Structural characteristics of redox-active pyridine-1,6-diimine complexes: Electronic structures and ligand oxidation levels

Abstract N-aryl and N-alkyl substituted pyridine-2,6-diimines (pdi) are useful tridentate ligands that are redox-active and can coordinate to main group elements, transition metal ions, lanthanides and actinides. The neutral (pdi)0 ligand can accept up to four electrons generating a monoanion (pdi )1− π-radical, singlet or triplet dianion (pdi)2− or (pdi )2−, a trianionic π-radical (pdi )3−, and a singlet tetraanion (pdi)4−. Upon this stepwise reduction the four C–N bond distances (Cpy–Npy and Cimine–Nimine) increase and at the same time the two Cpy–Cimine bond distances decrease. We show here that the single structural parameter Δ = [(d2 + d2′)/2 − (d1 + d1′ + d3 + d3′)/4] varies in a linear fashion with increasing reduction of the (pdi0)-ligand. Δ represents therefore a powerful structural parameter for the determination of the oxidation level of this ligand in a given complex provided the central metal ion does not exhibit significant π-backdonation effects (M → pdi0) as found in compounds with a neutral ligand and where M is a 2nd or 3rd row transition metal ion with dn configuration and n ≥ 6.