Group 9 Metallacyclopentadienes as Key Intermediates in [2+2+2] Alkyne Cyclotrimerizations. Insight from Activation Strain Analyses

Abstract

The intramolecular oxidative coupling converting a bis-acetylene complex of formula CpM (C H ) (Cp=C H ; M=Co, Rh, Ir) into a 16-electron metallacycle is studied in silico. This reaction is paradigmatic in acetylene [2+2+2] cycloaddition to benzene catalyzed by CpM fragments, being the step with the highest activation energy, and thus affecting the whole catalysis. Our activation strain and quantitative molecular orbital (MO) analyses elucidate the mechanistic details and reveal why cobalt performs better than rhodium and iridium catalysts outlining general principles for rational design of catalysts to be used in these processes.

Half‐Sandwich Metal‐Catalyzed Alkyne [2+2+2] Cycloadditions and the Slippage Span Model

“…The crucial step of the whole cycle is indeed oxidative coupling, as the degree of TOF control is almost one for all the analyzed catalysts. For this reason, a dedicated analysis of these species was recently performed . CpCo has the highest efficiency (Table ), in agreement with the experimental findings.…”

“…CpCo is the most rigid fragment: the metal remains almost perfectly centered with respect to the Cp anion during the whole catalytic cycle and only weak slippage occurs. The metal–Cp bonding strength explains the trend of CpIr and CpRh . As expected, IndRh, the intermediates and transition states of which are more slipped and overall show higher flexibility, is less performant than the other catalysts, that is, no indenyl effect is found.…”

“…This trend originates from the first reaction step, oxidative coupling, which involves both the TOF‐determining intermediate (TDI) and the TOF‐determining transition state (TDTS). Indeed, in this elementary step, the more rigid and, thus, almost perfectly η 5 CpCo complex is more efficient than the more slipped η 3 +η 2 CpRh and CpIr complexes …”

“…The CpIr catalyst has also been studied in silico, although interest in iridium has remained quite limited relative to interest in other metals such as Co, Rh, and Ru . The advantage of CpCo, with respect to heavier CpRh, in the initial oxidative coupling to form the metallacyclopentadiene was rationalized very recently and was ascribed to smaller slippage variation of the lighter metal along the reaction path …”

“…[28] The advantage of CpCo, with respect to heavierC pRh, in the initial oxidative coupling to form the metallacyclopentadiene was rationalized very recently and was ascribed to smaller slippage variationo ft he lighter metal along the reaction path. [34] One puzzling aspecte merging in the abovementioned DFT mechanistic studyo f2 007, [29] in which CpRh and IndRh were compared, is the lack of an indenyl effect, consistingo f" enhanced kinetic performance" of the latter fragment, which,due to the presence of the benzene ring fused with the Cp moiety, allowse asier metal slippage. This unexpected computational result led to the hypothesis of ad ifferent mechanism [35] inspired by the experimental work of Booth et al, [23] in which the real catalysts are LÀCpRh and LÀIndRh (L = CO).…”