Abstract
The electronic structure relevant to low spin (LS)↔high spin (HS) transitions in Fe(II) coordination compounds with a FeN6 core are studied. The selected [Fe(tz)6]2+ (1) (tz = 1H-tetrazole), [Fe(bipy)3]2+ (2) (bipy = 2,2′-bipyridine), and [Fe(terpy)2]2+ (3) (terpy = 2,2′:6′,2″-terpyridine) complexes have been actively studied experimentally, and with their respective mono-, bi-, and tridentate ligands, they constitute a comprehensive set for theoretical case studies. The methods in this work include density functional theory (DFT), time-dependent DFT (TD-DFT), and multiconfigurational second order perturbation theory (CASPT2). We determine the structural parameters as well as the energy splitting of the LS–HS states (ΔEHL) applying the above methods and comparing their performance. We also determine the potential energy curves representing the ground and low-energy excited singlet, triplet, and quintet d6 states along the mode(s) that connect the LS and HS states. The results indicate that while DFT is well suited for the prediction of structural parameters, an accurate multiconfigurational approach is essential for the quantitative determination of ΔEHL. In addition, a good qualitative agreement is found between the TD-DFT and CASPT2 potential energy curves. Although the TD-DFT results might differ in some respect (in our case, we found a discrepancy at the triplet states), our results suggest that this approach, with due care, is very promising as an alternative for the very expensive CASPT2 method. Finally, the two-dimensional (2D) potential energy surfaces above the plane spanned by the two relevant configuration coordinates in [Fe(terpy)2]2+ were computed at both the DFT and CASPT2 levels. These 2D surfaces indicate that the singlet–triplet and triplet–quintet states are separated along different coordinates, i.e., different vibration modes. Our results confirm that in contrast to the case of complexes with mono- and bidentate ligands, the singlet–quintet transitions in [Fe(terpy)2]2+ cannot be described using a single configuration coordinate.
Highlights
Switchable transition metal complexes are well-known candidates for high-density magnetic storage and other molecular devices.[1]
Our results indicate that in general, both density functional theory (DFT) and CASPT2 methods are suitable for the accurate estimation of ΔrHL (0.19−0.24 Å and ca. 0.2 Å calculated ΔrHL values were obtained for DFT and CASPT2, Article respectively, in agreement with experiments)
While hybrid density functionals (B3LYP and B3LYP*) and the CASPT2 method give reliable estimates to ΔrHL, they respectively overestimate and underestimate both rLS and rHS, compared to the experimental values. The former effect is due to the fact that the inclusion of exact (Hartree−Fock) exchange slightly weakens the Fe−N bond, to the HF method itself,13a while the latter one is probably due to the presence of basis set superposition error (BSSE).[39,59]
Summary
Switchable transition metal complexes are well-known candidates for high-density magnetic storage and other molecular devices.[1]. At the CASPT2 level in order to investigate the LIESST mechanism.17c,32,33 the breakdown of the single configuration mode of the LIESST model in [Fe(terpy)2]2+ has been suggested by a DFT study.13b This means that the breathing mode alone is insufficient for the adequate description of the system: a second coordinate corresponding to a bending mode of the terpyridine rings is required All these computational results are of high relevance, a systematic, density functional and multiconfigurational study on the same systems is necessary to gain new insights into the electronic structure as well as to better understand the performance of the available computational methods.
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