The mechanism of the trimerization of alkynes in the presence of an Ir complex bearing a hydrotris(pyrazolyl)borate (Tp) ligand has been studied using density functional theory calculations at the B3LYP and M06 levels. In this reaction, the initial oxidative coupling of two alkyne molecules yields an iridacyclopentadiene intermediate, which reacts with a third alkyne molecule to give a benzene TpIr complex. There are two possible mechanisms for the formation of the benzene complex in this reaction, including the intramolecular [4+2] cycloaddition and Schore mechanisms. The formation of unsubstituted benzene was initially investigated using acetylene molecules, and then the oxidative coupling reaction of 1,4-dimethyl-2-butyne-1,4-dioate (CH3OCOC≡CCOOCH3) followed by the formation of the substituted benzene complex with 2-butyne (H3CC≡CCH3) was studied. It has been possible to clarify the favorable reaction pathway and the effects of different substituents on the reaction mechanism. In the unsubstituted reaction of acetylene the [4+2] cycloaddition is more favorable than the Schore mechanism, whereas the reaction could proceed only via the Schore mechanism in the reactions involving substituted alkynes because of the effect of the substituents. Notably, the effects of additional water molecules on the stability of the reaction intermediates were also evaluated because the water complexes of several intermediates have been experimentally isolated and identified.

A mixture of ferrous ions and hydrogen peroxide, known as Fenton’s reagent, is an effective oxidant and has been widely used in various industrial applications; however, there is still controversy about what the oxidizing agents are and how they are produced. In this study, we have determined minimum free-energy paths (MFEPs) from Fenton’s reagent to possible oxidizing agents such as hydroxyl radicals and ferryl–oxo species by combining ab initio molecular dynamics simulations and an MFEP search method. Along the MFEPs, representative free-energy profiles of the Fenton reaction were elucidated. On the basis of the free-energy profiles, we revealed that the reaction producing ferryl–oxo species from Fenton’s reagent is more energetically favorable than that yielding a free hydroxyl radical, by 24.4 kcal mol–1, which indicates that the ferryl–oxo species is the primary oxidizing agent in reactions of Fenton’s reagent. Moreover, we clarified that the ferryl–oxo species is favorably formed via a two-step reaction pathway, which reaches the product through a dihydroxyiron(IV) intermediate. The energetics charting the free-energy profiles provided valuable information for a comprehensive understanding of Fenton reactions. We concluded that a ferryl–oxo species produced from Fenton’s reagent serves as the primary oxidizing agent in the Fenton reaction.

Using B3LYP level calculations, the reaction of acetonitrile with cobaltacyclopentadiene complex in singlet and triplet electronic states, to give pyridine cobalt complexes, was studied. While the most favorable path for the singlet reaction passes through a [4+2] cycloaddition transition state, the most favorable triplet reaction follows the reaction mechanism through azacobaltacycloheptatriene. Since the transition state for the singlet reaction path is lower in energy than those for the triplet reaction path, the singlet reaction seems to be more favorable. However, the reactant in the triplet state is more stable than that in the singlet state, suggesting that the two-state reactivity (TSR) mechanism with spin changes is followed. Determination of energy minimum crossing points between singlet and triplet energy surfaces led to the conclusion that the TSR mechanism is more favorable than both the singlet- and triplet-state (single-state reactivity) (SSR) mechanisms. Comparison of a reaction profile between trimerization of acetylene and cocyclotrimerization of acetonitrile with two acetylene molecules in the presence of the CpCo catalyst is also made.