Spectroscopy, electrochemistry, and nucleophilicity of nickel and cobalt complexes of 2′-pyridine carboxaldehyde 2′-pyridyl hydrazone (papyH)

Nickel and cobalt complexes of pyridine-2-carboxaldehyde-2′-pyridylhydrazone (papyH) and N-methyl(pyridine-2-carboxaldehyde-2′-pyridyl-hydrazone) (papyMe) have been fully characterized through electrochemistry, titration, and alkylation with methyl iodide and compared with existing data on zinc and iron complexes. Both rate constants for alkylation and ligand pKa depend on the coordinated metal and can be understood based on the influence of d electrons on the ligand molecular orbitals.

Rates of alkylation and pKa of metal papy complexes are dependent upon the metal ion as a result of the interaction between ligand orbitals and metal d orbitals.Figure optionsDownload as PowerPoint slideHighlights
► M3+ complexes of papyH are relatively strong acids with first pKa of the cobalt complex near 0.
► The Co3+/Co2+ couple for [Co(papyH)2]n+ is reduced by 0.37 V upon deprotonation of both ligands.
► The complexes [Co(papy)2]+ and Ni(papy)2 are alkylated on the ligand nitrogen by CH3I.
► The methylated Co3+ complex is reduced to Co2+ by the liberated iodide.