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Reger DL, Elgin JD, Foley EA, Smith MD, Grandjean F, Long GJ: Structural, magnetic, and Mossbauer spectral study of the electronic spin-state transition in [Fe{HC (3-Mepz) 2 (5-Mepz)}2](BF4) 2. Inorg Chem. 2009 Oct 5;48(19):9393-401.
The complex [Fe{HC (3-Mepz)(2)(5-Mepz)}(2)](BF (4))(2) (pz = pyrazolyl ring) has been prepared by the reaction of HC (3-Mepz)(2)(5-Mepz) with Fe (BF (4))(2) x 6 H (2) O. The solid state structures obtained at 294 and 150 K show a distorted iron (II) octahedral N (6) 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 A, 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 degrees, a much smaller twist than has been observed previously in similar complexes. At 150 K the Fe-N bond distances average 1.99 A, indicative of an almost fully low-spin iron (II) complex; the twist angle is only 1.3 degrees, 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 Mossbauer 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 Mossbauer 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. |
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