Allosteric Effects in Binuclear Homo- and Heterometallic Triple-Stranded Lanthanide Podates

This work illustrates a simple approach for deciphering and exploiting the various free energy contributions to the global complexation process leading to the binuclear triple-stranded podates [Ln₂(<b>L9</b>)]⁶⁺ (Ln is a trivalent lanthanide). Despite the larger microscopic affinities exhibited by the binding sites for small Ln³⁺, the stability constants measured for [Ln₂(<b>L9</b>)]⁶⁺ decrease along the lanthanide series; a phenomenon which can be ascribed to the severe enthalpic penalty accompanying the intramolecular cyclization around small Ln­(III), combined with increasing anticooperative allosteric interligand interactions. Altogether, the microscopic thermodynamic characteristics predict β_1,1,1^{La,Lu,<b>L9</b>}/β_1,1,1^{Lu,La,<b>L9</b>} = 145 for the ratio of the formation constants of the target heterobimetallic [LaLu­(<b>L9</b>)]⁶⁺ and [LuLa­(<b>L9</b>)]⁶⁺ microspecies, a value in line with the quantitative preparation (>90%) of [LaLu­(<b>L9</b>)]⁶⁺ at millimolar concentrations. Preliminary NMR titrations indeed confirm the rare thermodynamic programming of a pure heterometallic f-f′ complex