Jean-Claude Bunzli - Structural and dynamic properties of calixarene bimetallic complexes: solution versus solid-state structure of dinuclear complexes of Eu-III and Lu-III with substituted calix[8]arenes

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  Bunzli,J.C.G., Ihringer,F., Dumy,P., Sager,C., Rogers,R.D.   Journal of the Chemical Society-Dalton Transactions  1998 (3) 497-503

  Abstract: The solid state (Ln = Eu) and solution   (Ln = Lu) structure of the dinuclear complexes   [Ln(2)L(dmf)(5)].xsolv, where L is the hexaanion of   p-nitrocalix[8]arene (n-H8L) or p-tert-butylcalix[8]arene (b-H8L)   and dmf = dimethylformamide, have been analysed and compared. The   neutral [Eu-2(n-H2L)(dmf)(5)] species displays C-2 symmetry with   the axis perpendicular to the Eu-Eu internuclear axis. The   europium(III) ions are separated by 3.810(1) Angstrom and are   surrounded by eight O atoms, with six Eu-O distances in the range   2.2-2.4 Angstrom (two phenoxides, two mu-phenoxides and two dmf)   while the remaining Eu-O bond lengths are much longer (2.7-2.9   Angstrom, one phenol and one mu-dmf). The geometry of the   dinuclear assembly is comparable to that observed for   [Eu-2(b-H2L)(dmf)(5)].4dmf, both Ligands adopting a two-bladed   propeller conformation. The solution structure of [Lu2L(dmf)(5)],   L = b-H2L6- and n-H2L6-, has been investigated by H-1 NMR   spectroscopy in [H-2(7)]dmf. The dinuclear complexes retain a   two-fold symmetry element and the ligand conformation is   identical for both edifices and very close to the solid-state   conformation. An intramolecular racemisation process has been   evidenced and characterised at 294 K: k = 7.0 +/- 0.7 s(-1),   Delta G double dagger = 67.1 +/- 0.4 kJ mol(-1) (b-H8L) and k =   1.1 +/- 0.1 s(-1), Delta G double dagger = 71.6 +/- 0.4 kJ   mol(-1) (n-H8L)

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