Dioxygen model compounds

Fukuzumi S, Mochizuki S, Tanaka T. 1990. Efficient catal)dic systems for electron transfer from an NADH model compound to dioxygen. Inorg Chem 29 653. 

We shall return to the discussion of the orientation and electronic structure of the dioxygen ligand during the next section which is primarily concerned with recent advances in the synthesis and physical characterization (particularly X-ray analysis) of oxygencarrying metal porphyrin model compounds. 

Figure 4.13 Co(acacen), an early model compound exhibiting reversible dioxygen binding at low temperatures.

As previously discussed, the binuclear Cu(I) haemocyanins constitute another important class of dioxygen carriers. The first model compound whose structure in the oxygenated form resembles most closely that of oxyhaemocyanin is the dicopper(II) complex with the tris(3,5-diisopro-pylpyrazolyl)borate ligand shown in Figure 26.29... 

All successful myoglobin and hemoglobin model compounds provide steric bulk on the distal side of the porphyrin ring with a hydrophobic pocket for complexation of dioxygen as well as a bulky alkyl imidazole proximal ligand... 

Manganese is used by nature to catalyze a number of important biological reactions that include the dismutation of superoxide radicals, the decomposition of hydrogen peroxide, and the oxidation of water to dioxygen. The dinuclear manganese centers that occur in Lactobacillus plantar-aum catalase and Thermus thermophilus catalase have attracted considerable attention and many model compounds have now been synthesized that attempt to mimic aspects of these biological systems.The catalases have at least four accessible oxidation states (Mn Mn , Mn°Mn , Mn" Mn", and Mn Mn ) it is believed that the Mn"Mn"/Mn"Mn" redox couple is effective in catalyzing the disproportionation of water. 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

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