Volatility and High Thermal Stability in Tantalum Complexes Containing Imido, Amidinate, and Halide or Dialkylamide Ligands

Treatment of Ta(NtBu)Cl3(py)2 with 2 equiv of Li(iPrNCMeNiPr) or Li(tBuNCMeNtBu) afforded Ta(NtBu)(iPrNCMeNiPr)2Cl and Ta(NtBu)(tBuNCMeNtBu)2Cl in 63% and 61% yields, respectively. Treatment of Ta(NtBu)(iPrNCMeNiPr)2Cl or Ta(NtBu)(tBuNCMeNtBu)2Cl with LiNRR′ afforded Ta(NtBu)(iPrNCMeNiPr)2(NRR′) and Ta(NtBu)(tBuNCMeNtBu)2(NRR′) in 79−92% yields (R, R′ = Me, Et). Treatment of Ta(NtBu)(tBuNCMeNtBu)2Cl with AgBF4 afforded Ta(NtBu)(tBuNCMeNtBu)2F in 54% yield, while treatment of Ta(NtBu)(tBuNCMeNtBu)2Cl with BBr3 afforded Ta(NtBu)(tBuNCMe-NtBu)2Br in 68% yield. X-ray crystal structures of Ta(NtBu)(tBuNCMeNtBu)2F and Ta(NtBu)(tBuNCMeNtBu)2Br revealed that the amidinate ligands exhibit η2-coordination, and that the imido and halide ligands are cis to each other within the distorted octahedral structures. NMR studies indicated that the other complexes have analogous structures. Additionally, variable temperature NMR studies revealed that some of the complexes undergo amidinate rearrangement. The enthalpies, entropies, and free energies of activation for these rearrangements were calculated for Ta(NtBu)(tBuNCMeNtBu)2X (X = F, Cl, Br). When X = F, ΔH‡ = 9.1 ± 0.4 kcal/mol, ΔS‡ = −20.5 ± 1.6 cal/mol·K, and ΔG‡(298 K) = 15.3 ± 0.7 kcal/mol. For X = Cl, ΔH‡ = 12.4 ± 0.3 kcal/mol, ΔS‡ = −20.2 ± 0.8 cal/mol·K, and ΔG‡(298 K) = 18.4 ± 0.3 kcal/mol. When X = Br, ΔH‡ = 12.5 ± 0.5 kcal/mol, ΔS‡ = −21.7 ± 1.5 cal/mol·K, and ΔG‡(298 K) = 19.0 ± 0.7 kcal/mol. All of the complexes are volatile, and they sublime between 120 and 203 °C. In addition, Ta(NtBu)(iPrNCMeNiPr)2NMe2 has a decomposition point that is 65−160 °C higher than widely used film growth precursors and is therefore a promising candidate for use in chemical vapor deposition and atomic layer deposition film growth techniques.