Selective Isomer Formation and Crystallization-Directed Magnetic Behavior in Nitrogen-Confused C-Scorpionate...

The complex [Fe(^HL*)₂](OTf)₂, 1, where ^HL* = bis(3,5-dimethylpyrazol-1-yl)(3-1H-pyrazole)methane, was prepared in order to compare its magnetic properties with those of the analogous parent complex, [Fe(^HL)₂](OTf)₂, that lacks methyl groups on pyrazolyl rings and that undergoes spin crossover (SCO) from the low spin (LS) to the high spin (HS) form above room temperature. It was anticipated that this new semibulky derivative should favor the HS state and undergo SCO at a lower temperature range. During this study, six crystalline forms of 1 were prepared by controlling the crystallization conditions. Thus, when reagents are combined in CH₃CN, an equilibrium mixture of cis and trans isomers is established that favors the latter below 311 K. The trans isomer can be isolated exclusively as a mixture of solvates, LS trans-1·2CH₃CN and HS trans-1·4CH₃CN, by cooling CH₃CN solutions to -20 °C with the former being favored at high concentrations and short crystallization times. Subsequently, vapor diffusion of Et₂O into CH₃CN solutions of pure trans-1·2CH₃CN gives solvate-free HS trans-1. Subjecting trans-1·2CH₃CN to vacuum at room temperature gives microcrystalline trans-1·CH₃CN, identified by elemental analysis and its distinct powder X-ray diffraction pattern. If an isomeric mixture of 1 is subject to room-temperature vapor diffusion, then a crystalline mixture of HS isomers cis-1 and trans-1 is obtained. Finally, slowly cooling hot acetonitrile solutions of isomeric mixtures of 1 to room temperature gives large prisms of HS co-1, a species with both cis and trans isomers in the unit cell. The complexes trans-1, trans-1·CH₃CN, cis-1, and co-1 undergo SCO below 250 K while trans-1·xCH₃CN (x = 2, 4) solvates do not undergo SCO before desolvation.