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

摘要

N-aryl and N-alkyl substituted pyridine-2,6-diimines (pdi) are useful tridentate ligands that are redoxactive 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(center dot))(1-) pi-radical, singlet or triplet dianion (pdi)(2-) or (pdi(center dot center dot))(2-), a trianionic pi-radical (pdi(center dot))(3-), and a singlet tetraanion (pdi)(4-). Upon this stepwise reduction the four C-N bond distances (C-py-N-py and C-imine-N-imine 1 increase and at the same time the two C-py-C-imine bond distances decrease. We show here that the single structural parameter Delta = [(d2 + d2')/2 - (d1 + d1' + d3 + d3')/4] varies in a linear fashion with increasing reduction of the (pdi(0))-ligand. Delta 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 pi-backdonation effects (M - pdi(0)) as found in compounds with a neutral ligand and where M is a 2nd or 3rd row transition metal ion with d(n) configuration and n = 6. (C) 2018 Elsevier B.V. All rights reserved.