Formation, stability, and reactivity of a mononuclear nonheme oxoiron(IV) complex in aqueous solution

Article abstract of DOI:10.1039/b415507f

A mononuclear nonheme oxoiron(IV) complex bearing a pentadentate N5 ligand was prepared in aqueous solution; the pH dependence of its stability and reactivities was reported along with the mechanistic details of sulfide oxidation by the oxoiron(IV) species. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415507f

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Formation, stability, and reactivity of a mononuclear nonheme
oxoiron(IV) complex in aqueous solution{
Chivukula V. Sastri, Mi Sook Seo, Mi Joo Park, Kwan Mook Kim and Wonwoo Nam*
Received (in Cambridge, UK) 6th October 2004, Accepted 5th January 2005
First published as an Advance Article on the web 27th January 2005
DOI: 10.1039/b415507f
water, H218O (95% 18O-enriched), a mass peak corresponding to
A mononuclear nonheme oxoiron(IV) complex bearing a
[Fe(IV)(18O)(N4Py)(ClO4)]+ appeared at m/z of 540.1 (Fig. 1b,
pentadentate N5 ligand was prepared in aqueous solution;
inset), indicating that 1 exchanges its oxygen with H218O.7 Further,
the pH dependence of its stability and reactivities was reported
although 1 was stable for several days (t1/2 # 48 h) at 25 uC, the
intermediate reacted with thioanisole and reverted back to the
starting [FeII(N4Py)]2+ complex with a clear isosbestic point at
along with the mechanistic details of sulfide oxidation by the
oxoiron(IV) species.
520 nm (Fig. 1a). Product analysis of the reaction mixture revealed
that methyl phenyl sulfoxide was yielded quantitatively.
Mononuclear nonheme iron enzymes are involved in many
oxidative metabolic pathways in nature.1 Our understanding of
the catalytic transformations of the enzymes, especially the nature
of active oxidizing species, has increased dramatically with the
intensive mechanistic studies of the enzymes and their model
compounds.2 For example, a high-valent iron(IV)-oxo intermediate
has been characterized as an active oxidizing species in Escherichia
coli taurine: a-ketoglutarate-dependent dioxygenase (TauD).3 In
biomimetic studies, Mu¨nck, Nam, Que, and their co-workers have
reported the generation and isolation of mononuclear nonheme
oxoiron(IV) complexes bearing tetradentate N4 and pentadentate
N5 ligands.4 A high-resolution X-ray structure of a nonheme
We then investigated the dependence of the formation and
stability of 1 on the pH of reaction solutions, by carrying out
reactions of [FeII(N4Py)]2+ and KHSO5 at different pH values in
buffered H2O–CH3CN mixture (v/v 5 3 : 1) at 5 uC. The
formation of 1 was observed in the pH range 5–9," and the UV-vis
spectra of 1 in buffered solutions were identical to that observed in
oxoiron(IV)
intermediate,
[Fe(IV)(O)(TMC)(NCCH3)]2+
(TMC 5 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane),
has been obtained for the first time,4a and other nonheme
oxoiron(IV) complexes have been well characterized with various
spectroscopic techniques.4,5 In addition, it has been demonstrated
that nonheme oxoiron(IV) complexes are capable of oxygenating
various organic substrates such as PPh3, thioanisole, olefins, and
alkanes.4,5 Since nonheme oxoiron(IV) complexes reported so far
have been prepared only in organic solvents (e.g., CH3CN), we
decided to explore the chemistry of nonheme oxoiron(IV) species in
aqueous solution.6 In this communication, we report the first
example of a mononuclear nonheme oxoiron(IV) complex bearing
a
pentadentate
N5
ligand,
[(N4Py)FeIVLO]2+
(1)
(N4Py 5 N,N-bis(2-pyridylmethyl)-bis(2-pyridyl)methylamine),{
in aqueous solutions.
The reaction of [FeII(N4Py)(ClO4)2] with oxidants such as
peracetic acid (CH3CO3H), potassium monopersulfate (KHSO5),
iodosylbenzene (PhIO), and H2O2 at 25 uC in H2O–CH3CN
mixture (v/v 5 3 : 1) produces a pale green intermediate 1 with a
maximum absorption wavelength lmax at 680 nm (Fig. 1a, bold
red color).§4c The electrospray ionization mass spectrum (ESI MS)
of 1 exhibits a prominent ion at a mass-to-charge ratio (m/z) of
538.1, whose mass and isotope distribution pattern corresponds to
[Fe(IV)(O)(N4Py)(ClO4)]+ (calculated m/z of 538.1) (Fig. 1b).
When the identical reaction was carried out in isotopically labeled
Fig. 1 (a) UV-vis spectral change of 1 (2 mM) upon addition of 10 equiv.
thioanisole at 25 uC. Scan interval: 10 s (first scan was 10 s after the
addition of thioanisole). Inset shows time course of the decay of 1
monitored at 680 nm. (b) Electrospray ionization mass spectrum of 1
prepared in the reaction of FeII(N4Py)(ClO4)2 (2 mM) and CH3CO3H
(4 mM) at 25 uC. The mass peak at m/z of 220.3 corresponds to
[Fe(III)(OH)(N4Py)]2+. Insets show observed isotope distribution patterns
for [Fe(IV)(O)(N4Py)(ClO4)]+ (left panel) and [Fe(IV)(18O)(N4Py)(ClO4)]+
(right panel).
{ Electronic supplementary information (ESI) available: Experimental
details, Tables S1–S2, and Figs. S1–S3. See http://www.rsc.org/suppdata/
cc/b4/b415507f/
*wwnam@ewha.ac.kr
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the H2O–CH3CN mixture (Fig. 1a). Interestingly, the stability of 1
was found to depend significantly on the pH of reaction solutions
(Fig. 2a), in which 1 was stable at low pH (i.e., pH 5–6) but
decayed at a fast rate with increasing pH of the reaction solutions.
To the best of our knowledge, this study provides the first direct
observation that the stability of high-valent iron-oxo complex is
controlled by the pH of reaction solutions. Although it may be
suggested that the increased stability of 1 at low pH results from
the protonation of the iron(IV)-oxo group,8 more detailed
investigations are needed to understand the effect of pH on the
stability of 1.
us to determine kobs values to be 4.6(2) 6 1023 s21, 6.2(3) 6
1023 s21, and 7.3(3) 6 1023 s21 at pH 5, 6, and 7, respectively,
showing very little dependence on the pH of reaction solutions in
the oxo transfer from 1 to sulfide. When pseudo-first-order rate
constants determined in the reactions of 1 and various para-
substituted thioanisoles at pH 5 were plotted against sp, a good
correlation was observed with Hammett r value of 21.0(2)
(Fig. 2b; See ESI{, Table S1 for kobs values). The negative r value
indicates the electrophilic character of the oxo group of 1 in
oxygen atom transfer reactions.9 Further, the pseudo-first-order
rate constants increased proportionally with the thioanisole
concentration, leading us to determine second-order rate constants
for the para-substituted thioanisoles (See ESI{, Fig. S1 and
Table S2). By plotting the second-order rate constants against the
one-electron oxidation potentials (Eoox) of the sulfides, a linear
line with the slope of –2.0(3) was obtained (See ESI{, Fig. S2 and
Table S2). Such a small number indicates that the oxidation of
sulfides by 1 occurs via direct oxygen atom transfer mechanism
rather than involving an electron-transfer process.10 Finally, by
determining the rate constants for the oxidation of thioanisole by 1
from 283 to 308 K at pH 5, we were able to calculate activation
parameters of DH{ 5 13(2) kcal mol21 and DS{ 5 225(2) cal
mol21 K21 (See ESI{, Fig. S3).
The pH dependence of the reactivity of 1 in oxygen atom
transfer reactions was also examined in the pH range 5–7,I by
carrying out thioanisole oxidation by 1 in buffered H2O–CH3CN
mixture (v/v 5 3 : 1) at 10 uC. Upon addition of 10 equiv.
thioanisole to the solution of 1, the intermediate reverted back to
the starting [FeII(N4Py)]2+ complex with a clear isosbestic point at
520 nm (Fig. 1a), yielding the corresponding sulfoxide product
quantitatively. Pseudo-first-order fitting of the kinetic data allowed
In summary, we have reported the generation and reactivities of
a
synthetic mononuclear nonheme oxoiron(IV) complex at
different pH values in aqueous solution. By using the in-situ
generated oxoiron(IV) intermediate directly in oxygen atom
transfer reactions, we were able to provide mechanistic details of
sulfide oxidation in a nonheme iron model system.
This work was supported by the Ministry of Science and
Technology of Korea through the Creative Research Initiative
Program.
Chivukula V. Sastri, Mi Sook Seo, Mi Joo Park, Kwan Mook Kim and
Wonwoo Nam*
Department of Chemistry, Division of Nano Sciences, and Center for
Biomimetic Systems, Ewha Womens University, Seoul 120-750, Korea.
E-mail: wwnam@ewha.ac.kr; Fax: +82 2 3277 4441; Tel: +82 2 3277 2392
Notes and references
{ Although other nonheme oxoiron(IV) complexes such as
[(TMC)FeIVLO]2+ were generated in aqueous solutions, reactivity studies
were performed only with an oxoiron(IV) complex bearing a pentadentate
N5 ligand, to avoid a possible involvement of buffers as a ligand that may
change chemical properties of oxoiron(IV) species in different buffer
solutions.
§ It is worth noting that the formation of 1 was faster in aqueous solution
than in acetonitrile solution. For example, the formation of 1 in the
reaction of [FeII(N4Py)]2+ and CH3CO3H took y30 s and y25 min in
aqueous and acetonitrile solutions, respectively.
" Since the reaction solution became turbid below pH 5 and above pH 9
upon addition of oxidants to the solution of FeII(N4Py)(ClO4)2, the
reactivity studies were performed only in the pH range of 5–9.
I Due to the fast natural decay of 1 at high pH, we were unable to examine
its reactivity with substrate at pH 8 and 9 (see Fig. 2a).
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