Formation of a room temperature stable Fev(o) complex: Reactivity toward unactivated c-h bonds

Article abstract of DOI:10.1021/ja412537m

An Fe^V(O) complex has been synthesized from equimolar solutions of (Et₄N)₂[Fe^III(Cl)(biuret-amide)] and mCPBA in CH₃CN at room temperature. The Fe^V(O) complex has been characterized by UV-vis, EPR, M?ssbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol^-1 (cyclohexane) to 84.5 kcal mol^-1 (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.

Full text of DOI:10.1021/ja412537m

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Communication
Formation of a Room Temperature Stable Fe<V>(O)
Complex: Reactivity Towards Unactivated C–H bonds
Munmun Ghosh, Kundan K Singh, Chakadola Panda, Andrew C. Weitz, Michael
Paul Hendrich, Terrence J. Collins, Basab Bijayi Dhar, and Sayam Sen Gupta
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 Jan 2014
Downloaded from http://pubs.acs.org on January 6, 2014
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TOC for the communication
“Hydroxylation of C-H bond by FeV=O complex at Room Temperature”
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Formation of a Room Temperature Stable FeV(O) Complex:
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Reactivity Towards Unactivated C–H bonds
Munmun Ghosh,1 Kundan K. Singh,1 Chakadola Panda,1 Andrew Weitz,2 Michael P. Hendrich,2 Terꢀ
rence J Collins,2,3 Basab B. Dhar1* and Sayam Sen Gupta1*
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1Chemical Engineering Division, CSIRꢀNational Chemical Laboratory, Dr. Homi Bhabha Road, Pune, INDIA; 2Institute for
Green Science, Department of Chemistry, 3Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA, USA
Email : ss.sengupta@ncl.res.in; Fax: +9125902621; Tel: +9125902747; bb.dhar@ncl.res.in; Tel: +9125903207
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ABSTRACT: An FeV(O) complex has been synthesized
ent to physiological temperatures would have the potenꢀ
tial to advance insight into enzymatic oxidation processꢀ
es, and especially oxidations of the C–H bonds of unacꢀ
tivated alkanes which are among the most difficult of all
oxdiation processes to carry out in a controlled fashion.
from equimolar solutions of (Et4N)2[FeIII(Cl)(biuretꢀ
amide)] and mCPBA in CH3CN at room temperature. The
FeV(O) complex has been characterized by UV–vis, EPR,
Mössbauer and HRMS and shown to be capable of oxidizꢀ
ing a series of alkanes having C–H BDEs ranging from
99.3 kcal molꢀ1 (cyclohexane) to 84.5 kcal molꢀ1 (cumene).
Linearity in the BellꢀEvansꢀPolayni graph and the finding
of a large kinetic isotope effect suggest that hydrogen abꢀ
straction is engaged the rate determining step.
High valent ironꢀoxo intermediates play key roles in
enzymatic oxidations.1ꢀ5 For example, in the cytochrome
P450 enzymes, the high valent FeIV(O)(porphyrinꢀ
radicalꢀcation), isoelectronic with FeV(O), has been
shown to be the reactive intermediate in the selective
hydroxylation of camphor.2,6 In the Rieske dioxygenase
enzyme family1 an FeV(O) active intermediate has been
proposed.7,8 Several functional models of both heme9
and nonꢀheme10ꢀ13 ironꢀdependent monooxygenase enꢀ
zymes have been synthesized15ꢀ18 including the TAML
system which has provided fully functional, small moleꢀ
cule replicas of the peroxidase and shortꢀcircuited P450
enzymes.12,13 For nonꢀheme iron catalyzed oxidations,
FeIV=O active intermediates have been isolated and
structurally characterized and their reactivity towards C–
H bond hydroxylation has been studied in detail.9,14ꢀ16
Synthetic functional models of Rieske dioxygenase famꢀ
ily of enzymes have been postulated to engage an
FeV(O) reactive species in C–H and C=C bond oxidaꢀ
tions.17,18 Several complexes having FeV(O) have been
reported and they are thermally not stable above ꢀ40
°C.19,20 Que et. al. has described the formation of an
FeV(O) complex from [FeIV(O)(TMC)(MeCN)]2+ at ꢀ44
°C (t1/2 60 min)21. Recently, Costas et. al. has reported
formation of a purported FeV(O) at ꢀ60 °C (characterized
using mass spectroscopy) and studied its reactivity.18
Collins et. al first produced a nonꢀheme FeV(O) complex
by oxidation of an FeIII–TAML complex at ꢀ60 °C using
mꢀchloroperbenzoic acid (mCPBA) this was thoroughꢀ
ly characterized by UV–vis, ESIꢀMS, EPR, EXAFS and
Mössbauer spectroscopies.22 The FeV(O) intermediate
was shown to be reactive towards sulfoxidation and
epoxidation in the ꢀ40 to ꢀ60 °C range in nitrile solꢀ
vents.22,23 However, to date it has not been possible to
characterize and explore the ambient conditions for the
reactivity of an FeV(O) complex because no system has
been stable enough to permit this. The availability of an
FeV(O) complex that is both stable and reactive at ambiꢀ
Figure 1. Schematic presentation of FeV(O) formation and
reaction towards cyclohexane.
We have recently reported the first preparation and inꢀ
itial studies of an Fe(III) complex of a biuretꢀamide
based macrocyclic ligand, 1,24a a member of the broad
suite of catalysts called TAML activators that were inꢀ
vented by Collins in the midꢀ1990s.24b Complex 1 is the
first member of a fifth generation of TAML activators—
generation numbers mark the order of preparation of sets
of TAML activators with distinctive structural motifs.
Activator 1 differs from the prototypical first generation
TAML activator by substitution of the CMe2 moiety in
the sixꢀmembered macrocylic subꢀring25 with an –NMe
group (Figure 1). The planar 6ꢀmembered ring allows
electron donation from –NMe group and subsequent
delocalization of the electron density throughout the
ring.26 This is evident in electrochemical studies where
the Fe(IV)/Fe(III) couple of 1 is 230 mV higher24a than
the corresponding FeꢀTAML. This in addition to the fact
that –NMe group is situated far away from the Fe center
made us to believe that FeV(O) complex (2) should have
stablity at temperatures higher than ꢀ40 °C.22,23 In this
article, we report the formation and characterization of
the FeV(O) complex 2 from 1. Remarkably, 2 is suffiꢀ
ciently stable that it can be produced quantitatively and
its reactivity can be examined at room temperature. We
also demonstrate that 2 readily cleaves the unactivated
alkane C–H bond of cyclohexane –studies of the kinetics
lead to the conclusion that H atom abstraction by FeV(O)
is the rate determining step (r.d.s) in the resulting hyꢀ
droxylation process (Figure 1).
Complex 2 was prepared at 25 °C from the parent
TAML activator, (Et4N)2[FeIII(Cl)(biuretꢀamide)] 1, in
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CH3CN by adding equimolar amounts of metaꢀ
decrease characteristic 613 nm band of 2. The initial rate
of the decay was found to be first order with respect to 2
and the first order rate constants for the decay (k5/4,3
chloroperbenzoic acid (mCPBA).22 Addition of 0.5
equivalent of mCPBA (5 × 10ꢀ5 M) to 1 (10ꢀ4 M) at RT
in CH3CN with exclusion of O2 afforded a violet colꢀ
oured solution. Addition of a second half equivalent of
mCPBA to this violet solution resulted in the formation
of a green solution with distinct absorption maxima at
441 nm (ε = 4.35 × 103 Mꢀ1cmꢀ1) and 613 nm (ε = 3.42 ×
103 Mꢀ1cmꢀ1) (Figure 2A). The green solution was examꢀ
ined by mass spectrometry, EPR and Mössbauer specꢀ
troscopies. A sample of the green solution was split and
frozen in liquid nitrogen, and then analysed with EPR
and Mössbauer spectroscopies. The Xꢀband EPR specꢀ
trum at 21 K showed a rhombic S = 1/2 species with g =
1.983, 1.935, 1.726 (Figure 2D). Spin quantification
indicated quantitative conversion of the starting Fe(III)
complex (1) to the corresponding FeV(O) (2). The
Mössbauer spectrum of 57Fe enriched 2 at 4 K showed a
doublet with an isomer shift of ꢁ = ꢀ0.44 mm/s and
quadrupole splitting of ꢁEq = 4.27 mm/s (Figure SI 1).
A paramagnetic pattern is not observed, and the doublet
is broad due to intermediate relaxation of the spin sysꢀ
tem. The intermediate relaxation and broad EPR signal
suggest molecular aggregation. The gꢀvalues and Mössꢀ
bauer parameters are close to those of the FeV(O) comꢀ
plex reported previously.22 Both the EPR and Mössbauer
spectral analyses suggest quantitative conversion
(≥95%) of the starting 1 to 2 (Figure 2, SI 1). This is in
contrast to the FeV(O) prototype TAML where only 70%
of Fe(V) was produced from the starting FeIII complexes
at ꢀ40 °C (by Mössbauer spectroscopy).22 HRMS examꢀ
ination of 2 in CH3CN revealed one prominent ion at a
massꢀtoꢀcharge ratio of 429.0745 (calculated m/z
429.0730) the isotopic distribution pattern correspondꢀ
ed to that expected for 2 (Figure 2C). HRMS analysis of
a solution prepared by introduction of H218O (0.5 ꢂL)
into the solvent media during the synthesis of 2 showed
70 % formation of FeV(18O) (m/z 431.078, Figure SI 2).
Addition of 1 eq. of 1 to 2 also resulted in formation
of a violet solution, the UV–vis spectrum (Figure 2A) of
which was identical to the solution formed by addition
of 0.5 eq of mCPBA to 1. This common UV–vis specꢀ
trum with its conspicuous features at long wavelengths
was very similar to the known UV–vis spectra of ꢂꢀoxoꢀ
(FeIV)2 species of established FeꢀTAML complexes.23,27
Moreover, the violet solution was found to be EPR siꢀ
lent. Since they were diamagnetic, their 1H NMR spectra
was recorded (Figure SI 3) which again reproduced the
behavior of the parent (FeIV)2O TAML complexes.27 The
rate of comproportionation between 1 and 2 was deterꢀ
mined to be 1.00 × 105 Mꢀ1sꢀ1, twice the rate of the corꢀ
responding process for the prortotype TAML (NꢀMe
replaced by CMe2, Figure SI 4) consistent with the less
1
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)
were determined from the slope of the straight lines at
three different temperatures of 25, 10 and 4 °C (Figure
SI 5). The k5/4,3 value at 25 °C (4.45 × 10ꢀ5 sꢀ1) was simiꢀ
lar to the FeV(O) prototype TAML reported at ꢀ40 °C,23
showing that the biuretꢀsubstituted TAML ligand leads
to a much more stable FeV(O) complex. In fact, 2 is the
first example of an FeV(O) complex that is stable at
room temperature. The stability of the FeIV species (genꢀ
erated by reaction of 1 with 0.5 equivalent of mCPBA)
was determined by monitoring the decrease of a characꢀ
teristic band at 428 nm. Over time this proposed
(FeIV)2O slowly converted to 1.
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Figure 2. (A) UV–vis spectral changes upon addition of
0.5 eq of mCPBA (5 × 10ꢀ5 M) to 1 (10ꢀ4 M): Orange =
spectrum of 1, violet = proposed (FeIV)2O dimeric product.
(B) UV–vis spectral changes upon addition of 0.5 mcpba (5
× 10ꢀ5 M) to the preformed (FeIV)2O: Green = spectrum of
FeV(O). (C) HRMS spectra of green FeV(O). (D) EPR specꢀ
tra: Red = FeV(O), Blue = simulated spectrum.
The unprecedented high stability of 2 has allowed us
to examine its room temperature reactivity. Activation of
C–H bonds by 2 at room temperature were studied for a
range of hydrocarbons with bond dissociation energies
(BDE), spanning 85–100 kcal molꢀ1.28 Excess cyclohexꢀ
ane (1000 eq.) was added to a solution of 2 (10ꢀ4 M) in
CH3CN at 25 °C with exclusion of O2 and the reaction
was monitored by UV–vis spectroscopy. Upon addition
of cyclohexane, spectral scans showed the rapid forꢀ
mation of a mixture of FeIV and FeIII species which then
slowly converted to 1 (ESIꢀMS, UV–vis confirmation,
Figure 3). GC and GCꢀMS analysis indicated cyclohexꢀ
anol and cyclohexanone as products (Table 1) showing 2
is capable of effective oxygen atom insertion into an
unactivated C–H bond. Based on the concentration of 2,
the yield of the reaction was determined to be 44% with
an alcohol to ketone ratio of 7:1 (Table 1, Figure SI 6,
7). Reactions of 2 with substrates having stronger and
weaker C–H bonds than cyclohexane were also studied.
sterically encumbered nature of the biuret system.23
A
detailed study to elucidate the nature of this FeIV is curꢀ
rently underway.
The spontaneous reduction of FeV to FeIV/FeIII was
studied using UV–vis spectroscopy by monitoring the
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Addition of benzene (BDE of C–H ~110 kcal molꢀ1) to a
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cies did not lead any product formation on the timescale
(≥ 6 hr) of the experiments.
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solution of 2 showed no additional change in the UV–vis
spectrum and no product was observed by GCꢀMS. In
contrast, substrates having lower BDE like cumene,
Extensive kinetic studies were performed to ascertain
that nature of the hydroxylations by 2, using UV–vis
Table 1. Summary of data for the oxidation of different hydrocarbon by FeV(O)
Alkane (no. of equivalent H atom)
BDEC–H ( kcal molꢀ1)
k2 (Mꢀ1sꢀ1)
Products (eq./Fe)
PhC(OH)(CH3)2 (0.49 ); PhC(O)CH3 (0.07)
PhCH(OH)CH3 (0.3); PhC(O)CH3 (0.22)
PhCHO (0.30)
Con.
56%
52%
30%
46%
42%
PhCH(CH3)2 (1)
84.5
87
(7.91 ± 0.09) × 10ꢀ1
(2.84 ± 0.21) × 10ꢀ1
(1.36 ± 0.16) × 10ꢀ1
(3.50 ± 0.10) × 10ꢀ2
(2.26 ± 0.10) × 10ꢀ2
PhEt (2)
PhCH3 (3)
90
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2,3ꢀdimethylbutane (2)
Cyclohexane (12)
96.5
99.3
2,3ꢀdimethylꢀ2ꢀbutanol (0.46)
C6H11OH (0.4); C6H10O (0.02)
ethylbenzene, toluene and 2, 3ꢀdimethylbutane (DMB)
reacted more rapidly with 2 than with cyclohexane. In
case of DMB, 2 selectively hydroxylated the tertiary C–
H bond (BDE ~96 kcal molꢀ1, Table 1) in preference to
the primary C–H bond (BDE ~99 kcal molꢀ1), leading
exclusively to the formation of 2,3ꢀdimethylꢀ2ꢀbutanol.
spectroscopy at the isosbestic points for FeIII and FeIV
interconversions (353 and 400 nm) under pseudo first
order conditions. The pseudo firstꢀorder rate constant
(kobs) calculated from the absorbance vs. time traces at
both wavelengths were obtained from nonlinear curve
obst)]
fitting [(At = Aα – (Aα – Ao)e(ꢀk
(Figure SI 8, 9) and
exhibited good agreement in rate constant values within
5% error. The kobs values thus obtained correlated linearꢀ
ly with the substrate concentration to provide the second
order rate constant k2 (Figure SI 9) and it was observed
that the rate constant decreased with an increase in the
BDE of C–H bonds in the substrates. Linearity in Bellꢀ
EvansꢀPolanyi (BEP) relation was found from the plot of
log k2' (k2/ the number of equivalent Hꢀatoms) vs. BDE
(Figure 3)28 for all the substrates from cyclohexane to
cumene (slope of ꢀ0.17). This linearity supports hydroꢀ
gen abstraction (H˙) from a C–H bond by FeV(O) group
of 2 in the r.d.s as has been previously reported for
[FeIV(O)(N4Py)(CH3CN)] complexes.29 A significant
kinetic isotope effect (KIE) of 9 was observed for toluꢀ
ene/d8ꢀtoluene at 25 °C, supporting the conclusion that
the r.d.s entails abstraction of an H atom from the C–H
bond by the FeV(O) (Figure 3, inset) – KIEs of 2.1 and
6.5 have been found for typical porphyrin πꢀradical catiꢀ
on [(4ꢀTMPyP)+•FeIV(O)]+ and [(TMP)+•FeIV(O)(pꢀCH3ꢀ
PyO)]+ with xanthene and d2ꢀxanthene, respectiveꢀ
ly.9,29,30 Oxygen incorporation from FeV(O) into the cyꢀ
clohexane was followed by using 55% O18 labeled FeV
which resulted 35% O18 enriched product supporting a
rebound mechanism. (Figure SI 7).
Thus, the data supports the conclusion that the mechaꢀ
nism of C–H activation by 2 involves initial abstraction
of an H atom from the hydrocarbon substrate by FeV(O)
followed by rebound to yield the oxidized product toꢀ
gether with the regeneration of the parent FeIII complex
1 (Figure 3A and SI 10 for pictorial representation of the
proposed mechanism). The FeIII complex 1 thusꢀformed
undergoes rapid comproportionation (Figure SI 4) with 2
to form the observed ꢂꢀoxoꢀ(FeIV)2 product (UV–vis, 1H
NMR). Kinetic studies of the comproportionation reacꢀ
tion between 2 and 1 show that the second order rate
constant (1.00 × 105 Mꢀ1sꢀ1) (Figure SI 4) is at least 105
folds faster than the rate for C–H activation. Since this
comproportionation reaction is extremely fast, the direct
conversion of 2 to 1 is not observed in initial spectral
scans. This is also corroborated by the observation that
Figure 3. A. UV–vis spectral changes upon reaction of 2 (1
× 10ꢀ4 M) with cyclohexane (0.093 M). (Inset) the absorbꢀ
ance vs time plot at 400 nm (▪ indicates experimental data
point; the red line is the first order fit according to the
'
equation [(At = Aα – (Aα – Ao)e(ꢀk t)]. B. logk2 vs BDEC–H
of various hydrocarbons for the reactions with 2 at 25 °C.
(Inset) Plot of kobs/minꢀ1 vs [Toluene] (black line) and [d8ꢀ
toluene] (red line) showing a pronounced KIE at 25 °C.
obs
To confirm, if part of the product obtained was due to
the reaction with the FeIV species which is initially
formed upon addition of substrate to 2, the reactivity of
the FeIV species (generated by the reaction of complex 1
with 0.5 eq of mCPBA) towards CꢀH activation was also
explored. It was found from UVꢀvis, GC and GCꢀMS
studies that the reaction of cyclohexane with FeIV speꢀ
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(2) Meunier, B.; de Visser, S. l. P.; Shaik, S. Chem. Rev. 2004,
104, 3947ꢀ3980.
the product formation in all the reactions is less than
50% for all the substrates. Similar observations have
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(3) Que, L.; Tolman, W. B. Nature 2008, 455, 333ꢀ340.
(4) Rittle, J.; Green, M. T. Science 2010, 330, 933ꢀ937.
(5) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemley, J. N.;
Lee, S.ꢀK.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y.ꢀS.; Zhou,
J. Chem. Rev. 1999, 100, 235ꢀ350.
(6) Ortiz de Montellano, P. R. Chem. Rev. 2009, 110, 932ꢀ948.
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M. J. Am. Chem. Soc. 2003, 125, 13008ꢀ13009.
(8) Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M.; Krebs,
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been reported for sulphide oxidation by the parent
FeV(O) TAML activator. However, in this system, the
likely (FeIV)2O product formed by reacting 2 and 1 unꢀ
dergoes further slow reduction to finally yield 1. The
faster selfꢀreduction of our FeIV species is in contrast to
the prototype FeꢀTAML system, where the ꢂꢀoxoꢀ
(FeIV)2 is extremely stable and does not undergo reducꢀ
tion. Since the FeIV species is not reactive towards oxiꢀ
dation of alkanes, the formation of the oxidized product
in the reaction is solely due to the reaction of alkanes
with 2. Preliminary experiments performed in the presꢀ
ence of O2 show high amounts of ketone formation in
respect to the corresponding alcohol, indicating that the
radical formed after C–H abstraction is capable of reactꢀ
ing with O2 as has been proposed before.30
In conclusion, we have successfully synthesized an
FeV(O) complex of a biuretꢀcontaining TAML actvator
at room temperature. EPR and Mössbauer spectroscopic
studies show quantitative conversion of FeIII (1) to the
FeV(O) complex. This complex displays remarkably
higher stability at room temperature than any previously
reported FeVꢀoxo complex. This higher stability has alꢀ
lowed us to study oxidation reactions with alkanes havꢀ
ing strong C–H bond such as in cyclohexane (BDEC–H
99.3 kcal molꢀ1) at room temperature. This is the first
report of a wellꢀdefined FeV(O) species that has been
shown to react with strong CꢀH bonds. It was observed
that 2 oxidizes cyclohexane to cyclohexanol and cycloꢀ
hexanone with high reaction rates, k2 at 25 °C is (2.26 ±
0.10) × 10ꢀ2 Mꢀ1sꢀ1 (Figure SI 9).31 It is possible that the
subsequent oxygen atom incorporation may proceed by
either rebound mechanism or a dissociative mechanism
or a combination of both as has been recently proꢀ
posed.32 Studies aimed at further understanding the
complete reaction mechanism including efforts to crysꢀ
tallographically characterize 2 are underway.
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ACKNOWLEDGMENT
SSG
acknowledges
DAE
(BRNS;
Grant
no
2009/37/33/BRNS) for funding. M.G., K.K.S, C.P.
acknowledge CSIR (Delhi) for fellowship. Dr. B. B. Dhar
also acknowledges CSIR for SRA position. SSG and MG
thank Dr. A. Ryabov for discussion.
Supporting information
Experimental details, characterization data, details of kiꢀ
netic studies and proposed mechanism. This material is
available free of charge via the Internet at
http://pubs.acs.org.
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3635ꢀ3636.
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