Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

Article abstract of DOI:10.1021/ic7008495

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO) ₂]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K₁ is much larger than K₂. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K₁ = 267 and K₂ ≈ 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)₂] (C₅₂H ₄₄Fe-N₄O₂S₂): monoclinic, P2 ₁/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930-(4) A, β = 116.246(1)°, V = 1992.95(10) A³, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-N_p distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.