Oxygen-atom transfer reactivity of axially ligated mn(V)-oxo complexes: Evidence for enhanced electrophilic and nucleophilic pathways

Article abstract of DOI:10.1021/ja507177h

Addition of anionic donors to the manganese(V)-oxo corrolazine complex Mn^V(O)(TBP₈Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn^V(O)(TBP

Full text of DOI:10.1021/ja507177h

Article
pubs.acs.org/JACS
Open Access on 09/19/2015
Oxygen-Atom Transfer Reactivity of Axially Ligated Mn(V)−Oxo
Complexes: Evidence for Enhanced Electrophilic and Nucleophilic
Pathways
†
†
†
§
,§
Heather M. Neu, Tzuhsiung Yang, Regina A. Baglia, Timothy H. Yosca, Michael T. Green,*
‡
,‡
,†
Matthew G. Quesne, Sam P. de Visser,* and David P. Goldberg*
†
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, United States
Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States
§
‡
The Manchester Institute of Biotechnology and School of Chemical Engineering and Analytical Science, The University of
Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom
*
S Supporting Information
ABSTRACT: Addition of anionic donors to the manganese(V)−oxo
V
corrolazine complex Mn (O)(TBP Cz) has a dramatic influence on
8
oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-
V
−
−
−
coordinate anionic [Mn (O)(TBP Cz)(X)] complexes (X = F , N ,
8
3
−
−1
OCN ) exhibit a ∼5 cm downshift of the Mn−O vibrational mode
relative to the parent Mn (O)(TBP Cz) complex as seen by resonance
V
8
Raman spectroscopy. Product analysis shows that the oxidation of
thioether substrates gives sulfoxide product, consistent with single OAT.
A wide range of OAT reactivity is seen for the different axial ligands, with
the following trend determined from a comparison of their second-order
rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate <
cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT
calculations on the binding of the axial donors to the parent
V
V
−
Mn (O)(TBP Cz) complex. A Hammett study was performed with p-X-C H SCH derivatives and [Mn (O)(TBP Cz)(X)]
8
6
4
3
8
−
−
(
X = CN or F ) as the oxidant, and unusual “V-shaped” Hammett plots were obtained. These results are rationalized based
V
−
upon a change in mechanism that hinges on the ability of the [Mn (O)(TBP Cz)(X)] complexes to function as either an
8
electrophilic or weak nucleophilic oxidant depending upon the nature of the para-X substituents. For comparison, the one-
V
•+
electron-oxidized cationic Mn (O)(TBP Cz ) complex yielded a linear Hammett relationship for all substrates (ρ = −1.40),
8
consistent with a straightforward electrophilic mechanism. This study provides new, fundamental insights regarding the influence
of axial donors on high-valent Mn (O) porphyrinoid complexes.
V
INTRODUCTION
analogous to the highly unstable intermediates in enzyme
systems, including high-valent metal-oxo species. Regarding the
influence of axial donors on these species, a potential advantage
of model systems is the ability to rationally tune the nature of the
axial ligand by varying its structural and electronic properties in a
systematic fashion. The influence of axial ligands on high-valent,
iron-oxo porphyrin models has been investigated, including the
examination of the effects of axial donors on oxygen-atom
transfer (OAT) reactivity. For example, axial ligands were shown
■
A key structural factor that contributes to metal-oxo reactivity in
heme enzymes is the nature of the axial ligands coordinated trans
to the oxo group. The concept of axial ligand tuning is
exemplified in the powerful oxygenating enzyme Cytochrome
P450, in which the anionic, deprotonated Cys ligand is believed
to play a critical role. It has been suggested that the Cys in Cyt-
P450 plays an essential role in modulating the reactivity of
IV
•+
Compound-I (Cpd-I), Fe (O)(porph )(Cys), (porph =
13−21
to enhance the reactivity of Fe(O)(porph) in epoxidations.
A recent study on the effects of axial ligation on Cpd-I analogs,
porphyrin) which is the key intermediate prior to substrate
1
−7
oxidation.
Further evidence for the importance of the axial
IV
•+
Fe (O)(porph )(X), provided a detailed thermodynamic
analysis which led to a possible explanation for the trend in
donor lies in the fact that it is the major feature that distinguishes
broad classes of heme enzymes, with Cyt-P450 and chloroper-
oxidase (CPO) containing an axial Cys, peroxidase containing an
15
OAT reactivity versus axial donor. The mechanisms of these
reactions and especially the exact role played by the axial ligands
are still not well understood.
8
−10
11,12
axial His,
and catalase containing an axial Tyr ligand.
The preparation of synthetic analogs of heme enzymes has
provided a means to test mechanistic hypotheses regarding the
influence of axial donors. Synthetic models of heme active sites
are particularly useful for examining complexes that are
Received: July 15, 2014
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
In contrast to Fe(O) species, the influence of axial donors on
analogous Mn(O) porphyrinoid complexes has been much less
studied, with very few systematic investigations for a wide range
NMR spectrometer at 298 K. All spectra were taken in 5 mm o.d. NMR
tubes, and chemical shifts were reported as δ values from standard peaks.
V
Resonance Raman Sample Preparation. A solution of Mn (O)-
22−30
(TBP Cz) (1 mM) in CH Cl (250 μL) at 25 °C was combined with
8
2
2
of axial ligands reported to date.
One of the few examples
+
−
Bu N X (X = F, N , OCN at 1 M). Samples were loaded into an EPR
4
3
comes from Chang and co-workers, who showed that the
tube, immediately frozen in liquid nitrogen, and stored at 77 K until
measurements were performed.
V
reactivity of an Mn (O) corrole toward styrene can be enhanced
23
by 10-fold with an imidazole as an axial ligand. The effects of
Resonance Raman Spectroscopy. Resonance Raman spectra
were recorded on a triVista 555 triple monochromator (900/900/2400
gr/mm) equipped with a CCD camera (1340 × 100 pixels). A 413.2 nm
line of a krypton-ion laser was used for excitation. The laser power was
<15 mW at the sample to reduce the risk of photoreduction. 900/900/
axial donors on nonheme Mn(O) complexes such as Mn(O)-
31−34
(
salen)(X) have been reported.
However, direct structural,
spectroscopic, or reactivity information on these complexes is in
general quite limited because of the transient nature of these
−1
3
1
2
400 gr/mm gratings provided a resolution of 0.69 cm per CCD pixel
species.
at 413.2 nm. Samples were held in an EPR finger-dewar (77 K) in a 135°
backscattering arrangement. Raw spectra were analyzed using the
program Igor Pro for background subtraction. No smoothing
procedures were performed on the raw data.
Previously, we took advantage of our ability to isolate and
V
definitively characterize a relatively stable high-valent Mn (O)
V
complex Mn (O)(TBP Cz) (TBP Cz = octakis(p-tert-
8
8
3
−
butylphenyl)corrolazinato ), which contains a ring-contracted
porphyrinoid ligand, in order to examine the influence of cyanide
and fluoride ions on C−H activation. Indirect evidence was
obtained that suggested CN and F were weakly bound trans to
the terminal oxo ligand and caused significant increases in H-
atom abstraction rates. More recently, we used X-ray
absorption spectroscopy to characterize one of these complexes,
Product Analysis. In a typical reaction, a stirring solution of
V
Mn (O)(TBP Cz) (0.2 or 5 mM) in CH Cl (3 mL) was combined
8
2
2
+
−
with an anionic donor Bu N F (1.3 or 12 mM) at 25 °C. To initiate the
reaction, dimethyl sulfide (DMS): 11 M or triphenylphosphine (PPh ):
5 mM was added. Isosbestic conversion of [Mn (O)(TBP
λmax = 419, 634 nm) to [Mn (TBP Cz)(F)] (λ = 428, 471, 680
4
−
−
3
V
−
Cz)(F)]
8
3
5
III
−
(
8
max
nm) was observed by UV−vis spectroscopy. Upon completion of the
V
−
−
reaction, the solution was concentrated to ∼50 μL under vacuum. The
[
Mn (O)(TBP Cz)(CN)] , and showed that CN was indeed
8
31
V
phosphine reaction was dissolved in CDCl
3
and analyzed by P NMR.
coordinated to the metal while leaving the short Mn −O bond
intact. Addition of the CN donor caused a large rate
enhancement in OAT to thioanisole, giving the predicted two-
The yield was obtained by comparing the integration of the product
3
6
−
peak, triphenylphosphine oxide, to that of the reactant peak (PPh ). To
3
analyze the DMS reaction, n-decane was added as an internal standard,
and the solution was immediately analyzed by GC-FID. The product,
dimethyl sulfoxide (DMSO), was identified by comparison with a
standard sample. Yields were obtained by comparing the integration of
the product peak with the integration of the internal standard peak. All
yields are an average of two runs.
III
electron reduced Mn complex and sulfoxide product.
Herein we describe a comprehensive study regarding the
influence of a wide range of anionic axial ligands on the OAT
V
reactivity of an Mn (O) porphyrinoid complex. Significant rate
enhancements for OAT to thioether substrates are seen for
V
−
OAT Reaction of [Mn (O)(TBP Cz)(X)] with DMS and Dibutyl
8
different axial donors. The use of a series of p-X-C H SCH
6
4
3
V
Sulfide. In a typical reaction, a solution of Mn (O)(TBP Cz) (11 μM)
in CH Cl (2 mL) at 25 °C was combined with Bu N X (11 mM) (X =
N , OCN, SCN, NO ) or Bu N F ·3H O (0.03 M). Excess DMS or
8
substrates provides a range of OAT rates for the cyanide- and
+ −
V
2
2
4
fluoride-ligated Mn (O) complexes, revealing unexpected “V-
+ −
3
3
4
2
shaped” Hammett plots. These results point to a dramatic switch
in mechanism for OAT, depending upon the nature of the
substrate. This work provides new, fundamental insights
regarding the influence of axial donors on the OAT reactivity
dibutyl sulfide (DBS) was added to begin the reaction. The changes in
absorbance were monitored by UV−vis spectroscopy showing the
V
−
isosbestic decay of [Mn (O)(TBP Cz)(X)] (λ = 419, 634 nm) and
8
max
III
−
formation the of [Mn (TBP
spectrometer light and the sample to prevent bleaching of [Mn (O)-
TBP Cz)(X)] from UV light (<400 nm). The pseudo-first-order rate
Cz)(X)] . A filter was placed between the
8
V
V
of rarely observed Mn (O) porphyrinoid complexes.
−
(
8
constants, kobs, were obtained by nonlinear least-squares fitting (eq 1) of
EXPERIMENTAL SECTION
Materials. All reactions were performed under an argon atmosphere
III
−
■
the growth in absorbance of [Mn (TBP
decay of [Mn (O)(TBP Cz)(X)] at 634 nm versus time:
8
8
Cz)(X)] at 680−693 nm or
V
−
using dry solvents and standard Schlenk techniques. The
III
V
Mn (TBP Cz) and Mn (O)(TBP Cz) complexes were synthesized
Abst = Absf + (Abs0 − Absf ) exp(−kobst)
(1)
8
8
37
according to published methods. Solvents were purified via a Pure-
Solv solvent purification system from Innovative Technologies, Inc.
H2 O (97% O) was purchased from Cambridge Isotopes, Inc. All
other reagents were purchased from Sigma-Aldrich at the highest level of
purity and used as received.
where Abs = final absorbance, Abs = initial absorbance, and Abs =
f
0
t
1
8
18
absorbance at time (t). Second-order rate constants were obtained from
the slope of the best-fit line of the linear plot of kobs versus substrate
concentration.
Instrumentation. UV−vis spectroscopy was performed on a
Hewlett-Packard 8542 diode-array equipped with HP Chemstation
software. A filter was placed between the spectrometer light and the
Measurement of the Binding Constant for Fluoride to
V
V
Mn (O)(TBP
CH Cl
reaction was initiated by the addition of Bu N F ·3H O (1.7−59 mM).
4 2
Cz). A solution of Mn (O)(TBP Cz) (10 μM) in
8
8
(2 mL) at 25 °C was combined with DBS (2.7 mM). The
2
+
2
V
−
−
sample to prevent bleaching of [Mn (O)(TBP Cz)(X)] from UV light
8
(
<400 nm). The temperature-dependent kinetics were performed on a
The changes in absorbance were monitored by UV−vis spectroscopy
V
−
Hewlett-Packard 8453 diode-array spectrophotometer equipped with an
Unisoku thermostat cell holder. Gas chromatography (GC) was
performed on an Agilent 6850 gas chromatograph fitted with a DB-5
showing the isosbestic decay of [Mn (O)(TBP
8
Cz)(F)] (λmax = 419,
III
−
634 nm) and formation of [Mn (TBP
Cz)(F)] (λmax = 469, 680 nm).
8
The pseudo-first-order rate constants, kobs, were obtained by nonlinear
5
% phenylmethyl siloxane capillary column (30 m × 0.32 mm × 0.25
least-squares fitting (eq 1) of the growth in absorbance of
III
−
μm) and equipped with a flame-ionization detector. GC mass
spectrometry (GC-MS) was performed on an Agilent 6850 gas
chromatograph fitted with a DB-5 5% phenylmethyl siloxane capillary
column and equipped with a mass spectrometer. LDI-TOF was
conducted on a Bruker Autoflex III TOF/TOF instrument equipped
with a nitrogen laser at 335 nm using an MTP 384 ground steel target
plate. 31P NMR spectra were recorded on a Bruker Avance 400 MHz
[Mn (TBP
[F ] showed saturation behavior.
8
Cz)(F)] at 680 nm versus time. A plot of kobs versus
−
The data were modeled for a 1:1 binding event as shown in eq 2,
where the association constant K = k /k , k is the pseudo-first-order
1
−1
2
−
rate constant for a given concentration of DMS, and [F ] is the total
concentration of TBAF. A fit of 1/kobs and 1/[F ] plot is given in eq
total
3
8
−
total
3.
B
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
−
⎛
⎝
⎞
⎟
⎠
Scheme 1. Oxidation of Thioether and Phosphine Substrates
K[F ]
total
kobs = k ⎜
V
−
2
−
by [Mn (O)(TBP Cz)(F)]
1 + K[F ]
8
total
(2)
(3)
−
1
1 + K[F ]
1
1
k2
total
=
=
+
−
−
kobs
k K[F ]
k K[F ]
total
2
total
2
V
Linear Free Energy Correlations for OAT of [Mn (O)(TBP Cz)-
8
−
V
(
(
X)] to para-Substituted Thioanisoles. A solution of Mn (O)-
TBP Cz) (12 μM for CN ; 15 μM for F ) in toluene (X = CN ) or
CH Cl (X = F ) (2 mL) at 25 °C was combined with a para-substituted
thioanisole (0.13 M). The reaction was initiated by the addition of
−
−
−
8
−
2
2
+
−
−
−
Bu N X (12 mM, X = CN ; 15 mM, X = F ). The changes in
4
absorbance were monitored by UV−vis spectroscopy showing the
V
−
isosbestic decay of [Mn (O)(TBP Cz)(X)] (λ = 419, 634 nm) and
8
max
III
−
−
formation of [Mn (TBP Cz)(X)] (λ = 443, 492, 694 nm, X = CN ;
λmax = 470, 679 nm, X = F ). The pseudo-first-order (kobs) rate constants
were obtained as described above. The log(k /k ) for each para-
substituted thioanisole was graphed versus its Hammett constant (σ).
Linear Free Energy Correlations for OAT of Mn (O)(TBP Cz )
to para-Substituted Thioanisoles. Following an earlier report,
8
max
−
X
H
3
9
V
•+
8
4
0
a
V
solution of Mn (O)(TBP Cz) (11 μM) in CH Cl (2 mL) at 25 °C was
combined with a one-electron oxidant [(4-BrC H ) N ](SbCl ) (11
8
2
2
•+
−
6
4
3
6
μM) and monitored by UV−vis spectroscopy which showed the
V
isosbestic conversion of Mn (O)(TBP Cz) (λ = 419, 634 nm) to
8
max
V
•+
[
Mn (O)(TBP Cz )] (λ = 410, 780 nm). Once complete formation
of the Mn (O)(TBP Cz ) complex was observed the OAT reaction
8
max
•+
V
8
was initiated by the addition of a para-substituted thioanisole (0.13 M).
The changes in absorbance were monitored by UV−vis spectroscopy
V
•+
showing the isosbestic decay of [Mn (O)(TBP Cz )] (λ = 410, 780
8
max
IV
+
nm) and formation of [Mn (TBP Cz)] (λ = 433, 669, and 722 nm).
8
max
The pseudo-first-order (kobs) rate constants were obtained by nonlinear
least-squares fitting (eq 1) of the growth in absorbance of
IV
+
[
Mn (TBP Cz)] at 722 nm versus time. The log(k /k ) for each
8
X
H
para-substituted thioanisole was graphed versus its Hammet constant
39
(σ).
RESULTS AND DISCUSSION
■
V
−
OAT Reactivity of [Mn (O)(TBP Cz)(F)] with Thioether
and Phosphine Substrates. Kinetic analysis showed that the
oxidation of thioether substrates by Mn (O)(TBP Cz) was
8
V
8
−
3
relatively slow, with second-order rate constants of 2.0 × 10
−
1
−1
−4
−1 −1
40
M
s
for DMS and 3.8 × 10
M
s
for DBS. The addition
−
V
of CN to Mn (O)(TBP Cz) dramatically increased the rates of
8
−
1 −1
sulfoxidation, giving a second-order rate constant of 9.2 M
s
3
6
for DBS. We next sought to extend our study on the influence
of anionic donors and determine whether a correlation with
reactivity and the nature of the axial donor could be made.
Investigations were initiated by examining the reactivity of the
Figure 1. (a) UV−vis spectral changes (0−1.5 h) for the reaction of
V
+ −
Mn (O)(TBP
Cz) + Bu N F (6 equiv) (419, 634 nm) with excess
4
8
III
−
+
−
V
DMS to give [Mn (TBP Cz)(F)] (428, 471, 680 nm) in CH Cl at 25
fluoride anion. Addition of Bu N F (TBAF) to Mn (O)-
8
2
2
4
III
°
(
(
C. (b) UV−vis spectral titration of Mn (TBP Cz) (435, 685 nm)
(
TBP Cz) in the presence of excess DMS as substrate was
8
8
+
−
III
−
blue line) with Bu N F (0−1 equiv) to give [Mn (TBP Cz)(F)]
4
8
monitored in CH Cl at 25 °C (Scheme 1). The UV−vis spectra
are shown in Figure 1a and reveal isosbestic conversion of the
Mn -oxo complex to the axially ligated complex
2
2
430, 472, 681 nm) in CH Cl at 25 °C.
2
2
V
III
−
[Mn (TBP Cz)(F)] , with a split Soret band at 428 and 471
An alternate O-atom acceptor, PPh , showed similar reactivity,
3
8
V
−
nm. As previously observed, there is little change in the UV−vis
resulting in the rapid conversion of [Mn (O)(TBP Cz)(F)] to
8
V
−
III
−
31
spectrum for Mn (O)(TBP Cz) upon addition of F , but
[Mn (TBP Cz)(F)] . The P NMR spectrum (Figure S1) of
8
8
splitting in the Soret region of the corresponding
the reaction mixture identified OPPh as the major product in
89% yield (Scheme 1). Thus, the Mn (O) complex, in the
presence of TBAF, reacts rapidly and quantitatively with both
thioether and phosphine O-atom acceptors to give sulfoxide or
phosphine oxide products via a formal OAT process.
3
III
−
35,36
V
[
Mn (TBP Cz)(F)] complex is clearly seen in Figure 1b.
8
III
A related complex (Et N)[Mn (TBP Cz)(Cl)] was crystallo-
4
8
graphically characterized and exhibited a similar splitting in the
3
7,41
Soret band.
With the addition of excess TBAF, the OAT
reaction was complete in 1.5 h, and analysis by GC-FID revealed
Dependence of OAT Rates on Fluoride Concentration.
DMSO as the major oxidation product (85%). In comparison,
In earlier work we showed that the rates of H-atom abstraction by
V
V
the oxidation of DMS by the five-coordinate Mn (O)(TBP Cz)
Mn (O)(TBP Cz) with C−H substrates depended on the
8
8
35
is much slower, taking over 17 h for completion.
concentration of added TBAF. A plot of pseudo-first-order rate
C
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
constants (k ) versus [TBAF] for two different C−H substrates
the 413.2 nm krypton-ion excitation line at 77 K. A Mn−O
obs
V
gave a saturation curve that was successfully modeled as a 1:1
vibrational mode for the five-coordinate Mn (O)(TBP Cz)
8
35
−1
binding equilibrium. In the present study, the rates of
sulfoxidation of DBS under pseudo-first-order conditions were
similarly monitored by UV−vis, and the concentration of TBAF
was varied. Plots of the absorbance associated with the decay and
starting complex was identified at 981 cm , which downshifts to
939 cm− upon O substitution (Figure S3). This 42 cm shift
1
18
−1
is in good agreement with diatomic harmonic oscillator
4
2
predictions as well as previously published results. The spectra
V
III
−
−
−
growth of the Mn and Mn complexes, respectively, were well-
for the F , N , and OCN complexes are shown in Figure 3,
3
fit to a single exponential kinetic model (Figure S2). The
−
resulting kobs values were plotted versus [F ] (Figure 2a) and
1
6
V
Figure 3. Low-frequency resonance Raman spectra of O [Mn (O)-
−
−
−
−
(
TBP Cz)(X)] where X = no ligand, F , N , or OCN . The
8
3
−1
resonances near 900 cm were used to scale the spectra. Data were
collected using a 413.2 nm krypton-ion laser line.
together with the five-coordinate starting material. The Mn−O
mode clearly downshifts by 5 cm− to 976 cm relative to the
1
−1
−1
starting complex. The identity of the peak at 976 cm was
18
−
−
verified using O labeling for the F and N complexes (Figures
3
S4 and S5). These results provide strong evidence that the
anionic donors are coordinated trans to the terminal oxo ligand,
thereby inducing a subtle weakening of the Mn−O vibrational
mode. Similar effects on the metal-oxo stretch were seen upon
variation of trans axial donors in both heme and nonheme
Figure 2. (a) Dependence of the k values on the concentration of
obs
+
−
V
Bu N F for the reaction of Mn (O)(TBP Cz) + DBS (270 equiv) in
4
8
−
CH Cl at 25 °C. (b) Double-reciprocal plot of 1/k versus 1/[F ]
blue circles) and best fit line (black).
2
2
obs
IV
15,43,44
(
Fe (O) complexes.
Kinetic Studies of OAT for a Series of Anionic Axial
−
Donors. A broad series of anionic axial ligands (X ), in
conjunction with two thioether substrates (DMS and DBS), were
employed for kinetic studies (Scheme 2). An example of the
exhibit saturation behavior, indicative of a rapid pre-equilibrium
−
for the binding of F prior to the rate-determining step. A double
−
reciprocal plot (1/kobs versus 1/[F ]total) (Figure 2b) reveals
good linearity, with a best fit that gives a binding constant of
−
−1
Scheme 2. Kinetic Model for OAT to Thioether Substrates
K(F ) = 115 ± 4 M . This value is in good agreement with the
binding constants found previously for fluoride anion and the
V
Mn (O) complex in reactions with C−H substrates (163 ± 7 and
−
1
35
1
91 ± 10 M ). These results provide strong evidence that
−
addition of excess F leads to the formation of a six-coordinate
Mn (O) complex with the fluoride ligated relatively weakly trans
V
to the oxo group.
Resonance Raman Spectroscopy of the Six-coordinate
V
−
V
Mn (O) Complexes. The binding of X to Mn (O)(TBP Cz)
8
can be described according to the simple equilibrium in eq 4:
UV−vis spectral changes observed for these OAT reactions is
V
−
V
−
−
Mn (O)(TBPCz) + X ⇌ [Mn (O)(TBPCz)(X)]
shown in Figure 4, where X = N . Good isosbestic behavior is
8
8
3
III
five‐coordinate
six‐coordinate
seen, and the final spectrum matches that for [Mn (TBP Cz)-
N )] . Plots of absorbance versus time for the decay of the
Mn (O) complex (634 nm) and the appearance of the
8
(
4)
−
(
3
V
The six-coordinate complexes were examined by resonance
−
−
−
−
III
−
Raman spectroscopy for X = F , N , and OCN . Excess X was
Mn (N ) complex (685 nm) are shown in the inset, Figure
3
3
added to push the equilibrium in eq 4 to favor the six-coordinate
complexes in CH Cl . Frozen solution samples were excited with
4. These plots, along with the plots for the other axial donors,
were well fit by a single exponential model, consistent with the
2
2
D
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
V
Figure 4. UV−vis spectral changes for the reaction of [Mn (O)-
−
(
TBP Cz)(N )] (9 μM) (419, 634 nm) with DMS (4.0 mM) in
8 3
CH Cl at 25 °C. Inset: changes in absorbance versus time for the
2
2
III
−
growth of [Mn (TBP Cz)(N )] (685 nm) (open red circle) and the
decay of [Mn (O)(TBP Cz)(N )] (634 nm) (open blue square)
8
3
V
−
8
3
together with best fits (solid black line).
mechanism shown in Scheme 2. The resulting fits yielded
pseudo-first-order rate constants (k ), which were found to be
obs
linearly dependent on substrate concentration (Figure 5). The
best-fit lines of kobs versus [substrate] yielded second-order rate
constants (k) (Table 1) for the different axial donors.
As can be seen from Table 1, there is a remarkable
V
enhancement in OAT reactivity for the Mn (O) complex in
the presence of anionic donors. The second-order rate constant
−
for F reveals a dramatic rate enhancement of 1100-fold in the
V
oxidation of DMS compared to the five-coordinate Mn (O)
−
−
complex. A similar rate enhancement (1700) is seen for the
Figure 5. (a) Plots of k versus concentration of DBS for X = CN
obs
−
(solid blue triangle), F (solid red circle), and SCN− (solid green
diamond). Inset: expanded region from 0 to 0.05 M. (b) Plots of kobs
−
oxidation of DBS. Addition of CN as the axial donor has an even
more dramatic effect, revealing a rate enhancement of 24,000 for
V
−
−
−
−
versus concentration of DMS for X = F (solid red circle), N (solid
DBS oxidation by the [Mn (O)(TBP Cz)(CN)] complex.
3
8
−
−
purple diamond), OCN (solid blue square) and NO (solid green
Further examination of the other anionic axial donors in Table
3
−
triangle). Inset: expanded region from 0 to 0.045 M.
1
reveals some important trends. The addition of azide (N )
3
leads to a 70-fold rate enhancement over the five-coordinate
complex in the sulfoxidation of DMS. The cyanate (OCN )
−
Table 1. Second-Order Rate Constants for OAT to Thioether
Substrates
anion gives a more modest increase of only 1.5 times for reaction
−
with DMS, and the nitrate (NO ) anion showed essentially no
k (M−1 s−1
V
−
V
3
axial ligand substrate
)
k[Mn (O)(X)] /k[Mn (O)]
−
influence. The thiocyanate (SCN ) anion also has little effect
ab
none ,
3.8 × 10−4
9.2 ± 0.3
0.63 ± 0.02
DBS
DBS
DBS
DBS
DMS
DMS
DMS
DMS
DMS
over the reaction rate with DBS as substrate. Taken together, the
CN ,
ac
24,000
1700
0.4
kinetic data indicate the following trend regarding the influence
F
−
−
of the anionic donors on the rate of OAT: none ≈ SCN ≈ NO
−4
−3
3
SCN
(1.5 ± 0.1) × 10
(2.0 ± 0.2) × 10
2.3 ± 0.3
0.14 ± 0.01
(2.9 ± 0.3) × 10
(1.5 ± 0.1) × 10
−
−
−
−
<
OCN < N < F ≪ CN . This trend is supported by our
DFT calculations and suggests a possible structural explanation
for the relative influence of X on the reactivity of Mn (O)-
b
3
none
F
1100
70
−
V
N3
(
TBP Cz) (vide infra).
Influence of para-Substituents on the Reactivity of
−3
−3
8
OCN
NO3
1.5
0.75
Thioanisole Derivatives. The reactive cyanide-ligated
a
b
c
V
−
In toluene. Ref 40. Ref 36.
[
Mn (O)(TBP Cz)(CN)] complex was chosen for further
8
study with a series of p-X-C H SCH derivatives (Scheme 3).
6
4
3
III
−
The oxidation of C H SCH proceeds in good yield (84%) to
[Mn (CN)] product was seen for all substrates. The resulting
6
5
3
give the mono-oxygenated sulfoxide product, with 71% of the
pseudo-first-order rate constants (k ) for the different para-
X
1
8
V 18
−
O label in [Mn ( O)(TBP Cz)(CN)] transferring to the
substituted derivatives were used to construct the Hammett plot
shown in Figure 6. For the electron-donating substituents, there
is a good linear correlation with a clear negative slope of rho (ρ) =
−1.29. This trend is easily explained by a mechanism involving
8
3
6
sulfur atom. The reactions between the p-X-substituted
thioanisole derivatives and [Mn (O)(TBP Cz)(CN)] were
V
−
8
monitored by UV−vis spectroscopy, and good isosbestic
V
V
behavior for the conversion of the Mn starting material to the
electrophilic attack of the oxo group of the Mn (O) complex on
E
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Scheme 3. OAT to para-Substituted Thioanisole Derivatives
and p-NO substituents. Evidence for the contribution of a
2
similar quinoid-type resonance form in a p-nitrophenylthiolate−
II
47
Ni complex was observed by X-ray crystallography. The
quinoid-type resonance form shown in Figure 7 generates partial
positive charge on the sulfur center, making these relatively
electron-poor substrates potentially electrophilic in nature. A
different mechanism for sulfoxidation could then result for the p-
COOMe, p-CN, and p-NO substrates.
2
The two possible mechanistic scenarios are shown in Scheme
. Pathway A corresponds to the electron-donating para-X
4
Scheme 4. Possible Mechanistic Pathways for Electron-
Donating (Pathway A) and Electron-Withdrawing (Pathway
B) para-Substituted Thioanisole Derivatives
V
−
Figure 6. Hammett plot for the reaction of [Mn (O)(TBP Cz)(CN)]
8
and para-X-substituted thioanisole derivatives.
the sulfur center of the thioanisole derivatives. A negative ρ value
was also seen for H-atom abstraction by the five-coordinate
V
45
substituents and shows the sulfoxidation reaction formally
Mn (O) complex with a series of p-substituted phenols. These
V
broken into electron-transfer and O−S bond formation steps.
data indicate that the anionic, six-coordinate Mn (O) complex
The redox potentials of the thioether substrates (E° = 1.2−1.6
maintains significant electrophilic character. However, the
ox
V
V vs SCE) as compared to the parent five-coordinate Mn (O)
electron-withdrawing p-COOMe, p-CN, and p-NO substituents
2
37
complex (E1/2 = −0.05 V vs SCE) indicate that pure outer-
lead to a substantial increase in reaction rate and a positive rho
value of ρ = +1.22. Such a “V-shaped” Hammett plot suggests a
distinct change in mechanism for the electron-poor substrates.
For comparison, a V-shaped Hammett plot was observed for a
sphere ET between ArSR and the six-coordinate, anionic
V
Mn (O) complexes is likely to be thermodynamically highly
disfavored. Thus, a more reasonable mechanism, which we favor,
involves a single transition state with contributions from both the
ET and S−O bond formation steps formally depicted in Pathway
A. The rates for the electron-donating substituents correlate with
the trend expected from a significant contribution of the ET
process, which should get slower as the redox potential of the
thioether increases. In contrast, the electron-withdrawing
substituents may exhibit negligible contribution from electron-
transfer because of their very high redox potentials. For these
substrates, the quinoid-type resonance structure can enhance the
rate of S−O bond formation, taking advantage of the ability of
V
related Mn (imido)(corrole) complex, also implicating a
4
6
mechanistic switch. The generality of this change in
V
mechanism for the six-coordinate anionic Mn (O) complexes
V
was confirmed by examining the fluoride-ligated [Mn (O)-
−
(
TBP Cz)(F)] . Reaction of this complex with p-X-C H SCH
8
6
4
3
derivatives also revealed a clear V-shaped Hammett plot (Figure
S14).
A possible explanation for the V-shaped Hammett plots comes
from consideration of the contributing resonance structures
shown in Figure 7. The quinoid-type resonance forms may be
partly stabilized by the electron-withdrawing p-COOMe, p-CN,
V
−
−
−
the anionic [Mn (O)(TBP Cz)(X)] (X = CN or F ) complex
8
4
8,49
to serve as a potential weak nucleophile.
Thus, for pathway B,
the transition state is dominated by S−O bond formation. This
barrier would be lower for more electron-withdrawing substrates,
providing an explanation for the positive ρ value and the V-
shaped Hammett plot seen in Figure 6.
V
•+
The cationic complex Mn (O)(TBP Cz ) was previously
8
prepared by some of us and, to our knowledge, is the only
V
example of an Mn (O)(π-radical cation) complex. This cationic
complex functioned as an electrophilic oxidant toward phosphine
and thioether substrates and was an ideal candidate for
comparison of its OAT reactivity with the anionic axially ligated
Figure 7. Resonance structures for electron-withdrawing para-
subsituted thioanisole derivatives.
V
V
Mn (O) complexes. The Mn (O)(π-radical cation) complex
F
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
V
was generated by chemical oxidation of the neutral Mn (O)
starting material with the one-electron oxidant [(4-
has been shown to be the ground state in previous
3
5,40
calculations
and which is consistent with XAS measurements
•
+
−
−
36
BrC H ) N ](SbCl ) (Scheme 5). Isosbestic behavior was
for the CN complex. The optimized geometries for the six-
35
6
4 3
6
−
−
coordinate F and CN structures were calculated previously
but were repeated here for comparative purposes. The calculated
Scheme 5. Generation and OAT Reaction of the Cationic
V
+•
1
V
−
−
−
[
Mn (O)(TBP Cz )] Complex
structures for [Mn (O)(H Cz)(X)] (X = F , CN ) revealed
8
8
Mn−X distances (Table 2) indicating a binding interaction. The
Table 2. Select Bond Distances (Å) from DFT Calculations
none
1.549
CN−
F−
N3−
OCN−
NO3−
V
Mn −O
1.568
2.146
1.568
1.931
1.568
2.354
1.561
2.420
1.557
2.518
V
Mn −X
−
azide and cyanate anions also remained bound to the metal
following optimization but exhibit longer bond distances.
Interestingly, the nitrate anion binds only very weakly, and
observed for the reactions with the p-X-substituted thioanisole
derivatives, and in this case the two-electron reduced product of
thiocyanate (not shown in Table 2) dissociates completely from
V
•+
IV
IV
+
−
and SCN−
Mn (O)(TBP Cz ) is the Mn complex [Mn (TBP Cz)] , as
the metal upon geometry optimization. Both NO
3
8
8
expected following OAT. The resulting Hammett plot of log(kX/
kH) versus σ values for the para-X derivatives is shown in Figure
anions showed no influence on the rate of OAT. The
V
computational results show that the Mn −X distance correlates
8. A strong linear correlation with a slope ρ = −1.40 is found
well with the trend in reactivity for the axial donors, in which the
−
−
shorter distances seen for CN and F correspond to large rate
enhancements, whereas the other ligands exhibit either weak or
no influence on OAT rates.
CONCLUSIONS
■
−
V
Addition of anionic axial donors (X ) to an Mn (O) corrolazine
results in binding to the Mn center. Resonance Raman
spectroscopy confirms a subtle weakening of the Mn −O triple
bond upon addition of the X ligand, providing strong evidence
for coordination of X in a position trans to the terminal oxo
group. The axial donors CN and F lead to dramatic increases in
OAT reactivity to thioether substrates, and N also shows a
strong effect. The OCN , NO , and SCN anions, however,
V
V
−
−
−
−
−
3
−
−
−
3
exhibit little or no influence on OAT. These trends in reactivity
V
correlate well with the trend in the DFT-derived Mn −X
distances. A Hammett analysis with a range of p-X-C H SCH
6
4
3
V
+•
derivatives revealed an unusual V-shaped Hammett plot for
Figure 8. Hammett plot for the reaction of Mn (O)(TBP Cz ) with
8
Mn (O)(TBP Cz)(X)] (X = CN− or F ) complexes,
V
−
−
[
para-X-substituted thioanisole derivatives in CH Cl .
8
2
2
indicating a distinct change in mechanism for OAT to the
thioether substrates. This Hammett behavior strongly contrasts
V
throughout the range of para-X derivatives, in stark contrast to
the V-shaped Hammett plot seen for the anionic [Mn (O)-
the linear plot seen for the cationic complex Mn (O)-
V
•+
(TBP Cz ). These findings suggest a novel bifurcated
8
−
−
−
(
TBP Cz)(X)] (X = CN or F ). A single mechanism involving
mechanistic scenario in which the axially ligated anionic
complexes can function in either a purely electrophilic capacity
or in an initial nucleophilic manner when strong electron-
withdrawing substituents are present on the thioanisole
derivatives. Our results suggest that axial ligation is a key factor
8
V
•+
electrophilic attack of Mn (O)(TBP Cz ) (Pathway A) is
8
inferred. These findings provide strong support for the
conclusion that there is a mechanistic switch for the CN and
−
−
F complexes and the electron-poor substrates (Pathway B), and
−
−
V
it is the anionic nature of the CN and F species that likely
facilitates the opening of this unique reaction channel.
in the OAT reactivity of Mn (O) complexes, and the addition of
the appropriate axial donor can be a good strategy for
dramatically increasing OAT reactivity. Even though the axially
ligated complexes bear an overall negative charge, they function
as more powerful, electrophilic oxidizing agents as compared to
Density Functional Theory (DFT) Calculations. Compu-
tational studies were undertaken to help explain the trend in
reactivity for the different anionic axial ligands. Calculations were
performed at RIJCOSX-B3LYP/LANLDZ/6-31G level of
theory, and the optimized geometries revealed a variation in
Mn −X bond distances that were used for a qualitative
comparison of the binding interactions for the various axial
X ) donors. The TBP groups were replaced with H-atoms on
the Cz ligand to facilitate the computations. The calculations
were performed on the singlet state only for the Mn ion, which
V
the nonligated, neutral Mn (O) precursor complex. With the
strong electron-withdrawing p-X substituents on the thioanisole
V
V
derivatives, the nucleophilic properties of the anionic Mn (O)
complexes provide a further boost to their reactivity. These new
mechanistic insights should help in our understanding of axial
ligand effects in heme enzymes as well as in the potential design
of new synthetic oxidation catalysts.
−
(
V
G
dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
23) Liu, H. Y.; Zhou, H.; Liu, L. Y.; Ying, X.; Jiang, H. F.; Chang, C. K.
ASSOCIATED CONTENT
■
Chem. Lett. 2007, 36, 274−275.
*
S
Supporting Information
(24) Solati, Z.; Hashemi, M.; Hashemnia, S.; Shahsevani, E.; Karmand,
Computational methods, Table S1, Figures S1−S15, and
cartesian coordinates for the computational studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Z. J. Mol. Catal. A: Chem. 2013, 374, 27−31.
(25) Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc.
2007, 129, 12416−12417.
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Chem.Eur. J. 2009, 15, 11482−11489.
(27) Song, W. J.; Seo, M. S.; George, S. D.; Ohta, T.; Song, R.; Kang, M.
AUTHOR INFORMATION
■
J.; Tosha, T.; Kitagawa, T.; Solomon, E. I.; Nam, W. J. Am. Chem. Soc.
Corresponding Authors
dpg@jhu.edu
2
007, 129, 1268−1277.
(28) Latifi, R.; Tahsini, L.; Karamzadeh, B.; Safari, N.; Nam, W.; de
mtg10@psu.edu
sam.devisser@manchester.ac.uk
Visser, S. P. Arch. Biochem. Biophys. 2011, 507, 4−13.
(29) Park, S. E.; Song, W. J.; Ryu, Y. O.; Lim, M. H.; Song, R.; Kim, K.
M.; Nam, W. J. Inorg. Biochem. 2005, 99, 424−431.
30) Jin, N.; Lahaye, D. E.; Groves, J. T. Inorg. Chem. 2010, 49, 11516−
1524.
31) Wang, C.; Kurahashi, T.; Inomata, K.; Hada, M.; Fujii, H. Inorg.
Chem. 2013, 52, 9557−9566.
32) Kurahashi, T.; Kikuchi, A.; Shiro, Y.; Hada, M.; Fujii, H. Inorg.
Notes
(
1
(
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
This work was supported by the NIH (GM101153 to D.P.G).
The Kirin cluster at Johns Hopkins University School of Arts and
Sciences is thanked for CPU time to T.Y. The National Service of
Computational Chemistry Software is acknowledged for
providing CPU time. M.G.Q. thanks the BBSRC for a
studentship.
Chem. 2010, 49, 6664−6672.
(
33) Kurahashi, T.; Fujii, H. Bull. Chem. Soc. Jpn. 2012, 85, 940−947.
(34) Collman, J. P.; Zeng, L.; Brauman, J. I. Inorg. Chem. 2004, 43,
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672−2679.
(35) Prokop, K. A.; de Visser, S. P.; Goldberg, D. P. Angew. Chem., Int.
Ed. 2010, 49, 5091−5095.
(36) Neu, H. M.; Quesne, M. G.; Yang, T.; Prokop-Prigge, K. A.;
Lancaster, K. M.; Donohoe, J.; DeBeer, S.; de Visser, S. P.; Goldberg, D.
P. Chem. Eur. J., accepted; DOI: 10.1002/chem.201404349.
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dx.doi.org/10.1021/ja507177h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX