J. Am. Chem. Soc., 126, 16105-16116 (2004).
© 2004 American Chemical Society
William J. Leigh,* Cameron R. Harrington, and Ignacio Vargas-Baca
Contribution from the Department of Chemistry, McMaster University, Hamilton, ON Canada L8S 4M1
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 of 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (4a). Steady state (lamp) photolysis of 4a in the presence of methanol, acetic acid, isoprene, or triethylsilane affords the expected trapping products of Ph2Ge in chemical yields of >95% and a quantum yield of 0.55, while high intensity laser photolysis of 4a in the presence of millimolar concentrations of methanol affords the product of 1,2-addition of the alcohol to digermene 6a in addition to the germylene-alcohol adduct. Flash photolysis of 4a in dry deoxygenated hexane at 23 oC leads to the prompt formation of a transient assigned to Ph2Ge (lmax=500 nm; emax=1850 M-1cm-1), which decays with second-order kinetics (t ~ 3 ms). A rate constant of kdim = 1.2 x 1010 M-1s-1 for the dimerization of Ph2Ge has been estimated from the data. The decay of the germylene is accompanied over the same timescale by the formation of a second species assigned to digermene 6a (t ~ 40 ms; lmax = 440 nm), which also decays with second-order kinetics. Analogous results are obtained from 1,1-dimesityl-3,4-dimethylgermacyclopent-3-ene (4b), which affords the known Mes2Ge (t ~ 20 ms; lmax = 560 nm) and tetramesityldigermene (lmax = 410 nm); the results for this system are compared to the analogous ones from laser flash photolysis of hexamesitylcyclotrigermane, from which has been estimated an absolute rate constant of kdim = 6 x 109 M-1s-1 for the dimerization of Mes2Ge in hexane solution at 23 oC. The rates of decay of the signals due to Ph2Ge and Mes2Ge are accelerated upon addition of germylene scavengers such as n-butyl amine and acetic acid and the formation of the secondary (digermene) product is quenched; absolute rate constants for reaction with these reagents are reported. 1,1,3,4-Tetramethylgermacyclopent-3-ene (4c) has also been studied by (193 nm) laser flash photolysis, and yields transients assigned to Me2Ge (t ~ 2 ms; lmax = 480 nm) and tetramethyldigermene (6c; lmax = 375 nm) on the basis of comparisons to the known gas- or solution-phase spectra of these compounds. The spectra of the three germylenes and two of the digermenes are in excellent agreement with calculated spectra, derived from time-dependent DFT calculations.
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