Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 662

bridging O 2 moieties for n ¼ 3 or 6 because the required Co ... Co distance could not
be accommodated owing to restricted conformational flexibility of the peripheral
straps [Collman et al., 1994]. (Note, however, that the two-electron catalyst
(FTF6)Co 2 in benzonitrile solution forms a stable O 2 adduct, [(FTF6)Co 2 O 2 ] þ ,in
which the superoxo moiety bridges the two Co III ions [LeMest et al., 1997].) On
the other hand, the DPY (Y ¼ A, B, D, P, X) porphyrins are thought to be sufficiently
flexible to accommodate a wide range of metal - metal distances [Rosenthal and
Nocera, 2007].
Likewise, variations in selectivity within the pairs DPP versus DP tBu P (3.8 vs. 2);
DPD versus DPDM (3.6 vs. 3), and DPX versus DPXM (3.4 vs. 2.8) were interpreted
as a result of more substituted phenyl groups in the least selective member of the
pair interfering with the formation of the O 2 -derived bridges [Rosenthal and
Nocera, 2007; Park et al., 1995]. The poor selectivity of the (DP F P)Co 2 catalyst,
whose perfluorophenyl substituents' steric requirements are unlikely to differ
notably from those of the p-tolyl moieties in DPP, may illustrate the importance of
electronic factors (cf. Anson's work mentioned in Section 18.4.2 regarding the
enhanced four-electron selectivity of monomeric Co porphyrins bearing electron-
donating peripheral subsituents, including p-basic metal complexes). The two-
electron ORR catalysis by the cofacial phthalocyanines (DPcN)Co 2 and (DPcA)Co 2
(Fig. 18.13) was attributed to similar “electronic perturbations” [Kobayashi et al.,
1990]. In contrast, a derivative of (FTF4)Co 2 bearing a single Cl atom at a meso pos-
ition decreases the overpotential of O 2 reduction by about 40 mV and increases n av at
potentials ,0.5 V (where the parent catalyst becomes increasingly less selective)
[Collman et al., 1983b].
The stereoelectronic basis of the poor selectivity of (FTF4 )Co 2 , (FTF5)Co 2 ,
(C4)Co 2 , (C5)Co 2 (n av , 3; Fig. 18.15), which are structurally analogous to the
most selective cofacial bis-Co porphyrin ORR catalyst, (FTF4)Co 2 , remains unex-
plained. All five complexes bind O 2 in nonaqueous media exclusively in a bridging
fashion (forming m-peroxo or m-superoxo derivatives, depending on the oxidation
state; Section 18.5.2), and their electrochemical properties in the absence of a substrate
suggest that the two p-systems of the two macrocycles interact strongly, and hence
bimetallic cooperativity would be expected.
Much effort has been expanded in drawing mechanistic inferences from the obser-
vation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are
fairly selective catalysts (e.g., (DPA)CoM, where M ¼ Lu, Sc, Al, Ag, Pd, 2H, i.e.,
monometallic porphyrins; Fig. 18.15). At least two hypotheses have been proposed:
(i) polarization of the O - O bond in catalytic intermediates by the second ion (on
an N - H moiety) acting as a Lewis acid [Collman et al., 1987, 1994] and (ii) spatial
positioning of H þ donors especially favorable for proton transfer to the terminal O
atoms of coordinated O 2 [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
n 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).