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.

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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.