Synthesis of a Thiocarbamoyl Alkylidyne Complex and Caveats

Nov 2, 2010 - Synopsis. The successive reactions of [Mo(≡CBr)(CO)2(Tp*)] (1; Tp* ..... metal double and triple bond: Annual survey covering the year...
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Organometallics 2010, 29, 6482–6487 DOI: 10.1021/om1008296

Synthesis of a Thiocarbamoyl Alkylidyne Complex and Caveats Associated with the Use of [Mo(tCLi)(CO)2(Tp*)] (Tp* = Hydrotris(3,5-dimethylpyrazol-1-yl)borate) Annie L. Colebatch, Anthony F. Hill,* Rong Shang, and Anthony C. Willis Research School of Chemistry, Institute of Advanced Studies, Australian National University, Canberra, Australian Capital Territory, Australia Received August 25, 2010

The successive reactions of [Mo(tCBr)(CO)2(Tp*)] (1; Tp*=hydrotris(3,5-dimethylpyrazol-1yl)borate) with nBuLi and N,N0 -dimethylthiocarbamoyl chloride provides as intended, via formation of [Mo(tCLi)(CO)2(Tp*)] (2a), the thiocarbamoyl alkylidyne complex [Mo{tCC(dS)NMe2}(CO)2(Tp*)] (3). Although the yields are poor, analysis of the side products obtained provides insights into caveats to be considered when employing the lithium/halogen exchange protocol in these systems: the 1-pentylidyne [Mo(tCnBu)(CO)2{HB(pzMe2)3}] (4) would appear to arise from competition between n BuBr and the electrophile of choice, and Templeton’s nonclassical vinylidene [Mo2(μ-CCH2)(CO)4(Tp*)2] (5a) most likely arises under strictly anhydrous conditions from nBuBr acting as a proton donor (i.e., E2) rather than electrophile (SN2). The formation of the ethanediylidyne [Tp*(CO)2MotCCtMo(CO)2(Tp*)] (6) may be accounted for by single-electron-transfer (outersphere) oxidation of 2 to provide the radical carbido complex [Mo(tC•)(CO)2{HB(pzMe2)3}] (7), which dimerizes to provide 6. The dimer 6 is also formed alongside 5a in the reaction of 2 with [Fe(η-C5H5)2]PF6, supporting the intermediacy of 7 in the formation of 6. Introduction It is an essential feature of the classical Fischer carbyne (LnMtCR) synthesis1 and the subsequent Mayr modification2 that the ultimate carbyne substituent “R” is introduced in nucleophilic form, via attack at a metal carbonyl.3 Such a strategy imposes limitations upon the range of carbyne substituents “R” available via this methodology (alkyl, aryl, alkenyl, silyl, amino, alkynyl).3 Similarly, the various R-metalhydride elimination/abstraction approaches developed by Schrock4 do not proceed for alkyls with any but the most innocuous hydrocarbyl and silyl carbyne substituents, a notable exception being the recent progress in phosphonio-alkylidene *To whom correspondence should be addressed. E-mail: a.hill@ anu.edu.au. (1) Fischer, E. O.; Kreis, G.; Kreiter, C. G.; M€ uller, J.; Huttner, G.; Lorenz, H. Angew. Chem., Int. Ed. 1973, 12, 564. (2) (a) Mayr, A.; McDermott, G. A.; Dorries, A. M. Organometallics 1985, 4, 608. (b) McDermott, G. A.; Dorries, A. M.; Mayr, A. Organometallics 1987, 6, 925. (3) For reviews on alkylidyne complexes see: (a) Caldwell, L. M. Adv. Organomet. Chem. 2008, 57, 1. (b) Kim, H.-S.; Angelici, R. J. Adv. Organomet. Chem. 1987, 27, 51. (c) Mayr, A.; Hoffmeister, H. Adv. Organomet. Chem. 1991, 32, 227. (d) Mayr, A; Ahn, S. Adv. Transition Met. Coord. Chem. 1996, 1, 1. (e) Transition Metal Carbyne Complexes; Kreissl, F. R., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; NATO ASI Series C392. (f) Gallop, M. A.; Roper, W. R. Adv. Organomet. Chem. 1986, 25, 121. (4) (a) Schrock, R. R. Chem. Commun. 2005, 2773. (b) Schrock, R. R. Chem. Rev. 2002, 102, 145. (5) (a) Li, X.; Wang, A.; Sun, H.; Wang, L.; Schmidt, S.; Harms, K.; Sundermeyer, J. Organometallics 2007, 26, 3456. (b) Li, X.; Wang, A.; Wang, L.; Sun, H.; Harms, H.; Sundermeyer, J. Organometallics 2007, 26, 1411. (c) Li, X.; Schopf, M.; Stephan, J.; Kipke, J.; Harms, K.; Sundermeyer, J. Organometallics 2006, 25, 528. (d) Li, X.; Stephan, J.; Harms, K.; Sundermeyer, J. Organometallics 2004, 23, 3359. pubs.acs.org/Organometallics

Published on Web 11/02/2010

and -alkylidyne chemistry developed by Sundermeyer.5 Alkyne (RCtCR) cleavage by [W2(OtBu)6] offers considerable generality but again requires that the alkyne substituents (“R”, ultimately the carbyne substituent in RCtW(OtBu)3) are devoid of polar functional groups.6 With these limitations, it is not surprising that there is a dearth of carbyne complexes in which the carbyne substituent is electron withdrawing (π-acidic, negatively mesomeric). Lalor’s halocarbynes7 provide a late common intermediate en route to variously substituted carbyne complexes via nucleophilic halide substitution,8 in some cases metal-mediated,9 though again the carbyne substituent is introduced in nucleophilic form. We have recently described the reaction of the bromocarbyne complex [Mo(tCBr)(CO)2(Tp*)] (1; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) with nBuLi (THF, -78 C), (6) Listemann, M. L.; Schrock, R. R. Organometallics 1985, 4, 74. (7) (a) Lalor, F. J.; Desmond, T. J.; Cotter, G. M.; Shanahan, C. A.; Ferguson, G.; Parvez, M.; Ruhl, B. J. Chem. Soc., Dalton Trans. 1995, 1709. (b) Desmond, T.; Lalor, F. J.; Ferguson, G.; Parvez, M. J. Chem. Soc., Chem. Commun. 1983, 457. (8) (a) Lalor, F. J.; O’Neill, S. A. J. Organomet. Chem. 2003, 684, 249. (b) Chaona, S.; Lalor, F. J.; Ferguson, G.; Hunt, M. M. J. Chem. Soc., Chem. Commun. 1988, 1606. (c) Desmond, T.; Lalor, F. J.; Ferguson, G.; Parvez, M. J. Chem. Soc., Chem. Commun. 1984, 75. (d) Weber, L.; Dembeck, G.; Boese, R.; Blaeser, D. Organometallics 1999, 18, 4603. (e) Weber, L.; Dembeck, G.; Boese, R.; Blaser, D. Chem. Ber. 1997, 130, 1305. (f) Woodworth, B. E.; Templeton, J. L. J. Am. Chem. Soc. 1996, 118, 7418. (g) Jamison, G. M.; White, P. S.; Templeton, J. L. Organometallics 1991, 10, 1954. (h) Etienne, M.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1991, 113, 2324. (i) Cordiner, R. L.; Hill, A. F.; Wagler, J. Organometallics 2008, 27, 4532. (9) (a) Armitt, D. J.; Bruce, M. I.; Gaudio, M.; Zaitseva, N. N.; Skelton, B. W.; White, A. H.; Le Guennic, B.; Halet, J.-F.; Fox, M. A.; Roberts, R. L.; Hartl, F.; Low, P. J. Dalton Trans. 2008, 6763. (b) Bruce, M. I.; Cole, M. L.; Gaudio, M.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2006, 691, 4601. (c) Cordiner, R. L.; Gugger, P. A.; Hill, A. F.; Willis, A. C. Organometallics 2009, 28, 6632. r 2010 American Chemical Society

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Scheme 1. Reactions of the Lithiocarbyne Complex 2a with Electrophiles (E = H, Me, SiMe3, SPh, SePh, Fe(CO)2(η-C5H5))10

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Scheme 3. Reactions of 1 with nBuLi and ClC(dS)NMe2

Scheme 2. Templeton’s Synthesis of a Benzoylcarbyne Complex11

which provides in situ the lithiocarbyne complex [Mo(tCLi)(CO)2(Tp*)] (2a),9b,10 akin to Templeton’s previously reported tungsten analogue 2b.11 As with Cummins’ anionic carbido complex [Mo(C){NtBu(C6H3Me2-3,5)3][K(benzo-15-crown5)],12 the facile generation of 2a affords in principle a late common synthetic intermediate for the synthesis of a wide range of variously functionalized alkylidyne complexes via reactions with suitable electrophiles (Scheme 1).9c,10 We have therefore investigated the possibility of preparing carbyne complexes with electron-withdrawing (π-acidic) substituents and report herein the reaction of 2a with N,N-dimethylthiocarbamoyl chloride, not because the strategy was especially effective in this case (though successful), but because the complications experienced provide some insights into caveats associated with the use of 2a.

Results and Discussion Templeton has shown that the tungsten lithiocarbyne [W(tCLi)(CO)2(Tp*)] (2b) reacts with benzoyl bromide to provide the benzoyl carbyne complex [W{tCC(dO)Ph}(CO)2(Tp*)] (Scheme 2).11 Given that the reaction of [Mo(CO)3(Tp)]K with N,N-dimethyldithiocarbamoyl chloride results in the formation of the thiocarbamoyl complex [Mo(η2SCNMe2)(CO)2(Tp)],13 it was of interest to establish whether the molybdenum center of 2a would be the site of electro(10) Cordiner, R. L.; Hill, A. F.; Wagler, J. Organometallics 2008, 27, 5177. (11) (a) Enriquez, A. J.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 4992. (b) Jamison, G. M.; White, P. S.; Harris, D. L.; Templeton, J. L. In Transition Metal Carbyne Complexes; Kreissl, F. R., Ed.; Proceedings of the NATO Advanced Research Workshop on Transition Metal Carbyne Complexes, Wildbad Kreuth, Germany; Kluwer Academic: Dordrecht, The Netherlands, 1992; p 201. (c) Jamison, G. M.; Bruce, A. E.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1991, 113, 5057. (12) (a) Peters, J. C.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997, 1995. (b) Greco, J. C.; Peters, J. C.; Baker, T. A.; Davis, W. M.; Cummins, C. C.; Wu, G. J. Am. Chem. Soc. 2001, 123, 5003. (c) Agapie, T.; Diaconescu, P. L.; Cummins, C. J. Am. Chem. Soc. 2002, 124, 2412. (13) Anderson, S.; Cook, D. J.; Hill, A. F. Organometallics 2001, 20, 2468.

philic attack with ClC(dS)NMe2 or if previously unknown thiocarbamoyl carbyne complexes might be accessed. Treating a solution of 1 in tetrahydrofuran at -78 C with 1 equiv of nBuLi followed by addition of ClC(dS)NMe2 and warming to 0 C (30 min) and then room temperature affords after chromatographic workup, inter alia, four predominant compounds, 3-6 (Scheme 3). The complex 3 was the desired thiocarbamoyl carbyne complex [Mo{tCC(dS)NMe2}(CO)2 (Tp*)], obtained in disappointing yield (10%). The complexes 4-6 will be discussed below. The formulation of 3 rests on spectroscopic, analytical, and crystallographic data (vide infra, Figure 1). The infrared spectrum of 3 (CH2Cl2) includes two strong carbonyl-associated absorptions (νCO 2006, 1925 cm-1). Table 1 presents selected infrared data for representative alkylidyne complexes with a variety of more conventional carbyne substitutents, from which it is clear that the thiocarbamoyl carbyne is a potent π-acid ligand, consistent with the positive thiocarbamoyl Hammett parameter (σP = 0.23).18 (14) kCO = (2.0191  106)(νs2 þ νas2) N m-1: (a) Kraihanzel, C. S.; Cotton, F. A. Inorg. Chem. 1963, 2, 533. (b) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432. (15) Brower, D. C.; Stoll, M.; Templeton, J. L. Organometallics 1989, 8, 2786. (16) Jamison, G. M.; White, P. S.; Templeton, J. L. Organometallics 1991, 10, 1954. (17) Hart, I. J.; Hill, A. F.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1989, 2261.

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Colebatch et al. Chart 1. Valence Bond Descriptions of Carbyne Ligands Bearing (a) π-Donor (D) and (b) π-Acceptor (A) Substituents

Scheme 4. Alternative Routes for the Reaction of 2 with nBuBr (E2 vs SN2)

Figure 1. Molecular geometry of complex 3 in the crystal form (60% displacement ellipsoids at 200 K; pyrazolyl hydrogen atoms omitted, carbon atoms in gray, heteroatoms with octant hatching). Selected bond distances (A˚) and angles (deg): Mo1N11 = 2.292(3), Mo1-N13 = 2.217(3), Mo1-N15 = 2.234(3), Mo1-C1 = 1.808(4), Mo1-C11 = 2.006(5), Mo1-C12 = 2.013(5), S2-C2 = 1.668(4), N1-C2 = 1.335(6), N1-C3 = 1.453(6), N1-C4 = 1.455(6); N11-Mo1-N13 = 80.79(12), N11-Mo1-N15 = 83.58(12), N13-Mo1-N15 = 83.01(12), N13-Mo1-C1 = 102.65(15), N15-Mo1-C1 = 101.26(16), C1-Mo1-C11 = 81.13(18), C1-Mo1-C12 = 83.71(18), C11Mo1-C12 = 84.64(18), C2-N1-C3 = 121.8(4), C2-N1-C4 = 122.3(4), C3-N1-C4 = 115.9(4), Mo1-C1-C2 = 166.9(3), C1-C2-N1 = 117.7(4), C1-C2-S2 = 117.0(3), N1-C2-S2 = 125.4(3). Table 1. Illustrative Infrared Data for Selected Carbyne Complexes [Mo(tCR)(CO)2(Tp*)]a R

νs (cm-1)

νas (cm-1)

kCO3a,14 (Nm-1)

NEt28d Me15 OPh16 SPh8d SiMe311b Ph16 CtCtBu17 H (8)11 Cl7 C(S)NMe2 (3) DMAP(þ)8d

1949 1982 1982 1989 1993 1979 1993 2001 2005 2006 2012

1850 s 1889b 1889 1905 1908 1890b 1908 1913c 1921d 1925 1929e

14.56 15.14 15.14 15.29 15.35 15.36 15.37 15.47 15.57 15.58 15.66

c

a Data from CH2Cl2 solution unless otherwise indicated. THF. d C6H12. e DMAP(þ) = 4-(dimethylamino)pyridinium.

b

KBr.

The 1H NMR spectrum of 3 reveals two chemically distinct N-CH3 resonances (C6D6: δH 2.77, 2.89 ppm), indicating the anticipated restricted rotation about the thioamide bond, a feature also apparent in the 13C{1H} NMR spectrum (δC 40.2, 40.4). The low-field region of this spectrum includes resonances at 267.3, 227.3, and 191.7 ppm attributable to the carbyne, carbonyl, and thiocarbamoyl carbon nuclei, respectively. The appearance of a single carbonyl resonance is consistent with either rapid rotation of the carbyne ligand on the 13C NMR time scale or, alternatively, a static groundstate structure in which the thiocarbamoyl substituent lies in the mirror plane straddled by the Mo(CO)2(Tp*) group. In the solid state, this is indeed the geometry that is observed, as depicted in Figure 1, which also includes selected geometrical parameters. Structural data for poly(pyrazolyl)borate-ligated alkylidyne complexes have recently been compiled,3a and within (18) Creary, X.; Aldridge, T. E. J. Org. Chem. 1988, 53, 3888.

this context, those for 3 associated with the “Mo(CO)2Tp*” fragment are generally unremarkable and call for little comment. The “MotCC(dS)NMe2” group provides the focus of interest and is found to be essentially linear at C(1) (Mo-C(1)-C(2) = 166.9(3)). Departure from linearity in carbyne complexes is a commonly encountered phenomenon that is generally attributed to crystal-packing forces,3a with deformations as pronounced as 152 having been noted.19 The Mo-C1 separation of 1.808(4) A˚ is unremarkable, given that MotC bond lengths for hydrocarbyl alkylidyne complexes [Mo(tCR)(CO)2(Tp*)] typically span the range 1.801.81 A˚.3 Positively mesomeric carbyne substituents (Chart 1a), e.g., amino (NEt2, 1.853(3) A˚),8d thiolato (SPh, 1.820(3) A˚),8d and selenolato (SeMe, 1.823(2) A˚)10a,19 groups, typically lead to modest lengthening of the metal-carbon bond length relative to those for simple hydrocarbyl substituents: e.g., in the oft-cited “2-azavinylidene” valence bond descriptor.20 Strongly negatively mesomeric substituents might also be expected to lead to bond lengthening, although the charge localization is reversed. However, this is not evident for 3. The second reaction product was identified as the n-pentylidyne complex [Mo(tCnBu)(CO)2(Tp*)] (4) on the (19) (a) Caldwell, L. M.; Hill, A. F.; Rae, A. D.; Willis, A. C. Organometallics 2008, 27, 341. (b) Caldwell, L. M.; Hill, A. F.; Wagler, J.; Willis, A. C. Dalton Trans. 2008, 3538. (20) (a) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Michelin, R. A. Coord. Chem. Rev. 2001, 218, 43. (b) Pombeiro, A. J. L.; Fatima, M.; Guedes da Silva, C. J. Organomet. Chem. 2001, 617, 65. (c) Rosenblat, M.-C.; Henderson, R. A. Inorg. Chim. Acta 2002, 331, 270. (d) Filippou, A. C.; Portius, P.; Jankowski, C. J. Organomet. Chem. 2001, 617, 656. (e) Lungwitz, B.; Filippou, A. C. J. Organomet. Chem. 1995, 498, 91. (f) Gamble, A. S.; White, P. S.; Templeton, J. L. Organometallics 1991, 10, 693. (g) Kim, H. P.; Angelici, R. J. Organometallics 1986, 5, 2489. (h) Schubert, U.; Neugebauer, D.; Hofmann, P.; Schilling, B. E. R.; Fischer, H.; Motsch, A. Chem. Ber. 1981, 114, 3349. (i) Fischer, E. O.; Huttner, G.; Kleine, W.; Frank, A. Angew. Chem. 1975, 87, 781.

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Chart 2. Bridging (μ-η2:η2) Coordination of CAH2 (A = C, B) Ligands: (a) [M2(μ-CCH2)(CO)4(Tp*)2] (M = Mo (5a),W (5b)11); (b) [Fe4(μ-H)(μ-CBH2)(CO)12];25 (c) [FeRu5(μ-CCH2)(μ-SMe2)2(μ-PPh2)2(CO)13]a,26

Figure 2. Molecular geometry of the complex 5b in a crystal of 5b 3 CH2Cl2 (pyrazolyl hydrogen atoms omitted, 60% displacement ellipsoids at 200 K). Selected bond distances (A˚) and angles (deg): Mo1-C1 = 1.999(3), Mo1-C2 = 2.345(4), Mo2-C1 = 1.981(4), Mo2-C2 = 2.297(4), C1-C2 = 1.421(5); Mo1-C1C2 = 84.8(2), Mo2-C1-C2 = 83.2(2), Mo1-C2-Mo2 = 116.97(16).

basis of mass spectrometric and analytical data and by comparison of spectroscopic data with those reported for the related alkylidynes [Mo(tCR)(CO)2(Tp*)] (R = Me, Et, nPr).15 Typically, primary halides only undergo E2 reactions with sterically encumbered bases, which would appear to be the case for 2a. Keeping the solution of 2a below room temperature thus appears to favor the alternative SN2 reaction, with n BuBr acting as an electrophile rather than a (weak) acid (Scheme 4). The formation of small amounts of 8 could be observed (THF, νCO 2001, 1913 cm-1); however, this complex is thermally unstable and dimerizes to the nonclassical vinylidene complex [Mo2(μ,η2:η2-CCH2)(CO)4(Tp*)2] (5a) first described by Templeton from the reaction of [Mo(tCSiMe2Ph)(CO)2(Tp*)] with moist fluoride sources.11 The corresponding tungsten analogue 5b was structurally characterized by Templeton, and the results were interpreted with recourse to extended H€ uckel molecular orbital theory. The identity of 5a was established by comparison of spectroscopic data with those reported by Templeton and by crystallographic studies of the solvates 5a 3 CH2Cl2 and 5a 3 3/2C6H14. Structural data for 5a in a crystal of 5a 3 CH2Cl2 do not differ (21) (a) Pyykk€ o, P.; Riedel, S.; Patzschke, M. Chem. Eur. J. 2005, 11, 3511. (b) Pyykk€ o, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 186. (c) Pyykk€o, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 12770. (22) (a) Bailey, W. I., Jr.; Collins, D. M.; Cotton, F. A. J. Organomet. Chem. 1977, 135, C53. (b) Bailey, W. I., Jr.; Chisholm, M. H.; Cotton, F. A.; Rankel, L. A. J. Am. Chem. Soc. 1978, 100, 5764. (c) Curtis, M. D.; Klingler, R. J. J. Organomet. Chem. 1978, 161, 23. (d) Klingler, R. J.; Butler, W.; Curtis, M. D. J. Am. Chem. Soc. 1975, 97, 3535. (e) Kern, U.; Kreiter, C. G.; Mueller-Becker, S.; Frank, W. J. Organomet. Chem. 1993, 444, C31. (f) Doherty, N. M.; Elschenbroich, C.; Kneuper, H. J.; Knox, S. A. R. J. Chem. Soc., Chem. Commun. 1985, 170. (g) Bruce, M. I.; Low, P. J.; Werth, A.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1996, 1551. (h) Lang, H.; Blau, S.; Rheinwald, G.; Zsolnai, L. J. Organomet. Chem. 1995, 494, 65. (i) Malisza, K. L.; Girard, L.; Hughes, D. W.; Britten, J. F.; McGlinchey, M. J. Organometallics 1995, 14, 4676. (j) Ruffolo, R.; Decken, A.; Girard, L.; Gupta, H. K.; Brook, M. A.; McGlinchey, M. J. Organometallics 1994, 13, 4328. (k) Kim, D. H.; Khan, B. S.; Lim, S. M.; Bark, K.-M.; Kim, B. G.; Shiro, M.; Shim, Y.-B.; Shin, S. C. J. Chem. Soc., Dalton Trans. 1998, 1893. (l) Bruce, M. I.; Low, P. J.; Ke, M.; Kelly, B. D.; Skelton, B. W.; Smith, M. E.; White, A. H.; Witton, N. B. Aust. J. Chem. 2001, 54, 453. (m) Byrne, L. T.; Griffith, C. S.; Koutsantonis, G. A.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1998, 1575. (n) Mathur, P.; Dash, A. K.; Hossain, M. M.; Satyanarayana, C. V. V.; Rheingold, A. L.; Liable-Sands, L. M.; Yap, G. P. A. J. Organomet. Chem. 1997, 532, 189. (o) Stichbury, J. C.; Mays, M. J.; Davies, J. E.; Raithby, P. R.; Shields, G. P. J. Chem. Soc., Dalton Trans. 1997, 2309. (p) Abriel, W.; Baum, G.; Heck, J.; Kriebisch, K.-A. Chem. Ber. 1990, 123, 1767.

a

Each ruthenium center bears two carbonyl ligands, and the two Ru-Ru bonds carry phosphido bridges, omitted for clarity.

significantly from those for the hexane solvate and are summarized in Figure 2. The general geometric features of 5a are essentially comparable to those described by Templeton for the tungsten analogue, given the comparable covalent radii of tungsten (1.37 A˚) and molybdenum (1.38 A˚).21 The nonclassical vinylidene structure of 5a and its tungsten analogue 5b remains singular, and the preference for this geometry is in contrast with the more common dimolybdatetrahedrane motif.22,23 Geometric preferences for dimetallatetrahedranes have been investigated computationally at various levels of sophistication;24 however, these studies predated Templeton’s discovery and did not consider the avenue of alkyne-vinylidene rearrangements. Templeton’s theoretical study (EHMOT) describes the six-electron bonding in terms of a three-center, four-electron interaction between the dxy orbitals and a nonhybridized py orbital on the vinylidene carbon in addition to an orthogonal three-center, two-electron interaction between the dxz orbitals and the px orbital on the methylene carbon (taking the W-W vector as “x” and the C-C vector as “z”). It might perhaps be noted in passing that a similar binding mode is observed for the boravinylidene (CBH2) ligand in the cluster [Fe4(μ-H)(μ-CBH2)(CO)12] (Chart 2b),25 given the isolobality of the “CCH2” and “CBH2-” fragments. This mode of vinylidene binding has also been observed in the cluster [FeRu5(μ-CCH2)(μ-SMe)2(μ-PPh2)2(CO)13] (Chart 2c);26 however, in both of these cluster examples it should be appreciated that while the Ru2(μ-CCH2) and Fe2(μ-CBH2) geometries are similar to those in 5a,b, the lone “sp” pair on (23) Structural data are not available from the Cambridge Crystallographic Data Centre for the Tp*M-MTp* connectivity however two examples based on the more sterically demanding scorpionate hydrotris(3,5-diisopropylpyrazolyl)borate have been structurally characterized: (a) Kitajima, N.; Singh, U. P.; Amagai, H.; Osawa, M.; Moro-oka, Y. J. Am. Chem. Soc. 1991, 113, 7757. (b) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto, S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 1277. (24) (a) Calhorda, M. J.; Hoffmann, R. Organometallics 1986, 5, 2181. (b) Cotton, F. A.; Feng, X. Organometallics 1990, 5, 2181. (c) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc. 1982, 104, 3858. (d) Thorn, D. L.; Hoffmann, R. Inorg. Chem. 1978, 17, 126. (e) Aggarwal, R. P.; Connelly, N. G.; Crespo, M. C.; Dunne, B. J.; Hopkins, P. M.; Orpen, A. G. J. Chem. Soc., Chem. Commun. 1989, 33. (f) Bott, S. G.; Clark, D. L.; Green, M. L. H.; Mountford, P. J. Chem. Soc., Dalton Trans. 1991, 471. (25) Meng, X.; Rath, N. P.; Fehlner, T. P. J. Am. Chem. Soc. 1989, 111, 3422. (26) Adams, C. J.; Bruce, M. I.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1992, 3057.

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Colebatch et al. Scheme 5. Ethanediylidyne Formation via Catalytic Demercuration of a Bis(carbido) Mercurial27

(Tp*)], which is, in contrast, stable with respect to dimerization.29 This, in addition to the recent isolation of neutral terminal carbido complexes of ruthenium and osmium,30 lends support to the intermediacy of 7•. To test this hypothesis, the reaction of 3 with [Fe(η-C5H5)2]PF6 (a single-electron oxidant)31 was investigated and found to produce a mixture of 6 and the parent methylidyne complex [Mo(tCH)(CO)2(Tp*)] (8), which eventually evolves to 5a.

Conclusions Figure 3. Space-filling representations of (a) the dimolybdatetrahedrane [Mo2(μ-HCCH)(CO)4(η-C5H5)2] and (b) the vinylidene complex 5a. C5H5 and Tp* ligands are shown in blue, and the C2H2 unit is given in green.

the vinylidene carbon is directed toward a metal-metal bond (Chart 2). The unusual binding mode in 5 may be attributed to the steric pressures associated with accommodating two Tp* ligands on adjacent bound metals,23 thereby accounting for the reticence to adopt the more conventional dimetallatetrahedrane (μ^-alkyne) geometry observed for the related complexes [Mo2(μ^-RCCR)(CO)4(η-C5H5)2].22 This is illustrated by considering space-filling representations of the two molecules as depicted in Figure 3. The final product to be isolated was the ethanediylidyne complex [(Tp*)(CO)2MotCCtMo(CO)2(Tp*)] (6), which we have recently obtained more productively via an alternative synthesis involving the demercuration of the bis(carbido) mercurial [Hg{CtMo(CO)2(Tp*)}2] (9) that may be catalyzed by the complex [RhCl(CO)(PPh3)2] (Scheme 5).27 The formation of 6 may be explained by the N,N-dimethythiocarbamoyl electrophile reacting via outer-sphere singleelectron transfer to generate the thiocarbamoyl radical (or its dimer tetramethyldithiooxamide) and the 17-electron neutral carbido complex [•Mo(dC:)(CO)2(Tp*)]• (7), which is apparently prone to dimerization. Templeton has reported the one-electron oxidative dimerization of the anionic vinylidene complex [Mo(dCdCH2)(CO)2(Tp*)]Li to provide the butane-1,4-diylidyne complex [(Tp*)(CO)2MotCC2H4Ct Mo(CO)2(Tp*)].28 Ferrocenium oxidation of [Et4N][Mo(CO)3 (Tp*)] is known to provide the 17-electron species [•Mo(CO)3 (27) Colebatch, A. L.; Cordiner, R. L.; Hill, A. F.; Nguyen, K. T. H. D.; Shang, R.; Willis, A. C. Organometallics 2009, 28, 4394. (28) Woodworth, B. E.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1997, 119, 828.

The initially promising synthetic utility of 2 would appear to be not without caveats, despite numerous successes upon which we will report. Three possible complications are demonstrated by the side products of the reaction of 2 with ClC (dS)NMe2. The possibility of outer-sphere single-electron transfer will presumably be a feature of particular electrophiles, though perhaps a judicious choice of solvent may help to suppress this. The second issue, that of the reaction of 2 with the inevitable nBuBr side product, might in principle be obviated by the use of alternative transmetalating agents,32 an avenue we are currently exploring. However, it should be emphasized that these three side reactions have only rarely dominated in our ongoing work and that 2a generated in this manner remains a synthetically versatile reagent so long as these potential issues are borne in mind.

Experimental Section General Considerations. All manipulations were carried out under a dry and oxygen-free nitrogen atmosphere using standard Schlenk, vacuum-line and inert-atmosphere drybox (argon) techniques, with dried and degassed solvents which were distilled (29) Shiu, K. B.; Lee, L. Y. J. Organomet. Chem. 1988, 348, 357. (30) (a) Carlson, R. G.; Gile, M. A.; Heppert, J. A.; Mason, M. H.; Powell, D. R.; Velde, D. V.; Vilain, J. M. J. Am. Chem. Soc. 2002, 124, 1580. (b) Hejl, A.; Trnka, T. M.; Day, M. W.; Grubbs, R. H. Chem. Commun. 2002, 2524. (c) Caskey, S. R.; Stewart, M. H.; Kivela, J. E.; Sootsman, J. R.; Johnson, M. J. A.; Kampf, J. W. J. Am. Chem. Soc. 2005, 127, 16750. (d) Stewart, M. H.; Johnson, M. J. A.; Kampf, J. W. Organometallics 2007, 26, 5102. (31) We are unable with confidence to exclude the possibility that the 7b component arose from adventitious moisture or insufficiently dried [Fe(η-C5H5)2]PF6 rather than hydrogen abstraction. (32) We are grateful to the founding editor Professor Dietmar Seyferth, who alerted us to this possibility during his consideration of ref 10, including the possibility of employing tBuLi in place of nBuLi or using 2 equiv of nBuLi which, however, in this instance afforded no significant improvements.

Article from either calcium hydride (CH2Cl2) or sodium-potassium alloy and benzophenone (ethers and paraffins). NMR spectra were obtained at 25 C on a Varian Gemini 300BB spectrometer (1H, 299.95 MHz; 13C, 75.428 MHz; referenced to external SiMe4). Elemental microanalysis was performed by the microanalytical service of the Australian National University. Electrospray (ESI) mass spectrometry was performed by the Research School of Chemistry mass spectrometry service. Typically a sample was dissolved in dichloromethane and then diluted with methanol or acetonitrile. Data for X-ray crystallography were collected with a Nonius Kappa CCD diffractometer. The complex [Mo(tCBr)(CO)2(Tp*)] (1)7,10 was prepared according to the indicated published procedure. nBuLi was commercially available (Aldrich) as a 1.60 M solution in hexane. Reaction of [Mo(tCBr)(CO)2(Tp*)] (1) with nBuLi and ClC(dS)NMe2. A solution of [Mo(tCBr)(CO)2(Tp*)] (1; 2.97 g, 5.49 mmol) in tetrahydrofuran (115 mL) was cooled to ca. -78 C (dry ice/propanone) and treated dropwise with a solution of n-butyllithium (3.43 mL, 1.60 M in hexane, 5.49 mmol). The temperature was maintained at ca. -78 C while the resultant yellow-brown solution was stirred for 30 min. This was then treated with solid N,N-dimethylthiocarbamoyl chloride (0.845 g, 6.84 mmol) and stirred at -78 C for 30 min to produce a dark red solution, which was then warmed to 0 C and stirred for a further 30 min. The volatiles were removed under reduced pressure, and the resulting solid was extracted with dichloromethane. The combined extracts were filtered through diatomaceous earth, concentrated, and chromatographed on silica gel. (a). Compound 4. The first yellow fraction was collected by eluting with a mixture of dichloromethane and n-hexane (1:1) and dried using a rotary evaporator to afford 4 as an ocher powder. Yield: 0.37 g (13%, 0.71 mmol). IR (KBr, cm-1): 2526 w νBH, 1977 vs, 1885 vs (CO), 1543 s νCdN, 1203s νMotC. 1 H NMR (C6D6, 25 C): δH 0.78 (t, 3JHH = 7.5, 3 H, CH2CH3), 1.27 (tq, 3JHH = 7.5, 2 H, CH2CH3), 1.70 (tt, 3JHH =7.5, 2 H, MoCCH2CH2), 2.09 (s, 3 H, pzCH3), 2.12 (s, 6 H, pzCH3), 2.46 (s, 3 H, pzCH3), 2.58 (s, 6 H, pzCH3), 5.42 (s, 1 H, pzH), 5.60 (s, 2 H, pzH). 13C{1H} NMR (C6D6, 25 C): δC 308.7 (MoCR), 226.2 (CO), 151.3, 150.7, 144.4, 144.0 (C3,5(pz)), 106.6, 106.3 (C4(pz)), 50.0 (MotCCH2), 29.9, 23.0, 14.0 (CH2CH2CH3), 16.2, 14.8, 12.9, 12.7 (pzCH3). MS-ESI (þ): m/z 520 [M]þ, 464 [M - 2CO]þ, 427 [M - 2CO - nBu]þ. Anal. Found: C, 50.94; H, 5.62; N, 16.43. Calcd for C16H31BN6MoO2: C, 50.98; H, 6.03; N, 16.22. (b). Compounds 5a and 6. Subsequent elution with tetrahydrofuran afforded a brown fraction, which was collected and freed of volatiles to provide crude 3 as a brown solid. Fractional recrystallization of the crude material from dichloromethane and hexane at -18 C afforded initially traces of [Mo2(μCCH2)(CO)4(Tp*)2] (5a) and [Mo2(μ-CC)(CO)4(Tp*)2] (6), which were removed and characterized by X-ray crystallography and comparison of spectroscopic data with those previously published. In the case of 5a, characterization included crystallographic analyses of both 5a 3 CH2Cl2 (obtained by layering hexane upon a saturated solution of 5a in CH2Cl2) and 5a 3 3/2C6H14 (obtained by cooling (4 C) a solution of 5a in hexane). Crystal data for 5a 3 CH2Cl2: C36H46B2Mo2N12O4 3 CH2Cl2, Mr = 1009.27, monoclinic, P21/n, a = 12.5301(2) A˚, b = 17.2161(3) A˚, c = 20.5591(3) A˚, β = 101.1250(10), V = 4351.7(1) A˚3, Z = 4,

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Dc =1.540 Mg m-3, μ(Mo KR) = 0.753 mm-1, T = 200(2) K, brown plate, 0.11  0.16  0.31 mm, 80 604 measured reflections with 2θ e 55, 9971 independent reflections, 9943 absorptioncorrected data used in F2 refinement, 532 parameters, R1 = 0.0377, wR2 = 0.1049 for 5662 reflections with |I| > 2σ(|I|), CCDC 790459. Crystal data for 5a 3 3/2C6H14: C36H46B2Mo2N12O4 3 3/2C6H14, Mr = 1053.61, monoclinic, C2/c, a = 24.5979 (3) A˚, b = 25.1666(3) A˚, c = 17.0814(2) A˚, β = 101.4488(7), V 10363.8(2) A˚3, Z = 8, Dc = 1.350 Mg m-3, μ(Mo KR) = 0.536 mm-1, T = 200(2) K, dark green plate, 0.14  0.35  0.45 mm; 98 357 measured reflections with 2θ e 55, 11 871 independent reflections, 11 838 absorption-corrected data used in F2 refinement, 587 parameters, R1=0.0382, wR2 = 0.1049 for 8588 reflections with |I| > 2σ(|I|), CCDC 790458. Crystal data for 6 3 4CH2Cl2: C18H22BMoN6O2 3 4CH2Cl2, Mr = 630.02, monoclinic, P21/n, a = 11.1842(3) A˚, b = 15.2482 (4) A˚, c = 16.5134(4) A˚, β = 100.3897(17), V = 2770.00(12) A˚3, Z = 4, Dc = 1.511 Mg m-3, μ(Mo KR) = 0.888 mm-1, T = 200(2) K, purple-green dichroic plate, 0.12  0.23  0.25 mm, 51 449 measured reflections with 2θ e 55, 6352 independent reflections, 6226 absorption-corrected data used in F2 refinement, 307 parameters, R1 = 0.0484, wR2 = 0.1351 for 4592 reflections with |I| > 2σ(|I|), CCDC 734144. (c). Compound 3. Removal of the solvent from the filtrate then provided pure 3. Yield: 302 mg (10%). IR (CH2Cl2): 2006 vs, 1925 vs (CO), 1542s (CdN). IR (hexane): 1998 m, 1919 m (CO), 1544 m (CdN). 1H NMR (C6D6, 25 C): δC 2.04 (s, 3 H, pzCH3), 2.10 (s, 6 H, pzCH3), 2.32 (s, 3 H, pzCH3), 2.47 (s, 6 H, pzCH3), 2.77, 2.89 (s, 3 H  2, NCH3), 5.34 (s, 1 H, pzH), 5.60 (s, 2 H, pzH). 13C{1H} NMR (C6D6, 25 C): δC 267.3 (MotC), 227.3 (CO), 191.7 (CS), 151.8, 150.9, 144.8, 144.5 (C3,5(pz)), 106.8, 106.6 (C4(pz)), 40.4, 40.2 (NCH3), 16.0, 14.6, 12.6, 12.4 (pzCH3). MS-ESI (þ): m/z 574 [M þ Na]þ, 546 [M þ Na CO]þ, 518 [M þ Na - 2CO]þ, 496 [HM - 2CO]þ, 88 [CSNMe]þ. Anal. Found: C, 45.88; H, 4.32; N, 17.66. Calcd for C21H27BMoN7O2S: 45.92; H, 4.14; N, 17.85. Crystals of 3 suitable for X-ray diffractometry were obtained by slow evaporation of a solution of 3 in benzene. Crystal data for 3: C21H28BMoN7O2S, Mr = 549.32, monoclinic, P21/n, a = 11.2972(3) A˚, b = 17.1306(7) A˚, c = 14.0712(4) A˚, β = 107.119(2), V = 2602.52(15) A˚3, Z = 4, Dc = 1.402 Mg m-3, μ(Mo KR) = 0.615 mm-1, T = 200(2) K, brown plate, 0.06  0.10  0.13 mm, 32 448 measured reflections with 2θ e 50, 4594 independent reflections, 4594 absorption corrected data used in F2 refinement, 299 parameters, R1 = 0.0333, wR2 = 0.0871 for 2741 reflections with |I| > 2σ(|I|), CCDC 790457.

Acknowledgment. We gratefully acknowledge the financial assistance of the Australian Research Council (Grant Nos. DP0557815 and DP0881692). Supporting Information Available: CIF files giving crystallographic data for 3 (CCDC 790457), 5a 3 CH2Cl2 (CCDC 790459), 5a 3 3/2C6H14 (CCDC 790458), and 6 3 4CH2Cl2 (CCDC 734144). This material is available free of charge via the Internet at http:// pubs.acs.org.