Synthesis of cyclometalated aryltantalum(V) complexes: Amine C-H activation via [sigma]-bond metathesis

Cyclometalated aryltantalum(V) complexes containing a five-membered Ta-C-C-C-N chelate ring can be prepared by transmetalation reactions. The 1:2 reaction of [Zn(C[sub 6]H[sub 4]CH[sub 2]NMe[sub 2]-2)[sub 2]] with TaCl[sub 5] affords [TaCl[sub 4](C[sub 6]H[sub 4]CH[sub 2]NMe[sub 2]-2)] (1a) in high yield. [TaCl[sub 4][l brace]C[sub 6]H[sub 4]CH(Me)NMe[sub 2]-2[r brace]] (1b) (S enantiomer) and [TaCl[sub 4](C[sub 6]H[sub 3]CH[sub 2]NMe[sub 2]-2-Me-6)] (1c) can be prepared similarly. The reaction of 1a and 1c with a half-equiv of [Zn(CH[sub 2]Ph)[sub 2]] affords the monobenzyl substituted analogues [TaCl[sub 3](C[sub 6]H[sub 4]CH[sub 2]NMe[sub 2]-2)(CH[sub 2]Ph)] (2a) and [TaCl[sub 3](C[sub 6]H[sub 3]CH[sub 2]NMe[sub 2]-2-Me-6)(CH[sub 2]Ph)] (2c). The 2:1 reaction of [TaCl[sub 3](CH[sub 2]Ph)[sub 2]] with [Zn(C[sub 6]H[sub 4]CH[sub 2]NMe[sub 2]-2)[sub 2]] affords the dibenzyl species [TaCl[sub 2](C[sub 6]H[sub 4]CH[sub 2]CH[sub 2]NMe[sub 2]-2)(CH[sub 2]Ph)[sub 2]] (3a) in good yield. [TaCl[sub 2][l brace]C[sub 6]H[sub 4]CH(Me)NMe[sub 2]-2[r brace](CH[sub 2]Ph)[sub 2]] (3b) and [TaCl[sub 2](CH[sub 2]C[sub 6]H[sub 4]NMe[sub 2]-2)(CH[sub 2]Ph)[sub 2]] (3d) can be prepared similarly. An analogous transmetalation of [TaCl[sub 3](CH[sub 2]Ph)[sub 2]] with [1/2] [Li(1-C[sub 10]H[sub 6]NMe[sub 2]8)][sub 2] affords [TaCl[sub 2](1-C[sub 10]H[sub 6]NMe[sub 2]-8)(CH[sub 2]Ph)[sub 2]] (3e). The overall structure is distorted octahedral with the Ta-C-C-C-N chelate ring and both benzyl ligands in the equatorial plane. For 3b and 3e in solution rupture of the Ta-N bond (at ca. 60[degrees]C) leads to a (stereoselective) NMe C-H activation, preceeding via a [sigma]-bond metathesis step, that gives [TaCl[sub 2][l brace]C[sub 6]H[sub 4](CH(Me)N(Me)CH[sub 2])-2[r brace](CH[sub 2]Ph)(THF)] (4b) and [TaCl[sub 2][l brace]1-C[sub 10]H[sub 6](N(Me)CH[sub 2])-8[r brace](CH[sub 2]Ph)(THF)[sub 2]] (4e), respectively, in moderate yield. 41 refs., 6 figs., 7 tabs.