Synthesis and electronic structure of a series of first-row transition-metal pyrazine(diimine) complexes in two oxidation states

The synthesis, molecular and electronic structures, and redox properties of pyrazine(diimine) ((PDI)-D-z) metal dihalide complexes ((PDI)-P-DiPP-D-Z)MX2 (DiPP = 2,6-diisopropylphenyl; M = Mn, Fe, Co, Ni; X = Cl, Br) are presented and compared. The dihalide complexes are reduced to ((PDI)-P-DiPP-D-Z)MX monohalide complexes, and the role of ligand redox non-innocence in the electronic structure of the reduced compounds is examined and compared to related pyridine(diimine) (PDI) compounds. The compounds were characterized by NMR spectroscopy, UV-VIS spectroscopy, cyclic voltammetry, X-ray crystallography, and Evans method magnetometry. Similar to previously reported PDI compounds, the reduced monohalide complexes are best described as divalent metal centers coupled to a reduced radical anion ligand. Structural and computational analyses show that the ligand in ((PDI)-P-DiPP-D-Z)MX is reduced by one electron and bears significant spin density. Comparison across the series of first-row transition metals shows increasing metal-ligand covalency for the later metals. These results may be useful in the design of modified heteroarene(diimine) ligands for first-row transition-metal catalysis in the future.