We have described the design principles of high symmetry natural clusters, such as ferritin and the human rhinovirus, based on incommensurate interactions. [1a,b] We have extended this interpretation and shown that it provides a systematic method for the design and synthesis of analogues of these natural clusters using metal ± ligand interactions. [1,2] Here we present an example of the rational design of a M 4 L 6 tetrahedral cluster that exhibits dynamic exchange of guests within the supramolecular cluster cavity. [3] If the high symmetry clusters are viewed as truncated polyhedra, the interactions of the subunits must conform to the angles between the planes of the polyhedra, if a high symmetry cluster is to form. Synthesis of an M 4 L 6 cluster with tetrahedral symmetry requires that the incommensurate twoand threefold axes be rigidly fixed by the design. [4] A twofold symmetric bis-bidentate ligand interacting with an octahedral metal center (which generates the threefold axis) can lead to the formation of an M 2 L 3 helix (point group D 3 ) if the angle between the threefold and twofold axes can approach 908. [1c±e, 5] Entropic considerations dictate that if the lower stoichiometry M 2 L 3 complex can form, it will. [1c] Therefore, in order to favor the M 4 L 6 tetrahedron the geometry of the designed ligand must be correct and inflexible.The selectivity in the ligand H 4 L is achieved by a naphthalene spacer, which causes the two catechol binding units to be offset from one another when the ligand is in the conformation required for helicate formation (Scheme 1). [1e] Scheme 1. Helicate formation is disfavored by the use of a naphthalene spacer in the ligand H 4 L.Thus, the formation of a helicate becomes impossible, and the formation of the M 4 L 6 tetrahedron ( Figure 1) enabled. Computer modeling [6] of the M 4 L 6 cluster indicated that it would have T symmetry (all metal centers with the same chirality, all D or all L) and that there would be a substantial cavity inside the cluster.In this cluster design the planes of the ligands are coincident with the twofold planes of the truncated tetrahedron. As such, the elevation angle of the threefold axis (through the metal center) with the extended twofold ligand plane corresponds to the ªapproach angleº (Figure 1). [1f] This angle is calculated to be 35.38 and corresponds to the approach angle of a perfect Figure 1. a) An M 4 L 6 cluster can be thought of as a truncated tetrahedron with the planes of the polyhedron perpendicular to the symmetry axes. b) If a ligand is designed to lie on the twofold (blue) plane, then the elevation angle of the C 3 axis with the extended twofold plane represents the ªapproach angleº. octahedral metal complex. While catechol complexes of Ga III , Fe III , and Al III are typically distorted towards trigonalprismatic geometries [7] corresponding to an approach angle of 238, molecular mechanics calculations [6] indicated that slight out-of-plane twists in the ligand would compensate for this angle.The ligand H 4 L was syn...

The rigid tris- and bis(catecholamide) ligands H(6)A, H(4)B and H(4)C form tetrahedral clusters of the type M(4)L(4) and M(4)L(6) through self-assembly reactions with tri- and tetravalent metal ions such as Ga(III), Fe(III), Ti(IV) and Sn(IV). General design principles for the synthesis of such clusters are presented with an emphasis on geometric requirements and kinetic and thermodynamic considerations. The solution and solid-state characterization of these complexes is presented, and their dynamic solution behavior is described. The tris-catecholamide H(6)A forms M(4)L(4) tetrahedra with Ga(III), Ti(IV), and Sn(IV); (Et(3)N)(8)[Ti(4)A(4)] crystallizes in R3(-)c (No. 167), with a = 22.6143(5) A, c = 106.038(2) A. The cluster is a racemic mixture of homoconfigurational tetrahedra (all Delta or all Lambda at the metal centers within a given cluster). Though the synthetic procedure for synthesis of the cluster is markedly metal-dependent, extensive electrospray mass spectrometry investigations show that the M(4)A(4) (M = Ga(III), Ti(IV), and Sn(IV)) clusters are remarkably stable once formed. Two approaches are presented for the formation of M(4)L(6) tetrahedral clusters. Of the bis(catecholamide) ligands, H(4)B forms an M(4)L(6) tetrahedron (M = Ga(III)) based on an "edge-on" design, while H(4)C forms an M(4)L(6) tetrahedron (M = Ga(III), Fe(III)) based on a "face-on" strategy. K(5)[Et(4)N](7)[Fe(4)C(6)] crystallizes in I43(-)d (No. 220) with a = 43.706(8) A. This M(4)L(6) tetrahedral cluster is also a racemic mixture of homoconfigurational tetrahedra and has a cavity large enough to encapsulate a molecule of Et(4)N(+). This host-guest interaction is maintained in solution as revealed by NMR investigations of the Ga(III) complex.

There are examples of non-covalently linked molecular clusters in nature of stunning symmetry and beauty. Two are shown in Figure 1. The 24 individual protein subunits of apoferritin['.21 (the elipsoid-shaped subunits on the left of Fig. 1) cluster Fig. 1. Models of theapoferritin 24-mer (left) and human rhinovirus 60-mer (right). The view of ferritin IS down one of the C, axes. the other two C, axes of the octahedral cluster are horizontal and vertical, respectively. Along the body diagonal (between three of the elipsoid ends marked N) lies the C, axis of the octahedral cluster. The view, of the virus is down the C, axis, and the regions related by symmetry through the C2 axis and the Cj axis are shown in green and blue. respectively.together to form only this polymeric assemblage (symmetry group 0). The approximately spherical cavity can hold over 4000 iron atoms in the form of FeO(OH).['] When the apoprotein is dissociated into the individual protein subunits and then allowed to reassemble only the highly symmetrical 24-mer of apoferritin forms. Something similar happens with many viruses, in which non-covalently linked protein clusters of virus coats are used to protect the viral nucleic acid. The protein cluster coat of the human rhinovirus ( Fig. 1 contains 60 protein subunits, which spontaneously assemble to give an icosahedron (symmetry group I ) . Once again, it is generally true that dissociation and reassembly of the protein coat does not give a random polymer assembly but instead only the highly symmetrical 60-mer.What is it that leads to the high symmetry of such clusters? What is the relationship of this symmetry to the specificity seen in the formation of a single cluster type with unique cavity size and properties? In each case the individual molecular fragments (the protein subunits) are asymmetric units-the order of the pure rotation groups 0 and I are 24 and 60, respectively. Can this process be mimicked by using metal -ligand interactions as the driving force? This paper addresses these questions and shows that highly symmetrical clusters can be systematically designed by using the principles of symmetry-driven reactions deduced from natural structures.In considering the interaction of molecular subunits to make supramolecular assemblages[41 a useful, if extreme, simplification is to consider each intermolecular interaction around the symmetry axis as a molecular lock and key interaction.[5361 Such interactions that require different symmetries are incommensurate-they require different stoichiometries a t the site of interaction. A useful analogy can be made here with the interaction of incommensurate lattices (Fig. 2). As occurs with some natural silicates, the superposition of one layer upon another which has a slightly different lattice spacing soon results in a mismatch of the unit cells due to the incommensurate lattice spacings.171 The two spacings can be kept in register only by curling them, as shown in Figure 2. Similarly, two incommensurate coordination numbers can coexist only by a ...

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Nicht zu groß und nicht zu klein sollte der Gast im Hohlraum sein: Der homochirale tetraedrische Cluster [Ga4L6]12− weist eine hohe Selektivität für den Einschluß von Et4N+ gegenüber Pr4N+ auf, das seinerseits eingeschlossenes Me4N+ verdrängt (L=zweifach zweizähniger Ligand); dieser sukzessive Austausch von R4N+‐Ionen verläuft 1H‐NMR‐spektroskopischen Untersuchungen zufolge schnell und quantitativ (siehe unten). Der Einschluß von Et4N+ wurde auch im Festkörper nachgewiesen.