Spectroelectrochemical Characterization of Substituted Cobalt Nitrosyl Porphyrins

Full text of DOI:10.1021/ic9604223

6904
Inorg. Chem. 1996, 35, 6904-6906
first oxidation product [CoTPP(NO)]+ (1+) has been observed
using infrared spectroelectrochemistry, but the second oxidation
product has not been detected spectroscopically.7 This is
attributed to the rapid dissociation of NO from [CoTPP(NO)]2+
during controlled-potential electrolysis at the second oxidation
potential. Herein, we describe infrared spectroelectrochemical
studies on the series of cobalt nitrosyl porphyrins 1-3 and report
the observation of NO binding in the doubly oxidized com-
plexes, 12+-32+. Functionalization of TPP at its meso phenyl
para position was found to modulate the cobalt-nitric oxide
interaction. The attachment of electron-donating or -withdraw-
ing substituents on the porphyrin ring system alters the electronic
properties of the porphyrin ring and influences the binding of
nitric oxide to the metalloporphyrin.
Spectroelectrochemical Characterization of
Substituted Cobalt Nitrosyl Porphyrins
Abhay D. Kini, John Washington,
Clifford P. Kubiak,* and Bruce H. Morimoto*
Department of Chemistry, 1393 Brown Laboratory,
Purdue University, West Lafayette, Indiana 47907-1393
ReceiVed April 17, 1996
Introduction
Nitrosyl metalloporphyrins have been studied extensively over
the past 2 decades and characterized by numerous spectroscopic,
electrochemical, and structural techniques.1-5 The importance
of nitrosyl metalloporphyrins in human physiology gains greater
importance in light of the fact that nitric oxide has been found
to target heme-containing proteins.6 The cobalt nitrosyl por-
phyrin CoTPP(NO) (TPP ) tetraphenylporphyrin dianion) has
been used as a model for metalloporphyrin-nitric oxide
interaction.7 The para positions of the meso phenyl rings on
TPP are readily substituted, allowing the series of CoT(p-X)-
PP(NO) (X ) H, 1; OMe, 2; NO2, 3) compounds to be
synthesized.
Experimental Section
General Procedures. All moisture sensitive reactions were carried
out under nitrogen or argon in oven dried glassware using drybox or
standard Schlenk techniques. The syntheses of T(p-X)PPH2 (X ) H,
OMe, NO2) were adapted from published procedures.8-10 Metallopor-
phyrins were either purchased from Midcentury Chemicals or synthe-
sized by adaptations of literature procedures.11,12 Nitric oxide was
purchased from Matheson and passed through a column of KOH pellets
before use to remove higher oxides of nitrogen. Flash column
chromatography was performed using silica gel 60G (EM Science,
230-400 mesh) or alumina (Fisher Scientific, 80-200 mesh). Ultra-
violet-visible spectra were obtained using a Hitachi U-2000 spectro-
photometer. Infrared spectra were obtained as films in dichloromethane
(CH2Cl2) using a Mattson Research Series spectrometer.
Electrochemistry. Tetrabutylammonium hexafluorophosphate (TBAP)
was purchased from Aldrich, recrystallized from ethanol, dried at 120
°C under vacuum for 24 h, and subsequently stored in a drybox. Cyclic
voltammetry experiments were conducted in CH2Cl2 with 0.1 M TBAP
as the supporting electrolyte. Dichloromethane was distilled under
nitrogen from CaH2 prior to use.
Infrared Spectroelectrochemistry (SEC). The design of the
reflectance IR spectroelectrochemical cell used in the nitrosyl metal-
loporphyrin study was reported previously and is based on the cell
originally used by Mann and co-workers.13 Infrared spectral changes
accompanying thin-layer bulk electrolyses were measured using a flow-
through spectroelectrochemical cell. All spectroelectrochemical experi-
ments were carried out using 5 mM CH2Cl2 solutions of CoT(p-
X)PP(NO) with 0.1 M TBAP as the supporting electrolyte. All
solutions were prepared in a drybox and were degassed completely
before injection into the spectroelectrochemical cell. Blank CH2Cl2
solutions of 0.1 M TBAP were used for the FT-IR difference spectra.
A PAR Model 175 universal programmer with a PAR Model 176
Current Follower was used to effect and monitor thin layer bulk
electrolyses. The IR spectra were acquired using a Mattson Research
Series FTIR with an external sampling port and a MCT (mercury-
cadmium-telluride) detector.
Synthesis of CoT(p-X)PP(NO) (X ) H, 1; OMe, 2; NO2, 3). The
nitrosylated cobalt porphyrins CoT(p-X)PP(NO) (X ) H, 1; OMe, 2;
NO2, 3) were synthesized by adapting a procedure described by Scheidt
and Hoard.5 The cobalt porphyrins (CoT(p-X)PP; 200 mg) were
dissolved in dichloromethane and carefully degassed. Dry piperidine
(1.5 mL) was added and NO was bubbled through a potassium
hydroxide (KOH) scrubber and into the solution as a steady stream for
The unsubstituted species CoTPP(NO) (1) undergoes two
reversible oxidations in CH2Cl2 (eqs 1 and 2). The single
electron oxidation of 1 has also been studied by spectroelec-
trochemistry.7 The first oxidation is believed to be localized
CoTPP(NO) f [CoTPP(NO)]+ + e-
E1/2 ) 1.01 V Vs SCE
(1)
(2)
[CoTPP(NO)]+ f [CoTPP(NO)]2+ + e-
E1/2 ) 1.25 V Vs SCE
within the conjugated π system of the porphyrin ring, whereas
the second oxidation is postulated to be metal centered. The
(1) Wayland, B. B. O., L. W. J. Chem. Soc., Chem. Commun. 1973, 897.
(2) Wayland, B. B.; Olson, L. W. Inorg. Chim. Acta 1974, 11, L23.
(3) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17.
(4) Scheidt, W. R.; Hatano, K.; Rupprecht, G. A.; Piciculo, P. L. Inorg.
Chem. 1978, 18, 292.
(5) Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1973, 95, 8281.
(6) (a) Palmer, R. M. J.; Ferrige, A. G.; Moncada, S. Nature (London)
1987, 327, 524. (b) Ignarro, L. J.; Buga, G. M.; Wood, K. S.; Byrnes,
R. E.; Chaudhari, G. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 9265. (c)
Palmer, R. M. J.; Ashton, D. S.; Moncada, S. Nature (London) 1988,
333, 664
(8) Semeikin, A. S. K., O. L.; Berezin, B. D. Khim. Geterotsikl. Soedin.
1982, 10, 1046.
(9) Bettelheim, A. W., B. A.; Raybuck, S. A.; Murray, R. W. Inorg. Chem.
1987, 26, 1009.
(10) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976,
98, 3484.
(11) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
(12) Buchler, J. W. In Synthesis and Properties of Metalloporphyrins;
Buchler, J. W., Ed.; Academic Press Inc.: New York, 1978; Vol. I,
Part A.
(7) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27, 1489.
(13) Wittrig, R. E.; Kubiak, C. P. J. Electroanal. Chem. 1995, 393, 75.
S0020-1669(96)00422-3 CCC: $12.00 © 1996 American Chemical Society

Notes
Inorganic Chemistry, Vol. 35, No. 23, 1996 6905
Table 2. Spectroelectrochemical Data for CoT(p-X)PP(NO)
a
FT-IR (νNtO, cm-1
)
CoT(p-X)-
PP(NO)
[CoT(p-X)-
CoT(p-X)-
X
PP(NO)]+
PP(NO)]2+
1
2
3
H
OMe
NO2
1682
1680
1694
1726
1722
1732
1756
1746
1779
a Recorded in CH2Cl2 in spectrolectrochemical cell.
12 cm-1) in the ν(NtO) band compared to CoTPP(NO) whereas
the -OMe substituent resulted in only a small decrease (∆ν )
2 cm-1) in ν(NtO). The increase in ν(NtO) for 3, as
compared to 1 and 2, is attributed to a decrease in porphyrin to
cobalt electron donation. The decreased electron density at the
Co center reduces π back-bonding, and thereby increases the
NtO stretching frequency. A similar explanation can be
applied to the decrease in the NtO stretching frequency for 2.
In this case, the electron-donating -OMe group decreased
ν(NtO) by increasing the electron donating properties of the
porphyrin ring. Again, the magnitude and direction of the
changes in ν(NtO) observed with meso phenyl substitution are
in accord with Hammet σp values.14
Solution spectroelectrochemical studies were carried out on
1-3. The spectral changes observed at the two oxidation
potentials are shown in Figures 2 and 3, and the ν(NtO) data
for each oxidation state are collected in Table 2. The application
of potentials just anodic of the first oxidation potential to CH2-
Cl2 solutions of 1-3 resulted in the smooth conversion of CoT-
(p-X)PP(NO) to [CoT(p-X)PP(NO)]+, as monitored by infrared
spectroelectrochemistry (Figure 2).
Figure 1. Cyclic voltammogram of CoT(p-OMe)PP(NO) (2) in CH2-
Cl2 with 0.1 M TBAP as supporting electrolyte, V Vs SCE.
Table 1. Electrochemical Data for CoT(p-X)PP(NO)
X
E1/2(+/0)a
E1/2(+/2+)a
Hammetb σp
1
2
3
H
OMe
NO2
1.00 (1.01)c
0.99
1.38
1.37 (1.25)c
1.31
1.49
0
-0.28
0.81
a Cyclic voltammograms recorded in CH2Cl2, V Vs SCE. b March,
J. M. AdVanced Organic Chemistry: Reactions, Mechanisms, and
Structures; Wiley: New York, 1985; p 244. c Values in paretheses are
those obtained by Kadish et al.: see Kadish, K. M.; Mu, X. H.; Lin,
X. Q. Inorg. Chem. 1988, 27, 1489.
0.5 h. The solution was purged with nitrogen to remove excess NO
and subsequently heated to boiling. The nitrosyl porphyrin precipitated
after addition of degassed methanol. Products 1-3 were filtered and
dried under aspirator vacuum. The yields of the nitrosylated metal-
loporphyrins 1-3 were ca. 50%.
Results and Discussion
The observed spectral changes were fully reversible and, in
each case, single electron oxidation caused a ca. 40 cm-1
increase in ν(NtO). As expected, the ν(NtO) absorption band
for 3 was observed at the highest frequency (1732 cm-1) in the
singly-oxidized state, while ν(NtO) for 2 was detected at the
lowest frequency (1722 cm-1). The second oxidations of 1-3
were also studied and reasonably smooth conversions were
observed (Figure 3). A notable result of our studies is that
ν(NtO) absorption bands were observed for all doubly oxidized
[CoT(p-X)PP(NO)]2+ (X ) H, OMe, NO2) species. The
ν(NtO) band for the doubly oxidized [CoTPP(NO)]2+ had
eluded observation in an earlier study.7 The observed ν(NtO)
absorption bands of the doubly-oxidized species followed the
Hammet σp values with 2 having the lowest ν(NtO) frequency
(1746 cm-1) and 3 the highest (1779 cm-1). Upon oxidation
of 2 to [CoT(p-OMe)PP(NO)]2+ a broad ν(NtO) band was
observed while the two electron oxidations of 1 and 3 gave
rise to sharp, well-defined ν(NtO) bands. The second oxida-
tions for all the three porphyrins were reversible, although
intensity losses in the ν(NtO) absorption bands occurred when
solutions of 1-3 were maintained at the second oxidation
potentials for extended periods of time. This loss in intensity
was attributed to the instability and ensuing decomposition of
The cyclic voltammograms of the -OMe and -NO2-
substituted porphyrins displayed two reversible single electron
oxidations, similar to those reported for CoTPP(NO).7
A
representative cyclic voltammogram of CoT(p-OMe)PP(NO) is
shown in Figure 1 and the oxidation potentials for 1-3 are
collected in Table 1.
The most striking feature of the data found in Table 1 is the
large anodic shift in the first oxidation potential E1/2(0/+)
observed for the nitro-substituted porphyrin 3, compared to 1
and 2. The difference in E1/2(0/+) between 1 and 2 was small
(∆E ) 10 mV), and indicated that the change from CoTPP-
(NO) to CoT(p-OMe)PP(NO) only slightly perturbed the
electronic environment of the porphyrin ring. This is in sharp
contrast to the large 380 mV anodic shift in E1/2(0/+) observed
for CoT(p-NO2)PP(NO). These observations are consonant with
the differences in Hammet σp values for the meso phenyl
substituents (Table 1).14 The moderate electron-donating -OMe
group caused only a slight cathodic variation in E1/2(0/+) while
the strongly electron-withdrawing -NO2 group effected a large
anodic shift in E1/2(0/+). Comparatively smaller differences
in the second oxidation potentials E1/2(+/2+) for 1-3 were
observed. Thus, the data in Table 1 are consistent with the
conclusion that the first oxidation of CoT(p-X)PP(NO) is
porphyrin ring based.7,10 Substitution of the porphyrin primarily
affected the first oxidation potential while the second metal-
centered oxidation potential was little changed.
[CoT(p-X)PP(NO)]2+ 7
. The observed instability of [CoT(p-X)-
PP(NO)]2+ is expected in light of previous reports regarding
the oxidation of CoTPP(NO). Kadish has proposed a possible
decomposition pathway for [CoTPP(NO)]2+ in which NO
liberation is subsequently followed by further oxidation of
[CoTPP]2+ at the working electrode.7 The NO dissociation
pathway is plausible based on measured E1/2(2+/3+) oxidation
potentials for [CoT(p-X)PP]2+ (X ) H, +1.23 V; OMe, +1.21
V; NO2, +1.40 V Vs SCE). The potentials required to
electrochemically generate [CoT(p-X)PP(NO)]2+ are sufficiently
anodic to immediately oxidize [CoT(p-X)PP]2+, produced by
NO dissociation, to [CoT(p-X)PP]3+. The CofNO π back-
The infrared spectra of 1-3 (Table 2) also showed substituent
effects. The -NO2 group caused a significant increase (∆ν )
(14) (a) March, J. M. AdVanced Organic Chemistry: Reactions, Mecha-
nisms, and Structures; Wiley: New York, 1985; p 244. (b) Malinski,
T.; Chang, D.; Bottomley, L. A.; Kadish, K. M. J. Am. Chem. Soc.
1982, 21, 4248-4253. (c) Kadish, K. M.; Morrison, M. M. J. Am.
Chem. Soc. 1976, 98, 3326-3328. (d) Kadish, K. M.; Morrison, M.
M. Inorg. Chem. 1976, 15, 980-982.

6906 Inorganic Chemistry, Vol. 35, No. 23, 1996
Notes
Figure 2. IR spectral changes in the ν(NtO) region following
oxidation of CoT(p-X)PP(NO) to [CoT(p-X)PP(NO)]+: (a) X ) H, 1;
(b) X ) OMe, 2; (c) X ) NO2, 3.
Figure 3. IR spectral changes in the ν(NtO) region following
oxidation of [CoT(p-X)PP(NO)]+ to [CoT(p-X)PP(NO)]2+: (a) X )
H, 1; (b) X ) OMe, 2; (c) X ) NO2, 3.
Precedence for this observation is reported for iron nitrosyl
porphyrins whereby functionalization was shown to alter the
infrared ν(NtO) stretch.15 Reflectance infrared spectroelectro-
chemical experiments were undertaken to further investigate the
electrochemical properties. For all three compounds studied,
ν(NtO) bands were observed for the first and second oxidation
products, a feature previously unreported. The spectroelectro-
chemical studies also implied that the Co-NO bonding in the
CoT(p-X)PP(NO)]2+ species of 1-3 was mainly σ in character.
bonding contribution in [CoT(p-X)PP(NO)]2+ is expected to be
small due to the presence of a Co(III) center. This suggests
that the Co-NO bonding interaction in CoT(p-X)PP(NO)]2+
should be principally σ in character with the nitric oxide acting
primarily as a weak donor ligand. This is consistent with the
proposed lability of NO and the negligible variation in the
observed stabilities of [CoT(p-X)PP(NO)]2+ with meso phenyl
substituent. The instability of the doubly-oxidized forms of 1-3
enjoins the use of infrared spectroelectrochemsitry for the in
situ generation and subsequent spectroscopic studies of the
Acknowledgment. We would like to thank Dr. Mark Lipton
for providing advice and use of his laboratory for the porphyrin
syntheses. This work was supported in part by Grant NS33230
from the National Institute of Health (B.H.M.), by Grant
CHE9319173 from the National Science Foundation (C.P.K.),
by a predoctoral fellowship to A.D.K. from the American Heart
Association, Indiana Affiliate, by a postdoctoral fellowship to
J.W. from the Natural Sciences and Engineering Research
Council of Canada, and by funding to B.H.M. as a Cottrell
Scholar of the Research Corp.
chemical reactivity of [CoT(p-X)PP(NO)]2+
.
Summary
Nitrosyl metalloporphyrin interactions were modulated by
changing the functional group at the para position of the meso
phenyl ring in a series of compounds CoT(p-X)PP(NO) (X )
H, OMe, NO2). Cyclic voltammetry indicated that porphyrin
ring substitution primarily affected the first oxidation potential,
which is expected to be porphyrin ring based. Comparatively
smaller effects were observed for the second, metal-based,
oxidation potential. We also demonstrated that the strength of
the cobalt nitrosyl interaction, as monitored by ν(NtO), can
be controlled by varying the substituents on the porphyrin ring.
IC9604223
(15) Yoshimura, T. Bull. Chem. Soc. Jpn. 1991, 64, 2819.