CYTOCHROME P450 : A MONOOXYGENASE 379
complexes at 15 K showed a characteristic S = 3/2 spectrum with gz = 2, gy =
3.5 – 3.7, and gx = 4.2 – 4.4. The spin state of 3/2 arises from the two unpaired
electrons for a Fe 4+ – O 2 − bond plus the unpaired electron from the porphyrin
π - cation radical. [See Figure 7.21 for a molecular orbital (MO) diagram.] The
NMR data are similar to that found by Groves for [Fe IV = O(TMP + • )] + as shown
in Table 7.4. As can be noted in this table, the axial ligand, X, does affect the
shifts of the mesityl and pyrrole protons.
Resonance Raman (RR) data also show differences for the νFe = O stretching
frequency, depending on the axial ligand. For X = F − and Cl − we have νFe – O =
801 – 806 cm − 1 , whereas for CH 3 OH and ClO 4 − we have ν Fe = O = 831 – 835 cm − 1.
The reference 68 authors interpret these data as indicating weaker Fe = O
bonding when the axial ligands are electron - donating trans - donor F − and Cl −.
In the molecular orbital diagram of Figure 7.21 , one can see that donation of
bothσ - and π - electron density by the trans ligands will have the effect of
weakening the Fe = O bond. In this diagram, the low - spin Fe(IV) electrons, d^4 ,
in an approximate octahedral fi eld will have the electron confi guration:
(dxy )^2 ( dxz )^1 ( dyz )^1. A bond order of 2 is achieved by one two - electron
()22ce−−dpzz^2 σ - bond and two three - electron (2c – 3e) π - bonds, dxz – p x and
dyz – p y using the fi lled p - shell electrons of O 2 −. The reference 68 researchers
studied the kinetic profi les of the various [Fe IV = O(TMP + • )(X)] species, fi nding
that the 1 - X compounds underwent rapid intermolecular electron transfer to
a 3 - X intermediate, possibly a Fe(III) porphyrin radical with one electron
transferred to form a substrate radical. In the following rate - determining step,
intramolecular electron transfer would take place to form an [Fe IV (TMP)(O)(X)]
intermediate in intimate contact with the substrate radical. Fast transfer of
the oxygen atom to substrate completes the reaction to regenerate
the [Fe III (TMP)(X)] ( 2 - X) complex plus epoxide product. The authors present
NMR and EPR evidence that points to the 3 - X intermediate in reactions of
[Fe IV = O(TMP + • )(ClO 4 )] with styrene. The reaction with XClO= − 4 is slow
enough so that the 3 - X intermediate is detectable at low temperatures (see
Figure 7.22 ). In addition to the epoxide product, formed at 0 ° C and room
temperature, the product isomer phenylacetaldehyde is formed at − 78 ° C. The
authors postulate, in conclusion, that axial ligands X have most effect on the
TABLE 7.4 NMR Data for [Fe IV = O(TMP + • )] + and [Fe IV = O(TMP + • )(X)]
Complexes a
[Fe IV – O(TMP + • )(X)[( 1 - X)^68
Mesityl
meta - H Ortho - CH 3 Para - CH 3Pyrrole - H of
PorphyrinX = F − 71.4 26.9, 42.1 10.7 − 13.0
XClO= − 4 65.3 26.0, 23.5 11.1 − 26.9
[Fe IV = O(TMP + • )] +53a ∼ 70 ∼ 20 ∼ 10
a Referenced to tetramethylsilane (TMS).
Sources : References 53a and 68.