Navigated self-assembly of a Pd₂L₄ cage by modulation of an energy landscape under kinetic control

Kinetic control of molecular self-assembly remains difficult because of insufficient understanding of molecular self-assembly mechanisms. Here we report the formation of a metastable [Pd₂L₄]⁴⁺ cage structure composed of naphthalene-based ditopic ligands (L) and Pd(II) ions in very high yield (99%) under kinetic control by modulating the energy landscape. When self-assembly occurs with anionic guests in weakly cooordinating solvent then suitable intermedites and the metastable cage is formed. These conditions also prevent further transformation into the thermodynamically decomposed state. The cage formation pathways under kinetic control and the effect of the anions encapsulated on the self-assembly processes were investigated by QASAP (quantitative analysis of self-assembly process) and NASAP (numerical analysis of self-assembly process). It was found that the self-assembly with a preferred guest (BF₄ ^-) proceeds through intermediates composed of no more components than the cage ([Pd_aL_bX_c]^2a+ (a ≤ 2, b ≤ 4, X indicates a leaving ligand)) and that the final intramolecular cage-closure step is the rate-determining step. In contrast, a weaker guest (OTf^-) causes the transient formation of intermediates composed of more components than the cage ([Pd_aL_bX_c]^2a+ (a > 2, b > 4)), which are finally converted into the cage.