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Bispidine Ligand Effects on IronHydrogen Peroxide Chemistry.

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Angewandte
Chemie
Coordination Chemistry
Bispidine Ligand Effects on Iron/Hydrogen
Peroxide Chemistry**
Michael R. Bukowski, Peter Comba,*
Christian Limberg, Michael Merz, Lawrence Que, Jr.,*
and Tobias Wistuba
The long-term interest in dioxygen activation by heme centers
has recently been extended to non-heme iron systems,[1–3]
both of which can catalyze the stereoselective oxidation of
C H and C=C bonds. Key intermediates include FeIII h1hydroperoxo and FeIII h2-peroxo species, which have been
identified for cytochrome P450,[4] bleomycin,[5] and naphthalene dioxygenase.[6] A range of synthetic non-heme iron
complexes with amine/pyridine ligand combinations has been
[*] Prof Dr. P. Comba, Dr. M. Merz, Dr. T. Wistuba
Universitt Heidelberg, Anorganisch-Chemisches Institut
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+ 49) 6226-546617
E-mail: peter.comba@aci.uni-heidelberg.de
M. R. Bukowski, Prof. Dr. L. Que, Jr.
Department of Chemistry and Center for Metals in Biocatalysis
University of Minnesota
207 Pleasant Street SE, Minneapolis MN 55455 (USA)
Fax: (+ 1) 612-624-7029
E-mail: que@chem.umn.edu
Prof. Dr. C. Limberg
Humboldt-Universitt zu Berlin
Institut fAr Chemie
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
Fax: (+ 49) 30-2093-6966
[**] Financial support by the German Science Foundation (DFG to P.C.
and C.L.) and the U.S. National Institutes of Health (GM-33162 to
L.Q.) are gratefully acknowledged.
Angew. Chem. Int. Ed. 2004, 43, 1283 –1283
DOI: 10.1002/anie.200352523
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1283
Communications
found to catalyze hydrocarbon oxidations with H2O2 as
oxidant.[3] Extensive spectroscopic and labeling studies have
led to a thorough understanding of the properties of the FeIII
intermediates,[3] and support for a mechanistic proposal
involving high-valent iron-oxo species for the catalytic
alkane hydroxylation, olefin epoxidation, and olefin cisdihydroxylation has been obtained.
A set of iron complexes with pentadentate N5 ligands that
react with H2O2 to afford metastable low-spin FeIII h1-OOH
intermediates with hydroperoxo-to-FeIII charge-transfer
bands in the 500-nm region has been extensively studied.[7–9]
Deprotonation of these species affords their respective
conjugate bases with lower energy charge-transfer bands;
spectroscopic studies support their assignment as the corresponding high-spin FeIII side-on peroxo derivatives. To date,
no crystal structures are available for any of these FeIII
intermediates, so their precise structures remain to a large
extent a matter of speculation.
The rigid tetra- and pentadentate bispidine derivatives
L1–L3 have donor sets similar to tpa and N4py, the iron
complexes of which represent the most active oxidation
catalysts in non-heme iron chemistry (for ligand structures see
Scheme 1, for complex structures see Scheme 2).[7, 10, 11] L1–L3
provide a rigid ligand backbone and enforce cis-octahedral
(or square-pyramidal) coordination geometries for a range of
metal ions.[12, 13] The most relevant structural feature is that for
the two coligands (substrates) in [FeII(L1)(X)(Y)]n+ the
metal–ligand distances are significantly different, with that
trans to N3 being shorter than that trans to N7 (see Scheme 2).
With the two isomeric ligands L2 and L3, the coordination sites
trans to N3 and trans to N7, respectively, are blocked by an
additional pendant pyridine arm. This leaves one coordination site for H2O2 binding and thus provides us with the
opportunity to explore the effects of steric and electronic
constraints on the properties of the Fe–OOH and Fe–OO
units. Based on extensive structural, spectroscopic, electrochemical, and computational studies of CuII/I bispidine complexes,[13–16] the iron complexes with L2 and L3 are expected to
have strikingly different structural[12] and electronic properties. Here we report the spectroscopic properties of the FeIII
hydroperoxo and peroxo complexes; the reactivities of these
oxidation catalysts will be reported separately.[17]
Following the precedent for the observation of the
[Fe(OOH)(N4py)]2+ intermediate,[18] we have investigated
the reactions of the high-spin FeII complexes of L2 and L3 with
excess H2O2 in MeOH at 40 8C. Purple chromophores are
observed that can be assigned to the formation of low-spin
FeIII complexes with end-on hydroperoxo ligands. Thus,
addition of ten equivalents of H2O2[19] to [Fe(L3)(solvent)]2+[21] leads to a new species with a charge-transfer
transition at 561 nm (Figure 1), similar to those observed for a
Figure 1. Absorption spectra of [FeIII(OOH)(L3)]2+ (c; from the reaction of [FeII(L3)](BF4)2 with 100 equiv H2O2 in methanol at 40 8C) and
of [FeIII(O2)(L3)]+ (a; after addition of 14 equiv base).
Scheme 1. Molecular formula of the ligands discussed.
Scheme 2. Representations of the metal–ligand complexes.
1284
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
number of low-spin FeIII hydroperoxo complexes (see Table 1
for data of the iron bispidine and similar compounds).[22] The
low-spin character of [Fe(OOH)(L3)]2+ is confirmed by its
EPR signals at g = 2.18, 2.12, 1.95 (Table 1). Similar results
are obtained for the complex with L2 (Table 1, Figure 2).
Resonance Raman studies show that [Fe(OOH)(L2)]2+
and [Fe(OOH)(L3)]2+ give rise to the two expected features
that can be assigned to nFe-OOH and nO-O (Table 1). Isotopic
labeling studies (shown in Figure 3 for [Fe(OOH)(L3)]2+)
establish these assignments. The 18O and 2H isotope shifts for
the nFe-OOH vibration for the L2 and L3 complexes, unlike those
observed for [Fe(OOH)(N4py)]2+,[9, 18] are almost precisely as
predicted by a simple calculation based on Hooke's law,
assuming the hydroperoxo moiety to be a single mass unit.
Interestingly, while the nO-O vibrations of the two intermediates are identical, the nFe-OOH vibration for the L2 complex is
shifted to lower energy by 12 cm 1 relative to that of the L3
isomer, consistent with the expectation of a weaker bond (see
Scheme 2).
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Angew. Chem. Int. Ed. 2004, 43, 1283 –1283
Angewandte
Chemie
Table 1: Spectroscopic properties of [FeIII(h1-OOH)(L)]2+ and [FeIII(h2-OO)(L)]+ intermediates.
Ligand
[Fe(L)OOH]2+, S = 1/2
lmax [nm]
EPR gn(Fe-O)
(e [m 1 cm 1]) values
[cm 1]
(D18O)
[D2H]
L2
561 (1260)
L3
561 (1300)
N4py[9, 18] 548 (1100)
Me537 (1000)
tpen[25, 31]
tpa[32, 33]
538 (1000)
2.19,
2.13,
1.96
2.18,
2.12,
1.95
2.17,
2.12,
1.98
2.19,
2.12,
1.95
2.19,
2.15,
1.97
n(O-O)
[cm 1]
(D18O)
608
( 24)
[ 5]
620
( 22)
[ 6]
627
( 16)
[ 5]
617
804
( 44)
626
789
804
( 43)
790
( 44)
796
attribute these changes to the conversion of the low-spin FeIII h1OOH complex to a high-spin h2n(O-O)
1
[cm ]
peroxo species, as previously
observed for corresponding complexes of N4Py and R-tpen (see
Scheme 1). This notion is con780 ( 350)
0.24 not
not
observed observed
firmed by its resonance Raman
spectrum showing features at 493
721 (540)
0/0.33 493
827
and 827 cm 1 (see Table 1 and
Figure 3). These peaks can be
assigned, respectively, to the nFe-O2
685 (520)
0.11 495
827
and nO-O modes of [Fe(h2O2)(L3)]+, by analogy to the very
740 (500)
0.08 470
819
similar features observed for the
corresponding N4py and Me-tpen
complexes (Table 1).
not observed
The
conjugate
base
of
[Fe(OOH)(L2)]2+ appears to be
less stable. Unlike its L3 analogue,
[Fe(OOH)(L2)]2+ could not be converted to its conjugate base at 40 8C; addition of base affords
only an FeIII decomposition product. However, deprotonation
could be effected at 80 8C, and a new chromophore can be
observed with a broad absorption band centered near l =
760 nm (e 350 L mol 1 cm 1), which presumably is the
corresponding side-on peroxo complex for L2. This species
is associated with an EPR spectrum with g = 9.9, 8.9, 5.1, 4.3,
and 3.5, corresponding to an S = 5/2 species with E/D = 0.24
(Figure 2). As for the L3 complex, there is also a new but
minor S = 1/2 signal that appears when a significant amount of
[Fe(h2-OO)(L2)]+ has formed.[23] Unfortunately, attempts to
obtain a Raman spectrum of [Fe(h2-OO)(L2)]+ have not been
successful thus far. Although [Fe(OO)(L2)]+ cannot be
characterized as well as its L3 counterpart, it is clear that
[Fe(L)OO]+, S = 5/2
lmax [nm]
EPR
n(Fe-O)
(e [m 1 cm 1]) E/D
[cm 1]
Figure 2. X-band EPR spectra of [FeIII(OOH)(L2)]2+ at 25 K (top) and
[FeIII(OO)(L2)]+ at 2 K (bottom) in methanol. Conditions: 10 GHz modulation frequency, 0.3181 mW power.
The distinction between [Fe(OOH)(L2)]2+ and
[Fe(OOH)(L3)]2+ is emphasized by the properties of their
conjugate bases. Deprotonation of [Fe(OOH)(L3)]2+ in methanol at 40 8C (2 equiv NaOMe) leads to a blue solution with
an absorption maximum at l = 721 nm (Figure 1); this conversion is easily reversed by back titration with acid (2 equiv
HClO4).[23] All spectra go through an isosbestic point at l =
650 nm, indicating the presence of a fully reversible acid–base
equilibrium. This set of data allows the determination of the
extinction coefficients of the pure complexes (e560 =
1300 L mol 1 cm 1
for [Fe(OOH)(L3)]2+ and e721 =
1
1
540 L mol cm for [Fe(O2)(L3)]+), and an estimate of its
pK value ( 40 8C, MeOH, 4.5 < pK < 5.0).[24] When this
interconversion is monitored by EPR, the low-spin EPR
spectrum of [Fe(OOH)(L3)]2+ is replaced by a new signal at
g = 5.9 that corresponds to an axial high-spin FeIII species. We
Angew. Chem. Int. Ed. 2004, 43, 1283 –1283
Figure 3. Resonance Raman spectra of FeIII(L3)-peroxo complexes in
methanol. lex = 568.2 nm for the FeIII-OOH samples, lex = 647.1 nm for
the FeIII-O2 sample; 100 mW power.
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1285
Communications
the two complexes have quite different temperature-dependent stabilities and optical properties.
These differences give novel insights into the nature of
side-on peroxo complexes with FeIIIN5 chromophores, for
which there is as yet no crystal structure. For the related
[Fe(h2-OO)(Me-tpen)]+ species it has been suggested that its
formation from the FeIII h1-OOH precursor may involve the
dissociation of one of the pendant pyridine ligands to
generate a six-coordinate FeN4O2 complex;[25, 26] this structure
was postulated by analogy to that proposed for [FeIII(edta)(O2)]3 (edta = ethylenediamine tetraacetate) on the basis
of a comprehensive spectroscopic investigation.[11] On the
other hand, DFT calculations for [Fe(h2-OO)(N4py)]+ suggest that the N4py ligand remains pentadentate and that the
Fe–Namine bond lengthens from 2.1 to 2.4 E to accommodate
the side-on peroxo binding; the calculated structure affords
iron–ligand bond lengths fully consistent with those derived
from EXAFS analysis.[9] The significant difference in properties of the side-on peroxo complexes of L2 and L3 argues
against the dissociation of the third, additional pyridine donor
and leads us to consider seven-coordinate h2-O2 structures.
Seven-coordination for transition-metal complexes with
bispidine ligands has been observed before[13] and is supported for the high-spin FeIII h2-O2 complexes by DFT
calculations (UB3 LYP functional at the 6-31G(d) level; this
method reproduces the crystal structure of [FeCl(L2)]+
reasonably well).[28] The results of the structure optimiza-
tion[29] of the two peroxo complexes are shown in Figure 4.
The Fe(h2-O2)(L3) unit is quite symmetrical in the L3
complex; the peroxo group is coplanar with N3, the FeIII
center, and the two pyridine N donors (Figure 4). The two
Fe O bonds are close to identical and comparably short, and
the O O bond is relatively long. In [Fe(h2-O2)(L2)]+ the
peroxo group is coplanar with the FeIII center, N3, and N7.
The two Fe O bonds are relatively long and significantly
different from each other, and the O O bond is shorter than
in the complex with L3 (Figure 4 b). The structures of the
corresponding hydroperoxo complexes were also optimized,
and the Fe O bond length in [Fe(OOH)(L2)]2+ is 1.88 E while
that in [Fe(OOH)(L3)]2+ is 1.78 E. Hence, the low-spin
hydroperoxo (six-coordinate) and the high-spin peroxo complexes (seven-coordinate) have relatively weak Fe O bonds
in the case of L2 and stronger bonds in the case of L3. This is as
expected from structural studies with iron(ii)[12] and other
transition-metal bispidine complexes (Scheme 2).[13, 14, 31] The
strong interaction of the peroxo ligand with the FeIII center
coordinated to L3 leads to a weakening of the O O bond,
while the weaker interaction in the complexes with L2 gives
rise to an O O bond that is stronger and less activated.
Based on extensive studies of bispidine coordination
compounds, also including structural studies of FeII bispidines,
the isomeric pair of Fe complexes with L2 and L3 was
predicted to have significantly different hydrogen peroxide
chemistry. This expectation is largely fulfilled. Raman spectra
and computed structures indicate that the Fe O bonds are, as
expected, consistently weaker trans to N7 (L2), specifically for
the seven-coordinate high-spin peroxo complex. This also
indicates that the two complexes might differ considerably in
terms of their activity as oxidation catalysts.[17]
Received: July 31, 2003 [Z52523]
.
Keywords: coordination modes · iron · ligands · stereochemistry
Figure 4. Computed structures (UB3LYP/6-31G(d)) of the two highspin peroxo complexes. a) [FeIII(h2-O2)(L3)]+: Fe-N3 2.33, Fe-N7 2.24,
Fe-py1 2.29, Fe-py2 2.29, Fe-py3 2.16, Fe-O1 1.91, Fe-O2 1.91, O1-O2
1.41 L. b) [FeIII(h2-O2)(L2)]+: Fe-N3 2.21, Fe-N7 2.53, Fe-py1 2.23, Fepy2 2.27, Fe-py3 2.21, Fe-O1 1.97, Fe-O2 1.99, O1-O2 1.36 L.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[1] B. Meunier, Struct. Bonding (Berlin) 2000, 97.
[2] Biomimetic Oxidations Catalyzed by Transition Metal Complexes (Ed.: B. Meunier), Imperial College, London, 2000.
[3] K. Chen, M. Costas, L. Que, Jr., J. Chem. Soc. Dalton Trans.
2002, 672.
[4] R. Davydov, T. M. Makris, V. Kofman, D. E. Werst, S. G. Sligar,
B. M. Hoffman, J. Am. Chem. Soc. 2001, 123, 1403.
[5] R. M. Burger, Struct. Bonding (Berlin) 2000, 97, 287.
[6] A. Karlsson, J. V. Parales, R. E. Parales, D. T. Gibson, H.
Eklund, S. Ramaswamy, Science 2003, 299, 1039.
[7] “Metal-Oxo and Metal-Peroxo Species in Catalytic Oxidations”:
J.-J. Girerd, F. Banse, A. J. Simaan, Struct. Bonding (Berlin)
2000, 97, 145.
[8] K. B. Jensen, C. J. McKenzie, L. P. Nielsen, J. Z. Pedersen, H. M.
Svendsen, J. Chem. Soc. Chem. Commun. 1999, 1313.
[9] G. Roelfes, V. Vrajmasu, K. Chen, R. Y. N. Ho, J.-U. Rohde, C.
Zondervan, R. M. la Crois, E. P. Schudde, M. Lutz, A. L. Spek,
R. Hage, B. Feringa, E. MNnck, L. Que, Jr., Inorg. Chem. 2003,
42, 2639.
[10] L. Que, Jr., R. Y. N. Ho, Chem. Rev. 1996, 96, 2607.
[11] E. I. Solomon, T. C. Brunold, M. I. Davis, J. N. Kensley, S.-K.
Lee, N. Lehnert, F. Neese, A. J. Skulan, Y.-S. Yang, J. Zhou,
Chem. Rev. 2000, 100, 235.
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 1283 –1283
Angewandte
Chemie
[12] H. BOrzel, P. Comba, K. S. Hagen, M. Merz, Y. D. Lampeka, A.
Lienke, G. Linti, H. Pritzkow, L. V. Tsymbal, Inorg. Chim. Acta
2002, 337, 407.
[13] P. Comba, M. Kerscher, M. Merz, V. MNller, H. Pritzkow, R.
Remenyi, W. Schiek, Y. Xiong, Chem. Eur. J. 2002, 8, 5750.
[14] P. Comba, A. Lienke, Inorg. Chem. 2001, 40, 5206.
[15] P. Comba, A. Hauser, M. Kerscher, H. Pritzkow, Angew. Chem.
2003, 115, 4675 – 4679; Angew. Chem. Int. Ed. 2003, 42, 4536 – 4540.
[16] P. Comba, M. Kerscher, A. Roodt, Inorg. Chem., submitted.
[17] The tetra- and pentadentate ligand complexes (L1 vs. L2 and L3)
follow mechanistically different pathways (product distribution,
18
O-labeling experiments, aerobic vs anaerobic conditions).
Under standard conditions[3] the reactivity of the L1-based
catalyst is comparable to that with tpa. The catalyst based on L2
has, under aerobic conditions, a similar reactivity, that of L3 is
less reactive by a factor of 5–10.
[18] 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., B. L.
Feringa, Inorg. Chem. 1999, 38, 1929.
[19] 35 % aqueous solution, quantitative analysis by iodometric
titration.[20]
[20] D. H. R. Barton, V. N. L. Gloahec, H. Patin, F. Launay, New J.
Chem. 1998, 22, 559.
[21] BF4 salts, crystallized from CH3CN and fully characterized.
[22] These transitions are attributed to ligand-to-metal charge transfer from the hydroperoxo group.
[23] At high base concentrations there is formation of a new but
minor species not yet fully characterized, with a low-spin
electronic configuration (L3 : g1 = 2.13, g2 = 2.08, g3 = 1.98; L2 :
g1 = 2.14, g2 = 2.09, g3 = 1.98) and a low-energy ligand field. We
propose that this may arise from the end-on isomers [Fe(h1O2)(L3)]+ and [Fe(h1-O2)(L2)]+, respectively.
[24] The e-values of the hydroperoxo and peroxo FeIII complexes as
well as the corresponding acid–base equilibrium constant are
based on the assumption of 100 % conversion to the FeIII
peroxo–hydroperoxo system and full reversibility. Admittedly,
this is an oversimplification.[23] The pK value was calculated from
the intensities of the bands at l = 561 nm and l = 721 nm (and
the known e-values) at different amounts of acid (HClO4) added
to the fully deprotonated form.
[25] A. J. Simaan, S. DOpner, F. Banse, S. Bourcier, G. Bouchoux, A.
Boussac, P. Hildebrandt, J.-J. Girerd, Eur. J. Inorg. Chem. 2000,
1627.
[26] For a related tpen derivative a seven-coordinate peroxo complex
was proposed.[27]
[27] A. Hazell, C. J. McKenzie, L. Preuss Nielsen, S. Schindler, M.
Weitzer, J. Chem. Soc. Dalton Trans. 2002, 310.
[28] The comparison of the results of a structure optimization at that
level with those of a single-crystal X-ray diffraction study[12]
showed that all Fe N bond lengths are only slightly overestimated with a mean deviation of 2.5 %, while the Fe Cl bond
length is underestimated by 4.6 %; that is, the agreement
between experiment and theory is acceptable at that level.
[29] All structures were characterized as minima by frequency
analysis.
[30] However, the expected structural differences, which correlate
well with the different stabilities and reactivities, might have
been expected to also lead to significantly different spectroscopic properties. Reasons why this is not so are currently under
investigation.
[31] A. J. Simaan, F. Banse, P. Mialane, A. Boussac, S. Un, T. KargarGrisel, G. Bouchoux, J.-J. Girerd, Eur. J. Inorg. Chem. 1999, 993.
[32] Y. Zang, J. Kim, Y. Dong, E. C. Wilkinson, E. H. Appelman, L.
Que, Jr., J. Am. Chem. Soc. 1997, 119, 4197.
[33] R. Y. N. Ho, G. Roelfes, B. L. Feringa, L. Que, Jr., J. Am. Chem.
Soc. 1999, 121, 264.
Angew. Chem. Int. Ed. 2004, 43, 1283 –1287
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