Quantitative Assessment of rPM6 for Fluorine- and Chlorine-Containing Metal Complexes: Comparison with Experimental, First-Principles, and Other Semiempirical Results

Abstract

We report a reparameterization of PM6 parameters for fluorine and chlorine using our training set containing transition metal complexes. Spin unrestricted calculations with the resulting rPM6 (UrPM6) were examined quantitatively using two test sets: (i) the description of magnetic interactions in 25 dinuclear metal complexes and (ii) the prediction of barrier heights and reaction energies for epoxidation and fluorination reactions catalyzed by high-valent manganese-oxo species. The conventional UPM6 and UPM7 methods were also evaluated for comparison on the basis of either experimental or computational (the UB3LYP/SVP level) outcomes. The merits of UrPM6 are highlighted by both the test sets. As regards magnetic exchange coupling constants, the UrPM6 method had the smallest mean absolute errors from the experimental data (19 cm^-1), followed by UPM7 (119 cm^-1) and UPM6 (373 cm^-1). For the epoxidation and fluorination reactions, all of the transition state searches were successful using UrPM6, while the success rates obtained by UPM6 and UPM7 were only 50%. The UrPM6-optimized stationary points also agreed well with the reference UB3LYP-optimized geometries. The accuracy for estimating reaction energies was also greatly remedied.

Keywords: magnetic exchange coupling; oxidation reaction; semiempirical method.

Scheme 1

(a) Ethylene epoxidation catalyzed by ³[Cl–Mn^V=O(salen)] and (b) toluene fluorination catalyzed by ³[F–Mn^V=O(salen)]. The superscripts 3 and 5 on the left side denote triplet and quintet spin states, respectively.

Figure 1

Twenty-five dinuclear manganese and iron complexes (for details, see also Tables S1 and S2).

Figure 2

Potential energy profile with zero-point energy corrections (in kcal/mol) relative to the isolated ³[Cl–Mn^V=O(salen)] complex and ethylene (³R_a) obtained at the UB3LYP/SVP, UPM6, UrPM6, and UPM7 levels of theory. The superscript 3 on the left side denotes triplet spin state, and N.A. represents not available (Geometry optimizations failed).

Figure 3

Selected optimized structural parameters of critical points in the epoxidation reaction of ethylene catalyzed by ³[Cl–Mn^V=O(salen)], with bond lengths (in Å) obtained at the UB3LYP/SVP, (UPM6), {UrPM6}, and [UPM7] levels of theory, and with heavy-atom root-mean-squared deviation (RMSD) values from the reference UB3LYP-optimized structures. N.A. represents not available (The optimization failed to converge).

Figure 4

Potential energy profile with zero-point energy corrections (in kcal/mol) relative to the isolated ³[F–Mn^V=O(salen)] complex and toluene (³R_b) obtained at the UB3LYP/SVP, UPM6, UrPM6, and UPM7 levels of theory. The superscripts 3 and 5 on the left side denote triplet and quintet spin states, N.A. represents not available (Geometry optimization failed), and ISC is the acronym of intersystem crossing.

Figure 5

Selected optimized structural parameters of critical points in the epoxidation reaction of ethylene catalyzed by ³[F–Mn^V=O(salen)], with bond lengths (in Å) and angles (in deg.) obtained at the UB3LYP/SVP, UPM6, UrPM6, and UPM7 levels of theory, and with heavy-atom root-mean-squared deviation (RMSD) values from the reference UB3LYP-optimized structures. Only ⁵[F–Mn^IV(OH)(salen)] is shown for ⁵I_b. N.A. represents not available (The optimization failed to converge).