Modeling Non-Heme Iron Halogenases: High-Spin Oxoiron(IV)-Halide Complexes That Halogenate C-H Bonds

Article abstract of DOI:10.1021/jacs.5b11511

The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe^IV(O)(TQA)(Cl/Br)]⁺ (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O₂ to prevent halogenation, supporting an emerging notion that rapid rebound may not necessarily occur in non-heme oxoiron(IV) oxidations.

Full text of DOI:10.1021/jacs.5b11511

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Communication
Modeling Nonheme Iron Halogenases: High-Spin
Oxoiron(IV)-Halide Complexes that Halogenate C–H Bonds
Mayank Puri, Achintesh N. Biswas, Ruixi Fan, Yisong Guo, and Lawrence Que
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11511 • Publication Date (Web): 15 Feb 2016
Downloaded from http://pubs.acs.org on February 15, 2016
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Modeling Nonheme Iron Halogenases: High-Spin Oxoiron(IV)-
Halide Complexes that Halogenate C–H Bonds
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Mayank Puri,# Achintesh N. Biswas,# Ruixi Fan,‡ Yisong Guo,‡,* and Lawrence Que, Jr.#,*
#Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455,
United States
‡
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
Supporting Information Placeholder
methylguanidino)]ethyl}amine) obtained by halide exchange with
ABSTRACT: The nonheme iron halogenases CytC3 and SyrB2
catalyze C–H bond halogenation in the biosynthesis of some natuꢀ
the bound solvent of the precursor FeIV(O)(L)(solvent) complex.
The first two have S = 1 spin states, while the third has an S = 2
spin state, corresponding to that found for the halogenase interꢀ
mediates (Tables 1 and S1).10 While these complexes serve as
suitable spectroscopic models, no oxoiron(IV)ꢀhalide complex has
yet been demonstrated to have the ability to transform C–H bonds
into C–(Cl/Br) bonds. Some iron complexes capable of oxidative
ligand transfer are known, but the actual oxidants in these systems
have not been trapped and characterized.11,12,13 In this communiꢀ
cation, we report bona fide synthetic complexes that serve as both
spectroscopic and functional models for the oxoiron(IV)ꢀhalide
intermediates of nonheme iron halogenases.
We recently reported the generation of an S = 2 oxoiron(IV)
complex, [FeIV(O)(TQA)(MeCN)]2+ (1), that serves as both a
spectroscopic and a functional model of TauDꢀJ, the oxoiron(IV)
intermediate of the nonheme iron enzyme taurine dioxygenase
that effects taurine hydroxylation (Table 1).14 Addition of 1 equiv.
NBu4X (X = Cl or Br) to 1 in MeCN at ꢀ40 °C initiates ligand
exchange to afford oxoiron(IV) complexes [FeIV(O)(TQA)(Cl)]+
(2) and [FeIV(O)(TQA)(Br)]+ (3), respectively (Figure 1). UVꢀvis
titration experiments (Figure S1) show that 1 equiv. halide is sufꢀ
ficient to generate these new species fully. Complexes 2 and 3 are
less stable than 1, with halfꢀlives at ꢀ40 °C of approximately 5
minutes that correspond to a threeꢀfold faster selfꢀdecay rate.
ral products via S = 2 oxoiron(IV)ꢀhalide intermediates. These
oxidants abstract an Hꢀatom from a substrate C–H bond to generꢀ
ate an alkyl radical that reacts with the bound halide to form a C–
X bond chemoselectively. The origin of this selectivity has been
explored in biological systems but has not yet been investigated
with synthetic models. Here we report the characterization of S =
2 [FeIV(O)(TQA)(Cl/Br)]+ (TQA = tris(quinolylꢀ2ꢀmethyl)amine)
complexes that can preferentially halogenate cyclohexane. These
are the first synthetic oxoiron(IV)ꢀhalide complexes that serve as
spectroscopic and functional models for the halogenase intermeꢀ
diates. Interestingly, the nascent substrate radicals generated by
these synthetic complexes are not as shortꢀlived as those from
hemeꢀbased oxidants and can be intercepted by O2 to prevent
halogenation, supporting an emerging notion that rapid rebound
may not necessarily occur in nonheme oxoiron(IV) oxidations.
Nonheme iron halogenases are an important subset of the
family of αKGꢀdependent nonheme iron enzymes that utilize O2
to halogenate substrate C–H bonds in the biosynthesis of some
natural products.1 Two wellꢀstudied examples are CytC3 and
SyrB2, the enzymes responsible for the synthesis of cytotrienin
A/B and syringomycin E, which have antiꢀtumor and biosurfacꢀ
tant properties, respectively.2,3 The halogenase active site differs
from those of other αKGꢀdependent iron enzymes in having a
halide ligand in place of the carboxylate of the canonical 2ꢀHisꢀ1ꢀ
carboxylate facial triad that binds the iron.4 Despite this change,
the mechanism of dioxygen activation by nonheme iron halogenꢀ
ases is thought to be analogous to that of other αKGꢀdependent
nonheme iron enzymes. The active oxidant derived therefrom is
proposed to be a S = 2 oxoiron(IV)ꢀhalide species that abstracts a
Hꢀatom to form a hydroxoiron(III)ꢀhalide species and a substrate
radical. In the next step, the incipient radical can undergo rebound
with either the hydroxo or the halo ligand to afford respective Rꢀ
OH or RꢀCl products. In an elegant study, Matthews et al. showed
that substrate positioning controls the chemoselectivity of haloꢀ
genation over hydroxylation in SyrB2,5 which was corroborated
by HYSCORE studies on SyrB2ꢀsubstrateꢀNO complexes.6
2+/1+
O
N
N
Fe
N
X
N
[FeIV(O)(TQA)(X)]2+/1+
X = MeCN (1), Cl (2), Br (3)
There has been some effort to obtain synthetic models for the
oxoiron(IV)ꢀhalide intermediates of the halogenases. Notable
examples are FeIV(O)(L)(Cl/Br) complexes, where L = TPA,7
PyTACN,8 and TMG2dien9 (TPA = tris(pyridylꢀ2ꢀmethyl)amine;
PyTACN = 1ꢀ(pyridylꢀ2ꢀmethyl)ꢀ4,7ꢀdimethylꢀ1,4,7ꢀtriazacycloꢀ
Figure 1. UVꢀvisible spectrum of a 0.25 mM solution of 1 (black), 2 (red)
and 3 (blue) in acetonitrile at ꢀ40 °C, with a 1.0 mM solution of 1 (black),
2 (red) and 3 (blue) focused on the nearꢀIR region shown in the inset.
The electronic spectra of 2 and 3 display features at 370 nm
(ε ~ 5000 Mꢀ1 cmꢀ1), 625 nm (ε = 460 Mꢀ1 cmꢀ1) and 875 nm (ε =
nonane;
TMG2dien =
N-methyl-1,1ꢀbis{2ꢀ[N2ꢀ(1,1,3,3ꢀtetraꢀ
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110 Mꢀ1 cmꢀ1), which are blueꢀshifted relative to those of 1 (Figꢀ
Table 1. Spectroscopic properties of S = 2 oxoiron(IV) species.
1
2
3
4
5
6
7
8
ure 1). Similarly, a small blue shift is observed upon replacement
of the solvent ligand in the S = 2 [FeIV(O)(TMG2dien)(MeCN)]2+
complex with chloride.9 In contrast, S = 1 [FeIV(O)(L)(MeCN)]2+
complexes with L = TPA7 or PyTACN8 exhibit nearꢀIR features
that are red shifted by 50ꢀ80 nm upon halide addition (Table 1),
which can be rationalized by the fact that chloride and bromide
anions are weakerꢀfield ligands than acetonitrile. An explanation
for the observed blue shifts from 1 to 2 and 3 will require a better
understanding of the electronic spectra of this new family of S = 2
oxoiron(IV) complexes.
Evidence that the Fe=O unit remains intact upon formation of 2
and 3 is provided by resonance Raman spectroscopy. 514.5ꢀnm
excitation of frozen solutions of 2 and 3 reveals bands at 827 cmꢀ1
and 828 cmꢀ1, respectively, representing a downshift of ~10 cmꢀ1
relative to that of 1 (Figures S2a and 3a, respectively). 18Oꢀ
labeling of 2 and 3 elicits respective downshifts of 35 and 37 cmꢀ1,
consistent with the 36ꢀcmꢀ1 shift calculated for an Fe=O vibration
by Hooke’s law (Figures S2b and 3b, respectively). These
ν(Fe=O) values are remarkably close to the frequencies predicted
by DFT calculations for the FeIV(O)Cl and FeIV(O)Br intermediꢀ
ates of SyrB2 (824 cmꢀ1 and 825 cmꢀ1).15
The Mössbauer analysis of 2 and 3 at 4.2 K shows quadrupole
doublets for the oxoiron(IV) centers, respectively representing
65% and 70% of the iron in the samples, with parameters similar
to those of 114 (Table 1, Figures S4 and S5). Monoferric and diferꢀ
ric byproducts make up the balance of the iron in the samples.
Complexes 2 and 3 exhibit isomer shifts of 0.22 and 0.21 mm/s,
values within the range found for nonheme enzyme intermediates
with S = 2 oxoiron(IV) centers, (Tables 1 and S2). Other synthetic
S = 2 oxoiron(IV) centers have isomer shifts out of this range,
namely 0.38 mm/s for the [FeIV(O)(OH2)5]2+ ion characterized by
Bakac16 and 0.10 mm/s and below for trigonal bipyramidal oxꢀ
oiron(IV) complexes described by Borovik, Chang and Que (Taꢀ
ble 1, Table S1).17,18,19 Interestingly, the oxoiron(IV)ꢀhalide inꢀ
termediates of CytC3 and SyrB2 each exhibit two quadrupole
doublets with distinct isomer shifts, one at 0.30 mm/s that matchꢀ
es that of the prototypical TauDꢀJ enzyme intermediate and the
other at 0.23 mm/s, close to the values for 2 and 3 (Tables 1 and
S2). As has been pointed out, TQA is the only polydentate ligand
thus far that serves as a good electronic mimic for the iron coordiꢀ
nation environments of TauD, CytC3, and SyrB2.10
[FeIVO(L)X]
δ
∆EQ
ν(Fe=O) ref
λmax
(nm)
(mm/s) (mm/s) (cmꢀ1)
L/X
TQA/MeCN
380 (br)
650, 900
370 (br)
625, 875
370 (br)
625, 875
380,
805
385,
803,825
318
14
0.24
0.22
0.21
0.08
0.08
ꢀ1.05 838 (ꢀ35)
0.96b 827 (ꢀ35)
0.94b 828 (ꢀ37)
0.58 807 (ꢀ34)
(1)
TQA/Cl
(2)
a
TQA/Br
(3)
a
TMG2dien/
MeCN
9
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TMG2dien/
Cl
9
0.41 810 (ꢀ35)
ꢀ0.90 821 (ꢀ34)
20
TauDꢀJ
SyrB2ꢀClc
0.30
0.23
0.30
0.22
0.30
0.23
0.31
0.76b
824d
1.09b
3,15
318
318
CytC3ꢀCl
ꢀ0.70
ꢀ1.09
ꢀ0.81
2a
2b
CytC3ꢀBr
ꢀ1.06
b
c
a This work. The sign of ꢁEQ was not determined. The experimental
values were obtained from the reaction of SyrB2 with
Obtained from DFT calculations (ref 15).
LꢀThrꢀSꢀSyrB1.
d
Previously, we noted that 1 is the most reactive synthetic
nonheme oxoiron(IV) complex reported to date, being capable of
cleaving the C–H bonds of cyclohexane at ꢀ40 °C at a rate that
approaches that for substrate C–H bond cleavage by TauDꢀJ at 5
°C after adjustment for temperature, ultimately producing 35%
cyclohexanone as the oxidation product.14 In extending our reacꢀ
tivity study to 2 and 3, we found that their reactions with cycloꢀ
hexane at ꢀ40 °C afforded halocyclohexanes, cyclohexanol and
1
cyclohexanone as products, as identified by H NMR spectroscoꢀ
py (Figures S9 – S13). The halogenated products were obtained at
3ꢀ to 6ꢀfold higher yields than the oxygenated products (Table 2).
With toluene as substrate, more comparable yields of halogenated
and oxygenated products were observed. These results represent
the first examples of substrate halogenation by a bona fide synꢀ
thetic S = 2 FeIV(O)X complex, thereby mimicking the reactivity
of the oxoiron(IV)ꢀhalide intermediates of SyrB2 and CytC3.
Table 2. Product yields from the reactions of 1, 2 and 3 with cyꢀ
clohexane and toluene under N2 in CD3CN solution at ꢀ40 °C.a
Reaction
products
cyclohexane
toluene
1 b
ꢀ
ꢀ
2
3
1
2
3
33%
7%
R–X
R–OH
R=O
35%
7%
3%
41%
3%
4%
2% c
27%
15%
25%
13%
19%
35%
22%
a Reactions of 0.5 M substrate with 1 mM 1, 2, or 3 were carried out under
N2. The product yields are relative to Fe(TQA)(OTf)2 and reflect the averꢀ
age of three runs. Complexes 2 and 3 were obtained by treatment of 1 with
1 equiv. NBu4Cl/Br; 1 in turn was generated from the reaction of
[FeII(TQA)(OTf)2] and 2 equiv. ArIO (dissolved in CD2Cl2) in CD3CN at ꢀ
40 °C. b Results from ref 14. c The small amount of benzyl chloride obꢀ
served in the reaction of 1 with toluene likely derives from benzyl radical
interception by the CD2Cl2 used to dissolve ArIO. Parallel experiments
with trifluoroethanol in place of CD2Cl2 showed no halogenated product.
Figure 2. DFT calculated structure of 2, with a Fe=O distance of 1.64 Å
and a Fe–Cl distance of 2.36 Å, using the following color scheme: blue =
nitrogen; red = oxygen; green = chlorine; orange = iron; grey = carbon.
DFT calculations on 2 and 3 were performed to obtain
geometryꢀoptimized structures, and the calculated Mössbauer
isomer shifts match the experimental values. These afford Fe=O
distances of 1.64 Å for both 2 and 3 and respective Fe–Cl and Fe–
Br distances of 2.36 and 2.51 Å (Figure 2, Table S3). The latter
distances are consistent with the EXAFSꢀderived distances of 2.31
Å and 2.43 Å measured for the oxoiron(IV)ꢀhalide intermediates
of SyrB2ꢀCl and CytC3ꢀBr, respectively.2b,3
To gain more insight into the halogenation reactions, kinetic
studies were carried out on 2 and 3. For cyclohexane oxidation, 2
and 3 decayed at rates comparable to selfꢀdecay, so second order
rate constants could not be determined. On the other hand, toluene
was oxidized at significantly faster rates due to its weaker aliphatꢀ
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ic C–H bonds. Kinetic analysis showed that 2 and 3 reacted with
with the incipient FeIII(OH)X species in a rapid rebound step to
1
2
3
4
5
6
7
8
toluene at rates approximately 10ꢀfold smaller than 1 (Figures 3,
S7 and S8), reflecting the trend in decreased substrate C–H bond
cleaving rates observed for the oxoiron(IV) intermediates of the
halogenases SyrB2 and CytC3 versus TauDꢀJ.5 Interestingly, this
yield hydroxylated or halogenated products. Alternatively, the
radical may be longer lived than those found in heme systems and
escape from the solvent cage to react with other species in soluꢀ
tion,24 such as residual FeIV(O)X or radical traps such as O2. The
latter mechanistic outcome has been demonstrated experimentally
by Nam in studies of several S = 1 nonheme oxoiron(IV) comꢀ
plexes.25
reactivity trend is opposite to that for the
S
=
1
[FeIV(O)(PyTACN)(X)]+ complexes, where the halo variants are
2ꢀ3 fold faster at oxidizing 9,10ꢀdihydroanthracene (DHA) than
the parent MeCN complex.8 With these results in mind, there are
four important mechanistic points to clarify about the observed
halogenation: a) the nature of the species that cleaves the substrate
C–H bond, b) whether the classic fastꢀrebound mechanism applies
to 2 and 3, c) the role of the iron(IV) spin state and d) the
chemoselectivity between hydroxylation and halogenation.
To assess the lifetime of the nascent substrate alkyl radical
produced in the reactions of 2 and 3, O2 was introduced into the
reaction mixture to act as a radical trap (Figure 4), giving rise to
no halogenation and exclusive formation of oxygenated products
(Tables 3 and S5). These results show that C–X bond formation
with the alkyl radical is not as fast as its interception by O2,26,27
unlike for hemeꢀbased systems.23 Thus the observed halogenation
in the absence of O2 likely occurs by a relatively slow radicalꢀ
recombination step with either the FeIII(OH)(X) species within the
solvent cage or with remaining FeIV(O)(X) species following cage
escape. Either scenario can account for the ferric decay products
observed in Mössbauer spectra of frozen solutions of 3 after reacꢀ
tion with substrate (Figure S6), as the ferrous byproducts expected
from the first scenario readily comproportionate with 2 and 3. The
yields of halogenated products are consistent with this explanaꢀ
tion, but it is not as clear whether the oxygenated products arise
from the same mechanism. These observations concur with recent
results of Nam who has raised questions about the applicability of
the heme paradigm to highꢀvalent nonheme metalꢀoxo species.25
However, both these results are at odds with a recent report by
Maiti providing ESIꢀMS evidence for oxygen rebound in the reacꢀ
tions of an electronꢀrich S = 1 oxoiron(IV) complex.28 Clearly,
more effort is required to clarify what factors control the rate of
rebound in synthetic nonheme iron systems.
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Figure 3. Plots of pseudoꢀfirst order rate constants (kobs) versus
tolueneꢀh8/d8 concentration for 1 (black), 2 (red), and 3 (blue).
Solid lines indicate reactions with tolueneꢀh8 and dashed lines
indicate reactions with tolueneꢀd8.
To ascertain that the Fe=O unit in 2 and 3 was responsible
for initial C–H bond cleavage, we have measured the kinetic isoꢀ
tope effect with toluene as the substrate. As shown in Figure 2,
the oxidation of tolueneꢀh8 by 2 and 3 is significantly faster than
that of tolueneꢀd8, which occurs at a rate comparable to that of
selfꢀdecay (Figure 3, dashed blue and red lines). These results
show that C–H bond cleavage by the oxoiron(IV) species 2 and 3
must be the rateꢀdetermining step, with a KIE well above the clasꢀ
sical limit of 7. Other nonheme oxoiron(IV) complexes have been
found to exhibit large nonclassical KIE, implicating a significant
Figure 4. Proposed mechanisms for substrate hydroxylation and
halogenation by 2 and 3.
Table 3. Product yields (versus Fe(II) precursors) for the reacꢀ
tions of 2, 3, [FeIV(O)(TPA(Cl)]+ (4) and [FeIV(O)(TPA)(Br)]+ (5)
with cyclohexane in CD3CN under N2 and O2.
contribution from
a
tunneling mechanism for Hꢀatom
abstraction.14,21 A more accurate estimate of the KIE could be
obtained from the benzyl bromideꢀh7/d7 product ratio for the inꢀ
termolecular competitive halogenation of tolueneꢀh8 and tolueneꢀ
d8 by 3. A value of 20 was obtained (for experimental details, see
SI page S12 and Figure S13), which is comparable to the KIE of
25 reported for the oxidation of toluene by 1.14 The observed large
KIE also excludes the possibility of free radical bromination of
toluene, for which a much smaller KIE value of 4.9 has been reꢀ
ported in such a reaction with tolueneꢀαꢀd2;22 furthermore, the
yield of R–Br did not increase with the addition of excess bromide
ion and in fact decreased by 30% with 5ꢀ20 equiv. of added broꢀ
mide ion (Table S4). Even more importantly, the toluene KIE
value we measured for 3 is essentially identical to that reported by
Matthews et al. for oxidation of the C4ꢀprotio and C4ꢀdeuterio
native substrate in SyrB2.3 Taken together, these data support the
notion that the initial C–H bond cleavage step occurs via Hꢀatom
abstraction by the oxoiron(IV)ꢀhalide complexes 2 and 3.
S = 1 TPA (25 °C)
S = 2 TQA (ꢀ40 °C)
Oxidation
products
N2
35%
7%
O2
0%
22%
39%
N2
11%
0%
O2
0%
12%
4%
R–Cl
R–OH
R=O
3%
2%
41%
3%
4%
13%
0%
2%
0%
8%
24%
0%
6%
3%
R–Br
R–OH
R=O
To assess the role spin state may play on C–H bond halogenꢀ
ation, we carried out the oxidation of cyclohexane with related S =
1 complexes [FeIV(O)(TPA)(Cl)]+ (4) and [FeIV(O)(TPA)(Br)]+
(5) under the same conditions, except for a change in temperature
to 25 °C to compensate for their lack of reactivity at ꢀ40 °C. These
reactions showed the selective formation of halocyclohexane,
albeit in lower yields than with 2 and 3 (Table 3), indicating that
the S = 2 spin state is not an absolute requirement for halogenaꢀ
tion. Experiments with 4 and 5 under O2 also resulted in the comꢀ
plete loss of halogenation, suggesting that C–X bond formation is
Hꢀatom abstraction from the substrate should initially conꢀ
vert 2 and 3 into hydroxoiron(III)ꢀhalide species together with an
organic radical within a solvent cage (Figure 4). If the P450 paraꢀ
digm applies to this chemistry,23 the alkyl radical would then react
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slower than radical interception by O2 as well for S = 1 nonheme
oxoiron(IV) complexes.
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5. Matthews, M. L.; Neumann, C. S.; Miles, L. A.; Grove, T. L.; Booker,
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USA 2009, 106, 17723.
6. Martinie, R. J.; Livada, J.; Chang, W.ꢀC.; Green, M. T.; Krebs, C.;
Bollinger, J. M.; Silakov, A. J. Am. Chem. Soc. 2015, 137, 6912.
7. Rohde, J.ꢀU.; Stubna, A.; Bominaar, E. L.; Münck, E.; Nam, W.; Que,
L., Jr. Inorg. Chem. 2006, 45, 6435.
8. Planas, O.; Clemancey, M.; Latour, J.ꢀM.; Company, A.; Costas, M.
Chem. Comm. 2014, 50, 10887.
9. England, J.; Guo, Y.; Van Heuvelen, K. M.; Cranswick, M. A.; Rohde,
G. T.; Bominaar, E. L.; Münck, E.; Que, L., Jr. J. Am. Chem. Soc. 2011,
133, 11880.
Lastly, our results also shed some light on the chemoselectivꢀ
ity of C–X bond formation in the absence of a protein scaffold.
Table 3 shows halogenation to be favored over hydroxylation in
cyclohexane oxidation by all four complexes studied. These reꢀ
sults suggest that oxidative ligand transfer may be influenced by
electronic factors such as the oxidizability of the ligand, as Cl–
and Br– have lower oxidation potentials (1.36 V, 1.07 V, respecꢀ
tively) versus OH– (2.02 V).29 Steric factors seem less likely to
play a role, given that both TPA and TQA complexes favor haloꢀ
genation, despite the larger steric bulk of the quinoline donors.
Clearly, additional synthetic examples are needed to establish
whether this is a general trend and to rationalize the decreased
chemoselectivity observed in the oxidation of toluene by 2 and 3.
In contrast, the chemoselectivity of C–H bond functionalization in
SyrB2 has been shown to be determined by the positioning of the
organic substrate within the active site relative to the nascent oxꢀ
oiron(IV)ꢀhalide oxidant.5
In summary, we have reported the first examples for C–H
bond halogenation by synthetic oxoiron(IV)ꢀhalide complexes.
Although halogenation products are observed irrespective of the
spin state of the oxoiron(IV) unit, the S = 2 complexes 2 and 3 are
more effective halogenation agents and react much more rapidly.
Moreover, they are the only synthetic complexes to reproduce the
Mössbauer parameters of the S = 2 oxoiron(IV) intermediates of
the halogenases.3,5 Thus 2 and 3 are excellent spectroscopic and
functional models of the CytC3 and SyrB2 intermediates and
support Nature’s choice of the S = 2 spin state for the oxoiron(IV)
oxidants in the halogenases. These complexes may also serve as
models for the yet unobserved oxidants associated with the ironꢀ
catalyzed halogenations reported by Comba11 and Paine.12 Lastly,
our studies suggest that C–X bond formation by 2 – 5 is not as
rapid as C–O bond formation in hemeꢀmediated C–H bond oxidaꢀ
tions,23 in agreement with results reported by Nam on S = 1 nonꢀ
heme oxoiron(IV) complexes.25 These results raise the intriguing
question of whether the classic oxygen rebound mechanism apꢀ
plies to the highꢀvalent metal centers in nonheme iron oxygenasꢀ
es. To the best of our knowledge, there is no unequivocal stereoꢀ
chemical evidence reported thus far that requires fast rebound in
the catalytic cycles of these enzymes.
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10. Puri, M.; Que, L. Acc. Chem. Res. 2015, 48, 2443.
11. Comba, P.; Wunderlich, S. Chem. Eur. J. 2010, 16, 7293.
12. Chatterjee, S.; Paine, T. K. Angew. Chem. Int. Ed. 2016, DOI:
10.1002/anie.201509914
13. Sharma, A.; Hartwig, J. F. Nature 2015, 517, 600.
14. Biswas, A. N.; Puri, M.; Meier, K. K.; Oloo, W. N.; Rohde, G. T.;
Bominaar, E. L.; Münck, E.; Que, L. J. Am. Chem. Soc. 2015, 137, 2428.
15. Wong, S. D.; Srnec, M.; Matthews, M. L.; Liu, L. V.; Kwak, Y.; Park,
K.; Bell, C. B., 3rd; Alp, E. E.; Zhao, J.; Yoda, Y.; Kitao, S.; Seto, M.;
Krebs, C.; Bollinger, J. M., Jr.; Solomon, E. I. Nature 2013, 499, 320.
16. Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X.; Münck, E.; Que,
L., Jr.; Bakac, A. Angew. Chem. Int. Ed. 2005, 44, 6871.
17. Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.;
Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188.
18. Bigi, J. P.; Harman, W. H.; LassalleꢀKaiser, B.; Robles, D. M.; Stich,
T. A.; Yano, J.; Britt, R. D.; Chang, C. J. J. Am. Chem. Soc. 2012, 134,
1536.
19. England, J.; Martinho, M.; Farquhar, E. R.; Frisch, J. R.; Bominaar, E.
L.; Münck, E.; Que, L., Jr. Angew. Chem. Int. Ed. 2009, 48, 3622.
20. Sinnecker, S.; Svensen, N.; Barr, E. W.; Ye, S.; Bollinger, J. M., Jr.;
Neese, F.; Krebs, C. J. Am. Chem. Soc. 2007, 129, 6168.
21. Kleespies, S. T.; Oloo, W. N.; Mukherjee, A.; Que, L. Inorg. Chem.
2015, 54, 5053.
ASSOCIATED CONTENT
Supporting Information
Experimental details, ESIMS and Mössbauer data, DFT results
and insights, and kinetic data. This material is available free of
charge via the Internet at http:// pubs.acs.org.
22. Wiberg, K. B. Chem. Rev. 1955, 55, 713.
23. Groves, J. T. J. Chem. Ed. 1985, 62, 928.
24. Barton, D. H. R.; Doller, D. Acc. Chem. Res. 1992, 25, 504.
25. (a) Cho, K.ꢀB.; Wu, X.; Lee, Y.ꢀM.; Kwon, Y. H.; Shaik, S.; Nam, W.
J. Am. Chem. Soc. 2012, 134, 20222. (b) Cho, K.ꢀB.; Hirao, H.; Shaik, S.;
Nam, W. Chem. Soc. Rev. 2016 DOI: 10.1039/C5CS00566C.
26. Achord, J. M.; Hussey, C. L. Anal Chem. 1980, 52, 601 ꢀ 602.
27. Wu, D.; Bayes, K. D. Int. J. Chem. Kinet. 1986, 18, 547.
28. Rana, S.; Dey, A.; Maiti, D. Chem. Comm. 2015, 51, 14469.
29. CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC
Press: Florida, 1996; pp. 8ꢀ20.
AUTHOR INFORMATION
Corresponding Authors
larryque@umn.edu, ysguo@andrew.cmu.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the National Science Foundation for financial support (grant
CHEꢀ1361773 to LQ), the University of Minnesota for a doctoral dissertaꢀ
tion fellowship to M.P., and the IndoꢀUS Science & Technology Forum
(IUSSTF) for a postdoctoral fellowship to A.N.B. We acknowledge Ms.
Jale Ocal for sharing her results on the oxidation of cyclohexane by
[FeIV(O)(TPA)(MeCN)]2+.
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