Metal–Ligand Cooperativity Promoting Sulfur Atom Transfer in Ferrous Complexes and Isolation of a Sulfurmethylenephosphorane Adduct

The reaction of elemental sulfur with the cis-divacant octahedral complex [(pyrr2py)Fe(OEt2)] (1; pyrr2py2– = 3,5-tBu2-bis(pyrrolyl)pyridine) yields the iron dimer [(pyrr-1-S-pyrrpy)Fe]2 (2; pyrr-1-S-pyrrpy2– = 3,5-(tBu2-pyrrolyl)(1-S-3,5-tBu2-pyrrolyl)pyridine) resulting from a pyrr2py2– ligand based S-oxidation of one pyrrole arm. Addition of the phosphorus ylide H2CPPh3 to 1 forms the ylide adduct [(pyrr2py)Fe(CH2PPh3)] (3), which upon reaction with elemental sulfur produces a rare example of a sulfurmethylenephosphorane adduct, [(pyrr2py)Fe(SCH2PPh3)] (4). The sulfur-oxidized pyrrole group of the ligand pyrr-1-S-pyrrpy2– can be reversed, since complex 2 exhibits S atom transferability via the addition of 2 equiv of H2CPPh3 to yield a mixture of compounds 3 and 4. For all complexes reported, the ferrous ion remains S = 2. Complexes 2–4 were characterized by single-crystal X-ray diffraction as well as 1H NMR spectroscopy, solid and solution magnetic studies, and 57Fe Mossbauer spectroscopic measurements.