Effects of axial pyridine coordination on a saddle-distorted porphyrin macrocycle: stabilization of hexa-coordinated high-spin Fe(iii) and air-stable low-spin iron(ii) porphyrinates

Abstract

We have reported here the effect of axial ligand L (L: pyridine/substituted pyridine) on Fe(III)(tn-OEP)Cl/Fe(III)(tn-OEP)ClO(4) that first form high-spin Fe(III)(tn-OEP)(L)(2).X (X: Cl, ClO(4)) which, on longer exposure, spontaneously auto reduce to a series of air stable Fe(II)(tn-OEP)(L)(2) complexes. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin (tn-OEP), severely distorts the porphyrin macrocycle which enables the facile isolation of a rare family of high-spin Fe(III)(tn-OEP)(L)(2)(+) in a saddle distorted macrocyclic environment. The synthesis and characterization of high-spin Fe(III)(tn-OEP)(L)(2).X and low-spin Fe(II)(tn-OEP)(L)(2) are reported. The X-ray structures of Fe(II)(tn-OEP)(py)(2), Fe(II)(tn-OEP)(4-CNpy)(2) and Fe(II)(tn-OEP)(3-Clpy)(2) have been determined in which the axial ligands are orientated nearly perpendicular to each other. Electrochemical data obtained from cyclic voltammetric study for Fe(II)(tn-OEP)(L)(2) reveals the one electron oxidations at very high positive potentials which readily explains why the complexes are so stable in air. However, spectroscopic characterizations such as magnetic and EPR measurements in both solid and solution, and (1)H NMR in solution demonstrates the high-spin nature of Fe(III)(tn-OEP)(L)(2).X. Molecular orbital calculations using DFT for five coordinate Fe(III)(tn-OEP)Cl shows a(2u)-like HOMO that is expected for a saddle distorted porphyrin but for six coordinate Fe(III)(tn-OEP)(L)(2).X results in switch of the HOMO from a(2u) to a(1u). However, metal d(x(2)-y(2)) and porphyrin a(1u) bonding interaction is symmetrically unfavorable and thus responsible for high-spin nature of the complexes reported here. The porphyrin cores (tn-OEP) are found to be least distorted in Fe(III)(tn-OEP)(H(2)O)(2).ClO(4) with a core size of 2.061 A while, for Fe(II)(tn-OEP)(py)(2), the macrocycle is distorted most with lowest core size of 1.961 A; thus shows a significant and unprecedented core expansion of 0.1 A in the series.

Spontaneous Reduction of Mononuclear High‐Spin Iron(III) Complexes to Mononuclear Low‐Spin Iron(II) Complexes in Aqueous Media and Nuclease Activity via Self‐Activation

“…or strong Lewis acidity of iron(III) porphyrins causes the reduction to iron(II) porphyrins in presence of an axial ligand such as pyridine, imidazoles. Patra et al 24. also reported high‐spin [Fe III (tn‐OEP)(L) 2 .X] (X: Cl, ClO 4 ) porphyrins spontaneously autoreduced to low‐spin [Fe II (tn‐OEP)(L) 2 ] complexes depending upon axially bounded strong field ligand L (L: pyridine/substituted pyridine).…”

ESR analyses of picket fence MnII and 6th ligand coordinated FeIII porphyrins (S = 5/2) and a CoII(hfac) complex (S = 3/2) with sizable ZFS parameters revisited: a full spin Hamiltonian approach and quantum chemical calculations

“…We note, however, that this sizable degree of the asymmetry contradicts the experimental observation, whereas the order of the magnitude for the D-value and its sign seems reasonable, considering the accuracy of the theoretical method for the 3d 5 electron configuration of ferric iron cases. 11,51 As described above in (a) for the case of the Mn(II) porphyrins, the theory still overestimates the degree of the asymmetry. Importantly, the theoretical D-value of complex 3 + , +3.1241 cm -1 can be compared with the experimental one, which falls in the range of order of cm -1 to 10 cm -1 , as determined by the magnetic susceptibility analysis revisited in this work (see below and Fig.…”

“…18,52 Recently, hexa-coordinated (pseudo-octahedral) high spin cobalt(II) complexes with sizable ZFS parameters comparable to the microwave excitation energy of X-band ESR (9.5 GHz or 0.3 cm -1 ) have been studied. 18,28,[51][52][53][54][55][56][57][58][59][60][61][62][63][64] Palii and co-workers synthesized NH4[Co II (hfac)3] and investigated the electronic structures and magnetic characters as SMM by using of ESR spectroscopy and AC/DC magnetic susceptibility measurements. 51 From the theoretical viewpoint, the principal values of the magnetic tensors such as the g-, ZFS D-and hyperfine A-tensors for the 3d 7 electron configuration of high spin metal ions are still the focus of the current quantum chemical calculations.…”

Optimal Size Matching and Minimal Distortion Energy: Implications for Natural Selection by the Macrocycle of the Iron Species in Heme

“…Heme functions are highly sensitive to changes in the macrocycle because of its high flexibility , . Some earlier studies showed that the porphyrin core could regulate the spin state of nickel(II) ions and the electronic ground states of low‐spin (LS) iron(III) ions, , in a porphyrin on the basis of the expansion or contraction of the core. We found that the size‐matching rule could be used to obtain a combination of variable core and alterable iron species.…”