Synthesis, Reactivity, and DFT Studies of Tantalum Complexes Incorporating Diamido-N-heterocyclic Carbene Ligands. Facile Endocyclic C−H Bond Activation

The syntheses of tantalum derivatives with the potentially tridentate diamido-N-heterocyclic carbene (NHC) ligand are described. Aminolysis and alkane elimination reactions with the diamine−NHC ligands, ^Ar[NCN]H₂ (where ^Ar[NCN]H₂ = (ArNHCH₂CH₂)₂(C₃N₂); Ar = Mes, p-Tol), provided complexes with a bidentate amide−amine donor configuration. Attempts to promote coordination of the remaining pendent amine donor were unsuccessful. Metathesis reactions with the dilithiated diamido−NHC ligand (^Ar[NCN]Li₂) and various Cl_xTa(NR‘₂)_5-x precursors were successful and generated the desired octahedral ^Ar[NCN]TaCl_x(NR‘₂)_3-x complexes. Attempts to prepare trialkyl tantalum complexes by this methodology resulted in the formation of an unusual metallaaziridine derivative. DFT calculations on model complexes show that the strained metallaaziridine ring forms because it allows the remaining substituents to adopt preferable bonding positions. The calculations predict that the lowest energy pathway involves a tantalum alkylidene intermediate, which undergoes C−H bond activation α to the amido to form the metallaaziridine moiety. This mechanism was confirmed by examining the distribution of deuterium atoms in an experiment between ^Mes[NCN]Li₂ and Cl₂Ta(CD₂Ph)₃. The single-crystal X-ray structures of ^p-Tol[NCNH]Ta(NMe₂)₄ (3), ^Mes[NCNH]TaCHPh(CH₂Ph)₂ (4), ^p-Tol[NCN]Ta(NMe₂)₃ (7), ^Mes[NCCN]Ta(CH₂^tBu)₂ (11), and ^Mes[NCCN]TaCl(CH₂^tBu) (14) are included.