Supramolecular assemblies of crown-substituted dinickel and dicobalt complexes with guest cation binding

A new macrocyclic ligand (L1) having four crown ether moieties at the peripheral position was synthesised by a conventional Schiff-base condensation reaction. The corresponding dinickel (1) and dicobalt (2) complexes formed a unique face-to-face dimerised structure by complexation with a Cs+ ion, which was characterised by 1H NMR, UV-vis and electrospray ionisation (ESI)-MS. Multi-sandwich structures consist of a 2:1 interaction of the benzo-18-crown-6 ether with the Cs+ ion affording a significantly high overall formation constant (Kall = 25.9 ± 0.9 at 298 K). The redox behaviour of the assembled structure of the dicobalt complex was investigated by cyclic voltammetry. In the dimerised structure, two sequential reductions ascribed to the CoIICoII/CoIICoI and CoIICoI/CoICoI couples were observed at - 1.13 V (E1) and - 1.36 V (E2) vs. Ag/AgCl, and both potentials were separated by E = E1 - E2 = 0.23 V. A mixed valence state (CoIICoI) was stabilised in the complex with the comproportionation constant Kcom = 7.7 × 103 at 298 K.