Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides.

Article abstract of DOI:10.1021/jo010878o

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III

Full text of DOI:10.1021/jo010878o

1
506
J . Org. Chem. 2002, 67, 1506-1514
Ir on (III)-Sa len Com p lexes a s En zym e Mod els: Mech a n istic Stu d y
of Oxo(sa len )ir on Com p lexes Oxygen a tion of Or ga n ic Su lfid es
Veluchamy Kamaraj Sivasubramanian, Muniyandi Ganesan, Seenivasan Rajagopal,* and
Ramasamy Ramaraj*
School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India
seenirajan@yahoo.com
Received August 27, 2001
The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several
oxo(salen)iron (salen ) N,N′-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The
IV
•+
oxo complex [OdFe (salen)] , generated from an iron(III)-salen complex and iodosylbenzene,
effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of
IV
•+
[
OdFe (salen)] as the active oxidant is supported by resonance Raman studies. The kinetic
data indicate that the reaction is first-order in the oxidant and fractional-order with respect
to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the
substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the
product formation step determined using Michaelis-Menten kinetics correlate well with Hammett
σ constants, giving reaction constant (F) values in the range of -0.65 to -1.54 for different oxo-
(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by
substituted oxo(salen)iron complexes also correlate with Hammett σ constants, giving positive F
values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom
transfer from the oxidant to the substrate in the rate-determining step.
In tr od u ction
Heme enzymes peroxidases, catalases, and cytochrome
catalytic turnover of HRP involves reaction of the en-
zymes with H O to give a ferryl (Fe dO) porphyrin
2 2
IV
radical cation, two oxidation equivalents above the rest-
ing ferric state. This intermediate, known as compound
I, is normally reduced by sequential one-electron trans-
fers from different substrate molecules. The first of these
electrons reduces compound I and gives compound II, an
intermediate that is one oxidation equivalent above
the ferric state. Catalytic turnover of cytochrome P450
monooxygenases require two-electron reduction of mo-
lecular oxygen to a species that is thought to resemble
compound I of HRP, although it is not known if the
second oxidation equivalent resides on the porphyrin or
the protein. Despite the probable similarities in catalytic
species, cytochrome P450 differs from HRP in that it
routinely catalyzes the two-electron insertion of an
oxygen atom into its substrates rather than the removal
of a second electron.
P450s are present in most organisms, and they catalyze
a large variety of reactions such as oxidation, reduction,
and isomerization.1
-14
Horseradish peroxidase (HRP) is
a hemoprotein peroxidase capable of catalyzing the
oxidation of a large variety of organic compounds. The
*
To whom correspondence should be addressed. Fax: 91-0452-
59139.
1) Ortiz de Montellano, P. R., Ed. Cytochrome P450. Structure,
Mechanism and Biochemistry, 2nd ed.; Plenum Press: New York, 1995.
2) (a) Dunford, H. B. In Peroxidases in Chemistry and Biology;
8
(
(
Everse, J ., Everse, K. E., Giishman, M. B., Eds.; CRC Press: Boca
Raton, FL, 1991; Vol. II, pp 1-24. (b) Dunford, H. B.; Stillman, J . S.
Coord. Chem. Rev. 1987, 19, 187.
(3) (a) Watanabe, Y.; Groves, J . T. In The Enzyme; Sigman, D. S.,
Ed.; Academic Press: San Diego, CA, 1992; Vol. XX, Chapter 9. (b)
Bruice, T. C. In Mechanistic Principles of Enzyme Activity; Liebman,
F. F., Greenberg, A., Eds.; VCH: New York, 1988; p 227.
(4) (a) Metalloporphyrins in Catalytic Oxidations; Sheldon, R. A.,
To elucidate the mechanism of reactions catalyzed by
hemoproteins, cytochrome P450, and peroxidases, the
sulfoxidation of sulfides is a subject of current inter-
est.1 It has been established that, in the HRP-catalyzed
sulfoxidation, the reaction is initiated by the transfer of
one electron from the sulfide to the iron(IV) oxo porphyrin
Ed.; Marcel Dekker: New York, 1994. (b) Metalloporphyrins Catalyzed
Oxidation; Montanari, F., Casella, L., Eds.; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1994.
(
5) Meunier, B., Ed. Biomimetic Oxidations Catalyzed by Transition
Metal Complexes; Imperial College Press: London, 2000.
6) (a) Que, L., J r.; Ho, R. Y. N. Chem. Rev. 1996, 96, 2607. (b) Holm,
R. H. Chem. Rev. 1987, 87, 1401.
7) (a) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J . H. Chem.
Rev. 1996, 96, 2841. (b) Dawson, J . H. Science 1988, 240, 433.
3,15
(
(
•
+
IV
radical cation, [Por sFe dO], which is suggested to be
the active species in these reactions. The sulfoxide is then
formed by the reaction of the radical cation with Pors
(
(
(
8) Meunier, B. Chem. Rev. 1992, 92, 1411.
9) Feig, A.l.; Lippard, S. J . Chem. Rev. 1994, 94, 259.
10) Brothers, P. J . In Advances in Organometallic Chemistry; Stone,
IV
Fe dO (Scheme 1).
F. G. A., West, R., Eds.; Academic Press: New York, 2001; Vol. 46, p
64.
2
Though this mechanism holds in the HRP-catalyzed
reaction, the situation is not clear on the mechanism of
cytochrome P-450 oxidation of organic sulfides. Though
(
(
11) Mansuy, D. Coord. Chem. Rev. 1993, 125, 129.
12) Nam, W.; Han, H. J .; Oh, S. Y.; Lee, Y. J .; Choi, M. H.; Han, S.
Y.; Kim, C.; Woo, S. K.; Shin, W. J . Am. Chem. Soc. 2000, 122, 8677.
13) Goto, Y.; Matsui, T.; Ozaki, S.; Watanabe, Y.; Fukuzumi, S. J .
Am. Chem. Soc. 1999, 121, 9497.
14) (a) Groves, J . T.; Nemo, T. E. J . Am. Chem. Soc. 1983, 105,
(
(
(15) (a) Ozakl, S.-I.; Oritz de Montellano, P. R. J . Am. Chem. Soc.
1994, 116, 4487. (b) Ozaki, S.-I.; Oritz de Montellano, P. R. J . Am.
Chem. Soc. 1995, 117, 7056.
5
1
786. (b) Groves, J . T.; Subramanian, D. V. J . Am. Chem. Soc. 1986,
06, 2177. (c) Groves, J . T. J . Chem. Educ. 1985, 62, 928.
1
0.1021/jo010878o CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/08/2002

Iron(III)-Salen Complexes as Enzyme Models
J . Org. Chem., Vol. 67, No. 5, 2002 1507
Sch em e 1
type of salen complex in the epoxidation of simple
olefins.3
3,34
It is possible to introduce chiral substituents
at C3 and C3′ sites of the phenolic part of the salen ligand
cf. eq 1 for the structure of iron-salen complexes). These
(
stereogenic carbon atoms reside proximate to the metal
center, and this renders the salen ligand a promising
chiral template for the construction of an asymmetric
reaction site. To understand the mechanism of oxygen
transfer from oxometal ions, for example, oxometal
porphyrin and oxo(salen)metal complexes, organic sul-
fides seem to be better substrates than olefins because
of the efficient reactivities of sulfides and absence of other
an electron-transfer mechanism is possible, also viable
•
+
is a direct oxygen-transfer mechanism from the Por s
IV
13
Fe dO to sulfide as shown in Scheme 1, path c.
undesired reactions.
To establish the nature of the reactive intermediate
and the mechanism of cytochrome P450-catalyzed oxida-
tion of biological substrates, extensive studies have been
made using synthetic model iron-porphyrins in different
oxidation states and with various ligands with the aid of
With the aim of establishing the optimum conditions
for the synthesis of organic sulfoxides and the mechanism
of oxometal ion oxidation of organic sulfides, we have
initiated a systematic study on the kinetics of oxo(salen)-
metal ion oxidation of the above substrates and already
3
5-37
spectroscopic techniques including electronic absorption,
reported the mechanism of oxo(salen)manganese(V),
NMR, EPR, and resonance Raman (RR).1
3-32
Interest-
38
39
oxo(salen)chromium(V), and oxo(salen)ruthenium(V)
oxygenation of organic sulfides. We have initially pro-
posed single electron transfer from organic sulfide to the
oxometal ion as the rate-controlling step in the oxygen
atom transfer reaction from several cationic oxo(salen)-
manganese(V)35 complexes to sulfide. However, in a
subsequent study, by comparing the reactivity of organic
sulfides and sulfoxides toward the same oxidant, oxo-
(salen)manganese(V), a common mechanism involving
the electrophilic attack of the oxygen of the oxidant at
ingly no attempt has been made so far on the use of
iron(III)-salen complexes as catalysts for the oxidation
of biologically important organic substrates, particularly
sulfides.
Salicylidene-ethylenediamines, commonly known as
salens, can form stable complexes with transition metals.
Apart from the porphyrins, the most important synthetic
ligand systems, especially in the context of catalysis for
the asymmetric oxidation of organic substrates, are the
salens,3
3,34
since metal-salen complexes have features
37
the sulfur center of the substrate has been proposed.
in common with metalloporphyrins with respect to their
structure and catalytic activity. These metal-Schiff base
complexes have been developed as catalysts for the
epoxidation of olefins and oxidation of other organic
substrates, and a breakthrough has been made with this
Similarly, the selective oxidation of organic sulfides to
sulfoxides with oxo(salen)chromium(V) complexes pro-
ceeds through the electrophilic attack of oxygen at the
sulfur center of the organic sulfide.38
The oxygenation reaction of organic sulfides with oxo-
(salen)manganese(V) and oxo(salen)chromium(V) ions
(
(
16) Fujii, H. J . Am. Chem. Soc. 1993, 115, 4641.
17) Traylor, P. S.; Kim, C.; Richards, J . L.; Xu, F.; Perrin, C. L. J .
proceeds through clear second-order kinetics, first-order
each in the oxidant and substrate, and there is no pre-
coordination between the substrate and catalyst during
the course of reaction. Enzyme-catalyzed reactions achieve
high enantioselectivity, at least in part, by inducing
substrate precoordination to the catalyst prior to reac-
tion.40 The precoordination minimizes the degrees of
freedom in the critical transition state and maximizes
the selectivity-determining interactions between the
catalyst’s asymmetric environment and the substrate.
Many successful nonenzymatic asymmetric catalyst sys-
tems operate on this principle, though a number of
exceptions have also been reported.4
Am. Chem. Soc. 1995, 117, 3468.
(
(
18) Gross, Z.; Nimri, S. Inorg. Chem. 1994, 33, 1731.
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Biochem. 2000, 82, 141 and references therein.
(
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(
b) Guilmet, E.; Meunier, B. Tetrahedron Lett. 1982, 23, 2449.
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189.
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1
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24) (a) Smegal, J . A.; Schardt, B. C.; Hill, C. L. J . Am. Chem. Soc.
1
1
983, 105, 3510. (b) Smegal, J . A.; Hill, C. L. J . Am. Chem. Soc. 1983,
05, 3515.
1-44
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25) Mansuy, D.; Bortoli, J . F.; Momenteau, M. Tetrahedron Lett.
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1
(
(35) Chellamani, A.; Alhaji, N. M. I.; Rajagopal, S.; Sevvel, R.;
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University, 1999.
(40) Walsh, C. Enzymatic Reaction Mechanisms; W. H. Freeman and
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Rev. 1994, 94, 2483.
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therein.
(
3
3
(
28) Yun, C. H.; Miller, G. P.; Guengerich, F. P. Biochemistry 2000,
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29) (a) Sato, H.; Guengerich, J . Am. Chem. Soc. 2000, 122, 8099.
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(
8
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Chem. Soc. 1999, 121, 10178. (b) Moore, K. T.; Horvath, I. T.; Therien,
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(
32) Ogliaro, F.; Harris, N.; Cohen, S.; Filatov, M.; de Visser, S. P.;
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T. Bull. Chem. Soc. J pn. 1999, 72, 603.
34) J acobsen, E. N. In Comprehensive Organometallic Chemistry
(
(
(
II; Wilkinson, G., Stone, F. G. A., Abel, E. W., Hegedus, L. S., Eds.;
Pergamon: New York, 1995; Vol. 12, Chapter II.1.

1
508 J . Org. Chem., Vol. 67, No. 5, 2002
Sivasubramanian et al.
Ta ble 1. λm a x Absor p tion , Ma gn etic Mom en t, a n d Electr och em ica l Da ta of Sch iff Ba se Com p lexes of Ir on
magnetic moment
at 300 K (µB)
λmax (nm),a
iron salen
λmax (nm),a
oxo(salen)iron
complex
Fe(salen)Cl]
Fe(5,5′-(NO2)2salen)Cl]
Fe(5,5′-(Cl)2salen)Cl]
Fe(5,5′-(OMe)2salen)Cl]
Fe(7,7′-(Me)2salen)Cl]
Fe(3,3′,5,5′-(t-But)4salen)Cl]
E1/2 V (SCE)b
[
[
[
[
[
[
5.43 (5.37)c
5.11
470
463
470
500
468
503
450
459
462
466
461
495
-0.28
-0.15
-0.24
-0.35
-0.30
-0.68
5.19 (5.14)c
5.61
5.57
5.65
a
b
c
5
4
I
n
1
0
0
%
C
H
3
C
N
a
t
2
9
8
K
.
T
h
e
s
u
p
p
o
r
t
i
n
g
e
l
e
c
t
r
o
l
y
t
e
i
s
0
.
1
M
N
a
C
l
O
4
.
R
e
p
o
r
t
e
d
v
a
l
u
e
s
.
Despite the widespread interest in iron(III)-porphyrin-
other reagents were of AnalaR grade or were used after
purification. The kinetic study of the reaction was performed
after the purities of the reactants and solvents used in the
system were confirmed.
catalyzed oxygenation of organic sulfides,1
,6,13,45-53
so far
no attempt has been made to use iron(III)-salen com-
III
plexes as catalysts. Though salen complexes of Mn
,
III
III
III
Cr , Co , Ru , and other metal ions have been used
as catalysts, it is surprising that the catalytic role of
iron(III)-salen complexes has not been realized on the
oxygenation reactions of organic sulfides.33-39 The present
study seems to be the first attempt to use oxo(salen)iron
complexes as the oxidants for the sulfoxidation of organic
sulfides. Interestingly the reaction proceeds through
precoordination of the substrate to the oxidant, and the
synthetic and mechanistic details of oxo(salen)iron com-
plex oxygenation of organic sulfides are discussed here.
Meth od s. The magnetic moments of the iron(III)-salen
complexes were measured by the Guoy method, and the values
Exp er im en ta l Section
are collected in Table 1. The magnetic moment values are in
Ma ter ia ls. The iron(III)-Schiff base complexes IIIa -IIIf
were prepared by the reaction of stoichiometric amounts of
ligands with the ferric chloride in alcohol solution and recrys-
agreement with those reported.54 The equipment was cali-
brated by means of mercury(II) tetrathiocyanatocobaltate(II).
The IR spectra of the complexes were measured with a J ASCO
5
4-56
tallized using the appropriate solvents.
The corresponding
FTIR-410 spectrophotometer in CH CN medium. The ν(Cs
3
oxo(salen)iron complexes IVa -IVf were obtained by the fol-
O) band at 1252 cm-1 and ν(HsCdN) band at 1633 cm-1 were
lowing procedure. The stirring of a clear solution of iron(III)-
found for complex IIIb. Similar values were also obtained for
other complexes. The EPR spectra at room temperature for
complexes IIIa and IIId and their oxo forms IVa and IVd were
measured with a J EOL J ES-TE 100 x-band EPR spectrometer
3
salen complexes in CH CN with iodosylbenzene (PhIO) for
5
-15 min led to the formation of oxo(salen)iron species IVa -
IVf. The structures of iron(III)-salen and oxo(salen)iron
complexes used in the present study are shown in eq 1.
p-Methoxy- and p-methylphenyl thiols (Aldrich) were ob-
tained and converted to sulfides by the known procedures.5
p-Bromo-, p-cyano-, p-fluoro-, and p-nitrophenyl methyl sul-
in CH CN at room temperature. The redox potentials of
3
complexes IIIa -IIIf were measured in CH CN using an
3
EG&G Princeton Applied Research Potentiostat/Galvanostat
model 273A equipped with an X-Y-t recorder and are given
in Table 1.
7,58
fides (Aldrich) were used as received. The other sulfides were
prepared and purified by the reported procedures.5
9-61
All
Reson a n ce Ra m a n Mea su r em en ts. Resonance Raman
(RR) measurements were carried out at room temperature
(
45) (a) Kobayashi, S.; Nakano, M.; Goto, T.; Kimura, T.; Schaap,
using acetonitrile solvent with excitation of the 488 nm line
of an Ar laser (model Stabilite 2017, Spectra Physics). The
+
A. P. Biochem. Biophys. Res. Commun. 1986, 135, 166. (b) Kobayashi,
S.; Nakano, M.; Kimura, T.; Schaap, A. P. Biochemistry 1987, 26, 5019.
excitation wavelengths match the phenolate-to-Fe charge-
transfer (ArOFe CT) bands. A SPEX 1404 double monochro-
mator with two 600-groove gratings blazed at 500 nm was used
to disperse the scattered light. A liquid nitrogen cooled charge-
coupled device (Princeton Instruments) with 576 × 378 pixels
was used as the multichannel detector. The recorded Raman
spectra were calibrated using known solvent bands as refer-
(
46) Doerga, D. R.; Cooray, N. M.; Brewster, M. E. Biochemistry
991, 30, 8960.
47) Naruta, Y.; Tani, F.; Maruyama, K. Tetrahedron: Asymmetry
991, 2, 533; J . Chem. Soc., Chem. Commun. 1990, 1378.
48) (a) Duboc-Toia, C.; Menage, S.; Ho, R. Y. N.; Que, L., J r.;
1
1
(
(
Lambeaux, C.; Fontecave, M. Inorg. Chem. 1999, 38, 1261. (b) Duboc-
Toia, C.; Menage, S.; Lambeaux, C.; Fontecave, M. Tetrahedron Lett.
997, 38, 3727.
1
-
1
ence, and the spectral resolution is estimated as 5 cm . The
concentration of the metal complex and the oxo form in the
(
49) Groves, J . T.; Viski, P. J . Org. Chem. 1990, 55, 3628.
(50) Chiang, L. C.; Konishi, K.; Aida, T.; Inoue, S. J . Chem. Soc.,
-
3
Chem. Commun. 1992, 254.
51) (a) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S.; Ioele, M.;
Raman experiments was about 1 × 10 M. The relative
(
Raman intensities were obtained after normalization of the
Steenken, S. J . Am. Chem. Soc. 1996, 118, 8973. (b) Ioele, M.; Steenken,
S.; Baciocchi, E. J . Phys. Chem. A 1997, 101, 2979.
3
spectra with the CH CN band. The laser power of the sample
was about 25-40 mW. No changes were observed in the RR
spectra during the accumulation period (about 1 h), and the
metal complex and the oxo species were stable throughout the
experiment. This is in agreement with the observation that
the absorption spectra measured before and after the RR
experiments were the same.
(
52) Baciocchi, E.; Lanzalunga, O.; Marconi, F. Tetrahedron Lett.
994, 35, 9771.
53) Baciocchi, E.; Lanzalunga, O.; Pirozzi, B. Tetrahedron 1997,
3, 12287.
54) (a) Gerloch, M.; Lewis, J .; Mabbs, F. E.; Richards, A. J . Chem.
1
(
5
(
Soc. A 1968, 112. (b) Gullotti, M.; Casella, L.; Pasini, A.; Ugo, R. J .
Chem. Soc., Dalton Trans. 1977, 339.
Kin et ic Mea su r em en t s. The absorption spectra of
iron(III)-salen complexes and their oxo forms were obtained
using a J ASCO model 7800 UV-vis spectrophotometer. The
progress of the reaction was monitored spectrophotometrically
by following the decay of the respective absorbance maximum
of oxo complexes IVa -IVf at definite time intervals at 298 K.
The reaction was followed up to 50% conversion. The pseudo-
(
(
(
(
(
55) Gerloch, M.; Mabbs, F. E. J . Chem. Soc. A 1967, 1598.
56) Hobday, M. D.; Smith, T. D. Coord. Chem. Rev. 1972/73, 9, 311.
57) Suter, E. M.; Hansen, H. L. J . Am. Chem. Soc. 1932, 54, 1401.
58) Gilman, H.; Beaber, N. J . J . Am. Chem. Soc. 1925, 47, 1450.
59) Bourgeois, M. M., Ed.; Abraham, A. Recl. Trav. Chem. Pays-
Bas 1911, 30, 407.
60) Leandri, G.; Mangini, A.; Passerine, R. Gazz. Chim. Ital. 1954,
4, 3.
61) Burton, H.; Hu, P. F. J . Chem. Soc. 1946, 68, 498.
(
8
(
1
first-order rate constant (k ) value for each kinetic run was

Iron(III)-Salen Complexes as Enzyme Models
J . Org. Chem., Vol. 67, No. 5, 2002 1509
computed from the linear least-squares plots of log(absorbance)
vs time. The rate constant for the formation of products from
the oxidant-substrate complex and the Michaelis-Menten
constants were evaluated using double-reciprocal plots of k
1
and the concentration of the substrates.
P r od u ct An a lysis. In a typical experiment, 0.5 mM
substrate (ArSMe) was added to a 0.5 mM solution of oxo-
3
(salen)iron complex in 6 mL of solvent (CH CN). The solution
was stirred at 298 K for 1-3 h depending upon the nature of
the sulfide and complex. At the end, the solvent was removed,
and the organic product was then extracted with ether. The
2 4
ether extract was dried over anhydrous Na SO , and the
solvent was removed. Then the resulting residue was analyzed
by IR. The IR spectrum of the product was found to be identical
with that of the sulfoxide (ArSOMe), exhibiting the SdO
stretching frequency in the characteristic region 1070-1030
-
1
cm . The GC analysis of the product using a model GC17A
Schimadzu gas chromatograph indicated the formation of
sulfoxide as the only product under the present experimental
conditions. Sulfides and sulfoxides are identified by their
characteristic retention times and also by co-injection with
authentic samples.
-
4
F igu r e 1. (Left) (a) Absorption spectrum of IIIa (5 × 10
M) in CH CN. (b) Absorption spectrum of oxo(salen)iron IVa
3
-
4
(
5 × 10 M). (c) Absorption spectrum after the addition of
MPS to IVa . (Right) Absorption spectral changes (successive
Resu lts a n d Discu ssion
-3
scans) for the oxidation of MPS (5 × 10 M) by the oxo(salen)-
-4
iron IVb (5 × 10 M) in CH
3
CN (a-e are for an initial increase
In the metal-ion-catalyzed oxygenation of organic
substrates with terminal oxidants, in some reactions it
has been established that the formation of oxometal
species from the metal ion and the oxidant becomes the
rate-determining step, thereby eluding the opportunity
to understand the mechanism of more important oxygen
atom transfer from oxometal ion to organic substrates.62
To avoid this difficulty, in the present study, oxo(salen)-
iron complexes IVa -IVf have been synthesized from
iron(III)-salen complexes IIIa -IIIf and PhIO and then
used for the oxygenation of organic sulfides (eq 1).
Absor p tion Sp ectr a l Stu d ies. The parent iron(III)-
of absorbance, and f-z are for a decrease of absorbance with
time).
F igu r e 2. (a) EPR spectrum of IIIa . (b) EPR spectrum of oxo-
(salen)iron IVa . (c) EPR spectrum after the addition of MPS
salen complex shows characteristic absorption at λmax
70 nm. This absorption maximum is sensitive to the
nature of the substituent in the salen ligand. Introduction
of a 5-NO group shifts the λmax value from 470 to 463
)
4
to IVa in CH
3
CN.
2
2
.22, and 2.80.63,64 We presume that the coordination of
two phenolate ions to the iron(III) in the iron(III)-salen
complexes makes the Fe state low spin, giving g values
nm, whereas 5-OMe causes a large shift from 470 to 500
nm (Table 1). The formation of oxoiron complex IVa is
inferred from the following experimental observations:
III
of 2.02 and 2.81. When complex IIId was converted into
its oxo form IVd , the weak signal at 2.02 disappeared
and the other signal at g ) 2.81 was shifted to 2.53. To
understand the effect of adding substrate on the EPR
spectrum of the oxoiron complex, we have recorded the
EPR spectrum of IVa in the presence of MPS. Addition
of MPS to the oxo(salen)iron IVa resulted in the slow
reappearance of the weak signal at g ) 2.02 (Figure 2).
This observation indicates the reaction of oxo(salen)iron
with the MPS, resulting in the formation of the corre-
sponding iron(III)-salen complex. In a recent report,
Yan et al.65 observed similar behavior in the iron(III)-
porphyrin-catalyzed hydrogen peroxide oxidation of
naphthol.
(
i) the dark color of the iron(III)-salen solution faded
upon the addition of PhIO and (ii) there was a change in
the shape of the spectrum and a substantial shift in the
λ
max value (Table 1, Figure 1 (left)). For the parent
complex, λmax shifts from 470 to 450 nm, and in the
methoxy-substituted complex, λmax shifts from 500 to 466
nm. When the substrate methyl phenyl sulfide (MPS) is
added to the oxoiron ion, a shift of 15 nm is noticed, i.e.,
from 450 to 465 nm (Figure 1 (left)). The spectral changes
observed for complex IVb are shown in Figure 1 (right).
Further addition of substrate to the oxoiron species
increases the absorbance initially followed by a regular
decrease (Figure 1 (right)).
EP R Sp ectr a l Stu d ies. The EPR spectrum of IIId
showed a broad signal at g ) 2.81 and a weak signal at
g ) 2.02 (Supporting Information Figure S1). The obser-
vation of the EPR signal confirms that the iron(III)
Reson a n ce Ra m a n Stu d ies. Resonance Raman stud-
ies were carried out at room temperature using excitation
at 488 nm. This wavelength coincides with the phenolate-
4
8
complex is in the monomeric form in solution. The
presence of bases such as N
a low-spin environment for iron(III). Experimental g
values reported in the presence of N
-
-
-
(63) (a) Lippard, S. J .; Berg, J . M. Principles of Bioinorganic
Chemistry; University Science Books: 1994; p 305. (b) Drago, R. S.
Physical Methods for Chemists, 2nd ed.; Saunders College Publish-
ing: 1992; p 582.
3
, CN , and OH produces
-
3
species are 1.72,
(64) (a) Sobolev, A. P.; Babushkim, D. E.; Talsi, E. P. J . Mol. Catal.,
A 2000, 159, 233. (b) Bertini, I.; Luchinat, C.; Parigi, G. Eur. J . Inorg.
Chem. 2000, 2473.
(65) Yan, Y.; Xiao, F. S.; Zheng, G.; Zhen, K.; Fang, C. J . Mol. Catal.,
A 2000, 157, 65.
(
62) Finney, N. S.; Pospisil, P. J .; Chang, S.; Palucki, M.; Konsler,
R. G.; Hansen, K. B.; J acobsen, E. N. Angew. Chem., Int. Ed. Engl.
997, 36, 1720.
1

1
510 J . Org. Chem., Vol. 67, No. 5, 2002
Sivasubramanian et al.
oxidation of FeIII with PhIO. The decrease in intensity
-
1
of all peaks in the range 1300-1700 cm may be taken
as evidence for the formation of a ligand cation radical.
The RR spectra of phenoxyl radical-metal complexes
-1
display one prominent peak at 1505-1525 cm together
-1
with a small one at around 1590 cm . However, previous
studies indicate that the phenoxyl radical shows quinoid
character with substantial delocalization of the unpaired
electron over the respective phenyl ring. Thus, the
-
1
absence of a peak at 1505-1525 cm may be due to
the delocalization of the unpaired electron. Thus, on the
basis of the RR spectral study, the oxo(salen)iron complex
IV
•+
may be represented as [OdFe (salen)] , similar to
compound I.
E lect r och em ica l P r op er t ies of Ir on (III) Com -
p lexes. The cyclic voltammograms of all the iron(III)-
salen complexes were recorded, and the reduction poten-
3
tial (E1/2) values in CH CN are collected in Table 1. It is
interesting to note that the E1/2 value is highly sensitive
to the nature of the substituent and the E1/2 values of
iron(III) complexes with 5,5′-substituents in the salen
ligand correlate well with the Hammett substituent σ
constants (Supporting Information Figure S2). The re-
activity of oxo(salen)iron complexes is in accordance with
the redox potential values of iron(III) complexes (vide
infra). Electron-withdrawing substituents on the salen
ligand are expected to destabilize the oxo(salen)iron
intermediate, making it a more reactive oxidant. On the
contrary, the electron-releasing substituents are expected
to stabilize the oxoiron species, attenuating its reactivity
and thus forming a relatively less reactive oxidant. This
can be clearly verified if the redox potentials of oxo-
(salen)iron complexes are measured. But our attempts
to measure the redox potentials of oxoiron complexes did
F igu r e 3. (a) Resonance Raman spectrum of IIIa . (b) Reso-
nance Raman spectrum of oxo(salen)iron IVa in CH CN.
3
to-Fe charge-transfer (ArO- f Fe CT) transition. Con-
III
+
cerning the parent salen complex [Fe (salen)] and its
IV
•+
oxo complex [OdFe (salen)] , the strongest RR bands
are in the region between 1000 and 1700 cm (Figure
), which includes the internal ring modes of the pheno-
late ligands as well as the CsO stretch vibrations. The
latter mode gives rise to a band at 1311 cm . According
to previous studies on a number of ferric phenolate
complexes, a frequency at 1305-1307 cm corresponds
to a CsO bond length of 1.32 Å, which agrees very well
with the values obtained from the crystallographic
analysis of Fe complexes. Apart from the bands at 1311
cm , the RR spectrum is dominated by two more bands,
-
1
IV
V
not succeed due to the instability of Fe and Fe species
under the electrochemical reaction conditions.
Kin etic Resu lts. The kinetics of oxygenation of or-
3
6
6
-
1
3
ganic sulfides with oxo(salen)iron complexes in CH CN
were followed under pseudo-first-order conditions in the
presence of at least a 10-fold excess of substrate over the
oxidant by measuring the change in the absorbance at
the absorption maximum of the oxo(salen)iron complex
with time at 298 K. A sample run for the oxygenation of
MPS with IVb is shown in Figure 1 (right). The obser-
vance of an initial increase (Figure 1 (right, a-e) and
shift in the absorbance followed by a regular decrease
-
1
III
-
1
-
1
one in the range 1410-1445 cm and another in the
-
1
range 1600-1640 cm
.
An interesting observation in the RR spectrum of the
(Figure 1 (right, f-z) after the addition of sulfide to the
III
iron oxo complex compared to the Fe complex is that a
oxidant is interesting. This may be taken as supporting
evidence for the coordination of substrate to the oxidant,
and the spectral changes are shown in Figure 1 (right).
The initial increase in the absorbance of the oxidant due
to the addition of substrate deserves comment. The shift
in the λmax value of the oxo(salen)iron complex in the
presence of MPS from 450 to 465 nm indicates that the
interaction between the oxidant and substrate is of
charge-transfer type. It is known that organic sulfides
form charge-transfer (CT) complexes with oxidants such
dramatic decrease is observed in the intensities of all
-
1
modes in the range of 1300, 1400, and 1600 cm . This
III
behavior has previously been noted with Fe -porphyrin
complexes, and the reduced intensity is characteristic of
porphyrin π-cation formation. An additional point that
has to be noted with the iron oxo complex is the
appearance of a low-frequency band at 839 cm-1. It is
interesting to recall that the low-frequency spectrum
IV
•+
of [OdFe (TMP )]ClO exhibits a polarized ν(FedO)
4
-1
mode at 835 cm . Surprisingly the compound II ana-
logue [OdFe (TMP)] also has a band in this region,
ν(FedO) ) 843 cm
observed in the present study at 839 cm can be taken
68
as nitrosonium ion. Such CT complex formation is
IV
68
usually too fast to observe. In the present reaction we
-
1 67
.
Thus, the low-frequency band
are able to observe the initial increase only with complex
IVb and that too with less reactive sulfide MPS and aryl
methyl sulfides carrying electron-withdrawing groups. In
the presence of sulfides with electron-releasing groups
and other oxo(salen)iron complexes, the complex forma-
-
1
IV
as evidence for the formation of OdFe due to the
(
66) Hockertz, J .; Steenken, S.; Wieghardt, K.; Hildebrandt, P. J .
Am. Chem. Soc. 1993, 115, 11222.
67) Czarnecki, K.; Nimri, S.; Gross, Z.; Proniewicz, L. M.; Kincaid,
J . R. J . Am. Chem. Soc. 1996, 118, 2929.
(
(68) Bosch, E.; Kochi, J . K. J . Org. Chem. 1995, 60, 3172.

Iron(III)-Salen Complexes as Enzyme Models
J . Org. Chem., Vol. 67, No. 5, 2002 1511
Ta ble 2. Ra te Con sta n ts k a n d Mich a elis Con sta n ts KM Obta in ed fr om Mich a elis-Men ten Kin etics for Oxo(sa len )ir on
Com p lexes IVa -IVf Oxid a tion of Ar SR in CH3CN a t 298 Ka
a. Rate Constants
1
03k, s-1
X in
no.
p-XC6H4SCH3
IVa
4.74
13.3
6.69
3.03
1.57
1.46
0.56
0.49
0.32
IVb
IVc
8.91
20.4
15.6
6.49
3.66
2.59
1.61
1.05
0.84
IVd
0.52
IVe
0.63
IVf
0.32
1
2
3
4
5
6
7
8
9
H
17.5
32.1
22.9
15.2
12.2
11.5
8.97
4.49
4.04
OMe
Me
F
Cl
Br
COCH3
CN
NO2
1.13
0.95
0.47
0.33
0.30
0.18
0.13
0.09
1.04
0.87
0.55
0.42
0.41
0.33
0.25
0.21
0.82
0.56
0.25
0.15
0.12
0.06
0.03
0.02
F
r
-1.51 ( 0.09
0.989
-0.81 ( 0.05
0.987
-1.37 ( 0.08
0.988
-1.02 ( 0.03
0.997
-0.65 ( 0.03
0.991
-1.54 ( 0.05
0.997
b. Michaelis Constants
1
03 KM
X in
no.
p-XC6H4SCH3
IVa
IVb
IVc
IVd
IVe
IVf
1
2
3
4
5
6
7
8
9
H
121.0
88.1
55.7
76.4
42.1
45.9
20.6
30.2
16.7
42.9
0.769
14.5
38.8
32.0
36.8
39.4
17.4
21.4
46.7
21.0
68.2
41.8
28.2
26.3
14.7
26.8
17.2
37.9
26.9
18.3
17.8
8.31
4.41
2.38
11.8
1.78
15.8
11.3
9.23
9.83
12.2
10.2
9.04
14.0
16.4
18.5
9.85
13.0
10.7
5.77
3.66
6.00
OMe
Me
F
Cl
Br
COCH3
CN
NO2
9.31
26.9
a
General conditions: [OdFe(salen)Cl] ) 5 × 10-4 M.
tions, and the kinetic results are shown in Figure 4.
These results indicate that the rate of the reaction
increases with an increase in substrate concentration but
attains saturation at high substrate concentration. The
inference from this saturation kinetics is that the
substrate binds to the oxidant before the rate-control-
ling step. Thus, the redox reaction proceeds through
Michaelis-Menten kinetics (eqs 2 and 3).
oxidant + substrate h complex
(2)
(3)
k
complex
9
8 products
On the basis of the Michaelis-Menten formulation, the
observed rate constant is given by eq 4. The rearrange-
ment of eq 4 leads to eq 5,
k[substrate]
F igu r e 4. k
1
vs [sulfide] for the oxidation of p-XC
6
H
4
SMe with
k1 ) kobsd
)
(4)
(5)
KM + [substrate]
IVb. The points are referred to by the same numbers as in
Table 2a. [IVb] ) 5 × 10 M.
-
4
1
1
KM
)
+
tion seems to be fast and we are not able to observe the
initial increase in absorbance. Though the measurement
of the equilibrium constant for the formation of an
oxidant-substrate complex with the oxidant IVb and the
substrates mentioned above is possible, to maintain
uniformity, we did not attempt it at this stage. The
nature of the complex formed between the oxidant and
substrate is not known, and it requires further study,
which will be taken up in the future.
k1
k
k[substrate]
where K
rate of oxidation of the substrate.
From the plot of 1/k vs 1/[substrate] (Supporting
M
is the Michaelis-Menten constant and k is the
1
M
Information Figure S3) the values of k and K have been
evaluated and collected in Table 2.
Su bstitu en t Effects. The sulfoxidation is highly
sensitive to the nature of the substituent in the phenyl
ring of phenyl methyl sulfides and in the phenolic part
of the salen ligand (Table 2a). The introduction of
substituents in the para position of the phenyl ring of
phenyl methyl sulfide alters the rate appreciably; i.e., the
electron-releasing substituents accelerate the rate, and
the electron-withdrawing groups decelerate it. To under-
stand the extent of charge separation in the transition
The rate constant of the oxygenation reaction is
measured from the decrease in absorbance with time
(
Figure 1 (right, f-z). The reaction is found to be first-
order in the oxidant, which is evident from the linear log-
absorbance) vs time plots. The dependence of the reac-
(
tion on substrate concentration is studied by measuring
the rate of the reaction at different substrate concentra-

1
512 J . Org. Chem., Vol. 67, No. 5, 2002
Sivasubramanian et al.
Sch em e 2
to the formation of sulfoxide as shown in eqs 7 and 8. In
most of the other oxidation reactions of sulfides, kobsd
F igu r e 5. Hammett plot for the oxidation of para-substituted
phenyl methyl sulfides with IVb in CH CN at 298 K. The
points are referred to by the same numbers as in Table 2a.
3
depends linearly on the substrate concentration and no
13,35-38
saturation kinetics is observed.
This linear depen-
-
4
[IVb] ) 5 × 10 M.
dence on substrate was taken as evidence for large K
values (loose enzyme-substrate complexes). The satura-
tion kinetics observed here and low K values (Table 2b)
M
state, the k values collected in Table 2a are analyzed in
terms of the Hammett equation. The correlation of log k
values of aryl methyl sulfides with the Hammett constant
σ is good (r ) 0.987, n ) 9) (Figure 5), and the reaction
constant F is in the range -0.65 to -1.54 for different
oxo(salen)iron complexes. The F value for the reaction of
sulfides with each oxo complex is given at the bottom of
M
indicate strong binding of substrate with the oxidant.
Such saturation kinetics has been observed only in some
III 48
reactions involving Fe . The k/K
M
values are in the
range 0.2-1.6 with iron(III) complex IIIb, and these
values are better than the values observed with HRP and
human cytochrome P450 A12.2
8,70
+
-
Table 2a. The correlation is not improved if σ /σ values
As the iron(III)-salen complex synthesized in the
present study is a five-coordinate complex carrying a
chloride ion, the addition of PhIO leads to the formation
of the oxo complex IVa , which is six-coordinated.The
effect of added imidazole (Im) on the rate of the oxidation
reaction also provides a clue to the mechanism of the
reaction. In the presence of Im (0.05 M), the reaction is
not taking place between IVa and MPS. This behavior
of Im may be accounted for in two ways. Im forms a
hydrogen bond with the oxygen of the oxo species, thus
preventing the coordination of sulfide. We have already
emphasized that coordination of the substrate to the
oxidant is a necessary condition for the reaction to occur.
Alternatively Im may lead to the formation of oxo dimer
+
are used instead of σ (r ) 0.963, n ) 9, F ) -0.45) for
complex IVb. The effect of introducing substituents in
the salen ligand on the rate of reaction is also investi-
gated (Table 2a), the observed kinetic data are also
treated in terms of the Hammett equation (Supporting
Information Figure S4), and the F value is positive (F )
0
.80). When we compare this F value with the value
IV
obtained for the Ru dO oxidation of organic sulfides
(
F ) 0.49), we realize that the F value measured in the
69
present study is higher. An electron-transfer mecha-
nism has been postulated for the Ru dO oxidation of
IV
organic sulfides. It is interesting to compare the results
of log k vs Eox plots (Supporting Information Figure S5)
(
slope -1.70) obtained here for complex IVb with the
2
O[Fe(salen)Cl] , which is not able to oxidize the sulfide.
13
recent observations made by Goto et al. on the sulfoxi-
dation catalyzed by high-valent intermediates of heme
enzymes. They observed a slope of -10.5 when the
reaction proceeded through an electron-transfer mecha-
nism and -2.2 in the case of the reaction proceeding via
direct oxygen transfer.
Thus, the effect of Im also supports direct oxygen atom
transfer from the oxidant to the substrate as the mech-
anism of the reaction. Direct oxygen transfer from the
oxidant to the substrate has been postulated in the
iron(III)-porphyrin-catalyzed hydrogen peroxide oxida-
tion of organic sulfide.71 We also observe slopes of
Mech a n ism of th e Rea ction . The experimental
observations presented above can be interpreted with the
mechanism shown in Scheme 2.
-
1.70 to -2.92 in the plots of log k vs Eox (Supporting
13
Information Figure S5) as observed by Goto et al. for a
reaction proceeding via direct oxygen transfer. As the
The observations are that (i) the reaction is fractional-
order in the substrate and attains saturation kinetics at
high substrate concentration and (ii) the shift in the λmax
and the initial increase in the absorbance on the addition
of the substrate to the oxo(salen)iron complex indicate
the formation of a complex between the oxidant and
substrate. Thus, with the addition of sulfide, initial
complex formation takes place via oxygen, finally leading
3
reaction is carried out in cent percent CH CN and the
rate is not influenced by the presence of oxygen, it is
evident that the oxygen of the sulfoxide is only from oxo-
(salen)iron.
Selective Oxid a tion of Su lfid es to Su lfoxid es. To
check the utility of this system as a useful method for
(70) Savenkova, M. I.; Kuo, J . M.; Ortiz de Montellano, P. R.
Biochemistry 1998, 37, 10828.
(71) Marques, A.; di Matteo, M.; Ruasse, M. F. Can. J . Chem. 1998,
76, 770.
(
69) Acquaye, J . H.; Muller, J . G.; Takeuchi, K. J . Inorg. Chem. 1993,
3
2, 160.

Iron(III)-Salen Complexes as Enzyme Models
J . Org. Chem., Vol. 67, No. 5, 2002 1513
Ta ble 3. Selective Oxid a tion of p-XC6H4SCH3 in CH3CN
Sch em e 3
a
a t 298 K by IVa a n d IVb
IVa
reaction time
IVb
reaction time
%
%
b
sulfoxideb
X
(min)
sulfoxide
(min)
H
OMe
Me
F
Cl
Br
COCH3
CN
120
90
88
85
100
95
78
77
39
41
49
60
50
60
60
60
60
60
80
80
100
100
100
85
75
68
70
47
45
120
120
120
120
120
140
140
NO2
a
Analyzed by GC; error limit (5%. b Only sulfides and sulfox-
ides are present in the reaction mixture; the difference is that of
the unreacted sulfide only.
reactivity of similar reactants has already been proposed
in several cases.73
Com p a r ison w ith Oxo(sa len )Mn , Oxo(sa len )Cr ,
a n d Oxo(sa len )R u V Oxid a t ion s. The oxygenation of
organic sulfides with oxo(salen)metal complexes has been
V
V
the conversion of sulfides to sulfoxides and also to confirm
the mechanism of the reaction as direct oxygen atom
transfer rather than electron transfer, we have measured
the yield of sulfoxide formed by taking several substituted
phenyl methyl sulfides and two oxidants, IVa and IVb,
and the details of the percentage of sulfoxide formed are
given in Table 3. The conversion of sulfide to sulfoxide
is efficient and selective and depends on the nature of
the substituent in the substrate and the oxidant. The
high yield of sulfoxide also supports the proposed mech-
anism. If the reaction proceeds through an electron-
transfer mechanism, the yield of sulfoxide may be low.
It is pertinent to mention that Goto et al.13 could get only
extensively investigated in this laboratory, and the
35-37
results on oxo(salen)manganese(V),
oxo(salen)chro-
38
39
mium(V), and oxo(salen)ruthenium(V) oxidation of
sulfides have already been reported. It is worthwhile to
mention the differences between the results observed in
the present study and those reported earlier. The reac-
tions with oxo(salen)chromium(V) and oxo(salen)manga-
nese(V) complexes follow simple second-order kinetics,
first-order each in the oxidant and the substrate, and the
V
reactions involve large F values (-2.6 with Cr and -1.8
V
with Mn ). To account for these results an electrophilic
attack of the oxygen of the oxidant at the sulfur center
of the sulfide in the rate-controlling step has been
postulated. On the other hand, the results with oxo-
(salen)ruthenium ion oxidation of organic sulfides are
similar to those of the present study; i.e., both iron and
ruthenium complex oxidations proceed through satura-
tion kinetics, i.e., Michaelis-Menten kinetics, and the F
2
5% sulfoxide from the HRP compound I oxidation of
organic sulfide when an electron-transfer mechanism was
proposed and more yield when the reaction proceeded
through direct oxygen transfer. A mechanism similar to
this has also been proposed for the oxidation of alkanes
and alkenes catalyzed with iron porphyrins.7
,72
V
V 38
value (F ) -0.6 with Ru ) is low compared to that of Cr
Rea ctivity-Selectivity P r in cip le. When we com-
pare the reactivity (rate constant values) of the four oxo-
V 35-37
and Mn
oxidations. It is pertinent to mention that
the coordination of the substrate to the oxidant may
facilitate chiral induction in the product if the oxidant
carries a chiral center. Thus, the present redox system
may be tested as a good method for the preparation of
chiral sulfoxides if oxoiron complexes carrying chiral
ligands are used as the oxidants. Several chiral oxo-
(
salen)iron complexes IVa -IVd (as the substituents are
introduced in different positions in the salen ligand of
complexes IVe and IVf, they are not included in this
analysis) and their selectivity (the reaction constant F),
an interesting trend is observed, the exception being for
IVd . Among the three complexes IVa -IVc, there is an
inverse relationship between the reactivity and selectiv-
ity; i.e., the reactivity-selectivity principle (RSP) is
applicable to this redox system. The detailed results on
the application of the RSP to this redox system will be
reported separately. It is worthwhile to recall that the
RSP has been successfully applied to the reactions of oxo-
(
salen)manganese(V) complexes have successfully been
used for the synthesis of chiral sulfoxides. Recently,
Fontecave and co-workers48 have established that H
O -
2
2
dependent oxidations catalyzed by non-heme diiron
complexes can proceed through metal-based pathways
and can thus be made stereoselective. The synthesis of
chiral oxoiron complexes and their use in the synthesis
of chiral sulfoxides will be taken up in the future.
(salen)manganese(V) and oxo(salen)chromium(V) oxida-
3
6-38
tion of organic sulfides and sulfoxides.
Thus, all these
oxo(salen)metal complexes seem to proceed through a
common mechanism, i.e., direct oxygen transfer from the
oxidant to the substrate. The different behavior of IVd
may indicate the operation of a different mechanism for
the reaction. As this complex is least reactive among the
four complexes, it should have the highest F value. The
different behavior of IVd tempts us to postulate an
electron-transfer mechanism for the oxidation of organic
sulfides with IVd (Scheme 3). The change of mechanism
Ack n ow led gm en t. We thank Prof. C. Srinivasan
for reading the manuscript, offering valuable sugges-
tions, giving constant encouragement, and showing his
interest in this work. We thank Prof. S. Umapathy,
Department of Inorganic and Physical Chemistry,
Indian Institute of Science, Bangalore, and Prof. J .
Subramanian and Prof. Sambasiva Rao, Department of
Chemistry, Pondicherry University, Pondicherry, for
their help with the RR and EPR measurements. S.R.
thanks the CSIR and UGC-DRS program for financial
support, and R.R. acknowledges the partial financial
N
from S 2 to electron transfer with the change in the
(
72) Ogliaro, F.; Filatov, M.; Shaik, S. Eur. J . Inorg. Chem. 2000,
2
455.
(73) Marcus, R. A. J . Phys. Chem. A 1997, 101, 4072.

1
514 J . Org. Chem., Vol. 67, No. 5, 2002
Sivasubramanian et al.
support from the DST. V.K.S. and M.G. thank the
University Grants Commission, New Delhi, for selecting
them to do research under the Faculty Improvement
Program (FIP) and the management and principal of
Vivekananda College, Tiruvedakam West, for providing
FIP study leave. We also thank Prof. K. Pitchumani and
Dr. R. Sevvel for their help with the product analysis.
of E1/2 vs 2σ values of complexes IIIa -IIId , plots of 1/k
vs 1/[sulfide] for the oxidation of p-XC SMe with IVb,
1
6
H
4
Hammett plot for the oxidation of p-acetylphenyl methyl
sulfide by oxo(salen)iron complexes IVa -Ivd , and plots of log
k values for the reaction of oxo(salen)iron complexes IVa -IVd
with sulfides against the oxidation potential of sulfides, Eox
.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Su p p or tin g In for m a tion Ava ila ble: EPR spectra of
complex IIId and its oxo form in CH
3
CN, Hammett plot
J O010878O