Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 656

bis-Co complexes of FTF4, DPD, and DPP, with the average Ct - Ct distance (in
the absence of in-cavity ligands) of 3.42, 6.47, and 5.21 ˚ . Within this range, one
also finds the M - M distances in two crystal structures of cofacial porphyrins
whose bis-Co derivatives are essentially two-electron catalysts (C5 and FTF6).
† The DPD motif appears to be capable of accommodating the widest range of
M - M distances (8.63 - 3.49 ˚ ), followed by DPP (6.92 - 3.85 ˚ ); in both
cases, the flexibility is achieved by doming of the macrocycles. It is noteworthy
that (DPD)Co 2 and (DPP)Co 2 are among the best ORR catalysts discovered so
far (see Fig. 18.15 below).
† The two macrocycles appear to be largely parallel even in the DPD and DPP
series, where the structure of the linker would seem to favor a conformer in
which the planes of the macrocycles would be at an angle to each other. The
parallel arrangement of the two macrocycles is thought to result from the drive
to maximize p-pstacking.
In addition to the solid state structures, ultraviolet - visible (UV - vis), electron para-
magnetic resonance (EPR), and redox properties of cofacial metalloporphyrins pro-
vide information about the separation of the two macrocycles. Historically, cofacial
porphyrins have been separated into two groups [Collman et al., 1994]. In group 1
diporphyrins, the two macrocycles behave independently and similarly to monopor-
phyrins, whereas in group 2 diporphyrins, strong electronic interaction between the
two porphyrins results in UV - vis, EPR, and redox properties that are significantly
attenuated relative to monomeric porphyrins. For example, the Soret bands of group
2 diporphyrins are blueshifted relative to those of monoporphyrins or group 1 analogs.
Whereas redox transformations of group 1 diporphyrin derivatives proceed in two-
electron steps, those of group 2 are one-electron processes. Group 2 cofacial porphyr-
ins have molecular orbitals (MOs) that are delocalized over both macrocycles, whereas
MOs of group 1 are localized on each porphyrin. Thus, diamagnetism of doubly
oxidized group 2 diporphyrins (e.g., [(FTF4)Zn 2 ] 2 þ [LeMest et al., 1992]) is consist-
ent with removal of an electron pair from a nondegenerate highest occupied MO
(HOMO). In contrast, the magnetic properties of a group 1 metalloporphyrin,
[(FTF6)Cu 2 ] 2 þ , are close to those of noninteracting (por þ )Cu II moieties.
In metal complexes of FTF5 and DPA, both single two-electron and double one-
electron redox couples were observed. [(FTF5)Zn 2 ] 2 þ is diamagnetic, whereas the
EPR spectrum of [(DPA)Zn 2 ] 2 þ was interpreted as that of a triplet that was compli-
cated by aggregation. Why the ground electronic state of [(DPA)Zn 2 ] 2 þ is a triplet
remains to be established: a triplet porphyrin diradical was thought to be unique in por-
phyrin chemistry [LeMest et al., 1992].
As a result of strong electronic interactions between the two metalloporphyrin
units, there is a substantial uncertainty in assigning oxidation states in mixed-valence
group 2 complexes of redox-active metals, such as Co. Thus, although reduced neutral
Co 2 derivatives can be reasonably well described as those of Co II , the location (metal
versus porphyrin) of the electron hole(s) in the singly and doubly oxidized derivatives
is not known definitively, and may be very sensitive to the medium [LeMest et al.,
1996, 1997]. For example, in benzonitrile, the UV - vis spectrum of [(FTF4)Co 2 ] þ