Herein, we report for the first time polyoxometalate (POM)-based single-molecule magnets with mononuclear transition metal cores. The mononuclear Fe(III)-, Co(II)-, and Mn(III)-containing POMs have successfully been synthesized from a trivacant lacunary Keggin-type silicotungstate precursor TBA4H6[A-α-SiW9O34]·2H2O (TBA = tetra-n-butylammonium), and the highly distorted octahedral geometries of the incorporated metal cations resulted in the magnetic anisotropies and the single-molecule magnet behavior with mononuclear paramagnetic metal ions under the applied dc field.
Aqueous
solutions of polyoxometalates (POMs) have been shown to
have potential as high-capacity energy storage materials due to their
potential for multi-electron redox processes, yet the mechanism of
reduction and practical limits are currently unknown. Herein, we explore
the mechanism of multi-electron redox processes that allow the highly
reduced POM clusters of the form {MO
3
}
y
to absorb
y
electrons in aqueous solution,
focusing mechanistically on the Wells–Dawson structure X
6
[P
2
W
18
O
62
], which comprises
18 metal centers and can uptake up to 18 electrons reversibly (
y
= 18) per cluster in aqueous solution when the countercations
are
lithium
. This unconventional redox activity is
rationalized by density functional theory, molecular dynamics simulations,
UV–vis, electron paramagnetic resonance spectroscopy, and small-angle
X-ray scattering spectra. These data point to a new phenomenon showing
that cluster protonation and aggregation allow the formation of highly
electron-rich meta-stable systems in aqueous solution, which produce
H
2
when the solution is diluted. Finally, we show that
this understanding is transferrable to other salts of [P
5
W
30
O
110
]
15–
and [P
8
W
48
O
184
]
40–
anions, which
can be charged to 23 and 27 electrons per cluster, respectively.
The design of structurally well-defined anionic molecular metal-oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α-Dawson-type silicotungstate, TBA8[α-Si2W18O62]⋅3 H2O (II) that possesses a -8 charge was successfully synthesized by dimerization of a trivacant lacunary α-Keggin-type silicotungstate TBA4H6[α-SiW9O34]⋅2 H2O (I) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus-centered POM TBA6[α-P2W18O62]⋅H2O (III) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ3-oxygen atoms were increased by changing the central heteroatoms from P(5+) to Si(4+), thereby supporting the protonation of II. Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III.
The disassembly and reassembly of giant molecules are essential processes in controlling the structure and function of biological and artificial systems. In this work, the disassembly and reassembly of a giant ring-shaped polyoxometalate (POM) without isomerization of the monomeric units is reported. The reaction of a hexavacant lacunary POM that is soluble in organic solvents, [P2 W12 O48 ](14-) , with manganese cations gave the giant ring-shaped POM [{γ-P2 W12 O48 Mn4 (C5 H7 O2 )2 (CH3 CO2 )}6 ](42-) . This POM is a hexamer of manganese-substituted {P2 W12 O48 Mn4 } units, and its inner cavity was larger than any of those previously reported for ring-shaped polyoxotungstates. It was disassembled into monomeric units in acetonitrile, and the removal of the capping organic ligands on the manganese cations led to reassembly into a tetrameric ring-shaped POM, [{γ-P2 W12 O48 Mn4 (H2 O)6 }4 (H2 O)4 ](24-) .
An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn 4 }Lu 2 A 2 in TBA 5 [(A-α-SiW 9 O 34 ) 2 FeMn 4 O 2 {Lu-(acac) 2 } 2 A 2 ] (II A ; A = Ag, Na, K; TBA = tetra-n-butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A + into a paramagnetic {FeMn 4 }Lu 2 unit in TBA 7 [(A-α-SiW 9 O 34 ) 2 FeMn 4 O 2 {Lu(acac) 2 } 2 ] (I). POMs II A exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal (II Ag , 40.0 K; II Na , 40.3 K; II K , 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.
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