A Study of the Electronic Spin‐State Crossover in {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2

The synthesis, structural, magnetic, and Mössbauer spectroscopic properties of {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 are reported. The single‐crystal X‐ray structure results indicate that at 150 K {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 has a structure which is very similar to that observed at 220 K for the trigonally distorted octahedral, high‐spin {Fe[HC(3,5‐Me 2 pz) 3 ] 2 }(BF 4 ) 2 complex. Both the magnetic and Mössbauer spectroscopic results indicate that {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 is high spin between 160 and 296 K. Upon cooling, {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 exhibits a complete electronic spin‐state crossover from the high‐spin to the low‐spin state at approximately 110 K and remains completely low spin down to 4.2 K; upon subsequent warming from 4.2 K, the transition from the low‐spin to the high‐spin state occurs at 148 to 150 K. {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 exhibits a rather large thermal hysteresis of 38 K in its spin‐state crossover. Thus, {Fe[HC(3,4,5‐Me 3 pz) 3 ] 2 }(BF 4 ) 2 behaves differently from both {Fe[HC(3,5‐Me 2 pz) 3 ] 2 }(BF 4 ) 2 , which is known to show a unique spin‐state crossover of one‐half of its iron( II ) ions associated with a phase transition, and Fe[HB(3,4,5‐Me 3 pz) 3 ] 2 , which is known to remain high‐spin even upon cooling to 1.7 K. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)