Influence of calcium(II) and chloride on the oxidative reactivity of a manganese(II) complex of a cross-bridged cyclen ligand

Article abstract of DOI:10.1021/ic501342c

Available data from different laboratories have confirmed that both Ca²⁺ and Cl^- are crucial for water oxidation in Photosystem II. However, their roles are still elusive. Using a manganese(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca²⁺ on the oxidative reactivity of the manganese(II) complex and its corresponding manganese(IV) analogue were investigated. It has been found that adding Ca²⁺ can significantly improve the oxygenation efficiency of the manganese(II) complex in sulfide oxidation and further accelerate the oxidation of sulfoxide to sulfone. Similar improvements have also been observed for Mg²⁺, Sr²⁺, and Ba²⁺. A new monomeric manganese(IV) complex having two cis-hydroxide ligands has also been isolated through oxidation of the corresponding manganese(II) complex with H₂O₂ in the presence of NH₄PF₆. This rare cis-dihydroxomanganese(IV) species has been well characterized by X-ray crystallography, electrochemistry, electron paramagnetic resonance, and UV-vis spectroscopy. Notably, using the manganese(IV) complex as a catalyst demonstrates higher activity than the corresponding manganese(II) complex, and adding Ca²⁺ further improves its catalytic efficiency. However, adding Cl^- decreases its catalytic activity. In electrochemical studies of manganese(IV) complexes with no chloride ligand present, adding Ca²⁺ positively shifted the redox potential of the Mn^IV/Mn^III couple but negatively shifted its Mn^V/Mn^IV couple. In the manganese(II) complex having a chloride ligand, adding Ca²⁺ shifted both the Mn^IV/Mn^III and Mn^V/Mn^IV couples in the negative direction. The revealed oxidative reactivity and redox properties of the manganese species affected by Ca²⁺ and Cl^- may provide new clues to understanding their roles in the water oxidation process of Photosystem II.

Full text of DOI:10.1021/ic501342c

Article
pubs.acs.org/IC
Influence of Calcium(II) and Chloride on the Oxidative Reactivity of a
Manganese(II) Complex of a Cross-Bridged Cyclen Ligand
Zhan Zhang,† Katherine L. Coats,‡ Zhuqi Chen,† Timothy J. Hubin,*,‡ and Guochuan Yin*,†
†Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical
Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology,
Wuhan 430074, P. R. China
‡Department of Chemistry and Physics, Southwestern Oklahoma State University, 100 Campus Drive, Weatherford, Oklahoma
73096, United States
S
* Supporting Information
ABSTRACT: Available data from different laboratories have confirmed that both
Ca2+ and Cl− are crucial for water oxidation in Photosystem II. However, their roles
are still elusive. Using a manganese(II) complex having a cross-bridged cyclen ligand
as a model, the influence of Ca2+ on the oxidative reactivity of the manganese(II)
complex and its corresponding manganese(IV) analogue were investigated. It has been
found that adding Ca2+ can significantly improve the oxygenation efficiency of the
manganese(II) complex in sulfide oxidation and further accelerate the oxidation of
sulfoxide to sulfone. Similar improvements have also been observed for Mg2+, Sr2+, and
Ba2+. A new monomeric manganese(IV) complex having two cis-hydroxide ligands has
also been isolated through oxidation of the corresponding manganese(II) complex
with H2O2 in the presence of NH4PF6. This rare cis-dihydroxomanganese(IV) species
has been well characterized by X-ray crystallography, electrochemistry, electron
paramagnetic resonance, and UV−vis spectroscopy. Notably, using the manganese-
(IV) complex as a catalyst demonstrates higher activity than the corresponding
manganese(II) complex, and adding Ca2+ further improves its catalytic efficiency. However, adding Cl− decreases its catalytic
activity. In electrochemical studies of manganese(IV) complexes with no chloride ligand present, adding Ca2+ positively shifted
the redox potential of the MnIV/MnIII couple but negatively shifted its MnV/MnIV couple. In the manganese(II) complex having a
chloride ligand, adding Ca2+ shifted both the MnIV/MnIII and MnV/MnIV couples in the negative direction. The revealed
oxidative reactivity and redox properties of the manganese species affected by Ca2+ and Cl− may provide new clues to
understanding their roles in the water oxidation process of Photosystem II.
studies on a series of MMn3O4 (M = Ca2+, Sr3+, Zn2+, etc.)
INTRODUCTION
■
complexes, Agapie et al. proposed that Ca2+ may regulate the
Oxygen evolution through water oxidation in Photosystem II
redox potentials of manganese ions in the catalytic cycle.6
(PSII) is a crucial process in nature. The available data have
Pecoraro and Brudvig proposed that Ca2+ is possibly involved
clearly demonstrated that the oxygen evolution center (OEC)
in the O−O bond formation during oxygen evolution through
attack on the terminal MnVO functional group by a Ca2+OH
or Ca2+OH2; Brudvig et al. further suggests that Ca2+ serves as a
Lewis acid, which activates the substrate water molecule or
hydroxide anion to facilitate its nucleophilic attack on MnV
O.1b,7 Notably, on the basis of the resolved X-ray structure,
Ferreira et al. proposed that the monomeric manganese
appendage is the site of the water oxidation event (Figure
1).8 If the MnVO site proposed by Pecoraro and Brudvig for
O−O bond formation, the monomeric manganese appendage,
is correct, investigating the redox behaviors of OEC in water
oxidation with monomeric manganese models becomes
particularly valuable.
is comprised of a Mn4CaO5 core in which the manganese atoms
directly participate in water oxidation through a S0−S4 cycle.1
During the S0−S4 cycle, the oxidation states of manganese in
the Mn4CaO5 cluster change from 2+ to 4+, or even a proposed
5+, in a stepwise manner as the electron-transfer processes
proceed. Many synthetic models have been developed to help
understand the physicochemical properties of the Mn4CaO5
core, and additional small-molecule redox catalysts have been
explored for catalytic water oxidation to generate oxygen.2,3 As
part of the increased understanding of the manganese-catalyzed
water oxidation in PSII, both calcium(II) and chloride have
been recognized to play significant roles in the catalytic cycle,
even though their exact roles are still unclear.
For calcium(II), it has been generally believed that it is
essential for the S state to develop beyond S2.4 Moreover, van
Gorkom further suspected that Ca2+ is also required in all of the
S-state transitions.5 Particularly, with extensive electrochemical
Received: June 11, 2014
© XXXX American Chemical Society
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redox potentials of the manganese ion using a monomeric
manganese model having a cross-bridged cyclen ligand.
EXPERIMENTAL SECTION
■
MnII(Me2Bcyclen)Cl2, MnII(Me2EBC)Cl2, MnII(TPA)Cl2, and
MnII(BPMEN)Cl2 [Me2Bcyclen
= 4,10-dimethyl-1,4,7,10-
tetraazabicyclo[5.5.2]tetradecane; EBC = 4,11-dimethyl-1,4,8,11-
tetraazabicyclo[6.6.2]hexadecane; TPA = tris(2-pyridylmethyl)amine;
BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-dia-
mine] were synthesized according to literature procedures,20−22 and
the structures of these ligands are shown in Figure 2. All other
Figure 1. Structure of the OEC based on a recent X-ray structure. One
potential site for H2O binding and O2 formation is indicated.
In theoretical studies by quantum mechanical/molecular
mechanical calculations, it has been proposed that Ca2+ is
bound to the Mn4 cluster by a μ-O or μ-OH bridge, and the
Ca2+ activated water (Ca2+OH2) attacks the oxyl radical,
MnIVO•, rather than MnVO, to generate the O−O bond
before oxygen evolution.1a,9 Alternatively, Siegbahn et al.
suggest that oxyl radicals react with Mn−O−Mn bridges
instead of reacting with terminal Ca2+OH2 to form the O−O
bond.10 Compared with relatively extensive studies and an
increasing understanding of Ca2+ in water oxidation, the role of
chloride is even more elusive, and the precise binding site of
Cl− is still controversial.11 These limited studies have proposed
that Cl− may affect the electron-transfer events in the S-state
transitions by modulating the redox potential of the manganese
ions or neutralizing and stabilizing the accumulated positive
charge in the S states.12 Batista et al. have also proposed that
the role(s) of Cl− are possibly related to proton transfer in the
catalytic cycle.13 Notably, Yocum et al. suggested that Cl− is
required for the S-state transitions from S2 to S3 and from S3 to
S0,12b where Ca2+ is essential as well, implying that Ca2+ and
Cl− may synergistically participate in these S-state transitions.
Although versatile protocols have been applied to deplete
Ca2+ or Cl− for investigating their roles in the actual oxygen
evolution process in natural systems, similar studies using
synthetic models to investigate how Ca2+ and Cl− affect the
redox chemistry of the manganese ion are very limited. Borovik
et al. have reported that the presence of Ca2+ can accelerate
oxygen activation by their manganese(II) and iron(II)
complexes,14 and Agapie et al. revealed that the redox-inactive
metal ions, as Lewis acids, can affect the electrochemical
properties of their [MMn3O4] cubane clusters, which are
structurally related to the Mn4CaO5 core in the OEC.6
Figure 2. Structures of the ligands used in this study.
chemical reagents are commercially available. Electrochemical studies
were performed on a CS Corrtest electrochemical workstation
equipped with glassy carbon as both the working and counter
electrodes and saturated calomel as the reference electrode, and the
redox potentials were measured under argon in dry acetonitrile with
0.1 M tetrabutylammonium perchlorate as the supporting electrolyte.
Kinetic data were collected on an Analytikjena specord 205 UV−vis
spectrometer, and gas chromatography−mass spectrometry (GC−
MS) analysis was performed on an Agilent 7890A/5975C
spectrometer. Electron paramagnetic resonance (EPR) experiments
were conducted at 130 K on a Bruker A200 spectrometer.
Synthesis of [MnIV(Me2Bcyclen)(OH)2](PF6)2. A 2 mL aqueous
solution containing 0.10 g (0.28 mmol) of MnII(Me2Bcyclen)Cl2 and
0.093 g (0.56 mmol) of NH4PF6 was prepared. A total of 1 mL of 30%
H2O2 was added stepwise with stirring to the prepared solution over a
period of 2 h, which generates a blue-purple solution immediately. The
resulting solution was stirred until bubbles were no longer formed
(about 1 h). During this period, a blue-purple precipitate was gradually
generated, which was filtered on a glass frit and washed with H2O. The
product was collected and dried overnight under reduced pressure at
room temperature. A total of 0.074 g of crude product was obtained as
a blue-purple powder. Yield: 43%. To purify the crude product, it was
completely dissolved in 2 mL of H2O to generate a saturated aqueous
solution. Then, 1 mL of an aqueous solution saturated with 50 mg of
NH4PF6 was added stepwise to the solution with stirring, which
gradually led to the formation of a blue-purple precipitate. The
precipitate was filtered and dried under reduced pressure at room
temperature to obtain the purified blue-purple powder product. Yield:
60% (from the crude product). Anal. Calcd for MnC12H28N4O2P2F12:
C, 23.81; H, 4.66; N, 9.26. Found: C, 23.80; H, 4.66; N, 9.03. The X-
ray-quality crystal of [MnIV(Me2Bcyclen)(OH)2](PF6)2 was obtained
by diffusion of ether into an acetone solution containing
[MnIV(Me2Bcyclen)(OH)2](PF6)2 at 258 K.
Like redox-inactive Ca2+’s role in oxygen evolution, many
other redox-inactive metal ions may also play significant roles in
heterogeneous oxidations, like stabilizing the redox catalysts or
modulating their reactivity.15 Addressing how these redox-
inactive metal ions affect the reactivity of an active metal ion
has recently attracted significant attention in the biomimetic
community. The available results from different models have
revealed that adding Lewis acids like Sc3+, Al3+, or even BF3 or
Bronsted acid (H+) can accelerate active iron oxo (FeIVO) or
̈
Crystal Structure Analysis. A blue-purple prism-shaped crystal of
dimensions 0.66 × 0.50 × 0.40 mm was selected for X-ray structure
analysis. Intensity data for this compound were collected on a Rigaku
Mercury CCD diffractometer at 293(2) K using graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å) with a ω-scan method. The
centrosymmetric monoclinic space group P21/n was determined by
systematic absences and statistical tests and verified by subsequent
refinement. The structure was solved by direct methods and refined by
full-matrix least-squares methods on F2. One target molecule was
−
manganese oxo (MnIVO, MnVO, or MnO4 ) mediated
electron transfer, oxygenation, or hydrogen abstraction
rates.16−19 The acceleration effects are related to binding of
the Lewis acid to the redox metal oxo (Mn+O) or its
protonation by Bronsted acid, which leads to positively shifted
̈
redox potentials. These findings have greatly promoted the
understanding of how the redox-inactive metal ions affect the
oxidative properties of an active metal ion. However, up to
now, there has been no detailed example to probe the influence
of Ca2+ on the redox chemistry of the manganese ion in a
catalytic process.19h In this work, we report an example of how
Ca2+ and Cl− affect the catalytic oxidation properties and the
−
cocrystallized with 1 equiv of acetone. Both counteranions, PF6 , are
disordered. The hydrogen atoms H(1)O and H(2)O, which are bound
to the oxygen atoms O(1) and O(2), are located in the difference
Fourier synthesis and were refined semifreely with the help of a
distance restraint, while constraining their U values to 1.2 times the
B
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U(eq) values of corresponding oxygen atoms. All of the other
hydrogen atoms are placed in calculated positions and refined by using
a riding model.
Table 1. Catalytic Sulfide Oxidation by Manganese(II)
Complexes with Lewis Acid
a
conversion
(%)
yield of monoxide yield of dioxide
(%) (%)
Catalytic Sulfide Oxidation by Manganese(II) Complexes
with Lewis Acid. In 5 mL of dry acetonitrile containing 0.1 M
thioanisole, 0.33 mM manganese(II) complex, and 0.33 mM Lewis
acid, for example, Ca(OTf)2, 0.2 mL of 30% H2O2 was added to
initialize the reaction. The reaction mixture was stirred in a H2O bath
at 303 K for 6 h, and the product analysis was performed by GC using
the internal standard method. Control experiments using the
manganese(II) complex or Lewis acid alone as the catalyst were
carried out in parallel.
Catalytic Methyl Phenyl Sulfoxide Oxidation by Manganese-
(II) Complexes with Ca2+. In 5 mL of dry acetonitrile containing 0.1
M methyl phenyl sulfoxide, 0.33 mM manganese(II) complex, and
0.33 mM Ca(OTf)2, 0.2 mL of 30% H2O2 was added to initialize the
reaction. The reaction mixture was stirred in an ice H2O bath (273 K)
for 2 h, and product analysis was performed by GC using the internal
standard method. Control experiments using the manganese(II)
complex alone as the catalyst were carried out in parallel.
catalyst
additive
12.0
7.2
4.5
MnII
MnII
MnII
MnII
MnII
MnII
65.9
32.6
25.9
CaCl2
87.6
39.4
36.7
Mg(OTf)2
Ca(OTf)2
Sr(NO3)2
Ba(OTf)2
90.6 (17.8)
91.4 (18.9)
98.0 (18.6)
97.8 (16.7)
37.8 (5.1)
44.1 (8.1)
34.4 (7.9)
31.6 (6.1)
38.7 (4.4)
37.1 (5.3)
49.2 (3.7)
50.8 (3.6)
a
Conditions: solvent, acetonitrile 5 mL; thioanisole, 0.1 M;
manganese(II) catalyst, 0.33 mM; Lewis acid, 0.33 mM; H2O2, 0.2
mL; 303 K, 6 h. The data in parentheses represent the control
experiment without the manganese(II) catalyst.
with H2O2 provides only 7.2% sulfoxide and 4.5% sulfone as
products in 6 h, and the conversion of thioanisole is only
12.0%. The MnII(Me2Bcyclen)Cl2 catalyst alone is active, giving
65.9% conversion of sulfide with 32.6% and 25.9% yield of
sulfoxide and sulfone, respectively. Adding 1 equiv of Ca(OTf)2
to the MnII(Me2Bcyclen)Cl2 catalyst substantially improved its
oxygenation activity, providing 91.4% conversion and 44.1%
yield of sulfoxide with 37.1% of sulfone products. Using
Ca(OTf)2 alone as the catalyst yields results similar to those in
the control experiment. In another test, adding CaCl2 in place
of Ca(OTf)2 to the MnII(Me2Bcyclen)Cl2 complex generates
oxygenation improvements similar to those of Ca(OTf)2,
indicating that it is Ca2+, rather than OTf−, that accelerates the
oxidation reaction. In GC analysis of the products, some trace
peaks were observed in addition to the dominant sulfoxide and
sulfone products, which can be attributed to the formation of
C−S cleavage products (vide infra).
Catalytic Sulfide Oxidation by Manganese(IV) Complexes
with Ca2+. In 5 mL of dry acetonitrile containing 0.1 M thioanisole, a
0.33 mM manganese(IV) complex, and 0.33 mM Ca(OTf)2, 0.2 mL of
30% H2O2 was added to initialize the reaction. The reaction mixture
was stirred in an ice H2O bath (273 K) for 4 h, and product analysis
was performed by GC using the internal standard method. Control
experiments using the manganese(IV) complex alone as the catalyst
were carried out in parallel.
Stoichiometric Oxygenation by the Manganese(IV) Com-
plexes with Ca2+. In 5 mL of a dry acetonitrile solvent, 0.01 mmol of
the manganese(IV) complex, 0.01 mmol of Ca(OTf)2, and 0.01 mmol
of triphenylphosphine were added. The resulting reaction mixture was
stirred in a H2O bath at 298 K for 4 h, and product analysis was
performed by GC using the internal standard method. Control
experiment using the manganese(IV) complex alone was carried out in
parallel.
Catalytic Hydrogen Abstraction by Manganese(II) Com-
plexes with Ca2+. In 5 mL of dry acetonitrile containing 0.1 M 1,4-
cyclohexadiene, a 0.33 mM manganese(II) complex, 0.33 mM
Ca(OTf)2, and 0.2 mL of 30% H2O2 were added. The resulting
reaction mixture was stirred in a H2O bath at 303 K for 4 h, and
product analysis was conducted by GC using the internal standard
method. Control experiments using the manganese(II) complex or
Ca(OTf)2 alone as the catalyst were carried out in parallel.
Mg2+ generates an acceleration effect similar as to that of
Ca2+, while Sr2+ and Ba2+ are slightly more active than Ca2+. For
example, in the presence of Sr(NO3)2, thioanisole can be
almost completely converted, which provides 34.4% yield of
sulfoxide and 49.2% yield of sulfone, while Sr(NO3)2 alone is
also inactive like those in the control experiment. Because
Sr(OTf)2 is not commercially available, in situ generated
Sr(OTf)2 by reaction of SrCl2 with Ag(OTf) (after filtering
AgCl precipitate) gives similar results, that is, 34.5% yield of
sulfoxide and 47.1% yield of sulfone with 98.1% conversion,
supporting the theory that the acceleration effect is directly
related to Sr2+. Notably, adding Mg2+, Ca2+, Sr2+, or Ba2+ not
only accelerates MnII(Me2Bcyclen)Cl2-catalyzed sulfide oxida-
tion but also substantially improves sulfone formation. For
example, the MnII(Me2Bcyclen)Cl2 catalyst alone gives only
25.9% yield of the sulfone product under the described
conditions, while adding 1 equiv of Ca2+ improves the yield up
to 37.1% and Sr2+ gives an even higher 49.2% yield of sulfone.
In complementary experiments, it was found that adding Mg2+,
Ca2+, Sr2+, or Ba2+ to sulfide, sulfoxide, or sulfone does not shift
the UV−vis spectra, indicating that no interaction exists
between the added Lewis acid and substrate or products.
Interestingly, increasing the ratio of Ca2+ to manganese(II)
catalyst further improves the oxidation of thioanisole and
greatly increases the yield of sulfone. In the presence of 4 equiv
of Ca2+, the conversion of thioanisole reaches 98.5%, and the
yield of sulfone increases to 67.7%, with only 24.0% yield of
sulfoxide [see Table S1 in the Supporting Information (SI) for
details]. The catalytic kinetics further confirms that adding Ca2+
RESULTS AND DISCUSSION
■
Catalytic Sulfide Oxidation by Manganese(II) Com-
plexes with Ca2+. Because it has been hypothesized that the
water oxidation event may happen on the monomeric
manganese appendage of Mn4CaO5 in the OEC and Ca2+ is
possibly involved in the O−O bond formation for oxygen
evolution,7,8 investigating how Ca2+ and Cl− affect the redox
behavior of monomeric manganese model complexes becomes
informative to understanding water oxidation in the OEC.
Here, using a manganese model having a cross-bridged cyclen
ligand, MnII(Me2Bcyclen)Cl2, the influence of Ca2+ and its
analogues like Mg2+, Sr2+, or Ba2+ on the oxidative reactivity of
manganese was investigated using H2O2 as the terminal
oxidant, and the results are summarized in Table 1. Although
understanding the effects of Ca2+ and Cl− on the oxidation of
sulfide by a mononuclear manganese catalyst cannot provide
equal insight into OEC chemistry, the obtained knowledge
about how Ca2+ and Cl− affect the redox behavior of a
manganese ion may aid the eventual understanding of their
roles in OEC chemistry.
In the control experiment without both manganese
complexes and Lewis acid, oxidation of thioanisole at 303 K
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can accelerate the MnII(Me2Bcyclen)Cl2-catalyzed thioanisole
oxidation rate. As shown in Figure 3, in each data point
compared with using the manganese(II) catalyst alone, the
presence of Ca2+ can accelerate the substrate conversion and
product formations simultaneously.
Table 2. Oxidations of Different Sulfides by Manganese(II)
Complexes with Ca(OTf)2
a
conversion
(%)
yield of monoxide
(%)
yield of dioxide
(%)
substrate
thioanisole
91.4 (65.9)
88.2 (58.7)
93.1 (82.2)
44.1 (32.6)
47.9 (37.9)
21.7 (24.3)
37.1 (25.9)
26.7 (10.3)
50.4 (30.6)
diphenyl sulfide
benzyl phenyl
sulfide
a
Conditions: solvent, acetonitrile 5 mL; sulfide, 0.1 M; manganese(II)
catalyst, 0.33 mM; Ca(OTf)2, 0.33 mM; H2O2, 0.2 mL; 303 K, 6 h.
The data in parentheses represent the control experiment with
manganese(II) catalyst alone.
26.7% yield of diphenyl sulfone. Benzyl phenyl sulfide gives
93.1% conversion with 21.7% sulfoxide and 50.4% sulfone.
Significantly, in each case, using the manganese(II) catalyst
alone yielded less sulfoxide and sulfone products than obtained
by adding Ca2+, as shown in the parentheses of Table 2. In
addition to being a common substrate in sulfide oxidation,
benzyl phenyl sulfide is a unique substrate that can serve as a
probe to provide extra mechanistic information. In sulfide
oxidation, the reaction may proceed by concerted oxygen
transfer or electron transfer. In the case of electron transfer, the
sulfide radical cation intermediate, R−S•+−R, will be generated
from the substrate, and its instability leads to rearrangement
and formation of various C−S cleavage products.24 Here,
through GC−MS analysis, the C−S cleavage products,
including phenol, benzaldehyde, benzyl alcohol, benzyl phenyl
ether, 1,2-diphenylethane, etc., have been identified in benzyl
phenyl sulfide oxidation (see Figure S1 in the SI for details).
This result indicates the occurrence of an electron-transfer
process in its oxidation.
Experiments for Catalytic Hydrogen Abstraction by
Manganese(II) Complexes with Ca2+. In complementary
experiments, the influence of Ca2+ on catalytic hydrogen
abstraction reactions of manganese(II) complexes was also
investigated, although it is not directly related to oxygen
evolution in PSII. To simplify the reaction, 1,4-cyclohexadiene
was employed as the substrate because it would provide solely
benzene as the product. Without Ca2+, using H2O2 as the
oxidant in acetonitrile, the manganese(II) complex as the
catalyst provides 17.8% yield of benzene with 18.5%
conversion. Adding Ca2+ slightly improves the hydrogen
abstraction efficiency, giving 23.4% benzene with 34.4%
conversion of 1,4-cyclohexadiene.
Synthesis and Characterization of the Manganese(IV)
Complex. In the catalytic oxidation cycle, we suspected that a
manganese intermediate at high oxidation state would be
generated in situ under oxidative conditions. Particularly, in
PSII, it has been suggested that Ca2+ would affect the S-state
transitions beyond the S2 state, in which the oxidation state of
all of the manganese atoms in the Mn4CaO5 cluster will be 4+,
or even higher.1,4 Thus, to address how Ca2+ affects the
oxidative reactivity of these intermediates at high oxidation
state, it is essential to isolate the active manganese
intermediates in this study. Previously, a manganese(IV)
complex having a cross-bridged cyclam ligand, MnIV(Me2EBC)-
(OH)2(PF6)2 (Me2EBC = 4,11-dimethyl-1,4,8,11-
tetraazabicyclo[6.6.2]hexadecane), was synthesized using
H2O2.25 Following a similar preparation, its analogue,
MnIV(Me2Bcyclen)(OH)2(PF6)2, was also successfully synthe-
sized by oxidation of MnII(Me2Bcyclen)Cl2 with aqueous H2O2
in the presence of NH4PF6. A crystal suitable for X-ray analysis
Figure 3. Oxygenation kinetics of thioanisole by the manganese(II)
complex in the presence (red shapes) and absence (black shapes) of
Ca2+. Conditions: solvent, acetonitrile 5 mL; thioanisole, 0.1 M;
manganese(II) catalyst, 0.33 mM; Ca(OTf)2, 0.33 mM; H2O2, 0.2 mL;
303 K. Y.b% represents the yield of sulfoxide, and Y.c% represents the
yield of sulfone.
Adding Ca2+ may accelerate not only thioanisole oxidation
but also oxidation of sulfoxide to sulfone, which is indicated in
Table 1. Notably, oxidation of sulfoxide is faster than that of
sulfide. To determine the acceleration effect of Ca2+, oxidations
were performed under ice H2O conditions to slow the
oxidation rate. When using methyl phenyl sulfoxide in place
of thioanisole as the substrate, after 2 h of reaction in ice H2O
(273 K), the MnII(Me2Bcyclen)Cl2 catalyst alone provides
54.7% conversion of sulfoxide with 44.3% yield of sulfone.
Adding 1 equiv of Ca2+ to the catalyst achieved 64.9%
conversion of substrate with 51.5% yield of sulfone (see Table
S2 in the SI for details). Obviously, although Ca2+ and its
analogues are inactive for sulfide oxidation, they can
substantially accelerate manganese(II)-catalyzed oxidation.
In the literature, it has been reported that Sr2+ is the only
other redox-inactive metal ion that can functionally, but
partially, replace Ca2+ in PSII.23 Here, the metal ions Mg2+,
Sr2+, and Ba2+ all generated acceleration effects similar to that of
Ca2+. Possibly, in PSII, the protein environment surrounding
the OEC prohibits other metal ions from playing a role
identical with that of Ca2+, although active Sr2+ can recover up
to 40% function of Ca2+.23a,b However, with our small-molecule
catalyst, all of the redox-inactive metal ions may freely interact
with the manganese catalyst in bulk solvent. As a result, all of
these metal ions have demonstrated similar acceleration effects.
Certainly, the oxygenation mechanism of sulfide demonstrated
here is very different from oxygen evolution in PSII, which
requires stepwise electron transfer in the O−O bond formation
through the S0−S4 cycle. Therefore, a direct comparison
between the results obtained here with those in PSII is not
possible.
Oxidations of different sulfides were also investigated, and
the results are listed in Table 2. Under the identical conditions
of thioanisole oxidation by MnII(Me2Bcyclen)Cl2 in the
presence of 1 equiv of Ca2+, diphenyl sulfide oxidation provides
88.2% conversion with 47.9% yield of diphenyl sulfoxide and
D
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has also been isolated by diffusion of ether into an acetone
solution. The crystal data and structure refinement of the
manganese(IV) complex are listed in Table 3, with the selected
bond distances (Å) and angles (deg) listed in Table 4 and the
cation unit of the manganese(IV) complex shown in Figure 4.
Table 4. Selected Bond Distances (Å) and Angles (deg) of
2+
the MnIV(Me2Bcyclen)(OH)2 Cation
Mn−O
O(1)−Mn(1)
1.801(3)
1.799(3)
2.047(4)
2.028(3)
2.047(4)
2.042(4)
84.5(2)
82.4(2)
85.1(2)
85.5(2)
82.0(2)
163.1(2)
97.6(2)
173.8(2)
88.6(2)
89.3(2)
173.0(2)
97.2(2)
93.6(2)
93.4(2)
98.0(2)
O(2)−Mn(1)
Mn−N
N(1)−Mn(1)
N(5)−Mn(1)
N(8)−Mn(1)
N(12)−Mn(1)
Table 3. Crystal Data and Structure Refinement for
MnIV(Me2Bcyclen)(OH)2(PF6)2·CH3COCH3
N−Mn−N
N(5)−Mn(1)−N(12)
N(5)−Mn(1)−N(8)
N(12)−Mn(1)−N(8)
N(5)−Mn(1)−N(1)
N(12)−Mn(1)−N(1)
N(8)−Mn(1)−N(1)
O(2)−Mn(1)−O(1)
O(2)−Mn(1)−N(5)
O(1)−Mn(1)−N(5)
O(2)−Mn(1)−N(12)
O(1)−Mn(1)−N(12)
O(2)−Mn(1)−N(8)
O(2)−Mn(1)−N(1)
O(1)−Mn(1)−N(8)
O(1)−Mn(1)−N(1)
empirical formula
fw
C15H34F12MnN4O3P2
663.34
temperature (K)
wavelength (Å)
cryst syst
293(2)
0.71073
monoclinic
P21/n
space group
unit cell dimens
a (Å)
O−Mn−O
O−Mn−N
13.392(3)
10.614(2)
19.239(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
90.29(3)
90
γ (deg)
volume (Å3)
2734.7(9)
4
Z
density (calcd) (Mg m−3
)
1.611
abs coeff (mm−1
)
0.706
F(000)
1356
cryst color and habit
cryst size (mm3)
black block
0.66 × 0.50 × 0.40
θ range for data collection (deg) 3.23−27.46
index ranges
reflns collected
indep reflns
−17 ≤ h ≤ 17, −10 ≤ k ≤ 13, −18 ≤ l ≤ 24
12688
6176 [R(int) = 0.0382]
completeness to θ = 27.46° (%) 98.8
abs corrn
semiempirical from equivalents
max and min transmn
refinement method
data/restraints/param
GOF on F2
1.0000 and 0.7685
full-matrix least squares on F2
6176/675/454
1.119
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole (e
R1 = 0.0782, wR2 = 0.1587
R1 = 0.1274, wR2 = 0.1837
0.397 and −0.271
Figure 4. Structure of MnIV(Me2Bcyclen)(OH)22+. Ellipsoids are
drawn at the 30% probability level.
Å−3
)
manganese(III) ion is in a rhombic octahedral environment and
may experience pseudo-Jahn−Teller distortion. Moreover, the
2+
Selected bond lengths and angles for MnIV(Me2Bcyclen)-
MnIV−O lengths in MnIV(Me2Bcyclen)(OH)2 are also
shorter than 1.873(2) Å of the MnIII−O bond in five-
coordinated [MnIII(H3L)(OH)] (H6L = (tris[(N′-tert-butylur-
eayl)-N-ethyl)]amine) in which the OH ligand is hydrogen-
bonded to a urea nitrogen atom of the H3L ligand.28 The
average length of the Mn−N bonds is 2.041 Å in
2+
(OH)2 are summarized in Table 4. The Mn−O lengths in
MnIV(Me2Bcyclen)(OH)2 are 1.801(3) and 1.799(3) Å,
2+
clearly establishing the presence of MnIV−OH moieties.
These bond lengths are slightly shorter than the octahedral
1.830(4) Å MnIV−OH bond in trinuclear [MnIV3O4(OH)-
(bepa)3]3 [bepa = N,N-bis(2-pyridylmethyl)ethylamine]26 and
also shorter than the monoclinic 1.811(2) Å MnIV−O bond in
MnIV(Me2EBC)(OH)22+, in which hydrogen-bond formation
was found between the hydrogen atom in the hydroxo moiety
and water molecule.25 In the previously characterized
MnIV(Me2EBC)(OH)2(PF6)2 complex, the X-ray-quality crystal
was obtained from H2O under reduced pressure, while here the
crystal of MnIV(Me2Bcyclen)(OH)2(PF6)2 was obtained by
diffusion of ether into its acetone solution; thus, there is no
water molecule in its crystal structure. The Mn−O lengths in
MnIV(Me2Bcyclen)(OH)2 [2.047(4), 2.028(3), 2.047(4),
2+
and 2.042(4) Å, respectively], a value clearly shorter than
those observed in MnIV(Me2EBC)(OH)2 [2.110(3) and
2+
2.090(2) Å]. This difference is attributed to a more compact
coordination environment encompassed by the Me2Bcyclen
ligand than the Me2EBC ligand. The ease with which the cross-
bridged tetraazamacrocycle encompasses the metal varies with
the ligand size and is further demonstrated by the axial and
equatorial N−Mn−N bond angles. The Nax−Mn−Nax angles
decrease in the order of 175.7(1)° for MnIV(Me2EBC)(OH)2
2+
MnIV(Me2Bcyclen)(OH)2 are also slightly shorter than that
and 163.1(2)° for MnIV(Me2Bcyclen)(OH)22+, and the Nax−
Mn−Neq bond angles decrease in the same order: 91.0(1)° for
2+
[1.807(3) Å] of MnIII−O in [MnIII(PY5)(OH)]2+, which is
expected from the ionic radii Mn4+ < Mn3+.27 However, the
MnIV(Me2EBC)(OH)2 and 82.0(2)° for MnIV(Me2Bcyclen)-
2+
E
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(OH)222++. Thus, the manganese ion in MnIV(Me2Bcyclen)-
(OH)2 is less encompassed by the cross-bridged macrocycle
and more exposed to a potential solvent, oxidant, or substrate
in comparison with MnIV(Me2EBC)(OH)2
.
2+
Electronic Structure. The cyclic voltammogram of the
MnIV(Me2Bcyclen)(OH)2(PF6)2 complex is shown in Figure 5,
Figure 6. EPR spectra of the manganese(IV) complexes in dry
acetonitrile at 130 K.
(IV) complex catalyst alone at 273 K (ice H2O conditions) can
give similar results in only 4 h, that is, 85.3% conversion, 36.3%
yield of sulfoxide, and 45.0% yield of sulfone. Furthermore, in
the presence of 1 equiv of Ca2+ at 273 K, the manganese(IV)
complex gives complete conversion (99.0%) of substrate with
24.5% yield of sulfoxide and 65.7% yield of sulfone in 4 h.
Notably, the yield of sulfone is much higher than that of
sulfoxide, which is different from catalytic oxidation by the
manganese(II) complex in the presence of Ca2+. As disclosed
above, the manganese(IV) complex was quantitatively synthe-
sized by oxidation of the corresponding manganese(II)
complex with H2O2 in the presence of NH4PF6. Thus, a
similar manganese(IV) species should exist under the sulfide
oxidation conditions. One clear difference between the
manganese(II) complex and the corresponding manganese(IV)
complex as the catalyst, MnII(Me2Bcyclen)Cl2 vs
MnIV(Me2Bcyclen)(OH)2(PF6)2, is that the former has
chloride ligands, whereas the latter does not. In complementary
experiments, adding 2 equiv of Ag(OTf) to MnII(Me2Byclen)-
Cl2 to generate the active MnII(Me2Byclen)(OTf)2 catalyst in
situ provides 92.4% yield of sulfone with >99.9% conversion,
and no sulfoxide was detected at 303 K. [Note: Although
silver(0) formation is possible, which would generate a
manganese(III) complex, neither black silver(0) precipitate
nor characteristic brown manganese(III) catalyst color was
observed.] In the case of the addition of 1 equiv of Ag(OTf) to
generate in situ what could be represented stoichiometrically as
the MnII(Me2Byclen)Cl(OTf) catalyst (although we are not
claiming this is the actual solution structure), it provides 12.6%
yield of sulfoxide and 75.7% yield of sulfone with 99.6%
conversion. In another test using CaCl2 in place of Ca(OTf)2,
MnIV(Me2Bcyclen)(OH)2(PF6)2 as the catalyst provides only
25.6% yield of sulfoxide and 12.6% yield of sulfone with 41.2%
conversion at 273 K, which is almost identical with that using
MnII(Me2Bcyclen)Cl2 as the catalyst in the presence of
Ca(OTf)2, which provides 24.7% yield of sulfoxide and 10.7%
yield of sulfone with 37.7% conversion at 273 K. Apparently,
the presence of Cl− decreases the catalytic activity of these
manganese catalysts in sulfide oxidation, and the presence of
Ca2+ may assist dissociation of the Cl− anion, thus accelerating
sulfide oxidation. However, the role of Ca2+ cannot be solely
assigned to the abstraction of chloride from the manganese(II)
complex because adding Ca2+ to the manganese(IV) complex,
which has no chloride as a ligand, still improves its catalytic
efficiency and shifts its redox potential (vide infra).
Figure 5. Cyclic voltammogram for [MnIV(Me2Bcyclen)(OH)2]-
(PF6)2 in dry acetonitrile at 298 K (vs SCE).
and the redox potentials with peak separations are listed in
Table 5. The cyclic voltammogram of [MnIV(Me2Bcyclen)-
Table 5. Redox Potentials of
[MnIV(Me2Bcyclen)(OH)2](PF6)2 Complexes under Argon
a
(vs SCE)
redox couple
E1/2 (V)
peak separation (mV)
MnV/MnIV
MnIV/MnIII
MnIII/MnII
+0.893
+0.410
−0.793
90
128
285
a
Conditions: solvent, acetonitrile; manganese(IV) complexes, 2 mM;
tetrabutylammonium perchlorate, 0.1 M; 298 K.
(OH)2](PF6)2 in dry acetonitrile reveals a MnIII/MnII couple of
−0.793 V (vs SCE) with a peak separation of 285 mV, a MnIV/
MnIII couple of 0.410 V (vs SCE) with a peak separation of 128
mV, and a MnV/MnIV couple of +0.893 V (vs SCE) with a peak
separation of 90 mV. Apparently, this manganese(IV) complex
is a gentle oxidant, similar to its analogue, MnIV(Me2EBC)-
(OH)2(PF6)2. The EPR spectrum of [MnIV(Me2Bcyclen)-
(OH)2](PF6)2 in dry acetonitrile at 130 K reveals two broad
resonances at g values of 2.52 and 4.32, which further supports
its monomeric manganese(IV) oxidation state (Figure 6),
because these values are similar to those of its analogue,
MnIV(Me2EBC)(OH)2(PF6)2, and other monomeric
manganese(IV) complexes like [Mn(HB(3,5-Me2pz)3)2]-
(ClO4)4 (pz = pyrazolyl) reported in the literature.25,29
Catalytic Reactivity of the Manganese(IV) Complex
with Ca2+. Very interestingly, using the synthetic manganese-
(IV) complex in place of the corresponding manganese(II)
complex as the catalyst generates more efficient sulfide
oxidation. At 303 K, manganese(IV) alone as the catalyst
displays a very fast catalytic rate, which makes it difficult to
study with added Ca2+. Thus, a relatively low temperature, 273
K, was applied for this study to slow the oxidation rate. For
comparison, the results are listed in Table 6. Compared with
the MnII(Me2Bcyclen)Cl2 catalyst plus Ca2+, which needs 6 h at
303 K to achieve 91.4% conversion of thioanisole with 44.1%
yield of sulfoxide and 37.1% yield of sulfone, the manganese-
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Table 6. Comparison of Catalytic Thioanisole Oxidation by MnII(Me2Bcyclen)Cl2 and MnIV(Me2Bcyclen)(OH)2(PF6)2
a
Catalysts in the Presence of Ca2+
catalyst
additive
time (h)
temperature (K)
conversion (%)
yield of monoxide (%)
yield of dioxide (%)
MnII
MnII
MnIV
MnIV
MnII
MnIV
6
6
4
4
4
4
303
303
273
273
273
273
65.9
91.4
85.3
99.0
37.7
41.2
32.6
44.1
36.3
24.5
24.7
25.6
25.9
37.1
45.0
65.7
10.7
12.6
Ca(OTf)2
Ca(OTf)2
Ca(OTf)2
CaCl2
a
Conditions: solvent, acetonitrile 5 mL; thioanisole, 0.1 M; manganese(II) or manganese(IV) catalyst, 0.33 mM; Ca(OTf)2 or CaCl2, 0.33 mM;
H2O2, 0.2 mL.
Influence of Ca2+ and Cl− on the Redox Potentials of
Various Manganese Complexes. Although the role of Cl−
in oxygen evolution from PSII is still elusive, Sandusky and
Yocum found that Cl− is required for S2 → S3 and S3 → S0
transitions and proposed that Cl− is possibly related to the
electron-transfer event happening in the OEC.12a Batista et al.
also proposed that Cl− is possibly related to proton transfer in
the catalytic cycle.13a Up to now, there has been no reported
example to confirm the binding site of Cl− in PSII or to
demonstrate how Cl− affects the redox potential of the
Mn4CaO5 core in PSII. Generally, the thermodynamic driving
force of an active species for electron transfer is directly related
to its redox potential. Here, how Cl− and Ca2+ together affect
the redox potentials of a series of manganese ions in synthetic
models including MnII(Me2EBC)Cl2, MnII(Me2Bcyclen)Cl2,
MnII(TPA)Cl2, and MnII(BPMEN)Cl2 is investigated. Their
cyclic voltammograms are shown in Figures 7 and 9, and
related redox potentials are summarized in Tables 7 and 8.
Both manganese(IV) complexes of Me2EBC and Me2Bcyclen
have two hydroxide ligands, and there is no chloride present
under electrochemical conditions. One may see in Figure 7 that
Figure 7. Influence of Ca2+ on the electrochemical behaviors of
MnIV(Me2Bcyclen)(OH)2(PF6)2 and MnIV(Me2EBC)(OH)2(PF6)2 in
dry acetonitrile under argon at 298 K. Conditions: manganese(IV)
complexes, 2 mM; Ca(OTf)2, 2 mM; tetrabutylammonium per-
chlorate, 0.1 M; (a) MnIV(Me2EBC)(OH)2(PF6)2; (b)
MnIV(Me2Bcyclen)(OH)2(PF6)2. Black curves represent manganese-
(IV) complexes alone, and red ones represent manganese(IV)
complexes with Ca(OTf)2.
Table 7. Influence of Ca2+ on the Redox Potential of
a
Manganese(IV) Complexes (vs SCE)
E1/2 (V)
MnV/MnIV
MnIV/MnIII
MnIV(Me2EBC)(OH)2
MnIV(Me2EBC)(OH)2
+0.971
+0.959
+0.893
+0.855
+0.461
+0.483
+0.410
+0.455
2+
2+
Ca(OTf)2
Ca(OTf)2
MnIV(Me2Bcyclen)(OH)2
MnIV(Me2Bcyclen)(OH)2
2+
2+
adding 1 equiv of Ca2+ to the manganese(IV) complexes has
clearly changed their redox potentials. However, the influence
of Ca2+ on the MnV/MnIV and MnIV/MnIII couples is inverted.
For the MnIV(Me2Bcyclen)(OH)2(PF6)2 complex, the original
potentials of the MnIV/MnIII and MnV/MnIV couples are
+0.410 and +0.893 V (vs SCE), respectively. In the presence of
Ca2+, the potential of the MnIV/MnIII couple positively shifts
from 0.045 to +0.455 V (vs SCE), whereas for the MnV/MnIV
couple, it negatively shifts from 0.038 to +0.855 V (vs SCE).
However, varying the concentration of Ca2+ does not further
shift the potentials of the manganese(IV) complex, implying
that there is only a single bimetallic MnIV−Ca2+ adduct
generated, and no multimetallic adduct formation from adding
more Ca2+ (see Figure S2 in the SI for details). For comparison,
the influence of Ca2+ on the redox potentials of its analogue,
MnIV(Me2EBC)(OH)2(PF6)2, has also been investigated, and
similar shifts have been confirmed in the presence of Ca2+. That
is, adding Ca2+ negatively shifts the MnV/MnIV couple from
+0.971 to +0.959 V (vs SCE) but positively shifts the MnIV/
MnIII couple from +0.461 to +0.483 V (vs SCE). Clearly, there
are distinctly different influences of Ca2+ on the MnV/MnIV and
MnIV/MnIII couples. In complementary experiments, it was
found that other mild Lewis acid alkaline-earth metals, Mg2+,
a
Conditions: solvent, dry acetonitrile; manganese(IV) complexes, 2
mM; Ca(OTf)2, 2 mM; tetrabutylammonium perchlorate, 0.1 M;
under argon at 298 K.
Table 8. Influence of Ca2+ and Cl− on the Redox Potentials
of Manganese(II) Complexes
a
E1/2 (V)
MnV/MnIV
MnIV/MnIII
MnII(TPA)Cl2
+1.550
+1.482
+1.504
+1.480
+1.321
+1.274
+1.218
+1.174
+0.795
+0.775
+0.762
+0.737
+0.599
+0.563
+0.465
+0.444
MnII(TPA)Cl2
Ca(OTf)2
Ca(OTf)2
Ca(OTf)2
Ca(OTf)2
MnII(BPMEN)Cl2
MnII(BPMEN)Cl2
MnII(Me2EBC)Cl2
MnII(Me2EBC)Cl2
MnII(Me2Bcyclen)Cl2
MnII(Me2Bcyclen)Cl2
a
Conditions: solvent, dry acetonitrile; manganese(II) complexes, 2
mM; Ca(OTf)2, 2 mM; tetrabutylammonium perchlorate, 0.1 M;
under argon at 298 K.
G
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Sr2+, and Ba2+, have similar influences on the redox potentials of
the manganese(IV) complex as Ca2+ does (see Figure S3 in the
SI for details). While not included in this study of mild Lewis
acids focusing on Ca2+, the literature teaches us that much
stronger Lewis acids like Sc3+ and Al3+ shift the potentials of
managanese(IV) complexes more obviously, but they also make
the manganese(IV) species less stable.19h
Agapie and co-workers recently reported a series of
heterometallic manganese oxido cubane clusters in which
redox-inactive metal ions significantly affect the redox potential
of the manganese ions in clusters.6a,e Particularly, the synthetic
MnIV3CaO4 complex demonstrated a much more negative
potential compared with the all-manganese MnIV3MnIIIO4
cluster, suggesting that the presence of Ca2+ in the cluster
may facilitate access of the high oxidation state of manganese
centers at a relatively low potential level during the S-state
transition. Here, in this monomeric manganese(IV) model, the
presence of Ca2+ also shifts the potential of the MnV/MnIV
couple negatively. However, it also positively shifts the potential
of the MnIV/MnIII couple under electrochemical conditions.
The highest oxidation state of the manganese ions in the S4
state is still controversial in the OEC. In theoretical studies, it
generally proposed that all of the manganese ions in the S4 state
are in the 4+ oxidation state.9,10 Brudvig and co-workers have
suggested that the monomeric manganese appendage is in a
MnVO or MnIVO• form, which is proposed to be involved in
O−O bond formation in the S4 state.7a,b Here, the different
influences of Ca2+ on the redox potentials of the MnIV/MnIII
and MnV/MnIV couples may provide new clues for under-
standing the related oxidation development in the OEC.
The positively shifted potential of the MnIV/MnIII couple
upon the addition of Ca2+ may be expected to improve its
oxidation capability. For example, although manganese(IV)
alone, or in the presence of Ca2+, cannot quantitatively oxidize
sulfide to sulfoxide, adding Ca2+ can promote its oxygenation
efficiency on triphenylphosphine. In acetonitrile at 298 K,
oxygenation of triphenylphosphine by the MnIV(Me2Bcyclen)-
(OH)2(PF6)2 complex yields 54.5% of the oxide product in 4 h
based on the manganese(IV) complex added. Adding 1 equiv of
Ca2+ improves the yield up to 73.8%. The oxygenation kinetics
does not fit a simple first-order rate for the disappearance of the
manganese(IV) species, implying the formation of a
precomplex between the manganese(IV) complex and
triphenylphosphine prior to oxygenation. However, the kinetic
data still demonstrate that adding Ca2+ can accelerate the
oxygenation rate (see Table S5 and Figure S4 in the SI for
details). Although the Hammett plot of substituted triphenyl-
phosphine was not available because of the complicated
oxygenation kinetics, according to the oxygenation mechanism
of its analogue, MnIV(Me2EBC)(OH)2(PF6)2,19a,d it can be
rationally proposed that, here, the oxygenation of triphenyl-
phosphine also proceeds by electron transfer, and adding Ca2+
may substantially accelerate its electron transfer because of its
increased potential. Notably, the control experiment shows that
manganese(IV) alone is not indefinitely stable under the
reaction conditions, which is consistent with it being a gentle
oxidant having a potential of +0.41 V (vs SCE).
Figure 8. UV−vis spectra of the manganese(IV) complexes (0.33
mM) in the presence of different concentrations of Ca(OTf)2 in
acetonitrile at 298 K.
value of 777 M−1 cm−1. Interestingly, increasing the Ca2+
concentration beyond 1:1 changes only the intensity of the
new species and does not shift it further. A reasonable
explanation is that there exists an equilibrium between the free
manganese(IV) complex and a bimetallic MnIV−Ca2+ adduct.
Adding excess Lewis acid shifts this equilibrium toward the
bimetallic adduct, thus increasing its absorbance, but generates
no additional multimetallic adducts. This explanation (for-
mation of a bimetallic adduct with no further multimetallic
adduct formed by adding more than 1:1 Ca2+) is also consistent
with the results of the electrochemical studies (vide supra). In
complementary experiments, adding Mg2+, Sr2+, and Ba2+ also
blue-shifts the absorbance of the manganese(IV) species (see
Figure S5 in the SI in details). A similar blue shift was
previously observed upon the addition of Al3+ to the solution of
the MnIV(Me2EBC)(OH)2(PF6)2 complex, which caused its
characteristic absorbance of manganese(IV) species to shift
from 554 to 537 nm.19h Thereof, the bimetallic MnIV−Ca2+
adduct can be putatively proposed as forming a MnIV−O−Ca2+
bridge through Ca2+-assisted deprotonation of the MnIV−OH
moiety, similar to that formed by Al3+ with the related
MnIV(Me2EBC)(OH)2(PF6)2 complex.
To address the influence of both Ca2+ and Cl− on the redox
potentials of the manganese ion, the electrochemical behaviors
of a series of manganese(II) complexes having chloride ligand
including MnII(Me2EBC)Cl2, MnII(Me2Bcyclen)Cl2,
MnII(TPA)Cl2, and MnII(BPMEN)Cl2 were investigated, and
the cyclic votalmmgrams are shown in Figure 9 and their redox
potentials are summarized in Table 8. Unlike the above-
discussed manganese(IV) complexes having two hydroxide
ligands, the presence of Ca2+ negatively shifts the redox
potentials of all of the MnV/MnIV and MnIV/MnIII couples for
the four tested manganese(II) complexes having two chloride
ligands. For example, the original redox potentials of the MnV/
MnIV and MnIV/MnIII couples in the Mn(Me2Bcyclen)Cl2
complexes are +1.218 and +0.465 V (vs SCE), respectively.
In the presence of 1 equiv of Ca2+, its potential for the MnV/
MnIV couple negatively shifts by 0.044 to +1.174 V (vs SCE),
and the potential also negatively shifts by 0.021 to +0.444 V (vs
SCE) for the MnIV/MnIII couple. In addition, increasing the
Ca2+ concentration above 1:1 slightly shifts the potentials of the
manganese(II) complex further, possibly related to an
enhanced ability of Ca2+ in the removal of Cl− from the
manganese(II) complex (see Figure S6 in the SI for details).
The UV−vis spectrum provides evidence for the interaction
of Ca2+ with MnIV(Me2Bcyclen)(OH)22+. As shown in Figure 8,
MnIV(Me2Bcyclen)(OH)22+ alone demonstrates a characteristic
absorbance at 589 nm with a ε value of 682 M−1 cm−1. Adding
1 equiv of Ca2+ generates a blue shift of this absorbance of the
characteristic manganese(IV) species at 577 nm having a ε
H
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modulating its oxidative reactivity. Even in the absence of Cl−,
Ca2+ can also affect the reactivity of manganese through its
linkage to the high-oxidation-state manganese ion. Although
the influence of Ca2+ and Cl− on the redox potentials and
reactivity of the manganese complexes in this work may not
directly reflect their roles in the OEC, it may provide new clues
to understanding their roles in dioxygen evolution in the OEC
of PSII.
CONCLUSIONS
■
Through catalytic sulfide oxidations by the MnII(Me2Bcyclen)-
Cl2 complex in the presence/absence of Ca2+, it has been found
that adding Ca2+ substantially improves the oxygenation
efficiency of the manganese(II) catalyst and accelerates the
conversion of sulfoxide to sulfone. Similar acceleration effects
have also been observed for Mg2+, Sr2+, and Ba2+. Using an
isolated and well-characterized MnIV(Me2Bcyclen)-
(OH)2(PF6)2 complex as the catalyst, it was found that this
manganese(IV) catalyst demonstrates more efficient catalytic
activity than the corresponding manganese(II) complex.
Conclusively, adding Ca2+ further improves its efficiency, yet
adding Cl− to the manganese(IV) complex, which has no
chloride ligand, will decrease its catalytic activity. Detailed
electrochemical studies revealed that adding Ca2+ to mono-
meric manganese(IV) complexes, which have no chloride
ligand, shifts the MnIV/MnIII couple positively, whereas it shifts
the MnV/MnIV couple negatively. Notably, adding Cl− to the
manganese(IV) complex positively shifts its MnV/MnIV and
MnIV/MnIII couples, whereas further adding Ca2+ will reverse
that trend, negatively shifting both couples. In the tested
manganese(II) complexes, which contain chloride, adding Ca2+
shifts both the MnV/MnIV and MnIV/MnIII couples to the
negative direction, thus demonstrating different influences of
Ca2+ and Cl− on the redox behaviors of the manganese ions.
Figure 9. Influence of Ca2+ on the electrochemical behaviors of
manganese(II) complexes having chloride ligands. Conditions: solvent,
dry acetonitrile; manganese(II) complexes, 2 mM; Ca(OTf)2, 2 mM;
tetrabutylammonium perchlorate, 0.1 M; under argon at 298 K; (a)
MnII(TPA)Cl2; (b) MnII(BPMEN)Cl2; (c) MnII(Me2cyclam)Cl2; (d)
MnII(Me2Bcyclen)Cl2. Black curves represent manganese(II) com-
plexes alone, and red ones represent the manganese(II) complexes
with Ca(OTf)2.
Although the shifted values are not large, the identical shifting
tendency of four examined manganese(II) complexes makes
the trend convincing. In complementary experiments, it was
found that Mg2+, Sr2+, and Ba2+ have similar influences on the
redox potentials of Mn(Me2Bcyclen)Cl2. That is, all of them
negatively shift the potential of the MnV/MnIV and MnIV/MnIII
couples in the manganese(II) complex (see Figure S7 in the SI
for details).
Currently, the accurate binding site of Cl− in the OEC during
the catalytic cycle still remains elusive. The proposed potential
role of Cl− in the OEC is related to electron transfer, proton
transfer, regulation of the potential of the OEC, and activation
of substrate H2O.11−13 Here, the presence of Cl− also affects
the potentials of manganese complexes and even reverses the
influence of Ca2+ on the MnIV/MnIII couple. For example,
adding Ca2+ positively shifts the redox potential of the MnIV/
MnIII couple in the MnIV(Me2Bcyclen)(OH)2(PF6)2 complex
from +0.410 to +0.455 V (vs SCE), which is in the opposite
direction to the shift of the MnV/MnIV couple. In the
corresponding MnII(Me2Bcyclen)Cl2 complex, adding Ca2+
negatively shifts the MnIV/MnIII couple from +0.465 to
+0.444 V (vs SCE), demonstrating a tendency similar to that
of the MnV/MnIV couple with Ca2+ alone. Because chloride
salts like CaCl2, LiCl, and NaCl are not soluble in acetonitrile,
how Cl− affects the redox chemistry of the manganese(IV)
complex in acetonitrile is not accessible. Alternatively, LiCl is
soluble in acetone, and the influence of Cl− on the potentials of
the manganese(IV) complex was investigated in acetone. As
shown in Figure S8 in the SI, adding Cl− to the
MnIV(Me2Bcyclen)(OH)2(PF6)2 complex positively shifts
both of the MnV/MnIV and MnIV/MnIII couples from +0.930
and +0.503 V (vs SCE) to +1.014 and +0.528 V (vs SCE),
respectively, while subsequently adding Ca(OTf)2 to the
solution containing the manganese(IV) complex and LiCl
reverses the trend in the redox potential shift, that is, shifting
both potentials negatively from +1.014 and +0.528 V (vs SCE)
to +0.992 and +0.487 V (vs SCE), respectively. Taken together,
the presence of Ca2+ can affect the binding of the Cl− anion to
both manganese(II) and manganese(IV) complexes, thus
synergistically affecting the redox potentials of manganese and
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format, catalytic sulfide
oxidation under different conditions, GC−MS graphs of the
C−S cleavage products of benzyl phenyl sulfide in catalytic
oxidations, detailed electrochemical results, and detailed crystal
data for MnIV(Me2Bcyclen)(OH)2(PF6)2. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: tim.hubin@swosu.edu.
*E-mail: gyin@hust.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
GC−MS analysis was performed in the Analytical and Testing
Center of Huazhong University of Science and Technology.
This work was supported by the National Natural Science
Foundation of China (Grants 21273086 and 21303063). T.J.H.
acknowledges the donors of the American Chemical Society
Petroleum Research Fund for partial support of this research.
T.J.H. also acknowledges the Henry Dreyfus Teacher−Scholar
Awards Program for support of this work.
I
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Inorganic Chemistry
Article
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