Direct Time-Resolved Study of the Gas-Phase Reactions of Germylene with Ethyl- and Diethylgermane:  Absolute Rate Constants, Temperature Dependences, and Mechanism

Abstract

Time-resolved studies of germylene, GeH2, generated by the 193 nm laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reactions with ethyl- and diethylgermanes in the gas phase. The reactions were studied over the pressure range 1-100 Torr with SF6 as bath gas and at five temperatures in the range 297-564 K. Only slight pressure dependences were found for GeH2 + EtGeH3 (399, 486, and 564 K). The high pressure rate constants gave the following Arrhenius parameters: for GeH2 + EtGeH3, log A = -10.75 +/- 0.08 and Ea = -6.7 +/- 0.6 kJ mol-1; for GeH2 + Et2GeH2, log A = -10.68 +/- 0.11 and Ea = -6.95 +/- 0.80 kJ mol-1. These are consistent with fast, near collision-controlled, association processes at 298 K. RRKM modeling calculations are, for the most part, consistent with the observed pressure dependence of GeH2 + EtGeH3. The ethyl substituent effects have been extracted from these results and are much larger than the analogous methyl substituent effects in the SiH2 + methylsilane reaction series. This is consistent with a mechanistic model for Ge-H insertion in which the intermediate complex has a sizable secondary barrier to rearrangement.

What have we learnt about heavy carbenes through laser flash photolysis studies?

“…This is supported by much reduced pressure dependences for the reactions of SiH 2 with the methylsilanes 58 and GeH 2 with the ethylgermanes. 59 (b) Addition reactions. The prototype reactions:…”

A comparison of the reactivity of germylene and dimethylgermylene with some methylgermanes. Direct kinetic and quantum chemical studies

“…It should be noted that Et 2 GeH 2 does have a larger room temperature absorption cross section (4.3 Â 10 À19 cm 2 ) at 193 nm. 26 For each reaction it was independently verified during preliminary experiments that, in a given reaction mixture, k obs values were not dependent on the exciplex laser energy or number of photolysis shots. Because static gas mixtures were used, tests with up to 12 shots were carried out.…”

“…[9][10][11] The decrease in reactivity has been attributed to the electron withdrawing effect of the methyl groups in the silylene. 5,6 Since 1996 we 7,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and others [27][28][29] have been carrying out similar time-resolved kinetic studies of germylenes in the gas phase. While these studies have mainly focussed on GeH 2 , 7,13-24,26-29 some investigations of GeMe 2 have also been carried out.…”

Time-resolved gas-phase kinetic study of the germylene addition reaction, GeH2 + C2D2

“…4 In 1996, in collaboration with the group of Egorov and Nefedov, we reported the first rate coefficients for reactions of GeH 2 obtained by time-resolved means in the gas phase. 5 Since then we, [6][7][8][9][10][11][12][13][14][15][16][17][18] and the group of King and Lawrance, [19][20][21] have carried out a number of further detailed gas-phase kinetic studies of GeH 2 reactions, the results of which have been summarised in two recent reviews. 22,23 These investigations show that GeH 2 typically undergoes bond insertion reactions (into Ge-H, Si-H and O-H bonds), p-type addition reactions across CQC and CRC bonds and reaction with lone pair donor molecules such as ethers.…”