Light-Driven, Proton-Controlled, Catalytic Aerobic C-H Oxidation Mediated by a Mn(III) Porphyrinoid Complex

Article abstract of DOI:10.1021/jacs.5b00816

The visible light-driven, catalytic aerobic oxidation of benzylic C-H bonds was mediated by a Mn^III corrolazine complex. To achieve catalytic turnovers, a strict selective requirement for the addition of protons was established. The resting state of the catalyst was unambiguously characterized by X-ray diffraction as [Mn^III(H₂O)(TBP₈Cz(H))]⁺, in which a single, remote site on the ligand is protonated. If two remote sites are protonated, however, reactivity with O₂ is shut down. Spectroscopic methods revealed that the related Mn^V(O) complex is also protonated at the same remote site at -60 °C, but undergoes valence tautomerization upon warming.

Full text of DOI:10.1021/jacs.5b00816

Communication
pubs.acs.org/JACS
Light-Driven, Proton-Controlled, Catalytic Aerobic C−H Oxidation
Mediated by a Mn(III) Porphyrinoid Complex
†
‡
†
,†
†
‡
Heather M. Neu, Jieun Jung, Regina A. Baglia, Maxime A. Siegler, Kei Ohkubo,
,
‡,§
Shunichi Fukuzumi,* and David P. Goldberg*
†
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States
‡
Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology
Agency, Suita, Osaka 565-0871, Japan
§
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S Supporting Information
*
V
In previous work, we showed that a stable Mn (O) complex
prepared with a corrolazine ligand, which is a meso-N-substituted
ABSTRACT: The visible light-driven, catalytic aerobIiIcI
oxidation of benzylic C−H bonds was mediated by a Mn
corrolazine complex. To achieve catalytic turnovers, a strict
selective requirement for the addition of protons was
established. The resting state of the catalyst was
unambiguously characterized by X-ray diffraction as
analog of a corrole, could be synthesized via photoirradiation of a
III
7
a
Mn precursor, O (or air), and C−H substrates. This work
2
V
was the first example of the synthesis of a Mn (O) complex from
air, light, and a proton/electron source. The addition of toluene
7a
III
derivatives to the Mn /light/O system led to selective C−H
III
+
2
[
Mn (H O)(TBP Cz(H))] , in which a single, remote
7b
2
8
hydroxylation of benzylic C−H bonds. However, the inherent
site on the ligand is protonated. If two remote sites are
V
stability of the Mn (O) complex toward the toluene derivatives
protonated, however, reactivity with O is shut down.
2
V
limited this chemistry to a strictly stoichiometric process. Only in
Spectroscopic methods revealed that the related Mn (O)
the case of PPh , or dihydroacridine, which has a very weak C−H
3
complex is also protonated at the same remote site at −60
−1
7,8
bond (BDFE = 69 kcal mol ), was catalytic turnover obtained.
°
C, but undergoes valence tautomerization upon warming.
Herein we report the visible light-driven, catalytic aerobic
oxidation of benzylic C−H bonds via the proton-controlled
III
activation of a Mn corrolazine complex. The selective addition
he activation of molecular oxygen is a critical transformation
of a strong proton donor leads tothe formation ofnew mono-and
T
in biology, which is mediated by both heme and nonheme
III
1
diprotonated Mn complexes, which were characterized by
metal centers. Synthetic, biomimetic transition metal complexes
single crystal X-ray diffraction (XRD). The monoprotonated
complex is capable of reacting with air, light, and benzylic C−H
bonds to give a high-valent Mn−oxo complex. Under the right
conditions, the controlled addition of protons provides access to
a catalytic cycle. Low-temperature methods were used to trap a
aimed at activating O have been targeted by employing both
2
porphyrinoid and nonheme polydentate ligands, and some of
these efforts have resulted in important mechanistic insights and
2
catalysts forthe oxygenation of arange oforganic substrates. The
catalytic, aerobic oxidation of C−H bonds with first-row,
V
novel, proton-activated form of the Mn (O) complex which may
biologically relevant metal complexes remains a particularly
3
play a role in the catalytic cycle.
important, yet challenging goal. Although some Fe and Mn
Previously it was shown that photoirradiation (λ > 400 nm) of
metalloporphyrins, and related metallomacrocycles (e.g., phtha-
III
Mn (TBP Cz) (TBP Cz = octakis(p-tert-butylphenyl corrola-
8
8
locyanines), can catalyze the oxidation of C−H bonds with O ,
2
3−
zinato ) in the presence of excess O and hexamethylbenzene
2
drawbacks remain in most cases. Such drawbacks include the
need for stoichiometric coreductants, and the involvement of
(
HMB) in benzonitrile (PhCN) led to the selective oxidation of
2j,3
HMB to pentamethylbenzyl alcohol (PMB−OH) (87%) with
poorly controlled radical-type pathways. The mechanisms of
these systems, including the role of high-valent metal-oxo species,
is often poorly understood.
V
7b
V
clean formation of Mn (O)(TBP Cz). This Mn (O) complex
8
oxidizes relatively weak C−H bonds such as those found in
−1
dihydroanthracene (BDFE = 76 kcal mol ), but is essentially
Corroles are modified porphyrinoid ligands that stabilize high-
unreactive toward stronger C−H bonds such as those found in
valent metal-oxo species and can exhibit catalytic activity different
−
1
8
III
toluene (BDFE = 87 kcal mol ) or its derivatives. The selective
hydroxylation of HMB was therefore limited to strictly
from their porphyrin analogs. A Cr corrole catalyzes the aerobic
V
4
oxidation of PPh via a Cr (O) intermediate. Under visible light
3
7b
stoichiometric reactivity. We speculated that addition of a
strong proton donor might activate the Mn (O) complex toward
irradiation, diiron(IV) μ-oxo biscorroles have been shown to
catalyze the aerobic oxidation of phosphine and C−H substrates,
presumably through a disproportionation pathway that involves
V
oxidation of the stronger C−H bonds found in HMB, providing
entry into a catalytic oxidation pathway.
III
V
μ-oxo-bridge cleavage and leads to formation of Fe /Fe (O)
5
intermediates. Diiron(III)-μ-oxo-bridged porphyrins also medi-
ate photoactivated aerobic oxidations through similar μ-oxo
cleavage mechanisms.
Received: January 24, 2015
6
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00816
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Addition of the strong proton donor [H(OEt ) ] [B(C F ) ]
Communication
+
−
III
Mn ion with a water ligand located in the axial position, similar
9b
2
2
6 5 4
+
−
III
III
(
H [B(C F ) ] ) to Mn (TBP Cz) in benzene (C H ) caused
to the structure of a MeOH-bound Mn (MeOH)(TBP Cz).
8
6
5 4
8
6
6
III
an immediate color change of the brown Mn complex to a
brown-red solution. The addition of excess HMB (1000 equiv)
under ambient conditions to this solution, followed by
photoirradiation with visible light (λ > 400 nm), led to the
slow bleaching of the solution over 5.5 h. Monitoring of this
reaction by UV−vis confirmed the slow decay of the protonated
The axial H O ligands of two adjacent metallocorrolazines are H-
bond donors to the meso-N atoms of the neighboring Cz ligand
via a set of 1 + 1 O−H···N hydrogen bonds (i.e., only one H-bond
per axial H O ligand), resulting in H-bonded dimers of
2
2
III
Mn (H O) complexes in the solid state. Treatment of the
2
III
+
−
starting Mn complex with 1 equiv of H [B(C F ) ] in CH Cl ,
6
5 4
2
2
III
Mn complex, which exhibited distinct Soret and Q-bands at 446
followed by removal of solvent and recrystallization from
III
and 728 nm (Figure S3). Analysis of the reaction mixture by GC-
FID revealed the production of PMB−OH and the correspond-
ing aldehyde PMB−CHO (Scheme 1) with 18 turnovers for
toluene/heptane, led to dark brown crystals of [Mn (H O)-
2
(TBP Cz(H))][B(C F ) ]. The crystal structure reveals that
monoprotonation most likely occurs at one of the meso-N atoms
8
6 5 4
(
N1(H)) adjacent to the direct pyrrole−pyrrole bond of the Cz
III
Scheme 1
ligand. The axial H O ligand seen in the neutral Mn starting
2
complex and H-bonded dimers are retained. Dissolution of
III
crystalline [Mn (H O)(TBP Cz(H))][B(C F ) ] yields the
2
8
6 5 4
same UV−vis spectrum as seen for the in situ protonation of
III
+
−
the Mn complex via addition of 1 equiv of H [B(C F ) ] (λ
6
5 4
max
+
−
=
446, 730 nm) (Figure S6). If 2 equiv of H [B(C F ) ] are
added to the neutral Mn complex, a new complex in the form of
dark brown crystals can be isolated. XRD analysis shows that this
6 5 4
III
PMB−OH and 9 turnovers for PMB−CHO. Both oxidation
new species is doubly protonated with the molecular formula
products increase steadily over time, and catalytic activity appears
III
[
Mn (H O)(TBP Cz(H) )][B(C F ) ] (Figure 2). Both H-
2
8
2
6 5 4 2
only limited by catalyst stability. In the absence of light, O , or
2
III
atoms were unambiguously located on the opposite meso-N
atoms (N1 and N5) (Figure S2). Unlike the neutral and
monoprotonated structures, this complex exhibits no H-bonding
Mn complex, no oxidized products were detected. As
anticipated, the addition of H to the Mn catalyst leads to the
+
catalytic oxidation of HMB with only air and light as additives.
III
+
to nearest neighbors and crystallizes in isolated molecular units.
The independent reaction of Mn (TBP Cz) with H [B-
8
III
−
+
The UV−vis spectrum of crystalline [Mn (H
O)(TBP
2
Cz-
8
(
C F ) ] was investigated to determine the effect of H on the
Mn complex. These independent experiments were initially run
6 5 4
III
(H) )][B(C F ) ] redissolved in CH Cl matches that seen for
the in situ diprotonation of the Mn complex following
treatment with 2 equiv of H (λ = 470, 763 nm) (Figure
2 6 5 4 2 2 2
III
in CH Cl , as opposed to C H , because of the available UV−vis
2
2
6
6
+
9
,10
max
data on Mn corrolazine complexes in CH Cl .
Mn complex (435, 685 nm) was converted to a new brown-red
species upon addition of 1 equiv of H [B(C F ) ] (446, 730
The starting
2
2
III
S8). The metrical parameters for each of the structures show little
+
−
variation upon mono- and diprotonation, with the exception of a
6
5 4
III
slight shortening (Δd(Mn−O) = 0.028(3) Å) of the Mn −OH
2
nm) (Figure 1). This new spectrum (446, 730 nm) was almost
distance.
The preparation of crystalline mono- and diprotonated MnIII
complexes provided pure material for testing as possible catalysts
in the observed light-driven, aerobic C−H oxidation. Monop-
III
rotonated [Mn (H O)(TBP Cz(H))][B(C F ) ] was dissolved
2
8
6 5 4
in CH Cl or C H with excess HMB (1000 equiv) under aerobic
2
2
6
6
conditions. Photoirradiation of this solution for 30 min at 23 °C
followed by analysis by GC-FID showed both alcohol and
aldehyde products were formed, but only in stoichiometric
amounts (PMB−OH: TON = 0.4; PMB−CHO: TON = 0.7);
i.e., no catalysis was observed. Monitoring this reaction by UV−
vis showed isosbestic conversion of the spectrum for
III
III
+
Figure 1. UV−vis spectral changes for Mn (TBP Cz) (12 μM, black
8
[Mn (H O)(TBP Cz(H))] to a new spectrum with features
2 8
line) upon addition of 1 equiv (blue line) and 2 equiv (red line) of
H [B(C F ) ] in CH Cl (2 mL) at 25 °C.
at 418 and 784 nm. This new spectrum (Figure S12) matches that
+
−
IV
6
5
4
2
2
seen previously for the Mn -oxo π-radical cation complex
IV
•+
+
II
[
Mn (O)(TBP Cz )(LA)] (LA = Lewis acid = Zn , B-
8
V
identical to that seen for the resting state of the catalyst in C H .
(C F ) ), a valence tautomer of Mn (O)(TBP Cz) stabilized by
6 5 3 8
10
6
6
+
Introduction of a second equivalent of H caused a further change
to a second species with further red-shifted Soret and Q-bands at
Lewis acids. In support of this analysis, reaction of crystalline
III
[Mn (H O)(TBP Cz(H))][B(C F ) ] with the O-atom donor
2
8
6 5 4
IV
•+
4
70 and 763 nm. Spectral titrations (Figure S4) showed that no
further changes occur upon addition of up to 5 equiv of H .
PhIO leads to isosbestic conversion to [Mn (O)(TBP Cz )-
8
+
+
V
(H)] (Figure S13), instead of Mn (O)(TBP Cz), which is the
8
9b
III
Similar spectra were obtained in dry C H (Figure S10).
major product from [Mn (H O)(TBP Cz)] + PhIO. These
6
6
2
8
The UV−vis data in Figure 1 clearly show that there are two
results indicate that generation of the valence tautomer
III
IV
•+
+
+
distinct species accessible via protonation of the starting Mn
[Mn (O)(TBP Cz )(H)] occurs if 1 equiv of H is employed.
8
III
III
corrolazine. The three Mn complexes identified in Figure 1
The use of crystalline diprotonated [Mn (H O)(TBP Cz-
2
8
were successfully crystallized and unambiguously characterized
by XRD. The crystal structures of the neutral, mono- and
(H) )][B(C F ) ] as starting material, in contrast, leads to
2 6 5 4 2
catalytic turnover in the light-driven oxidation of HMB, giving
PMB−OH (TON = 16) and PMB−CHO (TON = 11) after 5 h
in C H at 23 °C. However, the initial spectrum for the
III
diprotonated Mn derivatives are presented in Figure 2. The
III
structure of the neutral Mn complex contains a five-coordinate
6
6
B
DOI: 10.1021/jacs.5b00816
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
III
III
+
Figure 2. (Top) Displacement ellipsoid plots (50% probability level) for Mn (TBP Cz)(H O) (left), [Mn (H O)(TBP Cz(H))] (center), and
[
8
2
2
8
III
2+
+
−
III
Mn (H O)(TBP Cz(H) )] (right) at 110(2) K. (Bottom) Addition of 1 and 2 equiv of H [B(C F ) ] to Mn (TBP Cz).
2 8 2 6 5 4 8
diprotonated complex (470, 763 nm) rapidly (≤2 min) converts
Scheme 2. Proposed Catalytic Cycle for the Oxidation of HMB
III
to a spectrum characteristic of the monoprotonated Mn species
(
446, 728 nm) under catalytic conditions, and no evidence for the
V
IV
formation of either the Mn (O) or Mn (O)(π-radical cation)
III
(Figure S14). The spectrum for the diprotonated Mn complex
+
−
can be maintained if a third equivalent of H [B(C F ) ] is added
6
5 4
prior to photoirradiation, but under these conditions no
oxidation products of HMB were obtained. Taken together,
+
these observations indicate that the presence of 2 equiv of H are
required for catalytic turnover, and it is the monoprotonated
III
Mn complex that is the catalytically active resting state.
+
Further insight regarding the role of 2 equiv of H came from
III
examining the diprotonated Mn complex in “wet” C H under
6
6
aerobic conditions, versus dry C H under an inert atmosphere. It
6
6
was found that residual H O, present under aerobic conditions,
2
acted as a weak base and led to the deprotonation of
III
2+
[
Mn (H O)(TBP Cz(H) )] to give the monoprotonated
2 8 2
complex, while under strictly anaerobic conditions (doubly
distilled C H , inert atmosphere), the diprotonated complex was
6
6
III
observed (Figures S9−S11). Thus, the monoprotonated Mn
complex is formed under the aerobic conditions of the catalytic
reaction.
solution caused significant decomposition (∼40% bleaching).
+
−
The addition of 2 equiv of H [B(C F ) ] to isolated
6
5 4
The data presented herein, together with previous femto-
V
Mn (O)(TBP Cz) in the presence of light and excess HMB
caused rapid (<10 s) formation of Mn , although organic
7
b
8
second transient absorption measurements, lead to the
proposed catalytic cycle shown in Scheme 2. The monoproto-
III
oxidation products could not be identified. The exact nature of
III
nated Mn complex is the resting state of the catalyst, which is
the catalytically active intermediate cannot be assigned, but it is
5
activated by photoexcitation to a tripquintet excited state ( T )
+
V
1
clear that 2 equiv of H rapidly destabilize the isolated Mn (O)
7
that rapidly converts to the O -reactive tripseptet ( T ) excited
+
2
1
complex, and these observations suggest that H may assist in
activating the Mn (O) complex to induce turnover under the
IV
state. Entry of O gives a putative Mn (superoxo) complex,
V
2
which can either undergo rapid back electron transfer to the
appropriate catalytic conditions.
ground state and release of O or can abstract an H-atom from the
V
2
Further examination of the reaction of Mn (O)(TBP Cz) and
8
IV
+
−
HMB substrate. The resulting Mn (OOH) complex can then
H [B(C F ) ] at low temperature (−60 °C) led to an isosbestic
6
5 4
hydroxylate the HMB radical via O−O cleavage and rebound of
change together with the formation of a distinct, new species with
λmax = 436, 650 nm (Figure S16). This spectral change is
reversible upon addition of 1 equiv of 1,8-bis(dimethylamino)-
naphthalene (proton sponge), consistent with an acid−base
•
V
OH , to yield PMB−OH and the Mn (O) complex. The
V
Mn (O) complex is activated in the presence of acid under the
catalytic conditions, leading to oxidation of HMB (or PMB−
OH) and completing the catalytic cycle.
V
transformation (Figure S17). The structure of the new Mn
Attempts were made to gain insights into the final step of the
species (436, 650 nm) was characterized by low-temperature 1D
V
1
catalytic cycle by reacting isolated Mn (O)(TBP Cz) with
and 2D NMR spectroscopy. The H NMR spectra (−50 to −60
8
+
−
V
H [B(C F ) ] in CH Cl or benzene. This reaction, when run
with 1 equiv of H at 23 °C, afforded the valence tautomer
Mn (O)(TBP Cz )(H)] (Scheme S1, Figure S15). This
°C) of the starting Mn (O) complex and the monoprotonated
6
5 4
2
2
+
derivative are compared in Figure 3. Both spectra are
IV
•+
+
2
V
[
diamagnetic, confirming that the low-spin (d ) Mn oxidation
state is retained at −60 °C (Figure 3). The most notable
difference between the two species is the appearance of a new
8
species was unreactive toward either excess HMB or PMBOH
1000 equiv) for up to 2 h at 23 °C, and photoirradiation of this
(
C
DOI: 10.1021/jacs.5b00816
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
AUTHOR INFORMATION
Corresponding Authors
dpg@jhu.edu
fukuzumi@chem.eng.osaka-u.ac.jp
■
*
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NIH (GM101153) and NSF
(CHE121386) to D.P.G. and an ALCA and SENTAN project (to
S.F.) from JST, Japan. We thank Prof. K. D. Karlin for
instrumentation use, and we thank Dr. A. Majumdar and Dr. J.
Tang for helpful NMR discussions.
1
Figure 3. Comparison of H NMR spectra (aryl region) for
V
+
V
[
(
Mn (O)(TBP Cz(H))] (red) at −60 °C and Mn (O)(TBP Cz)
8
8
blue) at −50 °C.
+
−
peak at 13.45 ppm following addition of the H [B(C F ) ] . The
6
5 4
REFERENCES
■
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(
1
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III
2
(
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aromatic and Bu regions (Figures S20). These perturbations are
t
consistent with a lowering of the molecular symmetry, as
expected for protonation at a single meso-N position adjacent to
the direct pyrrole−pyrrole bond of the Cz ligand. Definitive
assignment of the 13.45 ppm peak comes from H− H 2D
nuclear Overhauser effect spectroscopy (NOESY) (Figure S22).
Well-defined cross-peaks are observed between the new peak at
1
1
1
3.45 ppm and peaks at 8.04 and 7.70 ppm, indicating an
(h) Zhang, Q.; Gorden, J. D.; Goldsmith, C. R. Inorg. Chem. 2013, 52,
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1
1
C−H resonances. From the H− H 2D correlation (COSY)
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and 7.70 ppm as arising from protons on separate benzene rings
(
3) (a) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B. Science
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1
(Figure S23). Taken together, the NMR data show that a meso-N
atom closest to the direct pyrrole−pyrrole bond is protonated
+
−
V
̈
upon addition of H [B(C F ) ] to the Mn (O) complex at −60
6
5 4
Klemm, E.; Liebner, C.; Hieronymus, H.; Hsu, S. F.; Plietker, B.; Laschat,
V
°
C (Scheme S1). Thus, the Mn (O) complex can be protonated
S. ChemCatChem 2013, 5, 82.
III
at the meso-N position as seen for Mn , and this protonation site,
together with possible protonation at the oxo position, may play a
role in the proton-assisted, light-driven, catalytic aerobic
oxidation of HMB.
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III
In summary we have shown that a Mn porphyrinoid complex
(6) Rosenthal, J.; Luckett, T. D.; Hodgkiss, J. M.; Nocera, D. G. J. Am.
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III
achieve catalytic turnover. The Mn resting state of the catalyst
8
014. (b) Jung, J.; Ohkubo, K.; Prokop-Prigge, K. A.; Neu, H. M.;
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V
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6
(
223.
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+
protonated, and suggest a potential binding site for H along the
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V
the precise protonation state of the Mn complex under catalytic
T.; Quesne, M. G.; de Visser, S. P.; Goldberg, D. P. J. Am. Chem. Soc.
2014, 136, 13845. (b) Lansky, D. E.; Mandimutsira, B.; Ramdhanie, B.;
Clausen, M.; Penner-Hahn, J.; Zvyagin, S. A.; Telser, J.; Krzystek, J.;
Zhan, R.; Ou, Z.; Kadish, K. M.; Zakharov, L.; Rheingold, A. L.;
Goldberg, D. P. Inorg. Chem. 2005, 44, 4485.
conditions, taken together, the data indicate that 2 equiv of acid
are necessary for catalysis. This work furthers our knowledge
regarding the criteria for developing biomimetic heme complexes
for O activation catalysts and suggests a promising strategy in
2
(10) (a) Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; de Visser, S.
catalyst design that could involve the incorporation of remote
ligand sites for modification by noncovalent interactions.
P.; Goldberg, D. P. J. Am. Chem. Soc. 2012, 134, 10397. (b) Baglia, R. A.;
Durr, M.; Ivanovic-Burmazovic, I.; Goldberg, D. P. Inorg. Chem. 2014,
5
3, 5893.
ASSOCIATED CONTENT
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*
S
Supporting Information
CIF, experimental procedures, Scheme S1, Table S1, and Figures
S1−S23. This material is available free of charge via the Internet at
http://pubs.acs.org.
D
DOI: 10.1021/jacs.5b00816
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX