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)(2)C6H3N=](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 eta(2)-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 H-2 in bromobenzene to provide red-orange crystals of the eta(5)-cyclohexadienyl complex (Ar*=)[2-(eta(5)-2,4,6-Me3C6H3)-6-MesC(6)H(3)NH]Ta(OTD (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 H-2 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)(3)LiSi(SiMe3)(3), (THF)(2)LiSi((t)u)Ph-2, and (THF)(2)LiSiHMeS2 to provide the silyl chloride complexes (Ar*N=)(Ar*NH)Ta(SiR3)Cl (SiR3 = Si(SiMe3)(3), (Bu-t)Ph-2, SiHMeS2; 13a-c). Hydrogenolysis of 13a-c yields silane and a species with spectroscopic features similar to those of 11.