Jean-Claude Bunzli - Structural and Photophysical Properties of Pseudo-Tricapped Trigonal Prismatic Lanthanide Building-Blocks Controlled by Zinc(Ii) in Heterodinuclear D-F Complexes

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  Piguet,C., RivaraMinten,E., Hopfgartner,G., Bunzli,J.C.G.   Helvetica Chimica Acta 1995 78  (6) 1541-1566

  Abstract: An efficient combination of   electrospray mass spectrometry (ES-MS), spectrophotometric and   H-1-NMR titrations in solution is used to characterize the   assembly of the segmental ligand   2-{6-[1-(3,5-dimethoxybenzyl)-1H-benzimidazol-2-yl]   pyridin-2-yl}-1,1'-dimethyl-5,5'-methylene-2'-(5-methylpyridin-2-yl)bis[1H-benz imidazole]   (L(2)) with Zn-II and 4f metal ions, Ln(III). Ligand L(2) reacts   with Zn(ClO4)(2) in MeCN to give successively [Zn(L(2))(2)](2+),   where the metal ion is coordinated by the tridentate binding   units of the ligands, and the double-helical head-to-head complex   [Zn-2(L(2))(2)](4+). When L(2) reacts with Ln(ClO4)(3) (Ln = La,   Eu, Lu), La-III only leads to a well-defined cylindrical   C-1-symmetrical homodinuclear head-to-tail complex   [La-2(L(2))(3)](6+) in solution, while chemical-exchange   processes prevent the H-1-NMR characterization of   [Eu-2(L(2))(3)](6+), and Lu-III gives complicated mixtures of   complexes. However, stoichiometric amounts of Ln(III) (Ln = La,   Ce, Pr, Nh, Sm, Eu, Tb, Y, Lu), Zn-II, and L(2) in a 1:1:3 ratio   lead to the selective formation of the C-3-symmetrical   heterodinuclear complexes [LnZn(L(2))(3)](5+) under thermodynamic   control. Detailed NOE studies show that the ligands are wrapped   about the C-3 axis defined by the metal ions, and the separation   of dipolar and contact contributions to the H-1-NMR paramagnetic   shifts of the axial complexes (LnZn(L(2))(3)](5+) (Ln = Ce, Pr,   Nd, Sm, Eu) in MeCN establishes that Zn-II occupies the   pseudo-octahedral capping coordination site defined by the three   bidentate binding units, while Ln(III) lies in the resulting   'facial' pseudo-tricapped trigonal prismatic site produced by the   three remaining tridentate units. Photophysical measurements show   that [LnZn(L(2))(3)](5+) (Ln = Eu, Tb) are only weakly   luminescent because of quenching processes associated with the   C-3-cylindrical structure of the complexes. The use of 3d metal   ions to control and design isomerically pure 'facial' tricapped   trigonal prismatic lanthanide building blocks is discussed   together with the calculation of a new nephelauxetic parameter   associated with heterocyclic N-atoms coordinated to Ln(III)

  Address (URL): WOS:A1995RX60300011