Picket-fence porphyrins

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. ... 

Polycrystalline Material Versus Frozen Solution (Example Picket-Fence Porphyrin and Deoxymyoglobin)... 

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. 

The first test case was the ferrous high-spin state (Fe, S = 2) in the picket-fence porphyrin acetate complex [Fe(CH3COO)(TPpivP)] [13, 23], which is a model for the prosthetic group termed P460 of the multiheme enzyme hydroxyl-amine oxidoreductase from the bacterium Nitrosomonas europeae. Both the picket-fence porphyrin and the protein P460 exhibit an extraordinarily large quadrupole splitting, as observed by conventional Mossbauer studies [56]. 

Fig. 9.21 NFS spectra of the paramagnetic picket-fence porphyrin complex [ Fe(CH3COO) (TPpivP)] obtained at 3.3 K in a field of 6.0 T applied (a) perpendicular to both the synchrotron beam and the polarization vector of the radiation and (b) perpendicular to the synchrotron beam but parallel to the polarization vector of the radiation. The solid lines are simulations with the SYNFOS program using 5 = 2 and parameters described in the text. (Taken from [13])...

The NIS investigation of heme complexes includes various forms of porphyrins (deuteroporphyrin IX, mesoporphyrin IX, protoporphyrin IX, tetraphenylpor-phyrin, octaethylporphyrin, and picket fence porphyrin) and their nitrosyl (NO) and carbonyl (CO) derivatives, and they have been the subject of a review provided by Scheidt et al. [109]. 

A picket fence porphyrin system. Finally, examples of pendant donor groups attached to a porphyrin ring are known. A novel example of such a molecule is meso-a,a,a,a-tetrakis(o-nicotinamidophenyl)porphyrin (111) which is capable of binding two metal ions such that each has a square-planar environment with the square planes orientated coaxially to each other (Gunter et al., 1980). When a kinetically-inert metal ion such... 

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. 

I e model 2 (oxy form) Fe(TpiV)PP-(l-MeIm)02. The ligand is Fe(TpiV)PP or picket-fence porphyrin, meso-tetrai (a,a,a,a-o-pivalamidephenyl)porphyrin (see H2PF in Figure 4.23 of reference 20, Figure 16 of reference 8). In this chapter see Figures 4.17 and 14.18. 

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)... 

Figure 4.17 Picket-fence porphyrin Fe(Tpiv)PP, meso-fefralw(a,a,a,a-o-pivalamidephe-nyl)porphyrin described in references 5b, 30, 32, and 33.

Figure 4.18 Picket-fence porphyrin models. (A) Model for T-state. (B) Model for R-state. (Adapted with permission from Figure 5 of Collman, J. P. Inorg. Chem., 1997, 36, 5145-5154. Copyright 1997, American Chemical Society.)...

Modifying the porphyrin to sterically prevent addition of a large sixth ligand. The most well known version of these is the picket-fence porphyrin illustrated in Figure 4.17 (see also Figure 1 of reference 39), although many others have been synthesized.8,18... 

These strategies have been successful and have led to the X-ray crystallographic structure of the oxygenated picket-fence porphyrin described in Section 4.7. 

Despite the many model compounds that have been prepared, picket fence porphyrin and its many cogeners, first reported by the CoUman research group in 1974, remains the only porphyrin-type ligand system yielding... 

Fe model 1 (deoxy form) Fe(Tpiv)PP-(2-MeIm). The porphyrin ligand, Fe(Tpiv)PP, is the picket fence porphyrin, meso-tetrakis(a, a, a, a-o-pivalamidephenyl)porphyrin (Figure 16 of reference 9). In this chapter see Figure 7.9B. 

Figure 7.9 (A) Prevention of a-oxo dimer formation by distal-side porphyrin modifications. (Adapted with permission from the reference 19. Copyright 1985, Division of Chemical Edncation, Inc.) (B) Picket-fence porphyrin Fe(Tpjv)PP, meso-tetrakis(a, a, a, a-c>-pivalamidephenyl)porphyrin described in references 6,17,18, and 21.

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