Sharing the π-bonding. An iron porphyrin derivative with trans, π- accepting axial ligands. Synthesis, EPR and Mossbauer spectra, and molecular structure of two forms of the complex nitronitrosyl(α,α,α,α-tetrakis(o- pivalamidophenyl)porphinato)ferrate(II)

The reaction of the iron(II) picket fence porphyrin derivative [Fe(TpivPP)(NO 2)] - (Nasri et al. J. Am. Chem. Soc. 1991, 113, 717) with NO has been examined. The result is a new hexacoordinated iron(II) porphyrin complex, [Fe(TpivPP)(NO 2)(NO)] -. This compound has been characterized by UV-vis, IR, EPR, and Mossbauer spectroscopies. All data show that the species is low spin (S = 1/4 ). The EPR spectrum of [Fe(TpivPP)(NO 2)(NO)] -(frozen chlorobenzene solution) is similar to that of five-coordinate nitrosyl derivatives of iron(II) porphyrinates with g(x) = 2.085, g(y) = 2.032, g(z) = 2.01, a(x) ( 14N) = 24 G, a(y) = 16.5 G, and a(z) = 16 G. Two crystalline forms of [Fe(TpivPP)(NO 2)(NO)] - were obtained and their molecular structures determined. In both crystalline forms, the axial nitro ligand is inside the 'pocket' of the picket fence porphyrin derivative. In the first crystalline form isolated, there are two independent molecules, one of which has disorder in the nitro and nitrosyl groups while the other molecule has completely ordered axial ligands. In this first form, the plane of the nitro group is nearly perpendicular to the plane containing the nitrosyl group and the iron. In the second crystalline form, however, the two axial ligand planes are nearly parallel. Mossbauer investigations (solid state) show that the iron centers in the two different crystalline forms have different electronic properties. The Mossbauer parameters for the perpendicular form are δ = 0.22 mm/s and ΔE(q) = 1.78 mm/s (200 K) and for the parallel form δ = 0.35 mm/s and ΔE(q) = 1.2 mm/s (4.2 K). Mossbauer data are also presented for a related complex: five-coordinate [Fe(TPP)(NO)] (TPP = mesotetraphenylporphinato). IR spectra for the two forms also reveal distinctly different interactions of the trans (nitro) ligand.