Design, Formation and Properties of Tetrahedral M4L4and M4L6Supramolecular Clusters1

Abstract

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 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 H₆A forms M₄L₄ tetrahedra with Ga^III, Ti^IV, and Sn^IV; (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 = Ga^III, Ti^IV, and Sn^IV) 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 = Ga^III) based on an “edge-on” design, while H₄C forms an M₄L₆ tetrahedron (M = Ga^III, Fe^III) 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 Ga^III complex.