Dendritic Iron Porphyrins with Tethered Axial Ligands: New Model Compounds for Cytochromes

The novel dendritic iron porphyrins of generation zero ([ 1 ⋅Fe^III]Cl), one ([ 2 ⋅Fe^III]Cl), and two ([ 3 ⋅Fe^III]Cl) (Fig. 1) were prepared as models of cytochromes (Schemes 1 and 2). They feature controlled axial ligation at the iron center by two imidazoles tethered to the porphyrin core. Similar to the core compound [ 4 ⋅Fe^III]Cl, they are six‐coordinate low‐spin complexes as demonstrated by UV/VIS (Figs. 3 and 4) and EPR spectroscopy, as well as measurements of the magnetic moments by the Evans‐Scheffold method. The coordination environment does not change upon reduction to the corresponding iron(II) complexes. The dendritic iron porphyrins were purified by size‐exclusion chromatography and shown by matrix‐assisted laser‐desorption‐ionization mass spectrometry (MALDI‐TOF‐MS; Figs. 5 and 6) to be free of structural defects. With their triethyleneglycol monomethyl ether surface groups, the three dendritic mimics are soluble in solvents of widely differing polarity. Electrochemical studies (Figs. 7 and 8) and optical redox titrations (Fig. 9) revealed that the potential of the Fe^III/Fe^II couple in CH₂Cl₂, MeCN, and H₂O shifts strongly to more positive values (by as much as 380 mV) with increasing dendritic generation (Fig. 10). The redox potential of the second‐generation complex [ 3 ⋅Fe^III]Cl is, within experimental error, identical in all three solvents, which clearly demonstrates that the dendritic branching creates a unique local microenvironment around the isolated electroactive core. Whereas, in the organic solvents, the largest anodic potential shift is measured upon changing from generation zero to one, the largest shift in H₂O occurs only at the level of the second generation, when the dendritic superstructure is sufficiently dense to prevent access of bulk solvent to the electroactive core.