Jean-Claude Bunzli - Evidencing a reaction intermediate in the formation of the Eu-III bimetallic complex with p-tert-butylcalix[8] arene

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  Bunzli,J.C.G., Besancon,F. Physical Chemistry Chemical   Physics 2005 7 (10) 2191-2198

  Abstract: In view of the very few mechanistic   information available on the complex formation between lanthanide   ions and calixarenes, we study here the reaction mechanism   leading to the formation of the bimetallic complex between Eu-III   and p-tert-butylcalix[8]arene (b-LH8). We show that in dmf, the 1   : 1 complex [Eu(b-LH6)(NO3)(dmf)(4)] partially dissociates into a   compound A which is an important intermediate in the formation of   bimetallic [Eu-2(b-LH2)(dmf)(5)]. Several spectroscopic (ES-MS,   visible, luminescence, FT-IR) and conductometric techniques are   combined with variable-temperature and variable-pressure   experiments to demonstrate that in absence of base, intermediate   A is the cationic species [Eu(b-LH6)(dmf) (4)](+). The   thermodynamic parameters of the dissociation reaction, K-5(298) =   (4 +/- 1) x 10(-5) M, Delta H-0 = +16 +/- 3 kJ mol(-1), Delta S-0   = -30 +/- 11 J mol(-1) K-1, as well as the reaction volume Delta   V-0 = +9.4 +/- 0.9 cm(3) mol(-1) are evaluated and rationalized.   In presence of triethylamine, intermediate A deprotonates to   generate the neutral species A' = [Eu(b-LH5)(dmf)(4)], which, in   turn, further reacts with europium nitrate to yield the   bimetallic complex. In addition, the pK(a)s of the parent   calixarene LH8, of b-LH8 and of p-iso-propylcalix[8]arene are   determined in dmf, as well as the association constant between   b-LH8 and triethylamine, K-assoc = (9 +/- 1) x 10(-3) M-1

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