Reactions of Hydroxylamine with Metal Porphyrins

Article abstract of DOI:10.1021/ic9605783

The reaction of hydroxylamine with a series of metal porphyrins was examined in methanol/chloroform media. The reductive nitrosylation reaction was observed for the manganese and iron porphyrins, leading to a nitrosyl complex that precipitated out of the solution in good isolatable yield (80-90%). This reaction could be used synthetically for the generation of iron and manganese porphyrin nitrosyl complexes and was particularly useful for making isotopically labeled nitrosyl complexes. On the other hand, Co^II(TPP) and Cr(TPP)(CI) did not react with hydroxylamine under anaerobic conditions. With trace amounts of oxygen, the reaction of Co^II(TPP) with hydroxylamine led to the formation of a stable cobalt(III)-bis(hydroxylamine) complex. The infrared, resonance Raman, and proton NMR spectra were consistent with a cobalt(III)-bis(hydroxylamine) complex. The cyclic voltammetry and visible spectroelectrochemistry of this complex were examined. The one-electron reduction of Co^III(TPP)(NH₂OH)₂⁺ formed Co^III(TPP), for which there was no evidence for the coordination of hydroxylamine. Further reduction led to Co^I(.TPP)-, which reacted with the halogenated solvent to form a cobalt-alkyl complex. The difference in the reactivity of these four metal porphyrins with hydroxylamine correlated well with their E_1/2 values. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive nitrosylation reaction, while cobalt(II) and chromium(III) porphyrins are unreactive. The one-electron oxidation of the hydroxylamine complex with a M(IIT) porphyrin would be expected to oxidize the N-atom in the coordinated hydroxylamine. The oxidation of M^III(NH₂OH) with the loss of a proton would form M^II(N^IH₂O)⁺ by an internal electron transfer, which will eventually lead to M(NO). The relationship between the reductive nitrosyl reaction and the enzymatic interconversion of NO and hydroxylamine was discussed.

Full text of DOI:10.1021/ic9605783

Inorg. Chem. 1997, 36, 3113 3118
3113
Reactions of Hydroxylamine with Metal Porphyrins
In-Kyu Choi, Yanming Liu, Zhongcheng Wei, and Michael D. Ryan*
Chemistry Department, Marquette University, PO Box 1881, Milwaukee, Wisconsin 53201
Recei ed May 21, 1996X
The reaction of hydroxylamine with a series of metal porphyrins was examined in methanol/chloroform media.
The reductive nitrosylation reaction was observed for the manganese and iron porphyrins, leading to a nitrosyl
complex that precipitated out of the solution in good isolatable yield (80 90%). This reaction could be used
synthetically for the generation of iron and manganese porphyrin nitrosyl complexes and was particularly useful
for making isotopically labeled nitrosyl complexes. On the other hand, CoII(TPP) and Cr(TPP)(Cl) did not react
with hydroxylamine under anaerobic conditions. With trace amounts of oxygen, the reaction of CoII(TPP) with
hydroxylamine led to the formation of a stable cobalt(III) bis(hydroxylamine) complex. The infrared, resonance
Raman, and proton NMR spectra were consistent with a cobalt(III) bis(hydroxylamine) complex. The cyclic
voltammetry and visible spectroelectrochemistry of this complex were examined. The one-electron reduction of
CoIII(TPP)(NH2OH)2 formed CoII(TPP), for which there was no evidence for the coordination of hydroxylamine.
Further reduction led to CoI(TPP) , which reacted with the halogenated solvent to form a cobalt alkyl complex.
The difference in the reactivity of these four metal porphyrins with hydroxylamine correlated well with their E1/2
values. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive
nitrosylation reaction, while cobalt(II) and chromium(III) porphyrins are unreactive. The one-electron oxidation
of the hydroxylamine complex with a M(III) porphyrin would be expected to oxidize the N-atom in the coordinated
hydroxylamine. The oxidation of MIII(NH2OH) with the loss of a proton would form MII(NIH2O) by an internal
electron transfer, which will eventually lead to M(NO). The relationship between the reductive nitrosyl reaction
and the enzymatic interconversion of NO and hydroxylamine was discussed.
Introduction
The chemistry of hydroxylamine, which has been reviewed
by Wieghardt,5 shows a wide variety of behaviors. For example,
Electrochemical reduction of metal complexes can result in
electron transfer to either a metal or a ligand site. For mixed
ligand complexes (e.g., metal porphyrin axial ligand), there
can be three sites for the redox chemistry (as well as orbitals
that have extensive M L bonding or antibonding characteris-
tics). We have examined such systems where the coordinated
ligand, as well as the iron or porphyrin, may undergo redox
changes. Of particular interest has been the reduction of iron
nitrosyl and iron hydroxylamine complexes. Both ligands have
been either observed (nitrosyl) or postulated (hydroxylamine)
intermediates in assimilatory nitrite reductase.1
the oxidation of electrochemically generated Ti(III) by hydroxyl-
amine has been investigated by a variety of electrochemical
8
techniques.6 Alternatively, manganese(III) or cobalt(III) can
oxidize hydroxylamine to nitrate, dinitrogen or nitrous oxide,
11
depending upon the pH.9
Similar reactions have been
observed with heme-containing proteins. Hydroxylamine is a
substrate for hydroxylamine oxidoreductases,12 14 which oxidize
hydroxylamine to nitrite. Conversely, hydroxylamine is pre-
sumed to be an intermediate in the assimilatory reduction of
nitrosyl to ammonia. Bagchi and Kleiner15 have recently
reported on a hydroxylamine dismutase which catalyzes the
disproportionation of hydroxylamine to nitrite and ammonia.
Hemoglobin has been reported to decompose hydroxylamine
to form ammonia and dinitrogen, as well as some nitrous
oxide.16 In this work, we examined the chemistry of hydroxyl-
amine with metal porphyrins in order to determine the scope
The reaction of hydroxylamine with iron porphyrins has been
previously reported,2 and it has been shown to form a bis-
(hydroxylamine) complex with iron(II) at reduced temperatures.
When hydroxylamine was mixed with a ferric porphyrin, a
ferrous complex was rapidly formed, which, at room temper-
ature, decomposed further to form Fe(TPP)(NO), probably by
way of the disproportionation of hydroxylamine.2,3 This is a
well-known reaction of hydroxylamine, called reductive ni-
trosylation, where a coordinated hydroxylamine is oxidized to
nitric oxide (nitrosyl ligand) and an additional molecule of
hydroxylamine is reduced to ammonia. The use of alkylated
hydroxylamines prevents the reductive nitrosylation reaction,
and bis(alkyl) hydroxylamine complexes with iron porphyrins
have been reported.3 The reduction of hydroxylamine with
cob(I)alamins was reported by Balasubramanian and Gould.4
(4) Balasubramanian, P. N.; Gould, E. S. Inorg. Chem. 1984, 23, 824
828.
(5) Wieghardt, K. Ad . Inorg. Bioinorg. Mech. 1984, 3, 213 274.
(6) Blazek, A.; Koryta, J. Collect. Czech. Chem. Commun. 1953, 18, 326
336.
(7) Delahay, P.; Mattax, C. C.; Berzius, T. J. Am. Chem. Soc. 1954, 76,
5319 5324.
(8) Lingane, P. J.; Christie, J. H. J. Electroanal. Chem. 1967, 13, 227
235.
(9) Hlasivcova, N.; Novak, J. Collect. Czech. Chem. Commun. 1969, 34,
3995
(10) Sramkova, B.; Zyka, J.; Dolezal, J. J. Electroanal. Chem. 1971, 30,
169 175.
(11) Sramkova, B.; Sramek, J.; Zyka, J. Anal. Chim. Acta 1972, 62, 113
124.
(12) Erickson, R. H.; Hooper, A. B. Biochim. Biophys. Acta 1972, 275,
231 244.
(13) Hooper, A. B.; Maxwell, P. C.; Terry, K. R. Biochemistry 1978, 17,
2984 2989.
(14) Terry, K. R.; Hooper, A. B. Biochemistry 1981, 20, 7026 7032.
(15) Bagchi, S. N.; Kleiner, D. Biochim. Biophys. Acta 1990, 1041, 9 13.
* Author to whom correspondence should be addressed.
X Abstract published in Ad ance ACS Abstracts, June 1, 1997.
(1) Hughes, M. N. The Inorganic Chemistry of Biological Processes;
Wiley: New York, 1981; pp 204 211.
(2) Feng, D.; Ryan, M. D. Inorg. Chem. 1987, 26, 2480 2483.
(3) Newman, A. R.; French, A. N. Inorg. Chim. Acta 1987, 129, L37
L39.
S0020-1669(96)00578-2 CCC: $14.00 © 1997 American Chemical Society

3114 Inorganic Chemistry, Vol. 36, No. 14, 1997
Choi et al.
and generality of the reductive nitrosylation reaction in metal
porphyrins.
Experimental Section
Equipment. Cyclic voltammograms were obtained with an IBM
Instrument EC/225 voltammetric analyzer with a Hewlett Packard
7045A X-Y recorder. The reference electrode was a 0.1 M Ag/AgNO3
(in acetonitrile) electrode, and the working and auxiliary electrodes
were platinum. The UV visible spectra were recorded on a HP 8452A
diode array or a Perkin-Elmer 320 UV visible spectrophotometer. An
optically transparent thin-layer electrochemical (OTTLE) cell was used
for the spectroelectrochemical experiments.17 The NMR spectra were
obtained on a 60-MHz (proton) JEOL JNM-FX60Q Fourier transform
NMR spectrometer.
Chemicals. The porphyrin complexes (Fe(TPP)(Cl) (TPP tetra-
phenylporphyrin), Fe(OEP)(Cl) (OEP octaethylporphyrin), Co(TPP),
and Mn(TPP)(Cl)), hydroxylamine hydrochloride, methylene chloride,
and sodium methoxide were obtained from Aldrich Chemical Co, while
Figure 1. Visible spectra of Mn(TPP)Cl obtained after adddition of
hydroxylamine. The concentration of Mn(TPP)Cl
0.10 mM, con-
Fe(PPDME)(Cl) (PPDME
protoporphyrin dimethyl ester) was
centration of hydroxylamine 0.10 M, and the solvent is methanol.
Interval between spectra: 60 s. Spectrum just after mixing: dashed.
Spectrum after 360 s: solid. Intermediate spectra: dotted.
obtained from Porphyrin Products. Tetraphenylchlorin (H2TPC)18 and
Fe(TPC)(Cl)19 were synthesized by literature procedures. Tetrabutyl-
ammonium perchlorate (TBAP) was obtained from GFS Chemical Co.
15N-Hydroxylamine hydrochloride was obtained from MSD Isotopes.
Co(TPP)(NH2OH)2ClO4. Co(TPP) (0.30 g) was dissolved in 100
mL of chloroform. To that solution was added 100 mL of 2%
hydroxylamine in methanol, which contained 0.20 g of TBAP. After
mixing, the solvent was evaporated. The solid was then dissolved in
a minimum amount of chloroform and filtered to remove the excess
TBAP and hydroxylamine. The solvent was removed and mixed with
methanol. After filtration and drying, the solid was redissolved in
chloroform, the solution was filtered, and the solvent was removed.
The solid obtained was vacuum dried. The visible spectrum was
identical to that obtained from mixing Co(TPP) with hydroxylamine.
Yield: 82%. Anal. Calcd for C44H34ClCoN6O6; C, 63.12; H, 4.10;
N, 10.04; Co, 7.04. Found: C, 63.14, H, 4.07, N, 9.58; Co, 6.86.
Spectral characterization of the product is given in the Results and
Discussion section. As a precaution with all perchlorate salts,
quantities of the material were kept to a minimum for safety reasons.
Fe(TPP)(NO) and Mn(TPP)(NO). A 50 mg amount of Fe-
(TPP)(Cl) or Mn(TPP)(Cl) was dissolved in chloroform or methanol.
To this solution was added at least a 2-fold excess of hydroxylamine,
prepared as described below. The product nitrosyl complex precipitated
out of solution in a period of 30 min and was isolated by filtration.
Isolatable yields of 85 90% were obtained.
was dissolved in 10 mL of methanol (chloroform) and then was added
to 10 mL of 0.2 M hydroxylamine/methanol. The solution was purged
with argon for 30 min, and the ammonia was trapped with 4.0 mL of
0.01 M HCl. After the purging was complete, 5 drops of 0.003 M
manganese sulfate, 3.0 mL of cold alkaline phenol, and 1.5 mL of
chlorine solution were added. The mixture was shaken well and placed
in a boiling water bath for 5 min. The concentration of ammonia was
then determined from the visible absorbance at 624 nm.
Results
Iron Porphyrins. It has been previously shown that hy-
droxylamine reacts with Fe(TPP)(Cl) at room temperature by
the following net reaction:2
Fe(TPP)(NH2OH)2 f Fe(TPP)(NO) NH4
H2O
This reaction is quite rapid, and it is not clear if it is initiated
by uncoordinated hydroxylamine or by intramolecular electron
transfer. The reaction above can be used as a general synthetic
procedure with a mixed methanol/chloroform (1/1) solvent
system. For most porphyrin macrocycles, the Fe(P)(NO)
complex that was formed by the reductive nitrosylation reaction
was insoluble in this solvent system, and the nitrosyl complex
could be isolated in over 80% yield using small quantities of
iron porphyrin. Nearly quantitative yields were obtained for
Fe(P)(NO) complexes, where P tetraphenylporphyrin (TPP),
octaethylporphyrin (OEP), and tetraphenylchlorin (TPC). In
2% Hydroxylamine Solution. One equivalent of sodium methoxide
was added to a solution obtained by addition 200 mg of hydroxylamine
hydrochloride to 10 mL of methanol. Sodium chloride precipitated
and was removed by filtration. Fresh solutions were used for all
reactions. Solid hydroxylamine is unstable above 0˚C. As a precaution,
the volume of hydroxylamine solution was kept to a minimum. For
15N-labeled hydroxylamine and nitrosyl complexes, 15NH3OHCl was
used in the preparation of this solution.
order to precipitate Fe(PPDME)(NO) (PPDME
protopor-
Procedures. All the voltammetric solutions were deoxygenated by
deaerating the solution for 15 min with prepurified dinitrogen. The
dinitrogen was presaturated with the solvent in order to prevent
evaporation. The spectroelectrochemical data were obtained after the
current had decayed to the background. All the reactions between the
metal porphyrins and hydroxylamine were carried out under anaerobic
conditions in a glovebox, except as noted.
Analysis of Ammonia. The spectrophotometric analysis of am-
monia was carried out using Russell’s procedure with the phenolate
hypochlorite reagent.20,21 A 50 mg amount of MnTPPCl (FeTPPCl)
phyrin dimethyl ester) complex, an 80/20 methanol/chloroform
solution was used. This synthetic method is particularly
advantageous in the generation of 15N-labeled nitrosyl com-
plexes, starting with 15N-hydroxylamine. The UV/visible and
infrared spectra, as well as the voltammetric behavior, were
identical to authentic material synthesized from the reaction of
Fe(TPP)(Cl) with NO.22,23 In addition to Fe(TPP)(NO), am-
(21) Conway, E. J. Microdiffusion Analysis and Volumetric Error; C.
Lockwood: London, 1962; pp 109
(22) Lancon, D.; Kadish, K. M. J. Am. Chem. Soc. 1983, 105, 5610 5617.
(23) Olson, L. W.; Schaeper, D.; Lancon, D.; Kadish, K. M. J. Am. Chem.
Soc. 1982, 104, 2042 2044.
(24) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161
4167.
(25) Shirazi, A.; Goff, H. M. Inorg. Chem. 1982, 21, 3420 3425.
(26) Sugimoto, H.; Ueda, N.; Mori, M. Bull. Chem. Soc. Jpn. 1981, 54,
3425 3432.
(16) Bazylinski, D. A.; Arkowitz, R. A.; Hollocher, T. C. Arch. Biochem.
Biophys. 1987, 259, 520 526.
(17) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498 1501.
(18) Whitlock, H. W.; Hanauer, J. R.; Oester, M. Y.; Bower, B. K. J. Am.
Chem. Soc. 1969, 91, 7495 7499.
(19) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476
(20) Choi, I.-K.; Liu, Y.; Feng, D.; Paeng, K. J.; Ryan, M. D. Inorg. Chem.
1991, 30, 1832 1839.
(27) Gaughan, R. R.; Shriver, D. F.; Boucher, L. J. Proc. Natl. Acad. Sci.
U.S.A. 1975, 72, 433 436.

Reactions of Hydroxylamine with Metal Porphyrins
Inorganic Chemistry, Vol. 36, No. 14, 1997 3115
Table 1. Visible Spectra of Metal Porphyrin Complexes
1
compd
Co(TPP)
solvent
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
CH3OH
CH2Cl2
CH2Cl2
CHCl3
CH3CN
DMF
pyridine
CHCl3
CH3OH/CH2Cl2
pyridine
CH3OH/CH2Cl2
toluene/77 K
toluene/77 K
THF
λ, nm (ꢀ, cm 1 mM
)
ref
410 (295), 528 (16)
435, 549, 585sh
416, 540
24
25
Co(TPP)(pyridine)
Co(TPP)(NO)
CoIII(TPP)
this work
427 (193), 540 (19.2)
426 (95), 536 (6.9)
425 (95), 539 (7.6)
437 (100), 552 (9.1), 588 (4.0)
434 (132), 550 (9.8), 580 (4.5)
427 (93), 542 (9.2)
24
26
26
26
CoIII(TPP)(H2O)
CoIII(TPP)(pyridine)(Cl)
CoIII(TPP)(pyridine)2
CoIII(TPP)(NH2OH)2
26
this work
this work
this work
this work
27
this work, 28
27
312, 427, 544, 578s
416s, 432, 544, 582
436, 552, 590
MnIII(TPP)(Cl)
376 (43.5), 476 (76.5), 581 (7.5), 617 (8.5), 690 (0.6), 769 (0.50)
MnIII(TPP)(CH3OH)2
MnIII(TPP)(pyridine)2
MnII(TPP)(CH3OH)
MnII(TPP)
377, 399, 419, 465, 563, 596
380 (38.5), 476 (63.0), 578 (7.6), 613 (76), 690 (1), 826 (0.67)
434a
28
29
29
431, 537, 580, 617
434, 540, 568, 609
430, 540, 568, 606
MnII(TPP)(NO)
this work
a Only Soret band given in reference.
monia was also detected as a product of the reductive nitrosyl-
ation reaction. The yield of ammonia was well in excess of
the stoichiometric amount (≈10 times the expected amount),
indicating that the nitrosyl complex itself may also be catalyzing
the disproportionation of hydroxylamine.
Manganese and Chromium Porphyrins. In the absence
of hydroxylamine, Mn(TPP)(Cl) in methanol has an intense
Soret band at 466 nm, with additional bands at 378, 400, 418,
516, 562, and 598 nm. Upon addition of hydroxylamine, the
original bands between 350 and 470 nm decreased, and a new,
single Soret band appeared at 424 nm (Figure 1). In the longer
wavelength region, bands at 542, 572, and 602 nm appeared.
Several isosbestic points were observed in the spectrum during
the reduction (Figure 1), indicating that no intermediates could
be observed under these conditions in the conversion of
Mn(TPP)Cl to Mn(TPP)(NO). Later in the reaction, the
isosbestic points disappeared and the 424 nm band decreased
due to precipitation of the product. When the reaction was
complete (not shown in Figure 1), the original 350 and 470 nm
bands completely disappeared, and the bands in the final product
were identical to independently synthesized Mn(TPP)(NO)
(Table 1). The product of the reaction was isolated, and an
infrared spectrum of the product in KBr had a strong band at
1747 cm 1, which was characteristic of a nitrosyl complex.
Therefore, like the iron porphyrins, Mn(TPP)(Cl) underwent a
reductive nitrosylation reaction.
Figure 2. Visible spectrum of CoII(TPP) (dashed line) in methanol/
methylene chloride prior to the addition of hydroxylamine. After
addition of hydroxylamine, CoIII(TPP)(NH2OH)2 was formed (dotted
line). Also shown is the visible spectrum of authentic Co(TPP)NO
(solid).
investigated. The order of the reaction with respect to hydroxyl-
amine and Mn(TPP)Cl was investigated, using the initial rates
of the reaction, measured spectrophotometrically (the precipita-
tion of Mn(TPP)(NO) made it difficult to investigate the entire
kinetic curve). From the variation in the observed rate constants
as a function of hydroxylamine concentration and Mn(TPP)Cl
concentration, the reaction was found to be first order with
respect to both reactants with a rate constant of (6.2 0.9) ×
10 2 M 1 s 1. The first order nature of the reaction is consistent
with the reaction of Mn(TPP)Cl with hydroxyalmine being the
rate-limiting step. This rate-limiting step may be the electron
transfer from hydroxylamine to Mn(TPP)Cl to form Mn(TPP)
and NH2 radical or the initial formation of the complex followed
by an internal electron transfer. Unlike manganese and iron,
no reaction was observed when Cr(TPP)(Cl) was mixed with
hydroxylamine.
The ultimate product of the oxidizing agent (hydroxylamine)
was not established. The initial product of the one-electron
reduction of hydroxylamine is the NH2 radical (and hydrox-
ide).8,30 No detectable amounts of ammonia were generated in
the reaction, using the same procedure as was used with the
iron reaction, indicating an alternate fate for this radical.
Because the primary focus of this work was on the metallo-
porphyrin, the ultimate fate of the oxidizing agent was not
Cobalt Porphyrins. As was observed with Cr(TPP)(Cl), no
reaction was detected when Co(TPP) was mixed with hy-
droxylamine in a chloroform/methanol solution under strictly
anaerobic conditions. Outside the glovebox, small quantities
of dioxygen slowly leaked into the UV visible cell, oxidizing
Co(TPP). This caused the Co(TPP) bands at 410 and 528 nm
to shifted to 427 and 542 nm (Figure 2), indicative of a
cobalt(III) complex (Table 1). There was no evidence for the
formation of Co(TPP)(NO) nor was there any ammonia gener-
ated.
(28) Mu, X. H.; Schultz, F. A. Inorg. Chem. 1992, 31, 3351 3357.
(29) Wayland, B. B.; Olson, L. W.; Siddiqui, Z. U. J. Am. Chem. Soc.
1976, 98, 94 98.
(30) Albisetti, C. J.; Coffman, D. D.; Hoover, F. W.; Jenner, E. L.; Mockel,
W. E. J. Am. Chem. Soc. 1959, 81, 1489 1494.
(31) Andrews, M. A.; Chang, T. C.-T.; Cheng, C.-W. F. Organometallics
1985, 4, 268 274.
(32) Abraham, R. J.; Medforth, C. J. Magn. Reson. Chem. 1987, 25, 432
438.
(33) LaMar, G.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1790 1796.
(34) LaMar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1975, 97, 5103
5107.

3116 Inorganic Chemistry, Vol. 36, No. 14, 1997
Choi et al.
Table 2. Proton NMR Spectra of Various Cobalt Porphyrins
resonances, ppm
compd
solvent pyrrole
phenyl
ref
CoII(TPP)
CDCl3 15.7
CDCl3 12.5
13.1, 9.8, 8.95 this work
8.5, 8.33, 7.82 25
8.90 8.15, 7.76 31
CoII(TPP)(pyridine)
CoII(TPP)(NO)
CDCl3
CDCl3
CDCl3
CoIII(TPP)(NH2OH)2
CoIII(TPP)(pyridine)
9.1
8.2, 7.7
this work
8.99 7.81, 7.71
9.09 8.10, 7.71
8a,b
32
32
26
26
33
CoIII(TPP)(pyridine)2 CDCl3
CoIII(TPP)(H2O)2
CoIII(TPP)(H2O)(Cl)
CoII(p-CH3TPP)
CDCl3
CDCl3
9.0a 8.8,a 7.7a
CDCl3 15.40 12.9, 9.6c
a Resonance is broad. b Water resonance at 2.2 ppm. c Methyl at
4.0 ppm.
A cobalt(III) complex was isolated from the reaction solution,
as outlined in the Experimental Section. The elemental analysis
was consistent with a bis(hydroxylamine) complex. The visible
spectra of the isolated product in methylene chloride, aceto-
nitrile, and methanol were identical to the spectrum obtained
by the in situ reaction. Small shifts in the Soret band were
observed in THF, DMF, and DMSO (Table 1). Significant
spectral changes, though, occurred in pyridine, and the visible
Figure 3. Cyclic voltammetry of CoII(TPP) (A, 21 °C) and
CoIII(TPP)(NH2OH)2 (B, 21 °C; C, 25 °C; D, 40 °C) in methylene
chloride. Working electrode: platinum; 0.1 M TBAP. Scan rate: 100
mV/s.
spectrum was the same as that for CoIII(TPP)(pyridine)2
.
The proton NMR chemical shifts for the cobalt hydroxyl-
amine and other cobalt complexes are summarized in Table 2.
Four-coordinate CoII(TPP) has broad resonances at 15.7, 13.1,
9.8, and 8.9 ppm, due to the paramagnetic nature of the d7
complex. Upon addition of hydroxylamine, the resonances
shifted to their diamagnetic positions at 7.7, 8.2, and 9.1 ppm,
and a resonance for hydroxylamine was observed at 1.4 ppm.
This spectrum was typical of a diamagnetic Co(III) TPP
complex (Table 2) and was almost identical to the CoIII(TPP)-
(pyridine)2 complex.
The infrared spectra of the cobalt hydroxylamine complex
were obtained using 14N- and 15N-hydroxylamine. Two isotope-
sensitive bands were observed at 1678 (ν3) and 1261 cm 1 (ν5)
for the 14N-hydroxylamine complex which shifted to 1648 and
1
1250 cm for the 15N-hydroxylamine complex.35 The reso-
Figure 4. Visible thin-layer spectroelectrochemistry of CoIII(TPP)-
(NH2OH)2 in methylene chloride. Solid line: 0.11 V vs SCE. Dashed
line: 0.22 V vs SCE. Other lines: intermediate spectra.
nance Raman spectra were also consistent with a cobalt(III)
porphyrin complex. In acetonitrile, the ν2 band for the
hydroxylamine complex (CoII(TPP) bands in parentheses36
first wave was an ill-defined and irreversible wave with an Epc
of 0.78 V vs SCE. This wave was about 500 mV positive of
the Co(TPP) /Co(TPP) in the same solvent24 and very close to
the first reduction of Co(TPP)(pyridine)2 ( 0.77 V), Co(TPP)-
(4-picoline)2 ( 0.83 V), or Co(TPP)(piperidine)2 ( 0.81 V).38
Spectroelectrochemical reduction of Co(TPP)(NH2OH)2 at the
first wave in an OTTLE cell (Figure 4) led to a final spectrum
that was identical to CoII(TPP). Reversal of the potential did
not regenerate the cobalt hydroxylamine complex, confirming
the irreversibility observed in the cyclic voltammograms. These
results are consistent with the loss of hydroxylamine upon
reduction (eq 1). Previous studies using NMR25 and cyclic
occurred at 1569 cm (1563 cm 1) and the ν4 band at 1373
1
cm (1363 cm 1). Overall, the spectroscopic evidence was
1
consistent with the formation of a CoIII(TPP)(NH2OH)2
complex.
The formation of cobalt(III) porphyrins from cobalt(II) has
been observed in the presence of other bases, even under
anaerobic conditions. For example, CoII(PPDME) (PPDME
protoporphyrin dimethyl ester) reacted with methoxide in an
alcoholic solution to form CoIII(PPDME)(OCH3)2
.
This
37
reaction was repeated in this work with CoII(TPP) and methoxide
in a mixed methanol/chloroform solution. A cobalt(III) spec-
trum was generated, with a Soret band at 427 nm, indicative of
a CoIII(TPP)(OCH3)2 complex. It was possible to form Co-
(TPP)(NO) from the hydroxylamine product by heating the
solution or letting it stand for several days.
Co(TPP)(NH2OH)2
e f Co(TPP) 2NH2OH (1)
voltammetry38 have shown that pyridine complexes weakly with
Co(TPP), and it is unlikely that stoichiometric amounts of
hydroxylamine would lead to a stable cobalt(II) complex.
The second reduction wave was irreversible at room tem-
perature and was located close to the Co(TPP)/Co(TPP)
reduction wave (Figure 3). This wave became more reversible
as the temperature was reduced. Previous studies24 have shown
that Co(TPP) , the reduction product, was not stable in
The cyclic voltammetry of Co(TPP)(NH2OH)2 was carried
out in methylene chloride between 40 °C and room temper-
ature (Figure 3). Three reduction waves were observed. The
(35) Kharitonov, Y. Y.; Sarukhanov, M. A.; Baranovskii, I. B. Russ. J.
Inorg. Chem. 1967, 12, 82 87.
(36) Maclean, A. L.; Armstrong, R. S.; Kennedy, B. J. J. Raman Spectrosc.
1993, 24, 897 901.
(37) Pavlovic, D.; Asperger, S.; Ahmeti, X.; Cizmek, B. C.; Ju, B.; Veksli,
Z. Inorg. Chem. 1988, 27, 1515 1516.
(38) Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260 2267.

Reactions of Hydroxylamine with Metal Porphyrins
Inorganic Chemistry, Vol. 36, No. 14, 1997 3117
Table 3. Half-Wave Potentials for Metal Porphyrin Complexes
methylene chloride and reacted with the solvent to form
Co(TPP)(CH2Cl) (eqs 2 and 3). The third wave, which could
complex
solvent
E1/2, V vs SCE
ref
a
Fe(TPP)(Cl)
Fe(TPC)(Cl)
Fe(OEP)(Cl)
Fe(TPP)(NH3)2
Fe(TPP)
Co(TPP)(Cl)
Co(TPP)(NH2OH)2
Co(TPP)
EtCl2
EtCl2
0.29
0.28
0.52
0.09
1.07
0.06b
0.78b
0.85
0.29
1.06
42
42
43
this work
44
45
this work
46
47
48
a
Co(TPP) e f Co(TPP)
(2)
(3)
CH2Cl2
THF
Co(TPP)
CH2Cl2 f Co(TPP)(CH2Cl) Cl
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
not be seen at 40 °C, was due to the reduction of the cobalt
alkyl complex.24 At low temperatures, reaction 3 did not occur
fast enough, and the reduction wave for the cobalt alkyl
complex is not observed.
The cyclic voltammetry of Co(TPP)(NH2OH)2 in acetonitrile
was similar to the methylene chloride results. Unlike in
methylene chloride, though, the second reduction wave (forma-
tion of CoI) was reversible. The thin-layer spectroelectrochem-
istry of the hydroxylamine complex led to the formation of
CoII(TPP). Unfortunately, the product appeared to be insoluble
under the conditions studied and slowly precipitated out of
solution during the OTTLE scan. This precipitation was too
slow to be observed in cyclic voltammetry.
Mn(TPP)(Cl)
Cr(TPP)(Cl)
a EtCl2
reported.
ethylene chloride. b Wave was irreversible, Ep,c value
oxidation. On the basis of the voltammetry of Fe(P)(NO), this
process is both thermodynamically and kinetically favored for
iron.40
From the mechanism above, an easily reduced metal atom
would favor the internal electron transfer. The E1/2 values of a
number of metalloporphyrin complexes are shown in Table 3.
The easiest metals to reduce were iron(III) and manganese(III)
porphyrins, which were also the metals that underwent the
reductive nitrosylation reaction. In addition, their E1/2 values
were not significantly affected by coordination to N-ligands.
In fact, for the reduction of iron(III), the wave was generally
shifted to more positive potentials upon N-coordination (e.g.,
Discussion
The reductive nitrosylation reaction is particularly interesting
in that it couples the reduction of the metal with the dispro-
portionation of hydroxylamine:
E1/2 of Fe(TPP)(NH3)2
0.09 V). Similarly, the Mn(III)/
2NH2OH f NO NH4
H2O
e
Mn(II) potential was not shifted significantly in the presence
of nitrogen ligands because of the similar affinity of both
oxidation states to these ligand.49 On the other hand, the E1/2
of the Co(III)/Co(II) couple was shifted to much more negative
potentials when cobalt(III) was coordinated to nitrogen ligands.
For example, Co(TPP)(Cl) reduced irreversibly at 0.06 V,45
while Co(TPP)(pyridine)2 reduced at 0.77 V.38 Further
reduction of cobalt(II) porphyrins occurred at more negative
potentials ( 0.85 V), as did Cr(TPP)(Cl) ( 1.06 V).
The ability of the metal porphyrin to coordinate with
hydroxylamine was a less significant factor than the half-wave
potential. But, for an internal electron transfer to occur,
hydroxylamine must remain coordinated to the metal, and one
would expect reasonable affinity for the metal ion to hydroxyl-
amine. Selected formation constants for metal porphyrins
studied in this work with pyridine are given in Table 4. While
the mechanism proposed requires initial coordination of hy-
droxylamine, all the metalloporphyrins studied were strong
enough Lewis acids to form measurable amounts of the starting
complex. In examining Table 4, we see that ferric and ferrous
porphyrins coordinated well with pyridine and underwent
reductive nitrosylation. On the other hand, cobalt(II) porphyrin
formed weak complexes with pyridine and did not react with
hydroxylamine. Both manganese(III) and chromium(III) por-
phyrins coordinated weakly with pyridine, yet the former reacted
rapidly with hydroxylamine while no reaction was observed for
the latter. This was in spite of the fact that the CrIII(TPP)(Cl)
formation constant was about 100 times stronger than MnIII-
MIII(P) e f MII(P)
The presence of a reducible metal atom allows a pathway to
link 3-electron oxidation of hydroxylamine to NO with the
2-electron reduction of another hydroxylamine to ammonia. In
neutral to alkaline solutions, it is known that hydroxylamine
generally acts as an oxidizing agent.39 If we assume that the
initial step in the reaction is the one-electron oxidation of
MIII(P)(NH2OH) by free hydroxylamine to form MIII(P)-
(N0H2O) and NH2 (plus water), an internal electron transfer
would lead to MII(P)(NIH2O ). This species can readily lose
two protons to form MIIP(NO ), which can be further oxidized
to MII(P)(NO) by hydroxylamine or NH2 .
This process would be the reverse of the reduction of
FeII(P)(NO) to form FeI(P) and hydroxylamine in which the
key kinetically observed intermediate was FeII(P)(NH2O ).40,41
FeII(P)(NO) e f FeII(P)(NO)
(4)
(5)
FeII(P)(NO)
Fe(P)(NH2O)
2PhOH h Fe(P)(NH2O)
2PhO
PhOH 3 e f
Fe(P)
NH2OH PhO (6)
Here PhOH is phenol and PhO is the phenolate anion. For
the reductive nitrosylation reaction, the reverse of reactions 4
and 5 would occur: loss of 2 protons and a one-electron
(45) Huet, J.; Gaudemer, A.; Boucly-Goester, C.; Boucly, P. Inorg. Chem.
1982, 21, 3413 3419.
(46) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27, 1489
1492.
(39) Jones, K. Nitrogen. In Comprehensi e Inorganic Chemistry; Pergamon
Press: Oxford, U.K., 1973; pp 147 388.
(40) Liu, Y. M.; Ryan, M. D. J. Electroanal. Chem. 1994, 368, 209 219.
(41) Barley, M. H.; Takeuchi, K. J.; Meyer, T. J. J. Am. Chem. Soc. 1986,
108, 5876 5885.
(42) Choi, I.-K.; Ryan, M. D. New J. Chem. 1992, 16, 591 597.
(43) Stolzenberg, A. M.; Strauss, S. H.; Holm, R. H. J. Am. Chem. Soc.
1981, 103, 4763 4778.
(47) Kelly, S. L.; Kadish, K. M. Inorg. Chem. 1982, 21, 3631 3639.
(48) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1983, 22, 342 349.
(49) Kadish, K. M.; Kelly, S. Inorg. Chem. 1979, 18, 2968 2971.
(50) Kadish, K. M.; Bottomley, L. A. Inorg. Chem. 1980, 19, 832 836.
(51) Kadish, K. M.; Bottomley, L. A.; Beroiz, D. Inorg. Chem. 1978, 17,
1124 1129.
(52) Basolo, F.; Jones, R. D.; Summerville, D. A. Acta Chem. Scand. Ser.
A 1978, 32, 771 780.
(44) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1348 1357.

3118 Inorganic Chemistry, Vol. 36, No. 14, 1997
Choi et al.
Table 4. Coordination of Pyridine with Cobalt, Iron, and
Manganese Porphyrins
internal 2-electron transfer combined with the loss of hydroxide,
as was observed for cobalt(I).4 This could be facilitated by an
58
easily oxidized macrocycle (an isobacteriochlorin).53
compd
solvent
CH2Cl2
log K1
log â2
ref
FeII(TPP)
7.8
10.2
50
50
47
49
47
49
51
51
52
52
FeII(P)(NH2OH) f FeIII(P.)(NH2)
FeIII(P.)(NH2) H f FeII(P) NH3
OH
FeIII(TPP)(ClO4)
MnII(TPP)
CH2Cl2
a
EtCl2
4.55
2.70
4.08
1.10
2.90
MnII(TPP)(Cl)
MnIII(TPP)(ClO4)
MnIII(TPP)(Cl)
CoII(TPP)
CH2Cl2
EtCl2
CH2Cl2
CH2Cl2
CH2Cl2
20% THF/
80% toluene
2 e
6.99
15.6
In some model complexes, in fact, the oxidation of the
macrocycle has been observed before the oxidation of the ferrous
atom.53,54
CoIII(TPP)
CrIII(TPP)(Cl)
3.15
2.13a
a Ligand: 2-butylamine.
Conclusion
The reductive nitrosylation reaction of metalloporphyrins was
observed for those complexes where the metal was easily
reduced. Among the metals studied, iron and manganese
porphyrins formed nitrosyl complexes readily. Cobalt por-
phyrins were rapidly oxidized by air to form a stable bis-
(hydroxylamine)cobalt(III) complex. No reaction was observed
between chromium(III) porphyrins and hydroxylamine. These
results were consistent with the trend in E1/2 values of the
metalloporphyrins.
The one-electron oxidation of a metal hydroxylamine com-
plex, followed by an internal electron transfer between the metal
and the hydroxylamine, led to a complex which has the same
structure as the one observed kinetically in the voltammetric
reduction of Fe(P)(NO), FeII(P)(NH2O ). This would indicate
that the oxidation of hydroxylamine to NO or the reduction of
NO to hydroxylamine by metalloporphyrins proceeds through
a common intermediate.
(TPP)(Cl) (see Table 4). Finally cobalt(III) formed strong
complexes with hydroxylamine, yet underwent the reductive
nitrosylation reaction very slowly.
The reaction between hydroxylamine and cob(I)alamins has
been previously reported.4 In this case, there was no evidence
for the coordination of CoI with hydroxylamine, and the reaction
involves the oxidation of CoI to CoIII and the reduction of
hydroxylamine to ammonia. The mechanism for this reaction
was the nucleophilic attack of CoI on hydroxylamine. None of
the complexes studied in this work were sufficiently nucleophilic
for this mechanism to be feasible here. However, the products
of reaction 6 were Fe(P) and hydroxylamine. FeI is known to
be nucleophilic and could reduce hydroxylamine by the same
mechanism used by CoI. In the coulometric reduction of
Fe(P)(NO), only ammonia was observed as the ultimate product
of the reduction.40
The similarity of proposed intermediates in the oxidation of
hydroxylamine and the reduction of NO may have implications
for the mechanism of assimilatory nitrite reductases and
hydroxylamine oxidoreductases. For both of these enzymes,
the heme group (siroheme for most assimilatory nitrite reduc-
tases) carries out the transformations. While the structure of
the hydroxylamine oxidoreductases is not known, a similar
mechanism could occur if there is a basic amino acid side chain
near the active site that can abstract a proton while the prosthetic
group is oxidized. Conversely, acidic residues near the pros-
thetic group of nitrite reductases could add two protons
concurrent with the reduction of the NO group, leading to the
same intermediate, FeII(P)(NH2O) , which can be readily
reduced to FeII(P)(NH2OH). Axial coordination by sulfide may
make the iron siroheme nucleophilic enough to carry out an
The reductive nitrosylation reaction provides a convenient
method for the preparation of isotopically labeled nitrosyl or
the nitrosylation of small amounts of porphyrin. The yields
are nearly quantitative, and the product is insoluble in the solvent
used.
IC9605783
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Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 2652 2656.
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