Spectral study of the dioxygen and carbonyl adducts of ferrous porphyrins ligated by neutral S-donors

A series of the six-coordinated ferrous porphyrin complexes (L)Fe(II)TPP(O₂) and (L)Fe(II)TPP(CO) (L, (CH₃)₂S, or C₂H₅SH, TPP, meso-tetraphenylporphyrin dianion) was prepared. UV–vis and FTIR spectroscopy were applied to follow the reaction of O₂ and CO adducts of Fe(II) porphyrins with neutral thiol and thioether. Using sublimed layers methodology and low temperature technique, as well as isotope substitution, the formation of six-coordinate complexes was monitored in situ. The comparison of CO and O₂ stretching frequencies of thiol and thioether adducts indicates that the degree of electron donation is similar for these ligands. The complexes of Me₂S and EtSH with Fe(TPP)(O₂) are stable at 150 K and slowly decompose at elevated temperatures. In contrast, the carbonyl complexes of Fe(Por) with Me₂S and EtSH are relatively stable and retain six-coordination at room temperature. The electronic effects of axial S-donor ligands on the coordinated O₂ and CO are discussed and compared with the effects of N- and O-donors.