Photochemistry of Biologically Important Transition Metal Complexes. I. Cyanocobalamin and Related Corrin Complexes of Rhodium(III)

Abstract

Abstract The quantum yield of the well known photoaquation of cyanocobalamin (vitamin B 12 ) was determined (ϕ ˜ 10 −4 ) and was found to be independent of the irradiating wavelength. In addition cyanocobalamin did not show the characteristic corrin phosphorescence. In agreement with an earlier suggestion it is concluded that the excitation energy initially absorbed by the corrin ligand is rapidly transferred to a reactive excited LF state which lies below the lowest corrin triplet. This conclusion is supported by the behavior of corrin complexes of Rh(III). Contrary to cyanocobalamin the complex dicyanorhodium(III)‐corrin did not undergo a photoaquation but has been shown to emit a strong corrin phosphorescence. These observations are consistent with the assumption that the lowest excited LF state lies now well above the lowest corrin triplet. If both axial cyanide ligands which have a strong field are replaced by the weak‐field ligand chloride the energy of the lowest excited LF state should drop considerably. The behavior of dichlororhodium(III)‐corrin indicates that the lowest corrin triplet lies only slightly below the lowest excited LF state which is responsible for the photoreactivity. The reactive LF state may be populated from the lowest corrin triplet by thermal activation. At 77 K dichlororhodium(III)‐corrin showed a strong corrin phosphorescence. At room temperature this emission was essentially quenched and the complex underwent a photoaquation.