cl- bound porphyrin is at the bottom. It should be noted
that the same CoII/CoI redox process is seen for both
compounds at -0.87 V in PhCN, and the same UV-
visible spectrum is also obtained as a reduction product
independent of the oxidation state of the starting com-
pound. This is not surprising since the same (P)CoII
species is reduced to an identical product in each case,
that is, [(P)CoI]-.
UV-visible spectral changes for the stepwise conver-
sion of (P)CoIIICl to (P)CoII (Epc = -0.26 V) and then
to [(P)CoI]- (E1/2 = -0.87 V) in PhCN are shown in
Figures 2a,b. This figure also shows the spectral changes
for formation of the Co(III) π-cation radical during
controlled potential oxidation at 1.10 V (Figure 2c). A
summary of the absorption maxima is given in Table 2
which also includes results for (TPP)Co taken from the
(45) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamental
and Applications, 2nd ed.; John Wiley & Sons, Inc: New York, 2001.

1034 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
Table 2. UV-Visible Spectra Data (λmax, nm)a Showing Soret and near-IR Bands of the Neutral, Oxidized, and Reduced Cobalt Porphyrins in PhCN
λmax, nm
compound
TPPb
P
PQ
QPQ
PQ2
P-TA-Pc
446, 472
426, 459
369, 426, 780
372, 430, 793
PQ-QPc
[(Por)CoIII þ
]
441, 553, 588
417, 531
364, 428, 514
445, 559, 599
421, 533
365, 437, 513
349, 420, 468
414, 445
414, 445, 792
372, 435
360, 420, 486
411, 447
405, 454, 814
380, 407
353, 440, 508
410, 505
410, 441, 750
373, 410,
430, 476
423, 472
423, 472, 853
372, 421
(Por)CoII
[(Por)Co]-
[(Por)Co]2-
a Bold data for neutral compound. b Data taken from ref 35. c Data for reduction or oxidation of two porphyrin units.
Figure 3. UV-visible spectral changes of (PQ)CoII 3 during controlled potential reduction and oxidation in PhCN, 0.1 M TBAP.
and direct conversion of the neutral porphyrin to its
dication form was previously reported for (PQ)H2
and (PQ)Ni in PhCN and is also known to occur in
non-coordinating media for a variety of nickel porphyrins
with other macrocycles.46
oxidations are characterized by an initial one-electron
abstraction to give [(PQ)CoIII þ
(λmax = 349, 420, and
468 nm) followed by a global two-electron abstraction to
give the Co(III) porphyrin with a doubly oxidized macro-
cycle (an electrochemical EE mechanism as described
above).
A different site of electron transfer is proposed for the
first one-electron reduction of (PQ)CoII 3 compared to
that of (P)CoII 1. The singly reduced product of (P)CoII is
assigned as [(P)CoI]- and has a split Soret band at 365 and
437 nm (see Table 2). This contrasts with the UV-visible
spectrum of electrogenerated [(PQ)CoII]- which has a
split Soret band at 414 and 445 nm, as well as a broadband
in the near-IR region of the spectrum centered at 792 nm
(Figure 3a). This latter near-IR band is diagnostic of a
porphyrin π-anion radical,21 suggesting that the first
electron added to (PQ)CoII resides in part on the por-
phyrin π-ring system where the [(PQ)Co]- product is
assigned as a Co(II) π-anion radical with Co(I) porphyrin
character.
]
An overlapping of two porphyrin oxidations has been
observed in low dielectric constant media such as toluene,
heptane, or benzene47,48 and results from a stronger
binding of the doubly oxidized porphyrin with an anion
from the supporting electrolyte or the original counterion
in the case of porphyrin with M(III) central metal ions. In
the case of electrogenerated [(PQ)CoIII 2þ
] , the shift in
potential for the second oxidation amounts to 160 mV
when compared to compound 1.
UV-visible spectra were also monitored in a thin-layer
cell during each electron transfer process of (PQ)CoII in
PhCN, and the resulting data are shown in Figure 3 and
Table 2. The two reductions of (PQ)CoII are presented in
parts a and b of the figure, and the two oxidations in parts
c and d. Several isosbestic points are obtained, indicating
the absence of spectrally detectable intermediates. The
two reductions of (PQ)CoII each involve a one-electron
addition to give [(PQ)Co]- and [(PQ)Co]2- while the two
The controlled potential of reduction of (PQ)CoII at
-1.90 V then leads to (PQ)CoI]2-, and the resulting
spectral changes during this process are illustrated in
Figure 3b. The final spectrum at the completion of
electrolysis is characterized by a split Soret band at 372
and 435 nm. Similar absorption bands at 365 and 437 nm
are seen in the UV-visible spectrum of [(P)CoI]- (Table 2
and Figure 2b) suggesting the possibility of protonation
on the quinoxaline nitrogen to give [(PQH)CoI]- as
described in the literature for (PQ)Ni, (PQ)Cu, and
(46) Kadish, K. M.; Lin, M.; Van Caemelbecke, E.; Stefano, Q. D.;
Medforth, C. J.; Nurco, D. J.; Nelson, N. Y.; Krattinger, B.; Muzzi, C. M.;
Jaquinod, L.; Xu, Y.; Shyr, D. C.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem.
2002, 41, 6673–6687.
(47) Mu, X. H.; Lin, X. Q.; Kadish, K. M. Electroanlysis 1989, 113–116.
(48) Geng, L.; Ewing, A. G.; Jernigan, J. C.; Murray, R. W. Anal. Chem.
1986, 58, 852–860.

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1035
(PQ)SnX2 (X = OH- or Cl-) after reaction of the elec-
trogenerated dianion with trace water in CH2Cl2.5 This
type of reaction was not confirmed or characterized in the
case of doubly reduced (PQ)CoII because of the extremely
negative reduction potential needed for generation of the
anionic species in PhCN (-1.90 V).
(QPQ)CoIIICl 4 and (PQ2)CoIIICl 5. Cyclic voltam-
mograms of 4 and 5 are shown in Figure 1 which also
includes voltammetric data for 1-3. As seen in the figure,
reduction peak potentials for the non-coupled CoIII/CoII
process of (P)CoIIICl 2 and (QPQ)CoIIICl 4 are virtually
identical to each other and shifted negatively from
the reduction peak potentials of 1 and 3 where the product
of the one-electron oxidation is a Co(III) porphyrin
containing ClO4- from the TBAP supporting electrolyte.
In general, the larger the formation constant between
[(Por)CoIII]þ and an anionic axial ligand (Cl- or ClO4-),
the more negative will be the reduction potential,49 and
this is what is observed in the present case. For example,
the Epc = -0.24 V for the CoIII/CoII reaction of 4
indicates a stronger Cl- binding to [(QPQ)Co]þ as com-
pared to [(P)CoIII]þ which is reduced at Epc = -0.04 V.
(PQ2)CoCl 5 is reduced at Epc = -0.36 V (Figure 1e),
which indicates an even stronger Cl- binding in the case
of [(PQ2)Co]þ.
Figure 4. UV-visible spectral changes of (QPQ)CoIIICl 4 during reduc-
tions at (a) -0.50 V, (b) -0.90 V, (c) -1.70 V in PhCN, 0.1 M TBAP (the
initial spectrum is obtained at 0.70 V).
In contrast to the Co(III) reduction potentials which
depend on the counterion, the peak potentials for oxida-
tion of the Co(II) porphyrins to their CoIII-ClO4- form
are almost the same for all of the investigated compounds,
being equal to 0.64 to 0.66 V in 1, 2, 4, and 5 and 0.75 V in
3. An identical E1/2 is also obtained for the first ring-
centered oxidation of 2 and 4 (0.90 V), indicating that the
presence or absence of the two linear fused Q groups on
the macrocycle has no effect on the oxidation processes.
However, differences in potential do exist in the second
reduction of the same two Co(III) porphyrins which is
located at E1/2 = -0.87 V in 2 and -0.63 V in 4. Two
additional reductions of 4 are also seen at E1/2 = -1.61
and Epc = -1.91 V, neither of which can be measured
for (P)CoII 1 or (P)CoIIICl 2 in the PhCN solvent whose
cut off is approximately -1.90 V.
Thin-layer UV-visible spectral changes during the first
three reductions of (QPQ)CoIIICl 4 are shown in Figure 4,
and peak maxima for the neutral and electroreduced form
of the porphyrin are summarized in Table 2. Initially, the
potential was set at 0.70 V to oxidize any trace (QPQ)CoII
in solution, thus giving a pure spectrum of (QPQ)CoIIICl
which is characterized by three bands at 360, 420, 486 nm
in the Soret region. There is also a visible band at 669 nm.
The product of the first reduction at an applied potential
of -0.50 V exhibits a split Soret band at 411, 447 nm and a
visible band at 652 nm (Figure 4a). This spectrum is
characteristic of a (PQ)M species (M = CuII, ZnII, NiII,
PdII or AuII),4,7,8 thus confirming that the CoII form of
compound 5 is electrogenerated under the given solution
conditions.
CoII π-anion radical character. The third electroreduc-
tion of (QPQ)CoCl 4 at -1.70 V leads to a pure Co(I)
porphyrin based on the spectral changes illustrated in
Figure 4c. Finally, the spectral changes obtained during
the oxidation of (QPQ)CoCl at 1.00 and 1.20 V indicate
formation of a CoIII porphyrin π-cation radical and
dication, respectively, in PhCN, 0.1 M TBAP (Suppor-
ting Information, Figure S1).
Similar electrochemistry and spectroelectrochemistry is
observed for (PQ2)CoIIICl 5 which undergoes four reduc-
tions and two oxidations in PhCN (Figure 1e). The poten-
tials for all six redox processes of 5 are close to those of
(QPQ)CoIIICl 4, a result not observed for previously ex-
amined QPQ and PQ2 derivatives with other metal ions
where the differences in E1/2 are large.14 The spectral
changes observed during reduction and oxidation of (PQ2)-
CoCl 5 are also similar to those of (QPQ)CoCl 4 under the
same experimental conditions and are shown in Supporting
Information, Figure S2. Because of these similarities, the
same reduction/oxidation mechanism is proposed for this
compound as for 4 which is described above.
TA Linked Bis-cobalt Dyad 6. Co(P)-TA-(P)Co 6 un-
dergoes three oxidations and four reductions in PhCN as
illustrated in Figure 5. The first oxidation involves a non-
coupled CoII/CoIII transition like described earlier for the
other compounds, and this is followed by two reversible
ring-centered reactions at E1/2 = 1.10 and 1.30 V versus
SCE. The shape of the current-voltage curve for oxida-
tion of 6 and the potentials of each process are close to
those of (P)Co 1, as would be expected for two non-
interacting (P)CoII macrocycles linked to each other by a
fused TA group. The main difference in electrochemical
behavior between 6 and 1 is that each redox process
involves a global two-electron abstraction in the dyad
The final reduction product after the second controlled
potential electrolysis of (QPQ)CoCl at -0.90 V exhibits a
split Soret band at 405 and 454 nm, as well as a diagnostic
π-radical band at 814 nm (Figure 4b and Table 2). This
species is assigned as having both Co(I) porphyrin and
(49) Truxlllo, L. A.; Davis, D. G. Inorg. Chem. 1975, 47, 2260–2267.

1036 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
Figure 5. Cyclic voltammograms and UV-visible spectral changes for (a) oxidation and (b) reduction CoII(P)-TA-(P)CoII 6 in PhCN, 0.1 M TBAP.
with no difference in the potentials as compared to the
monocobalt macrocycle, indicating equivalent non-inter-
acting redox centers.
The UV-visible spectra obtained after each oxidation
of 6 in a thin-layer cell are shown in Figure 5a, with the
final oxidation products exhibiting bands found in both
electrooxidized (P)CoII 1 and (PQ)CoII 3. For example,
the global 2e- oxidation of 6 (eq 1) at 0.90 V leads to a
Co(III) spectrum with bands at 446 and 472 nm one of
which is comparable to a 445 nm band in the spectrum of
[(P)CoIII]þ (Table 1 and Figure 1) and the other to a 468
The anodic peak potential for conversion of Co(II) to
Co(III) is shifted from 0.64 V in 1 to 0.75 V in 6 while the
reverse Co(III)/Co(II) process appears at Epc = -0.05 V.
The mechanism for the CoII/CoIII process is the same as
shown in Scheme 1 with the only difference being the peak
potential for oxidation which is 0.75 V in 6 and (PQ)CoII
3, but 0.64 to 0.66 V for all of the other compounds (see
Figure 1c and Table 1). It should also be noted that the
two porphyrin ring-centered oxidations of 6 (eqs 1 and 2)
are shifted negatively by 30-50 mV with respect to E1/2
for the same two reactions of the monocobalt compound
1, consistent with a small electron-donating effect of the
fused TA unit which would lead to easier oxidations.
nm band in the spectrum of [(PQ)CoIII þ
Figure 3c).
] (Table 1 and
A following second 2e- oxidation of 6 at 1.12 V occurs
at the porphyrin macrocycle and leads to a spectrum
with a band at 446 nm. A similar band appears at 445 nm
in the spectrum of (P)CoCl 2 after the first ring-centered
oxidation. Finally, the third global two-electron oxida-
tion of 6 (eq 2) at 1.40 V leads to an almost total loss of
intensity in all bands, and this is seen for most porphyrin
dications.
The electrochemical and spectroscopic data for the
oxidation of 6 are thus self-consistent in showing a
lack of interaction between the two porphyrin units
of the dyad across the TA bridge, but this is not the
case for reduction as seen in Figure 5a, where four one-
electron reductions are observed as compared to
2þ
½CoIIIðPÞ-TAðPÞCoIIIꢁ / ½CoIIIðPÞ-TAðPÞCoIIIꢁ4þ þ 2e
ð1Þ
4þ
½CoIIIðPÞ-TAðPÞCoIIIꢁ / ½CoIIIðPÞ-TAðPÞCoIIIꢁ6þ þ 2e
ð2Þ

Article
Inorganic Chemistry, Vol. 49, No. 3, 2010 1037
in the following section. It is interesting to note that
peak potentials for the CoII f CoIII and CoIII f CoII
processes of 6 and 3 are virtually identical in the two
compounds, but this is not the case for the other
redox reactions of these same two compounds as seen in
Figures 5 and 6.
The similarity in oxidation potential for the CoII f CoIII
reactions of (PQ)CoII 3 and CoII(P)-TA(P)CoII 6 is
also reflected in the similar UV-visible spectra of
the two compounds (see Table 2). The spectrum of neu-
tral 6 is similar to that of the (PQ)CoII 3 derivative in
that it has a split Soret band (426 and 459 nm) like
most other (PQ)MII porphyrins.8 The 426 nm band of 6
is also comparable to a band at 421 nm in the spectrum
of (P)CoII (see Table 2). Generation of the half reduced
species at a controlled potential reduction at -0.70 V
(eq 3) then leads to a loss of the 459 nm band, a slight
increase in intensity of the band at 426 nm and the
appearance of a new Q-band at 551 nm. There are also
well-defined isosbetic points as seen in Figure 5b. The
spectrum of this singly reduced product closely resembles
the UV-visible spectrum of neutral (P)CoII (λmax = 421
and 533 nm), suggesting that the electron in the first
addition might be partially localized on the TA bridge
linking the two porphyrins.
Figure 6. Cyclic voltammograms of (a) CoII(P)-TA-(P)CoII 6, (b)
(PQ)CoII 3, and (c) CoII(PQ)-(QP)CoII 7 in PhCN, 0.1 M TBAP.
just one reduction in the case of (P)Co 1. The four
reductions of 6 are proposed to occur as shown in eqs
3-6 and give four different Co(P)-TA-(P)Co reduction
products, two of which are assigned as “half reduced”
dyads.
There is no spectral evidence suggesting reduction
of either the porphyrin macrocycle or the Co(II) center
in the first electron transfer process involving the
TA-linked dyad, but this is not the case for the
second controlled potential reduction of 6 at -1.10 V
(eq 4) where the UV-visible spectrum is characterized
by bands at 369, 426, and 780 nm. This spectrum resem-
bles in part the Co(I) spectrum of [(TPP)CoI]- (364
and 428 nm) or [(P)CoI]- (365 and 437 nm), and it
also resembles in part the product of the (PQ)CoII reduc-
tion which has bands at 414, 445, and 792 nm, and is
assigned as a Co(I) porphyrin with Co(II) π-anion radical
character. A diagnostic near-IR band of a π-anion radical
also appears in singly reduced (PQ)CoII (792 nm),
(QPQ)CoII (814 nm), and (PQ2)CoII (750 nm), all of
which also have a split Soret band (see Table 2), thus
suggesting a similar electron distribution in doubly re-
duced 6 where one electron resides in part on each of the
two cobalt centers and in part on each of the two
porphyrin macrocycles. This apparent distribution of
charge is consistent with 6 having the character of a
molecular wire where electron density is shared through-
out the molecule on both the metal center and the
macrocycle.
Single Bond Linked Bis-cobalt Dyad 7. Co(PQ)-(QP)Co
7 exhibits similar electrochemical behavior as the mono-
cobalt quinoxalinoporphyrin (PQ)Co 3. As seen from
Figures 6b and 6c, the first oxidation of 7 is 80 mV easier
than for oxidation of 3, but the second oxidations of 3 and
7 occur at identical potentials. The first E1/2 values for the
reduction of 3 and 7 are also identical, but the second
reduction of 7 is easier than in 3 and is split into two
peaks at -1.69 and -1.84 V, suggesting a weak interac-
tion between the two Co(PQ) units of the doubly reduced
dyad 7.
-
CoðPÞ-TA-ðPÞCo þ e / ½CoðPÞ-TAðPÞCoꢁ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
-
2 -
½CoðPÞ-TA-ðPÞCoꢁ þ e / ½CoðPÞ-TAðPÞCoꢁ
2 -
3 -
½CoðPÞ-TA-ðPÞCoꢁ þ e / ½CoðPÞ-TAðPÞCoꢁ
3 -
4 -
½CoðPÞ-TA-ðPÞCoꢁ þ e / ½CoðPÞ-TAðPÞCoꢁ
The difference in potentials between the first reduction
of 6 at E1/2 = -0.59 V and the second at E1/2 = -0.73 V
(eqs 3 and 4) amounts to 140 mV while the third and
fourth reductions (eqs 5 and 6) differ by almost 500 mV.
These separations can be compared to the ΔE1/2 values
which range from 150 to 300 mV for interacting redox
centers of bis-porphyrin50 and bis-corrole51,52 dyads
linked in a face-to-face arrangement, thus indicating a
stronger interaction between the singly and the doubly
reduced porphyrin units of 6 than for the dyad in its
neutral CoII form.
Although the electrochemistry of the dyad 6 can
be analyzed in terms of two (P)CoII macrocycles linked
by a fused TA bridge, comparisons should also be made
between the redox reactivity of 6 and that of 3 which
has a single fused quinoxaline group. This comparison
is shown in Figure 6 which also includes a cyclic voltam-
mogram of the single bond connected dyad 7 discussed
(50) Le Mest, Y.; L’Her, M.; Collman, J. P.; Kim, K.; Hendricks, N. H.;
Helm, S. J. Electroanal. Chem. 1987, 234, 277–295.
(51) Guilard, R.; Jerome, F.; Barbe, J.-M.; Gros, C. P.; Ou, Z.; Shao, J.;
Fischer, J.; Weiss, R.; Kadish, K. M. Inorg. Chem. 2001, 40, 4856–4865.
(52) Guilard, R.; Gros, C. P.; Barbe, J.-M.; Espinosa, E.; Jerome, F.;
Tabard, A.; Latour, J.-M.; Shao, J.; Ou, Z.; Kadish, K. M. Inorg. Chem.
2004, 43, 7441–7455.
UV-visible spectral changes obtained during the
three reductions and two oxidations of Co(PQ)-(QP)Co
7 are shown in Figure 7 and Supporting Information,

1038 Inorganic Chemistry, Vol. 49, No. 3, 2010
Zhu et al.
Summary
Seven cobalt(II) and cobalt(III) porphyrins with fused
quinoxaline rings at one or more β,β0-pyrrolic units of the
macrocycle were synthesized and characterized as to their
electrochemical properties in nonaqueous media. Each com-
pound could be cycled between its Co(III), Co(II), and Co(I)
forms under the application of a given oxidizing or reducing
potential. The electrochemical and spectroelectrochemical
properties of [(P)CoIII]þ, (P)CoII, and [(P)CoI]- are similar
to that for the well characterized TPP derivatives in the same
oxidation states. However, this is not the case for the
quinoxalinoporphyrins, where the one-electron reduction
of (PQ)CoII, (QPQ)CoII, and (PQ2)CoII leads to a product
with mixed Co(I) and porphyrin π-anion radical character
followed by generation of a pure Co(I) π-anion radical
species at more negative potentials. The position and number
of quinoxaline groups on the macrocycle has little or no
effect on the redox potentials for the Co(II) f Co(III) or
Co(III) f Co(II) processes, but this is not the case for
the other reactions where significant differences are seen
between the examined compounds. Significant interactions
are also observed between the two porphyrin macrocycles of
the laterally bridged dicobalt(II) bis-porphyrin dyad
Co(P)-TA-(P)Co in its singly and doubly reduced form, but
only weak interactions is seen between the two (PQ)Co units
of Co(PQ)-(QP)Co. The mixed character of the Co(I)
π-anion radical is apparent in both dyad architectures and,
in the case of Co(P)-TA-(P)Co, the sharing of charge between
metal and macrocycle is consistent with this class of molecule
being a candidate for molecular wire applications.
Acknowledgment. This work was supported by grants
from the Robert A. Welch Foundation (K.M.K., Grant
E-680) and the NSF of Jiangsu Province P.R. China
(BK2008226). Support was also provided by a Discovery
Research Grant (DP0208776) from the Australian Re-
search Council to M.J.C.
Figure 7. UV-visible spectral changes of CoII(PQ)-(QP)CoII 7 during
reductions and oxidations in PhCN, 0.1 M TBAP.
Figure S3, respectively. Two sets of spectral changes
can be observed for the first reduction at -0.90 V,
consistent with two overlapped one-electron reduction
processes. Comparison with the spectral changes for
(PQ)Co 3 under the same solution conditions leads to a
conclusion that the same redox mechanism is operative in
the case of 7.
Supporting Information Available: Figures S1-S3 illustrate
thin-layer UV-visible spectral changes obtained for
(QPQ)CoCl and Co(PQ)-(QP)Co during the oxidations and
for (PQ2)CoCl during the oxidation and reduction in PhCN
containing 0.1 M TBAP. This material is available free of charge
via the Internet at http://pubs.acs.org.