Polymorphism in Fe[(p-IC₆H₄)B(3-Mepz)₃]₂ (pz = Pyrazolyl):  Impact of Supramolecular Structure on an Iron(II) Electronic Spin-State Crossover

Abstract

The new ligands Na[(p-IC6H4)B(3-Rpz)3] (R = H, Me) have been prepared by converting I2C6H4 to IC6H4SiMe3 with Li(t)Bu and SiMe3Cl, and then to IC6H4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaO(t)Bu. 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.

The disentangling of hysteretic spin transition, polymorphism and metastability in bistable thin films formed by sublimation of bis(scorpionate) Fe(ii) molecules

“…39,40 Besides, the formation of novel crystal structures may result from the crystallization on a solid surface. 3 ] 2 produces a metastable surface--induced phase, which evolves toward the bulk phase by a thermally activated process.…”

para‐Bromophenyl[tris(pyrazolyl)]borate Complexes of Group 1 Metals, Thallium and Magnesium: Synthesis and Characterization of Transfer Agents for “Third‐Generation” Tp Ligands

“…The presence of the R group in RTp-containing complexes is known to extensively effect properties such a solubility, spin-state and solid-state structure. [14,15] In addition, RTp species have been employed in syntheses in which the [B-H] moiety is reactive. Furthermore, boronsubstituted ligands, in which R is suitably functionalized, provide a means to covalently attach other groups or whole molecules.…”

A tetragonal polymorph of bis[hydrotris(pyrazol-1-yl)borato]iron(II)

“…For iron(II) complexes with the Tp À derivative ligands, see : Janiak et al (2000); Reger et al (2005).…”

“…(2011). E67, m1033 [doi:10.1107 (Janiak et al, 2000;Reger et al, 2005). The crystal structure of the title compound has been reported previously (Oliver et al, 1980) which was measured at room temperature and crystallized in monoclinic space group of P2 1 /c (Z = 4).…”