1183 visible absorption spectra of porphyrin

Info icon This preview shows pages 311–314. Sign up to view the full content.

View Full Document Right Arrow Icon
-porphyrin complexes (see Figure 5.11).83 Visible absorption spectra of porphyrin complexes are due largely to 1T-1T* transitions of the porphyrin ligand. The bright green color is unusual for iron- porphyrin complexes, which are usually red or purple. (However, this green color has been seen for compound I of catalase and peroxidases; see Section VI below.) The unusually long-wavelength visible absorption bands that account for the green color result from the fact that the porphyrin ring has been oxidized by one electron. Similar visible absorption bands can be seen, for example, in other oxidized porphyrin complexes, such as CoIlI(p' -) +, formed by two-elec- tron oxidation of Co lI (p2-)(see 5.73).84 (5.73)
Image of page 311

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
290 1.2 1.0 0 <9 0 0.8 I I I I I I -3 -2 -1 0 2 3 0.6 velocity (mm/s) -'" E I:; E •• !pI:; <0 0.4 .. I:; -. 0.2 DO o 0 0 t 0 0 •• 0.0 * -0.2 0.0 1.0 2.0 3.0 4.0 ilE(mm/s) Figure 5.11 Comparative Mossbauer data for iron-porphyrin complexes: • high-spin Fe lIl , D low-spin Fe lIl , intermediate-spin (admixed) Fe lIl , 0 high-spin Fell, 6. low-spin Fell, • oxo- and imido-Fe IV, * dimethoxyiron(IV)TMP. Insert shows the zero-field Mossbauer spectrum of 57Fe-dimethoxyi- ron(IV)TMP at 4.2 K. (From Reference 83.) Oxidized porphyrin ligands also give characteristic proton NMR spectra, which are seen for the green porphyrin complex as well. 81 ,83 Magnetic measurements indicate that the green porphyrin complex contains three unpaired electrons. Detailed analysis of the Mossbauer spectra has indi- cated that the two unpaired electrons on the Fe IV ion are strongly ferro- magnetically coupled to the unpaired electron on the porphyrin, accounting for the resulting S = ! state. 81 ,83 S=1 ++ -it + p'- S=1/2 gives S=3/2 (5.74) Studies of the reactions of this species with P-4S0-type substrates demonstrate that this species is reactive enough to make it an attractive candidate for" active oxygen" in the enzymatic mechanism. 81,83 Synthetic analogues for two of the other candidates for "active oxygen" have also been synthesized and their reactivities assessed. For example, FellI
Image of page 312
V.OXYGENASES 291 and MnIII-porphyrin peroxo complexes analogous to 1a in Reaction (5.65) have been synthesized. The x-ray crystal structure of the Mn complex shows that the peroxo ligand is bound to the metal in a triangular, side-on fashion (see 5.75). The Fe complex is believed to have a similar structure. 85 ,86 0-0 \ / Fe --l ~ (5.75) Studies of this species indicate that 1a in Reaction (5.65) would not have the requisite reactivity to be a candidate for "active oxygen" in the cytochrome P- 450 mechanism, since it will not even oxidize triphenylphosphine, PPh 3 , to triphenylphosphine oxide, OPPh 3 , one of the more facile oxygenation reactions known. 87 Attempts to examine the protonated form, 1b in Reaction (5.65), how- ever, indicate that it is highly unstable, and its reactivity has not yet been thor- oughly examined. 87 Fe IV -oxo-porphyrin complexes analogous to 2 in Reaction (5.66) have also been prepared in solution and characterized by NMR. 60 ,61 Such complexes will react with PPh 3 to give OPPh 3 , but are relatively unreactive with olefins and totally unreactive with saturated hydrocarbons. Thus 2 is also ruled
Image of page 313

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 314
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern