Structural identification of the factors that prevent an electronic spin-state crossover in Fe[(C 6H 5)B(3-Mepz) 3] 2 (pz = pyrazolyl ring)

Abstract

The synthesis, structural, magnetic, and Mössbauer spectral properties of Fe[(C 6H 5)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[(C 6H 5)B(3-Mepz) 3] 2 ( 2) has a distorted octahedral iron(II) coordination environment with Fe–N bond distances that average ca. 2.18 Å, distances that clearly indicate the high-spin nature of the complex at these temperatures. Both the magnetic and Mössbauer spectral results indicate that Fe[(C 6H 5)B(3-Mepz) 3] 2 ( 2) remains high-spin down to 4 K. This result is surprising because the closely related Fe[( p-R-C 6H 4)B(3-Mepz) 3] 2 [R = I, C C H, C C SiMe 3, or C C C 6H 5] complexes do exhibit electronic spin-state changes on cooling. The absence of a spin-state crossover in Fe[(C 6H 5)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.