Synthesis, Characterization, and Spectroelectrochemistry of Cobalt Porphyrins Containing Axially Bound Nitric Oxide

Article abstract of DOI:10.1021/ic960031o

Several cobalt nitrosyl porphyrins of the form (T(plm-X)PP)Co(NO) (plm-X = p-OCH₃ (1), p-CH₃ (2), m-CH₃ (3), p-H (4), m-OCH₃ (5), p-OCF₃ (6), p-CF₃ (7), p-CN (8)) have been synthesized in 30-85percent yields by reaction of the precursor cobalt porphyrin with nitric oxide. Compounds 1-7 were also prepared by reaction of the precursor cobalt porphyrin with nitrosonium tetrafluoroborate followed by reduction with cobaltocene. Compounds 1-8 have been characterized by elemental analysis, IR and ¹H NMR spectroscopy, mass spectrometry, and UV-vis spectrophotometry. They are diamagnetic and display V_NO bands in CH₂Cl₂ between 1681 and 1695 cm^-1. The molecular structure of 1, determined by a single-crystal X-ray crystallographic analysis, reveals a Co-N-O angle of 119.6(4)°. Crystals of 1 are monoclinic, P2/c, with a = 15.052(1) A?, b = 9.390(1) A?, c = 16.274(2) A?, β = 111.04(1)°, V = 2146.8(4) A?³, Z = 2, T = 228(2) K, D(calcd) = 1.271 g cm^-3, and final R1 = 0.0599 (wR2 = 0.1567, GOF = 1.054) for 3330 observed reflections with I ≥ 2σ(I). Cyclic voltammetry studies in CH₂Cl₂ reveal that compounds 1-7 undergo two reversible oxidations and two reversible reductions at low temperature. This is not the case for compound 8, which undergoes two reversible reductions but an irreversible oxidation due to adsorption of the oxidized product onto the electrode surface. Combined electrochemistry-infrared studies demonstrate that each of the compounds 1-7 undergoes a first oxidation at the porphyrin π ring system and a first reduction at either the metal center or the nitrosyl axial ligand. The formulation for the singly oxidized products of compounds 1-7 as porphyrin π-cation radicals was confirmed by the presence of bands in the 1289-1294 cm^-1 region (for compounds 1-5), which are diagnostic IR bands for generation of tetraarylporphyrin π-cation radicals.

Full text of DOI:10.1021/ic960031o

6
530
Inorg. Chem. 1996, 35, 6530-6538
Synthesis, Characterization, and Spectroelectrochemistry of Cobalt Porphyrins Containing
Axially Bound Nitric Oxide
,
†
†
†
†
‡
George B. Richter-Addo,* Shelly J. Hodge, Geun-Bae Yi, Masood A. Khan, Tianshu Ma,
‡
‡
,‡
Eric Van Caemelbecke, Ning Guo, and Karl M. Kadish*
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman,
Oklahoma 73019, and Department of Chemistry, University of Houston, Houston, Texas 77204-5641
X
ReceiVed January 11, 1996
Several cobalt nitrosyl porphyrins of the form (T(p/m-X)PP)Co(NO) (p/m-X ) p-OCH3 (1), p-CH3 (2), m-CH3
(
3), p-H (4), m-OCH3 (5), p-OCF3 (6), p-CF3 (7), p-CN (8)) have been synthesized in 30-85% yields by reaction
of the precursor cobalt porphyrin with nitric oxide. Compounds 1-7 were also prepared by reaction of the
precursor cobalt porphyrin with nitrosonium tetrafluoroborate followed by reduction with cobaltocene. Compounds
1
vis spectrophotometry. They are diamagnetic and display νNO bands in CH2Cl2 between 1681 and 1695 cm .
The molecular structure of 1, determined by a single-crystal X-ray crystallographic analysis, reveals a Co-N-O
angle of 119.6(4)°. Crystals of 1 are monoclinic, P2/c, with a ) 15.052(1) Å, b ) 9.390(1) Å, c ) 16.274(2) Å,
1
-8 have been characterized by elemental analysis, IR and H NMR spectroscopy, mass spectrometry, and UV-
-
1
3
-3
â ) 111.04(1)°, V ) 2146.8(4) Å , Z ) 2, T ) 228(2) K, D(calcd) ) 1.271 g cm , and final R1 ) 0.0599 (wR2
0.1567, GOF ) 1.054) for 3330 “observed” reflections with I g 2σ(I). Cyclic voltammetry studies in CH2Cl2
)
reveal that compounds 1-7 undergo two reversible oxidations and two reversible reductions at low temperature.
This is not the case for compound 8, which undergoes two reversible reductions but an irreversible oxidation due
to adsorption of the oxidized product onto the electrode surface. Combined electrochemistry-infrared studies
demonstrate that each of the compounds 1-7 undergoes a first oxidation at the porphyrin π ring system and a
first reduction at either the metal center or the nitrosyl axial ligand. The formulation for the singly oxidized
products of compounds 1-7 as porphyrin π-cation radicals was confirmed by the presence of bands in the 1289-
-
1
1
294 cm region (for compounds 1-5), which are diagnostic IR bands for generation of tetraarylporphyrin
π-cation radicals.
Introduction
prominent attention for the binding and activation of nitric oxide.
For example, the water-soluble [(2-TMPyP)Co(NO)]4 (2-
TMPyP ) (N-methyl-2-pyridiniumyl)porphyrin) has been pre-
+
Studies into the binding of nitric oxide (NO) by metallopor-
phyrins have aided our understanding of how NO interacts with
heme-containing biomolecules, such as hemoglobin, myoglobin,
and soluble guanylate cyclase.1 Many iron nitrosyl porphyrins
5
+
pared and its [(2-TMPyP)Co] precursor shown to be a good
8
electrocatalyst for NO and nitrite reduction. Furthermore,
-4
(
TPP)Co (TPP ) the dianion of tetraphenylporphyrin) supported
5,6
have been examined as model compounds during such studies,
by TiO2 or imidazole/SiO2 is unusually active for the reduction
but other metalloporphyrins are also known to form nitrosyl
derivatives.7 Although they have not been studied as much as
their iron analogs, cobalt nitrosyl porphyrins have also gained
of NO by carbon monoxide or hydrogen, respectively.9
The substitution of iron by cobalt in heme and synthetic
porphyrins has also gained attention in a biological setting. For
example, (TPP)Co(NO) has been explored as an isoelectronic
†
University of Oklahoma.
University of Houston.
Abstract published in AdVance ACS Abstracts, October 1, 1996.
‡
10
model for oxygenated protoheme. The NO adducts of cobalt-
X
11
substituted myoglobin and hemoglobin are also known. Fajer
(
1) (a) Stone, J. R.; Marletta, M. A. Biochemistry 1994, 33, 5636 and
references therein. (b) Wang, J.; Rousseau, D. L.; Abu-Soud, H. M.;
Stuehr, D. J. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 10512. (c) Ignarro,
L. J. Semin. Hematol. 1989, 26, 63. (d) Craven, P. A.; DeRubertis, F.
R. Biochim. Biophys. Acta 1983, 745, 310. (e) Craven, P. A.;
DeRubertis, F. R. J. Biol. Chem. 1978, 253, 8433. (f) Hurshman, A.
R.; Marletta, M. A. Biochemistry 1995, 34, 5627.
(6) (a) Bohle, D. S.; Hung, C.-H. J. Am. Chem. Soc. 1995, 117, 9584. (b)
Scheidt, W. R.; Piciulo, P. L. J. Am. Chem. Soc. 1976, 98, 1913. (c)
Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17. (d)
Scheidt, W. R.; Brinegar, A. C.; Ferro, E. B.; Kirner, J. F. J. Am.
Chem. Soc. 1977, 99, 7315. (e) Nasri, H.; Haller, K. J.; Wang, Y.;
Huynh, B. H.; Scheidt, W. R. Inorg. Chem. 1992, 31, 3459. (f)
Yoshimura, T.; Ozaki, T. Arch. Biochem. Biophys. 1984, 229, 126.
(g) Wayland, B. B.; Olson, L. W. J. Am. Chem. Soc. 1974, 96, 6037.
(h) Waleh, A.; Ho, N.; Chantranupong, L.; Loew, G. H. J. Am. Chem.
Soc. 1989, 111, 2767. (i) Lipscomb, L. A.; Lee, B.-S.; Yu, N.-T. Inorg.
Chem. 1993, 32, 281. (j) Lancon, D.; Kadish, K. M. J. Am. Chem.
Soc. 1983, 105, 5610.
(
2) Yu, A. E.; Hu, S.; Spiro, T. G.; Burstyn, J. N. J. Am. Chem. Soc.
1994, 116, 4117.
(
(
3) Traylor, T. G.; Sharma, V. S. Biochemistry 1992, 31, 2847.
4) Selected reviews on the biology of NO: (a) Nitric Oxide: Biochem-
istry, Molecular Biology, and Therapeutic Implications; Ignarro, L.,
Murad, F., Eds.; Advances in Pharmacology 34; Academic: San
Diego, CA, 1995. (b) Moncada, S.; Palmer, R. M. J.; Higgs, E. A.
Pharmacol. ReV. 1991, 43, 109. (c) Ignarro, L. J. Annu. ReV.
Pharmacol. Toxicol. 1990, 30, 535.
(7) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls; Oxford University
Press: New York, 1992; Chapters 2 and 6.
(
5) (a) Scheidt, W. R.; Frisse, M. E. J. Am. Chem. Soc. 1975, 97, 17. (b)
Mu, X. H.; Kadish, K. M. Inorg. Chem. 1988, 27, 4720. (c) Choi,
I.-K.; Liu, Y.; Feng, D.; Paeng, K.-J.; Ryan, M. D. Inorg. Chem. 1991,
(8) Cheng, S.-H.; Su, Y. O. Inorg. Chem. 1994, 33, 5847.
(9) (a) Mochida, I.; Suetsugu, K.; Fujitsu, H.; Takeshita, K. J. Chem. Soc.,
Chem. Commun. 1982, 166. (b) Tsuji, K.; Imaizumi, M.; Oyoshi, A.;
Mochida, I.; Fujitsu, H.; Takeshita, K. Inorg. Chem. 1982, 21, 721.
(10) Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1973, 95, 8281.
(11) (a) Hori, H.; Ikeda-Saito, M.; Leigh, J. S.; Yonetani, T. Biochemistry
1982, 21, 1431. (b) Yu, N.-T.; Thompson, H. M.; Mizukami, H.;
Gersonde, K. Eur. J. Biochem. 1986, 159, 129.
3
0, 1832. (d) Yoshimura, T. Bull. Chem. Soc. Jpn. 1991, 64, 2819.
(e) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
1
1
993, 115, 9568. (f) Lan c¸ on, D.; Kadish, K. M. J. Am. Chem. Soc.
983, 105, 5610. (g) Wayland, B. B.; Olson, L. W. J. Am. Chem.
Soc. 1974, 96, 6037.
S0020-1669(96)00031-6 CCC: $12.00 © 1996 American Chemical Society

Cobalt Porphyrins Containing Nitric Oxide
Inorganic Chemistry, Vol. 35, No. 22, 1996 6531
and co-workers have prepared porphyrin and hydroporphyrin
complexes such as (OEP)Co(NO), (OEC)Co(NO), or (OEiBC)-
Co(NO) and have spectroscopically characterized their π-cation
radical oxidation products (OEP, OEC, and OEiBC are the
dianions of octaethylporphyrin, octaethylchlorin, and octaeth-
ylisobacteriochlorin, respectively).12 Importantly, Burstyn has
shown that (protoporphyrin-IX)Co(NO) activates soluble
guanylate cyclase to a greater extent than do the Fe and Mn
analogs.13
to use. Nitrogen of “ultrahigh-purity grade” for electrochemistry was
purchased from Trigas (Houston, TX).
Instrumentation. Infrared spectra were recorded on a Bio-Rad FT-
1
55 FTIR spectrometer or a Nicolet Magna-IR 550 FTIR spectrometer
-
1
with a resolution of 2-4 cm . Proton NMR spectra were obtained
on a Varian XL-300 spectrometer and the signals referenced to the
3
residual signal of the solvent employed (CDCl ). All couplings are in
hertz. FAB mass spectra were obtained on a VG-ZAB-E mass
spectrometer. UV-vis spectra were recorded on a Hewlett Packard
Model 8452A diode array instrument.
The upsurge of interest in cobalt nitrosyl porphyrins led us
to extend the data available on cobalt nitrosyl porphyrins
Cyclic voltammetry was carried out with an EG&G Model 173
potentiostat or an IBM Model EC 225 voltammetric analyzer. Current-
voltage curves were recorded on an EG&G Princeton Applied Research
Model RE-0151 X-Y recorder. A three-electrode system was used and
consisted of a platinum button working electrode, a platinum wire
counter electrode, and a saturated calomel reference electrode (SCE).
The reference electrode was separated from the bulk of the solution by
a fritted-glass bridge filled with the solvent/supporting electrolyte
mixture. Low-temperature measurements were performed by placing
the electrochemical cell in an acetone bath which was cooled at the
desired temperature by addition of dry ice. All potentials are reported
versus the SCE.
Thin-layer controlled-potential electrolyses were carried out with an
EG&G Model 173 potentiostat, and current-time curves were recorded
on an EG&G Princeton Applied Research Model RE-0151 X-Y
recorder. The design of the thin-layer cell has been described
elsewhere.17
10,12,14
containing a TPP or OEP macrocycle
and study how subtle
but systematic changes in porphyrin ring basicity might be
related to changes in the spectroscopic and electrochemical
properties of structurally related cobalt nitrosyl porphyrins. This
is examined in the present paper for eight different porphyrins
of the form (T(p/m-X)PP)Co(NO), where X is an electron-
donating or electron-withdrawing substituent on the para or meta
positions of the four phenyl rings of the porphyrin. The singly
oxidized compounds are also characterized with respect to the
site of oxidation and the effect of substituents on their
spectroscopic properties. The investigated compounds are
+
represented as (P)Co(NO) and [(P)Co(NO)] where P is the
dianion of a given porphyrin ring.
Infrared spectroelectrochemical measurements were carried out with
a Nicolet Magna-IR 550 FTIR spectrometer. The design of the thin-
Experimental Section
1
8
layer spectroelectrochemical cell is described in the literature. All
spectra were collected under a nitrogen atmosphere, and multiple scans
were taken over the desired frequency region in order to improve the
signal-to-noise ratio. Potentials for oxidation of each compound were
applied and monitored with an EG&G Model 173 potentiostat. The
concentration of each porphyrin was approximately 1 mM, while that
of the supporting electrolyte, which does not interfere in the investigated
All reactions were performed under an atmosphere of prepurified
nitrogen using standard Schlenk glassware and/or in an Innovative
Technology Labmaster 100 drybox. Solutions for spectral studies were
also prepared under a nitrogen atmosphere. Solvents were distilled
from appropriate drying agents under nitrogen just prior to use: CH
Cl (CaH ), benzene (Na), toluene (Na), ether (Na/benzophenone), THF
Na/benzophenone), hexane (Na/benzophenone/tetraglyme), and ac-
2
-
2
2
(
-
1
spectral region (1500-1900 cm ), was 0.2 M. Thin-layer cyclic
voltammograms were recorded in the spectroelectrochemical cell prior
to IR measurements in order to determine the stability of the
electrogenerated product. At the same time, the values of E1/2 under
these conditions were used in order to evaluate the appropriate potentials
for generating a given electrooxidized species in the thin-layer cell.
Synthesis of (T(p-X)PP)Co(NO) and (T(m-X)PP)Co(NO). The
investigated compounds were synthesized by three different methods.
The first two (methods A and B) involve a reaction of the precursor
etonitrile (Na). Anhydrous deaerated methanol was purchased from
Aldrich Chemical Co. and used as received.
Chemicals. (T(p-OCH
Aldrich Chemical Co. and used as received. The remaining para-
and meta-substituted tetraphenylporphyrins, T(p-X)PPH and T(m-X)-
PPH , were synthesized by condensation of the precursor benzaldehydes
3
)PP)Co and (TPP)Co were purchased from
2
2
1
5
and pyrrole using the method of Adler. The corresponding cobalt
derivatives, (T(p-X)PP)Co and (T(m-X)PP)Co, were obtained by
refluxing the free-base porphyrins with Co(CH
3
CO
methylformamide. Chloroform-d (99.8%, Cambridge Isotope Labo-
ratories) was vacuum-distilled from CaH under nitrogen prior to use.
2
)
2
‚4H
2
O in di-
(
P)Co complexes with NO, while the third (method C) involves
16
+
formation of [(P)Co(NO)] followed by chemical reduction with
2
cobaltocene to give (P)Co(NO). The synthesis of (T(p-OCH
NO) is described as a representative example for methods A and B,
while method C is presented in a later section under the synthesis of
(P)Co(NO)]BF . Caution: Nitric oxide is toxic, and reactions should
3
)PP)Co-
Elemental analyses were performed by Atlantic Microlab, Norcross,
GA. Nitric oxide (98%, Matheson Gas) was passed through KOH
pellets and a cold trap (dry ice/acetone, -78 °C) to remove higher
nitrogen oxides.
(
[
4
be performed in a well-Vented fume hood.
Method A. A Schlenk flask was charged with (T(p-OCH )PP)Co
3
2 2
Absolute dichloromethane (CH Cl ) over molecular sieves (for
electrochemistry) was obtained from Fluka Chemica and used without
further purification. Tetra-n-butylammonium perchlorate (TBAP) and
tetra-n-butylammonium hexafluorophosphate (TBAH) were purchased
from Fluka Chemica. TBAP and TBAH were recrystallized from ethyl
alcohol and ethyl acetate, respectively. Both salts, used as supporting
electrolytes, were dried under vacuum at 40 °C for a least 1 week prior
2 2
(0.200 g, 0.253 mmol) and CH Cl (20 mL). The mixture was stirred
to generate a purple solution, and NO gas was then bubbled through
the solution for 20 min. During this time, the color of the reaction
mixture turned red-purple. The solution was then heated to boiling
and the volume reduced in vacuo to ca. 5 mL. Anhydrous deaerated
3
CH OH (5 mL) was added, which led to precipitation of a purple solid.
The supernatant solution was discarded by careful removal with a
syringe, and the solid was dried in vacuo. The solid was then
recrystallized from toluene/hexane (10:1) to give (T(p-OCH )PP)Co-
3
(
12) (a) Fujita, E.; Chang, C. K.; Fajer, J. J. Am. Chem. Soc. 1985, 107,
665. (b) Fujita, E.; Fajer, J. J. Am. Chem. Soc. 1983, 105, 6743.
7
(
13) (a) Burstyn, J. N. In Abstracts of Papers, 210th National Meeting of
the American Chemical Society, Chicago, IL; American Chemical
Society: Washington, DC, 1995; INOR 254. (b) Reynolds, M. F.;
Graham, J. P.; Dierks, E. A.; Burstyn, J. N.; Bursten, B. E. Ibid., INOR
(
NO) (0.150 g, 0.183 mmol, 72% yield).
Method B. A Schlenk flask was charged with (T(p-OCH
0.316 g, 0.399 mmol), CH Cl (55 mL), and piperidine (1.5 mL). The
3
)PP)Co
(
2
2
0
95.
mixture was stirred to generate a purple solution, and NO gas was
bubbled through the solution for 30 min. During this time, the color
of the reaction mixture turned red-purple. Nitrogen was then bubbled
through the solution for 2 min to remove any unreacted NO. The
(
14) (a) Wayland, B. B.; Minkiewicz, J. V.; Abd-Elmageed, M. E. J. Am.
Chem. Soc. 1974, 96, 2795. (b) Groombridge, C. J.; Larkworthy, L.
F.; Mason, J. Inorg. Chem. 1993, 32, 379.
15) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(
(
16) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
(17) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
(18) Lin, X. Q.; Mu, X. H.; Kadish, K. M. Electroanalysis 1989, 1, 35.
1970, 32, 2443.

6532 Inorganic Chemistry, Vol. 35, No. 22, 1996
Richter-Addo et al.
solution was heated to boiling and CH OH (150 mL) added to
precipitate a purple solid. The solution was allowed to cool to room
temperature and the microcrystalline solid collected by filtration on a
3
m, 1545 w, 1500 w, 1399 w, 1351 m, 1107 w, 1002 s, 812 s, 799 m.
-
1
1
IR (CH
2
Cl
2
; cm ): νNO 1695. H NMR (CDCl
3
; δ, ppm): 8.84 (s,
8H, pyrrole H), 7.60 (d, J ) 8, 8H, m-H), 8.06 (d, J ) 8, 8H, o-H).
-
5
medium-porosity frit. This solid was then washed with CH
mL), recrystallized from CH Cl /hexane (ca. 30 mL, 1:1), and dried
under vacuum to give (T(p-OCH )PP)Co(NO) (0.208 g, 0.253 mmol,
3
OH (30
UV-vis (CH
2
Cl
2
; λ, nm (10 ꢀ)): 538 (1.55), 412 (11.50).
)PP)Co(NO). To
2
2
Alternate Preparation Method for (T(p-OCH
3
3
a stirred CH
2
Cl
2
(30 mL) solution of (T(p-OCH
3
)PP)Co (0.240 g, 0.303
+
-
6
3% yield).
The other complexes were obtained by both methods.
36 5 5
T(p-OCH )PP)Co(NO) (1). Anal. Calcd for C48H N O Co: C,
mmol) was added the C
5
H10NH
2
C
5 2
H10NN O
2
salt (0.140 g, 0.609
mmol).19 The solution was stirred for 30 min, during which the mixture
(
3
turned from red to red purple. The solution was then heated to boiling,
7
0.16; H, 4.42; N, 8.52. Found: C, 70.26; H, 4.50; N, 8.43. Low-
3
after which CH OH (100 mL) was added to precipitate a purple solid.
+
resolution mass spectrum (FAB): m/z 791 [(T(p-OCH
3
)PP)Co] . IR
The solution was allowed to cool to room temperature, and the
microcrystalline solid was collected by filtration on a medium-porosity
frit. This solid was then washed with MeOH (20 mL) and dried under
-
1
(KBr; cm ): νNO 1679 s; also 1607 m, 1547 vw, 1507 s, 1462 w,
1
441 w, 1351 m, 1249 vs, 1173 s, 1106 w, 1002 s, 801 m br. IR
-
1
1
(CH
2
Cl
2
; cm ): νNO 1682. H NMR (CDCl
3
; δ, ppm): 8.91 (s, 8 H,
vacuum to give (T(p-OCH
Synthesis of [(T(p-X)PP)Co(NO)]BF
BF Two methods were utilized for synthesis of the oxidized
3
)PP)Co(NO) (0.150 g, 0.183 mmol, 60%).
pyrrole H), 7.25 (d, J ) 8, 8 H, m-H), 8.06 (d, J ) 8, 8 H, o-H), 4.06
4
and [(T(m-X)PP)Co(NO)]-
-
5
(
(
s, 12 H, OCH
12.12).
3
). UV-vis (CH
2
Cl
2
; λ, nm (10 ꢀ)): 540 (1.31), 420
4
.
derivatives, after which the cationic compounds were reduced by
cobaltocene to give (P)Co(NO). The latter manipulation also provides
a third method (method C) for preparation of the neutral compounds.
The following examples are representative:
(
T(p-CH
3
)PP)Co(NO) (2). Yields: method A, 68%; method B,
OCo: C, 76.08; H, 4.79; N, 9.24.
6
7%. Anal. Calcd for C48H N
36 5
Found: C, 75.51; H, 4.83; N, 9.06. Low-resolution mass spectrum
+
-1
Method I (X * OCF , CF , CN). To a stirred CH Cl solution
(FAB): m/z 727 [(T(p-CH
3
)PP)Co] . IR (KBr; cm ): νNO 1678 s;
3
3
2
2
also 1597 m, 1508 w, 1451 w, 1351 m, 1109 w, 1005 s, 799 s. IR
3
(10 mL) of (T(p-OCH )PP)Co(NO) (0.061 g, 0.078 mmol) was added
-
1
1
AgBF (0.016 g, 0.080 mmol). The solution was stirred for ca. 5 min,
(CH
2
Cl
2
; cm ): νNO 1681. H NMR (CDCl
3
; δ, ppm): 8.90 (s, 8 H,
4
pyrrole H), 7.51 (d, J ) 8, 8 H, m-H), 8.03 (d, J ) 8, 8 H, o-H), 2.66
and an aliquot was then transferred under nitrogen into a sealed NaCl
-
5
IR cell Via a cannula.
(
(
s, 12 H, CH
10.85).
3
2 2
). UV-vis (CH Cl ; λ, nm (10 ꢀ)): 540 (1.50), 416
Method II (X * CN). To a stirred CDCl
)PP)Co (0.131 g, 0.180 mmol) was added NOBF
.265 mmol). The solution was stirred for ca. 5 min, and an aliquot
3
solution (10 mL) of
(
0
T(m-CH
3
4
(0.031 g,
(
T(m-CH
3
)PP)Co(NO) (3). Yields: method A, 85%; method B,
OCo: C, 76.08; H, 4.79; N, 9.24.
5
4%. Anal. Calcd for C48H N
36 5
was then transferred under nitrogen into the sealed IR cell Via cannula.
Found: C, 75.87; H, 4.82; N, 9.12. Low-resolution mass spectrum
+
-1
Attempts to generate [(T(p-CN)PP)Co(NO)]BF from either method
(EI): m/z 727 [(T(m-CH
3
)PP)Co] . IR (KBr; cm ): νNO 1681 s; also
4
-
1
I or method II resulted in the immediate formation of a highly insoluble
product that was not further characterized.
Chemical Reduction of Oxidized Compounds (Method C). Green
1
604 m, 1548 vw, 1452 w, 1004 m, 797 m. IR (CH
2
Cl
; δ, ppm): 8.91 (s, 8 H, pyrrole H), 7.98 (d, J
8, 4 H, o′-H), 7.97 (s, 4 H, o-H), 7.60 (dd, J ) 8/8, 4 H, m-H), 7.54
d, J ) 8, 4 H, p-H), 2.60 (s, 12 H, CH ). UV-vis (CH Cl ; λ, nm
2
; cm ): νNO
1
1
682. H NMR (CDCl
3
)
4 4
solutions of [(T(p-X)PP)Co(NO)]BF and [(T(m-X)PP)Co(NO)]BF (X
(
(
3
2
2
-
5
* CN; generated by method II above) were reacted with 1 equiv of
cobaltocene to generate the neutral (P)Co(NO) complexes in 32-67%
isolated yields (1 (40%), 2 (32%), 3 (39%), 4 (35%), 5 (44%), 6 (67%),
10 ꢀ)): 536 (2.01), 414 (13.84).
1
0
(
TPP)Co(NO) (4). Yields: method A, 71%; method B, 84%.
+
Low-resolution mass spectrum (FAB): m/z 671 [(TPP)Co] . IR (KBr;
cm ): νNO 1693 s; also 1592 m, 1438 w, 1345 s, 1168 s, 1068 m, 995
s, 798 s. IR (CH
-
1
7 (64%, from CH
Structural Determination of (T(p-OCH
crystal of (T(p-OCH )PP)Co(NO) was grown from a CH
2 2
Cl )).
-
1
1
3
)PP)Co(NO) (1).
Cl /CH
A
OH
2
Cl
2
; cm ): νNO 1683. H NMR (CDCl ; δ, ppm):
3
8
.89 (s, 8H, pyrrole H), 7.72 (d (app), J ) 8, 12H, m-H and p-H), 8.15
3
2
2
3
-
5
solution (ca. 1:1) kept at -20 °C for 2 weeks. X-ray data were collected
at 228(2) K on an Enraf Nonius CAD-4 diffractometer using mono-
chromated Mo KR radiation (λ ) 0.710 73 Å). Data were corrected
for Lorentz and polarization effects. No absorption correction was
applied since it was judged to be insignificant. The structure was solved
using the SHELXTL (Siemens) system and refined by full-matrix least-
(
4
2 2
d, J ) 8, 8H, o-H). UV-vis (CH Cl ; λ, nm (10 ꢀ)): 536 (2.51),
14 (17.70).
T(m-OCH
9%. Anal. Calcd for C48
(
3
)PP)Co(NO) (5). Yields: method A, 43%; method B,
Co: C, 70.16; H, 4.42; N, 8.52.
7
36 5 5
H N O
Found: C, 69.88; H, 4.51; N, 8.40. Low-resolution mass spectrum
+
-1
(FAB): m/z 791 [(T(m-OCH
3
)PP)Co] . IR (KBr; cm ): νNO 1677 s;
2
squares methods on F for all reflections. Weighted R factors, wR2,
also 1581 s, 1458 s, 1424 s, 1347 m, 1278 m, 1167 s, 1003 s, 796 s.
2
-
1
1
and goodness of fit are based on F . Details of crystal data and
IR (CH
H, pyrrole H), 7.76 (d, J ) 7, 4 H, o′-H), 7.71 (s, 4 H, o-H), 7.62 (dd,
J ) 7/8, 4 H, m-H), 7.30 (d, J ) 8, 4 H, p-H), 3.96 (s, 12 H, OCH ).
2
Cl
2
; cm ): νNO 1682. H NMR (CDCl ; δ, ppm): 8.94 (s, 8
3
refinement are given in Table 1, atomic coordinates and equivalent
isotropic displacement parameters are given in Table 2, and selected
bond lengths and angles are collected in Table 3.
3
-
5
UV-vis (CH
T(p-OCF
4%. Anal. Calcd for C48
Found: C, 55.49; H, 2.36; N, 6.69. Low-resolution mass spectrum
2
Cl
)PP)Co(NO) (6). Yields: method A, 56%; method B,
12Co: C, 55.56; H, 2.33; N, 6.75.
2
; λ, nm (10 ꢀ)): 536 (1.78), 414 (13.93).
(
3
Results and Discussion
5
24 5 5
H N O F
Nitric oxide reacts directly with the readily synthesizable four-
coordinate (P)Co compounds in CH2Cl2 at room temperature
to produce the corresponding five-coordinate (P)Co(NO) com-
plexes in 43-85% yields (eq 1) depending upon the specific
phenyl ring substituents.
+
-1
(FAB): m/z 1007 [(T(p-OCF
3
)PP)Co] . IR (KBr; cm ): νNO 1696 s;
also 1608 w, 1544 w, 1502 m, 1457 w, 1351 m, 1278 m, 1166 s, 1100
w, 1005 s, 825 m, 796 s. IR (CH
-
1
1
2
Cl
2
; cm ): νNO 1685. H NMR
(
8
4
CDCl ; δ, ppm): 8.88 (s, 8H, pyrrole H), 7.60 (d, J ) 8, 8H, m-H),
3
-5
2 2
.17 (d, J ) 8, 8H, o-H). UV-vis (CH Cl ; λ, nm (10 ꢀ)): 536 (1.77),
12 (14.77).
(P)Co + NO f (P)Co(NO)
(1)
(
T(p-CF
3
)PP)Co(NO) (7). Yields: method A, 63%; method B,
OF12Co: C, 59.21; H, 2.48; N, 7.19.
3
0%. Anal. Calcd for C48H N
24 5
During the course of addition of NO gas (usually 10-20 min),
the color of the reaction mixture changes from purple to red-
purple. However, all of the (P)Co(NO) compounds are purple
in the solid state, except the p-CN derivative, which is red-
purple. These five-coordinate complexes are moderately stable
in air in the solid state, showing no noticeable signs of
decomposition over a 2-3 week period, as judged by their IR
Found: C, 59.20; H, 2.52; N, 7.16. Low-resolution mass spectrum
+
-1
(FAB): m/z 943 [(T(p-CF
3
)PP)Co] . IR (KBr; cm ): νNO 1691 s;
also 1617 m, 1404 w, 1324 vs, 1166 s, 1107 m, 1003 m, 816 m, 798
w. IR (CH
-
1
1
2
Cl
2
; cm ): νNO 1690. H NMR (CDCl ; δ, ppm): 8.85
3
(
s, 8H, pyrrole H), 8.01 (d, J ) 8, 8H, m-H), 8.28 (d, J ) 8, 8H, o-H).
-
5
UV-vis (CH
2
Cl
2
; λ, nm (10 ꢀ)): 536 (1.76), 410 (14.10).
(
T(p-CN)PP)Co(NO) (8). Yields: method A, 55%; method B, 49%.
Anal. Calcd for C48 OCo: C, 71.91; H, 3.08; N, 15.72. Found:
C, 71.85; H, 3.08; N, 15.67. Low-resolution mass spectrum (FAB):
1
H N
24 9
and H NMR spectra. The parent (TPP)Co(NO) was previously
+
-1
m/z 771 [(T(p-CN)PP)Co] . IR (KBr; cm ): νNO 1684 s; also 1606
(19) Drago, R. S.; Karstetter, B. R. J. Am. Chem. Soc. 1961, 83, 1819.

Cobalt Porphyrins Containing Nitric Oxide
Inorganic Chemistry, Vol. 35, No. 22, 1996 6533
Table 1. Crystal Data and Structure Refinement Details for
3
(T(p-OCH )PP)Co(NO) (1)
formula
fw
T, K
λ, Å
crystal system
space group
48 5 5
C H36CoN O
821.75
228(2)
0.71073
monoclinic
P2/c
unit cell dimensions
a ) 15.052(1) Å, b ) 9.390(1) Å,
c ) 16.274(2) Å, R ) 90°,
â ) 111.04(1)°, γ ) 90°
2146.8(4)
V, Å3
Z
2
3
D(calcd), g/cm
abs coeff, mm
F(000)
1.271
0.451
852
-
1
Figure 1. Molecular structure of (T(p-OCH )PP)Co(NO). Only one
3
of the two disordered positions of the nitrosyl O1 atom is shown.
Hydrogen atoms have been omitted for clarity.
crystal size
θ range for data collection
index ranges
0.4 × 0.3 × 0.2 mm
2.17-26.56°
-17 e h e 16, 0 e k e 11,
the latter instance, a nitric oxide-amine adduct of the form
-1 e l e 16
+
-
(
R2NH2) [R2NN(O)NO] is the source of NO for metal-
no. of reflns collected
no. of independent reflns
refinement method
data/parameters
goodness-of-fit on F
final R indices [I > 2σ(I)]
R indices (all data)
3507
3348
2
0
nitrosyl formation. The reaction of (T(p-OCH3)PP)Co with
authentic (R2NH2) [R2NN(O)NO] (R2 ) C5H10), derived from
the reaction of piperidine with nitric oxide, resulted (after
workup) in the isolation of 1 in 60% yield. Compounds 1-7
could also be generated in CH2Cl2 by reaction of the respective
+
-
full-matrix least-squares on F2
1
9
3330/275
1.054
2
a,b
R1 ) 0.0599, wR2 ) 0.1567
a,b
R1 ) 0.0904, wR2 ) 0.2009
+
-
3
(P)Co derivatives with NO , followed by reduction with
largest diff peak and hole
0.842 and -0.281 e Å
cobaltocene (see following section).
a
b
2
2)2]/∑[wF 4
1/2
R1 ) ∑||F
o
| - |F
c
||/∑|F
o
|. wR2 ) {∑[w(F
o
- F
c
o
]} .
In CH2Cl2, the IR spectra of the neutral porphyrins show a
-1
strong band in the 1681-1695 cm region which is assignable
4
7
Table 2. Atomic Coordinates (×10 ) and Equivalent Isotropic
to νNO. These values are similar to those reported previously
2
3
-1
10,14
Displacement Parameters (Å × 10 ) for (T(p-OCH
3
)PP)Co(NO)
for (TPP)Co(NO) (1689 cm , KBr)
(1675 cm , CH2Cl2)
and (OEP)Co(NO)
-
1
12,14
Ueqa
and are close to the range of NO
x
y
z
frequencies reported for square pyramidal N-substituted
salicylideaminate complexes of cobalt containing a bent NO
Co(1)
O(1)
O(2)
O(3)
N(1)
N(2)
N(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
C(17)
C(18)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
0
2255(1)
4859(8)
3204(3)
479(4)
2500
45(1)
-232(5)
2017(2)
5530(2)
0
-576(2)
1155(2)
-1463(3)
-1610(3)
-836(3)
-185(3)
717(3)
3029(5)
-2333(2)
6652(3)
2500
1102(2)
2369(2)
578(3)
-409(3)
-499(3)
448(3)
76(2)
61(1)
83(1)
55(2)
44(1)
45(1)
49(1)
58(1)
56(1)
46(1)
46(1)
48(1)
54(1)
55(1)
51(1)
79(2)
57(1)
57(1)
48(1)
58(1)
60(1)
49(1)
49(1)
52(1)
104(3)
100(2)
59(1)
100(2)
77(2)
66(2)
-
1
21
ligand (1645-1675 cm , Nujol mull). Low-resolution mass
spectra of the (P)Co(NO) complexes are dominated by peaks
assignable to the [(P)Co] fragments, a feature consistent with
+
4230(6)
2106(3)
2042(3)
1942(4)
2065(5)
2344(5)
2316(4)
2390(4)
2639(4)
1633(5)
1850(5)
3088(5)
4524(6)
4115(5)
3874(5)
2149(4)
1934(5)
1691(5)
1791(4)
1732(5)
1429(5)
2479(6)
2197(6)
865(5)
the facile loss of the NO axial ligand under our measurement
conditions.
1
In CDCl3, the ambient-temperature H NMR spectra of all
eight (P)Co(NO) complexes consist of sharp peaks, which
indicates that each compound is diamagnetic. This diamagne-
tism was confirmed by solution measurements of the magnetic
635(3)
1041(3)
935(3)
-158(3)
-877(3)
-1579(3)
-1608(3)
-2437(4)
-919(3)
-209(3)
1551(3)
1746(3)
2676(4)
3088(3)
4038(3)
4726(3)
5010(5)
5662(5)
6011(3)
6986(5)
5719(4)
5080(4)
2
2
moment using the Evans method.
All of the complexes
display pyrrole H resonances in the 8.84-8.94 ppm region.
1258(3)
1708(3)
2406(4)
1811(3)
1476(3)
1332(3)
2255(3)
2632(3)
1964(3)
2114(3)
3032(3)
3662(4)
4505(4)
4718(3)
6178(4)
4111(3)
3278(3)
1
Furthermore, the H NMR spectra of the para-substituted
complexes show a doublet for the ortho and meta protons of
the substituted phenyl ring, a feature that suggests a rapid
rotation of the phenyl rings on the NMR time scale.23
The molecular structure of (T(p-OMe)PP)Co(NO) (1) is
shown in Figure 1, and selected bond lengths and angles are
shown in Table 3. The molecule is situated at the crystal-
lographic 2-fold center, and only half is unique. The Co(1)
and N(1) atoms lie on the 2-fold axis, and the nitrosyl O(1)
atom is disordered at two sites.
The most notable feature of the structure is that the Co-
N-O group is bent and has a bond angle of 119.6(4)°. The
NO ligand is oriented between two adjacent porphyrin nitrogens,
as shown in Figure 2a, and a similar feature is observed for the
1537(7)
-201(6)
94(5)
a
6a
Equivalent isotropic U is defined as one-third of the trace of the
orthogonalized Uij tensor.
related five-coordinate (P)Fe(NO) compounds. Nevertheless,
the structural observation of a bent NO ligand is consistent with
synthesized by Scheidt via reaction of (TPP)Co with NO in the
(
20) Piciulo, P. L.; Scheidt, W. R. Inorg. Nucl. Chem. Lett. 1975, 11, 309.
10
presence of piperidine and by Wayland in the absence of
base.1 We repeated the syntheses of each cobalt nitrosyl
complex in the presence of piperidine and, consistent with the
observation by Scheidt, also obtained the desired compounds
(21) Groombridge, C. J.; Larkworthy, L. F.; Marecaux, A.; Povey, D. C.;
4a
Smith, G. W.; Mason, J. J. Chem. Soc., Dalton Trans. 1992, 3125.
(
(
22) Grant, D. H. J. Chem. Educ. 1995, 72, 39 and references therein.
23) (a) Eaton, S. S.; Fishwild, D. M.; Eaton, G. R. Inorg. Chem. 1978,
17, 1542. (b) Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1977, 99,
(containing no piperidine axial ligand) in 30-79% yields. In
6594.

6534 Inorganic Chemistry, Vol. 35, No. 22, 1996
Richter-Addo et al.
Table 3. Bond Lengths (Å) and Angles (deg) for
(
T(p-OCH )PP)Co(NO)
3
Bond Lengths
Co(1)-N(3)
Co(1)-N(1)
Co(1)-N(2)
O(1)-N(1)
O(2)-C(10)
O(2)-C(9)
O(3)-C(21)
O(3)-C(22)
N(2)-C(1)
N(2)-C(4)
N(3)-C(16)
N(3)-C(13)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(13)
1.837(3)
C(5)-C(6)
1.551(7)
1.349(6)
1.467(6)
1.408(7)
1.355(6)
1.444(7)
1.435(7)
1.325(6)
1.431(7)
1.398(7)
1.481(7)
1.331(7)
1.374(7)
1.356(7)
1.363(7)
1.323(7)
1.340(6)
1.854(5)
2.128(4)
1.195(8)
1.408(6)
1.419(6)
1.342(5)
1.361(7)
1.309(5)
1.403(6)
1.372(5)
1.452(6)
1.545(7)
1.252(7)
1.501(6)
1.284(6)
1.463(6)
C(6)-C(12)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(11)
C(11)-C(12)
C(13)-C(14)
C(14)-C(15)
C(15)-C(16)
C(16)-C(17)
C(18)-C(19)
C(18)-C(24)
C(19)-C(20)
C(20)-C(21)
C(21)-C(23)
C(23)-C(24)
Bond Angles
96.26(10) C(9)-C(8)-C(7)
84.75(14) C(8)-C(9)-O(2)
N(3)-Co(1)-N(1)
N(3)-Co(1)-N(2)
N(1)-Co(1)-N(2)
C(10)-O(2)-C(9)
118.4(4)
114.3(4)
117.5(5)
128.2(4)
123.3(4)
120.3(4)
106.9(4)
119.5(5)
133.5(4)
93.77(9)
116.3(4)
C(21)-O(3)-C(22) 115.9(4)
C(8)-C(9)-C(11)
O(2)-C(9)-C(11)
C(9)-C(11)-C(12)
C(6)-C(12)-C(11)
C(14)-C(13)-N(3)
C(14)-C(13)-C(5)
N(3)-C(13)-C(5)
O(1)-N(1)-Co(1)
C(1)-N(2)-C(4)
C(1)-N(2)-Co(1)
C(4)-N(2)-Co(1)
119.6(4)
97.5(4)
129.3(3)
132.9(2)
C(16)-N(3)-C(13) 113.3(3)
C(16)-N(3)-Co(1) 120.4(3)
C(13)-N(3)-Co(1) 126.3(2)
C(13)-C(14)-C(15) 105.1(4)
C(16)-C(15)-C(14) 114.6(4)
N(3)-C(16)-C(15)
N(3)-C(16)-C(17)
99.9(4)
130.8(4)
N(2)-C(1)-C(2)
C(3)-C(2)-C(1)
C(2)-C(3)-C(4)
C(5)-C(4)-N(2)
C(5)-C(4)-C(3)
N(2)-C(4)-C(3)
C(4)-C(5)-C(13)
C(4)-C(5)-C(6)
C(13)-C(5)-C(6)
C(12)-C(6)-C(7)
C(12)-C(6)-C(5)
C(7)-C(6)-C(5)
C(8)-C(7)-C(6)
113.6(4)
110.3(4)
99.6(5)
C(15)-C(16)-C(17) 129.2(4)
C(18)-C(17)-C(16) 123.7(4)
C(19)-C(18)-C(24) 118.6(4)
C(19)-C(18)-C(17) 118.8(4)
C(24)-C(18)-C(17) 122.5(4)
C(18)-C(19)-C(20) 118.5(5)
C(19)-C(20)-C(21) 121.2(5)
121.7(4)
119.1(5)
119.0(4)
117.4(4)
115.7(4)
126.7(4)
114.8(4)
119.1(4)
126.1(4)
125.7(4)
C(23)-C(21)-O(3)
C(23)-C(21)-C(20) 121.1(4)
125.7(5)
113.2(4)
O(3)-C(21)-C(20)
C(21)-C(23)-C(24) 117.1(5)
C(23)-C(24)-C(18) 123.4(5)
Figure 2. (a) Top view of (T(p-OCH
3
)PP)Co(NO) showing the NO
ligand orientation. (b) Formal diagram of the porphyrinato core in (T(p-
8
(
T(p-OMe)PP)Co(NO) being formulated as a {Co(NO)} com-
OCH )PP)Co(NO). Positional displacements (in units of 0.01 Å) of
3
2
4
plex according to the Enemark-Feltham notation.
The
atoms from the 24-atom mean porphyrin plane are toward the axial
NO ligand.
previously determined structure of (TPP)Co(NO) also reveals
In comparison, the
related (P)Fe(NO) compounds are of the {FeNO} formulation,
a bent Co-N-O linkage (e128.5°).1
0,25
Electrochemistry. The electrochemistry of compounds 1-8
was examined at both room (≈22 °C) and low temperature (-60
°C) in CH2Cl2 containing 0.2 M TBAH or TBAP. Each
compound undergoes two oxidations and two reductions within
the solvent potential limit. The first reduction (eq 2) and the
two oxidations (eqs 4 and 5) are reversible for most compounds
at room and low temperatures, but the second reduction (eq 3)
is only reversible at low temperature.
7
and the known structures display Fe-N-O bond angles of ca.
31-151°.6
a-e
The Co-N(nitrosyl) bond length is 1.854(5)
Å, and the Co-N(porphyrin) bond lengths are 1.837(3) and
.128(4) Å. The Co atom is displaced by 0.20 Å from the mean
1
2
plane containing the 24-atom porphyrin ring toward the NO
ligand. The displacement features of the metalloporphyrin core
from the 24-atom mean plane are shown in Figure 2b. The
porphyrin ring exhibits a C2 ruffling distortion (also called B1u
ruffling in D4h symmetry), and as illustrated in Figure 1, all
four methoxy substituents are tilted up toward the side contain-
ing the NO ligand. The eight neutral compounds have similar
νNO bands in solution and in the solid state, suggesting that all
the (P)Co(NO) complexes have bent Co-N-O units under both
conditions.
-
-
(
P)Co(NO) + e h [(P)Co(NO)]
(2)
26
Examples of the resulting cyclic voltammograms illustrating
the room-temperature oxidations and low-temperature reductions
(
26) For recent discussions of porphyrin distortions, see: (a) Jentzen, W.;
Simpson, M. C.; Song, X.; Ema, T.; Nelson, N. Y.; Medforth, C. J.;
Smith, K. M.; Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon,
J.-C.; Takeuchi, T.; Goddard, W. A., III; Shelnutt, J. A. J. Am. Chem.
Soc. 1995, 117, 11085 and references therein. (b) Medforth, C. J.;
Hobbs, J. D.; Rodriguez, M. R.; Abraham, R. J.; Smith, K. M.;
Shelnutt, J. A. Inorg. Chem. 1995, 34, 1333. (c) Scheidt, W. R.; Lee,
Y. J. Struct. Bond. 1987, 64, 1. (d) Munro, O. Q.; Bradley, J. C.;
Hancock, R. D.; Marques, H. M.; Marsicano, F.; Wade, P. W. J. Am.
Chem. Soc. 1992, 114, 7218. (e) Ravikanth, M.; Chandrashekar, T.
K. Struct. Bonding (Berlin) 1995, 82, 105.
(
(
24) Feltham, R. D.; Enemark, J. H. Top. Stereochem. 1981, 12, 155.
25) The NO group in (TPP)Co(NO) displays 4-fold disorder, and solid
1
0
1
5
state N CPMAS NMR spectroscopy on this complex suggests a
dynamic motion involving swinging of the NO ligand between four
1
4b
sites.

Cobalt Porphyrins Containing Nitric Oxide
Inorganic Chemistry, Vol. 35, No. 22, 1996 6535
a
Table 4. Half-Wave Potentials (V vs SCE) for Oxidations and Reductions of (T(p/m-X)PP)Co(NO) in CH
2
Cl
2
, 0.2 M TBAP and TBAH
E
1/2
1
st oxidn
2nd oxidn
1st redn
TBAH
nd rednc
TBAH
2
compd no.
X
σb
TBAP
TBAH
TBAP
TBAH
TBAP
1
2
3
4
5
6
7
8
p-OCH
3
-0.27
-0.17
-0.07
0.00
0.12
0.35
0.93
0.97
0.98
1.01
1.01
1.11
1.15
1.20
0.89
0.91
0.93
0.96
0.97
1.06
1.14
1.15
1.19
1.23
1.23
1.20
1.23
1.23
1.27
1.28
1.27
1.20c
1.32
1.34c
1.37
-1.20
-1.19
-1.20
-1.18
-1.18
-1.12
-1.10
-1.02
-1.20
-1.20
-1.18
-1.16
-1.15
-1.10
-1.06
-1.05
-1.78
-1.75
-1.74
-1.73
-1.72
-1.63
-1.55
-1.50
p-CH
3
m-CH
3
H
m-OCH
p-OCF
p-CF
p-CN
3
3
3
0.55
0.63
a
X is a given substituent on each meso phenyl. b Hammett parameters taken from ref 23. c Potentials were determined at -60 °C.
-
-
2-
[
(P)Co(NO)] + e h [(P)Co(NO)]
(3)
(4)
(5)
+
-
(
P)Co(NO) h [(P)Co(NO)] + e
+
2+
-
[
(P)Co(NO)] h [(P)Co(NO)] + e
are shown in Figure 3. The two oxidations are well-defined
for six of the eight compounds (i.e compounds 1-6) at room
temperature, the exceptions being (T(p-CF3)PP)Co(NO) (7) and
(
T(p-CN)PP)Co(NO) (8). Both the first and second oxidations
of 7 become reversible at -60 °C, but this is not the case for
, where the singly oxidized product adsorbs at the electrode
8
surface, leading to a first oxidation process having an ill-defined
current-voltage curve. The separation in peak potentials, ∆Ep
)
|Epa - Epc|, ranges from 60 to 70 mV for the first oxidation
of each investigated compound and from 60 to 120 mV for the
second oxidation, the only exception being those of (T(p-CN)-
PP)Co(NO), which has an irreversible first oxidation. All of
the other compounds have an anodic to cathodic peak current
ratio, ipa/ipc, of ≈1.0 for both the first and second oxidations.
The overall electrochemical data thus suggest that the two
oxidations of compounds 1-7 involve diffusion-controlled
reversible to quasi-reversible one-electron transfers.
Half-wave potentials for oxidation and reduction of each
compound in CH2Cl2 containing either TBAH or TBAP are
2
7
given in Table 4, which also lists the Hammet σ parameters
associated with each substituent on the para or meta positions
of the four phenyl rings of the porphyrin. The changes in E1/2
as a function of substituent are as expected on the basis of linear
free energy relationships; i.e., those compounds with electron-
donating substituents are easier to oxidize and harder to reduce
than (TPP)Co(NO) (σ ) 0), while those with electron-
withdrawing substituents show an opposite effect.
Figure 3. Cyclic voltammograms showing (a) the room-temperature
oxidations and (b) low-temperature reductions of each (P)Co(NO)
derivative in CH Cl , 0.2 M TBAH. Scan rate ) 0.1 V/s.
2 2
The electrochemistry of metalloporphyrins in nonaqueous
media has been reviewed.28 For each of the eight compounds
in Table 4, the ∆E1/2 between the first and second oxidation
falls in the range 220-340 mV, a value that is much smaller
than the ∆E1/2 between the first and the second reductions, which
ranges from 450 to 580 mV. A similar result has been reported
for the redox reactions of (T(p-X)PP)Co29 and was explained
by a difference in the site of electron transfer; i.e., both the
second and third oxidations of (T(p-X)PP)Co were assigned as
being ring-centered processes, while their first reductions were
TBAH rather than TBAP is used as a supporting electrolyte.
This suggests that the singly oxidized product of a given (P)-
Co(NO) porphyrin is more stable in the presence of a weakly
coordinating counteranion. The same effect has been noted for
30
oxidation of several Ni(II) tetraphenylporphyrins and chlorins.
In general, substituent effects on redox potentials are larger
for porphyrin-ring reactions than for metal-centered reactions
2
8
in the case of planar TPP-type macrocycles. Figure 4 shows
plots of E1/2 vs the sum of the Hammet parameters (4σ), for
each electrode reaction of the compounds. Linear relationships
are obtained for the first reduction (F ) 44 mV), as well as the
first (F ) 76 mV) and second oxidations (F ) 34 mV) of the
compounds. These substituent effects on E1/2 can be compared
to those reported for the same types of electrode reactions
involving substituted cobalt(II) tetraphenylporphyrins not con-
2
9
assigned as being metal-centered, i.e. Co(II)/Co(I) reactions.
As also seen in Table 4, the ∆E1/2 between the first and
second oxidations of most of the eight compounds is larger when
(
27) (a) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. (b)
Zuman, P. Substituent Effects in Organic Polarography; Plenum: New
York, 1967.
(
(
28) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435.
29) Walker, F. A; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976, 98,
(30) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984,
23, 817.
3484.

6536 Inorganic Chemistry, Vol. 35, No. 22, 1996
Richter-Addo et al.
(
P)Co(NO) + AgBF f [(P)Co(NO)]BF + Ag (6)
4
4
Although we were not able to obtain the oxidized products
in an elementally pure form, these species could be characterized
by IR spectroscopy. As seen in Table 5, each chemically
oxidized species displays an NO stretching frequency which is
-1
about 42 cm higher in energy than that of its neutral precursor.
Not surprisingly, the cobalt porphyrins with electron-withdraw-
ing substituents on the macrocycle (complexes 6 and 7) are not
+
oxidized by AgBF4. The cyano derivative, 8, reacts with Ag ,
but this results in an immediate precipitation of a highly-
insoluble and as yet uncharacterized pink solid (νNO ) 1653
-
1
cm in Nujol), leaving a colorless supernate. Thus, a subtle
change in electronic effect of the peripheral phenyl substituents
brings about a marked difference in the oxidation chemistry of
the structurally related (P)Co(NO) porphyrins.
Interestingly, we have also been able to generate the cations
of (P)Co(NO) by reaction of the precursor (P)Co complexes
with NOBF4. In general, 1-1.5 equiv of NOBF4 is needed for
the reaction to proceed to completion. These cationic complexes
(
generated chemically) exhibit νNO bands that are similar to those
of the cationic complexes generated electrochemically (see Table
). In any event, further reaction of the cations with 1 equiv of
5
the one-electron reducing agent, Cp2Co, generates the neutral
complexes in 32-67% isolated yields (eq 7).
Cp Co
2
+
+
(
P)Co + NO f [(P)Co(NO)]
8 (P)Co(NO) (7)
The relatively small shift in νNO upon conversion of (P)Co-
(NO) to [(P)Co(NO)] is consistent with oxidation of the
complex involving the conjugated π ring system of the
+
Figure 4. Plot of E1/2 vs the Hammett substituent constants for each
electrode reaction of (P)Co(NO) in CH Cl , 0.1 M TBAH. The
compound numbers and half-wave potentials are listed in Table 4.
2
2
macrocycle rather than the metal center. In comparison, shifts
-
1
taining a nitrosyl axial ligand.29 Specifically, a F of 41 mV
of νNO as high as 170 cm are obtained for the metal-centered
5
b
oxidation of (TPP)Fe(NO). In CDCl3, the IR spectra of the
cationic complexes are also consistent with formation of species
which are oxidized at the macrocycle since they show bands in
was observed in the plot of E1/2 vs 4σ for the first metal-centered
reduction of (T(p-X)PP)Co, while a F of 76 mV was determined
for the second ring-centered oxidation of the same series of
compounds. A comparison of the F values thus indicates that
the first oxidation of (P)Co(NO) is ring-centered while the first
reduction is proposed to involve the central metal or the nitrosyl
axial ligand. The first oxidation of (T(p-X)PP)Co was shown
to have a F ) 34 mV,29 and this “reduced” substituent effect
on E1/2 is consistent with the site of electron transfer being the
metal center (as opposed to the conjugated π ring system of
the macrocycle) in these non-nitrosyl compounds. Although
an identical slope is observed for the second oxidation of (P)-
Co(NO) (Figure 4a), we cannot rule out that this latter electrode
reaction involves the NO ligand rather than the cobalt center.
The correlation in Figure 4 between half-wave potentials and
-
1
the 1289-1294 cm region that are characteristic of π-cation
3
4
radicals for TPP-type complexes. These marker bands are
3
5
described as pyrrole half-stretching vibrations. Due to the
solvent background, these bands are, however, not detected
when the oxidation takes place in CH Cl . The aforementioned
2
2
data are summarized in Table 5 and confirm analysis of the
E1/2 vs 4σ plots, i.e. that the site of oxidation of each (P)Co-
(NO) porphyrin (i.e., compounds 1-6 in CDCl3 and 1-7 in
CH2Cl2) involves the conjugated π ring system.3
6-38
The
(
32) Saheli, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K. J. Am.
Chem. Soc. 1986, 108, 5630.
(
33) (a) Ozawa, S.; Sakamoto, E.; Ichikawa, T.; Watanabe, Y.; Morishima,
I. Inorg. Chem. 1995, 34, 6362. (b) Ozawa, S.; Sakamoto, E.;
Watanabe, Y.; Morishima, I. J. Chem. Soc., Chem. Commun. 1994,
935. (c) Ozawa, S.; Fujii, H.; Morishima, I. J. Am. Chem. Soc. 1992,
4σ for the second reduction of (P)Co(NO) was made by utilizing
data obtained at -60 °C and consists of two separate linear
segments, one of which has a F of 36 mV and the other a F of
114, 1548.
1
05 mV. Such a nonlinearity in this correlation is consistent
(
34) Shimomura, E. T.; Phillippi, M. A.; Goff, H. M.; Scholz, W. F.; Reed,
with a change of mechanism which might involve differences
C. A. J. Am. Chem. Soc. 1981, 103, 6778.
in the site of electron transfer2 upon going from compounds
7b
(35) Hu, S.; Spiro, T. G. J. Am. Chem. Soc. 1993, 115, 12029.
(36) Further support for this view comes from our observation that the
singly oxidized product of (T(p-CH3)PP)Co(NO) in CH2Cl2 containing
TBAH shows a broad band centered at 776 nm, a value that is in the
1
-5 to compounds 6-8. However, confirmation of this could
not be obtained due to the difficulty in stabilizing the doubly
reduced species for spectroscopic characterization at -60 °C.
Chemical Oxidation. Compounds 1-5 are readily oxidized
at room temperature by AgBF4 to give the corresponding green,
37
range characteristic of TPP π-cation radicals.
(37) Fuhrhop, J.-H. Struct. Bonding (Berlin) 1974, 18, 1-67.
(
38) The ESR spectrum of singly oxidized (T(p-CH3)PP)Co(NO) is
characterized by an isotropic signal at g ) 2.009 with a peak-to-peak
separation of 18 G, a feature characteristic of cobalt tetraphenylpor-
phyrin cation radicals.39
31
air-sensitive [(P)Co(NO)]BF4 cations.
(
31) The use of silver salts for the generation of π-cation radicals of
(39) (a) Ohya-Nishiguchi, H.; Khono, M.; Yamamoto, K. Bull. Chem. Soc.
Jpn. 1981, 54, 1923. (b) Ichimori, K.; Ohya-Nishiguchi, H.; Hirota,
M.; Yamamoto, K. Bull. Chem. Soc. Jpn. 1985, 58, 623.
porphyrins, chlorins, and their nitrosyl derivatives has some
3
2-34
precedent.

Cobalt Porphyrins Containing Nitric Oxide
Table 5. NO Vibrational Frequencies (cm-1) of [(P)Co(NO)] (n ) 0 or +1) in CH
Inorganic Chemistry, Vol. 35, No. 22, 1996 6537
2
Cl , 0.2 M TBAH
n
2
ν
NO
singly oxidized
b
compd no.
substituent
p-OCH
p-CH
σ
E
1/2 (1st oxidn), V vs SCE
neutral
ECa
AgBF b
4
NOBF
4
c
c
c
c
c
1
2
3
4
5
6
7
8
3
-0.27
-0.17
-0.07
0.00
0.12
0.35
0.89
0.91
0.93
0.96
0.97
1.06
1.14
1.15
1679
1679
1679
1680
1682
1687
1690
1693
1724
1722
1718
1726
1722
1730
1730
1723
1724
1725
1725
1725
1724 (1293)
1725 (1291)
1726 (1294)
1726 (1293)
1727 (1289)
3
m-CH
3
H
m-OCH
p-OCF
p-CF
p-CN
3
d
3
1730 (...)
1733 (...)
e
3
0.55
0.63
a
Singly oxidized product generated by electrochemical means. b Singly oxidized product generated by chemical means. These bands in parentheses
c
d
-1
are also observed when the (P)Co compounds are reacted with NOBF
bands. The cation of compound 7 is produced in CH
4
in CDCl
3
.
The marker bands at ca. 1290 cm are obscured by porphyrin
e
2
Cl
2
but not in CDCl
3
.
previously reported electrochemistry of (TPP)Co(NO) in CH2-
Cl2 containing TBAP also supports this view.40
Morishima has reported that imidazole coordination to nitrosyl
Fe(II) chlorin and bacteriochlorin π-cation radicals produces
II
+
33c
Fe (NO ) complexes via valence isomerization.
Furthermore,
II
Chang and Babcock have oxidized (OEP)Co to the π-cation
radical [(OEP‚)Co ]ClO4, which is converted to [(OEP)Co -
ROH)2]ClO4 by addition of water or methanol. On the basis
II
III
32
(
of our IR studies, we have not as yet been able to detect any
valence isomerizations between the porphyrin-oxidized
+
+
[
(P‚)Co(NO)] and NO-oxidized [(P)Co(NO )]. However, this
effect cannot be excluded for our compounds, and further
investigations in this direction are now in progress.
IR Spectroelectrochemistry in CH2Cl2. The NO stretching
vibrations of each neutral and singly oxidized compound are
summarized in Table 5. The neutral complexes show a νNO
-
1
which falls in the range 1679-1693 cm , while the singly
oxidized derivatives show a νNO band between 1718 and 1733
-
1
cm , independent of the method used for generation of the
-
oxidized products or nature of the counteranion (PF6 in the
-
case of electrochemical generation and BF4 in the case of
chemical generation). A value of νNO for [(T(p-CN)PP)Co-
+
(
NO)] could not be determined since, as mentioned above, this
product adsorbs at the electrode surface. Attempts to character-
ize the doubly oxidized products of the (P)Co(NO) porphyrins
were also unsuccessful due to instability of the dications which
undergo a rapid loss of NO on the time scale of the spectro-
electrochemical experiments.
Time-dependent FTIR spectra were obtained during the first
electrooxidation of compounds 1-7, and a representative
example of the data is shown in Figure 5 for (T(m-
OCH3)PP)Co(NO) in CH2Cl2 containing 0.2 M TBAH. As seen
Figure 5. Time-dependent FTIR spectra measured between 1800 and
-1
3
1620 cm during oxidation of (T(m-OCH )PP)Co(NO) at a controlled
potential of 1.2 V in CH Cl , 0.2 M TBAH.
2
2
-
1
in this figure, the 1682 cm NO band of the initial complex
decreases in intensity upon oxidation as a new band grows in
upon going from (T(m-OCH )PP)Co(NO) to its singly oxidized
3
-
1
at 1722 cm . The νNO band of the singly oxidized product is
product suggests that the π-cation radical, [(T(m-OCH3)PP‚)-
-
1
4
0 cm higher than the νNO band of the neutral compound. A
+
Co(NO)] , should also exhibit a bent rather than linear Co-
similar difference between the νNO of the neutral compound and
the νNO of the singly oxidized product has been reported for
cobalt and iron nitrosyl porphyrins or porphyrin-like compounds
which are oxidized at the macrocycle,5b,12a,41 and the spectral
changes in Figure 5 suggest that (T(m-OCH3)PP)Co(NO)
undergoes a one-electron abstraction from the π ring system.
N-O unit. This result can be extended to the π-cation radicals
of compounds 1-4, 6, and 7, for which the νNO is only 39-46
cm higher than the νNO of their respective neutral precursors.
-1
The first oxidation of (T(m-OCH3)PP)Co(NO) is reversible
on the cyclic voltammetry time scale, and this is also the case
on the thin-layer spectroelectrochemical time scale, as shown
(T(m-OCH3)PP)Co(NO), like (T(p-OCH3)PP)Co(NO), contains
-
1
by the disappearance of the 1722 cm band of the singly
a bent Co-N-O unit, and the small shift of the NO frequency
-
1
oxidized product and a concomitant increase of the 1682 cm
band of the neutral compound as the potential is switched back
to 0.0 V and maintained at this value for a few minutes. (T(m-
OCH3)PP)Co(NO) can thus be regenerated upon re-reduction
(
40) (a) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27,
489. (b) Mu, X. H.; Kadish, K. M. Inorg. Chem. 1990, 29, 1031. (c)
1
Kelly, S.; Lan c¸ on, D.; Kadish, K. M. Inorg. Chem. 1984, 23, 1451.
41) Autret, M.; Will, S.; Van Caemelbecke, E.; Lex, J.; Gisselbrecht, J.-
P.; Gross, M.; Vogel, E.; Kadish, K. M. J. Am. Chem. Soc. 1994,
(
+
of [(T(m-OCH3)PP)Co(NO)] by both electrochemical and
116, 9141.
chemical means (using Cp2Co as a reducing agent). This result

6538 Inorganic Chemistry, Vol. 35, No. 22, 1996
Richter-Addo et al.
is also observed for all of the other compounds but
the authors thank Dr. V. Adamian for carrying out preliminary
electrochemical experiments on these compounds.
(T(p-CN)PP)Co(NO).
Supporting Information Available: A listing of experimental
Acknowledgment. G.B.R.-A. acknowledges the National
Institues of Health Institutional Development Award Program
Grant 1-P20-RR09180-01) for funding. The support of the
Robert A. Welch Foundation (K.M.K., Grant E-680) is also
gratefully acknowledged. S.J.H. acknowledges the Patricia
Roberts Harris Fellowship Program for a graduate stipend, and
3
crystallographic details for (T(p-OCH )PP)Co(NO), a drawing showing
the disorder of the nitrosyl group, and tables of anisotropic displacement
parameters, hydrogen coordinates and isotropic displacement param-
eters, and deviations from the porphyrin carbon and nitrogen planes (9
pages). Ordering information is given on any current masthead page.
(
IC960031O