STRUCTURAL AND PHOTOPHYSICAL PROPERTIES OF PSEUDO-TRICAPPED TRIGONAL PRISMATIC LANTHANIDE BUILDING-BLOCKS CONTROLLED BY ZINC(II) IN HETERODINUCLEAR D-F COMPLEXES

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-benzimidazole] (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).