The Prototype Ge−H Insertion Reaction of Germylene with Germane. Absolute Rate Constants, Temperature Dependence, RRKM Modeling and the Potential Energy Surface

Abstract

Time-resolved studies of germylene, GeH2, generated by laser flash photolysis of 3,4-dimethylgermacyclopentene-3, have been carried out to obtain rate constants for its bimolecular reaction with monogermane, GeH4. The reaction was studied in the gas phase over the pressure range 1−100 Torr, with SF6 as bath gas, at five temperatures in the range 292−520 K. The reaction of GeH2 with GeH4 to form digermane, Ge2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the following Arrhenius equation: log(k ∞ /cm3 molecule-1 s-1) = (−11.17 ± 0.10) + (5.2 ± 0.7 kJ mol-1)/RT ln 10. These Arrhenius parameters are consistent with a moderately fast reaction occurring at approximately one-fifth of the collision rate. RRKM modeling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure dependent curves, for a suitable choice of the critical energy, E o , for reverse decomposition of Ge2H6. The step size (energy removed in a down collision) was chosen by analogy with the corresponding system for Si2H6 (collisional efficiency (βc) of ca. 0.7 for SF6). The value obtained for E o was 155 kJ mol-1. Corrected for thermal energy and combined with the insertion activation energy this gives Δ H° = 166 kJ mol-1 for the decomposition of Ge2H6. There is no previous experimental determination of this quantity. From it we derive = 237 ± 12 kJ mol-1, in reasonable agreement with earlier estimates. From bond dissociation energy values the Divalent State Stabilization Energy (DSSE) of germylene (119 kJ mol-1) is larger than that of silylene (94 k J mol-1). Ab initio calculations at the correlated level reveal the presence of two weak complexes (local energy minima) on the potential energy surface corresponding to either direct or inverted geometry of the inserting germylene fragment. Surprisingly, the latter is the lower in energy, lying 25 kJ mol-1 below the unassociated reactants. These complexes rearrange to digermane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.

Gas phase kinetic and theoretical studies of reactions of germylenes and dimethylstannylene

“…The lack of reaction of GeH 2 with propane at room temperature on the time scale of the experiments allowed setting an upper limit to the rate constant. Kinetic experiments carried out in SF 6 with another precursor, MesGeH 3 , for two substrates, Me 3 SiH and buta 1,3 diene, 35 showed good agreement with the data obtained using DMGCP as the precursor (Table 1). However, when we generated GeH 2 from PhGeH 3 , the measured rate constants appeared signifi cantly lower than those obtained using DMGCP and MesGeH 3 .…”

“…34 In the time resolved studies, GeH 2 was generated by photodecomposition of 3,4 dimethyl germacyclopent 3 ene (DMGCP) 23 or phenylgermane, PhGeH 3 , 23, 24 although several experiments were also car ried out using MesGeH 3 . 35 Phenylgermane is a known photochemical source of GeH 2 . 36-38 By analogy with this source and also with the precursors of dimethylgermylene, MesGeH 3 and DMGCP were also tested as precursors of GeH 2 .…”

“…58 Each complex has an associated transition state, TS1 and TS2, linking it to digermane. 35 The passage via C1 and TS1 to digermane involves an inversion of con figuration of the GeH 3 group that is formed, while that via C2 does not. Complex C1 has a C 1 symmetry and exists as left (C1(l)) and right handed (C1(r)) forms, which are separated by a very low lying transition state TS0.…”

Gas‐Phase Chemistry and Mass Spectrometry ofGe‐,Sn‐ andPb‐Containing Compounds

“…The analogous reaction to 46, but using SiH 2 , reveals that silylene insertion in the Ge−H bond is faster than that of the corresponding germylene 237 . These fast insertion reactions display a negative activation energy 238 , and high level ab initio calculations 236 confirm the suggestion that these reactions proceed through a transition state where a hydrogen bridge between the two germanium atoms mediates formation of the final product in reaction 46.…”

“…More detailed studies on the insertion of GeH 2 have been obtained from the reaction with Me 3 SiH and GeH 4 (equations [45][46][47] 235,236 .…”