Spectroscopic characterization of diarylgermylene complexes with heteroatom-containing substrates

Time-resolved spectroscopic studies of the photochemistry of some diphenylgermylene (Ph2Ge:) precursors

“…It is noteworthy that photolysis of the dimesityl analogue of 2a (6) also affords the corresponding germylene (in this case dimesitylgermylene, Mes 2 Ge) quite cleanly, as shown by the results of steady-state trapping experiments, matrix photolysis experiments (1,16), and laser flash photolysis experiments in solution (7). In this case, the transient spectrum observed (and assigned to Mes 2 Ge, λ max~5 50 nm) in solution (7) matches that obtained in matrix experiments (1,16) or in solution from other precursors (7,9) quite closely.…”

“…Photolysis of the compound in cyclohexane solution in the presence of 2,3-dimethyl-1,3-butadiene (DMB) was found to afford hexamethyldisilane (Si 2 Me 6 ), trimethylphenylsilane (PhSiMe 3 ), and the diene trapping product 5 as the only detectable products (eq. [2]) (6), while laser flash photolysis of the compound afforded a transient that decayed over several hundred microseconds and exhibited λ max = 445 nm (2), blue-shifted somewhat relative to the spectrum recorded for this compound in the earlier matrix experiments (1,16). Absolute rate constants were determined for reaction of the species with various germylene scavengers, among them oxygen (1.02 × 10 8 (mol/L) -1 s -1 ), ethyldimethylsilane (k = 1.01 × 10 4 (mol/L) -1 s -1 ), 1-bromo-2-methylpropane (k = 6.36 × 10 5 (mol/L) -1 s -1 ), and 2,3-dimethyl-1,3-butadiene (2.75 × 10 4 (mol/L) -1 s -1 ) (2).…”

“…Finally, we examined the flash photolysis behavior of 2a in deoxygenated tetrahydrofuran (THF) solution, as Ph 2 Ge and other reactive germylenes are known to form Lewis acid-base complexes with ethers, with substantially blueshifted absorption maxima compared to those of the free species (16,17); for example, generation of Ph 2 Ge in a 2methyltetrahydrofuran glass at 77 K leads to a species absorbing at λ max = 325 nm, which was assigned to the germylene-ether complex (17). The transient spectra obtained by flash photolysis of 2a in THF solution are similar to those of Fig.…”

“…The rationale behind the latter statement is as follows. The rate constants for reaction of AcOH and MeOH with germene 8 (one of only two germenes of this type that have been studied (18)) are on the order of 4 × 10 5 mol/L -1 s -1 or less, three orders of magnitude lower than the homologous silene derivative (16) (18). The reactivity of 9 with nucleophiles can be expected to be significantly lower than that of 8 because of the presence of the trimethylsilyl substituent at Ge, based on analogies in transient silene chemistry; trimethylsilyl substitution at silicon in silenes reduces the re-activity of the Si=C bond by roughly a factor of 10 relative to a methyl substituent (35), which is similar to a phenyl group in its effects on Si=C reactivity (36,37).…”

“…In hexane solution at room temperature, diphenylgermylene exhibits λ max = 500 nm (⑀ = 1650 ± 390 (mol/L) -1 cm -1 ) and dimerizes at close to the diffusion-controlled rate to yield the transient 1,1,2,2-tetraphenyldigermene (Ph 2 Ge=GePh 2 , λ max = 440 nm, τ~40 µs) (9). None of the UV spectra previously assigned to Ph 2 Ge -including that reported by Ando et al in 3-MP matrices at 77 K (1,16,17) -match the one reported more recently in solution using 5 as the precursor.…”