Organogermanium Reactive Intermediates. The Direct Detection and Characterization of Transient Germylenes and Digermenes in Solution

Abstract

Diphenylgermylene (Ph2Ge) and its Ge=Ge doubly bonded dimer, tetraphenyldigermene (6a), have been characterized directly in solution for the first time by laser flash photolysis methods. The germylene is formed via (formal) cheletropic photocycloreversion of 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (4a), which is shown to proceed in high chemical (>95%) and quantum yield (phi = 0.62) by steady-state trapping experiments with methanol, acetic acid, isoprene, and triethylsilane. Flash photolysis of 4a in dry deoxygenated hexane at 23 degrees C leads to the prompt formation of a transient assigned to Ph2Ge (lambda(max) = 500 nm; epsilon(max) = 1650 M(-1) cm(-1)), which decays with second-order kinetics (tau approximately 3 micros), with the concomitant growth of a second transient species that is assigned to digermene 6a (tau approximately 40 micros; lambda(max) = 440 nm). Analogous results are obtained from 1,1-dimesityl- and 1,1-dimethyl-3,4-dimethylgermacyclopent-3-ene (4b and 4c, respectively), which afford Mes2Ge (tau approximately 20 micros; lambda(max) = 560 nm) and Me2Ge (tau approximately 2 micros; lambda(max) = 480 nm), respectively, as well as the corresponding digermenes, tetramesityl- (6b; lambda(max) = 410 nm) and tetramethyldigermene (6c; lambda(max) = 370 nm). The results for the mesityl compound are compared to the analogous ones from laser flash photolysis of the known Mes2Ge/6b precursor, hexamesitylcyclotrigermane. The spectra of the three germylenes and two of the digermenes are in excellent agreement with calculated spectra, derived from time-dependent DFT calculations. Absolute rate constants for dimerization of Ph2Ge and Mes2Ge and for their reaction with n-butylamine and acetic acid in hexane at 23 degrees C are also reported.

Re-examination of 7,7′-dimethyl-7-germanorbornadiene photocleavage in solution and in organic glasses

“…The value of e Me 4 Ge 2 is reported for the first time here, but it will be correlated with the value of the molar absorption coefficient of (2,4,6-Me 3 Ph) 4 Ge 2 , 20,000 M -1 cm -1 [23], but is four times larger than that of (2,4,6-Me 3 Ph) 4 Ge 2 . This difference can be explained by noting that tetramethyldigermene possesses more narrow bonds than (2,4,6-Me 3 Ph) 4 Ge 2 .…”

The photochemistry of a potential diphenylsilylene precursor. Time-resolved spectroscopic studies of the reactivity of a transient vinylsilirane

“…This is the only transient product that could be detected upon laser photolysis of 2; in particular, we could not detect signs of a short-lived transient absorbing in the 500 nm region, where SiPh 2 is known to absorb [17]. Our failure to detect transient absorptions assignable to SiPh 2 is not unexpected, given that the quantum yield for formation of the silylene from 2 is on the order of only SiPh2 ∼ 0.06, as estimated from the quantum yield for total consumption Given the typical laser intensities used in our experiments and assuming that the extinction coefficient of SiPh 2 at its absorption maximum is similar to that of GePh 2 (ε max = 1850 dm 3 mol −1 cm −1 [23]), this would result in an expected maximum transient absorbance of ca. 0.002, which would be barely distinguishable from noise under the conditions of our experiments.…”

“…Quenching by added acid is much slower; a similar treatment of the variation in the decay rate constant of 9 in the presence of 0.02-0.6 M methanesulfonic acid afforded a rate constant of k H+ = (2.6 ± 0.3) × 10 3 M −1 s −1 . The photochemical behavior of 2 contrasts sharply with that of the germanium homologue (1), whose photolysis yields GePh 2 and DMB cleanly and with a quantum yield = 0.55 under similar conditions (Equation (1)) [23]. It is difficult to determine with certainty whether or not the corresponding vinylgermirane (11a) is also formed in the photolysis of 1 in fluid solution; while no evidence for its formation could be obtained from steady state trapping studies, our subse- quent studies of the reactions of GePh 2 and GeMePh with isoprene and DMB [24,34] show that if it were formed as a primary photochemical product, it would dissociate to GePh 2 and DMB too rapidly for it to be trapped efficiently under typical experimental conditions [35].…”

“…Recently, we reported the results of a study of the photochemistry of 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (1) in hexane solution by steady state and laser flash photolysis methods (Equation (1)) [23]. Photolysis of the molecule proceeds very cleanly and with remarkably high efficiency ( ∼ 0.55), producing 2,3-dimethyl-1,3-butadiene (DMB) and the expected trapping products of diphenylgermylene as the only detectable photolysis products under steady state conditions.…”

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

“…Some of the con fusions over the solution studies have been clarified only recently. 41 The gas phase rate constants obtained allow compari son of the reactivity of silylenes and germylenes on a quantitative basis. The data presented in Table 3 show that GeH 2 is slightly less reactive than SiH 2 (usually, by factors of 2-4), but in general displays the same pattern of reactivity.…”