Synthetic Models of the Reduced Active Site of Superoxide Reductase

We report the synthesis, structural and spectroscopic characterization, and magnetic and electrochemical studies
of a series of iron(II) complexes of the pyridyl-appended diazacyclooctane ligand L8py2, including several that
model the square-pyramidal [FeII(Nhis)4(Scys)] structure of the reduced active site of the non-heme iron enzyme
superoxide reductase. Combination of L8py2 with FeCl2 provides [L8py2FeCl2] (1), which contains a trigonal-prismatic
hexacoordinate iron(II) center, whereas a parallel reaction using [Fe(H2O)6](BF4)2 provides [L8py2Fe(FBF3)]BF4 (2),
a novel BF4--ligated square-pyramidal iron(II) complex. Substitution of the BF4- ligand in 2 with formate or acetate
ions affords distorted pentacoordinate [L8py2Fe(O2CH)]BF4 (3) and [L8py2Fe(O2CCH3)]BF4 (4), respectively. Models
of the superoxide reductase active site are prepared upon reaction of 2 with sodium salts of aromatic and aliphatic
thiolates. These model complexes include [L8py2Fe(SC6H4-p-CH3)]BF4 (5), [L8py2Fe(SC6H4-m-CH3)]BF4 (6), and
[L8py2Fe(SC6H11)]BF4 (7). X-ray crystallographic studies confirm that the iron(II)−thiolate complexes model the square-pyramidal geometry and N4S donor set of the reduced active site of superoxide reductase. The iron(II)−thiolate
complexes are high spin (S = 2), and their solutions are yellow in color because of multiple charge-transfer
transitions that occur between 300 and 425 nm. The ambient temperature cyclic voltammograms of the iron(II)−thiolate complexes contain irreversible oxidation waves with anodic peak potentials that correlate with the relative
electron donating abilities of the thiolate ligands. This electrochemical irreversibility is attributed to the bimolecular
generation of disulfides from the electrochemically generated iron(III)−thiolate species.