Hydrogenolysis of Tantalum Hydrocarbyl Complexes: Intermediates on the Road to a Dinuclear Tantalum Tetrahydride Derivative

The synthesis, characterization, and reactivity with H₂ of a series of tantalum hydrocarbyl complexes are reported. The reaction of [NPN*]­TaMe₃ (<b>4</b>, where NPN* = PhP­(2-(<i>N</i>-mesityl)-5-Me-C₆H₃)₂) with dihydrogen (H₂, 4 atm) results in the formation of the dinuclear tetrahydride ([NPN*]­Ta)₂(μ-H)₄ (<b>5</b>), without the observation of intermediates. The preparations of two alkyne benzyl complexes of the formula [NPN*]­Ta­(alkyne)­(CH₂Ph) (where alkyne = BTA = bis­(trimethylsilyl)­acetylene (<b>6</b>), 3-hexyne (<b>7</b>)) are reported starting from the respective chloroalkyne complexes [NPN*]­Ta­(alkyne)­Cl, by addition of benzylpotassium. Hydrogenation of these two alkyne benzyl complexes ultimately results in the formation of the same dinuclear tetrahydride <b>5</b>; however, using lower pressures of H₂ and shorter reaction times results in the isolation of an intermediate in each case. Hydrogenation of <b>6</b> generates the alkene hydride complex [NPN*]­Ta­(<i>trans</i>-1,2-C₂H₂(SiMe₃)₂)H (<b>8</b>); addition of H₂ to <b>7</b> gives [NPN*]­Ta­(1-hexene)H (<b>9</b>), in which the 3-hexyne moiety has been partially hydrogenated and isomerized to the 1-hexene regioisomer. Both of these alkene hydride complexes can be converted to the dinuclear tetrahydride complex <b>5</b> by addition of H₂. A mechanism is proposed for the formation of the intermediates that involves hydrogenolysis of the alkyne moiety prior to the benzyl ligand; the formation of the <i>trans</i>-alkene units is suggested to be a result of a zwitterionic alkylidene intermediate that allows free rotation of a C–C single bond