Characterization of High-Spin and Low-Spin Iron(III) Quinoxalinotetraphenylporphyrin

The 1H NMR spectra of iron(III)
quinoxalinotetraphenylporphyrin
((QTPP)FeIIIXn), iron(III)
(methylquinoxalino)tetraphenylporphyrin
((MQTPP)FeIIIXn), and
iron(III) pyrazinotetraphenylporphyrin
((PTPP)FeIIIXn) have
been
studied to elucidate the impact of an aromatic extension of a single
pyrrole ring on the electronic structure of the
corresponding high- and low-spin iron(III) porphyrins. The
1H NMR spectra of the complexes with the
following
axial ligands have been reported:  chloride, iodide, cyanide,
pyridine-d5 (py-d5),
4-aminopyridine (4-NH2py), and
imidazole (ImH). Modification of the tetraphenylporphyrin by
addition of the quinoxaline (pyrazine) fragment
results in stabilization of the rare low-spin iron(III)
(dxzdyz)4(dxy)1
electronic ground state in the presence of axial
cyanide or pyridine ligands. The more common
(dxy)2(dxzdyz)3
electronic ground state has been established for
[(QTPP)FeIII(4-NH2py)2]+
and
[(QTPP)FeIII(ImH)2]+
species. To account for the substituent contribution,
the
Hückel linear combination of atomic orbitals (LCAO) method has
been used to determine the molecular orbitals
involved in the spin density delocalization. The deviation from
Curie law observed for
[(QTPP)FeIII(CN)2]-
suggests Boltzmann equilibrium
{(dxz)2(dyz)2(dxy)2(Ψ-1)1
↔
(dxz)2(dyz)2(dxy)1(Ψ-1)2}
⇌
(dxz)1(dyz)2(dxy)2(Ψ-1)2
⇌
(dxz)2(dyz)1(dxy)2(Ψ-1)2
where Ψ-1 is related to the a2u
orbital of a regular porphyrin. For the first time in
the
group of low-spin iron(III) tetraarylporphyrins, the sign reversal
of the isotropic shift was directly observed for
pyrrole-proton resonances. The structure of
(QTPP)FeIIICl was determined by X-ray
crystallography. (QTPP)FeIIICl crystallizes in the monoclinic space
group P21/c with a =
18.016(5) Å, b = 11.399(3) Å, c =
21.996(5)
Å, β = 112.22(5)°, and Z = 4. The
refinement of 548 parameters and 2696 reflections yields
R1 = 0.0654,
Rw2
= 0.1717. The (QTPP)FeIIICl presents features
of the high-spin five-coordinate iron(III)
tetraphenylporphyrin.
The quinoxalinotetraphenylporphyrin macrocycle assumes a
saddle-shape geometry.