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 Zn2+, and Cu2+, ions in solution, and bi- and tridentate N-coordination was confirmed by crystal structures of Cull 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 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(L-1)](2+) forms a 1:1 Zn complex. Spectrophotometric titrations suggest that [Fe(L-2)](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 L-1 and L-2. Contrary to expectation, Zn2+ and Cu2+ 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)