Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies

Article abstract of DOI:10.1021/acs.inorgchem.9b02923

High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the Fe^IV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of Fe^IV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k₂ = 3.6(1)-1.21(3) × 10⁴ M^-1 s^-1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O-H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex, Mn^IV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope for 1. The O-H bond dissociation energy (BDE) of the M^III(HO-H) complexes were estimated from a kinetic analysis to be 85 and 89 kcal mol^-1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are discussed in the context of how they influence the overall H atom transfer reactivity.

Full text of DOI:10.1021/acs.inorgchem.9b02923

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH)
Porphyrinoid Complexes: Mechanistic Insights from Experimental
and Computational Studies
Jan Paulo T. Zaragoza,† Daniel C. Cummins,† M. Qadri E. Mubarak,‡ Maxime A. Siegler,†
Sam P. de Visser,*,‡ and David P. Goldberg*,†
†Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
‡The Manchester Institute of Biotechnology and Department of Chemical Engineering and Analytical Science, The University of
Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom
S
* Supporting Information
ABSTRACT: High-valent metal-hydroxide species have been
implicated as key intermediates in hydroxylation chemistry
catalyzed by heme monooxygenases such as the cytochrome
P450s. However, in some classes of P450s, a bifurcation from
the typical oxygen rebound pathway is observed, wherein the
FeIV(OH)(porphyrin) species carries out a net hydrogen atom
transfer reaction to form alkene metabolites. In this work, we
examine the hydrogen atom transfer (HAT) reactivity of
FeIV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-
phenyl corrole, toward substituted phenol derivatives. The
iron hydroxide complex 1 reacts with a series of para-
substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-
DTBP; X = OMe, Me, Et, H, Ac), with second-order rate
constants k2 = 3.6(1)−1.21(3) × 104 M−1 s−1 and yielding linear Hammett and Marcus plot correlations. It is concluded that
the rate-determining step for O−H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that
include a KIE = 2.9(1) and DFT calculations. Comparison of the HAT reactivity of 1 to the analogous Mn complex,
MnIV(OH)(ttppc), where only the central metal ion is different, indicates a faster HAT reaction and a steeper Hammett slope
for 1. The O−H bond dissociation energy (BDE) of the MIII(HO−H) complexes were estimated from a kinetic analysis to be
85 and 89 kcal mol−1 for Mn and Fe, respectively. These estimated BDEs are closely reproduced by DFT calculations and are
discussed in the context of how they influence the overall H atom transfer reactivity.
systems have focused on the reactivity of metal-oxo complexes
and the thermodynamic analysis of the O−H bond of the
metal-hydroxide intermediate formed during the HAT
reaction. However, CYP monooxygenases, and the related
CYP peroxygenases, can perform desaturation and C−C bond
cleaving reactions in addition to hydroxylation, where their
pathways can bifurcate at the Cpd-II state between oxygen
rebound and HAT mechanisms. Desaturation reactions result
in the formation of toxic alkene metabolites (Scheme 1).
Conversion of valproic acid to 4-ene valproic acid,14−16 ethyl
carbamate to vinyl carbamate,17 testosterone to 17β-hydroxy-
4,6-androstadiene-3-one,18 lauric acid to 11-dodecenoic acid,19
and decarboxylation of fatty acids to terminal alkenes
(specifically by OleTJE)20−25 are just some of the examples
of these CYP-mediated desaturation reactions.
INTRODUCTION
■
The design and development of well-defined, observable
synthetic models of highly reactive metal−oxygen intermedi-
ates has helped elucidate the mechanisms of oxidation
reactions catalyzed by metalloenzymes.1,2 For example, syn-
thesis of biomimetic high-valent metal-oxo complexes and
examination of their hydrogen atom transfer (HAT) reactivity
has provided insights into the ground-state thermodynamics of
related biological oxidations, as well as information for the
design of synthetic catalysts.3−7 Heme enzymes are biological
catalysts that perform a large variety of oxidation reactions in
nature. Cytochrome P450 (CYP), in particular, is a heme
monooxygenase that activates inert C−H bonds via a high-
valent iron-oxo intermediate, Compound I (Cpd-I).8−13 The
rate-determining step for this reaction is the HAT from the C−
H substrate to Cpd-I to form protonated Compound II (Cpd-
II), an iron(IV)-hydroxide species (Scheme 1). The sub-
sequent step, which is rapid, is the rebound of the •OH group
to the incipient carbon radical to form the hydroxylated
substrate. Indeed, many of the studies of HAT involving model
While substrate positioning in the active site binding pocket
has been implicated to play a key role in selection of rebound
Received: October 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02923
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analogous Mn(OH)(ttppc) will be described as metal(IV)
hydroxide complexes throughout this work for simplicity. Since
our original report, the complex FeIV(OHn)(TAML) (TAML
= tetra-amido macrocyclic ligand) was reported,32 as well as a
nonheme FeIII(OH) complex that demonstrated both radical
rebound and hydrogen atom transfer reactivity.33
Scheme 1. Divergent Oxidation Pathways for CYP Cpd-II,
and Examples of Desaturation Reactions Mediated by CYPs
Herein, the H atom transfer reactivity of FeIV(OH)(ttppc)
(1) with a series of phenol derivatives is described, providing a
mechanistic analysis of HAT reactivity. The synthesis and
structural characterization of FeIII(H2O)(ttppc) (2), the
corresponding HAT product, was also accomplished. Hammett
and Marcus plot analyses, together with kinetic isotope effect
(KIE) measurements, provide detailed insight into how the
HAT reaction proceeds. A direct comparison with the
reactivity of MnIV(OH)(ttppc), combined with computational
analyses of the reaction trajectories of these two high-valent
metal hydroxide complexes, enabled assessment of how the
central metal ion (Mn vs Fe) tunes the H atom transfer
reactivity. The experimental and computational studies on 1,
along with work on the analogous Mn complex MnIV(OH)-
(ttppc), provide novel insights into the thermodynamic
properties that govern the reactivity of metal hydroxide species
toward H atom donors.
hydroxylation versus HAT pathways for protonated Cpd-II in
CYPs,22,26 the fundamental electronic and structural factors
that ultimately control these pathways remain poorly under-
stood. Ground state thermodynamic properties are often
invoked as key factors in controlling HAT by high-valent
metal-oxo complexes. Along these lines, the ground state
thermodynamics related to the HAT reactivity of protonated
Cpd-II can be assessed from the BDE(O−H) of the expected
ferric-aquo product (FeIII(H2O)(porph)(cys)). Few such BDE
values are available from experiment on heme enzymes, but a
BDE(O−H) of 90 kcal mol−1 was recently reported in one
case for the FeIII(H2O) species in CYP158.27 In contrast, the
FeIII(H2O) form of the heme enzyme aromatic peroxygenase
(APO) was characterized as having a much lower BDE of 81
kcal/mol, despite the similarity of this thiolate-ligated heme
active site with that of Cys-bound CYP. The corresponding
O−H bond strength in the ferric hydroxide form of
horseradish peroxidase (HRP), which contains a neutral His
axial ligand trans to the OH group, is 85 kcal/mol as derived
from the FeIV(O)/FeIII(OH) redox potential.28 Such a wide
range in similar porphyrin ferric O−H bond strengths indicates
that there are likely subtle, as yet unidentified factors regarding
the metal ion active site that can affect this value and ultimately
influence HAT reactivity.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of an
FeIII(H2O) Complex. The synthesis of FeIII(H2O)(ttppc)
(2), the expected H atom abstraction product of FeIV(OH)-
(ttppc) (1), was prepared by ligand exchange of the previously
synthesized FeIII(OEt2)2(ttppc)29 with H2O, followed by
fluorobenzene/n-pentane vapor diffusion to give single crystals
for X-ray diffraction. The molecular structure of FeIII(H2O)-
(ttppc) (2) is shown in Figure 1. The structure of 2 shows an
Given the challenges in characterizing these species in the
native, enzymatic systems, we have set out to develop well-
defined synthetic FeIV(OH)/FeIII(H2O) porphyrinoid com-
plexes that should allow for an examination of their reactivity
and related thermodynamics. Such compounds, in heme or
nonheme systems, are extremely rare. We previously reported
the synthesis of FeIV(OH)(ttppc) (ttppc = 5,10,15-tris(2,4,6-
triphenyl)-phenyl corrole) (1) and its reactivity toward para-X
substituted trityl radical derivatives in efforts to model the
radical rebound step leading to C−O bond formation.29
Kinetic studies indicated that the radical rebound step was a
concerted, charge neutral process. It should be noted that with
triarylcorrole ligands, the electronic ground state of formally
FeIV and MnIV corroles has been described as varying between
the two canonical valence tautomers, MIII(X)(corrole+•) and
MIV(X)(corrole), depending on the axial ligand X.30,31
Conclusive characterization of the ground electronic state of
these compounds is challenging, and complex 1 and the
Figure 1. Displacement ellipsoid plot (35% probability level) for
FeIII(H2O)(ttppc) (2) at 110(2) K. H atoms (except for those
attached to O1) are omitted for clarity.
Fe−O bond distance of 2.126(2) Å, which is much longer as
compared to that of 1 (Fe−O = 1.857(3) Å, Table S1). The
Fe−O bond distance for 2 is similar to other well-characterized
mononuclear FeIII(H2O) units in porphyrin (2.012(2)−
2.084(4) Å)34−36 and tetra-amido macrocyclic ligand scaffolds
(TAML) (2.097(2)−2.1102(18) Å).37,38 The X-ray structure
of 2 is the first example of an H2O-bound iron(III) corrole,
which are typically isolated with NO, Et2O, or pyridine axial
ligands.10−12 The frozen solution EPR spectrum of 2 shows an
intense signal at g = 4.29 consistent with an intermediate spin
(S = 3/2) FeIII metal center, similar to FeIII(OEt2)2(ttppc) in
toluene (Figure S1)29 and the related FeIII(OEt2)2(tpfc) (tpfc
= 5,10,15-tris(pentafluorophenyl) corrole).39,40 An Evans
method measurement of 2 gives μeff = 3.6(1) μB, (spin-only
B
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Figure 2. (a) Reaction of FeIV(OH)(ttppc) with para-X-substituted 2,6-di-tert-butylphenol derivatives. (b) Time-resolved UV−vis spectral changes
observed in the reaction of 1 (15 μM) with 4-OMe-2,6-DTBP (0.52 mM) in toluene at 23 °C. Inset: changes in absorbance vs time for the growth
of 2 (575 nm) (green circles) with the best fit line (black). (c) Plot of pseudo-first-order rate constants (kobs) vs [4-OMe-2,6-DTBP] for the OH
(black) and OD (blue) phenol isotopologues.
μeff = 3.87 μB for S = 3/2), further supporting the spin state
assignment. It should be noted that we are not able to
unambiguously determine the extent to which H2O remains
coordinated for the EPR and Evans method measurements in
toluene. For comparison, the analogous MnIII(H2O)(ttppc)41
has a much longer metal−oxygen bond distance of 2.2645(18)
Å, likely due to Jahn−Teller distortion for the high-spin MnIII
ion.42
thermodynamic properties of the phenol derivatives on the
H atom transfer reactivity (Figure 2a). A direct comparison
with the reactivity of MnIV(OH)(ttppc) can also be made,
providing an opportunity to assess how the central metal ion
(Mn vs Fe) affects the reactivity toward hydrogen atom
donors. Reaction of 1 with a series of para-substituted phenols
were carried out similarly to the reaction of 1 with 2,4-DTBP.
A solution of 1 and excess 4-methoxy-2,6-di-tert-butylphenol
(4-OMe-2,6-DTBP) (5.4 equiv) was analyzed by X-band
electron paramagnetic resonance (EPR) spectroscopy (16 K)
and gave EPR signals corresponding to FeIII(H2O)(ttppc) (2)
(g = 4.29) and phenoxyl radical (g = 2.0) (Figure S3a). An
additional signal (g = 5.3) appears in this reaction mixture,
which can also be observed in the EPR spectrum of
FeIII(H2O)(ttppc) (2) and excess 4-OMe-2,6-DTBP (possibly
due to formation of an FeIII-phenol adduct) (Figure S3b).
These data showed that 1 reacts with 4-OMe-2,6-DTBP in
accordance to the scheme in Figure 2a. The reaction kinetics of
1 with 4-OMe-2,6-DTBP was monitored by stopped-flow
UV−vis spectroscopy, showing isosbestic conversion of 1 to 2
within 1 s (Figure 2b). Pseudo-first-order conditions (>10-fold
equiv substrate) were employed and led to single exponential
curves that were fit up to five half-lives. The resulting observed
rate constant (kobs) varied linearly with phenol concentration,
yielding a second-order rate constant (k2) of 1.21(3) × 104
M−1 s−1. A kinetic isotope effect (KIE) was determined by
reaction of 1 with the ArOD isotopologue of 4-OMe-2,6-
DTBP. A KIE of 2.9(1) was measured, indicating that the rate-
determining step involves O−H cleavage.
Reaction of FeIV(OH)(ttppc) (1) with H Atom Transfer
Reagents. Initial studies on the HAT reactivity of 1 began
with C−H substrates. Reaction of 1 with 9,10-dihydroan-
thracene (DHA), a common H atom donor with a relatively
weak C−H bond (BDE = 76.3 kcal mol−1),43 was carried out.
Preliminary UV−visible spectroscopy measurements showed
that 1 reacts with DHA in toluene at 23 °C, although the
reaction is very slow (<5% decay to FeIII(H2O)(ttppc) (2)
over 5 h). In contrast, reaction of 1 with 2,4-di-tert-butylphenol
(2,4-DTBP) (toluene, 23 °C) results in rapid formation of 2,
as observed by UV−vis spectroscopy. Product analysis by gas
chromatography (GC-FID) led to detection of the bis(phenol)
dimer product 3,3′,5,5′-tetra-tert-butyl-(1,1′-biphenyl)-2,2′-
diol in good yield (67(1)%), consistent with a 1:1
stoichiometry for oxidation of the phenol substrate by the
iron complex 1.
Having demonstrated the proficiency of 1 in cleaving the
phenolic O−H bond in 2,4-DTBP, we employed a series of
para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-di-
t-Bu-C6H2OH; X = OMe, tBu, Et, Me, H, Ac) as H atom
transfer substrates. Variation of the para substituents allowed
us to measure the influence of the electronic and
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The second-order rate constants (k2) for the other para-
substituted 2,6-DTBP derivatives were obtained in a similar
fashion (Figures S4−S9) and are listed in Table 1 together
Table 1. Second-Order Rate Constants for the Reaction
Between FeIV(OH)(ttppc) (1) and para-X-Substituted
Phenol Derivatives, Hammett Constants (σ+ ), Redox
p
Potentials (Eox), and BDEs of 4-X-2,6-DTBP
b
c
Eox
BDE
a
X
k2 (M−1 s−1
)
σ+
(V vs Fc+/0
)
(kcal mol−1
)
p
−OMe
1.21(3) × 104
4.2(1) × 103
−0.78
−
0.53
0.59
78.3
−
−OMe
(OD)
−Me
−Et
−H
2.0(1) × 103
3.2(3) × 103
83(3)
−0.31
−0.29
0.00
−
0.81
0.88
1.07
−
81.0
−
82.8
−Ac
3.6(1)
84.4
a
b
c
Ref 44. In CH3CN, ref 45. In benzene, ref 46.
with their thermodynamic parameters (Hammett constants
σ+ , redox potentials, and BDEs). It should be noted that the
p
Eox values listed in Table 1 were obtained in acetonitrile.
However, we still obtain a good correlation between our
measured k2 values in toluene and the Eox values from CH3CN.
In each case, isosbestic conversion of 1 to the FeIII(H2O)
product 2 was rapid and varied over two orders of magnitude
(k2 = 3.6(1)−1.21(3) × 104 M−1 s−1).
Comparison of the Reactivity of FeIV(OH)(ttppc) (1)
and MnIV(OH)(ttppc) with Phenol Derivatives. Hammett
analysis of the kinetic data for 1 (Figure 3a, red circles) showed
a decrease in the rate constants with the more electron-
deficient phenol, with a slope of ρ+ = −2.6(8). This slope is
higher as compared to that of MnIV(OH)(ttppc) (ρ+
=
−1.4(2)) (Figure 3a, blue circles), implying a relatively greater
charge separation in the HAT transition state for the Fe versus
the Mn complex. A Marcus plot analysis, which provides
information on the involvement of electron transfer in the rate-
determining step of the reaction, was also performed. A slope
of approximately −0.5 is predicted for the plot of (RT/F) ln k2
versus Eox (where k2 is the second-order rate constant and Eox
is the redox potential of the substrate being oxidized) for
reactions with a rate-determining electron transfer step,
provided that ΔG°ET/λ (where λ is the reorganization energy)
is close to zero.47 A slope of −0.19(6) was obtained for 1
(Figure 3b, red squares), close to the values obtained for HAT
reactions of a Cu-superoxo complex (slope = −0.29)45 and
MnIV(OH)(ttppc) (slope = −0.12) (Figure 3b), in which a
concerted H atom transfer is invoked. Plotting the log k2 for 1
versus the bond dissociation energy (BDE) of the O−H bond
of the phenol substrate (Figure 3c) also shows a good
correlation, with a slope of −0.58(1). This slope is larger than
that for MnIV(OH)(ttppc) (slope = −0.25(3)), suggesting that
the FeIV(OH) complex is more sensitive to the changes in
BDEs of the O−H bond being broken. However, it must be
noted that the range of BDEs for the O−H substrates is small,
and therefore, caution must be used in the interpretation of
these data.
Figure 3. Comparison of (a) Hammett, (b) Marcus, and (c) log k2 vs
BDE plots for the reaction of FeIV(OH)(ttppc) (1) (red circles/
squares, in toluene) or MnIV(OH)(ttppc) (blue circles/squares, in
benzene) with 4-X-2,6-DTBP. Kinetic data for MnIV(OH)(ttppc)
from ref 41.
described.55−57 Taken together, the data suggest a rate-limiting
H atom transfer reaction for complex 1 with phenolic (O−H
bond) substrates. It is reasonable to conclude that the HAT
reaction proceeds in a concerted manner but with a partial
transfer of charge, rather than stepwise PT/ET or ET/PT
pathways.
Comparison of HAT Reactivity with Other Oxidants
and Estimation of BDEs from the Kinetics Data. The rate
constants for the oxidation of 4-X-2,6-DTBP (X = OMe, Me,
H) by FeIV(OH)(ttppc) (1) and MnIV(OH)(ttppc) can be
compared to the previously reported rate constants for
substituted phenol oxidation by various metal-oxo and metal-
hydroxo complexes, as well as a photolytically generated t-
BuO• radical (Table 2).48,50,58 It should be noted that the rate
constants in Table 2 are taken from reactions in different
solvents and temperatures, and thus close comparisons of the
values are not warranted. However, in general, the rate
The difference in slope implies that MnIV(OH)(ttppc) will
exhibit larger rate constants than 1 when reacting with O−H
substrates having sufficiently high BDEs (>ca. 85 kcal mol−1).
A similar reversal in relative rate of HAT for a nonheme
MnIV(O) versus an analogous FeIV(O) complex was recently
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Table 2. Comparison of Second-Order Rate Constants (M−1 s−1) and Hammett Slopes (ρ+) for the Oxidation of Substituted
Phenols, 4-X-2,6-DTBP (X = OMe, Me, H), by M(O(H)) Complexes and t-BuO•, and the Corresponding O−H BDEs of the
Reduced Complex
BDE (O−H)
oxidant
−OMe
1.21 × 104
510
1.44
11.4
26.4
29.8
−Me
2.0 × 103
150
1.20
0.18
1.84
1.84
412
−H
83.0
36
0.75
0.04
0.46
0.68
37.3
0.58
4.3 × 107
ρ+
conditions
reduced complex
(kcal mol−1
)
a
FeIV(OH)(ttppc) (1)
−2.6
−1.4
toluene, 23 °C
benzene, 23 °C
FeIII(HO−H)(ttppc)
MnIII(HO−H)(ttppc)
MnII(HO−H)(dpaq)+
FeIII(O−H)(TMC)(NCCH3)+
FeIII(O−H)(TMC)(CF3COO)
FeIII(O−H)(TMC)(N3)
MnIII(HO−H)(salen)
[RuV(L)(O)(O−H)]2+
t-BuO−H
89
b
a
MnIV(OH)(ttppc)
85
c
d
MnIII(OH)(dpaq)+
−0.88 CH3CN, 50 °C
−3.20 CH3CN, 25 °C
−2.30 CH3CN, 25 °C
−1.50 CH3CN, 25 °C
64
e
e
FeIV(O)(TMC)(NCCH3)+
78
e
e
FeIV(O)(TMC)(CF3CO2)
76.4
e
e
FeIV(O)(TMC)(N3)
77.4
f
MnIV(OH)(salen)
−
−
CH2Cl2, −70 °C
−
82.8
105
g
g
[RuVI(L)(O)2]2+
2.56 × 104
−
26
1.2 × 108
−2.90 CH3CN, 25 °C
−
h
i
t-BuO•
1:2 C6H6/C8H18O2,
22 °C
a
b
BDE values extrapolated from the correlation in Figure 4. BDE(O−H) refers to the bond dissociation energy of the reduced complex. Rate
c
d
e
f
constants from ref 41. Rate constants calculated from kobs (s−1) in ref 48. Ref 49. Rate constants calculated from kobs (s−1) values in ref 50. Rate
g
h
i
constants calculated from kobs (s−1) values ref 51. Ref 52. Ref 53. Ref 54.
constants for 1 are orders of magnitude larger than those for
Mn- and Fe-oxo and hydroxo complexes. Although the high-
valent FeIV(oxo) complexes in Table 2 may be among the
weaker FeIV(O) oxidants, it is still interesting to note that 1, a
terminal hydroxide complex, is a significantly more reactive
oxidant for the electron-rich phenols.
mechanism of FeIV(OH)(ttppc) (1) and MnIV(OH)(ttppc)
with the 4-H-2,6-DTBP substrate. Starting with the reactants,
both complexes are charge neutral and were studied with the
complete ttppc ligand included with odd (for Fe) or even (for
Mn) multiplicity. The [FeIV(OH)(ttppc)] complex is in a
triplet spin ground state with the quintet and singlet spin states
of ΔG = 3.7 and 15.4 kcal mol−1 higher in energy (Table S8).
With manganese as the central metal ion, the ground state is
the quartet spin state, while the doublet and sextet spin states
are ΔE + ZPE = 21.3 and 8.0 kcal mol−1 higher (Table S10).
The group spin densities of 3[Fe(OH)(ttppc)] indicate
An Evans−Polanyi plot (Figure 4) shows good correlation of
the 4-Me-2,6-DTBP reaction rates of these complexes with the
approximately three unpaired electrons on the iron (ρFeOH
=
2.79) antiferromagnetically coupled to one unpaired electron
on the ttppc ligand (ρtppcc = −0.80) in an a2u type orbital.
These spin densities are more consistent with the
3[FeIII(OH)(ttppc+•)] valence tautomer. This configuration
differs from the cytochrome P450s, which, in almost all cases,
3
is calculated to exhibit a [FeIV(OH)(heme)] as the ground
state.22,62 We attempted to calculate the energy difference
between the FeIII(OH)(ttppc+•) and the FeIV(OH)(ttppc)
structures by swapping the molecular orbitals to obtain a
structure with triplet spin configuration δx −y 2 π*xz1 π*yz σ*z
1
20
2
2
a1u a2u2, designated as [FeIV(OH)(ttppc)]. Unfortunately,
2
3
however, during the SCF convergence the state relaxed back to
3
the [FeIII(OH)(ttppc+•)] state. The ground state calculations
Figure 4. Plot of log k2 for the reaction of metal-oxo/hydroxo
complexes and tBuO• with 4-Me-2,6-t-Bu-phenol (22−25 °C) vs
BDE(O−H) of the O−H bond formed.
for the manganese complex gives spin densities consistent with
4
a similar description, i.e., [MnIII(OH)(ttppc+•)]. The bond
3
distances from the DFT optimized structures for [FeIII(OH)-
BDE(O−H) of the HAT reaction product. To calibrate this
relationship, a line was fit to the plot of rate constants (log k2)
of similar compounds whose BDE(O−H) values are
determined experimentally or calculated by DFT methods.
Based on this plot (Figure 4), BDEs of 85 and 89 kcal mol−1
are predicted for MnIII(HO−H)(ttppc) and FeIII(HO−H)-
(ttppc) (2), respectively. A similar linear correlation was
obtained when the log k2 versus BDE for 4-H-2,6-DTBP was
plotted (Figure S10); however, a slightly lower BDE(O−H)
value (∼87 kcal mol−1) was predicted for FeIII(HO−H)(ttppc)
(2) using this substrate.
Computational Studies on the HAT Mechanism.
Density functional theory (DFT) calculations were done
using extensively tested and benchmarked methods, which
have been shown to reproduce experimental BDEs within two
kcal mol−1.59−61 We began by calculating the reaction
(ttppc+•)] (Figure S11, Table S2) and 4[MnIII(OH)(ttppc+•)]
(Figure S12, Table S3), as well as the reaction products
4[FeIII(H2O)(ttppc)] (Figure S13, Table S4) and
3[MnIII(H2O)(ttppc)] (Figure S14, Table S5), are all in
good agreement with the experimentally obtained bond
distances from the X-ray crystal structures of these complexes.
While the DFT-optimized structures for 1 and the Mn analog
suggest a metal(III)(corrole+•) structure, a definitive electronic
structure assignment cannot be made from the available
structural and spectroscopic data.
The hydrogen atom transfer from the para-H 2,6-DTBP to
FeIV(OH)(ttppc) and MnIV(OH)(ttppc) complexes was
investigated, and the transition state structures are given in
Figure 5. The reaction is concerted with a single hydrogen
atom abstraction to form water and phenoxyl radical. We find
E
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4
5.3 kcal mol−1 for [Mn(OH)(corrole)] (Figure S15b). The
second-coordination sphere effect of the bulky phenyl groups
appears to increase the HAT barrier and facilitate the direct
observation of these kinetically transient chemical steps.
Calculation of Bond Dissociation Energies (BDEs)
from Density Functional Theory (DFT). Bond dissociation
energies (BDEOH) were calculated from the following reaction
of MIII(H2O)(L) → MIV(OH)(L) + H• + BDEOH and taken as
the energy difference of the electron affinity (EA) of the
metal(IV)-hydroxide complex, the experimental ionization
energy of a hydrogen atom (IEH),64 and the acidity (ΔGacid
)
of the metal(III)-aquo complex (Figure S16) in their
respective solvents (toluene for Fe and benzene for Mn).
Our calculated BDEOH values (with ZPE included) match the
predicted values well, with BDEOH (MnIII(H2O)(ttppc)) =
83.7 kcal mol−1 (in benzene) and BDEOH (FeIII(H2O)(ttppc))
= 88.5 kcal mol−1 (in toluene) (Tables S19 and S20). The
difference between these two values comes from a larger
electron affinity of the iron(IV)-hydroxo complex (by 12.1 kcal
mol−1), which is partially canceled by a larger acidity (by 7.3
kcal mol−1) of the iron(III)-aquo complex. This relative
reactivity for directly analogous Fe versus Mn complexes has
been similarly observed in comparisons of FeIV(O) versus
MnIV(O) nonheme complexes, where FeIV(O) species are
much more reactive toward HAT substrates, due to their more
positive redox potentials.65,66 Similarly, for porphyrin and
porphyrin-type complexes, FeIV(O)(porphyrin•+) species have
been shown to be much more reactive toward HAT reactions
versus the analogous MnV(O) species in the same ligand
environment.3 In contrast, previous thermodynamic analysis of
H atom transfer reactions of MnV(O) versus CrV(O)
corrolazines revealed that the basicity of the MIV(O) (M =
Mn, Cr) intermediate controls the H atom transfer reactivity
for these reactions.67 In the case of the H atom transfer
reactions of MIV(OH)(ttppc) (M = Fe, Mn) studied here, the
larger calculated redox potential of the FeIV(OH) complex
seems to have a significant effect on the faster H atom transfer
reactivity in comparison to MnIV(OH)(ttppc).
Figure 5. UB3LYP/BS2 optimized transition state geometries in
solvent for the hydrogen atom abstraction from para-H-2,6-DTBP by
FeIV(OH)(ttppc) and MnIV(OH)(ttppc). Bond lengths are in
angstroms, and energies contain zero-point energy and solvent
corrections.
hydrogen atom abstraction barriers of ΔE + ZPE‡ = 4.3 and
5.7 kcal mol−1 for the FeIVOH and MnIVOH complexes,
respectively (Figure S15a, Tables S8 and S10). These barriers
are low and hence predict a fast chemical reaction. The
difference in HAT barrier height for the FeIV(OH) versus
MnIV(OH) complexes implicates a rate enhancement of about
a factor of 10 for iron with respect to manganese from
transition state theory at 298 K, which is reasonably close to
the experimentally reported rate differences for this phenol
(Table 2).
Geometrically, the transition states are relatively central with
almost equal FeO−H versus H−OPh and MnO−H versus H−
OPh distances. In particular, short FeO−H/MnO−H distances
of 1.158/1.183 Å are found, while the H−O distance with the
phenol is 1.267/1.242 Å. Consequently, the iron and
manganese structures are very similar. The transition states
are early and have an electronic structure close to the reactants,
with only about 20% charge-transfer (ρSub = −0.18/−0.20 in
3TSHA,Fe/4TSHA,Mn). These results match the Hammett and
Marcus plots reported above that excluded a separation of
proton and electron transfer through either ET/PT or PT/ET
pathways. Indeed, the electron transfer is simultaneous with
proton transfer and indicates an HAT mechanism for the
reaction. These spin and charge distributions in the transition
states match those calculated previously for desaturation
reactions by CYPs.26,63
To examine the contribution of the secondary coordination
sphere effects of the sterically hindered ligand ttppc (5,10,15-
tris(2,4,6-triphenyl)phenyl corrole) on the HAT reaction, we
also ran transition state calculations with a hypothetical bare
corrole, in which the (2,4,6-triphenyl)phenyl substituents on
the 5,10,15- positions were removed and replaced with
hydrogen atoms. Based on the potential energy landscape for
the HAT reaction (Tables S12 and S14), the barrier drops
considerably by 3.6 kcal mol−1 for 3[Fe(OH)(corrole)] and by
CONCLUSION
■
The hydrogen atom transfer reactivity of FeIV(OH)(ttppc)
(1), a protonated Cpd-II analog, was described. It is capable of
the rapid oxidation of phenol derivatives by H atom
abstraction of the O−H bond and is more reactive than the
other high-valent metal-hydroxo oxidants and the weakly
oxidizing FeIV(O)(TMC) complexes given in Table 2 as tested
against the same substrates. It also reacts more rapidly than the
analogous MnIV(OH)(ttppc) complex, providing a direct
comparison of the oxidizing power of Fe(OH) versus
Mn(OH) species at the same oxidation level and in identical
ligand environments. The final HAT product, FeIII(H2O)-
(ttppc), was also definitively characterized by XRD.
The kinetic analyses and computational studies provide
strong support for a mechanism of phenol oxidation by 1, and
its Mn analog, that involves a concerted, rate-determining
HAT step. The calculations suggest a modest amount of charge
transfer in the transition state, and this finding is in line with
the weak Hammett and Marcus plot slopes. A plot of BDE(O−
H) values for M(O(H)) complexes versus log k2 for the
oxidation of 4-Me-2,6-di-tert-butyl phenol is linear and
provides a means to estimate the BDE(O−H) values for the
FeIII(H2O) and MnIII(H2O) ttppc complexes from kinetics
data. These estimated values are BDE(FeIII(HO−H)) = ∼89
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kcal mol−1 and BDE(MnIII(HO−H)) = ∼85 kcal mol−1, and
DFT calculations fully support these values. The estimated
BDE(O−H) for FeIII(H2O)(ttppc) is strikingly similar to the
BDE(O−H) recently reported for the FeIII(H2O) form of
CYP158 (90 kcal mol−1).27 However, it should be noted that
the BDE(O−H) calculations here are 2 kcal mol−1 at best
from the error in DFT calculations and kinetic measurements.
Taken together, these data suggest that 1 is a viable model for
protonated Cpd-II in CYP from both a structural and
thermodynamic perspective, although the electronic structure
of 1 may be closer to FeIII(OH)(corrole+•), as opposed to
FeIV(OH)(corrole).
stopped-flow spectrophotometer with a xenon light source and
Kinetic Studio software. 1H NMR spectra were recorded on a Bruker
Avance 400 MHz NMR spectrometer at 298 K and referenced against
residual solvent proton signals. Electron paramagnetic resonance
(EPR) spectra were recorded with a Bruker EMX spectrometer
equipped with a Bruker ER 041 X G microwave bridge and a
continuous-flow liquid helium cryostat (ESR900) coupled to an
Oxford Instruments TC503 temperature controller for low temper-
ature data collection. Laser desorption ionization mass spectrometry
(LDI-MS) was conducted on a Bruker Autoflex III MALDI ToF/ToF
instrument (Billerica, MA) equipped with a nitrogen laser at 335 nm
using an MTP 384 ground steel target plate. The instrument was
calibrated using peptide standards of known molecular weights. Gas
chromatography (GC-FID) was carried out on an Agilent 6890N gas
chromatograph fitted with a DB-5 5% phenylmethyl siloxane capillary
column (30 m × 0.32 mm × 0.25 μm) and equipped with a flame-
ionization detector. Elemental analysis was performed at Atlantic
Microlab, Inc., Norcross, GA. Cyclic voltammetry was performed on
an EG&G Princeton Applied Research potentiostat/galvanostat
model 263A with a three-electrode system consisting of a glassy
carbon working electrode, a Ag/AgNO3 nonaqueous reference
electrode (0.01 M AgNO3 with 0.1 M Bu4NPF6 in CH3CN), and a
platinum wire counter electrode. Potentials were referenced using an
external ferrocene standard. Scans were run under an Ar or N2
atmosphere at 23 °C using Bu4NPF6 (0.1 M) as the supporting
electrolyte.
The BDE(O−H) values for the FeIII(H2O) and MnIII(H2O)
complexes align with the relative HAT reactivity exhibited by 1
and the Mn analog and suggest that it is the overall
thermodynamic driving force for these reactions, determined
by the BDE(O−H) values, that is a key factor in controlling
the reaction rates of phenol oxidation. However, examination
of the log k2 versus the phenol BDE(O−H) plots for 1 and the
Mn analog show that 1 is much more reactive toward the
electron-rich phenols but is only modestly more reactive
toward the stronger O−H bond substrates. In fact, the relative
reactivities for Fe versus Mn should invert with substrates that
have O−H bonds much stronger than the 83 kcal mol−1
reported for the para-H derivative. This hypothetical switch in
relative reactivities, as expressed in the different slopes of log k2
versus BDE(OH) for Fe and Mn, suggest that the BDE(O−H)
of the metal-bound water ligand, and hence the thermody-
namics of the overall HAT reaction, are not the only predictors
of efficient H atom transfer reactivity for a high-valent metal-
oxo/hydroxo species.
FeIII(H2O)(ttppc) (2). Distilled H2O (0.5 mL) was added to a
solution of FeIII(OEt2)2(ttppc) (100 mg) in fluorobenzene (2 mL).
Vapor diffusion of pentane to this biphasic solution led to the slow
formation of dark red blocks (55 mg, 56% yield) after 1 week. UV−vis
(C6H6) λmax, nm (ε × 104 M−1 cm−1): 344 (3.29), 429 (7.27), 578
1
(1.35), 764 (0.24). H NMR (400 MHz, benzene-d6): 35.1 (s, br),
12.4 (s, br), 12.3 (s, br), 10.8 (s, br), 8.3 (s, br), 6.0 (s, br), 4.98 (s,
br), 4.66 (s, br), −19.0 (s, br), −35.1 (s, br), −61.6 (s, br), −69.0 (s,
br) δ (ppm). IR (KBr): 3580 (w, O−H), 3512 (vw, O−H), 3055
(m), 3028 (m), 1593 (s), 1556 (w), 1491 (s), 1443 (m), 1424 (w),
1394 (w), 1362 (w), 1332 (m), 1296 (m), 1214 (s), 1180 (w), 1154
(m), 1075 (m), 1045 (sh), 1020 (s), 984 (s), 916 (w), 885 (s), 851
(w), 821 (w), 794 (m), 750 (vs), 695 (vs), 637 (m), 609 (w), 574
(m), 537 (m), 499 (m), 443 (w). LDI-MS (m/z): isotopic cluster
centered at 1264.6 ([M − H2O]+). Anal. calcd for C91H59N4Fe·H2O·
2C6H5F: C, 81.33; H, 5.07; N, 3.91. Found: C, 81.53; H, 4.80; N,
4.05. μeff = 3.6(1) μB (Evans method). EPR: g = 4.29 (16 K).
Reaction of FeIV(OH)(ttppc) with 2,4-di-tert-Butylphenol:
Product Analysis. Under an inert atmosphere, a solution of
FeIV(OH)(ttppc) in benzene (2.1 mM, 500 μL) was combined
with 2,4-di-tert-butylphenol (0.132 mmol, 126 equiv), and 3.4 mg of
eicosane (6 mM) as an internal standard. The reaction mixture was
stirred and monitored by UV−vis spectroscopy, which showed rapid
and complete conversion of FeIV(OH)(ttppc)(1) to FeIII(H2O)-
(ttppc) (2). An aliquot of the reaction mixture was analyzed by GC-
FID. The phenol oxidation product 3,3′,5,5′-tetra-tert-butyl-(1,1′-
biphenyl)-2,2′-diol was identified by GC in comparison with an
authentic sample and quantified by integration of the peak and
comparison with a calibration curve constructed with eicosane. The
analysis was performed in triplicate. The average yield was equal to
67(1)% based on the reaction stoichiometry of 0.5 equiv of phenol
oxidation product per 1.0 equiv of FeIV(OH)(ttppc).
The FeIV(OH) complex 1 is capable of H atom abstraction,
as shown with the phenols in this study, and is also capable of
hydroxyl radical transfer to carbon radicals to give hydroxy-
lated product as seen in the model “rebound” reactions
described previously.29 These findings may give some insight
regarding the selection of iron, as opposed to other redox-
active, earth-abundant metals such as manganese, as the metal
of choice in heme monoooxygenases. The iron center appears
to be an efficient and versatile oxidant in a variety of oxidation
and protonation states.
EXPERIMENTAL SECTION
■
General Materials and Methods. All chemicals were purchased
from commercial sources and used without further purification unless
otherwise stated. Reactions involving inert atmosphere were
performed under Ar using standard Schlenk techniques or in an N2-
filled drybox. Toluene, dichloromethane, acetonitrile, and diethyl
ether were purified via a Pure-Solv solvent purification system from
Innovative Technologies, Inc. Benzene, ethyl acetate, and fluoroben-
zene were obtained from commercial sources. Deuterated solvents for
NMR were purchased from Cambridge Isotope Laboratories, Inc.
(Tewksbury, MA). The 4-X-2,6-di-tert-butyl-phenol derivatives were
recrystallized twice in ethanol (X = OMe, Me), acetonitrile (X = Et,
Ac), or n-pentane (X = H) and dried under vacuum. The
monodeuterated 4-OMe-2,6-di-tert-butylphenol was synthesized
using a previously published procedure.68 FeIV(OH)(ttppc) was
synthesized and purified as previously reported.29 The dark red
complex FeIII(OEt2)2(ttppc) was synthesized by reduction of
FeIV(Cl)(ttppc), following a previously published report.29
Kinetics. To a solution of FeIV(OH)(ttppc) (15 μM) in toluene,
varying amounts of 4-X-2,6-di-tert-butylphenol (X = OMe, Me, Et, H,
Ac) (0.1−3.8 mM) in toluene were added to start the reaction. The
spectral changes showed isosbestic conversion of FeIV(OH)(ttppc) to
FeIII(H2O)(ttppc). The pseudo-first-order rate constants, kobs, for
these reactions were obtained through nonlinear least-squares fitting
of the plots of absorbance at 575 nm (Abst) versus time (t) over 5
half-lives according to the equation Abst = Absf + (Abs0 − Absf)
exp(−kobst), where Abs0 and Absf are the initial and final absorbance,
respectively. Second-order rate constants (k2) were obtained from the
slope of the best-fit line from a plot of kobs versus phenol
concentration.
Instrumentation. Kinetics and other UV−vis measurements were
performed on a Hewlett-Packard Agilent 8453 diode-array spec-
trophotometer with a 3.5 mL quartz cuvette (path length = 1 cm). For
reactions with total reaction time of <10 s, stopped-flow experiments
were carried out using a HiTech SHU-61SX2 (TgK Scientific Ltd.)
G
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X-ray Crystallography. All reflection intensities for 2 were
measured at 110(2) K using a SuperNova diffractometer (equipped
with an Atlas detector) with Cu Kα radiation (λ = 1.54178 Å) under
the program CrysAlisPro (version 1.171.39.29c, Rigaku OD, 2017).
The same program was used to refine the cell dimensions and for data
reduction. The structure was solved with the program SHELXS-2014/
7 and refined on F2 with SHELXL-2014/7.69 The temperature of the
data collection was controlled using the system Cryojet (manufac-
tured by Oxford Instruments). An analytical numeric absorption
correction method was used involving a multifaceted crystal model
based on expressions derived elsewhere.70 The H atoms were placed
at calculated positions using the instructions AFIX 43 with isotropic
displacement parameters having values 1.2 Ueq of the attached C
atoms. The crystal lattice contains disordered and/or partially
occupied lattice solvent molecules (C6H5F and C5H12). Their
contributions were removed from the final refinement using the
SQUEEZE program. The crystallographic data for FeIII(H2O)(ttppc)
are summarized in Table S6.
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: dpg@jhu.edu. (D.P.G.)
*E-mail: sam.devisser@manchester.ac.uk. (S.P.V.)
ORCID
Jan Paulo T. Zaragoza: 0000-0002-2583-6571
Maxime A. Siegler: 0000-0003-4165-7810
Sam P. de Visser: 0000-0002-2620-8788
David P. Goldberg: 0000-0003-4645-1045
Notes
The authors declare no competing financial interest.
Computational Modeling. Calculations on the complexes
FeIV(OH)(ttppc) and MnIV(OH)(ttppc) and their reactions with
para-H-2,6-DTBP included all atoms for both metal complex and
phenol. Geometry optimizations, constraint geometry scans, and
vibrational frequencies were calculated with DFT in the Gaussian-09
software package.71 We utilized the unrestricted B3LYP hybrid
density functional method72,73 in combination with an LANL2DZ
basis set74,75 on iron/manganese (with core potential) and 6-31G on
the rest of the atoms (H, C, N, O), e.g., basis set BS1. Single points
using the LACV3P+ (with core potential) on Fe/Mn and 6-311+G*
on the rest of the atoms were done to correct the energies, e.g., basis
set BS2. The continuum polarized conductor model with a dielectric
constant mimicking toluene was applied to the system during the
geometry optimizations and frequency calculations.76 Single point
energy calculations were also done in benzene, e.g., basis set BS3, for
the BDE determination of MnIII(H2O)(ttppc) to match the
experimental conditions (Table S20). We initially optimized a
reactant complex of substrate and metal(IV)-hydroxo species in
close proximity and followed the pathway for H transfer through a
constrained geometry scan. Subsequently, the maximum in energy of
these scans was subjected to a full transition state search and the
obtained transition state was characterized with a frequency
calculation that gave an imaginary frequency for the correct mode.
These transition states were shown to be connected to reactants and
products through an intrinsic reaction coordinate (IRC) scan. As in
some cases, the transition states energies were lower than those of the
original reactant complexes, so the latter were reoptimized from the
final points of the IRCs, which gave slightly more stable isomers.
These methods were used in previous studies and were shown to
predict the correct spin-state orderings and barrier heights of reaction
mechanisms.26,77,78 The calculations of the BDE(O−H) values are
described in the Supporting Information (Tables S19 and S20).
ACKNOWLEDGMENTS
■
The authors would like to thank the NIH (GM101153)
(D.P.G.) for the financial support provided for this research.
M.Q.E.M. thanks the Ministry of Higher Education Malaysia
and Islamic Science University of Malaysia (USIM) for a
studentship. J.P.T.Z. thanks JHU for the Glen E. Meyer ’39
Fellowship.
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