Inorganic Chemistry, Vol.37, No.5, 956-963, 1998
Iron(III) complexes as superoxide dismutase mimics : Synthesis, characterization, crystal structure, and superoxide dismutase (SOD) activity of iron(III) complexes containing pentaaza macrocyclic ligands
Iron(III) complexes with four pentaaza macrocylic ligands, [Fe(L)Cl-2](PF6), have been synthesized, where the ligand L is 1,4,7,10,13-pentaazacyclopentadecane (L-1), 2R,3R,8S,9S-dicyclohexano-1,4,7,10,13-pentaazacyclopentadecane L-2), 2R,3R,8R,9R-dicyclohexano-1,4,7,10,13-pentaazacyclopentadecane (L-3), or 2S,5R,8S,11R,14S-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane (L-4), respectively. Conductivity measurements in acetonitrile are consistent with two chloro anions coordinated to iron(III). In acetonitrile solution, all four iron complexes exhibit a reversible or quasi-reversible redox couple in the experimental range -1.5 to +1.5 V vs EAg/Ag+, and the redox potentials for those four complexes are similar (from -0.15 to -0.19 V vs NHE). In aqueous solutions, the electrochemical properties of those complexes are different from these in acetonitrile solution; the redox peaks shift more than 0.5 V more positive. The complexes with L-1, L-2, and L-3 display a reversible redox at 0.35, 0.45, and 0.43 V vs NHE, respectively, while [Fe(L-4)Cl-2](PF6) shows a cathodic peak at 0.44 V and two anodic peaks at 0.31 and 0.14 V vs NHE, respectively. The base titration results reveal that two water molecules are coordinated to the iron(III) in these complexes, pK(a1) and pK(a2) : 3.46(7) and 7.31(7) for [Fe(L-1)Cl-2](PF6); 3.7(1) and 7.50(2) for [Fe(L-2)Cl-2](PF6); 4.1(1) and 7.73(2) for [Fe(L-3)Cl-2](PF6); 3.6(7) and 7.4(2) for [Fe(L-4)Cl-2](PF6). The superoxide dismutase (SOD) activity, assessed by stopped-flow experiments, reveals that all four metal complexes catalyze the fast disproportionation of superoxide in aqueous solution : at pH = 7.8, the catalyic rate constants for the complexes were 0.81 x 10(7), 1.42 x 10(7), 1.41 x 10(7), and 0.29 x 10(7) M-1 s(-1) for [Fe(L-1)Cl-2](PF6), [Fe(L-2)Cl-2](PF6), [Fe(L-3)Cl-2](PF6), and [Fe(L-4)Cl-2](PF6), respectively. The crystal structure of [Fe(L-2)Cl-2](PF6) was determined. Crystal data : triclinic, ; a = 7.367(2), b = 10.707(2), c = 17.632(2) Angstrom; alpha = 81.27(2), beta = 79.74(2), gamma = 83.38(2)degrees; R = 0.0465 for observed data (I > 2 sigma(I)). The iron is seven-coordinate with five nitrogen atoms from the pentaaza macrocylic ligand and trans-chloro anions in a pentagonal bipyramidal arrangement. The average bond lengths for Fe-N and Fe-Cl are 2.271 and 2.339 Angstrom, respectively.