Water exchange controls the complex-formation mechanism of water-soluble iron(III) porphyrins: Conclusive evidence for dissociative water exchange from a high-pressure 17O NMR study

Article abstract of DOI:10.1002/1521-3773(20010504)40:9<1678::AID-ANIE16780>3.0.CO;2-A

The water-exchange reactions of three modified iron(III) porphyrins were studied in acidic medium as a function of temperature and pressure using ¹⁷O NMR spectroscopic techniques. The positive activation entropies and activation volumes support a dissociative mechanism (see scheme showing the transition state; L = nucleophile, H₂O, NO). A comparison with available literature data reveals that the rate and mechanism of complex-formation reactions of such metal porphyrins are controlled by the water-exchange process.

Full text of DOI:10.1002/1521-3773(20010504)40:9<1678::AID-ANIE16780>3.0.CO;2-A

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performed on water-exchange reactions of water-soluble
porphyrin complexes using high-pressure NMR spectroscopic
techniques. Some mechanistic information on such reactions
was obtained indirectly from the effect of pressure on ligand-
substitution reactions of complexes of the type
[M(TPPS)(H2O)2]3 and [M(TMPyP)(H2O)2]5‡, where M ˆ
CoIII, RhIII, and CrIII.[18±20] In these reactions the rapid
substitution of water by thiourea and thiocyanate was
characterized by significantly positive volumes of activation,
from which a dissociative ligand-substitution mechanism was
concluded. However, a clear distinction between a dissocia-
tive interchange (Id) or a limiting dissociative (D) mechanism
was not possible.
Water Exchange Controls the Complex-
Formation Mechanism of Water-Soluble
Iron(iii) Porphyrins: Conclusive Evidence
for Dissociative Water Exchange from a
High-Pressure 17O NMR Study**
Thorsten Schneppensieper, Achim Zahl, and
Rudi van Eldik*
Transition metal porphyrins have received considerable
attention from many groups, especially as a result of their
catalytic activity in various oxidation processes.[1±3] Of funda-
mental interest is their high reactivity coupled to the ability to
undergo fast ligand-substitution reactions and the redox
cycling that forms an essential aspect of their solution-phase
chemistry.[4±6] The investigated iron porphyrins are known to
be model complexes for a number of enzymes and proteins.
These heme proteins serve a wide-range of functions, and the
nature of the axial ligand on the iron is thought to play a key
role in controlling the properties of the heme proteins. Of
particular interest are the oxidizing enzymes peroxidases,
cytochrome P-450ꢁs, and catalases.[7, 8] In addition, various
reports point to the important role of the metalloporphyrins
such as cytochrome oxidase,[9] nitrile hydrase,[10] and cata-
lase[11] as target molecules in mammalian biology. Mechanistic
studies on and the substitution behavior of various metal-
loporphyrins have been reviewed.[12, 13]
Quite recently, with the aid of laser flash-photolysis
techniques, Ford and co-workers[21] studied the rever-
sible binding of NO to [FeIII(TPPS)(H2O)2]3
and
[FeIII(TMPS)(H2O)2]3 , (H2TMPS ˆ meso-tetrakis(sulfonato-
mesityl)porphine). They suggested a dissociative mechansim
on the basis of significantly positive volumes of activation
found for both the ªonº and ªoffº reactions. Their mechanistic
interpretation of the data followed the concept of a dissocia-
tive (D) mechanism and consisted of reaction steps (1) and
(2), Porˆ porphyrin.
k1
[FeIII(Por)(H2O)2]3 „ [FeIII(Por)(H2O)]3 ‡ H2O
(1)
(2)
k
1
k2
[FeIII(Por)(H2O)]3 ‡ NO „ [FeIII(Por)(H2O)NO]3
k
2
In 1980 Hunt and co-workers[14] studied the water-exchange
reactions of two water-soluble FeIII porphyrins using 17O
NMR spectroscopic techniques. Both complexes, meso-tetra-
kis(N-methyl-4-pyridyl)porphine (H2TMPyP) and meso-tet-
rakis(p-sulfonatophenyl)porphine (H2TPPS), exhibited wa-
ter-exchange reactions that are several orders of magni-
tude faster than aquated FeIII ions in acidic medium.[15]
These complexes were considered to be model com-
pounds for aqueous porphyrin chemistry, since in aqueous
solution the axial interaction of water with the metal is of
fundamental importance in both ligand-substitution and
redox processes. The fast water-exchange reactions were
characterized by significantly positive activation entropies,
but no mechanistic conclusions were drawn from this
data.[14]
Under the selected experimental conditions, kon
ˆ
k1k2[NO]/k and koff ˆ k 2. The effect of pressure on kon
1
resulted in positive activation volumes, ‡13 Æ 1 and ‡8.3 Æ
1
1.5 cm3 mol , for porˆ TMPS and TPPS, respectively, under-
lining the overall dissociative nature of the binding reaction.
The complexity of the rate expression for kon, however, did
not allow a more detailed interpretation since DV=(kon) ˆ
DV=(k1) ‡ DV=(k2) DV=(k ). The authors argued, as in the
1
cases mentioned above,[18±20] that DV=(k2) DV=(k ) is ex-
1
pected to be small as a result of the rapid binding of NO and
H2O, respectively, such that DV=(kon) mainly presents
DV=(k1).
We have now studied the water-exchange reactions of
[FeIII(TPPS)(H2O)2]3 (1), [FeIII(TMPyP)(H2O)2]5‡ (2), and
[FeIII(TMPS)(H2O)2]3 (3), that is, reaction (1), in more detail
using high-pressure 17O NMR spectroscopic techniques.
In the meantime, the application of high-pressure kinetic
techniques has contributed significantly towards the elucida-
tion of inorganic reaction mechanisms in general,[16] and that
of solvent-exchange processes on transition metal centers in
particular.[17] As far as we know, no studies have been
[*] Prof. Dr. R. van Eldik, Dipl.-Chem. T. Schneppensieper,
Dr. A. Zahl
Institut für Anorganische Chemie
University of Erlangen-Nürnberg
Egerlandstrasse 1, 91058 Erlangen (Germany)
Fax : (‡49)9131-8527387
E-mail: vaneldik@chemie.uni-erlangen.de
[**] The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie,
and the Max-Buchner-Forschungsstiftung
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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The selected porphines TPPS, TMPyP, and TMPS vary in
steric hindrance and charge, and thus allow the influence of
these factors on the water-exchange process to be studied. The
results clearly support the operation of a dissociative mech-
anism in all cases and enable a more detailed interpretation of
the activation volumes reported for the complex-formation
reactions mentioned above.
suggests that the crucial factor for the exchange rate, besides
the steric decompression, is the negatively charged peripheral
substituents that increase the electron density on the metal
center and labilize the axial metal ± ligand (water) bonds.
Pressure dependence: From the temperature-dependence
data, appropriate temperatures were selected for the pres-
sure-dependence measurements. Plots of ln(1/T2r) versus
pressure for all three [FeIII(Por)(H2O)2] complexes are shown
in Figure 1. In all cases, the plots are linear within the
experimental error limits and the volume of activation could
be calculated directly from the slope (ˆ DV=/RT). Very
similar values are obtained for complexes 1 and 2, DV=ˆ
Temperature dependence: An analysis of the data for the
three investigated porphyrins (see Figures 2 ± 4 in the Sup-
porting Information) gave the activation parameters sum-
marized in Table 1. The fastest exchange rates are ob-
served for the negatively charged [FeIII(TMPS)(H2O)2]3
and [FeIII(TPPS)(H2O)2]3 complexes, with kex ˆ (2.1 Æ
1
‡7.9 Æ 0.2 and ‡7.4 Æ 0.4 cm3 mol , respectively, compared
1
1
1
0.1)  107 s and kex ˆ (2.0 Æ 0.1)  106 s at 298 K, respec-
to the significantly higher value of ‡11.9 Æ 0.3 cm3 mol for
tively. The decrease in rate constant for the TPPS complex is
the sterically demanding complex 3 (see Table 1).
associated with an increase in DH= from 61 Æ 1 kJmol
1
1
to 67 Æ 2 kJmol . The positive charged porphyrin
[FeIII(TMPyP)(H2O)2]5‡ is less labile with kex ˆ (4.5 Æ 0.1) Â
105 s at 298 K, for which DH= increases further to 71 Æ
1
1
2 kJmol . For all the investigated porphyrins almost an
identical entropy of activation, 100 Æ 10 JK 1 mol , was
1
found. This large positive value suggests that a dissociative
water-exchange mechanism is operative in all cases.
By way of comparison, Hunt and co-workers[14] found
significantly lower, but almost equal values for the activation
enthalpy for the oppositely charged complexes of TPPS and
TMPyP. In addition, they found kex to be 20 times larger for
the TPPS than for the TMPyP complex. These differences
were ascribed to an entropic effect since their values for the
activation entropy differed significantly. The data of Hunt and
co-workers[14] were calculated under the assumption that only
one water molecule is coordinated to the metal center in both
systems. However, according to data published later, two
water molecules are bound to the TMPS[22] and TPPS, as well
as to the TMPyP[23] complexes in an octahedral coordination
geometry. This assumption could have led to deviations in the
reported rate and activation parameters, since the number of
coordinated water molecules affects the data calculation. In
addition, as a 400 MHz NMR spectrometer was employed in
our study more accurate data can be expected.
On the basis of our data, steric hindrance on the porphyrin
increases kex by a factor of 10, as expected for the effect of
steric decompression on a dissociative activation process,
whereas the positively charged TMPyP complex has an
exchange rate that is approximately 50 times lower than that
of the TMPS complex. That kex for the negatively charged
complexes is significantly higher than for the TMPyP system,
Figure 1. Pressure dependence of T2r for water exchange on
[FeIII(TPPS)(H2O)2]3
(
, 288 K), [FeIII(TMPYP)(H2O)2]5‡ (Â, 318 K),
*
and [FeIII(TMPS)(H2O)2]3
(
~
, 279 K). For experimental conditions see
Figures 2 ± 4 in the Supporting Information.
It follows that the opposite charges on complexes 1 and 2 do
not affect the substitution mechanism of coordinated water,
and the reported DV= data support the operation of a
dissociative interchange (Id) mechanism.[16, 17] The introduc-
tion of steric hindrance through the bulky mesityl groups on
complex 3 enhances the water-exchange rate, and the more
positive DV= value suggests that substitution of coordinated
water follows a limiting dissociative (D) mechanism. In fact,
1
the reported value of ‡11.9 Æ 0.3 cm3 mol in the latter case
is very close to the limiting value of ‡13 cm3 mol 1 predicted
for water exchange on an octahedral metal complex following
a limiting dissociative mechanism.[17, 24, 25] This means that the
Table 1. Rate and activation parameters for water-exchange reactions of iron(iii) porphyrin complexes. The errors shown are standard deviations given by
the computer program.
298
ex
[a]
k
DH=
[kJmol
DS=
[JK 1 mol
DV=
[cm3 mol
kex
[s
Ref.
1
1
1
1
1
[s
]
]
]
]
]
[Fe(TPPS)(H2O)2]3
[Fe(TMPyP)(H2O)2]5‡
[Fe(TMPS)(H2O)2]3
1
(2.0 Æ 0.1) Â 106
(1.4 Æ 0.01) Â 107
(4.5 Æ 0.1) Â 105
(7.8 Æ 0.1) Â 105
(2.1 Æ 0.1) Â 107
67 Æ 2
99 Æ 10
84.5 Æ 1.3
100 Æ 6
‡ 7.9 Æ 0.2[b]
7.5 Â 105/8.5 Â 105
this work
[14]
this work
[14]
57.3 Æ 0.4
71 Æ 2
±
±
2
‡ 7.4 Æ 0.4[c]
±
‡ 11.9 Æ 0.3[d]
2.1 Â 106/2.7 Â 106
±
3.8 Â 106/3.9 Â 106
57.7 Æ 0.4
61 Æ 1
61.5 Æ 1.3
100 Æ 5
3
this work
[a] For comparison with the value of kex from the temperature-dependence data at normal pressure, the value of kex from the pressure-dependence data was
extrapolated to normal pressure at the temperature at which DV= was determined. [b] Measured at 288 K. [c] Measured at 318 K. [d] Measured at 279 K.
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[8] Y. Watanabe, J. T. Groves, The Enzymes, Vol. 20, Academic Press, San
Diego, CA, 1992, p. 405.
[9] M. W. J. Cleeter, J. M. Cooper, V. M. Darley-Usmar, S. Moncada,
A. H. V. Scapira, FEBS Lett. 1994, 345, 50.
[10] T. Noguchi, J. Honda, T. Nagamune, H. Sasabe, Y. Inoue, I. Endo,
FEBS Lett. 1995, 358, 9.
degree of bond cleavage in the transition state is significantly
higher for complex 3 than in 1 and 2.
Returning to the motivation for this work, our results on the
water-exchange mechanism of the porphyrins studied are in
excellent agreement with the mechanistic interpretation
offered by Ford and co-workers[21] for the complex-formation
[11] G. C. Brown, Eur. J. Biochem. 1995, 232, 188.
[12] D. K. Lavallee, Coord. Chem. Rev. 1985, 61, 55.
[13] M. Hoshino, L. Laverman, P. C. Ford, Coord. Chem. Rev. 1999, 187, 75.
[14] I. J. Ostrich, G. Liu, H. W. Dodgen, J. P. Hunt, Inorg. Chem. 1980, 19,
619.
[15] T. W. Swaddle, A. E. Merbach, Inorg. Chem. 1981, 20, 4212.
[16] a) A. Drljaca, C. D. Hubbard, R. van Eldik, T. Asano, M. V. Basilev-
sky, W. J. le Noble, Chem. Rev. 1988, 98, 2167; b) R. van Eldik, C.
Dücker-Benfer, F. Thaler, Adv. Inorg. Chem. 2000, 49, 1.
[17] L. Helm, A. E. Merbach, Coord. Chem. Rev. 1999, 187, 151.
[18] S. Funahashi, M. Inamo, K. Ishihara, M. Tanaka, Inorg. Chem. 1982,
21, 447.
[19] J. G. Leipoldt, R. van Eldik, H. Kelm, Inorg. Chem. 1983, 22, 4146.
[20] G. J. Lamprecht, J. G. Leipoldt, T. W. Swaddle, Inorg. Chim. Acta 1987,
129, 21.
[21] L. E. Laverman, M. Hoshino, P. C. Ford, J. Am. Chem. Soc. 1997, 119,
12663.
reactions of 1 and 3 with NO. Their reported activation
1
volumes of ‡8.3 Æ 1.5 and ‡13 Æ 1 cm3 mol
for these
reactions, respectively, are almost identical to those reported
for the water-exchange reactions in the present study. Their
conclusion that the observed DV= for the ªonº reaction with
NO mainly represents DV=(k1) for reaction (1), is perfectly
correct as shown by the data reported here. Thus the
formation of 1 and 3 is not only controlled by the rate but
also by the mechanism of the water-exchange process.
Depending on the structural and electronic situation this
process tends to occur according to an Id or D mechanism, the
complex-formation reactions with nucleophiles such as NO
follow the same mechanism.
[22] S.-H. Cheng, Y.-S. Chen, Y. O. Su, J. Chin. Chem. Soc. 1991, 38, 15.
[23] M. Ivanca, A. G. Lappin, W. R. Scheidt, Inorg. Chem. 1991, 30, 711.
[24] F. P. Rotzinger, J. Am. Chem. Soc. 1997, 119, 5230.
[25] M. Hartmann, T. Clark, R. van Eldik, J. Phys. Chem. A 1999, 103,
9899.
The water-exchange rate and associated activation enthalpy
of iron(iii) porphyrins are significantly affected by the charge
on the porphine and to a lesser degree by steric compression.
The mechanism of the process, however, is controlled by steric
factors and varies between a dissociative interchange and a
limiting dissociative mechanism. Thus the lability of the axial-
bound solvent molecules in these systems plays a key role in
the mechanism and substitution behavior of porphyrin- and
heme-based systems. High-pressure NMR spectroscopic tech-
niques present a powerful tool to add to the mechanistic
understanding of such processes, which could lead to a more
profound understanding of the reactions and processes in
biologically relevant macrocyclic systems such as metmyoglo-
bin and cytochrome P-450.
[26] E. B. Fleischer, J. M. Palmer, T. S. Srivastava, A. Chatterjee, J. Am.
Chem. Soc. 1971, 93, 3162.
Controlling the Lability of Square-Planar PtII
Complexes through Electronic Communication
between p-Acceptor Ligands**
Deogratius Jaganyi, Andreas Hofmann, and
Rudi van Eldik*
Experimental Section
Na3[FeIII(TPPS)(H2O)2] (Na3-1) was synthesized as described elsewhere.[26]
[FeIII(TMPyP)(H2O)(OH)](pts)4 (2-pts4), where pts ˆ p-toluenesulfonate,
and Na3[FeIII(TMPS)(H2O)2] (Na3-3) were purchased from Frontier
Scientific Ltd. Fine Chemicals Utah, USA, and used without further
purification. Ca. 20% enriched 17O-labeled water (D-Chem Ltd. Tel Aviv,
Israel) was used for the 17O NMR water-exchange measurements. NaClO4
(Aldrich) was used to adjust the ionic strength to 0.5m, and HClO4 (1 and 3)
and tosylic acid (2) were used to adjust the pH of the solution. No salt was
added in the case of 2 to avoid precipitation. The porphyrin samples were
prepared by combining weighted amounts of salt, perchloric or tosylic acid,
and water. The resulting solution was transferred to the NMR tube. The pH
was determined on identical samples prepared in ordinary water. The water
exchange measurements were performed at pH 3 (for 1 and 3) and pH 1.1
(for 2), where only the monomeric aqua forms of the porphyrins are present
in solution. The complex concentrations were 3.4 Â 10 2 m (1), 2.0 Â 10 2 m
(2), and 3.0 Â 10 2 m (3).
Dedicated to Professor Ernst-G. Jäger
on the occasion of his 65th birthday
Studies on the substitution mechanism of low-spin d8
square-planar complexes for many years centered around
the s trans influence or trans effect.[1] For PtII complexes this
has involved detailed systematic studies of different trans
groups[2] using a range of different nucleophiles.[3] Mechanistic
studies established that ligand-substitution reactions of PtII
complexes mainly occur by an associative process involving a
trigonal-bipyramidal intermediate. In recent years, volumes of
activation (DV=) obtained from high-pressure kinetic meas-
urements, have been used in distinguishing mechanistic
pathways of substitution reactions; negative values indicating
Received: November 7, 2000 [Z16050]
[*] Prof. Dr. R. van Eldik, Dr. D. Jaganyi, A. Hofmann
Institute for Inorganic Chemistry
[1] A. P. Hong, D. W. Bahnemann, M. R. Hoffmann, J. Phys. Chem. 1987,
91, 6245.
[2] B. Meunier, Chem. Rev. 1992, 92, 1411.
University of Erlangen-Nürnberg
Egerlandstrasse 1, 91058 Erlangen (Germany)
Fax : (‡49)9131-8527387
[3] G. Behra, L. Sigg, Nature 1990, 344, 419.
[4] V. Lepentsiotis, R. van Eldik, J. Chem. Soc. Dalton Trans. 1998, 999.
[5] V. Lepentsiotis, R. van Eldik, F. F. Prinsloo, J. J. Pienaar, J. Chem. Soc.
Dalton Trans. 1999, 2759.
[6] J. Lee, J. A. Hunt, J. T. Groves, J. Am. Chem. Soc. 1998, 120, 7493.
[7] H. B. Dunford, Adv. Inorg. Chem. 1982, 4, 41.
E-mail: vaneldik@chemie.uni-erlangen.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft,
Fonds der Chemischen Industrie, and the Alexander von Humboldt
Foundation (fellowship to D.J.).
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