rPM6 Parameters for Manganese and Application to Transition State Search for Oxidation Reactions of Cyclohexene by Manganese(IV)-Oxo Species

Abstract

We report a fast, easy-to-handle, and accurate semiempirical quantum mechanical method to find a transition state (TS) for Mn-catalyzed oxidation reactions. Following our previous work, the reparameterization of PM6 (rPM6) for Mn is performed based on our training set. The spin-unrestricted calculations with the resultant parameter sets (UrPM6) successfully provide acceptable TS structures similar to the UDFT-optimized geometries at much lower computational cost. Our method will lead to substantial savings of computational time spent in the TS search for transition-metal-containing species.Oxidation reactions are of great importance in medicinal and industrial fields. Extensive studies have been carried out, aiming at the rational design of catalysts with high reactivity and selectivity based on earth-abundant materials such as 3d transition metals. 13 Very recently, Nam and co-workers investigated the chemoselectivity between CH hydroxylation and C=C epoxidation in cyclohexene catalyzed by a mononuclear manganese complex, [Mn IV (O)(N4Py)] 2+ (N4Py: N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) (1). 4 They showed, for the first time, that 1 favors a hydrogen atom abstraction from the allylic CH bond over an oxygen atom transfer to the C=C double bond (Scheme 1). The spin-unrestricted density functional theory (UDFT) UB3LYP calculations 5 supported the experiment, providing reasonable reaction pathways with critical transition state (TS) structures (TS H , TS OC1 , and TS OC2 ).The fruitful interplay between experiment and theory has contributed to understanding plausible reaction pathways including key intermediates. Determination of TS structures with multiple spin states represents a computational bottleneck, although UDFT a provides good tradeoff between accuracy and feasibility.6 Semiempirical quantum mechanical (SQM) calculations 7,8 are considered as a likely alternative, and reparameterization and/or modifications make it possible to apply SQM methods to energy minimizations for metalcontaining complexes.9,10 We proposed a reparameterization of PM6 11 (called rPM6) targeting for open-shell molecules including 3d transition metal complexes, and the rPM6 parameters for several basic elements (i.e., H, C, N, O, P, and S) are now available. 1214 The applications of UrPM6 to various organic (di)radicals demonstrated its superior performance in both the electronic and geometric structures. However, to the best of our knowledge, there are no reports in which TS structures are located for 3d transition metals at the SQM levels.Following our previous work, here we report a reoptimized set of parameters for Mn on the basis of a training set consisting of seven mononuclear (ag) and two binuclear (h and i) Mn complexes with oxidation states ranging from +2 to +4, as shown in Figure S1 in the Supporting Information. Computational details for the reparameterization and the resulting set of parameters (Table S1) are also provided in the Supporting Information. Subsequently, we set out to apply UrPM6 to energy ...

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

“…Fluorine and chlorine both had 12 optimizable core-core parameters, resulting in the 18 and 21 parameters in total for fluorine and chlorine, respectively (for details, see Table 1). During the optimization, the parameters for H, C, N, O, Mn, and Fe remained frozen at the values determined in our previous studies [48,49,52].…”

“…Very recently, we proposed the rPM6 method [48,49,50,51,52], which is a reparameterization variant of PM6 [53], and tested the spin unrestricted PM6 (UrPM6) against organic diradicals and iron– and manganese–catalyzed C–H and C=C bond activation reactions. It turned out that UrPM6 outperforms the original UPM6 and spin unrestricted density functional-based tight binding (UDFTB) methods for calculating J values for organic diradicals.…”