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 KNO 2 yields the five-coordinate porphyrin species [Fe(TpivPP)(NO 2 )] - . The six-coordinate complexes, [Fe(TpivPP)(NO 2 )(PMS)] - and [Fe(TpivPP)(NO 2 )(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)(NO 2 )(PMS)] - and [Fe(TpivPP)(NO 2 )(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)(NO 2 )] - . The molecular structures of [Fe(TpivPP)(NO 2 )(PMS)] - and [Fe(TpivPP)(NO 2 )(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(NO 2 ) = 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(NO 2 ) = 1.951(5) Å, and Fe−N(Py) = 2.032(5) Å. In [Fe(TpivPP)(NO 2 )] -, Fe−N p is 1.970(4) Å while Fe−N(NO 2 ) is a very short 1.849(6) Å. The structural and spectroscopic data are interpreted as showing a significant difference in the [Fe−NO 2 ] π interaction on change in coordination number.