Spin Crossover in a Hexaamineiron(II) Complex: Experimental Confirmation of a Computational Prediction

Abstract

Single crystal structural analysis of [FeII(tame)2]Cl2⋅MeOH (tame=1,1,1‐tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high‐spin octahedral d 6 state and a low temperature low‐spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T 1/2=140 K. Single crystal, variable‐temperature optical spectroscopy of [FeII(tame)2]Cl2⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [FeII(tame)2]2+ during its de novo artificial evolution design as a spin‐crossover complex [Chem. Inf. Model. 2015, 55, 1844], offering the first experimental validation of a functional transition‐metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin‐passive structural components. A thermodynamic analysis based on an Ising‐like mean field model (Slichter–Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.

Temperature and Counterion Dependent Spin Crossover in a Hexaamineiron(II) Complex

“…This is supported by weak O−H…Br H−bonds (H…Br 2.75 Å; O…Br 3.30(2) Å) along the c axis. Given disorder of the MeOH over 6 symmetry related positions and the uncertainty in the position of the hydroxyl H−atom these assignments are not definitive, but appear to be correct and in contrast to our previous report [4] where the orientation of the MeOH was reversed.…”

“…This compound emerged as the most likely contender, from a series of hexaamineiron(II) compounds, to show SCO. This theoretical prediction proved to be correct and the compound [Fe(tame) 2 ]Cl 2 .MeOH showed a solid state, temperature‐dependent HS/LS interconversion [4] which validated a theoretical chemistry‐led approach to SCO compounds.…”

“…This results in two non–degenerate conformers of the [Fe(tame) 2 ] 2+ complex cation: D 3 symmetric rac (λλ which is enantiomeric with δδ) and S 6 symmetric meso (λδ which is identical to δλ). A recently published [4] DFT analysis of [Fe(tame) 2 ] 2+ and a much earlier molecular mechanics study of isoelectronic [Co(tame) 2 ] 3+ (low spin) [9] have concluded that the meso λδ conformer and the rac λλ/δδ conformer are close in energy. In fact the crystal structure of [Co(tame) 2 ] 3+ as its toluenesulfonate salt revealed both conformers within the same structure [10] .…”

“…Given the incomplete SCO in this case, no thermodynamic data associated with the spin transition were pursued through the so‐called Slichter‐Drickamer model [11] . By contrast at 60 K [Fe(tame) 2 Cl 2 .MeOH is >95 % LS on the basis of both temperature dependent crystallographic data and single crystal optical absorption spectroscopy [4] …”

“…The influence of counter ions on the coordinate bond lengths of [Fe(tame) 2 ] 2+ was considered previously with DFT calculations, [4] which indicated that the Fe–N bond lengths within the [Fe(tame) 2 ]Cl 2 .MeOH lattice were compressed by cation‐anion interactions that assist the HS→LS transition. The data in Figure 3 confirm that the HS Fe−N bond length in [Fe(tame) 2 ]Cl 2 .MeOH (ca.…”