A high-pressure iron K-edge X-ray absorption spectral study of the spin-state crossover in {Fe[HC(3,5-(CH3)2pz)3]2}I2 and {Fe[HC(3,5-(CH3)2pz)3]2}(BF4)2

The room temperature iron K-edge X-ray absorption near edge structure spectra of {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}I-2 and {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}(BF4)(2) have been measured between ambient and 88 and 94 kbar, respectively, in an opposed diamond anvil cell. The iron(II) in {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}I-2 undergoes the expected gradual spin-state crossover from the high-spin state to the low-spin state with increasing pressure. In contrast, the iron(II) in {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}(BF4)(2) remains high-spin between ambient and 78 kbar and is only transformed to the low-spin state at an applied pressure of between 78 and 94 kbar. No visible change is observed in the preedge peak in the spectra of {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}I-2 with increasing pressure, whereas the preedge peak in the spectra of {Fe[HC(3,5-(CH3)(2)pz)(3)](2)}(BF4)(2) changes as expected for a high-spin to low-spin crossover with increasing pressure. The difference in the spin-state crossover behavior of these two complexes is likely related to the unusual behavior of {Fe[HC(3,5-(CH3)(2)PZ)(3)](2)}(BF4)(2) upon cooling.