Saturation kinetics in phenolic O-H bond oxidation by a mononuclear Mn(III)-OH complex derived from dioxygen

Article abstract of DOI:10.1021/ic500943k

The mononuclear hydroxomanganese(III) complex, [Mn^III(OH)(dpaq)] ⁺, which is supported by the amide-containing N₅ ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [Mn^II(dpaq)] (OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O₂-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [Mn^III(OH)(dpaq)]⁺ reveals a short Mn-OH distance of 1.806(13) A, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [Mn^III(OH)(dpaq)]⁺ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [Mn^III(OH)(dpaq)]⁺ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn ^III(OH)(dpaq)]⁺ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn ^III(OH)(dpaq)]⁺ complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t- butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [Mn^III(OH)(dpaq)]⁺ and the phenol substrates.

Full text of DOI:10.1021/ic500943k

Article
pubs.acs.org/IC
Saturation Kinetics in Phenolic O−H Bond Oxidation by a
Mononuclear Mn(III)−OH Complex Derived from Dioxygen
Gayan B. Wijeratne, Briana Corzine, Victor W. Day, and Timothy A. Jackson*
†Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045,
United States
S
* Supporting Information
ABSTRACT: The mononuclear hydroxomanganese(III)
complex, [MnIII(OH)(dpaq)]+, which is supported by the
amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-
ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated
by treatment of the manganese(II) species, [MnII(dpaq)]-
(OTf), with dioxygen in acetonitrile solution at 25 °C. This
oxygenation reaction proceeds with essentially quantitative
yield (greater than 98% isolated yield) and represents a rare
example of an O2-mediated oxidation of a manganese(II)
complex to generate a single product. The X-ray diffraction
structure of [MnIII(OH)(dpaq)]+ reveals a short Mn−OH
distance of 1.806(13) Å, with the hydroxo moiety trans to the
amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless,
[MnIII(OH)(dpaq)]+ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week.
The [MnIII(OH)(dpaq)]+ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak
O−H and C−H bonds. For example, [MnIII(OH)(dpaq)]+ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-
ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic
isotope effect of 1.8 observed for this reaction implies a concerted proton−electron transfer pathway. The [MnIII(OH)(dpaq)]+
complex also oxidizes xanthene (C−H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t-butylphenol,
with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium
prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the
formation of a hydrogen-bonding complex between [MnIII(OH)(dpaq)]+ and the phenol substrates.
splitting.2 For both Mn-lipoxygenase and the OEC, hydroxo-
INTRODUCTION
■
manganese species have been proposed to participate in
Manganese-dependent enzymes play diverse biological roles,
proton-coupled electron-transfer (PCET) reactions.16,17 Sup-
ranging from the generation of deoxynucleotides in pathogenic
bacteria,1 to the water splitting reaction of plants, algea, and
cyanobacteria.2 Hydroxo-manganese adducts have been pro-
posed to play critical roles in the catalytic cycles of a small, yet
diverse, set of these manganese-dependent enzymes.2−6 For
example, experimental7 and computational8,9 studies have
port for a concerted proton−electron transfer (CPET) reaction
in Mn-lipoxygenase comes from the fact that the conversion of
α-linoleic acid to its hydroperoxy-derivative proceeds with a
temperature-independent18 kinetic isotope effect (KIE; kH/kD)
of 20−24.6 For the OEC, the proposition that a hydroxo-
manganese species participates in catalytically relevant PCET is
more tenuous, as there is substantial debate regarding the
mechanistic details of this system.2,19−23 Nonetheless, the
abstraction of a hydrogen-atom from a hydroxo- or aqua-
manganese moiety by a nearby tyrosyl radical, YzO•, has been
proposed as one of several possible mechanisms for stepwise
oxidation of the tetramanganese cluster during turnover.24−26
Terminal water ligands and bridging hydroxide ligands in
dimanganese(III,III), dimanganese(III,IV), and dimanganese-
(IV,IV) model complexes have O−H bond dissociation
enthalpies (BDEs) of 77−92 kcal/mol,27−29 a range which is
provided strong support for
a mononuclear
hydroxomanganese(III), MnIII−OH, adduct in the oxidized
state of manganese-superoxide dismutase (MnSOD). This
enzyme catalyzes the disproportionation of superoxide to
dioxygen and hydrogen peroxide, thereby defending aerobic
organisms from a reactive oxygen species.3,4 The MnIII−OH
unit of oxidized MnSOD is involved in a hydrogen-bonding
network with a nearby glutamine residue that plays a
controlling role in the functionally important redox-tuning
mechanism of the enzyme.10−12
There is also evidence for hydroxo-manganese species in the
active sites of Mn-lipoxygenase13 and the oxygen-evolving
complex (OEC).14,15 These enzymes respectively catalyze the
dioxygenation of polyunsaturated fatty acids5,6 and water
Received: April 22, 2014
Published: July 10, 2014
© 2014 American Chemical Society
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thermodynamically compatible with the transfer of hydrogen
atoms to YzO• (the O−H BDE of tyrosine is 86 kcal/mol).30
Although synthetic inorganic chemistry has provided
numerous examples of multinuclear manganese complexes
with bridging hydroxides,27,29,31−38 mononuclear manganese
complexes with terminal hydroxide ligands are comparatively
less common. Indeed, to the best of our knowledge, crystal
structures have been reported for only eight mononuclear
MnIII−OH complexes.39−46 In these structures, the Mn−OH
distances range from 1.81 Å to 1.86 Å, with the longer distances
observed for complexes where the hydroxo ligand is involved in
hydrogen bonding.40,41 In fact, intermolecular or intramolecular
hydrogen bonding is observed in many of the solid-state
structures.39−41,46 In several structurally characterized com-
plexes, the hydroxo ligand is sterically shielded by the
supporting ligand,40,44,45 which presumably disfavors formation
of hydroxo-bridged, multinuclear complexes. Notably, several
mononuclear MnIII−OH complexes were generated by treat-
ment of the corresponding manganese(II) species with
dioxygen.44−47
the expected CPET mechanism. Interestingly, the [MnIII(OH)-
(SMe2N4(tren))]+ complex was formed by hydrolysis of the oxo-
bridged species [MnIII2(μ-O)(SMe2N4(tren))2]2+, which itself
was generated through O2 oxidation of the
[MnII(SMe2N4(tren))]+ precursor complex.46 Because the
reaction of [MnIII(OH)(SMe2N4(tren))]+ with TEMPOH
regenerates [MnII(SMe2N4(tren))]+, this Mn system served as
an aerobic oxidation catalyst for TEMPOH, with at least 10
turnovers.46
In this present work, we report a new MnIII−OH complex,
[MnIII(OH)(dpaq)]+, featuring the previously reported mono-
anionic N5 ligand dpaq49−51 (dpaq = 2-[bis(pyridin-2-
ylmethyl)]amino-N-quinolin-8-yl-acetamidate). This complex
is generated by treatment of the corresponding [MnII(dpaq)]-
(OTf)2 species with dioxygen in MeCN at room temperature
and is remarkably stable, decaying by only 10% when stored in
MeCN solution at 25 °C for 1 week. [MnIII(OH)(dpaq)]+ is
capable of oxidizing TEMPOH by a CPET mechanism. The
oxidation of the hydrocarbon xanthene (BDFE = 73.3 kcal/mol
in dimethylsulfoxide) is also observed. In addition, this complex
can oxidize phenolic substrates with BDFEs (in MeCN) up to
78.5 kcal/mol. Intriguingly, saturation behavior is observed in
phenol oxidation by [MnIII(OH)(dpaq)]+ but not in TEMPOH
oxidation. A plausible explanation for these unusual results and
the relevance of this work within the broader context of
transition-metal-mediated CPET reactions is discussed.
While the number of known synthetic MnIII−OH complexes
is small, the number of such species that participate in PCET
reactions is even smaller. To the best of our knowledge, the
only reported example of C−H bond oxidation by a synthetic
MnIII−OH adduct is found in the [MnIII(OH)(PY5)]2+
complex of Stack and co-workers (Figure 1, left; PY5 = 2,6-
MATERIALS AND METHODS
■
All procedures, including the generation of [MnII(dpaq)](OTf) and
organic substrates, as well as kinetic experiments, were carried out
under an argon atmosphere, unless otherwise stated. Acetonitrile,
methanol, and ether were degassed and dried using a Pure Solv Micro
(2010) solvent purification system. These solvents were degassed in
airtight solvent reservoirs (4 L), by bubbling Ar gas through the
solvent for 20 min at room temperature. Acetonitrile and ether were
dried using airtight alumina columns, and methanol was dried using a
drierite column. Anhydrous dichloromethane was purchased from
Acros Organics (99.9% purity), and was degassed by four freeze−
pump−thaw cycles using Schlenk techniques. All solvents were taken
into an argon filled glovebox immediately after dispensing from the
solvent purification system or following the freeze−pump−thaw
cycles, and were stored in tightly sealed Schlenk glassware. The purity
of O2 gas used was >99% and was further purified by passage through
a column containing drierite and 5 Å molecular sieves prior to use.
TEMPOH, TEMPOD, and 2,4,6-tri-t-butylphenol-d (4‑t‑butylArOD)
were prepared according to literature procedures,52,53 and >99%
deuteration of TEMPOD and 4‑t‑butylArOD was confirmed by 1H NMR
experiments.
Figure 1. X-ray diffraction (XRD) structures of the cationic fragments
39
of [MnIII(OH)(PY5)](ClO4)2
(left) and [MnIII(OH)-
(SMe2N4(tren))](PF6)·H2O46 (right).
bis(bis(2-pyridyl)methoxymethane)pyridine).39 This complex
is capable of oxidizing hydrocarbon substrates with C−H BDEs
ranging from ∼75−88 kcal/mol by a CPET mechanism.
Kovacs and co-workers recently reported a MnIII−OH
adduct supported by the monoanionic N4S ligand SMe2N4(tren)
(Figure 1, right), which is capable of O−H bond oxidation of
TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-
ol)).46 TEMPOH is an ideal substrate for investigating CPET
reactivity, because it has a relatively weak O−H bond (in
MeCN, BDE = 70.6 kcal/mol; bond dissociation free energy
(BDFE) = 66.5 kcal/mol), and, when occurring separately,
both its deprotonation and one-electron oxidation are quite
unfavorable thermodynamically (pKa = 41 and E° = 0.71 V vs
Preparation of [MnII(dpaq)](OTf). [MnII(dpaq)](OTf) was
generated in high yield (>90%) by reacting the H-dpaq ligand with
MnII(OTf)2 in the presence of NaOtBu in MeOH under an inert
atmosphere. The MnII(OTf)2 salt was generated using a previously
reported method.54 The detailed metallation procedure is as follows.
To a stirred solution of 100 mg (0.26 mmol) of H-dpaq in 2 mL of
MeOH was added 92 mg (0.26 mmol) of MnII(OTf)2 in 2 mL of
MeOH, followed by 25 mg (0.26 mmol) of NaOtBu in 2 mL of
MeOH under an inert atmosphere. The orange-colored resultant
solution was stirred overnight and then the solvent was evaporated to
dryness under vacuum. The solid product was recrystallized using
MeOH/Et2O to yield orange-colored crystals of [MnII(dpaq)](OTf).
Crystals for X-ray diffraction (XRD) analysis were obtained by
subsequent recrystallization of the final solid product in the MeOH/
Et2O solvent system. [MnII(dpaq)](OTf) was further characterized by
ESI-MS and effective magnetic moment analysis by the 1H NMR
method of Evans55 in CD3CN at 298 K. ESI-MS (Figure S1 in the
Supporting Information): {[MnII(dpaq)]+} m/z = 437.0994 (calc.
437.1048). The effective magnetic moment (μeff) was found to be
FeCp2 /FeCp2 in MeCN).48 Thus, TEMPOH oxidation
+
proceeds by a CPET mechanism, unless it is reacting with an
exceptionally strong base or one-electron oxidant. The
[MnIII(OH)(SMe2N4(tren))2]+ complex quantitatively con-
verted TEMPOH to TEMPO, with a second-order rate
constant of 2.1 × 103 M−1 s−1 at 25 °C in MeCN.46
A
TEMPOH/TEMPOD KIE of 3.1 was observed, consistent with
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6.26μB, which compares well with the expected value (μeff = 5.92μB)
for mononuclear high-spin MnII centers. Elemental analysis
[MnII(dpaq)](OTf): C24H20F3MnN5O4S calc. (%): C 49.15, H 3.44,
N 11.94; found (%): C 48.96, H 3.51, N 11.79.
lized using MeCN/Et2O. Dark gold [MnIII(OH)(dpaq)](OTf)
crystals of X-ray crystallographic quality were obtained by repetitive
recrystallization, and were further characterized by ESI-MS and
1
effective magnetic moment (μeff) analysis by the H NMR method of
X-ray Diffraction Data Collection and Analysis for
[MnII(dpaq)](OTf). All XRD experiments were performed on a
Bruker Proteum Single Crystal Diffraction System equipped with
Helios multilayer optics, an APEX II CCD detector, and a Bruker
MicroSTAR microfocus rotating-anode X-ray source operating at 45
kV and 60 mA. The Bruker software package SHELXTL was used to
solve the structures using “direct methods” techniques. All stages of
Evans in CD3CN at 298 K. ESI-MS (Figure S1 in the Supporting
Information): {[MnIII(OH)(dpaq)]+} m/z = 454.1056 (calc.
454.1076). μeff found was 4.88 μB (at 298 K for 5 mM solution in
CD3CN) which compares well with the calculated value of 4.89 μB for
a monomeric high-spin d4 system. This value is also in close
comparison with previously reported solution magnetic susceptibility
values for mononuclear hydroxomanganese(III) complexes (μeff = 4.7
μB for [MnIII(OH)(PY5)]+ and μeff = 4.89 μB for [MnIII(OH)-
(SMe2N4(tren))2]+).39,46 Cyclic voltammetric studies of [MnIII(OH)-
(dpaq)](OTf) were conducted under an argon atmosphere in
acetonitrile (12.89 mg in 10 mL) at 298 K. A 0.1 M acetonitrile
solution of Bu4N(PF6) was used as the supporting electrolyte, and a
glassy carbon working electrode, a platinum auxiliary electrode, and a
AgCl/Ag reference electrode were used. Elemental analysis
[MnIII(OH)(dpaq)](OTf): C24H21F3MnN5O5S calc. (%): C 47.77,
H 3.51, N 11.61; found (%): C 47.26, H 3.65, N 11.62.
2
weighted full-matrix least-squares refinement were conducted using F0
data with the SHELXTL Version 2010.3−0 software package.56
Yellow single crystals of the triflate salt for the cationic polymer,
[Mn(C23H19N5O)+]n are, at 100(2) K, monoclinic, space group Cc-Cs4
(No. 9)57 with a = 28.6094(9) Å, b = 9.2197(3) Å, c = 28.5260(9) Å, β
=
100.550(1)°, V = Z = 4 ([Mn-
7397.1(4) Å3, and
(C23H19N5O)]3[CF3SO3]3) formula units {dcalcd = 1.577 g/cm3;
μa(Cu Kα) = 5.729 mm−1}. A full set of unique diffracted intensities
[5241 frames with counting times of 2−4 s and an ω- or ϕ-scan width
of 0.50°] was measured58 for a single-domain specimen using
monochromated Cu Kα radiation (λ = 1.54178 Å). Lattice constants
were determined with the Bruker SAINT software package using peak
centers for 9466 reflections. A total of 30828 integrated reflection
intensities having 2θ(Cu Kα) < 139.58° were produced using the
Bruker program SAINT;59 9683 of these reflections were unique and
gave Rint = 0.037. The data were corrected empirically for variable
absorption effects using equivalent reflections; the relative trans-
mission factors ranged from 0.836 to 1.000.
X-ray Diffraction Data Collection and Analysis for [MnIII(OH)-
(dpaq)](OTf). Dark gold single crystals of the CH3CN solvated salt,
[Mn(OH)(C23H20N5O)][CF3SO3], are, at 100(2) K, monoclinic,
space group P21/n (an alternate setting of P21/c−C2h (No. 14))57
5
with a = 9.2406(3) Å, b = 24.7263(8) Å, c = 12.2658(4) Å, β =
97.257(1)°, V = 2780.1(2) Å3, and Z = 4 formula units {dcalcd = 1.540
g/cm3; μa(Cu Kα) = 5.176 mm−1}. A full set of unique diffracted
intensities [5226 frames with counting times of 1 to 3 s and an ω- or
ϕ-scan width of 0.50°] was measured58 for a single-domain specimen
using monochromated Cu Kα radiation (λ = 1.54178 Å). Lattice
constants were determined with the Bruker SAINT software package
using peak centers for 9628 reflections. A total of 25663 integrated
reflection intensities having 2θ(Cu Kα) < 139.63° were produced
using the Bruker program SAINT;59 5017 of these were unique and
gave Rint = 0.039. The data were corrected empirically for variable
absorption effects using equivalent reflections; the relative trans-
mission factors ranged from 0.689 to 1.000.
The final structural model incorporated anisotropic thermal
parameters for all nonhydrogen atoms and isotropic thermal
parameters for all hydrogen atoms. Hydrogen atoms were included
in the structural model at idealized positions (sp2- or sp3-hybridized
geometry and C−H bond lengths of 0.95−0.99 Å) with isotropic
thermal parameters fixed at values 1.20 times the equivalent isotropic
thermal parameter of the carbon atom to which they are covalently
bonded.
A total of 1028 parameters were refined using 2 restraints, 9683 data
The triflate anion is 85/15 disordered between two closely spaced
sites in the asymmetric unit. This disorder produces a slightly
elongated anisotropic thermal ellipsoid for oxygen O(11) in the major-
occupancy triflate anion. The bond lengths and angles for the minor-
occupancy (15%) triflate were restrained to have values similar to
those of the major-occupancy anion. The final structural model
incorporated anisotropic thermal parameters for all nonhydrogen
atoms of the metal cation, major-occupancy triflate anion, and the
CH3CN solvent molecule. The nonhydrogen atoms for the minor-
occupancy triflate anion were incorporated with isotropic thermal
parameters. The hydrogen atoms were located from a difference
Fourier and included in the structural model as individual isotropic
atoms. The positional and thermal parameters for hydrogen atoms and
the minor-occupancy triflate were allowed to vary in least-squares
refinement cycles.
points, and weights of w = 1/[σ2(F2) + (0.0563P)2 + (12.3197P)],
2
where P = [F0 + 2Fc2]/3. Final agreement factors at convergence for
[Mn(C23H19N5O)+]n are R1(unweighted, based on F) = 0.037 for
9583 independent absorption-corrected “observed” reflections having
2θ(Cu Kα) < 139.58° and I > 2σ(I); R1(unweighted, based on F) =
0.037 and wR2(weighted, based on F2) = 0.096 for all 9683
independent absorption-corrected reflections having 2θ(Cu Kα) <
139.58°. The largest shift/s.u. was 0.001 in the final refinement cycle.
The final difference map had maxima and minima of 1.30 and −0.52
e−/Å3, respectively.
Preparation of [MnIII(OH)(dpaq)](OTf). [MnIII(OH)(dpaq)]-
(OTf) was formed by reacting a 2.5 mM [MnII(dpaq)](OTf) solution
in MeCN with excess O2 gas at room temperature. The formation of
[MnIII(OH)(dpaq)](OTf) was monitored by electronic absorption
spectroscopy, as the MnIII complex has characteristic features at 550
and 780 nm. A representative formation reaction is as follows. A 2.5
mM [MnII(dpaq)](OTf) solution (2.9 mg in 2 mL of MeCN) was
prepared under an inert atmosphere and transferred to a gas-tight
cuvette sealed with a pierceable septum. An excess of O2 gas was then
delivered to the solution by means of a syringe, and the formation of
[MnIII(OH)(dpaq)](OTf) was monitored by electronic absorption
spectroscopy. The formation was complete in ∼40 min and the
resulting dark gold solution was evaporated to dryness under reduced
pressure. The solid residue was then recrystallized using MeCN/Et2O
(3.1 mg/98% yield). Synthesis of [MnIII(OH)(dpaq)](OTf) on a
larger scale was undertaken to obtain suitable material for X-ray
crystallographic and kinetic experiments. In this procedure, O2 gas was
passed through an acetonitrile solution (20 mg in 5 mL) of
[MnII(dpaq)](OTf), and the completion of the formation of
[MnIII(OH)(dpaq)](OTf) was confirmed by ESI-MS and electronic
absorption spectroscopy. The resulting solution was evaporated to
dryness under reduced pressure, and the solid residue was recrystal-
A total of 509 parameters were refined using 19 restraints, 5017 data
points, and weights of w = 1/[σ2(F2) + (0.0291P)2 + (2.4398P)],
2
where P = [F0 + 2Fc2]/3. Final agreement factors at convergence are
R1(unweighted, based on F) = 0.031 for 4941 independent absorption-
corrected “observed” reflections having 2θ(Cu Kα) < 139.63° and I >
2σ(I); R1(unweighted, based on F) = 0.031 and wR2(weighted, based
on F2) = 0.079 for all 5017 independent absorption-corrected
reflections having 2θ(Cu Kα) < 139.63°. The largest shift/s.u. was
0.001 in the final refinement cycle. The final difference map had
maxima and minima of 0.49 and −0.58 e−/Å3, respectively.
Kinetic Studies of [MnIII(OH)(dpaq)](OTf) with Substituted
Phenols. For each kinetic experiment, a 1.25 mM [Mnlll(OH)-
(dpaq)]+ (1.6 mg, 2.5 × 10−3 mmol) solution was prepared in
acetonitrile (2 mL) within an argon-filled glovebox, transferred to a
gas-tight cuvette sealed with a pierceable septum. A solution of the
phenolic substrate was prepared in dichloromethane (300 μL) and
sealed in a 4 mL glass vial with a pierceable septum. Then, the cuvette
containing the [Mnlll(OH)(dpaq)]+ solution was inserted into a
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temperature-controlled cryostat (Unisoku), held at 50 °C, coupled to
an Agilent 8453 UV/Visible spectrophotometer. Upon achieving
thermal equilibrium (10 min), data collection was started, and 100 μL
of the substrate solution was injected into the cuvette using a gas-tight
syringe. Data collection times ranged from 3000 s to 6000 s. An
aliquot of the final reaction mixture was analyzed by perpendicular-
mode X-band electron paramagnetic resonance (EPR) spectroscopy at
5 K and revealed signals consistent with [MnII(dpaq)]+ and a phenoxyl
radical (see Figure S2 in the Supporting Information). The rates of the
reactions were calculated by applying initial rate approximation (initial
20% of the disappearance of [MnIII(OH)(dpaq)]+ was monitored at
800 nm) in order to prevent any interference by side reactions at
longer time scales. These initial rates (in absorbance units/time) were
converted into s−1 units by using the absorption coefficient (ε800 = 130
M−1 cm−1) and initial concentration (1.25 mM) of [Mnlll(OH)-
(dpaq)]+. The kinetic data for reactions with greater than 10 equiv
2,4,6-tri-t-butylphenol were collected to five half-lives and fit directly to
obtain a pseudo-first-order rate constant. In these cases, the directly
obtained pseudo-first-order rate constants were identical, within error,
to those obtained using the initial rate method.
distorted octahedral geometry (Figure 2), with two pyridine
ligands (N4, N5), the quinoline ligand (N1) and the tertiary
Figure 2. ORTEP (left) and space filling (right) diagrams showing two
cations (B and C) in the asymmetric unit of the X-ray structure of
[MnII(dpaq)](OTf). ORTEP diagram shows 50% probability thermal
ellipsoids. Hydrogen atoms and noncoordinating triflate counteranions
have been removed for clarity. Significant interatomic distances and
angles are listed in Table 1 and in Table S3 in the Supporting
Information.
Kinetic Studies of [MnIII(OH)(dpaq)](OTf) with TEMPOH.
Similar to the kinetic experiments with phenols, a 1.25 mM
[Mnlll(OH)(dpaq)]+ (1.6 mg, 2.5 × 10−3 mmol) solution was
prepared in acetonitrile (2 mL) within an argon-filled glovebox, and
was sealed in a gastight cuvette with a pierceable septum. The
TEMPOH solution was prepared in acetonitrile (100 μL) and sealed
in a 300 μL glass vial with a pierceable septum. Data collection was
started, and the TEMPOH solution was added into the [Mnlll(OH)-
(dpaq)]+ solution using a gas-tight syringe (at 25 °C). Data collection
times ranged from 200 s to 4000 s. A final reaction mixture analyzed
by perpendicular-mode X-band EPR spectroscopy at 5 K revealed
characteristic features consistent with the presence of the TEMPO
radical (see Figure S2 in the Supporting Information). Kinetic
experiments at variable temperatures (−15 °C to 45 °C (258−318 K))
were performed following the same procedure as described, allowing
the cuvette to achieve thermal equilibrium (10 min) in the cryostat,
prior to the addition of the substrate.
amine (N3) in the equatorial plane, and the amide nitrogen
(N2) occupying an axial position. The other axial site is
occupied by the amide oxygen atom (O1) of a second
[MnII(dpaq)](OTf) molecule, forming an overall polymeric
structure. Table 1 summarizes the structural properties for one
Table 1. Selected Bond Lengths and Angles for
[MnII(dpaq)](OTf) and [MnIII(OH)(dpaq)](OTf)
Bond Length
atom pair value
Bond Angle
angle
value
a
[MnII(dpaq)]+ (B)
Kinetic Studies of [MnIII(OH)(dpaq)](OTf) with Xanthene. For
each experiment, a 1.25 mM [Mnlll(OH)(dpaq)]+ (1.6 mg, 2.5 × 10−3
mmol) solution was prepared in acetonitrile (2 mL) within an argon-
filled glovebox. This solution was transferred to a gas-tight cuvette and
sealed with a pierceable septum. The xanthene solution was prepared
in dichloromethane (200 μL) and sealed in a 4-mL glass vial with a
pierceable septum. Then, the cuvette containing the [MnIII(OH)-
(dpaq)]+ solution was inserted into a temperature-controlled cryostat
held at 50 °C. Upon achieving thermal equilibrium, data collection was
started, and 100 μL of the xanthene solution was injected into the
cuvette using a gas-tight syringe. Data collection was carried out up to
∼900 min. The rates of the reactions were calculated by applying
initial rate approximation in order to exclude any interference by
secondary reactions.
Mn−O1A
Mn−N2B
Mn−N1B
Mn−N3B
Mn−N4B
Mn−N5B
2.079(2) Å
2.191(3) Å
2.214(3) Å
2.314(3) Å
2.244(3) Å
2.286(3) Å
O1A−Mn−N2B
N4B−Mn−N5B
N1B−Mn−N3B
N4B−Mn−N2B
N1B−Mn−N2B
N3B−Mn−N2B
164.88(10)°
147.89(11)°
151.40(10)°
94.97(10)°
74.46(10)°
77.52(10)°
[MnIII(OH)(dpaq)](OTf)
Mn−O2
Mn−N2
Mn−N1
Mn−N3
Mn−N4
Mn−N5
1.806(13) Å
1.975(14) Å
2.072(14) Å
2.173(14) Å
2.260(14) Å
2.216(15) Å
O2−Mn−N2
N4−Mn−N5
N1−Mn−N3
N4−Mn−N2
N1−Mn−N2
N3−Mn−N2
177.94(6)°
152.53(5)°
161.83(6)°
85.22(5)°
79.77(6)°
82.51(6)°
Catalytic Oxidation of TEMPOH with [MnIII(OH)(dpaq)](OTf).
For each experiment, [MnII(dpaq)]+ (initial concentration = 0.031
mM) and TEMPOH (initial concentration = 125 mM) were measured
into a 20-mL glass vial within an argon-filled glovebox and dissolved in
acetonitrile (2 mL). This solution was sealed in a gas-tight cuvette with
a pierceable septum, and the electronic absorption spectrum was
collected under ambient conditions (∼25 °C). Then, an excess of dry
dioxygen gas was bubbled through the solution, while electronic
absorption spectra were collected every ∼15 min. The solvent level of
the reaction mixture was closely monitored confirming a negligible
solvent evaporation during oxygenation.
a
Bond lengths and angles for one of the three [MnII(dpaq)]+ cations
in the asymmetric unit. Corresponding metric parameters for the other
two cations are given in Table S3 in the Supporting Information.
of the [MnII(dpaq)]+ cations in the asymmetric unit (cation B);
corresponding metric parameters for the other cations can be
found in Table S3 in the Supporting Information. For the three
cations, the angle between the axial ligands (O1−Mn−N2) is
∼165°−170° and the angles between the equatorial ligands
vary from ∼75° to ∼105°, indicating large deviations from
idealized octahedral geometry. Three triflate ions are also
present in the crystal structure but do not interact with the MnII
centers (Mn−O distances of ∼6.4 Å). All MnII−ligand
distances are in the range of 2.1−2.3 Å, which is typical for a
high-spin MnII center. Notably, the C10−O1 distance of the
amide ranges from 1.265(4) Å to 1.271(4) Å, which is typical of
RESULTS AND ANALYSIS
■
Structural Properties of [MnII(dpaq)](OTf). The XRD
structure of [MnII(dpaq)](OTf) shows three [MnII(dpaq)]-
(OTf) molecules in the smallest asymmetric unit. Although
these three molecules do show slight differences in bond
lengths and angles, each consists of a MnII center in a highly
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a CO double bond. Thus, the Mn−O1 interaction is likely
weak, and presumably the polymeric structure observed in the
crystal structure reverts to separate monomeric entities in
solution. Overall, the structure of [MnII(dpaq)](OTf)
compares well with that of a recently reported [Mn(dpaq)-
(NO)](ClO4) complex, although the latter species showed
shorter manganese-ligand bond lengths.49 The shorter bond
lengths in [Mn(dpaq)(NO)](ClO4) are expected given the
formulation of that complex as a low-spin MnII center
antiferromagnetically coupled to an NO radical.
(dpaq)](OTf) under these conditions is estimated to be 26
days.
Properties of [MnIII(OH)(dpaq)](OTf). The X-ray struc-
ture of [MnIII(OH)(dpaq)](OTf) contains a six-coordinate
MnIII center with a distorted octahedral geometry (Figure 4).
Formation of [MnIII(OH)(dpaq)](OTf). The absorption
spectrum of [MnII(dpaq)](OTf) in acetonitrile shows a very
weak band at 510 nm (ε = 24 M−1 cm−1). When treated with
excess O2 gas at 25 °C, the light orange-colored [MnII(dpaq)]-
(OTf) solution rapidly changed its color to dark gold, as new
absorption features grew in at 550 nm (ε = 320 M−1 cm−1) and
780 nm (ε = 130 M−1 cm−1) (Figure 3; the extinction
Figure 4. ORTEP (left) and space filling (right) diagrams of
[MnIII(OH)(dpaq)](OTf). ORTEP diagram shows 50% probability
thermal ellipsoids. Hydrogen atoms and noncoordinating triflate
counteranion have been removed for clarity. Significant interatomic
distances and angles are listed in Table 1.
Coordination of the dpaq ligand is the same as in
[MnII(dpaq)](OTf), with the exception that all Mn−N
bonds, other than Mn−N4, are shorter in [MnIII(OH)(dpaq)]-
(OTf) by 0.062−0.202 Å (Table 1). The hydroxide group in
[MnIII(OH)(dpaq)](OTf) is trans to the amide nitrogen, with
a Mn−O2 distance of 1.806(13) Å. Inspection of the extended
structure of [MnIII(OH)(dpaq)]+ reveals that the hydrogen
atom of the hydroxo ligand is involved in a hydrogen-bonding
interaction with the amide oxygen of an adjacent [MnIII(OH)-
(dpaq)]+ molecule (O···O and H···O separations of 2.7308(18)
and 1.95(3) Å, respectively). A free triflate ion and an
acetonitrile molecule are observed within the asymmetric
unit, but these are not associated with the metal center (Mn−O
and Mn−N distances of ∼6.2 and ∼4.8 Å for triflate oxygen
and acetonitrile nitrogen, respectively).
There have been several X-ray structures of mononuclear
MnIII−OH complexes, and these have revealed Mn−O(H)
distances within the range of 1.81−1.86 Å.39−46 The longest
distance of 1.86 Å is unusual, and represents hydrogen-bonding
interactions with an ordered cluster of water molecules in the
solid-state.41 The [MnIII(OH)(dpaq)]+ complex has a Mn−
O(H) distance of 1.806(13) Å, which is at the lower end of the
range of previously reported distances for this class of
compounds. The O2−Mn−N2 axis, which includes the
hydroxide oxygen and amide nitrogen ligands, represents the
compressed pseudo-Jahn−Teller axis. The observation of an
intermolecular hydrogen-bond involving the hydroxo ligand in
[MnIII(OH)(dpaq)]+ is not unusual. Four of the eight known
X-ray structures of MnIII−OH adducts show second-sphere
hydrogen-bonding interactions in the solid-state struc-
tures.39−41,46
Figure 3. Electronic absorption spectra of 2.5 mM [MnII(dpaq)]+
(solid blue trace) upon the addition of excess O2 at 25 °C in MeCN
under argon. Inset: Time evolution of absorption signals at 550 and
780 nm.
coefficients were obtained using recrystallized [MnIII(OH)-
(dpaq)](OTf)). Formation of these new electronic absorption
features is consistent with the oxidation of MnII upon reacting
with O2. Electronic absorption spectral signatures for other
hydroxomanganese(III) species vary in terms of their
energies,43,45,46 although the closely related [MnIII(OH)-
(PaPy2Q)]ClO4 complex (where PaPy2Q is the amide-
containing N5 ligand N,N-bis(2-pyridylmethyl)-amine-N-ethyl-
2-quinoline-2-carboxamide) has spectral features with striking
similarities to those of [MnIII(OH)(dpaq)](OTf).42 The
prominent absorption signatures for [MnIII(OH)(PaPy2Q)]-
ClO4 in acetonitrile are centered at 485 nm (ε = 280 M−1
cm−1) and 740 nm (ε = 120 M−1 cm−1). These similarities
between the electronic absorption features are anticipated given
the closely related primary coordination spheres of
[MnIII(OH)(PaPy2Q)]ClO4 and [MnIII(OH)(dpaq)](OTf)
(vide infra). The final product [MnIII(OH)(dpaq)](OTf) was
isolated in an essentially quantitative yield of greater than 98%.
The half-life (t1/2) of [MnIII(OH)(dpaq)](OTf) was estimated
by monitoring the electronic absorption spectrum of a 1.25 mM
acetonitrile solution at 25 °C. The concentration of
[MnIII(OH)(dpaq)]+ decreased by ∼10% after 6 days.
Assuming a constant decay rate, the half-life of [MnIII(OH)-
Cyclic voltammetric analysis of [MnIII(OH)(dpaq)](OTf) in
acetonitrile revealed a partially reversible wave with an E1/2 of
−0.60 V vs Fc+/Fc at a scan rate of 100 mV s−1 (ΔEp = 120
mV; see Figure S3 in the Supporting Information), which we
attribute to the MnIII/MnII couple. This potential value is
comparable to that reported for the reduction of [MnIII(OH)-
(SMe2N4(tren))2]+ (Ep,c = −0.6 V vs Fc+/Fc in MeCN).46
Previous studies have also described MnIII/MnII reduction
potentials for several other hydroxomanganese(III) complexes.
Stack and co-workers reported that [MnIII(OH)(PY5)]2+
(Figure 1, left) exhibited a quasi-reversible reduction wave at
E
1/2 = +0.17 V vs Fc+/Fc (reported as +0.81 V vs SHE; ΔEp =
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150 mV), which was attributed to the MnIII/MnII couple.39 The
higher reduction potential for this dicationic complex compared
to that of [MnIII(OH)(dpaq)]+ is anticipated given the
difference in overall charge of the two complexes. Furthermore,
a monoanionic hydroxomanganese(III) species supported by a
trianionic ligand has a MnIII/MnII couple at −1.51 V vs Fc+/Fc
(ΔEp = 380 mV).60 An outlier from this trend is [MnIII(OH)-
(OAc)(Me2EBC)]+ (Me2EBC = 4,11-dimethyl-1,4,8,11-
tetraazabicyclo[6.6.2]hexadecane), for which E1/2 = −1.33 V
vs Fc+/Fc (reported as −0.689 V vs SHE; ΔEp = 132 mV). This
deviation is presumably due to the preferential stabilization of
MnIII within the cavity of the macrocyclic supporting ligand.45
Reactivity of [MnIII(OH)(dpaq)]+ with TEMPOH. The
isolation of [MnIII(OH)(dpaq)]+ allowed us to investigate the
ability of this species to effect PCET reactions. Treatment of
[MnIII(OH)(dpaq)]+ with 100 equiv TEMPOH under an
argon atmosphere in MeCN at 25 °C lead to the rapid
disappearance of the electronic absorption bands of
[MnIII(OH)(dpaq)]+ (Figure 5). The final electronic absorp-
Figure 6. Pseudo-first-order rate constants, kobs (s−1) versus
TEMPOH and TEMPOD concentration for a 1.25 mM solution of
[MnIII(OH)(dpaq)]+. The second-order rate constant, k2 (M−1 s−1),
was calculated from the linear correlation of the observed rate constant
and substrate concentration.
in the rate-determining step. Similar KIE values of less than 2.0
have been observed for O−H bond oxidation by other
transition-metal species in cases where rate-determining O−
H/D bond cleavage has been proposed.62 Given the excep-
tionally high pKa of TEMPOH in MeCN (41),48 the KIE is
supportive of the expected CPET mechanism for this reaction.
Moreover, the relatively low reduction potential of
[MnIII(OH)(dpaq)]+ rules out a rapid electron transfer process.
An Eyring analysis of the reaction of [MnIII(OH)(dpaq)]+ with
TEMPOH from −15 °C to 45 °C (258−318 K), afforded
activation parameters ΔH⧧ and ΔS⧧ of 9.9(9) kcal/mol and
−35(3) cal/(mol K), respectively (Figure 7). Thus, at 25 °C,
the entropic contribution to the free energy of activation
(TΔS⧧ = −10 kcal/mol) is equal to the enthalpic contribution.
Figure 5. Electronic absorption spectra of 1.25 mM [MnIII(OH)-
(dpaq)]+ upon the addition of 100 equiv TEMPOH at 25 °C in
MeCN under argon. Inset shows the decay of the 550-nm absorption
signal.
tion spectrum appears essentially identical to that of
[MnII(dpaq)]+ in MeCN, showing a very weak band at 510
nm. Using the intensity of this band, and the extinction
coefficient for [MnII(dpaq)]+ (vide supra), the MnII complex
forms in 99% yield. The perpendicular-mode X-band EPR
spectrum of the final reaction mixture at 5 K shows
characteristic EPR features of the TEMPO radical centered at
g = 2.04 (Figure S2A in the Supporting Information).61
Together, these data demonstrate that TEMPOH reacts with
[MnIII(OH)(dpaq)]+ to generate TEMPO and [MnII(OH2)-
(dpaq)] quantitatively; i.e., the PCET reaction goes to
completion under these conditions.
Kinetic parameters associated with this process were
determined through a series of reactions between [MnIII(OH)-
(dpaq)]+ and 10−250 equiv TEMPOH (pseudo-first-order
conditions). In all cases, the decay of [MnIII(OH)(dpaq)]+
followed pseudo-first-order behavior to at least 5 half-lives
(Figure 5, inset). The pseudo-first-order rate constants increase
linearly as a function of TEMPOH concentration (Figure 6),
giving a second-order rate constant of 1.3(1) × 10−1 M−1 s−1 at
25 °C. Using deuterated TEMPOD, a kinetic isotope effect
(KIE) of 1.8 was observed (Figure 6). The KIE value, although
small, suggests that cleavage of the TEMPO−H/D bond occurs
Figure 7. Eyring plot showing ln(k/T) versus 1/T (K−1) for the
reaction of 1.25 mM [MnIII(OH)(dpaq)]+ with TEMPOH from −15
°C to 45 °C (258−318 K).
Reactivity of [MnIII(OH)(dpaq)]+ with Xanthene. The
addition of 250 equiv of xanthene to a 1.25 mM solution of
[MnIII(OH)(dpaq)]+ at 50 °C led to the slow decay of the
electronic absorption signals of the MnIII−OH complex (see
Figure S4 in the Supporting Information). Over the course of
15 h, after the addition of xanthene, 50% of [MnIII(OH)-
(dpaq)]+ was observed to disappear. Importantly, [MnIII(OH)-
(dpaq)]+ itself shows essentially no self-decay in the absence of
xanthene in this time period. A pseudo-first-order rate constant
of 8 × 10−4 s−1 was determined for the reaction of
[MnIII(OH)(dpaq)]+ with xanthene using the method of initial
rates.
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Reactivity of [MnIII(OH)(dpaq)]+ with Phenols.
least 3 half-lives. Between 10 and 50 equiv of added 4‑t‑butylAr-
OH, the pseudo-first-order rate constants (kobs) show an
almost-linear increase, with respect to 4‑t‑butylArOH concen-
tration. However, above 50 equiv 4‑t‑butylArOH, saturation
behavior was observed (Figure 9), indicating the formation of
[MnIII(OH)(dpaq)]+ also reacted with 2,4,6-tri-t-butylphenol
(
4‑t‑butylArOH), which has an O−H bond nearly 11 kcal/mol
stronger than that of TEMPOH (BDFE in MeCN of 77.1 and
66.5 kcal/mol, respectively). Monitoring the reaction of
[MnIII(OH)(dpaq)]+ with 4‑t‑butylArOH by electronic absorp-
tion spectroscopy showed that the decay of the [MnIII(OH)-
(dpaq)]+ complex is concomitant with the formation of the
corresponding phenoxyl radical, 4‑t‑butylArO• (λmax = 628 nm; ε
= 400(10) M−1 cm−1 in MeCN at 25 °C; see Figure 8).63 The
Figure 9. Observed pseudo-first-order rate constants (kobs) as a
function of 4‑t‑butylArOH and 4‑t‑butylArOD concentration for
[MnIII(OH)(dpaq)]+.
an intermediate prior to the rate-determining step (see Scheme
1). Accordingly, the relationship between kobs and the
Figure 8. Electronic absorption spectra of 1.25 mM [MnIII(OH)-
(dpaq)]+ upon the addition of 100 equiv of 2,4,6-tri-t-butylphenol
(
4‑t‑butylArOH) at 50 °C in MeCN under argon. Inset shows the time
Scheme 1. Reaction of [MnIII(OH)(dpaq)]+ with
4‑t‑butylArOH, Where an Initial Equilibrium (Keq) between
[MnIII(OH)(dpaq)]+ and 4‑t‑butylArOH Forms an
Intermediate, Which Converts to Products via a Rate-
Determining Step (k1)
evolution of single-wavelength traces at 628, 550, and 800 nm.
phenoxyl radical was formed in 80% yield, relative to the initial
[MnIII(OH)(dpaq)]+ concentration. A comparison of perpen-
dicular-mode X-band EPR spectra of [MnII(dpaq)]+,
[MnIII(OH)(dpaq)]+, and the final products of 4‑t‑butylArOH
oxidation provides evidence for the formation of [MnII(dpaq)]+
and the phenoxyl radical following the phenol oxidation
reaction (see Figure S2B in the Supporting Information).
The 80% yield of the phenoxyl radical was observed regardless
of the initial [MnIII(OH)(dpaq)]+:4‑t‑butylArOH ratio (see Table
S4 in the Supporting Information). It is possible that the 80%
yield is observed because the radical participates in side
reactions that prevent its full accumulation. However, the clean
isosbestic behavior observed in the reaction of [MnIII(OH)-
(dpaq)]+ with 4‑t‑butylArOH (Figure 8) is difficult to reconcile
with the disappearance of 20% of the radical product during the
timecourse of this reaction. The substoichiometric formation of
phenoxyl radical was also noted in the reaction of [MnIII(OH)-
(PY5)]+ (Figure 1, left) with 4‑t‑butylArOH.39 For that system,
the phenoxyl radical was formed after most of the MnIII−OH
complex was consumed, indicating a reaction between the
metal complex and the phenoxyl radical. In this present work,
the rate of formation of the radical is the same as the rate of
disappearance of [MnIII(OH)(dpaq)]+; thus, we assume that no
reaction occurs between the MnIII complex and the phenoxyl
radical. An alternative explanation for the 80% yield of the
phenoxyl radical would be that the equilibrium for the reaction
of [MnIII(OH)(dpaq)]+ and 4‑t‑butylArOH to give [MnII(OH2)-
(dpaq)]+ and 4‑t‑butylArO• does not lie very far to the right. This
can be discounted by the nearly invariant yield of the phenoxyl
radical over a range of initial [MnIII(OH)(dpaq)]+: 4‑t‑butylArOH
concentration of 4‑t‑butylArOH was fit using the following
equation: kobs = k1Keq[4‑t‑butylArOH]/1 + Keq[4‑t‑butylArOH],
where k1 is the rate constant (in units of s−1) for the rate-
determining step and Keq is the equilibrium constant describing
the formation of the intermediate from [MnIII(OH)(dpaq)]+
and 4‑t‑butylArOH (Scheme 1). This analysis afforded a Keq of
20(2) and a k1 of 1.2(1) × 10−3 s−1 (Table 2).
A similar analysis was performed for data collected using the
deuterated phenol 4‑t‑butylArOD (Figure 9). The Keq value for
the reaction using 4‑t‑butylArOD is quite similar to that observed
for 4‑t‑butylArOH (Table 2). Considering the standard deviations
for the Keq values, these parameters are almost overlapping. The
k1 values for the 4‑t‑butylArOD and 4‑t‑butylArOH substrates afford
a KIE of 1.4.
To determine if the observed saturation behavior is a feature
of the reaction of [MnIII(OH)(dpaq)]+ with phenolic substrates
in general, we explored the reactivity of this complex with a
series of phenols, including 4-methoxy-2,6-tri-t-butylphenol
(
4‑MeOArOH), 4-methyl-2,6-tri-t-butylphenol (4‑MeArOH), and
2,6-di-t-butylphenol (4‑HArOH). This series encompasses a
reasonable range of phenol O−H BDFEs (74−78.5 kcal/mol)
and pKa values (23−28), as summarized in Table 2. In reactions
with phenols with BDFEs greater than 79 kcal/mol (3,5-di-t-
butyl-4-hydroxybenzonitrile (4‑CNArOH) and 3,5-di-t-butyl-4-
concentrations (see Table S4 in the Supporting Information).
4‑t‑butyl
When [MnIII(OH)(dpaq)]+ was treated with excess
-
ArOH (10−125 equiv), the decay of [MnIII(OH)(dpaq)]+ and
formation of 4‑t‑butylArO• follow pseudo-first-order kinetics to at
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Table 2. Equilibrium and Rate Constants from Kinetic Experiments between [MnIII(OH)(dpaq)]+ and Phenols at 50 °C in
MeCN, and Substrate BDFEOH, pKa, and Reduction Potential Values
a
b
c
substrate
Keq
k1 (s−1
)
BDFEOH (kcal/mol)
pKa
E (V)
4‑MeOArOH
4‑MeArOH
4‑t‑butylArOH
4‑t‑butylArOD
4‑HArOH
>700
1.8(1) × 10−3
1.5(1) × 10−3
1.2(1) × 10−3
8.9(1) × 10−4
9.4(1) × 10−4
73.9(1)
76(1)
28
27
28
−0.806
−0.755
−0.645
18(1)
20(2)
15(2)
8(1)
77.1(1)
78.5(1)
b
27
−0.619
Data taken from Warren et al.48 or determined following the work of Waidmann et al.64 pKa values in MeCN; see ref 65. Potential for PhO•/
a
c
48
PhO− in DMSO vs Cp2Fe+/0
.
hydroxybenzaldehyde (4‑CHOArOH)), only a fraction of
[MnIII(OH)(dpaq)]+ was consumed. This implies that the
thermodynamic limit for the oxidative capability of this MnIII−
OH adduct is ∼80 kcal/mol.
are not shifted upon 4‑t‑butylArOH addition, and the absorption
intensity at 550 nm rises by only 12%. The observed increase in
intensity is wavelength dependent and, thus, is potentially due
to light scattering, which increases as a function of the fourth
power of the light frequency. Under the reasonable assumption
that the electronic absorption spectrum of [MnIII(OH2)-
(dpaq)]2+ should be distinct from that of [MnIII(OH)(dpaq)]+,
the similarities between the electronic absorption spectra before
and after the addition of the 4‑t‑butylArOH suggest that the
intermediate does not result from proton transfer between
4‑t‑butylArOH and [MnIII(OH)(dpaq)]+.
The reactions of [MnIII(OH)(dpaq)]+ with 4‑MeOArOH,
4‑MeArOH, and 4‑HArOH all showed saturation behavior, with
the pseudo-first-order rate constants leveling at higher phenol
concentration (Figure S5 in the Supporting Information).
Equilibrium and rate constants determined from fits to these
datasets are collected in Table 2. In the case of 4‑MeOArOH, the
observed rate was saturated even when using 10 equiv of
phenol (Figure S5 in the Supporting Information, top right).
Thus, only an estimate of the lower limit for Keq (700) could be
obtained for this substrate.
Catalytic Oxidation of TEMPOH with [MnII(dpaq)](OTf)
and O2. In the presence of a large excess of TEMPOH (4000
equiv), [MnII(dpaq)](OTf) catalytically converted TEMPOH
into TEMPO in continuously oxygenated MeCN solution at 25
°C. The catalytic process was followed over the course of 200
min by electronic absorption spectroscopy, monitoring the
characteristic features of TEMPO and [MnII(dpaq)](OTf) (see
Figure S7B in the Supporting Information). The direct
oxidation of TEMPOH in the absence of the catalyst was
investigated as a control (see Figure S7A in the Supporting
Information). Although oxidation of TEMPOH to TEMPO is
observed in the control reactions, ∼40% higher conversions are
observed in the presence of [MnII(dpaq)](OTf). Importantly,
separate control experiments using catalytic amounts of
MnII(OTf)2 gave results within error of the control experiments
lacking any MnII species. Thus, the increase in turnover
numbers is attributed to catalytic activity of the [MnII(dpaq)]-
(OTf) complex. Correcting for the background oxidation of
TEMPOH in the control reactions, a maximum of 1050
turnovers were observed using [MnII(dpaq)](OTf) as an
aerobic oxidation catalyst.
An Evans−Polanyi plot for the oxidation of phenolic
substrates with [MnIII(OH)(dpaq)]+ shows a linear depend-
ence of the rate-determining step (k1) as a function of BDFE of
the substrate (Figure 10). This relationship, along with the KIE
Figure 10. Evans−Polanyi plot of oxidation rate (k1) of phenolic
substrates versus the BDFE of the substrate.
DISCUSSION
■
value for the reaction of [MnIII(OH)(dpaq)]+ with 4‑t‑butylAr-
OH, provides strong evidence that the rate-determining step in
these reactions is a CPET between the phenol and the MnIII−
OH unit. Thus, the pre-equilibrium step shown in Scheme 1
potentially involves the formation of a hydrogen-bonded
reactant complex.
To further probe the nature of the intermediate formed in
the initial equilibrium reaction between [MnIII(OH)(dpaq)]+
and 4‑t‑butylArOH (Scheme 1), 400 equiv of the phenol were
added to a MeCN solution of [MnIII(OH)(dpaq)]+ at 50 °C.
Under these conditions, the intermediate species should
initially be dominant in solution. Electronic absorption spectra
collected 0.5−1.5 s after the addition of 4‑t‑butylArOH show very
minor perturbations relative to the spectrum of pure
[MnIII(OH)(dpaq)]+ (see Figure S6 in the Supporting
Information). Specifically, the observed absorption maxima
Synthetic complexes with mononuclear hydroxomanganese(III)
motifs are relatively rare,39−46,66−69 as the hydroxo ligand is
prone to bridge Mn centers.27,29,31−38 Of the known
mononuclear MnIII−OH complexes, only two[MnIII(OH)-
(PY5)]2+ and [MnIII(OH)(SMe2N4(tren))]+ (see Figure 1)
have been reported to perform substrate oxidation reac-
tions,39,46 and both these reactions proceed via a CPET
process. Given these limited examples, relatively little is known
concerning the factors affecting the CPET, or, more broadly,
PCET, reactions of mononuclear hydroxomanganese(III)
species. This is despite several biological examples where
MnIII−OH units are involved in important oxidation/reduction
reactions,2,4,6,19 and stands in clear contrast to the abundance of
studies focused on the PCET reactions of high-valent Mn-oxo
and Mn-hydroxo species.70,71 As noted by Mayer and co-
workers, the oxidation reactions of lower-valent MII/MIII
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Scheme 2. Possible Pathways for PCET Reaction of [MnIII(OH)(dpaq)]+ with Phenols
couples are overshadowed by studies of their high-valent
counterparts.72
results. For example, when the oxygenation of [MnII(dpaq)]-
(OTf) is carried out in the presence of 10 equiv PPh3, the
formation of O=PPh3 is not observed and the yield of
[MnIII(OH)(dpaq)]+ is unchanged. Nevertheless, the pathway
for O2-dependent formation of [MnIII(OH)(dpaq)]+ is
currently under investigation, utilizing different solvent systems
that may favor the formation of reaction intermediates.
[MnIII(OH)(dpaq)]+ is capable of oxidizing substrates with
O−H BDFEs of <79 kcal/mol. Saturation behavior was
observed in oxidation reactions with phenolic substrates. This
is only the second report of a mononuclear MnIII−OH adduct
that is capable of performing a CPET reaction with phenols,
and a relatively rare example of a Mn oxidant that shows
saturation behavior in reaction with substrates.75,76 Catalytic
oxidation of organic substrates using O2 as terminal oxidant is
another attractive property of [MnIII(OH)(dpaq)]+, which is
observed to perform over 1050 turnovers of TEMPOH in 3 h
under the conditions employed. This turnover number is over 2
orders of magnitude larger than that observed for catalytic
TEMPOH oxidation by [MnIII(OH)(SMe2N4(tren))]+,46
although the catalytic activity for that system was not
optimized.
Nature of Intermediate in Phenol Oxidation Reactions
of [MnIII(OH)(dpaq)]+. PCET reactions can occur via a
sequential pathway (i.e., initial proton transfer followed by
electron transfer or vice versa) or a concerted pathway (CPET
or HAT; see ref 16). The previously reported [MnIII(OH)-
(PY5)]2+ and [MnIII(OH)(SMe2N4(tren))]+ complexes (Figure
1), perform C−H and/or O−H bond oxidations by a CPET
process, where the proton and electron are transferred in a
single kinetic step.39,46 In contrast, a mononuclear MnIII-oxo
complex, [MnIII(O)(H3buea)]2− (H3buea = tris[(N′-t-butylur-
eaylato)-N-ethyl]aminato), featuring a trianionic supporting
ligand with a second-sphere hydrogen-bonding cavity around
In this work, we have described a new mononuclear MnIII−
OH adduct, [MnIII(OH)(dpaq)]+, which is remarkably stable
(t1/2 ≈ 26 days at 25 °C in MeCN) and is formed in essentially
quantitative yields from reaction of [MnII(dpaq)](OTf) with
O2. Because of the high intrinsic reduction potential of the
MnIII/MnII couple, reactivity with O2 is less common for
manganese(II) centers when compared to analogous iron(II)
centers.73 To the best of our knowledge, the formation of
[MnIII(OH)(SMe2N4(tren))]+ from O2 and the corresponding
MnII complex [MnII(SMe2N4(tren))]+ represents the only other
example of the quantative formation of a MnIII−OH adduct
from a MnII center and O2.46 In that case, a mono-oxo bridged
dimanganese(III,III) complex served as an intermediate. Other
examples of mononuclear MnIII−OH adducts generated by O2
oxidation of a MnII complex include [MnIII(OH)(OAc)-
(Me2EBC)]+,45 and a set of [MnIII(OH)(L)]− species, where
L is one of two trianionic tripodal ligands with a second-sphere
cavity shielding the bound hydroxo ligand.40,44 For one of the
latter complexes, mechanistic studies provided evidence that
the MnIII−OH complex resulted from the initial formation of a
peroxo-bridged-dimanganese(III,III) species that undergoes
O−O bond homolysis to give two mononuclear MnIV=O
intermediates.44 The MnIV=O adducts convert to the observed
MnIII−OH species by a PCET reaction with solvent. More
recently, Kovacs and co-workers have reported the first example
of a peroxo-bridged-dimanganese(III,III) species, lending
credence to this mechanism.74 Investigations of the reaction
of O2 with the [MnII(dpaq)](OTf) complex in MeCN using
electronic absorption spectroscopy at low temperatures (−40
°C) provide no evidence for the formation of any intermediates
en route to [MnIII(OH)(dpaq)]+. In addition, attempts to
detect transient MnIV=O species have led to only negative
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Table 3. Reaction Parameters for Oxidation of TEMPOH by Hydroxometal(III) Adducts
complex
concentration (mM)
k2 (M−1 s−1
)
T (°C)
solvent
ΔH⧧ (kcal/mol) ΔS⧧ (cal/(mol K)) ΔG⧧ (kcal/mol)
[MnIII(OH)(dpaq)]+
1.25
0.5
1.3(1) × 10−1
2.1 × 103
25
25
25
MeCN
MeCN
toluene
9.9(9)
8.2
−35(3)
−25.5
NR
20.3
a
[MnIII(OH)(SMe2N4(tren))]+
15.6
b
c
c
FeIII(OH)(TMP)
0.08
7.6(5) × 101
NR
15.0(1)
a
b
c
Data taken from ref 46. Data taken from ref 72. Not reported.
the oxo, performs C−H bond oxidation of dihydroanthracene
through a sequential mechanism, with rate-limiting proton
transfer prior to electron transfer.66 This sequential pathway is
favored for the MnIII-oxo because of its high basicity (the pKa of
the corresponding MnIII-hydroxo is 28.3 in DMSO66). In fact,
treatment of [MnIII(O)(H3buea)]2− with 4‑t‑butylArOH in
dimethylacetamide solvent resulted in a proton transfer
reaction; oxidation of 4‑t‑butylArOH was not observed.60
The saturation behavior in the PCET reaction of
[MnIII(OH)(dpaq)]+ with phenols could imply a sequential
mechanism, as the kinetic behavior indicates an initial
equilibration to an intermediate species prior to rate-
determining formation of [MnII(OH2)(dpaq)]+ and the
phenoxyl radical. In principle, there are several pathways
consistent with saturation behavior, as outlined in Scheme 2.
Pathway C, involving initial electron transfer, can be excluded
on the basis of the unfavorability of oxidizing the ArOH
substrates in the absence of their deprotonation, as well as the
low E1/2 value for [MnIII(OH)(dpaq)]+ (vide supra).48
There are several lines of evidence that the equilibrium step
does not involve an initial proton transfer but potentially
involves the formation of a hydrogen-bonded complex
(pathways B and A, respectively; see Scheme 2). First, there
is no correlation between the observed Keq values and the pKa
values of the phenols (Table 2). A correlation would be
expected if the equilibrium were due to a proton transfer from
the phenol to [MnIII(OH)(dpaq)]+.77 Second, the electronic
absorption spectrum of the intermediate, obtained within 0.5 s
of adding a large excess of 4‑t‑butylArOH to a MeCN solution of
[MnIII(OH)(dpaq)]+, is almost identical to that of
[MnIII(OH)(dpaq)]+ (Figure S6 in the Supporting Informa-
tion). Although the electronic absorption spectrum of a
putative [MnIII(OH2)(dpaq)]2+ species is not known, it is
reasonable to assume that the change from hydroxo to aqua
ligation would significantly perturb the ligand-field splitting of
the MnIII center (potentially causing a reorientation of the
pseudo-Jahn−Teller axis78), resulting in noticeable shifts of the
d−d transition energies. The minor changes observed
experimentally are more consistent with a hydrogen-bonding
interaction.79 Third, the KIE of 1.4 for the rate-determining
step in the reaction of [MnIII(OH)(dpaq)]+ with 4‑t‑butylArOH
suggests that phenol O−H bond breaking be part of this step.
Finally, k1 is related to the BDFE values of the phenol O−H
bonds, which is a hallmark of a CPET process. Given the above
considerations, we conclude that the initial equilibrium step
features the formation of a precursor complex, likely stabilized
by hydrogen-bonding interactions, and that the rate-determin-
ing step is a CPET.
adduct supported by a bulky salen derivative also showed
saturation behavior in reactions with substituted phenols.75,76
For the reaction with 4‑HArOH, in particular, a Keq of ∼5000
was determined, and this equilibrium was attributed to the
formation of a hydrogen-bonded complex. No initial
equilibrium was observed in the reaction of the corresponding
MnIV-oxo adduct with phenols.75,76 In addition, Costas and co-
workers observed the formation of a precursor complex in the
reaction of hydrocarbons with an oxohydroxomanganese(IV)
moeity supported by a neutral N4 ligand, but the forces
stabilizing the precursor complex remain unclear.76 Saturation
behavior was not observed in analogous reactions with the
corresponding di(hydroxo)manganese(IV) complex. Taken
together, these prior studies demonstrate that subtle changes
in properties of the metal-based oxidant can dramatically affect
whether or not the formation of a precursor complex is
observed.
Nevertheless, it is somewhat curious that saturation behavior
should be observed in the reaction of [MnIII(OH)(dpaq)]+
with phenols, but not with TEMPOH, which likewise contains
an O−H unit capable of hydrogen bonding. Indeed, TEMPOH
has been shown to form a hydrogen-bond-mediated precursor
complex, prior to rate-liming CPET, with [CoIII(Hbim)-
(H2bim)2]2+ (H2bim = 2,2′-biimidazoline).79 However, given
that the Keq values observed for reaction of [MnIII(OH)-
(dpaq)]+ with phenols reflect small stabilizations in terms of
free energy (−4.2 kcal/mol to −1.3 kcal/mol), seemingly
minor differences in the hydrogen-bond donating (or accept-
ing) ability of the phenol versus TEMOPH substrates, and/or
changes in solvent reorganization energies upon precursor
complex formation, could lead to a precursor complex being
less favorable in the case of TEMPOH substrate. Taken
together, this and other work highlights our nascent under-
standing of the optimal conditions that must be achieved to
observe formation of a precursor complex, and the impacts of
precursor complexes on the overall CPET process.75,76,81,82
CPET Reactivity of Mononuclear MnIII−OH Com-
plexes. To the best of our knowledge, [MnIII(OH)(dpaq)]+
is only the third hydroxomanganese(III) complex known to
facilitate either O−H or C−H bond oxidations. As both
[MnIII(OH)(dpaq)]+ and [MnIII(OH)(SMe2N4(tren))]+ (Fig-
ure 1, right) react with TEMPOH at 25 °C in MeCN, a direct
comparison can be made between these oxidants. Based on
these kinetic data, [MnIII(OH)(SMe2N4(tren))]+ is a more rapid
oxidant,46 showing a second-order rate constant for TEMPOH
oxidation 104-fold larger than that of [MnIII(OH)(dpaq)]+ (see
Table 3). The faster reaction for [MnIII(OH)(SMe2N4(tren))]+
is a consequence of a smaller enthalpy and entropy of activation
compared to [MnIII(OH)(dpaq)]+ (Table 3). In both systems
the negative ΔS⧧ values mark an associative process; the
substantially more negative ΔS⧧ for the reaction involving
[MnIII(OH)(dpaq)]+ implies a more ordered transition state.
The smaller ΔH⧧ for the reaction of [MnIII(OH)-
(SMe2N4(tren))]+ is tentatively attributed to an increased
nucleophilicity caused by the electron-donating thiolate ligand.
When CPET reactions are considered within the framework
of Marcus theory, a precursor (or encounter) complex is
expected to form prior to rate-determining proton−electron
transfer.80 While rare, precursor complexes have been observed
for other CPET processes, and these are often,75,81,82 but not
always,76 mediated by hydrogen-bonding interactions with an
O−H group on the substrate. For example, a MnIV-hydroxo
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A similar effect has been observed in CPET reactions of a
thiolate-ligated oxoiron(IV) complex.83 Also of relevance in
comparing TEMPOH reactivity with metal-hydroxo adducts is
a recent study of the hydroxoiron(III) complex FeIII(OH)-
(TMP), which features a tetramesitylporphyrin supporting
ligand.72 In that case, a second-order rate constant of 76(5)
M−1 s−1 was observed for the oxidation of TEMPOH at 25 °C
in toluene (Table 3).
detailed studies of such reactions to mid- and even low-valent
transition-metal oxidants.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystal data and structure refinement tables for [MnII(dpaq)]-
(OTf) and [MnIII(OH)(dpaq)](OTf); ESI-MS plots; yields of
4‑t‑butylArO• from 4‑t‑butylArOH oxidation by [MnIII(OH)-
(dpaq)]+; X-band EPR data for [MnII(dpaq)](OTf),
[MnIII(OH)(dpaq)](OTf) and oxidation reaction products;
cyclic voltammetry data for [MnIII(OH)(dpaq)]+; kinetic data
for reactions of [MnIII(OH)(dpaq)]+ with xanthene and
substituted phenols; electronic absorption spectra of
[MnIII(OH)(dpaq)]+ immediately following the addition of a
large excess of 4‑t‑butylArOH; results of catalytic TEMPOH
oxidation by [MnII(dpaq)](OTf). This material is available free
of charge via the Internet at http://pubs.acs.org.
Although both [MnIII(OH)(SMe2N4(tren))]+ and FeIII(OH)-
(TMP) react with TEMPOH much more rapidly than
[MnIII(OH)(dpaq)]+, neither of those complexes is able to
oxidize substrates with larger BDFEs. For example, FeIII(OH)-
(TMP) showed no reaction with 4‑t‑butylArOH over a period of
24 h,72 and the normal decay rate of [MnIII(OH)-
(SMe2N4(tren))]+ was unaffected by the presence of a large
excess of 1,4-cyclohexadience, dihydroanthracene, or toluene.46
In contrast, [MnIII(OH)(dpaq)]+ reacts with phenols with O−
H BDFEs up to 79 kcal/mol, and can also activate the weak C−
H bonds of xanthene (BDFE = 73.3 kcal/mol in dimethyl
sulfoxide). [MnIII(OH)(dpaq)]+ reacts with a large excess (250
equiv) of xanthene with a pseudo-first-order rate constant of 8
× 10−4 s−1 at 50 °C in MeCN, which is an order of magnitude
slower than that observed for [MnIII(OH)(PY5)]2+,39 the only
other MnIII−OH complex known to oxidize C−H bonds.
For the reaction of [MnIII(OH)(dpaq)]+ with phenols, the
rate of reaction is only weakly dependent on the phenol O−H
BDFE (Figure 10). In particular, the observed slope of the
log(k) versus BDFE correlation is only −0.07, which is
significantly lower than the expected Marcus theory value of
−0.5 for a reaction with a small thermodynamic driving force.80
However, this slope should be viewed within the caveat that the
linear-free energy relationship is only for a small range of
substrate BDFEs. In any case, values between −0.5 and −0.3
have been reported for transition-metal oxidants,80,84 including
oxoruthenium(IV)85 and oxomanganese(IV)70,76 species, and
are commonly interpreted as implying a symmetric transition
state structure. Notably, a similarly weak correlation between
rate and substrate BDE was observed for hydrocarbon
oxidation by [MnIII(OH)(PY5)]2+ (slope = −0.1), and this
was attributed to unusually large reorganizational energies of
the oxidant (ΔS⧧ = −36(5) cal/(mol K) for the reaction of
[MnIII(OH)(PY5)]2+ with dihydroanthracene).39 This inter-
pretation would be consistent with other instances were small
slopes have been attributed to large entropies of reorganization,
such that TΔS⧧ > ΔH⧧.82
AUTHOR INFORMATION
Corresponding Author
*Tel.: (785) 864-3968. Fax: (785) 864-5396 taj@ku.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation (No. CHE-1056470 to T.A.J.). B.C. was supported
by funds from an NSF-Research Experience for Undergraduate
program (No. CHE-1004897). The U.S. National Science
Foundation is also acknowledged for funds used for the
purchase of X-ray instruments (No. CHE-0079282) and the
EPR spectrometer (No. CHE-0946883). Prof. Mikhail Barybin
and Dr. John Meyers Jr. are acknowledged for their generous
assistance in the collection and interpretation of the electro-
chemical data.
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