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

Abstract

The 1H NMR spectra of iron(III) quinoxalinotetraphenylporphyrin ((QTPP)FeIIIX n ), iron(III) (methylquinoxalino)tetraphenylporphyrin ((MQTPP)FeIIIX n ), and iron(III) pyrazinotetraphenylporphyrin ((PTPP)FeIIIX n ) 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-d 5 (py-d 5), 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) (d xz d yz )4(d xy )1 electronic ground state in the presence of axial cyanide or pyridine ligands. The more common (d xy )2(d xz d yz )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 {(d xz )2(d yz )2(d xy )2(Ψ- 1)1 ↔ (d xz )2(d yz )2(d xy )1(Ψ- 1)2} ⇌ (d xz )1(d yz )2(d xy )2(Ψ- 1)2 ⇌ (d xz )2(d yz )1(d xy )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 R 1 = 0.0654, R w2 = 0.1717. The (QTPP)FeIIICl presents features of the high-spin five-coordinate iron(III) tetraphenylporphyrin. The quinoxalinotetraphenylporphyrin macrocycle assumes a saddle-shape geometry.

Electronic Structure of Iron Chlorins: Characterization of Bis(l-valine methyl ester)(meso-tetraphenylchlorin)iron(III)triflate and Bis(l-valine methyl ester)(meso-tetraphenylchlorin)iron(II)

“…47 Thus, there is a core-hole expansion of the macrocycle due to the reduced pyrrole. Such a situation has been previously reported for high-spin iron(III) quinoxalinotetraphenylporphyrin 21 and discussed in terms of molecular mechanics calculations. 48,49 The C (17) 28 The pyrrole and pyrroline rings are only slighly displaced above and below the mean plane of the chlorin [maximum displacement ) 0.15(2) Å for 2A and 0.15(2) Å for 2B].…”

Nitric oxide interaction with insect nitrophorins and thoughts on the electron configuration of the FeNO complex

“…There are two possible electron configurations of low-spin iron(III) porphyrinates, the more commonly observed (d xy ) 2 (d xz , d yz ) 3 , or d p , electron configuration, which is usually observed for the elecron-transferring ferricytochromes a, b, c, f and o [243][244][245][246][247], and the ''novel'' (d xz , d yz ) 4 (d xy ) 1 , or d xy , electron configuration, which is often observed in model hemes having axial ligands that are good p-acceptors, such as isocyanides [235,236,248], low-basicity pyridines [234], and under certain conditions, cyanide ions [249][250][251][252]. The d xy ground state is also believed to exist for some reduced hemes in biological systems, including heme d and d 1 and siroheme, but this appears to depend upon the nature of the axial ligands [253][254][255].…”

“…Thus, apart from model hemes having planar axial ligands in nearly perpendicular planes lying over the meso positions, most of the cases for which the porphyrinate ring is highly ruffled are those for which the iron(III) center has the electron configuration (d xz , d yz ) 4 (d xy ) 1 , i.e., the bis-isocyanide complexes of both (OEP)Fe(III) [236] and various (TPP)Fe(III) derivatives [235,236], and the bis-4-cyanopyridine complex of (TPP)Fe(III) [234], as well as the bis-cyanide complexes of meso-alkyl-substituted hemins [244,[249][250][251] and other modified porphyrin ring systems [252]. Complexes of this electron configuration have EPR g-values close to 2.0 [234][235][236]244], as well as unique NMR, Mö ssbauer and MCD spectra [244], that readily differentiate them from the more typical low-spin Fe(III) complexes that have the electron configuration (d xy ) 2 (d xz , d yz ) 3 , such as the vast majority of the cytochromes a, b, c, f and o [243,244].…”

Electronic ground states of low-spin iron(III) porphyrinoids

“…The spectral change can best be interpreted in terms of the hydrogen bonding between CD 3 OD and cyanide [37][38][39][40][41][42][43][44]; the hydrogen bonding lowers the energy level of the cyanide p à p orbital and consequently increases the (d xz , d yz ) 4 (d xy ) 1 ground state. Importantly, the negative slopes in Hammett plots are much larger in CD 3 OD solutions than in CD 2 Cl 2 solutions, which indicate that the electron donating groups at the meso positions stabilize the (d xz , d yz ) 4 (d xy ) 1 ground state while the electron withdrawing groups stabilize the (d xy ) 2 (d xz , d yz ) 3 ground state.…”