To obtain a better understanding of the effects of
ortho-halogen and related substituents on the phenyl rings
of
TPPH2 and (TPP)Fe(III) complexes, a series of
unsymmetrically substituted tetraphenylporphyrins have
been
synthesized. In each of these complexes one phenyl ring bears
halogen(s) on one (or both) ortho
position(s),
while the other three phenyl rings carry para-methoxy
substituents. The free-base porphyrins were
characterized
by UV−visible and 1H NMR spectroscopy. The resonance
of the pyrrole protons closest to the phenyl ring
bearing the ortho substituent, Ha, of the
free-base porphyrins shows a progressive shift to higher shielding as
the
atomic radius and Hammett substituent constant
σ
p
of the substituent increases. However,
the fact that 2,6-substitution has a similar effect as 2-substitution suggests that size
effects are more important than through-bond
electronic effects. Equilibrium constants,
β2
III, for addition of
N-methylimidazole to the series of complexes
(o-X)(p-OCH3)3(TPP)FeCl
and
(2,6-X2)(p-OCH3)3(TPP)FeCl
were measured in chloroform at 25 °C.
log(β2
III)
increases in the order F < Cl < Br < I < F2 <
CF3 < Cl2 < Br2, and all
β2
III values for mixed
substituent
porphyrins except the monofluoro-containing complex are larger than the
β2
III for the reaction between
the
symmetrical non-ortho-substituted parent compound,
(p-OCH3)4(TPP)FeCl, and
NMeIm. Hence, ortho-halogen
and -CF3 substituents increase the values of
β2
III in order of increasing size, with the
2,6-disubstituted phenyls
causing an increase of β2
III by more than a
factor of 2 over that for 2-substituted phenyls. In the strongly
solvating
solvent dimethylformamide, where dissociation of the anion has already
taken place, the opposite order of log(β2
III) is observed (F > CF3 >
Cl2 ≫ p-OCH3). Both sets of
equilibrium constant data suggest that ortho-halogens and -CF3 groups are electron donating, which we
believe is due to direct overlap between the electron
clouds of the halogens and the π system of the porphyrin. This
direct overlap of electron clouds decreases the
Lewis acidity of Fe(III), both making it easier for
Cl- to dissociate from the
Fe+Cl- starting material in
CHCl3
and making the complex less stable; the former contribution
is more important in CHCl3, while the latter
becomes
evident in DMF. The 1H NMR spectra of the same series
of low-spin Fe(III) complexes show a decrease in the
spread of the pyrrole-H resonances in the order of Cl2 >
F2 > Br2 ≫ F > Cl > Br ∼ CF3
≫ I. This order
suggests that the apparent electron-donating characteristics of the
substituents decrease in the listed order, which
is quite different from that derived from the values of
log(β2
III) measured in chloroform
solution. The differences
probably result from the fact that these two physical properties are
sensing different effects of the ortho
substituents: log(β2
III) values probe
changes in the σ basicity of the pyrrole nitrogens and hence the
Lewis acidity
of the metal, while the spread of pyrrole-H resonances probes changes
in the π el...
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