Accurate calculation of spin-state energy gaps in Fe(III) spin-crossover systems using density functional methods

Quantification of the thermodynamic effects of the low-spin - high-spin interaction in molecular crystals of a mononuclear iron(II) spin crossover complex

A method is proposed to estimate the energetic and entropic effects of spins of neighbouring molecules on the spin transition of a mononuclear spin crossover (SCO) complex in a molecular crystal. Density functional theory (DFT) methods have been used to model the SCO material [FeII(Lnpdtz)2(NCS)2] (Lnpdtz = 2-naphthyl-5-pyridyl-1,2,4-thiadiazole) exhibiting numerous π-π interactions using a 2D arrangement of 15 molecules. The modelling considers only the effects in the crystallographical ac plane with a particularly pronounced stacking but paves the way for future work with 3D arrangements which are computational much more costly. It involves the optimisation and normal mode calculation of the molecules in a rigid matrix of both low-spin (LS) and high-spin (HS) neighbours. This procedure has been used to calculate the previously defined cooperativity parameter Hcoop (S. Rackwitz, W. Klopper, V. Schünemann and J. A. Wolny, Phys. Chem. Chem. Phys., 2013, 15, 15450). For [FeII(Lnpdtz)2(NCS)] we obtain Hcoop = 11 kJ mol-1, a value which is comparable to those found for 3D polynuclear spin crossover materials. A normal mode analysis of the optimised centrally located molecule indicates that the vibrational entropy of the spin transition is somewhat higher (5 J K-1 mol-1) for the LS to HS transition in the LS matrix than in the HS one. The calculations show that the interactions with the neighbours influence the low-frequency modes with wave numbers <65-70 cm-1. These cause the main difference in the vibrational entropy of the spin transition for the vicinity of high- and low-spin molecules. Furthermore, a deformation of the coordination sphere of the central molecule is observed when the spins of the surrounding centres are switched. This deformation is accompanied by a change in the equatorial Fe-N bond lengths.

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction

Calculation of transition temperatures T1/2 for thermally driven spin-crossover in condensed phases is challenging, even with sophisticated state-of-the-art density functional approximations. The first issue is the accuracy of the adiabatic crossover energy difference ΔEHL between the low- and high-spin states of the bistable metal-organic complexes. The other is the proper inclusion of entropic contributions to the Gibbs free energy from the electronic and vibrational degrees of freedom. We discuss the effects of treatments of both contributions upon the calculation of thermochemical properties for a set of 20 spin-crossover materials using a Hubbard-U correction obtained from a reference ensemble spin-state. The U values obtained from a simplest bimolecular representation may overcorrect, somewhat, the ΔEHL values, hence giving somewhat excessive reduction of the T1/2 results with respect to their U = 0 values in the crystalline phase. We discuss the origins of the discrepancies by analyzing different sources of uncertainties. By use of a first-coordination-sphere approximation and the assumption that vibrational contributions from the outermost atoms in a metal-organic complex are similar in both low- and high-spin states, we achieve T1/2 results with the low-cost, widely used PBE generalized gradient density functional approximation comparable to those from the more costly, more sophisticated r2SCAN meta-generalized gradient approximation. The procedure is promising for use in high-throughput materials screening, because it combines rather low computational effort requirements with freedom from user manipulation of parameters.

First crystal structure of an Fe(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with Li+: synthesis, features of magnetic behavior and theoretical analysis

The iron(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with the lithium cation Li[Fe^III(thpy)₂]·3H₂O (1) has been synthesized and characterized by FTIR spectroscopy, powder and single crystal X-ray diffraction, direct current magnetic susceptibility measurements, and ⁵⁷Fe Mössbauer spectroscopy. Moreover, the molecular structure of the [Fe(thpy)₂]^- anion has been determined for the first time. The [Fe(thpy)₂]^- units in the triclinic P1̄ lattice of 1 are assembled into layers parallel to the bc plane. The Li⁺ cations and water molecules are located between the layers and the structure is stabilized by hydrogen bonding. The [Fe(thpy)₂]^- anions form interconnected dimer pairs through hydrogen bonds and short contacts with Fe⋯Fe separation of 6.7861(4) Å. According to dc magnetic measurements, compound 1 demonstrates an incipient spin-crossover transition from the LS (S = 1/2) to the HS (S = 5/2) state above 250 K. The Bleaney-Bowers equation for a model of an isolated LS dimer with a mean-field correction was applied to fit the experimental data of magnetic susceptibility dependence on temperature in the temperature range of 2-250 K. The intra-dimer J₁ = -1.79(1) K and inter-dimer J₂ = -0.24(3) K antiferromagnetic coupling constants were defined. The analysis of the ⁵⁷Fe Mössbauer spectra at 80 K and 296 K confirms the presence of the shortened distances between the iron nuclei. Moreover, the influence of the lithium cation on the stabilization of the LS state was shown for the [Fe(thpy)₂]^- anion. BS-DFT calculations for the optimized structure of two isolated [Fe(thpy)₂]^- anions also correctly predict a weak exchange J₁(calc) = -0.92 K. DFT calculations revealed the OPBE (GGA-type) functional that correctly predicts the spin-crossover transition for the iron(III) thpy compounds. Besides, the effect of the N₂O₄, N₂S₂O₂, and N₂Se₂O₂ coordination environments on the energy stabilization of the LS state of iron(III) anionic thpy complexes was noted as well.

Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes

We have explored the impact of electron-donating (methoxy) and electron-withdrawing (nitro) substituents on SalEen ligand based spin crossover (SCO) behavior of Fe(III) complexes. Thus, 3-X-substituted SalEen ligands were employed to prepare [Fe(3-X-SalEen)₂]·NCSe, where X = OMe (1), H (2), and NO₂ (3) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and N-ethylethylenediamine). The characteristic spin transition temperature (T _1/2) is shown to shift to a lower temperature when an electron-donating substituent (OMe) is used and to a higher temperature when an electron-withdrawing substituent (NO₂) is used. We used experimental and theoretical methods to determine the reasons for this behavior. The solid-state magnetic data revealed the transition temperatures for complexes 1, 2, and 3 to be 219, 251, and 366 K, respectively. The solution-state magnetic data also support this trend in T _1/2 values. UV-vis spectra analysis indicates that there is greater delocalization in the π-manifold of the ligand when the nitro group is the substituent. Theoretical studies through density functional theory methods suggest the methoxy substituent decreases the energy gap between the t_2g and e_g orbitals (explaining the lower T _1/2 value), while the nitro substituent increases the energy gap between the t_2g and e_g orbitals and thus increases the T _1/2 value.