Design, Formation and Properties of Tetrahedral M₄L₄ and M₄L₆ Supramolecular Clusters¹

The rigid tris- and bis(catecholamide) ligands H6A, H4B and H4C form tetrahedral clusters of the type M4L4 and M4L6 through self-assembly reactions with tri- and tetravalent metal ions such as GaIII, FeIII, TiIV and SnIV. 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 H6A forms M4L4 tetrahedra with GaIII, TiIV, and SnIV; (Et3N)8[Ti4A4] crystallizes in R3̄c (No. 167), with a = 22.6143(5) Å, c = 106.038(2) Å. The cluster is a racemic mixture of homoconfigurational tetrahedra (all Δ or all Λ 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 M4A4 (M = GaIII, TiIV, and SnIV) clusters are remarkably stable once formed. Two approaches are presented for the formation of M4L6 tetrahedral clusters. Of the bis(catecholamide) ligands, H4B forms an M4L6 tetrahedron (M = GaIII) based on an “edge-on” design, while H4C forms an M4L6 tetrahedron (M = GaIII, FeIII) based on a “face-on” strategy. K5[Et4N]7[Fe4C6] crystallizes in I 4̄3d (No. 220) with a = 43.706(8) Å. This M4L6 tetrahedral cluster is also a racemic mixture of homoconfigurational tetrahedra and has a cavity large enough to encapsulate a molecule of Et4N+. This host−guest interaction is maintained in solution as revealed by NMR investigations of the GaIII complex.