In silico design of heteroaromatic half-sandwich Rh^I catalysts for acetylene [2+2+2] cyclotrimerization: Evidence of a reverse indenyl effect

A mechanistic density functional theory study of acetylene [2+2+2] cyclotrimerization to benzene catalyzed by Rh^I half metallocenes is presented. The catalyst fragment contains a heteroaromatic ligand, that is, the 1,2-azaborolyl (Ab) or the 3a,7a-azaborindenyl (Abi) anions, which are isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, but differ from the last ones on having two adjacent carbon atoms replaced with a boron and a nitrogen atom. The better performance of either the classic hydrocarbon or the heteroaromatic catalysts is found to depend on the different mechanistic paths that can be envisioned for the process. The present analyses uncover and explain general structure-reactivity relationships that may serve as rational design principles. In particular, we provide evidence of a reverse indenyl effect. Indenyl running backwards? Rh^I complexes of the heteroaromatic ligands 1,2-azaborolyl (Ab) and 3a,7a-azaborindenyl (Abi), isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, are designed and their catalytic activity in acetylene [2+2+2] cyclotrimerization to benzene is studied in silico. Structure-reactivity insights emerge.