Ladder Structure of (^tBuCH₂)₂TaN]₅.middot.NH₃.middot.2C₇H₈ and Its Relationship to Cubic TaN

Ammonolysis of (^tBuCH₂)₃Ta=CH^tBu (1) at 20 °C produced an intermediate pentamer, [(^tBuCH₂)₂TaN]₅ (2, ∼40%); further reaction generated an orange precipitate. Thermolysis of the latter at 400 °C and then 820 °C produced cubic TaN (Fm3m, a = 4.300 (2) Å) at temperatures and pressures substantially below those of conventional nitriding procedures. Ammonolysis of (^tBuCH₂)₂Ta(NMe₂)₃ (3) provided an alternative (19%), route to 2. ¹H, ¹³C, and ¹⁵N NMR spectra of 2 indicated C_2v symmetry, and X-ray crystallographic characterization of [(^tBuCH₂)₂TaN]₅·NH₃·2C₇H₈ (2·NH₃·2C₇H₈) revealed a roughly planar ladder structure of alternating (^tBuCH₂)₂TaN units. The interior nitrogens adopt a T-shaped coordination geometry and an NH₃ is weakly bound to an exterior Ta atom. Crystal data (2·NH₃·2C₇H₈): triclinic P1, λ(Mo Kα), μ = 7.068 mm^-1, a = 11.616 (2) Å, b = 15.407 (3) Å, c = 23.172 (6) Å, α = 106.120 (17)°, β = 99.060 (18)°, Γ = 100.170 (16)°, T = -20 °C, Z = 2, V= 3829.6 (12) ų, R = 0.083, R_w = 0.098, 7346 (72.8%) reflections where |F₀| ≥ 3σ(F₀). The conformation of 2·NH₃·2C₇H₈ reflects the motif of cubic TaN, rather than the thermodynamically expected hexagonal form, hence the framework of this ceramic may be dictated by the geometry of the molecular precursor.