Inhibition of Alkyne Cyclotrimerization to Arenes on a Metal Site by Four‐Electron Donation through Simultaneous Sigma and Pi Bonding: The Tris(alkyne)Tungsten Carbonyls

Abstract

AbstractReactions of fac‐(CH3CN)3W(CO)3 with the alkynes RC≡CR are known experimentally to give the very stable (RC≡CR)3WL (L = CO or CH3CN) complexes without the complexed alkyne ligands on the tungsten site undergoing cyclotrimerization to the corresponding benzene derivatives C6R6. In order to evaluate the thermodynamics of these systems, the structures and energetics of the tris(alkyne)tungsten carbonyls (RC≡CR)3W(CO)n and the corresponding arenetungsten carbonyls (η6‐C6R6)W(CO)n (R = CH3, CF3; n = 3, 2, 1, 0) have been optimized using density functional theory. The cyclotrimerizations of the alkyne ligands in (RC≡CR)3W(CO)n (n = 3, 2) to a benzene ligand in (η6‐C6R6)W(CO)n are predicted to be very exothermic. However, the corresponding cyclotrimerizations of the alkyne ligands in (RC≡CR)3W(CO) and (RC≡CR)3W to a benzene ligand in (η6‐C6R6)W(CO) and (η6‐C6R6)W, respectively, are predicted to range from nearly thermoneutral (R = CH3) to highly endothermic (R = CF3). This can be related to the bonding mode of the alkyne ligands to the tungsten atom. Alkyne ligands bonded to tungsten as four‐electron donors (with one σ bond and one π bond) are resistant to cyclotrimerization reactions. However, alkyne ligands bonded to tungsten as two‐electron donors through only σ bonding of the C≡C triple bond are reactive towards cyclotrimerization reactions. The CF3 derivatives are found to be less reactive towards cyclotrimerization than the CH3 derivatives. The inhibition of alkyne cyclotrimerization on a metal site by bonding as a four‐electron donor through both σ and π bonding can be related to the fact that known alkyne cyclotrimerization catalysts are generally based on late transition metals of groups 8, 9, and 10, rather than early transition metals such as tungsten.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)