MODELS OF THE CYTOCHROMES - REDOX PROPERTIES AND THERMODYNAMIC STABILITIES OF COMPLEXES OF HINDERED IRON(III) AND IRON(II) TETRAPHENYLPORPHYRINATES WITH SUBSTITUTED PYRIDINES AND IMIDAZOLES
Mjm. Nesset et al., MODELS OF THE CYTOCHROMES - REDOX PROPERTIES AND THERMODYNAMIC STABILITIES OF COMPLEXES OF HINDERED IRON(III) AND IRON(II) TETRAPHENYLPORPHYRINATES WITH SUBSTITUTED PYRIDINES AND IMIDAZOLES, Inorganic chemistry, 35(18), 1996, pp. 5188-5200
The Fe-III/Fe-II and Fe-II/Fe-I reduction potentials of a series of mo
del hemes have been measured by cyclic voltammetry in dimethylformamid
e at 25 degrees C as a function of the concentration of added axial li
gands. The six porphyrinate ligands utilized were tetraphenylporphyrin
(TPP), tetramesitylporphyrin (TMP), and a series of ''hindered'' tetr
aphenylporphyrins having substituents (OCH3, F, Cl, Br) on both ortho
positions of each of the four phenyl rings. The perchlorato salts of t
he iron(III) porphyrinates were utilized for the titrations. The axial
ligands utilized were N-methylimidazole, 2-methylimidazole, and pyrid
ines of basicities ranging in pK(a)(PyH(+)) from 9.7 to 1.1. From the
electrochemical titration, the equilibrium constants (log beta(2) and
some log beta(1)) of each iron porphyrinate with each ligand were dete
rmined. The values of log beta(2) for the iron(III) complexes decrease
in the order Br > Cl > CH3 > OCH3 > H > F, while those for iron(II) d
ecrease in the slightly modified order Br > Cl > CH3 > F > OCH3 > H. R
ather than electronegativity or electron-donating/withdrawing characte
ristics, these results appear to follow primarily the physical size of
the ortho substituents: The iron porphyrinates with the largest ortho
substituents have the largest equilibrium constants. The hindered (TP
P)Fe-II complexes are usually, but not always, more stable than the co
rresponding hindered (TPP)Fe-III complexes. For both (TPP)Fe-III and t
he hindered (TPP)Fe-III complexes, the slope of the correlation of log
beta(2)(III) with the pK(a) of the conjugate acid of the pyridine lig
and is 1.0. The equilibrium constants for hindered (TPP)Fe-II complexe
s, however, have virtually no dependence upon the base strength of the
axial pyridines, while those for (TPP)Fe-II has a slight sensitivity,
with a slope of 0.15. In contrast to common belief, 2-methylimidazole
readily forms bis complexes with some iron(II) tetraphenylporphyrinat
es ((TMP)Fe-II, ((2,6-Cl-2)TPP)Fe-II, and ((2,6-Br-2)(4)(TPP)Fe-II), a
lthough it does not with (TPP)Fe-II and its (2,6-F-2)(4)- and (2,6-(OC
H3)(2))(4)- counterparts. N-Methylimidazole is unique among the axial
ligands of this study, in that the equilibrium constants for binding t
o both Fe(III) and Fe(II) are virtually identical for all porphyrinate
s studied.