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†

Abstract

The solid state (Ln = Eu) and solution (Ln = Lu) structure of the dinuclear complexes [Ln₂L(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 [Eu₂(n-H₂L)(dmf)₅] species displays C₂ symmetry with the axis perpendicular to the Eu–Eu internuclear axis. The europium(III) ions are separated by 3.810(1) Å and are surrounded by eight O atoms, with six Eu–O distances in the range 2.2–2.4 Å (two phenoxides, two µ-phenoxides and two dmf) while the remaining Eu–O bond lengths are much longer (2.7–2.9 Å, one phenol and one µ-dmf). The geometry of the dinuclear assembly is comparable to that observed for [Eu₂(b-H₂L)(dmf)₅]·4dmf, both ligands adopting a two-bladed propeller conformation. The solution structure of [Lu₂L(dmf)₅], L = b-H₂L^6– and n-H₂L^6–, has been investigated by ¹H NMR spectroscopy in [²H₇]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, ΔG^‡ = 67.1 ± 0.4 kJ mol^–1 (b-H₈L) and k = 1.1 ± 0.1 s^–1, ΔG^‡ = 71.6 ± 0.4 kJ mol^–1 (n-H₈L).