Neutral Heteroleptic Lanthanide Complexes for Unravelling Host-Guest Assemblies in Organic Solvents: The Law of Mass Action Revisited.

The binding of lanthanide containers [Ln(-diketonate)3dig] (dig = 1-methoxy-2-(2-
methoxyethoxy)ethane) to aromatic tridentate N-donor ligands (L) in dichloromethane produces
neutral nine-coordinate heteroleptic [LLn(-diketonate)3] complexes, the equilibrium reaction
quotients of which vary with the total concentrations of the reacting partners. This problematic drift
prevents the determination of both reliable thermodynamic stability constants and intrinsic host-guest
affinities. The classical solution theory assigns this behavior to changes in the activity coefficients of
the various partners in non-ideal solutions and a phenomenological approach attempts to
quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent
and contact-non-innocent contributors to the chemical potential. This assumption eventually predicts
an empirical linear dependence of the equilibrium reaction quotient on the concentration of the
formed [LLn(-diketonate)3] complexes, a trend experimentally supported in this contribution for
various ligands L differing in lipophilicity and nuclearity and for lanthanide containers grafted with
diverse beta-diketonate co-ligands. Even if the origin of the latter linear dependence is still the subject
of debate, this work demonstrates that this approach can be exploited by experimentalists for extracting reliable thermodynamic constants suitable for analyzing and comparing host-guest
affinities in organic solvents.