Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 654

Little is known about the stability of these porphyrins in O 2 reduction, how
this peripheral substitution affects O 2 affinity of the metalloporphyrin, how the periph-
eral metal complexes perturb the energetics of various intermediates, and/or the
kinetics of various steps or the mechanisms of O 2 reduction by these porphyrins. At
present, it remains to be seen if the strategy of coordinating metal complexes on the
periphery of a metalloporphyrin can be exploited in the rational design of new
ORR catalysts.
18.5 COFACIAL PORPHYRINS AND RELATED COMPOUNDS
In cofacial porphyrins, two porphyrin moieties are constrained in an approximately
stacked geometry by one to four covalent linkers (Fig. 18.13). The first cofacial
porphyrins were reported in the literature contemporaneously by several groups in
the late 1970s [Collman et al., 1994]. In their paper, Collman et al. first suggested
that such porphyrins could be good ORR catalysts. In the next 30 years, this idea
has been spectacularly demonstrated: certain Co and especially Ir derivatives appear
to be the best molecular catalysts for the ORR yet found. Cofacial bismetalloporphyrins
remain among the very few molecular catalysts definitely displaying bimetallic
catalytic cooperativity. As such, they hold the potential to help rational design of
efficient catalysts for the ORR and other multielectron processes. Recently, the concept
of cofacial diporphyrins was extended to corroles [Kadish, 2005], and it has served
as an inspiration for the design of new superstructured porphyrins [Rosenthal and
Nocera, 2007].
Certain aspects of electrocatalysis by cofacial porphyrins have been reviewed
previously [Collman et al., 1994, 2003a].
18.5.1 Molecular and Electronic Structure
Structurally, the cofacial porphyrins fall into two classes: in pillared (or Pacman)
porphyrins the face-to-face arrangement of the two macrocycles is enforced by a
single rigid linker, such as 1,2-phenylene, 1,8-biphenylene, or 1,8-anthracene, con-
nected to each macrocycle at its meso position. In strapped cofacial porphyrins, at
least two flexible chains connected at b-pyrrolic positions are used to hold the macro-
cycles together. Pillared cofacial porphyrins can typically be synthesized in fewer
steps and better yields, although, even in the best cases, more than 15 steps are
required, making these porphyrins prohibitively expensive for practical applications.
Strapped cofacial porphyrins, on the other hand, allow a systematic variation in the
length of the straps, potentially simplifying the acquisition and interpretation of struc-
ture/activity relationships.
Substantial effort has been invested in understanding how the linker(s) affect the
metal - metal (M - M) distances, since the capacity of a cofacial porphyrin to accom-
modate a bridging diatomic molecule is often thought to be critical for achieving
high catalytic activity. As of May 2007, 38 X-ray diffraction structures of cofacial
porphyrins in Fig. 18.13 had been reported, with the DPA, DPB, DPD, and DPX