Multiscale Experimental and Theoretical Investigations of Spin Crossover FeII Complexes: Examples of [Fe(phen)2(NCS)2] and [Fe(PM-BiA)2(NCS)2]

Abstract

For spin crossover (SCO) complexes, computation results are reported and confirmed with experiments at multiscale levels of the isolated molecule and extended solid on the one hand and theory on the other hand. The SCO phenomenon which characterizes organometallics based on divalent iron in an octahedral FeN6-like environment with high spin (HS) and low spin (LS) states involves the LS/HS switching at the cost of small energies provided by temperature, pressure or light, the latter connected with Light-Induced Excited Spin-State Trapping (LIESST) process. Characteristic infra red (IR) and Raman vibration frequencies are computed within density functional theory (DFT) framework. In [Fe(phen)2(NCS)2] a connection of selected frequencies is established with an ultra-fast light-induced LS → HS photoswitching mechanism. In the extended solid, density of state DOS and electron localization function (ELF) are established for both LS and HS forms, leading to characterizion of the compound as an insulator in both spin states with larger gaps for LS configuration, while keeping molecular features in the solid. In [Fe(PM-BiA)2(NCS)2], by combining DFT and classical molecular dynamics, the properties and the domains of existence of the different phases are obtained by expressing the potential energy surfaces in a short range potential for Fe–N interactions. Applying such Fe–N potentials inserted in a classical force field and carrying out molecular dynamics (MD) in so-called “semi-classical MD” calculations, lead to the relative energies of HS/LS configurations of the crystal and to the assessment of the experimental (P, T) phase diagram.

Iron(II) Spin Crossover (SCO) Materials Based on Dipyridyl-N-Alkylamine

“…However, in both cases, these values are in agreement with the expected value for a magnetically isolated Fe(II) ion in the HS state [17][18][19][20] with g factors of 2.12 and 2.15, respectively. Upon cooling, the χ m T value decreased gradually, in both cases, until approximately 260 K and then sharply decreased reaching a value of 0.02 cm 3 •K•mol −1 at 2 K, indicating the presence of a complete and gradual HS to LS transition which was accompanied, as expected, by a change of color observed for each single crystal (See Figure 4: orange to red for 2, yellow to red for 2').…”

“…The thermal dependences of the products of the molar magnetic susceptibility and the temperature (χmT) are shown in Figure 3 for complex 1 and in Figure 4 for the two polymorph complexes (2 and 2'). For compound 1, the χmT product of 3.205 cm 3 •K•mol −1 at 300 K, slightly higher than the spin only value calculated for an isolated metal ion with S = 2 (3.0 emu•K•mol −1 ), agrees well with the expected value for a magnetically isolated Fe(II) ion in the HS state (S = 2) ( Figure 3) [17][18][19][20]. Upon cooling, the χmT value decreased gradually until approximately 250 K and then sharply decreased, reaching a value of 0.024 cm 3 •K•mol −1 at 2 K, indicating the presence of a complete and gradual HS to LS transition at T 1/2 = 229 K, as also revealed by the thermoschromism (yellow at 296 K and red at 150 K) observed on single crystals (see Figure 3).…”

“…However, iron(II)-based SCO complexes, for which the transition takes place between the paramagnetic high-spin (HS) state (t 4 2g e 2 g , 5 T 2g , S = 2) and the diamagnetic low-spin (LS) state (t 6 2g e 0 g , 1 A 1g , S = 0) are, by far, the most studied switchable molecular materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. From the synthetic point of view, one of the relevant strategies to design original SCO systems is based on the use of appropriate polydentate rigid nitrogen-based ligands and simple anionic entities acting as terminal ligands, such as NCX (X = S, Se, BH 3 ) anions [16][17][18][19][20][21][22] or the more sophisticated ones such as cyanocarbanions exhibiting terminal or poly-bridging coordination modes [4,7,[23][24][25][26][27]. The latter are able to tune the ligand field energy and some SCO characteristics such as the transition temperature.…”

The Spin‐Crossover Phenomenon at the Coherent‐Domains Scale in 1D Polymeric Powders: Evidence for Structural Fatigability

“… Schematic overview of the different physical scales addressed by spin crossover within a molecular crystal including the associated main impacted features and the timescale of the phenomenon. Compiled and adapted from ref 2b,3,5. The results presented herein concern the coherent‐domain physical scale.…”

Chelating Mechanisms of Transition Metals by Bacterial Metallophores “Pseudopaline and Staphylopine”: A Quantum Chemical Assessment

“…Lastly, one should complementarily carry out multi-scale computations [33] along with further experimental findings, such as establishing crystal information files (CIFs) and more.…”