pubs.acs.org/IC Published on Web 09/10/2009 r 2009 American Chemical Society Inorg. Chem. 2009, 48, 9393–9401 9393 DOI: 10.1021/ic901259e Structural, Magnetic, and M :: ossbauer Spectral Study of the Electronic Spin-State Transition in [Fe{HC(3-Mepz) 2 (5-Mepz)} 2 ](BF 4 ) 2 Daniel L. Reger,* ,† J. Derek Elgin, †,‡ Elizabeth A. Foley, † Mark D. Smith, † Fernande Grandjean, § and Gary J. Long* , ) † Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, ‡ Department of Chemistry and Physics, Coastal Carolina University, Conway, South Carolina 29528, § Department of Physics, B5, University of Li  ege, B-4000 Sart-Tilman, Belgium, and ) Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010 Received June 30, 2009 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 3 6H 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°, 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°, 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 M :: ossbauer 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 M :: ossbauer 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. Introduction Spin-state crossover in iron(II) complexes that contain the FeN 6 coordination environment, first discovered in the 1960s, 1 is important because these complexes have potential applica- tions as optical elements in display devices, 1,2 as temperature/ pressure threshold indicators, 2 and as “intelligent” contrast agents for biomedical imaging. 3 Many of these types of iron(II) complexes are high-spin in the solid state at room temperature and show a cooperative spin-state crossover to the low-spin electronic state upon cooling. 1 Although a number of nitrogen based ligands have been used to prepare a wealth of complexes that show a spin-state crossover behavior, tris(pyrazolyl)borate complexes, which are of long-term interest to our research programs, were among the first iron(II) complexes studied; 4 their chemistry is still being developed. 5 More recently, we 6 and others 7 have shown that tris- (pyrazolyl)methane ligands also support unusual spin-state *To whom correspondence should be addressed. E-mail: reger@mail. chem.sc.edu. (1) (a) G :: utlich, P. In M :: ossbauer Spectroscopy Applied to Inorganic Chemistry; Long, G. J., Ed.; Plenum: New York, 1984; Vol. 1: p. 287. (b) G :: utlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. 1994, 33, 2024. (2) Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (3) Muller, R. N.; Van der Elst, L.; Laurent, S. J. Am. Chem. Soc. 2003, 125, 8405. (4) (a) Jesson, J. P.; Trofimenko, S.; Eaton, D. R. J. Am. Chem. Soc. 1967, 89, 3158. (b) Jesson, J. P.; Weiher, J. F.; Trofimenko, S. J. Chem. Phys. 1968, 48, 2058. (c) Long, G. J.; Grandjean, F.; Reger, D. L. In Spin Crossover in Transition Metal Compounds I;G :: utlich, P., Goodwin, H. A., Eds.; Springer: Berlin, 2004; p 91. (5) (a) Reger, D. L.; Gardinier, J. R.; Smith, M. D.; Shahin, A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. Inorg. Chem. 2005, 44, 1852. (b) Reger, D. L.; Gardinier, J. R.; Gemmill, W.; Smith, M. D.; Shahin, A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. J. Am. Chem. Soc. 2005, 127, 2303. (c) Reger, D. L.; Gardinier, J. R.; Elgin, J. D.; Smith, M. D.; Hautot, D.; Long, G. J.; Rebbouh, L.; Grandjean, F. Inorg. Chem. 2006, 45, 8862. (d) Reger, D. L.; Elgin, J. D.; Smith, M. D.; Grandjean, F.; Rebbouh, L.; Long, G. J. Polyhedron 2006, 25, 2616. (6) (a) Reger, D. L.; Little, C. A.; Rheingold, A. L.; Lam, M.; Concolino, T.; Mohan, A.; Long, G. J. Inorg. Chem. 2000, 39, 4674. (b) Reger, D. L.; Little, C. A.; Rheingold, A. L.; Lam, M.; Liable-Sands, L. M.; Rhagitan, B.; Concolino, T.; Mohan, A.; Long, G. J.; Briois, V.; Grandjean, F. Inorg. Chem. 2001, 40, 1508. (c) Reger, D. L.; Little, C. A.; Young, V. G., Jr.; Pink, M. Inorg. Chem. 2001, 40, 2870. (d) Reger, D. L.; Little, C. A.; Smith, M. D.; Long, G. J. Inorg. Chem. 2002, 41, 4453. (e) Reger, D. L.; Little, C. A.; Smith, M. D.; Rheingold, A. L.; Lam, K.-C.; Concolino, T. L.; Long, G. J.; Hermann, R. P.; Grandjean, F. Eur. J. Inorg. Chem. 2002, 1190. (f) Reger, D. L.; Little, C. A.; Semeniuc, R. F.; Smith, M. D. Inorg. Chim. Acta 2009, 363, 303. (g) Reger, D. L.; Elgin, J. D.; Smith, M. D.; Grandjean, F.; Rebbouh, L.; Long, G. J. Eur. J. Inorg. Chem. 2004, 3345.