[2+2+2] CYCLOADDITION OF ACETYLENE CATALYZED BY BIMETALLIC INDENYL FRAGMENTS: A MECHANISTIC DENSITY FUNCTIONAL STUDY

The mechanistic aspects of the [2+2+2] cycloaddition of acetylene mediated by IndRh (Ind=(C9H7 )-, indenyl anion) catalyst in the presence a second coordinated metal, i.e. Cr(CO)3, have been investigated. A detailed exploration of the potential energy surfaces (PESs) was carried out by identifying intermediates and transition states using density functional theory (DFT) methodologies. For comparison, the monometallic parent systems CpRh and IndRh are included in the analysis. [1] The active center is rhodium nucleus, where the cycloaddition occurs. An analysis of inter-metal cooperative effects has been done to assess the role of the second metal center, that is chromium. The coordination of this latter metal to the benzene moiety may occur in syn or in anti-conformation. [2] This affects the energetics of the cycle and also the mechanism. The reaction energies and barriers, the turn over frequency (TOF) [3] and the defined slippage parameter (a geometrical quantity related to the position of Rh with respect to the centroid of its coordinated cyclopentadienyl moiety) [4] of the complete catalytic cycles are discussed. In agreement with experiments [2], it emerges that the presence of the second metal favors the catalysis.