Organometallics 1995, 14, 1539-1541
1539
Germylene and Stannylene Anion Radicals: Generation and Electronic Structure Michael P. Egorov" and Oleg M. Nefedov N . D. Zelinsky Institute of Organic Chemistry, Leninsky Prospect 47, Moscow B-334, Russia
Tien-Sung Lin and Peter P. Gaspar" Department of Chemistry, Washington University, St. Louis, Missouri 63130-4899 Received July 6, 1994@ Summary: Germylene and stannylene radical anions [(Me3SihCHIzM- (M = Ge, Si) have been generated and their ESR spectra recorded. The a M ) hypefine coupling constants of both species clearly indicate a low degree of s-character associated with n-radicals.
reduction potentials of halogen-containing substrates and their reactivity toward dimethylgermylene was found.g For these reactions, an ion radical mechanism was suggested in which a key step is an electron transfer from dimethylgermylene that forms an ion radical pair. Electron transfer from the stable stanRadicals, ions, ion radicals, carbenes, and their ananylene [(MesSi)zNIzSnto organic halides has also been logs are well-known and intensively studied classes of proposed as the first stage of reaction between RzSn and reactive intermediates. The ion radicals of carbenes and RX.1oJ1 their analogs are an extension of this series and have Reactions that can be rationalized by electron transfer ofken, since the early 19709, been suggested as interfrom a reducing agent to a germylene have also been mediates in redox reactions of diazo compounds and reported, including the reaction between the GeClzother molecules, both in the liquid and gas phases.lrZ dioxane complex and a good reducing agent, hexamethThe first direct unequivocal ESR detection of a carbene ~ l d i t i n .This ~ did not result in insertion of dichlorogion radical, the diphenylcarbene cation radical was, ermylene into the Sn-Sn bond, but in Sn-Sn bond however, accomplished only very r e ~ e n t l y . Carbene ~ cleavage with concomitant chlorination. The products anion radicals have not yet been detected by ESR. were MesSnCl and (GeCl), oligomers. Very little is known about ion radicals of heavier Here we report the first successful generation of group 14 element carbene The silylene germylene and stannylene anion radicals and their anion radical SiH2'- has been produced in a lowdirect detection by ESR. The anion radicals were pressure discharge source upon admission of SiH4 and obtained by reduction of stable [(Me3Si)zCHIzE(E = Ge has been studied by laser photoelectron spectro~copy.~ (la),Sn (lb))by sodium: Photoionization mass spectrometry has been used to study silylene SiH$+ and germylene GeHz*+ cation [(Me,Si),CHI,E + Na 2o"c_ radical^.^^^ The 28SiH2'+ and zgSiH~*+ cation radicals THF E = Ge (la), Sn (lb) have been detected by ESR upon generation by photo[(Me,Si),CHI,E'Na+ (1) ionization of SiH4 in a neon matrix a t 4 KS7 E = Ge (2a),Sn (2b) In the cases mentioned above, the silylene and germylene ion radicals were generated by discharge or A 6 x M solution of stable germylene [(Meaphotoionization under severe conditions. However, the was allowed to react for 10S ~ ) Z C H I Z(la) G ~in~ THF ~ relatively low ionization potentials and appreciable s at 20 "C with a sodium mirror. The characteristic 15 electron affinities of a number of carbene analogs8 allow yellow-orange color of la (Amu = 410 nm in THF) them to participate in electron-transfer interactions immediately turned to green (Amm = 666 nm in THF), with a variety of electron acceptors and donors. Indeed, and a strong ESR signal appeared. The ESR spectrum the formation of germylene cation and anion radicals is displayed in Figure la. The 1:2:1 triplet with a as intermediates in the course of germylene reactions hyperfine splitting a = 2.6 G arises from two equivalent has recently been s ~ g g e s t e d .A ~ correlation between protons. The spectrum of Figure l a is shown at higher gain in Figure lb, in which four weak satellite lines Abstract published in Advance ACS Abstracts, January 15, 1995. appear on both sides of the central peak. These can only (1)Bethell, D.; Parker, V. D. Acc. Chem. Res. 1988,21,400. (2) McDonald, R. N. Tetrahedron l989,45, 3993. arise from the I = 9/2 nucleus of 73Ge(7.8% abundance) (3) Bally, T.; Matzinger, S.; Truttman, L.; Platz, M. S.; Admasu, A.; with aP3Ge) = 12.5 G. Two other satellite peaks of the Gerson, F.; Arnold, A.; Schmidlin, R. J . Am. Chem. SOC.1993,115, 7007. total of 10 expected overlap with the strong central peak (4)Kasdan, A.; Herbst, E.; Lineberger, W. C. J. Chem. Phys. 1976, and are not observed. Under even higher gain, each of 62, 541. the satellite lines splits further into a triplet due to ( 5 ) Berkowitz, J.; Greene, J. P.; Cho, H.; Ruscic, B. J. Chem. Phys. 1987,86,1235. coupling with two equivalent protons. The g-value of (6) Ruscic, B.; Schwarz, M.; Berkowitz, J. J . Chem. Phys. 1990,92, this radical species is 2.0125, which is typical for 1865. (7) Knight, L. B.; Winiski, M.; Kudelko, P.; M n g t o n , C. A. J . Chem. Phys. 1989,91,3368. (8) Nefedov, 0. M.; Egorov, M. P.; Ioffe, A. I.; Menchikov, L. G.; Zuev, P. S.; Minkin, V. I.; Simkin, B. Ya.; Glukhovtsev, M. N. Pure Appl. Chem. 1992,64, 265. (9) Egorov, M. P.; Gal'minas, A. M.; Basova, A. A.; Nefedov, 0. M. Dokl. Akad. Nauk. 1993,329, 594.
0276-7333/95/2314-1539$09.00/0
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(10)Gynane, M. J. S.; Lappert, M. F.; Miles, S. J.; Carty, A. J.; Taylor, N. J. J. Chem. SOC.,Dalton Trans. 1977,2009. (11)Lappert, M. F.; Misra, M. C.; Onyszchuk, M.; Rowe, R. S.; Power, P.P.; Slode, M. J. J . Organomet. Chem. 1987,330, 31. (12) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.; Thorne, A. J. J . Chem. SOC.,Dalton Trans. 1986, 1551.
0 1995 American Chemical Society
1540 Organometallics, Vol. 14, No. 3, 1995
Notes
I
-
I
Figure 2. ESR spectrum of [(MesSi)2CHI2Sno-(2b)at high gain.
IOG
believed to be formed from reaction of PhzGeH2 and n-BuLi.15 Silicon analogs of 3 and 4, MeszSi(K)Si(K)Mesz and MeszSiK2, respectively, have recently been obtained by potassium reduction of Me~zSi-SiMes2.l~ Stannylene anion radical [(Me3Si)&HI2Sn'- (2b)was obtained by brief contact (5-10 s) with a precooled Figure 1. (a) ESR spectrum of [(Me3Si)zCH12Ge'- (2a). (b) M solution of stable sodium mirror of a 6 x Same spectrum at increased gain. stannylene lb17in THF cooled to -80 "C. The reaction mixture was quickly transferred into a precooled ESR gennanium-centered radicals (g 2.0078-2.0100).13 On tube, and the ESR spectrum was recorded a t -80 "C. A the basis of the hyperfhe splitting patterns and the broad singlet (line width 7 G) was observed with a g-value, we can unequivocally assign it to [(MesSi)zg-value of 2.0177, similar t o that of the related neutral CHl2Ge'- anion radical 2a. radical [(Me3Si)2CHI3Sno(g = 2.0094).18 Under inAnion radical 2a is quite stable in solution at room creased gain, two broad unresolved satellites from 117temperature. It has a half-life tu2 = -1.5 h, which is Sn (I = V 2 , 7.7% abundance) and 119Sn(I = VZ, 8.7% however considerably shorter than the lifetime of the abundance) were observed with a splitting of a(117J19neutral species [(Me3Si)zCHlsGe*whose t112 is greater Sn) = 116 G (Figure 2). As in the case of the neutral than 4 months.22 The intensities of the ESR signal and radical, [(Me3Si)&HI3Sn*,the high-field satellite line is of the electronic absorption maximum at 666 nm considerably broader than the one a t low field. decrease at the same rate, consistent with the assignThe lack, probably caused by spectral broadening, of ment of the Am= = 666 nm absorption to anion radical an observed hyperfine splitting due to the two adjacent 2a. The disappearance of 2a,simultaneously monitored CH protons, makes the structural assignment for the by visible and ESR spectroscopy, obeys a second-order tin radical species less certain. The spectral broadening rate law, suggesting dimerization of 2a, a reaction typical for sterically nonoverloaded R3Ge' radi~a1s.l~ could arise from a slow tumbling motion of the radical at low temperatures, but is more likely due to a slow Calculations have indicated that dimerization of the exchange processes with excess neutral species. Unmethylene anion radical anion CH2'- is fea~ib1e.l~ fortunately the radical decomposes in minutes at 20 "C, and it was therefore not possible to obtain spectra at 2[(Me3Si),CHlZGe*higher temperatures. This is in sharp contrast to the 2a neutral [(Me3Si)2CHI3Sno,species which has a lifetime [(Me3Si)2CH12Ge"---"Ge[CH(SiMe3)212 (2) of 1 year.22 3 Nevertheless we can safely interpret the spectrum in Figure 2 as belonging to the stannylene anion radical Further contact (1-2 min) of solutions of 2a with [(Me3Si)2CHI2Snoon the basis of the small value of the sodium results in complete disappearance of the ESR tin hyperfine coupling constant a(117,119Sn)= 116 G, signal. Presumably the anion radical 2a is reduced by clearly indicative of the n-character of this radical. sodium to the diamagnetic dianion 4. In the ion radicals of carbenes and their analogs, an unpaired electron can occupy either a u- or norbital. 20 "C [(Me3Si),CHl,Ge'- Na+ + Na THF Experimental data on diphenyl~arbene~ and silylene7 2a cation radicals reveal that these species are u-radicals [(Me3Si)2CH12Ge:2*22Na+ (3) and have a 2A1 electronic ground state. Calculations 4
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Both 1,2-dianion3 and 1,l-dianion 4 have prototypes in products PhzGe(Li)Ge(Li)Phzand PhzGeLiz that are (13)Sakurai, H. Organomet. Chem. Rev. 1981,12, 267. (14)Davidson, R. B.; Hudak, M. L. J.Am. Chem. SOC.1977,99,3918.
(15) Cross, R. J.; Glockling, F. J. Chem. SOC.1964,4125. (16) S o h , H.; West, R. Abstracts of XXVZth Silicon Symposium, Indianapolis, 1993;P-30. (17) Davidson, P.J.; Harris, D. H.; Lappert, M. F. J. Chem. SOC., Dalton Trans. 1976,2268. (18)Davidson, P.J.; Hudson, A.;Lappert, M. F.;Lednor, P.W. J. Chem. SOC.,Chem. Commun. 1973,829.
Notes
Organometallics, Vol.14, No. 3, 1995 1541
predict that the unpaired electrons in methyleneig and silylene7 anion radicals should occupy a n-orbital. It is well-known that the a(M) splitting is proportional to the degree of s character in the orbital on the M atom occupied by the unpaired electron. Therefore a-radicals have considerably larger a(M) values than do n-radicals. For example, most 13C hyperfine coupling constants for carbon-centered a-radicals are in the range 100-140 G vs 24-26 G for n - r a d i ~ a l s .For ~ the PhzC'+ cationradical, which is a a-radical, a(13C) = 98.3 G.3 The is 301 G, experimentally observed U ( ~ ~for S ~SiH2'+ ) which is in the range of hyperfine coupling constants = 160-500 G found for pyramidal R3Si' radicah20 Recent calculations have indicated that the hybridization of the tricoordinate silicon atom of an Z3Si' radical is not directly related to the geometry a t the radical center.21 The widely accepted designation of Z3Si' radicals as o-radicals may need revision in light of the calculational result that the 3s character of the electron in the SOMO can vary from as low as 3.4% to as high as 41.8% with increasing electronegativity of the substituent Z, without substantial change in the nearly tetrahedral geometry.21 Germanium- and tin-centered radicals are also pyramidal, with a high degree of s-character in the orbital containing the odd electron. The range of hyperfine coupling constants aP3Ge)for R3Ge' is 70-220 G,13and a(117J19Sn) for R3Sn' occurs in the range 1400-1800 G.22 The value for [(MesSi)2CHI3Ge*is 92 G;23for [(Me3Si)2CHl3Sn' a P 7 S n ) = 1698 G, a(ligSn) = 1776 G.18,22Therefore the very small values of u ( ' ~ G = ~ )12.5 G for [(Me3Si)zCH12Ge*- and a(117J19Sn)= 116 G for [(MesSi)zCHIzSn'- clearly indicate the small degree of s-character of the unpaired electons in these germylene and stannylene anion radicals and point t o their 2B1 ground electronic states. Thus both 2a and 2b would traditionally be described as n-radicals, but given the reported lack of correlation between the hybridization of silyl radicals and their geometry,21one should have reservations about the use of the u,n-nomenclature.
Experimental Section ESR spectra were recorded on a Bruker ER-200D EPR spectrometer (X-band,9.4 GHz) equipped with an Oxford EPR cryostat (4.2-300 K). W-visible spectra were recorded on a Varian Cary 219 spectrometer. All reactions were carried out in an evacuated (10-4-10-6 Torr) sealed glass apparatus equipped with a quartz ESR tube or with both a quartz ESR tube and a quartz W cell for recording UV-visible spectra. Germylene l a and stannylene l b were synthesized as described in the literature.12J7 THF, distilled from sodium-benzophenone under an argon atmosphere, was degassed and than distilled in vacuum into a tube on the glass apparatus containing a sample of l a (lb). After an additional cycle of degassing of the resulting THF solution of la (lb),the apparatus was sealed. In another tube, isolated from the rest of the apparatus by a breakseal, there was a (19)Rodriquez, C.F.; Hopkinson, A. C. J. Phys. Chem. 1993,97, 849. (20)Rhodes, C.J. J. Chem. Soc., Perkin Trans. 2 1992,1475. (21)Guerra, M.J.Am. Chem. SOC.1993,115,11926. (22) Davies, A. G.; Smith, P.J. Comp. Organomet. Chem. 1982,2, 521.
loo
1
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r
0
0
100
300
200
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Figure 3. Plot of lhntensity of the ESR spectrum of [(MesSi)2CH]zGe*-(2a)vs time.
r
,
0 4
0
100
200
1
300
t, min
Figure 4. Plot of 1/OD at ,I,,, == 666 nm of a THF solution of [(MesSi)2CHI2Geo-(2a)vs time. sodium mirror obtained by sublimation of the metal in vacuum. After the breakseal was ruptured, the solution of 1 was allowed to briefly contact the sodium at 20 (la) or -80 "C (lb). The solution was then transferred to the ESR tube or the W cell, and spectra were recorded. The decomposition of 2a was monitored by measuring as a function of time the intensity of its absorption at ,Imu = 666 mn in W-visible spectra and the intensity of the ESR signal for the same sample. Slopes that differed by less than 3% were obtained for the linear plots over more than 2 half-lives ( R = 0.998 W-visible, 0.990 ESR) of 1/OD (UV-visible) and 1/Z ( I = ESR signal intensity in arbitrary units) vs time.
Acknowledgment. This work received financial support from the National Science Foundation under Grant CHE-9108130. We thank S. I. Weissman for advice and assistance. OM9405302 (23)Hudson, A.;Lappert, M. F.; Lednor, P. W. J.Chem. SOC., Dalton Trans. 1976,2369.
