The synthesis and characterization of isocyanide complexes of (porphyrinato)iron(III) species, [(Porph)-Fe(t-BuNC) 2 ]ClO 4 , Porph ) OEP, TPP, are reported. The crystal structures of [(TPP)Fe(t-BuNC) 2 ]ClO 4 and [(OEP)-Fe(t-BuNC) 2 ]ClO 4 have been determined. Consistent with the expected effect from the strong π-acceptor character of the axial tert-butyl isocyanide ligands, the X-ray structure of the complex shows that the porphyrinate ring is strongly ruffled. The spectroscopic properties of these complexes suggest the possibility of "blurring" of the definitions of the electron configurations of low-spin Fe(III) macrocycles having (d xy ) 1 electronic ground states, with the extreme possibilities being low-spin Fe(III)-(macrocycle) 2-, with the unpaired electron localized in the d xy orbital of the metal, and low-spin Fe(II)-(macrocycle) 1-• , with the unpaired electron localized on the macrocycle. EPR spectroscopy of the TPP and OEP complexes shows that the g-values (g ⊥ ) 2.20-2.28, g | ) 1.94-1.83) are consistent with an electron configuration that is (d xz ,d yz ) 4 (d xy ) 1 , the purest (d xy ) 1 ground state system with the most complete quenching of orbital angular momentum discovered thus far (∑g 2 as small as 13.5). Proton NMR spectra of [OEPFe(t-BuNC) 2 ]-ClO 4 in CD 2 Cl 2 , recorded over the temperature range -100 to +37°C, also support the (d xy ) 1 ground state, where ruffling of the porphyrinate ring makes it possible for unpaired electron spin delocalization to the 3a 2u (π) orbital of the porphyrinate ring. This orbital has very large electron density coefficients at the meso positions and hence explains the very large negative contact shift of the meso-H; its size indicates considerable (∼19%) spin delocalization from low-spin Fe(III) to the 3a 2u (π) orbital by porphyrin f Fe π donation. Mössbauer and IR spectral data are also consistent with the (d xy ) 1 ground state.

The reaction of the four-coordinate picket fence iron(II) porphyrin complex [Fe(TpivPP)] with cryptand-solubilized KNO2 yields the five-coordinate porphyrin species [Fe(TpivPP)(NO2)]-. The six-coordinate complexes, [Fe(TpivPP)(NO2)(PMS)]- and [Fe(TpivPP)(NO2)(Py)]-, are obtained when pentamethylene sulfide or pyridine is added anaerobically to the preformed five-coordinate porphyrin species. These novel species are characterized by UV−vis, IR, and Mössbauer spectroscopies as well as single-crystal structure determinations. The Mössbauer investigation shows that the isomer shifts and quadrupole splittings for [Fe(TpivPP)(NO2)(PMS)]- and [Fe(TpivPP)(NO2)(Py)]- are typical for six-coordinate, low-spin (S = 0) iron(II) porphyrinate complexes. This is in distinct contrast to the unusually large quadrupole splitting of the five-coordinate species [Fe(TpivPP)(NO2)]-. The molecular structures of [Fe(TpivPP)(NO2)(PMS)]- and [Fe(TpivPP)(NO2)(Py)]- show that the nitro groups are inside the “pocket” of the porphyrin trans to the neutral sulfur- or nitrogen-donating axial ligand. In all species, the projection of the nitrite ion onto the porphyrin plane bisects a Np−Fe−Np angle. In the pyridine derivative, the dihedral angle between the two axial ligand planes is 81.4°. In the pentamethylene sulfide derivative, Fe−Np = 1.990(6) Å, Fe−N(NO2) = 1.937(3) Å, and Fe−S(PMS) = 2.380(2) Å, while in the pyridine derivative Fe−Np = 1.990(15) Å, Fe−N(NO2) = 1.951(5) Å, and Fe−N(Py) = 2.032(5) Å. In [Fe(TpivPP)(NO2)]-, Fe−Np is 1.970(4) Å while Fe−N(NO2) is a very short 1.849(6) Å. The structural and spectroscopic data are interpreted as showing a significant difference in the [Fe−NO2] π interaction on change in coordination number.