Complex Stability and Energetics

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 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. 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

TS B14 leads to the complex B15. Here all the incipient changes found in TS B14 have completed N1 has two N-H bonds and is essentially localized on Zr2 and the N-N bond is completely broken. In almost every aspect, B15 is an analogue to BIO of the previous reaction path. This is true even energetically B15 is 36.6 kcal/mol more stable than reactants (B1 + H2) and 79 kcal/mol below TS B14 in the Zr-P frozen geometries. For the unconstrained structures, the corresponding stabilities are 33.6 and 72.8 kcal/mol. The similarity in developments along the A (first H2) and B (second H2) reaction paths is very clear, both structurally and energetically. 

Exciplex phosphorescence can be studied with complexes which are present in the ground state [118]. Equation (43) and its corollaries with respect to variations in A are applicable also to exciplex phosphorescence. For example, positive deviations (A > 0.18 eV) which have been found in the phosphorescence of complexes with (E — E d) > 2.75 eV are ascribed to the stabilizing interaction between the triplet CT state and energetically higher locally excited triplet states [108,119],... 

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