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 I2C6H 4 to IC6H4SiMe3 with Li tBu and SiMe3Cl, and then to IC6H 4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaOtBu. 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-IC6H 4)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 Mössbauer 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-IC6H 4)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-IC 6H4)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-IC 6H4)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.




National Science Foundation of Belgium
Ministere de la Region Wallonne
National Science Foundation (U.S.)
Petroleum Research Fund

Keywords and Phrases

Boron; Carbon; Dichloromethane; Hydrogen; Iron; Iron Derivative; Ligand; Poly(Pyrazolyl)borate Iron; Pyrazine Derivative; Pyrazolyl; Unclassified Drug; Chemical Reaction; Chemical Structure; Crystal Structure; Crystallization; Magnetism; Molecular Interaction; Mössbauer Spectroscopy; Nuclear Magnetic Resonance; Solid State; Synthesis; Temperature; X Ray Crystallography

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Article - Journal

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© 2005 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Mar 2005