On the Dehydrocoupling of Alkenylacetylenes Mediated by Various Samarocene Complexes: A Charming Story of Metal Cooperativity Revealing a Novel Dual Metal σ-Bond Metathesis Type of Mechanism (DM|σ-BM)

The prevailing reductive chemistry of Sm(II) has been accessed and explored mostly by the use of samarocene precursors. The highly reducing character of these congeners, along with their Lewis acidity and predominantly ionic bonding, allows for the relatively facile activation of C–H bonds, as well as peculiar transformations of unsaturated substrates (e.g., C–C couplings). Among other important C–C coupling reactions, the reaction of phenylacetylene with different mono- or bimetallic samarocene complexes affords trienediyl complexes of the type {[(C5Me5)2Sm]2(µ-η2:η2-PhC4Ph)}. In contrast, when t-butylacetylene is used, uncoupled monomers of the type (C5Me5)2Sm(C≡C–tBu) were obtained. Although this type of reactivity may appear to be simple, the mechanism underlying these transformations is complex. This conclusion is drawn from the density functional theory (DFT) mechanistic studies presented herein. The operating mechanistic paths consist of: (i) the oxidation of each samarium center and the concomitant double reduction of the alkyne to afford a binuclear intermediate; (ii) the C–H scission of the acetylinic bond that lies in between the two metals; (iii) a dual metal σ-bond metathesis (DM|σ-SBM) process that releases H2; and eventually (iv) the C–C coupling of the two bridged μ-alkynides to give the final bimetallic trienediyl complexes. For the latter mechanistic route, the experimentally used phenylacetylene was considered first as well as the aliphatic hex-1-yne. More interestingly, we shed light into the formation of the mono(alkynide) complex, being the final experimental product of the reaction with t-butylacetylene.