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

A series of bis(polypyrazolylborato)iron(II) complexes (with Tp(3CF3), Tp(4Me), Tp(4Br), and pzTp(4Me)) has been prepared and characterised by Fe-57 Mossbauer spectroscopy. In addition, the Tp(3CF3) has been studied and compared with Tp(3CH3) 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(Tp(3CF3))(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) Angstrom that are slightly longer than typical distances for high-spin iron(II) complexes. A close comparison of the Tp(3CF3) and Tp(3CH3) has revealed small but nonetheless significant differences as evidenced from low-temperature Mossbauer studies and from X-ray absorption near-edge structure approach. At variance with Fe(Tp(3CHF3))(2), the high spin state for Fe(Tp(3CF3))(2) is to be taken as the most stable one. (C) 2001 Elsevier Science B.V. All rights reserved.