Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 657

was interpreted as that of a p-cation radical [LeMest et al., 1997]. Frontier MOs in
some cofacial diporphyrins appear to have substantial coefficients both on the metal
ions and on the porphyrins [LeMest et al., 1997]. Therefore, oxidation state formalism
may not always be useful in thinking about the electronic structure of the mixed-
valence group 2 cofacial bismetalloporphyrins.
18.5.2 Chemistry of O 2 Adducts of Cofacial bis-Co Porphyrins
The dioxygen chemistry of group 2 biscobaltdiporphyrins (with the exception of
(FTF3)Co 2 ; Fig. 18.13) provides a remarkable example of bimetallic cooperativity,
and as a result appears to be strikingly different from that of metalloporphyrins
containing other Groups 8 and 9 metals. Like their monomeric analogs, Co II cofacial
metalloporphyrins bind O 2 reversibly only in the presence of a nitrogenous hetero-
cyclic base, such as imidazole. Most workers report that the resulting adducts are dia-
magnetic (their monomeric analogs have an S ¼ 2 ground state) and are typically
interpreted as a complex of two d 6 Co III ions bridged by a diamagnetic peroxo
moiety, O 22 [Liu et al., 1985; Collman et al., 1980, 1983a]. Despite their diamagnet-
ism, none of these adducts has ever been characterized by NMR. Controversially, a
species obtained by exposing a solution of (FTF4)Co 2 in benzonitrile in the presence
of N-methylimidazole to O 2 at low temperature was reported to give an EPR signal
[LeMest et al., 1997].
Although oxidized monomeric Co porphyrins are inert toward O 2 , singly and
doubly oxidized group 2 Co 2 derivatives bind O 2 reversibly with unexpectedly high
affinities. Even more surprising, a heterocyclic ligand trans to O 2 is not obligatory.
For example, in a noncoordinating solvent (CH 2 Cl 2 ), the half-saturation O 2 pressure
p 1/2 (O 2 ) for [(DPB)Co 2 ] þ is about 0.2 atm; it is about 0.01 atm in a weakly coordinat-
ing solvent, such as benzonitrile, whereas in the presence of 1,5-diphenylimidazole
(Ph 2 Im), the O 2 adduct could not be deoxygenated [Collman et al., 1992]. An excel-
lent ORR catalyst, [(FTF4)Co 2 ] þ , in benzonitrile solution manifests very high O 2 affi-
nity ( p 1/2 ¼ 0.4 Torr, about 5 10 24 atm), which exceeds that of some myoglobins
(0.4 - 10 Torr in aqueous buffers) [LeMest et al., 1997; Collman et al., 2004a].
Similarly unprecedented in metalloporphyrin chemistry is the fact that such adducts
can be protonated reversibly in benzonitrile. Such protonation has been proposed
[Rosenthal and Nocera, 2007; Fukuzumi et al., 2004] to be critical for the high cata-
lytic activity of group 2 cofacial bis-Co porphyrins in the ORR.
Consensus appears to exist in the literature that O 2 adducts of singly oxidized
bis-Co diporphyrins, [(dipor)Co 2 O 2 ] þ , contain bridging superoxide. At room temp-
erature, all of these adducts display isotropic or weakly anisotropic EPR signals cen-
tered at g 2 with a 15-line superhyperfine splitting indicative of spin coupling to two
equivalent 57 Co nuclei (nuclear spin 7/2). Collman and co-workers suggested that the
anisotropy of the EPR signal correlated with poor four-electron selectivity [Collman
et al., 1983a]: examples included pairs of structurally similar [(FTF4)Co 2 (O 2 )] þ
(n av 4) versus [(FTF4 )Co 2 (O 2 )] þ (n av 2) and [(C5)Co 2 (O 2 )] þ (n av 2.4)
versus [(C4)Co 2 (O 2 )] þ (n av . 3.2; see Fig. 18.15 below). In contrast, Le Mest and
co-workers reported that the EPR spectra of all dioxygenated group 2 Co 2 porphyrins