Variable π-Bonding in Iron(II) Porphyrinates with Nitrite, CO, and tert-Butyl Isocyanide:  Characterization of [Fe(TpivPP)(NO2)(CO)]

The addition of the strongly π-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV−vis, IR, and Mössbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, π-accepting ligands CO and nitrite compete for π density. The CO ligand however dominates the bonding. The Fe−N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) Å are lengthened compared to other nitrite species with either no trans ligands or non-π-accepting trans ligands to nitrite. The Fe−C(CO) bond lengths are 1.782(4) Å and 1.789(5) Å for the two anions. The two Fe−C−O angles at 175.5(4) and 177.5(4)° are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the Mössbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the Mössbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm-1 is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)]·1/2C6H5Cl: monoclinic, space group P21/c, Z = 8, a = 33.548(6) Å, b = 18.8172(15) Å, c = 27.187(2) Å, β = 95.240(7)°, V = 17091(4) Å3.