146343-96-6Relevant articles and documents
Molecular structures and magnetic resonance spectroscopic investigations of highly distorted six-coordinate low-spin iron(III) porphyrinate complexes
Ogura,Yatsunyk,Medforth,Smith,Barkigia,Renner,Melamed,Walker
, p. 6564 - 6578 (2001)
Three bis-axially ligated complexes of iron(III) octaethyltetraphenylporphyrin, (OETPP)FeIII, have been prepared, which are low-spin complexes, each with two axial nitrogen-donor ligands (N-methylimidazole (N-MeIm), 4-(dimethylamino)pyridine (4-NMe2Py), and 2-methylimidazole (2-MeImH)). The crystal and molecular structure of the bis-(2-MeImH) complex shows the macrocycle to be in a saddled conformation, with the ligands in perpendicular planes aligned at 14° to the porphyrin nitrogens so as to relieve the steric interaction between the 2-methyl groups and the porphyrin. The Fe-N(por) bond lengths are typical of nonplanar six-coordinate low-spin FeIII complexes, while the axial Fe-N(ax) bond lengths are substantially longer than those of [(TPP)Fe(2-MeImH)2]+ (2.09(2) A as compared to 2.015(4) and 2.010(4) A). The crystal and molecular structure of the bis-(4-NMe2Py) complex also shows the macrocycle to be in a mainly saddled conformation, but with a significant ruffled component. As a result, the average Fe-N(por) bonds are significantly shorter (1.951 A as compared to 1.974 A) than those of the bis-(2-MeImH) complex. One ligand is aligned at 9° to two trans porphyrin nitrogens, while the other is at 79° to the same porphyrin nitrogens, producing a dihedral angle of 70° between the ligand planes. The EPR spectrum of this complex, like that of the bis-(2-MeImH) complex, is of the large gmax type, with gmax = 3.29 and 3.26, respectively. However, in frozen CD2Cl2, [(OETPP)Fe(N-MeIm)2]+ exhibits both large gmax and normal rhombic signals, suggesting the presence of both perpendicular and parallel ligand orientations. The 1- and 2D 1H NMR spectra of each of these complexes, as well as the chloroiron(III) starting material, were investigated as a function of temperature. The COSY and NOESY/EXSY spectra of the chloride complex are consistent with the expected J-coupling and saddle inversion dynamics, respectively. Complete spectral assignments for the bis-(N-MeIm) and -(4-NMe2-Py) complexes have been made using 2D 1H NMR techniques. In each case, the number of resonances due to methylene (two) and phenyl protons (one each) is consistent with D2d symmetry, and therefore an effective perpendicular orientation of the axial ligands on the time scale of the NMR experiments. The temperature dependences of the 1H resonances of these complexes show significant deviations from Curie behavior, and also evidence of extensive ligand exchange and rotation. Spectral assignment of the eight methylene resonances of the bis-(2-MeImH) complex to the four ethyl groups was possible through the use of 2D 1H NMR techniques. The complex is fluxional, even at -90 °C, and ROESY data suggest that the predominant process is saddle inversion accompanied by simultaneous rotation of the axial ligands. Saddle inversion becomes slow on the 2D NMR time scale as the temperature is lowered in the ligand order of N-MeIm > 4-NMe2Py > 2-MeImH, probably due mainly to progressive destabilization of the ground state rather than progressive stabilization of the transition state of the increasingly hindered bis-ligand complexes.
Control of spin state by ring conformation of iron(III) porphyrins. A novel model for the quantum-mixed intermediate spin state of ferric cytochrome c' from photosynthetic bacteria
Cheng, Ru-Jen,Chen, Ping-Yu,Gau, Pong-Ren,Chen, Chun-Chia,Peng, Shie-Ming
, p. 2563 - 2569 (1997)
(2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphinato)iron(I II) chloride Fe(III)(OETPP)Cl and (2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphinato)iron( III) chloride Fe(III)(OMTPP)Cl complexes have been synthesized and characterized by 1H NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature 1H NMR studies confirm that the conformational distortions are maintained in solution with ΔG(paragraph) = 15.8 and 10.1 kcal/mol for ring inversion for Fe(OETPP)Cl and Fe(OMTPP)Cl, respectively. EPR (g(perpendicular to) = 5.2-5.3 at 77 K), magnetic moments (μ(eff) = 4.7-5.2 μ(B) at 300 K), and structural data (Fe-N(p) = 2.03 A?, Fe-Cl = 2.24-2.25 A?) all indicate that unlike high-spin Fe(III)(TPP)Cl and Fe(III)(OEP)Cl (S = 5/2), Fe(III)(OETPP)Cl and Fe(III)(OMTPP)Cl these complexes are of the uncommon quantum-mixed S = 5/2, 3/2 intermediate-spin state. Saddle-shaped ring deformations lower the symmetries of the complexes into C(2v). Other than the nonaxial symmetric EPR spectra of both complexes, 1H NMR spectrum of Fe(III)(OETPP)Cl shows large asymmetry to the methylene proton shifts. Certain cytochromes c' from photosynthetic bacteria reported to be of similar quantum-mixed intermediate-spin and showed EPR signals of rhombic symmetry have been noted to be with saddle-shaped deformations. These anomalous spin states and electronic structure asymmetry are ascribed to the ring deformation of the porphyrin macrocycle.
Spin-spin interactions in iron(III) porphyrin radical cations with ruffled and saddled structure
Kouno, Souhei,Ikezaki, Akira,Ikeue, Takahisa,Nakamura, Mikio
, p. 718 - 721 (2011)
Oxidation of essentially pure intermediate-spin iron(III) porphyrinates such as ruffled Fe(TiPrP)ClO4 and saddled Fe(OETPP)ClO4 produces the corresponding six-coordinate iron(III) porphyrin(por) radical cations [Fe(Por)(ClO4)2], where TiPrP and OETPP are dianions of 5,10,15,20-tetraisopropylporphyrin and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, respectively. Spin-spin interactions in these complexes are very much different; while ruffled [Fe(TiPrP)(ClO4)2] exhibits no antiferromagnetic coupling, saddled [Fe(OETPP)(ClO4)2] does exhibit it. The difference in magnetic behaviors has been explained in terms of the deformation mode and electron configuration of these complexes.
Metal-porphyrin orbital interactions in highly saddled low-spin iron(iii) porphyrin complexes
Ohgo, Yoshiki,Hoshino, Akito,Okamura, Tomoya,Uekusa, Hidehiro,Hashizume, Daisuke,Ikezaki, Akira,Nakamura, Mikio
, p. 8193 - 8207 (2008/10/09)
Substituent effects of the meso-aryl (Ar) groups on the 1H and 13C NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L2] +, where axial ligands (L) are imidazole (Hlm) and tert-butylisocyanide (tBuNC), have been examined to reveal the nature of the interactions between metal and porphyrin orbitale. As for the bis(Hlm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as well as meso and CH2 carbon shifts are fairly small in the bis(Hlm) complexes. Since these complexes adopt the (dxy)2(d xz, dyz)3 ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal dπ orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3eg-like orbitale. A fairly small substituent effect is understandable because the 3eg-like orbitale have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial Hlm is replaced by tBuNC. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(Hlm) complexes. Since the bis(tBuNC) complexes adopt the (dxz, dyz)4(dxy)1 ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled dxy orbital interacts with the specific porphyrin orbitale that have large coefficients on the meso-carbon atoms. Thus, we have concluded that the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (dxz, dyz) 4(dxy)1 ground state should take place between the d xy and a2u-like orbital rather than between the d xy and a1u-like orbital, though the latter interaction is symmetry-allowed in saddled D2d complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the dxy orbital due to a smaller energy gap between the d xy and dπ orbitale. In fact, the energy levels of the dxy and dπ orbitals are completely reversed in the complex carrying a strongly electron-withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (dxy)2(d xz, dyz)3 ground state despite the coordination of tBuNC.
Low-Spin Ferriheme Models of the Cytochromes: Correlation of Molecular Structure with EPR Spectral Type
Yatsunyk, Liliya A.,Carducci, Michael D.,Walker, F. Ann
, p. 15986 - 16005 (2007/10/03)
The preparation and characterization of the following bis-imidazole and bis-pyridine complexes of octamethyltetraphenylporphyrinatoiron(III), Fe(III)OMTPP, octaethyltetraphenylporphyrinatoiron(III), Fe(III)OETPP, and tetra-β,β′-tetramethylenetetraphenylporphyrinatoiron(III), Fe(III)TC6TPP, are reported: paral-[FeOMTPP(1-Melm)2]Cl, perp-[FeOMTPP(1-Melm)2]Cl, [FeOETPP(1-Melm)2]Cl, [FeTC6TPP(1-Melm)2]Cl, [FeOMTPP(4-Me2NPy) 2]Cl, and [FeOMTPP(2-MeHlm)2]Cl. Crystal structure analysis shows that paral-[FeOMTPP(1-Melm)2]Cl has its axial ligands in close to parallel orientation (the actual dihedral angle between the planes of the imidazole ligands is 19.5°), while perp-[FeOMTPP(1-Melm) 2]Cl has the axial imidazole ligand planes oriented at 90° to each other and 29° away from the closest Np-Fe-Np axis. [FeOETPP(1-Melm)2]Cl has its axial ligands close to perpendicular orientation (the actual dihedral angle between the planes of the imidazole ligands is 73.1°). In all three cases the porphyrin core adopts relatively purely saddled geometry. The [FeTC6TPP(1-Melm) 2]Cl complex is the most planar and has the highest contribution of a ruffled component in the overall saddled structure compared to all other complexes in this study. The estimated numerical contribution of saddled and ruffled components is 0.68:0.32, respectively. Axial ligand planes are perpendicular to each other and 15.3° away from the closest N p-Fe-Np axis. The Fe-Np bond is the longest in the series of octaalkyltetraphenylporphyrinatoiron(III) complexes due to [FeTC6TPP(1-Melm)2]Cl having the least distorted porphyrin core. In addition to these three complexes, two crystalline forms each of [FeOMTPP(4-Me2NPy)2]Cl and [FeOMTPP(2-MeHlm) 2]Cl were obtained. In all four of these cases the axial planes are in nearly perpendicular planes in spite of quite different geometries of the porphyrin cores (from purely saddled to saddled with 30% ruffling). The EPR spectral type correlates with the geometry of the OMTPP, OETPP and TC 6TPP complexes. For the paral-[FeOMTPP(1-Melm)2]Cl, a rhombic signal with g1 = 1.54, g2 = 2.51, and g 3 = 2.71 is consistent with nearly parallel axial ligand orientation. For all other complexes of this study, large gmax signals are observed (gmax = 3.61 - 3.27), as are observed for nearly perpendicular ligand plane arrangement. On the basis of this and previous work, the change from large gmax to normal rhombic EPR signal occurs between axial ligand plane dihedral angles of 70° and 30°.
Metal dependence of the nonplanar distortion of octaalkyltetraphenylporphyrins
Sparks,Medforth,Park,Chamberlain,Ondrias,Senge,Smith,Shelntt
, p. 581 - 592 (2007/10/02)
The biological activity of porphyrins and related tetrapyrroles in proteins may be modulated by nonplanar conformational distortions; consequently, two aspects of nonplanarity have been investigated in the highly nonplanar octaalkyltetraphenylporphyrins (OATPPs). In the first part, the effect of the central metal ion (M = Ni(II), Co(II), Cu(II), Zn(II), Co(III), Fe(III)) on the conformation of the OATPP macrocycle has been determined. Crystallographic studies reveal that the sterically encumbered, nonplanar porphyrin 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) remains sufficiently flexible to show a small decrease in nonplanarity for large metal ions. This decrease in nonplanarity for the OETPP metal complexes is predicted by using a molecular mechanics force field derived from structural and vibrational data for planar metalloporphyrins. A detailed analysis of the crystal structures of the Co(II) and Cu(II) complexes of OETPP reveals that the metal-dependent changes in bond lengths and bond angles are qualitatively similar to the changes observed for the OEP complexes. As the metal size increases, both OEPs and OETPPs exhibit expansion of the meso bridges (increases in the Cα-Cm bond length and the Cα-Cm-Cα bond angle) and a movement of the coordinating nitrogen atoms away from the metal atom (increases in the M-N bond length and the Cα-N-Cα bond angle and a decrease in the N-Cα bond length). Furthermore, the frequencies of several structure-sensitive Raman lines correlate with structural parameters obtained from these crystallographic studies. In the second part, a combination of molecular mechanics and INDO/CI molecular orbital calculations successfully predicts the optical spectra of a series of highly substituted OATPPs with increasing nonplanar distortion. The success of these calculations indicates the importance of including both the macrocycle conformation and the peripheral substituents in the INDO calculations.
