Porphyrin intermediate-spin states

This section provides a general introduction, with a few complexes cited in illustration. Many further examples will be mentioned at appropriate points in Sections 5.4.2 onward. For iron(II) the two spin states are high-spin ( T2 S = 2) and low-spin ( Ai S = 0). For iron(III) the common spin states are high-spin ( Ai S=5/2) and low-spin ( T2 S=l /2). However, the intermediate spin state of 3/2 is now known for a number of iron(III) complexes, while in a few cases the mixed spin state S = 3/2, 5/2 has been observed (see porphyrin complexes. Section 5.4.3.7.2). Section 5.4.5.4.3 details S values for several polynuclear complexes. 

The intermediate-spin state has been suggested both for ferrous (21) and ferric hemeproteins (22, 23). Direct participation of the intermediate-spin iron porphyrins in biological processes might be less probable however, any reasonable theoretical account of the varied electronic structure of iron in hemeproteins should explain the fact that the so-called intermediate-spin iron porphyrin can exist only when the axial ligand field is extremely weak. 

Structural Aspects. Four-coordinate iron(II) porphyrins are of intermediate spin state (5=1) and are in general square planar, with the iron placed in the center of the porphyrin core. However, the porphyrin ring is not always planar, as is observed for the OEPFe species. It can be ruffled as was found for TPPFe or OECFe derivatives, or saddled as was reported for a second crystalline form of TPPFe All complexes are characterized by relatively short Fe-Npor bond lengths (1.95-1.97 A), which are indicative of the depopulation of the d S = 1 spin state. 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

In contrast to the acid—base behavior of iron(III) porphyrin species, Fe (Pz) exists as an equilibrium mixture of monoaqua and diaqua complexes in acidic medium (46). The five-coordinate monoaqua iron(III) porphyrazine, [Fe (Pz)(H20)], which is the main species at low pH, is stabilized in the unusual intermediate spin state, S = 3/2. This behavior results from the fact that the porphyrazine ring is much smaller than that of the porphyrin and the iron(III) center has to be displaced out of the plane. At higher pH (pH = 10), a low-spin, six-coordinate aquahydroxido complex, [Fe (Pz)(H20)(0H)], is formed whereas a further increase in pH (pH = 13) results in the formation of the low-spin, dihydroxido complex, [Fe (Pz)(OH)2], for which no water exchange reaction could be observed (46). 

As might be expeeted, Mossbauer spectroscopy has been essential in understanding the electronic properties of porphyrin and porphyrin-like iron eomplexes. These complexes can be prepared in a variety of iron oxidation states and eleetronie spin states. Often for a given oxidation state, an intermediate spin state or an admixture of two spin states is observed and, as a consequenee, it is necessary to measure the Mossbauer speetra in an applied magnetie field. The resulting speetra are often very complex and the methods used to understand these spectra are discussed in Chapter 2.21. 

SPIN CROSSOVER IN IRON(III) PORPHYRINS INVOLVING THE INTERMEDIATE-SPIN STATE... 

Neutral nitrogen bases such as imidazole and pyridine are quite common axial ligands for iron(lll) porphyrin complexes. These complexes usually adopt six-coordinate low-spin state expressed as [Fe(Por)(L)2] [2]. However, the spin state of these complexes could change to the intermediate-spin state as shown in Fig. 10.1 b if the field strengths of nitrogen bases are weakened to a great extent. 

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