Cobalt coordinated “picket-fence

Oxygen transport has received considerable attention. "Picket fence" cobalt porphyrin complexes are selectively blocked at the hindered face so that imidazole bases may only coordinate from the open side. As seen in Scheme 11, the imidazole-coordinated picket fence complex has a vacant coordination site at the top. The small linear oxygen molecule can traverse the "fence" to coordinate reversibly with cobalt from the top side. By dispersing such complexes in films one can build membranes which facilitate oxygen transport. This type of chemistry is one approach to making artificial blood. 

Table 53 gives a list of a number of cobalt(III)-superoxo complexes which have been isolated as crystalline solids. There is a marked preponderance of complexes of the type [Co(SB)(B)(02)] (SB = SchifF base). The base adducts of simple Co11 porphyrins have low affinities for dioxygen at room temperature and consequently their 1 1 adducts with 02 are not isolable. In contrast, exposure of a solid sample of Collman s picket fence porphyrin system [Co(TpivPP)(iV-Meim)] to 1 atm of dioxygen for 24 h produces [Co(TpivPP)(A(-Meim)(02)] (200).654 The pivalamido pickets in this compound exercise control of solvation about the coordinated dioxygen moiety and the stability is comparable with that of CoMb02,655 where the globin protein environment performs the same function. 

---