Highly variable π-bonding in the interaction of iron(II) porphyrinates with nitrite

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 Mossbauer spectroscopies as well as single-crystal structure determinations. The Mossbauer 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 N(p) -Fe-N(p) angle. In the pyridine derivative, the dihedral angle between the two axial ligand planes is 81.4°. In the pentamethylene sulfide derivative, Fe-N(p) = 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-N(p) = 1.990(15) Å, Fe-N(NO₂) = 1.951(5) Å, and Fe-N(Py) = 2.032(5) Å. In [Fe(TpivPP)(NO₂)]^-, Fe-N(p) 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.