Oxygen-atom transfer between mononuclear nonheme iron(IV)-oxo and iron(II) complexes

Article abstract of DOI:10.1002/anie.200504422

(Figure Presented) Completing the move: The intermetal transfer of an oxygen atom from nonheme iron(IV)-oxo to iron(II) complexes (see example, purple: Fe, red: O, blue: N, gray: C) has been unambiguously evidenced spectrophotometrically, mass spectrometr

Full text of DOI:10.1002/anie.200504422

Communications
Enzyme Models
DOI: 10.1002/anie.200504422
Oxygen-Atom Transfer between Mononuclear
Nonheme Iron(IV)–Oxo and Iron(II)
Complexes**
Chivukula V. Sastri, Kyungeun Oh, Yoon Jin Lee,
Mi Sook Seo, Woonsup Shin, and Wonwoo Nam*
High-valent iron(IV)–oxo species have been implicated as the
key reactive intermediates in the catalytic oxidation of
organic substrates by mononuclear nonheme iron enzymes
and their model compounds.[1] Recently, such nonheme
iron(IV)–oxo species were observed directly in enzymatic
and biomimetic reactions.[2–5] For example, an intermediate
with a high-spin iron(IV)–oxo unit was identified and
proposed as an active oxidant in the catalytic cycle of
[*] Dr. C. V. Sastri,[+] K. Oh,[+] Y. J. Lee,[+] Dr. M. S. Seo, Prof. Dr. W. Nam
Department of Chemistry
Division of Nano Sciences
Center for Biomimetic Systems
Ewha Womans University, Seoul 120-750 (Korea)
Fax: (+82)2-3277-4441
E-mail: wwnam@ewha.ac.kr
Prof. Dr. W. Shin
Department of Chemistry and Interdisciplinary Program of Inte-
grated Biotechnology
Sogang University, Seoul 121-742 (Korea)
[+] These authors contributed equally to this work.
[**] This research was supported by the Korea Science and Engineering
Foundation through the Creative Research Initiative Program.
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E. coli taurine, namely in a-ketoglutarate dioxygenase
(TauD).[2,3] In biomimetic studies, mononuclear nonheme
iron(IV)–oxo complexes bearing tetradentate N4 and penta-
dentate N5 ligands were synthesized and characterized by
various spectroscopic techniques including X-ray crystallog-
raphy.[4,5] The iron(IV)–oxo species have shown reactivities in
a variety of oxidation reactions such as alkane hydroxyla-
tion,[4c,5b,c] olefin epoxidation,[4b,5b,c] alcohol oxidation,[4h] and
the oxidation of sulfides,[4e,i] dihydroanthracene,[4f,k] and
PPh3.[4a,e,j,5c]
Very recently, Golubkov and Gross reported the transfer
of a nitrogen atom between manganese complexes of salen
(N,N’-bis(salicylidene)ethylenediamine dianion), porphyrin,
and corrole ligands.[6] Prior to this work, intermetal transfer of
a nitrogen atom was well documented in metalloporphyrin
(M = Mn and Cr) reactions.[7] In addition to the transfer of
nitrogen atoms, the transfer of oxygen atoms between high-
valent metal-oxo and metal complexes has also been demon-
strated.[7,8] In the case of iron porphyrins, the reaction of
iron(IV)–oxo and iron(II) porphyrins resulted in the gener-
ation of m-oxo-bridged iron(III) porphyrin dimers (that is,
incomplete intermetal transfer of an oxygen atom) [Eq. (1),
Figure 1. UV/Vis spectral changes showing the formation of 2 (blue)
upon addition of 4 equiv [Fe(tmc)(CF3SO3)2] (2 mm, diluted in 50 mL
2+
CH3CN) to the solution of [(N4Py)FeIV O] (1, 0.5 mm; red) in
=
CH3CN (2 mL) at 258C.
transfer of an oxygen atom from 2 to [Fe(N4Py)]2+ does not
occur [namely, the reverse reaction of Eq. (3) does not occur].
The transfer of an oxygen atom between nonheme iron
complexes was also evidenced by changes in the electrospray
ionization mass spectra of the reaction solution recorded at
different times (Figure 2). Mixing equimolar amounts of
[Fe(tmc)]2+ and 1-18O resulted in a decrease in the signals
corresponding to [Fe(tmc)]2+ (m/z 461) and 1-18O (m/z 590)
with a concomitant increase in the signals corresponding to 2-
18O (m/z 479) and [Fe(N4Py)]2+ (m/z 572). Such mass spectral
½ðPorpÞFeIV¼OŠ þ ½FeIIðPorpފ ! ½ðPorpÞFeIII-O-FeIIIðPorpފ
ð1Þ
Porp = porphyrin].[7a,9] Although the oxidation of organic
substrates by nonheme iron(IV)–oxo complexes has been
demonstrated recently, the intermetal transfer of oxygen
atoms between nonheme iron complexes has never been
explored previously. Herein, we report that in contrast to iron
porphyrins,[9] nonheme iron(IV)–oxo complexes transfer their
oxygen atom to iron(II) complexes, thus affording the
corresponding iron(II) and iron(IV)–oxo complexes (that is,
complete intermetal transfer of an oxygen atom occurs)[7a]
[Eq. (2)].
2þ
2þ
½ðL1ÞFeIV¼OŠ þ ½FeIIðL2ފ
ð2Þ
2þ
2þ
! ½FeIIðL1ފ þ ½ðL2ÞFeIV¼OŠ
The oxygen-atom transfer reactions were carried out by
adding nonheme iron(II) complexes to in situ generated
nonheme iron(IV)–oxo complexes in CH3CN at 258C. The
2+
addition of [FeII(tmc)]2+ to the solution of [(N4Py)FeIV O]
=
(1) afforded changes in the absorption spectra which are
consistent with the transfer of an oxygen atom from 1 (lmax at
695 nm) to [FeII(tmc)]2+ [Eq. (3)], thereby producing
2þ
2þ
½ðN4PyÞFeIV¼OŠ ð1Þ þ ½FeIIðtmcފ
ð3Þ
2þ
2þ
! ½FeIIðN4Pyފ þ ½ðtmcÞFeIV¼OŠ ð2Þ
[(tmc)FeIV O] (2; lmax at 820 nm) and [FeII(N4Py)]2+ (lmax
at 450 nm; Figure 1; see the Supporting Information for
nonheme ligand structures).[10] Well-defined isosbestic points
were observed at 570 and 790 nm. The reaction was complete
within 20 minutes, and an increase in the concentration of the
reactants diminished the reaction time (data not shown). The
latter results indicate the reaction to be first order in each of
the reactants. In contrast, the intermediate 2 remained intact
upon addition of [Fe(N4Py)]2+, thus indicating that the
2+
=
Figure 2. Changes in the electrospray ionization mass spectra in the
reactants [FeII(tmc)(CF3SO3)]+ (m/z 461) and [FeIV(N4Py)(18O)-
(CF3SO3)]+ (1-18O; m/z 590) as well as the products [FeIV(tmc)(18O)-
(CF3SO3)]+ (2-18O; m/z 479) and [FeII(N4Py)(CF3SO3)]+ (m/z 572) in
the reaction of equimolar amounts of [FeII(tmc)]2+ (0.5 mm) and
[(N4Py)FeIV O]2+ (1; 0.5 mm) in CH3CN at 258C.
=
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changes strongly demonstrate the transfer of an oxygen atom
disappearing and the regeneration of the redox wave at
+ 0.35 V which corresponds to the [FeIII(tmc)(N3)]2+/[FeII-
(tmc)(N3)]+ couple (data not shown); the disappearance of 2-
N3 is the result of the oxygenation of PPh3.[4j] On the basis of
the results described above, we conclude that the irreversible
wave at ꢀ0.63 V results from the reduction of 2-N3.[11–13]
Similarly, we found that the reduction peak of 1 appears at
ꢀ0.44 V (see the Supporting Information for the CVand DPV
traces of 1 and 2-N3).[13] The transfer of an oxygen atom from 1
to [FeII(tmc)(N3)]+ was followed by monitoring the changes in
the DPV trace of the reactant (1) and the product (2-N3) in
the reaction solution [Eq. (3)]. Addition of [FeII(tmc)(N3)]+
to the solution of 1 resulted in the peak current at ꢀ0.44 V
corresponding to 1 disappearing gradually, whereas the
reduction peak current at ꢀ0.63 V corresponding to 2-N3
increased (Figure 4). The decrease and increase in the peak
from 1 to [Fe(tmc)]2+ which results in the formation of
[Fe(N4Py)]2+ and 2 [Eq. (3)]. Furthermore, the observation of
the 18O transfer from 1 to 2 indicates that molecular oxygen
was not involved in the oxygen atom transfer reaction.
Further evidence for the transfer of an oxygen atom was
obtained by electrochemical methods. First, cyclic and differ-
ential pulse voltammograms (CV and DPV) of nonheme
iron(II) complexes [Fe(N4Py)]2+ and [FeII(tmc)(N3)]+ as well
as their iron(IV)–oxo complexes 1 and [FeIV(tmc)(O)(N3)]+
(2-N3) were recorded. The [Fe(N4Py)]2+ and [FeII(tmc)(N3)]+
complexes exhibit reversible and quasireversible redox waves
for the FeIII/FeII couple at + 0.64 and + 0.35 V, respectively
(see the Supporting Information). We then attempted to
identify voltammetric responses corresponding to the
iron(IV)–oxo species, by carrying out titration experiments
by adding different amounts of PhIO to solutions of the
iron(II) complexes. Incremental addition of PhIO to the
solution of [FeII(tmc)(N3)]+ resulted in a decrease in an
oxidation peak at + 0.35 V for the [FeIII(tmc)(N3)]2+/[FeII-
(tmc)(N3)]+ couple with a proportionate increase in a new
reduction peak at ꢀ0.63 V (Figure 3a). The new reduction
peak is irreversible in nature, and the increase in the peak
current was proportional to the amount of 2-N3 formed in the
reaction of [FeII(tmc)(N3)]+ and PhIO (see UV/Vis spectral
data in Figure 3b).[4j] Furthermore, the addition of PPh3 to the
solution of 2-N3 resulted in the reduction peak at ꢀ0.63 V
Figure 4. Differential pulse voltammograms of 1 (red) and 2-N3 (blue)
upon addition of [FeII(tmc)(N3)]+ (1 mm) to a reaction solution
containing 1 (1 mm) and nBu4NPF6 (0.1m) in CH3CN at 258C.
currents of 1 and 2-N3, respectively, demonstrate again that
the oxo group of 1 was transferred to [FeII(tmc)(N3)]+, thus
giving rise to the formation of 2-N3.
The transfer of an oxygen atom was also examined
2+
between [(Bn-tpen)FeIV O] (3)[10] and [FeII(tmc)]2+ as well
=
as between 3 and [FeII(N4Py)]2+. Addition of [FeII(tmc)]2+
(2.0 mm) to the solution of 3 (0.5 mm) in CH3CN at 258C
resulted in changes in the electronic absorption spectra,
namely the disappearance of 3 and the formation of 2
[Eq. (4)] (data not shown). Addition of [FeII(N4Py)]2+ to the
2þ
2þ
½ðBn-tpenÞFeIV¼OŠ ð3Þ þ ½FeIIðtmcފ
ð4Þ
2þ
2þ
! ½FeIIðBn-tpenފ þ ½ðtmcÞFeIV¼OŠ ð2Þ
solution of 3 resulted in the disappearance of 3 and the
concomitant formation of 1 in the electronic absorption and
ESI mass spectra [Eq. (5); see the Supporting Information].
Figure 3. a) Cyclic voltammograms and b) UV/Vis spectral changes for
the conversion of [FeII(tmc)(N3)]+ into [FeIV(tmc)(N3)(O)]+ (2-N3) upon
addition of different amounts of PhIO. Reaction conditions: PhIO
(black, 0 equiv; red, 0.4 equiv; green, 0.8 equiv; blue, 1.2 equiv) was
added to a reaction solution containing [FeII(tmc)(N3)]+ (1 mm) and
tetrabutylammonium hexafluorophosphate (nBu4NPF6, 0.1m) in
CH3CN at 258C.
2þ
2þ
½ðBn-tpenÞFeIV¼OŠ ð3Þ þ ½FeIIðN4Pyފ
ð5Þ
2þ
2þ
! ½FeIIðBn-tpenފ þ ½ðN4PyÞFeIV¼OŠ ð1Þ
In contrast, the reverse reactions such as the transfer of an
oxygen atom from 2 to [FeII(Bn-tpen)]2+ and from 1 to
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[FeII(Bn-tpen)]2+ did not take place [that is, the reverse
reactions of Eqs. (4) and (5)]. The transfer of an oxygen atom
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ꢀ
was not observed between 2-X (X = CH3CN and N3 ) and
[FeII(tmc)(X)]. The latter results may be ascribed to the fact
that the accessibility of an iron(II) ion toward an iron(IV)–
oxo group is not favorable because of a steric interaction
between the tmc ligands of the iron complexes.[4a,g]
In conclusion, we have reported the first example of the
transfer of an oxygen atom between mononuclear nonheme
iron complexes. The transfer of an oxygen atom from
iron(IV)–oxo to iron(II) complexes was clearly evidenced
by spectrophotometric, ESI mass spectrometric, and voltam-
metric techniques. The results (summarized in Scheme 1)
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[10] Abbreviations used: tmc 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-
cyclotetradecane; N4Py N,N-bis(2-pyridylmethyl)-N-bis(2-pyri-
dyl)methylamine; Bn-tpen N-benzyl-N,N’,N’-tris(2-pyridylme-
thyl)ethane-1,2-diamine.
Scheme 1. Summary of the transfer of an oxygen atom between non-
heme iron(IV)–oxo and iron(II) complexes.
show that the transfer of the oxygen atom was dependent on
the oxidizing power of the iron(IV)–oxo complexes; the
oxidizing power of iron(IV)–oxo complexes was determined
to be in the order 3 > 1 > 2 in the oxidation of organic
substrates.[4c,h,j] Detailed investigations of mechanistic aspects
(e.g., complete transfer of an oxygen atom in nonheme iron
models versus incomplete transfer in iron porphyrins)[8,9] and
kinetic parameters of the transfer from nonheme iron(IV)–
oxo to iron(II) complexes are currently underway.
Received: December 13, 2005
Revised: April 2, 2006
Published online: May 10, 2006
Keywords: coordination modes · enzyme models · iron ·
.
N ligands · oxygen
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and will be presented elsewhere.
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