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

The synthesis, structural, magnetic, and Mössbauer 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 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[(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, CCH, CCSiMe3, or CCC6H5] 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 C3v coordination symmetry, a twisting distortion that favors the high-spin form of the complex with longer Fe–N bond distances.

Graphical abstractIn contrast to other Fe[(p-R-C6H4)-B(3-Mepz)3]2 (R = I, CCH, CCSiMe3, or CCC6H5) derivatives, magnetic, Mössbauer spectral, and X-ray crystallographic studies clearly demonstrate that Fe[(C6H5)B(3-Mepz)3]2 remains high spin down to 4 K. Analysis of the data indicate that the degree of twisting of the pyrazolyl rings from ideal C3v coordination symmetry (see figure) explains the lack of the spin-state crossover for Fe[(C6H5)B(3-Mepz)3]2.Figure optionsDownload full-size imageDownload as PowerPoint slide