Thermodynamic characterization of the self-assembly process of a three component heterobimetallic bisporphyrin...

The molecular self-assembly of macrocycle 4 is induced by the simultaneous coordination of two molecules of 4-pyridyldiphenylphosphine 3, a highly selective ditopic ligand, to Zn-bisporphyrin 1 and a square-planar Pd(II) complex 2.COD. We report a detailed thermodynamic characterization of the assembly process based on the quantification of each one of the two metal(Zn,Pd)-ligand(N,P) pairwise binding interactions implicated in the supramolecular macrocycle and its effective molarity value (EM). The experimental values of the pairwise metal-ligand interactions have been derived from UV-vis, NMR titrations, and isothermal titration calorimetry experiments of reference model systems. In turn, an EM = 1 x 10(-2) M has been determined by relating the experimental overall stability constant determined for the cyclic assembly with the equation to evaluate the statistical (noncooperative) self-assembly equilibrium constant. We used numerical methods (SPECFIT program) to predict the solution behavior (speciation) of two mixtures of the three molecules 1, 3, and 2.COD in a 1:2:1 relative stoichiometry at two different overall concentrations. The method uses the overall stability constant values and the stoichiometries of eleven species (complexes) implicated in the multicomponent equilibrium self-assembly of 4. Estimated stability constants of some of the species were statistically determined. The agreement observed between the theoretical simulations and the experimental data validates the suitability of the theoretical treatment of self-assembly macrocyclization in a three component strategy.