Pnictogen-hydride activation by (silox)₃Ta (silox = ^tBu₃SiO); Attempts to circumvent the constraints of orbital symmetry in N₂ activation

Activation of N₂ by (silox)₃Ta (1, silox = ^tBu₃SiO) to afford (silox)₃Ta=N-N=Ta(silox) ₃ (1₂-N₂) does not occur despite ΔG°_cald = -55.6 kcal/mol because of constraints of orbital symmetry, prompting efforts at an independent synthesis that included a study of REH₂ activation (E = N, P, As). Oxidative addition of REH ₂ to 1 afforded (silox)₃HTaEHR (2-NHR, R = H, Me, ⁿBu, C₆H₄-p-X (X = H, Me, NMe₂); 2-PHR, R = H, Ph; 2-AsHR, R = H, Ph), which underwent 1,2-H₂- elimination to form (silox)₃Ta=NR (1=NR; R = H, Me, ⁿBu, C₆H₄-p-X (X = H (X-ray), Me, NMe₂, CF ₃)), (silox)₃Ta=PR (1=PR; R = H, Ph), and (silox) ₃Ta=AsR (1=AsR; R = H, Ph). Kinetics revealed NH bond-breaking as critical, and As > N > P rates for (silox)₃HTaEHPh (2-EHPh) were attributed to (1) ΔG°_calc(N) < ΔG° _calc(P) ∼ ΔG°_calc(As); (2) similar fractional reaction coordinates (RCs), but with RC shorter for N < P∼As; and (3) stronger TaE bonds for N > P∼As. Calculations of the pnictidenes aided interpretation of UV-vis spectra. Addition of H₂NNH₂ or H₂N-N(^cNC₂H₃Me) to 1 afforded 1=NH, obviating these routes to 1₂-N₂, and formation of (silox)₃MeTaNHNH2 (4-NHNH₂) and (silox) ₃MeTaNH(-^cNCHMeCH₂) (4-NH(azir)) occurred upon exposure to (silox)₃Ta=CH₂ (1=CH₂). Thermolyses of 4-NHNH₂ and 4-NH(azir) yielded [(silox)₂TaMe](μ- N_αHN_β)(μ-N_γHN _δH)[Ta(silox)₂] (5) and [(silox)₃MeTa] (μ-η²-N,N: η¹-C-NHNHCH₂CH ₂CH₂)[Ta(κ-O,C-OSi^tBu₂CMe ₂CH₂)(silox)₂] (7, X-ray), respectively. (silox)₃Ta=CPPh₃ (1=CPPh₃, X-ray) was a byproduct from Ph₃PCH₂ treatment of 1 to give 1=CH ₂. Addition of Na(silox) to [(THF)₂Cl₃Ta] ₂(μ-N₂) led to [(silox)₂ClTa](μ-N ₂) (8-Cl), and via subsequent methylation, [(silox) ₂MeTa]₂(μ-N₂) (8-Me); both dimers were thermally stable. Orbital symmetry requirements for N₂ capture by 1 and pertinent calculations are given.