ABSTRACT
The binding of lanthanide containers [Ln(ß-diketonate)3dig] [dig = 1-methoxy-2-(2-methoxyethoxy)ethane] to aromatic tridentate N-donor ligands (L) in dichloromethane produces neutral nine-coordinate heteroleptic [LLn(ß-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 nonideal solutions, and a phenomenological approach attempts to quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent and contact-noninnocent 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(ß-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 ß-diketonate coligands. 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.
ABSTRACT
Since its identification as an independent topic after the first world war, the chemistry of (bio)polymers and macromolecules rapidly benefited from intense synthetic activities driven by contributors focusing on formulation and structural aspects. Satisfying rationalization and predictions concerning polymer organization, stability, and reactivity were, however, delayed until the late fifties, when physical chemists set the basis of an adapted thermodynamic modeling. The recent emergence of metal-containing (bio)organic polymers (i.e., metallopolymers) thus corresponds to a logical extension of this field with the ultimate goal of combining the rich magnetic and optical properties of open-shell transition metals with the processability and structural variety of polymeric organic scaffolds. Since applications as energy storage materials, drug delivery vectors, shape-memory materials, and photonic devices can be easily envisioned for these materials, the development of metallopolymers is faced with some urgency in producing novel exploitable structures, while the rational control of their formation, organization, and transformation remains elusive. Caught between the sometimes antagonistic requirements of economic efficiency on one side and of scientific pertinence on the other side, the ongoing achievements in the control of the metal loadings of multi-site polymers are highlighted here with some tutorial discussions of luminescent lanthanidopolymers as proof-of-concept.
ABSTRACT
Access to reliable values of the thermodynamic constants ß1,1H,G , which control simple host-guest ([HG]) association, is crucial in medicine, biology, pharmacy, and chemistry, since the optimum concentration of an effector (i.e., a drug) acting on a receptor is set to 1ß1,1H,G . Intermolecular association between charged species in polar solvents, for which water is the archetype, largely obeys this principle. Any deviation from ideality, which alters the speciation in solution, is mastered by the Debye-Hückel theory of ionic atmosphere. Much less is known for related association reactions involving neutral species in non-polar (lipophilic) media such as membranes, bilayers, or organic polymers. Taking the intermolecular association between [La(hfa)3 dig] guest (hfa=hexafluoroacetylacetonate, dig=2-{2-methoxyethoxy}ethane) and tridentate polyaromatic host receptors L1-L3 in dichloromethane as a proof-of-concept, we show that the progress of the association reactions, as measured by the increase in the mole fraction of occupied sites of the receptors, disrupt the chemical potential of the solvent to such an extent that ß1,1H,G may seemingly be shifted by two orders of magnitude, thus leading to erroneous dose-response prescriptions. A simple chemical model, which considers a subset of solvent molecules in surface contact with the partners of the association reaction, restores reliable access to true and interpretable thermodynamic constants. The concomitant emergence of a concentration-dependent corrective parameter reestablishes satisfying dose-dependent response under real conditions. This "complement" to the law of mass action offers a simple method for safely taking care of the non-predictable variations of the activity coefficients of the various partners when host-guest reactions are conducted in non-polar media.
