Theoretical Investigation of the Electronic Structure of Fe(II) Complexes at Spin-State Transitions

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.

Characterizing the Solvated Structure of Photoexcited [Os(terpy)2]2+ with X-ray Transient Absorption Spectroscopy and DFT Calculations

“…Frontier Kohn-Sham orbitals were extracted from the converged wave function. In order to obtain accurate electronic energies of the GS and 3 MLCT states, the DFT optimizations were repeated using the B3LYP* [30]/ZORA-TZVP method, which provided accurate spin-state energetics for related transition metal complexes [31,32].…”

Computational approach to the study of thermal spin crossover phenomena

“…Nowadays, there is not a definitive DFT functional able to compute ?H HL with sufficient precision for different series of Fe(II) complexes and, in fact, this issue is currently debated in the literature. [45][46][47][48][49][50][51][52][53][54][55][56][57] Multiconfigurational wave function based methods, on the other hand, have proved to be capable of giving accurate values for the LS-HS energy difference but they are computationally much more expensive, particularly in the calculation of optimized geometries and vibrational frequencies. 26,28,29,51,52,[58][59][60][61][62] The aim of this work is to determine through calculations the key parameters of the thermal SCO process, i.e., the zeropoint corrected energy difference between the LS and HS states, ?H HL , the entropy change associated to the spin transition, ?S HL , and an estimation of the transition temperature, T 1/2 , for a set of Fe(II) compounds with ligands of different nature.…”

From Mononuclear to Dinuclear: Magnetic Properties of Transition Metal Complexes

“…Two conclusions were obtained with this analysis: (i) the wave function of the ground state does The good results obtained with the B3LYP* functional (see Section: Exchange-Correlation Functionals in Chapter 2) in the treatment of the excited states of the Fe(III) complexes discussed in the next chapter and the systems described in Ref. [62] made us come back to the [Fe(phen) 3 ] 2+ complex after the publication of the results reported in this chapter. Moreover, the increase of the computational power allowed us to redo the TD-DFT/PBE0 calculations with many more roots.…”

“…This is one of the reasons why more than twenty years after its discovery, LIESST e↵ect is still at the very center of the SCO research. The advances in short pulsed lasers enables researchers to extract information on the deactivation mechanism [15][16][17][18][19][20][21][22][23] and theoretical studies also contributed to the understanding of LIESST in octhedral Fe(II) complexes [60][61][62]. For Fe(III) complexes, the light irradiation induces ligand-tometal charge transfer transition (LMCT).…”

“…This functional has been used previously, giving good results for the geometrical parameters for related SCO complexes [62,160]. The molecular orbitals were expanded with the default triple-⇣ basis set with one polaritzation function (def2-TZVP) [161].…”