Synthesis of ionisable [2 x 2] grid-type metallo-arrays and reversible protonic modulation of the optical properties of the [Co4(II)L4]8+ species.

The new bis-hydrazone based ligands A and B form ionisable [2 x 2] grid-type transition metal complexes whose properties may be modulated by multiple protonation/deprotonation as shown by the reversible change in optical properties of the [Co2(II)L4]8+ complexes depending on their protonation state.