Complexation of alkali- and alkaline-earth-metal cations by a conjugated tetraimine macrocyclic ligand: synthesis, sodium-23 nuclear magnetic resonance spectra, and the X-ray structure of a twelve-co-ordinate barium(II)‘sandwich’ complex

Abstract

Reaction of 2,5-diformylfuran with o-phenylenediamine in methanol in the presence of a salt of K⁺, Ca²⁺, Ba²⁺, or Pb²⁺ yields complexes of the 18-membered conjugated macrocyclic tetraimine L² derived from the cyclic (2 + 2) condensation of 2 mol of dialdehyde with 2 mol of diamine. The complexes all have a 1 : 1 ligand : metal stoicheiometry except those of Ba²⁺ which contain two macrocyclic ligands per metal atom. Crystals of [BaL²₂][BPh₄]₂ are monoclinic with a= 23.25(1), b= 14.92(1), c= 23.69(1)Å, β= 101.0(1)°, Z= 4, and space group A2/a(no.15). 1 468 Reflections above background were measured by diffractometer and refined to R 0.10. In the [BaL²₂]²⁺ cation the metal atom is sandwiched between two almost parallel ‘N₄’ planes 2.6 Å apart, the furan rings being tilted by 24.2 and 26.3° with respect to these planes. The barium atom is twelve-co-ordinate being bonded to the four nitrogen and two oxygen atoms of each ring [Ba–N 2.96(3)–3.08(3), Ba–O 2.91(2)–3.02(3)Å]. The smaller ions Li⁺, Na⁺, and Mg²⁺ are ineffective as templates for the macrocycle synthesis but complexes of Na⁺(and of [NH₄]⁺) could be prepared by neutralisation of the diacid salt [H₂L²][ClO₄]₂ with aqueous NaOH (or aqueous NH₃). The free macrocycle could not be isolated although ‘recrystallisation’ of the complexes of Na⁺ or [NH₄]⁺ from hot alcohol afforded the metal-free alcohol addition products L²·2MeOH and L²·EtOH. The complexes have been characterised by i.r., u.v.–visible, mass, and ¹H n.m.r, spectra. Sodium-23 n.m.r. spectra of equimolar solutions of NaL²(ClO₄) and NaClO₄·H₂O in [²H₆]dimethyl sulphoxide showed separate resonances at temperatures below about 30 °C indicating slow exchange of Na⁺ between macrocycle and solvent. The free energy of activation for exchange was estimated to be 59 kJ mol^–1. The stoicheiometries and relative stabilities of the complexes are interpreted in terms of the sizes of the metal ions in relation to the fixed hole size of the macrocycle.