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)<inf>2</inf>]Cl·MeCN·H<inf>2</inf>O, [Fe(qsal-OMe)<inf>2</inf>]Cl·2MeOH·0.5H<inf>2</inf>O, [Fe(qsal-OMe)<inf>2</inf>]BF<inf>4</inf>·MeOH, [Fe(qsal-OMe)<inf>2</inf>]NCS·CH<inf>2</inf>Cl<inf>2</inf>, [Fe(qsal-F)<inf>2</inf>]NCS, [Fe(qsal-Cl)<inf>2</inf>]NCS·MeOH, [Fe(qsal-Br)<inf>2</inf>]NCS·MeOH, [Fe(qsal-I)<inf>2</inf>]OTf·MeOH, and [Fe(qsal)<inf>2</inf>]NCS⋅CH<inf>2</inf>Cl<inf>2</inf> 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)<inf>2</inf>]⁺ 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).