Influence of ligand architecture on oxidation reactions by high-valent nonheme manganese oxo complexes using water as a source of oxygen

Article abstract of DOI:10.1002/anie.201409476

Mononuclear nonheme Mn^IV=O complexes with two isomers of a bispidine ligand have been synthesized and characterized by various spectroscopies and density functional theory (DFT). The Mn^IV=O complexes show reactivity in oxidation reactions (hydrogen-atom abstraction and sulfoxidation). Interestingly, one of the isomers (L¹) is significantly more reactive than the other (L²), while in the corresponding Fe^IV=O based oxidation reactions the L²-based system was previously found to be more reactive than the L¹-based catalyst. This inversion of reactivities is discussed on the basis of DFT and molecular mechanics (MM) model calculations, which indicate that the order of reactivities are primarily due to a switch of reaction channels (σ versus π) and concomitant steric effects.

Full text of DOI:10.1002/anie.201409476

Angewandte
Chemie
DOI: 10.1002/anie.201409476
Reactive Intermediates
Influence of Ligand Architecture on Oxidation Reactions by High-
Valent Nonheme Manganese Oxo Complexes Using Water as a Source
of Oxygen**
Prasenjit Barman, Anil Kumar Vardhaman, Bodo Martin, Svenja J. Wçrner,
Chivukula V. Sastri,* and Peter Comba*
IV
Abstract: Mononuclear nonheme Mn =O complexes with
two isomers of a bispidine ligand have been synthesized and
characterized by various spectroscopies and density functional
high-valent metal-based intermediates and optimize their
reactivity.
[3]
In PS II, the high-valent manganese species of the OEC
uses water as a source of oxygen. In biomimetic studies, the
formation of high-valent ruthenium oxo complexes has been
IV
theory (DFT). The Mn =O complexes show reactivity in
oxidation reactions (hydrogen-atom abstraction and sulfox-
idation). Interestingly, one of the isomers (L ) is significantly
more reactive than the other (L ), while in the corresponding
Fe =O based oxidation reactions the L -based system was
previously found to be more reactive than the L -based
catalyst. This inversion of reactivities is discussed on the
basis of DFT and molecular mechanics (MM) model calcu-
lations, which indicate that the order of reactivities are
primarily due to a switch of reaction channels (s versus p)
and concomitant steric effects.
1
IV
3+
achieved with the strong oxidants Ce or [Ru(bpy) ] (bpy =
3
2
[4]
IV
IV
2,2’-bipyridine). Similarily, Fe =O and Mn =O complexes
IV
2
have also been generated using water as a source of oxygen
1
IV
[2a,5]
and Ce as a one-electron oxidant.
Herein, we report the
IV
generation of mononuclear nonheme Mn =O complexes
with water as an oxygenation source and Ce as a one-
IV
electron oxidant. Detailed spectroscopic and preliminary
DFT studies were employed to evaluate the reactivity of the
IV
IV
Mn =O and the corresponding Fe =O intermediates.
The inherently rigid adamantane-derived bispidine
moiety has been used extensively as a widely variable ligand
platform for transition-metal complexes, in particular also for
H
igh-valent manganese oxo complexes with heme and
nonheme ligands have attracted much interest in the fields of
bioinorganic and oxidation chemistry in the past decades. This
interest is due to their importance as intermediates in the
oxidation of organic substrates as well as in water oxidation
by the oxygen-evolving complex (OEC) in photosystem II
[
6]
ferryl-based oxidation catalysts, and derivatives of the two
1
isomeric pentadentate ligands L (X = N, Y = CH, i.e.,
2
methylpyridine at N3) and L (X = CH, Y= N, i.e., methyl-
[7]
pyridine at N7) have been of particular interest (Figure 1).
[
1]
(
PS II). To reveal the chemical and physical properties of
high-valent manganese oxo intermediates, a number of
IV
V
nonheme Mn =O and Mn =O complexes have been
reported.
[
2]
The ability to control the reactivity of metal oxo
complexes has important consequences in a variety of
chemical processes. The selective oxidation of hydrocarbon
compounds and the oxidation of organic substrates by
oxygen-atom transfer are fundamental transformations. One
of the challenges associated with improving these processes is
the choice of the ligand architecture, which helps to stabilize
II
1
Figure 1. Plots of the X-ray structures of [Mn (MeCN)(L )] (left (1),
methylpyridine at N3, X=N, Y=CH, see drawing in the middle for X,
II
2
Y), and [Mn (MeCN)(L )] (right (2), methylpyridine at N7, X=CH,
Y=N). Counterions and hydrogen atoms omitted for clarity (see the
Supporting Information for further details and ORTEP plots of the two
structures).
[
*] P. Barman, Dr. A. K. Vardhaman, Dr. C. V. Sastri
Department of Chemistry, Indian Institute of Technology Guwahati
Assam, 781039 (India)
E-mail: sastricv@iitg.ernet.in
Dr. B. Martin, S. J. Wçrner, Prof. Dr. P. Comba
Anorganisch-Chemisches Institut and Interdisciplinary Center for
Scientific Computing (IWR), Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: peter.comba@aci.uni-heidelberg.de
These ligands enforce square-pyramidal geometries, and the
open coordination site, occupied by the oxo group in high-
n+
1
2
valent M =O complexes, is trans to N3 and N7 for L and L ,
respectively. This geometric variation is known to lead to
striking differences in structural and electronic properties,
stabilities, and reactivities, because N7 is part of a rather
flexible six-membered chelate ring, while N3 is part of a very
[
**] Research support was provided by the Department of Science and
Technology, India (SR/S1/IC-02/2009) and the Council for Scientific
&
Industrial Research (01(2527)/11/EMR-II) to C.V.S. Support by
the University of Heidelberg is gratefully acknowledged.
[
6a,8]
II
1
rigid five-membered chelate.
[Mn (L )(ClO ) ] (1) and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409476.
4 2
II
2
[Mn (L )(ClO ) ] (2) were prepared in analogy to earlier
4
2
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.
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Communications
reports and were structurally characterized (see the Support-
[
8c]
ing Information).
The high-valent Mn =O intermediates, [Mn (O)L ] (3;
IV
2+
IV
1 2+
IV
2
2
mm) and [Mn (O)L ] (4; 2 mm) were generated from
their manganese(II) precursors, using cerium(IV) ammonium
nitrate (CAN; 8 mm) as a one-electron oxidant in either
acetonitrile/water (9:1) or acetone/water (9:1) at 278 K. The
addition of CAN to the colorless solution of the mangane-
se(II) complexes produced pale green solutions. Complex 3
exhibits an absorption spectrum with an intense band at l =
À1
À1
5
45 nm (e ꢀ 400m cm ) and a weak absorption in the near
À1
À1
IR region at l = 970 nm (e ꢀ 35m cm ). Similarly, complex
À1
À1
4
has electronic transitions at l = 570 nm (e ꢀ 400m cm )
À1 À1
and at l = 975 nm (e ꢀ 103m cm ; see Figure S7 in the
Supporting Information). The electrospray ionization mass
spectra (ESI-MS) of 3 and 4 exhibit prominent peaks with
m/z 332, and the isotope patterns show that these correspond
IV
1,2 2+
to [Mn (O)L ] (see Figures S8 and S9).
The reactivity of 3 and 4 was investigated with two-
electron oxidation reactions such as the oxidation of the sulfur
atom of thioethers. Upon addition of thioanisole to the
IV
corresponding Mn =O species at 278 K, the intermediates
decayed immediately and gave methyl phenyl sulfoxide as the
major product (Figure 2a). The pseudo-first-order rate con-
stants of the decay of 3 and 4 increased linearly with
increasing thioanisole concentration, thus giving second-
order rate constants, for the sulfoxidation reactions with 3
À1 À1
À1 À1
and 4 as oxidants, of 0.120(2)m
s
and 0.012(1) m s ,
respectively (see Figure 2b and Table S10). Therefore, it
appears that 3 reacts with thioanisole about ten times faster
than the isomeric oxidant 4. To further elucidate the reaction
Figure 2. a) UV/Vis spectral changes of 4 (2 mm) upon addition of
thioanisole (160 mm) in CH CN/H O (9:1) at 278 K (the inset shows
the time trace at l=570 nm for the decay of Mn =O). b) Second-
3
2
pathway, second-order rate constants (k ) for the oxidation of
IV
X
various para-substituted thioanisole substrates (kH is the
second order rate constant for thioanisole; see the Supporting
Information) with 4 were also determined (see Table S11 and
Figure S12): a plot of the logarithm of the rate constant ratio
order rate constants determined in the reaction of 2 mm of 3 (blue )
&
and 4 (red *) in CH
thioanisole at 278 K. c) Second order rate constants determined in the
reaction of 2 mm of 3 (blue
) and 4 (red *) in CH CN/H O (9:1)
against various concentrations of 2,4-di-tert-butylphenol at 233 K.
CN/H O (9:1) against various concentrations of
3
2
&
3
2
[
log(k /k )] against the one-electron oxidation potentials
X H
0
(E OX) of the sulfides gave a linear correlation with a slope of
À10.8 (Figure 3a). Similarly large slopes were also obtained
in Hammett correlations (see Figures S13 and S14). Such
a large negative slope implies that the electron transfer from
the sulfides to 4, rather than a group transfer, is the rate-
[
9]
determining step.
To further evaluate the reactivity of the two isomeric
IV
Mn =O oxidants, H-atom abstraction reactions were also
studied. The oxidation of 2,4-di-tert-butylphenol (2,4-
tBu C H OH) with 3 and 4 yielded 2,2’-dihydroxy-3,3’,5,5’-
2
6
3
tetra-tert-butylphenol. The second-order rate constants reveal
that the reactivity of 3 is again larger (ca. 40 times) than that
of 4 (Figure 2c and Table S10).
The OÀH bond cleavage in phenols can be achieved by
a H-atom abstraction or by a proton-coupled electron-trans-
[10]
fer (PCET) mechanism. PCET can be discarded when two
substrates with similar OÀH bond strengths and pK values,
a
but with substituents with different steric demand, such as 2,6-
À1
tert-Bu C H OH [BDE(O-H) = 81.65 kcalmol ; pK = 11.70,
2
6
3
a
aqueous solution] and 2,4-tert-Bu C H OH [BDE(O-H) =
0
2
6
3
Figure 3. a) Plot of log (k /k ) against E values of para-X-thioanisoles
X H ox
À1
8
1.85 kcalmol ; pK = 11.64, aqueous solution] show differ-
a
by 4 at 278 K. b) Correlations of second-order rate constants,
log(k /k ), of 4 against BDE of p-X-2,6-tBu C H OH at 273 K.
[10b,11]
ent reactivities.
The observed approximate 50-fold rate
X
H
O-H
2
6
2
2
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enhancement in the reaction of 4 with 2,4-tBu C H OH
different pathways: 1) the s pathway [linear M-O-H(sub-
strate) orientation] involving the iron dz orbital, and 2) the
p pathway [bent M-O-H(substrate) orientation] involving
2
6
3
À1 À1
[
[
0.950(1) m s ] as compared to 2,6-tBu C H OH,
2
2
6
3
À1 À1
0.017(2) m s ; see Figure S16] suggests an H-atom abstrac-
[10,12,13]
[19]
3
IV
tion pathway.
a d orbital (d , d ).
In contrast, for the d Mn =O
p
xz
yz
To further substantiate the H-atom abstraction reaction
with phenol, we evaluated the corresponding second-order
rate constants with a series of para-substituted 2,6-di-tert-
butylphenol substrates (X = OMe, Me, H, CN; see Table S17
and Figure S18). A plot of the logarithm of the rate constant
system, specifically in its S = 1/2 electronic configuration,
the p pathway is favored. As therefore expected, all relevant
optimized iron-based transition states are linear, and those of
the manganese-based system are bent (M-O-H angles of
[
20]
approx. 1758 vs. 1308; see Figure 4 and TableS29).
+
ratio [log(k /k )] as a function of s shows an excellent
X
H
p
Hammett correlation with a 1 value of À1.6. Such a linear
relationship has been used as evidence for an H-atom
abstraction pathway in the oxidation of phenol OÀH bonds
V
VI
2+
by [Mn (O)(Cz)] (where Cz is corrolazine), [(L)Ru (O) ] ,
2
IV
2+
and [Fe (O)(TMC)]
(where TMC is tetramethyl
As further support, we have also plotted log
[
10b,12,14]
cyclam).
(
k /k ) values against the phenol OÀH bond dissociation
X
H
energy (BDE), which afforded a good correlation with a slope
of À0.40 (Figure 3b), thus matching well with earlier
[
10b,12,14,15]
reports.
An interesting observation is that the corresponding iron
Figure 4. Orbital plots of the H abstraction intermediates:
a) Fe (O)(L )] (aSOMOÀ1); b) Mn (O)(L )] (aSOMO);
isovalues=0.04.
IV
2
2+
IV
2 2+
1
2
oxidation chemistry with derivatives of L and L (methyl
1
2
instead of benzyl groups at N7 for L and N3 for L ) follows
IV
quite a different pattern, that is, the corresponding Fe =O
complexes of L are more efficient oxidants than those of
2
The computed spin densities suggest that the ferryl
1
[16]
IV
L , and this is in sharp contrast to the corresponding Mn = oxidants in their S = 1 ground state have about equal spin
IV
O system reported here. While the substitution pattern (i.e.
benzyl versus methyl) might have some influence (in a study
density on the Fe and O centers. In Mn =O (S = 3/2) most of
the unpaired electron density is on the metal center (see
Table S29). In the transition states most of the spin density is
on the S = 2 Fe and on the S = 1/2 Mn centers (Figure 4). That
is, the ferryl system in its ground state is further on its way
II
on Cu complexes, the steric influence was studied in detail),
IV
a substantial influence is not expected since in the Fe =O
system, where the high-spin state is pseudo-Jahn–Teller
active, the larger substituents should not lead to a decrease
III
towards an Fe -oxyl-radical description than the manganese
2
of the relative reactivity of the L -based ferryl complex, and
system and, in both cases, partial electron transfer is well on
its way.
IV
the corresponding Mn =O systems are not pseudo-Jahn–
Teller active, that is, in terms of the relative reactivities, we do
Both isomers are very similar in their behavior with both
metal ions. The computed energy barriers for the rate-
determining H-abstraction steps are in very good agreement
with the experimental data for the iron-based system, but less
[
17,8c]
not expect a large effect.
difference between the Fe =O and Mn =O systems, we have
To understand the striking
IV
IV
done a preliminary computational study, based on DFT
[
18a]
[18b]
2
À1
(
B3LYP/TZVP/Def2-TZVP
and B3LYP/LANL2DZ
)
clear cut for the manganese-based system (Fe-L : 33 kJmol ,
[
18c]
1
À1
1
À1
2
À1
and force-field calculations (MOMEC force field ), to
analyze the steric and electronic factors which determine the
reactivity (CÀH activation with cyclohexane as substrate,
Fe-L : 44 kJmol ; Mn-L : 71 kJmol , Mn-L : 65 kJmol ;
Table S27): while DFT predicts the correct order of reactiv-
IV
À1
ities for Fe =O by 11 kJmol , the two isomers are (within
IV
using the methyl-substituted ligands; see the Supporting
Information for details). We have optimized the metal oxo
starting structures, the preequilibrium of the reactants with
the substrate, the transition states for CÀH abstraction, and
the error limit) identical for the Mn =O oxidants (energy
difference of 6 kJmol ). While this is not satisfactory, one
À1
needs to appreciate that the error limit for the iron systems,
À1
with the theoretical setup used, is on the order of 10 kJmol .
III
IV
the emerging intermediates (M ÀOH, for M = Mn, Fe, both
For Mn =O, where less thorough bench marking is available,
1
2
isomers (L , L ) and all relevant spin states).
the results therefore are within the error limit also in
[
21,22]
For the iron-based oxidants, the ferryl complexes are all in
agreement with the experimental data.
Importantly,
the S = 1 intermediate spin state (stabilized by approx.
there is a constant shift of relative energies, approximately
2–3 kJmol , between the two basis sets used. That is, the
À1
À1
2
0 kJmol with respect to the S = 2 state), and the reaction
crosses to the S = 2 high-spin surface in the H abstraction step,
activation barriers of the two isomers are approximately
identical and largely independent of the basis set. The main
result from the DFT analysis is, therefore, that the two
that is, the transition state has an S = 2 configuration
À1
(
stabilized by approx. 50 kJmol ; see Table S27). For the
IV
IV
IV
manganese-based system, the ground state of Mn =O for
both isomers is S = 3/2 (stabilized by approx. 25 kJmol ) and
systems (Fe =O versus Mn =O) follow two different
À1
reaction channels (s versus p) and that the relative activation
IV
the transition state is low-spin, that is, S = 1/2 (stabilized by
barriers of the s channel for the pair of Fe =O isomers are
À1
IV
approx. 20 kJmol ; see Table S27). For the S = 2 ferryl
accurately predicted, while those for the pair of Mn =O
species, the electrophilic attack may proceed through two
isomers following the p channel are not.
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From the transition-state structures (linear versus bent;
see Figure 4 for two examples) it emerges that steric strain
might be a decisive factor for the energy of the four transition-
state structures (L , L , linear and bent). We therefore have
performed the four corresponding force-field calculations
along the torsional rotation around the M=O bond (van-der-
expected: the N3 and N7 methyl (or phenyl) substituents are
a major reason for blocking the approach of the substrate, and
it is a known and well-understood feature of bispidine
coordination geometry that the metal–N7 bonds are generally
1
2
[6]
more flexible and longer than the metal–N3 bonds. We
suggest that this is the reason for the observations emerging
from Figure 5 and the strikingly different activation barriers
for the p pathway of the manganese-based systems. Clearly,
what emerges is an entropy effect, which is not captured by
the DFT analysis presented here. A quantitative analysis of
this effect would require an (ab initio) molecular dynamics
analysis, which is outside the scope of this study.
Waals energy as a function of the torsional angle, rigid
n
geometry of the L M-O-H substructure, fully refined sub-
strate and substrate orientation at each point of the torsional
coordinate; see Figure 5 and the Supporting Information for
IV
The important DFT-based prediction is that the Fe =O
IV
systems react through the s pathway and the Mn =O
complexes through the p channel. Therefore, for the sterically
less demanding iron-based oxidations, the DFT-predicted
relative activation barriers are accurate. For the manganese-
based system, the situation is more complex: the DFT-based
and the MM-based steric analysis are in good agreement and
predict similar activation barriers for the two isomers.
However, the MM analysis also suggests that the approach
of the substrate is, as predicted from extensive structural work
Figure 5. Plot of the van der Waals energies of the transition-state
structures (Mn complexes, rotation of the cyclohexyl substrate around
2
1
[6]
the MnÀO bond, for details see Supporting Information). a) L -based
of bispidine complexes, favored for the L -based system.
1
complex. b) L -based complex.
The main point emerging from the current report is that
between the manganese and iron systems, there are drastic
changes in subtle mechanistic features, which are responsible
for the switch in reactivity between the two isomers based on
details). It emerges that the lowest energy orientations are
those found in the DFT optimizations, thus suggesting that
steric strain is of importance for the choice of the pathway.
Importantly, for the linear structure, there is basically free
rotation of the cyclohexyl substrate around the M=O axis, and
1
2
L and L : while the iron systems may evade steric strain
through the s pathway, the manganese systems do not have
this possibility and react through the p pathway, that is, the
sterically demanding bent structures. While these pathways
have been appreciated before, they have rarely been used to
interpret subtle differences in reactivities as observed here. It
appears that other striking and so far not satisfactorily
explained reactivity differences might find similar interpre-
1
2
both isomers (L and L ) are, in terms of steric energy,
degenerate. That is, in the case of the iron-based systems,
where both isomers react along the s pathway, there are
2
electronic reasons for the enhanced reactivity of the L -based
[
3c]
system. Interestingly, the reactivities of the iron-based com-
tations.
plexes have been experimentally shown to be correlated with
IV/III
the Fe
redox potential, and this then suggests that the
Received: September 25, 2014
Revised: November 17, 2014
Published online: && &&, &&&&
reactivities are correlated to the driving force of the electron-
[3c]
transfer step.
For the manganese-based bent system, the minimum-
energy transition-state structures are the lowest energy
minima on the steric energy surface, but for the L -based
Keywords: density functional calculations · isomers ·
manganese · oxidation · structure elucidation
.
1
system the barrier is significantly smaller and there is also
À1
another minimum, which is less than 10 kJmol higher in
energy (Figure 5). The minimum steric strain imposed by the
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Angew. Chem. Int. Ed. 2006, 45, 3446 – 3449; Angew. Chem.
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[
[
[
[
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1
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5
[20] We have not completed a computational analysis for the
sulfoxidation reaction. From published work on the iron-based
systems it emerges, however, that depending on the isomer (oxo
group trans to either N3 or N7), the lowest energy transition
[
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[
16b]
state (S = 2) has either linear or bent structures,
thus
suggesting that in the iron system, there might be changes in
structure and concomitantly in reactivity resulting from subtle
changes in the steric demand of the substrate.
5
236 – 5239; c) P. Comba, S. Fukuzumi, H. Kotani, S. Wunder-
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2
[21] S. P. de Visser, M. G. Quesne, B. Martin, P. Comba, U. Ryde,
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Reactive Intermediates
Choosing a path: Two isomers of mono-
nuclear nonheme Mn =O complexes
IV
P. Barman, A. K. Vardhaman, B. Martin,
S. J. Wçrner, C. V. Sastri,*
have been synthesized and characterized.
Interestingly, one of the isomers is sig-
nificantly more reactive than the other,
P. Comba*
&&&& — &&&&
IV
while in the corresponding Fe =O-based
Influence of Ligand Architecture on
Oxidation Reactions by High-Valent
Nonheme Manganese Oxo Complexes
Using Water as a Source of Oxygen
oxidation reactions the opposite reactivity
is observed. Theoretical calculations
indicate that the order of reactivity is
primarily due to a switch in reaction
pathways (s versus p) with concomitant
steric effects.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!