Investigation of fuel production using metalloporphyrin-based complexes as catalysts and electron-transfer intermediates: Annual report, April 1987--December 1988

Four and six coordinate forms of a nickel(I1)-hydrocorphinate model of cofactor F,,, of the methyl-coenzyme M methylreductase were investigated using resonance Raman spectroscopy. complexes, formed in various coordinating solvents and in noncoordinating solvents with added organic bases, have Raman spectra showing only a single species at room temperature and at 7 7 " K in frozen solutions. The six-coordinate complexes are characterized by the two highest frequency, strong lines in the 1550-1560 and 1620-1630 cm-' regions. There is some variation in the frequencies of these two marker lines with the particular axial ligand, but the separation of the lines remains relatively constant at 6 7 7 4 cm-1. The separation of these two lines is an indicator of the nickel coordination number for the hydrocorphinate. The separation for a five-coordinate complex is 81 cm-1, and for the major four-coordinate form the separation is about 93 cm-1. Raman spectra of four-coordinate nickel hydrocorphinates show evidence of multiple species both at room temperature and at 77" K. Besides the previously identified four-coordinate form, we have now identified two other forms. One of the newly identified species is detected in room temperature Raman spectra taken with pre-resonance excitation and is observed also in frozen solutions. The other new four-coordinate form is positively detected only in frozen samples. The new room-temperature form has a red shifted absorption spectrum based on resonance enhancement of its Raman scattering with excitation to the red of the absorption maximum. observed 20-nm red shift in the absorbance maximum upon freezing at 7 7 " The six-coordinate This red shift is consistent with the I I I I 1 1 I I 1 I I I I I I I I I I K, under which