A Simple Thermodynamic Model for Rationalizing the Formation of Self-Assembled Multimetallic Edifices:  Application to Triple-Stranded Helicates

Reaction of the bis-tridentate ligand bis{1-ethyl-2-[6‘-(N,N-diethylcarbamoyl)pyridin-2‘-yl]benzimidazol-5-yl}methane (L2) with Ln(CF3SO3)3·xH2O in acetonitrile (Ln = La−Lu) demonstrates the successive formation of three stable complexes [Ln(L2)3]3+, [Ln2(L2)3]6+, and [Ln2(L2)2]6+. Crystal-field independent NMR methods establish that the crystal structure of [Tb2(L2)3]6+ is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative ( ) characterization of the heterobimetallic helicates [(Ln1)(Ln2)(L2)3]6+. A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N6O3 site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (ΔE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln3(L5)3]9+