STRUCTURAL CHARACTERIZATION OF LOW-SPIN IRON(III) COMPLEXES OF OCTAETHYLOXOPORPHYRIN

Low-spin (S = 1/2), SiX-coordinate iron(III) complexes of the octaethy loxoporphyrin anion (OEOP) have been obtained by treating high-spin (S = 5/2), six-coordinate (OEOP)Fe(III)Cl2 with strong-field ligands. Th ese nucleophiles do not attack the oxoporphyrin ligand, which remains intact. The cyano complex (OEOP)Fe(III)(CN)2.1.6CH2Cl2 has been isolat ed and studied by X-ray crystallography. It crystallizes in the monocl inic space group P2(1)/c with a = 12.750(4) angstrom, b = 8.069(3) ang strom, c = 19.728(8) angstrom, and beta = 99.67(3)-degrees at 130 K wi th Z = 2. Refinement of 1250 reflections with F > 6.0 sigma(F) and 123 parameters yields R = 0.076, R(w) = 0.078. The complex is centrosymme tric with the six-coordinate iron located at the center of symmetry. H ence, there is disorder in the location of the meso-oxygen atom. The F e-N distances are 1.976(9) and 1.955(9) angstrom. The magnetic moment (2.4 mu(B) at 20-degrees-C in dichloromethane) and ESR spectrum (g = 2 .4, 2.2, 1.8) confirm the low-spin formulation. The H-1 NMR spectrum h as been analyzed by one- and two-dimensional studies. Stepwise binding of 4-methylimidazole (4-MeIm, which coordinates as 5-MeIm) has been f ollowed by H-1 NMR measurements at -60-degrees-C. Evidence for the suc cessive formation of two low-spin species, [(OEOP)Fe(5-MeIm)Cl]+ and [ (OEOP)Fe(5-MeIM)2]2+, is presented. Addition of excess amounts of thes e strong-field ligands results in the reduction of Fe(III) to Fe(II).