Toward an Allosteric Metallated Container

Abstract

Polytopic ligands L¹ and L² in which three 2,2′-bipyridine units are linked to a central tris(pyrid-2-yl)amine (L¹) or tris(pyrid-2-yl)methanol (L²) moiety by alkyl spacers were prepared by multistep organic syntheses. The parent tris(pyrid-2-yl)-type ligands were shown to be modest-to-good chelators for Zn²⁺ and Cu²⁺ ions in solution, and bi- and tridentate N-coordination was confirmed by crystal structures of Cu^II and Ru^II complexes, respectively. Fe^II and Ru^II smoothly form stable, cage-like 1:1 complexes with L¹ and L², in which the metal ion is coordinated to the tris(bpy) site of the ligands. The vacant tris(pyrid-2-yl) site of these complexes is, however, a poor donor site for Zn²⁺ and Cu²⁺ ions. In addition, Fe^II modulates the coordination behaviour of the tris(pyrid-2-yl) site toward Zn²⁺: Whereas tris(5-methylpyrid-2-yl)amine forms a 2:1 complex with Zn²⁺ in CH₂Cl₂, [Fe(L¹)]²⁺ forms a 1:1 Zn complex. Spectrophotometric titrations suggest that [Fe(L²)]²⁺ forms a polynuclear Zn²⁺ complex in CH₂Cl₂, possibly involving bridging coordination of the alcohol OH group, which contrasts the smooth formation of a 2:1 complex of the parent tris(pyrid-2-yl)-type ligand with Zn. Fe^II might therefore be considered as an allosteric effector, which modulates the metal binding properties of the second tris(pyrid-2-yl) site of L¹ and L². Contrary to expectation, Zn²⁺ and Cu²⁺ appear to associate weakly with donor atoms directed toward the exterior of the cage-like complexes [Fe(Lⁿ)]²⁺ and [Ru(L¹)]²⁺, rather than locating in the interior of the container by tripodal coordination to the tris(pyrid-2-yl) site.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)