Deduced that the single unpaired electron resides

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deduced that the single unpaired electron resides primarily on the dioxygen moiety. 104a,105 From other experiments 107 it is appar- ent that net transfer of electron density from the metal onto the dioxygen varies
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206 4 / BIOLOGICAL AND SYNTHETIC DIOXYGEN CARRIERS considerably, from about O.le- to about 0.8e-. For example, it is found for a given COIl Schiff base, Co(bzacen), that the redox potential of the cobalt-Schiff- base center LCo, measured by cyclic voltammetry, E1/ 2 , B 2 LCo 11I + e - ( ) B 2 LCo" B = substituted pyridine (4.39) is a linear function of log K(02) as the axial base B is varied. The more easily the COIl center may be oxidized, the higher is the O 2 affinity, 103 as illustrated in Figure 4.18A. The dioxygen affinity also increases as the basicity of the axial nitrogenous ligand increases. 104a This effect is illustrated in Figure 4.18B. Be- cause of differing steric requirements, dimethylformamide (DMF) , substituted imidazole, and piperidine (pip) ligands do not fall on the correlation defined by the series of substituted pyridine species. Note the synergistic nature of dioxy- gen binding: in general, the more electron density that is pumped onto the metal by the axial base, the more electron density is available for donation into the 7T* orbitals of the dioxygen ligand. El/ 2 and log K(02) are also correlated, al- though more weakly, for a number of hemoglobins (Figure 4.18C).108 Here the porphyrin and axial base remain constant, but presumably the surroundings of the heme group and O 2 binding site vary in a manner that is less well-defined than in the model systems of Figure 4.18A and B. Notwithstanding these var- ious perturbations to the metal center, the 0-0 stretch occurs at about 1140 cm -1, placing all 1: 1 cobalt and iron-dioxygen complexes of nitrogenous and other hard ligands into the superoxo class. * Cobalt(II) porphyrins and their adducts with diamagnetic molecules invari- ably have spin S = i. (See Figure 4.16, but add one electron.) Thus the struc- tural changes are less pronounced than for corresponding iron(II) systems. 110,111 From the similarities in geometries and differences in electronic structures be- tween cobalt- substituted and native hemoglobins and their models, many in- sights have been gained about the factors that determine oxygen affinity as well * Because the 0-0 stretch may be coupled with other ligand modes, 109 its value should not be used to estimate superoxo character, although in a series of fL-superoxo and fL-peroxo complexes of carefully con- trolled stereochemistry, small changes in 1'(0-0) have been correlated with the pK a of the suite of ligands 66 Figure 4,18 (facing page) Linear free-energy relationships: (A) Correlation of the O 2 affinity at 21°C of Co(bzacen)L with the COllI ;;::::: COil cyclic voltammetric wave of Co(bzacen)L 2 species,I04a (B) Correlation of ligand affinity with pKa- Squares (0) pertain to the binding of L to Co(PPIX) at 23°e. Circles (0) pertain to the O 2 affinity at - 45°C of Co(PPIX)L species, Filled shapes pertain to substi- tuted pyridines; the least-squares lines shown are calculated from these data only, 104a (C) Corre-
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