Resonance Raman evidence for the interconversion between an [Fe(III)-η1-OOH]2+ and [Fe(III)-η2-O2]+ species and mechanistic implications thereof

Article abstract of DOI:10.1039/a905535e

The deprotonation of [Fe(III)(N4Py)(η¹-OOH)]²⁺ 1 gives [Fe(III)(N4Py)(η²-OO)]⁺ 2, as unequivocally demonstrated by resonance Raman spectroscopy, and leads to the loss of alkane hydroxylation activity by 1.

Full text of DOI:10.1039/a905535e

Resonance Raman evidence for the interconversion between an
III
1
2+
III
2
+
[
Fe –h -OOH] and [Fe –h -O ] species and mechanistic implications
2
thereof
a
b
c
c
b
Raymond Y. N. Ho, Gerard Roelfes, Roel Hermant, Ronald Hage, Ben L. Feringa* and
Lawrence Que, Jr.*a
a
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minnesota 55455, United
States of America. E-mail: que@chem.umn.edu
Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG
b
Groningen, The Netherlands. E-mail: feringa@chem.rug.nl
Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands.
c
Received (in Bloomington, IN, USA) 8th July 1999, Accepted 21st September 1999
III
1
2+
The deprotonation of [Fe (N4Py)(h -OOH)]
1 gives
III
2
+
[
Fe (N4Py)(h -OO)] 2, as unequivocally demonstrated by
resonance Raman spectroscopy, and leads to the loss of
alkane hydroxylation activity by 1.
Iron–peroxo species have been established or postulated in the
chemistry of dioxygen activation at mononuclear iron centres in
biology.1 For example, activated BLM,† the form of the
antitumor drug bleomycin responsible for its DNA cleavage
,2
activity,3 is formulated as a low spin Fe –h -OOH species.
,4
III
1
5
Scheme 1 Proposed structures for 1 and 2.
III
1
Similarly, an Fe –h -OOH moiety is strongly implicated in the
mechanism of cytochrome P450 and considered the key
intermediate immediately prior to the cleavage of the O–O bond
2.3, 2.1 and 1.9 to one with an intense isotropic signal at g =
III
4.3, which is typical of a mononuclear rhombic high-spin Fe
6
III
1
III
to generate the high valent iron–oxo oxidant. Both Fe –h -
center (E/D = 1/3). In the case of the Fe (N-R-trispicen)
III
OOH species are proposed to derive from the one-electron
complexes, the addition of base to the Fe –OOH species
2+
III
reduction of oxy adducts ([Fe–O
2
+
] ) and subsequent protona-
converts them to high-spin Fe complexes with more axial EPR
III
2
III
2
+
16
tion, possibly via an [Fe –h -O
2
] species. Such [Fe –h -O
2
]
spectra (E/D ≈ 0.08).
species are also postulated to be the source of nucleophilic
peroxides that are implicated in substrate oxidations by heme
Electrospray ionisation mass spectrometry provides the
elemental composition of 2. As reported earlier, the ESMS of 1
shows a prominent feature at m/z 555, whose mass and isotope
7
8
enzymes such as aromatase and heme oxygenase. A related
III
2
+
[
Fe –h -O
2
] species has been suggested to account for the cis-
distribution pattern unequivocally assigns this ion as {[Fe(N-
9
+ 13a
dihydroxylation chemistry of the Rieske dioxygenases. Sig-
nificant effort has been devoted in the characterisation of small
molecule analogues of such peroxo intermediates. Two high
4Py)(OOH)]ClO
4
} .
The ESMS of 2, on the other hand,
exhibits a prominent feature at m/z 455, a value which
corresponds to the loss of HClO , relative to that of 1. The
4
III
2
+
III
22 10,11
2
spin [Fe –h -O
and [Fe(porphyrin)O
examples of synthetic Fe –h -OOH species have just appeared
2
] species, namely [Fe (EDTA)(O
)]
isotope distribution pattern associated with this feature matches
–
12
III
3–15
+
2
] , have been identified, while the first
well with that calculated for, [Fe(N4Py)(OO)] , thus supporting
1
the notion that 2 is the conjugate base of 1.
in the last few years.1
precedent for the interconversion between an [Fe OOH]
We have sought to establish a
Resonance Raman spectroscopy provides further insight into
this transformation. Excitation into the peroxo-to-iron(iii)
charge transfer band of 1 generates a Raman spectrum with
III
2+
III
+
2
species and its deprotonated form [Fe O ] , implicit in the
2
1
mechanisms of bleomycin and cytochrome P450. Such an
features at 632 and 790 cm
[Fig. 1(a)].‡ The latter is
III
18
21
equilibrium was recently reported for Fe (N-R-trispicen)
unequivocally assigned to n(O–O) by its O shift of 246 cm ,
complexes (R = Me, Et, Bn),16 but no Raman data was
while the former is associated with a coupled Fe–OOH mode.
14
III
+
provided to establish the binding mode of the [Fe O
2
] species.
These features are considered the Raman signature for a low
spin Fe(iii)–OOH species. Conversion to 2 results in a
significantly different Raman spectrum with prominent peaks at
2
Here, we provide unequivocal Raman evidence for the h -
III
+
2
peroxo binding in the [Fe (N4Py)O ] species, obtained from
III
1
2+
21
18
the deprotonation of the [Fe (N4Py)(h -OOH)] species. In
495 and 827 cm [Fig. 1(b)].§ The use of H
features to 478 (Dn = 217) and 781 (Dn = 246) cm
respectively [Fig. 1(c)], which are fully consistent with their
2 2
O shifts the two
2
1
addition we show for the first time that protonation of the
,
III
+
[
Fe O
2
] species is essential for alkane oxidation.
The purple [Fe (N4Py)(OOH)] complex 1 (lmax 547 nm,
III
21
2+
21
assignments as n(Fe–O ) (Dncalc 218 cm , based on Hooke’s
2
3
21
e
M
1300 dm mol cm ) is generated at 220 °C by addition
Law for diatomic harmonic oscillators) and n(O–O) (Dncalc
III
21
of 5 equiv. H
4
2
O
2
to a methanolic solution of [Fe (N-
247 cm ), respectively. Consistent with these assignments,
2+
II
2+
54
21
Py)(OMe)] , obtained by pretreating [Fe (N4Py)(MeCN)]
with 5 equiv. H
at room temperature.15 Upon addition of 5
equiv. NH (aq) at 245 °C, the purple chromophore converts to
a blue species 2 (Scheme 1) with an absorption maximum at 685
the introduction of Fe in 2 does not affect the 827 cm
21
21
O
2 2
vibration but upshifts the 495 cm feature by 3 cm (Dncalc
1 14
2
3
+3 cm ) [Fig. 1(d)]. Furthermore, unlike for 1, the use of
2
2 2
H O does not affect the Raman spectrum of 2. Thus the
Raman spectra support the notion that 2 is an [Fe –O ]
2
3
21
21
III
+
nm (e
M
520 dm mol cm ). Addition of perchloric acid
reverses this colour change and quantitatively regenerates the
original purple solution. The conversion of 1 to 2 can also be
monitored by EPR spectroscopy. Addition of base converts the
complex.
2
Strong evidence for the h -peroxo binding mode has been
obtained from the Raman spectrum of 2 with H
2
O
2
containing
III
18
characteristic low-spin Fe EPR spectrum of 1 with g values at
61% O (Fig. 2).¶ Although the various isotopic components of
Chem. Commun., 1999, 2161–2162
This journal is © The Royal Society of Chemistry 1999
2161

the peroxo species activates it for participation in the oxygen
activation mechanisms by iron enzymes.6
,11
We acknowledge C. M. Jeronimus-Stratingh and A. P. Bruins
for advice and assistance with mass spectrometric measure-
ments and financial support from Unilever (Vlaardingen, The
Netherlands) and the National Institutes of Health (Grant GM-
3
3162).
Notes and references
†
Abbreviations used: BLM = bleomycin; N4Py = N-[bis(2-pyr-
idyl)methyl]-N,N-bis(2-pyridylmethyl)amine; N-R-trispicen = N-alkyl-N,
NA,NA-tris(2-pyridylmethyl)ethane-1,2-diamine.
‡
Resonance Raman spectra were collected on an Acton AM-506
spectrometer (2400-groove grating) using Kaiser Optical holographic
super-notch filters with a Princeton Instruments liquid N -cooled (LN-
100PB) CCD detector with 4 or 2 cm spectral resolution. Spectra were
Fig. 1 Resonance Raman spectra of 1 and 2. (a) 1 generated by the addition
2
III
2+
of 5 equiv. H
2
O
2
to a methanolic solution of [Fe (N4Py)(OMe)] (10 mM)
(aq) to the
was used. (d) Same
as (b) except that the Fe complex was used. All spectra were obtained with
a backscattering geometry on liquid-N frozen samples using 568.2 nm laser
21
1
at 235 °C. (b) 2 generated by the addition of 5 equiv. NH
3
2
obtained by back-scattering geometry on liquid N frozen samples using
1
8
solution of 1 at 235 °C. (c) Same as (b), except H
2 2
O
5
68.2 nm laser excitation from a Spectra Physics 2030-15 argon ion laser
5
4
and a 375B CW dye (Rhodamine 6G). Raman frequencies were referenced
2
to indene.
excitation at 20 mW power at the sample. The Raman frequencies were
referenced to indene.
21
§
6
Aside from the features at 495 and 827 cm , weaker features at 625 , 645,
64 cm are also observed and are sensitive to O labeling. However the
21
18
intensities of these features vary relative to the intensities of the 495 and 827
cm features with different samples, suggesting that these minor features
are not associated with 2. We have ruled out the possibility that these minor
) feature centred at 486 cm2 are unresolved, three
1
21
the n(Fe-O
2
peaks associated with the n(O–O) feature are readily discerned
2
1
21
at 781, 802 and 826 cm and can be fitted with peaks having
approximately equal linewidths. The fact that the peak arising
features are associated with 1 since the features at 626, 648, and 668 cm
2
1
are slightly shifted from those of 1 and a corresponding 790 cm feature is
not observed. The assignment of these features is currently under
investigation.
16 18
from the O O isotopomer has a linewidth equal to those of
16
16
18 18
2
O O and O O isotopomers strongly implies an h -peroxo
1
8
(90% 18O-enriched, 2% solution in H
ICON Services Inc. The statistical mixture of H
by the reduction of O
16
¶
H
2
O
2
2
O) was obtained from
(61% 18O) was prepared
(statistical mixture with 61% O from ICON
III
2
10,11
binding mode, as found for [Fe (EDTA)(h -O
2
)]
and
O
III
+ 16
2 2
suggested for [Fe (N-R-trispicen)(O
2
)] . At this point, we do
18
2
not have enough information to determine whether the iron
centre in 2 becomes seven-coordinate, as illustrated in Scheme
, or remains six-co-ordinate by a decrease in the denticity of
Services Inc.) according to a literature procedure (A. J. Sitter and J. Terner,
J. Labelled Compd. Radiopharm., 1984, 22, 461).
1
the N4Py ligand. The latter may come about by detachment of
one of the pendant pyridines or, by breaking the already weak
1 M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson, Chem. Rev.,
1996, 96, 2841.
2 L. Que, Jr. and R. Y. N. Ho, Chem. Rev., 1996, 96, 2607.
2
tertiary amine bond as the h -peroxo ligand pulls the iron centre
3
S. A. Kane and S. M. Hecht, Prog. Nucl. Acid Res. Mol. Biol., 1994, 49,
13.
further out of the plane defined by the four pyridine ligands to
achieve a coordination geometry similar to that found in the
3
4
5
J. Stubbe and J. W. Kozarich, Chem. Rev., 1987, 87, 1107.
J. W. Sam, X.-J. Tang and J. Peisach, J. Am. Chem. Soc., 1994, 116,
2
– 17
2
structure of [Mn(TPP)(h -O )] .
With the peroxo binding mode of 2 established, we now
compare the relative abilities of 1 and 2 to oxidise substrates.
5
250.
6
7
D. L. Harris and G. H. Loew, J. Am. Chem. Soc., 1998, 120, 8941.
M. J. Coon, A. D. N. Vaz and L. L. Bestervelt, FASEB J., 1996, 10,
428.
II
2+
We have previously found that [Fe (N4Py)(MeCN)] can
catalyse the hydroxylation of cyclohexane with H in acetone
, we observe 1.6 and
.4 turnovers of cyclohexanol and cyclohexanone, respectively.
2 2
O
via intermediate 1.1 With 5 equiv. H
3a
8 P. R. Ortiz de Montellano, Acc. Chem. Res., 1998, 31, 543.
9 D. Ballou and C. Batie, in Oxidases and Related Redox Systems, ed. T.
E. King, H. S. Mason and M. Morrison, Alan R. Liss, New York, 1988,
p. 211.
2
O
2
0
3
In CD OD the number of turnovers is lower, 0.8 and 0.4
turnovers, respectively, owing to a competing oxidation of the
solvent to D CNO (1.4 turnovers). When 5 equiv. NH (aq) is
added prior to or after the formation of 1 in CD OD, no
1
0 S. Ahmad, J. D. McCallum, A. K. Shiemke, E. H. Appelman, T. M.
Loehr and J. Sanders-Loehr, Inorg. Chem., 1988, 27, 2230.
1 F. Neese and E. I. Solomon, J. Am. Chem. Soc., 1998, 120, 12 829.
2 E. McCandlish, A. R. Miksztal, M. Nappa, A. G. Sprenger, J. S.
Valentine, J. D. Stong and T. G. Spiro, J. Am. Chem. Soc., 1980, 102,
4268.
2
3
3
1
1
oxidation of cyclohexane is observed. These results suggest
that, whereas 1 is capable of activating the O–O bond to oxidise
alkanes, 2 is unreactive towards such substrates, resembling the
III
2
11,18
inertness of other [Fe (h -O
2
)] species.
Our demonstration
13 (a) M. Lubben, A. Meetsma, E. C. Wilkinson, B. Feringa and L. Que,
Jr., Angew. Chem., Int. Ed. Engl., 1995, 34, 1512; (b) I. Bernal, I. M.
Jensen, K. B. Jensen, C. J. McKenzie, H. Toftlund and J. P. Tuchagues,
J. Chem. Soc., Dalton Trans., 1995, 22, 3667; (c) C. Kim, K. Chen, J.
Kim and L. Que, Jr., J. Am. Chem. Soc., 1997, 119, 5964; (d) M. E. de
Vries, R. M. La Crois, G. Roelfes, H. Kooijman, A. L. Spek, R. Hage
and B. L. Feringa, Chem. Commun., 1997, 1549.
III
1
III
2
–
of an [Fe (h -OOH)]/[Fe (h -OO )] interconversion allows
us to support unequivocally the hypothesis that protonation of
1
4 R. Y. N. Ho, G. Roelfes, B. L. Feringa and L. Que, Jr., J. Am. Chem.
Soc., 1999, 121, 264.
1
5 G. Roelfes, M. Lubben, K. Chen, R. Y. N. Ho, A. Meetsma, S.
Genseberger, R. M. Hermant, R. Hage, S. K. Mandal, V. G. Young, Jr.,
Y. Zang, H. Kooijman, A. L. Spek, L. Que, Jr. and B. L. Feringa, Inorg.
Chem., 1999, 38, 1929.
1
6 A. J. Simaan, F. Banse, P. Mialane, A. Boussac, S. Un, T. Kargar-Grisel,
G. Bouchoux and J.-J. Girerd, Eur. J. Inorg. Chem., 1999, 993; K. B.
Jensen, C. J. McKenzie, L. P. Nielsen, J. Z. Pedersen and H. M.
Svendsen, Chem. Commun., 1999, 1313.
2 2
Fig. 2 Raman spectrum of 2 generated with a statistical mixture of H O
1
8
isotopomers (61% O). The dashed lines represent the curve fit of the
features associated with the n(O–O) peaks. The curve was fitted with three
1
8
18
21
peaks with the
O
O feature constrained to a frequency of 781 cm and
17 R. B. VanAtta, C. E. Strouse, L. K. Hanson and J. S. Valentine, J. Am.
Chem. Soc., 1987, 109, 1425.
18 M. Selke and J. S. Valentine, J. Am. Chem. Soc., 1998, 120, 2652.
2
1
18
+
a linewidth of 16 cm as found for [Fe(N4Py)(
the other two peaks at 802 and 826 cm with linewidths of 16.3 and 15.9
cm , respectively. The latter matched well the properties of the n(O–O)
feature in [Fe(N4Py)(
O
2
)] . The fitted data gave
21
21
1
6
+
O
2
)] .
Communication 9/05535E
2162
Chem. Commun., 1999, 2161–2162