Theoretical studies on acetylene cyclotrimerization into benzene catalyzed by CpIr fragment

•The mechanisms of the CpIr-catalyzed [2 + 2 + 2] cyclotrimerization of acetylene are computed.•An intramolecular [4 + 2] cycloaddition for yielding CpIr(η4-benzene) is proposed.•The aromatic planar iridacycloheptatriene species is found as a trap on the catalytic cycle.•The mechanisms found for CpIr, CpRh and CpCo are compared and discussed.

Using the B3LYP density functional theory method we have computed the potential energy surface of the (η5-C5H5)Ir-catalyzed acetylene cyclotrimerization. The first step is the oxidative addition which forms the iridacyclopentadiene complex (2) from the bisacetylene complex (η5-C5H5)Ir(η2-HCCH)2 (1), and this is the rate-determining step. On the potential energy surface, acetylene addition to complex (2) forms the iridacyclopentadiene(acetylene) complex (3), and this step is barrier-free and highly exergonic; and complex (3) forms the benzene complex (4) via an intramolecular [4 + 2] cycloaddition. Alternatively, complex (2) and acetylene form the iridabicyclo[3.2.0]heptatriene complex (5) via an intermolecular [2 + 2] cycloaddition, and complex (5) can further isomerize into the aromatic iridacycloheptatriene complex (6a), which is a trap on the potential energy surface. This potential energy surface is qualitatively close to that for the CpRh-catalyzed reaction, but differs strongly from that for the CpCo-catalyzed reaction, which undergoes a spin crossing (or non-adiabatic) mechanism. The differences among these mechanisms as well as the relative stability of their intermediates have been compared and discussed accordingly.

Graphical abstractFor CpIr-catalyzed [2 + 2 + 2] cyclotrimerization of acetylene B3LYP calculations show that the CpIr(η4-benzene) complex (4) is formed from iridacyclopentadiene(acetylene) complex (3), and alternatively, an intermolecular [2 + 2] addition between singlet iridacyclopentadiene complex (2) and an acetylene molecule forms iridabicyclo[3.2.0]heptatriene (5), which can isomerize into the aromatic iridacycloheptatriene complex (6a).Figure optionsDownload full-size imageDownload as PowerPoint slide