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 about 80% high-spin and 20% low-spin iron(II). At 294 K the twisting of the pyrazolyl rings from the ideal C_3v symmetry averages only 2.2°, 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 1.3°, 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 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.