Nickel(II) and Iron(II) Complexes with Tetradentate NHC/Amide Hybrid Ligands

Two methylene-bridged bis(imidazolium) salts [H4L1](PF6)(2) and [H4L2](PF6)(2) with appended amide groups have been synthesized which, after deprotonation, may serve as potentially tetradentate ligands providing two bis(imidazole-2-ylidene) and two amide donors. Using [H4L1](PF6)(2), a square-planar nickel(II) complex [NiL1] and a six-coordinate bis(ligand) iron(II) complex [Fe(HL1)(2)] have been isolated and structurally characterized. Their low-spin states have been confirmed spectroscopically, and their redox properties have been studied by cyclic voltammetry. Oxidations are metal-centered to give Ni-III and Fe-III species, respectively.