Reversible Structural Changes Accompanying the Two-Electron Redox Reaction of Pt(tacn) (tacn: 1,4,7-triazacyclononane) Complexes

Platinum complexes with 1,4,7-triazacyclononane (tacn) as facial ligands, [PtIVCl3(tacn)](ClO4), [PtIVCl(bpy)(tacn)](ClO4)3 ([1](ClO4), and [2](ClO4)3, bpy: 2,2′-bipyridine) were prepared, and their hexa-coordinated octahedral structures were determined via single-crystal X-ray analysis. In aqueous solutions containing NaCl as the electrolyte, [2]3+ exhibited a quasi-reversible two-electron redox wave at +0.22 V (vs. SCE) with a large peak separation (ΔE = Epc − Epa = 0.28 V). In addition, controlled-potential reduction of [2]3+ at −0.30 V in aqueous solutions containing NaCl afforded [PtII(bpy)(tacn)]2+ ([3]2+), which was then re-oxidized to regenerate [2]3+. X-ray analysis of [3]2+ revealed a tetra-coordinated square-planar structure in which both tacn and bpy were bound to the central Pt as bidentate ligands. The redox potential of this reaction is coupled with dissociation/association of Cl− from/to Pt in the solution. MO/DFT calculations indicate that structural changes in tacn promote the reversible two-electron redox reaction between the octahedral Pt(IV) and square planar Pt(II) complexes.