Adsorption and Catalytic Properties of the Inner Nanospace of a Gigantic Ring‐Shaped Polyoxometalate Cluster

Abstract

Porous materials with structurally well-defined nanoscale cavities (e.g., carbon nanostructures, zeolites, mesoporous silica, and coordination compounds) [1][2][3] are of great interest because of their unique properties such as adsorption, separation, exchange, and catalysis, which are all associated with the presence of a functional nanospace. In this respect, polyoxometalate clusters, which are nanosized metal-oxide macroanions built from molecular precursors with a range of unique redox and acidic properties, are ideally suited for the development of functional nanospaces but have not yet been thus explored. [4][5][6] Indeed, ring-shaped POMs have a great deal of potential for allowing creation of an exceptionally well defined and accessible nanosurfaces and nanospaces. These ring clusters date back to 1995, when the synthesis and structural analysis of gigantic mixed-valent ring-shaped POM cluster (NH 4 ) 28 [Mo 154 (NO) 14 O 448 H 14 (H 2 O) 70 ] ({Mo 154 } ring) was first reported by Müller et al., [5a] and later this was followed by a much more facile synthesis of the analogous compound Na 14 [Mo 154 O 462 H 14 (H 2 O) 70 ]·400 H 2 O (1, Scheme 1; [5b-d] for further information on ring properties, see Ref.[5f-j]) The diameter of the POM ring and inner void space are about 3.9 and 1.7 nm, respectively, while the height of ring is about 1.8 nm. [7] Despite the great potential of ringshaped POMs to form porous materials, POM clusters have been greatly under-utilized and -investigated, probably due to poor stability. [8] Because of this fundamental limitation, we opted to examine the stability and functionality of a {Mo 154 } ring encapsulated by dimethyldioctadecylammonium (DODA) cations (Scheme 1), because the DODA cation, with a length of about 2.5 nm, introduces amphiphilic-like properties into the POM-cation hybrid, and POM-DODA hybrids have shown some interesting properties, for example, for formation of Langmuir-Blodgett films. [9,10] Further, we hypothesized that encapsulation by DODA will also help to increase the stability of the nanoscale wheel cluster. Not only will nanoscale molecular assembly of the hybrid material be controlled by electrostatic interactions between the {Mo 154 }-ring polyanion and DODA cations, hydrophobic interactions between the alkyl chains should contribute to stabilization of the hybrid material in the solid state. As reported previously, [9] the POM-DODA hybrid shows high stability in CHCl 3 solution, while the ring structure of 1 is not robust in aqueous solution and the solid state (see Figures S1-S4, Supporting Information).Here we report on the inner nanospace functionality of POM-DODA hybrid (DODA) 23 [Mo 154 O 462 H 5 ]·n H 2 O (2). [11] Initially, dynamic light scattering (DLS) experiments were conducted to confirm the presence of the {Mo 154 } ring of 2 in solution and showed that the particle diameter in a solution in THF is about (4.0 AE 0.6) nm, similar to the size of the {Mo 154 } ring ( Figure S5, Supporting Information). Hence, these data are consistent wit...

Investigating Cation Binding in the Polyoxometalate‐Super‐Crown [P8W48O184]40−

“…Investigation of the cationbinding characteristics of {P 8 W 48 } is crucial for any potential applications that exploit the intrinsic porosity of the material, such as catalysis. [21] The use of ITC in POM chemistry may be extended to become as integral for studying hostguest relationships as in other areas of supramolecular chemistry. Additionally, characterization of the sites of highest nucleophilicity in {P 8 W 48 } may allow us to explain the formation of discrete anionic cluster compounds as opposed to extended framework materials when reacted with transition metals, and allow selective functionalization of the internal cavity for guest encapsulation, or externally for network reticulation (see Figure S7 in the Supporting Information).…”

“…The preparation of Li 17 -A C H T U N G T R E N N U N G (NH 4 ) 21 [7] have enabled us to study cation binding in the cyclic heteropolyanion and investigate the effect these supramolecular interactions have on the {P 8 W 48 } unit. The ability to characterize binding sites in the cluster would be an important step towards designing materials based on {P 8 W 48 }, crucially allowing control over internal and external functionalization.…”

Amphiphilic hybrids containing inorganic constituent: More than soap

“…The surfactant shell improves the stabil ity of the embedded POM and its solubility in nonpolar, aprotic organic solvents. Our group presented a unique system [22e], which was some years later "adapted" by Cronin and coworkers [25]. Giant, donut shaped POMs were treated with a cationic, double tailed surfactant.…”

“…Self organization into columnar lyotropic liquid crystal (LLC) phases was reported. Noro et al used a very similar system to demonstrate gas and vapor adsorption and catalytic proper ties in addition to the self organization properties [25]. In recent time surfactant complexes with smaller POM cores like the Keggin structure have moved into the focus of research.…”

Study of the first antibacterial agent pipemidic acid modifying Keggin polyoxometalate

“…Researchers have found versatile structures [5][6][7][8][9][10] and numerous potential applications in catalysis, materials science, magnetism and medicine [11][12][13][14][15]. Recently, a large number of POMs have been reported in the literatures, and formed a comprehensive review of their physical, chemical and others properties.…”