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Organometallics 2001, 20, 1230-1242

Activation of Terminal Alkynes at the Sulfur-Rich Bimetallic Site [MoIII2Cp2(µ-SMe)3]+: Alkyne-Vinylidene Conversion and C-S and C-C Couplings Promoted by Addition of Unsaturated Substrates (RCtCH, RNtC, SdCdS). Crystal Structures of µ-η1:η2-Vinylidene, µ-η1:η2-Acetylide, and µ-η1:η3-Vinyl-Thioether Compounds Philippe Schollhammer, Nolwenn Cabon, Jean-Franc¸ ois Capon, Franc¸ ois Y. Pe´tillon,* and Jean Talarmin UMR CNRS 6521, Chimie, Electrochimie Mole´ culaires et Chimie Analytique, Faculte´ des Sciences, Universite´ de Bretagne Occidentale, BP 809, 29285 Brest-Ce´ dex, France

Kenneth W. Muir* Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Great Britain Received November 14, 2000

Reactions of the bis(nitrile) compound [Mo2Cp2(MeCN)2(µ-SMe)3](BF4) (1) with terminal alkynes in a 1:1 ratio in dichloromethane at room temperature led to the alkyne adduct [Mo2Cp2(µ-SMe)3(RCCH)](BF4) (2: R ) Tol (2a), Ph (2b), CH3CdCH2 (2c), nPr (2d), CO2Me (2e), CF3 (2f)). Compounds 2a-d were readily deprotonated with Et3N to give neutral acetylide derivatives [Mo2Cp2(µ-η1:η2-CtCR)(µ-SMe)3] (3). Protonation of 3 afforded exclusively the vinylidene complexes [Mo2Cp2(µ-η1:η2-CdCHR)(µ-SMe)3](BF3) (4). Reaction of 1 with an excess of terminal alkyne RCCH in dichloromethane gave either the six-membered metallacycle compounds [Mo2Cp2(µ-η2:η4-CRdCHCRdCHSMe)(µ-SMe)2](BF4) (5) or the S-methylthiophenium derivatives [Mo2Cp2(µ-η2:η4-C4H2R2SMe)(µ-SMe)2](BF4) (6), depending on the nature of the R groups. Further reactions of the alkyne adducts 2 with isocyanide and carbon disulfide led to the vinyl-thioether complexes [Mo2Cp2(µ-η1:η3-CRdCR′-SMe)(µ-SMe)2(RNC)](BF4) (7, 8) and to their CS2 adducts [Mo2Cp2(S2C-CR′dCRSMe)(µ-SMe)2](BF4) (9, 10), which arise from regioselective C-S coupling. A mechanism is proposed for the formation of the cyclic thiometalla compounds 5, 9, and 10 and of the thiophenium species 6 which assigns a key intermediate role to vinyl-thioether species. The molecular structures of 3a, 4b, and 7a have been established by X-ray diffraction studies. Introduction This study of the interaction of alkynes with the sulfur-rich dimolybdenum system Cp2Mo2(µ-SMe)3 was aimed at eliciting cooperative activation by the adjacent metals, leading to chemical transformations unattainable with a single metal atom.1 Such studies also afford simple structural and chemical precedents for the behavior of less accessible natural or industrial catalyst systems.2 Bimetallic complexes offer an attractive compromise between polymetallic activity and structural simplicity.3 The incorporation of bridging groups inhibits (1) (a) Catalysis by Di- and Polynuclear Metal Cluster Complexes; Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998. (b) Braunstein, P.; Rose, J. In Chemical Bonds-Better Ways to Make Them and Break Them; Bernal, I., Ed.; Elsevier: Amsterdam, 1989; p 5. (c) Hidai, M.; Mizobe, Y. In Transition Metal Sulfur Chemistry-Biological and Industrial Significance; Stiefel, E. I., Matsumoto, K., Eds.; ACS Symposium Series 653; American Chemical Society: Washington, DC, 1996; p 310. (d) Hidai, M.; Mizobe, Y.; Matsuzaka, H. J. Organomet. Chem. 1994, 473, 1 and references therein. (2) Henderson, R. A. J. Chem. Soc., Dalton Trans. 1995, 503.

fragmentation but may also affect the activity of the metal site.4 Though sulfur-containing bridges can stabilize robust polymetallic complexes, their activity has been relatively little explored because of the notoriety of sulfur derivatives as catalyst poisons.4b,c In this respect, the sulfur-rich environment of the cofactors of some metalloenzymes5 and the well-developed chemistry of thiolato-bridged diruthenium and dimolybdenum systems are noteworthy.1c,d,4b,c Recently, Rakowski DuBois and co-workers reported that the thermal lability of the thioether bridge in [Mo2Cp2(µ-S2CH2)(µ-SMe)(µ-SMe2)]+ gave rise to sub(3) For example: (a) Curtis, M. D. Polyhedron 1987, 6, 759. (b) Winter, M. J. Adv. Organomet. Chem. 1991, 29, 101. (4) For example: (a) King, J. D.; Mays, M. J.; Mo, C.-Y.; Raithby, P. R.; Rennie, M. A.; Solan, G. A.; Adatia, T.; Conole, G. J. Organomet. Chem. 2000, 601, 271 and references therein. (b) Rakowski DuBois, M. Polyhedron 1997, 16, 3089. (c) Rakowski DuBois, M. J. Cluster Sci. 1996, 7, 293. (5) (a) Richards, R. L. New J. Chem. 1997, 21, 727. (b) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460. (c) Coucouvanis, D. J. Bio. Inorg. Chem. 1996, 1, 594.

10.1021/om000966j CCC: $20.00 © 2001 American Chemical Society Publication on Web 02/22/2001

Activation of Terminal Alkynes

stitution and desulfurization reactions. They speculated that the active intermediate, which could be neither isolated nor characterized, was the unsaturated species [Mo2Cp2(µ-S2CH2)(µ-SMe)]+, stabilized by coordinated solvent molecules.6,7 We have reported stoichiometric reactions which occur at a similar tris(µ-thiolato) bimetallic site in the µ-chloro complex [Mo2Cp2(µ-Cl)(µSMe)3].8 This complex in MeCN solution afforded the stable and tractable bis(acetonitrile) species9 [Mo2Cp2(MeCN)2(µ-SMe)3](BF4) (1), which is similar to the unsaturated intermediate postulated by Rakowski DuBois. Recently, we have shown that 1 can add two terminal alkynes to its coordination sphere: formally regioselective head-to-tail and tail-to-tail linkage of the alkynes and their coupling to the sulfur atom of a thiolate bridge gave six-membered metallacycles or thiophenium derivatives.10 These results suggest that 1 may undergo formal insertion of alkynes into a Mo-S bond. The rarity of such processes, which are known only for certain mono- and bimetallic complexes containing fluorinated alkynes11 and for diruthenium compounds,12 and the opportunity to develop new routes to S-heterocycles13 have led us to investigate the reactivity of 1 toward terminal alkynes RCtCH. We shall show that 1 reacts with various alkynes to afford the dinuclear alkyne adducts [Mo2Cp2(µ-SMe)3(RCCH)]+ (2), which are intermediates in the formation of thiametallacycles at the {Mo2Cp2(µ-SMe)3}+ site, and in which the alkyne group may also undergo an 1,2-H shift, giving rise to vinylidene species. Furthermore, addition of unsaturated molecules (e.g., isocyanides) to 2 results in the formation of vinyl-thioether derivatives with an unprecedented µ-η1:η3-coordination. A preliminary communication of a part of this work has already appeared.10 (6) Gabay, J.; Dietz, S.; Bernatis, P.; Rakowski DuBois, M. Organometallics 1993, 12, 3630. (7) Tucker, D. S.; Dietz, S.; Parker, K. G.; Carperos, V.; Gabay, J.; Noll, B.; Rakowski DuBois, M.; Campana, C. F. Organometallics 1995, 14, 3630. (8) (a) Schollhammer, P.; Pe´tillon, F. Y.; Poder-Guillou, S.; Saillard, J. Y.; Talarmin, J.; Muir, K. W. Chem. Commun. 1996, 2633. (b) Schollhammer, P.; Gue´nin, E.; Pe´tillon, F. Y.; Talarmin, J.; Muir, K. W.; Yufit, D. S. Organometallics 1998, 17, 1922. (9) Barrie`re, F.; Le Mest, Y.; Pe´tillon, F. Y.; Poder-Guillou, S.; Schollhammer, P.; Talarmin, J. J. Chem. Soc., Dalton Trans. 1996, 3967. (10) Capon, J. F.; Schollhammer, P.; Pe´tillon, F. Y.; Talarmin, J.; Muir, K. W. Organometallics 1999, 18, 2055. (11) (a) Davidson, J. L.; Sharp, D. W. A. J. Chem. Soc., Dalton Trans. 1975, 2283. (b) Carlton, L.; Davidson, J. L.; Miller, J. C.; Muir, K. W. J. Chem. Soc., Chem. Commun. 1984, 11. (c) Carlton, L.; Davidson, J. L.; Ewing, P.; Manojlovic-Muir, L.; Muir, K. W. J. Chem. Soc., Chem. Commun. 1985, 1474. (d) Azelee Wan Abu Bakar, W.; Carlton, L.; Davidson, J. L.; Manojlovic-Muir, L.; Muir, K. W. J. Organomet..Chem. 1988, 352, C54. (e) Agh-Atabay, N. M.; Canoira, L. J.; Carlton, L.; Davidson, J. L. J. Chem. Soc., Dalton Trans. 1991, 1175. (f) Carlton, L.; Agh-Atabay, N. M.; Davidson, J. L. J. Organomet. Chem. 1991, 413, 205. (g) Pe´tillon, F. Y.; Le Floch-Pe´rennou, F.; Guerchais, J. E.; Sharp, D. W. A. J. Organomet. Chem. 1979, 173, 89. (h) Guerchais, J. E.; Le Floch-Pe´rennou, F.; Pe´tillon, F. Y.; Keith, A. N.; Manojlovic-Muir, L.; Muir, K. W.; Sharp, D. W. A. J. Chem. Soc., Chem. Commun. 1979, 410. (12) (a) Nishio, M.; Matsuzaka, H.; Mizobe, Y.; Hidai, M. J. Chem. Soc., Chem. Commun. 1993, 375. (b) Nishio, M.; Matsuzaka, H.; Mizobe, Y.; Tanase, T.; Hidai, M. Organometallics 1994, 13, 4214. (c) Koelle, U.; Rietmann, C.; Tjoe, J.; Wagner, T.; Englert, U. Organometallics 1995, 14, 703. (d) Adams, H.; Allott, M.; Bancroft, M. N.; Morris, M. J. J. Chem. Soc., Dalton Trans. 2000, 4145. (13) (a) Spencer, J.; Pfeffer, M.; De Cian, A.; Fischer, J. J. Org. Chem. 1995, 60, 1005. (b) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259. (c) Linford, L.; Raubenheimer, H. G. Adv. Organomet. Chem. 1991, 32, 1. (c) Kondo, T.; Mitsudo, T. A. Chem. Rev. 2000, 100, 3205.

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Results and Discussion 1. Reaction of [Mo2Cp2(MeCN)2(µ-SMe)3](BF4) (1) with an Equimolar Amount of Terminal Alkyne RCtCH. A red solution of 1 in dichloromethane reacted quantitatively with 1 equiv of RCCH (R ) Tol, Ph, C(CH3)dCH2, nPr, CO2Me) and with 2-3 equiv of CF3CCH at room temperature within 30 min to give brown or green solutions. Addition of diethyl ether precipitated complexes 2a-f (eq 1) as brown or green

solids. Under the same conditions no reaction occurred with either the more sterically hindered tBuCtCH or with disubstituted alkynes RCtCR. The complexes 2 were unambiguously formulated by spectroscopic studies as the MoIII-MoIII alkyne adducts [Mo2Cp2(RCCH)(µ-SMe)3](BF4), but the mode of coordination of the alkyne has not been reliably established, since monocrystals of diffraction quality could not be obtained. At room temperature the 1H NMR spectra of 2 displayed both the resonances expected for the alkyne substituent, R, and a low-field signal (between 12 and 14 ppm) for the CH end of the alkyne, indicating coordination of an RCCH ligand to the molybdenum atoms. The single broad resonance which was observed for the two cyclopentadienyl groups suggested that a dynamic process was operative in solution at room temperature for complexes 2a-d. Two cyclopentadienyl resonances were observed at room temperature for complexes 2e,f, which contain alkynes with the more electron-withdrawing groups COOMe and CF3. 13C NMR spectra were recorded to get more information about the mode of coordination of the alkyne in 2. The two resonances of the sp-hybridized carbon atoms of the alkyne RCCH display some of the largest downfield chemical shifts yet reported for M2(alkyne) complexes (between 254 and 291 ppm),14 lying in the carbene range but slightly lower than expected for four-electron-donor alkynes in mononuclear compounds.15 Such data do not allow the nature of the alkyne-metal bonding to be defined without ambiguity, especially as the strong deshielding observed in 2 may partially reflect the electronic behavior of the thiolato-bridged bimetallic cation. Nevertheless, they suggest that the alkyne acts as a four-electron donor with a significant π⊥ contribution,15 as required by the electron-counting rules to saturate the {Mo2Cp2(µSMe)3}+ core. The alkyne coordination in 2 may not be completely described by either the classical tetrahedrane µ-η2:η2 coordination mode (I; Chart 1)16 or the dimetallacyclobutene µ-η1:η1 coordination mode (II; (14) (a) Bott, S. G.; Clark, D. L.; Green, M. L. H.; Mountford, P. J. Chem. Soc., Dalton Trans. 1991, 471. (b) Feng, Q.; Green, M. L. H.; Mountford, P. J. Chem. Soc., Dalton Trans. 1992, 2171. (15) Templeton, J. L. Adv. Organomet. Chem. 1991, 29, 1.

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Chart 1. Suggested Limiting Form Contributions to the Structure of the Alkyne Adduct Complexes 2

Scheme 1

Chart 1)17 but may also involve contributions from the µ-η2:η1 or µ-η1:η1 bis(carbene) limiting forms (III and IV; Chart 1);18a III resembles structures found in ynamine complexes.18b,c A final possibility is the unbridged fourelectron-donor alkyne V (Chart 1).15,19 The 1JCH value (156.5 Hz) suggests a significant C(sp) f C(sp2) rehybridization in 2e (1JCH ) 156 Hz in CH2dCH2 and 249 Hz in HCtCH).15,16 Interactions of alkynes with dinuclear complexes containing tripositive group 6 metals have been reported:14,20 they generally give rise to adducts whose structures deviate from the ideal perpendicular bridge geometry, with an unsymmetrical M2(µ-C2R2) unit and a C-C bond slightly twisted with respect to the M-M axis. The internal carbon chemical shifts of the alkyne bridge carbon atoms in such complexes are 2σ(I) abs cor Tmax, Tmin no. of data/params goodness of fit on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data) Flack param ∆F range, e Å-3 a

3a

4b

7a

C22H26Mo2S3‚0.5C5H12 614.56 monoclinic Cc 14.1852(8) 22.5726(10) 7.9974(3) 90.695(4) 2560.6(2) 4 1.594 1.234 1244 0.47 × 0.16 × 0.11 2.9-30.4 -20 e h e 20 -19 e k e 32 -11 e l e 6 8598 5221 (Rint ) 0.015) 4835 analytical 0.896-0.669 5221/255 1.033 0.022, 0.056 0.028, 0.058 0.07(3) +0.43 to -0.30

C21H25BF4Mo2S3 652.28 monoclinic P21/c 15.7730(15) 8.5305(6) 17.7108(7) 90.285(6) 2383.0(3) 4 1.818 1.354 1296 0.30 × 0.28 × 0.13 2.3-27.0 -20 e h e 2 -10 e k e 1 -22 e l e 22 6768 5206 (Rint ) 0.035) 3502 ι-scans 0.769-0.709 5206/284 0.918 0.042, 0.073 0.101, 0.083

C27H36BCl2F4Mo2NS3 820.34 monoclinic P21/c 10.2490(6) 12.2493(12) 25.9798(17) 91.212(5) 3260.9(4) 4 1.671 1.167 1648 0.30 × 0.13 × 0.05 2.3-26.9 -13 e h e 13 -2 e k e 15 -3 e l e 33 8766 7037 (Rint ) 0.020) 5077 analytical 0.950-0.906 7037/365 1.010 0.039, 0.093 0.071, 0.106

+0.66 to -0.50

+0.85 to -0.88

All measurements were made at 20 °C with Mo KR radiation, λ ) 0.710 73 Å, on a Nonius CAD4 diffractometer.

and afforded in the presence of an equimolar amount of tBuNC (v ) 35 µL) the complex 8 as an orange powder Yield: 204 mg, 92%. 7a. 1H NMR (CD2Cl2, 25 °C; δ): 7.37 & 7.0 (m, 5H, C6H5), 6.76 (s, 1H, MeSCRdCH), 5.21 (s, 5H, C5H5), 5.11 (s, 5H, C5H5), 2.16 (s, 3H, SCH3), 1.96 (s, 3H, SCH3), 1.59 (s, 3H, SCH3), 1.36 (s, 9H, C(CH3)3). 13C{1H} NMR (CD2Cl2, 25 °C; δ): 166.3 (CNC(CH3)3), 151.8 (MeSCRdCH), 139.0, 129.2, 128.9, 127.9 (C6H5), 111.7 (MeSCRdCH), 93.8 (C5H5), 91.4 (C5H5), 59.1 (C(CH3)3), 29.9 (C(CH3)3), 23.9 (SCH3), 22.4 (SCH3), 14.1 (SCH3). IR (KBr pellets; cm-1): 2120 (s) ν(CN), 1150-1050 (s) ν(BF). Anal. Calcd for C26H34NBF4Mo2S3‚CH2Cl2: C, 39.5; H, 4.4; N, 1.7. Found: C, 39.4; H, 4.4, N, 2.2. 7b. 1H NMR (CD2Cl2, 25 °C; δ): 7.4-6.9 (m, 8H, C6H3(CH3)2 and C6H5), 6.86 (s, 1H, MeSCRdCH), 5.27 (s, 5H, C5H5), 5.26 (s, 5H, C5H5), 2.37 (s, 6H, C6H3(CH3)2), 2.28 (s, 3H, SCH3), 1.88 (s, 3H, SCH3), 1.67 (s, 3H, SCH3). 13C{1H} NMR (CD2Cl2, 25 °C; δ): 187.1 (CNC6H3(CH3)2), 150.8 (MeSCRdCH), 139.0127.0 (C6H3(CH3)2 and C6H5), 111.7 (MeSCRdCH), 94.2 (C5H5), 92.1 (C5H5), 24.9 (SCH3), 23.1 (SCH3), 19.5 (CNC6H3(CH3)2), 14.5 (SCH3). IR (KBr pellets; cm-1): 2060 (s) ν(CN), 11501050(s) ν(BF). Anal. Calcd for C30H34NBF4Mo2S3‚CH2Cl2: C, 42.8; H, 4.1; N, 1.6. Found: C, 42.6; H, 4.1, N, 1.7. 8. 1H NMR (CDCl3, 25 °C; δ): 6.83 (s, 1H, CRdCHSCH3), 5.45 (s, 5H, C5H5), 5.11 (s, 5H, C5H5), 3.58 (s, 3H, CO2CH3), 2.37 (s, 3H, CRdCHSCH3), 2.26 (s, 3H, SCH3), 1.71 (s, 3H, SCH3), 1.41 (s, 9H, C(CH3)3). 13C NMR (CDCl3, 25 °C; δ): 175.6 (CO2CH3), 164.2 (CNC(CH3)3), 129.6 (CRdCHSCH3), 100.2 (CRdCHSCH3), 93.7 (C5H5), 91.5 (C5H5), 58.5 (C(CH3)3), 51.8 (CO2CH3), 29.6 (C(CH3)3), 23.6 (SCH3), 21.7 (CRdCHSCH3), 13.4 (SCH3). Reaction of [Mo2Cp2(µ-SMe)3(RCCH)](BF4) (2a,b,e) with CS2. A solution of the complex [Mo2Cp2(µ-SMe)3(RCCH)](BF4) (0.2 g: 2a, 0.30 mmol; 2b, 0.31 mmol; 2e, 0.31 mmol) in CH2Cl2 (15 mL) was stirred with excess CS2 (2 mL) for 2 h at room temperature. The solution turned brownish red. The solvent was then concentrated, and Et2O was added. Brown solids precipitated. They were collected by filtration and then washed with pentane. Yields: 9a (R ) Tol), 184 mg, 80%; 9b (R ) Ph), 186 mg, 85%; 10 (R ) CO2Me), 189 mg, 86%.

9a. 1H NMR (CD3COCD3, 25 °C; δ): 7.31-7.26 (m, 4H, C6H4), 6.44 (s, broad, 5H, C5H5), 6.08 (s, 1H, -CHdCRSCH3), 5.76 (s, broad, 5H, C5H5), 2.36 (s, 3H, C6H4CH3), 2.19 (s, 3H, SCH3), 2.14 (s, 3H, SCH3), 2.06 (s, 3H, -CHdCRSCH3). 13C{1H} NMR (CD Cl , 25 °C; δ): 146.6 (-CHdCR-SCH ), 2 2 3 142.6 (CS2), 140.0, 135.2, 130.1, 129.3 (C6H4), 133.3 (-CHd CRSCH3), 103.1 (C5H5), 94.6 (C5H5), 26.2 (SCH3), 21.1 (C6H4CH3), 16.8 (-CHdCRSCH3), 10.3 (SCH3). Anal. Calcd for C23H27BF4Mo2S5‚CH2Cl2: C, 34.7; H, 3.7. Found: C, 34.8; H, 3.5. 9b. 1H NMR (CD3COCD3, 25 °C; δ): 7.42 (m, 5H, C6H5), 6.28 (s, broad, 5H, C5H5), 6.09 (s, 1H, -CHdCR-), 5.76 (s, broad, 5H, C5H5), 2.14 (s, 3H, SCH3), 2.10 (s, 3H, SCH3), 2.05 (s, 3H, SCH3). 13C{1H} NMR (CD2Cl2, 25 °C; δ): 147.6, 142.3, 137.3, 132.3, 129.7, 129.2, 129.0 (C6H5 + -CHdCR- + CS2), 102.2 (C5H5), 93.9 (C5H5), 26.5 (SCH3), 19.7 (SCH3), 10.9 (SCH3). IR (KBr pellets; cm-1): 1150-1050 (s) ν(BF). Anal. Calcd for C22H25BF4Mo2S5‚CH2Cl2: C, 33.9; H, 3.3. Found: C, 34.2; H, 3.5. 10. 1H NMR (CD3COCD3, 25 °C; δ): 7.93 (s, 1H, CH3SCHd CR-), 6.34 (s, broad, 5H, C5H5), 5.74 (s, broad, 5H, C5H5), 3.72 (s, 3H, CO2CH3), 2.66 (s, 3H, CH3SCHdCR), 2.17 (s, 3H, SCH3), 2.15 (s, 3H, SCH3). 13C{1H} NMR (CD3COCD3, 25 °C; δ): 152.0 (CH3SCHdCR-), 163.3 (CO2CH3), 142.5 (CS2), 127.1 (CH3SCHdCR-), 102.9 (C5H5), 94.4 (C5H5), 52.6 (CO2CH3), 25.7 (CH3SCHdCR), 20.5 (SCH3), 10.2 (SCH3). Anal. Calcd for C18H23BF4Mo2O2S5: C, 30.4; H, 3.2. Found: C, 30.2; H, 3.3. Crystal Structure Determinations of 3a, 4b, and 7a. Pertinent data are summarized in Table 3. In general, the structures were refined with anisotropic displacement parameters for non-hydrogen atoms and hydrogen atoms were subject to riding constraints during refinement.35 However, the hydrogen atoms attached to C5 in 4b and to C9 in 7a were located in difference syntheses and their parameters were then (35) Programs used: Sheldrick, G. M. SHELX97-Programs for Crystal Structure Analysis (Release 97-2); Institu¨t fu¨r Anorganische Chemie der Universita¨t, Tammanstrasse 4, D-3400 Go¨ttingen, Germany, 1998. Farrugia, L. J. WinGX-A Windows Program for Crystal Structure Analysis. J. Appl. Crystallogr. 1999, 32, 837.

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successfully refined. The crystal of 3a contained highly disordered solvent, believed to be n-pentane, which was modeled approximately by including the seven most highly populated atomic sites in the structure factor calculations as C atoms with 50% occupancy. The position of each site was taken from a difference synthesis, and subsequently, only its Uiso parameter was refined. Crystals of 7a contain ordered dichloromethane.

Acknowledgment. We are grateful to N. Kervarec and R. Pichon for the recording of two-dimensional NMR spectra on a Bruker DRX 500 (500 MHz) spectrometer.

Schollhammer et al.

We thank the CNRS, the EPSRC, Glasgow University, and the University of Brest for financial support. Supporting Information Available: For 3a, 4b, and 7a, tables giving details of structure determinations, non-hydrogen atomic positional parameters, all bond distances and angles, anisotropic displacement parameters, and hydrogen atomic coordinates. This material is available free of charge via the Internet at http://pubs.acs.org. OM000966J