Coordination and electronic characteristics of a nitrogen heterocycle pincer ligand

The Fe(II) coordination chemistry of bis(pyrazole-3-yl)pyridine ligands with both proton or methyl substituents on pyrazole nitrogen are investigated, including the willingness of the ligand to undergo redox change. Protons on the pyrazole nitrogen promote intermolecular hydrogen bonding and lead to redox irreversibility; N methylation of those nitrogens eliminates those intermolecular interactions and leads to reversible outer-sphere reducibility. The resulting anion radical of the N-methylated ligand has more spin in the pyridine moiety than in the pyrazolyl pincer ligand arms, but also detectably delocalized into the electron withdrawing pyrazolyl pincer ligand arms; EPR and density functional calculations assist in characterizing the ligand radical anion, as its potassium complex. (C) 2016 Published by Elsevier B.V.