Controlled reactivity tuning of metal-functionalized vanadium oxide Clusters

Article abstract of DOI:10.1002/chem.201501049

Controlling the assembly and functionalization of molecular metal oxides [M_xO_y]^n- (M=Mo, W, V) allows the targeted design of functional molecular materials. While general methods exist that enable the predetermined functionalization of tungstates and molybdates, no such routes are available for molecular vanadium oxides. Controlled design of polyoxovanadates, however, would provide highly active materials for energy conversion, (photo-) catalysis, molecular magnetism, and materials science. To this end, a new approach has been developed that allows the reactivity tuning of vanadium oxide clusters by selective metal functionalization. Organic, hydrogen-bonding cations, for example, dimethylammonium are used as molecular placeholders to block metal binding sites within vanadate cluster shells. Stepwise replacement of the placeholder cations with reactive metal cations gives mono- and difunctionalized clusters. Initial reactivity studies illustrate the tunability of the magnetic, redox, and catalytic activity.

Full text of DOI:10.1002/chem.201501049

DOI: 10.1002/chem.201501049
Communication
&
Polyoxometalates
Controlled Reactivity Tuning of Metal-Functionalized Vanadium
Oxide Clusters
[a]
[a]
[b]
[b]
[b]
Katharina Kastner, Johannes Forster, Hiromichi Ida, Graham N. Newton, Hiroki Oshio,
[a]
and Carsten Streb*
Dedicated to Prof. Dr. Rolf W. Saalfrank on occasion of his 75th birthday
control, however, is required for the targeted synthesis of ma-
[4]
terials with specific structure and chemical reactivity.
Abstract: Controlling the assembly and functionalization
nÀ
One notable exception in POM chemistry where controlled
functionalization is possible are lacunary clusters: these species
are obtained by controlled hydrolysis of the parent anion, for
of molecular metal oxides [M O ] (M=Mo, W, V) allows
x
y
the targeted design of functional molecular materials.
While general methods exist that enable the predeter-
mined functionalization of tungstates and molybdates, no
such routes are available for molecular vanadium oxides.
Controlled design of polyoxovanadates, however, would
provide highly active materials for energy conversion,
4À
example, the Keggin anion [SiW O ] or the Dawson anion
12 40
6
À
[
P W O ] . In the case of the Keggin anion, lacunary clusters
2 18 62
8À
such as the monovacant [SiW O ]
or the divacant
11 39
8À
[2c]
[
SiW O ] are formed, which feature one or several vacant
10 34
coordination sites. These binding sites can be functionalized
(
photo-) catalysis, molecular magnetism, and materials sci-
nÀ
with metal cations, giving species such as [M(H O)SiW O ]
ence. To this end, a new approach has been developed
that allows the reactivity tuning of vanadium oxide clus-
ters by selective metal functionalization. Organic, hydro-
gen-bonding cations, for example, dimethylammonium
are used as molecular placeholders to block metal binding
sites within vanadate cluster shells. Stepwise replacement
of the placeholder cations with reactive metal cations
gives mono- and difunctionalized clusters. Initial reactivity
studies illustrate the tunability of the magnetic, redox,
and catalytic activity.
2
11 39
nÀ
[2c,5]
and [{M(H O)} SiW O ] (M: metal cation).
Using this ap-
2
2
10 38
[6]
proach, functional molecular materials for energy conversion,
[7]
[8]
(
photo-)catalysis,
molecular magnetism,
supramolecular
[9]
[10]
chemistry and materials science
have been developed.
[2c,3b]
However, to-date, this approach is limited to tungstate-
[
11]
and molybdate-based lacunary clusters, and no analogous
[12]
strategy exists in vanadium oxide chemistry. Controlled and
predictable access to metal-functionalized vanadates, however,
would allow researchers to exploit their potential as tunable,
highly reactive molecular materials in solar energy conver-
[12d,f,g]
[13a]
[13b]
sion,
magnetism
redox catalysis,
molecular electronics,
and
[13c]
[13d]
as well as metal oxide nanostructures.
In supramolecular chemistry, the predictable synthesis of
a target compound is difficult to achieve because complex
self-assembly mechanisms often do not allow a high degree of
We have recently taken the first steps towards controlled
metal functionalization in vanadium oxide cluster chemistry:
a
chloride-templated
dodecavanadate
cluster,
[
1]
3À
reaction control. In inorganic supramolecular chemistry, this
is particularly obvious for molecular metal oxides, or polyoxo-
metalates (POMs). POMs are anionic transition metal oxo clus-
ters formed by the spontaneous self-assembly of oxometalate
(DMA) [V O Cl]
(={V }; DMA=dimethylammonium) was
12
2
12 32
developed that is capable of selective metal-ion binding. {V }
1
2
features two hexagonal metal binding sites that are blocked
[14]
by hydrogen-bonded DMA placeholder cations (Figure 1).
Reaction of {V } with transition metals allows the selective re-
[
2]
precursors and a variation of the reaction conditions gives
12
[2,3]
access to a vast number of different cluster architectures.
placement of one DMA cation, giving the mono-functionalized
nÀ
III
II
II
However, this synthetic approach inherently lacks the precise
control required for predetermined cluster assembly, and often
the resulting architectures cannot be predicted. This level of
cluster anions [M(L)V O Cl] (={MV }, M=Fe , Co , Cu , and
12
32
12
À
II
Zn ; L=ligand, for example, MeCN, Cl ), see Figure 1. Density-
functional theory calculations suggested that the second DMA
[14]
cation can in principle be replaced also.
Here, we show how stepwise metal incorporation into {V }
[
a] K. Kastner, J. Forster, Prof. Dr. C. Streb
Institute of Inorganic Chemistry, Ulm University
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
E-mail: carsten.streb@uni-ulm.de
1
2
can be used to control cluster reactivity; examples in redox,
magnetic, and catalytic activity are given. To this end, the
DMA-placeholder-blocked {V } was allowed to react with
Homepage: http://www.strebgroup.net
12
[
b] H. Ida, Dr. G. N. Newton, Prof. Dr. H. Oshio
MnCl ·4H O in acetonitrile solution at room temperature. Crys-
2
2
Graduate School of Pure and Applied Sciences, University of Tsukuba
Tennodai 1-1-1, Tsukuba 305-8571 (Japan)
tallization of the mother liquor gave green block crystals, and
single-crystal X-ray diffraction (SC-XRD, see the Supporting In-
formation) confirmed the formation of the mono-manganese-
functionalized species (nBu N) (DMA)[(MnCl)V O Cl]·EtOAc
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501049 (synthetic, analytical, crystallo-
graphic, spectroscopic, magnetometric and catalytic details).
4
3
12 32
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&
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Communication
tween the manganese and the chloride template (d(Mn···Cl) ca.
3.2–3.4 ꢁ) are observed; the Mn coordination environment is
completed by a terminal chloro ligand. Bond valence sum
V
II
(
BVS) calculations gave the expected oxidation states (V , Mn );
no protonation of the cluster shell was found.
In the next step, we investigated whether {MnV } can be
12
II
functionalized further by a second Mn center. Initial studies
had suggested that the dimanganese-functionalized cluster is
formed as a minor product in solution (based on ESI mass
spectrometry); however, no crystalline product was obtained.
It was hypothesized that the product yield might be increased
by reduction of the vanadate shell, as this should improve the
electrostatic interactions between the cluster anion and the
II
Mn cation. Therefore, {MnV } was allowed to react with
12
MnCl ·4H O (4 equiv) in acetonitrile solution in the presence of
2
2
the reducing agent N H ·H O (0.5 equiv). Diffusion of ethyl ace-
2
4
2
tate into the reaction solution gave one crystalline product
and SC-XRD (see Supporting Information) showed that the di-
functionalized species (nBu N) [(MnCl) V O Cl] (={Mn V })
Figure 1. Top : {V12} featuring two DMA cations (left) and detailed illustration
of the DMA-blocked binding site (right). Bottom: the mono-functionalized
2
+
[14]
4
4
2
12 32
2
12
{
MV12} (left, here: M=Zn
)
and detailed illustration of the metal coordi-
was obtained (yield ca. 40%). Structural analysis shows two
nation mode (right). Color scheme: V: teal; Cl: light green; Zn: brown; O:
red, N: blue; C: gray; H: white.
II
Mn centers coordinated to the metal binding sites; their
mode of coordination is virtually identical to the binding mode
described above for the monofunctionalized {MnV }. Structur-
12
al overlay of the metal oxide framework of {V }, {MnV }, and
12
12
{
Mn V } showed that significant changes in the vanadium
2 12
II
oxide framework geometry are observed upon Mn coordina-
tion (Figure 2 and Supporting Information); the structural flexi-
bility of the vanadate shell suggests that incorporation of
larger cations (e.g. f-block metal cations) should also be possi-
ble. ESI mass spectrometry showed that both {MnV } and
1
2
{
Mn V } are stable in solution and can be transferred into the
2 12
gas phase without de-metalation of the manganese ions from
the cluster shell (see the Supporting Information).
Charge balance considerations and BVS calculations indicate
IV
the presence of one delocalized V center in {Mn V }; charge
2
12
delocalization is not unexpected as all V centers are in identical
[15]
square-pyramidal [VO ] coordination environments. This was
5
further substantiated by the observation of inter-valence
charge-transfer (IVCT) transitions between lꢀ 800–1600 nm
(
see the Supporting Information). SQUID magnetometry (T=
1
.8–300 K, see the Supporting Information) confirmed the pres-
IV
ence of one reduced V center in {Mn V } and gave c T
values (at 300 K) of 9.16 emumol K for {Mn V } and 4.4 emu
mol K for {MnV }; the results are in line with the expected
spin-only values.
2
12
m
À1
2
12
À1
12
[16]
Further, antiferromagnetic exchange was
II
observed for {Mn V }, highlighting that magnetic interactions
Figure 2. a) (1) Mono-functionalization of {V12} with Mn giving {MnV12};
2
12
II
(
(
2) di-functionalization by reacting {MnV12} with Mn , giving {Mn
3) one-pot di-functionalization of {V12} giving {Mn V12}. Color scheme: see
2
Figure 1. b) Overlay of the metal oxide frameworks of {V12} (red), {MnV12
2
V
12};
within the cluster shell are tunable by incorporation of the de-
[16]
sired paramagnetic metal ions.
Electrochemical analyses
}
show that manganese incorporation allows tuning of the elec-
2
(green) and {Mn V12} (blue).
tronic and redox properties: {MnV } shows three quasi-reversi-
12
ble redox-processes while {Mn V } features six quasi-reversible
2
12
(
=(nBu N) {MnV }·EtOAc) in yields of about 39% (see Figure 2
redox couples (see the Supporting Information), making the
4
3
12
[6c]
and the Supporting Information). {MnV } is virtually isostruc-
tural to the {MV } species described above and also features
one hydrogen-bonded DMA placeholder cation. The Mn ion is
coordinated to the metal binding site by four V-O-Mn coordi-
nation bonds, and long-range electrostatic interactions be-
materials interesting for multi-electron transfer and storage.
12
[
14]
We were further able to show that the one-pot difunctionali-
12
II
zation of {V } is also possible (Figure 2): to this end, {V } was
12
12
allowed to react with MnCl ·4H O (4 equiv) and N H ·H O
2
2
2
4
2
(0.5 equiv) in acetonitrile at room temperature. Diffusion crys-
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Communication
tallization of the reaction mixture gave single crystals of
Mn V } in yields of about 49%; product purity was confirmed
tions measured, 36924 unique (Rint =0.0502), 2076 refined parame-
ters, R =0.0581, wR =0.1984, GooF=1.151.
{
1
2
2
12
by SC-XRD, EA, ICP-MS, ESI-MS, FT-IR, and UV-Vis methods.
Based on their (electro-) chemical properties, the clusters
were considered promising candidates for oxidative CÀH-acti-
Crystallographic data and structure refinement for {Mn V12}:
2
Monoclinic space group, P2 /c, a=24.324(5), b=16.727(3), c=
1
3
À3
2
4.438(5) ꢁ, b =94.71(3)8, V=9909(3) ꢁ , Z=4, 1=1.535 gcm
m(MoKa)=1.470 mm , T=150(2) K, 161699 reflections measured,
,
À1
[
17]
vation catalysis. Initial catalytic tests used acetonitrile solu-
tions of the respective cluster anion (1 mol%), the substrate
2
0263 unique (Rint =0.0796), 1134 refined parameters, R =0.0571,
1
wR =0.1408, GooF=1.088.
2
9,10-dihydroanthracene, and the oxidant tBuOOH (12 equiv).
CCDC-1038467 ({MnV }) and CCDC-1038466 ({Mn V }) contain the
1
2
2 12
Both manganese-functionalized vanadates showed CÀH activa-
tion reactivity, and the formation of the product anthracene
was observed. Moderately higher zeroth order rate constants
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
À1
were observed for {Mn V } (k ({Mn V })=1.2 mmh ) com-
2
12
obs
2
12
À1
pared with {MnV } (k ({MnV })=1.0 mmh ). The non-func-
12
obs
12
Acknowledgements
tionalized {V } showed no catalytic activity and simple manga-
12
nese salts (e.g. MnBr ·4H O, Mn(NO ) ·H O) showed significantly
2
2
3 2
2
This work was supported through the Fonds der Chemischen
Industrie (C.S.) and a doctoral fellowship from the Deutsche
Bundesstiftung Umwelt (K. K.). Financial support by the Deut-
sche Forschungsgemeinschaft (DFG) (grant no: STR1164/4-1),
the DFG Graduate School GRK 1626 “Chemical Photocatalysis”,
the University Erlangen-Nuremberg and Ulm University is
gratefully acknowledged. M. Dꢂrr and Prof. Dr. I. Ivanovic-Bur-
mazovic are acknowledged for ESI-MS measurements.
lower oxidative activity, even when employed at 15 mol% (see
Figure 3 and the Supporting Information).
Keywords: mass spectrometry
· metal oxides · metal-
functionalization · polyoxometalates · self-assembly
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Published online on && &&, 0000
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Communication
COMMUNICATION
Stepwise functionalization of a vanadi-
um oxide cluster by one or two func-
tional metal centers is achieved by
using a molecular placeholder ap-
proach. The novel strategy provides
access to redox-, catalytically, and mag-
netically active molecular materials.
&
Polyoxometalates
K. Kastner, J. Forster, H. Ida, G. N. Newton,
H. Oshio, C. Streb*
&
& – &&
Controlled Reactivity Tuning of Metal-
Functionalized Vanadium Oxide
Clusters
Chem. Eur. J. 2015, 21, 1 – 5
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5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ