Assessment of electronic structure methods for the determination of the ground spin states of Fe(ii), Fe(iii) and Fe(iv) complexes

Abstract

Our ability to understand and simulate the reactions catalyzed by iron depends strongly on our ability to predict the relative energetics of spin states. In this work, we studied the electronic structures of Fe ion, gaseous FeO and 14 iron complexes using Kohn-Sham density functional theory with particular focus on determining the ground spin state of these species as well as the magnitudes of relevant spin-state energy splittings. The 14 iron complexes investigated in this work have hexacoordinate geometries of which seven are Fe(ii), five are Fe(iii) and two are Fe(iv) complexes. These are calculated using 20 exchange-correlation functionals. In particular, we use a local spin density approximation (LSDA) - GVWN5, four generalized gradient approximations (GGAs) - BLYP, PBE, OPBE and OLYP, two non-separable gradient approximations (NGAs) - GAM and N12, two meta-GGAs - M06-L and M11-L, a meta-NGA - MN15-L, five hybrid GGAs - B3LYP, B3LYP*, PBE0, B97-3 and SOGGA11-X, four hybrid meta-GGAs - M06, PW6B95, MPW1B95 and M08-SO and a hybrid meta-NGA - MN15. The density functional results are compared to reference data, which include experimental results as well as the results of diffusion Monte Carlo (DMC) calculations and ligand field theory estimates from the literature. For the Fe ion, all functionals except M11-L correctly predict the ground spin state to be quintet. However, quantitatively, most of the functionals are not close to the experimentally determined spin-state splitting energies. For FeO all functionals predict quintet to be the ground spin state. For the 14 iron complexes, the hybrid functionals B3LYP, MPW1B95 and MN15 correctly predict the ground spin state of 13 out of 14 complexes and PW6B95 gets all the 14 complexes right. The local functionals, OPBE, OLYP and M06-L, predict the correct ground spin state for 12 out of 14 complexes. Two of the tested functionals are not recommended to be used for this type of study, in particular M08-SO and M11-L, because M08-SO systematically overstabilizes the high spin state, and M11-L systematically overstabilizes the low spin state.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

“…The M06 functional has been shown to perform well for barrier calculations on a large data set of organic reactions . However, for transition‐metal complexes, the ordering of alternative multiplicities appears reliable with B3LYP …”

“…[23] However,f or transition-metal complexes, the ordering of alternative multiplicities appears reliable with B3LYP. [24] Optimization criteria were set so that forces on any atom will be below 0.0045 a.u. and the root mean square (rms) force across all atoms will be below 0.003 a.u.…”

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

“…Small frequencies below 100 cm À1 were raised to 100 cm À1 in order to avoid breakdown of the rigid rotor harmonic oscillator (RRHO) model. [29] Based on ar ecent benchmark study [30] we selected the B3LYP, [31] M06-L, [32] MN15 [33] and PW6B95 [34] functionals, which performed well for the determination of the spin ground states for iron complexes, in addition to the TPSS-D3(BJ) combination. Using the geometries obtained at the TPSS-D3(BJ)/def2-TZVP level of theory single point energies were computed using al ocally modified version [28] of Gaussian 09.…”

Performance of density functional theory for describing hetero‐metallic active‐site motifs for methane‐to‐methanol conversion in metal‐exchanged zeolites

“…The good performance of OLYP for predicting the ground spin states for these transition metal complexes is also not at all surprising, given existing literature experience. In a recent study of Fe(II), Fe(III), and Fe(IV) complexes, it was found that OLYP predicts the correct ground state in 12 out of 14 cases . Harding et al computed the enthalpy associated with conversion between the high‐ and low‐spin states of a Fe(III) spin crossover complex .…”