Spin-crossover behavior of bis[dihydrobis(4-methylpyrazol-1-yl-borate)]-(2,2'-bipyridine)iron and analogous complexes in the bulk and in thin films: Elucidating the influence of π-π-interactions on the type of spin transition

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

Crystal structure of bis(5-bromo-1,10-phenanthroline-κ2 N,N')bis-[di-hydro-bis-(pyrazol-1-yl)borato-κ2 N 2,N 2']iron(II) toluene disolvate

The structure determination of the title compound was undertaken as part of a project on the modification and synthesis of new spin-crossover (SCO) compounds based on octa-hedral FeII bis-(pyrazol-yl)borate complexes. In the course of these investigations, the compound [Fe(C6H8BN4)2(C12H7BrN2)] was synthesized, for which magnetic measurements revealed an incomplete spin-crossover behaviour. Crystallization of this compound from toluene led to the formation of crystals of the toluene disolvate, [Fe(C6H8N4B)2(C12H7N2Br)]·2C7H8. Its asymmetric unit comprises two discrete metal complex mol-ecules and two toluene solvent mol-ecules. One of the latter is severely disordered and its contribution to the diffracted intensities was removed using the SQUEEZE routine [Spek (2015 ▸). Acta Cryst. C71, 9-18]. In each complex mol-ecule, the FeII cation is coordinated by the two N atoms of a 5-bromo-1,10-phenanthroline ligand and by two pairs of N atoms of chelating di-hydro-bis(pyrazol-1-yl)borate ligands in the form of a slightly distorted octa-hedron. The discrete complexes are arranged in columns along the a-axis direction with the toluene solvate mol-ecules located between the columns. The 5-bromo-1,10-phenanthroline ligands of neighbouring columns are approximately parallel and are slightly shifted relative to each other, indicating π-π inter-actions.

Crystal structure of bis-{(3,5-di-methyl-pyrazol-1-yl)di-hydro-[3-(pyridin-2-yl)pyrazol-1-yl]-borato}iron(II)

The structure determination of [Fe(C13H15BN5)2] was undertaken as part of a project on the modification of the recently published spin-crossover (SCO) complex [Fe{H2B(pz)(pypz)}2] (pz = pyrazole, pypz = pyridyl-pyrazole). To this end, a new ligand was synthesized in which two additional methyl groups are present. Its reaction with iron tri-fluoro-methane-sulfonate led to a pure sample of the title compound, as proven by X-ray powder diffraction. The asymmetric unit consists of one complex mol-ecule in a general position. The FeII atom is coordinated by two tridentate N-binding {H2B(3,5-(CH3)2-pz)(pypz)}- ligands. The Fe-N bond lengths range between 2.1222 (13) and 2.3255 (15) Å, compatible with FeII in the high-spin state, which was also confirmed by magnetic measurements. Other than a very weak C-H⋯N non-classical hydrogen bond linking individual mol-ecules into rows extending parallel to [010], there are no remarkable inter-molecular inter-actions.

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity

Two polymorphic modifications (1-I and 1-II) of the new spin crossover (SCO) complex [Fe{H₂B(pz)(pypz)}₂] (pz = pyrazole, pypz = pyridylpyrazole; 1) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T_1/2 values of 390 K (1-II) and 270 K (1-I). An analysis of the crystal structures of 1-II and the corresponding Zn(II) (2) and Co(II) (3) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I. In agreement with these findings, investigations of thin films of 1, where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I. These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.