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 REH2 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, CF3)), (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-NαHNβ)(μ-NγHNδH)[Ta(silox)2] (5) and [(silox)3MeTa](μ-η2-N,N:η1-C-NHNHCH2CH2CH2)[Ta(κ-O,C-OSitBu2CMe2CH2)(silox)2] (7, X-ray), respectively. (silox)3TaCPPh3 (1CPPh3, X-ray) was a byproduct from Ph3PCH2 treatment of 1 to give 1CH2. Addition of Na(silox) to [(THF)2Cl3Ta]2(μ-N2) led to [(silox)2ClTa](μ-N2) (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.