Barriers to Inversion of Nonplanar Ferric Porphyrin Radical Cations: Comparison with the Corresponding Neutral Complexes

Assessment of the intramolecular magnetic interactions in the highly saddled iron(iii) porphyrin π-radical cations: the change from planar to saddle conformations

The intramolecular magnetic interactions in one-electron oxidized iron(iii) porphyrin π-radical cations, [Fe(OETPP˙)Cl][SbCl6] (1), [Fe(OMTPP˙)Cl][SbCl6] (2) and [Fe(TPP˙)Cl][SbCl6] (3), have been compared by means of X-ray crystallography, SQUID magnetometry, cyclic voltammetry, UV-Vis spectroelectrochemical analysis, NMR spectroscopy analysis and unrestricted DFT calculations. Unlike a generally recognized antiferromagnetic coupling dxy↑dxz↑dyz↑dz2↑dx2-y2↑P˙+(a2u)↓ (S = 2) state via a weak bonding interaction as in (3), we have disclosed that a strong bonding interaction among iron dx2-y2 and porphyrin a2u orbitals forms in (1) into a highly delocalized Ψπ = [P˙+(a2u) + FeIII(dx2-y2, dz2)] orbital that is able to accommodate two spin-paired electrons to form the Ψπ2dxy1dxz1dyz1, dz21 (S = 2) ground state. Concurrently, the spin polarization effect is exerted on the paired spins in the Ψπ orbital by magnetic induction from the remaining unpaired electrons in the iron d orbitals. The interpretation mentioned above is further verified by the diamagnetic nature of the saddled copper(ii) porphyrin π-cation radical, CuII(OETPP˙)(ClO4) (S = 0), where the strong bonding interaction leads to the Ψπ2dxy2dxz2dyz2dz22 (S = 0) ground state but no spin polarization exists. Thus, the magnetic nature of the iron(iii) porphyrin π-radical cation is tuneable by saddling the ring planarity.

Effects of porphyrin deformation on the 13C and 1H NMR chemical shifts in high-spin five- and six-coordinate manganese(III) porphyrin complexes

As an extension of our study to reveal the effect of porphyrin deformation on the [Formula: see text]C and 1H NMR chemical shifts, both five- and six-coordinate high-spin (S [Formula: see text] 2) Mn(III) complexes such as Mn(Por)Cl and [Mn(Por)(CD3OD)2]Cl have been prepared, where Por is a porphyrin dianion such as TPP, OMTPP, and T[Formula: see text]PrP. Molecular structures of five-coordinate Mn(OMTPP)Cl and Mn(TiPrP)Cl have been determined by the X-ray crystallographic analysis. As expected, Mn(OMTPP)Cl and Mn(TiPrP)Cl have exhibited a highly saddled and highly ruffled porphyrin core, respectively. The [Formula: see text]C NMR spectra have revealed that these complexes generally exhibit the [Formula: see text]-pyrrole signals at the downfield positions and [Formula: see text]-pyrrole an. meso signals at the upfield positions. The results suggest that the spin polarization of Mn(III)–NP σ bonds, which occurs in all the high-spin Mn(III) complexes, is the major factor to determine the chemical shifts of the porphyrin carbon signals (Cheng, R.-J.; Chang, S.-H.; Hung, K.-C. Inorg. Chem. 2007; 46: 1948–1950). Although th. meso and [Formula: see text]-pyrrole signals are observed at the upfield and downfield positions, respectively, these signals are widely dispersed depending on the deformation mode of the porphyrin ring. The results have been explained in terms of the strong spin polarization of the Mn–NP bond together with the specific metal-porphyrin orbital interactions such as: (i) the a2u-dz2 interaction in five-coordinate complexes, (ii) the a2u-dxy interaction in ruffled complexes, and (iii) the a2u-dx2-y2 interaction in saddled complexes.

Electronic structure of five- and six-coordinate iron(III) tetraazaporphyrin complexes: pyrrole-Cαchemical shift as a useful probe

Electronic structure of a series of five-coordinate Fe ( OArTAzP ) X ( OAr = octaaryltetraazaporphyrin , X = Cl-, Br-, I-; Ar = 4-tert-butylphenyl) have been examined on the basis of1H NMR,13C NMR, and EPR spectroscopy as well as SQUID magnetometry. These complexes adopt the intermediate-spin state as in the case of analogous complexes reported by Fitzgerald et al. (Inorg. Chem. 1992; 31: 2006-2013) and Stuzhin et al. (Inorg. Chim. Acta 1995; 236: 131-139). The13C NMR studies using13C -enriched complexes at the pyrrole α positions have revealed that the pyrrole- Cαsignals appear at extraordinary upfield positions, i.e. -130 to -250 ppm at 273 K, due to the dz2-a2 uand dπ-3 eginteractions. The Curie plots of the pyrrole- Cαsignals have further revealed that the iodide complex adopts a much purer intermediate-spin state than the bromide and chloride complexes. In contrast to the case of Fe ( OArTAzP ) X , six-coordinate [ Fe ( OArTAzP )( CN )2]-showed the pyrrole- Cαsignal at 47 ppm at 273 K, which indicates that the complex adopts the low-spin state with the ( dxy)2( dxz, dyz)3electron configuration. Thus, the13C NMR chemical shift of the pyrrole- Cαsignal turns out to be quite a good probe to elucidate the spin state and electron configuration of iron(III) tetraazaporphyrins, where the1H NMR spectroscopy is less useful because of the absence of the hydrogen atoms as well as the alkyl or aryl groups directly attached to the meso positions.