Structural, Magnetic and Mössbauer Spectral Study of the Electronic Spin-state Transition in {Fe[HC(3-Mepz)2(5-Mepz)]2}(BF4)2

The complex {Fe[HC(3-Mepz)2(5-Mepz)]2}(BF4)2 (pz = pyrazolyl ring) has been prepared by the reaction of HC(3-Mepz)2(5-Mepz) with Fe(BF4)2·6H2O. The solid state structures obtained at 294 and 150 K show a distorted iron(II) octahedral N6 coordination environment with the largest deviations arising from the restrictions imposed by the chelate rings. At 294 K the complex is predominately high-spin with Fe–N bond distances averaging 2.14 Å, distances that are somewhat shorter than expected for a purely high-spin iron(II) complex because of the presence of an admixture of ca. 70 (I get 80 from both mag and X-ray, 3/15, where 3 is the subtraction of 2.14 and 2.17 and 15 1.99 and 2.14 and from Figure 3b) percent high-spin and 30 (20) percent low-spin iron(II). At 294 K the twisting of the pyrazolyl rings from the ideal C3v symmetry averages only 2.2o, a much smaller twist than has been observed previously in similar complexes. At 150 K the Fe–N bond distances average 1.99 Å, indicative of an almost fully low-spin iron(II) complex; the twist angle is only 1.3o, as expected for a complex with these Fe–N bond distances. The magnetic properties show that the complex undergoes a gradual change from low-spin iron(II) below 85 K to high-spin iron(II) at 400 K. The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin iron(II) complex but, upon further warming, the iron(II) begins to undergo spin-state relaxation on the Mössbauer time scale such that, at 155 and 315 K, the complex is 7.5 and 65 percent high-spin in the absence of any adjustment for the differing low-spin and high-spin recoil-free fractions. I would replace the previous sentence with the red. I see no reason to give the % from the Mössbauer in the abstract as it is likely a bit low as discussed in detail – neither the mag data nor X-ray data have the recoil issue. The last sentence in the abstract is the key information. OK The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin-iron(II) complex but, upon further warming above 85 K the iron(II) begins to undergo spin-state relaxation between the low- and high-spin forms on the Mössbauer time scale. At 155 and 315 K the complex exhibits spin-state relaxation rates of 0.36 and 7.38 MHz, respectively, and an Arrhenius plot of the logarithm of the relaxation rate yields an activation energy of 670 ± 40 cm–1 for the spin-state relaxation