Influence of an extremely negatively charged porphyrin on the reversible binding kinetics of NO to Fe(III) and the subsequent reductive nitrosylation

Inorg. Chem. 2007, 46, 3336−3352

Influence of an Extremely Negatively Charged Porphyrin on the

Reversible Binding Kinetics of NO to Fe(III) and the Subsequent

Reductive Nitrosylation

Joo-Eun Jee,^†Siegfried Eigler,^§Norbert Jux,*^,§Achim Zahl,^†and Rudi van Eldik*^,†

Institute for Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Egerlandstrasse 1, 91058

Erlangen, Germany, and Institute for Organic Chemistry, UniVersity of Erlangen-Nu¨rnberg,

Henkestrasse 42, 91054 Erlangen, Germany

Received September 13, 2006

The polyanionic, water-soluble, and non-

tetra-t-butyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,20⁶-octakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin), (P¹⁶^-)Fe^III,

µ

-oxo dimer-forming iron porphyrin (hexadecasodium iron 5⁴,10⁴,15⁴,20⁴-

with 16 negatively charged meso substituents on the porphyrin was synthesized and fully characterized by UV

−

vis

1

and H NMR spectroscopy. A single pK_a1value of 9.90

± 0.01 was determined for the deprotonation of coordinated

water in the six-coordinate (P¹⁶^-)Fe^III(H₂O)₂and as attributed to the formation of the five-coordinate monohydroxo-

ligated form, (P¹⁶^-)Fe^III(OH). The porphyrin complex reversibly binds NO in aqueous solution to yield the nitric

oxide adduct, (P¹⁶^-)Fe^II(NO⁺)(L), where L

)

H₂O or OH^-. The kinetics for the reversible binding of NO were

studied as a function of pH, temperature, and pressure using the stopped-flow technique. The data for the binding

of NO to the diaqua complex are consistent with the operation of a dissociative mechanism on the basis of the

significantly positive values of

∆

S^qand

∆

V^q, whereas the monohydroxo complex favors an associatively activated

3.1

°

C, determined for the NO binding to (P^16-)Fe^III(OH) at higher pH, is significantly lower than the

mechanism as determined from the corresponding negative activation parameters. The rate constant, k_on

)

×

1

10⁴M^-s^-at 25

corresponding value measured for (P¹⁶^-)Fe^III(H₂O)₂at lower pH, namely, k_on

)

11.3 × °C. This

10⁵M^-s^-at 25

1

decrease in the reactivity is analogous to that reported for other diaqua- and monohydroxo-ligated ferric porphyrin

complexes, and is accounted for in terms of a mechanistic changeover observed for (P¹⁶^-)Fe^III(H₂O)₂and

(P¹⁶^-)Fe^III(OH). The formed nitrosyl complex, (P^16-)Fe^II(NO⁺)(H₂O), undergoes subsequent reductive nitrosylation

to produce (P¹⁶^-)Fe^II(NO), which is catalyzed by nitrite produced during the reaction. Concentration-, pH-, temperature-,

and pressure-dependent kinetic data are reported for this reaction. Data for the reversible binding of NO and the

subsequent reductive nitrosylation reaction are discussed in reference to that available for other iron(III) porphyrins

in terms of the influence of the porphyrin periphery.

Introduction

Nitric oxide is very reactive to Fe(II) and Fe(III), for which

different electronic and structural factors of the iron center

influence the chemical properties of the resulting nitrosyl

Nitric oxide (NO) is a versatile signaling molecule, and

its interaction with metal-centered proteins plays an important

role. In the human system, NO is associated with biological

functions ranging from vasodilatation of vascular smooth

muscle cells to neurotransmission, cytotoxic immune re-

sponse, inflammation, and regulation of cell death.¹It is

generated in vivo by NO synthase in various organs from

arginine with the aid of molecular oxygen and NADPH.^1,2

(1) (a) Moncada, S.; Higgs, E. A. Br. J. Pharmacol. 2006, 147, S104. (b)

Valsova, I. I.; Tyurin, V. A.; Kapralov, A. A.; Kurnikov, I. V.; Osipov,

A. N.; Potapovich, M. V.; Stoyanovsky, D. A.; Kagan, V. E. J. Biol.

Chem. 2006, 281, 14554. (c) Feelisch, M.; Stamler, J. S. Methods in

Nitric Oxide Research; John Wiley and Sons: Chichester, U.K., 1996.

(d) Verma, A.; Hirsch, D. J.; Glatt, C. E.; Romnett, G. V.; Snyder, S.

H. Science 1993, 259, 381. (e) Ribeiro, J. M. C.; Hazzard, J. M. H.;

Nussenzveig, R. H.; Champagne, D. E.; Walker, F. A. Science. 1993,

260, 539. (f) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharmacol.

ReV. 1991, 43, 109. (g) Moncada, S.; Radomski, L. W.; Palmer, R.

M. Biochem. Pharmacol. 1988, 37, 2495. (h) Palmer, R. M. J.; Ferrige,

A. G.; Moncada, S. Nature. 1987, 327, 524.

* To whom correspondence should be addressed. E-mail: vaneldik@

chemie.uni-erlangen.de (R.v.E.); norbert.jux@organik.uni-erlangen.de (N.J.).

^†Institute for Inorganic Chemistry.

^§Institute for Organic Chemistry.

3336 Inorganic Chemistry, Vol. 46, No. 8, 2007

10.1021/ic061732g CCC: $37.00

Published on Web 03/22/2007

Influence of an Extremely NegatiWely Charged Porphyrin

product, as well as the NO binding and dissociation rate

constants. The reactivity of iron porphyrins is finely regulated

by a variety of structural and electronic features, for example,

the nature of the axial ligands, the type of substituents on

the porphyrin periphery, the polarity of the reaction medium,

and other factors. To investigate the role of these factors,

numerous spectroscopic, structural, and mechanistic studies

are reported on the interaction of NO with synthetic model

complexes.³

ferric porphyrins such as TPPS (tetra-(4-sulfonatophenyl)-

porphyrinato), TMPyP (meso-tetrakis(N-methyl-4-pyridyl)

porphyrinato),^6,16,17P⁸⁺(5,10,15,20-tetrakis-(4′-t-butyl-

2′,6′-bis(4-t-butylpyridine)phenyl)-porphyrinato),¹⁸

and

P

^8-(5⁴,10⁴,15⁴,20⁴-tetra-t-butyl-5²,5⁶,15²,15⁶-tetrakis-(2,2-bis-

carboxylato-ethyl)-5,10,15,20-tetraphenylporphyrinato)¹⁸were

systematically studied in aqueous medium, in which the

resulting nitrosyl adducts ((P)Fe^III+ NO f (P)Fe^II-NO⁺)

-

interact with a nucleophile, namely, OH^-and NO₂, to yield

Mechanistic studies on the reactions of NO with heme

proteins or model ferric porphyrins have been performed by

the application of laser flash photolysis and stopped-flow

techniques. Studies on the reversible binding of NO to

synthetic iron(II) and iron(III) porphyrins^4,5and iron(III)

heme proteins,^6a,7-10such as cytochrome P450 and met-

myoglobin, have been performed in which distinctive features

of the active site of heme proteins were used to develop

useful biomimetic model systems.

The binding of NO to an oxidizing metal (Mⁿ⁺) induces

intramolecular electron transfer, leading to the formation of

(P)M^(n-1)+-NO⁺, followed by nitrosylation of a nucleophile

(Nu^-) to produce (P)M^(n-1)+and Nu^--NO⁺. The observed

reaction involves reductive nitrosylation as expressed in

reaction 1. Recent reports have supported that reductive

nitrosylation is proposed as a viable pathway in which

hemoglobin binds NO to the cys â-93 residue and forms

S-nitrosohemoglobin in the transport and metabolism of

NO.^11,12

ferroheme proteins. Recent studies performed in our labo-

ratories clearly revealed that the observed rate constant for

NO reduction depends on the concentration of NO and OH^-

and suggested that the porphyrin environment involving

oppositely charged substituents has a crucial influence on

the observed rate constants and mechanistic features of

nitrite-catalyzed reductive nitrosylation.¹⁸

The goal of these investigations is to understand the

influence of the iron porphyrin microenvironment on the

reactivity with NO and the stability of the resulting (P)Fe^II-

(NO⁺) species toward subsequent reaction in aqueous solu-

tion. In this context, the reported studies were undertaken

to investigate the systematic influence of the porphyrin

environment on the properties and reactivity of water-soluble

ferric porphyrins.^3-7We now report the synthesis and

spectroscopic characterization of an extremely negatively

charged iron(III) porphyrin, (P^16-)Fe^III(L)₂(hexadecasodium

iron 5⁴,10⁴,15⁴,20⁴-tetra-t-butyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,20⁶-

octakis[2,2-bis(carboxylato)ethyl)]-5,10,15,20-tetraphenylpor-

phyrin) (Figure 1), and evaluate the detailed kinetics of its

interaction with NO. In the latter context, variable pH-,

temperature-, and pressure-dependent stopped-flow measure-

ments provided a detailed kinetic and mechanistic description

of the reversible binding of NO to (P^16-)Fe^III(H₂O)₂and

(P^16-)Fe^III(OH) present in aqueous solution at low and high

(P)Mⁿ⁺+ NO {^K^NO} (P)M^(n-1)+- NO⁺

^-8

+Nu

(P)M^(n-1)++ Nu^--NO⁺(1)

On the basis of spectroscopic and kinetic data, the NO

reduction of ferric-heme proteins (viz., ferric cytochrome c,

metmyoglobin, and methemoglobin)^13-15and synthetic model

(9) (a) Frank, A.; Jung, C.; van Eldik, R. J. Am. Chem. Soc. 2004, 126,

4181. (b) Suzuki, N.; Higuchi, T.; Urano, Y.; Kikuhchi, K.; Uchida,

T.; Mukai, M.; Kitagawa, T.; Nagano, T. J. Am. Chem. Soc. 2000,

122, 12059. (c) Rich, A. M.; Armstrong, R. S.; Ellis, P. J.; Lay, P. A.

J. Am. Chem. Soc. 1998, 120, 10827.

(10) Newman, K. E.; Meyer, F. K.; Merbach, A. E. J. Am. Chem. Soc.

1979, 101, 1470.

(11) Gladwin, M. T.; Ognibene, F. P.; Pannell, L. K.; Nochols, J. S.; Pease-

Fye, M. E.; Shelhamer, J. H.; Schechter, A. N. Free Radical Res.

2000, 97, 9943.

(12) (a) Weichsel, A.; Maes, E. M.; Andersen, J. F.; Valenzuela, J. G.;

Shokhireva, T. K.; Walker, F. A.; Montfort, W. R. Proc. Natl. Acad.

Sci. U.S.A. 2005, 102, 594. (b) Gladwin, M. T.; Lancaster, J. R., Jr.;

Freeman, B. A.; Schechter, A. N. Nat. Med. 2003, 9, 496. (c)

Luchsinger, B. P.; Rich, E. N.; Gow, A. J.; Williams, E. M.; Stamler,

J. S.; Singel, D. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 461. (e)

Stamler, J. S.; Jia, L.; Eu, J. P.; Mcmahon, T. J.; Demchenko, I. T.;

Bonaventura, J.; Gernert, K.; Piantadosi, C. A. Science 1997, 276,

2034. (f) Stamler, J. S.; Simon, D. I.; Osborne, J. A.; Mullins, M. E.;

Jaraki, O.; Michel, T.; Singel, D. J.; Loscalzo, J. Proc. Natl. Acad.

Sci. U.S.A. 1992, 89, 444. (g) Tran, D.; Skelton, B. W.; White, A. H.;

Lavermann, L. E.; Ford, P. C. Inorg. Chem. 1998, 37, 2505.

(13) Yoshimura, T.; Suzuki, S.; Nakahara, A.; Iwasaki, H.; Masuko, M.;

Matsubara, T. Biochim. Biophys. Acta. 1985, 831, 267.

(14) (a) Gow, A. J.; Luchsinger, B. P.; Pawloski, J. R.; Singel, D. J.;

Stamler, J. S. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 9027. (b) Chien,

J. C. W. J. Am. Chem. Soc. 1969, 91, 2166.

(15) Yoshimura, T.; Suzuki, S. Inorg. Chim. Acta. 1988, 152, 241.

(16) Tran, D.; Skelton, B. W.; White, A. H.; Lavermann, L. E.; Ford, P.

C. Inorg. Chem. 1998, 37, 2505.

(2) (a) Bredt, D. S.; Hwang, P. M.; Glatt, C. E.; Lowenstein, C.; Reed,

R. R.; Snyder, S. H. Nature (London) 1991, 351, 714. (b) Steven, P.

S.; Lewis, C. B.; David, A. K.; Hunter, C. C.; Michael, L. T.; Sandra,

F.; Kavita, V. E.; Mark, D. K.; Susan, N. T.; Anne, C.; Joshua, H.;

Gary, G. J. Am. Med. Assoc. 2006, 295, 58. (c) Moncada, S.; Palmer,

R. M.; Higgs, E. A. Biochem. Pharmacol. 1989, 38, 1709. (d) Hibbs,

J. B., Jr.; Tainter, R. R.; Vabrin, Z. Science. 1987, 235, 473.

(3) (a) Ford, P. C.; Laverman, L. E.; Lorkovic, I. M. AdV. Inorg. Chem.

2003, 54, 2003. (b) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002,

102, 993, and references therein. (c) Hoshino, M.; Laverman, L. E.;

Ford, P. C. Coord. Chem. ReV. 1999, 187, 75. (d) Laverman, L. E.;

Ford, P. C. J. Am. Chem. Soc. 2001, 123, 11614. (e) Ford, P. C.;

Fernandez, B. O.; Lim, M. D. Chem. ReV. 2005, 105, 2439. (f)

Meunier, B. Chem. ReV. 1992, 92, 1411.

(4) (a) Wolak, M.; van Eldik, R. J. Am. Chem. Soc. 2005, 127, 13312

and references cited therein.

(5) (a) Jee, J.-E.; Eigler, S.; Hampel, F.; Jux, N.; Wolak, M.; Zahl, A.;

Stochel, G.; van Eldik, R. Inorg. Chem. 2005, 44, 7717. (b) Jee, J.-

E.; Wolak, M.; Balbinot, D.; Jux, N.; Zahl, A.; van Eldik, R. Inorg.

Chem. 2006, 45, 1326 and references cited therein.

(6) (a) Theodoridis, A.; van Eldik, R. J. Mol. Catal. A. 2004, 24, 197. (b)

Trofimova, N. S.; Safronov, A. Y.; Ikeda, S. Inorg. Chem. 2003, 42,

1945. (c) Hoshino, M.; Maeda, M.; Konishi, R.; Seki, H.; Ford, P. C.

J. Am. Chem. Soc. 1996, 118, 5702.

(7) (a) Franke, A.; Stochel, G.; Suzuki, N.; Higuchi, T.; Okuzono, K.;

van Eldik, R. J. Am. Chem. Soc. 2005, 127, 5360. (b) Wolak, M.; van

Eldik, R. Coord. Chem. ReV. 2003, 230, 263. (c) Schneppensieper,

T.; Zahl, A.; van Eldik, R. Angew. Chem., Int. Ed. 2001, 40, 1678.

(8) (a) Wyllie, G. R. A.; Scheidt, W. R. Chem. ReV. 2002, 102, 1067. (b)

Ellison, M. K.; Schulz, Chapter E.; Scheidt, R. W. J. Am. Chem. Soc.

2002, 124, 13833.

(17) Fernandez, B. O.; Lorkobic, I. M.; Ford, P. C. Inorg. Chem. 2004,

43, 5393.

(18) Jee, J.-E.; van Eldik, R. Inorg. Chem. 2006, 45, 6523.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3337

Jee et al.

vigorously. The green (organic) layer was shaken once again with

2 M HCl (50 mL) and twice with water (50 mL each). After

neutralization with a saturated NaHCO₃solution (50 mL) and a

final wash with brine (50 mL), the organic layer was dried with

MgSO₄, and the solvent was removed under reduced pressure. The

compound was further cleaned by column chromatography (silica

gel, CH₂Cl₂/ethyl acetate 9:1) and was obtained as a purple powder.

1

Yield: 76 mg (98%, 0.038 mmol). H NMR (300 MHz, CDCl₃,

rt): δ 8.71 (s, 8H, â-pyrr-H), 7.45 (s, 8H, o-Ar-H), 3.16 (t, 8H,³J

) 8.2 Hz, CH), 3.12 (s, 48H, CH₃), 2.79 (d, 16H,³J ) 8.2 Hz,

Ar-CH₂), 1.53 (s, 36H, CH₃), -2.51 (s, 2H, NH). ¹³C NMR (75

MHz, CDCl₃, rt): δ 168.6, 151.7, 139.1, 137.8, 124.4, 115.7, 51.9,

51.8, 34.8, 33.7, 31.5. MS (FAB, NBA): m/z 1991 (M^+.), 1932

(M^+.- COOCH₃). IR (KBr): ν [cm^-1] 2957, 2907, 2872, 1733,

1606, 1559, 1436, 1343, 1278, 1227, 1197, 1150, 1061, 1027, 965,

887, 868, 803, 737. UV-vis (CH₂Cl₂): λ [nm] (ꢀ [mol^-1cm^-1])

422 (4.31 × 10⁵), 517 (1.67 × 10⁴), 550 (2.3 × 10³), 593 (3.7 ×

10³), 646 (1.0 × 10³).

Figure 1. Structure of (P^16-)Fe^III(L)₂where L ) H₂O or OH^-.

pH, respectively. The subsequent reductive nitrosylation was

also studied as a function of concentration, temperature, and

pressure. The results are discussed in reference to kinetic

data and mechanistic information reported for other water-

soluble iron(III) porphyrins.

Experimental Section

Chloroiron(III) 5⁴,10⁴,15⁴,20⁴-Tetra-t-butyl-5²,5⁶,10²,10⁶,15²,-

15⁶,20²,20⁶-octakis[2,2-bis(methoxycarbonyl)ethyl)]-5,10,15,20-

5,10,15,20-tetraphenylporphyrin (5). FeCl₂(230 mg, 1.81 mmol)

was added to a solution of 4 (230 mg, 0.12 mmol) in THF (30

mL), and the mixture was heated under reflux for 24 h. The solvent

was evaporated, and the residue was dissolved in CH₂Cl₂and

washed with 6 M HCl. The organic layer was separated and washed

twice with water. After it was dried over MgSO₄, the compound

was precipitated to give a dark brown powder. Yield: 189 mg (73%,

0.088 mmol). ¹H NMR (300 MHz, CDCl₃, rt): δ 82.6 (br s, â-pyrr-

Synthesis and Characterization of (P^16-)Fe^III. The chemicals

and solvents employed for the synthesis of (P^16-)Fe^IIIwere used

as received unless otherwise noted. Solvents were dried using

standard procedures. Column chromatography was performed on

1

silica gel 60, 32-63 µm, 60 Å (MP Biomedicals). Standard H

and ¹³C NMR spectra were recorded on a Bruker Avance 300

spectrometer. FAB mass spectrometry was performed with Micro-

mass Zabspec. Standard UV-vis spectra were recorded on a

Shimadzu UV-3102 PC UV-vis NIR scanning spectrophotometer.

IR spectra (KBr pellets) were recorded with a FT-IR IFS 88 infrared

spectrometer (Bruker Analytische Messtechnik GmbH). Elemental

analyses were carried out on a CHN-Mikroautomat (Heraeus). Thin-

layer chromatography (TLC) was carried out on E. Merck silica

gel 60 F254 plates. Zinc(II)-5⁴,10⁴,15⁴,20⁴-tetra-t-butyl-5²,5⁶,10²,-

10⁶,15²,15⁶,20²,20⁶-octabromo-5,10,15,20-tetraphenylporphyrin (2)

was synthesized as described previously.¹⁹

H), 16.6, 14.6 (br s, aryl-H). MS (FAB, NBA): m/z 2046 (M⁺

-

Cl). IR (KBr): ν [cm^-1] 2957, 2907, 2872, 1733, 1606, 1436, 1227,

1197, 1150, 1031, 995, 883, 837, 802, 725. UV-vis (CH₂Cl₂): λ

[nm] (ꢀ [mol^-1cm^-1]) 391 (5.85 × 10⁵), 427 (9.39 × 10⁵), 517

(1.38 × 10⁴), 579 (3.8 × 10³), 692 (2.7 × 10³).

Hexadecasodium Hydroxoiron(III) 5⁴,10⁴,15⁴,20⁴-Tetra-t-bu-

tyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,20⁶-octakis[2,2-bis(carboxylato)ethyl)]-

5,10,15,20-tetraphenylporphyrin (1-OH). NaOH (1.50 g, 37.5

mmol) was added to a solution of 5 (150 mg, 0.072 mmol) in

ethanol (20 mL), and the reaction mixture was heated under reflux

for 2 h. After the mixture was cooled to room temperature, the

precipitate was filtered, washed with ethanol (200 mL), and dried

under reduced pressure. Gel permeation chromatography (Sephadex

LH20) in methanol and the subsequent precipitation with diethyl

ether gave a dark brown powder which, according to the mi-

croanalysis, contains sodium hydroxide in the lattice. Yield: 230

mg (86%, 0.061 mmol, based on formula obtained by microanaly-

Zinc(II)-5⁴,10⁴,15⁴,20⁴-tetra-t-butyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,

20⁶-octakis[2,2-bis(methoxycarbonyl)ethyl)]-5,10,15,20-tetra-

phenylporphyrin (3). KH (24 mg, 0.60 mmol) was suspended in

DMF (20 mL) under a N₂atmosphere. Dimethyl malonate (160

mg, 1.20 mmol in 20 mL DMF) was added dropwise at room

temperature (rt). After an additional hour, porphyrin 2 (100 mg,

0.061 mmol) was added, and the solution was stirred for 4 h at rt.

The reaction mixture was poured onto an ice/water mixture to

precipitate the product. The solid was filtered and washed thor-

oughly with water. The purple residue was dissolved in CH₂Cl₂,

dried over MgSO₄, and concentrated in vacuum. The product was

further purified by column chromatography (SiO₂, CH₂Cl₂/EtOAc

10:1) and was obtained as a purple solid. Yield: 104 mg (83%,

1

sis). H NMR (300 MHz, pD ) 7, rt): δ 48.6 (br s, â-pyrr-H),

9.71, 3.75, 2.81. IR (KBr): ν [cm^-1] 2964, 2876, 1563, 1405, 1359,

1332, 1290, 1065, 999, 876, 837, 802. UV-vis (H₂O, unbuf-

fered): λ [nm] (ꢀ [mol^-1cm^-1]) 324 (2.4 × 10⁴), 401 (7.5 × 10⁴),

510 (1.0 × 10⁴). Anal. Calcd for C₉₂H₇₇FeN₄Na₁₆O₃₃38NaOH:

C, 29.78; H, 3.12; N, 1.51. Found: C, 29.48; H, 3.24; N, 1.27.

Materials. Mes, Bis-Tris, Taps, sodium borate, Caps, and NaOH

were used as buffer solutions and were purchased from Sigma-

Aldrich. All chemicals used in this study were of analytical grade

reagent. The NO gas (Messer Griesheim or Riessner Gase, g99.5

vol %) was purified from impurities of nitrogen oxides such as

NO₂and N₂O₃by being passed through an Ascarite II column

(NaOH on silica gel from Sigma-Aldrich) and concentrated NaOH

solution.

1

0.050 mmol). H NMR (400 MHz, CDCl₃, rt): δ 8.64 (s, 8H,

â-pyrr-H), 7.43 (s, 8H, o-Ar-H), 3.18 (s, 48H, CH₃), 3.07 (t, 8H,³J

) 8.3 Hz, CH), 2.79 (d, 16H,³J ) 8.3 Hz, Ar-CH₂), 1.52 (s, 36H,

CH₃). ¹³C NMR (75 MHz, CDCl₃, rt): δ 169.0, 151.2, 150.1, 139.3,

139.0, 131.4, 124.6, 115.7, 52.2, 51.9, 34.8, 34.1, 31.6. MS (FAB,

NBA): m/z 2055 (M^+.). IR (KBr): ν [cm^-1] 2957, 2907, 2872,

1737, 1436, 1332, 1285, 1227, 1200, 1150, 1065, 1027, 992, 883,

799, 725. UV-vis (CH₂Cl₂): λ [nm] (ꢀ [mol^-1cm^-1]) 430 (3.94

× 10⁵), 561 (1.27 × 10⁴), 599 (1.1 × 10³).

5⁴,10⁴,15⁴,20⁴-Tetra-t-butyl-5²,5⁶,10²,10⁶,15²,15⁶,20²,20⁶-octakis-

[2,2-bis(methoxycarbonyl)ethyl)]-5,10,15,20-tetraphenylporphy-

rin (4). HCl (6 M, 50 mL) was added to a solution of 3 (80 mg,

0.039 mmol) in CH₂Cl₂(50 mL), and the two layers were shaken

Solution Preparation. All solutions were prepared under strict

oxygen-free conditions with Milli-Q water and handled in gastight

glassware because of the high oxygen-sensitivity of NO and the

nitrosyl complexes. Oxygen-free Ar and N₂were used to prepare

(19) Jux, N. Org. Lett. 2000, 2, 2129.

3338 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

deoxygenated solutions. Buffered solutions of the appropriate pH

for stopped-flow measurements were prepared with Mes, Bis-Tris,

Taps, sodium borate, and Caps buffers at a concentration of 0.05M.

The pH of the solution was controlled with HClO₄and NaOH. The

ionic strength was kept constant at 0.1 M with NaClO₄.

Measurements. pH measurements were carried out on a

Metrohm 623 pH meter equipped with a Sigma glass electrode. A

NO electrode (World Precision Instruments isolated nitric oxide

meter, model ISO-NO) was used to determine the concentration of

NO gas in aqueous solution. The NO electrode was calibrated with

a freshly prepared KNO₂solution according to the method suggested

by the manufacturer. UV-vis spectra were recorded in gastight

cuvettes on a Shimadzu UV-2100 spectrophotometer equipped

with a thermostated cell compartment CDS-260. UV-vis spectra

at pressures up to 150 MPa were recorded in a custom-built high-

pressure optical cell.²⁰

Stopped-flow kinetic measurements on the reaction of NO with

(P^16-)Fe^IIIat pH 6.5 and 12.7 were carried out with an Applied

Photophysics SX-18MV stopped-flow spectrophotometer. Deoxy-

genated aqueous solutions of (P^16-)Fe^IIIwere rapidly mixed in

varying volume ratios with a saturated NO solution in a gastight

syringe to obtain the appropriate NO concentration (0.2-2 mM).

The kinetics of the reaction were monitored at 430 and 435 nm

where the change in absorbance is a maximum for the nitrosyl

adducts at pH 6.5 and 12.7, respectively. The rates of NO binding

(k_on) and release (k_off) were determined from the slope and intercept

of linear plots of k_obsversus [NO], respectively, as described in

more detail in the Results and Discussion. More accurate NO

dissociation rates at different temperatures and pressures were

measured directly by the NO-trapping method, in which [Ru^III-

(edta)(H₂O)]^-was used as an efficient NO scavenger. This involved

rapid mixing of a (P^16-)Fe^II(NO⁺)(L), L ) H₂O or OH^-, solution

(2 × 10^-5M, L ) H₂O and OH^-at pH 6.5 and 12.7, respectively)

containing a small excess of NO with aqueous solutions of

[Ru^III(edta)(H₂O)]^-(1-2 mM) to give [Ru^III(edta)(NO)]^-and

(P^16-)Fe^III(L)₂, as shown by the observed spectral change. The

kinetics of NO release was followed in the stopped-flow spectro-

photometer at 430 (pH 6.5) or 435 nm (pH 12.7). We also measured

the rate constants for the conversion of (P^16-)Fe^II(H₂O)(NO⁺) to

(P^16-)Fe^II(NO). The observed rate constants and activation param-

eters for the reactions with NO/nitrite were monitored at 431 nm

where the change in absorbance is at a maximum. All kinetic

experiments were performed under pseudo-first-order conditions,

that is, with at least a 10-fold excess of NO over the iron porphyrin.

Reported rate constants are the mean values of at least five kinetics

runs, and the quoted uncertainties are based on the standard

deviation. High-pressure stopped-flow studies were performed on

a custom-built instrument (from 10 to 130 MPa).²¹Kinetic traces

were recorded on an IBM-compatible computer and analyzed with

the OLIS KINFIT (Bogart, GA) set of programs.

D-Chem Ltd) was added to each solution. A sample containing

the same components without the (P^16-)Fe^IIIcomplex was used as

a reference. Variable-temperature and -pressure Fourier transform

¹⁷O NMR spectra were recorded at a frequency of 54.24 MHz on

a Bruker Advance DRX 400WB spectrometer. The temperature

dependence of the ¹⁷OH₂line broadening was studied in the range

of 278-353 K. A homemade high-pressure probe²²was used for

the variable-pressure experiments performed at the selected tem-

perature (at 293 K at pH 6.5) and in the pressure range of 1-150

MPa. The sample was placed in a standard 5 mm NMR tube cut to

a length of 45 mm. Hydrostatic pressure was transmitted to the

sample by a movable macor piston, and the temperature was

controlled as described elsewhere.²²The reduced transverse relax-

ation times (1/T_2r) were calculated for each temperature and pressure

from the difference in the line widths observed in the presence and

absence of the metal complex, (∆ν_obs- ∆ν_solvent). The reduced

transverse relaxation time is related to the exchange rate constant

k_ex) 1/τ_m(where τ_mis the mean coordinated solvent lifetime) and

to the NMR parameters by the Swift and Connick equation (2a),^22,23

1

T_2r

1

P_m

) π

(∆υ_obs- ∆υ_solvent) )

T_2m^-2+ (T_2mτ_m)^-1+ ∆ω²

1

τ_m

m

1

T_2os

+

(2a)

(T_2m^-1+ τ_m^-1)²+ ∆ω²

{

}

m

where P_mis the mole fraction of water coordinated to the Fe(III)

ion, T_2osrepresents the outer-sphere contribution to T_2rarising from

long-range interactions of unpaired electrons of Fe(III) with the

water outside the coordination sphere, T_2mis the transverse

relaxation time of water in the inner coordination sphere in the

absence of chemical exchange, and ∆ω_mis the difference in the

resonance frequency of ¹⁷O nuclei in the first coordination sphere

of the metal and in the bulk solvent. In the present system, the

contribution of 1/T_2osto 1/T_2rand 1/T_2mis negligible, so that eq 2a

can be reduced to eq 2b. Taking into account that the temperature

1

T_2r

1

P_m

) π

(∆υ_obs- ∆υ_solvent) )

T_2m^-2+ (T_2mτ_m)^-1+ ∆ω²

1

τ_m

m

(2b)

(T_2m^-1+ τ_m^-1)²+ ∆ω²

{

}

m

dependence of k_exis given by eq 2c (taken from transition state

theory), the NMR and kinetic parameters were calculated by the

use of a nonlinear least-squares method applied to eq 2b in which

1/τ_mwas replaced by eq 2c.

1

τ_m

k_ex

)

) (k_BT/h)exp {(∆S^q_ex/R) - (∆H^q_ex/RT)}

(2c)

The temperature dependence of ∆ω_mwas assumed to be a simple

reciprocal function A/T,^8,24where A was determined as a parameter

in the treatment of the line-broadening data. The exchange rate

constant is assumed to have a simple pressure dependence given

¹⁷O NMR Water Exchange Measurements. Rate constants for

water exchange on the paramagnetic (P^16-)Fe^IIIcomplex and the

corresponding activation parameters, ∆H^q_ex, ∆S^q_ex, and ∆V^q_ex, were

measured at pH 6.5 and 12.7 using a ¹⁷O NMR line-broadening

technique. Aqueous solutions of (P^16-)Fe^III(20 mM) were prepared

at pH 6.5 (Mes buffer solution adjusted with HClO₄) and 12.7 (with

NaOH). Both solutions were kept at 0.1 M NaClO₄for the

approximate ionic strength, in which 10% of the total sample

volume of enriched ¹⁷O-labeled water (normalized 19.2% ¹⁷O H₂O,

0

by eq 2d where k_exis the rate constant for solvent exchange at

1

τ_m

k_ex

)

) k_ex⁰exp{(-∆V^q_ex/RT )P}

(2d)

atmospheric pressure. The pressure-dependent measurements were

performed at a temperature close to the optimal exchange region

(i.e., around the maximum of the plot of ln(1/T_2r) versus 1/T). The

(20) Spitzer, M.; Gartig, F.; van Eldik, R. ReV. Sci. Instrum. 1988, 59,

2092.

(21) van Eldik, R.; Gaede, W.; Wieland, S.; Kraft, J.; Spitzer, M.; Palmer,

D. A. ReV. Sci. Instrum. 1993, 64, 1355.

(22) Zahl, A.; Neubrand, A.; Aygen, S.; van Eldik, R. ReV. Sci. Instrum.

1994, 65, 882.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3339

Jee et al.

Scheme 1. Synthesis of (P^16-)Fe^III

reduced relaxation time, T_2r, and the value of ∆ω_m(calculated using

the value of A determined from the temperature dependence and

assumed to be pressure independent) were substituted into eq 2b

to determine k_exat each pressure. The resulting plot of ln(k_ex) versus

pressure was linear, and the volume of activation was calculated

Results and Discussion

Synthesis of (P^16-)Fe^III(1). We choose porphyrin 2 as a

starting material¹⁹because its bromomethyl groups can be

substituted easily with potassium malonates to yield the

malonate ester porphyrin 3. Demetalation with concentrated

hydrochloric acid transformed 3 into the free-base porphyrin

4 as shown in Scheme 1. Subsequent reaction with ferrous

chloride in THF gave the neutral iron(III) porphyrin octa-

malonic ester system 5. ¹H NMR and UV-vis spectroscopic

data clearly show that 5 is a paramagnetic iron(III) porphyrin

with a single chloro axial ligand. The â-pyrrole ¹H resonance

at 82.6 ppm (half-width of ∼300 Hz) indicates the overall

0

directly from the slope (-∆V^q_ex/RT). The value of k_exobtained

from the plot of ln(k_ex) versus P by extrapolation to atmospheric

pressure was in good agreement with the corresponding value for

k_exfrom the temperature-dependent measurements at ambient

pressure. In analogous temperature-dependent ¹⁷O NMR measure-

ments performed for the (P^16-)Fe^IIIcomplex at pH 12.7 (0.05M

NaOH), the line-width differences in the presence and absence of

the metal complex were very small, indicating the absence of a

significant water-exchange process for the (P^16-)Fe^IIIform present

at higher pH. Because of the small observed differences, the data

obtained in the variable-temperature study did not enable a reliable

fit to the Swift and Connick equation.

Electrochemical Measurement. Cyclic voltammetry measure-

ments were carried out using an Autolab PGSTAT 30 system (Eco

Chemie). A conventional three-electrode arrangement was employed

consisting of a gold disk working electrode (geometric area ) 0.07

cm²), a platinum wire auxiliary electrode, and a Ag/AgCl, NaCl (3

M) reference electrode, manufactured by Metrohm. The measure-

ments in aqueous solution were performed in 0.1 M Bis-tris

electrolyte and 25 °C. The complex concentration was 1 mM. The

solution was initially thoroughly degassed with purified nitrogen

(15 min), and a stream of N₂gas was passed over the sample

solutions during the measurements.

C

_4Vsymmetry of the porphyrin and proves the S ) 5/2 spin

state.²⁵Saponification of the malonic esters of 5 with sodium

hydroxide in ethanol leads to precipitation of a brownish

material that is soluble in water but totally insoluble in apolar

solvents. Gel permeation chromatography (Sephadex LH20)

in methanol and subsequent precipitation with diethyl ether

gave a solid material containing about 75% of 1 (the

impurities being NaOH and H₂O) as was found earlier for

(P^8-)Fe^IIIin a similar way.^5aStandard NMR spectra mea-

sured in unbuffered D₂O have shown that under these

conditions 1 exists as a high-spin monohydroxo complex

(P^16-)Fe^III(OH) (â-pyrrole ¹H at 80.0 ppm, see also further

text).

(23) Schneppensieper, T.; Seibig, S.; Zahl, A.; Tregloan, P.; van Eldik, R.

Inorg. Chem. 2001, 40, 3670.

(24) (a) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307. (b)

Swift, T. J.; Connick, R. E. J. Chem. Phys. 1964, 41, 2553.

(25) (a) Woon, T. C.; Shirazi, A.; Bruice, T. Inorg. Chem. 1986, 25, 3845.

(b) Cheng, R.-J.; Latos-Grazynski, L.; Balch, A. Inorg. Chem. 1982,

21, 2412.

3340 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

Figure 2. UV-vis spectral changes observed for aqueous solutions of (P^16-)Fe^IIIin the pH ranges of (a) 1-6 and (b) 6-13. The inset shows a plot of the

absorbance at 420 nm versus the pH in the range of 1-13. Experimental conditions: [(P^16-)Fe^III] ) 1.1 × 10^-4M, temp ) 25 °C, I ) 0.1 M (with NaClO₄).

Two structural details which are important with regard to

the investigations presented in this paper can be extracted

from the previously shown crystal structure of the zinc

precursor of (P^8-)Fe^IIIcrystallized from THF.^5aFirst, the

malonate groups are located above and below the porphyrin

plane, thus not allowing for the formation of µ-oxo-dimer

formation for steric and electrostatic reasons (as confirmed

by in-depth spectroscopic studies described below). Second,

the malonate groups cannot coordinate to the metal center

in an intramolecular fashion without extreme distortion of

the molecule (the average distance of the malonate oxygen

atoms from the zinc atom is ∼7 Å, and the closest distance

is ∼4 Å). Nevertheless, ester or carboxylate groups of

malonate substituents may interact with axial ligands coor-

dinated to the metal center.

changing the pH and suggests that the possible formation of

a µ-oxo dimer does not occur in the present system. This

interpretation is further supported by the NMR data reported

below. Two pK_avalues were determined from the plot in

Figure 2b, from which the first pK_aof ∼2 in the range of 1

< pH < 6 is attributed to the deprotonation of the carboxylic

acid groups (reaction 3) on the modified porphyrin in

reference to the pK_avalues for benzylmalonic acid of 2.56

and 5.22.⁵The second pK_a1) 9.90 ( 0.01 in the pH range

of 6-13 is attributed to the deprotonation of coordinated

water as described by reaction 4.

(H₁₆P)Fe^III(H₂O)₂a (P^16-)Fe^III(H₂O)₂+ 16 H⁺

1 < pK_a(RCOOH)< 6 (3)

Studies on (P^16-)Fe^III. The speciation of (P^16-)Fe^IIIas a

function of pH in aqueous medium was studied by UV-

(P^16-)Fe^III(H₂O)₂a (P^16-)Fe^III(H₂O)(OH^-) + H⁺

pK_a1) 9.90 ( 0.01 (4)

1

vis, H NMR, and ¹⁷O NMR techniques. A spectrophoto-

metric titration of a (P^16-)Fe^IIIsolution in the pH range of

1-13 at 0.1 M ionic strength (adjusted with NaClO₄) resulted

in the spectral changes presented in Figure 2. The spectral

changes are separated into two parts, and the corresponding

plot of absorbance at 420 nm versus pH is shown in the

inset of Figure 2b. In the pH range of 1-6, the absorbance

continuously increases over the whole spectral range as a

result of the deprotonation of the carboxylic acid groups upon

increasing the pH which is accompanied by an increase in

the solubility of the porphyrin complex (see Figure 2a). As

can be seen in Figure 2b, an increase in pH from 6 to 13

leads to a gradual shift of the peaks at 398 (ꢀ ) 6.4 × 10⁴

M^-1cm^-1) and 532 nm (ꢀ ) 1.04 × 10⁴M^-1cm^-1) to 418

(ꢀ ) 7.2 × 10⁴M^-1cm^-1) and 606 nm (ꢀ ) 7.1 × 10³M^-1

cm^-1). The observed pattern is similar to that reported for

other water-soluble iron(III) porphyrins and typical for the

formation of monohydroxo-ligated species from the corre-

sponding diaqua-ligated (P)Fe^III(H₂O)₂.^25,26No further spec-

tral changes occurred in solution at high pH upon extended

standing, and the observed spectral changes in the pH range

of 6-13 were fully reversible. This indicates rapid inter-

conversion of species present at high and low pH upon

On the basis of the data summarized in Table 1, the pK_a1

value for (P^16-)Fe^III(H₂O)₂is the highest value reported for

water-soluble iron(III) porphyrins so far and must be related

to the extremely negative charge on the porphyrin substit-

uents, which increases the electron density on the iron(III)

center in (P^16-)Fe^III(H₂O)₂and, as a result, increases the pK_a1

value. Another reason for the high pK_a1value can be related

to the through-space interaction of deprotonated carboxylate

groups of the flexible malonate substituents on the porphyrin

with the coordinated water molecules stabilized by hydrogen

bonding, as reported for other negatively charged water-

soluble ferric porphyrins, namely, (P^8-)Fe^III(H₂O)₂and

(TanP^4-)Fe^III(H₂O)₂.^5a,27

¹H NMR measurements were carried out for (P^16-)-

Fe^III(H₂O)₂and (P^16-)Fe^III(H₂O)(OH^-) at pD values of 6.5

(26) (a) El-awady, A. A.; Wilkins, P. C.; Wilkins, R. G. Inorg. Chem. 1985,

24, 2053. (b) Miskelly, G. M.; Webley, W. S.; Clark, Ch. R.;

Buckingham, D. A. Inorg. Chem. 1988, 27, 3773. (c) Kobayashi, N.

Inorg. Chem. 1985, 24, 3324. (d) Tondreau, G. A.; Wilkins, R. G.

Inorg. Chem. 1986, 25, 2745. (e) Zipplies, M. F.; Lee, W. A.; Bruice,

T. C. J. Am. Chem. Soc. 1986, 108, 4433.

(27) Imai, H.; Yamashita, Y.; Nakagawa, S.; Munakata, H.; Uemori, Y.

Inorg. Chim. Acta. 2004, 357, 2503.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3341

Jee et al.

1

Table 1. pK_a1Values and â-pyrrole H NMR Chemical Shifts of Synthetic Water-Soluble Iron(III) Porphyrins

d

^aThe quoted charge represents the overall charge on the meso substituents in a given porphyrin ligand. ^bRef 36. ^cReferenced to TMPS. This work.

^eRef 5a. ^fRef 4. ^gRef 28a, NA ) not assigned. ^hRef 5b.

1

and 12.7. H NMR spectra of complexes of the type (P)-

Fe^III(L)_n(n ) 1 and 2) have extensively been analyzed and

serve as an excellent means of identification of the product

being formed,²⁸in which the â-pyrrole proton signal is

sensitive to the spin and ligation state of the iron center by

hyperfine interaction specially for paramagnetic iron(III)

porphyrins. The chemical shift of the â-pyrrole resonance

in the purely high-spin state (S ) 5/2) iron(III) porphyrin

has a value of approximately +80 ppm, characteristic for

the (P)Fe-X type (XdOH^-, Cl^-) complex,²⁵whereas in the

case of pure intermediate spin state (S ) 3/2) iron(III)

Information) and 12.7. Detailed variable-temperature and

-pressure measurements gave the water exchange rate

constants (k_ex) and activation parameters (∆H^q_ex, ∆S^q_ex, and

∆V^q_ex). The positive values of ∆S^q) +66 ( 10 J mol^-1

ex

K^-1and ∆V^q) +6.5 ( 0.3 cm³mol^-1at pH 6.5 support

ex

the operation of a dissociative interchange (I_d) mechanism

for the water-exchange process on (P^16-)Fe^III(H₂O)₂, which

is similar to that found for other diaqua-ligated porphyrins

(I_dor D).^5,7cFrom the analysis of the data, the obtained rate

constant for water exchange, k_ex) 4.2 × 10⁶s^-1at pH 6.5

and 25 °C, is in the similar range of k_exvalues reported for

complexes with negatively charged meso substituents on the

porphyrin environment. Conversely, the smaller k_exvalues

determined for the positively charged iron porphyrins of

TMPyP⁴⁺and P⁸⁺suggest that the lability of the metal center

is decreased by the influence of the positive porphyrin

periphery, thus, stabilizing the Fe^III-H₂O bond through an

inductive effect. Although there is a trend of increased lability

for the P^16-and P^8-complexes as compared to the TMPS^4-

complex, the water exchange process for the P^16-and P^8-

complexes is slower than for the TMPS^4-complex, which

could be attributed to the steric hindrance of the bulky

negative carboxylate substituents that prevent the exchange

of coordinated water in a dissociative manner. The data

obtained for (P^16-)Fe^III(H₂O)₂and related results for other

iron(III) porphyrins are summarized in Table 2. In analogous

¹⁷O NMR measurements performed at pH 12.7, the bulk

¹⁷OH₂signal as a function of temperature resulted in only

small changes in the line width in the presence and absence

of the (P^16-)Fe^III, which suggests that (P^16-)Fe^IIIat high pH

forms a five-coordinate monohydroxo-ligated iron porphyrin,

(P^16-)Fe^III(OH), that exhibits no measurable water exchange

reaction.

1

porphyrins, the â-pyrrole H resonance moves upfield to

approximately -60 ppm. The â-pyrrole resonance in the

intermediate spin admixed state (S ) 3/2, 5/2) is in between

+80 and -60 ppm. Our ¹H NMR data indicates a â-pyrrole

resonance at +45.6 ppm for pD ) 6.5 that shifts downfield

to +80 ppm at pD ) 12.7. The broad â-pyrrole resonance

at +80 ppm at high pH is diagnostic for a monomeric high-

spin monohydroxo-ligated iron(III) porphyrin, consistent with

the other reported water-soluble ferric porphyrins.^4,5,25The

absence of a â-pyrrole resonance at ∼13 ppm, characteristic

for the ((P)Fe^III)₂O dimer, excludes the formation of the

µ-oxo-bridged binuclear species at high pH. The observed

signal for the diaqua-ligated iron(III) porphyrins exhibits

variable δ_â-pyvalues in the range from 43-70 ppm, which

indicate a varying contribution of S ) 3/2 in the spin-

admixed mixture (S ) 3/2, 5/2) from which it follows that

Int % is 24.5 for (P^16-)Fe(H₂O)₂, similar to that found for

other (P^n-)Fe(H₂O)₂. The observed data for (Pⁿ⁺)Fe^III(H₂O)₂

shows a marked downfield shift of the â-pyrrole resonance

toward the high-spin value with the increasing electron-

withdrawing capability of the porphyrin substituents.

Temperature- and pressure-dependent ¹⁷O NMR measure-

ments were performed at pH 6.5 (see Figure S1 in Supporting

Earlier literature reports showed that high-spin monohy-

droxo-bound iron(III) porphyrins may exist as five-coordinate

(P)Fe^III(OH) species in non-coordinating solvents or as six-

coordinate (P)Fe^III(H₂O)(OH) species in aqueous solution as

supported by ¹⁷O NMR measurements.^{4 17}O NMR water

exchange studies indicated that no water exchange reaction

(28) (a) Ivanca, M. A.; Lappin, A. G.; Scheidt, W. R. Inorg. Chem. 1991,

30, 711. (b) La Mar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A.

J. Am. Chem. Soc. 1973, 95, 63. (c) La Mar, G. N.; Walker, F. A. J.

Am. Chem. Soc. 1973, 95, 6950. (d) La Mar, G. N.; Walker, F. A. J.

Am. Chem. Soc. 1975, 97, 5103. (e) Snyder, R. V.; La Mar, G. N. J.

Am. Chem. Soc. 1976, 98, 4419.

3342 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

Table 2. Rate Constants (at 298 K) and Activation Parameters for Water Exchange Reactions of (P)Fe^III(H₂O)₂Complexes

iron(III)

porphyrin

k_ex

∆H^q

∆S^q

∆V^q

ex

Int%^a

(s^-1

)

(kJ mol^-1

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(P^16-)Fe^b

24.5

24

20

26

7

(42 ( 5) × 10⁵

(77 ( 1) × 10⁵

(20 ( 1) × 10⁵

(210 ( 10) × 10⁵

(4.5 ( 0.1) × 10⁵

(5.5 ( 0.1) × 10⁴

55 ( 3

61 ( 6

67 ( 2

61 ( 1

71 ( 2

53 ( 3

+66 ( 10

+91 ( 23

+99 ( 10

+100 ( 5

+100 ( 6

+28 ( 9

+6.5 ( 0.3

+7.4 ( 0.4

+7.9 ( 0.2

+11.9 ( 0.3

+7.4 ( 0.4

+1.5 ( 0.2

(P^8-)Fe^c

(TPPS^4-)Fe^d

(TMPS^4-)Fe^e

(TMPyP⁴⁺)Fe^d

(P⁸⁺)Fe^f

10

^aContribution of the intermediate spin state (S ) 3/2) in the spin-admixed (P)Fe^III(H₂O)₂porphyrins, compare ref 4b. ^bThis work. ^cRef 5a. ^dRef 7c.

^eRef 4. ^fRef 5b.

was observed for iron porphyrins with negatively charged

substituents, although another water molecule could bind to

the complex to form (P^n-)Fe^III(H₂O)(OH) in aqueous solu-

tion. The observations can be explained in terms of the

presence of five-coordinate (P^n-)Fe^III(OH) complexes under

such conditions as a result of the strong electron-donating

substituents in the porphyrin meso position and the trans

labilizing effect of coordinated hydroxide that results in an

electron-rich character of the iron(III) center. In contrast,

however, (Pⁿ⁺)Fe^III(OH) complexes with positively charged

substituents tend to exist as six-coordinate (Pⁿ⁺)Fe^III(H₂O)-

(OH) species such that line-broadening could be observed

in the ¹⁷O NMR experiments. The latter data are summarized

later in the text in Table 6.

change in coordination number, which involves movement

of the Fe^IIIcenter from out-of-plane into the porphyrin plane

to yield (P)Fe^II(OH)(NO⁺).

On the basis of the reported spectroscopic results,

(P^16-)Fe^III(H₂O)₂represents the six-coordinate spin-admixed

(S ) 3/2, 5/2) species in the range of 6.0 < pH < 9.9,

whereas (P^16-)Fe^III(OH) is predominantly present as a high-

spin (S ) 5/2) five-coordinate complex at pH >9.9 with an

out-of-plane metal center toward the OH^-ligand.³¹No

indication for the formation of a µ-oxo-bridged dimer or

dihydroxo species was found in the pH range studied.

Reaction of (P^16-)Fe^III(H₂O)₂with Nitric Oxide. When

gaseous NO is added to a solution of (P^16-)Fe^III(H₂O)₂, rapid

spectral changes occur as shown in Figure 3, indicating the

formation of a typical low-spin iron nitrosyl complex in

which electron transfer from NO to Fe(III) produces

(P^16-)Fe^II(H₂O)(NO⁺). The maximum of the Soret- and

Q-bands shifts from 398 and 532 nm to 428 (ꢀ ) 9.8 × 10⁴

M^-1cm^-1) and 541 nm (ꢀ ) 1.3 × 10⁴M^-1cm^-1),

respectively, during formation of the final product according

to reaction 5. The binding of NO appears to be reversible as

1

It is well established from the H and ¹⁷O NMR data

summarized in Table 2 that the rates of water exchange

correlate with the contribution of the intermediate spin state

S ) 3/2 (Int %) in the spin admixed state (S ) 3/2, 5/2) of

ferric porphyrins, where the contribution of S ) 3/2 was

estimated from the proton â-pyrrole signal.²⁹The results

show that the calculated Int % value increases with the

increasing lability of the water molecule coordinated to the

iron(III) center, which is consistent with an increasing water

exchange rate (k_ex) resulting from the lengthening of the

Fe^III-OH₂bond. The population of the S ) 3/2 spin state

(P^16-)Fe^III(H₂O)₂+ NO {^k^on}

k_off

(P^16-)Fe^II(H₂O)(NO⁺) + H₂O K_NO) k_on/k_off(5)

2

arises from destabilization of the d_{x -y}orbital, resulting in

an increased electron density on the nitrogens of porphyrin

in (P^n-)Fe^III(H₂O)₂as compared to that in (Pⁿ⁺)Fe^III(H₂O)₂.

The observed effects also affect the reactivity of the

complexes toward NO as will be discussed in more detail

later.

determined from an experiment in which the bubbling of an

inert gas through the product shifted the equilibrium back

to the reactant side. The thermodynamic equilibrium constant,

K_NO) (1.7 ( 0.2) × 10⁵M^-1was determined from the

spectral changes observed for various NO concentrations

measured in solution with an NO electrode, as shown in the

inset of Figure 3.⁵

The kinetics for the reversible binding of NO were

investigated by stopped-flow technique for the “on” and “off”

reaction, respectively. The observed reaction proved to be

follow pseudo-first-order kinetics with an at least 10-fold

excess of NO, and the observed rate constant, k_obs, is

expressed by eq 6. Accordingly, k_obsdepends linearly on

Inspection of the structural features reported for 5-coor-

dinate high-spin, 6-coordinate spin-admixed, and 6-coordi-

nate low-spin (P)Fe^III(L) systems⁸suggest that minor struc-

tural changes are demanded for the formation of (P)Fe^II-

(H₂O)(NO⁺) from 6-coordinate (P)Fe^III(H₂O)₂,³⁰in which in

both cases the Fe^IIIcenters remain in the porphyrin plane.

However, NO binding to 5-coordinate (P)Fe^III(OH) com-

plexes requires some structural changes as a result of a

k_obs) k_on[NO] + k_off

(6)

(29) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 6225.

(30) (a) Scheidt, W. R.; Reed, Ch. A. Chem. ReV. 1981, 81, 543. (b)

Simonato, J.-P.; Pecaut, J.; Le pape, L.; Oddou, J.-L.; Jeandey, C.;

Shang, M. R.; Wojaczynski, J.; Wolowiec, S.; Latos-Grazynski, L.;

Marchon, J.-C. Inorg. Chem. 2000, 39, 3978. (c) Cheng, B.; Scheidt,

W. R. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1995, 57,

1271. (d) Safo, M. K.; Schmidt, W. R.; Grupa, G. P.; Orosz, R. D.;

Reed, Ch. A. Inorg. Chim. Acta. 1991, 184, 251. (e) Ko¨rber, F. C. F.;

Linsay Smith, J. R.; Prince, S.; Rizkallah, P.; Reynolds, C. D.;

Shawcross, D. R. J. Chem. Soc., Dalton Trans. 1991, 3291.

[NO] with a slope of k_on) (33.6 ( 0.5) × 10⁴M^-1s^-1and

an intercept of k_off) 4 ( 3 s^-1at 14.7 °C. The values of k_off

(31) (a) Swepston, P. N.; Ibers, J. A. Acta Crystallogr., Sect. C: Cryst.

Struct. Commun. 1985, 41, 671. (b) Hoffman, A. B.; Collins, D. M.;

Day, V. W.; Fleischer, E. B.; Srivastava, R. S.; Hoard, J. L. J. Am.

Chem. Soc. 1972, 94, 3620.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3343

Jee et al.

Figure 3. Spectral changes recorded for the binding of NO to (P^16-)Fe^III(H₂O)₂. The inset shows a plot of (A - A₀)^-1versus [NO]^-1(where A₀

)

absorbance at 398 nm at [NO] ) 0 and A) absorbance at 398 nm at a given NO concentration). Experimental conditions: [(P^16-)Fe^III] ) 1.34 × 10^-5M,

pH ) 6.5 (0.05M Mes), temp ) 25 °C, I ) 0.1 M (with NaClO₄)

Table 3. Rate and Activation Parameters for the Binding and Release

of NO to (P^16-)Fe^III(H₂O)₂at pH 6.5 by Stopped-Flow and

NO-Trapping Methods

a

temp

(°C)

pressure

(MPa)

k_on

k_off

(×10⁴M^-1s^-1

)

(s^-1

)

(s^-1

)

4.7

6.3

9.7

12.5

14.7

20

0.1

10

9.6 ( 0.2

12.2 ( 0.4

18.3 ( 0.3

25.9 ( 0.3

33.6 ( 0.5

0.6 ( 0.4

0.9 ( 0.5

2 ( 1

3 ( 2

4 ( 3

2.3 ( 0.2

3.2 ( 0.2

6.1 ( 0.1

10.1 ( 0.1

13.7 ( 0.3

31.4 ( 0.2

65.5 ( 0.5

1.7 ( 0.1

1.3 ( 0.1

1.0 ( 0.1

0.7 ( 0.1

25

2.5

10.5 ( 0.3

8.6 ( 0.4

7.0 ( 0.4

5.9 ( 0.5

50

90

130

∆H^q(kJ mol^-1

∆S^q(J mol^-1K^-1

∆V^‡(cm³mol^-1

)

80 ( 1

+138 ( 4

+10.8 ( 0.2

117 ( 13

+173 ( 24

110 ( 2

+161 ( 8

+16.9 ( 0.3

)

^aData obtained by the NO-trapping method with the use of [Ru^III(edta)-

(H₂O)]^-, compare to Figure S3 (Supporting Information).

Figure 4. Plots of k_obsversus [NO] for the reaction of (P^16-)Fe^III(OH₂)₂

with NO in the restricted temperature range of 4.7-14.7 °C measured with

the stopped-flow technique. Experimental conditions: [(P^16-)Fe^III] ) 2.0

× 10^-5M, pH 6.5 (0.05 M Mes), λ_det) 430 nm, I ) 0.1 M (with NaClO₄).

data, K_NO) k_on/k_off) (1.7 ( 0.1) × 10⁵M^-1at 25 °C, is in

an excellent agreement with the thermodynamic value of K_NO

determined from spectral changes as a function of [NO],

namely, (1.7 ( 0.2) × 10⁵M^-1.

are small and subjected to large error limits when determined

by extrapolation of plots of k_obsversus [NO] to [NO] ) 0.

More reliable k_offvalues were obtained in a direct way by

using an NO trap, namely, [Ru^III(edta)(H₂O)]^-, an efficient

scavenger for NO. Fast NO trapping by an excess of

scavenger from (P^16-)Fe^II(H₂O)(NO⁺) gives the original

complex (P^16-)Fe^III(H₂O)₂and [Ru^III(edta)(NO)]^-, as indi-

cated by the observed spectral changes. The kinetic traces

were recorded at 430 nm and could be fitted with a single-

exponential function. The rate constant for NO release is

the rate-limiting step under the selected conditions (see

reaction 7) since the observed rate constant does not depend

on the concentration of [Ru^III(edta)(H₂O)]^-employed since

it is a much faster reaction than either k_onor k_off. The values

of k_offdetermined in this way differ considerably from those

extrapolated from the data in Figure 4, as shown in Table 3.

The overall equilibrium constant obtained from the kinetic

The binding (k_on) and release of NO (k_off) rate constants

were determined as a function of temperature in the restricted

range of 4.7-14.7 °C because of the limitations of the

stopped-flow technique (see Table 3 and Figure 4). Eyring

plots (see Figures S2 and S3, in Supporting Information)

were constructed from which the activation parameters, ∆H^q

3344 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

reaction at low pH (viz., 428 and 541 nm at pH 6.5) and

points to the formation of a deprotonated form of the nitrosyl

product at high pH as given in eq 8.

(P^16-)Fe^III(OH) + NO {_k^k^on} (P^16-)Fe^II(OH)(NO⁺)

off

K_NO) k_on/k_off(8)

For the reversible binding of NO to (P^16-)Fe^III(OH) at pH

12.7, K_NO) (4.1 ( 0.4) × 10³M^-1at 25 °C was determined

from the spectral changes as a function of the [NO] shown

in Figure 6.⁴Detailed kinetic measurements for the reversible

binding of NO to (P^16-)Fe^III(OH) were carried out in a

manner similar to that employed for (P^16-)Fe^III(H₂O)₂. k_on

) (31.5 ( 0.2) × 10³M^-1s^-1and k_off) 8.0 ( 0.1 s^-1at 25

°C were obtained from the slope and intercept of a linear

plot of k_obsversus [NO], respectively. The thermodynamic

equilibrium constant is close to the value calculated from

the kinetic data, namely, K_NO) k_on/k_off) (3.9 ( 0.2) × 10³

M^-1at 25 °C. Variable-temperature and -pressure experi-

ments were performed to determine the activation parameters

in a manner similar to that for the diaqua complex. The

results are reported in Table 5 and Figures 7, S5, and S6

and are compared with a series of related systems in Table

6. The reported activation volumes for the on and off

reactions were used to construct the volume profile reported

in Figure 8.

As mentioned above, the spectra reported in Figure 9a

show differences for the nitrosyl product formed at pH 6.5

and 12.7. The plot of absorbance at 434 nm versus pH in

Figure 9b, fitted with a sigmoidal function, resulted in pK_a(NO)

) 9.68 ( 0.03 for the deprotonation of a coordinated water

molecule in the nitrosyl product (P^16-)Fe^II(NO⁺)(H₂O) as

given in reaction 9. The observed reactions can be sum-

marized as outlined in Scheme 2.

Figure 5. Volume profile for the reversible binding of nitric oxide to

(P^16-)Fe^III(H₂O)₂according to reaction 5.

and ∆S^qfor the on and off reactions, were determined.

Despite the large difference in the k_offvalues, the calculated

activation parameters, ∆H^qand ∆S^q_off, obtained with the

off

stopped-flow and NO-trapping techniques are rather similar

and do not affect the mechanistic interpretation. For a better

understanding of the underlying reaction mechanism, activa-

tion volumes were determined from the effect of pressure

up to 130 MPa on k_onand k_off(see Figure S4 in Supporting

Information) and are included in Table 3. The reported

activation volumes were used to construct a volume profile

for reaction 5 as shown in Figure 5. The obtained rate and

activation parameters are summarized in comparison with

other (P)Fe^III(H₂O)₂complexes in Table 4.

Reaction of (P^16-)Fe^III(OH) with Nitric Oxide. As

reported in our earlier papers,⁵spectroscopic and kinetic

studies on the reaction of NO with iron(III) porphyrins at

pH . pK_aindicate that the nature of the resulting nitrosyl

species and the NO binding mechanism differ from those

observed for the diaqua-ligated iron porphyrins. Figure 6

shows the spectral changes that accompany the reaction of

(P^16-)Fe^III(OH) with NO at pH 12.7, where the decrease in

the absorbance maximum at 418 nm is accompanied by the

appearance of new bands at 433 (ꢀ ) 9.7 × 10⁴M^-1cm^-1)

and 547 nm (ꢀ ) 1.2 × 10⁴M^-1cm^-1) for the formation of

the nitrosyl product, (P^16-)Fe^II(OH)(NO⁺). The latter UV-

vis spectrum differs from that observed for the corresponding

(P^16-)Fe^II(NO⁺)(H₂O) a (P^16-)Fe^II(NO⁺)(OH^-) + H⁺

pK_a(NO)) 9.68 (9)

In comparison to the binding and release rate constants at

pH 6.5 (viz., 1.84 × 10⁵M^-1s^-1and 6.1 s^-1, at 10 °C, see

Table 3) the corresponding values at pH 12.7 and 10 °C are

1.91 × 10⁴M^-1s^-1and 1.0 s^-1, respectively, indicating that

both NO binding and release rates for (P^16-)Fe^III(OH) are

significantly slower than those for (P^16-)Fe^III(H₂O)₂. A

Table 4. Rate Constants (at 298 K) and Activation Parameters for Reversible Binding of NO to a Series of Diaqua-Ligated Water Soluble Iron(III)

Porphyrins

NO binding

NO release

∆S^q

iron(III)

porphyrin

k_on

∆H^q

∆S^q

∆V^q

k_off

∆H^q

∆V^q

off

on

off

pK_a1Int % (×10⁴M^-1s^-1

)

(kJ mol^-1

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(s^-1

)

(kJ mol^-1

117 ( 13

65.5 ( 0.5^g110 ( 2^g

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(P^16-)Fe^a

9.8

24.5

113 ( 5^f

80 ( 1

+138 ( 4

+10.8 ( 0.2

22 ( 3

+173 ( 24

+161 ( 8^g

+140 ( 7

+94 ( 10

+60 ( 11

+150 ( 12

+61 ( 14

+16.9 ( 0.3^g

+16.8 ( 0.4

+17 ( 3

+18 ( 2

+16.6 ( 0.2

+9.3 ( 0.5

(P^8-)Fe^b

9.3

6.9

7.0

24

26

20

7

82 ( 1

280 ( 20

50 ( 3

2.9 ( 0.2

1.5 ( 0.1

51 ( 1

57 ( 3

69 ( 3

67 ( 4

77 ( 3

+40 ( 2

+6.1 ( 0.1 220 ( 2

101 ( 2

84 ( 3

76 ( 6

108 ( 5

83 ( 4

(TMPS^4-)Fe^c

(TPPS^4-)Fe^c

+69 ( 11

+95 ( 10

+67 ( 13

+94 ( 12

+13 ( 1

+9 ( 1

+3.9 ( 1.0

900 ( 200

500 ( 400

59 ( 4

(TMPyP⁴⁺)Fe^d5.5

(P⁸⁺)Fe^e

5.0

10

+1.5 ( 0.3 26.3 ( 0.5

^aThis work. ^bRef 5a. ^cRef 3d. ^dRef 6a. ^eRef 5b. ^fCalculated for 298 K from Eyring plots based on data from Table 3. ^gData measured with

[Ru^III(edta)(H₂O)]^-as NO scavenger.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3345

Jee et al.

Figure 6. Spectral changes resulting from the binding of NO to (P^16-)Fe^III(OH). The inset shows a plot of (A - A₀)^-1versus [NO]^-1(where A₀

)

absorbance at 435 nm at [NO] ) 0 and A ) absorbance at 435 nm at a given NO concentration). Experimental conditions: [(P^16-)Fe^III] ) 1.38 × 10^-5M,

pH ) 12.7 (0.05M NaOH), temp ) 25 °C, I ) 0.1 M (with NaClO₄).

Table 5. Rates and Activation Parameters for the Binding and Release

pH 6.5 undergoes a subsequent reaction on a longer time

scale in which the characteristic bands at 428 and 541 nm

disappear with a concomitant appearance of new bands at

of NO for (P^16-)Fe^III(OH) at pH 12.7 by Stopped-Flow and

NO-Trapping Methods

a

temp

(°C)

pressure

(MPa)

k_on

k_off

416 and 612 nm, respectively (see Figure 11). The obtained

kinetic trace for this reaction can be fitted with a single-

exponential function as shown in the inset of Figure 11. The

final spectrum resembles that of a five-coordinate ferrous

nitrosyl product and is the same as that observed for the

product formed in the reaction of NO with reduced (P^16-)Fe^II.

This suggests that (P^16-)Fe^II(H₂O)(NO⁺) is converted to

(P^16-)Fe^II(NO) by a slow redox process as outlined in

reactions 10-12. The observed spectral changes are similar

to that reported for other negatively charged water-soluble

iron(III) porphyrins.^17,18

(×10³M^-1s^-1

)

(s^-1

)

(s^-1

)

5

10

15

20

25

2.5

0.1

10

50

90

130

14.8 ( 0.2

19.1 ( 0.9

21.8 ( 0.5

26.3 ( 1.2

31.5 ( 0.2

4.5 ( 0.2

5.3 ( 0.3

6.3 ( 0.5

7.4 ( 0.4

0.32 ( 0.04

1.1 ( 0.1

2.0 ( 0.3

4.1 ( 0.6

8.0 ( 0.1

0.6 ( 0.1

1.0 ( 0.1

1.8 ( 0.1

2.9 ( 0.1

4.8 ( 0.1

0.33 ( 0.04

0.25 ( 0.03

0.19 ( 0.02

0.15 ( 0.03

∆H^q(kJ mol^-1

∆S^q(J mol^-1K^-1

∆V^q(cm³mol^-1

)

23 ( 1

-82 ( 4

-9.4 ( 0.2

108 ( 7

+136 ( 19

84 ( 1

+53 ( 4

+15 ( 1

)

^aData obtained by the NO-trapping method with the use of [Ru^III(edta)-

(H₂O)]^-as a trap for NO (see Figures S5 and S6).

(P^16-)Fe^III(H₂O)₂+ NO {^K^NO} (P^16-)Fe^II(H₂O)(NO⁺) + H₂O

(10)

k_red

comparison of these rate constants along with those for a

series of complexes studied before is given in Table 7. From

the data shown in Figure 10a, it can be seen that the rates of

NO binding as a function of pH in the range of 6.5-12.7

resulted in k_onvalues that decrease with increasing pH (see

Figure 10b). A fit of the data resulted in a pK_avalue of 9.83

( 0.03, which is similar to that determined from a spectro-

photometric titration (viz., pK_a1) 9.90), suggesting that the

rate of NO binding is controlled by the axial ligand bound

to the metal center and reflects differences in the kinetics of

NO binding at low and high pH. In the case of NO

dissociation rates, the k_offvalues are rather small and include

large errors when extrapolated from plots of k_obsversus [NO].

More reliable values were obtained for k_offusing the NO-

trapping method mentioned above. A similar observation was

shown previously for other water-soluble iron(III) porphyrins.

Spectroscopic and Kinetic Studies on the Subsequent

Reactions. The nitrosyl complex (P^16-)Fe^II(H₂O)(NO⁺) at

(P^16-)Fe^II(H₂O)(NO⁺) _{+H O}8

2

(P^16-)Fe^II(H₂O) + NO₂^-+ 2H⁺(11)

fast

(P^16-)Fe^II(H₂O) + NO 8 (P^16-)Fe^II(NO) + H₂O (12)

The effect of the NO concentration on the observed

reductive nitrosylation reaction at pH 6.5 and 25 °C is

reported in Figure 12a, from which it follows that k_obs

increases with increasing [NO] in a nonlinear way and

reaches a constant value at high [NO]. In agreement with

the rate law (eq 13) for the above given reaction scheme,

the plot of k_obsvs [NO]^-1is linear as displayed in Figure

12b, from which the values for K_NOand k_redwere calculated

to be (1.4 ( 0.2) × 10⁵M^-1and (1.7 ( 0.3) × 10^-4s^-1,

respectively. This value for K_NOis close to the equilibrium

-1

constant calculated from the kinetic data, namely, K_NO

k_on/k_off) (1.7 ( 0.1) × 10⁵M^-1at 25 °C, and the value of

)

3346 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

Figure 7. (a) Plots of k_obsversus [NO] for the reaction of (P^16-)Fe^III(OH) with nitric oxide in the temperature range of 5-25 °C measured with the

stopped-flow technique. (b) Corresponding Eyring plots for the on and off reactions. Experimental conditions: [(P^16-)Fe^III] ) 2.0 × 10^-5M, pH 12.7 (0.05

M NaOH), λ_det) 435 nm, I ) 0.1 M (adjusted with NaClO₄).

Table 6. Rate Constants (at 298 K) and Activation Parameters for Water Exchange and Reversible Binding of NO to Monohydroxo-ligated Iron(III)

Porphyrins

NO binding

NO release

∆S^q

iron(III)

porphyrin

k_ex

k_on

∆H^q

∆S^q

∆V^q

k_off

∆H^q

∆V^q

off

on

off

(×10⁵s^-1

)

(×10⁴M^-1s^-1

)

(kJ mol^-1

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(s^-1

)

(kJ mol^-1

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(P^16-)Fe^a

e

3.1 ( 0.4

23 ( 1

-82 ( 4

-9.4 ( 0.2

8.0 ( 0.1

7.1 ( 0.1^h

108 ( 7

+136 ( 19

+53 ( 4^h

+136 ( 7

+77 ( 3

+25 ( 7

+12 ( 5

84 ( 1^h

107 ( 2

90 ( 1

78 ( 2

72 ( 2

+15 ( 1^h

+17 ( 3

+7.4 ( 1.0

+9.5 ( 0.8

+2.6 ( 0.2

(P^8-)Fe^b

e

f

g

5.1 ( 0.2

1.46 ( 0.02

0.36 ( 0.01

0.16 ( 0.01

34.6 ( 0.4

28.1 ( 0.6

41.4 ( 0.5

41 ( 1

-39 ( 1

-128 ( 2

-38 ( 5

-45 ( 2

-6.1 ( 0.2 11.4 ( 0.3ⁱ

(TMPS^4-)Fe^c

(TMPyP⁴⁺)Fe^d

(P⁸⁺)Fe^d

-16.2 ( 0.4 10.5 ( 0.2^j

-13.7 ( 0.6

-13.8 ( 0.4

3.2 ( 0.1

6.2 ( 0.1

24 ( 6

^aThis work. ^bRef 5a. ^cRef 4. ^dRef 5b. ^eNo water exchange process was detected, compare ref 5a. ^fAlthough the effect of (TMPS^4-)Fe^III(OH) on the

bulk water line width is observable in temperature dependent ¹⁷O NMR studies, it was too small to determine k_exand the corresponding activation parameters

for the water exchange reaction, compare ref 4. ^gFormation of a µ-oxo dimer at porphyrin concentrations required for NMR measurements precludes reliable

studies on the water exchange process. ^hData measured with [Ru^III(edta)(H₂O)]^-as a scavenger for NO. ⁱAt 297 K. ^jCalculated for 298 K from the Eyring

plots, compare ref 5b.

rins (P)Fe^IIIas summarized in Table 9. In general, slow

reduction rates are observed for the negatively charged

complexes with values for k_redof approximately 10^-4s^-1at

25 °C. In contrast, the positively charged porphyrins show

a much faster reductive nitrosylation, which suggests that

the nucleophilicity of coordinated NO⁺on the Fe^IIcenter is

affected by the porphyrin environment in controlling the

stability of the intermediate (Pⁿ)Fe^II(H₂O)(NO⁺), which

affects the rate of the subsequent redox reaction.

The observed reductive nitrosylation reaction is catalyzed

by nitrite and was systematically studied as a function of

pH, nitrite concentration, temperature, and pressure. The

suggested sequence for the nitrite-catalyzed reaction is given

in reaction 14, for which the observed rate constant can be

expressed as shown in eq 15.

Figure 8. Volume profile for the reversible binding of nitric oxide to

k_nit

8

(P^16-)Fe^III+ NO {^K^NO} (P^16-)Fe^II(NO⁺) + NO₂^-9

(P^16-)Fe^III(OH) according to reaction 8.

(P^16-)Fe^II(NO) + other products (14)

k_redis in a good agreement with that measured directly at

high [NO], namely, k_obs) 1.56 × 10^-4s^-1.

-

k_nitK_NO[NO₂][NO]

k_obs

)

(15)

k_redK_NO[NO]

1 + K_NO[NO]

k_obs

)

(13)

1 + K_NO[NO]

The observed rate constant shows a linear dependence on

the nitrite concentration as shown in Figure 13 for which

the second-order rate constant is (1.3 ( 0.2) × 10^-2M^-1

The observed value for k_redfor (P^16-)Fe^II(H₂O)(NO⁺) is

the smallest among the series of water-soluble iron porphy-

Inorganic Chemistry, Vol. 46, No. 8, 2007 3347

Jee et al.

Figure 9. (a) Spectral changes accompanying the reaction of (P^16-)Fe^IIIwith NO at pH 6.5 and 12.7. (b) A plot of absorbance at 434 nm versus pH in

buffered aqueous solutions in the pH range of 6.5-12.7. Experimental conditions: [(P^16-)Fe^III] ) 7 × 10^-5M, [NO] ) 1 mM, temp ) 25 °C, I ) 0.1 M

(with NaClO₄). Buffers: pH 6.5, 0.05 M Mes; pH 7, 0.05 M Bis-Tris; pH 7.5-8.8, 0.05 M Taps; pH 9.1, 0.05 M borate; pH 9.5-11.6, 0.05 M Caps; pH

12.0-12.7, NaOH.

Scheme 2

Table 8. Effect of Temperature and Pressure on the Nitrite-Induced

Reductive Nitrosylation Reaction of (P^16-)Fe^II(H₂O)(NO⁺) with Nitrite

Ion at pH 6.5^a

temp

(°C)

pressure

(MPa)

10³k_obs(s^-1

)

10²k_nit(M^-1s^-1

)

25

30

35

40

45

27

0.1

10

50

90

130

0.22 ( 0.02

0.37 ( 0.04

0.58 ( 0.05

1.09 ( 0.05

1.53 ( 0.07

0.33 ( 0.06

0.39 ( 0.05

0.45 ( 0.06

0.54 ( 0.07

1.4 ( 0.1

2.5 ( 0.3

4.1 ( 0.4

8.1 ( 0.5

11.9 ( 0.5

2.2 ( 0.3

2.7 ( 0.5

3.1 ( 0.4

3.6 ( 0.7

∆H^q(kJ mol^-1

)

80.4 ( 1.4

-10.8 ( 3.1

-9.8 ( 0.4

∆S^q(J mol^-1K^-1

)

Table 7. Ratio of Rate Constants Observed for the NO Binding

∆V^q(cm³mol^-1

^a[NO₂^-] ) 16 mM

)

H₂O OH

OH

off

(k /k ) and Release (k^H_off^O/k ) for Selected Water-Soluble Iron(III)

2

on

Porphyrins

OH

on

OH

off

(P)Fe^III

Int %

k^H_on^O/k

k^H_off^O/k

2

the observed reaction was studied in the pH range of 6.0-

8.4 where a water molecule is still bound to the metal center.

Since the plot gives a straight line with a nonzero intercept,

k_obscan be formulated as given in eq 16.

(P^16-)Fe

24.5

24

26

7

36

16

192

9.1

19.3

85

18.4

4.2

(P^8-)Fe

(TMPS^4-)Fe

(TMPyP⁴⁺)Fe

(P⁸⁺)Fe

8.0

9.4

10

k_obs) k_OH[OH^-] + k_{H O}

(16)

2

s^-1, which presents k_nitK_NO[NO]/(1 + K_NO[NO]). Since K_NO

and [NO] are known under the selected experimental

conditions, the values of k_obscould be converted to the

corresponding k_nitvalues (summarized in Table 8) as a

function of temperature and pressure at pH 6.5. The

activation parameters were estimated in the usual way for

which the corresponding plots are reported in Figure S7

(Supporting Information). The results are summarized along

with the rate constants and activation parameters for the other

investigated complexes in Table 9. On the basis of these data,

the nitrite-induced reductive nitrosylation is the slowest for

(P^16-)Fe^II(H₂O)(NO⁺) in comparison to all other studied

porphyrins, suggesting that the overall negative charge

reduces the electrophilicity of the coordinated NO⁺and slows

The obtained slope and intercept of the plot are k_OH) 1.0

× 10³M^-1s^-1and k_{H O}) 1.1 × 10^-4s^-1, respectively. The

2

value of k_OHis consistent with that measured for metallo-

proteins and the ferric heme complex, (P^8-)Fe^II(H₂O)(NO⁺),

for which analogous measurements resulted in k_OH) 4.5 ×

10³M^-1s^-1and k_{H O}) 1.1 × 10^-4s^-1, respectively.^3,6This

2

-

means that OH^-and H₂O, as well as NO₂, contribute to

the reductive nitrosylation of (P^16-)Fe^II(H₂O)(NO⁺) to yield

(P^16-)Fe^IIspecies. The base-catalyzed reaction is much more

effective than the nitrite-catalyzed reaction as shown by k_OH

) 1.0 × 10³M^-1s^-1and k_nit) (1.3 ( 0.2) × 10^-2M^-1s^-1,

respectively.

The overall reaction is accompanied by an electron-transfer

process in which (P^16-)Fe^II(NO) is formed from (P^16-)-

Fe^II(NO⁺). Potential measurements using cyclic voltammetry

resulted in an irreversible ∆E_1/2((P^16-)Fe^II-NO⁺/(P^16-)Fe^II-

NO) of 0.20 V versus Ag/AgCl at pH 6.5. This value fits

-

down the direct binding of NO₂.

The observed reaction is also catalyzed by hydroxide ions.

The [OH^-] dependence of the reductive nitrosylation reaction

is reported in Figure S8 (Supporting Information), in which

3348 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

Figure 10. (a) pH dependence of the rate constants for the reaction of NO with (P^16-)Fe^III. (b) Plots of k_onversus pH, where k_onwas determined from the

slopes of k_obsversus [NO] plots measured in the pH range of 6.5-12.7. Experimental conditions: [(P^16-)Fe^III] ) 2.0 × 10^-5M, temp ) 10 °C, λ_det) 435

nm, I ) 0.1 M (with NaClO₄).

Figure 11. Spectral changes observed during the subsequent reaction following the binding of NO to (P^16-)Fe^III(H₂O)₂. The inset shows the kinetic trace

of absorbance versus time fitted to a single-exponential function. Experimental conditions: [(P^16-)Fe^III] ) 1 × 10^-5M, [NO] ) 1 mM, pH ) 6.5 (0.05M

Mes), λ_det) 431 nm, temp ) 25 °C, I ) 0.1 M with NaClO₄. The first ten spectra were recorded every 6 min, the next 10 spectra every 9 min, and the rest

every 15 min to give k_obs) 1.56 × 10^-4s^-1

.

Figure 12. (a) Plot of k_obsversus [NO] for the reductive nitrosylation of (P^16-)Fe^II(H₂O)(NO⁺). (b) Plot of k_obs^-1versus [NO]^-1. Experimental conditions:

[(P^16-)Fe^III] ) 2.0 × 10^-5M, pH 6.5 (0.05 M Mes), temp. ) 25 °C, λ_det) 431 nm, I ) 0.1 M NaClO_4.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3349

Jee et al.

Table 9. Rate and Activation Parameters for Nitrite-Induced Reductive Nitrosylation of (Pⁿ)Fe^II(H₂O)(NO⁺) and (Pⁿ)Fe^II(OH)(NO⁺) at 25 °C

slope^b

k_nit

∆H^q

∆S^q

∆V^q

a

iron(III)

porphyrin

k_red

(s^-1

pK_a

pH

)

(M^-1s^-1

)

(M^-1s^-1

)

(kJ mol^-1

)

(J mol^-1K^-1

)

(cm³mol^-1

)

(P^16-)Fe^c

9.9

9.3

7.0

5.5

5.0

6.5

7

5

4.5

2

4

(1.7 ( 0.2) × 10^-4

(2.8 ( 0.2) × 10^-4

(4.6 ( 0.4) × 10^-4

7 × 10^{-3 g}

(1.3 ( 0.2) ×10^-2

1.6 ( 0.1

2.2 ( 0.1

15.0 ( 0.1

55 ( 3

(1.4 ( 0.2) ×10^-2

2.1 ( 0.2

3.1

80 ( 1

60 ( 2

-11 ( 3

-36 ( 7

-9.8 ( 0.4

-8.6 ( 0.4

(P^8-)Fe^d

(TPPS^4-)Fe^e

(TMPyP⁴⁺)Fe^f

(P⁸⁺)Fe^d

42 ( 3

88 ( 2

90 ( 3

73 ( 2

+92 ( 6

+99 ( 10

+92 ( 7

+8.8 ( 0.1

+7.2 ( 0.5

+12.3 ( 0.7

(4.0 ( 0.2) × 10^-2

155 ( 5

89 ( 1

242 ( 7

^aCalculated values using eq 12 when no nitrite was added. ^bValue calculated from k_obsversus [NO₂^-] at constant [NO]. ^cThis work. ^dRef 18. ^eMeasured

at 15 °C, ref 17. ^fRef 6a. ^gObserved rate constant (k_obs) for the studied reaction.

Figure 13. Nitrite concentration dependence of the reductive nitrosylation of (P^16-)Fe^II(H₂O)(NO⁺). Experimental conditions: [(P^16-)Fe^III] ) 3.0 × 10^-5

M, [NO] ) 1 mM, temp. ) 25 °C, λ_det) 431 nm, pH 6.5 (0.05M Mes), I ) 0.1 M (with NaClO₄).

excellently to the linear free-energy relationship (LFER)

reported for the nucleophilic addition (OH^-) rate constant

and the redox potential for a series of trans-tetrapyridine

ruthenium nitrosyl complexes.³²For the k_OH) 1.0 × 10³

M^-1s^-1value found for the present complex, the reported

correlation predicts a redox potential of 0.21 V, which is in

perfect agreement with our experimental value. According

to the data referenced, the ruthenium complexes have lower

rate constants because of the steric hindrance of pyridine

ligands which are free to rotate. In the present case, the

malonate substituents on the porphyrin cause sufficient steric

hindrance such that the (P^16-)Fe^II(H₂O)(NO⁺) complex also

fits the reported LFER.

Suggested Mechanisms and Comparison with Other

Iron(III) Porphyrins. A. Reactivity of (P^16-)Fe^III(H₂O)₂

and (P^16-)Fe^III(OH) toward NO. Mechanistic information

on the on and off reactions of NO with (P^16-)Fe^III(H₂O)₂can

best be obtained from the volume profile presented in Figure

5. The positive activation volumes for the binding and release

of NO favor a dissociative mechanism which is analogous

to that reported for a series of diaqua-ligated porphyrins

summarized in Table 4. The overall volume change observed

for the reaction is not only the result of the displacement of

water by NO but also of a change in spin state from S )

3/2, 5/2 for the diaqua complex to S ) 0 for the Fe^IInitrosyl

complex. The data in Table 4 show that the rates for NO

binding correlate to some extent with the contribution of the

intermediate spin state (Int %) in the spin-admixed state (S

) 3/2, 5/2). As mentioned above, increasing Int % correlates

with increasing lability of the ligand on the metal center as

influenced by the increasing electron-donating properties of

the meso substituents on the porphyrin. The trend in ∆V^q

on

along the series of complexes suggests a gradual changeover

from a dissociative mechanism for the anionic complexes

to a dissociative interchange mechanism for the cationic

complexes. Almost similar trends are observed for the water

exchange reactions for the series of (P)Fe^III(H₂O)₂complexes

summarized in Table 4. The magnitude of the water exchange

rate constant and the close agreement between the volumes

of activation for the water exchange process and the binding

of NO, suggest that the rate and mechanism of the latter

process is controlled by the water exchange process of the

diaqua complexes.^4,5,7c,33Thus, electron-donating substituents

on the porphyrin induce a dissociative mechanism by

weakening the Fe-OH₂, whereas electron-withdrawing sub-

stituents strengthen the Fe-OH₂bond and tend to favor a

dissociative interchange mechanism.

The rate constants for the dissociation of NO from the

nitrosyl complex (P)Fe^II(H₂O)(NO⁺) summarized in Table

4 show for some of the complexes the trend that the nitrosyl

complex is stabilized by positively charged as compared to

(32) (a) Videla, M.; Jacinto, J. S.; Baggio, R.; Garland, M. T.; Singh, P.;

Kaim, W.; Slep, L. D.; Olabe, J. A. Inorg. Chem. 2006, 45, 8608 (b)

Roncaroli, F.; Ruggiero, M. E.; Franco, D. W.; Estiu, G. L. Olabe, J.

A. Inorg. Chem. 2002, 41, 5760

(33) (a) Helm, L.; Merbach, A. E. Chem. ReV. 2005, 105, 1923. (b) Richens,

D. T. Chem. ReV. 2005, 105, 1961.

3350 Inorganic Chemistry, Vol. 46, No. 8, 2007

Influence of an Extremely NegatiWely Charged Porphyrin

negatively charged substituents on the porphyrin. This

observation is in agreement with results from Raman

addition mechanism for NO binding and show slower binding

rates in contrast to a dissociatively activated mode observed

for (P)Fe^III(OH₂)₂complexes. Furthermore, the reported

values for k_onvary by a factor of approximately 10²for the

diaqua-ligated porphyrins and appear to correlate with the

contribution of the intermediate spin state, S ) 3/2. In

comparison, however, differences in the NO binding rate

constants are rather small for all studied (P)Fe^III(OH)

complexes. This difference is because the lability of the Fe^III

complex controls the rate of NO binding to the six-coordinate

(P)Fe^III(OH₂)₂complexes, which does not play a role in the

case of the five-coordinate (P)Fe^III(OH) complexes.

resonance and DFT studies.³⁴In general, the value of ∆V^q

off

is more positive than of ∆V^q_onsince the off reaction involves

bond cleavage, formal oxidation of Fe^IIto Fe^III, accompanied

by a spin-state change (S ) 0 to S ) 3/2, 5/2), and solvent

reorganization resulting from the charge neutralization during

Fe^II-NO⁺bond cleavage.³

The reversible binding of NO at higher pH revealed a

different kinetic behavior for the monohydroxo-ligated

(P^16-)Fe^III(OH) complex compared to that of the (P^16-)-

Fe^III(H₂O)₂complex, as indicated by the negative activation

parameters, namely, ∆S^q_on) -82 ( 4 J mol^-1K^-1and ∆V^q

The volume profile for the binding of NO to (P^16-)Fe^III(OH)

is shown in Figure 8, from which it can clearly be seen that

on

) -9.4 ( 0.2 cm³mol^-1. It was in general concluded that

the slow rate constant for NO binding to complexes of the

type (P)Fe^III(OH) is mainly controlled by the Fe^II-NO⁺

bond-formation step and the accompanying electronic and

structural changes, rather than the lability of the metal

center.^4,5Electronic changes for the formation of the Fe^II-

NO⁺bond are accompanied by reorganization of the spin

arrangement in the iron(III) center from high spin (S ) 5/2)

to low spin (diamagnetic, S ) 0). In addition, structural

changes as a result of the iron center that moves from out-

of-plane to in-plane during bond formation are also expected.^5b

These conclusions suggest that the larger spin and structural

changes for the monohydroxo-ligated species (P)Fe^III(OH)

demand a higher activation barrier for NO binding and

release, such that lower reaction rates were observed. The

observed results are consistent with the earlier findings for

related complexes summarized in Table 6.⁵The rate-

determining step for NO binding to (P^16-)Fe^III(OH) via an

associative addition mechanism is mainly controlled by

electronic changes on the Fe^IIIcenter and the accompanying

structural rearrangement. Larger spin and structural changes

during the formation and breakage of the Fe^II-NO⁺bond

accompanied by higher activation barriers account for the

slower reactions in the case of the monohydroxo ligated

species (see rate data in Table 6).

the overall reaction volume (∆V ) ∆V^q- ∆V^q_off) has a

on

value of -24.4 ( 0.6 cm³mol^-1, which is close to a value

of approximately -23 cm³mol^-1found for several other

hydroxo complexes (except for (P⁸⁺)Fe^III, see data in Table

6). This large overall volume collapse for the binding of NO

is partially caused by the formation of the Fe-NO bond and

the change in spin state from a five-coordinate high-spin

hydroxo reactant to a six-coordinate, diamagnetic (low-spin)

nitrosyl product. The activation volumes reported for

(P^16-)Fe^III(OH) and (P^8-)Fe^III(OH) suggest an “early” transi-

tion state for the on reaction and a “late” transition state for

the off reaction, compared to the opposite trend for the other

complexes cited in Table 6. This is also in the value of K_NO

in terms of the more effective binding of NO.

From the data summarized in Table 7, it is clearly seen

how the rate constants for the binding and release of NO

decrease on going from (P)Fe^III(OH₂)₂to (P)Fe^III(OH) for

the series of complexes. This is throughout ascribed to the

role of the electronic barriers involved in the reactions with

the (P)Fe^III(OH) complexes. These different decreases arise

from the characteristic electronic and structural properties

among the complexes studied (P)Fe^III(OH₂)₂. In purely high-

spin five-coordinate (P)Fe^III(OH) complexes, it normally

demands larger reorganization of the spin state of the metal

center and structural changes with rather small variation in

We propose in the mechanistic scheme outlined in reaction

16 that NO binding to five-coordinate (P^16-)Fe^III(OH)

involves the formation of a diffusion-controlled encounter

complex {(P^16-)Fe^III(OH) || NO}, followed by the activation

step for the formation of the Fe^II-NO⁺bond, similar to that

suggested for high-spin 5-coordinate monohydroxo ligated

iron(III) porphyrins.³⁵

OH

k_onfor the studied complexes. While larger ratios for the

NO binding rate constants (k_on^{H O}/k_on^OH) are observed for

2

(P^n-)Fe^IIIcomplexes, resulting from mainly high k_onvalues,

smaller ratios are observed for the (Pⁿ⁺)Fe^IIIcomplexes.

B. Subsequent Reductive Nitrosylation of (P^16-)Fe^II-

(H₂O)(NO⁺). Our report¹⁸on the reactivity of ferric nitrosyl

complexes as a function of the nature of the porphyrin

substituents, indicates that kinetic and mechanistic features

of the subsequent reductive nitrosylation reaction of nega-

tively charged porphyrins, (P^n-)Fe^II(H₂O)(NO⁺), differs from

that found for positively charged porphyrins, (Pⁿ⁺)Fe^II(H₂O)-

(NO⁺). The observed reduction of coordinated NO⁺is

catalyzed by the addition of nitrite to the reaction solution.

It was further concluded that the rate of nitrite-catalyzed

reduction of (P)Fe^II(H₂O)(NO⁺) depends on the electrophi-

licity of coordinated NO⁺and the nature of the reactant,

(P^16-)Fe^III(OH) + NO {_K^k^D} {(P^16-)Fe^III(OH) || NO}

9

^k8

-D

[(P^16-)Fe^III(OH)‚‚‚‚NO]^qf (P^16-)Fe^II(OH)(NO⁺) (17)

On the basis of the results summarized in Tables 4 and 6,

(P)Fe^III(OH) complexes follow an associatively activated

(34) (a) Linder, D. P.; Rodgers, K. P.; Banister, J.; Wyllie, G. R. A.; Ellison,

M. K.; Scheidt, W. R. J. Am. Chem. Soc. 2004, 126, 14136. (b) Linder,

D. P.; Rodgers, K. R. J. Am. Chem. Soc. 2005, 44, 1367.

(35) (a) Franke, A.; Stochel, G.; Jung, Ch.; van Eldik, R. J. Am. Chem.

Soc. 2004, 126, 4181. (b) Laverman, L. E.; Ford, P. C. J. Am. Chem.

Soc. 2001, 123, 11614 and references therein.

-

namely, NO₂or HONO. The available results (see Table

9) favor the operation of an inner-sphere electron-transfer

process between nitrite and coordinated NO⁺. Charge

(36) Gruhn, N. E.; Lichtenberger, D. L.; Ogura, H.; Walker, F. A. Inorg.

Chem. 1999, 38, 4023.

Inorganic Chemistry, Vol. 46, No. 8, 2007 3351

Jee et al.

neutralization during bond formation between NO₂^-and NO⁺

on positively charged (Pⁿ⁺)Fe^II(H₂O)(NO⁺) complexes largely

accounts for the positive activation entropies and volumes

associated with this process. In contrast, negative values for

these activation parameters observed for the reaction with

the negatively charged (P^n-)Fe^II(H₂O)(NO⁺) complexes

suggest that bond-formation and charge concentration ac-

count for the observed data. These conclusions are in good

agreement with the experimental data found for the nitrite-

induced reductive nitrosylation of (P^16-)Fe^II(H₂O)(NO⁺) as

porphyrin complex, (P^16-)Fe^III. The speciation of (P^16-)Fe^III

in aqueous solution depends on pH and involves the

formation of diaqua- and monohydroxo-ligated complexes.

The experimental data obtained for (P^16-)Fe^III(H₂O)₂in

comparison with the reported reactivity pattern for other

water-soluble (P)Fe^III(H₂O)₂porphyrins demonstrate that the

number and nature of charged substituents on the porphyrin

affect the dynamics of the water exchange reaction and the

reversible binding of NO, in which both processes follow a

dissociative substitution mechanism. For the five-coordinate

(P^16-)Fe^III(OH) complex, the addition of NO follows an

associative bond formation process which is in agreement

with that found for a series of (P)Fe^III(OH) porphyrins. The

nitrosyl adduct (P^16-)Fe^II(H₂O)(NO⁺) is subjected to slow

reductive nitrosylation that is catalyzed by nitrite to yield

(P^16-)Fe^II(NO) as product via an inner-sphere redox pathway.

The rate-limiting step of this process shows that the reaction

is controlled by the electrophilicity of coordinated NO⁺.

Importantly, the potential catalytic activity of hydroxide ions

during reductive nitrosylation was found to be several orders

of magnitude larger than for the nitrite-catalyzed process.

-

shown in Table 9. The rate of NO₂coordination (k_nit) to

the nitrosyl complex is faster for (Pⁿ⁺)Fe^II(H₂O)(NO⁺) than

that for (P^n-)Fe^II(H₂O)(NO⁺). For example, the observed rate

constants for the binding of NO₂^-to (P^16-)Fe^II(H₂O)(NO⁺)

decrease by a factor of 10⁴compared to that observed for

(P⁸⁺)Fe^II(H₂O)(NO⁺) and are even 10²times slower than

those for (P^8-)Fe^II(H₂O)(NO⁺). This again indicates that the

crucial factor that controls the rate of reductive nitrosylation

is the overall negative charge on the porphyrin that increases

the electron density on metal center and reduces the

electrophilicity of coordinated NO⁺. Thus direct nucleophilic

attack of nitrite on coordinated NO⁺is suggested as rate-

determining step to form a (P)Fe^II-N₂O₃intermediate that

subsequently releases N₂O₃and rapidly reacts with NO to

form (P)Fe^II-NO. From the data in Table 9, an increase in

activation volume for nitrite binding to (P)Fe^II(H₂O)(NO⁺)

on going from (P^16-)Fe^IIIto (P⁸⁺)Fe^IIIclearly shows that the

contribution of bond formation is facilitated by negatively

charged meso substituents, while a decreasing electrostriction

prevails over bond-formation in porphyrins containing

electron-donating substituents.

Acknowledgment. The authors gratefully acknowledge

financial support from the Deutsche Forschungsgemeinschaft

as part of SFB 583 on Redox-Active Metal Complexes and

the European Commission within AQUACHEM Research

Training Network Contract MRTN-CT-2003-503864.

Supporting Information Available: Figures S1-S7 that

present the ¹⁷O NMR measurement and temperature and pressure

dependencies of the individual reactions studied and Figure S8

showing the hydroxide concentration dependence of the reductive

nitrosylation reaction of (P^16-)Fe^II(H₂O)(NO⁺). This material is

available free of charge via the Internet at http://pubs.acs.org.

Conclusions

The presented results provide mechanistic information on

the reaction of NO with an extremely negatively charged

IC061732G

3352 Inorganic Chemistry, Vol. 46, No. 8, 2007

Article Doi

Influence of an extremely negatively charged porphyrin on the reversible binding kinetics of NO to Fe(III) and the subsequent reductive nitrosylation

DOI: 10.1021/ic061732g

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Article abstract of DOI:10.1021/ic061732g

Full text of DOI:10.1021/ic061732g

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