Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

18B/2004G239

Stabilization of aluminium polycations by aminocarboxylic acid ligands to give
Al15 clusters with zeotypic supramolecular structure
Wolfgang SCHMITT*, Jonathan P. HILL
National Institute for Materials Science, ICYS, Tsukuba, 1-1 Namiki, Ibaraki 305 0044, Japan
Introduction
Aluminium-based materials find wide applications inthe field of heterogeneous catalysis. Barrer andlater Vaughan, for instance, investigated pillared clays
where cations in the interlayer spaces of montmorillonite
were replaced by aluminium Keggin-polycations. Thesesystems are catalysts for the production of ethers andesters and show shape selectivity unlike their precursor
acid-clays [1-3]. Of particular relevance for possible
future applications is the ability to construct openframework hybrid organic-inorganic materials from welldefined nanoscale alumina-derived building blocks.
Synthetic concept
In our scientific studies we succeeded in encapsulating{Al7(OH)12(H2O)12}9+ polycation mineral fragments withfour dimeric {Al2hpdta}-units [4]. The three-dimensional
orderoftheresulting
Al15-clustersin(pipH2)(H3O)[Al15(µ3-O)4(µ3-OH)6(µ-OH)14(hpdta)]·pip·
41H2O (1) shows similarities to that of zeolites (Fig. 1).
The clusters pack in the crystal in such a way that two
different types of channels are created, which run parallelto the crystallographic c-axis. The larger channels (Ø ca.
11 Å), contain exclusively solvent molecules and thesmaller channels (Ø ca 5 Å) the piperazinium ions.
(H5hpdta: HOCH2[CH2N(CH2COOH)2]2.
Experimental, Results and Discussion
At beamline 18B we investigated new types of theabove described compound where we exchanged thecounterions in 1 with ethylenediamine or alkali metal ions(e.g. potassium ions). The materials crystallize from
aqueous solution after the reaction of the organic ligand
with four equivalent of aluminium nitrate under alkaline
reaction conditions. The crystal analysis reveals that bysubstituting the counterions in 1 the packing of theclusters changes and we observe a transformation fromthe orthorhombic space group Pccn to the tetragonal,space-group P42/ncm. (in e.g. (enH2)(H3O)[Al15(µ3O)4(µ3-OH)6(µ-OH)14(hpdta)4]·dioxane·19H2O(colourless octahedral crystals: 0.2 mm x 0.2mm x 0.2mm,
cell dimensions a = 27.981(3), b = 15.627(3), Z = 4, T =200 K, R1(F) = 8.51.). Similar results were also obtainedwith alkali metal ion substituted compounds.
In addition to these experiments, we exchanged thesolvent in the larger channels of the structure with a seriesorganic substrates (e.g. dioxane, acetone, ethylacetate,THF etc.). We investigated by measuring the crystal
structures at a temperature of T = 150 K how thesesubstrates interact with the framework of the zeotypiccompound and how they bind to the mineral core in thecluster. In order to evaluate the potential of this hybridorganic-inorganic structure for hydrolytic activations orfor catalysis, further investigations will be performed.

Figure 1. Encapsulation of {Al7(OH)12(H2O)12}9+ mineral fragments with four dimeric [Al2(hpdta)(H2O)4]+units to form a [Al15(µ3-O)4(µ3-OH)6(µ-OH)14(hpdta)4]3–
complex (Al: yellow, O: red, C: grey, N: blue). Lower
Left: The supramolecular array of Al15 aggregatestemplated around the protonated base (piperazinium) inthe smaller channels. Further solvent and base molecules
have been omitted from the larger channels for clarity(view in [001]). Lower Right: Polyhedral representation.
The blue Al15-clusters are half a unit cell length below theplane containing the red clusters (view in [001]).

References[1] J. M. Thomas, Angew. Chem., Int. Ed. Eng.100, 1735
(1988).
[2] I.E. Maxwell, J.E. Naber, Catal. Lett. 12, 105. (1992).
[3] J. M. Thomas, Perspectives in Catalysis: A ChemistrystCentury,
Blackwell
Scientificfor
the21
Publications/IUPAC, London, 1992.
[4] W. Schmitt et al., Angew. Chem., Int. Ed. Eng. 40,3577, (2001).
* schmitt.wolfgang@nims.go.jp

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