Surprisingly Slow Reaction of Dimethylsilylene with Dimethylgermane:  Time-Resolved Kinetic Studies and Related Quantum Chemical Calculations

Abstract

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm3 molecule(-1) s(-1)) = -13.25 +/- 0.16 and E(a) = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.

Kinetic studies of reactions of organosilylenes: what have they taught us?

“…A considerable number of rate constants have been measured at room temperature 10,43,44 and a selected number of these over an extended temperature range. [45][46][47][48][49] Rate constants for SiMe 2 reactions are considerably less pressure dependent than those of their SiH 2 analogues in the practical range of measurement. This is because products (or association complexes) have more vibrational modes over which to distribute the energy released upon their formation.…”

“…The low A factors and high negative activation energies (for the substituted silylenes compared with SiH 2 ) would all be hard to explain if intermediate complexes were not involved. However any lingering doubts about such a mechanism should be removed by the recent striking and unexpected result that both SiMe 2 45 and ClSiH 27 insert more slowly into Ge-H bonds than into Si-H bonds. The rate constants on which this is based are given in Table 4.…”

Time-resolved gas-phase kinetic studies of the reaction of dimethylsilylene with triethylsilane-1-d: kinetic isotope effect for the Si–H insertion process

“…While there have been no previous calculations of the energy surface for the reaction of SiMe 2 with Me 3 SiH, calculations of the energy barriers for other Si-H insertion processes of SiMe 2 have been carried out earlier by ourselves in conjunction with our Russian colleagues. 12 These show the same characteristics, viz. the presence of an intermediate complex and a low barrier transition state.…”

“…There is a similar, but reduced discrepancy for the reaction of SiMe 2 with MeSiH 3 . 12 While there may be enough uncertainty in experiment, kinetic analysis and theory to accommodate these differences, this remains a problem needing further consideration. On the positive side, it should be noted that the predictions of the overall reaction enthalpy change, À214 kJ mol À1 (G3) and À215 kJ mol À1 (G3B3) (Table 3) are in good agreement with the value of À218 kJ mol À1 obtained by use of the Allen bond additivity scheme based on the calorimetric value of DH o f (Si 2 Me 6 ).…”

“…Starting in 1987, absolute rate coefficients became available through the employment of techniques of timeresolved laser flash photolysis. By this means more detailed kinetic and mechanistic information about the behaviour of SiMe 2 both in the gas phase [9][10][11][12][13] and in solution [14][15][16][17][18][19][20] has been obtained in recent years. We recently reviewed this growing database.…”