The effect of the 3-trifluoromethyl substituent in polypyrazolylborato complexes on the iron(II) spin state; X-ray diffraction and absorption and Mössbauer studies

A series of bis(polypyrazolylborato)iron(II) complexes (with Tp3CF3, Tp4Me, Tp4Br, and pzTp4Me) has been prepared and characterised by 57Fe Mössbauer spectroscopy. In addition, the Tp3CF3 has been studied and compared with Tp3CH3 by X-ray absorption investigation. The X-ray crystal structure of iron(II)bis(3-trifluoromethyl-1H-pyrazol-1-yl)borato has been resolved. In Fe(Tp3CF3)2 the coordination geometry around the Fe centre is distorted octahedral, with the FeN bond distances in the range 2.219(3)–2.251(3) Å that are slightly longer than typical distances for high-spin iron(II) complexes. A close comparison of the Tp3CF3 and Tp3CH3 has revealed small but nonetheless significant differences as evidenced from low-temperature Mössbauer studies and from X-ray absorption near-edge structure approach. At variance with Fe(Tp3CHF3)2, the high spin state for Fe(Tp3CF3)2 is to be taken as the most stable one.