Structural and dynamic properties of calixarene bimetallic complexes:: solution versus solid-state structure of dinuclear complexes of EuIII and LuIII with substituted calix[8]arenes

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).