Dramatic influence of an anionic donor on the oxygen-atom transfer reactivity of a Mnv-Oxo complex

Article abstract of DOI:10.1002/chem.201404349

Addition of an anionic donor to an Mn^V(O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn^V(O)(TBP₈ Cz)(CN)]^- was generated from addition of Bu⁴N ⁺ CN^- to the 5-coordinate Mn^V(O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives Mn-O = 1.53 ?, Mn-CN = 2.21 ?. In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN^- complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e^--reduced Mn^III(CN) ^- complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives ΔH± = 14 kcalmol^-1, ΔS± = -10 cal mol^-1 K^-1 . Computational studies fully support the structures, spin states, and relative reactivity of the 5- and 6-coordinate Mn^V(O) complexes.

Full text of DOI:10.1002/chem.201404349

DOI: 10.1002/chem.201404349
Communication
&
Metal–Oxo Reactivity
Dramatic Influence of an Anionic Donor on the Oxygen-Atom
V
Transfer Reactivity of a Mn –Oxo Complex
[a]
[b]
[a]
[a]
Heather M. Neu, Matthew G. Quesne, Tzuhsiung Yang, Katharine A. Prokop-Prigge,
[
c]
[c]
[c, d]
[b]
Kyle M. Lancaster, James Donohoe, Serena DeBeer,*
Sam P. de Visser,* and
[
a]
David P. Goldberg*
Heme enzymes can be differentiated by the protein-derived
axial ligand which binds the heme cofactor through one of the
axial positions of the central metal ion. For example, peroxidas-
V
Abstract: Addition of an anionic donor to an Mn (O) por-
phyrinoid complex causes a dramatic increase in 2-elec-
tron oxygen-atom-transfer (OAT) chemistry. The 6-coordi-
[1]
[2]
es employ a histidine, catalases employ a tyrosinate, and
Cytochrome P450s (CYP450) utilize a cysteinate ligand to coor-
V
À
nate [Mn (O)(TBP Cz)(CN)] was generated from addition
8
+
À
V
of Bu N CN to the 5-coordinate Mn (O) precursor. The
[3]
4
dinate to the iron center. It has been suggested that the
cyanide-ligated complex was characterized for the first
time by Mn K-edge X-ray absorption spectroscopy (XAS)
and gives MnÀO=1.53 ꢀ, MnÀCN=2.21 ꢀ. In combination
with computational studies these distances were shown
to correlate with a singlet ground state. Reaction of the
anionic Cys donor in CYP450 plays an important role in modu-
lating the electronic features and reactivity of the high-valent
iron–oxo species Fe (O)(porph C)(Cys) (Compound-I: Cpd-I),
IV
+
which is the critical intermediate responsible for substrate oxi-
[3]
dation.
À
CN complex with thioethers results in OAT to give the
The influence of axial ligands on the reactivity of synthetic
Cpd-I analogues has been examined for several reaction
classes, including alkyl and arene hydroxylation, and alkene ep-
À
III
À
corresponding sulfoxide and a 2e -reduced Mn (CN)
complex. Kinetic measurements reveal a dramatic rate en-
hancement for OAT of approximately 24000-fold versus
the same reaction for the parent 5-coordinate complex.
[4]
oxidation. These previous studies have shown that both
anionic and neutral donors have had a significant influence on
the reactivity of the Cpd-I analogues, with reaction rates typi-
cally being influenced by 1–3 orders of magnitude. However,
the fundamental origins of these axial ligand effects are still
not well understood, and some of the explanations that were
offered provide conflicting conclusions. The analogous influ-
ence of axial ligands on high-valent Mn–oxo complexes has
been less well studied, in part because Cpd-I analogues
°
À1
°
An Eyring analysis gives DH =14 kcalmol , DS =
À1 À1
À10 calmol K . Computational studies fully support the
structures, spin states, and relative reactivity of the 5- and
V
6
-coordinate Mn (O) complexes.
Determining the factors that control the inherent reactivity of
metal–oxo complexes is of critical importance for understand-
ing the function of both heme and nonheme metalloenzymes.
IV
+
V
0
Mn (O)(porph C), or the equivalent Mn (O)(porph ), are typical-
[5,6]
ly highly unstable and difficult to observe. We previously ex-
amined axial ligand effects on a high-valent manganese(V)–
V
oxo complex, Mn (O)(TBP Cz), [TBP Cz=octakis(p-tert-butylphe-
8
8
[
a] H. M. Neu, T. Yang, Dr. K. A. Prokop-Prigge, Prof. D. P. Goldberg
Department of Chemistry, The Johns Hopkins University
Baltimore, MD (USA)
3À
nyl)corrolazinato ], which is a rare example of a stable and
V
[7,8]
isolable Mn (O) porphyrinoid species.
Reactivity towards CÀ
E-mail: dpg@jhu.edu
H substrates was examined, and dramatic increases in hydro-
gen-atom transfer (HAT) rates were observed upon addition of
[
b] Dr. M. G. Quesne, Dr. S. P. de Visser
Manchester Institute of Biotechnology and
À
À
anionic donors (e.g., F , CN ). It was concluded that the anion-
ic donors were weakly coordinating trans to the oxo ligand,
and based on density functional theory (DFT) calculations, the
axial ligands likely caused an increase in the basicity of the in-
School of Chemical Engineering and Analytical Science
The University of Manchester, 131 Princess Street, Manchester (UK)
E-mail: sam.devisser@manchester.ac.uk
[
c] Prof. K. M. Lancaster, J. Donohoe, Prof. S. DeBeer
Department of Chemistry and Chemical Biology, Cornell University
Ithaca, New York (USA)
IV
–
cipient [Mn (OH)] intermediate and a strengthening of the OÀ
H bond. This thermodynamic influence, in turn, led to a greater
overall driving force for HAT, which correlated with a lowering
[
d] Prof. S. DeBeer
Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse
Mꢀlheim an der Ruhr (Germany)
E-mail: serena.debeer@cec.mpg.de
[8a]
of the reaction barrier. An alternative explanation for the
HAT reactivity based on additional DFT calculations was recent-
ly offered, and involves spin cross-over from the low-spin (ls)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404349.
V
–
singlet (S=0) state for [Mn (O)(TBP Cz)(CN)] to a high-spin
8
ꢁ
2014 The Authors. Published by Wiley-VCHVerlag GmbH & Co. KGaA. This
[9]
(
hs) triplet (S=1) state.
is an open access article under the terms ofthe Creative Commons Attribu-
tion License, which permits use, distribution andreproduction in any
medium, provided the original work is properly cited.
In this work we provide novel insights regarding the axial
V
ligand effects on the OAT reactivity of a high-valent Mn (O)
Chem. Eur. J. 2014, 20, 14584 – 14588
14584 ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
À
complex. Addition of the mono-anionic donor CN leads to
V
the formation of the 6-coordinate complex [Mn (O)-
À
(
TBP Cz)(CN)] . X-ray absorption spectroscopy (XAS) measure-
8
ments provided structural parameters and an oxidation state
assignment for this complex. The oxygen-atom-transfer (OAT)
reactivity of this complex was examined for thioether sub-
strates, and a dramatic rate acceleration of approximately
24000-fold over the 5-coordinate parent complex was ob-
served. This increase in reactivity is orders of magnitude larger
than that seen in similar studies on the axial ligand influence
in either heme- or nonheme high-valent metal–oxo complexes
[
4,5,10]
of biological relevance.
Computational (DFT) methods led
to optimized geometries that were in good agreement with
the XAS studies, and these experimentally validated computa-
tional results help shed light on the ground spin states for the
V
Mn (O) complexes. Further DFT calculations on the OAT reac-
tion profiles fully support the experimentally observed increase
in reaction rate.
+
In an earlier study we showed that addition of excess Bu N
4
À
V
CN to Mn (O)(TBP Cz) in CH Cl led to a large increase in HAT
8
2
2
V
Figure 1. A) Comparison of the normalized Mn K-edge XAS data for Mn (O)-
[
8a]
V
reactivity. The UV/Vis spectrum of the Mn (O) complex is not
(
TBP
8
Cz) with zero (red), 10 (dark blue) and 100 (light blue) equivalents of
À
+
À
sensitive to addition of CN , but LDI-MS (neg. mode) showed
added Bu N CN in benzonitrile. B) EXAFS data (solid lines) and fits (dashed
lines) for Mn (O)(TBP
Bu N CN (blue). C) The corresponding FTs (solid) and fits (dashed).
4
V
V
V
8
Cz) (red) and Mn (O)(TBP
8
Cz)+100 equivalents of
clear evidence for a complex matching the formula [Mn (O)-
+
À
À
4
(
TBP Cz)(CN)] , consistent with formation of a 6-coordinate
8
complex as seen in Scheme 1. However, direct structural infor-
mation on the cyanide-ligated complex was lacking, in part
due to the instability of this complex. We now report the struc-
ces a structural perturbation at the Mn site. The overall beat
pattern of the EXAFS is altered and the relative intensity of the
FT peaks is also modified. Based on our EXAFS fits, these
trends are best interpreted as resulting from direct coordina-
V
À
tural characterization of [Mn (O)(TBP Cz)(CN)] by XAS. The Mn
8
V
V
K-edge XAS spectra of Mn (O)(TBP Cz) and [Mn (O)-
8
À
À
V
V
(
TBP Cz)(CN)] (prepared by addition of 10 and 100 equivalents
tion of CN to Mn (O)(TBP Cz). The 5-coordinate Mn (O)-
8
8
+
À
of Bu N CN ) are shown in Figure 1a. All three spectra are
(TBP Cz) is best fit by inclusion of one MnÀO vector at 1.54 ꢀ
4
8
V
consistent with a Mn oxidation state assignment, based on
and 4 MnÀN(Cz) interactions at 1.87 ꢀ, and multiple scattering
(MS) interactions from the Cz ring are needed to fit the outer-
shell contributions to the FT (see the Supporting Information
for full fit parameters). These data match well with an earlier
report, and were repeated here for the direct comparison with
the approximately 6553 eV position of the rising edge. Upon
À
addition of CN , the edge shifts by about 0.3 eV to lower
IV
energy, but not enough to suggest reduction to Mn . Most no-
À
tably, upon addition of CN , the pre-edge remains at high
V
À [8b]
V
À
energy (~6542 eV), but the pre-edge intensity decreases rela-
[Mn (O)(TBP Cz)(CN)] .
For [Mn (O)(TBP Cz)(CN)] , the data
8
8
V
tive to the 5-coordinate Mn –oxo complex. These data are con-
are best fit by one MnÀO at 1.53 ꢀ, four MnÀN(Cz) interactions
at 1.87 ꢀ, and a 2.21 ꢀ MnÀC/N interaction consistent with
V
sistent with formation of a 6-coordinate Mn –oxo species. Co-
À
À
À
ordination of a trans axial CN ligand would move the Mn fur-
a coordinated CN . The coordination of CN is further support-
ed by the increase in the outer-shell FT amplitude (despite the
decrease in the first shell amplitude). The increase in outer-
shell multiple scattering intensity at approximately 2.5 ꢀ is at-
tributed to the presence of linear scattering from the nitrogen
À
of coordinated CN , in addition to Cz ring scattering.
V
Reactivity studies with the cyanide complex [Mn (O)-
À
(
TBP Cz)(CN)] were undertaken to determine the influence of
8
anionic axial donors on OAT reactivity. This species was allowed
to react with dibutyl sulfide (DBS) in toluene at 258C
(Scheme 2). This reaction was monitored by UV/Vis spectrosco-
py as shown in Figure 2, and exhibited rapid isosbestic conver-
V
À
8
Scheme 1. Formation of 6-coordinate [Mn (O)(TBP Cz)(CN)] .
V
À
ther into the Cz plane, decreasing the amount of metal 3d–4p
sion of [Mn (O)(TBP Cz)(CN)] to a species with a split Soret
8
mixing, and thus decreasing the observed pre-edge intensity.
band at 443 and 492 nm, and a Q-band at 694 nm. This final
V
À
III
Figure 1b shows the EXAFS data for Mn (O)(TBP Cz) and
spectrum can be assigned to the 2e -reduced Mn complex
8
V
À
+
À
III
À
[
Mn (O)(TBP Cz)(CN)] (at 100-fold excess of Bu N CN ) with
[Mn (TBP Cz)(CN)] , as confirmed by independent generation.
8
4
8
III
the corresponding Fourier transforms (FTs) presented in Fig-
The related chloride-ligated complex (Et N)[Mn (TBP Cz)(Cl)]
4
8
À
ure 1c. These data clearly show that addition of CN introdu-
also exhibits a similar split Soret band and has been crystallo-
Chem. Eur. J. 2014, 20, 14584 – 14588
www.chemeurj.org
14585 ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[
8b,c]
graphically characterized.
The reaction was complete within
a single exponential model. The resulting fits yielded the
pseudo-first-order rate constants kobs, and plots of kobs at 694
or 634 nm versus [DBS] were linear. The kinetics are consistent
a few minutes, as compared to several hours for the reaction
V
of the 5-coordinate Mn (O) complex with the same substrate
[
8d]
V
at much larger concentrations.
These data clearly demon-
with the overall second-order rate law: –d[Mn (O)(X)]/dt=k-
À
V
strate that addition of CN leads to a large increase in reactivi-
ty towards DBS.
[Mn (O)(X)][DBS], and the slope of the best-fit line of the
À1 À1
second-order plot for 694 nm gives k=9.2(Æ0.3)m s . A
À
comparison of the rate constant for the CN complex with that
V
reported for the 5-coordinate Mn (O) complex reveals a dramat-
ic rate enhancement of 24000-fold for OAT with DBS as sub-
strate. This change can be compared to recent work by Fujii,
which showed that modifying the nature of the axial ligand in
an iron–oxo porphyrin Cpd-I analogue resulted in a 30-fold dif-
[4a]
ference in OAT rate.
To obtain further information on the mechanism of OAT for
V
À
V
À
[Mn (O)(TBP Cz)(CN)] and DBS, a temperature-dependent
8
Scheme 2. Reaction of [Mn (O)(TBP
8
Cz)(CN)] with thioether substrates.
study of the rate constants (À20 to 408C) was performed. An
Eyring plot of ln(k/T) versus 1/T (Figure 3) shows a linear de-
pendence that yields the activation parameters listed in
V
Table 1. The activation parameters for the 5-coordinate Mn (O)
V
complex, and the one-electron-oxidized analogue Mn (O)-
+
(
TBP Cz C), are included in Table 1 for comparison, measured
8
[8d]
previously for the same substrate and solvent. The enthalpy
of activation for the cyanide-ligated complex is lower than that
°
found for the 5-coordinate complex, while the DS is more
positive. These differences both contribute to a lowering of
À
the reaction barrier for the CN complex. The overall negative
°
À
DS value for the CN complex is consistent with a bimolecular
°
mechanism, while the unusually large and negative DS seen
for the positively charged Mn (O)(TBP Cz C) is not observed for
the negatively charged CN complex. From these data, it ap-
V
V
+
Figure 2. UV/Vis spectral changes (0–80 s) for the reaction of [Mn (O)-
8
À
À
(
[
TBP
Mn (TBP
8
III
Cz)(CN)] (11 mm) (419, 634 nm) with excess DBS (500 equiv) to give
À
8
Cz)(CN)] (443, 492, 694 nm) in toluene at 258C. Inset: changes in
absorbance versus time for the growth of [Mn (TBP Cz)(CN)] (694 nm) and
8
À
V
pears that the addition of CN to the Mn (O) complex does
not cause a significant change in the mechanism of OAT, but
does induce a significant lowering of the reaction barrier
through a combination of both enthalpic and entropic effects.
III
À
V
À
8
the decay of [Mn (O)(TBP Cz)(CN)] (634 nm).
A second substrate, thioanisole (PhSMe), was tested to ex-
À
amine the generality of the CN influence over OAT reactivity
with thioethers, and provides a convenient substrate for meas-
uring quantitative yields of sulfoxide product by GC-FID. The
V
À
reaction between PhSMe and [Mn (O)(TBP Cz)(CN)] led to the
8
expected conversion of the starting material to the cyanide-li-
III
gated Mn product, and the reaction mixture was analyzed di-
rectly by GC-FID. The anticipated methyl phenyl sulfoxide was
obtained in high yield (84%; Scheme 2). Labeling of the termi-
18
18
nal oxo ligand with O from H2 O, followed by reaction with
1
8
PhSMe led to 71% incorporation of O in the final sulfoxide
V
À
Figure 3. Eyring plot for the reaction of [Mn (O)(TBP
DBS (21 mm) in toluene from À20 to 408C.
8
Cz)(CN)] (11.5 mm) with
V
product as seen by GC-MS. These data show that [Mn (O)-
À
(
TBP Cz)(CN)] carries out direct OAT to thioethers to give the
8
À
2
e -oxidized sulfoxide as the major product.
V
À
Table 1. Kinetic and activation parameters for OAT reactions with dibutyl
The kinetics of OAT for [Mn (O)(TBP Cz)(CN)] with DBS as
8
sulfide.
substrate were quantitatively analyzed for comparison with the
V
V
[a]
V
+
[a]
V
À
previously examined 5-coordinate Mn (O) complex (second-
Mn (O)(TBP Cz)
Mn (O)(TBP Cz C)
[Mn (O)(TBP Cz)(CN)]
8
8
8
À4 À1 À1
[8d]
order rate constant of 3.8ꢂ10
m
s
for DBS). Reactions
[b]
À4[c]
À3
k
3.8ꢂ10
(6.2Æ0.2)ꢂ10
9.2Æ0.3
14Æ0.4
°
[d]
were run under pseudo-first-order conditions (175–750 equiv
of DBS) and monitored by UV/Vis. Isosbestic behavior was ob-
served as seen in Figure 2 for all concentrations of DBS, and
plots of absorbance versus time for the decay of the
DH
DS
DG
16Æ1
7.0Æ0.8
À45Æ3
°
[e]
À20Æ4
À10Æ0.8
°
[d]
[f]
[f]
[f]
22Æ2
20Æ2
17Æ0.5
À1 À1
[a] From Ref. [8d]. [b] Values in m
s
at 298 K. [c] Extrapolated from
a ln(k/T) versus 1/T plot. [d] Values in kcalmol . [e] Values in calK mol .
À1
À1
À1
V
À
[
Mn (O)(TBP Cz)(CN)] complex (634 nm) and the appearance
8
III
À
[f] At 298 K.
of the [Mn (TBP Cz)(CN)] complex (694 nm) were well-fit by
8
Chem. Eur. J. 2014, 20, 14584 – 14588
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14586 ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Computational studies were employed to gain further in-
V
À
sight into the structure and reactivity of [Mn (O)(TBP Cz)(CN)] .
8
V
Previous calculations on the 5- and 6-coordinate Mn (O) com-
[
8d]
plexes utilized an abbreviated Cz core with hydrogen atoms
on all b-carbon positions (H Cz) for computational conven-
8
ience. To determine the influence of the peripheral substitu-
ents on the computational results, a set of DFT calculations
with H Cz, octamethyl (Me Cz), and octamethylphenyl
8
8
(
(MePh) Cz) corrolazine ligands were performed for both the
8
V
V
À
Mn (O) and [Mn (O)(CN)] complexes. Three different density
functional methods were also employed for all complexes, pro-
viding a comprehensive DFT analysis of optimized geometries
[
11]
and spin ground states. Although a certain degree of fluctu-
ation in spin state ordering and relative energies is obtained
between the different DFT methods, the majority of calcula-
tions give a singlet spin ground state. The MnÀO bond is
shortest in the closed-shell singlet spin ground state; with
MnÀO=1.55 ꢀ at B3LYP-D3/SDD/6-31G(d) level of theory, as
Figure 4. Potential energy profile calculated at UB3LYP/BS2 level of theory
for OAT reactions involving [Mn (O)(H Cz)] and [Mn (O)(H Cz)(CN)] with
8 8
DMS. Energies (DE+ZPE+Esolv) are given. Also shown are optimized transi-
tion state geometries (right-hand side) with bond lengths in angstroms and
the imaginary mode in wave numbers.
V
V
À
2
expected for a low-spin d ion with both p*(MnO) orbitals un-
V
À
occupied. The excited triplet spin states for the [Mn (O)(CN)]
À
complexes exhibit different electronic configurations depend-
ing upon which functional is employed. For B3LYP and M06,
spin densities indicate an electronic configuration that can be
(O)(H Cz)(L)] with L=no ligand (NL) or CN . Figure 4 shows
8
the potential energy profile and rate determining transition
state geometries for the sulfoxidation reaction. Similar to previ-
IV
best described as a hs-Mn (oxyl radical), whereas BP86 results
ous studies, sulfoxidation is a concerted reaction process with
V
1
1
[8d,12]
in a configuration closer to a hs-Mn (d p* state). The oxyl rad-
ical states give highly elongated MnÀO bonds (1.80–1.82 ꢀ),
a single SÀO bond formation transition state, TS .
The en-
L
thalpy of activation (with solvent corrections included) is low-
ered dramatically upon addition of an axial ligand, in good
agreement with the large rate enhancement observed for the
V
and although the hs-Mn state has a shorter distance of ap-
proximately 1.67 ꢀ, all of these MnÀO distances are significant-
ly longer than that observed by EXAFS. Thus the DFT-opti-
mized geometries for the singlet spin states provide the best
match for the structure obtained by EXAFS. Time-dependent
DFT calculations on the singlet states for the 5- and 6-coordi-
nate complexes also reproduce the qualitative trend in the
6-coordinate complex. A close inspection of the transition
1
state geometries reveals that TS
is earlier on the potential
SO,CN
1
energy surface than TS
with a longer SÀO distance and
SO,NL
shorter MnÀO distance. Often early transition states corre-
spond to lower energetic barriers than late transition states.
The lowering of the barrier may also be related to the stabiliza-
[
13]
À
lowering of the pre-edge peak intensity for the CN complex.
III
À
From our combined spectroscopic and computational studies,
tion of the Mn product via coordination of CN . This compu-
tational result is in agreement with Fujii’s recent analysis that
the increase in oxidative reactivity of Cpd-I analogues with dif-
ferent axial ligands can be ascribed to the energetics of axial
V
À
we conclude that [Mn (O)(TBP Cz)(CN)] has a low-spin singlet
8
ground state. This state is the same as that seen for the five-
V
À
coordinate Mn (O) complex, and these data indicate that CN
III
[4]
ligation does not perturb the spin ground state.
ligand stabilization of the Fe (porph) products. Future experi-
mental and computational work is warranted to determine the
origins of the lowering of the reaction barrier in Figure 4.
Our combined experimental and computational studies
demonstrate that the addition of an anionic axial ligand to an
To further establish the electronic ground state and orbital
occupations of the lowest energy singlet and triplet spin states
V
À
of [Mn (O)(H Cz)(CN)] we ran a series of CASSCF-NEVPT2 cal-
8
culations on the B3LYP optimized geometries. We find a singlet
spin ground state with the triplet higher in energy by 13 kcal
V
Mn (O) porphyrinoid complex results in a remarkable increase
À1
mol . This singlet–triplet energy gap is considerably larger
in OAT reaction rate with thioether substrates. The XAS data
and DFT calculations indicate that the singlet ground state,
and associated short MnÀO distance, for the 5-coordinate
than that found from DFT, and indicates that reactivity on the
triplet spin state surface is highly unlikely. Interestingly, the
NEVPT2 calculated triplet spin state gives spin densities of 2.1
on Mn and À0.1 on O, whereas the DFT result gives spin densi-
ties of about 3 on Mn and À1 on O instead. These results im-
plicate that the DFT calculated triplet spin state may be a spuri-
ous artefact that has no realistic electronic structure. Further
studies are needed to establish the exact nature of the triplet
spin state and its reactivity, and therefore we will focus on the
singlet spin state only.
V
Mn (O) complex does not change upon addition of an anionic
axial ligand. The OAT mechanism appears to be a concerted
À
III
À
2e process, leading to the smooth formation of [Mn (CN)]
and sulfoxide products. The DFT studies reproduce well the ob-
served increase in reaction rate by a significant stabilization of
1
V
À
the reaction barrier for the [Mn (O)(TBP Cz)(CN)] complex.
8
Future studies will be aimed at determining the generality of
V
the axial ligand effects on OAT reactivity for Mn (O) complexes.
To gain insight into the mechanism of OAT, we then did
V
a set of DFT studies on dimethyl sulfide sulfoxidation by [Mn -
Chem. Eur. J. 2014, 20, 14584 – 14588
www.chemeurj.org
14587 ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
1
2479; i) N. Hessenauer-Ilicheva, A. Franke, D. Meyer, W. D. Woggon, R.
Acknowledgements
van Eldik, Chem. Eur. J. 2009, 15, 2941–2959.
[
5] a) D. Balcells, C. Raynaud, R. H. Crabtree, O. Eisenstein, Inorg. Chem.
This work was supported by the NSF (CHE0909587 and
CHE121386 to D.P.G.) and the NIH (GM101153 to D.P.G). BBSRC
is acknowledged for a studentship to M.G.Q. and the National
Service of Computational Chemistry Software (NSCCS) is
thanked for cpu time to S.d.V. Portions of this research were
carried out at SSRL, a national user facility operated by Stan-
ford University on behalf of the US DOE, BES. We thank Prof.
K. D. Karlin for instrumentation use. The Kirin cluster at Johns
Hopkins University School of Arts and Sciences is thanked for
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Received: July 10, 2014
Published online on September 26, 2014
Chem. Eur. J. 2014, 20, 14584 – 14588
www.chemeurj.org
14588 ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim