Highly charged iron porphyrins

Interactions of nitric oxide with highly charged iron(lll) porphyrins... 

Table 2 Rate constants (at 298 K) and activation parameters for reversible binding of NO to a series of diaqua- and monohydroxido-ligated highly charged iron(lll) porphyrins (21-23)... 

Unlike the situation for charge transfer energies, excitations are expected to be quite sensitive to spin state (triplet vs. quintet) and the presence of axial ligands. Calculations of transitions for high-spin five-coordinate iron porphyrins will be presented elsewhere. 

The large number of electronic configuration of iron porphyrins with oxidative states of 2+ or 3+, high and low spin forms, and charge transfer states with different axial ligands offer the possibility of a number of non-radiative decay pathways ( ). In... 

A third approach for synthesizing high-spin iron(II) porphyrins includes the binding of anions, such as imidazolate, thiolate, or acetate, to form complexes with an overall negative charge. " ... 

Since the porphyrin ring neutralizes only two of the positive charges of the metal, Fe porphyrins must have an associated counterion. This counterion may be directly bound to the metal, as in five-coordinate high-, intermediate-, and low-spin Fe porphyrins, or may exist as a separate species that forms an ion-pair with the iron porphyrin complex, as in all six-coordinate complexes of Fe porphyrins with neutral ligands. We will survey the structures, properties, and reactions of Fe porphyrins of each of the possible spin states. 

A relatively high contribution of the S=3/2 spin state in the spin-admixed state of the studied highly n atively charged iron(III) porphyrins is nicely reflected in the kinetic behavior of the latter complexes concerning... 

Williams (28) suggested that the band found at about 16 kK in high-spin systems was a charge transfer transition, in which the excited state arises from a configuration in which an electron has been transferred from a porphyrin 71-orbital to a metal -orbital. Such transitions frequently occur in iron complexes. His alternative suggestion that the band could be an intra-metal d—d transition mixed with charge transfer was considered most unlikely on intensity grounds (88). 

Most molecular orbital calculations on metalloporphyrins have focussed attention on the porphyrin ji-orbitals, while the metal is regarded simply as a positive charge at the centre of the porphyrin ring. However, Zemer et al. (93) have recently performed calculations based on the Wolfsberg-Hdmholtz model (94) in which the metal orbitals are explicitly included in the basis set. The results of such calculations must be treated with caution (95) but we shall occasionally appeal to this paper where it is necessary to estimate a quantity such as an orbital or transition energy, and where no better evidence is available. An interesting result of this calculation was that the displacement of the iron atom out of the haem plane was an essential condition for a high-spin compound. 

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