156970-82-0Relevant academic research and scientific papers
Axial ligand orientation in iron(III) porphyrinates: Effect of axial π-acceptors. Characterization of the low-spin complex [Fe(TPP)(4-CNPy)2]C1O4
Safo, Martin K.,Walker, F. Ann,Raitsimring, Arnold M.,Walters, W. Patrick,Dolata, Daniel P.,Debrunner, Peter G.,Scheidt, W. Robert
, p. 7760 - 7770 (2007/10/02)
The preparation and characterization of the low-spin bis(pyridine)iron(III) porphyrinate complex [Fe(TPP)-(4-CNPy)2]ClO4 is reported. Consistent with the expected effect from the strong π-acceptor character of the axial 4-cyanopyridine ligands, the X-ray structure of the complex shows that the two axial ligands have relative perpendicular orientations along with an extensively S4-ruffled porphyrin core. The S4 ruffling is among the largest found for bis(pyridine) complexes and leads to the extremely short average Fe-Np bond distance of 1.952(7) A?. The axial Fe-N bond distances average to 2.002(8) A?. Molecular mechanics calculations indicate that the ruffling observed in this and other bis(pyridine) complexes of Fe(III) porphyrins is not simply a result of steric interactions between the phenyl rings and the pyridine ligands, since the minimized energies of bis(pyridine) complexes of the non-phenyl-containing system, [Fe(porphine)(pyridine)2]+, as a function of the angular orientation of two perpendicularly aligned pyridine ligands with respect to the N-Fe-N axes of the porphyrin ring, are within experimental error of those of [Fe(TPP)-(pyridine)2]+. Therefore, the observed strong S4 ruffling of the porphyrinato core must be due to electronic rather than steric factors. This electronic contribution is likely the partial delocalization of the dxy unpaired electron into the α2u(π) orbital of the porphyrin ring, which is made possible by the twisting of the nitrogen pz orbitals out of the mean plane of the porphyrin ring as a result of the strong S4 ruffling. The Mo?ssbauer spectrum of the complex has an isomer shift of 0.19(1) mm/s and an unusually small quadrupole splitting (ΔEq) of 0.65(1) mm/s. The EPR spectra in both solid and solution phases are axial, with g⊥ ≥ 2.62 and g∥ ≤ 0.92 at 4.2 K, Σg2 ～ 14.6. The [Fe(TPP)(4-CNPy)2]ClO4 complex is thus a case in which a large amount of the d orbital angular momentum of the metal is quenched, and hence Σg2 is much lower than 16, and is, in fact, midway between that value and the minimum possible value of 12 that is expected for a pure (dxy)1 unpaired electron. Single crystal EPR spectra show a strong broadening of the EPR signal near the g∥ turning point that is indicative of what has been called by some researchers g-strain, as was previously observed in the large gmax type of rhombic EPR signals of low-spin Fe(III) porphyrin systems having perpendicularly aligned planar axial ligands and a (dxy)2(dxz,dyz)3 electronic ground state (Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am. Chem. Soc. 1986, 108, 5288-5297). The inability to observe the minimum g-value in this axial system is a result of the strong dependence of g∥ on the field strength of the ligands, as measured by the tetragonality (Δ/λ). All physical properties are consistent with an iron(III) ion that has the unusual ground-state configuration (dxy,dyz)4(dxy)1. Crystal data for [Fe(TPP)(4-CNPy)2]ClO4·CH2Cl 2: a = 11.187(11) A?, b = 20.208(11) A?, c = 21.815(18) A?, orthorhombic, space group F212121, V = 4931.5 A?3, Z = 4, no. obsd data = 5662.
