Mono(pentamethylcyclopentadienyl)tantalum complexes containing a structurally versatile enediamido(2-) ligand

Treatment of Cp*TaCl4 with 1 equiv of Li-2(R-2-dad) in THF afforded Cp*Ta(R-2-dad)Cl-2 (R = Pr-i (1), Bu-t (2)). The chloride ligands of 1 and 2 reacted smoothly with 2 equiv of LiC=(CBu)-Bu-t, MeMgI and (LiSBu)-Bu-t, resulting in Cp*Ta(R-2-dad)(C=(CBu)-Bu-t)(2) (R = Pr-i (3), Bu-t (4)), Cp*Ta(R-2-dad)Me-2 (R = Pr-i (5), Bu-t (6)), and Cp*Ta(Bu-t(2)-dad)((SBu)-Bu-t)(2) (7), respectively. The X-ray analyses of 1, 3, 4, 5, and 7 have been carried out. In the structures of 1, 3, 4, and 5, the R-2-dad Ligand is bound to the metal center with a sigma(2)(N, N'), pi(C, C')-coordination mode, which opts for a supine conformation with respect to the Cp* group for 1 and 3, while an alternative prone conformation is adopted for 4 and 5. In contrast, the tBu2-dad ligand of 7 assumes an intriguing eta(2)(N, C) coordination geometry. Treatment of 7 in toluene at 80 degrees C was found to induce C-S bond cleavage of the thiolates generating Cp*Ta(Bu-t(2)-dad)(S) (8), in which the dad ligand resumes a sigma(2), pi-coordination geometry with a prone conformation.