title: Stability and Size-Discriminating Effects in Mononuclear Lanthanide Triple-Helical Building Blocks with Tridentate Aromatic Ligands.
abstract: The planar aromatic tridentate ligand 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (L(1)) reacts with Ln(III) (Ln = La-Lu) in acetonitrile to give the successive complexes [Ln(L(1))(n)()](3+) (n = 1-3). Stability constants determined by spectrophotometry and potentiometric competitive titrations with Ag(I) show that the 1:1 and the 1:2 complexes display the usual thermodynamic behavior associated with electrostatic effects while the 1:3 complexes exhibit an unusual selectivity for the midrange Ln(III) ions (Delta log K(3)(Gd-Lu) approximately 4). A detailed investigation of the solution structure of [Ln(L(1))(3)](3+) (Ln = La-Dy) reveals that the closely packed triple-helical structure found in the crystal structure of [Eu(L(1))(3)](3+) is retained in acetonitrile for the complete series. A sharp control of the coordination cavity results from the interstrand pi-stacking interactions which appear to be optimum for Gd(III). For Yb(III), for instance, a 1:2 complex only could be isolated, which crystallizes as a hydroxo-bridged dimer [Yb(OH)(L(1))(2)](2)(ClO(4))(4)(HClO(4))(0.5)(CH(3)CN)(7.32)(L(1))(0.5) (triclinic, P&onemacr;, a = 13.250(2) Å, b = 16.329(2) Å, c = 27.653(3) Å, alpha = 99.941(9) degrees, beta = 93.394(9) degrees, gamma = 108.114(9) degrees, Z = 2). The binding of bulky substituents to the nitrogen atoms of the benzimidazole side arms in L(4) (i) severely affects the wrapping process, (ii) leads to less stable triple-helical building blocks, and (iii) removes the size-discriminating effect. The last can however be restored if a strong electron-donor group is connected to the central pyridine ring in L(8). Stability and solution structure data for [Ag(2)(L(i)())(2)](2+) (i = 1, 4, 8) are also reported and discussed.
authors.affiliation: Institute of Inorganic and Analytical Chemistry, BCH, and the Institute of Crystallography, BSP, University of Lausanne, CH-1015-Lausanne, Switzerland, Department of Inorganic, Analytical and Applied Chemistry, Sciences II, University of Geneva, CH-1211 Geneva-4, Switzerland, and Pharma Division, Hoffmann-La Roche, CH-4070 Basle, Switzerland.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Petoud
authors.forename: Jean-Claude G.
authors.initials: JC
authors.lastname: Bünzli
authors.forename: Fabien
authors.initials: F
authors.lastname: Renaud
authors.forename: Claude
authors.initials: C
authors.lastname: Piguet
authors.forename: Kurt J.
authors.initials: KJ
authors.lastname: Schenk
authors.forename: Gérard
authors.initials: G
authors.lastname: Hopfgartner