Tantalum Alkyl and Silyl Complexes of the Bulky (Terphenyl)imido Ligand [2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃N]^2- ([Ar*N]^2-). Generation and Reactivity of [(Ar*N)(Ar*NH)Ta(H)(OSO₂CF₃)], Which Reversibly Transfers Hydride to an Aromatic Ring of the Arylamide Ligand

Tantalum complexes containing the sterically demanding imido ligand [2,6-(2,4,6-Me3C6H2)2C6H3N]2- ([Ar*N]2-) are reported. The dimethyl complex (Ar*N)(Ar*NH)TaMe2 (4), prepared by the reaction of 2 equiv of Ar*NHLi (3) with TaMe3Cl2, was structurally characterized. Compound 4 forms adducts with donor ligands such as trimethylphosphine and pyridine and readily reacts with CO to give a monoinsertion product, the η2-acyl complex 5. The dichloride complex (Ar*N)(Ar*NH)TaCl2 (7), formed by the reaction of 2 equiv of Ar*NH2 (2) with TaMe3Cl2, reacted cleanly with NpLi (Np = Me3CCH2) to give the dineopentyl species (Ar*N)(Ar*NH)TaNp2 (8), which was structurally characterized. Unlike 4, complex 8 does not form adducts with donor ligands. The cationic complex [(Ar*N)(Ar*NH)TaMe][MeB(C6F5)3] (9), prepared by the reaction of 4 with B(C6F5)3, slowly converted to [(Ar*N)(Ar*NH)TaMe][HBC6F5)3] in the presence of PhSiH3. Treatment of 4 with AgOTf (OTf = OSO2CF3) provided the methyl triflate complex (Ar*N)(Ar*NH)TaMeOTf (10), which reacts cleanly with H2 in bromobenzene to provide red-orange crystals of the η5-cyclohexadienyl complex (Ar*N)[2-(η5-2,4,6-Me3C6H3)-6-MesC6H3NH]Ta(OTf) (11, Mes = 2,4,6-Me3C6H2). Complex 11, which was structurally characterized, represents a previously unobserved arene hydrogenation intermediate formed by endo transfer of a single hydride from the metal center to an arene ring. The hydride intermediate [(Ar*N)(Ar*NH)Ta(H)OTf] (A) was not observed spectroscopically, but was trapped by exposure of 10 to H2 in the presence of excess 1-hexene to give (Ar*N)(Ar*NH)Ta(Hex)OTf (12). Complex 11 is in equilibrium with A, which is the more reactive isomer, as indicated by its ability to catalyze olefin hydrogenation, diene cyclization, and silane deuteration. Complex 7 reacts cleanly with (THF)3LiSi(SiMe3)3, (THF)2LiSi(tBu)Ph2, and (THF)2LiSiHMes2 to provide the silyl chloride complexes (Ar*N)(Ar*NH)Ta(SiR3)Cl (SiR3 = Si(SiMe3)3, (tBu)Ph2, SiHMes2; 13a−c). Hydrogenolysis of 13a−c yields silane and a species with spectroscopic features similar to those of 11.