Comparison of density functionals for the study of the high spin low spin gap in Fe(III) spin crossover complexes

Abstract

A detailed investigation of the accuracy of different quantum mechanical methods for the study of iron(III) spin crossover complexes is presented. The energy spin state gap between the high and low spin states; ΔE(HS‐LS) of nine iron(III) quinolylsalicylaldiminate complexes were calculated with nine different DFT functionals, then compared. DFT functionals: B3LYP, B3LYP‐D3, B3LYP*, BH&HLYP, BP86, OLYP, OPBE, M06L, and TPSSh were tested with six basis sets: 3‐21G*, dgdzvp, 6‐31G**, cc‐pVDZ, Def2TZVP, and cc‐pVTZ. The cations from the X‐ray crystal structures of [Fe(qsal‐OMe)2]Cl·MeCN·H2O, [Fe(qsal‐OMe)2]Cl·2MeOH·0.5H2O, [Fe(qsal‐OMe)2]BF4·MeOH, [Fe(qsal‐OMe)2]NCS·CH2Cl2, [Fe(qsal‐F)2]NCS, [Fe(qsal‐Cl)2]NCS·MeOH, [Fe(qsal‐Br)2]NCS·MeOH, [Fe(qsal‐I)2]OTf·MeOH, and [Fe(qsal)2]NCS⋅CH2Cl2 were used as starting structures. The results show that B3LYP, B3LYP‐D3, OLYP, and OPBE with a 6‐31G**, Def2TZVP, and cc‐pVTZ basis set give reasonable results of ΔE(HS‐LS) compared with the experimental data. The enthalpy of [Fe(qsal‐I)2]+ calculated with an OLYP functional and cc‐pVTZ basis set (1.48 kcal/mol) most closely matches the experimental data (1.34 kcal/mol). B3LYP* yields an enthalpy of 5.92 kcal/mol suggesting it may be unsuitable for these Fe(III) complexes, mirroring recent results by Kepp (Inorg. Chem., 2016, 55, 2717–2727).

Effect of Ligand Field Strength on the Spin Crossover Behaviour in 5‐X‐SalEen (X=Me, Br and OMe) Based Fe(III) Complexes

“…al . where they have explored the appropriate functionals and basis sets to accurately model spin crossover behaviour …”

Spin-Crossover from a Well-Behaved, Low-Cost meta-GGA Density Functional

“…It is not our purpose to survey the literature. For that, see refs − The last-mentioned of these is particularly relevant. It presented a database of 20 molecules in which SCO arises from a first-row transition metal.…”

Light-Induced Spin Crossover in an Intermediate-Spin Penta-Coordinated Iron(III) Complex

“…The geometry and spin-state energetics of the complex (PMe 3 ) 2 FeCl 3 were recently investigated 72 employing a series of density functionals, where all functionals except for BLYP and B3LYP provided good agreement of the calculated structural parameters with that reported in the crystal structure, 70 and the adiabatic energy differences between the intermediate-spin and high-spin state of the complex were found to be largely dependent on the choice of functionals, which is commonly observed in different systems. 73,74 As the present work aims to explore the excited-state curve-crossings at geometries far from equilibrium, the methyl groups of the complex are replaced by hydrogen atoms to reduce computational cost. This results in the model complex (PH 3 ) 2 FeCl 3 , which was optimized in its high-spin (sextet), intermediatespin (quartet), and low-spin (doublet) ground electronic states employing DFT/BP86 functional and def2-TZVP basis set.…”