A Reactive Manganese(IV)-Hydroxide Complex: A Missing Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo Porphyrinoid Compounds

Article abstract of DOI:10.1021/jacs.8b00350

High-valent metal-hydroxide species are invoked as critical intermediates in both catalytic, metal-mediated O₂ activation (e.g., by Fe porphyrin in Cytochrome P450) and O₂ production (e.g., by the Mn cluster in Photosystem II). However, well-characterized mononuclear M^IV(OH) complexes remain a rarity. Herein we describe the synthesis of Mn^IV(OH)(ttppc) (3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand allowed for isolation of 3. The complexes Mn^V(O)(ttppc) (4) and Mn^III(H₂O)(ttppc) (1·H₂O) were also synthesized and structurally characterized, providing a series of Mn complexes related only by the transfer of hydrogen atoms. Both 3 and 4 abstract an H atom from the O-H bond of 2,4-di-tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield (3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate constant of k₂ = 2.73(12) × 10⁴ M^-1 s^-1, which is ～3 orders of magnitude larger than 4 (k₂ = 17.4(1) M^-1 s^-1). Reaction of 3 with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k₂ = 510(10)-36(1.4) M^-1 s^-1 and led to Hammett and Marcus plot correlations. Together with kinetic isotope effect measurements, it is concluded that O-H cleavage occurs by a concerted H atom transfer (HAT) mechanism and that the Mn^IV(OH) complex is a much more powerful H atom abstractor than the higher-valent Mn^V(O) complex, or even some Fe^IV(O) complexes.

Full text of DOI:10.1021/jacs.8b00350

Article
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
A Reactive Manganese(IV)−Hydroxide Complex: A Missing
Intermediate in Hydrogen Atom Transfer by High-Valent Metal-Oxo
Porphyrinoid Compounds
Jan Paulo T. Zaragoza, Maxime A. Siegler, and David P. Goldberg*
Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: High-valent metal-hydroxide species are invoked as critical
intermediates in both catalytic, metal-mediated O2 activation (e.g., by Fe
porphyrin in Cytochrome P450) and O2 production (e.g., by the Mn cluster in
Photosystem II). However, well-characterized mononuclear MIV(OH) com-
plexes remain a rarity. Herein we describe the synthesis of MnIV(OH)(ttppc)
(3) (ttppc = tris(2,4,6-triphenylphenyl) corrole), which has been characterized
by X-ray diffraction (XRD). The large steric encumbrance of the ttppc ligand
allowed for isolation of 3. The complexes MnV(O)(ttppc) (4) and
MnIII(H2O)(ttppc) (1·H2O) were also synthesized and structurally charac-
terized, providing a series of Mn complexes related only by the transfer of
hydrogen atoms. Both 3 and 4 abstract an H atom from the O−H bond of 2,4-
di-tert-butylphenol (2,4-DTBP) to give a radical coupling product in good yield
(3 = 90(2)%, 4 = 91(5)%). Complex 3 reacts with 2,4-DTBP with a rate
constant of k2 = 2.73(12) × 104 M−1 s−1, which is ∼3 orders of magnitude
larger than 4 (k2 = 17.4(1) M−1 s−1). Reaction of 3 with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-
DTBP; X = OMe, Me, tBu, H) gives rate constants in the range k2 = 510(10)−36(1.4) M−1 s−1 and led to Hammett and Marcus
plot correlations. Together with kinetic isotope effect measurements, it is concluded that O−H cleavage occurs by a concerted H
atom transfer (HAT) mechanism and that the MnIV(OH) complex is a much more powerful H atom abstractor than the higher-
valent MnV(O) complex, or even some FeIV(O) complexes.
Fujii,22 and both were prepared with nonheme ligands (Chart
1).
The interest in high-valent, terminal, hydroxo-metal species
comes from their central roles in biological and synthetic
INTRODUCTION
■
Terminal, high-valent, metal-hydroxide (Mn+(OH)) complexes
(n ≥ 3) with first-row transition metals are relatively rare, in
part because of their tendency to form hydroxo- or oxo-bridged
catalytic processes. One key role is in the oxidation of C−H
(M−O(H)−M) structures. Redox instability, especially for the
substrates. Homolytic cleavage of C−H bonds by high-valent
tetravalent (n = 4) species, is also a significant destabilizing
metal-oxo species (Mn(O)) is an essential step in the
factor, leading to reduction or disproportionation of the desired
metal(IV) complex.1−4 In contrast, the synthesis and isolation
functionalization of organic molecules mediated by enzymes
and synthetic catalysts, and leads to Mn−1(OH) and carbon
of related high-valent, metal-oxo (Mn+(O)) species have been
more successful.5−9 However, both heme and nonheme
Mn+(OH) species have emerged recently as desirable targets,
with some limited successes being reported.10−14 Studies
involving FeIII(OH) complexes have examined their reactivity
in H atom transfer (HAT) or proton-coupled electron-transfer
(PCET) reactions.11−13,15 Similarly, a few terminal MnIII(OH)
complexes have been prepared, and their reactivity toward
HAT has been investigated.16−18 However, Fe(OH) and
Mn(OH) complexes with formal oxidation states greater than
+3 are exceedingly rare. The first Fe(OH) complex outside of a
protein19 at the formal +4 oxidation level was recently reported
by our laboratory.20 The first report of an isolated MnIV(OH)
complex came from Busch in 2006, which described a nonheme
MnIV complex with two terminal OH ligands.21 Since 2006,
only two examples of isolated monohydroxo-MnIV complexes
have been reported, one from Borovik10 and the other from
radical (R•) intermediates. The fate of the metal-hydroxide
determines the ultimate outcome of these reactions. Typically,
R• recombines with the metal-bound OH moiety to give a
hydroxylated (ROH) product. The heme monooxygenase
Cytochrome P450 (CYP) provides a well-established example
of this hydroxylation pathway (Scheme 1).23−28 However,
deviations from this mechanistic scenario occur, originating
from the divergent reactivity of FeIV(OH)(porph)(cys)
(protonated “Compound II”). In certain CYPs, FeIV(OH)-
(porph)(cys) can react as an H atom abstractor, resulting in
desaturation of the substrate, as opposed to hydroxylation.29−31
The CYP enzyme known as OleT from Jeotgalicoccus sp.
provides a fascinating example of the latter chemistry, in which
Received: January 10, 2018
© XXXX American Chemical Society
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Chart 1. Examples of MnIV(OH)n (n = 1, 2) Complexes
Mn-bound H2O/OH/O species. These studies in turn have
provided information to help delineate plausible water
oxidation scenarios for the OEC.43−51
Scheme 1. Net H-Atom Transfers in CYP and the OEC
Of the few MnIV(OH)n (n = 1, 2) complexes that are
known10,21,22,52,53 (Chart 1), there are no examples of
crystallographically characterized monohydroxo complexes.54
Previously, we reported the generation of a metastable
MnIV(OH) corrolazine complex55 but were unsuccessful in
growing crystals of this complex for characterization by X-ray
diffraction (XRD), and a detailed study of the reactivity of this
complex was not performed. Herein we report the synthesis
and crystal structure of MnIV(OH)(ttppc), which, to our
knowledge, is the first MnIV(OH) complex characterized by
single-crystal XRD. The related MnV(O) and MnIII(H2O) ttppc
complexes were also synthesized and characterized by XRD,
demonstrating the versatility of the ttppc ligand to house
Mnn+(OHx) (n = 3−5; x = 2, 1, 0) species in three different
oxidation and protonation states in an identical porphyrinoid
ligand environment. The reactivity of the MnIV(OH) complex
in HAT reactions with O−H substrates was examined, and it
was found to exhibit dramatically enhanced rates of HAT as
compared to its higher oxidation state MnV(O) relative.
Hammett and Marcus analyses, together with kinetic isotope
effects, provide detailed insight into the mechanism of HAT for
these MnIV(OH) and MnV(O) complexes.
desaturation of long chain fatty acid substrates leads to C−C
bond cleavage and production of terminal alkenes, as opposed
to C−H hydroxylation.32,33 The latter reactivity has potential
consequences for the development of biofuel.34,35 A mono-
nuclear, terminal iron-hydroxide also plays a similarly important
role in the nonheme iron enzyme soybean lipoxygenase (SLO).
An FeIII(OH) in SLO abstracts an H atom from the bisallylic
C−H bond of linoleic acid substrates. The details of this
process, and especially the observation of gigantic kinetic
isotope effects (KIEs) for C−H cleavage by the FeIII(OH)
intermediate, have been the focus of several studies.36−38
The reverse reaction, in which an Mn+(OH) species releases
a net H•, may play an important role in the water oxidation
process carried out by the Mn-containing oxygen-evolving
cluster (OEC) in photosystem II.39−42 Proposed mechanisms
for this process involve the sequential oxidation and
deprotonation of a MnIII(H2O) species, to give MnIV(OH)
and ultimately MnV(O) species (Scheme 1). The latter species
may then facilitate O−O bond formation, leading to O2
production. However, despite much effort to unravel the
mysteries of Mn-mediated water oxidation, many questions
remain about the mechanism, and possible role(s), of
MnIV(OH) and MnV(O) intermediates. Synthetic analogues
of the Mn cluster allow for detailed correlations to be made
between geometric and electronic structure and the reactivity of
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of
MnIV(OH)(ttppc). Our previous synthesis of FeIV(OH)-
(ttppc)20 suggested that the analogous MnIV(OH)(ttppc)
could be prepared by the same method involving ligand
metathesis of a metal-chloride precursor. The ttppcH3 ligand
was metalated with Mn(OAc)2 to give MnIII(ttppc) (1) in 88%
yield according to a literature method.56 Aerobic oxidation of
dark green 1 in dichloromethane in the presence of excess
HCl(aq) led to the formation of brown MnIV(Cl)(ttppc) (2),
which was monitored by UV−vis spectroscopy. After workup,
the crude reaction mixture was purified by column chromatog-
raphy to yield 2 as a brown solid (40% yield). Single crystals of
2 were obtained from CH2Cl2/n-hexane, and the structure is
shown in Figure 1. The structure reveals a 5-coordinate Mn
B
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(ttppc) (3), which were analyzed by single-crystal X-ray
diffraction (XRD).
The structure of 3 (Figure 3) shows that the axial chloride
ligand of 2 has been replaced by hydroxide, as expected. While
Figure 1. Displacement ellipsoid plot (35% probability level) for
MnIV(Cl)(ttppc) (2) at 110(2) K. Hydrogen atoms omitted for clarity.
with an axial Cl− ligand, and charge balance requires a formal
+4 oxidation state at Mn. The MnIV−Cl distance of 2.2754(11)
Å is similar to other MnIV(Cl) corroles.57,58 This complex
served as starting material for preparation of the target
MnIV(OH) species.
Figure 3. Displacement ellipsoid plot (35% probability level) for
MnIV(OH)(ttppc) (3) at 110(2) K. Hydrogen atoms (except H1)
omitted for clarity.
Titration of the chloride complex 2 with Bu4NOH (0−1.5
equiv) in toluene/CH3CN (100:1) at 23 °C was followed by
UV−vis spectroscopy (Figure 2). Isosbestic conversion of 2 to
the hydrogen atom of the OH− group in 3 could not be
accurately located from the difference Fourier maps due to the
disorder of the Mn−OH unit, charge balance and the Mn−O
bond distance are consistent with the presence of an axial OH−
ligand. The Mn−O bond distance (from the major component
of the disorder) for the terminal hydroxide ligand in 3 is
1.881(2) Å, which is slightly longer than those reported for the
few other MnIV(OH)n (n = 1, 2) complexes (1.793(2)−
1.830(4) Å)10,21,22,52,53 but much longer than a typical
MnIV(O) distance (1.58−1.71 Å).22,61−64 The Mn−O distance
in 3 is quite close to that seen for FeIV(OH)(ttppc) (Fe−O =
1.857(3) Å).20
The UV−vis and X-band EPR spectra (Figure S1) of
crystalline 3 match those seen in the spectral titrations in Figure
2. The structural and spectroscopic data confirm that 3 is a rare
example of a terminal MnIV(OH) complex and is the first
example, to our knowledge, of a monomeric MnIV(OH)
complex characterized by X-ray crystallography.
Synthesis and Structural Characterization of MnV(O)-
(ttppc). Corroles and corrolazines are known to stabilize high-
valent MnV(O) complexes. The first crystal structure of an
MnV(O) porphyrinoid complex was obtained with an octaaryl-
substituted corrolazine.65 The in situ generation of MnV(O)-
(ttppc) was described previously, and it was shown to mediate
oxygen atom transfer to alkene substrates.56 However, this
complex was not isolated as a pure solid, and no structural
information was obtained.
Figure 2. UV−vis spectral titration of 2 with NBu4OH (0, 0.33, 0.66,
and 1.0 equiv) in toluene/CH3CN (100:1 v/v) at 23 °C. Inset:
changes in the X-band EPR spectra of 2 (blue line) upon titration with
Bu4NOH (0−1.0 equiv) in toluene/CH3CN (4:1 v/v) at 12 K (final
spectrum, orange line) (* = radical impurity (S = 1/2)).
a new species with λmax = 420, 553, and 670 nm was observed
upon addition of 1 equiv of OH−, after which no further
changes in spectral features were seen. This reaction was also
followed by EPR spectroscopy, as seen in the inset of Figure 2.
The X-band EPR spectrum of 2 at 12 K reveals a well-resolved
axial spectrum with g = 4.1, 2.0 and A(55Mn)ave = 84, 60 MHz,
which can be assigned to a high-spin MnIV (d3, S = 3/2)
complex. Addition of Bu4NOH leads only to a broadening of
the g1 signal near 4.1, displaying surprisingly little change for a
change in the first coordination sphere of the Mn ion. However,
other MnIV complexes have exhibited EPR spectra that are
similarly insensitive to the identity of the axial ligand.22,59,60
The UV−vis and EPR titrations suggested that the chloride
ligand in 2 could be quantitatively displaced by hydroxide.
Following the two-phase synthesis of FeIV(OH)(ttppc),20 the
Mn(OH) complex was prepared on a bulk scale by the addition
of aqueous LiOH to a toluene solution of 2. Recrystallization
from toluene/n-pentane yielded brown crystals of MnIV(OH)-
We set out to crystallize MnV(O)(ttppc) and definitively
characterize the structure of this complex by single-crystal
XRD. The synthesis of MnV(O)(ttppc) (4) followed the earlier
report from Chang, with minor modifications.56 The original
method involved the addition of iodosylbenzene (PhIO) to
MnIII(ttppc) in CH2Cl2, followed by flash chromatography on
basic alumina. However, the resulting MnV(O) complex was
only metastable in CH2Cl2 at 23 °C (t1/2 ∼ 7 h). We examined
other solvents for the synthesis and purification of 4 in order to
improve stability. Performing the PhIO oxidation and
purification on basic alumina in ethyl acetate resulted in
much better stability, with 4 showing little or no decay in this
solvent for weeks at 23 °C. Vapor diffusion of n-pentane into a
concentrated solution of 4 in ethyl acetate led to the growth of
dark-red blocks after a few days. Crystals of 4 suitable for X-ray
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structure determination were obtained from ethyl acetate/n-
pentane, and the structure of 4 is shown in Figure 4.
Figure 5. Displacement ellipsoid plots (50% probability level) for the
Mn−O unit in 4 at 110(2) K, from crystals grown over different time
periods. The structures were obtained from crystals of 4 grown for 1
week (4a), 2 weeks (4b), and 1 month (4c). All bond distances are
given for the major component of the Mn−O group which is
disordered over two positions (∼7:3 occupancy) above and below the
plane of the corrole.
Figure 4. Displacement ellipsoid plot (35% probability level) for
MnV(O)(ttppc) (4) at 110(2) K. H atoms omitted for clarity.
The structure of 4 revealed a five-coordinate Mn ion, with
Mn−Npyrrole distances between 1.8970(12) and 1.9249(11) Å
and an apparent Mn−O bond distance of 1.6332(16) Å. This
Mn−O distance is significantly longer than that seen for
MnV(O) corrolazine (1.5455(18) Å)65 or for other nonheme
MnV(O) complexes (1.548(4)−1.558(4) Å).66−68 This elon-
gated Mn−O bond could possibly be consistent with a high-
spin MnV(O) complex (S = 1) based on the DFT-optimized
geometries of excited state (S = 1) MnV(O) corroles and
corrolazines,69 although all of the former MnV(O) species were
characterized experimentally as exhibiting low-spin (S = 0)
ground states. Crystals of 4 dissolved in benzene-d6 gave a
to the 425 nm peak with crystallization time, as seen in Figure
S7. These changes are consistent with an increase of the
MnIV(OH) impurity relative to MnV(O)(ttppc), with the pure
MnIV(OH) complex exhibiting a single Soret band at 420 nm as
seen in Figure 2. In fact, a linear correlation between the
absorbance ratio (A365/A425) and the Mn−O bond distance is
observed (Figure S8). The apparent bond elongation in 4 is
similar to the bond elongations observed by Parkin for the
Mo−O bond in Mo(PMe2Ph)3OCl2, which arises from a
compositional disorder via cocrystallization with the isostruc-
tural derivative Mo(PMe2Ph)3Cl3, leading to unresolved site
disorder of Mo−O/Mo−Cl groups with significantly different
bond lengths.70
1
sharp, diamagnetic (S = 0) H NMR spectrum (Figure S2),
ruling out an S = 1 ground state for this complex.
Photoreduction by the X-ray beam was also considered, but
UV−vis spectra of crystals of 4 dissolved in ethyl acetate
showed no change after X-ray irradiation (Figure S3).
We next considered a possible site disorder in the crystal
lattice with an MnIV(OH)(ttppc) impurity, which could be
present from minor reduction of MnV(O) during prolonged
crystallization times. Magnetic susceptibility measurements
(Figure S4) on crystalline 4 revealed the presence of a small
amount of a paramagnetic species (16% MnIV), consistent with
an MnIV(OH) impurity. EPR analysis of crystalline 4 also
revealed a weak signal indicative of the presence of a minor
MnIV component (Figure S5). These data were consistent with
a minor amount of MnIV(OH) cocrystallizing with the MnV(O)
complex.
Further confirmation that the MnV−O bond distance is
shorter than the apparent bond lengths seen in Figure 4 comes
from resonance Raman (RR) spectroscopy. The Mn−O stretch
for MnV(O)(ttppc) in CH2Cl2 is ν(Mn−O) = 952 cm−1.56 This
vibration can be compared with other metal-oxo and related
species in a Badger’s rule plot,71 which relates stretching
frequency with bond distance. From the Badger’s rule plot,
normalized for differences in atomic masses, an Mn−O distance
of 1.59 Å is predicted for MnV(O)(ttppc). This distance is in
line with other MnV(O) complexes and is clearly shorter than
the apparent distances seen by XRD in Figure 5. We conclude
that 4 cocrystallizes with varying, small amounts of the
isostructural MnIV(OH)(ttppc) derivative, depending on
crystallization time, and gives rise to an apparent elongated
MnV−O bond distance due to a small amount of MnIV(OH) in
the crystals.
Synthesis and Structural Characterization of
MnIII(H2O)(ttppc). Crystals of 1·H2O were grown from
addition of a small amount of H2O (50 μL) to a solution of
1 in fluorobenzene (1 mL), followed by removal of excess H2O
and slow vapor diffusion of n-pentane. Complex 1·H2O
cocrystallizes with the metal-free corrole (ttppcH3), which
presumably forms during the crystallization process. The
occupancy factor of the major component 1·H2O refines to
0.7114(15), and its structure is shown in Figure 6. The Mn−O
bond distance in 1·H2O is 2.2645(18) Å, and the two H atoms
of the H2O molecule were located from the difference Fourier
maps (Figure S9). This Mn−O bond distance is also
comparable to that reported for MnIII(H2O) corrolazine
(2.143(2) Å).72 The structural characterization of 1·H2O
To determine if the amount of MnIV(OH) impurity
correlated with crystallization time, XRD data were collected
on crystals of 4 isolated after standing in solution for 1 week
(4a), 2 weeks, (4a), and one month (4c). The apparent
lengthening of the Mn−O bond did indeed increase with
longer crystallization times, as seen in Figure 5. Efforts to
model the disorder of MnV(O)/ MnIV(OH) were not
successful due to the disorder of the Mn−O group above and
below the corrole plane, as shown by the contoured Fourier
maps in Figure S6. The observed elongation of the Mn−O
bond distance in 4 most likely results from the averaging of the
mixtures of the MnV(O) and MnIV(OH) bond distances in the
crystal over time and space.
The single crystals that were used for the structure
determinations of 4a−c were each removed from the X-ray
diffractometer immediately following data collection and
dissolved in ethyl acetate for UV−vis spectroscopy. The
intensity of the peak at 365 nm decreases in intensity relative
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complexes.73−75 However, 4 reacts with DHA only very slowly
(<5% reacted over 8 h). Slow kinetics for C−H cleavage were
also seen with the related MnV(O) corrolazine, which reacts
with DHA with k2 = 1.8 × 10−5 M−1 s−1.76 In contrast, it was
found that complex 4 is capable of abstracting H atoms from
aryl O−H bonds with relative ease. Addition of excess 2,4-di-
tert-butylphenol (2,4-DTBP) to a solution of 4 in benzene at 23
°C caused complete isosbestic conversion of 4 (λmax = 365, 425,
565 nm) to the MnIII-aquo complex 1·H2O (λmax = 412, 433,
660 nm) within 5 min (Figure 7a,b). Analysis of reaction
mixtures by GC-FID revealed the formation of the phenol
radical coupling product in 91(5) % yield (Figure S10),
according to the stoichiometry shown in Figure 7a. The
coupled dimer is the expected product following homolytic
ArO−H bond cleavage, which initially should give a transient
phenoxyl radical and the MnIV(OH) complex 3 after HAT.
However, complex 3 was not detected, and only the MnIII
product was observed.
The kinetics of the reaction between 4 and 2,4-DTPB was
monitored by UV−vis spectroscopy under pseudo-first-order
conditions. Addition of excess 2,4-DTBP to 4 led to first-order
decay of 4 and concomitant production of 1·H2O, as seen in
Figure 7b. Satisfactory fits of the single-wavelength data (inset,
Figure 7b) with a first-order rate expression gave pseudo-first-
order rate constants (kobs) that correlated linearly with substrate
concentration (Figure 7c). The slope from Figure 7c gives a
second-order rate constant of kH = 17.4(1) M−1 s−1 for 2,4-
DTBP. The deuterated phenol derivative, 2,4-DTBP(OD), was
prepared and measured under the same conditions, giving kD =
16.5(2) M−1 s−1 and leading to a kinetic isotope effect (KIE) of
kH/kD = 1.05 for ArO−H cleavage by 4. The reactivity of 4 can
be compared with MnV (O)(tpfc) (tpfc = tris-
(pentafluorophenyl)corrole), which was examined by Abu-
Omar and co-workers in reactions with 2,4-DTBP.77 This
complex oxidized 2,4-DTBP with k2 = 45 M−1 s−1, similar in
magnitude to 4. A KIE value of kH/kD = 1.3 was observed,
which is larger than that seen for 4, although still smaller than
most other metal-oxo-mediated HAT reactions.76,78,79 From
product analysis, it is clear that 4 reacts with 2,4-DTBP via a net
H atom abstraction, although the small KIE suggests that the
rate-determining step may not be a concerted HAT, and
instead a mechanism closer to PCET may be operative.
The absence of the formation of MnIV(OH)(ttppc) (3) in
the reaction between 4 and 2,4-DTBP leads to two possible
mechanisms (Scheme 2). The first mechanism (A) involves a
MnIV(OH) intermediate that is much more reactive as an H
atom abstractor than the starting MnV(O) complex, giving a
steady-state condition for MnIV(OH)(ttppc). We have
suggested a similar mechanism for the HAT reactivity of an
analogous MnV(O) corrolazine complex.76 Alternatively,
mechanism B involves rapid disproportionation of MnIV(OH)-
(ttppc) to MnV(O)(ttppc) and MnIII(H2O)(ttppc), with the
MnV(O) complex serving as the sole oxidant in the system.
These mechanistic possibilities for reactions mediated by high-
Figure 6. Displacement ellipsoid plot (35% probability level) for
MnIII(H2O)(ttppc) (1·H2O) at 110(2) K. H atoms (except for those
attached to O) omitted for clarity.
completes the series of MnV(O), MnIV(OH), and MnIII(H2O)
complexes related by single H atom transfer steps.
Density Functional Theory (DFT) Calculations. DFT
calculations at the B3LYP/LANL2DZ/6-31G level of theory
were performed on complexes 1·H2O, 3, and 4 to support the
structural assignments. The DFT calculations were performed
with starting structures that contained truncated forms of the
triarylcorrole ligand, with the outer phenyl substituents
removed from each of the meso-phenyl groups to facilitate
the calculations. The experimental and calculated Mn−O, Mn−
Nave, and Mn−Nplane distances are given in Table 1.
The DFT-calculated Mn−O distances for both MnIV(OH)-
(ttppc) and MnIII(H2O)(ttppc) are in good agreement with
those determined by single-crystal XRD, as are the Mn−Nave
and Mn−Nplane distances. Complexes 1·H2O and 3 thus
provide a good experimental calibration of our DFT methods.
In contrast to 1·H2O and 3, the DFT-derived Mn−O bond
length for 4 (1.560 Å) deviates significantly from the
experimentally obtained value (1.6332(16) Å), although it
does fall in line with other structurally characterized MnV(O)
complexes.65−68 The discrepancy for the MnV(O) complex 4
further supports the conclusion that the apparent Mn−O bond
length in 4 observed by single-crystal XRD is elongated because
of compositional disorder with a small amount of the
MnIV(OH) complex 3.
H Atom Transfer Reactivity of MnV(O)(ttppc) (4). With
the Mn complexes 1·H2O, 3, and 4 in hand, it is clear that the
ttppc ligand provides an excellent platform for the isolation of
manganese in three different oxidation states, together with
sequential protonation at a terminal oxo ligand. The conversion
from MnV(O) (4) to MnIV(OH) (3) to MnIII(H2O) (1·H2O)
involves the formal transfer of hydrogen atoms. These
complexes thus provide the rare opportunity to study
sequential H atom transfer (HAT) at a single terminal oxo
ligand of a high-valent Mn, while holding the remaining ligands
constant.
Initial attempts to study the HAT reactivity of 4 began with
9,10-dihydroanthracene (DHA), a common substrate em-
ployed to examine HAT reactions with high-valent metal-oxo
Table 1. Comparison of Select Experimental and Calculated Metrical Parameters for 1·H2O, 3, and 4
MnV(O)(ttppc) (4)
X-ray
MnIV(OH)(ttppc) (3)
MnIII(H2O)(ttppc) (1·H2O)
DFT
X-ray
DFT
X-ray
DFT
Mn−O
Mn−Nave
Mn−Nplane
1.6332(16)
1.9093(12)
0.527
1.560
1.916
0.535
1.881(2)
1.9051(14)
0.372
1.867
1.941
0.408
2.2645(18)
1.9077(14)
0.218
2.254
1.922
0.196
E
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Figure 7. (a) Reaction of MnV(O)(ttppc) (4) and 2,4-DTBP. (b) Time-resolved UV−vis spectral changes for the reaction between (4) (15 μM) and
2,4-DTBP (0.5 mM) in benzene at 23 °C. Inset: change in absorbance vs time for the growth of MnIII(ttppc) (660 nm) (green circles) with the best
fit line (black). (c) Plot of pseudo-first-order rate constants (kobs) versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares). (d)
Reaction of MnIV(OH)(ttppc) (3) and 2,4-DTBP. (e) Time-resolved UV−vis spectral changes for the reaction between (3) (15 μM) and 2,4-DTBP
(0.42 mM) in benzene at 23 °C. Inset: change in absorbance vs time for the growth of MnIII(ttppc) (660 nm) (green circles) with the best fit line
(black). (f) Plot of pseudo-first-order rate constants (kobs) versus [2,4-DTBP-OH] (black circles) and [2,4-DTBP-OD] (red squares).
Scheme 2. Possible Mechanisms for H-Atom Abstraction by
an MnV(O) Complex
hydroxide intermediate. In addition, the general HAT reactivity
of an MnIV(OH) porphyrinoid complex is not known and
could be examined here for the first time.
MnIV(OH)(ttppc) (3): A Reactive HAT Intermediate?
The reactivity of 3 with phenol derivatives was examined.
Addition of excess 2,4-DTBP to 3 in benzene at 23 °C results
in the rapid, isosbestic conversion of 3 to the MnIII species
1•H2O (Figure 7e). Analysis by GC-FID revealed the
formation of the bis(phenol) dimer in good yield (90(2)%),
consistent with a single HAT to MnIV(OH) to give the
MnIII(H2O) product. The rate of this reaction was measured by
UV−vis spectroscopy, as done previously for 4, and led to a
second-order rate constant of k = 2.73(12) × 104 M−1 s−1.
Comparison of the rate constant for the deuterated derivative
of 2,4-DTBP gave a KIE = 1.40 for the abstraction of hydrogen
by MnIV(OH). Although this KIE is relatively small, it is similar
to what is seen for MnV(O)(tpfc),77 and combined with our
other data, it is reasonable to conclude HAT is the rate-
determining step for this reaction.
valent metal-oxo corroles and porphyrins have been debated for
some time.1−4 The isolated MnIV(OH)(ttppc) complex 2
provides a rare opportunity to address this mechanistic
question by direct observation of the reactivity of the metal-
F
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The kinetic data clearly show that the MnIV(OH) complex 3
reacts much more rapidly with phenol derivatives than the
higher-valent MnV(O) complex 4. The HAT reaction with 2,4-
DTBP occurs with an ∼1570-fold rate increase and points to
mechanism A in Scheme 2. Further support for mechanism A
comes from the stability of 3 in solution. There is no evidence
of disproportionation of 3 (mechanism B) upon standing in
C6H6 for hours, even over a wide range of concentrations (20
μM−3 mM). We conclude that mechanism A is operative, and
the MnIV(OH) complex is a much more potent H atom
abstractor than the higher-valent MnV(O) with O−H bonds.
While hydrogen atom transfer reactions have been widely
studied for metal-oxo complexes, less is known about the
hydrogen atom transfer reactivity of high-valent metal-hydroxo
complexes. There are several reaction pathways by which net
hydrogen atom transfer from phenols to 3 could occur: (1)
electron transfer (ET) followed by proton transfer (PT), (2)
PT followed by ET, or (3) concerted HAT. Kinetic studies
were performed on a series of para-substituted 2,6-di-tert-
butylphenol substrates (4-X-2,6-DTBP, X = OMe, Me, tBu, H)
in benzene at 23 °C. Isosbestic conversion of 3 to 1 was
observed upon addition of excess phenols, and a plot of the
first-order rate constant (kobs) versus substrate concentration
gave second-order rate constants, ranging from 5.1(1) × 102
M−1 s−1 for the −OMe derivative to 36.3(1) M−1 s−1 for the
−H derivative. Hammett analysis of the second-order rate
constants obtained from the reaction of 3 with 4-X-2,6-DTBP
(Figure 8a) shows a decrease in the rate constants with an
increase in the electron-withdrawing nature of the para
substituent, giving ρ+ = −1.4(2), with good linearity (r2 =
0.975). Slightly smaller ρ values were obtained for the H atom
abstraction of 4-X-2,6-DTBP by MnV(O) corrolazine (ρ+ =
−1.26)76 and of substituted phenols by FeIV(O)(T2MPyP)
(T2MPyP = tetra(2-N-methylpyridyl)porphyrin) (ρ+
=
−1.10),81 which were indicative of a concerted H atom
transfer. A plot of log k vs the bond dissociation energy (BDE)
of the phenolic O−H bond (Figure S17) similarly gives a fairly
linear correlation (r2 = 0.975) with a slope of −0.25(3). This
linear relationship is indicative of an HAT mechanism;82,83
however, due to the small BDE range (∼4 kcal/mol) studied,
care should be taken in interpreting this linearity.
Further support for a concerted H atom transfer mechanism
comes from a Marcus analysis of the second-order rate
constants for 3. A linear free energy relationship between
(RT/F) ln k vs Eox can be derived from the Marcus
equation,20,84 wherein a slope of ∼−0.5 is expected for a
simple rate-determining ET process. A plot of (RT/F) ln k vs
E
ox (k = second-order rate constant, Eox = redox potential of 4-
X-2,6-DTBP derivative) affords a linear correlation with slope =
−0.12(1) (Figure 8b). This slope is much smaller than the
expected −0.5 for a rate-limiting ET reaction and is comparable
to the slope (−0.05) for the reaction of cumylperoxyl radical
and phenols, which is a clear example of concerted HAT.85,86
The reactivity of 3 toward the phenol derivatives can be
compared with other high-valent metal-oxo and metal-hydroxo
complexes (Table 2). The MnIV(OH) complex 3 is by far the
most reactive, oxidizing the phenols at rates that are 1−3 orders
of magnitude greater than the other FeIV(O), MnV(O), and
MnIII(OH) complexes in Table 2. Moreover, the Hammett
slopes for the other complexes given in Table 2 are all negative
and relatively close to that obtained for 3. These Hammett
trends are consistent with only small charge separation in the
transition state and point to a concerted HAT mechanism as
described for 3. Thus, the increased reactivity for 3 is likely not
due to a significant difference in the mechanism of phenol
oxidation for 3 as compared to the other complexes in Table 2.
Interestingly, the only other MnIV(OH) complex in Table 2,
MnIV(OH)(salen), also exhibits good reactivity (at −70 °C)
with the same phenol substrates.59
The observed superior HAT reactivity of MnIV(OH)(ttppc)
compared to MnV(O)(ttppc), as well as the other complexes in
Table 2, deserves comment. The MnV(O) complex might be
expected to be a more reactive oxidant than the MnIV(OH)
complex based simply on oxidation state, and comparison of
the oxidative reactivity of the few well-characterized pairs of
MV(O)/MIV(O) complexes (M = Mn, Fe) agrees with this
prediction.87−89 The enhanced reactivity of 3 contrasts this
trend, indicating that the obvious predictions based on
oxidation state regarding HAT reactivity do not fit for the
MnV(O)/MnIV(OH) ttppc complexes. A comparison with the
analogous heme enzyme intermediates FeIV(O)(porph•+)/
FeIV(OH)(porph), which are at the same relative oxidation
levels and protonation states as 4 and 3, is also informative. A
direct comparison of the HAT reactivity of these species has
recently been made for aromatic peroxygenase (APO), a
thiolate-ligated heme enzyme.90 In APO, protonated Cpd-II
abstracts hydrogen from benzylic C−H bonds much more
slowly than Cpd-I, consistent with the former species being a
milder oxidant. Protonated Cpd-II is also generally less reactive
than Cpd-I in the oxidation of phenolic O−H bonds, although
the difference is much less apparent (k(Cpd‑I) = 1.1−4.8 × 107
Figure 8. Hammett (a) and Marcus (b) plots for the reaction of
MnIV(OH)(ttppc) (3) and 4-X-2,6-DTBP (X = −OMe, −Me, −tBu,
−H) in benzene at 23 °C.
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Table 2. Comparison of Second-Order Rate Constants (M−1 s−1) for the Oxidation of the Phenol Derivatives 4-X-2,6-DTBP (X
= OMe, Me, tBu, H) by M(O) and M(OH) Complexes
4-X-DTBP
MnIV(OH)(ttppc) (3)
−OMe
−Me
−tBu
−H
36
ρ+
conditions
510
1.44
0.45
11.4
26.4
29.8
nd
150
105
−1.4
benzene, 23 °C
CH3CN, 50 °C
CH2Cl2, 25 °C
CH3CN, 25 °C
CH3CN, 25 °C
CH3CN, 25 °C
a
MnIII(OH)(dpaq)+
1.20
0.02
0.18
1.84
1.84
0.02
0.96
0.01
0.22
0.90
1.02
0.007
0.75
0.008
0.04
0.46
0.68
0.002
−0.88
−1.26
−3.20
−2.30
−1.50
nd
b
MnV(O)(TBP8Cz)
c
FeIV(O)(TMC)(NCCH3)+
c
FeIV(O)(TMC)(CF3COO)
c
FeIV(O)(TMC)(N3)
d
MnIV(OH)(salen)
CH2Cl2, −70 °C
a
b
c
d
From ref 16, calculated from reported kobs (s−1) values. From ref 76. From ref 80, calculated from reported kobs (s−1) values. From ref 59.
M−1 s−1 vs k(protonated Cpd‑II) = 0.05 − 8.0 × 107 M−1 s−1). Here
again, the relative reactivity of the MnV(O)/MnIV(OH)
complexes appears to contradict these trends. Further
investigations of 3 and 4 are warranted to determine the
influence of other factors (e.g., spin state, redox potentials,
proton affinities) on the HAT reactivity of these species.
discovery. In the meantime, the development of new high-
valent metal-hydroxide species in synthetic models will allow
for correlations to be made between their structural and
electronic properties and their reactivity. Such work is ongoing
in our laboratory.
EXPERIMENTAL SECTION
■
SUMMARY AND CONCLUSIONS
Materials. All chemicals were purchased from commercial sources
and used without further purification unless otherwise stated.
Reactions involving inert atmosphere were performed under argon
gas using standard Schlenk techniques or inside a nitrogen-filled
drybox. Toluene, dichloromethane, acetonitrile, and diethyl ether were
purified via a Pure-Solv solvent purification system from Innovative
Technologies, Inc. 2,4-Di-tert-butylphenol was recrystallized twice in n-
pentane. The monodeuterated 2,4-di-tert-butylphenol was prepared
following a published procedure.91 The phenol derivatives of formula
4-X-2,6-di-tert-butylphenol were purchased from Sigma-Aldrich and
then recrystallized twice in an appropriate solvent, ethanol (X = OMe,
Me), acetonitrile (X = tBu), or n-pentane (X = H), and dried under
vacuum. The metal-free ligand 5,10,15-tris(2,4,6-triphenylphenyl)-
corrole (ttppcH3) was synthesized following a modified procedure.20
Iodosylbenzene (PhIO) (>95%) was obtained from TCI chemicals.
Deuterated solvents for NMR spectroscopy were purchased from
Cambridge Isotopes, Inc.
■
The synthesis and characterization of a rare MnIV(OH)
complex (MnIV(OH)(ttppc) (3)) was described, which is also
the first example of a monohydroxo-MnIV complex to be
characterized by XRD. The related MnV(O)(ttppc) (4) and
MnIII(H2O)(ttppc) (1·H2O) complexes were prepared, com-
pleting a series of Mn complexes in three different oxidation
states that could be interconverted by a single HAT within the
same overall ligand environment. It was shown that 3 was
capable of the facile oxidation of phenol derivatives, and kinetic
analysis indicated that the mechanism of oxidation was best
described as a concerted HAT mechanism. A key result
obtained from these studies is the observation that the
MnIV(OH) complex 3 is a much more powerful oxidant
toward phenol substrates than the higher-valent MnV(O)
complex 4. This finding contradicts the expectation that the
higher oxidation state complex should be the better oxidant but
falls in line with the observation that the MnV(O) complex
reacts with phenols via HAT to give MnIII(H2O), without the
observation of a MnIV(OH) intermediate. These results help
settle the debate regarding the mechanism of MnV(O)
oxidations in favor of Mechanism A in Scheme 2, which
includes a highly reactive MnIV(OH) intermediate. The
MnIV(OH) complex 3 is also a more powerful oxidant than
FeIV(O) complexes with the same series of phenol substrates,
despite the fact that FeIV(O) species are widely invoked as
potent oxidants in synthetic catalysts and enzymes.
Instrumentation. UV−vis spectroscopy was performed on a
Hewlett-Packard Agilent 8453 photodiode-array spectrophotometer.
Samples were contained in a quartz cuvette (3.5 mL, path length = 1
cm) fitted with a septum. UV−vis stopped-flow spectroscopy was
carried out for reactions with total reaction times of less than ∼10 s,
using a HiTech SHU-61SX2 (TgK Scientific Ltd.) stopped-flow
spectrophotometer with a xenon light source and Kinetic Studio
software. 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 chromatog-
raphy (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. 1H NMR spectra were recorded on a Bruker
Avance 400 MHz NMR spectrometer at 298 K and referenced against
residual solvent proton signals. Elemental analyses were performed at
Atlantic Microlab, Inc., Norcross, GA. 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. 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 atmosphere at 23 °C using
Bu4NPF6 (0.1 M) as the supporting electrolyte.
Water oxidation can be considered as the microscopic reverse
of HAT to MnV(O) to give MnIII(H2O) (Scheme 1). The
ability of MnIV(OH) to rapidly abstract an H atom from
phenols implies that the reverse concerted H atom abstraction
from an MnIII(H2O) to give an MnIV(OH) intermediate may
be an unfavorable pathway in water oxidation. Thus, separated
proton/electron transfer steps, or multiproton/multielectron
transfers, as opposed to a single, concerted HAT beginning
with MnIII(H2O), may be more favored for water oxidation by
the Mn cluster in the OEC.
From the results presented here, it is clear that high-valent
metal-hydroxide complexes can function as potent oxidizing
species in their own right, in addition to the better known
oxidants comprised of high-valent metal-oxo species. The
nonheme enzyme SLO relies on such a species for C−H
cleavage, and perhaps other enzymatic examples await
H
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Synthesis of MnIII(H2O)(ttppc) (1·H2O). Complex 1 was
synthesized by metalation of ttppcH3 with Mn(OAc)2·4H2O following
a published procedure.56 After purification of 1 by column
chromatography in Et2O/basic alumina, the H2O-bound complex 1·
H2O was synthesized by addition of a small amount of distilled H2O
(50 μL) to 1 dissolved in fluorobenzene (1 mL) followed by stirring
for 2 h. After manually separating the excess water, vapor diffusion of
n-pentane led to the slow formation of dark green crystals of
MnIII(H2O)(ttppc) (1·H2O) after 2 weeks.
Reaction of 4 with 2,4-Di-tert-butylphenol. Product Anal-
ysis. To a solution of 4 in benzene (500 μL, 3.3 mM) was added 2,4-
di-tert-butylphenol (16.5 μmol, 10 equiv) and the internal standard
eicosane (6 mM). The reaction was monitored by UV−vis
spectroscopy, which showed complete conversion of 4 to 1. The
reaction mixture was directly injected onto the GC for analysis. 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 an internal standard (eicosane). The reaction
was performed in triplicate. Average yield = 91(5)% based on the
reaction stoichiometry of 1.0 equiv of phenol oxidation product per 1.0
equiv of 4.
Synthesis of MnIV(Cl)(ttppc) (2). A solution of 1 (0.250 g, 0.198
mmol) in CH2Cl2 (100 mL) was stirred with aqueous HCl (100 mL,
1.0 M) for 1 h. The organic layer was washed with water (3 × 50 mL)
and dried over anhydrous Na2SO4. The solvent was removed, and the
residue was purified by column chromatography on silica gel (4:1
CH2Cl2/hexanes, Rf = 0.40) to obtain 2 as a pure brown solid (0.103
g, 40% yield). The relatively low yield was due to the dematalation of
1, giving ttppcH3 which was recovered as a green solid (Rf = 0.74).
Crystals of 2 suitable for XRD were obtained by layering a CH2Cl2
solution of 2 with n-hexane, followed by slow evaporation of this
mixture over 1 week. This method gave 2 as dark brown blocks. UV−
vis (benzene) λmax, nm (ε × 10−4 M−1 cm−1): 420 (2.95), 460 (2.64),
710 (0.25). LDI-MS (m/z): isotopic cluster centered at 1263.4 (M −
Cl+) (calcd 1263.4). Anal. Calcd for C91H59MnN4Cl: C, 84.15; H,
4.58; N, 4.31. Found: C, 84.79; H, 4.89; N, 4.33.
Kinetic Studies. Reaction of MnV(O)(ttppc) with 2,4-Di-tert-
butylphenol. To a benzene solution of MnV(O)(ttppc) (4) (2 mL,
15 μM) at 23 °C, varying amounts of 2,4-di-tert-butylphenol or
monodeuterated 2,4-di-tert-butylphenol (0.5−2.8 mM) were added to
start the reaction. The spectral changes showed isosbestic conversion
of 4 (λ = 365, 425, 565 nm) to 1 (λ = 412, 433, 660 nm). The pseudo-
first-order rate constants, kobs, for these reactions were obtained by
nonlinear least-squares fitting of the plots of absorbance at 365 or 660
nm (Abst) versus time (t) 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 (M−1 s−1),
were obtained from the slope of the best-fit line from a plot of kobs
(s−1) vs 2,4-di-tert-butyl phenol concentration (M).
Synthesis of MnIV(OH)(ttppc) (3). To a dark brown solution of 2
(20.0 mg, 15.4 μmol) in toluene (5 mL) was added an aqueous
solution of LiOH (5 mL, 5% w/v), and the biphasic mixture was
stirred for 1 h. Completion of the reaction was observed by UV−vis
spectroscopy. The toluene layer was extracted, washed with water (3 ×
5 mL), and dried over anhydrous sodium sulfate. The brown solution
was then passed through a short plug of celite and dried in vacuo to
give 3 as a dark brown solid. Crystals of 3 suitable for XRD were
obtained by vapor diffusion of n-pentane into toluene over 1 week
under anaerobic conditions, giving 3 as brown blocks (12.6 mg, 64%
yield). UV−vis (benzene) λmax, nm (ε × 10−4 M−1 cm−1): 420 (3.62),
553 (0.60), 670 (0.27). LDI-MS (m/z): isotopic cluster centered at
1280.6 (M+) (calcd 1280.4). Anal. Calcd for C91H60MnON4: C, 85.36;
H, 4.72; N, 4.38. Found: C, 84.45; H, 4.86; N, 4.26.
Kinetic Studies. Reaction of MnIV(OH)(ttppc) with 2,4-Di-tert-
butylphenol and 4-X-2,6-DTBP Derivatives. In a typical reaction,
MnIV(OH)(ttppc) (3) (15 μM) was reacted with varying amounts of
2,4-di-tert-butylphenol, monodeuterated 2,4-di-tert-butylphenol (0.4−
1.7 mM), or 4-X-2,6-DTBP (X = OMe, Me, tBu, H) in benzene at 23
°C. The spectral changes showed isosbestic conversion of 3 (λ = 420
nm) to 1 (λ = 412, 433, 660 nm). The same kinetic analysis was used
as employed for kinetic studies with 4, following the growth in
absorbance at 660 nm corresponding to production of 1. Plot of the
resulting kobs (s−1) values versus substrate concentration (M) were
linear and yielded second-order rate constants, k2 (M−1 s−1).
Computational Methods. Calculations on the manganese
1
4
5
complexes [MnV(O)(tpc)], [MnIV(OH)(tpc)], and [MnIII(H2O)-
(tpc)] (tpc = 5,10,15-triphenylcorrole) utilized density functional
theory (DFT) as implemented in the Orca program package.92
Geometry optimizations and frequency calculations were performed
using the unrestricted hybrid density functional B3LYP with the
LANL2DZ93 basis set on Mn and the 6-31G94 basis set on the
remaining atoms. The initial geometries of the manganese complexes
were obtained from the X-ray crystal structures of the complexes, with
the 2,4,6-triphenyl substituents on each meso-phenyl group replaced
with hydrogen atoms. Independent calculations have shown that the
addition of electron-donating substituents, or sterically bulky groups,
to the meso-phenyl groups does not affect the Mn−O bond distances
for the manganese complexes discussed. Optimized geometries were
used for frequency calculations at the same level of theory to ensure
the local minimum nature of the geometry (without any imaginary
frequencies).
Synthesis of MnV(O)(ttppc) (4). The complex was synthesized
following a modification of a published procedure.56 To a solution of 1
(70 mg, 55.4 μmol) in ethyl acetate (5 mL) was added PhIO (120 mg,
0.55 mM), and the solution was stirred for 5 min, after which a color
change from green to red was observed. The solution was passed
through a short plug of basic alumina to remove unreacted PhIO.
Vapor diffusion of this solution with n-pentane gave 4 as dark-red
blocks (17 mg, 24% yield) after 1 week (4a). For crystals of 4 with
variable crystallization time, crystals were grown in ethyl acetate/n-
pentane for 2 weeks (4b) or in benzene/n-heptane for 1 month (4c).
UV−vis (benzene): λmax, nm (ε × 10−4 M−1 cm−1): 365 (3.30), 425
(4.20), 565 (0.81). 1H NMR (400 MHz, C6D6) δ (ppm): 8.71 (d, J =
4.5 Hz, 2H), 8.49 (dt, J = 14.5, 4.9 Hz, 6H), 8.20−8.10 (m, 6H), 7.80
(dt, J = 6.6 Hz, 6H), 7.38 (td, J = 7.4, 3.2 Hz, 6H), 7.33−7.24 (m, 3H),
7.07 (d, J = 7.5 Hz, 4H), 6.94 (d, J = 7.5 Hz, 4H), 6.90 (d, J = 7.4 Hz,
2H), 6.82 (d, J = 7.4 Hz, 2H), 6.53−6.12 (m, 18H). LDI-MS (m/z):
isotopic cluster centered at 1279.5 (M+) (calcd 1279.4). Anal. Calcd
for C91H59MnN4O: C, 85.43; H, 4.65; N, 4.38. Found: C, 85.39; H,
4.79; N, 4.33.
Reaction of 3 with 2,4-Di-tert-butylphenol. Product Anal-
ysis. To a solution of 3 in benzene (500 μL, 3.3 mM) was added 2,4-
di-tert-butylphenol (10 μmol, 6 equiv) and the internal standard
eicosane (6 mM). The reaction was monitored by UV−vis
spectroscopy, which showed complete conversion of 3 to 1. The
reaction mixture was directly injected onto the GC for analysis. 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 an internal standard (eicosane). The reaction
was performed in triplicate. Average yield = 90(2)% based on the
reaction stoichiometry of 0.5 equiv of phenol oxidation product per 1.0
equiv of 3.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b00350.
■
S
Kinetics studies, UV−vis, 1NMR, and X-band EPR
spectra, magnetic susceptibility measurements, GC-FID
traces and parameters, single-crystal X-ray crystallog-
raphy analysis and data, summary of selected bond
lengths, and coordinates of the DFT-optimized struc-
tures (PDF)
Data_1_OH2 file (CIF)
Data_2 file (CIF)
Data_3 file (CIF)
I
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Journal of the American Chemical Society
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Data_4b file (CIF)
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AUTHOR INFORMATION
Corresponding Author
*dpg@jhu.edu
ORCID
Jan Paulo T. Zaragoza: 0000-0002-2583-6571
David P. Goldberg: 0000-0003-4645-1045
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(33) Grant, J. L.; Mitchell, M. E.; Makris, T. M. Proc. Natl. Acad. Sci.
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■
The authors gratefully acknowledge research support of this
work by the NIH (Grant GM101153 to D.P.G.). J.P.T.Z. would
like to thank JHU for the Glen E. Meyer ’39 Fellowship. We
also thank Jennifer Morey and Prof. Tyrel McQueen for the
solid-state magnetic measurements. Computer time was
provided by the Maryland Advanced Research Computing
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