Characterization and alkane oxidation activity of a diastereopure seven-coordinate iron(III) alkylperoxo complex

Article abstract of DOI:10.1039/b706604j

Spectroscopic characterization and alkane oxidation studies of a diastereopure seven-coordinate high-spin iron(iii) alkylperoxo complex based on the chiral N,N′,N-bis(l-prolinate)pyridine ligand Py(ProMe)₂ (1) are reported. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b706604j

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Characterization and alkane oxidation activity of a diastereopure
seven-coordinate iron(III) alkylperoxo complex†
Silvia Gosiewska,a Hjalmar P. Permentier,b Andries P. Bruins,b Gerard van Kotena and
Robertus J. M. Klein Gebbink*a
Received 1st May 2007, Accepted 24th May 2007
First published as an Advance Article on the web 26th June 2007
DOI: 10.1039/b706604j
Spectroscopic characterization and alkane oxidation stud-
ies of a diastereopure seven-coordinate high-spin iron(III)
alkylperoxo complex based on the chiral N,Nꢀ,N-bis(L-
prolinate)pyridine ligand Py(ProMe)2 (1) are reported.
on (R,R)-BPMCN,8d we have set out to explore the reactivity
of enantiopure iron(II) complexes with alkylperoxides and the
potential of possible Fe(III)–OOtBu intermediates in selective
oxidation reactions. Previously, we reported the synthesis and the
solid state and solution studies of diastereopure, high-spin Fe(II)
complex [Fe(OTf)2(1)] (2) with an unusual pentagonal bipyrami-
dal geometry (pbp) (Scheme 1).11 Solution studies revealed the
integrity of complex 2 in solution with the Py(ProMe)2 ligand
(1) retaining its pentagonal planar coordination of the NN’NOO
donor atoms around the Fe(II) center as was found in the solid
state. The triflate anions which are coordinated at the axial
positions of the pbp in 2 can be substituted for solvent molecules
like acetonitrile (3) or water. The well-defined ligand coordination
around the iron center and the facile replacement of the axial
triflate anions prompted us to investigate the reactivity of this
complex as a catalyst in the oxidation of alkanes with tert-butyl
hydroperoxide (TBHP). In the course of this study we observed
the formation of a purple-colored, transient species 4 and here
we report the result of its independent synthesis (Scheme 1),
spectroscopic characterization, and catalytic activity.
In recent years, iron(III)-peroxo species have been observed
as intermediates in many biological oxidations catalyzed by
mononuclear, non-heme iron enzymes1–3 and in the antitumor
drug bleomycine.4 In addition, high-valent iron(IV) and iron(V)
oxo-species have been postulated as possible products of iron(III)-
peroxo species decomposition by homolysis or heterolysis of
the O–O bond.5 Several alkylperoxo iron(III) intermediates of
enzyme model compounds were trapped at low temperatures and
studied spectroscopically.6 These low-spin7 and high-spin8 alkyl
peroxoiron(III) complexes display very different spectroscopic
features, notably in their Raman, EPR and Mo¨ssbauer spectra.
Theoretical calculations on [Fe(OOtBu)(OHx)(L)]x+ (x = 1 or 2)
revealed different decomposition pathways for a low-spin (L =
TPA, tris(2-pyridylmethyl)amine) and a high-spin complex (L =
6-Me3TPA).9 This seems to be the consequence of the high energy
barrier of the LFeO–OtBu homolytic cleavage process of the
high-spin Fe(III) alkylperoxo complex as compared to that of
the process with low-spin intermediates. Hence, the reactivity
of the alkylperoxo intermediates depends on the spin state of
Fe(III) ion. The homolytic LFe–OOtBu bond cleavage yielding
back the starting Fe(II) complex and tBuOO• for the high-
spin [FeIII(OOR)(6-Me3-TPA)(O2CAr)]+ complex was proposed
by Que et al. as the reasonable alternative decay pathway of the
high-spin intermediates.10 However, more recently the decomposi-
tion of the high-spin [FeIII(OOtBu)(HOOtBu)BPMCN]2+complex
(BPMCN
=
(R,R)-N,Nꢀ-bis-(2-pyridylmethyl)-N,Nꢀ-dimethyl-
1,2-cyclohexane diamine) via O–O homolytic cleavage of
the FeO–OtBu bond to yield
a high-valent iron species
[FeIV(OH)(OOtBu)BPMCN]2+ was reported.8d These observations
show that the fate of high-spin iron(III) alkylperoxo species is not
straightforward.
As most of the studied alkylperoxo iron(III) intermediates
contain achiral ligands, except for the recently published studies
Scheme 1 Synthesis and structure of intermediate 4 in acetonitrile.
aOrganic Chemistry and Catalysis, Utrecht University, Padualaan 8, 3584,
CH Utrecht, The Netherlands. E-mail: r.j.m.kleingebbink@chem.uu.nl;
Fax: +31 30252 3615; Tel: +31 30253 3054
Addition of three equivalents of TBHP (solution in decane) to
an acetonitrile solution of 2 results in the immediate formation of
the purple species 4, which has a characteristic UV-Vis absorption
at kmax = 582 nm (e = 1334 M−1 cm−1, Fig. 1a) arising from a
alkylperoxo-iron(III) LMCT band12 and which is persistant for
about 5 min at room temperature and for several hours at −40 ◦C.
bUniversity of Groningen, Mass Spectrometry Core Facility, A. Deusinglaan
1, 9713, AV Groningen, The Netherlands; Fax: +31 50363 8347; Tel: +31
50363 3262
† Electronic supplementary information (ESI) available: Experimental
synthetic and catalytic procedures and physical measurements, ESI-MS,
EPR, IR and catalytic results. See DOI: 10.1039/b706604j
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The UV-Vis, Raman and EPR spectroscopic features of inter-
mediate 4 correspond well with the spectroscopic data reported
for other high-spin iron(III) alkylperoxo species.8 Additionally,
the m(CO) around 1670 cm−1 in the IR spectrum of 4 and the
detection of the doubly substituted ions ([Fe(OH)(OOtBu)L]+
and ([Fe(OH)2L]+ by ESI-MS indicate the retention of the seven-
coordinate geometry around the iron center and the planar
coordination mode of ligand 1 as in 2 with two ligands in the axial
positions of the pbp, i.e. one tBuOO− anion and a second ligand
X (X = OH2, HO−, or MeCN). The pentagonal coordination
of the ligand donor atoms corroborates with the formation of a
single diastereoisomer with the same RNRNSCSC configuration as
in diastereopure 2.
The additional high-spin Fe(III) features as detected by EPR
spectroscopy most probably originate from other seven-coordinate
complexes with different axial ligands (a shoulder at 1684 cm−1 in
the IR spectra). The observation of a mixture of species by EPR,
whereas these additional species were not observed in the UV-
Vis and Raman spectra, indicates that these species are formed in
relatively low concentrations. The presence or absence of O2 during
the generation of species 4 did not influence its spectroscopic
characteristics.
Fig. 1 Spectroscopic features of intermediate 4; (a) UV-Vis spectra,
(b) Raman spectra and (c) ESI-MS spectra.
The Raman spectrum of 4 shows four features at 885, 850,
648 and 470 cm−1 (Fig. 1b), which is characteristic of a high-
spin iron(III) alkylperoxo complex.8 Based on the assignment of
1
Lehnert et al.9 for terminal g -alkylperoxo coordination, the two
features above 800 cm−1 correspond to the O–O stretch that is
strongly mixed with the C–C stretch vibration of the tBu group.
The band at 648 cm−1 is assigned to the Fe–O vibration and the
feature around 470 cm−1 to a combined d(OCC)/d(CCC) bending
vibration of the tBu group.
During the decay of 4 we did not spectroscopically observe the
formation of high-valent iron oxo-intermediates. Isolation of the
iron complex after reaction with oxidant, demetalation (with 30%
aqueous NH3), and 1H NMR analysis of the organic component
demonstrated that ligand 1 stays intact, i.e. no racemisation or
oxidation occurred. All our attempts to crystallize the transient
purple species from acetonitrile solution failed. Attempts to
crystallize 4 from CH2Cl2, in which it is stable for several days
below −40 ◦C, resulted in the crystallization of the decomposition
To gain more information about the ligand coordination in 4,
IR spectra were recorded in acetonitrile at −40 ◦C 15 min after the
addition of TBHP. Only a slight shift of the m(CO) of the prolinate
ester moieties from 1684 cm−1 in 3 to 1670 cm−1 in 4 was observed,
which points to the retention of coordination of both carbonyl
groups of ligand 1 to the Fe(III) center (Fig. S2†). In addition, the
observation of four sharp vibrations of the triflate anions indicates
that they are no longer coordinated to iron in 4.13
14
product [Fe(1)(OH2)2](OTf)2 which further corroborates the
structural integrity of the Fe-coordination mode upon generation
and deterioration of 4, and the robustness of chiral ligand 1.
Complex 2 in acetonitrile, i.e. complex 3, catalyzes the oxidation
of hydrocarbons in the presence of TBHP with reasonably high
TON’s after 1 h (4.2–9.5) and with high oxidant consumption (42–
94%) in the case of cyclohexane, adamantane, and ethylbenzene.
However, for sterically hindered 1,2-dimethylcyclohexane a much
lower TON (1.7) was found. Data for the oxidation of different
alkanes with a combination of 2 and TBHP/decane are sum-
marized in Table 1.15 Large excesses of substrate were used to
minimize over-oxidation of alcohols to ketones (ratio 2 : substrate :
oxidant = 1 : 500 : 10). The alcohol to ketone ratio (A/K) obtained
in the oxidation of both cyclohexane and ethylbenzene is <1. A
normalized 3◦/2◦ ratio of 12.3 was obtained in the oxidation of
adamantane. In the oxidation of cis-1,2-dimethylcyclohexane, the
stereo-retained product, (S,S & R,R)-1,2-ol formed in addition
to the product with loss of the original stereochemistry, (R,S &
S,R)-1,2-ol. In the oxidation of ethylbenzene, a 6.5% enantiomeric
excess was obtained for one of the reaction products, 1-phenyl
ethanol.
Positive electrospray ionization mass spectrometry on 4 showed
four main ions at m/z 655, 583, 253 and 217. These mass values and
+
their isotope patterns correspond to {[FeIII(OOtBu)(1)](OTf)}
(m/z = 655), {[FeIII(OH)(1)](OTf)} (m/z = 583), (Fig. 1c left
+
and right, respectively) and {[FeIII(OOtBu)(1)])} (m/z = 253)
+2
and {[FeIII(OH)(1)]} (m/z = 217). Ions with the stoichiometry
+2
{[FeIII(OOtBu)(OH)(1)]} (m/z 523) and {[FeIII(OH)2(1)]} (m/z
451) are also present in the spectra, but with lower intensities
(Fig. S3†). The observation of ions with one tBuOO− or HO−
anion as well as of ions substituted with two of these anions,
points to the formulation of the intermediate species 4 as
[FeIII(OOtBu)(X)(1)](OTf)y (X = OH2, MeCN or HO− and y = 1
or 2) regardless of the TBHP solution (decane or water) used in its
generation. The hydroxyl anions observed in the mass spectra of 4
can originate from fragmentation of either tBuOO− or of an aquo
ligand8b introduced by the oxidant solution or during ambient
sample handling.
+
+
EPR spectra of frozen solutions of 4 show a mixture of high-
spin Fe(III) species with S = 5/2 (Fig. S4†). The major feature
at g = 7.7, 4.26 and 2.01 with E/D = 0.07 represents a high-
spin Fe(III) center with a small distortion from axial symmetry.
Regrettably, we were not able to calculate the E/D value for the
other minor high-spin features or to simulate these features with
sufficient accuracy.
Much lower yields of the oxidation products were found when
these oxidations were performed under Ar (Table 1) and in
the presence of radical traps like acetone, CCl4, or 2,6-di-tert-
butyl-4-methylphenol (Table S5†). The A/K ratio in cyclohexane
oxidation remained lower than one also when the reaction was
performed in acetone or in the presence of radical traps. Only a
small amount of 1-chloro-adamantane was detected in GC-MS,
3366 | Dalton Trans., 2007, 3365–3368
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Table 1 Oxidation of alkanes catalyzed by complex 2 with TBHP–decanea
Substrate
Product
TONb
Conv. oxidant
42% (5%)
Remarks
Cyclohexanec,d
Adamantanec
Cyclohexanol
Cyclohexanone
Adamant-1-ol
1.6 (0.5)
2.6 (n.o.)
6.9 (0.6)
1.1 (n.o.)
0.5 (n.o.)
1.2
0.5
4
5.4
A/K = 0.6 (0.5)
3◦/2◦ = 12.6 (1.8)
85% (6%)
Adamant-2-ol
Adamant-2-one
(R,R & S,S)-1,2-ol
(R,S & S,R)-1,2-ol
1-Phenyl ethanol
Acetophenone
cis-1,2-Dimethylcyclohexane
17%
94%
RCe = 41%
Ethylbenzene
ee% = 6.5
A/K = 0.74
a Reaction conditions: 0.5 mL of 70 mM tBuOOH (5–6 M decane solution) solution in MeCN was added slowly over 30 min to a stirred solution of
2.8 mL MeCN containing 2 (3.5 lmol) and substrate (1.75 mmol) at ambient temperature, stirring was continued for another 30 min; ratio of cat :
oxidant : substrate = 1 : 10 : 500. The reported values are the average of two independent runs. b Moles of product/moles of catalyst. c The values in
parentheses were obtained under Ar, n.o. = not observed. d Cyclohexyl tert-butylperoxide was only observed in GC-MS when 150 eq of oxidant were
used (results not shown). e Retention of configuration (RC) of 3◦-ol = 100 × [(RR + SS) − (RS + SR)]/(3◦-ol).
while no cyclohexylchloride or cyclohexyl tert-butylperoxide was
observed under similar conditions in the oxidation of cyclohexane.
The low A/K ratios, observed for cyclohexane and ethylbenzene
indicate that O-centered alkylperoxy radicals,7c,7d,16 which can dis-
proportionate into products via Haber–Weiss reactions17a and Rus-
sell termination steps,17b most likely participate in the oxidation
process. The formation of the 1,2-ol product with the loss of the
stereochemistry in the oxidation of cis-1,2-dimethylcyclohexane
can be explained by the formation of substrate alkyl radicals with
a lifetime sufficiently long to allow epimerization at the radical
site. These observations together with the significant decrease of
the oxidation products in the presence of radical traps imply that
alkyl radicals are to a certain extent involved in the oxidation of
the alkanes with 2 and TBHP.16 On the other hand, the normalized
3◦/2◦ ratio (12.3) for adamantane oxidation is significantly larger
than that found for Gif-type oxidations (2.7) or oxidations
attributed to HO• (2) or tBuOO• (6-10) radicals,7c,18 and is in the
range of oxidations with PhIO catalyzed by P450 mimics19 and
close to the values obtained with [FeII(TPA)(MeCN)2]/H2O2.20
The high 3◦/2◦ ratio indicates that an oxidant more selective than
HO• or tBuO• is involved.
In conclusion, we have demonstrated by several spectroscopic
techniques the formation of a diastereopure, mononuclear, high-
spin iron(III) alkylperoxo complex with seven-coordinate geometry
which is unique for Fe(III) alkylperoxo complexes.7,8 Complex 2
shows catalytic activity in the oxidation of non-activated alkanes
with high TON’s and the possibility that a metal-based oxidant
is involved in the oxidation cannot be excluded in view of the
high 3◦/2◦ ratio for the oxidation of adamantane, even though
O-radicals and C-radicals are apparently generated during these
reactions.
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12 Complex 4 is also formed when an aqueous solution of TBHP (70%)
is added to an acetonitrile solution of 2 with kmax = 568 nm (e =
1046 M−1 cm−1, Fig. S1†) showing essentially identical ESI-MS spectra.
13 For detailed analysis of the IR vibrations of coordinated and uncoor-
dinated triflate anions to the iron, see ref. 11.
The authors thank Dr Gerard Roelfes (Stratingh Institute,
University of Groningen), Dr Ernst E. van Faassen (Department
of Surfaces, Interfaces and Devices, Utrecht University), Dr
Eli Stavitski and Dr Kaisa Kervinen (Inorganic Chemistry and
Catalysis, Utrecht University). This work was supported by the
National Research School Combination-Catalysis (NRSC-C).
Notes and references
1 For lipoxygenases see: E. Skrzypczak-Jankun, R. A. Bross, R. D. Caroll,
W. R. Dunham and M. O. Funk, Jr., J. Am. Chem. Soc., 2001, 123,
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14 The synthesis and X-ray structural determination of [Fe(1)(OH2)2]-
(OTf)2 was reported earlier by us, see ref. 11.
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15 Generally higher TON’s were observed with TBHP–decane then with
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3368 | Dalton Trans., 2007, 3365–3368
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