A Tripodal Tetradentate Ligand with a Sulfonamide Bridgehead Nitrogen and Flexible Charge

A first in class tripodal tetradentate ligand containing a secondary sulfonamide group in the bridgehead position is described. Iron, nickel, and vanadium complexes of neutral N-(di-(pyridin-2-yl)-methyl)-pyridine-2-sulfonamide (dpmpsH) or its anionic deprotonated form (dpmps) were prepared. All complexes exhibit the desired tripodal coordination around the metal ions. The ligand is neutral in [M^II(dpmpsH)-Cl₂] M = Ni-(II), Fe-(II), and [Ni^II(dpmpsH)-Cl-(H₂O)]₂(ClO₄)-Cl, while in complexes with higher metal ion oxidation states, the ligand is deprotonated. This is the case for [Fe^III(dpmps)-Cl₂] and [V^VO-(η²-O₂)-(dpmps)] for which the tripodal topologies are closely comparable. The Fe^III/Fe^II reduction potential for [Fe^III(dpmps)-Cl₂] (0.08 V vs SCE) falls exactly between those for analogous iron complexes of neutral tetradentate tripodal ligands with bridgehead tertiary amine donors and those with one monoanionic terminal donor on a pendant arm. Facile methoxylation of [Fe^III(dpmps)-Cl₂] in methanol to produce [Fe^III(dpmps-OMe)-Cl₂] indicates acidity of the tertiary C-H group adjacent to the N-coordinated sulfonamido is high. In turn, this increases the susceptibility of high valent complexes to undergo oxidative cleavage of the C-C and C-H bonds around this carbon under aerobic conditions or at high electrochemical potentials.