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

The conversion of pentamer [( t BuCH 2 ) 2 TaN] 5 ( 2 ) and ammonia to cubic TaN 0.8 ( 5 ) proceeds via precipitate 1/ n [TaC 1.41 H 3.90 N 1.90 ] n ( 3 ) and amorphous TaN ( 4 ). Precursors other than ( t BuCH 2 ) 3 Ta CH t Bu ( 1 ) and NH 3, which provided 2, such as Ta(NMe 2 ) 5 ( 7 ) and NH 3, (Me 2 N) 3 Ta N t Bu ( 9 ) and NH 3, also afforded 5 . Extended Hückel molecular orbital calculations were conducted to examine the electronic changes that appear upon oligomerization of hypothetical Me 2 TaN to (Me 2 TaN) ∞, 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 15 N NMR experiments that probe the local environments of the various nitrogen atoms. In combination with 15 N-labeling studies, the spin-echo spectra were used in an effort to analyze the possibility of a direct relationship between the 2-dimensional Ta 2 N 2 “squares” of the pentamer ( 2 ) and the 3-dimensional Ta 4 N 4 “cubes” of the ultimate product, cubic TaN 0.8 ( 5 ).