AMMONOLYSIS OF TANTALUM ALKYLS - FORMATION OF CUBIC TAN AND A TRIMERIC NITRIDE, [CP-STAR-METAN]3

Ammonolyses of precursor alkyl complexes have been employed to generate nitrides of tantalum. Treatment of (*BuCH2)3Ta=CHtBu (1) with NH3led to the formation of an orange precipitate, best formulated as an oligomer, [TaN2.14H1.35], (2), on the basis of ammonia uptake, neopentane loss, and combustion analysis. Upon thermolysis of 2 at 400 °C (24 h), a ~6% weight loss occurred and amorphous TaN was generated; crystalline cubic TaN (Fm3m, 95%) was formed after further heating at 820 °C (3 days). Under ambient light, a similar ammonolysis of 1 afforded another oligomer, [TaC, 41H3.90N1.90.]„ (2'); thermolysis of 2' (400 °C, 24 h) resulted in a ~ 14% weight loss and amorphous TaN that was subsequently annealed to pure, crystalline cubic TaN (820 °C, 3 days, XRD, Fm3m). Cp*TaMe4(3, Cp* =?5-C5Me5) was exposed to excess NH3in order to model the ammonolysis process. The uptake of 1 equiv of ammonia/equiv of Ta was noted, 3 equiv of CH4was released, and [Cp*MeTaN]3(4) formed in 90% yield. Cyclic trimer 4 contains equivalent TaN distances (1.887 (17) Å) akin to those of related phosphazenes. Crystal data: monoclinic, P21/c, a = 16.951 (5) Å, b = 8.920 (3) Å, c = 23.141 (6) Å, 0 = 91.47 (2)°, Z = 4, T =-100 °C. EHMO calculations revealed why a structure containing alternating double and single TaN bonds was not favored. The low-lying LUMO of 4 was predicted to be nonbonding; consequently, 4 was reduced with Na/K to yield [K-nEt2O]+[|Cp*MeTaN|3]~ (5). A reversible reduction wave at E°' =-2.5 V vs SSCE was also observed. Similar IR spectra of 4 (v(TaNTa) = 960 cm-1) and 5 (v(TaNTa) = 964 cm-1) support the contention that the LUMO is nonbonding. The relationship of 4 to the solid-state nitrides produced via ammonolysis is addressed. © 1990, American Chemical Society. All rights reserved.