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

Abstract

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, (F 8)Fe III(Cl) (F 8 is tetrakis(2,6-difluorophenyl)porphyrinate) with a series of [(L)Cu I] + complexes (L are tripodal tetradentate pyridylalkylamine ligands varying in effective chelate ring sizes: TMPA, PMEA, PMAP) to form (F 8)Fe II and [(L)Cu II(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 ( k obs) [Cu I(pmea)] + > [Cu I(pmap)] + > [Cu I(tmpa)(thf)] +. However, the redox potentials as related to Δ G ∘ for the reaction, [ E 1/2(oxidant) − E 1/2(reductant)], predict the trend [Cu I(tmpa)(CH 3CN)] + > [Cu I(pmea)] + > [Cu I(pmap)] +. A detailed description of Cu I-to-Cu II structural changes is provided and these likely influence the observed [(L)Cu I] +/(F 8)Fe III(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 F 8-like heme. Kinetic/mechanistic insights were obtained from transient absorption spectroscopic monitoring in CH 3CN following photoejection of carbon monoxide from [( 6L)Fe II(CO)⋯Cu II(Cl)].