Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl):: Impact of supramolecular structure on an iron(II) electronic spin-state crossover

The new ligands Na[(p-IC6H4)B(3-RPZ)(3)] (R = H, Me) have been prepared by converting I2C6H4 to IC6H4SiMe3 with (LiBu)-Bu-t and SiMe3Cl, and then to IC6H4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of (NaOBu)-Bu-t. These new ligands react with FeBr2 to give either purple, low-spin Fe[(p-IC6H4)B(PZ)(3)](2) or colorless, high-spin Fe[(p-IC6H4)B(3-Mepz)(3)](2). Depending upon the crystallization conditions, Fe[(p-IC6H4)B(3-Mepz)(3)](2) can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by variable-temperature X-ray crystallography, magnetic susceptibility, and Mossbauer spectroscopy has revealed different electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing, independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC6H4)B(3-Mepz)(3)](2) has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC6H4)B(3-MePZ)(3)](2) has a stacked two-dimensional supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure of the methylene chloride solvate of Fe[(p-IC6H4)B(3-Mepz)(3)](2) has a similar stacked two-dimensional supramolecular structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the long-range solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions, has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spin-crossover temperatures and more abrupt spin-crossover behavior.