Generation of low-valent tantalum species by reversible C-H activation in a cyclometallated tantalum hydride complex.

The cyclometallated tantalum(v) hydride complex {ArNC(Me)CHC(Me)N[2-(CHMeCH2)-6-(i)Pr-C6H3]}Ta(N(t)Bu)H (2) was prepared from hydrogenolysis of (BDI)TaN(t)BuMe2 (BDI = N,N'-diaryl-β-diketiminate, aryl = 2,6-(i)Pr2-C6H3). Based on mechanistic studies, formation of 2 likely proceeds through a dihydride intermediate generated from successive σ-bond metathesis steps. Compound 2 was found to undergo reductive elimination under certain conditions to form trivalent tantalum species. Coordination of DMAP to 2, followed by gentle heating under H2, gives (MAD)Ta(N(t)Bu)(NAr)(DMAP), which is produced through reductive C-N bond cleavage of the BDI ligand in a Ta(iii) intermediate. Ta(iii) dicarbonyl derivatives have been accessed by either introducing CO atmosphere to the DMAP adduct of 2 at room temperature, or by directly adding CO to 2 at low temperature.