Sulfoxide as a Ligand in Iron(II) Porphyrinates: S- or O-Bound?

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] (C52H44Fe−N4O2S2):  monoclinic, P21/c, a = 11.2580(3) Å, b = 15.9262(5) Å, c = 12.3930(4) Å, β = 116.246(1)°, V = 1992.95(10) Å3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin
bis-TMSO ligated species. The average Fe−Np distance is 1.999(4) Å. 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) Å.