Structural identification of the factors that prevent an electronic spin-state crossover in Fe[(C6H5)B(3-Mepz)3]2 (pz = pyrazolyl ring)

Article abstract of DOI:10.1016/j.poly.2006.03.014

The synthesis, structural, magnetic, and M?ssbauer spectral properties of Fe[(C₆H₅)B(3-Mepz)₃]₂ (2, pz = pyrazolyl ring) are reported. The single crystal X-ray structural results indicate that at both 294 and 90 K Fe[(C₆H₅)B(3-Mepz)₃]₂ (2) has a distorted octahedral iron(II) coordination environment with Fe-N bond distances that average ca. 2.18 ?, distances that clearly indicate the high-spin nature of the complex at these temperatures. Both the magnetic and M?ssbauer spectral results indicate that Fe[(C₆H₅)B(3-Mepz)₃]₂ (2) remains high-spin down to 4 K. This result is surprising because the closely related Fe[(p-R-C₆H₄)B(3-Mepz)₃]₂ [R = I, {single bond}C{triple bond, long}C{single bond}H, {single bond}C{triple bond, long}C{single bond}SiMe₃, or {single bond}C{triple bond, long}C{single bond}C₆H₅] complexes do exhibit electronic spin-state changes on cooling. The absence of a spin-state crossover in Fe[(C₆H₅)B(3-Mepz)₃]₂ (2) is the result of its molecular structure in which the three pyrazolyl rings of each tridentate ligand are highly twisted away from an ideal C_3v coordination symmetry, a twisting distortion that favors the high-spin form of the complex with longer Fe-N bond distances.