Models of Factor 430. Structural and Spectroscopic Studies of Ni(II) and Ni(I) Hydroporphyrins

The reductive chemistry of a series of progressively more saturated Ni(II) porphyrins, derived from anhydromesorhodochlorin XV methyl ester, has been examined as models of F430. Cyclic voltammetry, spectroelectrochemistry, electron paramagnetic resonance, and X-ray absorption studies are used to characterize the parent compounds and their reduction products. Within the Ni(II) porphyrin, chlorin, isobacteriochlorin (iBC), and hexa- and octahydro porphyrin series, only the iBCs are reduced to Ni(I). The other compounds yield π anion radicals or π radicals with some metal character. The structure of one of the iBCs in the series has been determined by single-crystal X-ray diffraction and used to validate EXAFS results. Crystallographic data for Ni(II) anhydromesorhodoisobacteriochlorin methyl ester with rings C and D reduced, 3, are the following: space group P2₁ a = 13.688 (1) Å, b = 8.124 (1) Å, c = 14.178 (1) Å, β = 111.83(1)°, V= 1463.6 ų, and Z = 2. The structure was refined against 1226 data points to R_F = 0.058 and R_wF = 0.056. Saturation of the macrocycles affects their electronic configurations as evidenced by changes in redox and optical properties as well as sites of reduction. In addition, structural factors emerge as significant determinants of the chemistry of the Ni compounds. Their ability to form Ni(I) or hexacoordinate high-spin Ni(II) requires that the macrocycles be flexible enough to accommodate the conformational changes that accompany reduction to Ni(I) or axial ligation of square-planar Ni(II). Thus, interdependent electronic and structural factors control the reactivity of the models considered here and, by analogy, that of F430 as well.