Jean-Claude Bunzli - Extended rodlike polyaromatic receptors with bent tridentate units complexed to lanthanide metal ions

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  Nozary,H., Piguet,C., Rivera,J.P., Tissot,P., Bernardinelli,G.,   Vulliermet,N., Weber,J., Bunzli,J.C.G. Inorganic   Chemistry 2000 39 (23) 5286-5298

  Abstract: A synthetic strategy is developed to   attach semirigid lipophilic sidearms to the 6-positions of bent   aromatic tridentate 2,6-bis(benzimidazol-2-yl)pyridine cores to   produce U-shaped ligands, L(6,7). Differential scanning   calorimetry (DSC) reveals that entropic contributions severely   affect the isotropization processes of these flexible receptors,   but no mesomorphism is detected. The attachment of oxygen linkers   to the 5- or 6-positions of the benzimidazole sidearms lowers the   ligand-centered (1)pi pi* and (3)pi pi* excited states, and the   semiempirical ZINDO method assigns this effect to a   destabilization of the **** orbitals resulting from pi   -interactions. Reactions of L6 With Ln(NO(3))(3)(.) xH(2)O   provide the rodlike 1:1 complexes [Ln(L(6))(NO(3))(3)] (Ln =   La-Lu), which are stable in the solid state but partially   dissociate in acetonitrile. The crystal structure of   [Lu(L(6))(NO(3))(3)]. CH(3)CN (18a, LUC(63)H(84)N(9)O(13),   monoclinic, P2(1)/n, Z = 4) reveals an I-shaped arrangement of   the ligand strand arising from the meridional complexation of the   bent tridentate unit to nine-coordinate Lu(III). The replacement   of nitrate anions with trifluoroacetate anions gives the   centrosymmetric dimer [Lu(L(6))(CF(3)CO(2))(3)](2) (23,   Lu(2)C(134)H(162)N(10)O(20)F(18), triclinic, P (1) over bar, Z =   1), in which the symmetry-related Lu atoms are connected by two   bridging carboxylates, leading to an II-shaped dimetallic   edifice. These complexes [Ln(L(6))(NO(3))(3)] and   [Ln(L(6))(CF(3)CO(2))(3)](2) fulfill the geometrical criteria   required by precursors of calamitic metallomesogens, but no   mesomorphism can be detected, while photophysical studies   indicate that the low energies of ligand-centered (3)pi pi*   excited states drastically limit the luminescence of Eu(III)   complexes. The relationships between structural and electronic   properties resulting from 5- or 6-substitutions of the   benzimidazole rings and the effects of these substitutions on   photophysical and thermal properties are discussed

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