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

• Published in: Organometallics Vol. 34; no. 14; pp. 3546 - 3558
• Main Authors: Parker, Kyle D. J, Nied, Dominik, Fryzuk, Michael D
• Format: Journal Article
• Language: English
• Published: American Chemical Society 27-07-2015
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.5b00344

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.