Polyoxometalate Frameworks Derived from Arylstibonic Acids

Article pubs.acs.org/Organometallics

New Sb12 and Sb14 Polyoxometalate Frameworks Derived from Arylstibonic Acids: [LiH3(p-MeC6H4Sb)12O28]4− and [BaH10(pMeC6H4Sb)14O34] Brian K. Nicholson,*,† Christopher J. Clark,‡ Cody E. Wright,† Shane G. Telfer,§ and Tania Groutso∥ †

Chemistry Department, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand Bioengineering Technologies, Plant and Food Research, Private Bag 3230, Hamilton 3240, New Zealand § MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Private Bag 11-222, Palmerston North 4442, New Zealand ∥ Chemistry Department, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand ‡

S Supporting Information *

ABSTRACT: The lithium salt of p-tolylstibonic acid contains the anion [LiH3(p-MeC6H4Sb)12O28]4−, in which the Li+ is fully enclosed within an {Sb12O28} cage with γ Keggin ion geometry; the idealized overall formula is Li5[LiH3(p-MeC6H4Sb)12O28]Br·18H2O. In contrast, Ba2+ induces the formation of the unprecedented {Sb14O34} bowl-shaped polyoxostibonate [BaH 10(p-MeC6H4Sb)14O34]·4H2O. Compounds were characterized by single-crystal X-ray crystallography and by ESIMS.

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standard conditions in negative ion mode, with samples made up in MeCN immediately before infusion. Assignment of ions was aided by matching the characteristic patterns generated by the 121Sb (57%) and 123 Sb (42%) isotopes. Peaks are reported as the m/z values with the greatest intensity in the isotopic envelope. EDAX measurements on single crystals were performed on a Hitachi S-4700 field emission scanning electron microscope. The reported compounds do not melt at 2σ(I)), wR2 = 0.1386 (all data), GOF on F2 = 1.062. Solution and Refinement for [BaH10(p-MeC6H4Sb)14O34]·4H2O. The structure was solved by direct methods to reveal the heavy atoms in the asymmetric unit. Subsequent difference maps revealed the remainder of the complex anion. Three of the aryl rings were disordered equally over two orientations; this was successfully modeled. All phenyl rings were constrained as rigid hexagons. The Sb, Ba, and framework O atoms were treated anisotropically, while the aryl ring C atoms and the oxygen atoms of four lattice water molecules were refined isotropically, since the quality of the data did not warrant anisotropic treatment. Two of the lattice water molecules appeared partially occupied (sof values of 0.8 and 0.5, respectively). H atoms were included for the ordered aryl rings only. There was only a small void space remaining (ca. 46 Å3, 2σ(I)), wR2 = 0.2429 (all data), GOF on F2 = 1.053, final Δe = +1.27/ −1.81 e Å−3.

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RESULTS AND DISCUSSION

Our earlier studies focused on the formation of arylstibonic acid salts neutralized with NaOH or KOH in the presence of organic cations, such as tetraphenylphosphonium or benzyltrimethylammonium ions, to encourage the formation of insoluble complexes.1,2 However, once it became apparent that the oxometalate cage was of fixed geometry and that inclusion of organic cations in a crystallization matrix was not obligatory, a more comprehensive set of experiments was designed to investigate the impact of metal cations with a range of size and charge. The structures established by this approach demonstrate that changes do occur when alternative metallic cations are present. A sample of p-tolylstibonic acid (nominally MeC6H4SbO3H2 but with a more complex structure2) was neutralized with LiOH in the presence of LiBr in aqueous solution. The residue after slow evaporation was recrystallized from MeCN/H 2O (100/1) to give colorless crystals. X-ray crystallography showed 6613

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these to have the overall idealized formula Li 5[LiH3(pMeC6H4Sb)12O28]Br·18H2O. The core of the anion consists of a closed triskaidecahedral arrangement of 12 Sb atoms, where the Sb atoms define 2 pentagonal faces, 3 nearly square faces and 8 triangular faces, with an overall C2v symmetry (Figure 1). Each Sb atom is 6coordinate with 1 terminal aryl group (average Sb−C = 2.110 Å) and 5 framework O atoms.

Figure 2. Polyhedral representation of the structure of [LiH 3(pMeC6H4Sb)12O28]4−.

Figure 1. Structure of the anion [LiH3(p-MeC6H4Sb)12O28]4−. H atoms are omitted. Legend: Sb, blue; Li, yellow; O, red; C, black.

case. The Li(6) site (and the attached H2O, O(11)) appeared to be partially occupied (sof value of 0.41 for Li and 0.64 for the O). The remaining two Li+ ions are in general lattice positions as regular [Li(H2O)4]+ species, with one site possibly partially (80%) occupied. The crystal lattice is completed by a Br − ion (the presence of which was confirmed by EDAX measurements on single crystals showing a Br to Sb ratio of ca. 1:12) and a further 10 H2O molecules (some with partial occupancy) in an H-bonded network between the cations and anions. Charge considerations based on the idealized formula require there to be three H+ associated with the complex anion, undoubtedly attached to framework O atoms, but these were not assigned to particular sites. The partial occupancy of the Li(6) and Li(5) sites leave a net charge deficit of ca. 0.8. This is presumably compensated by extra H+ attached either to a framework O atom or as a H3O+ within the lattice water network, but there was no way of distinguishing between these possibilities. A third possibility would be from partial occupancy of the Br − site, but that seems unlikely, since refinement of the sof value converged to 0.96. There is no residual void volume; therefore, the structure description is unusually complete for this type of crystal. The cluster anion retains its integrity in MeCN solution, as shown by electrospray mass spectrometry, where the most intense peak at m/z 3032.457 corresponds to [Li4H3(MeC6H4Sb)12O28]− (calcd 3032.453) within a closely matched isotope envelope. The formation of this type of cluster with Li+ ions is also seen with p-chlorophenylstibonic acid, where the solid formed gave an ESI-MS spectrum dominated by a peak centered at m/z 1641.407 which corresponds to

The 28 framework O atoms are of two types: 24 which doubly bridge two Sb atoms (average Sb−O = 1.996 Å) and 4 which triply bridge three Sb atoms (average Sb−O = 2.122 Å). The latter are arranged tetrahedrally and are also coordinated to one Li+ ion encapsulated in the center of the core (average Li− O = 1.958 Å, O−Li−O′ = 110.2°). A polyhedral representation of the Sb−O framework indicates that the structure has fundamental building blocks of composition R3Sb3O10 (Figure 2). Each unit has three Sb−O octahedra, where each octahedron shares two adjacent edges with each of the other two octahedra. Four such units joined at a single vertex per unit (the four triply bonded O atoms) result in formation of the completed oxometalate cluster, with the Li + ion trapped in the interior. This overall geometry corresponds closely to the rare γ isomer of the Keggin ions 13,14 [XMo12O40]x−, where Li+ takes the place of the X atom, Sb forms the cage instead of Mo, and the Sb−C of the aryl−Sb(V) act as surrogates for the MoO for Mo(VI). Other Keggin geometries (δ and ε) were identified as part of more complex structures by Baskar et al. for polyoxostibonates incorporating Mn2+ or Zn2+ ions.3 Zhang et al. have recently performed DFT calculations which show that the order of stability of the various α, β, γ, δ, and ε isomers of the Keggin ions for the Mo or W cages is reversed for polyoxostibonates.14 There are three other Li+ cations associated closely with the polyoxometalate core. Li(2) lies above one of the square faces of the polyhedron and is five-coordinate, with four framework O atoms making up the base and one H2O the apex of a squarepyramidal arrangement. Li(3) and L(6) are above the two pentagonal faces of the core and have distorted-tetrahedral coordination from three cluster O atoms and one H2O in each 6614

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[Li2H4(ClC6H4Sb)12O28(H2O)]2‑ (calcd 1641.388). The complex envelope arising from the isotopes of both Sb and Cl matches completely between experiment and simulation. There are also weaker peaks at m/z 1093.935 for [Li2H3(ClC6H4Sb)12O28(H2O)]3− (calcd 1093.923) and at m/ z 1123.251 for [Li3H3(ClC6H4Sb)12O28(H2O)Br]3− (calcd 1123.234). We have noted before that under comparable mass spectrometry conditions the p-chlorophenylstibonic acid oxyanions favor multiply charged ions more so than the p-tolyl equivalents.2 When the arylstibonic acid was neutralized in the presence of Ba2+ ions, the crystals isolated gave a completely different structure, with overall formula [BaH10(pMeC6H4Sb)14O34]·4H2O. The core of this cluster consists of an open 12-sided bowl formed by 14 Sb atoms, where the Sb atoms define two pentagonal faces (coplanar to within ±0.33 Å), two quadrilateral faces (coplanar to within ±0.04 Å), and eight triangular faces, with overall C2 symmetry (Figure 3).

Figure 4. Polyhedral representation of the structure of [BaH 10(pMeC6H10Sb)14O34].

but the positions of the other 6 are not determined. The lattice is completed by four H2O molecules which link the main species together by H-bonding. This present Sb14 structure is intimately related to the enclosed Sb 16 polyoxostibonate framework cluster [(pXC6H4Sb)16O28(OH)8] of Jami et al.5b Their structure is described as consisting of a ring of four triads capped top and bottom by Sb2 dimers. For our Sb14 structure, removal of the Ba ion, followed by addition of a further pair of edge-sharing octahedra to the top of the bowl, would lead directly to the Sb 16 framework described. Alternatively, the Sb14 polyoxometalate can be described as a lacunary version of the closed Sb16 one. Interestingly, the Sb16 structure of Jami et al. is depicted as being empty, possibly because simple inorganic ions were not present as part of the synthetic process. 5b A detailed comparison of the bond parameters for [BaH 10 (pMeC6H4Sb)14O34] with those of [(p-ClC6H4Sb)16O28(OH)8]5b shows a close correspondence with average Sb−C distances of 2.09 and 2.14 Å, μ2-O−Sb distances of 1.99 and 1.98 Å, and μ3O−Sb distances of 2.10 and 2.09 Å, respectively. ESI-MS shows that the [BaH10(p-MeC6H4Sb)14O34] cluster remains intact in MeCN solution, giving a strong envelope at m/z 3670.29 (cf. m/z 3670.23 for [BaH9(pMeC 6 H 4 Sb) 14 O 34 ] − ) and the corresponding signal for [BaH 8 (p-MeC 6 H 4 Sb) 14 O 34 ] 2− at m/z 1834.613 (calcd 1834.611). The crystals used for the structural study were fractionally crystallized from a mixture of products which formed from a reaction that included Co2+ ions as well as Ba2+ ions (see the Experimental Section). However, subsequent reactions between just the arylstibonic acid and BaO showed that the [BaH 10(pMeC6H4Sb)14O34] complex also forms in the absence of the Co2+ ions, as a major component in an inseparable powder that contained unreacted acid and also an Sb16 species [H8(pMeC6H4Sb)16O36] corresponding to that reported by Jami et al.5b An analogous reaction between p-chlorophenylstibonic acid and BaO under the same conditions gave a powder that ESIMS showed contained the corresponding Ba-bowl compound [BaH10(p-ClC6H4Sb)14O34] and the Sb16 aggregate [H8(pClC6H4Sb)16O36] in qualitatively similar amounts, judging from the relative intensities of the peaks, though these will also be affected by the relative ease of ionization of the different species.

Figure 3. Structure of [BaH10(p-MeC6H4Sb)14O34]. H atoms are omitted. Legend: Sb, blue; Ba, yellow; O, red; C, black.

Each Sb atom is again six-coordinate with one terminal aryl group (average Sb−C = 2.103 Å) and five framework O atoms. The bowl is held together by 6 μ3-O (average Sb−O = 2.102 Å) and 24 μ-O atoms (average 1.997 Å), with the remaining 4 atoms being terminal ones (presumably hydroxyls, average Sb− O = 1.975 Å). Contained within this bowl is the Ba2+ cation, coordinated to the 4 terminal O atoms at the top of the bowl (Ba−O = 2.964 Å), to 4 μ3-O (Ba−O = 2.889 Å) and 2 μ-O (Ba−O = 3.103 Å) from the middle, and to 1 μ-O from the bottom of the bowl (Ba−O 2.709 Å), giving 11-coordination overall. The 4 terminal O atoms are coplanar, and the Ba 2+ ion is located 0.68 Å beneath this plane inside the bowl. A polyhedral representation of the Sb−O framework indicates that the same R3Sb3O10 building blocks employed in the Li structure are utilized here (Figure 4). Four units (as described above) link together to define the sides of the bowl, with two vertices along one edge of an octahedron in one unit each linking to a single vertex on two separate octahedra in the neighboring unit, nose-to-tail around the cluster. They are supplemented by a single pair of edgesharing octahedra on the base. The overall structure is neutral, which means there must be 10 H+ associated with the cluster; 4 will be as the terminal Sb−OH groups at the top of the bowl, 6615

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Article

CONCLUSION

Whereas the medium-sized cations Na+ and K+ template polyoxostibonates with a [(RSb)12O30] core with one of the cations in a 10-coordinate site within the hexagonal antiprismatic channels, as previously reported,1,2 the smaller Li+ encourages a more condensed [(RSb)12O28] anion with the cation entirely encapsulated in a tetrahedral site. On the other hand, the larger 11-coordinate Ba2+ is cradled in a more open [(RSb)14O34] bowl-shaped structure.

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ASSOCIATED CONTENT S Supporting Information * Tables, figures, and CIF files giving full details of the crystal structure determination and extra structure diagrams. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS We thank Anna Jagger for providing a sample of p-tolylstibonic acid and Dr. Helen Turner for the EDAX measurements. REFERENCES

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