In the presence of o 2 nonbiological copperl and

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In the presence of O 2 , nonbiological copper(l) [and iron(II)] complexes are often susceptible to ligand degradation, which may give the illusion of O 2 bind- ing. 102 The mechanisms by which this reaction occurs remain essentially un- known. Iron-porphyrin systems are rather more robust. Nonetheless, there are now several well-characterized copper(l) systems that reversibly bind dioxy- gen,ISb,l03 at least at low temperature. One that has been structurally character- . /0- ...... /cu Ized features a Cu ° dicopper(II)-peroxo moiety, l03f while a second, with more properties in common with oxyhemocyanin, features a
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II. SELECTED CHEMISTRY OF DIOXYGEN, IRON, COPPER, AND COBALT 205 D. General Aspects of the Chemistry of Cobalt Many parallels exist between the chemistry of Fe II_ and Co II-porphyrinato sys- tems. Dioxygen binds to many COil complexes to give mononuclear 1: 1 Co:0 2 complexes with a bent geometry (4.37) and dinuclear 2: 1 Co:0 2 complexes, 64,66 analogous to those described for Fe II systems in Reactions (4.29a) and (4.29b). Indeed, these dinuclear systems were the first nonbiological oxygen carriers to be isolated. The geometry of the diox- ygen moiety, spanning two metals, may be cis or trans: /0-0" Co Co (4.38) However, whereas these dinuclear cobalt species are invariably octahedral, din- uclear copper-peroxo species are tetrahedral or distorted square pyramidal. 40,41 In the late 1960s, 1: 1 Co:0 2 species were first isolated by use of a combi- nation of low temperatures and specific Schiff-base ligands. 104 It was found that cobalt corrins, such as vitamin B 12n also formed 1:1 dioxygen adducts,105 al- though this chemistry is not known to be utilized by living systems. 103 Cobalt(Il) porphyrins also form 1: 1 adducts but with low O 2 affinity, especially in nonpo- lar, aprotic solvents. Thus hemoglobin and myoglobin may be reconstituted from a cobaltoheme with preservation not only of dioxygen-binding capabilities but also of cooperativity. 106 The synthetic 1: 1 Co:0 2 complexes have proven to be very useful in increasing our understanding of factors that determine oxygen affinity for cobalt systems and by extrapolation for iron systems. Two important differences make COil systems more accessible. First, in contrast to iron sys- tems, the cleavage reaction (4.29c) and redimerization to a fL-OXO species (Re- action 4.29d) do not occur (see Figure 4.15). Thus COli complexes of O 2 are stable in solution at room temperature without the need for protection illustrated in Figure 4.14. Second, for CoII-porphyrinato systems, the equilibrium constant for the addition of a second axial base, such as pyridine or I-methylimidazole, is small. Thus the disproportionation to four-coordinate and six-coordinate spe- cies that occurs for corresponding Fe 11 systems (Reaction 4.33) does not occur. This difference simplifies the interpretation of spectral changes that are used to obtain thermodynamic and kinetic parameters of which there are now volumi- nous examples. 66 Moreover, the 1: 1 Co-0 2 complexes are paramagnetic. From the small 59CO hyperfine splitting, it is
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