Jean-Claude Bunzli - Influence of bulky N-substituents on the formation of lanthanide triple helical complexes with a ligand derived from bis(benzimidazole)pyridine: Structural and thermodynamic evidence

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      Muller,G., Bunzli,J.C.G., Schenk,K.J., Piguet,C., Hopfgartner,G.   Inorganic Chemistry 2001 40  (12) 2642-2651

      Abstract: The planar aromatic tridentate ligand   2,6-bis(1-S-neopentylbenzimidazol-2-yl)pyridine (L-11) reacts   with Ln(III) (Ln = La-Lu) in acetonitrile to give the successive   complexes [Ln(L-11)(n)](3+) (n = 1-3). However, stability   constants determined by spectrophotometry and NMR titrations show   that formation of the tris complexes is not favored, log K-3   being around 1 for La(III) and Eu(III), while no such species   could be evidenced for the smaller Lu(III) ion. The X-ray   structures of L-11 (monoclinic, P2(1), a = 13,3850(12) Angstrom,   b = 12.0243(1 1) Angstrom, c = 16.4239(14) Angstrom, beta   =103.747(7)degrees),   [La(ClO4)(2)(L-11)(2)](3)[La(ClO4)(2)(H2O)(L-11)(2)](ClO4)(4).   15MeCN (1a, monoclinic, P2(1), a=21.765(4) Angstrom, b =   30.769(6) Angstrom, c 21.541(5) Angstrom, beta =   116.01(3)degrees), and [Eu(L-11)(3)](ClO4)(3).4.28MeCN (5a,   monoclinic, P1, 14.166(3) Angstrom, b = 19.212(4) Angstrom, c =   21.099(4) Angstrom, alpha = 108.91(3)degrees beta =   98.22(3)degrees, gamma = 108.30(3)degrees) have been solved. In   la, two different types of complex cations are evidenced, both   containing 10-coordinate La(III) ions. In the first type, both   perchlorate anions are bidentate, while in the second type, one   perchlorate is monodentate, the 10th coordination position being   occupied by a water molecule. In 5a the three ligands are not   equivalent. Ligands A and B are wrapped in a helical way and are   mirror images of each other, while ligand C lies almost   perpendicular to the two other ones. This stems from the steric   hindrance generated by the bulky neopentyl groups with the   consecutive loss of any stabilizing interstrand pi -stacking   interactions. This explains the low stability of the tris   complexes and the difficulty of isolating them and points to the   importance of the steric factors in the design of self-assembled   triple helical lanthanide-containing functional edifices   [Ln(L-i)(3)](3+)

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