1992, 33. 3409; the authors also report on a reaction of (k)-lO-ammo- 10-tetrahydrohenzo[a]pyrene with a silylated 2-

fluorodeoxyinosine derivative but characterization of the products and yields are not given.

[lo] S. Monnerjahn. A. Seidel. P. Steinberg. F. Oesch, M. Hinz, J. J. Steaowski. A. Hewer. D. H . Phillips, H . R. Glatt in Proc. Int. Meet. Postlabelling Methodsfor the Drlection o/ D N A Addutts, Lyon (France), 1992, in press.

[ l l ] a) L. Latif. R. C. Moschel, K. Hemminki. A. Dipple, Cheni. Res. Torrcol. 1988, I, 364; b) K. Pongracz, S Kaur, A. L. Burlingame, W. J. Bodell, Curcinogenesis 1992, 13, 31 5.

[12] For the stereoselective synthesis of (+)-lo, see M. Lakshman, D. V. Nadkarni, R. E. Lehr. J Org. Chm?. 1990, 55,4892. A 0-N migra- tion of an acyl group was not observed under these conditions.

[13] M. K. Lakshman. J. M. Sayer. D. M. Jerina, J Am. Chem. Soc. 1991,113, 6589.

[14] a) S. C. Cheng, B. D. Hilton, J. M. Roman, A. Dipple, Chem. Re>. T o r i d . 1989. 2, 334; b) J. M. Sayer. A. Chadha. S. K. Agarwal, H. J. C. Yeh, H. Ydgi, D. M. Jerina, J Org. Chern. 1991,56. 20.

[15] A. K. Basu. J. M. Essigmann. Chem. Res. Toricol. 1988. 1, 1.

all-carbon network 3.[41 Here, we report that Hay coupling of If leads not only to the [18]annulene 2a but also to the [12]annulene 4a and discuss the very different properties of these macrocycles.

Stable [12]- and [18]Annulenes Derived from Tetraethynylethene"" By John Anthony, Carolyn B. Knobler, and FranGois Diederich*

In memoriam Franz Sondheimer

We recently described the synthesis of tetraethynylethene (la)"] along with geminally bisdeprotected (lb, c),"] monodeprotected (1d),I2I and trans-bisdeprotected (le)"] derivatives and now wish to report the completion of the series with the first synthesis of a cis-bisdeprotected te- traethynylethene (lf).f3] Compound If with a free cis- enediyne unit is of special interest because it could lead, by oxidative cyclization, to the hexaethynylhexadehydro- [18]annulene 2a and after deprotection to 2c, to the planar

H H H H Ph Ph H H TIPS TIPS H H

d TIPS TIPS TIPS H e Ph H H Ph f TIPS H TIPS H

R 9 TMS H TMS H

R3

2 1 R = TIPS 2b R = TMS 2c R = H

R \ - - _ _ TIPS = Si(iPr13, TMS SiMe,

R =

[*] Prof. F. Diederich, J. Anthony Laboratorium fur Organische Chemie ETH-Zentrum Universititstrasse 16, CH-8092 Zurich (Switzerland) Dr. C. B. Knohler Department of Chemistry and Biochemistry University of California Los Angeles, CA 90024- 1569 (USA)

[**I We thank the U S . National Science Foundation, the American Chemical Society (A.C.S. graduate student fellowship to J. A.) and the ETH (set-up funds) for support of this work. According to IUPAC nomenclature te- traethynylethene should he called 3.4-diethyny1-3-hexene-lS-diyne. The trivial name tetrdethynylethene is easier to remember, however, and equal- ly unambiguous.

4a R = TIPS 4b R = TMS

cis-Enediyne If proved to be far more difficult to prepare than the tetraethynylethene derivatives l b e previously syn- thesized. The simplest route to If started from cis-2,3- dichloro-2-buten-I ,4-dIol(5), obtained by exhaustive reduc- tion of mucochloric acid,r51 and followed the pathway previously used for the synthesis of the trans derivative l e (Scheme I).['] However, this route was abandoned after oxi-

TIPS

CI Cul. benzene, 40% .-lliO" 5 OH TIPS

TIPS-H, >6 2 equiv PDC, CHzCIz

BUNHz, [Pd(PPh3)&1zl,

6 m.p. 51-53 OC

7 (= 30 Yo m.p. 49-50 Oc)

8 (= 10 70, highly unstable)

Zn, CHzCIz

T I P S Z ;

Br Br

4 9

Scheme 1. Attempted direct synthesis of cis-enediyne If from precursor 9. TIPS = triisopropylsilyl, PDC = pyridinium dichromate.

dation of cis-diol6 led preferentially to the dialkynyl lactone 716] and since dialdehyde 8 could not be transformed into tetrabromide 9.

To avoid the problems associated with the preparation and reactivity of dialdehyde 8, a route to If was chosen by which the key intermediate 9 was prepared by sequential introduction of the two dibromomethylene residues (Scheme 2). The key steps in this synthesis are the prepara- tion of orthoester 11 and its selective reduction to the monoprotected diol 12.17' Since compound 13 and all other intermediates with dibromomethylene groups on the way to If underwent facile cis-trans isomerizations in the presence of acid, acidic reaction and workup conditions had to be strictly avoided. As an example, removal of the methoxy-

406 ((3 VCH Verlug.sge.rrll~s~hu~r mhH, W-6940 Weinheim. IY93 0570-0833/93/0303-0406 $10.00 + ,2510 Angew. Chem. Inr. Ed. Engi. 1993, 32, No. 3

TIPS

- (CH30)3CH, "'x& TIPS-H

62 % Cul, benzene, 91%

5 OMe

10 m.p. 52-53 "C

BuNHz, [Pd(PPh3)zCIzl.

11

CSA. CHpCIz. CI

TIPS

1) PDC, CH&, *

2) CBr4, PPh,,

TIPS CHzCI,, - 78 'C, 1 h. 9 4 % TIPS 12

LDA (7 equiv), 9 * 11

NH,CI, 90%

TIPS

CHzCIz, 65 - 70 Yo

TIPS

Scheme 2. Synthesis of cis-enediyne If. Except for 10. all compounds are oils. CSA = camphorsulfonic acid, DIBAH = diisobutylaluminum hydride, LDA = lithium diisopropylamide.

bered cycle 2a is a stable yellow solid (m.p. 21 5 "C decomp.), and its I3C and 'H NMR data strongly support the cyclic trimeric structure. In addition, the characteristic electronic absorption spectrum of 2a (Fig. 2) closely resembles those of other hexadehydro[ 1 8lannulenes with planar aromatic perimeters.['41 Based on this strong spectral correlation, we also assign aromatic character to the cyclic [4n + 21 Ir-elec- tron system in 2a.

The [12]annulene 4a (m.p. 200 "C, decomp) crystallizes readily from pentane to give dark red needles suitable for X-ray crystallography (Fig. 3).['01 Except for the isopropyl groups, 4a is planar with a mean out-of-plane deviation of the conjugated 20-carbon framework of 0.089(6) A. Con- siderable strain in the 12-membered ring is expressed by the

1 . m

0.7500

1 0.5000

A methylene protecting group in 13 with bromotrimethyl- silane[*] afforded 14 in addition to a significant amount of the corresponding lrans-enediyne, whereas no isomerization

("B-bromocatecholborane") as the deprotecting agent.['] Tetrabromide 9 was obtained as a yellow oil which, upon

standing on the lab bench, yielded pale yellow crystals. Sur-

trans-tetrabromide 15 (m.D. 78 "C, Fig. I)![''' Further study

of 14 was detected with B-bromo-1,3,2-benzodioxaborole 0.2500

0.ooOC 2( prisingly, X-ray analysis of these crystals showed them to be

-_ . . . . . . . . .._.__. -. . - - ._. . .. . - -. .-- 00 3m.00 400.00 500.00 600.00

Ahml - ~~ -

Fig. 2. Electronic absorption spectra of Za (----) and 4a (-) recorded in showed that cis-tetrabromide 9 can be stored safely for sever- al days in solution in the dark at - 20 "c when traces of acid = 298 K, = cm, = 4,23 l o - b for 2a and ~ = .024 ,o- 5

are strictly eliminated.

Fig. I . X-ray crystal structure of 15

Upon treatment of 9 with an excess of LDA at - 78 "C, a deep purple solution of the cis-di(1ithium acetylide) formed, which was quenched at 0°C with saturated NH,CI. The free cis-enediyne If was obtained as a yellow oil which is quite stable a t room temperature in dilute solution. When a solu- tion of If in benzene was heated at reflux for 6 h, 80 YO of the compound was recovered; heating the neat oil to 60 "C gave immediate decomposition. Whether intramolecular Berg- man cycIizations[' ' ] or intermolecular processes are respon- sible for the decomposition of If is currently under investiga- tion." '1

Hay coupling of If (0.002 M in acetone) yielded the [12]annulene 4a (28%) and the [18]annulene 2a (12%) as

for the shorter wavelength and 2.56 x tions (Insert) of 4a. Inset is scaled up 25 x , A = Absorbance.

M for the longer wavelength absorp-

strong bend in the two butadiyne unit^[^“^^] and by the reduction of the inner C-C=C bond angles to 117.6 and 118.5". Despite this ring strain, the molecule is quite stable. At room temperature, dilute pentane solutions remain un- changed over weeks and crystals can be kept in air for several days without decomposition. However, if a solution of 4a is concentrated without crystallization, significant decomposi- tion occurs.

Both [18]annulene 2a and [12]annulene 4a have planar rigid chromophores, and correspondingly, their electronic absorption bands show considerable vibrational fine struc-

readily separable and isolable compounds.[' 31 The 18-mem- Fig. 3. X-ray crystal structure of 4a.

AnRew. Clwm. I n f . Ed. EngI. 1993, 32. No. 3 &:, VCH Verla~.s~cscllscliafr mhH, W-6940 Weinheinz, I993 (~5?o-OXs3193j03o3-o407 $ IO.OOf .25/0 407

ture. The spectra of the two compounds are otherwise strik- ingly different (Fig. 2). The end absorption in 2a occurs near 480 nm (2.57 eV), while 4a shows a characteristic weak ab- sorption band at longer wavelengths which extends to 660 nm (1.87 eV) (Fig. 2). Solutions of 2a are bright yellow, whereas compound 4a gives purple solutions that, when di- lute, look remarkably similar to solutions of buckminster- fullerene C,, .

Although the absence of protons in proximity to the [l 2lannulene perimeter of 4a prevents a direct experimental determination of its paratropic character by 'H NMR spec- troscopy, a comparison of optical properties suggests that 4a is antiaromatic. The strong redshift of the end absorption in the UVjVIS spectrum of 4a as compared to Za, can be ex- plained by the smaller HOMO-LUMO gap in an antiaro- matic system. The spectrum of 4a is indeed extremely similar in shape and structure to the spectrum of [12]annulene 16 which has an end absorption at approximately 620 nm (1.99 eV) and is paratropic according to its 'H NMR spec- trum.['6s'71 In contrast, [12]annulene 18 does not exhibit a paramagnetic ring current as a result of the benzannelation and shows optical properties that differ entirely from those of 4a and 16.['5a1 Compound 18 is pale yellow and its end absorption in the UVjVIS spectrum already occurs a t 450 nm (2.74 eV).['*I Together with the results of previous comparative studies on [18]- and [24]ann~lenes[~* 1 4 9 191 these findings suggest that in series of planar, structurally related annulenes, careful correlations of electronic absorption spec- tra can be used to evaluate their aromatic or antiaromatic character.

m

16 17

0 .: . 18

The deprotection of 2a to give compound 2c and the preparation of planar all-carbon networks by oxidative polymerization are now under investigation.

Experimental Procedure 12: To a solution of 7.35 g (15.0 mmol) of 11 in CH,CI, (150 mL) at -78 'C was added slowly 45 mL of a 1.0 M solution of DIBAH in hexanes (45 mmol). After 15 min, the solution was warmed rapidly to 0 ° C and stirred at that temperature for 15 min. The solution was quenched by careful addition of water until the entire mixture solidified. The product was dissolved in 50 mL of CH,CI,. and the resulting solution was dried over MgSO,. The grainy solid was removed by filtration, and the product-containing solution was concentrated to yield crude (96% pure) 12 (7.0 g, 94 % yield) as a pale yellow oil which was used in the next reaction without further purification. Analytically pure 12 was obtained as a colorless oil by flash chromatography (SO,, CH,CI,): IR (neat): i.[cm-'] = 3440 (0-H), 2146, 2127 (C=C); 'H NMR (200 MHz, CDCI,): 6=1.07 (s, 42H), 3.38 ( s . 3H), 4.24 (s, 2H). 4.66 (s. 2H); I3C NMR (50.32MHz,CDC13):6 =11.40,18.86,55.70,62.04,66.15,95.86,99.48,101.02, 105.63, 106.55, 126.80, 132.49. MS (EI, 20 eV) mjz 492.3 ( M ' ) . Za/4a: To a solution of I f (0.89 g, 0.002 mol) in 500 mL of acetone was added 1 mL of Hay catalyst (prepared by stirring CuCl(2.5 g) and TMEDA (1.25 mL) in acetone (45 mL) for 30 min). After 1 h, the black solution was poured into pentane and extracted first with 0.1 N HCI, then with water and saturated NaCl

solution. The solution was filtered with pentane through a short silica gel plug. The rapid elution of the deep purple fraction of4a (0.24 g, 28%) was followed by elution of the bright yellow fraction of 2a (0.104 g, 12%). Compound 4a: dark red needles (pentane), m.p. % 200°C (decomp); IR (KBr) i[cm-'] = 2107 ( C r C ) ; ' H N M R (360 MHz. CDCI,) b =1.00 (s. 84H); I3C NMR (90.6MH2, CDCI,): d =11.06. 18.51. 87.80, 94.05, 99.67, 106.79, 124.37; MS (Desorption Electron Impact (DEI)). 20 eV) mi; 868 (M +): UVjVIS (pentane) i.,,, (ti) = 614 (270). 598 (sh) (190). 564 (300). 546 (3701, 531 (sh) (290). 506 (290). 491 (254), 477 (225), 461 (195). 407 (1 540) 402 (sh) (565). 385 (1690). 374 (sh)(3110). 349(32650). 335(44700), 326(36900), 276(13200). 262(7210), 233 (sh) (19600). 220 nm (sh) (35200). Compound 2a: m.p. 215 C (decomp). 1R (neat): i.[cm-']=2122 (CEC); ' H N M R (360MHz, CDCI,) 6=1.17 (s, 126H), "C NMR (90.6 MHz. CDCI,); 6 = 11.31, 18.69. 83.81. 85.09, 102.25, 107.38. 118.00. MS (DEI. 20 eV) mi; =1303.7 (M '). UVjVIS (pentane) i.,,, (6) = 453 (56000). 438 (53700). 425 (sh) (22500), 405 (189700). 383 (84300). 375 (59400). 367 (35400). 356 (sh) (24100). 323 nm (17600).

Received: November 13, 1992 [Z 5686 IE] German version: Angels. Chem. 1993, 105, 437

[ I ] Y. Rubin. C. B. Knobler, F. Diederich. Angeir. Chem. 1991, 103, 708-710; Angen. Chem. h r . Ed. Engl. 1991. 30. 698-700.

[2] A. M. Boldi. J. Anthony, C. B. Knobler, F. Diederich, Angew. Chem. 1992, 104. 1270-1273; Angen. Chem. In!. Ed. Engl. 1992, 31, 1240-1242.

[31 a) Y Hori, N. Noda, S . Kobayashi, H. Taniguchi. Tetrahedron Lett. 1969. 3563-3566; b) H. Hauptmann, Angew. Chem. 1975,87,490- 491; Angew. Chem. Int. Ed. EngI. 1975, 14, 498-499; c) H. Hauptmann, Tetrahedron Lett. 1975,1931 - 1934; d) H. Hopf, M. Kreutzer, Angew. Chem. 1990,102, 425-426; Angen. Chem. Int. Ed. Engl. 1990.29.393-395;e) H. Hopf, M. Kreutzer. P. G. Jones, Chem. Ber. 1991, 124, 1471-1475.

I41 F. Diederich, Y. Rubin, Angew. Chem. 1992, 104, 1123-1146; Angels'. Chem. Int. Ed. Engl. 1992. 31, 1101-1123,

151 B. A. Arburov. E. N. Klimovitskii, M. B. Timirbaev, Zh. Uhshch. Khim. 1986, 13. 150-155.

[61 All new compounds were fully characterized by 'Hand "C NMR, IR. and UVjVIS spectroscopy, and mass spectrometry. Correct elemental analyses were obtained for all stable derivatives; high-resolution mass spectra were measured for instable compounds.

171 M. Takasu. Y. N a m e . H. Yamamoto, Tetruhedron Lett. 1988, 29, 1947- 1950.

181 S. Hanessian. D. Delorme, Y. Dufresne, Tetruhedron Lett. 1984, 25, 251 - 254.

[91 R. K. Boeckman. Jr. , J. C. Potenza, Tetrahedron Lett. 1985, 26, 1411- 1414.

[lo] X-ray crystal dard for 15 (C2aH+,Br.,Si2); M, =756.45, triclinic space group Pi, Z = 1 (centrosymmetric molecule); u = 8.310(2), h = 9.514(2),

861 A3, Mo,, radiation, 2 6 5 50". 3029 unique reflections; the structure was solved by heavy-atom methods, yielding R = 0.054, R, = 0.072 for 1439 independent reflections with I z 3u(I). X-ray crystal data for 4a (C56H84Si4), M, = 867.64, triclinic space group Pi, Z = 1 (centrosym- metric molecule); u =11.739(3), h =12.445(3), c =12.438(3)& OL = 101.262(8). 0 = 105.936(7), 7 = 117.635(7)", V = 1433 A', Cu,, radiation, 2 0 5 11 5 ', 3929 unique reflections; the structure was solved by statistical methods (SHELXS90), yielding R = 0.080, R, = 0.101 for 2827 inde- pendent reflections with I > 3a(I). Further details of the crystal structure investigations are available on request from the Director of the Cambridge Crystallographic Data Centre, 12 Union Road, GB-Cambridge CB2 1EZ (UK). on quoting the full journal citation.

I l l ] T. P. Lockhart. P. B. Comita, R. G. Bergman. J. Am. Chem. SOC. 1981, 103,4081 -4090.

[121 The TMS-protected cis-enediyne lg was also obtained as a colorless oil following the synthetic route shown in Scheme 2. The TMS-protected [12]annulene 4b (ready decomposition) and [lXlannulene 2b (m.p. 170°C. decomp) have also been prepared and their spectral properties closely resemble those of the two TIPS derivatives 4a and 2a, respectively; J. Anthony, F. Diederich, unpublished results.

1131 The yield of the oxidative cyclization reaction is strongly dependent on concentration as was shown in more systematic studies with the TMS-pro- tected cis-enediyne lg: Whereas a 0.006 M solution of l g in acetone gave a product ratio of 12: 5 : 1 (4b:Zb: larger macrocycles), a 0.045 M solution led to a higher yield of the larger cycles [product ratio: 1 :4:2 (4b:Zb: larger macrocycles)]. Generally, the overall yield of isolable cyclic products in- creased with increasing concentration of Ig. In the coupling of the TIPS derivative If, higher cyclic oligomers beyond 2a are not formed pre- sumably for reasons of steric crowding of the TIPS groups in the larger macrocycles

(141 a) F. Diederich, Y. Rubin, C. B. Knobler, R. L. Whetten, K. E. Schriver. K. N. Houk, Y Li, Science (Washington. D.C.) 1989, 245. 1088-1090; b)Y. Rubin. C. B. Knobler, F. Diederich. J. Am. Chem. Soc. 1990. 112, 1607-1617

c =11.899(3) A, I =112.204(7), = 97.872(7). ;I = 90.175(7)", V =

408 Q VCH Verlugsgesell.~chaji mhH. W-6940 Weinheim, 1993 05?0-0833/93j0303-0408 S 10.00+ .25/0 Angew. Chem. Int. Ed. Engl. 1993. 32, No. 3

[15] a ) O . M. Behr, G. Egiinton. A. R. Galbraight, R. A. Raphael, J. Chem. Soc. 1960, 3614-3623; b) W. K. Grant, J. C. Speakman, Proc. Chem. Soc. London 1959, 231.

1161 a) K. G. Untch. D. C. Wysocki. J . Am. Chem. Soc. 1966.88. 2608-2610; b) F. Sondheimer. R. Wolovsky, P. J. Garratt, I. C. Calder, hid. 1966.88. 2610.

[17] Some degree of paratropicity has been assigned to 17 by using the 'H NMR criterion, although the conformational flexibility of the cyclo- hexene rings seems to influence the rigidity of the 12-membered ring as suggested by the reduced vibrational fine structure in the electronic ab- sorption spectrum which extends to approx. 550 nm (2.24 ev); G. M. Pilling, F. Sondheimer, J . Am. Chem. Sor. 1971, 93, 1970-1977.

[I81 For the cancellation of the paramagnetic ring current in other benzan- nelated [12]annulenes, see H. A. Staab. R. Bader, Chem. Ber. 1970. 103, 1157-1167 and references therein.

[19] Y. Li, Y. Rubin. F. Diederich, K. N. Houk, J. Am. Chem. Soc. 1990. If?, 161 8 - 1623.

[Mo,(CCH),(PMe,),], the Parent Dimetallapoly yne** By Timothy C. Stoner, Steven J Geib, and Michael D. Hopkins*

We are presently exploring the syntheses, structures, bond- ing, and properties of transition metal complexes and transi- tion metal-containing polymers with conjugated electron systems.['-31 As part of this work, we recently reported the preparation of alkynyl-substituted complexes with quadru- ple bonds of the types [M,(CCR),(PMe,),] (M = Mo, W; R = alkyl, C,H,, SiMe,)[" and [I ,I-W,(CCMe),Cl,- (PMe,),J,['] and we noted that the electronic absorption spec- tra of these dimetallapolyynes manifest G(MM)-x(CC) con- jugation. Unfortunately, we were neither able to characterize the [M,(CCR),(PMe,),] class by X-ray crystallography,[41 nor to prepare the parent [M,(CCH),(PMe,),J derivatives by the otherwise general synthetic route to these compounds. The study of the parent ethynyl compounds, which possess the electronically simplest alkynyl R group with the highest symmetry, is crucial to understanding the dimetallapolyyne class as a whole, since their bonding and properties provide benchmarks against which the symmetry and inductive ef- fects of organic and inorganic R groups can be evaluated. We now wish to report the synthesis and crystal structure of the dimetallapolyyne 1, as well as spectroscopic evidence that 1

is, in fact, electronically distinct from other [Mo,(CCR),- (PMe,),] complexes.

We have successfully prepared a variety of [M,(CCR),- (PMe3)4] complexes from the reaction between [M,CI,- (PMe,),] and LiCCR (4 equiv).['. In contrast, no tractable products were obtained under comparable conditions from the reactions between [Mo,CI,(PMe,),] and either LiCCH . (H,NCH,CH,NH,) or MgCl(CCH). Given that protodesilylation of alkynylsilanes is a standard synthetic route to terminal a lkyne~, [~] it seemed plausible to us that 1

['I Prof. M. D. Hopkins, T. C . Stoner, Dr. S. J. Geib Department of Chemistry, University of Pittsburgh Pittsburgh, PA 15260 (USA)

[**I This research was supported by the National Science Foundation, the Camille and Henry Dreyfus Foundation. and the David and Lucile Pack- ard Foundation (M. D. H.), and by an Andrew W. Mellon Graduate Fel- lowship (T. C. S . ) .

might be similarly generated from 2.['] Although the use of

[Mo,(CCSiMe,),(PMe,).+] 2

this method to generate transition metal ethynyl complexes has not, to our knowledge, been reported,[61 and there is scant precedent in the chemistry of complexes with quadru- ple bonds for ligandlocalized rea~tivity,~'~ the reaction of 2 with [~Bu,N][HF,][~~ in dimethoxyethane at room tempera- ture affords 1 cleanly in high yield. The use of [nBu,]F '

3H,O as the protodesilylation reagent slows down the reac- tion, and lowers the yield of 1. Compound 1 was identified by elemental analyses, 'H, 13C, and 31P NMR spectroscopy, and X-ray crystallography.

R I

L I

A 1:R=H 2: R = SiMes L = PMe3

3

Crystals of 1 are free of disorder, unlike those of all other [M,(CCR),(PMe,),] compounds that we have examined,[41 allowing the first high-quality single-crystal X-ray diffrac- tion study for this class of compounds.r91 The structure of 1 (Fig. 1) is typical of those of the [M,X,(PMe,),] class:['. "1 it possesses idealized D,, symmetry, and the bond lengths and angles of the Mo C,P, core lie within the expected ranges. The CEC (1 . I 8 A) and C-H (0.93 A) bond lengths of 1 are similar to those of [Fe(qs-C,Me,)(CO),(CCH)] (1.17

HI21

H(41

Fig. 1. Molecular structure of 1 (ORTEP. thermal ellipsoids at the 50% prob- ability level). For clarity, all hydrogen atoms are omitted except H(2) and H(4). which are represented by spheres of arbitrary size. Selected bond lengths [A] and bond angles ['I (standard deviations in parentheses): Mo-Mo' 2.134(1). Mo-P(l) 2.509(1), Mo-P(2) 2.515(1), Mo-C(1) 2.153(4), Mo-C(3) 2.161(4), C(l)-C(2) 1.183(6), C(3)-C(4) 1.174(6), C(2)-H(2) 0.950(39), C(4)-H(4) 0.90905); Mo'-Mo-P(l) 103.2(1), Mo'-Mo-P(2) 103.1(1), Mo'-Mo-C(I) 110.5(1). Mo'-Mo-C(3) 1 lO.l(l), Mo-C(l)-C(2) 171.7(3). Mo-C(3)-C(4) 171.9(4). C(1)-C(2)-H(2) 172 l(24). C(3)-C(4)-H(4) 166.7(24), Mo-Mo' torston angle = 1.1.

Angew. Chem. Int. Ed. Engl. 1993. 32. No. 3 $3 VCH Vcrlr~~sgesell.~chuf~ mhH, W-6940 Wemheini. 1993 057o-o833193jo303-04n9 $ 10.00 t ,2510 409