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 octaethylisobacteriochlorin (OEiBC) are investigated as part of an effort to understand the chemistry of factor 430 (F430), the nickel hydrocorphinoid cofactor of methyl coenzyme-M reductase. Both Ni^IIand Cu^II(OEiBC) undergo one-electron reductions to yield metal(I) complexes. The nature of the starting materials, the anionic metal(I) reduction products, and their ligand-binding properties are probed with electron paramagnetic resonance (EPR) and X-ray absorption techniques. Comparison between the structures of the isoelectronic Ni^Iand Cu^IIcomplexes reveals that the marked distortions observed in the Ni^I—macrocycle core environment, which has two Ni—N distances of 1.91(2) Å and two Ni—N distances of 2.07(2) Å, are not apparent for Cu^II, which has four Cu-N distances of 2.00(2) Å. Thus, the distortion of the Ni^Ienvironment does not result from an electronic configuration effect 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^Ihydroporphyrins 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^Icomplex that has unchanged Cu—N distances and two sodium ions coordinated on opposite sides of the OEiBC ring at Cu—Na distances of 2.89(4) Å. The sodium ions can be sequestered using the crown ether 18-crown-6. Direct comparison of Cu^IIand the non-ion-paired Cu^Icomplexes, which have the same geometry and coordination environment, establishes that the average Cu-N distance increases 0.06 Å upon reduction. The structural differences between the Ni^Iand Cu^Icomplexes may account for their different reactivities toward alkyl halides and oxygen.