Structure, Bonding, and Reactivity of Tantalum Amides Containing o-Naphthyl- and o-Indenylphenoxide Ligation

The reaction of [Ta(NMe 2 ) 5 ] with the o -(2,3-dihydro-1-naphthyl)-, o -(1-naphthyl)-, and o -(inden-3-yl)phenols [HOC 6 H 2 Ar-2-Bu t 2 -4,6] (Ar = C 10 H 9 ( 1 ), C 10 H 7 (Np; 2 ), C 9 H 7 ( 3 )) has been investigated. In all three cases initial displacement of 1 equiv of dimethylamine occurs, yielding mono(aryloxides) [(ArO)Ta(NMe 2 ) 4 ]. Structural studies of the o -(2,3-dihydro-1-naphthyl) and o -(1-naphthyl) compounds 4 and 5 show they both adopt geometries best described as square pyramidal with an apical dimethylamido ligand and basal aryloxide oxygen. The Ta−O−Ar angles are 162° in both compounds, with no metal interaction with the ortho substituents. Compound 4 reacts with 2,3,5,6-tetraphenylphenol to form the corresponding bis(aryloxide) 6 . The intermediate o -(inden-3-yl)phenoxide [(ArO)Ta(NMe 2 ) 4 ] 7 thermally eliminates a further 1 equiv of HNMe 2 with formation of the tris(amido) compound [Ta(OC 6 H 2 {η 1 -Ind}-2-Bu t -4,6)(NMe 2 ) 3 ] 8 . The coordination geometry about tantalum in 8 is best described as trigonal bipyramidal, with an oxygen and amido group in the axial positions: O−Ta−N = 170°. The carbon atom of the indenyl ring bound directly to the phenoxide nucleus is metalated, leading to a five-membered metallacycle. Hence, both deprotonation (CH bond activation) and tautomerization of the original inden-3-yl ring has occurred. The Ta−C(121) distance of 2.285(9) Å is consistent with an η 1 -indenyl ring being present in 8 . Replacement of the dimethylamido ligands in 8 by chloride groups was achieved by reaction with SiCl 4 . Structural analysis of the 4-phenylpyridine adduct [Ta(OC 6 H 2 {η 3 -Ind}-2-Bu t -4,6)(NC 5 H 4 Ph-4)Cl 3 ] 9 showed the presence of an η 3 -indenyl interaction with the tantalum metal center. The bonding parameters for the η 1 - and η 3 -indenyl rings are compared with those of related η 5 -cyclopentadienyl derivatives of niobium and tantalum.