Toward an Allosteric Metallated Container

Polytopic ligands L 1 and L 2 in which three 2,2′‐bipyridine units are linked to a central tris(pyrid‐2‐yl)amine (L 1 ) or tris(pyrid‐2‐yl)methanol (L 2 ) 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 Zn 2+ and Cu 2+ 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 L 1 and L 2 , 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 Zn 2+ and Cu 2+ ions. In addition, Fe II modulates the coordination behaviour of the tris(pyrid‐2‐yl) site toward Zn 2+ : Whereas tris(5‐methylpyrid‐2‐yl)amine forms a 2:1 complex with Zn 2+ in CH 2 Cl 2 , [Fe(L 1 )] 2+ forms a 1:1 Zn complex. Spectrophotometric titrations suggest that [Fe(L 2 )] 2+ forms a polynuclear Zn 2+ complex in CH 2 Cl 2 , 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 L 1 and L 2 . Contrary to expectation, Zn 2+ and Cu 2+ appear to associate weakly with donor atoms directed toward the exterior of the cage‐like complexes [Fe(L n )] 2+ and [Ru(L 1 )] 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)