DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 2607–2609 2607 Desulfurisation of trithiocarbonates at a dimolybdenum centre: an unexpected insertion into a co-ordinated alkyne Harry Adams, Christopher Allott, Matthew N. Bancroft and Michael J. Morris*,† Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF On reaction with the dimolybdenum alkyne complex [Mo2- {m-C2(CO2Me)2}(CO)4(h-C5H5)2], dialkyl trithiocarbonates (RS)2C]] S were dismantled into sulfido (m-S), thiolate (m-SR) and CSR fragments; remarkably the last of these inserts into the middle of the alkyne to produce a dimetalla-allyl species with a m-C(CO2Me)C(SR)C(CO2Me) ligand.The desulfurisation of organic molecules is an important process which finds widespread application in organic synthesis and materials chemistry, in particular the formation of tetrathiafulvalenes by phosphite-induced coupling of two sulfur heterocycles. 1 Recently we described the unusual reaction of the dimolybdenum alkyne complex [Mo2{m-C2(CO2Me)2}(CO)4- (h-C5H5)2] 1 with cyclic trithiocarbonates (1,3-dithiole-2- thiones) S]] CS2C2R2 (R = CO2Me, SMe or SCOPh) to produce complexes 2, in which the C]] S bond had been cleaved to produce a m-sulfido ligand and a complex organic fragment derived by ring opening of the heterocycle and coupling with the alkyne ligand (Scheme 1).2 Subsequently we showed that further reaction of 2 with sulfur produced dithiolene complexes in which the C2R2 backbone of the original heterocycle was incorporated into the dithiolene ligand.3 Here we describe the reaction of 1 with acyclic dialkyl trithiocarbonates which sheds further light on the mechanism of the processes involved.Treatment of 1 with 1 equivalent of the trithiocarbonates (RS)2C]] S (R = Me, Pri or Bu) in refluxing toluene for 48 h followed by column chromatography aVorded green complexes 3 as the only isolable products (Scheme 1), though yields in general were lower than those of 2.Spectroscopic characterisation‡ indicated that 3 had a structure very similar to 2, though with certain diVerences. Crystals of 3a suitable for X-ray diVraction were obtained from dichloromethane and diethyl ether solution. § The structure is shown in Fig. 1, with selected bond lengths given in the caption. There are two independent molecules in the unit cell, but the only significant diVerence between † E-Mail: M.Morris@sheYeld.ac.uk ‡ Spectroscopic data (NMR in CDCl3, all signals are singlets unless otherwise stated). Satisfactory elemental analyses were obtained for all new compounds. 3a: green solid, 51% yield, m.p. 184–200 8C (decomp.). 1H NMR: d 5.94 (10 H, C5H5), 3.67, 3.53 (both 3 H, CO2Me), 2.18, 1.97 (both 3 H, SMe). 13C NMR (260 8C): d 176.3, 175.5 (both CO2Me), 141.2 (CSMe), 102.7 (CCO2Me), 99.6 (C5H5), 98.0 (CCO2Me), 52.8, 52.1 (both CO2Me), 46.3 (m-SMe), 13.7 (SMe). MS: m/z 603 (M1). 3b: green solid, 24% yield, m.p. 132–133 8C. 1H NMR: d 5.92 (10 H, C5H5), 3.66 (3 H, CO2Me), 3.52 (spt, 1 H, J = 6.6, CH), 3.49 (3 H, CO2Me), 1.19 (d, 6 H, J = 6.6, Me), 1.09 (d, 6 H, J = 6.7, Me), 0.49 (spt, 1 H, J = 6.7 Hz, CH). 13C NMR: d 175.3, 174.8 (both CO2Me), 140.9 (CSPri), 103.1 (CCO2Me), 99.2 (C5H5), 98.1 (CCO2Me), 69.3 (CH), 52.1, 51.6 (both CO2Me), 34.5 (CH), 28.4, 23.8 (both Me). MS: m/z 659 (M1). 3c: green oil, 26% yield. 1H NMR: d 5.92 (10 H, C5H5), 3.66, 3.52 (both 3 H, CO2Me), 2.70 (t, 2 H, J = 7.3, CH2), 1.81 (t, 2 H, J = 7.6, CH2), 1.55–1.08 (m, 8 H, CH2), 0.88 (t, 3 H, J = 7.2, Me), 0.78 (t, 3 H, J = 7.2 Hz, Me). 13C NMR: d 175.3, 174.9 (both CO2Me), 140.8 (CSBu), 102.9 (CCO2Me), 99.2 (C5H5), 98.7 (CCO2Me), 63.5 (CH2), 52.0, 51.5 (both CO2Me), 37.7, 31.2, 29.7, 21.9, 21.8 (all CH2), 13.7, 13.6 (both Me). MS: m/z 688 (M1). them lies in the position of C(13), which in one molecule is almost equidistant from the two metal atoms, but in the other is displaced towards one molybdenum; the former molecule is shown in Fig. 1 and the bond lengths quoted refer to this. The basic structure is indeed very similar to that of 2 and incorporates the now familiar quadruply-bridged MoIV motif. The two molybdenum atoms are joined by a bond of 2.5605(10) Å, even shorter than the 2.5825(7) Å observed in 2a, and are bridged symmetrically by the sulfido ligand S(3) and by the methanethiolate sulfur S(1). The methyl substituent on the bridging thiolate group is pointing away from the dimetallaallyl ligand, whereas in 2 it is constrained to point towards it by the linking chain.The main point of interest however lies in the dimetalla-allyl ligand itself. By analogy with 2 we had expected the carbon atom derived from the trithiocarbonate to be situated at the terminus of this ligand, i.e. m-C(SMe)C(CO2- Me)]] C(CO2Me). Instead, remarkably, it occupies the central position in a m-C(CO2Me)C(SMe)]] C(CO2Me) arrangement. Although in the molecule shown all three carbons of this ligand are equidistant from both metal atoms within experimental error, the bonds from the metals to the central carbon C(13) are much longer than those to the terminal carbons C(11) and C(12).The 13C NMR spectrum of 3a contains three peaks at d 141.2, 102.7 and 98.0 assigned to the carbons of the dimetalla-allyl ligand. This can be compared with 2a–2c, where all three peaks occur with very similar shifts in the region d 108–113.Since the two terminal carbons C(11) and C(12) are Scheme 1 C Mo C O OC Mo CO CO C MeO2C CO2Me S C C S C C C S Mo Mo R R MeO2C CO2Me S S S R R S R MeO2C C C C S Mo Mo RS CO2Me RS RS S 1 toluene, heat 3a: R = Me 3b: R = Pri 3c: R = Bu 2a: R = CO2Me 2b: R = SMe 2c: R = SCOPh toluene, heat § Crystal data for 3a: C19H22Mo2O4S3, M = 602.43, monoclinic, space group P21/n (a non-standard setting of P21/c, C5 2h, no. 14), a = 10.086(3), b = 13.077(3), c = 33.601(7) Å, b = 95.00(2)8, U = 4415(2) Å3, Z = 4, Mo-Ka radiation (l � = 0.710 73 Å), m(Mo-Ka) = 1.442 mm21, T = 293(2) K; 10 070 reflections measured, 7770 independent reflections (Rint = 0.0331), R1 = 0.0406 for 7765 unique data. CCDC reference number 186/1070.See http://www.rsc.org/suppdata/dt/1998/2607 for crystallographic files in .cif format.2608 J. Chem. Soc., Dalton Trans., 1998, Pages 2607–2609 Fig. 1 Molecular structure of one of the two independent molecues of complex 3a in the crystal (50% probability ellipsoids, H atoms omitted for clarity).Selected bond lengths (Å): Mo(1)]Mo(2) 2.5605(10), Mo(1)]S(1) 2.476(2), Mo(2)]S(1) 2.466(2), Mo(1)]S(3) 2.318(2), Mo(2)]S(3) 2.322(2), Mo(1)]C(11) 2.173(5), Mo(2)]C(11) 2.152(5), Mo(1)]C(12) 2.204(6), Mo(2)]C(12) 2.203(6), Mo(1)]C(13) 2.564(5), Mo(2)]C(13) 2.594(6), C(11)]C(13) 1.429(7), C(12)]C(13) 1.393(8) Scheme 2 Mo Mo SR S R S MeO2C MeO2C Mo Mo SR S MeO2C MeO2C Mo C MeO2C CO2 Me S C SR SR Mo C MeO2C CO2 Me C SR S R S SR or 3 alkyne bond breaks C–C(SR) bond breaks 2 C Mo C Mo in very similar environments, we assign the low field 13C NMR signal to the central CSMe feature.In the molecule shown C(13) is within bonding distance of both metals, but in the second molecule there is one short distance, Mo(1A)]C(13A) [2.509(6) Å] and one which is much longer, Mo(2A)]C(13A) [2.645 Å]. As in complex 2, the observation of equivalent h-C5H5 ligands in the NMR spectra of 3, even at low temperature, implies that a fluxional process is occurring in solution in which the central carbon of the dimetalla-allyl ligand is flipping back and forth between the two metal atoms, rendering them equivalent.The observation of two molecules in the unit cell which diVer only in the position of C(13) (and its associated substituent) provides additional evidence that this trajectory is plausible, since the energy diVerence between these two positions is evidently small. In our previous paper we hypothesized that the first step of the reaction mechanism was loss of a CO ligand, co-ordination of the thione group and cleavage of the C]] S bond to give a dithiocarbene.2 This was followed by coupling of the carbene carbon to the alkyne and cleavage of one of the C]S bonds.Obviously this mechanism is inadequate to explain the foation of 3, though it may still be correct for 2. We now propose that, after carbene formation, cleavage of the C]S bond occurs either before or after coupling with the alkyne, leading ultimately to the formation of a three-membered ring (Scheme 2).Cleavage of the C]C bond of the alkyne would then provide the observed product 3. Positioning of the CSR group in the centre of the dimetalla-allyl fragment is not possible in the case of 2 because it is anchored to the thiolate bridge through the spacer group, but this mechanism could still account for the formation of 2 by scission of one of the two C(CO2Me)]C(SR) bonds of the three-membered ring.The apparent insertion of carbyne ligands into the centre of an alkyne is not without precedent. For example, treatment of [WFe2(m3-CC6H4Me)(CO)8(h-C5H5)] with C2Ph2 gave two dimetalla-allyl complexes, one of which was [WFe- {m-CPhC(C6H4Me)CPh}(CO)5(h-C5H5)] with a rearranged chain.4 Moreover the compound [W2(m-CSiMe3)(m-CMe- CMeCSiMe3)(CH2SiMe3)4], reported by Chisholm et al., was found to undergo a fluxional process in which the substituents ofJ.Chem. Soc., Dalton Trans., 1998, Pages 2607–2609 2609 the bridging ligand changed places, i.e. CMeCMeCSiMe3 interconverted with CMeC(SiMe3)CMe, for which a similar threemembered ring intermediate was proposed.5 Other examples involving dimetalla-allyl ligands are known on trinuclear metal centres.6 Further studies on the reactivity of the unusual species 2 and 3 are currently in progress in our laboratory. Acknowledgements We thank the EPSRC for a studentship (to M.N. B.). References 1 M. Narita and C. U. Pittman, jun., Synthesis, 1976, 489; A. Krief, Tetrahedron, 1986, 42, 1209; J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini and M.-H. Whangbo, Organic Superconductors: Synthesis, Structure, Properties and Theory, Prentice-Hall, Englewood CliVs, NJ, 1992. 2 H. Adams, M. N. Bancroft and M. J. Morris, Chem. Commun., 1997, 1445. 3 A. Abbott, M. N. Bancroft, M. J. Morris, G. Hogarth and S.P. Redmond, Chem. Commun., 1998, 389. 4 F. G. A. Stone, J. C. JeVery, K. A. Mead, H. Razay, M. J. Went and P. Woodward, J. Chem. Soc., Dalton Trans., 1984, 1383. 5 M. H. Chisholm, J. A. Heppert and J. C. HuVman, J. Am. Chem. Soc., 1984, 106, 1151. 6 E. Sappa, A. Tiripicchio and A. M. Manotti Lanfredi, J. Chem. Soc., Dalton Trans., 1978, 552; M. J. Morris, Ph.D. Thesis, University of Bristol, 1984. Received 2nd June 1998; Communication 8/04138EJ. Chem. Soc., Dalton Trans., 1998, Pages 2607–2609 2609 the bridging ligand changed places, i.e.CMeCMeCSiMe3 interconverted with CMeC(SiMe3)CMe, for which a similar threemembered ring intermediate was proposed.5 Other examples involving dimetalla-allyl ligands are known on trinuclear metal centres.6 Further studies on the reactivity of the unusual species 2 and 3 are currently in progress in our laboratory. Acknowledgements We thank the EPSRC for a studentship (to M. N. B.). References 1 M. Narita and C. U. Pittman, jun., Synthesis, 1976, 489; A. Krief, Tetrahedron, 1986, 42, 1209; J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini and M.-H. Whangbo, Organic Superconductors: Synthesis, Structure, Properties and Theory, Prentice-Hall, Englewood CliVs, NJ, 1992. 2 H. Adams, M. N. Bancroft and M. J. Morris, Chem. Commun., 1997, 1445. 3 A. Abbott, M. N. Bancroft, M. J. Morris, G. Hogarth and S. P. Redmond, Chem. Commun., 1998, 389. 4 F. G. A. Stone, J. C. JeVery, K. A. Mead, H. Razay, M. J. Went and P. Woodward, J. Chem. Soc., Dalton Trans., 1984, 1383. 5 M. H. Chisholm, J. A. Heppert and J. C. HuVman, J. Am. Chem. Soc., 1984, 106, 1151. 6 E. Sappa, A. Tiripicchio and A. M. Manotti Lanfredi, J. Chem. Soc., Dalton Trans., 1978, 552; M. J. Morris, Ph.D. Thesis, University of Bristol, 1984. Received 2nd June 1998; Communication 8/04138E