Understanding, Controlling and Programming Cooperativity in Self-Assembled Polynuclear Complexes in Solution

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions Delta E-M,E-M, whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of Delta E-M,E-M with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and lieterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn2+ (a spherical d-block cation) and Lu3+ (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (Delta E-1-4(Lu,Lu) > Delta E-1-2(Lu,Lu)), ii) of larger magnitude when Zn2+ replaces Lu3+ (Delta E-1-2(Zn,Lu) > Delta E-1-2(Lu,Lu)) and iii) attractive between two triply charged cations held at some specific distance (Delta E-1-3(Lu,Lu) < 0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.