Study of the High-temperature Spin-State Crossover in the Iron(II) Pyrazolylborate Complex Fe[HB(pz)₃]₂

A study of the infrared and Mössbauer spectra and magnetic properties of Fe[HB(pz) ₃ ] ₂ indicate that this nominally low-spin iron(II) compound undergoes a spin-state crossover to the high-spin state accompanied by a crystallographic phase change at about 400 K. The crystallographic phase transition shatters the crystals and leads to a large hysteresis in the magnetic moment upon cooling from 460 to about 250 K. The analyses of the Mössbauer spectra, obtained as a function of temperature during both heating and cooling, indicate the presence of spin-state relaxation on the Mössbauer time scale of 10 ^-8 s. The activation energy for this relaxation process is 7300 cm ^-1 for freshly sublimed Fe[HB(pz) ₃ ] ₂ and 1760 cm ^-1 after the crystals have undergone the phase transition. Both the magnetic moments and Mössbauer spectra indicate that between 295 and 430 K the ground state of Fe[HB(pz) ₃ ] ₂ has a temperature-dependent population of both the high-spin and low-spin electronic configurations. The optical absorption spectrum provides further support for the spin-state crossover and DSC and volume expansion studies indicate the presence of the phase transition at the spin-crossover. Analysis of the far-infrared spectrum utilizing ⁵⁴ Fe/ ⁵⁷ Fe substitution at both room and high temperatures allows unambiguous assignment of the Fe-N stretching bands both in high-spin and low-spin forms. The low-spin Fe-N stretching bands appear at 459, 434.5, and 399.5 cm ^-1 , whereas upon heating the high-spin Fe-N stretching bands appear at 257.5 and 223 cm ^-1 . Measurement of the area of these bands as the temperature is increased reflects the change in the population of the low-spin and high-spin states.