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Anion−Solvent Dependence of Bistability in a Family of Meridional N-Donor-Ligand-Containing Iron(II) Spin Crossover Complexes

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journal contribution
posted on 15.10.2007, 00:00 by Ben A. Leita, Suzanne M. Neville, Gregory J. Halder, Boujemaa Moubaraki, Cameron J. Kepert, Jean-François Létard, Keith S. Murray
Five mononuclear spin crossover iron(II) bis-meridional ligand complexes of the general formula [Fe(L)2](X)2·solvent, have been synthesized, where X = BF4- or ClO4-; L = 2-(1-pyridin-2-ylmethyl-1H-pyrazol-3-yl)-pyrazine (picpzpz) or 2-(3-(2-pyridyl)pyrazol-1-ylmethyl)pyridine) (picpypz); solvent = MeOH or EtOH. The magnetic and structural consequences of systematic variation of meridional ligand, solvent, and anion, including a desolvated species, have been investigated. The complex [Fe(picpzpz)2](BF4)2·MeOH, 1·MeOH, displays several unique properties including a two-step spin transition with a gradual higher-temperature step (1T1/2 = 197 K) and an abrupt low-temperature step with hysteresis (2T1/2 = 91/98 K) and a metastable intermediate spin state below 70 K with quench-cooling. Removal of the solvent methanol results in the loss of the abrupt step and associated hysteresis (T1/2 = 150 K). The complexes [Fe(picpzpz)2](BF4)2·EtOH (1·EtOH), [Fe(picpzpz)2](ClO4)2·MeOH (2·MeOH), [Fe(picpzpz)2](ClO4)2·EtOH (2·EtOH), and [Fe(picpypz)2](BF4)2·MeOH (3·MeOH) all show gradual one-step spin transitions with T1/2 values in the range 210−250 K. Photomagnetic LIESST measurements on 1·MeOH reveal a near-quantitative excitation of high-spin sites and a unique two-step relaxation process related to the two-step thermal spin transition (1T(LIESST) = 49 K and 2T(LIESST) = 70 K). The structural consequences of the unusual spin transition displayed by 1·MeOH have been investigated by single-crystal X-ray diffraction structural analyses between 25 and 293 K. Detailed characterization of the unit cell parameter evolution vs temperature reflects both the gradual high-temperature step and abrupt low-temperature step, including the thermal hysteresis, observed magnetically.