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. PipNCN pincer and terpyridine ligands are used to support square planar Pt(ii) and octahedral Pt(iv). For the Pt(ii) complexes, [Pt(Z-pipNCN)(R-tpy)] (Z = NO, MeO, H; R = H, tertyl butyl, tolyl), H NMR spectroscopy shows that the Z-pipNCN 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-pipNCN)(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 / = 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).