Intermolecular versus intramolecular electron-/atom- (Cl) transfer in heme-iron and copper pyridylalkylamine complexes

While outer-sphere electron-transfer reactions come with a firm experimental and theoretical basis, less is known about the redox reactions occurring by atom-transfer. In the present study, relevant reactions occur upon mixing, (F8)FeIII(Cl) (F8 is tetrakis(2,6-difluorophenyl)porphyrinate) with a series of [(L)CuI]+ complexes (L are tripodal tetradentate pyridylalkylamine ligands varying in effective chelate ring sizes: TMPA, PMEA, PMAP) to form (F8)FeII and [(L)CuII(Cl)]+. The electron-/atom- (Cl) transfer process is characterized by electrochemical measurements as well as UV–Vis, 1H NMR, and EPR spectroscopies. Stopped-flow UV–Vis spectroscopy in THF indicate the following relative rates (kobs) [CuI(pmea)]+ > [CuI(pmap)]+ > [CuI(tmpa)(thf)]+. However, the redox potentials as related to ΔG∘ for the reaction, [E1/2(oxidant) − E1/2(reductant)], predict the trend [CuI(tmpa)(CH3CN)]+ > [CuI(pmea)]+ > [CuI(pmap)]+. A detailed description of CuI-to-CuII structural changes is provided and these likely influence the observed [(L)CuI]+/(F8)FeIII(Cl) reaction rates. Analogous chemistry is described for a heme–copper system utilizing a heterobinucleating ligand (6L), which is comprised of a TMPA like moiety tethered at the 6-position of one of the pyridyl donors to a F8-like heme. Kinetic/mechanistic insights were obtained from transient absorption spectroscopic monitoring in CH3CN following photoejection of carbon monoxide from [(6L)FeII(CO)⋯CuII(Cl)].