Iron picket-fence porphyrin

One of the representative models is the picket fence iron porphyrin reported by Coltman and his coworkers (Figure 4). The picket fence porphyrin as a structural model for myoglobin has four bulky pivaloyl groups at the peripheral positions to prevent two of these iron porphyrins from coming together to form a /u.-oxo dimer. The first significant result in their study was to demonstrate the 3D structure of the stable... 

Figure 4 Molecular structure of O2-bound picket fence iron porphyrin complex. The iron porphyrin has four pivalamide groups forming a hydrophobic cavity for the ligand O2. The axial ligand is 1,2-dimethylimidazole. X-ray crystal structure shows the end-on and bent coordination of O2 to the heme-iron...

The possible applications of the models include blood substitutes for clinical use in humans [274]. An important recent advance was made by Tsuchida et al. [291,292], who prepared membrane-bound picket-fence iron porphyrins for in vivo testing. The dioxygen binding rate and affinity were found to be similar to those of red blood cells [293] or erythrocytes in suspension [294]. 

As to inorganic species, MWCNT-metal porphyrins hybrids were used to detect chlorate, bromate, and iodate ions (with iron(III) meso-teraphenylporphyrin [224]), sulfite ion in beverages (with picket fence [iron (III) 5,10,15,20-tetrakis(a,a,a,a-2-pivalamidopheny)porphyrin], FeTpivPP [225], Cu(II) [with 5-(4-benzylacrylamido)-10,15,20-tiis(4-benzylamino) porphyrin [226]] and Hg(II) cations [with 5,10,15,20-tetrakis(3-hydroxyphenyl) porphyrin] [227]). 

Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

Originally, this temperature variation of AEq was attributed to the dynamic distribution of the terminal oxygen of the Fe02-moiety, as suggested by X-ray structural results for picket-fence porphyrins [26,27]. This view is now supported by NFS studies which provide more information on dynamic processes in iron-containing molecules. 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

---