Use of Protected Binding Sites for Nitrite Binding in Iron(III) Porphyrinates. Crystal Structure of the Bis(nitro)(α,α,α,α-tetrakis(o-pivalamidophenyl)porphinato)iron(III) Anion

The reaction of nitrite ion with iron(III) picket-fence porphyrin species leads to the synthesis of low-spin bis(nitro) complexes. These compounds have been characterized by EPR, IR, UV-vis, and NMR spectroscopy. The crystal structure of one derivative, [K(18-C-6)(H2O)][Fe(NO2)2(TpivPP)], has been determined. The complex has one N-bound nitrite within the ligand-binding pocket of the pickets; the other N-bound nitrite is on the open side of the porphyrin plane but is protected from reaction by the formation of a tight ion pair with the [K(18-C-6)(H2O)]+cation: there are two K−O(NO2) bonds = 2.9 Å. Equatorial Fe-N bond distances average to 1.992 (1) Å while the axial Fe-N distances average to 1.985 (22) Å. Crystal data: tetragonal system, FeKO15N10C76H2O, space group P4, a = 16.691 (6) Å, c = 13.534 (3) −B, Z = 2. A total of 4184 observed data were used in the structure solution and refinement with final values of R(F0) = 0.069 and Rw(F0) = 0.066. Ligand-binding studies, followed by NMR or EPR spectroscopy, were performed in an attempt to assign the spin state of the intermediate mono(nitro) complex, which appears to be a high-spin species. Association constants for the formation of mono- and bis(nitrite) complexes are reported; differences as a function of counterion are consistent with significant ion-pairing differences. © 1990, American Chemical Society. All rights reserved.