Mixed-Valent Mn16-Containing Heteropolyanions: Tuning of Oxidation State and Associated Physicochemical Properties

Article abstract of DOI:10.1021/acs.inorgchem.5b02503

The two 16-manganese-containing, Keggin-based 36-tungsto-4-silicates [Mn^III₁₀Mn^II₆O₆(OH)₆(PO₄)₄(A-α-SiW₉O₃₄)₄]^28- (1) and [

Full text of DOI:10.1021/acs.inorgchem.5b02503

Article
pubs.acs.org/IC
Mixed-Valent Mn16-Containing Heteropolyanions: Tuning of
Oxidation State and Associated Physicochemical Properties
Ali Haider,†,□ Masooma Ibrahim,†,☆,□ Bassem S. Bassil,†,‡ Akina M. Carey,† Anh Nguyen Viet,†
Xiaolin Xing,† Wassim W. Ayass,† Juan F. Minambres,† Rongji Liu,§ Guangjin Zhang,§ Bineta Keita,∥,△
̃
Valeriu Mereacre,⊥ Annie K. Powell,⊥,# Kamil Balinski,∇ Alpha T. N’Diaye,○ Karsten Kupper,∇
̈
Han-Yi Chen,◆,¶ Ulrich Stimming,◆,¶,◇ and Ulrich Kortz*,†
†Department of Life Sciences and Chemistry, Jacobs University, P.O. Box 750 561, 28725 Bremen, Germany
‡Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon
§Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, 100190, Beijing,
China
∥Universite
́
Paris-Sud, Laboratoire de Chimie-Physique, UMR 8000 CNRS, Orsay, F-91405, France
⊥Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz Platz 1, 76344
Eggenstein-Leopoldshafen, Germany
#Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany
∇Department of Physics and Center of Physics and Chemistry of New Materials, Osnabruck University, D-49069 Osnabruck,
̈
̈
Germany
○Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
◆TUM CREATE, 1 CREATE Way, #10-02 CREATE Tower, 138602 Singapore
¶Department of Chemistry, Technische Universitat Munchen, Lichtenbergstraße 4, 85748 Garching, Germany
̈
̈
◇School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom
S
* Supporting Information
ABSTRACT: The two 16-manganese-containing, Keggin-
based 36-tungsto-4-silicates [MnIII10MnII O6(OH)6(PO4)4(A-
6
α-SiW9O34)4]28− (1) and [MnIII4MnII12(OH)12(PO4)4(A-α-
SiW9O34)4]28− (2) have been prepared by reaction of the
trilacunary Keggin precursor [A-α-SiW9O34]10− with either
Mn(OOCCH3)3·2H2O (for 1) or MnCl2·4H2O (for 2), in
aqueous phosphate solution at pH 9. Polyanions 1 and 2
comprise mixed-valent, cationic {MnIII10MnII O6(OH)6}24+ and
6
{MnIII4MnII12(OH)12}24+ cores, respectively, encapsulated by
four phosphate groups and four {SiW9} units in a tetrahedral
fashion. Both polyanions were structurally and compositionally
characterized by single-crystal XRD, IR, thermogravimetric
analysis, and X-ray absorption spectroscopy. Furthermore,
studies were performed probing the magnetic, electrochemical, oxidation catalytic, and Li-ion battery performance of 1 and 2.
POMs have been reported as promising electrode materials for
energy storage applications such as lithium-ion batteries (LIBs),
sodium-ion batteries, and supercapacitors.3 Removal of one or
more MO6 octahedra from plenary structures such as the Keggin
ion (e.g., [PW12O40]3−) or the Wells−Dawson ion (e.g.,
[P2W18O62]6−) results in so-called lacunary (vacant) derivatives
(e.g., monolacunary [PW11O39]7−, trilacunary [P2W15O56]12−),
which can be considered as inorganic multidentate ligands,
allowing for stabilization of most d- and f-block metal ions.1,2
INTRODUCTION
■
Condensation reactions of simple oxoanions of early transition
metals in high oxidation states (e.g., VV, MoVI, WVI) in acidic,
aqueous medium give rise to the formation of polynuclear
structures known as polyoxometalates (POMs).1 In this process,
many factors, such as pH, temperature, concentration and ratio
of reagents, as well as counter cations are crucial. Such
phenomenon leads to a large number of novel POM structures
with a vast variety in terms of shape, size, and composition,
leading to interesting combinations of physiochemical properties
with potential applications in areas ranging from heterogeneous
and homogeneous catalysis to biomedical sciences.2 Recently,
Received: October 30, 2015
© XXXX American Chemical Society
A
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First row transition metal-containing heteropolytungstates are
probably the largest subclass of POMs due to their structural,
electronic, redox, magnetic, optical, and catalytic properties,
resulting in numerous studies on such compounds in many
different areas of science.2c,4 POMs can be synthesized using
conventional, “open-beaker” conditions or by hydrothermal
methods, with the latter frequently leading to extended POM-
based solid-state networks, rather than discrete molecular
polyanions.4
As a continuation of our work on {M16}-containing
polyanions, we report herein the synthesis and characterization
of two mixed-valence Mn16-containing 36-tungsto-4-silicates.
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. The precursor salt
Na10[A-α-SiW9O34]·23H2O was prepared according to the published
procedure and characterized by FT-IR spectroscopy.23 All other
reagents were used as purchased without further purification. Infrared
spectra were recorded on a Nicolet Avatar 370 FT-IR spectropho-
tometer using KBr pellets. The following abbreviations were used to
assign peak intensities: w = weak, m = medium, and s = strong.
Thermogravimetric analysis was carried out on a TA Instruments SDT
Q600 thermobalance with a 100 mL/min flow of nitrogen; the
temperature was ramped from room temperature to 500 °C at a rate of 5
°C/min. Elemental analysis for NaCs-1 and NaRb-2 was performed by
CNRS, Service Central d’Analyze, Solaize, France.
In particular, the class of manganese-containing heteropoly-
tungstates is well-established and comprises many members. The
number of incorporated manganese centers within these POMs
ranges from 1 to 40, including some mixed-valent species (mostly
common oxidation states II, III, or IV). Some prominent
examples include {MnIIP2W17},5a {MnII/MnIII/MnIVAlW11},5b
{MnII(SiW10)2},5c {MnII(PW7)2},5d {MnIII(SiW9)2},5e
{MnIII2SiW10},6a {MnII As2W19},6b {MnII W5(P2W12)3},6c
Synthesis of Na19.5Cs8.5[{MnIII10MnII6O6(OH)6(PO4)4}(A-α-
SiW9O34)4]·90H2O (NaCs-1). To a solution of Mn(OOCCH3)3·2H2O
(0.32 g, 1.19 mmol) in 20 mL of H2O, 0.50 g (0.17 mmol) was added
Na10[A-α-SiW9O34]·23H2O, and the mixture was stirred until a clear
dark solution was obtained. The pH of the mixture was then raised to 9
by adding solid Na3PO4, and the mixture was stirred for 2 h at room
temperature. The turbid black-brown solution was centrifuged (10 min
at 4000 rpm) and then decanted. A few drops of 1 M CsCl solution were
added to the clear solution, which was then allowed to evaporate in an
open vial at room temperature. After about a week, a brown crystalline
product appeared, which was collected by filtration and air-dried. Yield
140 mg (57%). IR (2% KBr pellet, ν/cm−1): 1090(m), 944(w), 943(m)
742(m), 899(m), 798(w), 723(w), 524(w); see Figure S1. Elemental
analysis (%) for Na19.5Cs8.5[{MnIII10MnII6O6(OH)6(PO4)4}(A-α-
SiW9O34)4]·90H2O, calculated (found): Na 3.31 (3.57), Cs 8.33
(8.40), P 0.91 (0.96), Mn 6.48 (6.20), W 48.79 (48.30), Si 0.83 (0.86).
Synthesis of Na18Rb10[MnIII4MnII12(OH)12(PO4)4(A-α-SiW9O34)4]·
75H2O (NaRb-2). MnCl2·4H2O (0.24 g, 1.21 mmol) was dissolved in
20 mL of H2O, and then Na10[A-α-SiW9O34]·23H2O (0.50 g, 0.17
mmol) was added. The pH of the solution was adjusted to 9 by addition
of solid Na3PO4, and then the resulting dark brown solution was stirred
for 1 h at room temperature. After filtration, 3−4 drops of 1.0 M RbCl
solution were added to the filtrate, which was allowed to evaporate in an
open vial at room temperature. Dark brown crystals started to appear
after a week, which were then collected by filtration and air-dried. Yield
120 mg (53%). IR (2% KBr pellet, ν/cm−1): 1096(m), 988(w), 938(w),
891(m), 806(m), 714(w), 518(w); see Figure S1. Elemental analysis
(%) for Na18Rb10[MnII12MnIII4(OH)12(PO4)4(A-α-SiW9O34)4]·75H2O
calculated (found): Na 3.20 (3.48), Rb 6.57 (6.84), P 0.95 (1.00), Mn
6.76 (6.44), W 50.95 (50.00), Si 0.86 (0.89).
Synthesis of SBA-15. 8.0 g of P123 gel (Sigma-Aldrich) was added to
40 mL of 2 M HCl and 208 mL of H2O. This mixture was stirred initially
for 2 h in a water bath at 35 °C until it was completely dissolved. Then,
18 mL of tetraethylorthosilicate (TEOS) was added dropwise, and the
mixture was kept stirring for 4 h. Afterward, the mixture was heated in an
oven at 95 °C for 3 days. The white precipitate was collected by
filtration, washed, and then air-dried. Finally, a calcination step was
performed and the as-synthesized material was heated to 550 °C at a rate
of 1−2 °C min−1 and kept at this temperature for 6 h to remove the
templates.
Synthesis of SBA-15-apts. 1.61 mL of 3-aminopropyltriethoxysilane
(apts) was added to 3 g of SBA-15 in 90 mL of toluene. This mixture was
refluxed for 5 h and then filtered at room temperature. The obtained
modified SBA-15-apts was then heated at 100 °C for 5 h.
2
2
{MnII2(PW9)2},6d {MnIIMnIIISiW10},6e {MnIII2W17},6f
{MnII2(SiW8)2},6g {MnII3SiW9},7a,g {MnII3(SiW11)3},7b
{MnII (AsW9)2},7c {MnIIMnIII2SiW9},7d {MnIII3(PW9)2},7e
{MnII3I3SiW9},7f,g {MnII4(PW9)2},8a {MnII4(P2W15)2},8b
{MnII3MnIII/MnIIMnIII3(PW9)2},8c {MnII4(As2W15)2},8d
{MnII4(XW9)2} (X = Si, Ge),8e,f {MnII4(SiW10)2},8g
{MnIII3MnIVP2W15},8h {MnII4P8W48},8i {MnII2MnIII
-
2
(P2W15)2},8j {MnIII3MnIVSiW9},7g,8k {MnIII5(SiW9)2},9
{MnII (PW9)2(PW6)},10a {MnII MnIII4(XW9)2} (X = Si,
6
2
Ge),10b {MnII6(GeW10)3},10c {MnII6GeW6(GeW9)2},10d
{MnII4MnIII2(SiW8)(SiW9)(SiW10)},10e {MnII3MnIII
-
3
(SiW10)3},10f {MnII (XW9)2(XW8)2} (X = Ge, Si),11a,b
7
{MnIII6MnIV(P2W15)2},11c {MnII7(Si2W8)2(Si2W10)2},11d
{MnII P8W48},12a {MnII (GeW10)3},12b {MnII10(SiW9)4},11d
8
8
{MnII MnIII6(SiW9)2(SiW6)2},13 {MnII MnIII6MnIV2(SiW6)2},14
4
4
{MnI I MnI I I 1 3 (PW9 )4 }, 1 5 a {MnI I I 6 MnI V 8 W4 8 }, 1 5 b
{MnII19(SiW10)6},16 and {Mn40(P8W48)(P2W14)4(P2W15)8}.17
Moreover, other mixed-metal POMs containing manganese
together with other 3d or 4f metal ions are also known.18
Recently, Kortz and Bonchio have shown that the acetate-
capped, mixed-valence tetramanganese-containing 9-tungstosili-
cate [MnIII3MnIVO3(CH3COO)3(A-α-SiW9O34)]6− displays
photocatalytic water oxidation properties.19 In the same year,
Su and co-workers showed that the dimanganese(III)-incorpo-
rated δ-Dawson ion [(WO5)3W14MnIII2O44Cl2]12− catalyzes
hydrogen evolution.6f
Our group has been interested for many years in the synthesis
and characterization of discrete-molecular heteropolytungstates
containing high-nuclearity 3d-metal assemblies. For example, we
have reported the {Co16}-containing 36-tungsto-4-phosphate,
-silicate, and -germanate [{Co4(OH)3PO4}4(A-α-XW9O34)4]32−
(X = P, Si, Ge), each composed of four {Co3XW9} units
connected in a tetrahedral fashion to a {Co4O4} cubane unit via
four {PO4} groups.20 These assemblies proved to possess single-
molecule magnet (SMM) behavior. Meanwhile, Wang and co-
workers have shown that these compounds can act as water
oxidation catalysts.21 Recently, we reported the iron(III)
analogues [Na2Fe14(OH)12(PO4)4(A-α-XW9O34)4]20− (X = Si,
Ge), where the inner cubane is heterometallic, being occupied by
two iron and two sodium ions.22 All of these polyanions were
synthesized in the presence of phosphate ions and in basic
aqueous medium, which is somewhat unusual for the synthesis of
transition metal-containing POMs, as mostly acidic conditions
are preferable.
Synthesis of SBA-15-apts-1 and SBA-15-apts-2. Both compounds
(NaCs-1 and NaRb-2) (10 wt %) were separately dissolved in water,
resulting in brown solutions. While stirring, SBA-15-apts (90 wt %) was
slowly added, and the solutions were kept stirring for 24 h. The solutions
were then filtered and washed 3 times with water. The filtrates were
colorless, indicating that both POMs were quantitatively loaded on the
support. The obtained products were then collected and air-dried.
Additionally, the observed decrease of the BET surface area shown in
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Figure 1. Top left: Combined polyhedral/ball-and-stick representation of polyanions 1 and 2. Top right: Representation of various tetrahedral building
units in 1 and 2. Color code: WO6 octahedra (white), MnII/MnIII (yellow), P (pink), Si (green), O (red).
Table S3 is a clear indication that apts was grafted in the mesochannels
of SBA-15, followed by immobilization of 1 and 2, respectively.
X-ray Crystallography. Single crystals of NaCs-1 and NaRb-2 were
mounted on a Hampton cryoloop in light oil for data collection at 100 K.
A Bruker D8 SMART APEX II CCD diffractometer with kappa
geometry and Mo−Kα radiation (graphite monochromator, λ = 0.71073
Å) was used for indexing and data collection. Data integration was
performed using SAINT,24 and routine Lorentz and polarization
corrections were applied. Multiscan absorption corrections were
performed using SADABS.25 Direct methods (SHELXS97) successfully
located the tungsten atoms, and successive Fourier syntheses
(SHELXL2013) revealed the remaining atoms.26 Refinements were
full-matrix least-squares against |F2| using all data. In the final refinement,
all non-disordered heavy atoms (Na, Cs, Rb, Mn, Si, W) were refined
anisotropically; oxygen atoms and disordered counter-cations were
refined isotropically. No hydrogen atoms were included in the models.
Single-crystal XRD refinements showed the presence of one potassium
cation in the solid lattice of NaCs-1, which was not detected by
elemental analysis. We believe this to be a localized impurity within the
crystal, as no potassium cation has been used throughout the synthesis
and isolation of the product. Crystallographic data are summarized in
Table S1 (Supporting Information). Further details on the crystal
structure investigations may be obtained from the Fachinformations-
zentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen,Germany (fax:
(+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the
depository numbers 429913 (for NaCs-1) and 429914 (for NaRb-2).
XAS Measurements. The Mn L2,3-edge X-ray absorption spectros-
copy (XAS) was performed at beamline 6.3.1 of the Advanced Light
Source at Lawrence Berkeley National Laboratory, USA. The spectra
were taken with the sample at room temperature. Linear polarized light
and total electron yield as detection mode were used.
temperature. The solutions were deaerated thoroughly for at least 30
min with pure argon and kept under a positive pressure of this gas during
the experiments.
Catalytic Oxidation. Oxidation of cyclohexene (49 mmol, 5 mL)
under a constant airflow (about 5 mL/min) and in the presence of 45 mg
of catalyst (1 and 2 supported on the modified mesoporous support
SBA-15-apts) was carried out in a temperature-controlled glass vessel at
70 °C in toluene (10 mL). After the reaction, the solution was allowed to
cool to room temperature and then samples were taken for GC analysis.
A GC-2010 Shimadzu instrument equipped with a flame ionization
detector and HP-FFAP column (50 m × 0.32 mm) was used. The carrier
gas was He. The analysis of substrate loss was below 5%.
Battery Performance. The composite electrodes were prepared by
mixing the active material (NaCs-1 and NaRb-2, respectively), carbon
black (super P), and poly(vinylidene fluoride) (PVDF) binder in a
weight ratio of 60:20:20 with N-methylpyrrolidinone (NMP) to form a
slurry. The mixture was coated onto copper foil using a doctor blade
with a height of 75 mm and then dried in air at 80 °C to remove the
solvent. The electrodes were roll-pressed and then punched into pieces
of diameter 16 mm. These punched electrodes were dried at 110 °C
under vacuum for 4 h before transferring to a glovebox. Then, the
electrodes were assembled in 2016 coin cells with circular metallic
lithium metal (diameter 16 mm) as counter electrode, glass fiber
(Whatman, diameter 19 mm) as separator, and 1 M LiPF6 in a mixture of
ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 wt %) as
electrolyte. Cyclic voltammetry was performed with a Biologic VMP3
potentiostat in the range of 0.01−2.0 V vs Li/Li+ with a scan rate of 0.1
mV s−1. A Neware battery tester was used for galvanostatic charge/
discharge tests in the range of 0.01−2.0 V vs Li/Li+ at a current density
of 100 mA g−1.
Magnetic Susceptibility Measurements. The magnetic suscept-
ibility measurements were obtained on a Quantum Design SQUID
magnetometer MPMS-XL. This magnetometer works between 1.8 and
400 K for dc applied fields ranging from −7 to +7 T. Measurements were
performed on polycrystalline samples. The magnetic data were
corrected for the sample holder and the diamagnetic contribution.
Electrochemical Measurements. The electrochemical setup was a
CHI660E driven by a PC with the CHI software. Potentials are quoted
against a saturated calomel electrode (SCE). The counter electrode was
a platinum gauze of large surface area. The working electrode was an
ITO plate (0.3 cm2). All experiments were performed at room
RESULTS AND DISCUSSION
■
Synthesis and Structure. Reaction of Mn(OOCCH3)3·
2H2O and MnCl2·4H2O with Na10[A-α-SiW9O34]·23H2O23 in
7:1 and 6:1 ratios, respectively, resulted in the 16-manganese-
containing, mixed-valent, Keggin-based 36-tungsto-4-silicates
[MnIII10MnII O6(OH)6(PO4)4(A-α-SiW9O34)4]28− (1) and
6
[MnIII4MnII12(OH)12(PO4)4(A-α-SiW9O34)4]28− (2), respec-
tively. Both reactions were carried out in aqueous medium at
pH 9 and in the presence of phosphate ions. Polyanion 1 was
C
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Figure 2. Ball-and-stick representation of the cationic manganese-oxo cores {MnIII10MnII O6(OH)6(PO4)4}12+ in 1 (left) and {MnIII4MnII12(OH)12-
6
(PO4)4}12+ in 2 (right). Color code: MnII (yellow), MnIII (indigo), O (red).
isolated as the hydrated sodium-cesium salt Na19.5Cs8.5-
[{MnIII10MnII6O6(OH)6(PO4)4}(A-α-SiW9O34)4]·90H2O
(NaCs-1), whereas polyanion 2 was isolated as the sodium-
rubidium salt Na18Rb10[MnIII4MnII12(OH)12(PO4)4(A-α-
SiW9O34)4]·75H2O (NaRb-2). The chemical composition
including counter cations and crystal waters was confirmed by
elemental and thermogravimetric analyses (see the Experimental
Section). Single-crystal X-ray diffraction analyses revealed that
polyanions 1 and 2 crystallized in the orthorhombic Pmmn and
monoclinic C2/c space groups, respectively. Both polyanions 1
and 2 are isostructural (Figure 1), each containing 16 Mn centers,
but the exact number of Mn3+ and Mn2+ ions is different (see
Figure 2): 10Mn3+/6Mn2+ in 1 vs 4Mn3+/12Mn2+ in 2 (vide
infra). The structures of 1 and 2 comprise a central {Mn4O4}
cubane unit capped by four trimanganese-substituted {Mn3(A-α-
SiW9)} Keggin units, the overall assembly being connected by
four phosphate {PO4} linkers (Figure 1).
It deserves attention that the manganese-oxo clusters in 1
({MnIII10MnII O6(OH)6}24+) and 2 ({MnIII4MnII12(OH)12}24+
)
6
have the exact same positive charge of 24+, in spite of different
numbers of Mn3+ and Mn2+ ions, but different degrees of
protonation for 1 (6 protons) and 2 (12 protons) counterbalance
this effect. It is also worth noting that the {Mn4O4} cubane cores
in both polyanions 1 and 2 contain exclusively Mn2+ ions,
resulting in a neutral assembly. This is fully consistent with our
earlier reported {Co16} and {Fe14Na2} analogues, which have
{CoII O4} and {FeIII2Na2O4} cores, respectively.20,22
4
The different mixed-valence nature of polyanions 1 and 2 has
structural consequences, leading to a pronounced distortion
within the {Mn16} cores as well as the polyanions overall, as seen
in Figure 2. Polyanion 2 displays an overall “twisted” tetrahedral
arrangement, as compared to the more ideal tetrahedral one for
polyanion 1. This reduction in symmetry is also reflected in the
monoclinic space group C2/c for NaRb-2, compared to the
orthorhombic space group Pmmn for NaCs-1, and the former
also has more IR signals, resulting in a broader spectrum (see
Figure S1).
All manganese centers in 1 and 2 exhibit a distorted octahedral
coordination geometry, and the respective MnII−O bond lengths
are in the range of 2.066(17)−2.413(11) Å for 1 and 2.070(16)−
2.463(14) Å for 2, whereas the MnIII−O bond lengths are in the
range of 1.896(4)−2.325(8) Å for 1 and 1.902(15)−2.367(14) Å
for 2, which indicates the expected shorter bond lengths for
manganese in the higher oxidation state. Moreover, the
respective trans O−Mn−O bond angles are in the range of
157.9(3)−176.8(4)° for 1 and 160.7(5)−175.2(6)° for 2,
whereas the cis O−Mn−O bond angles are in the range of
70.0(4)−103.0(4)° for 1 and 70.6(5)−107.5(5)° for 2.
Bond valence sum (BVS) calculations27 (Table S2) indicated
that, for polyanion 1, the four manganese centers in the central
{Mn4O4} cubane unit all have a 2+ oxidation state. In the capping
{Mn3(SiW9)} Keggin units, 10 out of 12 manganese ions have a
3+ oxidation state, with the 2 Mn2+ ions being located in two
different Keggin units, which are related to each other by a mirror
plane. Hence, 1 contains a total of 10 Mn3+ and 6 Mn2+ ions.
Furthermore, in each {Mn3(SiW9)} Keggin unit of 1, the oxygen
atoms bridging the manganese centers all have a BVS value of ca.
1.5, which we assigned as 50:50% oxo/hydroxo, resulting in a
cationic {MnIII10MnII O6(OH)6}24+ core, and an overall charge
6
of 28− for polyanion 1, which is balanced in the solid state by
19.5 sodium and 8.5 cesium counter cations, as confirmed by
elemental analysis (see the Experimental Section).
BVS on polyanion 2 revealed that all four manganese centers
in the central {Mn4O4} cubane unit have a 2+ oxidation state, in
analogy with 1. Each of the four capping {Mn3(SiW9)} units
contains 1 Mn3+ and 2 Mn2+ ions, so polyanion 2 has a total of 4
Mn3+ and 12 Mn2+ ions (Table S2). All oxygen atoms bridging
the manganese centers in the {Mn3(SiW9)} Keggin units of 2 are
monoprotonated and hence hydroxo ligands. This results in a
cationic {MnIII4MnII12(OH)12}24+ core, and an overall charge of
28− for polyanion 2, which is balanced in the solid state by 18
sodium and 10 rubidium counter cations, as confirmed by
elemental analysis (see the Experimental Section).
With respect to the synthetic procedures of 1 and 2, three
points were crucial in order to successfully synthesize and isolate
the desired products in best yield. First, solid Na3PO4 had to be
added slowly to the reaction mixture, after dissolution of all other
reactants, until the pH reached 9, as otherwise the yield is lower.
Also, if phosphate is already present before addition of the other
reagents, precipitation is induced. It seems that the presence of
phosphate ions at a later stage of the reaction process directs the
structural reorganization of the {Mn16} assembly, which is
consistent with our previous work.15a,20−22,28 Second, the ratio of
lacunary POM precursor to manganese ions needs to be 1:7 for 1
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and 1:6 for 2 in order to obtain the highest yields, rather than the
stoichiometric ratio of 1:4. Third, although both polyanions 1
and 2 contain mixed-valence (Mn2+/Mn3+) manganese-oxo
clusters, the manganese reagents for the respective syntheses of 1
and 2 need to contain exclusively Mn3+ for 1 and Mn2+ for 2. Any
attempt to reproduce 1 and 2 by using mixtures of Mn2+ and
Mn3+ salts resulted in unidentified precipitates. It should be
noted that the exact number of Mn2+ and Mn3+ centers is
perfectly reproducible, as we have verified by performing
numerous experiments.
multiplet structure and less sharp features. The Mn L2,3-edge
XAS of NaRb-2 has its absolute maximum in intensity at 640 eV
(A), confirming that the major fraction of the Mn ions in 2 is
divalent. However, feature B, corresponding to the Salox L3 edge
maximum is significantly more pronounced compared to
MnStar, indicating also the presence of trivalent Mn ions in 2.
In contrast, the Mn L3 multiplet structure of NaCs-1 is more
similar to the spectrum of Salox, confirming that the majority of
the Mn ions in 1 are in a trivalent state. However, a more intense
feature A and the presence of a significant feature C as compared
to Salox suggest that also some divalent Mn ions are present in 1.
Magnetic Measurements. Temperature-dependent dc
magnetic susceptibility measurements were carried out on solid
samples of polyanions 1 and 2 in the temperature range 1.8−300
K in a field of 1000 Oe (Figure 4).
Synthesis of 1 with exclusively Mn3+ ions implies that some
manganese ions must be reduced in situ during the course of the
reaction, whereas the synthesis of 2 with exclusively Mn2+ ions
implies that some manganese ions must be oxidized in situ during
the course of the reaction. It should be noted that both reactions
were performed in open beakers. Autoxidation and autoreduc-
tion of manganese ions in aqueous solution during reactions with
lacunary POMs is not unprecedented. A literature survey on
manganese-containing heteropolytungstates demonstrated sev-
eral examples of mixed-valence manganese centers in the final
products, with manganese in a single oxidation state being used
in the reactions.8j,10d,14 However, preformed mixed-valence,
multinuclear manganese complexes have also been used for the
synthesis of mixed-valence Mn-containing POMs.7d,e,g,8h,k
Finally, the isolation process of polyanions 1 and 2 as bulk
pure crystalline solids in maximum yields required the presence
of cesium ions in the case of 1 and rubidium ions for 2.
XAS Measurements. X-ray absorption spectra on NaCs-1
and NaRb-2 were carried out at room temperature using
beamline 6.3.1 of the Advanced Light Source for linear polarized
light with total electron yield as detection mode. The spectra
were compared with those of the reference compounds
Figure 4. Temperature dependence of the χT product for polycrystal-
line samples of 1 (squares) and 2 (circles) at 1000 Oe.
The χMT value of polyanion 1 progressively decreases from
44.7 cm3 K mol−1 at 300 K to 22.8 cm3 K mol−1 at ∼40 K before
falling rapidly to 9.6 cm3 K mol−1 at 1.8 K. For polyanion 2, the
χMT values decrease from 52.8 cm3 K mol−1 at 300 K to 8.9 cm3 K
mol−1 at 1.8 K. The spin-only χMT value for polyanion 1
comprising 10 noninteracting Mn3+ and 6 Mn2+ ions is 56.25 cm3
K mol−1, and for polyanion 2 comprising 4 Mn3+ and 12 Mn2+
ions is 64.50 cm3 K mol−1, and hence, Figure 4 is indicative of an
appreciable antiferromagnetic exchange between the Mn3+ and
Mn2+ centers in 1 and 2. For polyanions 1 and 2, the total spin
value ranges would vary from 0 to 35 and from 0 to 38,
respectively. To determine their spin ground states, magnet-
ization data were collected in the temperature range 2−5 K and
in the magnetic field range 0−70000 Oe (Figure S4). The
magnetization data for both compounds do not show any
saturation even at the highest field measured. Possible reasons
could be the presence of magnetic spin frustration or significant
zero-field splitting in the spin ground state. Low-lying excited
states are also anticipated for such big ions which generally have a
high density of molecular spin states.
Also, ac susceptibility measurements were carried out in zero
and nonzero dc fields, but no peaks down to 1.8 K and no
frequency-dependent in-phase and out-of-phase signals were
observed (Figures S5 and S6). The latter results indicate that
polyanions 1 and 2 are not single-molecule magnets.
Electrochemistry. The redox behavior of 1 and 2 was
studied in pH 7 media. The UV−visible stability carried out in
these media revealed that both polyanions are sufficiently stable
to stand electrochemical characterization (see the SI for details).
Ever since the pioneering work of Pope and co-workers,6a,8c,31a,b
numerous examples were reported on the electrochemical
generation of high-valent manganese center(s) within poly-
[MnII (C12H9N2O)6](BF4)2·2CH3CN·H2O (“MnStar”, only
4
Mn2 + -containing)2 9 and [MnI I I 6 (μ3 -O)2 (Salox)6 -
(CH3CH2OH)4{(CH3)3CCOO}2] (“Mn salox”, only Mn3+-
containing);30 see Figure 3.
Figure 3. Manganese L2,3-edge X-ray absorption spectra of NaCs-1 and
NaRb-2. For comparison, the spectra of the exclusively Mn2+-containing
“MnStar”29 and the exclusively Mn3+-containing “Salox”30 are also
displayed.
The Mn L3 edge of MnStar comprises three distinct features,
namely, an absolute maximum of absorption at 640 eV (labeled
A), and shoulders around 641.5 eV (B), and 643.5 eV (C),
whereas the L3 X-ray absorption of Salox peaks at 641.5 eV (B)
(Figure 3). The Mn XAS of Salox also shows a feature around
640 eV, which can be attributed to charge transfer excitations
(d5L states). The L2 edge (spanning the range of ∼650−655 eV)
of MnStar comprises two small, but rather sharp, peaks. In
contrast, the Mn L2 edge of Salox consists of a rather broad
E
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oxotungstates.6e,7d−g,13,16,18a,b,19,31c−f These redox processes are
well-known to depend on various parameters including solvent,
pH, and composition of electrolyte.31c,d,f The electrochemical
generation of higher-valent multinuclear manganese complexes
is an important issue because most of them can act as earth-
abundant metal electrocatalysts for several challenging oxidation
processes including the oxygen-evolving reaction
(OER).7e,16,19,31c,e Figure 5A shows the redox activity of the
region where appreciable oxidation current does not appear on
the CV recorded with the electrolyte alone. The variation of the
Mn oxidation peak current as a function of the square root of the
potential scan rate is shown in Figure 5B. The linearity of this
curve demonstrates that the CVs feature a diffusion-controlled
process. The Mn centers redox processes and the OER
associated with 2 are also featured by well-behaved waves
(Figure S8), but the oxidation potential of Mn2+ to Mn4+ in 2 is
shifted in the positive direction (by 0.070 V) compared to 1,
which means that the latter is slightly easier to oxidize. It is worth
noting that reaching the OER wave has no deleterious effect on
the redox processes associated with the Mn and W centers.
However, the determination of the nature of the true
electrocatalyst and the study of its stability during the OER are
among the main challenges and will be investigated in the future.
The W6+ centers in 1 and 2 exhibit two quasi-reversible and
well-defined reduction waves with the same general shape
(Figure 6).
Figure 6. Cyclic voltammograms of the quasi-reversible W6+ waves of 1
and 2, respectively, run in a pH 7 (1 M Li CH3COO + CH3COOH)
medium. The scan rate was 100 mV s−1, and the reference electrode was
a saturated calomel electrode (SCE). The concentration of the
polyanions was 50 μM.
However, when compared to 1, a slight anodic shift of ca. 0.035
V was observed for both W reduction waves of 2 (at ca. −0.993
and −1.067 V vs SCE, respectively). It is well-known that the
voltammetric behaviors of most POMs depend on several
parameters including the basicity of the reduced species and,
occasionally, of the fully oxidized species.32 The overpotential
required for the onset of the OER (425 mV) is lower than that
observed for [MnIII3MnIVO3(CH3COO)3(A-α-SiW9O34)]6−
(530 mV),19 but the smallest value reported for a manganese-
Figure 5. Redox activity of the Mn3+ and Mn4+ centers of 50 μM 1 in a
pH 7 medium (1 M LiCH3COO/CH3COOH). The scan rate was 100
mV s−1, and the reference electrode was a saturated calomel electrode
(SCE). (A) Dotted pink line: background current; full blue line: cyclic
voltammogram recorded in the presence of 1. (B) Variation of the Mn
oxidation peak current intensity as a function of the square root of the
scan rate.
Mn centers of 50 μM 1 and that of the supporting electrolyte
alone between −0.880 and +1.350 V versus SCE at a scan rate of
100 mV s−1. The reduction of the Mn3+ centers to Mn2+ is
represented by a broad wave peaking at −0.040 V versus SCE.
On the reverse scan, the CV pattern features one quasi-reversible
oxidation wave (at +0.548 V versus SCE) preceded by a low
intensity prewave and followed by a large irreversible wave. In
agreement with previous reports, these waves are assigned to the
stepwise oxidation of Mn2+ to Mn4+ and the OER,
respectively.7e,16,31e The rereduction of the Mn4+ centers to
Mn3+ is also featured by a well-defined wave located at +0.286 V
vs SCE. The remarkable outcome is that the presence of 1 makes
it perfectly possible to observe the OER wave in a potential
containing POM was observed for [Mn19(OH)12-
(SiW10O37)6]34− (330 mV).16 It is worth noting that the
electrolytes used in these reports are more acidic (pH 5)
compared with the pH 7 medium used here. Therefore,
polyanion 1 is potentially a good candidate for efficient
electrocatalysis of the OER.
Catalysis. The heterogeneous catalytic activity of 1 and 2 for
cyclohexene oxidation was carried out by loading both
polyanions on a modified mesoporous support of SBA-15-apts
(see details in the Experimental Section).33 The BET surface area
of these supported catalysts was measured and determined to be
around 350 m2/g (Table S3; see the SI). According to our
experiments, the desired reaction (Scheme 1) did not occur in
F
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the blank test without any catalyst or in the presence of the
support alone, whether SBA-15 or SBA-15-apts.
cycles.3d The main oxidation peaks can be observed at 1.07 and
0.17 V vs Li/Li+, and the corresponding reduction peaks are
located at 0.30 and 0.01 V vs Li/Li+. These peaks might result
from the redox transition of Mn and W, and the insertion/
extraction of Li ions with good reversibility. This can be further
proven by X-ray photoelectron spectroscopy (XPS) and in situ
X-ray absorption spectroscopy (XAS), which are not discussed
here.
Scheme 1. Cyclohexene Allylic Oxidation Pathway
Figure 7C,D shows the galvanostatic charge−discharge
(GCD) curves of the NaCs-1 and NaRb-2 electrodes with
initially 2 cycles at a current density of 100 mA g−1. Two well-
defined plateaus can be observed on the discharge/charge curves
where the potentials of the plateaus are similar to the redox peaks
in the CV curves. The discharge capacity of the NaCs-1 and
NaRb-2 electrodes in the first cycle are 908 and 727 mAh g−1,
and the discharge capacities in the second cycle are 430 and 316
mAh g−1, respectively. These results are comparable with
graphite (theoretical capacity: 372 mAh g−1), which is the
most commonly used anode material in LIBs.35
The NaCs-1 and NaRb-2 electrodes exhibit high cycling
stability with excellent Coulombic efficiencies of ∼100% up to 50
cycles; see Figure 7E,F. The capacity retention for the NaCs-1
electrode is 77% and for the NaRb-2 electrode is 85% (calculated
using discharge capacity from 2nd to 50th cycle). The above
results suggest that NaCs-1 and NaRb-2 electrodes are
potentially interesting anode materials for real LIB applications.
The homogeneous and heterogeneous catalytic oxidation of
cyclohexene has been previously studied with several transition
metal-modified mesoporous materials and various transition
metal-containing POMs (including manganese) under different
oxidizing conditions, where the product distribution varied
accordingly.34 In some cases, the corresponding epoxide of
cyclohexene dominated, whereas the allylic products did in other
cases.
Here, the selectivity toward 2-cyclohexene-1-one was higher
than that toward 2-cyclohexene-1-ol, reaching a maximum of
52−55% and 32−38%, respectively. Such product distribution,
where allylic oxidation products are dominant, is typical for a
homolytic oxidation mechanism.34e,f
As shown in Table 1, cyclohexene conversion increased from
50% to 63% when 2 was used as a catalyst instead of 1, and the
Table 1. Air Oxidation of Cyclohexene by Polyanions 1 and 2
a
Immobilized on SBA-15-apts
CONCLUSIONS
■
bc
,
selectivity (%)
We have successfully synthesized the two 16-manganese-
containing 36-tungsto-4-silicates [MnIII10MnII O6(OH)6(PO4)4-
6
(%) conv.
cyclohexene allylic ketone allylic alcohol TOF (h−1
d
(A-α-SiW9O34)4]28− (1) and [MnIII4MnII12(OH)12(PO4)4(A-α-
SiW9O34)4]28− (2) by careful tuning of the synthetic conditions.
Interestingly, polyanions 1 and 2 comprise mixed-valent, cationic
catalyst
)
SBA-15-apts-1
SBA-15-apts-2
50
63
55.5
32.4
38.4
3100
3740
52.26
{MnIII10MnII O6(OH)6}24+ and {MnIII4MnII12(OH)12}24+ cores,
a
Reaction conditions: cyclohexene (49 mmol, 5 mL), toluene (10
mL), catalyst amount: 45 mg, 70 °C, 24 h, airflow: 5 mL/min. GC
yield based on cyclohexene consumed. Some other unidentified
6
b
respectively, which have exactly the same charge. Synthesis of 1
and 2 using exclusively Mn3+ and Mn2+ ions, respectively, implies
autoxidation and autoreduction of manganese ions in situ. The
hydrated mixed alkali salts of 1 and 2 were investigated in the
solid state by single-crystal X-ray diffraction, FT-IR spectroscopy,
thermogravimetric and elemental analyses, as well as X-ray
absorption spectroscopy. Magnetic studies showed that poly-
anions 1 and 2 do not act as molecular magnets and possess a
complex spin ground state which does not saturate even at 70000
Oe. Both 1 and 2 enhance the OER as shown by electro-
chemistry, and they also act as heterogeneous catalysts in the
oxidation of cyclohexene. The difference in activity can be related
to the different numbers of Mn3+ and Mn2+ ions and possibly a
resulting subtle structural variation and warrants further
investigation. Furthermore, both 1 and 2 have been shown to
act as potential anode candidates for LIBs with an initial
discharge capacity of 908 and 727 mAh g−1, respectively, and
excellent Coulombic efficiencies ∼ 100% up to 50 cycles.
In summary, we have expanded the class of M16-containing
tetrahedral polyanions to manganese with the discovery of two
isostructural, mixed-valent Mn2+/3+ derivatives. Such compounds
are structurally appealing, but more importantly, they contain a
multitude of interesting electronic, redox, magnetic, and catalytic
properties, which we have all explored. In future work, we plan to
investigate the catalytic properties of 1 and 2 in more detail,
including water oxidation, and we will also try expanding this
POM class further structurally and compositionally.
c
d
products up to 12%. TOF = (moles of consumed cyclohexene)/
(moles of POM × time).
former also has a higher turnover frequency (TOF) than the
latter (3740 h−1 vs 3100 h−1). It appears that not only the total
number and oxidation state of the manganese centers is
important, but also the detailed structural arrangement of these
metal ions in the polyanion structure. It should be remembered
that the number of Mn3+ and Mn2+ ions is different in polyanions
1 and 2, and the latter also exhibits a small structural distortion
(see Figure 2), compared to the more ideal tetrahedral situation
for 1.
Battery Performance. We decided to test the concept of
NaCs-1 and NaRb-2 as anode materials in lithium ion battery
(LIB) applications. The battery performances were investigated
by galvanostatic charge−discharge (GCD) cycling and cyclic
voltammetry (CV) in a half-cell configuration as described in the
Experimental Section. CVs were measured for the first three
cycles at a scan rate of 0.1 mV s−1 over a potential range of 0.01−
2.0 V vs Li/Li+ in order to understand the electrochemical
properties of NaCs-1 and NaRb-2 electrodes, as shown in Figure
7A,B. The NaCs-1 and NaRb-2 electrodes exhibited similar
behavior in the CV curves. The solid electrolyte interphase (SEI)
formed in the first cycle and generated an irreversible reduction
peak at ∼0.7 V vs Li/Li+ which disappeared in the following
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Figure 7. Battery performance of electrodes based on 1 and 2 in 1 M LiPF6/EC:DEC (1:1) in a half-cell configuration with Li metal as counter electrode:
cyclic voltammograms of (a) NaCs-1 and (b) NaRb-2 electrodes measured at a scan rate of 0.1 mV s−1; the galvanostatic charge−discharge curves of (c)
NaCs-1 and (d) NaRb-2 electrodes measured at a current density of 100 mA g−1; the cycling performance and the Coulombic efficiency of (e) NaCs-1
and (f) NaRb-2 electrodes measured at a current density of 100 mA g−1.
Present Address
ASSOCIATED CONTENT
■
☆Institute of Nanotechnology, Karlsruhe Institute of Technol-
ogy (KIT), Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-
Leopoldshafen, Germany.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02503.
Author Contributions
□A.H. and M.I. contributed equally to this work.
Crystallographic data for NaCs-1 (CIF)
Crystallograpic data for NaRb-2 (CIF)
X-ray crystallographic table, selected bond valence sum
values, surface area measurements of the support and
supported catalysts, IR spectra, TGA curves, additional
magnetism figures, and UV−vis spectra (PDF)
Notes
The authors declare no competing financial interest.
△B.K. is retired.
ACKNOWLEDGMENTS
■
U.K. thanks the German Science Foundation (DFG, KO 2288/
20-1), the CMST COST Action CM1203 (PoCheMoN), and
Jacobs University for research support, and kindly acknowledges
the Chinese Academy of Sciences President’s International
Fellowship Initiative (Grant No. 2015VMA041). A.H. thanks
DAAD for a doctoral fellowship. M.I. thanks DFG and Institute
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: u.kortz@jacobs-university.de. Fax: (+49)421-200-
3102. Homepage: www.jacobs-university.de/ses/ukortz.
H
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4132. (e) Car, P. E.; Spingler, B.; Weyeneth, S.; Patscheider, J.; Patzke,
G. R. Polyhedron 2013, 52, 151−158. (f) Jiao, Y. Q.; Qin, C.; Wang, X.
L.; Liu, F. H.; Huang, P.; Wang, C. G.; Shao, K. Z.; Su, Z. M. Chem.
Commun. 2014, 50, 5961−5963. (g) Winter, R. S.; Long, D. L.; Cronin,
L. Inorg. Chem. 2015, 54, 4151−4155.
of Nanotechnology, Karlsruhe Institute of Technology (KIT),
for a postdoctoral fellowship, and she also thanks DAAD and the
Higher Education Commission of Pakistan for a doctoral
fellowship (Jacobs University). She also acknowledges the
University of Balochistan, Quetta, Pakistan, for allowing her to
pursue Ph.D. (Jacobs University) and postdoctoral (KIT) studies
in Germany. The Advanced Light Source is supported by the
Director, Office of Science, Office of Basic Energy Sciences, of
the U.S. Department of Energy, under Contract No. DE-AC02-
05CH11231. G.Z. thanks the National Science Foundation of
China (No. 21371173) for the funding and research support. H.-
Y.C. and U.S. thank TUM CREATE, National Research
Foundation of Singapore, and Newcastle University for research
and funding support. Figures 1 and 2 were generated by
Diamond, Version 3.2 (copyright Crystal Impact GbR).
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