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

Abstract A series of bis(polypyrazolylborato)iron(II) complexes (with Tp 3CF 3 , Tp 4Me , Tp 4Br , and pzTp 4Me ) has been prepared and characterised by 57 Fe Mossbauer spectroscopy. In addition, the Tp 3CF 3 has been studied and compared with Tp 3CH 3 by X-ray absorption investigation. The X-ray crystal structure of iron(II)bis(3-trifluoromethyl-1 H -pyrazol-1-yl)borato has been resolved. In Fe(Tp 3CF 3 ) 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) A that are slightly longer than typical distances for high-spin iron(II) complexes. A close comparison of the Tp 3CF 3 and Tp 3CH 3 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 3CHF 3 ) 2 , the high spin state for Fe(Tp 3CF 3 ) 2 is to be taken as the most stable one.