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Stable Soluble Conjugated Carbon Rods with a Persilylethynylated Polytriacetylene Backbone * * John Anthony, Corinne Boudon, Frangois Diederich,* Jean-Paul Gisselbrecht, Volker Gramlich, Maurice Gross, Markus Hobi, and Paul Seiler

Rigid acetylenic frameworks are being used with increasing fre- quency for the fabrication of molecular nanostructures.[’] Here we report the preparation of the conjugated, nanometer-sized molec- ular rods[’] 2-6 through end-capping polymerization of trans-bis- (triisopropylsily1)-protected tetraethynylethene 1 (Scheme 1) . [ 3 3 41

,TIPS

1 ) CUCI, TMEDA, 0 2 , 2 28% 3 33% CHC13, 20 ‘C, 2 d

34.3 A

Scheme 1. Preparation and length of the 2 -6 . TIPS = triisopropylsilyl. TMEDA methylethylenediamine.

Oligomers 2-6 are polytriacetylenes and are thus intermediate between p~lyacetylene[~] and polydiacetylene,[61 and carbyne (Scheme 2) .[’I Derivatives of 2-6 could exhibit interesting non- linear optical properties[81 and function as molecular wires.[g1

trans-Bis(triisopropylsily1)-protected tetraethynylethenes are readily prepared in multigram quantities by the general synthetic route shown in Scheme 3. Both the diester 14 and the free di- ethynyl derivative 1 were characterized by X-ray crystallogra- phy (Figs. 1 and 2).[Io1 In the crystal, the carboxyl groups of 14 are twisted out of the plane of the tetraethynylethene backbone because of steric hindrance between the ester residue and the

polymerize rapidly upon concentration of solutions.[’ An analysis of the crystal packing shows that the stability of 1 is related to the “insulating effect” of the TIPS groups, which we had previously found to stabilize crystals of a persilylethynylated tetradehydro[l2]annulene.[’’] The molecules of 1 align in a her- ringbone fashion with the reactive free ethynyl groups pointing directly towards the silyl groups of neighboring molecules.

End-capping reactions are used in polymer chemistry to con- trol polymer length.[’31 When phenylacetylene was added to the oxidative polymerization of 1 (Scheme I), a mixture of oligomers was obtained, from which compounds 2-6 were isolated by chro- matography (SiO, , hexanes). If a large excess of phenylacetylene

JPS

(6 equiv) was added at the beginning of the reaction, 2 was the only compound isolated. If phenylacetylene (2 equiv) was added after 6 to 8 h, oligomers 2-4 were isolated in fair yields. For the preparation of the larger derivatives 5 and 6 the oxidative cou- pling had to be allowed to run for at least 24 h before the end- capping reagent was added.

polyacetylene neighboring TIPS group. The angle between the least-squares planes passing through C(2), C(1), 0(1), O(2), and the tetra-

quite ethynylethene remarkable, fragment since bisdeprotected is 42(1)”. The stability trans-enediynes of crystalline typically 1 is --$++]- - [*I Prof. F. Diederich. J. Anthony, M. Hobi. P. Seiler

Ldboratorium fur Organische Chemie ETH-Zentrum

1 ‘1 n polydiacetylene

Universititstrasse 16, CH-8092 Zurich (Switzerland) Telefax- Int. code + (1)261-3524 Dr. V Gramlich Institut fur Kristallographie und Petrogrdphie ETH-Zentrum Sonneggstrasse 5. CH-8092 Zurich (Switzerland) Prof M . Gross. C. Boudon. J.-P. Gisselbrecht Labordtoire d’Electrochimie et de Chimie Physique du Corps Solide

+j+$ - ~

- n

polytriacetylene R

U.R.A. au C.N.R.S. no 405 Faculte de Chimie, Universite Louis Pasteur 1 et 4. rue Blaise Pascal B.P. 296 F-67008 Strdsbourg Cedex (France)

[**I This work was supported by the Swiss National Science Foundation. Scheme 2. Progression of polymeric backbones from polyacetylene to carbyne

Angrw. Cliern. Inr. Ed. Engl. 1994. 33. N o . 7 0 VCH I/erlugsgcsellsohuff mhH. 0.69451 Wernheim, 1994 U57U-0833/94/0707-0763 3 10.00+ 2j10 763

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Table 1. Selected physical data of compounds 2-6[a-c]. 1) DIBAL-H, CHpCIz, 0 "C 95% TIPS-SnBus zeT"'

t 2: Golden yellow crystals, m.p. 142-143 T . FT-IR (KBr): v = 2220 (C=C) cm-' ; -5 2) PCC, CHpClp. 85% OMe [(PPh3),PdCl2], THF

M e O k

9 d, 92 % ilps n "

7 8, m.p. 55 "C

Br ps

CBr4, PPh3, Zn ' 1) LOA(6 equiv), THF, -78 "C

CHpC12, 75% 5- 2) electrophile 11, 20 "C * *

TIPS Br Br 9, m.p. 45 "C 10, m.p. 85-87 "C

R Yield [XI M.P. YCl

95 76 23 88-90 81 104-106

Scheme 3. Synthesis of rruns-bis(triisopropylsily1)-protected tetraethynylethene de- rivatives. 10 was metalated and then treated with the following electrophiles Hi . I,, acetaldehyde. isobutyl chloroformate. fluorenone. DIBAL-H = diisohutyldlu- minum hydride, PCC = pyridinium chlorochromate. LDA = lithium diisopropyl- amide.

Fig. 1. Crystal structure of 14. Selected bond lengths [A] and angles ['I: C(2)-C(3) 1.173(8), C(3)-C(4) 1.436(7). C(4)-C(5) 1.426(6), C(5)-C(6) 1.204(6). C(4)-C(4A) 1.353(11); C(l)-C(2)-C(3) 175.9(6), C(2)-C(3)-C(4) 176.7(5), C(3)-C(4)-C(4A) 121.4(4), C(3)-C(4)-C(5) 116.5(5). C(4A)-C(J)-C(5) 122.0(5). C(4)-C(5)-C(6) 178.2(6), C(S)-C(6)-Sl(l) 174244)

C 14

C8 Fig 2. Crystal structure of 1. Selected bond lengths [A] and angles ['I : C(l)-C(2) 1.194(4), C(2)-C(3) 1.431(4), C(3)-C(3A) 1.368(5), C(3)-C(4) 1.437(3). C(4)-C(5) 1.207(3), C(5)-Si(l) 1.850(2); C(l)-C(2)-C(3) 177.3(3), C(2)-C(3)-C(3A) 121.9(3), C(2)-C(3)-C(4) 116.8(2), C(3A)-C(3)-C(4) 121.3(3), C(3)-C(4)-C(5) 177.1(3). C(4)- C(5)-Si(1) 175.4(2).

Oligomers 2-6 are amazingly stable, high-melting materials (Table 1) that remain unchanged for months at room tempera-

' H N M R (200 MHz): 6 = l . i 6 (s, 42H), 7.37 (m, 6H). 7.50 (m, 4H); ' T N M R (125.76MHz): 6=11.24, 18.62, 74.16. 76.99, 84.56, 86.91, 101.8, 104.2, 119.8. 121.5, 128.5, 129.9, 132.5; UVjVIS (CHCI,): Amex [nm] = 256 ( E = 36700). 275 (28100). 290 (25300), 306 (20000). 325 (15600), 394 (34000). 422 (35500). 479 (1 600)): MALDI-TOF-MS: m:z 637 ( M - ) . 3: Orange-yellow crystals, m.p. 180'C (decomp.). FT-IR (KBr): v = 2200 (CGC) cm- ' : 'HNMR (200 MHz). 6 =1.16 (s, 42H), 1.17 (s, 42H). 7.36 (m, 6H) , 7.50 (m. 4H): l3CNMR (125.76MHz): 6 =11.17, 11.22, 18.57, 18.60, 74.17, 78.86. 83.83. 84.29, 85.04, 87.24, 101.4, 101.7, 104.4. 105.0, 119.5, 120.6. 121.5, 128.5, 129.6, 132.5; UVWIS (CHCI,): E.,,, [nm] = 270 (6 = 21 OOO), 290 (21 000). 309 (19000), 329 (18000). 384 sh (15000), 440 (280001, 478 (31000): MALDI-TOF- MS: m:: 1073 ( M - ) . 4: Red crystals. m.p. 245 C (decomp.). FT-IR (KBr): v = 2179, 2177, 2133 (C=C) cm- I ; 'H NMR (200 MHz): 6 = 1.11 (s. 42H), 1.14 (s. 42H). 1.16 (s. 42H). 7.37 (m. 6H). 7 50 (m. 4H): "CNMR (125.76 MHz): 6 =11.16, 11.21, 18 56, 18.60. 74.15. 78.85. 83.90. 84.15. 84.25. 84.75, 85.10, 87.29, 101.3, 101.4, 101.7, 104.5, 105.1, 105.2. 119.4. 120.1, 120.7, 121.5. 128.5. 129.6. 132.5; UVWIS (CHCI,): E.,,,

(63800). 506 sh (51 500): MALDI-TOF-MS: mi; 1507 ( M - ) , 1463 ( M - - iso- propyl), 1419 ( M - ~ 2isopropyl). 5 : Red crystals, m.p. 250 C (decomp.). FT-IR (KBr): Y = 2200,2177,2133 (CEC) cm-' ; ' H N M R (200 MHz): 6 = 1.117 (s. 42H), 1.119 (s, 42H), 1.123 (s, 42H), 1.14 (s, 42H). 7.37 (m. 6H). 7.50 (m, 4H); "CNMR (125.76 MHz): 6 =11.19. 11.25. 18.54, 18.57. 74.17, 78.89. 83.90, 84.20, 84.21, 84.25, 84.67. 84.79, 85.10, 87.29, 101.31, 101.35, 101.5. 101.7. 104.5, 105.1, 105.3. 105.35, 119.3, 120.0, 120.1, 120.7, 121.5, 128.5. 129.6, 132.5; UViVlS (CHCI,): I,,, [nm] = 272 (c = 40900). 291 (40300), 317139400). 327 (38000). 366 (28300), 392 (28900), 416 sh (30600), 484 (53600) 511 sh (44600); MALDI-TOF-MS: mi. 1937 ( M - ) . 6: Dark red crystals. m.p. 250'C (decomp.). FT-IR (KBr): II = 2199, 2168, 2152, 2130 (C=C) cm-' ; ' H N M R (200MHz)- 6 =1.116 (br. s, 84H). 1.118 (s, 42H), 1.123 (s. 42H). 1.142 (s, 42H). 7.37 (m, 6H). 7.50 (m, 4H); 13CNMR (125.76MHz): 6 =11.23, 11.30, 11.46. 18.58. 18.61, 74.21. 78.93. 83.899. 84.191, 84.229. 84.289, 84.646, 84.665. 84.737. 84.801, 85.108, 87.296, 101.299 (2 x ) , 101.337, 101.443. 101 712, 104.506, 105.128. 105.329, 105.344, 105.394. 119.33. 119.98. 120.11, 120.20. 120.69. 121.56, 128.5, 129 6, 132.5; UVjVlS (CHCI,): A,,, [nm] = 290(& = 25300),320(25800),329 (25700),367sh(18200).390sh(17800), 419 sh (19700). 490 (375001, 514 sh (38700): MALDI-TOF-MS: mi; 2376 (M-).

[nm] = 274 (6 = 40500). 290 (41 000). 311 (40100), 330 (39900), 368 (28600), 480

[a] All NMR spectra were recorded in CDCI,. [h] Matrix for MALDI-TOF mass spectrometry: 9-nitroanthracene; we thank L. lsaacs for recording these spectra. [c] All new compounds were fully characterized by 'H and I3C NMR, FT-IR, and UV/VIS spectroscopy. El-MS or MALDI-TOF MS. and correct elemental analysis.

ture when exposed to air and light. The TIPS groups both stabi- lize the extended conjugated chromophores and render them soluble in common solvents. The shorter derivatives 2-4 dissolve in hexane, while all five molecular rods are soluble in chloroform,

C

Fig. 3. Crystal structure of 2. Selected bond lengths [A] and angles r]: Si-C(l0) 1.853(6), C( 10)-C( 11) 1.190(6), C(ll)-C( 12) 1.426(6), C(12)-C( 12A) 1.368(7), C(12)-C(13) 1.429(6), C(13)-C(14) 1.1 88(6). C(14)-C(15) 1.376(6), C(l5)-C(16) 1.190(6), C(16)-C(J 7) 1.431(6); Si-C(l0)-C(l1) 178.6(3), C(l0)-C(1 1)-C(12) 177.9(4), C( 1 1)-C( 12)-C( 13) 118.2(3), C( 11 )-C(12)-C(12A) 121.4(4), C(l2A)-C( 12)- C(13) 120.4(4), C(12)-C(13)-C(14) 179.2(4). C(14)-C(15)-C(16) 178.1 (4). C(15)- C(16)-C(17) 176.5(4).

764 Q VCH Verlagsgesrllschuft mbH, 0-69451 W&heim. I994 0570-0833j94j0707-0764 $ /0.00+ ,2510 Angen. Chrm. Inr. Ed. Engl. 1994, 33, No. 7

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which allowed their full analytical characterization (Table 1). Mass spectra of 2-6 with the molecular ions as parent ions were obtained by matrix-assisted laser desorption ionization time-of- flight mass spectrometry (MALDI-TOF-MS) with detection in the negative ion mode. Upon recrystallization from hexane, the shorter rods 2 and 3 formed crystals suitable for X-ray analysis (Figs. 3 and 4). The conjugated carbon frames of both molecules, including the phenyl rings, are nearly perfectly planar with a maximum deviation from the least-squares plane passing through the entire carbon chromophore of 0.22 A in 2 and only 0.03 A in 3.131 Distances between the puru-phenyl C atoms at the ter- mini of the rods are 19.37 A (in 2) and 26.84 A (in 3), which is in agreement with the calculated values in Scheme l . [ I4 ]

Fig. 4. Crystal structure of 3. Selected bond lengths [A] and angles ["I: C(1)-C(1A)

1.175(2). C(6)-Si(l) 1.852(2), C(4)-C(16) 1.424(2), C(16)-C(17) I .192(2), C(17)-

C(26)-C(27) 1.187(2). C(27)-Si(2) 1.844(2): C(l)-C(Z)-C(3) 176.4(2), C(2)-C(3)-C(4) 119.5(2), C(3)-C(4)-C(S) 121.8(1). C(4)-C(5)-C(6) 179.6(1), C(5)-C(6)-Si(l) 175.9(8), C(3)-C(4)-C(16) 121.9(2), C(4)-C(16)-C(17) 179.6 (2). C(16)-C(17)-C(IX) 178.1(2). C(17)-C(lX)-C(19) 178.0(2). C(18)-C(19)-C(20) 178.4(2), C(2)-C(3)-C(26) 118.1(1). CO)-C(26)-C(27) 176.0(1). C(26)-C(27)-Si(2) 175.8(1), C(4)-C(3)-C(26) 122.4( 1 ) .

1.368(3). C(2)-C(3) 1.432(2), C(3)-C(4) 1.352(2), C(4)-C(5) 1.452(2), C(5)-C(6)

C(18) 1.371(2). C(18)-C(19) 1.1 97(2), C( 19)-C(20) 1.429(2), C(3)-C(26) 1.441(2).

In the electronic absorption spectra of 2-6 (Fig. 5 ) the wave- length of the end absorption increases with the length of the oligomeric chain. It is well known that the band gap E, of a polymer can be determined from plots of the longest wavelength UVjVIS absorption for a series of related oligomers versus the

1.5 h 3

4

5

6

0 250 300 350 400 450 500 550 600

Fig. 5. Electronic absorption spectra of 2-6 in CHCI,, c = 2.3 x 293 K.

M, T =

length of the o l igomer~. [ '~] When the end absorptions of 2-6 were plotted against the inverse of the oligomeric length, a straight line was obtained ( r z = 0.996) with an intercept a t 540 nm (2.3 eV), which corresponds to the longest wavelength absorption cutoff for an infinite polymer in solution, in other words, its solution band gap. This band gap is comparable to that of many polydiacetylenes (E, =. 2.1 eV).

Of particular interest were the redox properties of the conju- gated rods, which were examined by polarography and cyclic voltammetry in T H F with Bu,N+PF, (0.1 M) as the supporting electrolyte.['61 While none of the oligomers 2-5 could be oxi- dized below + 1 .0 V (vs ferrocene), which helps explain their stability in air, they were all readily reduced with the number of reversible one-electron reduction steps corresponding to the num- ber of tetraethynylethene moieties in each rod." 'I For longer oligomers the first reduction occurred at increasingly less negative potentials (Fig. 6). A plot of reduction potential vs l/(oligomer length) gave nearly identical first and second reduction potentials of - 1.01 V and - 1 .OS V, respectively, for an infinite polymer.r1s1

-2.00

-1.75

t Ell2 'v' -1,50

-1.25

-1.1)O

- 1.99

2 3 4 5

Oligomer

Fig. 6. Reduction potentials E, ,2 (V, vs ferrocene)for 2-5 in THF wlth Bu,N 'PF; (0.1 M) as the electrolyte.

A closer inspection of the reduction potentials shows an inter- esting trend (Fig. 6). The monomeric compound 2 undergoes one reversible one-electron reduction at - 1.57 V. The first reduction potential of dimeric 3 is significantly less negative (-1.32 V) and the second reduction step occurs at a potential ( - 1.60 V) similar to that for the reduction of 2. The trimeric compound 4 undergoes a facile first ( - 1.19 V) and second (- 1.43 V) reduc- tion, but the third reduction occurs at significantly higher energy (- 1.96 V). Tetramer 5 undergoes its fourth one-electron reduc- tion (- 1.99 V) at nearly the same potential as the third one-elec- tron reduction in 4 ( - 1.96 V). This "painvise" reduction behav- ior is best explained by especially stable dianions and tetraanions. In the first one-electron reduction, a radical anion forms which is delocalized over the entire backbone (Scheme 4). We propose that in the dianion the anionic centers are separated by a [5]cumulene unit. In such a structure the anionic centers are held far apart, minimizing repulsive electrostatic interactions. Further- more, the negative charges are delocalized over what are essen- tially dialkynylmethyl moieties, which have been shown previ- ously to form quite stable anions." 91 In the trimer the negative

Angew. Chm. In/. Ed. Engl. 1994, 33, No. 7 0 VCH Kdagsgesell.schaft mhH, 0-69451 Wernheim, 1994 0570-0R3319410707-0765 $ 10.00 + .25,'0 7-65

TIPS

TIPS TIPS / / @ -

TIPS

Scheme 4. Proposed structures of the rddicdl anion of 3 (top) and the dianions of 3 (middle) and 4 (bottom).

charges of a dianion are localized on the dialkynylmethyl moi- eties on opposite ends of the molecule (Scheme 4). A third elec- tron would have to add to the central dicumulenic unit, which explains the difficulty in forming the trianion.

Because of their remarkable stability, oligomers such as 2-6 could become useful in the field of molecular electronics. Other substituents may also be attached to the end of the rods by chang- ing the end-capping reagent. The properties of these nanometer- scale materials could be easily altered by exchanging the silyl groups for other substituents. This study of new oligomeric rods has allowed us to predict interesting properties of the corre- sponding polymers, on which we will report in due course.

Received: November 6. 1993 [Z6482IE] German version. Angew. Cliem 1994, 106. 794

[i] Z Wu, S. Lee. J. S. Moore, J. Am. Chmi. Soc. 1992. 114, 8730-8732. [2] a) J.-M. Lehn. Angew. Cliem. 1988, 100. 91 ~ 1 1 6 ; Angeir. C17en7. Int. Ed. Engl.

1988. 27, 89 112; b) P. Kaszynski. A. C. Friedli, J. Michl. J. A m Cheni. Soc. 1992. 114. 601-620. and references cited therein; c) J. Miiller. K . Base, T. F. Magnera, J. Michl. ihrd. 1992, f f 4 . 9721-9722. d) X. Yang, W. Jiang, C. B. Knobler, M. F. Hawthorne. did. 1992. 114.9719-9721: ej J. Guay, A. Diaz. R. Wu. J. M. Tour. ihid. 1993, 1f5, 1869-1874.

[3] A. M . Boldi. J. Anthony. C. B. Knobler. F. Diederich. Angew. Chem. 1992, 104, 1270-1273; Angeiv. Chiw~. In / . Ed. Engl. 1992, 31. 1240-1242.

[4] The potential of substituted tetraethynylethenes as monomers for new poly- mers has been independently recognized: H. Hopf. M. Kreutzer. P. G. Jones, Chem. Ber. 1991. 124. 1471 ~ 1475.

[5] A. J. Heeger, A. G. McDiarmid in T17e Ph~sics and Chemitrry of L O N Dimrn- riof7ul Solids (Ed.: L. Alcacer), Reidel. Dordrecht. 1980, pp. 353-391

[h] a) G. Wegner, Angeic. Clzeni. 1981, 93, 352 371. Angew. Clzem. In/. Ed. Engl 1981, 20. 361 381: bj G. Adler. Mol. C r W . Liy. Cry.tt (Purr A ) 1983. 96, 1-41',

[7] A. M. Sladkov, Sov. Scf. Rev. Sccr. B 1981, 3, 75-110. [XI H. S. Nalwa, A h . M u m . 1993. 5, 341 - 358. [Y ] a) F. L. Carter, .Molecular Elecirontc Devices II, M. Dekker, New York. 1987;

bj .Vunostrirt/ureA Bused on Moliwdur Murerrals (Eds. : W. Gopel. C. Ziegler). VCH. Weinheim. 1992.

[lo] X-ray crystal data for 14 (C,,H,,O,Si,, M, = 637.0): Monoclinic space group P2,:c (No. 14). p, , , ,=l .O46gcm~', 2 = 2 . u=13.896(12). b=10.810(10), c=14.999(11).&. /j=116.16(6)', V=2022(3)Ai, Mo,, radiation, 20<40 ' , 1900 unique reflections. T = 293 K. The structure was solved by direct methods (SHELXTL PLUS) and refined by full-matrix least-squares analysis using experimental weights (heavy atoms anisotropic. H atoms fixed. whereby H positions are based o n stereochemical considerations). Final R ( F ) = 0.047. II.R(F) = 0.060 for 199 variables and 1307 observed reflections with F > 4n(F ) . 1 (C2RH44Si2. M, = 436.8): Monoclinic space group P2,,'c (No. 14) pL,, ,=1.03gcm-' . Z = 2 . u=l3.443(4). h=11.762(5), c=8.904(1)A, /l= 92.85(0)'. V = 1406.13 A'. Mo,, radiation, 2 8 5 50', 2447 unique reflections.

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766 ?' VCH Verla~.;ge.srll.~rhufr inhH, 0-69451 Weinlzeim. 1994 0570-0833/94j0707-0766 $10.00+ .25!0 Angrw. Chem. Inr. Ed. Engl. 1994, 33, No. 7

T = 100K. The structure was solved by direct methods (SHELXTL PLUS) and refined by full-matrix least-squares analysis using an isotropic extinction coef- ficient and an exponentially modified weight factor with r = 5 ,&'(heavy atoms anisotropic, H atoms isotropic, whereby H positions are based on stereochem- ical considerations). Final R(F) = 0.038. wR(F) = 0.039 for 159 variables and 1578 observed reflections with I z 3 o ( I ) -2 (C4*Hs2Si2, M , = 637.0): Mono- clinic space group P2,ic (No. 14), pCaI, =1.046 gcm-', Z = 2, a = 8.40(2), h = 15.570(6). < = 15.57(4) A, 0 = 96.7(2)", Y = 202317) A', Mo,, radiation. 20 S 45-. 2635 unique reflections. T = 293K. The structure was solved by direct methods (SHELXTL PLUS) and refined by full-matrix least-squares arialysis using experimental weights (heavy atoms anisotropic. H atoms fixed, whereby H positions are based on stereochemical considerations). Final R(F) = 0.052. n.R(F) = 0.050 for 208 variables and 2008 observed reflections with F > 4ojF) . - 3 (C,>Hg4Si*, A4, ~ 1 0 7 1 . 8 ) : Triclinic space group Pi, p,,,, = 1 . 0 2 9 g c m ~ ' , Z = l , r r = 1 2 . 0 2 1 ( 6 ) . b =12.249(7).c=13.597(5)A.r= 105.07(4), =109.46(3), ;' =101.53(4)'. V=1730.2(15) A3, Mo,, radiation, 2 0 5 45'. 4537 unique reflections, T = 293K. The structure was solved by direct methods (SHELXTL PLUS) and refined by full-matrix least-squares analysis using unit weights (heavy atoms anisotropic, H atoms fixed, whereby H positions are based on stereochemical considerations). Final R ( F ) = 0.052, i vR(F) = 0.053 for 343 variables and 1280 observed reflections with F > 4u(F). We thank 1. Steiner. A. Kramer, and S. Zurcher for help with the X-ray crystal structure analyses. Further details of the crystal structure investigations are available o n request from the Director of the Cambridge Crystallographic Data Centre, 12 Union Road, GB-Cambridge CB2 IEZ (UK), o n quoting the full journal citation.

[i 11 J. A. Walker, S. P. Bitler, F. Wudl, J Org. Cltem. 1984, 49, 4733-4734. [I21 a) J. Anthony, C. B. Kuobler. F. Diederich, Angew. C h e m 1993, 10s. 437-440:

Angew. Chem. I n f . Ed. Engl. 1993, 32, 406-409: b) J. Anthony, F. Diederich, unpublished.

[13) See: T. X. Neenan. M . R. Callstrom. L. M. Scarmoutzos, K. R. Stewart, G. M Whitesides. V. R. Howes, Matromolrrules 1988, 21, 3525 -3528.

[14] CVFF force field. Biosym Technologies, San Diego. [IS] J. L. Bredas, R. Silbey. D S. Boudreaux, R. R. Chance, J. Am. Chem. Soc. 1983,

105. 6555 - 6559 [16] In the electrochemical investigations of 2-5 polarography and cyclic voltam-

metry gave concurring results. In T H F with 0.1 M Bu,N+PF; as the supporting electrolyte the accessible potential range on mercury was +0.15 to -3.4 V vs ferrocene. In a standard three-electrode cell. the working electrode was a drop- ping mercury electrode, the counter electrode a mercury pool, and the pseudo- reference electrode a silver wire dipped in a solution of supporting electrolyte. Under these conditions, ferrocene, used as an internal standard, is oxidized at 0.87 V vs Ag. Similar results were also obtained in T H F with a platinum working electrode (available potential range + 1.0 to -2.2 V vs ferrocene). A detailed account on the electrochemical properties of 2-5, including spec- troelectrochemical studies, will be given elsewhere.

[17] a) J. Heinze, J. Mortensen, K . Mullen, R. Schenk. J Chem. Soc., Chem. Com- mun. 1987.701 -703: b) R. Schenk. H. Gregorius, K. Meerholz, J Heinze, K. Miillen, J . A m Chem. SOC. 1991, 113. 2634-2647.

[18] P. Biiuerle. Adv. Malrr.1992, 4, 102-107. 119) H. Hauptmann, Terrahrdron Lrrr. 1974, 40, 3593-3596.

1,l-Dimeth yl-1 H-silirene ** M a r t i n Trommer , Wolfram Sander,* and Claudia Marquard

Silyldiazo compounds are frequently used as precursors for the synthesis of silaethenes (silenes). For instance, 1 ,I ,2-trimethyl- silene (l), the first silene characterized spectroscopically, was obtained by irradiation of trimethylsilyldiazomethane (2) in an argon matrix.['] Carbene 3, formulated as an intermediate in this reaction, is produced only in a very low stationary concentra- tion and could be detected by ESR but not by IR spec-

[*] Prof. Dr. W. Sander."' Dip1.-Chem. M. Trommer, C. Marquard lnstitut fur Organische Chemie der Technischen Universitat Hagenring 30, D-38106 Braunschweig (FRG)

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Fonds der Chemischen Industrie. [**I This work was supported by the Deutsche Forschungsgemeinschaft and the