Communication pubs.acs.org/IC
[MnIIIMnIV2Mo14O56]17−: A Mixed-Valence Meso-Polyoxometalate Anion Encapsulated by a 64-Nuclearity Silver Cluster Jian-Yu Wang,† Kuan-Guan Liu,† Zong-Jie Guan,† Zi-Ang Nan,† Yu-Mei Lin,*,† and Quan-Ming Wang*,†,‡ †
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen, 361005, People’s Republic of China Department of Chemistry, Tsinghua University, Bejing, 100084, People’s Republic of China
‡
S Supporting Information *
anion, namely, [Ag64(CCtBu)38(CF3COO)8(MnIIIMnIV2Mo14O56)](OH)·10CH3CN·2H2O (1). The templating [MnIIIMnIV2Mo14O56]17− anion is interesting because (a) it contains a mixed valence of Mn(III) and Mn(IV); (b) it is a rare inorganic meso anion built up by D-{MnIVMo7} and L-{MnIVMo7} units connecting together through a {MnIII} fragment; and (c) the formation performs a reassembly process for increasing nuclearities from {MnMo9} to {Mn3Mo14}. The unique POM anion is encapsulated in a new silver cage with 64 nuclearities. Herein, we report the synthesis and characterization of the silver cluster. An aqueous solution of (NH4)6[MnMo9O32] was added to the suspension of AgCCtBu with AgCF3CO2 in acetonitrile, leading to the formation of an orange solution, which was solvothermally treated at 80 °C for 20 h. The filtrate was evaporated to give dark-red crystals of 1. The composition of 1 is supported by elemental analysis. The powder X-ray diffraction pattern (PXRD) matches the simulated one from single-crystal data (Figure S1). The elements of Ag, Mo, and Mn are also confirmed by X-ray photoelectron spectroscopy (XPS) investigations (Figure S2). An IR band at 2009 cm−1 confirms the presence of the CC group in 1. The bands at 847, 721, 613, and 585 cm−1 are assigned to the Mo−O vibrations, and the band at 1664 cm−1 is due to the trifluoroacetic ligand (Figure S3). The 1H NMR spectrum shows three signals at 1.27, 1.41, and 1.55 ppm, which are attributed to the general contribution of the alkynyl ligands coordinated at different silver centers (Figure S4). Single crystal X-ray structural analysis revealed that 1 consists of 64 silver atoms peripherally bridged by 38 alkynyl ligands and eight trifluoroacetate ligands with a core [MnIIIMnIV2Mo14O56]17− anion (Figure 1). This cationic cluster is shaped as a peanut-like skeleton with an Ag64 shell and a [MnIIIMnIV2Mo14O56]17− core. The templating core adopts a strip-like arrangement, while the 64 silver atoms assemble in an irregular ellipsoidal shape. The [MnIIIMnIV2Mo14O56]17− anion acts as the template for the formation of the silver cluster, which is built by two {MnIVMo7} units bridging together via a {MnIII} unit. Both Mo and Mn metals are six-coordinated to O atoms with an octahedral geometry. The Mo−O bond distances vary from 1.726(6) to 2.322 (6) Å. The MnIV−O bond lengths vary from 1.889(6) to 1.924(6) Å, which are
ABSTRACT: A 64-nuclearity silver cluster encapsulating a unique POM anion [MnIIIMnIV2Mo14O56]17− has been synthesized. The formation of the templating core performs a reassembly process for increasing nuclearities from {MnMo9} to {Mn3Mo14}. It represents a rare inorganic meso anion containing mixed-valent Mn that is built up by D-{MnIVMo7} and L-{MnIVMo7} units connecting together through a {MnIII} fragment.
P
olyoxometalates (POMs) are anionic oxide clusters with structural versatility and fascinating properties.1 Recently, integrating POMs with silver alkynyls or sulfides has attracted great attention, which leads to the formation of a family of POMs with templated high-nuclearity silver assemblies.2−11 The selection of a POM precursor is very important, because it helps control the size and shape of the cluster and also affects the physical properties of the product.2 Various POM anions such as Keggin type [CoW12O40]4−;4 and its derivatives [PW9O34]9−;5 [H2SiW10O36]4−;3b Dawson type [P2W15Nb3O62]9−;6 Lindquist type [M6O19]2−;7 and other polyoxoanions, [Ho(W 5 O 18 ) 2 ] 9− , 8 [Mo 7 O 24 ] 6− , 3a,9 and [Mo8O26]4−,10 have been used. Some POM precursors retain virtually their original compositions and structures when participating in the formations of silver clusters.4−6,8 Transformations of POM anions into new forms are also observed in some cases.3,7,9 The strategy of encapsulation of a POM into a silver cage generates a series of fascinating structures and also offers the clusters new applications. For example, new POM species, such as icosamolybdate [Mo20O66]12−, are in situ formed and stabilized in a silver cage;7b acid−base triggered structural transformation of a polyoxometalate core inside nanoscale silver clusters occurs;11e single SMM molecules are separated from each other via the encapsulation of silver cages;8 and core−shell electronic communication is observed in the POM templating cluster.3,5 The Waugh-type [MnMo9O32]6− anion is a chiral polyoxoanion which has two enantiomers, a left- and a right-handed one.12 However, resolving the D/L polyoxoanion in solution is a big challenge.13 We are interested in testing whether it is possible to resolve the racemic mixture through the encapsulation by a silver cage. Surprisingly, our trials reacting racemic (NH4)6[MnMo9O32] with AgCCtBu and AgCF3CO2 under solvothermal conditions led to the formation of a 64nuclearity silver cluster encapsulating an unprecedented POM © XXXX American Chemical Society
Received: April 12, 2016
A
DOI: 10.1021/acs.inorgchem.6b00901 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
proposed in Scheme 1. The strip-like product {Mn3Mo14} consists of an L-{MnMo7} and a D-{MnMo7} unit which are Scheme 1. Illustration of meso-{Mn3Mo14} Related to the Precursor D/L-{MnMo9}
bridged together by a linker {Mn} fragment. The two D/L{MnMo7} units can be formally derived from the elimination of three sets of {MoO6} octahedra from the enantiomers D{MnMo9} and L-{MnMo9}, which is followed by recombination with one free {MoO6} octahedral unit. Although the detailed formation of a meso anion is not clear, the precursor [MnMo9O32]6− is a lacunary POM anion which has the capacity to reorganize in solution, facilitating the formation of different POM building blocks. The chemical reaction equation involving the formation of 1 is proposed (Scheme S1). It is conceivable that the self-assembly undergoes a decompositionreconstitution process and results in the increase in nuclearities from {MnMo9} to {Mn3Mo14}. Another intriguing part of the templation anion is that it contains a mixed valence of Mn (one MnIII and two MnIV). The Mn-substituted POMs (MnII, MnIII, or mixed MnII−MnIII) are interesting and have been widely reported.14 However, POMs containing MnIII−MnIV-based mixed-valence cores are very rare.15 The oxidation states of the Mn atoms in 1 are established by a combination of charge balance considerations, inspection of bond lengths, bond valence sum (BVS) calculations,16 and also EPR spectrum. BVS values for the central Mn atom and Mn in the {MnMo7} unit are 3.154 and 3.904, indicating that the respective valences are +3 and +4, respectively. The values for all Mo atoms are in the range of 5.808 to 6.098, suggesting Mo atoms maintain +6 valence states (Table S1). The ESR spectrum of 1 showed two signals at g = 2.67 and 2.00, also confirming the existence of a mixed valence of Mn(III) and Mn(IV)17 (Figure 2). The solid UV−vis diffuse reflectance spectrum of 1 shows a broad absorption band around 520−650 nm and a near-IR absorption band around 720 nm (Figure S5). The absorption
Figure 1. (a) The templating core [MnIIIMnIV2Mo14O56]17−. (b) Silver shell. (c) Molecular structure of the cationic part of 1. Color legend: pink, Ag; turquiose, Mo; plum, Mn; red, O; green, F; gray, C. Hydrogen atoms omitted for clarity.
shorter than the central MnIII−O distances from 1.905(6) to 2.188(1) Å. The 64 silver atoms surround the surface of the [MnIIIMnIV2Mo14O56]17− core through the connection of Ag atoms with terminal or bridge O atoms and the Ag−Ag interactions, which are reflected by the Ag−O bond lengths varying from 2.097(7) to 2.597(1) Å and the Ag−Ag separations in the range 2.837(2)−3.372 (1) Å, respectively. The whole silver shell is further protected by 38 tBuCC− ligands and eight CF3CO2− anions. The coordinations of alkynyl ligands are diverse, which adopt five types of coordination modes, 22 in μ3-η1, η1, η2; two in μ3-η1, η1, η1; eight in μ3-η1, η2, η2; four in μ4-η1, η1, η1, η2; and two in μ4-η1, η2, η2, η2. Eight trifluoroacetic anions are bound to silver atoms with the Ag−O distances in the range of 2.331(12)−2.584(6) Å. The cluster is nanosized with approximate dimensions of 3.3 × 2.3 nm, which is comparable with the reported Ag60 and Ag62 clusters.3a,7b The precursor [MnMo 9O32]6− was transformed into [MnIIIMnIV2Mo14O56]17− under the reaction conditions. It is reported that the Waugh-type [MnMo9O32]6− anion is labile and can be decomposed above pH 5.12 In the synthesis of 1, the reaction was carried out under nearly neutral conditions at 80 °C. This is helpful for the transformation process. Transformations of POM precursors to the corresponding templates with higher charge densities and more surface oxygen atoms are shown to improve the coordination ability of POM and favor the binding with silver atoms.3,7c The precursor [MnMo9O32]6− anion has 18 terminal oxygen atoms, six μ2-bridging oxygen atoms, two μ3-bridging oxygen atoms, and one central μ4-O atom. It is notable that the [MnIIIMnIV2Mo14O56]17− anion has much higher charge densities which possess 17 negative charges and has more terminal oxygen atoms, that is, 32 terminal oxygen atoms, eight μ2-O atoms, 10 μ3-O atoms, and six μ4-O atoms. The racemic mixture [MnIVMo9O32]6− precursor transformed into the meso species [MnIIIMnIV2Mo14O56]17− via a self-assembly process. For a better structural analysis of the changes in the precursor and product, an illumination was
Figure 2. ESR spectrum of compound 1. B
DOI: 10.1021/acs.inorgchem.6b00901 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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properties of 1 are similar to the precursor POM, suggesting the absorption bands arise from the templating POM core. It is plausible that the band around 520−650 nm is ascribed to spinallowed transition in MnO6 groups and the weak band around 720 nm is assigned to spin-forbidden transition12 (Figure S6). The wide absorption in the visible region is consistent with the dark-red color of its crystals. The spectrum of 1 in the CH3CN solution displays a weak absorption band at ca. 510 nm. The voltammetric behavior of 1 shows two couples of redox peaks within the testing potential range (Epa: −0.28(I), 0.50 V(II); Epc: −1.40(I′), 0.30 V (II′)). The precursor (NH4)6[MnMo9O32] exhibits two couples of redox peaks (Epa: −0.66(i), 0.37 V(ii); Epc: −1.00(i′), −0.12 V (ii′); Figure S7). The redox peaks of 1 are not similar to those of (NH4)6[MnMo9O32] and AgBF45, suggesting that the mixedvalence POM core possess different redox behavior and the outer silver shell also participates in the redox process. In summary, we have synthesized and structurally characterized a 64-nuclearity silver cluster enclosing a unique POM anion. The template [MnIIIMnIV2Mo14O56]17− is an inorganic meso anion, which is constructed by two D/L-{MnMo7} units connecting together via a {Mn} fragment. It represents a rare POM anion containing a mixed valence of Mn(III) and Mn(IV). The [MnMo9O32]6− precursor undergoes a decomposition−reconstitution process, and the interplay between the POM anion and silver ions results in the formation of 1. The encapsulation of an anion within a silver cage provides an effective strategy for discovery and stabilization of new POM species which are not accessible by a traditional synthetic route.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00901. Synthesis, powder XRD, IR, XPS, UV−vis−NIR, 1H NMR spectra, cyclic voltammograms, single-crystal X-ray diffraction, bond valence sum (BVS) calculations (PDF) X-ray crystallographic data for CCDC-1472778 (1) (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86-592-2184185. Fax: +86-592-2183047. E-mail:
[email protected]. *Tel.: +86-592-2184185. Fax: +86-592-2183047. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the 973 Program (2014CB845603) and the Natural Science Foundation of China (21390390, 21301145, 21473139, and 20720150038).
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REFERENCES
(1) Miras, H. N.; Yan, J.; Long, D. L.; Cronin, L. Chem. Soc. Rev. 2012, 41, 7403−7430. (2) Wang, Q.-M.; Lin, Y.-M.; Liu, K.-G. Acc. Chem. Res. 2015, 48, 1570−1579. C
DOI: 10.1021/acs.inorgchem.6b00901 Inorg. Chem. XXXX, XXX, XXX−XXX
