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.

For 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 as PowerPoint slide