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

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)A, 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 A 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)A]. 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.