EPR spectroscopic trapping of the active species of nonheme iron-catalyzed oxidation

Article abstract of DOI:10.1021/ja902659c

(Graph Presented) The key intermediate of a bioinspired iron catalyst for selective hydrocarbon oxidation based on hydrogen peroxide and an iron complex with a tetradentate aminopyridine ligand was trapped by EPR. On the basis of EPR and reactivity data this intermediate is tentatively proposed to be an oxoiron(V) complex.

Full text of DOI:10.1021/ja902659c

Published on Web 07/20/2009
EPR Spectroscopic Trapping of the Active Species of Nonheme
Iron-Catalyzed Oxidation
Oleg Y. Lyakin,† Konstantin P. Bryliakov,† George J. P. Britovsek,‡ and Evgenii P. Talsi*,†
Siberian Branch of the Russian Academy of Sciences, BoreskoV Institute of Catalysis, NoVosibirsk 630090, Pr.
LaVrentieVa 5, Russia, and Department of Chemistry, Imperial College London, London SW7 2AY, Exhibition Road, U.K.
Received April 3, 2009; E-mail: talsi@catalysis.ru
Recently, substantial research efforts have been aimed at the
development of catalyst systems capable to induce oxygen-transfer
reactions similar to those of typical monooxygenase enzymes,
including alkene epoxidation and alkane hydroxylation.1,2 The most
efficient iron-based bioinspired olefin epoxidation catalysts presently
known, which use H2O2(CH3CO3H) as oxidant, are iron complexes
with aminopyridine ligands, [(TPA)FeII(CH3CN)2](ClO4)2 (1) and
[(BPMEN)FeII(CH3CN)2](ClO4)2 (2) (Chart 1).3 Despite intensive
research efforts, the nature of the critical oxidant(s) in catalytic
systems such as 1/H2O2(CH3CO3H) and 2/H2O2(CH3CO3H) remains
Self-decay of 1a follows first-order kinetics with an apparent rate
constant of (1.6 ( 0.2) × 10-3 s-1 at -70 °C (Figure 1). The rate
of this decay increases by a factor of 5 in the presence of 12 equiv
of cyclohexene, thus indicating that 1a is reactive toward cyclo-
hexene even at -70 °C (Figure S1, Supporting Information (SI)).
The only reaction product formed after storing the sample above
at -70 °C for 1 h is cyclohexene oxide (yield 80% with respect to
1; see SI for experimental details). Decay of 1a is accelerated with
increasing cyclohexene concentration. Addition of electron-deficient
olefins to the 1/m-CPBA system does not substantially affect the
decay rate of 1a (Table S1). The same complex 1a was observed
by EPR in the system 1/CH3CO3H. The maximum concentration
of 1a amounts to 7% of [Fe]total (Table S2).
elusive.4 Oxoiron(IV) complexes of the type [(L)FeIVdO(S)]2+
,
which are reactive toward alkenes, were observed in the catalytic
systems 1,2/CH3CO3H (L ) TPA or BPMEN, S ) H2O or
CH3CN).5 However, the independently determined selectivity of
[(TPA)FeIVdO(S)]2+ toward epoxidation of olefins is poor and
cannot explain the observed yield of epoxide in the catalyst system
1/CH3CO3H/cyclooctene.3 Therefore, another more selective species
should drive epoxidation. Here, we report on the EPR detection of
these very unstable and highly reactive iron-oxygen intermediates
in the catalyst systems 1,2/m-CPBA, 1,2/CH3CO3H, and 2/H2O2,
(m-CPBA ) meta-chloroperoxybenzoic acid). Several related
systems, 3-6/m-CPBA and 3-6/CH3CO3H, have been included in
the present study (Chart 1).
Chart 1
Figure 1. X-band EPR spectra (-196 °C) of the sample 1/m-CPBA frozen
after mixing the reagents during 30 s at -60 °C in a 1.7:1 CH2Cl2/CH3CN
mixture ([1] ) 0.04 M, [m-CPBA] ) 0.08 M) and storing at -70 °C for
(a) 0, (b) 2, (c) 6, (d) 10, (e) 16, and (f) 26 min. Signals denoted by asterisks
belong to still unidentified minor species. (g) Relative concentration of 1a
(c/c0) vs time calculated from the decay of EPR signals of 1a in (a)-(f).
The catalytic systems 1/CH3CO3H and 2/CH3CO3H display
similar reactivity patterns toward epoxidation of cyclooctene.3 EPR
studies on the systems 2/CH3CO3H and 2/m-CPBA give results
similar to those for systems based on complex 1. EPR spectra of
2/CH3CO3H and 2/m-CPBA systems, frozen to -196 °C at various
moments of time after mixing the reagents at -60 °C and storing
at -70 °C, shows that complex 2a displays a rhombic EPR
spectrum (g1 ) 2.69, g2 ) 2.42, g3 ) 1.70) (Figure S2). The
intensities of these three peaks change in parallel. The apparent
rate constant evaluated for the self-decay of 2a at -70 °C is (5.4
The EPR spectrum of a sample frozen to -196 °C after mixing
1 with m-CPBA at -60 °C for 30 s and storing at -70 °C ([1] )
0.04 M, [m-CPBA]/[1] ) 2) shows signals of a new complex 1a
with g-values of g1 ) 2.71, g2 ) 2.42, g3 ) 1.53. The maximum
concentration of 1a amounts to 15% of total iron concentration.
† Boreskov Institute of Catalysis.
‡ Imperial College London.
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10.1021/ja902659c CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
( 1.3) × 10-4 s-1. The rate of this decay increases by a factor of
10 in the presence of 12 equiv of cyclohexene. Thus, both 1a and
2a are reactive toward cyclohexene even at -70 °C.
Synthesis of the first and so far only bona fide example of an
oxoiron(V) complex, [(TAML)FeVdO]-, where TAML is a mac-
rocyclic tetraamide ligand, was reported in 2007. The EPR spectra
observed for 1a and 2a differ distinctly from the EPR spectrum
reported for [(TAML)FeVdO]- (g1 ) 1.99, g2 ) 1.97, g3 ) 1.74).7
This difference can be caused by diverse ligand fields. The most
specific spectral feature of [(TAML)FeVdO]- (distinguishing it
from FeIII and FeIIIFeIV species found in the systems based on 1
and 2) is the high-field peak at g ) 1.74. This peak is much broader
than those at g ) 1.97 and 1.99.7 Similar peaks at g ) 1.70 and
1.53 were observed for [(BPMEN)FeVdO(S)]3+ and
[(TPA)FeVdO(S)]3+, respectively (Figures 1 and S2). These peaks
are much broader than the corresponding low-field peaks, as in the
case of [(TAML)FeVdO]-. However, it is evident that further
studies are needed for the unambiguous assignment of the observed
intermediates to the oxoiron(V) species.
To establish that intermediate 2a does indeed drive the catalytic
cyclohexene epoxidation by catalyst system 2/H2O2, we have
compared the yield of cyclohexene oxide formed in the catalyst
system 2/H2O2/cyclohexene at -70 °C ([2] ) 0.027 M, [H2O2] )
0.27 M, [cyclohexene] ) 0.81 M) with that expected from the
kinetic data for 2a. The EPR spectra of the above catalyst system
in the absence of cyclohexene show that the concentration of 2a is
practically constant during 1 h at -70 °C and amounts to (2.0 (
0.8) × 10-3 M (Figure S3). In order for the concentration of 2a to
reach a steady state, its rate of formation, WF(2a), must equal its
rate of decay, WF(2a) ) k × [2a], where k is the rate constant of
self-decay of 2a at -70 °C, and [2a] is the steady state concentration
of 2a in the absence of cyclohexene. In the presence of cyclohexene,
2a cannot be detected, in agreement with its high reactivity toward
cyclohexene at -70 °C (Figure S4). Hence, the rate of formation
of cyclohexene oxide is expected to be equal to the rate of 2a
formation (WF(epoxide) ) WF(2a)). The expected concentration of
cyclohexene oxide formed during t seconds of the catalytic reaction
is given by following equation
The main value of this paper is EPR spectroscopic trapping of
the actual epoxidizing agents of bioinspired catalyst systems based
on nonheme iron complexes and hydrogen peroxide. Despite the
nature of these iron-oxygen species is not entirely clear, their key
role in epoxidation has been reliably established.
Acknowledgment. The authors thank the Russian Foundation
for Basic Research, Grants 06-03-32214 and 09-03-00087, for
financial support. We are grateful to Prof. Dr. Hans Brintzinger
for fruitful comments.
[cyclohexene oxide] ) k × [2a] × t
(1)
Values of k ) (5.4 ( 1.3) × 10-4 s-1 and [2a] ) (2.0 ( 0.8) ×
10-3 M, derived from our measurements, would thus predict that
the cyclohexene oxide formed during 1 h of the catalytic reaction
at -70 °C (eq 1) should reach a concentration of (3.9 ( 2.5) ×
10-3 M. Experimentally, a concentration of cyclohexene oxide of
7.4 × 10-3 M was determined by gas chromatographic product
analysis (see SI for details). That the predicted and experimental
epoxide yields are close strongly supports the key role of 2a in
selective epoxidation. The results of catalytic cyclohexene epoxi-
dation by systems 1,2/CH3CO3H(m-CPBA) are presented in Table
S3.
To view the role of intermediates such as 1a and 2a in a broader
perspective, we have compared the reactivity patterns of complexes
1-6 with regard to cyclohexene epoxidation by H2O2. The results
of these studies (Table S4) show that only the systems 1-4/H2O2
demonstrate epoxidation activity at 25 °C. Interestingly, intermedi-
ates of the type 1a, 2a, 3a, and 4a, with EPR parameters presented
in Table S5, were observed only in systems 1-4/CH3CO3H and
1-4/m-CPBA (Figures 1, S2, and S5). In the systems 5,6/m-CPBA
and 5,6/CH3CO3H, the only unidentified signal near g ) 4 is
observed.
Supporting Information Available: Detailed experimental proce-
dures, EPR spectra, Table S1 of the influence of olefin addition on the
decay of 1a, Table S2 of maximum concentration of 1a and 2a, Tables
S3 and S4 of epoxide yields for cyclohexene oxidations, Table S5 of
1
EPR spectroscopic parameters for S ) /2 iron-oxygen species. This
material is available free of charge via the Internet at http://pubs.acs.org.
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1
) /2 species. Several iron-oxygen intermediates, most notably
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