Tuning two-electron transfer in terpyridine-based platinum(ii) pincer complexes

An important factor in obtaining reversible multi-electron transfer is overcoming large changes in coordination geometry. One strategy is to use ligands that can support the geometries favored before and after the electron transfer. Pip₂NCN^- pincer and terpyridine ligands are used to support square planar Pt(ii) and octahedral Pt(iv). For the Pt(ii) complexes, [Pt(Z-pip₂NCN)(R-tpy)]⁺ (Z = NO₂, MeO, H; R = H, tertyl butyl, tolyl), ¹H NMR spectroscopy shows that the Z-pip₂NCN^- ligand is monodentate whereas the R-terpyridyl ligand is tridentate. The availability of flanking piperidyl groups of the monodentate pincer ligand is essential for the stabilization of the metal center upon oxidation. Pt(Z-pip₂NCN)(R-tpy)⁺ complexes undergo two-electron platinum centered oxidation near 0.4 V and two Pt(tpy) centered reductions near -1.0 V and -1.5 V. An estimate of n _ox/n _red = 1.8 is consistent with an oxidation that involves two-electron transfer per Pt center. Variation in the pincer-(Z) and terpyridine-(R) substituents allows for tuning of the oxidation process over a 260 mV range and the two reduction processes over ranges of 230 mV (first reduction) and 290 mV (second reduction step).