Synthesis and Characterization of a Superoxo Complex of the Dicobalt Cofacial Diporphyrin [μ-O₂)Co₂(DPB)(1,5-diphenylimidazole)₂][PF₆], the Structure of the Parent Dicobalt Diporphyrin Co₂(DPB), and a New Synthesis of the Free-Base Cofacial Diporphyrin H₄(DPB)

Chemical oxidation of the dicobalt cofacial diporphyrin Co₂ ^II/^II(DPB), followed by exposure to dioxygen affords the bridged superoxo complex [(μ-O₂)Co₂(DPB)] [PF₆], This μ-superoxo complex has been implicated in possible mechanisms of four-electron dioxygen reduction by dicobalt cofacial diporphyrins but had not been isolated and fully-characterized previously. Although the superoxo complex is unstable with respect to loss of dioxygen, addition of 2 equiv of 1,5-diphenylimidazole yields [(μ-O₂)Co₂ ^III/^III(DPB)(1,5-diphenylimidazole)₂] [PF₆], a μ-superoxo complex that is stable toward loss of dioxygen. Analytically pure samples of the latter complex have been prepared, and their spectral and electrochemical properties are described. The crystal structure of the parent dicobalt cofacial diporphyrin Co₂ ^II/II(DPB) is reported. The Co-Co distance (3.726 (1) Å) and other structural features are compared to those of Co₂(FTF4) and of other known metallo-DPB structures. A new, improved total synthesis of the free-base porphyrin H₄(DPB) is presented. The combination of new reaction sequences, increased reaction scales, and improved product yields allows for large scale synthesis (gram quantities) of the free-base porphyrin needed to develop fully the coordination chemistry of the cofacial metallodiporphyrins.