Conversion of [(^tBuCH₂)₂TaN]₅ to Cubic TaN: Related Syntheses, EHMO Calculations, and MAS and Spin Echo ¹⁵N NMR Spectroscopies

The conversion of pentamer [(^tBuCH₂)₂TaN]₅ (2) and ammonia to cubic TaN_0.8 (5) proceeds via precipitate 1/n[TaC_1.41H_3.90N_1.90]n (3) and amorphous TaN (4). Precursors other than (^tBuCH₂)₃Ta=CH^tBu (1) and NH₃, which provided 2, such as Ta(NMe₂)₅ (7) and NH₃, (Me₂N)₃-Ta=N^tBu (9) and NH₃, also afforded 5. Extended Hückel molecular orbital calculations were conducted to examine the electronic changes that appear upon oligomerization of hypothetical Me₂TaN to (Me₂TaN)_∞, as a model for pentamer 2 formation. The relationship between pentamer 2 and the remaining nitrides, 3-5 was probed via magic angle spinning (MAS) and spin-echo solid-state ¹⁵N NMR experiments that probe the local environments of the various nitrogen atoms. In combination with ¹⁵N-labeling studies, the spin-echo spectra were used in an effort to analyze the possibility of a direct relationship between the 2-dimensional Ta₂N₂ "squares" of the pentamer (2) and the 3-dimensional Ta₄N₄ "cubes" of the ultimate product, cubic TaN_0.8 (5).