Reactions of Alkynes and Olefins with Tantalum Hydrides Containing Aryloxide Ancillary Ligation: Relevance to Catalytic Hydrogenation

Abstract

The reactivity of the three hydride compounds [Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2] (1), [Ta(OC6H3Pri2-2,6)2(H)2Cl(PMe2Ph)2] (2), and [Ta(OC6H3But2-2,6)2(H)2Cl(PMePh2)] (3) toward olefins and alkynes has been investigated. The reactivity observed is highly dependent on the nature of the ancillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC6H3Ph2-2,6)2(η2-CH2CHPh)Cl(PMe3)] (5). In contrast, 1 reacts with 3-hexyne to eliminate H2 along with formation of the analogous alkyne complex 6. Structural studies of 5 and 6 show a square-pyramidal geometry with an axial olefin (alkyne) unit lying along the Cl−Ta−P axis. Structural parameters support a tantalacyclopropane (tantalacyclopropene) bonding picture for these molecules. Compound 5 is converted back into 1 under H2 along with formation of PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H2, and the dehydrogenation product [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PMe2Ph)2] (7). The related adduct [Ta(OC6H3Pri-η2-CMeCH2)(OC6H3Pri2-2,6)Cl(PEt3)2] (9) was isolated by treatment of [Ta(OC6H3Pri2-2,6)2Cl3] with PEt3/Bu3SnH and was structurally characterized. Labeling studies show that the H2 generated comes exclusively from the aryloxide o-Pri group which was dehydrogenated. Both hydrides initially attached to the metal are transferred to the olefin substrate. In the case of the 2,6-di-tert-butylphenoxide compound 3, reaction with styrene generates the mono-cyclometalated compound [Ta(OC6H3ButCMe2CH2)(OC6H3But2-2,6)(CH2CH2Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group. The related trans-phenylvinyl compound 10 is produced when 3 is reacted with phenylacetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis(iminoacyl) derivative 11, in which xyNC has inserted into the cyclometalated carbon as well as the Ta−CHCHPh bond in 10. Structural studies of 11 confirmed the trans arrangement of the hydrogen atoms in the phenylvinyl group. Mechanistic studies of the formation of 10 and 11 show the presence of two competing pathways. The first involves direct elimination of H2 from the dihydride and formation of an intermediate olefin/alkyne adduct. The product then arises by CH bond activation of the aryloxide with the hydrogen transferring to a carbon atom of the tantalacyclopropane (tantalacyclopropene) ring. The second pathway involves insertion of olefin/alkyne into a Ta−H bond followed by CH bond activation by the remaining hydride.