Evaluation of Octaethyl-7,17-dioxobacteriochlorin as a Ligand for Transition Metals

The propensity of octaethyl-7,17-dioxobacteriochlorin toward the formation of transition metal complexes was evaluated. A variety of M^II ions (M = Co, Ni, Cu, Zn, Pd, Ag, and Cd) and Fe(III) could be inserted using standard methodologies or, more often, using more forcing conditions. The stable products were spectroscopically characterized. The solid-state structures of the Ni(II), Cu(II), Pd(II), and Ag(II) complexes could also be determined by single crystal X-ray diffractometry, whereby the [7,17-dioxobacteriochlorinato] chromophore was found to be largely planar in all cases. The rate of Zn(II) insertion into octaethyl-7,17-dioxobacteriochlorin was less than half that into the corresponding 7-oxochlorin, which itself was about half the rate into the parent octaethylporphyrin. These rate differences reflect the relative decreased basicity of the β-oxo-substituted chromophores and possibly also their decreased conformational flexibility. We compare the basicity of the dioxobacteriochlorin to that of a range of related products of varying reduction state (porphyrin, chlorin, bacteriochlorin), an isomer, and the absence or presence of oxo-functionality, like oxochlorin, chlorin, oxobacteriochlorins, and bacteriochlorin, quantifying the effects of these macrocycle modifications. The work rationalizes earlier reports of the inability of tolyporphin A, a natural product possessing a 7,17-dioxobacteriochlorin chromophore, to form metal complexes and provide a more quantitative understanding of the degree of modulation that β-oxo groups have on the coordination properties of porphyrinoids.