The reaction between silylene and ammonia: Some gas-phase kinetic and quantum chemical studies

Abstract

Time-resolved kinetic studies of the reaction of silylene, SiH 2 , generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH 3 . Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF 6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ/pulse) fitted the Arrhenius equation:Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10-100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ/pulse. Ab initio calculations at the G3 level of theory, show that SiH 2 +NH 3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH 2 +NH 3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH 2 decays are better explained by reaction of SiH 2 with the amino radical, NH 2 , formed by photodissociation of NH 3 at 193 nm. The mechanism of this previously unstudied reaction is discussed.

Kinetics and mechanisms of the reactions of transient silylenes with amines

“…The noncatalytic components of the calculated pathways for the reaction of SiMe 2 with MeNH 2 and MeOH are generally analogous to those reported in previous theoretical studies of the reactions of SiH 2 with ammonia, water, and MeOH . This aspect of the results for the SiMe 2 + MeOH system is also in reasonable agreement with those reported by Su from calculations at the CCSD(T)/LANL2DZdp//B3LYP/LANL2DZ level of theory, but the G4 method predicts a greater stabilization of the SiMe 2 –MeOH complex (C O ) and the transition state for unimolecular H‐migration (TS O ) by 2–3 kcal mol –1 , and of the net insertion product by about 6 kcal mol –1 , compared to the earlier calculations.…”

“…Thus, the increase in stability of about 4 kcal mol –1 that results from replacing the first H in ammonia with Me is followed by additional stabilization of only 2 kcal mol –1 upon replacing the second, and a decrease in stability of about 1 kcal mol –1 upon replacing the third. The calculated enthalpy of complexation of SiMe 2 with ammonia (−19.5 kcal mol –1 ) can be compared to the G3‐value of −25.3 kcal mol –1 reported by Becerra et al for the SiH 2 +NH 3 system . The difference is similar to those reported for other reactions of SiH 2 and SiMe 2 with common substrates …”

“…The intercepts of the plots gave values of k decay = (7 ± 1) × 10 4 s –1 and (3.6 ± 0.4) × 10 4 s –1 , respectively , and define the upper limits of the rate constants for unimolecular H‐migration in the complexes. These values are indicative of free energy barriers in excess of 11 kcal mol –1 at 25 °C, which can be compared to the theoretically‐predicted values of 33–38 kcal mol –1 for the corresponding process in the SiH 2 –NH 3 complex …”

“…These reactions have been well‐studied theoretically, and as a result are generally considered to proceed with the initial rapid formation of a Lewis acid–base complex between the silylene and the substrate, followed by rate‐controlling migration of hydrogen from oxygen or nitrogen to silicon to form the net insertion product. Indeed there is abundant experimental evidence that, in general, silylenes are ardent participants in complexation processes with Lewis bases and that complexation with the basic site in the substrate is the first step in the reactions with water and alcohols, as well as ammonia and alkylamines . In the case of the oxygenated substrates, this proceeds at rates close to the encounter‐controlled limit in both the gas phase and in solution, for both SiH 2 and the dimethyl‐derivative (SiMe 2 ) in the gas phase and for SiMe 2 and simple (transient) arylalkyl‐ and diarylsilylenes in solution .…”

“…The complexation of SiMe 2 and SiMes 2 with amines and various other donors was studied in solution and low temperature matrixes in several early seminal studies of the chemistry of these prototypical silylene derivatives, and one study in particular explored the effects of donor‐complexation on the rates of several representative silylene reactions . However, the mechanisms of the uni‐ and bimolecular reactions of the complexes have only been studied theoretically . The actual course of the reactions of SiMe 2 with Et 2 NH and other (primary and secondary) amines was reported in the early 1980s by Gu and Weber…”