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 (4, where NPN* = PhP­(2-(N-mesityl)-5-Me-C6H3)2)
with dihydrogen (H2, 4 atm) results in the formation of
the dinuclear tetrahydride ([NPN*]­Ta)2(μ-H)4 (5), 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 (6), 3-hexyne (7)) 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 5; however, using lower pressures of H2 and shorter
reaction times results in the isolation of an intermediate in each
case. Hydrogenation of 6 generates the alkene hydride
complex [NPN*]­Ta­(trans-1,2-C2H2(SiMe3)2)H (8); addition of H2 to 7 gives [NPN*]­Ta­(1-hexene)H (9), 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 5 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 trans-alkene units is suggested to be a result of a zwitterionic
alkylidene intermediate that allows free rotation of a C–C
single bond.