Metalloporphyrins spin states

Detailed investigations of different spin states in metalloporphyrins have been performed by number of groups (47,48,56,60,72-76) and results relevant to Fe(II)P are summarized in the Table I. 

The second and the third row transition metalloporphyrins, in general, take the lower spin states due to the larger energy gap between the metal d orbitals. 

The ligand field of the macrocycle is also modified by ring distortion.154 The metal to alkylated nitrogen distance is 2.33 A (Fe) to 2.53 A (Zn) which is considerably longer than the normal value of 2.0-2.1 A for the first row transition metalloporphyrins in a high spin state, while the extension is small for the other three M—Npor bonds. The reduced and distorted ligand field of the macrocycle makes it possible for even the Co complexes as well as other metals to take a high spin state. 

Coordination of metal ions often has a dramatic effect on the n delocalization in porphyrins and porphyrinoids. It has particularly conspicuous influence on the electronic spectra of metalloporphyrins, which show a dependence on the identity of the metal ion, axial ligation, oxidation level, and spin state. In regular porphyrins, metal coordination reduces the number of observed Q bands from four to two, reflecting the higher symmetry of the chromophore relative to the free base. However, detailed quantitative information on the Jt-electron delocalization is more easily accessible from other physical methods. 

A book including a survey of NMR studies of porphyrins and metalloporphyrins has appeared. (637) Iron porphyrins may involve Fe(ii) and Fe(iii) with both high and low spin states of iron. All are paramagnetic except those of low-spin Fe(ii). 

In the past 15 years, a number of reviews have appeared. Two general reviews appeared in the mid 70s Both of these reviews attempted to comprehensively survey the topic of porphyrin stereochemistry up to the time of publication. These two reviews are appropriately consulted for complete information of all work completed to that time. In addition, there have been a number of more specialized reviews pertaining to tetrapyrrole macrocyclic structure. An excellent article by Glusker has detailed the structural work on vitamin B12 derivatives. An early classic review examined the stereochemistry of hemes (iron porphyrinates) and their relationship to the function of the hemoproteins A review of trends in metalloporphyrin stereochemistry as a function of electronic state and position in the periodic table was written by the author in 1977 There are also two subsequent reviews in which the senior author has participated a 1983 article (with Martin Gouterman) that attempted to reach an understanding of control of spin state in metalloporphyrins and a 1981 article (with Christopher A. Reed) that catalogues spin-state/stereochemical relationships of the iron porphyrinates and the implications of these structures for the hemoproteins. Articles by Hoffinan and Ibers have discussed the use of oxidized porphyrins and phthalocyanine derivatives as molecular metals. It is not the intention of the present review to attempt to supplant any of these earlier reviews but rather to extend them when appropriate, new information is available. Further, we will review some additional topics that have not been considered previously. 

W.R. Scheldt, M. Gouterman, Ligands, spin state, and geometry in hemes and related metalloporphyrins, in Iron Porphyrin, Part I, A. B.P. Lever, H.B. Gray, eds., Addison-Wesley, Reading, MA, 1983, pp. 89-139. 

J.R. Bolton We have not carried out any experiments as yet on metalloporphyrins linked to quinones. The spin selection rules should be operative in the radical pair. The singlet state of the radical pair should be able to return to the ground state with no spin inhibition however, the triplet state of the radical pair can return to the ground state only via spin interconversion or via the triplet state of the porphyrin. 

As discussed in detail in Section III, in metalloporphyrins states of different spin multiplicities are close in energy in... 

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