Jean-Claude Bunzli - Stability and size-discriminating effects in mononuclear lanthanide triple-helical building blocks with tridentate aromatic ligands

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  Petoud,S., Bunzli,J.C.G., Renaud,F., Piguet,C., Schenk,K.J.,   Hopfgartner,G. Inorganic Chemistry 1997  36 (25) 5750-5760

  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)(m)](3+) (n = 1-3). Stability constants   determined by spectrophotometry and potentiometric competitive   titrations with Ag-1 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)approximate to 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)-(ClO4)(4)(HClO4)(0.5)(CH3(C)N)(7.32)(L-1)(0.5)   (triclinic, P (1) over bar, a = 13.250(2) Angstrom, b = 16.329(2)   Angstrom, c = 27.653(3) Angstrom, alpha = 99.941(9)degrees, beta   = 93.393(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(Li)(2)](2+) (i = 1, 4, 8) are   also reported and discussed

  Address (URL): WOS:A1997YK96400018