Synthesis and Reactivity of Alkyl, Hydride, and Silyl Derivatives of the (Terphenyl)imido Fragments Cp*(Ar^MesN)Ta (Cp* = η⁵-C₅Me₅; Ar^Mes = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃) and Cp*(Ar^TripN)Ta (Ar^Trip = 2,6-(2,4,6-ⁱPr₃C₆H₂)₂C₆H₃)

Abstract

The syntheses and reactivities of tantalum alkyl, hydride, and silyl complexes containing a sterically demanding, mixed Cp*-(terphenyl)imido (Cp* = η 5 -C 5 Me 5 ) ligand set are reported. The dichloride complex Cp*(Ar Mes N )TaCl 2 ( 1 ), prepared by the reaction of Ar Mes NHLi (Ar Mes = 2,6-(2,4,6-Me 3 C 6 H 2 ) 2 C 6 H 3 ) with Cp*TaCl 4 in the presence of a large excess of NEt 3, was structurally characterized. Complex 1 reacted readily with KSi(SiMe 3 ) 3 to give the silyl chloride compound Cp*(Ar Mes N )Ta[Si(SiMe 3 ) 3 ]Cl ( 2 ), and with MeLi (2 equiv) to yield the dimethyl complex Cp*(Ar Mes N )TaMe 2 ( 4 ). Compound 2 reacted with PhSiH 3 via the intermediate complex Cp*(Ar Mes N )Ta(SiH 2 Ph)Cl to give the hydrido chloride complex Cp*(Ar Mes N )Ta(H)Cl ( 3 ). While the analogous compound of the [DippN ] 2 - (Dipp = 2,6- i Pr 2 C 6 H 3 ) ligand is a dimer with bridging hydrides, compound 3 contains a terminal hydride, as indicated by the downfield 1 H NMR resonance of the hydride ligand (15.28 ppm). Treatment of 4 with AgOTf (OTf = OSO 2 CF 3 ) provided the methyl triflate compound Cp*(Ar Mes N )Ta(Me)OTf ( 6 ), which reacted cleanly with H 2 in bromobenzene to yield pale yellow crystals of the hydrido triflate complex Cp*(Ar Mes N )Ta(H)OTf ( 7 ). Complexes 4, 6, and 7 reacted with PhSiH 3 at high temperatures (85−100 °C) to give silane products arising from σ-bond metathesis and redistribution of the substituents at silicon. The neopentyl hydride complex Cp*(Ar Mes N )Ta(CH 2 CMe 3 )H ( 8 ), prepared by treating 7 with LiCH 2 CMe 3, reacted with PhSiH 3 at room temperature via the silyl hydride intermediate species Cp*(Ar Mes N )Ta(SiH 2 Ph)H to yield the dihydride dimer [Cp*(Ar Mes N )TaH(μ-H)] 2 ( 5 ). In an effort to stabilize a monomeric dihydride complex, the more sterically demanding [Ar Trip N ] 2 - (Ar Trip = 2,6-(2,4,6- i Pr 3 C 6 H 2 ) 2 C 6 H 3 ) ligand was employed. The dichloride complex Cp*(Ar Trip N )TaCl 2 ( 12 ) and the dimethyl complex Cp*(Ar Trip N )TaMe 2 ( 13 ) were prepared using procedures analogous to those used for the syntheses of 1 and 4 . Treatment of 13 with H 2 in bromobenzene yielded the hydrido bromide complex Cp*(Ar Trip N )Ta(H)Br ( 14 ), which formed via solvent activation by the intermediate dihydride species Cp*(Ar Trip N )TaH 2 . When D 2 was used in place of H 2, Cp*(2-[2-( i Pr- d 6 )-4,6- i Pr 2 C 6 H 2 ]-6-Trip-C 6 H 3 N )Ta(D)Br ( 14- d 7, Trip = 2,4,6- i Pr 3 C 6 H 2 ) was obtained. Compound 14- d 7 arises from the intramolecular C−H bond activation of the methyl groups of one of the isopropyl substituents of the [Ar Trip N ] 2 - ligand. Hydrogenolysis of the neopentyl hydride complex Cp*(Ar Trip N )Ta(CH 2 CMe 3 )H ( 15 ) in bromobenzene- d 5 allowed observation of the corresponding dihydride complex, which was observed to slowly convert to the hydrido bromide species 14 .