DPA-O2

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

According to Scheme 27, the rate of formation of ferricenium ions is given by Eq. 28, where the catalytic rate constant (/ cat) corresponds to 2A et- The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] (DPA = bisporphyrin with anthrocene spacer, DPB = bisporphyrin with biphenylene spacer, DPD = bisporphpyrin with dibenzofuron spacer) also catalyze the reduction of O2 by Fe(CsH4Me)2, but the amount of [Fe(C5H4Me)2] formed is <4 equiv of O2 (168). Thus, the clean four-electron reduction of O2 by [Fe-(C5H4Me)2]... 

FcII(DPAH)2 + O2 (DPAH)(DPA)FeiV(30H. v 2 FeJH(DPA)(DPAH)... 

Here too, charge density calculations (lS-16) reflect the structural properties of the material. Charge density plots for the Individual states near Ep demonstrate the 2D nature of Cu2-O2-03 dpa bands and the ID nature of the Cul-01-04 dpa bands. The Ionic Y (or R = rare earth) atoms act as electron donors and do not otherwise participate. Also, the partial DOS at Ep for Y give extremely low values for the conduction electrons (the same Is true for Gd). 

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