Tetrafluorosilane–amine adducts: stoicheiometry, thermodynamics of dissociation, stability, and hydrolysis

Abstract

Tetrafluorosilane combines with the following amines under varying conditions to give 1 : 2 adducts: NH_xMe_{3 –x}(x= 0–3), NH_xEt_{3 –x}(x= 0–2), NH₂R (R = Prⁿ, Buⁿ, Bu^s, Bu^t, or Ph), NHR₂(R = Prⁿ or Buⁿ; R₂= C₄H₈ or C₅H₁₀), NC₅H₅, NMe_xEt_{3 –x}(x= 1 or 2), and with NHR·[CH₂]_n·NHR (n= 2, R = Me or Et; n= 3, R = Me) to give 1 : 1 adduct. Solid tetrafluorosilane also reacts with a deficit of liquid pyridine to yield an approximately 1 : 1 adduct. Solid–vapour or liquid–vapour equilibria are reported for adducts of NH_xMe_{3 –x}(x= 0–3), NH_xEt_{3 –x}(x= 0 or 1), NMeEt₂, and NC₅H₅; the extent of dissociation in the vapour phase is estimated, and values of ΔH, ΔG, and ΔS for dissociation from the condensed phase are calculated. Competition experiments indicate the order of adduct stability: NH₃ NH₂Me > NH₂Ph > NHMe₂ > NMe₃. First-order rate constants for hydrolysis of SiF₄,2NH₃ and SiF₄,2NH₂Me are 1·0 × 10^–3 and 1·9 × 10^–3 s^–1 at 25 °C; qualitatively hydrolysis becomes more rapid in the order ammonia < primary amines < secondary amines < tertiary amines. Ease of dissociation and hydrolysis are related to hydrogen-bonding and steric factors.