The Catalytic Effect in Opening an Organoactinide Metal Coordination Sphere:  Regioselective Dimerization of Terminal Alkynes and Hydrosilylation of Alkynes and Alkenes with PhSiH₃ Promoted by Me₂SiCp‘ ‘₂Th<i>ⁿ</i>Bu₂

This contribution describes the catalytic effect observed by opening the coordination sphere at an organoactinide complex. Replacing the pentamethylcyclopentadienyl ligand in Cp*2ThCl2 (Cp* = C5Me5) by the bridge ligation [Me2SiCp‘ ‘2]2-2[Li]+ (Cp‘ ‘ = C5Me4) affords the synthesis of ansa-Me2SiCp‘ ‘2ThCl2. The X-ray structure of this bridged complex coordinated to LiCl salt and solvent is presented, indicating the large coordinative unsaturation of the bridge organoactinides. This dichloro complex reacts with 2 equiv of BuLi, affording the corresponding dibutyl complex ansa-Me2SiCp‘ ‘2Th(CH2CH2CH2CH3)2, which was found to react extremely fast for the dimerization of terminal alkynes and also in the hydrosilylation of terminal alkynes or alkenes with PhSiH3. Besides the rapidity of the processes using the bridge organoactinide, as compared to Cp*2ThMe2, the chemo- and regioselectivity of the products were increased, allowing the production of only the gem-dimer, the trans-vinylsilane, and the 1-silylated alkane for the dimerization, hydrosilylation of alkyne, and hydrosilylation of alkene processes, respectively. In the latter process, the corresponding alkane is always obtained as a byproduct. The rapidity of the processes is a consequence of the opening of the coordination sphere at the metal center, whereas the chemoselectivity and regioselectivity were achieved due to the hindered equatorial plane, attributed to the disposition of the methyl groups in the bridge ligation, forcing the incoming substrates to react with a specific regiochemistry. For the dimerization of alkynes the kinetic rate law is first order in organoactinide and exhibits two domains as a function of the alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order, whereas at higher alkyne concentrations a zero order is observed. The turnover-limiting step is the carbon−carbon triple bond insertion of the terminal alkyne into the actinide acetylide bond to give the corresponding dimer. For the hydrosilylation of terminal alkyne or alkenes, the rate law for both processes follows a first order in catalyst and silane concentrations, although an inverse order is observed for either alkyne or alkene, respectively. D2O quenching experiments between the organoactinide complex in the presence of 1-octene under starving PhSiH3 conditions indicate the presence of a π-alkene complex responsible for the inverse order in all three processes. Plausible mechanistic scenarios are proposed.