Generation of cluster capsules (I h ) from decomposition products of a smaller cluster (Keggin-T d ) while surviving ones get encapsulated: species with core–shell topology formed by a fundamental symmetry-driven reaction Achim Müller,* Samar K. Das, Hartmut Bögge, Marc Schmidtmann, Alexandru Botar and Adrian Patrut Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany. E-mail: a.mueller@uni-bielefeld.de Received (in Cambridge, UK) 27th November 2000, Accepted 19th February 2001 First published as an Advance Article on the web 19th March 2001 A novel and fundamental reaction system of matter follow- ing a type of ‘supramolecular Darwinism’ leads to the formation of giant spherical nano-sized cluster capsules as kinetically controlled destination having the highest possible symmetry (I h ) and formed directly from the decomposition products of the well known but less symmetrical Keggin anions (T d ) in aqueous medium in the presence of Fe III – acting as a type of environmental influence–under condi- tions where Keggin anions are known to be extremely stable; remarkably the remaining non-decomposed Keggin anions finally get (non-covalently) encapsulated protected by the formed spherical capsules of the new supramolecular compound [PMo 12 O 40 7 {(Mo VI )Mo VI 5 } 12 Fe III 30 O 252 - (H 2 O) 102 (MeCO 2 ) 15 ]·xH 2 O 1 · 1a ·xH 2 O (x ≈ 120). The fact that self-assembly processes–based on simple (mainly highly symmetrical) building blocks or preorganized units– preferentially lead to higher symmetrical reaction products is an interesting phenomenon, e.g. in cluster chemistry which has not been explored in detail until now. Examples are the well known Keggin type anions, like [PMo 12 O 40 ] 32 (T d ) formed from molybdate and phosphate, models for the Fe 4 S 4 type ferredoxin, like [Fe 4 S 4 (SH) 4 ] 22 (T d ), the molybdenum-oxide-based giant wheels ( ≈ D 7d ) as well as spheres (I h ). Therefore, the question arises as to why highest (possible) symmetrical species are the targets, and furthermore, whether we can elucidate this phenomenon or solve the problem by means of a model reaction system. In a novel fundamental type of reaction, which can be related to a type of ‘Supramolecular Darwinism’ (see below), it can be shown that tetrahedral Keggin anions ‘lose–at least partly–the competition’ with higher symmetrical, i.e. icosahe- dral capsule type species as kinetic target in aqueous solution even under conditions where they are quantitatively formed. The formation of the latter occurs at the expense of the former. In the presence of Fe III (FeCl 3 ·6H 2 O), i.e. formally in a kind of ‘environmental attack’, the Keggin anions 1 decompose in solution with the formation of the pentagonal {(Mo VI )- Mo VI 5 O 21 } type building blocks which get linked by {Fe III (H 2 O) 2 } 3+ groups. This leads finally to the formation of novel composites consisting of discrete icosahedral nanocluster capsules with the encapsulated Keggin anions [PMo 12 O 40 ] 32 , which are abundant in the new compound [PMo 12 O 40 7 {(Mo VI )Mo VI 5 } 12 Fe III 30 O 252 (H 2 O) 102 (MeCO 2 ) 15 ]·xH 2 O 1 · 1a·xH 2 O (x ≈ 120).† Upon drying, 1 shows a fast solid-state reaction with the consequence that the composites 1a get covalently linked leading to the formation of {PMo 12 O 40 7 H 4 [(Mo VI )Mo VI 5 } 12 Fe III 30 O 254 (H 2 O) 98 (MeCO 2 ) 15 }·xH 2 O 2 · 2a·xH 2 O (x ≈ 60), see refs. 2–4. This condensation process is only important in the present context as the relevant dry (!) product can be more easily structurally characterized compared to 1. Using Fe II instead of Fe III the related compounds [H y PMo 12 O 40 7 {(Mo VI )Mo VI 5 } 12 Fe III 30 O 252 (H 2 O) 102 (Me- CO 2 ) 15 ]·xH 2 O 3 · 3a ·xH 2 O (x ≈ 120) and {H y PMo 12 O 40 7 H 4 {(Mo VI )Mo VI 5 } 12 Fe III 30 O 254 (H 2 O) 98 (MeCO 2 ) 15 }·xH 2 O 4 · 4a·xH 2 O (x ≈ 60) are obtained which contain the one- or two- electron reduced Keggin anions [H y PMo 12 O 40 ] 32 .‡ Compound 4 can be obtained not only by the present new fundamental type of reaction but also in a facile synthesis starting from the simple ingredients phosphate, molybdate, acetate, and Fe II . 2 Compound 2 with the cross-linked composites having core– shell topology and partly also 1, i.e. the ‘corresponding’ non- dried crystals, were characterized by elemental analyses, thermogravimetry (to determine the crystal water content), single-crystal X-ray structure analysis§ [including the calcula- tion of bond valence sums in order to distinguish between (terminal) O and OH 2 ligands] and spectroscopic methods (IR, Raman, UV–VIS, NIR) as well as magnetic measurements.¶ Whereas the complete structural characterization of 4, obtained with another reaction, has already been reported (see ref. 2), the corresponding non-dried new compound 3 was characterized, like 1, spectroscopically and by its crystal data (see below).∑ Compounds 1 and 3 with discrete cluster units (space group P2 1 /n) as well as 2 and 4 with their corresponding layer structures (space group Cmca) are isostructural and have, as expected, practically the same unit cell dimensions. The crystal structures of 2 and 4 show the icosahedral capsule/nucleus type composites, abundant in 1 and 3, cross-linked to 2D type assemblies via the formation of four Fe III –O–Fe III bonds per unit (see footnote ∑). Whereas the capsules of 1 and 2 have, as mentioned above, non-covalently bonded classical non-reduced Keggin anions [PMo 12 O 40 ] 32 (Fig. 1) those of 3 and 4 have the reduced Keggin anions [H y PMo 12 O 40 ] 32 encapsulated. The acetate ligands, which are highly disordered, are located inside the spheres and coordinate as bidentate ligands bridging Mo and Fe sites. As the non-dried compounds 1 and 3 with the discrete cluster composites and different electron populations have, as expected, the same space group and practically the same unit cell dimensions as the compound [{(Mo VI )Mo VI 5 } 12 Fe III 30 O 252 (MeCO 2 ) 10 {Mo 2 O 7 (H 2 O)}{H 2 Mo 2 O 8 (H 2 O)} 3 (H 2 O) 91 ]·xH 2 O 5 (x ≈ 140) containing the same {(Mo)Mo 5 O 21 } 12 Fe 30 cluster capsules without Keggin ions, and for which the complete single crystal X-ray analysis has been performed, 8 1 and 3 can easily be identified from the relevant crystal data.§ (Note that in the present case the non-dried crystals of 1 and 3 do not diffract sufficiently.) The important result of this investigation is that the Keggin ions are not stable in the presence of Fe III (or Fe II and air) as they decompose, while {(Mo VI )Mo VI 5 O 21 } type pentagons are formed which are subsequently linked by {Fe III (H 2 O) 2 } 3+ groups. The remarkable fact is that the remaining non- decomposed Keggin ions (reduced or not reduced) appear in the reaction product encapsulated. Interestingly, the Keggin ions even seem to accelerate the formation of their cage around them as templates. This models the observation that assembly processes of simple linkable units lead preferably to highly symmetrical species. The process can be correlated with a general symmetry formalism or symmetry-evolution principle for a quasi isolated (!) system; in this respect, the second law of thermodynamics and the symmetry-evolution principle are isomorphic (see ref. 6). The degree of symmetry cannot decrease as the system evolves, but either remains constant or This journal is © The Royal Society of Chemistry 2001 DOI: 10.1039/b009518b Chem. Commun., 2001, 657–658 657 Published on 19 March 2001. Downloaded by University of Illinois at Chicago on 28/10/2014 17:16:56. View Article Online / Journal Homepage / Table of Contents for this issue