How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity

Article abstract of DOI:10.1021/jacs.9b04729

Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O₂ complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn^III-OOR complexes containing N-heterocyclic (N^Ar) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn^II complex containing a more electron-rich, less sterically demanding N^Ar ligand scaffold, and compare it with previously reported Mn^II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O₂ binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N^Ar ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (k_H/k_D = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected Mn^IIIMn^IV bimetallic species, and 0.5 equiv after its formation.

Full text of DOI:10.1021/jacs.9b04729

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Article
How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier
to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity.
Penny Chaau Yan Poon, Maksym A. Dedushko, Xianru Sun, Guang Yang, Santiago Toledo, Ellen C. Hayes,
Audra Johansen, Marc C. Piquette, Julian A. Rees, Stefan Stoll, Elena Rybak-Akimova, and Julie A. Kovacs
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04729 • Publication Date (Web): 03 Sep 2019
Downloaded from pubs.acs.org on September 3, 2019
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How Metal Ion Lewis Acidity and Steric Properties
Influence the Barrier to Dioxygen Binding, Peroxo O-O
Bond Cleavage, and Reactivity.
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Penny Chaau Yan Poon,‡ Maksym A. Dedushko,‡ Xianru Sun,
Guang Yang, Santiago Toledo,⊥ Ellen C. Hayes,‡ Audra Johansen,‡
Marc C. Piquette, Julian A. Rees,§ Stefan Stoll,‡ Elena Rybak-
Akimova*,,& and Julie A. Kovacs*,‡
‡ Department of Chemistry, University of Washington, Campus Box 351700,
Seattle, WA 98195-1700.
⊥The Department of Chemistry, St. Edward’s University, 3001 South Congress,
Austin, Texas 78704-6489.
 Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford,
Massachusetts, 02155.
§Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720
Received date: (to be automatically inserted once manuscript is accepted)
Title running head: Barrier to O2 Binding and Peroxo O-O Bond Cleavage
Corresponding author: J. Kovacs, tel. (206) 543-0713, FAX (206) 685-8665,
kovacs@uw.edu , ORCID 0000-0003-2358-1269
&Deceased.
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Abstract
Herein we quantitatively investigate how metal ion Lewis acidity and steric properties
influence the kinetics and thermodynamics of dioxygen binding versus release from
structurally analogous Mn-O2 complexes, as well as the barrier to Mn peroxo O-O bond
cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that
the steric and electronic properties of MnIII-OOR complexes containing N-heterocyclic
(NAr) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths,
and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen
reactivity of a new MnII complex containing a more electron-rich, less sterically
demanding NAr ligand scaffold, and compare it with previously reported MnII complexes.
Dioxygen binding is shown to be reversible with complexes containing the more
electron-rich metal ions. The kinetic barrier to O2 binding, and peroxo O-O bond
cleavage is shown to correlate with redox potentials, as well as the steric properties of
the supporting NAr ligands. The reaction landscape for the dioxygen chemistry of the
more electron-rich complexes is shown to be relatively flat. A total of four intermediates,
including a superoxo and peroxo species, are observed with the most electron-rich
complex. Two new intermediates are shown to form following the peroxo, which are
capable of cleaving strong X-H bonds. In the absence of a sacrificial H-atom donor,
solvent, or ligand, serves as a source of H-atoms. With TEMPOH as sacrificial H-atom
donor, a deuterium isotope effect is observed (kH/kD = 3.5) implicating a hydrogen atom
transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced
prior to the formation of an EPR detected MnIIIMnIV bimetallic species, and 0.5 equiv
after its formation.
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Introduction. Fundamental scientific research is needed in order to develop
methods for efficiently capturing sunlight, and converting its energy into storable fuels.
Nature accomplishes this via
9
photosynthesis, which converts
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solar energy into energy that is
stored in a chemical bond by
extracting electrons from H2O to
form O2.1-4
The sluggish
kinetics of H2O oxidation has
been a major concern for
existing fuel cells.5 Nature had
a few billion years to refine its
Figure 1. Low temperature reaction between 1Mepy and O2 affords 6Mepy
via observable superoxo 2Mepy and peroxo 3Mepy intermediates.
Mn-containing photosynthetic H2O oxidation catalyst, therefore understanding the
mechanism by which it facilitates this reaction, or its microscopic reverse, would be of
value. However, despite its relevance to photosynthesis,6-9 the dioxygen chemistry of Mn
remains relatively unexplored,6,10-12 relative to that of Fe and Cu.13-19 Very little is known
about the Mn-induced O-O bond forming step,20 because it occurs following the rate-
determining step.21-25 An unobserved peroxo intermediate is proposed to form during
photosynthesis,22-24 which then readily evolves O2.26
Previously we showed that coordinatively unsaturated [MnII(LMepy)]+ (1Mepy, LMepy= (6-
Me-DPEN)N4Me2S–) reacts with O2 at low temperatures (-40 ˚C) to form dioxygen-bound
[Mn(LMepy)(O2)]+ (2Mepy), en route to the first example of a crystallographically
characterized binuclear peroxo-bridged dimer, {[MnIII(LMepy)]2(µ-O2)}2+ (3Mepy).6 Dioxygen
binding to 1Mepy was shown to occur on the millisecond time-scale.6 A thiolate was
incorporated into our ligand scaffold in order to 1) provide a chromophore that facilitates
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the spectroscopic observation of metastable intermediates,6,27-31 2) lower the activation
barrier to O2 binding, and 3) provide stability to M-O2 species.32-33 The rate at which
peroxo-bridged 3Mepy forms is slow enough at -40 ˚C to monitor using a benchtop UV/vis
spectrometer. However, with the LMepy ligand system it was not possible to determine
the mechanism by which the O-O bond of peroxo-bridged 3Mepy cleaves to afford mono-
oxo bridged MnIII-O-MnIII (6Mepy, Figure 1), because this step was shown to be rate-
determining.6 Mechanisms for O-O bond cleavage with Fe and Cu have been shown to
involve either protonation of a M-OOH at the distal oxygen,34 or the addition of a second
metal ion to the distal oxygen to form a bridging peroxo. Ideally an η2,η2-side-on peroxo
forms so as to maximize overlap with the empty σ*(O-O).14,19 In contrast, mechanistic
details regarding Mn-promoted O-O bond cleavage versus formation remain relatively
unexplored.35-41 An end-on peroxo complex is expected to be more stable than a side-on
peroxo complex, perhaps explaining why we were able to crystallize 4. Prior to our
work,6 there were no characterized examples of binuclear end-on or side-on MnIII-
peroxos, and there were only a handful of examples of monomeric η2-side-on MnIII-
peroxos,10,42-45 only one of which is derived from O2.10 An end-on MnIII-OOH was shown
to form upon protonation of the corresponding η2-side-on MnIII-peroxo compound,
however it was not crystallographically characterized.46
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A series of structurally analogous five-coordinate, thiolate–ligated MnII complexes
(e.g., 1Mepy of Figure 1) was previously reported by our group, which incorporate readily
derivatized N–heterocycles amines (NAr = 6-H-pyridine (1py), 6-Me-pyridine (1Mepy), and
quinoline (1Quino)), providing us with a method for tuning their steric and electronic
properties.6,47 In order to obtain more information regarding the mechanism of the O-O
bond cleaving step, we explore herein the low-temperature dioxygen chemistry of
quinoline-ligated [MnII(LQuino)]+ (1Quino),48 in addition to that of a new MnII complex,
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[MnII(LMeOpy)]+ (1MeOpy), containing a more electron donating substituent on the pyridine
ring. We will show that the barrier to selected steps of the reaction, including O2 binding
and release, and O-O bond cleavage, can be adjusted by changing the electron donor
character of the ligand.47
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Results and Discussion. Rationale for the ligand design of the complexes studied
herein stems in part from a previous study in which a correlation between peroxo O-O
bond lengths and Mn•••NAr distances (NAr = N-heterocycle) was observed with a series of
alkylperoxo Quino,MepyLMnIII-OOR (R = tBu, Cm) compounds.49 We refer to the interaction
between the Mn ion and the NAr ligand as Mn•••NAr, because, although not within
bonding distance, the NAr ligand was shown (by X-ray crystallography) to point directly
towards the metal ion, and (by spectroscopic methods) to influence properties of the
complex in a systematic way. A decrease in the mean Mn•••NAr distance (from 2.510 Å
to 2.411 Å) was found to elongate the peroxo O-O bond (from 1.431 Å to 1.468 Å).6,7 In
addition, the kinetic barrier to alkylperoxo O-O bond cleavage (∆H‡) was shown to
directly correlate with the Mn•••NAr distance. At ambient temperatures, dioxygen was
shown to react with the MnII precursors (1) to form fairly stable bimetallic mono oxo-
bridged MnIII complexes, (e.g., 6Mepy of Figure 1) in high yields (96-98%).6,48 Comparison
of the mean Mn•••NAr distance (Table 1) in these mono oxo-bridged products provides us
with a predictive metric that potentially reflects the O-O bond cleaving properties of the
ligand. The Mn•••NAr distance was shown to depend upon the steric properties of the NAr
ligand scaffold.11,49 Inspection of space filling models shows that the substituent in the 6-
position of the pyridine ring clashes with one of the gem-dimethyls adjacent to the
thiolate sulfur. This prevents the pyridine nitrogen from approaching the metal to the
optimum MnIII-NAr bond distance, if the substituent is larger than a hydrogen. With a
hydrogen in the 6-position, the Mn-NAr distance is within the normal bonding range
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Table 1. Mean Crystallography-Determined
(Table 1), and thus the use of a single
line to represent a bond. With larger
substituents, the Mn•••NAr separation is
well outside the normal bonding range.
For example, with a Me-group, the
Mn•••NAr Distance for Bimetallic Mono Oxo-
Bridged MnIII Complexes, 6.
NAr
Complex Mn•••NAravg (Å)
6-H-pyridine
6py
2.23(1)
2.413(3)
2.5(1)
6-MeO-pyridine 6MeOpy
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Quinoline
6Quino
6Mepy
6-Me-pyridine
2.56(2)
Mn•••NAr separation is 0.33 Å longer
avg
than that of the less sterically encumbered 6-H derivative, 6py (Table 1), resulting in a
more Lewis acidic metal ion. If this is extrapolated to the corresponding peroxo
compounds, the more Lewis acidic metal ion associated with bulkier substituents would
facilitate 휋 back-donation of electron density out of the peroxo antibonding 휋*(O-O) to
the metal ion, thereby strengthening the peroxo O-O bond.49 Conversely, the shorter
MnIII-NAr bond associated with 6-H-pyridine would be more electron-rich, thereby
weakening the peroxo O-O bond, and facilitating its cleavage. Consistent with this, no
intermediates are observed in the reaction between O2 and [MnII(Lpy)]+ (1py),48 even at
temperatures as low as -78 ˚C,
whereas peroxo compound
3Mepy (Figure 1) is stable
enough to crystallize. In order
to obtain more information
regarding the O-O bond-
cleaving step, we turned to
ligand systems (Scheme 1) that
support a Mn•••NAr separation in
the
crystallographically
Scheme 1. Low temperature reaction between 1 and O2 affords mono
oxo bridged 6 via observable superoxo, 2, and peroxo, 3, intermediates.
L= pyridine, 6-Me-pyridine, 6-MeO-pyridine, and quinoline.
characterized binuclear oxo-
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bridged, {[MnIII(LNAr)]2-(μ-O)}2+ (6), which is shorter than 6-Me-pyridine 6Mepy, but longer
than pyridine 6py. Distances in 6 were used as a predictive parameter, since we do not
have structures for all of the peroxo compounds. Quinoline {[MnIII(LQuino)]2-(μ-O)}2+
(6Quino)48 and 6-MeO-pyridine {[MnIII(LMeOpy)]2-(μ-O)}2+ (6MeOpy) both fit this criterion (Table
1). Below, we examine the temperature-dependent kinetics of the reaction between O2
and the MnII precursors to 6Quino and 6MeOpy, [MnII(LQuino)] + (1Quino) and [MnII(LMeOpy)] +
(1MeOpy), respectively (Scheme 1). We will show that the relative stability and reactivity of
metastable intermediates, and the barriers to O2 binding or release, and peroxo O-O
bond cleavage, is highly dependent on the supporting ligands. By changing the solvents
and corresponding solvent C-H bond strength, or by adding a sacrificial H-atom donor,
we are able to spectroscopically observe two new intermediates, each of which is
capable of abstracting H-atoms from strong X-H bonds (X = C, or O).
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Reactivity of 1Quino with Dioxygen. The low-temperature (-73 ˚C) reaction between
colorless [MnII(LQuino)]+ (1Quino 48
)
and O2
green
affords
a
metastable
intermediate, 3Quino
,
which rapidly
converts to mono oxo-bridged 6Quino
(λmax = 580 nm).48 The stability of 3Quino
is temperature-dependent. In CH3CN
(f.p. = -40 ˚C, C-H BDE = 93 kcal/mol),
metastable species 3Quino decays in ~
30 seconds at -40 ˚C, and is therefore
Figure 2. Electronic absorption spectrum of putative peroxo
3Quino (green) generated via the addition of O2 to 1.0 mM 1Quino
in CH2Cl2 at -73 °C, and 3Mepy (black) generated via the
addition of O2 to 0.9 mM 1Mepy and O2 in CH3CN at -40 °C.
best observed using a stopped-flow instrument. In CH2Cl2 (f. p. = -90 ˚C, C-H BDE = 98
kcal/mol), on the other hand, intermediate 3Quino is more stable at -90 ˚C (t3Quino→6Quino = 7
min) making it possible to obtain a spectrum using a benchtop UV-vis spectrometer
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(Figure 2). Given the close similarity of its electronic absorption spectrum, to that of
peroxo-bridged 3Mepy (λmax = 640 nm),6 it was deemed likely that 3Quino is also a peroxo-
bridged dimer, {[MnIII(LQuino)]2(μ-O2)}2+ (3Quino). This is supported by a low-resolution
crystal structure (Figure S1), the connectivity of which indicates that there are two
bridging oxygens (Table S1). In addition, 0.48 equiv. of H2O2 are released per Mn ion
upon the addition of 1.34 equiv of H2SO4 to 3Quino (see assay conditions in SI) consistent
with a peroxo-bridged dimer. Metrical parameters (Table S2) of the DFT optimized
structure (Figure 3), calculated using the spin-unrestricted B3LYP hybrid functional and
the 6-311G basis set, are consistent with a trans-μ-1,2-peroxo-bridged MnIII2 dimer (O-O
= 1.516 Å, Mn(1)-O(1) = 1.856 Å, Mn(2)-O(2) = 1.846 Å, Mn-Savg = 2.35 Å, Mn(1)•••Mn(2)
= 4.244 Å, Mn-O-Oavg = 101˚).
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The time-dependent DFT
(TD-DFT)
electronic
calculated
absorption
spectrum (Figure 3) is in good
agreement
with
the
experimental
spectrum
(Figure
2).
Transition-
Figure 3. Time-dependent density functional theory (TD DFT) calculated
electronic absorption spectrum of peroxo-bridged 3Quino and its DFT
optimized structure (using a B3LYP spin-unrestricted hybrid functional,
and 6-311G basis set).
difference density plots show
that these bands correspond
to charge transfer transitions, and involve a mixed peroxo/thiolate → Mn(d) transition at
higher energies and a thiolate → Mn(d) transition at lower energies (Figure S2). The
calculated exchange coupling constant, Jcalc = 3.8 cm-1, indicates that the two MnIII ions
are essentially uncoupled. The calculated peroxo O-O bond length of 3Quino is 0.06 Å
longer than the crystallographically determined O-O bond of peroxo-bridged 3Mepy
(1.452(5) Å),6 indicating that this bond should be more readily cleaved in 3Quino
.
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Temperature-Dependent Kinetics for the Formation of Peroxo 3Quino. Kinetics
data for the formation of peroxo 3Quino and its subsequent conversion to mono-oxo
bridged 6Quino (Scheme 1), were collected at low temperatures using a stopped-flow
instrument. The build-up and decay of 3Quino was followed at 749 nm, where interference
from 6Quino is minimal. As illustrated in the time-resolved electronic absorption spectra of
Figure S3, intermediate 3Quino forms in less than 10 sec at -20 ˚C (blue trace, λmax = 652
nm) en route to the purple mono oxo bridged product, 6Quino (pink trace, λmax = 565 nm).
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No additional intermediates, prior to 3Quino, or in between 3Quino and 6Quino, were detected
with this particular ligand system (vide infra). The general kinetic scheme reflecting the
two spectrophotometrically observed processes is shown in equations (1) and (2) below.
Under pseudo first-order conditions, in the presence of excess O2, kinetic traces could
be fit to the bi-exponential equation (3), affording the pseudo-first-order rate constants
Quino
(k1obs
’
and k3obsQuino) of equations (1) and (2), respectively. Residuals were slightly
larger for fits to a single exponential relative to fits to a bi-exponential. The observed
Quino, was found to increase linearly (Table S3) with
rate constant for equation (1), k1obs
’
increasing O2 concentration (Figure S4), indicating that the reaction is 1st order with
respect to [O2]. The second-order rate constant (k1’Quino) was obtained from the slope of
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Quino
k1obs
’
vs [O2] plots (Table 2) at four different temperatures. The non-zero, and
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increasing value of the, y-intercept with temperature in Figure S4, indicates that either
peroxo 3Quino formation (equation (1)), or a step prior to peroxo formation, is reversible.
The final absorbance values associated with the maximum accumulation of 6Quino were
found to be independent of [O2], providing support for irreversible formation of peroxo
3Quino. Together these
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Table 2. Experimentally obtained rate constants and activation
suggest that a step
parameters for the formation of peroxo-bridged 3Quino (k1’Quino), from
1Quino (0.30 mM) + O2 (4.1 mM) in CH3CN, and its conversion to
prior to peroxo 3Quino
formation, i.e. O2
binding, is reversible
(vide infra). The
mono oxo-bridged 6Quino (k3Quino).
observed pseudo first-
order rate constants,
k3obsQuino, for equation (2) were found to be independent of [O2] over the temperature
range examined (Figure S5) indicating that the rate of 3Quino to 6Quino conversion is zero-
order with respect to [O2].
In the next set of stopped-flow experiments, the concentration of 1Quino was varied
(0.2 – 0.8 mM after mixing) while maintaining a fixed excess concentration of O2 (4.1 mM
after mixing) at -10 C, allowing us to verify the proposed mechanism, and determine the
Quino
reaction order with respect to MnII 1Quino. The observed rate constants (k1obs
and
k3obsQuino) were obtained by fitting kinetic traces to equation (3). The observed pseudo-
first-order rate constant for the formation of 3Quino (k1obsQuino) displays a linear
dependence on the concentration of 1Quino in the presence of excess [O2] (Figure S6),
confirming second-order dependence on MnII overall. This would be consistent with
3Quino being a binuclear peroxo (Figures 3 and S1). A large non-zero intercept (Figure
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S6) indicates that a process that is first order in MnII (e.g., equation (4)) contributes to
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the rate-determining step. Observed rate constants, k3obsQuino, for the conversion of
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peroxo 3Quino to oxo-bridged 6Quino, in the presence of excess [O2], are independent of
[1Quino] indicating that this step (equation (6)) is zero-order with respect to Mn (Figure S7).
Collectively, the O2 and MnII concentration-dependence experiments described above
are consistent with the proposed stepwise mechanism, outlined in equations (4) through
(6) above. This reaction sequence is analogous to that previously established for the
reaction between MnII 1Mepy and O2. Although not directly observed
spectrophotometrically, superoxo [Mn(LQuino)(O2)]+ (2Quino) is proposed to form as a
transient intermediate (eqn (4)), prior to the formation of the observable peroxo
intermediate 3Quino (eqn (5)). The analogous 6-Me-pyridine superoxo 2Mepy (Figure 1) is,
in contrast, directly observed by stopped-flow.6 The rate laws for each step in the
mechanism are provided in equations (7) through (9) below. The first two rate laws,
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equations (7) and (8), are analogous to that previously established for [MnII(LMepy)]
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(1Mepy, Figure 1).6 Information regarding the rate of peroxo O-O bond cleavage was not
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available for the 6-Me-pyridine system, however, because this step was too slow.6
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Table 3. Calculated rate constants, obtained from global fits to time-resolved spectra, and activation
parameters for the formation of putative superoxo 2Quino (k1calcQuino), peroxo-bridged 3Quino (k2calcQuino),
and oxo-bridged 6Quino (k3calcQuino) in the reaction between 1Quino and O2 in CH3CN
The proposed mechanism for the formation of quino peroxo 3Quino and its conversion
to mono oxo-bridged 6Quino (Scheme 1), summarized in equations (4)-(6), was verified
using global fits to the time-resolved spectra using ReactLab. The rate constants
Figure 4. Dioxygen reactivity of MnII 1Quino, showing rate constant labeling scheme, as well as observed
versus unobserved intermediates.
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(k1calcQuino, k3calcQuino) obtained from these global fits (Table 3) show that the rate at which
O2 binds to 1Quino (k1calcQuino, Table 3) is roughly equivalent to the rate at which peroxo
3Quino is observed to form (k1’Quino, Table 2, Figure 4). In other words, the rate-
determining step in peroxo 3Quino formation involves O2 binding. Consistent with this, the
rate at which superoxo 2Quino converts to peroxo 3Quino (k2Quino) is two orders of
magnitude faster (Table 3) than the rate at which it forms (k1Quino), making it impossible
to observe (Figure 4). Calculated spectra (Figure S8) are in good agreement with the
experimentally measured spectrum (Figure 2).48 A comparison with our previously
reported system,6 shows that the rate at which O2 binds to 1Quino (30.0(8) M-1 s-1, Table 3)
is two orders of magnitude slower than the rate at which O2 binds to 1Mepy (3.8(2) x 103
M-1 s-1), at 263 K. The rate at which superoxo 2Quino converts to peroxo 3Quino (1.6(6) x
104 M-1 s-1, Table 3), on the other hand, is two orders of magnitude faster (at 263 K) than
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the conversion (Figure 1) of superoxo 2Mepy to
Table 4. Temperature-Dependent Rate-
Constant for O2 Release* from 2Quino, and
peroxo 3Mepy (k2Mepy = 4.17(3) x 102 M-1 s-1).6 It is
Temperature-Dependent
Equilibrium
Constant for Reversible O2 Binding to
1Quino
Temperature
(K)
273.15
263.15
253.15
243.15
Quino
worth noting that the rate constant k2
is less
Quino
Quino
k–1
(s-1)
Keq
(M-1)
reliable than k3Quino, since it was calculated using
9.8 x 10-2 640
6.1 x 10-2 890
3.3 x 10-2 1500
1.9 x 10-2 1900
global fits. The non-zero, and increasing value
of the y-intercept with temperature in the
Quino
k1obs
’
versus [O2] plots of Figure S4 indicates that the rate-determining step, i.e., O2
binding to 1Quino, is reversible. One can obtain the rate constant associated with the
release of O2 from the intercept of these plots, and then calculate the temperature-
Quino
dependent equilibrium constants, Keq
= k1Quino/k-1Quino, summarized in Table 4.
Previously reported superoxo 2Mepy, on the other hand, does not release O2 once it
binds.6 Thermodynamic parameters for O2 binding to 1Quino (ΔH = -20 kJ mol-1 and ΔS =
-23 J mol-1 K-1) were obtained from a van’t Hoff plot (Figure S9). The negative entropy
value is consistent with an associative process. The activation parameters for O2
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release from 2Quino Ea = 30(5) kJ mol-1, ΔH‡ = 28(3) and ΔS‡ = -160(9), were obtained
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from Arrhenius (Figure S10) and Eyring (Figure S11) plots, respectively. Although the
negative entropy of activation is not what one would expect for a dissociative process, it
indicates that the barrier associated with the bond rearrangement required for electron
transfer (Mn(III)→Mn(II)) is large enough to offset the entropy gained via O2 release.
The Keq for O2 binding to 1Quino is four orders of magnitude larger than Keq for O2 binding
to [(py)MnII(TPP)] (Keq(298 K)= 2.3 x 10-2 M-1),41 but comparable to Keq for O2 binding to
[CoII(SalMeDPT))] (7, Keq(233 K)= 1.63 x 103 M-1).50 The kinetic barrier to O2 binding,
ΔH‡, is 3.4 kJ mol-1 higher than that of 7,50 but 7 kJ mol-1 lower than O2 binding to
MbFe(II) (Mb= myoglobin).51 The barrier to O2 release is 13 kJ mol-1 higher than that of
7-O2,50 and 9 kJ mol-1 higher than that of MbFe-O2.51
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Activation parameters (Tables 2 and 3) for peroxo 3Quino formation (Figures S12 and
S13), O2 binding to 1Quino (Figures S14 and S15), as well as superoxo 2Quino → peroxo
3Quino conversion (Figures S16 and S17), were obtained from Eyring and Arrhenius plots.
The largest errors are associated with the conversion of superoxo 2Quino to peroxo 3Quino
,
since the superoxo intermediate is not directly observed. The enthalpy of activation
(∆H1‡Mepy) associated with O2 binding to 1Mepy (26(2) kJ/mol)6 is 1.6 times greater than
that for O2 binding to 1Quino (16(3) kJ/mol, Table 3). A negative entropy of activation
(∆S1‡Mepy, Table 3) is seen with both 1Mepy and 1Quino, consistent with an associative
process. With the 6-Me-pyridine ligand system, the barrier to the conversion of
superoxo 2Mepy to peroxo 3Mepy (Ea = 49 kJ mol-1) is significantly higher than the barrier to
O2 binding (Ea = 26(2) kJ mol-1), explaining why the superoxo intermediate is observed.
For the quinoline system, both of these barriers are significantly lower (Ea = 18(3) kJmol-
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1, Figure S15), and Ea = 17(5) kJ mol-1 (Figure S17), respectively), and the reaction
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landscape is relatively flat, explaining why it is more difficult to trap superoxo 2Quino
.
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The Barrier to Peroxo O-O Bond Cleavage. With the 6-Me-pyridine ligand
scaffold, cleavage of the peroxo O-O bond of 3Mepy was determined to be slow and rate-
limiting, making it difficult to obtain information regarding the mechanism of 3Mepy to
mono oxo-bridged 6Mepy conversion (Figure 1).6 In contrast, the rate at which the less
stable quinoline peroxo, 3Quino converts to mono-oxo bridged 6Quino (Table S5) is an
order of magnitude faster (vide infra), allowing us to obtain stopped-flow kinetics data for
this step. Calculated rate constants (k3calcQuino) obtained from global fits (Table 3) are in
good agreement with the experimentally determined rate constants (k3Quino, Table 2).
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Activation parameters (Table 2)
for peroxo O-O bond cleavage
were obtained from Eyring and
Arrhenius plots (Figures 5 and
Figures S18 - S20). Although
activation parameters are not
available for comparison to other
Figure 5. Eyring plot for cleavage of the peroxo O-O bond of 3Quino
Mn-peroxo
compounds,
the
and its conversion to mono oxo-bridged 6Quino in CH3CN, from which
Quinoǂ
activation parameters H3
= 13(1) kJ mol–1 and S3Quinoǂ = −
barrier to cleavage of the peroxo
Quino
220(4) J mol–1 K–1, were obtained. First-order rate constants k3
were obtained from stopped-flow experiments.
O-O bond of 3Quino is comparable
to the barrier to homolytic O-O bond cleavage for Fe hydroperoxo [(TMC)FeIII-OOH]2+.52
If Mn peroxo 3Quino follows a pathway (Figure 4) analogous to that of Cu- and Fe-
peroxo compounds,13-19 then homolytic O-O bond cleavage would afford a high-valent
bis-oxo-bridged compound, LQuinoMnIV(µ-O)2MnIVLQuino (4Quino). Two electrons, and two
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protons, or, two H-atoms, would then be required to convert this high-valent bis-oxo
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intermediate to mono-oxo bridged LQuinoMnIII (µ-O) (6Quino). We were unable to observe
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intermediates following quinoline peroxo 3Quino, however, even at temperatures as low as
-120 ˚C (in Me-THF). This implies that O-O bond cleavage is the rate-determining step
in this case. In order to increase the stability of high-valent intermediates and increase
the likelihood of observing intermediates beyond the peroxo, we synthesized a more
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electron-rich ligand system, containing a 6-MeO-pyridine substituent, LMeOpy
.
Synthesis of a New MnII Complex Containing a Less Sterically Encumbered,
More Electron-Donating Pyridine Substituent. The pentadentate ligand LMeOpy, which
is a structural derivative of previously reported LMePy, was synthesized according to the
procedure outlined in the supplemental material. This ligand features a 6-methoxy-
pyridine substituent, which is simultaneously more electron-donating and less sterically-
encumbering than LMePy (A-value = 0.6 versus 1.7 for MeO and Me, respectively).53 The
2+
corresponding MnII complex, [MnII(LMeOpy)]2 (1MeOpy), was synthesized via a metal-
templated Schiff-base condensation between
N,N-bis(6-methoxy-2-pyridilmethyl)ethane-1,2-
diamine and 3-methyl-3-mercapto-2-butanone, as
previously described for 1Mepy and 1Quino 48
.
Crystallographic characterization showed that the
MnII complex derived from this ligand, 1MeOpy, is
dimeric in the solid state (Figure S21). However,
its magnetic moment (μeff = 5.88 B.M., CH3CN,
Figure 6. ORTEP diagram of mono oxo
bridged 6MeOpy formed in the reaction
between 1MeOpy and O2. Hydrogens have
been omitted for clarity.
298 K), EPR spectrum (hyperfine splitting 90 G, Figure S22), and solution-state ESI-MS
(Figure S23), and cyclic voltammogram (Figure S24) all indicate that it is monomeric in
solution.
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Reactivity of 1MeOpy with
Dioxygen. Like complexes
9
1Mepy and 1Quino, reduced 1MeOpy
rapidly reacts with O2 (Scheme
1) under ambient conditions to
ultimately afford binuclear mono
oxo-bridged {[MnIII(LMeOpy)]2(µ-
O)}2+ (6MeOpy), characterized by
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Figure 7. Electronic absorption spectrum of peroxo
intermediate 3MeOpy formed in the reaction between
1MeOpy and O2 (5 mL dry O2 gas) in CH3CH2CN at -73 ˚C.
its λmax = 565 nm (흴 = 1030 M-1 cm-1) in the electronic absorption spectrum (Figure S25),
the ORTEP of which is shown in Figure 6. The mean Mn•••NAr separation in 6MeOpy is
0.15 Å shorter than that of 6-Me-pyridine 6Mepy, and 0.18 Å longer than that of 6-H-
pyridine 6py (Table 1), indicating, based on previously established trends,54 that the
corresponding peroxo derivative should have a weaker O-O bond, but will likely be
observable (vide supra). In addition, the more
electron-rich metal ion should be capable of
stabilizing high-valent intermediates (vide infra).
Consistent with this, four metastable intermediates
are observed en route to mono oxo-bridged 6MeOpy
.
At low temperatures (-73 ˚C) a metastable green
intermediate, 3MeOpy, (λmax(CH3CH2CN) = 612 nm,
λmax(CH2Cl2) = 630 nm) is observed in the reaction
between 1MeOpy and O2 (Figure 7). Given the
similarity of this spectrum (Figure 7) to that of
Figure 8. DFT optimized structure of
peroxo-bridged 3MeOpy. O-O = 1.518 Å,
Mn-Oavg = 1.86 Å, Mn-Savg = 2.33;
Mn(1)•••Mn(2) = 4.361 Å, Mn-O-Oavg
106˚.
=
peroxo-bridged 3Mepy (λmax = 640 nm, Figure 2)6 and peroxo-bridged 3Quino (λmax = 660 nm,
Figure 2), this intermediate was tentatively assigned as a peroxo-bridged dimer,
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{[MnIII(LMeOpy)]2(μ-O2)}2+ (3MeOpy). DFT and TD-DFT calculations were found to support
this assignment. Metrical parameters (Table S6) for the DFT optimized structure,
calculated using the spin-unrestricted B3LYP hybrid functional and the 6-311G basis set,
are consistent with 3MeOpy being a trans-μ-1,2-peroxo-bridged MnIII dimer (Figure 8),
analogous to 3Mepy and 3Quino. The TD-DFT calculated electronic absorption spectrum
(Figure S26) is in good agreement with the experimental spectrum of Figure 7, and the
calculated exchange coupling constant, Jcalc = –0.53 cm-1, indicates that the two MnIII
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ions are essentially uncoupled.
The
calculated O-O bond length (1.518 Å) is
0.02 Å longer than that of 3Quino, and 0.07
Å longer than that of 3Mepy (O-O = 1.452(5)
Å),6 indicating that this bond should cleave
more readily in 3MeOpy. Consistent with this,
3MeOpy is less stable than 3Mepy and 3Quino
,
Figure 9. Time-resolved electronic absorption spectra, for
dioxygen binding to 1MeOpy in CH3CH2CN at -80 ˚C,
obtained using a stopped-flow instrument, under pseudo
1st order conditions with excess O2 ([1MeOpy] = 0.375 mM,
[O2] = 8 mM after mixing). Inset: kinetic trace obtained at
and is not readily observed at
λ
max = 504 nm and 612 nm.
temperatures above -73 ˚C unless a
stopped-flow instrument is used.
Kinetics data for O2 binding to MnII 1MeOpy and the formation of putative peroxo 3MeOpy
were collected at low temperatures using a stopped-flow instrument. The build-up and
decay of putative superoxo 2MeOpy was followed at 504 nm. As illustrated in the time-
resolved electronic absorption spectra of Figure 9, 2MeOpy forms in approximately 3 sec at
-80 ˚C, en route to putative peroxo 3MeOpy (λmax = 612 nm). The electronic absorption
spectrum of 2MeOpy is similar to that of our previously reported superoxo, 2Mepy (Figure 1,
λmax = 515 nm),6 which forms en route to crystallographically characterized 3Mepy
,
suggesting that 2MeOpy is a superoxo, [Mn(LMeOpy)(O2)]+ (2MeOpy). In contrast to 2Mepy
,
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however, 2MeOpy is only cleanly observed if ≥ 20-fold excess [O2] (8 mM [O2] vs 0.375
mM [1MeOpy]), and lower temperatures (-80 ˚C to -62 ˚C) are used (Table S7, Figure S27).
These conditions are likely required in order to ensure that MnII 1MeOpy is consumed by
the excess O2, thereby slowing down the conversion of 2MeOpy to binuclear peroxo-
bridged 3MeOpy (Figures 10 and S27) and increasing its lifetime. The general kinetic
scheme used to analyze the data was the same as that used for 1Mepy.6 Reactions were
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run under pseudo first-order conditions, in the presence of excess O2. Pseudo-first-
MeOpy
order rate constants, k1obs
and k2obsMeOpy, as a function of [O2], temperature, and
[1MeOpy] are assembled in Tables S7 – S9, respectively. The observed rate constant
MeOpy
k1obs
was found to increase linearly (Figure S28) with increasing O2 concentration,
indicative of a 1st order reaction with respect to [O2]. The second order rate constant for
MeOpy
O2 binding k1
was obtained from the slope, and the first order rate constant for O2
release at -80 ˚C, k–1MeOpy, was obtained from the intercept of Figure S28. Intermediates
2MeOpy and 3MeOpy were found to be too unstable at temperatures above -80 ˚C if lower
concentrations of dioxygen (i.e., [O2] < 8.8 mM) are used, requiring that, at higher
MeOpy
temperatures, k–1
parameters be obtained from Global fits. Second order rate
constants for the conversion of superoxo 2MeOpy to peroxo 3MeOpy, k2MeOpy, were also
Figure 10. Dioxygen reactivity of 6-MeO-pyridine 1MeOpy, showing rate constant labeling scheme, and observed
intermediates.
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obtained from Global fits to the ([O2] = 8.8 mM) data using ReactLab. Superoxo 2MeOpy is
not observed at temperatures above -62 ˚C.
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9
Temperature-dependent rate constants for the reaction between 1MeOpy and O2
(Scheme 1) are summarized in supplemental Tables S7-S8, and Tables S11-S12. The
rate constant numbering scheme and observed intermediates are shown in Figure 10.
Dioxygen binding to 1MeOpy (k1MeOpy (211 K) = 70.9(8) M-1 s-1, Table S9, Figure S28) is two
orders of magnitude slower than O2 binding to 1Mepy (k1Mepy (263 K) = 3.8(2) x 103 M-1 s-
1),6 and approximately twice as fast as O2 binding to 1Quino (k1calcQuino (263 K) = 30.0(8) M-1
s-1, Table 3), albeit, at lower temperatures, where entropy favors an associative binding
process. This step is rate determining, with respect to formation of peroxo 3Quino and
3MeOpy, but not 3Mepy. Dioxygen binds reversibly to 1MeOpy, as well as 1Quino (vide supra),
but irreversibly to 1Mepy. Dioxygen release from 2MeOpy is three orders of magnitude
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MeOpy
MeOpy
slower ((k-1
= 2.4(2) x 10-2 s-1, Table S10) than O2 binding (k1
= 70.9(8) M-1 s-1)
at -80 C. Due to the instability of superoxo 2MeOpy at temperatures above -80 C, and the
limitations of the low-temperature stopped-flow instrument, the error associated with the
kinetic parameters, k-1MeOpy, for O2 release is too large to reasonably obtain equilibrium
constants. The apparent low affinity for O2 (based on the intercept of Figure S28), and
the reversibility of O2 binding to 1MeOpy indicates that, consistent with our design, the
electron donating LMeOpy ligand decreases the metal ion Lewis acidity of the 6-MeO-
pyridine dioxygen derivatives. This is reflected in the MnII/III redox potentials for 1MeOpy
versus 1Quino and 1Mepy (vide infra). This is not only an electronic effect, but also a steric
effect since the decreased steric bulk of the 6-MeO-pyridine substituent favors shorter
Mn•••NAr distances, which should increase electron density at the metal ion (vide supra,
Table 1).11,49
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Eyring and Arrhenius plots were used to obtain activation parameters for O2 binding
to 1MeOpy (Figures S29 – S30), O2 release from 2MeOpy (Figures S31 – S32), and the
conversion of superoxo 2MeOpy to peroxo 3MeOpy (Figures S33 – S34), and are compared
with the corresponding parameters for 1Mepy and 1Quino in Table S10. An analysis of the
factors that govern the magnitude of these kinetic barriers shows that both metal ion
Lewis acidity and metal ion accessibility play a role. For example, there is a strong
correlation (R2= 0.999) between the barrier to O2 binding and the cathodic peak potential,
Ep,c,48 an experimental parameter
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that reflects metal ion Lewis acidity:
the higher the redox potential, the
higher the barrier to O2 binding
(Figure 11). This indicates O2
binding is coupled with the
oxidation of the metal ion, as one
Figure 11. Correlation between the activation barrier (Ea) to O2
would expect for a process that
binding and the cathodic peak potential, Ep,c
.
•–
converts O2 to superoxide, O2 .
Peak potentials, as opposed to E1/2 values are compared in Figure 11, since the cyclic
voltammogram wave associated with the 1Quino is irreversible.55 In addition, comparison
of the steric properties shows that the activation barrier decreases as the metal ion
becomes
more
accessible (Figures
12, 13, and S34).
The Mn(II) ion is
more accessible in
1MeOpy, less so in
Figure 12. The steric properties of the ligand influence metal ion
(shown in purple) accessibility, as shown by space filling diagrams of
1Mepy (left), 1Quino(center), and 1MeOpy(right).
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1Quino, and least accessible in the
more sterically encumbered 1Mepy
.
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A similar correlation is observed
between the redox potential and the
kinetic barrier to the conversion of
superoxo 2 to the corresponding
peroxo 3 (Figure 13).
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Figure 13. Correlation between cathodic peak potential,
Ep,c, and the kinetic barrier to the conversion of superoxo
2 to peroxo 3.
Barrier to O-O Bond Cleavage and the Observation of Additional Intermediates.
As noted in the introduction, the steric properties of the substituted N-heterocycles differ
dramatically, and have been shown to influence the barrier to alkylperoxo O-O bond
cleavage.6 The least sterically encumbered complexes contain shorter Mn•••NAr
distances and weaker alkylperoxo O-O bonds. This is because the metal ion becomes
less Lewis acidic as the Mn•••NAr
distance decreases, and this
decreases π-back bonding from the
filled peroxo π*(O-O) orbital to the
metal ion. X-ray emission (XES)
spectroscopically
calibrated
calculations showed that peroxo
Figure 14. Conversion of peroxo 3MeOpy to a metastable
species, 4MeOpy, with 휆max = 805 nm monitored by
transient absorption spectroscopy in CH3CH2CN at -44
˚C. [O2] = 8.8 mM, [1MeOpy] = 0.375 mM, after mixing.
3Mepy adheres to this correlation, as
well.11
Presumably this also
applies to the other dioxygen-derived peroxos, 3Quino and 3MeOpy, described herein
(Scheme 1). Kinetic studies were used to determine whether this was indeed the case.
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Observation of peroxo 3MeOpy, and the metastable intermediates that follow, is
optimized when excess amounts of O2 ([O2] = 8.8 mM vs [1MeOpy] = 0.375 mM) and
higher temperatures (-44 ˚C), are used (Figure 14). These conditions speed up the
9
formation of superoxo 2MeOpy and its conversion to peroxo 3MeOpy
.
Under these
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conditions, peroxo 3MeOpy forms within 1.2 s (Figure 14), and then converts within 21 s to
a metastable species, 4MeOpy. There
is a clean isosbestic point at 730 nm,
indicating that at most two species
contribute to the absorbance at
higher wavelengths. However, at
wavelengths below 525 nm, the
isosbestic points are less clear
Figure 15. Low temperature (6.6 K) perpendicular-mode
X-band (9.645 GHz) EPR spectrum of the Mn(III)Mn(IV)
intermediate, 15, observed following peroxo 12.
because several species (including
microwave power= 0.2 mW, modulation amplitude = 0.75
mT
superoxo 2MeOpy, and peroxo 3MeOpy
)
contribute to this region of the spectrum. Based on global fits to the data 4MeOpy
possesses absorption bands at 805 nm and 497 nm, and a shoulder at 532 nm (Figure
14).
According to the reaction scheme of
Figure 10, the species most likely to form
following peroxo 3MeOpy, would be a high-
valent bis -oxo, [((LMeOpy)MnIV)2(µ2-O)2]2+
(4MeOpy). Conversion of 3MeOpy to 4MeOpy
would involve peroxo O-O bond cleavage,
Figure 16. Comparison of the rates of formation
explaining why higher temperatures are
required. Rate constants for the
and yield of 6MeOpy in CH2Cl2, both in the absence
(green), and presence, of sacrificial H-atom donors
CHD (red) or TEMPOH (blue).
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conversion of 3MeOpy to 4MeOpy were obtained from Global fits to the data. A comparison
of the half-lives of 3MeOpy (k3MeOpy (193 K) = 0.06(3) s-1, t1/2 = 11.5 s), 3Quino (Table S10),
and 3Mepy (Table S10), shows that there is a correlation between the barrier to peroxo O-
O bond cleavage and the experimentally measured redox potential, Ep,cMnIII/II. This effect
is enhanced if one takes into account the fact that these half-lives were measured at
different temperatures, due to the fact that peroxo-bridged 3MeOpy is not observed above
233 K, and temperatures below 233 K were required for an accurate determination of
rate constants for 3Quino due to its instability. The higher the redox potential, the more
Lewis acidic the metal ion, and the more 휋 back-donation from the filled peroxo 휋* orbital
is facilitated. Redox potentials for the MnIII/II couple are used, as opposed to the MnIV/III
couple, because our Mn(IV) derivatives are too reactive (vide infra) to obtain the latter.
The metastable, reactive nature of 4MeOpy contrasts with the majority of bis-oxo bridged
MnIVMnIV dimers,56-58 likely reflecting the electron donating properties of the thiolate.
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Following its formation, putative high-valent bis oxo MnIVMnIV (4MeOpy, λmax = 805 nm)
converts more slowly (Table S9) to metastable red species with λmax = 505 nm, and a
reproducibly observed multi-line ⟂-mode EPR signal (Figure 15). The spectrum of
Figure 15 is well simulated with an S = 1/2 system with a rhombic g tensor with principal
values 1.994, 2.004, and 2.016 coupled to two 55Mn nuclei with isotropic hyperfine
couplings of 325 MHz and 180 MHz.59 These values are consistent with an
antiferromagnetically coupled MnIIIMnIV dimer.60-66 If the reaction between 1MeOpy and O2
is carried out in the presence of 100 equiv. of 1,4-cyclohexadiene (CHD), and then
quenched immediately upon the formation of the red, EPR-active intermediate, then 0.5
equiv. of benzene is detected by GC-MS (Figure S35). This indicates one H atom is
abstracted en route to the red intermediate. If the same reaction is followed to its
completion (Figure 10), then 1.0 equiv. of benzene is detected, indicating that two H
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atoms in total, one before, and the other after the red intermediate (Figure S36), are
abstracted en route to mono oxo-bridged 6MeOpy. In the absence of a sacrificial H-atom
donor the yield of 6MeOpy decreases if it is carried out in CH2Cl2 solvent (Figure 16). In
this solvent, with its strong C-H bonds (98 kcal/mol), near quantitative yields of 6MeOpy
are only observed when sacrificial H-atom donor TEMPOH is added. If TEMPOD is used
in place of TEMPOH, then a deuterium isotope effect (kH/kD = 3.5) is observed (Figure
17). Together these results indicate that the O2-promoted conversion of 1MeOpy to 6MeOpy
requires two e– and two H+ (Figure 10).
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Conclusions. With this study, we have shown that the relative stability of
metastable dioxygen intermediates can be tuned by adjusting the supporting ligands.
Whereas O2 binds irreversibly to
1Mepy, kinetics data shows that O2
binding is less favored for the less
Lewis acidic complexes 1Quino and
1MeOpy
,
and the microscopic
reverse process involving O2
release from superoxo 2Quino and
2MeOpy, is more favorable. The MnII
complexes 1Mepy and 1Quino are
Figure 17.
TEMPOH(D) is used as a sacrificial H-atom donor for the
conversion of 5MeOpy to 6MeOpy
Deuterium isotope effect observed when
.
complementary in that the initial O2 binding step to form a superoxo intermediate is
observable with 1Mepy, but not with 1Quino. Information regarding the barrier to peroxo O-
O bond cleavage can be obtained for peroxo 3Quino, but not for 3Mepy, or 3MeOpy because
they are too slow, or too fast, respectively. The incorporation of a more electron rich,
less sterically encumbering ligand (6-MeO-pyridine) ligand decreases the barrier to O2
binding and release, as well as peroxo O-O bond cleavage, relative to the other
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derivatives. A strong correlation between the MnII/III redox potential and the ligand-
dependent activation barrier to O2 binding and the conversion of superoxo 2 to peroxo 3
is observed. The reaction landscape for the dioxygen chemistry of 1Quino and 1MeOpy is
shown to be relatively flat, compared to that of 1Mepy, with respect to the barriers
separating metastable intermediates. With a more electron-donating, less sterically
encumbered 6-MeO-pyridine ligand, a total of four intermediates are observed at low
temperatures, including two reactive intermediates following peroxo 3MeOpy, en route to
the final mono oxo-bridged dimer product, 6MeOpy. Each of the intermediates following
the peroxo 3MeOpy is shown to cleave strong X-H bonds (X = C, O). Two electrons and
two protons are shown to be required for the O2-promoted conversion of MnII 1MeOpy to
binuclear mono oxo-bridged 6MeOpy. In the absence of a sacrificial H-atom donor, such as
1,4-cyclohexadiene (BDE(C-H) = 76.0 kcal/mol) or TEMPOH (BDE(O-H) = 70.6
kcal/mol) this reaction presumably involves the abstraction of H-atoms from the C-H
bonds (BDE = 94 kcal/mol) of CH3CH2CN solvent. With TEMPOH, a deuterium isotope
effect is observed (kH/kD = 3.5) implicating HAT in each of these steps. The highly
reactive nature of putative MnIVMnIV contrasts with the majority of MnIVMnIV dimers
reflecting the electron donor properties of the thiolate ligand.
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Acknowledgments. This manuscript is dedicated to the memory of Professor Elena
Rybak-Akimova, whose joyful and generous spirit will be with us always. Funding from
the NSF (CHE-1664682) is gratefully acknowledged.
Supporting Information is available free of charge on the ACS Publications
website, and includes an Experimental Section, crystallographic tables for 1MeOpy, 6MeOpy
,
and 3Quino; kobs and DFT optimized metrical parameter tables, kobs vs [O2] or [Mn] plots,
Van’t Hoff plot for O2 binding to 1MeOpy, Eyring and Arrhenius plots, DFT calculated
structures and Mulliken charges, TD-DFT calculated spectra for 3Quino and 3MeOpy
,
ORTEP, ESI-MS, and EPR of 1MeOpy, quantitative UV/vis of 6MeOpy
.
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