Among the plethora of parameters controlling the stability and structures of lanthanide coordination complexes, it is often difficult to decipher their relative importance in the global complexation processes. The combination of the bond valence method (for analyzing solid state structures) with the thermodynamic site binding model (for unravelling complexation reactions occurring in solution) appears to be an efficient tool for specifically addressing interligand effects, which affect the output of the coordination process. When applied to the reaction of the tridentate aromatic scaffolds 2,2':6',2''-terpyridine (L1) and 2,6-bis(benzimidazol-2-yl)pyridine (L2) with trivalent lanthanides, Ln(III), we demonstrate that the successive fixation of ligands, eventually leading to the triple-helical complexes [Ln(Lk)3]3+, is anticooperative both in the solid state and in solution, with a special sensitivity to the nature of the counteranion and to the peripheral substitution for L2. Consequently, in addition to the classical entropic driving forces resulting from the use of specific metal/ligand ratio, the stoichiometry of the final complex can be tuned by a judicious choice of interligand interactions, as exemplified by the unusual isolation of stable complexes with Ln/L = 2:3 ratios.
A series of 10 different mesomorphic semidendrimeric tridentate ligands L5-L14 grafted with terminal cyanobiphenyl groups have been synthesized. Upon reaction with Ln(NO(3))(3) (Ln = trivalent lanthanide), the central 2,6-bis(N-ethylbenzimidazol-2-yl)pyridine unit is meridionally tricoordinated to the metal to give rodlike monomeric [Ln(Lk)(NO(3))(3)] and H-shaped dimeric [Ln(2)(Lk)(2)(NO(3))(6)] complexes. For the small Lu(III) cation, the monomeric complexes are quantitatively formed in a noncoordinating CD(2)Cl(2) solution. For larger cations (Ln = Eu, Pr), the thermodynamic equilibrium 2[Ln(Lk)(NO(3))(3)] ↔ [Ln(2)(Lk)(2)(NO(3))(6)] can be evidenced across the complete ligand series. Detailed thermodynamic studies show that the dimeric complexes result from the formation of primary intermetallic nitrate bridges whose strength depends on the metallic size. For each complex, secondary nonspecific interstrand van der Waals interactions produce nonartifactual enthalpy/entropy compensation. In the absence of solvent, only the complexes with the most extended ligands L5 and L6 produce thermotropic mesophases. Layered organizations are dominant (smectic A) with the induction of nematogenic behavior at high temperature when interstrand interactions are modulated by methyl substitutions. Correlations between the trend of dimerization and the sequences of thermotropic mesophases are attempted.
Reaction of the Single Molecule Magnet [Mn(12)O(12)(CH(3)CO(2))(16)(H(2)O)(4)] (Mn(12)) with mesogenic dendritic ligands Li (i = 4, 5) quantitatively yields functional clusters [Mn(12)O(12)(Li-H)(16)(H(2)O)(4)] (i = 4, 5) that self-organize into a thermotropic SmA-type liquid crystalline phase. The perturbation of the molecular interface by methylation of the terminal mesogenic cyanobiphenyl groups induces a significant decrease of the clearing temperature without affecting the magnetic properties and the supramolecular organization of the Mn(12)-based clusters.
Lipophilic methyl‐substituted cyanobiphenyls can be considered as molecular salmon that jump out of the condensed phase, but are limited by an increase in size of their rigid core. The concept of cohesive free‐energy densities applied to melting processes correlates the molecular volumes of the constituents with the macroscopic enthalpic and entropic changes that accompany the phase transitions of the bulk materials. For more details see the Full Paper by E. Terazzi, C. Piguet et al. on
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