EFFECTS OF METAL-CENTERED REDUCTION ON THE STRUCTURAL, ELECTRONIC, AND COORDINATION PROPERTIES OF NICKEL AND COPPER OCTAETHYLISOBACTERIOCHLORINS

The structural properties of nickel and copper complexes of octaethyli sobacteriochlorin (OEiBC) are investigated as part of an effort to und erstand the chemistry of factor 430 (F430), the nickel hydrocorphinoid cofactor of methyl coenzyme-M reductase. Both Ni-II and Cu-II(OEiBC) undergo one-electron reductions to yield metal(I) complexes. The natur e of the starting materials, the anionic metal(I) reduction products, and their ligand-binding properties are probed with electron paramagne tic resonance (EPR) and X-ray absorption techniques. Comparison betwee n the structures of the isoelectronic Ni-I and Cu-II complexes reveals that the marked distortions observed in the Ni-I-macrocycle core envi ronment, which has two Ni-N distances of 1.91(2) Angstrom and two Ni-N distances of 2.07(2) Angstrom, are not apparent for Cu-II, which has four Cu-N distances of 2.00(2) Angstrom. Thus, the distortion of the N i-I environment does not result from an electronic configuration effec t such as a Jahn-Teller distortion. X-ray absorption near-edge studies of OEiBC complexes at reduced temperatures demonstrate for the first time that Ni-I hydroporphyrins can bind a single axial ligand. Optical and EPR spectra are found to be insensitive to the axial binding for these cases. Chemical reduction of Cu-II(OEiBC), using sodium amalgam, affords a Cu-I complex that has unchanged Cu-N distances and two sodi um ions coordinated on opposite sides of the OEiBC ring at Cu-Na dista nces of 2.89(4) Angstrom. The sodium ions can be sequestered using the crown ether 18-crown-6. Direct comparison of Cu-II and the non-ion-pa ired Cu-I complexes, which have the same geometry and coordination env ironment, establishes that the average Cu-N distance increases 0.06 An gstrom upon reduction. The structural differences between the Ni-I and Cu-I complexes may account for their different reactivities toward al kyl halides and oxygen.