Laser Flash Photolysis Studies of Some Dimethylgermylene

Organometallics 2009, 28, 3239–3246

3239

Laser Flash Photolysis Studies of Some Dimethylgermylene Precursors Farahnaz Lollmahomed and William J. Leigh* Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1 ReceiVed February 3, 2009

The laser flash photolysis of dodecamethylcyclohexagermane (1) and dimethylphenyl(trimethylsilyl)germane (5), two known photochemical precursors to dimethylgermylene (GeMe2), has been reinvestigated. Laser flash photolysis of 1 in hexane solution affords markedly different results than were reported in an earlier study of this compound in cyclohexane. The present study shows it to yield two primary transient products, one exhibiting λmax ) 490 nm and lifetime τ e 10 ns and a second that exhibits λmax ) 470 nm and decays on the microsecond time scale with second-order kinetics, with the concomitant growth of absorptions centered at λmax ) 370 nm due to tetramethyldigermene (Ge2Me4). The spectrum and absolute rate constants for quenching of the 470 nm species by acetic acid, 2,3-dimethyl-1,3-butadiene (DMB), CCl4, and oxygen in hexane, as well as the transient behavior observed in the presence of millimolar concentrations of THF, are all quite similar to those established recently for GeMe2 using 1-germacyclopent-3-ene derivatives as precursors. Two different transient products dominate the time-resolved spectra obtained from laser flash photolysis of 5 under similar conditions, one exhibiting λmax ) 300 nm and τ ≈ 20 µs and a second exhibiting λmax ) 430 nm and τ ≈ 4 µs. The latter species was assigned to GeMe2 in earlier studies; it is re-assigned in the present work to the conjugated germene (6) derived from 1,3trimethylsilyl migration into the ortho-position of the phenyl ring in 5, on the basis of comparisons of the spectrum and absolute rate constants for quenching of the species by oxygen, DMB, CCl4, acetic acid, and acetone to those reported previously for the homologous silene derivative (11) and the corresponding 1,1-diphenyl-substituted analogues of the germene (12) and the silene (13). Weak transient absorptions consistent with the postpulse formation of Ge2Me4 are also detectable from 5 in deoxygenated hexanes, but GeMe2 itself cannot be detected. Introduction The chemistry and spectroscopic properties of dimethylgermylene (GeMe2) have been studied extensively over the past few decades in the gas phase,1,2 in solution,3,4 and in lowtemperature matrixes,5 and much of what is known today of germylene reactivity in solution is derived from product studies carried out with thermal and/or photochemical precursors to this particular germylene derivative.3,4 The species was first detected directly by UV-vis spectrophotometry in low-temperature matrixes,6 where its longest wavelength electronic absorption is centered at 420-430 nm in hydrocarbons at 77 K5-14 and * Corresponding author. E-mail: [email protected]. (1) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Krylova, I. V.; Nefedov, O. M.; Becerra, R.; Walsh, R. Russ. Chem. Bull. Int. Ed. 2005, 54, 483. (2) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2817. (3) Neumann, W. P. Chem. ReV. 1991, 91, 311. (4) Neumann, W. P.; Weisbeck, M. P.; Wienken, S. Main Group Metal Chem. 1994, 17, 151. (5) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Nefedov, O. M. In The Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; John Wiley and Sons: New York, 2002; Vol. 2, pp 749839. (6) Sakurai, H.; Sakamoto, K.; Kira, M. Chem. Lett. 1984, 1379. (7) Ando, W.; Tsumuraya, T.; Sekiguchi, A. Chem. Lett. 1987, 317. (8) Tomoda, S.; Shimoda, M.; Takeuchi, Y.; Kajii, Y.; Obi, K.; Tanaka, I.; Honda, K. Chem. Commun. 1988, 910. (9) Ando, W.; Itoh, H.; Tsumuraya, T. Organometallics 1989, 8, 2759. (10) Kolesnikov, S. P.; Egorov, M. P.; Dvornikov, A. S.; Kuz’min, V. A.; Nefedov, O. M. Organomet. Chem. USSR 1989, 2, 414.

λmax ≈ 405 nm in argon at 12-18 K;15 similar spectra in hydrocarbon matrixes were obtained by photolysis of at least six different precursors. The direct detection of GeMe2 in solution by laser flash photolysis methods has been much more problematic. Several such studies were reported in the late 1980s and early 1990s, using many of the same precursors that were employed in the low-temperature matrix studies.8,11-13,16-18 These placed the absorption maximum of GeMe2 at various wavelengths between 420 and 500 nm in hydrocarbon solvents, with the spectrum, lifetime, and measured rate constants for reaction of various substrates with the species assigned to GeMe2 varying with the precursor employed. The spectrum in the gas phase, as established in experiments employing 193 nm laser photolysis of two different GeMe2 precursors, is centered at λmax ≈ 480 nm,19 red-shifted quite significantly from that observed in low-temperature matrixes. (11) Wakasa, M.; Yoneda, I.; Mochida, K. J. Organomet. Chem. 1989, 366, C1. (12) Mochida, K.; Yoneda, I.; Wakasa, M. J. Organomet. Chem. 1990, 399, 53. (13) Mochida, K.; Kanno, N.; Kato, R.; Kotani, M.; Yamauchi, S.; Wakasa, M.; Hayashi, H. J. Organomet. Chem. 1991, 415, 191. (14) Mochida, K.; Tokura, S.; Murata, S. Chem. Commun. 1992, 250. (15) Barrau, J.; Bean, D. L.; Welsh, K. M.; West, R.; Michl, J. Organometallics 1989, 8, 2606. (16) Bobbitt, K. L.; Maloney, V. M.; Gaspar, P. P. Organometallics 1991, 10, 2772. (17) Mochida, K.; Kikkawa, H.; Nakadaira, Y. J. Organomet. Chem. 1991, 412, 9. (18) Mochida, K.; Tokura, S. Bull. Chem. Soc. Jpn. 1992, 65, 1642.

10.1021/om9000814 CCC: $40.75  2009 American Chemical Society Publication on Web 05/06/2009

3240 Organometallics, Vol. 28, No. 11, 2009

Most of the early solution-phase assignments were made in conjunction with studies of the photochemistry of various group 14 catenates, such as the six- and five-membered cyclogermanes 113 and 2,18 respectively, phenyl-substituted digermanes17,20 and trigermanes11,12 (e.g., 3 and 4), and silylgermanes such as 5.16,17 In most cases, trapping experiments were consistent with the formation of GeMe2 as a major (transient) photoproduct, and this guided transient assignments in laser flash photolysis studies. However, germylene formation was often accompanied by the products of other, minor photolytic pathways, particularly with phenylated systems. For example, the photolysis of silylgermane 5 was shown to yield trapping products consistent with the formation of GeMe2 (55-78%16) and its coproduct (PhSiMe3), germene 6 (20-28%16), and trace amounts of the dimethylphenylgermyl and trimethylsilyl radicals (eq 1).16,17 Flash photolysis of the compound in cyclohexane was found to afford a longlived transient with λmax ≈ 430 nm, which was assigned to GeMe2 on the basis of the similarity of the spectrum to the 77 K hydrocarbon matrix spectrum, the indication from product studies that the germylene is the major transient photoproduct, and in one of the papers,16 an exhaustive list of rate constants for reaction of the species with dienes and various other substrates. We have suggested that the spectroscopic and kinetic behavior of the species assigned to GeMe2 in these studies is in fact more reasonably attributable to the corresponding germene (6), which is derived from photochemical [1,3]-silyl migration into the ortho-position of a phenyl ring in the precursor.21 Silenes of this general structure are well known to be formed in significant yields upon photolysis of phenylated di- and trisilanes in solution at ambient temperatures22 and similarly confounded early attempts to detect and study transient phenylsilylenes by flash photolysis in solution23,24 owing to their strong (π,π*) absorptions throughout the 400-500 nm spectral range.25

Lollmahomed and Leigh

products consistent with the formation of GeMe2 in high yields in both cases, but they afforded markedly different results in low-temperature matrix and solution-phase flash photolysis experiments.13,18 For example, flash photolysis of 1 in cyclohexane afforded a species decaying with second-order kinetics and exhibiting λmax ≈ 450 nm; the decay of the species was accompanied by the growth of absorptions assignable to tetramethyldigermene (Ge2Me4; λmax ) 370 nm), and it was therefore assigned to GeMe2.13 Flash photolysis of 2 under similar conditions was reported to yield a transient exhibiting λmax ≈ 500 nm, which was also assigned to GeMe2 based on its kinetic behavior and the characteristic delayed formation of the absorptions due to Ge2Me4.13,18 The spectra obtained upon low-temperature matrix photolysis of the two compounds were also quite different.13,18 Our own work in this area has employed the 1,1-dimethylgermacyclopent-3-ene derivatives 7a-c as photochemical precursors to GeMe2 (eq 2).26-29 These compounds distinguish themselves from most of the earlier precursors by the fact that they afford GeMe2 in essentially quantitatiVe chemical yields and with quantum yields of ca. 50%.26,27 Laser flash photolysis of all three compounds in hexane affords a species with a welldefined UV-vis spectrum centered at λmax ) 470 nm26,27 (ε ) 730 ( 300 dm3 mol-1 cm-1),27 which decays on the microsecond time scale with second-order kinetics, coincident with the growth of the characteristic absorptions due to Ge2Me4. The spectrum of the first-formed species and the absolute rate constants for its reaction with a variety of characteristic germylene scavengers all correspond closely to the gas phase data of Walsh and co-workers for GeMe2.1,19 The data also correlate in rational ways with those for other transient germylenes such as GeMePh30 and GePh2,31 as well as the silicon homologue, SiMe2.32-35 They do not agree with those assigned to GeMe2 in any of the earlier solution-phase studies.

While the assignment of GeMe2 in our laser photolysis experiments with 7a-c seems compelling enough, it remains to be ratified with corresponding spectral and kinetic data obtained using a different, structurally unrelated precursor. We note that a species with qualitatively similar spectral characteristics has been detected in a recent reinvestigation36 of the Some of the other early reports are more difficult to rationalize. For example, photolysis of the cyclic GeMe2 oligomers 1 and 2 in cyclohexane was found to yield trapping (19) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Lee, V. Y.; Nefedov, O. M.; Walsh, R. Chem. Phys. Lett. 1996, 250, 111. (20) Mochida, K.; Wakasa, M.; Nakadaira, Y.; Sakaguchi, Y.; Hayashi, H. Organometallics 1988, 7, 1869. (21) Leigh, W. J.; Toltl, N. P.; Apodeca, P.; Castruita, M.; Pannell, K. H. Organometallics 2000, 19, 3232. (22) Ishikawa, M.; Kumada, M. AdV. Organomet. Chem. 1981, 19, 51. (23) Gaspar, P. P.; Boo, B. H.; Chari, S.; Ghosh, A. K.; Holten, D.; Kirmaier, C.; Konieczny, S. Chem. Phys. Lett. 1984, 105, 153. (24) Gaspar, P. P.; Holten, D.; Konieczny, S.; Corey, J. Y. Acc. Chem. Res. 1987, 20, 329. (25) Leigh, W. J.; Moiseev, A. G.; Coulais, E.; Lollmahomed, F.; Askari, M. S. Can. J. Chem. 2008, 86, 1105.

(26) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem. Soc. 2004, 126, 16105. (27) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R. Organometallics 2006, 25, 2055. (28) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald, J. M. Organometallics 2006, 25, 5424. (29) Lollmahomed, F.; Huck, L. A.; Harrington, C. R.; Chitnis, S. S.; Leigh, W. J. Organometallics 2009, 28, 1484. (30) Leigh, W. J.; Dumbrava, I. G.; Lollmahomed, F. Can. J. Chem. 2006, 84, 934. (31) Leigh, W. J.; Harrington, C. R. J. Am. Chem. Soc. 2005, 127, 5084. (32) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics 1989, 8, 1206. (33) Yamaji, M.; Hamanishi, K.; Takahashi, T.; Shizuka, H. J. Photochem. Photobiol. A: Chem. 1994, 81, 1. (34) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6268. (35) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6277. (36) Gorner, H.; Lehnig, M.; Weisbeck, M. J. Photochem. Photobiol. A: Chem. 1996, 94, 157.

Studies of Some Dimethylgermylene Precursors

Organometallics, Vol. 28, No. 11, 2009 3241

Figure 1. Transient absorption spectra recorded by laser flash photolysis of deoxygenated solutions of dodecamethylcyclohexagermane (1) in (a) hexanes (80-112 ns (O) and 2.0-2.1 µs (0) after the pulse) and (b) hexanes containing 15 mM THF (80-112 ns (O) and 5.38-5.41 µs (0) after the pulse). The insets show transient growth/decay profiles at 470, 370, and 310 nm, from the data sets used to construct the spectra. The sharp spikes present at the beginning of the 470 nm decays are due to an ultrashort-lived species that decays with the laser pulse; its spectrum (recorded at the peak of the laser pulse) is shown as the dashed lines in (a) and (b).

laser flash photolysis behavior of benzogermanorbornadiene 8,37 its absorptions superimposed on the much stronger, longer-lived ones due to the triplet state of the coproduct of GeMe2 extrusion, 1,2,3,4-tetraphenylnaphthalene.36 Unfortunately however, no kinetic data have been reported that might establish its identity as the same species observed in gas-19 and solution-phase26,27 experiments with 1-germacyclopent-3-ene derivatives as GeMe2 precursors. Given the difficulties that we38 and others36 have experienced in detecting transient germylenes from precursors of this general structure, we decided to revisit two of the other GeMe2 precursors that were reported in earlier studies.

In this paper, we report the results of a reinvestigation of the laser flash photolysis of dodecamethylcyclohexagermane (1), carried out with the intent of refining (or otherwise explaining) the transient spectrum that was published previously from this compound.13 Considering the success that a number of groups have had in laser flash photolysis studies of SiMe2 using the silicon homologue of 1 as precursor,32-35,39,40 the lack of agreement between the behavior reported for 113 and that of the principle transient product in our experiments with 7a-c26-29 is particularly difficult to rationalize. We have also studied the flash photolysis behavior of silylgermane 5, with the goal of confirming our preliminary reassignment of the transient product that dominates the time-resolved spectra obtained from this compound21 and to see whether GeMe2 might in fact be detectable in laser flash photolysis experiments, buried under the much stronger absorptions due to the other transient products.

Results and Discussion 1. Dodecamethylcyclohexagermane (1). Dodecamethylcyclohexagermane (1) was synthesized by the method of Carberry (37) Kaletina, M. V.; Plyusnin, V. F.; Grivin, V. P.; Korolev, V. V.; Leshina, T. V. J. Phys. Chem. A 2006, 110, 13341. (38) Harrington, C. R.; Leigh, W. J.; Chan, B. K.; Gaspar, P. P.; Zhou, D. Can. J. Chem. 2005, 83, 1324. (39) Shizuka, H.; Tanaka, H.; Tonokura, K.; Murata, K.; Hiratsuka, H.; Ohshita, J.; Ishikawa, M. Chem. Phys. Lett. 1988, 143, 225. (40) Levin, G.; Das, P. K.; Lee, C. L. Organometallics 1988, 7, 1231.

et al.41 and was obtained in yields of 6-18% after purification by column chromatography and multiple recrystallizations from acetone. The procedure afforded the compound in g96% purity, contaminated with e4% of cyclopentagermane 2, as determined by 1H NMR spectroscopy and GC/MS. Unfortunately, a pure sample of the latter compound could not be isolated from the reaction mixtures in sufficient amounts to enable its further study. Laser flash photolysis of a flowed, deoxygenated solution of 1 (ca. 4.2 mM) in anhydrous hexanes, with the pulses from a KrF excimer laser (248 nm, 25 ns, ca. 100 mJ/pulse), afforded three distinct transient absorptions: one centered at λmax ) 490 nm that decayed with the laser pulse (τ e 10 ns), a second centered at λmax ) 470 nm that decayed on the microsecond time scale with second-order kinetics, and a third centered at λmax ) 370 nm that grew in over the first ca. 2 µs after the pulse and then decayed over ca. 100 µs, also with approximate second-order kinetics. Figure 1a shows the time-resolved UV-vis spectra of these species, recorded over various time windows after the laser pulse, along with transient decay/growth profiles at selected monitoring wavelengths to illustrate their temporal behaviors. The spectra of the two longer lived transients, recorded 80-112 ns and 2.0-2.1 µs after the laser pulse, match almost precisely (in the region above ca. 320 nm) those assigned by us previously to GeMe2 and Ge2Me4, respectively, generated from the germacyclopenten-3-enes 7a-c.26,27 The present spectra also contain long-lived absorptions in the 270-320 nm region that cannot be reliably assigned. On the other hand, there are (at best) only qualitative similarities between the spectra of Figure 1a and the previously reported transient spectra from this compound;13 the latter showed significantly more intense transient absorptions below 400 nm than were observed in the present work, and a secondary band centered at 450 nm, as mentioned above. The ultrashort-lived transient centered at ca. 490 nm, whose spectrum is shown as the dashed line in Figure 1a, was evidently not detected in the earlier study.13 A few quenching experiments were carried out to enable further comparisons between the characteristics of the 470 nm transient absorptions observed for 1 in the present work and those reported using 7a-c26,27 as GeMe2 precursors. Addition of 15 mM tetrahydrofuran (THF) quenched the 470 nm (41) Carberry, E.; Dombek, B. D.; Cohen, S. C. J. Organomet. Chem. 1972, 36, 61.



Figure 2. Plots of kdecay vs [Q], for quenching of the 470 nm transient from laser photolysis of dodecamethylcyclohexagermane (1) by 2,3-dimethyl-1,3-butadiene (DMB; O) and acetic acid (AcOH; 0) in hexanes at 25 °C. The solid lines are the least-squares fits of the data to eq 3.

absorption almost completely, replacing it with the longer lived absorption centered at λmax ) 310 nm due to the GeMe2-THF Lewis acid-base complex28 and causing a significant lengthening of the growth time of the Ge2Me4 signal; the relevant series of transient spectra is shown in Figure 1b. The very weak signal at 470 nm that remains detectable in the presence of THF (see Figure 1b, inset) may be due to free GeMe2, present in equilibrium with the complex (Keq ) 9800 ( 3800 M-1).28 These results are in excellent agreement with those reported previously using 7b as precursor to GeMe2.28 Addition of acetic acid (AcOH; 0.05-1 mM), 2,3-dimethyl-1,3-butadiene (DMB; 0.1-1 mM), or carbon tetrachloride (CCl4; 1-65 mM) to hexane solutions of 1 caused enhancements in the decay rate of the 470 nm absorption and quenched the formation of the 370 nm (Ge2Me4) absorption; the decay at 470 nm followed clean firstorder kinetics once sufficient substrate had been added to reduce the maximum signal intensity at 370 nm to ca. 50% or less of its value in pure hexanes. Plots of the pseudo-first-order decay rate constant (kdecay) vs substrate concentration were linear; analysis of the data according to eq 3 (where kQ is the secondorder rate constant for quenching by the substrate, Q, and k0 is the (hypothetical) pseudo-first-order decay rate constant in the absence of Q) afforded slopes of kAcOH ) (9.4 ( 0.9) × 109 M-1 s-1, kDMB ) (1.4 ( 0.1) × 1010 M-1 s-1, and kCCl4 ) (9 ( 1) × 107 M-1 s-1 (see Figure 2 and Figure S1, Supporting Information). Saturation of the solution with oxygen ([O2] ) 15.7 mM42) had similar effects on the 370 and 470 nm signals and afforded an estimate of kO2 ≈ 1.4 × 108 M -1 s-1 for the rate constant for reaction of O2 with the 470 nm transient, from the pseudo-first-order decay rate constant of the 470 nm absorption (kdecay ) 2.1 × 106 s-1). The values of these four rate constants are also in excellent agreement with those assigned previously to GeMe2, using 7b and 7c as precursors under similar conditions.26,27

its yield due to precursor quenching. Whatever the interaction responsible for the effect, it is possible to say only that it occurs with a bimolecular rate constant of ca. 2 × 109 M-1 s-1 or greater. The intensity of the GeMe2 absorption at 470 nm also appeared to be reduced modestly in O2-saturated hexanes compared to deoxygenated solution, suggesting that the 490 nm species might be the direct precursor to (singlet) GeMe2, but it is again difficult to be certain of this because of the overall weakness of the transient signals. Two reasonable possible assignments for the 490 nm species are an excited state of 1 and the 1,6-biradical derived from Ge-Ge bond homolysis in 1 (i.e., 9; see eq 4). Better time resolution will be required for even a tentative identification to be made of the short-lived transient species detected in these experiments. It is worth noting that Wakasa and co-workers recently reported evidence from laser photolysis and product studies for the formation of a related (1,4-) biradical from the photolysis of octa(isopropyl)cyclotetragermane; the species exhibits a long-wavelength absorption band centered at λmax ) 550 nm and lifetime τ ) 50 ( 5 ns in cyclohexane at 25 °C.43

The intensity of the short-lived 490 nm absorption appeared to be reduced significantly in O2-saturated hexanes, though because its decay coincides with that of the laser pulse, it is not clear whether this is due to a shortening of the lifetime of the species responsible for the absorption or to a reduction in

In any event, it is clear that the species that was assigned to GeMe2 in the earlier flash photolysis study of 113 is not the same as the one observed in the present work from the same precursor; as indicated above, the latter corresponds closely to the primary transient product obtained in similar experiments with 7a-c in every respect.26,27 In addition to the difference in the UV-vis spectra, there is also no correspondence between the quenching rate constants reported for the (450 nm) species detected in the earlier work by O2 (k ) 9.7 × 108 M-1 s-1), CCl4 (k ) 4.9 × 108 M-1 s-1), and DMB (k ) 2.2 × 107 M-1 s-1)13 and those reported by us for quenching of GeMe2 by the same substrates in hexane at 25 °C, in both the present and previous studies.27 It is difficult to speculate on the true identity of the reactive intermediate that was reported in the earlier work, or its origins. We note that the earlier experiments were evidently collected using static samples to which several excitation pulses were delivered, thus allowing the buildup of photoproducts that could very well lead to transient photoproducts of their own. In our experience, flowed sample delivery and strictly anhydrous conditions are both essential for the successful detection of transient germylenes in solution. 2. Dimethylphenyl(trimethylsilyl)germane (5). Laser flash photolysis of a flowed, deoxygenated solution of 5 (ca. 0.6 mM) in anhydrous hexanes led to the formation of prominent transient absorptions centered at λmax ≈ 300 nm and λmax ) 430 nm, which decayed on the microsecond time scale with mixed-order kinetics (see Figure 3). The behavior is in reasonably good agreement with that reported previously for this compound in cyclohexane at ambient temperatures.16,17 Interestingly, an

(42) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data 1983, 12, 163.

(43) Wakasa, M.; Takamori, Y.; Takayanagi, T.; Orihara, M.; Kugita, T. J. Organomet. Chem. 2007, 692, 2855.

kdecay ) k0 + kQ[Q]

(3)


Figure 3. Transient absorption spectra from laser flash photolysis of a solution of PhMe2GeSiMe3 (5; ∼0.6 mM) in deoxygenated hexanes at 25 °C, recorded 0.06-0.13 µs (O) and 8.88-8.94 µs (0) after the laser pulse. The inset shows transient decay traces recorded at selected monitoring wavelengths between 300 and 480 nm.

absorbance-time profile recorded at 370 nm (the absorption maximum of Ge2Me4) showed a slight hint of an initial growth and decayed over a significantly longer time scale than the absorptions at 300 and 430 nm, which is suggestive of the formation of Ge2Me4 subsequent to the excitation pulse via dimerization of GeMe2; the associated absorption band is (barely) discernible at ca. 360 nm in the 8.9 µs spectrum of Figure 3. The growth component disappeared upon addition of 0.5-3.0 mM AcOH, consistent with the assignment of the species to Ge2Me4, but decay traces recorded at 470-480 nm showed no evidence of a minor short-lived component that could be attributed to absorption by the germylene, at any AcOH concentration studied. It can be concluded that the transient absorptions due to GeMe2 are too weak relative to those of the other transient products for the germylene to be detected in experiments with this compound. Transient absorption spectra recorded in hexanes containing 3.0 mM AcOH (Figure S2, Supporting Information) verified that the two prominent absorption bands in the spectra of Figure 3 are due to two distinct species, one that is unaffected by AcOH (λmax ) 300 nm; τ ≈ 20 µs) and one (λmax ) 430 nm) that is quenched by the carboxylic acid. An upper limit of kAcOH < 1 × 108 M-1 s-1 was estimated for the rate constant for quenching of the latter species by AcOH, from lifetimes measured at low laser intensities in order to minimize second-order contributions to the decay. Transient decay traces recorded at 300-310 nm showed a minor fast decay component in the presence of AcOH, suggesting that a small component of the transient absorption in the 280-320 nm range may be due to a higher energy absorption band associated with the 430 nm species. Mochida and co-workers assigned the prominent component of the shortwavelength absorption to the dimethylphenylgermyl radical,17 based on comparisons of the spectrum and the reactivity of the species (with DMB, CCl4, and O2) to previously reported data for this and other germyl-centered radicals.20,44,45 Transient spectra recorded in air- and O2-saturated hexane solution afforded somewhat different results from those expected based on the earlier studies of 5.16,17In our hands, (44) Chatgilialoglu, C.; Ingold, K. U.; Lusztyk, J.; Nazran, A. S.; Scaiano, J. C. Organometallics 1983, 2, 1332. (45) Mochida, K.; Wakasa, M.; Sakaguchi, Y.; Hayashi, H. J. Am. Chem. Soc. 1987, 109, 7942.


Figure 4. Plots of kdecay vs [Q], for quenching of the 430 nm transient from laser photolysis of 5 by DMB (O) and CCl4 (0) in hexanes at 25 °C. The solid lines are the least-squares fits of the data to eq 3.

the presence of oxygen resulted in reductions of the initial signal intensities of both the 300 and 430 nm absorptions, the effect on the 300 nm absorption appearing to be considerably greater than that on the longer wavelength one. As a result, while both species remained detectable in airsaturated solution, only the 430 nm transient could be detected in O2-saturated hexanes (see Figure S3, Supporting Information). A 3-point plot of kdecay vs [O2] for the 430 nm transient was linear, affording a slope of kO2 ) (3.8 ( 0.2) × 108 M-1 s-1 (Figure S4, Supporting Information), roughly 5 times smaller than that reported for the same process in the earlier studies.16,17 A bigger discrepancy exists in regards to the effect of oxygen on the 300 nm species, for which a quenching rate constant of kO2 ) 1.0 × 109 M-1 s-1 was reported.17 This leads to an expected lifetime of τ ≈ 300 ns in air-saturated hexanes, which is significantly shorter than what we observe (see Figure S3a, Supporting Information). The different effects of O2 on the intensities of the 300 and 430 nm absorptions could be the result of considerably faster quenching of the 300 nm species than what was originally reported17 or could be due to the two species originating from different excited states of the precursor, one more efficiently quenched by oxygen than the other. We did not pursue these possibilities further. As was reported previously,16,17 the lifetime of the 430 nm transient was also shortened in the presence of CCl4 and DMB. Plots of kdecay vs substrate concentration (Figure 4) were linear, affording values of kCCl4 ) (1.3 ( 0.1) × 108 M-1 s-1 and kDMB ) (1.4 ( 0.1) × 107 M-1 s-1. The rate constant for quenching by the diene is in good agreement with the earlier reported values,16,17 while that for CCl4 quenching is 2-4 times smaller. The lifetime of the 430 nm species was also shortened by added acetone, and a plot of kdecay vs [Q] was linear with slope kacetone ) (2.1 ( 0.5) × 106 M-1 s-1 (see Figure S8, Supporting Information). Flash photolysis of 1 in deoxygenated THF led to quite similar transient absorption spectra and lifetimes to those obtained in hexanes under similar conditions (Figure S5a, Supporting Information). In particular, the position of the 430 nm absorption band remained unchanged in THF relative to that in hexane, indicating little or no susceptibility of the species responsible toward complexation with the ether solvent. This eliminates the possibility that the 430 nm absorption is due to a weakly bound complex of GeMe2 with the aromatic



Table 1. Absolute Rate Constants for Quenching of Dimethyl- and Diphenyl-Substituted 1-Metallahexatrienes in Hydrocarbon Solvents

a

In hexanes at 25 °C (this work). b In isooctane at 21 °C (ref 46). c In hexane at 23 °C (ref 21).

precursor (5),14 since such a species could not possibly survive in THF, itself a potent complexing agent with transient germylenes such as GeMe2.9,28 Interestingly, saturation of the solution with O2 led to almost no change in the relative signal intensities at 300 and 430 nm (Figure S5b, Supporting Information), unlike the behavior in hexane, where the intensity at 300 nm was reduced preferentially in the presence of O2 (Vide supra). This is due in part to the presence of an additional component of the 300 nm absorption that was not present in the spectra recorded in O2-saturated hexanes and which exhibits a lifetime of τ ≈ 5 µs. Though we note that the lifetime of this component is similar to that observed for the GeMe2-THF complex under these conditions,29 such an assignment would be difficult to prove because of the complexity of the absorptions in the 300 nm region. The lifetime of the 430 nm species was shortened to τ ≈ 160 ns in O2-saturated THF ([O2] ) 10.1 mM42), consistent with a quenching rate constant of kO2 ≈ 6 × 108 M-1 s-1, similar to the value obtained for the species in hexanes (Vide supra). Table 1summarizes the absolute rate constants for reaction of O2, AcOH, CCl4, acetone, and DMB with the 430 nm transient from laser photolysis of silylgermane 5 in hexanes at 25 °C. An assignment of this species to germene 6 can be made on the basis of a comparison of these data to the analogous values for reaction of the same substrates with the homologous silene 11,46,47 the major product of photolysis of disilane 10 (eq 5),22,48 coupled with a more general knowledge of the quantitative differences in the reactivities of homologous silenes and germenes;49,50 the requisite kinetic data for silene 11 are included in Table 1 to facilitate the comparison. The trends can be expected to parallel those found in our earlier study of germene 12,21 which was identified on the basis of similar comparisons, so we have included the corresponding data for it and its silicon homologue (13)21,46 in the table as well. The products of (46) Leigh, W. J.; Sluggett, G. W. Organometallics 1994, 13, 269. (47) Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1994, 116, 10468. (48) Steinmetz, M. G. Chem. ReV. 1995, 95, 1527. (49) Toltl, N. P.; Leigh, W. J. J. Am. Chem. Soc. 1998, 120, 1172. (50) Leigh, W. J.; Potter, G. D.; Huck, L. A.; Bhattacharya, A. Organometallics 2008, 27, 5948.

these reactions have all been documented for the two silenes; in the cases of the germenes the same is true only for DMB, for which the (ene-) reactions with 616,17 and 1121 proceed in identical fashion to those with 12 and 13.25,46

The first set of comparisons to be made relates to the effects of phenyl-for-methyl substitution on the spectra and reactivity of silenes and germenes of otherwise identical structures. In silenes, phenyl-for-methyl substitution at the silenic silicon atom results in a substantial red-shift in the lowest energy (π,π*) absorption band and a moderate decrease in reactivity,51 as exemplified by the differences in the UV-vis spectra of silenes 11 and 13 and the rate constants for reaction with each of the five substrates considered in this work. Comparison of the corresponding parameters for the germenes (6 and 12) shows that they follow the same general trends as do the homologous silenes. The red-shift in the absorption spectrum of 12 relative to that of 6 is somewhat smaller than that exhibited by the silenes, which can be explained as being due to poorer π-overlap between the phenyl substituents and the GedC bond compared to the situation with the silenes. This appears to be a general phenomenon, as the same differences are also evident in the UV-vis absorption spectra of homologous disilenes and digermenes, as well as silylenes and germylenes.34 The second point of comparison focuses on the differences in reactivity within the two homologous pairs of metallaenes, 6/11 and 12/13. The differences in the rate constants within the two pairs of compounds mirror each other quite closely, and are broadly similar to those observed previously with simpler (51) Morkin, T. L.; Leigh, W. J. Acc. Chem. Res. 2001, 34, 129.


metallaene homologues such as Ph2SidCH2 and Ph2Ged CH2.49,50,52,53 In both cases, the germene is consistently less reactive than the homologous silene; the difference is largest for reactions in which the polarity of the MdC bond and Lewis acidity at the metal center play the dominant role in affecting the rate constant, such as that with acetone and acetic acid,50,54 and smallest for reactions that proceed through radical or diradical intermediates, such as those with CCl4 and O2.25,47 It should be noted that the presence of the cyclohexadienyl substituent at the metallaenic carbon in silenes of this type leads to considerably greater reactivity toward O2 and CCl4 than is typically observed for simpler silenes without radical-stabilizing substituents at this position,55 and the rate constants vary much more modestly with substituent-induced changes in SidC bond polarity than do those for reaction with polar substrates such as alcohols, acetic acid, and acetone.25 Thus, little difference in reactivity between homologous germenes and silenes is expected toward these two substrates, as is indeed observed. The differences are somewhat larger for the ene reaction with DMB, which proceeds with transfer of the allylic hydrogen in the cyclohexadienyl substituent and aromatization of the ring (see eq 6), but is still relatively small compared to the effects on the kinetics of reactions with O-donors. With silenes, the latter reactions are initiated by Lewis acid-base complexation at the silenic silicon atom, and thus their rate constants are strongly affected by substituent-induced changes in SidC bond polarity.25 In general, reactions of this type are dramatically slower with germenes than with the corresponding silene homologues, owing to the substantially lower Lewis acidities of germenes compared to silenes of otherwise identical structures.49,50 The ca. 5000-fold smaller values of the rate constants for quenching of 6 and 12 by acetone compared to the corresponding values for the homologous silenes, as well as the extremely low reactivity of 6 toward ethanol,16 are thus also consistent with what has been reported previously for simpler derivatives.49,50 The substantial difference in the Lewis acidities of germene 6 and silene 11 is also manifested in their UV-vis spectra in THF and hexane; the absorption maximum of 11 is shifted from 425 nm in hexane to 460 nm in THF,46 while that of 6 is unaffected by the change in solvent.

Summary and Conclusions Laser flash photolysis of dodecamethylcyclohexagermane (1) in hexane solution allows the detection of the characteristically weak UV-vis absorption spectrum due to GeMe2, which is centered at λmax ) 470 nm and decays over several microseconds with second-order kinetics, coincident with the growth of the longer lived absorption centered at λmax ) 370 nm due to the (transient) germylene dimer, tetramethyldigermene (Ge2Me4). The spectrum and basic kinetic behavior, as well as the absolute rate constants for reaction of the 470 nm species with O2, acetic (52) Leigh, W. J. Pure Appl. Chem. 1999, 71, 453. (53) Toltl, N. P.; Stradiotto, M. J.; Morkin, T. L.; Leigh, W. J. Organometallics 1999, 18, 5643. (54) Mosey, N. J.; Baines, K. M.; Woo, T. K. J. Am. Chem. Soc. 2002, 124, 13306. (55) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: New York, 2001; Vol. 3, pp 949-1026.


acid, DMB, and CCl4, and its behavior in the presence of millimolar concentrations of THF all agree quite closely with those observed in similar experiments with 1,1-dimethylgermacyclopent-3-ene derivatives as GeMe2 precursors.26-28 The results suggest that the photolysis of 1 yields GeMe2 in reasonably high quantum efficiency, with the only transient sideproduct being an ultrashort-lived species whose UV-vis spectrum overlaps with that of (singlet) GeMe2. The identity of this species is not yet clear, but it is sufficiently short-lived (τ e 10 ns) that its presence does not interfere greatly in kinetic studies of the reactions of GeMe2 with added substrates. Laser flash photolysis of silylgermane 5 under similar conditions allows the detection of two main transient products, as reported in the earlier studies of this compound in cyclohexane.16,17 One of these was assigned previously to the dimethylphenylgermyl radical (λmax ) 300 nm),17 while the other (λmax ) 430 nm) was assigned to GeMe2.16,17 The latter species is re-assigned in the present study to the transient 1,1-dimethyl1-germahexatriene derivative 6, formed via [1,3]-trimethylsilyl migration into an ortho-position of the phenyl ring in the lowest excited singlet state of 5. The germene (6) assignment for this species is based on comparisons of its UV-vis spectrum and reactivity with those of the homologous silene derivative (11),46as well as with those of the analogous 1,1-diphenyl1-metallahexatriene homologues, germene 12 and silene 13.21 While GeMe2 is known to be formed as a major product of photolysis of 5,16,17 the species cannot be detected at all in laser photolysis experiments with this compound because of the overlapping, much stronger absorptions due to the minor transient coproduct, 6. The various discrepancies that have been noted between the flash photolysis behavior exhibited by 1 and 5 in the present study and those reported in the earlier studies of these compounds are most likely due to differences in the way the experiments were carried out. In our experience, flowed sample delivery is absolutely essential in experiments involving siliconand germanium-containing reactive intermediates (of any type), to avoid the buildup of reaction products that are very frequently also photoreactive and which lead to complications in experiments requiring multiple laser shots for the acquisition of data. A second complicating factor results from the high sensitivity of transient germylenes to water and other hydroxylated substrates, due to rapid and reversible Lewis acid-base complexation. The kinetic and thermodynamic details of these processes make (free) transient germylenessGeMe2 in particulars uniquely difficult to detect by UV-vis spectroscopy unless strictly anhydrous conditions are employed. These crucial experimental details could not have been appreciated at the time the original experiments were carried out.

Experimental Section 1

13

H NMR and C NMR spectra were recorded on a Bruker AV200 spectrometer in CDCl3 solution and are referenced relative to tetramethylsilane using the solvent residual proton or 13C peak as calibrant. GC/MS analyses were carried out on a Varian Saturn 2200 GC/MS/MS system equipped with a VF-5 ms capillary column (30 m × 0.25 mm; 0.25 µm; Varian Inc.). Hexanes (EMD Omnisolv) was dried by passage through a Solv-Tek solvent purification system and contained

Laser Flash Photolysis Studies of Some Dimethylgermylene

Recommend Documents